Issuu

Geological Society of Australia

ABSTRACTS Number

CRATONIC

MARGINS

STRUCTURAL AND TECTONIC PROCESSES

SGTSG CONFERENCE MARGARET RIVER WESTERN AUSTRALIA SEPTEMBER 30 - OCTOBER 4 1991

GEOLOGICAL SOCIETY OF AUSTRALIA INC Abstracts Number 31

Cratonic Margins: Structural and Tectonic Processes Edited and Compiled by Dr A.C. Duncan

Abstracts of a conference organised by the Specialist Group in Tectonics and Structural Geology. Held at Margaret River, Western Australia, September 30 - October 4, 1991 ISSN 0729 - OllX

Acknowledgments: The Specialist Group in Tectonics and Structural Geology wishes to thank the School of Applied Geology, Curtin University of Technology for its support in providing computing and laser printing facilities for the production of this abstracts volume. Editor/Compiler Dr Andrew C.Duncan, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6001, Australia

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TABLE OF CONTENTS M.E. Barley and T.S. Blake

THE MOIMT BRUCE SUPERGROUP AND EASTERN YILGARN CRATON EXAMPLES OF LATE ARCHAEAN TO EARLY PROTEROZOIC DIVERGENT AND CONVERGENT CRATON MARGINS AND CONTROLS ON MINERAUS ATION

R. Batson

SYN- AND POST-DEPOSmONAL FAULTING IN THE SOUTHERN MYALLY SHELF, MOUNT ISA INUER, NW QUEENSLAND.

T,H. Bell and A. Forde

STRUCTURAL PRESERVATION OF THE DIRECTION OF RELATIVE PLATE MOTION DURING OROGENESIS

R. Blewett

PROTEROZOIC GRANITES OF THE COENINLDER, CAPE YORK PENINSULA? NEW STRUCTURAL AND METAMORPHIC EVIDENCE

D.R. Byrne and L.B. Harris

PROTEROZOIC TECTONOTHERMAL EVOLUTION OF THE NORTHAMPTON BLOCK, DARLING MOBILE BELT WESTERN AUSTRALIA

S.F. Cox and M.S. Paterson

EXPERIMENTAL DEFORMATION OF QUARTZ AGGREGATES BY DISSOLUTION-PRECIPITATION PROCESSES AT HIGH TEMPERATURES

P. Darvall, P.B. Cans and G.S. Lister

RAPID, LARGE MAGNTTUDE TERTL\RY EXTENSION IN THE ELDORADO MOUNTAINS OF SOUTHEASTERN NEVADA, U.S.A

M. Drury and J. Braun

THE INFLUENCE OF MANTLE SHEAR ZONES ON THE RHEOLOGY OF MOBILE ZONES IN CONTINENTAL LTTHOSPHERE

M. Economo and B.V^ Logan

GENESIS OF CARBONATE LITHOFACIES IN DILATIONAL ENVIRONMENTS OF A CRATONMARGIN FAULT SYSTEM, CANNING BASIN, WESTERN AUSTRALIA

M.A. Etheridge

LITHOSPHERIC EXTENSION AND REPEATED FAULT REACTIVATION IN THE STRUCTURAL EVOLUTION OF THE WESTERN AUSTRALIAN MARGIN

J.D. Fitz Gerald and T.M Harrison

DIFFUSION DOMAINS IN K-FELDSPAR THERMAL HISTORY FROM AR40/AR39 STEP-HEATING

H.J. Franzke, P. Bankwitz, E. Bankwitz

THE FORMATION OF RUPTURE ZONES IN THE HERC YNIAN FOLD BELT IN EASTERN GERMANY

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H.J. Gibson

LATE CRETACEOUS AND CENOZOIC DEFORMATION OF THE MURCHISON BASIN NEW ZEALAND TECTONIC HISTORY EVALUATION USING APATITE HSSION TRACK ANALYSIS

L. B. Harris and C. P. Delor

TECTONIC EVOLUTION OF THE ERASER MOBILE BELT, WESTERN AUSTRALL\

22 24

E.J. Hill

EASTERN PAPUA NEW GUINEA PALEOGENE SHEARING AND UPLIFT OF ECLOGITES AND ASSOCIATED METAMORPHIC ROCKS IN AN EXTENSIONAL SEFITNG

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I. Jackson, M.S.Paterson and J.D. Fitz Gerald SEISMIC WAVE DISPERSION AND ATTENUATION IN AHEIM DUNTTE AN EXPERIMENTAL STUDY

R J . Korsch, K.D, Wake-Dyster and D.W. Johnstone SEISMIC IMAGING OF EXTENSIONAL AND CONTRACTIONAL PHASES IN THE BOWEN AND SURAT BASINS, EASTERN AUSTRALIA

J.H. Kruhl THE STRUCTURAL STATE OF A FORMER LOWER CONTINENTAL CRUST (NORTHERN SERRE, CALABRIA, S.ITALY)..

B. Lafrance EVOLUTION OF MICROSTRUCTURES AND PETROFABRICS IN CALCITE MYLONITES FROM NEWFOUNDLAND

P.G. Lennox, J. Roberts, R. Offler and P. Schmid HASTINGS TERRANE EMPLACEMENT AND ASSEMBLY OF THE NEW ENGLAND OROGEN

Z.X, Li and C.McA Powell MAGNETIC FABRIC IN THE MID-CAMBRIAN ROCKS OF THE CENTRAL FLINDERS ZONE APPUCATION TO REGIONAL STRUCTORAL AND TECTONIC ANALYSIS

T.A. Little, R J . Holcombe, G.M. Gibson, R. Offler, P.B. Gans and M.O. McW^illiams STRUCTURAL AND THERMAL HISTORY OF BLUESCHISTS FROM THE NORTH D'AGUILAR BLOCK OF SOUTHEAST QUEENSLAND IMPUCATIONS FOR TECTONICS OF THE NEW ENGLAND OROGEN

B.W, Logan DILATIONAL MACROSTRUCTURES C'CARBONATE DILAFORMS") ASSOCIATED WITH CRATONMARGIN FAULT SYSTEMS

S.K. Matthai STRUCTURAL CONTROLS AND TIMING OF MESOTHERMAL GOLD MINERAUSATION CASE STUDIES FROM THE PINE CREEK INLDER, NORTHERN TERRTTORY

M. F. Middleton THE RELATIONSHIP BETWEEN EXTENSION AND THERMAL SUBSIDENCE APPUCATION TO THE PERTH BASIN, WESTERN AUSTRAUA

J,S. Myers NORTHWEST MARGIN OF THE YILGARN CRATON

A-K. Pahl THE SCALE AND DISTRIBUTION OF DUCTILE STRAIN IN HENDRY'S CREEK LOWER PLATE OF A METAMORPHIC CORE COMPLEX, NEVADA, U.S.A

A-K. Pahl, G. S. Lister and J. Lee PARTITIONING OF DEFORMATION IN CRUSTAL SHEAR ZONES THE CASE OF METAMORPHIC CORE COMPLEXES

C.McA

Powell BREAKUP OF EASTERN GONDWANALAND A REVIEW OF SEAFLOOR SPREADING AROUND AUSTRAUA

C.McA Powell and Z.X.Li NEW EVIDENCE FOR THE AGE OF DEFORMATION ALONG THE SOUTHERN MARGIN OF THE HAMERSLEY PROVINCE RELEVANCE TO THE PALAEOGEOGRAPHIC EVOLUTION AND TIME OF IRON-ORE FORMATION

M.S.

Rattenbury THE MARY LANE SHEAR AND A FOLD-THRUST MODEL FOR THE DEFORMATION OF THE TENNANT CREEK GOLD-FIELD, NORTHERN AUSTRALIA

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M.S, Rattenbury, M.B. Duggan, A. Whitaker POLYPHASE DEFORMATION IN THE MOUNT IDA GREENSTONE BELT, AND THE STATUS OF THE EASTERN C»OLX)FIELDS-SOUTHERN CROSS PROVINCE BOUNDARY, WESTERN AUSTRAUA

D.L.Scott ARCHTTECrURE OF THE QUEENSLAND TROUGH IMPLICATIONS FOR THE STRUCTURE AND TECTONICS OF THE NORTHEASTERN AUSTRALIA MARGIN

J. Scott and A. Stein ILLUSTRATION OF STRUCTURAL STYLES OFFSHORE PERTH BASIN INTERPLAY OF EXTENSION AND STRIKE SUP

M.M.

Scott STRUCTURAL HISTORY OF THE TUGLOW DISTRICT, LACHLAN FOLD BELT, NSW

Scott EXTENSIONAL STRUCTURES IN THE RAWHIDE MOUNTAINS, ARIZONA, U.S A. IMPUCATIONS FOR DETACHMENT FAULTS

R.H.

Sibson CYCLIC FLUID FLOW RELATED TO FAULT LOADING IN DIFFERENT TECTONIC REGIMES

J. Streit FLUID-ROCK INTERACTION DURING SHEAR ZONE DEVELOPMENT IN LATE PROTEROZOIC GRANFTOEDS ON KING ISLAND, TASMANIA P.G.

Stuart-Smith THE GILMORE FAULT ZONE -- THE DEFORMATIONAL HISTORY OF A POSSIBLE TERRANE BOUNDARY WITHIN THE LACHLAN FOLD BELT, N.S.W

I.M. Tyler, T.J. Griffin and R.D. Shaw EARLY PALAEOZOIC TECTONISM AND REACTIVATION OF PRE-EXISTING BASEMENT STRUCTURES AT THE MARGINS OF THE KIMBERLEY CRATON, WESTERN AUSTRALL\EOZOIC TECTONISM AND REACTIVATION OF PRE-EXISTING BASEMENT STRUCTURES AT THE MARGINS OF THE KMBERI^Y CRATON, WESTERN AUSTRALIA

D. van der Wal and R.L.M. Vissers DEFORMATION PROCESSES IN MANTLE PERIDOTFTES THE RONDA PERIDOTTTE RECORD

R.H. Vernon, W J , Collins and S.R. Paterson PREDEFORMATIONAL PORPHYROBLASTS IN LOW PRESSURE-HIGH TEMPERATURE METAMORPHIC TERRANES

R.H. Vernon, S.R. Paterson and D. Foster GROWTH AND DEFORMATION OF PORPHYROBLASTS IN THE FOOTHILLS TERRANE, CENTRAL SIERRA NEVADA, CAUFORNL\ TESTING SOME MODERN HYPOTHESES

J.N. Wang, A. Ord and B. Hobbs NEWTONIAN VISCOUS CREEP OF A QUARTZITE AT LOW STRESSES

P.P. Williams and R.H. Vernon TRANSPRESSIVE DEFORMATION AT BROKEN HILL, AUSTRALIA AND THE PROBLEM OF VERTICAL LINEATIONS IN TRANSCURRENT SHEAR ZONES

P.P. Williams, A.R. Norman and R.H. Vernon REGIONAL-SCALE SHEATH FOLDING IN THE ARUNTA BLOCK, CENTRAL AUSTRAUA

S.Zhang, S.F. Cox and M.S.Paterson POROSITY EVOLUTION DURING EXPERIMENTAL COMPACTION AND DEFORMATION OF CALCITE ROCKS

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Y. Zhang, B. Hobbs and A. O r d PREFERRED ORIENTATION DEVELOPMENT OF POLYCRYSTALS WITH ONE SLIP SYSTEM AND Wrra GRAIN BOUNDARY SLIDING

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THE MOUNT BRUCE SUPERGROUP AND EASTERN YILGARN CRATON: EXAMPLES OF LATE ARCHAEAN TO EARLY PROTEROZOIC DIVERGENT AND CONVERGENT CRATON MARGINS AND CONTROLS ON MINERALISATION M.E. Barley and T.S. Blake Department of Geology, The University of Western Australia, Nedlands WA 6009, Australia

The Late Archaean to Early Proterozoic Mount Bruce Supergroup overlies and is restricted to the Pilbara Craton, and comprises five, mostly unconformity-bounded, sequences that formed in different, but related, tectonic settings. The lower three sequences, comprising dominantly tholeiitic flood basalt and terrigenous clastic sedimentary rock (with only minor placer gold and uranium), formed during the breakup of a Late Archaean craton between about 2.77 and 2.70 Ga. The fourth sequence is dominated by mudrock, carbonate sedimentary rock, iron-formation and was deposited in a divergent marginal setting bordering an open ocean between about 2.69 and 2.64 Ga. The fifth sequence, comprising clastic sedimentary rock, iron-formation, carbonate sedimentary rock and felsic volcanic rock, was deposited between about 2.47 Ga and <2.44 Ga in a convergent marginal setting. Between 2.69 Ga and <2.44 Ga, iron-formation was deposited during periods of limited clastic sedimentation as the terrane evolved from a divergent marginal setting (shallow shelf or platform) to foredeep sedimentation. The Archaean-Proterozoic boundary is probably represented by a ca 170 Ma lacuna which separates the forth and fifth sequences. In the eastern Yilgarn Craton, the 2.7 to 2.6 Ga Norseman-Wiluna Belt is intensely mineralised with mesothermal Au and Komatiite-associated Ni sulphide deposits. This belt, which is divided into tectonostratigraphic domains or terranes by a system of craton-scale faults and shear zones, is interpreted as a Late Archaean orogen (similar to those in the modem Pacific Rim), formed by the accretion of volcanic arcs and related basins to the proto-Yilgarn Craton. In contrast, greenstone belts in the western Yilgarn appear to be dominated by older >2.9 Ga volcano-sedimentary assemblages. Gold mineralisation occurred during the late stages of this orogeny which involved transfer of strain from the eastern Yilgarn, limited crustal thickening, metamorphism, and granitoid emplacement. Accretion into orogenic belts provides the best chance of preserving mineralisation formed in deep-marine environments such as komatiite-associated Ni and volcanogenic massive sulphide mineralisation, and explains the common close spatial association of these deposits with gold mineralisation in greenstone belts. Models for Archaean tectonics and mineralisation have generally emphasised granite-greenstone and gneiss terranes as forming in a premobile regime with the diachronous development of continental crust leading to an early form of plate tectonics in the Proterozoic. However, there is mounting evidence that granite-greenstone terranes formed at convergent plate margins and evidence from the Pilbara and Kaapvaal Cratons indicates that those cratons were parts of larger pieces of continental crust which had started to break up by ca 2.8 Ga. The existence of both convergent and divergent craton margins at this time provides empirical evidence that some form of Wilson cycle tectonics has operated for at least 2.8 Ga.

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SYN- AND POST-DEPOSITIONAL FAULTING IN THE SOUTHERN MYALLY SHELF, MOUNT ISA INLIER, NW QUEENSLAND. R. Batson

Department of Earth Sciences, Monash University, Victoria, 3168, Australia

Several different styles of faulting due to successive generations of syn- and post-depositional faulting are displayed in the Myally Shelf of the Mount Isa Inlier: these include a) growth faulting; b) thrusting; c) subvertical shearing associated with a shcMtening event; and d) transpressional wrench faulting which is responsible for the formation of positive flower structures. The basal sequence of the Myally Shelf provides a dramatic contrast with the Leichhardt River Fault Trough immediately to the west: the oldest unit, Quilalar Formation, is underlain by basement on the Myally Shelf and in the Leichhardt River Fault Trough, by 10,000 m of sediment. Thus the Quilalar Fault which separates the two tectonic units, originated as a graben-bounding normal fault during a major rift event at about 1790 Ma. More wide-spread regional subsidence and deposition of the Quilalar Formation was associated with a subsequent sag-phase. Spectacular growth faulting is displayed on the Myally Shelf within the Surprise Creek Formation. Deposition of the formation was associated with a younger episode of rifting (1678 Ma) and was preceded by a period of volatile tectonism and volcanism. This episode of rifting affected much of the northern part of Australia, witli units of the MacArthur Basin (N.T.) correlating closely with units in the Mount Isa Inlier. Rift sequences are locally 6 km thick in the MacArthur Basin, but the Surprise Creek Formation is only 2 km thick, as are most other Mount Isa Inlier and MacArthur Basin correlatives. Mild tectonic activity accompanied deposition of the Surprise Creek Formation, with several features indicating this: a) syn-depositional normal faulting (ie. growth faulting) accompanied deposition in the Myally Shelf; b) the relationship with the overlying sag-phase Mount Isa Group is locally unconformable but elsewhere gradational; c) while the Myally Shelf subsided during this phase, the Leichhardt River Fault Trough just to the east was uplifted. Despite the tectonism and extension which accompanied deposition of the Surprise Creek Formation, it was very minor. The basin was very broad and stratigraphic thickness changes were slight: the dominant cause of subsidence is therefore, believed to have resulted from thermal contraction due to the preceding volcanic and thermal event associated with the initiation of rifting. The main sag-phase, in which all extension had ceased, was responsible for deposition of the overlying Mount Isa Group (1670 Ma). The first phase of post-depositional deformation in the Myally Shelf was responsible for bedding parallel faults which involve narrow discrete zones of cataclasis. Faulting is responsible for a stratigraphic repetition from south to north and with geometries indicating they are thrust faults. Although the faults are present mainly within the Quilalar Formation, a lack of soft-sediment deformation indicates rocks were well lithified and that thrusting was post-depositional.

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A phase of east-west shortening is responsible for the a pervasive regional cleavage, S2, which provides a structural correlation with the rest of the inlier. Open to tight folding is associated with the fabric. An episode of shearing has resulted in shear zones generally sub-parallel to S2, but locally overprinting it. It is responsible for spectacular L-tectonitic fabrics in volcanic breccias, and which indicate west-side-up sub-vertical movement. The main shear zone, the Saint Paul Fault, bounds the eastern margin of the Myally Shelf, and has juxtaposed greenschist facies sediments of the Myally Shelf against amphibolite facies granite of the Kalkadoon Leichhardt Belt. D2 folds have been irregularly deformed in proximity to the Quilalar Fault. A strong mica fabric and chevron folding is associated with the fault, and displacement of regional structures across it indicates displacement has involved sinistral smke-slip movement. The irregular deformation of D2 folds and closely associated tear faults which splay off the Quilalar Fault are best explained by transpressional wrench faulting, resulting in the formation of positive flower structures. Positive flower structures involve localised zones of shortening and uplift which result from transpression at constrictional fault bends on a strike-slip fault. The Quilalar Fault, which was initiated as a major normal fault bounding a rift basin, the Leichhardt River Fault Trough, was reactivated during this later phase of regional strike-slip faulting. The Myally Shelf with its proximity to major tectonic boundaries (viz. the Quilalar and Saint Paul Faults), is in a unique position to shed light on a number of tectonic stages which have affected the Mount Isa Inlier.

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STRUCTURAL PRESERVATION OF THE DIRECTION OF RELATIVE PLATE MOTION DURING OROGENESIS T.H. Bell and A. Forde Department of Geology, James Cook University, Townsville, Qld 4811, Australia

The lack of relationship between relative directions of plate motion and mesoscopically preserved linear structures in foliated rocks in the Alps, is a product of the direction of movement on all dirusts above the basal detachment during gravitational collapse and spreading being solely a function of the local bulk topographic slope. Microscopically preserved foliation intersection axes in porphyroblasts with a multiple growth history result from the intersection of alternating non-parallel tectonic foliations that form on the edge and get incorporated within growing porphyroblasts. These axes form at 90® to the bulk compression direction, which is the relative direction of plate motion operating at the time of porphyroblast growth. For porphyroblasts in the central European Alps with sigmoidal or spiral shaped inclusion trails, these axes cluster about three distinct orientations that resulted from three successive periods of growth as determined from matrix-inclusion trail relationships. Direct correlation of this succession of foliation intersection axes with changes in the direction of relative plate motion during Tertiary Alpine orogenesis derived from magnetic stripes, indicates that they have tracked the motion of Africa relative to Europe during this period. Around 35 Myr, precise correlation ceases, suggesting that the Adria Plate began to move as a separate plate with respect to Africa at this time. Hence, foliation intersection axes preserved in porphyroblasts provide a unique tool for directly determining the relative direction of plate motion in ancient orogenic belts.

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PROTEROZOIC GRANITES OF THE COEN INLIER, CAPE YORK PENINSULA? NEW STRUCTURAL AND METAMORPHIC EVIDENCE. R. Blewett Bureau of Mineral Resources, Geology and Geophysics, GPO Box 378, Canberra, ACT, 2601 Australia

Recent (1990) mapping and associated studies by the BMR and Queensland Department of Resource Industries (QDRI) in the Proterozoic Coen Inlier (Cape York) have produced a better understanding of the complex chronology of deformation, metamorphism and age of granite emplacement in the Inlier, which appear to be distinct from the evolution of the Georgetown Inlier to the south. The Coen Inlier (far North Queensland), comprises essentially Proterozoic Coen, Sefton and Holroyd Metamorphics, and Siluro/Devonian granites of the CYPB (Willmott et al., 1973; BMR Bulletin 135). The metamorphics range from sub to low greenschist silt/sandstones and slates to upper amphibolite grade sillimanite gneiss, schist and quartzite. The Sefton Metamorphics also contain quartz-haematite schist at Iron Range. The Cape York Peninsula Batholith (CYPB) was thought to be partly Proterozoic in age, and the Rb/Sr ages (Cooper et al., 1975: J., OS A, 22, 285-310) were considered to be reset ages. Preliminary U/Pb zircon dating appears to confirm Cooper et al., (1975) and the relationship between the timing of granite emplacement and deformation in the Coen Inlier is distinct from the Georgetown Inlier F i folds are commonly tight to isoclinal, gently E-plunging, upright and have an axial planar Si schistosity with local associated shallow lineations. In many areas. Si is transposed by D2 to a NNW-trending sub-vertical orientation. Prograde metamorphic climax is associated with D i , shown by common sillimanite (now pseudomorphed by retrograde muscovite) and local kyanite. F2 folds are upright, asymmetric, have variably plunging chevron-like hinge zones and a well developed steeplydipping NNW to N-trending S2 crenulation cleavage. Mesoscopic folds are common, while macroscopic folds are generally confined to the Holroyd and Sefton Metamorphics. These plunge gently and have large amplitude to wavelength ratios. D2 structures overprint some of the granites of the CYPB. Retrogression is associated with M2 metamorphism. Major ductile shear zones include the Coen (CSZ), Ehagoola (ESZ) and Lukin River Shear Zones which are oriented just north of NNW are common in the Coen Inlier; many of them have a spatial relationship with gold mineralization. A well developed restraining bend and strike-slip duplex is developed north of the Archer River. New BMR aeromagnetics define the western edge of the inlier and show that the shear zones are truncated against a magnetically featureless area (possibly granite). These shear zones are discrete en-echelon belts up to 3 km wide, and have well developed S-C mylonites all of which display a sinistral-west-over-east sense of shear. Mylonites dip steeply, on average to ENE (86^ to 067®), and presently define oblique-slip normal shear zones. Stretching lineations on Sfn pitch moderately to steeply NNW (average 65® to 333®). An intense L-S tectonite fabric (Dm) is visible in many steeply-dipping rhyolite dykes along the CSZ and ESZ. These are commonly invaded by sub-parallel steeply-dipping quartz veins and a costean SW of Coen reveals that the quartz vein and

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rhyolite dyke are F2 folded and overprinted by S2. This demonstrates that at least one generation of quartz vein and rhyolite are not simple linear bodies; but are complexly folded. Most of the shear zones appear, however, to be rectilinear and the mylonitic foliation is locally a composite of at least two generations of movement (all sinistral-west-over-east). The timing of shear zone development is post-Dj because mylonites overprint granites that cross-cut Si. Mylonites are also clearly overprinted by the N-S trending S2 crenulation cleavage with an Sshaped asymmetry viewed north. This crenulation of 'Sml' is itself strongly transposed into a second generation of mylonitization with a sinistral sense of shear. The S shaped asymmetry and steep plunge of F^ folds suggest maintenance of the sinistral shearing with reduced dip-slip movement. They also point to progressive deformation, where foliations were formed and being deformed within an active shear zone. Open to tight F3 folds and a (less well developed) S3 crenulation cleavage are common in the Holroyd and Sefton Metamorphics and to a lesser extent in the Coen Metamorphics. These are typically mesoscopic, upright, E-W to NE-SW trending structures. Rare low-angle F4 axial surfaces with shallow E-plunges overprint D3. Di in the Coen Inlier may correlate with Dj in the Georgetown Inlier. The Coen Inlier, however, has a Di climax and D2 retrogression as well as pre and post-D2 ductile shearing. In the Georgetown Inlier, Dj and D2 were prograde events (probably not separated by a long period of quiescence) which are older than 1550 My (Ian Withnall pers. comm., 1991), and D3 was a period of retrogression and ductile shearing (the absolute timing of which is in doubt). D2 structures overprint the Siluro/Devonian granites of the CYPB. Overprinting by the major shears zones of the Permian (?) Wolverton Adamellite (Willmott et al., 1973: BMR Bulletin 135) and felsic dykes (Permo-Carboniferous?), subject to confirming these dates, implies that the shear zones may be as young as Mesozoic. The Nd model ages for the Coen Metamorphics are within the range of other north Australian Proterozoic Inliers (T^d between 1940-2130 My; McCulloch, 1987: American Geophysical Union, Geodynamic Series, 17,115130), and inherited U/Pb zircon ages in Siluro/Devonian granites in the Coen Inlier record Proterozoic thermal events (Lance Black pers. comm., 1991). D\ in Coen may correlate with the 1570 Ma. Georgetown events, but the deformational and metamorphic quiescence in the former until the Mesozoic (?) is in conu-ast with the more prolonged deformational history in Georgetown. This raises questions of whether the two Inliers form a single province (Henderson 1980: Geology & Geophysics of NE Australia, 1-26), are they fundamentally different, why the long period of quiescence, is there an incomplete structural chronology for both or either inlier, and where are the Proterozoic granites in the Coen Inlier?

