Issuu

Geological Society of Australia

ABSTRACTS Number

2001: A Structural Odyssey

Specialist Group in Tectonics and Structural Geology Ulverstone, Tasmania 12-16 February 2001

Geological Society of Australia

ABSTRACTS Number

2001: A Structural Odyssey Editors: Garry Davidson and June Pongratz

Specialist Group in Tectonics and Structural Geology Ulverstone, Tasmania 1 2 - 1 6 February 2001

UNIVERSITY OF TASMANIA

Produced by Centre for Ore Deposit Research and the School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001

February 2001

ISBN 0 85901 941 1

CODES SRC

TASMANIA

Centre for Ore Deposit Research

MINERAL RESOURCES TASMANIA

another Pongratz Production 2001 Primed by Uniprini, Hoban

Foreword

This foreword is our opportunity to get history straight. Firstly, although all on the organising committee made their contributions, the human powerhouse behind this year's conference has been Ron Berry, from conception, to organisation, to implementation. (We only say this so that no one blames the rest of us!). The themes of the conference essentially established themselves through the registrant contributions. The exceptions to this were Chris Powell's "Continent colUsion processes" symposium, and a promise we have kept to dwell on the correlations between Tasmania and mainland Australia. It is interesting to think, therefore, that the contributions that follow are a reasonable reflection of the focii of Australian structural geology today. We hope you judge the first SGTSG conference of the millenium to be a success. May there be many more 'Odder Seas' in the future! . y f ^ ^

Your organising committee was: Ron Berry Convenor & excursion leader (University of Tasmania) Stuart Bull Committee member and wine specialist (University of Tasmania) Garry Davidson Abstracts editor (University of Tasmania) Oliver Holm Excursion leader (University of Tasmania) June Pongratz Abstracts editor and desktop publisher (University of Tasmania) Alistair Reed Excursion leader (Mineral Resources Tasmania) David Seymour Treasurer (Mineral Resources Tasmania) Robert Scott Secretary and excursion leader (University of Tasmania) Nick Turner Committee member (Independent) Garry Davidson and June Pongratz (editors) January 2001

SGTSG abstracts, February 2001

Contents

Ailleres, L., N J . Archibald and G.S. Lister The Banda Arc: a 500 km amplitude fold of the lithosphere? Baxter, J.L, P.C. Sauter and N.C. Laubsch *Examination of the structural control of mineralisation in the Kalgoorlie district, Western Australia Berry, R. *Arc-continent collision: a comparison of Timor and Tasmania Betts P. and D. Giles *1.6-1.5 Ga orogenies of Eastern Australia Blewett, R.S. Evidence for mid Archaean continental collisions in the development of the dome-and-basin geology of the North Pilbara Craton Bodorkos, S., M. Sandiford, N. Oliver and P. Cawood *High-r, low-P metamorphism as the middle crustal response to a mantle-related transient thermal pulse: a numerical model and its application to the Palaeoproterozoic Halls Creek Orogen, northern Australia Buick, I.S., J.A. Miller, LS. Williams, M. Hand and J. Mawby Deep holes in the Centralian Superbasin, SHRIMP zircon constraints on the depositional age of granulites in the eastern Arunta Block Cawood, P.A. and E.G. Leitch Terra Australis Orogen: Rodinian breakup and subduction initiation in the proto-Pacific Ghen, S.R Kinematics and tectonic roles of large-scale shear zones in the Yilgarn Graton, Western Australia Gho, M., S.R. Lee and K. Yi *Permo-Triassic exhumation of mid-crustal granulites, central Korea: relationship with continental collision in the Dabie-Sulu belt, Ghina Gobb, M.M., PA. Gawood, PD. Kinny and LG.W. Fitzsimons *SHRIMP U-Pb zircon ages from the Mullingarra Complex, Western Australia: Isotopic evidence for allochthonous blocks in the Pinjarra Orogen and implications for East Gondwana assembly Collins, A , B. Windley, L Fitzsimons and B. Hulscher The tectonic architecture of central Madagascar - Using tectonostratigraphic principles to unravel the deep expression of a continental collisional zone Collins, A. *The tectonic evolution of the Lycian Allochthon and implications for the evolution of SW Turkey Collins, W.J. Nature of extensional accretionary orogens Cox, S.R, K. Ruming, PT. Nguyen and W.E. Stone The behaviour of faults, fluids and gold in a crustal scale shear system, St Ives Goldfield, WA — A case of golden aftershocks? Gunneen, J. *Neogene fault reactivation and structural styles in the Timor Sea, northwest Australia Daczko, N.R., K.A. Klepeis and G.L. Clarke Thermo-mechanical evolution of the crust during convergence and mid-crustal pluton emplacement in the Western Province of Fiordland, New Zealand Das, P.K., A. Cummings and N. Lemon Seismic expression of layer-bound fault system in the Late Tertiary of the Bass Basin: indications of syneresis of colloidal sediments Ding, P *Major gold deposits in the Tanami region and their associated structural geology and tectonics Ding, P Field evidence for frequent Paleoproterozoic orogenies in northern and western Arunta Otogenic Province,

SGTSG abstracts, February 2001

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Direen N.G. and A J . Crawford A tectonic odyssey-the hunt for Australia's Rodinian boundary, and significance of the "Tasman Line" Dong, Y., C. Zhang and K. Guo *Structure and tectonic evolution of Southern Altay, Xinjiang and its relationship with gold deposit Duboz, C., S. Pisarevsky, C.McA. Powell and G. Lister *Global sea level prediction for the past 300 Myr using PlatyPlus technology Fergusson C.L. and ODP Leg 190 Shipboard Scientific Party ODP Leg 190 Nankai Trough - stratigraphic/structural results and implications for the tectonostratigraphic development of accretionary prisms Fitzsimons, LC.W., M.M. Cobb, PA. Cawood and PD. Kinny Proterozoic accretion and reactivation along the Darling Mobile Belt of Western Australia Forster M.A. and G.S. Lister *The method of asymptotes and limits applied to the interpretation of apparent age spectra produced in 40Ar/39Ar step heating experiments Foster, D., D.R. Gray and C.V. Spaggiari *Ar-Ar geochronology and timing of deformation in the Tasmanides: What do we know? Foster, D.A., M.C. Fanning, S. Coyner and A. Raza Evolution and exhumation of the Bitterroot metamorphic core complex, Montana and Idaho, USA Fowler, A. *New interpretation for terrane development of western New Zealand, based on detrital zircon age distributions and geochemistry Gallen, M J . , S.M. Reddy and PA. Cawood ^Deformation history in the hinge region of a chevron fold Giles, D., P. Betts and G. Lister A continental back-arc setting for the Early to Middle Proterozoic basins of north-eastern Australia Goscombe, B. and J. Everard *Tectonic evolution of Macquarie Island Goscombe, B. and M. Hand *Metamorphic model for the Inland Branch of the Damara Orogen: A Paired Metamorphic Mountain Belt Goscombe, B. and M. Hand *Tectonometamorphic Evolution of the Kaoko Belt, Namibia Goscombe, B. and C. Passchier *Boudin trains as a kinematic tool kit Gray, D.R. and R.T. Gregory Ophiolite obduction, Oman: Passive or non-passive margin behaviour? Gray, D.R., J.McL. Miller and R.T. Gregory *Structure, strain and kinematic constraints on the evolution of the Saih Hatat fold-nappe beneath the Samail ophiolite nappe, Oman Guo, K., C. Zhang and Y. Dong The two ophiolite belts in Qinling orogen and their constraints on the evolution of the Qinling-Dabieshan orogen, China Haines, PW., S.P Turner, S.P Kelley and J. Wartho Comparison of detrital muscovite and detrital zircon dating in the Adelaide fold-thrust belt: Implications for the tectonics of the source areas Hand M. and J. Mawby *The Larapinta Event: early Ordovician rifting in central Australia Hand, M., B. Bendall and J. Mawby *The metamorphic expression of the Alice Springs Orogeny Hansen, D., Q. Hills, M. Krabbendam and D. Giles Syn-extensional deposition of the Upper Etheridge Group: an explanation for heterogeneous deformation intensity across the Georgetown Inlier, Queensland Harbort, T , R.J. Holcombe, P. Vasconcelos and C.R. Fielding *Latest Permian emplacement of the Marlborough Block duplex: the major mountain-building phase of the Hunter-Bowen Orogeny in the northern NEFB Harris, A.C. *Structural controls on vein emplacement in porphyry-style mineralisation, Goonumbla, NSW Hills, Q.G., D. Giles, G. Rosendam, C. Forbes and G.S. Lister The Eldee Structure: A kilometre-scale sheath fold at the northwestern margin of the Broken Hill Block, NSW; Australia

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SGTSG abstracts, February 2001

Hoadley, E., M. Rubenach, D. Coleborn, I. Sisois and M. Fanning Significance of an amphibolite facies shear zone event synchronous with the emplacement of the Sybella Bathoiith, Mount Isa 83 Hobbs, B.E., H-B. Muhlhaus, A. Ord and L.N. Moresi Metamorphic differentiation 84 Holcombe, R.J., R. Gordon, C. Pratt, M. Carder *Anatomy of the Mount Isa Fault system: thrust exhumation of middle crustal elements of the Mount Isa Rift basement during the late Isan Orogeny 85 Hollingsworth, D.A., P.A. Cawood & B. Monek ^Reconstruction of an Archaean granite-greenstone terrane: an example from the Sylvania Inlier, WA 87 Holm, O., R. Berry and D. Steele Defining Tasmania's deformation history - geochronology of metamorphic monazites 89 Hulscher, B., A.S. Collins, K.L. Dahl, I.C.W. Fitzsimons, S.P. Johnson, M.K. Jonsson, A.R. Passmore, C.McA. Powell Evidence for 800 Ma and possibly older Deformation and Plutonism in Madagascar 91 Huynh, T.H. and L. Ailleres Three-dimensional geometrical, petrophysical and geophysical modelling of the Cannington deposit, eastern Mount Isa Inlier 93 James, P. and C. Conor *Multiscale ductile and brittle shear response to Olarian and Delamerian deformation in the eastern Weekaroo Inlier, Olary Domain, South Australia 95 Johnson, S.L. *High-pressure metamorphism and the production of yoderite in the Usagaran Belt, central Tanzania 97 Johnson, S.P. Are whiteschists indicators of A' type high-pressure (HP)-ultra-high-pressure-metamorphic (UHPM) belts? Implications for continental subduction and collision in the Zambezi Belt and the amalgamation of Sub-Saharan Africa 99 Jones, S.A., R. Berry and Boliden Geological Staff Recognition of early growth structures after multiple deformation episodes at Myra Falls VHMS camp, Vancouver Island, BC, Canada 101 Keep, M. Neogene collision: the response of Australia's northwest margin 103 Kruhl,J.H. and M. Peternell *Sutured grain boundaries of dynamically recrystallized quartz: Crystallographic orientation and temperature control 105 Lau, I. and P. James *Differentiation of Delamerian and Holocene thrusting, shearing and faulting in the southern Mount Lofty Ranges using remote sensing data and GIS analysis 107 Lennox, P.G., R. Trzebski, A. Miiller, W. Siebel and R. Armstrong Crustal evolution of the northern Eastern Lachlan Fold Belt 108 Li, Z.X. *Understanding the Precambrian tectonic events in Tasmania: clues from south China 110 Lister, G.S., M.A. Forster and T.J. Rawling Formation and exhumation of high-pressure metamorphic terranes as the result of the operation of lithospheric dislocations that reverse their shear sense 112 Litde, T.A., R.J. Holcombe and B.R. Ilg Middle crustal processes of oblique collision in the central Southern Alps, New Zealand: What record is provided by ductile fabrics in the Alpine Schist? 114 Marshall, L.J. and N.H.S. Oliver Regional brecciation and relationships to metasomatism and magmatism at the Gilded Rose breccia type area. Northwest Queensland 116 Meffre, S., R.E Berry, M. Hall and A. McNeill The structural style of Cambrian Metamorphic Complexes in Tasmania: SW Tasmanian examples 118 Miller, J.MCL., C.J.L. Wilson and B.A. Witham The Magdala Lode System, Stawell, southeastern Australia: structural style and relationship to gold mineralisation across the western Lachlan Fold Belt 121 Mohajjel, M., C.L. Fergusson, M.R. Sahandi *Cretaceous continental collision of the Sanandaj-Sirjan Zone, western Iran 123 Morand, V.J., R.A. Cayley, K.E. Wohlt and D.H. Taylor Ultramafic rocks in the Glenelg Zone of western Victoria: tectonic implications 124

SGTSG abstracts, February 2001

Murphy, J.B., R.D. Nance, R.A. Strachan, K.D. Parker and M.B. Fowler Global-scale constraints from small terranes: a 1.2-1.0 Ga tectonothermal event in Avalonia (Appalachian Orogen) and constraints for the evolution of Rodinia Noble, M.P. and G.S. Lister *Discovery of a kilometre-scale early extensional shear zone that deformed the Broken Hill Pb-Zn-Ag orebody, NSW, Australia Norris, R.J. and RO. Koons How appropriate are the Southern Alps of New Zealand as a model for continental collision? Ord, A., L. Moresi and B.E. Hobbs Modelling magma ascent Passmore, A.R. and C.McA. Powell *Polyphase deformation of the Itremo Group in the Kiangara Mountains, central Madagascar Pisarevsky, S.A. and C.McA. Powell Palaeomagnetic constraints for the building blocks of Rodinia Potts, G J . and S.M. Reddy *The systematic analysis of porphyroblast-matrix relationships in deformed rocks Powell, C.McA., M.T.D. Wingate, R.D. Tucker, I.C.W. Fitzsimons and L.D Ashwal *The Mania River traverse across central Madagascar Powell, C.McA., K.L. Dahl, B. Hulscher, S.P Johnson, A.R. Passmore, A.S. Collins, I.C.W. Fitzsimons and M.K. Jonsson *The significance of 800 Ma or possibly older deformation in central Madagascar Powell, C.McA. and S.A. Pisarevsky *The map of Rodinia Powell, C.McA. Continental collision: Processes, consequences and speculations Preiss, W.V. Neoproterozoic extension and Cambrian plate convergence in South Australia; relationships with Tasmania — Reddy, S.M. and S.A. Occhipinti Discrimination of transpression and reactivation in high-strain zones Reddy, S.M., G.J. Potts and S.P Kelley *Microstructure and ^^Ar/^^Ar apparent ages in deformed K-feldspar: Evidence for a microstructural control on Ar isotope systematics Reid, A. and P. James *The structural configuration and evolution of Lower Lead Lode and the 2 Lens Dropper, Broken Hill, NSW Rey, PF. Collisional processes through time Richardson, J. and P Betts *The evolution of Proterozoic rocks of the Peake and Denison Inlier: implications for tectonic setting Packard, M.J. History of structural geology (an incomplete and personal view) Rickard, M.J. *The Tilba Tilba metamorphic and dyke complex — a new discovery Rosenbaum, G., G.S. Lister and C. Duboz Tectonic reconstruction of the Western Mediterranean and the Alpine chain since the Oligocene Rudge, T , C. Duboz, M. Norvick, G. Lister, D. Fitzgerald, B. Kohn, A. Gleadow and R. Brown Use of the PlatyPlus reconstruction software for geometric reconstruction of the evolution of sedimentary basins on the Australian Craton between 150-100 Ma Ruming, K. and S.F. Cox Structural evolution, fluid flow and gold mineralisation in the Victory Thrust Complex, St Ives Russell-Head, D. and C.J.L. Wilson *Automated Fabric Analyser system for quartz and ice Sandiford, M. and A. Gleadow An intraplate response to plate margin orogeny: the SE Australian neotectonic record Schaubs, P. *Chevron folding and fluid-flow in the Bendigo gold field, central Victoria: Constraints on gold mineralisation from numerical models Schellart, W.P and M . W Jessell The role of potential energy in the collapse of mountain belts

Vii 126

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137 139 140 142 144

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SGTSG abstracts, February 2001

Viii

Scott, R J . a n d R.F. Berry *Structural data from oriented and unoriented drill core 165 Scott, R J . , S.W. Bull and D. Selley Megabreccias in the McArthur Basin, N.T.: earthquake-triggered collapse of a carbonate-platform succession — 167 Scrimgeour, L, J.G. Raith and W. Frank 40^.39Ar constraints on north-vergent Palaeozoic intraplate deformation and exhumation in the northeastern Selley, D. Origin and tectonic significance of Broken Formation, Dundas region, western Tasmania Sheppard, S., S.A. Occhipinti, and LM. Tyler Links between regional-scale deformation and granite generation: the Yarlarweelor gneiss complex, Western Sims, J.R, M. Hand, C J . L . Wilson and C.M. Fanning * Cambrian-aged ultra-high temperature metamorphism in the Rauer Group, east Antarctica Sorjonen-Ward, R, R Cow, Y. Zhang, B. Drummond, B. Goleby Ore inspiring structures - some perspectives from seismic surveys and numerical modelling Spaggiari, C.V., D.R. Gray, D.A. Foster and E. Harris *The Governor Fault: A structural and geophysical perspective of the boundary between the western and central Lachlan orogen Stallard, A.R. and K.H. Hickey Can Zand A^help us to understand fold mechanisms in schistose rocks? Taylor, D.H. The Dimboola Igneous Complex in western Victoria: The crash test dummy for a Delamerian arc-continent Tong, L., J J . Vassallo and C J . L . Wilson ^Tectonic significance of the P-T-D path of the Sleaford Complex granulites at Fishery Bay, Eyre Peninsula, South Australia Trzebski, R., RG. Lennox and D. Palmer *A new gravity traverse across the northern Wyangala Batholith (Eastern Lachlan Fold Belt) Tyler, LM. Collision orogeny during the Palaeoproterozoic in Western Australia Vandenberg, L.C., M.A. Hendrickx, A.J. Crispe, K.R. Slater and A.A. Dean Unravelling polydeformed Palaeoproterozoic basement structure of the Tanami gold province, NT: 5% outcrop, 100% geophysics Vassallo,J.J. and C J . L Wilson Regional non-coaxial deformation of Archaean-Palaeoproterozoic basement in Eyre Peninsula, South Australia Wijns, C., L. Moresi, B. Davies and A. Ord Understanding an evolving orogen Willetts, G., B. Hobbs and A. Ord Geodynamic modelling of Irish-type base metal deposits of the central Irish midlands, and genetic comparisons with the Rosebery lead-zinc deposit, Tasmania Willman, C.E., A.H.M. VandenBerg, B.A. Simons, C. Quinn and V J . Morand ^Structure of the Tabberabbera Zone in Victoria Witham, B., J.McL. Miller, C J . L . Wilson and G. Phillips ^Influence of hydrothermal activity on the Federal-Albion area of the Magdala Mine, Stawell, Victoria Zhang, C., K. Guo and Y. Dong The ophiolites in Mian-Lue zone and Mian-Lue-Ning Block in South Qinling, China and their tectonic significance Zhang, Y, B.E. Hobbs, A. Barnicoat, A. Ord and J.L. Walshe Faulting related deformation and fluid flow Zwingmann, H., T Wilson and R. Offler K-Ar dating of clay-rich fault gouge, northern Sydney Basin Email addresses Index of authors

* Poster presentation

SGTSG abstracts, February 2001

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The Banda Arc: A 500 km amplitude fold of the lithosphere? L AilleresS N.J. Archibald^and G.S. Lister^ ^ Australian Crustal Research Centre, Dept Earth Sciences, PO Box 28E, Monash University, Victoria, Australia 3800 2 Fractal Graphics Pty Ltd, 39 Fairway, Nedlands, WA, Australia 6009

The modern geodynamic evolution of the northern margin of the Australian plate started with the break up of Gondwana from late-Carboniferous to earlyPermian, in the west (Metcalfe, 1996), to Cretaceous, further east (Lister et al., 1991; Lister & Etheridge, 1989). Rifting during the break up separated slivers of continental crust from the Indo-Australian plate (Metcalfe, 1996; Rawling, 1998). Since then, the area has suffered a very complex geodynamic history involving (1) rifting and extension resulting in the opening of three successive oceans (PaleoTethys, Meso-Tethys and Ceno-Tethys during the Paleozoic, Mesozoic and Cenozoic respectively), and (2) subduction and convergence at plate margins resulting in the closure of these oceans through time. The tectonics of the area was, and still is, controlled by the interaction between major tectonic plates (Australian Plate, Pacific Plate, the Caroline Plate and the Philippines Sea Plate) and further away by the collision of India into Eurasia. At a smaller scale, more variations appear due to the way by which convergence at plate boundaries is expressed. These variations range from "normal" subduction of oceanic crust, subduction of buoyant continental slivers, arc-arc collision, arccontinent collision, etc. Recent tectonic syntheses have been compiled (Hall & Blundell, 1996 and included references) and kinematic reconstructions have been attempted (Rangin et al., 1990; Daly et al., 1991; Lee & Lawler, 1994; Hall & Blundell, 1996). These reconstructions have been made in two dimensions. They assume that tectonic plates behave as mechanical rigid-bodies. The reconstructions do not take into account the third dimension where changes in the shape and location of the subduction zone may occur. McCaffrey (1996) recognised that most of the subduction zones in the SW Pacific did not have a slip direction perpendicular to the trend of the subduction zone, but rather that strain was partitioned. This illustrates the need for a better three-dimensional understanding of convergence at plate boundaries. This paper presents a preliminary integration of multiple datasets (topography, bathymetry, gravity, present-day seismicity) on a global scale through of a three-dimensional model of the area. At this point,

we will focus on recent seismicity (Figure 1). The hypocentre dataset, available through the USGS/ National Earthquake Information Centre (http:// www.neic.cr.usgs.gov), lists all recorded earthquakes from 1973 until today. Hypocentre coordinates (usually including depth), magnitude and time of events are available. Earthquakes for which the depth could not be determined (and consequendy assigned to an arbitrary 33 km) have been rejected for the purpose of the modeUing. A total of 12,360 earthquakes were used with relevant information for the area along the trenches from Sumatra to Irian Jaya and around the Banda Arc. Figure 1 shows a total of approximately 50,000 hypocentres recorded with all relevant information. The 3D visuahsation of the dataset allows the following observations: 1. There is the usual seismic gap in the 300-400 km depth zone; 2. The subduction zone is very steep and overturns at depth; 3. Deep seismic activity (600-650 km) is common along the subduction zone but stops abruptly on the western side of Java. Does this mean that the slab is missing? 4. The Banda Arc subduction zone is a continuous arcuate subduction zone and does not result from the combination of two opposite-vergence subduction zones respectively marked by the Timor and Seram trenches; 5. The slab under the Banda Arc is continuously folded around a non-cylindrical fold which defines the keel shape of the seismicity enveloppe. These observations support the interpretation of the area where roll-back is responsible for the arcuate shape of the Banda Arc subduction zone. The pattern of seismicity in the hinge area and beneath the Banda Arc itself suggests that the lithosphere was actively folded during the roll-back. This resulted in a fold of the lithosphere in a structure of 500 km of amplitude and wavelength. The asymmetry of the subduction zone, from India to Irian Jaya, probably accounts for differences in lithospheric behavior at both extremities of the subduction zone. On the Indian side, it seems that SGTSG abstracts. February 2001

the slab detached from India whereas on the eastern side, the slab is pinned to the Birds Head continental fragment, - ^ f f ^

Acknowledgement This work is supported through the ARC/SPIRT scheme in partnership with Fractal Graphics.

Figure 1. Geodetic projection of recent (> 1973) earthquake hypocentres in the northern margin of the Australian Plate area. Depth of hypocentres is color coded from shallow (20-40 km) in light gray to deep earthquake (600-700 km) in black.

SGTSG abstracts, February 2001

Examination of the structural control of mineralisation in the Kalgoorlie district, Western Australia J.L. Baxter\ P.C. Sauter^ and N.C. Laubsch^ ^ Continental Resource Management Pty Ltd, P.O. Box 307, Belmont, WA, Australia 6984 2Homestake Gold of Australia Ltd, Locked Bag 12, Cloisters Square, WA, Australia 6850 3 W M C Resources Ltd, P.O. Box 91, Belmont, WA, Australia 6984

Mineralisation, stratigraphy and structure of the giant Kalgoorlie ore deposit has been examined with particular reference to the geometry of the rock units and disruptive structures. Integration of the results with published geochronological data and regional geology indicates that the mineralisation in the Golden Mile is related to the earliest D^ structures while that at Mt. Charlotte is associated with the D3 deformation some 30 m.y. later. The convergence of the geological, geochronological and structural data provides substantial support for this interpretation. Structural information was obtained from geological mapping, oriented drill core and historical information from underground mapping on the Golden Mile and Mt. Charlotte. The information was compiled into a digital database and 3-D model as part of a study of the Kalgoorlie district. The data was interpreted by identifying groups of surfaces that plotted along a single girdle on the stereographic net, had consistent metamorphic and kinematic indicators and also had observable crosscutting relationships with other groups of surfaces. The resulting interpretation is shown for the Golden Mile Fault data in Figure 1. This method of analysis is particularly appropriate for mesothermal and epithermal shear zone hosted mineralisation, where multiple seismic and hydrothermal events occurred during the development of mineralised shear zones, faults and veins. Heterogeneous deformation in the Kalgoorlie district is associated with three coherent shear systems, all of which can be identified by their kinematic, geometric and stereographic net attributes. Each of the systems has been mineralised to some degree. Thrusting, recumbent folding, mylonitic shearing and late brittle faults accompanying mineralisation (Fig. 1) formed during D j at a time when the tectonic architecture of the Kalgoorlie was created. The Golden Mile mineralisation (including the Oroya Shoot) was mainly deposited at this time (ca 2629 ± 9Ma, Kent & M^Dougall, 1995, 1996). Left-lateral strike-slip cataclastic shearing and localised steep north-plunging

open folds formed during D2. Minor remobilisation of mineralisation occurred including the deposition of the Cross Lodes at the Golden Mile and minor quartz vein hosted mineralisation in the northern part of the district. Extensional oblique dip-slip reactivation of pre-existing flat faults accompanied by brittle stockwork vein development as at Mt. Charlotte is characteristic of D3. The age of development of D3 is 2602±8Ma (Kent & M^Dougall, 1995,1996),about 30Ma younger than the formation of D^. . ^ ^ f f ^

References Kent, A.J.R. & McDougall, I., 1995.40Ar-39Ar and U-Pb age constraints on the timing of gold mineralisation in the Kalgoorlie gold field, Western Australia. Economic Geology 90: 845-859. Kent, AJ.R. & McDougall, I., 1996. ^0Ar-39Ar and U-Pb age constraints on the timing of gold mineralisation in the Kalgoorlie gold field. Western Australia - a reply. Economic Geology 91: 795-799.

Golden Mile Fault data Srjcturetypo : F^/urt.

Figure 1

SGTSG abstracts, February 2001

Table 1. Deformation history — Kalgoorlie district DBformation and descrip^on

Charaoteristic locations

Thrusting, recumbent folding, shearing with development of mylonite, late brittle failure accompanying mineralisation including shoot development and telluride mineralisation

Golden Mile Fault, Mount Hunt Thrust, Kalgoorlie Syncline, Kalgoorlie Anticline, Later Golden Pike, Charlotte, Baikal Faults, Flanagan-style faults, main Lode, Gaunter lodes, porphyries.

Left-lateral strike-slip shearing, mainly cataclastic 1 deformation, associated with steep north-plunging fold hinges and mineralised shoots

Boomerang and Mount Hunt Anticline; Boulder, Mount Hunt, Eastem Boundary and Western Boundary Faults; Quartz-hosted mineralisation, cross lodes, porphyries.

Extensional oblique dip-slip movement accompanied by normal movement, but local reverse movement. Associated with stockwork vein mineralisation

Reactivation on early structures; e.g. Flanagan Fault, vein arrays in all stockworks including Charlotte, Reward, Golden Pike, Drysdale, Ondola

Event Di

1 •2

! D3

SGTSG abstracts, February 2001

Arc-continent collision: a comparison of Timor and Tasmania R. Berry School of Earth Sciences University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001

Timor represents one of the best exposed modern examples of arc-continent collision. The collision started about 8 Ma and has progress to a stage of arcreversal (Berry & McDougall, 1986; Breen & Silver, 1989). The age of initiation was diachronous with collision starting 3 million years earlier in the east than the west. The area of East Timor is characterised by flake tectonics with the passive continental margin delaminating and over-riding the fore-arc. In contrast West Timor has a more classic pattern for arc-continent collision with the structure dominated by thrusts dipping towards the arc. The structure of Tasmania has been interpreted as a result of Cambrian arc-continent collision (Berry & Crawford, 1988). While the subsequent history has strongly modified the Cambrian structural elements, there is still a number of characteristic features that are best explained by this style of orogenesis. The aim here is to compare these structural elements with those exposed in Timor (Table 1). While there are many detailed differences between the Timor and Tasmanian situation there are also very many similarities which appear to distinguish this style of collision from the Taiwan style arc-continent collision (Huang et al., 2000). In particular the complexity of the geology is related to rapid along strike variability. There is no recognisable accretionary complex in the classic sense. In its place there is a scaly clay matrix melange which contains a wide range of blocks. One style of block that the especially distinctive is the complexly faulted metamorphic blocks which contain a disordered array of high and low strain, high and low metamorphic grade continental sourced siliciclastics and forearc-sourced ophiolite. Blueschists are very rare, the level of crustal thickening is limited with good preservation of many of the pre-collisional passive margin stratigraphic relationships. The volume of obducted ophiolite did not reach the levels of Oman. Some of these features may relate to the very young age of the oceanic arc involved in the collision. At least for Timor the collision is strongly affected by the shape of the continental margin and by along strike variation in the crustal response to collision. These types of complex geometric relations cannot be uniquely

determined for Tasmania but some of the complex relationships may be explained in this way. References Berry, R.E & Crawford, A J . , 1988. The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Austr. J. Earth Sci. 35: 523-533. Berry, R.F. & McDougall, I., 1986. Interpretation of 40Ar/39Ar and K/ Ar dating evidence from the Aileu Formation, East Timor, Indonesia. Chemical Geology (Isot. Geosc. Sect.) 59: 45-58. Breen, N.A. & Silver, E A , 1989. The Wetar back arc thrust belt, Eastern Indonesia: the effect of accretion against an irregularly shaped arc. Tectonics 8: 85-98. Huang, C.-Y., Yuan, RB., Lin, C.-W., Wang, T K . & Chang, C-P., 2000. Geodynamic processes of Taiwan arc-continent collision and comparisons with analogs in Timor, Papua New Guinea, Urals and Corsica. Tectonophysics 325: 1-21.

SGTSG abstracts, Febaiary 2001

Feature

Timor

Tasmania

Pre-passive margin basement

Lolotoi block in the Ladubar-Cribas area with Permian unconfomiity

; Passive margin ! stratigraphy showing rift/drift pattem

Permian-Jurassic rift style stratigraphy followed by Cretaceous to Miocene open shelf stratigraphy

King Island ?Strathgordon Mmc Complex overlain by Wedge River Bed s - overlain by Middle Cannbrian Island Road. Neoproterozoic shallow water stratigraphy with a late Neoproterozoic change to deeper water with associated tholeiitic volcanism

I Ophiolites and arc volcanic rocks

Three styles a) coherent forearc ophiolite sequence: Atapupu b) thin slices intimately related to metamorphic rocks in the Lolotoi allochthonous units c) isolated blocks along f aultzones Bobonaro Scaly clay: complex association of Pleistocene clays canning a huge range of blocks on all sizes across West Timor. Not very obvious and much more restricted in extent in East Timor. Includes mass flows, diapiric intrusions and tectonic fabrics

Lolotoi Massifs: A range of metamorphic and non metamorphic blocks characterised by rapid changes in nrretamorpNc grade in no particular stacking order. The stratigraphy of all these units matches the stratigraphic elements of Sundaland and the source is assumed to be the Sumba fragnent isolated in the forearc. Metamorphic ages are 25 million years older than collision. The metamorphic ages are inconsistent with the lithostratigraphic relationships originally reported for these rocks. Aileu "Formation": while the coastal metamorphic complex can now be seen as syn-collisional metamorphism there are many bodies throughout Timor that have an ambiguous origin. The sack term of Lolotoi Complex has been used for most of these. Some areas that have been called Lolotoi Complex may be related to the Aileu "Fonriation"

Complex ass ociation of high and low grade bbcks | in thin structurally bound units. These blocks appear ; to all match the local stratigraphy except the Bowry Formation. Metamorphic ages are apparently the same as the age of collision. The metamorphic ages are totally inconsistent with the original interpretation that these blocks form the crystalline basement to Tasmania.

Syn-orogenic i extensional structures Overprinting wrench faults

E-W basin in West Timor (at 3 Ma). Normal faulting on southern margin of Timor Trough.

Dundas Trough at 505 Ma

NE striking faults in West Timor

Pre- Late Cambrian wrench faults are known from Osmiridium Beach, on the South Coast, and the Boat Harboir Fault in NW Tasmania

i Arc reversal and

Arc reversal began at 3 Ma. No new

Extensive post collisional magmatism along the Dundas Trough. Operates from 505-500 Ma largely in a submarine environment Cratonisation after minor inversion and N-S folding in the Late Cambrian

Melange zones

1 !

J Allochthonoushigh strain metamorphic 1 blocks 1

Syn-collisional 1 metamorphism i j i

magmatism Post colliaonal inversion and j cratonisation 1

SGTSG abstracts. February 2001

magmatic event yet. Arc reversal can only be a short term solution as the Banda Sea is narrow and plate complexities will require a new geometry within 10 Ma

b) c)

i 1

larger coherent ophiolite masses of fore-arc affinity: Heazlewood River Complex structurally bound slices in metamorphic rocks: : Forth Metamorphics isolated blocks along fault zones: Hibbs Shear zone

Ragged Basin Complex, Cleveland-Waratah Complex both undemeath ultramafic rocks

?northern Margin of the Tyennan - cannot easily distinguish these from the allochthonous units at this ; stage. It is also very difficult to distinguish older metamorphics from Cannbrian metamorpNcs. The present Tyennan Complex almost certainly contains ; both of these elements.

1.6-1.5 Ga orogenies of Eastern Australia p. Betts and D. Giles Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton, Australia 3800

Evidence of Mesoproterozoic (c. 1.6-1.5 Ga) orogenesis is preserved v^ithin several Eastern Australian Proterozoic terranes, including Mount Isa Inlier (Isan Orogeny), Georgetown Inlier, Willyama Inlier (Olarian Orogeny), and the northern Gawler Craton (Kararan Orogeny) (Figure 1). There are significant differences in the details of the deformation history, the local strain associated with orogenesis, as well as the crustal geometry within and between these terranes. Despite this, a common feature of the eastern Australian Proterozoic orogens is that they appear to have undergone a two stage evolution. The first stage (c. 1.60-1.56 Ga) is characterised by high strain, thinskinned lateral translation. The second stage (c. 1.561.50 Ga) involved shortening of deeper crustal levels and is often characterised by upright to inclined folding, and brittle faulting. The most complete record of this two stage otogenic evolution is preserved in the Mount Isa terrane. In the Eastern Fold Belt, the earliest stages of the Isan Orogeny involved thin-skinned tectonism and westward lateral translations of relatively young cover sequences along sub-horizontal detachments (1.581.56 Ga) (O'Dea et al., 1997; Betts et al., 2000). Metamorphism associated with this stage has been dated at c. 1.58 Ga (Page & Sun, 1998; Giles, 2000). As the orogen matured deeper crustal levels were shortened and upright folding and basement penetrating reverse and strike-slip faults developed (MacCready et al., 1998; O'Dea et al., 1997) (stage II). Peak metamorphism in the Western Fold Belt has been dated at c. 1.54 Ga (Connors &C Page, 1995). The deformation style and strain pattern in this part of the terrane is more typical of that expected in an inverted intracontinental basin. In the Georgetown Inlier crustal shortening and metamorphism occurred at c. 1.55 Ga (i.e., stage II) (Black et al., 1998), although the paucity of geochronological data precludes meaningful interpretation of earlier deformation evolution. In the Willyama Inlier, crustal shortening (c. 1.60-1.59 Ga) involved the development of highly non-cylindrical recumbent folds. Recumbent folds are overprinted by syn-orogenic granites (c. 1.59 Ga) (Page et al., 2000) and by regional upright folds (c. ?1.58-1.50: stage II?). The buried Goober Pedy Ridge and Mabel Creek Ridge in the northern Gawler

Craton preserves the remnants of the Kararan Orogen (Daly et al., 1998). The evolution and age of this poorly understood orogen has been constrained from geophysical interpretation and U-Pb SHRIMP dating of metamorphic zircon from drill core. The earliest stage of the Kararan Orogen is preserved in the Goober Pedy Ridge and involved the thin-skinned, southdirected thrusting and recumbent folding. Peak metamorphism in this part of the orogen has been dated at c. 1.565 Ga (Daly et al., 1998) (stage I). Peak metamorphism in the Mabel Creek Ridge has been dated at c. 1.54 Ga (stage II) and geophysical interpretation and modelling indicates that deformation is characterised by upright to inclined folding. Eastern Proterozoic orogens of Australia are interpreted to have formed a continuous mountain belt formed as a consequence of west-dipping subduction and collision between the Australian and North American continents during the Mesoproterozoic (Giles and Betts, 2000). . y f f ^ References Beets, P.G., Ailleres, L., Giles, D. & Hough M., 2 0 0 0 . Deformation history of the Hampden synform in the Eastern Fold Belt of the Mount Isa terrane. Australian Journal of Earth Science 47. Black, LP., Gregory, P., Withnall, I . W & Bain, J.H.C., 1998. U-Pb zircon age for the Etheridge Group, Georgetown region, North Queensland; implications for relationship with the Broken Hill and Mt Isa sequences. Australian Journal of Earth Sciences 45: 925-935. Daly, S.J., Fanning, C . M & Fairclough, M . C . , 1 9 9 8 . Tectonic evolution and exploration potential of the Gawler Craton. A G S O Journal of Geology and Geophysics 17: 1 4 5 - 1 6 8 . Giles, D. & Betts, RG., 2 0 0 0 . Beyond Rodinia: The Early to Middle Proterozoic amalgamation of Australia and North America. Australian Crustal Research Centre Technical Publication 85. Giles, D., 2 0 0 0 . Tectonic Setting of Broken Hill type mineralisation, the Cannington Perspective. P h D Thesis. Monash University (unpublished). MacCready, T , Goleby, B.R., Goncharov, A., Drummond, B.J. & Lister, G.S., 1 9 9 8 . A framework of overprinting orogens based on interpretation of Mount Isa Deep Seismic Transect. Economic Geology 93: 1 4 2 2 - 1 4 3 4 . O'Dea, M.G., Betts, RG. MacCready, T. & Ailleres, L , 1 9 9 7 . Sequential development of a mid-crustal fold-thrust complex in the Eastern Mount Isa Inlier, Australia. Australian Crustal Research Centre Technical Publication 47. Page, R.W. & Sun, S., 1 9 9 8 . Aspects of the geochronology and crustal evolution in the Eastern Fold Belt, M t Isa Inlier. Australian Journal of Earth Sciences 4 5 : 3 4 3 - 3 6 1 . Page, R.W., Stevens, B.P.J & Gibson, G.M., 2 0 0 0 . New SHRIMP zircon results from Broken Hill: Towards robust stratigraphic and event timing. Geological Society of Australia Abstracts 59: 375.

SGTSG abstracts, February 2001

Olarian Orogeny Isan Orogeny (Mount Isa terrane (Willyama Inlier)

WFB

Jana Orogeny (Georgetown)

Kararan Orogeny (North Gawler)

I EFB

Post-orogenic granite emplacment

1.51

HPost-orogenic granite emplacment

1.52 -1.53

Reconstructed shortening direction

f 0)

Reconstructed shortening direction®

CO E-W shortening

-1.54 -1.55

uprigt folding basement penetrating shear zones and faults

NW-SE shortening^ upright folding

Croydon Volcanics

¥ t

Peak metamorphbm <Mab€il CreoH Ridge)

N-S shortening crustal-scale upright folding & reverse faulting

Forsayth Batholith^

-1.56 .1.57

N-S shortening basin inversion 0

1.58 3

1.59

NW-SE shortening thin-skinned thrusting

Peak metamorphl$m (CooberPecJy Ridge

Reconstructed shortening direction

N-S shortening thin-skinn^ thwjting

Mundi Mundi Granite

Peak ^ metamorphisnr

Early Fabric Development

^NE-SW shortening non-cylindrical recumbent folding

Hiltaba Granite Suite rn Ll

^ G a w l e r Range U Volcanics

Figure 1. Cladogram of the Proterozoic Eastern Australian Orogenic events. Dark arrows show the interpreted shortening directions with respect to present day coordinates. Reconstructed shortening directions (shown in white arrows) assume the Palaeoproterozoic configuration of the South Australian Craton by Giles & Betts (2000).

SGTSG abstracts. February 2001

Evidence for mid Archaean continental collisions in the development of the dome-and-basin geology of the North Pilbara Craton R.S. Blewett Minerals Division, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, Australia 2601

One of the major controversial questions for Archaean geology concerns what deformation drivers operated prior to 2500 Ma. Current theories include both uniformitarianism, i.e., plate tectonic (or similar) processes operating as far back as 3.8 Ga, and nonuniformitarianism, i.e., the crust and mantle dynamics were radically different in the Archaean in comparison to modern systems, with different tectonic processes (e.g., vertical tectonics or diapirism). The Pilbara Craton is a granitoid-greenstone terrane located in N W Australia, and is generally regarded as one of the best-exposed examples of Archaean crust. The Pilbara is therefore an excellent study area for early earth processes, and has been the focus of a number of studies on the formation of the domeand-basin style geology, for which the Pilbara is famous. In the Pilbara Craton rocks range in age from 3.652.85 Ga, with older inheritance implying even earlier continental crust. The long geological history of the Pilbara Craton resulted in the development of eleven, generally synformal, volcano-sedimentary greenstone packages. Many packages are separated by regional or local unconformities. Some of the greenstone packages were developed during extensional periods (collapse?) following far-field driven compressional deformation events; others formed during transpression/ transtension. These packages make up greenstone belts that mostly envelop domal composite granitoid batholiths. The batholiths are up to 120 km in diameter (average 50 km diameter), and have been repeatedly intruded by up to twelve magmatic pulses. Many of the intrusive events into the batholiths were coeval with volcanism in the greenstone belts. Detailed and regional structural studies, literature compilations and published geochronology have resulted in the definition of up to thirteen phases of penetrative deformation over an 800 Ma period (at micro- to mega-scales), with multiple overprinting relationships mostly about steeply dipping planar fabric elements. The correlation of these early to mid Archaean deformation events across the whole Pilbara implies far-field horizontal compression tectonics occurred at that time.

Were horizontal forces enough to derive the present geometrical arrangements on their own, and if so why did they not transpose the geology into linear belts? The answer may be that horizontal compression was mostly partitioned into the batholith margins and adjacent greenstones, leaving the large batholiths as essentially "mega-porphyroclasts". What is happening in the third dimension? Geophysical studies show that the dome-and-basin geometry extends down to the mid-crust at about 14 km. The batholiths are steep to vertical sided, and the enveloping greenstone synforms mirror these steep attitudes. The fold amplitudes of the greenstone synforms are extreme, with some greenstone belts only a few kilometres across, and with inferred closures at 14 km or more in depth. This suggests that crossfolding models are not the entire solution to forming the dome-and-basin geometry, and that steep shear zones are also important. The Archaean has generally been regarded as a period of high mantle heat flow, and this, coupled with the presence of heat-producing granitoids, would have thermally softened the upper crust. The style of deformation would be expected to be more ductile, even under relatively low amounts of stress. Such thermal softening may have facilitated (1) gravitational readjustment between dense greenstone and less dense granitoid, (2) attenuation of fold geometries, and (3) formation of ductile shear zones. The early and mid Archaean record in the Pilbara Craton reflects repeated horizontal shortening across much of its preserved continental fragment. However, these horizontal shortening events were without widespread terrane accretion, and may have reflected far-field or outboard collisions. In this paper, a model is presented where the dome-and-basin geometry is developed and controlled by large-scale fold interference during multiple deformation events from far-field compression, and later modified by fold tightening and amplification (steepened and deepened) by periodic 'diapirism'. This modification was possibly enhanced by a regime of elevated thermal gradients. The challenge for structural geologists working in SGTSG abstracts, February 2001

the most ancient orogens is to separate the effects of the obvious repeated horizontal shortening events from the more subtle vertical accommodation due to diapirism.

SGTSG abstracts, February 2001

High-r, low-Pmetamorphism as the middle crustal response

to a mantle-related transient thermal pulse: a numerical model and its application to the Palaeoproterozoic Halls Creek Orogen, northern Australia S. BodorkosS M. Sandiford^ N. Oliver^ and P. Cawood^ ^ Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA, Australia 6845 2 School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia 3010 3 Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Qld, Australia 4811

temporal or spatial association with the highestOver the last thirty years, a wide variety of heat sources grade rocks; have been proposed as potential mechanisms for highr, low-P metamorphism {T> GOO^'C, P < 500 MPa) (ii) field and isotopic evidence for a protracted thermal event affecting the high-grade mid-crustal section in otogenic belts worldwide (De Yoreo et al., 1991). exposed, with temperatures probably remaining These include voluminous mid-crustal plutonism (e.g. above -400°C for -30 million years (Bodorkos et Lux et al., 1986), fluid flow (e.g. Chamberlain & al., 1999); Rumble, 1989), accretion of heat-producing elements (iii) a large number of layered mafic-ultramafic at the base of the crust during continental collision intrusions emplaced at various crustal levels in a (e.g. Jamieson et al., 1998), and crustal heating due to 30 million-year interval straddling the time of peak attenuation or removal of the mantle lithosphere metamorphism; and during extension or compression (e.g. Houseman et (iv) low values of surface heat flow (<50 mWm'^) and al., 1981; Loosveld Etheridge, 1990). This paper surface heat production throughout the exposed aims to evaluate the role of transient mantle-related area of the orogen (Cull, 1982). heat sources as a potential mechanism for high- T, lowIn order to account for this combination of P metamorphism in the middle crust, with specific geological and thermal characteristics, we model the reference to terranes displaying: (i) below-average surface heat flow values; (ii) high-T, low-P transient thermal perturbation experienced by a lithospheric section with uniform and time-invariant metamorphic rocks cropping out continuously over a thickness (and within which heat is transported wide area, and (iii) evidence for a prolonged thermal primarily by conduction), following the instantaneous event, of the order of tens of millions of years in removal of the underlying mantle lithosphere. The duration, during which the rocks cooled slowly at (temporary) partial or total removal of the lithospheric depth. mantle may occur through a variety of processes The Palaeoproterozoic (-1915-1800 Ma) Halls (including convective thinning, detachment, extension Creek Orogen provides an excellent example of a or slab break-off) and the long-term thermal evolution metamorphic belt characterised by this combination of such scenarios is well understood by analogy with of features, which is difficult to explain in the context cooling of newly-formed oceanic lithosphere at a midof a primary heat source dominated by magmatism or ocean ridge (e.g. McKenzie, 1967). radiogenic heating. This otogenic belt represents the -1860-1800 Ma collisional interface between the Implementation of the numerical model simulating Kimberley and North Australian cratons, and contains this process involves the use of a finite-element areally extensive, prograde andalusite-sillimanite facies algorithm which artificially adjusts the thermal rocks with peak metamorphic 700-750°C and P conductivity k within the lithospheric domain that is - 400-500 MPa (Bodorkos et al., 1999; Oliver et al., removed. Using the boundary conditions of the "plate" 1999). Specific geological and thermal properties of model for the thermal structure of oceanic lithosphere the orogen that support the operation of transient ( r = 0 at z = 0, T= both = 0 and z = Z), we thermal anomalies in the mantle and/or preclude other utilise the idea that in a conductive regime, an increase obvious heat sources include: in k of several orders of magnitude within a specified domain results in the almost instantaneous (i) voluminous pre- to syn-metamorphic felsic homogenisation of the temperature field in that plutonism which does not show any systematic SGTSG abstracts, February 2001

<10 million years, even in the immediate vicinity of domain. Consequently, we generate a thermal anomaly very large (30 km wide, 4 km thick) mafic intrusive in the lithosphere by setting = 10^k within a bodies ( T - 1100°C). However, the interaction of restricted lithospheric domain for the interval of time 0 < t < tY), corresponding to the interval over which pluton- and mantle-related thermal effects has the the mantle anomaly is dynamically maintained. At ^ = potential to maintain host rock temperatures in excess of 400-450°C for -30 million years in some parts of k is reset to its correct value, with the result that the mid-crust. The results of the numerical model the thermal anomaly decays back to the initial support the notion that transient mantle-related heat condition with a characteristic time-scale governed by sources have the capacity to contribute significantly the thickness and thermal diffusivity of the lithosphere. to the thermal budget of metamorphism in high-T, The amplitude of temperature increases in the low-P metamorphic belts, especially in those middle crust (and the time at which the peak characterised by low surface heat flow, very high peak temperature is attained) depend strongly on the metamorphic geothermal gradients and abundant intensity of the mande thermal anomaly (which is a mafic intrusions, where obvious alternative regional function of the vertical thickness of lithosphere initially heat sources are absent. removed, the lateral extent of significantly thinned lithosphere and the duration of the interval over which References the associated elevated heat flux is maintained). In Bodorkos, S., Oliver, N.H.S. & Cawood, P.A., 1 9 9 9 . Thermal evolution addition, the thermal properties of the model crust of the central Halls Creek Orogen, northern Australia. Australian (such as the thermal conductivity, and the abundance Journal of Earth Sciences 46: 4 5 3 - 4 6 5 . Chamberlain, C.R & Rumble, D.I., 1989. The influence of fluids on the and distribution of radiogenic heat-producing thermal history of a metamorphic terrane: New Hampshire, elements) exert significant control on the magnitude USA. In: Evolution of Metamorphic Belts (eds. Cliff, J. S. & Yardley, B. W. D.) Geological Society of London Special of the maximum temperatures attained. However, this Publication 43: 2 0 3 - 2 1 4 . Geological Society of London. is primarily due to their influence on the steady-state Cull, J. P., 1 9 8 2 . An appraisal of Australian heat-flow data. BMR Journal of Australian Geology and Geophysics 7: 1 1 - 2 1 . (background) thermal gradient, and the amplitudes of De Yoreo, J.J., Lux, D.R. & Guidotti, C.V., 1 9 9 1 . Thermal modelling in temperature increases in the crust are broadly low-pressure/high-temperature metamorphic belts. Tectonophysics 188: 2 0 9 - 2 3 8 . independent of such "thermal baseline" effects. Houseman, G.A., McKenzie, D.R & Molnar, P., 1 9 8 1 . Convective The steady-state crustal geotherm in the Halls instability of a thickened boundary layer and its relevance for the thermal evolution of continental convergent belts. Journal Creek Orogen in the Palaeoproterozoic (-1800 Ma) of Geophysical Research 86: 6 1 1 5 - 6 1 3 2 . may be estimated using the present-day crustal Jamieson, R.A., Beaumont, C., Fullsack, R & Lee, B., 1 9 9 8 . Barrovian contribution to surface heat flow and area-averaged regional metamorphism: where's the heat? In: W h a t Drives Metamorphism and Metamorphic Reactions? (eds. Treloar, R J. volumetric heat production rate (-15-35 mWm-^ and & O'Brien, R J.) Geological Society of London Special -2.25 respectively), which equate to -20-45 Publication, 138: 2 3 - 5 1 . Geological Society of London. Loosveld, R.J.H. & Etheridge, M.A., 1990. A model for low-pressure mWm-2 and -3.2 jLLWm-^ when recalculated to 1800 facies metamorphism during crustal thickening. Journal of Ma. Using a depth-integrated heat production value Metamorphic Geology 8: 2 5 7 - 2 6 7 . Lux, D.R., De Yoreo, J.J., Guidotti, C.V. & Decker, E.R., 1 9 8 6 . Role of of -30-35 mWm-2 for the 1800 Ma model crust and plutonism in low-pressure metamorphic belt formation. Nature a uniform thermal conductivity ^ = 3 Wm'^^C-^ within 323: 794-797. McKenzie, D., 1967. Some remarks on heat flow and gravity anomalies. a thermal boundary layer 40 km in thickness, Journal of Geophysical Research 72: 6 2 6 1 - 6 2 7 3 . conductive dissipation of the mantle temperature Oliver, N.H.S., Bodorkos, S., Nemchin, A A . , Kinny, RD. & Watt, G.R., anomaly results in the elevation of mid-crustal (z = 17 1 9 9 9 . Relationships between zircon U-Pb SHRIMP ages and leucosome type in migmatites of the Halls Creek Orogen, km) temperatures by -150-400°C within - 1 0 - 2 0 Western Australia. Journal of Petrology 40: 1 5 5 3 - 1 5 7 5 . million years of instantaneous lithospheric removal. Maximum temperatures decrease sharply for mande thinning episodes less than - 5 million years in duration, and/or anomaly half-widths less than -100 km. Mid-crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Pluton-related thermal effects are typically short-lived and are often significantly dissipated within

SGTSG abstracts. February 2001

Deep holes in the Centralian Superbasin, SHRIMP zircon constraints on the depositional age of granulites in the eastern Arunta Block

I.S. Buick\ J.A. Millers I.S. WilllamsS M. Hand^ and J . Mawby^ ^ Department of Earth Sciences, La Trobe University, Bundoora, Victoria, Australia 3086 2 Department of Geological Sciences, University of Cape Town, Rondebosch 7700, Cape Town, South Africa 2 Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia 0200 ^ Department of Geology and Geophysics, Adelaide University, SA, Australia 5005

The eastern Arunta Block forms a metamorphic corridor between the structurally remnant Amadeus and Georgina basins, which were separated during the p r o f o u n d basin inversion associated with the Devonian-Carboniferous Alice Springs Orogeny. This basin inversion event broke up the northern fragment of the Centralian Superbasin, a vast Neoproterozoic to early Palaeozoic intracratonic basin. In the Harts Range (eastern Arunta Block), the metamorphic rocks can be divided into two major lithostratigraphic associations: the regionally extensive Irindina Supracrustal Assemblage (ISA) and the structurally underlying Palaeoproterozoic Entia Gneiss Complex. Within the Arunta Block, the Irindina Supracrustal Assemblage has been a problematic package in that it does not easily fit with any of the other lithostratigraphic associations. The granulite-to uppermostamphibolite-grade Irindina Supracrustal Assemblage has a structural thickness of around 7 km, and consists of structurally lowermost marble and quartzite (Naringa Calcareous Member), intermediate-level intercalations of metapelite (Irindina Gneiss) and metabasic units (Harts Range Meta-Igneous Complex which consists of basaltic flows or volcaniclastics, and associated anorthosite and ultramafic intrusives), and a structurally uppermost metapelite- and calcsilicatedominated unit (Brady Gneiss). Its vertical lithological succession, and the chemistry of mafic rocks, is consistent with deposition in a continental rift environment (e.g. Sivell & Foden, 1985; James & Ding, 1988). Until now it has been universally accepted that deposition of the ISA occurred sometime in the PalaeoMesoproterozoic and therefore constitutes part of the Arunta basement to the Centralian Superbasin. However SHRIMP U-Pb zircon and cathodoluminescence analyses of zircons from: (1) granulitefacies metapelite (Irindina Gneiss) and mafic volcaniclastic from the Harts Range Meta-Igneous Complex, and (2) a granite cross-cutting these units in the northernmost ISA (Mallee Bore area), show that the depositional age of the ISA is much younger than previously thought. Both the metapelite and metabasite contain near-concordant zircon populations at c. 1000-1300 Ma and c. 650 Ma, and individual zircon grains that may be as young as c.

510-520 Ma (Fig. 1). The individual populations contain a range of igneous and metamorphic zircon types, suggesting that they are detrital in origin. Inherited zircon with similar age groups to the detrital populations are found in late to post-kinematic (Corkwood Hill) granite (Figure 1). The presence of such young detrital and inherited zircon populations is inconsistent with a Palaeoproterozoic depositional age for the ISA precursors. Instead, the age data indicate that the protoliths to the granulites must be late Neoproterozoic, or possibly even early Palaeozoic in age. A similar range of detrital ages has also been obtained from quartzite (Naringa Calcareous Member) and metapelite (Brady Gneiss) at the bottom and top, respectively, of the ISA in the central Harts Range, c. 30 km to the south of Mallee Bore. While the detrital populations provide an upper age limit for deposition, a lower limit is provided by the timing of granulite facies metamorphism, which occurred at around 470 Ma (Mawby et al., 1999; Hand et al., 1999; Buick et al., 2001). The detrital zircon age data from the Harts Range has profound implications for the evolution of the Centralian Superbasin, since it effectively means that the granulite-grade ISA is actually part of the superbasinal succession, and is not part of their basement as has been universally accepted. In addition, the detrital zircon age data also means that deposition of a major part of the eastern Arunta Block is younger than much of the unmetamorphosed Neoproterozoic sequences that occur in the surrounding Amadeus and Georgina basins. In this context, the Irindina Supracrustal Assemblage and associated metamorphic rocks do not constitute part of the Arunta Inlier per say. Furthermore, the existence of the Harts Range Meta-Igneous Complex indicates that significant basaltic volcanism occurred sometime during the late Neoproterozoic to Cambrian in the basinal linkage between the Amadeus and Georgina basins. The detrital age distribution from the ISA metasediments is quite similar to that of the late Cambrian Goyder Formation in the Amadeus Basin (Fig. 1), whose abundance of late Meso- to Neoproterozoic zircons may reflect transport of sediment derived from the Musgrave Inlier to the southwest (Zhao et al., 1992). If this is the case, SGTSG abstracts, February 2001

sequences such as the Goyder Formation, in the Amadeus Basin and the Cambrian Arrinthurunga Formation in the Georgina Basin may represent lateral equivalents of a rift sequence that accumulated in what is termed here the Irindina sub-basin, which was located between the presently preserved Amadeus and Georgina basins. T h e development o f the Irindina sub-basin probably occurred sometime during the Cambrian, and appears to represent part of a widespread and longlived interval of extension in parts of central Australia. This extension was associated with eruption of the continental Antrim Plateau and Table Hill Basalts, and the development of a system o f sub-basins along the northern margin of the Amadeus Basin (Lindsay & Korsch, 1991). In part, extension coincided with convergent deformation that was most obviously expressed in the formation of the Petermann Orogen, which shed sediment northwards into developing subbasins (Lindsay Korsch, 1 9 9 1 ) that probably included the Irindina sub-basin. T h e apparent synchronous development of localised extensional and compressional systems in central Australia at that time suggests the existence o f a c o n t i n e n t a l - s c a l e transcurrent (dextral) regime. The total thickness of sediment that accumulated in the Irindina sub-basin is difficult to estimate, however we speculate that as much as 30-35 km of sediment was accommodated. This is based on palaeogeographic reconstructions which indicate that throughout the Cambrian and up until the midOrdovician, the region now occupied by the eastern Arunta was marine (Lindsay & Korsch, 1991). This means that attainment of pressures of > 10 kbar (equivalent to 3 0 - 3 5 km o f burial) during early Ordovician granulite facies metamorphism of the ISA

Detrital zircon: metabasic granulite (metavolcanic): Harts Range Igneous Complex Metamorphic age = 471.0± 7.1 Ma (not shown)

'iliki-ilU 500

500

1000 1500 Inferred age 151 (Ma) 1000

2000 500

Inherited zircon: small granitic plug in ISA Emplacement age = 386.9 ± 3.5 Ma (not shown)

iiA

1000

1500

Inferred age (Ma)

SGTSG abstracts. February 2001

2000 500

(Mawby et al., 1999; Buick et al., 2001) must have been largely the consequence of sedimentation within the Irindina sub-basin, and any younger overlying subbasins. I f this is true, the now exposed Irindina Supracrustal Assemblage in the Harts Range region of the Arunta Block represents the basal sequences of an exceptionally deep sub-basin within in the broader extent of the Centralian Superbasin. One slightly sobering aspect o f the Cambrian interval in the Amadeus and Georgina basins, is that apart from modest local increases in sedimentation rates (Lindsay & Korsch, 1991), the sequence development appears relatively unremarkable, and does not obviously point to the development of localised basins of the magnitude of Irindina sub-basin, or its obvious association with mafic and ultramafic magmatism.

References

Buick, I.S., Miller, J.A., Williams, I.S. & Carcwright, L, 2001. Journal of Metamorphic Geology (accepted). Hand, M., Mawby, J., Miller, J., Ballevre, M., Hensen, B., Moller, A. & Buick, I.S., 1999. Geological Society of Australia, Specialist Group in Geochemistry, Mineralogy and Petrology Field Guide, 4:73. Lindsay, J.E & Korsch, R.J. 1991. BMR Bulletin 236: 7-32. James, RR. & Ding, R, 1988. Precambrian Research, 40/41: 199-216. Mawby, J., Hand, M. & Foden, J., 1999. Journal of Metamorphic Geology 17: 653 - 668 Sivell, W. J., & J. Foden 1985. Precambrian Research 28: 223-252. Zhao, J.X., McCulloch, M.T. & Bennett, V.C., 1992. Geochimica et Cosmochimica Acta 56: 921-940.

Detrital zircon: metapelitic granulite (Irindinia Gneiss) Metamorphic age = 460.5± 4.3 Ma (not shown)

Figure 1. Comparison between relative probability distributions o f detrital/inherited zircon components in granulite-facies metasediments fi:om the ISA and a late cross-cutting granite, and the C a m b r i a n Goyder

1000

1500

2000

Inferred age (Ma)

Basin.

