Abstracts No.58 - 9th International Conference on Fission Track Dating & Thermochron, 2000, Lorne

Fission

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International Conference on Fission Track Dating and Thermochronology Lome, Australia February 6 - 11, 2000 SA

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS SERIES # 5 8

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International Conference on Fission Track Dating and Thermochronology Lome, Australia February 6-1 1, 2000 Edited by: W. P. Noble, P. B. O'Sullivan and R. W. Brown GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS SERIES # 5 8

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ISSN 0729-01IX © Geological Society of Australia Incorporated 1999 Copies of this publication may be obtained from the Geological Society of Australia Inc., Suite 706, 301 George Street, Sydney, NSW, 2000 Australia. Example citation for papers in this volume:

Arne, D., 2000. 'Cooling of thrust sheets.' In W. P. Noble, P. B. O'Sullivan and R. W. Brown eds. International Conference on Fission Track dating and Thermochronology, Lome, 2000. Geological Society of Australia Abstracts No. 58, 9-10.

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ORGANISING COMMITTEE

Co-convenors Professor Andrew Gleadow Dr Barry Kohn

University of Melbourne University of Melbourne

Melissa Anderson Dr Dennis Arne Mr David Belton Dr Roderick Brown Dr David Foster Dr Paul Green Professor Peter Kamp Dr Geoffrey Laslett Dr Wayne Noble Dr Paul O'Sullivan Dr Asaf Raza

University of Melbourne Gurtin University University of Melbourne University of Melbourne University of Melbourne Geotrack International Pty Ltd University of Waikato GSIRO Mathematical 8l Information Sciences University of Melbourne University of Melbourne University of Melbourne

Members

Other Assistants Matthias Raab Mattevs Lorencak Ursula Weber Timothy Carter Steven Spencer

University of Melbourne University of Melbourne University of Melbourne University of Melbourne La Trobe University

Scientific Program Committee Dr Barry Kohn, Dr Paul Green Professor Peter Kamp Dr Geoffrey Laslett

University of Melbourne Geotrack International Pty Ltd University of Waikato GSIRO Mathematical & Information Sciences

Editorial Committee Dr Barry Kohn, Dr Paul Green Dr Tony Hurford Dr Diane Seward Dr Paul Fitzgerald

Geological Society of Austral

University of Melbourne Geotrack International Pty Ltd University College, London ETH, Zurich University of Arizona

bstracts Number 58

FT2 T H E ORGANISING COMMITTEE OF THE 9 ™ INTERNATIONAL CONFERENCE ON FISSION TRACK DATING AND THERMOCHRONOLOGY WOULD LIKE TO EXPRESS OUR THANKS TO THE FOLLOWING SPONSORS

AUSTRALIAN GEODYNAMICS COOPERATIVE RESEARCH CENTRE UNIVERSITY OF MELBOURNE FISSION TRACK RESEARCH GROUP AUTOSCAN SYSTEMS GEOTRACK INTERNATIONAL PTY. LTD. F T S T A G E SYSTEMS DONELICK ANALYTICAL INC. ZEISS EUROTRACK GEOLOGICAL SOCIETY OF AUSTRALIA (VICTORIA DIVISION) DEPARTMENT OF NATURAL RESOURCES AND ENVIRONMENT

International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

CONTENTS

ABSTRACTS (IN ALPHABETICAL ORDER BY FIRST AUTHOR)

PAGE 1 AUTHOR INDEX PAGE 361

Geological Society of Australia - Abstracts Number 58

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EXHUMATION HISTORY OF THE NORTHERN APENNINES

E.Abbatei, M.L. Balestrierii, G. Bigazzi^ G.A. Pim3, B.Ventura3, M. Zattin3 and G.G. 2iiffa3 1 Dipartimento di Scienze della Terra, Universita di Firenze,Via La Pira 4,50121 Firenze, Italy. 2 Istituto di Geocronologia e Geochimica Isotopica del CNR, Via C. Maffi 36, 52100 Pisa, Italy. 3 Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, Via Zamboni 67,40127 Bologna, Italy

Nucleation of Alps and northern Apennines (Fig. 1) occurred with collision of Europe with the Adria microplate in the Middle Eocene after Upper Cretaceous oceanic subduction. Collision led to the formation of an orogenic wedge (Ligurian units) and was followed by geothermal rise, uplift, extension and sedimentation on the wedge (epi-Ligurian successions) during Late Eocene-Early Oligocene. From Late Oligocene to Early Miocene opening of the western Mediterranean basin caused detachment and counterclockwise migration of the Sardinia-Corsica continental block from the European plate and west-dipping ensialic subduction of the Adria plate. The Ligurian units were emplaced and progressively thrust onto the Adria margin (Fig. 2). Marine sedimentation of the epi-Ligurian successions continued on top of the migrating thrust system. Outgrowth of foreland basins ahead of the thrust system allowed deposition of thick turbidite successions (Macigno, Mt. Cervarola and Marnosoarenacea Fms) which deposited in elongated foredeeps migrating towards the east and were mainly supplied longitudinally from the Alps. Within this tectonic framework the sedimentary cover and part of the crystalline basement of the Adria continental margin, including the foredeep clastic wedges, became progressively incorporated in the evolving orogen and gave rise to distinct structural units (metamorphosed and unmetamorphosed Tuscan units, Cervarola unit and Romagna-Umbria fold and thrust belt). After the emplacement of Corsica-Sardinia block in its final and present N-S orientation (Early Miocene) the orogenic wedge underwent uplift and erosion. Extension affected the orogenic wedge from middle Miocene in relation with the opening of the Tyrrhenian Sea, and exhumation of the orogenic wedge since Tortonian was accompanied by the eastward, further drifting of the compressive front toward the Adria plate.

Unmetamorphosed Tuscan unit Metamorphic Tuscan unit u m

Cervarola unit

5 - 1 0 m.y. > 1 0 - 2 0 m.y. > 20 m.y.

Romagna-Umbria fold and thrust belt

Figure 1. Geologic sketch of northern Apennines. Age symbols refer to one or groups of samples.

The data from more than 100 samples presented here are related to several transects across the chain. Samples from the Ligurian and Tuscan units were dated using the Population Method whereas those from the Mt. Cervarola and Marnoso-Arenacea Fms by the External Detector Method. Apatite ages are commonly younger than the depositional age of the host rock, thus the apatite fission-track clock must have been reset at least once since deposition. Tracks are partially annealed Geological Society of Austral

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NE

not to scale ^

present day mean topographic level

Figure. 2. Schematic geologic profile across the northern Apennines. The structural units 1,3,4,5 include crystalline basement. 1, Ligurian units; 2, unmetamorphosed Tuscan unit; 3, metamorphic Tuscan unit; 4, Cervarola unit; 5, Umbria-Romagna fold and thrust belt.

only in the easternmost part of the chain and locally in the uppermost structural units (Ligurian units). All the zircon ages are older than the depositional age of the host rocks. Samples from Late Cretaceous to Paleocene sandstones of the Ligurian units yielded ages (6 to 20 Ma) significantly younger than the Late Cretaceous to Eocene subduction in which these sediments were involved. On the contrary two zircon ages (199, 155 Ma) are older. These data indicate that during subduction sandstones reached temperatures to totally anneal fission tracks in apatite, but not in zircon. Stratigraphic evidence shows that after the Middle Eocene collision, the Ligurian units were exhumed (some of them almost to the surface). They cooled and started again to retain tracks. Fission-track data allow recognition of a further burial likely due to the epi-Ligurian sedimentary cover and a final denudation event starting since Late Miocene. Cooling ages from successions deposited on the continental margin of the Adria plate indicate a diachronous exhumation of the chain. The Tuscan unmetamorphosed unit (Fig. 1) yielded apatite ages ranging mostly between 8 and 11 Ma, whereas the ages from the Cervarola and the Marnosoarenacea Fms are significantly younger, mostly between 5 and 7 Ma. All the analysed samples show a mean track length of 13-14 pm indicating a moderate to rapid exhumation similar in all the units. These data suggest that exhumation began in the western part of the chain and affected its central and eastern parts a few million years later. It should be noted that this trend is not detectable inside each structural unit. Differential exhumation is evident across the tectonic contact between the unmetamorphosed Tuscan unit and the Cervarola unit. Besides the suggestion of a younging of the exhumation age towards the east, fission-track data also indicate that extensional tectonics was active during/after exhumation as youngest ages are present in the footwall of the frontal thrusts of the main structural units. The presence of extensional structures inside each tectonic unit is indicated also by the occurrence of anomalous ages, significantly younger than the general exhumation phases. Normal faulting reactivated and offset older compressive structures. A clear example is given by the Apuan Alps tectonic window where the apatite ages of the metamorphic core are some m.y. younger than ages of the surrounding unmetamorphosed Tuscan unit.

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LATE JURASSIC TO M I D CRETACEOUS UNROOFING AND NEOGENE TOPOGRAPHIC EVOLUTION OF THE COASTAL CORDILLERA, NORTHERN C H I L E , RESPONSE TO THE EASTWARD MIGRATION OF THE ANDEAN SUBDUCTION R E G I M E ASSESSED BY F T ANALYSES

P.A.M.Andriessen Faculty of Earth Sciences, Free University, Amsterdam,The Netherlands

The Andes are a 8000 km long mountain chain along the western margin of South America. Characteristic features are the offshore nearby (<150 km) presence of the Peru-Chile trench and eastward dipping subduction zone, the corresponding active volcanic arc onshore and active east-vergent fold and thrust belt along the eastern margin of the mountain chain. The study area of northern Chile and Argentina includes a coastal batholith, cut by the 1000 km long strike-slip Atacama fault system. To the east of the fault, but still within the Coastal Range, are allochthonous Paleozoic crystalline rocks and a Jurassic-Cretaceous basin of volcaniclastic deposits, up to 10 km thick. The fate of the Coastal Cordillera of northern Chile, the domain of a Jurassic-Early Cretaceous magmatic arc and occupying today a fore-arc position at a plate boundary, has been either to be pushed up and eroded or to be moved sideways away from the collision. The Coastal Cordillera is characterized by sub-vertical faults that separate blocks exposing different levels, a clear evidence for differentiated vertical block movements. Geological field relations show that vertical displacement along E-W orientated faults may be significant, but displacement reduces to almost zero within only a distance of 25 km to the east. The distribution in time and space of the volcanic activity at a convergent margin is indicative for changes in oceanic plate directions and rates, and angle of subduction that have taken place in the past. At the north Chilean margin the centre of magmatic activity has moved eastward from Jurassic time to present. The present active zone is situated some 250 to 300 km east of the trench. At the onset of subduction, in the Early Mesozoicum, an equally long distance between the trench and the location of the Jurassic magmatic activity, occupied by a subduction wedge, can be assumed. However, today no subduction wedge is present in front of the Jurassic magmatic arc and this is explained by tectonic erosion at the base of overriding plate. Progressive subduction accommodates the missing material at the margin of the continental plate and this mechanism would then cause the continental margin to be uplifted. This process explains the non-occurrence of a continental shelf along the Pacific coast of South America: the plate margin is tectonically so active that only steep coastal escarpments are formed. The steep coastal slopes have a mean surface elevation of about 1000 m, but reach maximum elevation of about 2000 m, north of the city of Taltal. The geodynamic setting of the Coastal Cordillera has changed from an intra-arc to a fore-arc position and as such it has been exposed to the tectonic and deformation forces associated with successive subduction regimes. Of particular interest are the implications of the changed/changing geodynamics for the tectonothermal evolution of the lithosphere and the extent to which fission track analysis register these effects. The effect on global atmospheric circulation and climate of one of the most prominent and highest mountain chains in the world has long been known. It is therefore important to have constraints on the timing and quantification of the evolution of the topography. The creation of the topography of the Cordillera, up to a maximum of some 2000 m a.s.l. is perhaps not older than 20 Ma, because the Brazilian Shield in the east was still dewatering towards the west before that time. Miocene erosion products of plutonic rocks from the Coastal Cordillera region lay discordantly on top of the Lower Cretaceous, indicating that the Coastal Cordillera has indeed been tectonically active during the Miocene.

Geological Society of ^ H M l i a - Abstracts Number 58

FT2^ee Previous Ar-Ar, K-Ar, Rb-Sr and FT dating studies have shown that an important phase of tectonic uplift with local amounts of unroofing up to 10 km took place in the Early-Mid Cretaceous (Maksaev, 1990; Scheuber and Andriessen, 1990; Andriessen and Reutter, 1994). Sediments related to the uplift of the Coastal Cordillera are found to the east, in the back-arc sedimentary basin at that time. Several zircon, sphene and apatite FT ages yielded, within error, concordant at 120-130 Ma and applying the closure temperature concept it was concluded that the FT ages registered a period of fast cooling and unroofing during Early-Mid Cretaceous. Apatite FT length distributions revealed for some samples high mean track length of about 14 |LLm, but also lower mean fission track lengths, between 12 and 13 |im, were obtained, indicative for a more prolonged stay in the PAZ and a more recent passing through the PAZ to surface temperatures. In this study new FT data of apatite, sphene and zircon from the Coastal Cordillera, between Antofagasta and Taltal are presented. The FT ages are in general similar to the ones obtained in the previous studies, ranging from 60 to 140 Ma. Fission track length measurements revealed mean track lengths between 12.7 to 13.7 |im. Hardly any correlation is detected between altitude and FT apparent apatite age, but this may be due to the sampling of different tectonic block units. Two apatite samples belonging to the same tectonic block and some 2000 m apart in elevation revealed one of the oldest ages (124.5 ± 7.6 Ma) for the highest sample (at 2000 m a.s.l.) and one of the youngest ages (76.4 ± 6.2 Ma) for the sample at sea-level). Thermal modelling using apatite FT age and length data clearly indicates a accelerated cooling rate for the older apatite samples during Late Jurassic-mid Cretaceous and a prolonged period of slow continuous cooling till Miocene. Only in the Miocene the apatite passed through the partial annealing zone to surface temperature. The samples with the younger apatite ages show a moderate to slow continuous cooling from mid-Cretaceous to Neogene. The rocks pass through the PAZ to surface temperatures in the last 10 m.y. It is therefore concluded that FT dating of zircon, sphene and some apatites form the Coastal Cordillera in northern Chile register an important unroofing event during the Late Jura-mid Cretaceous, but all samples outcropping today were still at temperatures of 120 to 90°C, corresponding to a depth of 4 to 5 km, depending on the geothermal gradient). Apatite FT thermochronology shows that the rocks reached the surface and elevation they are outcropping today in the Miocene and or even in more recent times, supporting the idea that topography of the Coastal Cordillera was created in the Neogene. Maksaev, V., 1990: Metallogeny, geological evolution, and thermochronology of the Chilean Andes between latitudes 21 and 16 south, and the origin of major porphyry copper deposits. PhD thesis, Dalhousie University, Halifax, Nova Scotia, Canada. (554 pp). Scheuber, E and Andriessen , RA.M. 1990. The kinematic and geodynamic significance of the Atacama fault system. J. Struct Geol. 12, 243-257. Andriessen, RA.M. and Reutter, K-J. 1994. K-Ar and fission track mineral age determinations of igneous rocks related to multiple magmatic arc systems along the 23 S latitude of Chile and NW Argentina. In: Tectonics of the southern central Andes; structure and evolution of an active continental margin (editors K.-J. Reutter, E. Scheuber and P.J. Wiggers) Springer Verlag, 121-141. Acknowledgments This research is part of NSG and ISES

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TRACKING CHANGES IN EXHUMATION R A T E S U S I N G L O W - T E M P E R A T U R E THERMOCHRONOMETRY: AN EXAMPLE FROM THE WASATCH MOUNTAINS, UTAH

(USA)

P.A.Amistrongi,T.A. Ehlers2, RJJ. Kamp3, K.A. Farley^ and D.S. Chapman2 1 Department of Geological Sciences, California State University, FuUerton, CA, USA. 2 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT, USA. 3 Dept. of Earth Sciences, University of Waikato, Hamilton, New Zealand. 4 Division of Geological and Planetary Sciences, California Institute of Technology Pasadena, CA, USA.

The Wasatch Mountains, Utah, are a classic example of a continental rift margin uplift. The Wasatch fault, located to the west of the north-south trending mountain range, marks the boundary between the relatively stable Colorado Plateau province to the east and the active Basin and Range province to the west. Dramatic topographic relief, ~2 km, and abundant corroborative geologic information make the Wasatch an ideal locale to (1) study exhumation processes of extensional settings using lowtemperature thermochronometers and (2) develop and test thermal models that are used to deduce uplift and exhumation rates. We utilise 2-D and 3-D transient thermal models that account for lateral heat flow, uplift and erosion of the footwall, burial of hanging-wall, and thermal influences of highrelief topography. The central part of the Wasatch contains a series of Tertiary intrusive rocks that range in age from 30.5 to ~38 Ma. Fluid inclusion data, stratigraphic and structural reconstructions, and metamorphic mineral assemblages suggest that intrusion emplacement depths in the central Wasatch ranged from 11 km at the range front to ~1 to the east (John, 1989); these depths imply differential exhumation of -10 km across a distance of 35 km. Parry and Bruhn (1987) used fluid inclusion data from hydrothermally altered footwall rocks to infer 11 km of exhumation at the range front in the last 17 m.y., which implies an exhumation rate of 0.7 mm/yr. Two earlier studies (Evans etal, 1985; Kowallis et al, 1990) reported AFT ages that range from 5 Ma at the Wasatch fauk to 12 Ma on the ridge crest (Fig. la). Using simple 1-D, horizontal isotherm thermal models, they suggested a time-averaged exhumation rate of -0.7 mm/yr for the last 12 m.y. Like the eadier studies, our new central Wasatch AFT ages increase from ~5 Ma at the Wasatch fauk (1500 m elev.) to -10 Ma at the highest point of the range front (Lone Peak at 3450 m elev.) (Fig. la). This slight age increase also correlates with distance from the Wasatch fault, to a distance of -15 km into the footwall. The ages, therefore, reflect at least two variables: elevation and distance into the footwall. Farther east, the ages increase to a maximum of about 30-38 Ma at a distance of 30 km; the oldest ages are concordant with the emplacement ages for the intrusive belt. The corresponding elevation profile shows that east of -15 km into the footwall, ages continue to increase even as elevation decreases (Fig. la). Track lengths, though few for these young-age samples, are relatively long (14.0-15.5 |im) and suggest rapid cooling. We interpret the oldest AFT ages to reflect cooling ages of shallow intrusions in the easternmost part of the Wasatch intrusive bek. AFT ages of 20-30 Ma and long track lengths for deeper intrusions located 25-30 km east of the range front reflect rapid exhumation -20-30 Ma in this area (and possible the entire central Wasatch). These rocks then remained close to the surface as the range tilted eastward. Our 2-D thermal modelling suggests that the average exhumation rate for the last -10 Ma was -0.5 mm/yr at the range front, which is 30% lower than suggested previously. (U-Th)/He ages show considerable scatter in age-elevation space, with ages ranging from 1.6 Ma at the range front to 4-10 Ma at -3000 m elevation (Fig. lb). Like the AFT ages, (U-Th)/ Fie ages increase with increasing distance from the range front. (U-Th)/He ages for samples from topographic high regions between major E-W canyons (ridges) consistently increase with elevation (Fig. lb), but continue to increase in age at the same elevation beyond about 7 km from the range front. Samples from locations in the major canyons are consistently older than corresponding elevation samples from Geological Society of Australia - Abstracts Number 58

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Figure 1. Apatite fission track (a) and (U-Th)/He (b) ages versus elevation for the central Wasatch. Thick dashed lines in (b) show model (U/Th)/He age profiles from thermal model simulations at different exhumation rates in mm/yr.

ridges that are closer to the range front. The ridge/canyon (U-Th)/He age variations, therefore, partially reflect at least three variables: elevation, distance from range front, and topographic region (ridge vs. canyon). Thermal model runs that track ridge samples through helium closure temperature (75°C) depths for samples at different distances from the range front and different ultimate elevations assuming steady state Wasatch topography and constant uplift/exhumation rate. They suggest considerable difference in ridge exhumation rates. At low elevations along the Wasatch fault but on the major ridge salient, exhumation rates were >1 mm/yr for the last 2 m.y. (Fig. lb). However, 7-10 km farther east on the ridge, exhumation rates are <0.5 mm/yr. These rate variations reflect as much as 10 degrees of eastward tik of the central Wasatch in the last 2 m.y. and a doubling of range front exhumation rate over that time relative to the average rate for the last 10 m.y. The apparently high rate of range front exhumation and eastward range tilt in the last 2 m.y. may have caused the conspicuous convex outward shape of range-front ridge profiles for this part of the Wasatch.

The (U-Th)/He ages are corrected for eastward range tilt based on a profile of average age versus distance from the Wasatch fault. Tilt-corrected (U-Th)/He ages along a N-S profile (normal to the main canyons) increase systematically northward from 3 Ma at the south end of the profile to 6 Ma 15 km northward. This age implies higher exhumation rates to the south and possibly a component of northward tilt of the central Wasatch. Geomorphically, northward tilt may have caused south-directed headward erosion of secondary N-S canyons, resulting in southward drainage-basin asymmetry of the major E-W canyons. In summary, combining more realistic multidimensional thermal models with AFT and (U-Th)/He ages allow new constraints to be placed on exhumation rates and topographic development of the central Wasatch Mountains. Our models suggest average exhumation rates have been -0.5 mm/yr along the western range front for the last 10 m.y. In the last 2 m.y., range front exhumation rates have increased to >1 mm/yr, but have been roughly half that at ~7 km into the range resulting in overall eastward tilt. Evans, S.H., Parry, W. T., and Bruhn, R. L. 1985. Thermal, mechanical, an chemical history of Wasatch fault cataclasite and phyllonite, Traverse Mountains area, Salt Lake City, Utah: age and uplift rates from K/Ar and fission track measurements. U.S. Geologic Survey Open File Report 86-31, 410-415. John, D.A. 1989. Geologic setting, depths of emplacement, and regional distribution of fluid inclusions in intrusions of the central Wasatch Mountains, Utah. Economic Geology 84, 386-409. Kowallis, B. J., Ferguson J., and Jorgensen G. J. 1990. Uplift along the Salt Lake segment of the Wasatch fault from apatite and zircon fission track dating in the Little Cottonwood Stock. Nuclear Tracks Radiation Measurements 17, 325-329. Parry, W. T. and Bruhn R. L. 1987. Fluid inclusion evidence for minimum 11 km vertical offset on the Wasatch fault, Utah. Geology 15, 67-70.

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International Conference on Fission Track Dating and Thermochronology

COOLING OF T H R U S T

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D.Arne Western Australian School of Mines, Curtin University of Technology, Kalgoorlie,WA, Australia

Fold and thrust belts are thermally complex regions, being subject to the effects of both regional and tectonic exhumation, as well as groundwater circulation. They are also host to economic accumulations of petroleum in many regions of the world. While the timing of maximum paleotemperatures during burial relative to the formation of structural traps is important for the accumulation of hydrocarbon deposits, the absolute timing of folding and thrusting places important constraints on tectonic models. Thermochronological techniques, such as apatite fission track analysis, can be used at relatively shallow erosional levels to constrain the time of cooling of thrust sheets, but the significance of this cooling is controversial. The timing of peak burial temperatures relative to the formation of structural traps can be determined using paleotemperature indicators such as vitrinite reflectance. Offset of isoreflectance contours indicates that deformation post-dated maximum burial temperatures and, by implication, hydrocarbon generation. Isoreflectance contours that cross structures with little or no offset indicate that hydrocarbon generation post-dated deformation. At shallow erosional levels, most strata in fold and thrust belts underwent maximum burial temperatures prior to deformation. However, at deeper levels in the same region, peak temperatures may post-date deformation, particularly where this has involved tectonic burial by over-riding thrust sheets (Deming and Chapman, 1989). The extent of post-tectonic, regional exhumation determines the crustal level presently exposed at surface. In general, deeper crustal levels are exposed in pre-Tertiary orogenic belts, either due to the effects of post-tectonic isostatic rebound, or exhumation in response to subsequent, unrelated events such as rifting. Thus, apatite fission track data from Paleozoic fold and thrust belts typically reveal only regional-scale exhumation (e.g. Arne, 1992). A number of end-member, conceptual models can be used to describe cooling in fold and thrust belts. Tectonically-driven exhumation during thrusting can lead to cooling of the hanging wall sequences. This model infers that faulting was accompanied by erosion, but the latter is controlled by a number of factors, including climate, local relief and changes in base level (Kooi and Beaumont, 1996). Thus, there may be a lag between tectonic movement and cooling of the thrust sheet due to erosion. Syntectonic sediments in the foot walls of thrust faults can provide an independent constraint on the timing of hanging wall erosion, but these coarse-grained, generally alluvial deposits are often difficuk to date stratigraphically, and their precise relationship to thrusting is controversial (Beck et al, 1988). On a broad scale, fold and thrust belts are subject to regional exhumation due to isostatic rebound following the cessation of thrust stacking and erosion of thrust sheets (Beaumont, 1981). This process will affect all rocks within a fold and thrust bek, regardless of structural position (i.e. hanging wall or foot wall), as well as the adjacent foreland basin with which it is isostatically linked (Omar et al., 1994). Thermal maturity data from the foot walls of thrust faults thus provide a control on the extent of regional cooling due to this effect. A drop in the geothermal gradient in the hanging wall of a thrust sheet may also produce cooling, either due to a juxtaposition of rock packages with contrasting thermal conductivites, or due to changes in heat flow. The latter may result from climatic changes induced by surface uplift that produce an increase in precipitation, and thus the establishment of ground water recharge zones. This effect is likely to be of a regional nature and would not be expected to affect an individual structure (Burtner and Nigrini, 1994)

Geological Society of Australia - Abstracts Number 58

Vitrinite reflectance data can be used to predict if apatite fission track data from the hanging walls of thrust sheets will provide evidence for tectonically-driven exhumation (Arne and Zentilli, 1994). Total annealing of fission tracks in apatite from the previously undisturbed foot wall of a thrust fault indicates significant post-tectonic cooling and, unless there has been reactivation of the structure subsequent to regional cooling, a contrast in fission track age across the structure would not be expected. By contrast, a marked change in vitrinite reflectance data across a thrust fault, with evidence for total annealing of fission tracks in the hanging wall but only partial annealing in the foot wall, indicates that not only has thrusting post-dated peak burial temperatures, but that fission track data from the hanging wall can be used to constrain the timing of its exhumation. These principles are illustrated by fission track data from the eastern margin of the Tibetan Plateau in China (Arne et al, 1997), the foothills region of the Rocky Mountains (Arne and Zentilli, 1994), and the Innuitian Orogen of the Canadian Arctic (Arne et al, 1998). In each case a significant contrast in apatite fission track age occurs at relatively constant elevations across major thrust faults. Apatite fission track data from the hanging walls of individual thrust faults indicate total annealing of fission tracks prior to thrusting, so that the onset of track retention during cooling may be inferred through forward modelling. In two of the three examples for which data are available, vitrinite reflectance data also show a significant break across the structure and were used to determine the suitability of the area for detailed thermochronology. Apatite fission track data can be used to constrain the time of cooling of thrust sheets in certain circumstances. The amount of post-tectonic exhumation should be minimal, as inferred from thermal maturity data in the foot wall of the fault, unless later fault reactivation is suspected. Offsets in thermal maturity data can be used to indicate thrusting post-dated maximum burial temperatures, so that fission track data from the hanging wall of the structure can be directly related to cooling during tectonically-driven exhumation. Erosion may lag behind thrusting in some circumstances, but this is unlikely to be significant where kilometres of section have been removed by faulting. Although the examples presented here refer to apatite fission track and vitrinite reflectance data, the approach is applicable to thermochronological techniques in general. Arne, D.C., 1992. Evidence from apatite fission-track analysis for regional Cretaceous cooling in the Ouachita Mountain fold belt and Arkoma Basin of Arkansas. The American Association of Petroleum Geologists Bulletin, 76, 392-402. Arne, D.C. and Zentilli, M., 1994. Apatite fission track thermochronology integrated with vitrinite reflectance; Re-evaluation of Vitrinite Reflectance as a Maturity Parameter - Applications and Limitations, P.K. Mukhopadhyay and W.G. Dow (eds.), American Chemical Society , pp. 249-268. Arne, D., Worley, B., Wilson, C.J.L., Chen, S., Luo, Z., Liu, S. and Dirks, P., 1997, Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau. Tectonophysics, 280, 239-256. Arne, D.C., Zentilli, M., Grist, A. and Collins, M., 1998. Constraints on the timing of thrusting during the Eurekan Orogeny, Canadian Arctic Archipelago: an integrated approach to thermal history analysis. Canadian Journal of Earth Science, 35, 30-38 Beaumont, C., 1981. Foreland basins. Geophysical journal of the Royal Astronomical Society, 65, 291-329. Beck, R.A., Vondra, C.F., Filkins, J.E. and Olander, J.D., 1988. Syntectonic sedimentation and Laramide basement thrusting, Cordilleran foreland; Timing of deformation. Geological Society of America Memoir 171, 465-487. Burtner, R.L. and Nigrini, A., 1994. Thermochronology of the Idaho-Wyoming thrust belt during the Sevier Orogeney: A new, calibrated, multiprocess thermal model. The American Association of Petroleum Geologists Bulletin, 73, 1455-1471. Deming, D. and Chapman, D.S., 1989- Thermal histories and hydrocarbon generation: Example from Utah-Wyoming thrust belt. Geological Society of America Bulletin, 73, 1455-1471. Kooi, H. and Beaumont, C., 1996. Large-scale geomorphology: Classical concepts reconciled and integrated with contemporary ideas via a surface process model. Journal of Geophysical Research, 101, 3361-3386. Omar, G.L, Lutz, T.M. and Giegengack, R., 1994. Apatite fission track evidence for Laramide and post-Laramide uplift and anomalous thermal regime at the Beartooth overthrust, Montana-Wyoming. Geological Society of America Bulletin, 106, 74-85. Acknowledgements The concepts described here were first developed while the author was employed at Geotrack International, and were subsequently refined and tested during a Killam Post-doctoral Fellowship at Dalhousie University, Halifax.

International Conference on Fission Track Dating and Thermochronology

EXHUMATION A G E S AND BLOCK-FAULTING ON THE EASTERN FLANK OF THE S E R C H I O GRABEN ( N O R T H E R N A P E N N I N E S )

M.L. Balestrieri Department of Earth Sciences, University of Florence, Italy

The northern Apennines are a Tertiary thrust and nappe belt produced by the collision of the European and Adria plates. Its Neogene - Quaternary evolution has been interpreted as an eastward migrating compressive external front, coupled with an extension in the hinterland related to the formation of the Tyrrhenian sea. Numerous extensional basins have developed in the internal part of the chain while extension moved eastward following the front. The Serchio Basin is one of these basins and is particularly interesting due to the fact that it corresponds to a graben bounded by remarkably high relief structures: on the western side the Apuan Alps, with outcrops of the lowermost structural unit, and on the eastern side the highest peaks on the present-day divide of the northern Apennines (Fig.l). On the latter the Macigno Fm (Middle/Late Oligocene - Early Miocene), a thick turbidite succession that constitutes the roof of the Tuscan Nappe, is exposed from 200 m (Garfagnana valley) to c. 2000 m of altitude (Mt. Gomito). Three Macigno sandstone samples were collected from an undeformed succession stretching from the top of the Mt. Gomito to 1200 m altitude towards east. The two highest ones gave apatite fission-track ages of c. 10 Ma, while the lower one yielded an age of c. 6 Ma. Further five samples were collected at decreasing elevations moving westwards from Mt. Gomito along the eastern flank of the Serchio graben that is dissected by several normal faults. Their ages cluster around 10-9 Ma. Two samples, a Macigno pebble and its surroundings sandy matrix, were collected in the Pliocene fluvio-lacustrine filling of the Garfagnana valley and yielded ages of c. 8 Ma. Two other samples were from the granite outcrop of Camporgiano on the western side of the valley. This granite is an olistolith included in the Ligurian sedimentary successions and represents a remnant of the Hercynian continental crust involved in the opening of the Jurassic Ligurian-Piedmont ocean. These samples from an high structural level in the northern Apennine pile yielded the oldest fission-track age found in this area, c. 13 Ma. All the apatite fission-track ages from the Macigno sandstones are 15-20 m.y. younger than the stratigraphic age of this formation. This indicates that a fission track total resetting took place after the deposition of the apatites in the Macigno sedimentary basin and that a restarting of the fission-track recording occurred in more recent times when the samples were brought to the surface. This interpretation is supported also by the track length distributions; although it was possible to measure only a limited number of confined tracks for each sample, the lack of shortened tracks indicates that ages

Apuan Alps 5 km

Garfagnana valley Serchio Basin

_

Mt Gomito 10 6 ^ 5km

_ 0

5 km

5 km

Plio - Quaternary sediments

Tuscan Nappe

Ligurian units

Metamorphic Tuscan unit

Geological Society of Austral

bstracts Number 58

5 km

FT2^ee represent a rapid cooling event. This implies that all the samples, before being exhumed to the surface, were buried at temperatures exceeding 120°C to completely anneal fission-tracks in apatite. Considering a gradient of 25°C/km and a paleo-mean surface temperature of 15°C, they must have been buried at c. 4 km depth. Their cover, now removed, was constituted by a portion of the Macigno Fm plus the Ligurian units and, likely, their associated epi-Ligurian successions. Similar consideration can be applied also to the granite samples that show track length distributions with mean lengths over 14 pm. Thus, also this lower stratigraphic level in the Ligurian successions had to be buried below c. 4 km of cover before finally cooled in the Serravallian. Further information can be obtained looking in detail the data from the northern Apennines ridge. Ages of samples collected in the undeformed block at Mt. Gomito show a correlation with elevation. A rough estimate of the mean rate of exhumation of this block is of 0.15 mm/yr. It is worth noting that these samples were collected moving down topographically but along the same stratigraphic level thus, the attitude of the strata (20°NE) was produced by a deformation phase preceding exhumation. Ages from the eastern flank of the Serchio graben do not show correlation with elevation since similar ages are found from Mt. Gomito summit down to the foot of the valley. This trend can be due to the presence of the normal faults that dissected the flank in a series of downgoing blocks toward the valley axis. Therefore faulting, which led to the formation of the graben, results to be subsequent to the cooling/exhumation event.

International Conference on Fission Track Dating and Thermochronology

2 - D

A N A L Y S I S AND T H E I M P O R T A N C E O F A P A T I T E ( U - T H ) / H E CONVERGENT O R O G E N S : T H E OLYMPIC

F T 2 '

D A T I N G IN

MOUNTAINS

G.E. Batt and M.T. Brandon Department of Geology and Geophysics Yale University PO Box 208109 New Haven CT 06520-8109 USA

Simple 1-D interpretation of thermochronological data as indicators of exhumation can underestimate the unique contribution of apatite (U-Th)/He analysis to the constraint of orogenic evolution. Active orogenic regions are characterized by spatial variation in erosion conditions and crustal character, leading to lateral variation in thermal structure. Convergent motion strongly influences the exhumation rates and thermal conditions experienced during passage through such a region, and the overall thermal history experienced during residence in the orogen is thus dependant on both vertical and horizontal motion In this context, the physical significance of a thermochronological age depends on the extent of lateral variation in conditions experienced between closure at depth and exposure at the surface, and hence on closure temperature. The higher the closure temperature of a system, the earlier during exhumation it will be set, and hence the longer the interval over which it integrates varying conditions. The lower the closure temperature in contrast, the less variability it can experience prior to exposure, and hence the closer the system comes to reflecting conditions at a particular point in space and time. Different thermochronometers thus constrain varying aspects of orogenic character. While higher temperature chronometers (zircon fission track ages, K-Ar ages etc) provide insight into the lateral material path through a deforming region and long-term average regional behaviour, the low (65-75°C) closure temperature of the apatite (U-Th)/He chronometer leave it optimally positioned to constrain the active surface processes across a deforming region. This sensitivity to local conditions also allows apatite (U-Th)/He to provide good insight into the thermal impacts of topography and local variations in crustal character We illustrate the novel constraint offered by apatite (U-Th)/He dating through analysis of the Olympic Mountains of Washington State. We employ a steady state kinematic model to specify the thermal history of accreted materials in the Cascadia accretionary wedge. The 2-dimensional nature of the model also allows spatially discrete data values to be interpreted in a consistent physical context, thus contributing to an overall picture of the evolution of the wedge. This analysis is used to test the hypothesis of steady state flux through the Olympic Mountains, segment of the wedge since 14 Ma, as proposed by Brandon et al (1998), with rates of accretion balanced by rates of erosion of material from the wedge. This is distinct from an evolutionary steady state, where all features of the wedge remain constant with time. Accretionary and erosional fluxes are used to define a steady state velocity field, which forms the kinematic basis of our thermal model. Thermal calculations are defined by a flux of heat across the base of the wedge from the underlying oceanic lithosphere (after Molnar and England, 1995), frictional heating along the decollement, heat production by radioactive decay within the wedge, and the advection of heat by material transport. This coupled modelling approach allows the derivation of thermal histories through assessing the interaction of material paths through the orogen with the evolving thermal structure. Through comparison to the thermal response of different chronometers (fission track annealing or diffusive transport of radiogenic daughter products), these thermal histories provide a means of predicting patterns of thermochronological ages associated with a given physical scenario, which can then be tested against observed ages.

Geological Society of Australia - Abstracts Number 58

FT2 This model indicates that exhumation rates across the Olympics have been steady since 14 Ma. Assuming the subduction geometry of the subduction zone beneath the Olympics has remained unchanged, this supports the hypothesised conditions of steady-state flux over this period. Furthermore, the model shows an across-strike balance in fluxes, so that little or no flux is indicated parallel to the strike of the margin. The results of this analysis illustrate the relative strengths of the various thermochronometers employed. High closure temperature (zircon fission track) ages offer little insight into local conditions in this setting, but are sensitive indicators of style of deformation and overall material paths. Apatite (U-Th)/He ages from this region in contrast are insensitive to lateral motion but provide constraint on local exhumation conditions. Brandon, M. T., M. K. Roden-Tice, and J. I. Garver (1998). Late Cenozoic exhumation of the Cascadia wedge in the Olympic Mountains, northwest Washington state. Geological Society of America Bulletin 110, 985-1009. Molnar, P., and P. England (1995). Temperatures in zones of steady-state underthrusting of young oceanic lithosphere, Earth and Planetary Science Letters 131, 57-70.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

STRATEGY FOR F I S S I O N - T R A C K RECOGNITION VIA DIGITAL IMAGE PROCESSING, AND COMPUTER-ASSISTED TRACK MEASUREMENT

EE Bellonii, N. Keskes2 and AJ. Hurfordi 1 London Fission-Track Research Group, University College London, London, United Kingdom. 2 Dept. IMA3G,CSTJF Elf EP, Pau, France.

Introduction The Fission-Track (FT) community has been interested in the possibility of Digital Image Processing (DIP - vision) for decades. The large amount of information required for any serious statistical analysis, linked to the difficulty and tedium of data procurement, has led it to look at any opportunity to automate such work. A complete processing of FT through DIP techniques can only work if it is possible to develop an algorithm that will recognise, and extract the tracks from the global image of a natural sample. The aim of this contribution is to introduce our current strategy to evolve a process that will achieve this objective. The problem is structured via the classic scheme of DIP that separates vision into three stages, filtering-segmentation-analysis, each relying on different significant properties of tracks, which are introduced for each step. Difficulties in setting up such a task may put off the FT analyst leading him away from other valuable techniques not based upon image processing but using the advantages of a digital camera associated to a computing system (Dc/C couple). Such computer-assisted techniques themselves offer strong support to the FT operative for many of the procedures of FT analysis independent of the eventual success, or failure, of the track recognition process. We discriminate between full digital image processing of FT and computer-assisted track measurement (CATRAM). Classic scheme in DIP, filtering, segmentation and analysis Image processing is usually separated into three steps subsequent to the image grabbing, a) Filtering or pre-processing has the objective of enhancing the local significant information. Filtering can include any or all of thresholding, contrast enhancement, noise reduction, image erosion and dilation, grey level (GL) histogram expansion amongst others. The result of such a process is still an image (i.e. a n-D array of elements), b) Segmentation aims to extract from the filtered image some primitive information (e.g. geometric objects such as a list of connected elements, area, boundaries or lines). The result is no longer the original or filtered image but is comprised of these detected elements providing a second, higher level of information than an image. Finally, c) the analysis step uses for each detected element the global properties derived from the original image to compare and reconstruct the individual elements for comparison with the desired objective. Each step is processed according to a model that explains at its own level what are the properties of the object being looked for - here a fission-track. Extracting and reconstructing tracks, step by step Local variation of the intensity of the grabbed picture, in particular around the track location may be explained as the presence of an interface between the crystal and air (each with its specific refractive index) upon the light path. If the incident angle between the light and this interface is smaller than a critical value (Snell's law [1])), then light passes through. Above this critical angle, an internal reflection results, which also generates local diffraction wavelets [2]. Because fission-track shape is not restricted to one plane, the grabbed pictures must integrate certain of the 3D information by optical slicing, that is serial horizontal sections at different depths of focus (_z-axis). In each section the internal reflection that is locally represented by diffraction waves appears in a specific position as the light travels along the z-axis. These diffraction waves (upon the local reflection plane) can be experimentally enhanced through an anisotropic implementation (that gives a stronger importance to the z-axis^ of a specific family of non-linear filters, the Edge-Preserving Smoothing Filters (EPSF, e.g. the Geological Society of Austral^^^bstracts Number 58

FT2 Laplacian-Like Enhancement or LLE [3]). Finally, the detection and the projection of the best intensity variation along the z-axis upon a 2D plane allows automatic determination of a threshold between two classes in the GL histogram (e.g. the Fisher binary level [4]). The resulting image of this full filtering line represents the presence of the local reflection plane, with a high incident angle upon the light path, that should be linked to track boundaries. A "labelling" algorithm can be used to segment the filtered picture into clusters of connected elements, but each cluster is far away from representing completely one specific track as tracks may be discontinuous or overlapped with other tracks or connected with any other defects. Likewise, each element of these clusters is associated with a vector of attributes (gradient, intensity, altitude of the best maxima along the z-axis before the projection), computed on the grabbed and/or filtered images, and the partition of these vectors into homogeneous classes leads to an over-segmentation of these clusters into primary elements [5]. This particular process is self-directing (i.e. fully unsupervised) and so does not require a large number of parameters to be set up in strong contrast to other clustering procedures. These primary elements may then be linked together according to their intrinsic properties in order to reconstruct higher-order features such as fission tracks, cracks or artifacts. This analysis stage compares and associates these elementary bodies, two by two and iteratively, in order to reconstruct some more complex items whose characteristics match a list of pre-defined models. Such a processing line has obviously to be checked and validated upon a wide range of natural samples in order to calibrate it. Moreover, even if this process works efficiently, this is only the first stage before a complete and automatic line of FT recognition and analysis via DIP is achieved. If this step cannot be established then a full-automated system will remain a dream! Is Digital camera and computing system helpful without image processing? Currently track lengths are measured only if the track lies in a plane that is perpendicular to the observation, as analysts cannot readily integrate the 3D information of a dipping track. A Dc/C system may record easily a 3D block of data through an optical slicing, and the or y-axis of this data block can be directly calibrated via the usual stage micrometer. The z-axis may be set up via a motorised z-axis (piezoelectric or stepper motor) that could be calibrated precisely using focus functions [6]. Then the operative will get a 3D set of data that is fully calibrated. He is able to measure any distance (e.g. a confined track length) inside this block via any suitable interface that allows him to explore this set. The precision of such a system is not linked to any particular advance in processing images of FT as this block is not processed, but it is only related to the ability of the operative to determine the beginning and the end of confined tracks (as in the current 2D measurement) and so, it is better to talk about a computerassisted system. Such a Dc/C system may offer a significant improvement in FT results principally by increasing the number of measurable confined tracks even if a robust process of track recognition cannot be established. The aim of this study is to present both our strategy to recognise FT features inside natural apatite samples via DIP techniques, and to introduce other computer-assisted possibilities (CATRAM). This strategy represents a first step to an automatic FT analysis system much of which remains to be validated. Recognition that FT thermochronology is useful in a wide range of geoscience applications has led to a rapid increase in the number of analysts, laboratories and data. The time is over-due to establish a robust programme to see how far the tasks of FT analysis method can be automated. This study introduces a few answers to that problem. (1) Bueche F. J. 1975. Introduction to physics for scientists and engineers, 2nd edition. McGraw-Hill Book Company, Chap. XXX, 584-607. (2) Born M. and Wolf E. 1993. Principles of optics, electromagnetic theory of propagation, interference and diffraction of light, 6th (corrected) edition. Pergamon Press pic, 370- 400. (3) Guillon S. Baylou P. Najim M. and Keskes N. 1998. Adaptive nonlinear filters for 2D and 3D image enhancement. Signal processing 67, 237-254.

FT2 (4) Fisher W. D. 1958. On grouping for maximum homgeneity. Journal of the American statistical association 53, 284, 789-798. (5) Coleman G. B. and Andrews H. C. 1979. Image segmentation by clustering. Proceedings of the IEEE 67, 5, 773-785. (6) Boddeke F. R. Van Vliet L. J. and Young I. T. 1997. Calibration of the automated z-axis of a microscope using focus functions. Journal of microscopy 186, 3, 270-274. Acknowledgements This work is partially supported by Elf EP.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

THE FIRST QUANTITATIVE EROSION RATE ESTIMATES FROM

"•..THE OLDEST PERSISTING...LANDFORMS IN THE WORLD." D.X. Beltoni, R.W. Browni, B.R Kohn2 and D. Fink3 1 School of Earth Sciences,The University of Melbourne, Victoria 3010, Australia 2 Australian Geodynamics Cooperative Research Centre 3 Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia

Introduction Extraordinary longevity has been inferred for some subaerial landforms on the Australian continent (e.g. Stewart et ah, 1986). These interpretations are founded on the assumption that the interior of Australia has been tectonically and geomorphically stable for at least the last ~600 m.y. or longer. The persistence of subaerial surfaces over these time scales necessitates extremely low rates of weathering and denudation (Stewart et al 1986; Oilier et al, 1988; Gale, 1992). Current geochronological techniques enable us to test the validity of such models by directly measuring denudation rates over a range of time scales. Here, we present quantitative estimates of long- and short-term denudation rates in the Davenport Ranges, central Australia, derived from combined apatite fission track (AFT) thermochronology and cosmogenic isotope analysis (CIA), and show that the denudation history is incompatible with the extreme subaerial longevity suggested for landforms in this region of Australia. Background The concept of cratonic interiors as regions characterised by long-term structural, thermal and erosional stability, has gained some currency in recent years. It is a model frequently applied to landscape evolution in Australia. However, recent studies in the Brazilian, Kaapvaal and Canadian cratons (Brown et al, 1996; Harman et ciL, 1998; Osadetz et ciL, 1998) suggest that denudation of several kilometres has occurred over the last 600 m.y., with episodes of particularly high denudation rates during the last 200 m.y.. In many cases the data suggest relatively recent reactivation of pre-existing, shallow, crustal structures in response to mantle deformation (e.g. Harman et al, 1998). This evidence is difficult to reconcile with the prevailing view of cratonic stability.

Sites 6 & 9 , A & B

Adapted from Stewart et a!. ( 1 9 8 6 )

Figure 1: Geometry of paiaeovalieys hi the Davenport Ranges: "A" denotes the Ashburton Surface (Hays, 1967).

A decade ago Stewart et al (1986) proposed that ridge tops in the Davenport province of central Australia are remnants of an ancient land surface, the Ashburton surface (Hays, 1967), which have existed as subaerial landforms since at least the Cambrian - a period dating back nearly 600 m.y. This would make them the oldest persistent landforms on the continent, if not on Earth. Extreme ages have also been assigned to landforms elsewhere in Australia (Oilier et al, 1988; Gale, 1992; Nott, 1995).

Geological Society of Australia - Abstracts Number 58

FT2^ee In this study we have directly tested the antiquity hypothesis of the Ashburton surface remnants by obtaining quantitative estimates of the regional, long-term and local short-term denudation rates within the Davenport province. We have combined two independent and complementary methodologies; AFT and in-situ CIA, to measure respectively the long-term (>10 m.y.) and short-term (<1 m.y.) denudation rates of these key landscape features. AFT thermochronology is an established technique for estimating broad, regional scale patterns of denudation and is most useful for detecting and measuring amounts of denudation of at least several hundred metres which occurred over time scales of the order of 10 m.y. For estimating denudation rates over shorter time scales, specifically the last ~1 m.y., the limitations of the AFT technique can be overcome by complementary analysis of in-situ produced cosmogenic isotopes such as 26a1 (half life - 0.7 m.y.) and i^Be (half life - 1.5 m.y.). This technique provides a unique means of estimating local, time-averaged denudation rates over timescales ideal for geomorphological investigations (Lai, 1991). If the landscape features described by Stewart et al (1986) have indeed existed as subaerial landforms since the Cambrian then both the short and long-term denudation rates must have been negligible. Measuring these rates therefore provides a direct test of the antiquity hypothesis: if either or both techniques indicate that significant rates of denudation have occurred since the Cambrian then the hypothesis must be rejected. Results A suite of 24 samples was collected from key sites within the Davenport Range, Northern Territory, specifically for AFT and CIA. Sampling localities were chosen to coincide exactly with sites documented by Stewart et al (1986) as examples of the Ashburton surface and associated conglomerate deposits (descriptions and coordinates for these sites were provided by Stewart). Apatite fission track analysis results Of the ten samples processed for AFT analysis, four had sufficient countable grains to provide statistically robust apparent fission track ages and track length measurements. These apparent ages ranged from 270 to 370 Ma, with mean track lengths between 11 and 12 pm and standard deviations around 2 jjm. Thermal histories derived by forward modelling (Gallagher, 1995) of these data show that samples reached a maximum palaeotemperature of ~90°C at -190 Ma before undergoing moderate to rapid cooling over the following ~50 m.y. Subsequent cooling to the present surface temperature (~20°C) occurred at much lower rates. Present heat flow measurements for the Tennant Creek area (c. 80 mW.m-2) (Cull, 1982) and an assumed thermal conductivity of 2.5 W.m'i.K'i indicate a present geothermal gradient of about 32°C.km-i. If this thermal gradient is representative of the last 190 m.y. then the total 70°C of cooling since -190 Ma predicted by the fission track modelling suggests denudation of some 2.2 km of section, at an effective average long-term denudation rate of -12 m.Ma-i. More detailed analysis of the thermal modelling results indicates that about 1 km of this section was removed between Early Jurassic-Early Cretaceous time, resulting in an effective average denudation rate of approximately 20 m.Ma-i. Since the Early Tertiary, the average denudation rate has been at least an order of magnitude lower. Cosmogenic isotopic results Two samples from each of three sites have been analysed for i^Be. Two of the sites were located on the flat surface exposure of an isolated remnant of poorly bedded conglomerate, preserved as a narrow, elongated outcrop located within a valley and situated a few metres below the interfluve (Fig. 1). Quartzite cobbles exposed at the surface of the conglomerate were sampled for CIA. This style of occurrence of the conglomerate was interpreted by Stewart et al (1986) as evidence for deposition of the sediments as terrace deposits within palaeovalleys. Our field observations within the Davenport Ranges suggest that the extent of the sedimentary cover was originally more extensive, and was not restricted to terrace deposits within palaeovalleys. The third CIA site was located on a ridge of

FT2 Precambrian quartzite forming the Ashburton surface (Hays, 1967). Average i^Be denudation rates were calculated using the steady-state erosion model of Lai (1991) and are less than 5 m.Ma-i for the sites underlain by the conglomerate, and some two orders of magnitude less for the site on the Ashburton surface itself. Variation within sample pairs for each site was within analytical error. Discussion Both the local, short-term and regional, long-term denudation rates presented here indicate that landscape evolution models for the Davenport province implying negligible denudation rates persisted over periods of the order of 600 m.y. or longer cannot be sustained. Even if the lowest i^Be denudation rates determined for the Ashburton surface site persisted over a period of 600 m.y. at least 180 m of denudation is indicated. Extrapolating the short-term rates determined from the conglomerate deposits over 600 m.y. suggests 1.4-2.5 km of denudation. However, the long-term average rates determined from the AET results suggest a discrete period of accelerated denudation which removed ~1 km of sedimentary cover during the Early Jurassic-Early Cretaceous followed by much lower average rates through the Tertiary. This landscape history is compatible with that determined from the CIA data, and suggests that the short-term denudation rates determined are probably representative of most of the Tertiary. We emphasise that the denudation rates estimated here are time-averaged values and that changing climatic, lithologic and tectonic regimes may have caused fluctuations unresolved by our current measurements. Eield relationships of the conglomerate deposits indicate that some contemporary valleys are indeed Cambrian palaeovalleys. Our data suggest these have been exhumed from beneath a substantial sedimentary cover during the Mesozoic. The order of magnitude difference, between the average denudation rates determined for the ridge tops and the valley fill, indicates that palaeovalley exhumation is a viable, and probable, process. Furthermore, our results seriously question the current paradigm (e.g. Oilier et al., 1988; Gale, 1992; Nott, 1995) of continental landforms being able to exist subaerially, essentially intact, for hundreds of millions of years. Brown, RW et al., 1996. Extraordinary denudation rates for cratons inferred from apatite fission track data: Implications for tlie long-term stability of cratonic lithosphere. Eos, Trans. Am. Geophys. Union, 77 No.22, W152. Cull, JP, 1982. An appraisal of Australian heatflow data. BMR Jnl. of Australian Geol. and Geophys, 7, 11-21 Gale, SJ, 1992. Long-term landscape evolution in Australia. Earth Surf. Processes and Landforms, 17, 323-343. Gallagher, K, 1995. Evolving temperature histories from apatite fission-track data. EPSL. 136(3-4)-.421-435. Harman, R, et aL, 1998. Accelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceous., Journal of Geophysical Research, v. 103, p. 27,091-27,105. Hays, J, 1967. Land surfaces and laterites in the north of the Northern Territory. In Jennings, JN and Mabbutt, JA (eds.), Landforms studies from Australia and New Guinea, ANU Press, Canberra. 182-210. Lai, D, 1991. Cosmic ray labelling of erosion surfaces: in-situ production rates and erosion models. EPSL. 104, 424-439. Nott, J, 1995. The antiquity of landscapes on the North Australian Craton and the implications for theories of long-term landscape evolution. Journal of Geology, 103, 19-32. Oilier, CD et al, 1988. The Kimberley Plateau, Western Australia: a Precambrian erosion surface? Zeitschrift fur Geomorphology 32, 239-246. Osadetz, KG, et al, 1998. Thermotectonics of the Williston Basin and environs: variations in heat flow and hydrocarbon generation. In JE Christopheret et al, eds.. Eighth International Williston Basin Symposium, Saskatchewan Geological Society Special Publication No. 13, p. 147-165. Stewart, AJ, Blake, DH and Oilier, CD, 1986. Cambrian river Terraces and ridgetops in Central Australia: oldest persisting landforms? Science, 233, 758-760. Summerfield, MA and Hulton NJ, 1994. Natural controls of fluvial denudation rates in major wodd drainage basins. Journal of Geophysical Research-Solid Earth. 99(B7):13871-13883. Acknowledgments This work was funded by the Australian Geodynamics Cooperative Research Centre (AGCRC) and the Australian Institute of Nuclear Science and Engineering (AINSE Grant Nos. 98/023 and 99/047). This work is published with the permission of the Director, AGCRC. DXB was supported by an AINSE postgraduate scholarship and RWB acknowledges the support of a University of Melbourne Research Career Establishment Grant.

FT2

22

International Conference on Fission Track Dating and Thermochronoiogy

FT2'

AXIAL SHORTENING OF FISSION TRACKS BY VOLUME DIFFUSION OF LATTICE DEFECTS: EXPLAINING EMPIRICAL ANNEALING DATA WITH A PHYSICO-CHEMICAL MODEL

D.X. Beltoni, K. GaUagher2 and R.W. Browni 1 School of Earth Sciences,The University of Melbourne, Australia. 2 Imperial College of Science,Technology and Medicine, London, UK.

Introduction Fission track analysis is well established as an analytical methodology and the technique has been calibrated against an extensive suite of empirical data (e.g. Green et al, 1986; Duddy et al, 1988) on the kinetics of fission track annealing. However, despite the techniques widespread use in addressing tectonics problems, basin studies and landscape evolution, there is still no widely accepted quantitative physical description of the phenomenon of thermally driven track shortening (e.g. Carlson, 1990). We present here a preliminary physical model of fission track annealing based on thermally driven bulk diffusion of lattice defects within the crystal lattice. Most AFT practitioners are familiar with the good correlation between fission track lengths measured after a series of experiments, and those predicted using empirical mathematical relationships described in Green et al. (1986). The equations therein are the basis of contime (uncalibrated) time (seconds) temporary thermo-chronological modelling O • A « data points from Green et al., 1988 with apatite. There are limitations with this V " Xv curvt>8 calculated using our model Y'A X approach, however. For example, published models of annealing fail to address the prob• 1 \3ODay8 X z O Hin lOOMa 1014a lem of mixed ages occurring within a sample. . . 1 . \ . J_J 1 100 200 This variability has been associated with Temperature(°C) chemical variations in grains. Analysts are Figure 1. Comparison of time dependence and isothermal anneal^^ increase in chlorine content ing results from our model with empirically established models.

^

^

^

,

i

i.^r

leads to reduced annealing (i.e. slower diffusion) resulting hi older apparent fission track ages. Gleadow and Duddy (1981) illustrated this convinchigly in their Otway Basin samples. However, recent work (O'Sullivan and Parrish, 1995), where age and chlorine content are very poorly correlated, suggests that this is not the only mechanism at work. This is the type of question, we hope to address with an operational kinetic model. In order to develop this model we fu-st establish a physico-chemical basis for the process we will be attempting to replicate. We also define the geometry of the fission track for our purposes. The track geometry although poorly understood and notoriously difficult to knage, is described in terms of energy-loss phenomena of highly charged particles in solids. We then apply Fick's second law of diffusion to this pre-defined geometry in order to establish a relationship between the diffusion of our "model" defect species, and time for the "annealing" of the track. Modelling the solid state processes The variation in annealing and as a result, apparent age, described in Gleadow and Duddy (1981) has been attributed to inhibition of chlorine and fluorhie transport along the hexad axis m apatite (Green etal, 1986). The small F ion passes along this channel easily in comparison to the larger Cl ion. This view is in accord with observations of both chemical and crystallographic control on track annealing. Implicit in this approach is the relationship between halogen concentration and the effect on annealing or diffusion. Consideration also needs to be given to the equally significant atomic abundance of Geological Society of Australia - Abstracts Number 58

FT2 the rare earth elements (REE) in apatite. Thus far, the likely kinetic contribution of the rare earth elements to the diffusion problem has been largely ignored. The halogen diffusion mechanism appears to be a function of ion size and thus the energy required to mobilise the ion. In contrast, the mechanisms involving the REE are more a function of valency and charge balance within the lattice. While the halogen ions are located in the hexad channel of apatite, the REE^+ substitute for Ca2+ in either of two lattice sites. Charge balance typically requires the presence of either monovalent cations (e.g. Na+) or vacancies. The presence of significant amounts of REE - and by implication a high proportion of vacancies - may play a crucial role in diffusion processes in apatite. Lattice vacancies are central to what Borg and Deines (1988) describe as the "most general and important of all the diffusion mechanisms". Lasaga (1981) argues that both activation energy and vacancy concentration (which is a reflection of impurity content) are very sensitive to temperature. In addition, the geological environment for apatite geochronology is low temperature (<150''C) putting annealing process in the "extrinsic" region for apatite. This is the temperature range within which lattice defects make a significant contribution to the diffusive process. Modelling a track geometry Axial and radial geometry of fission tracks are important parameters in the diffusion model. Although poorly described for apatite, these parameters needed to be realistic in order to support a practical model. Extensive work has been carried out on track morphology in mica and other materials (e.g. Albrecht et al, 1982). The relationship between defect density and energy loss of fission fragments (dE/dx) has been summarised mathematically by Spohr, 1990. Here, ionisation of the solid approximates the energy loss of the projectile ion and this in turn is proportional to the number of defects produced. We were able to make a comparison between (dE/dx) and ionisation for materials with known track parameters, and apatite - for which track morphology is poorly understood (Paul and Fitzgerald, 1992). In addition, detailed small angle neutron diffraction work (Albrecht et al, 1982) shows an essentially linear correlation between defect density and cross-sectional area of tracks in mica and m.y.lar, hence a power law governing track radius as a function of length. This approach established a simple but realistic geometry for our model track. This includes the track shape (essentially a prolate ellipsoid - major axis of ~16 pm, minor axis -10 nm) and the density of defects along the track - both crucial to describing the annealing process. Assembling the model: In the model, we treat the diffusing species as vacancies only, since this species appears to dominate the track core (Spohr et and detailed studies by others (Borg and Deines, 1988) confirm that vacancies have higher migration energies and thus diffuse more slowly through the lattice than interstitial atoms. The migration of atoms into the track core can be treated as equivalent to the motion of vacancies out of the track - a concept called "complementarity". This approach is used because the basic transient diffusion equations are easier to handle. Complete annealing is controlled by the slowest moving species, so that the assumption of vacancy diffusion was considered plausible starting point. Our model is further based on the assumption of defect diffusion in apatite being largely a first order process, and high temperature diffusion experiments would support this, although clustering of defects is likely to produce a second order component. Following passage of a fission fragment through the solid, a disrupted core of defects is produced. At any given point along the track axis the parameters of defect density and radius are functions of the energy imparted (dE/dx) by the fission fragment. These provide the initial conditions for the transient diffusion problem. The solid is treated as an infinite medium with an instantaneous source (i.e. due to the fission event). Analytically, the diffusion problem can be simplified to a cylindrical one since the relative dimensions of a fission track (length ca. 16 |im, and diameter ca. 10 nm) are such that we remove axial diffusion from the problem. Thus at each point along the track, we calculate the radial diffusion as for an infinite cylinder with a radius of the track at that point. We subsequently chose a finite difference solution using the Crank-Nicholsen Method to automate the model for more extensive modelling over a range of time

FT2 frames. Code for the finite difference solution is still under development however the initial results suggest that the model may provide some useful insight into the problem of variable rates of track reduction observed under isothermal annealing (Green et al. 1986). Results Preliminary results suggest that the model may be able to simulate the annealing kinetics of fission tracks in apatite. One promising aspect of the results is the form of time-dependence produced for track annealing (Fig. 1). In Figure lA, Green et afs (1988) figure shows the time dependence of track length for a given temperature. This plot matches the observations of initial rapid track shortening, followed by an approximately linear variation between. Subsequently the tracks show a very rapid shortening to zero. Although our input parameters for the both the diffusion coefficient and activation energy are poorly constrained, the model plot (Fig. IB) appears to show a similar length/time response to that based on the empirical equations of Laslett et al. (1987). The time-dependence plot is also a significant improvement on an earlier attempt by Zimmerman and Gaines (1978) to fit a simple first order annealing equation to track density data. We argue that the complex time dependence seen in fission track annealing is, to a first approximation, the result of combining the first order process of defect diffusion with both a non-linear track geometry and a non-linear axial defect distribution. In a simulated annealing experiment, our model also compares favourably with the Green et al.'s (1986) isothermal annealing data (Fig. IC). The model curves have bracketed their values, though the results again suggest our input parameters need refining. These parameters are crucial since small variations can significantly effect the predicted annealing time for a track. Nevertheless, our model is capable of extrapolation to geological time periods without producing unreasonable time/temperature estimates. The model is in its very early stages of development, and key parameters need to be more tightly constrained. The results suggest that further work will help advance our understanding of these processes. Albrecht et al, 1982. Small-angle neutron scattering observations from orientated latent nuclear tracks. Rad. Effects 65: 145-148. Borg, RJ and Deines, GJ, 1988. An Introduction to Solid State Diffusion, Academic Press. Crank, J, 1975. The Mathematics of Diffusion (2nd ed.) Oxford, Clarendon Press. Carlson, WD, 1990. Mechanisms and kinetics of apatite fission track annealing. Am. Min., 75: 1120-1139 Duddy et al, 1988. Thermal annealing of fission tracks in apatite 3. Variable temperature behaviour. Chem.Geol. 73: 25-38. Fleischer et al, 1975. Nuclear Tracks in Solids, Uni. of California Press. Gleadow, AJW, and Duddy IR, 1981. A natural longterm annealing experiment for apatite. Nucl. Tracks, 5: 169-174. Green, et al, 1986. Thermal annealing of fission tracks in apatite 1. A qualitative description. Chem. Geol. 59: 237-253. Green, et al, 1988. Can fission track annealing in apatite be described by first order kinetics? EPSL 87:216-228. Lasaga, AC., 1981. The atomistic basis of kinetics: defects in minerals. Rev. Min. 6: 261-319. Laslett et al, 1987. Thermal annealing of fission tracks in apatite 2. A quantitative analysis. Chem.Geol. 65: 1-13. O'Sullivan PB, and Parrish, RR, 1995. The importance of apatite composition and single grain ages when interpreting fission track ages from plutonic rocks: a case study from the Coast Ranges, British Columbia. EPSL 132: 213-224. Paul, TA, and Fitzgerald, PG, 1992. Transmission electron microscope investigation of fission tracks in fluorapatite. Am. Min., 77:336-344. Spohr, R, 1990. Ion Tracks and Microtechnology: principles and applications. Vieweg, Germany. Zimmerman, RA and Gaines, AM, 1978. A new approach to the study of fission track fading. USGS Open File Rept.78-701: 467468.

Acknowledgments AINSE has provided financial support for this project by means of an AINSE Postgraduate Award to DXB.

FT2^ee

International Conference on Fission Track Dating and Thermochronoiogy

F T 2 '

TESTING DETRITAL F I S S I O N - T R A C K ANALYSIS ON M O D E R N R I V E R SEDIMENTS OF THE EUROPEAN ALPS

M. Berneti, J.L Garver2 and M.T. Brandoni 1 Department of Geology and Geophysics, Yale University, New Haven, CT, USA 2 Geology Department, Union College, Schenectady, NY, USA

Fission-track (FT) dating of detrital zircons provides a method for studying the exhumationai histories of mountain belts. A critical assumption is that a sample distribution of zircon fission-track grain ages (FTGA) grant a useful representation of basement cooling ages in the erosional source at the time of deposition. This assumption is tested using detrital zircons from modern rivers draining the southern flank of the European Alps, where FTGA distributions can be compared to a relatively dense suite of low-T bedrock cooling ages (Wagner et al 1977, Wagner et al, 1979, Hurford, 1986, Hunziker et al, 1992, etc).

Figure 1. Drainage areas of Ticino, Adda, Adige, Brenta and Piave, Northern Italy.

The Ticino, Adda, Adige, Brenta and Piave rivers drain large areas of the Central, Southern and Eastern Alps (Fig. 1). Exposed bedrock consists of Paleozoic granites, pre-Alpine metamorphic rocks, and Mesozoic clastic and carbonate sediments. About 100 zircons per sample were dated from modern sediment collected from the foothill reach of each drainage. Zircon fractions were mounted in equal portions in PEA Teflon, polished, and etched in a KOH-NaOH eutectic melt at 228°C. Depending on the amount of available sample material, 1-3 mounts per sample were assembled for differential etching for 7, 15 and 24 h. The etched mounts were irradiated at the Oregon State nuclear reactor using a nominal fluence of 2 x 10^5 neutrons cm-2. The internal gradient within the irradiated package was estimated by interpolating track densities for fluence monitors (CN5) placed within the irradiation tube. Fission tracks were counted at Union College at 1250 x (100 objective, 1.0 tube factor, 12.5 oculars) on an Olympus BH-2 microscope. Ages were calculated using an mean weighted zeta CN5 of 332.87 ± 6.31 (± ISE, M.B.). All zircon FTGA distributions show a close match to bedrock cooling ages. The Ticino drainage provides the best comparison because it has the densest set of bedrock cooling ages. The Ticino sample contains 4 resolvable peaks: 9, 16, 26, 140 Ma, which closely match the area distribution of cooling ages provided by 42 low-temperature bedrock ages in the

Geological Society of Australia - Abstracts Number 58

FT2 drainage. Table 1 gives the data for the Ticino, Adda and Adige samples. The given peak ages were determined by binomial peak fitting (Brandon, 1996). Some interesting regional trends are also apparent. The youngest detrital cooling ages are found in the Ticino drainage, indicating that exhumation is locally fastest there in the Lepontine Alps. The FTGA distributions show an E-to-W increase in the proportion of young grain ages, which indicates that young exhumation involves a larger fraction of area in the more western drainages. These information are lost if only the central age of the samples would be used. Peak fitting allows to determine different grain age components and also the source areas of the zircons. Table 1. Zircon FT peak ages for modem river sediments, Italy

Sample

Age (Ma) 0

n

PI

P2

P3

P4

15.6± 25.7±2.4 140.1± 19.C W=0.20 1.8 W=0.17 8.4% 19.7% W=0.17 56.6% 0 92.4±3.8 30.6± 125 17.9± 98MB03 W=0.18 1.6 0.9 Adda W=0.17 31.4% W=0.20 29.4% 39.3% 0 118 24.2± 13.6± 45.5±2.9 101.5±7.9 98MB04 W=0.21 1.0 1.8 W=0.25 Adige W=0.21 W=0.20 13.0% 30.5% 21.0% 35.5% Ages are given ± 2 SE, W is the peak width. The fractions of grains that belong to the individual peaks are given in percent. 98MB02 Ticino

95

8.6± 1.4 W=0.23 15.4%

The given examples demonstrate the ability of zircon FTGA distributions to provide a quick overview of the low-T history of a large drainage area. We are working now to extend our study back in time by dating detrital zircons from stratigraphically coordinated foreland-basin samples. The objective is to resolve a detailed long-term record of exhumation of the Alpine orogen. Brandon, M. T. (1996): Probability density plot for fission-track grain-age samples. Radiation Measurements, v. 26, 663-676. Hunziker, J. C., Desmond, J. and Hurford, A. J. (1992): Thirty-two years of geochronological work in the Central and Western Alps: A review on seven maps. Mem. de Geol. (Lausanne) 13, 1-59. Hurford, A. J. (1986): Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line. Contrib Mineral Petrol, 92, 413 - 427. Wagner, G. A., Reimer, G. M. and Jager, E. (1977): Cooling ages derived by apatite fission-track, mica Rb-Sr and K-Ar dating: the uplift and cooling history of the Central Alps. Mem. 1st. Geol. Min. Univ. Padova. Wagner, G. A., Miller, D. S. and Jager, E. (1979): Fission-track ages on apatite of Bergell rocks from Central Alps and Bergell boulders in Oligocene sediments. Earth and Planetary Science Letters, 45, 355-360.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

F I S S I O N - T R A C K TEPHROCHRONOLOGY IN ITALIAN S E R I E S : A REVIEW

G. Bigazzii, F.P. Bonadonna^ and G. Zanchetta^ 1 Istituto di Geocronologia e Geochimica Isotopica, C.N.R, Pisa, Italy. 2 Dipartimento di Scienze della Terra, Universita di Pisa, Pisa, Italy.

Numerous distal tephra were recognised in Italian sedimentary series deposited since Late Oligocene up to Holocene. Glass is the only datable phase of many of these tephra. Some of them yielded other mineral phases datable using the fission-track dating method, such as apatite and zircon. Many of these volcanics do not allow application of other dating techniques. For this reason, although fissiontrack dating often produces low precision ages compared with other methods, specially in case of young rocks, this technique significantly contributed to the reconstruction of the chrono-stratigraphy of Italian sedimentary series. Due to poor stability of spontaneous tracks in glass during geological times, this phase commonly yields a reduced age (apparent age), unless an age-correction technique is applied. For this reason, in principle ages measured on heavy minerals have to be preferred, as apatite or zircon separated from volcanics commonly retain undisturbed tracks. Nevertheless, although application of age-correction techniques (the size-correction method and the plateau method) in principle may produce less precise and/or less accurate ages, glass maintains its significance due to its large diffusion and abundance in tephra beds. Fission-track ages determined in Italian tephra during several years are synthetically shown in Table 1. When available, plateau ages have been preferred to size-corrected ages for their higher precision. Track-size measurements on glass, that were used since late sixties for correcting thermallylowered ages, may yield precious information on thermal histories. For example, a typical bimodal distribution of track sizes in three Messinian glasses of Table 1 was interpreted as the imprint of a Upper Pliocene-Lower Pleistocene tectonic activity (Bernardes et al, 1986; Bigazzi et al, 1999). We can divide the studied tephra in two major groups: the first around twenty million years old, the second younger than around six million years. Between these two groups we have not found distal tephra within sedimentary series. The younger group can be divided in four subgroups, the younger of them may be linked to the Roman volcanism, that is younger than half million years, always found in continental, marsh-lake, series. These tephra are trachyte and belong to alkaline (ultrapotassic) magma series. The second and the third ones belong to the range Pliocene-Pleistocene (2-4 Ma). These tephra are widely scattered in central-southern Italy, in the Adriatic side as well as in the Tyrrhenian one. The sediments in which they are enclosed are sandy clays of marine neritic environment. They are rhyolites of sub-alkaline magma series. Their stratigraphic placing, controlled by biostratigraphy, is embraced in Globorotalia crassaformis and Globorotalia inflata zones. The temporal interval of the fourth subgroup is confined in all the Messinian age (Upper Miocene), but its areale distribution is larger than the Pliocene-Pleistocene subgroup. This tephra is found from Ancona, Marche region, until Cosenza, Calabria region. We remember, furthermore, the tephra of similar age dated near Nador, Melilla region (Morocco) that marks the Tortonian-Messinian boundary (Arias et al., 1976). The Messinian Italian tephra is a well-defined ash level in the "Flysch della Laga Formation" that marks the Miocene-Pliocene boundary. The oldest group of Italian dated tephra is significantly older than the tephra described above. They belong to the calcalkaline magma series and consist of some volcanic ash intercalations in marine sandy-clay sediments Oligocene-Miocene in age, in central-northern Apennines. The thickness of these levels, more than 1 metre, is bigger than the Pliocene-Pleistocene tephra, which are always of decimetric or centime trie dimensions.

Geological Society of ^ H M i a - Abstracts Number 58

F T 2 ^ e e Table 1. Fission-track ages of Italian distal tephra

Phase Location

PH OJ a v u c/D

(u a S 2 . s

1

Apparent Age

Formation Age

Trino Vercellese (VC) Cava Campani (PI) Poggio al Vento (PI) Collesalvetti (PI) Montopoli (PD

0.29 ±0.05^

0.35 + 0.10^

0.46 ± 0.04^

0.62 ± 0.07^ 0.59 ± 0.08^

Valle Ricca (RM) Bellante 1 (I'li) Bellante 1 (TE) Bellante 2 (TE) Mosciano (IE) Vrica (CS)

1.38 ± 0 . 1 4 ^ 1.69 + 0.17^

2.03 ± 0.26^ 2.08 ± 0 . 1 9 ^

1.68 ± 0 . 1 7 ^ 1.52 ±0.15"^ 1.37 ± 0 . 0 9 ^

2.10 ± 0 . 2 1 ^ 2.17 ± 0 . 1 9 ^ 2.20 ± 0.20^

Cimino 18 (VT) Marco Simone (RM) Pisticci (MT)

2.24 ± 0 . 1 6 ^ 1.88 ± 0 . 1 4 ^

3.75 ± 0 . 4 3 * 3.18 ±0.31^ 2.60 ± 0.2^

S. Maria in Carpineto (AN) Maccarrone (MC) S. Ginesio (MC) Colle Piccione (AP) Civitella del Tronto (TE) Colle Addina (PE) San Fili (CS)

2.77 ±0.30^ 2.71 ± 0 . 2 0 ^ 1.84 + 0.11^ 2.82 + 0.21^ 2.78 ± 0.24^ 1.64 + 0.17^ 2.89 ±0.31^

6.12 + 0.62^ 5.38 ± 0 . 4 2 ^ 5.43 ± 0.69"^ 5.65 ± 0 . 6 1 ^ 5.91 + 0.57^ 5.75 ± 0.68^ 6.87 ± 0.75^

Tripoli di Contignago (PR) 10.7 ± 0 . 4 ^ 6.25 ± 0 . 5 6 ^ Torrente Tarugo (AN)

22.7+1.3^ 19.7 ± 2 . 2 ^

Apatite (a) Zircon (z)

0.46 ± 0.05 (a)* 0.48 ± 0.05 (a)*

1.8 ± 0 . 3 ^ 2 ) 2.l6±0.22\a) 2.0010.16^2)

=

w a s g

"V 4 §

t

Glass

y s

Formation age: we report here size-corrected or plateau ages, excepted for Collesalvetti and Cimino 18 glasses that did not require application of age correction techniques. (VC): identifies the Italian province. 1: Arias et al, 1981b; 2: Bigazzi et al, 1994; 3: Arias et al, 1981a; 4: Bigazzi et al, 1999; 5: Obradovich et al1982; 6: Bigazzi and Bonadonna, 1988; 7: Bemardes et al, 1986; this work. Glasses S. Ginesio - Colle Addina: samples CTU 9, 8, 7 and 6 in Bemardes et al, 1986.

The succession of these dated tephra allows us to note a showy temporal gap between the two main groups. A similar gap, even if more short, is also shown by the Apennine magmatism. This gap may be explained with the geodynamic evolution of the Apennines: very roughly we may say that the two main tephra groups represent the volcanic activity before and after continent collision. The different

FT2 length between the time gap between the two main groups of tephra and the real gap of magmatism probably depends on the need for deposition of distal tephra of very big ignimbritic, or anyway explosive, eruptions that are not always the rule for the volcanic activity. The last consideration regards similarity of age between the Pliocene-Pleistocene group and the age of the tectonic activity recorded by the Messinian tephra mentioned above. The explanation of this coincidence is again in the geodynamic evolution of the Apennines. These tephra are clearly connected to the last big tectonic push that contributed to give birth to the Apennine chain. Arias C. Bigazzi G. and Bonadonna FP. 1981a. Size corrections and plateau age in glass shards. Nuclear Tracks and Radiation Measurements 5, 129-136. Arias C. Bigazzi G. and Bonadonna FP. 1981b. Studio cronologico e paleomagnetico di alcune serie sedimentarie dell'Italia appenninica. Contributi preliminari alia realizzazione della carta neotettonica d'ltalia. Pubblicazione n. 356. Progetto Finalizzato Geodinamica - Sottoprogetto Neotettonica, 1441-1448. Arias C. Bigazzi G. Bonadonna FP. Morlotti E. Radicati di Brozolo F. Rio D. Torelli L. Brigatti M.F. Giuliani O. and Tirelli G. 1976. Chronostratigraphy of Izarorene section (North Eastern Morocco) in the Melilla basin. Bollettino della Societa Geologica Italiana 95, 1681-1694. Bernardes C. Bigazzi G. Bonadonna FP. Centamore E. Lattes C.M.G. and Hadler N. J.C. 1986. Fission track dating on glass from "Flysch della Laga" formation: a very interesting and problematic application. Nuclear Tracks and Radiation Measurements 12, 901-904. Bigazzi G. and Bonadonna FP. 1988. Fission track dating of a volcanic ash layer near Pisticci (Basilicata, Italy). II Quaternario 1, 127-129. Bigazzi G. Bonadonna F.P. Centamore E. Leone G. Mozzi M. Nisio S. and Zanchetta G. 1999. New radiometric dating of volcanic ash layers in Periadriatic foredeep basin system, Italy. Palaeogeography, Palaeoclimatology, Palaeoecology (in press). Bigazzi G. Bonadonna FP. Cioni R. Leone G. Sbrana A. and Zanchetta G. 1994. Nuovi dati geochimici, petrografici e geocronologici su alcune cineriti Plio-Pleistoceniche del Lazio e della Toscana. Memorie Descrittive della Carta Geologica d'ltalia XLEK, 135-150. Obradovich J.D. Naeser C.W. lazlett G.A. Pasini G. and Bigazzi G. 1982. Age constraints on the proposed Plio-Pleistocene boundary stratotype at Vrica, Italy. Nature, 298, 55-59.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

APPLICATION OF NEUTRON DOSIMETRY BY NATURAL URANIUM AND THORIUM THIN FILMS IN FISSION TRACK DATING

G. Bigazzii, S. Guedes O.2, J.C. Hadler N.2, PJ. Iunes2, S.R. Paulo3 and C.A.TeUo S ^ 1 Istituto de Geocronologia e Geochimica Isotopica, CNR, 56010 Ghezzano, Pisa, Italy. 2 Institute de Fisica "Gleb Wataghin", Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil 3 Departamento de Fisica, Institute de Ciencias Exatas e da Terra, Universidade Federal de Mate Grosso, UFMT, 78060-900, Cuiaba, MT, Brazil 4 Institute de Geociencias e Ciencias Exatas, Universidade Estadual Paulista, UNESP, 13506-900, Rio Clare, SP, Brazil

Introduction When a mineral is irradiated with neutrons in a nuclear reactor, the induced fission track density it presents, pj, can be written as (Bigazzi et aL, 1999): PJ

=£N^R M

where Rm - ^u +

(1)

Nr

R Th

where Ru - ^235^235 + ^38^238 and R n = ^232

(2) (3)

(4)

£ is a detection efficiency factor representing the ratio between the number of fission tracks observed per unit surface and the number of fission per unit volume that took place inside the mineral; Ny (N^h) is the number of uranium (thorium) atoms per unit volume; C235 (C238) is the 235u (238u) isotopic abundance in natural uranium and A235, A238 and A232 are the number of fissions per 235u^ 238U and 2321^]^ target nucleus, respectively. The goal of neutron dosimetry inside the fission track dating (FTD) is the determination of R^. 238U and 232Th fissions are not expected to occur in well thermalized neutron facilities then the second term both of equation 2 and 3 vanish. Besides, the first term of equation 3 can be simplified and RM can be obtained employing usual activation monitors. However, the use of such irradiation facilities is not always possible, mainly in countries where the research reactors are rare. In this work, we show that FTD can be used as an absolute calibration even in the case where low thermalized neutron facilities are employed. This can be achieved if natural uranium and thorium thin films are utilized as neutron dosimeters: an absolute dosimetry which leads to the numerical values of Ru, Rxh ai^d N^h/Nu (Bigazzi eta/., 1999). The samples we dated were 4 mono-crystals (CI, B4, C2 and B3) from the Durango apatite (Green, 1985). Results and discussion In Table 1, the results concerning neutron dosimetry, Th/U ratio and the dating data are shown. The neutron irradiations were carried out in three nuclear reactors: IPEN/CNEN-Sao Paulo, Brazil (irradiation 1-27, 1-29, 1-32 and 1-33), LENA-Pavia, Italy (irradiation P-4) and IPEN-Lima, Peru (irradiation L1, L-2 and L-19). The values of R'j^ were obtained through thin thorium films whose alpha activity was measured in nuclear emulsion coupled to them for suitable time intervals (Bigazzi et aL, 1995). Ru values were obtained analysing uranium doped glasses (CNl, CN2, CN5 and IRMM-540) calibrated through irradiations where thin uranium films were also present (Bigazzi et aL, 1999). Such uranium films had been previously calibrated by their alpha activities (Bigazzi et aL, 1995).

Geological Society of Australia - Abstracts Number 58

FT2 Table 1

Irrad

Irrad

distal

Cd L-1

L-2

(3)

(2)

1-29

1-33

(4.5)

(4.5)

1-27

1-33

(4.5)

(4.5)

P-4

1-33

(6.5)

(4.5)

L-19

CI

B4

C2

1-32

1-33 (4.5)

^Th

(10-9)

(10-9)

2781

0.1694

C±29,%)

Ct3.0%) CM8,%)

1.702

2.10

6.29

0.0966

C140.%) Cb63,%)

1.626

0.0966

2.70

5.56

CW4,%)

Ct6.5%)

1.42

14.11

Ct5,9%)

Ct5,4%) 3.74

1.42

0.709

CM0,%) Ct6,2%)

C2

1.840

0.0966

Ot4.0«)

Ci4.0%) Ot62,%)

C2

0.854

0.674

B3

2.450

Ct5,8%)

C±59,%) 1.42

0.673

3.81

Ct5,9%)

Ci5,7%)

0.0966

25.0

Ctl4,5%)

Ps/Pi

Lg/Lj

1.42

1.12

Ct4.o%) Ct5.8%)

4.70

Ci5.6%)

C±59.%)

^AP ^COR (Ma)

(Ma)

3.31

0.159

0.719

0.915

28.6

31.3

CM8,%) Ctl5.6%)

Ct5,7%)

Ctl.8%) Ct8,0%) CB.0%)

Otl2,9%)

Ct5,8%) Ctl5.3%)

7.06

0.109

0.349

0.890

29.7

33.3

27.5

1.821

0.107

1.155

0.900

25.3

28.1

0.960

0.874

23.7

27.1

0.855

22.9

26.8

0.893

28.1

31.5

28.4

Ctl2.6%)

Ct5,0%) Ctl4.9%)

Ct3,9%)

0t6.0%)

2.05

0.102

Ctl23%)

Ci5,0%) Ctl44%)

29.3

1.051

0.188

1.81

0W5,%) Ctll.7%)

Ct7,4%)

2.85

0.141

0.819

31.1

C±85,%)

Ct6,6%)

Ct3.1%)

K/Ri

Ntl/Nu (10"9)

Ci4,8%)

Ci2,6%)

Pj/Pc

Cd

Rill

03.9%)

(3)

(4.5)

Cd

35.9

Ctl2.0%)

C±41.%) Ctl3.9%)

Ct6,4%)

Ct5.3%)

01:1,6%) C±76,%) Ct8,6%)

Ctl.6%) Ct8.3%) Ct9,2%) ctl.8%) Ct8.7%) C±55,%) C±24,%) et9,i%) ctio,i%) ctl.8%) Ct7,3%) Ct8.4%)

The N-Th/Nu ratios were obtained irradiating the samples inside (L-2 and 1-33) and outside (L-1, 1-29, 1-27, P-4 and 1-32) cadmium boxes. Adapting a equation obtained in Bigazzi et el, 1999, these ratios can be described as:

RuTh

Nr,

—

P -

R

Cd

PI

p -

where the superscript Cd indicates irradiation carried out inside a cadmium box. Durango apatite age determination, T, was obtained from the spontaneous track density, Ps:

A

where X = 1.55125 x lO-io a-i (Lederer and Shirley, 1978), C238 = 0.99275 (Lederer and Shirley, 1978) and ?If = ©.35 ± 0.24) x lO-i^ a-i (Guedes et al., 1999). This resulted from 5 determinations where thin uranium films were employed as neutron dosimeter. In the case of the apparent age, T^pp, e238/£ ^ ^ s taken as 1. In the case of the corrected age, T^or, £238/8 = (1.00 ± 0.037) X (L^/Lj). This value was obtained annealing the monocrystal B4 of irradiation 1-29 at two different temperatures so that the measured L/L^ are in the range where the Durango Lg/Lj values have been found. The mean value of N^h/Nu for the Durango monocrystals studied here was (29.9 ± 1.7), which is relatively close to ~=25 obtained by Hurley and Fairbairn (1957) but considerably greater than Young et al's (1969) value, 18. Analysing our N^h/Nu individual determinations, = 4 54 for 4 degree of freedom was obtained, indicating that ^0.35.

FT2 It should be noted that the influence of thorium fissions on R^ (shown in Table 1 as K/R^, where K = (NTh/Nu) R-rh) significant for the Durango apatite for the irradiation facilities employed in this work. For instance, for an irradiation carried out under R^jCAu) = 3 (number within parenthesis in columns "irrad" and "irrad Cd" of Table 1), irradiation L-19, K/R^ = (18.2 ± 2.2) % and in the case where a relatively well thermalized facility was employed (R^/Au) = 6.5), irradiation P-4, K/R^ = (10.2 ± 1.5)%. We found (30.0 ± 1.1) Ma as the mean value for the corrected age, that is compatible with (31.4 ± 0.5) Ma indicated by Green (1985) as the mean age of the Durango deposits. Analysing our T^^^^ ii^dividual measurements, = 5 j for 5 degree of freedom was obtained, implying that ~ 0.40. Besides the results shown above, we have also experimental results (2 different irradiations) by which the neutron dosimetry by thin uranium film is compared to the neutron dosimetry proposed by the Belgian group (Ingelbrecht et al, 1996). The latter is based on the calibration of natural uranium doped glasses (in the case of IRMN-540) through fluence measurements carried out using activation reactions in Au and Co, mainly. The ratio between the RU measured through IRMN-540 calibrated against Au and Co and Ru through thin natural uranium films presented a mean value of (1.093 ± 0.034), i.e. the fluence obtained with the Belgian calibration is significantly greater than the one obtained with thin uranium films. Bigazzi, G., S. Guedes O., J.C. Hadler N., P. J. lunes, M. Oddone, A. M. Osorio A., S.R. Paulo and A. Zuniga (1999), Rad. Meas., 31, 651-656. Bigazzi, G., J.C. Hadler N., PJ. lunes, T.C.W.P. Mello, L.M.S. Navia, S.R. Paulo and A. Zuniga G. (1995a), Brazilian J. Phys., 25, 246-251. Bigazzi, G., J.C. Hadler N., P.J. lunes, M. Oddone, S.R. Paulo and A. Zuniga G. (1995b), Nucl. Instrum. Meth. Phys. Res. A, 352, 588-591. Lederer, C.M. and Siriey, V.M. (1978), Table of Isotopes, seventh edition, Wiley-interscience, New York, USA. Guedes O., S., J.C. Hadler N., R J. lunes, S.R. Paulo and A. Zuniga (1999), submitted for publication in J. Radioanal. Nucl. Chem. Green, RF. (1985), Chem. Geol. (Isot. Geosci. Sect.) 58, 1-22. Hurley, RM. and Fairbairn, H.W. (1957), Trans. Am. Geophys. Union,38, 939-944. Young, EJ., Myers, A.T., Munson, E.L. and Conklin, M.N. (1969), U.S.G.S. Prof. Papers 650, D84. Bellemans, F., De Corte, R, Van den haute, R, Ingelbrecht, C. and Nicholl, C. (1996) International Fission Track Workshop, Gent, Belgium, abstracts, 6. Acknowledgments The authors are grateful to Dr. C. W. Naeser for providing the Durango apatite samples employed in this work, to FAPESP, Brazil, for sponsoring S. Guedes and C. Tello S. and to Dr. Massimo Oddone, from LENA reactor, Pavia, Italy, Dr. Agustin Zuiiiga, from IPEN rector, Lima, Peru, and Dr. Marina Koskinas, from IPEN/CNEN reactor, Sao Paulo, Brazil, for performing the neutron irradiations.

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

STRAIN PARTITIONING AND TOPOGRAPHIC RESPONSE OF FAULT BLOCKS ALONG THE SAN ANDREAS FAULT, SOUTHERN CALIFORNIA, FROM APATITE FISSION TRACK, ( U - T H ) / H E AND DEM

ANALYSES

A.E. Blythei, M.A. House2, J.A. Spotila3, K.A. Farley2 and D.W. Burbank^ 1 Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA 3 Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg,VA, USA 4 Department of Geosciences, Pennsylvania State University, University Park, PA, USA

The Transverse Ranges of Southern California formed and continue to undergo deformation as the result of compression associated with the 'big bend' in the San Andreas Fault (SAF). In a previous Study, Blythe et al (in review) used fission track and (U-Th)/He analyses to constrain the structural and topographic evolution of the central Transverse Ranges (San Gabriel and San Bernardino Mountains). Prior to ~7 Ma, the thermal history of the San Gabriel Mountains (SGM) appears to have been nearly identical to many of the core complexes in the Basin and Range of southeastern California with a major phase of cooling from ~60 to 40 Ma and a more recent phase beginning at - 2 3 Ma and continuing until - 1 0 Ma. During this time period the SGM were adjacent to the Chocolate and Orocopia Mountains. The most recent phase of cooling in the SGM began at ~7 Ma, as the result of the onset of contractional deformation associated with the SAF. In the San Bernardino Mountains (SBM), the two phases of cooling documented by the FT data occurred from - 6 5 to 55 Ma, and from - 1 8 Ma to the present, with the timing on the second phase very poorly constrained. Blythe et al (in review) documented variations in the total amounts and rates of cooling between different fault-bounded blocks within the SGM since 7 Ma. Comparisons of topographic features with denudation rates suggested that orographic effects in rainfall along the southern margin of the SGM, as well as more rapid bedrock uplift in the eastern part of the range, strongly affected the presentday physiography. Despite a higher mean elevation, the SBM were much less dissected than the SGM, suggesting either that the most recent phase of cooling and bedrock uplift began much later than that in the SGM or that localised climatic conditions have led to less erosion. A recent study of the SBM using (U-Th)/He thermochronometry supports the onset of surface uplift in the SBM in the last few million years (Spotila et al 1998). In this study, new FT data from fault blocks adjacent to the SAF are used to more closely examine the relationship between cooling rate and topography. Samples from these blocks consistently yield FT ages less than 10 Ma, and in combination with (U-Th)/He ages, relatively robust cooling rates can be estimated. The block undergoing the fastest cooling rate in the study area is the Yucaipa block, which is located south of the SBM along the SAF. This block has yielded the youngest FT ages (2.7 to 1.7 Ma) as well as He ages (1.4 to 1.2 Ma; Spotila et al, submitted) of any region within the central Transverse Ranges. Spotila et al (submitted) has suggested that this block is at its maximal sustainable elevation. Therefore, we use the topographic characteristics of the Yucaipa block (slopes and mean elevation), measured with a DEM, as a benchmark for comparison with fault blocks in the region undergoing slower rates of cooling. These fault blocks include the Wilson Creek block ( - 1 0 Ma FT age) to the northwest of the Yucaipa block, and Table Mountain and Circle Mountain Ridge (both with FT ages of - 8 Ma), which are located on the northeastern side of the San Gabriel Mountains. With these correlations, we hope to provide a better understanding of the interplay of tectonic and erosional processes, at least in the Southern California region. Blythe, A.E., Burbank, D.W., Farley, KA., and Fielding, E.J. in review. Structural and topographic evolution of the central Transverse Ranges, California, from apatite fission-track, (U-Th)/He and DEM analyses. Basin Research, spotila, J.A., Farley, K.A., and Sieh, K. 1998. Uplift and erosion of the San Bernardino Mountains associated with transpression along the San Andreas fault, California, as constrained by radiogenic helium thermochronometry. Tectonics 17, 360-378. Spotila, J.A., Farley, K.A., Yule, J.D., and Reiners, P.W. submitted to 1999 AGU Fall Meeting. Rapid, long-lived exhumation along the transpressive San Andreas Fault Zone in Southern California, based on U-Th/He dating. Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

H E R C Y N I A N T H E R M O T E C T O N I C E V E N T AT T H E N E

F T 2 '

M A R G I N OF THE PANAFRICAN

BASEMENT,

E A S T E R N D E S E R T , E G Y P T : E V I D E N C E F R O M F I S S I O N T R A C K D A T A ON Z I R C O N AND S P H E N E

A.-V Bojar and H. Fritz Institut fuer Geologie und Palaeontologie, Karl-Franzens Universitaet, Graz,Austria

Major portions of the Pan-African orogen, in tlie Eastern Desert of Egypt were formed during crustai consolidation in the Late Proterozoic. In the study area, between Safaga and Marsa Alam the basement is covered by Late Proterozoic volcanic arc rocks referred as the "Pan-African nappe complex". The basement domains are exposed within metamorphic core complexes bounded by NW trending sinistral shear zones. They form domal structures arranged in a NW-SE direction parallel with the Red Sea strike. Late Proterozoic molasse sediments discontinuously cover the volcanic arc complex. As the Paleozoic sedimentary cover is largely missing the thermotectonic history after the Panafrican event was constrained from fission track data on zircon and sphene. Sphene ages range from 340 to 416 Ma, zircon from 302 to 367 Ma. Single zircon grain age distributions show a high chi-square probability in comparison with the single grain age data on sphene. Zircon ages are interpreted to show a major cooling event during the Early Carboniferous time. In contrast, sphenes are considered to be only partially reset. Considering an annealing temperature for zircon of 240°C, and a normal geothermal gradient of 20°C/km, the method request that the actual section was buried at depth of 11-13 km at that time. Another option is that the fission track data indicate an high thermal gradient predating cooling and erosion. As at the end of Late Proterozoic the area was consolidated the ages are not constraining cooling after an orogenic event. More probably they are related to intraplate heat transfer, uplift and erosion starting with Late Devonian. This event could reflect structural reorganisation induced by the first contact between Gondwana and Laurasia.