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PROTEROZOIC TECTONOTHERMAL EVOLUTION OF THE NORTHAMPTON BLOCK, DARLING MOBILE BELT WESTERN AUSTRALIA D.R. Byrne and L.B, Harris Department of Geology, University of Western Australia, Nedlands, 6009, Australia

The Northampton Block occurs as an isolated basement inlier surrounded by Phanerozoic sediments of the Perth Basin in the northern part of the Darling Mobile Belt., W.A. The block is largely composed of pelitic and psammitic paragneisses, quartzites, mafic granulites and a porphyritic granite. Previous Rb-Sr dating of the paragneisses indicate that peak metamorphism occurred prior to about 1000-1100 Ma, while the granite intruded between 900 Ma and 1150 Ma. Sedimentary structures can be observed in well exposed sections, with bedding being accentuated by the development of reverse grading due to the coarser grainsize of garnet in the more pelitic portion of graded beds. The meta-sedimentary layering has been folded into inclined to recumbent, tight to isoclinal folds verging westward with associated limb thrusts. Elongate cordierite, and the preferred orientation of sillimanite and biotite define an axial planar foliation (Si). These folds are refolded by open to close upright folds, usually trending NW and plunging shallowly to the SE. An axial planar foliation (S2) is defined by the crenulation of Si biotite, or newly crystallised oriented biotite. Intrusion of porphyritic granite during this event is indicated by the alignment of microcline phenocrysts defining a foliation striking NW parallel to S2. Peak metamorphism appears to have taken place prior to the first folding event. Prograde metamorphic reactions in the meta-pelites are suggested by the inclusions contained within garnet porphyroblasts and cordierite. Fine grained sillimanite, and rounded inclusions of biotite and quartz suggest the following reaction: biotite+sillimanite+quartz->(gamet, cordierite)+K-feldspar+H20

(1)

The inferred peak metamorphic assemblage included hercynite and quartz, although in thin section they are not observed in mutual contact due to the following retrograde reactions where hercynite is either rimmed by garnet, cordierite or sillimanite: hercynite4-quartz+H20->Fe-cordierite

(2)

hercynite+quartz-^almandine+sillimanite

(3)

Two more inferred retrograde reactions are a reversal of reaction 1, producing biotite, and: gamet+sillimanite-i-quartz-x:ordierite

(4)

Reactions 2, 3, 4, and the reverse of 1 were instrumental in producing the cordierite, coarse sillimanite and biotite that define S i . While Si generally wraps around garnet porphyroblasts, coarse Si sillimanite is sometimes included in their margins. This suggests that the growth of garnet occurred mostly prior to, and ceased during the formation of Si before biotite formed. The reverse of reaction 1 also occurs in the porphyritic granite where garnet is partially rimmed by biotite.

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From the above reactions the P-T path followed by the rocks of the Northampton Block is neither distinctly clockwise nor anticlockwise, but is rather a backtracking path (when errors in P & T are considered). From reactions 2,3, and 4 metamorphic conditions during the initial formation of Si are estimated at being 900-1000^ C at 5-6 kb, while those during the development of S2 were SOO-TW C at about 4 kb. The retrograde portion of the P-T path corresponds essentially to isobaric cooling at around 4 kb. To explain the high-temperature low- to medium-pressure granulite facies metamorphism, a crustal thinning model is proposed in which an increased geothermal gradient is achieved by mantle upwarping. This implies Middle Proterozoic regional-scale crustal extension has occurred on the western margin of the Archaean Yilgam Craton, reflecting a possible rifting episode between Greater India and Australia. No regional-scale structures formed during this event have yet been recognised. Closure of the extended region is indicated by westwards verging folds and minor thrusting (i.e. vergence away from the Yilgam Craton towards Greater India). Subsequent folding represents a change in the regional compressional regime to NE-SW shortening across the block.

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EXPERIMENTAL DEFORMATION OF QUARTZ AGGREGATES BY DISSOLUTION-PRECIPITATION PROCESSES AT HIGH TEMPERATURES S.F. Cox and M.S. Paterson Research School of Earth Sciences, The Australian National University, GPO Box 4, Canberra, ACT. 2601, Australia

Creep tests and hydrothermal isostatic compaction (HTIC) treatments have been conducted in a gas medium apparatus on porous quartz aggregates which have initial median grainsizes ranging from 250 mm to 2mm. Experiments have been performed at 1200"" K, confining pressures up to 300 MPa and at a pore water pressure of 250 MPa. During HTIC treatment under these conditions, compaction from an initial porosity of 35% occurs at rates between 10"^ s"^ and 10'^ s"^ and is associated with a progressive increase in the strength of grain aggregates. Creep tests on compacted aggregates having initial porosities around 15% have achieved up to 20% shortening, with creep rates ranging from 10"^ s"^ to 10"^ s'^ at stress differences between 40 MPa and 150 MPa. Final porosities as low as 6% have been achieved. Creep is usually characterised by a steady decrease in creep rate with time, although anomalous transient behaviour can be associated with loading after extended periods of HTIC treatment. Initial results suggest that although creep rate has a marked stress dependence, there may not be a simple relationship between creep rate and grainsize. Changes in grain shapes, grainsize distribution, pore geometry and the microstructure of grain surfaces, together with an absence of substantial plastic strain or intragranular microcracking, indicate that deformation has occurred largely by dissolution-precipitation processes. Interpenetration of grains and grain truncation is well-developed at grain contacts. Rounded to faceted overgrowths are present in pores. Complex microstructures at grain contacts indicate that the fluid distribution on actively dissolving interfaces has a dynamic structure. The experimental results indicate that the presence of reactive pore fluids can lead to a time-dependence of the mechanical behaviour and fluid transport properties of porous grain aggregates. This has implications especially for the mechanics of active fault zones.

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RAPID, LARGE MAGNITUDE TERTIARY EXTENSION IN THE ELDORADO MOUNTAINS OF SOUTHEASTERN NEVADA, U.S.A. P. Darvall^ P.B. Gans^ and G.S. Lister^ W.I.E.P.S., G.P.O. Box 2729X. Melbourne 3001, Australia ^Geology Dept.. Stanford University, CA 94305, U.S.A.

The Eldorado Mountains lie at the northern end of the north-south trending Lower Colorado Extensional Corridor, 50 km SE of Las Vegas, Nevada. East and west rotated blocks characterise the corridor which has a present east-west extent of 100 km, and is flanked by relatively unextended ranges. Integrated field work and high precision "^^Ar/^^Ar geochronology have enabled the timing and rate of east-west directed extension in the Eldorados to be tightly constrained, and related to extension in other ranges in the corridor. An approximately 5000 m thick volcanic succession directly overlies Pre-Cambrian crystalline basement. The section consists, in ascending order, of: 1 Patsy Mine Volcanics, including a lower member of predominantly intermediate lavas (18.7 to 15.50 Ma), a middle member of rhyolite to dacite lavas and sediments (15.5015.30 Ma), and an upper member of basaltic andesite lavas (15.30-15.20 Ma); 2 The Tuff of Bridge Spring, a 15.20 Ma composite sheet of dacitic ash-flow tuff; 3 The 15.20 to «14 Ma Mt Davis Volcanic sequence of basalt to rhyolite lavas and tuffaceous sediments. A large plutonic body marks the southern boundary of the mapped area. Crosscutting relationships, compositional variation within the pluton, and K-Ar ages indicate it is a high level, composite body emplaced synchronously with extrusive units. Other intrusive rocks include numerous north-south trending dykes of dominantly Mt Davis affinity. East-west directed extension has resulted in a number of variably east rotated blocks separated by west dipping normal faults, accommodating offsets up to 2.5 km. Consistent bedding to fault angles indicate faults formed as planar structures, and crosscutting relations indicate movement on all faults was not synchronous. High bedding to fault angles, an intimate association of dikes and faults, and other geometric evidence indicate these faults formed vertically. The field area may be conveniently subdivided into three structural domains, which will be encountered in any west to east transect. The western and eastern parts of the range are characterised by steeply east dipping to vertically dipping units, and subhorizontal faults and dikes. In the central domain units dip 40-55"^ cast against «50® west dipping faults. Transitions between the three domains occur over short distances (< 300 m), and arc marked by curved faults and dykes, rapid changes in bedding attitudes, and high angle (60-70'') west dipping normal faults that crosscut earlier structures. Significant local warping/doming in the range is inferred to be responsible for these features, and occurred synchronously with extension. The bulk of > 8 km of extension in the range is restricted to between 15.0 and 14.5 Ma, with a maximum of 10^ tilting prior to this time. Lava flows and clastic deposits of Middle Mt Davis Volcanics thicken against several - 10 -

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faults, and arc capfxxl by shallowly (<10'') cast dipping, 14.5 Ma upper Mt Davis basalts. Extension is extremely heterogeneously distributed. Most extension is acxommodated within the eastern and western domains where units are more steeply dipping. Average extension across the range is 150% and an average extensional strain rate of 6 X 10"^^ s'^ applies across the whole range. This translates to a separation rate of «18 cm/year for the extensional corridor assuming an initial width of 40 km. The close spatial and temporal association of extensional faulting, volcanism, and plutonism, evidence for faults forming as vertical tension fractures intimately associated with dykes, and significant local warping suggests that rapid, large magnitude extension in the Eldorado Mountains occurred in the roof of an active magma chamber. A series of plutonic bodies, including the pluton in the Eldorado Mountains correspond to a continuous, linear geophysical anomaly along the extensional corridor. Timing of events indicate that the locus of maximum extension in the extensional corridor has migrated northwards; from the Mohave Mountains (19.5-18.5 Ma), through the Eldorados (1514.5 Ma) to the River Mountains (12-9 Ma). This work reinforces our developing concepts suggesting a basic and fundamental relation between plutonism and extension in the Basin and Range Province.

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THE INFLUENCE OF MANTLE SHEAR ZONES ON THE RHEOLOGY OF MOBILE ZONES IN CONTINENTAL LITHOSPHERE M. Drury and J. Braun Research School of Earth Sciences, The Australian National University, GPO Box 4 Canberra ACT 2601, Australia

Conventional models of the rheology of mobile zones in the continental lithosphere are based on two types of deformation mechanism; frictional faulting and dislocation creep. However, recent studies on mantle peridotites [1-3] suggest that after a critical strain most of the deformation in the uppermost mantle is accommodated by an array of mylonitic shear zones. Microstnictural studies [2,4] indicate that grain-size-sensitive (GSS) mechanisms (diffusion creep and grain boundary sliding creep) are important in these shear zones. GSS creep becomes predominant over dislocation creep after large strains at low temperature and high stress. The onset of GSS creep may induce shear localisation. We have developed a model for the rheology and microstructure of the upper-most mantle which includes frictional faulting, dislocation creep and GSS creep in shear zones. The model includes grainsize reduction by dynamic recrystallisation during dislocation creep and grain growth during GSS creep. The exjx^rimental data used are from a self-consistent set on "wet" olivine from the ANU deformation laboratory [5,6]. The model has been subjected to homogeneous constant-strain-rate extension with simultaneous conductive cooling (figure 1). GSS creep results in drastic weakening of the lithosphere and locally in an increase of stress with increasing depth. After moderate strains a rheological stratification develops. At T > 950 ± C dislocation creep is always dominant. At intermediate temperatures grainsize reduction and growth cycles result in alternate cycles of dislocation creep and GSS creep. The shear zone material is softened during the initial part of the GSS creep cycle but grain growth then induces hardening. At T < 700 ± 50^ C, after an initial stage of dislocation creep, GSS creep dominates to very large strains. The weakest part of the upper mantle is bounded roughly by the 550 and 850® C isotherms. This depth range may therefore act as a detachmcnt horizon and in consequence the Moho will not always be a rheological discontinuity. The results are very sensitive to the relationship between dynamic recrystallised grainsize and deformation conditions and the recrystallisation rate. Better experimental constraints on these arc required. In future, the effects of hydration reactions in shear zones and grainsize reduction during cataclasis will also be included. A two dimensional finite-element version of the model is being developed to study the influence of GSS creep in shear zones on the bulk rheology and lateral growth rates of extensional mobile zones. This study is at an early stage, however, our preliminary results show that GSS creep in shear zones can result in a complex strain and time dependence for lithosphere rheology.

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log(Stross)

log(Stress)

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log(Stre$s)

Moho

Base of the Lithosphere

Figure (1). Results from the rheological/microstructural model of the upper mantle. Deformation at constant strain rate of s"^ to a total extensional strain of 200%, with initial geotherm of 15^ C / km and the base of the lithosphere defined by the 1300® C isotherm. Stress/depth profiles are shown at 1%, 50%, 100% and 150% extension. Shading indicates the dominant deformation mechanism; white = frictional faulting, grey = dislocation creep and black = grain-size sensitive creep. The graph shows the change of integrated lithosphere strength with strain. A rheological effect related to changes in deformation mechanisms and a geometric effect related to lithosphere thinning are distinguished. GSS creep results in a gradual decrease of integrated lithosphere strength with increasing strain. The stress profiles show that this weakening is concentrated in the depth range where T = 550® to 950® C. This region has a complex layered rheology at large strains and may act as a detachment horizon or zone during extensional tectonics. RgfQrQnces: [1] Hoogerduijn Strating 1991 PhD thesis University of Utrecht, Geologica Ultraiectina 74; [2] Drury et al. 1990 Geologic en Mijnbouw 69, 3; [3] Vissers et al, 1990 Terra Abstracts 2,21; [4] Rutter and Brodie 1988, Geol Rund, 77,295.; [5] Karato et al 1986 JGR 91, 8151; [6] Karato 1989 Tectonophys, 168,255

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GENESIS OF CARBONATE LITHOFACIES IN DILATIONAL ENVIRONMENTS OF A CRATON MARGIN FAULT SYSTEM, CANNING BASIN, WESTERN AUSTRALIA M, Economo and B.W Logan Department of Geology, The University of Western Australia, Nedlands, W A 6009, Australia

Dilational processes associated with deviatory fluid pressures are recognised as responsible for lithogenesis in Devonian carbonate sequences that lie adjacent to a craton-margin fault system near Geikie Range, Canning Basin, Western Australia. A distinct lithofacies characterised by vugular rock, veined rock and breccioid rock plus associated cavity-filling materials has been produced in dilational environments of the fault system. Petrologic analysis of cavity-filling material in veins, vugs and breccia lithotypes provides understanding of processes operating in dilational domains. Cavity-filling materials may be classified as either flocculate, infiltrate, injectate, metacolloidate or crystal late. Flocculate: generated as a crystal suspension or dispersed slurry following a rapid drop in fluid pressure, coeval with dilation. Fine flocculates are transported and deposited as the fluids invade dilating cavities. Infiltrate: material characterised by plumose structures and inclined and graded bedding, indicating mechanical transport and deposition in cavity networks. Injectate: material that was injected as slurry under high fluid pressure. Injection results in abrasion and fragmentation of precursor material and cavity walls. Metacolloidate: expulsion of CO2 during pressure spraying and droplet impact results in precipitation of crustose linings of fine carbonate gels. These gels harden and crystallise to form metacolloidate. Crystallate: well-organised mineral mosaics that have crystallised from solution. Three crystallate types are: 1) equant, 2) columnar, and 3) fibrous. The spectrum of cavity-filling materials is related to variable conditions of fluid pressure gradient and flow. Genesis involves interacting processes of fluidisation, injection, exhalation and precipitation. Variations in dilation rate, pressure gradient, flow velocity and host rock determine the type of cavity-filling material. Dilational lithogenetic systems are transitory since the requisite deviatory fluid pressures are dissipated by outflow. However, they developed as the result of processes operating during compressional episodes. Thus a prevailing compressional regime is punctuated by dilation events. In the study area, five deformational phases are identified:- three compressional and two dilational. The second compressional phase and third dilational event are recorded by strong overprints and the macrostructural framework was probably shaped during these. During compression, compaction, pressure solution and shear fracture are dominant processes with consequent net volume reduction and porosity diminution, leading to increasing fluid pressure in the domain. Generation ot CO2 and carbonate-enriched fluids during compression leads to rapid carbonate precipitation and CO2 exhalation when fluid pressure falls as a consequence of dilation.

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LITHOSPHERIC EXTENSION AND REPEATED FAULT REACTIVATION IN THE STRUCTURAL EVOLUTION OF THE WESTERN AUSTRALIAN MARGIN M.A. Etheridge Research School of Earth Sciences, The Australian National University, GPO Box 4 Canberra, ACT 2601, Australia and Tectonex Geoconsultants Pty Ltd

The Western Australian continental margin, especially the Northwest Shelf, has long been regarded as a classical passive continental margin formed by fairly simple rift-drift processes during the Mesozoic break-up of this part of Gondwanaland. In this scenario, continental extension (rift stage) is generally considered to have taken place in the Triassic to early Jurassic, followed by thermal subsidence (drift stage) through the remainder of the Mesozoic and into the Cainozoic. The break-up unconformity separating those two stages has been dated from mid-Jurassic to early Cretaceous, according to location and author. It is also widely accepted that the majority of structures found along the margin, especially those that control petroleum entrapment in the Carnarvon and Bonaparte Basins, are simple normal faults formed during the rift stage of margin development. This classical passive margin model is, however, a very substantial oversimplification of what has been a complex, multi-event history involving extensional, compressional and wrench tectonics from the midPalaeozoic until the present. The most important conclusion to emerge from this study is that both the gross architecture and much of the structural grain of the Western Australian margin formed during a major extensional event in the late Carboniferous to early Permian. It is unclear whether this extensional event proceeded to the stage of sea-floor spreading. The limited deep seismic data available from the western Australian margin suggests that large extensional strains developed beneath much of the margin at that time. For example, extrapolation of the major Permian detachment and the Moho from nearshore to beneath the Dampier sub-basin suggests that the pre-Permian crust has been thinned from about 35 km onshore in the Pilbara region to less than 10 km and perhaps as little as 5 km beneath the Lewis Trough. Despite its relatively poor definition and deep burial, the Permo-Carboniferous extensional event proved fundamental to subsequent structural evolution and petroleum entrapment along the western margin because it established its structural grain. Consequently, virtually all subsequent structuring events led to reactivation of the Permo-Carboniferous faults and followed the same trends. The tectonic transport direction during the PermoCarboniferous extension was NW-SE along the whole margin. The northern part of the margin underlying the Carnarvon, Browse and Bonaparte Basins underwent approximately orthogonal extension, with the formation of NE-SW trending normal faults and NW-SE trending transfer faults. The major transfer faults compartmentalised the basin and correspond in large part to the present day basin and sub-basin boundaries. The Perth Basin was initiated by oblique extension (transtension) because of the interaction between NW-SE extension and the major pre-existing lithospheric weakness represented by the Darling Fault system. Correlation with onshore sequences in the southern Carnarvon and northern Perth Basins suggests that extension commenced in the late Carboniferous and continued into the early Permian. Seismic evidence from the

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Carnarvon, Perth and Bonaparte Basins indicates that extension gave way to basin-wide thermal subsidence before the end of the Permian. Thermal subsidence along the whole of the Western Australian margin continued throughout the Triassic. There is no evidence that there was significant syn-depositional faulting, and there was certainly no major crustal extension during the deposition of the Triassic sequence, whose thickness and facies varies only gradually both within and between basins along the margin. Thermal subsidence was interrupted by a regional deformation event of latest Triassic to early Jurassic age. Although this event was widespread and it dominates much of the seismic structure, especially in the Carnarvon and Perth Basins, it was of relatively low strain magnitude. It led primarily to wrench reactivation of the underlying NE and NW trending extensional fault systems - sinistral on the NE trend and dextral on the NW trend, indicating ^proximately N-S compression. The N-S compression direction is consistent with deformation in the Canning Basin at the same time. The key structures formed during this event, especially from the petroleum viewpoint, were the gas and oilbearing structures of the Rankin and Madeleine trends in the Dampier sub-basin. In addition, the Lewis trough, Madeleine high, Kendrew trough and Rankin high within the Dampier sub-basin are interpreted to be crustal scale faulted folds whose positions and orientations reflect variations in crustal thickness resulting from the Permo-Carboniferous extension. By the same reasoning, the Exmouth Plateau is a Permo Carboniferous feature, although somewhat modified by subsequent structural and thermal events. Perhaps the most enigmatic feature of the Western Australian margin is how little structural impact the initiation of the Indian Ocean sea-floor spreading had on the present continental shelf. It is limited to sporadic, small displacement extensional reactivation of the Permo-Carboniferous faults in the mid to late Jurassic. However, the thermal effects of the initiation and subsequent departure of an active spreading ridge did have substantial consequences for subsidence and thermal histories on the shelf. The Cretaceous and Tertiary history of the margin remained one of periodic reactivation of the underlying PermoCarboniferous extensional structures, usually with a significant compressional wrench component. N-S compression in the Cretaceous gave rise to very similar fault reactivation and crustal scale buckling in the Bonaparte and Browse Basins to that which dominated the Carnarvon Basin in the early Jurassic. Features such as the Cartier Trough and the Jabiru trend are considered to be analogous to the Lewis Trough and Rankin (or Legendre) trend respectively. The most recent event to substantially influence the Western Australian margin has been the mid Tertiary to Recent collision along the northern margin of the Australian plate. Once again, the stresses resulting from this collision have led principally to reactivation of the now deeply buried Permo-Carboniferous fault systems. The peak of fault activity occurred in the Miocene and is recorded as far south as the southern Carnarvon Basin. The geometry of the Miocene faulting indicates that compression was predominantly E-W, consistent with the present day stresses in that part of the Australian plate.

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In summary, the Western Australian margin was localised along a pre-existing Permo-Carboniferous extensional basin system. The thinned and highly faulted crust formed in that extensional event has then deformed repeatedly in response to changes in the regional stresses. These stress changes can be related to plate-scale to global tectonic events, and a structural event history erected which relates to the global plate tectonic framework and provides a basis for understanding the structural evolution of this and other complex passive margins.

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DIFFUSION DOMAINS IN K-FELDSPAR: THERMAL HISTORY FROM AR40/AR39 STEP-HEATING J.D. Fitz Gerald^ and T.M Harrison^ ^Research School of Earth Science, The Australian National University, GPO Box 4, Canberra A.C.T. 2601, Australia 2e&SS, UCLA, Los Angeles, CA 90024, U.S.A.