Goyder Formation: Amadeus Basin Depositional age = 520 Ma (data after Zhao et aL, 1992)

1000

1500

Inferred age (Ma)

Formation in the Amadeus

2000

Terra Australis orogen: Rodinian breakup and subduction initiation in the proto-Pacific

P.A. Cawood^ and E.G. Leitch^ Tectonics Special Research Centre, Curtin University, GPO Box U1987, Perth, WA, Australia 6001 ^School of Environmental Science, University of Technology, Sydney, PO Box 123, Broadway, NSW, Australia 2007

The proto-Pacific Ocean formed through Neoproterozoic rifting of Rodinia and was bounded by the conjugate Laurentian and Gondwanan margins. Despite a long history of plate convergence this ocean has never subsequently closed, and is manifest today in the Pacific Ocean. The record of inception of convergent plate interaction in the proto-Pacific is preserved in a Neoproterozoic to late Paleozoic otogenic belt here termed the Terra Australis orogen. The orogen forms a fiindamental crustal element along the Pacific margin of Gondwana and is preserved in the now dispersed Adelaide fold belt and Tasman orogen of east Australia, Ross orogen of the Transantarctic Mountains, andTuhua orogen of New Zealand. Older rocks of the Terra Australis orogen are divisible, from west to east, into four main stratotectonic assemblages, a continental margin assemblage, a volcanic assemblage, an ocean margin assemblage and an intra-oceanic assemblage. The continental margin assemblage developed on continental crust stabilized within Rodinia, whereas the latter two developed in an oceanic realm formed following breakup of Rodinia and are differentiated on their relative isolation from the continental margin. The volcanic assemblage developed at or near the continent/ocean boundary and may have included crust of transitional affinities. The ocean-margin assemblage is linked to cratonic Gondwana by widespread siliciclastic detritus, but the intra-oceanic elements show no evidence for a continental influence until the late Paleozoic. Ocean margin and intraoceanic elements were progressively incorporated onto the Gondwana margin during Paleozoic otogenic movements (e.g. Tabberabberan, Tuhua) culminating in the Permian Hunter-Bowen event which marks the termination of the Terra Australis orogen and a stepping out of the plate margin to a Permian to Mesozoic otogenic tract extending through New Guinea, New Zealand, Marie Byrd Land, the Antarctic Peninsula and into South America (Gondwanide Orogen). Relations w i t h i n and between the various assemblages indicate that the end Neoproterozoic marks a fundamental change in tectonic style along

the Terra Australis orogen. Available data indicate that subduction was established at or close to the Gondwana margin by 530 Ma but an earlier, and perhaps ephemeral, phase may have commented at around 570-590 Ma. The presence of magmatic arc rocks within the continental margin assemblage as well as the presence of siliciclastic strata in stratigraphic continuity with the oceanic substrate in the ocean margin sequences indicates that at least the main phase of convergent plate margin activity likely formed close to the Gondwana continent where relatively old, dense oceanic lithosphere was susceptible to subduction. Disruption and deformation of the continental margin successions and the initiation of subduction and convergent plate margin activity within the Terra Australis orogen between 590 and 530 Ma corresponds with a period of global plate reorganization. This involved opening of the lapetus Ocean, final assembly of Gondwana through closure of the Mozambique Ocean, and rifting of Siberia from northern Laurentia. The temporal equivalence of this series of end Neoproterozoic Wilson cycle stages suggests they may be interdependent and we consider that their far-field effects could have influenced subduction initiation in the proto-Pacific. The margins of the Mozambique Ocean were aligned approximately orthogonal to the trend of the East Gondwana margin preserved in the Terra Australis orogen. With collision of East and West Gondwana along the Mozambique Belt, continued plate motion may have been transferred to the East Gondwana - proto-Pacific ocean margin resulting in strike slip deformation and margin decoupling. The initiation of subduction along the Pacific margin of Gondwana by 530 Ma and possibly as early as --590-570 Ma argues against models which suggest breakup of Rodinia and generation of conjugate Laurentian and Gondwana margins did not occur until the end Neoproterozoic at around 560 Ma. The initiation of subduction in the late Neoproterozoic to Early Cambrian marks the inception of the Pacific 'ring of fire, yet throughout the Phanerozoic the Pacific has remained a major ocean basin. This indicates that the longevity of the Pacific SGTSG abstracts, February 2001

and its antecedents is a result of continued production of oceanic lithosphere throughout the Phanerozoic, rather than a delayed onset of subduction. Although the Pacific has been cited as an example as the declining stage of the Wilson cycle of ocean basins, its protracted history of ongoing subduction, and by inference oceanic crust generation, contrasts with the clear evidence for opening and closing of oceans preserved in the lapetus/Atlantic and Tethyan realms, . y f ^

SGTSG abstracts, February 2001

Kinematics and tectonic roles of large-scale shear zones in the Yilgarn Craton, Western Australia

S.F. Chen

Geological Survey of Western Australia, Kalgoorlie Regional Office, P.O. Box 1664, Kalgoorlie, WA, Australia 6430 The Archaean Yilgarn Craton is subdivided into a series of granite-greenstone belts by regional- to craton-scale shear zones that trend N N W in the Eastern Goldfields Province and NE in the Murchison Province. Both NW- and NE-trending shear zones are present in the Southern Cross Province. They are typically high-strain zones characterised by strongly foliated to mylonitic granitoid, amphibolite, and various schists derived from ultramafic, mafic and felsic volcanic rocks and metasedimentary rocks. Some shear zones separating greenstones with contrasting lithostratigraphic associations were possibly basin-bounding extensional structures that were reactivated as reverse faults and strike-slip shear zones during subsequent deformation. Some acted as foci for igneous, tectonic and hydrothermal activity, and controlled gold mineralisation. Although some of these structures had long and complex deformation histories, the last substantial movement was ductile strike-slip that played different tectonic roles in the Eastern Goldfields Province and the Southern Cross Province. In the Eastern Goldfields Province, the NNWtrending shear zones preserve evidence for reverse movement overprinted by sinistral transpression. Reverse movement is indicated by stratigraphic repetition and by locally preserved, steeply plunging mineral lineations and striations on a NNW-trending, pervasive foliation that is axial planar to large- and small-scale folds. Deposition or preservation of molasse-type polymictic conglomerates in faultbounded, elongate basins (e.g. the Pig Well graben) was probably related to reverse movement on the bounding structures. The structural framework in the northern Eastern Goldfields Province was largely shaped by transpression that involved sinistral strikeslip on the N N W - t r e n d i n g shear zones and contemporaneous compression in the adjoining greenstone belts. Sinistral movement was recorded by (1) locally preserved S—C fabrics and asymmetric porphyroclasts; (2) small-scale, S-shaped asymmetric folds; (3) the structural deflection patterns of bedding, foliation, fold axes and reverse faults against the shear zones; and (4) numerous shallowly plunging mineral lineations and striations. Local contraction induced

by sinistral strike-slip on the NNW-trending, rightstepping regional shear zones has led to the development of N- to NNE-trending folds, axial planar foliations, and reverse faults within the Laverton, Duketon and Yandal restraining jogs, and at the N W termination of the Kilkenny shear zone. Each of these restraining jogs contains a diagonal transfer shear zone (namely Laverton shear zone in the Laverton jog, C u t l i n e shear zone in the Duketon jog, and Ockerburry shear zone in the Yandal jog) that merges with the bounding shear zones at a small to moderate angle. In the Southern Cross Province, various kinematic indicators (such as numerous asymmetric feldspar porphyroclasts, well-developed S - C fabrics, and the drag patterns of adjacent structures), together with a prominent shallowly plunging mineral lineation, clearly demonstrate sinistral movement on NWtrending shear zones and dextral movement on NEtrending shear zones. These NW-trending sinistral and NE-trending dextral shear zones do not intersect each other as conjugate pairs, but are subparallel to macroscopic folds and are linked by N-trending reverse faults, folds and flattening fabrics, forming regionalscale arcuate structures. The Evanston-Mount Dimer, Youanmi—Yuinmery, and Edale-Evanston arcuate structures share a number of common geological features that have implications for their origin: (1) they are convex towards greenstones and concave towards large areas of granitoid; (2) the granitoid centres were less strained and the greenstone belts were wrapped around the granitoid blocks; (3) simple shear strain was largely partitioned along the granitoid margins where shear zones commonly dip steeply away from the greenstones; (4) adjacent to the shear zones earlier folds in greenstones rotated into parallelism with the shear zones, while new folds and flattening fabrics developed at the broad apex of the lozenge-shaped granitoid bodies; and (5) folding and reverse faulting in greenstone belts were most intense within the Ntrending linking zones that accommodated local shortening induced by horizontal displacements on the NW-trending sinistral and NE-trending dextral shear zones. These geometric and k i n e m a t i c SGTSG abstracts, February 2001

characteristics suggest that the arcuate structures were generated by impingement of competent granitoid blocks into the less competent greenstone belts during progressive, inhomogeneous E - W shortening. This recently recognised deformation style may have important tectonic implications for other Archaean granite-greenstone terrains and where rock heterogeneity and competency differences are significant.

-rr

SGTSG abstracts, February 2001

Permo-Triassic exhumation of mid-crustal granulites, central Korea: relationship with continental collision in the DabieSulu belt, China

M. Cho^ ^ S.R. Lee 2 and K. YP ^ School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-742, Korea 2 Geological Research Division, Korea Institute of Geology, Mining and Materials, Taejon, 305-350, Korea ^ Now at: Tectonics Special Research Centre, University of Western Australia, Nedlands, WA, Australia 6907

The Triassic growth of the east Eurasian landmass is primarily governed by the continental collision between the Sino-Korean and Yangtze cratons along the Dabie-Sulu otogenic belt (Fig. 1). This collisional belt is characterized by the occurrence of ultrahighpressure rocks, but its continuation towards Korean Peninsula is still controversial (e.g., Yin & Nie, 1993; Li, 1994;Reeetal., 1996; Chen&Jahn, 1998). Some recent workers (Ernst & Liou, 1995; Ree et al., 1996; Kim et al., 2000; Lee et al., 2000) suggested that the Dabie-Sulu belt apparently passes through central Korea along the Imjingang belt and the Gyeonggi massif Although the east-trending structures are prominent in the western part of the Imjingang belt (Cho et al., 1995; Ree et al., 1996), their continuation to the east has not been documented by detailed mapping. In contrast with the common assumption

Suture 303N M^feult

. Yangtze cnaton ^

Nangrim Massif

Imjingang Belt 100

?00 km

Eist Ses

' i ^tUKfyarea

Gyeonggi • ^ : Massif

Fig. (A) Tectonic map of northeastern Asia; (B) Location oAhe study area. S1 and S2 are possible positions of the suture (Yin and Nie. 1993; U. 1994)

of the continuity of east-trending structures and lithologies, we have found a coherent granulite complex in east-central Korea, formerly regarded as a part of the eastern Imjingang belt (Lee et al., 2000). This granulite complex (HGC) experienced two cycles of tectonometamorphic events. The first event corresponds to crustal thickening, followed by peak granulite-facies metamorphism at 7.2-8.8 kbar and 785-840°C that induced widespread partial melting in pelitic rocks and produced syn- to post-tectonic, (para-) autochthonous leucogranites. The quasiisobaric cooling path, suggested by the occurrences of secondary garnet and kyanite, indicates substantial residence time at mid-crustal levels prior to exhumation of the HGC. The timing of peak metamorphism was dated from the unzoned overgrowth rims on zircons in a migmatitic granulite at 1872 ± 7 (2s) Ma, using an ion microprobe. T h e second t e c t o n o m e t a m o r p h i c event is characterized by the growth of cordierite mantling garnet and kyanite in pelitic granulites, and the development of hornblende-plagioclase symplectites around garnet in mafic granulites and amphibolites. This decompressional event could have been responsible for the final exhumation of the H G C towards the surface. P - T estimates from both pelitic and mafic rocks suggest a quasi-isothermal decompression. The age of exhumation is interpreted to be Permo-Triassic on the basis of available isotopic data: e.g., the electron-microprobe chemical age of monazite (245 ± 3 Ma) from a sillimanite-garnet gneiss (Cho et al., 1996) and the ^OAr /39Ar ages of hornblende (226 ± 8 Ma) from a garnet amphibolite. Moreover, our preliminary result on chemical ages of monazite grains from both granulites and biotite gneisses defines two distinct age groups: early Proterozoic and Permo-Triassic. The latter age appears to be prominent in high-strain zones of the marginalzone gneiss complex (MZGC), surrounding the HGC. The structural patterns during the granulite formation are ambiguous, primarily because of SGTSG abstracts. February 2001

widespread partial melting in pelitic rocks. However, mineralogical evidence such as the occurrence of relict kyanite suggests that the compressional deformation led to the development of HGC during the early Proterozoic. The major structural features observed in the Hwacheon area are the product of a younger Permo-Triassic tectonothermal event that has resulted in southwestward juxtaposition of the HGC over the MZGC. The overthrusting of HGC was accompanied by tight to isoclinal folds with east-plunging lineations, Ljj, that are prominent in the MZGC. This deformation is generally absent in the internal part of the HGC, but mylonitic gneisses are common in highstrain zones near the boundary between HGC and MZGC. Subsequent gravitational readjustment of the thickened crust is probably responsible for the widespread top-down-to-the-north (normal) shear sense in the semi-circular, extensional shear zones. This shear sense is manifested by almost all shear criteria, including north-plunging stretching lineations, Lj^+iBoth structural and temporal relationships responsible for the exhumation of the HGC in the northeastern Gyeonggi massif are compatible with

SGTSG abstracts. February 2001

those reported in the western Imjingang belt by Ree et al. (1996). It is thus likely that the HGC represents the reworked crystalline basement exhumed during the Permo-Triassic collisional orogeny. This thermotectonic event is coeval with the continental collision between the Sino-Korea and Yangtze cratons at ca. 210240 Ma (e.g., Hacker et al., 2000). Hence, both the Imjingang belt and the Gyeonggi massif corresponding to thin- and thick-skinned orogens, respectively, are probably the eastward continuation of the Chinese continental collision belt in the Korean Peninsula. As a corollary, the Gyeonggi massif is correlative with the Yangtze craton.

References

Chen, J., and Jahn, B., 1998. Tectonophysics 284: 1 0 1 - 1 3 3 . Cho, D.-L. et al., 1996. J. Earth Planet. Sci., Nagoya Univ. 43: 4 9 - 6 5 . Cho, M. et al., 1995. J. Petrol Soc. Korea 4: 1 - 1 9 . Ernst, W. G., and Liou, J. G., 1995. Geology 23: 353-356. Hacker, B.R. et al., 2000. Jour. Geophys. Res. 105: 13339-13364. Kim, J.-N. et al., 2000. Jour. Geol. 108: 4 6 9 - 4 7 8 Lee, S.R. et al., 2000. Jour. Geol. 108: 729-738. Li, Z. X., 1994. Geology 22: 7 3 9 - 7 4 2 . Ree, J.-H. et a l , 1996. Geology 24: 1 0 7 1 - 1 0 7 4 . Yin, A., and Nie, S., 1993. Tectonics 12: 8 0 1 - 8 1 3 .

SHRIMP U-Pb zircon ages from the Mullingarra Complex, Western Australia: Isotopic evidence for allochthonous blocks in the Pinjarra Orogen and implications for East Gondwana assembly

M.M. Cobb, P.A. Cawood, P.D. Kinny and I.C.W. Fitzsimons Tectonics Special Research Centre, Departnnent of Applied Geology, Curtin University of Technology, GPO Box U 1987, Perth, WA, Australia 6845

The Pinjarra Orogen extends along the western margin of Australia and is exposed as three inliers within the Paleozoic to Tertiary Perth Basin. These are the Mullingarra, Northampton and Leeuwin complexes. The orogen truncates the Yilgarn Craton, and Capricorn and Albany-Fraser orogens, and is separated from them by the Darling Fault. Prior to opening of the Southern Ocean, the orogen extended south into the Denman Glacier region of Antarctica. The northern extent of the orogen is unconstrained. New geologic and geochronologic data for the previously little studied Mullingarra Complex integrated with data from the Northampton Complex indicate that these blocks are probably contiguous and were formed, deformed and metamorphosed at a site well removed from the current Yilgarn margin. The Mullingarra Complex crops out in the northern Perth Basin between the Darling and Urella faults. Remapping of the northern part of the complex indicates it is divisible into two major units; a suite of high-grade metasedimentary rocks and related amphibolites, and an undeformed monzogranite. Age and tectonometamorphic data indicate that the metasedimentary unit structurally overlies the granite, with the latter forming a local basement to the complex. Undeformed and unmetamorphosed gently dipping siliciclastic sedimentary rocks of the Yandanooka Group are inferred to unconformably overly the complex along its eastern margin. The U-Pb SHRIMP zircon age calculated for the monzogranite is Paleoproterozoic at 2181 ± 10 Ma (2s error). The overlying metasedimentary sequence is characterized by a pervasive foliation (Si) parallel to relict lithologic layering and axial planar to submetre scale rootless isoclinal folds. S y n - D l , amphibolite-facies metamorphism reached an estimated pressure and temperature of 5.9 kbar and 666°C. A later greenschist-facies metamorphic event (M2) is related to the emplacement of felsic pegmatitic sheets parallel to the foliation in the paragneiss. A second deformational event has resulted in meso- to

macro-scale folding of both the metasedimentary unit and its faulted contact with the underlying granite. S H R I M P age data from detrital zircons of the paragneiss and quartzite samples indicate derivation of sediment from dominandy Proterozoic sources, with peaks in the age spectrum at ca 1200, 1300-1450, and 1600-1800 Ma. There is a very minor component of Archaean detritus present in both samples. The youngest detrital grain is 1 1 1 3 + 2 6 Ma (2s error). The age of metamorphism for the metasedimentary rocks has been calculated at 1058 ± 83 Ma (2s error), based on thin metamorphic overgrowths on detrital zircon grains in both the paragneiss and quartzite. Similarities between the metasedimentary units in the M u l l i n g a r r a Complex and those of the Northampton Complex in terms of lithologic assemblage, age signature of detrital zircons, deformational sequence and timing of high-grade metamorphism suggest the two constitute a single tectonostratigraphic package. The absence of Mesoproterozoic high-grade metamorphism in the Yilgarn Craton indicates that the development of the metasedimentary rocks of both the Mullingarra and Northampton complexes occurred at some distance from their current position. The detrital age signature of metasedimentary units in the Mullingarra and Northampton complexes overlaps the age range of rock units in the Capricorn and Albany-Fraser orogens, suggesting that these two orogens could have provided detritus for metasedimentary protoliths in the Pinjarra Orogen. However, the tectono-metamorphic evidence for significant separation of the M u l l i n g a r r a and Northampton complexes from the Yilgarn Craton at the time of Pinjarra metamorphism would also suggest a large separation between these complexes and current exposures of the Capricorn and Albany-Fraser orogens at the time of sediment deposition. Although it has been demonstrated elsewhere that detrital grains can be deposited great distances from their source terranes (Rainbird et al., 1992), it is difficult to explain how SGTSG abstracts, February 2001

such detritus could comprise a major component from the Capricorn and Albany-Fraser orogens yet lack any significant input from the intervening Yilgarn Craton. We favour an alternative model, in which metasedimentary protoliths in the Pinjarra Orogen were derived from source terranes that had very similar ages to the Capricorn and Albany-Fraser orogens but were situated some distance from current Western Australian exposures of these orogens. These source terranes could have been direct along-strike exposures of both the Capricorn and Albany-Fraser orogens that were subsequently displaced by movement along the Darling Fault, or they could have been unrelated terranes that happened to be of the same age. The end Mesoproterozoic paleogeographic reconstruction of East Gondwana is too poorly constrained to confirm specific source regions, although one model is proposed by Fitzsimons et al. (2001). We note, however, that both the Mullingarra and Northampton metasedimentary units contain a small component of detritus derived from a 2200 Ma source. Rocks of this age are largely absent from Western Australia but are represented by the monzogranite in the Mullingarra Complex. Thus it is possible that this unit constituted part of the source terrane for the metasediments which has subsequently been incorporated with them during later assembly of the Pinjarra Orogen. The requirements for an allochthonous origin to the Mullingarra and Northampton metasedimentary rocks, and also for the assembly of these with the Paleoproterozoic monzogranite, is consistent with the recent work of Fitzsimons (2000) suggesting PanAfrican otogenic activity in East Antarctica represents Neoproterozoic assembly of East Gondwana and not reactivation in a much older, stable Mesoproterozoic continent. The constraining age of 750 Ma from the Mundine Well dyke swarm (Wingate and Giddings, 2000) requires that assembly and juxtaposition of the Northampton and Mullingarra against the Yilgarn be complete by this time. This being the case, the 500600 Ma period in which the Prydz-Denman-Darling mobile belt of Fitzsimons (2000) was active needs to be extended further back into Neoproterozoic time, until at least 750 Ma.

SGTSG abstracts, February 2001

References Fitzsimons, I.C.W., 2 0 0 0 . Grenville-age basement provinces in East Antarctica: Evidence for three separate collisional orogens. Geology 28: 8 7 9 - 8 8 2 . Fitzsimons, I.C.W., Cobb, M.M., Cawood, PA. & Kinny, P.D. 2 0 0 1 . Proterozoic accretion and reactivation along the Darling Mobile Belt of Western Australia. This volume. Rainbird, R.H., Heaman, L M . & Young, G., 1 9 9 2 . Sampling Laurentia: Detrital zircon geochronology offers evidence for an extensive Neoproterozoic river system originating from the Grenville orogen. Geology 20: 3 5 1 - 3 5 4 . Wingate, M.T.D. & Giddings, J.W., 2 0 0 0 . Age and palaeomagnetism of the Mundine Well dyke swarm, Western Australia: implications for an Australia-Laurentia connection at 7 5 5 Ma. Precambrian Research 100: 3 3 5 - 3 5 7 .

The tectonic architecture of central l\/ladagascar - Using tectonostratigraphic principles to unravel the deep expression of a continental collisional zone

A. C o l l i n s ' , B. W i n d l e y ^ I. F i t z s i m o n s ' a n d B. Hulscher"^ ^Tectonics SRC, Curtin University of Technology, GPO Box U1987, Perth, WA, Australia 6845 2 Department of Geology, University of Leicester, University Road, Leicester, UK ^Tectonics SRC, Departnnent of Geology and Geophysics, University of Western Australia, Nedlands, WA, Australia 6907

Many Phanerozoic zones o f continental collision (e.g.

unravel the amalgamation history o f the orogen. In

the Alpine-Himalayan chain, Variscan Orogeny) are

this presentation we use techniques more commonly

characterised by a complicated history o f early

used to unravel the complexities o f upper crustal

ophiolite obduction, collision o f microcontinental and

collisional zones to attempt to identify tectonic units

island arc terranes with an active margin, continental

in the deeply eroded East African Orogen in central

collision and otogenic collapse. These processes are

Madagascar and produce a testable model for the

often diachronous over the extent of the orogen.

formation o f central and north Madagascar.

In deeply e x h u m e d o r o g e n s ,

such as the

Mozambique belt o f the East African Orogen, these

B y e x a m i n i n g the s t r u c t u r a l ,

geochemical,

geochronological and lithological history of central and

complications are often missed due to high grades o f

north Madagascar five major tectonic units are picked

metamorphism and pervasive ductile deformation that

out. These are summarised in the table and figure

tend to blur the boundaries o f original tectonic units.

below.

However, by careftilly integrating field and laboratory data, individual tectonic units (blocks o f rock that

Laterally extensive shear zones separate each tectonic unit.

experienced similar t e c t o n i c processes) can be

The Tsaratanana sheet has a mylonite zone at its

recognised. Once isolated, it is possible to investigate

base that separates it from the Antananarivo block. As

the bounding zones o f these tectonic units and to

both the Tsaratanana sheet and the Antananarivo block

Tectonic Unit Bemarivo orogenic belt Itremo sheet

Tsaratanana sheet

; Antananarivo block

Antongil block

• • • • • • • • • • • • • • • • • • • • • • • • • •

Major tectonic events 717-754 Ma granite magmatism 715 Ma rhyolite extrusion coeval with deposition of sandstones and conglonnerates 510-520 Ma granulite-grade metamorphism coeval with south-west directed thrusting Extensional deformation Post 1850 Ma deposition of quartzites, mudrocks and carbonates Deformation into large recumbent isoclinal folds 1 800-790 Ma intrusion of gabbros, syenites and granitoids Deformation into upright, open folds 572-534 Ma post-tectonic granite intrusion possibly coeval with:j Extensional deformation along the eastern boundary (the Betsileo shear zone) 2747-2494 Ma granitoid intrusion through cmst dating back to 3260 Ma Deformation and emplacement over Antananarivo block 787-779 Ma gabbro magmatism coeval with high-temperature metamorphism in the country rock 637-627 Ma granitoid intrusion Intense deformation transposing earlier rocks into gneissic tectonites. 549 Ma late diorite magmatism Deformation -2500 Ma crust formation 2188.5-1006.9 Ma zircon xenocrysts in later granites, whose significance is unknown. 824-719 Ma supra-subduction zone gabbro and granitoid intrusion 633-561 Ma granitoid magmatism pre-550 Ma granulite-grade metamorphism followed by (or coeval with) extension-related retrogressive metamorphism Intrusion of post-tectonic granitoids 561-530 Ma 3200-2500 Ma continental crust formation and defomriation 2500 Ma Granite intrusion Palaeozoic sediments deposited on erosion surface

SGTSG abstracts, February 2001

are cut by 825-719 Ma igneous rocks, formation of this mylonite zone and tectonic emplacement of the Tsaratanana sheet probably occurred before -820 Ma. The Itremo sheet is locally east to west thrust imbricated with the Antananarivo block. However, at its eastern boundary the Betsileo shear zone, a crustalscale extensional detachment that juxtaposes upper amphibolite-grade footwall gneiss with greenschistfacies hangingwall metasedimentary rocks, separates the overlying Itremo sheet from the Antananarivo block. Sediments within the Itremo sheet contain detrital zircons that indicate deposition after 1850 Ma. Large-scale (<20 km amplitude) recumbent folds formed before intrusion of -800 Ma gabbro and granitoid plutons. The Antananarivo block was thrust east over the Antongil block between -700 and 550 Ma. A paragneiss belt with peridotite bodies marks the boundary zone between these tectonic units. This belt is interpreted as a strand of the Mozambique Ocean suture (the Betsimisaraka suture) that separated the Dharwar craton (i.e. the Antongil block) from the Antananarivo block. The Bemarivo orogenic belt truncates the other units and was metamorphosed to granulite-facies conditions in Cambrian times. It

preserves evidence for contractional, top-to-the-south deformation overprinted by later extensional deformation and represents a terminal stage of mountain building in the East African orogenic cycle. By integrating the history preserved within the tectonic units with the kinematic evidence preserved in their bounding shear zones the following conclusions are made about the tectonic history of the East African Orogen in central Madagascar: • Central Madagascar experienced contractional deformation prior to the -800 Ma magmatic event. • Between 820 and -750 Ma central Madagascar formed a continental magmatic arc that overlay a west-dipping subduction zone now represented by the Betsimisaraka suture zone. • The Antongil Block (correlated with the Dharwar craton of East Gondwana) collided with the Antananarivo block between 630 and 532 Ma with intense deformation focussed along the Betsimisaraka suture. • The Bemarivo orogeic belt was thrust south over amalgamated central Madagascar at -520 Ma. ^ ^ f f ^

Bemarivo orogenic bell

AnforiQr! block

Antananarivo block with

Tsaratanana sheet

Southern tir^rno ! ^^adagascaf SS » Setsimisaraita suture zone BSZ « Betsileo st^em- zonQ » Ranotsara shear zone ToC « Tropic of Capncom A s Antananarivo

Figure 1. The tectonic units of cent ral and northern Madagascar,

SGTSG abstracts, February 2001

The tectonic evolution of the Lycian Allochthon and implications for the evolution of SW Turkey A. Collins Tectonics SRC, Curtin University of Technology, GPO Box U 1987, Perth, WA, Australia 6845

The Lycian Allochthon is made up of the far-travelled remnants of the northern passive margin to the Menderes-Tauride block along with the upper plate of an intra-Tethys subduction zone. Grossly, the Lycian Allochthon forms a 100 km-scale tectonic outlier thrust southeastwards over both the Bey Daglari foreland and the Menderes Metamorphic Complex. The Lycian Allochthon is divided into four first order tectonic units: (1) Supra-allochthon sediments; (2) the Lycian Peridotite Thrust Sheet; (3) the Lycian Melange; (4) and the Lycian Thrust Sheets (in broad top-bottom structural superposition; see Figure 1). Each unit formed in a different tectonic environment and is separated from the others by a shear zone or a regionally significant unconformity. A number of second-order tectonic units are also identified that formed in related (but distinguishably different) tectonic environments. The table below summarises the hierarchy of these tectonic units. 1" order tectonic unit

order tectonic unit

Supra-allochthon sediments Lycian Peridotite Thrust Sheet Lycian Melange

Ophiolitic Melange Layered Tectonic Melange

Lycian Thrust Sheets

Koycegiz Thrust Sheet Teke Dere Thrust Sheet Karadag Thrust Sheet

Yavus Thrust Sheet

The Karadag T h r u s t Sheet records Upper Carboniferous, Early and Late Permian continental shelf/lagoonal deposition. The overlying Teke Dere Thrust Sheet includes a rift succession of Late Permian age that was subaerially exposed during much of the Triassic; a marine transgression followed in the Early Jurassic succeeded by subsidence that formed a continental slope from Mid Jurassic to Palaeocene times. The overlying Koycegiz Thrust Sheet records Late Triassic oceanic crust (along a rifted margin), overlain by an Early Jurassic carbonate platform; this then subsided to form a continental slope, that survived until Late Cretaceous times.

Kinematic evidence and sedimentary facies indicate that the Lycian Thrust Sheets can be restored as a north-facing Mesozoic rift and passive margin. Mainly deep-water sediments of Triassic to Late Cretaceous age, preserved as blocks within the Layered Tectonic Melange and Ophiolitic Melange, are interpreted as deep-water sediments deposited on Mesozoic (Neotethyan) oceanic crust. Subduction of this ocean basin proceeded from north to south, beginning with accretion of oceanic-derived melange and disrupted thrust sheets. Debris was shed into a continentwardmigrating flexural foredeep, initially located along the distal edge of the continental margin in CampanianMaastrichtian times; this foredeep then propagated southwards in stages over more proximal continental crust (including an intra-platform basin). The first main stage of southeastward propagation was in Palaeocene and Eocene times followed by a second stage in Oligocene-Miocene times. The Lycian Allochthon was finally emplaced over the most proximal (south-easterly) foredeep (i.e. the Kas basin) in Late Miocene time. A model linking S W Turkey with the Cyclades and the Aegean subduction system is presented by integrating the evidence preserved in the Lycian Allochthon with recent work from the Menderes Metamorphic Complex (e.g. Gessner et al., in press; Ring et al., 1999). In this model Eocene continental collision of the Menderes/Tauride block with Eurasia choked the subduction system in S W Turkey. The buoyancy of the Menderes-Tauride Block forced a 200300 km southward jump in subduction that may have forced a similar retreat in the Aegean subduction system. References Gessner, K., Ring, U., Passchier, C.W. & Giingdr, T , in press. The Cyclades in Turkey: Evidence for Eocene post-high-pressure emplacement of the Cycladic blueschist unit onto the Menderes nappes, Anatolide belt, western Turkey Journal of the Geological Society of London. Ring, U., Gessner, K., Gungor,T. & Passchier, C.W., 1999. The Menderes Massif of Western Turkey and the Cycladic Massif in the Aegean - do they really correlate. Journal of the Geological Society of London 155:3-6.

SGTSG abstracts, February 2001

Domus Dagi Layefed Teclonic subunrl yelange Supra-allochtrnon

Lycian Peridotite Thrust Sheet

yenderes Metarnorphic Complex / ^ /Bey Daglari Platform / \<

/ / /

Figure 1: The structural relationships between the various tectonic units in SW Turkey.

CuilHtfjulc Jcfr? ';Jhtmt

Litbc aicoilcj. znd

uitjtct

rir^ciiAtcs ivish

iiftii

rti-THs;*-?!

Aiijifi

>ijc ciry

m, ^

I im^umi: Kumdai Thrust Shiiiif

.SHi tbsnncli^J

The Ivtimi

Aumhthi^n

Figure 2: Structural relationships and lithologies of the Lyican Thrust Sheets

SGTSG abstracts, February 2001

Nature of extensional accretionary orogens W.J. Collins School of Geosciences, University of Newcastle, Newcastle, NSW, Australia 2308

Extensional orogens form above continuously

increasing P-wave velocities through the lower crust.

extending lithosphere, while the crust undergoes

Slab rollback and lithospheric thinning during

intermittent contraction. Extensional accretionary

accretion provides a mechanism for rapidly producing

orogens initiate at intraoceanic convergent margins and

large extensional orogens, for keeping them hot during

grow by protracted sediment (turbidite) and magma

crustal contraction, for generating voluminous mafic

additions in the backarc, arc and accretionary prism,

igneous lower crust (and granites) during crustal

during slab rollback. Transient, localised contractional

thickening, and for maintaining a shallow Moho

events, during arrival of thickened buoyant oceanic

throughout the otogenic history. .^^JFf^

crust (plateau) in the subduction zone, thrust the prism toward the thermally softened arc and backarc regions, which preferentially deform to produce normalthickness continental crust. If the buoyant oceanic crust is only moderately thick, it is offscraped and the slab continues to subduct steeply, resulting in ongoing lithospheric extension and asthenospheric melting below the developing orogen during crustal thickening, leading to extensive syn-collisional arc magmatism. Following offscraping and associated thickening, crustal extension is re-established and another cycle of arc/backarc magmatism and sedimentation occurs before arrival of the next oceanic plateau. All rocktypes are preserved at low-metamorphic grade and are intruded extensively by granites. Evidence of large-scale "terrane accretion", doubly-vergent thrust systems, exposure of high-grade basement, and narrow plutonic belts, which characterises contractional accretionary orogens like the North American Cordillera, is lacking. A diagnostic feature is the emplacement of primitive mafic rocks throughout the otogenic history, which have oceanic arc or backarc geochemical signature. In the vast Paleozoic Lachlan Orogen of eastern Australia, these rocks only acquired an intraplate, "continental" character (reflecting lithospheric thickening), some 50 Ma after initial crustal thickening, during the final major deformation event. At this stage, stable continental crust was produced in the Lachlan. Synorogenic mafic magmatism occurs because counter flow in the mantle wedge during slab retreat ensures the sub-arc lithosphere remains thin, thereby promoting adiabatic decompression melting of the asthenosphere and generation of basaltic magma, irrespective of whether the crust is contracting or extending. Most of the mafic input is by underplating, reflected by a dominance of lower crustal, cumulate mafic xenoliths and seismic refraction profiles showing

SGTSG abstracts, February 2001

28 The behaviour of faults, fluids and gold in a crustal scale shear system, St Ives Goldfield, WA — A case of golden aftershocks? S.F. Cox\ K. Burning^, P.T. Nguyen^ and W.E. Stone^ ^ Centre for Advanced Studies of Ore Systems, Department of Geology and Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia 0200 2 Department of Geology, The University of Newcastle, Callaghan, NSW, Australia 2308 3 Agnew Gold operations, WMC Resources Ltd, PMB 10, Leicester, WA, Australia 5437 ^ St Ives Gold operations, WMC Resources Ltd

Fluid flow during the formation of Archaean, greenstone-hosted gold deposits in the St Ives Goldfield, near Kambalda (WA), was localised within arrays of low displacement faults and shear zones which form part of the N N W trending, crustal-scale BoulderLefroy fault system. The ore-hosting structures are kinematically related to sinistral to oblique-reverse slip on the Playa Fault, which is a 20 km long driving structure splaying from the 200 km long BoulderLefroy Fault. Net slip on the Playa Fault is inferred to be up to one kilometre. The Playa Fault was severely misoriented with respect to east-west crustal shortening during formation of the ore-hosting structures and concurrent fluid flow and gold mineralisation. The distribution of the low displacement structures which hosted fluid flow and gold mineralisation was controlled largely by the development of a kilometrescale contractional jog in the Playa Fault. Gold mineralisation occurs predominantly in: 1. An imbricate thrust fan within the contractional jog (Victory complex); 2. Arrays of optimally-oriented, N-S trending thrusts in a belt up to one kilometere wide, immediately west of the Playa Fault and N W of the Victory complex; 3. Mixed reverse to strike-slip faults up to several kilometres west of the Playa Fault; 4. Rare E-W trending normal faults and shear zones; 5. Fracture networks in competent rock units within and adjacent to the Playa Fault. The mechanical evolution of the low displacement, gold-hosting structures was influenced by repeated fluid pressure fluctuations associated with fault-valve behaviour, and by reaction-weakening and reactionhardening processes during progressive hydrothermal alteration. Fast, possibly seismic, brittle shear failure events are interpreted to have been driven by episodic fluid pressure build-up to supralithostatic values in parts of the low displacement shear network. Slower SGTSG abstracts, February 2001

creep events occurred during lower pore fluid pressure intervals between fast slip events. Lower fluid pressure periods were probably controlled by partial draining of the hydrothermal system after major slip events on the larger driving structures in the Boulder-Lefroy system. By analogy with modern seismogenic systems, the low displacement, gold-hosting structures in the St Ives Goldfield are interpreted as aftershock structures whose evolution was controlled by major slip events on the Boulder-Lefroy fault system. We have conducted 3D finite element modelling of static stress changes associated with large slip events on the Boulder-Lefroy and Playa Faults. For cases where major northward propagating ruptures are arrested at the Victory jog, the modelling indicates that most of the gold mineralisation in the St Ives Goldfield is localised within a domain whose proximity to failure was increased by Coulomb stress transfer during major slip events in the Playa-Boulder-Lefroy fault. The modelling points to other areas which may also contain potentially gold-hosting aftershock structures. We propose that the distribution of low displacment, gold-hosting structures around major, crustal-scale shear systems may be substantially influenced by stress transfer driven by large slip events. However, the distribution of aftershock structures will also be modulated by changes in effective stress states due to transient fluid migration though shear systems after large slip events.

Neogene Fault reactivation and structural styles in the Timor Sea, northwest Australia

J. Cunneen Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA. Australia 6907

Neogene deformation in the Timor Sea has caused significant left-lateral transtensional reactivation of Mesozoic rift structures (Shuster, 1998; Nelson, 1993; Keep et al., this volume). Given that some hydrocarbon traps in the Timor Sea have been charged in the past 3 Ma, the Neogene structural history and fault linkage is crucial in understanding structural controls on trap formation and breaching in this area. This paper is based on interpretation of a regional 2D seismic dataset covering the greater Timor Sea region, and a 3D dataset covering the Laminaria basement high area (see Figure 1 for location). Fault patterns The regional fault pattern consists of right-stepping en-echelon faults at all scales, indicating left-lateral movement. Despite these fault patterns, there are no large offsets visible in 3D seismic data from the Nancar Trough/Laminaria area (De Ruig, 2000). This suggests that any lateral movement is below seismic resolution, and the strain is spread over numerous small faults (Walsh & Watterson, 1991). Fault aspect ratios (the ratio of fault length to depth) (Willemse, 1997) from faults in the Laminaria area indicate a high degree of fault interaction and a strike-slip stress field. The large amount of fault interaction also suggests a significant amount of sub-seismic strain. Sub-seismic strain, often referred to as ductile strain, can account for up to 4050% of the total strain (Walsh et al., 1996). Fault styles Fault growth curves from the Laminaria basement high area show several pulses of movement during the Neogene, with the youngest faults occurring adjacent to the Timor Trough. Structural styles include net normal and left-lateral strike-slip faults, as well as controversial "hourglass" faults. The hourglass faults, which appear to cross within the Tertiary horizons, may be related to detachments within the Neogene section, with the crossover points representing areas of high strain. I have classified hourglass faults into two broad types determined by size, intersection point, complexity and amount of fault displacement. Their

distribution depends on the magnitude of Neogene strain and the amount of Tertiary sedimentation. Hourglass faults The first type of hourglass structures formed from a single phase of extension and occur mainly on the Sahul and Ashmore Platforms, where there is thin Tertiary cover. They converge between the Early Miocene and Early Pliocene horizons and show minor normal displacement (Figure 2). The second type of hourglass structures were formed by at least two stages of movement and are the result of linking faults which nucleate above Mesozoic horst structures and propagate in both directions, converging in the early Eocene horizons. They occur in areas of high strain and thick Tertiary cover, such as the Laminaria area and the Carrier Trough (Figure 3). A decrease in displacement towards the intersection point is attributed to sub-seismic strain. Detailed analysis of 3D seismic data from the Laminaria area suggests that in areas of high strain, hourglass structures become discontinuous detached faults with an area of subseismic faulting at the crossover point Spatial relationships Although most of the faulting is confined to the Neogene section, some faults form a hard link with reactivated Mesozoic structures. The proportion of hard linkage increases with the amount of strain and is dominant on the northern margins of basement highs that flank the Timor Trough. The spacing of these faults is directly proportional to the amount of Neogene strain. On the shelf margin there are many with spacing of less than 2 km, while on the platforms (areas of low strain) they can be more than 100 km apart. Contemporary stress orientations also vary locally around the Laminaria high, and the effects of present day stress in reactivating faults is important in understanding the structural controls on the Laminaria/Corallina fields. Regional stress analyses from borehole breakout data show the maximum horizontal stress is oriented NE-SW (Hillis, 1998),

SGTSG abstracts, February 2001

but there is large variation in local measurements (De Ruig et al., 2000), indicating a high degree of strain partitioning and a complex strain regime.

Summary Neogene deformation in the Timor Sea has produced right-stepping en-echelon faults at all scales, some of which formed a hard link with reactivated Mesozoic structures. Hourglass faults occur in several regions and have been studied in detail in the Laminaria area. Strain at the crossover point results in extensive subseismic faulting and decreased apparent offset of horizons. A detachment may form near the crossover point in areas of high Neogene strain. Local stress orientations from borehole breakout data reflect the complex strain regime, which mimics the broader-scale tectonics influences on this part of the North West Shelf (see Keep, this volume), . y f ^

References Clough, M., Keep, M. & Longley, I. Neogene Structural Evolution of the Timor Sea Region, Northwest Australia: evidence for an 8 Ma event. In review at Australian Journal of Earth Science. De Ruig, M., Trupp, M., Bishop, D J., Kuek, D. & Castillo, D., 2000. Fault Architecture and the Mechanics of Fault Reactivation in the Nancar Trough/Laminaria area of the Timor Sea, Northern Australia. The APPEA Journal 40: 174-193. Hillis, R.R., 1998. The Australian Stress Map, Petroleum Exploration Society of Western Australia News, December/January: 40-43. Keep, M. Neogene Collision: the Response of Australia's Northwest Margin. This volume. Nelson, A. 1993. Wrench and inversion structures in the Timor Sea region. PESA Journal, July: 3-30. Shuster, M . W , Eaton, S., Wakefield, L L , & Kloosterman, H.J., 1998. Neogene Tectonics, Greater Timor Sea, Offshore Australia: Implications for Trap Risk, The APPEA Journal 38(1): 351379. Walsh, J.J., and Watterson, J., 1991. Geometric and kinematic coherence and scale effects in normal fault systems. In: Roberts, A.M., Yielding, G. and Freeman, B. (eds) The Geometry of Normal Faults. Geological Society, London, Special Publications 56:193203. Walsh, J.J., Watterson, J., Childs, C. and Nicol, A. 1996. Ductile strain effects in the analysis of seismic interpretations of normal fault systems. In: Buchanan, PG. and Nieuwland, D.A., Modern Developments in Structural Interpretation, Validation and Modelling. Geological Society, London, Special Publication 99: 27-40. Willemse, E.J., 1997. Segmented normal faults: Correspondence between three-dimensional mechanical models and field data. Journal of Geophysical Research 102(B1): 675-692.

Figure 1. Location of basement highs in the Timor Sea, northwest Australia.

Figure 2. Type 1 hourglass structure. Figure 3. Type 2 hourglass structure.

SGTSG abstracts. February 2001

Thermo-mechanical evolution of the crust during convergence and mid-crustal pluton emplacement In the Western Province of Flordland, New Zealand

N.R. Daczko^*, K.A. Klepeis^ and G.L. Clarke^ ^ School of Geosciences, Building F05, University of Sydney, NSW, Australia 2006 2 Department of Geology, University of Vermont, Burlington, VT 05405-0122, USA

Fiordland, New Zealand contains exposures of Early Cretaceous high-P (12-14 kbar) granulite facies orthogneisses whose tectonic evolution is controversial. A large portion of the granulite facies assemblages exposed in Fiordland occurs within a composite monzodioritic to gabbroic batholith called the Western Fiordland Orthogneiss. We present new structural, kinematic, and metamorphic data from two fiords in central Fiordland (George Sound and Caswell Sound) that show the emplacement of the batholith occurred in the middle crust within a convergent tectonic regime. The contact aureole of the batholith exposed at George Sound contains diatexites that formed during emplacement of the batholith at conditions of P^ 7-9 kbar. Following its emplacement, the batholith experienced tectonic loading and burial to lower crustal depths of > 45 km. Data from Caswell Sound indicate that burial was accomplished via imbrication of a series of thrust sheets that deformed the uppermost (westernmost) contact of the batholith at conditions of P = 9 kbar, T> 750°C. The Caswell thrust sheets

form part of a well exposed, two-sided fold-thrust belt that represents the first structural evidence of Cretaceous tectonic thickening of the middle crust via thrust faulting in western Fiordland. Samples collected across the pluton/country rock contact at Caswell Sound document microstructural changes that are consistent with an amphibolite to granulite facies thermal gradient across 500 m. Garnet-biotite-Kfeldspar assemblages in granulite facies thrusts give way to garnet-chlorite-epidote amphibolite facies assemblages greater than 500 m distal to the pluton contact. We propose a kinematic model of fold-thrust belt evolution that includes the ductile extrusion of Paleozoic metasediments between forward-breaking (east-vergent) thrust ramps and west-vergent backthrusts followed by thrust sheet propagation via a rigid translation of rocks above a buried decollement surface that deforms the western contact of the Western Fiordland Orthogneiss batholith.

SGTSG abstracts, February 2001

32 Seismic expression of layer-bound fault system in the Late Tertiary of the Bass Basin: indications of syneresis of colloidal sediments P.K. Das, A. Cummings and N. Lemon The National Centre for Petroleum Geology and Geophysics, The University of Adelaide, SA, Australia 5005

High-resolution 2D seismic data reveal an extremely high-density of normal extensional faults in the middle Oligocene to late Miocene section in the entire offshore Bass Basin. This paper documents for the first time, a complex, layer-bound, fault system in the Bass Basin. The very fine grained Late Tertiary calcareous clay and marl-dominated succession of the basal Torquay Group exhibits a complex system of small-scale extensional faults with layer-bound deformation without displacement transfer to the underlying basement structures (Fig. 1). The entire sequence is pervasively deformed by very small-scale normal faults with average fault spacing ranging between 60-500 m and fault throws ranging between 1 0 - 4 0 m. The stratigraphy overlying and underlying this sequence is undisturbed and is characterised by highly continuous reflection patterns. The seismic expression of this intensely faulted, vertically isolated section observed in the close-grid 2D data set suggests that the faults may be polygonal in nature. The exact fault dimensions and geometry of the polygonal fault pattern can only be adequately mapped using a 3D seismic data set considering the close fault spacing and inherent aliasing problems of 2D data (Cartwright, 1996). The reflection characteristics of this deformed sequence varies from low amplitude-low frequency to high amplitude-high frequency content and there are three distinct units (tiers) defined in the succession. The low frequency-low amplitude middle unit obscures the definition of the faulting whereas the upper and lower units have clear indications of the structural style of the complex layer-bound fault system. Some faults have connected the middle unit with the top and bottom units respectively. Close scrutiny of seismic data from all parts of the basin suggests that the upper two units are pervasively faulted throughout the whole of the basin while the bottom unit is intensely faulted in some places and undisturbed elsewhere. This change in deformation style of the bottom unit can be attributed to lateral facies variation, if compared to studies from the North Sea basin where fault intensity correlates positively with both clay fraction and smectite content. When the facies is very SGTSG abstracts, February 2001

fine grained with high smectite clay content, faulting is intense but diminishes as the facies becomes coarse grained and smectite poor (Dewhurst et al., 1999). The development of polygonal fault systems by three-dimensional volumetric contraction of muddy sediments during early burial was first reported in literature from central North Sea basin (Cartwright, 1994; Cartwright & Lonergan, 1996). Layer-parallel volumetric contraction observed in seismic sections in the North Sea data has recently been attributed to a process called syneresis of colloidal smectitic gels during early compaction history of the sediments (Dewhurst et al., 1999). Syneresis results from the spontaneous contraction of sedimentary gel without evaporation of constituent pore fluid. This condensation process occurs due to the domination of interparticle attractive forces in marine clays and is governed by the change of gel permeability and viscosity with progressive compaction. A similar process is attributed to the complex layer-bound fault system in the Bass Basin. Large-scale modification of reservoir geometry of the Alba field in the North Sea through sand remobilization and sand withdrawal (Cartwright, 1999) during early burial is seen to be associated with changes in the polygonal fault density and pattern in the hemipelagic mudstones surrounding the reservoir. Exploration for subtle sand bodies capped by finegrained mudrocks in the Late Tertiary of the Bass Basin will require similar attention to the details of development of the polygonal fault system, References Cartwright, J.A., 1 9 9 4 . Episodic basin-wide hydrofracturing of overpressured Early Cenozoic mudrock sequences in the North Sea basin. Marine and Petroleum Geology 11: 587-607. Cartwright, J. A., 1996. Polygonal fault systems: a new type of fault structure revealed by 3 D seismic data from the North Sea basin. AAPG Studies in Geology 42: 225-230. Cartwright, J. A. & Lonergan, L., 1996. Volumetric contraction during the compaction of mudrocks: a mechanism for the development of regional scale polygonal fault systems. Basin Research 8: 183-193. Dewhurst, D. N., Cartwright, J. A. & Lonergan, L., 1 9 9 9 . The development of polygonal fault systems by syneresis of colloidal sediments. Marine and Petroleum Geology 16: 793-810. Lonergan, L & Cartwright, J. A., 1999. Polygonal faults and their influence on deep-water sandstone reservoir geometries, Alba filed, UK Central North Sea. AAPG Bulletin 83: 410-432.

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w»c iDu

Z ill

A.) Uninterpreted : m^m

mi wn vtAr ti

I 4mn ftsi 4«in -naa «uii v.

B) interpreted Figure 1. Seismic expression of the mddle Oiigocene - late Miocene internal shovy/ing Nghly discontinuous pattern of reflections, bounded above and below by oftremely continuous reflection inteivals. The entire section can be formally divided into 3 units, A B & C. The units A & C are marked by reasonably high ampliitude-high frequency reflections whereas the middle unit B is characterised by very lowfequency-lcw amplitude reflection& Units A & Bare pervasively deformed by large number of small throw extensional (normal) faults with spacing rangir^ between 60nv500m. The fault throw varies between 10fn40m. Though the defornrwtion style of unit C is similar, the intensity of deformatbn is however, restricted mainly to the central and right hand side of the section. The lack of reflectivity in unit B nray be due to loss of acoustic contrast in a^erpressured shales.Oetails about the possible genesis of such faults is given in text. Seeinsetfor location of the seismic line

—

vi6t6ftiA' MELBOURNE

General setting of the Bass Basin and localion of the seisrtilcline

SGTSG abstracts, February 2001

34 Major gold deposits in the Tanami region and their associated structural geology and tectonics p. Ding Ding Pty Ltd, 41 Nelson Road, Valley View, SA, Australia 5093

T he most remote Tanami Desert has become the major active gold producing region in Northern Territory of Australia since 1990. The major deposits include the multi-million-ounce Callie deposit (>3 Moz), the Dead Bullock Soak (DBS) deposit (>1 Moz), the Granites deposit (>1 Moz) and the Tanami Mine deposit (Otter) (>1.52 Moz). Other smaller deposits include theTitania deposit (0.42 Moz) and the Ground Rush deposit (0.46 Moz). Evidence shows that all these gold deposits are structurally controlled. The "Tanami Complex", which was used by David Blake (1970) to include all stratigraphic units older than the Mount Winnecke Formation, has been further studied and subdivided by geologists of the historical North Flinders Exploration since 1990. The oldest Paleoproterozoic strata in the region is named the Tanami Group, which is a metamorphosed sedimentary sequence unconformably overlying the Late Archaean gneissic basement ( - 2500 ma). The Tanami Group recorded five major events of ductile deformation (D1 to D5) in addition to a complex preorogenic (or pre-Dl) deformational history during a prolonged depositional period. The first deformation (Dl) was the orogeny of the Tanami Orogen, whereas the D2 to D5 were the influence of later orogenies. Each deformation created different structures, which in places hosted gold mineralisation. The most significant discovery of my research in the Tanami Group was the pre-Dl deformation and the pre-Dl mineralisation. Evidence shows that the main phase of gold mineralisation in The Granites and DBS deposits occurred during the subsiding period of the Tanami Orogen. A prolonged syn-depositional subhorizontal shearing caused bedding parallel schistosity, mineral and stretching lineation, and discrete subhorizontal shear zones, which in places became traps for mineralising fluid and formed strataform ore bodies. Within the subhorizontal shear zones, details of progressive deformational features have also been recognised. The pre-Dl deformation was associated with regional metamorphism, magmatism, hydrothermal alteration and veining. All these bedding parallel structures, mafic sills, veins and ore bodies have SGTSG abstracts, February 2001

been folded by D l and formed a northeast trending regional fold belt. The D l structures also made contribution to gold mineralisation. The Tanami Group is overlain unconformably by the Pargee Sandstone, which is the basal unit of the younger stratigraphic sequence in a cycle 2 orogen. The Pargee Sandstone was intensely deformed by the second regional orogeny (D2) and formed a NW trending fold belt. The tight F1 folds in the Pargee Sandstone (cycle 2-Fl) extended into its basement and became the F2 folds in the Tanami Group (cycle 1F2). The NE trending F1 folds in the Tanami Group (cycle 1-Fl) were refolded by the NW trending F2 folds (cycle 1-F2) during D2. The D2 structures in the Tanami Group also made contribution to gold mineralisation, e.g. the Footwall Lode of The Granites Gold Deposit and the mineralisation of the East Ptylotus Prospect. In the Halls Creek region, cycle 2 Ding Dong Downs Volcanics (-1910 Ma) is overlain unconformably by the cycle 3 Halls Creek Group (18601880 Ma), which was intensely deformed by the Halls Creek Orogeny and formed a NNE trending fold belt. The Tanami Mine Succession (TMSS) in the Tanami region appears younger than the cycle 2 Pargee Sandstone and has been correlated to the cycle 3 Halls Creek Group on the basis of structural evolution. The TMSS was deformed by the third regional orogeny (D3) and formed NE trending folds, which were the F1 folds in the TMSS (cycle 3-Fl), but the time equivalent of F3 folds within the Tanami Group (cycle 1-F3) and F2 folds within the Pargee Sandstone (cycle 2-F2). The gold mineralisation in the Tanami Mine was controlled by the northeast trending fractures, which are interpreted as D3 structures. A leucocratic tonalite dyke, which has been folded, foliated and boudinaged by D3, gives zircon age of -1870 Ma. A syn-D3 tonalite dyke, which was metamorphosed, foliated and boudinaged, and occurred in the axial plane schistosity of cycle 1-F3 fold, gives zircon age of - I 8 6 0 Ma. The Halls Creek Group is unconformably overlain by the Whitewater Volcanics (-1855 Ma), which has been intensely deformed and developed east-west

trending vertical schistosity During D4. In theTanami region, a sequence of acid volcanic and sedimentaryrocks can correlated to the Whitewater Volcanics and informally named the cycle 4 Wilson Creek Group. D4 deformation caused F3 folds in the Pargee Sandstone and F2 folds in the Tanami Mine Succession. D4 deformation also caused east-west trending cycle 1-F4 folds in the Tanami Group, and east-west trending foliation in cycle 4 tonalite, which gives zircon age of -1845 Ma. The cycle 1-F4 folds in the Tanami Group controlled the Callie Deposit and the Titania Deposit The strata of the youngest orogen in the region are informally named the cycle 5 Birthday Creek Group, which is represented by the Mount Winnecke Formation (1830-1810 Ma) and the Supplejack Down Sandstone. Angular unconformity has also been recognised between the Supplejack Downs Sandstone and the cycle 4 Wilson Creek Group in the Tanami region. D5 orogeny caused NNW trending tight cycle 5-Fl folds in the Birthday Creek Group but cycle 1F5 in the Tanami Group. The following table shows relationship between regional events of deformation and fold generations in different cycles. A tectonic model is presented here to interpret the sequential otogenic evolution in the Tanami region. (1) At the beginning of an otogenic cycle, heat induced ascending currents within the mantle asthenosphere were diverged underneath the mantle lithosphere and caused lateral elongation and vertical thinning of the lithosphere. (2) Part of the strain in the lithosphere

would be transmitted to the crust above. (3) A subsiding basin, which hosted sediments from land, would occur above the thinned lithosphere and crust. (4) After a certain period of heat loss, the ascending currents within the asthenosphere weakened or stopped and the strained lithosphere would be restored to its original dimensions. (5) The restoration of lithosphere caused orogeny in the crust. (6). The orogeny induced uplifting and erosion, which at times exposed the metamorphosed root of the fold belt to the surface. The eroded material from the fold belt would have been transported to subsiding basins in neighboring areas. (7) After a certain period, new ascending convection currents caused a new, usually smaller subsiding basin. A new cycle of otogenic evolution began. The strata in the new basin always show angular unconformity relationship with the basement below. (8) When the new basin was ended by an orogeny and formed a new fold belt, the rocks below the unconformity would have recorded one more deformational event than that above the unconformity. (9) After five such otogenic cycles, the Tanami region was generally cratonised with only very gently deformed younger platform cover sequences.

Fold generations in strata of dififerent orogens

Regional events of deformation Tanami

Pargee

Tanami Mine

Wilson Creek

Birthday Creek

Group

Sandstone

S.S.

Group

Cycle 2-F4

Cycle 3-F3 Cycle 3-F2 Cycle 3-Fl

Cycle 4-F2 Cycle 4-Fl

Cycle 5-Fl

Cycle 2-F3 Cycle 2-F2 Cycle 2-Fl

D5 D4

Cycle 1-F5 Cycle 1-F4

D3 D2 D1

Cycle 1-F3 Cycle 1-F2 Cycle 1-Fl

SGTSG abstracts, February 2001

36 Field evidence for frequent Paleoproterozoic orogenies in northern and western Arunta Orogenic Province, central Australia p. Ding Ding Pty Ltd, 41 Nelson Road, Valley View, SA, Australia 5093

The Paleoproterozoic rocks in northern and eastern Arunta were divided by BMR in early 1980s into three stratigraphic Divisions, which are separated by two unconformities. The intense deformation and metamorphism in the Paleoproterozoic rocks were assigned to Strangways Orogeny (-1780-1745 Ma) and Chewings Range Orogeny (--1600 Ma). I proposed Harts Range Group and Harts Range Orogeny in 1983, and provided evidence for the unconformable relationship between the Harts Range Group and the Strangways Orogenic Belt. Up to date, more than ten sequential Paleoproterozoic orogenic cycles can be recognised in central and northern Australia (including Kimberley and Mount Isa regions). Each cycle started with deposition in a regional basin and ended with orogeny. The sequential cycles were separated by angular unconformity. However only the deformation of eight early cycles, and strata of five cycles have been recognised in northern and eastern Arunta. The deformations associated with each of these orogenies have been temporarily labelled as D1 to D8 in a younging order. Different orogenies are of variable intensities, and even the intensity of each orogeny varies from place to place. The Paleoproterozoic orogenies and their associated strata or intrusive rocks in northern and eastern Arunta are presented in Table 1, which also shows correlation with strata in other regions. The cycle 1 stratigraphy is temporarily named the Yundurbulu Range Group (Ay), which comprises mainly granulite facies meta-sediments and mafic rocks with different degree of migmatisation. It exposes in fault blocks over a large area, and is unconformably overlain by cycle 2 and cycle 3 strata in places. The massive -1880 Ma Ngadarunga Granite intruded into the migmatitic gneisses of Ay near Yuendumu. The cycle 2 stratigraphy is newly recognised during this study and named the Woodforde River Group (Aw), which unconformably overlies the Ay in the southeast part of the Reynolds Range, but is unconformably overlain by the cycle 3 strata. The Aw comprises mainly metamorphosed mature sediments (cyclic quartz sandstone, shale and carbonate rocks) with SGTSG abstracts, February 2001

widespread amphibolite facies lithology but localized upper greenschist and lower granulite facies lithlogy. The metamorphic grade of Aw is generally lower than that of the Ay. The Cycle 3 stratigraphy is named the Lander River Group (Al), which comprises mainly a metamorphosed turbidite sequence with few interbeds of quartz-rich meta-sandstone or quartzite, but lack of shale and carbonate rocks. Its basal beds are pebbly sandstone with occasional conglomerate, which is unconformably overlying Aw or Ay in places. This Group shows constant greenschist facies regional metamorphism, which mainly caused recrystallisation in fine-grained sediments and in the matrix of wacke and sandstone. No cycle 4 and cycle 5 strata have been recognised in the area up to date. However there are evidence of deformation and magmatism of the two cycles. The cycle 4 granitoid had been pervasively deformed and metamorphosed by D4 before the intrusion of cycle 5 granitoid (-1820 Ma), which was partially deformed and metamorphosed. The Cycle 6 stratigraphy is the Hatches Creek Group at the northern margion of Arunta and the Coniston Group (Ac) at the Giles and Reynolds Ranges. The Coniston Group comprises mainly greenschist facies metarhyolite and tuffaceous sandstone with a basal subunit of conglomerate and sandstone, which unconformably overlies the cycle 3 Al. This Group has been intruded by micrographic granite (-1785 Ma), which was intensely deformed and retrogressed to greenschist facies schist. No cycle 7 strata have been recognised in the area, but deformation and metamorphosed cycle 7 granitoid (-1765 Ma to -1775 Ma) have well been reported. The cycle 8 stratigraphy includes the amphibolite facies Harts Range Group (Ahr), and the greenschist facies informally named Giles Range Group (Ai). The Harts Range Group unconformably overbed the cycle 7 gneiss (-1765 Ma) but was intruded by -1745 Ma granitoid. A significant angular unconformity separated the Ai from the Ac, Al and cycle 4 metamorphosed granitoid. This Ai is mainly very low-grade meta-sediments. An tectonic modal has been developed to explain the observed cyclic orogenies.