Geological Society of

ia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

EXHUMATION H I S T O R I E S FROM DETRITAL ZIRCON

F T 2 '

THERMOCHONOMETRY

M.T. Brandoni,J.I. Garver2, M. Bemeti and J.A.Vance^ 1 Kline Geology Laboratory, Yale University, New Haven, CT, USA 2 Department of Geology, Union College, Schenectady, NY, USA 3 Department of Geological Sciences, University of Washington, Seattle, WA, USA

Fission-track dating of detrital zircons from unreset sandstones provides information about the thermal evolution of the source area for the sediment, including volcanism, normal faulting, and erosion. The rate of exhumation within the orogenic source region is indicated by the lag time of the fissiontrack grain age (FTGA) distribution. Lag time is the difference between the time of cooling and the time of deposition. Most of the lag time is associated with exhuming the source rocks from the zircon FT closure depth; sedimentary transport time is considered to be negligible in comparison. The geomorphic/erosional evolution of an orogen can be divided into constructional, steady state, and decay phases, as determined by the difference between the accretionary flux into the orogen and the erosional flux out of the orogen. The constructional phase would produce a decrease in the lag time of the FTGA distribution with decreasing depositional age of the sandstone sample (i.e. upsection decrease in lag time). Erosional steady state would appear as a constant lag time with decreasing depositional age (i.e. steady lag time). The decay phase would appear as an upsection increase in lag time. Lag time can also be used to assess tectonic exhumation. Unroofing of metamorphic core complexes by normal faulting can quench a large volume of the upper crust to a common zircon FT cooling age. Sediments derived from this type of setting would show an abrupt upsection decrease in lag time, roughly coincident with the time of tectonic exhumation, followed by a long interval where lag time increased at a 1:1 rate with the depositional age. The upsection increase in lag time marks the progressive erosion of the quenched crust, with zircon FT cooling ages coeval with tectonic exhumation. Eocene through Miocene sandstones derived from the Eocene metamorphic core complexes of the eastern North America Cordillera (eastern Washington State, Idaho, and southern British Columbia) provide a good example of this relationship. A more direct method for examining the geomorphic/erosional state of a convergent orogen is to convert zircon FTGA distributions into erosion rate distributions. There are several steps to this approach. The first is integrate results from zircon FT mounts with different etch times to reduce sampling biases related to the correlation of etch time with FT cooling age. The second is to convert the FTGA sample into lag times, which requires precise estimates of the depositional age of the sample. The third step is to convert the lag times into equivalent erosion rates. For this step, we use a lag time/erosion rate relationship that includes the influence of erosion rate on the thermal profile and the effective FT closure temperature for zircon as a function of cooling rate. This approach assumes that the zircon FT ages are primarily related to erosional cooling, and that the upper crust was at thermal steady state at the estimated erosion rate. The calculation requires an estimate of the initial thermal gradient before crustal thickening, and the thickness of the actively deforming crust. The result is an erosion rate estimate for each grain age. The erosion rate distribution is normalised to area by scaling for the increase in sediment yield (i.e. zircon yield) with increasing erosion rate. This information is especially useful for studying the geomorphic/erosional evolution of non-volcanic convergent orogens, such as the European Alps, Himalayas, Taiwan Alps, and New Zealand Alps. We have applied this approach to the zircon FTGA sample of Cerveny et al (1988) collected in the modern Indus River in the foothills of the western Himalayas. The erosion-rate distribution indicates an average long-term erosion rate of 0.5 km/m.y. for the upper Indus River, which is remarkably low given the high erosion rates estimated for the Nanga Parbat massif. However, it is useful to note that the upper Indus also drains a large part of southern Tibet where erosion rates are probably much

Geological Society of Australia - Abstracts Number 58

lower due to the orographic effect of the Himalayas. The erosion-rate distribution does show 10% of the drainage area with maximum erosion rates between 1 to 1.8 km/m.y. We are currently converting stratigraphically older zircon FTGA samples from Cerveny et al. (1988) to erosion rate distributions, and will present those results and discuss their implications for the geomorphic/erosional evolution of the Himalayas over the last 18 m.y.

International Conference on Fission Track Dating and Thermoclironology

F T 2 '

T H E R M O B A R O M E T R I C DATA F R O M A F O S S I L Z I R C O N P A R T I A L ANNEALING Z O N E IN H I G H P R E S S U R E - L o w T E M P E R A T U R E R O C K S OF E A S T E R N C R E T E ,

GREECE

M.R. Brixi, B. Stockherti, E. Seidel2,T.Theye3 and S.N.Thomsoni 1 Ruhr-Universitat Bochum, Institut fur Geologie, Mineralogie und Geophysik, D - 44780 BOCHUM, Germany 2 Universitat 2u Koln, Mineralogisch-petrologisches Institut, Ziilpicher Str. 49B, D - 50674 KOLN, Germany 3 Institut fiir Mineralogie und Kristallchemie der Universitat, Azenbergstr. 18, D - 70174 STUTTGART, Germany

The island of Crete represents a horst in the fore-arc of the Hellenic subduction zone. Its internal structure developed during the collision of a microcontinent in the subducting African plate with the active European continental margin in Oligocene/Miocene times. Wide areas on the island expose rocks of the Phyllite-Quartzite Unit (PQ) representing the Permian and Triassic sedimentary cover of the microcontinent. These rocks were buried, metamorphosed under high pressure - low temperature (HP-LT) conditions and exhumed by buoyant escape within only a few million years. The zircon fission track (FT) ages of samples from the PQ across the whole island display wide differences ranging from detrital through partially reset to completely reset values. The resetting results from a single heating period related to the HP-LT metamorphism. Correlation with independent thermobarometric, microstructural and new experimental annealing data has allowed a better assessment of the limits of the zircon partial annealing zone (PAZ). In eastern Crete, where rocks have experienced 300±50°C and 8±3 kbar, zircon FT ages range from 4l4±24 Ma to 158±12 Ma. Ages above 300 Ma occur mostly near the east coast of the island and probably represent a pre-Variscan source. The zircon FT ages of many samples, however, are younger than their depositional age. This implies that the zircon fission tracks in these samples have experienced temperatures reaching well into the PAZ after deposition. The common occurrence of detrital and partially reset ages in rocks with independent thermobarometric indication of at least 250°C, but probably more, makes this temperature a minimum value for the lower boundary of the PAZ in the given situation. The lack of completely reset ages in the rocks of the PQ in eastern Crete implies that since their deposition they were not exposed to temperatures significantly above the lower boundary of the zircon PAZ, i.e. only to temperatures in the lower part of the error range indicated by the thermobarometric data. Another factor which may be responsible for this only partial annealing is the short period of time which has to be assumed for the peak metamorphism in eastern Crete. According to Tagami et al (1998) complete annealing should occur at 350±50°C over a period of 10 m.y.. Based on zircon and apatite FT data as well as overlying Neogene sediments, for the PQ on Crete a cooling rate of about 45°C/m.y. over 5 m.y. has to be taken into account. West of a fault in the Dikti Mountains, a sudden decrease to younger ages ranging from l6.8±1.3 to 22.3±1.3 Ma is observed. Such ages are found consistently throughout the central and western part of the island. Thermobarometric data for this area indicate maximum temperatures of 400±50°C and pressures of 10±2 kbar. Thus the complete resetting of the ages is compatible with the Tagami et al (1998) data and confirms that no fission tracks in zircons survive over geological times at temperatures exceeding 350°C. Additional thermobarometric data on the stability in zircons have been obtained from annealing experiments at 5 kbar and different temperatures. The results fit curves obtained by Tagami et al (1998) at ambient pressures, thus indicating that pressure has no significant influence on the stability of fission tracks in zircon. Tagami, T. et al. (1998). Solid Earth Sci. Libr., 10, p. 99 - 112 (Kluwer).

Geological Society of A u s t r a l i P ^ b s t r a c t s Number 58

B

FT2

International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

F T 2 '

T H E P R O B L E M , T H E DATA, SOME M O D E L S AND T H E I R P U R P O S E

R.W. Browni, K. Gallagher2 and C. Johnson2 1 School of Earth Sciences,The University of Melbourne, Victoria 3010, Australia 2T.H. Huxley School of Environment, Earth Science and Engineering, Imperial College of Science,Technology and Medicine, London, SWT 2AS, England

The problem All interpretations of all data involve the use of models-always. And within the earth sciences a core component of the models is how they handle what happens at unsampled locations-that is, how they tackle interpolation and extrapolation of the model behaviour between and beyond data. The models may take many forms, being sometimes simply a vague hunch the analyst uses, possibly unconciously, as a navigation aide in exploring the data. For example, plotting boomerang plots of fission track age against mean track length-why do you think we do this? Interpolation schemes, like models, are ubiquitous and are usually informal. How often do you see publications entitled, "The thermotectonic history of 23 sample sites within the Whatsisname Range" versus, "The thermotectonic history of the Whatsisname Range'^ Usually, though, the models, unlike interpolation schemes, are explicit, and mostly probabilistic. This is largely because problems within the earth sciences are seldom sufficiently well understood to justify the use of deterministic models. It also significantly simplifies the statistical treatment of the data and the calculation of uncertainties. An apposite example of a disarmingly candid statement of the problem can be found in Geostatistics with Mo and Ed (Isaaks and Srivastava, 1989, p. 200). "In a probabilistic model, the available sample data are viewed as the result of some random process. From the outset, it should be clear that this model conflicts with reality. The processes. ..are certainly extremely complicated, and our understanding of them may be so poor that their complexity appears as random behaviour to us, but this does not mean that they are random; it simply means that we are ignorant. Unfortunately, our ignorance does not excuse us from the difficult task of making predictions about how apparently random phenomena behave where we have not sampled them." In this paper we explore some approaches to tackling this task with regard to interpreting thermochronologic data in the broad context of landscape evolution studies. The approaches can be conveniently grouped into the traditional divide-and-conquer genre, where the modelling and interpolation tasks are explicitly separated, or an interpolate-and-solve approach, where the interpolation and modelling tasks are treated as integral sub-tasks of a higher order, unified model. The performance of the second approach remains to be thouroughly tested, but in theory it has numerous advantages over the first. The data Simply put, there are never enough, they're not usually where we'd like them to be and they're too complicated. Earth scientists' staple diet consists of heterogeneous, sparse, 3-dimensional data sets and qualitative guesses. Complicated and heterogeneous in the sense that we have to consider, for example, such varied factors as the geographical extent, age and continuity of lithological units and unconformities along with a variety of quantitative point estimates (e.g. track length distributions and U-Th/He ratios in apatite). Sparse in the sense that we're invariably seeking information about whole orogens, rifts or cratons using data from a handful of sample sites. It is ironic, however, that for many areas of the world these 'sparse' data sets are already unmanageably large, further emphasising that Geological Society of Australia - Abstracts Number 58

FT2^ee we need to devise more efficient ways of managing and analysing them. Qualified guesses are an integral and important type of geological data. For example, the expectation that palaeotemperature estimates determined for an array of samples should always increase with depth at a particular time may be construed as a reasonable guess. Or, that the amount of estimated erosion must be less than or equal to the total thickness of the crust. And we have to contend with the added fact that all these data are time dependent and possibly discontinuous. A more optimistic view is that we have a ready array of complimentary data types at our disposal each of which contains unique information. Which begs the question-Are there models capable of explicitly utilising the diversity of data types?

Some models and their purpose The purpose of probabilistic models, with or without interpolation, is to estimate parameters from the available data which can be compared with the predictions of those parameters made by conceptual or theoretical models of hypothetical geological processes. In this context estimating the variation of palaeotemperature with time and space is a key objective. Once this is accomplished other, possibly more interesting, derivative parameters may then follow. For example, the pattern and amount of denudation is clearly of interest to understanding how landscapes develop and change over geological time scales. Estimates of palaeotopography, based on the prior estimates of denudation combined with tectono-isostatic models, could then be used to examine palaeodrainage patterns and hence transport of the eroded material. Standard hydrological algorithms are easy to implement and they have properties which can be adapted to sediment transport modelling on geological time scales. We already have well constrained empirical models which describe the behaviour of a range of thermochronologic systems. We also have a range of well understood stochastic methods capable of deriving thermal histories from these individual systems, such as heuristic guided search algorithms using GA's for example. These models are normally combined to derive thermal history information for single samples independently and the results are interpolated either implicitly or explicitly using a favoured formal interpolation scheme. Kriging, which is a generic name for a whole family of generalised least squares regression algorithms, is a versatile and powerful approach to interpolation. It also has some very attractive properties in this context in that it readily allows for the incorporation of secondary data, both continuous and categorical, which are used to guide the estimation of the primary variable of interest. For example, using topography as a secondary variable to guide the estimation of palaeotemperature. We also like the fact that the method was invented by a South African, Danie Krige, who was a Masters student at the time (Krige, 1951). The logical, and relatively straightforward, next step is to adapt these divide-and-conquer methods to explicitly incorporate the full range of thermochronologic systems for which we have empirical models. However, valuable information, which resides in the geometric distribution of sample locations, is ignored by the divide-and-conquer approach. A second step is therefore to devise modelling schemes that determine thermal histories for a region, rather than a point, by explicitly incorporating the spatial distribution of a series of data into the model. It is this second step that necessitates making the interpolation task an integral part of the modelling procedure, and hence suggests that a new interpolate-and-solve approach to the problem may be more rewarding, and well worth the extra computational effort. The basic concept here is to find a thermal history for the 3 dimensional region containing the sample locations that simultaneously satisfies all the data. This can be accomplished by populating a 4dimensional model ix,y,z,i) with palaeotemperature estimates and deriving thermal histories for each sample location using a 4-dimensional interpolation scheme (such as a multi-dimensional natural neighbour algorithm). A GA approach could easily be adapted to search the 4-D space using a collective fitness function calculated over the interpolated thermal histories for all sample locations. This interpolate-and-solve approach could be modified to incorporate a two-stage interpolation of geothermal gradient and sample depth (relative to present sample depth) in place of palaeotemperature.

FT2 The advantage being that this approach would allow, and estimate, transient changes in geothermal gradient and that sample locations are allowed to move relative to their present positions (i.e. discontinuities are allowed and should arise naturally if the data require them). In fact, there is no theoretical reason for restricting sample movement to be vertical. There may, however, be very good reasons for doing this due to the sparseness and limited resolution of the data. But for orogenic and transform plate boundary environments it may be a useful property. Finally, there is clearly tremendous scope for harnessing Geographical Information System (GIS) and data base technology here. Not only for house keeping tasks like managing the input data and the model results, but for deriving constraints on the thermal history model from geological data. For example, GIS covereages of unconformities and stratigraphic units could be readily used to automate this process.

a priori

Isaaks, E.D. and Srivastava, R.M. 1989, An Introduction to Applied Geostatistics, Oxford University Press, New York, 561 p. Krige, D.G. 1951, A Statistical Approach to Some Mine Valuations and Allied Problems at the Witwatersrand, Masters thesis. University of the Witwatersrand.

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

THE SIGNIFICANCE OF INHERITED FISSION-TRACKS IN APATITE ON MODEL PREDICTED THERMAL HISTORIES A. Carteri, K. Gallagher2 1 Research School of Geological and Geophysical Sciences, Birkbeck College and University College, London, Gower Street, London WC1E6BT, UK 2T.H. Huxley School of Environment, Earth Science and Engineering Imperial College of Science,Technology and Medicine South Kensington London SWT 2AS, UK

The development of quantitative 'annealing' models that describe the thermal dependence of fissiontrack annealing in apatite has enabled reconstruction of low temperature thermal histories via various forward model and optimisation procedures. The level to which any thermal history can be resolved is dependent on the quality and type of sample data, choice and construction of annealing model and type of modelling procedure. Although there is a general awareness of these issues, currently, most effort is devoted to understanding the impact and significance of variations in the chemistry of apatite grains. In this study we have considered how the type of data (e.g. form of length distribution, time difference between pre- and post-depositional history), may affect the inference and resolution of the optimal thermal history. A commonly encountered scenario during thermal history modelling is where a sample, having experienced some earlier form of thermal history, encounters a later phase of reheating and/or cooling. For example: detrital grains in a sedimentary basin can contain a fission-track signature diagnostic of the source region's thermal evolution. Subsequent track accumulation, burial/heating, and erosion/cooling will modify this 'inherited' record. Where the level of post-depositional heating is low inherited tracks will experience only minor modification and some provenance signature will be preserved in model predicted thermal histories. With increased post-depositional annealing, the record of provenance diminishes and the subsequent thermal history dominates. Thus, there is a transition from provenance-dominated to post-depositional-dominated situations. Consequently, there is also potential for the provenance-related thermal history information to bias model predicted thermal histories. We examined the conditions under which a provenance signal can cause significant disruption to resolution of model predicted thermal histories. Two clearly significant factors are identified; the shape of the inherited length distribution and the time difference between pre-depositional track accumulation and subsequent re-heating. These issues will be discussed in the context of data driven modelling procedures.

Geological Society of Austral^^^bstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

A COMBINED F I S S I O N - T R A C K AND COSMOGENIC ISOTOPE ANALYSIS APPROACH TO QUANTIFYING L O N G - T E R M LANDSCAPE CHANGE: AN EXAMPLE FROM THE CENTRAL NAMIBIAN CONTINENTAL MARGIN

H.A.R COCKBUMI'2, R.W. Brown2, M.A. Summerfieldi and M.A. SeidP 1 Department of Geography,The University of Edinburgh, Drummond St., Edinburgh, EH8 9XP, United Kingdom 2 School of Earth Sciences,The University of Melbourne, Parkville, 3052, Australia 3 Pew Charitable Trusts, One Commerce Square, 2005 Market St., Suite 1700, Philadelphia, PA 19103-7017, USA

Numerous studies have demonstrated that fission-track thermochronology yields valuable information on regional variations in long-term (10^ - 10^ a) denudational records across a range of morphotectonic settings, including passive continental margins (Brown et al., 1994). However, fission-track data alone generally have insufficient spatial and temporal resolution to identify particular modes of landscape development and constrain denudation rates over recent geological time. By contrast, cosmogenic isotope analysis, which involves the measurement of the depth-dependent, in-situ accumulation in near-surface materials of particular isotopes through interactions of target elements with cosmic radiation, can provide site-specific estimates of denudation rates over shorter time scales of 10^ - 10^ a. Combining fission-track thermochronology with cosmogenic isotope analysis therefore provides a powerful strategy for constraining denudation rates across a broad range of geological time scales and for characterising modes of long-term landscape development. Here we illustrate the integration of apatite fission-track thermochronology (AFTT) and i^Be and cosmogenic isotope analysis in a study of the Namibian passive continental margin of southern Africa (Fig. 1), and show that by combining these techniques it is possible to test contrasting models of passive margin landscape evolution.

22° S

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^ Cosmogenic sample site Drainage divide Escarpment zone O Fission track sample site Topographic contours (m) 88-151 Map area © FT age only (Haack, 1983) Rivers (non-perrenial)

Geological Society of Austral

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The main topographic elements of the central Namibian margin are typical of a high-elevation passive margin. Along our sampled transect (~lat. 22-24° S) there is a well-defined major escarpment with a relief of up to -1000 m (Fig. 2). It is located 170 km inland of the present coastline and separates a gently inclined coastal plain from an interior plateau with a mean elevation of -1800 m. The escarpment is generally coincident with a major drainage divide and reaches a maximum elevation of 2347 m at the Gamsberg, a flat-topped residual. Present climate ranges from hyper-arid at the coast to semiarid at the base of the escarpment (mean annual precipitation <15 to 100 mm) and is strongly affected by the prevailing sub-tropical high pressure system and the presence of the cold Benguela current. Terrestrial and offshore palaeoenvironmental data indicate that arid to semi-arid conditions have persisted across the coastal plain for at least the past 10-12 m.y. under the influence of the adjacent Benguela current, but also possibly throughout much of the Cenozoic (Ward, 1987). Bedrock samples, predominantly of granite-gneiss, were collected from 20 locations for apatite fission track analysis. Quantitative thermal histories were modelled from the AFTT data for each of the fission-track sample locations, with mean denudation rates being estimated from these modelled palaeotemperatures (Gallagher, 1995). Between the time of break-up and margin formation (-130 Ma) and the end of the Eocene mean denudation rates for the coastal plain average -40 m Ma-i, but fall to a mean of - 5 m Ma ^ up to the present. Inland of the escarpment mean rates of denudation since break-up remain relatively constant at around 10 m Ma-i (Fig. 2). Sampling sites for cosmogenic isotope analysis were selected in order to characterise typical rates on key landform components. In the escarpment zone 13 granite and quartzite surface samples were collected along an 800 m profile from flanking slopes, the escarpment free face and the flat summit of the Gamsberg. Six samples from three representative biotite-granite bornhardts across a zone 40-80 km from the coast were collected to constrain rates of denudation on the coastal plain. The steadystate erosion model of Lai (1991) was used to calculate denudation rates from concentrations of cosmogenic i^Be and ^^M. This model assumes that isotope concentrations have reached secular equilibrium and that denudation has occurred in increments that are small in comparison to the cosmic ray attenuation length (-0.6 m). Although these assumptions are supported by the very close agreement in some samples between the denudation rates estimated from the two isotopes and therefore should be considered as maximum rates, the Be/Al ratios in a few samples indicate more complex exposure histories, possibly involving episodic detachment of relatively thick layers of rock. Given these uncertainties, we treat our results as approximate mean denudation rates.

FT2 The mean rate of escarpment retreat determined from combined results for the free face and flanking slope samples at the Gamsberg is -10 m Ma-i, while the summit samples yield very low denudation rates with a mean value of 0.42 ± 0.1 m Ma-i. The mean denudation rate for the coastal plain bornhardts is 5.07 ± 1.1 m Ma-i. The assumption of prolonged denudation and secular equilibrium inherent in the steady-state erosion model means these rates are necessarily integrated over the past 1-2 X 105 a for the bornhardt samples, ~8 x 10^ a for the escarpment samples, and -10^ a for the slowly eroding Gamsberg escarpment summit samples. However, given the apparent persistence of arid conditions similar to those of the present throughout the Quaternary, and possibly throughout much of the Tertiary, it is probable that rates broadly similar to these have prevailed throughout the late Cenozoic. The AFTT data support the extrapolation of our average cosmogenic denudation rates since they indicate a mean rate of denudation across the coastal plain of ~5 m Ma-i since the end of the Eocene, and a somewhat higher rate of denudation in the zone of recent escarpment retreat (Fig. 2). Our combined AFTT and cosmogenic isotope data are consistent with an initial phase of rapid denudation in the coastal zone until the end of the Eocene, promoted by the high local relief generated by rifting and the establishment of new lower base levels, followed, at the end of the Eocene, by a period of much lower denudation rates. Existing morphotectonic models of high-elevation passive margins, such as that of central Namibia, have generally assumed that landscape development is dominated by the retreat of a major escarpment initially formed along the coast at the time of breakup, but recent surface process modelling studies have questioned this style of landscape development (Gilchrist and Summerfield, 1990 and Beaumont et al, 1999). Given the age of break-up (-130 Ma) for Namibian margin and the present position of the escarpment, this would require a mean retreat rate of >1000 m Ma-i. The difference of two orders of magnitude between this hypothesised retreat rate and that estimated here for the Gamsberg sector of the escarpment strongly suggests that the landscape has not evolved through the progressive inland migration of a major escarpment. Given the magnitude of this difference it is also highly unlikely that the very slow retreat rates we estimate for the past ~8 x 10^ yr can be explained by variations in lithological resistance or climate. The regional pattern of post-break-up denudation revealed by the AFTT data, combined with the constraints on the escarpment retreat and erosion surface downwearing provided by the cosmogenic isotope data, are consistent with a model of landscape evolution in which any initial escarpment that may have been formed at the coast at the time of break-up was degraded by river systems flowing from an inland drainage divide and adjusting to the new base level at the coast. We speculate that the initial location of the present escarpment was controlled by a major inland drainage divide separating low-gradient interior drainage from the higher gradient river systems flowing to the newly formed South Atlantic margin. The escarpment probably originated only a few kilometres oceanward of its present location and its subsequent slow rate of retreat has been controlled by pinning at the drainage divide, possibly enhanced by flexural isostatic rebound. This interpretation is supported by recent surface process modelling of passive margin landscape evolution which indicates the importance of drainage divides in controlling the location and evolution of major escarpments. The denudation rate data reported here challenge previous assumptions about the significance of escarpment retreat in passive margin landscape evolution and illustrate the potential for an integrated cosmogenic isotope analysis and AFTT approach to quantifying long-term denudation rates and determining modes of landscape development. It is clear the incorporation of the low temperature thermochronology that can be provided by the (U-Th)/He system would further enhance the kind of approach we have proposed here by providing information on depths of crustal stripping intermediate between AFTT and cosmogenic isotopes analysis. Beaumont C. Kooi H. and Willet S. 1999. Progress in coupled tectonic-surface process models with application to rifted margins and collisional orogens. In Summerfield M. A. (ed) Geomorphology and Global Tectonics, Wiley, Chichester 29-55. Brown R. W. Summerfield M. A. and Gleadow A. J. W. 1994. Apatite fission track analysis: Its potential for the estimation of denudation rates and implications for models of long-term landscape development. In Kirkby M. J. (ed) Process Models and Theoretical Geomorphology, Wiley, Chichester 23-53.

FT2 Gallagher K. 1995. Evolving temperature histories from apatite fission-track data. Earth and Planetary Science Letters 136, 421-435. Gilchrist A. R and Summerfield M. A. 1990. Differential denudation and flexural isostasy in formation of rifted margin upwarps. Nature 246, 739-742. Haack, U. 1983. Reconstruction of the cooling history of the Damara Orogen by correlation of radiometric ages with geography and altitude. In Martin, H. and Eder, F. W. (eds) Intracontinental Fold Belts, Springer-Verlag, Beriin, 873-884. Lai D. 1991. Cosmic ray labelling of erosion surfaces: in-situ nuclide production rates and erosion models. Earth and Planetary Science Letters 104, 424-429. Ward J. D. 1987. The Cenozoic succession in the Kuiseb Valley, central Namib Desert. Geological Survey of Namibia Memoir 9. Acknowledgments: We thank A. L. Hubbard for exposure geometry corrections, R. Finkel (Lawrence Livermore National Laboratory) for AMS analyses, and the Geological Survey of Namibia for logistical support. This research was supported by De Beers Mining Company Ltd., the Natural Environment Research Council (HAPC/MAS) and through grants to A. J. W. Gleadow from the Australian Institute of Nuclear Science and Engineering and the Australian Research Council (RWB).

International Conference on Fission Track Dating and Thermochronology

F T

VISUALISING TECTONIC DISPLACEMENTS ACROSS A REACTIVATED TERRANE USING APATITE FISSION TRACK DATA: THE SNOWY MOUNTAINS, SOUTHEASTERN AUSTRALIA

SJ.D. Coxi, B.P. Kohn2 and AJ.W. Gleadow2

Australian Geodynamics Cooperative Research Centre 1 CSIRO Exploration and Mining, Nedlands,WA 6009, Australia 2 School of Earth Sciences, University of Melbourne, Victoria 3010, Australia Introduction

A lengthy period of tectonic quiescence leads to the establishment of a stable thermal regime in which a well-defined partial annealing gradient is developed, within the upper few kilometres of crust. Subsequent denudation brings the rocks successively out of the annealing zone, such that those higher in elevation will display an older apparent apatite fission track (AFT) age. The AFT parameters form a profile characterising paleodepth thermochronological markers within the rocks which can be used as an "invisible" stratigraphic tool, particularly in crystalline terranes. Considered over an area, segments of the profile maybe offset relative to each other across important structures, enabling the estimation of relative uplift between blocks and the amount of throw on bounding faults, and even the detection of structures where no visible expression of such a disruption is now apparent. The Snowy Mountains

The Snowy Mountains of the southeastern Australia passive margin are centred around Mt. Kosciuszko (2228 m), mainly comprising Paleozoic granitoid rocks of the Lachlan fold belt. A striking feature of the region is the contrast between the stepped fauk block morphology of the uplifted Kosciuszko massif and the old, gently dipping, erosion surface of the Monaro tableland to the east. The mountain blocks rise up to km above the tableland surface. Remnants of an erosion surface are also preserved at higher elevations in the Kosciuszko uplift, a combination of high relief, and suitable apatite-bearing lithologies provide ideal thermochronological markers which can be used to reconstruct the tectonic disruption of the palaeo surface. Maps published by Wyborn et al. (1990), and a regional compilation of linear features from air-photos by Browne (1969) allow us to identify a large suite of potential faults. We have assembled an AFT data set comprising 115 samples from the Kosciuszko region including nine vertical profiles which together cover over -1700 m of elevation, from wide areal sampling and along tunnels excavated through the mountains during the course of the Snowy Mountains hydroelectric scheme. Modelling

AFT Profiles, master profile Three subsets of the AFT data set were designed explicitly to sample extended elevation ranges. Plotting the i'th sample elevation zi against its apparent AFT age t^ reveals a set of parallel curves, with a monotonically increasing section between ~90 Ma to 230 Ma, and constant ages for lower elevations. We assume that the offset between these curves corresponds to differential vertical movement between blocks containing the different traverses. Correcting the curves by adding a constant value Azj to the each of the elevation values for samples from the 7 th profile brings the suite of measurements into approximate coincidence and establishes a profile Z(t) of pre-offset depths, for which we set Z(100 Ma) (arbitrarily) to zero (Kohn et al, 1999). Geological Society of A u s t r a l R P ^ b s t r a c t s Number 58

FT2 The surface of relative vertical

displacements

If the apparent AFT age is t-tdti then the corresponding pre-offset depth is Z(tp}, within limits Z(tfdt^, Z(t^+5t^. The displacement for each sample point, relative to its pre-offset depth, is rji = z^-ZCtp) with limits of ZfZCtfk^), z^-ZO^+St^. The hi's give us a consistent estimate of vertical displacements since the time when the master profile Z(t) was uniform through the area. If the movements of the blocks containing the sample sites are primarily vertical since reactivation, then these are samples, at a set of locations within a surface H(x,y) giving the relative vertical displacements of the entire study area. This surface may be approximated based on the constraints

We used two methods:

1. A smooth and continuous surface over the whole area was calculated using the regularised spline with tension (RST) method (Mitas and Mitasova, 1995). Two adjustable parameters, corresponding to the stiffness or thickness of the plate or membrane, and to the weighting given to each control point, are available. We set control point weighting to be inversely proportional to the errors St^, so that the continuous surface H^, passes within the limits corresponding to 28t^ at each point. This method produced an irregular surface with many locally steep gradients. 2. The area was divided into 13 blocks based on mapped faults. The model minimises the total number of blocks consistent with the fault population, whilst grouping sample points with similar rj^ into the same block. The RST method was used to estimate the surface, using the same control parameters, but run separately for the set of points within each block. The resulting patches were then merged to produce an estimate of the complete surface H^ which has a set of discontinuities located at the block margins, but with mainly flat patches within each block. Discussion The results of the modelling shows that the general topography is predicted quite well. High elevations near Mt Kosciuszko and the elongate Barney's Range are particularly well resolved, the latter particularly in the discontinuous model. However, there are a number of areas where the relative elevations do not appear to be directly related to uplift that can be measured in the AFT parameters. In the south of the area the apparent ages are greater than 200 Ma which is at the extreme end of the profile, and thus do not allow the elevations to be very well resolved. In the block to the south of the Crackenback Fauk a topographic high is not seen in the AFT predictions, and to the north high elevations around Mt Jagungal are not resolved, mainly due to the sample distribution. Several considerations influence which approximation of H is preferred. The continuous version is compatible with deformation is distributed in shear-zones and folds. The discontinuous version models relative vertical movement of blocks, assuming that bounding fauks are vertical with dip-slip displacements, deformation within blocks is minimal, and the AFT profile within each block is uniform. In the simplest case the latter would result in flat, horizontal patches but this has not been achieved. Some possible reasons for this are: (a) the fauk network selected is wrong; (b) the blocks are not internally rigid; (c) the pre-deformation thermal profile was not spatially uniform. Unfortunately, despite being one of the most densely sampled areas available, the data is probably insufficient to resolve (a) or (b), in particular as some of the blocks contain very few sample points. Overall, the network was selected in an ad hoc manner to be largely consistent with the a priori geological information, but certainly only represents one possible solution. The continuous version He is certainly derived in a more objective manner, but omits an important known component of the local tectonics; viz. the major high angle faults.

FT2 Conclusions We have shown how thermochronologic information relating to the upper layers of continental crust can be used for quantitative spatial modelling of tectonics in a crystalline terrane. We have developed a method which includes the conversion of age data to a spatial parameter (displacement), appropriate ways of spatially interpolating this in an area where some prior geological information is also available, and the use of visualisation techniques in evaluating the results. Applied to a small area of high relief in south-eastern Australia, we find that the present day topography is predicted quite well from the assumption that local deformation over the last 100 Ma has been mainly vertical, after a previous period of stability which included the development of an erosion surface and a stable and uniform thermal profile. Browne, W.R., 1969. Geomorphology of New South Wales. Journal of the Geological Society of Australia, 16, 559-569. Kohn, B.P., Gleadow, A.J.W. and Cox, S.J.D. 1999. Denudation history of the Snowy Mountains: constraints from apatite fission track thermochronology. Aust. Journal of Earth Sciences. 46, 181-198. Mitas, L. and Mitasova, H., 1995. Interpolation by regularized spline with tension: I. Theory and implementation. Mathematical Geology, 25, 641-655. Wyborn, D., Owen, M., and Wyborn, L., 1990. Geology of the Kosciusko National Park (1:250 000 scale map) Bureau of Mineral Resources, Canberra. Acknowledgments Funding for this project was provided by the Australian Institute of Nuclear Science and Engineering and the Australian Geodynamics Cooperative Research Centre (AGCRC). The work reported here was conducted as part of AGCRC project 2005LO, and this work is published with the permission of the Director, AGCRC. For more information see http://www.agcrc.csiro.au/projects/2005LO/.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

PERFORMANCE CAPABILITIES OF THE C S I R O U A ' H - H E THERMOCHRONOLOGY FACILITY

RY Crowhursti, B.LA. Mclnnes^, D.B. Patterson^ and NJ. Evans^ 1 CSIRO Petroleum Resources, PO Box 136, North Ryde, NSW, 2113,Australia 2 CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW, 2113,AustraHa 3 School of Earth Sciences, University of Melbourne, Parkville,Vic, 3052, Australia

The CSIRO helium extraction and analysis facility comprises an all-metal helium extraction and gas handling line connected to a dedicated on-line quadrupole mass spectrometer. Gas extraction is performed using a single vacuum resistance furnace, where samples are heated to ~1050°C for -20 minutes. The line and furnace are evacuated to ~10-8 Mbar. Active gases, particularly hydrogen are removed using SAES getters. ^He abundances are determined by isotope dilution using a pure ^He spike, which is calibrated on a daily basis against an independent ^He standard tank. ^He hot blanks (or re-extracts) are performed routinely before and after each sample. Up to 6 capsules can be loaded into the sample holders. If the ^He standard and blank levels are acceptable a sample capsule is dropped into a ceramic crucible within the furnace. After the heating and purification procedure the extracted gas is handled and measured via a fully automated computer controlled system. The sample is repeatedly reheated until an acceptable blank level is reached and only then is the next sample analysed. When all of the samples are completed they are removed from the crucible and the U-Th concentration is determined on the same aliquot. The U and Th content of degassed apatite samples are determined by ICP-MS. 235u and 23(yrh spike solutions are added to the samples, which are then dissolved in concentrated nitric acid. Standard solutions Qohnson Matthey) are similarly spiked and acidified. Blanks are prepared by adding an equivalent amount of nitric acid to washed, empty capsules. The blanks, standards and samples are all diluted to 5% nitric acid just prior to analysis. Based on replicate analysis of standard solutions, precision for 235u/238u ^nd ^^^Th/^^^Th determination is 0.5% and 1%, respectively. In order to test whether the facility was able to produce valid ages, multiple aliquots of a crushed standard apatite (Durango) were analysed and the resultant ages were consistent with ages determined at the California Institute of Technology. The average age calculated from 5 sample runs at Caltech yielded 30.4 ± 0.6 Ma and 19 Durango analyses from the CSIRO facility yielded an age of 29.8 ± 1.1 Ma. In addition, we have developed the capability to analyse single apatite crystals and provide reproducible cooling ages for samples <1 Ma.

Geological Society of AustralHP^bstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronoiogy

FT2'

INTERPLAY OF INTRAPLATE TECTONICS AND SURFACE PROCESSES IN CENTRAL SPAIN, ASSESSED BY APATITE FISSION TRACK ANALYSIS

C.H. de Bruijne and P.A.M.Andriessen Isotope Geochemistry, Faculty of Earth sciences, Vrije Universiteit, Amsterdam, the Netherlands

The Iberian peninsula is a microplate situated between Africa and Europe. The Alpine plate tectonics effecting the peninsula have been controlled by the convergence between Africa and Europe and the related opening of the Atlantic. After the extensional collapse of the thickened Hercynian crust during the Permian, the peninsula experienced an asymmetrical rift opening of the Pyrenean and Iberian basins. The Eocene to Mid Miocene N-S Pyrenean compression induced inversion of these two basins, the present day manifestations being the Pyrenees and the Iberian range (Fig. la). The Oligocene to Recent NW-SE Betic compression enhanced further compression in Central Spain. Apatite fission track (AFT) analysis is used as a tool to relate the surface expression to tectonics and to constrain, in time and magnitude, the far-field effects of the Alpine plate tectonics, expressed by reactivation lineaments in the Hercynian basement. Central Spain comprises the Tertiary Madrid basin and surrounding mountain ranges. The more or less triangle-shaped continental basin is bounded by the Sierras de Guadarrama and Credos to the north-west to west, the Toledo Mountains to the south, and to the east the Iberian Range. 57 granitic and gneissic samples were analysed from the Hercynian basement, 3 Bundsandstein-samples from the Mesozoic cover, and 2 boulders from the deformed Paleogene alluvial fan deposits along the rim of the Tertiary continental Madrid Basin.

Figure 1. Minimum amounts of denudation during the Tertiary have been estimated from the modelled thermal histories of 5 samples. Errors range from 0.4 to 1.1 km. From these amounts of denudation, the minimum amounts of absolute Tertiary uplift were deduced, with a correction for eustatic sea level change between the C-T boundary and the present day. Maximum min. amount of Tertiary uplift is 7.5±2.2 km. Geological Society of Australia - Abstracts Number 58

FT2 In this study, two major periods of accelerated cooling were found: A middle Cretaceous cooling event, and a Mid-Miocene to recent event. Late Cretaceous and Late Eocene to Early Oligocene accelerated cooling events are restricted to smaller parts of Central Spain. From AFT analysis of two Bundsandstein samples from the northern border of the Madrid basin, we found evidence for Jurassic sedimentation prior to the Cretaceous cooling event. At least 3 km of Jurassic sedimentation preceded the deposition of the Utrillas fm. and Upper Cretaceous deposits. The erosion of the Jurassic sediments occurred during a regional tectonic event around 100 Ma with cooling rates of 20°C/m.y., interpreted as a flexural response to extension in the Iberian basin, related to the opening of the Atlantic. This tectonic cooling event can be detected across the entire Sierra de Guadarrama, and in the easternmost part of the Sierra de Credos. Since the western part of the Iberian range experienced tectonic subsidence in the same period of time (van Wees et al, 1998), the tectonic cooling is interpreted as a flexural response to extension in the Iberian basin, related to the opening of the Atlantic. Both Sierras also reveal a Mid-Miocene to recent (up to 100°C in 3 m.y.) accelerated cooling event, congruent with the neotectonic setting as described by De Vicente et al (1996) from structural- and earthquake-data. The greatest period of accelerated cooling occurred in the Pliocene and Quaternary and affected almost the entire Sierra de Guadarrama, parts of the Sierra de Credos and even a part of the Extremadura. A greatest minimum Pliocene denudation rate of 1.2 mm yr^ is calculated. The Pliocene uplift and denudation constitutes most of the Tertiary uplift and denudation (Fig. lb). This is explained as a superposition of the tilting of eastern and central Spain towards the SE, due to the opening of the Valencia trough, on top of the uplift of the Sierra de Guadarrama and to a lesser extent the Sierra de Credos, due to the NE-SW compression induced by convergence in the Betics. Minimal absolute uplift in the Tertiary is greater in the south-eastern part of the Sierra de Guadarrama compared to the north-western part, with the greatest uplift occurring in the central part: 7.5±2.2 km. We explain this with a progressive basinward stepping reverse fault, bordering the Madrid basin, from the Lower to the Middle Miocene. None of the events can be detected in the modelled histories from the Toledo mountains. The samples from this area only show very slow cooling from the Permian to the present day, except for the easternmost sample, revealing Jurassic accelerated cooling. The Sierra de Guadarrama, the Madrid basin, the Iberian Range, and to a lesser extent the Sierra de Credos are shaped by the Alpine intraplate tectonics. The topography of the of the Toledo mountains is controlled by Hercynian morphostructures, except for the step towards the Madrid basin, which is only a minor topographic feature. De Vicente G. Giner J. L. Munoz-Martin A. Gonzalez-Casado J. M. and Undo R. 1996. Determination of present-day stress tensor and neotectonic interval in the Spanish Central System and Madrid Basin, central Spain. Tectonophysics 266, 405-424. van Wees J. D. Arche A. Beijdorff C. G. Lopez-Gomez J. and Cloetingh S. A. P. L. 1998. Temporal and spatial variations in tectonic subsidence in the Iberian Basin (eastern Spain): inferences from automated forward modelling of high-resolution stratigraphy (Permian-Mesozoic). Tectonophysics 300, 285-310.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

A P A T I T E F I S S I O N - T R A C K T H E R M O C H R O N O L O G Y O F T H E ALTAI M O U N T A I N S , TELETSKOYE REGION, SOUTH SIBERIA: PRELIMINARY

LAKE

RESULTS

J. De Grave and P. Van den haute Geological Institute, University of Gent, Gent, Belgium

Introduction The Altai mountain range is situated in the border zone of Russia, Mongolia, China and Kazakhstan. It forms part of the extensive Central Asian orogenic belt. The Russian Altai terrain has a complex blocky-mozaic structure (Fig.), resulting from accretion throughout the Palaeozoic, of different units onto the southern edge of the Siberian platform (e.g. Dobretsov et al., 1996 and Sengor et al., 1993). Frontal collision of the Kazakhstan and Siberian continents resulted in the closure of the Ob-Zaysan part of the Paleoasian Ocean in Permo-Triassic times. At the end of the Triassic a period of tectonic stability commenced that may have lasted until the Paleogene (200-53 Ma).

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Figure. Schematic geological map of the Lake Teletskoye area in the Altai Mountains. The positions of the collected samples is indicated. I: Altai-Mongolian terrain, II: Gorny Altai (GA) unit, III: Teletslk (TL) unit and IV: West Sayan (WS) unit, (after Buslov, unpublished).

Lake Teletskoye is located along the contact between the Teletsk (TL), the West Sayan (WS) and the Gorny Altai (GA) geodynamic units (Fig.). The main part of the lake lies within the so-called TeletskBashkauss fault zone that separates the WS from the TL unit. The lake occupies a tectonic depression with graben morphology. It is believed that the lake basin is forming in an active extensional regime created by an eastward movement of the WS relative to the GA, which was possibly accompanied by counterclockwise block rotation. The tectonic movements occur along Paleozoic structures that were reactivated since Paleogene times. The lake basin is the most recent expression of this evolution and Geological Society of Australia - Abstracts Number 58

F T 2 ^ e e is believed to have formed just 1 or 2 m.y. ago. All these younger movements must be regarded in the broader tectonic framework of the India-Eurasia collision. Samples and techniques In order to understand the tectonics of the area better, an AFT study was initiated. A total of 37 rock samples were collected during the summers of 1998 and 1999 (see Figure for sample location). Apatite concentrates were separated using conventional magnetic and heavy liquid techniques. The apatite etching conditions were 70 sec in a 2.5% HNO3 solution at room temperature. In a first run, a package of 13 samples and apatite standards (Durango and Fish Canyon Tuff) was irradiated in the Thetis reactor facility of the Institute of Nuclear Sciences (INW, University of Gent). The standards were embedded together with IRMM-540 dosimeter glass fragments (De Corte et al, 1998). The induced tracks were recorded in an external detector (ED). Etching conditions for the mica were 20 min in a 48% HF solution at room temperature. Track counting was done under an Olympus BH-2 using lOOx dry objectives.

Sample (Altitude)

Lat. (N) Long. (E)

n

Ps(NO

Pi (Ni)

Ps/Pi

SHI

51°45'13"

50

2.033 (2177)

95%

102±4

87°55'40" 51°44'42" 87°55'44" 51°44'32" 87°55'44" 51°44'58" 87°55'55" 51°45'31" 87°55'11"

2.551 (2718)

1.269

1850 m SH2 1950 m SH3 2010 m SH4 2100 m SH5 1720 m

50

4.013 (4148) 1.618 (2961) 6.291 (3304) 1.442 (2746)

3.089 (3167) 1.293 (2350) 4.542 (2324) 1.160 (2200)

1.324

39%

106i:4

1.287

72%

103±4

1.429

80%

115t4

1.296

81%

104±4

52 50 51

Age (Ma)

Table: Sample details and AFT data for the Teletskoye samples, pg and pj are the spontaneous and induced (mica ED) track density respectively (given in lO^cm'^). N^ and N^ are the number of counted tracks.n the number of counted grains. Age calculated using (|) data and Q-factor.

Age calibration The thermal neutron fluence was monitored using Au and Co foils that were placed on top, bottom and the middle of the sample package. An average % value of 2.781 ± 0.042 1015 n cm-^s'i and a mean Co/Au fluence ratio of 1.017 ± 0.013 were obtained, which is regarded to be consistent (Van den haute etal, 1988 and De Corte et al, 1991). An average B-factor (B = x/pd) of 4.98 ± 0.03 109 neutrons per track was obtained for the IRMM-540 dosimeter glass. In addition, a conventional ^-factor was determined using the Durango and FCT standards (Hurford and Green, 1983) and the IRMM-540 glass. Our preliminary CiRMM-540-value amounts to 291 ± 10. This corresponds to a value of 327 ± 11 for CSRM612 ^^^ 111 ± 4 for taking into account the 235u concentrations in these glasses (De Corte et al, 1998 and Hurford, 1998). Based on the absolute % value, a ?i(|)-value of 8.46 lO-^^a ^, a cross-section of 235u thermal neutron induced fission a = 570.8 b we determined the procedure factor Q (Wagner and Van den haute, 1992), for which we obtained a preliminary value of 1.19 ± 0.04.

FT2 First results and discussion At the time of writing only five samples (SHI to SH5, diorite, see Fig.) have been dated (ED method, 50 grains per sample). The ages were calculated using our % and Q values reported above. They vary between 102 ± 4 Ma and 115 ± 4 Ma (Table) and fall in the mid-Cretaceous. They can be interpreted in two possible ways. Either they are mixed ages between the onset of the tectonic stability (Triassic) and Cenozoic reactivation, or they suggest an earlier start of this reactivation. The latter interpretation corresponds with observations made by van der Beek et al (1996). These authors determined an Early Cretaceous cooling in the Baikal terrain, which is adjacent to our study area. This event was interpreted as the result of closure of the Mongol-Okhotsk Ocean. The effect of this event has been recognised before in the Altai range, albeit more pronounced in the Mongolian Altai (Dobretsov et al, 1996). Track length measurements are being carried out to establish which of the two interpretations will be correct. De Corte F. Van den haute P. De Wispelaere A. and Jonckheere R. 1991. Calibration of the fission-track dating method: is Cu useful as an absolute thermal neutron fluence monitor? Chemical Geology (Isotope Geoscience Section) 86, 187-194. De Corte F. Bellemans F. Van den haute P. Ingelbrecht C. and Nicholl C. 1998. A new U doped glass certified by the European Commission for the calibration of fission-track dating. In : Van den haute P. and De Corte F. (eds.) Advances in fissiontrack geochronology, Kluwer Academic Publishers, Dordrecht, 67-78. Dobretsov N.L. Buslov M.M. Delvaux D. Berzin N.A. and Ermikov V.D. 1996. Meso- and Cenozoic tectonics of the Central Asian mountain bek : effects of lithospheric plate interaction and mantle plumes. International Geology Review 38, 430-466. Hurford A.J. 1998. Zeta: the ultimate solution to fission-track analysis calibration or just an interim measure? In : Van den haute P. and De Corte F. (eds.) Advances in fission-track geochronology. Kluwer Academic Publishers, Dordrecht, 19-32. Hurford A.J. and Green P.F. 1983. The zeta age calibration of fission-track dating. Isotope Geoscience 1, 285-317. Sengor A.M.C. Natal,in B.A. and Burtman V.S. 1993. Evolution of the Altaid tectonic collage and Paleozoic crustal growth in Eurasia. Nature 364, 299-307. Van den haute P. Jonckheere R. and De Corte F. 1988. Thermal neutron fluence determination for fission-track dating with metal activation monitors : a re-investigation. Chemical Geology (Isotope Geoscience Section) 73, 233-244. van der Beek P.A. Delvaux D. Andriessen P.A.M. and Levi K.G. 1996. Early Cretaceous denudation related to convergent tectonics in the Baikal region, SE Siberia. Wagner G.A. and Van den haute P. 1992. Fission-track dating. Kluwer Academic Publishers, Dordrecht, 285 pp. Acknowledgements We wish to express our gratitude to the Flemish Institute for Enhancement of Scientific-Technological research in the Industry (IWT grant SB981199) that offered financial support for this study. Prof. Dr. G.A. Wagner and Dr. R. Jonckheere are gratefully acknowledged for the fruitful discussions and the help in sample preparations at the Max Planck Institute (Heidelberg, Germany). Also genuine appreciations to Prof. Dr. F. De Corte and A. De Wispelaere for the help with the irradiations and activity measurements at the INW (University of Gent). Finally a word of gratitude to the researchers, especially Prof. Dr. J. Klerkx and Prof. Dr. M.M. Buslov, at the Geological Departments of the Royal Museum of Central Africa (Tervuren, Belgium) and the University of Novosibirsk (Russia) for the help with fieldwork and sampling.

FT2^ee

F T 2 '

International Conference on Fission Track Dating and Thermochronology

HOT AND COLD CRUSTS: NOT BREAKFAST, BUT THERMAL HISTORY CONSTRAINTS FROM A F T A ® AND

irrx^"' IN EXTREME THERMAL REGIMES

I.R. Duddy i, RE Greeni and RC. van de Kamp2 1 Geotrack International Pty Ltd, 37 Melville Road, Brunswick West Victoria, 3055, Australia 2 40385 Queener Drive, Scio, Oregon, United States of America

This paper presents AFTA® Apatite Fission Track and ZFTA^"' Zircon Fission Track results from drill holes in two extreme crustal environments, and demonstrates the unique constraints on key aspects of the geological history obtainable using these techniques. The Gravberg-l driU hole, Siljan Ring impact structure, Sweden At around the Devonian Carboniferous boundary (~370 to 340 Ma; Aberg et aL, 1989), the Siljan Ring impactor struck a thin Late Proterozoic to Silurian sedimentary sequence overlying Precambrian

granitic basement, producing an annular structure with uplift core over 50 km in diameter. Heating associated with the impact has been interpreted as causing local oil generation from organic-rich Ordovician sediments (Vlierbloom et al, 1986). The -6.8 km (TVD) deep Gravberg-l drill hole was drilled at about the centre of the impact structure and penetrated a basement sequence of various Precambrian granites and associated dolerites.

GRAVBERG-l —

•

Siljan Granite

h

R1 dolerite

Jama Granite

10-

• 20

20-

*

• 30

s

4|4

' Impact event -340-370 Ma

*

• •

?migmatite

•10

f T.

1

1

1 00

1

200

1

1

300

r-| Zircon 1 1

400

1

500

1

30/— ' s • 40 U 40• 50 50• 60

u

•

GO-

• 70

2

TO-

• 80

s

80-

0) • 90 H

90-

• 1 00

100-

•

• -a-flh

•110

llCj 1

1 20

12C^

600

Fission Track Age (Ma)

6

8

10 12 14

16

Mean track length (|Lim)

Figure 1. AFTA parameters plotted against sample depth and present temperature for samples from the Gravberg-l well, Siljan Ring impact structure, Sweden. The variation of stratigraphic age with depth is also shown, as the solid line in the central panel.

The research in Gravberg-l was initiated to provide information on the regional thermal history and as a possible test for apatite fission track annealing kinetics in a "cold" shield setting. The present-day in-situ geothermal gradient in Gravberg-l determined from BHT measurements is ~l6°C/km (for a surface temperature of 3.5°C). Eight samples from near surface to -6.4 km vertical depth were analysed by AFTA and one sample from -6.4 km was subjected to ZFTA (Fig. 1). Key findings of the Gravberg-l research are: 1. Fission tracks in apatite are totally annealed at a depth of -6.4 km, where the present temperature is between -100 and 110°C (Fig. 1). Total annealing at this temperature is remarkably consistent with the predictions of the laboratory-based apatite annealing kinetics for the appropriate apatite compositions. Geological Society of i

ia - Abstracts Number 58

FT2 2. The fission track age of the shallowest AFTA sample (Fig. 1) is consistent with the impact age determined by K-Ar analyses on psedotachylite melt samples from outcrop (Aberg et al, 1989) and kinetic modelling incorporating the track length parameters indicate rapid cooling below ~110°C occurred at this time. The zircon fission track age of the deepest sample (from the 1635 Ma Dala Granite) is also consistent with the impact age (Fig. 1) and, at the limits of the age, suggests that this part of the well cooled below ~300°C at some time between ~320 and 500 Ma. If the fission track data from these two samples -6.0 km apart reflect the same thermal event, then the paleogeothermal gradient at around the Devonian-Carboniferous was ~30°C/km. 3. Detailed kinetic modelling of the AFTA sample suite reveals two additional thermal episodes throughout the drill hole: A Triassic-Jurassic (-210 to l60 Ma) episode in which peak paleotemperatures were around 70-80°C higher than present temperatures and a Late Cretaceous-Recent (-75 to 0 Ma) episode in which peak paleotemperatures were around 30-40°C higher than present temperatures. The paleogeothermal gradient for the Late Cretaceous-Recent episode is well controlled and is similar to the present-day value (~l6°C/km). The paleogeothermal gradient for the Triassic-Jurassic episode is less well controlled and could vary from similar to, to about twice, the present-day value. 4. The results clearly demonstrate considerable cooling has occurred since the Early Mesozoic with perhaps half of the total cooling during the Tertiary. Simple extrapolation of the low paleogeothermal gradients measured in the basement section to estimate the erosion required to explain the cooling results in geologically unrealistic magnitudes. However, consideration of the likely low thermal conductivity nature of a possible sedimentary cover, allows lower more appropriate erosion estimates to be made. 5. Application of fission track technologies in this region has revealed two major and unexpected thermal episodes in a -400 Ma time gap represented by an unconformity from the Silurian to the present day. These episodes have important implications for the tectonic evolution of this shield area since the Late Paleozoic. The Steamboat Geothermal area Research in the Steamboat Hills area of the Carson segment of the northern Walker Lane Belt was initiated to provide a regional thermal history framework and to investigate the age of the active hydrothermal system in the area. Seven outcrop samples, representing ?Cretaceous granodiorite and ?Triassic Peavine sequence metamorphosed volcanic flow and volcaniclastic rocks plus six samples of Peavine rocks in vertical sequence from the 0.8 km deep ST-13-05 geothermal corehole have been analysed using AFTA and 2FTA. AFTA results from the regional outcrop samples

indicate:

1. The rocks mapped as PCretaceous granodiorite are represented by at least two distinct phases; one, in the western Steamboat Hills, is Late Cretaceous or older (>-80 Ma); whereas the other, in the eastern Steamboat Hills, is Miocene (-22 to l6 Ma). Intrusions of Miocene age were not known in the area prior to this study, but granodiorite intrusions and volcanism of similar age is well known in the Cenozoic magmatic arc of western Nevada and California. 2. A major cooling period followed "Laramide" tectonism, with the older granodiorite and PTriassic Peavine rocks cooling from peak paleotemperatures of >-100°C commencing at some time in the period -90 to 60 Ma.

FT2 AFTA and ZFTA results from the ST-13-05 geothermal well indicate: 3. Fission tracks in apatite are totally annealed at a depth of only -0.64 km, where the present temperature is ~180°C, as the average present thermal gradient in the well is ~250°C/km (Fig. 2).

Core hole ST-13-5 10

10

30

30 H O

60 ^ 60 3 80 o 80 100 I

loa

120 S 12a 140 E 14a

200

Fission Track Age (Ma)

160 ^ 180 :2oo :220 -240

16a 180; 2a 22i 24a

"260

26a

250

6 8 10 12 14 16 Mean Track Length (|im)

Figure 2. AFTA and ZAFTA parameters plotted against sample depth and present temperature for samples from the ST-13-5 corehole, Steamboat Hills, Nevada. The variation of stratigraphic age with depth is also shown, as the sloid line in the central panel.

Consideration of apatite annealing kinetics for the appropriate apatite compositions present in the sample suggests that the current phase of hydrothermal activity (present thermal gradient in the well is ~250°C/km over 0.8 km) was initiated between 30,000 and 100,000 years BR This suggests an average heating rate between 1° and 3°C per 1000 years. However, actual heating was probably much more rapid as hot fluids invaded contemporaneous fractures. 4. Integration of AFTA and ZFTA resuks shows that the ?Triassic metamorphic rocks in the well cooled below ~300°C between ~90 and 80 Ma and below ~110°C by ~60 Ma. This thermal history is consistent with contact heating associated with local "Laramide" granodiorite intrusion rather than regional metamorphism, followed by regional kilometre-scale uplift and erosion. Aberg G. Collini, B. and Schmitz B. 1989 K-Ar isotope analyses from the Siljan Ring meteorite impact structure, Sweden. Geologiska Foreningens Stockholm Forhandlingar V.III (4), 355-360. Vlierbloom FW. Collini B. and Zumberge J.E. 1986. The occurrence of petroleum in sedimentary rocks of the meteor impact crator at lake Siljan, Sweden. Organic Geochemistry vol. 10, 153-161.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

EARTHQUAKES AND ACTIVE MOUNTAIN BUILDING ALONG THE SAN ANDREAS FAULT S Y S T E M IN THE SAN FRANCISCO BAY AREA

T. Dumitrui and R. Biirgmann^ 1 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. 2 Department of Geology and Geophysics, University of California, Berkeley, California 94720, USA

In California, the San Andreas transform fault system forms the modern plate boundary between the North American and Pacific plates. Although the predominate motion on the system is clearly rightlateral strike-slip, important subsidiary vertical motions also occur. These vertical motions are the principle drivers for Pliocene to Recent mountain building in coastal California. Much of this mountain building represents fault slip accumulated over the last several million years during repeated thrust and transpressional earthquakes that are subsidiary to the predominantly strike-slip earthquakes along the San Andreas system. Several such contractional earthquakes have caused major damage in recent times, including the 1989 Loma Prieta (San Francisco area, M=7.1) and 1994 Northridge (Los Angeles area, M=6.6) events. This contraction deformation may be ascribed to either of two end-member mechanisms, (1) a component of plate margin contraction (transpression) induce on the San Andreas system by changes in the relative motions between the Pacific and North American plates in latest Miocene(?) time, and (2) contraction or extension at bends, steps, or other irregularities in the San Andreas system. In the San Francisco Bay Area, modern motion along the San Andreas system is partitioned along several major active strike-slip faults, including the San Andreas, Hayward, Calaveras, and San Gregorio faults. Young mountains in the area, with elevations up to about 1200 m, are closely associated with these fauks, and the locations of lowlands such as San Francisco Bay are also structurally controlled by these faults. The Santa Cruz Mountains, including Loma Prieta ('Dark Mountain'), are located along the San Andreas fault to the southwest of the Bay and are closely associated with a contractional left bend along the San Andreas. The Loma Prieta area on the northeast side of the San Andreas is underlain by fault-bounded blocks that rise along active, deeply rooted, high-angle reverse and oblique-slip faults. Transects of samples across this area yield apatite fission track ages averaging 4.6±0.5 Ma near the San Andreas, with a zone of progressively older partially annealed ages farther from the San Andreas. The 4.6 Ma ages record a minimum of about 3 km of unroofing over the last 4.6 m.y., whereas the partially annealed ages recording lesser unroofing toward the northeast toward the adjacent Santa Clara Valley. The zone of young, totally reset ages is roughly 4 km wide and at least 40 km long parallel to the San Andreas. To further define the extent and distribution of this young uplift, we used morphometric analyses of the youthful topography of the area. Steep drainage slope and high local relief indicate that the area northeast of the San Andreas forms a well-defined zone of high uplift on the 1,000 to 100,000 year time scale that correlates fairly well with the young fission track ages. In contrast, the region on the southwest side of the San Andreas is characterized by broad upwarping and folding, more subdued topography, and old fission track ages. Geodetic data since the 1906 San Francisco indicate that southern Santa Cruz Mountains repeatedly rise and subside through a complex sequence of deformation events associated with interactions between the major active faults in the Bay Area. Loma Prieta actually subsided about 10 cm during the 1989 earthquake. An additional deformation element not represented in the 94 year geodetic record is needed to explain the youthful uplift around Loma Prieta. This element would involve reverse slip averaging 2-3 mm/yr along the thrust system underlying the mountain. Recurrence intervals for possible major earthquakes (M=6-7) on this system would be on the general order of 300 to 1000 years. Such contractional earthquakes are thus a possible significant hazard, but much less important as a hazard than the stronger and more frequent strike-slip events expected along the San Andreas and Hayward faults. Geological Society of Australia - Abstracts Number 58

Major uplift in the Santa Cmz Mountains has been restricted to a remarkably narrow belt on the order of 5 km wide adjacent to the San Andreas fauk. Although uplift is apparent along faults elsewhere in the Bay Area, fission track data suggest long term (several million years) total exhumation there has been less than 2-3 km. The strong, protracted uplift in the Santa Cruz Mountains probably derives from the significantly left bend along the San Andreas in the area, which focuses major contraction, and from the incompetent nature of the Franciscan accretionary complex sediments which underlie the area, which permit major shortening to be accommodated within a relatively narrow zone.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

H E L I U M D I F F U S I O N IN APATITE R E V I S I T E D : I S THE PREVIOUSLY I N F E R R E D CHANGE IN H I G H T E M P E R A T U R E D I F F U S I O N MECHANISM AN ARTIFACT OR A R E A L I T Y ?

TJ. Dimai Faculteit derAardwetenschappen,Vrije Universiteit,Amsterdam,The Netherlands

Data from previous studies of helium diffusion in apatite (Wolf et al, 1996; Zeitler et al, 1987) suggest that helium diffusion at low temperatures (<290°C) has a different activation energy than diffusion at higher temperatures. Above 290°C the diffusivity deviates from linearity toward lower activation energies. Wolf et al. (1996) suggested a reversible change of the physical mechanism of helium diffusion as a cause for that change. For pure chlorapatite there is indeed a monoclinic-hexagonal phase transition occurring between 250 and 330°C (Bauer and Klee, 1993), which however is absent in flourapatite (Bauer and Klee, 1993). As the apparent change in activation energies was observed in fluorapatites (Wolf et al, 1996; Zeitler et al, 1987) the phase transition cannot be used as a potential explanation. Fluorapatite has no known phase transitions in the temperature range relevant to the above diffusion studies. In the absence of a probable physical cause usually invoked to explain changes in diffusion behaviour (phase transitions, exsolutions, twinnings, etc.) the question arises whether the deviation from linearity at higher temperatures is actually a reality or an experimental/mathematical artifact. The results of this study demonstrate that the change in activation energy at higher temperatures is indeed an artifact. In both studies in which the effect was observed (Wolf et al, 1996; Zeitler et al, 1987), the experiments used to derive the activation energy were stepwise heating experiments, i.e. a sample is successively degassed in incrementally increased temperature steps and the helium released during each step is analysed. Wolf et al (1996) use the approximation equations of Fechtig and Kalbitzer (1996) to consider the effects of the perturbed diffusion profiles and to calculate the diffusion coefficients. Inherently the approximation equations deviate slightly from the exact solution, in the case of stepwise heating experiments these slight deviations might add up to a significant deviation. Furthermore, at diffusion losses >50% the diffusion parameters deviate from the ideal volume diffusion due to depletion of He from the apatite (Zeider et al, 1987). In this context it is interesting to note that the previously observed deviations predominately occur in steps after >50% of the helium was extracted. To avoid the potential methodological pitfalls mentioned above, I conducted a new series of experiments using similar samples but a different approach. The experiments are still ongoing and in the following I report preliminary results. Gem quality apatite (Durango, Mexico) was crushed and wetsieved. Aliquots of the 160-180 micron fraction were individually heated in isothermal experiments and then subsequently fully degassed. Thus with the two measurements of an aliquot the diffusion coefficient at a given temperature is determined by the fraction of helium released (equation and procedure in (Dunai and Roselieb, 1996)). This approach has the advantage that the isothermal experiments can be designed to keep the fractional release well below 50%, thus avoiding any effects arising from depletion of helium from the grains. Moreover the equations that describe diffusion exactly, and not as an approximation, can be used (Crank, 1957; Dunai and Roselieb, 1996). At temperatures at and below 300°C the diffusion coefficients obtained are indistinguishable of those of Wolf et al (1996). However at higher temperatures up to at least 450°C (the range covered by the new experiments so far), I do not observe the deviation from linear relationship in the Arrhenius diagram as described by Wolf et al (1996) and Zeitler et al (1987). In contrast I observe a perfectly linear relationship, the correlation coefficient of the linear regression of all data points, at both sides of the transition point at 290°C proposed by (Wolf et al, 1996), is better than 0.9999. The activation energy obtained, 35±1 kcal/mol, is indistinguishable of those derived by Zeitler et al (1987) and Wolf et al (1996) for apatites from Durango, i.e. 38.5±8 and 36±1 kcal/mol, respectively. While my value for the activation energy confirms the low temperature experiments of Wolf et al (1996) and Zeitler et al (1987) it clearly disproves the existence of a change of physical diffusion mechanisms in apatite at Geological Society of ^ H I H i i a - Abstracts Number 58

B

FT2^ee 290°C as it was derived from measurements including those above 290°C. The deviations that can be seen in the data of Wolf et al (1996) and Zeitler et al (1987) are therefore a experimental/mathematical artifact, most likely attributable to the methodological pitfalls described above. The results of the new diffusion study do not alter the application of apatite as a low-temperature thermochronometer as the relevant low temperature data of Wolf et al (1996) is confirmed. The importance of the finding that there is no change of physical diffusion mechanisms above 290°C lies in the practical application of the method. Before the contrary is established, it has to be assumed that volume diffusion of helium in apatite is dependent on its chemistry, notably its ion-porosity (i.e. not exclusively the Cl/F-ratio) as is probably true for all diffusing species in minerals (see e.g. Fortier and Giletti, 1989). Therefore it would be advisable to characterise the diffusion parameters of each apatite population investigated, until general rules can be derived. If the transition at 290°C would be real, the necessary diffusion experiments would have to be conducted at lower temperatures. This would require excessive experiment durations in the order of several days to weeks to characterise a single apatite population (Wolf et al, 1996). Now that the change at 290°C is disproved, experiments can be safely conducted at higher temperatures, and a population can be characterized in 1-2 days. Experiments using the same experimental approach as applied to the Durango apatite above, are scheduled for other apatites as well as on other minerals as e.g. titanite. The corresponding results will be presented at the conference. Bauer M. and Klee W. E. (1993) The monoclinic-hexagonal phase transition in chlorapatite. Eur. J. Min. 5, 307-316. Crank J. (1957) The mathematics of diffusion. Clarendon Press. Dunai T. J. and Roselieb K. (1996) Sorption and diffusion of helium in garnet: implications for volatile tracing and dating. Earth Planet. Sci. Lett. 139, 411-421. Fechtig H. and Kalbitzer S. (1966) The diffusion of argon in potassium-bearing solids. In Potassium-Argon dating (ed. O. A. Schaefer and J. Zahringer). Springer. Fortier S. M. and Giletti B. J. (1989) An empirical model for predicting diffusion coefficients in silicate minerals. Science 245, 1481-1484. Wolf R. A., Fadey K. A., and Silver L. T. (1996) Helium diffusion and low-temperature thermochronometry of apatite. Geochim. Cosmochim. Acta 60, 4231-4240. Zeitler P. K., Herczeg A. L., McDougall I., and Honda M. (1987) U-Th-He dating of apatite: a potential thermochronometer. Geochim. Cosmochim. Acta 51, 2865-2868.

International Conference on Fission Track Dating and Thermochronoiogy

F T 2 '

I M P A C T O F T H E V O L C A N I S M ON A G E - P R O V E N A N C E S T U D I E S - T H E P E R I A D R I A T I C E V E N T IN THE ALPINE

MOLASSE

I. Dunkl, C. Spiegel, J. Kuhlemann and W. Frisch Institute of Geology, University of Tubingen, Germany

The Oligocene Periadriatic magmatic activity is one of the most important thermotectonic events in the evolution of the European Alps. Tonalite bodies arranged in a 800 km long chain indicate the huge amount of magma formed in a relative short period. The late magmatic dikes have higher geochemical variability than the granodiorite-tonalite intrusive suites, the composition of dikes ranges from dacitic to lamprophyric. The accelerated erosion removed nearly all extrusive and effusive products and only the deep root zone of the volcanoes are exhumed to the surface in the s.s. Alps. A few volcanic edifices are preserved and buried by younger sediments in the Po basin. The Late- and PostOligocene intense tectonics and the postmagmatic fluids produced high heat flow and strong transformation in some places, thus the thermochronometers often show Miocene mixed or reset ages. Thus, the investigation of the pyroclastic products and volcanogenic pebbles of the Peri-Alpine sediments provides more information on the magmatic- and exhumation history of the Periadriatic belt than the study of the dikes emplaced in Alpine crystalline formations. The usually 30 Ma old Periadriatic calc-alkaline igneous rocks are rich in well developed, clear, colourless zircon crystals of mainly S7, SI2, S22 morphotype. The presence of this zircon population in the Molasse indicates the contribution of disintegrated, decomposed Periadriatic magmatic material in many Late Oligocene-Middle Miocene sedimentary formations. Sandy facies rocks may contain high proportion of Periadriatic zircons, when no any other petrographic indication of igneous contribution is preserved. The euhedral, clear accessoric apatite crystals are also very characteristic, but, due to the smaller hardness they are less durable and loosing the crystal edges and faces during the sediment transport. Usually, when the apatites remained euhedral in the sand fraction, the pebble fraction contains detectable amounts of andesite or tonalite pebbles. Volcanogenic pebbles and the above mentioned, characteristic apatite-zircon content of the sandstones can be found in many Peri-Alpine sedimentary units, in the Apennines, Lombardian Flysch, northern foreland basin, Slovenian basin fragments and in the Pannonian basin (Fig. 1). Volcanogenic material occurs also in those sedimentary basins, which has recent catchment areas without volcanic dikes. This indicates the northern migration of the main water divider of the Eastern Alps since Oligocene time. By FT dating of Oligo-Miocene strata of the Peri-Alpine Molasse, we can draw the following conclusions on the timing of the volcanic activity: • The first traces of the volcanic activity were found in Paleocene sequences (~57 Ma). • There were two intense cycles in Eocene time, (44 and 39 Ma). • The main amount of pyroclastic material formed at around 32 Ma. • In several cases apatite gives younger FT ages than the K/Ar age of the volcanogenic material (27 vs. 32 Ma). It indicates long-lasting volcanic activity and thermal overprint of the former phases by subsequent ones. • The cessation of the volcanic activity was around 27 Ma. The easily datable Periadriatic zircon and apatite grains hides the age signal of the erosion of the nonmagmatic material. To reveal the exhumation history of the crystalline basement when the young, Geological Society of Australia - Abstracts Number 58

FT2^ee igneous component is predominant we are usually manufacturing crystal mounts from the 'ugly' zircon and/or apatite crystals, beside the unselected, representative mounts. In this way increases the probability of dating sufficient amount of properly etched grains derived from tectonic units composed of metasedimentary rocks.

Cs.

r'

^^ Carpathrans

/

^

'

Pannonlan^^

basin

^

300 km Figure 1. Schematic map of the Alps and adjacent areas. 1: mountains, 2: basins with Tertiary sediments, CA: Central Alps, thick line: Periadriatic lineament, stars: main intrusive bodies or dike groups, dark gray areas: Late Oligocene sedimentary formations with volcanogenic contribution, Cs: Csatka formation, B: Baustein beds, H: Howerplatten sandstone, L: Lombardian Flysch, M: Macigno formation

Several Late Oligocene sediments (stratigraphic age: 28-25 Ma) contain significant amount of rounded zircon grains derived from metasedimentary rocks having FT ages around 30-27 Ma. It means, that at the time of magmatic activity the zircon FT chronometer were reset in huge volumes of the Austroalpine host rock. These rock bodies underwent a fast uplift and their erosion have started only a few Ma after the setting of the cooling ages. The recent occurrence of such areas marks a narrow bek at around the Periadriatic Lineament and at the frame of the Lepontine Dome of the Central Alps. This arrangement gives an important aid for the age-provenance research and thus, in paleogeography, for the reconstruction of the origin of Alpine derived siliciclastic material.

International Conference on Fission Track Dating and Thermochronology

(U-TH)/HE

D A T I N G : A R E V I E W O F T H E T E C H N I Q U E AND C U R R E N T

APPLICATIONS

K.A. Farley Div. Geological and Planetary Sciences, Caltech, Pasadena, CA, USA

Introduction After decades of dormancy the (U-Th)/He dating technique has enjoyed a resurgence of interest following the suggestion by Zeitler et al [1] that He ages may be useful for low temperature thermochronometr^. To rigorously evaluate its feasibility we have established techniques for helium dating of apatite, titanite, and zircon [2]. We have also performed extensive He diffusivity measurements to establish the temperature sensitivity of the (U-Th)/He system in these minerals [2]. Many applications of the He technique, especially using apatite, have recently been completed [3]. The results confirm that helium thermochronometry is a viable method to constrain cooling histories. In particular apatite He dating, with a closure temperature of 70°C, provides unique new insights to the lowest temperature history of rocks, and is an excellent complement to apatite fission track (FT) methods. Principles, Methods, Precision and Accuracy ^He is produced by the decay of U and Th series nuclides, and although these isotopes are seldom found in concentrations above a few hundred ppm, the multiple parents (238u, 235u, 232^11) and multiple daughters produced by series decay (8, 7, and 6 alpha particles, respectively), coupled with an excellent analytical detection limit, make the method well-suited for high precision age determinations even in very young samples. The major complications with obtaining precise and meaningful He ages include: 1. Grain to grain heterogeneity in parent isotope concentrations. This difficulty can be dealt with simply by analysing parent and daughter abundances on the same aliquot. 2. Ejection of high energy alpha particles. The -20 pm stopping distance of alpha particles is comparable to the size of typical grains to be dated, causing a substantial fraction of alphas to be ejected. By making reasonable assumptions this phenomenon can be corrected for [4]. Smaller grains require larger corrections; most grains we have investigated require corrections of about 20%. This is a large correction, and although we believe the correction procedure is generally robust, ejection likely limits the ultimate accuracy of the technique to perhaps 5% (2-sigma). 3. For grains with very young crystallisation ages (e.g. <1 Ma), U-series disequilibrium may cause erroneous He age estimates. For older grains, this effect is negligible. We routinely measure between 1 and 25 grains of U and Th bearing phases. He is measured by isotope dilution quadrupole mass spectrometry after thermal outgassing. Grains are then retrieved from the vacuum furnace, dissolved, and analysed for parent abundance by isotope dilution inductively coupled plasma mass spectrometry (ICPMS). Propagated uncertainties in age are typically 2-3% (2sigma). -1% accuracy is assured through the use of primary gravimetric and manometric standards. Verification of the technique is provided by routine analyses of the Durango apatite standard, from which we have obtained a He age of ~32 Ma with a 6% 2-sigma spread on -40 replicates over the last 2 years. This age is in good agreement with independent estimates of the age of this standard. The age range accessible by the method is likely to be very large. At the young end. He blanks are the limiting factor. For typical sample sizes and parent concentrations, it is now possible to date samples with a He age of 150 kyr with an uncertainty of -10% (excluding possible errors arising from secular disequilibrium, see above). Larger sample sizes could reduce this limit substantially. There is no obvious upper age limit to the technique, though radiation damage effects might become significant at high dosages. Geological Society of / • • M i a - Abstracts Number 58

B

FT2 Thermochronometry While there are some applications of He dating of quickly cooled samples (e.g. tephrachronology [5D, the major interest in the technique is for low temperature thermochronometry. Of the phases we have examined, apatite is by far the best developed and of the greatest applicability both because of the ubiquity of apatite and because it has the lowest closure temperature of all known thermochronometers. Laboratory experiments reveal little variability in the diffusion behaviour of He in fluorapatite. At temperatures < 265°C He diffusion obeys a highly linear Arrhenius relationship indicating a closure temperature (Tc) of 70°C assuming a cooling rate of 10°C/m.y. The total range in closure temperature obtained from a large suite of apatites is < ± 10°C. This range may be analytical in origin; there is no indication that Tc varies with composition in fluorapatite. Above 265°C there is an incompletely understood and irreversible change in He diffusivity, but this is likely of no consequence in the natural setting. Diffusivity scales with the inverse square of grain dimension, consistent with the diffusion domain being the physical grain, i.e. sub-grain domains do not dominate He diffusion. As a consequence, Tc increases with grain size, but only weakly. For example, increasing grain dimension from 50 to 150 pm increases Tc by just 10°C. Work on crystallographically controlled wafers indicates that He diffusion is very nearly isotropic, so in apatite the relevant dimension is most commonly prism diameter. Based on these measurements it is possible to predict apatite He ages given any time-temperature path [6]. For comparison purposes it is useful to consider the distribution of He ages expected in a thermally-static crustal block, such as a borehole. As shown in Figure 1, apatite He ages are expected to define a pattern analogous to the fission track partial annealing zone (FTPA2). Above ~80°C, He ages are nearly zero. Between this temperature and ~40°C is the He partial retention zone (HePRZ), where He ages change very rapidly with temperature. He is quantitatively retained cooler than ~35°C. The HePRZ is similar in shape but lies at ~35°C lower temperature than the FTPAZ. The two zones have essentially no overlap, i.e. apatites can reside at temperatures where fission tracks are completely retained but He is not.

0) 13

-I—•

o a.

E

.CD

20

40

60

80

100

120

Apatite Helium or Fission Track Age (Ma) Figure 1. He and FT ages in apatites lield in a thermally static geothermal gradient for 120 m.y. See (6) for details.

A critical question is whether laboratory diffusion measurements are applicable in the natural environment. Borehole apatites from the Otway Basin show a well-developed HePRZ at about the expected temperature, although the exact position is extremely dependent on the assumed temperature history of the basin [7]. Under certain circumstances it is possible to compute model-independent diffusivities; these are in good agreement with laboratory data. The ability to measure He and FT ages on the same separate suggests that comparison of apatite He and FT ages can be used to verify the He diffusivity measurements, and also to provide additional

FT2 detail on cooling histories at very low temperatures. We have combined apatite FT and He ages from about 100 apatites from many different settings. As expected from laboratory kinetic data, the He ages are almost invariably younger than corresponding FT ages. In some cases, near-zero He ages are associated with high FT ages, confirming the prediction that the HePRZ and FTPAZ do not overlap. Nearly indistinguishable He and FT ages have been found in several rapidly cooled bodies, demonstrating good age cross-calibration of the methods. Occasional samples yield He ages older than FT ages; these likely arise either from mineral inclusions yielding anomalously old He ages (see below), or unknown errors in the FT age measurement. The biggest technical difficulty we have identified with the apatite method is U and Th rich mineral inclusions, usually zircon, in the apatites. These inclusions contribute He during outgassing, but because we dissolve the apatites only in nitric acid, they do not contribute U and Th to the ICPMS measurement. Erroneously high He ages thus result. We have adopted two protocols for eliminating this problem. Prior to analysis, grains are inspected with a high-power binocular microscope. Under x-polars, inclusions are easily detected by their birefringence contrast with apatite. However, even with this check occasional inclusion-bearing grains apparently escape detection. We have discovered that these inclusion-bearing grains can be identified during the He outgassing step. Inclusion free apatites almost invariably release all He when held at 950°C for 20 minutes. In contrast, inclusionbearing grains continue to release helium when heated to this temperature a second time. We routinely "re-extract" samples to test for this effect. Along with replication of ages, we believe these protocols are sufficient to yield highly reliable He ages. Conclusions

Although the present database is limited, apatites from igneous, metamorphic, and sedimentary environments have yielded geologically reasonable and reproducible He ages that are consistent with the lab-based closure temperature of 70°C. At this point it would be useful to undertake a systematic comparison of He and FT ages (and track length distributions) on apatites with "well-known" temperature histories to verify that the two techniques are fully consistent. Less is known about titanite and zircon He dating, but preliminary results are promising and suggest Tc's ~100°C higher than in apatite. Combined with existing FT and Ar/Ar techniques, the new He dating methods may lead to substantially improved documentation of the cooling history of rocks. 1. Zeitler et al. Geochimica et Cosmochimica Acta 51, 2865-2868 (1987). 2. Wolf etal. Geochimica et Cosmochimica Acta 60, 4231-4240 (1996); Reiners et al. Geochimica et Cosmochimica Acta in press (1999); Reiners and Farley,in prep. 3. House et al Nature 396, 66-69 (1998) and references therein, for example. 4. Farley et al Geochimica et Cosmochimica Acta 60, 1-7 (1996). 5. Kohn et al, this volume. 6. Wo\{etal Chemical Geology 148, 105-114 (1998). 7. House et al Earth Planet. Sci. Lett. 170, 463-474 (1999).

FT2^ee

International Conference on Fission Track Dating and Thermochronology

T I M I N G OF WEATHERING BY 4 0 A R / 3 9 A R DATING OF K - M N G Y M P I E , QUEENSLAND,

p T 2 '

O X I D E S F R O M M A R Y VALLEY,

AUSTRALIA

YX. Feng and P.M. Vasconcelos Department of Earth sciences, University of Queensland, Brisbane, QLD 4072, Australia

Southeastern Queensland is a geomorphological complex area where narrow coastal plains are separated by intervening structural highs. These elevated areas consist of upthrusted Palaeozoic blocks of highly folded and metamorphosed sedimentary and granitic intrusions and Tertiary volcanic rocks, forming a belt of fold mountains 200-500 km wide along the coast. Its climate is humid at present. Most of the area is covered with forest, grass land, or cultivated lands. Though discontinuously distributed and varying in elevation, the presence of moderately deep (3050 m) lateritic profiles overlying volcanic rocks in the region indicate that periods of intense weathering must have followed the extrusion of Tertiary volcanic rocks. To determine the geological and climatological factors possibly responsible for the formation and preservation of these weathering profiles, we are studying these profiles on a regional scale. One area chosen for close scrutiny is the Mary Valley region, southwest of Gympie. Mary Valley is an area of mountainous, moderately high relief, ranging from 6l m above sea level along the Mary River flats to 632 m at Mt. Allan. Though many low hills in this area have gentle slopes, many high mountains are rather steep, narrow ridges, contrasting to the commonly flat top mesas typical of the Australian inland landscape. The geology in the area is composed of slate, mudstone, chert, andesite, tuff, schist and jasper, which belong to the Permo-Carboniferrous Amamoor Beds (Martin, 1975; O'Flynn, 1976). These lithologies were accreted and have been exposed to weathering and erosion since the late Permian (Beckmann and Stevens, 1978). In addition. Early Cainozoic volcanic activity occurred on the western side of Mary Valley region. The Mn-bearing metasediments are often covered with incipient to well-developed weathering profiles varying from less than one meter to tens of meters in depth. Samples from Amamoor occur within mottled or bleached zones. The Mn oxides close to the surface display massive textures often enveloped by botryoidal overgrowths. The Mn-oxides were precipitated during weathering of Mn-silicates. In Imbil, Upper Kandanga, Kenilworth and Mt. Allen, samples were collected from supergene Mn-oxide deposits or prospects that are thought to be weathering products of marine sedimentary Mn-deposits (Cranfield, 1990). Manganese oxides generally display massive textures crosscut by later generation veins. Usually, two to four generations of Mn-oxides may be identified. Some samples also display botryoidal textures. All hand specimens were sliced, micro-drilled or cut to produce approximately 5 mm samples. These samples were subsequently cut or crushed into smaller grains (ranging from 0.002 to 0.014 g). Representative grains were mounted for scanning electron microscopy and electron microprobe analysis, powdered for x-ray diffractometry and thermo-gravimetric analysis, and picked for irradiation for 40Ar/39Ar analyses. A total of 490 grains from 123 samples of cyptomelane-hollandite were analysed by step-wise laser-heating 40Ar/39Ar dating on a MAP-215-50 mass spectrometer at UQ-AGES (University of Queensland Ar Geochronology in Earth Sciences laboratory). Sample preparation and analytical procedures follows Deino and Potts (1990), Deino et al (1990), and Vasconcelos (1999). Results are shown as ideogram plots in this abstract, and ideogram age peaks are confirmed by plateau ages. Geological Society of Australia - Abstracts Number 58

FT2 Fig.1 Ideogram f o r A m a m m o r

400 350 ^

300

£1 to

250

3.15

JQ 200 O b 150 a, 100

2.74 .. i 2.25 AM-1 (155m) J-'-

50 0

1

-1

: :3.44

I.03-1 94

3.07

2

3

4-17 ^^•yl4(208m)

4

Fig.2 Ideogram for Imbil

500 400

-Q

300 1

Si O 200 k. Q. 100

Fig.3 Ideogram forKandanga, Kenilworth and Mt Allen -r

- 1 0

1

2

3

Age (Ma) Results and discussion Amamoor Seventy-four grains from 17 samples were analysed and the results are presented in Fig. 1. Grains of pure cryptomelane yield well-defined plateau ages varying from 0.73 to 4.5 Ma. Samples collected at higher elevations (AM-4, 208 m elevation), yield the greater plateau ages (4.2-4.5 Ma) but also show younger results (2.7-2.92, 2.17, 1.07, and 0.75 Ma) suggesting a prolonged period of Mn-oxide precipitation. Samples collected at lower elevation sites (AM-2 (180m elevation, 2.25 Ma, 2.74 Ma, 3.15 Ma, 3.36 Ma) and AM-1 (155 m elevation, 0.59 Ma, 0.85 Ma, 1.07 Ma, 1.69 Ma)) yield lower age groups, lacking the 4.2-4.5 Ma group.

FT2 Imbil In general, geochronological results for K-Mn oxides (226 grains from 57 samples) from Imbil are basically identified as two large groups (Fig. 2): 0.22 Ma to 1.32 Ma and 3.06 to 4.42 Ma, though for a single site, age ranges may be larger. No direct relationship is found between elevation and age distribution, contrasting with the relationships obtained for the Amamoor area. The major ore deposit (IM-1, or Imbil West) was formed during 0.37-0.77 Ma (peak position). In addition, plateau ages of 6.9 ± 0.6 Ma, 5.9 ± 1.2 Ma occur in two grains (IM-98-19-4B) from IM-5. Though to be confirmed, this may be the greatest age of weathering in this area. Upper Kandanga

(KA-4)

Results for 104 grains from 24 samples cluster around 0.30 Ma (Fig. 3). The span of age at half position of the peak is about 0.2 to 0.4 Ma, a feature confirmed by plateau ages. Similarly to the Imbil West area, the Upper Kandange results indicate that supergene ore formation in this area happened in quite recent times. Mt. Allan Thirteen grains extracted from four samples from the Mt. Allen Mn-deposit, located at 525 m about sea level, show age peaks ranging from 2.3 to 2.9 Ma (Fig. 3), clearly indicating that, in this region, weathering profiles at higher elevations are not necessarily older than those at lower elevations. Kenilworth Results for 76 grains from 19 samples from Kenilworth (KE-1, 105 m elevation) display four main age peaks: 0.37, 0.52, 0.58, 0.68 Ma (Fig. 3), similar to IM-1 (Imbil West), though they occur at quite different altitudes. We interpret the geochronology results as indicative that the formation of supergene Mn-deposits in this area (IM-1, KA-4) is a rather recent process. The K-Mn oxides were precipitated in the Mary Valley weathering profiles from 6.9 Ma to the present, mainly during two periods (0.2-1.32 Ma, 2.74-4.42 Ma). These periods of weathering in eastern Queensland are much younger than those obtained for western Queensland, Northern Territory, and western Australia (Dammer et ah, 1996; Vasconcelos, 1996; Dammer et al, 1999; Vasconcelos, 1999). It is suggested that differences in climate and tectonic stability may be the major cause for the younger weathering profiles in eastern Queensland. In addition, the resuks do not show a direct relationship between the age distribution and akitude in the Mary Valley area. This means that formation and preservation of weathering profiles in this region may depend on Late Cainozoic epeirogenic (warping) movements (so-called the Plio-Pleistocene Kosciusko Uplift by Browne (1969)), which resulted in the differential uplift of the region. The lack of age vs. elevation patterns seen in other regions of Australia (Vasconcelos, 1999) may also reflect differential erosion. In tectonic stable belts (i.e. northwestern Queensland, Northern Territory and Western Australia), hardcaps of silcrete and Mn-Fe oxides formed within the weathering profiles usually promote the preservation of these profiles as plateaus or mesas. In tectonically active belts with sub-tropical humid climates (as in coastal area in southeast Queensland), few hardcaps are found and the topography consists of relatively narrow ridges, suggesting that the potential for preserving older weathering profiles is lower. Even if older supergene minerals were formed in the region, relatively faster erosion during the late Cenozoic removed the weathering products older than late Miocene, and only recently formed supergene minerals are preserved. The results of this study suggest that supergene K-Mn oxides from humid belts can be useful indicators of Late Cenozoic paleoclimatic and tectonic processes.