Ar diffusion through mineral grains has long been appreciated be complex, though a process well enough understood for Ar^0/Ar39 analyses to be one of the routine methods of thermochronology. One aspect of the process's complexity is the scale over which diffusion occurs through *bulk' crystal, both in nature and in the laboratory. NcMmally, the pattern of Ar release in the laboratory indicates that volume diffusion of Ar in minerals actually involves domains that are much smaller than the physical grain or fragment size. It is considered that Ai diffuses slowly within each domain, but relatively quickly in domain boundary regions. In laboratory degassing experiments Ar, which has naturally diffused throughout the domain network during geological time, is liberated on heating by diffusion of Ar through domains and domain boundaries out to the surface of each grain or fragment Such behaviour requires that networks of domain boundaries occur throughout mineral grains in rocks and it has been speculated but never proven that sti-uctures such as exsolution or twin boundaries constitute the necessary domain boundary structures. Traditionally, thermal history has been determined by tedious measurement of a series of ages from minerals with different closure temperatures. (In simple cases, the age of a body determined using radioactive decay will indicate the time at which the body last dropped below its closure temperature since closure temperature of a material is 'defined' as the temperature below which diffusion of both parent and daughter species effectively ceases.). Recently Lovera, Harrison and others have demonsu-ated that the low temperature thermal history of slowly cooled samples can be determined by analysing the complexities of Ar gas release during step heating of alkali feldspar alone. These analyses involve both measurement and modelling of argon release from grains that must show some range in domain size to satisfy a requirement for domains of different closure temperatures to exist within each sample. MH-10 is the K-feldspar from a granodiorite in the Chain of Ponds Pluton, Maine, that was used by Harrison, Lovera and colleagues for detailed studies of Ar diffusion kinetics in "^^Ar/^^Ar thermochronometry. A microstructural study has been carried out using light (LM) and electron microscopy (TEM) to search for defects corresponding to domain boundaries in this K-feldspar. Both 'virgin' K-feldspar, and samples heated in vacuum for 70 mins at temperatures of 750, 950 or llOO^C, have been examined. The K-feldspar does not possess networks of dislocations in a subgrain structure. However, three classes of substructure arc present, with apparently only the third affected at all by laboratory heating:- (1) Cross-hatched extinction at the l -10|im scale (LM, maximum extinction angles ±18^) is common but variable. TEM reveals almost no Albite/Pericline twinning associated with the optical cross-hatching, only tweed microstructure at the 5nm scale. (2) Turbid zones exist, but only 5-10 vol.% of MH-10 is affected. Micropores, <1 - 2 jxm, charactcri/e these regions, also often blebs of albite, 2 - 40 |J.m diameter. These 'modified' zones are complex with intricate twin and tweed

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structures in TEM at the sub-micron scale and numerous dislocation and strain features. Although LM indicates many of the structures have originated at fractures, adjacent pores are effectively isolated from one another and do not define continuous channels. (3) Exsolution lamellae. Albite lamellae (<1 x 20 |im, separated by 1 |im, LM) arc rare (-2%). TEM shows submicroscopic lamellae in - 20% of K-feldspar. These lamellae, 0.01 x 0.2-1 |im, separated by 0.1-0.5 ^m, are disk-shaped and related semi-coherently to the host K-feldspar. Submicroscopic lamellae are removed by heating to 950 and 1 lOOX. Reasonable agreement between diffusional and microstructural analysis has been found. The large domain size identified by diffusion analysis was order 50 pjn. This length scale corresponds to the size of K-feldspar blocks defined by the network of fractured/turbid zones in MH-10. The small domain size from diffusion analysis is -0.5 M.m, corresponding to the separation between submicroscopic exsolution lamellae. Such lamellae should disappear in the time-scale of laboratory heating, a phenomenom recognised during the kinetics analysis. However, some problems remain. No candidate has been found for an intermediate domain. Also, it is unclear whether defect microstructures of the types identified would be able to enhance Ar diffusion sufficiently for them to act as domain boundaries both by virtue of the nature of each boundary (eg. does a semi-coherent exsolution interface have a diffusivity much higher than the crystal lattice?) and its geometry (eg. does the population of disk-shaped lamellae possess sufficient connectivity to define continuous boundaries?). Furthermore, the diffusional modelling requires that domains of different sizes be spatially independent, whereas the large and small domains described above are almost certainly nested, small domains inside larger ones. Work is continuing to resolve these differences.

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THE FORMATION OF RUPTURE ZONES IN THE HERCYNIAN FOLD BELT IN EASTERN GERMANY H.J. Franzke, P. Bankwitz, E. Bankwitz Zentral Institut Stir Physik Der Erde, 0-1561 Potsdam, Germany

The Central Eurq)ean Variscides are positioned between the Pre-Phanerozoic deformed cratonic region of Baltica in the north and the Gondwana Continent in the south. Finally all the tectonic processes in the Variscides are interpreted as interactions between both these continents during Palaeozoic time. The strip-like zonation of the Variscides was firstly discovered by KOSSMAT and is principally valid up to the present. The lithofacial development and facial distribution between these zones and their individual tectonomagmatic evolution hint to the interpretation of these zones as terranes. After the Cadomian consolidation, acting in two phases (approximately 670 Ma., 600-570 Ma.), the Phanerozoic evolution had started with a long term subsidence of parallel to the zonal distribution (SW-NE) aligned sedimentary troughs up to the Lower Carboniferous. Abrupt facies differentiations and extrusions of mafic rocks refer to strike-slip movements parallel to the trough alignments, especially during Devonian time. The periods of crustal stretching and the subsidence of sedimentary troughs are to subdivide in two periods in Cambro-Ordovician and Upper Devonian/Lower Carboniferous partly forming rift systems in back arc position with basaltic volcanism. Metamorphic peaks are to recognise during the Ordovician (480 Ma.) and in the Upper Devonian/Lower Carboniferous (330 Ma.). Remnants of Pre-Phanerozoic granulitic rocks are outcropping in the Saxonian granulite massif and refer to widespread distribution of granulitic facies in the middle and lower crust. Hercynian tectonics acted under the condition of an acid crust clarified by means of P-wave velocities. The crust beneath the Central European Variscides underwent deep long term crustal stacking processes producing increasing higher specific heat production as a source for generating acid melts in the crust. The suture zones of the Variscides suffered under increased deformation forming shear zones welding the neighbouring zones together as it was promoted by intrusions of granitic bodies. Gravitational steered nappe tectonics acted mainly in the outer zones of the Variscides in Eastern Germany and seems to be absent in the inner Moldanubian and Saxothuringian zones. Plutonic bodies of S-type granites were common products of crustal slacking and followed to the metamorphic peaks with a time gap of about 20-30 Ma.There are relations between the amount of melted material in the middle and upper crust and the intensity of the deformations in the upper crust. The Variscan metamorphic front propagated during Carboniferous from the Internides (Moldanubian zone) towards the Extemides (Rhenohercynian zone) with a time-span of about 20 Ma expressed by different ages of flysch sedimentation,late-postkinematic granites and the beginning of the Late Variscan inversion tectonics. Remarkable is the fact,that in the Moldanubian zone of the Hercynides gneisses were formed up to the Upper Carboniferous and simultaneously rigid strike-slip tectonics could act in the outer zones of the Variscides .That might be a question of vertical zonation of the structural processes. These dextral N\V-SE trending shear zones contribute to the crustal shortening especially in the Elbe zone the sedimentary fillings were very intensely

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shortened by iranspressionai shearing. The evolution of tectonometamorphic processes in seleciccl /ones will demonstrate in more detiiil. H A R Z Mts. (sw)

SAXOTHURINGIAN ZONE A C C R E T I O N /RHENOHERCYNIAN ZONE WEDGE f \f \y

tlOO #

/l50

200

250 km

BANKWITZ.1989 The figure is showing the geological interpretation of a N-S striking refraction seismic profile crossing the boundary between the Saxothuringian and Rhenohercynian zones of the Central European Hercynides. One of the most important and dominating feature are flat lying and toward SE dipping refractures which we interpret as thrust sheets belonging to the Hercynian stacking processes. Main thrust zones were formed in the welded collision zones between the Variscan terranes, also marked by increasing magmatic activities. The thrusting was directed toward the outer zone of the Hercynides (from SE to NW). The tectonometamorphic evolution took place as a retrograde polyphase process starting under amphibolite facies conditions and accompanied by the formation of partial anatectical melts and finishing under cataclastical conditions during the Late Hercynian compression in the Upper Carboniferous. Based on this concept the Harz Mountains form an accretion wedge with outcropping thrusts, gravitational nappes, intense deformations and upper crustal shortening. Younger steep normal faults, caused by Late Hercynian up to Mesozoic inversion tectonics restored the in Variscan time disturbed isostatic balance of the crust. These normal fault sets are contouring NW-SE shaped blockfields and were responsible for the generation of widespread hydrothermal circulation cells in the fundament and in the cover rocks of Mesozoic. These deep reaching fault activities have displaced even the MOHO-discontinuity.

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LATE CRETACEOUS AND CENOZOIC DEFORMATION OF THE MURCHISON BASIN NEW ZEALAND: TECTONIC HISTORY EVALUATION USING APATITE FISSION TRACK ANALYSIS. H.J. Gibson Earth Science Department, University of Melbourne, Parkville, 3052 Victoria, Australia

The Murchison Basin is located 60 kilometres east of Westport, in the Basin and Range Province, South Island of New Zealand. The Province is west of the boundary of colliding continental crust of the Australian and Pacific plates. From the Late Cretaceous to Early Miocene, rifting occurred in the region, opening the Tasman Sea and forming deep basins proximal to the New Zealand continent (Nathan et. al. 1986). Normal growth faulting is proposed to have aided basin subsidence at this time. In the Miocene (10 to 15 Ma ago), oblique compression commenced across the Alpine Fault, which forms the modem plate margin (Allis 1981). Compression, continuing to the present day, has resulted in considerable uplift on the New Zealand continent, particularly in the Southern Alps. In the Murchison Basin, compression has been accommodated by strong folding of basin sediments and by reverse faulting. The basin is now 100 kilometres long and up to 20 kilometres wide, trending NNE. 7000 metres of Eocene to early Quaternary sediment fill the deepest part of the basin, and a further 2000 metres is thought to have been eroded from above the preserved sequence. Forming basement to the basin, are Early Permian basic rocks, intruded by Early Cretaceous granitoids (outcropping in the east), and Carboniferous granitoids (outcropping in the wesO. Dominant folding styles observed in the basin sediments are tight, upright folds with NNE trending fold axes. Hexural slip and parasitic folding are frequently observed in fold limbs. Reverse faulting is generally steep, NNE trending and displays a component of dextral stike-slip movement. Relative displacement on major fault planes is difficult to determine in the field, but appears to be only tens or perhaps hundreds of metres. Where normal movement is observed, it is sympathetic with a compressional regime.

Scate 1:250 000

Probctad from 12 Km

0 Km ^ b«k>w 3 L 3 WWt» Cr99k FatM

TataW Fault

Figure 1. Sctwmatle section of the Murchieon Baein. Relative movement on the

Tutaki Faulta aa proposed by Suggate 1984 .

From previous mapping in the region, workers proposed two kilometres of normal (synrift) displacement on the Maunga Fault near the western margin of the basin, and three kilometres of reverse (syn-uplift) movement on the Tutaki Fault on the eastern margin of the basin (figure 1, Suggate 1984). While such magnitudes of normal

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faulting may be acceptable given the depth of the basin, field mapping alone cannot determine such displacements. Furthermore, the record of normal faulting has been overprinted by uplift and reverse faulting. Other techniques must be employed to determine differential uplift patterns throughout the basin and hcnce ihe amount of relative displacement across faults. In this study. Apatite Fission Track Analysis (AFTA) will be used in conjunction with seismic interpretation to provide insight into: 1. The burial, uplift and erosion history of the basin sequence. 2. The extent of normal growth faulting during basin subsidence. 3. The amount of syn-uplift movement on reverse faults. The technique, reliant on the temperature sensitive nature of fission tracks in apatite, uses apparent fission track ages and length data to reconstruct the palaeo-geothermal temperature profile (Gleadow et. al. 1983). It allows us to see that a section of crust has been hotter in the past and provides estimates of the time elapsed since cooling. If all heating and cooling can be directly contributed to burial and subsequent exhumation, temperatures can relate to depths and AFTA can also estimate timing, amount and rate of uplift and erosion. Preliminary results from this study include an AFTA age of 79±4.5 Ma from a west coast granite. This age probably indicates the sample has not been significantly heated since the Late Cretaceous. A basal sediment from the inverted Paparoa Basin, west of the Murchison Basin, gives a much younger age of 19± Ma and a high vitrinite reflectance of 1.4 %. The combined information indicates burial resulted in almost total resetting of the apatite age prior to uplift at around 19 Ma ago. Preliminary AFTA results from the Murchison Basin give a range; from reset ages in the centre of the basin, to preserved stratigraphic (or older) ages on the eastern margin of the basin. This trend of increasingly reset ages is supported by vitrinite reflectance values which increase from the margin to the centre of the basin. The preliminary AFTA results are encouraging because they indicate that a large palaeo-crustal profile is now exposed, due to differential uplift across the basin. Regional and local outcrop samples, and samples from two wells in the basin are therefore expected to provide a reliable insight into the tectonic history Murchison Basin. Refwnp^: Allis, R.G 1981. Continental underthrusting beneath the southern Alps of New Zealand, Geology, 9, 303-307. Gleadow A. et al 1983. FTA: a new tool for evaluation of thermal hist & hydrocarb potential. APEA 7. 23, 93102.

Nathan S. et al. 1986. (eds) Basin Studies 1. Sed. Basins of West Coast Region. NZ Geol. Surv. & Gcophys, DSIR. Suggate, R P. 1984. Mangles Valley Mapsheet M29 AC. 1:50 000. NZ Geol Surv, Govern. Printer, Wellington, N Z.

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TECTONIC EVOLUTION OF THE ERASER MOBILE BELT, WESTERN AUSTRALIA L. B. Harris and C. P. Delor* Department of Geology, University of Western Australia, Nedlands 6009 W.A., Australia.

The Fraser Mobile Belt is situated along the southeastern margin of the Archaean Yilgam craton in Western Australia within the Albany Fraser Province. The mobile belt may be subdivided into four domains based on consistency of lithology, deformational style, metamorphic facies and aeromagnetic expression : (i)

The Reworked Archaean Domain in the southwestern sector comprises granitic to granodioritic gneisses with isolated outcrops of acid and basic granulites, Archaean greenstone sequences, minor gabbroic dykes, meta-sediments and late-tectonic granitoids The boundary with the Yilgam Block is marked by ductile shear zones clearly visible on aeromagnetic imagery. Our re-defined Fraser Mobile Belt boundary is at considerable variance with Myers and Hocking (1988) who include this domain as part of the Yilgam Craton. Strong Proterozoic overprinting of Archaean gneisses (including foliation development in ductile shear zones) along with amphibolite facies metamorphism necessitates the inclusion of this domain in the Fraser Mobile Belt.

(ii)

The Granulite-Gneiss Domain comprises a --30 km wide, regional mylonite belt. Highly magnetic Stype granitoids and remnants of magnetite bearing granulites are responsible for the strong magnetic signature.

(iii)

The Fraser Complex comprises tectonically interleaved metagabbros, gneisses and metasediments. Both low- and intermediate pressure two-pyroxene basic assemblages may locally be juxtaposed, indicating significative vertical displacement along the intemal shear zones. Amphibolite facies metamorphism was contemporaneous with the tectonic emplacement of granulitic slices.

(iv)

The Orthogneiss-Granite Domain is an approximately 100 km wide area of syn- to late-tec tonic, Proterozoic S-type granitoids showing locally remnants of strongly deformed granulitic- and amphibolitic facies assemblage. Orthogneisses, migmatites and granitic sheets have been folded on a regional scale and intruded by late-kinematic granites. The Mount Ragged quartzo-feldspathic metasediments lie in structural continuity with the surrounding orthogneissic basement and define a major synformal structure.

An initial phase of ensialic rifting resulted in the intrusion of a gabbroid dyke suite and layered intrusions along northeasterly trending normal faults, basin development and sedimentation.

* present address:

D6panement cartes et syntheses gcologiqucs, BROM, 45060 Orleans, Francc. - 24 .

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Dcxtral transpression along the Yilgam craton margin is the dominant, mid-Proterozoic tectonothemmal event in the Fraser Mobile Belt, pre-dating deformation in the adjacent Albany Mobile Belt. Archaean greenstone belts in the eastern Yilgam Block may be traced into the mobile belt where they have been displaced dextrally by --SOkm. Sinuous foliation trends also attest to dextral displacements along the craton margin. Microstructures in mylonites of the Granulite-Gneiss Domain also indicate dextral movement. Gabbros, metasediments and gneisses of the Fraser Complex have been tectonically interleaved during the early stages of this event, forming a structure analogous to a positve flower structure. The Fraser Complex has then acted as a rigid indentor, deforming ductile shear zones on the craton margin during its progressive translation to the SW. Westerly directed thrusts and tight folds with N-S trending axial surfaces formed during this event in the southern reworked Archaean domain away from the influence of the rigid indenting body. Sediments of the Mount Barren Group may have been deposited in a pull-apart basin during this event NE-SW shear zones are either cut by sinistral ductile shear zones and /or are reactivated with reverse movement during the second tectonothermal event in the Fraser Mobile Belt contemporaneous with dextral transpression in the Albany Mobile Belt (at -1.1-1.2 Ga). Near the junction of the two mobile belts, progressive foliation and fold development indicates NW thrusting of metasediments of the Mount Barren Group onto the Yilgam Craton. Granitoids were emplaced in the late stages of this event, recording varying degrees of deformation. The Fraser Mobile Belt was cross-cut by regional brittle and brittle-ductile shear zones during the early Cambrian. Field exposures show that NE-SW striking foliations are reactivated or cut at low angles by cataclasite shear zone arrays with dextral displacements. Dolerite dykes bisect conjugate shear zones; a change in trend of dykes from the mobile belt into the Yilgam craton reflects a refraction of stresses across the craton margin.

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EASTERN PAPUA NEW GUINEA: PALEOGENE SHEARING AND UPLIFT OF ECLOGITES AND ASSOCIATED MET AMORPHIC ROCKS IN AN EXTENSIONAL SETTING. E.J. Hill Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia

Goodenough and Fergusson Islands, in eastern Papua New Guinea, are made up of a number of mountainous domes (fig. 1) of complexly deformed metamorphic rocks, including gneisses and migmatites. This metamorphic basement has been intruded by large bodies of granodiorite and lies in fault contact with overlying mafic and ultramafic rocks believed to be part of the Papuan Ultramafic Belt. The domes are bounded by curved shear zones dipping outwards from the centres of the domes at angles of around 0® to 45^. The outer shear zones are up to 1000 m in width. These dome-like structures are similar to the metamorphic core complexes found in the western United States Cordilleran region. The metamorphic basement can be divided into two structural zones: a core zone in the centre of the domes and an outer sheared zone (the area affected by the dome-bounding shears). The core zones contain a gneissic layering which is overprinted by a foliation and fold forming event. Deformation in the outer sheared zones is intense and overprints the core zone structures, all earlier fabrics have been rotated into parallelism with the mylonitic fabric in the outer sheared zone. At least three sets of structures can be recognised using overprinting criteria, these probably represent an evolving sequence of structures developed during uplift of the basement. The deformation becomes progressively more localised and changes from ductile to brittle in nature. The outer sheared zones around each dome can be divided up into a number of intersecting shear zones on the basis of the orientation of the shear zones and shear zone fabrics, including a well developed mineral lineation. Within each of these shear zones the orientation of the mineral lineation is consistent in orientation throughout the sequence of shear zone development, i.e. in the broad shear zones as well as later narrow shear zones which overprint them; in addition, slickenslides found on a late fault are parallel to the mineral lineation in the adjacent shear zone. Movement indicators on both the wide and narrow shear zones indicate a complex movement geometry which does not fit the models proposed by earlier workers. The geometry and kinematics of the shear zones indicate that movement on the shear zones could have accommodated extension of the crust and localised uplift of the domes of metamorphic rock. In addition, many of the shear zones appear to have acted as transfer zones. The rocks in the cores of the domes have experienced peak metamorphic conditions of eclogite facies, these rocks have undergone retrograde metamorphism in the outer sheared zones to amphibolite and greenschist facies. The application of geothermometers and geobarometers to the rocks of the core zone and shear zones allows the uplift and thermal history to be related to the deformation of the metamorphic rocks. The most extreme metamorphic conditions experienced by the basement rocks are preserved in the eclogites from the core zone. Eclogites may occur as dykes which cross-cut the core zone gneisses, these show no evidence of having been tectonically introduced into the gneisses, hence metamorphic conditions calculated for the eclogites are believed to also represent the peak metamorphic conditions attained by the gneisses. Highest minimum pressures calculated for eclogites from the core zone are around 21 kbar (using the methods of Holland, 1980 and Gasparik & Lindsley, - 26 .

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1980). Temperatures for co-existing clinopyroxene and garnet were calculated at 730-9(X)''C (using the method of Ellis & Green, 1979). Highest grades in shear zone rocks indicate equilibration at temperatures of around 570730^C (co-existing garnet and biotite, Ferry & Spear, 1978) and pressures of around 10 kb (using the following barometers: GASP, Ghent, 1976 and Anovitz & Essene, 1987; GRIPS, Bohlen & Liotta, 1987 and Anovitz & Essene, 1987; Hoisch, 1990). The results indicate that the earliest stages of uplift from at least 70 km up to around 35 km were accompanied by a decrease in temperature. This first stage of uplift encompasses deformation in the core zone which post-dates the gneissic layering, until the beginning of deformation in the outer sheared zones. Subsequent metamorphic reactions occuring during deformation in the outer sheared zones include the development of hercynite (Fe-rich spinel) in shear zone rocks and a change from the kyanite to the sillimanite stability field in aluminous rocks. When plotted on a PT diagram these reactions indicate that temperatures increased during later stages of uplift. Field observations indicate that large bodies of granodiorite were intruding the basement rocks during deformation in the outer sheared zones, hence this thermal event can be related to the intrusion of these granitic bodies. It is proposed that during the later stages of uplift of the D'Entrecasteaux region crustal extension associated with the Woodlark Basin seafloor spreading system initiated deformation in the outer sheared zones, resulting in late stage uplift being localised into domes ("metamorphic core complexes"). Furthermore deformation in the outer sheared zones was accompanied by the introduction of large bodies of granitic melt into the crust resulting in overall heating accompanying uplift and deformation. This timing implies that the introduction of granitic melt into the crust was also triggered by the onset of extension. dome-bounding fault

late granodiorite J

cover rocks

early granodiorite

north

5 km

core zone outer sheared zone

Figure 1: Cross-section from north to south through the Goodenough Island dome.