Table 1 Orogenic cycles

Correlation with stratigraphy in other

Stratigraphy in northern and

regions ! eastern Arunta |-|3P[(5 Kange urogeny (Uo) — ~ Cycles Harts Range Group (Ahr) in Harts Range Orogen; Giles 750-71700 Ma Range Group (Pi) in NW I i Arunta Qfpoi j — oiraingways urogeny \ u i ) — ' Cycle 7 780-1750 Ma

Tomkinson Creek Group in Northem Tennant Creek region

No strata but the -1775 Ma to

-1765 Ma granitoid or 1 gneisses udvenpuri vjruyeiiy ^uoj ~ —

Previously published siratigraphic names Harts Range Group & part of Reynolds Range Group in NW Arunta

Tomkinson Creek beds

Cycles

Hatches Creek Group (Ah)

Hatches Creek Group in Davenport

Hatches Creek Group &

-1810-1780 Ma

and Coniston Group (Ac)

Province \ange urogeny (uoj —

Coniston Schists

No strata but the -1820 Ma

Birthday Creek Group inTanami region

Mount Winnecke Formation and Supple Jack Downs Sandstone

Cycle 5 -1830-1810 Ma

i granitoid or gneisses

Cycle 4

1 No strata but sheared cycle 4 granitoid in 04 shear zones or gneisses

1 Ware Ranae Oroaenv Wilson Creek Group in Tanami region; Whitewater Volcanics and Panton River Formation in Kimberley region

Nanny Goat Creek beds; White Water Volcanics and part of the mapped Olympic Formation

Barramundi Orogeny (03) - Lander River Group (Al) in Lander River Orogen

Wan-amunga Group in Tennant Creek region;

Low grade Lander Rock beds

Revnolds Ranae Oroaenv ( 0 2 ) —

Halls Creek Group in Kinrterley region; Mount Partridge Group to Finniss River Group in Pine Creek region. ~

-1855-1830 Ma

1 Cycle 3 i -1900-1855 ma

—

: Cycle 2 -72000-1900 Ma

! Woodforde River Group (Aw) ^ in Napperby Orogen

Pargee Sandstone inTanami region; Ding Dong Downs Volcanics in Kimberiey region

Wickstead Creek beds & part of Reynolds Range Group

Yuendumu Oroaenv (01) Cycle 1 >72000 Ma

Tanami Group inTanami region; Sophie Yundurbulu Range Group (Ay) Downs Group in Kimberley region i in Arunta Orogen Angular unconformity

Mount Stafford beds and Weldon Metamorphics

-2500 Ma Archaean basement

SGTSG abstracts, February 2001

38 A tectonic odyssey — the hunt for Australia's Rodinian boundary, and significance of the 'Tasman Line" N.G. Direen^'2 ^^^ A.J. Crawford^ ^ Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79 Hobart, Tasmania, Australia 7001 2 Australian Geological Survey Organisation, GPO Box 378 Canberra ,ACT, Australia 2601

In Australia, recent debate has centred on timing of Rodinia breakup, with evidence indicating breakup between 777 Ma and 755 M a (Wingate, 1998). Breakup is beUeved to have been preceded by basaltic volcanism recorded in the Curnamona Province (Little Broken Hill Gabbro), Gawler Craton (Gairdner Dyke Swarm) and Adelaide "Geosyncline" (Boucaut Volcanics). The timing for these events is pulsed at C.830 Ma and 780 Ma (Wingate, 1998). Near-coincident gravity and magnetic anomalies apparent in the Australian national gravity and magnetic datasets have been interpreted as the signature of the Rodinian rifted margin (Gunn, 1996; Powell, 1996), based on consideration of image data alone. These features include an apparently rectilinear "fault system" truncating the Mt Isa block in Queensland, and sigmoid anomalies in western NSW and western Victoria. This system of anomalies has been linked with the "Tasman Line" - a contentious feature with a long and chequered history of interpretation. Three schools of thought regarding the "meaning" of the Tasman Line can be identified in the literature: a depositional boundary, related to rift basin formation (e.g. Hill, 1951; Gunn, 1996); a deformation boundary related to an otogenic event (e.g. Scheibner, 1992; Murray et al., 1989); or part of a transitional zone between overprinted fold-belts of different ages (Zhou et al. 1990; Mills, 1992). The last definition demands re-evaluation of geophysical anomalies in the area encompassed by the "Tasman Line", due to the possible complexities involved. We review new evidence acquired in SE Australia to determine the age and character of anomaly sources, and find that the anomalies are caused by diverse packages of rocks. NSW: Bancannia Trough and Koonenberry Belt Direen (1998) has shown that the Bancannia magnetic anomalies are most likely caused by Late Neoproterozoic Mt Arrowsmith Volcanics (MAV) — transitional tholeiitic to alkaline lavas, feeder dykes and plugs, that range in composition from wehrlite to rhyolite. The volcanics are interbedded with shales and limestones, and contain pillow lavas, indicating eruption in a submarine setting. The MAV crop out SGTSG abstracts, February 2001

55 km northwest of the Bancannia Trough as a vertically dipping massif c. 4.5 km thick. They have high magnetic susceptibilities due to primary magnetite. These susceptibilities are increased markedly where olivine-rich variants have been serpentinised along faults. The gravity low over the Bancannia Trough is attributable to a combination of c. 300 m of low density (2.00 t/m^) semi-consolidated lake sediments overlying a c. 4000 m package of Devonian red beds with densities c. 2.22 t/rnP (Direen 1998). Magnetic anomalies immediately east of the Bancannia Trough occur over east-dipping listric slices of MAV (Direen, 1998) e.g. Conns Creek, where there is serpentinised wehrlite at the base of a deformed slice of MAV that has been transported up the Conns Creek Thrust. The gravity high over the Koonenberry Belt is incompletely explained by the thickness of deformed packages of outcropping Late Neoproterozoic and Cambrian metasediments and volcanics (density 2.70 t/m^). An underlying foundered block of Adelaidean metasediments, also with densities around 2.70 t/m^, is consistent with model solutions. Magnetic anomalies in the eastern Koonenberry have three distinct sources: • magnetic phyllite within the ?Early Cambrian Teltawongee beds turbidites; • thin, post-Early Cambrian and pre-Mindyallan (Late Cambrian) dolerite sills and dykes, deformed with the Teltawongee beds; • magnetic phyllite and tholeiitic metabasalt within multiply-deformed, steeply east-dipping thrust slices of the Late Cambrian Ponto Complex. SA: western Murray Basin Drillcore (Rankin et al., 1991) indicated that these anomalies are sourced by greenschist and amphibolite grade metadolerites, and undeformed granitoids. These intrude deformed metasediments of the Kanmantoo Group. Geochemistry suggests that the mafic dykes are tholeiitic, with primitive MORB characteristics. The undeformed (post-tectonic) magnetic granites are I-type, and are unusual, as previous work in the Adelaide Fold Belt has shown I-type granites to be almost exclusively syn-tectonic; conversely, other post-

tectonic granitoids in eastern South Australia are Atype (Foden et al., 1990). In any case, these granites are much younger (?Early Ordovician) than the proposed Rodinian break-up. Victoria: Dimbooia and Ozenkadnook Subzones Drillcore (Maher et al., 1997; Direen, 1999), suggests two main sources for high amplitude magnetic anomalies in western Victoria. The oldest are serpentinised alkaline and picritic volcanics of inferred Late Neoproterozoic age. These have striking similarity to C.600 Ma mafic volcanics of King Island and northwestern Tasmania. Both Victorian and Tasmanian rocks have within-plate characteristics, similar to the MAV, and all three groups include rare, highly refractory picrite lavas. A thick wedge of this material may also underlie the Dimbooia Subzone at depth. The second possible sources of anomalies are serpentinised boninite slices in drillcore from both the eastern and western margins of the Dimbooia Subzone. These highly unusual rocks range in composition from ultramafic to andesitic, and have strong affinities to boninites in central Victoria and western Tasmania. Modelling presented in Direen (1999) suggests that further volumes of these rocks may also exist as a thin slice under the Dimbooia Subzone. Weak magnetic anomalies in this region are due to slices of Middle-Late Cambrian calc-alkaline volcanics exposed within the Black Range, Mt Stavely and Mt Dryden Belts. Rocks of this type have also been intersected in drillholes described by Maher et al., (1997) and Direen (1999). These rocks have medium to high-K geochemical characteristics, and strong similarities in age, geochemistry and tectonic setting to the Mt Read Volcanics of western Tasmania. Synthesis Geophysical anomalies and lineaments thought to define a "Tasman Line" of specific age and tectonic origin can be shown to be related to diverse rocks that vary in age, protolith, setting of eruption and/or deposition, degree of metamorphism, style and intensity of deformation. There is thus no basis for the interpretation of a coherent, supracrustal Tasman Line, or Rodinian "margin" from geophysical imagery. Instead, there are at least five distinct tectonic events represented by: • a widespread episode of Late Neoproterozoic rifi:ing, which produced magnetic packages of unusual alkaline and picritic rocks, and voluminous tholeiites;

• establishment of an Early Cambrian turbiditedominated passive margin, which underwent a second episode of rifting forming a rifiied marginal basin; • Mid-Late Cambrian collision of the rifi:ed passive margin with a boninitic forearc that had formed in the Middle to Late Cambrian; • Collapse of the collisional collage, followed by MidLate Cambrian calk-alkaline volcanism; • Intrusion of a series of Cambro-Ordovician synorogenic tholeiites and post-orogenic granitoids. All the rocks created by these events were extensively modified by thrusting, and by further magmatism, during the Silurian, Devonian and Carboniferous; later effects were not limited to, or bounded by, the earlier features in any observable way. It is thus overly simplistic to represent these geophysical anomalies as representing a fossil rife, or fold belt boundary, without extensive qualification.

References Direen, N.G., 1 9 9 8 . The Palaeozoic Koonenberry Fold and Thrust Belt, far western New South Wales: a case study in applied gravity and magnetic modelling. Exploration Geophysics 2 9 , 3 3 0 - 3 3 9 . Direen, N.G., 1 9 9 9 . Geology and geophysics of the Koonenberry Belt, far western New South Wales, and eastern Australian correlates. PhD thesis, University of Tasmania (Unpubl.). Foden, J.D., Turner, S.P. & Morrison, R.S., 1990. Tectonic implications of Delamerian magmatism in South Australia and western Victoria. In J A G O J.B. & M O O R E R S. eds. The evolution of a late Precambrian-early Palaeozoic rift complex: the Adelaide geosyncline. Geological Society of Australia Special Publication 16, 4 6 5 - 4 8 2 . Gunn, P. J., 1996. The location and nature of the Proterozoic/Palaeozoic boundary in eastern Australia. Geological Society of Australia Abstracts 4 1 , 173. Hill, D., 1 9 5 1 . Geology. In Mack, G. ed.. Handbook of Queensland: 13-24. Maher, S., Moore, D. H., Crawford, A. J., Twyford, R. & Fanning, C.M., 1997. Test drilling of the southern margin of the Murray Basin. Victorian Initiative for Minerals and Petroleum Report 52. Mills, K.J., 1 9 9 2 . Geological evolution of the Wonominta Block. Tectonophysics 2 1 4 : 57-68. Murray, C.G., Scheibner, E. & Walker, R.N. 1 9 8 9 . Regional geological interpretation of a digital coloured residual Bouguer gravity image of eastern Australia with a wavelength cut-ofF of 2 5 0 km. Australian Journal of Earth Sciences 36: 4 2 3 - 4 4 9 . Powell, C.McA., 1 9 9 6 . Breakup and dispersal of the Rodinia supercontinent: implications for resource exploration. Geological Society of Australia Abstracts 4 1 : 3 5 1 . Rankin, L.R., Clough, B.J., Farrand, M. G., Barnett, S., Ublack, K., Gatehouse, C . G . & Hough, L P , 1 9 9 1 . Murray Basin basement transect project: 1 9 9 0 well completion reports. Dept of Mines and Energy, South Australia Unpublished Report Book 91/15. Scheibner, E., 1 9 9 2 . Influence of detachment-related passive margin geometry on subsequent active margin dynamics: applied to the Tasman Fold Belt System. Tectonophysics 2 1 4 : 4 0 1 - 4 1 6 . Wingate, M.T.D., 1 9 9 8 . Isotopic and paleomagnetic constraints on the timing of Neoproterozoic breakup of Rodinian Australia. Geological Society of Australia Abstracts 50: 7 0 - 7 1 . Zhou, B., Mills, K.J. & Liu, S.E, 1 9 9 0 . The boundary between the Tasman Fold Belts and the Australian Craton: a reappraisal from studies of mafic rocks. Geological Society of Australia Abstracts 26: 7.

SGTSG abstracts, February 2001

40 Structure and tectonic evolution of Southern Altay, Xinjiang and its relationship with gold deposit metallogensis Y. Dong\ C. Zhang^ ^gnj K. Guo^ 1. Nanjing Institute of Geology and Mineral Resources, Nanjing, PRC 210016 2. Guiyang Geochemistry Institute, CNNC, Guizhou, Guiyang, PRC 550002

In this paper, a three-layer structural model of the crust in Southern Altay, Xinjiang, is proposed based on the study of the crustal formation, deformation characteristics, etc. The three layers include a Precambrian base structural layer, an up-layering sedimentary structural layer and a Mesozoic-Neozoic post-orogeny molass, foreland sedimentary and regenerated foreland sedimentary structural layer. Such crust structure recorded the different tectonic system in different stages. The authors suggest that although growth occurred between 1000 Ma and 300 Ma, gold deposit metallogenesis was intimately related to the postorogenic collapse and crustal growth at around 300 Ma. The extensional collapse generated an abnormal geothermal gradient, which drove the orebearing fluid to favourable tectonic zones, where gold was deposited. Post-orogeny crust-mantle evolution plays an important role in gold metallogenesis.

SGTSG abstracts, February 2001

Global sea level prediction for the past 300 Myr using PlatyPlus technology

C. Duboz\ S. Pisarevsky^, C.McA. PowelP and G. Lister^ ' Australian Crustal Research Centre, Monash University, Wellington Road, Clayton, Victoria, Australia 3800 2 Tectonics Special Research Centre, The University of Western Australia, Nedlands, WA, Australia 6907

The ability to predict sea level variation is very important to the petroleum exploration industry, as hydrocarbon production and maturation depends on the formation of relatively shallow basins with anoxic conditions. It is also potentially important to the minerals industry for the prediction of eustatic controls on metallifeorus basins in the Palaeozoic and Precambrian. There is considerable knowledge of sealevel changes from -200 Ma until the Present, based on preserved passive margin sequences and seafloor magnetic anomalies. However there is relatively little information available before the Triasic. This project aims to develop global reconstructions using plate tectonic principles to predict quantitatively first-order features of global sea-level variation for the past 300 Myr, and to compare these predictions with published global sea-level curves (e.g. Vail et al., 1977, 1984; Haq et al., 1984). A control experiment can be run for the last 200 Myr. The PlatyPlus software can then be used to extend the quantitative prediction of global sea-level variation back to 300 Ma. The main plate-tectonic factor that affects sea level is the average age of the seafloor over the whole Earth, weighted for area. The average age of the oceanfloor depends on the rate of creation of new oceanic crust at the mid-ocean ridge and the age of the oceanfloor being subducted. Young oceanic lithosphere is more buoyant and has relative shallow ocean above it while older oceanic lithosphere is colder and denser with relatively deep ocean above it. Important secondary features are changes in the area of the continental crust, which can increase during continental extension and decrease during crustal shortening. Continental glaciation also modulates sealevel on a shorter-term duration. We need to model the effects of these factors in a global reconstruction framework to constrain the variation of sea level through time.

in the reconstruction. These points can carry information such as elevation, lithospheric thickness, and crustal age at each reconstruction step using agedependent ftinctions. By keeping track of the position of the clouds of points on each plate, we can calculate as a first approximation sea level evolution through time. Initially, we assume that there is no change in area of continental bocks or in the total global volume of water, and that the variation of sea level depends only on ocean spreading. Secondary factors are to be added later, ^ ^ f ^

The PlatyPlus software enables production of global reconstructions where each data object of the reconstruction can carry special information. One of the strengths of the PlatyPlus software is in the use of clouds of points associated with each entity involved

SGTSG abstracts, February 2001

42 ODP Leg 190 Nankai Trough - stratigraphic/structural results and implications for the tectonostratigraphic development of accretionary prisms C.L. Fergusson^ and ODP Leg 190 Shipboard Scientific Party^ ^ School of Geosciences, University of Wollongong, NSW, Australia 2522 2 See below

During Leg 190 of the Ocean Drilling Program we drilled two reference sites in the Nankai Trough and drilled four sites into lower trench-slope sediments and the underlying Nankai Trough Accretionary Prism. The Nankai Trough occurs in the northwestern Pacific Ocean adjacent to southwest Japan and is a shallow trench formed at a subduction zone between overriding Eurasia and the underthrust Philippine Sea Plate. The Nankai Trough Accretionary Prism is considered to be one of the "type" examples of a sand-rich accretionary complex. The main objectives of Leg 190 were to provide constraints on fluid flow, mechanical properties, deformational styles and to test models of accretionary prism evolution. Leg 190 sites in combination with earlier deep-sea drilling sites have contributed to the development of two transects, 125 km apart, across the Nankai Trough Accretionary Prism. The Ashizuri Transect occurs in the east (-150 km southeast of Cape Ashizuri in southwestern Shikoku, Sites 583, 582, 298, 1177) whereas the Muroto Transect occurs in the west (-100 km southeast of Cape Muroto in southeastern Shikoku, Sites 808, 1173, 1174, 1175, 1176, 1178). The incoming stratigraphy consists of basement and overlying Shikoku Basin deposits of the Philippine Sea Plate and trench deposits of the Nankai Trough. Basement to the Philippine Sea Plate in this region consists of basalt of probable early Miocene age (-23 Ma) in the Ashizuri Transect and probable mid Miocene age (-15 Ma) in the Muroto Transect. At Site 1177 (Ashizuri Transect) the basement is overlain by an early Miocene volcaniclastic unit with hemipelagic mudstone, silicic volcanic ash layers and fme-grained siliciclastic beds. Above this unit is the Shikoku Basin which in the Ashizuri Transect consists of a lower unit with 327 m of early to late Miocene turbidites. Sands within this unit are siliciclastic and dominated by quartz, feldspar and rock fragments with some beds rich in wood and plant material. In the Muroto Transect the basalt basement is overlain by a mid Miocene volcaniclastic unit that contains thick silicic volcanic ash layers interbedded with variegated SGTSG abstracts, February 2001

mudstone in Site 808 but had only low recovery at Site 1173 and is not recognised in the Ashizuri Transect. Above the volcaniclastic unit in the Muroto Transect is the Miocene to Pleistocene Shikoku Basin dominated by hemipelagic mudstone with glassy volcanic ash layers preserved in the upper part whereas in the lower part these layers have been diagenetically altered. The Miocene turbidite unit is not present. In the Ashizuri Transect the Pliocene-Pleistocene part of the Shikoku Basin is similar to that from the Muroto Transect. Thick Pleistocene trench deposits occur along both transects and consist of an upward thickening and coarsening succession with sands derived from the rapidly uplifting volcanic and metasedimentary rocks in the Honshu-Izu collision zone of central Japan. Three slope sites (1175, 1176, 1178) have been drilled in the Muroto Transect and are amongst the relatively few deep-sea drilling sites anywhere that have penetrated slope sediments into underlying accretionary prism. At Sites 1175 and 1176 we cored through a trench-slope basin with an upper unit of -200 m of Pleistocene-Holocene hemipelagic mud, volcanic ash layers and silt-to-sand turbidites. Folds and chaotic bedding are distributed throughout and indicate that the unit contains many slump accumulations. The underlying units consist of PliocenePleistocene gravels, sands, muds, volcanic ashes that include the transition from the trench-slope basin to the underlying accretionary prism. The young age of the slope deposits (< 1-1.5 Ma) indicates that the underlying accretionary prism has formed rapidly with at least 40-km growth of the prism during the Pleistocene. This was a totally unexpected outcome and requires rethinking about our perceptions of the growth rates of accretionary prisms. Site 1178 is the highest site on the lower trench slope with a PliocenePleistocene unit with -200 m of hemi-pelagic mud and minor sand, gravel and ash. This unit had a considerably lower sedimentation rate than the upper unit at Sites 1175 and 1176 and an exceptionally thin Pleistocene section indicating that part of the section has been removed by slumping. Contorted

stratification occurred and is consistent with gravitational down-slope movement of unlithified sediment. The lower unit consists of Miocene to Pliocene hemipelagic mudstone, silt and sand turbidites that have formed in the trench (outer trenchwedge and axial trench wedge facies) and are now part of the accretionary prism. Duplication of axial trench wedge facies indicates that thrusts are present. Thrust faults had previously been intersected near the toe of the accretionary prism at Sites 298 and 808. The basal decollement had been intersected at Site 808. On Leg 190 we cored the basal decollement at Site 1174 where it consisted of 33 m of mainly finely comminuted fault breccia in amongst some more coherent material. This is some 13-m thicker than that found at Site 808 despite Site 1174 occurring 3 km southeast of Site 808. Additionally deformation bands occurred and some fault zones were intersected but we failed to find a proto-thrust predicted from the seismic section although it may well occur higher in the section. The decollement is stratigraphically controlled and occurs within a 5-7 Ma horizon with distinctive magnetic susceptibility within the late Miocene part of the Shikoku Basin in both transects in spite of the stratigraphic differences between them. Coring at the sites on the lower trench slope have intersected accretionary prism sediments at depths of > 200 mbsf At Sites 1175 and 1176 few structures have been identified in the accretionary prism sediments although this in part reflects lower core recovery in the deeper parts of these holes. Core at Site 1178 in the interval 400-506 mbsf has beddingparallel fissility, foliation oblique to bedding, bedding dips up to 55° and fracture sets. A zone with shearing (scaly surfaces with downdip slickenlines) probably marks a prism thrust fault and other zones with increased deformation occur deeper in the hole and probably indicate the presence of minor thrust faults. The regional differences between the Muroto and Ashizuri Transects probably reflect the nature of the incoming Philippine Sea Plate stratigraphy and underthrusting of the Kinian Seamounts beneath the Muroto Transect. In the Ashizuri Transect the prism has a steeper wedge taper compared to that of the Muroto Transect and we felt that this is at least partly related to the presence of the Miocene turbidite succession in the former. Some mudstones in and above the Miocene turbidite unit of the Ashizuri Transect have higher smectite contents than Shikoku Basin mudstones at an equivalent stratigraphic level in the Muroto Transect.

In general, the main implications of Leg 190 for accretionary prism growth are the stratigraphic control over the location of the decollement during accretion and the previously unanticipated rapid rates of prism growth, . y ^ j ^ f ^

2 Details of Leg 190 scientific results and Leg 190 Shipboard Scientific Party are available on the Ocean Drilling Program/ TAMU Website {http://www-odp.tamu.edu/ click on Publications, Leg Summaries).

SGTSG abstracts, February 2001

44 Proterozoic accretion and reactivation along the Darling Mobile Belt of Western Australia I.C.W. Fitzsimons, M.M. Cobb, P.A. Cawood and P.D. Kinny Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA, Australia 6845

The north-south trending DarUng Mobile Belt is located at the western edge of the Australian continent, where it truncates the Yilgarn Craton (3.7--2.6 Ga), Capricorn Orogen (1.9-1.6 Ga) and Albany-Fraser Orogen (1.4-1.1 Ga). Basement rocks within the mobile belt are exposed as three gneissic inliers (the Northampton, Leeuwin and Mullingarra Complexes) draped by Phanerozoic sedimentary rocks of the Perth and Carnarvon Basins. These inliers have been collectively called the Pinjarra Orogen, but preliminary age data indicate that they preserve evidence for at least two or three distinct periods of tectonism and here we use the term Darling Mobile Belt to allow for the possibility of multiple orogenic cycles. The Northampton and Mullingarra Complexes comprise metasedimentary gneiss and granitic intrusive rocks. Metamorphic zircon in gneissic samples from both complexes yield SHRIMP U-Pb ages that are within error of one another (1079±3 and 1058±83 Ma) and detrital zircons in psammitic gneisses have indistinguishable age distributions with major peaks at 1.9-1.6 and 1.4-1.1 Ga with variable evidence for minor 2.2 Ga and Archaean components (Bruguier et al., 1999; Cobb et al., 2001). A moderately-deformed granite pluton in the Northampton Complex has an emplacement age of 1068±13 Ma (Bruguier et al., 1999) and was intrusive into the Complex, as was an undeformed pegmatite emplaced at 989±2 Ma, whereas an u n d e f o r m e d granite body in the Mullingarra Complex crystallized at 2181±10 Ma (Cobb et al., 2001) and has a faulted contact with overlying metasedimentary rocks. The ages of structures that post-date regional metamorphism are less well constrained. Dolerite dykes in the Northampton Complex have yielded K-Ar ages of 750 and 550 Ma, with the older age taken as the time of dyke emplacement and the younger age correlated with greenschist- to amphibolite-facies shear zones that locally offset the dykes. This is consistent with the work of Wingate & Giddings (2000) who showed that the Northampton dykes have a palaeomagnetic pole indistinguishable from the 755 Ma Mundine Well dyke swarm of the Pilbara Craton, suggesting that the SGTSG abstracts, February 2001

Northampton C o m p l e x (and presumably the Mullingarra Complex) were more-or-less in their present position adjacent to the Yilgarn Craton by 750 Ma. The Leeuwin Complex comprises felsic orthogneiss, ranging from granodiorite to alkali granite in composition, interleaved with anorthosite, leucogabbro, and subordinate mafic lenses which are probably remnants of mafic dykes. Recent SHRIMP U - P b zircon ages (Nelson 1999) have revealed orthogneiss protoliths with emplacement ages of 1091±8 and 1091±17 Ma and xenocrystic zircons with ages of 1200-1150 Ma. Previous geochronology had identified younger orthogneiss protoliths with ages of 780-680 and 570-530 Ma and a 525 Ma post-tectonic felsic dyke (Nelson, 1996). Although the Leeuwin Complex is lithologically distinct from the Northampton and Mullingarra Complexes, all three preserve evidence of tectonism at 1100-1000 Ma, and we believe that this correlates with limited evidence for tectonism at 1040±50 Ma in the Obruchev Hills of East Antarctica (Sheraton et al., 1995) which would have been immediately alongstrike from the Darling Mobile Belt before Gondwana break-up. This tectonism is notably younger than the last recorded tectonism in the Albany-Fraser Orogen (1130 Ma) and the crustal blocks preserving this younger event are aligned in a distribution that truncates the western margin of the Albany-Fraser Orogen. This distribution could either reflect in situ tectonism at 1100-1000 Ma along the CapricornYilgarn—Albany-Fraser margin, in which case the truncation of this margin was achieved in the short time interval between 1130 and 1100 Ma, or transport of these blocks along the margin some time after they were deformed and metamorphosed. The available isotopic data support the latter model. Rb-Sr biotite ages decrease towards the northwestern margin of Yilgarn Craton, but they are invariably Palaeoproterozoic (Libby et al., 1999) with no evidence of the pervasive 1.1-1.0 Ga high-grade tectonism that characterizes the Northampton and Mullingarra Complexes. Given that the Mullingarra

Complex is only 15 km from the craton margin, it seems highly unlikely that it was metamorphosed in its present position (Cobb et al., 2001). Further evidence for an allochthonous origin comes from detrital zircon grains in the metasedimentary gneisses of the Northampton and Mullingarra Complexes. The dominant 1.9-1.6 and 1.4-1.1 Ga detrital populations closely match the age of basement rocks in the Capricorn and Albany-Fraser Orogens respectively, but it is difficult to account for the absence of a significant Archaean zircon population from the Yilgarn Craton if the sedimentary protoliths were deposited in their present location. At first sight, detrital populations in the metasediments may seem contradictory, with potential source terrains both to the north and south of their current position, but a review of isotopic age data reveals that all the necessary age components are exposed in the Albany-Fraser Orogen and the correlative Wilkes Province of East Antarctica (e.g. Nelson etal., 1995; Sheraton et al., 1995; Clark etal., 2000). Tectonism in the Albany-Fraser-Wilkes Orogen occurred in two pulses at 1350-1250 and 1200-1130 Ma, reworking older protoliths dated at 3.1, 2.7-2.6, and 1.7-1.6 Ga. Additional protoUth ages of 2.2-2.1 and 1.8 Ga are suggested by detrital grains in metasediments from the Albany-Fraser Orogen. We conclude that sedimentary protoliths in the Northampton and Mullingarra Complexes were eroded from source terranes within the Albany-FraserWilkes Orogen sometime between 1113±26 Ma (the youngest detrital grain) and 1079±3 Ma (the age of metamorphism in the Northampton Complex). We assume that the depositional basin lay to the south of the Albany-Fraser Orogen, such that detritus was dominated by reworked material of Antarctic afiPmity rather than material derived from the Yilgarn Craton. These sedimentary rocks were then deformed and metamorphosed to high grade within 50 Myr of their deposition. The Albany-Fraser Orogen, Yilgarn Craton and Capricorn Orogen were truncated along their present-day western margin sometime after the highgrade tectonism in the Northampton and Mullingarra Complexes, and this same truncation event may have been responsible for translation of these complexes northwards where they were accreted onto the northwest Yilgarn Craton. The Northampton palaeomagnetic data require that this translation was complete by 750 Ma. This model requires some 750-1000 km of early Neoproterozoic dextral movement along the western margin of the Yilgarn Craton. This displacement of allochthonous blocks would pre-date the late

Neoproterozoic ductile shearing described by Harris (1994), which was associated with the 570-530 Ma plutonism in the Leeuwin Complex and a widespread 500-400 Ma resetting of Rb-Sr biotite ages in the southwest Yilgarn Craton and western Albany-Fraser Orogen (Libby and De Laeter 1998). Further north, this later phase of ductile deformation was focused to the west of the Northampton and Mullingarra Complexes, which remained attached to the Yilgarn Craton. This complex history of deformation, accretion and reactivation supports the suggestion by Fitzsimons (2000) that the Darling Mobile Belt is part of a significant Neoproterozoic tectonic boundary. Unfortunately this orogen is very poorly exposed and consequently poorly understood. The western half in Greater India has been subducted beneath the Himalaya and the southern extension is largely buried beneath the Antarctic ice sheet, leaving the three West Australian inliers as the most accessible outcrop. Further detailed study of these outcrops is needed to develop the models discussed here, and constrain the regional setting of repeated tectonism along the Darling Mobile Belt, ^ ^ f f ^

References

Bruguicr, O., Bosch, D., Pidgcon, R.T., Byrne, D.R. & Harris, L.B., 1999. Contrib. Miner. Petrol. 136: 2 5 8 - 7 2 . Clark, D.J., Hensen, B.J. & Kinny, P.D., 2000. Prccambrian Res. 102: 155-83. Cobb, M.M., Cawood, PA., Kinny, RD. & Fitzsimons, I.C.W., 2001. This volume. Fitzsimons, I.C.W., 2000. Geology 28: 8 7 9 - 8 2 . Harris, L.B., 1994. J. Geol. Soc. Lond. 151: 9 0 1 - 4 . Libby, W.G. & De Laeter, J.R., 1998. Austral. J. Earth Sci. 45, 6 2 3 - 3 2 . Libby, W.G., De Laeter, J.R. & Armstrong, R A . , 1999. Austral. J. Earth Sci. 46: 8 5 1 - 6 0 . Nelson D.R., 1996. Western Austral. Geol. Surv. Record 1996/5. Nelson D.R., 1999. Western Austral. Geol. Surv. Record 1999/2. Nelson, D.R., Myers, J.S. & Nutman, A.R, 1995. Austral. J. Earth Sci. 42: 4 8 1 - 4 9 5 . Sheraton, J.W., Tingey, R.J., Oliver, R.L. & Black, L.R, 1995. Austral. Geol. Surv. Org. Bull. 244. Wingate, M.T.D. & Giddings, J.W., 2000. Prccambrian Res. 100: 3 3 5 57.

SGTSG abstracts, February 2001

46 The method of asymptotes and limits applied to the interpretation of apparent age spectra produced in "^^ArP^Ar step heating experiments M.A. Forster and G.S. Lister Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne, Australia 3168

The Argon Partial Retention Zone is that region of the Earths crust where minerals retain most of their radiogenic argon, but not all. Temperatures are sufficient to facilitate recrystallisation, but the bulk of the original grain may still be preserved. Similarly, temperatures are sufficient to allow partial gas loss, but not sufficient to completely reset the argon "clock". Heterogeneity in an apparent age spectrum will commonly result if a sample is deformed and recrystallized in the Argon Partial Retention Zone (Baldwin & Lister 1996). We present data to illustrate typical spectra from the Argon Partial Retention Zone. There are no welldefined "plateau ages" in many of the age spectra illustrated, and there is considerable scatter between individual measurements in the step-heating sequence. Such data might well be discarded, except that several samples have been measured, and many individual steps across the total aggregate of samples have similar ages. What is interesting is that these frequently measured ages can be predicted by defining asymptotes and limits to individual sequences and trends in the apparent age spectrum. This paper is the result of further investigation of this apparent coincidence, and the realisation that it was possible to provide the basis of a new method for the systematic interpretation of complex ^^Ai/^^Ai apparent age spectra obtained from separates of minerals in metamorphic tectonites. Traditional methods for the interpretation of apparent age spectra sit uncomfortably with theoretical predictions when rocks are deformed and/or metamorphosed in the Argon Partial Retention Zone. The concept of closure temperature has limited validity under these circumstances, and the recognition (and the interpretation) of plateaux in the apparent age spectra is often uncertain. The new method is based on the theory of mixing gas from different reservoirs. It relies on the definition of asymptotes and limits in step-heating experiments with many increments of gas release. The method is particularly applicable to the analysis of complex apparent age spectra produced from minerals from metamorphic tectonites. Such rocks can be subject to several episodes of deformation and metamorphism, and may contain several microstructural reservoirs each with distinct diffusion characteristics in respect to radiogenic argon. SGTSG abstracts, February 2001

Frequently measured ages The phenomenon of Frequently Measured Ages (FMAs) defined by the limits and asymptotes to age sequences in complex ^Âr/^Âr apparent age spectra is not uncommon (for example in apparent age spectra measured from the Lachlan Fold Belt, in Eastern Australia, Foster et al. 1998). These authors infer the presence of plateaux, and have taken averages of the apparent age over arbitrary gas release intervals to obtain a 'meaningful' age. Such methods of interpreting ^Âi/^Âr apparent age spectra are not entirely satisfactory since the definition of plateaux can be unconvincing. In some cases the interval over which apparent ages were averaged is part of a trend with gradually increasing apparent ages, for example. It is difficult to justify selection of an arbitrary segment of an essentially smooth trend to average the apparent age, and to thus imply the existence of plateaux in the apparent age spectrum. In the other cases the average is taken over three steps in a "saddle". There is no theoretical basis for the ages thereby obtained, and other information potentially of value in the apparent age spectrum is ignored. An alternative way to obtain meaningful' ages is to determine the asymptotes and limits for the same dataset, and to determine the significance of the FMAs thereby obtained. It is curious that the ages thereby obtained are remarkably consistent across the spectra examined. The sample size has to be large enough to determine whether or not the identified asymptotes cluster around particular FMAs to any degree of statistical significance. The point of the example is to illustrate that the phenomenon exists, and that the method of asymptotes and limits is capable of being applied in a general way to aid in the interpretation of apparent age spectra.

Mixing gas from different reservoirs If a rock is deformed and metamorphosed in the Argon Partial Retention Zone (Baldwin & Lister, 1996) the apparent age spectra it produces during a step-heating experiment can be interpreted in terms of the effects of partial argon loss from particular gas reservoirs as the result of specific thermal and/or deformation events during its geological history. Where more than one gas reservoir can be identified, the apparent age spectra can be interpreted in terms of mixing of gas released

progressively from the dififerent reservoirs during the course of a step-heating experiment. During a step-heating experiment gas can be released sequentially from different reservoirs as the result of several different processes, including solidstate diffusion, dehydroxylation, or simple degassing from low-retentivity adsorption sites. This release process is capable of providing useful information in respect to geochronology, as long as the age structure of the dififerent reservoirs can be constrained in some way. For example there may be two or more generations of white mica that have grown during different metamorphic and/or deformational episodes in a micaceous schist or a fine-grained slate. During a stepheating experiment gas may be released progressively from both reservoirs, or episodically. One reservoir may release gas before the other, or it may dominate the early stages of gas release. We will illustrates dififerent mixing models for a material with two distinct gas reservoirs. The ideal case in which a retentive reservoir releases its gas after a much less retentive reservoir has degassed is first shown. This may be contrasted with progressive release from both reservoirs, initially as the result of a transition as the step-heating experiment reaches intermediate temperatures. Geochronologically useful information is still available. However if gas is released as the result of a smooth transition considerably less information is provided. It need not be assumed that gas released in the stepheating experiment is the result of solid-state dififusion. For example dehydroxylation may result in complete mixing, or episodic sampling from one reservoir or the other. Another complication is provided if there is early release of gas from the most retentive reservoir prior to degassing of less retentive domains. This might be the result if one reservoir releases gas as the result of solid-state diffusion, while the other reservoir is tapped as the result of breakdown of the mineral structure during dehydroxylation. Only in the case of complete mixing is a result obtained however that precludes gaining of useful information from the stepheating experiment. Three types of asymptotes can be used to estimate the "age" of the gas in the individual reservoirs. The upward bounding asymptote is often recognized in data from K-feldspar, where the release of gas from the least retentive domain often has a saddle-shape due to the interference of excess argon. The downwardbounding asymptote is often recognized in apparent age spectra obtained from phengitic white-mica, where it provides a useful minimum constraint on the minimum "age" of the reservoir. Intermediate changes of slope are more difficult to use, since they may obtain contributions from multiple reservoirs.

Apparent age spectra from convergent settings The best way to illustrate practical use of the method of asymptotes and limits is to provide some detailed examples of its potential use. The example provided comes apparent age spectra measured by M.A. Forster in the course of her PhD thesis research, in the Otago Schist, and in the Aegean. The FMAs thereby determined appear to have geological significance. The question is whether we can provide alternative confirmation that these ages are significant, and therefore demonstrate our hypothesis.

Conclusion There is no doubt that the method of asymptotes and limits is fraught with potential difficulties, but it appears to work, and therefore deserves further consideration. As it stands now the method of asymptotes and limits provides an alternative strategy for the interpretation of apparent age spectra produced from minerals separated from metamoprhioc tectonites. In the absence of detailed understanding of the tectonothermal evolution of a particular region, the method of asymptotes and limits allows identification of potential events. Thereafter it is a matter of interpreting the apparent age spectra in terms of the gas released from different reservoirs, and if possible, using microstructural and other data to identify potential candidates for the reservoir site. References Foster, D A . , Gray, D.R., Kwak,T.A.P. & Bucher, M., 1998. Chronology and tcctonic framework of turbidite hosted gold deposits in the western Lachlan Fold Belt, Victoria: 40Ar-39Ar results: Ore Geology Reviews 13: 2 2 9 - 2 5 0 . Baldwin, S . L & Lister, G.S., 1998. Thermochronology of the South Cyclades Shear Zone, los, Greece - Effects of ductile shear in the argon partial retention zone. Journal of Geophysical Research-Solid Earth. 103(B4): 7 3 1 5 - 7 3 3 6 .

SGTSG abstracts, February 2001

48 Ar-Ar geochronology and timing of deformation in the Tasmanides: What do we know? D.A. Foster\ D.R. Gray^ and C.V. SpaggiarP ^ Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA 2 Department of Earth Sciences, Monash University, Melbourne, Victoria, Australia 3800

There presently exist over 400 Ar-Ar analyses of white mica and whole rock phyllite/slate samples from deformed parts of the Lachlan Orogen and other adjacent belts in the Tasmanides. A majority of these data are published in journal articles, PhD theses, and reports, while others are in manuscripts under review. The vast majority of the samples used for these analyses are from structural zones or fault zones where metamorphism exceeded the upper anchizone (IC <0.3), and indeed most exceeded epizone (IC <0.25) conditions. The Ar-Ar results and their implications for the timing of deformation, cleavage formation, quartz veining, and exhumation have formed part of the basis for an improved understanding of the tectonic history and setting of eastern Australia. This has been done using the geochronological and thermochronological data in conjunction with recent structural compilations, detailed mapping, metamorphic petrology, and recognition of the significance of local blueschist facies metamorphism and duplex structures in the oceanic basement crust. Ar-Ar data from the Lachlan can be broadly summarized as follows:

Western Lachlan-Stawell and BendigoBallarat Zones • Concordant plateau ages at about 455 Ma and 440 Ma, and more minor plateau segments at about 420 are given by white mica separates, strongly strained whole rock phyllites/slates, and biotite separates. The plateau ages are interpreted to give the growth age of the metamorphic mica. • Some less deformed whole rock slate samples give age gradients between about 420 or 440 Ma and 500 Ma indicating mixing between metamorphic and detrital mica • Coarse detrital micas from sandstones give ages of c. 500 Ma.

Western Lachlan - Melbourne Zone • Most samples give complex age spectra with apparent ages between 380 and 500 Ma due to mixing between very low-grade metamorphic mica and detrital mica, and ^^Ar recoil. SGTSG abstracts, February 2001

• Some whole-rock samples from the Mount Wellington fault zone give plateau ages of about 390-385 Ma for part of the gas release. Plateau ages up to - 4 1 0 Ma are given by sericite separates from deformed mineral deposits in Cambrian fault slices, and --440 Ma from slate in fault slices of turbidite.

Central Lachlan - Tabberrabbera Zone • Slate samples give plateau dates of about 413 to 440 Ma. Not enough data exist to establish if there is more than one episode of metamorphic mica growth >413 Ma or how much detrital contamination could be effecting the older samples.

Central Lachlan - southern Omeo Zone • Separates of coarse muscovite and biotite from shear zones and mylonitic granitoids give plateau ages of about 410-400 Ma. These ages record the relatively rapid exhumation of high-grade metamorphic rocks.

Eastern Lachlan - Monaro and Molong Zones • White mica and biotite separates give plateau ages of 3 9 0 - 3 8 0 M a that record exhumation of metamorphic complexes and shearing. Two wholerock slate age spectra give similar plateau ages. • Other analyses of white mica (separates) give plateau ages of 370 Ma, and some are pardy reset at 360-340 Ma. These record later events.

Eastern Lachlan - Hill End Zone • Muscovite and biotite mineral analyses give plateau ages of -360 and 340 Ma. • Whole rock slate analyses give plateau ages concordant with the mica separates.

Eastern Lachlan - South Coast Zone • Published results from this zone and unpublished data (Fergusson & Phillips, in review) exhibit variable mixtures of ages from low-grade slates due to mixing with detrital mica, and ^^Ar recoil. The most highly cleaved samples with bedding-parallel fabrics give more internally concordant results, one with a plateau age of - 4 4 5 Ma. Samples lacking

this early fabric (Fergusson, pers. com.) have been interpreted to be -430 Ma The question that begs to be addressed is — how robust and reliable is the data set as a whole and what data are better than others? The most potentially problematic data are those from the lowest grade structural zones where the IC values are low and grain sizes are very small. These low-grade whole-rocks may suffer from: ^Âr recoil and "^Âr mixing from detrital mica and plagioclase, both of which cause an increase the apparent age and produce discordant age spectra. Moreover, because the micas are fine-grained and have low IC values they are less retentive for "^Âr and may loose argon at relatively low temperature 150°-250°). Many samples from the Melbourne Zone and some from the South Coast Zone fall into this group. These difficulties diminish at higher metamorphic grade and for most of the Lachlan structural zones a very high degree of consistency is revealed by multiple analyses of both phyllites/slates and mica mineral separates. Complications at higher metamorphic grades may arise from multiple generations of metamorphic mica. Fortunately, with the dominant micas being phengitic there is a tendency, unlike other hydrous phases (biotite, hornblende, some muscovite), for "^Âr gradients to be preserved during sample degassing. Where this occurs two plateau segments are occasionally revealed on age spectra with surprising consistency between samples and over a region, e.g. Stawell and Bendigo-Ballarat zones. Even so, for whole-rock slates and phyllites, in our experience from hundreds of analyses from the Lachlan, only data from the most highly cleaved samples are generally meaningful and consistent, even in structural zones with upper anchizonal and epizonal metamorphic conditions. Less deformed samples may not experience rejuvenation of residual detrital grains by thermal diffusion alone. The highly deformed, strongly cleaved slates presumably are fully rejuvenated by a combination of mechanical, mineralogical, and thermal mechanisms.

In summary, the Ar-Ar results from the Stawell, Bendigo-Ballarat, Hill End, Omeo, Monaro, Molong zones, as well as our unpublished data from northern Tasmania show excellent internal consistency between pure white mica mineral separates and whole-rock phyllites/slates. Unfortunately, the same advantage of comparison with mica separates is not possible for much of the Melbourne, Tabbarabbera, and South Coast Zones. Further analyses are being obtained to try and rectify this uncertainty,

SGTSG abstracts, February 2001

50 Evolution and exhumation of the Bitterroot metamorphic core complex, Montana and Idaho, USA D.A. Foster\ M.C. Fanning^, S. Coynei^ and A. Raza^ ^ Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA 2 Research School of Earth Sciences, Australian National University, Canberra, ACT Australia 0200 ^ Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA ^ School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia 3010

The Bitterroot metamorphic core complex exposes a large middle crustal plutonic and metamorphic terrain of the Cordilleran hinterland. Rocks within the complex include plutons of the central IdahoBitterroot batholith, metamorphosed Proterozoic Belt Supergroup strata and possible pre-Belt basement. Compressional and extensional deformation, metamorphism, partial melting, and plutonism in the complex occurred from >100 Ma to <45 Ma, and were often synchronous with each other. A major phase of high-grade metamorphism (-0.65-0.75 GPa and -650°-750°C) in the northeastern part of the complex was coincident with intrusion of quartz diorite plutons at -75-80 Ma (SHRIMP U-Pb zircon). The metamorphism locally caused partial melting in quartzofeldspathic gneiss based on zircon dates U-Pb of migmatites. Widespread partial melting and metamorphism of the lower and middle crust also occurred between -65-52 Ma (SHRIMP U-Pb zircon), producing the "main-phase" granitic plutons of the central Idaho-Bitterroot batholith and migmatites at the present level of exposure. The youngest mid-crustal granitic intrusions in the complex include diorite dikes, and I-type granitoids that overlap the onset of regional extension at -53 Ma based on SHRIMP U-Pb zircon dates. Because these plutons were intruded during the early phases of extension they are strongly stretched and transposed at the structural top of the footwall where they are caught up within the extensional shear zone. The younger partial melting could either have resulted from voluminous magmatism beneath the area or rapid decompression due to the onset of extension. Apatite and zircon fission track data from the Bitterroot mylonite footwall suggest that the western part of the complex was at temperatures <150°-100°C, at -5048 Ma. These data are consistent with relatively lowT deformation mechanisms on linked high- and lowangle normal faults that place Eocene volcanics and epizonal plutons on top of amphibolite facies gneiss. At the same time (-50-48 Ma), amphibolite facies metamorphism and mylonitization continued in the eastern part of footwall at 15-20 km depth. SGTSG abstracts. February 2001

Deformation of in the deeper part of the shear zone produced amphibolite and greenschist facies S-C mylonite from granites and paragneisses. Eastward decreasing ages of all thermochronometers (apatite and zircon fission track, and biotite, muscovite and Kfeldspar Ar-Ar) are also consistent with significandy deeper Eocene structural levels in the east during extensional exhumation. Apatite and zircon in the western part of the footwall give fission track ages of a -48 (apatites have long mean track lengths), recording very rapid cooling of the shallow structural levels of the complex. In the eastern part of the footwall the zircon fission track ages are -42-35 Ma and the apatite fission track ages are -38-20 Ma. These thermochronologic data record progressive exhumation from shallower crustal levels in the west to deeper crustal levels in the east f r o m - 5 1 - 3 8 M a and slower exhumation between 38 and 20 Ma. Tectonic exhumation is also recorded in the transition from amphibolite facies, to greenschist facies, to brittle fabrics in the Bitterroot mylonite. ^ f ^

New interpretation for terrane development of western New Zealand, based on detrital zircon age distributions and geochemistry

A. Fowler School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia 3010

Constraints provided by U-Pb dating of detrital zircons shed new light on the terrane division in New Zealand. Comparison of age distributions and geochemistry from turbidites in western New Zealand argue for separate but interrelated histories between two terranes, which begs the question — are they really distinct terranes? The New Zealand continental fragment comprises the North and South Islands and large areas of submerged continental crust (Fig. 1). It is composed of eight terranes, which are divided into two provinces -the Eastern and the Western Provinces. The Eastern Province is entirely subduction related, being mostly composed of sedimentary rocks. On the whole it is younger than the Western Province. The Western Province has been divided into two terranes: theTakaka Terrane to the East and the Buller Terrane to the West (Cooper, 1989). The Takaka Terrane contains two major assemblages (Munker & Cooper, 1999). One comprises Mid to Late Cambrian island arc volcanics and arc derived sediments. The other assemblage has no genetic relationship to the island arc, but comprises deep sea turbidites. It was derived from Gondwana from the Mid Cambrian to the Early Ordovician and deposited alongside the arc (Fowler, unpub. data). One of these turbiditic units from the Takaka Terrane is the Junction Formation, which contains detrital mica and potassium feldspar. Its petrography, geochemistry and zircon age data indicates it was derived from Gondwana (Fowler, unpub. data). One trilobite locality suggests the maximum age of deposition was probably no later than Middle Cambrian (Munker & Cooper, 1999). U-Pb dating of detrital shows deposition can be no earlier than -540 Ma (Fowler, unpub. data). Another unit from the Takaka Terrane that was formerly turbiditic is the Balloon Melange. The Balloon Melange is a highly disrupted unit that includes abundant sandstone plus exotic blocks, in a muscovite-bearing, muddy matrix (Pound, 1993). Petrographically, the occurrence of potassium feldspar together with detrital mica in the sandstone, is the main distinguishing feature of both the Junction Formation and the Balloon Melange. All

authors to date have inferred that the Balloon Melange is a highly deformed version the Junction Formation (with minor exotic components), based on the strong petrographic similarities. This interpretation required that formation of the melange occurred in the Late Cambrian, soon after deposition of the Junction Formation. U-Pb dating of detrital zircons from the Balloon Melange sandstone, however, place its maximum age of formation squarely in the Early Ordovician, -480 Ma (Fowler, unpub. data). The Buller Terrane is predominantly composed of Cambro-Ordovician, quartzose, deep-sea turbidites (Cooper & Tulloch, 1992). These turbidites are mature, graded, metagreywacke-dominated sequences that are generally unfossiliferous. They are tentatively assigned a Late C a m b r i a n - E a r l y Ordovician depositional age based on one graptolite locality. This report presents a new interpretation for the relationship between the Takaka and Buller Terrane turbidites. Petrography does not provide significant evidence to support correlation of Takaka Terrane turbidites with Buller Terrane turbidites but geochemistry and zircon ages do. The literature suggests a relationship between the turbidite units of theTakaka and Buller Terranes was not previously considered. This report, however, shows that the evidence in favour of correlation of the Takaka Terrane turbidites with the Buller Terrane turbidites outweighs the evidence against. Additionally, correlation of Takaka Terrane turbidites with other turbidite deposits of the Gondwana region is attempted. In all other fragments of the now dismembered paleo-Pacific margin of Gondwana, the CambroOrdovician turbidite sequences are clearly separated from the Cambrian island arcs by terrane/zone boundaries. This situation provides a useful basis for classifying the rocks of western New Zealand. Following the associations found in Antarctica and Australia, I suggest the turbidites of the Takaka Terrane be considered as quite separate to the island arc rocks that make up the rest of the Takaka Terrane, and be equated with the Buller Terrane rather than theTakaka Terrane. The unfortunate occurrence of the Junction SGTSG abstracts, February 2001

Formation and Balloon Melange protolith in the Takaka Terrane does not fit in well with the existing etwo-terranei model of western New Zealand, which assumes the Takaka and Bullet Terranes had distinct geological histories. Perhaps it is better to consider the Junction Formation as a fault-bounded island of Buller Terrane and the Balloon Melange an intruded blob of Buller Terrane within the Takaka Terrane and to recognise the two terranes had different but interrelated histories,

References

Cooper, 1989. J. Roy. Soc. N.Z. 19: 7 3 - 1 1 2 . Cooper &Tulloch, 1992. Tectonophysics 214: 129-144. Miinker & Cooper, 1999. N.Z.J.G.G. 42: 415-445. Pound, 1993. unpub. PhD thesis.

SGTSG abstracts, February 2001

Deformation history in the hinge region of a chevron fold.

M.J. Gallen, S.M. Reddy & P.A. Cawood Tectonics Special Research Centre, School of Applied Geology, Curtin University, G.P.O. Box U1987, Perth, Western Australia, Australia 6845

Folding in the Hamersley Province of Western Australia is associated with mineralisation but a clear understanding of folding at all scales in this region, has not been developed. Here we present a detailed study of fold formation from the Hamersley Province, which has implications for chevron fold development and for models of ore body formation. The sample, from the Joffre Member of the Hamersley Group, is composed of layers of chert and banded iron (magnetite ± haematite) with sub-mm to cm scale bands. The sample is approximately 17.5 cm high and 13 cm wide and was taken from of a close fold with a wavelength of 85 cm and amplitude of 120 cm. The axial plane strikes at 260°/86°S and the fold axis plunges at 10° towards 256°. The orientation of the fold and the geometry of structures nearby suggests that it developed during the Palaeoproterozoic Opthalmian Orogeny (Gallen, 1998), Tylers (1991) regional D2 deformational event related to the collision of the Yilgarn and Pilbara Cratons. A systematic assessment of age relationships (Potts & Reddy, 1999) has been used to systematically analyse the development of this chevron fold. The technique shows that deformation developed sequentially with no evidence for repetition or synchronous development of structures. The fold has a well developed angular hinge but the shape of individual layers varies with lithology. Deformation was concentrated in the hinge regions of the fold with only minor deformation in the limbs. A competent chert layer near the base of the sample in the inner arc of the hinge shows a more rounded form and would be classified as a Class IB fold in Ramsays (1967) classification scheme. This band overlies a layer of fine-grained magnetite and minor quartz, which is also rounded. The top layer of the sample is a magnetite dominated layer with minor fine-grained quartz and is typically 6 mm thick although it has been locally thickened to more than 25 mm. The central portion of the sample contains a combination of chert and magnetite bedding layers and bedding parallel fibrous quartz veins. The thickness of this portion of the sample ranges from 30 mm in an area of significant dissolution to >120 mm in the hinge of the antiform. The deformation history of the sample, which documents a complex series of interactions between structures at a small scale, includes: bedding parallel fibrous quartz vein formation, multiple folding events, dissolution of quartz, thrusting and faulting. The

bedding parallel fibrous quartz veins are the oldest deformation related structures in the sample. The veins are particularly common in the hinge region but they are also present in the limbs. The veins are interpreted to have grown antitaxially between micro-bedding layers and the sense of curvature in some veins indicates a south over north sense of movement, whilst in other veins it indicates a north over south sense of movement. This is interpreted to be a product of different amounts of compression during layer parallel shortening in early stages of the D2 deformation event. The sense of curvature, and hence the movement direction, does not change on either side of the axial plane of small scale F2 folds, indicating that the veins are older than the folds. The veins were folded by F2 folds of the D2 deformation event, which are represented in the sample by vertical to sub-vertical axial planes. Some axial planes can not be traced through zones of quartz dissolution indicating that the folds were older than dissolution and cleavage development. Quartz dissolution was typically axial planar and was identified by discontinuities in bedding and axial planes and in the concentration of magnetite ± haematite. Significant dissolution of quartz post-dated the quartz vein formation. Two small thrusts, indicating movement consistent with the regional south over north sense of movement are located on the southern edge of the sample. These thrusts are identified as planes that define discontinuities in bedding layers and discontinuities in the axial planes of folds from the main folding event. The two small faults in the sample are located on the northern edge and indicate material on the northern side of the faults moved downwards. The faults show mm offset of bedding layers and layer parallel fibrous quartz veins. A later stage of folding that refolds all previously described structures, except for the thrusts and faults, produces angular to open folds with subhorizontal to shallowly dipping axial planes. A deformation history was constructed utilising a technique developed by Potts & Reddy (1999). All of the data was entered into a single large younging table (Potts & Reddy, 1999) and all observed relationships were noted. The structures were ordered from oldest (at the bottom) to youngest (at the top). The size of the younging table was reduced by removing the 41 structures that had no observed relationships (Potts & Reddy, 1999). The reduced younging table was divided into seven mutually exclusive groups of structures. SGTSG abstracts, February 2001

No relative age relationships were observed for any two structures within the same structural group (for example the fibrous quartz grains). Consequently the relative age relationships between the different fibrous quartz grains are unknown. However, all of the relative ages relationships observed between fibrous quartz veins and structures from other groups were consistent (for example F2 folds were always younger than fibrous quartz veins). A consequence of this observation was that the fibrous quartz veins could be placed in any order in the younging table (eg ascending; descending; random), with no effect on the validity of the younging table (all younging structures would still point in the same direction (Potts & Reddy, 1999)). This relationship was true for fibrous quartz veins, F2 folds, zones of quartz dissolution, and the late stage folds. Because of this consistent relationship between structures of different groups, it was interpreted that the structures within each of the groups were synchronous (ie all fibrous quartz veins were synchronous, all F2 folds were synchronous etc) and that each group of structures occurred systematically with no repetition or synchronous development of structures. A number of deformation related structures can be observed in the hinge region of the fold at a grain scale and sub-grain scale. These include sutured grain boundaries, optical discontinuity of fibrous quartz veins and boudinage in magnetite bedding layers. All of these features are observed in the central portion of the sample and are consistent with the sequence of deformation recognised on the basis of meso-scale features. The presence of sutured grain boundaries and the preferential growth of grains at the expense of their neighbours is indicative of grain boundary migration and is most commonly observed in fibrous quartz grains that have been folded by F2 folds. Undeformed fibres that grew in a bent shape are optically continuous whereas deformed, or mechanically bent, fibres may or may not have been bent before deformation. The mechanical bending of fibrous quartz grains results in bent fibres that are optically discontinuous. Magnetite bedding layers commonly deform as britde, competent layers and boudinage in these layers is common in the limbs of small-scale F2 folds. A model of deformation has been developed that incorporates all of the structural features described above in a systematic deformation sequence. Early compression, possibly related to the Opthalmian D2 deformation event, led to the formation of the fibrous quartz veins. Continued compression led to the folding of bedding layers and bedding parallel fibrous quartz veins. This is shown by the optical discontinuity of some bent fibres, evidence for grain boundary migration and the consistent sense of movement indicated by some veins on both limbs of small scale F2 folds. Dissolution of both primary quartz in bedding layers and vein quartz followed this episode

SGTSG abstracts, February 2001

of folding, and becomes more significant with increasing distance from the axial plane of the fold. Evidence for dissolution is concentrated in the limbs of the fold, and in the limbs of small-scale folds in the hinge region of the fold. The final folding event, which folds all previously described structures, may or may not be related to the Opthalmian D2 deformation and may reflect a late stage local deformation event. The faulting is younger than the bedding parallel fibrous quartz veins, and the thrusting is younger than the F2 folds, but no relative age relationships have been observed with any other structures. The systematic development of deformation in the hinge region of the fold, with no evidence for repetition or synchronous development of structures, indicates that rocks in the Hamersley Province may respond to stress in a predictable and sequential manner. Models of ore body formation in the Hamersley Province incorporate elements of quartz dissolution and concentration of iron-oxides (Powell et al., 1999). The precipitation of quartz during fibrous quartz vein formation prior to the dissolution of quartz during cleavage development has important implications for models of ore body formation, such models being highly influenced by the solubility of quartz (Powell etal., 1999). References Gallen, M J . , 1998. Structural styles in the southern Hamersley Province in the vicinity of Coondawanna Ridge. BSc (Hons) thesis, Curtin University of Technology (unpublished). Potts, G.J. & Reddy, S.M., 1999. Construction and systematic assessment of relative deformation histories. Journal of Structural Geology 21: 1,245-1,253. Powell, C.M., Oliver, N.H.S., Xheng-Xiang, L., Martin, D.M. & Ronaszeki, J., 1999. Synorogenic hydrothermal origin for giant Hamersley iron oxide ore bodies. Geology 27: 175-178. Ramsay, J.G., 1967. Folding and fracturing of rocks: McGraw Hill, New York: 99, 248. Tyler, I.M., 1991. The geology of the Sylvania Inlier and the southeast Hamersley Basin. Geological Society of Western Australia Bulletin 138.

A continental back-arc setting for the Early to Middle Proterozoic basins of north-eastern Australia

D. Giles, P. Betts and G. Lister Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne, Australia 3800

Between 1.8 and 1.6 Ga a series of partially interconnected "intracratonic" volcano-sedimentary basins developed in northern and eastern Australia. The deformed and metamorphosed remnants of these basins are preserved in the Mount Isa Inlier, the McArthur Basin, the Georgetown Inlier and the WillyamaInlier (Fig. 1). The depositional and thermal evolution of the basins is remarkably coherent and is characterised by two cycles of episodic extension and magmatism (c. 1.8-1.73 Ga and 1.73-1.59 Ga) separated by a major regional unconformity. Major tectono-thermal events within the basins also coincide with significant tectonic events in the Arunta Inlier and the Gawler Craton (Fig. 1). The Arunta Inlier and the Gawler Craton comprise a sequence of deformed sedimentary basins (e.g. Kimban Orogen in the Gawler Craton), accreted continental fragments (e.g. the Archaean nucleus of the Gawler Craton) and magmatic-arc terranes (e.g. the "Strangways" arc in the Arunta Inlier, Zhao & McCulloch, 1995, and the Ifould Complex in the western Gawler Craton, Teasedale, 1997) that formed between 1.8 and 1.6 Ga. These terranes represent the remnants of a complex convergent plate boundary that developed on the southern margin of the proto-Australian continent. Episodes of Cordilleran arc-style magmatism at the plate margin (c. 1.78-1.75 Ga, Zhao & McCulloch, 1995; 1.74-1.67 Ga, Teasedale, 1997) coincide with extensional tectonism in the continental interior (c. 1.79-1.75 Ga Leichhardt and Myally rift events, O'Dea et al., 1997; c. 1.73-1.67 Ga, Mount Isa Rift Event, Betts et al., 1998). High temperature-low pressure metamorphism at the plate margin (c. 1.781.77 Ga, Early Strangways orogeny, Collins and Shaw, 1995) coincides with voluminous mafic volcanism possibly related to asthenospheric upwelling (c. 1.78 Ga, Eastern Creek Volcanics, O'Dea et al., 1997) in the continental interior. Cordilleran-style orogenesis at the plate margin (c. 1.74-1.73 Ga, Late Strangways orogeny, Collins & Shaw, 1995) coincides with a transient period of basin inversion (c. 1.74-1.73 Ga, Mount Isa Inlier mid-basin inversion, Betts, 1999) in the continental interior. The correlation of these events implies a paired evolution of the intracratonic basins

and the convergent plate margin, whereby thermal events were shared and plate margin stresses were propagated (up to 1000s of kilometres) into the plate interior. We propose a tectonic model in which the 1.8-1.6 Ga northeastern Australian basins occupied a wide region of intermittently extending continental crust in the over-riding plate of a long-lived subduction system. They are interpreted as continental back-arc basins (Fig. 1). The combined effects of slab roll-back, accretion, enhanced sub-continental convection and asthenospheric upwelling produced an environment of episodic extension, transient shortening, elevated heat flow and magmatism. . y f ^ f ^

References Betts, RG., 1999. Palacoproterozoic mid-basin inversion in the northern Mt Isa terrane, Queensland: Australian Journal of Earth Sciences, 46: 735-748. Betts, P.G., Lister, G.S. & O'Dea, M.G., 1998. Asymmetric extension of the Middle Proterozoic lithosphere, Mount Isa terrane, Queensland, Australia. Tectonophysics 296: 293-316. Collins, W.J. & Shaw, R.D., 1995. Geochronological constraints on orogenic events in the Arunta Inlier; a review. Precambrian Research 71: 315-346. O'Dea, M.G., Lister, G.S., MacCready, T., Betts, P.G., Oliver, N.H.S., Pound, K.S., Huang, W. & Valenta, R.K., 1997. Geodynamic evolution of the Proterozoic Mount Isa terrain. In Burg, J.R, and Ford, M., eds.. Orogeny Thro ugh Time. Geological Society Special Publication: 99-122. Teasdale, J., 1997. Methods for understanding poorly exposed terrances: the interpretive geology and tectonothermal evolution of the Western Gawler Craton. Unpublished PhD thesis. University of Adelaide. Zhao, J.-X. & McCulloch, M.T., 1995. Geochemical and Nd isotopic systematics of granites from the Arunta Inlier, central Australia; implications for Proterozoic crustal evolution. Precambrian Research 71: 265-299.

SGTSG abstracts. February 2001

accretion and arc maginatism

continental back-arc extension

backj^rc basins %

6eo/gelowi! /Inlier . Wiihama

Western Gawler Eastern Gawler 500 km 1000 km

Figure 1. The interpreted relationship between the ca 1.8-1.6 Ga northeast Australian "intra-cratonic" basins and the active continental margin terranes of the Arunta Inlier and the Gawler Craton.