FT2 Cranfield, R. G., 1990. The Gumpie group and other Permian strata of the Gympie composite Terrain. Qld Grovt Mining J. 92:151 Demo AL, Potts R. 1990. Single-crystal 40Ar/39Ar dating of the Olorgesailie Forma-tion, southern Kenya Rift. J. Geophys. Res. 95:8453-70 Dammer, D., A. R. Chivas, et al 1996. Isotopic dating of supergene manganese oxides from the Groote Eylandt Deposit, Northern Territory, Australia. Economic Geology and the Bulletin of the Society of Economic Geologists 91(2): 386-401. Dammer, D., I. McDougall, et al. 1999. Timing of weathering-induced alteration of manganese deposits in Western Australia; evidence from K/ Ar and 40Ar/39Ar dating. Economic Geology and the Bulletin of the Society of Economic Geologists 94(1): 87-108. Martin, J. E., Willmott, W. F., 1975 and O'Flynn, M. L., 1976-1977, Geological Survey of Queensland, 1:100,000 series, Nambour. Vasconcelos, P. 1996. Geochronological evidence for the preservation of Cretaceous weathering profiles in Northwestern Queensland. Mesozoic geology of the eastern Australia Plate conference. Anonymous. Sydney, N.S.W., Australia, Geological Society of Australia. 43: 543-544. Yashvili LP, Gukasyan RK. 1974. Use of cryp-tomelane for potassium-argon dating of man-ganese ore of the Sevkar-Sarigyukh Deposit, Armenia. Trans. Acad. Sci. USSR Dokl., Earth Sci. Sect. 212:49-51

International Conference on Fission Track Dating and Thermochronology

F T 2 '

L o w T E M P E R A T U R E THERMOCHRONOLOGY OF THE PRAGUE BASIN AND SURROUNDING A R E A , CZECH R E P U B L I C

J. Filipi, U.A. Glasmacher2, G.A.Wagner2,V Suchyi,V Mann3 and H.Volk3 1 Institute of Geology, Academy of Sciences, Czech Republic, Rozvojova 135 165 02, Prague 2 Forschungsstelle Archaometrie der Heidelberger Akademie derWissenschaften am Max-Planck-Institut fur Kernphysik, P.O. Box 103980,69029 Heidelberg, Germany 3 Institut fur Chemie und Dynamik der Geosphare, Forschungszentrum Jiilich, Germany

The NE-SW trending Prague Basin, in the central part of the Czech Republic, represents the unmetamorphosed Cambrian to Middle Devonian 5800 m thick sedimentary and volcanic strata of the TeplaBarrandian. Cambrian to Middle Devonian siliciclastic and carbonate rocks, v^hich discordantly overlay Proterozoic metamorphic rocks, are the dominant lithological units (Chlupac et al, 1998). Basaltic tuffs as well as diabase dikes and basaltic to rhyolitic lavas are intercalated in the Lower Paleozoic and Lower Devonian strata (Chlupac et al, 1998). Surrounding the Prague Basin from north to southeast a 9 km thick strongly deformed volcano-siliciclastic sequence of Proterozoic age represents the metamorphic part of the Tepla-Barrandium. In the NW a Permo-Carboniferous sequence (900 m) of mainly siliciclastic rocks overlay the metamorphic units of the Tepla-Barrandium. Thick coal seams as well as thin pyroclastic horizons of basaltic affinity are typical for the Carboniferous strata. Erosional remains of marine Cretaceous sediments discordantly overlay the western part of the TeplaBarrandian. North and east of Prague these Cretaceous sediments (1000 m) cover the Proterozoic to Permian strata of the Tepla-Barrandian completely. The marine Cretaceous sedimentation started at about 95 Ma. During the Variscan compressional cycle the deformational history of the Prague Basin is characterised by folding and nappe tectonics (Chlupac et al., 1998, Melichar and Hladil, 1999) and might have been influenced by extensional tectonics during the post-orogenetic collapse. Towards the north and south-east the Upper Proterozoic rocks (samples H24, H25) experienced weak regional metamorphism of the prehnite-pumpellyite facies (Chab and Bernardova, 1974). According to Schiffman and Day (1999) this would relate to a temperature between 200°C-350°C and a pressure between 0.12-0.2 GPa. Maturation of the organic matter indicates values between Ro: 0.59% and 4.0 % for the Paleozoic sedimentary sequence. Reflectivity of organic matter was measured using vitrinite and vitrinite-like material. Apatite grains of nine samples from the Tepla-Barrandian area were dated (Table 1). Sampling was done along a NW-SE transect (~30 km) crossing all major structural elements, especially the NW dipping Ockov thrust. Also four samples were taken from the drill-core of the drill-hole Tobolka 1 which intersects the Ockov thrust in 1200 m depth. Only two of the Tobolka 1 samples revealed apatite grains. Hanging wall of Ockov fault In the hanging wall of the Ockov thrust a basaltic tuff sample (H8) of Lower Silurian age (-430 Ma) shows a pooled age of 313±10 Ma and a mean confined track length distribution (m.c.t.l.d.) of 13.33±1.16 pm. Single grain ages of a Middle Devonian (~380 Ma) sandstone (H21), which failed the X2-test, have a mean age of 308±14 Ma with a broad single grain age distribution. The m.c.t.l.d. is 12.33±1.47 pm. Single grain ages in the range of the depositional age indicate that the sample was only partly annealed. Footwall of Ockov fault The Upper Ordovician (-440 Ma) greywacke sample (H9) in the footwall of the Ockov thrust is characterised by a pooled age of 257±10 Ma (m.c.f.l.d.: 12.45±1.06 pm). Another greywacke sample (H18) Geological Society of AustralHP^bstracts Number 58

FT2 of upper Ordovician age (-440 Ma) in the footwall of the Ockov thrust which failed the %2-test gave a mean age of 253±14 Ma (m.c.f.l.d.: 12.29±1.4l pm). Single grain ages of this sample range from 311±48 Ma to 183±23 Ma. Interpretation of the fission-track pooled ages has to consider that the stratigraphic distance between the Upper Ordovician (H9, H18) and the Lower Silurian sample (H8) is only 50 m. The Middle Devonian (~380 Ma) sandstone sample (H20) of the same horizon, but in the footwall of the Ochkov thrust, revealed a pooled apatite fission-track age of 268±10 Ma and a m.c.f.l.d. of 12.24±1.56 ]im. The pooled age difference between the hanging wall and the footwall of the Ockov fault, as indicated by sample H8 (313 Ma) and H9 (257 Ma) as well as sample H21 (308 Ma) and sample H20 (268 Ma), has interesting tectonic implications: a) Since the investigated horizons (H20, H21) were deposited at similar times (Middle Devonian) and the stratigraphic distance between H8 (Lower Silurian) and H9 (Upper Ordovician) is only 50 m apart, H8 and H9 as well as H20 and H21 were at the same depth levels during those times. b) In the time between 310 and 257 Ma sample H9 and H20 must have been buried to a significant deeper level than samples H8 and H21 as indicated by the apatite fission-track age difference. c) After 257 Ma these samples were exhumed again to the same depth level. Tectonic investigations done by Melichar and Hladil (1999) indicate movement of the hanging wall of the Ockov fault towards the south-east. Such movements might explain the longer burial of those samples from the footwall of the Ockov fault. Further to the south-east, two samples of Upper Proterozoic age were taken in the metamorphic area of the Tepla-Barrandian. Whereas the single grain ages of sample (H25) failed the %2-test (mean age of 260±20 Ma), apatite of the other sample (H24) revealed a pooled age of 270±10 Ma. Single grain ages of sample H25 range from 376±114 Ma to l65±29 Ma. The samples (TB-1-2, TB-1-3) of the Tobolka drill-hole are characterised by pooled ages of l66±12 Ma and l65±7 Ma. If a geothermal gradient of about 30°/km is assumed, 2 km of overlaying sedimentary rocks must have been eroded since the Upper Jurassic. An Upper Carboniferous (305 Ma) tuff sample ( H l l ) from the Permo-Carboniferous basin north-west of the Prague basin revealed an pooled age of 259±10 Ma. Chab, J. and Bernardova E. (1974) Prehnite and pumpellyite in Upper Proterozoic basalts of the NW part of the Barrandian. Krystalinikum, 10 53-65. Chlupac, I., Haviicek, V., Kriz, J., Kukal, Z. and Storch, P. (1998) Paleozoic of the Barrandian (Cambrian to Devonian). Pub. Of Czech Geological Survey 1998, 183 pp. Melichar, R. and Hladil J. (1999) Resurrection of the Barrandian Nappe Structures (Central Bohemia). Geolines, Praha, 8, 48 - 50. Schiffman, P. and Day, H.W. (1999) Petrological methods for the study of very low-grade metabasites. In: M. Frey and D. Robinson 1999, Low-Grade Metamorphism 108 - 142.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

A S Y M M E T R I C EXHUMATION A C R O S S THE PYRENEAN INTRAPLATE COLLISIONAL

OROGEN

R G . F i t z g e r a l d i , J . A . M u n o z ^ a n d S.L. B a l d w i n i 1 Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA 2 Group de Geodinamica i Analisi de Conques, Department de Geodinamica i Geofisica, Universitat de Barcelona, Zona Universitaria de Pedrables, Barcelona 08028, Spain

Introduction and geological setting The Pyrenean mountain belt is an intraplate collisional orogen formed by Late Cretaceous to early Miocene convergence between the Afro-Iberian and European plates. Late Cretaceous to EoceneOligocene thrusting was accommodated along pre-existing extensional structures originally formed during Triassic to Cretaceous rifting and transtension associated with opening of the Central Atlantic Ocean and rotation of Iberia to open the Bay of Biscay. The Pyrenean orogen comprises a central axial zone (AZ) of Hercynian basement, flanked north and south by fold and thrust belts developed in Mesozoic and Cenozoic sedimentary cover rocks. The thrust belts are flanked to the north (Aquitane Basin) and south (Ebro Basin) by foreland basins. The AZ is a complex south-vergent duplex structure that culminates in an antiformal stack of three upper crustal stacked basement thrust sheets. The antiformal stack is bounded to the north by the North Pyrenean Fault (NPF), regarded as the boundary between the Iberian plate and Europe. Timing of events in the foreland fold and thrust belts are well constrained due to the unusual preservation of synorogenic deposits with well exposed structural relationships that constrain their tectonic evolution. However, the variation in timing, rate and amount of exhumation across the range was relatively unconstrained. We collected vertical sampling profiles from Hercynian granitic plutons along the ECORS transect, a deep-seismic reflection transect across the uplifted central core of the central Pyrenees. Profiles were collected north of the NPF (Lacourt, Trois Segnieurs, Castillion Gneiss Dome), within the Nogueras thrust sheet (Les Bourdes, Riberot, Salau, Marimafia) and within the Orri thrust sheet (Maladeta). The main objective of this study is the quantification of the exhumation history across the Pyrenees in order to: (1) constrain the vertical component of its tectonic evolution and hence tectonic models for its formation, (2) provide information on the rate of sediment supply to help constrain the stratigraphic record of foreland basins, (3) address the question of climatic versus tectonic control on exhumation, and (4) constrain the unusual post-orogenic evolution of the central Pyrenees southern flank, which besides being distinct from that of the northern flank, is somewhat unique amongst convergent orogens in general. Eocene and OUgocene exhumation Results yielded an asymmetric pattern of AFT ages that young from north to south across the Pyrenees, indicating more exhumation to the south. Exhumation is a result of rock uplift due to IberiaEuropean plate convergence in the Paleogene with different crustal levels exposed at each profile due to varying amounts of exhumation. North of the NPF, the onset of rapid exhumation at ~50 Ma is recorded in the Lacourt profile. The Riberot profile just south of the NPF records Eocene exhumation at a rate of -173 m/m.y. from 44 to 36 Ma. Exhumation at this rate was followed by extremely rapid exhumation in the Early Oligocene (starting at ~35 Ma) concentrated on the southern flank of the AZ as recorded in the Maladeta profile. The pattern of exhumation is not controlled by the position of major thrust faults between antiformal nappes, although there is evidence for increased exhumation along the NPF, and for offset of AFT ages on faults north of the NPF. In the Eocene, relatively uniform exhumation occurs across the Pyrenees, changing in the Oligocene to significantly more exhumation on the southern flank of the AZ. This variation in exhumation pattern is controlled by a change in how convergence is accommodated within the Pyrenean doublewedge (Beaumont et al., submitted). Accommodation of thrusting on relict extensional features that Geological Society of Australia - Abstracts Number 58

led to inversion dominated thrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internal deformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumation on the southern flank in the Oligocene. The asymmetric exhumation pattern invalidates lithospheric models that call for symmetric exhumation centred on the NPF or models with lower crustal stacking below the northern A2. Late Oligocene burial of the southern Pyrenees and post-Miocene re-excavation A change in slope at 32-30 Ma in the Maladeta profile signals a dramatic slowing or cessation of exhumation in the middle Oligocene, marking the apparent end of major tectonic activity in the Maladeta region and likely also the central Pyrenees. We suggest that abrupt slowing of the erosion rate at ~30 Ma was due to a change in base level and a significant reduction of local relief. Below the change in slope in the Maladeta profile, the preservation of an exhumed PAZ indicates relative tectonic and thermal stability from ~30 to at least 20 Ma, but this profile must have been exhumed to its present elevation <20 Ma, most likely since the Middle Miocene (10-5 Ma) as shown by forward modelling of the Maladeta profile. The Maladeta profile thus support the sequence of events as postulated by Coney et al. (1996) who suggested that closure of the Ebro Basin resulted in burial of the active fold and thrust belt, filling of the basin, and burial of paleo-drainages and canyons along the southern flank of the A2 resulting in a change in base level. This was followed by a long period of relative quiescence and subsequent reexcavation of that landscape. The AFT data records when rapid exhumation of the AZ ceases (32-30 Ma), the formation of a PAZ during that time of relative stability (~30 to 10-5 Ma), when reexcavation began (10-5 Ma, from forward modelling) and its magnitude (2-3 km). Syn-tectonic conglomerates on the southern flank of the Pyrenees At Serra de Cis southwest of the Maladeta profile, granitic clasts were collected from two levels within syn-tectonic conglomerates. The "lower" level yielded AFT ages of 46-49 Ma, while the "uppermost" level of the conglomerates gave ages of 27-28 Ma. All samples had track length distributions indicative of rapid cooling. The 46-49 Ma ages confirm that rapid exhumation of the AZ was underway in the mid-Eocene. The younger ages of 27-28 Ma are more interesting as they suggest rapid exhumation was still ongoing at this time as compared to the Maladeta profile that indicate exhumation on the southern flank of the AZ had ceased by ~30 Ma. These results thus suggest that exhumation patterns along the AZ may have been diachronous, with cessation of Oligocene exhumation and burial of the foothills of the AZ possibly younging to the west. Tectonic versus climatic influence on exhumation We consider that the asymmetric exhumation pattern across the Pyrenees in the Early Oligocene resulted from tectonic forces as described above rather than as an orographic response to growing Eocene-Oligocene topography. The Ebro Basin was characterized by a warm and humid climate in the Eocene when it was open to the Atlantic Ocean. Once it became closed to the Atlantic due to uplift of the Cantabrian Ranges, climate became arid with reduced rainfall. As it is today, the prevailing wind direction for the Pyrenees in the Oligocene was probably from the northwest and thus the northern flank rather than the southern flank of the Pyrenees should have undergone orographic induced exhumation. The southern flank of the AZ was probably in a rain shadow that was less likely to facilitate rapid erosion unless controlled by other (e.g. tectonic) forces. Vertical profiles versus forward modelling of single samples These AFT results interpreted using the vertical sampling profile approach are compared to the approach utilising forward modelling of single samples. Morris et al (1998), using the genetic algorithm approach of Gallagher (1995), modelled the AFT data of Yelland from throughout the Pyrenees,

FT2 assuming a constant geothermal gradient to calculate exhumation rates. Their results suggested that exhumation rates along the ECORS profile remained relatively uniform (-100-300 m/m.y.) from the Eocene to the mid-Miocene. In contrast, the vertical profile approach suggests a two stage EoceneOligocene asymmetric pattern with post-orogenic exhumation of 2-3 km on the southern flank. While the two data sets (Yelland and this study) are compatible, the different interpretations reveal the strengths and weaknesses of each approach. The vertical profile approach has the potential to provide more precise information related to the thermal history and exhumation through time, but can be limited in its geographical coverage as more samples must be processed to reveal the age-elevation patterns. While the single age-modelling approach allows derivation of generalised thermal histories over a larger geographic area, it lacks precision in determining the thermal and exhumation history through time. Ideally, the combination of both approaches provides both precision and regional coverage. Beaumont, C., Munoz, J.A., Hamilton, J., and Fullsack, P., submitted, Factors controlling the Alpine evolution of the central Pyrenees from a comparison of observations and geodynamical models: Journal of Geophysical Research. Coney, P.J., Munoz, J.A., McClay, K., and Evenchick, C.A., 1996, Syn-tectonic burial and post-tectonic exhumation of an active foreland thrust belt, southern Pyrenees, Spain: Journal of the Geological Society, London, v. 153, p. 9-16. Morris, R.G., Sinclair, H.D., and Yelland, AJ., 1998, Exhumation of the Pyrenean orogen: implications for sediment discharge: Basin Research, v. 10, p. 69-85.

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International Conference on Fission Track Dating and Thermochronology

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S L I P H I S T O R Y AND T E C T O N I C SIGNIFICANCE OF A M A J O R O F F - A X I S FAULT ARRAY IN THE SOUTHERN GULF EXTENSIONAL P R O V I N C E : EVIDENCE FROM LOW-TEMPERATURE

THERMOCHRONOLOGY

J.M. Fletcheri, B.P. Kohn2, D.A. Foster3,AJ.W. Gleadow^ and R. Mendoza-Borundai 1 Centre de Investigacion Cientifiica y de Educacion Superior de Ensenada, Baja California, Mexico 2 Victorian Institute of Earth and Planetary Sciences, School of Earth Sciences, University of Melbourne, Parkville,Victoria 3052, Australia 3 Department of Geology, University of Florida, Gainesville, EL 326II, USA

The peninsula of Baja California is a 1200 km long sliver of the North American plate that has been undergoing transfer to the Pacific plate since the middle Miocene when the cessation of Farallon subduction and the southward passage of the Rivera triple junction put the two plates in direct contact with each other (e.g. Atwater, 1970; Bohanon and Parsons, 1995). The "captured" Baja microplate is separated from the Mexican mainland by a complex system of faults that has accommodated more than 300 km of dextral transtension. Deformation along most of plate margin, has been evolving from broadly distributed continental rifting to a more focused system of integrated transform faults and nascent oceanic spreading centres. Perhaps one of the best places to characterise this complex tectonic history and evolution from subduction-related tectonism to continental rifting to seafloor spreading is in La Paz-Los Cabos region, which is cut by several major transpeninsular faults and lies adjacent to a spreading centre that has been producing magnetically lineated oceanic crust for the past 3.6 m.y. (DeMets, 1995). However, like many structurally important sites in the Gulf Extensional Province, the La Paz-Los Cabos region is largely composed of Mesozoic crystalline basement that contains few structural or paleo-horizontal markers that can be used to define fault offset and/or tectonic-block geometry. An extensive sample suite has been obtained from crystalline basement fault blocks in the La Paz Los Cabos region which are being analysed for zircon and apatite fission track (FT) data, apatite (U-Th)/He ages and multiple diffusion domain modelling of K feldspar ^^Ar/^^Ar release spectra. Samples were collected on several transects across the structural grain of the NNW-striking transpeninsular faults as well as along the strike in the immediate footwall of some faults with the largest topographic escarpments and inferred offset (Fig. 1). Preliminary FT ages and thermal modelling indicate strongly asymmetric cooling and exhumation of the Los Cabos block, which is the largest granitic massif in the La Paz-Los Cabos region (Fig. 1). The La Paz fault is inferred to lie along the western margin of the Los Cabos block and separate granitic basement in the La Paz-Los Cabos region from Miocene volcanogenic strata that dominate most of the southern Baja Peninsula (Fig. 1). The La Paz fault is unquestionably the most widely cited structure in the southern Gulf Extensional Province. It is thought to have accommodated 50 km of left-lateral displacement (Hausback, 1984) and even a proposed post-Pliocene accretion of the Los Cabos block to the Baja Peninsula (Anderson, 1971). Consequently the Los Cabos block is commonly identified as a distinct tectonostratigraphic terrane that is fundamentally different from the rest of the peninsula (e.g. Sedlock et aL, 1993). However, recent field studies have not confirmed the existence of the La Paz fault. Moreover, the western margin of the Los Cabos block is very sinuous and does not show many of the characteristics of a fault-controlled range front. In contrast, the eastern margin of the Los Cabos block is defined by the San Jose del Cabo (SJC) fault, which, despite it's lack of recognition in peer-reviewed literature, is an impressive structure that extends more than 80 km along strike and forms a topographic escarpment with more 1000 m of relief (Fig. 1). Apatite and zircon FT data demonstrate a marked difference in Neogene tectonism between the two margins of the Los Cabos block. Thermal modelling of the data indicate that samples from the western margin, in the footwall of the proposed La Paz fault, experienced rapid Paleocene cooling (~20°C/m.y.), followed by essentially slow monotonic cooling (~2-3°C/m.y.) through the Tertiary. Geological Society of Austral^^^bstracts Number 58

FT2 In contrast, samples from the eastern margin, in the footwall of the SJC fault, record rapid cooling (up to ~45°C/m.y.) related to tectonic exhumation across the fault commencing in mid-Miocene (-10-12 Ma) when the sampled rocks were at ~150°C. The SJC fault accommodated 2-6.5 km of exhumation at rates that reached as high as ~1.5-2 mm/yr, averaged 0.4-0.7 mm/yr, but significantly diminished since the late Pliocene. Western fiexico

study Af'^a Isia ^ . Espiritu L \ ^ : Santo ^ Pv ^ Isla V >' ^ ^ / Cerralvop^ '

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24°N

Todos Santos Rock Units I Quaternary alluvium ^ I Miocene volcanogenic ^ strata Ull't Cretaceous metamorphic\ ' rocks V j Cretaceous plutonic rocks 0

10 20 30

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San Jose

'.^delCabo

Kilonoeters Figure 1. Schematic geologic map of the La Paz-Los Cabos region showing distribution of FT samples (triangles, blue = analysed, green = in progress) and ages (apatite = italic, zircon = regular, *mean of two measurements) from the Los Cabos block. Stars are epicenters of earthquakes with Ms> 5. Focal mechanisms indicate normal slip on NNW-striking planes. Bathymetric escarpments shown with diagonal hatch patterns. EC = El Carrizal fault, LP = proposed La Paz fault, SJP = San Juan de los Planes fault, SJC = San Jose del Cabo fault.

These data demonstrate that at a very basic level the tectonic framework of the southern Gulf Extensional Province needs to be significantly reevaluated. The slow monotonic cooling history throughout most of the Cenozoic makes it highly unlikely that any Neogene structure of regional importance exists on the western margin of the Los Cabos block. Therefore, we do not consider valid any hypothesised Neogene accretion of the La Paz-Los Cabos region to the rest of the peninsula of Baja California. Although it may be valid to describe the crystalline basement of the La Paz-Los Cabos region as a distinct tectonostratigraphic terrane, such characterisations exaggerate differences between this area and the rest of the peninsula. Instead, we propose that crystalline basement similar to that found in the La Paz-Los Cabos region likely underlies Tertiary strata that makes up most of the rest of the southern half of the peninsula. The reset apatite FT ages and rapid cooling history recorded by rocks from eastern margin of the Los Cabos block establish the SJC fault as one of the dominant structures of the southern Gulf Extensional Province. Additionally these data suggest that the granitic massifs of the La Paz-Los Cabos region are significantly tilted (up to 20° on a regional scale) towards the west as a result of east-side down normal faults like the SJC fault.

FT2 Conclusions This study is one of a growing number that demonstrates the power of low-temperature thermochronology for detailed characterisation of the tectonic evolution of the Gulf Extensional Province. In particular for: 1) identifying important fault zones, 2) defining limits of highly extended regions, 3) characterising slip history of major faults including critical time periods such as the onset of faulting and changes in slip rate, and 4) characterising along-strike displacement gradients. Continental rifting along the western margin of the southern GEP has produced an extensive array of NNW-striking normal faults that are presently active. Zircon and apatite FT data indicate that the onset of continental rifting coincided approximately with the southward passage of the Rivera triple junction (-12 Ma), which implies that crustal extension most likely was triggered by far-field stresses generated by lithospheric-plate kinematics and possibly coupling between the Pacific and North America plates through partially subducted microplates of the former Farallon slab (e.g. Bohannon and Parsons, 1995). The Gulf margin normal faults in the La Paz-Los Cabos region accomplish ENE extension at a high angle to the rift margin and the kinematics of faulting have not changed throughout many inferred reconfigurations of plate motion including: the northward rotation of the relativemotion vector at 8 Ma (Atwater and Stock, 1998), the eastward migration of Pacific-North America wrenching from the Tosco-Abreojos fauk to the GEP at ~6 Ma (Lonsdale, 1989) and the onset of seafloor spreading at 3.6 Ma (DeMets, 1995). However, decreasing slip rates on the SJC fauk in the late Pliocene may reflect the onset of seafloor spreading in the Gulf. Anderson, D. L., 1971, The San Andreas Fault: Scientific American, v. 225, p. 52-66. Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geological Society of America Bulletin, v. 81, p. 3513-3536. Bohannon, R. G., and Parsons, T., 1995, Tectonic implications of post-30 Ma Pacific and North American relative plate motions: Geological Society of America Bulletin, v. 107, no. 8, p. 937-959. DeMets, C., 1995, A reappraisal for seafloor spreading lineations in the Gulf of California: Implications for the transfer of Baja California to the Pacific plate and estimates of Pacific-North America plate motion: Geophysical Research Letters, v. 22, p. 3545-3548. Hausback, B. P., 1984, Cenozoic volcanic and tectonic evolution of Baja California Sur, Mexico, in Frizzell, V. A., ed.. Geology of the Baja Peninsula: Bakersfield, California, Society of Economic Paleontologists and Mineralogists, Pacific Section, p. 219-236. Lonsdale, P., 1989, Geology and tectonic history of the Gulf of California, in Winterer, E. L., etal, ed., The eastern Pacific Ocean and Hawai: Boulder, Colorado, Geological Society of America, p. 499-521. Sedlock, R. L., Ortega-Gutierrez, F., and Speed, R. C., 1993, Tectonostratigraphic Terranes and Tectonic Evolution of Mexico: Boulder, Colorado, The Geological Society of America, Inc., 153 p. Acknowledgments This work was funded by Consejo Nacional de Ciencia y Tecnologia, (grants #26750-T, #4345 P-T), the Australian Research Council and the Australian Institute of Nuclear Science and Engineering.

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International Conference on Fission Track Dating and Thermochronology

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T H E EVOLUTION OF SEDIMENTARY BASIN EXPLORATION

S.S. Poland The Geological Society of America, Boulder, Colorado, USA

Explorationists attempt to define petroleum systems within sedimentary basins to determine the level of uncertainty for each of the four critical elements (source, reservoir, trap, and seal) necessary for the creation and accumulation of hydrocarbons. Basin analysis will lead to a calculation of trap size and ultimately an estimate of the amount of trapped resources. In a known productive area, this is an exercise in analogy and extrapolation from producing fields into undrilled or underdrilled areas. In frontier basins (those with few or no wells) other methods must be applied to determine the level of uncertainty for the four critical elements. Outcrops provide information about potential source rocks, reservoir suites and seals in frontier areas. Vitrinite reflectance studies provide a measure of maximum paleotemperature. Knowing maximum temperature allows prediction of porosity generation/preservation, seal integrity and an estimate of source rock maturity. Reflection seismic surveys and potential field studies identify subsurface traps and trap configuration. But the most important petroleum system factor and (timing) is more difficult to determine and often ignored. Since the late 70's, apatite fission-track analysis has been used successfully to provide both temperature and timing information allowing a reconstruction of the thermal history of the basin from deposition to deformation through uplift and erosion. The temperature range for fission-track annealing corresponds to the onset of liquid hydrocarbon generation and to low-temperature, burial diagenetic processes allowing the timing of source maturation and porosity/cement formation to be determined. The timing of deformation and trap formation can also be determined and compared to the estimate of when the source matured and migration may have occurred. As exploration costs continue to rise and frontier areas become ever more remote it is critical that a systems approach be used in basin analysis. Fission-track analysis allows an estimate of timing the critical fourth dimension to be integrated into the basin analysis providing temporal constraints and improving one's estimate of exploration risk. At any scale of geologic analysis (sedimentary basin, petroleum system, play or prospect) the stratigraphic, geographic and temporal context must be determined. A basin-wide analysis typically focuses on the tectonic and/or structural evolution of the area. However, as more geochemistry is incorporated into the analysis, the ability to predict and quantify the occurrence of petroleum increases (Tissot, et al, 1987). When basins with straightforward geologic histories are studied (Green River Basin, Wyoming or the Anadarko Basin, Oklahoma) a basin analysis methodology with limited geochemistry works well. In areas with a more complex history (Ouachita fold and thrust belt, Oklahoma and Arkansas or the offshore basins of California) standard basin analysis techniques fall short. In these instances, where the timing of critical events source maturation, trap formation and migration must be determined and related to one another a detailed burial history chart is required (Magoon and Dow, 1994). To determine the age of critical events and associated temperatures, fission-track analysis along with other time-temperature techniques (e.g. Ar/Ar) is needed. Magoon L. B. and Dow W. G. eds. 1994. The petroleum system - from source to trap. American Association of Petroleum Geologists Memoir 60, 655p. Tissot B. P. Pelet R. and Ungerer P. 1987. Thermal history of sedimentary basins, maturation indices, and kinetics of oil and gas generation. American Association of Petroleum Geologists Bulletin 71, 1445-1466.

Geological Society of Australia - Abstracts Number 58

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International Conference on Fission Track Dating and Thermochronology

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TECTONIC EXHUMATION OF THE EOCENE B I T T E R R O O T METAMORPHIC C O R E

COMPLEX,

M O N T A N A AND I D A H O

D.A. Foster^ and A. Raza^ 1 Department of Geology, University of Florida, Gainesville, Florida 32611, USA also at: Department of Earth Sciences, LaTrobe University, Melbourne, Victoria 3083 Australia 2 School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052, Australia

The Bitterroot metamorphic core complex, in western Montana and eastern Idaho is an exhumed, mid-crustal, plutonic-metamorphic complex that formed during crustal thickening and subsequent extension in the hintedand of the Cordilleran Orogen. In the Bitterroot complex U-Pb zircon ages indicate that high-grade metamorphism (~0.65-0.75 GPa and ~600-750°C) was coincident with intrusion of syntectonic quartz diorite plutons at ~75-80 Ma. Between -65-53 Ma major partial melting of the lower and middle crust took place, leading to the intrusion of the voluminous "main-phase" granitic plutons of the Idaho-Bitterroot batholith as thick (3-4 km) sills. Intrusion of the main-phase was accompanied by renewed upper amphibolite facies metamorphism and partial melting production migmatites at -0.65 GPa pressure. The youngest mid-crustal granitic intrusions are about the same age as initial collapse of the orogen and extension at ca. 52-50 Ma. Extension was accommodated mainly by normal slip on the Bitterroot mylonite zone that deforms the younger intrusive as well as the older high-grade rocks. Crustal collapse in this sector of the Cordilleran Orogen appears to have been focussed where partial melting and plutonism was most intense and long-lived. Exhumation is revealed by the transition from amphibolite facies mylonitization, to greenschist facies shearing, to brittle faulting, that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 51-38 Ma, based on Ar-Ar and fission-track thermochronology. Apatite and zircon in the western part of the footwall give fission track ages of a - 4 8 Ma (apatites have long mean track lengths), recording very rapid cooling of the shallow structural levels of the complex. In the eastern part of the footwall the zircon fission track ages are -42-35 Ma and the apatite fission track ages are -38-20 Ma. Mean track lengths on the apatites from the eastern part of the footwall vary but are shorter than those from the west. The data in the east reflects the younger exhumation of the structurally deeper parts of the footwall as well as the effects of a second stage of extension dominated by basinand-range-type faulting in Oligo-Miocene time. The thermal history of the upper plate rocks contrasts sharply with the footwall rocks. Granitic plutons that intruded the hanging wall during Late Cretaceous time give apatite fission track ages of -65 Ma and long mean track lengths. The thermochronologic data record the very different cooling and exhumation histories of the upper and lower plates of the core complex, which helps to define the geometry and slip rates on the Bitterroot mylonite zone.

Geological Society of AustralRP^bstracts Number 58

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International Conference on Fission Track Dating and Thermochronology

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WEATHERING AND LANDSCAPE EVOLUTION OF THE G R E A T E R B R I S B A N E A R E A ,

SE

QUEENSLAND

D.M. Franks Department of Earth Sciences, University of Queensland,Qld, Australia.

Introduction The combination of regolith stratigraphy and regolith geochronology is useful in unravelling the history of evolving landscapes. The study of regolith stratigraphy can be attained through the description and chemical analysis of weathering profiles and through regolith mapping. Regolith geochronology involves the determination of the age of supergene (weathering-produced) minerals by absolute dating methods. The most promising technique suitable to weathering geochronology involves the application of K-Ar and ^^Axft^Ax dating in the analysis of K-bearing Mn-oxides and sulphates precipitated during weathering. To exploit the potentially useful combination of regolith stratigraphy and regolith geochronology to unravel the history of weathering in a region, 81 samples from 13 different hollandite-rich weathering profiles in the Greater Brisbane area have been collected and prepared for analysis. Detailed local and regional mapping of the regolith distribution in the region has been combined with previous geological investigations to determine possible controls on the weathering history and landscape evolution within this region. Brisbane Area Geology and Geomorphology Brisbane is situated along the lowermost reaches of the Brisbane River (Fig. 1), sprawling north and south along the coast and westwards between a gap in the resistant accretionary sediments that make up the coastal range. The remnants of Tertiary shield volcanoes bound the area to the west and south and define the major drainage divides for the region. The landscape of the Greater Brisbane Area is predominantly lithologically controlled and reflects a diverse and relatively complex geological history. Devonian-Carboniferous accretionary rocks (South DAguilar, Beenleigh and Yarraman blocks) provide the foundation for the region and occur in northwest trending terranes, juxtaposed by large fault systems (Sliwa, 1995) (Fig. 1). Uplift of the accretionary terranes, thought to have occurred during the Late Permian (Beckmann and Stevens, 1978), resulted in the development of mountain ranges and these provided a source for accumulation of the freshwater sediments of the Early-Middle Triassic Esk Trough, and the Late Triassic Ipswich basin. These accretionary terrains are thought to have been continuously exposed in the area, and still persist today as elevated features of the landscape, illustrating the longevity of some of the land-surfaces within the field area. The D'Aguilar and Yarraman blocks are bounded by faulted relics of Late Permian marine sediments (Northbrook Beds and Cressbrook Creek Group) and have been intruded by Permian-Triassic granitoid plutons, which have subsequently eroded to form distinctive physiographic features. The EarlyMiddle Triassic was also the period in which andesitic and tuffaceous volcanic units were extruded. This volcanism was post-dated by further basin development. As an extension of the Surat Basin, the Clarence-Moreton basin began forming during the Late Triassic initially by subsidence of the topographic surface (Oilier and Haworth, 1994). The Nambour Basin also formed at this time and sedimentation was continuous with the Moreton basin via the Brisbane Strait (Murphy et al, 1976). Fluvial processes with minor marine incursions dominated the depositional environment for both of these basins. No deposition is recorded in the Brisbane region during the period from Early Middle Jurassic to Early Tertiary and this period may be a time of erosion and deep weathering. During the Cainozoic, the landscape in the region is controlled by large variations in sea level and the consequent accumulation of sediments during high seastands and erosion during sea level drop. In addition, during the Geological Society of Austral^^^bstracts Number 58

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TARONG BASIN

YARRAMAN BLOCK

CRESSBROOK BUARABA BLOCK

Toowoomba MORBTON BASIN

Figure 1. Paleozoic and Mesozoic blocks and sedimentary basins (modified after Cranfield et a., 1976, Fig. 4.)

Mid to Late Cainozoic, renewed tectonic activity and the extmsion of volcanic rocks occurred over much of SE Queensland. The late Tertiary landscape was thus controlled by the emplacement of several volcanic centres and the erosion of the volcanic edifices, which in most cases are still apparent (Tweed, Main Range and Bunya Mountain Volcanoes). Alluvial terraces have formed along many of the major rivers (e.g. Brisbane and North Pine Rivers) during the Quaternary, whilst reworking of parabolic dunes during the Holocene contributed to the formation of a system of beach ridges and sand islands. Deep lateritic weathering profiles are preserved on various surfaces of low and high relief within the Greater Brisbane area and these record periods of intense chemical weathering and low denudation rates. Conspicuous occurrences are preserved on basalt extruded over much of the region during the Tertiary (especially between 25-20 Ma). Iron-oxide rich paleosols are preserved prior to, and between many of the successive flows, thus indicating intense weathering during this time period. Intense weathering continued after the cessation of volcanic activity and is preserved as extensive profiles on the Toowoomba and coastal basaltic occurrences (Redcliffe and Redland bay). Akin to other areas of Tertiary basalt along the east coast of Australia (e.g. the Monaro region; Taylor et ah, 1992), deep weathering has facilitated the production of bauxite and this has been preserved at several locations, predominantly along the dividing range in the west of the field area (Toowoomba, Maryvale, Yarraman and Mt Tamborine). The ages of the basalts pose some upper constraints, but the precise age and duration of this deep weathering is unknown. The occurrence of the lateritic weathering profiles along the coastline in Moreton Bay (Redcliffe to Redlands) is also problematic as their present landscape position is not conducive to the formation of deep weathering profiles. Furthermore, eroded remnants of the weathering profiles lie submerged under much of the bay area and only outcrop as scattered occurrences on various bay islands (e.g. Coochiemudlo, Stradbroke, and St Helena Islands). It is uncertain whether the factors contributing to the present location of the weathering profiles are the result of neotectonic movements within the landscape or a rise in mean sea level subsequent to their formation. Resolution of this issue will require more detailed regolith mapping and the location of supergene minerals applicable to geochronological study. Despite the obvious associations between basalt flows and weathering

FT2 profile development, lateritic weathering is not only confined to basaltic rocks and extends across large sections of the Late Triassic Nambour basin sediments, Late Triassic-Early Jurassic Woogaroo Sub-Group and the Early Jurassic Marbug Formation. Sampling Strategy and Analytical Procedures Mn-oxides occur as the product of weathering of Mn-rich sediments in the various units of the accretionary terranes. The abundance and preservation of suitable Mn-oxides in these units provide the focus of our geochronological study. The occurrences are located predominantly on dissected surfaces of relatively high relief. In-situ weathering of Mn-silicates and carbonates has released soluble Mn2+ within the regolith and promoted the precipitation of hollandite-group Mn-oxides. The Mn' oxides range in morphology from massive bands to thin botryoidal surface coatings and fracture fills. One occurrence of Mn-oxide replacing a root cast was also sampled. At slightly lower elevations, Mnoxides were preserved as cemented pisoliths and concretions associated with channel deposits. Several samples were collected at each site to enable a comprehensive analysis of the weathering history. Recording the landscape position and elevation for each sample provided a rigid framework for confident interpretation of results. Visual inspection of the collected samples determined the presence of one or more generations of Mn-oxide. When several generations were found to be present in a hand specimen or when botryoidal growth bands were identified (indicating prolonged periods of weathering and mineral precipitation) the sample was sectioned into a flat slab. An image of this slab was either captured through a video camera or scanned in a flat bed scanner to provide a record of the textural context from which each sample was derived. When several generations of Mn-oxides were present, each generation was individually sawed or micro-drilled from the slab. This ensured that the geochronological results could be correlated to specific events of Mn-oxide precipitation and that the weathering reaction dated was unambiguously identified. Samples were then individually crushed and several grains, ranging from 0.2-2 mm, were hand-picked for irradiation and 40Ar/39Ar laser step-heating analysis. Petrographic descriptions, x-ray diffraction analysis of selected phases, and electron microprobe analysis of Mn-oxides was also undertaken to provide geochemical and mineralogical information to better understand the weathering processes involved. The use of Mn-oxides from the Greater Brisbane area can provide valuable information about the weathering history of the region, which may be used to quantitatively constrain the processes controlling landscape evolution. Since manganese dissolution and precipitation reactions reflect specific geochemical reactions they may also act as palaeochemical indicators of past environmental conditions. Beckmann G. G. and Stevens N.C. 1978. Geological history of the Brisbane river system. Proc. R. Soc. Qd 89, 77-85. Cranfield L. C. Schwarzbock H. and Day R. W. 1976. Geology of the Ipswich and Brisbane 1:250 000 sheet areas. Geological survey of Qld. Report 95 Murphy P. R. Scharzboch H. Cranfield L. C. Whitnall 1. W. and Murray C. G. 1976. Geology of the Gympie 1:250,000 sheet area. Geological Survey of Qld. Report 96 Oilier C. D. and Haworth R. J. 1994. Geomorphology of the Clarence Moreton basin. In: Geology and petroleum potential of the Clarence-Moreton basin, New South Wales and Queensland, ed. A. T. Wells and P. E. O-Brien. AGSO Bulletin, 291-302 Sliwa R. 1995. Regional structural geology of the central north D'Aguilar block, southeast Queensland. PhD thesis. Univ. Qld. 207pp Taylor G. Eggleton R. A. Holzhauer C. C. Maconachie L. A. Gordon M. Brown M. C. and McQueen K. G. 1992. Cool climate lateritic and bauxitic weathering. J. Geology 100, 669-77

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THE THERMOTECTONIC HISTORY OF THE FRENCH WESTERN ALPS: A COMBINED STRUCTURAL AND MODELLING APPROACH

B. Fiigenschuhi, D. Seward^, M. Ford3 and Y Podladchichov2 1 Geologisch-Palaontologisches Institut, Universitat Basel, Bernoullistr. 3 2 , 4 0 5 6 Basel 2 Geologisches Institut, ETH-Ziirich, Sonneggstr. 5,8092 Zurich 3 Ecole Nationale Superieure de Geologie, CRPG-CNRS, Rue du Doyen Marcel Roubault BP40, 54501 Vandoeuvre-les-Nancy Cedex, France.

In the last decade numerous fission-track data from the Western Alps have become available. We will focus on the area extending from the Mont-Blanc massif in the north to the Pelvoux massif in the south in the light of both published and our own ongoing fission-track work. The paleogeographical domains and tectonic units addressed include (from external to internal): the external massifs and their cover, the North Penninic Valais zone and the Middle Penninic "Zone Houillere". These three domains are separated by two first order tectonic lineaments, namely the Penninic Front and the Houiller Front as evidenced by the ECORS-CROP seismic traverse (Nicolas et al., 1990). The external massifs are characterized by relatively young apatite ages, between 2 and 10 Ma and zircon ages of -10 to 20 m.y.. Based on an age-altitude correlation and an assumed geothermal gradient Soom (1990) calculated cooling rates on the order of 10 to 20°C/m.y. (last 20 m.y.) and exhumation rates of 1 km/Ma (for the last 7 m.y.). Slightly lower cooling rates have been calculated for the Belledonne massif in the Maurienne area by Lelarge (1993). From the available data, the cooling and exhumation histories of the external massifs seems quite similar and no overall trend can be observed. Samples from the turbiditic Champsaur sandstone (age of deposition Early Oligocene) and the underlying Pelvoux basement indicate that after deposition of the sandstone, burial temperatures reached 120°C to 250°C. Fission-track ages from the North Penninic Valais zone south of the Aosta valley display a similar age pattern to the external massifs. Both zircons and apatites are slightly older with the zircons yielding very consistent ages of around 17 Ma. This indicates that the both these units underwent a common cooling and exhumation history with the Valais zone being slightly less exhumed. No jump in ages could be observed across the Penninic Front. Significantly older ages of >10 Ma for apatite and >20 Ma for zircon have been determined for the Middle Penninic Zone Houillere. By combining the geochronological information with detailed structural mapping it becomes clear that some of these changes in cooling ages can directly be attributed to discrete faults. An attempt to model an area that has both a highly variable topography and a complex geological history reveals that: time-temperature slices as obtained in many studies on passive margins (e.g. Gallagher et al., 1998 and references therein) only provide valuable information if the constraints on the tectonometamorphic evolution are included. For the Western Alps temperature-time slices older than upper Miocene seem highly speculative due to large amounts of non-vertical displacement (e.g. more than 100 km NW-SE shortening since Oligocene time). Gallagher, K; Brown, R. and Johnson C. Fission track analysis and its applications to geological problems. Annu. Rev. Earth. Planet. Sci. 26: 519-572, 1998 Lelarge, N. Thermochronologie par la methode des traces de fission d'une marge passive (Dome de Ponta Grossa, Bresil) et au sein d'une chame de collision (zone externe de I'Arc alpin, France). Unpubl. Ph.D. thesis, Grenoble, 252 pp, 1993. Nicolas, A.; Polino, R.; Hirn, A.; Nicolich, R.; and ECORS-CROP working group. ECORS-CROP traverse and deep structure of the western Alps: a synthesis. In: Deep Structure of the Alps. Eds: Roure, F.; Heitzmann, P.; Polino, R. Memoire de la Societe geologique de France 156: 15-28, 1990. Soom, M. Abkiihlungs- und Hebungsgeschichte der Internmassive und der penninischen Decken beidseits der Simplon-Rhone Linie seit dem Oligozan: Spaltspurdatierung an Apatit/Zirkon und K/Ar datierungen an Biotit/Muskovit (Westliche Zentralalpen). Unpubl. Ph.D. thesis, Berne, 120 pp, 1990.

Geological Society of Australia - Abstracts Number 58

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International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

S T A T I S T I C A L A S P E C T S OF F I S S I O N T R A C K D A T I N G A N D

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THERMOCHRONOLOGY

R.E Galbraith Department of Statistical Science, University College London, London UK.

Fission track thermochronology is now well established as a scientific discipline. There have been many success stories of geological dating, thermal history estimation, provenance studies and other applications. Of course there have also been controversies and there are still many unresolved questions, but these are an expected part of the scientific process. This healthy state has resulted mainly from the careful methodological, laboratory and field work done by many people, but it is also partly due to the existence of a convincing mathematical basis for quantitative models -perhaps a more substantive basis than for other dating methods. At its foundation, fission track analysis is inherently statistical. Tracks (spontaneous or induced) may be regarded as line segments, of varying lengths and random orientations, located at points of a spatial Poisson process in three dimensions. Observing tracks by chemical etching of a crystal surface corresponds to sampling by plane section. This representation, based on physical considerations and spatial probability theory, has proved successful for explaining observed data, thereby adding credence to both the model and the observation process. It has led to a deeper understanding of the fission track age equation and of the variation to be expected for different types of measurement (confined, truncated and projected lengths and angles to the c-axis) and different experimental designs (external detector and population methods and variants). It has also provided the basis for several quantitative methods that are now used routinely. Heating tracks over geological time corresponds to changing their length distribution, according to the particular thermal history experienced, and generally in a manner that depends on the track orientations. In particular, it is the statistical distribution of track lengths, not just their mean, that reflects the thermal history. Annealing experiments are aimed at determining the relation between track length, temperature and time, in the presence of other sources of variation. Estimates of geological parameters, such as maximum temperatures or times of cooling, generally require data from both track lengths and densities. The combination of such data is often done informally, but it is possible in principle to specify the joint likelihood of all of the data (track lengths, orientations and densities, and indeed projected lengths) in a single statistical model. Further work has been aimed at determining complete thermal histories from observed track lengths and densities. Statistical mixture distributions arise naturally, not only for track lengths but also for single grain ages. Finite mixtures arise in provenance studies where a sample may contain grains from more than one source, while more complex processes of differential annealing may produce mixtures with many components. Statistical mixture modelling and estimation is not always straightforward. In particular, there may be an ambiguity between the number of component age groups and the variation within age groups, so it is important to try to obtain external information on one of these. I will elaborate on some of the above remarks and try to provide an overview of how statistical thinking can aid the scientific process in fission track analysis. I will stress the importance of good experimental design, and I will try to point to areas where more methodological work is needed.

Geological Society of Australia - Abstracts Number 58

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T H E LONG AND S H O R T OF F I S S I O N T R A C K ANNEALING - FROM LABORATORY TO GEOLOGICAL T I M E S C A L E S

K. Gallagher T.H. Huxley School of Environment, Earth Science and Engineering Imperial College of Science,Technology and Medicine South Kensington London SW7 2AS England

Over the last 15 years, a large quantity of laboratory-based experiments have been undertaken to characterise fission track annealing in apatite and zircon. Kinetics-based or pseudo-kinetic models have been calibrated from such data, relying on temperature and time as the key physical parameters. More recent models for annealing of fission tracks in apatite have addressed the role of chemical variations, and primarily consider halogen (Cl, F), hydroxyl and REE substitutions. To date, the models formulated for predicting how track lengths shorten with temperature and com-

position rely on empirical calibrations, rather than being soundly based on the relevant physical processes. Calibrations of annealing susceptibility against factors such as ionic porosity are well known, and certainly imply that atomic level processes are critical. However, given the complexity of the atomic scale processes, an empirical approach is pragmatic but is commonly considered to be unsatisfactory by, for example, the mineral physics community. However, as this community seems to agree that little is known about track formation and the structure of fission tracks, there are clearly major steps required to develop an ab initio type formulation of track annealing. Therefore, the geological applications of fission track data are likely to rely on empirical calibrations in the foreseeable future. The resulting annealing models are dependent on data quality, sample preparation, observer bias and the parameterisation of the model. At worst, developing an annealing model comes down merely to a curve fitting exercise. This can lead to problems when extrapolating laboratory calibrations to longer time scales, where small differences or uncertainties in the laboratory calibration can be magnified many times. A further limitation of laboratory experiments is the limited range in time scale it is possible to consider. Experiments need to be run at higher temperatures for shorter times, and consequently the rate of any process is much greater than in the geological setting. Similar problems exist in constraining rates of many processes in geology. Experimental rock deformation, for example, is one field in which it is well known that there is more than one competing mechanism for atomic scale deformation. Thus, researchers in this field recognise that the dominant process may vary depending on the time scale or rate. Such caveats may be applicable to annealing mechanisms for fission tracks from the laboratory to geological time scales. One of the current issues for long term annealing is the resolution of low temperature annealing, evidenced by the inference that mean track lengths on apparently unannealed samples are typically 11.5 |Lim shorter than induced tracks. This raises the question of other external factors, as alluded to earlier, as well as the possibility that induced tracks behave differently to spontaneous tracks and reveals a need to characterise the initial track length appropriately. Various attempts have been made to calibrate models on geological time scales, but these approaches will always suffer variably from a degree of circularity in that it is necessary to infer the thermal history of the host rock. In practice, we can never be totally convinced of the validity of the assumed thermal history, not least because of uncertainties in burial histories, palaeo heat flow and the typically poor quality constraints on the present day thermal state of a basin. In principle, the integration of fission track data with other thermochronological data suites such as "^^Ar/^^Ar for feldspar or (U-Th)/He for apatite has great promise in constraining at least an internally consistent model for the various techniques.

Geological Society of Australia - Abstracts Number 58

FT2 Overall, progress in understanding in fission track annealing will require a combination of theoretical, experimental and applications-based studies. The mineral physics community will consider the first approach in more detail, while the geological community will tend to focus on the latter two aspects. All three approaches are required to characterise the annealing behaviour of fission tracks in natural mineral samples and should be considered equally to produce a consistent predictive model.

International Conference on Fission Track Dating and Thermochronology

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PRACTICAL CONSIDERATIONS FOR USING DETRITAL ZIRCON FISSION-TRACK THERMOCHRONOLOGY FOR PROVENANCE, EXHUMATION STUDIES, AND DATING SEDIMENTS

J.L Garveri, M.T. Brandon2, M. Bemet2,1. Brewer4,A.V Soloviev3, PJJ. Kamp5 and N. Meyeri 1 Geology Department, Olin Building, Union CoUege, Schenectady NY 12308-2311, USA 2 Geology and Geophysics, Yale University, New Haven CT, 06511 USA 3 Institute of the Uthosphere, Russian Academy of Sciences, Staromonetny per. 22, Moscow, 109180 Russia 4 Department of Geosciences, Pennsylvania State University, University Park, PA, USA 5 Earth Sciences, University of Waikato, Hamilton, New Zealand

Overview Detrital fission-track thermochronology involves using FT ages of single grains for stratigraphic correlation, provenance analysis, dating unfossiliferous sediments, and exhumation studies. Unlike a conventional FT age where dated grains from a single source have a common thermal history, FTGA distributions contain many single grain ages that originate from a variety of thermotectonic source terrains (Garver and Brandon, 1994a,b; Carver et al, 1999). Revelation of fission tracks in zircon is affected by total accumulated alpha damage. Therefore, because this damage varies from grain to grain, detrital suites require an etching procedure designed to reveal a spectrum of grain-damage characteristics (Gleadow, 1978; Naeser et al, 1987). We use the multi-mount technique with a pre-etch in controlled conditions (228°C ± 1 in covered Teflon in a laboratory oven). Different etch times are needed to capture the full range of grain ages, especially in suites with a wide range of radiation damage. An important goal in these studies is to obtain a data set that has as little bias as possible in order to capture a full and quantitative grain-age spectrum (see Bernet et al, this volume). Accumulated radiation damage, mainly alpha damage, affects the etchability of a zircon so that optimal etch times are not uniform from grain to grain. Radioactive decay of 238u ^nd 232xh results in a-damage, which is mainly a function of uranium, thorium, and grain age. Because thorium decays at a slower rate and because it is typically less than half the concentration of uranium, almost all the a-damage in a typical zircon grain can be ascribed to uranium decay, a-damage facilitates track revelation; for typical zircons with FT ages between 1 to 1000 Ma the required etch time varies between 5 and 100 hours. Methodology Our methodology employed in the samples presented for this paper is similar to that widely used for the external detector method. Non-magnetic zircon grains are mounted in 2 cm2 squares of PFA Teflon™ using a glass-sandwich technique on a hotplate at ~330°C. Mounts are etched for 5 to 60 hours in a NaOH:KOH eutectic at exactly 228°C in covered Teflon™ in an laboratory oven. A 1 hr pre-etch forces disintegration of metamict grains, and other possible contaminants including iron sulfides which affect etching efficiency (Kowallis etal, 1996). This pre-etchant is then discarded and the etch continues with fresh etchant. Although there are a number of methodologies for extracting component populations, we use the methodology of Brandon (1996) in these studies. Examples Young suites of zircons (0-10 Ma) result from rapid exhumation or volcanic activity. Recent detrital zircon shed from the Southern Alps in New Zealand and those shed from the Himalayas are revealed with etch times from 30 to 60 hr. These suites of zircons are challenging because the zircons have a strong etching anisotropy. Uranium concentrations for datable grains is about the same as for typical detrital zircons as determined from U/Pb analyses (~300 to 500 ppm) suggesting little, if any, bias. Rivers draining both the Himalayas and the Southern Alps (both non-magmatic) have young Geological Society of Australia - Abstracts Number 58

FT2 populations of about 5 Ma or younger implying local exhumation rates in excess of 1500 to 2000 m/m.y. (see Brandon et al, this volume). Young to moderate zircon suites (FT ages between 10 to 40 Ma) are fully etched in 15 to 30 hr. These suites are ideal because the etching response is nearly uniform from grain to grain. Most countable grains in these suites have a mean uranium content between about 100 and 300 ppm. Detrital zircon suites from modern rivers draining the European Alps provide an excellent example of the full range of datable grains as applied to understanding the exhumation history of this mountain range (Bernet et al this volume). Moderate to old FT grain ages (40 to 400 Ma) also require multiple mounts, but very little difference exists in FT ages between the different etch times. Our etch time varies between 10 and 25 hours. We illustrate these suites with an example from Eocene flysch from the Kamchatka Peninsula of Russia where zircon FT ages are used to constrain the age of deposition and to reveal the exhumation history of the source (Soloviev et al, 1998; Garver et al, in review). In this study, the goal is full revelation of a young population that corresponds to the age of deposition ( - 8 8 to 44 Ma) and a second population of grain ages that tracks the progressive exhumation of a batholithic complex (-130 to 75 Ma). Both populations are fully revealed in a wide range of etch conditions. Uranium content ranges from 100 to 300 ppm. Old to very old suites (400 to 2000 Ma) are analysed only with great difficulty and patience while counting. Etch time varies between 5 and 10 hours but care needs to be taken to ensure that the etchant is not degraded by metamict or severely damaged grains which rapidly disintegrate in the early stages of etching. A strong bias exists in these data sets because only grains with low radiation damage (< about 100 ppm uranium) are countable with the standard external detector method. Jurassic sandstones of the Taurin Suite in the Crimea (-150 to 1000 Ma) and sediments in the Mohawk River in eastern North America (300 to 850 Ma), both of which have a partial source in Paleozoic sediments and Precambrain cratonal rocks, illustrate the relation between etch time, uranium, and FT age. Note that because typical detrital suites have a number of zircons with <50 ppm uranium, detrital work in unreset Precambrain strata is possible as long as this bias is factored into interpretation. Samples with a full spectrum of grain ages (young to very old) require 3 to 5 separate mounts to fully reveal the total grain-age population. We illustrate this rather complicated situation with data from the Mississippi River which drains much of North America. In this case, grain ages range from 10 to 1800 Ma and shorter and shorter etch times clearly capture older and older grains which on average have lower average uranium concentrations. If the number of under-etched, well-etched, and overetched grains can be assessed for each mount, the data can be mixed and treated quantitatively.

Summary. Studies using FT ages of detrital zircon allow for important insight into problems associated with sediment provenance, and exhumation of orogenic belts. To obtain a nearly unbiased sampling of an entire population of detrital grains, etching procedures must fully account for variation in alpha damage which affects track revelation. Using the mukimount technique and etch times of 5, 10, 15, 30, and 60 hours, a full spectrum of grain ages between 1 and 2000 Ma can be obtained. Brandon, M.T., 1996, Probability density plot for fission-track grain-age samples: Radiation Measurements, v. 26, no. 5, p. 663-676. Garver, J.I., and Brandon, M.T., 1994a, Fission-track ages of detrital zircons from Cretaceous strata, southern British Columbia: Implications for the Baja BC hypothesis, Tectonics, v. 13, n. 2, p. 401-420. Garver, J.L, and Brandon, M.T., 1994b, Erosional denudation of the British Columbia Coast Ranges as determined from fissiontrack ages of detrital zircon from the Tofino Basin, Olympic Peninsula, Washington, Geological Society of America Bulletin, V. 106, n. 11, p. 1398-1412. Garver, J.L, Brandon, M.T., Roden-Tice, M., and Kamp, P.J.J., 1999, Erosional denudation determined by fission-track ages of detrital apatite and zircon, in Ring, U., Brandon, M.T., Willett, S., and Lister, G. (editors). Exhumation Processes: Normal Faulting, Ductile Flow, and Erosion, Geological Society of London Special Publication 154, p. 283-304.

FT2 Garver, J.L, Soloviev, A.V., Bullen, M.E., and Brandon, M.T., Volcanism and exhumation of the Okhotsk-Chukotka arc (Russia) revealed by detrital fission-track ages of zircon; Physics and Chemistry of the Earth, (in review). Gleadow, A.J.W., 1978, Comparison of fission-track dating methods: effects of anisotropic etching and accumulated a-damage; in R.E. Zartman (ed.) Short papers of the 4th International conference on geochronology, cosmochronology and Isotope geology, Snowmass Colorado, August 1978, U.S. Geological Survey, Open File report 78-701. Naeser, N. D., Zeitler, P. K., Naeser, C. W., and Cerveny, P. K, 1987, Provenance studies by fission-track dating-etching and counting procedures: Nuclear Tracks and Radiation Measurements, v. 13, p. 121-126. Kowallis, B.J., Naeser, C.W., Queensbury, J.B., and Tingey, D.G., 1996, The effects of minor iron sulfide contamination on the etching of fission tracks in zircon from the fish Canyon Tuff. International Workshop on FT dating, Gent, Belgium, Abstracts, p. 69. Soloviev, A.v., Brandon, M.T., Garver, J.L, Bogdanov, N.A., Shapiro, M.N., and Ledneva, G.L., 1998, Collision of the Olyutor Island Arc with the Eurasian Continental margin: Kinematic and age aspects; Doklady Earth Sciences, v. 361, n 5, p. 632-634.

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M I D D L E C R E T A C E O U S D E N U D A T I O N IN T H E S O U T H E A S T H I G H L A N D S O F A U S T R A L I A : P O S S I B L E R E C O N C I L I A T I O N O F F I S S I O N T R A C K AND G E O M O R P H O L O G I C A L

RESULTS

D.L. Gibsoni, RB. 0'Sumvan2 and R.A. Chani 1 Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Australian Geological Survey Organisation, PO Box 378, Canberra,ACT 2601 AustraHa 2Australian Geodynamics Cooperative Research Centre, School of Earth Sciences,The University of Melbourne, Parkville, Victoria 3052 Australia

Apatite fission track (AFT) data indicate that rocks now exposed at many locations within the highlands of southeast Australia (Fig. 1), resided at elevated paleotemperatures in excess of 70-110°C prior to the middle Cretaceous, at which time they underwent rapid cooling commencing at ~95±5 Ma (e.g. O'Sullivan et al., 1996a). In the Sydney Basin located to the east of the highlands (Fig. 1), thermal indicators such as AFT, vitrinite reflectance, and thermal resetting of paleomagnetic directions, also suggest that rocks within the b a s i n h a v e e x p e r i e n c e d a significant d e g r e e o f c o o l i n g since the middle

Cretaceous (e.g. Schmidt and Embleton, 1981; Branagan, 1983; O'Sullivan et aL, 1996b). Detailed interpretation of the AFT data suggests these rocks experienced rapid km-scale denudation starting during the middle Cretaceous. However, the thermal signature of this middle Cretaceous event becomes weaker westward from the highlands and finally disappears altogether. Therefore, AFT data from Parkes in the central Lachlan Fold Bek (LFB) (Fig. 1), do not record middle Cretaceous cooling and instead record a major Permo/Triassic cooling event, followed by local evidence of reheating in the late Mesozoic, and widespread latest Cretaceous to early Tertiary cooling (e.g. O'Sullivan et al., in press).

SE Highlands

Eromanga Basin

\

ClarenceMoreton Basin Basins

Basement 29

26

Magnetic Stripes ^ ^ Tasmania

• Tasmania Basin

Eastern Highlands

300 km

Figure 1. Regional map of southeastern Australia, showing many of the major features mentioned in the text.

Geomorphologists have long studied rates of erosion in the highlands, mostly based on incision since the extrusion of Tertiary basalt flows or the deposition of dateable Tertiary sediments. Results from most of these studies suggested that maximum rates of erosion during the Tertiary in rocks of the LFB and the adjoining Sydney Basin were of the order of a few tens of metres per million years (m/m.y.). Such rates for the Tertiary are significantly lower than the middle Cretaceous denudation rates of up to -400 m/m.y. for some regions of the highlands determined from AFT data (O'Sullivan etaL, 1996a). Geological Society of Australia - Abstracts Number 58

FT2 Thus an apparent conflict developed between some geomorphologists who wanted to extrapolate their low rates back in time, and those generating the AFT data. High rates similar to those determined from AFT are presently observed only in tectonically active mountain building terrains, or high relief areas of poorly consolidated sediment. To address the apparent divergent rates of erosion since the Cretaceous this paper proposes a model for part of the evolution of the southeast highlands, based on interpretation of both geological and thermal data and models, which might explain the thermal data. This model also explains some of the gross drainage patterns present in southeast Australia. The Eromanga, Surat and Clarence Morton Basins in eastern Australia (Fig. 1), clearly have been eroded since close of sedimentation. Raza (unpublished data, in press) clearly shows that the rocks near the southern margin of the Surat Basin have undergone cooling equivalent to removal of one or more km of sediment, assuming palaeo-thermal gradients similar to those of today. In addition, erosional outliers of Surat Basin rocks are present throughout the Parkes area south of the main basin margin (Fig. 1) . The age of some of these has been confirmed as Late Jurassic by recent palynological studies (Gibson and Chan, 1999). AFT data from the Parkes area also suggest late Mesozoic deposition, though no rocks of this age are preserved in the immediate area (O'Sullivan et aL, in press). Consideration of the above data, the apparent superimposition of the middle reaches of major river drainages across the structural grain of the LFB, and earlier published models (e.g. Veevers, 1984) lead to the model presented by Gibson and Chan (1999) that the highlands area was previously covered with sediment of a southern extension of the Surat Basin, connecting with the Gippsland Basin to the south. It was the rapid erosion of this newly deposited sediment, rather than well indurated rocks of the LFB, which gave rise to the middle Cretaceous cooling recorded in parts of the highlands. However, we have to ask why were several kilometres of sediment deposited on top of the highlands, when there is little evidence of extensive deposition at this time further west? A possible answer lies in a tectonic model of viscous corner flow during convergence (e.g. Waschbusch et aL, 1999). They consider that the Surat Basin may have previously been a tectonic downwarp parallel to a west-subducting convergent margin somewhere to the east. The amount of downwarp would initially be a balance of downward viscous coupling force, and upward isostatic force. As sediment was trapped in the downwarped basin, first terriginous clastics, and then marine volcanolithic muds and sands sourced from a Mesozoic island arc to the east, isostatic sinking continued, allowing the build up of several kilometres of sediment. The initiation of cooling during the middle Cretaceous is consistent with a major change in plate motion, which included "switching off the viscous coupling which initially created the downwarp. Without the downward force from viscous coupling, the basin began to rise from isostatic force, forming an arched upland of poorly consolidated sediment, which was rapidly eroded by newly formed and modified rivers draining roughly normal to the uplift axis. Tectonic denudation may also have taken place, by gravity driven decollement. Some of the eroded sediment was deposited on the shoulders of the uplift, as the youngest rock units in the Eromanga Basin, and now completely eroded sediment further south in the Parkes area. There is no record of sedimentation in the east, as it probably mostly occurred over Pacifica, which subsequently separated from Australia at 80-65 Ma. As sediment was removed, isostatic rebound continued as upwarp, ensuring continued rapid erosion. When the rivers eroding the upwarped area eroded through the sediment and into the underlying LFB rocks, rates of incision slowed, due to the well-cemented nature of the older rocks, and waning isostatic rebound. This resulted in the exhumation, in isostatic balance, of the pre-existing landscape. The courses of the main rivers were superimposed onto LFB rocks at this time. This landscape was probably further affected by tectonically driven shifts in isostatic balance during the subsequent breakup of eastern Gondwana, such as underplating, crustal thinning, etc., to form the basis of the modern highlands.

FT2 The scenario of vertical uplift of a basin near an active plate margin is somewhat similar to the modern situation in Papua New Guinea, where Miocene-Pliocene and Mesozoic marine sediments of the Papua Basin/Aure Trough have been up warped along the Kubor anticline, with Miocene and Pliocene marine sediments now present at elevations of up to 2000 m, on either side of the uplift axis. Late Palaeozoic basement rocks have been exhumed along the up warp axis, by both erosion and decollement, and form ranges up to nearly 4000 m elevation and with 2000 m relief. Modern deposition in the Gulf of Papua is analogous to deposition of the post-95 Ma age sediments of the Eromanga Basin. It is possible that the late Mesozoic was the second time that a deposition/erosion/exhumation cycle occurred in southeast Australia. The findings of O'Sullivan et al (in press) at Parkes indicate substantial deposition during the Late Carboniferous to Early Permian, followed by erosion in the Late Permian/Early Triassic to exhume an Early Carboniferous weathering profile, which is still preserved. The late Palaeozoic sediment was most probably contiguous with the preserved Sydney Basin, which was preserved as a downwarped area by convergence of the New England Fold Belt. The model, although perhaps containing tenuous assumptions about tectonic conditions and mechanisms, accounts for the following: 1) High temperatures prior to the middle Cretaceous in LEB rocks, due to burial in the JurassicCretaceous. 2) Rapid cooling due to rapid denudation of poorly cemented sediment after upwarp at ~95 Ma. 3) Relatively slow erosion of LEB rocks since exhumation in the Late Cretaceous. 4) The courses of rivers draining west superimposed from drainage flowing over Mesozoic sediment and eroding the western flank of the uplift. Branagan, D. R, 1983. The Sydney Basin and its vanished sequence. Journal of the Geological Society of Australia 30, 75-84. Gibson, D.L. and Chan, R.A., 1999. Aspects of palaeodrainage in the north Lachlan Fold Belt. Regolith 98 Conference, New Approaches to an Old Continent, Proceedings, CRC LEME, Perth, 23-37. O'Sullivan, P. B., Foster, D.A., Kohn, B.P., and Gleadow, A.J.W., 1996a. Tectonic implications of Early Triassic, and middle Cretaceous denudation in the eastern Lachlan Fold Belt, NSW, Australia. Geology 6. 563-566. O'Sullivan, P. B., Coyle, D. A., Gleadow, A. J. W., and Kohn, B. P., 1996b. Late Mesozoic to Early Cenozoic thermotectonic history of the Sydney Basin and the eastern Lachlan Fold Belt, Australia. Geol. Soc. Aus. Extended Abstracts No. 43, Mesozoic 96-Mesozoic Geology of the Eastern Australian Plate, September 23-26, Brisbane. 424-432. O'Sullivan, P. B., Gibson, D. L., Kohn, B. P., Pillans, B., and Pain, C. F., in press. Long-term Landscape Evolution of the Northparkes Region of the Lachlan Fold Belt, New South Wales: Constraints from Apatite Fission Track and Paleomagnetic Data. Journal of Geology. Schmidt, P. W., and Embleton, B. J. J., 1981. Magnetic overprinting in southeastern Australia and the thermal history of its rifted margin. J. Geophys. Research 86, 1981, 3998-4008. Veevers, J. J. (Ed), 1984. Phanerozoic Earth History of Australia. Clarenden Press, Oxford University Press. Waschbusch, P., Beaumont, C. and Korsch, R. J., 1999. Geodynamic modelling of aspects of the New England Orogen and adjacent Bowen, Gunnedah and Surat Basins. Regional geology and metallogenesis of the New England Orogen. University of New England, 203-210. Acknowledgements We thank Asaf Raza for access to unpublished AFT data, Russell Korsch for ideas on viscous corner flow during subduction, and John Veevers and others for publishing ideas on Mesozoic palaeogeography. This work was made possible with funding by the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), the Australian Geodynamics Cooperative Research Centre (AGCRC) and the Australian Institute of Nuclear Science and Engineering (AINSE). Work reported here was conducted as part of ongoing cooperation between the CRC LEME and AGCRC. This paper is published with the permission of the Director, CRC LEME and the Director, AGCRC. D. Gibson and R. Chan publish with the permission of the Executive Director, Australian Geological Survey Organisation.

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

MULTIPHASE COOLING AND EXHUMATION OF THE SOUTHERN ADELAIDE F O L D B E L T REVEALED FROM APATITE F I S S I O N T R A C K DATA AND THERMAL H I S T O R Y M O D E L L I N G

HJ. Gibsoni and K. Stiiwe^ 1 Geotrack International, 37 Melville Road, Brunswick West, Victoria, 3055, Australia. 2 Institut fur Geologie und Palaontologie, Universitat Graz, Heinrichstr. 26,A-8010 Graz,Austria.

Data from apatite fission tracks were gathered from 20 outcrop samples along two transects in the southern Adelaide Fold Belt, South Australia (Fig. 1). Simple multiphase thermal histories which are consistent with these data have been discerned using numerical models. All samples considered, fission track age and length distributions reveal three discrete cooling events: an event beginning some time between 85 and 0 Ma (Late Cretaceous to Recent) which was characterised by cooling throughout the study area from roughly 50-70°C, and an event beginning some time between 300 and 270 Ma (Late Paleozoic) which was characterised by cooling from temperatures >120°C in all areas except for the Mt Lofty Ranges and Murray Bridge region where peak temperatures were only 95-115°C prior to cooling. Some samples from this sub-region of relatively cool Late Paleozoic temperatures also retain evidence for an even earlier cooling event from temperatures >120°C, beginning some time prior to 350 Ma. We interpret the post-85 Ma cooling event as the consequence of region-wide exhumation from a depth of 1.0-1.6 km (Table 1). This event is more widespread and of larger magnitude than the degree of localised surface uplift which is thought to have formed the Mt Lofty and Flinders Ranges (-0.4 km) at about the same time (Wellman and Greenhalgh, 1988). However, since AFTA is based on thermally activated processes, our results can not be used to draw any firm conclusions about surface uplift, rather only about amounts of erosion, i.e. our results neither support nor rebuke the surface uplift estimates of previous workers. The Late Paleozoic cooling event (beginning some time between 300 and 270 Ma) is interpreted to have been caused by the coupled processes of uplift and exhumation, and may correlate with the final stages of the Alice Springs Orogeny. This event is classically linked only to the evolution of central Australia. However, similar Late Paleozoic effects as those which we observe have also been recognised by fission track workers in the northern Flinders Ranges and adjacent areas (Mitchell et aL, 1998; Foster et al., 1994), and hence the effects of the Alice Springs Orogeny are now thought to extend to southern Australia (Mitchell et al., 1998). For the earliest cooling event recorded (onset some time >350 Ma), evidence from fission track data is retained only by samples from the Mt Lofty Ranges and Murray Bridge areas. However, we nonetheless suggest that the >350 Ma-event affected the entire study area, and that many samples do not retain any evidence, because they were totally annealed prior the subsequent 300 to 270 Ma event. We propose that the 350 Ma cooling event may relate to exhumation which either accompanied the latest stages of inversion of the Kanmantoo Trough during the Cambro-Ordovician Delamarian Orogeny (Jenkins and Sandiford, 1992), or perhaps accompanied the final unroofing of this region some time after intrusion by Early Paleozoic granites (now outcropping east of Mt Lofty and comprising samples 1, 2 and 3, see Fig. 1). However, we emphasise that the AFTA data only provide a younger limit to the age of this cooling episode (>350 Ma), and hence the tectonic interpretation here is only speculation. Results from this study highlight the importance of regional, episodic post-orogenic exhumation of Paleozoic fold belts where some times, due to the lack of preserved younger strata, conventional methods have erroneously suggested relatively long term stability.

Geological Society of Australia - Abstracts Number 58

33°00'

34°00'

35°00'

36°00' 137°00'

140°00'

139°00'

138°00'

Figure 1. Generalised geological map of the southern Adelaide Fold Belt (modified after Jenkins and Sandiford, 1992), show-

ing the locations of AFTA samples analysed in this study (circles surrounding sample numbers). Light grey shading = Cenozoic rocks; hatched areas = Permian rocks including glacial tills; dark grey = Cambrian/Ordovician intrusives; open-spaced dots = undifferentiated Cambrian; close-spaced dots = undifferentiated Pre-Cambrian; black lines = faults; arcuate lines with double arrows = major anticlinorium of the Mt Lofty Ranges Table 1 Sub-area of the study region

Maximum/peak Paleotemperature

Estimated former burial depths for possible paleogeothermal gradients 20

35

55

(°C/km) (°C/km) CC/km) (°C) For the Late Cretaceous-Recent exhumation event (onset some time <85 Ma): 0.6-1.0 all five sub-areas: 50-70 1.8-2.8 1.0-1.6 For the Late Paleozoic exhumation event (onset some time between 300 & 270 Ma): Mt Lofty Ranges & Murray Bridge area: 95-115 4.0-5.0 2.3-2.9 1.4-1.8 Northern Yorke Peninsula, Clare Valley & south-east, & Yorke Penin. to sth of Adelaide: > 120

>3.0

>1.9

For the pre-Mid Paleozoic exhumation event (onset some time >350 Ma): all five sub-areas: >120 >5.3 >3.0

>1.9

>5.3

Table 1. Estimated former depths of burial prior to exhumation in sampled areas of the southern Adelaide Fold Belt in the

pre-mid Paleozoic, Late Paleozoic and Late Cretaceous-Recent. These estimated burial depths (i.e. amounts of section removed) depend on the AFTA-derived estimated paleotemperatures, which in turn depend on assumed heating and cooling rates of 1 and 5°C/Ma, respectively. These estimates are calculated with respect to the paleo-surface and are independent of past or present sample elevation. Paleogeothermal gradients are unconstrained in this study, but the range investigated here (20 to 55°C/km), is thought to include geologically reasonable values.

FT2 Foster, D.A., Murphy, J.M. and Gleadow, AJ.W. (1994) Middle Tertiary hiydrothermal activity and uplift of the northern Flinders Ranges, south Australia: Insights from apatite fission-track thermochronology. Aust. J. Earth Sci., 41, 11-17. Jenkins, RJ.F. and Sandiford, M. (1992) Observations on the tectonic evolution of the southern Adelaide Fold Belt. Tectonophys., 214, 27-36. Mitchell, M.M., Kohn, B.P. and Foster, D.A. (1998) Post-orogenic cooling history of eastern South Australia from Apatite FT Thermochronology. In: Advances in Fission - Track Geochronology (Ed. by P. Van den Haute and F. De Corte), pp. 207224. Kluwer Academic Publishers, The Netherlands. Wellman, P. and Greenhalgh, S.A. (1988) Flinders/Mount Lofty Ranges, South Australia. Their uplift, erosion, and relationship to cnjstal structure. Trans. Roy. Soc. South Austral., 112, 11-19.

Acknowledgments The authors gratefully acknowledge the full financial and technical support which was given to this research by Geotrack International and its staff (unfortunately too numerous to list here). Geoff Laslett is also acknowledged for his part in developing the interpretive methods used in this study. We thank Melinda Mitchell, formerly of La Trobe University, for collecting sample 5, and for supplying the fission track ages and mean track lengths of this sample.

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International Conference on Fission Track Dating and Thermochronology

VISUALISATION OF LATENT A L P H A - R E C O I L - T R A C K S

(ART)

F T 2 '

IN D A R K M I C A VIA ATOMIC

FORCE MICROSCOPY

U.A. Glasmacheri, F. Ohnesorge^, G.A.Wagneri and R. Neumann^ 1 Forschungsstelle Archaometrie der HeidelbergerAkademie derWissenschaften am Max-Planck-Institut fur Kernphysik, P.O. Box 103980,69029 Heidelberg, Germany 2 Gesellschaft fur Schwerionenforschung (GSI), Planckstrasse 1, D-64291 Darmstadt, Germany

Dark micas contain uranium, thorium and their radioactive daughters in trace concentrations. Their alpha-decay releases energies of several MeV, part of which is transferred to the daughter nucleus as recoil energy (-lO^ keV). The alpha-recoil nucleus slows down when interacting with the lattice atoms producing -10^ lattice defects which equals one single alpha-recoil track. The size of a latent ART in dark mica is expected to be in the range of 30 to 50 nm (Gogen, 1999). Phlogopite from Fornicher Lava/Eifel volcanic field in Germany, which is characterised by an K/Ar-age of 180±20 Ma, 18 ng/g U and 40 ng/g Th revealed an average surface track density of about 2x103 ART/mm (Gogen, 1999; Gogen and Wagner, 1999). Therefore, biotite from the southern Ural, Russia with a known time of -150 m.y. below 60°C was chosen for the visualisation experiment (Glasmacher et al., this volume). If this biotite would have accumulated ARTs during the whole time, a surface track density of 10« ART/mm is expected. This high surface track density cannot be visualised anymore by light microscopic inspection of the strongly enlarged etched tracks. However, the latent ARTs may be directly detectable for an atomic force microscope which is addressed here: Both topographic and lateral force images of a cleaved sample show surface details roughly having the expected size and density distribution (size 20-50 nm, 10^ ART/mm, Fig. 1), most likely representing the latent ARTs. Their relatively uniform appearance, partly even triangular in shape, suggests that they should not originate from intercalates in the biotite. Selective but very gende etching of the tracks in situ (Glasmacher et al, this volume), i.e. during AFM imaging, would provide reliable conclusive evidence for those surface structures actually being the latent ARTs, and these experiments are under way.

mLciu vuncicncuy iVd^ i i i i i \ i i i i i ikely identifying the Figure 1. Al M una^^^o latent ARTs. A: Constant force ("topographic") image (image size 0.9 pm x 1 pm), B: Lateral ("friction") force image (size 0.9 pm X 1 pm). Surface track density is about 10® tracks/mm Glasmacher, U.A., Wagner, G.A. and Gogen, K. this volume. Alpha-recoil track dating (ART-D) of dark micas - a potential tool for tephrochronology and thermochronology Gogen, K. 1999. Die Alpha-RiickstoS-Spuren Datierungsmethode: Theoretische und experimentelle Entwicklung und die Datierung von Glimmer, unpub. Dr. Arbeit, 166 p. Gogen, K. and Wagner, G.A. 1999. Alpha-recoil track dating of Quaternary volcanics. Chem. Geol. in print, 13 pages.

G e o l o g i c a l Society of Australia - Abstracts IMumber 58

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International Conference on Fission Track Dating and Thermochronology

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CRETACEOUS TO T E R T I A R Y T E C T O N O - T H E R M A L EVOLUTION OF THE LAURENTIAN M A R G I N IN Q U E B E C , CANADA

U.A. Glasmacheri,A.Tremblay2 and M. Zentilli^ 1 Forschungstelle Archaometrie der HeidelbergerAkademie derWissenschaften am Planck Insti-tut fiir Kernphysik, P.O. 103980, 69029 Heidelberg, Germany 2 INRS-Georessources-Quebec, Geosciences Centre, Ste-Foy, Quebec, GIV 4C7, Canada 3 Department of Earth Sciences, Dalhousie University, Halifax, NS, B3H 3J5, Canada

Along two E-W transects around Quebec City an apatite fission track study was conducted. Samples were taken in an 4 to 6 km spacing. The aim of the study was to evaluate and quantify the postTaconian thermal history of the Laurentian margin and possibly determine the influence of the opening of the North Adantic Both transects extend from the Precambrian metamorphic basement of the Canadian Shield into the Taconian external zone of the Appala-chian Orogen. Apatite grains from Cambrian and Ordovician sandstones, Precambrian mangerites, metasediments and an amphibolite were dated with the external detector method and confined tracks were measured to model the thermal history of the region. The geochemical composition of dated single apatite grains was used to interpret the single grain ages distribution and the confined track length data. The T-t path for all samples was modelled by using the inverse model of Willet (1992) and Issler (1996) with the Durango annealing model of Laslett et al. (1987), a random determination of the heating and cooling events with 5 m.y. substeps, and very open upper and lower temperature limits. All grains are dominated by flouroapatite with negligible chlorine content. The oldest grains in nearly all samples are low in fluorine. Two age groups of the amphibolite are due to variation in fluorine content. The decrease of the pooled ages of apatite grains with similar F-Cl-composition from east (191±15.2 Ma) to west (127±11.8 Ma) towards Quebec City was caused by an increase of local thermal heat flow. The difference in pooled ages of apatite grains with a similar chemical composition on both sides of the Montmorency fault indicates exhumation of the western side (Laurentian shield) between 160 and 140 Ma. Variations in pooled ages further west point to a differential exhumation of the Laurentian shield. The younger pooled ages of the eastern side of the Montmorency Fauk either indicate younger reverse movement along the fault or higher heat flow between 140 and 120 Ma. In comparison with regional trends similar age groups (200-155 Ma, 135-85 Ma) are described for the magmatic province of the Northern New England States and Southern Quebec (Eby, 1984; McHone and Butler, 1984) which is related to the opening of the North Atlantic. The gas occurrences between Quebec City and Montreal might have been caused by the change in heat flow during Cretaceous time. Eby, G.N. 1984. Geochronology of the Monteregian Hills alkaline igneous province, Quebec. - Geology 12, 468-470. Issler, D.R. 1996. An Inverse model for extracting thermal histories from apatite Fission Track data: Instructions and software for the Windows 95 Environment. - GSC Open File 2325, 84 p. Laslett, G.M., Green, P.F., Duddy, I.R. and Gleadow, A.J.W. 1987. Thermal annealing of fis-sion tracks in apatite 2. A quantitative analysis. - Chem. Geol. (Isot. Sec.) 65, 250-269. McHone, J.G. and Butler, J.R. 1984. Mesozoic igneous provinces of New England and the opening of the North Atlantic Ocean. - GSA Bull. 95, 757-765. Willett, S.D. 1992. Modelling thermal annealing of fission tracks in apatite. - Min. Ass. Can. Short Course V 20, Appendix 1, 190-201.

Geological Society of Australia - Abstracts Number 58

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International Conference on Fission Track Dating and Thermochronology

A L P H A - R E C O I L T R A C K DATING ( A R T - D )

F T 2 '

OF D A R K M I C A S - A P O T E N T I A L T O O L F O R

TEPHROCHRONOLOGY AND THERMOCHRONOLOGY

U.A. Glasmacheri, G.A.Wagneri and K. Gogen2 1 Forschungsstelle Archaometrie der Heidelberger Akademie derWissenschaften am Max-Planck-Institut fur Kernphysik, P.O. Box 103980,69029 Heidelberg, Germany 2 Biethsstr. 14,69121 Heidelberg, Germany

Alpha-recoil-tracks (ART) in mica are first described by Huang and Walker (1967). They un-covered a background of small and shallow etch pits as they tried to etch spontaneous fission tracks in mica. Caused by the alpha decay of 238u (io6 times more frequent then spontaneous fission), 235u, ^^^Th and daughter products, the surface density of ARTs would allow to date Quaternary geological events as well as archaeological materials. Since then many attempts were made to develop a dating technique based on the accumulation of alpha-recoil-tracks but essentially failed. Dark mica contain uranium and thorium in trace concentrations (IQi-lO^ ng/g). Their alpha-decay releases energies of several MeV, part of which is transferred to the daughter-nucleus as recoil-energy (-10^ keV). The alpharecoil nucleus slows down when interacting with the lattice atoms producing -lO^ lattice defects which equals one single al-pha-recoil track (Glasmacher et aL, 2000). The size of latent ART in dark mica is in the range of 30 to 50 nm. To visualise ARTs, dark mica (grain size > 200 \xm) was etched with 40% HF at 25°C between 1 and 25 sec. This etching procedure creates small shallow etch pits at the surface of the cleavage plane (Fig. 1). The etch pits are visible and countable in the light of phase-contrast microscopy. Increase of surface density of ART etch pits with etching time (t^) as described by Huang and Walker (1967), Huang et aL (1967) and Hashemi-Nezhad and Durrani (1981) was also observed for phlogopite and biotite from volcanic rocks of the Eifel volcanic field, Germany, the Long-Valley-Caldera, California and the East-African-Rift system. In the t^-range of incipient visual appearance of individual etched tracks (min. ^ = r^ -0.5 pm) in the phase-contrast microscope (magnification: 40x obj. x lOx oc.) and the overlapping of individual etch pits the relation between etching time and surface density of ART is a linear relationship. Based on the experiments on phlogopite from volcanic rocks of the Eifel region, Germany, Gogen and Wagner (1999) proposed a model for the accumulation of ARTs in mica. The age equation combines the volumetric density (r) of ART with the uranium and thorium contents. The volumetric density is determined from the linear increase of the surface density of etched tracks as a function of the etching time, and can be derived either from the slope (m = v^ . r) or the Y-intersection (b = r . (R^ - r^ . v/vi,), whereby Re is the etchable length of latent ART) of the etching curve. Techniques have been developed to determine the etching velocity perpendicular (v^) and parallel (Vh) to the c-axes of dark mica. Age (ka):

K/Ar = 150±401

40Ar/39Ar = 12.9±0.62

ESR: 245±753

r (lO^Art/mm):

6.764±0.888

0.714±0.046

3.44710.385

U (ng/g):

20±1.6

17±1.4

13±1.0

Th (ng/g):

56±4.5

49±3.9

12±1.0

Comparison of both etching velocity with chemical composition of phlogopite indicates a positive linear correlation between the AlVI, Fe3+, Fe2+, Ti content per formula unit at the octahedral positions and vh as well as a negative linear correlation between Mg-content at the octahedral positions and y^ (Fig. 2). A similar correlation between v^ and the occupation of the octahedral position is less well defined.

Geological Society of Australia - Abstracts Number 58

FT2

Figure. 1. Microphotograph of fossil ART etch pits in phlogopite (etched for 6 min at 25°C in 40% HF) observed with an phase contrast microscope (Gogen 1999, sample: Bausenberg and Laacher See, Eifel, Germany; Eisenbuhl, NW-Bohemia, Germany). Age reference: 1: Frechen and Lippolt (1965), 2: Van den Bogaard (1995), 3: Wagner et al. (1998).

Applying the fission track technique (to determine U) and the ICP-MS-technique (for U and Th) phlogopites of the Eifel region and the East African Rift system have values between 13 and 20 ng/g U and 12-56 ng/g Th. The detection limit is 2 ng/g and the error 8% (Eig. 1). The applicability of ARTda^i^ty tprhnimip to .^^mnles has been initiallv tested bv usine nhlosooite from Quaternary all ilinc \ 1 ini i 1 I ilic 1 ikl i m l 1+u ^ 1 i m u+ ii 1 i; f Mi

im

• • ^ • l l l

III B P

I

lfm

I

H

it

v^

*

m

Wr

•n ^BlillM Figure. 2. Comparison of both etching velocities with chemical composition (octahedral positions).

FT2 The age revealed by the ART-technique shows a good correlation with K/Ar and ^^Ar/^^Ar ages. In general two possibilities arise from the recent data. First a correlation curve between uranium- and thorium-normalised volumetric density of ART and independently determined ages can be used to date samples with unknown age. Second, the age can be determined by using the age equation. Investigation of the thermal stability of the ART by Gogen (1999) were performed using phlogopite from the Eifel volcanic field. The results imply that in phlogopite at 50°C over a time of 1 m.y. 50% of tracks are lost. Glasmacher, U.A., Ohnesorge, R, Wagner, G.A. and Neumann, R. (2000) Visualisation of latent Alpha-Recoil-Tracks (ART) in dark mica by means of atomic force microscopy. This volume Hashemi-Nezhad, S.R. and Durrani, S.A. (1981) Registration of alpha-recoil track in mica: The prospects for alpha-recoil dating method. Nucl. Tracks, 5, pp 189-205, 1981. Huang, W.H. and Walker, R.M. (1967) Fossil alpha-particle recoil tracks: a new method of age determination. Science 155, 11031106, 1967. Huang, W.H., Maurette, M. and Walker, R.M. (1967) Observation of fossil a-particle recoil tracks and their implication for dating measurements. In: Radioactive dating and Methods of Low-level Counting. IAEA, Vienna, 415-429, 1967. Frechen, J., and Lippolt, J. (1965) Kalium-Argon- Daten zum Alter des Laachersee Vulkanismus der Rheinterrasen und der Eiszeiten . Eiszeiten und Gegenwart, 16, 30, 1965 Gogen, K. (1999) Die Alpha-RiickstoE-Spuren Datierungsmethode: Theoretische und experi-mentelle Entwicklung und die Datierung von Glimmer, unpub. Dr. Arbeit, l66p. Gogen, K. and Wagner, G.A. (1999) Alpha-recoil track dating of Quaternary volcanics. Chem. Geol. in print, 13 pages. Van den Bogaard, P. (1995) 40Ar/39Ar ages of sanidine phenocrysts from Laacher See Tephra (12,900 yr BP): chronostratigraphic and petrological significance. EPSL 133, 163-174, 1995. Wagner, G.A., Gogen, K., Jonckheere, R., Kampf, H. Wagner, I. and Woda, C. (1998) The age of quaternary volcanoes Zelezna Hurka and Komurni Hurka (western Eger Rift), Czech Republic: Alpha-recoil Track, Tl, ESR and Fission Track chronometry. Magmatism and Rift Basin Evolution; Liblice-Czech, 7-11 September 1998

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

THERMO-TECTONIC EVOLUTION OF THE W E S T E R N F O L D - A N D - T H R U S T BELT, SOUTHERN URALS, RUSSIA

U.A. Glasmacheri, GA.Wagneri and VN. Puchkov2 1 Forschungsstelle Archaometrie der Heidelberger Akademie derWissenschaften am Max-Planck-Institut fur Kernphysik, P.O. Box 103980,69029 Heidelberg, Germany 2 Ufimian Geoscience Centre, Russian Academy of Sciences, Ufa, Russia

A multidisciplinaiy approach (Apatite-Fission-Track, 40Ar/39Ar, K/Ar, IC, Ro) was applied to quantify tiie thermo-tectonic history of Precambrian and Paleozoic strata along a NW-SE-Transect (AC-TS^96) in the western fold-and-thrust belt of the southern Urals, Russia (Fig. 1). In this contribution we will describe the results of the fission-track study in detail and combine with results of the other techniques. The transect is located 60 km north of the URSEIS '95-Transect and extends from Devonian to Permian sedimentary units of the Pre-Uralian foredeep (P-U-F), crossing the Precambrian siliciclastic and carbonate strata of the Bashkirian Mega-Anticlinorium (BMA) and the metamorphic complex of Beloretzk into the Paleozoic units of the Zilair Nappe (2-N). The BMA was subdivided into the structural units Ala-Tau-Anticline (A-T-A), Inser Syncline (I-S), Yamantau Anticline (Y-A) and the metamorphic complex of Beloretzk (MCB). These units show different tectonic inventories and tectono-metamorphic histories. They are separated by thrusts or faults. Also the Tirlyan Nappe which represents the northern extension of the Zilair Nappe was included.

AC-TS 96

Figure 1. Geological situation along the AC-TS'96 transect with apatite fission-track pooled ages and metamorphic data (Matenaar et al, 1999).

To reveal apatite for fission-track dating, Paleozoic and Precambrian sandstones and mafic to andesitic dikes were sampled in a spacing of 4 km along the 120 km long transect. In relation to the metamorphic grade of the rocks these ages quantify mostly the lower temperature part of the cooling history. Geological Society of Australia - Abstracts Number 58

FT2 The fission-track analysis lead to five age groups. The oldest age group at about 300 Ma (Up. Carboniferous) characterises the low-temperature history of the A-T-A and the western part of the IS. Lower Triassic ages of about 245 Ma are typical for the MCB and the eastern part of the I-S. The Zilair Nappe as well as the eastern part of the Yamantau Anticline is dominated by ages at about 210 Ma (~75°C). In comparison, the Tirlyan Nappe is characterised by older apatite fission track ages (-300 Ma, ~80°C) than the Zilair Nappe. Ages of about 120 Ma (~75°C) at the western side of Uralian-Neoproterozoic thrusts in the Yamantau Anticline clearly indicate a Cretaceous tectonic reactivation and an exhumation of ~2 km. The Upper Triassic age (219±18 Ma) on the SW-side of a NW-SE striking fault in the Inser Syncline also indicates a tectonic reactivation and an exhumation of ~2 km. The T-t path for all samples was modelled by using the inverse model of Issler (1996) with the Durango annealing model of Laslett et al (1987), a random determination of the heating and cooling events with 5 Ma substeps, and very open upper and lower temperature limit (Tmax: l60°C, Tmin: 0°C, tmax: 300 Ma and 200 Ma, tmin: 0 Ma). A relative fast exhumation is indicated by the T-t-simulation in all cases. Post-depositional temperature history of Middle Devonian sediments from the Ala-Tau Anticline and Permian sediments from the Pre-Uralian Foredeep did not lead to a complete annealing of fission tracks. The oldest apatite grains of Up. Vendian and Up. Riphean (Ukl) sediments from the Ala-Tau Anticline also indicate a partial annealing. In the footwall of the Zilmerdak thrust (eastern limb of the A-T-A) within the Upper Vendian (i.e. ~560-570 Ma) sedimentary rocks, Neoproterozoic-Cambrian post-depositional thermal events are recorded by 40Ar/39Ar microcline age spectra. The interbedded slates in this unit indicate an thermal overprint up to the diagenesis - anchizonal transition (~200°C), a possible explanation for the Neoproterozoic-Cambrian overprint of microcline age spectra (Glasmacher etal, 1999, Glasmacher etal, subm.). An orthoclase from one of these samples and one from the slightly younger Zigan horizon (~560 Ma) yielded an ^OAr/^^Ar age spectrum with older ages (ca. 590 to 640 Ma) and with smaller apparent thermal overprints. The stratigraphic age of the Upper Riphean Zilmerdak 1 conglomerate in the hanging wall of the Zilmerdak thrust (western BMA) is much older than 938 Ma but younger than 1226 Ma (Kozlov et al, 1995, Maslov et al, 1997). Thus the plateau-like segment in the age spectrum of the orthoclase pebble (at ~910 to 950 Ma) from that horizon may indicate a thermal overprint of the sediments, possibly one that correlates with a decrease in the subsidence rate of the Upper Riphean basin (Kozlov et al, 1995, Maslov et al, 1997). The interbedded shales in this unit indicate an upper diagenetic grade, a possible explanation for the mild Neoproterozoic-Cambrian overprint in the spectrum. Glasmacher, U.A., Bauer, W., Giese, U., Reynolds, P., Kober, B., Puchkov, V., Stroink, L., Alekseyev, A. and Willner, A. (subm.) The metamorphic complex of Beloretzk, SW Urals, Russia - a terrane with a polyphase Meso- to Neoproterozoic thermodynamic evolution. Submitted to Precambrian Research. 32 pages Glasmacher, U.A., Reynolds, P., Alekseev, A., Puchkov, V., Taylor, K., Gorozhanin, V. and Walter, R. (1999). 40Ar/39Ar Thermochronology west of the Main Uralian Fauk, Southern Urals, Russia. Geol. Rundschau 87: 515-525. Issler, D.R. (1996): An Inverse model for extracting thermal histories from apatite Fission Track data: Instructions and software for the Windows 95 Environment. - GSC Open File 2325, 84 p. Kozlov, V.I., Sinitsyna, Z.A., Kulagina, E.I., Pazukhin, V.N., Puchkov, V.N., Kochetkova, N.M., Abramova, A.N., Klimenko, T.V. and Sergeeva, N.D. (1995). Guidebook of excursion for the Paleozoic and Upper Precambrian sections of the Western slope of the Southern Urals and Preuralian regions, Russian Academy of Sciences: 1-165 Laslett, G.M., Green, P.F., Duddy, I.R. and Gleadow, A.J.W. (1987): Thermal annealing of fis-sion tracks in apatite 2. A quantitative analysis. - Chem. Geol. (Isot. Sec.) 65, 250-269. Maslov, A.V., Erdtmann, B.D., Ivanov, K.S., Ivanov, S.N. and Krupenin, M.T. (1997). The main tectonic events, depositional history and the palaeogeography of the southern Urals during the Riphean-early Palaeozoic: Tectonophysics 276: 313-335. Matenaar, I., Glasmacher, U.A., Pickel, W., Giese, U., Pazukhin, V.N., Kozlov, V.I., Puchkov, V.N., Stroink, L. and Walter, R., 1999. Incipient metamorphism between Ufa and Beloretzk, western fold-and-thrust bek, southern Urals, Russia. Geol. Rundschau 87: 545-560.

International Conference on Fission Track Dating and Thermochronology

FISSION TRACK THERMOTECTONIC AND DENUDATION IMAGING OF THE AUSTRALIAN CONTINENT

AJ.W. Gleadowi, B.R Kohni, RB. O'SuUivani, R.W. Browni and K. Gallagher2 1 Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, University of Melbourne, Australia 2 T.H. Huxley School of Environment, Earth Science and Engineering, Imperial College, London, UK

The Utility of apatite fission track thermochronology (AFTT) for examining the thermal history of rocks in the low-temperature environment of the upper few kilometres of the continental crust is now well established. Techniques to reconstruct thermal histories of rocks below about 120°C have found important applications in studies of sedimentary basin analysis, the evolution of convergent orogenic belts and continental extension tectonics. The results of such studies, mostly on a local or regional scale, have provided an important new source of information about crustal evolution, but often in a form which has been difficult for non-specialists to interpret. More recently, quantitative modelling has brought a new dimension to this work but the implications have often remained difficult to visualise and apply over large areas. For each individual sample analysed, numerical forwardmodelling procedures are used to reconstruct the "best fit" thermal history, which, in most cases will trace the sample's movement towards the landsurface as overlying material is gradually removed by denudation. The Australian Geodynamics Cooperative Research Centre (AGCRC) is a collaboration between Melbourne, Monash and La Trobe Universities, the CSIRO Division of Exploration and Mining and the Australian Geological Survey Organisation. A major project was set up within the AGCRC six years ago to produce a fission track thermotectonic image of the whole of the Australian continent. The concept of just what this might entail has evolved considerably over this period, especially through additional collaboration with Kerry Gallagher at Imperial College and Rod Brown. In addition to compiling a continental-scale coverage of apatite fission track data for the first time, this project has also aimed to develop new methodologies for interpreting and visualising the results in a format that could be combined with other continental data sets. The extension of earlier quantitative thermal modelling techniques to handle large regional data arrays now means that fission track data can be accessed in a variety of novel formats over broad areas of the crust. The first stage of the imaging process is simply an interpolation of the measured fission track age and mean track length to provide a view of the regional variations in these parameters. The most direct images derived from the thermal history modelling are those that reveal the palaeotemperature of present-day surface rocks at various times in the past. In a digital format, these images can be viewed sequentially to show how palaeotemperature has varied with time. Cooling through the upper crustal zone is mostly controlled by proximity to the land surface so that, in many cases, a first-order interpretation of these images can be made in terms of the amount of denudation that has occurred over particular time periods. Consequently, from this palaeotemperature information, a second group of images can be generated which describe the amount of surface denudation based on estimates of the prevailing thermal gradients. In turn, these estimates of the amount of removed section can be combined with digital topographic data, and isostatically adjusted, to show how the palaeotopography has evolved. These images provide a striking new perspective on crustal processes and landscape evolution over time scales up to hundreds of millions of years. The fission track images are not only valuable for visualising an otherwise rather intractable data set, but also allow a whole new range of quantitative measurements to be made on the virtual landscapes constructed. For example, a direct consequence of the denudation models is to predict sediment volumes and to trace the evolution of drainage basins, at least on a broad scale. This opens up a new range of mass-balance calculations on the amounts of eroded material and sediment accumulation in appropriate depocentres. The patterns of sediment movement down the reconstructed drainage lines can also give new insight into the migration of economic mineral deposits, such as diamond placers. Geological Society of Australia - Abstracts Number 58

FT2 The development of quantitative thermal modelling and regional imaging techniques for large data sets now means that fission track analysis can be used to investigate patterns and timing of denudation, and post-orogenic tectonic activity over broad areas of the crust. In addition to the already widespread application of apatite fission track data in petroleum exploration and sedimentary basin analysis, these new approaches are expected to find application in exploration programs for commodities where the relative position of deposits to a palaeolandsurface is an important factor. Fission track imaging can also reveal discontinuities associated with major lineament features, possibly indicating the presence of long-lived fundamental faults. In practice, the regional coverage of the Australian continent has focussed on exposed basement terranes rather than the younger cover of sedimentary basins. The project began with a total database of just over 700 apatite analyses from older studies, which has increased now to over 2,700 analyses. The density of data coverage is heterogeneous, however, being greatest in southeastern Australia and least in northwestern and northern Australia. However at the end of this project in 2000 we are able to provide at least a reconnaissance scale coverage of the entire continent and many interesting patterns within the thermal evolution of the crust are revealed. The outcome is a completely new perspective of the evolution of Australia from the period of maximum assembly of the Gondwana Supercontinent in the late Palaeozoic, through subsequent breakup until the present day. Acknowledgements This work has been supported from its inception by the Australian Geodynamics Cooperative Research Centre under Project 2005LO, and this work is published with the permission of the Director, Dr Graham Price. Some of the data used were obtained from several earlier projects supported by the Australian Research Council, and all irradiations have been supported by the Australian Institute of Nuclear Sciences and Engineering. This large project has depended on the contributions of many people, and has included input from a number of recent PhD projects by Asaf Raza, Melinda Mitchell, Richard Spikings and Ursula Weber, whose contributions are gratefully acknowledged. Additional sample material was kindly provided though AGSO, state geological surveys in Tasmania, South Australia, Western Australia, Queensland and New South Wales, the Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Monash University, the Australian National University, and Curtin University of Technology. We are also grateful to many colleagues within the AGCRC, especially Dr Simon Cox of the CSIRO Division of Exploration and Mining, and Dr Russell Korsch at the Australian Geological Survey Organisation, and former members of the Melbourne Fission Track Research Group for valuable discussions and support.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

A F T A REVEALS ELEVATED EARLY T E R T I A R Y HEAT F L O W IN N O R T H W E S T ENGLAND

RE Green Geotrack International Pty Ltd, 37 Melville Road, Brunswick West, Victoria 3055, Australia

Green (1986) reported AFTA data in samples of outcropping Caledonian basement from the Lake District block of Northwest England which revealed that these rocks reached paleotemperatures between 80° and 110°C during the Early Tertiary (with cooling beginning at ~60 Ma). Lewis et al (1992) later reported that sediments of Carboniferous to Triassic age throughout northwest England and the adjacent Irish Sea all showed evidence of cooling from paleotemperatures around 100°C or more beginning in the Early Tertiary. Green (1986) and Lewis et al (1992) concluded that the most likely explanation of the observed Early Tertiary paleotemperatures was deeper burial, with cooling due to kilometre-scale uplift and erosion. For a paleogeothermal gradient of 30°C/km, the observed paleotemperatures required ~3 km of former cover to have been removed over much of the region since the Early Tertiary. These resuks were in marked contrast to the prevailing consensus view of the Lake District Block as tectonically stable since the Late Paleozoic, and led to a reassessment of the geological evidence for the amount of former Late Paleozoic and Mesozoic cover across Northern England. Holliday (1993) considered 3 km of removed section unacceptably high when compared to the evidence of section preserved in adjacent Mesozoic basins, and estimated a probable range of 700 to 1750 metres for the amount of former Mesozoic cover over the Lake District and Pennine blocks. Subsequently, an increasing focus on hydrocarbon exploration in the Irish Sea and adjacent regions, coupled with major improvements in thermal history interpretation of AFTA data (involving identification of compositional effects on annealing rates and improved methods of extracting information on the magnitude and timing of cooling events from AFTA data), led to major advances in definition of thermal history styles across the region (Green et al, 1997). In addition to recognising the occurrence of Mesozoic paleo-thermal episodes, this work resulted in the first real constraints on the nature of processes responsible for Early Tertiary paleo-thermal effects across the region. In particular, AFTA and VR data from the onshore West Newton-1 well (Green et al 1997), located on the coastal fringe of the Lake District Block, suggested the possibility of an elevated Early Tertiary paleogeothermal gradient around ~50°C/km, compared with the present-day gradient of ~35°C/km. This implies that the Early Tertiary heat flow may have been up to 50% higher than the present-day value. Some aspects of these results remained equivocal, however, and alternative interpretations were possible. New AFTA data in a series of outcrop samples from an elevation section around Sea Fell (England's highest mountain with a summit at just under 1000 metres a.s.l.) in the Lake District, confirm this interpretation, and finally provide a plausible explanation of the Early Tertiary paleotemperatures observed throughout Northern England and the northern Irish Sea. Fission track ages from Sea Fell, located only - 2 5 km to the south of West Newton-1, decrease from over 300 Ma at the summit to ~60 Ma at the base of the section at an elevation of -100 metres a.s.l. Thermal history interpretation of these data reveals a major cooling episode which began between 65 and 50 Ma. Paleotemperature estimates as a function of elevation (Fig. 1) define an Early Tertiary paleogeothermal gradient of ~6l°C (42 to 79°C at ±95% confidence limits). Extrapolation of the West Newton-1 paleotemperature profile suggests that around 1.75 km of post-Early Triassic section has been removed by Tertiary exhumation at that location, while similar analysis of the Sea Fell results suggests around 0.84 km of removed section (0.54 to 1.43 km at ±95% c.L). The difference in removed section between these two sites is accounted for almost exactly by the ~950 metre difference in elevation between the location of the West Newton-1 well and the summit of Sea Fell.