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SEISMIC WAVE DISPERSION AND ATTENUATION IN AHEIM DUNITE : AN EXPERIMENTAL STUDY I. Jackson, M.S.Paterson and J.D. Fitz Gerald Research School of Earth Sciences, The Australian National University, GPO Box 4, Canberra ACT 2601, Australia

A machine has been developed which provides for the study, by observation of forced torsional oscillations, of both shear modulus dispersion and internal friction in geological materials at low frequencies (10 - 1000 mHz) and stram amplitudes (<10"^) under conditions of high pressure (to 300 MPa) and temperature (to > 1000°C). This equipment has been utilised in work on an olivine-rich rock from Aheim, Norway, directed towards the eventual understanding of the dispersion and attenuation of seismic shear waves in the Earth's upper mantle. Measurements have been made on cylindrical specimens of this rock which contains, in addition to olivine, about 10% pyroxene and 5-10% of hydrous silicate phases dominantly clinochlore, serpentine and talc. The specimens were either previously fired under controlled oxygen fugacity at 1200'^C for 24 hours in order to effect complete dehydration within the olivine stability field, or simply oven-dried. Linearity of the mechanical behaviotu* has been demonstrated by the amplitude independence of the results for the strain amplitude range 10"^ -10"6, and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the Kramers-Kr5nig relations of linear theory. At room temperature, dispersion of the shear modulus G does not exceed 0.2% between 3 and 100 s oscillation periods, and the modulus increases markedly with increasing pressure for all specimens presumably in response to the suppression of crack porosity. The relatively low and strongly anisotropic shear moduli of the oven-dried specimens are explained semi-quantitatively by the presence of the highly aligned sheets of clinochlore, a layer silicate phase with very low resistance to shear across its basal plane. Similarly low moduli and pronounced elastic anisotropy in the prefired specimen are attributed to the presence of lens-shaped voids which open normal to the clinochlore basal plane during its dehydration. For all specimens, the internal friction Q-1 decreases with increasing pressure up to 200 MPa, thereafter becoming pressure independent at a value near 0.001. For the oven-dried specimens tested below 600''C, the temperature derivative of the shear modulus dO/dT is indistinguishable from the 'intrinsic' value expected for an ideal olivine aggregate of zero porosity. At higher temperatures, the temperature dependence of G is more marked (fig. 1), reflecting the creation of porosity by dehydration in situ. For the prefired material, IdG/dTI is less than the intrinsic value throughout the temperature range, presumably as a consequence of the reduction of porosity by sintering during mechanical testing. The internal friction behaves similarly for all specimens whether oven-dried or prefired, suggesting that the anelastic losses are strongly concentrated within, or less probably at the boundaries between, the olivine grains. Q'^ is mildly frequency dependent, increasing with increasing oscillation period TQ at 300 MPa and 1000°C approximately as To^ with a values of about 1/6. Q"^ increases approximately exponentially with increasing temperature (fig. 2) from 0.001 at 25X and 300 MPa to about 0.01 and 0.02 at lOOO^C, 300 MPa and 3 and 100 s oscillation period, respectively. These results suggest that solid-state anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the Earth's upper mantle, although the mechanistic basis for the observed anelasticity remains to be established.

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Fig. 1 Shear modulus versus temperature for Aheim dunite.

100 03

'intrinsic' (F0IOO)

(D (O

oven-dried, || foliation -

•D O

prefired, L foliation

oven-dried L foliation

05 CD

sz CO

Berckhemer et al j_L

•

500

•

1000

1500

2000

Temperature / C Fig. 2 Internal friction versus temperature. 1 Berckhemer et al

Aheim dunite

.deformed

OJ c

undeformed'

Forsterite (single crystal) • Gueguen el al

.001 500

1000

1500

Temperature / C

2000

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SEISMIC IMAGING OF EXTENSIONAL AND CONTRACTIONAL PHASES IN THE BOWEN AND SURAT BASINS, EASTERN AUSTRALIA R.J. Korsch, K.D. Wake-Dyster and D.W. Johnstone Onshore Sedimentary & Petroleum Geology Program, Bureau of Mineral Resources, GPO Box 378,Canberra, ACT 2601, Australia

The Bowen and Sural basins in eastern Australia form a sedimentary basin system that was initiated in the latest Carboniferous or earliest Permian and in which sediments were deposited until the Late Cretaceous. The basin system has had a complex evolutionary history, with early phases of extension and u-anstension being followed by later phases of contraction and transpression. The geometries of the phases are best observed in selected industry seismic data and in long regional BMR deep seismic reflection profiles recorded across the basins in 1984 and 1989. Seismic data from the Denison and Arbroath troughs along the western margin of the Bowen Basin indicate that they were initiated in the Early Permian as extensional half-graben bounded by north to north-northwest striking faults. The polarity of the half-graben change along strike, with the bounding faults in the Denison Trough being mainly in the west and those in the Arbroath Trough being mainly in the east. Farther east, the Taroom Trough is the main depositional centre. In the vicinity of seismic line BMR84.14, the Taroom Trough also displays a half-graben geometry but only a limited amount of extension (20 km maximum) can be accommodated on the steep bounding fault at its eastern margin, which is the northsouth striking BurungaLeichhardt-Moonie-Goondiwindi Fault. This trough is considered to have developed by oblique extension (? transtension) with the bounding fault having a significant strike-slip component. Reactivation in the (?) mid Triassic of the bounding faults in the Denison Trough has led to tectonic inversion and the development of contractional structures. At the same time, to the east of the Denison Trough in the central Bowen Basin, major shortening was accommodated by thin-skinned thrusting on a series of lisu-ic thrust faults that dip to the east. These thrusts root in a major east-dipping detachment that appears to flatten in the middle crust. These features are displayed in seismic line BMR89.B01. Three previously recognised tcctonic zones, the Blackwater and Yarrabee zones and the Dawson Fold Zone (Hobbs, 1985) exhibit distinct structural styles in response to the shortening. The Blackwater Zone has displacements of up to 5 km along bedding-plane thrusts and is dominated by subhorizontal reflections. The Yarrabee Zone is characterised on the surface by dome and basin structures. The seismic data indicate that the structures are controlled by listric thrust faults that have an imbricate fan geometry. The Dawson Fold Zone consists of tightly folded Late Permian sedimentary rocks at the surface. Because of the steep dips, reflectivity is low but faults with moderate easterly dips occur, suggesting that the folds on the surface are the upper level response linked with thrusting at depth. The major east-dipping detachment defines the base of this zone. In the Taroom Trough in the vicinity of seismic line BMR84.14, Jurassic sediments of the Surat Basin are affected by structures such as thrusts, folds and positive flower sU^uctures. Compared to the central and northern Bowen Basin, the zone of deformation is extremely localised and is confined to the vicinity of the basin - 30 -

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bounding faults. These structures are interpreted to result from transpression associated with reactivation of the strike-slip faults. Thus seismic data from the Bowen and Surat basins provide excellent images of early extensional and transtensional structures and also of the later contractional structures in the Denison Trough and central Bowen Basin and the transpressional structures associated with the major bounding faults in the east. Refgr^ngg: Hobbs, B.E., 1985. Interpretation and analysis of structure in the Bowen Basin. Geological Society of Australia, Abstracts, 17, 151.

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THE STRUCTURAL STATE OF A FORMER LOWER CONTINENTAL CRUST (NORTHERN SERRE, CALABRIA, SJTALY) J.H.

Kruhl

Geologisch-Palaontologisches Institut, UniversitSt Frankfurt, D-6000 Frankfurt/M.ll, Germany

Lack of penetrative overprint allows to reconstruct the structural state of the Calabrian granulite facies rocks, as it has been during the former lower crustal position of these rocks. The micro structures are mainly influenced by high-T defamation as well as by rerecrystallization and recovery processes. Ortho- and clinopyroxene as well as cordierite recrystallize at minimum conditions of 5-6 Kbar and TCWC. Basis parallel subgrain boundaries of quartz are developed in lower levels of the lower crust and disappear toward higher levels at about 730°C. This marks the minimum conditions of dominance of prism-c glide. Preferred mineral orientations indicate the long axis of the paleostrain ellipsoid which shows constant NNE-SSW orientation during prograde high-T metamorphism as well as throughout the entire crustal section. Shear indicators during prograde metamorphism are rare or even absent, pointing to more or less co-axial flattening conditions. It may be speculated that this indicates continuous subsidence of the rocks from higher to lower crustal levels rather than a tectonic transport (Kruhl & Huntemann 1991). Geophysical investigations during the last decade have shown that commonly the lower continental crust is highly reflective, but the reasons are still under debate (Percival & Berry 1987). In the former lower crustal position of the Calabrian granulite facies rocks compositional layering of felsic granulites and sillimanite-garnetrich metapelites with strong seismic velocity differences (Kern & Schenk 1985) could cause reflectivity. Deformation structures have probably not caused the reflectivity. Neither shear zones of prograde metamorphism with a clearly higher texture intensity than neighbouring rocks and zones of superplasticity with a lower texture intensity have been detected, nor occur strong differences in strain orientation and, therefore, in texture orientation between neighbouring layers of the same or different composition. Moreover, plagioclase, the most abundant rock constituent, shows fairly weak preferred crystallographic orientations with a corresponding small effect on velocity anisotropy and shear wave splitting (Siegesmund & Kruhl 1991). The situation might be different, however, if lower crustal rocks aquire structural anisotropy on different scales during tlieir uplift to upper crustal levels. Refgrepces: Kern, H. & Schenk, V. 1985. Elastic wave velocities in rocks from a lower crustal section in southern Calabria (Italy).- Phys.Earth Planet Int., 40, 147-160. Kruhl, J.H. & Huntemann, T. 1991. The structural state of the former lower continental crust in Calabria (S.Italy).- GeolRdsch., 80/2. Percival, J.A. & Berry, M.J. 1987. The lower crust of the continents. In: K.Fuchs & C.Froidevaux (eds.), Composition, structure and dynamics of the lithosphere-asthenosphere system.- Geodynamics Series, Vol.16, 33-59, Am.Geophys.Union. Siegesmund, S. & Kruhl, J.H. 1991. The effect of plagioclase textures on velocity anisotropy and shear wave splitting at lower crustal levels.- Tectonophysics 191.

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EVOLUTION OF MICROSTRUCTURES AND PETROFABRICS IN CALCITE MYLONITES FROM NEWFOUNDLAND. B. Lafrance School of Earth Sciences, Macquarie University, 2109, Sydney, New South Wales, Australia

An horizon of Ordovician limestone, which outcrops on Farmer's Island in north central Newfoundland, has been mylonitized at temperatures and pressures less than 500"^ C and 3.5 Kbars, respectively. The mylonite zone is 30 m wide on western Farmer's Island, but is only 30 cm wide on the eastern coast of the island. Protomylonites and mylonites occur on the west coast of the island whereas only ultramylonites are found on the east coast. A shear foliation parallel to the local stratigraphy is observed on the field. In the protomylonites and mylonites, the shear foliation is defined by elongated grey porphyroclasts surrounded by a laminated recrystallized white to grey matrix. In the ultramylonites, the porphyroclasts are completely recrystallized and the shear foliation is a strongly differentiated foliation of alternating black, grey, and white layers. The orientations of crystallographic C-axes of recrystallized grains and porphyroclasts were measured separately on two sections perpendicular to the shear foliation using a conventional U-stage. The crystallographic preferred orientations (CPO) of r-planes in the mylonites and ultramylonites were measured using the Starkey X-ray Fabric Camera, and the orientations of a-axes were calculated based on the preferred orientation of the r-planes. Slip systems active during deformation were determined by transmission electron microscopy. During mylonitization, the porphyroclasts underwent both deformation and recrystallization. Deformation occurred by twinning, microfracturing, and intracrystalline slip. Recrystallization occurred dominantly by twin boundary migration and rotation recrystallization. Twins and fractures are rare in the recrystallized grains. This indicates that stress concentrations along grain boundaries were relieved by grain boundary sliding. In the mylonites and protomylonites, recrystallized grains are equant, and show dihedral angles of approximately 120^ between adjacent faces. These rocks underwent dynamic recrystallization in a regime dominated by slow rates of grain boundary migration, and progressive misorientation of subgrains. By contrast, the ultramylonites underwent recrystallization in a regime dominated by fast grain boundary migration. The recrystallized grains have irregular boundaries, and define an oblique grain shape fabric that is oriented anticlockwise with respect to the shear foliation. In the protomylonites and mylonites, recrystallized and porphyroclasts have similar C-axes CPO. The C-axes CPO form a diffuse concentration normal to the shear foliation. The C-axes CPO of porphyroclasts is stronger than the C-axes CPO of recrystallized grains because 1) strongly twinned porphyroclasts are removed by recrystallization, and 2) rotation recrystallization and grain boundary sliding has weakened the CPO of recrystallized grains. In the ultramylonites, the C-axes of recrystallized grains form a strong point maximae which is oriented clockwise to the normal to the shear foliation. Poles to r-planes have a small circle distribution with two strong point maximae. The a-axes form a strong point maxium that is contained within the two great circles normal to the two r-planes point maximae. This suggests that a-slip in r-planes was important during the - 33 -

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deformation of the recrystallized grains. This is consistent with results obtained by electron microscopy where bi slip (i.e. a-axes) and b3 slip on r-planes were determined by contrast analysis. C-axes CPO of recrystallized grains and porphyroclasts are similar to the low temperature experimental fabrics that have twinning as the dominant deformation mechanism. However, twins are rare within the recrystallized grains, and intracrystalline slip accompanied with grain boundary sliding are the dominant deformation mechanisms. The ultramylonites have stable end C-axes CPO. Their fabric is equivalent to high temperature experimental falMics where metastable C-axes orientations were removed by dynamic recrystallization. Although the asymmetry of natural petrofabrics with a strong point maximum have been compared to experimental low temperature petrofabrics to determine the sense of shear, this can only give erroneous results since most low temperature natural petrofabrics correspond to high temperature experimental petrofabrics with intense recrystallization.

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HASTINGS TERRANE EMPLACEMENT AND ASSEMBLY OF THE NEW ENGLAND OROGEN P.G.

L e n n o x ^ , J. R o b e r t s ^ R. O m e r ^ and P. Schmid^ 1 Department of Applied Geology, UNSW ^Department of Geology, University of Newcastle ^Division of Exploration Geoscience, CSIRO

The Hastings Terrane (HT) in the southern New England Orogen, which consists of fault-bounded fore-arc sequences, is surrounded by accretionary complex rocks in the Nambucca, Yarrowitch and Port Macquarie Terranes. The emplacement of the HT into this anomalous position provides constraints on the timing and nature of tectonism in the Southern New England Orogen. Questions to be addressed include; where did this terrane originate?, when did it reach its present position?, how did it get there, and does it contain distinct subterranes?. The HT contains Late Devonian to Early Carboniferous deepwaler facies which change in the Late Carboniferous to shallow marine to continental coal-bearing rocks. Within the Late Carboniferous facies variations indicate deep water to the northwest, north and east in contrast with the generally accepted easterly direction for the Tamworth Terrane. In the north and northeast there is a hiatus before deposition of local conglomerates and then regionally extensive but thin limestones in the Early Permian, but in the west the succession may be conformable with Permian rocks. The cold water limestones are overlain by an overlap assemblage contiguous with the Nambucca Trough in the north, consisting of siltstone and diamictitie. Modal analyses of the Late (?) Devonian to Early Permian arenites indicate an arc setting for the source of these rocks. Late (?) Devonian arenites are feldspar-rich. Carboniferous arenites become more lithic-rich with time and Early Permian arenites are quartz-rich, particularly those in the upper part of the sequences. The increasing quartz content probably reflects the stripping back of the volcanic arc and a quartz component from the accretionsubduction complex. Overall the Tamworth Terrane sequences exhibit a similar trend for rocks of the same age range, although in detail the actual path is somewhat different. The northern HT is a dome caused by interference between D i and D5 folds, with Triassic granites having intruded the sub-domes within the main structure. The upright folds and poorly developed slaty cleavage contrasts with the structures within Nambucca Terrane to the north which was subjected to the same Late (?) Permian deformation and is characterised by occasional overturned folding, excellent crenulation cleavages and many mesoscopic folds. Most of these differences in structural style can be attributed to the more silty character of the Nambucca Terrane rock sequence compared with the highly siliceous, mudstone-poor character of the HT sequences. The middle and southern parts of the HT have a distinctly different structural history compared with the north. The Yarras Suture Zone between the Hastings and Yarrowitch Terranes is defined by numerous serpentinite slivers which contain S and C microstructures and shear band fabrics suggesting sinistral strike-slip and late stage dextral strike-slip faulting. Petrographic studies and K-white mica crystallinity ("illite crystallinity") determinations indicate that the grade is low (zeolite to prehnite-pumpellyite facies) in the HT.

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Lower greenschist facies assemblages also have been noted in rocks within one fault bounded block, bo cell parameters of the K-white micas show that a moderate to high T/low P imprint occurs in the Late Devonian (?) Carboniferous rocks, contrasting with the higher P imprint in Early Permian Kempsey Beds in the north. In the southern HT, bo values of detritial micas in Early Permian shales and siltstones, suggest that the accretionsubduction complex sequences to the east were the source of the detritus for these rocks. Non-buffered assemblages in Late Devonian meta-basalts and sandstones, indicate P = 150 - < 300 MPa and T = 225 - 340^ C. Overall, the grade in the rocks from the Hastings and Tamworth Terranes is similar, suggesting a common origin for them. However, they show slightly different metamorphic imprints, the reasons for which are not yet undCTStood. Preliminary palaeomagnetic studies have been carried out on the Late Carboniferous Majors Creek and Kullatine Formations, Early Permian Yesabah Limestone, microdiorite and serpentinite. Samples of Kullatine Formation from near Mooneba on the Macleay River revealed both polarities of remanence, indicating that an ancient geomagnetic direction has been isolated. The in situ direction has an easterly declination and a moderate upward inclination. If the remanence is secondary and post- folding in age, it is difficult to reconcile with known directions from any other Australian rocks of Late Carboniferous or younger age. The declination is anomalous and suggests up to 50"" dextral rotation. If this remanence predates the tectonic folding, which would necessitate correcting the direction before calculating a palaeomagnetic pole, up to 130"^ of dextral rotation is suggested. This rotation would be consistent with models suggesting a change from a Late Carboniferous convergent margin to an Early Permian margin subject to dextral transpression. Thin skinned tectonic models for the HT like those being advocated for the Lachlan Fold Belt require detachment at 8-10 km depths. This model seem inconsistent with the upright, symmetric, modified dome and basin folding, sub-vertical faulting and regionally extensive diagenetic alteration and lower prehnite-pumpellyite facies metamorphism, however similar features are present in the Ballarat Terrane and this has been shown to be detached at a similar level from its basement. The HT structures, metamorphic signature and preliminary palaeomagnetic results seems more likely to be consistent with thick skinned deformation. It is envisaged that the HT was originally contiguous and east of the existing Tamworth Terrane. During the Late Carboniferous Early Permian it was translated approximately 150 km with up to 130"" of dextral rotation during dextral transpression. The New England Orogen was evolving from a convergent margin to a transpressional margin during the Late Carboniferous - Early Permian. After emplacement the HT was overlapped by Early Permian sediments of the Nambucca Terrane, polydeformed in the Late (?) Permian and intruded by granites in the Triassic.

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MAGNETIC FABRIC IN THE MID-CAMBRIAN ROCKS OF THE CENTRAL FLINDERS ZONE: APPLICATION TO REGIONAL STRUCTURAL AND TECTONIC ANALYSIS Z.X. Li and C.McA Powell Department of Geology, The University of Western Australia, Nedlands WA 6009, Australia

A magnetic fabric study of the mid-Cambrian sedimentary rocks in the Central Hinders Zone of the Adelaide Fold Belt, South Australia, indicates that the tectonic strain level in the clastic rocks ranges from being undetectable to the pencil-structure stage. Most of the clastic rocks investigated have a well-developed magnetic lineation. A negative fold test on a soft-sediment fold suggests that the magnetic lineation is not of depositional origin. Rather, it is interpreted as the result of two interfering generation of magnetic fabric: one is a compactional bedding-parallel magnetic foliation, and the other is a magnetic foliation of tectonic origin, defined by the girdle distribution of the site-mean lineation directions. This interpretation suggests that as well as ENEWSW directed compression producing the regional NNW-trending folds, there was also a phase of NNW-SSE directed tectonic compression in the Central Flinders Zone during the Delamerian Orogeny. Two phases of regional tectonic compression directing 90® apart have been previously proposed to interpret the structure of the Adelaide Fold Belt (e.g. Preiss 1987). Alternatively, the two phases of tectonic compression in the Central Flinders Zone, as well as the arched structure in the southern half of the Adelaide Fold Belt, could have been caused by local stress fields reoriented by basement shear zones in a southward migrating, but consistently oriented regional NW-SW tectonic compression. R^fgrgncos: Preiss, W.V. 1987. Tectonics of the Adelaide Geosyncline. In: The Adelaide Geosyncline - Late Proterozoic Stratigraphy, Sedimentation, Palaeontology and Tectonics (compiled by Preiss, V.W.) Bulletin of the Geological Survey of South Australia, 53, 255-281.

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STRUCTURAL AND THERMAL HISTORY OF BLUESCHISTS FROM THE NORTH D'AGUILAR BLOCK OF SOUTHEAST QUEENSLAND: IMPLICATIONS FOR TECTONICS OF THE NEW ENGLAND OROGEN. T.A. Little^, R.J. Holcombe^ G.M, G l b s o n 2 , R Offler^, P.B. Gans^ and M.O. McWilliams'^ ^Department of Geology and Mineralogy, University of Queensland, QLD 4072, Australia ^University College of Southern Queensland, Toowoomba, QLD 4350, Australia ^Department of Geology, University of Newcastle, Newcastle, N.S.W. 2308, Australia ^School of Earth Sciences, Stanford University, Stanford, California 94305 U.S.A.

Most pre-Permian rcKks east of the Peel-Yarrol fault system are weakly metamorphosed, graywacke-rich assemblages that are thought to have been offscraped against an east-facing subduction complex in mid to Late Carboniferous time. Located just north of Brisbane, the North D* Aguilar block (NDB) occurs as a metamorphic culmination in this subduction complex, and contains the largest exposure of blueschists yet described in Australia. The NDB is an arch consisting of several nappes bounded by low-angle faults. A lower plate of blueschistgreenschist transitional facies rocks form the core of the arch. These rocks include metabasaltic flows and fragmental rocks (Rocksberg greenstone) and siliceous metasediments. Mafic Mi assemblages include crossite, epidote, albite, chlorite, phengite, sphene, ± quartz, ± garnet, ± Na-pyroxene (-20 mole % jadeite). Preliminary geobarometry indicates pressures of over 6 kb at temperatures of over SSO^'C. Phengite in 4 pelitic samples has a mean Kubler index of 0.23 (epizone conditions) and a mean bo of 9.044, indicative of high pressures. Mi minerals define an originally steeply dipping foliation, Si. This fabric was pervasively crenulated and retrogressed during a middle greenschist facies overprint, M2. This overprint was synkinematic with a gently dipping foliation, S2 that is warped across the lower plate to define a broad foliation arch. A carapace of metamorphosed serpentinite-matrix melange caps the lower plate. Blocks in this ophiolitic melange include harzburgite, garnet amphibolite, blueschist and greenschist facies metabasalt and metagabbro, pelitic schist and metachert, and marble. A low-angle fault, the Mt. Mia fault, separates coarsely crystalline schists from an upper plate of very low-grade metamorphic rocks. The upper plate consists chiefly of broken formation in which lenses of chert and siltstone are encased in an argillaceous matrix. These rocks are cut by a single, steep slaty cleavage, S], and distended by brittle normal faults. Illite crystallinity and bo studies on 8 pelitic samples indicate anchizone conditions (mean Kubler index = 0.30) and moderate pressures (mean bo = 9.029). Subordinate pillow basalt is metamorphosed in the pumpellyite-actinolite facies. A fault slice of foliated and locally mylonitic biotite granodiorite with a synkinematic amphibolite facies aureole rests in brittle fault contact above the broken formation unit. Middle Triassic andesite lavas unconformably overlie all of the above metamorphic units and are probably correlative with Early-Middle Triassic strata in the Esk Trough to the west. The Mt. Mia fault is interpreted as a westward-dipping normal fault that juxtaposes an underplalcd occ^inic massif from an higher level fragment of the subduction complex. Evidence for this interpretation includes ihc contrast in . 38 -

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metamorphic grade and structural style across the ML Mia fault, a marked eastward thickening of the serpentinite and broken formation units, and top-to-the west kinematic indicators in mylonitic fabrics below the Mt. Mia fault. The Brisbane-Eromanga seismic reflection transect, shot just south of Brisbane, has imaged a westward dipping boundary between a thin, seismically transparent upper plate (South D'Aguilar and Beenleigh blocks) and a strongly reflective, layered lower plate (Greenbank layered sequence) that Finlayson et al. (1990) interpret as a thrust fault. An alternate explanation consistent with our surface mapping is that this feature is correlative with the Mt Mia fault. High-precision ^^Ar/^^Ar step-heating experiments on 6 samples of phengite from blueschists and (retrogressed) greenschists from the northern part of the NDB all yield plateau ages between 296 and 300 Ma, with four of the plateaux in the range of 298 to 299 ±1-2 Ma (2a). Two hornblende separates from the granodiorite's aureole yield plateaux of 306 ± 2-4 Ma. Muscovite in the pluton yielded steps as old as 295 ± 1 Ma, and orthoclase a high-temperature plateau of 290 ±1.7 Ma. These data are interpreted as follows. Schists in the lower plate cooled below '-330'^C after the peak of the M2 metamorphic event at about 298 Ma. This event probably corresponds to their partial exhumation along the Mt. Mia fault. The granodiorite was intruded into the lower plate near the peak of the M2 event at --306 Ma, and cooled from -500 to '-330''C at '-20''C/Ma. Later eastdirected thrust faulting emplaced the granodiorite above the upper plate in pre-Middle Triassic time. The high P/r, Ml event must be older than than 306 Ma. Exposure of the NDB had occurred by Early Permian time, as foliated granitic and metamorphic pebbles are present in the Marumba beds along the faulted margin of the NDB. The Marumba beds are probably an early fill of the Esk extensional basin. A major change in tectonic setting of the New England orogen occurred in Late Carboniferous-Early Permian time. This transition included a major eastward out-stepping of the subduction zone, and intrusion of S-type granites within the fossil subduction complex, which became a site of backarc extension and magmatism. Data from the NDB suggest that influx of heat into the subduction complex had begun by about 306 Ma, and that this metamorphic event was synkinematic with crustal extension.