SGTSG abstracts, February 2001

Tectonic evolution of l\/lacquarie Island B. Goscombe^ and J. Everard^ ^ Department of Geology and Geophysics, Adelaide University, Adelaide, SA, Australia 5005 2 Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania, Australia 7018

Macquarie Island is a 150 km^ exposure on the Macquarie Ridge that approximately coincides with the plate margin between the Indo-Australian and Pacific oceanic plates. Macquarie Island is comprised entirely of oceanic crust (Varne et al., 1969) and is the only sub-aerial exposure of oceanic crust on Earth that is still within an ocean basin. Because of tilting and differential block uplift, all crustal levels down to, and including, the mantle lithosphere are well exposed. Consequently, Macquarie Island offers a unique opportunity to study, by conventional geological mapping techniques, oceanic crust that has not been affected by obduction and is still within its oceanic basin context. Furthermore, Macquarie Island is the only exposure of the Macquarie Ridge, a unique plate boundary type, in that it is a transpressional mountain belt at the margin between two oceanic plates, that is in transition to becoming a subduction zone. The sector of oceanic crust containing Macquarie Island, between Tasman Sea oceanic crust and continental crust of the Campbell Plateau, was generated from 39 to 10.5 Ma at the Proto-Macquarie Spreading Ridge (PMSR), that propagated from the Pacific/Antarctic spreading ridge. The PMSR evolved with time from long ridge segments of NNE-trend to short ridge segments of E-trend in the vicinity of Macquarie Island and generated during the latest episodes of seafloor spreading (Lamarche et al., 1997). The overall trend of the PMSR was NNE-SSW throughout and roughly coincided with the present day Macquarie Ridge. Plate reconstructions by Molnar et al. (1975) and Lamarche et al. (1997) suggest coincident seafloor spreading and strike-slip movements at the PMSR between 14 and 10.5 Ma. The present day Indo-Australian/Pacific plate margin is coincident with the Macquarie Ridge, an arcuate 2100 km long crustal fracture system connecting the Pacific/Antarctic and Indo-Australian/Antarctic spreading ridges with the Alpine Fault system in New Zealand. The Macquarie Ridge evolved from a dextral strike-slip system (10.5 to 5 Ma) to a transpressional plate margin (5 Ma to present) (Frohlich et al., 1996). The complex evolution of Macquarie Island crust.

from crust formation to the present day, can be assigned to three distinct tectonic periods with different crustal stress fields (Goscombe & Everard, 1999). Z); was a protracted period of N-S extension, encompassing initial crust formation at the PMSR, overprinting by late-stage gabbro and dolerite dykes, and a wide range of extensional and dilational structures formed in the near- to off-axis environment. Extensional structures include; ductile and semiductile shear zones, dilational serpentine veinlet foliations, dilational fracture-cleavages, fissures and slots, hydrothermal veins, small-scale vein-filled faults, major growth faults, differential block uplift (up to 4 km), block tilting and formation of scarps and associated talus deposits. All formed in a stress regime with sub-vertical Sj and sub-horizontal N-trending s^ and are consistent with pure extension in a spreading ridge environment. A minor set of orthogonal fractures and serpentine veinlets in the mantle lithosphere harzburgite suggest a component of along-axis extension during Dj. This pattern may reflect drag on the lower lithosphere by flow in the asthenosphere, indicating complex mantle mantle flow at the spreading ridge, with both divergent flow away from the ridge and flow along the ridge away from mantle diapirs. Early-Z)j tilting of 20°-58° around horizontal axes parallel to the ridge axis, accompanied growth faults and major differential block uplifts in the nearaxis environment. Vergence of tilting indicate that the Macquarie Island Crust was generated on the Pacific Plate side of the PMSR. D2 involved NE-SW extension and gave rise to rare, late-stage dolerite dykes, hydrothermal veins and possibly also late-stage extrusives. Z)^ is interpreted as a transtensional period during transition from D^ extension to D^ transcurrent tectonics. D^ encompassed the entire period of transcurrent and transpressional tectonics, after cessation of extension at the PMSR. D3 gave rise to major (0.3-100 m wide) gouge and breccia faults and associated cleavages, rare thrust faults, neotectonic fault scarps and rotation of crustal blocks around vertical-axes. Palaeo-stress analysis of D^ faults and neotectonic fault scarp SGTSG abstracts, February 2001

geometry indicate dominantly strike-slip movements and rare thrust events, both with NE-trending sj , these geometries being compatible with dextral transpression. Sharp angular discordances in the palaeoseafloor fabric {Dj dyke trend), document clockwise rotation (up to 65°) of km-scale crustal blocks around vertical axes during D^. Consistent clockwise rotation and increasing degrees of rotation towards the plate margin, immediately (2-5 km) east of Macquarie Island, is entirely consistent with dextral transpression at the plate margin. All D^ structures formed in response to dextral strike-slip and transpression at the Indo-Australian/Pacific plate margin from approximately 10.5 Ma to the present day.

References

Frohlich, C., Coffin, M. R, Massell, C., Mann, P., Schuur, C. L , Davis, S. D., Jones, T. & Karner, G.,1996. Constraints on Macquarie Ridge tectonics provided by Harvard focal mechanisms and telescismic earthquake locations. Journal of Geophysical Research 102(B3): 5029-5041. Goscombe, B.D. & Everard, J . L , 1999. Macquarie Island mapping reveals three tectonic phases. EOS 80(5): 50 & 55. Lamarche, G., Collot, J-Y., Wood, R A , Sosson, M., Sutherland, R. & Delteil, J., 1997. The Oligocene-Miocene Pacific-Australian plate boundary, south of New Zealand: Evolution from oceanic spreading to strike-slip faulting. Earth and Planetary Science Letters 148: 129-139. Molnar, P., Atwater, T , Mammerick, J. & Smith, S.M., 1975. Magnetic anomalies, bathymetry and the tectonic evolution of the South Pacific since the Late Cretaceous. Geophysical Journal of the Royal Astronomical Society 40: 383-420. Varne, R., Gee, R.D. & Quilty, RG.J., 1969. Macquarie Island and the Cause of Oceanic Linear Magnetic Anomalies. Science 166: 230232.

SGTSG abstracts, February 2001

Metamorphic model for the Inland Branch of the Damara Orogen: A paired metamorphic mountain belt (PMMB)

B. Goscombe and M. Hand Department of Geology and Geophysics, Adelaide University, Adelaide, SA, Australia 5005

The Damara Orogen is a Neoproterozoic fold and thrust belt with a triple junction geometry, consisting of two coastal arms and a ENE-trending Inland Branch which is the focus of this study (Fig. 1). The Damara Orogen experienced a single otogenic cycle of PanAfrican age (550-520 Ma) and can be divided into some 16 zones that are lithologically, structurally and metamorphically distinct, although a coherent Neoproterozoic stratigraphy can be recognised throughout. This study, for the first time, rationalizes a large body of new and existing metamorphic data in order to characterise the P-T evolution and average thermal gradient* in each tectono-metamorphic zone and map out the average metamorphic field gradient across the entire Inland Branch of the orogen (Fig. 2). Metamorphic conditions have been estimated from P-T calculations on 74 samples using the average P-T approach of Powell & Holland (1994). Because only one thermobarometric method and thermodynamic dataset have been used, the P-T results are probably directly comparable throughout the sampled region, and give some useful insights into variation in metamorphic style in different parts of the Damara otogenic system. The Inland Branch of the Damara Orogen consists of two terranes with contrasting metamorphic "style" (a composite of the average thermal gradient*, P-T path and metamorphic grade), that are confined to either the upper, or lower-plate domains of a crustalscale over-thrust system (Fig. 1). Goscombe & Hand (2000) have referred to this type of otogenic architecture as a Paired Metamorphic Mountain Belt (PMMB). The Inland Branch displays some degree of symmetry in structural and metamorphic style; a high-grade, granite-rich Central Zone is flanked by the Northern and Southern Zones which are both high-P, Barrovian metamorphic sequences dominated by outward thrusting onto the cratonic basement in their forelands (Fig. 1). At both margins, the Central Zone occupies the upper-plate position of a crustalscale over-thrust system, and thus gives the appearance of being an extruded otogenic core. The Central Zone experienced high-T/low-P (35-50°C/km*) metamorphism (700°-800°C, 3.5-6 kbar; Figure 2)

characterised by isobaric cooling paths. Both the Northern and Southern Zones occupy lower-plate positions and experienced moderate-T/high-P (1525°C/km*; Figure 2) Barrovian metamorphism and clockwise P-T paths. Both zones contain an inverted metamorphic sequence with the highest metamorphic grade occupying the highest structural levels immediately below the contacts with the Central 2^ne (Figure 1). Aside from contrasts in the style of metamorphism between the core and margins of the Inland Branch of the Damara Orogen, the Central Zone is characterised by large volumes of granite (30-60% of the current erosion surface). An important question that needs to be addressed is whether the granites are the cause, or the consequence, of the high thermal gradient regime in the Central Zone. The granites are derived from melting of the Damara Sequence (McDermott et al., 1996), suggesting they are a consequence of the high thermal gradient conditions. There is no obvious evidence of mafic magmatism, or a significant volume of highly radigenic heat producing rocks, and the ultimate source of the high temperature conditions is still unresolved. However we speculate that the formation of low-P/high-T conditions in the Central Zone of the Inland Branch is a consequence heat advection during rapid mass flow within the orogen in which a relatively hot lower crust was extruded upwards and outwards, and across the Northern and Southern zones. In such a scenario it is possible that all earlier high-P parageneses and the record of isothermal decompression would be obliterated during pervasive ductile flow of the hot terrain as it decompressed (giving rise to decompression melting and pervassive granitoids), and then underwent rapid (isobaric) coohng. The spatial metamorphic patterns with respect to gross structural architecture of the Inland Branch, are very similar to those developed in the Eastern Himalayas (Goscombe & Hand, 2000). Both of these examples are PMMBs that record high-T/low-P metamorphism in the upper-plate and Barrovian metamorphism in the lower-plate. This suggests that development of PMMB patterns may be SGTSG abstracts, February 2001

a common feature of collisional orogens with crustal-

References

scale over-thrust architectures.

Goscombc, B. & Hand, M., 2000. Journal of Petrology 41. McDermott, R, Harris, N.B.W. & Hawkcsworth, C.J., 1996. Contrib. Min. Petrol. 123: 406-423. Powell, R. & Holland, T.J.B., 1994. American Mineralogist 6: 173-201.

Kalahari Craton

Figure 1

Figure 2

*Averagc thermal gradient is simply the c a l c u l a t e d metamorphic t e m p e r a t u r e d i v i d e d by the calculated depth assuming a density of 2 . 8 g c m - l . It should not be confused with the instantaneous thermal gradients in the vicinity that an assemblage formed, or imply that the thermal gradients are time equivalent.

SGTSG abstracts, February 2001

Tectonometamorphic Evolution of the Kaoko Belt, Namibia

B. Goscombe and M. Hand Department of Geology and Geophysics, Adelaide University, Adelaide, SA, Australia 5005

The Kaoko Belt is the NNW-SSE-trending northern coastal arm of the Neoproterozoic-Palaeozoic Damara Orogen that extends into Angola (Fig. 1). The Kaoko Belt is poorly understood and largely unmapped because of its remoteness and the security problems in northern Namibia throughout the 1970s and 1980s. In the Kaoko Belt the Damara Sequence consists of meta-greywacke, quartz-mica schists, meta-arenites and meta-turbidites with subordinate meta-diamictite, amphibolite schist, quartzite and carbonate units. This supracrustal assemblage unconformably overlies a basement complex composed of by Palaeoproterozoic paragneisses, 1977-1970 Ma granitoid orthogneisses, and late Archaean (2585-2645 Ma) rocks (Seth et al., 1998). The Kaoko Belt can be divided into three parallel zones. (1) The East Kaoko Zone (EKZ) which consists of sub-greenschist Damara platform sequences dominated by carbonates and resting on the Palaeoproterozoic Kamanjab Inlier. (2) The Central Kaoko Zone (CKZ) which is strongly deformed and ranges from lower greenschist in the east to upper amphibolite grade in the west. (3) The West Kaoko Zone (WKZ) which is an igneous complex consisting of656 Ma and 552-580 Ma Damaran granitoids (Seth et aL, 1998) and subordinate upper-amphibolite to granulite-grade Damara Sequence. In the Kaoko Belt, the Damara Orogeny involved a single progressive orogenic cycle {D2'D^) (approximately over the interval 580-552 Ma). D2 was responsible for a regionally pervasive and intense S2'L2 fabric and small-scale isoclinal folds colinear with an L2 mineral aggreagte lineation, throughout the WKZ and CKZ. D2 fabrics formed during sinistral oblique transpression along shallow NNW-plunging stretching lineations, in a non-coaxial shear environment approaching simple shear. Bulk strain was highly constrictional, with aspect ratios of deformed clasts averaging 4.6: 1.0:0.3 and ranging up to 11.0:1.0:0.3. D2 appears to have progressed continuously into D^, through intermediate fold events and formation of Wto NW-plunging mineral lineations (Z3). D^-D^ shortening produced the regional-scale, sub-horizontal, east verging tight to isoclinal folds that dominate the Kaoko Belt. 1)3-!)^ also occurred in a non-coaxial shear environment, but with lower vorticity than D2, tending towards pure shear. D2 and D^-D^ deformational

episodes have tectonic transport directions at high angles to each other. Later deformations are insignificant and involved limited N-S shortening with formation of rare conjugate kinkbands and E-W trending upright folds (D^), and rare normal faults and dolerite dykes during the Cretaceous break up of South America and Africa. The Kaoko Belt is divided into two metamorphic terranes of contrasting metamorphic style, by a median crustal-scale shear zone called the Purros Mylonite Zone (PMZ). The PMZ formed during D2 sinistral transpression, delineates the margin between the CKZ and WKZ (Fig. 1) and is a 2.5 km wide ductile shear zone composed entirely of mylonite with at least 11 ultra-mylonite zones, each 10-100 m wide. Throughout most of it s length, metamorphic grade increases rapidly towards the west across the PMZ into the WKZ. However, the polarity of metamorphic grade is reversed across a portion of the PMZ in the north (Fig. 1), with low-grades immediately west of the PMZ, suggesting a complex and poorly understood interaction between shearing in the PMZ and regional metamorphic isograds. The WKZ occupies the upper-plate position in an oblique over-thrusting orogenic system, and experienced high-T/moderate-P (30°-35°C/km) metamorphism with peak metamorphic conditions calculated to be around 800^-850®C and 8 kb. These results are consistent with mafic gneiss assembalges of orthopyroxene-clinopyroxene-garnet-hornblendeplagioclase. Post-peak assemblages (and spinelcorundum-plagioclase symplectites) and compositional data from mineral rims give P-T conditions around 600®C and 4.5 kb, suggesting that the WKZ underwent a clockwise P-T evolution. The CKZ occupies the lower-plate position, to the east of the PMZ, and experienced Barrovian metamorphism of moderateT/high-P (15°-25°C/km) (Fig. 1). Metamorphic grade in the CKZ increases towards the west to higher structural levels giving the appearance of an inverted metamorphic sequence. Prograde mineral compositions in the CKZ are preserved in mineral cores that equilibrated at conditions of 450°-550°C and 5.56.5 kb. Rim analyses from the matrix assemblage equilibrated at the peak-T (550°-650^C) and peak-P conditions (8.0-9.5 kb). Thus the core to rim SGTSG abstracts, February 2001

compositional zonation preserves a prograde path documenting burial and accompanying heating. The retrograde path involved decompression with cooling accompanying crystallization of hydrous retrograde phases and plagioclase coronas on garnet. In contrast to the Inland Branch of the Damara Orogen (Goscombe & Hand, this volume), the upper- and lower-plate metamorphic terranes of the Kaoko Belt show similar P-T paths (clockwise). However, both the Inland Branch and Kaoko Belt contain two

SGTSG abstracts. February 2001

metamorphic terranes with significant differences in average thermal gradient, a lower-plate with an inverted Barrovian metamorphic sequence and an upper-plate that experienced high-T metamorphism and contains volumous granitoids, ^ ^ f ^ References Sech, B., Kroner, A., Mczger, K., Nemchin, A.A., Pidgeon, R.T. & Okrusch, M., 1998. Precambrian Research 92: 341-363.

Boudin trains as a kinematic tool k\X

B. Goscombe^ and C. Passchier^ ^ Department of Geology and Geophysics, Adelaide University, Adelaide, SA, Australia 5005 2|nstitut fuer Geowissenschaften, Johannes Gutenberg Universitaet, Becherweg 21, Mainz, Germany

We have compiled a comprehensive dataset of 1800 boudin structures from a wide variety of geological provinces worldwide. This dataset has permitted the construction of a robust geometric classification scheme in which all natural boudin structures can be sub-divided into; shearband, domino, sygmoidal, torn and drawn morphological groups (Fig. 1). Shearband boudins are asymmetric with inter-boudin surfaces at low angles (<30°) to the enveloping surface of the boudin train and with curved, sigma-shaped boudin blocks, typically with tapering wings. Domino boudins are also asymmetric, have angular blocky shaped boudins with inter-boudin surfaces at high angles to the enveloping surface (>50°) and in contrast to shearband boudins, often with dilation across the interboudin surface. Sygmoidal boudins are very similar to domino boudins, differing only in having inter-boudin surfaces with sygmoidal trace (either of curved tensiongash-type or angular forked-type). Both torn and drawn boudins are symmetric; Torn boudins have angular, blocky shapes with inter-boudin faces at high angles to the enveloping surface of the boudin train. Drawn boudins have lenticular, ellipsoidal and tapering boudin shapes with appearances suggestive of been ductilely drawn (Fig. 1). Recognition of these morphological groups has permitted the testing of correlations between boudin geometry and numerous kinematic parameters. For example: 1. By comparison with associated stretching lineations (mineral and mineral aggregate lineations), it was found that the extension axis associated with boudin structures (orthogonal to the long axis or neck line of the boudin and contained within the layer enveloping surface) was in all cases sub-parallel to the stretching lineation. Thus boudin structures can be employed to accurately indicate the orientation of the principal axis of the strain ellipsoid (X-axis) in terranes otherwise devoid of stretching lineations, such as in low-grade, lowstrain or fine-grained terranes. 2. There are three kinematic classes by which a layer can be boudinaged; symmetrically without slip on the inter-boudin surface called no slip boudinage (NSB), and asymmetrically with slip on the interboudin surface that is either synthetic or antithetic

with respect to bulk shear sense (Fig. 1). Antithetic slip boudinage (ASB) and synthetic slip boudinage (SSB) have mirror-image symmetries, thus we have investigated the geometry of boudins developed by ASB or SSB as indicated by other independent shear sense indicators. We found that the geometry of asymmetric boudins, in boudin trains parallel to the fabric attractor, were indeed different depending on whether they formed by ASB or SSB. We refer to these two geometric classes as "shearband-type" (formed by SSB in 100% of cases) and "domino-type" (formed by ASB in 98% of cases), which have been quantitatively defined and readily recognised in the field and thus can be employed as shear sense indicators (Fig. 1). In contrast, we found that in all cases where the boudin train is oblique to the fabric attractor, bulk shear sense is synthetic with "forward-rotation" of the boudin train towards the fabric attractor and that all asymmetric boudin geometries (both shearband- and domino-types) formed by SSB. 3. Considering only two aspects of boudinage; geometric class and obliquity of the boudin train with respect to the fabric attractor, we found that the suite of boudins developed in different terranes, or deformational episodes, that experienced different flow regime (ie. pure shear, simple shear, transpressional general shear, transtensional general shear), are markedly distinct in many cases. It was found that symmetric boudin trains form in both coaxial and non-coaxial progressive deformation where the X-Y plane of the strain ellipsoid remains coincident with the enveloping surface of the boudin train throughout deformation, domino boudins have been produced experimentally in coaxial progressive deformation while both shearband and domino boudins have been formed experimentally in simple shear and noted in transpressional terranes. Oblique boudin trains with shearband boudin geometry have been modelled in simple shear and noted in transpressional terranes. However, oblique boudin trains with domino boudin geometry have not been found in transpressional terranes but are developed in pure shear regimes, implying that domino SGTSG abstracts, February 2001

boudins form by either coaxial progressive deformation or by non-coaxial progressive deformation only where the boudinaged layer is approximately coplanar with the fabric attractor. Not-with-standing the problems of applying such findings in the field (ie. evolution in the flow regime with progressive deformation, and how to interpret a suite containing different boudin-types developed in close proximity), boudin geometry in combination with boudin train obliquity can in some cases indicate flow regime, . ^ f ^ f ^

Kinematic

and

geometric

Kinematic Classes

No Slip Boudinage (NSB)

Geometric Classes

Symmetric-types

Morphological Groups

Drawn boudins

]

Fig.1 classification

Tom boudins

boudinage

monoclinic

symmetry.

Antithetic Slip Boudinage (ASB) |

| Synthetic Slip Boudinage (SSB) |

Domino-types

Domino boudins

/./-

SGTSG abstracts. February 2001

| Sygmoidal boudins

Sheartaand-types

| Shearband boudins

Ophiolite obduction, Oman: Passive or non-passive margin behaviour?

D.R. Gray^ and R.T. Gregory^ ^ Department of Earth Sciences, Monash University, Melbourne, Australia 3800 2 Stable Isotope Laboratory, Department of Geological Science, SMU, Dallas, TX 75275

The popular view of Samail Ophiolite emplacement involves Late Cretaceous closure of the former Tethys Ocean by "piggy-back" thrusting with northeast to southwest emplacement of successively lower "thrust slices" from an external oceanic domain over and towards a "passive" Arabian continental margin. The Samail ophiolite nappe, the structurally highest and farthest travelled "thrust-sheet", now overlies "thrustsheets" of Permian-Mesozoic ocean floor and continental rise sediments (Haybi and Hawasina units), and time equivalent Permian-Mesozoic slope and shelf facies carbonates (Sumeini and Hajar carbonate units) that form para-autochthonous "thrust slices" at the base of the pile. The recognition of major NE-facing isoclinal fold closures in these carbonates of both the Saih Hatat and Hawasina windows however, provides a major contradiction for the Oman Mountains. Isoclinal folds that verge away from the Arabian craton have been previously recognised, but have been simply related to back-thrusting. These folds are extensively developed however, have regional scale and extent, and are the major structures of the Saih Hatat dome. Their presence (1) indicates that for at least part of the history the platform to slope carbonates sequences were "thrust" to the northeast, and (2) requires non-passive margin behaviour for part of the ophiolite obduction. In Saih Hatat the upper limb of a major, antiformal fold-nappe is pinned to the relatively undeformed Permian to Cretaceous shelf carbonates of the autochthon, whereas the lower limb is isoclinally folded and intensely deformed. The Hatat Schist (basement of the Arabian margin) is folded into the core of this fold-nappe and the schistosity, stretching lineation, and degree of strain match those of the intensely deformed rocks of the nappe lower limb. The intensity of deformation in the Hatat Schist is relatively uniform across the window, particularly in the northern half of the dome. This necessitates a fault break between Hatat Schist and the relatively undeformed on the N W side of the dome (i.e. upper limb of the fold-nappe). This break must either root into an undeformed basement cover contact to the southwest (i.e. towards the craton), or is part of a

through-going crustal-scale shear that appears to deepen and intersect the Moho to the southwest. The kinematics for the development of the major antiformal fold-nappe requires underthrusting beneath the pinned upper limb, that is the para-autochthonous carbonate of the margin (i.e. underthrusting towards the margin). Ar-Ar geochronology from the Saih Hatat window requires Late Cretaceous ( - 7 6 to 70 Ma) movement of the para-autochthonous Arabian margin rocks to the NE due to underthrusting of the margin. This is an important part of the tectonic evolution (and ophiolite obduction) that has not been previously recognised. Therefore, models involving cratondirected thrusting with domal culminations related to deep-seated, footwall and lateral ramps are too simplistic for the overall tectonic evolution of this part of the Arabian Peninsular. Such models may however, be more applicable to the Tertiary structure and Tertiary evolution of the Mountains.

SGTSG abstracts, February 2001

66 Structure, strain and kinematic constraints on the evolution of the Saih IHatat fold-nappe beneath the Samail ophiolite nappe, Oman D.R. Gray\ J.McL. Miller^'^and R.T. Gregory^ ^ Department of Earth Sciences, Monash University, Melbourne, Australia 3800 2 Stable Isotope Laboratory, Department of Geological Science, SMU, Dallas, TX 75275 3 now at School of Earth Sciences, University of Melbourne, Melbourne, Australia 3052

The Saih Hatat domal culmination of NE Oman defines a major, regional-scale, window into rocks beneath the Samail Ophiolite an obducted slab of former Tethyan oceanic crust. Structurally this culminaton is dominated by a Late Cretaceous antiformal fold-nappe. This fold-nappe is NE facing and closing within Hatat Schist basement and carbonates of the Hajar Supergroup. The upper limb of the fold (southwest side of the dome) consists of relatively undeformed Permian to Cretaceous carbonates of the autochthon, whereas the lower limb (northeast side of the dome) has inverted stratigraphy, is isoclinally folded and intensely deformed. This structure is responsible for a deepening structural level towards the NE, and has produced an apparent NEincrease in deformation across the structural dome (c.f Le Metour et al. 1990). This deformation gradient is reflected by strong fabric (L-S tectonite) development and increasing pressure of metamorphism, culminating in a pervasive schistosity and regional fold-nappes that have isoclinal, sheath-like fold geometry and markedly attenuated stratigraphy along fold limbs. Deformation associated with these early recumbent closures has produced a consistent N- to NE-trending stretching lineation in the upper plate. This is defined by pressure shadows on framboidal pyrites, the long axes of deformed clasts in conglomerate units, mineral lineations defined by white mica, and more rarely by the long axes of pencils that form in units adjacent to the Saiq 2v volcanics. In the upper plate there is a significant increase in strain towards a major structural break where X/Z strain ratios change from -16:1 to over 100:1; X, Y and Z are the maximum, intermediate and minimum principal stretches respectively. Pressure shadows on pyrite are very common in limestones, dolomites, and some quartzites and mafic schists of the upper and lower plates. In the foliation (XY plane) pressure shadows are long, straight to slightly arcuate, with tapered form. When fibres are curved they show a 20° to 40° rotation (commonly clockwise sense) of X during pressure shadow development. In XZ sections

SGTSG abstracts. February 2001

pressure shadow fibre tails are also generally long and straight, although those with slight curvature indicate top to the north shear sense. Strain magnitudes clearly vary with lithology; limestone > dolomite » mafic schist > quartzite. The lower plate has a pervasive schistosity and stretching lineation associated with the formation of regional isoclinal folds with sheath-like form and hinges that are subparallel to the regional stretching lineation. These regional isoclines fold the earlier higher-grade assemblages. Deformed conglomerates and calc-schists within the lower plate reflect a strong component of flattening accompanied by marked stretch in X, producing flattened "cigar-like" forms (constrictional strain) and extensive shear bands that indicate top to the NE shear sense. Strains in the lower plate are more uniform than in the upper plate, and range from 20:1 to 30:1. Strain magnitude and kinematic considerations based on pressure shadows requires Saih Hatat fold-nappe development to involve (1) underthrusting with fold-nappe development in the hanging wall, (2) a pinned, essentially undeformed upper limb, and (3) a rolling hinge' where the upper limb cycles through the hinge into a zone of intense shear strain on the fold lower limb (e.g. Morcles nappe development in the Helvetic Alps),

The two ophiolite belts in Qinling orogen and their constraints on the evolution of the Qinling-Dabieshan orogen, China

K. Guo, C. Zhang and Y. Dong Nanjing Geology Institute, CGEB, 534 East Zhongshan Road, Nanjing, PRC 210093

The Qinling-Dabieshan orogen in central China is one of the most important and well-studied orogens in China. Though the timing of the subduction and collision of the two plates (the Yangtze plate and north China plate) is controversial, most geologists suggest that the Shangdan zone represents the early Paleozoic suture between the north China plate and the Yangtze plate, and that north Qinling was part of the Caledonian orogen (soft collision included). In south Qinling, the recently discovered Mianlue ophiolite is believed to be the Triassic suture zone between Yangtze plate and the south Qinling orogen. Geologists here and abroad are confident that the eclogite in Dabieshan was formed during the Triassic ( 2 0 0 - 2 4 0 M a ) continent-continent collision between the Yangtze and north China plates. The late Paleozoic Mianlue ophiolite provides great evidence to support their Triassic evolution models concerning Dabieshan orogen. In this paper, we have collected the previous data concerning the Shangdan zone and the Mianlue zone and present our own study in Qinling. We would like to propose our interpretation of the timing of formation of the ophiolite, the evolutionary model and the constraints on the evolution of the QinlingDabieshan orogen. These include: 1. A suitable source for the Phanerozoic ophiolite did not exist in the north Qinling orogen, the Songshugou ophiolite was formed before lOOOMa and thrust into the Qinling Group at about 980Ma, the ultramafic and mafic rocks found in the Dangfeng Group, and the Erlangping Group are not typical ophiolite and they may have formed in a micro ocean in the Proterozoic. 2. The ophioUte in the Mianlue zone is Meso- to Neoproterozoic in age and is coeval from the same source as the ophiolite in Mian-Lue-Ning block. 3. The Phanerozoic radiolaria chert in Qinling was formed on a continental base rather than an oceanic one. 4. The new evolutionary model of the QinlingDabieshan orogen suggested in this paper has

displayed the plate tectonic evolution from the Mesoproterozoic to Neoproterozoic, and the Phanerozoic intra-continental A-type-subduction orogeny of Qinling.

SGTSG abstracts. February 2001

68 Comparison of detritai muscovite and detrital zircon dating in the Adelaide fold-thrust belt: Implications for the tectonics of the source areas P.W. Haines\S.P. Turner^, S.P. Kelley^ and J. Wartho^ ^ School of Earth Sciences, University of Tasmania, GPO Box 252-79 Hobart, Tasmania, Australia 7001 2 Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom 3 Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom

Detrital mineral dating is an important tool in provenance studies. Its robustness and near ubiquity in sandy sediments has made zircon one of the most popular minerals for such investigations. Although less utilised, muscovite Ar-Ar dating is also gaining importance in this field. Muscovite is a very common detrital mineral and can remain isotopically closed during weathering, erosion, transport, deposition and diagenesis. However, its different origin and much lower isotopic closure temperature (ca. 350°C), when compared to zircon, provide different but complimentary information about the source terrain. Zircon is ultimately derived from magmatic (plutonic and volcanic) or high-grade metamorphic sources. The ages are generally crystallisation ages, but can be complicated by multiple age zoning. Because of its resistance to physical abrasion and chemical attack, and its high closure temperature, zircon can survive many cycles of erosion and deposition, potentially pass through multiple otogenic events while retaining its original age. In contrast, most detrital muscovite is of medium-grade metamorphic origin, and the dates obtained are cooling ages imparted as the terrain cooled through the closure temperature; rapid cooling is usually facilitated by active uplift and exhumation. Muscovites are much less likely to survive multiple otogenic events without destruction or isotopic resetting. While detrital zircon age spectra will reflect the long-term magmatic and high-grade metamorphic history of the greater source terrain, muscovite age spectra may reveal more about the immediate tectonic history of this area. A significant age peak close to the age of sedimentation suggests rapid exhumation and erosion from a contemporaneous otogenic source. A broad spread of older ages suggests more gradual uplift of a slowly denudating and cooling terrain (or multiple sources). Also significant is the age of any distinct ceiling on the muscovite ages, suggestive of a regional thermal resetting event. Where muscovites and zircons are derived from :he same otogenic event, muscovite ages should typically by younger than plutonic zircon ages, but older than contemporaneous volcanic zircon ages.

SGTSG abstracts, February 2001

Combining the information from zircon and muscovite detrital data sets therefore provides the potential for robust interpretations of provenance history during basin evolution. In this study we selected a suite of detrital muscovite-bearing samples spanning the Neoproterozoic and Early Cambrian of the Adelaide Fold Belt. For comparative purposes we deliberately selected samples from formations (or their assumed equivalents) from which detrital zircon ages have already been published (Ireland et al., 1998). The sampled units were Niggly Gap beds (ca. 800 Ma), Marino Arkose (ca. 660 Ma), Bonney Sandstone (ca. 560 Ma), Carrickalinga Head Formation (ca. 525 Ma) and Billy Creek Formation (ca. 520 Ma). No samples have exceeded lowest greenschist facies metamorphism. Dating of single detrital muscovite grains separated from the samples was carried out using an Ar-Ar laser probe at the Open University, UK. The Niggly Gap beds of the Central Flinders Ranges lie close to the base of the Neoproterozoic sequence and were deposited shortly after the initial opening of an intracontinental rift. Detrital zircon data suggest a Gawler Craton-like source with a dominating c. 1600 Ma peak best explained by the extensive Gawler Range Volcanics (and associated granitoid intrusives), with a smaller contribution from the underlying/peripheral Lincoln Complex. Older peaks can be explained by known older basement, or may be multi-cycle grains. Interestingly, the muscovites are wholly younger than the zircons, with ages starting essentially where the zircons ages stop. As very litde muscovite would come from the Gawler Range Volcanics themselves, it is assumed that these grains are derived from older metamorphic basement. The coincidence between the muscovite ceiling age and the Gawler Range magmatic event suggests that this thermal event was effective in resetting muscovite ages throughout the source area. There is a spread of younger muscovite ages, but all significantly older that the age of sedimentation. This is interpreted in terms of slow and regionally heterogeneous cooling and denudation of the terrain and is not supportive of any

single significant orogenic event in the region during this time, broadly consistent with the known tectonic history of the Gawler Craton region. The Marino Arkose is anomalous unit in terms of provenance. Its immaturity is suggestive of a single cycle basement source. The detrital zircon data is consistent with partial derivation from the Gawler Craton (similar to Niggly Gap beds), but there is a very significant younger (c. 1150 Ma) source, possibly the distant Musgrave or Albany-Fraser Orogens. The muscovite data can also be tentatively interpreted in terms of two sources, the older and dominant being similar to the Niggly Gap muscovite source, the younger reflected only by a single c. 1000 Ma grain. However, more grains need to be analysed before any in depth interpretation can be made of this sample. The Bonney Sandstone is a thick, red, immature sandstone of fluvio-deltaic origin. The sedimentology is consistent with a contemporaneous orogenic source. Age constraints indicated that it was deposited coevally with the Petermann Ranges Orogeny of central Australia, an event that reactivated of the c. 1100-1200 Ma Musgave Orogen, but involved little or no contemporaneous igneous activity. The zircon data are entirely consistent with this source, being dominated by 1100-1200 Ma peaks. The bulk of the muscovite ages are in the 1100-950 Ma range consistent with the cooling phase of this orogenic event. A small, essentially deposition-aged peak (c. 600 Ma), is consistent with exhumation and closure during the Petermann Ranges Orogeny. The Carrickalinga Head Formation is the basal unit of the Kanmantoo Group, and the only unit in that group to locally escape significant Delamerian metamorphism. The zircon data suggests that the Kanmantoo Group had a very different provenance from older sequences in the fold belt, and this is corroborated by our new muscovite data. The zircon ages display a major peak of late Neoproterozoic to Early Cambrian age, and a broad 'Grenvillian peak centred around 1050 Ma. Scattered older zircons back to c. 3500 Ma are presumably multi-cyclic. Although details of the provenance are uncertain, the main peak probably reflects Ross Orogeny magmatism in Antarctica, the initiation of which apparendy preceded that of the contiguous Delamerian Orogeny in Australia. The Carrickalinga Head muscovite age spectra is much cleaner than the zircon spectra, showing one major and one minor peak, essentially coincident with, but sharper than the main zircon peaks. Interestingly, the main muscovite peak is slighdy older than the main zircon peak, suggesting that most of the zircons are from contemporaneous volcanism rather than being plutonic/metamorphic. Uplift/

cooling apparently began around 600 Ma, but as some muscovites ages are very close to that of sedimentation, exhumation was very rapid. The smaller c. 1000 Ma peak indicates that this unidentified 'Grenvillian source was not totally resent by the Ross Orogeny, whereas the older history has been erased. The Billy Creek Formation, an Early Cambrian red bed unit in the Flinders Ranges, has traditionally been correlated with the Kanmantoo Group, but could be slightly younger. No detrital zircon data is available for the unit as such. The muscovite age spectra seems to confirm some affiliation with the Kanmantoo Group, or at least affiliation with a similar source. In this case there are two young peaks, but no muscovites older than about 680 Ma. The older c. 1000 Ma source has disappeared; perhaps by now depleted by continued exhumation such that only reset muscovites are being supplied. A local basement source for the Billy Creek Formation, as hypothesised by several earlier authors, is clearly not present. The Kanmantoo zircon age spectra closely resembles that of younger Palaeozoic sequences of the Lachlan Fold Belt in eastern Australia, suggesting a similar provenance and/or reworking of material via the southern Delamerian Orogen. In an earlier Ar-Ar detrital muscovite study of the basal Lachlan Fold Belt sediments. Turner et al. (1996) concluded that the data supported rapid exhumation of a source in RossDelamerian Orogen around the time that convergent deformation ceased in the Early Ordovician. The current study makes a logical extension of the Ar-Ar detrital muscovite database back to the Neoproterozoic for southeastern Australia. We conclude that the technique can provide useful constraints on the tectonics of source areas and makes a useful compliment to other detrital mineral dating studies.

References Ireland, T.R., Flottmann, T., Fanning, C.M., Gibson, G.M. & Prciss, W.V., 1998. Development of the early Paleozoic Pacific margin of Gondwana from detrital-zircon ages across the Delamerian orogen. Geology 26: 243-246. Turner, S.R, Kelley, S.P., VandenBerg, A.H.M., Fodc,n J.D., Sandiford, M. & Flottmann, T., 1996. Source of the Lachlan fold belt flysch linked to convective removal of lithospheric mantle and rapid exhumation of the Delamerian- Ross fold belt. Geology 2 4 : 9 4 1 944.

SGTSG abstracts, February 2001

70 The Larapinta Event: early Ordovician rifting in central Australia M. Hand and J. Mawby Department of Geology and Geophysics, Adelaide University, Adelaide, SA Australia 5005

Until recently the early to mid Palaeozoic interval in central Australia was thought to reflect a relatively quiescent tectonic regime in which marine sediments were deposited in gently subsiding sub-basins within the broader context of the Centralian Superbasin (e.g. Lindsay & Korsch, 1991). However the recent recognition of widespread early to mid-Ordovician lower crustal granulite and upper amphibolite metamorphism and deformation in the eastern Arunta Block (Hand et al., 1999; Mawby et a l , 1999; Buick et al., 2000) has placed the early Palaeozoic evolution of central Australia in a new context. In the Harts Range region of the eastern Arunta Block (Figure 1), which is located within the metamorphic corridor that separates the Amadeus and Georgina basins, up to 4 kbar ( > 1 2 km) of nearisothermal decompression of early Ordovician granulites that formed at > SOO^C at depths of 3035 km occurred at around 475 ± 4 Ma - 467 ± 8 Ma (Figure 2). The decompression was associated with the development of regionally recumbent upper amphibolite gneissic mylonitic fabrics that formed during NE-SW directed tectonic transport. During decompression of the lower crustal rocks, at least four generations of basaltic dykes, as well as a number of gabbroic plugs were emplaced as well as sheets of megacrystic granite. The early Ordovician basaltic magmatism is by far the youngest recognised in central Australia, and appears to be a continuation of a magmatic province that also produced early Ordovician basalts in the Warbuton Basin at the eastern margin of the central Australian region. Palaeogeographic reconstructions for early Ordovician time slices for central Australia indicate that the high grade Ordovician rocks in the eastern Arunta Block were located in the deep crust beneath the Larapintine Seaway, which extended across central Australia, linking the eastern and western margins of the continent. During the early Ordovician, deposition of sequences belonging to the Larapinta Group (Figures 1,2) in the Amadeus Basin was marked by slow subsidence rates and few breaks in the sequence. Initially sedimentation was primarily located close to, and to the north of the current northern margin of the Amadeus Basin. However with time the basin expanded southward (Figure 1), and sedimentation rates in the northern part of the basin appear to have increased (Shaw, 1991). Although the style of sedimentation during deposition of the Larapinta Group appears to have been unremarkable (e.g. Nicoll SGTSG abstracts, February 2001

et al., 1991), when considered in the context of the magnitude and style of the early Ordovician deformation in the Arunta Block, it is apparent that parts of the Larapinta Group (specifically the Horn Valley Siltstone and Stairway Sandstone) and equivalents in the Georgina Basin, are syn-rift sequences. We propose the term 'Larapinta Event' to identify the phase of early Ordovician rifting in central Australia that was associated with the deposition of the Larapinta Group. The timing and direction of the early Ordovician extension in central Australia is similar to that associated with the initiation of the

Canning Basin in northwestern Australia, and suggests that the Larapintine seaway makes the location of an early Ordovician extensional corridor across Australia. While early Ordovician tectonism produced dramatic features in the lower crust, the upper crustal expression appears to have been far more subtle, at least in the regions that are now preserved. The Larapintine sequences are remarkably uniform and mature sediments, with very occasional growth fault development (Schroder & Gorter, 1984) and no evidence ofvolcanism or coarse clastic input. This lead to the proposal that the Larapinta Group was a thermal sag-phase sequence (e.g. Korsch & Lindsay, 1989) in which the expansion of the basin reflected the on-going thermal decay. The shallow marine character of the Larapinta Group in the Amadeus Basin, combined with very slow over all subsidence rates, which saw - 2km of sediment accumulate over around 35 Ma (Figures 1, 2; Lindsay & Korsch, 1991), suggest that the Larapintine rift axis was not located within the preserved Amadeus Basin. Instead we suggest that the locus of rifting was located within the now exhumed eastern Arunta Block between the Amadeus and Georgina basins. The evolv ng pattern of sedimentation during deposition of the Larapinta Group and the orientation of coeval mineral stretching lineations in the deep crust, suggest that the Larapintine rift was oriented east-west to slightly southeast-northwest. During the subsequent mid-Palaeozoic Alice Springs Orogeny, the rift was inverted, removing the presumed Ordovician intrarift sequences and exposing mid-crustal (> 20 km) metamorphic rocks. The profound inversion of the early Ordovician rift system during the Alice Springs Orogeny has created a remarkable juxtaposition, where the upper and lower crustal domains of an intraplate rift can be viewed side by side. The development of the Larapintine rift appears

to m a r k a c o n t i n u a t i o n o f t h e previously unsuspected

References

deep sub-basin d e v e l o p m e n t a n d basaltic m a g m a t i s m

Buick I.S., Miller, J. Williams, I.S. & Cartwright, L, 2000. Journal of Metamorphic Geology (accepted). Hand, M . Mawby, J., Kinny, P. & Foden, J., 1999. Journal of the Geological Society of London 156: 715-730. Korsch, R.J. & Lindsay, J.R 1989. Basin Research 2, 3-25. Lindsay, J.R & Korsch, R.J. 199L In: Geological and Geophysical studies in the Amadeus Basin, central Australia. BMR Bulletin 236: 732. Mawby, J., Hand, M . & Foden, J., 1999. Journal of Metamorphic Geology 17: 653-668. Nicoll, R.S., Gorter, J.D. & Owen, M. 1991. In: Geological and geophysical studies in the Amadeus Basin, central Australia. BMR Bulletin 236: 277-284. Schroder, R.J. & Gorter, J.D. 1984. The APEA Journal, 19-30. Shaw, R.D. 1991. In: Geological and Geophysical studies in the Amadeus Basin, central Australia. BMR Bulletin 236: 429-462.

that was initiated at a r o u n d 5 2 0 - 5 3 0 M a (Buick et al., this v o l u m e ) b e t w e e n t h e n o w p r e s e r v e d A m a d e u s a n d G e o r g i n a basins. I n total, t h e localised rift basin(s) a c c u m u l a t e d i n excess o f 3 0 k m o f s e d i m e n t i n a h i g h l y compartmentalised system, in w h i c h the sub-basin(s) appear

to have had

little in c o m m o n

with

the

surrounding shallow marine sequences in the A m a d e u s a n d G e o r g i n a basins.

f ^

Figure 1

Upper crust

450

Carmictiael Ss

460

Stokes SItstn

470

Stairway Ss

\ '

Horn Valley SItstn

480

490

Lower crust

\ \

Pacoota Ss

\ \

500 Figure 2 510

Larapinta Group

Upper crustal and lower crustal expressions o f early Ordovician rifting in central

Australia. In (A) the Larapinta G r o u p has been excised in the eastern part o f the Amadeus Basin b y basin inversion marking the onset of the Alice Springs Orogeny.

Unlabelled

areas indicate gaps in the sequence.

SGTSG abstracts, February 2001

The metamorphic expression of the Alice Springs Orogeny M. Hand, B. Bendall and J. Mawby Department of Geology and Geophysics, Adelaide University, SA, Australia 5005

The Alice Springs Orogeny refers to the protracted period of intraplate deformation in central Australia that spanned from around 450 Ma to 310 Ma. The principal expressions of the deformation were the exhumation of the Arunta Block from beneath the cover of the intracratonic Centralian Superbasin, and the deposition of syn-orogenic sediments in the structurally remnant basins (Amadeus, Georgina, Ngalia and Wiso) that flank the Arunta. Given its status as one of the v^orld's classic intracratonic otogenic systems, the affects of the Alice Springs Orogeny have been the subject of a considerable number of studies. These have mainly focussed on the structural, sedimentological and geophysical consequences of the deformation, and also on the primary large-scale strain localisation mechanisms. In contrast, comparatively little is known about the metamorphic evolution of the otogenic belt. To some extent the paucity of metamorphic information reflects the difficulty in identifying mid-Palaeozoic metamorphic features against the complex background metamorphic record of the Arunta Block. However a growing geochronological database means that the metamorphic affects of the mid-Palaeozoic tectonism in central Australia are becoming increasingly identifiable. The metamorphic consequences of the Alice Springs Orogeny are best expressed in the southeastern part of the Arunta Block in a SE-trending corridor that encompasses the Reynolds-Anmatjira Ranges in the central Arunta Block, and the Strangways and Harts Range region further east (Figure 1). Within this corridor, which forms the core of the otogenic belt, mid-Palaeozoic mineral growth ranges from greenschist to upper amphibolite grade, and is primarily restricted to shear zones, which are steeply dipping in the northwest, and regionally reclined to recumbent in the Harts Range. Within this system of shear zones there is the suggestion of discrete metamorphic episodes, suggesting a rather complex thermal picture that may point to the "Alice Springs Orogeny" being a series of events, rather than a single overall otogenic cycle.

SGTSG abstracts, February 2001

450-440 Ma In the Harts Range region, Sm-Nd isotopic data from garnet-bearing shear zones indicates that convergent deformation began at around 449 ± 1 0 Ma (Hand et al., 1999). Deformation occurred under mid-crustal upper amphibolite (6 kbar, 650°C) conditions and was associated with an up-pressure path. In the adjacent Strangways Range, a similar U/Pb, zircon age (Moller et al., 1999) from an amphibolite grade shear zone indicates widespread late Ordovician to early Silurian metamorphism in the eastern part of the Alice Springs Orogen. The convergent deformation associated with 450-440 Ma metamorphism marked the beginning of inversion of a deep, multi-stage Late Neoproterozoic-early Ordovician rife system that was located in the region now occupied by the eastern Arunta Block. This inversion event (the Rodingan Movement; e.g. Shaw et al., 1992), resulted in a major rearrangement of sedimentation patterns in the Amadeus and Georgina basins, with the deposition of clastic sequences (Carmichael Sandstone and Ethabuka Sandstone), and the development of an east-west trending trough in the northern Amadeus Basin, which accommodated the Meerenie Sandstone. This reorganised basin structure signified the initial development of a foreland. T h e relatively high-grade late Ordovician metamorphism appears to mark a continuation of the high-temperature conditions associated with early Ordovician (480-460 Ma) rifting (Hand et al., 1999), suggesting that the onset of mid-Palaeozoic convergent deformation was in part localised by the existing thermal structure. At this stage it is not clear whether the 450-440 Ma metamorphism in the eastern Arunta represents cooling of the early Ordovician hightemperature regime, or the affects of a new thermal event. However what is clear, is that during the following - 1 2 5 Ma, the region affected by early Ordovician rifting was the site of c o n t i n u e d amphibolite to upper amphibolite-grade metamorphism.

390-380 Ma In the northern Strangways Range, Sm/Nd isotopic data from garnet ± kyanite-bearing assemblages developed in east-west-trending, south over north shear zones give ages around 380 ± 24 Ma (this study) and 381 ±7 Ma, (Ballevre et al., 1999). The garnets preserve prograde zonation, and reaction textures are consistent with prograde up-pressure metamorphism to peak conditions of around 600°C and 6 kbar. Further east in the Harts Range, metamorphism occurred under somewhat higher average thermal gradients, with prismatic sillimanite-bearing assemblages indicating temperatures around 650°C and 6kbar, and the generation of locally derived early Devonian granites (Buick et al., this volume). There is some suggestion that the early Devonian metamorphism followed a limited high-T decompression path, with sillimanite overprinting kyanite-bearing assemblages in the northern Strangways Range. The early Devonian metamorphism was coeval with the deposition of the lower sections of the syn-orogenic Pertnjara Group in the northern Amadeus Basin (Pertnjara Movement, see Shaw et al., 1992). 340-320 Ma During the mid-late Carboniferous, regional amphibolite-grade metamorphism affected a large part of the southeastern Arunta Block. In the Reynolds and Anmatjira Range in the central Arunta, LPHT metamorphism (550°-600®C, 4-5 kbar) produced andalusite and sillimanite-bearing assemblages that underwent near-isobaric cooling, resulting in the growth of late kyanite and chlorite-bearing assemblages. The pattern of mid-Carboniferous isograds closely mimics 1580 Ma LPHT isograds in the same area, implying the existence of a long-lived energetic crustal heat source that may reside within voluminous Palaeoproterozoic granites. In the southern Strangways Range, Sm/Nd isotopic data indicates that prograde mid-amphibolite metamorphism reaching a peak of around 600°C, 6 kbar occurred at around 324 ± 6 Ma. The timing of prograde metamorphism is some 60 Ma younger than the strikingly similar metamorphism in the northern Strangways Range, and points to significant metamorphic diachroneity within the anastomosing shear zone system that traverses the Strangways Metamorphic Complex. At this stage it is not known if the locus of metamorphism in the Strangways area shifted very slowly southward with time, or if there was a major hiatus which separated unrelated events in the northern and southern Strangways region respectively. However the '-50 Ma

gap between metamorphism in the southern and northern Strangways Range also roughly corresponds to a long gap in the syn-orogenic sedimentological record in the surrounding basins. Further east in the southern Harts Range, U/Pb monazite and zircon ages indicate that midamphibolite facies metamorphism (600°C, 6 kbar) occurred at around 343±8 Ma (Hand et al., 1999), and that LPHT (>550°C, 4.5 kbar) conditions (associated with voluminous pegmatite emplacement) persisted until at least 330±6 Ma (Hand et al., 1999). In contrast to the mid-Carboniferous metamorphism in the Reynolds and Anmatjira region, petrological evidence and P-T data from the southern Strangways and Harts Ranges indicates that those regions underwent relatively high-T decompression. A reasonable suggestion is that the decompression was linked to exhumation in the hanging wall of the southvergent crustal-scale thrust system (Harry Creek-Two Mile Bore - Illogwa system) that borders the southern margin of the Strangways and Harts Ranges Although the geochronological database is still rather sparse, the emerging age groups correspond reasonably well to the "Tectonic Movements" (e.g. Shaw et al., 1992) inferred from the sedimentological record in the surrounding basins. The recognition of widespread Carboniferous metamorphism in the eastern part of the Alice Springs Orogen has important implications for the patterns and timing of exhumation associated with the overall mid-Palaeozoic intraplate deformation in central Australia. The P-T-t data from the southern Harts and Strangways region indicates that around 20 km of regional denudation occurred after the mid-late Carboniferous, suggesting that the Alice Springs Orogeny was not a simple, long lived orogenic system characterised by a slow strain rate and steady rates of denudation. Rather the style and conditions of Carboniferous metamorphism point to relatively rapid strain rates leading up to the terminal stages of the deformation. An important question that needs to be addressed in order to develop some kind of mechanistic understanding of what drove the midPalaeozoic intracratonic deformation, is whether the 450 -310 Ma tectonic interval represents a single longlived process, or a number of shorter and potentially unrelated events. In the latter case, the superposition of up to three "events" in regions like the HartsStrangways Range region may point to important longterm controls on the location of thermal events in central Australia.

SGTSG abstracts, February 2001

References Ballevre, M. Henscn, B.J. & Moller, A., 1999, In: Hand ct al., Specialist Group in Geochemistry, Mineralogy and Petrology, Field Guide No. 4., Geological Society of Australia: 5-10. Hand M. Mawby, J., Miller, J. & Buick, I., 1999, In: Hand et al., Specialist Group in Geochemistry, Mineralogy and Petrology, Field Guide No. 4., Geological Society of Australia: 11-27. Cartwright, I, Buick, I.S. Foster, D.A. & Lambert, D.D., 1999. Australian Journal of Earth Sciences 46: 355-363. Moller, A. Williams, I.S. Jackson, S., & Hensen, B.J., 1999. Geological Society Australia Abstracts 54: 71-72. Shaw, R. D., Zeitler, PK., McDougall. I. &Tingate, PR., 1992. Journal of the Geological Society of London 149: 937-954.

Figure 1. Metamorphism in the Alice Springs Orogen. Age data: (1) this study, (2) Ballevre et al., (1999); (3) Hand et al., (1999); (4) Buick et al., this volume; (5) Cartwright et al., (1999).

SGTSG abstracts, February 2001

Syn-extensional deposition of the Upper Etheridge Group: an explanation for heterogeneous deformation intensity across the Georgetown Inlier, Queensland

D. Hansen, Q. Hills, M. Krabbendam and D. Giles Australian Crystal Research Centre, Monash University, Victoria, Australia 3800

New interest is being generated in the Proterozoic Georgetown Inlier of north Queensland as recent publications draw parallels between it and the mineralisation-rich provinces of Broken Hill and Mount Isa (Laing, 1996; Laing & Beardsmore, 1986). Recent reconstructions place the Georgetown Inlier at the margin of intracontinental rifting in northern and eastern Australia between 1.8 and 1.6 Ga, with rifting leading to the development of a new basin to the east of the Georgetown Inlier (Giles & Betts, 2000). The reconstructions place the Mount Isa and Broken Hill terranes in contact with each other during Early to Middle Proterozoic, near the centre of the rift in northern and eastern Australia. The Georgetown Inlier is an excellent place to study structural and metamorphic relationships. Here, metamorphism and deformation are contemporaneous with 1550 Ma granites, which are coeval with the Croydon Volcanic Group, a massive extrusion of rhyolite. Therefore, in the western part of the inlier it is known precisely where the Proterozoic surface level was during orogenesis. This should enable the construction of a profile through the Proterozoic orogen from the surface in the west, to mid-crustal levels in the east. However, the situation is complicated by structural isolation of the eastern, central and western parts of the inlier due to faulting or granitoid emplacement.

Changing deformation intensity A notable feature of the Georgetown Inlier is the strong contrast in metamorphic grade and deformation intensity across the inlier. At Stockyard Creek in the western part of the inlier, metamorphic grade in the Upper Etheridge Group metasediments reached greenschist facies. This metamorphic event M l g (Stockyard) ^ ^ coeval with the major deformation event D15 to affect the western part of the inlier. This northsouth shortening event producing large scale (4-12 km) tight folds with an axial planar slaty cleavage S i s defined by biotite. The area was also weakly affected by a second deformation event D25 responsible for the development of localised crenulations. The Lower Etheridge Group metasediments within the Robertson River area in the central part of the inlier record four fabric forming events. The earliest fabric

S1R (Robertson River) ^^ only Seen as quartz inclusion trails

(within garnet porphyroblasts) that are perpendicular to the external schistosity S2j^. The garnet prophyroblasts grew during the first metamoprhic event MIR which outlasted the first fabric forming event DIR. The second deformation event D2R produced a pervasive schistosity S2R defined by chlorite + biotite + muscovite + quartz. The second metamorphic event M2R produced biotite, staurolite and garnet prophyroblasts that preserve quartz inclusion trails continuous with the external schistosity, indicating M2R occurred syn-D2i^. This mineral assemblage suggests middle amphibolite facies conditions. Two later crenulation cleavages S3R and S4J^ were also recognised, with S3R being stronger and more kinklike than S4R. In the east around Einasleigh, metamorphic grade reached granulite facies based on the preservation of two pyroxenes in amphibolites of the Cobbold Metadolerite Suite intruded into the Lower Etheridge Group (Withnall, 1996). More commonly, the layered gneiss complex of the Einasleigh Metamorphics preserve upper amphibolite facies metamorphic grades based on the presence of abundant partial melts and sillimanite. Three fabrics were evident in the field study. What appears to be the earliest fabric Sl£ (£jjjaslcigh) ^^ represented by the alignment of amphiboles within hornfelsed metasediments. The second fabric S2£ comprises the dominant gneissic, commonly migmatitic foliation defined by biotite + sillimanite + melt. S2£ is observed to be variably crenulated, producing the third fabric, S3£. Two generations of melt were observed: (1) first melting event is characterised by pre-boudinage lenses and layers parallel to S2£, and occurred prior to D2£ (2) second melting event comprises syn-boudinage clots in boudin necks and melt pods in fold hinges, and occurred synD2£. Some, but not all melt segregations had adjacent melanosome layers, suggesting both in-situ melting and melt migration occurred.

Correlating across the inlier Deformation phases within the Georgetown Inlier have been both N-S and E-W directed, with at least two N-S shortening events occurring. This makes correlations on the basis of structure between isolated SGTSG abstracts. February 2001

areas problematic. Currently Slg and S2R are considered equivalent (Withnall, 1996) based on the presence of rocks transitional between slates and schists, which this study confirms. If this is the case, the central and eastern parts of the inlier have undergone at least one extra deformation prior to the S1 forming event in the west-central Georgetown Inlier. As yet, it is uncertain if SI are equivalent to the S2£, the dominant fabric in the east. SIr and Sl£ may be equivalent; neither show any clear relationships to folding. Explanations for changing deformation intensity

The increase in structural complexity and metamorphic grade from west to east across the inlier can be explained by several hypotheses. The first and second hypotheses relate to strain partitioning within the crust. One alternative is that more deformation was occurring in the east than in the west, essentially horizontal strain partitioning. This implies that the Robertson River area represents a Proterozoic deformation front. This is the current favoured hypothesis to explain the heterogeneous nature of deformation intensity across the inlier. The other alternative is that the Upper Etheridge Group sediments were not deforming significantly at the surface while the Lower Etheridge Group sediments were deforming plastically and complexly at depth. This hypothesis implies strong vertical partitioning of deformation between upper- (west) and mid- crustal (east) levels. Strain is typically partitioned within the crust by decoupling different crustal levels via detachment faults. During contractional orogenesis deformation is typically focused at upper crustal levels (Harry et al., 1995), while during transtensional tectonics it has been found that deformation can be focused at mid-crustal levels (Krabbendam & Dewey, 1998). This would imply that a detachment fault should exist between the Upper and Lower Etheridge Group, for which there is currently no evidence. The third hypothesis suggests that the Upper Etheridge Group sediments were not deposited until after significant deformation of the Lower Etheridge Group sediments had occurred. This implies that an unconformity should exist between the Upper and Lower Etheridge Groups, of which there is currently little or no evidence. A fourth and preferable possibility is that the Upper Etheridge Group sediments were being deposited during extension of the Lower Etheridge Group. This hypothesis implies that the earliest foliation (and possibly S l£) is the result of a regional syn-depositional extensional basin-forming event. This may provide an explanation for why the Cobbold Metadolerite Suite

SGTSG abstracts, February 2001

is only intruded into the Lower Etheridge Group. It also implies that a paraconformity should exist between the Upper and Lower Etheridge Groups, which would be difficult to identify as it would be parallel to compositional layering and be further obscured by subsequent deformation and metamorphism. Gibson (2000) and Noble (2000) have recently proposed that the earliest fabric SI at Broken Hill was the result of an extensional event during deposition of the Willyama Supergroup, rather than as a result of a shortening event. Extension in the Broken Hill Block occurred -1690 Ma as seen by the Rasp Ridge Gneiss (1682 ± 3 Ma) and the Alma Gneiss (1704 ± 3 Ma) (Noble, 2000). Within the Georgetown Inlier extension may be constrained by the two pulses of Cobbold Metadolerite Suite intruded at 1655 Ma and 1675 Ma (Black et al., 1998). References Black, L.P., Gregory, P., Withnall, I.W. & Bain, J . H . C , 1998. U-Pb zircon age for the Etheridge Group, Georgetown region, north Queensland: implications for relationship with the Broken Hill and Mount Isa sequences. Australian Journal of Earth Sciences, 45: 925-935. Gibson, G.M., 2000. Tectonic evolution of the Paleoproterozoic Willyama Supergroup, Broken Hill: the early years. AGSO Record 2000/ 10: 45-47. Giles, D. & Betts, P.G., 2000. Beyond Rodinia: The Early to Middle Proterozoic amalgamation of Australia and North America. Australian Crustal Research Centre Publication No. 85: 9 pp. Harry, D.L., Oldow, J.S. & Sawyer, D.S., 1995. The growth of otogenic belts and the role of crustal heterogeneities in decollement tectonics. Geological Society of Amereica Bulletin, 107: 14111426. Krabbendam, M. & Dewey, J.R, 1998. Exhumation of UHP rocks by transtension in the Western Gneiss Region, Scandinavian Caledconides. In: Holdsworth, R.E., Strachan, R.A. & Dewey, J.R (eds) 1998. Continental transpressional and transtensional tectonics. Geological Society, London, Special Publications, 135: 159-181. Laing, W.P., 1996. The Diamantina orogen linking the Willyama and Cloncurry Terranes, eastern Australia. In: New developments in Broken Hill Type deposits, CODES Special Publication 1: 67-72. Laing, W.P. & Beardsmore, T.J., 1986. Stratigraphic rationalisation of the Eastern Mount Isa Block, recognition of key correlations with Georgetown and Broken Hill Blocks in an eastern Australian Proterozoic terrain, and their metallogenic implications. Gelogical Society of Australia Abstracts, 15: 114115. Noble, M., 2000. Geology of the Broken Hill Synform, New South Wales, Australia. Unpublished MSc thesis, Monash University. Withnall, I.W, 1996. Stratigraphy, structure and metamorphism of the Proterozoic Etheridge and Langlovale Groups, Georgetown region. North Queensland. AGSO Record 1996/15: 114 pp.

Latest Permian emplacement of the Marlborough Block duplex: the major mountain-building phase of the HunterBowen Orogeny in the northern NEFB

T. Harbort, R.J. Holcombe, P. Vasconcelos and C.R. Fielding The University of Queensland, Brisbane, Qld, Australia 4072

The Hunter-Bowen Orogeny of the northern New England Fold Belt in Queensland is now fairly well understood to have been produced by westwardly propagating fold-thrust belts that initiated at the eastern margin of the continent at about the beginning of the Late Permian ( - 2 7 0 Ma) and persisted episodically until about the end of the Middle Triassic (-235 Ma). Although, locally, syntectonic sediments accumulated in front of, and were subsequently incorporated into, the westwardly-stepping fold-thrust fronts (Fielding et al., 1997) it wasn't until the latest Permian that a major foreland basin system developed to the west of the then active otogenic belt A thick terrestrial sequence then accumulated through the Early and Middle Triassic (Fielding et al., 2000). Toward the end of the Late Permian the active front of the thin-skinned fold-thrust belts was at least as far west as the Gogango-Overfolded Zone (GOZ)/ Connors Arch which was the focus of crustal thickening by thrust duplication (Fergusson, 1991) to that point. Subsequent to development of the first wave of fold-thrust belts, a substantial crustal thickening occurred to the east of the GOZ by an outof-sequence thrust duplex forming the Marlborough Block (Fig. 1). The horses of the Marlborough duplex

are themselves remnants of at least two cycles of earlier, thrust duplicated, slices from progressively shallower crustal sources. Strongly ductile amphibolite-facies shear zones separate sheets of metagranite/metasedimentary schist from an ophiolitic suite. A further set of greenschist facies ductile shear zones emplaces this higher grade sandwich against lower grade metasedimentary rocks of the accretionary complex. Final semi-brittle emplacement of the Marlborough Block has produced at least two tilted duplex horses separated by brittle thrusts and a brittle floor thrust. Erosion has removed the full extent of this complex as well as any overlying rocks. The wider occurrence of magnesite as lag deposits on exhumed Tertiary erosion surfaces suggest that the influence of weathering of ultramafic rocks extended at least 30-50 km southwest of the current western margin of the Marlborough Block. The mechanics of emplacing a thin (1-3 km) Marlborough Block as a thrust nappe sheet over a basal surface of 80-100 Km extent are problematical. We believe that a simple solution is if the Marlborough Block represents the basal duplex detachment zone below a more substantial major nappe sheet (Fig. 2) similar to the small-scale duplexed detachment shown

High Grade Lower amphibolite facies ductile thrust Lower greenschist facies ductile thrust Brittle basal detachment

Low Grade

Figure 1. Schematic E-W cross-section of the Marlborough Block showing the cyclic arrangement of internal thrust sheets. The ophiolitic component is Neoproterozoic MORB cratonised during eadier (Delamerian?) orogenesis

SGTSG abstracts, February 2001

as the cover photograph of the March 1998 issue of

Geology. The duplication of crustal section in such a structure would produce a major NEFB mountain structure and is reminiscent of plateau-building phases in the Andes foreland that also involve superimposed thrust duplication. The timing of f i n a l e m p l a c e m e n t of the Marlborough duplex is provided by the age of Late Permian sediments in the underlying imbricate thrust stacks, and by the age of intrusive rocks that either are cut by the basal detachment or pierce both the detachment and the internal thrusts. Twenty previously published K/Ar ages and three Rb/Sr ages have been supplemented by twenty two new "^^Ar/^^Ar stepheating data. These data constrain emplacement of the Marlborough Block to be - 2 5 3 Ma, that is, latest Permian and that emplacement of the duplex nappe sheet, and intrusion by a substantial granitic complex, occurred over an interval of less than 8 my.