Geological Society of Australia - Abstracts Number 58

Paleotemperature (°C) 20

40

60

80

100 120 140 160 180 200

Borrowdale Volcanics (Ordovician)

Figure 1. Estimates of Early Tertiary paleotemperature derived from AFTA data in outcrop samples from Sea Fell, plotted against sample elevation. These results define a paleogeothermal gradient of 6l°C/km compared with a present day value around 30°C/km.

Thus, these new data confirm the occurrence of an episode of elevated heat flow in the Early Tertiary in the Lake District region and, in combination with the West Newton-1 data, provide a consistent framework within which existing AFTA data from the Lake District block can finally be understood. The amounts of removed section required to explain these results are entirely consistent with the conclusions of Holliday (1993) based on regional geological trends. Thus, an extensive database involving detailed sampling, combined with improved interpretation methods, have resulted in a consensus on the post-Paleozoic evolution of the Lake District block, ending years of geological debate and uncertainty. The cause of the elevated Early Tertiary heat flow in the region remains open to speculation. Bray, R., Green, P.F. and Duddy, I.R. 1992. Thermal history reconstruction in sedimentary basins using apatite fission track analysis and vitrinite reflectance: a case study from the east Midlands of England and the Southern North Sea. In: Hardman, R.F.P. (Ed) Exploration Britain: Into the next decade. Geological Society, London, Special Publications, 67, 3-25. Green P.F. 1986. On the thermo-tectonic evolution of Northern England: evidence from fission track analysis. Geological Magazine, 123, 493 - 506. Green, P.F., Duddy, I.R. and Bray, R.J., 1997. Variation in thermal history styles around the Irish Sea and adjacent areas: implications for hydrocarbon occurrence and tectonic evolution. In: N.S. Meadows, S. Trueblood, M. Hardman and G. Cowan (eds). Petroleum Geology of the Irish Sea and Adjacent Areas, Geological Society, London, Special Publications, 124, 73-93. Holliday, D.W., 1993, Mesozoic cover over northern England: interpretation of apatite fission track data. Journal of the Geological Society of London, 150, 657-660. Lewis, C.L.E., Green, P.P., Carter, A. and Hurford, A.J. 1992. Elevated late Cretaceous to Early Tertiary paleotemperatures throughout North-west England: three kilometres of Tertiary erosion? Earth and Planetary Science Letters, 112, 131-145.

International Conference on Fission Track Dating and Thermochronology

FT2'

APATITE FISSION TRACK CONSTRAINTS ON A WIDESPREAD EARLY LATE CRETACEOUS HEATING E P I S O D E IN THE CANADIAN ATLANTIC MARGIN: ADDED EFFECT OF A CLIMACTICALLY PERTURBED GRADIENT?

A.M. Grist and M. Zentilli Department of Earth Sciences, Dalhousie University, Halifax, NS, B3H 3J5

Previous apatite fission track (AFT) studies have determined that the Atlantic margin of Canada, including the Maritimes Basin located beneath the Gulf of St. Lawrence (Fig. 1) was buried under at least 4-5 km of strata in late Paleozoic times, and that erosion removed much of this cover by Triassic/Jurassic times (e.g. Ravenhurst et al, 1990; Grist et al, 1995). Organic maturation parameters (Ro 0.44%) of Cretaceous lignite in exposed sinkholes within Windsor Group strata (e.g. Davies et al., 1984), and AFT data on surface rocks at Gays River in mainland Nova Scotia suggested a post-Albian heating event (Arne et al, 1989). Maturation parameters in the Jurassic sequence of the TriassicJurassic Fundy Basin also imply substantial heating in late Mesozoic times (Wade et al, 1996). Our AFT studies on the Scotian Basin offshore Nova Scotia, discovered a post-Albian heating pulse in most wells implying that the strata were at one time hotter (20° to 70°C) than their present temperature in the wells (Grist et al, 1992; Li et al, 1995).

Figure 1. The Maritime Provinces of Canada showing the locations of selected basins and sample selection sites.

New AFT data, and time-temperature modelling on samples from outcrops (pre-Carboniferous basement, Permian-Triassic sediments, and Mesozoic dikes) and wells onshore and offshore Nova Scotia indicate that the Cretaceous heating event is widespread. Modelling (using the methodology of Willett, 1997) of new data from the 1.5 km deep Digby D1 drillhole in the South Mountain Batholith (Fig. 2) is also compatible with surface rocks (close to a Triassic unconformity) having been heated to temperatures between 60° and 70°C at ca. 100-80 Ma (Late Albian to Early Campanian). Possible explanations for the recorded post-Albian heating episode are considered below: 1) An increase in the paleo-geothermal gradient, perhaps related to magmatic activity. Magmatic activity of Mesozoic age has been recognised from the New England offshore to the Montreal region, and in the Orpheus Graben offshore Nova Scotia, but this activity was generally pre-Albian (Jansa and PePiper, 1988). Furthermore, higher than normal paleo-geothermal gradients should probably be expected to accompany these mantle-derived igneous events. Yet our AFT data for the Digby D1 drillhole data are compatible with a (normal) paleo-geothermal gradient of ca. 25°C/km during AlbianCampanian time, for samples taken between depths of 60 and 1437 m. Geological Society of Australia - Abstracts Number 58

FT2 Digby D-1 60 m (granite)

Mic Mac J-77 2314-2343 m

Observed and calculated TL distributions for the exponential mean solution at the .05 significance level observed age = 244 Ma predicted age = 242 Ma

0.51

observed age = 64 Ma predicted age = 63 Ma

8 12 16 length (microns)

8 12 16 length (microns)

20

Mean thermal history at the .05 significance level aaslett/Durango model) 2001

150

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200 100 time (Ma)

300

200 100 time (Ma)

Figure 2. Modelling of the Digby D-1 and Mic Mac J-77 wells using AFTINV (Willett, 1997). The upper panels show histograms of the measured TL distributions overlain by the model distributions for the exponential mean of 250 solutions which fit the data at the 95% confidence level. Also shown are the upper and lower bounding envelopes for the solutions. The lower panels show the calculated thermal histories for the mean solutions and the upper and lower bounding envelopes. Both data sets are consistent with a thermal heating event during Late Cenomanian -Early Turonian time.

2) Circulation of warm fluids at depth in response to a structural/tectonic process. Hydrocarbon migration, accumulation and gas overpressuring in the Sable Basin offshore Nova Scotia may have coincided with the post-Albian heating event in the late Cretaceous (e.g. Williamson and Smyth, 1992; Li et al, 1995). It is difficult to envisage why basinal tectonics offshore caused a concurrent thermal event in the remote Digby D-1 drillhole in the South Mountain Batholith, but Stea and Pullan (1997) have recently documented post-Albian faulting in the intervening region. 3) Basin inversion. In the offshore record for Nova Scotia, several unconformities are recognised within the Cretaceous (Wade and Maclean, 1990) reflecting variations in subsidence rates and sediment supply. However, an unconformity reflecting 2-3 km of erosion, necessary to account for cooling following subsidence in mid-Cretaceous times is not specifically discussed in the literature. 4) Sub-surface heating in response to a long-term increase in the paleo-mean annual surface temperature. Thermal calculations using heat flow theory and various case histories demonstrate that changes of more than ca. 10°C in the mean-annual surface temperatures (Ts), acting over geological time scales (1-10 m.y.) can penetrate kilometres deep into the crust (e.g. Beck, 1977; O'Sullivan and Brown, 1998). Karstification and deep lateritic weathering of Late Aptian and younger age have been documented in the Atlantic margin of North America (e.g. Davies et al., 1984). Fossil vertebrate assemblages from the high Canadian Arctic imply that polar climates were warm (mean annual temperatures exceeding 14°C, rather than near freezing) during the Turonian-Coniacian (e.g. Tarduno et al., 1998). This anomalously warm event would have also affected the mean-annual surface temperatures

FT2 (Ts) at lower latitudes. If so, the Nova Scotia AFT data could be explained by migration of this thermal pulse deep into the crust. This hypothesis deserves serious testing because its validity would have far-reaching implications. Further T-t modelling after integration with geological/geomorphological constraints and maturation parameters will test these hypotheses. For instance, the Aptian-Albian erosional surface of Davies et al. (1984) and Arne et al (1989) may have cooled the Digby D-1 area as well (Fig.2). In any case post-Triassic/Jurassic and post-Albian heating would have influenced oil and gas generation and migration from all types of source rocks including Carboniferous coal. It also has implications for lowgrade metamorphism and fluid migration through the Mesozoic successions of the Bay of Fundy and the formation of zeolite, amethyst and native copper concentrations. Arne, D.C., Duddy, I. R. and Sangster, D.F., 1989. Thermochronologic constraints on ore formation at Gays River Pb-Zn deposit, NS, Canada, from AFT analysis. CJES, 27, 1013-1022. Beck, A.E., 1977. Climactically perturbed temperature gradients and their effect on regional and continental heat-flow means. Tectonophysics, 41, 17-39. Davies, E.H., Akande, S.O., and Zentilli, M. , 1984. Early Cretaceous deposits in the Gays River lead-zinc mine. Nova Scotia (in Curr.Res., part A., Geol. Surv. Canada., Paper 84-lA, 353-358. Grist, A.M., Reynolds, P.H., Zentilli, M. and Beaumont, C., 1992. The Scotian Basin offshore Nova Scotia: thermal history and provenance of sandstones from apatite fission track and 40Ar/39Ar data. CJES, 29, p. 909-924. Grist, A.M., Ryan, R.J., and Zentilli, M., 1995. The thermal evolution and timing of hydrocarbon generation in the Maritimes Basin of Eastern Canada: evidence from apatite fission track data. CSPG Bull., 43, p. 145-156. Jansa, K. and Pe-Piper, G.A., 1988. Middle Jurassic to Early Cretaceous igneous rocks along the eastern North American continental margin. AAPG Bull., 72, 347-366. Li, G., Ravenhurst, C. and Zentilli, M., 1995. Implications of AFT analysis for the thermal history of the Scotian Basin, offshore Nova Scotia, Canada. CSPG Bull., 43, p. 127-144. O'Sullivan, P.B. and Brown, R.W. 1998. Effects of surface cooling on apatite fission-track data: evidence for Miocene climatic change, North Slope, Alaska. In: Advances in Fission Track Geochronology (Eds. P. Van den haute and F. De Corte) Kluwer, Dordrecht, p. 255-267. Ravenhurst, C.E., Zentilli, M., Reynolds, P.H., Donelick, R., and Beaumont, C., 1990. A fission track pilot study of the thermal effects of rifting on the onshore Nova Scotia margin, Canada. Nuclear Tracks and Radiation measurements, 17, 373-378. Stea, R.R., and Pullan, S.E., 1997. Post-Early Cretaceous faulting in the Musquodoboit Valley, Nova Scotia. NSDNR, Report of Activities 1997, 135-143. Tarduno, J.A., Brinkman, D.B., Renne, P.R., et.al., 1988. Evidence for extreme climatic warmth from late Cretaceous Arctic vertebrates. Science 282, 2241-2244. Wade, J.A. and Maclean, B.C., 1990. The geology of the southeastern margin of Canada. Geological Survey of Canada, Geology of Canada, 2, 167-238. Wade, J.A., Brown, D.E., Traverse, A., and Fensome, R.A., 1996. The Triassic-Jurassic Fundy Basin, eastern Canada. Atlantic Geology, 32, 189-231. Willett, S.D., 1997. Inverse modelling of annealing of fission tracks in apatite I. American Journal of Science, 297, 939-969. Williamson, M. and Smyth, C., 1992. Timing of gas and overpressure generation in the Sable Basin offshore NS: implications for gas migration dynamics. CSPG Bull., 40, p. 151- 169

FT2

138

International Conference on Fission Track Dating and Ttiermochronology

F T 2 '

RECOGNITION OF CRETACEOUS, PALEOCENE AND PLIO - PLEISTOCENE ACTIVITIES, THROUGH A F T A , IN PRECAMBRIAN AREAS OF THE SOUTHEAST BRAZIL: ASSOCIATION WITH THE SOUTH ATLANTIC OCEAN OPENING

S. Guedes2, R Hackspacheri J.C. Hadler N2., PJ. Iunes2, S.R. Paulo3, L.F.B. Ribeiroi and C.A.Tello S2 1 Institute de Geociencias e Ciencias Exatas, Universidade Estadual Paulista, UNESP, 13506- 900, Rio Clare, SP, Brazil 2 Institute de Fisica "GlebWataghin", Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil 3 Departamente de Fisica, Institute de Ciencias Exatas e da Terra, Universidade Federal de Mate Gresse, UFMT, 78060-900, Cuiaba, MT, Brazil

Introduction Different tectonic and geomorphological reactivation have been found in Precambrian areas in the South American Platform, specially in the Brazilian Southeast. In the Brazilian Southeast it is possible to make a reconstruction of the process, in which, the Gondwana continent starts drifting apart in the Mesozoic, promoting reactivation of faults, intrusion of basic and acid rocks and uprising of the Ponta Grossa Arch. Such features are related to the Parana-Etendeka flood volcanic province with age of 134 Ma (Renne et al, 1997). At this time the southern area of Mantiqueira Mountain Range presented intense tectonism forming northeast-southwest tranpressional structures, responsible for the regional uprising. Since then, we have the registration of a gradual thermal history presenting uprising and erosion in the whole region, with uprising rates that reached 4 km from the Cretaceous (Eoaptian) to the present days (Gallagher etal, 1994; Harman et ^/.,1998). In this work AFTA was used to obtain the ages and thermal histories of 14 samples analysed in groups with similar geological features. The samples were collected from different Precambrian units in Mar and Mantiqueira mountain ranges, close to (±10 cm) and far from brittle structures such as faults and fractures. Cretaceous (Eoaptian), Paleocene and Plio - Pleistocene periods were identified systematically in different parts of the studied region.

Figure 1. Location of the Mantiqueira and Mar mountain ranges in southeast Brazil

Geological Society of Australia - Abstracts Number 58

Methodology The thermal histories were obtained using the empirical model elaborated by Australian researchers (Green etal, 1986; Lasllet etal, 1987; Duddy etal, 1988) and implemented by Lutz and Omar (1991). We have used this model, with several modifications, considering the hypothesis that a certain number of samples that have similar geological features should have a similar thermal history. This procedure can provide conclusive results when analysing a reduced number of samples (Amaral et al, 1995; Hadler ^^ a/.,1995). In this work the samples were grouped considering common geological constraints, such as type of rock, in situ tectonic data (faults) and the regional geomorphology from where the samples were collected. Another constraint considered among the samples which belong to a same group, was the apparent age. By this way, we started to analyse 14 samples distributed in 5 groups, the first belongs to Mar mountain range (TF-17; TF-22; TF29; TF-30), the second belongs to Mantiqueira mountain range above 1000 m (TF-1; TF-9; TF-10), the third belongs to Mantiqueira mountain range below 1000 m (TF-12; TF13; TF-5), the fourth belongs to Jundiai region (TF-11 and TF-25) and the fifth belongs to Itupeva region (TF-24 and TF-31). The later two are located in the south of Mantiqueira mountain range (~50 km). Area

Tap±a (Ma)

test

Tcorr+la

(Ma) Apparent age (Tap) fv

Mar Mountain 49 ± 3

62 ± 4

71 ± 4

89 ± 5

132 ± 6

17218

Mantiqueira <1000 m Mantiqueira> 1000m

0.902 (v = 3) 2.067 (v = 4) 1.199 (v = 4)

Corrected age (Tcorr) P(fv)

P(%'v)

-0.40 «0.10 «0.30

1.388 (v = 3) 2.875 (v = 4) 1.514 (v = 3)

-0.25 -0.03 -0.20

Table 1. Results for the wighted mean values of the apparent and corrected ages and the x^ test.

Results and Discussion The results have pointed similar thermal histories between the second and fourth groups and the same has been found to the third and fifth groups. From this result, it was decided that the fifth and third groups should be joined as well as the second and fourth groups. This was done because the apparent age of the samples belonging to each new group are statistically compatible according to the x2 test (see Table 1). For that reason, from the 5 initial groups only three were defined: upper Mantiqueira (>1000 m) (TF-1; TF-9;TF-10 TF-11 and TF-25), lower Mantiqueira (<1000 m) (TF-12; TF13; TF-5; TF-24 and TF-31) and Mar mountain range (TF-17; TF-22;TF-29 and TF-30). From Table 1 it can be observed that the weighted mean values for the apparent and corrected ages increase sensibly from the approximately 2 km from shoreline (Mar mountain range) to interior (~60 km) (Mantiqueira montain range). The thermal histories obtained from each group are shown in Fig. 2, from which it can be observed that there was an important tectonic event in the Mantiqueira mountain range during the Eoaptian. The Upper Mantiqueira suffered a smaller erosion rate than the lower Mantiqueira between 100-120 Ma ago. During this tectonic event, that could be linked to the South Atlantic drifting apart, the fission tracks in the upper mountain range were kept in the partial annealing zone whereas in the lower mountain range the fission tracks were completely annealed. It must be emphasised that almost all samples collected in the upper and lower Mantiqueira mountain range were located inside faults

FT2 (-10 cm).This could imply that two faults generation affected the region, one before 180 Ma and a second one at 120 Ma. That last caused a heating over all the region followed by a slow cooling until the Plio-Pleistocene related with the South American geomorphologic surface. During this last period a fast cooling was observed and it could be related to an uprising followed by an erosion of the whole Mantiqueira mountain range. In the Mar mountain range a heating was observed in the Paleocene that can be related with an important tectonic event that originated the mountain range (Almeida and Carneiro, 1998). •M;Eiiit.iqueiid > 1 dooms'

u rvfentiqudra < 1000mT(°C) 75 150 200

0

I

I —1

1

1

1

1

1

1

1

t (Ivia) 100

1

1

1 —

- Mai- Ivlountdu-i Rjitige

75 t

LJ

(m)

100

0

I5ri 200

1

1

1

1

1

0

Figure 2. Thermal histories obtained in each study region. Almeida F.F.M. and Carneiro C.D.R. 1998. Origem e evolugao da Serra do Mar. Revista brasileira de Geociencias 28, 135-150. Amaral G., Born H., Hadler N. J.C., Junes P.J., Kawashita K., Machado Jr. D.L., Oliveira. E.P., Paulo S.R. and Tello S. C.A. 1995. Fission track analysis of some Brazilian apatites. Radiation Measurements 25, 499-502. Amaral G., Born H., Hadler N. J.C., Junes P.J., Kawashita K., Machado Jr. D.L., Oliveira E.P. Paulo S.R. and Tello S. C.A. 1997. Fission track analysis from Sao Francisco Craton and Mesozoic Alcaline - Carbonatite Complexes from Central Southeastern Brazil. Journal of South American Earth Science. 10, 285-294. Duddy J.R., Green P.F. and Laslett G.M. 1988. Thermal annealing of fission tracks in apatite . 3.Variable temperature behaviour. Chem. Geol. (Jsot. Geosci. Sect.) 73, 25-38. Green RF., Duddy I.R., Gleadow AJ.W., Tingate RR. and Laslett G.M. 1986. Thermal annealing of fission tracks in apatite, 1. A qualitative description. Chem. Geol. ( Jsot. Geosci. Sect.), 59, 237-253. Hadler N. J.C., Junes P.J., Paulo S.R. and Tello S. C.A. 1995. Obtainment of thermal histories for sets of geologically compatible localities by using apatite fission track analysis. Revista Brasileira de Fisica Aplicada e Jnstrumentagao.lO, no. 1, 1-10. Harman R. Gallagher K. Brown R. Raza A. Bizzi L. 1998. Acelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceus. J. Geophys. Res. 103. Bll. 27,091-27,105. Laslett G.M., Green P.F., Duddy J.R. and Gleadow AJ.W. 1987. Thermal annealing of fission tracks in apatite 2. A quantitative analysis, Chem. Geol. (Jsot. Geosci. Sect.), 65, 1-13. Lutz, T.M. and Omar G. (1991). An inverse Method of modelling thermal histories from apatite fission-track data. Earth and Planetary Science Letters. 104, 181-195. Renne P.R. 1997. Geocronology of the Parana Etendeka Igneous Province. South Amearican Symposium on Jsotope Geology (Ssagi). 220-223 Acknowledgments This work is part of the C. Tello S. post doctoral research sponsored by FAPESP, S. Guedes doctoral research sponsored by FAPESP and L.F. Ribeiro sponsored by CNPq. The authors would like to express their gratitude to Dr. Luiz Paulo Geraldo from JPEN/CNEN, SP, Brazil, for the irradiation of the samples used in this work.

FT2^ee

International Conference on Fission Track Dating and Thermochronology

FTl2'

A S T U D Y OF APATITE F I S S I O N T R A C K ANNEALING: E X P E R I M E N T A L M E A S U R E M E N T S OF D E N S I T Y AND LENGTH IN BASAL AND RANDOMLY O R I E N T E D FACES

J.C. Hadler N.i, RJ. lunesi, S.R. Paulo2 and C.A.Tello S.i 1 Institute de Fisica "Gleb Wataghin", Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil 2 Departamento de Fisica, Institute de Ciencias Exatas e da Terra, Universidade Federal de Mato Grosso, UFMT, 78060-900, Cuiaba, MT, Brazil

Introduction The apatite fission track annealing process has been intensively studied in the last decades. The first researches were made by measuring densities, r, while the length reduction, L, of confined induced tracks was focused on more recent studies (Green et al, 1985, 1986; Crowley, 1991; Donelick, 1991). Annealing models were developed by using the latter type of data (for example, Laslett et al, 1988; Carlson, 1990; Crowley et al, 1991). In this work, the fission track annealing was studied in a single crystal (A-1) collected in Brazil. In order to determine the annealing rate of induced fission tracks, 60 isothermal treatments were carried out in temperatures between 150 to 600°C for times of 1, 10, 100 and 1000 hours. Besides measuring the track lengths, the fission track densities were also measured This work resulted from C.A Tello S. doctoral thesis (1998).

1,0

• d1h • d10h A d100h • d1000h ROF Faces ® d1h « d10h A d100h •V dIOOOh

0,8 H

0,6-

•O 0,4-

0,2-

300

400

500

600

0,0

100

200

T(iC)

300

400

500

600

T(iC)

Figure 1. Data comparison for r and d in the basal face and in tne random oriented faces (ROF).

Experimental procedures The chemical analysis of the sample A-1 has revealed that the Chlorine (Cl) / Fluorine (F) rate is -0.03. The sample annealing behaviour was compared with Durango (Cl/F -0.10) at different heating temperatures between 2 L 6 - 3 8 0 ° C for one hour. This comparison has shown that the curves for r ( L / L q ) and also for d (r/r^) in the basal faces and in the randomly oriented faces are similar. Before each irradiation, grains of - 2 0 0 |im in diameter from the samples were heated for 10 hours at 450°C to obtain total annealing. The sample A-1 and Durango have been irradiated at nominal fluence of 5x1015 and 2x10^5 neutrons/cm^ respectively. After each annealing experiment the samples were mounted on epoxy resin, polished and etched in a solution of HNO3 at 20°C for 50 seconds. The length (confined tracks parallel to the surface of observation) and density measurements have been done under an optical microscope at a nominal magnification of 12.5x100 in oil. The

G e o l o g i c a l Society of A u s t r a l i a - Abstracts N u m b e r 58

identification of the basal face has been done under a nominal magnification of 12.5x25 by mapping the grains which presented a polished surface with thick and wide scratches after the etching (P surfaces according to Jonckheere and Van Den Haute, 1996). Among those P surfaces, only the grains that presented tracks whose surface openings were close in shape to regular hexagons were selected. Approximately 13% of the grains were in the basal face where the measurements were taken.

a) 1.0-,

1 •

\-L.\

T\

dl

\

V

\

V Vi

u Model: Carlson \ Chi2 = 10.45 \ PI 39.66 - 12.13 I P2 52.63 - 1.29 T P3 0.30 - 0.031 1

150

200

P1 0.1510386600 - 0.5442420605 P2 0.7828624640 - 0.8910324004 P3 -5.386161183 - 4.144128315 P4 1.93158888E-4 - 1.66208799E-4 \ P5 0.00059-0.00167 P6 0.00118 -0.00027

150

250

T(°C)

200

TCO

b) i.a 1 •

r|

0.8-

0.6-

Model Carlson Chi2 = 3.24188 P1 3.30-0.82 P2 43.00 - 1 . 6 3 P3 0.157-0.009

o.a

0.2-

400

Model: Crowley Chi2 = 2.85 PI 0.49070 -1.26830 P2 4.29961 -3.68078 P3 -2.9957 -7.2057 P4 8.0349E-5 -2.773E-4 P5 0.0026 -0.01063 P6 -0.00011 -0.00156 250

300

350

400

T(°C) F i g u r e 2 . Data adjustment for Carlson and Crowley models: a) d ( r / r ^ ) ; b) r ( L / L Q)

Results and Discussion The curves obtained for data r and d in the basal and randomly oriented faces are shown in Figure 1. From that figure, it can be observed that both r and d on the basal face are systematically smaller than those on the randomly oriented face. However, the differences are not very marked. That is, despite its importance, the annealing anisotropy does not seem to be the most intricate part of

FT2 models that search for the physical meaning of the adjust parameters of modelling curves. From the same figure we can notice that the less meaningful points in r can be neglected based on curve d. Another very important fact referring to the curve fitting is that despite there are no r values under 0.5, d values cover the whole range. This happens mainly due to the fact d values are not strongly influenced by gap effects. In the density measurements the small tracks are counted even in samples that did not suffer annealing. Thus, if we consider that the gaps do not have dimensions comparable to the track length, they do not influence significantly the measurements of d. To describe our experimental results taking into account Crowley et al (a generalisation of Laslett et a O and Carlson models (see Fig. 2) we proceeded: i) r values are those not repeated and/or where the d curve indicate a cut, ii) the values of d were normalised from the values where d >0.5, iii) the fitting, by y} minimisation, was made only on the randomly oriented faces because the shape of the basal curve is practically the same. Our r results reinforce the data set of Green et al, (1986) however is also compatible with Crowley et al (1991) data set in some regions. The track density data were very useful to a better interpretation of the r ones. This was possible because the problems concerned with track counting efficiency were minimised carrying out a d normalisation through r data. Laslett et al (1987) and Crowley et al (1991) models describe well both r as d data. Carlson model describes well d data although its r description is unsuitable if a new model for r < 0.6 is not introduced to incorporate track segmentation effects. Carlson W. D. 1990. Mechanisms and kinetics of apatite fission track annealing. Amer. Mineralogist. 75. 1120-1139. Crowley K.D. Cameron M. and Shaefer R. L. 1991. Experimental studies of annealing of etched fission tracks in apatite. Geoch. Cosmoch. 55. 1449-1465 Donelick R.A. 1991. Crystalographic orientation dependence of mean eatchable of fission track in apatite: An empirical model and experimental observations. Amer. Mineral. 76. 83-91. Green P.P. Duddy I.R. Gleadow A.J.W. Tingate P.R. and Laslett G.M. 1986. Thermal annealing of fission track in apatite, 1. A qualitative description. Chem. Geol. (isot. Geosci. Sect.). 59. 237-253. Jonckheere R. and Van Den Haute P. 1996. Observations on the geometry of etched fission track en apatite: Implications for models of track revelations. Amer. Mineralogist. 81. 1476-1493. Laslett G.M. Green P.P. Duddy LR. and Gleadow A.J.W. 1987. Thermal annealing of fission tracks in apatite, 2. A quantitative analysis. Chem. Geol. (Isot. Geosci. Sect.). 65. 1-13. Tello S. C.A. 1998. Estudo de annealing de tragos de fissao em apatitas, tanto em se^oes basais como em s e 0 e s sem orientagao prefrencial a partir do comprimento e da desnsidade de tragos de fissao. Disertagao de doutorado. UNICAMPCampinas-SP-Instituto de Pisica Gleb Wataghin. 103 pp. Acknowledgments The authors thank Dr. G. Bigazzi from IGGI, Pisa, Italy, for supplying the Durango sample used in this work and Dra. I. Sato from IPEN-SP, Brazil, for the chemical analysis of the samples.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 v W " 0 0

T H E EVOLUTION OF THE EAST TO SOUTHEAST GREENLAND MARGIN AS REVEALED FROM FISSION TRACK STUDIES

K. Hanseni and C.K. Brooks2 1 Geological Institute Oster Voldgade 10, Dk-1350 Copenhagen K, Denmark.

A comprehensive dataset of mainly apatite fission track analyses from eastern Greenland has been achieved over time at the fission track laboratory in Copenhagen. The dataset covers the East Greenland margin between c. TS^'N (Geographical Society Island), 72''N (Staunings Alper) and c. 63°N (Skjoldungen). It consists of data from basement, sediments and magmatic rocks. The FT work (KH) in southern part of East Greenland is still in progress. The range of apatite fission track apparent ages is shown in Figure 1. The studied areas represent different types of thermal histories. Major aspects of thermal history are highlighted and used to constrain current hypotheses about the evolution of the Eastern Greenland continental rim.

ir

' M 2-25 Mai

Figure 1. Summary of geology of East Greenland. Also shown are the apatite fission track age ranges obtained in the different regions.

Morphological observations show a rugged young relief in the central Kangerlussuaq area with uplifted Cretaceous to Early Tertiary marine sediments and highly dissected gneisses and Tertiary intrusions in the core. Also the Tertiary Scoresby Sund basalts to the north were uplifted on a regional scale, and southwards from Kangerlussuaq and inland the relief shows relics of an uplifted peneplain (Soper etaL, 1976; Brooks, 1973; Brooks, 1979; Larsen and Marcussen, 1992; Hansen 1996). Thus Neogene uplift in south and southeast Greenland is well established. The overall pattern of apatite fission track ages in the crystalline basement is one of young ages (~50 Ma) along the oceanic coast at sea level increasing inland and upwards to 100-800 Ma reflecting Post-Caledonian uplift and erosion between the Scoresby Sund area (Hansen, 1992) and in Skjoldungen. In the Scoresby Sund basement (Hansen,; 1992) apatite ages (c. 125 to 275 Ma) increased inland along a horizontal line that is ascribed to exhumation and tilting inland associated with increased erosion along the escarpment. Furthermore, the lack of a well-defined lower single grain age-limit along the coast south of Kangerlussuaq suggest ongoing erosion without relaxation of the rift margin following the opening of the Northeast Atlantic Ocean. Length distributions from the different areas are skewed to complex with different proportions of short old tracks due to a stepwise thermal evolution. Geological Society of Australia - Abstracts Number 58

FT2 The evolution of the Kangerlussuaq area is overprinted by Tertiary magmatic activity. FT analyses of apatite, zircon and titanite from sediments at Kangerlussuaq (This study; Gleadow and Brooks, 1979; Hansen, 1996) indicate slow exhumation to the surface in the Cretaceous and a temperature increase at peak Tertiary magmatism. Just north of Kangerlussuaq still younger apatite FT ages are found. The Icelandic plume that is supposed to have passed below Kangerlussuaq in early Tertiary time is not directly revealed by the FT data as the timing (and position) of maximum cooling (c. 30 m.y. later) does not fit the suggested time for the first appearance of the plume below Kangerlussuaq. In the Jameson Land sedimentary basin apatite FT ages of less than 25 Ma are restricted to the northeastern part of the basin (this study; Hansen, 1988). The late evolution in north east Jameson Land and Traill 0 coincides with spreading and both off and onshore extensive magmatism (Larsen, 1990). Apatite FT ages are found to increase westwards (this study; Thomson et aL, 1999) and upwards (Wegener Halv0 area to the north east of Jameson Land excepted). The close association between the thermal pattern and the magmatic activity suggests a magmatic influence on thermal evolution in this part of the Jameson Land basin. Cooling in the area may then be a combination of heat equilibration with surroundings and exhumation. The regional evolution of the Jameson Land basin is one of subsidence, before faulting and differential movements, and exhumation. Along the eastern margin of the basin temperatures were close to total annealing for apatite from Permo-Triassic samples. Along its western margins temperatures were slightly less. In the central part of the basin apatites from the Jurassic sediments were only slightly annealed. Maximum burial of Permian to Triassic sandstones occurred along the eastern margin earlier than 20 Ma in the northeast and later than 55 Ma in the south before differential vertical movements. The reduced number of long tracks and thermal modelling using the approach of Jensen et al. (1992) suggest final exhumation below the PAZ to the present surface in the last 5-10 Ma. The thermal pattern agrees well to that inferred by thermal maturity studies of other workers (e.g. Surlyk et aL, 1986; Mathiessen et al., 1995; Krabbe, 96). Brooks, C.K. 1973- Rifting and doming in southern East Greenland. Nature (London) Phys. Sci. 244, 23-25. Brooks, C.K. 1979. Geomorphological observations at Kangerlussuaq, East Greenland. Meddr. Gronland. Geosci. 1, 21pp. Hansen, K. 1988. Preliminary report of fission track studies in the Jameson Land basin, East Greenland. Geol. Surv. Greenland Rep. 140, 85-89. Hansen, K. 1992. Post-orogenic tectonic and thermal history of a rifted continental margin: the Scoresby Sund area. East Greenland. Tectonophysics 216, 309-326. Hansen, K. 1996. Thermotectonic evolution of a rifted continental margin: fission track evidence from the Kangerlussuaq area, SE Greenland. Terra Nova 8, 458-469. Jensen, P.K., Hansen, K. and Kunzendorf, H. 1992. A numerical model for the thermal history of rocks based on confined horizontal fission tracks. Nucl. Tracks 20, 349-359. Krabbe, H. 1996. Biomarker distribution in the lacustrine shales of the Upper Triassic-Lower Jurassic Kap Stewart Formation, Jameson Land, Greenland. Marine and Petroleum Geology 13, 741-754. Larsen, H.C. 1990. The East Greenland shelf. In Grantz, A., Johnson, L. and Sweeney, J.F. (eds.) The Geology of North America, L, The Arctic Ocean region, 185-210. Larsen, H.C. and Marcussen, C. 1992. Sill Intrusion, Flood basak emplacement and deep crustal structure of the Scoresby Sund region. East Greenland. In Storey, B.C., Alabaster, T. And Pankhurst, RJ. (eds.) Magmatism and causes of continental breakup, Geological Society Special Publication No. 68, 365-386. Mathiesen, A., Christiansen, E.G., Bidstrup, T., Marcussen, C., Dam, G., Piasecki, S. and Stemerik, L. 1995. Modelling of hydrocarbon generation in the Jameson Land Basin, East Greenland. First Break 13, 329-341. Soper, NJ., Higgins, A.C., Downie, C., Matthews, D.W. and Brown, P.E. 1976. Late Cretaceous-Early Tertiary stratigraphy of the Kangerlussuaq area. East Greenland, and the opening of the north-east Atlantic. J. Geol. Soc. London 132, 85-104. Surlyk, F., Hurst, J.M., Piasecki, S., Rolle, F., Stemerik, L. and Thomsen, E. 1986. The Permian of the western margin of the Greenland Sea - a future exploration target. In: Halbouty, M.T. (ed.) Future petroleum provinces of the worid. AAPG Memoir 40, 629-660. Thomson, K., Green, P.F., Whitham, A.G., Price, S.P. and Underbill, J.R. 1999. New constraints on the thermal history of NorthEast Greenland from fission-track analysis. Geol. Soc. Am. Bulletin 111, 1054-1068. Acknowledgements KH was financed by University of Copenhagen, Geological Institute, Danish Natural Science Foundation, Nordic Ministers Council, EFP-88, the Commission for Scientific Work in Greenland (Danish Polar Centre), Novo Nordisk Fonden and Danish Lithosphere Centre.

International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

F T 2 '

DEVELOPMENTS IN K - A R THERMOCHRONOLOGY T.M. Harrison Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, USA

Introduction Many geophysical processes, such as magmatism, rifting, thrust faulting, and denudation, impart a characteristic thermal signature to the crust. Reading the record of these events preserved in the form of isotopic variations in minerals permits insights into the timing and rates of dynamic processes that might otherwise be unobtainable. However, many of these heat flow disturbances are too subtle to be revealed by conventional thermochronometric methods; i.e. interpolation of temperature-time data obtained from bulk measurements. Rather, the highest resolution thermal histories require harnessing knowledge of the concentration distribution of the daughter product within the solid of interest. In the case of the K-Ar system, by far the best opportunity to obtain detailed information regarding internal radiogenic argon distributions is by application of the 40Ar/39Ar step-heating method to Kfeldspars. K-feldspar is ideal in this role as it is widespread in the continental crust, contains high concentrations of parent ^^K, and is remarkably stable during laboratory heating. Two distinct sources of information are available from a K-feldspar ^^Arft^Ax step-heating experiment: the age spectrum and Arrhenius plot. Model ages are calculated from the flux of radiogenic argon (40Ar/39Ar*) (assuming trapped argon of atmospheric composition) relative to the reactor produced ^^Ar evolved during discrete laboratory heating steps. With the additional assumption that 39Ar is uniformly distributed within the sample, we can infer the spatial distribution of the daughter product. The associated Arrhenius plot, derived by plotting the diffusion coefficient (obtained from the inversion of the ^^Ar release function assuming a single domain size) against the inverse temperature of laboratory heating, are a convolution of the parameters which characterise the total diffusion properties of the sample (whether dictated by varying length scale, energetics, etc.). All 40Ar/39Ar age spectra for slowly cooled K-feldspars are significantly different from model age spectra calculated assuming a single diffusion-domain size. Furthermore, Arrhenius plots show departures from linearity that are inconsistent with diffusion from domains of equal size. Lovera et al (1989) obtained an internally consistent explanation for the commonly observed features of K-feldspar age spectra and their associated Arrhenius plots by extending the closure model to apply to minerals with a discrete distribution of diffusion domain sizes. This led to the recognition that virtually all basement K-feldspars contain discrete argon retentivities that are well-modelled as a distribution of diffusion domain sizes. What are the features that impart this behaviour? Planar defects due to exsolution largely control diffusive transport at small (0.1-10 |im) length scales whereas irregular surfaces produced during subsolidus reactions appear to define the larger length scales (10-100 |Lim). Multi-diffusion Domain Model In our formulation of the multi-diffusion domain (MDD) model, the form of the Arrhenius plot and age spectrum are a function of the diffusion parameters for each discrete domain (activation energy, E, and frequency factor, D^), the domain distribution parameters (domain size, p, and volume fraction, (|)), and the thermal history. Because the form of the Arrhenius plot varies with laboratory heating schedule for samples containing a domain size distribution, w e developed an alternate form of data display termed the log ( r / r j plot (Richter et al., 1991) in which the log of the deviation from the diffusion law for the earliest released argon (r^) is plotted against the cumulative %39Ar released. Since the diffusion parameters may be obtained directly from the Arrhenius plot and w e have two independent measures of p and (j) (i.e. the log (r/r^) plot and age spectrum), we have sufficient information to obtain a unique thermal history solution for the case of monotonic cooling.

Geological Society of Australia - Abstracts Number 58

FT2^ee MDD theory contains a number of assumptions regarding the diffusion characteristics of K-feidspar. The main assumption is that the loss of argon is a thermally activated diffusion process described by the Arrhenius equation. The excellent fit of model diffusion results to the Arrhenius plot obtained from laboratory data is evidence of the general validity of this assumption. Other assumptions include: 1) argon diffusion in the laboratory is controlled by the same boundaries and mechanisms as in nature, 2) the domains can be approximated by a simple geometry, 3) the number of domains used in the model must be equal to or greater than those present in the sample, 4) the domain boundaries are maintained at zero argon concentration, 5) the domains are non-interacting, and 6) the parent (4ok) is uniformly distributed within each domain. We have systematically tested all these underlying assumptions and, with only minor exceptions, found them to be generally valid. The most important of these assumptions, that argon diffusion in the laboratory is controlled by the same boundaries and mechanisms as in nature, can be directly assessed by comparing the natural gradients of ^^Ar revealed in 40Ar/39Ar age spectra, imparted over timescales of tens to hundreds of millions of years, with log (rAo) plots which are produced in a matter of hours to days. The consistently clear correlations we observe between these two plots, in some cases approaching that of autocorrelation, necessitates that the features that controlled argon loss in nature are the same that define diffusion boundaries in the laboratory (McDougall and Harrison, 1999). The robust nature of the Arrhenius parameters calculated in this way is underscored by a plot of E vs. log CDjr^^) for a large database (n = 115) of Kfeldspars which shows a remarkably high degree of correlation (r = 0.97) (Lovera et al., 1997). One conclusion that can be drawn from systematic analysis of this large population is that the diffusion behaviour predicted by the MDD model is observed for virtually all K-feldspars. Recent Developments The discovery that isothermal duplicate heating steps permits the identification of Cl-correlated components of excess argon (Ar^), hosted by fluid inclusions (Harrison et al, 1994), provides the basis for correction of Ar^ commonly observed in the initial stages of K-feldspar age spectra. Since the early gas release corresponds to the lowest closure temperatures, K-feldspar thermochronometry has not overlapped with temperatures associated with apatite fission track dating. The Method for Removal of CL-correlated Excess ArgoN (MR CLEAN) can routinely recover thermal history information over the first few percent of 39Ar release, typically corresponding to Tc's between 100° and 150°C. Although a unique T-t history can be obtained for the case of monotonic cooling by inverting the Kfeldspar step-heating data (McDougall and Harrison, 1999), a unique solution is not possible if that constraint is relaxed (i.e. the sample experienced a re-heating episode). However, the MDD model can be generalised to apply to samples that have experienced combinations of slow cooling and reheating. We have developed a variational approach in which an initial arbitrary thermal history is iteratively varied until an optimal statistical fit to the age spectrum is obtained (Quidelleur et al, 1997). This method permits all thermal history solutions which satisfy the minimisation criteria to be converged upon. While the quality of the thermal history information derived in this way is diminished relative to that obtained assuming monotonic cooling, objective information can be routinely obtained approaching temperatures associated with apatite fission track dating. Other Models A related but distinctly different conceptual approach to the migration of argon in solids is to assume some form of non-diffusive exchange (implemented by adding non-Fickian terms to the diffusion equation) which results in regions of enhanced Ar transport. Although there are a number of ways to parameterize this additional term, the most developed of these multi-path models is that described by Lee (1995) in which exchange occurs between two transport media defined as lattice and short-circuit pathways. With the exception of the interaction terms, this formulation is mathematically equivalent to the MDD model with two domains. However, because the process that permits mass transfer between the two regions is not specified, thermal history information cannot be obtained.

FT2 Fortunately, predictions of this model regarding the form of laboratory Arrhenius plots have been tested using K-feldspar step-heating data and found to be highly inconsistent with the empirical evidence. A third type of model assumes interactions between heterogeneously distributed regions with differing diffusion properties and can be simply implemented by using a spatially variable scaling of the Arrhenius law. Although a fundamentally different formulation than the MDD model (which assumes no interaction between diffusion domains), calculations show that this model predicts similar age spectra and log ( r / r j plots. While the relative merits of this model have not been completely evaluated, we can ask the question: Is the MDD model a suitable proxy for the heterogeneous diffusion model in recovering thermal histories? We have addressed this by creating a synthetic age spectrum from an assumed spatial distribution of diffusivities and thermal history. Modelling these results in terms of the MDD model faithfully recovers the input temperature history indicating that recovery of the thermal history is essentially independent of the nature of the model used, provided that the transport process is Fickian diffusion. Summary Our fundamental premise is that the most informed opinion on the argon diffusion behaviour in a solid comes from interrogation of the argon molecule itself and that the most important piece of information we can obtain is the loss of argon as a function of temperature and time during laboratory treatment. With these data, progressively more and more complex transport models can be tested until one is found that explains all the observations with the minimum of assumptions. Our work to date indicates that the MDD model is able to account for the vast majority of 40Ar/39Ar data obtained from basement K-feldspars. Harrison T.M. Heizler M.T. Lovera O.M. Chen W. and Grove M. 1994. A chlorine disinfectant for excess argon released from Kfeldspar during step-heating. Earth and Planetary Science Letters 123, 95-104. Lee J.K.W. 1995. Multipath diffusion in geochronology. Contributions to Mineralogy and Petrology, 120, 60-82. Lovera O.M. Richter P.M. and Harrison T.M. 1989. '^^Ar/^^Ar geothermometry for slowly cooled samples having a distribution of diffusion domain sizes. Journal of Geophysical Research 94, 17917-17935. McDougall L and Harrison T.M. 1998. Geochronology and Thermochronology by the 40Ar/39Ar Method. 2nd ed., Oxford University Press, New York. 265 p. Quidelleur X. Grove M. Lovera O.M. Harrison T. M. Yin A. and Ryerson F.J. 1997. The thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet. Journal of Geophysical Research 102, 2659-2679. Richter P.M. Lovera O.M. Harrison T.M. and Copeland P. 1991. Tibetan tectonics from a single feldspar sample: An application of the 40Ar/39Ar method. Earth and Planetary Science Letters 105, 266-276.

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International Conference on Fission Track Dating and Thermochronoiogy

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E F F E C T S O F R E M N A N T F I S S I O N T R A C K D A M A G E ON E T C H I N G AND ANNEALING C H A R A C T E R I S T I C S O F F I S S I O N T R A C K S IN Z I R C O N

N. Hasebei, S. Morii andT.Tagami2 1 Dept. Earth Sci., Fac. Sci., Kanazawa Univ., Japan. 2 Div. Earth Planet. Sci., Grad. Sch. Sci., Kyoto Univ.,Japan.

Fission track (FT) method is now widely adopted to reveal the thermal histories of rocks by counting the number of tracks and measurement of track lengths. Because these two determinants depend on both the etching and thermal annealing characteristics of tracks in a particular mineral, understanding of them is the most fundamental need for FT method. Although difficulties arise from their dependence on several factors, some of those factors are already well-known and are introduced into the routine work of FT method. The most famous example among those factors is the alpha-damage accumulation that affects the etching efficiency for case of zircon. The chemical composition of apatite is also an important factor that affects the annealing temperature of tracks. In this study, we examined the effect of remnant FT damage on etching and annealing characteristics of FTs in zircon. Zircon crystals were obtained from a young volcanic rock with an original track density of -0.5 xlOVcm2. They were divided into six aliquots, and irradiated at the reactor for 0, 0.5, 1, 3, 1 and 20 hours, respectively, to give different amount of FT damage. Resultant induced track densities are -0.5, 4, 7, 20, >50, and >150 xlOVcm^, respectively. Then they were annealed at 1044±2°C for 2 hours to remove all induced fission track damages. The samples were again irradiated at the reactor to produce induced tracks of -TxlOVcm^, and track lengths were measured after the annealing treatment for one hour at different temperatures. As a result, samples with short-term irradiation show relatively short mean track length after the annealing at ~700°C. Track length distributions do not show the evidence that etching of 'gap zone' which appears within tracks at late stage of annealing to block etchant penetration, play a role in observed short mean track length. Hence samples with heavy irradiation damage could have high resistability against thermal annealing. The observed phenomenon could explain unexpected short tracks found in sedimentary rocks that are believed to be totally annealed. Each grain could have its own apparent ages reflecting its original track density. Future experiments with higher annealing temperatures, and/or with other crystals are required to confirm this result.

Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

M A P P I N G OF THE S O U T H W E S T JAPAN USING F I S S I O N T R A C K M E T H O D : UNDERSTANDING OF ACCRETIONARY

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TOWARDS

PROCESSES

N. Hasebei, H.Watanabei,A.Yamagiwa2 andT.Tagami^ 1 Dep. Earth Sci., Fac. Sci., Kanazawa University, Japan. 2 Div. Earth Planet. Sci., Grad. Sch. Sci., Kyoto Univ., Japan

The Japanese Islands consist fundamentally of late Paleozoic to Cenozoic accretionary complexes that formed in a subduction zone along the East Asian continental margin. Understanding of the evolution of accretionary complexes is important because of its critical role in a horizontal growth of continental crusts and its potential as a recorder of the geological events occurred in subduction zones. Recent studies by seismic reflection data and ocean drilling in several active trenches lead understanding of structure and forming processes in frontal part of accretionary complexes, where sediment thicken to form imbricate structure and resultant wedge shape (Moore et al, 1988; Taira et al, 1992). In spite of continuous subduction of oceanic plates beneath the continents, studies on ancient accretionary complexes reveal that these orogenic rocks formed episodically, as evidenced by discontinuous matrix ages of accretionary complexes (Nakajima, 1997). The original accretion structures are gradually disturbed as time passes caused by back-arc basin extension, fore-arc transformation, intra-continent collision and tectonic erosion (Kimura, 1997). The Median Tectonic Line (MTL) is a prominent strike-slip fault running through southwest Japan. It divides southwest Japan into two zones: the 'Inner Zone' on the back-arc side and the 'Outer Zone' on the fore-arc side. Ancient accretion structures were less disturbed at the Outer Zone than the Inner Zone, and thus analysis of the Outer Zone is helpful to understanding of accretionary processes. We tried age mapping based on fission track thermochronological data in southwest Japan. Sandstone samples were mainly collected from five regions, which are the Kyushu, Shikoku, Kii, Gifu and Shizuoka regions from west to east. The results from the Outer Zone indicate that basically northern samples reached the higher maximum temperatures during accretionary processes, which is expressed by zircon resetting ages completely younger than depositional ages inferred from microfossils. Northern older samples have stayed in the exhumation stage relatively long resuking in the exposure of samples with deeper maximum burial. Cooling times through the closure temperature are contemporaneous. Most apatite ages are of -10 Ma except for that of ~35 Ma found in northern Kii area. This suggests the exhumation process since the Miocene could be limited in the region except for the northern Kii area and probably northern area of other regions where we don't obtain apatite data. This leads to the idea that the exhumation could be caused by the rapid subduction of the Shikoku Basin that affected southern part of the Outer Zone. In several narrow regions (west Sambagawa, central Kii, northern Shizuoka), anomalously young zircon ages (Miocene) are obtained. Shizuoka region records the thermal effect by the granitic intrusion and the uplifting caused by the collision of Izu-Bonin arc to Japan arc. The reason for other regions, young ages is still under the discussion. Samples from the Inner Jurassic accretionary complex (Mino sedimentary rocks, Gifu region) yield older fission track zircon and apatite ages (-100 Ma for zircon, ~60 Ma for apatite) than that from the Outer Zone. Time discrepancy between the Inner Zone and Outer Zone suggests that the exhumation rate of Inner accretionary complex was slower than that of the Outer Zone, or the exhumation had stopped after the exposure of present surface material with the past exhumation rate similar with that of the Outer Zone accretionary complexes. Apatite ages could alternatively be explained as a result of thermal effect by intrusive Ryoke granitoids in this Gifu region.

Geological Society of Australia - Abstracts Number 58

FT2 Kimura, K., 1997. Offscraping, underplating and out-of-sequence thrusting process of an accretionary prism: On-land example from the Mino-Tamba Belt, central Japan. Bull. Surv. Japan, 48: 313-337. Moore, J.C., Mascle, A. and Others, 1988. Tectonics and hydrogeology of the northern Barbados ridge: results from Ocean Drilling Program Leg 110. GSA Bull., 100: 1578-1593. Nakajima, T., 1997. Regional metamorphic belts of the Japanese Islands. The Island Arc, 6: 69-90. Taira, A., Byrne, T. and Ashi, J., 1992. Photographic atlas of an accretionary prism - Geologic structures of the Shimanto Belt, Japan. Univ. of Tokyo Press, Tokyo, 124 pp.

International Conference on Fission Track Dating and Thermochronology

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P O S T - P L U T O N I C UNROOFING AND M O R P H O G E N E S I S OF THE CYCLADIC CRYSTALLINE COMPLEX (AEGEA,

GREECE)

E. Hejli, H. RiedP and H.Weingartner2 1 Institut fur Geologie und Palaontologie, Universitat Salzburg, Austria. 2 Institut fur Geographic und Angewandte Geoinformatik, Universitat Salzburg,Austria.

Geologic models concerning the Cycladic crystalline complex have changed fundamentally during the last 30 years. The former concept of a rigid microcraton within the Hellenic orogen has been disproved by geochronologic data (Diirr et al, 1978; Altherr et al, 1982; Andriessen et al, 1979). New findings have shown that the Cycladic complex is an alpidic nappe pile which was formed by thrust faulting and normal faulting. It comprises three structural units separated by low angle tectonic contacts (Avigad and Garfunkel, 1991, 1993). An Eocene high-P unit with blueschist occurrences was overthrusted upon a para-autochthonous unit of low-grade metamorphics. Minor allochthonous rocks, including ophio-lithic material, sediments and low-P metamorphics form the uppermost unit, which rests on the blueschist unit. Unroofing of the blueschist unit occurred in Oligocene-Miocene times and was accompanied by a regional greenschist facies overprint and finally by Miocene granitic plutonism (Papanicolaou, 1993). The unroofing process was mainly governed by low-angle normal faulting and extensional thinning of the overburden, thus allowing the emergence of the underlying blueschist unit (Avigad and Garfunkel, 1991, 1993). The outlined geodynamic evolution is in clear contradiction to the idea of long lasting peneplanation on rigid crustal areas, which has been supported by Philippson (1959) according to the ancient concept of Davis (1899). Miocene intrusion ages of plutonites - being now exposed at the surface - and subsequent tectonic movements imply a landscape development during rather short time under specific climatic conditions (cf. Riedl, 1982, 1984; Weingartner, 1994). The distribution of paleo-planation surfaces often exhibits a steplike arrangement which might be the result of a regional piedmont-staircase development or of local horst-graben tectonics dissecting a former peneplain. Summerfield and Brown (1998) have outlined the geomorphological potential of apatite fission-track analysis, which indicates a maximum age of preserved paleo-relief features. Within the Hellenides, the fission-track method has been applied only to few areas (for example Hejl et al, 1998, Thomson et at., 1998). Since April 1998 we have collected a total of 65 samples from the islands of Tinos, Myconos, Syros, Naxos, Paros, los, Serifos, Ikaria and Samos. Until now (September 1999), 14 apatite fission-track ages have been determined by the grain population method. The preliminary results can be summarized as follows: Four samples from Tinos, taken at elevations between 340 and 506 m, yielded very similar cooling ages between 8.44 ± 0.71 and 9.48 ± 0.76 Ma. These ages are positively correlated with altitude. A fifth sample from Tinos, which was taken at 210 m above sea level, yielded an age of 13.08 ± 4.36 Ma. Five samples from Myconos, taken at elevations between 50 and 370 m, have fairly consistent cooling ages ranging from 7.59 ± 0.98 until 8.89 ± 1.01 Ma. There is no clear correlation with altitude. These apatite fission-track ages are close to the intrusion age of the Myconos granodiorite (10 ± 0.3 Ma) and therefore imply a very fast unroofing between 10 and 8 Ma ago. The corresponding uplift rates are too rapid to be achieved only by erosion. Four samples from Serifos have apatite cooling ages between 5.29 ± 0.62 and 6.68 ± 0.8 Ma that are negatively correlated with altitude. Obviously the samples belong to two different blocks being displaced by a young fault.

Geological Society of Australia - Abstracts Number 58

FT2^ee Altherr, R., Kreuzer, H , Wendt, I., Lenz, H., Wagner, G. A, Keller, J., Harre, W. and Hohndorf, A, 1982. A Late Oligocene/Early Miocene high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece). Geol. Jb. E23, 97-164. Andriessen, P., Boelrijk, N., Hebeba, E. Priem, N., Verduermen, E. and Vershure, R., 1979. Dating the events of metamorphism and granitic magmatism in the alpine orogen of Naxos (Cyclades, Greece). Contrib. Mineral. Petrol. 69, 215-225. Avigad, D. and Garfunkel, Z., 1991. Uplift and exhumation of high-pressure metamorphic terrains: the example of the Cycladic blueschist belt (Aegean sea). Tectonophysics 188, 357-372. Avigad, D. and Garfunkel, Z., 1993. The role of extension in unroofing the Cycladic blueschist belt. Geol. Soc. Greece 28/1, 57-69, Athens. Davis, W. M., 1899. The Geographical Cycle. Geographical Journal 14, 481-509. Diirr, St., Keller, J., Okrusch, M. and Seidel, E., 1978. The Median Aegean cristalline Belt: Stratigraphy, Stmcture, Metamorphism, Magmatism. In: Closs et al (eds.): Alps, Apennines, Hellenides; I.U.C.G. Sc. Rept. 38, 455-477, Stuttgart. Boger, H., 1983. Stratigraphische und tektonische Verknupfungen kontinentaler Sedimente des Neogens im Agais-Raum. Geol. Rundschau 72, 771-814. Hejl, E., Weingarner, H., Vavliakis, E. and Psilovikos, A., 1998. Macrorelief features and fission-track thermochronology of the Rila-Rhodope massif (Eastern Macedonia, Greece). Z. Geomorph. N. E. 42, 517-530. Papanicolaou, D., 1993: Geotectonic evolution of the Aegean. Bull. Geol. Soc. Greece 28/1, 33-48, Athens. Philippson, A., 1959. Die griechischen Landschaften. Band IV: Das agaische Meer und seine Inseln. 523 pp., Frankfurt/Main. Riedl, H., 1982. Vergleichende Untersuchungen zur Geomorphologie der Kykladen unter besonderer Beriicksichtigung der Insel Naxos. Salzburger Exkursionsberichte 9, 33-84. Riedl, H., 1984. Die Reliefgenerationen Griechenlands. Osterreichische Osthefte 26, 52-72. Summerfield, M. A. and Brown, R., 1988: Geomorphic factors in the interpretation of fission-track data. In: R Van den haute and E. De Corte (eds.): Advances in Eission-Track Geochronology, 269-284, Kluwer, Dordrecht. Thomson, S. N., Stockhert, B., Rauche, H. and Brix, M. R., 1998. Apatite fission-track thermochronology of the uppermost tectonic unit of Crete, Greece: Implications for the post-Eocene tectonic evolution of the Hellenic subduction system. In: P. Van den haute and E. De Corte (eds.): Advances in Eission-Track Geochronology, 187-205, Kluwer, Dordrecht. Weingartner, H. 1994: Die Insel Thasos. Eine physisch-geographische Synthese. Salzburger geogr. Arb. 24, l66 pp., Salzburg.

International Conference on Fission Track Dating and Thermochronology

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MORPHOTECTONIC EVOLUTION OF NORTHERN SCANDINAVIA FROM FISSION TRACK THERMOCHRONOLOGY

B.W.H. Hendriks and RA.M.Andriessen Faculty of Earth Sciences, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands

The geomorphology of western Scandinavia (Norway and Sweden) and the almost complete absence of post-Paleozoic sediments was recognised as evidence for large scale Tertiary uplift and exhumation decades ago. Features both on- as well as offshore that are thought to be related to Tertiary uplift include the formation of domes and arches offshore mid-Norway, domal uplift and exhumation of Fennoscandia and the deposition offshore of young, Neogene, large prograding wedges. Also the glaciation and deglaciation cycles throughout the late Neogene belong to the complex Tertiary uplift history. The rugged morphology of the western Scandinavian Caledonide orogen, the preservation of planation surfaces at high altitudes in the southern Norwegian and in the more elongated northern Scandinavian dome and the absence of a crustal root under the orogen, indicate that the morphotectonic evolution of Fennoscandia was strongly affected by Meso-ZCenozoic tectonic and climatic events, rather than being the result of the Caledonian orogeny itself. In order to elucidate the uplift and exhumation history of the southern Norwegian dome, Rohrman (1995), applied Apatite Fission Track Thermochronology to the Caledonian and Sveconorwegian basement rocks in this area. The dome-shaped topography of southern Norway is incised almost to its centre by the Sognefjord. AFT ages there range from 106 ± 12 Ma at sealevel to l67 ± 11 Ma at 2465 m altitude (Jotunheimen). AFT ages of surface samples increase radially outwards from the centre of the dome to ~160 Ma on the west coast and -200 Ma on the south and east coast. Only a slight perturbation of this pattern exists in the Oslo Rift area. Vertical profiles through the dome-shaped topography, show a strong correlation with (mean) elevation, and AFT isochron maps clearly indicate a slightly assymetric domal uplift, with a steeper gradient on the NW than on the SE side. As inferred from modelling the FT data, the observed distribution of FT ages over the area, is the result of two uplift and exhumation events in the Triassic-Jurassic and Neogene. Triassic-Jurassic denudation amounted to 2.4 ± 1.1 km and the rapid Neogene tectonic uplift was in the order of 1 - 1.5 km. The Neogene uplift and exhumation event in the North Atlantic domain has, besides from western Scandinavia, also been reported for East Greenland, Spitsbergen, Faroer Islands and the northern part of the UK. Because of the obvious similarities in space and time, a common cause seems plausible. FT data exist for most of these areas, but until now the northern Scandinavian elongated dome has not been studied in detail with FT analysis. The present study therefore focusses on the elongated northern Scandinavian morphological dome and on the (borders of the) Barents Sea (Kola, Bj0rn0ya and Spitsbergen). Over 50 samples have been selected from this area and these include 3 transects from the Norwegian coastline far into Sweden and Finland and 4 vertical profiles. The first results come from a transect from the Lofoten islands far into Sweden, and include two vertical profiles which combined span -2300 m (from -340 m in the Kiruna mine to approximately 2000 m on Kebnekaise, the highest peak in the northern Scandinavian Caledonides). In the Lofoten area, AFT ages from samples at sealevel are all close to 150 Ma and their mean FT lengths range from 12.76 ± 1.64 in the north (Andenes) to 13.62 ± 1.62 in the south (near A). On Kebnekaise, AFT ages obtained so far, range from 101 ± 11 Ma at 575 m to 212 ± 24 Ma at 1085 m altitude. From these first results, a regional pattern quite similar to that of southern Norway emerges, but with a FT age/mean elevation gradient that is not as steep. Denudation amounts are expected to be of the same order of magnitude as in southern Norway. Geological Society of Australia - Abstracts Number 58

FT2^ee Geomorphological studies have also revealed that the southern and northern domes evolved differently (Lidmar-Bergstrom, 1999). The northern dome is more dissected than the southern dome, and also the relief on their eastern flanks is quite different. Plains and residual hills form the eastern flank of the northern dome, while the eastern flank of the southern dome displays an undulating hilly relief which was protected there during uplift by a Cretaceous cover. The new FT data from northern Scandinavia will be integrated with new FT data from mid Norway (Huigen and Andriessen, this abstract volume), Finland (Murrell and Andriessen, this abstract volume) and geophysical datasets from the offshore areas of the Norwegian margin. Together with the already existing FT data from Scandinavia, this will allow for a better reconstruction and understanding of the uplift and exhumation history of the Scandinavian mainland, its relation to the observed features offshore, and associated thermomechanical processes on the lithospheric scale. Lidmar-Bergstrom, K. 1999. Uplift histories revealed by landforms of the Scandinavian domes. London Geol. Soc, Special Publication. In press. Rohrman, M. 1995. Thermal evolution of the Fennoscandian region from fission track thermochronology - An integrated approach. Ph.D. thesis, Vrije Universiteit Amsterdam Supported by Statoil, Norsk Hydro, Saga Petroleum and the Norwegian Petroleum Directorate. This research is part of NSG and ISES.

International Conference on Fission Track Dating and Thermochronology

NEW GUINEA ARC-CONTINENT COLLISION: F I S S I O N T R A C K

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CONSTRAINTS

K.C. HiUi and A. Raza^ 1 Australian Geodynamics Cooperative Research Centre, Earth Science, LaTrobe University, Melbourne,Australia 3083 2 Australian Geodynamics Cooperative Research Centre, Earth Science,The University of Melbourne. Melbourne, Australia 3053

Pleistocene to Recent Collision of the Finisterre Arc with Papua New Guinea (PNG) is well documented geophysically, but arc-continent collision has been ongoing through the Neogene forming the Mobile Belt and Fold Belt that make up the PNG orogen. The Neogene collision was known to have involved Oligo-Miocene post metamorphic cooling and Middle Miocene intrusion of the Maramuni Arc in the Mobile Belt, as well as deformation and uplift of the orogen. However, the time-space relationships of these events were not well known, leading to a variety of models for arc-continent collision. Neogene time-temperature paths have been determined by 40 new apatite and zircon fission track analyses of Mobile Belt rocks previously dated by K-Ar and Rb-Sr analyses. The MesozoicTertiary igneous, metamorphic and sedimentary rocks analysed are from the Mobile Belt adjacent to the Finisterre Arc. This area was formerly the leading NE corner of the Australian Plate that converged obliquely with the Pacific Plate, so is a sensitive recorder of changes in plate motions. The Paleogene arc along the southern margin of the Caroline Plate was juxtaposed against PNG in the Early Miocene, coeval with locking up of the west-dipping Solomon's subduction zone by the Ontong Java Plateau. These events initiated wrenching along the northern PNG margin and increased westward subduction of the Solomon Sea Plate beneath the eastern margin. The PNG Mobile Bek underwent extension above the downgoing slab with rapid cooling of metamorphic rocks at 17 Ma, immediately prior to emplacement of the Maramuni Arc from 17-12 Ma. A change in plate motion at -12-10 Ma terminated the arc and caused PNG-Caroline Plate convergence, creating the orogenic belt in New Guinea from 12-4 Ma. This resulted in ~2-4 km of uplift, denudation and cooling of the entire Mobile Belt from 10-7 Ma, propagating westwards along the Mobile Bek at 8-5 Ma and southwards into the Fold Belt at 5-4 Ma. The compression caused thrusting of Miocene strata within the Mesozoic type section. A further change in plate motion at 4-3 Ma returned the margin to transpression with local compression along strike-slip faults and ongoing collision of the Finisterre Arc terrane. At present within the zone of arc-continent collision, there is ongoing convergence with the Finisterre Ranges, transpression in the Mobile Belt and Irian Jaya Fold Bek and extension at both ends of the fold belt in the Birds Neck and adjacent to the Woodlark Spreading Centre. This spatial juxtaposition of different styles is also recorded in time. The thermal histories demonstrate that arc-continent collision spanned 25 m.y. and involved arc accretion, wrenching, extension, -10 m.y. of oblique subduction beneath the continental margin generating a volcanic arc, ~8 m.y. of folding and thrusting, then transpression with ongoing convergence of the Finisterre Arc.

Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

F X 2 '

C O O L I N G H I S T O R Y V A R I A T I O N O F C R E T A C E O U S G R A N I T I C R O C K S IN S O U T H W E S T J A P A N C O N S T R A I N E D B Y F I S S I O N T R A C K M E T H O D AND I T S I M P L I C A T I O N T O R I D G E

SUBDUCTION

O. Himenoi, K.Watanabei and O. Ohira2 1 Department of Earth Resources Engineering, Kyushu University, Fukuoka, Japan. 2 Department of Geoscience, Shimane University, Matsue, Japan.

Igneous rocks of Cretaceous (partly Jurassic) to Paleogene are widely distributed in Southeast China, Korean Peninsula and Sikhote Aline, along the eastern continental margin of East Asia. A series of igneous rocks of Cretaceous to Paleogene age are also seen in southwest Japan (Fig. 1). Time and space distribution of these igneous activities are strongly related with the subduction of oceanic plate beneath east Asia since Jurassic. Based on reconstruction of relative plate motion and regional geology around Japan, the ridge subduction has been suggested (Maruyama and Seno, 1986). Cretaceous igneous rocks are mainly composed of pulutonic rocks of granodiorite-granite and extrusives of andesite-rhyolite in the San'you zone, southwest Japan. In the south of the San'you zone, high-temperature type metamorphic rocks of Cretaceous age are distributed and these are called Ryoke Granite (Fig. 2). Nearly one-hundred rediometric ages have been reported for Cretaceous granitoids in southwest Japan. After the examination of along-arc lateral variation, reported ages (mostly biotite K-Ar cooling ages) seem to become systematically younger from the east (about 95 Ma around Karatsu City in Fig. 2) to the west (65 Ma). This eastward age reducing trend was considered as a result of migration of magmatism caused by ridge subduction (Nakajima etal, 1990; Kinoshita , 1999). On the other hand, the ages for Cretaceous volcanic rocks do not show any systematic trend after the increase of determined ages and seem to indicate two main peaks of magmatism at 110-90 Ma and 85-80 Ma in southwest Japan (e.g. Matsumoto et al, 1994). This confusion suggested a reconsideration of eastward magmatism migration model and we expected that low temperature thermo-chronologic data help to explain the age reducing trend for Cretaceous granitic rocks. Here we report a result of fission track (FT) study.

Figure 1. Left - Map showing plate boundaries and main fault systems in East Asia. Right - Paleo-tectonic reconstmctions of the Japanese Islands at Late Cretaceous

Geological Society of Australia - Abstracts Number 58

Zircon and apatite grains were separated using the standard magnetic and heavy liquid techniques. Grain mounts were polished to expose internal surface and attached with mica detectors (Gleadow, 1981). 27 zircon and 23 apatite FT ages were determined using the zeta calibration method (Hurford and Green, 1982). Cretaceous granitic rocks sampling points are indicated in Fig. 2. Zeta values determined using SRM 6l2 glass are 394±9 for zircon and 323±9 for apatite. Apatite track length were also measured and thermal history inversion analysis were carried out. Chlorine content were also determined for apatite age determined grains.

I STL

Karatsu City

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Sampling point 100 km

Figure 2. Geological map showing distribution of Cretaceous volcanic rocks and Cretaceous-Paleogene plutonic rocks in Southwest Japan. Fission track study sampling points are also indicated.

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Figure 3. Zircon and apatite fission track ages plotted against distance from Karatsu City projected along the MTL.

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10 Length "T"!—I—I—I—i—\—\—I I I I I I—\—I—\—I—I—]—I I I I—I—I—r 10 £0 30 40 50 Apatite FT age (Ma)

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Figure 4. Apatite fission track ages plotted against mean track length and one standard deviation.

Figure 3 indicates the zircon and apatite FT ages plotted against the distance from Karatsu City projected along the Median Tectonic Line (MTL). Biotite K-Ar age reduction trend after Kinoshita (1999) is also indicated. Zircon FT ages clearly reduce from west to east. Taking into consideration of no systematic age trend for Cretaceous volcanic rocks, zircon FT cooling ages suggest the eastward migration of uplift centre, maybe caused by the ridge subduction. Apatite FT ages have relatively large error and doesn't clearly show the systematic trend and seem to fluctuate around 50-55Ma. Several apatite ages from Fukuoka area indicated about 15 Ma reset ages (Fig. 4), which temporary corresponds to the clockwise rotation of Southwest Japan resulted from opening of the Japan Sea (Otofuji and Matsuda, 1984). Gleadow A.J.W. 1981. Fission-track dating method: what are the real alternatives ? Nucl. Tracks 5, 3-14. Hurford A.J. and Green P.F. 1982. A user's guide to fission track dating calibration. Earth Planet. Sci. Lett. 59, 343-354. Kinoshita O. 1999. A migration model of magmatism explaining a ridge subduction and its details on a statiatical analysis of the granite ages in Cretaceous Southwest Japan. The Island Arc 8, 181-189. Matsumoto I. Sawada Y. and Kagami H. 1994. Rb-Sr isochron ages of Cretaceous Kisa Volcanics and granitoids in the central Chugoku district, Southwest Japan, and their geological significance. Jour. Geol. Soc. Japan 100, 399-407. Maruyama S. and Seno T. 1986. Orogeny and relative plate motions: Example of the Japanese Islands. Tectonophysics 127, 305-329. Nakajima T. Shirahase T. and Shibata K. 1990. Along-arc lateral variation of Rb-Sr and K-Ar ages of Cretaceous granitic rocks in Southwest Japan. Contib. Mineral. Petrol. 104, 381-389. Otofuji Y. and Matsuda T. 1984. Timing of rotational motion of Southwest Japan inferred from paleomagnetism. Earth Planet. Sci. Lett. 70, 373-382.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

THRUST FAULTS FORMATION DURING THE EVOLUTION OF AN ACCRETIONARY P R I S M ; A R E T H E R E ANY PAST EARTHQUAKES FAULTS?

H. Hoshino and N. Hasebe Department of Earth Sciences, Faculty of Science, Kanazawa University, Kanazawa, Japan.

Accretionary prisms develop along subducted continental margins by the offscraping and underplating of the mixture of oceanic sediments and trench fill continental deposits. Because subduction zones, where accretionary prisms develop, is the area of large earthquakes at depths of 10-30 km, the thermal energy radiated by earthquakes could cause various geological events to surrounding accretionary complexes, as is detected by the fission track (FT) method from rocks around the active Nojima Fault, Awagi Island, central Japan. The purpose of this study is to distinguish past-earthquake faults from faults related accretionary complex development by focusing on thermal anomaly along faults, v^hich enables us to investigate materials on earthquake faults and to understand faulting mechanism. The Shimanto accretionary complex in northeastern Kyusyu, southwest Japan, is divided into four units based on microfossiles and rock facies. They are Morotsuka and Makimine units of Cretaceous, and Kitagawa and Hyuga units of Paleogene, from northwest to southeast. Unit boundaries are lowdipping thrusts. The southern most Hyuga unit is adjacent to both Makimine and Kitagawa units along the Nobeoka thrust. The northern hanging wall, the Makimine unit, slips over the Hyuga unit southeastward. The thickness of the thrust gouge is 10-20 cm. The amount of displacement is estimated geologically to be more than 10 km. The metamorphic grade of the Makimine unit is greenschist facies and that of Hyuga unit is lower grade than prehnite-pumpellyite facies. There is a grate gap in the illite crystallinity between the hanging wall and the foot wall. The Makimine and Kitagawa units are divided by the Furue thrust fault. In spite of the difference in their depositional ages, they show similar rock facies and metamorphic grade. Samples were collected near and around thrust faults and analysed by the FT method. The thermal disturbance by intrusive rocks at about 15 Ma is also examined as well as thermo-tectonic evolution since Miocene. FT ages from one of intrusive rocks indicate that the time of igneous activity is coeval with the exhumation of the basement Hyuga unit. Using FT method, we investigate role of thrust faults during the evolution of an accretionary prism.

Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

F T S ^ ^ O O "

( U - T H ) / H E THERMOCHRONOMETRY IN SOUTHEASTERN AUSTRALIA: CONFIRMATION OF LABORATORY DIFFUSION EXPERIMENTS AND INSIGHTS INTO THE CENOZOIC THERMAL HISTORY OF THE OTWAY BASIN

M.A. Housei, B.R Kohn2, K.A. Farleyi and A. Raza^ 1 Division of Geological Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA 2 Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, University of Melbourne, Victoria 3052,Australia

Recent revival of interest in the (U-Th)/He thermochronometer (e.g. Zeitler eta/., 1987; Lippolt, 1994; Wolf et aL, 1996) has demonstrated the value of this technique to low-temperature studies of the uppermost crust (e.g. House et al., 1998). This technique also represents one of the most important means to place independent limits on the lower temperature portion of thermal histories derived from apatite fission-track length models. However, before this can be done with confidence, the success of extrapolating laboratory helium diffusion data to geologic problems must be demonstrated. Here we report results of such a study from the Otway Basin, Australia that confirm that laboratory data can be used with confidence in natural settings, thus permitting us to refine existing thermal models for this region. To date, laboratory diffusion experiments from the Noble Gas Laboratory at Caltech suggest that the blocking temperature of helium in apatite is ~75°C (given a cooling rate of 10°C/m.y., for grain size of 100 micron minimum dimension; Wolf et aL, 1996), and that diffusion is sensitive over an interval between approximately 45°-75°C (the helium partial retention zone; Wolf et aL, 1996; 1998). Our experiments also indicate that helium diffusion in apatite is largely compositionally independent, although other results suggest there may be a compositional dependence (e.g. Warnock et aL, 1997). In spite of the high quality of the experimental data however, uncertainties persist in applying the laboratory diffusion data to geologic settings. In particular, extrapolations over time (from laboratory experiments over days to natural diffusion over millions of years) and temperature (typical diffusion experiments are greater than 80°C, while natural diffusion occurs at temperatures as low as 45°C) introduce unquantifiable uncertainties into the significance of helium age data. The confidence with which such extrapolations can be made depends on the precision and accuracy of the diffusion data, the magnitude of the requisite extrapolation, and whether the mechanism of diffusion investigated by vacuum experiments is similar to that in nature. The best way to evaluate the laboratory diffusion data is to turn to a region where the thermal history is well known by independent means. Forward modelling of well-constrained thermal history and laboratory diffusion data can produce model helium ages that can in turn be compared to observed ages. The degree of misfit can then be viewed as a measure of the accuracy of the laboratory diffusion data. However, the lower temperature portions of thermal histories are rarely that well known, in part because many are based on apatite fission track (AFT) annealing models which are not well constrained below ~60°C, and so may have potentially large uncertainties in the temperature regime most important for helium diffusion. However, if the thermal history can be shown, with reasonable confidence, to be isothermal over a characteristic time, then helium diffusion and production in apatites will reach a steady state, where ^He ingrowth balances loss by diffusion (Wolf et aL, 1998), yielding an equilibrium age, t^q = (l/15)(D/a2>i. This age is achieved over characteristic time of approx. 5t*, where t* = 1.5(teq). Thus, helium ages that can be reasonably argued to be in steady state can be used to calculate empirical diffusivities completely independently from any laboratory calibrations. In order to evaluate laboratory diffusion data, we have collected helium ages for samples from four boreholes across the Otway Basin, southeastern Australia. The Otway Basin was selected as the study site because it is historically one of the most important natural laboratories for the establishment of

Geological Society of Australia - Abstracts Number 58

FT2 AFT thermochronometry. Structural, burial and thermal history data generated as a by-product of hydrocarbon exploration provide constraints on basin evolution that are among the best in the world. In addition, a widespread sedimentary horizon, the Otway Supergroup, contains abundant volcanic detritus bearing minerals suitable for FT analysis (apatite and zircon largely from a single source with a single crystallisation age). Following initial subsidence and deposition of the Otway Supergroup through early Cretaceous time to ~95 Ma, the burial and thermal histories of the eastern and western regions of the Otway Basin diverged. While the tectonic history of the western basin is, to first order, characterized by monotonic burial and heating, that of the eastern basin is marked by at least two periods of structural inversion and uplift (mid-Cretaceous and Late Tertiary). Published thermal models for the eastern Otway Basin disagree as to the age of the latter inversion event by as much as 30 m.y., (Duddy, 1994; Cooper and Hill, 1997). Because of these discrepancies, the thermal histories cannot be used with confidence to evaluate helium diffusivities. Despite the differences in the burial histories, both across the basin and within the eastern Otway basin itself, AFT and vitrinite reflectance models and subsidence studies from both regions indicate that, as a whole, the Otway Supergroup arrived at its present depths and temperatures sometime during the last 10-40 m.y. Thus, the basin can be regarded to be in "steady-state" during this time, (albeit with possible temperature uncertainties as large as 10°C), with basin sediments residing at temperatures close to those of modern day. Helium ages were obtained from four boreholes (Anglesea-1, Fergusons Hill-1, Eumeralla-1 and Heathfield-1) as well as two surface localities in the Otway Ranges. Downhole apatite helium ages define a broad band of values from 58-65 Ma at the surface (13.5±1°C) to zero at depths corresponding to ambient temperatures of ~67-83°C (House et al 1999). This swath results from at least three effects: (1) uncertainties in corrected borehole temperatures, (2) differences in the thermal history experienced by the various boreholes, and (3) possibly slightly different helium diffusivities among the detrital apatite samples. In the eastern Otway Basin, the shape and position of the helium profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for Durango apatite coupled with published thermal histories for this part of the basin. In contrast, helium ages are much younger than predicted in the western Otway Basin. Based on measured ages from Otway sediments, which have been essentially isothermal over the last few million years (within ~5°C), an empirical diffusivity (D/a^) of 2 x lO'i^ s'l can be assigned to apatites residing at downhole temperatures of ~67-97°C (ages of approx. 1 Ma, requiring roughly 7.5 m.y. of thermal stability). This empirical diffusivity is consistent with laboratory diffusion measurements, demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings. We have also carried out detailed AFT measurements and chlorine analyses on the same aliquots from which the helium measurements were made (Kohn et al, 1999). The AFT data for individual samples show a wide range of apparent ages and compositions, whereas helium measurements on coexisting grains usually fall within a restricted age range. Although not conclusive, this finding supports previous suggestions that composition does not affect the sensitivity of the helium closure temperature. Given the confirmation of the laboratory diffusion data by this study, the discrepancy between the observed and modelled helium age profiles in the western Otway Basin suggests that these sediments recently experienced higher temperatures than presently supposed. Comparison of published thermal histories for the eastern Otway Basin reveals possible discrepancies in modern downhole temperature estimates (possibly up to ~10°C). Such uncertainties, are very significant in forward modelling of helium ages. Cooper G. T and Hill K. C., 1997. Cross-section balancing and thermochronological analysis of the Mesozoic development of the eastern Otway basin. APPEA Journal, 37, 390-414. Duddy I. R., 1994. The Otway basin: thermal, structural, tectonic and hydrocarbon generation histories. NGMA/PESA Otway basin symposium extended abstracts, 14, 35-42.

FT2 House M. A. Farley K. A. and Kohn B. P., 1999. An empirical test of helium diffusion in apatite: borehole data from the Otway Basin, Australia. EPSL, 170, 463-474. House M. A. Wernicke B. P. and Farley K. A., 1998. Dating topography of the Sierra Nevada, California, using apatite (U-Th)/He ages. Nature, 396, 66-69. Kohn B. P. House M. A. Farley K. A. and Raza A., 1999. A comparative study of helium diffusion and fission track annealing in apatites: borehole data from the Otway Basin, Australia. Geological Society of Australia Abstr. Series, 53, 137-138. Lippolt H. J. Leitz M. Wernicke R. S. and Hagedorn B., 1994. (U+Th)/He dating of apatite: experience with samples from different geochemical environments. Chem. Geol., 112, 179-191. Warnock A. C. Zeitler P. K. Wolf R. A. and Bergman S. C., 1997. An evaluation of low-temperature apatite U-Th/He thermochronometry. Geochim. Cosmochim. Acta, 61, 5371-5377. Wolf R. A. Fariey K. A. and Kass D. M., 1998. Modelling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer. Chem. Geol., 148, 105-114. Wolf R. A. Farley K. A. and Silver L. T., 1996. Helium diffusion and low temperature thermochronometry of apatite. Geochim. Cosmochim. Acta, 60,4231-4240. Zeitler P. K. Herczig A. L. McDougall I. and Honda M., 1987. U-Th-He dating of apatite: a potential thermochronometer. Geochim. Cosmochim. Acta, 51, 2865-2868.

Acknowledgments

This study was supported by the Donors of the Petroleum Research Fund, administered by the American Chemical Society and grants from the Australian Research Council, Australian Institute of Nuclear Science and Engineering, and the National Science Foundation. Part of this study was also conducted as part of the Australian Geodynamics Cooperative Research Centre and this work is published with the permission of the Director, AGCRC.

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

L O N G - T E R M M O R P H O L O G I C E V O L U T I O N IN M I D - S C A N D I N A V I A ; Q U A N T I F I C A T I O N AND T I M I N G O F V E R T I C A L M O T I O N S U S I N G C O N S T R A I N T S F R O M F I S S I O N T R A C K ANALYSES

YD. Huigen and P.A.M. Andriessen Department of Isotope Geology, Vrije Universiteit Amsterdam, the Netherlands.

In recent years, it has become apparent that rifted margins may record significant vertical motions related to pre-, syn- and postrift events. Constraints of these events are important to incorporate in tectonic models explaining the formation and evolution of rift basins and the development of the morphology of the continent. In conjunction with the margin uplift it is important to quantify the accelerated postrift subsidence in the various sedimentary basins. The North Atlantic Domain (i.e. Norway, Sweden, Finland, Svalbard, V0ring Basin, Barents Sea) contains all the facets for studying these features related to rifted margin evolution. The almost complete absence of a post-Paleozoic sedimentary record onshore have led geoscientists to suggest a large-scale Tertiary uplift related to the opening of the Northern Atlantic. Also, the absence of a crustal root suggests that the morphotectonic evolution of Scandinavia was strongly affected by Meso-, Cenozoic tectonic and climatic events, rather than solely the result of Caledonian orogeny and the formation of the North Atlantic passive margin. However, the exact timing and mechanism of uplift and erosion, the formation of two morphological domes in northern and southern Norway, as well as the influence of Quartenary glaciations, remain a matter of controversy. Fission Track (FT) analysis has proven to be a very strong tool to unravel the geological history of this region (e.g. Rohrman, 1995). This study aims to constrain and date the different vertical movements of Mid-Norway and Sweden and the adjacent Voring Basin. The sedimentary record in the Voring Basin in front of the Norwegian coast, shows accelerated subsidence and increased sedimentary influx in Jurassic, Cretaceous, Tertiary and Neogene times. To reconstruct the geological evolution of the continental margin, FT age provenance analysis of offshore sediments will be correlated in time and space with uplift and denudation processes of basement rocks onshore. Onshore FT analysis is used to reconstruct the paleo-thermal history of the Caledonian and preCambrian basement. Two domes are expressed in the geomorphology of north and south Norway. Rohrman (1995) conducted FT analysis on the southern dome and found two main periods of uplift and exhumation: a Triassic-Jurassic event with 2.4 ± 1.1 km. of denudation, related to the extension tectonics prior to the opening of the North Atlantic and an uplift of 1-1.5 km. in Neogene time. Since Neogene time the northern and southern dome have a saddle-shape loweringand' in between. Geomorphological studies done in Sweden indicates differences in uplift history between these two domes (K. Lidmar-Bergstrom, 1996), which is explained due to protection of the basement rocks in the south by a Cretaceous sedimentary cover. The morphology of Mid-Scandinavia shows the saddleshape (350 by 200 km) between the two domes. FT analysis in this region is performed to get information on the timing and constraints of the uplift- denudation history. The FT data will be used together with gravity data, a proxy for the lithospheric composition, to understand the relationship between the occurrence of the saddle-shape morphology and underlying lithosphere. The first preliminary FT results are correlated with the phases of denudation deduced from geomorphological studies onshore and the increased sedimentary influx in the offshore basins. For FT analysis of this saddle-shape and the way these two domes are connected some 60 samples are taken along 4 profiles, parallel and perpendicular to the Caledonides of Mid-Scandinavia. Along the profiles two vertical profiles were sampled. One with a difference in altitude of 1040 m in the centre of the morphological saddle (near Are, Sweden) and one at the southern border of the Northern dome with a difference in altitude of 940 m (near Trofors, Norway). FT ages parallel to the coast of Norway

Geological Society of Australia - Abstracts Number 58

FT2^ee and in Sweden range from 233 ± 25 Ma to 250 ± 41 Ma, with mean track lengths between the 13.64 ± 1.31 pm to 13.52 ± 1.52 pm. Modelling these data, using the Monte-Trax model of Gallagher, shows an accelerated cooling rate in the Triassic-Jurassic period and subsequent slow continuous cooling till now. Lidmar-Bergstrom K. 1996. Long term morphotectonic evolution in Sweden. Geomorphoiogy l6, 33-59. Rohrman M. 1995. Thermal evolution of the Fennoscnadian region from fission track thermochronology. An intergrated approach. Thesis, Vrije Universiteit Amsterdam. Gallagher, 1990-1992. Dept. of Earth Sciences, The Open University, MiltonKeynes and Dept. of Geological Sciences, UCL, London Acknowledgments Supported by NWO/ALW, This research is part of NSG, ISES.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

ACID AND CHLORO-TINCLES, R A R E EARTHS AND DIFFERING ANGLES, J U S T WHAT M I G H T BE IMPORTANT IN UNDERSTANDING TRACK ANNEALING IN APATITE?

AJ. Hurfordi,A. Carteri,J. Barbarandi and F.Walgenwit22 1 Research School of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK. 2 Elf EP, CSTJF, avenue Larribau, F-64018 Pau, France.

A major strength of fission-track (FT) thermochronology is the ability to provide the geoscience community with time-integrated temperature pathways for upper crustal rocks. The prediction of plausible thermal histories for geologic and geomorphic studies in orogenic belts, passive rifted margins, sedimentary basins and stable cratonic blocks is predicated on a precise and accurate understanding of the annealing behaviour of fission tracks in apatite. Three stages are essential in deriving such a thermal history: an empirical assessment of track length and density reduction at elevated temperatures for different time periods; derivation of an appropriate algorithm to describe the measured data and to permit extrapolation to geological time; and a numerical modelling system which utilises the annealing algorithm to predict a possible tT scenario which best-fits sample FT data. The non-uniqueness of a predicted tT solution testifies both to the need to consider all known geological constraints and yet avoid bias in the modelling procedure. In this presentation we consider a broad range of experimental factors which can critically affect the determination of track length and hence the estimation of annealing level. Certain of these factors may have been neither appreciated nor evaluated. We also describe new annealing data on a range of apatite mega-crysts of differing composition. Finally we speculate on reasons for variation in the susceptibility for track annealing. Revelation and observation of track length are fundamental to FT study. Most workers use 5 M HNO3 at room temperature as an apatite track etchant. But comparison with weaker 0.8 M HNO3 shows the stronger 5 M etch to be biased toward tracks at higher angles to the c-axis, which tracks themselves show preferential levels of annealing. Whilst the 0.8 M etch has a greater etching isotropy, ovoid etch pits are the result, making difficult the identification of the c-axis direction and thus sections of high etching efficiency. Multiple analysis, preferably by different analysts, is essential in gathering annealing data to construct an annealing algorithm which subsequently may be used by many workers in predicting thermal history. Results from 3 analysts give our data a baseline for the estimation of the precision of measurement variation with analyst. Tint and tincle confined tracks possess greatly differing levels of bias. Separate tint and tincle analysis in our study, each at widely differing levels of annealing show tincles biased toward longer lengths and thus potentially less sensitive as a measure of annealing. Experimental data require a baseline against which the level of annealing can be assessed, typically the unannealed induced track length, 1^. We compare of confined tracks from apatites of different composition, and in induced track populations produced in irradiations over a 15 year period and subsequently held, unetched, at ambient laboratory temperatures, finding that variation in IQ exceeds 1 pm. The assumption that FT annealing in apatite, with its multiple cation and anion substitutions, could be described by a single kinetic model has long been questioned, primarily by the not-infrequent presence of anomalous FT single-grain ages. Assessment of the Otway Basin data suggested older ages may correlate with higher chlorine content (Green et al , Chem. Geol. Isot. Geosci. Sect., 59, 237, 1986) and, by implication, greater resistance to annealing. Annealing may thus be related to composition and therefore reliance on a single, e.g. Durango-derived annealing dataset could be less valid for apatites of differing composition. Just how different the composition needs to be remains undefined. Chlorine substitution is cited by many workers in explanation of difficult data, sometimes supported by limited microprobe measurement. An upper boundary of 1 wt% chlorine is often suggested for compliance with Durango-type annealing. At the time of writing, no published systematic study Geological Society of Australia - Abstracts Number 58

FT2^ee of the dependence of annealing on composition exists. Uncertainty thus remains as to the accuracy of predicted thermal histories, denudation rates, thicknesses of lost section and volumes of potential sediment flux. The assemblage of more than 75 apatite samples with differing compositions provided l 6 candidates for a series of new laboratory annealing runs. Repeat WDS and EDS electron microprobe analysis show the variation in sample composition: fluorine contents vary between 0 and 4 wt%, chlorine contents between 0 and 5 wt.%; extensive cation substitution is also apparent. Experimental design is critical for annealing studies with a need for precise and accurate monitoring of both time and temperature, especially temperature variation between samples in the annealing furnace, and over the (possibly long) experimental times. Use of multiple calibrated thermocouples linked to a temperature logging system safeguards against periodic temperature excursions producing anomalous annealing runs. Time measurement for one hour anneals is inherently less precise than for 100 hour runs, favouring longer annealing runs. A single experiment using multiple aliquots of the Durango apatite has provided an empirical assessment of analytical precision. Experimental runs were selected using the Durango-based algorithm (Laslett et al, Chem. Geol. Isot. Geosci. Sect., 65, 1, 1987) to predict timetemperature pairs which might yield mean track lengths at 1 pm intervals from 15 to 7 pm. In total 45 annealing runs are being undertaken. Results obtained at the time of writing clearly reinforce the supposition that samples with >1 wt.% chlorine are more resistant to track annealing; but are there also correlations with other substitutions, either singly or through some more complex interaction? Using ion and electron probe data we look for correlations between other variations in apatite composition and systematic patterns of FT annealing. Finally we report the determination of XRD data on our suite of apatites to define unit cell parameters and to seek relationships with compositional substitutions. We speculate on the inter-relationship of cation and anion substitution, structural change and FT annealing.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

C O N S T R A I N T S ON T H E T H E R M A L H I S T O R Y O F T H E I N T E R I O R B A S I N S O F T H E SULTANATE O F OMAN USING APATITE F I S S I O N T R A C K ANALYSIS

(AFTA)

S.L. IndreUd and J.MJ.Terken Petroleum Development Oman (Author 2 now with Shell International E and P)

The thermal history of Oman is still relatively poorly understood despite decades of hydrocarbon exploration and production. The thermal history is important for understanding the hydrocarbon systems - is there recent charge? Or did excellent (mainly salt) seals lock in early charge? Key problems are: a) the fact that much of sedimentary column is Early Paleozoic in age means that maximum temperatures are poorly constrained by traditional maturity indicators (in particular vitrinite does not occur). b) the lack of clearly identified source areas for many of the oils means that oil maturity data is only a very weak constraint on the thermal history. Furthermore, even when the oils are in situ (intra-salt oils) the error bars on maturity interpretation, and lack of understanding of the source rocks themselves still leads to difficulty in interpreting the maximum temperatures attained by the rocks. c) there are very large gaps in the stratigraphic record, and it is not clear how much section is missing. d) the early tectonic history of the main basins is still not fully understood, requiring thermal predictions from geotectonic models to address several different scenarios. A long-standing debate centres on the possibility of Precambrian/Cambrian rifting. Apatite fission track analysis (AFTA) forms the primary constraint in interpreting the thermal history of Oman - by allowing estimation of the timing of cooling periods, and the timing and magnitude of maximum temperatures. AFTA data have been acquired extensively by PDO over the last 11 years around 340 samples from 53 wells and 5 outcrop areas, most of which have been analysed for composition, as well as track length and age. Zircon fission track data has been acquired for 8 wells where high present day temperatures limit the applicability of AFTA. Approach An in-house study (unpublished) of those samples presently at annealing temperatures (75 - 125°C) suggested that a single kinetic model is not applicable to all grains. Being aware of the effect of compositional variations on the annealing behaviour of apatites a) allows a more detailed and accurate thermal history to be extracted from a single apatite sample b) reduces the error that would occur if samples with unusual compositions were treated as "normal". The inferred maximum paleotemperature could be in error by as much as 20°C in the range 80 125°C. Because oil generation occurs within this temperature range this error can affect predicted maturity significantly. Review of apatite composition with fission track age and length data showed that: • Chlorine content is a key control on how easily the grains are annealed, and thus the temperature at which full annealing resets the fission track age - in many samples most of the older grains are relatively chlorine-rich (i.e. ages not reset). • The fluorine content also has an impact. In general this is related to the chlorine content as the two substitute for one another. However, comparing grains with very low/zero% chlorine, the more

Geological Society of Australia - Abstracts Number 58

FT2^ee fluorine-rich (F > 4%) grains are apparently reset at lower temperature than the less fluorine-rich grains (F < 4%). • The hydroxyl concentration also seems affect the resistance to annealing: the grains with high OH content are more resistant to annealing. • Uranium content may also affect resistance to annealing with more uranium resulting in grains which are less resistant to annealing. Neat compartmentalisation of annealing behaviour (especially ages) versus composition does not occur for all samples - there are a substantial number of cases where there is apparently no relation between composition and age. Possibly this may represent a bias in the detrital ages of the apatite grains in the sample. Radial plots of single grain ages have been interpreted in terms of the compositional differences to derive time estimates for different temperatures (Fig. 1). This approach has been used to derive age trends (with associated temperature ranges where the compositions show a good divide) for the PDO AFT database. When interpreted temperatures and age trends for samples at various depths in a well are combined together, an approximate temperature history for the well can be derived. This is compared to the thermal history derived by Geotrack for the same samples, using forward modelling based of thermal histories to fit the track length distribution. In many cases the results are consistent, with the compositional based model perhaps filling in some detail. The two approaches are thus considered complementary. Thermal history of the interior basins of Oman There is a major divide between the severely - fully annealed Lower Paleozoic Haima samples and weakly annealed Permo-Carboniferous Haushi samples, even within the same well. The main trends observed in the data are: There is pronounced cooling during the Tertiary (~20°C on average), probably between 50 and 20 Ma. This is particularly pronounced on the eastern margin, where emplacement of the Batain nappes and Masirah ophiolite caused uplift, with associated hydrothermal effects. Gradual, phased cooling occurred during the Mesozoic with temperatures 0 - 60°C higher than present day. Single grain ages of 370 - 450 Ma probably record maximum temperatures (and sometimes also maximum burial). Maximum temperatures were much higher (20 - 80°C) than present day, with a bigger difference in the east than the west. There is no direct evidence from AFTA that the thermal gradient during the Paleozoic was particularly high, posing difficulties for the interpretation of the South Oman Salt Basin as a rift system. Peak temperatures were followed by gradual late Paleozoic cooling. Dates as young as 320 - 280 Ma may represent a final phase of rapid cooling associated with glaciation and inflow of cool water. Ages of -500 Ma are seen in a few samples but temperature history prior to -450 Ma is poorly constrained due to severe/total annealing of older samples. These ages may represent either the age of deposition (within error) or an early cooling phase (possibly associated with uplift and/or rifting). The high paleotemperatures inferred from apatite fission track analysis would imply early oil generation, with oil locked in place for several million years! The result has thus been subject to challenge. Fiowever high paleotemperatures are supported independently by:

FT2

150- / / 'V 125-

/ /

'

'

>

100- : . ^ a 755025" 0

Confidence Figure 1. Example 1 of compositional effect on fission track single ages and PDO interpretation. The sample was annealed

early (410 - 300 Ma) with temperatures > 115 - 120°C at that time. The fluorine rich/chlorine poor (square) and intermediate (triangle) crystals were subsequently annealed at some time, before cooling during the Tertiary (40 Ma). As this did not anneal the chlorine rich (circle) crystals a maximum temperature < 115°C and > 90°C is inferred for that time period. Note: Unfilled figures denote crystals which are so Uranium poor that the ages are extremely error prone.