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DILATIONAL MACROSTRUCTURES ("CARBONATE DILAFORMS") ASSOCIATED WITH CRATON-MARGIN FAULT SYSTEMS. B.W. Logan Department of Geology, The University of Western Australia, Nedlands, W A 6009, Australia

The term "dilaform" is applied to decameter- to kilometer-size carbonate masses that are composed dominantly of veined rock, vugular rock and dilational breccia, plus cavity-filling materials. The structures are the result of inflation that occurs as a consequence of interrelated dilation, fluidisation and injection processes. Dilational lithogenetic systems also include precipitation, exhalation, fractionation and mineralisation processes which are significant in determining dilaform comj)osition and dynamics. Inflationary growth requires a negative geopressure balance wherein fluid pressure exceeds solid pressure components. Such deviatory pressure balances are associated with fault systems and develop either as a result of endogenous processes or by pressure transmission from exogenous sources. Distinction between endogenous and exogenous inflation is important since there are different flow patterns and hence different potentials for import of solute, slurries and gas. Inflationary growth reflects volume increase and mass gain due to dilation, injection and precipitation. Long-term preservation of the inflated condition requires stabilisation by introduction of cavity-filling material (various precipitates, injectates, hydrocarbons, sulphides). States of partial inflation are maintained when materials settle into partly supportive configurations of asperite, propped, vugular and fenestral fabrics. Fixed inflation may be demonstrated as 50% to 80% of precursor volume and can exceed it by orders of magnitude when there have been multiple dilations. Such super-inflated dilaforms consist mostly of introduced material and fragments of cavityfilling materials. Lithofacies gradations into a dilational domain evolve concurrently with inflation: PRECURSOR Layaed Limestone Dolostone C-breccia Stylobreccia

D-FACIES 1 Residual precursor Fenestral rock (ii-mm) Vugular rock (mm-da) Veined rock (mm-he) (with host support)

D-FACIES 2 Vugular rock Veined rock D-bieccia Injectate Res. precursor

D-FACIES 3 Injectate D-breccia D-breccia (injectate)

The lithofacies gradation is modified by precursor susceptivity and local structural anomalies and is further complicated in cases involving multiple dilational events. Dilaform shape is related to stress state and also is influenced by media features:- layer boundaries; faults and joints; and differentials in strength and permeability. Other determinative factors arc the event paraniclcrs:geopressure balances; spatial and temporal pressure gradients;delivery portal geometry; pressure seals and injectate supply. The evolving geometric form trends from lensoid through hemispheroidal to cylindroidal. There is additional impetus for upward growth if significant quantities of CO2 and/or methane are exhaled during inflation and propagation rates also are accelerated in near surface levels. - 40 -

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Dilaforms stand out as lithologically distinct idens in host facies but the boundaries arc highly variable as functions of complex interactions between factors that include host lithology and structure; growth mode and stage; presence and configuration of pressure seals; and later deformation. Sealed growth is within encapsulating high-tensile, impermeable materials. It involves compression of seal materials and boundaries are therefore sharply demarcated with transition across narrow zones characterised by crowded stylolites, shear fractures, drag folds, laminite and breccia. There usually is thinning and wedging along the flanks and consequent dip steepenings. Unsealed growth occurs as the dilational environment propagates through responding materials (with permeation, intersertion and mobilisation stages) and boundaries involving lithofacies gradation (above) are set by fluid-pressure gradients, tensile strength and material supply. Concurrent inflation and lithologic transformation is reflected in erasure of precursor sedimentary, and other layering. Thus precursor structures (eg. folds) defined by such surfaces fade into dilational zones. Dips on residual surfaces and structural axes also steepen and this can result in reversals of dip and plunge even as precursor macrostructure fades. A corollary of inflation is the reshaping of the macrostructure in which the dilaform is set. This comes about because the inflating dilaform applies compressional force to adjacent formations. These yield by compaction, pressure solution, shear fracture or ductile flow and thereby thin across more dilatant sectors. If fluid pressure exceeds gravitational load the overburden uparches into antiformal configuration. The antiform weakens upward as stress is adsorbed but structural closure increases as growth continues. Adjustments also occur in the roof materials by fracture across brittle layers and shear along layer boundaries.The faults can become conduits for transmission of deviatory fluid pressure (plus fluids and slurries) to higher levels. This facilitates the extension of the inflationary domain.

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STRUCTURAL CONTROLS AND TIMING OF MESOTHERMAL GOLD MINERALISATION: CASE STUDIES FROM THE PINE CREEK INLIER, NORTHERN TERRITORY S.K, Matthai Research School of Earth Sciences, The Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601

In the Pine Creek Inlier, gold is associated with four types of retrograde, mesothermal quartz veins that occur in thermal aureoles of middle Proterozoic I-type granites: (1) A system of bedding-concordant crack-seal veins that are linked by discordant massive veins and occur in the subvertical limbs of anticlines; (2) within-fault quartz sheets with bedding concordant branches; (3) tension gashes in coarse grained sandstones in the vicinity of reverse faults and (4) tension gash swarms in vertical, elongate hydrothermal alteration zones that intersect anticlines subparallel to the axial cleavage and broaden in certain lithologies. The four types of veins are hosted by an Early to Mid-Proterozoic succession including carbonaceous metapelites, overlying pyroclastics and greywackes. These are intruded by quartz tholeiite sills and were folded prior to granite intrusion which induced dominantly static contact metamorphism (cordierite, almandine, andalusite, and biotite; T > 520''C, P > 200 MPa). Aplite dikes associated with granite emplacement are postdated by pegmatitic tourmaline veins. The Au-quartz vein systems truncate these and formed during biotite grade host rock temperatures. The Au-vein systems were deformed to varying extents by a second period of folding. Later reverse faulting imbricated the new complex fold structures. Subsequent strike slip deformation was accompanied by local retrograde alteration, sheath folding, quartz vein formation and remobilisation of gold where fault zones cut the earlier auriferous vein systems. Stringer sulphides were precipitated and quartz-carbonate-chlorite crack-seal veins formed at a temperature below 240X. Normal faults and vuggy tension gashes overprint all these structures. The microstructure of the auriferous quartz veins varies with their structural style. Although incipient dynamic recrystallization has obliterated some of the primary features, the Au-vein textures usually indicate episodic, antitaxial vein growth. Vein growth was coupled with wallrock alteration and the gold content of the veins is strongly dependent on the chemical and/or physical hostrock properties.

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THE RELATIONSHIP BETWEEN EXTENSION AND THERMAL SUBSIDENCE: APPLICATION TO THE PERTH BASIN, WESTERN AUSTRALIA M. F. Middleton Dept. Exploration Geophysics, Curtin University of Technology, GPO Box U1987, Perth W.A 6001

Thermal subsidence of a sedimentary basin occurs after heating associated with extension and rifting. It has been recognised for some time that thermal subsidence is related to the amount of lithospheric extension (McKenzie, 1978, Royden, 1985). A new model of lithospheric extension is proposed that entails two layers: an upper layer consisting of semi-brittle crust, and a deeper layer that deforms by pure-shear. This model is a compromise between the pure-shear models (McKenzie, 1978; Royden, 1985) and the simple-shear (complete lithospheric detachment) models (Lister et a l , 1986; Voorhoeve and Houseman, 1988). The model lithosphere is comprised of 1) an upper brittle layer of thickness C, which is not "thinned" by extension, and 2) a lower layer of thickness (H-C), which is "thinned" to a thickness (D-C) after extension by pure-shear (fig. 1). Stretching in the upper layer is accomodated by rotation of fault blocks along listric faults, which sole to the base of the brittle layer at depth D, and entail plastic deformation at depth and sediment in-fill at the surface. Accomodation of extension in the upper layer can also take place by strike-slip movement within the layer (fig. 1). In this model, the total thermal subsidence (S) is related to the extension parameter (P) by: S = K.G (1 - 1/P), where p = (H-C)/(D-C), G = [(H-C)/H] x [(H-C)/H] and K is a constant in the range 5-8 km, depending on assumed physical parameters of the lithosphere. Further, the extension parameter (P) can be related to the length of stretching in the upper layer (dL) by the relationship: ( l . l / p ) = [dL/(L + dL)], where L is the original length of the stretched lithosphere. These equations, can provide an estimate of sedimentary basin extension in the absence of deep seismic or other information. The Perth Basin, Western Australia, was an extensional basin during most of the Mesozoic and Early Tertiary. The present method of extensional analysis is applied to various seismic profiles across the southern part of the basin. The seismic data (Luck and lasky, 1989) suggest that there are at least two zones of thermal subsidence within the southern part of the Perth Basin: 1) Southeastern Zone (Bunbury Trough) with total thermal subsidence of about 0.4 seconds (-500 m), and 2) Northwestern Zone (Vlaming Sub-basin) with total thermal subsidence of about 1-2 seconds (-1500 m). The seismic data shows that the two zones, also recognised as separate tectonic entities (Playford et al., 1986), appear to be separated by a wrench system. These results, which contrast extension parameters (P and dL) versus the fault block rotation angle (w), are consistent within the two zones: small extension and rotation within the Eastern Zone (Bunbury Trough), and larger within the Western Zone (Vlaming Sub-basin). The fault block rotation data are not consistent with the "block rotation model" of Le Pichon and Sibuet (1981), who maintain that P-l/cos(w), but concede that their method may give smaller values of extension (P) than measured by other methods. The above data do, however,

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suggest that (within the constraints of data studied in the present basin) the amount of stretching dL is proportional to the square of the angle of fault block rotation. The results of the extensional model applied to these two zones are: SE Zone NW Zone Bunbury T. Vlaming S/b S=500m S=1500 m K=8km K=8km C=15km C=15km H=100km H=100km L+dL=50kin L+dL=47km w=6 degrees w= 12 degrees p=1.529 b=1.095 dL=16.3km dL=4.3km

Source of data OBSERVED FROM SEISMIC ASSUMED ESTIMATED FROM SEISMIC ASSUMED OBSERVED FROM SEISMIC OBSERVED FROM SEISMIC CALCULATED CALCULATED

Le Pichon, X, & Sibuet, J-C, 1881, Passive margins: a model of formation, JGR, v. 86, 3708-3720. Lister, G.S., Etheridge, M.A, & Symonds, P.A., 1986, Detachment faulting and the evolution of passive continental margins. Geology, v. 14, 246-250. Luck, G.R., & lasky, R.P., 1989, Geophysical review of the offshore Perth Basin, IN: Offshore Perth Basin Seminar, Western Australian Department of Mines (Special Report), 5 October 1989. McKenzie, D.P., 1978, Some remarks on the development of sedimentary basins. Earth Planet Sci. Lett., v. 40, 25-32. Playford, P.E., Cockbain, A.E., and Low, G.H., 1976, Geology of the Perth Basin , Western Australia, Geol. Survey West. Aust, Bull. 124. Royden, L., 1986, A simple method for analysing subsidence and heat flow in extensional basins, IN: Burrus, J. (Ed.), Thermal Modeling in Sedimentary Basins, Editions Tech., Paris, p. 49-73. > 1 ^ 1 2 + ciL2->i

-Li

' V V 'f '/ / '/ V / ' y y y y y y y '^y . • / X X X • X' •y^y y y y y y -y . \ \ \ \ \ \ \ \ y y y y y y y y \y. •\ y\ y\ y\ y\ y\ y\ y\ yyyyy y y yy \y\y y\ :y \y\ y\ y\ \y \y \y\^\ y\ yNy \y \. X ,yy'yyyyyyyy y y y y y y y yV y y• yy\yy\yy\yysyysyy\yyVyy\yy Nyy yyNyy\ yy\yy \ yNs' \. ,y\y \y \y \y \y sy vyyyyyyyy'^y y -y . yyyyyyyy'^y y'yyyyyyyy A y'yyyyyyyy^ y'yy y y y y yyyyyyyy'.y y y y y y y y • 'y y'y\ yNy \y \y \y \. _ \s \\ \\ \s•\ y\ y'yyyyyyyy ' y y y , V ^ v y ,y'yyyyyyyy \ : \ \ \ s \ s s \ '\ y\ y\ y\ , y y y \y s s \ Ny y y y \y ,yy nyy :yy \yy\ yy\ \yy\ yy\ \yy %yy 'yy '\\ y\\ yN\ ,yy\\ yy\\ yy\syy S\y-y\> yy yy yy yy yy yy yy yy yy ^yy \ \ sx • s \ \^'\ yy\ yyNyy \.y, y y y '.y f y? 1y y y y y y y ^\\ v.\y \\y y\\ . v V >1/ fs:y \y \y \y \y y\ fy y y y yy y y y y y y y y \y f\:y \y \y \y \y \y . . ysy\ yNy \y \y sy \y 'y ,y yy yy yy yy yy yy yy ^yyyyyyyy'-y ^ y y y y y y y y '.y H \ \ \ \ \ \ \ > ; \

>. \

^ > >

>; \

Figure 1: Model of extension thai entails 1) variable lateral extension and 2) little ihinnin^^ at ihc base of the

upjK^T briltlc layer.

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NORTHWEST MARGIN OF THE YILGARN CRATON: LATE ARCHAEN AND EARLY PROTEROZOIC TECTONICS J.S. Myers

Geological Survey of Western Australia, 100 Plain Street, Perth, Australia

The northwest margin of the Yilgam Craton reflects two major episodes of continent-continent collision. During the c. 2.65 Ga Yilgam Orogeny, the Murchison granite-greenstone terrane collided and amalgamated with the ancient Narryer gneiss terrane. The suture is marked by the Yalgar Fault, a zone of repeated intense deformation and granite emplacement. The rocks of both terranes were intercalated along thrusts and folded into large-scale recumbent structures (Di). These structures were refolded by folds with steep east-west axial surfaces (D2) and the rocks were metamorphosed in granulite or amphibolite facies. They were refolded by folds with steep northsouth axial surfaces (D3). The D3 deformation was generally intense and was associated with metamorphism in amphibolite facies. Older structural elements were mostly transposed into the D3 fabric which forms the main tectonic grain of the Murchison and Narryer terranes. During the c. 1.7 Ga Capricorn Orogeny, the Narryer terrane, as the leading edge of the Yilgam Craton, was in collision with the Pilbara Craton to the north and intervening island arcs and back arc basins. The latter were obducted onto the margin of the Yilgam Craton whilst the leading edge of the craton was subducted beneath the margin of the Pilbara Craton. The Narryer terrane was sliced up by thmsts that transported successively deeper levels southward, over formerly higher levels of the terrane, shortening and thickening the cratonic margin, and extensively converting granite and gneiss into schist.

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THE SCALE AND DISTRIBUTION OF DUCTILE STRAIN IN HENDRY'S CREEK: LOWER PLATE OF A MET AMORPHIC CORE COMPLEX, NEVADA, U.S.A A-K. Pahl VIEPS, Dept. of Earth Sciences, Monash University, Clayton 3168, Australia

This Study examines the partitioning of deformation in the quartz mylonites and schists of Hendry's Creek, a canyon which cuts through the lower plate of the northern Snake Range Metamorphic Core Complex. Field and microstructural observations, including U-Stage fabric analysis, indicate coeval development of coaxial and noncoaxial ductile strain. A pervasive, subhorizontal mylonitic fabric has been interpreted as the earliest fabric formed during a period of Tertiary extension. This is overprinted, in turn, by an extensional crenulation cleavage, several generations of kinkbands, shearbands, boudinage, normal faults and joints, at all scales. The timing of the later, dominantly brittle structures is at least partly coeval with the final stages of ongoing ductile deformation, as is evident by the offsetting and overprinting of some joints by the mylonitic foliation. From regional mapping of relative movement on the decollement, and asymmetric shear indicators (boudinage and east-dipping normal faults) mapped on the mesoscopic scale, the bulk strain picture of the area is known to be a top-to-the-east ductile shear zone. Ductile and semi-brittle fabrics at the microscopic scale, however, show a considerable variation in both the strain intensity and degree of non-coaxiality. Partitioning of coaxial and noncoaxial deformation is obvious in the quartzites. Here domains of ribbon grains formed by stretching of quartz pebbles alternate with domains of (rotation) recrystallized quartz. In the micaceous units, development of symmetric and asymmetric shearbands also indicate that strain varied from coaxial to non-coaxial. The microscopic sense of shear does not always consistent with the macroscopic shear sense. The average S-C angle increases (from 15® up to

moving eastwards and structurally upwards towards the

decollement (fig. 1.). If the S-C angle is assumed to represent a bedding-cleavage angle, which is a measure of finite strain, then an increase in this angle is at odds with the previous interpretation of a decollement as representing the centre of a shear zone. It is possible that a previous decrease in angle has been reset. The mylonitic surface is close in orientation to the C-surfaces. The oblique foliation, or schistocity, is defined by recrystallized quartz. Recrystallization of quartz may be continuous, overprinting previous rotations of the schistosity into parallelism with the decollement and mylonitic foliation. The increase in S-C angle may then be interpreted as due to a greater degree of dynamic recrystallization at higher structural levels. It is also possible however, that the formation of the present decollement was unrelated to development of an earlier formed, ductilc shear zone in the lower plate of the northern Snake Range . The degree and type of shear accommodated in Hendry's Creek is determined by microscopic, rather than mesoscopic fluctuations in both compositional and foliation induced anisotropics. It appears that the strain path in the lower plate of a Metamorphic Core Complex is not simply a case of either coaxial or non-coaxial shear,

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nor need one strain regime necessarily overprint the other, as previous workers have suggested. Both coaxial and non-coaxial deformation probably occur synchronously, with deformation partitioned so that the most highly anisotropic domains accommodate most strain by simple shear. This work has been supported by the Australian Research Council grant "Continental Extension Tectonics" (Lister, Houseman and Gleadow)

contour interval^OOm scale approx. 1:^3 000 EZI Lower Plate ^ E 3 Prospect Mt QuartziteBI Osceol la Argil lite •

Mylonitic Quartzitel g l / s t / b i / m u s c / c h l / q z schist2 O Mylonitlc Quartzite3 , gt/st/bi/musc/chl/qz schist4 Mylonitic Quartzite5

Guilmette Formation Mylonitic foliation & min.elongation lineation 4WD track

Figure 1. Summary Diagram of variation in S-C angle in Hendry's Creek

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PARTITIONING OF DEFORMATION IN CRUSTAL SHEAR ZONES: THE CASE OF METAMORPHIC CORE COMPLEXES A-K. Pahl, G. S. Lister and J. Lee VIEPS, P.O Box 2729X, Melbourne 3001, Australia

Recent evidence from the study of metamorphic core complexes in the Basin and Range Province, U.S.A. shows that it is an over simplification to assume that uniform sense shear occurs on the crustal scale. It is generally accepted that shear zones are integral in accommodating extension in the ductilely deformed lower plate of MCCs. The nature of this deformation has however been widely debated. The main question is whether simple shear zones form synchronously with ductilely stretching crust, whether they cut previously stretched crust, or whether a combination of these endmember processes will occur. Early workers on the strain path in MCCs suggested that the bulk strain history varied from one MCC to another, and that either pure shear or simple shear deformation occurred at any one time. More recently, quartz peti-ofabric studies of the quartzites and schists in the lower plate of the northern Snake Range MCC have shown that both pure and simple shear may have occurred concurrently in the region of highest strain (Pahl, 1991). Further, Lee et al. (1987) have shown that simple shear postdated pure shear in the regions of lower strain to the northwest. Movement zones in MCCs likely evolve in space and time. Microscale studies in the northern Snake Range considered the recrystallization pattern of quartz, and overprinting relationships of foliations defined by quartz and mica. These are consistent with mesoscopic development of several generations of extensional crenulations, and folds with axes oblique to the mylonitic foliation. The data suggests that mylonitization resulted from more than one stage of deformation, and that a pre-existing shear zone was folded. The evolution of a shear zone may be considered to result from heterogeneity or discontinuity in flow, so strain is likely to be partitioned (eg. Lister & Williams, 1983). In the most highly strained quartzites of the northern Snake Range, microscopic variations in the degree of coaxiality occur both parallel and perpendicular to the foliation; i.e. deformation is partitioned into roughly lenticular domains. Similar lenses exist also on the macroscopic scale in both quartzites and schists beneath the decollement. These are joined by anastomosing shear zones along which noncoaxial strain appears to be concentrated in the later stages of deformation. At all scales, lenses appear to undergo internal deformation coeval with external strain concentration. A pattern of deformation partitioning which recurs in many MCCs is that of ductile shear zones involving high strains forming synthetic to the bulk shear sense, while separating zones of low strain with antithetic shear sense. This can be explained as "domino" type rotation of "rigid" bodies, with the bulk shear sense in the intervening domains remaining synthetic. Such a pattern is evident in the gneisses of the Arizona MCCs, where veins and dykes formed in tension mode fracture are in the appropriate orientation for rotation to occur. In the northern Snake Range, where lower plate fabrics are dominated by anisotropy and heterogeneity, boudins are bounded by zones of simple shear. In the more competent units, boudins have undergone an internal component of rigid body rotation, to balance the overall shear induced vorticity. It is possible that deformation accrues - 48 -

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homogeneously on all scales (luring the initial stages of deformation, during which time, material strain hardens. After reaching a critical point (the "yield surface peak"), the nature of deformation changes from strain hardening to strain softening. Discrepancies in strain distribution due to the developing anisotropies are amplified, and coaxial deformation is differentially concentrated throughout the rock. If temperatures remain high enough during mylonitization to facilitate continuous recrystallization, as thermochronological (Lee, 1990) and microstructural data (Pahl, 1991) suggest, it is possible that this cycle of strain hardening and softening may be repeated over and over. On the geologic timescale then, development of the shear zone and stretching of the crust will be essentially synchronous. Further, the shear surfaces in crustal scale shear zones of MCCs are not always parallel to their overlying decollements. This observation can also be interpreted in terms of the evolution of the shear zone. Ar/Ar data show that the northeastern part of the northern Snake Range has cooled at a much earlier stage in the extensional history of the Range than the southeast (Lee, 1990). The overprinting relationships of meso- and microscopic structures in this region are such that the decollement is considered to have evolved in two (Lee, 1990) or possibly more stages. The formation of the currently exposed decollement was thus probably not coeval with the development of the ductile shear zone, but may be interpreted as a late stage, brittle expression of the cooling movement zone. It is clear that determination of the overall strain picture of a MCC can not enable accurate description of the kinematics involved in deformation of the lower plate. Whether pure or simple shear deformation is considered to be the dominant process during extension is scale dependent in both time and space. The concepts of strain partitioning and cyclic deformation can help to explain the nature and distribution of structural features observed. RQf^r^ng^: Lee, J. (1990) Structure and Metamorphism in the northernmost Snake Range, Nevada, PhD. Thesis, Stanford Univ., Stanford, CA. Lee, J. Miller, E. L. & Sutter, J. F. (1987) Duclile strain and metamorphism in an extensional tectonic setting: A case study from the northern Snake Range, Nevada, U.S.A. In Coward, M. P., Dewey, .J.F;. and Hancock, P.L. (Eds.) Continental Extension Tectonics. Geol. Soc. Am. Sp. Publ. 28, 267-298. Lister, G. S. & Williams, P. F. (1983) The partitioning of deformation in flowing rock masses. Tectonophysics 92, 1-33. Pahl, A-K. (1991) The Scale and Distribution of Ductilc Strain in Hendry's Creek: Lower Plate of a Metamorphic Core Complex, Nevada, U.S.A. GSA Abstr. SGTSG, Margaret River, W.A,