Mariborough Block

The timing of emplacement of the Marlborough Block coincides with the rapid foreland loading of the Bowen Basin at the end of the Permian. We believe this event was triggered by the development of a substantial mountain system along what is now the coastal section of Queensland. Although the Marlborough Block is the only place where a major allochthonous sheet has been recognised, the odd juxtaposition of other terranes in the northern NEFB may also reflect large scale allochthoneity, driven by the same event.

References Fcrgusson, C.L., 1 9 9 1 . Thin-skinned thrusting in the northern New England Orogen, central Queensland, Australia. Tectonics 10: 797-806. Fielding, C.R., Stephens, C.J. & Holcombe, R.J., 1 9 9 7 . Submarine masswasting deposits as an indicator of the onset of foreland thrust loading - Late Permian Bowen Basin, Queensland, Australia. Terra Nova 9: 1 4 - 1 8 . Fielding, C.R., Sliwa R., Holcombe, R.J. & Kassan, J., 2000. A new palaeogeographic synthesis of the Bowen Basin of Central Queensland. Bowen Basin Symposium.

Accretionary nits

Figure 2 . M a r l b o r o u g h B l o c k m o d e l l e d as a t h i n basal d e t a c h m e n t z o n e consisting o f d u p l e x e d slices o f c r a t o n a n d accretionary elements o n w h i c h an o v e r l y i n g n a p p e w a s t r a n s p o r t e d w e s t w a r d s .

SGTSG abstracts, February 2001

structural controls on vein emplacement In porphyry-style mineralisation, Goonumbia, NSW

A.C. Harris University of Queensland, Brisbane, Qld, Australia 4072

Porphyry-style deposits are characterised by an abundance of temporally separated fracturing events, which vary in alteration and mineralisation styles. The brittle fracturing of high level porphyries and the surrounding host rocks is influenced by changing physiochemical properties of hydro thermal fluids and the surrounding tectonic regime. The Endeavour 26 North (E26N) porphyry-style deposit is the largest of several significant copper producing resources of the Late Ordovician Goonumbla Volcanic Complex, in central New South Wales. This deposit will produce some 1.2 Mt Cu and 36.9 t Au. Mineralisation is related to the emplacement of quartz veins distributed around a central quartz monzonite stock. Analysis of vein and fault data from the center of the deposit has led to a structural model for the emplacement of veining during porphyry-style deposit development. In a rock mass, such as at E26N, where the deformation is entirely brittle, the bulk plastic' strain (bulk strain ellipsoid) is accumulated by both orientation dependent volume changes and fault displacements. Separate analysis of the mineralised vein and post-mineralisation fault systems allow the contribution of each of these brittle components to be assessed independently. That is, the bulk strain can be partitioned into volumetric and fault-displacement components. At E26N, quartz veining is sheeted and subparallel, and as such exhibits a preferred orientation at -340. Vein morphology indicates a simple crack-seal dilation history with incremental stretching occurring perpendicular to the vein wall following the development of a serrate microfracture. Therefore, veining represents the introduction of a volume of hydrothermal fluid focused into discrete fracture meshes with a preferred orientation (Figure la). The maximum stretching due to vein emplacement occurs perpendicular to this preferred orientation, where the maximum stretching axis (X') is oriented at 10-265, an intermediate axis (Y') at 32-175 and the Z'-axis oriented at 58-324 (Figure lb). The X', Y' and T axes are equivalent to the bulk finite X, Y and Z axes only if there has been no component of bulk shear along

any of these directions. It is evident that faulting occurred synchronous with veining, however, the displacement along individual veins is so minor as to be not measurable. With the establishment of the orientation of the principal axes of dilational strain, it is possible to quantify the bulk dilatational shape change of the region during quartz vein emplacement. The sum of the quartz vein widths that lie within X, Y, and Z orientation domains have been extracted. Longitudinal strain (elongation) of the volcanic unit in the X'direction is 12.92% while in the Y'-direction longitudinal strain is 2.12% and in the Z'-direction longitudinal strain is 0.45%. These values then constrain the minimum volume of quartz veining at E26N to be -I6,124,800m3. Post-mineralisation faulting at E26N occurs at a deposit scale and exhibits preferred orientations along which consistent slip directions can be measured. Faults are generally high angle, trending towards 290. Faults in this orientation are dextral oblique-slip. Faults trending towards the north have sinistral oblique-slip movement, while those at approximately 340 are normal dip-slip faults. It is inferred that the faulting accompanying mineralised vein systems was equivalent to post mineralisation faulting, such that veining was accomplished by fluid flow occurring into pre-existing (and synchronous) faults. The pumping of fluids into faults resulted in the dilation of the fault arrays normal to the plane. The orientation of high-angle postmineralisation normal faults is consistent with the preferred orientation of veining. It is concluded that fluids have infiltrated the fault arrays at E26N with some preference towards normal faults as they are at a high angle to the bulk extension direction. Associated with vein emplacement is a switch in the Y and Z-axes of incremental finite strain from their positions of the finite strain produced by faulting. Note that the total accumulated finite strain axes will only switch once the amount of SE-NW extension due to NE-SW trending veins exceeds the SE-NW shortening produced during faulting. The apparent switching of the strain axes occurs because throughout SGTSG abstracts. February 2001

t h e b r i t t l e d e f o r m a t i o n at E 2 6 N , t h e v e r t i c a l

axis, but during veining both the other two directions

dimensions remain almost unchanged. D u r i n g pure

undergo extension leaving this vertical axis as the

faulting this makes the vertical axis the intermediate

m m i m u m axis.

Width-Azimuth

Sectorsi2e = 10® Vector mean = 163 Circular variance =0.47 Mean resultant = 0.53 Circular std. dev. = 65° Maximum 10.4% [Width 3095] n=6330 Total width =29705 cm A.

•

>1%

0 3 >2%

Directbn of maxinum diatton (X-axis)

Max 4J09% X'-axis

n = 6330 C.

Plane of maximum stretch X Frequency rosette of the normal to dipping veins in tie preferred orientation

Figure 1. (a) Width azimuth rosette plot of all quartz veins mapped from the 9800RL mine development, illustrating a strong preferred orientation. This plot is constructed as to reduce noise of thin networked veins. It represents a modification of a standard length-azimuth rosette which weights the orientation of the fractures against fracture length; however, due to the inherent thickness of a vein, the orientation of a vein may be weighted against thickness to establish the direction at which veining thickness is maximum, (b) Equal area stereographic projection of6330 poles of normals to quartz vein orientations, uncorrected for traverse bias. The trend of the X-axis is the normal to the preferred vein orientation while its plunge is the intersection of the normal to the preferred orientation and the girdle defined by the poles. The Y-Z plane has the X-axis as its normal. The girdle of the poles to defined as the X-Y plane such that the b-axis is the Z-axis. The intersection of the newly constructed X-Y and Y-Z plane is the Y-axis, (c) Block diagram showing the relationship of the preferred orientation of veining, its dilations and the X-axis.

SGTSG abstracts, February 2001

The Eldee Structure: A kilometre-scale sheath fold at the northwestern margin of the Broken Hill Block, NSW, Australia

Q.G. Hills, D. Giles, G. Rosendam, C. Forbes and G.S. Lister Australian Crustal Research Centre, Earth Sciences Dept, Monash University, Melbourne, Victoria, Australia 3083

Detailed structural mapping in the Eldee region, adjacent to the Mundi Mundi fault in the northwest of the Broken Hill Block revealed the presence of a macro-scale sheath fold. The structure has an elliptical outcrop pattern of approximately 3 . 5 x 2 km (Fig. la). This pattern is defined by a - 5 0 0 m wide layer containing abundant amphibolite and pegmatite within a package of partially melted metasediments (migmatite, sillimanite-biotite-garnet schist and metaquartzite). Previous interpretations (Hobbs et al., 1984) of the Eldee and adjacent Mount Robe structures suggested that they are the result of kilometre-scale dome and basin fold interference. Hobbs et al. (1984) concluded that both structures are doubly plunging synformal anticlines resulting from interference between upright,

N-S trending and NE-SW folds superimposed on the lower limb of an earlier macroscopic recumbent fold. In contrast, Venn (pers. comm.) has suggested that the Mount Robe structure is a modified "sheath" or "condom" fold, which plunges to the south at its northern and southern closures. Macroscopic sheath folds have not previously been recognised in the Broken Hill Inlier. Their occurrence has important implications for the geometry and structural evolution of the region. The Eldee structure is a well exposed feature with a simple elliptical outcrop pattern that is an ideal field location to test the sheath fold hypothesis. Our mapping showed that the northern closure of the Eldee structure is a moderately southwest plunging synform and the southern closure is a moderately

Figure 1. (a) Lithological and (b) structural maps of the Eldee Structure, N W region of the Broken Hill Block (modified after Willis, 1989). SGTSG abstracts, February 2001

southwest plunging antiform (Fig. 1 b). These closures are connected by steeply west clipping limbs defining a slightly flattened cylindrical geometry, which is consistent with the sheath fold hypothesis. The dominant fabric in the Eldee Structure is an L-S mineral fabric that is parallel to compositional and migmatitic banding in metasedimentary rocks and is folded around the Eldee Structure (Fig. 1 b). The fabric is defined by sillimanite (often replaced by sericite) and biotite in metasedimentary rocks and coarsegrained (2-5 mm) amphibole and feldspar in the amphibolites. S-C structures are common, providing evidence for strongly non-coaxial deformation. Whilst the dominant fabric is folded by and predates the Eldee Structure, mineral lineations within the metasediments predominantly plunge to the southwest parallel to the overall plunge of the sheath structure (Fig.lb). These lineations were probably rotated into the extension direction during formation of the sheath fold. In contrast, lineations in the more competent amphibolites were shielded from pervasive reorientation during sheath folding. Instead, they show a range of orientations as a result of bulk rotation of the amphibolite bodies during deformation. Compositional layering and the dominant fabric are folded by similar-style folds with northeast trending axial planes and moderate southwest plunges, most common in the northern and southern closures of the sheath structure. These folds have an axial planar fabric that ranges from a crenulation cleavage to a differentiated cleavage defined by alternating bands of biotite-muscovite and quartz. We interpret the Eldee Structure as a moderately southwest plunging anticlinal sheath fold that has been partially flattened during subsequent NW-SE directed shortening. A pre-existing fabric, parallel to compositional layering in the metasedimentary rocks, is folded around the Eldee Structure, whereas mineral lineations of the same generation were probably rotated toward the extension direction during formation of the sheath fold. Sheath fold geometries appear to provide a better explanation for the elliptical structures in the NW of the Broken Hill Block than dome-andbasin fold interference. References Hobbs, B., Archibald, N.J., Etheridge, M.A. & Wall, V J . , 1984.Tcctonic history of the Broken Hill Block, Australia. In: Kroner, A. & Greiling, R. (Eds). PrecambrianTectonics Illustrated. Stuttgart, Germany. Willis, LL., 1989. Broken Hill Stratigraphic Map. New South Wales Geological Survey, Sydney.

SGTSG abstracts. February 2001

Significance of an amphibolite facies shear zone event synchronous with the emplacement of the Sybella Batholith, l\/lount Isa

E. Hoadley^ M. Rubenach\ D. Coleborn\ I. Sisois^ and M. Fanning^ ^ School of Earth Sciences, James Cook University, Townsville, Qld, Australia 4811 2 Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia 0200

The Sybella Batholith is a sheeted complex of granite and dolerite, exhibiting abundant evidence of magma mingling and mixing that took place prior to and synchronous with the final level of emplacement. Such evidence includes abundant mafic enclaves in granite,

hybridization (including granite-derived phenocrysts in dolerite), crenulate contacts, net veining, and rapakivi texture. The rocks have been heterogeneously deformed by a sequence of events, all of which were previously interpreted as having occurred in the Isan Orogeny (^1610-1500 Ma, e.g.. Bell & Hickey, 1998). Previous age dating placed granite plutons in the Mount Isa area as having intruded in the period 16711655 Ma (Connors & Page, 1995). Geochemical studies indicate that all the granites in the Mount Isa area belong to a coherent suite. A key outcrop in the Kitty Plains area northwest of Mount Isa shows a localized strongly foliated enclave-rich granite (50 m wide) truncated by leucocratic granite possessing a weaker younger foliation. U-Pb dating of zircons from the younger granite revealed an intrusion age of 1673 ±2.5 Ma, indistinguisable from the main phase granite (1671 ± 8 Ma, Connors Page, 1995). The age of the shear zone is therefore constrained as late syn to postemplacement of main phase granites. In the Slaughter Yard Creek and Mica Creek areas, strongly foliated xenoliths of typical porphyritic main phase granite occur in leucocratic granite, along with foliated and folded xenoliths of quartzite and gneiss very similar to the adjacent May Downs Gneiss. In the Mica Creek area, weakly deformed leucocratic granite is in contact with strongly foliated and folded calcsilicate gneiss and amphibolite of the lowermost Eastern Creek Volcanics. It is therefore concluded that at least the lower sections of the Haslingden Group were deformed in the synSybella shear zone event. The new results have implications not only for the emplacement of Sybella Batholith, but also for the development of the cover sequence 3 sedimentary basins, . y f ^

References Bell, T.H. & Hickey, K A . , 1998. Multiple deformation with successive subvertical and subhorizontal axial planes in the Mount Isa Region: Their impact on geometric development and significance for mineralization and exploration. Economic Geology 93: 1369-1389. Connors, K.A. & Page, R.W., 1995. Relationship between magmatism, metamorphism and deformation in the western Mount Isa Inler, Australia. Precambrian Geology 71: 131-153

SGTSG abstracts, February 2001

84 Metamorphic differentiation B.E. Hobbs, H-B. Muhlhaus, A. Ord and L.N. Moresi CSIRO Division of Exploration and Mining, 39 Fain/vay, Nedlands, WA, Australia 6009

We examine the influence of the migration of chemical constituents, driven by gradients in normal stress, upon the evolution of fold geometry in anisotropic, linearlyviscous, multi-layered materials. The layering consists of an alternating sequence of high viscosity, chemically mobile rock and lower viscosity, chemically immobile rock. As a natural analogue, the high viscosity, chemically mobile layers could be quartz rich whilst the low viscosity, chemically immobile layers could be mica rich The deformation is assumed to be incompressible, with temporarily and spatially evolving volume fractions of the mobile and immobile constituents. The conservation equation for the chemically mobile rock relates the material rate of the volume fraction to the divergence of the in-layer gradient of the layer-normal traction. The latter expresses the assumption that the layer interfaces are the main conduit for mass transport. We present a large deformation formulation for a layered, viscous material. Geological modeling frequently presents the need to track material composition, pressure, temperature and stress histories through extreme levels

SGTSG abstracts, February 2001

of deformation including thermal convection. We illustrate our model in a linear instability analysis first and subsequently in a number of particle-in-cell (PIC) simulations. The particle-in-cell finite element method combines many of the advantages of traditional finite elements with the geometrical flexibility of pure particle methods. The method is particularly suitable for problems involving very large deformation without the need for re-meshing. The PIC simulations show the initial development of short wavelength crenulations' in the multi-layered, anisotropic, viscous materials with no obvious development of large wavelength buckling instabilities. At a later stage in the deformation, these short wavelength crenulations are distorted to form the axial plane structure for larger wavelength folds. Chemical migration of the mobile phase now accentuates this axial plane structure with the progressive development of a metamorphic differentiation layering oblique to the initial layering. At high shortening a new phenomenon takes over with the development of chemical boudinage' which is rapidly amplified by continuing deformation.

Anatomy of the Mount Isa Fault system: thrust exhumation of middle crustal elements of the Mount Isa Rift basement during the late Isan Orogeny

R.J. Holcombe, R. Gordon, C. Pratt, M. Carder The University of Queensland, Brisbane, Qid, Australia 4072

The contractional Isan Orogeny in the Mount Isa Inlier of western Queensland, is generally regarded as a multiphase event spread episodically over a -100 my interval from -1600 Ma to -1500 Ma. It terminated a protracted extensional interval in which rocks of the Haslingdon and Mount Isa Groups accumulated in the superposed Leichhardt River and Mount Isa Rifts at -1800 Ma and -1650 Ma, respectively. An extensive fault system, the Mount Isa Fault, separates high-grade rocks of the Haslingden Group (and older rocks) to the west from lower-grade equivalents to the east, as well as from rocks of the overlying Mount Isa Group. The complex system of faults associated with this boundary is well-defined in deep seismic imagery. We present the results of detailed mapping and geometrical analysis (based on -6000 field observations) of the structure across the Mount Isa Fault system -10 km southwest of Mount Isa in the Mica Creek area. Multiple generations of metamorphic structural fabrics associated with two discrete crustal events characterise the rocks. Many of the results simply confirm previous work but major conclusions that emerge from this study are: 1. The existence of a low Isan strain window of middle crustal remnants of the ^-1650 Ma Mount Isa Rift system. The earliest fabric recognised is a stratigraphy-parallel (subhorizontal) foliation associated with mid-crustal emplacement of syntectonic granitoid bodies (Sybella Granite) at -1650 Ma. We believe that these rocks represent ductilely stretching middle crust at the brittle-ductile transition immediately below the evolving brittle Mount Isa Rift system. Constrictional fabric geometry and kinematics are consistent with development in a transtensional system with dextral shear on northwesterly diverging N-S oriented rift boundaries. 2. The Mount Isa Fault system, separating low grade rocks in the east from high grade rocks in the west, is a system of east vergent, eastwardly propagating, thrusts. All remaining fabrics and folds are associated with the sequential eastward develop-

ment of three, increasingly less ductile, fabrics, each associated with the forward propagation of an eastwardly verging, thin-skinned thrust system (and associated folds). The Mount Isa Fault system itself is a distincdy more brittle and a more thick-skinned, reverse separation structure, and may have a significant post-Precambrian displacement component. The oldest of the fault-related ductile fabrics ($2) is most intensely developed around the eastern faulted margin of the Sybella Granite and overprints the early syn-intrusion fabric. We interpret this fabric as related to a local ramp in a thin-skinned east vergent thrust system. We attribute theological problems associated with moving the 1-2 km thick Sybella granite sill through this ramp as the possible driving force for the subsequent development of kilometre-scale eastvergent asymmetric folds, and development of the stepforward imbricate thrust fault system, that becomes progressively more thick-skinned to the east. A wide- , spread west-dipping foliation (S3) accompanies the formation of the macroscopic fold system, which, with its associated faults, provides the dominant control on the structural grain of the area. Within 2 km of the more steeply west dipping Mount Isa Fault system proper, a spaced cleavage (S4) accompanies gentle refolding (wavelengths of 100s of metres) of the earlier faults and macroscopic folds. The surface expression of the Mount Isa Fault itself is almost entirely brittle, although weak (F5) folds occur locally. 3. The Pre-Isan orogeny basin geometry consisted of westerly-tilted half-grabens. Palinspastic reconstruction of the thrusts and folds indicates that, prior to thrusting, the entire Haslingdon Group sequence dipped shallowly to the west (-20®), thus accounting for the subsequent juxtaposition of different metamorphic grades at approximately the same stratigraphic level (e.g. Eastern Creek Volcanics) across the Mt Isa fault system. We suggest that this tilting is related to the geometry of the evolving Isan rift system at -1650 Ma. The westward tilt suggests half-graben geometry with a depocentre toward the

SGTSG abstracts, February 2001

west. Similar tilted strata are apparent in the published deep seismic transect, as are early east-dipping faults that may be the half-graben fault margins. 4. Structures associated with Isan orogenic contraction did not appear in the western part of the Mount Isa Inlier (around Mount Isa mine) until late in the reported range of Isan ages (at -1535 Ma) and continued perhaps for an interval of '-30-40 Ma. The oldest of the fault-related ductile fabrics (S2) correlates with both peak metamorphism and pegmatitic partial melt accumulations in the granite. Zircon ages of -1532 Ma from this generation of peak-metamorphic pegmatite have previously been obtained by Connors 6c Page, 1995. Other pegmatite generations span the subsequent ductile fabric, and these authors report zircon ages as young as 1480 Ma. Our findings are consistent with these results and suggest that the brittle Mount Isa fault system is no older than this age (and may have considerably younger components of movement). ^^^IFf^

References

Connors, K.A., 1992. Tectonothermal evolution of the Mount Novit Ranges, Mount Isa Inlier, Australia. Unpublished PhD thesis, Monash University. Connors, K.A. & Page, R.W., 1995. Relationship between magmatism, metamorphism and deformation in the western Mount Isa Inlier, Australia. Precambrian Research 7 1 : 131-153. Drummond, B.J., Goleby, B.R., Gocharov, A.G., Wyborn, L A I . , Collins, C.D.N., MacCready, X , 1998. Crustal-scale structures in the Mount Isa Inlier north Australia: their response and influence on mineralisation. Tectonophysics 228: 43-45.

SGTSG abstracts, February 2001

Reconstruction of an Archaean granite-greenstone terrane: an example from the Sylvania Inlier, WA

D.A. Hollingsworth, P.A. Cawood & B. Monek Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA, Australia 6845

The Sylvania Inlier is a part of the Pilbara granitegreenstone terrane that is exposed to the south of the Hamersley Province of Western Australia (Fig. 1). The Inlier represents a basement high that was structurally active at around 2.2 Ga during the Ophthalmian Orogeny; however, until recently the structural history prior to this event was poorly understood. Recent work within the Jimblebar Greenstone Belt in the eastern part of the inlier (Fig. 2), has revealed a long and complex deformation history prior to 2.8 Ga, and the identification of sedimentary structures (Monek, 1999) and igneous layering in mafic plutons (Tyler, 1991) has enabled the recognition of a coherent stratigraphy. A pre-existing basement of diorite and granodiorite is inferred from its presence both as clasts within sedimentary rocks of the greenstone sequence and as xenoliths within the surrounding granitoids. The lowermost mappable unit comprises quartz-garnetamphibole schist, for which an intermediate volcanic protoUth is inferred. This unit is succeeded by a sequence of siliciclastic and chemical sedimentary rocks, interleaved with mafic and ultramafic volcanic flows. This supracrustal sequence was then intruded by large volumes of mafic to ultramafic magma, resulting in formation of layered intrusions. Relict igneous textures are preserved in isolated low-strain zones throughout the belt, allowing the recognition of both intrusive and extrusive mafic to ultramafic rocks. Sedimentary rocks present within the belt include conglomerate, quartzite, pelitic schist, chert and banded iron formation. Within the Jimblebar Greenstone Belt and adjacent. granitoid rocks of the Sylvania Inlier, six deformation events are recognised that pre-date deposition of the -2775-2300 Ma Mount Bruce Supergroup. The relative deformation chronology was determined from consistent crosscutting and overprinting relations observed throughout the greenstone belt and adjacent granitoid rocks. These relationships permit the progressive restoration of successive deformation events, providing insight into the original depositional relationship of the rock units. This allows the initial assembly of the granite-greenstone terrane to be accurately reconstructed. The first three of these deformation events were ductile compressive events, with the first event (Dj) producing thrusts, sheath folds and a pervasive fabric.

The remaining ductile events produced open to tight folding (D2 and D3). The remaining three events are fault- and shear-related events. The major faulting event (D4) in the greenstone belt pre-dates emplacement of the ^Ivania granitoids, and the D5 and D5 shearzone events are confined to the granitoid rocks and pre-date the 2.8 Ga unconformity with the overlying Mount Bruce Supergroup. The first stage in reconstruction of the Jimblebar Greenstone Belt involves the removal of the eflFects of post-Archaean deformation, which chiefly comprises: (i) minor offset of the greenstone stratigraphy across a series of northwest-trending oblique-slip faults, and (ii) Ophthalmian reactivation of many of the D4 faults in the greenstone belt, plus shearing of the western contact of the greenstone belt with the adjacent granite. The extent of D4 reactivation is quantifiable using offset on the post-D4 granitoids that pre-date the Ophthalmian Orogeny. The youngest Archaean deformation events (D5 and D^) in the restoration of the Jimblebar Greenstone Belt are restricted to the postD4 granitoids that surround the belt and need not be considered in the restoration of the Jimblebar Greenstone Belt. Emplacement of the granitoid rocks resulted in stoping and wedging of the greenstone stratigraphy and their removal requires only minor reorientation of the greenstone belt. Restoration of the greenstone belt to its pre-D4 state was achieved by realigning stratigraphic units to the north and south of the major E-W trending D4 fault. Restoration of the D3 event required both the unfolding of the F3 structures around the mean F3 axis and minor changes to unit thickness as a result of fold limb thickness variations. After unfolding of the F3 structures throughout the greenstone belt, a macroscopic F2 syncline is clearly evident and its upright, symmetrical nature strongly suggests that the greenstone sequence was essentially horizontal prior to D2. Restoration of the Dj event was achieved via the removal of drag folds and repeated stratigraphic units associated with thrusting. The age and complex deformation history preserved in many Archaean granite-greenstone terranes oft:en renders structural restoration difficult or impossible, and the amenity of the Sylvania Inlier to such reconstruction has important implications for the evolution of polydeformed greenstone sequences SGTSG abstracts. February 2001

elsewhere. In addition, unraveling of such deformation histories provides insight into the nature o f deformation styles and processes during the Archaean. The crustal architecture of the Sylvania Inlier contrasts with the older (pre-3.0 Ga) dome-and-keel provinces of the northern Pilbara (e.g. Hickman, 1984), and the existance o f early structures in the Jimblebar Greenstone Belt which suggest compression in a ductile deformation regime possibly indicates a significant horizontal componant in tectonic force. The Jimblebar Greenstone Beltis older than most Archaean provinces for which terrane accretion models have been proposed (e.g. eastern Yilgarn; Campbell & Hill, 1992) and the possibility that the Jimblebar Greenstone Belt represents an intermediary between such provinces and Early Archaean dome-and-keel terranes has important

implications for the evolution of tectonic processes through the Archaean,

References

Campbell, I.H. & Hill, R.L, 1992. A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area. Western Australia. Earth and Planitary Science Letters 90: 11-25. Hickman, A.H., 1984. Archaean Diapirism in the Pilbara Block, Western Australia. In: Kroner, A. & Greiling, R., eds. Precambrian Tectonics Illustrated: 113-127. E. Schweizerbartische Verlagbuchhandlung, Stuttgart. Monek, B., 1999. Structural history of the Jimblebar Greenstone Belt, Pilbara Craton, Western Australia. B.Sc (Hons) thesis, Curtin University of Technology. Tyler, I.M., 1991. The geology of the Sylvania Inlier and the Southeast Hamersley Basin. Geological Survey of Western Australia, Bulletin 138.

Figure 1.

. !

. ; l„Figure 2

Regional m a p showing the location o f the Sylvania

Inlier in relation to the major exposure o f the Pilbara granitegreenstone terrane.

Mount Bruce Supergroup , (2775 - 2300 Ma)'

Sylvania Inlier Post-D basins

J Mount Bmce Sypei-gEX>up Unconformtty

Post«D intnisions

1 Megacrystic Granitoid i Granitoid

SGTSG abstracts, February 2001

Jimbiebar Greenstone Belt (pre-D )

Layered Intrusions Ullramafic Gabbro Fau.lt

F ^ UndifTefentiated F

Sediments Basalt tntermediale volcanics

Figure

A simplified

map

showing the distribution o f rock units within, and adjacent to, the Jimblebar Greenstone Belt.

Defining Tasmania's deformation history - geochronology of metamorphic monazites

O. Holm^ R. Berry^ and D. Steele^ ^ Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79 Hobart, Tasmania, Australia 7001 2 Central Science Laboratory, University of Tasmania, GPO Box 252-74, Hobart, Tasmania, Australia 7001

Monazite is a light rare earth element (LREE) phosphate ((Ce, La, Th, Nd) PO4) that occurs widely as an accessory mineral in variably metamorphosed pelites (Overstreet, 1967; Parrish, 1990). Monazites have been shown to form in sub-greenschist to granulite facies conditions, being evident in slates, schists and upper amphibolite to granulite facies rocks

of pelitic composition (Overstreet, 1967; Bons, 1988; Parrish, 1990). The abundance of metamorphic monazite increases with progressive metamorphism (Overstreet, 1967; Smith & Barreiro, 1990), and they are being increasingly used to date metamorphic events (Eusden & Barreiro 1988, Smith & Barreiro 1990). Monazites are thought to have lower closure temperatures than zircon and therefore are useful thermochronometers of medium to high temperature metamorphic events (Kingsbury et al., 1993). In comparison to zircon though, they are still relatively under-utilised in U-Pb geochronological studies. As monazites do not incorporate appreciable common lead during growth, the Pb present is radiogenic, having been derived from the decay of Th and U (eg. Scherrer et al., 2000). Whilst certain micro-analytical methods (eg. ion microprobe, LA-ICP-MS) allow dating of relatively young monazites (<200 Ma), the electron microprobe (EMP) enables the dating of older monazites relatively easily and cheaply (Scherrer et al., 2000). The grains can be quickly located (using a back scattered electron detector) and analysed (in the EMP) in thin section. The EMP has the additional advantages that small grains 10-50 |ULm in diameter can be analysed non-destructively (and hence repeatedly), unlike with the ion microprobe and by LA-ICP-MS. The EMP monazite dating technique described above has been utilised in this study to analyse numerous samples from northwest Tasmania together with samples from offshore dredges on the South Tasman Rise (Figure 1). The aim of this work is to aid the resolution of several contentious aspects of the Mesoproterozoic to Devonian geological history of Tasmania. These include the age and significance of the deformational events, and by comparison with other regions, will aid in determining the proximity of Tasmania to the Australian mainland during this

period. Whilst these issues have been partly resolved by recent workers (Berry et al., 1997; Black et al., 1997; Turner et al., 1998; Calver, 1998; Calver & Walter, 2000; Mefifre et al., in press), their findings leave many questions unanswered. There are some striking similarities between the depositional and deformational history of theTasmanian mainland, King Island and southeast Australia from the Neoproterozoic onwards. These include stratigraphic correlation of the segments of the rift-related sedimentation in the Neoproterozoic, and the Middle Cambrian Tyennan/ Delamerian Orogeny. Importantly though, there are major differences between the different regions during and prior to the major Neoproterozoic sedimentation represented by the Adelaide Rift Complex in South Australia, the Grassy Group on King Island and the Ahrberg and Togari Groups in Tasmania. Intermixed with the Neoproterozoic rift sediments on King Island and in northwest Tasmania are large volumes of rift-related basalts, unseen in the Adelaide Rift Complex. Large amounts of 760-780 Ma granitoid together with associated amphibolite facies metamorphism and multiple folding events are present on King Island. In northwest Tasmania, a thin discontinuous unit of 780 Ma granitoid is present in an allochthonous slice of the Arthur Lineament. Other than a low angle unconformity, no significant deformation is thought to represent the 760-780 event in Tasmania. In southeast Australia, the only igneous event of this age

Rocky Cape Gp Arthur .. (Figure 2a) uneament Togari Gp;;. . / yk.ni.inrsr^H Prr

Ahrtjerg Gp^ (Figure 2b)

Figure 1 SGTSG abstracts, February 2001

is the occurrence of a 7 8 0 Ma rhyolite below the base of the Burra Group (Preiss, 2000). Small monazites (5-80mm in diameter), interpreted to be metamorphic in origin occur in lower greenschist to lower amphibolite facies mudstones over a wide area in northwestern Tasmania. By analysing these grains using the EMP technique it is possible to distinguish Neoproterozoic events (Figure 2a) from those of Cambrian age (Figure 2b) across Tasmania. Within the Rocky Cape Group mudstones (Figure 2a) two populations of monazites appear to be present, possibly reflecting the overprinting of the Neoproterozoic metamorphism by the Cambrian event. Not only does this help in defining the regional extent of different deformational events, it has major implications for Tasmania's position at the time of breakup of Rodinia.

References Berry, R.F, Meffre, S. & Kreuzer, H., 1997. Metamorphic rocks from the southern margin of Tasmania and their tectonic significance. Australian Journal of Earth Sciences 44: 609-619. Black, L.P., Seymour, D.B. & Corbett, K.D., 1997. Dating Tasmania's oldest geological events. Australian Geological Survey Organisation Record 1997/15. Bons A.J., 1988. Intracrystalline deformation and slaty cleavage development in very low^ grade slates from the central Pyrenees. Geologica Ultraiectina 56.

Calver, C.R., 1998. Isotope stratigraphy of the Neoproterozoic Togari Group, Tasmania. Australian Journal of Earth Sciences 45: 865874. Calver, C.R. & Walter, M.R., 2000. The late Neoproterozoic Grassy Group of King Island, Tasmania: correlation and palaeogeographic significance. Precambrian Research 100: 299-312. Eusden, J.D. & Barreiro, B., 1988. The timing of peak high grade metamorphism in central-eastern New England. Maritime Sediments and Atlantic Geology 24: 241-255. Kingsbury, J.A., Miller, C.R, Wooden, J.L. & Harrison, T M . , 1993. Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, USA: implications for thermochronometry. Chemical Geology 110: 147-167. Meffre, S., Berry, R.R & Hall, M., 2000. Cambrian metamorphic complexes in Tasmania: tectonic implications. In press. Overstreet, W.C., 1967. The geological occurrence of monazite. United States Geological Survey, Professional Paper 530. Parrish, R.R., 1990. U-Pb dating of monazite and its applications to geological problems. Canadian Journal of Earth Sciences 27: 1431-1454. Preiss, W.V., 2000. The Adelaide Geosyncline of South Australia and its significance in Neoproterozoic Continental Reconstruction. Precambrian Research 100: 21-63. Smith, H.A. & Barreiro, B., 1990. Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists. Contributions to Mineralogy and Petrology 105: 602-615. Scherrer, N.C., Engi, M., Gnos, E., Jakob, V. & Liechti, A., 2000. Monazite analysis; from sample preparation to microprobe dating and REE quantification. Schweiz. Mineral. Petrogr. Mitt. 80: 93-105. Turner, N.J., Black, L.P &Kamperman, M., 1998. Dating of Neoproterozoic and Cambrian orogenies in Tasmania. Australian Journal of Earth Sciences 45: 789-806.

Figure 2a

Figure 2b

SGTSG abstracts. February 2001

Evidence for 800 Ma and possibly older Deformation and Plutonism in Madagascar

B. HulscherS A.S. Collins^, K.L. DahP, I.C.W. FitzsimonsS S.P. Johnson\ M.K. Jonsson^, A.R. Passmore\ C.McA. PowelP Tectonics Special Research Centre, The University of Western Australia, Nedlands, WA, Australia 6907 ^Tectonics Special Research Centre, Curtin University of Technology, GPO Box U 1987, Perth, WA, Australia 6845

Madagascar occupies a key tectonic position for unravelling tectonic events that occurred during the break-up of Rodinia and the subsequent amalgamation of Gondwanaland. Central Madagascar consists of extensively reworked -2.5 Ga Archaean crust overlain by a belt of Proterozoic supracrustal rocks, the Itremo Group: a succession of quartzite, pelite and carbonate that has been metamorphosed at sub-greenschist to upper amphibolite-facies in the eastern half of the belt (Moine, 1968). The age of deposition is broadly constrained only, between 1855 and -800 Ma (Cox et al., 1998): the maximum age is given by detrital zircons in the Itremo Group and the minimum by the intrusion of 804-779 Ma gabbros and granitoids, which are part of a -450-km-long belt throughout central and north Madagascar (Handke et al., 1999). A second phase of dominantly granitoid emplacement occurred at 570-539 Ma (Tucker et al., 1996), and is generally considered to be related to the final stages of Gondwanaland amalgamation. A major task in Madagascar is to separate the Gondwanaland assembly deformation from earlier tectonic events. A detailed study of the tectonic setting of the -800 Ma plutons sheds light on the timing and nature of early deformation in the Itremo Group, the presence of which was hinted at by Cox and others (1998), but not documented in detail. We present structural and metamorphic evidence for an early deformational event that was synchronous with greenschist- to lower-amphibolite-facies regional metamorphism. This deformation locally predated the intrusion of -800 Ma granitic and gabbroic plutons, and is overprinted by high-T-low-P metamorphism in metasediments with peak conditions > 650°C and < 3 . 5 kbar during intrusion. The Itremo map sheet (Moine, 1968) delineates three large - 8 0 0 Ma elongate batholiths whose relationship to the supracrustal sediments has been controversial, as many contacts are now tectonic. The plutons range from little deformed to highly strained, the deformation generally being attributed to crustal shortening in the 6 3 0 - 5 5 0 Ma interval during

Gondwanaland assembly. However, clear intrusive relationships can be observed at several localities along the eastern margin of the Imorona batholith (-1200 km2) where the 791.4 ± 1.3 Ma porphyritic K-feldspar East Imorona Granite and coeval 793.1 ± 1.5 AmbohitsaonyGabbro (Handke etal., 1999) cause contact metamorphism along steep contacts with country rocks. The contacts are parallel to a weak magmatic/tectonic foliation in the granitoid and the aureole contains a weakly foliated actinolite/plagioclase hornfels, with the amphibole both forming and weakly overgrowing the Sj fabric. In a nearby metapelite, coarse muscovite overgrows a fine-grained foliation of muscovite, biotite and quartz. Some biotite wraps around the coarse muscovite and locally forms a disjunctive secondary cleavage. Fibrolitic sillimanite and tourmaline, both associated with the granitoid intrusion, overprint the S^ biotite foliation. At another contact of the granitoid and metapelite, a metamorphic assemblage of muscovite, biotite and quartz forms a well-developed S^ foliation that also contains fibrolitic sillimanite. This fabric is folded at high angles by open to tight F2 folds, which locally develop a disjunctive secondary cleavage. The metamorphic growth locally outlasted D2 as some of the sillimanite occurs as random fibrolite crystals and aggregates in both hinges and limbs of F2 folds. The fibrolite is temporally associated with growth of metamorphic K-feldspar, implying that contact metamorphism occurred above the second sillimanite isograd. Cordierite is also present. Field evidence supports both the high-grade nature and the syn-tolate D2 timing of contact metamorphism as there is widespread in-situ relict melt in the country rock adjacent to the granitoid. The relict melt veins crosscut a pre-existing penetrative fabric in the country rock. Furthermore, the granitoid contains compositionallylayered enclaves of surrounding calc-silicate rocks (skarn), which contain early K-feldspar-bearing melt veins that have been folded. Additional evidence for syntectonic intrusion during D2, is found in the contact aureole of a SGTSG abstracts. February 2001

continuation of the ca. 792 Ma granitoid. Calc-silicate rocks of the Itremo Group in the aureole are diopsidescapolite-bearing skarns. Several enclaves, up to 50 cm long, are found in the granitoid -160 m away from the contact. They have sharp boundaries and contain a weak fabric that is both defined and weakly overgrown by actinolitic amphibole. The similarity of the textural relationship and the composition to that of the hornfels suggests that the enclaves also had an Itremo Group protolith. One - 3 m long, foliated enclave forms a rootless, type-3 refolded isoclinal fold. The axial planes of the refold are parallel to a weakly developed foliation in the granitoid, defined by biotite wrapping around weakly aligned porphyritic Kfeldspar grains. A felsic vein crosscuts the enclave fabric at high angles and is folded by open folds parallel to the granitoid fabric. Some euhedral prismatic Kfeldspar phenocrysts lie at high angles to this weak, macroscopic foliation. Textural relationships in quartz and feldspar indicate only minor solid-state deformation, which occurred at temperatures greater than 500°C. The relationships in the enclaves and contact aureoles suggest three major conclusions: 1. Intrusion of the c. 792 Ma granitoid occurred after the formation of a penetrative fabric in the Itremo Group metasediments. 2. Intrusion produced sillimanite and muscovite, and sillimanite, K-feldspar and cordierite assemblages in metapelites, and diopside + scapolite assemblages in calc-silicates, indicating local peak temperatures of at least 650°C and pressures <3.5 kbar. 3. The D2 deformation of the granitoid and the country rock sediments was broadly coeval with intrusion. The deformation in the Itremo Group platform sediments around -800 Ma and possibly earlier has implications for late Mesoproterozoic and Neoproterozoic continental configurations. Did Madagascar lie at the edge of Rodinia facing the Mozambique Ocean one billion years ago or did it lie in the interior of a continent that broke up around 800 Ma? The nature of the early deformational event also remains enigmatic. Regionally, stratigraphic overturning of the sedimentary succession indicates the presence of large-scale lower limbs of early recumbent folds that are locally older than the granitoid and gabbro intrusions. More work is needed to establish whether these folds resulted from thickening during early crustal shortening, or from thinning along crustal-scale, low-angle extensional shear zones during continental break-up. . y f ^ ^

SGTSG abstracts, February 2001

References Cox, R., Armstrong, R.A. & Ashwal, L.D., 1998. Sedimcntology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana paleogeography. Jour. Geol. Soc. London 155: 1 0 0 9 - 1 0 2 4 . Handke, M.J., Tucker, R.D. & Ashwal, L D . , 1999. Neoproterozoic continental arc magmatism in west-central Madagascar. Geology 27:351-354. Moine, B., 1968. Carte du Massif Schisto-Quartzo-Dolomitique. 1/ 200.000. Antananarivo, Madagascar: Service Geologique de Madagasikara, 1 sheet. Tucker, R.D., Handke, M.J. & Ashwal, L.D., 1996. New isotopic ages and geological perspectives of the Precambrian rocks of North and north-central Madagascar. Geol. Soc. Amer. 28th meeting. Abstracts. 28(7): 230.

Three-dimensional geometrical, petrophysical and geophysical modelling of the Cannington deposit, eastern Mount Isa Inlier

T.H. Huynh and L. Ailleres Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton, Victoria, Australia 3800

The Cannington deposit of silver, lead and zinc is located in the immediate environs of the Eastern Fold Belt, approximately 10 km southeast of the exposed Mount Isa terrane, and is completely obscured beneath an unconformably thick surface of Cretaceous and recent sediments. The deposit exhibits a discrete yet complex magnetic signature from which aeromagnetic data has indicated a broad correlation with lithomagnetic equivalents of the Soldiers Cap Group (Walters & Bailey, 1998). The nature of this distinct anomalous feature, which led to the deposit s discovery, has however remained poorly understood. We present a three-dimensional forward modelling approach of structural, petrophysical and geophysical data in which to compare the interpreted potential field response of the model with the actual survey results. This model provides a greater understanding of the deposit s lithological and structural relationship to the distribution of its petrophysical properties within the internal geometry. The observed patterns of anomaly asymmetries and shapes from the magnetic field response show strong correlation with the interpreted subsurface geometry of the deposit. This suggests that the mineralisation, which is associated with magnetite-rich lithologies, may be structurally controlled. This assumption is crucial in our attempt to model the magnetic response of the deposit based on induced magnetisation, which is assumed to be parallel with the geomagnetic field. The modeUing included capture of an existing interpretive geological dataset from the mine, which provided the initial direction for this study. This spatial dataset is an inventory of all lithological, geochemical and resource valuation data gathered during feasibility and is represented in plan and cross-sectional views. We refer to this dataset as our starting model because it is an approximation or interpretation based on incomplete information. The principal rock properties examined in this investigation are rock density and magnetic susceptibility. These properties were determined by direct measurement of samples from drillcore and, where possible, from in-situ underground exposures. The

acquisition of over 25,000 magnetic susceptibility measurements and more than 6100 density estimates were obtained which sampled all the major host rocks and mineralised lithologies throughout areas exposed by drilling. The three-dimensional geometrical model of the deposit was constructed using gOcad™. This model consists of surfaces that define the boundaries of interpreted lithological contacts and include structural features such as major faults. A corresponding threedimensional regular grid model, which approximates the in-fill volume of the generated surfaces, was likewise constructed. The addition of a third geometrical model, where the grid volume elements are created curvilinear with the bounding surfaces (gOcad™ stratigraphic' grid), was similarly created to allow recognition of possible anisotropic patterns in orientation and geometry within the modelled structure. The external drillhole constraints of density and magnetic susceptibility were implemented using geostatistics for the petrophysical modelling aspect. These measured properties were interpolated via kriging estimation within individual stratigraphic grids of unit lithologies, thereby providing a local recognition of spatial variability of the property values. This petrophysical model demonstrates that the anisotropic distribution of magnetic susceptibility values, and to a lesser extent the density values, is approximately concordant with the litho-stratigraphy. The spatially interpolated properties provided the framework for the geophysical modelling scheme. A synthetic anomaly image of gravity and magnetic fields was generated from the petrophysical model through Noddy (Jessell et al., 1993). The modelled magnetic field image showed good correlation with the surveyed data while the generated gravimetric field image exhibited strong dissimilarities. The recognition of the internal anisotropic distribution of magnetic susceptibility trend is thought to have significantly influenced the form of the observed magnetic anomalies. These three-dimensional models have yielded far greater correlation with the target data than with initial SGTSG abstracts, February 2001

models, which assumed the homogeneous distribution of properties. The models provide a framework for the estimation of mining and geotechnical provisions, the economic extraction of mineral resources and ultimately, the conceptual near-mine exploration of additional ore.

Acknowledgements The authors wish to thank BHP Cannington for their support of this study and for permission to publish. This contribution is released with the permission of the Director of the Australian Crustal Research Centre.

References Jessell, M.W., Valenta, R.K., Jung, G., Cull, J. & Geiro, A., 1993. Structural Geophysics. Exploration Geophysics 24: 599-602. "Walters, S.J. & Bailey, A., 1998. Geology and mineralisation of the Cannington Ag-Pb-Zn deposit: An example of Broken Hill-Type mineralisation in the Eastern Succession, Mount Isa Inlier, Australia. Economic Geology,!93: 1307-1329.

Figure 1. Observed total magnetic field response of the Cannington deposit.

SGTSG abstracts, February 2001

Figure 2 . Generated magnetic anomaly response f r o m the model.

Multiscale ductile and brittle shear response to Olarian and Delamerian deformation in the eastern Weekaroo Inlier, Olary Domain, South Australia

P. James and C. Conor Department of Geology and Geophysics, University of Adelaide, Adelaide, SA, Australia 5005 Office of Mineral and Energy Resources, Primary Industries and Resources SA, Adelaide, SA, Australia 5000

The Eastern Weekeroo Inlier (EWI) in eastern South Australia is a small isolated ovoid (30 km N-S by 15 km E-W) block of Palaeoproterozoic Curnamona Province surrounded by Neoproterozoic Adelaidean strata. Its eastern boundary is a clear unconformity with easterly dipping basal Burra Group conglomerates overlapping highly deformed and sheared gneisses and schists of the inlier. The western contact is more complex with apparent Delamerian Orogeny (Cambrian) thrusting causing the inlier to override both Adelaidean rocks and the adjacent central Weekeroo Inlier. The Palaeoproterozoic basement units, comprise para- and orthogneisses (-1;D5 Ma), schists, calc silicates and pelites, migmatites and at least three sets of granitoid intrusions. In the northern part of the Inlier, the basement lithologies include an overturned metamorphic sequence comprising metasediments, metabasites, felsic metavolcanics and granites. The variably foliated granite complex comprises at least two magma varieties including a marginal gneissic metagranodiorite sheet and a younger central peraluminous granite. Similar granites dated elsewhere give ages of -1705 Ma for the older and -1580 Ma for the younger. This gneissic and granitic sequence is deformed by at least three early ductile "Olarian" deformations (fold phases OD F1 to 3), the earliest of which includes a pervasive high-grade layer-parallel foliation, extensive migmatisation and recumbent isoclinal folds. Subsequent events imposed an overall luniformgeometry of superposed interference patterns. Post Adelaidean W-E extension was responsible for the emplacement of N-S mafic-ultramafic dykes. One of the dykes, which corresponds with the eastern boundary of the EWI, was sheared during Delamerian (DD1) compression. Folding of these dykes is evidence for later NW-SE shortening {DDI), which tightened the Olarian (0D3) NE trending folds. Recently acquired total magnetic intensity and radiometric images of the EWI show that the gneissic sequences can be subdivided into lithotectonic

packages separated by major ENE and WNW trending shear zones. Some zones acted as decollement or glide surfaces separating major planar layered and foliated units from others which are more complexly folded. These zones offset and truncate earlier lithological units and structures and comprise retrograde schists and phyllonites (i.e. typical retrograde shear zones). Field relationships indicate that several of these zones affected the Adelaidean and therefore were active during the Delamerian. In contrast the Walter-Outalpa Shear Zone (WOSZ), one of the largest and most obvious shear zones, is overlain by the Adelaidean unconformity. The WOSZ was therefore unequivocally initiated prior to Adelaidean deposition and thus is of Palaeo- Mesoproterozoic age. However, Delamerian reactivation is indicated by deformation and offset the abovementioned mafic dykes. The timing of reactivation is supported by Sm-Nd determinations on garnet-mica pairs from parts of the WOSZ in which garnet was stabilised by Mn (as well as by FeOx, Au and Cu) mineralisation. The Walter Outalpa Shear Zone (WOSZ) is a major, discrete WNW trending, linear feature, which has a trace length of about 10 km and is up to a few hundred metres in width. Within the shear zone, the low angle SE pitching mineral and stretching lineation, together with a clockwise fabric rotation and internal kinematic indicators indicate oblique slip transpressional displacement. The WOSZ separates the abovementioned overturned sequence in the north, from a sequence to the south, which is interpreted to be right-way-up. Up to 3.5 km of pre-Adelaidean displacement has been calculated from large-scale marker offset and scaling-up of slip vectors measured on minor faults. The continuity of the Adelaidean unconformity indicates that the Delamerian Orogeny produced no lateral displacement. However, presumably the previously mentioned Mn garnet and mica grew in response to the NW-SE DDI shortening Internal deformation within the shear zone is by both brittle minor imbrication and ductile flow. Minor

SGTSG abstracts. February 2001

structures include arrays of bookshelf-tectonic style riedel and antiriedel fractures and asymmetric foliation boudinage. Protomylonitic and mylonitic fabrics contain a range of S-C composite foliations, shear folds and asymmetric winged porphyroclast microstructures. All of these support the overall dextral sense of displacement on the zone. Detailed analysis of the minor structures at selected localities within the WOSZ indicate that the minor brittle fault arrays which form domino-type boudin trains formed early in the development of the zone and may reflect Olarian shear. Subsequent shear band type boudin trains and enwrapping mica schists/phyllonites may reflect the influence of the later (Delamerian?) more ductile reactivation, ^^jf^^f^

SGTSG abstracts, February 2001

High-pressure metamorphism and the production of yoderite in the Usagaran Belt, central Tanzania

S.L Johnson Tectonics Special Research Centre, Department of Geology & Geophysics, The University of Western Australia, Nedlands, WA, Australia 6907

The mineral yoderite has been reported to occur naturally at only two localities: Mautia Hill, central Tanzania (McKie, 1959) and the Chewore Inliers, northern Zimbabwe (Johnson & Oliver, 1998). It is therefore an interesting and unusual phase, of which relatively little is known about its field occurrence. Yoderite, so far, has been found to occur within highpressure whiteschists of MFASH composition, and is therefore associated with kyanite and talc. Despite the uniqueness of this mineral, no work has yet been reported on the regional tectonic framework that led to the formation of yoderite at Mautia Hill. This is important because not only does the tectonic setting help us understand the paragenesis of yoderite, but it also has implications for the northerly limit of 1.8 Gaaged Usagaran tectonometamorphism and whether this belt has been subsequently overprinted by the Pan African-aged, Gondwana related, collisional event. Recent mapping shows that Mautia Hill comprises an interlayered metasedimentary sequence. From base to top, the structural sequence consists of paragneiss, marble, t a l c - k y a n i t e schist, y o d e r i t e - b e a r i n g whiteschist, quartzite, feldspathic gneiss and a thin, intrusive, concordant metabasic unit, found between the paragneiss and marble. No stratigraphic younging has been determined. T h e structural and metamorphic evolution of Mautia Hill can be divided into two stages: 1. The development of an early regional fabric (both foliation and lineation) associated with at least midamphibolite-facies metamorphism. The Sj foliation strikes 140°, and the associated L^ lineation plunges m o d e r a t e l y ( 2 0 - 3 0 ° ) t o w a r d 0 9 0 ° . I n the paragneisses at the base of the sequence, kyanite defines the L^ lineation and occurs in association with a peak assemblage of biotite-garnet-muscoviteplagioclase-quartz. T h e metabasic lithology comprises amphibole-oligoclase, with minor (max. 1 m m diameter) garnet porphyroblasts. T h e kyanite-talc whiteschist near the top of the sequence comprises a peak assemblage of kyanite-talchematite-quartz. Kyanite porphyroblasts (up to 23 cm in length) define the L^ lineation. Thus, the

whiteschist metamorphism is a result of this early tectonometamorphic event. 2. The second phase of evolution is seen on the map scale as a large (wavelength of c. 100 m) synform with an upright axial plane, which strikes 109°. The production of an axial planar cleavage, $2, and associated mineral alignment lineation, L2, parallel to the ESE plunging fold hinge, overprints the Sj fabrics in the fold hinge. Yoderite is seen as a purple rim to large (up to 3 cm) kyanite crystals. The kyanite blades, inferred to have formed during D j , have been rotated into parallelism with the fold axis and thus delineate The metabasic lithology shows $2 and L2 overprinting the earlier fabric. The lineation is illustrated by the alignment of amphibole parallel to the fold axis. Marble, found below the kyanite and yoderite schists, is deformed, in the fold hinge, into discrete elongate lenses (boudins) of dolomitic marble, the long axes of which are parallel to the L2 lineation. The matrix of the m a r b l e lithology comprises talc and t r e m o l i t e , w i t h no evidence of d i o p s i d e / wollastonite. Considering that other lithologies show a peak P / T of mid-amphibolite facies for the early, D^ event (1), the presence of diopside/ wollastonite would be expected. Since diopside/ wollastonite is not seen, it is logical to conclude that the second phase of deformation is retrogressive. The yoderite, therefore, is the product of decreasing pressures and temperatures of a talckyanite whiteschist, which is evident from the formation of yoderite at the expense of kyanite. Fockenberg and Schreyer (1994) proposed that the yoderite is produced from a reaction between talc, kyanite and hematite. P / T estimates for this reaction have been experimentally constrained between 6 0 0 ° C / 1 2 . 5 k b a n d 6 8 0 ° C / 1 5 kb (Fockenberg & Schreyer, 1994) and that yoderite formation is due to retrogression (Schreyer &Yoder, 1968; M r u m a & Basu, 1987; Fockenberg & Schreyer, 1994). This stage of metamorphism is associated with regional fluid infiltration and is most evident in the metabasic unit, where the fluids

SGTSG abstracts, February 2001

have produced a quartz-epidote-garnet association. The garnet porphyroblasts are large (up to 3 cm across) and the assemblage occurs in veins that are oriented parallel to the $2 foUation. Continued deformation following the folding resulted in the attenuation of the northern limb, with displacement of lithologies southward over the fold. While stages 1 and 2 are not contemporaneous, they probably represent different stages of a P/T path for one progressive tectonothermal event and, hence, are probably not separated by a large interval of time. Retrogression to the yoderite-quartz stability field means that the peak P/T for stage 1 must have been higher than the conditions determined for this assemblage. While temperatures are likely to have been similar (i.e. -600°C), the pressure would have been higher than 12.5-15 kb, based on Fockenberg & Schreyer (1994). Temperatures of around 600°C combined with pressures of above 12-15 kb, suggest a geothermal gradient approaching 10-15°C/km, i.e. subduction/suture zone settings. Although no lithologies at Mautia Hill display eclogite or relict eclogite-facies mineral assemblages, eclogites interpreted to be related to an Usagaran-aged subduction zone (Moller et al., 1995) occur only 100 km to the southwest. Mautia Hill and surrounding metasediments, therefore, may represent an accretionary prism that had been buried to at least 50 km and subsequendy uplifted, ^^y^p^f^

References

Fockenberg, T. & Schreyer, W., 1994. Stability ofyoderite in the absence and in the presence of quara: an experimental study in the system M g 0 - A l 2 0 3 - F e 2 0 3 - S i 0 2 - H 2 0 . Journal of Petrology 35(5): 1341-1375. Johnson, S. P. & Oliver, G. J. H., 1998. A second natural occurrence of yoderite. Journal of Metamorphic Geology 16: 809-818. McKie, D., 1 9 5 9 . Yoderite, a new hydrous magnesium iron aluminosilicate from Mautia Hill, Tanganyika. Mineralogical Magazine 32: 282-307. Moller, A., Appel, R, Mezger, K. & Schenk. V., 1995. Evidence of a 2 Ga subduction zone. Eclogites in the Usagaran Belt, Tanzania. Geology 23(12): 1067-1070. Mruma, A. H. & Basu, N. K., 1987. Petrology of the talc-kyaniteyoderite-quartz schist and associated rocks of Mautia Hill, Mpwapwa District, Tanzania. Journal of African Earth Sciences 6(3): 3 0 1 - 3 1 1 . Schreyer, W. & Yoder, H. S., 1968. Yoderite: Synthesis, stability and interpretation of its natural occurrence. Carnegie Institute Washington Yearbook 66: 376-380.

SGTSG abstracts, February 2001

Are whiteschists indicators of 'A' type high-pressure (HP) ultra-high-pressure-metamorphic (UHPI\/I) belts? implications for continental subduction and collision in the Zambezi Belt and the amalgamation of Sub-Saharan Africa

S.P. Johnson Tectonics Special Research Centre, The University of Western Australia, 35 Stirling Highway, Crawley, WA, Australia 6009

The term whiteschist was penned by Schreyer (1974) to describe the stable, high pressure, equilibrium assemblage of talc and kyanite. It can be noted that such assemblages only occur naturally (or experimentally) in rocks, or sub-domains of rocks, with the simple chemical composition of MgO, Fe203/Fe0, AI2O3, Si02 and H2O, i.e., the M[F]ASH system. The term whiteschist has now been expanded to describe not only the talc- and kyanite-bearing assemblages but those assemblages which derive from the high-pressure reaction between talc and kyanite, or assemblages which comprise only M[F]ASH system HP minerals such as orthoamphibole, cordierite, sapphirine, chlorite, kornerupine, garnet and yoderite etc. Since the lower pressure stability of talc and kyanite has been experimentally located at >5-6 and the upper stability limit located at +35 kbar (at moderate temperatures of 500''-800°C), it has been speculated that whiteschists represent a chemically restricted equivalent to subduction related blueschists and hence indicate the presence of an 'A' type HP - UHPM belt. The occurrence of whiteschist in nature is extremely rare with only a few documented examples e.g., Chewore Inliers (northern Zimbabwe); Chilapila Hill and other isolated occurrences (southern and central Zambia); Sar e Sang (Afghanistan); Mautia Hill (Tanzania); Kokchetav Massif (Kazakhstan); Modum Complex (southern Norway); Dora Maira Massif (European Alps) and the Lyell Highway (Western Tasmania). These occurrences define a three-fold division either as kilometre-scale boudins within a predominantly HP-UHPM basement (Chewore Inliers, Chilapila Hill, Kokchetav and Dora Maira); metre-scale layers within amphibolite and upper amphibolite facies sedimentary sequences (Mautia Hill, Sar e Sang and Lyell Highway) or as millimetrescale subdomains within variably metasomatised amphibolite gneiss terranes (Modum Complex). The longest standing problem surrounding all of these occurrences is the nature of their protolith, since primary (sedimentary, or igneous) M[F]ASH rocks are extremely rare/absent in nature. All occurrences require the extreme metasomatic alteration (removal of all mobile elements - K, Na, Ca, Mn including Fe, and a relative increase in Si, Mg and Al) of the parent

lithology which may range from meta-basalt (Chewore Inliers, Chilapila Hill, Kokchetav Massif?, Dora Maira Massif); to pelitic sediments (Mautia Hill, Sar e Sang and Lyell Highway) and felsic gneisses (Modum Complex). Such extreme, open system metasomatic

processes are unusual and must be controlled by an extraordinary metasomatic fluid and/or some exceptional external conditions.

Whiteschist and fluid geochemistry All of the above whiteschist examples are geochemically constrained within the MASH system ± Fe (up to 13 wt%) and ± K and Na (<2 wt%, as phengite and tourmaline respectively). Where any multivalent ions such as Fe or Mn are present (Chewore Inliers, Chilapila Hill, Mautia Hill and the Modum Complex), they always occur (with the exception of the Modum Complex) in the highest oxidation state, i.e. or indicating the presence of excess O2 in the metasomatic fluid (a high /O2 fluid). Where such multivalent ions are absent (Sar e Sang, Kokchetav, Dora Maira Massif and Lyell Highway) it is difficult to determine the relative /O2 of the fluid. From fluid inclusion studies it is possible to determine the relative water activity (^H20) at the time of formation of the whiteschist assemblage. This is especially important since a fluid with a low ^H20 (i.e. a fluid rich in CO2) may shift the stability limit of some of the whiteschistforming reactions toward much lower PTs. It is evident that the whiteschists containing trivalent/tetravalent ions (Chewore Inliers, Chilapila Hill and Mautia Hill) i.e., those with a known high /O2, have undergone metamorphism where the water activity was close to unity. Those where the /O2 is known to be much lower (Sar e Sang, the Modum Complex and Dora Maira M a s s i f ) are all characterised by lower ^ H 2 0 concentrations, however; the additional fluid phase is not that of CO2 but an exotic phase such as CH4 or B and hence unlikely to substantially alter the stability of the whiteschist reaction curves. Fluid compositions and oxygen ftigacities are yet to be determined for the Kokchetav and Lyell Highway examples. The common characteristic of these whiteschists seems to be the absence of CO2 in the metasomatic fluid at moderate to high pressures (i.e., where talc SGTSG abstracts. February 2001

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and kyanite are stable). Even when the a H 2 0 of the fluid is moderate to low, the accompanying phase is exotic. If CO2 were present at these PT s one might expect the onset of dehydration with the production of eclogite-facies minerals, and hence the termination of metasomatism. Since the whiteschists are characterised by extreme metasomatism, it is likely that the production of the whiteschists is due to prolonged metasomatic activity of an H2O dominated (CO2 deficient) fluid at depths equivalent to that of the mid/ base of the continental crust. The relative /O2 of the fluid may also have a significant effect on this process. Are whiteschists indicators of 'A' type HPUHPM belts? With the recent discovery of coesite and microdiamonds in continental crustal rocks, there has been considerable investigation into HP and UHPM belts around the world. Maruyama et al., (1996) have subdivided such belts into W type, which are characterised by the continental margin nature of the protoliths, lack of an extensive granite belt and very high metamorphic pressures (>12 kbar) and are associated with continental subduction and collision; 'B' type belts are characterised by the shallow levels of subduction of accretionary complexes. Of the known occurrences of whiteschist, a threefold subdivision is here devised. 1. Those whiteschists that contain, or are associated with coesite and microdiamond (Kokchetav Massif and Dora Maira Massif). 2. Moderate-pressure whiteschists that record evidence of a low-moderate ^H^O and fO^ metasomatic fluid (Sar e Sang, the Modum Complex and the Lyell Highway?). 3. High-pressure whiteschists that record a total ^H^O and fO^ metasomatic fluid but are not associated with coesite or diamond (Chewore Inliers, Chilapila Hill and Mautia Hill). Type [1] whiteschists record pressures in excess of 35 kbar and occur as kilometre-scale boudins in highly tectonised and dismembered UHPM terranes. Their association with coesite and microdiamond indicate an origin in W type UHPM belts. Type [2] whiteschists record peak pressures of 57 kbar and predominantly occur as m-scale layers in intact sedimentary sequences or as mm-scale subdomains in amphibolite facies country rocks. Such relationships indicate that they did not develop as a result of continental subduction and collision processes. Their association with unbroken sedimentary sequences (Sar e Sang and Lyell Highway) or as mm-scale subdomains (Modum Complex) indicates that they result from very localised, exceptional, fluid conditions. The Sar e Sang example is clearly a result of extensive metasomatism in

SGTSG abstracts, February 2001

association with evaporite deposits (which provide a rich and exotic C02-deficient, low ^H20 fluid). Type [3] Whiteschists and the Amalgamation of Sub-Saharan Africa—These whiteschists record peak pressures of >12kbar and occur as km-scale boudins, or as m-scale lenses within highly attenuated, mafic eclogite-bearing, moderate-P terranes. The Zambezi Belt and Lufilian Arc of central, southern Africa can now interpreted as the crustal roots of a Pan Africanaged (520-550 Ma), Himalayan-style otogenic belt and is the site of one of the longest (150 km x 40 km) HP terranes in the world. The belt contains numerous (over 100 examples) mafic eclogite bodies and rare whiteschist occurrences (Chewore Inliers, Chilapila Hill and many other unstudied examples). Both the Chewore Inliers and Chilapila Hill examples record peak P7"conditions of between 15-23 kbar at moderate temperatures of 650®C and subduction related geotherms of -- 10°C km'^. Their interpreted parental origin as oceanic basalts, peak/Tconditions and their association with MORB-type basic eclogites attests to the deep (+50 km) subduction of these oceanic-type rocks. The highly oxidising, total ^H20 metasomatic fluid associated with these types of whiteschists, are not usually associated with subduction zones, but more commonly with near-surface, continental crustal processes such as at hydrothermal ridges. Such a metasomatic fluid must have a continental crustal source and may result from the dehydration of deeply subducted continental lithosphere. Since whiteschists are not the normal result of oceanic subduction they may be a unique indicator of this process. If the Zambezi Belt represents a Pan African-aged A' type U H - U H P M belt resulting from the continental subduction and collision, then amalgamation of the Zimbabwe/Kalahari and Congo Cratons could not have occurred until this time. The ZambeziLufillian-Damara Belts therefore represent a significant Pan African suture in West Gondwana. Conclusions Most whiteschist examples are indicators of A' type HP-UHPM belts, but are most unique in that they result directly from the collision and subduction of continental crust. Lower-pressure whiteschists are mainly associated with unbroken sedimentary sequences and may not be the result of continental subduction processes but attest to locally derived exotic fluids. References Maruyama, S.. Liou, J. G., &Terabayashi, M., 1996. Int. Gcol. Rev. 38: 4 9 0 - 596. Schreycr, W., 1974. Geol. Rundsch. 63: 597 - 609.