Organic maturity (measured as biomarker maturity of in situ oils, Tmax in source rocks) is generally higher than would be expected for the present day temperatures. Aqueous fluid inclusions from an outcrop in South Oman imply temperatures of at least 173±19°C (unpublished data, Shell Research), although this area has been uplifted much more than the interior basins, and may also have been affected by high temperatures associated with Gulf of Aden rifting in Yemen. A study of illite crystallinity (unpublished) suggests that several western margin wells have been significantly hotter in the past.

FT2^ee Igneous rocks, possibly associated with rifting are known at several locations in Oman (Worthing et al in prep.) K-Ar and Ar-Ar dating of outcrop dykes suggests a Paleozoic age (470 - 370 Ma). Oil to gas cracking is strongly suggested by diamondoid analysis (Dahl et al 1999) in several intra-sak hydrocarbon accumulations. In this case, oil to gas cracking is thought to start at temperatures of around 130°C, yet reservoir temperatures at present day in these wells is around 90°C. Independent constraint on the timing of maximum paleotemperatures is more limited: Argon-Argon spectrum analysis of a Precambrian sample in central Oman gives a suite of ages: 455 Ma and 350 Ma are interpreted by FM consultants as "overprinting by a tectonic, thermal or hydrothermal event". Uplift (and hence cooling) can also be inferred from unconformities in the stratigraphic record of Oman. For example the base Al Khlata unconformity cuts through the thick (known) Raima sequence, and reworked palynomorphs suggest that Devonian sediments were relatively widespread (Penney, pers. com.) suggesting that >lkm of sediments have been removed. Although early (Paleozoic) maximum temperatures are inferred over most of Oman, this does not mean that maximum paleotemperature everywhere in Oman occurred early: in the west-central area and much of north Oman the AFT data do not require that temperatures have been higher in the past. However, it should be noted that they also do not rule it out. In particular, the foreland basin area, which underwent rapid subsidence in the Cretaceous and Tertiary, is likely to be at maximum temperature at present day.

Summary Apatite fission track data has been instrumental in understanding the thermal history of the interior basins of the Sultanate of Oman, particularly in timing maximum temperatures. Apatites are now routinely used in PDO as an important constraint in charge modelling in the interior basins of Oman. Dahl J.E., Moldowan J.M., Peters K.E., Claypool G.E., Rooney M.A., Michael G.E., Mello M.R. and Kohnen M.L. Diamondoid hydrocarbons as indicators of natural oil cracking. Nature v399 p54-57. Worthing M.A., Oterdoom H. and Partington M. (in prep.) Petrological and tectonostratigraphic evidence for a mid Ordovician rift pulse on the Arabian Peninsula.

International Conference on Fission Track Dating and Thermochronology

T H E R E C O R D O F O N S H O R E M E S O Z O I C E R O S I O N AND S E D I M E N T F L U X T O T H E B A S I N S OF THE N E

GREENLAND CONTINENTAL

OFFSHORE

MARGIN

C Johnson and K. Gallagher T.H. Huxley School, Imperial College of Science,Technology and Medicine, UK

The continental margin of NE Greenland is perhaps the least studied of the North Adantic margins. This margin is characterised by a long history of post-Caledonian rifting, commencing in the late Devonian and ending with the onset of sea floor spreading at ~55 Ma. The continent-ocean boundary (COB) was formed oblique to the axis of Mesozoic rifting. Therefore, sediments derived from erosion of one of the margins of this part of the North Atlantic may now reside in basins attached to its conjugate partner. This is likely to be particularly true of the East Greenland margin between 72°-74° N given the proximity of the COB to the present-day coast in this sector of the North Atlantic. Prior to break-up, three of the most important frontier regions in the NE Adantic, the V0ring, More, and Faroes-Shedand Basins, were adjacent to the NE Greenland continental margm between latitudes 66° and 75°N. These basins may now be the repository for substantial volumes of Jurassic and Cretaceous sediments that were derived, not from the Norwegian margin, but from East Greenland. Consequently, the distribution of reservoir quality sands in the currently distal parts of these basins is likely to be controlled primarily by the nature of the eroding source region on the east Greenland margin during the Mesozoic and Tertiary. The spatial and temporal distribution of onshore erosion can be constrained with apatite fission track thermochronology. A suite of 200 rock samples have been collected from NE Greenland between 68° and 75°N from the coast to 200 km into the continental interior The sampling strategy was designed specifically to characterise the pre-break-up denudation chronology. In particular, 20 vertical profiles have been sampled, taking advantage of the relief created by glacial erosion. This very recent erosion has exposed samples from different levels in the upper few kilometres of the crust. Samples from individual vertical profiles clearly will experience similar thermal histories, with the samples now at lower elevations being exposed to higher temperatures as they resided deeper in the pre-glacial crustal section. Further sampling localities were selected to maximise the constraints on pre-beak-up movement of fault blocks. The new data has been integrated with regional scale structural data and what is known of the record of Mesozoic and early Tertiary sedimentation to provide a detailed picture of the onshore erosion and offshore deposition of the North Adantic margins prior to break-up. Results indicate that maximum temperatures in the Devono-Carboniferous basins were reached at the end of the Carboniferous to early Permian. This timing lends support for the contention that a significant section of the East Greenland 'Devonian' basin is Carboniferous in age. Maximum paleo-temperatures of presently outcropping rocks are estimated to range between 100° and 130°C. Temperatures of this magnitude imply cumulate erosion from these basins of 3 to 4 km when combined with thermal conductivity values measured on a representative selection of DevonoCarboniferous sediments and mean Early Permian Earth surface temperature for the East Greenland region. Following these maximum temperatures, regional cooling occurred during the Permian. This is linked to well recognised Permian erosion and the development of a widespread mid-Permian peneplain. The next important phase of cooling commenced in the Early Jurassic and predates the Middle Jurassic marine transgression and deposition of the Vardekloft Formation. Cooling is interpreted as recording erosion resulting in initial deposition in depocentres presently located in Early Jurassic basins to the east of the present-day onshore Jurassic outcrops. The onshore margin to the north of Kong Oscar Fjord and east of the Post-Devonian Main Fault was exposed at this time and the erosion produced a peneplain of Permian and Triassic sediments and Caledonian basement. A third phase of cooling in the Late Jurassic is linked to the break-up of the relatively wide fault blocks of the earlier Jurassic rift system (~90 km) into the narrower blocks seen today (5-30 km). Erosion related cooling is spatially linked to fault movement on the Gauss Halv0 Fault system. A final phase Geological Society of Australia - Abstracts Number 58

FT2 of Mesozoic cooling occurs in the interval 108 to 80 Ma (mid-Cretaceous). This cooling is related to renewed rifting in the Albian and is best resolved in those sections that lie in the footwall of the Gauss Halv0 Fault system. Sediments derived from the subarial margin are thought to have bypassed the Cretaceous shelf and slope, where sedimentation was predominantly fine grained, through south facing relay ramps giving rise to deep marine sand rich depositional systems near the rift axis.

International Conference on Fission Track Dating and Thermoclironoiogy

A M O N T E - C A R L O CALCULATION OF THE DIMENSIONS OF LATENT ALPHA R E C O I L TRACKS

R.Jonckheere, K. Gogen and G.Wagner 1 Max-Planck-Institute Fiir Kernphysik, Heidelberg, Germany

Natural minerals contain trace amounts of Uranium, Thorium and their radioactive daughters. Their alpha-decay sets free several MeV, which is transferred as kinetic energy to the alpha-particle (-103 keV) and the recoil-nucleus (-lO^ keV). The alpha-particle dissipates most of its energy through electronic interactions over a range of -20 |Llm, and produces -lO^ isolated atomic displacements near the end of its trajectory through nuclear interactions. The recoil nucleus loses most of its energy through nuclear interactions over a range of 30-40 nm, producing a highly localised displacement cascade resulting in -10^ lattice defects, which, together, constitute an alpha-recoil track. The number of alpha-recoil tracks increases with time, and is thus a measure for the age of the mineral [1, 2]. In micas, natural recoil-tracks can be revealed by etching and observed under a phase-contrast microscope. The number of alpha-recoil tracks intersecting a unit surface can be measured accurately with modern image-analysis equipment [2]. One of the problems in recoil-track dating is to relate the areal densities of surface-intersecting recoil-tracks to their volumetric densities. The size of latent recoiltracks is a central parameter in this relationship. The size of a single recoil track, and, in consequence, the size distribution of a population of recoil tracks, is time-dependent because a single track can result from a single or up to eight spatially correlated but temporally separated decays. The timedependent size-distribution of alpha-recoil tracks was calculated using numerical techniques. The size of a track was, to this end, defined as the maximum distance between the successive positions taken in by the decaying nucleus, and depends on the starting isotope and on the length of the decay-chain. For a particular isotope and chain-length, the size of the recoil track is calculated with a Monte-Cario method, that takes as input the mean vector ranges and straggling calculated beforehand using a binary collision approximation (TRIM-98) [31. The number of tracks with chain-lengths of 1, 2, etc., produced by this isotope at a given point in time is given by the relative concentrations of its daughters. If we neglect branching ratios of less than 0.1%, the decay series reduce to single nonbranching chains for which the Bateman-equations [4] allow to calculate the time-dependent concentration of these daughters. The total number of recoil tracks produced by the different isotopes depends on their initial relative concentrations and their respective rates of decay. For secular equilibrium, these concentrations are fixed, and the number of recoil-tracks from each isotope follows from the laws of radioactive decay. The time-dependent recoil-track size-distributions are obtained by combining these results. Interestingly, the recoil-track population resulting from the decay of the Uranium-series isotopes is built up of two sub-populations. The first consists of tracks resulting from a single decay. They have a fairly narrow range of sizes with a mean of ~30 nm. The second population essentially consists of tracks that are the result of the complete decay of 238u and 234u to stable 206pb. They have a broad range of sizes with a mean of -125 nm. It is also interesting to observe that the first population increases non-linearly with time and saturates around - 1 Ma, whereas the second shows an approximately linear increase in the entire time interval between 100 ka and 1 Ma. (1) HUANG W.H. and WALKER R.M. 1967. Fossil alpha-particle recoil tracks: a new method of age determination. Science 155, p.1103-1106 (2) WAGNER G.A. 1998. Dating of young rocks and artifacts: physical and chemical clocks in quaternary geology and archaeology. springer Verlag, 350 pp. (3) ZIEGLER J.F., BIERSACK J.P. and LITTMARK U. 1985. The stopping and range of ions in solids. Pergamom, New York. (4) BATEMAN H. 1910. Solution of a system of differential equations occurring in the theory of radioactive trans-formations. Proc. Cambr. Phil Soc. 15, 423.

Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

POST-OROGENIC EXHUMATION OF THE ULTRAHIGH PRESSURE CONTINENTAL CRUST IN EAST-CENTRAL CHINA: TECTONICS AND FT-THERMOCHRONOLOGY

R. Jonckheerei, J.C. Schmid2, L. Ratschbacher2,A. Blythe3, S. Dong^, S. Uu\ B.R. Hacker^ and G.Wagneri 1 Max-Planck-Institut fiir Kernphysik, Heidelberg, Germany. 2 Institut fur Geologie, Universitat Wiir2burg,Wurzburg, Germany. 3 Department of Earth Sciences, University of Southern California, Los Angeles, USA. 4 Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, P.R. China. 5 Changsha Institute of Geotectonics, Changsha, PR. China. 6 Department of Geology, University of California, Santa Barbara, USA.

The Qinling-Dabie-Sulu orogen stretches from east to west through east-central China and sutures the Sino-Korean craton in the north and the Yangtze craton in the south (Fig. 1). This metamorphic belt is the result of the attempted northward subduction of the Yangtze craton beneath the Sino-Korean craton during Triassic time and comprises the largest exposure of ultrahigh-pressure (UHP) metamorphic rocks in the world. Petrological and geochemical data show that a segment of continental crust was subducted to depths in excess of 100 km, followed by exhumation. Understanding the exhumation mechanism of the UHP and related metamorphic rocks is crucial for understanding the processes involved in continental subduction. Here we present the post-orogenic cooling and exhumation history of the Hong'an-Dabie segment of the orogen based on apatite and titanite fission-track dating, integrated with structural and geobarometric data, and U/Pb (zircon) and Ar/Ar (hornblende, mica, K-feldspar) thermochronology.

EI] Upper Cretaceous-Eocene red beds Lower Cretaceous Granitoids Upper Jurassic-lower Cretaceous voicanics/red beds m Paleozoic-Jurassic sediments m] low grade metamorphic rocks (not differentiated) Yangtze crystalline basement (sensu lato)

f

Hehuai Basin

iir + 32-

i + 31S S +

t

metamorphic/tectonic boundary fault

68.4 ± 6.6

70.7±9.0 80.2 ±7.5

Paleocene - Eocene FT-age (#65 Ma) Upper Cretaceous FT-age ($65 Ma) sample location, FT-dating in progress

+ 30-

US-

Fig. 1: Map of the Tongbai-Hong'an-Dabie area. Note the widespread intrusions of lower Cretaceous age. Lower Cretaceous transtension facilitated the formation of the the syn-to posttectonic magmatic-metamorphic complex of the Northern Orthogneiss Unit Upper Cretaceous - Eocene red beds surround the orogen in the E and S, and in the N, in the Hehuai Basin, reach several km thickness. Cooling in the latest Cretaceous resultedfromen-block updoming centered at the ultrahigh-pressure-unit (indicated here as CE: coesite eclogite). QE: quartz eclogite, EA: eclogitic amphibolite, AM: amphibolite, GS: greenschist, BS: blueschist, XMF: Xiaotian-Mozitang fault.

Geological Society of Australia - Abstracts Number 58

FT2^ee The orogenic architecture of the Hong'an and Dabie belt is dominated by Cretaceous and Cenozoic structures that contributed to its exhumation from up to 30 km depth. Cretaceous magmatic crustal recycling, estimated at ~50%, and heating (>250° to >700°C) were most prominent in the Dabie. Exhumation, magmatism, and cooling were all controlled by Cretaceous transtension. Exhumation was essentially accomplished by an asymmetric Cordilleran-type extensional complex in the northern Dabie (Northern Orthogneiss Unit) between 140 and 120 Ma. Cretaceous reactivation occurred within a regional transtensional strain field as a result of far-field collisions and Pacific subduction. The onset of crustal extension was preceded and possibly facilitated by a re-heating of the Hong'an and Dabie crust (~140 Ma) coeval with the onset of voluminous magmatism in eastern China (~145 Ma) which resulted from a change in Pacific subduction from highly oblique to frontal. The structural control on regional cooling vanished at 120-110 Ma and in the latest Cretaceous the entire orogen exhumed as an updoming, mostly rigid block with the central UHP region in the Dabie showing the youngest cooling (Fig. 1). Temperature-time path modelling based on track-length measurements in apatite indicate a monotonous slow uplift (<100 m/Ma) of the Dabie Shan. The same uplift style is suggested by the track length distribution in samples from the North of the Tongbai block. The late cooling probably followed regional reheating at 85-60 Ma. Traces of the Triassic-Jurassic exhumation history of the UHP orogen are preserved (1) in the basement units of the orogenic foreland, (2) in the Hong'an to the west of Dabie, and (3) in Triassic and Jurassic clastic sediments of the eastern and southern foreland.

International Conference on Fission Track Dating and Thermochronology

F I S S I O N T R A C K AGE CALIBRATION - T H E N E X T

F T 2 '

GENERATION

R. Jonckheerei, P. van den Haute2, F. De Corte^ and G.Wagneri 1 Max-Planck-Institute Fiir Kernphysik, Heidelberg, Germany 2 Geological Institute, University of Gent, Belgium 3 Insittute for Nuclear Sciences, University of Gent, Belgium

From 1963 to 1988, geochronologists used the method of their choice to determine a fission-track age. A series of conflicting results brought to light the existence of methodological difficulties, that were ascribed to uncertainty about the value of the decay constant (A.O) and difficulties in determining the absolute thermal neutron fluence ((|)) [1,2]. Errors resulting from the methods for measuring the fossil and induced track densities (p^, p^) received less attention. In 1988, the fission track community adopted the recommendation that henceforth the z-calibration should be used for determining fission-track ages [3]. An exception was made for apatite, that, under certain conditions, could still be dated with the "absolute" method (i.e. independent of age standards). This was significant because it drew attention to the measurement of p^ and Pi as a possible cause of methodological problems. Although it was acknowledged that a ^-age is not an independent age, because the fossil and induced track densities only serve to relate the unknown age to the K-Ar, Ar-Ar or Rb-Sr age of a standard, the ^-recommendation was widely followed and had an overwhelming success. New labs and people got involved in fission-track analysis, resulting in a vastly increased number of ages and a wider acceptance of the results. More than ten years later, the situation has changed. Thermal neutron fluence measurements using metal activation monitors and the calculation of the induced-fission rate of 235u ^^n be performed with an accuracy and precision that cannot be matched by ^ [4,5,6]. Problems related to track registration, revelation and observation have become much better understood [7,8,9,10,11]. The ^-factor, initially believed to be a constant [12], was shown to be different for different minerals [13]. For a given mineral it depends on the techniques used for revealing and counting the spontaneous and induced tracks (POP, ED), and ^ could even be influenced by microscope settings. The apatite ^-factors from different laboratories using the same method (ED) and monitor glasses (NIST-SRM612-613) range from 290 to 360 a.cm^, revealing systematic differences related to the procedure for track counting. Thus, although the ^-method is useful in producing a factor that links an unknown age to that of a standard, it has to be acknowledged that it has not proved effective when it comes to testing the techniques of track counting or the abilities of the analyst. But aren't such tests asked for from all newcomers in the field? Is it excluded that these errors propagate into the fission-track age of an unknown sample? In other words, does ^ really guarantee universally comparable ages, as it is believed? Together with the introduction of the ^-method, fission-track studies started to focus on thermochronology. Such studies [14,15] showed that the spontaneous-track-length reduction in apatite is ubiquitous, and that it should have a proportional effect on the fission-track age [I6]. Thus, it is uncertain that the FT-ages of the standards are equal to their independent reference ages. It has been argued that this is not required as long as they exhibit the undisturbed volcano-type track-length distribution, but this nevertheless implies that track length measurements enter into age-calibration. Partial annealing of the fossil tracks is also ubiquitous in glass. In particular, only certain specimens of the Moldavite glass, from specific localities (Jankov, Habri), can be expected to give the reference age for this age-standard [17]. Plateau-age experiments do appear to correct effectively for partial annealing, but does this imply that, in practice, a "plateau-^" calibration will have to be developed before other glasses can be dated using the ^-method. Analyses of apatite age-standards and putative standards (Fish Canyon, Durango, Mount Dromedary), both with the "absolute" and with the ^-method, reveal the limitations of the ^-calibration. We suggest that, considering the advances that have been made in the determination of (j) and in the

Geological Society of Australia - Abstracts Number 58

FT2^ee identification of systematic errors related to measurement of ps and pi, a more liberal recommendation should be considered, that also allows other calibration systems, provided that the guarantees for the accuracy of the result are equal to those offered by the ^-method. (1) HURFORD AJ. and GREEN P.F. (1981) Standards, dosimetry and the uranium-238 If decay constant: a discussion. Nucl. Tracks 5, 73-75. (2) HURFORD AJ. and GREEN P.F (1982) A users' guide to fission tmck dating calibration. Earth Planet. Sci. Lett. 59, 343-354. (3) HURFORD A.J. (1990) Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the I.U.G.S. Sub-comission on Geochronology. Chem. Geol. (Isot. Geosc. Sect.) 80, 171-178. (4) HURFORD A J . (1998) Zeta: the ultimate solution to fission-track analysis calibration or just an interim measure? In: Solid Earth Sciences Library, 10, Advances in fission track geochronology (P. Van den haute and F. De Corte eds.), 19-32. (5) VAN DEN HAUTE P., DE CORTE F , JONCKHEERE R. AND BELLEMANS F (1998) The parameters that govern the accuracy of fission-track age determinations: a re-appraisal. In: Solid Earth Sciences Library, 10, Advances in fission track geochronology (P. Van den haute and F. De Corte eds.), 33-46. (6) DE CORTE F , BELLEMANS F , VAN DEN HAUTE P., INGELBRECHT C. AND NICHOLL C. (1998) A new U doped glass certified by the European Commission for the calibration of fission-track dating. In: Solid Earth Sciences Library, 10, Advances in fission track geochronology (P. Van den haute and F. De Corte eds.) 67-78. (7) IWANO H., KASUYA M., DANHARA T. YAMASHITA T. AND TAGAMI T. (1993) Track counting efficiency and unetchable track range in apatite. Nucl. Tracks Radiat. Meas. 21, 513-517 (8) JONCKHEERE R. (1995) De absolute ouderdomsbepaling van apatiet gebaseerd op uranium-fissiesporen. Ph.D. Thesis, University of Gent, 504 pp. (9) JONCKHEERE R. and VAN DEN HAUTE R (1996) Observations on the geometry of etched fission tracks in apatite: Implications for models of track revelation. Amer. Mineral. 81, p. 1476-1493. (10) JONCKHEERE R. and VAN DEN HAUTE R (1998) On the frequency distributions per unit area of the dimensions of fission tracks revealed in an internal and external mineral surface and in the surface of an external detector. Rad. Meas. 29, 135-143 (11) JONCKHEERE R. and VAN DEN HAUTE R (1999) On the frequency distributions of the projected and etchable lengths of surface-intersecting fission tracks: Influences of track revelation, observation and measurement. Rad. Meas. 30, 155-179. (12) HURFORD A J . and GREEN R F (1983) The zeta age calibration of fission track dating. Isot. Geosci. 1, 285-317. (13) GREEN P.F. (1985) Comparison of zeta calibration baselines for fission-track dating of apatite, zircon and sphene. Chem. Geol. (Isot. Geosc. Sect.) 58, 1-22. (14) GLEADOW AJ.W., DUDDY I.R., GREEN R F and LOVERING J . F (1986) Confined fission track lengths in apatite: a diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol. 94, 405-415. (15) GREEN R F , DUDDY I.R., LASLETT G.M., HEGARTY K.A, GLEADOW AJ.W. and LOVERING J . F (1989) Thermal annealing of fission tracks in apatite. 4. Quantitative modelling techniques and extension to geological timescales. Chem. Geol. (Isot. Geosc. Sect.) 79, 155-182. (16) GREEN P.F. (1988) The relationship between track shortening and fission track age reduction in apatite: combined influences of inherent instability, annealing anisotropy, length bias and system calibration. Earth Planet Sci. Lett. 89, 335-352. (17) BALESTRIERI M.L., BIGA2ZI G., BOUKA V , LABRIN E., HADLER NETO J.C., KITADA N., OSORIO A., POUPEAU G., WADATSUMI K. and ZUNIGA A. (1998) Potential glass age standards for fission-track dating: An overview In: Solid Earth Sciences Library, 10, Advances in fission track geochronology (P. Van den haute and F De Corte eds.), 287-304.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

ANNEALING O F F O S S I L AND I N D U C E D F I S S I O N T R A C K S IN T I T A N I T E P U Z Z L I N G

FIRST

RESULTS

R.Jonckheere and G.Wagner Max-Planck-Institute Fiir Kernphysik, Heidelberg, Germany

Annealing of fission tracks in titanite is not a continuous process but proceeds in three stages: [1] an initial stage of rapid annealing, [2] a second, plateau-stage and [31 a third stage at which annealing resumes at a rate that increases with the annealing temperature. The position of the plateau stage is independent of the annealing temperature. The spontaneous and induced tracks show different annealing behaviour with respect to the position of the plateau stage. As far as the track densities are concerned, this apparent difference might be an artefact, resulting from normalisation. The fact that the same trend appears in the length data, that have not been normalised, leads us also to interpret the different density reduction of spontaneous and induced tracks as a real trend. Titanite undergoes phase transitions at 215°C and between 530°C and 630°C [1, 2]. The intermediate phase is either not a real phase, but consists of short-range-order domains that gradually merge into a new phase, or it is a real thermodynamic phase, that undergoes a continuous phase transition at the high-temperature limit [2]. This gradual transition could interfere with the annealing of the fission tracks over a finite length of time. DTA have identified two exothermic stages (300°-500°C and 500°800°C) in titanite that have been related to the annealing of the alpha-recoil tracks [31. A sharp increase in the XRD peak between 600°C and 700°C has also been ascribed recoil-track annealing [31. A contraction of the recoil-track diameter takes place from 400°C upwards, followed by a decrease of the recoil-track density between 500°C and 600°C and complete annealing at 700°C [4]. The annealing of recoil damage in titanite thus overlaps the interval of fission-track annealing. Interference between both processes may account for the observed trend in the reduction of track lengths and densities. On the other hand, this trend could also arise from the nature of the lattice damage along the tracks themselves [5]. However it then remains to be explained why the fossil and induced tracks show different annealing behaviour The relationship between track length and density shows an initially proportional decrease up to the break in slope at 35% reduction, followed by a rapid decrease of track density without a corresponding shortening of track length. The initial relationship is not 1:1. The fact that part of the tracks escape observation is a significant but maybe not the only reason for this deviation. Anisotropic track length reduction could account for the remaining deviation, although confined track length measurements provide no evidence of significant length anisotropy in titanite in this interval. The break in slope and the trend below the break are probably due to unetchable gaps. The fission-track ages for pairs of simultaneously annealed samples define a plateau. Their mean age (31.8±0.6 Ma) is however -15% higher than the known age of the Fish Canyon tuff titanite. Of the three potential sources of systematic error, [1] the absolute thermal neutron fluence ([), [2] neutron absorption or shielding, and [31 Ps/Pi, the latter appears a priori the more likely. The factors that can cause the measured values of p/pj to be systematically in error are diverse, but, insofar that they are understood, they cannot explain how p/p^ can be overestimated over the entire range of annealing conditions. It is therefore conceivable that the p/pj-values are accurate, when corrected for track length, and that other potential sources of error must be re-examined. (1) SALJE E.K.H., SCHMIDT C. and BISMAYER U. 1993. Structural phase transition in titanite CaTiSi05: A raman-spectroscopic study. Phys. Chem. Min., 19, 502-506. (2) ZHANG M., SALJE E.K.H., BISMAYER U., UNRUH H.G., WRUCK B. and SCHMIDT C. 1995. Phase transition(s) in titanite CaTiSi05: An infrared spectroscopic, dielectric response and heat capacity study. Phys. Chem. Min., 22, 41-49. (3) VANCE E.R. and METSON J.B. 1985. Radiation damage in natural titanites. Phys. Chem. Min., 12, 255-260. (4) LUMPKIN G.R., EBY R.K. AND EWING R.C. 1991. Alpha-recoil damage in titanite (CaTiSi05): Direct observation and annealing study using high resolution transmission electron microscopy. J. Mater. Res., 6, 560-564. (5) DARTYGE E. DURAUD J.R LANGEVIN Y and MAURETTE M. 1981. New model for particle tracks in dielectric minerals. Phys. Rev. B23, 5213-5229. Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronoiogy

F T 2 ^ E E

A NON-STANDARD PROCEDURE FOR THE ANALYSIS OF EXTERNAL-DETECTOR SAMPLES INTRODUCED AT THE HEIDELBERG FISSION-TRACK LAB R.Jonckheere and G.Wagner Max-Planck-Institute Fiir Kernphysik, Heidelberg, Germany

The Standard procedure for counting the fossil and induced tracks in fission-track dating with the external detector method consists in fixing the grain mount and external detector, track side up, next to each other on a microscope slide, and to switch back and forth between the grains, for counting of the fossil tracks, and the prints, for counting the induced tracks. They form mirror images, and finding the exact position on the external detector involves a complicated co-ordinate transformation and requires reference points on the mount and external detector. The task of switching between a grain and its print has now been taken over by computer-controlled motorised stages and dedicated software, but, in our experience, an exact match between the areas to be counted in the grains and prints is often hard to achieve. A simple alternative, tested at the Heidelberg fission-track lab, is described here. The principle is simple, even obvious. It must have been tried before, although we didn't find it mentioned in the literature. The muscovite external detector is repositioned, track side down, on the grain mount after etching the induced tracks. The detector is manipulated into the exact same position as during the irradiation with cotton-tipped sticks at a magnification of -lOOx. The fossil tracks are counted by focussing on the grain surface through the external detector, and the induced tacks by focusing on the underside of the detector itself. Even without spacers (cover glasses, which were used at the start), the distance between both is sufficient for there to be no interference between both images. There is some deterioration of the microscope image at high magnification but it is not disturbing when using a thin (<50 \xm) detector and an objective with cover-glass correction. This simple method has certain practical advantages: (1) there is no need for reference points; in particular in standards the mounds around the reference points tend to locally deteriorate the contact between mount and detector during later irradiations; (2) the reliance on motorised stages and software is reduced and there is no need for a cumbersome calibration procedure; (3) it is possible to count the fossil and induced tracks in exactly matching areas (Fig. lA-C); this is sometimes difficult to achieve with the standard mirror-image method in grains with an inhomogeneous uranium distribution (Fig. 2A -C), and can lead to serious errors; (4) the attainable positioning precision is very high; Figure 3, for example, shows the exact alignment of the induced tracks in an annealed and irradiated test-sample and the continuation of the same tracks in a co-irradiated external detector; (5) we observed that in rare grains the distributions of fossil and induced tracks did not seem to match; we suspect that this is due to uranium-rich inclusions/areas in the part of the grain that was removed by polishing; with this procedure such mismatches can be detected at a glance and possible errors can so be avoided; (6) the analysis of low-uranium samples becomes much easier if the detector can be positioned accurately; this is possible by using grains richer in uranium as a reference or by spiking the grain mount with a few zircon grains. Two other minor changes proved worthwhile: (1) small shards of the CN5 uranium-glass were embedded in the same mounts as the apatite age-standards (Fig. 4); this reduces the amount (activity) of glass that is irradiated, it avoids the necessity of handling naked irradiated glasses and increases the number of samples that can be packed in the irradiation container; at the same time it ensures close contact between the glass and the age standard and simplifies the determination of the ^-factor (same detector); (2) a final polishing step with 0.04 jam Si02-suspension has been added on to the polishing procedure (1/4 |im diamond paste); this ensures microscopically smooth apatite surfaces, even after track revelation (Figures 1, 2, and 5); in Fig. 5B defects intersecting a {l,0,T,0}-surface are seen revealed as triangular etch pits; the track openings are also triangular. Thus neither sharp polishing scratches nor elongated hexagonal track openings are characteristic of all prismatic surfaces. Geological Society of Australia - Abstracts Number 58

FT2^ee

Apatite grains With homogeneous ffiguje 1] and Inhomogeneous uranmm distribution [Figure 2] fccussed on gmin [A] and on detector [8] composfte miage [C] Figure 3. track-to track match mg in apatite and detector Figure 4 glass embedded with apatite age-standafd. Figure 5. prismatic apatite surfaces alter track, rsveiation.

International Conference on Fission Track Dating and Thermochronology

THE K T B

F T 2 '

APATITE F I S S I O N - T R A C K P R O F I L E : T H E SIGNIFICANCE OF B O R E - H O L E IN F I S S I O N - T R A C K

DATA

ANALYSIS

R.Jonckheere and G.Wagner Max-Planck-Institute Fiir Kernphysik, Heidelberg, Germany

The KTB is the most extensively sampled borehole to date and offers a unique opportunity for testing the predictions of fission-track thermal-history analysis. The apatite fission-track age and length versus depth profiles are at first glance characteristic for rapid uplift at the end of the Cretaceous followed by a period of tectonic stability lasting till the present. There is a well-developed partial annealing zone in situ topped by what appears to be a total stability zone. Contrary to these initial appearances, the assumption of tectonic stability is not in perfect agreement with the numerical predictions of modern thermal-history analysis. This leads us to consider two alternatives. (A) If a different geological scenario gives a better fit to the data, does it agree with the independent geological evidence? (B) If the scenario of tectonic stability can at all be maintained, what are the implications for fissiontrack thermal-history analysis, and are these supported or contradicted by independent methodical evidence? A different geological scenario does provide a better fit to the apatite profiles, and must involve all four geological processes identified before [1,2]: an uplift episode, a burial episode, crustal stacking in the top section of the profile and a drop of the geothermal gradient. The methodological difficulties with this scenario are: (A) a significant drop of the geothermal gradient must be assumed at the time that crustal slabs from greater depth replace the cooler top section of the profile, (B) the confined track length distributions in the upper section of the profile, that is in essence a stack of uplifted partial annealing zones, do not clearly show a mixed or bimodal character. Thus, although standard fission-track analysis provides a unique solution, it is not one that sits comfortably with the independent evidence. The scenario of uplift at the end of the Cretaceous followed by tectonic stability can also be maintained, provided that we assume that, at low temperatures and over geological periods of time, there can take place a limited reduction of the length of fossil fission tracks in apatite without a proportional reduction of the fission track age [31. This implies a process of ineffective annealing, operating in parallel with the process of effective annealing known from annealing experiments. A re-examination of the Otway data of Gleadow and Duddy [31 together with the KTB-data [1,2,4], indicates that there probably exists such a process. The independent methodical evidence also supports the existence of a process of ineffective annealing, and the arguments against it [5] are inconclusive. The KTBdata further suggest that the mean length of fossil fission tracks, annealed in the natural environment over a geological period of time, can decrease below ~8 |im without the tracks breaking up into segments separated by unetchable gaps. There is no hard methodical evidence in support of this assumption, and the case rests essentially on the absence of evidence to the contrary and on the agreement in the KTB-profiles between the measured ages in the lower part of the partial annealing zone and the calculated ages for tliis scenario. This discussion underlines the crucial importance of borehole data for the further development of apatite fission-track analysis. In the framework of the International Continental Scientific Drilling Program (ICDP), deep boreholes are being drilled in rocks of different ages and in different tectonic settings. The KTB, precursor to the ICDP, has been important in broadening our understanding of fission-track annealing. The new boreholes will no doubt provide new insights in the low-temperature annealing of fission tracks over geological periods of time as well as raise new questions, and will contribute to establishing a more secure basis for fission-track thermal-history analysis.

Geological Society of Australia - Abstracts Number 58

FT2 (1) COYLE D.A., WAGNER G.A., HEJL E., BROWN R. and VAN DEN HAUTE P. 1997. The Cretaceous and younger thermal history of the KTB site (Germany): apatite fission-track data from the Vorbohrung. Geol. Rundsch. 86, 203-209. (2) WAGNER et al 1997. Postvariscan thermal and tectonic evolution of the KTB site and its surroundings. J. Geophys. Res. 102, 18221-18232 (3) GLEADOW and DUDDY 1981. A natural long-term track annealing experiment for apatite. Nucl. Tracks Radiat. Meas. 5, 169-174. (4) HEJL, E., COYLE, D., NAND LAL, VAN DEN HAUTE, R AND WAGNER, G.A. 1997. Fission-track dating of the western border of the Bohemian massif: thermochronology and tectonic implications. Geol. Rundsch. 86, 210-219. (5) GREEN 1988. The relationship between track shortening and fission-track age reduction in apatite: combined influences of inherent instability, annealing anisotropy, length bias and system calibration. Earth Planet. Sci. Lett. 89, 335-352.

International Conference on Fission Track Dating and Thermochronology FISSION-TRACK ANALYSIS APPLIED TO THE CATALAN COASTAL RANGES ( N E SPAIN): TIMING AND QUANTIFYING UPLIFT AND DENUDATION RATES AND UNRAVELING THE TECTONIC EVOLUTION

JJuez-Larre and RA.M.Andriessen

Isotope and Geochemistry, Faculty of Earth Sciences, Free University, Amsterdam,The Netherlands

The Iberian microplate collided against the Eurasian plate during the Upper Cretaceous and after a Mesozoic extensional period. The Catalan Coastal Ranges (CCR), located at the most northeastern part of the Iberian plate, record the complex tectonic evolution of this convergence (Alpine orogen). After this compressional phase (during Upper Cretaceous-late Paleogene) the area underwent an extensional phase related to the west-Mediterranean rifting (during late Oligocene-Miocene). The present day topography is a surface expression of the compressional and extensional forces related to plate tectonics since Mesozoic times. The CCR are composed of two NE-SW oriented mountain chains. They include several similarly oriented Neogene basins. The Pyrenees and its foreland basin (Ebro Basin) forms the northwestern boundary, while the southeastern boundary comprises the submerged Valencia Trough. To the southwest the CCR are linked with the NW-SE oriented Iberian range (Fig. 1). Two mam fault directions affect the area: longitudinal (NE-SW) and transversal (NW-SE), some of the faults have been active since Hercynian time. These basement faults subdivide the ranges into structural units and control the movements during the entire tectonic evolution. Fission-track analysis has been applied in order to obtain information on vertical block movements, thereby unraveling the tectonic and topographic evolution of the CCR in relation to the plate tectonic events.

Geological Society of Australia - Abstracts Number 58

FT2 Forty samples are collected transversally to the CCR, one series from the coast, Barcelona City land inwards and another perpendicular to the Priorat horst (Fig. 1). For FT analyses basement rocks and samples comprising lithologies from late Paleozoic, Paleogene and Neogene sediments, were collected. Apatite FT ages from the Barcelona basement vary from ~92 to ~40 Ma inland with mean track length between 12.76-13.5 microns. The ages around 92 Ma are interpreted to record the first alpine impulse during the Upper Cretaceous. Thermal modelling (Montetrax, Gallagher., 1995) shows that during the uppermost Cretaceous and Paleogene, at the time the Pyrenees were built, cooling rates decrease, indicating a less intense tectonic phase. FT apatite ages between 55 and 40 Ma suggest that the block movement was reactivated during the Eocene, creating a large NE-SW massif oblique to the Pyrenees. The unroofing of this massif (pre-existent CCR) during Eocene is reported by the presence of large alluvial fans situated in the northwest of the CCR and infilling the Ebro basin. Samples from one of the largest alluvial fans (some 500 meters in profile) yielded FT ages between 40 and 50 Ma, close to the Eocene rapid cooling event, indicating that those fans were not significantly buried afterwards. Regarding to the West-Mediterranean rifting, basement samples don't display record of this event indicating that since late Oligocene-Miocene, uplift and denudation must have been less than ~2 kilometres (assuming a geothermal gradient of 25°C/km). To the southwest, the Priorat cross-section records an older history. The basement rocks yield similar FT ages of about 170-160 Ma (mean track length between 12.92-12.85 microns). There is evidence for Triassic differential block movements in the area related to the large Mesozoic extension (Anadon et al, 1979). This differential subsidence compartmentalized the basement in several sedimentary basins that were infilled by fluvial Bundsandstein facies. However, thermal modelling of basement rocks only shows a rapid Late Triassic-Upper Jurassic cooling rate that decreased from late Jurassic until late Eocene. From late Eocene-Oligocene a fast cooling is detected again, indicative for a compressive phase during the Alpine orogeny, as has been detected in the Barcelona cross-section. The Eocene rapid cooling continues until the present-day. The FT apatite data do not show evidence for the West-Mediterranean rifting (upper Oligocene- Lower Miocene until present), for the same reason as explained for the Barcelona area. A particular interest is given to the Bundsandstein facies outcropping at different altitudes (maximum difference of altitude is ~700 meters) in the Priorat horst, presenting a dome structure. The apatite FT ages obtained, so far range from 192 to 156 Ma (younger than the stratigraphic age) and mean track length between 12.52-12.17 microns, with one low age of -105 Ma (11.4 microns), suggesting (partial) annealing of the sediment. The single crystal FT analyses shows a large spread in grain ages and decomposing the age population, using Gallagher's Mix program (Sambridge, 1995) yields two age groups for all samples: ~140 Ma and -180 Ma. A first thermal modelling attempt is performed in order to compare the result to the cooling history of the surrounding basement. A similarity between the sediment and the source rock become obvious but further analysis is needed. Anadon, P., Colombo, F., Esteban, M., Marzo, M., Robles, S., Santanach, P., and Sole Sugranes, Ll. 1979. Evolucion tectonoestratigrafica de la Catalanides. Acta Geologica Hispanica, Homenatge a Lluis Sole i Sabaris, v. 14, 242-270. Gallagher, K. 1995. Evolving temperature histories from apatite fission-track data. Earth Planet. Sci. Lett. 136, 421-35. Sambridge, M.S. and Compston, W. 1994. Mixture modelling of multi-component data sets with application to ion-probe zircon ages. Earth and Planetary Sciences Letters 128, 373-390.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

KINEMATICS AND DYNAMICS OF TOPOGRAPHY DEVELOPMENT IN THE CATALAN COASTAL RANGES

JJuez-Larrei J.M. Gaspar-Escribano^, M. Steehouweri, S.A.P.L. Cloetingh2, P.A.M.Andriesseni,TJ. Dunaii and M. ter Voorde^ 1 Isotope and Geochemistry Department, Faculty of Earth Sciences, Free University, Amsterdam,The Netherlands 2 Tectonic department, Faculty of Earth Sciences, Free University, Amsterdam,The Netherlands

The Catalan Coastal Ranges, located in the NE of Spain, represent the NE-SW rift flank of the CatalanBalearic basin. After Mesozoic extension there was a compressional stage during the Paleogene. Since late Oligocene the Catalan Coastal Ranges were submitted to extension again. Nowadays, the extension is still ongoing as is indicated by present seismic activity, subrecent alkaline basaltic volcanism and significant vertical movements. The aim of this project is to quantify erosion and denudation rates as an interplay between climate and tectonics in the Catalan Coastal Ranges. This will be done by looking at the geomorphic evolution of the Catalan Coastal Ranges and the infilling of the adjacent basins. In order to determine rates of these short and long-term processes different methods will be applied in this project: (1) Fission track thermochronology for long term processes (2) Surface exposure age dating for short term processes (3) (U-Th)/He thermochronometry for long and short term processes The results of these studies will be integrated as constraints in (4) 3D dynamic numerical modelling of the Catalan Coastal Ranges

Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

F T 2 '

THERMOCHRONOLOGY OF THE TORLESSE ACCRETIONARY COMPLEX, NEW ZEALAND

PJJ.Kamp Department of Earth Sciences,The University of Waikato, New Zealand

The Torlesse Complex comprises several Late Paleozoic-Mesozoic accretionary prism complexes together forming continental basement over large parts of New Zealand. Previous studies of the Torlesse rocks have concluded that the complex formed at successive convergent plate margins, and that it probably includes several distinct terranes. In its lithological and structural characteristics, the Torlesse Complex is similar to other large sandstone-dominated accretionary complexes in the circumPacific region, including the Franciscan Complex in California and the Shimanto Belt in Japan. A current issue with respect to the Torlesse and associated rocks is the timing within the Cretaceous when subduction terminated along the Pacific margin of New Zealand. This study focuses on the thermal history of relatively low grade greywacke rocks exposed in a transect in southern North Island that crosses the structural grain of the Torlesse Complex, including its older and younger parts, but reference will also be made to windows into the Torlesse in Raukumara Peninsula (North Island) and parts of Canterbury (South Island). Zircon fission track (FT) ages for the Late Triassic Rakaia Terrane near Wellington, which is the most inboard of the accretionary complexes, are partially annealed, some possibly reset, and indicate early Cretaceous (134 ± 10 Ma) cooling from maximum temperatures (Tj^ax)^ probably related to amalgamation of the Pahau and Rakaia Terranes. Numerical modelling of the zircon FT ages and published "^^Ar/^^Ar muscovite and biotite ages for the Rakaia Terrane suggest T ^ ^ values of 265-320°C and exhumation from depths of 10 12 km. Zircons from sandstone blocks within the Rimutaka Melange have similar FT ages as those in the Rakaia Terrane, and may have experienced similar T ^ ^ values. The rocks underlying the Aorangi Range farther to the east and involving the youngest accretionary complex (Waioeka Terrane), have experienced much lower values of <210° and >110°C, bracketed by reset apatite FT ages and detrital zircon FT ages. The occurrence of a c.lOO Ma component of zircon FT ages in both the weakly and highly indurated rocks beneath the Aorangi Range, as well as in remnants of an overlying Albian accretionary slope basin (Whatarangi Formation), imply multistorey accretion and incorporation of sediment into the youngest prism. This c.lOO Ma zircon FT age component also places a maximum age on the termination of formation of the sparsely fossiliferous prism, and thus termination of the subduction of Phoenix Plate beneath the New Zealand region; a minimum age on this event of 70 ± 4 Ma is indicated by a component of zircon FT ages obtained from a greywacke sample adjacent to an undeformed lamprophyre dike. The young c.lOO Ma detrital zircon FT age component indicates that the rocks in the Aorangi Range are better classified as Omaio facies of the Waioeka Terrane, rather than Pahau Terrane, as assumed previously. This change emphasises the North Island extent of the Waioeka Terrane, being the least exhumed of the Torlesse accretionary complexes. The occurrence of reset apatite FT ages across the whole of the Wellington transect indicates at least 4 km of exhumation occurred during the Miocene. The greater total exhumation of the more inboard terranes is a pattern that was imposed by Cretaceous tectonics and exhumation. The results of this study suggest that the application of apatite and zircon fission track thermochronology can advance considerably understanding about the origin and evolution of accretionary complexes. 1) In sparsely fossiliferous sequences the ages of detrital zircons reflect the cooling history in source areas and provide maximum depositional ages, which in the youngest part of the prism constrains the age of termination of subduction. 2) The distribution of zircon grain ages from different parts of a prism can demonstrate contemporaneous underplating, accretion at the toe of the prism and deposition upon the prism in accretionary slope basins. This multi-storey development of a prism arises fundamentally from the lateral accretion processes. 3) More inboard parts of Geological Society of Australia - Abstracts Number 58

FT2 prisms can show zircon FT parameters that are partially or totally overprinted, yielding information via modelling about the maximum paleotemperatures experienced, and the timing of cooling, which can relate to the amalgamation of terranes. 4) Apatite FT parameters derived for the same samples as analysed for zircon, will, in exhumed accretionary prisims, usually reveal information about the paleotemperatures experienced, and hence constraints on the amounts of exhumation, and the timing of the last major phase of exhumation. This may relate to the contemporary subduction regime, or a later tectonic environment and orogeny. 5) Where stratigraphy is preserved, such as in forearc basins, changes in the geothermal gradient, arising for example from the termination of subduction, can be established from the interpretation of apatite FT parameters.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

T H E POTENTIAL U S E OF SECONDARY URANIUM M I N E R A L S IN WEATHERING GEOCHRONOLOGY

S. Keay and P. Vasconcelos Department of Earth Sciences, University of Queensland, St Lucia, Queensland, Australia

The application of radiogenic isotope dating techniques to aid our understanding of earth surface processes has led to the emergence of a new field of research that can be broadly described as weathering geochronology. Currently, most weathering geochronology focuses on K-bearing minerals because of their abundance in the surficial environment. Preliminary results show that an interesting pattern of global weathering patterns is emerging, with episodic precipitation of minerals in weathering profiles over time-scales of more than 70 million years (Vasconcelos, 1999). There is a growing need to expand the range of dateable minerals from the mantle of weathered rock covering the Earth's continents, both to test new theories of land surface development and to aid reconstruction of past global environmental conditions (Koch, 1998). The occurrence of naturally formed secondary uranium minerals in weathering profiles suggests that these phases are suitable for weathering geochronology. The development of timescales for such minerals will supply information on weathering profile development and landscape evolution as well as constraining the mobility of uranium through time in the natural environment. The formation of secondary uranium minerals is controlled by the mobility of uranium in the surficial environment. In the Earth's crust uranium often occurs in the form of minerals such as uraninite (UO2) and coffinite (USi04), which contain relatively insoluble making these minerals stable until they undergo oxidation in the surficial environment where converts to 11^+. Secondary uranium minerals form in the surficial environment by oxidation of primary uranium minerals. For this reason secondary uranium minerals are mainly concentrated in close proximity to uranium ore deposits, although they also occur associated with U-rich granites. Uranium is a strongly lithophile and oxyphile element and is never observed in the native state, instead forming the highly mobile uranyl ion (1102^"^) according to the reaction: U02(s) + H2O + l / 2 0 2 ( a q ) ">

+ 2(OH>

The uranyl ion forms stable complexes with carbonate, sulfate, fluoride and chloride (effective uranyl chelating compounds), making uranium one of the most soluble and mobile trace elements in the surficial environment (Smith et al., 1982). Leaching of primary uranium compounds leads to the formation of secondary uranium minerals as pseudomorphous replacements, in veins and as surface coatings forming hydrous compounds of uranium. The distribution of secondary uranium minerals may form a zoned pattern relative to the land surface and ground water table, possibly reflecting drops in the paleowater table. Dating each level of secondary uranium mineral concentration would allow the migration of weathering "fronts" to be traced out in the surficial environment (McKenzie et al, 1992) as well as providing direct evidence of the rates at which weathering processes operate. Secondary uranium minerals may provide valuable information on the timing and rate of uranium mobility in the geologic past. Their time of formation can be directly related to the geochemical evolution of weathering profiles, and can hence be used to increase our understanding of landscape evolution and the geochemistry of ancient environments. Certain environmental conditions are able to cause intense uranium mobilisation and precipitation, e.g. humid temperate climates in the early stages of erosion, or under arid conditions in mature landscapes (Dall'Aglio, 1974; Langford, 1978). The high solubility of some secondary uranium minerals makes them ideally suited to dating subtle climatic transitions, particularly the onset of aridity in the Late Tertiary history of Australia (Kemp, 1978). In addition, the distribution of different secondary uranium minerals can yield information about the transport distance of different elements associated with uranium such as Au and Cu.

Geological Society of Australia - Abstracts Number 58

FT2 Although secondary uranium minerals are amenable to dating (Wendt and Carl, 1985), their time of formation has rarely been related to climate and geomorphology (for a notable exception, see Dill, 1985). The geochronology of these minerals could provide a useful tool for studying pathways of meteoric solution migration in U-bearing weathering profiles. Understanding these fluid migration paths and the history of dissolution-reprecipitation of secondary uranium minerals in the surficial environment has important implications in the consideration of safe mechanisms for disposing nuclear waste. A thorough understanding of uranium geochemistry at low temperatures, over lengthy timescales and the impact of different climatic conditions is required to predict areas of extreme uranium mobility. An improved understanding of the conditions, timing and rates of formation of low temperature minerals during weathering is essential to constrain past climates, develop models of landscape evolution and trace element mobility in the surficial environment, all important aspects of weathering geochronology. Dall'Aglio, M., Gragnani, R., and Locardi, E. 1974. Geochemical factors controlling the formation of the secondary minerals of uranium. In: Formation of uranium ore deposits, 33-48. Dill, H. 1985 Genesis and timing of secondary uranium mineralization in northern Bavaria (F.R. of Germany), with special reference to geomorphology. Uranium 2 (1), 1-16. Kemp, E.M. 1978. Tertiary palaeogeography and the evolution of Australian climate. In: Archer, M. and Clayton, G. (eds) Vertebrate zoogeography and evolution in Australasia (animals in space and time), Hesperian Press. Carlisle, West Aust., Australia, 61-67. Koch, P. 1998. Isotopic reconstruction of past continental environments. Annual Reviews of Earth and Planetary Sciences 26, 573-613. Langford, F.F. 1978. Mobility and concentration of uranium in arid surficial environments. In: Uranium Deposits, Their Mineralogy and Origin (ed. M. M. Kimberley), pp. 383-394. McKenzie, A.B., Scott, R.D., Linsalata, P. and Miekeley, N. 1992. Natural decay series studies of the redox front system in the Pocos de Caldas uranium mineralization. Journal of Geochemical Exploration 45 (1-3), 289-322. Smith, D.K., Scheetz, B.E., Anderson, C.A.F., and Smith, K.L., 1982. Phase relations in the uranium-oxygen-water system and its significance on the stability of nuclear waste forms, Uranium 1(1), 79-110. Vasconcelos, P.M. 1999. K-Ar and ^^Ar/^^Ar Geochronology of Weathering Processes. Annual Reviews of Earth and Planetary Sciences, 27, 183-229. Wendt, I. and Cari, C. 1985. U/Pb dating of discordant 0.1 Ma old secondary U minerals. Earth and Planetary Science Letters 73, 278-284.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

A N E W M U L T I K I N E T I C C A L I B R A T I O N F O R F I S S I O N - T R A C K ANNEALING IN A P A T I T E R.A. K e t c h a m i , R.A. D o n e l i c k ^ a n d W.D. C a r l s o n i 1 Department of Geological Sciences, University of Texas, Austin, TX, USA 2 Department of Geology and Geological Engineering, University of Idaho, Moscow, ID, USA

We have collected an extensive new data set of 434 new laboratory annealing experiments for induced confined horizontal fission tracks in apatite (Carlson et al, 1999). The experiments were performed under tightly controlled heating and etching conditions on 15 different apatites spanning a wide compositional range. The experiments were conducted so that the scientist measuring the length distributions had no knowledge of the annealing conditions. Only TINT fission tracks were utilized. These data form the basis of a new apatite annealing model intended for use in determining timetemperature paths based on fission-track ages and length distributions, that also accounts for changes in kinetic behaviour stemming from compositional variability. Our approach incorporates a number of new methods and discoveries. First, we use an empirical conversion that projects randomly oriented horizontal confined fission tracks onto a uniform crystallographic orientation, parallel to the apatite c-axis (Donelick et al, 1999). This procedure eliminates a number of uncertainties that arise from the use of mean lengths, including the increase in angular anisotropy as annealing progresses. Non-elliptical length reduction of high-angle tracks at advanced stages of annealing suggests that they may shorten by a different mechanism than occurs for lesserannealed or lower-angle tracks; use of c-(^x/5-projected fission tracks eliminates this second mechanism from consideration. It is preferable that the annealing equation only has to describe a single mechanism. Second, we found that the relative annealing behaviour of any two apatites on laboratory time scales can be modelled extremely well with a simple two-parameter equation (Fig. 1), which may even simplify to one parameter (Ketcham et al., 1999). Annealing models simultaneously fit to a number of apatites using this relation yield very similar predictions on geological time scales to individual models fit to each apatite. Thus, we infer that there is no practical penalty to assuming that the conversion equation holds over geological time scales. We calculate sets of parameters for all of the apatites in this study for both mean and c-axis-projecied lengths, allowing them all to be fitted to an annealing model simultaneously. The fitted parameters can be related to measurable factors, such as composition or etching characteristics, that allow the annealing model to be specifically configured to any type of apatite. Third, we derive revised and somewhat simplified statistical fitting methods that explicitly incorporate measured and estimated errors. In particular, we use a chi-squared minimisation technique that utilizes measurement uncertainty and estimated uncertainty in temperature determination, which can be of comparable or greater magnitude in highly annealed populations. Fourth, we select our preferred model based not only on statistical goodness-of-fit, but also on how well it matches well-characterized geological observations. There are few data sets that are optimal for this purpose, as they require fission-track data to be combined with additional measurements that allow the kinetic behaviour of each apatite analysed to be estimated, and they require independent knowledge of the geological and thermal history of their environment. High-temperature benchmarks can be used to constrain the conditions under which fission tracks become fully annealed, while low-temperature benchmarks are useful for quantifying the amount of annealing expected at near-surface conditions. The model selected based on these criteria was not a "fanning Arrhenius" model (e.g. Laslett et al, 1987; Crowley etal, 1991; Laslett and Galbraith, 1997), but one in which contours of constant annealing have some curvature (Fig. 2). This curvature may be a result of fission-track defects having a Geological Society of Australia - Abstracts Number 58

FT2 c.mod

Figure 1.

Figure 2.

continuum of activation energies for annealing. The resulting model does an excellent job of reproducing both high-temperature and low-temperature field-based data for fluorapatite, and is capable of explicitly taking apatite kinetic variability into account. Our modelling suggests that kinetic differences can cause apatite closure temperature to vary by 100°C or more, although compositions defining apatites at the high end of this range are rare.

FT2^ee We examine a number of compositional variables to see how well they serve as predictors of resistance to annealing. Our data show an unexpected lack of correlation between chlorine content and predicted closure temperature; although increased chlorine content does lead to increased resistance to annealing, the relationship shows a high degree of scatter in the apatites we studied. Certain cation substitutions, most notably iron and manganese, also produce very large changes in annealing behaviour. We also examine hydroxyl content and multi-compositional approaches to this problem. We conclude that there is not yet sufficient data to provide a model that is reliable for all cases, but the current estimate should nevertheless be useful for most. Carlson W.D. Donelick R.A. and Ketcham R.A. 1999. Variability of apatite fission-track annealing kinetics I: Experimental results. American Mineralogist 84, 1213-1223. Crowley K.D. Cameron M. and Schaefer R.L. 1991. Experimental studies of annealing etched fission tracks in fluorapatite. Geochimica et Cosmochimica Acta 55, 1449-1465. Donelick R.A. Ketcham R.A. and Carlson W.D. 1999. Variability of apatite fission-track annealing kinetics II: Crystallographic orientation effects. American Mineralogist 84, 1224-1234. Ketcham R.A. Donelick R.A. and Carlson W.D. 1999. Variability of apatite fission-track annealing kinetics III: Extrapolation to geological time scales. American Mineralogist 84, 1235-1255. Laslett G.M. Green P.P. Duddy I.R. and Gleadow A.J.W. 1987. Thermal annealing of fission tracks in apatite 2. A quantitative analysis. Chemical Geology (Isotope Geoscience Section) 65, 1-13. Laslett G.M. and Galbraith R.P. 1996. Statistical modelling of thermal annealing of fission tracks in apatite. Geochimica et Cosmochimica Acta 60, 5117-5131.

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( U - T H ) / H E AND F I S S I O N T R A C K DATING OF THE P L E I S T O C E N E RANGITAWA T E P H R A , N O R T H ISLAND, N E W ZEALAND: A COMPARATIVE S T U D Y

B.P. Kohni, K.A. Farley2 and B. Pmans3 1 School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052, Australia 2 California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125, USA 3 Research School of Earth Sciences,The Australian National University, Canberra, ACT 0200, Australia

Radiometric dating of Late Tertiary to Quaternary volcanic rocks provides critical data to address a range of important topics in the Earth Sciences, such as, the geomagnetic polarity time scale, the tempo of regional volcanism, the age calibration of stratigraphic sequences, the time frame for hominid evolution and the timing of palaeoclimate change. For volcanic deposits <100 ka the most commonly used methodologies are K-Ar and "^^Aift^Kv dating on K-bearing phases and fission track dating of glass and zircon. Although K-Ar and ^^Ar/^^Ar dating may achieve ages down to ~5 ka where suitable K-rich minerals are available (McDougall, 1995), such favourable conditions are relatively rare. Early studies employing fission track dating on volcanic rocks used rhyolitic obsidian and glass shards with seemingly good correlation to palaeomagnetic stratigraphy. However, with the gradual understanding of annealing, it was realised that most volcanic glass, even at ambient temperatures, is readily subject to track-length shortening and hence to a reduction in age (e.g. Seward, 1979; Naeser et al, 1980). In an attempt to overcome this problem the application of fission-track dating to zircons from tephra was successfully tested (e.g. Hurford et al, 1976; Seward, 1979; Gleadow, 1980). Zircon has a much higher annealing temperature than glass, is strongly resistant to weathering and fission-track dating applied to this mineral allows individual grains to be dated and assessed for suitability. Later, studies on glass applied a correction for annealing using the isothermal plateau technique (ITPFT) (Westgate, 1989) yielding ages that are in accordance with palaeomagnetic studies and which fall within the error limits (at ±lcj) of single feldspar crystal 40Ar/39Ar ages. We have analysed (U-Th)/He ages on zircon and apatite from the Rangitawa Tephra at its type section in the North Island, New Zealand to evaluate this system for the numerical dating of young volcanic rocks. Rangitawa Tephra is a widespread and well-documented mid-Pleistocene marker in marine and terrestrial sequences in New Zealand, and adjacent ocean basins (up to several lOO's km offshore) (Kohn et al, 1992; Pillans et al, 1996). It has also been the subject of several dating studies principally using the fission track technique. Previously published fission track ages from the type section are: 380±40 ka (glass) (Seward, 1976), 390±10 ka (glass) (Boellstorff and Te Punga, 1977), 370±70 ka (glass) and 370±50 ka (zircon) (Pillans and Kohn, 1981), 400±90 ka (zircon) (Kohn et al., 1992) and 370±70 ka (ITPFT on glass) (Alloway et al., 1993). In general, the fission track dating of glass and zircon from the Rangitawa Tephra yields errors (at ±la) of >10%. However, the precision may be improved, either by dating many more grains in a sample, or by dating several samples of the same tephra (at different sites), but both these approaches are very labour intensive (Kohn et al., 1996). (U-Th)/He dating of coexisting apatite and zircon from the same section yields a grand mean for four fractions (duplicate analyses for each mineral) of 370.5±4.5 ka, with an estimated accuracy of better than 2%. Each of the four fractions consisted of about 20 individual grains. Our preliminary data reveal that Rangitawa Tephra shows concordant ages (within analytical error) for apatite and zircon, minerals having very different Th/U ratios. This suggests no substantial 230Th/238u disequilibrium in either of these mineral phases and this is being further investigated over an age range of samples, as is the lowest age range that can be measured reliably for each mineral.

Geological Society of Australia - Abstracts Number 58

FT2^ee We anticipate that (U-Th)/He dating will complement other available methods for dating young volcanic deposits but in some cases will be the only possible high precision method available. This will specifically apply to dating of zircon, where other radiometric methods may be unsuitable due to weathering, lack of suitable minerals or where deposits are thin. The establishment of a further high precision geochronological tool for dating young volcanic rocks will be keenly sought and will complement available techniques, particularly as the subdivision of the Pleistocene using oxygen isotope stages defined in deep sea cores means that Pleistocene studies increasingly demand time resolution of at least ±20 ka (about half a 41 ka obliquity cycle) over the whole of the past ~2 m.y. Alloway, B. V., Pillans, B. J., Sandhu, A. S. and Westgate, J. A., 1993. Revision of the marine chronology in the Wanganui Basin, New Zealand, based on the isothermal fission-track dating of tephra horizons. Sedimentary Geology, 82: 299 - 310. Boellstorff J.D and Te Punga, M.T., 1977. Fission track ages and correlation of middle and lower Pleistocene sequences from Nebraska and New Zealand. N.Z.J. Geol. Geophys., 20: 47-58. Gleadow, A.J.W., 1980. Fission track age of the KBS Tuff and associated hominid remains in northern Kenya. Nature, 284: 225230. Hurford, A.J., Gleadow, A.J.W. and Naeser, C.W., 1976. Fission track dating of pumice from the KBS Tuff, East Rudolf, Kenya. Nature, 263: 738-740. Kohn, B.P., Pillans, B., and McGlone, M.S., 1992. Zircon fission track age for middle Pleistocene Rangitawa Tephra, New Zealand. Palaeogeography, Palaeoclimatology, Palaeoecology, 95: 73-94. Kohn, B.P., Pillans, B., and Alloway, B.V., 1996. Fission track age estimates for Mount Curl Tephra - Letter to the Editor (reply). N.Z. J. Geol. Geophys., 39: 333-335. McDougall, L, 1995. Potassium-Argon Dating in the Pleistocene. In, Rutter, N.W. and Catto, N.R. (eds). Dating Methods for Quaternary Deposits, Geological Association of Canada - Geotext 2, Newfoundland, p. 1-14. Naeser, C.W. , lazlett, G.A. and Obradovich, J.D. 1980. Fission-track and K-Ar ages of natural glasses. U.S. Geol. Surv. Bull., 1489, 31 p. Pillans, B. and Kohn,B.P,. 1981. Rangitawa Pumice: a widespread (?) Quaternary marker bed in Taranaki -Wanganui. Geology Department Victoria University Pub., 20: 94-104. Pillans, B., Kohn, B.P., Berger, G., Froggatt, P., Duller, G., Alloway, B., and Hesse, P., 1996. Multi-method dating comparison for mid-Pleistocene Rangitawa Tephra, New Zealand. Quaternary Science Reviews, 15: 641-653. Seward, D. 1976. Tephrostratigraphy of the marine sediments in the Wanganui Basin, New Zealand. New Zealand Journal of Geology and Geophysics, 19: 9-20. Seward, D., 1979. Comparison of zircon and glass fission track ages from tephra horizons. Geology, 7: 479-482. Westgate, J.A., 1989. Isothermal plateau fission track ages of hydrated glass shards from silicic tephra beds. Earth Planet. Sci. Letts., 95: 226-234. Acknowledgments This study was supported by the Australian Research Council.

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PHANEROZOIC T H E R M O - T E C T O N I C H I S T O R Y OF THE NORTHERN MARGIN OF THE ARABO-NUBIAN S H I E L D A C R O S S THE D E A D S E A TRANSFORM

B.R Kohni, S. Feinstein2, M. EyaP, M.S. Steckler3, K. Ibrahim^ and B. Khalil^ 1 School of Earth Sciences,The University of Melbourne, Parkville,Victoria 3052, Australia 2 Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva 84105, Israel 3 Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964, USA 4 Department of Geology,The Hashemite University, Zarqa, Jordan 5 Department of Applied Mineralogy and Field Geology, Natural Researches Authority, P.O. Box 7, Amman 111 18, Jordan

We present a preliminary attempt to reconstruct the paleoburial and exhumation history at the northern margin of the Precambrian Arabo-Nubian Shield (ANS). Shield regions are usually thought of as relatively old, thick and stable parts of the lithosphere. Yet, epeirogenic movements of considerable wave length and amplitude are a common mode of deformation found in such regions. The mechanism for epeirogeny where no major lateral deformation is apparent, is quite enigmatic. The geology of the study region is dominated by the Dead Sea Transform (DST) which intersects the northern margin of the ANS into the Arabian plate and Sinai sub-plate (Fig. 1). The southern part of the DST is occupied by the Dead Sea Rift (DSR) and its elevated margms, and is characterized by prominent structural and morphological asymmetries with a markedly greater uplift of the eastern flank (Wdowinsky and Zilberman, 1997). However, these characteristics are pertinent only since evolution of the DST, as part of the Red Sea system during the Tertiary. At earlier stages, throughout most of the Phanerozoic, the Sinai sub-plate was part of the Arabian Plate to the east. The pre-DST Phanerozoic geology is characterised by prolonged, mainly platform, sedimentation interrupted by several regional erosional events. The most prominent unconformities in the stratigraphic section are basal Cambrian, basal Permian, basal Cretaceous (Fig. 2) and in the Oligocene.

29Paleozoic lilllil Precambrian l l E H l

Figure. 1. Study area and distribution of Precambrian crystalline basement and Palaeozoic rocks in the Sinai sub-plate (Israel and Sinai) and in Jordan across the DST (modified after Weissbrod, 1981).

Geological Society of Australia - Abstracts Number 58

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N Beer-Sheva

Ramon

Elat

Basal Cretaceous truncation

Figure 2. Pre-Cretaceous sequences of the Negev and the major unconformities (modified after Weissbrod, 1969; Garfunkel and Derin, 1988).

In this study we have applied apatite fission track (FT) thermochronology in an attempt to reconstmct the paleoburial and exhumation history retained by the paleothermometric signals in the preserved rocks at the northern margin of the ANS in Israel, Sinai and Jordan across the DSR (Fig. 1). Preliminary data gathered in this study together with data from earlier studies of Precambrian crystalline basement and Cambrian formations in Sinai and southern Israel reveal three major Phanerozoic cooling events. Zircon FT data suggest temperatures attained in early-middle Palaeozoic were at least ~240°±40°C whereas models of ^^Kvft^Ax K-feldspar age spectra indicate that samples were never heated to >~240°C over times of 10^ yrs. For a similar heating time the spectrum of one crystalline basement sample within the Gulf of Suez indicates maximum temperatures of ~300°C at 350 Ma. These palaeotemperatures, combined with regional stratigraphic evidence imply that geothermal gradients may have reached >50°C/km. This thermal event was followed by significant uplift and erosion, and suppression of the palaeothermal gradient in Late Devonian-Early Carboniferous time (Kohn etal, 1992, 1997). This episode, which possibly involved ~2-3 km of erosion, appears to be the deepest regional Phanerozoic erosional event in the study area. It may mark the earliest events arising from the collision between Gondwanaland and Laurasia to form the supercontinent of Pangea. Nevertheless, this event which left a marked thermochronological signature, is not recorded in the preserved section due to removal of section during later erosional episodes (Fig. 2). Forward models of apatite FT data suggest a Late Jurassic-Early Cretaceous cooling episode. This is probably related to widespread cmstal heating, magmatism, uplift and erosion associated with the passage of the Darfur hot spot across the Levant at this time (Garfunkel and Derin, 1988; Gvirtzman and Garfunkel, 1997). The forward models obtained, in conjunction with burial history, suggest that this event was also accompanied by elevated palaeogeothermal gradients. The time-temperature models also suggest a further episode of cooling during Oligoene-Miocene time. Precambrian basement rocks in the Timna area in southernmost Israel have been exposed to maximum mid-Miocene temperatures of ~65°-70°C. Moreover, this temperature range provides an upper limit for the acquisition of a Miocene chemical remanent magnetisation in some basement rocks and the crystallisation of authigenic apatites within Cambrian sediments. These phenomena and the recorded cooling event are probably related to the initiation of the DST and associated rift flank uplift (Marco et al, 1993).

FT2 Garfunkel, Z., and Derin, B. 1988. Re-evaluation of the latest Jurassic-Early Cretaceous history of the Negev and the role of magmatic activity. Israel Journal of Earth Science 37, 43-52. Gvirtzman, Z. and Garfunkel, Z. 1997. Vertical movements following intracontinental magmatism: an example from southern Israel. Journal of Geophysical Research 102, 2645-2658. Kohn, B.P., Eyal, M., and Feinstein, S. 1992. A major late Devonian-early Carboniferous thermotectonic event at the NW margin of the Arabian-Nubian Shield: Evidence from zircon fission track dating. Tectonics, 11, 1018-1027. Kohn, B.P., Feinstein, S., Foster, D.A., Steckler, M.S., and Eyal, M. 1997. Thermal history of the eastern Gulf of Suez: II Reconstruction from apatite fission track and ^^Ar/^^Ar K-feldspar measurements. Tectonophysics, 283, 219-239. Marco, S., Ron, H., Matthews, A., Beyth, M., and Navon, O. 1993- Chemical remanent related to the Dead Sea rift: evidence from Pan African igneous rocks in Timna, southern Israel. J. Geophys. Res. 98, l6001-l6012. Wdowinsky, S. and Zilberman, E. 1997. Systematic analysis of the large topography and structure across the Dead Sea Rift. Tectonics l6, 409-424. Weissbrod, T. 1969. The Paleozoic of Israel and adjacent countries. Ministry of Development, Geological Survey Bulletin No, 47, 1-24. Weissbrod, T. 1981. The Paleozoic of Israel and adjacent countries (Lithostratigraphic study). Israel Geol. Surv. Rep. M.P. 600/81 (in Hebrew with English Abstract) 276 p. Acknowledgments This research was supported by the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel and the Australian Institute of Nuclear Science and Engineering.

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International Conference on Fission Track Dating and Thermoclironology

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PHANEROZOIC DENUDATION HISTORY OF SOUTHWEST AUSTRALIAN CRYSTALLINE TERRANES INFERRED FROM APATITE FISSION TRACK THERMOCHRONOLOGY

B.R Kohn, RB. O'Sullivan andAJ.W. Gleadow Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, University of Melbourne, Victoria 3010,Australia

We report the results of apatite fission track (AFT) analyses and numerical forward modelling to constrain the low-temperature thermal history of the southwestern Archaean Yilgarn Craton, the Proterozoic Albany Mobile Belt and the Neoproterozoic to early Phanerozoic Leeuwin Complex in southwestern Australia (Fig. 1). This approach is particularly powerful for reconstructing thermotectonic histories of the upper continental crust of crystalline terranes, especially where the use of traditional stratigraphic and structural parameters is severely limited. The sampling strategy employed involved a series of traverses that were designed to take into account earlier regional geochronological studies (Fig. 1). Traverses extended eastwards for >200 km from the western margin of the Yilgarn Craton, at approximately the latitude of Perth, and along the southern coast, covering areas where previous Rb/Sr biotite age trends have been reported by De Laeter and Libby (1993) and Libby and De Laeter (1998). These biotite ages increase eastwards from ~400-600 Ma in a 30-100 km wide western zone which widens southwards, through a 20-40 km wide transition zone, to an eastern biotite domain yielding ages of -2100-2500 Ma across the Yilgarn Craton and -1000-1130 Ma in the Albany Mobile Belt. One explanation proposed for the reset biotite ages within the western zone is in response to the later denudation of topography created by tectonic loading resulting from Pan African thrusting of Proterozoic sedimentary rocks over the southwestern Australia margin (Libby and De Laeter (1998). A further north-south traverse closely followed the western boundary of the Yilgarn Craton, marked by the Darling fault system separating the craton from the Perth basin. The basin contains >14 km of Silurian to Recent strata, but most of the fill is Upper Triassic to Lower Cretaceous. The Perth basin was formed by a series of rifting events leading to the break-up of Gondwana which culminated in the Early Cretaceous. This traverse was designed to reveal any thermotectonic affects on the Yilgarn Craton related to the development of flank topography and denudation associated with the development of the Perth basin and the southern margin of the Albany Fraser Province. The AFT data set (75 samples), yields ages ranging from 185-320 Ma and mean track lengths of ~1214 |im. The youngest samples of this data set (-185-230 Ma), which also include the shortest mean track lengths, are located: 1) on the Yilgarn Craton, within a few kms of the Darling fault, 2) in the Leeuwin Complex and 3) along the south coast of the Albany MobUe Belt. Forward modelling of data indicates that samples have generally experienced at least -40-50°C of Late Palaeozoic (Late Carboniferous-Permian) regional cooling. Thermal modelling also suggests later cooling possibly during the Late Jurassic-Early Cretaceous and/or Tertiary. These later episode/s are required by the modelling but are poorly constrained temporally since most of the cooling occurred from temperatures <60-70°C, a range over which the thermal effects of fission tracks in apatite are less sensitive. Heat flow measurements for the study area (Howard and Sass, 1964; Cull, 1982) and an assumed thermal conductivity of 2.5 W.m-i.K'i indicate a present geothermal gradient of -ll±3°C.km-i. If this thermal gradient was linear and representative of the last 250-300 m.y. then the minimum of 40-50°C of Late Palaeozoic cooling predicted by the fission track modelling suggests denudation of an average of -3.6-4.5 km of section during this episode. Assuming a similar gradient as above (and a surface temperature of 20°C), then the later event/s would in total have involved a further -3-4 km of denudation. In areas, where AFT ages are youngest and track lengths shorter, post Late Palaeozoic cooling would have occurred from slightly deeper crustal levels (-15-20°C hotter) suggesting an additional - 1 km of exhumation in this area, which was probably related to rift margin topography denudation.

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Figure 1. Locality map showing crystalline terranes studied in southwestern West Australia, distribution of apatite fission track samples, and Rb/Sr biotite domain limits (thick lines) of Libby and De Laeter (1998).

Independent evidence for regional late Palaeozoic regional cooling/denudation is supported by studies from the fault-bounded Collie, Wilga and Boyup basins located within the Yilgarn Craton -ISOISO km SSE of Perth. The largest, the Collie basin, contains -1400 m of Permian tillite and verticallystacked coal measures (Le Blanc Smith, 1993). Reported vitrinite reflectance measurements (Sappal, 1986) and the stratigraphy indicate that maximum coal burial depth was possibly up to ~95-100°C, and that several km's of the missing section were removed in the Permian, with a later erosional cycle removing more section between Permian and pre Early (but not earliest) Cretaceous time - in total -6.5 km of cover-rock (Le Blanc Smith, 1993). These observations combined with the AFT data suggest a substantial thickness of late Palaeozoic sediment extended across the crystalline rocks of the study area, and that the Collie and adjacent basins are preserved outliers of this accumulation. This possibility is supported by a recent study of detrital zircons in Western Australia placer deposits which have been recycled from Perth basin sediments (Sircombe and Freeman, 1999). These show a predominance of Mesoproterozoic to Neoproterozoic zircon ages suggesting that, despite its close

FT2 proximity, the Archaean Yigarn craton itself was not a major provenance for Perth basin sediments. Alternatively, the late Palaeozoic cooling could be related to the final removal of the Pan African related compressional topography, but the cooling cuts across all of the Rb/Sr biotite zones previously reported, making this possibility unlikely. Considering that: 1) the above estimates for removed overburden are based on present-day geothermal gradients taken from the cratonic rocks, and 2) the results/interpretation suggesting that much of the removed overburden may have involved at least some sedimentary rocks overlying the crystalline basement - then the paleogeothermal gradients would have been considerably higher within the package of sedimentary rocks. If this was the case then the calculated amount of overburden removed would have been considerably less. Killick (1998) calculated the volume of sediment deposited in basins marginal to the Western Shield (Yilgarn and Pilbara Cratons and intervening Proterozoic basins) between Early Ordovician to end Cretaceous time, after which time clastic sedimentation effectively ceased. This led to an estimate that -4.09 km of basement has been removed from the Western Shield since the onset of basin development and sedimentation in the Early Ordovician at an average denudation rate of 8.87 m/m.y. The present study indicates that denudation has been episodic and we calculate an average late Palaeozoic to Present denudation rate of -25 m/m.y. Further, consideration of the distribution of Rb/Sr biotite ages in the study area suggests exhumation from temperatures >-320°C at a considerably higher average Phanerozoic denudation rate for the western part of the area investigated (domain 1 in Fig. 1). All these denudation estimates are significantly higher than the rates of 0.1-0.2 m/m.y. previously reported for the Yilgarn Craton based on geomorphic evidence (Fairbridge and Finkl, 1980). The existence of significant regional episodic Phanerozoic cooling requires rethinking of the longterm thermotectonic stability of the Yilgarn Craton and highlights the need to further examine the role of tectonic reactivation and denudation patterns in this terrane. Cull, J.P., 1982. An appraisal of Australian heat-flow data. BMR Journal of Aust. Geology and Geophysics 7, 11-21. De Laeter, J.R. and Libby, W.G., 1993. Early Palaeozoic biotite Rb-Sr dates in the Yilgarn Craton, near Harvey, Western Australia. Australian Journal of Earth Sciences 40, 445-453. Fairbridge, R.W., and Finkl, C.W. Jr., 1980. Cratonic erosional unconformities and peneplains. Journal of Geology 88, 69-86. Howard, L.E., and Sass, J.H., 1964. Terrestrial heat flow in Australia. J. Geophys. Research 69, I 6 l 7 - l 6 2 6 . Killick, M.F., 1998. Phanerozoic denudation of the Western Shield of Western Australia. Geological society of Australia Abstracts No. 49, p. 248. Libby, W.G., and De Laeter, J.R., 1998. Biotite Rb-Sr age evidence for Early Palaeozoic tectonism along the cratonic margin in southwestern Australia. Australian Journal of Earth Sciences 45, 623-632. Le Blanc Smith, G., 1993. Geology and Permian coal resources of the Collie Basin, Western Australia. Geological Survey of Western Australia Report 38, 86 p. Sappal, K.K., 1986. Petrography of Collie Coal, Collie Basin, Western Australia. Western Australian Mining and Petroleum Research Institute Report 26, 227 p Sircombe, K.N., and Freeman, M.J., 1999. Provenance of detrital zircons on the Western Australia coastline - Implications for the geologic history of the Perth basin and denudation of the Yilgarn craton. Geology 27, 879-882.

Acknowledgments This work was funded by the Australian Geodynamics Cooperative Research Centre (AGCRC) and the Australian Institute of Nuclear Science and Engineering. Some samples/mineral splits used in this study were supplied by John Lovering (University of Melbourne), Bob Pidgeon and Alexander Nemchin (Curtin University of Technology) and Peter Fleming and Peter Jackson (La Trobe University). Some apatite mounts previously studied by Kirrian Ferguson were also reexamined in this study. Colin Patterson of the Griffin Coal Mining Company facilitated access to sampling in the Muja Open Cut of the Collie Basin. This work is published with the permission of the Director, AGCRC.

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International Conference on Fission Track Dating and Thermochronology

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ANNEALING OF FISSION TRACKS IN TITANITE BJ. Kowallis, N. Hu, D.T. Griffen and E.H. Christiansen Department of Geology, Brigham Young University, Provo, Utah, USA

Previous studies of fission tracks in titanite (CaTiSi05) suggested tliat variations in composition might affect the annealing behaviour (Naeser and Paul, 1969; Gleadow, 1978), similar to compositional effects observed with apatite. The composition of titanite is quite variable with significant substitutions occurring for Ca (Y, Mn, RPPs, Pe2+), Ti (Pe3+, Pe2+, Al, Nb, Ta), and O (P, OH). Our initial investigation centred around the substitutions of Pe and Al into the octahedral Ti-site, because they are the most common and abundant elements involved in substitution into the titanite structure. Mossbauer spectrometry shows that most of the Pe is Pe3+ with minor amounts of Pe2+ (Holenyi and Annersten, 1987). Al and Pe may occupy as much as 50% (0.5 atoms per formula unit (apfu)) of the Ti-site, although the vast majority of titanites have Al + Pe less than 0.2 apfu. The ionic radii of Pe5+ and Ti4+ are almost the same, while A13+ is about 25% smaller Therefore substitution of Al into the Ti-site has a substantial effect on reducing the average interatomic octahedral oxygen distance (Oberti et al, 1991). Coupled with the substitution of Pe and Al into the Ti-site is the substitution of OH- or P- for 02- (Pnami et al, 1993; Pranz and Spear, 1985) or the substitution of Y3+ or LREPs3+ for Ca2+, thus maintaining molecular charge balance (Pig. 1).

0.1

0.2

Fe + Al - pfu Figure 1. Substitution of trivalent Fe and Al into the Ti-site, is coupled with subsubstitution of Y and LREEs into the Ca-site, Nb and Ta in to the Ti site, and F (as well as OH) into an O-site. The alaskite sample had a higher total annealing temperature than the other samples, i = plutonic titanite, m = metamorphic titanite, v = volcanic titanite, h = hydrothermal titanite, p = pegmatitic titanite, z = miarolitic titonite.

To determine if Pe and Al substitution affected the annealing of fission tracks in titanite, we obtained titanites with a range of Pe (0 to 0.1 apfu) and Al (0.02 to 0.17 apfu) contents and ran them through a series of one-hour annealing experiments. The titanites are from 14 localities, including Magnet Cove, Arkansas (provided by C.W. Naeser) and Mount Painter, Australia (provided by A.J.W. Gleadow) previously shown to have different one-hour total annealing temperatures (625°C and 750°C, respectively). Samples were first completely annealed by heating in an oven at 750°C for 12 hours, and then irradiated to produce induced tracks. Splits from each sample were then heated for one hour at temperatures of 400°, 500°, 600°, 650°, 675°, 700°, 725°, 750°, 775°, and 800°C. Geological Society of Australia - Abstracts Number 58

FT2 In all of our samples, except one, total annealing of fission tracks occurred at about 750°C for one hour of heating and seemed to have no correlation with Fe or Al content (Fig. 2). The one titanite separate that was different came from an alaskite (leucocratic granite) from Ontario, Canada. It had a one-hour total annealing temperature of 800°C. Although this sample did have the highest total Fe+Al content (about 0.25 apfu with 0.14 apfu Al), it is not significantly different in Fe and Al from one of the other samples that had about 0.22 apfu Fe+Al (with 0.l6 apfu Al). It is unusual, however, in that it has twice as much Y+LREEs (0.11 apfu versus <0.05 apfu) replacing Ca as any other sample we studied. Both the high Fe+Al and the high Y+LREEs in this sample may effect its annealing behaviour. Future work will focus on locating and studying the annealing behaviour of additional titanites with variable amounts of substitution into the Ca-site and of samples with higher Al contents.

0

^

^

^

^

r

100 200 300 400 500 600 700 800 1 hour aniiealuig T

Figure 2. Typical one-hour annealing curve for most titanite samples in this study (open circle) where track density is reduced to zero by about 750°C. Alaskite sample (closed triangle) still has 50% of its tracks at 750°C, but is completely annealed by 800°C.

Enami M. Suzuki K. Liou J. G. and Bird D. K. 1993. Al-Fe3+ and F-OH substitutions in titanite and constraints on their P-T dependence. European Journal of Mineralogy 5, 219-231. Franz G. and Spear F. S. 1985. Aluminous titanite (sphene) from the eclogite-zone, south-central Tauern Window, Austria. Chemical Geology 50, 33-46. Gleadow A. J. W. 1978. Anisotropic and variable track etching characteristics in natural sphenes. Nuclear Track Detection 2, 105-117. Holenyi K. and Annersten H. 1987. Iron in titanite: a Mossbauer-spectroscopy study. Canadian Mineralogist 25, 429-433. Naeser C. W. and Faul H. 1969. Fission track annealing in apatite and sphene. Journal of Geophysical Research 74, 705-710. Oberti R. Smith D. C. Rossi G. and Caucia F. 1991. The crystal-chemistry of high-aluminum titanites. European Journal of Mineralogy 3, 777-792.

International Conference on Fission Track Dating and T h e r m o c h r o n o i o g y

F T 2 '

FISSION TRACK STUDIES IN NORTHERN VICTORIA LAND - PASSIVE MARGIN EVOLUTION VERSUS UPLIFT OF THE TRANSANTARCTIC MOUNTAINS F. Lisker FB Geowissenschaften, University of Bremen, Bremen, Germany

Although the evolution of most passive continental margins of Gondwana has been fairly well constrained in the last few years and the Transantarctic Mountains of southern and northern Victoria Land have become a key area for the interpretation of fission-track data, the long-term landscape development of the neighbouring Oates Land and westernmost Victoria Land has so far received little attention. Northern Victoria Land in the Pacific sector of Antarctica has traditionally been divided into three distinct units separated by NW-SE-trending faults: the Robertson Bay Terrane, the Bowers Terrane and the Wilson Terrane. (Fig. 1). These terranes comprise a metasedimentary basement made up by the Robertson Bay Group to the east (a lower Paleozoic turbiditic sequence), the Bowers Supergroup in the central part (early Paleozoic volcanic rocks with intercalated sediments) and the Wilson Group to the west (late Precambrian high-grade metamorphic rocks) The basement is intruded by two suites of granitic intrusions (the Cambro-Ordovician Granite Harbour Intrusives, and Devonian Admiralty Intrusives). After the Ross Orogeny the Devonian to Early Jurassic intracratonic Beacon Basin developed in the region. This is characterized by a sequence of glacial and alluvial sedimentary rocks. Initial fragmentation of Gondwana in the Early Jurassic was associated with magmatic activity of the Ferrar event andwas followed by a fundamental reorganisation of the regional plate pattern. The Mesozoic landscape evolution in northern Victoria Land is peculiarly complicated due to the interaction of three different modes of tectonics and exhumation: the rifting between northern Victoria Land and Australia starting at ~140 Ma, a stage of episodic continental rifting perpendicular to the evolving continental margin ('Rennick Rifting') in the mid-Cretaceous, and the formation of the West Antarctic Rift System including uplift of the Transantarctic Mountains since the Late Cretaceous. Due to these tectonic events several different structural levels of the upper crust are now exposed at the surface. Tessensohn (1994) has proposed the formation of three major structural features, roughly coinciding with the Paleozoic terranes: the Admiralty Block, Rennick Graben and Outback Shoulder (Fig. 1). The exhumation history of the Admiralty Block is controlled by the up to 6 km uplift of the Transantarctic Mountains that occurred in three stages at -125, ~95 and 50-45 Ma, with the main uplift closely related to the development of the West Antarctic Rift System (e.g. Fitzgerald and Gleadow, 1988; Fitzgerald and Stump, 1997). In contrast, little information exists about the exhumation of the Rennick Graben and Outback Shoulder. 44 apatite fission track data from the backside of the Transantarctic Mountains have been previously obtained by Fitzgerald and Gleadow (1988), Arne et al (1993), Lisker (1996) and Schafer (1998). These fission track ages, ranging from - 4 7 0 Ma on the cratonic margin to - 4 0 Ma in the vicinity of the Rennick Graben, display three regional trends. They show a more or less systematic increase (a) from the Rennick Glacier towards the margin of the East Antarctic Craton, (b) landwards from the continental margin of northern Victoria Land (?), and (c) from the major thrust zones towards the tectonically undisturbed basement of Oates Land (Fig. 2). Generally, the apatite ages of basement rocks from the Outback Shoulder range from - 4 7 0 Ma to - 2 0 0 Ma, and the mean track lengths vary between 11.5 and 13.5 pm with corresponding standard deviations between 1.5 and 2.5 pm. This pattern indicates slow cooling at this time, suggesting long-lasting regional tectonic stability in the Oates Land region between the early Paleozoic and the Early Jurassic. Only in the immediate surrounding of the - 1 8 0 Ma Ferrar extrusive rocks (and the major thrust zones) are apatite ages <250 Ma obtained. Here, partial or complete annealing of fission tracks indicates reheating to or cooling from paleotemperatures between >120° and -80°C. The Mesozoic cooling along these major structures was probably

Geological Society of Australia - Abstracts Number 58

FT2

Figure 1. Schematic map of northern Victoria Land including the geology of the basement complexes. Rennick Graben after Tessensohn (1994). The arrows display the trends of increasing apatite fission track ages given in Fig. 2.

500

(D

400 300 LO

I

100

A

iS • |dd

t 200

A

/

300

®

250 Ol

i

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Lisker, 1996

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O Schafer, 1998

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00

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• h • 111 •• • ijIj III 0 0 100 200 300 400 0 100 200 300 Distance Rennick Glacier - craton [km] Distance coastline - craton [km] •

OD ® t o

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0

O Fitzgerald & Gieadow, 1988

0

5 10 15 20 25 Distance to thrust zones [km] A Arne et a!., 1993

Figure 2. Relationship between regional distances and apatite ages of the Outback Shoulder (Oates Land/ westernmost northern Victoria Land). Regional trends as indicated in Figure 1 (see text) may be partially biased by different sample elevations. AFTA - apatite fission track age. (a) Distance Rennick Glacier - East Antarctic Craton. The amounts of denudation decrease towards the craton. (b) Distance coastline - continental interior. A crude tendency of increasing ages may be indicated by the oldest ages of the samples far inland, (c) Distance from the thrust zones. The large thrust zones of Oates Land (Wilson Thrust: white, Exiles Thrust: gray) are characterized by decreasing fission track ages.

caused by either thermal advection due to hydrothermal activity along the thrusts or by increased denudation. In the latter case crustal extension might have weakened the upper crust, with locally increased erosion and isostatic re-equilibration leading to the exhumation of rocks from lower crustal levels. The samples from the Rennick region are distinctively younger. Ages <90 Ma and track lengths of ~14 pm with standard deviations between 1.0 and 1.5 pm are very similar to the fission track results from the Transantarctic Mountains. It thus appears that the Rennick Graben was also affected by the

FT2 Cenozoic uplift seen in the Transantarctic Mountains and probably occupies a transitional position between the Ross Sea-rifting determined evolution of the Admiralty Block and the slow exhumation of the Outback shoulder. Based on this data overview, a new fission track investigation is planned to refine the pre-breakup history of the former Pacific cratonic margin of Gondwana (Beacon evolution), the formation of the passive continental margin of northern Victoria Land, the mode of reactivation of the major thrust zones of Oates Land during Ferrar event and 'Rennick Rifting' and the potential influence of the uplift of the Transantarctic Mountains and the rifting of the West Antarctic Rift System on the exhumation of the Rennick Graben and Outback Shoulder. Arne D. C. Kelly R R. Brown R. W. and Gleadow A. J. W. 1993. Reconnaissance apatite fission-track data from the East Antarctic Shield. In Findlay R. H. Unrug R. Banks M. R. and Veevers J. J. (eds.) Gondwana Eight: assembly, evolution and dispersal, Balkema, Hobart, 605-611. Fitzgerald P. G. and Gleadow A. J. W. 1988. Fission track geochronology, tectonics and structure of the Transantarctic Mountains in northern Victoria Land, Antarctica. Chemical Geology (Isotope Geoscience Section) 73, 169-198. Fitzgerald P. G. and Stump E. 1997. Cretaceous and Cenozoic episodic denudation of the Transantarctic Mountains, Antarctica: new constraints from apatite fission track thermochronology in the Scott Glacier region. Journal of Geophysical Research 102, 7747-7765. Lisker F. 1996. Geodynamik des Westantarktischen Riftsystems basierend auf Apatit-Spaltspuranalysen. Berichte zur Polarforschung 198, 108pp. Schafer T. 1998. Thermo-tektonische Entwicklung von Oates Land und der Shackleton Range (Antarktis) basierend auf ApatitSpaltspuranalysen. Berichte zur Polarforschung 263, 107pp. Tessensohn F. 1994. The Ross Sea region, Antarctica: structural interpretation in relation to the evolution of the southern ocean. Terra Antartica 1, 553-558.