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1991

BREAKUP OF EASTERN GONDWANALAND: A REVIEW OF SEAFLOOR SPREADING AROUND AUSTRALIA. C.McA Powell Department of Geology, The University of Western Australia, Nedlands WA 6009, Australia

Studies of the pattern of seafloor spreading around Australia (Veevers et al. 1991) which led to the breakup of Eastern Gondwanaland, show that there are four phases of seafloor spreading. The first phase, from 160 Ma (Late Jurassic) to 132.5 Ma (Mil, Early Cretaceous) involved separation of a continental mass, Argo Land, of unknown extent, from the northwestern margin of Australia. This separation was coeval with the rotation of East Gondwanaland (Australia, India with Madagascar, and Antarctica) from West Gondwanaland (South America and Africa) leading to narrow oceanic areas separated by long transform faults along the eastern margin of Africa. Continental-crust extension between Australia and Antarctica began at this time, but no ocean floor was generated until much later. The second phase, from 132.5 Ma until 96 Ma (mid-Cretaceous) involved the rotation of India northwestward from Australia-Antarctica. This phase was coeval with the initiation of the South Atlantic Ocean, and saw Gondwanaland split into four separate plates, viz. South America, Africa, India with Madagascar, and AustraliaAntarctica. Over 300 km of continental extension between Australia and Antarctica occurred, and the continental crust in places was thinned to less than 10 km thick. A major change in global sea-floor spreading patterns around 96 Ma ushered in the next phase of continental breakup of East Gondwanaland, with the onset of ocean-floor spreading between Australia and Antarctica, and opening of the Tasman Sea as the continental strip of New Zealand-Lord Howe Rise rotated eastwards from Australia. Up to this time, Australia's Pacific margin had been an Andrean type of margin with a calc-alkaline magmatic belt reflecting westward subduction of the neo-Pacific beneath Eastern Gondwanaland. This third phase of spreading was associated with India's rapid northward flight from the southern Hemisphere towards the Equator, and its separation from Madagascar, now attached to Africa. The third phase of sea-floor spreading lasted until 45 Ma (A20y, middle Eocene), and was relatively slow between Australia and Antarctica. Spreading between the New Zealand - Lord Howe Rise and Australia ceased during the third phase, as did spreading in the Coral Sea, which was initiated during this interval. The fourth phase of sea-floor spreading commenced around 45 Ma, at the time India collided with the southern margin of Asia. There was a global realignment of sea-floor spreading at this time, which saw the rate of spreading between Australia and Antarctica increase to its present 2 to 3 cm per year. Spreading between India and Australia all but died in the middle Eocene, and from late Eocene onward, India and AusU-alia were a single plate. In the last 25 million years Australia has entered a new phase of continental margin generation. The leading northern edge of the Australian continent began to collide with island arcs along the projected eastward extent of

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the Sunda Arc, the location of the subducting margin of the India-Australia Plate. Exotic terranes have been accreted in Papua-New Guinea between 25 and 10 Ma and are still being accreted in the Banda Sea. The Australian continent crossed the line of the inferred eastward extension of the Sunda Arc about 15 Ma ago (mid-Miocene), and this led to the formation of the complex double-loop of the Banda Arcs. The mid-Miocene also saw considerable reactivation of old lines of geological weakness within the Australian continent, and could be related to the attempt by Australia to rotate counter-clockwise within the oceanic matrix of the IndoAustralian plate. In recent time, there is considerable earthquake activity at the latitude of Sri Lanka, and it appears that the Indo-Australia plate is now in the process of breaking into two. Refo^nggg: Veevers, J.J., Powell, C.McA. & Roots, S.R. 1991. Review of seafloor spreading around Australia - 1. Synthesis of the patterns of spreading. Aust. Jour, Earth Sci. 38 (in press for September).

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1991

NEW EVIDENCE FOR THE AGE OF DEFORMATION ALONG THE SOUTHERN MARGIN OF THE HAMERSLEY PROVINCE: RELEVANCE TO THE PALAEOGEOGRAPHIC EVOLUTION AND TIME OF IRON-ORE FORMATION C.McA Powell and Z.X.Li Department of Geology, The University of Western Australia, Nedlands WA 6009, Australia

Stratigraphic and structural relationships along the southern margin of the Precambrian Hamersley province have led to apparently conflicting conclusions about the timing and nature of deformational events. An angular unconformity at the base of the Beasley River Quartzite, the lowest formation in the Wyloo Group, brings the Beasley River Quartzite to rest on various units of the Mt. Bruce Supergroup, and locally cuts down to the Fortescue Group. Cobble clasts in the Beasley River Quartzite include jasperoid banded iron formation (BIF), dolerite and finely microbanded chert, but no ore-grade mineralisation has been found. Higher in the Wyloo Group, at the base of the McGrath Formation, there is another unconformity, above which conglomerate clasts include hematitic ore-grade BIF. Thus, it has been concluded (Morris, 1985, Tyler & Thome, 1990) that enrichment to ore grade did not occur until after deposition of the Beasley River Quartzite, but before deposition of the McGrath Formation. Structural mapping in the southeastern Opthalmia Fold Belt show that the dominant east-trending folds in the Turee Creek Syncline affect rocks up to, and including, the Cheela Springs Basalt, which lies above the Beasley River Quartzite but below the McGrath Formation. Tyler and Thome (1990) accordingly concluded that the easttrending folds in the southeastern Opthalmia Fold Belt are younger than the Beasley River Quartzite but older than the McGrath Formation. Structural relationships in the Hardey River Syncline in the southwestern Opthalmia Fold Belt have been interpreted (Trendall, 1979; Seymour et al. 1988; Tyler & Thome, 1990) as indicating that 098''- trending folds there affect all units up to the Turee Creek Group, but pre-date deposition of the Beasley River Quartzite. Thus, there arises a dilemma in that 098''-trending folds in the southwestem part of the Opthalmia Fold Belt were believed to be older than folds with the same strike along trend in the southeastern Opthalmia Fold Belt. New mapping in the Hardey River Syncline shows two important relationships: (1) There is a well-exposed angular unconformity between the Turee Creek Group and the Beasley River Quartzite, which, after restoration of the latter to horizontal, leaves the Turee Creek Group with residual dips conforming to llO^'-trending fold structures, and (2) The 098''-U-ending folds, with a well-developed axial-surface cleavage in pelite intervals, affect both the Turee Creek Group and Beasley River Quartzite. This 098''-trending set of folds, which corresponds to the principal cleavage-producing deformation in the Tumer and Brockman Synclines, is thus of the same orientation and age as the east-trending fold-and-thmst belt mapped in the southeastern Opthalmia Fold Belt. This new structural and stratigraphic information confirms the conclusion that upgrading of the BIF to hematite-ore grade occurred after the deposition of the Beasley River Quartzite, but before deposition of the McGralh

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Formation (c.f. Thornc & Seymour, 1991). It lends support to the inference (Powell, 1991) that formation of ore-grade iron deposits is related to the east-trending folding and thrusting. The new information uncovers an additional 110^-trending deformation along the southern margin of the Hamersley province, a defcMination which pre-dates the main east-trending folds-and-thrusts which dominate the outcrop pattern of the Opthalmia Fold Belt. It accounts for Horwitz's (1982) observation that the greatest uplift prior to deposition of the Beasley River Quartzite was along the southern margin of the Hamersley province, an observation supported by palaeocurrent directions measured in fluvial facies of the Turee Creek Group indicating a southwesterly source for jasper clasts, presumably derived from uplifted Hamersley BIF. It also confirms previously established constraints on the time of formation of ore-grade hematite deposits as being after deposition of the Beasley River Quartzite and before deposition of the McGrath Formation, but limits the amount of rock available to generate burial metamorphic reactions as the cause of iron-ore formation to the thickness of the Beasley River Quartzite and Cheela Springs Basalt. The regional extent of the early 110''trending deformation, however, has yet to be mapped. RQfgrgncgs: Horwitz, R.C. 1982. Geological history of the Early Proterozoic Paraburdoo Hinge Zone, Western Australia. Precambrian Research 19,191-200. Morris, R.C. 1985. Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes - A conceptual model. In: Handbook of Strata-bound and Stratiform Ore Deposits, 13 (ed. K. Wolf), Elsevier, Amsterdam, 73-235. Powell, C.McA. 1991. A polyphase deformational origin for the domes and basins of the southern Hamersley province (in prep.) Seymour, D.B., Thome, A.M. & Blight, D.F. 1988. Wyloo, Western Australia - 1:250,000 Geology Series (2nd ed.) West. Aust. Geol Surv. Explan, Notes, SF 50-10. Thome, A.M. & Seymour, D.B. 1991. Geology of the Ashburton Basin. Bull. West. Aust. Geol. Surv. 139, 141 pp. Trendall, A.F. 1979. A revision of the Mount Bmce Supergroup. West. Aust. Geol. Surv. Ann. Rep. 1978, 6371. Tyler, I.M. & Thome, A.M. 1990. The northern margin of the Capricom Orogen, Western Australia - an example of an Early Proterozoic collision zone. J. Struct. Geol. 12, 685-701.

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THE MARY LANE SHEAR AND A FOLD-THRUST MODEL FOR THE DEFORMATION OF THE TENNANT CREEK GOLD-FIELD, NORTHERN AUSTRALIA. M.S. Rattenbury Bureau of Mineral Resources. GPO Box 378, Canberra, ACT 2601, Australia

The Mary Lane Shear is a major structural and magnetic lineament which trends east-west to WNW-ESE through the central-southern Tennant Creek goldfield. Foliation within the Mary Lane Shear dips steeply south and contains steeply plunging stretching mineral lineations. Various sense of shear criteria indicate south up over north (reverse) movement during low temperature brittle-ductile shear. Highly strained conglomerates show a strong flattening component, whereas highly strained porphyry lithologies show a tendency towards a prolate or constrictional strain. This variation in strain type is interpreted to reflect partitioning of strain in space and/or time through the shear zone. Total vertical displacement across the Mary Lane Shear is probably between 1-2 km, based on an offset porphyry sill. The Mary Lane Shear has been reactivated during later sinistral strike-slip faulting. Similar shear zones occur elsewhere in the Tennant Creek field, defined by magnetic lineaments (usually steeply north-dipping gradients) and the occasional outcrop structure, including S-C fabrics developed within the Tennant Creek Granite. Microstructural criteria and detailed structure and stratigraphy across these shear zones show also south up over north movement sense. Analysis of a large structural database of bedding and cleavage measurements from the southern Tennant Creek field indicates the folding is slightly asymmetric with a south up over north vergence. Reconstruction of an undeformed section through the Warramunga Group by removing shortening effects dominated by cleavage, folding and, to a lesser extent faulting, suggests a minimum of 54% total shortening has occurred. The ?granitic basement, however, probably shortened predominantly by thrust faulting. A major thrust system is interpreted to underlie the Warramunga Group, and various splinter thrusts curve upwards. Most of these thrusts terminated below the present erosion surface and propagated anticlinal structures above their tiplines. Other thrusts continued above the present erosion surface, like the Mary Lane Shear. The north-directed polarity of this fold-thrust belt is consistent with preliminary gravity interpretations of the basement structure, and basement topography may have influenced the position of some of the larger thrusts/reverse faults. The slight discordance between the regional fold axes and the Mary Lane Shear suggests slightly oblique (dextral) movement during predominantly reverse faulting. The initiation of the Mary Lane Shear oblique to the fold axes may also reflect basement topography, particularly the upper surface of the Tennant Creek Granite pluton. The Mary Lane Shear is neither unusually mineralised nor altered compared to other areas in the Tennant Creek goldfield. Massive hematite-magnetite-chlorite-quartz "ironstones", which are host to Au-Cu-Bi mineralisation, formed during the regional folding and cleavage development event. The basal thrusts were likely conduits for ironstone and ore mineralising fluids at depth. The lack of significant alteration along the Mary Lane Sheai suggests the mineralising fluids may have been generally unreactive to the shear zone lithologies, and/or - 54 -

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channelled into pipelike conduits within the thrust fault plane, and/or may have preferentially moved vertically upwards through the hanging wall of the Mary Lane Shear.

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POLYPHASE DEFORMATION IN THE MOUNT IDA GREENSTONE BELT, AND THE STATUS OF THE EASTERN GOLDFIELDS-SOUTHERN CROSS PROVINCE BOUNDARY, WESTERN AUSTRALIA. M.S. Rattenbury, M.B. Duggan, A. Whitaker Bureau of Mineral Resources, GPO Box 378, Canberra, ACT 2601, Australia

The Mount Ida greenstone belt straddles the historical boundary between the mafic and ultramafic metavolcanicdominated Eastern Goldfields Province (Kalgoorlie terrane), and the BIF-rich Southern Cross province (Callion terrane). The nature of the contact between these contrasting lithological provinces, or terranes, is largely conjectural, but has been interpreted to be a major fault (e.g., the Ida Fault of Swager et al. 1990). The bifurcation of the Mount Ida greenstone belt is due to a regional scale south plunging D2 fold, the Kurrajong Anticline, which closes southward around a strongly lineated granite. The Kurrajong Anlicline is well defined by folding of several distinctive lithological layers within the mafic-ultramafic stratigraphy, notably an anorthositic gabbro/dolerite, and farther south, cumulate ultramafics. The eastern limb of the Kurrajong Anticline has been truncated by a north-south trending D3 ductile shear zone, the Ballard Shear, which comprises strongly foliated mylonitic gneiss with a pervasive gently south-plunging stretching lineation. The curvature and attenuation of the eastern limb of the Kurrajong Anticline, and the slight anticlockwise rotation of the Kurrajong Anticline axial trace into the Ballard Shear suggests sinistral movement of the shear zone. Discrete east-west trending faults with apparent sinistral displacements have been identified from aeromagnetic interpretation of the gneisses. The faults do not offset the Ballard Shear, and may be Reidal (antithetic) shears to dextral D4 brittle movement of the Ballard Shear. The amount of strike-slip displacement is uncertain. Mafic-ultramafic metavolcanics do not outcrop on the east side of the Ballard Shear within the mapped area. The Lawlers-Agnew mafic-ultramafic sequence 100 km to the north occurs to the east of the Waroonga Shear, a possible extension of the Ballard Shear. The Ballard Shear is probably the northward extrapolation of the Zuleika Shear (Swager et al. 1990), and the nearest mafic-ultramafic metavolcanic sequence occurring east of the Zuleika Shear is near Ora Banda, 70 km to the southeast. There is no structural or metamorphic grade evidence to support earlier dip-slip component movement in the Ballard Shear zone, therefore large displacement strike-slip faulting is preferred to explain the apparent offsetting of the mafic-ultramafic metavolcanics. Within the hinge area of the Kurrajong Anticline, complexly interlayered ultramafics and basalt show ramp-like structures, and isoclinal folds. Pre-D2 restoration, by unfolding the Kurrajong Anticline, shows some abrupt lateral terminations of distinctive lithologies, for example, cumulate ultramafics to chert and quartzite across Bottle Creek. These terminations are most simply explained as low angle Di faults. Isoclinal folds occur elsewhere on a macroscopic 2 km scale and as intrafolial folds in sedimentary lilhologies. The lithologies of western fork of the Mt Ida greenstone belt differ distinctly from those of the eastern fork. Banded iron formation, quartzite and tholeiitic basalt predominate, and ultramafic cumulates and komatiites arc absent. The boundary between these contrasting lithologies has been historically defined as separating the Eastern Goldfields and Southern Cross Provinces. The banded iron formation is conspicuously folded wiih an earlier tight - 56 -

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10 isoclinal phase refolded by an upright more open phase, thus sharing the Di and D2 elements of the maficultramafic volcanics to the east. Shearing at the easternmost banded iron formation shows evidence for dextral strike-slip, although the degree of deformation is mild (compared to that of the Ballard Shear), and a major tectonic boundary is not suggested at this location. The presence of BIF-rich lithologies more likely reflects a different part of the greenstone stratigraphy, with relatively minor fault juxtaposition against the maficultramafic volcanics.

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ARCHITECTURE OF THE QUEENSLAND TROUGH: IMPLICATIONS FOR THE STRUCTURE AND TECTONICS OF THE NORTHEASTERN AUSTRALIA MARGIN D.L.Scott

Research School of Earth Sciences. The Australian National University, GPO Box 4, Canberra, ACT 2601, Australia

The Queensland Trough is a ISO"" trending bathymetric deep located just seaward of the Great Barrier Reef of NE Australia. The trough reaches a maximum depth of 2800 m and defines the boundary between the continental mainland and the submerged Queensland Plateau and therefore lies wholly within continental crust. This preliminary interpretation of the underlying structure of the trough uses 3700 km of 1970's vintage seismic data. The interpretation of these data is supplemented by gravity and magnetic data collected during the same surveys. The main data grid has a spacing of approximately 50 km. However, another grid shot in a zig-zag pattern provides line spacing as close as a few kilometres locally. Three main acousto-stratigraphic packages can be recognised. The post-rift section is comprised of flat-lying, very continuous reflections and extends up to two (2) seconds below the water bottom. The syn-rift section is comprised of moderately dipping reflections which are semi-continuous separated by zones of chaotic reflections and diffractions. The reflection separating the syn- and post-rift packages is generally quite distinct, characterised by angular discordances and truncated reflections. The pre-rift or basement section is commonly chaotic and indistinct, but steeply dipping reflections are recognisable locally. No wells have penetrated the syn-rift package. Seismic profiles of all orientations reveal tilted basement fault blocks. Many bounding faults are clearly listric. These half-graben form a series of syn-rift depocenters with up to 5 km of syn-rift infill (Panel B). Syn-rift isopach cells appear to be elongate along the axis of the rift suggesting rift parallel bounding faults and orthogonal extension. No structures parallel or perpendicular to the rift axis have been recognised. Most of the syn-rift cells are composed of two or more smaller "deeps". Concepts from two rifting models are applied to the available data to provide syn-rift structural interpretations. In Panel C a mapping wherein curvilinear faults define a series of half-graben is provided. Accommodation zones and half-graben polarity switches arc identified from the profiles and as a consequence of the mapping. This interpretation is based on a model derived from the East African Rift where the tectonic transport direction has been shown to be oblique to the rift axis. In Panel A, a detachment model structural style is invoked. Nearly rectilinear faults striking at approximately 290"" and 020"^ predominate. Both of these trends are oblique to the rift axis and "transfer faults" appear to compartmentalise the syn-rift isopach cells into the "deeps". It is concluded that the structure underlying the Queensland trough is the product of oblique rifting.

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30 < NE AUSTRALIA Queensland Trough Rectilinear Fault Map

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CO CO

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ILLUSTRATION OF STRUCTURAL STYLES OFFSHORE PERTH BASIN: INTERPLAY OF EXTENSION AND STRIKE SLIP J. Scottl and A. Stein2 ^School of Applied Geology, Curtin University of Technology. GPO Box U1987, Perth WA 6001, Australia ^Dolan & Associates. 3 Old Lodge Place. St Margarets, Twickenham, TWl IRQ, England

The Perth Basin is an elongate, north-south trending, compartmentalised rift which forms the western continental margin of Australia. The subsidence history of the basin can be related to two "rift-to-drift" cycles associated with the development of the neo-Tethyan and Indian Oceans. Subsidence associated with the first rift-cycle began during the Permian. A mixed clastic sequence was deposited into a series of extensional half-graben which are imaged on regional seismic profiles in the offshore parts of the basin. The rifting event culminated in the late Permian with continental break-up along the northern margins of Australia to form the Tethys II Ocean. Active rifting in the Perth Basin ceased and during the Triassic a classic transgressive/regressive sequence was deposited. At this time the Perth Basin was an embayment to the passive margin facing the spreading Tethys Ocean to the north. The second rift cycle was associated with increasing instability between the Indian and Australian fragments of Gondwana. Active faulting in the Perth Basin appears to have begun in the mid-Jurassic. The extension direction was orientated NW-SE resulting in the development of an oblique slip basin dominated, in the offshore, by NNE-SSW extensional faults and NW-SE trending strike-slip transfer faults. These major structures sub-divided, or compartmentalised, the basin into a series of linked half-graben. The NW-SE transfer faults appear to have reactivated a fundamental basement fabric which is present within the Yilgam Craton to the east. This basement fabric has also played a role in the partial reactivation of the Darling Fault in the onshore which is illustrated by the relationship between the Darling Fault and the Urella Fault where they intersect. The basins bounded by these faults underwent rapid subsidence and contain thick sequences of synrift clastics. The South Perth Basin in particular contains in excess of 8,000 m of middle Jurassic to Lower Cretaceous continental and marginal marine clastics. Patterns of sedimentation within the active graben were strongly fault controlled and for the first time we have been able to make predictions about facies distribution within the basin using both the sparse data available from the offshore and from recent studies of analogous rift basins in other parts of the world. These predictions can be used to demonstrate that the areas containing the best potential source rocks and possibly with enhanced reservoir characteristics have, as yet, been largely un-tested by the drill. Immediately prior to, or during, continental break-up between Australia and India, in the early Crctaceous, there was a period of right-lateral N-S directed stress which resulted in transpressional inversion across many of the structures in the basin. This is overprinted upon a period of regional uplift which has resulted in the development of the Neocomian "Break-up Unconformity". Transpressional structures formed during the inversion event form some of the most spectacular features within the basin and many of these constitute valid exploration targets. Examples illustrated here include: the collapsed inversion anticline in the South Perth Basin (Gage Roads-1,

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Charlotlc-l and Quinns Rock-1), transprcssional flower structures (Sugarloaf-l) and reactivated strike-slip faults (South Turtle Dove-1). In the North Perth Basin more subtle inversion structures similar to that which contains the Mount Homer oilfield has been identified in the offshore. The Perth Basin is at a very early stage in its exploration history. Only 20 offshore wells have been drilled in the basin so far and most of these were located using data and play concepts from the 1960's and 70's. The application of modem data and new play concepts such as those presented in our 1989 study should result in some exciting results during the most recent phase of exploration which has just started. The authors would like to acknowledge permission to publish data from the following non-exclusive report: Dolan & Associates, Petroleum Geological Analysis Ltd., WA Centre for Petroleum Exploration (1989) Perth Basin, Western Australia. Petroleum Geology, Offshore Exploration Potential and Economic Evaluation. Non Exclusive Report, 6 volumes plus enclosures.

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1991

STRUCTURAL HISTORY OF THE TUGLOW DISTRICT, LACHLAN FOLD BELT, NSW. M.M.