Recognition of early growth structures after multiple deformation episodes at Myra Falls VHMS camp, Vancouver Island, BC, Canada

101

S.A. Jones', R. Berry' and Geological Staffs ' Centre for Ore Deposit Research, GPO Box 252-79, Hobart, Tasmania, Australia 7001 2 Boliden-Westmin Ltd, IVIyra Falls Operations, PO Box 8000, Campbell River, BC V9W5E2, Canada

Myra Falls VHMS orebodies are hosted by the Devonian Sicker Group, a volcano-sedimentary package which forms the basement rocks of Vancouver Island, and are part of the Wrangellia Terrane. The Cu-Pb-Zn deposits are located in two mineralized horizons, which have a strike length of 5-6 km (Juras & Pearson, 1990). The lower ore horizon is located on or near the contact with the footwall, Price Andesite, and is hosted by the HW horizon, a rhyolitic package, 5 to 200 m thick, ofvolcaniclastics, sediments and quartz-feldspar porphyry bodies. The HW horizon is overlain by a mixed volcano-sedimentary sequence, up to 450 m thick, of andesite, dacite and minor rhyolite. The upper ore horizon is hosted by a discontinuous rhyolite sequence near the top of the mixed volcano-sedimentary package. At the top of this sequence there is a relatively sharp change into finely laminated muds, silts and andesitic sandstones of the Thelwood Formation. Early ductile deformation is overprinted by several distinct episodes of brittle deformation. The brittle deformation events are separated on the basis of fault geometry, kinematics, morphology and crosscutting relations. The deformation history is as follows: • D l : is the early growth structures associated with basin formation and VHMS orebodies. • D2: Open, upright folds with strong axial planar cleavage zones variably developed. A stretching lineation plunges gently to the NW, parallel to D2 fold axes. • D3: is a two-stage evolution of strike slip faulting (D3a-D3b) resulting from NE-SW compression followed by E-W compression, similar to the MidLate Jurassic and later Cretaceous structures noted by Nixon, et al., (1994), in northern Vancouver Island. The D3a N striking dextral faults and E striking sinistral faults have coarse quartz-chloriteepidote fibres and no fault gouge. They are crosscut by SE striking, NE and SW dipping, D3b faults, which are wavy, anastomosing faults with minor gouge. • D4: NE to E and SE striking normal faults. These

structures are predominantly gouge-free, with fine steep quartz fibres and are often associated with quartz-carbonate veining. This event commonly reactivates D3 faults. • D5: E to SE striking, shallow NE dipping oblique reverse-sinistral faults (top to west displacement) and steep E-W oriented sinistral strike-slip faults. These structures are gouge-rich, wavy-anastomosing, often associated with clear quartz veining. These faults have a major affect on the Myra Falls sequence, with two of the faults having 300 m offsets. They are most likely associated with major Eocene thrusting, and could be correlated with the north dipping thrust faults, recorded in southern Vancouver Island, in the Cowichan uplift (England et al., 1997). • D6: SE to E striking normal faults with abundant gouge, such as the North Fault, are responsible for the large downthrow in the northern part of the property. Figure 1(a) shows the contoured top surface of the Price Andesite, representing the deformed paleoseafloor (contour maps and isopach maps were constructed from drillhole pierce points). The property was subdivided into a number of structural blocks separated by large structures. Within these blocks the geology is relatively coherent, with most fault displacements being less than 2-3 m. The footwall contour map shows a large, linear structural high to the southwest of the orebodies, representing a D2 anticline, while a rapid drop in elevation in the northeastern part of the property, is a result of late normal faulting (D6). The Myra-Price Fault, an Estriking fault with 300 m sinistral displacement, occurs on the eastern side of the property, while the LynxPhillips fault, a N-striking fault, with about 350 m dextral displacement, occurs on the western side of the property. To restore the Devonian seafioor topography, the effects of later deformation were removed. We assumed that the Thelwood Formation was deposited during a time of subdued relief Contour maps of the base of SGTSG abstracts, February 2001

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this formation show that the contact has a consistent elevation of 3200 m over the HW orebody and parts of the Battle orebody, even though there is marked irregularity in the underlying Price Andesite in these areas. The vertical displacements during the D2, D4 and D6 events, were calculated by using an original elevation of 3200 m, for the base of the Thelwood Formation. This removed the large dip slip offset of the North Fault and the structural high due to the D2 anticline. The large strike-slip displacements on the Myra-Price and Lynx-Phillips Faults were also removed (Figure lb). Growth faults were recognised by rapid thickening of the mixed volcano-sedimentary package, facies variations and topographic changes of the paleoseafloor. Figure lb shows several growth structures associated with the NW-SE trending ridge to the north of the VHMS orebodies. Displacement along these structures was up to 100-150 m. The growth structures

are spatially associated with the VHMS orebodies and most likely act as conduits for ore fluids. Fine-grained facies, such as argillite, silt and fine sandstone accumulated in topographic lows, mostly to the south of the ore deposits but also in smaller depressions above the Battle and HW mines.

References England, T.DJ., Currie, L.D., Massey, N.W.D., Roden-Tice, M.K. & Miller, D.S., 1997. Apatite fission-track dating of the Cowichan fold and thrust system, southern Vancouver Island, British Columbia. Canadian Journal of Earth Sciences 34: 635-645. Juras, S.J. & Pearson, C.A., 1990. Mineral deposits of the southern Canadian Cordillera, in Guidebook for Fieldtrip B2: Vancouver 90 Geological Association of Canada-Mineralogical Association of Canada Joint Meeting: 1-21. Nixon G.T., Hammack, J.L., Koyanagi, V.M., Payie, G.J., Panteleyev, A., Massey, N.W.D., Hamilton, J.V & Haggart, J.W., 1994. Preliminary geology of the Quatsino-Port McNeill map areas. Northern Vancouver Island (92L/12,11). EC Geological Survey Branch Paper 1994-1: 63-85.

Myi»-PxicePtah

MineN<Hth NosihF^t Magnetic Noctibi

HW orebody Battle orebody

Lynx-Phillips Fault

MineN Fxgm

1«: Fcete&t day i i u c ^ costoun of Fdce Andeiite (or paleofesfloor) sbowisig tbie effects of defonoitioa (VHMS orebodies • grey)

MineN

Hgme lb: Restored ptleoseafloor (at time of deposition of Thelwood Ponztatioxt) showing die position of growtib fnihs and VHMS orebodies

SGTSG abstracts, February 2001

Neogene collision: the response of Australia's northwest margin

103

M. Keep Tectonics Special Research Centre, University of Western Australia, Perth, WA. Australia 6009

Plate collision pulses along Australias nortliwest margin at approximately 25, 8 and 3 Ma have resulted in a diverse suite of reactivation and inversion structures in the offshore Bonaparte, Browse and Carnarvon basins (Fig. 1). From the northeast to the southwest across the northwest margin, Neogene left-lateral transtensional reactivation yields to right-lateral transpression and inversion. Complicated linkage between Neogene and older faults, the relationship of Neogene faults to the contemporary stress field, and the structural control by basement structures all result in a suite of structures including pull-apart basins, restraining bends, localised inversion and complex reactivated fault systems. This paper documents some of the Neogene structural styles from the Bonaparte, Browse and Carnarvon Basins.

Bonaparte Basin The offshore Bonaparte Basin, often referred to as the Timor Sea, hosts a number of oil and gas discoveries. Neogene deformation in the Timor Sea exhibits a number of styles, reflecting the partitioning between flexure and strike slip as the dominant controls (Clough et al., 2001). Reactivated faults, which cluster around the margins of structural highs (e.g. Ashmore and Sahul platforms and Laminaria High), exhibit overall net normal displacement, although this commonly masks a complex structural history (Clough et al., 2001). Fault styles change from flexure- to leftlateral wrench-dominated from northeast to southwest (Sunrise/Troubadour to Kelp). In the area outboard of the Ashmore Platform these controls are ftirther complicated by a change in margin orientation, making the area a locus of complex deformation. Basement highs in the Timor Sea, and around the North West Shelf, play an important role in distributing strain from the plate collision. Most fault reactivation in the Timor Sea occurs adjacent to these highs, with the structural style dependent on the proportion of flexure versus strike-slip control in any one area.

Browse Basin The Barcoo Sub-basin of the Browse Basin lies in a narrow neck of the North West Shelf, between the Scott Plateau and Leveque Shelf basement highs. The Barcoo Fault System, which lies along the LynherLombardina trend, exhibits positive inversion over 180 km of strike, with structures apparently younging diachronously to the southwest. Structures along the zone change from a 20 km-wide zone of overall net normal slip at the northeast end of the system, through

narrow zones (< 5 km) of net strike slip (in the vicinity of the Sheharazade-1 well) to a 20 km-wide transpressional anticline zone at the southwestern end, where the fault trend changes direction from NEstriking to N-striking. Pulses of inversion have been identified as early Miocene and middle Miocene. Near the Trochus-1 and Lynher-1 wells the seafloor is affected (Keep et al., 2001). Neogene deformation along the LynherLombardina trend has been linked to the reactivation of Triassic horst blocks at depth, in the vicinity of the informally named Obi High (Keep & Moss, 2000). Some deep-seated faults bounding the Obi High appear to link through to the inverted Neogene section. However, similar apparent linking faults in the Nancar Trough area of the Timor Sea have been proposed to actudly detach, although Neogene deformation is still located above older horst blocks (De Ruig et al., 2001).

Carnarvon Basin The Carnarvon Basin, is situated on the corner of the Australia craton, and as such records a rather different deformation history than its counterparts to the northeast. Neogene right-lateral reactivation in the Carnarvon basin modified pre-existing Cretaceous right-lateral structures. Strongly compartmentalised fault systems occur in the Exmouth, Barrow, Dampier and Beagle sub-basins, with accommodation and transfer zones proposed to be compartment boundaries. Detailed analysis of Neogene faults along one such transfer zone has revealed a Neogene extensional duplex system, with strain tightly partitioned along a few major structures. Onshore in the Cape Range area, Neogene deformation appears more intense than in immediately adjacent offshore areas.

Discussion Structural styles in the Timor Sea, Browse and Carnarvon basins reflect significant control by Neogene tectonic influences. Initial plate collision between the Australian plate and the southern margin of the Eurasian plate occurred in the early Miocene. Continuing plate convergence along this curvilinear plate margin (Fig. 1) for the last 25 Ma has resulted in a number of structural and reactivation styles (Keep et al., 1998). Predominant left-lateral reactivation in the Timor Sea changes to dominant right-lateral reactivation in the Barcoo and Barrow sub-basins. Reactivation is controlled primarily by the relative angle of plate convergence at any given point along the collisional margin, and the location and orientation SGTSG abstracts. February 2001

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of basement structures. A number of basement highs along the outer margins of the North West Shelf act to partition strain, and the proportion of dip-slip versus strike-slip components of fault movement changes along strike of reactivated fault systems. The change in deformation style from right lateral in the Barcoo Sub-basin to left-lateral in the Timor Sea coincides with the location of a marked changed in seafloor bathymetry (e.g. O'Brien et al., 1999). This bathymetric change causes a noticeable seafloor lineament, which extends along strike from the island of Sumba. Keep et al. (in press) proposed that the lineament marks the boundary between continental fragments derived from the Eurasian plate, and the edge of the Australian plate.

Summary

The Bonaparte, Browse Basin and Carnarvon basins exhibit a complex array of Neogene structural styles resulting from the collision of the Australian plate with the southern margin of the Eurasian plate in the last 25 Ma. Plate-scale and far field controls, including plate flexure, left-lateral strike-slip and the location of basement fragments, dominate local reactivation patterns, and styles of Neogene reactivation vary significantly with the transition between these controlling elements. These varying Neogene tectonic influences have significant effects on petroleum systems in these areas.

References De Ruig, M. J., Trupp, M., Bishop, D., Kuek, D. & Castillo, D., 2 0 0 1 . Fault architecture and the mechanics of fault reactivation in the Nancar Trough/Laminaria area of the Timor Sea, Northern Australia, The APPEA Journal, 40: in press. Clough, M., Keep, M. & Longley, I., 2 0 0 1 . Neogene tectonic and structural evolution of the Timor Sea region, NW. Australia: evidence for an 8 Ma event. In review at Petroleum Geoscience Keep, M., Powell, C. McA. & Baillie, R W., 1998. Neogene deformation of the North West Shelf, Australia. In: Purceil, RG. & Purcell, R.R. (Eds), The Sedimentary Basins of Western Australia 2. Proceedings of the Petroleum Exploration Society of Australia, Perth: 81-91. Keep, M. & Moss, S. J., 2000. Basement reactivation and control of Neogene structures in the Outer Browse Basin, North West Shelf Exploration Geophysics 31: 424-433. Keep, M., Bishop, A. & Longley, I., 2001. Neogene wrench reactivation of the Barcoo Sub-basin, northwest Australia: Implications for Neogene tectonics of the northern Australian margin. Petroleum Geoscience: in press. O'Brien, G. W , Etheridge, M. A., Willcox, J. B., Morse, M., Symonds, R, Norman, C. & Needham, D. J., 1993. The structural architecture of the Timor Sea, northwestern Australia: implications for basin development and hydrocarbon exploration. The APEA Journal 33(1): 258-278.

Figure 1. Tectonic elements m a p s o f the N o r t h West Shelf, Australia, showing major basins, generalised structural trends, relative a n d absolute plate m o t i o n , location o f the collisional boundary, and the supracrustal thrust zone adjacent to the island of Timor. Keep et al. ( 1 9 9 8 ) .

SGTSG abstracts, February 2001

From

Sutured grain boundaries of dynamically recrystallized quartz: Crystallographic orientation and temperature control

105

J.H. Kruhl and M. Peternell General, Applied and Engineering Geology, Technische Universitat Munchen, Germany

The shape, pattern and crystallographic orientation of grain boundaries represent important characteristics of crystalline material and contain information about its deformation and annealing history. It has been known for a long time that grain boundaries in monophase materials may be crystallographically controlled under certain conditions and that, for example, during deformation and strain-induced grain boundary migration as well as during grain coarsening in foam textures, the straight segments of quartz-quartz boundaries are preferentially oriented along rhombohedral planes (Voll, 1969; Kruhl, 2001). This crystallographic control results in preferred frequencies of the angles between the poles of the segments and quartz-[c] ranging from ca. 40° to 70°. Consequently, sutured boundaries develop an approximately rectangular pattern that is especially common in rocks from high-grade metamorphic regions or from contact metamorphism. Measurements on dynamically recrystallized quartz from metamorphic rocks with different maximum temperature conditions, ranging from lowermost greenschist facies over amphibolite facies to conditions of syntectonic granitoids, on one hand, confirm the general crystallographic control on grain boundary orientations and, on the other hand, reveal more details of such a control. (1) Dynamically recrystallized and sutured quartz grain boundaries show similar patterns of preferred crystallographic orientations than foam textures, however, generally more accentuated. (2) At higher temperatures, the preferred orientations of the grain boundary segments are more accentuated than at lower temperatures (Fig.l). (3) The preferred crystallographic orientations are preferentially occupied by relatively long grain boundary segments. These results again point to the crystallographic control on the development of grain boundaries during stressfree grain coarsening as well as during stress-induced migration and suturing of boundaries. Moreover, they suggest that the stabilization of grain boundaries in distinct crystallographic orientations is temperaturecontrolled, i.e. controlled by the different energy of these different positions. More detailed studies on the development of grain boundaries in metamorphic

rocks appear promising with regard to better understanding of the restructuring of crystalline material due to variable (mainly physical) conditions, . y f ^ f ^ References Kruhl,J.H., 2 0 0 1 . Crystallographic control on the development of foam textures in quartz, plagiclase and analogue material. International Journal of Earth Sciences, in press. Voll,G., 1 9 6 9 . Klastische Mineralien aus den Sedimentserien der Schottischen Highlands und ihr Schicksal bei aufsteigender Regional- und Kontaktmetamorphose.- Habilitationsschrift, Fak Bergb Hiittenw, T U Berlin, D 8 3 : 3 6 0 pp.

SGTSG abstracts. February 2001

106

.53 3 -

mil

t i f i l 'itill 111

30 40 50 « AogiefioundeiyPole - Qgariz [c]

2Qa0 40 5 0 M Aii^ Boundety P(^-Quariz [o]

2-J

if 11

/ 0

6-1

I-

Figure 1. Frequenq^ distribution of angles between quartz-c and grain boundary poles of dynamically recrystallized quartz from a lowermost greenschist fades rock (A) and a syntectonic granitoid (B).

SGTSG abstracts, February 2001

Differentiation of Delamerian and Holocene thrusting, shearing and faulting in the southern l\/lount Lofty Ranges using remote sensing data and GIS analysis

107

I. Lau and P. James Department of Geology and Geophysics, University of Adelaide, SA, Australia 5005

The southern Mount Lofty ranges contains an array of low angle imbricate thrusts and shears. These discontinuities form the northern continuation of major west to northwestward displacing detachments which form the western boundary of the Delamerian fold-thrust belt of the Fleurieu "oroclinal" arc. The Fleurieu arc, however, does not appear to be related to curved Delamerian trends but rather is an artifact created by Neotectonic activity associated with overprinting seismically active faults. These Recent faults are steeply dipping with weakly curved NNE to NE trends and normal or reverse minor displacements. They have caused the uplift of the Mt Lofty Ranges and erosional exhumation of the earlier Delamerian imbricate stack. The different faults and shears in the Southern Mt Lofty Ranges are poorly exposed due to urban development, agriculture and native vegetation. Individual fault strands are identified by linear stream channels and stream capture, scarps and uplifted blocks, as well as zones of intense brecciation, veining and melange and occasional fault plane outcrop. Significant disruption, truncation and offset of stratigraphic marker horizons also allows recognition and geometric restoration of fault zones. Analysis of the southern Mount Lofty Ranges using integrated digital datasets including, Landsat TM, Radar, aeromagnetic data and a digital elevation model has proved useful for identifying major faults and thrusts, especially where large lateral displacements have occurred. A number of image enhancements techniques were trialed to assess the ability of the data to extract lithological and structural features. Edge enhancement of principal components, band ratios and radar images proved best at emphasising lineaments. A significant correlation was found between the Stonyfell Quartzite marker and native vegetation growth. Identification of the unit was performed using the colour ratio composite 7/5 7/4 4 (red, green, blue). The Barossa Complex basement gneisses and the Emeroo Subgroup units were also able to be extracted from the other lithologies in the ERS-1 radar imagery due to textural contrasts.

Orientation analysis of previously mapped faults compared with extracted lineaments found PC4 best at identifying known features. This study showed that the use of Landsat T M imagery integrated with other datasets such as ERS-1 SAR can be effective for lithological and structural mapping in vegetated and pastoral regions,

SGTSG abstracts, February 2001

108 Crustal evolution of the northern Eastern Lachlan Fold Belt P.G. Lennox,\ R. Trzebski\ A. Miiller^, W. SiebePand R. Armstrong^ ^ School of Geology, University of New Soutli Wales, Sydney, Australia 2052 2IGDL, University of Gottingen, Goldschmidtstr. 3, 37077, Gottingen, Germany 3 Institut fur Mineralogie Petrologie and Geochemie, University of Tubingen, Wilhelmstrasse 56, 72074, Tubingen, Germany ^ PRISE, RSES, Australian National University, Canberra, ACT, Australia 0200

New structural and fabric data combined with new dating of key events in the Molong-Wyangala Zone suggest that the Eastern Lachlan Fold Belt (ELFB) underwent at least three deformation stages following the Benambran event in the earliest Silurian. The Carcoar (CGd) and Barry (BGd) granodiorites and the Sunset Hills Granite (SHG) simultaneously intruded Late Ordovician Adaminaby Group and the Coombing Formation in the late Early Silurian (Figs 1, 2a). The increase in more ductile features from the north to the south and the presence of a second crenulation cleavage within the Adaminaby Group are consistent with exposure of deeper crustal levels to the south. Cathodoluminescence analysis shows two generations of quartz in the CGd and BGd, whereas three generations of quartz are present in the SHG due to its deeper emplacement. Hornblende barometry indicates that the CGd and the BGd were emplaced at 6 ± 2.6 km and the SHG was accumulated at 1012 km. The granites were subsequently deformed and uplifted kilometres during three subsequent events.

Event 1: Opening of the Hill End Zone (HEZ) and granite emplacenfient The opening of the HEZ was caused by NE-SW extension (Fig. 2a). The forerunners of the meridional Carcoar and Copperhannia faults were right stepping, en echelon faults. During opening of the HEZ both faults underwent sinistral, strike-slip displacement with bridging, NW-SE trending zones of weakness shown as dotted lines on Fig. 2a. The three granites intruded at different depths near the faults using these zones of weakness around 434 Ma.

Event 2: Granite deformation and uplift The granites were deformed during the BowningBindian event (Fig. 2b). After deformation the granodiorites became rigid and acted as buttresses

resolution gravity studies indicate that the Wyangala Batholith has a root zone between its surface exposure in the east and the SHG (see Trzebski et al., this volume).

Event 3: Movement between granites and ? further uplift The Reedy Creek Shear Zone (RCSZ) between the Barry and Sunset Hills granites was active during the Tabberabberan event with possible west-over-east thrusting of the Barry Granodiorite and its translation up to five kilometres to the east (Glen et al., 1999; Fig. 2c). The Sunset Hills Granite underwent deformation and uplift during this event. It is possible that the Carcoar and Godolphin Faults underwent dextral strike-slip motion at this time, whilst the Copperhannia Thrust underwent thrusting and some dextral strike-slip motion. Resolution of these movements across the region would have resulted in the block containing these granites between the major faults undergoing anticlockwise motion as shown by the semi-circular, dashed line with arrowhead in Fig. 2c. This may have caused uplift of the southern SHG region consistent with the exposure of more ductile structures in this region. The margins of the Carcoar Granodiorite show granite up, country rock down senses of movement suggesting the development of a pop-up at this time. The change from meridional trends of bedding and the first foliation in the south within the Adaminaby Group (AG) to northeast-trends for these same foliations further north within the Coombing Formation (CF) suggest possible ?Tabberabberan dextral strike-slip motion along this east-west trending boundary (Fig. 2c). It is possible this boundary was initially a thrust surface during the Benambran event, which was re-activated as a strike-slip fault during the Tabberabberan event.

during subsequent events. The northern section of the Wyangala Batholith appears to have been uplifted at least 10km possibly at this time, with apparent dextral translation of the Coombing Formation-Adaminaby Group boundary about 7-8 km (Fig. 2b). New highSGTSG abstracts, February 2001

Event 4: Megakinking of the country rock foliations During the Kanimblan event north-south shortening resulted in megakinking of the foliations in the country

rocks along the mainly northwest-trending zones o f

Reference

weakness originally used during granite emplacement

Glen, R.A., Lennox, P.G. & Foster, DA., 1999. ^0Ar.39Ar dating of deformations west of the Hill End Trough, Lachlan Orogen, New South Wales. Quarterly Notes of the Geological Survey of New South Wales 110: 13-22.

(Fig. 2d), mylonite development in the granites and further movement on the Copperhannia Thrust near

109

the S H G . - f T ^

Acknowledgements T h e Deutsche Forschungsgemeinschaft over many years and a Small A R C Grant ( R M S 3 2 0 3 ) in 2 0 0 0 supported our research in the northern part o f the Eastern Lachlan Fold Belt.

, Wyang

. ^ * * * * y Figure 1.

, ^

y. . . > J"*" * * * * \ .

r •'ISiluro-Devonian »|granites Siluro-Devonian m e t a s e d I m e n t s, volcanics Ordovician

(a) Locality map of the Bathurst region in Australia and New South

Wales and (b) a generalised geology map with the dashed region shown schematically in Figure 2.

Event 1

Event 2

Events

Event 4

Piguje 2: ScheftKilic Illustrations or the empkacement (2A). ctetonmaSbn {2A.D) and upHft {28.C) of the Carcocar GfonodtoJJe ICgorj. BGBT/ Graoocfiojtte (Bgd). Sunset ms Gram JSHGJ one .tne VivongatoficjthoSitnfV^} cnct counlry rocJs m 1he the northem EccSefn lachlan FokJ (see 1ext tor fufflher explanatton). AG » Adamlnaby Gfoip, Cf » Coonnblng ftafmalkxi RCS2 ^ Reecy Creek S?«ar Zone. SCSI Biowns Creek Shear Zone, CS2 « Carcoor Snecff Zone ond ISSCZftocky Brtdge Qeek S ^ r Zone.

SGTSG abstracts, February 2001

110 Understanding the Precambrian tectonic events in Tasmania: clues from south China Z.X. Li Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, Australia 6907

The tectonic setting of Precambrian Tasmania and King Island has long been an enigma for Australian geologists: • The region is the only one in the Tasman Fold Belt that has Precambrian outcrops; • Neoproterozoic little-metamorphosed successions are mostly comparable with that in the adjacent Adelaide Rift Complex, South Australia (e.g., Calver & Walter, 2000), yet • The late Precambrian orogenic(?) events identified from the region, i.e., the Wickham/Penguin Orogenies (e.g.. Turner et al., 1998), are not found in South Australia (Powell, 1992; Leaman et al., 1994); • It is the only place in Australia where c. 780 Ma and 760 Ma granites have been reported (Turner et al., 1998). The widespread Cambro-Ordovician tectonic activities led to the belief that Tasmania was welded into the Australian craton during the Ross-Delamerian Orogeny (e.g., Findlay, 1987; Crawford & Berry, 1992; Powell, 1992), although others suggested much younger ages for the accretion (e.g., Elliott & Gray, 1992). A Late Cambrian palaeomagnetic pole from the Smithton Basin agrees very well with the apparent polar wander path of Australia and Gondwanaland, and thus disfavours younger accretion ages (Li et al., 1997). Recent tectonic and geochronological analyses in South China have revealed similar features to those in Tasmania and southeastern Australian craton: • Sedimentary provenance analysis of Mesoproterozoic metasediments along southern Yangtze Craton of South China (work in progress) revealed detrital zircon populations similar to those in the metasediments from both sides of the Author Lineament, and the inherited zircon polulations in the granites/dykes, in Tasmania and the King Island (Black et al., 1997; Turner et al., 1998); • The major zircon populations in the Tasmania and King Island rocks, particularly the c. 1800, 1400 and 1300-1000 Ma populations, are prominent in the basement of the Cathaysia Block in South

SGTSG abstracts, February 2001

China. The c. 1400 Ma population is most significant because to my best knowledge, the only other place where it is prominent is in the basement of southern Laurentia; • A major hiatus in the inherited zircon and crystallisation ages in Tasmania/King Island, between c. 1100 Ma and the c. 780 Ma Neoproterozoic magmatic event (Turner et al., 1998), could be correlated with the early-Neoproterozoic hiatus in both South China and mainland Australia (Li et al., 1999); • Widespread c. 750 - 780 Ma syn-rifting granites in the Yangtze Craton (work in progress) are matched by the 777 ± 7 Ma granite in the Arthur Lineament, and the 760 ± 1 2 Ma King Island granite ENRfu (Turner et al., 1998), with 777 ±7 Ma rift volcanics (i.e., the Boucaut Volcanics; M. Fanning, pers. comm., 1998) occurring in South Australia; C. 780—750 Ma magnetism is also common in Laurentia, India, and East Antarctic. The 780 Ma radiating dyke swarm in western Laurentia has been interpreted to be of mantle plume origin (Park et al., 1995). Although the contact relationship between the granites and the Neoproterozoic rift successions in Tasmania/King Island, which are broadly correlatable with those in the Adelaide Rift Complex (Calver & Walter, 2000), is unclear, elsewhere the granite/syenite intrusions were often accompanied by rapid unroofing and/or continental rifting. It is thus suggested that: 1. The basement ofTasmania and King Island was between East Gondwanaland (adjacent to South Australia), South China and western Laurentia in the supercontinent Rodinia; 2. The so-called Wickham or Penguin Orogenies in the region may in fact reflect crustal melting and lithospheric doming above a mantle plume, with excessive heat from the plume causing crustal melting and hence granitic intrusions, and the lithospheric doming causing regional angular unconformaties; 3. The metasediments that give detrital zircon ages of c. 1100 Ma and older could be either foreland-basin

deposits formed during the late-Mesoproterozoic assembly of Rodinia, or sediments correlatable with the Heavitree Quartzite in Central Australia;

111

4. This mantle plume interpretation of the Neoproterozoic tectonothermal events in Tasmania/ King Island negates the necessity for the region to be allochthonous at that time. Rather, it could have remained attached to East Gondwanaland during the breakup of Rodinia; 5. The proposed c. 825 Ma mantle plume (Li et al., 1999;Wingateetal., 1998; Zhao etal., 1994) and the c. 780 Ma mande plume (Park et al., 1995) may all be part of a superplume beneath Rodinia. This superplume, which started sometime after c. 900 Ma and lasted over 100 million years, led to the breakup of Rodinia.

References

Black, L.P., Seymour, D.B., Corbett, K.D., Cox, S.E., Strcit, J.E., Bottrill, R.S., Calvcr, C.R., Everard, J.L., Green, G.R., McClcnaghan, M.R, Pemberton, J., Taheri, J. & Turner, N J . , 1997, Dating Tasmania's oldest geological events. AGSO Record 1997/15: 57 PPCalver, C.R. & Walter, M.R., 2000, The late Neoproterozoic Grassy Group of King Island, Tasmania; correlation and palaeogeographic significance. In M. Walter, ed., Neoproterozoic of Australia, Precamb. Res. 100: 299-312. Crawford, A.R. & Berry, R.R, 1992, Tectonic implications of Late Proterozoic-Early Palaeozoic igneous rock associations in western Tasmania. Tectonophysics 214: 37-56. Elliott, C.G. & Gray, D.R., 1992. Correlations between Tasmania and the Tasman-Transantarctic orogen: Evidence for easterly derivation of Tasmania relative to mainland Australia. Geology 20: 621-624. Findlay, R.H., 1987. A review of the problems important for interpretation of the Cambro-Ordovician paleogeography of northern Victoria Land (Antarctica), Tasmania, and New Zealand. Gondwana Six: Structure, Tectonics and Geophysics, Geophys. Monogr. Ser. 40: 49-66. Leaman, D.E., P W. Baillie & Powell, C.M., 1994. PrecambrianTasmania: A thin-skinned devil? Explor. Geophys 25: 19-23. Li, Z.X., Baillie, P W & Powell, C.M., 1997. Relationship between northwestern Tasmania and East Gondwanaland in the Late Cambrian/Early Ordovician: Paleomagnetic evidence. Tectonics 16: 161-171. Li, Z.X., Li, X.H., Kinny, PD. & Wang, J., 1999. The breakup of Rodinia: did it start with a mantle plume beneath South China? Earth Planet. Sci. Lett. 173: 171-181. Park,J.K.,Buchan, K.L. & Harlan, S.S., 1995. A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca.780 Ma mafic intrusions in western North America. Earth Planet. Sci. Lett. 132: 129-139. Powell, C.M., 1992. New perspectives onTasmanian geology. Geol. Surv. Tasm Bull 70: 177-187. Turner, N.J., Black, L.P & Kamperman, M., 1998. Dating of Neoproterozoic and Cambrian orogenies in Tasmania. Aust. J. Earth Sci. 45: 789-806. Wingate, M.TD., Campbell, I.H., Compston, W. & Gibson, G.M., 1998. Ion micropobe U-Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup of Rodinia. Precamb. Res. 87: 135-159. Zhao, J.X., Malcolm, M . T & Korsch, R.J., 1994. Characterisation of a plume-related - 800 Ma magmatic event and its implications for basin formation in central-southern Australia. Earth Planet. Sci. Lett. 121:349-367.

SGTSG abstracts, February 2001

112 Formation and exhumation of high-pressure metamorphic terranes as the result of the operation of iithospheric dislocations that reverse their shear sense G.S. Lister, M.A. Forster and T.J. Bawling Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne, Australia 3800

Renewed roll-back of subducting oceanic lithosphere in front of an otogenic zone after an accretion event has the capacity to exhume very deep levels of the Earths crust or lithosphere, because the crust and lithosphere can be severely extended in such environments. Since this exhumation mechanism does not depend on buoyancy, it is possible to readily exhume even ultrahigh-pressure (UHP) metamorphic rocks, or even deeper (microdiamond bearing) mantle rocks. Extension is all that is required. We discuss the role of Iithospheric dislocations in this process, and how Iithospheric dislocations that reverse their shear sense can be recognized. With few exceptions (e.g., Dewey et al., 1993; Krabbendam, 1997) all current models consider that rocks that will be eventually metamorphosed at high pressures begin their descent into the Earth attached to a subducting Iithospheric slab. For example Piatt (1986) writes that: "The initial process of deep burial can be explained in terms of plate convergence: material on the subducting plate will be subjected to progressively greater load pressures as it is carried beneath the upper plate." In this paper we suggest that high-pressure metamorphism is not necessarily the result of subduction, but in fact may be the result of large-scale thrusting taking place in front of a major subduction zone, in the over-riding plate, in a back-arc envirormient. In a previous paper (Lister et al., in press) it has been suggested that individual accretion events in SW Pacifictype tectonic settings lead to an episode of high-pressure metamorphism followed immediately by an episode of extreme crustal (or Iithospheric?) extension. This is because convergence may continue irrespective of whether or not an individual subduction zone is able to continue to operate. The back-arc environment is thus first subject to a period of compressional orogenesis, accompanied by large-scale thrusting, and then to a period of extensional tectonism wherein the same Iithospheric dislocations may reverse their shear sense. The earlier formed marginal basins close because they are mechanically the weakest part of the system. The thrusts that are initiated will lead to the emplacement of oceanic lithosphere over the adjacent continental crust, and the resultant large scale overthrusting can lead SGTSG abstracts. February 2001

to very high pressures. Because the geotherm will quickly adjust after such movements, even a high initial geotherm will be quickly subdued, and the conditions at depth will be such as to allow relatively low temperatures at high pressure. Coherent slices of rocks from surficial levels can thus be rapidly depressed to deep levels in the Earth, where they form the crustal roots of the newly formed mountain belt, and are subject to high-pressure low-temperature metamorphism. We propose that crustal shortening during such accretion events can produce exceptionally thick crust (with thicknesses that exceed 100-150 km). In contrast present day crustal thicknesses rarely exceed -80 km. High-pressure metamorphism can take place in the crustal roots of the mountain belt that results from such collision processes, if exceptionally thick crust can be produced (Dewey et al., 1993; Krabbendam, 1997; Ryan & Dewey, 1997). The high-pressure terranes were never attached to a subducting Iithospheric slab. The accretion model developed by Lister et al. (in press) has the capacity to explain a number of significant aspects of the geology in back-arc settings of SW Pacific-type. The models presented allow an alternative explanation of the formation of coherent high pressure metamorphic terranes (and their subsequent exhumation). No sooner has a mountain belt with such a vastly overthickened crust been formed than it will be torn apart, if for no other reason than a highly unstable situation has resulted. Renewed rollback of the subducting Iithospheric slab in front of the otogenic zone will result in large-scale extensional tectonism. As a result coherent high-pressure metamorphic terranes are exhumed shordy after their formation, as the crustal roots of the now-destroyed mountain belt are wholly or partially exposed. It is envisaged that the entire process takes place in the over-riding plate to a major subduction zone, and that it can occur within a timeframe of 1 -3 Myr. This includes the time take to accomplish burial (during the accretion event) and the time required to allow subsequent exhumation of the high-pressure metamorphic rocks from the crustal roots of the resultant orogen. Is the possibility of thick continental crust mechanically feasible? Two facts are important: (a) progressive metamorphism increases the density of

continental crust subject to increasing pressure, and these metamorphic transformations gradually increase the density of continental crust; (b) the stacking of thrust slices greatly reduces the geothermal gradient and this will greatly increase the mechanical strength of the lithosphere. The reduction in geothermal gradient will occur quite rapidly (over a time spans of 0.1-1.0 Myr depending on the thicknesses of the overthrust sheets). The accretion model developed by Lister et al. (in press) also allows explanation of the close temporal and spatial association between the emplacement of ophiolites and the formation and/or exhumation of coherent eclogite-blueschist terranes. Ophiolite sheets are found in structurally high positions in the tectonic succession. They often overlie eclogite-blueschist terranes that have been exposed as the result of continental extension. These terranes are also often found in the over-riding plate of a major subduction zone, but distal to a coevally developing accretionary prism. The story that may be told in respect to the emplacement of ophiolite sheets is similar in all respects to the model envisaged above for the formation of coherent high-pressure metamorphic terranes. The emplacement of ophiolite sheets takes place because crustal shortening leads to the closure of marginal basins in the over-riding plate above a major subduction zone. Newly formed oceanic crust (and the associated oceanic lithosphere) is thrust over continental ribbons adjacent to the subduction zone. However this major episode of crustal shortening is closely followed by a significant period of extensional tectonism, during which time lithospheric dislocations that formerly acted as thrusts reverse their movement, and become extensional detachments, Again lithospheric dislocations that reverse their sense of movement play a fundamental role. Extensional detachments that spawn from these fundamental structures strand the remnants of the emplaced nappe, to form the ophiolite sheets we recognize today. The key to the above model lies in developing an ability to recognize lithospheric dislocations that switch their shear sense. It turns out that this is a dificult task, but not impossible. In many cases the "upper plate" will have the tectonothermal history of the "upper plate" of a thrust complex. It will contain highpressure rocks that are relatively well-preserved, and diat have escaped the altering effects of later extensional tectonism. The "lower plate" will have the tectonothermal characteristics of the "lower plate" of a later formed extensional system. The rocks now exposed in the "lower plate" remained at temperature at depth, until they were exhumed from beneath the now cooled rocks of the "upper plate". They are thus extensively affected by thermal/fluid events during the later part of the geological history. In contrast, the rocks

of the "upper plate" occupied a relatively surficial level 113 in the Earths crust at the time of exhumation of the lower-plate (which of course is one reason that they are so well-preserved). The effects of these contrasting tectonothermal evolutions are most evident when comparing the metamorphic history, the evolution of microstructure, and ^^Ar/^^Ar apparent age spectra measured from the same samples (e.g. in the Aegean metamorphic core complexes, Baldwin & Lister, 1998; Vandenberg & Lister, 1996; Baldwin, 1996). Both the "upper plate" and the "lower-plate" cool rapidly. However the upperplate is emplaced earlier, and cools (rapidly) early. The rocks of the "lower plate also display the effects of rapid cooling, but this occurs at a later time than the rocks of the upper plate. .^^JPf^ Acknowledgment The work forms part of the "Mountains and Metals" initiative of the Australian Crustal Research Centre. It was supported by a grant from the Australian Research Council. References Baldwin, S.L., 1996. Contrasting P-T-t histories for blueschists from the western Baja Terrane and the Aegean; effects of synsubduction exhumation and backarc extension. In: Bebout, G.E., Scholl, D.W., Kirby, S.H., Piatt, J.R (eds). Subduction top to bottom, Geophysical Monograph 96: 135-141. Baldwin, S.L.B. & Lister, G.S., 1998. Thermochronology of the South Cyclades Shear Zone, los, Greece: the effects of ductile shear in the argon partial retention zone (PRZ). Jour. Geophys. Res. 103: 7315-7336. Dewey, J.R, Ryan, RD. & Andersen, T.B., 1993. Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites. Geol. Soc. Spec. Publ. 76: 325343. Krabbendam, M., 1997. Structural and metamorphic evolution of eclogite gneisses during exhumation in S W Norway. Unpublished Ph.D. thesis, Oxford University: 195pp. Lister, G.S., Forster, M.A. & Rawling, T.J., in press. Episodicity in orogenesis. In: Special Publication, Geological Society of London. Piatt, J.P., 1986. Dynamics of orogenic wedges and the uplift of highpressure metamorphic rocks. Geological Society of America Bulletin 97: 1037-1053. Ryan, RD. & Dewey, J.R, 1997. Continental eclogites and the Wilson Cycle. Journal of the Geological Society of London 154: 437442. Vandenberg, L C . & Lister, G.S., 1996. Structural analysis of basement tectonites from the Aegean metamorphic core complex of los, Cyclades, Greece. Journal of Structural Geology 1 8 : 1 4 3 7 - 1 4 5 4 .

SGTSG abstracts, February 2001

114 Middle crustal processes of oblique collision in the central Southern Alps, New Zealand: What record is provided by ductile fabrics in the Alpine Schist? T.A. Little, R.J. Holcombe and B.R. Ilg School of Earth Sciences, Victoria University of Wellington, Wellington New Zealand

Middle crustal rocks in the hangingwall of the obliqueslip Alpine fault near Franz Josef Glacier have been uplifted and exhumed during the past --2-3 M a providing a rare sample of the ductile underbelly of modern obliquely convergent orogen (e.g., Holm et al, 1989). The Southern Alps exposes a crustal section through the western edge of the Pacific Plate, a SEtilted slab of rocks that have been up-ramped along the Alpine fault (e.g., Norris et al., 1990). The basal - 1 km of this slab was mylonitized in the late Cenozoic in proximity to the Alpine Fault at depth. The Alpine Schist includes amphibolite-facies rocks at its structural base near the Alpine Fault, and chlorite-zone rocks - 1 5 km to the east of that structure near the Main Divide of the Southern Alps. Seafloor data indicate that S A Ma this part of the Australia-Pacific plate boundary changed from a dextral-slip transform margin to an obliquely convergent one with an interplate convergence angle of - 2 0 ° (Sutherland, 1995). Since then, - 9 0 km of continental crust of the Pacific Plate has been removed by convergence-related uplift and erosion across the Southern Alps, and - 2 3 0 km of dextral-slip motion has accrued between the two plates (Walcott, 1998). The Alpine fault has a late Quaternary strike-slip rate of25-30 mm/yr, about 5580% of the total plate motion (Cooper and Norris, 1994; Sutherland, 1994). In this talk we will use the Alpine Schist as a "natural laboratory" to examine how ductile deformation is imprinted on rocks in a wellunderstood, modern oblique collision zone, and to predict how such processes may be preserved in the fabric record of ancient orogens. (1) What is the evidence that ductile fabrics in the Alpine Schist have recently been exhumed from the middle crust of an oblique continental collision zone? In the Southern Alps, inherited Mesozoic fabrics bearing no relationship to the present Southern Alps orogen are widely preserved, even in the mylonites. The overprint of late Cenozoic deformation constructively reinforced pre-existing fabrics making the two difficult to distinguish. Near vertical and striking obliquely to the Alpine Fault, the pre-existing "Alpine foliation" was predisposed in attitude to SGTSG abstracts, February 2001

accommodate a transpressive overprint in the modern oblique collision zone. The older foliation lay in the extensional sector of the late Cenozoic incremental strain, and was "re-used" rather than being crenulated or transected by a new foliation. Upright Mesozoic folds and crenulations which had resided at depth in the middle crust for tens of millions of years were tightened during the late Cenozoic under only slightly different metamorphic conditions. The resultant composite fabric is diachronous. This diachroneity is also expressed in the growth history of syntectonic garnet porphyroblasts, the cores of which nucleated in the Late Cretaceous, but the rims of which resumed syntectonic growth during crustal thickening much later in the Cenozoic. In the central Southern Alps, thermochronometers, including hornblende ^^Ar/^^Ar are widely reset at deep structural levels recording their rapid cooling from depth during uplift and unroofing since the Pliocene, but retain older ages along strike and at higher levels above the partial retention zones for these minerals. This results in a complex spatial distribution of ages across the collision zone. Evidence for late Cenozoic transpressive ductile flow in the Alpine Schist includes deformation of young veins and analysis of a fossil brittle-ductile transition zone (BDTZ) within the exhumed section. Brittle-ductile shears within the - 1 km thick BDTZ are the youngest ductile fabric element identifiable in the nonmylonitic Alpine Schist. These strike parallel to the Alpine Fault and vertically crosscut the older "Alpine foliation" with a dextral-oblique sense of slip. Below the BDTZ, changes in quartz microstructure reflect the superposition of a late increment of ductile strain across those deeper parts of the Pacific Plate. As predicted by modelling of transpressive flow (e.g., TikofiF & Fossen, 1999) strain shape was distincdy oblate and the finite stretching lineation is down-dip. Above the BDTZ, polygonal quartz grain-shape fabrics are relict from the period of schist burial predating the present collisional phase. (2) What do these ductile febrics and surface geodetic observations tell us about crustal-scale mechanisms of oblique collision and uplift in New Zealand?

GPS-derived velocities of the Pacific Plate relative to the Australian plate across the central South Island suggest a pattern of oblique convergence parallel to the plate motion vector (Beavan et al., 1999). Every mm of convergence is matched by - 3 mm of dextral motion. This deformation is approximately uniformly distributed over a -150 km wide zone. Any material moving west towards the Alpine Fault must first decelerate through an eastern zone of oblique shortening. Extending geodetic and plate motion rates back in time, rocks presently exposed at the surface along the Alpine Fault would have originated in the middle crust -50 km to the east. During the next -2 m.y, they would have moved westward through the outboard part of the orogen, shortening and thickening by - 6 % . Seismic data image a gentle westward thickening in the present-day crust (Davey et al., 1998). Ductile fabrics support the idea that the rocks have undergone a phase of transpressive flow associated with vertical thickening (Holcombe and Little, 2000). This deformation was accompanied by reinforcement of the steep "Alpine foliation" and development of a down-dip stretching lineation. After - 2 m.y, the rocks would have been transported onto the SE-dipping Alpine Fault ramp. Field observations indicate that the ramping process involved passage of the rocks through a sequentially activated, escalator-like array of nearly vertical, oblique-slip shear zones that are upthrown to the west. Passage through this fixed shear zone tilted the delaminated Pacific Plate crust to the SE. This process would structurally thin the section by -25%. Such "back-shearing" is obvious in the BDTZ, where britde-ductile shears form a systematic, closely spaced array; the same sense of shear pervades downward into the section of ductilely deformed rocks beneath the BDTZ. A final phase of deformation was chiefly confined to the Alpine mylonite zone, but extended up to -5 km away from it, as expressed by a drag-like oversteepening and shallowing of foliation towards the Alpine Fault. (3) What are the implications of these observations for recognition of ancient oblique collisions zones? Other than the Alpine fault, its mylonite zone, and the late shears in the BDTZ, evidence for Cenozoic oblique collision is sparse in the Alpine Schist. In the non-mylonitic part of the section, post-biotite measures of incremental strain suggest only -30-40% of foliation-orthogonal shortening during the late Cenozoic, a magnitude that accords with our strain modelling which incorporates available data on geodetic strain rates and ramp kinematics. This strain takes the form of a modest constructive overprint

which might be overlooked in an ancient orogen. Because of its small magnitude, older ductile fabrics are widely preserved, providing the unwary with opportunities for misinterpretation. The "soft footprint" of collision-related Cenozoic finite strain reflects the following important point: oblique collision in the New Zealand orogen is dominated by translation, erosion, and interplate slip on the Alpine Fault. To the east of that structure, rocks migrate rapidly through the deforming zone, preventing the accumulation of large finite strains.

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References

Beavan, J., Moore, M., Pearson, C., Henderson, M., Parsons, B., Blick, G., Bourne, S., England, P, Walcott, R. I., Darby, D. & Hodgkinson, K., 1999. Crustal deformation during 1994-1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault. Journal of Geophysical Research 104(B11), 25: 233-25,255. Cooper, A.F. & Norris, R. J., 1994. Anatomy, structural evolution, and slip-rate of a plate-boundary thrust: the Alpine Fault at Gaunt Creek, Westland, New Zealand. Geological Society of America Bulletin 106: 627-633. Holcombe, R.J. & Little, T.A., 2000. A sensitive vorticity gauge using rotated porphyroblasts, and its application to rocks adjacent to the Alpine Fault, New Zealand. Journal of Structural Geology. Holm, D.K., Norris, R.J. & Craw, D., 1 9 8 9 . Brittle and ductile deformation in a zone of rapid uplift: Central Southern Alps, New Zealand. Tectonics 8: 153-168. Norris, R.J., Koons, PO. & Cooper, A.F., 1990. The obliquely convergent plate boundary in the South Island of New Zealand: Implications for ancient collision zones. Journal of Structural Geology 12: 715-726. Sutherland, R., 1994. Displacement since the Pliocene along the southern section of the Alpine fault, New Zealand. Geology 22: 327330. Sutherland, R., 1995. The Australia-Pacific boundary and Cenozoic plate motions in the S W Pacific: Some constraints from Geosat data. Tectonics 14: 8 1 9 - 8 3 1 . Tikoff, B. & Fossen, H., 1999. Three-dimensional reference deformations and strain facies. Journal of Structural Geology 21: 1497-1512. Walcott, R.I., 1998. Modes of oblique compression: Late Cenozoic tectonics of the South Island of New Zealand. Reviews of Geophysics 36: 1-26.

SGTSG abstracts, February 2001

116 Regional brecciation and relationships to metasomatism and magmatism at the Gilded Rose breccia type area, Northwest Queensland L.J. Marshall and N.H.S. Oliver Economic Geology Research Unit, James Cook University, Townsville, Qld, Australia 4811

Possibly the most widespread breccia occurrence on the planet, widespread metasomatism, and numerous iron oxide copper-gold deposits are hosted in the eastern succession of the Mt Isa Block, northwest Queensland. Regional scale brecciation is largely confined to calcsilicates of the Mary Kathleen Group. Locally breccias form important structural hosts to many of the regions ore deposits in the Mary Kathleen Group (eg. Starra), as well as the Soldiers Cap group (eg. Osborne), intrusions of the Williams and Naraku batholiths (eg. Lightning Creek prospect) and the Mt Fort Constantine felsic volcanics (eg. Ernest Henry). Recent work in the region has concentrated largely on the relationships between alteration, mineralization and magmatism (eg. Mark & Crookes, 1999; Mark & Foster, 2000; Perring et al., 2000; Williams et al., 1999). This paper is a preliminary report on the structural controls on hydrothermal brecciation in the Cloncurry district, and the interplay between brecciation, metasomatism and magmatism, with specific reference to the Gilded Rose breccia type area (Fig. 1). Corella (marble-matrix) Breccia Genesis of the regionally extensive Corella Breccia was the result of the mixed brittle and ductile response of calcsilicate and marble beds respectively, to regional deformation. The breccia commonly consists of clasts of calcsilicate rocks "floating" in a relatively intact, ductilely deforming carbonate rich matrix. The breccia is quite different from fault or hydrothermal breccias in which the matrix consists of ground rock fragments and/or mineral precipitates. Gilded Rose Breccia Less widespread than the Corella Breccia, occurrences of the Gilded Rose Breccia are strongly controlled by major dilatant brittle structures. The breccia is a hydrothermal breccia, and exhibits extreme mixing and milling of clasts derived from both the Corella and adjacent formations. Clasts and matrix exhibit intense sodic-calcic and/or potassium-iron rich alteration assemblages. Structural setting At the Gilded Rose type area, the main breccia body is localized at a dilatant jog in a major N-S trending fault SGTSG abstracts, February 2001

(Fig. 2). An offshoot to the main body lies along an inferred NE-SW trending fault, which is possibly a reactivated D2 structure. A mapped lateral offset of approximately one kilometre, is consistent with sinistral displacement on the N-S fault in response to late D3 (brittle) compression. During D3, the NE-SW trending fault would not have been in a dilatant setting. This may be reflected by the difference in metasomatic assemblages present in the NE-SW trending offshoot versus the main breccia body, as described below. In map view, the breccia body is completely surrounded by Toole Creek volcanics and Mt Noma Quartzite of the Soldier's Cap group. Regional mapping suggests the Soldier s Cap group is unconformably overlying Corella formation in this area. Breccia clasts are largely derived from Corella formation calcsilicates and marbles, with an increasing content of Soldiers Cap derived clasts towards the margins of the breccia body. This, combined with field evidence that the breccia body is significantly larger than the dilatant gap that would have been created by slip on the N-S trending fault, suggests diatreme-like behavior for intrusion of the main breccia body. Metasomatism and the Gilded Rose Breccia The entire breccia body is affected by albite + actinolite + quartz + magnetite alteration. The portion of the breccia surrounding the dilatant jog is subsequendy overprinted by intense hematite + k-feldspar "red-rock" alteration. While different fluids may cause the two alteration styles, alteration could reflect a progressive enrichment in potassium in the fluid (change from albite stable to k-feldspar stable), and a drop in temperature and/or pressure (change from magnetite to hematite stable). This sequence of alteration styles is consistent with that observed in previous studies, particularly along the Cloncurry fault metasomatic zone (eg. de Jong & Williams, 1995). The breccia body locally grades into nearly 100% pure magnetite + hematite ironstones. While the breccia body and ironstones here lack significant sulphide mineralization, the alteration assemblages related to brecciation are otherwise similar to those associated with many of the district's breccia-hosted Fe-oxide-Cu-Au deposits. As such, the Gilded Rose breccia-related metasomatic fluids may play an important role in ore genesis, but lack a key mechanism

for sulphide precipitation at the Gilded Rose type area. As such, occurrences of Gilded Rose breccia are considered prospective for Fe-oxide-Cu-Au mineral exploration.

Magmatism and the Gilded Rose Breccia Published 1:100 000 maps (Ryburn et al., 1988), indicate an occurrence of Naraku granite within the Gilded Rose breccia type area. Recent field mapping has revealed six distinct occurrences of felsic intrusives within the breccia body. The intrusives consist of granodiorite affected by varying intensities of albite ± actinolite ± magnetite alteration and later K-feldspar + hematite alteration. The location of the intrusions is strongly coincident with the dilatant fault jog believed to be the locus for brecciation (Fig. 2). Around the margins of the intrusive rocks, granitic clasts are commonly incorporated into the breccia body As such, intrusion is either syn-brecciation, or, given the lack of evidence for granite cutting breccia, pre-brecciation. Because of the very strong spatial relationship between the granites, and the dilatant fault jog, a genetic link between Gilded Rose style brecciation and felsic intrusion should not be ruled out. This relationship will be further investigated through the use of stable isotope geochemistry and other techniques.

mineralization and sodic-calcic alteration. Economic Geology 95: 1067-1089.

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Phillips, W.J., 1972. Hydraulic fracturing and mineralization. Journal of the Geological Society of London 128: 337-359. Ryburn, R.J., Grimes, K.G. et al., 1988. Cloncurry (1:100 000 scale map). Bureau of Mineral Resources, Canberra. Williams, P.J., Dong, G., Pollard, PJ., Perring, C.S., Ryan, C.G. & Mernagh, T.R, 1999. Fluid inclusion geochemistry of Cloncurry (Fe)-Cu-Au deposits, in Stanley, C.J., ed., Mineral Deposits: Processes to Processing: 111-114.

Discussion The main stage of hydrothermal brecciation at the Gilded Rose type area is coincident with potassiumiron rich metasomatism, and is consistent with rapid intrusion of breccia, triggered by slip on a major D3 brittle structure. Fluid overpressures required for hydrothermal brecciation may be driven in part by felsic intrusion at depth. Earlier sodic-calcic alteration may reflect more brittle-ductile conditions (early D3?), prior to major brittle fault slip. A near instantaneous drop from lithostatic to hydrostatic fluid pressure gradients may be in part responsible for a change in alteration assemblages. As of yet, it is unclear whether brecciation reflects true diatreme processes, whereby the breccia pipe reached surface, or if brecciation was in response to changes in fluid pressure gradients and rapid fluid flux within a fault that did not reach the paleosurface (Phillips, 1972). ^ ^ f f ^

Figure 1. Location of the Gilded Rose breccia type area within the Eastern Fold belt, Mount Isa Inlier.

References de Jong, G. & Williams, PJ., 1995. Giant metasomatic system formed during exhumation of mid-crustal Proterozoic rocks in the vicinity of the Cloncurry fault, northwest Queensland. Australian Journal of Earth Sciences 42: 281-290. Mark, G. & Crookes, R.A., 1999- Epigenetic alteration at the Ernest Henry Fe-oxide-(Cu-Au) deposit, Australia, in Stanley, C.J., ed., Mineral Deposits: Processes to Processing: 185-188. Mark, G. & Foster, D.R.W., 2000. Magmatic-hydrothermal albiteactinolite-apatite-rich rocks from the Cloncurry district, N W Queensland, Australia. Lithos 51: 223-245. Perring, C.S., Pollard, P.J., Dong, G., Nunn, A.J. & Blake, K.L., 2000. T h e Lightning Creek Sill C o m p l e x , Cloncurry district, northwest Queensland: a source of fluids for Fe oxide Cu-Au

Kilometers Gilded Rose breccia, K-Fe and Na-Ca alteration Gilded Rose breccia, Na-Ca alteration a w Felsic Intrusions Soldier's Cap, metabasite and metadolerite Soldier's Cap, metasediments

Figure 2. Geological map of the Gilded Rose type area. SGTSG abstracts, February 2001

118 The structural style of Cambrian Metamorphic Complexes in Tasmania: SW Tasmanian examples S. Meffre\ R.F. Berry\ M. HalP and A. McNeilP ^ Centre for Ore Deposit Research, University of Tasnnania, GPO Box 252-79, Hobart, Tasmania, Australia 7001 2 Department of Earth Sciences, Monash University, Clayton, Victoria, Australia 3168 ^Pasminco Exploration, Rosebery, Tasmania, Australia 7470

Cambrian metamorphic complexes containing amphibolite to eclogite-grade rocks are present throughout western and northwestern Tasmania. These complexes contain mostly quartz-albite-biotite schists, garnet-quartz-albite-biotite schists and mafic amphibolite lenses (up to 1 km long). The chemistry of these rocks is similar to unmetamorphosed, Late Neoproterozoic tholeiitic basalts and continentalderived siliciclastics. The rocks within these complexes were probably located on the edge of a thin Late Neoproterozoic passive margin which was partially subducted during a Cambrian arc-continent collision

and uplifted during post-collisional crustal reequilibration. The structural style of these complexes is poorly constrained. Most of the examples are poorly exposed or have been intensely overprinted. In particular the faulted boundaries between the units have never been properly investigated. The Port Davey Metamorphic Complex complex crops out along the rugged and remote southwest coast ofTasmania (Fig 1) (McNeUl 1985; Williams 1982). The complex is made up of fault bound slices of low grade metasedimentary rocks (thin bedded and graded quartz-dominated meta-sandstone and phyllite;

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Rocky Cape block

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SGTSG abstracts, February 2001

50 km

graphitic phyllite and quartzite) and amphibolite facies metamorphic rocks (albite dominated schist with minor thin bedded quartz-rich metasedimentary rocks, garnet bearing schist and gneisses with amphibolite boudins to 5m, massive quartz albite rocks, and albiterich rocks with garnet amphibolite bands. Pelitic schist from Port Davey consists of alternating layers of quartz and plagioclase-rich rocks, with varying amounts of garnet and mica. Kyanite also occurs, with one sample from Nye Bay bearing kyanite rimmed with sillimanite. Amphibolite and garnet amphibolite lenses (5-500 m long) occur throughout the complex. They are mainly massive with little mineralogical variation within the lenses. Pressure and temperature estimates for peak metamorphism are 600°C and 750 MPa. Metamorphism occurred during the Cambrian. The amphibolite facies schists have undergone at least four phases of deformation of which the first two have produced schistosity defined by muscovite and biotite. The early deformations are closely associate with mylonite zones both internally and on there contacts. The mineral lineations feldspar, muscovite, S1/S2 intersection lineations and mylonitic lineations are all shallowly plunging. Internally all these rocks are dominated by very high strains as indicated by the complete disruption of amphibolites which are always

Fig.2 Geology of the Mulcahy Bay area

found as widely separated boudin blocks which are intrafolial. It is interpreted here that the parallelism of fold axes and stretching lineations is a fiinction of the rotation of all structures. The greenschist facies rocks share some of this structural history as indicated by the juxtaposition of the two rock types by mylonite zones with a strong sinistral sense of shear. Away from the mylonite zones the meta-sandstones have a strong cleavage and often also a crenulation cleavage with a shallowly plunging S1 /S2 intersection. Some shear zones are visible within the meta-sandstone but these are not always easy to recognise. The metamorphic grades from chlorite zone to biotite zone. Albite is widespread and often forms porphyroblasts. The early cleavages are overprinted by several generations of kink style folds which are mosdy closely associated with small fault zones. Some of these fault zones also effect the Ordovician stratigraphy All contacts between the higher grade rocks and the chlorite stable metasediments are faulted. These contacts can be grouped as:

119

a) mylonitic contacts The well exposed contact between chlorite zone metasedimentary rocks and the amphibolite facies schists that occurs south of Mulcahy Bay at 396521E 5224834N is outstanding because it is free of any overprinting cataclastic deformation. The contact is occupied by a 10 m wide zone of muscovite albite quartz mylonite. The mylonite has a strong L/S fabric and contains dismembered vein quartz fish which have asymmetric boudin shapes and has strong shallowly south plunging grooves along the edges. The schistose rocks have very common shear band geometry visible in outcrop. Oriented thin sections support a sinistral sense of shear with shear band geometry, s^ shapes on albite porphyroblasts and subgrain textures in quartz grains all supporting this sense of shear. A very similar rock type is exposed at the north side of Mulcahy Bay on the edge of the amphibolite facies rocks but here the block to the east of the mylonite is not exposed and there is a much stronger overprint by later deformation with intense chlorite replacement. However the exposure of amphibolite facies rocks have a mylonitic fabric with sinistral sense of shear. (b) high strain ductile faults overprinted by a brittle dip slip event South of Elliott Point at 392044E 5226734N the contact zone is marked by a narrow cataclastic zone overprinting a phyllitic fabric. Within the cataclasite SGTSG abstracts, February 2001

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the lineation pitches 80°S. East of this contact is a 20 m wide zone of phyllites with a very strong L/S fabric. One metre east of the contact the phyUites have a recognisable early L/S fabric with a lineation plunging 10°S. One metre west of the contact a weaker L/S fabric is present in the felsic metamorphic rocks and the feldspar lineation plunges 2°S. Samples from 20 m into the phyllites have an intense cleavage but are not mylonitic. The material from close to the fault surface may have a mylonitic fabric but this was not recognisable through the late dip slip cataclastic texture. This fault boundary is also exposed on the coast north of Elliot Point but at this locality the fabric was dominated by intense folding and fracturing along with retrograde metamorphism and cataclasite formation. No zone of strong L/S fabric was recognised. Within the feldspathic rocks of Elliott point a contact is exposed between garnet schist and the more massive garnet-absent rocks on the point. The zone has a 20 m zone of strong L/S schists with a shallow plunging lineation but within 1 m of the contact there is a preponderance of dip slip lineations and a 2 cm cataclasite forms the contact surface Quartz fibre veins near the contact are west dipping and plunging. On the fault surface there are a number of horizontal kink folds which we correlate with the proposed normal movement on this zone. (c) cataclastic zones dominated by west side down (normal) dip slip textures North of Nye Bay, at 391200nE 5232800 mN, a 60 m wide zone of highly sheared rocks, some of which have an L-S fabric, separate the high and low grade metamorphic complexes. The foliation in this zone is subparallel to Si of both the phyllites and adjacent schists. The dominant lithology in this zone has a black matrix with quartz and calcite-rich porphyroclasts. In section, cut parallel to the lineation and perpendicular to the cleavage, these features indicate a normal sense of movement. At the south end of Mulcahy Bay (Fig. 2) the Owen Conglomerate is faulted against amphibolite facies rocks across a steep shear zone. Within the shear zone the first 10 m are intensely foliated cataclastic rocks in which a dip slip lineation is visible. The rocks have intense chlorite sericite retrogression. Small quartz fibre veins indicate a dip slip movement and a west side down sense of shear is indicated by fibre veins and the down throw of the Owen Conglomerate which only 300 m to the north is unconformable of these rocks. The Cambrian metamorphic rocks of SWTasmania have an early history of intense deformation with a

SGTSG abstracts, February 2001

strong N-S stretching lineation. They are juxtaposed with lower grade metasedimary rocks on fault zones which have a strong ductile component the strain. These faults, in their present orientation, are steeply dipping sinistral shear zones. However this orientation has been modified by post-Cambrian folding. Where the Ordovician unconformity surface is known, as in the southern contact, restoration of the unconformity surface to horizontal, implies that the mylonites were dipping 40°SSW with a lineation plunging 36/180 and with a top to south (normal) sense of movement, in the Late Cambrian. We conclude the original transport direction was to the south on shallowly dipping fault planes. The steeply west dipping normal faults in this area are not related to the Cambrian tectonics . y f ^

References

McNeill, A.W., 1985. The structure and petrology of the Nye Bay area south west Tasmania. BSc (Hons) Thesis, University ofTasmania, Hobart (unpubl.). Williams, P.R., 1982. Geological atlas 1:50 000 series. Sheet 91 (801 IS). Davey. Explanatory Report, Geological Survey ofTasmania , Hobart.