FT2^ee

International Conference on Fission Track Dating and T h e r m o c h r o n o l o g y

F T 2 ^

TIMING OF THE LATER STAGES OF THE EXHUMATION OF THE SHUSWAP CORE COMPLEX, BRITISH COLUMBIA, CANADA: A STUDY USING FISSION-TRACK ANALYSIS

M. Lorencak, D. Seward and J.P. Burg Institute of Geology, ETH Zentrum, CH-8092 Zurich

Nine zircon and eighteen apatite fission-track analyses have been made in order to determine the late stage thermotectonic history of the Shuswap Metamorphic Core Complex, western Canada. The research area extends west and south of Revelstoke including the Thor-Odin dome; sampling was carried out along two profiles across the complex. The apatites vary in apparent age from 48.5 ± 3-2 to 27.7 ± 3.4 Ma, and the zircons from 53-9 ± 5.6 to 37.5 ± 5.0 Ma (Fig. 1). The ages and the combination of ages fall into four groups: 1) In a first group, between the Columbia River Fault (CRF) and the Victor Creek Fault (VCF), from Mt Odin NNW through to Victor Creek, zircon ages range from 53 to 43 Ma, while the apatite ages lie between 44 and 28 Ma. The apatites display a clear age-altitude correlation with the youngest ages at the lowest elevations, implying there has been no disturbance since the rocks were at a temperature of approximately 100°C. Estimates of the cooling rates, assessed from paired data sets, show a sharp decrease in cooling/exhumation at ~38 Ma. 2) West of the VCF and the Thor-Odin migmatites, apparent apatite ages (49 - 43 Ma) are clearly older at comparable elevations and are essentially statistically identical. From the age-altitude relationships an identical exhumation rate to that of the Thor-Odin region is found. Zircon ages range from 49.0 ± 4.4 Ma to 42.8 ± 4.6 Ma, and are also statistically identical to each other and to the apatite ages within this unit. This unit continued to cool very rapidly, evidenced from the similarity in zircon and apatite ages, slowed down at approximately 45 Ma and since cooled slowly until the present. On the other hand the lower units (1) continued to be exhumed and their cooling rate did not change until late at approximately 40 Ma. This suggests that the controlling factor was the VCF, which has allowed the youngest unit to be brought into juxtaposition with the middle unit. The VCF must have been initiated after the units had cooled through 250°C, i.e. at about 45 Ma. 3). Another jump in ages (27.7 to 39.6 Ma) occurs on the eastern side of the Monashee Complex in the region of Revelstoke, where the younger apatite ages again occur in the footwall of the CRF. This implies that the exhumation is controlled on this side by the CRF. 4) The Selkirk Allochthon in the hanging wall of the Okanagan detachment (OD) has a distinctly different apatite age. Even though there is only one age, we are confident that this is meaningful and that the Selkirk Allochton has clearly undergone a different thermal history. Apatites from a Ladybird Granite clast from sediments in the Enderby Basin, lying on the hanging wall of the OD, have an apparent age of 44.5±6.4 Ma. They are overlain by basaltic lava flows dated between 49.2-42.2 Ma (K-Ar, Matthews 1981). Modelling the thermal history of this clast shows that, after rapid exhumation, which brought the clast to the surface, burial/ heating followed. At 45-25 Ma exhumation/ denudation/ cooling took place. This is similar to the history of the underlying middle unit and is considered to be a reflection of the effects of the underlying tectonics. Thus, on the basis of the fission-track data, we suggest that the region can now be divided into four thermotectonic units resulting of differing tectonic controls during regional extension (Fig. 1). The boundaries of two of these units are no longer controlled by the detachment faults of the core complex but by later normal faults that cross cut the lithological units. The proposed units are: a) the upper outer hanging wall of the OD; b) the upper outer hanging wall of the CRF, the Clachnacudainn; c) a middle unit bounded by the OD and the VCF, and d) a lower unit bounded by the VCF and in the east by the CRF in the east. Due to lack of data, the structural position of the Selkirk Allochthon, east of the Columbia Detachment and south of Clachnacudainn cannot be included here. Geological Society of Australia - Abstracts Number 58

FT2

119 00'

Hanging wall of detachments

upper unit 11111 sedimentary basins | I volcanics PV^

Shuswap MCC middle unit I FT! Clachnacudainn I ) youngest unit

Sample site • Apatite age |42^j4.6 I Zircon age ( g g ^ J ) all ages are central ages (Ma); Error = 2a (after Galbraith and Laslett, 1993)

Figure 1. Location and geological setting of the research area, including the locations of fission track samples. The map shows the tectonic setting as seen from the results of the fission track analysis. CRF and VCF represent the Columbia River and the Victoria Creek Faults, respectively. Map modified after Vanderhaeghe and Teyssier, 1997.

A complete cooling history of the Shuswap core complex can now be reconstructed with constraints from U-Pb, Rb-Sr, K-Ar and Ar/Ar age data (Vanderhaeghe and Teyssier 1997 and references therein, Vanderhaeghe 1998) as well as the fission track results presented here. GALBRAITH R.F., LASLETT G. M. 1993: Statistical models for mixed fission track ages. Nuclear Tracks Radiat. Measurements 21, p. 421-470 MATTHEWS W.H. 1981: Early Cenozoic resetting of potassium-argon dates and geothermal history of north Okanagan area, British Columbia. Can. J. Earth Sci.18, p. 1310-1319 VANDERHAEGHE O., TEYSSIER C 1997: Formation of the Shuswap metamorphic core complex during late-orogenic collapse of the Canadian Cordillera: Role of ductile thinning and partial melting of the mid- to lower crust. Geodinamica Acta 10, 2, p. 41-58 VANDERHAEGHE O., TEYSSIER C., MCDOUGALL I., DUNLAP J.W. 1998 (?): Cooling and exhumation of the Shuswap metamorphic core complex constrained by ^^Ar/^^kv thermochronology. Submitted to Tectonics

International Conference on Fission Track Dating and Thermochronology

F T 2 '

MEASUREMENTS OF THE URANIUM DISTRIBUTION AND ABSOLUTE CONCENTRATION IN CALCITE F R O M GERMAN TRAVERTINES AND T H E I R POTENTIAL SIGNIFICANCE FOR U R A N I U M - S E R I E S DISEQUILIBRIUM DATING

R. Mallicki, R. Jonckheere2, H. Hannen3, H. Petrie3,A. Manginii and G.Wagner2 1 Institut Fur Umweltphysik, Universitat Heildelberg, Heildelberg, Germany 2 Max-Planck-Institut Fiir Kernphysik, Heildelberg, Germany 3 Forschungszentrum JulichJiilich, Germany

This experiment aims to refine and optimise conventional sampling procedures for the purpose of Uranium series disequilibrium dating of travertines using a Thermalised Ion Mass Spectrometer (TIMS). Travertines from four localities in the Thiiringer Basin in Germany were studied with polarisation and cathodoluminescence microscopes. Three distinct carbonate phases were identified: a) micrite, b) sparite and c) pore cement. Their trace-element concentrations of Mg, Sr, Fe and Mn were determined using an electron microprobe. Areas rich in micrite were selected, marked on the travertine thin sections and sampled with a micro-drill for uranium series disequilibrium dating. The uranium distribution in samples from the micrite and sparite phases was examined with the fission-track method. Twelve grain separates were mounted in epoxy, polished, covered with low-uranium muscovite external detectors and irradiated with a thermal neutron fluence of -IQi"^ cm-2 at the research reactor in Jiilich in order to produce induced fission tracks. The neutron fluence was measured using 0.1%Au-Al and Zr-wire monitors, certified by the European Institute for Reference materials and Measurements (IRMM). The principal aim is to establish whether uranium is homogeneously distributed throughout the micrite and sparite phases or whether it is bound to certain features, e.g. dislocations, cracks ... etc. The results allow to select suitable areas from which to take sample material for uranium series disequilibrium dating and to determine whether the micrite carbonate phase, which is currently used for dating, is the most suitable. The absolute uranium concentrations were calculated, using a corrected formula, and compared with those obtained from thermalised ion mass spectrometry.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

F T 2 '

M I D MIOCENE COOLING IN H E X I CORRIDOR-QILIAN SHAN REGION, Q I N G H A I - T I B E T PLATEAU

SJ. MarshaUseai,A.D. George2, K-H.WyrwolP,J. Chen^ andY Lu^ 1 Geotrack International, Melbourne,Australia 2 Department of Geology and Geophysics, University of Western Australia, Perth, Australia 3 Department of Geography University of Western Australia, Perth, Australia 4 Institute of Geology, State Seismological Bureau, Beijing, China

The Qinghai-Tibet Plateau has been the focus of numerous tectonic studies for many years. This study reports apatite fission track and vitrinite reflectance results from the northeastern thrust margin of the Plateau, encompassing the Qilian Shan (Mountains) and the Hexi Corridor, a narrow foreland basin proximal to the mountains (see Fig. 1). The Qilian Shan is a currently active mountain range, up to 4 km high, with rocks varying in age from Pre-Mesozoic, Mesozoic, Tertiary and Quaternary exposed in thrust-bounded slices. Jurassic strata are also uplifted in the Hexi Corridor area and locally; the older rocks are overlain by Cretaceous strata. Erosion of these pre-Cretaceous sediments provided the sediment infill to the Oligocene to Pliocene basins of the Hexi Corridor. These basins have been subsequently reworked to form extremely thick Pleistocene gravel successions.

CHINA

Qinghai-TibetQ Plateau

Figure 1. The location of the Qilian Shan and Hexi Corridor along the northeast margin of the Qinghai-Tibet Plateau

About 20 samples were collected for AFTA and VR with Paleozoic and Mesozoic samples collected across the Qilian Shan and the more northern Kuan Tai Shan with Tertiary and Quaternary samples collected from the Hexi Corridor. The following outlines the AFTA and VR results from a preliminary batch of samples. AFTA results from Mesozoic and Paleozoic samples reveal that the samples have experienced multiple paleo-thermal episodes. AFTA data from the Paleozoic samples show evidence for two paleo-thermal episodes, with samples reaching paleotemperatures at least ~115°C prior to cooling beginning at some time between 115 and 90 Ma (mid-Cretaceous). This Cretaceous episode is also revealed by AFTA data from Mesozoic samples with paleotemperatures of 70° to 80°C reached during this episode. All AFTA samples also show evidence for a later cooling episode from peak paleotemperatures of between 50° and 70°C for Cretaceous samples and between 80° and 120°C for the Paleozoic samples. AFTA results from all samples constrain the time of cooling to beginning at some time between 20 and 10 Ma (mid-Miocene).

Geological Society of Australia - Abstracts Number 58

VR results from a Cretaceous sample suggests maximum paleotemperatures of ~105°C prior to cooling at some time after deposition (until AFTA results are available for this sample it is uncertain when these paleotemperatures were reached). VR results from a Jurassic sample also suggest the possibility of a third earlier paleo-thermal episode with AFTA results constraining the timing of cooling to beginning some time prior to 120 Ma (mid-Cretaceous). This eariier paleo-thermal episode may have also affected the Paleozoic samples but any evidence for this episode in these samples has been masked by subsequent paleo-thermal events. Recognition of this episode is based on limited data and awaits analysis of additional AFTA and VR samples to be more fully resolved. In summary the following paleo-thermal episodes have been recognised from AFTA and VR data. • 20 to 10 Ma (mid-Miocene) • 115 to 95 Ma (mid-Cretaceous) • prior to 120 Ma (pre-mid-Cretaceous?) It is not possible from these isolated outcrop data to determine the paleogeothermal gradient prior to cooling from maximum paleotemperatures, or in turn to assess the nature or cause of the heating event responsible for maximum paleotemperatures and subsequent cooling. However, in most geological situations heating can usually be ascribed to increased depth of burial with cooling due to uplift and erosion on an unconformity and it would seem reasonable to assume the paleotemperatures reached by these samples were the result of heating due to burial and with subsequent cooling due to uplift and erosion. Mid-Miocene paleo-thermal episode AFTA samples have cooled by between 40 and 110°C from mid-Miocene paleotemperatures to a present temperature of 10°C. Assuming that heating was a result of increased depth of burial, with cooling due to uplift and erosion, and a geologically reasonable geothermal gradient of 30°C/km, these amounts of cooling necessitate kilometre-scale removal of section since cooling began in the midMiocene. Lower geothermal gradients would necessitate a greater amount of removal of section and vice versa. Cretaceous paleo-thermal episodes Mesozoic samples cooled by between 60° and 70°C and Paleozoic samples by at least 105°C from mid Cretaceous paleotemperatures to a present temperature of 10°C. Assuming that heating was a result of increased depth of burial, with cooling due to uplift and erosion these amounts of cooling also necessitate kilometre-scale removal of section since cooling began in the mid-Cretaceous. Discussion Current ideas on the development of the Qilian Shan mountains suggests that they are the youngest mountain range of the Tibet Plateau region, being currently active and having risen in less than 6 million years (Meyer et aL, 1998). The initial AFTA and VR results presented here do not conflict with this idea, but they do indicate that if cooling from peak paleotemperatures was associated with denudation the process began somewhat earlier, in the mid-Miocene. The mid-Miocene period of significant cooling identified from AFTA is consistent with the mid-Miocene collision of the Indian and Asian plates. Further integration of the AFTA and VR results with the tectonic development of the region, including understanding of the Cretaceous and possible earlier cooling episodes, awaits further work. Meyer, B., Tapponier, P., Bourjot, L., Metivier, R, Gaudemner, Y., Peltzer, G., Guo Shunmin and Chen Zhitai, 1998, Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction and oblique, strike-slip controlled growth of the Tibet Plateau. Geophysical Journal International, 135, 1-47.

International Conference on Fission Track Dating and Thermoclironology

F T 2 '

P O S T - O R O G E N I C E X H U M A T I O N O F T H E ULTRAHIGH P R E S S U R E CONTINENTAL C R U S T IN E A S T - C E N T R A L C H I N A - DATA F R O M T H E F O R E L A N D

B. MAYERI'2, L. Ratschbacheri, R. Jonckheere^, J. Schmidi, B. Hacker3 and G.Wagner^ 1 Institut Fiir Geologic, UniversitatWur2burg,Wur2burg, Germany 2 Max-Planck-Institut Fiir Kernphysik, Heildelberg, Germany 3 Department of Geological Sciences, UC Santa Barbara, USA

The Dabie-Shan mountains, part of the Qinling-Dabie-Sulu orogen in east central China, contain the worlds largest exposure of ultra-high-pressure (UHP) metamorphic rocks. They are the result of the north-directed attempted subduction and subsequent isostatic exhumation of the leading edge of the Yangtze continent to the south underneath the Sino-Korean continent to the north, during TriassicJurassic times. The deformation mechanism of the Dabie orogen has been investigated in detail (Hacker et al, in press; Ratschbacher et al, in press) and seems now to be clear: continental crust has been subducted to depths in excess of 100 km, metamorphosed, and then rapidly exhumed along a detachment fault. During the Early Cretaceous there was intense magmatism and concomitant reheating in the entire Dabie Shan. A transtensional stress field was created as a result of far-field collisions and a change of the Pacific subduction from highly oblique to perpendicular. In the Late Cretaceous and Early Cenozoic, this resulted in a quasi en-bloc updoming of the Dabie-Shan, which overprinted pre-existing tectonic structures. The mechanisms of continent-continent collisions are still not well understood. In the case of the Qinling-Dabie-Sulu orogen, it is of great importance to better understand the tectonic evolution of the Yangtze foreland fold-and-thrust belt to the south and east (Shmid et al, in press). We present results related to the eastern foreland. Apatite fission-track analysis of Middle Triassic to Middle Jurassic clastics and Upper Jurassic to Lower Cretaceous (?) magmatites from the eastern fold-and-thrust belt is used to reconstruct the pre-(?) and post-depositional thermal history of the foreland sediments. The following questions are addressed : • It is certain that there was continued sedimentation during the Cretaceous. Was the thickness of these sediments enough to completely reset the apatite ages in the underlying Middle Triassic to Middle Jurassic sediments? • If not, can these samples provide information relating to the main exhumation episode that has been erased in the Dabie Shan region itself by Early Cretaceous thermal overprinting and Late Cretaceous uplift Qonckheere et al, this volume)? • What information relating to tectonic deformation and/or re-heating by intrusions in the foreland has been registered in the apatite fission-track system? The analysis of magmatites will provide the most direct answer to this question. The results of apatite temperature-time path modelling and age-component analysis are discussed in the light of these questions. HACKER, B. R., RATSCHBACHER, L., WEBB, L., MCWILLIAMS, M., IRELAND, T., CALVERT, A., DONG, S.: Exhumation of Ultra-high-Pressure Continental Crust in East-Central China: Late Triassic-Early Jurassic Tectonic unroofing. - J. Geo-phys. Res., in press. JONCKHEERE, R., RATSCHBACHER, L., SCHMID, J., BLYTHE, A., DONG S., LIU S., HACKER, B. and WAGNER, G.: Post-orogenic exhumation of the ultrahigh pressure continental crust in East-central China: Tectonics and ET-thermo-chronology. Ninth International Conference on Fission-Track Dating and Thermochronology, Lome, Australia, this volume. RATSCHBACHER, L., HACKER, B.R., CALVERT, A., WEBB, L., IRELAND, T., MCWILLIAMS, M., DONG, S., WENK, H.-R.: Exhumation of the Ultrahigh-pressure Continental Crust in East-Central China: Cretaceous and Cenozoic unroofing and the TanLu Fault. - J. Geophys. Res., in press. SCHMID, J. C., RATSCHBACHER, L., HACKER, B. R., GAITZSCH, L, DONG, S.: How did the foreland react? Exhumation of the Dabie Shan ultrahigh-pressure continental crust and the Yangtze foreland fold-and-thrust belt (eastern China). - Terra Nova, in press. Geological Society of Australia - Abstracts Number 58

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International Conference on Fission Track Dating and Thermochronology

ZIRCON FISSION-TRACK ANALYSIS OF NOJIMA FAULT BOREHOLE SAMPLES M. Murakami!,T.Tagamii and N. Hasebe^ 1 Department of Geology and Mineralogy, Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Japan 2 Department of Earth Sciences, Kanazawa University, Japan

We analysed zircon separates from borehole samples of Nojima fault by using the fission track method. This fault was activated during the Hyogoken-Nanbu earthquake (M7.2) on January 17th, 1995. Several boreholes penetrated into this fault for various geological and geophysical investigations. Samples were collected from Cretaceous granitic rocks using '500 m borehole' at Toshima, 'GSJ (the Geological Survey of Japan) borehole' at Hirabayashi, and outcrops nearby these boring sites. Tagami et al (1999) made a preliminary FT analysis of samples from '500 m borehole', and found shortened tracks in zircons near the fault at 389 m depth (along-core depth). In this study, we made the first analysis of samples from 'GSJ borehole (Ito et al, 1996; Tanaka et al, 1999)' (the fault at 625 m) and outcrops nearby the boring site, including one adjacent (<30 cm) to the fault outcrop. In addition, we also made the further detailed analysis of '500 m borehole'.

Depth (m)

GSJ borehole

sample localities

500m borehole Depth (m)

150

195

250 245 350

450

295

550 Fault (625m 650

345

725 3

5

7

9

11

Track length (/u. m)

13

3

5

7

9

_ _ _

Fault

11

(389ni)

13

Track length (Mm)

M 20H

15-20

• •

10-15 5-10

•

0-5

Figure 1. Relative frequency distribution of fission track lengths in zircon

Near the fault within the 'GSJ borehole', we found both shortened (6-7 micrometer) and preserved (10-12 micrometer) tracks in a sample (Fig. 1). As a result, the track length distribution shows bimodality, similar to the case of '500 m borehole' section. On the other hand, away from the fault, very few shortened tracks were found at the middle depth (370-550 m depth). Further away from the

G e o l o g i c a l Society of A u s t r a l i a - Abstracts N u m b e r 58

FT2 fault (150-260 m depth), we found a large proportion of preserved tracks with a small number of shortened ones. In samples of outcrops the tracks are totally preserved even at the nearest site. These results, combined with '500 m borehole' data, suggest thermal anomaly localised around the fault at depth. Besides, in 'GSJ borehole' samples, we found that the locality of highest degree of track shortening (i.e. 650 m depth) is deeper than the main shear zone (i.e. 625 m). H. Ito et al 1996. Structure and Physical Properties of the Nojima Fault by the Active Fault Drilling. SEGJ Journal 49, 522-535. H. Tanaka et al. 1999. Distribution, deformation and alteration of fault rocks along the GSJ core penetrating the Nojima Fault, Awaji Island, Southwest Japan. Jour. Geol. Soc. Japan 105, 72-85. T. Tagami et al. 1999. Thermal anomaly around Nojima fault as detected by the fission-track analysis of Ogura 500 m borehole samples. The Island Arc in press.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

LONGTERM MORPHOTECTONIC EVOLUTION IN FINLAND: CONSTRAINTS ON QUANTIFICATION AND T I M I N G OF VERTICAL M O T I O N S FROM F I S S I O N T R A C K THERMOCHRONOLOGY

G.R. Murrell and P.A.M.Andriessen Faculteit derAardwetenschappen,Vrije Universiteit van Amsterdam,The Netherlands

Traditional ideas relating to the geological evolution of the Finland part of the Baltic shield are that the craton has remained largely inert throughout most of its history. These ideas mainly stem from the lack of recording features in the crust, as the craton displays little sedimentary cover, has very little relief and exhibits little or no surface expressions of crustal movements. While Finland itself may have been more or less stable since the end of the Svecokarelian orogeny (-1.7 Ga), areas surrounding the craton have experienced several significant plate tectonic events. These events being the Caledonian orogeny, the various phases of the opening of the North Atlantic and the formation of the North Atlantic margin. Since these events occurred some distance from the Finland craton their far field influences would be relatively subtle and the vertical crustal movements associated to them small. For this reason, the unique low temperature sensitivity of Apatite Fission Track Thermochronology (AFT) means that it is an ideal tool to apply to the analysis of the geological landform evolution of the craton and the resolution of these far field processes. A previous AFT study conducted in the Svecokarelian intrusive rocks of southern Finland (Lehtovaara, 1976), produced FT ages ranging from 520 - 980 Ma. Lehtovaara concluded that in general the craton experienced a Precambrian cooling event and attributed the spatially variable ages to differential movements in the crust. While the application of AFT to Finland is rare there is fortuitously a large geochronological database covering Finland containing a substantial set of radiometric dates. An 40Ar/39Ar study conducted in southern Finland by Heeremans and Wijbrans (1999) yielded ages between 550 Ma and 1750 Ma with a significant number being less than 1000 Ma, providing further evidence for Late Proterozoic tectonic events. A total of 84 samples have been collected from a largely grid-like pattern covering the greater part of Finland. There are several ideas in particular that will be investigated through the application of AFT to this sample set. Firstly the investigation of the thickness and spatial extent of a Caledonian foreland basin will be facilitated by the construction of several regional transects across and along Finland. These transects will also help to determine the existence of any relationship between the general domal shape of south central Finland with similar landforms in Sweden and Norway. Higher density sampling has been conducted across selected lineaments present in the landscape, as well as the geology, in the hope of detecting any differential vertical movements in the crust and/or structural control to these lineaments. With the aim of quantifying a denudation rate for the craton, an elevation profile has also been sampled. The high track density in single apatite grains presents the opportunity to perform apparent age - FT length thermal modelling on individual crystals. At the same time we are aware of the so-called aging effect and crystal damage caused by a-recoil. Preliminary results of the present study confirm a Precambrian cooling event with ages ranging from 312±34 to 730±70 Ma. There is also a spatial variance in the ages obtained, but whether this is due to differential movements in the crust or other reasons remains to be confirmed. First length measurements from 11 samples scattered throughout southern Finland show mean lengths varying from 11.11±1.68 |Lim to 12.86±1.22 |Lim with normally few long (>14.5 |im) tracks. This is surprisingly low for 'old' ages indicating that these samples have remained a relatively long time in the partial annealing zone. Spatially the 11 samples exhibit a general increase in mean track length from east to west. These results seem to suggest a later cooling in the east possibly due to the existence of a Caledonian foreland basin. However the results are preliminary and as more data comes available, more substantiated conclusions can be made.

Geological Society of Australia - Abstracts Number 58

Heeremans M. and Wijbrans J. R. 1999. Late Proterozoic thermal events in southern Finland, constrained by ^^Ax/^^Ai incremental heating and single spot fusion experiments on K-feldspars. Terra Nova (in print). Lehtovaara J. 1976. Apatite fission track dating of Finnish Precambrian intrusives. Annales Academiae Sciantiarum Fennicae. Series A, III. Geologica - Geographica. 117.

Acknowledgments This research is supported by NSG and ISES.

International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

F T 2 '

DETERMINATION OF THE GEOLOGICAL AGE OF THE MOUNTAINOUS MINERALS OF NORTH TAJIKISTAN K.H. Murtazaeyi and DJ. Salomov^ 1 Khujand State University, 735700 Khujand, Republic of Tajildstan 2 Tajik National University, 734000 Dushanbe, Republic of Tajikistan

The determination of the rock age is of great importance for Tajikistan, as 93% of the territory is occupied with mountains. According to the data obtained from geologists the mountain formation of Northern Tajikistan took place rather earlier in comparison with the Pamirs mountains, for example. For the determination of the age of the samples of the mountains (mica, transparent minerals) from the Kuramin, the Turkestan and the Zeravshan mountains we used the method of a solid track detector. The selected samples were wrought out in chemical solution HF of definite density during 90 minutes under 300°C temperature. Then by means of an optical microscope under magnifying (300400)x we calculated the quantity of tracks got through the spontaneous fission of uranium. After this the paragons of minerals were radiated from a standard source of californium and determined the density of tracks caused by induced fission of uranium. By means of comparison of the quantity of tracks of spontaneous and induced fission of uranium we determined the age of the minerals. For the micas of biotite and flogopite from the abovementioned mountains the age is equal to 200 Ma, for transparent minerals it is 150 Ma. According to the data obtained we may say that mountain formation in Northern Tajikistan took place 150-180 m.y. earlier than that one which took place in the south of our republic.

Geological Society of Australia - Abstracts Number 58

FT2

236

International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

F T 2 '

PALEOZOIC TO RECENT COOLING HISTORY OF THE APPALACHIAN BLUE RIDGE PROVINCE IN NORTH CAROLINA, TENNESSEE, AND VIRGINIA, EASTERN UNITED STATES, FROM APATITE AND ZIRCON FISSION TRACK ANALYSIS

N.D. Naeser, C.W. Naeser, B.A. Morgan, III,A.P. Schultz and C.S. Southworth U.S. Geological Survey, 926A National Centre, Reston,VA 20192, U.S.A.

Apatite and zircon fission-track (F-T) analysis of Middle Proterozoic to Cambrian granitic and metasedimentary rocks is defining the cooling history of the Blue Ridge Province in the Central and Southern Appalachians. Zircons from the highlands of the Great Smoky Mountains (North Carolina and Tennessee) (King et al, 1968), from the upper plate of the Blue Ridge fault in the James River gorge area (Virginia) (Spencer, 1994), and from the upper plate of a thrust fault in central Shenandoah National Park (Virginia) (Gathright, 1976) yield remarkably uniform F-T ages of -285-300 Ma that show no significant change over elevation differences as much as 1.25 km. The data suggest that rocks from these high structural positions, from the Great Smoky Mountains north to Shenandoah National Park (over 550 km), underwent relatively rapid cooling through the zircon F-T closure temperature (~225°C) in latest Carboniferous-earliest Permian time, most likely related to emplacement of major thrusts during the Alleghanian orogeny. In contrast, rocks from the western foothills of the Great Smoky Mountains, the lower plate of the Blue Ridge fault in the James River gorge, and most samples from the lower plate of the thrust fault in central Shenandoah National Park yield significantly older (Middle Proterozoic to Devonian) zircon F-T ages. Some of these rocks may never have been buried deeply enough during the Paleozoic to obtain temperatures sufficiently high (>225°C) to totally reset their zircon F-T ages. Apatites yield mid-Jurassic to Cretaceous ages that define very similar age-elevation trends for all three study areas. Mean track lengths range from 13.04 pm to 14.40 pm, with negatively skewed, unimodal track-length distributions and standard deviations of the track-length distribution between 1.09 pm and 1.98 pm. In a given section, change in mean track length with elevation is insignificant. Mean pit width of tracks in the dated grains indicates an annealing susceptibility comparable to Durango apatite. Preliminary modelling of the apatite age and track-length data, using Monte Trax (Gallagher, 1995; K. Gallagher, written commun., 1996) with Laslett et al's (1987) annealing model for Durango apatite, indicates that most Blue Ridge rocks cooled through the apatite F-T partial annealing zone (from =110°C to 60°C) in Jurassic to Cretaceous time at rates typical of relatively slow-cooled, thermally undisturbed basement rocks. Since that time, the rocks have remained at temperatures <60°C as they cooled to present-day surface temperatures. Offset in the apatite ages suggests Cretaceous or younger displacement (southeast side up) on the Gatlinburg fault in the Great Smoky Mountains and possibly on the Blue Ridge fault in Virginia. In contrast, there is no evidence for significant post-Jurassic movement on the fault that separates sample sites in central Shenandoah National Park. Gallagher K. 1995. Evolving temperature histories from apatite fission-track data. Earth and Planetary Science Letters 136, 421435. Gathright T. M. II 1976. Geology of the Shenandoah National Park, Virginia. Virginia Division of Mineral Resources Bulletin 86, 93 p. King P. B. Neuman R. B. and Hadley J. B. 1968. Geology of the Great Smoky Mountains National Park, Tennessee and North Carolina. U.S. Geological Survey Professional Paper 587, 23 p. Laslett G. M. Green P. F. Duddy I. R. and Gleadow A. J. W. 1987. Thermal annealing of fission tracks in apatite~2. a quantitative analysis. Chemical Geology (Isotope Geoscience Section) 65, 1-13. Spencer E. W. 1994. Structure of the Blue Ridge at the James River Gap, in Schultz A. and Henika B., eds., Fieldguides to southern Appalachian structure, stratigraphy, and engineering geology. Virginia Tech Department of Geological Sciences Guidebook 10, 283 p.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Tliermochronology

F T 2 '

M I D D L E T O L A T E T E R T I A R Y C O O L I N G O F T H E G O R E AND W E S T E R N F R O N T CENTRAL COLORADO, U S A ,

F R O M APATITE F I S S I O N - T R A C K

RANGES,

DATA

C.W. Naeseri, B. Bryant2, K. Kellogg2 and WJ. Perry, Jr.3 1 926A National Centre, U.S. Geological Survey, Reston,VA 20192 USA 2 MS-913, U.S. Geological Survey, P.O. Box 25046 Denver, CO 80225 USA 3 MS-939, U.S. Geological Survey P.O. Box 25046 Denver, CO 80225 USA

Apatite fission-track (AFT) data from Proterozoic crystalline rocks and Paleozoic sedimentary rocks of the mountains flanking the Blue River graben (western Front Range to the east and the Gore Range to the west), in west-central Colorado, USA, indicate significant Neogene cooling. AFT ages of rocks from these ranges are between 5 and 40 Ma. The AFT ages from both mountain blocks increase with altitude. Ages from the western Front Range are significantly older than those from similar altitudes in the eastern Gore Range, suggesting asymmetrical uplift, erosion, and cooling. For example, samples at 3,000 m altitude in the eastern Gore Range have an apatite age of Ma, whereas AFT ages from directly across the valley at 3,000 m altitude in the western Front Range are - 2 5 Ma. Apatite ages from the western Gore Range are older than those obtained at similar altitudes on the east side of the range ( - 2 2 Ma at 3,000 m). Apatite track lengths are all greater than 13 pm, and most of the samples have track lengths greater than 14 pm. These long lengths, and the AFT ages, indicate total track annealing followed by relatively rapid cooling in the Neogene. The AFT data suggest that all of the rocks that flank the Blue River graben were at temperatures >110°C well into middle, and in some cases. Late Tertiary time. Stratigraphic reconstruction suggests that the basement rocks in the northern, lower, part of the Gore Range should have yielded Laramide ages or older ages. The AFT data indicate that Neogene heating has removed evidence of these older ages. Zircon from Mesoproterozoic-crystalline rock yields a fission-track age of -1,300 Ma, indicating that this part of the Colorado basement has not been heated to temperatures >200°C since Mesoproterozoic time. In a regional context, the AFT ages along the flanks of the Blue River graben are significantly younger than AFT ages farther to the east in the central and eastern Front Range and to the west in the White River uplift. In both of these areas, the apatite ages suggest Laramide cooling. The western Front Range-Gore Range zone of young AFT ages extends southward along the axis of the Rio Grande rift through southern Colorado and New Mexico. These young ages result from a combination of elevated heat flow, uplift, and erosion along the axis of the Rio Grande rift during Neogene time.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

F T 2 v w O O

C H A R A C T E R I S A T I O N O F ION T R A C K S IN S O L I D S B Y N E A R - F I E L D M I C R O S C O P Y AND

T E M

R. Neumann Gesellschaft fur Schwerionenforschung (GSI), Planckstr. 1,64291 Darmstadt, Germany

A swift heavy ion propagating through a solid, successively transfers its kinetic energy in tiny portions and induces very rapidly developing processes. These processes lead to changes such as point defects, a discontinuous sequence of disordered zones, or even a continuous damage trail. A deeper and eventually comprehensive understanding of the mechanisms of damage creation and track development requires a long-term effort of data collection from numerous different materials. The results of various experimental methods must be combined and compared with continuously improved theoretical models. The analytical tools used for this purpose can be separated roughly into two groups: A number of techniques including small-angle scattering of X-rays or neutrons and optical spectroscopy collect information by simultaneous interaction with a large amount of latent ion tracks (of order lO^-lO^o per cm2). Based on model assumptions, specific parameters as for example a track diameter can be extracted. The microscopy methods, representing the second group, are capable of revealing and characterising single individual tracks. Since latent tracks have diameters of a few to some tens of nm, only methods providing very high magnification are of major interest: transmission electron microscopy (TEM) and near-field (scanning probe) microscopes, in particular scanning tunnelling microscopy (STM) and scanning force microscopy (SFM). Besides the resolution of ion-induced damage on a subnanometer scale, TEM provides also images of strain zones in the vicinity of a track, and SFM gives access to track properties such as modified surface friction and elasticity. This contribution presents an overview of recent microscopy investigations on ion tracks in solids of different classes (for pioneering work see for example [1-3]). The report addresses the advantages as well as the limits in comparison to other techniques and illustrates the aim to achieve information complementary to the results of the more classical analytical methods. Due to limited space, this abstract displays only a few examples of high-resolution microscopy: SFM of ion tracks in lithium fluoride (LiF) single-crystals and in polyimide (Kapton) as well as TEM of tracks in polyethylene and mucovite mica. The irradiation was performed with heavy ions of 11.4 MeV/u at the UNILAC linear accelerator of GSI. LiF samples were examined by SFM [4] to complement studies performed with optical spectroscopy, small-angle X-ray scattering, chemical etching, and surface profilometry (monitoring bulk swelling) [5]. Figure 1 presents a lateral-force micrograph of a (100) surface plane exposed to ion irradiation under grazing incidence. It contains the area in which the projectiles vanished, probably because of a slight tilt angle between surface and direction of ion propagation. Each sequence of hillock-like defects supposedly represents the track of a single ion. Until now, the chemical nature of the hillocks, for example, whether Figure 1. Lateral-force micrograph (5x5 they consist of metallic Li colloids is not known. SFM pm^) of a single-crystal (100) surface irradiatrevealed latent tracks also on the surface of polyimide foils ed at grazing incidence by Au-ions (1x1 O^^ (thickness 25 ]xm) after normal irradiation [6,7]. Figure 2 ions/cm2) [4]. shows a topographic image recorded with sample surface and force sensor both immersed in water, applying forces of the order of lO-^o N. The single tracks are ring-shaped with an average diameter (measured at the maximum height) of about 15-20 nm, the height ranging from 5-10 nm.

Geological Society of Australia - Abstracts Number 58

F T 2 ^ e e Latent ion tracks in a polymer have

been

imaged by TEM for the first time in semi-crystalline high-density polyethelene [8]. Samples of thickness 60-100 nm, prepared with a cryoultramicrotome, were treated with chlorosulphonic acid and OsO^ prior to

irradiation.

Besides crystalline lamellae, the TEM image in Figure 3 contains the cross sections of heavyion tracks. Due to a contrast enhancement by staining with OSO4, the tracks appear as bright spots.

Obviously,

the

OSO4

concentration

diminished along the ion trajectories and the damaged material became more transparent for the electron beam. Only recently, images of track cross sections in muscovite mica were imaged by TEM with very high magnification [9]. The micrographs demonstrate that the material has been completely amorphized within a cylindrical volume, exhibiting a sharp boundary between damaged zone and intact crystalline lattice. Figure 2. Topographic SFM image of a polyimide surface irradiated with Au-ions (5xlOio ions/cm^) [6,7].

Figure 3. TEM image (1.1x0.7 pm^) of high-density polyethylene, irradiated with Xe-ions (5xlOio ions/cm2) [8].).

Figure 4. TEM image (62x52 nm^) with Pb ion tracks in muscovite mica [91.

(1) Thibaudau F. Cousty J. Balanzat E. and Bouffard S. 1991. Atomic-force-microscopy observations of tracks induced by swift Kr-ions in mica. Physical Review Letters 67, 1582-1585. (2) Kopniczky J. Reimann C. T. Hallen A. Sundqvist B. U. R. Tengvall P. and Erlandsson R. 1994. Scanning-force-microscopy study of MeV-atomic-ion-induced surface tracks in organic crystals. Physical Review B 49, 625-628. (3) Dunlop A. Henry J. and Jaskierowicz G. 1998. Characterisation by various microscopic techniques of the damage created by MeV C^o jons in amorphous Ni3B. Nuclear Instalments and Methods B 146, 222-232. (4) Miiller A. Miiller C. Neumann R. and Ohnesorge F. 1999. Scanning force microscopy of heavy-ion induced damage in lithium fluoride single-crystals. Nuclear Instruments and Methods B, accepted for publication.

FT2 (5) Trautmann C. Schwartz K. Costantini J. M. Steckenreiter T. and Toulemonde M. 1998. Radiation defects in lithium fluoride induced by heavy ions. Nuclear Instruments and Methods B 146, 367-378. (6) Ohnesorge F. Muller A. and Neumann R. 1999. Latent heavy-ion tracks in polyimide (Kapton) imaged with scanning force microscopy under liquid. GSI Scientific Report 1998, 123. (7) Ohnesorge F. Muller A. and Neumann R. 1999. Scanning force microscopy in a liquid on single latent ion tracks: Towards applications in polymers and atomic resolution on crystals. Nuclear Instruments and Methods B, accepted for publication. (8) Vetter J. Michler H. G. and Naumann I. 1998. TEM observation of latent tracks of heavy ions in semi-crystalline polymers. Radiation Effects and Defects in Solids 143, 273-286. (9) Vetter J. Scholz R. Dobrev D. and Nistor L. 1998. HREM investigation of latent tracks in GeS and mica induced by high energy ions. Nuclear Instruments and Methods B 141, 747-752.

FT2

International Conference on Fission Track Dating and Thermochronology

P O S T P A N A F R I C A N T E C T O N I C E V O L U T I O N OF E A S T E R N A N APATITE F I S S I O N T R A C K

AFRICA:

STUDY

W.R Noble, D.A. Foster and AJ.W. Gleadow School of Earth Sciences,The University of Melbourne, Parkville, Victoria 3052 AUSTRALIA

The present geomorphological and structural architecture of eastern Africa has been profoundly affected by continental extensional tectonics since the Late Carboniferous. This study looks at the effects of prolonged tensional stress and provides valuable insights into the formation and propagation of rift valleys and the stability of Archaean cratons. The unique low temperature sensitivity of apatite fission track thermochronology facilitates estimations of amount, rate and chronology of denudation and is helpful for addressing the Phanerozoic development of Eastern Africa. Apatite fission track data highlight several important expressions of extension: (i) Post Late Carboniferous extension resulted in at least four episodes of accelerated cooling during the Middle Jurassic, Early to mid Cretaceous, Late Cretaceous/Palaeocene and Late Eocene/Oligocene. Accelerated cooling is linked to an increase in the rate of denudation manifested as influxes of sediment into basins. The periods of accelerated denudation are related to changes in base level and generation of topographic relief during fault reactivation. The timing of accelerated cooling/denudation in eastern Africa is contemporaneous with periods of extensional reactivation across the African plate. (ii) Prolonged extension produced broad regions of block faulted and mildly extended crust within which nascent development of rift valleys occurs. The focussing of extension in a rift valley appears to take place where zones of weakness, defined by favourable orientation of regional scale Proterozoic structures, are present and the amount of extension is sufficient. (iii) Post Carboniferous extension and related denudation was not confined to Proterozoic mobile belts because data indicate that the Tanzanian and Zimbabwe cratons have undergone kilometre scale denudation and fault reactivation over this time interval. Timing of reactivation on the cratons is concomitant with that recorded from the surrounding mobile belts. Evidence for significant Phanerozoic tectonic reactivation has implications for how cratons react to plate boundary stresses.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

F T 2 '

CONSTRAINING L O N G - T E R M LANDSCAPE EVOLUTION USING F I S S I O N T R A C K AND PALEOMAGNETIC D A T A : A N E X A M P L E FROM THE LACHLAN F O L D BELT, A U S T R A L I A

P.B. O'SuUivani, D.L. Gibson2, B.R Kohni, B. PiUans3 and C.E Pain2 1 Australian Geodynamics Cooperative Research Centre, School of Earth Sciences,The University of Melbourne, Parkville, Victoria 3052 AUSTRALIA 2 Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601AUSTRALLV 3 Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 AUSTRALIA

This paper presents the first known study integrating apatite fission track thermochronology (AFTT) and paleomagnetic (PM) results in order to constrain a region's long-term landscape evolution; in this case the Late Palaeozoic to Cenozoic landscape evolution around the Northparkes copper-gold deposit in east-central New South Wales (Fig. 1) within the western Lachlan Fold Belt (LFB). This was accomplished by documenting periods of significant thermal change (i.e. cooling/denudation or heating/burial) as well as periods during which the landscape must have remained relatively stable (i.e. insignificant burial/heating or denudation/cooling) thus allowing thick weathering profiles to develop.

Crest of Eastern Highlands Mountain Belt Figure 1. Regional map of southeastern Australia, showing major features including: LFB, Lachlan fold belt; CMB, ClarenceMoreton Basin; NEFB, New England fold belt; SB, Surat Basin; SBB, Sydney-Bowen Basin; and MB, Murray Basin. The regions labelled ELFB (eastern LFB) and SM (Snowy Mountains) outline areas which O'Sullivan et al. (1995, 1996) and Kohn et al (1999) respectively have reported previous AFTT results.

Until recently, the thermotectonic history and related evolution of the surface landscape of the LFB following the end of orogenesis in the Early Carboniferous was largely unconstrained because of the lack of adequate geological control. Then, in an attempt by the Australian Geodynamics Cooperative Research Centre (AGCRC) to use low-temperature geochronological data to constrain the tectonic and landscape development of the relatively rugged Eastern Highlands, AFTT analyses were carried out on samples collected throughout the eastern LFB (Fig. 1). As a result, O'Sullivan et al (1995, 1996) and Kohn et al. (1999) have proposed that two distinct episodes of post-orogenesis rapid km-scale

Geological Society of Australia - Abstracts Number 58

FT2^ee denudation occurred in parts of the eastern LFB. The first occurred at some time between ~260-240 Ma, possibly in response to uplift associated with far-field affects related to compressional deformation resulting from oblique convergence associated with the Permo-Triassic Hunter-Bowen Orogeny. The second occurred at ~95±5 Ma, presumably in response to the onset of continental extension in the Tasman Sea to the east. Furthermore, the AFTT results indicated a period between these episodes of rapid denudation during which the landscape remained relatively stable. In contrast to the eastern LFB, the present-day landscape of the western LFB, including the region of the Northparkes mine, is relatively flat with little expression of the underlying rock. This has previously been interpreted to indicate long-term stability of the region since the end of LFB orogenesis. This was presumably borne out by PM analyses from thick weathered horizons within open pits at the mine (Fig. 2), which suggested significant periods of weathering, and hence relative landscape stability during the Early to middle Carboniferous and at some time during the Cenozoic. However, results from AFTT analyses indicate the region experienced significant episodes of cooling/denudation during the Late Permian to Early Triassic and during the Early Cenozoic, as well as episodes of heating/burial during the Late Carboniferous to mid-Permian and during the Late Mesozoic. When combined, the AFTT and PM results are in fact consistent and indicate that since the Late Palaeozoic the landscape of the LFB around the Northparkes deposit has evolved through multiple episodes of denudation and deposition, as well as periods of relative stability during which the thick weathering horizons formed (Fig. 3).

^

Present-day landscape = flat ^ M Sediment fill (Tertiary??) T

In situ

weathered m saprolite v.\\;.Porphyry copper-gold \\

\

\

\

\

\

\

\

\

\

yyyyyyyyy. \yyyyyyyyy. \ \\\ \\\ \\ \y \y \y \y \y \ y^ \\\ \ ^y \y \y \y \y ^

85PM^amples rrom: ~ saprolite & sediment fill

. ;

Quartz monzonitev^vv NP-a^vK^^s (Late Ordovician) vvv

\ \ \ \ \ \ \ \ \ •/ y y y\y \y y^ NP-4\ \ \ \ \ \ \ \ \ \ yyyyyyyyyyyyyyyyyyy

\ \ \ \ \ \ \ \ \ \yyyyyyyyyyyyyyyyyyy/ \ \ \ yyyyyyyyy.' \\\\\ \Vy\ yy\ yy\ yX\ Xy\ \yy yy\ yz-INr-0 KlP.ft yyyyyyyy yy-< ,,^_yyyyyy yyyyyyyyy \y\y \y \y ^i^NP-7"-\ XXy y y y I>lr-# ^ y y y y x x y y y y / y

yyyyyyyyyyyyyyyyyyyyy

'

Figure 2. Diagrammatic view of the geologic relationships exposed in the open pits at the Northparkes mine where Late Ordovician monzonite intrusions hosting copper-gold porphyries are weathered to at least 30 m (in situ saprolite) and are unconformably overlain by up to 25 m of weathered sediment-fill deposits. A total of 85 samples for paleomagnetic analysis were collected from both the weathered regolith (saprolite) and sediment-fill (Pillans et al 1999). Seven drill core samples for AFTT analysis were collected from the Late Ordovician monzonite at depths between -400-1000 m.

Together these results provide important constraints on: (1) the timing and amount of burial and denudation that has occurred within this region of low relief and poor outcrop since the last known recorded episode of deformation, and (2) the timing of major episodes of weathering which indicate distinct periods of relative erosional and denudational stability, as well as defining the relative position of the weathered material to the landscape surface. Furthermore, in cases similar to the western LFB where geological controls are minimal, this dual approach constrains previously lacking details of the region,s thermotectonic history and reveals the long-term evolution of the regional landscape in response to ongoing regional tectonics.

FT2 Time (Ma)

O o O <D

E 100AAAA

>250_

Figure 3. Proposed time/temperature histories of rocks in tlie Northparkes region based on integrated AFTT and PM data. Kohn, B. P., Gleadow, A. J. W., and Cox, S. J. D., 1999. Denudation history of the Snowy Mountains: Constraints from apatite fission track thermochronology. Australian Journal of Earth Sciences 46. 181-198. O'Sullivan, P. B., Kohn, B. P., Foster, D. A., and Gleadow, A.J. W., 1995. Fission track data from the Bathurst Batholith: Evidence for rapid middle Cretaceous uplift and erosion within the eastern highlands of Australia. Australian Journal of Earth Sciences 42. 597-607. O'Sullivan, P. B., Foster, D.A., Kohn, B.P., and Gleadow, AJ.W., 1996. Tectonic implications of Early Triassic, and middle Cretaceous denudation in the eastern Lachlan Fold Belt, NSW, Australia. Geology 6. 563-566. Pillans, B., Tonui, E, and Idnurm, M., 1999. Paleomagnetic dating of weathered regolith, in Taylor, G., and Pain, C., eds.. New Approaches to an Old Continent, Proceeding of Regolith 98 Conference, Kalgoorlie, May 1998, Co-operative Research Centre for Landscape Evolution and Mineral Exploration, Perth. 237-242. Acknowledgements This work was made possible with funding by the Australian Geodynamics Cooperative Research Centre (AGCRC), the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), and the Australian Institute of Nuclear Science and Engineering (AJNSE). Work reported here was conducted as part of ongoing cooperation between the AGCRC and CRC LEME. This paper is published with the permission of the Director, AGCRC and the Director, CRC LEME. D. Gibson and C. Pain publish with the permission of the Executive Director, Australian Geological Survey Organisation. We are pleased to acknowledge the support of North Limited in funding the original paleomagnetic study as well as providing sample material for the AFTT analyses.

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MESOZOIC TO CENOZOIC THERMOTECTONIC EVOLUTION OF TASMANIA: RECONCILING GEOLOGICAL OBSERVATIONS WITH THERMOCHRONOLOGY DATA FROM WITHIN A T R I P L E R I F T SYSTEM

RB. O'SulUvani, B.R Kohni,AJ. 0'Sumvan2 andAJ.W. Gleadowi 1 Australian Geodynamics Cooperative Research Centre, School of Earth Sciences,The University of Melbourne, Parkville, Victoria 3052 AUSTRALIA 2 Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Clayton, Victoria 3168 AUSTRALIA

Previous studies of the rifted margins of southeastern mainland Australia have suggested that denudation along the margins occurred in association with rifting and separation of Australia from Antarctica and from the Lord Howe Rise during the Cretaceous to Tertiary. For instance, apatite fission track (AFT) studies from New South Wales and Victoria suggest that ~ 1.5-2 km of denudation occurred along the coast in response to rifting while ~4 km of denudation occurred within parts of the southeastern Highlands located inland (e.g. O'Sullivan et al, 1999; Kohn et al, 1999). Furthermore, since the two major rift systems responsible for Cretaceous to Early Tertiary denudation on the mainland continue southward along the eastern and western coasts of Tasmania, and since the highlands continue across Bass Strait into Tasmania (Fig. 1), it is proposed that rocks in Tasmania would have experienced similar amounts of denudation in response to rifting.

Figure 1. Regional map of southeastern Australia showing the locations of the three major rift systems surrounding Tasmania, including the failed Bassian Rift through Bass Strait, the Otway Rift between Australia and Antarctica, and the Tasman Rift between Australia and the Lord Howe Rise/New Zealand. Other features include: OB, Otway Basin; BB, Bass Basin; GB, Gippsland Basin; KIH, King Island High; BR, Bassian Rise; and OR, Otway Ranges. The Otway, Bass, and Gippsland basins contain extensive Lower Cretaceous syn-rift sedimentary rocks deposited during the initial failed episode of rifting between Antarctica and Australia.

The post-Paleozoic geologic evolution of the Tasmanian landscape has previously been described as being relatively simple (e.g. Baillie et al., 1989), based primarily on geologic observations. In particular, there is only a limited preserved section of post-Carboniferous-aged rocks throughout the island. This includes a relatively thin preserved Permo-Triassic section (<1-1.5 km thickness) intruded by extensive Early Jurassic (-170-180 Ma) shallow-level dolerite sills. As these sills are now exposed across much of the eastern half of the state, it has been assumed that little has occurred since the time of their intrusion except for the removal of the limited Permo-Triassic overburden (Baillie etal., 1989). Therefore, if the geologic observations are correct and little has disrupted the Tasmanian landsurface since the Early Jurassic, then rocks exposed in Tasmania today must have experienced a considerably different Mesozoic and Cenozoic thermotectonic history than rocks exposed on the mainland just

Geological Society of Australia - Abstracts Number 58

FT2 across Bass Strait (Fig. 1). Alternatively, if rocks in Tasmania experienced a similar Mesozoic and Cenozoic thermotectonic history as those on the southeastern mainland, then there are aspects of the geologic evolution of Tasmania which have not been previously recognised due to the limited crosscutting relationships. In order to constrain both the Mesozoic and Cenozoic geologic evolution of Tasmania as well as a model for the extensional tectonics of southeastern Australia, >250 AFT analyses from a wide range of rocks throughout Tasmania have now been generated. Tasmania is an ideal site to study the complexities associated with the continental extension tectonics of southeastern Australia as it has rifted margins on all three sides. These include the Early Cretaceous failed rift to the north between Antarctica and the mainland, which at that time also included Tasmania, the middle Cretaceous to Paleocene rift between Antarctic and Australia along the west coast, and the middle Cretaceous to Paleocene rift between New Zealand and Australia along the east coast. Furthermore, the distance between Tasmania and its adjacent rifts is minimal due to a very narrow continental shelf; hence any rift-related effects should also be evident onshore. The results show that all AFT ages are younger than their sample ages (Paleozoic to Triassic), with more than 200 having AFT ages <120 Ma. However, in apparent conflict with the geologic observations, the AFT results indicate that the rocks experienced at least three major cooling episodes since the Early Jurassic. These include an episode during the Early-Middle Jurassic, another during the middle Cretaceous, and a third during the Paleocene to early Eocene (Fig. 2). The first event is recorded Age (Ma) 200

175

150

125

INLAND REGIONS ^ (cooling following dolerite intrusion)

100

75 -R

___

50

25

n""

•

^

COASTAL REGIONS (Margin flank denudation) Figure 2. Schematic proposed time-temperature histories for the different regions of Tasmania inferred from APT data. Solid lines along cooling paths represent times when thermal history is constrained by the data; dashed lines represent times when thermal history is less constrained.

by a small number of samples from inland regions (>20 km from any coastline) in the southeastern part of the island. These experienced a major cooling episode -190-170 Ma from temperatures >110°C to temperatures <60°C (Fig. 2), and thereafter have remained at temperatures <60°C. We propose that these samples record rapid cooling following heating associated with the intrusion of the Jurassic dolerites. However, most samples from inland Tasmania experienced a major rapid cooling episode during the middle Cretaceous (-100-85 Ma) from temperatures >110°C, or from >95°C for older samples with AFT ages >120 Ma, to temperatures <60°C (Fig. 2). This cooling was most likely in response to kmscale denudation following the onset of continental extension along eastern Australia at - 9 6 Ma, possibly as a result of underplating inward of the rift, as has been proposed for much of the southeastern highlands of Australia. Furthermore, in many localities throughout eastern Tasmania, PermoTriassic sediments presently exposed beneath Jurassic dolerites and intruded by them, yield apatite ages which indicate rapid cooling from paleotemperatures >110°C during the middle Cretaceous, long after the emplacement of the Jurassic sills.

FT2Wee These relationships suggest two possibilities. First, the dolerites were intruded at much deeper levels (>3-4 km) than previously proposed and that the entire km-scale section of overburden has subsequently been removed since the middle Cretaceous. This is possible, as it is difficult from the limited geologic relationships to constrain neither the exact depth at which the dolerites were originally intruded nor their total original thickness. However, the majority of samples with AFT ages recording rapid cooling following localised elevated heat-flow in response to dolerite intrusion, were collected from some of the highest elevations in the state. Therefore, this would require a much thicker package of dolerites than is presently preserved. Alternatively, following shallow level intrusion, the Jurassic rocks were deeply buried by a thick (>3-4 km) Middle Jurassic to middle Cretaceous sedimentary sequence which must have covered much of Tasmania to varying depths prior to middle Cretaceous rifting. Since the oldest AFT ages are found in rocks from the highest elevations, as would be expected if resetting of the AFT ages was in response to deeper burial, we suggest the second option is more likely. The next episode of rapid cooling occurred at some time during the Paleocene to early Eocene (~6050 Ma). Evidence for this event is restricted to the eastern and western coastal areas, and throughout the Furneaux Islands (Fig. 1). It is probable that other regions of Tasmania, located inland from present-day coastal areas, also experienced accelerated cooling during this time. However, due to the low temperatures from which cooling occurred this proposal is difficult to constrain. During the Early Cenozoic rifting between Antarctica and Australia along the western coast of Tasmania resulted in higher than normal heat flow. The AFT data probably record the subsequent relaxation of geotherms as rifting progressed beyond the region, possibly in combination with some denudation. However, it is not clear at this time what is responsible for the cooling ages along the eastern coast of Tasmania. Finally, by ~30 Ma, most of the rocks presently at the surface had cooled to paleotemperatures <45°C and have not been further heated since that time. In conclusion, the AFT results from Tasmania indicate that the post-Paleozoic thermotectonic history of Tasmania was significantly different than previously suggested based on the regional geologic observations and limited cross-cutting relationships. We suggest that as a result of continental extension along all of its three margins, Tasmania has experienced a complex thermotectonic history, which included at least three major episodes of cooling/denudation since the Early Jurassic. Baillie, P.W., (and many contributions), 1989. Jurassic-Cainozoic. In Burrett, C.F., and Martin, E.L. (eds.), Geology and Mineral Resources of Tasmania. Special Publication Geological Society of Australia 15. 339-409. Kohn, B. P., Gleadow, A. J. W., and Cox, S. J. D., 1999. Denudation history of the Snowy Mountains: Constraints from apatite fission track thermochronology. Australian Journal of Earth Sciences 46. 181-198. O'Sullivan, P. B., Orr, M., O'Sullivan, A. J., and Gleadow, A. J. W., 1999. Episodic Late Palaeozoic to Recent denudation of the Eastern Highlands of Australia: evidence from the Bogong High Plains, Victoria. Australian Journal of Earth Sciences 46: 199-216. Acknowledgements This work was made possible with funding by the Australian Geodynamics Cooperative Research Centre (AGCRC), and this paper is published with the permission of the Director, AGCRC. AGSO, the Tasmania NGMA Project ("TASGO"), and Geotrack International Pty. Ltd also provided financial and field support. Some sample material was provided by the Tasmanian Geological Survey from their registered rock collection. Costs for neutron irradiations were supported by an AINSE grant to the Fission Track Research Group.

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International Conference on Fission Track Dating and Thermochronology

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POST-MINERALIZATION THERMOTECTONIC EVOLUTION OF THE R E G I O N OF THE R E D P B - Z N - A G M I N E , NORTHWEST

DOG

ALASKA

RB. O'SuUivani, K.D. Kelley2 and S. Jennings3 1 Australian Geodynamics Cooperative Research Centre, School of Earth Sciences,The University of Melbourne, Parkville, Victoria 305,2 AUSTRALIA 2 U.S. Geological Survey Box 25046, MS964, Denver, CO 80225, USA 3 Cominco Alaska Inc., Red Dog mine, RO. Box 1230, Kotzebue,AK 99752, USA

Mineralized deposits at and around the Red Dog mine site in northern Alaska (Fig. 1), occur within a substantial belt of Pb-2n-Ag deposits hosted in Devonian-Pennsylvanian(?) clastic sedimentary rocks. A study was initiated in 1998 to better understand the geologic history of Red Dog, including: 1) the geologic conditions and depositional environment of the basin (and host rocks) in which the deposits formed, 2) the geochemistry of mineralizing fluids, 3) the source and pathways for ore fluids and mechanisms for deposition of sulfides, 4) the timing and duration of mineralization, and 5) the timing of subsequent uplift and denudation. Understanding these factors may ultimately aid in identifying favourable areas for additional large sediment-hosted base-metal sulfide deposits.

PACIFIC OCEAN Sediment Isopachs (km) Fold and Thrust Belt

Figure 1. Generalised location map of the Red Dog mine in the northwestern Brooks Range relative to the major tectonic features present throughout Alaska.

To address the timing of post-mineralization uplift and denudation of the region surrounding the Red Dog deposit, 25 apatite fission track (AFT) and 9 zircon fission track (ZFT) analyses have been completed. Original samples were collected from Lower Mississippian-Upper Devonian and Neocomian sedimentary rocks exposed in multiple allochthonous thrust sheets mapped in the region of the mine, and from moderately folded Albian-aged sedimentary rocks located to the north of the present-day mountain front. Importantly, these results also constrain a model for the compressional tectonics responsible for shaping the regional landscape. In particular, the data help constrain whether the allochthons mapped in the Brooks Range have traveled a significant distance (e.g. Mull, 1982; Mayfield etal., 1988), or whether they were in fact locally derived as suggested by Kelley and Brosge (1995).

Fission Track Results The AFT ages range between 67±4 and 42±3 Ma ( ± l a ) (Fig. 2), mean track lengths range between 14.9±0.2 and 13.9±0.1 \xm ( ± l a ) , and standard deviations typically range between 1.30 and 0.98 pm. All of the AFT ages are significantly less than the stratigraphic ages of the samples, which indicates substantial age reduction by thermal annealing following deposition. Furthermore, as shown in Figure 2, all of the reset samples yield AFT ages that define two distinct groups, an older group (n=10) Geological Society of Australia - Abstracts Number 58

FT2^ee with a weighted mean age of ~60±4 Ma (±2a) and a younger group (n=15) with a weighted mean age of ~45±1 Ma (±2a). These ages also define three distinct groups when plotted with respect to location (Fig. 2). Younger AFT ages of -45 Ma are found to the far north and around the mine site (labelled the northern and southern zones on Fig. 2). The middle zone in between these two contains AFT ages of ~60 Ma. Taken together, the AFT ages and track-length data indicate that rocks throughout the region of the Red Dog mine experienced rapid cooling in the Cenozoic during at least two distinct periods. The initial phase occurred during the Paleocene at ~60 Ma, while the second phase occurred during Middle Eocene at -45 Ma. Modelling of these results along with the track length data indicate that the samples cooled rapidly from elevated paleotemperatures ~110°C to -50°C at the time suggested by each of the AFT ages.

162°30'00"

68°30'00"

67°45'00"

Figure 2. Location and results of samples collected for fission track thermochronology in the region of the Red Dog mine. Stars represent the locations of the Red Dog mine (4) and related mineralized deposits (1, Su-Lik; 2, Suds; 3, Alvinella). (A) AFT ages ( ± i a ) for the 25 samples that yielded adequate apatite. (B) ZFT ages ( ± i a ) for the 9 samples analysed.

The ZFT ages range between 231±l6 and 130±9 Ma (Fig. 2). The majority of these ages are equal to or older than the stratigraphic ages of the samples, which indicates that the zircon results are giving primarily provenance information. Two exceptions to this are core samples from Lower MississippianUpper Devonian sedimentary rocks at the mine site. The ages for these samples are -225 Ma, which are distinctly younger than their depositional ages, thus indicating substantial age reduction by thermal annealing following deposition. Furthermore, the younger ZFT ages represent either the time of rapid cooling below temperatures of ~240°C (as in the case of samples from Lower MississippianUpper Devonian sedimentary rocks) or representing provenance information (as in the case of samples from Albian sedimentary rocks). These results also suggest that rapid cooling below -240°C occurred in some of the regional thrust sheets during the Early Cretaceous, and zircons reset during this event were subsequently redeposited into younger units out in front of the range.

FT2 Summary 1) Zircon ages of -225 Ma from Mississippian-aged rocks at the mine site represent partial resetting of older zircons. It is likely that this partial resetting occurred due to deep burial - probably related to stacking of the thrust sheets during Late Jurassic to Early Cretaceous orogenisis. The fact that these ages were not totally reset to Early Cretaceous ages indicates that: 1) burial temperatures during stacking did not exceed ~240°C, and 2) burial depths during stacking did not exceed ~9 km (assumes a prevailing gradient of ~25°C/km and a mean annual surface temperature of ~5°C). 2) Early Cretaceous zircon ages from the Lower Mississippian-Upper Devonian indicate that these rocks, which are presently exposed in the upper-most thrust sheets above the mine site, were themselves buried under a significant thickness prior to rapid cooling during the Early Cretaceous. This cooling occurred presumably in response to the recognised Late Jurassic to Early Cretaceous deformation. These results are doubly important as they indicate that even the upper-most thrust sheets still preserved today were themselves buried under many higher sheets, which have subsequently been removed due to denudation. Such an interpretation is difficult to explain unless a great deal of shortening occurred resulting in the stacking of numerous thrust sheets as the mountain belt propagated northward. A model of relatively minor shortening (e.g. Kelley and Brosge, 1995), can not adequately explain the totally reset ages in overlying thrust sheets, whereas the lower sheets have not seen equivalent temperatures - the possible dip angles on these thrusts indicate that significant shortening must have occurred to produce these relationships. Therefore, it is proposed that: 1) well to the south of the mine site the upper-most thrust sheets still preserved today were first overthrust by a significant number of thrust sheets (now removed) - this resulted in deep burial and exposure to paleotemperatures >240°C and resetting of the ZET ages for these rocks; 2) continued compression resulted in the propagation of the thrust sequence northward into the Red Dog region; 3) denudation during thrust propagation resulted in a total thickness of rocks thrust over the Red Dog region being significantly less than what was originally thrust over the upper sheets still preserved. 3) Mixed zircon ages present in the Neocomian and Albian-aged rocks, which contain single-grain ages ranging from Triassic to Cretaceous, indicate that these rocks were sourced by rocks containing totally reset zircon ages as well as rocks containing partially reset zircon ages. Since deposition during the Cretaceous, these rocks have not been buried to temperatures needed to significantly reduce the zircon ages. 4) Two distinct populations of apatite ages are present. The older population gives ages of ~60 Ma, while the younger gives ages of - 4 5 Ma. When combined with the track length data the resuks indicate that two distinct episodes of rapid km-scale denudation must have shaped the region during the Cenozoic. As compression during the Cenozoic was directed from the south (unlike from the north during Jurassic to Early Cretaceous orogenisis) it is likely that this was due to a totally different orogenic event. Importantly, the entire Red Dog region was still buried under >4 km of material at the beginning of the Cenozoic, and the actual mine site was still buried under at least 4 km of material into the Eocene. Kelley, J.S. and Brosge, W.P., 1995. Geologic framework of a transect of the central Brooks Range: Regional relations and an alternative to the Endicott Mountains Allochthon. AAPG Bulletin 79. 1087-1116. Mayfield, C.F., Tailleur, I.L., and Ellersieck, I., 1988. Stratigraphy, structure, and palinspastic synthesis of the western Brooks Range, northwestern Alaska, in Gryc, G., ed.. Geology and exploration of the National Petroleum Reserve in Alaska, 1974 to 1982. U.S. Geological Survey Professional Paper 1399. 143-186. Mull, C.G., 1982. The tectonic evolution and structural style of the Brooks Range, Alaska: An illustrated summary, in Powers, R.B., ed.. Geologic studies of the Cordilleran thrust bek. Volume 1. Denver, Colorado, Rocky Mountain Association of Geologists. 1-45. Acknowledgements This work was made possible with funding by the U.S. Geological Survey and Australian Geodynamics Cooperative Research Centre (AGCRC). This paper is published with the permission of the Director, AGCRC. Costs for neutron irradiations were supported by an AINSE grant to the Fission Track Research Group.

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

C L U E S T O T H E O R I G I N , T E C T O N I C S AND H Y D R O C A R B O N G E N E R A T I O N H I S T O R Y O F

WiLLisTON B A S I N F R O M A P A T I T E F I S S I O N T R A C K T H E R M O C H R O N O L O G Y K.G. Osadetzi, B.R Kohn2, S. Feinstein3 and RB. 0'Sumvan2

1 Geological Survey of Canada, 3303 33rd St. NW, Calgary, Alberta T2L 2A7, Canada 2 School of Earth Sciences, University of Melbourne, Victoria 3010, Australia 3 Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, P.O. Box 6 5 3 , 8 4 120 Beer Sheva, Israel

The Williston Basin (Fig. 1) is a sub-circular epicratonic Phanerozoic basin, approximately 800 km in diameter. It is included within, and is contiguous with, the Williston Basin region, a geographically more extensive Phanerozoic succession in Manitoba, Saskatchewan, and the adjacent USA (Gerhard etal, 1982). This complicated terminology reflects a stratigraphic continuity that extends well beyond the Williston Basin proper (Burgess et al, 1997). The Williston Basin region overlies portions of the western Canadian Shield, specifically the Paleoproterozoic Trans-Hudson collisional orogen, as well as the margins of the Archean Superior and Hearne/Wyoming cratons (Burwash et al, 1984; Leclair et al, 1997). The Williston Basin region contains several significant petroleum provinces and mineral deposits which are unevenly distributed both geographically and stratigraphically (e.g. Burrus et al, 1996). Group I Example: Richmound {1-31-18-28W3) 2 2 6 7 metres depth

TCC>«,

Group li Example: Balidon (2-11-15-2evV2) 2 3 5 7 metres d e ^ h

IVsck Length (microns)

Figure 1. Regional setting, major tectonic elements and petroleum provinces of Williston Basin. Basin form-lines are indicated by the depth to the base of Carboniferous Madison Gp. Figure 2. Best match thermal history models for Group I (A), undisturbed exposed Shield or shallowly buried Williston Basin region, and Group II (B), deeply buried Williston Basin proper, samples as defined by Osadetz et al. (1998). Models for Group I samples record regional Late Precambrian cooling, whereas Group II models commence with Late Paleozoic cooling. Note the similarity of Mesozoic and Cenozoic thermal histories for both groups

Geological Society of Australia - Abstracts Number 58

FT2^ee Apatite fission-track (AFT) thermochronology and organic maturity data (Crowley etal, 1985; Crowley and Kuhlman, 1988; Kohn et al, 1995; Osadetz et al, 1998) indicate spatial and temporal variations in the thermal history of Williston Basin and adjacent Precambrian Shield of North America (Figs. 1, 2). On the Canadian Shield and under thin portions of the Williston Basin region AFT data typically record Late Precambrian cooling followed by intervals of Phanerozoic heating and cooling, the pattern of which mimics the Phanerozoic sedimentation history. Elsewhere, the earlier thermal history is overprinted by resetting of AFT clocks during the Late Paleozoic. The region affected by this Late Paleozoic thermal event includes both a broad portion of the "central basin", extending more than 300 km from the basin centre where the Phanerozoic succession is up to 5 km thick, and along a linear trend, at shallower depths, stretching from the Williston to Athabasca Basins (Fig. 2). The extent of this event follows patterns of Devonian Elk Point Group subsidence.

400

300

200

Age (Millions of Years)

100

0

gpo

'

600

4^0

^

T I M E (Ma)

Figure 3. Representative Williston Basin burial history model similar to that used to constrain thermal history models. Notice the acceleration of subsidence accompanying Kaskaskian and Foreland Basin sedimentation in the Late Paleozoic and early Mesozoic, respectively. Comparison with Figure 3 indicates that the total resetting of AFT clocks during the Late Paleozoic precedes maximum burial in the Paleogene, a clear indication that basal heat flow was higher in the Late Paleozoic than in the Paleogene. Figure 4. Generalised thermal history models for Group I (undisturbed exposed Shield or shallowly buried Williston Basin region), Group II (deeply buried Williston Basin proper) and select Group III (High Rock Lake and Lake St. Martin impact crater samples generally restricted to the eastern limits of Williston Basin region, as defined by Kohn et al., 1995) sample locations. Group II is distinguished from Group I by the total resetting of AFT clocks in Late Paleozoic time. Notice that the subsequent history of Group II and Group III samples is similar to Group I samples during the Mesozoic and Cenozoic.

Our analysis distinguishes the Late Paleozoic event extending into Williston Basin region from the Early Paleozoic origin of Williston Basin proper (Figs. 3, 4). We infer that the Williston Basin proper formed as the result of at least two'distinct thermo-mechanical processes. Initial Ordovician-Silurian subsidence is generally contemporaneous with the origin of other North American intracratonic basins (Sloss, 1984). Work by others attribute this subsidence interval to the thermal contraction of a large sill intruded into the lower crust (e.g. Hamdani et al., 1994; Ahern and Mrkvicka, 1984). An upward migration of the lithosphere-asthenosphere boundary (LAB) in Devonian time provides a plausible mechanism for linking Late Paleozoic and Early Mesozoic tectonics in the Williston Basin region. These events include: accelerated Middle Devonian to Carboniferous subsidence, a Late Paleozoic increase in basal heat flow, followed by Late Paleozoic-Early Mesozoic uplift and erosion. This was succeeded by Triassic and Jurassic subsidence signifying the decay of the heat flow anomaly. Whereas accelerated Devonian-Carboniferous subsidence and the Late Paleozoic thermal event conform broadly to the outline of the Devonian Elk Point Basin, the Early Mesozoic subsidence is restricted to that region where the Elk Point and Williston Basins are superimposed. The restriction of Triassic-Jurassic subsidence to the region of Williston Basin proper, even though the Late Paleozoic event affects a broader region, is inferred to indicate a mechanical response, probably a phase change, involving the igneous sill responsible for the initial Ordovician-Silurian subsidence of Williston Basin.

FT2^ee We conclude that the accelerated Devonian-Carboniferous subsidence and the succeeding Late Paleozoic thermal event are both responses to thermal thinning of the continental lithosphere, temporally separated by the thermal inertia of the continental lithosphere. The differing response between areas within the Williston Basin proper, and those outside, may be due to the effects of Late Paleozoic heating on the intrusion inferred responsible for initiating Williston Basin subsidence in Early Paleozoic time (Ahern and Mrkvicka, 1984). Therefore the origin and pattern of the Williston Basin subsidence is inferred to be episodic and fortuitous as opposed to continuous and discrete. Other epicratonic basins (e.g. Illinois Basin) exhibit either accelerated subsidence or thermal anomalies in late Paleozoic time (e.g. Kominz and Bond, 1991). Together these may define a previously unrecognized, but widespread and coordinated interaction with the sublithospheric mantle. A final phase of subsidence contemporaneous with the formation of the Cretaceous-Paleogene Interior Seaway and Laramide orogeny (Fig. 3) may be due to other processes interacting with pre-existing lithospheric structure (Burgess et al., 1997), but without a change in basal heat flow (Fig. 4). The Late Paleozoic thermal event identified in this study has great significance for potential petroleum systems in the Sauk and Tippecanoe sequences, both of which reached the oil window for Type II organic matter at that time (Osadetz et al., 1998). Within the region affected by the Late Paleozoic thermal event is a persistent region of elevated heat flows associated with crustal structure, specifically the North American Central Plains Conductivity Anomaly and the Nesson Anticline (Majorowicz et al., 1988; Fig. 1). Along the Nesson Anticline oil windows in Paleozoic strata occur 750-1250 m higher in comparison to other regions affected by the Late Paleozoic heating (Osadetz et al., 1989). This persistent geographical heat flow variation is attributed to crustal compositional differences originating during Precambrian Trans-Hudson orogeny (Morel-a-l'Hussier et al, 1990). Together these effects suggest early opportunities for hydrocarbon generation, migration and entrapment in the Williston Basin which are not indicated by analysis of the Devonian and Carboniferous petroleum systems alone (Osadetz et al, 1998). Ahern, J. L., and Mrkvicka, S. R., 1984, A mechanical and thermal model for the evolution of the Williston Basin. Tectonics, 3 (1): 79-102. Burgess, P., Gurnis, M., and Moresi, L., 1997, Formation of sequences in the cratonic interior of North America by interaction between mantle, eustatic, and stratigraphic process. Geological Society of America, Bulletin, v. 108, p. 1515-1535. Burrus, J., K. G. Osadetz and Wolf, S., 1996a, Geochemical and numerical modelling constraints on oil expulsion and accumulation in the Bakken and Lodgepole petroleum systems of the Williston Basin (Canada-USA), Bulletin of Canadian Petroleum Geology, v. 43, p. 429-445. Burwash, R. A., McGregor, C. R., and Wilson J., 1994. Chapter 5 ~ Precambrian basement beneath the Western Canada Sedimentary Basin; in, G. Mossop and I. Shetsen (compilers); Geological Atlas of the Western Canada Sedimentary Basin. Canadian Society of Petroleum Geologists and Alberta Research Council, Calgary and Edmonton, p. 49-56. Crowley, K.D., Ahern, J.L. and Naeser, C.W. 1985, Origin and epeirogenic history of the Williston Basin: evidence from fissiontrack analysis of apatite. Geology v. 13, p. 620-623. Crowley, K.D. And Kuhlman, S. L. 1988, Apatite thermochronometry of western Canadian Shield: implications for the origin of Williston Basin. Geophysical Research Letters. V. 15, no. 3, p. 221-224. Gerhard, L. C., S. B. Anderson, J. A. LeFever and C. G. Cadson, 1982, Geological development, origin, and energy mineral resources of Williston Basin, North Dakota, American Association of Petroleum Geologists Bulletin, v. 66, p. 989-1020. Grieve, R., Rupert, J., Smith, J., and Therriault, A. 1995. The record of terrestrial cratering. GSA Today, v. 5, p. 189-196. Hamdani, Y., Mareschal, J.-C., and Arkani-Hamed, J., 1994, Phase change and thermal subsidence of the Williston Basin. Geophysical Journal International, v. 116, p. 585-597. Kohn, B.R, Osadetz, K.G., and Bezys, R.K. 1995, Apatite fission-track dating of two crater structures in the Canadian Williston Basin. Bulletin of Canadian Petroleum Geology, v. 43 no. 1, p. 54-64. Kominz, M. A. and Bond, G. C., 1991, Unusually large subsidence and sea-level events during middle Paleozoic time: New evidence supporting mantle convection models for super continent assembly. Geology, v. 19, p. 56-60. Leclair, A. D., Lucas, S. B., Broome, H. J., Viljoen, D. W., and Weber, W., 1997, Regional mapping of Precambrian basement beneath Phanerozoic cover in southeastern Trans-Hudson orogen, Manitoba and Saskatchewan. Canadian Journal of Earth Sciences, v. 34, p. 618-634. Majorowicz, J. A., Jones, F. W., and Osadetz, K. G., 1988, Heat flow environment of the electrical conductivity anomalies in the Williston Basin, and occurrence of hydrocarbons. Canadian Bulletin of Petroleum Geology, v. 36, no. 1, p. 86-90. Morel-a-l'Huissier, P., Green, A. G., Jones, A. G., Latham, T., Majorowicz, J. A., Drury, M. J., and Thomas, M. D. 1990, The crust beneath the Williston Basin from geophysical data; in B. Pinet and C. Bois eds., The Potential of Deep Seismic Reflection for Hydrocarbon Exploration. Edition Technip, Paris, p. 141-160. Osadetz, K. G., Kohn, B. R, O'Sullivan, P., Feinstein, S., Hannigan, P. K., Everitt, R. A., Gilboy, C. R, Bezys, R. K., and Stasiuk, L. D., 1998, Thermotectonics of the Williston Basin and environs: variations in heat flow and hydrocarbon generation; in

FT2 J. E. Christopher, C. F. Gilboy, D. F. Paterson and S. L. Bend, eds., Proceedings of the Eighth International Williston Basin Symposium, Regina, October 18-20, 1998, no. 13, p. 147-165. Osadetz, K. G., Snowdon, L. R., and Stasiuk, L. D., 1989, Association of enhanced hydrocarbon generation and crustal structure in the Canadian Williston Basin; in Current Research, Part D, Geological Survey of Canada, Paper 89-lD, p. 35-47. Sloss, L. L., 1984, Comparative anatomy of cratonic unconformities; in J. S. Schlee ed.. Interregional Unconformities and hydrocarbon accumulation. American Association of Petroleum Geologists, Memoir 36, p. 1-6.

Acknowledgments This study was supported by the Australian Research Council and the Australian Institute of Nuclear Science and Engineering and Geological Survey of Canada Project 950003. Manitoba Energy and Mines staff and Saskatchewan Energy and Mines staff, especially Dr. D. F. Paterson, and Ms. F. M. Haidl, facilitated the collection of basement samples from petroleum boreholes.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

P H A N E R O Z O I C T H E R M O T E C T O N I C S O F T H E CANADIAN S H I E L D : A O N E K I L O M E T R E A P A T I T E F I S S I O N T R A C K T H E R M O C H R O N O L O G Y P R O F I L E AT CANADA'S R E S E A R C H L A B O R A T O R Y , PINAWA,

DEEP

UNDERGROUND

MANITOBA

K.G. Osadetzi, B.P. Kohn2, R.A. Everitt3, S. Feinstein^ and PB. 0'Sullivan2 1 Geological Survey of Canada, 3303 33rd St. NW, Calgary,AlbertaT2L 2A7, CANADA 2 School of Earth Sciences, University of Melbourne, Victoria 3010, AUSTRALIA 3 AECL,Whiteshell Laboratories, Pinawa, Manitoba, ROE ILO, CANADA 4 Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, P.O. Box 6 5 3 , 8 4 120 Beer Sheva, ISRAEL

The suitability of igneous plutons as sites for the underground disposal of nuclear fuel waste depends on numerous factors, among which is the fracturing history of the host pluton. There are indications that some plutons in the Canadian Shield have remained sparsely fractured despite protracted and complicated epeirogenic histories (Everitt et al, 1998). The formation and propagation history of fractures in igneous plutons is often problematical because the fracture filling minerals are not suitable for radiometric dating techniques. The low temperature thermal history of such plutons provides both, information that characterises the Phanerozoic tectonic history of the Canadian Shield, and a potential constraint on the formation and propagation of fractures filled with minerals containing fluid inclusions. Atomic Energy of Canada Limited's Underground Research Laboratory (URL) at Pinawa, Manitoba provides an ideal test site for the application of apatite fission-track (AFT) thermochronology to a profile through a pluton in the Archean Superior Province of the Canadian Shield (Everitt et al, 1996; Fig. 1). The combination of mine excavations and boreholes at the URL provide a continuous section extending over a kilometre below the peneplained Shield (Fig. 2). The URL lies - 1 0 km east of the outcrop margin of the Phanerozoic Williston Basin (Fig. 1). This proximity, combined with gradual rates of stratal thickening provide constraints on Phanerozoic burial and thermal history that assists with the interpretation of the AFT profile from the granitic Lac du Bonnet batholith (2665±20 Ma; Krogh et al, 1976) at the URL. LEGEND

Subprovmce' boursdaj-s" ^ R ^ ^ F A N S U B P R O \ 1 N< F T^ PE

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Figure 1. Regional setting of the Lac du Bonnet batholith (LDBB), host pluton of the Underground Research Laboratory (URL). BR - Bird River Subprovince. WR - Winnipeg River Subprovince. After Everitt et al (1998). Geological Society of Australia - Abstracts Number 58

FT2 Fission Track Age (Ma)

Legend • AFT Age

1000

• AFT Mean HCTL

1200

I ) I I I I i I I j i ! n I I M I [ M i I I I H i I I M I I I • i I [ M I I { I I i I

0

11

12

13

14 15

Mean HCTL (microns) Figure 2. Profile of AFT ages and mean HCTL,s for the profile at the URL. Qualitative indications of AFT age and HCTL variations with depth are indicated by dotted and dashed lines, respectively.

To the southwest of the URL, Lower Cretaceous sandstones and carbonaceous sandstones lie unconformably on Precambrian basement in an Early Paleozoic (Late Ordovician-Silurian) impact crater at West Hawk Lake (Short, 1970). West Hawk Lake lies - 6 5 km southeast of the URL and ~30 km east of the outcrop margin of the Phanerozic succession. This indicates that the Williston Basin Paleozoic succession was km thick and once contiguous with the Paleozoic succession in Hudson Bay, was probably completely eroded from the URL site prior to subsidence and burial by the Cretaceous to Paleogene succession (McCabe and Barchyn, 1982). These inferences are consistent with other regional stratigraphic relationships and analysis. The Cretaceous to Paleogene succession, now eroded from the URL site is constrained by coalification data to have been several kilometres thick under basinal geothermal gradients similar to those of the present day, ~20°C/km.

The AFT profile at the URL exhibits a progressive decrease in age from ~380 Ma at the surface to -250 Ma at depths of -1100 m (Fig. 2). The 'dog-legged' mean horizontal confined track length (HCTL) profile decreases from -11.9 microns at the surface to -11.1 microns just below 500 m then increases to -12.2 microns just above 650 m below which it decreases progressively to about 11.2 microns at -1100 m (Fig. 2). AFT age and length parameters at the present surface suggest that the URL has a thermal history similar to that of much of the exposed Canadian Shield and portions of the shallowly buried basement below the Williston basin region, away from the Williston Basin proper (Osadetz et al, 1998; Kohn etal, 1995; Crowley and Kuhlman, 1988; Crowley etal, 1985). Characteristically AFT ages at the basement surface in such regions are greater than 300 Ma, with mean HCTL's between -11-12 microns. Elsewhere forward models of AFT parameters suggest that such samples preserve a protracted thermal history that begins with the Late Precambrian epeirogeny, that formed the basement surface onlapped by Cambrian successions, through the Phanerozoic to the Present. Thermal history models indicate a low temperature history heating and cooling that mimics the Phanerozoic burial history inferred for the URL site. Such models suggest that the base of the profile reached maximum Phanerozoic temperatures, approaching 100°C, at about mid-Carboniferous time, coincident with maximum Paleozoic burial and an interval of elevated basal heat flow. The time of highest basal heat flow appears to be coincident with a Late Paleozoic thermal event inferred from basement samples in Williston Basin proper, the effects of which are typical only of samples lying approximately 300 km west of the URL. The formation of the 'dog-leg' in the HCTL profile at the URL is the strongest indication for cooling following such an event. A subordinate temperature maximum was reached in the Paleogene at the time of maximum burial by sedimentation in the Larmide Interior Seaway of North America (i.e. -3-4 km in the study area, assuming present-day geothermal gradients). At this time the base of the URL profile reached temperatures approaching 95°C. It is notable that the base of the URL profile appears to record slightly higher temperatures in Late Paleozoic time rather than in Paleogene time, the latter being the time of maximum regional Phanerozoic sedimentary burial.

FT2 AFT data from the URL profile at Pinawa confirm a protracted and episodic Phanerozoic history of burial, uplift and erosion for the Lake du Bonnet batholith. There appears to be a link between temporal variations in heat flow among samples lying in Williston Basin proper and those comprising the URL profile, especially during the late Paleozoic. This linkage is not readily discerned elsewhere on the Shield or beneath shallow portion of Williston Basin (Osadetz et al, 1998). The study of the Lac du Bonnet batholith for the characterisation of its low temperature tectonic history demonstrates that AFT thermochronology can be applied to the assessment of plutons, elsewhere, that may be considered for the underground disposal of nuclear fuel waste. Crowley, K.D., Ahern, J.L. and Naeser, C.W. 1985, Origin and epeirogenic history of the Williston Basin: evidence from fissiontrack analysis of apatite. Geology v. 13, p. 620-623. Crowley, K.D. And Kuhlman, S. L. 1988, Apatite thermochronometry of western Canadian Shield: implications for the origin of Williston Basin. Geophysical Research Letters, v. 15, no. 3, p. 221-224. Everitt, R., Brown, A., Ejeckam, R., Sikorsky, R., and Woodcock, D., 1998. Litho-structural layering within the Archean Lac du Bonnet batholith, at AECL's Underground Research Laboratory, southeastern Manitoba. Journal of Structural Geology, v. 20, no. 9/10, p. 1291-1304. Everitt, R. A., McMurry, J., Brown, A., and Davidson C. C., 1996. Geology of the Lac du Bonnet batholith, inside and out: AECL's Underground Research Laboratory, southeastern Manitoba. Field Trip Guidebook B5, Geological Association of Canada/Mineralogical Association of Canada. 1996 Annual Meeting Winnipeg, Manitoba May 27-29, p. 6 l . Kohn, B.R, Osadetz, K.G., and Bezys, R.K. 1995, Apatite fission-track dating of two crater structures in the Canadian Williston Basin. Bulletin of Canadian Petroleum Geology, v. 43 no. 1, p. 54-64. Krogh, T. E., Davis, G. L., Ermanovics, I. And Harris, N. B. W., 1976. U-Pb isotopic ages of zircons from the Berens block and English River gneiss bek. Proceedings of the 1976 Geotraverse Convference, University of Toronto 12-1, p. 46. McCabe, H. R., and Barchyn, D., 1982. Paleozoic stratigraphy of southwestern Manitoba, Field Trip Guidebook 10, Geological Association of Canada/Mineralogical Association of Canada. 1982 Annual Meeting Winnipeg, p. 48. Osadetz, K. G., Kohn, B. R, O'Sullivan, R, Feinstein, S., Hannigan, R K., Everitt, R. A., Gilboy, C. F., Bezys, R. K., and Stasiuk, L. D., 1998, Thermotectonics of the Williston Basin and environs: variations in heat flow and hydrocarbon generation; in J. E. Christopher, C. F. Gilboy, D. F. Paterson and S. L. Bend, eds.. Proceedings of the Eighth International Williston Basin Symposium, Regina, October 18-20, 1998, no. 13, p. 147-165. short, N. M., 1970. Anatomy of a meteorite impact crater: West Hawk Lake, Manitoba, Canada. Geological Society of America, Bulletin, v. 81, p. 609-648.

Acknowledgments This study was supported by the Australian Research Council and the Australian Institute of Nuclear Science and Engineering, Geological Survey of Canada Project 950003, and as part of the AECL Used Fuel Disposal Program, funded by Ontario Power Generation. We also thank URL staff for their assistance during sample collection activities.

FT2

International Conference on Fission Track Dating and Thermochronology

M A P P I N G T H E E X T E N T O F L A T E C R E T A C E O U S M O R P H O T E C T O N I C R E A C T I V A T I O N IN NORTHERN

NAMIBIA

M Raabi'2, R.W. Brown2, K. GaUagher3,A. Carter^ and K.Weberi 1 Institut fiir Geologie und Dynamik der Lithosphare, Goldschmidtstrasse 3,37077 Gottingen, Germany 2 School of Earth Sciences,The University of Melbourne, Melbourne 3010, Australia 3 TH. Huxley School of Environment, Earth Science and Engineering, Imperial College of Science,Technology and Medicine, South Kensington, London, SW7 2AS, England 4 Research School of Geological and Geophysical Sciences, Birkbeck College and University College, Gower Street, London, WCIE 6BT, United Kingdom

Summary Apatite fission track data from the Atlantic margin and hinterland of northern Namibia has previously been used to document a discrete phase of tectonism, occurring about 50 m.y. after continental break-up, involving reactivation within an intracontinental zone defined by major NE-SW trending crustal-scale structures (e.g. Brown et al, 1999; Gallagher and Brown, 1999). Here we present data from an additional set of 72 outcrop samples from northern Namibia which provide improved constraints on the chronology of tectonism and the geometry and extent of this zone of intracontinental reactivation. The new data set can be split into two major groups: samples with younger apparent ages (c. 90-60 Ma) and relatively long mean track lengths (MTL) of c. 13.5 pm and a second group with older apparent ages (>200 Ma) with MTL <12 pm. Thermal histories derived from the FT data by forward modelling indicate a discrete phase of accelerated crustal cooling between c. 90-60 Ma. Estimates of maximum palaeotemperatures immediately prior to the onset of cooling range from over 110°C near the coast to less than 60°C within the highlands in the interior. The highest palaeotemperatures, and consequently higher cooling rates, occur within a c. 150 km wide coastal zone and generally decrease systematically inland. However, this regional pattern is modified by a well defined NE-SW trending zone of higher palaeotemperatures J20 140 jgo 20° (lower apparent ages) which cross-cuts the 16° 16° regional pattern and coincides closely with the intracontinental arm of the Damara Angola mobile belt (Fig. 1). Geological and Tectonic Setting Namibia 250

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Contours represent fission track ages In Ma

22°

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O new constraint apatite fission tracl< ages • previous apatite fission track ages by Brown and Haad< Waifish Bay

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The crustal structure in northern Namibia is dominated by the intracontinental branch of the Pan African Damara mobile belt. It separates the Congo and Kalahari cratonic terranes and is divided into several tectonos22^ tratigraphic zones (Miller, 1983). Regional lineaments form southwest to northeast striking boundaries and extend the 24^ Precambrian transcontinental Mwembeshi shear zone (Coward and Daly, 1984). Deep erosion of the pan-African Damara Orogen, tectonic activity along the eastern margin of

Figure 1. Contour-plot of all available apatite fission track age data for northern Namibia. Note the well defined NE-SW trending intracontinentaUone of younger apparent apatite FT age (higher mum palaeotemperatures) which is aligned with the regional tectonic fabric of the Damara mobile belt in this region and coincides closely with the Central and Northern Zone (Miller, 1983).

Africa, deposition of the Nama Group sediraentS and the Karoo megasequence affect^^

Permo-Carboniferous

times to mid-Jurassic times. Early Mesozoic tectonic reactivation of some of the regional

Geological Society of Australia - Abstracts Number 58

FT2 lineaments is recorded by coarse clastic sediments deposited within northeast striking half-graben structures between the Otjohorongo Thrust and the Omaruru Lineament-Waterberg Thrust. Continental break-up and initial sea-floor spreading in the South Atlantic during the Early Cretaceous was accompanied by eruption of large amounts of continental flood basalts and extensive mafic dike swarms on both sides of the Atlantic Ocean. Twenty six Early Cretaceous alkaline intrusions (137-124 Ma) were preferentially emplaced along the Mesozoic half-graben-structures and form the Damaraland Igneous Province (DIP). The morphology of the Atlantic margin can be characterized by an interior highland region separated from a dissected coastal zone by a prominent erosional escarpment. The escarpment diminishes in the area of the DIP. Remnants of Permo-Carboniferous glaciogenic deposits indicate that parts of the present land surface in the north and in the highland regions corresponds with the Permo-Carboniferous land surface (Martin, 1975). New AFT Results The new apatite fission track ages range from 59 to 550 Ma with mean track lengths ranging from 9.8 to 14.5 pm. The age vs. elevation diagram (Fig. 2 A) reveals a very strong correlation of age with elevation with the older ages occurring at higher elevations. The younger apatite FT ages (c. 90-60 Ma) in the region of the Damaraland Igneous Province are significantly younger than the intrusion (or reset) age of c. 137-124 Ma of igneous complexes within the DIP. This means that the currently exposed level of these igneous bodies remained buried beneath a substantial cover for up to 50 m.y. before being exposed in the Late Cretaceous. The oldest ages from the Precambrian Kamanjab basement inlier in the north and the interior (Damara and pre-Damara) highland region are associated with strongly annealed MTL between 9-11 pm. Track length distributions and thermal modelling suggests that these samples cooled quickly in the Late Cretaceous (70-60 Ma) from maximum palaeotemperatures of 90±5°C to near surface temperatures.

o

g

1000

-

100

200 300 400 Apparent Apatite Age (Ma)

500

600

200 300 400 Apparent Apatite Age (Ma)

600

Figure 2. The relationship between apatite FT age and elevation (A), and mean track length (B). All apatite apparent ages are much younger than the stratigraphic age of the host rock. Younger ages are represented by both intrusive Mesozoic and Damara metamorphic rocks. Error bars are ±2G for ages and have been omitted on the track lengths for clarity.).

Discussion Direct estimates of the palaeogeothermal gradient during the Late Cretaceous are unavailable, but the lack of any igneous rocks of this age and the c. 50 m.y. period since the last major igneous activity suggests that the present day gradient of c. 20°C.km-i is representative of the Late Cretaceous. However, even if palaeogeothermal gradients were significantly elevated, the estimated palaeotemperatures (in some places >110°C) reflect a major geomorphic response to the Late Cretaceous reactivation along the margin and interior. The over all picture leads to the conclusion that a substantial

FT2 amount of Mesozoic cover must have been eroded since break up. Secondly, the onset of accelerated erosion is very well recognised in all samples with an apparent younging from 80-60 Ma in the south to c. 90 Ma in the north. Our new data enables the extent of the intracontinental zone of reactivation to be clearly defined and confirms the existence of a significant morphotectonic episode during the latest Cretaceous in northern Namibia. These observations add weight to the proposal that plate kinematic changes between South America, Africa and Antarctica, recorded by complex fracture zone geometries within the South Atlantic and West Indian ocean basins between magnetic anomalies C34 and C24 (84-56 Ma), are linked to this period of intracontinental tectonism in Africa and South America (Brown et al., 1997, 1999; Harman et aL, 1998). Brown, R.W., Gallagher, K., Gleadow, AJ.W. and Summerfield, M.A. 1999. Morphotectonic evolution of the South Atlantic margins of Africa and South America., In (Ed.), Summerfield, M.A., Geomorphology and Global Tectonics, John Wiley and Sons Ltd., Chichester, 257-283. Brown, R.W., Gallagher, K. and Gleadow, A.J.W. 1997. Plate kinematics of the South Atlantic and Indian Ocean basins and intracontinental tectonics of southern Africa, Abstract, Chapman Conference on The History and Dynamics of Global Plate Motions, American Geophysical Union, Marshall, California, June 17-22. Coward, M.P. and Daly, M. C. 1984. Crustal lineaments and shear zones in Africa: their relationship to plate movements, Precambrian Research 24, 27-45. Gallagher, K. and Brown, R.W. 1999. Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa, Philisophical Transactions of the Royal Society, London, 357, 835-859. Harman, R. Gallagher, K., Brown, R., Raza, A. and Bizzi, L. 1998. Accelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceous, Journal of Geophysical Research 103, 27,097-27,105. Martin, H. 1975. Structural and paleogeographical evidence for an Upper Paleozoic sea between South Africa and South America, in lUGS 3rd Gondwana Symp., 37-51, Canberra. Miller, M.R. 1983. The Pan-African Damara Orogen of South West Africa/Namibia, Special Publication of the Geological Society of South Africa 11, 431-515. Acknowledgements:

This research was partly funded by the Deutsche Forschungsgemeinschaft (DFG), the Deutscher Akademischer Austauschdienst (DAAD) as a part of the tertiary higher education special program, and the Natural Environment Research Council (Grant GR9/1573). MR thanks the Fission Track Research Group in Melbourne for their encouragement and extremely helpful and friendly support. Fission track research at The University of Melbourne is supported by grants from the Australian Institute of Nucelar Science and Engineering and the Australian Research Council.