Scott

Department of Geology and Geophysics, University of Sydney, NSW, 2006, Australia The Tuglow district lies in the northeastern Lachlan Fold Belt. The Late Ordovician-Early Silurian Triangle Group consists of a conformable sequence of black shales (Bubalahla Formation) and overlying quartzose turbidites (Hurra Hurra Creek Formation). Middle Silurian volcaniclastics are conformably overlain by fossiliferous Late Silurian limestones, siltstones and tuffaceous sandstones. The Late Silurian sequence was partly eroded before deposition of silicic volcanics and subsequent intrusion of dolerite dykes and sills during the Early-Middle Devonian. Erosion preceded deposition of interbedded quartzose sandstones and siltstones of the Late Devonian Lambie Group. Early Silurian F i folds in the Hurra Hurra Creek Formation have no associated cleavage (Si), plunge gently WNW with north to upright vergence. An area of downward-facing mesoscopic Fi and F2 folds in the Hurra Hurra Creek Formation possibly formed on the overturned limb of a macroscopic Fi recumbent nappe. Fi folds are transected by a later cleavage (S2) 82 dips steeply to moderately to the NW or W, approximately parallel to the axial plane of subhorizontal to gently plunging F2 folds. Cleavage in the Hurra Hurra Creek Formation is refracted with changes in grainsize, from steep in sandstones to shallow in siltstones. Facing can be determined from the refraction of cleavage, which is convex-upward in upright beds and concave-upward in overturned beds. F2 are truncated by latitudinaltrending low angle faults and the W dipping meridional-trending Jaunter Fault and Hollanders Fault (Zone). The trend of F2 folds, S2 and the Jaunter Fault swing from NE in the south to N in the north of the district. The Middle Silurian to Late Devonian sequences are down-faulted into the Triangle Group, in fault blocks between the Jaunter Fault and Hollanders Fault and fault slices of the Hollanders Fault Zone. Separation of the major meridional faults increases eastwards, from the Jaunter Fault to the Hollanders Fault to the Taralga Fault. The hangingwall blocks of the reverse Jaunter Fault and Taralga Fault have been transported eastwards. The normal Hollanders Fault has allowed different relative movement between the footwall block of the Jaunter Fault and hangingwall block of the Taralga Fault The major deformation produced F2 folding, S2, faulting and mesoscopic kinking (F3) of the Ordovician to Devonian units during the Early Carboniferous. Later in the Carboniferous, undeformed quartz porphyry dykes and sills intruded adjacent to faults, and the undeformed Kanangra Granite was emplaced.

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EXTENSIONAL STRUCTURES IN THE RAWHIDE MOUNTAINS, ARIZONA, U.S.A.: IMPLICATIONS FOR DETACHMENT FAULTS R. Scott Victorian Institute of Earth and Planetary Sciences, Monash University, Victoria 3168, Australia

Detachments faults mantling metamorphic core complexes in southwestern U.S.A. have long been the subject of controversy in extension tectonics. If a i is vertical during continental extension, the formation of, or slip along, very low angle normal faults is mechanically implausible. Furthermore, there are no records of earthquakes for low angle normal faults in areas currently undergoing continental extension (Jackson & White 1989). Models in which detachment faults originate as moderate to steeply dipping normal faults but flatten out and become inactive as the footwall is progressively unloaded (Wernicke & Axen 1988, Buck 1988), incorporate this geophysical data but are inconsistent with some fundamental field relationships. In all these models the detachment fault forms a permanent boundary between the upper and lower plates which undergo distinctly different styles of deformation. The mylonitic fabric in the lower plate should conform to the shape of the detachment, and originally flat lying strata in the hanging wall should dip steeply above the flattened section of the fault. A detailed study of the Rawhide fault in the Rawhide Mountains of western Arizona, however, indicates that this detachment fault is the youngest major structure developed during Miocene extension, is oblique to the mylonitic fabric in the footwall, and did not produce significant rotation of hanging wall strata. The Rawhide fault was an initially gently dipping normal fault that was active at very shallow levels in the crust. In the southwest of the study area, the Rawhide fault dips very shallowly and is exposed in a 3 km wide trough between linear ridges of lower plate rocks, extending 20 km southwest into the Buckskin Mountains. At the northeastern end of the trough the Rawhide fault is offset by the Lincoln Ranch fault, the largest of several reverse faults which post-date Miocene extension. In the study area the detachment fault separates an upper plate dominated by syn-extensional, Miocene sedimentary rocks and lesser preTertiary basement, from amphibolite to greenschist facies mylonitic rocks of Proterozoic to Tertiary age in the lower plate. Upper plate rocks in the Rawhide Mountains are cut by numerous northwest trending, high and low angle normal faults, which have both top-to-the-northeast and top-to-the-southwest displacement. Many faults have listric and ramp-flat geometry with roll-over and fault bend folds developed in the hanging walls. Although the upper plate is extensively faulted and the Rawhide fault is gently dipping, Miocene sedimentary rocks southwest of the Lincoln Ranch fault are not steeply tilted (average dip < 25''). The Rawhide fault truncates the bases of tilted fault blocks and folds in the upper plate. Normal faults which offset the Rawhide fault are rare and displacements are small, indicating the detachment is the youngest major extensional structure. The regionally extensive late Miocene Sandtrap Conglomerate is the youngest upper plate unit cut by the detachment fault This unit contains abundant clasts derived from the lower plate and is locally steeply dipping adjacent to the detachment. Not only does this indicate that detachment faulting continued after the lower plate had breached the surface, but it shows either that listric faults which merged with the detachment tilted the conglomerate, or that several generations of detachment faults are present

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In the Whipple Mountains in southeastern California and the South Mountains in central western Arizona, the mylonitic fabric in the lower plate diverges from parallelism with the detachment fault at the southwest dipping *mylonitic front*. The mylonitic front also marks a rapid transition from mylonitic to non-mylonitic rocks in the lower plate (Davis & Lister 1988). In the study area the orientation of the mylonitic foliation is extremely variable but generally dips moderately southwest and is cut by the detachment. Similar relationships exist elsewhere in the Rawhide and Buckskin Mountains. At Clara Peak in the central Buckskin Mountains moderately dipping, gently folded mylonites are truncated by the flat detachment fault (Woodward 1981). At Battleship Peak (Marshak & Vander Meulen 1989) and Planet Peak (this study), at the southwestern end of the core complex, the structurally highest mylonites generally dip moderately southwest, oblique to the interpreted projection of the (now eroded) detachment fault If the moderate southwest dip of the mylonites is maintained along the length of the core complex, the 20 km exposed thickness of greenschist to amphibolite facies mylonites is too great to represent a single gently dipping midcrustal shear zone. It is more likely that the apparent thickness of the mylonites is due to repetition across a series of tilted fault blocks in the lower plate. This is significant because it may represent the first example of lower plate tilt blocks truncated upwards by flat detachment faults. Upper plate tilt blocks truncated downward by flat detachment faults have been recognised for some time, although the geometric significance of this phenomenon has largely been ignored. The picture of detachment faulting that is emerging from studies in the Rawhide Mountains is radically different to that proposed by Wernicke & Axen (1988). Although the mechanics of the process still defy explanation, it is clear that detachment faults are active at, and probably form at, low angles. RgfwngQS Buck, W.R. 1988, Flexural rotation of normal faults. Tectonics 7, 959-973. Davis, G.A. & Lister, G.S. 1988, Detachment faulting in continental extension; Perspectives from the southwestern U.S. Cordillera. Geol. Soc. Am. Sp. Paper 218, 133-159 Jackson, J.A. & White, NJ. 1989, Normal faulting in the upper continental crust: observations from regions of active extension. J. Struct. Geol. 11, 15-36. Marshak, S. & Vander Meulen, M. 1989, Geology of the Battleship Peak area, southern Buckskin Mountains, Arizona: structural style below the Buckskin detachment fault. In Spencer, J.E. & Reynolds, S.J., (Eds.) 1989 Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona. Ariz. Geol. Surv. Bull, 198, 51-66. Wernicke, B. & Axen, GJ. 1988, On the role of isostacy in the evolution of normal fault systems. Geology 16, 848-851. Woodward, R.J. 1981, The structural geology of the Swansea area, east-central Buckskin Mountains, Yuma County, Arizona. Unpubl. M.Sc. thesis, Los Angeles, U.S.C., 106p.

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CYCLIC FLUID FLOW RELATED TO FAULT LOADING IN DIFFERENT TECTONIC REGIMES R.H. Sibson Depl. of Geology, University of Otago, P.O. Box 56. Duncdin, New Zealand

Cyclic loading of faults to seismogenic failure within the upper crust leads to fluid redistribution near fault zones, the pattern of induced flow depending on the type of faulting. Frictional shear resistance of an existing fault may be approximated by: Tf= C + iisan' = C + lis(an-Pf) (1) where C is the cohesive or cementation strength (small for an active fault), p-s is the static friction coefficient (typically -0.75), Pf is the fluid pressure and an is the normal stress on the fault. Reshear occurs when the shear stress on the fault, x = if, and is generally assumed to result from rising tectonic shear stress. However, failure may also also be induced by reducing normal stress or increasing fluid pressure (as in the case of earthquakes triggered by fluid injection), or through some combination of these factors. In general, shear stress on faults cannot be increased without changing the normal stress, which in turn affects the shear resistance of the fault. Normal stress is directly related to the level of mean stress affecting the dilatation of a rock mass, so that fluid redistribution in response is controlled by the manner in which normal stress changes as shear stress rises on a fault. Because fluid pressure may also vary during loading, and shear resistance depends on the effective normal stress, an' = (an - Pf), the parameter: (aanVax) = (3an/aT - dPf/dx) (2) is a useful discriminant distinguishing different modes of fault loading. Two time-scales need to be considered in relation to the earthquake cycle; long-term accumulation of shear stress through the interseismic period lasting tens to perhaps many thousands of years, and the rapid drop in shear stress during seismic rupture lasting only a few seconds at any one place. Significant stress cycling around a seismogenic fault will be felt through a response zone whose extent will compare broadly with characteristic rupture dimensions, perhaps extending laterally for distances of 10-15 km from ruptures that occupy the full depth of the continental seismogenic zone. Consider first, the loading of faults within dry crust in the three basic Andersonian stress regimes (fig. 1). In these circumstances, an and i remain directly coupled to each other. The vertical stress or overburden pressure, ay is taken to stay constant through successive loading cycles and is one of the three principal compressive stresses (ai > a2 > as). In a compressional regime involving pure reverse dip-slip, where ay = as, faults are loaded to failure by increasing horizontal ai. so that normal stress, mean stress and shear resistance all increase as shear stress rises during loading, but decrease during the drop in shear stress accompanying failure (i.e. 3an/3T > 0). In contrast, for pure normal dip-slip in an extensional regime where ay = ai. faults are loaded to failure by decreasing horizontal a3. Normal stress, mean stress and shear resistance then all decrease as shear stress rises, but increase during the shear stress drop at failure (3an/3x < 0). In both situations the magnitude of 3an/3x is determined by the dip of the faults. In the case of strike-slip faulting where ay = a2. the loading parameter may be either positive or negative, depending whether an increases or decreases as shear stress builds towards failure. - 65 -

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This suggests a stress-based definition of transpression (3an V3ct > 0) and transtension (3an V3a < 0) founded on the fault loading parameter. In fluid-saturated crust, consider initially the case where the rock mass remains effectively drained during loading, so that fluid pressure stays close to constant (and hydrostatic). For compressional/transpressional

tectonic

regimes (3an'/3cr > 0), fluid will move out of the response zone in response to rising mean stress as shear stress accumulates, but will be drawn back in after failure. In contrast, within extensional/transtensional

settings

(3an73ci < 0), fluid will be drawn into the response zone during loading and expelled postfailure. The situation is more complicated within imperfectly drained crust. Fluid pressure will tend to increase during loading in compressional/transpressional

settings, decreasing the time to failure, but will decrease in the vicinity of

extensionalltranstensional faults, increasing time to failure. Sudden changes in normal stress accompanying the shear stress drop at failure will be partly counteracted by the instantaneous response of pore-fluid pressure; the overall effect is to delay the strength changes at failure predicted by the dry fault analysis. Fluid redistribution from fault loading and sudden pressure changes at failure may lead to mineralisation in epithermal environments where background fluid pressures are approximately hydrostatic. At greater depths where fluid pressure is suprahydrostatic, loading-related flow may be masked by other processes such as fault-valve action.

iE UJ

CO V.

o CO

TIME Fig. 1- Variation of shear stress, x, and shear resistance, Tf, (tracking the normal stress) during the loading to failure at 1 km depth of dry, cohesionless thrust and normal faults that are optimally oriented for reactivation. Constant stress drop arbitrarily selected.

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FLUID-ROCK INTERACTION DURING SHEAR ZONE DEVELOPMENT IN LATE PROTEROZOIC GRANITOIDS ON KING ISLAND, TASMANIA J . Streit Research School of Earth Sciences. The Australian National University, GPO Box 4, Canberra ACT 2601, Australia

Late Proterozoic granitoids which form the basement of the Lachlan Fold Belt on King Island are cut by N-S trending shear zones and fault zones that are up to 30 m wide. These structures have been active during upper to lower greenschist facies conditions and exhibit mixed transcurrent and extensional displacement histories. At least one of these zones, the Currie Fault Zone (CFZ) may have been active during Cambrian regional crustal extension and contains evidence for substantial fluid infiltration during deformation. The CFZ is localised along the contact zone between a major granitoid sheet and low grade metasediments. The granite and the metasediments have been intruded by mafic magmas both prior to and during fault zone development. High shear strain, cataclasis, hydrothermal alteration and veining tend to be localised at contacts between mafic bodies and granitoids in the shear zone. Early displacement of the CFZ has led to pervasive development of mylonitic fabrics within the granite in a zone up to 30 metres wide adjacent to its contact with the metasediments. Retrograde alteration during mylonitisation has produced a quartz - k-feldspar - albite - phengite - epidote - chlorite - titanite mineral assemblage. Shear sense indicators provide evidence for dextral displacement. Quartz is plastically deformed; feldspars and micas are brittley deformed. Epidote and titanite can form foliation parallel layers. Mylonitisation has been associated with intense veining. Cataclasites overprint the granitic mylonites and occur in a zone up to 50 m wide which extends into the previously undeformed granite. Cataclasite bands contain evidence for repeated failure events and are associated with locally intense hydrothermal alteration. The presence of injection cataclasites and multiple episodes of veining indicate that the onset of dominantly brittle deformation in the CFZ was associated with at least transiently near-lithostatic fluid pressures. Microstructural and microchemical studies are used to place constraints on the geometry and scale of fluid transport and its influence on fault mechanics during development of the CFZ.

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THE GILMORE FAULT ZONE - THE DEFORMATIONAL HISTORY OF A POSSIBLE TERRANE BOUNDARY WITHIN THE LACHLAN FOLD BELT, N.S.W. P.G.

Stuart-Smith

Bureau of Mineral Resources, Australia, PO Box 378 Canberra, ACT 2601, Australia

The Gilmore Fault Zone, also refered to as the Gilmore Suture (Scheibner, 1985), is a major north-northwesttrending tectonic feature extending for several hundred kilometres in the southeastern part of the Lachlan Fold Belt, southeastern Australia. In its southern part, the zone forms a long-lived imbricate fault system separating Ordovician metasediments of the Wagga Metamorphic Belt in the west from Ordovician-Early Silurian volcanosedimentary sequences of the Tumut Block to the east. 5-C fabrics and minor structures within the fault zone indicate dominantly sinistral transpressional movements during regional deformation in the Siluro-Devonian and mid-Devonian and/or Carboniferous. These movements, in response to lateral compression, resulted in the Wagga Metamorphic Belt being thrust over the Tumut Block. In addition dextral strike-slip movement may be inferred during Early Silurian regional deformation and subsequent extension. Common structural and metamorphic histories, and lithological correlation of rock units straddling the fault zone indicate that the Gilmore Fault Zone was not a terrane boundary in the Late Ordovician or Early Silurian as suggested by some previous workers. Differences in geophysical expression and crustal composition across the southern portion of the zone can be explained by the the zone being a reactivated basement fault which may correspond, in part, to an older terrane boundary. The fault zone is interpreted as splay off a gently west-dipping mid-crustal detachment (fig. 1). Reference: Scheibner, E., 1985 - Suspect terranes in the Tasman Fold Belt System, eastern Australia. I n Tectonostratigraphic terranes in the Circum-Pacific region. Circum-Pacific Council for Energy and Mineral Resources - Earth Science Series 1,493-514.

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WAGGA METAMORPHIC BELT

TUMUT SYNCLINORIAL ZONE TUMUT X JINDALEE. BLOCK BLOCK

GILMORE FAULT ZONE

.GOOBARRAGANDRA BLOCK

i m i p a t MOONEY MOONEY FAULT FAULT ZONE

LONG PLAIN FAULT

Moho

= 1

Early-Late Silurian volcanics

Coolac Serpentinite

Late Silurian granitoids

Cambrian-Ordovician greenstone sequence

Ordovician-Early Silurian strata

Figure 1. Schematic crustal profile for the Tumut region, showing relationship of major faults to interpreted mid-crustal detachment, based on the Tumut seismic traverse.

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EARLY PALAEOZOIC TECTONISM AND REACTIVATION OF PREEXISTING BASEMENT STRUCTURES AT THE MARGINS OF THE KIMBERLEY CRATON, WESTERN AUSTRALIA I.M. Tyler

T J . Griffin ^ and R.D. Shaw2

^Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, Australia ^Bureau of Mineral Resources, Geology and Geophysics, GPO Box 378, Canberra 2601, Australia

The Kimberley Craton consists of unexposed ?Archaean or earliest Proterozoic basement underlying the Early Proterozoic (c. 1840 - 1800 Ma) Kimberley Basin. Its margins are defined by the King Leopold and Halls Creek Orogens, which have a long history of deformation and metamorphism with the periodic reactivation of faults and shear zones established in the Early Proterozoic and oriented subparallel to the margins of the Kimberley Craton. Structural and stratigraphic relationships, together with recent K - Ar geochronology from the King Leopold Orogen, date two periods of late Proterozoic to late Cambrian (c. 560 Ma and c. 500 Ma) tectonism. In the King Leopold Orogen, the first period of deformation produced a fold and thrust belt (the Precipice Fold Belt) at the southwest margin of the Kimberley Basin. As well as deforming Kimberley Basin rocks it also deforms rocks of a Late Proterozoic glacigene sequence. Movement to the southwest initially took place on the unconformity surface at the base of the Kimberley Basin succession (the Inglis Fault). Later thrusting developed "out-of-sequence" at higher structural levels above a pre-existing footwall ramp, with the associated development of local backthrusting. The Precipice Fold Belt extends to the northwest as far as Mount Hart. In the Yampi area limited southwest-directed movement has taken place on the Kimberley Basin unconformity, together with open refolding of earlier fold structures, and the development of the the Mount Page Fault Arc system, which trends from northeast to southeast and has a dominantly sinistral movement sense. Several major southwest-dipping faults in the King Leopold Orogen were reactivated under lower greenschist facies metamorphic conditions. Associated fold and fault orientations are consistent with a sinistral transpression. In the zone where the King Leopold and Halls Creek Orogens meet, fault movements are complex. Dextral movement on the Stoney Creek Shear, and sinistral movement on the Glidden Fault accompanied westward movement of the Kimberley Basin rocks and Late Proterozoic glacigene rocks between the two faults, resulting in the formation of a north-trending, west-directed fold and thrust belt. In contrast, east-directed thrusting has affected Late Proterozoic rocks between the Glidden Fault and the Pinnacles Fault. In the Halls Creek Orogen, sinistral movement took place on the Greenvale and Halls Creek Fault systems. The pattern of fault movements can be explained by southwestward movement of the Kimberley Craton during the latest Proterozoic (c. 560 Ma - the King Leopold Orogeny) causing compression in the King Leopold Orogen and sinistral strike-slip faulting in the Halls Creek Orogen. Sinistral transpression developed in the King Leopold Orogen as the Kimberley Craton was subjected to a clockwise rotation. The presence of a weak fabric asymmetry and down-dip stretching lineation on southwest-dipping fault structures such as the Speilers Shear, suggests that strike-slip movements were followed by northeast-directed thrusting in the late Cambrian (c. 500 Ma - the Spielers event). - 70 -

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Tectonism of this age, typically reactivating major Proterozoic orogenic belts, is widespread throughout Australia. In the Kimberley region deformation occurred before and after rifting in the Ord and Bonaparte Basins, which is related to the breakup of a Late Proterozoic "supercontinent". The orogens comprise: Early Proterozoic igneous and metamorphic rocks (c. 1900 - 1840 Ma), exposed in the Hooper and Lamboo Complexes; deformed Kimberley Basin rocks at the margins of the orogens; Middle Proterozoic rocks at the northwest margin of the Birrindudu Basin and in a series of basins within the orogens (Carr Boyd, Glidden and Oscar Range Groups); Late Proterozoic glacigene rocks; and Palaeozoic rocks marginal to the Bonaparte and Ord Basin.

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DEFORMATION PROCESSES IN MANTLE PERIDOTITES: THE RONDA PERIDOTITE RECORD. D. van der Wal and R.L.M. Vissers Institute of Earth Sciences, University of Utrecht, P.O. Box 80021, 3508TA Utrecht, The Netherlands

Advanced modelling applied to dynamics of the lithosphere acquires accurate insight into the geometry of deformation and rheological parameters such as grain size, a , e and pressure-temperature conditions during deformation. Such information is commonly derived from the combination of detailed field- and microstructural studies and laboratory investigations into the mechanical properties of quartz, feldspar and olivine rocks, being the major constituents of the lithospheric column. In particular, the dynamics of olivine rocks (peridotites) is critical in understanding orogeny and basin development, because the upper mantle is believed to be the strongest lithospheric layer and therefore governing the mechanical properties of the lithosphere. Growing evidence appears from seismic reflection experiments of major shear-zones penetrating well into the upper mantle, which is inconsistent with assumptions of a homogeneously stretching upper mantle during orogeny and basin evolution as applied to lithosphere modelling. Direct information to the mechanical state of the upper mantle during orogeny can be derived either from mantle xenolith suites or from Iherzolite massifs exposed in orogenic belts such as the Alpine suture. A detailed (micro)structural investigation of the Ronda peridotite (Betic Cordilleras, Southern Spain) has been performed for the above reasons and as a part of investigation of upper mantle deformation processes. Among Iherzolite massifs exposed in the Alpine suture, the Ronda peridotite occupies a unique position for the occurrence of all three peridotite facies (garnet-, spinel- and plagioclase peridotite) within 300 km2 coherent outcrop of peridotite. This allows du-ect insight in tectono-metamorphic and microphysical processes in an upper mantle section subjected to successive equilibration at different conditions during uplift and emplacement. The first step in understanding dynamics of the upper mantle during closure of the Alpine suture in the western Mediterranean is to constrain the tectono-metamorphic evolution during uplift and emplacement of the Ronda peridotite and is presented here as such. Three different structural domains have been recognised in the Ronda peridotite: 1.

In the NW part of the massif, km-scale gneissic spinel peridotites (tectonites) occur with garnet pyroxenites transposed into parallelity with the foliation. They show a typical porphyroclastic micros true ture with olivine, pyroxene and spinel aligned to the foliation. Olivine deformed by intracrystalline slip on the [100](010) slip system. At the NW periphery of the massif and also locally within the massif, the tectonites arc transected (at low angle) by up to 500 m thick mylonitic spinel and garnet bearing peridotites. Occasionally, spinel is rimmed by garnet (at a later stage transformed to kelyphite), suggesting that the mylonites developed during compressional tectonics at conditions of high and increasing pressure.

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Coarse grained, virtually undeformed (i.e. protogranular) peridotites define a second structural domain. They occur in a dome-shaped body in which annealing recrystallization led to grain growth of olivine, pyroxene and spinel. Annealing recrystallization partly reduced the rock's anisotropy as described under (1), but preserved the preferred orientation of pyroxenites and compositional banding and the olivine [100](010) petrofabric. Garnet pyroxenites recrystallised to spinel pyroxenites, indicating intermediate pressure conditions during annealing recrystallization in the Seiland subfacies of spinel peridotite.

South of the granular domain, spinel is partly transformed to plagioclase in km-scale gneissic peridotites (plagioclase tectonites) which occur in a complex low angle shear zone system. Spinel pyroxenites recrystallised to olivine gabbros. Characteristically, olivine gabbros and compositional banding occur oblique to the foliation. Plagioclase tectonites probably accommodated emplacement of the Ronda peridotite to mid- to upper crustal levels during opening of the Alboran basin in an extensional setting.