The Magdala Lode System, Stawell, southeastern Australia: structural style and relationship to gold mineralisation across the western Lachlan Fold Belt

121

J.McL. Miller, C.J. L. Wilson and B.A. Witham School of Earth Sciences, The University of Melbourne, Victoria, Australia 3010

The internationally significant Victorian goldfields in the Palaeozoic Lachlan Fold Belt of southeastern Australia have produced approximately 2500 tonnes of gold. The Lachlan Fold Belt is an approximately 700 km wide belt of deformed marine turbidites that overlie a mafic substrate with a variable composition. The western Lachlan Fold Belt is divided into three structural zones namely (going from west to east), the Stawell, Bendigo-Ballarat and Melbourne Zones (Gray and Foster, 1997). These have deformed at different times and contain turbidite sequences of different ages (Cambro-Ordovician to Devonian) and have predominantly north- and northwest-trending structural grains. All of the structural zones are bounded by steep west-dipping reverse faults. Recently published ^Âr/ 39Ar data sets (Foster et al., 1998; Bierlein et al., 1999; Foster et al., 1999) have provided critical information on the timing of deformation and gold mineralisation. Much of the interpretation of this ^Âil^Âi data set is still controversial. This paper provides critical new field based structural information that resolves some key issues with respect to the relative timing of deformation and gold mineralisation in western Victoria. Gold mineralisation at Stawell is hosted within both deformed turbidites, and the underlying mafic successions (volcanogenics), making it atypical to the slate-hosted gold deposits of the same age found elsewhere in the Lachlan Orogen. Lodes have developed on the western flank of a large, doublingplunging, basalt dome (the Magdala Antiform) during northeast-southwest and east-west shortening. The irregular shape of the basalt dome, and the stratigraphic contacts between different turbidite packages, are the key controls on the location of the gold lodes. Regionally the flow of gold-bearing fluids appears to have been driven by upward flow along the flanks of this dome with lateral flow being focussed by flow parallel to fold hinges. The combination of a unique structural geometry, and rocks with the appropriate chemistry (volcanogenics), resulted in the precipitation of gold at all stages of fault-valve behaviour. Much of the gold is associated with sulphide-rich gold lodes

(mainly pyrite, pyrrhotite, and arsenopyrite) with fine disseminated gold that is trapped as inclusions in arsenopyrite or pyrite. Early ductile deformation form is defined by a series of upright to slightly inclined closures that have produced the antiformal basalt geometry. Nearly all of the mica growth occurs during, or prior to, this folding event and this is believed to represent peak regional metamorphic conditions. These upright folds are overprinted by a second folding event that has produced a flat-lying to northwest-dipping cleavage. Not only does gold mineralisation at Stawell post-date this ductile deformation, the main stage of gold mineralisation occurred late in the brittle deformation history at Stawell, well after the formation of a series of major brittle faults that cross cut all earlier ductile features and contain laminated veins up to 10 m thick. In contrast, the initial stages of mineralisation in the Bendigo-Ballarat Zone occurs much earlier in the deformation history. This is reflected by mineralised saddle reefs that formed just prior to fold lock-up immediately before the development of mineralised west- and east-dipping brittle faults that cut across the simply folded stratigraphy. These timing relationships suggest deformation initiated in the Stawell region and propagated to the east, with major gold mineralisation occurring at the same time in both gold fields (at 455440 Ma; Foster et al., 1998; Bierlein et al., 1999; Foster et al., 1999), but at a different point in the relative structural evolution of both systems. This supports the proposed deformation chronology of Foster et al. (1998), Gray & Foster (1997) and Foster et al. (1999). Later gold lodes at Stawell developed as a result of the reactivation of the system during sinistral obliqueslip. This sinistral wrenching can be correlated with at least three other gold deposits (Percydale Fields, Tarnagulla and Fosterville) across western Victoria that are younger than the main stage of mineralisation (Foster et al., 1998). Later gold lodes, associated with a strong south-directed transport, formed between 399 and 413 Ma (termed 'Wonga and 'Scotchmans Fault Zone' at Stawell, and 'Cross Course' faults in the Bendigo-Ballarat Zone). All of these deposits predate SGTSG abstracts, February 2001

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the Melbourne Zone mineralisation which occurred at shallower crustal levels in an east-west shortening regime between 380 and 370 Ma (Foster et al., 1998). This style of gold mineralisation is associated with stibnite and is not solely confined to the Melbourne Zone occurring sporadically in the Bendigo-Ballarat Zone (e.g., Toolleen; Foster et al., 1998). In total four stages of mineralisation in western and central Victoria have been identified, each of these has a distinct structural/geochemical signature. The variation in shallow level features related to major gold mineralisation in the Stawell and BendigoBallarat Zones (e.g., structural style/histories, host rocks) contrasts with the synchroneity in the absolute timing of mineralisation (455-440 Ma) and similar ore fluid compositions (low salinity and H2O-H2SCO2 rich; e.g. PhiUips and Hughes, 1996). There is also an eastward younging in gold deposits and plutonism (that reflects deeper level thermal events) between the Stawell/Bendigo-Ballarat Zones and the Melbourne Zone. These features suggest that there is a unifying deep crustal and/or mantle source of goldbearing fluids for the major stages of gold mineralisation in the Stawell and Bendigo-Ballarat Zones (at 455-440 Ma) and possibly the Melbourne Zone (at 380-370 Ma),

References Bierlcin, RP, Foster, D.A., McKnight, S. & Arnc, D.C., 1 9 9 9 . Timing of gold mineralisation in the Ballarat goldfields, central Victoria: constraints from "^^Ar-^fAr results. Australian Journal of Earth Sciences 46: 3 0 1 - 3 0 9 . Foster, D A . , D.R. Gray, TA.P. Kwak & M. Bucher, 1 9 9 8 . Chronology and tectonic framework of turbidite hosted gold deposits in the western Lachlan Fold Belt, Victoria:^®Ar-3^Ar results. Ore Geology Reviews 13: 2 2 9 - 2 5 0 . Foster, D A . , D.R. Gray & M. Bucher, 1 9 9 9 . Chronology of deformation within the turbidite-dominated, Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana. Tectonics 18: 4 5 2 - 4 8 5 . Gray, D.R. & D A . , Foster 1 9 9 7 . Orogenic concepts n application and a definition: Lachlan Fold Belt, eastern Australia. American Journal of Science 297: 8 5 9 - 8 9 1 . Phillips, G.N. & Hughes, M.J., 1 9 9 6 . The geology and gold deposits of the Victorian gold province. Ore Geology Reviews 1 1 : 2 5 5 - 3 0 2 .

SGTSG abstracts, February 2001

Cretaceous continental collision of the Sanandaj-Sirjan Zone, western Iran

123

M. MohajjeP ^ C.L. Fergusson^ and M.R. SahandP ^ School of Geosciences, University of Wollongong, Wollongong, NSW, Australia 2522 2 Geological Survey of Iran, Tehran, Iran 3 Present address: Geology Department, University of Tarbiat Modarres, PO Box 14155-4838, Tehran, Iran

A Cretaceous continental collision developed in the Sanandaj-Sirjan Zone of western Iran. The collision formed when a fragment from the breakup of Gondwana collided with the central Iranian microcontinent. The major tectonic elements of western and southwestern Iran include the Urumieh-Dokhtar Magmatic Arc, the Sanandaj-Sirjan Zone and the Zagros Fold-Thrust Belt. The magmatic arc includes rocks of Eocene to Quaternary age that formed during the Tertiary and present-day collision between central Iran and the Arabian Peninsula and its leading edge in the Zagros Fold-Thrust Belt. The Sanandaj-Sirjan Zone is subdivided into five sub-zones that from northeast to southwest are the complexly deformed sub-zone, the marginal sub-zone, the ophiolite sub-zone, the Bisotun sub-zone and the radiolarite sub-zone. The radiolarite sub-zone consists of southwest-vergent folded units and thrust sheets with Late Triassic to Late Cretaceous shallow-marine limestones, deep-marine radiolarites and some limestone megabreccias. The Bisotun sub-zone has a Late Triassic to Late Cretaceous shallow- to deepmarine carbonate succession thrust to the southwest. The ophiolite sub-zone has several Late Cretaceous ophiolites that were formed shortly prior to their emplacement as is the case of the related Samail ophiolite of Oman and the United Arab Emirates. The marginal sub-zone consists of a Late Jurassic—Early Cretaceous andesitic to silicic volcanic succession that formed in a shallow marine setting. The complexly deformed sub-zone consists of a late PalaeozoicMesozoic passive margin succession related to the formation of Tethys and overlain by deep-marine turbidites interpreted to have formed in a convergent margin setting.

collision zone. Closure of the Tethys was associated with an early deformation in the complexly deformed sub-zone and the Late Jurassic-Early Cretaceous continental margin arc preserved in the marginal subzone. A regional deformation formed in the Late Cretaceous related to the continental collision and formed northwest trending intense folds and axial planar cleavage. The ophiolite, Bisotun and radiolarite sub-zones formed major thrust slices that were emplaced to the southwest during the Cretaceous continental collision. Tertiary and present-day collision in western Iran has involved a regime of dextral transpression and has resulted in additional uplift: of the Sanandaj-Sirjan Zone with thin-skinned thrusting and folding in the Zagros Fold-Thrust Belt and magmatic activity in the Urumieh-Dokhtar Magmatic Arc. ^ ^ ^ ^ ^

Continental rifting to form one of the arms of the Tethyan Ocean occurred in the latest Triassic in the Sanandaj-Sirjan Zone as indicated by the rapid deepening in the complexly deformed sub-zone at this time. Apart from minor mafic volcanic rocks in the radiolarite sub-zone no oceanic crust formed in the opening of Tethys is preserved in the Cretaceous SGTSG abstracts, February 2001

124 Ultramafic rocks in the Gleneig Zone of western Victoria: tectonic implications V.J. Morand, R.A. Cayley, K.E. Wohit and D.H. Taylor Geological Survey of Victoria, PO Box 500, East Melbourne, Victoria, Australia 3002

New mapping (Morand et al., in prep) provides more details of the Proterozoic to Cambrian Delamerian Fold Belt of The Gleneig Zone of western Victoria. It consists of low grade marine metasediments and volcanics in the west (Nolan Subzone) and medium to high grade equivalents of these rocks in the east (Gleneig River Metamorphic Complex — GRMC). Numerous syn- to post-tectonic plutons intruded the zone during and after the Middle to Late Cambrian Delamerian Orogeny. Structural trends are north to northwest, with upright folds in low grade rocks and structural complexity increasing eastwards into the high grade rocks. The Yarramyljup Fault forms the eastern edge of the zone, juxtaposing sillimanite zone rocks of the Gleneig Zone against lower greenschist facies rocks (probably Middle to Late Cambrian) of the Grampians-Stavely Zone to the east. Small outcrops of ultramafic rocks occur in widely separated places in the Gleneig Zone, all of them metamorphosed to the same grade as the surrounding regional metamorphic rocks and thus pre- or synDelamerian. Two compositional groups are present: a normal group and an alkalic group. The largest body is the Hummocks Serpentinite, belonging to the normal group and forming three discrete bodies on the western edge of the GRMC. It is mainly serpentinised cumulate textured harzburgite surrounded by sheared talc-rich schist. Olivine and pyroxene are the main igneous minerals recognised, although they are pseudomorphed by serpentine and talc. The very low Ca contents revealed by whole rock analyses (Turner et al., 1993) indicate the pyroxene was orthopyroxene rather than Ca-bearing clinopyroxene, and hence the original rock was harzburgite. Chrome spinels are also present, with Cr# 71-93 and Mg# 7 - 2 8 (Turner et al., 1993). The Hummocks Serpentinite is considered to lie within a shear zone, the Hummocks Fault Zone. On the eastern edge of the body normal shearing along a moderately west-dipping fault is indicated by S-C fabrics, but these fabrics are locally folded, indicating a complex fault history. An origin as oceanic mantle has been suggested for the Hummocks Serpentinite by Turner et al. (1993). SGTSG abstracts, February 2001

To the east in Steep Bank Rivulet, two small fault slivers of metaperidotite occur within chlorite zone slate and metagreywacke. These ultramafic rocks also have a cumulate texture dominated by pseudomorphed olivine, but other preserved igneous minerals are clinopyroxene, kaersutite, phlogopite, chrome spinel, ilmenite and apatite. These minerals indicate an alkalic magma, and the original rock was probably a wehrlite. Metamorphic minerals are serpentine, talc, chlorite, magnesite, tremolite and magnetite, with talc-rich schist forming the sheared margins of the bodies. Chrome spinel compositions are different to those from the Hummocks (Cr# 33-65, Mg# 33-47), and they are similar to chrome spinels from xenoliths in basalts from Hawaii. The high Ti contents (1-3% Ti02) are similar to spinels from continental layered intrusions. In the Wando Vale area within the GRMC a few thin bands of ultramafic schist are interlayered with garnet- and staurolite-bearing schist and amphibolite. They are talc-actinolitic hornblende rock and actinolite-chlorite schist. Few igneous minerals are preserved and their affinities are uncertain. However, one sample has minor brown amphibole (probably kaersutite) and thus appears to belong to the alkalic group. The presence of Ca-amphibole after Capyroxene indicates the original rocks were Iherzolite or wehrlite. In Young and Robsons creeks in the south of the GRMC, northeast of Coleraine, two small bodies of ultramafic rock with relic coarse igneous texture crop out within migmatite. They are mainly actinolitic hornblende, chlorite and talc, with some phlogopite, suggesting they belong to the alkalic group. Another set of ultramafic rocks occurs within the Woodlands Shear Zone cutting sillimanite zone schist near the eastern edge of the Gleneig Zone, south of Balmoral. These rocks are mosdy phyllonite consisting of actinolitic hornblende and minor chlorite, but one sample has a relic plutonic texture with olivine inclusions (now talc + magnetite) within large pyroxenes (now actinolitic hornblende) and some phlogopite interpreted as an igneous mineral. This also belongs to the alkalic group. These bodies in the high grade part of the G R M C were probably clino-

pyroxenites as they are dominated by Ca-bearing amphiboles. North of Balmoral, and along strike from the Woodlands Shear Zone, CRAE drillhole DD88BL213 went through migmatite and intersected chlorite-magnetite-?taIc schist with high Ni and Cr values consistent with it being an ultramafic rock. The alkalic ultramafic rocks have a within-plate signature, suggesting a kinship with the Early Cambrian Truro Volcanics. These are mainly basaltic volcanics interbedded with marine clastic sediments and carbonates of the Normanville Group, equivalents of which crop out in the Glenelg Zone. Truro Volcanics erupted during the early stages of rifting of Precambrian continental crust which was followed by deposition of the Neoproterozoic to Cambrian passive margin sequence that was deformed during the Delamerian Orogeny to produce the exposed southern Delamerian Fold Belt. The alkalic ultramafics are possibly cumulates from mid-crustal magma chambers which gave rise to the Truro Volcanics. These magma chambers may have been thrust upwards into the overlying sedimentary rocks during Delamerian deformation. Their wide distribution in the Glenelg Zone may result partly from extensional faulting during rifting bringing them close to the surface before Delamerian contraction. The normal ultramafics, as exemplified by the Hummocks Serpentinite, are slices of mantle incorporated into the crust early in the Delamerian Orogeny. Although described as the basal part of an ophiolite by Turner et al. (1993) on the basis of its strongly depleted MORB-normalised incompatible element contents, there are no remnants of the oceanic crust (layered gabbros, sheeted dykes, pillow lavas) and overlying sediments (chert and argillite) in the region. Metasedimentary rocks adjacent to the Hummocks Serpentinite include continentally derived sandstone and siltstone and dolomitic shale, consistent with a passive continental margin setting. Two possible origins for the Hummocks Serpentinite are presented here. The first assumes the Hummocks Fault Zone is a steep fault penetrating a significant thickness of crust, along which the serpentinite has been emplaced from lower crustal depths, perhaps from a thrust plane within the mid-crust. Serpentinisation occurred before faulting, producing a buoyant and slippery rock that could be squeezed up the fault, without the necessity of major vertical displacement across the fault, for which there is no evidence. A second possibility is that the Hummocks Fault Zone was a subhorizontal thrust that has been since been folded and faulted, and largely removed by erosion, with the Hummocks Serpentinite

constituting a klippe sitting on top of the thrust plane. 125 In this scenario, the Hummocks Serpentinite could be outlying remnants of the Dimboola Igneous Complex—an oceanic fore-arc complex that was thrust westwards during arc-continent collision, overriding the passive margin sequence of the Glenelg Zone on the down-going plate to cause the Delamerian Orogeny (see Taylor & Crawford, this volume). Such a scenario explains the apparent truncation of the Hummocks Fault Zone along strike, and the similarity of lithotectonic character on each side of the Hummocks Serpentinite. However, the bulk of the Dimboola Igneous Complex proper occurs in the Grampians-Stavely Zone 70 km east of the Hummocks and would thus require erosional removal of a huge sheet of ultramafic and mafic rock during and afi:er the Delamerian Orogeny, for which there is litde record in the sedimentary history of the region. ^

References

Morand, V J . , Wohk, K.E., Kemp. A.I.S., Cayley, R A . Taylor, D.H. & M^art, A.P., in prep. Glenelg special map area report. Geological Survey of Victoria map report. Turner, S.P., Adams, C.J., Flottmann, T. & Foden, J.D., 1 9 9 3 . Geochemical and geochronological constraints on the Glenelg River Complex, western Victoria. Australian Journal of Earth Sciences 40: 2 7 5 - 2 9 2 .

SGTSG abstracts, February 2001

126 Global-scale constraints from small terranes: a 1.2-1.0 Ga tectonothermal event In Avalonia (Appalachian Orogen) and constraints for the evolution of Rodlnia J.B. Murphy\ R.D. Nance^ R.A. Strachan^ K.D. Parker^ and M.B. Fowler^ ^ Tectonics Special R e s e a r c h Centre, T h e University of W e s t e r n Australia , N e d l a n d s , Perth, W A , A u s t r a l i a 6009 2 Department of Geological Sciences, O h i o University, A t h e n s , Ohio, 4 5 7 0 1 , U S A 3 Geology (BMS), O x f o r d Brookes University, Oxford, 0 X 3 O B P UK

Neoproterozoic crustal evolution was profoundly influenced by the amalgamation of the supercontinent Rodinia, its subsequent breakup, and "Pan-African" (650-550 Ma) collisional events that resulted in the formation of Gondwana (e.g. Powell et al., 1993; Dalziel, 1997). However, the timing remains controversial, as does the configuration of Rodinia (e.g., Karlstrom et al., 1999; Wingate et al., 2000). Most tectonic interpretations for the Neoproterozoic are based on the near-field effects of these events. However, global-scale geodynamic linkages implies that far-field effects should also be preserved. The Neoproterozoic evolution of Avalonia, the largest terrane in the Appalachian orogen of North America, is thought to have been geodynamically linked to the amalgamation and dispersal of Rodinia. Similar SmNd isotopic signatures for different periods of arc activity suggest that Avalonian basement, or protoAvalonia, was generated in a series of primitive oceanic island arcs between 1.2 and 1.0 Ga. Since this interval coincides with the amalgamation of Rodinia, protoAvalonia is inferred to have been located in a Panthalassa-like peri-Rodinian ocean. An early (760660 Ma) phase of Avalonian arc activity is attributed to renewed subduction in the peri-Rodinian ocean following the breakup of Rodinia, which caused the accretion of Avalonian terranes to the Gondwanan margin by c. 650 Ma. Further subduction along the margin occurred outboard of these terranes and resulted in the onset of main-phase Avalonian volcanism at 630 Ma. The diachronous cessation of arc magmatism is attributed to ridge-trench collision and the generation of a continental transform. The geodynamic linkage between Avalonia and Rodinia is analogous to that between the Mesozoic dispersal of Pangea and the tectonothermal evolution of western North America. This event also resulted in the accretion of outboard terranes and in arc-related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the San Andreas transform. The model SGTSG abstracts, February 2001

implies that the Neoproterozoic evolution of Avalonia and other peri-Gondwanan terranes provide important constraints on the tectonic history of a large portion of the Rodinian continental margin More generally, by comparing the isotopic characteristics of different generations of magmatic activity in a terrane, the relative contributions of juvenile and ancient crust may be evaluated. Similar Sm-Nd isotopic signatures for Neoproterozoic and Early Paleozoic tectonothermal events within Avalonia suggest that 1.2 to 1.0 Ga depleted mantle model ages represent a tectonothermal event rather than mixing between older crust and more juvenile magma and that subsequent events recycled this crust,

References Dalziel, I.W.D., 1 9 9 7 . Overview: Neoproterozoic-Paleozoic geography and tectonics: review, hypotheses and environmental speculations. Geological Society of America Bulletin 109: 16-42. Karlstrom, K.E., Williams, M . L , McLelland, J., Geissman, J.W. & Ahail, K-I., 1 9 9 9 . Redefining Rodinia: Geologic evidence for the Australia-Western U.S. connection in the Proterozoic. G S A Today 9(10): 1-6. Powell, C.McA., Li, Z.X., McElhinney, M . W , Meert, J.G. & Park, J.K., 1 9 9 3 . Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana. Geology 2 1 : 8 8 9 - 8 9 2 . Wmgate M . T D . & Giddins, J . W , 2 0 0 0 . Age and paleomagnetism of the Mundine Well dyke system. Western Australia: implications for an Australia-Laurentia connection at 7 5 5 Ma. Precambrian Research 100: 3 3 5 - 3 5 7 .

Discovery of a km-scale early extensional shear zone that deformed the Broken Hill Pb-Zn-Ag orebody, NSW, Australia

127

M.P. Noble and G.S. Lister Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne, Victoria, Australia 3800

The commercially valuable Broken Hill Pb-Zn-Ag orebodies have been attributed to many causes. The most popular theories for their origin consider that they formed in a sedimentary basin during rifting. However, the rocks of the Broken Hill area have been subjected to extreme tectonism, and this makes any assertion in regard to the early stages of the geological evolution speculative. We do not know what the geometry was during the stage of basin formation. This project therefore set out to provide a 3D solid geology model for the region adjacent to the Broken Hill orebody. The strategy applied was to use the excellent lithological maps already in existence as base maps, to which structural mapping was superimposed to develop an accurate 3D solid geology (gOcad™) model. The veracity of this model was tested by attributing physical properties to individual rock volumes, and computing potential field datasets that were compared with those observed. This systematic approach has proven to be readily achievable. Detailed structural analysis, geological mapping and interpretation of regional aeromagnetic and gravity datasets has now been carried out, and a 3D model of the geology has been constructed using gOcad™. The geometry proposed provides good correlation with measured data. The geometry and lithologies of the Broken Hill synform appears to be the result of an early high temperature extensional shear zone that has subsequently been folded and overprinted by at least one, if not two, generations of high temperature shear zones (which operated at middle to upper-amphibolite facies conditions). The area has subsequently been folded and faulted (during several episodes spanning the Late Proterozoic and Paleozoic eras) and now assumes a complex geometry. There is no evidence of the early rift geometry presumed by previous workers. The early extensional shear zone more or less coincides with the Cues Formation and the Rasp Ridge Gneiss. These units are defined largely on the basis of the concentration of amphibolites, deformed metagranitoids, quartz-magnetite rocks, composite gneisses and metasediments. The amphibolites and metagranitoids are the deformed and metamorphosed

relicts of a -1685 Ma bimodal dyke and sill swarm produced during an early stage of crustal extension (Raetz, pers. comm., 2000). Contrasts in stress and temperature across this shear zone may be the reason for the localisation of the dyke and sill swarm at this crustal level. Once the dykes and sills were emplaced they were deformed and metamorphosed in the extensional shear zone. The effects of crustal extension during the period 1710-1600 Ma is now represented by a tectonostratigraphy that simply comprises rocks below the extensional shear zones (partially molten at -1600 Ma), rocks in the shear zone (coinciding here with the --1685 Ma bimodal sill swarm) and rocks above the shear zone (comprising sediments of coevally developing sedimentary basins, beginning with the -1690 Ma Sundown Group, and passing into the younger -1640 Ma Paragon Group). Rocks structurally below the shear zone in this area of the Broken Hill terrane are partially melted migmatites and gneisses (the Alma Gneiss), and the rocks structurally above the shear zone comprise lesser deformed and lesser metamorphosed metasediments (pelites, psammopelites and metavolcanics) of the Sundown Group. Rocks in the shear zone in this area are largely coincident with the locus of the -1685 Ma regionally developed sill swarm, comprising hypabyssal mafic rocks and granitoid sills intruded as the result of bimodal igneous activity during extension. The extensional shear zone is regionally developed. Large-scale inversion of the Broken Hill terrane took place at -1600 Ma (Lister, pers. comm., 2000) and as a result the extensional shear zone was thrown into km-scale lobate recumbent folds, cored by partially molten rock (Venn, pers. comm., 2000). The geometry of the Broken Hill synform can be explained as the down-folded side of one of the oblate recumbent sheath folds that was formed during this event. The synform is cored by gneiss and migmatite. There is no corresponding Broken Hill antiform. Another (quite distinct) high-temperature shear zone crops out immediately adjacent (west) of the (now-folded) extensional shear zone, and transects the structure. This younger high-grade shear developed at SGTSG abstracts. February 2001

128

the same time that the older extensional shear zone was thrown into gneiss-cored sheath folds. This (younger) shear zone is also marked by intense ductile deformation, and also operated under hightemperature conditions (upper amphibolite facies). The younger high-temperature shear zone is of immediate interest in that the Broken Hill orebodies are now located within it. The shape of the orebodies are not consistent with the stretching direction that can be inferred for the operation of the this younger high temperature shear zone. The stretching lineations in this shear zone plunge steeply south. The Broken Hill orebodies now have the shape of elongate bent and flattened cigars within the younger high-grade shear zone. The southern extent of the ore zone has an orientation that is consistent with deformation in the younger hightemperature shear zone. The elongation of the northern part of the orebodies is in a direction that is shallowlydipping southwards. This is a direction quite different to that of the stretching direction within the younger high-temperature shear zone. Therefore we can infer that the shape of the ore deposit is only in part related to stretching in the younger high-temperature shear zone. The younger high-temperature shear zone (that which now contains the Broken Hill orebodies) truncates the older (now folded) extensional shear zone. The direction of relative movement is such that one might expect to discover lenses of the older shear zone preserved within the younger shear zone. It should be possible to distinguish these lenses by the orientation of the stretching lineations contained therein. Lenses of the older shear zone should contain a stretching lineation that was gently plunging north. Note that since the older shear zone has been deformed and folded into its present orientation, the orientation of these lineations is no longer an accurate indicator of the direction of relative motion when this shear zone operated. The present orientation of the older stretching lineation varies with its position on the younger folds. In the east it is gently plunging towards the south. As it folds around the younger hinge it is gently plunging east. On the truncated western limb it is gently plunging north. The northern extent of the deformed lodes of the Broken Hill orebodies have an orientation that is consistent with deformation in the older (now-folded) extensional shear zone. We can therefore argue that parts of the older high-temperature (extensional) shear zone have been preserved as lenses within the younger shear zone, and that these lenses contain the Broken

SGTSG abstracts. February 2001

Hill orebodies. These lenses have been wrapped around by the anastomosing fabrics of the younger hightemperature shear zone. This observation provides important constraints as to the timing of mineralisation at Broken Hill, and provides a significant clue as to the active tectonic environment in which ore deposition took place.

Summary

The structural data are consistent with the existence of a folded high-temperature shear zone, of extensional origin. This early extensional shear zone has been folded and truncated by a later high-temperature shear zone, in which the Broken Hill orebody is currendy located. The shape of the Broken Hill orebodies (doubly plunging elongate flattened cigars) suggests that the orebodies were deformed with the early extensional shear zone, and that they too were caught up in the later cross-cutting high-temperature shear zone. The early high temperature shear zone is thus the oldest structure (apart from sedimentary bedding) to which the Broken Hill orebodies can be related. This suggests alternative models for the origin and location of Broken Hill mineralisation that need to be considered, . y f ^ f ^

How appropriate are the Southern Alps of New Zealand as a model for continental collision?

129

R.J. Norris and P.O. Koons Department of Geology, University of Otago. PO Box 56, Dunedin, New Zealand

The Southern Alps of New Zealand's South Island are the result of oblique convergence between the Australian and Pacific plates. The interplate vector currently is oriented 15-20® to the average trend of the plate boundary with a strike-parallel component of c. 35 mm/yr and a boundary-normal component of c. 10 mm/yr. 70—90 km of total convergence have taken place across the boundary, mainly within the last 6 Ma. This has resulted in a doubling of the thickness of the continental crust beneath the Southern Alps, uplift of a range of mountains reaching over 3000m in altitude, and exhumation of metamorphosed rocks from the mid to lower crust. The Southern Alps are bounded on the western side by the Alpine Fault that currently accommodates approximately 75% of the interplate motion. The deforming zone may be modelled as a two-sided critical wedge, with intense erosion on the western side focussing deformation along the Alpine Fault. The Southern Alps are highly instructive in understanding processes of continental collision. However, they have a number of features which make them distinct from the classical concept of a continental collision zone. • The Southern Alps evolved from a zone of oblique extension to a zone of oblique convergence over time due to changes in plate rotations. Thus the two continental plates have always been in contact and no ocean has been subducted between them during the history of the present plate boundary. A preexisting Andean-type margin with its associated topography does not therefore influence the Southern Alps. This is in marked contrast to most models of continental collision. The Southern Alps may, however, be a useful model for understanding ancient collision zones that appear to originate within a single "plate". • The motion between the two plates is highly oblique. Although cross-sections of the belt are commonly interpreted solely in terms of the convergent component of motion, the major part of interplate displacement is perpendicular to the sections. This leads to many structures oblique to the plate boundary and to bending due to

distributed transpressional strain. It also probably results in substantial differences in the deep structure of the belt. The section of plate boundary that juxtaposes two continental fragments as a collision zone is relatively short (c. 300 km) and getting shorter. "Edge" effects from the two oblique subduction zones at either end extend throughout the collisional zone, creating marked changes along the length of the belt. These changes are mirrored by the pattern of faulting and crustal deformation east of the Alpine Fault, the width of the deforming zone, and topographic and drainage patterns within the Pacific plate. Although current strike-slip rates on the Alpine Fault are relatively constant along its length at c. 75% of the total interplate rate, dip-slip rates vary from a maximum of c. 10 mm/yr in the central part, to c. 6 mm/yr (50% of the convergence rate) at the northern end and to zero at the southern end. This slip distribution indicates variable slip partitioning along the length of the orogen and is mirrored by the structural characteristics of the plate boundary. The oceanic setting of South Island athwart the Roaring Forties results in extreme rainfall gradients across the island and a highly asymmetric and narrow orogen. The high erosion on the western side means that rock sequences must undergo rapid transit through a deforming zone in which their major velocity changes direction from horizontal to vertical. Different climatic settings are likely to result in very diflferent styles of deformation. Lack of strong erosional contrasts will lead to a wider, more symmetric orogen with distributed deformation and slow rates of movement of rocks through the system. The total amount of convergence in the Southern Alps is relatively small at c. 70-90 km. Modelling suggests that the development of stacked sections of crust in the form of nappes will take place during collision and appear at the surface after larger amounts of convergence. These have not yet appeared in the Southern Alps in which the surface geology is in effect a single crustal section, whereas SGTSG abstracts. February 2001

130

in belts with greater total convergence, repeated structural sequences are likely. In summary, the Southern Alps are a small, narrow, highly oblique, intracontinental collision zone developed from a transform boundary. They exhibit distributed transpression and variable slip partitioning along their length. Intense asymmetric erosion results in highly focussed asymmetric deformation. They exhibit rapid changes along the orogen both spatially and temporally. Clearly the Southern Alps have significant differences from the classic model of continental collision and may not be a perfect model for many zones of continental collision. Nevertheless, they provide considerable insight to the processes of continental collision and, because the signals are so

strong, allow assessment of the effects of many different boundary conditions. They raise important questions regarding the interpretation of features observed in ancient collisional zones,

Figure 1. South Island, New Zealand, showing the main basement rock units, the Alpine Fault and related structures, and the distribution of smaller faults active during late Cenozoic

SGTSG abstracts, February 2001

Modelling magma ascent

131

A. Ord, L. Moresi and B.E. Hobbs CSIRO Exploration and Mining, PO Box 437, Nedlands, WA, Australia 6009

We aim to provide quantitative constraints to geological interpretations regarding magma ascent throughout the lithosphere of the Earth. We do this through the application of numerical modelling techniques to the various processes, both active and passive, inferred to occur during magma ascent. This requires consideration both of the boundary conditions, for example of pressure and temperature, present throughout the lithosphere, and of the mechanical behaviour, or rheology, of the rock under these conditions. The mathematical description of the geometry, the rheology and the boundary conditions of the system through numerical codes applies rigorous constraints to our geological interpretations regarding magma ascent. For example, many treatments of the mechanics of intrusive behaviour assume the magma is a viscous fluid with constitutive behaviour that does not include elasticity or yield; that is, the magma is incapable of supporting a shear stress without flowing. The driving force for intrusion is buoyancy alone. Although the assumption of a lack of yield stress is perhaps reasonable for melts which contain no crystals the assumption of no elastic behaviour is quite unreasonable since such melts are likely to have an elastic bulk modulus exceeding 1 P a . For a crystal mush or crystal bearing magma the assumption of lack of yield behaviour is also unreasonable. For magmas with such behaviour an additional driving force for intrusion is the shear stress induced by magma pressure differences or by deformation of the country rock. We examine here various approaches to modelling intrusive behaviour including: (i) some particle-in-cell' modelling procedures where both the magma and the country rock have only viscous behaviour. Diapirs develop and we explore the complex entrainment and mixing geometries that develop once the diapir approaches and reaches the level of neutral buoyancy. Such modelling is perhaps representative of magma intrusion in early Archaean greenstone belts. It is also possible to include elasticity and yield in such particle-in-ceir codes and we examine the differences in behaviour of magmas given these extra theological properties over and above pure viscosity. We also examine the

feedback between boyancy driven melt and melting ahead of the melt package as a mechanism for transporting melt through the mande (ii) pure particle' modelling procedures where both the magma and the country rock have (different) elastic-plastic constitutive behaviour and explore the intrusion shapes and country rock fracture systems that arise from this intrusive action. Such behaviour is perhaps representative of high level plutons especially those that rise from larger plutons that have been emplaced in the crust at or near their level of neutral boyancy and where the driving mechanism is magma pressure together with buoyancy. (iii) a continuum hydrofracture process whereby the magma rises due to the fluid pressure gradient and hydrofractures the country rock ahead of itself The country rock has elastic-plastic behaviour with the capacity to hydrofracture whereas the magma is elastic-viscous with no yield behaviour. We examine the shapes that can develop for such intrusions. The behaviour is perhaps representative of mid to high level magmas with little crystal content.

SGTSG abstracts, February 2001

132 polyphase deformation of the Itremo Group in the Kiangara l\/lountains, central Madagascar A.R. Passmore and C.McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, Australia 6907

Structural mapping of the Proterozoic metasedimentary Itremo Group in the Kiangara Mountains of Central Madagascar has identified four phases of folding and an intrusive event around 800 Ma (Cox et al., 1998; Moine, 1968). A porphyritic K-feldspar granite pluton dated at 791.4 ± 1.3 Ma and a gabbro pluton 793.1 ± 1.5 Ma are part of a 450-km long magmatic belt intruded into the Itremo Group and its underlying basement (Handke et al., 1999). Field data and microstructural evidence indicate at least one deformation event occurred prior to the intrusion of the granite and gabbro plutons. This early deformation (D^) involved the formation of regionalscale isoclinal folds, with F^ hinges plunging gently west-southv^est. Stratigraphic younging and fold asymmetry indicate that the Itremo Group metasediments in the Kiangara Mountains are part of the lower limb of a regional F^ recumbent fold. When Fj hinges are restored to their original orientation, F^ folds verge to the south. During Dj, the Itremo Group metasediments underwent greenschist-facies metamorphism, although locally metamorphic mineral assemblages indicate metamorphism to upperamphibolite facies grade. A pervasive foliation (S^) related to D^ is defined by the alignment of fine-grained muscovite, biotite and quartz. This is folded by tight to open F2 microfolds, which are overgrown by coarse muscovite, cordierite and segregations of fibrolitic sillimanite. The presence of coarse muscovite and fibrolitic sillimanite overgrowing the F2 folds indicates that the second deformational event (D2) occurred locally before the peak temperatures related to the granite and gabbro intrusions were reached Segregations of K-feldspar and quartz crosscut the S^ foliation in Itremo Group biotite schist near the granitoid contact. These segregations are relict partial melt of country rock, which formed during intrusion of the ca. 792 Ma granitoid, and provides additional evidence that the pervasive S^ foliation is older than the intrusions. Structures related to a third set of folds (F3) are locally crosscut by the granite pluton. At other localities, the relative timing relationships are less clear. SGTSG abstracts, February 2001

F3 trends north to north-northwest and could have pre-dated or been associated with intrusion of the plutons. Alternatively, the reason for the folding not being evident in the granite and gabbro is that there is a rheology contrast between the igneous rocks and sediments. Solid-state deformation fabrics in the Kfeldspar granite together with a weak foliation in the associated contact aureole, and folding in the metasediments indicate a fourth deformation (D4) younger than the emplacement of the ca. 792 Ma granite and gabbro plutons. The recumbent Fj and S^ in the Itremo Group in the Kiangara Mountains are of regional extent, and could be either early-formed parts of deformation associated with emplacement of the 800 Ma granitoid and gabbroic plutons, or they could be a separate older deformation (Powell et al., 2000). F2 and possibly F3 are temporally related to the intrusions, and F4 is likely to be related to the 630-550 Ma contractional deformation associated with the assembly of Gondwanaland. ^^f^f^

References Cox, R., Armstrong, R.A. & Ashwal, L.D., 1998. Scdimcntology, geochronology and provcnancc of the Protcrozoic Itremo Group, central Madagascar, and implications for pre-Gondwana paleogeography. Jour. Geol. Soc. London 155: 1 0 0 9 - 1 0 2 4 . Handke, M.J., Tucker, R.D. & Ashwal, L.D., 1999. Neoproterozoic continental arc magmatism in west-central Madagascar. Geology 27: 3 5 1 - 3 5 4 . Moine, B., 1968. Carte du Massif Schisto-Quartzo-Dolomitique. 1/200.000. Antananarivo, Madagascar. Service G^ologique de Madagasikara, 1 sheet. Powell, C.McA., Dahl, K.L., Hulscher, B, Johnson, S.P, Passmore, A.R., Collins, A.S., Fitzsimons, I.C.W. & Jonsson, M.K., 2001. The significance of 8 0 0 Ma or older deformation in Central Madagascar. Geol. Soc. Aust. Abstracts (this volume).

Palaeomagnetic constraints for the building blocks of Rodinia

133

S.A. Pisarevsky and C.McA. Powell Tectonics Special Research Centre, The University of Western Australia, Department of Geology and Geophysics, Nedlands, WA. Australia 6907

There are several versions of the composition and configuration of the late Proterozoic supercontinent Rodinia, for instance: Hoffman, 1991; Dalziel, 1997; Weil et al., 1998. The main features are the same in all of them: Laurentia, the core of Rodinia, is surrounded by other blocks. East Gondwanaland in all three models was connected to its W-SW margin, according to SWEAT hypothesis of Moores (1991). However, other histories of the Laurentias western margin were proposed recently (e.g. Karlstrom et al., 1999; Burrett and Berry, 2000; Sears and Price, 2000). Amazonia-Rio de La Plata-West Africa craton (or cratons) and Baltica were attached to the east, southeast and south margins, and Siberia - to the northern margin (in Laurentia present-day orientation); however, the latter is a matter of discussion (e.g. Ernst et al., 2000). The positions of other cratons — Kalahari, Congo/San-Francisco, North and South China and others — are less certain. The palaeomagnetic data provide quantitative constraints for the Precambrian palaeoreconstructions. We have used McElhinny and McFaddens (1999) analysis of the Laurentian data, and their APWP as a framework for Rodinia reconstructions. Both SWEAT and AUSWUS models are possible according to Australian palaeomagnetic data. However, we prefer the modified SWEAT fit mainly because of the new analysis of Kalahari poles which leads to the "reversed" position of Kalahari with its Namaqua-Natal foldbelt facing away from the core of Rodinia. The comparison of Grenvillian and Sveconorwegian Loops leads to the conclusion that Baltica was not attached to East Greenland (Dalziel, 1997), but further to the south with the Rockall Plateau in between, which fits the models of Park (1992) and Starmer (1996). . y f f ^

References Burrett, C & Berry, R., 2000. Proterozoic Australia-Western United States (AUSWUS) fit between Laurentia and Australia. Geology 28: 103-106. Dalziel, I.W.D., 1997. Neoproterozoic-Paleozoic geography and tectonics: review, hypothesis, environmental speculation. GSA Bulletin 109: 16-42. Ernst, R.E., Buchan, K.L., Hamilton, M.A., Okrugin, A.V:&Tomshin, M . D . , 2 0 0 0 . Integrated paleomagnetism and U-Pb geochronology of mafic dikes of the Eastern Anabar shield region, Siberia: implications for Mesoproterozoic paleolatitude of Siberia and comparison with Laurentia. The Journal of Geology 108: 381-401. Hoffman, P. R, 1991. Did the breakout of Laurentia turn Gondwana inside out? Science 252: 1409-1412. Karlstrom, K.E., Harlan, S.S., Williams, M.L., McLelland, J., Geissman, J.W & Ahall, K.-L, 1999. Refining Rodinia: geologic evidence for the Australia-Western U.S. connection in the Proterozoic. GSA Today 9(10): 1-7. McElhinny, M.W. & McFadden, P.L, 1999. Paleomagnetism: Continents and Oceans. Academic Press, San Diego: 386 p. Moores, E.M., 1991. Southwest US - East Antarctic (SWEAT) connection: a hypothesis. Geology 19: 425-428. Park, R.G., 1992. Plate kinematic history of Baltica during the Middle to Late Proterozoic: a model. Geology 20: 725-728. Sears, J.W. & Price, R.A., 2000. New look at the Siberian connection: no SWEAT. Geology 28: 423-426. Starmer, I.C., 1996. Accretion, rifting and collision in the North Adantic supercontinent, 1700-950 Ma. In: Brewer, T.S.(cd), Precambrian crustal evolution in the North Adantic region. Geological Society Special Publication 112: 219-248. Weil, A.B., Van der Voo, R., Mac Niocaill, C & Meert, J.G., 1998. The Proterozoic supercontinent Rodinia: paleomagnetically derived reconstruction for 1100 to 800 Ma. Earth Planet. Sci. Lett. 154: 13-24.

SGTSG abstracts. February 2001

134 The systematic analysis of porphyroblast-matrix relationships in deformed rocks G.J. Potts^ and S.M. Reddy^ ^ Dept. of Earth Sciences, Liverpool University, 4 Brownlow Street, Liverpool L69 3GP, UK 2 Tectonics SRC, Dept. of Applied Geology, Curtin University of Technology, Perth, WA, Australia 6845

The analysis of porphyroblast-matrix relationships forms a critical part of deformation studies in metamorphic terrains. Porphyroblast-matrix systems provide both kinematic and temporal information. Recent research has focussed on the kinematic significance of porphyroblast-matrix relationships and the temporal significance has been neglected. Here we present a comprehensive analysis of the age relationships and deformation histories that can be extracted from porphyroblast-matrix systems in single and multiple sites. For a simple system of one foliation and one continuous period of porphyroblast growth there are 11 possible deformation histories. These histories display various combinations of pre- syn- and postfoliation porphyroblast growth and they can be investigated by considering, (a) the patterns of age relationships associated with the various histories, (b) the morphology of the porphyroblasts (the presence or absence of a particular phase of growth) or, (c) the exact geometry of a porphyroblast and its inclusion fabric and matrix. Analysis of the age relationships associated with the 11 possible histories indicates that the system is highly ambiguous with several histories sharing one or more pieces of evidence. The degree of ambiguity is reduced when morphologies rather than relationships are considered. The 11 histories generate six morphologies, three of which are unique. Unfortunately the remaining morphologies are common to two sets of two and one set of four possible deformation histories. For a single site with uniform porphyroblasts this discrepancy can be explained in terms of missing relationships/periods of growth. T h e possible deformation histories that are supported by a particular collection of observations have been identified. Thus, although the interpretation of porphyroblast-matrix relationships is ambiguous these ambiguities can be recognised easily. In a single location individual porphyroblasts may have grown for only part of the history and this can lead to the presence of more than one type of porphyroblast. From knowledge of the systematics of

SGTSG abstracts, February 2001

the 11 histories, groups of compatible and incompatible porphyroblasts have been identified. Compatible porphyroblasts can form parts of a single deformation history whereas incompatible porphyroblast cannot. Incompatible porphyroblasts indicate that the deformation history is more complex involving either two foliations or two distinct periods of porphyroblast growth. The patterns of ambiguity are different to those arising from single sites with uniform porphyroblasts but again, these ambiguities have been identified. Where porphyroblasts have been observed in more than one location in partially exposed regions the correlation of structures may lead to further ambiguity. Using insights gained from the systematics of porphyroblast-matrix relationships in single sites the effects of correlating porphyroblasts (rather than foliations) can be distinguished from those that arise from correlating foliations. This information has been used to generate lists of possible deformation histories that are supported by a given set of observations.

The Mania River traverse across central Madagascar

135

C.McA. Poweir, M.T.D. WingateS R.D. Tucker^, I.C.W. Fitzsimons^ and LD Ashwal^ ^ Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, Australia 6907 2 Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130-4899, USA 3 Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box 1987, Perth, Australia 6001 ^ Department of Geology, Rand Afrikaans University, Auckland Park, 2006, South Africa

Madagascar occupies a key position in the understanding the configuration of Rodinia, its midNeoproterozoic breakup and the latest Precambrian formation of Gondwanaland. In Rodinia, Madagascar is commonly shown as lying at the edge of the supercontinent a billion years ago, whereas it lies near the middle of Gondwanaland, along the extension of the East African orogen, 530 million years ago. The structural and tectonic events that led to this change of palaeogeography are potentially recorded in the Itremo Group, a Proterozoic supracrustal quartzitepelite-carbonate succession in central Madagascar. The age of the Itremo Group is constrained to be younger than 1855 Ma, the age of the youngest detrital zircon grain yet analysed (Cox et al., 1998), and older than 800 Ma, the age of a bimodal suite of gabbro and monzonitic to syenitic granitoids that intrude the succession (Handke et al., 1999). The Itremo Group has been multiply deformed, as shown on Moines (1968) regional map. Little systematic mapping has been done since I960, when the French geological surveys in Madagascar were discontinued. Thus, there is a dearth of information about the structural relationships of the Itremo Group to its substrate, and to the plutons that intrude it. Accordingly, in July 1998, we conducted a 400-km raft trip down the Mania River to provide a 200-km-long structural transect across the Itremo Group. This was the first time the Mania River has been rafted. The Itremo Group is a succession of quartzite, pelite and carbonate rocks, with mafic sills intruding the pelite and carbonate rock. Primary sedimentary structures such as ripple marks, cross-bedding, mudcracks, graded bedding and stromatolites show that the quartzite is the oldest unit and the carbonate the youngest, with facies transitions between units. Structurally, the Itremo Group can be divided into two parts: an eastern domain where there is a dominantly meridional regional trend to the structure, as seen in the outcrop pattern on Moines 1:200,000 (1968) map, and a western domain where there are many domes and basins with a variety of fold axial

traces. The domains are separated by a north-trending set of folds and faults that runs past the village of Itremo. In the eastern domain, outcrop patterns suggest refolded folds are present, although the orientation and trend of the folds not trending north-south is unclear. Two large granitoid batholiths and a large gabbroic batholith are elongated north-south, with the outcrop patterns suggesting that the batholiths are folded by the north-trending set of folds. The metamorphic grade is generally low to upper greenschist, with sillimanite and muscovite present only on the southeastern and western margins of the domain. O n the Mania River traverse, three deformations were identified in the eastern domain: 1. Fp Early recumbent structures giving rise to belts of overturned or downward-facing structures. 2. F2: Upright, open to tight, locally overturned folds with N- to NNW-trending axial traces and axial surfaces dipping steeply westward. 3. F3: Open kink-like folds with steep-axial fold hinges and NW-trending axial surfaces. F2 is younger than the ca. 800 Ma batholiths, and F3 is related to late-stage sinistral offsets in the meridional fold trends. The western domain is dominated by moderate to shallow west-dipping foliation, which in places is parallel to bedding. There is a pronounced lineation plunging towards 260° on average with fluctuation from 290° to 230°. Tight to isoclinal folds with reclined hinges and the prominent foliation are present at scales from centimetres to hundreds of metres, and in the far west the whole outcrop pattern is dominated by reclined folds on the scale of kilometres. This Wdipping strongly-lineated foliation pervades all outcrops of the metasediments in the western domain. Sheath folds could be present. In contrast with the eastern domain, the western domain is at high metamorphic grade. Sillimanite and muscovite, or sillimanite and K-feldspar, are present in rocks of appropriate composition, but no kyanite and very little garnet was seen. Remnants of quartzoSGTSG abstracts, February 2001

136

feldspathic rocks partially melted at temperatures above 650-700°C are present. In places, the Itremo Group quartzite is recrystallised into aggregates of coarsegrained rose quartz with tourmaline and quartz simplectite filling potential sites of dilation. Westdipping mullions abound. The boundary between the eastern and western domains appears to be a zone of E-directed thrusts with a marked jump in metamorphic grade from East to West. The contact between the Itremo Group quartzite and the granitoid plutons just east of the Itremo thrust zone are mylonitic, with the gneissic foliation in the Imorona Granite becoming increasingly finer-grained and mylonitic as the quartzite contact is approached. Both east and west domains contain essentially undeformed pink granitoids that cross-cut F2 in the eastern domian, and the W-dipping foliation in the western domain. These structural and metamorphic relations enable us to establish the following structural history for the Itremo Group. 1. Early F^ folds, possibly recumbent but of unknown facing or vergence. 2. Intrusion of a bimodal suite of granitoids and gabbro/norite around 800 Ma (Handke et al., 1999). 3. E-directed thrusting and folding related to the imbrication of the Itremo Group. 4. Late-stage intrusion little-deformed granite at 550 Ma (Handke et al., 1999) 5. Sinistral wrenching along NW to NNW-trending steep shear zones. Whether the early recumbent folds are significantly older than the ca. 800 Ma intrusions, or approximately coeval is an open question. The E-directed folds and thrusts are probably related to the 630-550 Ma closure amalgamation of Africa and India during formation of Gondwanaland. The 550 Ma granites are essentially post-tectonic, but we note a weak fabric in one of these granites, the Carion Granite east of Antananarivo. Collins and others (2000) have reported a large extensional detachment along the eastern edge of the Itremo Group, which they related to extensional collapse after the E-directed imbrication and thrusting. This down-to-the west detachment would have been one of the youngest events in the region, ^ y f ^

SGTSG abstracts, February 2001

References Collins, A.S., Razakamanana,T. & Windley, B.R, 2000. Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African Orogen. Geol. Mag. 137: 3 9 51. Cox, R., Armstrong, R.A. & Ashwal, L.D., 1 9 9 8 . Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana paleogeography. Jour. Geol. Soc. London 155: 1 0 0 9 - 1 0 2 4 . Handke, M.J., Tucker, R.D. & Ashwal, L.D., 1999. Neoproterozoic continental arc magmatism in west-central Madagascar. Geology 27: 3 5 1 - 3 5 4 . Moine, B., 1968. Carte du Massif Schisto-Quartzo-Dolomitique. 1/200.000. Antananarivo, Madagascar. Service G^ologique de Madagasikara, 1 sheet.

The significance of 800 Ma or possibly older deformation in central Madagascar

137

C.McA. Poweir, K.L. DahP, B. Hulscher\ S.P. Johnson\ A.R. Passmore\ A.S. CollinsS I.C.W. Fitzsimons^ and M.K. Jonsson^ ^ Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, Australia 6907 ^Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box 1987, Perth, Australia 6001

Many reconstructions of the late Mesoproterozoic supercontinent, Rodinia, place Madagascar attached to India in its younger Gondwanaland fit on the eastern side of a wide Mozambique Ocean that separates India-Madagascar-East Antarctica from the CongoSao Francisco block of Africa (e.g. Dalziel, 1997). W h e n Rodinia broke up during the midNeoproterozoic, the Mozambique Ocean is postulated to have contracted by subduction beneath either or both of the India-Madagascar and east African margins while the Palaeo-Pacific Ocean grew between Australia-Antarctica and Laurentia. Final closure of the Mozambique Ocean is thought to have occurred in the late Neoproterozoic during the end-Precambrian formation of Gondwanaland. The expectation from this palaeogeographic sequence is that Madagascar should contain the evidence of: • late Mesoproterozoic continental sediments formed at the edge of the Mozambique Ocean, • mid- to late Neoproterozoic subduction-related magmatism and deformation related to closure of the Mozambique ocean, and • continent—continent collision when east Africa collided with India, closing the Mozambique Ocean Whether there was a late Mesoproterozoic continental margin in Madagascar is debatable. The possible candidate is the Proterozoic Itremo Group a succession of quartzite, pelite and carbonate rocks more than 2 km thick deposited as a platform sequence. The depositional age of the Itremo Group is constrained only broadly between 1855 and - 8 0 0 Ma (Cox et al., 1998). As neither thickness variations nor lateral facies changes have been documented, the Itremo Group could represent the deposits of an epicontinental basin. There is no direct field evidence for coeval continental slope and rise deposits, necessary if Madagascar lay at the edge of Rodinia. The Itremo Group is intruded by a 450 km-long meridional belt of gabbroic and granitoid batholiths, many of which are composite with large gabbroic cores

enclosed by thinner sleeves of monzonite and syenite. The gabbros and granitoids are coeval and formed between 804 and 779 Ma (Handke et al., 1999). These plutons are part of a much longer belt of bimodal volcanic rocks and alkalic granitoids that can be traced over 1000 km through Seychelles Islands into the Malani Igneous Province of N W India. The magmatic belt in Madagascar has been interpreted by Handke and others (1999) to be the roots of a former c. 800 Ma continental magmatic arc formed by eastward subduction of the Mozambique Ocean beneath Madagascar. Kroner and others (2000) suggested that, on lithological and structural grounds, the subduction zone dipped east from a trench that separated central Madagascar from southern India. An alternative to a subduction origin is that the magmatic belt represents the bimodal products of continental extension during breakup of this part of Rodinia around 800 Ma. Our discovery of a penetrative deformation with isoclinal recumbent folds and an early layer-parallel cleavage, overprinted by low-P-high-T minerals in the aureoles of the plutons (Hulscher et al., 2000) indicates that either there was an episode of deformation before the plutons were emplaced, or that progressive deformation had occurred during intrusion. The first possibility raises the question of whether there is any Grenville-aged late Mesoproterozoic deformation, which has been reported from the Mozambique belt (Costa etaL, 1994) but not recognized in Madagascar before. If the penetrative early deformation were of this age, we would expect to find its distribution unrelated to the c. 800 Ma plutons. Whether the early large recumbent folds are spatially related to the granitoids is yet to be tested. The second possibility is that the isoclinal folds with penetrative fabrics formed early during the same tectonic event that emplaced the c. 800 Ma plutons. There are two possibilities. One is that the penetrative deformation is related to upper-crustal imbrication of the postulated continental magmatic arc (Handke et al., 1999) during contractional deformation, analogous SGTSG abstracts, February 2001

138

to retro-arc thrusting in the Patagonian Andes. In this model, any foreland basin deposits would have lain above the platformal Itremo Group, and subsequently been eroded. The recumbent folds would have been associated with upper-crustal imbrication. The second option is to consider that the early deformation could be part of crustal extension, with heating locally outlasting deformation. The high geothermal gradient implied by the P-T conditions (= 60® per km), together with the attenuated metamorphic facies distribution, are to be expected with this model. The implications of the last interpretation are very important. Continental extension around 800 Ma has been interpreted in adjacent regions in the Zambezi belt (Dirks &: Sithole, 1999), the Zambian Copper Belt (Unrug, 1988), and in the East African orogen farther north (Stern, 1994). If the c. 800 Ma deformation in Madagascar were related to continental extension, it is possible that the Antananarivo block of Central Madagascar (Kroner et al., 2000) was originally part of the African collage of continental blocks in Rodinia. This possibility also raises the question of whether India, Congo and the Kalahari cratons were joined in Rodinia, in which case Madagascar was nowhere near the edge of Rodinia one billion years ago.

References Costa, M., Cadoppi P., Sacchi R. & Fanning, C.M., 1994. U - P b SHRIMP dating of zircons from Mozambique gneiss. Boll. Soc. Geol. Italy 113: 173-178. Cox, R., Armstrong, R.A. & Ashwal, L.D., 1998. Sedimcntology, gcochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana paleogeography. Jour. Geol. Soc. London 155: 1009-1024. Dalziel, I.W.D., 1997. Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, and environmental speculation. Bull. Geol. Soc. Amer. 108: 16-42. Dirks, PH.G.M. & Sithole, TA., 1999. Eclogites in the Makuti gneisses of Zimbabwe: implications for the tectonic evolution of the Zambezi Belt in southern Africa. J. Metamorphic Geol. 17: 593612. Handke, M.J., Tucker, R.D. & Ashwal, L.D., 1999. Neoproterozoic continental arc magmatism in west-central Madagascar. Geology 27: 351-354. Hulscher, B., Collins, A.S., Dahl, K.L., Fitzsimons, I.C.W., Johnson, S.P, Jonsson, M.K., Passmore, A.R. & Powell, C.McA., 2001. Evidence of 800 Ma and possibly older deformation in Central Madagascar. SGTSG Abstracts (this volume) Kroner, A., Hegner, E., Collins, A.S., Windley, B.R, Brewer, T.S., Razakamanana, T. & Pidgeon, R.T., 2000. Age and magmatic history of the Antananarivo Block, central Madagascar, as derived from zircon geochronology and Nd isotopic systcmatics. Amer. J.Sci. 300: 251-288. Stern, R.J., 1994. Arc assembly and continental collision in the Neoproterozoic Eat African Orogen: implications for the consolidation of Gondwanaland. Ann. Rev. Earth Planet. Sci. 22:319-351. Unrug, R., 1988. Mineralisation controls and Source of Metals in the Lufilian Fold Belt, Shaba (Zaire), Zambia, and Angola. Economic Geology 93: 1247-1258

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139

The map of Rodinia C.McA. Powell and S.A. Pisarevsky Tectonics Special Research Centre, The University of Western Australia, Department of Geology and Geophysics, Nedlands, WA, Australia 6907

There are several versions of the composition and configuration of the late Proterozoic supercontinent Rodinia. The main features are the same in all of them: Laurentia, the core of Rodinia, is surrounded by other blocks. The palaeomagnetic data from Laurentia and Australia support both SWEAT and AUSWUS models. However, we prefer the modified SWEAT fit mainly because of the new analysis of Kalahari poles which leads to the "reversed" position of BCalahari with its Namaqua-Natal foldbelt facing away from the core of Rodinia. Amazonia-Rio de La Plata-West Africa combined plate was attached to the southeast and south margins, and Siberia - to the northern margin (in Laurentia present-day orientation); however, the latter is a matter of discussion. The comparison of Grenvillian and Sveconorwegian Loops leads to the conclusion that Baltica was not attached to East Greenland, but further to the south with the Rockall Plateau in between.