FT2

International Conference on Fission Track Dating and Ttiermochronology

F T 2 '

FISSION-TRACK ANNEALING IN "ZERO-DAMAGE" ZIRCONS: FIELD CONSTRAINTS AND AN EMPIRICAL MODEL

M.K. Rahni, M.T. Brandon2, G.E. Batt2 and J.I. Garver3 1 Institut fiir Mineralogie, Petrologie iind Geochemie, University of Freiburg, Germany 2 Department of Geology and Geophysics,Yale University, New Haven, CT, USA 3 Geology Department, Union College, Schenectady, NY, USA

Radiation damage, mainly due to alpha recoil tracks, has a distinct influence on fission-track (FT) annealing in zircon (Kasuya and Naeser, 1988, Yamada et al, 1995). It has generally been assumed that this radiation damage effect is annealed at temperatures just below the FT partial annealing zone (PAZ). However, our observations and others reported in the literature indicate that that the physical results of radiation damage, such as changes in colour, broadening of x-ray peaks, and thermoluminescence, persist to temperatures comparable or perhaps even greater than the FT PAZ. We propose that the persistence to high temperatures of various types of radiation damage in zircon is responsible for the conflicting estimates for TPAZ, the temperature range for FT PAZ in zircon. To address this problem, we have examined the internal consistency of available experimental annealing studies of FTs in "zero-damage" zircons (Kasuya and Naeser, 1988, Carpena, 1992, Yamada et al, 1995), created by first annealing all radiation damage at high temperatures and then neutron irradiaton to produce induced FTs. This analysis shows that all available experimental data give very similar results. They are fit with similar significance by both the "parallel" and "fanning" annealing equations, but the best-fit fanning equation is preferred because it appears to be slightly better constrained. Comparison with FT annealing data for natural zircons with varying amounts of alpha damage (Tagami et al, 1990, Yamada et al, 1995, Galbraith and Laslett, 1997, Tagami et al, 1998) shows that TPAZ and Tc are greatest for the zero-damage zircons. Compilations of geologic constraints on TPAZ and effective closure temperature Tc for zircon FTs do not or only partly overlap with curves of proposed models for the zircon partial annealing zone and closure temperature. The deviation from experiment-based curves is consistent with the fact that natural zircons have a more complex evolution in their temperature history and damage state. Zircons with young FT ages or low U content come closest to the predictions of the zero-damage model, whereas those with older FT ages (>1030 Ma) show much lower TPAZ and Tc (e.g. Zaun and Wagner, 1985). The zero-damage model can be used to predict: 1) TPAZ and Tc in cases of very rapid cooling from high temperatures, and 2) the temperature needed to ensure complete annealing of FTs in a mixed zircon population (e.g. detrital zircons). The model also provides a starting point for development of a more comprehensive model of FTs in natural zircons with evolving radiation damage. Carpena, J. 1992. Fission track dating of zircon: zircons from Mont Blanc Granite (French-Italian Alps). Journal of Geology 100, 411-421. Galbraith, R. F., Laslett, G. M. 1997. Statistical modelling of thermal annealing of fission tracks in zircon. Chemical Geology 140, 123-135. Kasuya, M., Naeser, C. W. 1988. The effect of a-damage of fission-track annealing in zircon. Nuclear Tracks and Radiation Measurements 14, 477-480. Tagami, T., Ito, H., Nishimura, S. 1990. Thermal annealing characteristics of spontaneous fission tracks in zircon. Chemical Geology 80, 159-169. Yamada, R., Tagami, T., Nishimura, S, Ito, H. 1995. Annealing kinetics of fission tracks in zircon: an experimental study. Chemical Geology 122, 249-258. Zaun, P. E., Wagner, G. A. 1985. Fission track stability in zircons under geological conditions. Nuclear Tracks and Radiation Measurements 10, 303-307.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

( U - T H ) / H E AND F T A P A T I T E D A T I N G A L O N G A 1 6 EUROPEAN

F T 2 '

K M T U N N E L C R O S S S E C T I O N IN T H E

ALPS

M.K. Rahni and M.A. House2 1 Institut fiir Mineralogie, Petrologie und Geocheinie,Albert-Ludwigs-Universitat, 79104 Freiburg, Germany 2 Division of Geological and Planetary Sciences, California Institute of Technology MSlOO-23, Pasadena, CA 91125, U. S. A.

Studies of helium diffusion in apatite have established the (U-Th)/He method as a new dating tool with a wide variety of potential applications (Zeitler et al, 1987; Lippolt et al, 1994; Wolf et al, 1996; Warnock et al, 1997). The very low closure temperature of the helium system in apatite (75°C, provided a cooling rate of 10°C/m.y.; Wolf et al, 1996), suggests this method will provide data that is vital to investigations into the thermal development of the uppermost crust, as well as constraints on landscape evolution (e.g. House etal, 1998). Until recently, such constraints were mainly based on apatite fission track thermochronometry (e.g. Rahn and Grasemann, 1998), but the higher closure temperature of this system precluded detailed integration of time-temperature histories with surface processes. In this study, we integrate these two techniques to investigate the thermal and surface evolution of a moderately to fast exhuming orogen of great historical significance: the Central Alps of Europe. We have chosen to focus on a classic Alpine cross-section along the l6 km Gotthard road tunnel and overlying Gotthard Pass. Schar et aL, (1975) and Wagner et al., (1977) showed that the Gotthard region of central Switzerland has been continuously exhuming since approx. 18 Ma. Exhumation rates deduced from AFT data approach today's surface uplift rates (0.4-0.7 mm/yr, Schar et al., 1975). Contoured AFT age-elevation profiles suggest that steady exhumation occurred between 10-6 Ma, and that growing topographic relief has exhuded a moderate influence on the geometry of near surface isotherms during this time. However, the subsequent exhumation history remains poorly known, as does the evolution of the developing topographic relief. In order to obtain additional limits on the rate of Late Cenozoic exhumation, as well as first order constraints on the evolution of the Alpine landscape during this time, we are gathering AFT data and (U-Th)/He ages in apatite from samples taken from along the Gotthard tunnel, as well as along the overlying landsurface. Our sample transects correspond to a range of 1.6 km in elevation over a lateral distance of 16 km, and so should provide the maximum possible range in elevation at the surface, while cutting the deepest available point under the Gotthard pass, revealing the most protracted exhumation history, and the magnitude of the influence of topographic relief on the geometry of near surface isotherms. Our preliminary results are of special interest in the context of the development and duration of thermal anomalies due to fluid flow along fracture systems in the crystalline rock and deep-rooted sedimentary synclines associated with a planned 54 km railway base tunnel below the Gotthard Pass. Farley, K., Wolf, R., Silver, L. 1996. The effects of long alpha-stopping distances on (U-Th)/He dates. Geochimica et Cosmochimica Acta 60, 4223-4229. House, M.A., Wernicke, B.P., and Farley, K.A. 1998. Dating topography of the Sierra Nevada, California, using apatite /U-Th)He ages. Nature 396, 66-69. Lippolt, H.J., Leitz, M., Wernicke, R.S., and B. Hagedorn, B. 1994. (U+Th)/He dating of apatite: experience with samples from different geochemical environments. Chemical Geology 112, 179-191. Rahn, M.K., Grasemann, B. 1999. Numerical and Monte Trax modelling on fission track data from the Glarus Alps: thermal and tectonic evolution of a thrust plane during metamorphism and exhumation. Earth and Planetary Science Letters 169, 245-259. Schar, J.P., Reimer, G.M., Wagner, G.W. 1975. Actual and ancient uplift rate in the Gotthard region, Swiss Alps: A comparison between precise levelling and fission-track apatite age. Tectonophysics 29, 293-300. Wagner, G.W., Reimer, G. M., Jager, E. 1977. Cooling ages derived by apatite fission-track, mica Rb-Sr and K-Ar dating: The uplift and cooling history of the Central Alps. Memorie degli Istituti di Geologic e Mineralogia dell'Universita di Padova 30, 27p. Wolf, R.A., Farley, K.A., Silver L.T. 1996. Helium diffusion and low-temperature thermochronometry of apatite. Geochimica et Cosmochimica Acta 60, 4231-4240.

Geological Society of Australia - Abstracts Number 58

FT2 Wolf, R.A., Farley, K.A., Silver, L.T. 1997. Assesment of (U-Th)/He thermochronometry: The low-temperature history of the San Jacinto mountain, California. Geology 25, 65-68. Zeitler, P.K., Herczig, A.L., McDougall, I., Honda, M. 1987. U-Th-He dating of apatite: a potential thermochronometer. Geochimica et Cosmochimica Acta 51, 2865-2868.

International C o n f e r e n c e on Fission Track Dating and T h e r m o c h r o n o l o g y

F T 2 '

A REVISED STUDY OF THE EXHUMATION OF THE CENTRAL ALPS: FIRST RESULTS FROM A COMPILATION OF MORE THAN 5 0 0 , PUBLISHED AND NEW APATITE FISSION TRACK ANALYSES M.K. Rahni and D. Seward2 1 Institut fur Mineralogie, Petrologie imd Geochemie,Albert-Ludwigs-Universitat,Albertstrasse 23b, 79104 Freiburg, Germany, 2 Institut fiir Geologic, ETH-Zentrum, Sonneggstrasse 5 , 8 0 9 2 Zurich, Switzerland

The Central European Alps were among the first orogenic belts to be saidied using apatite fissiontrack analysis (Wagner and Reimer, 1972, Wagner et al, 1977). They have since remained an area for ongoing investigations (e.g. Hurford 1986, Michalski and Soom, 1990, and others). Within 30 years, this very considerable data set has verified that the Alpine exhumation history is complex down to a local scale. The development of the understanding and the meaning of track-length distributions on the age interpretation on one hand, and of the topography on very late cooling patterns on the other, suggests that the older data sets should be carefully re-evaluated. A new study seeks to overcome the above problems by adding missing track-length data to samples analysed earlier, as well as adding to this suite with new samples from areas where the exhumation needs to be understood more clearly. More than 150 new analyses have been added to the 350 previously published apatite ages in order to completely cover space and time for the apatite FT constraints. To the north, the data set of Alpine apatite ages is bordered by the northern Alpine thrust. In the Subalpine Molasse north of it, apatite ages are only partially reset by the Neogene Alpine overprint, but vertical profiles from bore holes indicate movements along these faults within the last few m.y. that are linked to the exhumation of the Alpine border chain. In the southern Alps all apatite ages, from Variscan to Cretaceous sequences yield Tertiary ages and most represent complete resetting before or at this time. The youngest apatite ages, are not found in the Lepontine dome, the area of maximum total exhumation, but in two other domes around Chur (East) and Martigny (West), where FT ages of less than 2 Ma have been realised. The data suggest a coupled interplay between erosion along the Rhone and Rhine river valleys (crossing the dome structures) and the recent tectonothermal evolution. A direct comparison of the ages between and within different data sets is complicated due to the fact that the Central Alps have altitude differences of up to 4000 m and are crosscut by several Neogene active thrust and normal faults (e.g. Grasemann and Mancktelow, 1993, Seward and Mancktelow, 1994); both parameters add severe complications to the interpretation of an "age". Filling the gaps between existing sample localities, in particular along and across Neogene active faults, has recently been started (e.g. Martin et al, 1998). We are now able to look at the Alps as a whole and from this detailed data set we can view the regional exhumation both from a local and regional point of view. For example to the west of the Lepontine dome, a prominent normal fault (Simplon Fault) is responsible in part for the exhumation of the dome by tectonic unroofing. To the east of the Lepontine dome, however, a similar structure has not yet been identified. Preliminary results suggest that extension is possibly distributed over several sub-structures, e.g. within the Penninic (Turba mylonite), the Austroalpine-basal thrust, and the Mesocco and Forcola zone between Adula and Tambo nappe. (Schlunegger and Willett, 1999). The presence or absence of extension along these faults are an example of the type of work currently being carried out now by fission-track analysis. Grasemann, B., Mancktelow, N.S. 1993. Two-dimensional thermal modelling of normal faulting: the Simplon Fault Zone, Central Alps, Switzerland. Tectonophysics 225, 155-165. Hurford, A.J. 1986. Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and age of vertical movement on the Insubric fault line. Contributions to Mineralogy and Petrology 92, 413-427. Martin, S., Bigazzi, G., Zattin, M., Viola, G., Balestrieri, M. L. 1998. Neogene kinematics of the Giudicarie-fault (Central Eastern Alps, Italy): new apatite fission-track data. Terra Nova 10, 217-221.

Geological Society of Australia - Abstracts Number 58

FT2 Michalski, I., Soom, M. 1990. The Alpine thermo-tectonic evolution of the Aar and Gotthard massifs, Central Switzerland: Fission track ages on zircon and apatite and K-Ar mica ages. Schweizerische Mineralogische und Petrographische Mitteilungen 70, 373-388. Schar, J.P., Reimer, G.M., Wagner, G.W. 1975. Actual and ancient uplift rate in the Gotthard region, Swiss Alps: A comparison between precise levelling and fission-track apatite age. Tectonophysics 29, 293-300. Schlunegger, R, Willett, S. 1999. Spatial and temporal variations in exhumation of the central Swiss Alps and implications for denudation mechanisms. Journal of the Geological Society London, Special Volume 154, 157-180. Seward, D., Mancktelow, N. 1994. Neogene kinematics of the central and western Alps: Evidence from fission-track dating. Geology 22, 803-806. Soom, M.A. 1990. Abkiihlungs- und Hebungsgeschichte der Externmassive und der penninischen Decken beiderseits der Simplon-Rhone-Linie seit dem Oligozan: Spaltspurdatierungen an Apatit/Zirkon und K-Ar-Datierungen an Biotit/Muskovit (westliche Zentralalpen). Ph. D. thesis, University of Berne, 64p. Wagner, G.A., Reimer, G.M. 1972. Fission track tectonics: the tectonic interpretation of fission track apatite ages. Earth and Planetary Science Letters 14, 263-268. Wagner, G.W., Reimer, G. M., Jager, E. 1977. Cooling ages derived by apatite fission-track, mica Rb-Sr and K-Ar dating: The uplift and cooling history of the Central Alps. Memorie degli Istituti di Geologic e Mineralogia dell'Universita di Padova 30, 27p.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

PHANEROZOIC THERMAL H I S T O R Y OF TECTONOSTRATIGRAPHIC E L E M E N T S OF NORTHLAND, NEW

ZEALAND

A. Raza and R.W Brown School of Earth Sciences, University of Melbourne, Victoria, Australia

The New Zealand microcontinent has a long history of association with convergent margin tectonics. A number of discrete tectonostratigraphic terranes either accreted or developed along its eastern margin during Permian to mid-Cretaceous subduction. Northland peninsula in the northern most part of the North Island comprises of four such pre-mid-Cretaceous units. These are 1) Mount Camel suspect terrane in the far north, 2) forearc sediments of Murihiku Group in the west 3) Dun Mountain Ophiolite Belt in the centre and 4) Waipapa Group forming the eastern edge. Both the Murihiku Group and Dun Mountain Ophiolite Belt are concealed under the post-mid-Cretaceous sedimentary cover. The Permian to Jurassic Waipapa Group is the oldest basement exposed on North Island and represents an accretionary prism. It consists of assemblages of tectonically accreted terrigenous sediments and incorporated oceanic material. The lithological heterogeneity of Waipapa Group has long been recognised but more recently Black (1994) has subdivided it into a number of subterranes and correlated them with the well defined terranes of South Island. Sproli (1978) identified three phases of deformation in the Waipapa terrane. The first two involve imbrication and folding of strata, and formation of melange zones either related to or predating the Early Cretaceous Rangitata Orogeny. The third phase characterised by open folding with plunging axes has an unknown age, but predates the unconformably overlying Eocene strata. The rocks of Waipapa Group are metamorphosed up to pumpellyite-actinolite grade and based on mineral chemistry and mineral phase relationships is considered that peak metamorphic conditions were achieved at P = 3 kbar, T = 250°C (Black, 1989). An abrupt change in tectonic regime occurred during the mid-Cretaceous c. 105 Ma. Subduction ceased owing to the collision of the Pacific-Phoneix ridge with the subduction zone and was closely followed by extension leading to the separation of New Zealand from Australia. Seafloor accretion in the Tasman sea commenced at 80 Ma. Between 80 and 25 Ma New Zealand underwent a prolonged period of relative tectonic stability and thermal subsidence during which thin shallow water sequences were deposited over the present land mass. However, on the Pacific seaboard a thick passive margin sedimentary wedge accumulated - the Northland Allochthon. At - 2 5 Ma a convergent tectonic regime was re-established with the inception of subduction system first along the Northland which then migrated to the south at its present location. Two noteable events related to the Northland geology during this time are: 1) the obduction of thrust sheets of Late Cretaceous to Oligocene Northland Allochthon (between 24-22 Ma) from the northeast and 2) initiation of the inter-arc Waitemata Basin. Sedimentological evidence from the Waitemata Basin suggests that apart from contemporaneous volcanic activity, both allochthonous and Waipapa Group rocks were the provenance to basinal sediments. The thermal effects of these events on Waipapa basement rocks and overlying younger sequence is not well known. Apatite and zircon fission track analysis from outcrop samples from Waipapa Group, Northland Allochthon and Waitemata Basin is in progress to determine their thermotectonic evolution. Apatite ages from Waitemata Basin outcrop samples reveal that the basin sediments were shallowly buried (<60°C) prior to denudation (Raza et al, 1999). This conclusion is consistent with the measured VR values from the basin. Statistical analysis of detrital apatite fission track (AFT) ages identify four dominant sources of sediments to the basin. These are: contemporaneous volcanic activity; metagraywacke rocks of Waipapa Group; the Northland Allochthon; and an unidentified source that cooled at - 6 3 Ma south of the basin. Geological Society of Australia - Abstracts Number 58

FT2 AFT data from the Northland Allochthon indicate that sediments have not been exposed to temperatures exceeding 60°C. This implies either that the original thickness of the allochthon sedimentary pile was relatively thin or that prevailing geothermal gradient in the basin was low and subsequent tectonic thickening did not cause any significant annealing of apatites. Preliminary data from the Waipapa Group indicate a complex cooling history which involved a three stage cooling from peak palaeotemperatures during Jurassic, Cretaceous and Oligocene/Miocene time. Black P.M., 1989, Regional metamorphism in basement Waipapa Group, Northland, New Zealand. In: Sporli, K.B.and Kear, D. (eds.). Geology of Northland-accretion, allochthon and arcs at the edge of the New Zealand microcontinent. Royal Society of New Zealand Bulletin 26, 15-21. Black RM., 1994. The "Waipapa Terrane", North Island, New Zealand: subdivision and correlation. Geoscience Reports, Shizuoka University 20, 55-62. Raza, A., Brown, R.W., Ballance, RF., Hill, K.C.and Kamp, P.J.J., 1999. Thermal history of the early Miocene Waitemata Basin and adjacent Waipapa Group, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 1999, 42: 169-188. Spodi, K.B., 1978. Mesozoic tectonics. North Island, New Zealand. Geological Society of America Bulletin 89, 415-425. Acknowledgement We acknowledge the financial support provided by Australian Institute of Nuclear Science and Engineering to meet the irradiation cost.

International Conference on Fission Track Dating and Thermochronology

AN INSIGHT INTO THE THERMAL EVOLUTION OF NEOGENE-QUATERNARY PANNONIAN B A S I N , HUNGARY, AS UNRAVELLED BY N E W APATITE F I S S I O N T R A C K DATA

A Razai, B Kohni and M. Hamor-Vido^ 1. School of Earth Sciences, University of Melbourne, Victoria, 3010, Australia 2. Geological Institute of Hungary, Budapest, Hungary

The Miocene to Recent Pannonian Basin situated in eastern central Europe is a large depocentre formed during the Miocene major extensional phase within the eastern Alpine-Carpathian orogenic system. The Eastern Alps and Dinarides lie at its western and south western edge while on all other sides it is wrapped by an arcuate belt forming the Carpathians. Syn-rift sedimentation commenced at -19 Ma, continued up to 13.5 Ma and was followed by a post-rift phase of sedimentation through to the present. The oldest basinal sequence rests unconformably on the Palaeozoic to Mesozoic basement except where local Palaeogene strata exist. The thickness of the basin fill is variable but in deeper parts is up to 7 km. Two post-rift compressional events have been recognised in the basin sequence (Horvath, 1995). The first commencing at -10 Ma resulted in the inversion of thick syn-rift fill troughs and their moderate erosion. The second episode which persists to the present day began in latest Pliocene (-2.5 Ma), resulted in uplift of the eastern and western flanks of the basin and also provided a sediment source to the continuously subsiding Great Hungarian Plain in the central Pannonian Basin. Published maximum vitrinite reflectance (VR) values from the Pannonian Basin exceed 2.0% and strongly suggest that the basin fill has been exposed to relatively high temperatures. Basinal geothermal gradients are in the high range (often 40-60°C/km) and have been attributed to high heat flow caused by crustal thinning during extension (Royden et al, 1983). However, is not clear at what time maximum basin temperatures were achieved. Such temporal information is particularly important as it can provide a useful constraint on the timing of hydrocarbon generation in the basin. A number of previous studies suggest that hydrocarbon generation was initiated at -10-12 Ma (Szalay, 1988; Horvath et al., 1988; Hetenyi, 1992) and this maybe ongoing to the present day as basin sediments are currently at maximum temperatures. To shed further light on the question of the timing of hydrocarbon generation in the basin we have examined 21 core samples for apatite fission track (AFT) analysis from five select boreholes. The Szirak-2 and Iharosbereny-1 wells are located in the north and the west of the basin respectively. The remaining three, Algyo-29, Algyo-105, Algyo-K-1 are situated on a basement high on the western flank of the Mako Trough which hosts over 6 km of sediments (Fig. 1). All samples from Szirak-2 and Iharosbereny-1 wells, regardless of depth (<2 km), yielded AFT ages between -11 - 6l Ma, and these are older than the ages of their host Miocene to Quaternary strata. This indicates that these apatites have not been annealed since their deposition and suggests that the samples have remained at temperatures ~60±10°C. This inference is consistent with the VR data from these and other wells which suggest low maturity (<0.45%) of organic matter at depths up to 2 km in the basin (Horvath et al., 1988). The AFT data also revealed some useful information about provenance of the basinal sediments. Data from the deepest sample at 1921 m (Karpatian age: -17.5-16.5 Ma) from Szirak-2 yielded apatite ages with a distinct 16.5±1 Ma population peak. This age range coincides closely with rift-related volcanic activity in the basin resulting from crustal extension at that time (Horvath, 1995; Peresson and Decker, 1997). On the other hand the three uppermost samples (within depths of 570 m) from Iharosbereny-1 belong to the unconsolidated Upper Pannonian section. These samples yielded individual AFT grain ages with a dominant population of 10±0.4 Ma. In these samples, >90% of the single grain ages belong to this population indicating sediments were mainly derived from a source which had previously cooled Geological Society of Australia - Abstracts Number 58

FT2

Figure 1. Location map of Pannonian Basin and surrounding tectonostratigraphic units. Bold dots indicate the locations of boreholes studied by fission track analysis. 1: Alpine-Carpathian foredeep; 2: Alpine-Carpathian flysch belt; 3: Inner AlpineCarpathian and Dinaric mountains; 4: Neogene cal-alkaline volcanics; 5: Quaternary subsidence. VB: Vienna Basin; DB: Danube Basin; SB: Styrian Basin; TB: Transylvanian Basin; TD: Transcarpathian Depression. Map modified after Horvath (1995).

rapidly at - 1 0 Ma. In terms of regional tectonics, - 1 0 Ma was a time of major E-W compression associated with the collision of the European plate with the overriding Adriatic Plate (Peresson and Decker, 1997). Consequently basin configuration changed and the subsidence was driven by foreland loading which continued through to the present day. Analysed samples from the wells Algyo-29, Algyo-105, Algyo-K-1 are from depths >1985 m. AFT ages range from greater than depositional age at shallow levels to <1 Ma from deeper parts of the basin. The shallower samples have mean track lengths of 9-12 pm and often no track data was available from the younger samples. Ages as low as - 1 Ma suggest that these samples are currently residing at their maximum temperatures. Recently acquired VR data from Algyo-29, Algyo-105, Algyo-K-1 range from 0.41% at 1985 m to 1.05% at 3500 m corresponding to temperatures of ~75°C to l60°C respectively assuming a maximum heating time of 1 Ma. The progressive rise in VR values with depth indicates continuous exposure to elevated temperatures due to continuous burial from Late Miocene to Recent. The estimated maximum temperatures inferred from the VR data correspond well with the observed annealing of apatite and calculated geothermal gradients from the VR data are -4l-45°C/km which also equate well with the present day regional thermal regime. By applying an integrated approach, we conclude that AFT and VR data indicate that sediments in the studied boreholes are presently experiencing peak temperatures. These temperatures were achieved solely through continuous Miocene to Recent burial. The sequence at <2000 m depth has been exposed to temperatures <75±10°C i.e. it remained just within the lower temperature limit of the apatite partial annealing zone. Application of the maturity limits for oil of VR 0.6-1.3% as determined by Szalay, 1988 for the Pannonian Basin, suggest that sediments now buried >2600 m in these wells are presently within the oil generation window.

FT2 Hetenyi, M. 1992. Organic geochemistry and hydrocarbon potential of Neogene sedimentary rocks in Hungary. Journal of Petroleum Geology, vl5 (1), 87-96. Horvath, R, Divebtu, P., Szalay, A., and Royden, L.H. 1988. Subsidence, thermal, and maturation history of the Great Hungarian Plain. AAPG Memoir 45, 355-372. Horvath, F. 1995. Phases of compression during the evolution of the Pannonian and its bearing on hydrocarbon exploration. Marine and Petroleum Geology, v. 12 (8), 837-844 Peresson, H. and Decker, K. 1997. Far-field effects of Late Miocene subduction in the eastern Carpathians: E-W compression and inversion of structures in the Alpine-Carpathian-Pannonian region. Tectonics, v. 16, 38-56. Royden, L., Horvath, F., Nagymarosy, A. and Stegena, L. 1983. Evolution of the Pannonian Basin system 2. Subsidence and thermal history. Tectonics, v. 2, 91-137. Szalay, A. 1988. Maturation and migration of hydrocarbons in the southeastern Pannonian Basin. AAPG Memoir 45, 347-354. Acknowledgments This research is a collaboration between Geological Institute of Hungary and School of Earth Sciences, Melbourne University, Australia. We acknowledge the financial support provided by Australian Institute of Nuclear Science and Engineering to meet the irradiation cost.

FT2

International Conference on Fission Track Dating and Thermochronology

H E L I U M D I F F U S I O N AND ( U - T H ) / H E T H E R M O C H R O N O M E T R Y O F Z I R C O N

RW Reinersi and K.A. Farley2 1 Department of Geology, Washington State University, Pullman, WA 99164, USA 2 Division of Geological and Planetary Science, California Institute of Technology, Pasadena, CA 91125, USA

Recent technical and interpretational advances in (U-Th)/He dating of apatite and titanite have produced a surge of interest in the low-temperature thermochronometric capabilities of He dating. A wide range of applied studies using apatite He dating (and fewer using titanite) has now confirmed the practical utility of the method in constraining the timing and rates of orogenic exhumation and topographic development. Other diverse applications of He dating also hold potential, including tephrachronology, the timing of ore deposition and hydrothermal processes, craton stabilization, and cross-calibration with both fission-track and feldspar multidomain Ar dating. Because apatite and titanite are not present in all rock types, and because the properties of He diffusion (and therefore closure temperature) differ between minerals, establishing He dating methods for other minerals is clearly an important next step for (U-Th)/He dating. A number of characteristics of zircon make it an attractive candidate for (U-Th)/He thermochronometry. U contents typically two to three orders of magnitude higher than in apatite or titanite yield high He in short time spans, making it potentially suitable for dating young events. Additionally, high abundances of zircon in sedimentary rocks suggest that this method may be quite useful in constraining a range of problems not as easily accessible by apatite and titanite dating, including sediment provenance and thermal histories of sedimentary basins. Potential complications associated with zircon He dating include the possibility of U-series disequilibrium in young (<1 Ma) samples, and the effects of strong radiation damage (and low annealing rates of zircon) on He diffusion characteristics (Damon and Kulp, 1957). Here we report He diffusion experiments and age determinations on zircons from a variety of geologic environments that illustrate potential utilities and complications of the method. We performed a series of cycled step-heating diffusion experiments (Farley et al, 1999) on zircons from a variety of areas and time-temperature paths. In each experiment, 10-20 individual zircon crystals (or an equivalent mass in crushed and sieved zircon fragments) were held at temperatures between 250 and 600°C, in steps ranging from 15 to 420 minutes. Zircons from the quickly cooled, 28 Ma Fish Canyon tuff show the simplest and mostly easily interpretable Arrhenius trends for He diffusion. Two experiments on whole Fish Canyon tuff zircons (approximately 120 by 60 |Lim) yielded activation energies E^, of 35 and 36 kcal/mol, frequency factors Df/a^, of 2200 and 5000, and closure temperatures T^, of 135''C and 145°C (for a cooling rate of 10°C/m.y.). Fragments of Fish Canyon Tuff zircons sieved to a 44-74 |im size range yielded an E^ of 37 kcal/mol, a much higher D(/a^ of 3 x 10^, and a T^ of 123°C. The lower T^ of the crystal fragments and correlation between inverse grain size and suggests that, as for both apatite and titanite, the diffusion domain for He is the crystal (or crystal fragment) itself. Zircons from older rocks yield more complicated Arrhenius plots, with initial up-temperature steps that are generally more erratic, lower in slope, and show higher apparent diffusivity than later steps that follow heating above about 450°-500°C. Relatively large zircons from the early Paleozoic Chain of Ponds pluton in Maine (sample MH-10 of Heizler et al, 1988) show E^ of 37 kcal/mol and D(/a^ of 870 for well-correlated steps following heating to 500°C, indicating a T^ of l67°C. Diffusion experiments on 1.45 Ga zircons from the Gold Butte block, an exhumed >15-km crustal section in southern Nevada, provide preliminary evidence for a relationship between time-integrated temperature of zircon and its He diffusion characteristics. Zircons from the structurally highest position in the block (~3.5 km paleodepth) yielded highly irregular and erratic diffusivities in the initial up-temperature steps. Following heating at high temperature steps (up to 500°C) however, diffusion steps form a well-correlated Arrhenius trend with E^ of 38 kcal/mol, yielding a of 151°C. Zircons Geological Society of Australia - Abstracts Number 58

FT2 from the same pluton, but the structurally lowest position in the block (-15 km paleodepth) contained very little '^He, but also yielded an E^ of 36 kcal/mol and a T^ of ISS^'C. Interestingly, the structurally lowest zircons did not yield irregular and erratic diffusivities in the initial up-temperature steps. Comparison of these two experiments may indicate that prolonged residence at low temperatures (less than about 350°C since 1200 Ma, for the case of the structurally highest sample) causes the irregular and erratic He diffusivity prior to reheating to 450°-500°C. Taken as a whole these results indicate that zircon has an activation energy for He diffusion of about 37 kcal/mol, intermediate between that of apatite (32.6 kcal/mol) and titanite (44.6 kcal/mol), and a closure temperature of about 140°-160°C, also intermediate between apatite (TO^'C) and titanite (200°C); (Fariey, 1998; Reiners and Farley, 1998; in press). Our data also clearly show that older zircons (with presumably higher radiation dosages) and prolonged low-temperature histories show anomalous and unsystematic He diffusivity at temperatures lower than about 450°-500''C prior to heating to these temperatures. Following heating to these temperatures however, the diffusion characteristics of these crystals are very similar to those of the much younger Fish Canyon tuff zircons. This may mean that, in contrast to initial observations of titanite (Reiners and Farley, 1998; in press), crystal defects in zircon caused by radiation damage have a significant effect on He diffusion, and that these defects are annealed on several hour-timescales during heating to 450°-500°C. If, as these preliminary results suggest, He diffusivity in zircon is strongly affected by radiation damage, reliable interpretations of (U-Th)/He dates of this phase may be restricted to samples that have not resided at low temperatures for hundreds of millions of years. In addition to diffusion studies, we have also determined (U-Th)/He ages on a number of zircon samples. We measured the dimensions of 3-4 grains containing either no, or only minor, mineral inclusions and outgassed them in a stainless steel capsule at approximately 1100°C. Liberated ^He contents were measured to within 1% by either 3He dilution and peak ratioing on a quadropole mass spectrometer, or comparison of peak heights to standards on a magnetic sector instrument. Gas re-extractions were also performed to insure full extraction of ^He. Grains were retrieved, spiked and dissolved, and U and Th contents were measured on a Finnegan Element ICP-MS with a precision better than 0.1-0.2%. Calculated raw ages were then corrected for alpha-ejection using Farley et aVs (1996) model modified for the higher density and tetragonal prism morphology of zircon. Two separate samples of Fish Canyon Tuff zircons yielded ages of 26.4 and 28.1 Ma, within the range of ages derived by other methods. We are also currently measuring zircon He ages of a series of samples from the Gold Butte block, an exhumed 18 km thick section of Proterozoic crust in southern Nevada for which detailed age-paleodepth relations are known for five separate chronometers, including apatite and titanite (U-Th)/He, which show well-preserved fossil He partial retention zones (PR2). The position of the zircon He PR2 is expected to be intermediate between those of apatite and titanite, providing a test of the 150°C T^ indicated by the diffusion experiments. These results will also address whether or not zircon (U-Th)/He ages remain interpretable despite radiation-damage induced diffusivity variations, in these old (1.45 Ga) rocks. Damon RE. and Kulp J.L. 1957. Determination of radiogenic helium in zircon by stable isotope dilution technique. Transactions American Geophysical Union 38, 945-953. Fadey K.A. Wolf R.A.and Silver L.T. 1996. The effects of long alpha-stopping distances on (U-Th)/He ages. Geochimica et Cosmochimica Acta 60, 4223-4229. Fariey K.A. House M.A. and Kohn B.P. 1998. Laboratory and natural diffusivity calibrations for apatite (U-Th)/He thermochronometry. Mineralogical Magazine 62A, 426-427. Farley K.A. Reiners P.W. and Nenow V. 1999. An apparatus for measurement of noble gas diffusivities from minerals in vacuum. Analytical Chemistry 79, 2059-2061. Heizler M.T. Lux D.R. and Decker E.R. 1988. The age and cooling history of the Chain of Ponds and Big Island Pond Plutons and The Spider Lake Granite, west-central Maine and Quebec. American Journal of Science 288, 925-952. Reiners P.W. and Fariey K.A. 1998. Helium diffusion and (U-Th)/He thermochronometry of titanite. Mineralogical Magazine 62A, 1249-1250. Reiners P.W.and Farley K.A. 1999. Helium diffusion and (U-Th)/He thermochronometry of titanite. Geochimica et Cosmochimica Acta. In press.

International Conference on Fission Track Dating and Thermochronology

C O O L I N G AND D E N U D A T I O N OF T H E M E N D E R E S M A S S I F , S . W .

F T 2 '

TURKEY

U. Ringi and C Johnson^ 1 Institut fiir Geowisenschaften, Universitat Mainz, 55099 Mainz, Germany 2 T.H. Huxley School of Environment, Earth Science and Engineering, Imperial College, London, UK

The central Menderes Massif of southwest Turkey defines a symmetric metamorphic core complex (CMCC = central Menderes core complex) that is bounded to the north by the NNE-vergent Kuzey detachment and to the south by the SSW-vergent Guney detachment. The Kuzey detachment is cut to the north by the Gediz graben. Motion on both detachments started in the Miocene. The Giiney detachment is cut to the south by the Buyiik Menderes Graben. Apatite fission track thermochronology (AFTT), zircon fission track thermochronology, and Ar/Ar and Apatite U-Th/He data obtained from a NNE-SSW transect across the entire Menderes Massif that parallels the direction of maximum late ductile-brittle finite strain, reveal a cooling history that has a broadly symmetrical form, defining the CMCC. The outer Menderes submassifs above the two detachments are characterised by cooling that had commenced at the latest by the late Eocene, and which became relatively rapid (>50°C/Ma) during the middle to late Oligocene. The AFTT data constrain this phase of cooling to have resulted in rocks presently exposed in the outer Menderes submassifs as having been close to the palaeo-Earth surface during the late Oligocene to early Miocene. In contrast, little of the core, or Inner Menderes submassif, had cooled to temperatures <110°C by the beginning of the late Miocene, and in the case of the northern Inner Menderes submassif, temperatures remained >300°C until the end of the Miocene. The asymmetry of the cooling histories across the detachments and grabens that seperate the CMCC and outer Menderes submassifs is thought to be reflected in the structural asymmetry of these features. The master shear zone of the Kuzey detachment and the Gediz graben roots to the north, while that of the Giiney detachment and the Buyiik Menderes graben roots to the south, placing the Inner Menderes submassif in the 'footwall' of two conjugate crustal scale detachment/normal fault systems. These two lines of evidence suggest that a regionally significant phase of cooling in the Oligocene of uncertain cause but possibly related to an early period of extension, was followed by a second phase of extension in the Late Miocene resulting in the main phase of motion on the Kuzey and Guney detachments, a two phase extensional history similar in its timing to that obtained for nearby Aegean extensional province.

Geological Society of Australia - Abstracts Number 58

FT2

International Conference on Fission Track Dating and Thermochronology

E V O L U T I O N O F AN A C T I V E O R O G E N W I T H I N A C O N T I N E N T A L I N T E R I O R E X A M P L E S F R O M T H E T I E N S H A N AND N E -

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POSITION:

AND C E N T R A L P A M I R S IN C E N T R A L A S I A

M. Schwabi, L. Ratschbacher2,A. Kuhlemanni, 1. Dunkli and W. Frischi 1 Geological Institute, University of Tuebingen, Germany 2 Geological Institute, University of Wuerzburg, Germany

From active continental margin to continental interior position During most of the Late Paleozoic and Mesozoic time, the area north of the Pamirs was part of the active continental margin of Eurasia. The Pamir region formed due to successive accretion of continental fragments and magmatic arcs building up the recent three-division in Northern-, Central-, and Southern Pamirs. The Early Eocene India-Asia collision started more than 1000 km south of the Pamirs and brought the Pamir range in an interior continental position. The area was intensely reactivated, so that older structural relationships have been obscured but also previously deeply buried units were exhumed. The recent total convergence between India and Eurasia is at very high rate of about 45 mm/yr (DeMets et al, 1990), resulting in crustal deformation in the Alpine chains, but also far north in Tibet, the Pamirs and the Tien Shan mountains. Crustal shortening across the Tien Shan is found at a rate of about 20 mm/yr (Abdrakhmatov et al, 1996), implying that convergence is transferred across the entire Pamir range. Questions arise, where and how the crustal deformation is distributed across the Tien Shan and Pamir region from the time of India-Eurasia collision until today. To constrain the propagation of deformation and to estimate how crustal shortening might be translated into uplift, we collected structural data and samples for geochronology along a N-S traverse from S-Tien Shan to NE- and Central Pamirs. The presentation will be focused on the areal distribution of the low temperature data in combination with structural analyses. The aim is to demonstrate how the tectonic activity post-dating the IndiaAsia collision is partitioned over a wide range of Central Asia.

Sampling strategy and methods Sampling was concentrated on E-W striking magmatic belts, which are associated to Late Paleozoic, Triassic-Jurassic and Early Cretaceous suture zones. To constrain the timing of intrusion/crystallisation of the magmatic rocks, the U-Pb dating method on single zircon minerals has been applied. The high to medium temperature cooling of these rocks as well as from metamorphic basement rocks were determined with the Rb-Sr dating method on white mica and the Ar-Ar dating method on hornblende and biotite. The fission track thermochronology on zircons and apatites was used to evaluate the low temperature cooling history. Sediment samples from Cretaceous to Tertiary intramontane basins were analysed to obtain either information about the exhumation and erosion of the hinterland and/or to decipher the burial history and overprint of these basins.

Localization of deformation and thermochronological results 1) In the transitional zone of the Central- and Southern Pamirs (Early Cretaceous suture zone) Ar-Ar biotite ages from granitoids demonstrate regional cooling below 300°C in Late Cretaceous time. The apatite fission track (AFT) ages cluster around 11 Ma. The structural analyses show N-S shortening and E-W extension with segmented dextral transpressional strike slip faults in the far field of the Karasu- and Karakorum fauk zones. 2) The Central Pamirs forms a high strain region with basement domes metamorphosed to amphiboGeological Society of Australia - Abstracts Number 58

FT2 lite grade and intrusions of syn- to post- tectonic granites (31 Ma U-Pb zircon age). Age determinations (Ar-Ar amphibolite and biotite, FT zircon and apatite) and thermal modelling on this granites reflect a very fast cooling in Miocene and then a change to a more gradual cooling until today. The Miocene exhumation of the metamorphic domes developed during postcollisional extensional tectonics. The domes have been, at least on their southern margin, overprinted by compression. Tertiary sediments from an intramontane basin south of the dome show low grade metamorphic overprint and youngest K-Ar and FT cooling ages which correlate with the cooling ages of the dome. 3) The Lake Karakul area (Northern Pamirs) is a low strain region with a rifting structure and several Jurassic/Cretaceous granite intrusions. The Ar-Ar biotite cooling ages are close to the intrusion ages (210 Ma U-Pb zircon age), whereas the AFT ages spread from Paleocene ages in the south to Miocene in the north of the Lake. Crustal deformation in the Pamirs seems to be concentrated in narrow E-W striking zones. Deformation is not simply propagating from south to north; more likely already weakened zones became reactivated and overprinted, so that a "out of sequence" deformation pattern can be recognised. These regions cooled faster from Miocene to recent, than areas with lower strain rates. DeMets C. Gordon R.G. Argus D.F. and Stein S. 1990. Geophys. J. Int. 101, 425-478. Abdrakhmatov K.Y. et al 1996. Nature 384, 450-453.

International Conference on Fission Track Dating and Thermochronoiogy

P O S T P A N - A F R I C A N E V E N T S IN M A D A G A S C A R : I N F E R E N C E S APATITE F I S S I O N - T R A C K

F T 2 '

FROM

ANALYSIS

D. Sewardi, D. Grujic2 and G. Schruers3 1 Geological Institute, ETH-Zentrum, CH-8092 Zurich, Switzerland 2 Geological Institute,Albert-Ludwigs University, D-79104 Freiburg i. Br., Germany 3 Geological Institute, University of Bern, CH-3012 Bern, Switzerland

Madagascar, covering an area of approximately 627,000 km^, occupied a central position within Gondwana, sandwiched between Africa and the Seychelles-India block. The eastern two thirds of the island is formed dominantly of amphibolite to granulite grade Precambrian rocks with occasional Cretaceous to Neogene volcanics, while the western third is composed of two large sedimentary basins, the Morondava and Mahajanga containing Palaeozoic to Tertiary sequences. Most of the Precambrian basement underwent intense deformation and metamorphism in the Late Proterozoic/Early Palaeozoic (ca. 800-550 Ma) times. The Precambrian basement sequences tend to form the higher topography, while the two large basins essentially form the western dip slope. The eastern coastal belt is narrow and straight as are its offshore isobaths which trend parallel to the present coast. Several large shear zones cut the high grade Precambrian basement. The most prominent of these is the sinistral Bongolava Ranotsara shear zone which trends NW-SE and may possibly divide two different crustal terranes. This shear zone should be traceable into India but there is some disagreement as to which shear zone in southern India is the correlative. The oldest sea floor anomaly between Madagascar and the African continent is M22 (approximately 149 Ma) and that between Madagascar and India in the Mascarene Basin is 34 (the Cretaceous quiet period, with a broad age span from approximately 120 to 84 Ma). These represent the oldest identified sea floor between the different continental masses. Associated with the separation of the IndiaSeychelles mass in the Cretaceous, are numerous basalts, particularly along the eastern coast; the Volcan d'Androy in the south east; and inland, subparallel to the northwestern coast. The ages of these basalts are reported as 87.6 ± 0.6 Ma (Storey et al, 1995). These authors suggested that the lavas at Volcan d'Androy in the southeast marked the focal point of the hot spot at approximately 88 Ma. Further, they present a postulated track of the hot spot back to 120 Ma. Apatite fission track analysis has been completed on samples from the Precambrian succession. The apparent ages range from 68 Ma along the southeast coast to 397 Ma at an altitude of 2460 m. As in other passive margin studies (see review in Gallagher et al, 1998), there is a very broad trend in the age altitude relationship, as well as an ill defined shallow "boomerang" plot for the age - mean track length relationship. A regional overview of the ages, reveals the island has not remained stable since the break up of Gondwana. Some Pan African faults have been reactivated. The most obvious of these is recognised by a jump in the apatite ages either side of the Ranotsara Bongolava shear zone. Here the ages to the south of the zone are less than 200 Ma, whereas to the north they are older. This implies that there has been post Pan African vertical movement with south side up. This is supported by the recent study (Lardeaux etal, 1999) in which they showed brittle normal fauk displacement along this former ductile zone. Other examples of reactivation are present further north, such as along the trace of the Itremo Fault, where younger ages lie to the north. Since most previous studies in this Precambrian region of Madagascar have concentrated in understanding the geology of that time, then the recognition of such reactivation has probably been by-passed. This means that smaller brittle systems have not been mapped and this results in the rather broad age altitude plots that are common in passive margin studies. Along the eastern coast there is a rough trend in ages from old (120 Ma) to younger (68 Ma) from the central region to the south. The coincidence of these ages to the timing of the movement of the Geological Society of Australia - Abstracts Number 58

FT2 Marion Hot Spot should not be ignored. However, they all occur along the coastal plain where they also represent exhumation ages produced by denudation during scarp retreat. Hence they may be a result of the two processes. A sequence of altitudinally controlled samples was analysed from Mt Pic Boby, to the north of the Bongolava Ranotsara Zone, ranging from 930 m 2658 m. The apatite apparent ages range from 201 Ma at the base to 397 Ma. There is no obvious evidence in this section of variable structural relationship, yet the ages from similar altitudes have a spread as large as 100 Ma. A correlation with rock type is obvious. All samples have been analysed for variation in chemistry. But at the time of writing, there is no obvious, common, single element that may be showing control. Most apatites are simple fluorapatites with generally less than 0.5% chlorine. Gallagher K. Brown, R. and Johnson, C., 1998. Fission track analysis and its applications to geological problems. Annual Reviews of Earth and Planetary Sciences 26, 519-572. Lardeaux J. M. Martelat J. E. Nicollet C. Pili E. Rakotondrazafy R. and Cardon H. 1999. Metamorphism and tectonics in southern Madagascar: an overview. Gondwana Research 2, 355-362. Storey M. Mahoney J. J. Saunders A. D. Duncan R. A. Kelley S. P. and Coffin, M. F. 1995. Timing of hotspot related volcanism and break up of Madagascar and India. Science 267, 852-854.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

CENOZOIC EXHUMATION OF THE K Y R G Y Z T I A N SHAN

E.R. Sobel Institut fuer Geowissenschaften, Universitaet Potsdam, Potsdam, Germany

New apatite fission track thermochronology (AFTT) samples provide constrains on the time of initiation and subsequent development of the Kyrgyz Tian Shan, one of the best examples of an active intracontinental mountain belt. The present phase of mountain building is certainly a consequence of the India-Eurasian collision, but its evolution is far from clear. This study also addresses discrepancies between previously published thermochronologic and geodetic GPS measurements of when deformation commenced in the range. To address these questions, samples were collected for AFTT from transects across every major range between the northern margin of the Kyrgyz Tian Shan south almost to the border with China. In simple terms, the present-day structure of the reverse- and thrust- fault bounded Tian Shan consists of roughly E-W-trending ranges separated by roughly parallel sedimentary basins. Zones of shallow seismicity surround several basins, and active faults mark most range fronts. Thus, unlike the Himalaya or subduction zones, convergence seems not to be localised on one main thrust fault, but distributed over a broad zone. Topographic boundaries between ranges and basins are either gradual, where the erosional unconformity dips beneath the accumulating sedimentary cover of the basins, or sharp and marked by thrust faults. Paleozoic and older units, deformed in collisions of continental fragments with southern Asia in both Early and Late Paleozoic time, crop out in the Tian Shan. Deformation continued into Mesozoic times, as documented by the distribution and facies of Mesozoic strata and inferred paleocurrent directions within those strata (Hendrix et al, 1992) and by long narrow basins of thick Jurassic rock along the Talas-Ferghana fault that suggest pull-apart basins and Mesozoic slip on that fault (Burtman, 1964; Sobel, 1999). However, tectonic activity near the close of the Mesozoic Era seems to have been modest, if not insignificant. Upper Cretaceous and Lower Cenozoic strata unconformably overlie deformed older units on a widely recognised denudational surface (e.g. Sadybakasov, 1990). This erosional unconformity provides an important marker within the Tian Shan, traceable over large areas. Cenozoic structural relief on this surface exceeds 8000 m (Cobbold et al, 1996). Thick Late Cenozoic sedimentary basins are located adjacent to all of the major ranges. These, coupled with the lack of Neogene igneous activity and scarcity of Upper Cretaceous to Paleogene basakic magmatism (Sobel and Arnaud, in press), suggest that exhumation linked to thrust or oblique slip faulting is the primary mechanism for Late Cenozoic cooling within the ranges. Early to middle Miocene AFTT ages have been obtained from two ranges, located at the southern and northern ends of the transect. To date, no Cenozoic ages have been obtained from the intermediate ranges. The results will be described from north to south. The Kyrgyz range is located on the northern margin of the Tian Shan. Structurally, this range appears to be a large, thrust-faulted anticline, with a present topographic relief of ~3200 m in the vicinity of the transect. The core of the anticline lies on the north side of the range. Interpretation of map-scale structures suggest that exhumation of the range is linked to a south-dipping thrust fault on the north side of the range as well as a high angle backthrust (?) just north of the pass. A sample from the northern margin of the range yielded a partially reset central age of 62 ± 7 Ma. This sample fails the chi squared test. Only 10 track lengths were measured, with a mean track length of 10.5 pm. The next sample going south could not be counted due to extremely low uranium content. A sample with a reset AFTT pooled age of 13 ± 2 Ma was collected slightly north of the crest of the range. The sample passes the chi squared test. Unfortunately, only 5 confined track lengths could be measured. A higher elevation sample yielded an age of 19 ± 3 Ma; track length data are unavailable, and a sam-

Geological Society of Australia - Abstracts Number 58

FT2 pie collected on the south side of the pass and south of the backthrust (?) fault passed the chi squared test and yielded an age of 67 ± 5 Ma (Ann Blythe, pers com., 1997). This suite of observations suggest that there is an exhumed partial annealing zone on the north side of the pass. The region of maximum cooling roughly corresponds with the anticlinal structure rather than the frontal thrust on the northern margin of the range. Samples were collected in several area across the next two ranges to the south. These ranges all preserve fragments of the erosional unconformity. Samples were collected to maximize the structural relief below this surface. All samples analysed to date, however, yield Late Paleozoic through Mesozoic ages. Track length data and distribution of single grain ages suggest that the apatite ages have been partially reset, suggesting that there has not been sufficient Cenozoic exhumation to expose fully reset apatite samples. This interpretation is supported by the relatively small magnitude of structural relief exposed below the Late Cretaceous unconformity surface. Unfortunately, AFTT does not provide good chronologic constraints for this exhumation. The Atbashi range lies about 60 km north of the Chinese border. Two elevation transects were collected from the northern side of the range and one from the southern margin. Samples collected from Permian sandstone and Carboniferous granitoids on the north margin of the range yield Jurassic ages which pass the chi squared test. Track lengths are -12.3 pm with a standard deviation of -1.5 pm. The core of the range is composed of metasediments and schists; eclogite outcrops along the northern edge of the metamorphic series. Metamorphic apatites recovered from the metasediments have no crystal shape and contain about 1 ppm uranium, making analysis quite difficult and imprecise; confined track lengths are quite rare. One sample from the northern half of the range yields an age of 21± 8 Ma. Other samples from the southern side of the range yield older, Cenozoic ages. A single felsic intrusive sampled within the core of the range, near the southern margin, yielded good-quality uranium-bearing apatite. This sample passes the chi squared test with a pooled age of 16 ± 1 Ma. The mean track length is 13.5 pm with a standard deviation of 2.1 pm; a small number of short (5-8 pm) tracks are observed. Geologic observations coupled with the AFTT ages suggest that there is a large north-vergent Cenozoic thrust on the northern margin of the range. The middle Miocene sample on the southern margin of the range, coupled with older, imprecise AFTT ages from higher elevations suggests that there is a second Neogene, south-vergent thrust on the southern margin of the range. However, this fault does not outcrop and is covered by the Late Cenozoic sediments in the Aksai basin. Along the southern margin of the Chinese Tian Shan, directly south of the Atbashi range, four detrital Oligocene-Miocene (25 ± 4 to 13 ± 2 Ma) fission track ages were obtained from a Miocene stratigraphic section; the sediment was apparently sourced from rapidly exhumed ranges to the north and east in the Tian Shan (Sobel and Dumitru, 1997). Younger thrusting has propogated southward into the Tarim basin. The Oligocene-Miocene cooling ages from the Tian Shan are interpreted to reflect an exhumation event which slightly post-dates the initiation of significant Cenozoic thrusting and therefore crustal thickening within the range. The new resuks presented herein suggest that the locus of exhumation has shifted from one range to another within the Kyrgyz Tian Shan throughout the Neogene. The present shortening rate across the Tian Shan is a large fraction of India's current northward motion of 40-50 mm/yr with respect to Eurasia (DeMets et al., 1990). Repeated GPS geodetic measurements reveal consistent north-south shortening across the Tian Shan, between the stable Kazakh platform on the north and sites near the southern edge of Kyrgyzstan, at a rate of 13 ± 2 mm/yr (Abdrakhmatov et al., 1996). Because the network does not cross the entire range, this rate underestimates the total convergence rate across the Tian Shan. High ranges and active seismicity south of the network attest to rates of active deformation comparable to that studied further to the north. Thus the estimated -20 mm/yr shortening rate across the Tian Shan, which exceeds previous estimates by 50-100%, accounts for nearly half of India's penetration into Eurasia at 44 mm/yr in this area.

FT2 Crustal shortening within the Tian Shan accounts for only a small fraction (-200 km, Avouac et ai, 1993) of India's 2000-3000 km of penetration since the collision. Thus, the current high rates of convergence documented by GPS imply that shortening has accelerated and suggests that deformation in the Tian Shan has a relatively short history. Assuming roughly 200 km of shortening are necessary to account for the present crustal thickness and material eroded from the Tian Shan, and assuming a roughly constant rate of convergence, shortening at 20 mm/yr suggests that the entire range was built in -10 m.y. (Abdrakhmatov et al, 1996). This resuk is significantly younger than that deduced from fission track studies. The discrepancy between the Oligocene - early Miocene initiation of significant exhumation documented herein and the estimated 10 Ma initiation predicted from geodetic measurements suggests that the range has undergone a complex shortening history. Track length distributions show that the early episode of cooling did not bring the samples all the way to the surface at that time. Therefore, there must have been a decrease in exhumation rate in the middle to late Miocene. Possibly the locus of active deformation migrated towards the present centre of the Tian Shan. The structures bounding the Atbashi and Kyrgyz ranges may have been reactivated more recently in order to produce the young and ongoing deformation which is clearly recognisable within the Tian Shan. Abdrakhmatov, K. Y., and 16 others, 1996, Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates. Nature 384, 450-453. Avouac, J. P., Tapponnier, P., Bai, M., You, H., and Wang, G., 1993, Active thrusting and folding along the northern Tien Shan and late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan. Journal of Geophysical Research 98, 6755Burtman, V. S., 1964, The Talaso-Fergana strike-slip fault. Nauka, Moscow, Trudy Geological Institute, Akademi Nauk, USSR, 143 p. Cobbold, P. R., Sadybakasov, E., and Thomas, J. C., 1996, Cenozoic transpression and basin development, Kyrgyz Tien Shan, central Asia in Roure, R, Ellouz, N., Shein, V. S., and Skvortsov, I., ed., Geodynamic Evolution of Sedimentary Basins. Paris, Editions Technip, 181-202. DeMets, C., Gordon, R.G., Argus, D.F., and Stein, S., 1990, Current plate motions. Geophysics Journal International, 101, 425478. Hendrix, M. S., Dumitru, T. A., and Graham, S. A., 1994, Late Oligocene-Early Miocene unroofing in the Chinese Tian Shan: An early effect of the India-Asia collision. Geology 22, 487-490. Hendrix, M. S., Graham, S. A., Carroll, A. R., Sobel, E. R., McKnight, C. L., Schulein, B. J., and Wang, Z., 1992, Sedimentary record and climatic implications of recurrent deformation in the Tian Shan; evidence from Mesozoic strata of the north Tarim, south Junggar, and Turpan basins. Northwest China. Geological Society of America Bulletin 104, 53-79. Sadybakasov, I. 1990, Neotectonics of High Asia (in Russian), Nauka, Moscow, 176 p. Sobel, E. R., 1999, Basin analysis of the Jurassic - Lower Cretaceous southwest Tarim basin, NW China. Geological Society of America Bulletin 111, 709-724. Sobel, E. R., and Arnaud, N., in press, Cretaceous - Paleogene basaltic rocks of the Tuyon basin, NW China and the Kyrgyz Tian Shan: the trace of a small plume. Lithos. Sobel, E. R., and Dumitru, T. A., 1997, Exhumation of the margins of the western Tarim basin during the Himalayan orogeny. Journal of Geophysical Research 102, 5043-5064.

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

DECIPHERING EROSION HISTORY AND THERMOTECTONIC EVOLUTION OF THE SWISS ALPS BY FISSION TRACK DATING ON CLASTIC SEDIMENTS OF THE FORELAND BASIN

C. Spiegel, 1. Dunkl J. Kuhlemann and W. Frisch Institute for Geology and Paleontology, University ofTuebingen, Frisch AG, Sigwartstr. 10, D-72076 Tuebingen, Germany

The clastic sediments of a foreland basin record the exhumation and erosion of the evolving mountains in the hinterland. Dating these sediments by the fission track method gives evidence of the thermotectonic evolution of an orogen. The Alps provide ideal conditions for this approach because of their dense geochronological data set and their well-constrained foreland basin stratigraphy. The Alps were formed in response to the Cretaceous to recent collision between the African and the European continent. In the present situation most of the Eastern Alps are covered by Austroalpine Units that represented the upper (African) plate during continent-continent collision. The Austroalpine units show no zircon fission track ages younger than Cretaceous to Paleocene. In contrast, the Swiss Alps are dominated by the Lepontine Dome, a large metamorphic core complex that belongs to the Penninicum, the remnants of the subducted ocean between Africa and Europe. Penninic basement nappes show almost exclusively Zr FT ages younger than 30 Ma. The north Alpine foreland basin is a flexural basin consisting of marine to alluvial conglomerates, sandstones and mudstones with Oligocene to Miocene sedimentation ages. To deduce the thermotectonic evolution of the Swiss Alps different stratigraphic levels of the main dispersal systems of the Swiss molasse basin were sampled. Three different approaches have been used: (1): Dating detrital zircons from molasse sandstones: Paleorivers collected and mixed detritus from different tectonic units. The 2r FT age distribution provides an overview of the cooling history of the main units exposed in the catchment area. (2): Dating of pebble populations (PPD, Dunkl, et al, 1998): 60-100 pebbles of the same lithotype were sampled, mixed together and dated. This method allows to decipher cooling histories of single distinct lithologies. The age distribution shows if they represent a homogenously cooled source area or if they belong to different tectonic units. Comparing the age spectra of PPD and sandstone shows the significance of the dated lithotypes contribution to the overall erosion history. (3): Dating of single pebbles: The apatite and zircon fission track method can be completed by the K/Ar-method on white mica and/or biotite, resulting in paleo-cooling paths of the rocks exposed by that time in the hinterland. In the Swiss molasse basin Rupelian to Aquitanian sandstones from the different dispersal systems yield very similar age patterns with age clusters around 230 - 220 Ma, 140 - 130 Ma and around 80 Ma. Ages clustering around 230 - 220 Ma are related to the enhanced heat flow during crustal extension due to a Permo-Triassic rifting event. A pronounced age group between 140 and 130 Ma points to a thermal or tectonic event by that time that reset the Zr FT ages of large rock volumes. In the recent Alpine surface these ages only occur in the very east of the Eastern Alps. The prominent age cluster around 80 Ma reflect the cooling period after Eoalpine Cretaceous metamorphism. A minor age cluster around 50 Ma is more difficult to explain because again in the presently exposed rocks no cooling event by that time is recorded. However, zircon grains of that age are also detected in the Paleogene Rhenodanubian Flysch (Trautwein et al, this volume), so they might be attributed to some Early Tertiary fast exhumation event or volcanism whose remnants are completely destroyed today. No younger grains occur in the age spectra, so in conclusion the older molasse sandstones display an exclusively Austroalpine age signature. Two flysch PPD show an almost identical age distribution

Geological Society of Australia - Abstracts Number 58

FT2 as the molasse sandstones. Also, different granite PPDs match with age clusters of the molasse sandstone.Therefore, until around 20 Ma, the entire hinterland of the Swiss molasse basin was composed of Austroalpine basement and flysch nappes. Burdigalian (around 20 Ma) sandstones from the eastern dispersal systems still display an Austroalpine age signature, while the sandstones from the western fan show a very distinct peak at 30 Ma. The postcollisional extension period of the entire Alps started around Aquitanian to Burdigalian times (Frisch et al, GSA, in press.) causing the exhumation of the Eastern Alps^ Penninic domains (e.g. the Tauern window) and very probably also the exhumation of the Penninic Lepontine Dome. Therefore, the 30 Ma age peak in the Burdigalian molasse sandstone can be related to the beginning erosion of the Lepontine Dome or of its thermally overprinted orogenic lid. However, a minor contribution of grains from Periadriatic intrusive bodies or volcanism (Dunkl et al, this volume) cannot be excluded. Age spectra from the youngest molasse sandstones (14 Ma sedimentation age) show young peaks in both the eastern and the western dispersal systems. But while sandstones from the western fan display a very distinct peak at 20 Ma, the eastern fan shows a more diffuse pattern with grain ages clustering around 33 Ma. Therefore, deeper Penninic units of the Lepontine Dome were already exposed in the catchment area of the western dispersal system while towards the east only higher Penninic units or the orogenic lid of the Lepontine Dome were eroded. Dunkl I. Frisch W. Kuhlemann J. and Briigel A. 1998. Pebble population dating: A new method for provenance analysis. Terra Nostra 98/1, 45

International Conference on Fission Track Dating and Thermochronology

F T 2 '

AND APATITE F I S S I O N - T R A C K THERMOCHRONOLOGY OF PALAEOPROTEROZOIC CRATONS IN N E AUSTRALIA

R.A. Spikingsi, B.P. Kohn2 and D.A. Foster3 1 Geologisches Institut, ETH Zentrum, CH-8092 Zurich, Switzerland 2 Australian Geodynamic Cooperative Research Centre, Dept. of Geology, University of Melbourne, Parkville, Vic. 3052, Australia 3 Dept. of Geology, University of Florida, PO Box 112120, Gainsville, FL 32611-2120, USA

The paucity of preserved Neoproterozoic and Phanerozoic units within the Palaeoproterozoic Mt. Isa, Murphy and Georgetown Inliers of north eastern Australia (Fig. 1) renders it difficult to determine their post orogenic (-1550 Ma) tectonic histories. However, a combination of various different thermochronological methods provides a means to assess this problem. Hornblende, white mica, biotite and alkali feldspar 40Ar/39Ar data and apatite fission-track (AFT) data reveal several distinct periods of post -1550 Ma cooling. Plateau and weighted mean 40Ar/39Ar ages from the Mt. Isa Inlier range between 1580 - 850 Ma and the Georgetown Inlier records ages between 740 - 380 Ma. AFT ages from the Mt. Isa and Murphy Inliers (Fig. 1) vary from 390 - 218 Ma and corresponding mean track lengths range between 13.55 and 10.94 |im. AFT ages and mean track lengths from the Georgetown Inlier (Fig. 1) range between 258 - 132 Ma and 13.11 - 11.36 |im respectively. These results record a nonlinear, protracted cooling history below ~550°C.

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Sydney

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Forward modelling of the 40Ar/39Ar (Lister and Baldwin, 1996) and AFT (Gallagher, 1995; using the quantitative kinetic description of Laslett et al, 1986; unnannealed track length set to 15.8 |Lim) data reveals a significant and previously unrecognised post-orogenic thermal record of the assembly and break-up of Rodinia and Gondwana (Fig. 2). Periods of cooling revealed by the data occurred at -1400 Ma, 1160-1100 Ma, 750 - 700 Ma, 550 - 500 Ma, 400 - 300 Ma and post -100 Ma (Fig. 2). These periods correspond with major tectonic episodes associated with the assembly and break-up of Rodinia and Gondwana and were responses to intra-plate and plate margin tectonism focused elsewhere in the contemporaneous Australian plate. Assuming a constant geothermal gradient of 20°C/km (Cull 1982), the events resulted in the denudation of -20 km of crust from the Mt. Isa Inlier since the Isan Orogeny (1550 Ma). Some prevailing geomorphological interpretations of long-term landscape development of intra-plate, cratonic terranes, which envisage repeated phases of denudation and burial suggest maximum denudation rates Geological Society of Australia - Abstracts Number 58

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FT2 of 0.1 - 0.2 m/m.y. (Fairbridge and Finkl, 1980). However, the cratonic inliers studied here experienced considerably higher denudation rates that varied between 0.9 - 0.02 km/m.y. and were greatest during the -1400 Ma event when up to 14 km of crust was denuded from various sub-regions of the Mt. Isa Inlier. Therefore, it is possible that denudation rates of cratonic interiors have generally been significantly under estimated in previous geomorphological studies. The spatial variation of 40Ar/39Ar and AFT data across the present erosion surface of individual inliers indicates that major structurally defined sub-regions show similar thermal histories on a regional scale. However, smaller scale fault-bounded blocks demonstrate variable cooling histories (Fig. 2). This suggests that reactivation of favourably oriented faults within the inliers, including segments of major faults, have occurred since -1550 Ma. Variations in ^^Ar/^^Ar data from central regions of the Mount Isa Inlier suggest shearing and up to 5 km of vertical displacement across a major fault zone during -1160 - 1150 Ma. Cull J.P. 1982. An appraisal of Australian heat-flow data. Bureau of Mineral Resources Journal of Australian Geology and Geophysics 7, 11-21. Fairbridge R.W. and Finkl C.W. Jr. 1980. Cratonic erosional unconformities and peneplains. Journal of Geology 88, 69 - 86. Gallagher K. 1995. Evolving thermal histories from fission-track data. Earth and Planetary Science Letters 136, 421 - 435. Laslett G.M. Green P.F. Duddy I.R. and Gleadow AJ.W. 1987. Thermal annealing of fission tracks in apatite 2: A quantitative analysis. Chemical Geology 65, 1 - 1 5 . Lister G.S. and Baldwin 1996. Modelling the effect of arbitrary P-T-t histories on argon diffusion in minerals using the Mac Argon program for the apple macintosh. Tectonophysics 253, 83 - 109. Acknowledgments This document is released with the permission of the Director, Australian Geodynamics Cooperative Research Centre.

FT2^ee

International Conference on Fission Track Dating and Thermochronology

F T 2 '

POST-PALAEOCENE THERMO-TECTONIC HISTORY OF THE NORTHERN CORDILLERA REAL, ECUADOR: INSIGHTS FROM ZIRCON AND APATITE FISSION-TRACK ANALYSIS

R.A. Spikings, D. Seward and W.Winkler Geologisches Institut, ETH-Zentmm, Zurich CH-8092, Switzerland

Introduction The poly-deformed Cordillera Real in Ecuador (Fig. 1) originated during the Early Cretaceous accretion of terranes at ~140 - 120 Ma, otherwise referred to as the Peltetec Event. Progressive cratonward migration of the orogenic wedge and unroofing of the orogen since the Peltetec Event has lead to the exposure of greenschist and lower grade metamorphic rocks. The post-Peltetec geological evolution is only constrained by a series of partially and completely reset K/Ar ages (e.g. Litherland et al, 1994). We present zircon and apatite fission-track (ZET and AET respectively) data from three traverses across the northern Cordillera Real (labels a, b, c; Eig. 1) that provide a quantitative framework for the post-Palaeocene tectonothermal evolution of the orogen.

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Figure 2. A selection of AFT age and length data from three traverses across the Cordillera Real,

Results and Interpretation AET ages range between 4 5 - 8 Ma and a majority of mean AET lengths are <14 |im long and have standard deviations >1.4 |im. These data record a protracted cooling history below ~110±20°C. However, some samples yield concordant 2FT and AET ages that cluster at ~10±2 Ma, 20±3 Ma and 42±3 Ma, with mean AET lengths of ~14 |im (Eig. 2). These samples cooled rapidly through ~29060°C at these times. Thermal modelling of the samples with mean AET lengths of <14 |im (e.g. Laslett et al, 1987; Gallagher, 1995) confirms the significantly elevated cooling rates at -10, 20 and - 4 0 Ma at temperatures <~110±20°C. While variations in fission-track annealing kinetics may be partly responsible for geographical differences in the ET ages, it is more likely that these differences are a consequence of variable, postPalaeocene, cooling histories. No igneous activity is documented within the Cordillera Real at 10 Ma, 20 Ma and 40 Ma, hence cooling has been directly attributed to crustal denudation. Geological Society of Australia - Abstracts Number 58

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The data clearly show that denudation depths and rates within and between individual traverses differ at any particular time, and are not delineated by Cretaceous terrane boundaries. However, distinct periods of relatively high denudation rates can be placed into three broad groups, each of which is chronologically synchronous with major tectonic rearrangements and correlates with sedimentary facies development within the Amazon Foreland Basin (Fig. 3). A. 0 - 10 Ma: All regions were denuded at relatively high rates during this period although they were greatest in northern Ecuador (Fig. 3), where they peaked at -1.3 km/m.y. This time period may be synchronous with the subduction of the Carnegie Aseismic Ridge, as well as increased half-spreading rates in the equatorial Atlantic.

FT2 B. 15 - 25 Ma: Fault bounded units from the most northern and southern traverses (traverses, a, and c) were denuded at average rates of <1.2 km/m.y. and local variations are significant (Fig. 3). These increased rates may have been driven by the break-up of the Farallon plate and a re-orientation of the subducting plate vector from ENE to E at - 2 5 Ma. C. 30 - 43 Ma: Concordant 2FT and AFT ages imply that various faulted blocks were denuded at rates between - 0 . 4 - >1 km/m.y. in central regions of Ecuador. This time period corresponds with the accretion of oceanic terranes to the west (e.g. Pinon Terrane; Fig.l).

Conclusions 1. ZFT and AFT data suggests that up to 10 km of crust may have been denuded from local faultbounded blocks, at rates comparable to the European Alps, since the Palaeocene. This demonstrates that the Early Cretaceous Cordillera was an active tectonic belt - 1 0 0 m.y. after its initial development. 2. Elevated denudation rates persisted for periods of - 1 0 m.y. and were partially synchronous with the tectonic phases of Steinmann (1929) for the Peruvian Andes. 3. Brittle deformation has not been restricted to terrane boundary faults but was prevalent at the intraterrane scale. Gallagher K. 1995. Evolving thermal histories from fission-track data. Earth and Planetary Science Letters 136, 421 - 435. Laslett G.M. Green P.F. Duddy I.R. and Gleadow A.J.W. 1987. Thermal annealing of fission tracks in apatite 2: A quantitative analysis. Chemical Geology 65, 1 - 15. Litherland M. Aspden J. and Jemieleta R.A. 1994. The metamorphic belts of Ecuador. Overseas Memoir 11, British Geological Survey, pp. 147. Noblet C. Lavenu A. and Marocco R. 1996. Concept of continuum as opposed to periodic tectonism in the Andes. Tectonophysics 255, 65 - 78. Steinmann G. 1929. Geologie von Peru. Karl Winter, Heidelberg, pp. 488.

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

INTERCALIBRATION AND INTEGRATION OF APATITE ( U - T H ) / H E AND F I S S I O N - T R A C K THERMOCHRONOMETERS ON AN EXHUMED EXTENSIONAL FAULT B L O C K , W H I T E MOUNTAINS, EASTERN CALIFORNIA,

U.S.A.

D.F. STOCKUI'2, K.A. Farley2 andT.A. Dumitrui 1 Department of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115, UASA 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

(U-Th)/He dating of apatite has recently attracted considerable interest as a potential new low temperature thermochronometer (Zeitler et al, 1987; Farley et al, 1995; Wolf et al, 1996). Laboratory He diffusion experiments suggest that this system should be sensitive at crustal temperatures of 45°C to 85°C (Wolf et al, 1996, 1998), lower than any other known thermochronometer. Assuming a typical Earth surface temperature of 10°C and geothermal gradient of 25°C/km, this is equivalent to depths of 1.2 to 3 km beneath the Earth's surface. Thus the system can potentially be applied to investigate a variety of geologic processes in the uppermost part of the crust, such as kilometre-scale fault slip and landscape development. In order for the (U-Th)/He system to be applied with confidence, however, it is essential to empirically demonstrate that its temperature sensitivity range is properly calibrated and that measured (UTh)/He ages reliably date cooling events. Warnock et al (1997) and House et al (1999) have attempted to test the validity of laboratory derived diffusion data through the analysis of samples collected from boreholes with known downhole temperatures, broadly confirming the expected decrease in apparent age with increasing depth and downhole temperature. Another approach for testing the (U-Th)/He thermochronometer is to apply it to exhumed rocks whose time-temperature histories have been constrained by other thermochronometers. In this study, we apply (U-Th)/He methods to samples from the White Mountains in eastern California where the thermal history is constrained by apatite fission-track dating. The northern White Mountains comprise a virtually intact range-scale crustal block that has been tilted eastward by about 25° during Basin and Range extension in Miocene time (Stockli, 1999). This tilting has exhumed and exposed rocks that were at paleodepths of 0 to 7 km before extension began. This near-ideal setting permits (U-Th)/He and fission-track samples to be analysed from a range of pre-extensional paleodepths and paleotemperatures and compared to evaluate and calibrate the (U-Th)/He dating method. This study on an exhumed Basin and Range footwall block confirms that He diffusion parameters derived from short-time-scale, laboratory heating experiments may be validly extrapolated to geologic time scales. The reconstruction of the pre-extensional thermal state of the crust using the apatite fission-track method confirms that the (U-Th)/He thermochronometer is characterized by a partial retention zone between about 45° and 85°C (Fig. 1). This is in good agreement with laboratory He diffusion experiments (WoMetal, 1996, 1998) and boreholes calibration studies (Warnock etal, 1997; House et al, 1999), lending support to the potential of the (U-Th)/He system to retain precise thermochronometric information. This study also shows that the two low-temperature thermochronometers yield consistent estimates for paleogeothermal gradients (~17°C/km) and for the time of onset of rapid footwall exhumation of the White Mountains fault block (-12 Ma). In interpreting (U-Th)/He apparent ages, it is important to remember that they are total gas ages, much like conventional K/Ar ages. Partial diffusive loss of He substantially reduces the initial age of a sample which resided at temperatures between 45° and 85°C. Such partially reset (U-Th)/He ages cannot simply be interpreted as directly dating when a sample cooled through a specific closure temperature. In this case study, only five of fifteen (U-Th)/He samples directly date the timing of cooling and footwall exhumation, whereas most samples yield partially reset ages that do not directly date a specific cooling event. The proper interpretation of a single (U-Th)/He age is thus possible only with corroborating evidence, such as apatite fission-track data from the same sample and/or (UTh)/He versus paleodepth sample arrays.

Geological Society of Australia - Abstracts Number 58

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Figure 1. Summary diagram of the integrated (U-Th)/He and apatite fission track age data from the northern White Mountains. Apatite fission track age and length data from the northern White Mountains establish a thermal reference system that allows for the empirical evaluation of the validity and applicability of the (UTh)/He thermochronometer. The (U-Th)/He partial retention zone (PRZ) is well-defined and ranges from -85-45 °C using the thermal reference frame deduced from the apatite fission track data. Rapidly uplifted samples from below the exhumed PAZ/PRZ are invariant in age and directly date the time of inception of footwall cooling and extensional faulting. Several anomalously old apparent (U-Th)/He ages are the result of excess He due to zircon and fluid inclusion. Younger apparent ages (3-4 Ma) at the base of the profile reflect renewed Pliocene uplift (discussed in Stockli, 1999).

15

FT2 In conclusion, this study demonstrates that a wide variety of important geologic processes in the upper 1 to 5 km of the crust can potentially be investigated by integrated apatite (U-Th)/He and fission-track thermochronological methods, constraining thermal histories over the interval 45°-120°C. Farley, K. A., Wolf, R. A., and Silver, L. T., 1995, (U-Th)/He dating; overview of the system and analytical techniques: Abstracts with Programs - Geological Society of America, v. 27, no. 6, p. 37. House, M. A., Farley, K. A., and Kohn, B. P., 1999, An empirical test of helium diffusion in apatite: borehole data from the Otway Basin, Australia, in press. Stockli, D. F., 1999, Regional timing and spatial distribution of Miocene extension in the northern Basin and Range Province. Unpubl. Ph.D. thesis, Stanford University, 270 p. Warnock, A. C., Zeitler, P. K., Wolf, R. A., and Bergman, S. C., 1997, An evaluation of low-temperature apatite U-Th/He thermochronometry: Geochimica et Cosmochimica Acta, v. 6l, no. 24, p. 5371-5377. Wolf, R. A., Farley, K. A., and Silver, L. T., 1996, Helium diffusion and low-temperature thermochronometry of apatite, Geochimica et Cosmochimica Acta, v. 60, p. 4231-4240. Wolf, R. A., Farley, K. A., and Kass, D. M., 1998, Modelling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer: Chemical Geology, v. 148, no. 1-2, p. 105-114. Zeitler, P. K., Herczeg, A. L., McDougall, I., and Honda, M., 1987, U-Th-He dating of apatite. A potential thermochronometer: Geochimica et Cosmochimica Acta, v. 51, p. 2865-2868.

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International Conference on Fission Track Dating and Thermochronology

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CosMOGENic ISOTOPE CONSTRAINTS ON GEOMORPHIC PROCESSES AND LANDSCAPE EVOLUTION

J.O. Stone Department of Geological Sciences and Quaternary Research Centre, University of Washington, Seattle, Washington, U.S.A.

Cosmogenic isotopes are formed by high energy cosmic ray reactions in the uppermost 10-20 m of the crust. A broad spectrum of nuclides is produced in common rock-forming minerals. The most widely used are i^Be (tl/2 = 1.5 m.y.) and ^^Al (0.7 m.y.), produced from O and Si in quartz, (0.3 m.y.) produced from Ca, K and 35cl in calcite, K-feldspar and whole rock samples, and the stable noble gas isotopes ^He and ^iNe, produced from Na, Mg, A1 and Si in olivine and feldspar. Isotope production rates depend strongly on depth, so isotope concentrations reflect a sample's shielding history in the uppermost crust. This leads to a range of applications in geomorphic studies of erosion, sediment transport and landscape evolution, in stable and tectonically active terrains. Both measurement techniques and our knowledge of isotope production systematics - production rates and their dependence on target composition, latitude, altitude and depth - have advanced enormously in the past decade. As a result, geological variance and the overly simplistic geological assumptions used to interpret cosmogenic isotope data now probably outweigh systematic errors in most studies. Erosion rates The dominant cosmogenic isotope production process, nuclear fragmentation by high energy cosmic ray neutrons, decreases sharply with depth in the topmost 2-3 meters of the crust. Below this, production by muon reactions decreases more gradually, reaching negligible levels below a few tens of meters. Isotope accumulation thus responds to erosion, which controls how long rock spends in this zone en route to the surface. The higher the erosion rate, the shorter the exposure time to cosmic radiation in the uppermost crust and the lower the eventual isotopic concentration. An isotopic steady state is reached after erosion of 3-5x the characteristic attenuation depth for isotope production. Hence erosion rates recorded by cosmogenic isotopes pertain only to removal of the last few meters of rock (~2-5 m, depending on the muon contribution). This should be contrasted with thermochronological methods, which average erosion rates over removal of the last 2-3 km. The depth sensitivity of cosmogenic-isotope-based erosion estimates (m) lies between that of thermochronological methods (km) and modern process measurements such as stream gauging (mm), but nonetheless is far shorter than the characteristic relief of most landscapes. Extrapolating erosion rates measured with cosmogenic nuclides to the vertical scale of landscape relief therefore requires geomorphic models or assumptions. The required extrapolation is least severe m ancient, slowly-evolving, low relief landscapes. A growing body of data from cratonic landscapes is revealing low rates of erosion, on the order of a few meters per million years, consistent with very long-term preservation of ancient landforms. Recent data from northern Australia indicate that indurated surfaces formed by deep weathering, such as silcretes, ferricretes and laterites have very low erosion rates, which in turn accords with old (early Tertiary/late Mesozoic) 40Ar/39Ar ages for the weathering profiles [Ij. It is noteworthy, however, that erosion rates of zero have only been recorded in Antarctica, on surfaces unlikely to have been exposed to liquid water for millions of years [2]. Erosion rates can be measured with cosmogenic nuclides on individual outcrops, or by analysing sediment shed from the ensemble of outcrops and soil-covered surfaces comprising a drainage basin (e.g. [3]). Sediment measurements give the catchment-averaged lowering rate without revealing the spatial distribution of erosion. Catchment-scale measurements rely on (i) steady-state throughout the sediment delivery system, (ii) uniform distribution of the target mineral (generally quartz) in the eroding rocks, (iii) absence of long-term storage and sediment recycling in the catchment and (iv) the availability of a truly representative sediment sample. These criteria may not be met in large catchments with low sediment delivery ratio, but can probably be satisfied in low-order catchments. The approach holds great promise, but is still at an exploratory stage of development. Geological Society of Australia - Abstracts Number 58

FT2^ee Scarp retreat Denudation rates can be measured on surfaces in any orientation, provided the zenith-angle distribution of the cosmic ray flux is taken into account. Cosmogenic isotope measurements can thus be used to measure back-wearing rates of scarp faces or the recurrence time between scarp failures. Measurements on the Drakensberg Escarpment in southern Africa [4] indicate slow retreat - an order of magnitude slower than suggested by models in which the scarp position results from steady backwearing of an ancestral rift margin near the present coast. Neotectonics and active geomorphic process studies: Erosion in tectonically active regions can reach rates of mm/yr. At such rates, the effective cosmic ray exposure time of material at the surface is less than 103 yr, resulting in concentrations below detection limits for some isotopes. An alternative approach to gauging erosion in these terrains is measurement of river incision rates by exposure dating of bedrock strath terraces. Neotectonic activity can also be monitored by exposure dating of marker surfaces displaced by faulting, or direct dating of hard-rock fault scarps. Several studies have now dated basalt flows and alluvial fan surfaces offset by normal, thrust and strike-slip faults, giving average slip rates over 10^ - 10^ year periods. More detailed displacement histories can be obtained by direct dating of normal fault scarps. Co-seismic unroofing of the footwall block results in an isotope profile on the scarp face from which earthquake ages and displacements can be calculated. Examples from the Basin and Range province indicate that multiple events can be dated and used to assess the recurrence behaviour and likely strain state of faults. Palaeoaltimetry One of the most promising potential applications of cosmogenic isotopes is true palaeoaltimetry. The cosmic ray flux approximately doubles with each kilometre in altitude, making isotope production a very sensitive recorder of exposure altitude. Development of cosmogenic isotope palaeoaltimetry requires more robust calibration of isotope production rates, and can only be applied to independently dated surfaces. The greatest potential problem, however, is that shielding changes due to erosion mimic changes due to uplift, and many regions where akitude changes are likely to be large and interesting are also prone to rapid erosion. Arid areas such as the Tibetan Plateau, Altiplano and Transantarctic Mountains are exceptions and work in these areas may be able to place constraints on altitude changes over the past 10^ - 10^ years (e.g. [2]). (1) Vasconcelos P. and Stone J. 1999. Ancient landscapes in northern Australia in prep. (2) Schafer J.M., Ivy-Ochs S., Wieler R., Leya I., Baur H., Denton G. and Schluchter C. 1999. Cosmogenic noble gas studies in the oldest landscape on earth: surface exposure ages of the Dry Valleys, Antarctica. Earth and Planetary Science Letters 167, 215-226. (3) Granger D.E., Kirchner J.W. and Finkel R. 1996. Spatially averaged long-term erosion rates measured from in-situ produced cosmogenic nuclides in alluvial sediment. The Journal of Geology 104, 249-257. (4) Fleming A., Summerfield M.A., Stone J.O., Fifield L.K. and Cresswell R.G. 1999. Denudation rates for the southern Drakensberg escarpment, SE Africa, derived from in-situ-producQd cosmogenic ^^Ch initial results. Journal of the Geological Society, London 156, 209-212. Acknowledgements I wish to acknowledge the continued support and collaboration of numerous colleagues in the ANU accelerator mass spectrometry group and the Centre for Accelerator Mass Spectrometry, Lawrence Livermore National Lab. Supported in part by grant EAR9805132 from the US National Science Foundation.

International Conference on Fission Track Dating and Thermochronoiogy

OBSIDIAN FISSION-TRACK DATING USING LUTETIUM, ZIRCONIUM AND GOLD NEUTRON MONITORS T. Suzuki Department of Geology, Faculty of Education, Kagoshima University, Kagoshima, Japan

Zeta calibration of fission track dating (Hurford and Green, 1983) has been applied to crystal minerals like zircon and apatite. However, for fission track dating of volcanic glass, such as obsidians, no age-standard materials have been found at the present. Furthermore, fission track dating of younger obsidians has to be carried out frequently with samples, from which the low density of spontaneous tracks and additionally large counting errors are derived. In such cases, a direct method of the fission track age calculations would be useful also to check their experimental results. From these points of view, absolute determination of fission reaction rate of dosimeter glasses has been carried out using activation reaction rates of neutron monitors, gold, lutetium and zirconium. Characteristics of irradiation fields revealed by Au and Zr monitors high purity gold (5N), lutetium (4N) and zirconium dioxide (4N75) were used as neutron monitors for the irradiation runs of CNl and CN5 dosimeter glasses. For evaluation of neutron spectra at standard states of irradiation fields, various nuclide monitors, Co, Cu, Mo, Zn, Ni, In, and Al, were also used mostly in a form of their infinitive thin condition. These monitors were irradiated individually and continuously for a series of reactor runs. Neutron spectra at non-disturbed state without any effect of other nuclide were obtained from these various monitors. Activation rate of Au monitors irradiated with standard glasses at a same capsule was estimated from cadmium ratios and alpha values which were calculated from a calculation programme written with F-Basic (Win95A5C^in98) as the step-by-step approximation to the formula of De Corte et al. (1981). These calculated values of cadmium ratio for Au and Zr are in good agreement with the values directly measured with cadmium capsule, which was made of a 1 mm thick cadmium plate (Table 1). These reactor runs were carried out at research reactors of Japan: TRIGA Mark II, Rikkyo University, Yokosuka and JRR-3M, Japan Atomic Energy Research Institute, Tokai.

Table 1. Cadmium ratio at PN2, JRR-3M (Feb.98) Nuclide Au-197 Zr-94 Zr- 96

Cadmium ratio Calculated Measured 11.29 122.15 2.74

14.84 70.39 2.71

Table 2. Parameters for CN5 dosimeter glass (])th(Au) = ( 3 . 7 2 9 ± 0 . 0 5 1 ) x l 0 ^ W / s e c pi(CN5)/sec = ( 5 . 4 0 0 ± 0 . 0 1 5 ) x l 0 W / s e c B(0)CN5= 4.039x10-^2 Z(0)CN5= 418.9

Table 3. Analytical Data for Fission Track Dating of JRl obsidian Uncorrected ps/pi: 3.56x10 ^ Plateau ps/pi : 5.83x10 ^ pi(CN5)=(4.07±0.13)x 1 OVcm' Themially Corrected Age: 99.4x10'a

;if = 7.03xl0-^7a Geological Society of Australia - Abstracts Number 58

FT2 Calibration of dosimeter glasses Before zeta calibration was introduced, B-value (Hurford and Gleadow, 1976) has been used. 0= B,'pd However, I would like to use B(0) value instead of B-value because this is independent from neutron spectra in the neutron irradiation fields. B(0) =

/pd

then 2(0) = B(0) pi / Xi Parameters for CN5 estimated from this work are shown in Table 2. . An obsidian fission-track age by Iso-Thermal Plateau method JR-1 obsidian, one of Geochemical Reference Materials issued from Geological Survey of Japan has been used for absolute calculation of the fission track age. Iso-thermal Plateau method was used for thermally correction of track densities of JR-1. The annealing experiments of JRl obsidian, irradiated at HIFAR research reactor, Sidney, Australia, were carried out at the fission track laboratory of Department of Earth Sciences, La Trobe University, Melbourne, using with an aluminium furnace at 151 degrees Centigrade in the duration from 3 hours to 4 weeks. The experimental results are shown in Table 3- The age of JRl estimated in this work is in good agreement with K-Ar ages previously reported (Kaneoka and Suzuki, 1970) from similar intrusive bodies regarded as same as JRl obsidian. De Corte, F. et al. (1981): Jour. Radioanal. Chem., 62, 209-255. Hurford, A.J. (1990): Chem. Geo!., 80, 171-178. Hurford, A.J. and Gleadow, AJ.W. (1977): Nucl. Tracks, 1, 41-48. Hurford, A.J. and Green, P.F. (1983): Isot. Geosci., 1, 285-317. Kaneoka, I. and Suzuki, M. (1970): J. Geol. Soc. Japan, 76,309-313. Acknowledgement special thanks are due to Prof. A.J.W.Gleadow, Dr.B.Kohn and the members of Fission Track Group, La Trobe University. The author also appreciates Dr. H.W.H.Schreurs (Corning) and Dr. A. Ando (GSJ) for their kind providing dosimeter glasses and JRl obsidian, respectively. This work was in part financially supported by the Visiting Researchers Program at Rikkyo University Reactor and Inter-University Programme for Common use of JAERI Facilities.

International Conference on Fission Track Dating and Thermochronology

F T 2 '

T H E TECTONIC R E S P O N S E OF THE SOUTHERN CHILEAN ANDES TO A SUBDUCTING M I D - O C E A N R I D G E : AN INVESTIGATION USING F I S S I O N T R A C K THERMOCHRONOLOGY

S.N.Thomsoni, E Herve2, B. Stockherti, M.R. Brixi and A.Adriasola2 1 Institut fur Geologie, Mineralogie und Geophysik, Ruhr-Universitat Bochum, D-44780 Bochum, Germany 2 Departamento de Geologia, Universidad de Chile, Casilla 13518 Correo 21, Santiago, Chile

Introduction Since about 14 Ma, the Chile Rise, an active mid-oceanic spreading system, has been subducting beneath Andean-type continental lithosphere of the southernmost Chilean Andes. Presently active subduction of this ridge is situated at the Chile Triple Junction (CTJ) at about 46°S on the Peru-Chile Trench. The response of the overriding plate to this subduction has included the creation of an arc volcanic gap, surface uplift and erosion of the main Andean Cordillera, development of a back-arc fold and thrust belt, eruption of large areas of back-arc plateau basalts, obduction of an ophiolite and the development of a major trench parallel dextral strike-slip fault (the Liquifie-Ofqui Fault or LOF) that extends some 1000 km northward from present day Golfo de Penas (ca. 47°S). The aim of our work in the region, as part of an international collaborative program in the southern Chilean Andes, has been to apply fission track (FT) thermochronology to try to better assess the low temperature cooling and denudation history of the region. A particular interest has been to study the effects of subduction of the Chile Rise on the overriding plate to subduction. Additionally, other factors such as possible climate controlled differential E-W erosion across the southern Andes, and local effects of block movements related to the LOF have also been investigated. 100 FT ages (47 apatite and 53 zircon) and 29 apatite length distributions have been obtained from the region between ca. 42°S and 48°S (Fig. 1). A further 49 samples collected from the Chilean fjord region between 48°S and 52°S are presently being processed.

75° W

74° W

73° W

72° W

Figure 1. Apatite (a) and zircon (b) fission track ages from various rocks of the southern Chilean Andes between 42°S and 48°S Geological Society of Australia - Abstracts Number 58

FT2 Apatite FT results The apatite FT ages show a wide variation between 1±0.3 to 86±4 Ma. These ages can be subdivided into several groupings. Young ages between 4 and 12 Ma with long track lengths >14 pm indicative of rapid cooling through the apatite partial annealing zone (APAZ) are obtained along the main N-S topographical high of the Andes, suggesting that erosion related to relief and topography increased significantly along the main Andean Cordillera after about 12 Ma. The very youngest apatite ages (<3 Ma) occur close to the LOF north of Puerto Cisnes (44°30'S). It is likely that these ages are related to local high heat flow along the main fault trace as indicated by present day thermal springs and volcanism. Significant recent denudation close to the fault may also have played an important role. Much older ages of over 40 Ma with mean track length distributions between 11 pm and 13 pm are obtained east of the main Andean divide. These samples indicate that very little denudation (<34 km) has occurred here since Late Cretaceous and Early Tertiary times. The west to east decrease in total denudation during the Tertiary may be explained by changes in precipitation values linked to erosion rates across the Andean divide at these latitudes. Present day rainfall west of the Andean divide reaches up to 8 m/yr, whereas to the east it is practically zero. Such a westerly directed precipitation pattern has probably existed in the area at least since the opening of the Drake Passage in the Early Oligocene. In the region of Puerto Aysen (45°30'S) a sudden west to east increase in apatite FT ages from <5 Ma to ages between 17 and 72 Ma is observed across a poorly studied major fault (the Azul-Tigre Fault) suggesting this fault was active since ca. 12 Ma (see discussion of zircon FT results). Finally, west of the LOF and in the western part of Canal Baker (ca. 48°S) ages of between 24 and 30 Ma with long track lengths (>14 pm) occur, indicating an accelerated cooling event at this time. This can be correlated with a significant increase in the convergence rate between the Nazca and South American plates identified between ca. 28 and 26 Ma (Somoza, 1998). This event is well known for correlating with the surface uplift and formation of the Altiplano and Central Andes (Giese et al, 1999). However, these apatite FT results reveal that the southern Andes were also affected by this increase in convergence rate. The different climatic conditions of the southern Andes probably meant that any increase in topography and relief of the Cordillera caused increased denudation on its western margin, close to the subduction zone. Zircon FT results The zircon FT ages range from 3±1 to 264±l4 Ma. Although quite varied, most of the zircon FT ages obtained from the Patagonian Batholith are approximately 10 to 30 m.y. younger than the local 'intrusion' ages determined by Rb-Sr (whole rock) and U-Pb (zircon) - e.g. Pankhurst et al (1999). A zircon FT closure temperature of 280±30°C requires that many of the granitoid bodies of the batholith cooled at rates of ca. 20°C/m.y. following intrusion. This most likely represents in situ cooling of the individual plutons intruded into relatively cool high crustal level country rock. Cooling related to denudation was probably small, as large amounts of denudation were unlikely to have been localised at different times to individual plutons. In contrast to this general age pattern, zircon FT ages from batholithic rocks in the western part of Canal Baker (ca. 48°S) of between 66 to 42 Ma are up to 80 m.y. younger than the U-Pb (zircon) ages from rocks in the same area (Bruce et al, 1991). Several explanations can be proposed for these ages. Most likely is that they represent much slower cooling to below the zircon FT closure temperature following intrusion due to these rocks being emplaced into relatively hot deep crustal level country rocks. This implies that the western part of the batholith represents has been more deeply eroded. Alternatively, the younger ages may represent partial resetting related to subduction of a mid-oceanic ridge separating the Farallon and Aluk plates at around 40 to 50 Ma (Ramos and Kay, 1992). Very young zircon ages (<12 Ma) are found in Cretaceous-aged batholithic rocks to the west of Puerto Aysen and close to the LOF, north of Puerto Cisnes. These ages signify substantial cooling from temperatures of >300°C since the Miocene. In the Aysen region a jump in zircon FT ages (from 12 Ma to 70 Ma, west to east) occurs in the same position as that seen in the apatite FT results and further supports postulated activity along the Azul-Tigre Fault since ca. 12 Ma.