The above results are inconsistent with previous models applied to the facies development in the Ronda peridotite. It was suggested that the garnet- to plagioclase peridotite facies developed in a dynamically cooling rising diapir, in which garnet peridotites developed at the periphery of the ascending diapir, at conditions of low temperature and high pressure, whereas plagioclase peridotites developed during the final stages at conditions of low pressure and high temperature in the core of the ascending diapir. Alternatively, our results suggest that the Ronda peridotite facies distribution is closely related to deformation at various pressure and temperature conditions during uplift and emplacement of the Ronda peridotite. The high pressure peridotite facies developed during a compressional tectonic event involving strain localisation and mylonitisation during early stages of equilibration of the massif. This assemblage was partly removed in the spinel peridotite facies and involved an episode of extensive annealing recrystallization. Finally, the development of low pressure plagioclase bearing assemblages is ascribed to emplacement of the peridotite to mid- to low crustal levels in an extensional shear zone system. The above peridotite facies therefore preserve the cumulative imprints of upper mantle deformation mechanisms and processes operating during uplift and emplacement.

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PREDEFORMATIONAL PORPHYROBLASTS IN LOW PRESSURE-HIGH TEMPERATURE METAMORPHIC TERRANES R.H. Vernon^ W.J. Collins^ and S.R. Paterson^ ^School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia ^Department of Geology, University of Newcastle, Newcastle, NSW 2308, Australia ^Department of Geological Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

Collins & Vemon (1991, 1992) have suggested that in low pressure-high temperature (LPHT) metamorphic terranes of the "regional aureole" type in the Proterozoic of central Australia and the Palaeozoic of southeast Australia, the initiation of heating may precede and even promote deformation. If so, the rocks concerned may show evidence of predeformational mineral growth, as well as growth during deformation. Most porphyroblasts in schists in LPHT terranes show microstructural evidence of early syndeformational growth (e.g.. Bell et a l , 1986), which suggests that heating began relatively eaily in the deformational history of such areas. However, some porphyroblasts in LPHT terranes have small, random inclusions that indicate growth before the formation of a foliation in the matrix, and consequently, before the occurrence of appreciable penetrative deformation (e.g., Zwart, 1960; Fleming & Offler, 1968). Less commonly, "arcs" rich in mica and graphitic material extend into a non-foliated matrix from all faces of idioblastic porphyroblasts, which is also consistent with predeformational growth of the porphyroblasts. Examples of predeformational porphyroblasts of cordierite, andalusite, K-feldspar, garnet and staurolite occur in low-pressure regional metamorphic rocks in the Kanmantoo Group (Adelaide Fold Belt, South Australia), the Arunta block (central Australia), the Broken Hill block (western New South Wales, Australia), and the Foothills terrane (Western Metamorphic Belt, central Sierra Nevada, California, USA). If the predeformational interpretation is correct, the inference is that, at least in some LPHT terranes, heating precedes appreciable deformation and may promote it. The evidence provided by porphyroblast-matrix microstructural relationships supports other observations consistent with predeformational or early syndeformational heating in LPHT areas, such as the delineation of all foliations by high-grade mineral assemblages and melt leucosomes. The situation is broadly similar to that in some contact metamorphic aureoles, in which heating of the wall rocks by the intrusion occurs before final emplacement, so that deformation largely post-dates growth of contact metamorphic porphyroblasts. In both the contact and regional situations, heating may be largely responsible for the deformation, especially by promoting metamorphic reactions that liberate water at closely distributed reaction sites, thereby assisting deformation at the grain scale. RgfwngQs: Bell, T. H., 1986. Foliation development and refraction in metamorphic rocks: reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning. Journal of Structural Geology, 4, 421-444. Bell, T. H., Fleming, P. D. & Rubenach, M. J., 1986. Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development. Journal of Metamorphic Geology, 4, 37-67.

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Collins, WJ. and Vcmon, R.H., 1991. Orogeny associated with anticlockwise P-T-t paths: evidence from lowP, high-T metamorphic terranes in the Axunta Inlier, central Australia. Geology, in press. Collins, W.J. and Vernon, R.H., 1992. Palaeozoic arc growth, deformation and migration across the Lachlan Fold Belt, southeastern Australia. Tectonophysics, submitted. Fleming, P. D. & Offler, R., 1968. Pre-tectonic metamorphic crystallization in the Mt. Lofty Ranges, South Australia. Geological Magazine, 105, 356-359. Zwart, H. J., 1960. Relations between folding and metamorphism in the central Pyrenees, and their chronological succession. Geologie en Mijnbouw, 39,163-180.

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GROWTH AND DEFORMATION OF PORPHYROBLASTS IN THE FOOTHILLS TERRANE, CENTRAL SIERRA NEVADA, CALIFORNIA: TESTING SOME MODERN HYPOTHESES. R.H.

V e r n o n ^ S.R. Paterson^ and D. Foster^

^School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia ^Department of Geological Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA ^Earth Science Board, University of California, Santa Cruz, CA 95064, USA

The main porphyroblastic minerals in schists and phyllites of the Foothills terrane. Western Metamorphic Belt, central Sierra Nevada, California, are cordierite and andalusite (mostly chiastolite). Less commonly biotite, muscovite, chlorite, garnet or staurolite are also present as porphyroblasts. The variety of porphyroblast and matrix microstructures in these rocks makes them suitable for testing three modem hypotheses on growth and deformation of porphyroblasts,

namely:

(1) porphyroblast

growth

is always

syndeformational;

(2) porphyroblasts nucleate only in low-strain, largely coaxially deformed, quartz-rich (Q) domains of a crenulation foliation and are dissolved in active high-strain, non-coaxially deformed, mica-rich (M) domains, the spacing between which limits the size of the porphyroblasts; and (3) porphyroblasts generally do not rotate, with respect to geographical co-ordinates, during deformation, provided they do not deform internally, so that they may be used as reliable indicators of the orientation of former regional structural surfaces, even on the scale of orogenic belts. Many porphyroblast-matrix relationships in the Foothills terrane are inconsistent with hypotheses 1 and 2. However, some are, although alternative explanations are generally possible. For example, in many rocks it cannot be determined whether the porphyroblasts grew in rocks in which the strain had akeady been partitioned into M and Q domains, or whether the porphyroblasts caused this partitioning. Observations that do not support the general application of hypotheses 1 and 2 to rocks of the Foothills terrane include: (a) lack of evidence of residual cienulations in many strain-shadows (and alternative explanations where they arc present); (b) absence ol porphyroblasts smaller than the distance between nearest mica-rich domains; (c) nucleation of many crenulations on existing porphyroblasts, rather than nucleation of porphyroblasts between existing crenulations; (d) presence of micaceous "arcs" against some porphyroblasts in an undifferentiated, non-foliated matrix, suggesting static growth; and (e) absence of evidence of crenulations in porphyroblastic rocks showing sedimentary bedding. In addition, though most porphyroblasts are syndeformational, as predicted by hypothesis 1, porphyroblasts with very small, random inclusions are probably predeformational. Similarly, porphyroblasts that have overgrown sets of crenulations and porphyroblasts with micaceous "arcs" in foliated matrix are probably post-deformational, at least on the scale of a large thin section and probably over much larger areas, judging from mesoscopic structural evidence. Such porphyroblasts are too far away from possible active micaceous domains to have received nutrient components from them, as required by the growth model of hypothesis 2. Although the evidence is inconclusive, some porphyroblasts in the Foothills terrane do not appear to have rotated, with respect to geographical co-ordinates, during matrix deformation, at least on the scale of a large thin section, which supports hypothesis 3. However, other porphyroblasts evidently have rotated. In some instances,

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this appears to be due to mutual interference, but many apparently rotational porphyroblasts are too far apart to have interfered with each other, which indicates that the porphyroblast rotation was associated with deformation of the matrix. The occurrence of porphyroblasts with oblique inclusion trails, about which foliation surfaces are locally folded, without folding of nearby thin beds, suggests rotation of the porphyroblasts during non-coaxial flow parallel to bedding, rather than static porphyroblasts with crenulation relics that have been obliterated elsewhere by later deformation. Thus it appears that porphyroblasts may rotate during deformation if the matrix is relatively homogeneous, so that the strain is effectively non-coaxial. In zones of strong multiple deformation (e.g. the Bear Mountains fault zone), this rotation may have occurred after homogenisation of the matrix in response to the strongest degree of crenulation folding, whereas the same porphyroblasts may have been inhibited from rotating previously, when strain accumulation was partitioned in the matrix. However, this does not apply to porphyroblastic rocks outside this zone, in which the deformation has been much less intense.

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NEWTONIAN VISCOUS CREEP OF A QUARTZITE AT LOW STRESSES J.N. Wangl, A. Ord^ and B, Hobbs^ ^Department, of Earth Sciences, Monash University, Clayton, Vic. 3168 Australia ^CSmO Division of Geomechanics, P.O.Box 54, Mt. Waverley, Vic. 3149 Australia

Heavitree quartzite with an average grain size of 200ti has been experimentally deformed in creep at 700 to 900X, ISOOMPa confining pressure, and 100 to llOOMPa differential stress. Specimens are sealed in silver capsules together with a small amount of solid oxygen buffer and excess water with the aim of controlling the chemical environment The buffers utilised are Ta/Ta205, M0/M0O2, Ni/NiO. Cu/Cu20 and Mn304/Mn203, which generate a wide range of oxygen, water, and hydrogen fugacities. The creep rate of the specimens varies with temperature, stress (fig. a), and chemical environment(fig. b). The steady state creep rate is plotted logarithmically against the stress as shown in Fig. c. The plotted results exhibit two distinct regimes of behaviour in all imposed chemical environments: at low stresses (a<300MPa), the stress exponent n is about unity; at high stresses (<T>300MPa), it is about 2.5 (fig. c). Therefore, the quartzite behaves as a Newtonian material at low su-esses, but as a non-Newtonian material at high stresses.The activation energy for the creep in the Newtonian regime is found to be the same as that for the creep in the non-Newtonian regime within experimental errors. In the Newtonian flow regime, microstructures in specimens deformed in environments with high f02 and fH2 , but low fH2 (buffered by Mn and Cu) are characterised by few deformation lamellae, formation of subgrains, development of intragranular recrystallization, and flattening of both original and recrystallized grains at high strains. Microstructures in specimens deformed in environments with low f02 and fH20 but high fH2 (buffered by Ta and Mo) are characterised by extensive deformation lamellae, homogeneous flattening, rare formation of subgrains and absence of intragranular recrystallization. C-axes in the Mn buffered specimens develop a small circle girdle about the compressive direction(fig. d), and those in the Ta buffered specimens develop a maximum parallel to the compressive direction(fig. d). Based on the rheological data, microstructural observations, and c-axis orientation measurements, the Newtonian viscous behaviour at low stresses is believed to be induced by a dislocation process which operates in HarperDom creep.

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50-

Cu buffer

300MPa

40c

lOOMPa^

30-

200MPa

5 55 2010-

800®C

(a)

TimexlO ^ (seconds) 100 200

TimexlO

300

500

700

(seconds) (MPa)

800®C n: stress exponent -4' I cT

n-101. n-1.07 ^ n-0.96 «• • ^• ji n-1.19

(c)

n-2.57

1 • • •

-7'

2.0

2.2

.I'** #

• Mn buffer A Cu buffer o Mo buffer w Ta buffer 2.4

2.6

2.8

3.0

log(stress) (MPa)

Mn buffer. Ta buffer 50% strain,158 grains 30% strain, 205 grains C-axis preferred orientations in Mn and Ta buffered specimens deformed at lOOMPa stress and 800X, compression direction vertical, equal area, contours( /1% area).

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TRANSPRESSIVE DEFORMATION AT BROKEN HILL, AUSTRALIA AND THE PROBLEM OF VERTICAL LINEATIONS IN TRANSCURRENT SHEAR ZONES P.F. Williams^ and R.H, Vernon2 ^Department of Geology, University of New Brunswick, Fredericton, N.B., Canada ^School of Earth Sciences, Macquarie University, N.S.W. 2109, Australia The Little Broken Hill area south of Broken Hill is characterised by sillimanite grade metamorphic rocks including sillimanite gneiss and schist, garnet biotite gneiss, large deformed pegmatite bodies, amphibolites, banded ironstones and Broken Hill type mineralisation. The regional foliation ( S i ) is a product of intense transposition and dips generally at moderate angles in a northwestly direction. A prominent mineral lineation that is best developed in the sillimanite bearing rocks lies in the foliation and plunges shallowly in a southwestly du^ction. The early transposition fabric is overprinted by a set of vertical, northwesterly trending dolerite dykes and numerous approximately vertical shear zones that trend northeasterly and northwesterly. Both sets of shear zones are characterised by a new foliation and a stretching lineation that plunges parallel to their mutual line of intersection. There is no consistent overprinting relationship between the shear zones which suggests that they are contemporaneous. Their conjugate geometry with a vertical intersection, large horizontal separation of markers and general fabric symmetry indicate that despite the orientation of the stretching lineation the shear zones were transcument. The northwestly trending zones can be divided into two groups: (a) Simple zones with dextral horizontal separation, sympathetic "drag" of S i and a vertical, sigmoidal shear zone foliation (S2). The latter is inclined to the shear zone boundaries by a little less than 45'' in the margins of the zones and is approximately parallel to the zones in their centres, (b) Complex zones with sinistral horizontal separation, dextral "drag" and a strongly folded S2. The axial planes of the folds are approximately parallel to the zone boundaries and the folds plunge parallel to the stretching lineation. There is no evidence of sheath folding. Where a dolerite dyke occurs in a simple shear zone it is undeformed, but where the dykes occur in the complex zones they are deformed. The northeasterly trending shear zones are approximately parallel to the general trend of S 1 and perpendicular to the dolerite dykes. The dykes are consistently offset dextrally. Where the shear zones are inclined to S1 the horizontal separation of marker layers is mostly sinistral but may be dextral. There is no consistent "drag" associated with these zones and S2 is approximately parallel to the zone boundaries. All of the data can be explained in terms of the following history: First a conjugate pair of transcurrent shear zones developed in response to horizontal minimum and maximum compressive directions with the maximum direction trending north/south. This caused the dcxU"al "drag" and displacement on the northwest trending zones and the sinistral displacement on the northeast trending zones. The maximum compressive direction then rotated to approximately northwest and the dolerite dykes were emplaced. At this time the minimum compression was probably still horizontal. Then the maximum compressive axis rotated to a westerly direction and the minimum

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axis switched to a vertical orientation. The existing shear zones, behaving as zones of weakness, were reactivated and accommodated the east-west shortening and vertical extension was accommodated by slip on S ] . Consequently the shear zones behaved as transpressive stretching faults with shear zone-parallel extension occurring vertically. The reactivation reversed the sense of movement on the shear zones and in many the net result is the opposite sense of displacement to that of the original movement.

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REGIONAL-SCALE SHEATH FOLDING IN THE ARUNTA BLOCK, CENTRAL AUSTRALIA P.F. Williams^, A.R, Norman^ and R.H. Vernon^ ^Department of Geology, University of New Brunswick, Fredericton, N.B., Canada ^School of Earth Sciences, Macquarie University, N.S.W., Australia

Complex fold patterns in the Strangways Metamorphic Complex around Cadney Creek at the eastern end of the Central Zone of the Arunta Block indicate at least three generations of macroscopic structures. All are shown to have sheath-fold geometry, a pattern that is well known in the region. A prominent stretching lineation plunges parallel to the sheath-fold axes and both have an unusually constant plunge (approximately

towards the NE)

throughout the area mapped. On a regional scale the direction of plunge shows greater variation but is still predominantly in the NE quadrant throughout an area approximately 50 x 100 km (Goscombe, 1991). The area is cut by numerous shear zones which vary in trend but all intersect in the stretching lineation which is well developed within them. The shear zones may be tens of metres wide but in general do not significantly displace markers. They are recognised by sudden changes in the orientation of foliation; both schistosity and layering within the shear zones are approximately parallel to the shear zone boundaries. They are also characterised by their fine grain size which is generally at least an order finer than that of the country rock. Both characteristics suggest high strains and it has to be asked therefore why the shear zones do not produce significant displacement of markers. The answer is that, since the markers are simply layering and since movement, as indicated by the stretching lineation, is parallel to the sheath-fold axes the movement vector is parallel to the intersection of shear zone and marker layers. It would therefore require very considerable movement to cause any recognisable offset of the markers, except where a sheath terminal closure is brought to the surface. Such a situation has been recognised and as might be expected the markers are displaced both dextrally and sinistrally by the same shear zone. Despite the fine grain size there is no evidence that the shear zone rocks are any lower in grade than the general granulite facies metamorphism of the area. This and the fact that they share the same movement vector as the regional sheath-folds suggests that they represent a late stage in the deformation that produced the latter. The fine grain size suggests rapid exhumation immediately after deformation. The lack of variation in the metamorphic grade suggests that the folds are essentially in the orientation in which they developed (Goscombe, 1991). Goscombe (ibid) has interpreted them as having formed as drag folds during progressive deformation in a steep zone (of the order of 50 km wide) of non-coaxial strain resulting from crustal shortening. The same explanation has been proposed elsewhere for more shallowly plunging sheath-folds (e.g. Williams and Zwart, 1977). The width of the Strangways zone however, requires that the displacement across the zone be unrealistically large if the necessary strain magnitudes are to be produced in this manner; some other explanation seems necessary.

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As an alternative hypothesis it is suggested that the sheath-folds did develop in such a crustal shear zone but that they developed in response to the shortening of a very heterogeneous system. Strong bodies of rock moved within the shear zone so as to occupy the minimum space in a direction parallel to the bulk shortening direction and in the process draped the regional layering around them and in doing so formed sheath-folds. Given the right starting geometry much smaller strains are required to produce sheath folds by this model than by the dragfold model. R^f^nces: Goscombe, B., 1991. Intense non-coaxial shear and the development of mega-scale sheath folds in the Arunta Block, Central A u s t r a l i a , o f Structural Geology, v. 13, pp. 299-318. Williams, P.F., & Zwart, H.J., 1977. A model for the development of the Seve-Koli Caledonian nappe complex. In: Energetics of Geological Processes, Saxena, S.K., & Bhattacharji, S., eds.. Springer, New York, 168-187.

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POROSITY EVOLUTION DURING EXPERIMENTAL COMPACTION AND DEFORMATION OF CALCITE ROCKS S.Zhang, S.F. Cox and M.S.Paterson Research School of Earth Sciences. The Australian National University, GPO Box4, Canberra ACT 2601, Australia

Synthetic calcite aggregates with initial average grain size in the range 38-53 pm have been isostatically hotpressed (HIP) in a gas confining medium apparatus. Temperature ranged up to SOO'^C; confining pressure ranged from 200MPa to 300MPa. No pore fluid was used in the hot-pressing experiments and specimens were vented to air. At temperatures up to 400''C during HIP, porosity reduction from initial values of 20% is controlled by plastic deformation and grain crushing and results in porosities falling to slightly less than 10% in 50 minutes. Under these conditions , high connectivity of porosity is maintained along high aspect ratio pores at grain interfaces and along irregular grain edge pores. At temperatures higher than 400''C. microstructural analysis indicate that more substantial porosity reduction and progressive decrease of pore connectivity is associated with increased plasticity, grain boundary migration, the development of polygonal grain fabrics and grain growth. The formation of isolated, lenticular pores on grain interfaces and along grain edges appears to be controlled by equilibrium wetting angles. The effects of deformation on the porosity (<()) of hot-pressed calcite polycrystals and Carrara marble have been examined during constant strain rate £ (10"^ s"^) shortening at a confining pressure of 200MPa, argon pore pressures ranging from 50MPa to ISOMPa, and at temperatures up to 400°C. At room temperature the rate of the change of dilatancy with axial strain (d<t)/de) increases with increasing pore fluid pressure (decreasing effective pressure) and decreases with decreasing grain size. Microstructural studies indicate that in coarse-grained samples, transgranular cracks are dominant and crack aspect ratios are larger than 30. In contrast, in fine-grained material, intergranular cracks are well developed, and the crack length is comparable to the grain size. The average aspect ratio of the grain boundary cracks is about 10. At elevated temperature and high pore pressure, crack aspect ratios tend to be lower than at room temperature and cracks are not well connected. The experimental results illustrate how fluid transport in the earth's crust should depend on the competition between deformation - induced dilatancy and porosity- reducing processes such as plastic flow and crack-healing.

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PREFERRED ORIENTATION DEVELOPMENT OF POLYCRYSTALS WITH ONE SLIP SYSTEM AND WITH GRAIN BOUNDARY SLIDING Y. Z h a n g l , B. Hobbs^ and A. Ord^ ^Department.of Earth Sciences, Monash University, Clayton, Vic. 3168 Australia ^CSIRO Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149 Australia

Preferred orientation development in polycrystals with one slip system has been examined by using a finite difference code FLAG (Cundall & Board, 1988). The model assumes that dislocation glide is the major strain accommodating mechanism and that dislocation climb is a supplementary one. The model does not assume homogeneity of stress or strain so that each grain deforms inhomogeneously with the formation of zones of diffuse or localized deformation. Strain compatibility and a geometry where grain boundaries remain in contact throughout deformation are maintained. This differs from Etchecopar's (1977) model which results in the formation of large gaps and overlaps. Each grain is modelled as an elastic-perfectly plastic material. Three types of finite-difference meshes represent three types of numerical polycrystal specimens as starting configurations. They are: type I with fine grains, type II with square grains and type III with hexagonal grains, each of these has an initially random and an initially uniform orientation distribution for slip planes. The specimens are subjected to axial shortening, axial extension, pure shearing and simple shearing deformation histories. The preferred orientations of slip plane normals are well developed in these deformed polycrystals. Axial shortening: for specimen I at 30.8% overall shortening (OS), a broad maximum is situated parallel to the OS direction; specimens II and III at 28.6% OS both possess two maxima symmetrical about the OS direction at 30'' and 25'' respectively, while a third maximum, approximately parallel to the OS direction, exists for specimen II. Axial extension: for specimen I at 47% and II and III at 33.3% overall extension, the slip plane normals are all preferentially oriented around the bulk shortening direction. Pure shearing: with a constant shortening rate for specimens I and II at 38.5% and 39% OS respectively, one broad maximum appears around the OS direction; for specimen III at 38.1% OS, two maxima develop symmetrically about the OS direction at 15^. Simple shearing: for specimen I (Y= 0.94), two maxima are at 20'' to and symmetric about the bulk shortening direction (40° and 80^" oblique to the shearing direction respectively); for specimens II and III (y =0.715), the results show that one maximum is approximately normal to the shearing direction, and a second maximum is symmetric to the first about the bulk shortening direction and oblique to the shearing plane. A third maximum exists for specimen III, which is close to the bulk shortening direction. Grain shape fabrics are also well developed. A general trend is that elongate grains show preferred concentration of long axes normal to the bulk shortening direction. Some different types of microstructures are observed within grains, such as kinks, subgrains and shear bands.

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1991

The preferred orientation results show good agreement with that observed in experimentally sheared ice and flattened peridotites and are basically consistent with Etchecopar's results. The obliquity of one maximum to a shear plane may be used as an indicator of shear sense. Also considered here are situations where strain is partly accommodated by grain boundary sliding (fig. 1). In these models, g ^ s at grain boundaries develop as one of the strain accommodation mechanisms. Rgf^rgpcgg: Cundall, P.A. and Board, M., 1988. A Microcomputer Program for Modelling Large strain Plasticity Problem. 6th Int. Conf. on Numerical Methods in Geomechanics. Innsbruck, Austria, 11-15 April. Etchcopar, A., 1977. A plane kinematic model of progressive deformation in a polycrystalline aggregate. Tectonophysics.,39:121-139.

Figure 1. The traces of slip planes for experiments with grain boundary sliding incorporated, (a) axial shortening, 28.5% OS; (b) pure shearing, 39.5% OS; (c) simple shearing, y = 0.714.

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