FT2 These younger ages are also probably related to increased activity along the LOF at around 12 Ma marked by plutonism and increased denudation and more recent high heat flow and volcanism. Two old zircon FT ages of 252 and 264 Ma have been obtained from metamorphic basement (phyllites and quartzites) east of the main Patagonian Batholith. Such ages indicate that either metamorphism of these rocks was pre-Permian in age or that the conditions of metamorphism never reached temperatures >ca. 230°C. A zircon FT age of 140±8 Ma from low grade metamorphic sandstone of probable Triassic age in the Chonos Archipelago (ca. 45°20'S) indicates that these rocks have experienced temperatures of >230°C since their deposition. This age therefore represents a minimum age for the metamorphism in these rocks prior to their intrusion by granitoids at ca. 135 Ma Bruce, R.M., Nelson, E.P., Weaver, S.G. and Lux, D.R. (1991). Temporal and spatial variations in the southern Patagonian batholith; constraints on magmatic arc development. Geological Society of America Special Paper, 265, p. 1-12. Giese, P., Scheuber, E. and The ANCORP Research Group (1999). The formation of the Central Andes - controlled by varying spreading rates of the SE-Pacific Rise. Terra Abstracts, 11, p. 414. Pankhurst, R.J., Weaver, S.D., Herve, F. and Larrondo, P. (1999). Mesozoic-Cenozoic evolution of the North patagonian Batholith in Aysen, southern Chile. Journal of the Geological Society of London, 156, p. 673-694. Ramos, V.A. and Kay, S.M. (1992). Southern Patagonian plateau basalts and deformation: backarc testimony of ridge collisions. Tectonophysics, 205, p. 261-282. Somoza, R. (1998). Updated Nazca (Farallon) - South America relative motions during the last 40m.y.: implications for mountain building in the central Andean region. Journal of South American Earth Sciences, 11, p. 211-215. Acknowledgement Fieldwork in Chile was supported by Catedra Presidencial en Ciencias and Eondecyt Grant 1980741 to FH and DFG Grant Stol96/ll-l to BS. SNT is supported by DFG Stipendium Th 573/2-1.

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International Conference on Fission Track Dating and Thermochronology

APATITE F I S S I O N T R A C K DATA F R O M THE E A S T E R N O F F I C E R

F T 2 '

BASIN

P.R.Tingatei and I.R. Duddy2 1 National Centre for Petroleum Geology and Geophysics, University of Adelaide, South Australia, Australia. 2 Geotrack International Pty Ltd, Brunswick West, Victoria, Australia.

The study was funded by the Petroleum Group in Primary Industries and Resources South Australia (PIRSA) in order to obtain information about the petroleum generation history of the eastern Officer Basin. The boreholes analysed included Manya-2, -5 and -6; SMD5001 and Giles-1. The samples analysed come from formations Proterozoic to Mesozoic in age. Within the basin succession there are several major unconformities representing uncertain amounts of erosion. Erosional events occur between 560 and 540 Ma (Petermann Ranges Orogeny), approximately 500 - 490 Ma (Delamerian Orogeny), 375 - 290 (Alice Springs Orogeny), 260 - 150 Ma, and 95 - 35 Ma. Only relatively minor erosion has probably occurred since the development of Late Eocene - Early Miocene (~35 Ma) Cordillo surface. All Proterozoic and Cambrian samples show evidence of cooling from temperatures =100°C during the Alice Springs Orogeny. The AFTA data does not constrain palaeotemperatures from earlier events. All samples appear to have cooled from elevated temperatures of 90-100°C prior to cooling between 110 and 70 Ma with most having cooled from temperatures ~25°C higher during the last 50 Ma. Elevated temperatures associated with the last two events are probably related to increased depth of burial but may also be associated with increased temperatures caused by aquifers with heated water. The Proterozoic and Cambrian samples analysed were within and possibly passed through the oil window prior to cooling during the Alice Springs Orogeny between 360 and 300 Ma. The basal part of the 500 m thick section of Permian and Mesozoic section in Manya-2 appears to have entered the 'early mature' part of the oil window prior to cooling between 110 and 70 Ma.

Geological Society of Australia - Abstracts Number 58

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International Conference on Fission Track Dating and Thermochronology

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F T STUDIES ON THE RHENODANUBIAN FLYSCH - T H E K E Y TO EXHUMATION PROCESSES IN THE EASTERN ALPS

B.TrautweinJ. Kuhlemann, 1. Dunkl and W. Frisch Geological Institute, University of Tubingen, Sigwartstr. 10,72076 Tubingen, Germany

The fission-track dating method on apatite and zircon was applied to the Late Cretaceous to Eocene turbiditic sequence of the Rhenodanubian flysch basin of the Eastern Alps. The Rhenodanubian flysch basin situated at the northern margin of the Eastern Alps stretches E-W over 520 km. The entire turbiditic sequence has an approximate thickness of 2000 m. In the west the sequence embraces the early Lower Cretaceous to the late Maastrichtian. To the east it becomes successively younger and extends into the lower Eocene. Using single-grain FT geochronology, the thermotectonic processes in the hinterland and the exhumation of the accreted basin sediments could be deciphered. Thermotectonic processes in the hinterland of the Rhenodanubian flysch basin To elucidate the provenance of the Rhenodanubian flysch and gain knowledge about the thermotectonic processes in the hinterland, the fission-track method has been applied on zircon. Zircon fission-track age distributions with clusters at around 80, 120 and 180 to 255 Ma are characteristic for the Cenomanian to Eocene sedimentary rocks. The two young age clusters reflect the cooling after the Cretaceous Eoalpine metamorphism with a clear source from the Alpine chain (Austroalpine and related units further east). The Jurassic to Permian ages are an expression of the break-up of Pangea and rifting, starting off in Permian times, with an enhanced heat flow. Such ages are known from Alpine areas (mainly from the western part of the Austroalpine). An additional source area for the Permian zircons could be the Variscan European continent. All zircon samples of the Rhenodanubian flysch zone show a low percentage of Variscan ages, reflecting only poor exposure of bodies which have solely undergone Variscan orogeny. The zircon population of the Eocene sedimentary rocks is characterised by an additional age cluster around 60 Ma. The colourless zircons clustering around 60 Ma are of euhedral to subhedral shape. These ages give evidence either for a volcanic source area or a fast exhumed orogenic body in Early Tertiary times. According to our FT data and paleogeographic considerations, the main source area of the Rhenodanubian flysch basin from Cenomanian to Eocene times was the evolving Eoalpine orogen. The European continent was only acting as a minor supplying area. Successive accretion and differential exhumation along the Eastern Alpine front The thermal evolution of the Rhenodanubian flysch zone was deciphered using apatite fission-track geochronology. In the Rhenodanubian flysch zone three nappes can be distinguished. Only the lowest nappe extends along the entire Eastern Alps. The two higher nappes are restricted only to the eastern part of the flysch zone. They are actually located further south of the lowest nappe. The apatite FT ages strongly differ in the lowest nappe along strike. In the west, the ages indicate total annealing by burial underneath the APA2. In the central part only the Cenomanian rocks are overprinted while the apatite fission-track ages in Maastrichtian rocks are not rejuvenated. Some apatite fission-track ages of the central area reflect partial resetting due to burial. In the east, the rocks stayed above the APA2 and show no overprint. The ages, which show overprint, indicate Miocene cooling in the west, and Eocene to Oligocene cooling in the central part of the Rhenodanubian flysch zone. Bimodal track-length distributions illustrate residence in the apatite partial annealing zone (APAZ) followed by relatively fast exhumation. The partially overprinted rocks of the central part Geological Society of Australia - Abstracts Number 58

FT2^ee experienced burial into the APA2 for only a short time and were exhumed above the APAZ, while the older sandstones at the same time still remained within the APAZ. The two higher nappes consistently yield post-sedimentary thermally overprinted apatite fission-track ages. All formations independently from their stratigraphic age reveal Late Oligocene to Early Miocene apatite fission-track ages. The apatite FT ages and the track-length distributions indicate fast exhumation of the higher nappes at that time. Keeping in mind the stratigraphy and reconstructing the evolution of an accretionary wedge on the base of the thermal history revealed by apatite fission-track dating, we can draw the following conclusion for the Rhenodanubian flysch zone. In the east, sedimentation was terminated in the southernmost basin in Paleocene times by the thrust movement of the Northern Calcareous Alps over this basin, while there was still ongoing sedimentation in the northern two basins. In Eocene times the northern nappes were accreted in sequence. The two southern nappes have been buried below the apatite partial annealing zone into the total annealing zone, while the northern nappe stayed above the APAZ. In Late Oligocene/Early Miocene times the exhumation of the two southern nappes was performed by erosion and extensional strike-slip movement of the Northern Calcareous Alps with respect to the underlying Rhenodanubian flysch due to lateral extrusion of the whole Eastern Alps. At that time thrusting over the lowest nappe and the foreland was still going on. The western area underwent burial in Eocene times. In the central section of the Rhenodanubian flysch zone, the southern margin was buried much deeper and partly suffered total annealing, while the northern margin experienced only partial annealing in Eocene times. The exhumation driven by the lateral extrusion of the Eastern Alps evolved from the central area in the Oligocene and prograded to the east and west in late Oligocene/early Miocene times. The central area is characterized by the protrusion of a basement high beneath the foreland Molasse, and large-scale extension in the orogenic body to the south.

International Conference on Fission Track Dating and Thermochronology

FT2'

LINKING LATE MESOZOIC AND CENOZOIC ONSHORE DENUDATION IN THAILAND WITH OFFSHORE SEDIMENT ACCUMULATION: CONSTRAINTS FROM F T ANALYISIS D.R. Upton,A. Carter, C.S. Bristow andAJ. Hurford London Fission Track Research Group, Research School of Geological and Geophysical Sciences, Birkbeck and University Colleges London, London, United Kingdom. The successive collisions of Gondwana-derived fragments have had a major impact on the regional evolution of Indochina throughout the Late Mesozoic and Cenozoic. The findings of this fission track study have been able to place tighter constraints upon the timing of regional deformation events associated with plate collisions and the progressive deformation linked to the northwards translation of the Indian indenter. In addition, the AFT data have been successfully used to link the timing and area of sediment source to the solid phase accumulation rates (SPAR) in the Gulf of Thailand, and speculate upon the sediment loading of intermontane basins. Apatite fission track (AFT) data from eastern Thailand show near total annealing during the Late Cretaceous/Early Tertiary (~70-50 Ma). The timing at which the stratigraphic section reached maximum thickness lies approximately between 74 Ma and 39 Ma. Quantitative modelling of the AFT track data implies that the Khorat Group in eastern Thailand has cooled at an average rate of 1.5±0.5°C Ma-i. The Phu Phan Uplift experienced 2.9-4.0 km section loss (denuded at 45-62 m/m.y.) following 275-440 m of tectonic uplift. Tectonic uplift (700±100 m) of the Frontal Monocline and LoeiPetchabun Fold Belt initiated isostatically-compensated erosive denudation averaging 50-68 m/Mam.y. and 35-50m/m.y.

AFT constraints (this study) Eastern Thailand E T ^ -steady erosive dcnudatiOE Western Thailand W1 m ^erosivedei W2 ^ -nqrid tectooic denudation W3 m -R^rid tectonic denudation? -eutisostatic erosive /\

denudation late

Sediment accummulaion constraints ^ ZlJ

solidphase accumulation rate CMetlvleretaL, 1999)

Structural constraints

(f^Ar/^Ar, Tarawto et al, 1997). Three Pagodas fwilt zone TPF2 Oeft-lateral motion) TPF2 (tight-lateral motion)

Wang Chaofeultzone

• WCFZ (left-lateral motion) >. WCFZ (rigbt^ateral motion)

80

70

60

50

40 30 Time (Ma)

20

10

0

Figure 1. Synoptic plot to show the link between the timing of onshore tectonic deformation as constrained by AFT derived cooling histories and the offehore solid-phase accimmlation rates (SPAR). Figure la shows the onshore denudation rate obtained from the AFT cooling histories (thick black line). Figure lb, shows the SPAR (dotted boxes) for the Gulf of Thailand (Metivier et al., 1999). Grey coloured bands mark the timing of denudation inferred by the AFT cooling histories. Horizontal solid and dashed alines show the timing of left-lateral and right-lateral motion along the Wang Chao and Three P^odas strike-slip faults (Lacassin et al., 1997).

Geological Society of Australia - Abstracts Number 58

FT2 In western Thailand, samples from the Lampang-Nan-Uttaradit (LNU) region show similar thermal histories to eastern Thailand. The timing of the onset of cooling is relatively well defined between 7050 Ma (96-44 Ma, at the 95% confidence level). Since inversion, both regions have cooled steadily through the partial annealing zone until the present-day, with cooling rates of 1.85±0.55°C km-i in the west and 1.5±0.5°C Ma-i in the east. It is inferred that the concordant timing of inversion between the western LNU region, and eastern Thailand signify a regional deformation event to the west, such as the 'soft-docking' of the Western Burma Plate onto Shan-Thai. The continued northward motion of the Indian Plate and subduction of oceanic crust beneath Sundaland caused sufficient tectonic uplift in eastern Thailand to initiate several kilometres of isostatically-driven erosive denudation and palaeodrainage reorganisation. Continued subduction compressed smaller platelets outboard of the ShanThai Plate until the system 'locked' and the Western Burma Plate underwent 'hard collision' with Sundaland during the Late Cretaceous/Early Tertiary (70-50 Ma). Eocene 'hard collision' of the Indian Plate initiated E-W extension and rift basin formation throughout the Sunda Shelf region. AFT central ages (18-29 Ma) in the Wang Chao (WC)-Three Pagodas (TP) region signify moderate rates of cooling (8.5°C Ma-i to 25°C Ma-i, constraining rate and timing of denudation after left-lateral motion along the WC-TP faults (TP, 36-33 Ma and WC, 33-30 Ma). These conclusions are consistent with the findings of Lacassin et al, (1997), whose samples from the Bhumibol Dam have 40Ar-39Ar ages ranging between 29 Ma and 23 Ma. The rocks show no evidence of shearing and therefore Lacassin et al, (1997) conclude that the ages must be a consequence of rapid cooling, mostly likely caused by dip-slip motion on the east dipping Sam Ngao normal fault. A switch in stress regime appears to be a major contributory factor in the regional tectonic development of Thailand. Continued northward indentation of India initiated right-lateral strike-slip motion along the WC-TP causing tectonic denudation of the hanging wall meta-sedimentary rocks from above the crystalline footwall core complex (at 22-19 Ma) and rapid unroofing of the Doi-Inthanon-Doi Suthep crystalline complex with section loss (5.7-8.5 km) denuded at 1900-8500 m/m.y. The rapid rate of tectonic denudation (22-19 Ma) associated with the unroofing via detachment faulting at the beginning of the Miocene is sufficiently high to draw-up the crustal isotherms so that the rocks further east remained hotter for longer (17-14 Ma). The central core of the dome located near the Doi Inthanon summit would accommodate the maximum displacement along the eastern limb low-angle normal fault and, consequently, have the greatest amount of cooling and denudation. The samples collected from the Doi Suthep mountain, which forms the faulted western margin of the relatively large Tertiary, Chiang Mai basin, show that entry into the partial-annealing zone occurred during the Middle Miocene (~16±2 Ma). Figure l b shows the averaged rate of denudation throughout Thailand as constrained by the AFT cooling history data and solid phase accumulation rates (SPAR) in the Gulf of Thailand from Metivier et al, (1999). There is a relatively good fit between calculated rates of denudation and SPAR indicating that sedimentation in the Gulf of Thailand is directly linked to onshore denudation, both of which show a sharp increase following tectonic movements in the Oligocene to Miocene. Approximately 1 km of section loss at 50 m Ma-i is expected during the late Cretaceous to Eocene ( - 2 0 Ma) time interval, but the solid-phase accumulation rate (SPAR) in the Gulf of Thailand (see Fig. lb) does not show a corresponding increase. The lithology of the section removed and the timing of basin formation explain the apparent incompatibility of the two data. Much of the upper section of the Khorat Group in eastern Thailand comprises evaporites of the Maha Sarakham formation; hence, the early section loss enters solute-phase and is therefore not recorded by the SPAR. The averaged denudation rate shown in Figure lb does not show a marked increase related to the phase of tectonic denudation in western Thailand during the Early-Miocene, because the size of the region affected by the unroofing is comparatively small when compared to the complete data set coverage. The timing of tectonic uplift in western Thailand has implications for the supply of sediment to contemporary Tertiary basins in onshore and the Gulf of Thailand. The intermontane basins of

FT2 Northern and Central Thailand show two phases of clastic fill. The relief generated by the tectonic movement caused accelerated erosion that resulted in increased sediment supply to adjacent Tertiary basins. Basin fill switched from lacustrine environments, representing underfilled (subsidence>sediment supply) basins to fluvial clastic dominated deposits (sediment supply>subsidence). It is suggested that the small increase in offshore SPAR rates in the Mid-Miocene (l6±2 Ma) be attributed to the increased connectivity of fluvial systems linking intermontane basins and the continued supply of sediment from the Doi Suthep-Doi Inthanon region. Lacassin, R., Maluski, H., Leloup, P. H., Tapponnier, P., Hinthong, C., Siribhakdi, K., Chuaviroj, S. and Chareonreavat, A. 1997. Tertiary diachronic extrusion and deformation of western Indochina: Structural and 40Ar/39Ar evidence from NW Thailand. Journal of Geophysical Research, 102, 10013-10037. Metivier, R, Gaudemer, Y., Tapponier, P. and Klein, M. 1999. Mass accumulation rates in Asia during the Cenozoic. Geophys. J. Int., 137, 280-318.

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324

International Conference on Fission Track Dating and Thermochronology

F T 2 '

P O S T - B R E A K - U P LANDSCAPE EVOLUTION ON THE S E AUSTRALIAN R I F T E D MARGIN AS I N F E R R E D FROM F I S S I O N T R A C K THERMOCHRONOLOGY, GEOMORPHOLOGY, AND NUMERICAL M O D E L S

RA. van der Beek Laboratoire de Geodynamique des Chaines Alpines, Universite Joseph Fourier, Grenoble, France

The rifted continental margin of southeastern Australia has generated considerable interest and controversy within the earth sciences community over much of this century. During the last 20 years, the region has become perhaps the best-studied high-elevation rifted margin worldwide, with vast amounts of fission track thermochronological and geomorphological data available to constrain its denudation and landscape evolution history. Despite (or perhaps because oO the large amounts of data collected, the uplift and denudation history of southeastern Australia remains highly controversial. During the better part of this century, the evolution of the southeastern highlands of Australia has been cast in cyclical models of landscape evolution. The highlands were thought to have originated by a Pleistocene phase of uplift affecting a pre-existing peneplain that had formed close to sealevel. This uplift phase was thought to have led to widespread drainage capture that was inferred from the complicated drainage patterns observed in the region. The 1970's and 1980's saw a gradual but nearly complete move away from these concepts, as geomorphological evidence for the considerable antiquity of large parts of the highlands' morphology mounted (cf. Bishop, 1988). Although most workers now agree on a pre-Cenozoic origin of the highlands, the debate during the last decade has centred on the timing, style and driving mechanism of uplift in SE Australia and its relationship with rifting in the Tasman Sea (Lambeck and Stephenson, 1986; Wellman, 1987; Lister and Etheridge, 1989; Oilier and Pain, 1994), as well as on the amount, timing and rate of denudation (O'Sullivan et al, 1996; Bishop and Goldrick 1999). The fission track database collected by the Melbourne group indicates that most of the highlands cooled from temperatures >110°C to <60°C during the Triassic (250-200 Ma) whereas large parts of the coastal region cooled through this temperature range in the mid-Cretaceous (100-90 Ma) (e.g. Dumitru et al, 1991; O'Sullivan et al, 1996). Some high-standing regions within the highlands core (e.g. the Bathurst region, O'Sullivan et al, 1996; the Snowy Mountains, Kohn et al, 1999) also show evidence for rapid cooling and km-scale denudation during mid-Cretaceous - Paleocene times, possibly related to fault reactivation. In contrast, the preservation of early-mid-Cenozoic basalts, paleosols and sediments throughout SE Australia indicates that denudation rates both within the highlands and in the coastal region have been minimal (<5 m/m.y.) since these times (cf. review by Bishop and Goldrick 1999). The preserved geology of the Sydney Basin indicates ~1 km of differential denudation between the highlands and the coastal strip, and provides evidence for minimal tilting of the margin during rifting and break-up. We have attempted to elucidate the mode of, and the controls on, long-term landform evolution in SE Australia using a numerical surface processes model that includes fluvial transport, hillslope diffusion and bedrock landsliding (Braun and Sambridge, 1997; van der Beek and Braun, 1999). Our modelling approach is heuristic: we impose an initial topography and tectonic uplift history and let the model predict the resulting landscape evolution, denudation, and isostatic rebound history. Models were designed to study the Meso-Cenozoic evolution of the whole of SE Australia (van der Beek et al, 1999) as well as the controls on post-break-up evolution on a more regional scale (van der Beek and Braun, 1999) The models predict present-day topography and drainage, as well as total denudation, isostatic rebound and denudation rates through time. They therefore allow to assess, within the limits of model

Geological Society of Australia - Abstracts Number 58

FT2 assumptions and simplifications, to what extent the different proposed scenarios for the uplift and evolution of the southeastern highlands are compatible with the available thermochronological, geomorphological and geological data. The models also suggest ways to reconcile the apparent contradiction in thermochronologically and geomorphologically derived estimates of denudation in SE Australia. The main insight we have gained from the models is that the initial (pre-break-up) topography of the highlands provides a fundamental control on their subsequent evolution. In particular, the observed highlands morphology (with the continental drainage divide located 50-100 km inland of the escarpment) requires that the drainage divide was established at its present location before opening of the Tasman Sea. Models that predict either rift related uplift of the coastal region and inland tilting of the highlands (Dumitm et al., 1991; van der Beek et aL, 1995) or downwarping of the coastal strip and seaward tilting of the highlands (Oilier and Pain, 1994) are incompatible with the observed morphology, drainage patterns and geology of the Sydney Basin. This result places tight constraints on the amount of differential denudation between the highlands and the coastal region. Our preferred model features a gently seaward dipping plateau (<1° slope) seaward of an initial drainage divide; there is no tectonic uplift nor subsidence during break-up, but a base-level drop down to sealevel at the seaward side of the model. Within the context of this model, it is gorge extension rather than escarpment retreat that appears to be the fundamental mechanism eroding the highlands. The observed drainage pattern is imprinted on the model from the start and does not evolve through capture. These results are consistent with inferences from recent quantitative geomorphological studies (e.g. Nott et aL, 1996). The model indicates that widespread mid-Cretaceous rift-related uplift of the highlands is not necessary to explain their subsequent evolution, but it does not exclude such uplift. All of our models show large spatial and temporal variations in denudation rates and suggest that much of the controversy concerning the amounts and rates of denudation in SE Australia arises from unwarranted extrapolation or comparison of different data sets. Denudation rates predicted by our models appear to be grossly consistent with both the fission track and the geomorphological data. Our models predict total post-mid-Cretaceous denudation of the coastal region to lie between 1 and 2 km. This amount is consistent with the fission track data if Late Mesozoic - Early Cenozoic upper crustal geothermal gradients were significantly higher than present. Vitrinite reflectance and fluid inclusion data from the Sydney Basin (e.g. Middleton, 1993) support a scenario of moderate denudation (1.5-2.5 km) under temporally high geothermal gradients (40-60°C/km). Thermomechanical models suggest that it is very difficult to explain these increased upper crustal geothermal gradients by either orthogonal or oblique spreading; a more appropriate explanation may be blanketing by now removed sediments with low thermal conductivities and/or heat advection by fluid flow. We therefore suggest that such local effects on the pattern of fission track ages observed in SE Australia may have been underestimated. Bishop, P. 1988. The eastern highlands of Australia: the evolution of an intraplate highland belt. Progress in Physical Geography 12, 159-182. Bishop, P. and Goldrick, G. 1999. Geomorphological evolution of the East Australia continental margin. In Summerfield, M.A. ed. Global Tectonics and Geomorphology. Wiley, Chichester, pp. 227-255. Braun, J. and Sambridge, M. 1997. Modelling landscape evolution on geological time scales: a new method based on irregular spatial discretization. Basin Research 9, 27-52. Dumitru, T.A., Hill, K.C., Coyle, D.A., Duddy, I.R., Foster, D.A., Gleadow, A.J.W., Green, P.F., Kohn, B.P., Laslett, G.M. and O'Sullivan, A.J. 1991. Fission track thermochronology: Application to continental rifting of south-eastern Australia. Australian Petroleum Exploration Association Journal 31, 131-142. Kohn, B.P., Gleadow, A.J.W., and Cox, S.J.D. 1999. Denudation history of the Snowy Mountains - Evidence from apatite fission track thermochronology. Australian Journal of Earth Sciences 46, 181-198. Lambeck, K. and Stephenson, R. 1986. The post-Palaeozoic uplift history of south-eastern Australia. Australian Journal of Earth Sciences 33, 253-270. Lister, G.S. and Etheridge, M.A. 1989. Detachment models for uplift and volcanism in the Eastern Highlands, and their application to the origin of passive margin mountains. In Johnson, R.W. ed. Intraplate Volcanism in Australia and New Zealand, Cambridge University Press, New York, pp. 297-312.

FT2 Middleton, M.R 1993. Thermo-tectonic influences on the Sydney Basin during the breakup of Gondwana. In Findiay, R.H., Unrug, R., Banks, M.R. and Veevers, J.J. eds. Gondwana Eight; Assembly, Evolution and Dispersal, Balkema, Rotterdam, pp. 613-622. Nott, J.F., Young, R.W. and McDougall, I. 1996. Wearing down, wearing back and gorge extension in the long-term evolution of a highland mass: Quantitative evidence from the Shoalhaven catchment, southeast Australia. Journal of Geology 104, 224-232. O'Sullivan, RB., Foster, D.A., Kohn, B.R and Gleadow, A.J.W. 1996a. Multiple post-orogenic denudation events: an example from the eastern Lachlan Fold Belt, Australia. Geology 24, 563-566. Oilier, C.D. and Pain, C.F. 1994. Landscape evolution and tectonics in southeastern Australia. AGSO Journal of Australian Geology and Geophysics 15, 335-345. van der Beek, RA., Andriessen, RA.M., and Cloetingh, S. 1995. Morphotectonic evolution of rifted continental margins: Inferences from a coupled tectonic-surface processes model and fission track thermochronology. Tectonics 14, 406-421. van der Beek, RA., Braun, J. and Lambeck, K. 1999. The post-palaeozoic uplift history of southeastern Australia revisited : Results from a process-based model of landscape evolution. Australian Journal of Earth Sciences 46 157-172. van der Beek, RA. and Braun, J. 1999. Controls on post-mid-Cretaceous landscape evolution in the southeastern highlands of Australia : Insights from numerical surface process models. Journal of Geophysical Research 104, 4945-4966. Wellman, R 1987. Eastern Highlands of Australia: Their uplift and erosion. BMR Journal of Australian Geology and Geophysics 10, 277-286.

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328

International Conference on Fission Track Dating and Thermochronology

CAN G E O C H R O N O L O G Y O F W E A T H E R I N G P R O F I L E S C O N S T R A I N T S ON E R O S I O N

F T 2 '

IMPOSE

RATES?

P.M. Vasconcelos Department of Earth Sciences, University of Queensland, Brisbane, QLD, Australia.

The history of weathering and landscape evolution in tectonically quiescent parts of southern hemisphere cratons has been attributed to repeated periods of alternating weathering-prone and erosionprone conditions. During weathering-prone conditions, relatively stable tectonic conditions permit the formation of deeply weathered profiles, while erosion rates are minimal. Tectonic reactivation or climatic change towards erosion-prone conditions promotes the partial or total erosion of the weathering blanket by scarp-retreat, surface denudation, or a combination of these two end-member processes. Repeated cycles of these processes, acting for tens of millions of years, result in the stepped landscape that characterise the relatively stable cratonic areas of the southern hemisphere. Researchers identified and correlated several of these land surfaces across and between South America, Africa, and Australia. In these studies, land surfaces were identified based on stratigraphic and topographic relationships, surface morphology, and the nature of the soils and weathering profiles they hosted. The highly subjective nature of such correlations and the lack of an identifiable and quantifiable process for the generation of these surfaces raises serious doubts on the validity of such correlations. In addition, the formation of such land surfaces requires continuous exposure of continents for tens of millions of years, implying tectonic stability and low average denudation rates. The assumed tectonic stability and low erosion rates have been seriously questioned by recent apatite fission track results. These results indicate that many of the assumed stable parts of southern hemisphere cratons may have undergone significant erosion in the recent past. Spikings et al (1997) propose 1.22.0 km of denudation for the Mount Isa Region, in Queensland, during the last 100 Ma. Harman et al (1998) suggest 3-4 km of denudation in the Guapore Shield, Amazon, Brazil during the past 80 Ma. I will present results from detailed weathering geochronology studies in the Mount Isa Region, Australia, and the Carajas Region, Amazon, that pose some constraints on the cyclical landscape evolution model. The geochronology results in these studies indicate that the Mount Isa and the Carajas regions have been continuously exposed to weathering for a minimum of ca. 70 Ma. The landscape in these areas show a clear correlation between age and topographic position of weathering profiles, and maximum erosion rates obtained by assuming some reasonable depth of weathering (100-200 m) at the time of datable mineral precipitation is constrained at 2-4 mMa ^. These erosion rates are consistent with rates obtained from cosmogenic isotope studies in the same region, but they are not consistent with the amount of denudation implied by the apatite fission track modelling results for these areas. Harman R. Gallagher K. Brown R. Raza A. and Bizzi L. 1998. Accelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceous. Journal of Geophysical Research 103 ( B l l ) , 27091-27105. Spikings R.A. Foster D.A. and Kohn B.R 1997. Phanerozoic denudation history of the Mount Isa Inlier, northern Australia; response of a Proterozoic mobile belt to intraplate tectonics. International Geology Review 39, 107-24.

Geological Society of Australia - Abstracts Number 58

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International Conference on Fission Track Dating and Thermochronology

F T 2 '

EVIDENCE OF EXTENSIONAL TECTONICS FROM APATITE FISSION-TRACK DATA: AN EXAMPLE FROM THE NORTHERN APENNINES (ITALY) B.Ventura Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, Italy

Apatite fission-track analysis was carried out on 24 samples from the Lower-Middle Miocene Mt. Cervarola Sandstone, a synorogenic foreland succession of the northern Apennines. The northern Apennines are part of the peri-Mediterranean system of mountain chains that formed as a result of the ongoing collision between African and European plates and later post-collisional events. This section of the Apennines consists of a series of stacked tectonic units accreted onto the Adriatic foreland by north-east thrusting active mainly during the Neogene. Within this stack, the Ligurian units constitute the uppermost structural unit, and comprise remnants of a Tethyan oceanic crust and associated sediments. In addition, a sub-Ligurian succession was deposited on the thinned continental crust of the Adriatic microplate during the Late Oligocene-Early Miocene. The Ligurian units overly the Tuscan units, a group consisting of a carbonate Mesozoic succession deposited on the Adriatic continental crust and including the Late Oligocene Macigno Formation and Mt. Modino Sandstone. To the east, the Tuscan units overthrust the Lower-Middle Miocene turbiditic deposits of the Mt. Cervarola Sandstone and Middle-Late Miocene Marnoso-arenacea Formation. Starting from Late Miocene, extensional tectonics related to the opening of the Tyrrhenian Basin affected the western part of the chain. By the middle Pleistocene, thrusting in the northern Apennines fold-and-thrust belt ceased and the tensional stress regime migrated eastwards to the northern Apennines foothills. The investigated area is approximately 200x40 km and covers almost entirely what is generally referred to as Mt. Cervarola Sandstone. Samples were mainly collected along horizontal profiles, perpendicular to the tectonic structures, and along one vertical profile with an altitude difference of 1 km. Several samples from the sub-Ligurian succession, Macigno Formation and Mt. Modino Sandstone were analysed for comparison. Apatite ages range from 2.7±0.8 to 12.0±2.0 Ma (all uncertainties quoted ± l a ) . All samples pass the X^-test with values higher than 50%, indicating that all grains belong to one single age population. AJl the measured apatite ages are significantly younger than the stratigraphic age. This, combined with the high probability, indicates that samples underwent complete annealing as a consequence of the exposure to temperatures of at least 120°C. Mean track lengths, varying between 13 and 15 |Lim, indicate moderate to rapid cooling. Thus, the fission track ages represent the time the samples last cooled through to temperatures lower than ~60-110°C. The presence of mean track lengths less than full length (15-16 |im) indicates that the samples resided in the temperature range of ~60-110°C long enough to partially shorten tracks during exhumation. The majority of samples from the Mt. Cervarola and Mt. Modino Sandstone yield ages between 5.0±0.5 and 7.4±0.7 Ma. Samples from the Mt. Cervarola Sandstone with young ages (2.7±0.8, 3.9±0.8 Ma), and samples with older ages from the sub-Ligurian succession (12.0±2.0 Ma) and Macigno Formation (8.7±1.3, 9.8±1.1 Ma) are located in proximity to major tectonic structures and indicate time differences of 2-7 m.y. across these structures. The fission track data record a phase of moderate to rapid cooling of the western part of investigated area in the Middle Miocene, and similar cooling of the eastern part in the Late Miocene up to the Miocene-Pliocene boundary. For the Mt. Cervarola Sandstone, an average exhumation rate of 0.4 mm/yr between 2.6±0.5-4.1±0.5 Ma can be inferred from the vertical profile. However, the ages along the horizontal profiles suggest that the vertical movement does not follow a simple model. The offset of fission-track ages along the horizontal profiles may be explained by the activation of major, SW-dipping, listric normal faults, partially overprinting older compressive structures. The displace-

Geological Society of Australia - Abstracts Number 58

FT2 ments would juxtapose rocks that cooled in different times to temperatures lower than ~60-110°C. Additional tilting may be related to downward flexure of the hangingwall and upward flexure of the footwall accompanying normal faulting. On the basis of the apatite data, the onset of normal faulting in the study area does not appear to have occurred prior to the late Pliocene. The decrease of reset fission-track ages from ~8-12 Ma in the west to ~3-5 Ma in the east is interpreted to represent the eastward migration of exhumation across the northern Apennines.

International Conference on Fission Track Dating and Thermochronoiogy

P A R T I C L E - T R A C K DATING: A LONG STONY

F T 2 '

ROAD

G.A. Wagner Forschungsstelle Archaometrie der Heidelberger Akademie der Wissenschaftenam Max-Planck-Institut fiir Kernphysik, Postfach 103980,69029 Heidelberg, Germany

In the early days of fission-track dating, some 35 years ago, a very disturbing fact turned up: the fission-track ages were commonly too young when compared to independent age control. This phenomenon was observed in particular for tektites (Fleischer and Price, 1964; Wagner, 1966). Its cause soon was spotted: the insufficient thermal stability of the latent tracks under natural conditions. Apparently, the new clock did not seem really to work. However, failure and progress come often together The southern Australian tektites, the australites, gave the decisive hint: the younger their apparent fission-track ages, the smaller the fission-track etch pits. This opened the way of using the size of partially faded tracks to correct the lowered ages. Australites with fission-track ages as low as 0.13 Ma gave, after correction, the expected age of 0.7 Ma (Storzer and Wagner, 1969). But the next failure was not far. For most apatites, another material liked already by the early fission trackers, the correction procedure did not help, the fission-track ages stayed much too low, because under most geological conditions they are stable only below ca. 100°C (Wagner, 1968; Naeser and Faul, 1969). This was a conceptual turning point in the development of the fission-track dating method: fission tracks can be used to reveal, instead of the time of formation, the cooling history of geologic materials. Fission-track thermochronoiogy was born and has since then developed to a powerful and indispensable tool, as is amply evident from the presentations at this conference. Another story was not so successful. The discovery of fossil alpha-recoil tracks (Huang and Walker, 1967), originally celebrated as breakthrough because they are by three orders of magnitude more common than fission tracks, did not lead to any practical dating technique in spite of several serious attempts. But alpha-recoil tracks do exist, at least in mica. Based on recent experimental evidence, we believe that they are the basis for a reliable and valuable clock for Quaternary tephrochronology (Gogen and Wagner, 1999). Beyond that age range one encounters the problem of having too many tracks to be counted under an optical microscope. Modern observation techniques, such as electron scanning and transmission microscopy as well as atomic force microscopy, may be the better instruments for alpha-recoil track revelation. Furthermore, the thermal fading of alpha-recoil tracks in dark mica seems to be significant more sensitive than that of fission tracks. This observation might be the starting point for another low-temperature tool in thermochronoiogy (Glasmacher and Wagner, 2000). Looking back to the thorny story of fission-tracks, the effort seems worthwhile. Fleischer R.L. and Price RB. 1964. Fission track evidence for ttie simukaneous origin of tektites and other natural glasses. Geochimica et Cosmochimica Acta 28, 755-760. Glasmacher, U. and Wagner G.A. 2000. Alpha-recoil track dating of dark mica - a potential tool for tephrochro-nology and thermochronoiogy. This volume. Gogen K. and Wagner G.A. 1999. Alpha-recoil track dating of Quaternary volcanics. Chemical Geology, in print. Huang W.H. and Walker R.M. 1976. Fossil alpha-particle recoil tracks: a new method of age determination. Science 155, 11031106. Naeser C.W. and Faul H. 1969. Fission track annealing in apatite and sphene. Journal of Geophysical Research 74, 705-710. Storzer D. and Wagner, G.A. 1969. Correction of thermally lowered fission track ages of tektites. Earth and Planetary Science Letters 10, 463-468. Wagner G.A. 1968. Fission track dating of apatites. Earth and Planetary Science Letters 4, 411-415. Wagner G.A. 1966. Altersbestimmungen an Tektiten und anderen natiirlichen Glasern mittels Spuren der spontanen Kernspaltung des Uran238 ("fission-track"-Methode). Zeitschrift fur Natur-forschung 21a, 733-745.

Geological Society of Australia - Abstracts Number 58

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334

International Conference on Fission Track Dating and Thermochronoiogy

N U C L E A R T R A C K S IN E X T R A T E R R E S T R I A L F R O M LUNAR S A M P L E S TO ANCIENT

F T 2 '

MATERIALS: STARDUST

R.M.Walker McDonnell Centre for the Space Sciences,Washington University, One Brookings Drive, St. Louis, MO 63130

This topic is particularly fitting for a symposium in honour of John Lovering. I have a vivid memory of John at the first Apollo science conference where he was sounding me out to see whether we had discovered a new U-rich mineral in the lunar samples. We had not, but he had [1]. As I remember it, he was not displeased with this state of affairs! The new mineral was independently found, and characterized, by a number of researchers and given the name, "tranquillityite" [2]. Extraterrestrial samples played a key role in the development of track research. The fact that rock samples would be returned from the moon led to the idea that nuclear tracks might be present and could be used to study the history of particle irradiations in space. This would require that tracks be stable for long times. A test of stability would be to find ancient spontaneous fission tracks in the vicinity of U-rich inclusions in micas. At that time tracks had only been seen in the electron microscope. We (foolishly) believed that it would be necessary to cleave the mica in the immediate vicinity of an inclusion to a thickness of -1000 A to look for fossil tracks. Our colleague, W.G. Johnston, at the G.E. Research laboratory, where Buford Price and myself were then employed, successfully performed this challenging experiment. However, Johnston was so little impressed with the potential of fossil track studies that he declined to co-author the original discovery paper in Nature [31. Part of Johnston's reluctance was due to the fact that he was a leading expert on dislocation etch pits. I remember a discussion in which he convinced me that it was silly to try to etch tracks to be seen in an optical microscope because we would be swamped by dislocation etch pits. The argument lasted ~lhr - the experiment would have taken minutes. Price and I were later approached by a G.E. engineer who asked if we could produce a mica sheet containing just one hole several microns in size. We said yes, and set Ethel Fontanella to work on the problem. I remarked that we might find a background of holes due to fossil tracks and that she should etch both unirradiated and neutron-irradiated mica samples. She did this and put a series of photographs on my desk. At the time I was getting ready to leave for a sabbatical year in France and shuffled the photographs off to one side. Days later, while driving to work, I suddenly realized the implications of those photos. It was with great excitement that we looked in an optical microscope and saw our first etched fossil fission tracks! My first fission track dates were obtained in collaboration with a graduate student at the University of Paris, Michel Maurette, and a mineralogist, Paul Pellas, of the Paris Museum of Natural History [4]. The results were published in French and probably little noticed. This paper also discussed track annealing and the possibility of using track length measurements to correct for track fading. Maurette subsequently learned to etch tracks in olivine, a major mineral in meteorites, and became the first person to see fossil tracks in an extraterrestrial sample. Turning to more topical work, I now discuss induced fission track studies of ancient stardust. As described in a recent popular article [5] and in more extensive technical reviews [e.g. 6,7], it has proven possible to isolate small grains of ancient stardust from primitive meteorites. The presolar grains are identified by their anomalous isotopic compositions. The effects are so large, amounting in some cases to over a thousand-fold difference in isotopic ratios relative to solar system values, that they can only be attributed to nuclear effects occurring at different stages of stellar evolution. Although over a half-dozen different kinds of circumstellar phases have been identified, I will mention only SiC. Several isotopic subsets of SiC grains are also known but I will discuss only two - "mainstream (MS)" grains that constitute over 90% of all SiC grains and "X-grains" which comprise - 1 % of the total. The grains range from sub-micron to several microns in size. The isotopes in those pm

Geological Society of Australia - Abstracts Number 58

F T 2 ^ e e can be measured using an ion microprobe. Mainstream grains lie along a slope 1.4 correlation line on a Si three-isotope plot. X-grains are characterized by enrichments in ^^Si and lie in a completely different region of such a diagram. Isotopic measurements in the MS grains show signatures of s-process nucleosynthesis in which neutrons are added slowly to seed nuclei producing species that decay back to the valley of nuclear stability before additional neutrons are added. The s-process is thought to occur in red giant stars undergoing nuclear shell burning. Astronomical observations also show that such stars account for most of the dust produced by stars. For these and other reasons, it is believed that the MS grains are condensates formed in the atmospheres of red giant stars [8]. In contrast, a variety of isotopic effects in the X-grains require synthesis by the rapid addition of neutrons (r-process). Supernovae explosions are the only known source of such exposures and we believe that X-grains are supernova condensates [9]. The actinide elements are formed only by r-process nucleosynthesis. Any actinides present in the original mix of isotopes from which different red giant stars formed should have decayed away substantially during the stellar lifetimes. Since the half-life of 235u is considerably shorter than that of 238u^ reduced 235/238 ratios are expected in MS grains. The reduction depends on the lifetime of the precursor star, which in turn varies as the inverse cube of the starting mass. As a reference, our sun is expected to evolve into a red giant star some lO^o y after it was formed. Precursor red giant stars with <1.3 solar masses should produce dust with negligible amounts of 235u. An X grain should have the initial production value of 235u/238u the r-process (taken as 1.5 [10]), modified by the passage of time since the grain was originally formed. Even if most of the U in a given X-grain came from the exterior of the exploding star, and not from the interior r-process site, the short lifetime of the precursor massive star might be expected to lead to a 235u/238u j-^tio close to the original solar system value (235u/238u = 0.54). An important test of these ideas will be afforded by fission track mapping of a suite of SiC grains following both slow neutron and fast particle bombardments. The idea is to locate specific grains from which fission tracks have emanated and then perform detailed isotopic measurements on those same grains using an ion microprobe. In the case of bombardment with well-thermalized neutrons, all grains producing tracks are grains that contain 235u. From the arguments given above we would expect that the proportion of X-grains in this population should be much higher than the 1% seen in the total grain population. Fast particle bombardment will predominantly fission 238u and 232xh. The data should allow one to calculate "model ages" for the precursor stars, thereby introducing a time parameter into the study of presolar grains. Many fundamental aspects of the problem, e.g. the U concentrations in the MS and X grains, are not known. I estimate that a U concentration of -1.3 x lO'i^ at/at can be measured to 10% in a SiC separate from a starting sample of 10 g of the Murchison meteorite. The actual U concentration is probably much higher. The proposed experiment will also benefit from the forthcoming addition of a new type of ion microprobe - the NanoSIMS - to our laboratory [11]. A number of the techniques involved are commonly used by the fission track dating community and I look forward to receiving valuable advice during this conference. 1. 2.

3.

Levering, J. R, and Kleeman, J. D., Proc. of the Apollo 11 Lunar Sci. Conf., 627-631 (1970). Lovering, J. R, Wark, D. A., Reid, A. R, Ware, N. G., Keil, K., Prinz, M., Bunch, T. E., El Goresy, A., Ramdohr, R, Brown, G. M., Peckett A., Phillips, R., Cameron, E. N., Douglas, J. A. V., and Plant, A. G., Proc. of the Second Lunar Sci. Conf. M. L T. Press, 1971, pp. 39-45. Price, R B., and Walker, R. M., Nature 196, 732-734 (1962).

FT2 Maurette, M., Pellas, P., and Walker, R. M., Bull. Soc. fran?. Miner. Crist.LXXXVII, 6-17 (1964). Bernatowicz, T., and Walker, R. M., Physics Today 50, 26-32 (1997). Astrophysical Implications of the Laboratory Study of Presolar Materials, edited by T. Bernatowicz and E. Zinner, AIP Conference Proceedings 402, New York: American Institute of Physics, 1997, 750. This is a collection of 29 refereed papers discussing various aspects of the field. 7. Zinner, E., Ann. Rev. Earth and Planet. Sci. 26, 147-188 (1998). Hoppe, P., Amari, S., Zinner, E., and Lewis, R., Astrophys. J. 430, 870-890 (1994). 9. Amari, S., and Zinner, E., in Astrophysical Implications of the Laboratory Study of Presolar Materials, edited by T. Bernatowicz and E. Zinner, AIP Conference Proceedings 402, New York: American Institute of Physics, 1997, pp. 287-305. 10. Meyer, B. S., and Schramm, D. N., Astrophys. J. 311, 406- (1986). 11. Stadermann, K J., Walker, R. M., and Zinner, E., Lunar Planet. Sci. XXX, Abstract #1407 (1999); Met. and Planet. Sci. 34, A111-A112 (1999); New Frontiers in Isotope Geoscience, Lome, (Feb. 2000).

Acknowledgement I am indebted to Andrew Gleadow and the other conference organizers for their kind invitation to participate in Fission Track 2000. This work was partially supported by NASA grant NAG5-8336.

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FISSION TRACK ANALYSIS OF D R I L L H O L E SAMPLES FROM ORDOS BASIN OF NORTH CHINA

S.Wangi,T. Kangi, L.Wangi and 2. Ren^ 1. Department of Nuclear Analysis, Institute of High Energy Physics, Academia Sinica 2 Department of Geology, Northwest University, USA

The Ordos Basin is located in the west of North China platform, which is divided into six tectonic units. The Yimeng uplift is in the north; Weibei uplift is in the south; Among these two units are Xiyuan Obduction plate, Tienhuan Syncline, Yishan Clinothem and Jinxi Folded Belt from west to east. Fission track analysis was applied to study the thermal evolution in the Ordos Basin. Fission track ages of apatite and zircon were measured using external detector method and Zeta constant method. Fission track lengths of apatite were measured on confined fission tracks on apatite. Thirty seven apatite samples were from 10 wells and their straitigraphic ages are all older than 140 Ma. However their apatite fission track ages are very young: for most samples their fission track ages are younger than 30 Ma; even the oldest fission track age is not older than 140 Ma, which indicate that their strata have experienced temperature during burial high enough to anneal all pre-existing fission tracks in apatite. Inverse modelling based on Monte Carlo random sampling was used to reconstruct the temperature-time history. The results show that the samples from the Ordos basin have experienced maximum paleotemperatures of >120°C by 60 Ma; subsequently cooling took place. Eight Zircon samples were from 4 wells in three tectonic units. Zircon fission track ages and their distributions of single grain ages vary obviously from west unit to the east units. The zircon samples from Xiyuan Obduction Plate and Tienhuan Syncline all have fission track ages from the early Jurassic period to the late Triassic period with P(x^) >5%, which are all younger than their straitigraphic ages. It is shown that at sometime after deposition all pre-exiting tracks in zircon have been erased, which suggests that these samples have been heated to temperature above 200°C. The samples from Yishan Clinothem have ages of the Paleozoic era with P(%2) <50/0 ^ which are older than their straitigraphic ages. It suggests that a proportion of the observed tracks were formed prior to deposition. Thus the Permian system and the Carboniferous system in this unit did not experience temperature high enough to anneal fission tracks in zircon severely (<175-180°C). This interpretation is supported by their single grain age data as well. The data of zircon fission track ages suggest that the paleotemperature during the late Mesozoic era are much higher in the west part of the Ordos Basin than in the middle part. It might be due to the strong tectonic movement in the west edge of the basin.

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APATITE FISSION T R A C K THERMOCHRONOIOGY OF THE NORTHERN W E S T E R N

SHIELD,

AUSTRALIA

U.D.Weberi, B.R Kohni, D.R. Nelson2 and A J. W. Gleadowi lAustralian Geodynamics Cooperative Research Centre, School of Earth Sciences,The University of Melbourne,Victoria 3010, Australia 2Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

As part of an ongoing investigation of the thermotectonic evolution of the northern part of the Precambrian Western Shield of Western Australia we have carried out a regional reconnaissance apatite fission track (AFT) study (Fig. 1). The study area encompasses an area of -1x10^ km^ and includes the Archaean rocks of the Pilbara Craton, the Sylvania Inlier, and the northern part of the Yilgarn Craton including the Narryer Gneiss Complex, and the Proterozoic Gascoyne, Rudall and Northampton complexes. The Palaeo- to Mesoproterozoic Hamersley, Ashburton, Bangemall and Glengarry basins and the Neoproterozoic Savory and Officer basins to the east of the study area developed on the Pilbara and Yilgarn cratons.

Northampton ] Complex

Perth Basin

Fig. 1: S t u d y area in W e s t e r n A u s t r a l i a , b l a c k p o i n t s are s a m p l e l o c a t i o n s

Geological Society of Australia - Abstracts Number 58

FT2 Previous U-Pb zircon SHRIMP data (e.g. Nutman et al, 1993; Nelson, 1998) and ^^Ax/^^Ax analyses of hornblende, muscovite and biotite (e.g. Zegers, 1996; Davids et al, 1997) from the Pilbara craton and the northwestern part of the Yilgarn craton indicate mainly middle to late Archaean ages. Ferguson (1981) reported Mesoproterozoic sphene fission track ages from the Pilbara craton which were viewed as transient points on a slow cooling curve. All these geochronological data suggest that there have been no major cooling events which have affected the cratons since late Archaean time and give credence to the notion that these ancient continental nuclei have remained relatively stable since their formation. Sixty eight AFT analyses reported here (including five samples previously analysed by Ferguson, 1981) were mostly sampled from the Pilbara and the northern Yilgarn Cratons, the Gascoyne and Northampton complexes. AFT data yield ages ranging between 260±8 Ma and 380±20 Ma, with the majority falling between 280-350 Ma. Mean confined horizontal track lengths fall between -12 and 13 pm and usually show a simple Gaussian distribution with standard deviations ranging from 1.1-2.2 pm. The possible effect of chlorine content on the AFT data has not yet been evaluated. Forward modelling of time-temperature history paths (Gallagher, 1995) for representative samples reveal a period of regional cooling of at least ~50°C between 350 and 280 Ma. Most paths also show a second period of <25°C of cooling from temperatures <~80-85°C. The timing of this later episode occurred in the Mesozoic, but is less well constrained. Sparse heat flow measurements for the study area (Cull and Denham, 1979) fall in the range 40-50 mW.m-2. Assuming a thermal conductivity of 2.5 W.m-l.K-i suggests a present-day average geothermal gradient of ~18±2°C.km-i. If this thermal gradient was representative of the Phanerozoic then the minimum of ~50°C of late Palaeozoic cooling predicted by the fission track modelling suggests denudation of an average of -2.5-3.1 km of section during the late Paleozoic and a further -1.2-1.5 km at some time during the Mesozoic. The northern Western Shield is bordered by Phanerozoic basins (Fig. 1), to the north and west, where the basins may continue offshore and form complexes of platforms and sub-basins. The basins are characterised by Early Ordovician to Late Cretaceous clastic sediments and are likely to have been depocentres for the denudational unloading recorded by the AFT data in the crystalline terranes. To the west, the N-S trending Darling fauk forms a sharp boundary between the Shield and the Perth and Carnarvon basins (Fig. 1) which each contain up to -15 km of sediments. The Perth basin contains sediments of mainly Permian ( - 3 km) and Triassic-Jurassic (>10 km) age. The Carnarvon Basin comprises several sub-basins, those on-shore mainly contain Paleozoic strata, while those off-shore mainly Mesozoic and Cainozoic strata. Ghori (1999) reported on the thermal history (based on AFT and VR data) of sediments from deep wells in the southern Carnarvon basin, and identified the late Paleozoic as the peak time for petroleum expulsion from lower Paleozoic source rocks, in response to maximum burial. In the northern Carnarvon basin Devonian and Carboniferous sequences attain a maximum thickness of - 2 km, the Permian up to - 3 km and the Triassic-Jurassic sequence - 8 km (Hocking, 1994). The Canning basin borders the Pilbara craton to the north and contains up to 18 km of sediment ranging in age from Ordovician to Recent, with a Carboniferous to Early Permian sequence over 10 km thick. Killick (1998) calculated the volume of sediment deposited in basins marginal to the Western Shield (Yilgarn and Pilbara Cratons and intervening Proterozoic basins) between Early Ordovician to end Cretaceous time, after which time clastic sedimentation effectively ceased. This led to an estimate that -4.09 km of basement has been removed from the Western Shield since the onset of basin development and sedimentation in the early Ordovician. A possible causative event which could be linked to the observed late Paleozoic cooling is uplift caused by the collision of Gondwanaland with Laurussia forming the supercontinent Pangea. The formation of Pangea closed off an equatorial seaway and caused a world-wide orogenesis around 330 Ma (Baillie et al, 1994). This collision might also have triggered the main Gondwana glaciation which covered most of Australia and lasted through to the earliest Permian (Powell and Veevers, 1987).

FT2 Baillie, P.W., Powell, C.McA., Li, Z.X. and Ryall, A.M., 1994. The tectonic framework of Western Australia,s Neoproterozoic to recent sedimentary basins. In: Purcell, P.G. and Purcell, R.R (eds). The sedimentary basins of Western Australia. Proceedings of Petroleum Exploration Society of Australia Symposium, Perth, 45-62. Cull, J.P., and Denham, D., 1979. Regional variations in Australian heat flow. BMR Journal of Australian Geology and Geophysics, 4, 1-13. Davids, C., Wijbrans, J.R. and White, S.H., 1997. 40Ar/39Ar laserprobe ages of metamorphic hornblendes from the Coongan Belt, Pilbara, Western Australia. Precambrian Research 83, 221- 242. Ferguson, K.U., 1981. Fission track dating of shield areas, Australia: Relationships between tectonic and thermal histories and fission track age distributions. M.Sc. thesis. University of Melbourne, 194 p. Gallagher, K., 1995. Evolving thermal histories from apatite fission track data. Earth Planet. Sci. Letters 136, 421-435. Ghori, K.A.R., 1999. Silurian-Devonian petroleum source-rock potential and thermal history, Carnarvon Basin, Western Australia, Geological Survey of Western Australia Report 72, 88 p. Hocking, R.M., 1994. Subdivisions of Western Australian Neoproterozoic and Phanerozoic basins. Geological Survey of Western Australia Record 1994/4, 84 p. Killick, M.F., 1998. Phanerozoic denudation of the Western Shield of Western Australia. Geological Society of Australia Abstracts No. 49, p. 248. Nelson, D.R., 1998. Compilation of SHRIMP U-Pb zircon geochronology data 1997, Geological Survey of Western Australia Record 1998/2, 242 p. Nutman, A.P., Kinny, P.D. and Price, R., 1993. Large-scale cmstal stmcture of the Northwestern Yilgarn Craton, Western Australia: Evidence from Nd isotopic data and zircon geochronology. Tectonics 12, 971-981. Powell, C.McA. and Veevers, J.J., 1987. Namurian uplift in Australia and South America triggered the main Gondwanan glaciation. Nature 326, 177-179. Zegers, T.E., 1996. Structural, kinematic and metallogenic evolution of selected domains of the Pilbara greenstone-terrain; implication for mid Archaean tectonic regimes. PhD thesis. University of Utrecht, 208 p. Acknowledgments This work was funded by the Australian Geodynamics Cooperative Research Centre (AGCRC) and the Australian Institute of Nuclear Science and Engineering. This paper is published with the permission of the Director, AGCRC.

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POST-COLLISIONAL UPLIFT-EXHUMATION OF THE DABIE MASSIF, EAST CHINA: EVIDENCE FROM PETROLOGICAL AND THERMOCHRONOLOGICAL STUDY ON GRANITOIDS

C.H. Xui, Z.Y ZHOUi and C.Q. Ma2 State Laboratory of Marine Geology, Tongji University, Shanghai, 200092 Faculty of Earth sciences, China University of Geosciences, Wuhan 430074

Dabie massif, which is enclosed by Tan-lu, Mozitan-Xiaotian, Tuan-Ma and Dawu-Xishui faults in the Dabie orogenic belt, is characterized by the widespread distribution of Mesozoic orogenic granitoids (syn-uplifting granitoids). The cooling history of these granitoids, whose ^^Aift^Ai hornblende ages range from 90 Ma to 200 Ma.(Chen et al, 1995; Ma, 1995), might provide valuable information on the post-collisional uplift-exhumation history of the orogenic belt In the present paper, on the basis of evidences of petrology and thermochronology, we'll exemplify the process of differential upliftexhumation of the Dabie massif (with special emphasis on the western area). This process in turn throws new light to our understanding of the exhumation mechanism of HP and UHP rocks in the post-collisional period. The chemical composition-variations of these granitoids in the multi-element diagrams (spider diagrams) are almost identical and mainly show the genetic type of late-orogenic or syn-uplifting granitoids. Moreover, the cooling curves of Mesozoic granitoids and thermo-altered metamorphic rocks, plotted from data of 40Ar/39Ar hornblende and biotite cooling ages, fission-track zircon and apatite ages, against their corresponding closure temperatures, suggest several thermo-evolution units in the Dabie massif. Within each unit, the cooling process of granitoids accorded with the cooling process of the altered metamorphic rocks. The boundaries of these units often comply with tectonic boundaries in the Dabie massif. Thus, it can be said that the same heat source and tectonic background controlled thermo-evolution of post-collisional granitoids and thermo-altered metamorphic rocks. This phenomena may be related to the regional mantle adjustment and asthenosphere uplift in the postcollision period (Kay et al.,1995] Zhang et aL, 1995). In other words, during post-collisional period, the uplifting asthenosphere heated the mid-lower crust and drove the uplift-exhumation of the orogenic belt. Consequently, Mesozoic granitoids resulting from the partially-meked crust, as well as the thermo-altered metamorphic rocks, cooled gradually during the process of uplift-exhumation of the Dabie massif. Therefore, the thermal history of altered rocks (including granitoids and thermo-altered metamorphic rocks) can provide useful constrains on the process of uplift-exhumation. The uplift of the massif often, or inevitably, concurred with its exhumation, and the difference between uplift and exhumation can't be directly recognised if only depending on the cooling history of rocks. Here, the term "uplift-exhumation" is employed in the discussion on the post-collisional tectonic evolution of the Dabie massif. Parameters to describe the process of upliftexhumation include: • the total depths of regional differential uplift-exhumation and their time-sequences; velocities of regional differential uplift-exhumation and their corresponding depths in different stages of upliftexhumation; • the contrasts among regional parameters of uplift-exhumation and their corresponding tectonicstyle. These parameters of uplift-exhumation correspond fairly well with those from the cooling history of thermo-altered rocks, that is, crystallisation pressures Qohnson et aL, 1989) of thermo-altered rocks (already reached minerals balance) within the revealed section can stand for minimum degree of uplift-exhumation of different units, while their crystallisation ages (central values from "^^Ar/^^Ai hornblende) can stand for different time-sequences of regional differential uplift-exhumation; • the cooling age vs. closure temperature curves for thermo-altered rocks can be attained and divided into several sections. Assuming a geothermal gradient of 40°C/km for the Dabie orogenic belt Geological Society of Australia - Abstracts Number 58

FT2^ee (Chen et al, 1995), the cooling rate of each curve-section can be converted directly into the corresponding velocity and magnitude of uplift-exhumation. Thus, the uplift-exhumation for each unit can be described clearly; • by comparing parameters of regional uplift-exhumation with vertical deformation conditions, the outline of regional differential uplift-exhumation and its corresponding tectonic-style can be clarified. On the basis of crystallisation pressures of altered metamorphic rocks and Mesozoic granitoids, the Dabie massif can be divided, from north to south, into three arch-shaped NW-SE trending, SW protruding units of different exhumation magnitudes. The minimum magnitude of uplift-exhumation for each unit increases southward from 14.0 km, 16.6 km to 21.0 km. These units generally suggested one concordant pattern of regional differential uplift-exhumation in the Dabie massif during the postcollisional period. The characteristics of regional differential uplift-exhumation are reflected in the uplifting of isothermal surface from 600°C to 300°C. The altered rocks cooled quickly and their cooling rates changed greatly over an area ranging from 4.l6°C/m.y. to 187.5°C/m.y. According to the converted velocities of uplift-exhumation, the Dabie massif can be divided, from north to south, into four arch-shaped NW-SE trending units. The velocities of the units I and II are 0.38 km/m.y and 3.30 km/m.y, respectively. Unit III can be further divided, from north to south, into three sub-units, with their corresponding velocities of uplift-exhumation being 0.45 km/m.y, 0.30 km/m.y and 0.94 km/m.y, respectively. The velocity of the unit IV is 0.17 km/m.y. Within the Dabie massif, the northern part (unit I) and the southern part ( units III and IV) uplifted with a low velocity, while the central section (unit II) uplifted at a much faster speed. The northern and southern parts of the Dabie massif started to fall into differential uplift-exhumation earlier than the central section. The concrete resuks are listed as follows: units I and III are about 156 Ma and 150 Ma. and unit IV is about 210 Ma; but the unit II is about 128 Ma The boundaries between different units, protruding towards southwest, indicate that the early process of uplift-exhumation, which satisfied brittle-ductile deformation conditions, was accompanied by the southwestward horizontal displacement. Within the Dabie massif, the arch-shaped units corresponds fairly well with the thermo-evolution units attained from the cooling curve of the altered rocks. Moreover, the distribution of these arch-shaped boundary-lines intimately accords with Yuanxi Arch, Luotian Arch (Suo et al, 1993) and the boundary between the South Dabie Massif and the North Dabie Massif (Dong etal, 1993; Wang etal, 1992). Geophysical data show that these boundaries dip northwards. Thus, it can be concluded that the present units in the Dabie massif with different exhumation magnitude, might be formed rapidly during Jurassic and early-Cretaceous periods by regional differential thrust towards southwest. And those concrete parameters describing this thrust process can be attained easily by converting vertical parameters of regional uplift-exhumation with the thrust angle. In addition, some Mesozoic intrusive granitoids also played an important role in the formation of dome-shaped structures. During late-Cretaceous and Tertiary periods, the regional differential uplift-exhumation of the Dabie massif was exhibited by the following characteristics. • The thermo-altered rocks cooled slowly and their cooling rates changed slightly from 1.44°C/m.y to 10.9°C/m.y. Correspondingly, converted velocities of differential uplift-exhumation changed slightly between 0.04 km/m.y and 0.27 km/m.y across the geothermal gradient 35°C/km (higher than normal geothermal gradient of 33°C/km). • The units expressing regional differential uplift-exhumation can't be recognised clearly, and the process of uplift-exhumation was very slow. • During this period, the Dabie massif uplifted towards the surface where brittle deformation is favourable. The above evidences also show that the Dabie massif underwent fault-block style deformation during the late-Cretaceous and Tertiary periods (Yang et al., 1991).

FT2 Chen, J., Xie, Z., Liu, S., Li, X. And Poland, K.A., 1995. Determination of cooling ages of the Dabie orogenic belt by 40Ar-39Ar and fission track dating. Science in China (ser. B), 25,1086-1091. Dong, S., Sun, X., Zhang, Y., Huang, D., Wang, G., Dai, S., Yu,B.,1993. The basic tectonic-style of the Dabie orogenic belt. Chinese Science Bulletin, 38, 543-544. Kay, R.W. and Key, S.M., 1993. Delamination and delamination magmatism. Tectonophysics, 219, 177-189. Johnson, M.C. and Ruther, M.J., 1989. Experimental calibration of the aluminan-in-hornblende geobarometer with application to Long Valley Caldera (California) volcanic rocks. Geology, 17, 837-841. Ma, C., 1995. Coupling between continental lithosphere and asthenosphere: a breakthrough point in continental dynamical study. Earth Science Frontiers, 2,159-164. Suo, S., Sang, L., Han, Y., 1993. The petrology and tectonics in Dabie Precambrian metamorphic terranes. Central China. China University of Geosciences Press, Wuhan, 71-74 . Wang, G., Yang, W., 1996. Structural and choronological evidence of the luotian dome in the core of the eastern Dabie Mountains, central China. Earth Science, 21, 524-528. Wang, X., Liou, J.G., Shigenori Maruyama, 1992. Coesite-bearing eclogites from the Dabie Mountains, Central China: petrogenesis, P-T paths, and implications for regional tectonics. Journal of Geology, 100, 231-250. Yang, W., Yang, S., 1991. Modern theories and study methods of the architecture and evolution of orogenic belts: an analysis of the East Qinling orogenic belt. China University of Geosciences Press, Wuhan, 163-166. Zhang, G., Zhang, Z. and Dong, Y, 1995. Nature of the main tectono-lithostratigraphic units of the Qinling orogen: implications for the tectonic evolution. Acta Petrologic Sinca, 1995,11,110-113.

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DENUDATIONAL HISTORY OF THE EASTERN MARGIN OF THE TIBETAN PLATEAU: CONSTRAINTS FROM FISSION TRACK THERMOCHRONOLOGY

G. Xu and PJJ. Kamp Department of Earth Sciences,The University of Waikato, Hamilton, New Zealand.

In this poster we report and interpret new apatite and zircon fission track thermochronogical data from a 300 km-long corridor down the eastern margin of the Tibetan Plateau and on either side of the Xianshuihe Fault. This fault is the northernmost of a series of arcuate left-lateral strike-slip faults splaying off the Tibet Plateau into southern China. The extensive Xianshuihe-Xiaojiang fault system extending from eastern Tibet to central Yunnan, China, is a major left-shear structural boundary, which, during the past 4 m.y., has accommodated the clockwise rotation of crustal rocks between the Eastern Himalayan syntaxis and the South China Block. These FT data may be useful to help constrain models of the evolution of the Tibetan Plateau, particularly its eastern margin. We report zircon and apatite fission track data for 111 samples of basement collected from both sides of the northern part of the Xianshuihe Fault, where the mean elevation of the plateau drops from 3500 to 1500 m above sea level. The zircon FT ages define two fossil partial annealing zones at different elevations, one fossilised at c.130 Ma, and the other at c. 21 Ma as a result of cooling probably via regional denudation. The apatite FT ages are mostly less than 25 Ma, but a few granitoids higher up on the plateau retain Late Cretaceous apparent ages. Within the apatite FT data with Neogene ages there may be several partial annealing zones, with mixture modelling identifying age components suggestive of discrete cooling phases at c. 22, 14, 1 and 2 Ma. The late Miocene and late Pliocene-Pleistocene phases of cooling are clearly associated with emplacement, cooling, denudation and exhumation of the Gongga Shan granite, which has a published U-Pb igneous age of 12.8 ± 1.4 Ma. Broad limits on the paleotemperatures of the host rocks propagated through to the amount of Neogene denudation, including associated uncertainties, are estimated from the combined apatite and zircon FT data for a section across the plateau margin northeast of the Xianshuihe Fault. The first order pattern of the minimum amount of Neogene denudation superimposed on the present day profile of mean surface elevation, suggests a relatively uniform 4-6 km has been eroded from the inner part of the plateau margin, possibly being as little as 3.5 km at Dandu (mean elevation 3400 m), increasing to 9-12 km at Kangding (mean elevation 2100 m), where the oldest rocks (Precambrian) are exposed, and then decreasing markedly into Sichuan Basin. The peak in the denudation profile coincides with a marked decrease in mean surface elevation and a peak in annual precipitation, suggesting coupled lithosphere - atmosphere processes have produced some of the characteristics of the margin. There is no FT evidence for relative vertical offset on the Xianshuihe Fault at its northeastern end (Dandu and Luhuo), but possibly 1-2 km of throw up to the east has occurred in central segments (Daofu to Moxi) with concurrent erosion. The episodic Neogene denudation of the eastern Tibetan Plateau in the vicinity of the Xianshuihe Fauk identified here, with phases starting at c. 22, 14 and 7-2 Ma, coincide exactly with the timing of phases of cooling/denudation evident from mixture modelling undertaken here of the published FT data for the other margins of the Tibetan Plateau. The plateau-wide thermochronological evidence for sychronous early Miocene initiation of denudation indicates that the present extent of the plateau was defined by this time, but not necessarily its present elevation.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

F T 2 '

FISSION TRACK STUDY ON THE METALLOGENETIC EPOCH AND THERMAL HISTORY OF XIAYINGFANG GOLD DEPOSIT, EASTERN HEBEI, CHINA W.Yuan, S.Wang and L.Wang Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 2732,100080

This paper aims at studying the hydrothermal metallogenetic epoch and thermal history of Xiayingfang gold deposit, Hebei province, China, based on integrated apatite fission track and zircon fission track analyses for different alteration zones. The measured fission track ages of both zircon and apatite from Xiayingfang gold deposit range from 153.9 Ma to 103.3 Ma, indicating a more than 50 m.y. metallogenetic duration. The age data and quantitative AFT modelling show that there are two stages of gold mineralization, in which the first stage is of higher temperature and more rapid cooling rate than the second stage and the time and temperature of the turning point between them is about 120 Ma and 100°C. The first stage of mineralization resulted from intrusion of a granite-porphyry body and took place in about 150 Ma, and second stage of the mineralization was related to both rhyolite-porphyry and magma cryptoexplosive breccia and occurred at about 135 Ma. Combined with the minerogenetic temperature of 370°, 290°, 230°, 170°, 150 and 80°C in vanward-, main- and late-mineralization period the ZFT and AFT ages correspond to the main- and late-mineralization period respectively. Since the samples from different places are heated in different degree by the metallogenetic hydrothermal solution, the less the distance from the thermal source and the long the heating time is, the lower the measured fission track age. It is demonstrated that the quartz-sericitization alteration is earlier and lasts longer than the potassic alteration. Therefore, we consider fission track analysis of the minerals is useful to researching the metallogenetic epoch and thermal evolution history of hydrothermal deposits.

Geological Society of Australia - Abstracts Number 58

FT2^ee

International Conference on Fission Track Dating and Thermochronology

T H E R M O C H R O N O L O G Y OF A FORELAND S E D I M E N T A R Y

F T 2 '

SUCCESSION

(MARNOSO-ARENACEA F M , NORTHERN APENNINES,

ITALY)

M. Zattin Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna, Bologna, Italy

Application of apatite fission-track (AFT) analysis to sedimentary successions is a powerful tool to investigate the range of burial temperatures reached by the sediments. This study concerns the postdepositional evolution of the Marnoso-arenacea clastic wedge, which represents an ideal case study because of a very well known geological setting. The Marnoso-arenacea Fm. (MA) is a composite turbidite, wedge-shaped succession deposited in a migrating foredeep basin located on the front of the Apennine belt from Langhian to late Tortonian. Its stratigraphy and post-depositional history has been carefully described by numerous authors (e.g. Ricci Lucchi, 1986; Landuzzi, 1994) thanks to the occurrence of basin-wide marker beds, the more important of which is the so-called "Contessa bed". The depositional history of its clastic supply has been investigated in detailed petrographic studies which indicate a main provenance from the central-western part of the Alpine chain (Gandolfi et al, 1983). The present tectonic setting consists of nearly-parallel and regularly spaced NE-verging thrusts, which are detached around the base of the MA. This structural setting is the product of a complex compressional tectonics which started probably at the end of the Miocene. The present state of stress is characterized by extension, as documented by steep normal faults, often northeast dipping and with vertical throws up to a few hundred metres (Bertotti et al., 1997). Fifty samples have been dated by using the external detector method and the zeta calibration approach. The main result is a gradual decrease of the annealing degree from the internal to the external tectonic units (i.e. from SW to NE), as demonstrated by the relationships between individual grain ages and the sample stratigraphic ages. In fact, all the single grain ages are younger than the stratigraphic age in the internal units, whereas they are older than the depositional age in the external units. For an intermediate grade of annealing, the main feature consists of a spread distribution of single grain ages. In order to better constrain the maximum post-depositional temperatures reached by the sediments, forward modelling with the Monte Trax program (Gallagher, 1995) has been carried out where a sufficient number of lengths was recorded. In order to avoid mixture of data coming from different thermal histories, in each modelled sample, the length measurements have been carried out only in the youngest population of grains. Thus, the length measurement in possible Clrich apatites (more resistant to annealing) has been also avoided. The resulting burial temperatures range from about 130°C in the internal tectonic units to less than 75°C in the very external unit, with a gradual and uniform decrease across all the main thrust fronts. This values are well constrained by vitrinite reflectance data which span from more than 0.7% to 0.2% (Reutter et al., 1983). The geological information has been significantly improved by conversion of temperatures into depths. Estimation of the geothermal gradient has been made through a careful analysis of the available geothermics data and models. By using a geothermal gradient of 20-25°C/km, a gradual decrease of the burial depths from 5000-4000 m to less than 2800 m has been obtained. These values cannot be justified by the thickness of the eroded MA sedimentary succession and of the late Miocene-to-Recent deposits. A conceivable explanation requires an extensive covering of the Miocene succession by a tectonic nappe (the Ligurids), still preserved as a klippen at the centre of the outcropping bek. The onset of the exhumation of the MA and the consequent erosion of the Ligurid nappe and of part of the Miocene succession can be framed in the late Messinian-early Pliocene, as demonstrated by AFT ages-elevation relationships along a vertical profile. The north-eastward younging of cooling ages in totally annealed samples collected at the same altitude follows the main exhumation phase of the axial part of the Apennine chain (see Abbate et al, this volume) and seems to be related to the late Pliocene-to-Recent extensional tectonic phase.

Geological Society of Australia - Abstracts Number 58

FT2^ee Bertotti G., Capozzi R. and Picotti V. 1997. Extension controls Quaternary tectonics, geomorphology and sedimentation of the N-Apennines foothills and adjacent Po Plain (Italy). Tectonophysics, 282, 291-301. Gallagher K. 1995. Evolving temperature histories from apatite fission-track data. Earth and Planetary Science Letters, 136, 421435. Gandolfi G., Paganelli L. and Zuffa G.G 1983. Petrology and dispersal pattern in the Marnoso-arenacea Formation (Miocene, Northern Apennines). Journal of Sedimentary Petrology, 53/2, 493-507. Landuzzi A. 1994. Relationships between the Marnoso-arenacea Formation of the Inner Romagna units and the Ligurids (Italy). Memorie Societa Geologica Italiana, 48, 523-534. Reutter K.J., Teichmuller M., Teichmuller R. and Zanzucchi G. 1983. Coalification studies in the Northern Apennines and palaeogeothermal implications. Geologische Rundschau, 72, 861-894. Ricci Lucchi F. 1986. The Oligocene to Recent foreland basins of the northern Apennines. Special Publications Int. Ass. Sediment., 8, 105-139.

International Conference on Fission Track Dating and Thermochronology

U - T H / H E DATING R I D E S

F T 2 '

AGAIN

RK. Zeitler Department of Earth and Environmental Sciences Lehigh University Bethlehem, PA 18015 USA

The accumulation of helium from the decay of uranium and thorium is perhaps the most venerable of all isotopic dating methods. However, early in the development of geochronology (before even the days of Lovering), He dating was largely supplanted by other methods which appeared to offer more reliable ages, particularly for the higher-temperature systems which were then of primary interest. Sporadic attempts to revisit the method were not particularly encouraging due to ages that were too young, problems with metamictization in zircon, and analysis of materials that were marginal hosts for U, Th, and He (e.g. bone, coral). The resurgence of interest in He dating was motivated in part by increased interest in environmental change, surficial processes, and understanding linkages between the solid-Earth and surficial realms. A natural outgrowth of this interest was the desire to address lower-temperature problems in thermochronology, which the fission-track method had shown to be an area rich in potential. At ANU in the late 1980's, our foray into helium dating of apatite stemmed from the observation that the low closure temperature for fission tracks in this mineral must reflect rapid diffusion, even at low temperatures. Given that He is small, it seemed plausible that apatite might have a very low closure temperature for this species. Our experiments with Durango apatite showed that while the mineral can quantitatively retain He for tens of millions of years. He loss occurs by systematic volume diffusion at very low temperatures, equivalent to a closure temperature that looked to be well below 100°C. Most recently, work by Farley and coworkers has corroborated and refined this suggestion and provided the analytical procedures and basic understanding of He systematics required to obtain consistent dating results at the percent level or better. The He system offers many advantages: measurement of He is analytically simple and He can be detected with great sensitivity; blanks are easy to minimise because of the low abundance of atmospheric He and because He is mobile and easily removed by modest pumping and baking (certainly so compared to other noble gases such as argon); the effective decay constants for U and Th to He are fast, 6-8 times that of decay to Pb; and, as noted above, the closure temperatures for He are low in common accessory minerals. Thus, with good precision of perhaps a percent possible for even young ages and with a closure temperature of perhaps 70°C for typical grain sizes (and a closure range of about 50° to 90°C), a prime application of He dating is in very low temperature thermochronology. However, the method also holds great promise in dating very young volcanics, given the high He production rates and the sensitivity with which He can be analysed (e.g. only a milligram of typical apatite 200 Ka in age easily contains enough ^He for analysis; analysis is possible with a relatively inexpensive quadrupole mass spectrometer). At this point in its development. He dating does face some challenges, challenges that those familiar with the development of fission-track dating will recall, perhaps all too vividly. Experience has shown that it will be important to have at least several labs involved in a program of calibration, standardization, and methods development if controversy and backlash are to be avoided: the community must avoid a rush into magic-bullet mode. There is a need to understand diffusion systematics in detail, and to have a firm handle on the issue of alpha-recoil and required corrections. For applications to very young materials, we need to understand U- and Th-series systematics in apatite and zircon and examine the magnitude of disequilibrium effects. Perhaps of most importance, we need to think differently about the range of surficial and shallow-crustal processes that have the potential to influence closure at very low temperatures; these issues represent both technical challenges and targets of great opportunity. In particular, practitioners of He thermochronology should be open to applications to muted, low-relief systems which until now have been beyond the reach of thermochronology. Finally, Geological Society of Australia - Abstracts Number 58

FT2^ee U-Th/He dating should not be viewed as a threat to existing methods; because they cannot provide more than a single time-temperature point, He dates will be most useful when combined with apatite fission-track and K-feldspar Ar-Ar measurements which provide segments of cooling paths and a higher-temperature context.

International Conference on Fission Track Dating and Thermochronology

P R E L I M I N A R Y A P A T I T E F I S S I O N T R A C K A N A L Y S I S ON

F T 2 '

MESO-CENOZOIC

SEDIMENTARY R O C K S FROM NORTH JIANGSU BASIN,

CHINA

Z.Y Zhoui, D. Seward2, J. Qian3 and Q. Laoi 1 Laboratory of Marine Geology,Tongji University, Shanghai, China 2 Geology Institute, ETHZ, 8092 Zurich, Switzerland 3 Jiangsu Bureau of Petroleum Exploration, Jiangdu, Jiangsu, China

The North Jiangsu sedimentary basin is located to the north of Yangtze River in East China's Jiangsu province. It is bounded in the west by Tanlu fault and in the north by Jiaonan Mesozoic orogenic belt. The basin covers an area of 3-32 km^. The age range in the basin is from Late CretaceousPaleocene to Pliocene. The thickness of the Tertiary sedimentary sequence is of the order of 9 km. The six source rocks in the basin range in age from Late Cretaceous to Eocene. A well-constrained thermal history of the sedimentary rocks in the basin is of significant implication to this hydrocarbonrich basin. This abstract introduces preliminary fission-track data that we have obtained from core samples from this basin. Standard apatite fission track (AFT) analysis has been carried out on 7 core samples from the basin. The samples were obtained from 4 wells (JSGl, JSHl, JSLl and JSC3). The stratigraphic ages of the samples are from Late Cretaceous to Early Eocene. All samples were analysed by external detector method according to the recommendations of Hurford and Green (1982). Irradiation was carried out at the ANSTO Facility, Australia. Central ages range from 163 to 67 Ma. All samples fail the y2 test at 5% level (Table 1) suggesting that more than one age population is present. The most likely explanation is of course that the sediments were derived from different provenances. In order to identify grain age populations and assign individual grains to specific age populations, the method of "mixed modelling" (Sambridge and Compston, 1994) was applied. The result shows that each sample is made up dominantly of two components. Some component is less than 10% and as a result, is not listed in Table 1, such as 72.7 Ma component of sample JSH. Confined track lengths were measured, but the number found tended to be rather low. The mean lengths vary from 12.4 to 10.3 |Lim. The following preliminary information can be obtained from this data set: 1. The mean AFT ages of samples from bore holes, JSC, JSL, JSH and JSC are older than their stratigraphic ages. This suggests that the sediments have not experienced temperatures higher than approximately 110°C since deposition. However, the mean confined fission-track lengths of samples from hole JSC decreases from 12.16 |im to 10.6 |im over a depth change of 1213 m, which suggests that there may have been some partial resetting (unless the incoming apatites had by coincidence these length parameters as they entered the basin). Support for the existence of some resetting comes from the fact that the lower two samples from hole JSC were taken from source rock horizons, at depths below surface of 1217 and 2037 m. 2. The split age populations of JSGl, 2 and 3, JSL, JSH andJSC-1 fall consistently into two groups, i.e. 68-95 Ma and 139-179 Ma. Sample JSG-3 has a younger age than those that lie stratigraphically above it, in the same drill hole. This is further evidence that there has been at least some partial annealing especially at depths greater than 1500 m. An attempt to find source areas from partially annealed detrital apatites is not really feasible but since the annealing seems minimal we have attempted to look at the local regional geology. To the east of the Jiangsu Basin is a volcanic arc, which was being rapidly exhumed during the Late Cretaceous. It is possible that the younger apatites may have been sourced from there. To the west and northwest of the basin is the Zhang Ba Ling-Jiaonan orogenic belt, where rapid uplift occurred from 172-196 Ma (Chen et aL, 1992) as a result of transpressive processes along the NNE trending Tanlu fault. Although the older apatites have also suffered some annealing and hence have an age reduction, we suggest that they may have been derived from this region. Geological Society of Australia - Abstracts Number 58

Table 1. Apatite fission track data from North Jiangsu basin, East China Sample Depth Irradiation Number oi Standard grains track density number below number surface x 10 cm" (m) (counted) eth-114-6 (12) 147.9 (5937) JSG-1* -824 eth-114-7 (16) 141.9 (5937) 4

2

xl0 cm_2^ (counted) 62.0 (280) 121.8 (826) P s

4

U xl0 content P(X ) % cm" ppm (counted) 148.2 12.5 4.2 (669) 215.5 19.0 0 (1461) 4

P i

2

2

JSG-2*

-1217

eth-114-8 eth-114-3

(33) (30)

135.9 (5937) 198.8 (1806) 165.9 (5937) 226.1 (2407)

423.8 (3908) 324.3 (3453)

39 24.4

0 0

JSG-3*

-2037

eth-114-4 eth-114-5

(31) (22)

159.9 (5937) 213.4 (1976) 153.9 (5937) 99.8 (707)

434.8 (4026) 397.2 (2815)

34.0 32.3

0 0

JSL JSH JSC-1 JSC-2

-1234 -946 -1873 -2207

eth-115-5 eth-115-6 eth-115-2 eth-115-3

(50) (41) (36) (20)

121.2 (4298) 119.1 (4298) 127.6 (4298) 125.5 (4298)

282.6 (4818) 321.0 (2453) 337.4 (5441) 275.7 (2243)

29.2 33.7 33.0 27.5

0 0 0 0

157.1 (2678) 249.0 (1903) 220.0 (3549) 228.7 (1861)

MCTL (nm, la) (number of tracks)

Std. dev. la (fim)

Central age (Ma, ± 2a)

12.06±0.43 12.38+0.58 (5) 12.16±0.15 (16) 12.41±0.22 (25) 10.30t0.34 (20) 11.78±0.96 (45) 10.56i£>.33 (22) 10.6310.26 (18) 10.60+0.13 (40) 10.79+0.30 (16) 11.62±0.18 (37) 12.0410.24 (29) 12.3810.40 (16)

1.42 1.29 1.35 1.04 1.51 1.62 1.60 1.12 1.41 1.20 1.11 1.30 1.61

106.619.8 162.6119.3

an

* Split age Group1 (Ma, 12a)

*Split age Group2 (Ma, 1 2a)

156.8112.7

73.2+5.1

177.814.64

72.3+3.0

178.617.5 139.115.9 163.115.8 146.6+5.6 236.8116.4

68.112.2 78.513.4 72.7+8.0 94.716.0 134.616.5

130.0+10.0 195.1112.6 134.3111.8 66.815.2 119.916.4 163.418.9 143.818.3 178.9113.1

p and pj represent sample spontaneous and induced track densities; numbers in parentheses are number of tracks counted. P(x ^ is the probability of y} for v degrees of freedom where v = no.of crystals - 1. Note that all samples fail this test. MCTL=mean confined track length. Xp = 1.55125 x 10"^. Ages calculated using dosimeter glass CN5 with £ ^ =342±10. Samples were irradiated at the ANSTO facility, Australia. * Results of two separate irradiations that are combined for the mixture modelling. s

2

FT2 3. In sample JSC-2, Cretaceous source rock, the split age populations are older and different from those above. Additionally, these are deeper samples and must have undergone more annealing such that their ages are impossible to interpret. Although we have not studied the chemistry of these apatites yet, and cannot therefore totally refute the idea that the two age groups are due to various annealing rates from crystals with variable chemistry we do suspect that the AFT ages obtained from this study suggest that there was more than one provenance for the Meso-Cenozoic detrital rocks of the North Jiangsu basin. The most probable provenances are the volcanic arc to the east and the Zhang Ba Ling-Jiaonan orogenic belt to the west and northwest of the basin. Chen Wenji, Harrison, T.M. and Heizerler, M.T. 1992. The cooling history of mekange zone in north Jiangsu-South Shandong region: Evidence from multiple diffusion domain ^^hi/^^Kr thermal geochronology. Acta Petrologica Sinica 8(1), 1-17. Hurford, A. J. and Green, P. F. 1982. A users' guide to fission track dating calibration. Earth and Planetary Science Letters 59, 343-354. Sambridge, M.S. and Compston, W. 1994. Mixture modelling of multi-component data sets with application to ion-probe zircon ages. Earth and Planetary Science Letters 128, 373-390. Acknowledgements Mineral separation, track counting and length measurement were carried out at ETH, Zurich, Switzeriand. This is part of a research project supported by the National Science Foundation of China (grant No.: 49876013).

FT2

F T 2 W E E

AUTHOR INDEX

Abbate, E.

1

De Corte, F.

Adriasola, A.

313

De Grave, J.

Andriessen, P.A.M.

3, 61, 159, 173, 195 Donelick, R.A. 197, 233 Dong, S.

Armstrong, P.A.

Duddy, I.R.

Arne, D.

9

Baldwin, S.L.

87

Balestrieri, M.L.

1, 11

Barbarand, J.

175

Batt, G.E.

13, 271

Belloni, F.R

15

Belton, D.X.

19, 23

187 63 203 185 67, 317

Dumitru, T.A.

71, 305

Dunai, T.J.

73, 197

Dunkl, I.

75, 287, 295, 319

Ehlers, T.A.

5

Evans, N.J.

59

Eyal, M.

209

Everitt, R.A.

263

Bernet, M.

27, 41, 109

Bigazzi, G.

1, 29, 33

Blythe, A.E.

37, 185

Bojar, A.-V.

39

Feng, Y.X.

81

Bonadonna, F.P.

29

Filip, J.

85

Brandon, M.T. Brewer, I. Bristow, C.S. Brix, M.R.

Farley, K.A. Feinstein, S.

13, 27, 41, 109, 271 Fink, D.

321

Fletcher, J.M.

91

43, 313

Foland, S.S.

95

147

19, 23, 45, 51, 131 Foster, D.A. 267, 277

Bryant, B.

239

Ford, M.

Franks, D.M.

Burbank, D.W.

37

Burg, J.P.

223

Burgmann, R.

71

Carlson, W.D.

203

Chapman, D.S.

19

Fitzgerald, RG.

Brown, R.W.

Chan, R.A.

209, 259, 263

109

Brooks, C.K.

Carter, A.

5, 37, 77, 169, 207 283, 305

Frisch, W.

5

103 91, 97, 245, 297 99

75, 287, 295, 319

Fritz, H.

39

Fiigenschuh, B.

103

Galbraith, R.R

105

Gallagher, K.

23, 45, 49, 107, 131 181, 267

Garver, J.I.

27, 41, 109, 271

49, 175, 267, 321 113

87

Gaspar-Escribano, J.M.

197

Chen, J.

227

George, A.D.

227

Christiansen, E.H.

217

Gibson, D.L.

113, 247

Cloetingh, S.A.RL.

197

Gibson, H.J.

Cockbur, H.A.R

51

Glasmacher, U.A.

85, 121, 123, 125, 129

Cox, S.J.D.

55

Gleadow, AJ.W.

Crowhurst, RV.

59

55, 91, 131, 213, 245 251, 341

de Bruijne, C.H.

61

117

Gogen, K.

125, 183

Green, P.F.

67, 133

FT2 Griffen, D.T.

217

Kuhlemann, A.

287

Grist, A.M.

135

Kuhlemann, J.

75, 295, 319

Grujic, D.

289

Lao, Q.

357

Guedes, S.

33, 139

Lisker, F.

219

Hacker, B.R.

185, 229

Liu, S.

185

Lorencak, M.

223

Hackspacher, P.

139

HadlerN., J.C.

33, 139, 143

Lu, Y.

227

Hamor-Vido, M.

279

Ma, C.Q.

345

Hannen, H.

225

Mallick, R.

225

Hansen, K.

147

Mangini, A.

225

149

Mann, V.

85

Marshallsea, SJ.

227 229

Harrison, T.M. Hasebe, N.

153, 155, 167, 231

Hejl, E.

157

Mayer, B.

Hendriks, B.W.H.

159

Mclnnes, B.LA.

59

Herve, R

313

Mendoza-Borunda, R.

91

Hill, K.C.

161

Meyer, N.

109

Himeno, O.

163

Morgan, B.A.

237

Hoshino, H.

167

Mori, S.

153

House, M.A.

37, 169, 273

Munoz, J.A.

87

217

Murakami, M.

231

Huigen, Y.D.

173

Murrell, G.R.

233

Hurford, AJ.

15, 175, 321

Murtazaev, K.H.

235

Ibrahim, K.

209

Naeser, C.W.

Indrelid, S.L.

177

Naeser, N.D.

237

33, 139, 143

Nelson, D.R.

241

Jennings, S.

255

Neumann, R.

121, 241

Johnson, C.

45, 181, 285

Noble, W.P.

245

Hu, N.

lunes, P.J.

Jonckheere, R.

183, 185, 187, 189, 191 193, 225, 229

Juez-Larre, J.

195, 197

Kamp, P J J .

5, 109, 199, 349

237, 239

O'Sullivan, AJ.

251

O'Sullivan, P.B.

113, 131, 213, 247, 251 255, 259, 263

Ohira, O.

163

Kang, T.

339

Ohnesorge, F.

121

Keay, S.

201

Osadetz, K.G.

259, 263

Kelley, K.D.

255

Pain, C.F.

Kellogg, K.

239

Patterson, D.B.

Keskes, N.

15

Paulo, S.R.

Ketcham, R.A.

203

Perry, WJ.

239

Khalil, B.

209

Petrie, H.

225

Kohn, B.P.

19, 55, 91, 131, 169 207, 209, 213, 247, 251 259, 263, 279, 297, 341

Pillans, B.

207, 247

Kowallis, B J .

217

Pini, G.A. Podladchichov, Y.

247 59 33, 139, 143

1 103

FT2 Puchkov, V.N.

129

Tingate, RR.

317

Qian, J

357

Trautwein, B.

319

Raab, M

267

Tremblay, A.

123

271, 273, 275

Upton, D.R.

321

Rahn, M.K. Ratschbacher, L. Raza, A.

185, 229, 287

van de Kamp, P.C.

67

97, 161, 169, 277, 279

Van den haute, P.

63, 187 325

Reiners, RW.

283

van der Beek, P.A.

Ren, 2.

339

Vance, J.A.

Ribeiro, L.F.B.

139

Vasconcelos, P.M.

Riedl, H.

157

Ventura, B.

Ring, U.

285

Volk., H.

Salomov, D.J.

235

Wagner, G.A.

Schmid, J.C.

185, 229

Schmers, G.

289

Schultz, A.R

237

Schwab, M.

287

Seidel, E.

43

Seidl, M.A.

51

Seward, D.

103, 223, 275, 289, 301 357

Sobel, E.R.

291

Soloviev, A.V.

109

Southworth, C.S.

237

Spiegel, C.

75, 295

Spikings, R.A.

297, 301

Spotila, J.A.

37

Steckler, M.S.

209

Steehouwer, M.

197

Stockhert, B.

43, 313

Stockli, D.R

305

Stone, J.O.

309

Stiiwe, K.

117

Suchy, V.

85

Summerfield, M.A.

51

Suzuki, T.

311

Tagami, T.

153, 155, 231

TelloS., CA.

33, 139, 143

ter Voorde, M.

197

Terken, J.M.J.

177

Theye, T. Thomson, S.N.

43 43, 313

41 81, 201, 329 1, 331 85 85, 121, 125, 129, 183 185, 187, 189, 191, 193 225, 229, 333

Walgenwitz, R

175

Walker, R.M.

335

Wang, L.

339, 351

Wang, S.

339, 351

Watanabe, H.

155

Watanabe, K.

163

Weber, K.

267

Weber, U.D.

341

Weingartner, H.

157

Winkler, W.

301

Wyrwoll, K-H.

227

Xu, C.H.

345

Xu, G.

349

Yamagiwa, A.

155

Yuan, W.

351

Zanchetta, G. Zattin, M.

29 1, 353

Zeitler, RK.

355

Zentilli, M.

123, 135

ZHOU, Z.Y.

345, 357

Zuffa, G.G.

1

FT2

112E

16S '

120 E

128 E

136 E

144 E

152 E

144 E

152 E

—

24 S

32 S

40 S

16 S '

+

24 S

32 S

Apatite FT Age (Ma) 40 S 50

112 E

A G C R C

120 E

150

250

350

128 E

450

136 E

Maps showing the distribution of sample sites (top) and apatite fission tracl< age (bottom) arising from the Australian Geodynamics Cooperative Research Centre Project 2005LO (http://www.agcrc.csiro.au/projects/2005LO). Digital elevation data are from the GTOP030 global data set (http://edcwww.cr.usgs.gov/landdaac/gtopo30/gtopo30.html) and images were created using Generic Mapping Tools (GMT v3.3.3, available at http://www.soest.hawaii.edu/gmt/).

T H E ORGANISING COMMITTEE OF THE 9 ™

INTERNATIONAL CONFERENCE

FISSION TRACK DATING AND THERMOCHRONOLOGY W O U L D LIKE TO OUR THANKS TO THE FOLLOWING

ON

EXPRESS

SPONSORS

AUSTRALIAN GEODYNAMICS COOPERATIVE RESEARCH CENTRE UNIVERSITY OF MELBOURNE FISSION TRACK RESEARCH G R O U P A U T O S C A N SYSTEMS G E O T R A C K I N T E R N A T I O N A L PTY. LTD. F T S T A G E SYSTEMS D O N E L I C K A N A L Y T I C A L INC. ZEISS EUROTRACK GEOLOGICAL SOCIETY OF AUSTRALIA (VICTORIA DIVISION) DEPARTMENT OF NATURAL RESOURCES AND

THE UNIVERSITY OF

MELBOURNE

ENVIRONMENT

A G C R C

FT ST Age Systems

Eurotrack

Natural Resources and Environment AGRICULIURt

ZEISS

Fission

Track

CONSERVATION LANO

MANAGEMEN

ISSN 0 7 2 9 - 0 1 I X