SGTSG abstracts, February 2001

140 Continental collision: Processes, consequences and speculations C.McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, Australia 6907

Collision between continents is a necessary consequence of global tectonics where continental fragments travel as passengers on plates composed of both oceanic and continental lithosphere. Conventional wisdom is that oceanic lithosphere subducts beneath younger oceanic and continental lithosphere along subduction zones, with island arcs or Andean-type magmatic arcs being formed on the overriding plate. When continental crust arrives on the down-going plate at an ocean-ocean subduction zone, the continent crosses the subduction zone, displacing the locus of convergence between the plates. If the overriding plate is continental lithosphere, continental collision occurs. The convergence of Australia with Southeast Asia is an example of the former configuration, and the collision of India with the southern margin of Asia is an example of the latter. The details of what happens during continental collision are not clear. The Himalayas are widely recognised as the result of India's collision with Asia, and are commonly cited as the "type" example of a continent-continent collision. However, whether the Himalayas are typical of what happens when continents collide has not been addressed, and the more general philisophical question of whether it is scientifically sound to erect a behaviour class on one example is not addressed. There are two features of the Himalayan collision which have been known for nearly thirty years, but which are for most people just as puzzling today as when they were first discovered (Powell & Conaghan, 1973). The first is why the collision between India and Asia around 55 to 50 Ma did not lead to the formation of the Himalayas for 25 to 30 m.y, and the second is why the Himalayas did not form above the Indus-Tsangpo suture where the continents collided, but some 100 to 150 km to the south within the Indian continental lithosphere. The sequence of events related to the breakup of India from Australia-Antarctica and its subsequent collision with Asia is reasonably well documented, and can be considered in ten stages. 1. 135-99 Ma: Greater India (i.e. the currently exposed Indian subcontinent enlarged at its northern and eastern edges by continental lithosphere now possibly subducted beneath Tibet) began slow seafloor spreading away from the West Australia-Antarctic continental margin in a SGTSG abstracts. February 2001

7. 8.

northwesterly direction relative to Australia. India was at moderate southerly latitude. 99-70 Ma: India underwent very rapid northward rotation at rates just under 20 cm per year. An Andean-type magmatic arc was established along the southern margin of Asia. The Oman ophiolite was obducted onto Arabia from between 87 and 76 Ma (Searle & Cox, 1999). 70-55 Ma: India collided with a latitudinal system of island arcs that were separated from Asia by marginal seas. Ophiolites were obducted onto the leading edge of NW India (Mahmood et al., 1995). 55—40 Ma: India collided with the continental margin of Asia, with the ocean possibly closing earlier (55-50 Ma) in the northwest than in the northeast (45-40 Ma). Widespread Middle Eocene shallow-marine limestone was deposited above the suture zone in the northwest, and also in an arm of the Peri-Tethys sea which extended from Western Europe across central Asia to the Pamir-Kun Lun-Tarim Basin. 40-22 Ma: Topographic relief in northern India remained low, although there could have been mountains in southern Tibet, where the Kialas conglomerate was deposited as a piedmont to the Gandise Mountains that rose above the site of the Cretaceous-Paleogene Andean-type magmatic arc in southern Tibet. 22-18 Ma: Onset of continental subduction within Indian lithosphere along the Main Central Thrust, with coeval development of a large downto-the-north normal fault along the northern margin of the High Himalayas (Burchfiel & Royden, 1985). 18-10 Ma: Continued continental underthrusting and subduction along the Main Central Thrust. 10-4.5 Ma: Development of north-trending graben in the High Himalayas, and, by the earliest Pliocene, more widely in the southern half ofTibet. The southeast Asian monsoon started around 8 Ma, and the return dry winter monsoon deposited red clay in the northern Loess Plateau from c. 8.3 Ma (Qiang et al., in review). 4.5-2.7 Ma: Widespread uplift of the northern margin ofTibet (Zheng et al., 2000), with at least 3 km of relative relief being generated along the northwestern Kun Lun. Rapid uplift along the

northern margin of Tibet was accompanied bystrengthening of the dry northwestern winter monsoon at 3.5 Ma before the Northern Ice Age began at 2.7 Ma (Qiang et al., in review). 10. 2.7-0 Ma: Continued rise of Tibet to its present elevation, accompanied by an increase in the rate of sediment shed south from the Himalayas, the deterioration of climate in central Asia, and strong, dry, northwesterly winter winds depositing the thick blanket of yellow silt and clay in the Loess Plateau. When the two dilemmas about why the Himalayas did not form until 25 m.y. after the India-Asia collision, and then not over the site of the continental collision, were first raised, conventional wisdom was that continental crust could not be subducted (McKenzie, 1969; Molnar & Gray 1979). Many examples of supracrustal continental rocks that have been subducted to depths of 150 km or more are now known. Deep seismic reflection profiles show that Indian lithosphere currently lies beneath Tibet for at least 300 km north of the Indus-Tsangpo suture (Nelson et al., 1996). Xenohths brought to the surface of northern Tibet in Neogene volcanic rocks contain metamorphosed sediments that have been subducted to at least 45 km. We need to re-examine the why early workers did not consider subduction of continental crust to be possible. The main argument against subduction of continental crust relates to buoyancy (McKenzie, 1969; Molnar & Gray 1979). Molnar & Gray (1979) showed that by considering the relative densities of continental crust and the asthenosphere, no more than 130 km of continental crust could be subducted. Molnar and Grays (1979) calculations were based on the fiill length of the subduction zone being occupied the subducting continent; they noted that if the continent occupied only a part of the subduction zone, the buoyancy force would be proportionally reduced. In India's case, the 3000 km width of the subcontinent was arguably only 20% of the 12,000-km-long early Neogene subduction front along the northern edge of the India-Australia Plate, which extended from the Owen Fracture Zone to near Fiji. Thus, the limit on how much continent could be subducted using the 1979 calculations of Molnar & Gray (1979) could be increased to 500 km (Powell, 1986); this distance is close to the width of Tibet up to the Kun Lun Shan, measured parallel to the NNE-direction of subduction. Thus, buoyancy alone is not sufficient to prevent continental crust from being subducted for hundreds of km. The second issue relates to why intracontinental subduction commenced along the site of the Main Central Thrust, which, prior to 25 Ma, lay 100150 km south of the northern edge of the Indian

subcontinent. From a mechanical perspective, there appears to be no reason for subduction to commence within the Indian lithosphere, unless there were a preexisting weakness that localised the strain. A recent observation may provide the clue, viz: a line of c. 500 Ma plutons in the High Himalayas lies along the northern side of, and parallel to, the Main Central Thrust. These plutons are S-type and reflect melting of the Indian lower crust in the Late Cambrian. Powell and Singh (in prep.) speculate that the plutons reflect melting during crustal extension along the thennorthern margin of Gondwanaland, with the displacement being northern side down. The line of c. 500 Ma plutons marks the southern edge of the extensive thick Palaeozoic succession of the Tethyan Himalayas, which accumulated along the northern ocean-facing margin of Gondwanaland. At the very least, the line of S-type plutons reflects a deep-seated lithospheric dislocation that subsequently became the locus of strain during the continuing post-collision convergence between India and Asia. Oligocene cooling ages in some metamorphic rocks in the Main Central Thrust zone indicate that movement and heating along this dislocation could have accumulated during the Oligocene until frictional resistance to underthrusting on the zone as a whole was overcome in the earliest Miocene enabling continental underthrusting to commence. The inferencefiromthis is that the conception of the Himalayas was during continental extension almost 450 m.y. before India collided with Asia,

141

References

Burchfiel, B.C. & Royden, L.H., 1985. North-south extension within the convergent Himalayan region. Geology 13: 6 7 9 - 6 8 2 . Mahmood, K., Boudier, R, Gnos, E., Monie, P. & Nicolas, A., 1995. ^^Ai/^'^Ar dating of the emplacement of the Muslim Bagh ophiolite, Pakistan. Tectonophysics 250: 1 6 9 - 1 8 1 . McKenzie, D.R, 1969. Speculations on the consequences and causes of plate motions. Geophys. J. Roy. Soc. 18: 1 - 3 2 . Molnar, R & Gray, D., 1979. Subduction of continental lithosphere: some constraints and uncertainties. Geology 7: 5 8 - 6 2 . Nelson, K.D. and 27 others, 1996. Partially molten middle crust beneath southern Tibet: Synthesis of Project INDEPTH results. Science 274: 1 6 8 4 - 1 6 8 8 . Powell, C. McA. & Conaghan, P.J., 1973. Plate tectonics and the Himalayas. Earth Planet Sci. Letts 20: 1 - 1 2 . Powell, C. McA., 1986. Continental underplating model for the rise of the Tibetan Plateau. Earth Planet. Sci. Letts 81: 7 9 - 9 4 . Powell, C. McA. & Singh, D., (in prep.). The significance of ca. 500 Ma magmatism in controlling the site of the Neogene Himalayan Main Central Thrust. Qiang, X.K., An, Z.S., Li, Z.X., Powell, C.McA. & Zheng, H.B., (in review). Magnetostratigraphic record of the Late Miocene onset of the East Asia monsoon, and Pliocene uplift of northern Tibet. Earth Planet. Sci. Lett. Searle, M. & Cox, J., 1999. Tectonic setting, origin, and obduction of the Oman ophiolite. Bull. Geol. Soc. Amer. I l l : 1 0 4 - 1 2 2 . Zheng, H.B., Powell, C. McA., An, Z.S., Zhou, J. & Dong, G.G., 2000. Pliocene uplift of the northern Tibetan Plateau. Geology 28 (8): 7 1 5 - 7 1 8 .

SGTSG abstracts, February 2001

142 Neoproterozoic extension and Cambrian plate convergence in South Australia; relationships with Tasmania W. V. Preiss Office of Mineral and Energy Resources, PIRSA, GPO Box 1671, Adelaide, SA, Australia 5001

The Adelaide Geosyncline of eastern South Australia is one of the better studied Neoproterozoic to Cambrian rift complexes in the world, yet its tectonic setting is still debated. While contacts with older Australian cratons (Gawler Craton to the west and Curnamona Province to the northeast) are relatively clear, its southeastern boundary relationships are obscured by later Palaeozoic tectonic events as well as extensive younger cover. Whereas the effects of the 0.5 Ga Delamerian Orogeny on the sedimentary fill, extending south into Antarctica as the Ross Orogen, are well documented, the role of potential colliding or accreting crustal blocks is less clear. The basement beneath the Adelaide Geosyncline underwent protracted and extreme subsidence to provide accommodation space for 10-15 km of sedimentary fill comprising the Neoproterozoic Callanna, Burra, Umberatana and Wilpena Groups, as well as Early to Middle Cambrian deposits. The distribution and orientation of mafic dykes, shifting depocentres, and disposition of syn-sedimentary extensional faults all help to define a series of distinct rifted troughs suggesting the changing orientation of the extensional stress with time, from pre-break-up intracontinental rifting, through breakup, to postbreakup rifts. The Torrens Hinge Zone (THZ) marks the eastern limit of unattenuated Precambrian crust, and is expressed as a zone of extensional faulting during deposition of the Burra Group, flexuring during later phases of deposition, and a zone of gentle, open folding in the Delamerian Orogeny, west of the main thrust front.

Record of Neoproterozoic extension events and the timing of continental breakup --830 Ma. Arkaroola Subgroup of the Callanna Group. Early NE-SW extension manifested by the Gairdner Dyke Swarm within the Gawler Craton followed a brief period of stable, epi-continental sedimentation in a shallow epeiric sea and did not lead to the formation of major depocentres. Equivalent mafic lavas, originally widespread, were deeply buried by later rift deposits, and are only locally preserved or brought to the surface in diapirs. SGTSG abstracts, February 2001

--800 Ma. Curdimurka Subgroup of the Callanna Group. The first major rift valleys, formed by mostly NW-trending extensional faults, were restricted basins with mixed immature clastics and carbonates deposited under evaporitic conditions, but only minor local felsic and mafic volcanism. These troughs underlie and are mostly obscured by younger, more extensive Neoproterozoic strata, but to some extent their distribution can be inferred from the distribution of diapirs resulting from the mobilisation of the evaporitic sequences. ^-780 Ma. Burra Group. Rifting at this time resulted from generally E-W extension, bounded to the west by the THZ. Major grabens and horsts formed adjacent to the THZ in the Mid-North and Willouran Ranges, with thick, coarse-grained, basal fluvial facies (Rhynie Sandstone) and local mafic volcanism. The bimodal Boucaut Volcanics (rhyolite component 777 Ma), south of the Curnamona Province, probably resulted from the same extension event. The remainder of the fill includes lacustrine to paralic carbonates (e.g. Skillogalee Dolomite), followed by clastic sediments deposited during marine transgression into the rift basins. These troughs were probably still intracontinental, with both eastern and western sediment provenances, and sedimentation did not extend onto adjacent cratonic areas (Gawler Craton, Curnamona Province). --700 M a . Yudnamutana Subgroup of the Umberatana Group. New rifts trending N W to E-W were established around the margins of the Curnamona Province, where very thick Sturtian glacial successions were deposited. Iron-rich facies, comparable to those of the Rapitan in the NW Canadian Cordillera, were deposited in the Baratta Trough south and west of the Curnamona Province, but not in the equivalent Yudnamutana Trough to the north. Post-glacial marine transgression onto the Gawler Craton is recorded by the Tapley Hill Formation, marking the onset of the first major sag-phase sedimentation and probably the start of continental separation. Younger strata of the Umberatana and Wilpena Groups (including deposits of the Marinoan glaciation) represent shallow to deep shelf environments, with fiirther minor rifting likely

to have continued. However, there is little evidence from sediment thickness and facies patterns for major grow^th faulting, except along the THZ during deposition of the Wilpena Group. --590 Ma. The Koonenberry Belt of western NSW contains thick basalts interlayered in mostly fine clastics, provisionally correlated with volcanics at Mount Wright dated at - 5 9 0 Ma. Similar late Neoproterozoic mafic volcanism, reflecting major crustal extension, is nowhere recorded in equivalents within the Adelaide Geosyncline in South Australia, but may be represented on King Island. As no basement to the Koonenberry Belt is known, by which to link it to the Australian craton, the autochthoneity of this crustal block is open to question.

crustal block from the southeast with the formerly 143 passive continental margin may be inferred as a cause of this compressive deformation. These early structures extend only about 100 km north of Adelaide, and are expressed mainly as a bedding-parallel foliation and rarer isoclinal folds in higher-grade metasediments east of Adelaide. More extensive are the second-generation folds that trend generally north-south and fold the early foliation in the south, but fold only bedding in areas further north, unaffected by the first deformation. Most of the I and S-type granitic plutons accompanying the Delamerian Orogeny intruded late during or after this second fold event, while posttectonic A-type granite intrusives post-date substantial uplift and erosion.

Early Cambrian rifting and the nature of the Kanmantoo Trough Early Cambrian sedimentation (Hawker Group, Normanville Group) the in the Adelaide Geosyncline was dominantly in shelf to ramp settings, dominated by carbonates and reefs, but generally deepening upward. In the late Early Cambrian, the Kanmantoo Trough formed by rifting across the trend of older troughs, truncating the THZ in the south, where it swings into E-W orientation on Kangaroo Island. The trough is characterised by rapid subsidence and infill with turbidites in two megasequences, each culminating in shallow-water sandstone. The sedimentary fill is ensialic, overlying Neoproterozoic sediments east of the THZ and probably directly overlying basement where the trough forms a reentrant into the Gawler Craton on Kangaroo Island. Local, dominantly mafic volcanism ( - 5 2 0 Ma), immediately predating formation of the trough, extends under the Murray Basin into the Padthaway Ridge of southeast SA, and possibly into the Glenelg River region of Victoria, where Kanmantoo equivalents have long been inferred.

Position of Tasmania in the Neoproterozoic The evidence from northwest Tasmania and King Island is equivocal. On the one hand, diamictite and overlying cap dolomite strongly resemble (including by carbon isotopes) deposits of the Marinoan glaciation on the mainland, and stromatolites in the Black River Dolomite compare with those in the Skillogalee Dolomite; these have been taken to suggest proximity to the Adelaide Geosyncline. On the other hand, metasediments possibly forming the basement to these deposits are deformed and intruded by syntectonic granite dated at -760 Ma (the Wickham Orogeny) and are unlike any basement province on mainland Australia. Northwest Tasmania thus underwent orogeny while deposition was occurring in intracontinental rift basins in South Australia and its position in relation to Australia-Antarctica at that time is uncertain. Delamerian structures in southern SA suggest initial NW-SE plate convergence followed by transpressive deformation propagating northward. Collision may have involved a micro-continent of distant origins, of which NW Tasmania could have been a part, and Cambrian island arc volcanics in western Victoria, ^ f f ^

Delamerian Orogeny In South Australia the first evidence of compressive deformation is the onset of the Delamerian Orogeny, around the early Middle Cambrian; all earlier deformation relates to diapirism and rotation of extensional fault blocks. It is likely that sedimentation of the mostly Middle Cambrian Lake Frome Group redbeds in the central Flinders Ranges region was still proceeding while north-west directed thrusting was under way in the Mount Lofty Ranges region, where extensional faults, especially those defining the Kanmantoo Trough, were inverted. Collision of a

SGTSG abstracts, February 2001

144 Discrimination of transpression and reactivation in high-strain zones S.M. Reddy' and S.A. OcchipintP ^ Tectonics SRC, School of Applied Geology, Curtin University of Technology, Perth, WA, Australia 6845 2 Geological Survey of Western Australia, Dept. of Minerals and Energy, 100 Plain Street, Perth, WA, Australia 6004

High-strain zones, often referred to as shear zones, are localised areas of deformation that are common in the Earths crust. In the past, geologists have used the geometries of foliations and lineations within highstrain zones to establish the kinematic history of rock deformation. Such information has been used to establish the relative displacements of different parts of the crust and thereby construct tectonic histories. The basis for this approach has often been the assumption that shear zones deform by simple shear. This situation allows lineations on c-fabrics within the zone to be related to transport directions, while the sense of shear may be inferred from the geometry of structures that are asymmetric across the shear plane. Numerous detailed field studies of high-strain zones have recognised complexities that cannot be ascribed to a single, simple-shear deformation (Fig. 1). During natural deformation the operation of only simple shear deformation is unlikely and the deformation may involve combined pure and simple shear components, referred to as general shear' or transpression if the pure shear component is compressional perpendicular to the plane of the zone. Recent theoretical modelling of transpression has considered the geometry and magnitude of both finite and incremental strain in systems involving different magnitudes and orientations of pure and simple shear components. These models predict patterns of deformation that are far more complex than those produced by simple shear alone. An important conclusion of these models is that the finite strain axes associated with transpression may switch as deformation progresses. This will affect foliation and lineation orientations and, as a result, mineral stretching lineations in transpressional highstrain zones need not correspond to tectonic transport directions (Fig. Ic). Heterogeneous localisation of pure and simple shear components within the high-strain zone will also lead to fiirther complexities in foliation and lineation orientations. In addition, high-strain zones are rheologically weak and may undergo reactivation during subsequent phases of deformation. The structures preserved in high-strain zones may therefore reflect a polyphase deformation history (Fig. lb). As a result complex structural relationships SGTSG abstracts, February 2001

in high-strain zones may reflect single deformation events involving simultaneous components of both pure and simple shear or polyphase deformation histories. Discriminating between these transpression and reactivation scenarios is difficult but is crucial if correct interpretations of tectonic histories and processes are to be made. Within the Capricorn Orogen ofWestern Australia, Palaeoproterozoic high strain zones developed during the convergence and subsequent collision of the Archaean Pilbara and Yilgarn Cratons between 18301780 Ma. Here we present a preliminary structural study of a high strain zone from the southern margin of the Capricorn Orogen with the aim of ultimately establishing the relative importance of transpression or reactivation in the deformation history. The Capricorn Orogen comprises a number of amalgamated terranes. These include the Yarlarweelor Gneiss Complex (YGC), originally part of the Yilgarn Craton but re-worked and intruded by voluminous felsic magmatism during the Capricorn Orogeny; lowgrade sedimentary and volcano-sedimentary basins (e.g. the c. 2.0 Ga Bryah and Padbury Groups); and the Gascoyne Complex, interpreted as the "high-grade core" to the Capricorn Orogen. The Archaean to Palaeoproterozoic YGC lies in faulted contact with the Bryah and Padbury Groups. The faulted contact is arcuate in structure with its southern edge defined by the east-trending Kerba Fault. Detailed structural mapping of high-strain zones spatially associated with the Kerba Fault illustrates a high degree of structural complexity within the contact zone. Ductile deformation associated with foliation development is heterogeneous and more intense towards the tectonic contact between the YGC and mafic schists of the Bryah Group. Foliations lie parallel to the tectonic contact and have a strong monoclinic symmetry that defines a sub-vertical vorticity vector associated with a dextral simple shear component. Mineral lineations associated with these foliations are variably developed. Large areas of the high-strain zone record no welldeveloped lineation. In contrast, where mineral stretching lineations are developed they generally show a bimodal distribution in orientation with one set being

parallel to the sub-vertical vorticity vector whereas a second set of sub-horizontal lineations lies orthogonal to the vorticity vector in the plane of the foliation. Foliations are folded by tight folds with axial planes that lie parallel to the foliation and hinges that are parallel to both sets of mineral lineations. Overprinting these structures are a series of brittle faults and fractures. Faults are commonly foliation-parallel and can only be recognised in areas where stepping of the detachment across the foliation leads to hanging wall or footwall cut-offs. These faults again appear to be associated with dextral shearing. Orientation of tension and shear fractures indicate a coincidence of the $2 - S3 plane with the earlier regional foliation, the axial surface of the folds and the foliation-parallel detachments. A wide variety of microstructures can be observed within the high-strain zones of the YGC. Feldspars accommodate stretching orthogonal to the vorticity vector by fracturing and subsequent rotation of feldspar fragments. Quartz preserves crystallographic preferred orientations and therefore records evidence of more ductile deformation mechanisms. In detail however, quartz microstructures show variable degrees of dynamic recrystallisation and recovery although stronger quartz fabrics and reduced grain sizes tend to be preserved towards the contact of the YCG with the mafic schists of the Bryah Group. Although foliation development on the scale of the shear zone is heterogeneous, all microstructures are characteristic of deformation at greenschist facies conditions and there is no apparent difference in metamorphic grade between the higher and lower strain zones. In summary, the high-strain zones examined in this study record a steeply dipping vorticity vector associated with dextral shear and structures that are consistent with shortening at high angles to the foliation and contact zone. This contrasts previous models for high-strain zone formation in the southern Capricorn Orogen, which have previously been related to north over south thrusting. Instead the high-strain zones suggest formation during a progressive, dextraltranspressional deformation. Theoretical models of transpression clearly predict the switching of finite strain axes from horizontal to vertical during strongly oblique convergence and the development of mineral stretching lineations parallel to the vorticity vector. Application of this model to the southern Capricorn high-strain zones studied here requires that the variations in finite strain in different parts of the orogen reflect heterogeneous and potentially non-steady-state deformation. However, the data are also consistent with

a polyphase deformation history involving reactivation. We demonstrate that structural evidence alone is not diagnostic and that to differentiate between transpression and reactivation end-members absolute ages of different fabrics within the high-strain zone are required. This tenet forms the crux of future investigations in the orogen.

145

Fig. 1. Schematic diagram illustrating three end member cases of highstrain zone deformation, a) simple shear, b) polyphase deformation and c) combined pure and simple shear (tran^ression). Geometrically the three case s are similar but mineral stretching lineations (heavy lines) show a single orientation in (a) but more complex patterns in (b) & (c). The orientation of the vorticity vector (V) is constant in (a) & (c) but varies in (b) depending on the nature and orientation of superposed defomiations. Structural criteria may be used to differentiate between examples (a), (b) & (c)but in more complex natural deformation structural observations alone may not be diagnostic. Discriminating between these different cases requires determination of absolute deforaiation age s of different fabrics in combination with structural criteria.

SGTSG abstracts, February 2001

146 M'icrostructure and "^^ArP^Ar apparent ages in deformed K-feldspar: Evidence for a microstructural control on Ar isotope systematics S.M. Reddy\ G.J. Potts^ and S.P. Kelley^ ^ Tectonics SRC, Dept. of Applied Geology, Curtin University of Technology, Perth, WA, Australia 6845 2 Department of Earth Sciences, The University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, UK 3 Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK

Detailed field and microstructural studies have been combined with high spatial resolution ultra-violet laser 40^/39Ar dating of naturally deformed K-feldspar to investigate the direct relationship between deformation-related microstructure and Ar isotope systematics. The sample studied is a c. 1000 Ma Torridonian arkose from Skye, Scotland, that contains detrital feldspars previously metamorphosed at amphibolite-facies conditions c. 1700 Ma. The sample was subsequently deformed c. 430 Ma ago during Caledonian orogenesis. The form and distribution of deformation-induced microstructures within three different feldspar clasts has been mapped at a range of scales, using atomic number contrast and orientation contrast imaging, to identify intragrain variations in composition and lattice orientation. These variations have been related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar clasts. One hundred and forth-three in situ ^Ârl^Âr analyses measured using ultraviolet laser ablation record a range of apparent ages (317-1030 Ma). The K-feldspar showing the least strain records the greatest range of apparent ages from 420 to 1030 Ma, with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary. The most deformed K-feldspar yields the youngest apparent ages (317-453 Ma) but there is no spatial relationship between apparent age and the detrital grain boundary. Within this feldspar, the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped "^Âr component resides. Orientation contrast images at a similar scale to the Ar analyses illustrate a significant deformation-related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling Ar systematics of feldspars at both the inter- and intragrain scales even at relatively low 'bulk' strains. The data show that Ar loss and trapping within the deformed K-feldspars reflects the presence of a deformationSGTSG abstracts, Febaiary 2001

induced population of small diffusion domains in combination with 'short-circuit' diffusion along deformation-induced defects. The complex history of microstructures induced in the K-feldspars during their cooling, alteration, erosion, and sedimentation do not appear to be as significant as deformation-induced microstructures in controlling the distribution of apparent ages at the grain scale.

The structural configuration and evolution of Lower Lead Lode and the 2 Lens Dropper, Broken Hill, NSW

147

A. Reid^ and P. James^ ^ Department of Geology, University of Melbourne, Victoria, Australia 3000 2 Department of Geology and Geophysics, University of Adelaide, SA, 5005

The Broken Hill Pb-Zn-Ag deposit occurs within the Willyama Supergroup, western New South Wales and has undergone deformation at peak metamorphic conditions of upper amphibolite to granulite facies. During deformation, sulphide ore was mobilised into non-conformable, elongate, shear hosted bodies termed droppers. Structural mapping and microscopic fabric analysis was conducted within Lower Lead Lode, Broken Hill Mine, to determine the structural configuration and deformation history of the area and thereby to asses the structural evolution of a specific dropper, the 2 Lens Dropper. Within the ore host sequence three main microstructural fabrics were recognised: granoblastic polygonal, protomylonite and muscovite rich mylonite. Each of the fabric types have distinct mineralogical features and grain size and shape characteristics which enable the reconstruction of the deformation history within samples of each. Early deformation induced grain size reduction through dynamic recrystallisation and initially folded the sequence. Post-peak deformation involved grain area reduction through static annealing at high temperatures and later ductile shearing at amphibolite facies conditions, forming a distinctive asymmetric extensional crenulation cleavage. The structural configuration of the study area is a composite effect of the action of both folding and shearing during continuous deformation and bears strong resemblance to a fold-related thrust structure. The favoured model for dropper formation states that during folding, rupture of the ore-host contact mobilised sulphide ore across the stratigraphy, thereby forming a localised shear zone. Subsequent amphibolite facies shearing developed along zones of prior weakness, such as the earlier localised shear zones. The merging of the earlier and later shears provided an opportunity for further plastic and chemical ore mobilisation into zones of dilation within the larger shear, thus forming a dropper ore zone. Thus dropper structures such as the 2 Lens Dropper are also

considered a function of the composite effect of both folding and shearing.

SGTSG abstracts, February 2001

148 Collisional processes through time P.R Rey School of GeoSciences, The University of Sydney, Sydney NSW 2006

Parameterization of collisional orogen The evolution of collisonal orogens is controlled by (i) the balance between gravitational, compressional, and basal traction forces, and (ii) the balance between radiogenic heating, heat advection, and heat diffusion. A few dimensionless numbers characterize both the balance of force and the heat balance.

Changes through time Progressive temporal changes in the thermal and density structure of the continental lithosphere strongly impacted on the ratio defined above. One can therefore expect significant difference in collisional processes during the earlier stages of the Earth history. With Moho temperatures in excess of 800°C due to a higher concentration in radiogenic elements, the Archaean continental geotherm was higher than its Phanaerozoic counterpart. Such a high temperature geotherm dramatically reduces the lithospheric strength to merely that of the upper continental crust. Secondly, the Sub-Continental Lithospheric Mande was buoyant (Griffin et al., 1998) and thick (120 to 150 km). This implies that, for the same amount of lithospheric thickening, the gravitational force was much stronger in the Archaean than in the Phanaerozoic, where a negatively buoyant SCLM balances a positively buoyant continental crust. These changes increase very significantly the following parameters: An Rm, Pe, and DIV,

Force balance •Argand number (Ar): The ratio of the gravitational stress (related to lateral variations in crustal thickness) to the compressive stress (normal viscous stress required to deform the crust at a given strain rate) (England & McKenzie, 1982). •Ampferer number (Am): The ratio of the basal traction stress (related to the subduction of sub-orogenic lithospheric mantle) to the compressive stress (Ellis etal., 1995). •Ramberg number (Rm): The ratio of gravitational stress to basal traction stress (Weijermars & Schmeling, 1986).

Consequences for orogenesis The large increase in Ar and Rm shows that gravitational force played a major role in the Archaean, a role that impedes the formation of mountain belts. To test this hypothesis, we have performed ID thermo-mechanical modelling in which an Archaean Uthosphere (crust: 42 km, SCLM: 158 km, T^Moho: 820°C) and a Phanaerozoic lithosphere (crust: 35 km, SCLM: 69 km, T^Moho: 540^C) was deformed under the same conditions (figure below). Preliminary results

Heat balance •Peclet number (Pe): The ratio of the rate of advective heat transport to the rate of conductive heating (Turcotte & Schubert, 1982) •Damkdhler numbers (Boucher & Alves, 1959): Dili: The ratio of the rate of radiogenic heating to the rate of advective heat transport. DTV: The ratio of the rate of radiogenic heating to the rate of conductive heating.

Time (Ma) 35 40

150

^200 E

^250 ^300

SGTSG abstracts. February 2001

• Moho '

100

350

Archaean

Time (Ma) 35 40

show that during the Archaean buoyancy-driven lateral flow prevents significant thickening, and a plateau stage

149

is reached very early in the thickening history. Indeed, with a Moho temperature of about 820°C, the total crustal thickening an Archaean continental lithosphere can sustain is only a fraction of the initial crustal thickness. This result is broadly consistent with (1) the subaerial nature of volcanogenic materials in Greenstone sequences, (2) the rare occurrence of voluminous detrital sediments, (3) the relative homogeneity of the erosional level now exposed at the surface of many Archaean cratons, (4) the virtual absence of significant age gradients in the tectonic, metamorphic and magmatic activities in many cratons, and (5) the relative homogeneity of strain across large areas.

References

Boucher, D.R & Alvcs, G.E., 1959. Dimensionless numbers for fluid mechanics heat transfer, mass transfer and chemical reaction. Chemical Engineering Progress 55(9): 55-64. Ellis, S., Fullsack, P. & Beaumont, C., 1995. Oblique convergence of the crust driven by basal forcing: implications for length-scales of deformation and strain partitioning in orogens. Geophysical Journal International 120: 24-44, England, PC. & McKenzie, D.P., 1982. A thin viscous sheet model for continental deformation. Geophys. J. R. Astron. Soc. 70: 295-

321.

Griffin, W.L., O'Reilly, S.Y., Ryan, C G . , Gaul, O. & lonov, D.A., 1998. Secular variation in the composition of subcontinental lithospheric mantle. In: J. Braun, J. C. Dooley, B. R. Goleby, R. D. van der Hilst and C.T. Klootwijk, (eds). Structure & Evolution of the Australian Continent Amer. Geophys. Union, Washington D.C., Geodynamics Volume 26: 1-26. Turcotte, D.L. & Schubert, G., 1982. Geodynamics: Applications of Continuum Physics to Geological Problems. John Wiley & Sons, New York: 450 pp. Weijermars, R. & Schmeling, H., 1986. Scaling of non-newtonian fluid dynamics without inertia for quantitative modelling of rock flow due to gravity (including the concept of rheological similarity). Phys. Earth Planet. Interiors 43: 316-330.

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150 The evolution of Proterozoic rocks of the Peake and Denison Inlier: implications for tectonic setting J. Richardson and P. Betts Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton, Victoria, Australia 3800

Correlating and interpreting the tectonic evolution of sparsely outcropping Proterozoic rocks is subjective because of poor geochronological constraints and structural analysis. In the case of the Peake and Denison correlations with the Mount Isa Inlier as well as the southern Gawler Craton have been made. Consequently, the place of the Peake and Denison Inlier in the Australian Proterozoic jigsaw is poorly constrained. The timing of sedimentation within the Proterozoic parts of the Peake and Denison Inlier is poorly constrained. Bimodal volcanics near the base of the stratigraphy have yielded extrusion ages of 1780 ± 12 Ma (Hopper, 1998), providing a maximum age of sedimentation. These volcanics have been correlated with the Aygylla Formation and the Bottletree Formation in the Mount Isa terrane (Wyborn et al., 1987). If this correlation is correct then Proterozoic rocks of Peake and Denison Inlier may be correlated with supracrustal sequences of the northern Australian Craton (c. 1780-1595 Ma: (Page et al., 2000)). Contemporaneous with sedimentation was the collision between the Archaean Gawler Craton and the Northern Australian Craton during the Kimban Orogeny (c. 1.78-1.70 Ga) (Giles & Betts, 2000). We speculate, given this temporal relationship, that the Early Proterozoic sediments of the Peake and Denison Inlier were deposited behind a magmatic arc during this collision event. Pearce plots (Pearce et al., 1984) of the Wirriecurrie Granite (1793±8 Ma) indicate volcanic arc affinities (Hopper, pers. comm). Subsequent to deposition Early Proterozoic sequences were deformed and metamorphosed. Three Proterozoic deformational events are recognized. The first deformation event produced a layer parallel foliation SI, with no recognized associated folding. The SI foliation is defined by the alignment of muscovite and chlorite. During the second deformation event, east-west oriented isoclinal F2 folds developed. An S2 axial planar layer-parallel crenulation cleavage is defined by muscovite ± biotite. During the third deformation event coplanar east-west oriented F3 isoclinal folds formed. On the limbs of F3 folds

S G T S G abstracts, February 2001

the axial planar S3 cleavage, defined by muscovite and chlorite, is composite with S2. Without detailed syn-deformation geochronology it is difficult to correlate deformation events in the Peake and Denison Inlier with the Kimban Orogeny (Daly et al., 1998) or the Olarian Orogeny (c. 1.601.50 Ga) (Giles & Betts, 2000). Several undeformed granite dykes at Lagoon Hill have yielded emplacement ages of 1533±6 Ma (Hopper, 1998), placing a minimum age of deformation. This age broadly coincides with the cessation of orogeny in Mount Isa and the Broken Hill Block. Our preferred interpretation is that the deformation in the Peake and Denison Inlier occurred during the Mesoproterozoic Orogeny along the eastern margin of the Australian

Figure I. TTie four Precambrian Inliers of the Peake and Denison Inlier. (After Ambrose et al, 1980).

Continent (Betts & Giles, this volume). This interpretation is supported by the recently published reconstruction of the Australian Proterozoic (Giles &

151

Betts, 2000) which places the Peake and Denison Inlier along a north-south otogenic belt,

References Daly, S. J., Fanning, G.M. & Fairclough, M.C, 1998. Tectonic evolution and exploration potential of the Gawler Craton, South Australia. AGSO Journal of Australian Geology and Geophysics 17: 145-168. Giles, D. & Betts, RG., 2000. Beyond Rodinia: The Early to Middle Proterozoic amalgamation of Australia and North America. Australian Crustal Research Centre Publication. 85: 9 pp. Hopper, D.C., 1998. Crustal Evolution of Early to Mid-Proterozoic Basement in the Peake and Denison Ranges, Northern South Australia. Geolo^cal Society of Australia, Abstracts. 49: 213. Page, R.W., Stevens, B.P.J. & Gibson, G.M., 2000. New SHRIMP zircon results from Broken Hill: towards robust stratigraphic and event timing. 45: 375. Pearce, J. A., Harris, N.B.W. & Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic TocVs. Journal of Petrology. 25: 956-983. Wyborn, L. A. I., Page, R.W. & Parker, A.J., 1987. Geochemical and geochronological signatures in Australian Proterozoic igneous rocks. In: Pharaoh,T.C., Beckinsale, R.D., and Rickard, D. (eds). Geochemistry and Mineralisation of Proterozoic Volcanic Suites. Blackwell Scientific Publications: 377-394.

SGTSG abstracts. February 2001

152 History of structural geology (an incomplete and personal view) M.J. Rickard University of Newcastle, Nelson Bay, NSW, Australia 2315

Although there was a long period of development with detailed work arising from general descriptive tectonic studies, the flowering of structural geology as a separate discipline with specific university courses began in the 1950s using early texts such as Hills (1953), Billings (1954), and DeSitter (1956). Bruno Sander's classic account of the fabric of deformed rocks and methods of geometric analysis was published in German in 1930, and popularised in the USA by Knopf & Ingerson (1938) and Fairbairn (1949). Important early work on cleavage in the Appalachians was published by Leith (1905), Mead (1940); Gilbert Wilson, one of the pioneers in the UK, was later attracted to study in this 'Wisconsin School'. Three important techniques laid the foundations for modern structural analysis:(i) The recognition of way up from sedimentary structures, imported from the USA by E. B. Bailey in 1930 was used to great effect to establish the nappe structures of the Scottish Dalradians. (ii) Fold facing introduced by C u m m i n s & Shackleton (1955) from work in the Dalradian nappes. This technique was used by Mel Stauffer (1966) to establish the first nappe structures in the Lachlan Fold Belt. (iii) Fold vergence originally meaning the up-dip direction of a fold axial plane, was taught by Gilbert Wilson (1961) as a sense-of-movement method of determining fold-hinge directions, and formalised by Wood (1963) to help unravel the complications of the Otago Schists. Early work was greatly influenced by studies in the Swiss Alps where structure was displayed in three dimensions; the 'alpine nappe structures' illustrated by Heim (1922) for example, were exported and became the basis for generalised models of orogenic belts. Wegmanns 'Swiss School', attended by both Gilbert Wilson and Donald Mclntyre, taught the useful but simple concept of viewing folds down plunge, and structural contours were used to good effect in Greenland gneiss terrains (Berthelsen, I960).

Survey (Clough in Peach et al., 1907). In 1954, Reynolds & Holmes published a series of sections through double-folded plasticene models, that explained some of the bewildering complexities of deformed gneissic terrains. In Australia, Joklik (1995) published maps with clear double fold patterns from the Arunta complex in the Harts Ranges NT, but he failed to realise the full significance of the structures, interpreting them as ' a ' and 'b' folds related to thrust movements. A surge of activity followed the setting out of the complex geometry of 'double folds' by the Imperial College School led by John Ramsey (1958), and the Edinburgh School led by Donald Mclntyre (Weiss & Mclntyre, 1957). A good selection of early accounts was published by Whitten (1966). Eventually all possible configurations were established by computer studies (Thiessen & Means, 1980). These principles were first applied in Australia at Broken Hill by Sydney students with Bruce Hobbs (1966), and they emphasized, perhaps overly, the process of transposition whereby bedding could be sheared out into the axial-plane schistosity. By then many mine geologists were avidly studying the new methods as set out in Turner & Weiss (1963) and later Ramsey (1967). Lionel Weiss visited Australia several times, and field excursions provided lively exchanges; these were precursors of our SGTSG meetings. Microstructural studies of quartz fabrics were an integral part of many early structural works, based largely on Fairbairn's publications (e.g. 1949). The meaning of these patterns was far from clear until computer simulations showed the relationship to strain mechanisms (Lister & Hobbs, 1980), by which time such studies had more-or-less ceased! It is surprising that such micro-studies were commonly carried out on rocks for which the meso-structure was imperfecdy known! (Rickard, 1991). Important studies relating general metamorphic textures to deformation were published by Hank Zwart (I960), Allan Spry (1969) and later by Ron Vernon (1983), a strong Australian contribution.

That folds could be formed at high angles to each other (cross folds) or more complex had long been recognised, for example by the Scottish Geological

Following this development phase structural geology expanded rapidly, and studies in thrust geometry and mechanisms, strain, fold mechanisms,

SGTSG abstracts, February 2001

cleavage, mylonites, extension and metamorphic core

153

complexes, and recendy fault mechanisms and the role of fluids have become, in turn, popular research topics. In the future we may expect more computer simulations of deformational processes and a clearer understanding of the relationship between structure and other tectonic processes.

References

Bailey, E.B.,1930. New light on sedimentation and tectonics. Geol. Mag. 67: 77-92. Berthelsen, A., 1960. Geology of theTovqussap Nuna. Medd Groenland. 123. Billings, M.P.,1954. Structural Geology (2nd edn) Prentice-Hall: 514 pp.

Cummins W A . & Shackleton R.M. 1955. Ben Lui recumbent syncline (SW Highlands). Geol. Mag. 92: 353-363. De Sitter, L.U.,1964. Structural Geology (2nd edn) McGraw-Hill: 551 pp. Fairbairn, H.W., 1949. Structural Petrology of Deformed Rocks. AddisonWesley: 344 pp. Heim, A., 1922. Geologie der Schweiz. vol l.Tauchnitz. Hills, E.S., 1953. Outlines of Structural Geology. Wiley: 182 pp. Hobbs, B.E., 1966. The structural environment of the northern part of the Broken Hill ore body J1 geol. Soc. Austr. 13: 315-338. Joklik, G.F., 1955. The geology and mica fields of the Harts Range, central Australia. Bur. Min. Res. Bull. 26: 226 pp. Knopf, E.B. & Ingerson, E., 1938. Structural petrology. Mem. Geol. Soc. Am. 6: 270 pp. Leith, C.K ,1905. Rock cleavage. Bull. U.S. Geol. Surv. 239: 216 pp. Lister, G.S. & Hobbs, B.E., 1980. The simulation of fabric development during plastic deformation and its application to quartzite etc. Jour. Struct. Geol. 1: 355-370. Mead, W.J., 1940. Folding rock flowage and foliate structures. Jour. Geol. 48: 1007-1021. Peach, B.N. et al., 1907. The geological structure of the North-West Highlands of Scotland. Mem. Geol. Surv. U.K.: 668 pp. Ramsey, J.G., 1958. Superimposed folding at Lough Monar, Invernessshire and Rossshire. Quart. Jour. Geol. Soc. Lond. 113: 271307. Ramsey, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill: 568 pp. Reynolds, D.L. & Holmes, A., 1954. The superposition of Caledonian folds on an older system in the Dalradians of Malin Head, Co. Donegal. Geol. Mag. 91: 417-444. Rickard, M.J., 1961. Stratigraphy and structure of the CowansvilleSutton-Mansonville area in the Appalachians of southern Quebec. Geol. Surv. Can. Pap. 88-27: 67 pp. Sander, B., 1930. Gefungekunde dergesteine. Springer: 352 pp. Spry, A.,1969. Metamorphic Textures. Pergamon: 350 pp. Stauffer, M.R. & Rickard, M.J., 1966. The establishment of recumbent folds in the Lower Palaeozoic near Qucanbeyan, New South Wales. Jour. Geol. Soc. Aust. 13: 419-438. Thiessen, R.L. & Means, W.D., 1980. Classification of fold interference patterns: a re-examination. Jour. Struct. Geol. 2: 311-316. Turner, F.J. & Weiss, L.E., 1963. Structural Analysis of Metamorphic Tectonites. McGraw-Hill: 545 pp. Vernon, R.H., 1983. Metamorphic Processes. Allen & Unwin: 247 pp. Weiss, L.E. & Mclntyre, D.B., 1957. Structural geometry of Dalradian rocks at Loch Leven, Scottish Highlands. Jour. Geol. 65: 575602. Whitten, E.H.T., 1966. Structural Geology of Folded Rocks. Rand McNally: 663 pp. Wilson, G., 1961. The tectonic significance of small scale structures, and their importance to the geologist in the field. Ann. Soc. Geol. Belg. 84: 423-548. Wood, B.L., 1963. Structure of the Otago Schists. N.Z. Jour. Geol. & Geophys. 6: 641-680. Zwart, H.J., 1960. The chronological succession of folding and metamorphism in the central Pyrenees. Geol. Rdsch. 50: 203-218.

SGTSG abstracts. February 2001

154 The Tilba Tilba metamorphic and dyke complex — a new discovery M.J. Rickard University of Newcastle, Nelson Bay, NSW, Australia 2315

Tucked into the southeastern corner of the Mount Dromedary igneous complex (Eggleton, 1987) are two outcrops of dyke-intruded spotted schists of the Ordovician Adaminaby Group located east of the Tilba Tilba trig, point. The structures are upright N-S tight F1 folds with a strong stripy cleavage cut by a weaker second phase with a slightly less steep S2 crenulation cleavage. They are located north of Bermagui, where SI is horizontal (Powell, 1983) and south of Mystery Bay where F3 kink folds are strongly developed (Stubley, 1989). The south coast Ordovician rocks are generally of sub greenschist facies, but at Tilba the spotted schists contain altered cordierite, rare andalusite, perthitic feldspar, biotite and muscovite. Curved SI inclusions in the cordierite porphyoblasts and wrap around S2 micas indicate that metamorphism was syn to post F2. The coarser grain size and higher grade minerals are very different from the spotted hornfelses that surround the nearby Dromedary plutons. Five sets of dykes intrude these schists: (i) inclusionrich tonalites, (ii) plagioclase-rich porphyritic granodiorites, (iii) microcline bearing adamellites, (iv) thin pegmatites and (v) syenite dykes of the Dromedary complex. In places the dykes are so rich in inclusions that they resemble the Cooma diatexites. The inclusions are strongly aligned in several dykes, parallel to dyke margins where the dykes are parallel to bedding and across the dykes in the S2 direction where the dykes are crosscutting. Some granite dykes appear to be folded by F2, whereas, the pegmatites cross cut the F2 structures. Thus it is clear that granitic intrusion brackets the F2 deformation. Petrographically the dykes consist of phenocrysts of oscillatory zoned altered plagioclase set in a groundmass of quartz, plagioclase, microcline, biotite and muscovite, with variable amounts of pelitic schist inclusions. They are unlike the A-type granites associated with the Boyd Volcanic Complex but resemble rocks in the Moruya and Bega batholiths, both only 25 km away (Collins, pers. comm.). Regional F2 deformation was pre early-Late Devonian (Powell, 1983), thus the dykes and metamorphism are likely to be Silurian to Mid Devonian in age, which

SGTSG abstracts, February 2001

accords well with the age of the Moruya and Bega batholiths. However, there are difficulties in relating to the Moruya batholith as it occurs in a different structural zone on the opposite side of the major Narooma anticHnorium (Miller & Gray, 1996). Lack of outcrop precludes the mapping of the full extent of this complex; it is probably less than a few km long and 1 km wide. The structural relations and the presence of quartz-rich granitic dykes preclude an association with the Dromedary monzonite complex. The mineralogy suggests high-temperature, lowpressure conditions similar to Cooma and other metamorphic belts to the west. It was probably a similar hot spot accompanying granitic intrusion at the northern end of the Bermagui strong F2 fold zone. Movements associated with the nearby Comerong rift (Carboniferous) and the D r o m e d a r y plugs (Cretaceous) may have subsequently uplifted the area. The regional significance of this litde metamorphic complex is that it extends the occurrence of local high thermal gradients to a younger period, and eastward to the inferred subduction zone, where it probably accompanied the Mid Silurian to Mid Devonian shortening of the accretionary wedge (Miller & Gray, 1997). . ^ f f ^

References Eggleton, A., 1987. M t Dromedary the golden volcano — a field guide to the geology of the Tilba region, NSW. Includes Map. Geology Department, Australian National University. Miller, J.McL. & Gray, D.R., 1997. Subduction related deformation and the Narooma anticlinorium, eastern Lachlan Fold Belt, southeastern New South Wales. Australian Journal of Earth Sciences 44: 237-251. Powell, C.McA., 1983. Geology of the New South Wales South Coast and adjacent Victoria with emphasis on the pre Permian structural history. Geological Society of Australia, Specialist Group in Tectonics and Structural Geology. Field Guide 1 : 1 1 8 pp. Stubley, M.P, 1989. Structural analysis of Mystery Bay area. New South Wales. Australian Journal of Earth Sciences 36: 479-493.

Tectonic reconstruction of the Western Mediterranean and the Alpine chain since the Oiigocene

155

G.Rosenbaum, G.S. Lister and C. Duboz Australian Crustal Research Centre, Monash University, Victoria, Australia 3800

The formation of the Alpine belt in Europe is generally considered to represent a suture zone between the African and the Eurasian plates. In details, however, Cenozoic deformation associated with Alpine orogenesis affected large areas of Europe, the Mediterranean Sea and northern Africa. These features can not be explained by a simple (two plate) model, in which Africa has been continuously moving towards Eurasia. More conceivable tectonic reconstructions should take into account numerous microplates and continental ribbons that interacted with newly formed extensional basins of the western Mediterranean. Here we present a tectonic reconstruction that aims to elucidate the inevitable relationships between Alpine deformation and western Mediterranean tectonics. The Alpine orogen is a part of the AlpineHimalayan belt, which accommodated collisions of southern continents with stable Eurasia during the Cenozoic. The earliest deformation and metamorphism in the Alps took place in the Upper Cretaceous, possibly due to the commencement of convergence between Africa and Europe 90-80 Ma ago (Dewey et al., 1989). This tectonic framework has led many geologists to describe Alpine orogenesis as a gradual process driven by continuous convergence (e.g., Hsii, 1989). Alternatively, a more episodic behavior (Lister et al., in press) can be proposed on the basis of the following observations: • Plate motions have been subjected to considerable changes. For example, Africa effectively stopped moving towards Europe between 65-51 Ma (Dewey et al., 1989). • Dating of high-pressure metamorphism in the Alps shows clusters of ages at 65 Ma, 44 Ma and 35 Ma (Gebauer et al., 1997; Rubatto et al., 1998; Rubatto et al.,1999). It therefore seems likely that events of high-pressure metamorphism occurred in a few otogenic episodes of great magnitude (Lister et al., in press). An additional aspect in the Late Cenozoic evolution of the Alpine belt is the widespread extension that took place in the Mediterranean Sea. Back-arc basins, such as the Ligurian, the Tyrrhenian and the Alboran basins, developed since the Oiigocene caused by the roll-back

of subduction zones. The role of subduction retreat is fundamental for understanding the tectonic evolution of the Alpine orogen since the Oiigocene. Thus, reconstruction of the western Mediterranean jigsaw puzzle is the first aim of this project. Modeling was done by the use of PlatyPlus, a software package for interactive plate reconstruction (Duboz et al., 1998). Available palaeomagnetic and kinematic data were given to PlatyPlus as an input file, whereas other motions were found by trial and error until geological constraints were satisfied. Further development of PlatyPlus will allow internal deformation and self-consistent crustal thickness calculations. Results of the tectonic reconstruction are presented as a movie and in a poster format. The most prominent feature in our reconstruction is the widespread extension in the immediate vicinity of the Alpine orogen. As a result, continental fragments previously involved in the Alpine orogeny subsequently drifted over great distances to their present positions. The reason subduction roll-back commenced at c. 30 Ma is not clear, but it was probably related to rapid consumption of oceanic lithosphere in a north dipping subduction zone. The reconstruction shows that before subduction roll-back started, numerous continental fragments had been positioned in a NE-SW striking belt adjacent to present-day southern France and NE Spain. The continental fragments included Corsica, Sardinia, the Balearic islands, the Rif-Betic cordillera (southern Spain and northern Morocco), the Kabylies (northern Africa) and Calabria (southern Italy). Rifting and the subsequent opening of the Gulf of Lion were responsible for the anticlockwise rotation of Corsica and Sardinia, whereas opening of the Valencia Trough accounts for the clockwise rotation of the Balearic Islands. The Kabylies drifted further south due to the opening of Liguro-Proven9al Basin, until they eventually collided with Africa in the Middle Miocene. This collision led to the cessation of the subduction roll-back in North Africa, and the subsequent segmentation of the subduction zone to an eastern (Tyrrhenian) segment and a western (Alboran) segment. The Alboran Sea is interpreted here as a back arc SGTSG abstracts, February 2001

156

basin formed due to a Miocene subduction retreat of the Alboran arc (Royden, 1993; Lonergan & White, 1997). This interpretation does not agree with other models that suggested delamination or sinking of a Uthospheric root (e.g., Seber et al., 1996); however, it provides a better explanation for the palaeomagnetic rotations in the Betic and Rif oroclines (Lonergan & White, 1997). Extension in the Tyrrhenian Sea commenced in the middle Miocene when the eastern segment of the subduction zone further retreated eastward. This was accompanied by crustal shortening in the Apennines and by the anticlockwise rotation of the Adriatic plate. The reconstruction presented here demonstrates that the motions of the different continental fragments in the Mediterranean Sea and the Late Cenozoic development of the Alpine orogen were strictly associated with subduction roll-back.

References Dewey, J.R, Helman, M . L , Turco, £., Hutton, D.H. W. & Knott, S.D., 1989. Kinematics of the western Mediterranean. In: Coward, M.P., Dietrich, D. & Park, R. G.(eds), Alpine tectonics. Geol. Soc. Lon. Spec. Pub. 45: 265-283. Duboz, C., Lister, G. S. & Jessel, M., 1998. PlatyPlus project: A discrete approach to 3D reconstruction. In: 4D Modeling of Natural objects: A Challenge for the 2000s Conference to come, Nancy, France. Gebauer, D., Schertl, H. P., Brix, M. & Schreyer, W. 1997. 35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif, Western Alps. Lithos 41(1-3): 5-24. Hsii, K.J., 1989. Time and place in Alpine orogenesis; the Fermor Lecture. In: Coward, M.P, Dietrich, D. & Park, R.G. (eds), Alpine tectonics. Geol. Soc. Am. Spec. Pub. 45: 421-443. Lister, G.S., Forster, M.A. & Rawling, T., in press. Episodicity during orogenesis. Geol. Soc. Lon. Spec. Pub. Lonergan, L. & White, N., 1997. Origin of the Betic-Rif mountain belt. Tectonics 16(3): 504-522. Royden, L.H., 1993. Evolution of retreating subduction boundaries formed during continental collision. Tectonics 12(3): 629-638. Rubatto, D., Gebauer, D. & Compagnoni, R., 1999. Dating of eclogitefacies zircons; the age of Alpine metamorphism in the SesiaLanzo Zone (Western Alps). Earth and Planetary Science Letters 167(3-4): 141-158. Rubatto, D., Gebauer, D. & Fanning, M., 1998. Jurassic formation and Eocene subduction of the Zermatt-Saas-Fee ophiolites; implications for the geodynamic evolution of the Central and Western Alps. Contributions to Mineralogy and Petrology 132(3): 269-287. Seber, D., Barazangi, M., Ibenbrahim, A. & Demnati, A., 1996. Geophysical evidence for lithospheric delamination beneath the Alboran Sea and Rif-Betic mountains. Nature 379: 785-790.

SGTSG abstracts, February 2001

Use of the PlatyPlus reconstruction software for geometric 157 reconstruction of tlie evolution of sedimentary basins on the Australian Craton between 150-100 Ma T. Rudge\ C. Duboz^ M. Norvick^, G. Lister\ D. Fit2gerald^ B. Kohn^ A. Gleadow^ and R. Brown^ ^ Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton, Victoria, Australia 3800 ^Latrobe University, Dept. of Earth Sciences, Bundoora, Victoria, Australia 3 Des Fitzgerald and Associates ^ School of Earth Sciences, The University of Melbourne, Parkville, Australia 3010

From the period 150 to 100 Ma (Aptian-Albian) the eastern half of continental Australia was progressively covered by a thin veneer of sediments, predominantly derived from volcanogenic activity. At its maximum extent this basin may have stretched from the present day extent of the Eromanga Basin across Bass Strait to Tasmania. A major volcanic province existed along the present eastern Australian margin during the early Cretaceous. From 132 to 95 Ma, a large amount of volcanic and intrusive activity took place, and it is suggested that these arcs provided the sedimentary fill for most of this basin. Volcanogenically derived sediments in the Otway Basin are geochemically correlated with the Whitsunday group. Bryan et al. (1997) estimates a sediment volume greater than 10^ km^ derived from volcanogenic sediments. The extent of the sedimentary basin at 100 Ma nearly covered the eastern half of Australia. However after volcanic activity ceased, the sediments were slowly stripped away through erosion. The use of Apatite Fission Track Analysis (AFTA) provides evidence of the timing of heating episodes, as well as maximum paleo-temperatures, for which the thermal history of sedimentary basins can be constrained. AFTA of rocks exposed at locations within the highlands of southeast Australia resided at elevated paleo-temperatures in excess of 70°-l 10°C prior to the mid-Cretaceous. Rapid cooling commenced at -95 ± 5 Ma, which indicates the close of sedimentation of the Aptian-Albian sedimentary basin (Gibson et al., 2000). Interpretation ofAFTA indicates a rapid kilometrescale denudation starting during the mid-Cretaceous. This is assuming a similar paleo-thermal gradient to the present day. AFTA of rocks from the Lachlan Fold Belt (LFB) indicates that the present day Surat/ Eromanga basins were connected to the south with the Gippsland Basin. This does involve the superimposition of a major river drainage system through the Lachlan Fold Belt.

The sedimentary rocks that were deposited on the highlands, were then eroded as uplift occurred and are recorded in the Eromanga Basin as the younger sequences. This AFT model can account for the following events (Gibson et al., 2000): 1. High temperatures prior to the Middle Cretaceous in LFB rocks, due to burial in the JurassicCretaceous period. 2. Rapid cooling due to a rapid denudation of poorly cemented sediment after uplift at - 9 5 Ma. 3. Relatively slow erosion of LFB rocks since sedimentation in the Late Cretaceous. 4. The courses of rivers draining west superimposed from drainage following the Mesozoic sediment and eroding the western flank of the uplift. The PlatyPlus reconstruction software allows the evolution of this sedimentary basin to be modelled, and a match to be made between the volume of sediments deposited, and the volume of crust eroded (based on the fission track calculation). Achieving this objective requires the definition of several PlatyPlus data objects defined by closed polygons bounding contiguous space. Upon creation of these objects, they can be deformed, mass can be added (sediment influx) or taken away (erosion). This is achieved through the definition of a cloud of mass points associated with each PlatyPlus data object and visually represented by mapping point cloud density as crustal thickness. This use of the PlatyPlus software provides an example of its capabilities in respect to modelling and visualising geological processes, . y f ^

References Bryan, S.E., Constantinc, A.E., Stephens, C.J., Ewart, A., Schon, R.W. & Parianos, J., 1997. Early Cretaceous volcano-sedimentary successions along the eastern Australian margin: Implications for the break-up of Gondwana. Earth and Planetary Science Letters, 153: 85-102. Gibson, D.L., O'Sullivan, RB. & Chan, R.A., 2000. Middle Cretaceous Denudation in the south-east highlands of Australia: Possible Reconciliation of Fission Track and Geomorphological Results, Geol. Soc. Aust. Abstracts 58: 1 1 3 - 1 1 5 . SGTSG abstracts, February 2001

158 Structural evolution, fluid flow and gold mineralisation in the Victory Thrust Complex, St Ives K. Burning^ and S.R Cox^ The University of Newcastle, Callaghan, NSW, Australia 2308 ^Schoolf of Earth Sciences, Australian National University, ACT, Australia 0200

The Victory Thrust Complex is located within a contractional jog on the Playa Shear System (PSS), a regional NNW trending, strike-slip shear zone in the Eastern Goldfields of Western Australia. The PSS has a strike length of over 20 km and an inferred displacement of up to 1 km associated with sinistral strikeslip to oblique-slip movement. It is developed within metamorphosed Archaean greenstones (2.72-2.68 Ga) composed of komatiitic and basaltic lavas, and minor sedimentary rocks. A dolerite sill and felsic and lamphrophyric dykes intrude the greenstones. Four main deformation episodes are recognised regionally. Hydrothermal alteration assemblages and fluid inclusion data indicate the deformation and associated mineralisation in the PSS occurred at depths of 1012 km at approximately 400°C after mid-greenschist facies peak metamorphism. The Victory Complex consists of two broad structural domains: the eastern and western. The eastern domain is dominated by a set of N N W trending, east-dipping, listric thrust faults (Repulse, Victory, Britannia and Sirius), whereas the western domain comprises gently SE-dipping reverse shear zones (the 30,31, 32, 33 shears) and a series of smaller interconnected shear zones and breccias horizons (the 34, 35, 36, 37 shears). Initial development of the Victory Complex occurred during D2, but continued during D3 and D4. The D2 event was approximately synchronous with peak metamorphism at about 2.65 Ga in the Kambalda region, and was responsible for the formation of the gross structure of the eastern domain. NNW trending listric thrusts faults, a mine-scale fault-bend fold, and stratigraphic stacking occurred during D2. Shear zone development was associated with extensive quartzcarbonate veining and the formation of two distinct, kinematically related NNW trending foliations, S2s and S2c. The S2s is a composite shear zone foliation that has been crenulated by the sub-vertical, S2c foliation that is axial planar to gently, SE plunging folds with SW vergence. These foliations have formed during NE-SW directed, sub-horizontal compression. Gold mineralisation was not associated with this event.

SGTSG abstracts. February 2001

During D3, the formation of the major SE dipping shears in the western domain, and reactivation of the D2 structures in the eastern domain was associated with major fluid flow and gold mineralisation within the Victory Thrust Complex at approximately 2.63 Ga. Fluid flow and gold mineralisation was associated with extensive hydrothermal alteration and formation of quartz-carbonate-albite veins and breccias within D3 shear zones. Vein and foliation data indicate west-directed thrusting on shears in the western domain during D3. Stretching lineations and stratigraphic separations in reactivated D2 thrusts in the eastern part of the Complex, indicate sinistral shear-sense, consistent with overall displacement on the PSS. D4 structures correspond to a change from contractional style deformation to one of crustal extension and vertical flattening. Minor gold mineralisation is associated with this event. NE-E striking, SE-S dipping, ductile shear zones with SE trending mineral stretching lineations and sub-vertical, NE striking extension veins are consistent with extension directed in a SE-NW direction. Low displacement, SW dipping, reverse brittle faults with SW plunging slicken-fibres formed during D5. These faults are associated with gypsum veins, however they are not associated with gold mineralisation. Based on lineation and slicken-fibre orientations, the D5 event involved NE directed thrusting. The structure of the Victory Complex illustrates the structural evolution of a long-lived, strike-slip shear system that formed near the boundary of the brittleplastic transition in the continental crust. Major fluid flow associated with gold mineralisation was localised within the dense network of new and reactivated, high permeability thrusts that formed within an imbricate thrust system in a major contractional jog on the Playa Shear System.

Automated Fabric Analyser system for quartz and ice

159

D. Russell-Head and C.J.L. Wilson School of Earth Sciences, The University of Melbourne, Victoria, Australia 3010

A new fully automated data acquisition system that can be used for the collection of c-axis crystailographic fabric data in hexagonal mineral systems has been developed. It collects a comprehensive set of crystailographic measurements, including digital images of a thin section. The f-axis measurement sites can be selected either interactively or in an automatic stepping mode. This is a portable acquisition system that uses a modified Zeiss Axioskop microscope (the most compact in the range of Zeiss p e t r o g r a p h i c microscopes) and a PC laptop computer. The total system weighs less than 16 kg. The computer controls the microscope and a digital C C D camera system, and contains a software package that is responsible for the data acquisition with the whole system operating on a 12-volt power source. The microscope stage can accommodate thin sections of up to 100 x 100 m m with motors controlling translations in the X and Y directions to 10 mm resolution. No tilting of the thin section is required for the analysis. The software system on the computer controls the microscope stage and captures images directly from the camera. T h e image and photometric information is analysed to determine the f-axis orientation. If the crystal element being tested is not sufficently uniform (due to impurities, grain boundaries, or bubbles), a search is made close around the selected site for clear crystal material. The software incorporates a quality control system that tests for and rejects doubtful results. The process yields spatial information describing individual grains and their orientation within a sample. Each r-axis is recorded with its XY position, orientation and relationship within the captured sequence. The results can then be analysed using a variety of discriminators to sort and select different collections of candidate grains to prepare fabric and AVA diagrams or analyse nearest neighbour grain relationships. To ensure the system is producing optimum results and accuracy it has been assessed by comparing measured orientations with c-axes obtained using a conventional Universal Stage from single crystals of quartz. Currently the system is being applied to a set of experimentally deformed polycrystalline ice

aggregates with highly deformed and recrystallised grains. It is proposed to establish a set of benchmark quartzite samples that can be made available to other researchers, who may purchase such a Fabric Analyser, so they can compare their results with known standards, .y^jj^f^

SGTSG abstracts. February 2001

160 An intraplate response to plate margin orogeny: the SE Australian neotectonic record M. Sandiford and A. Gleadow School of Earth Science, University of Melbourne, Melbourne, Victoria, Australia 3010

Because intraplate settings are generally characterised by very low amplitude tectonic response, they potentially provide subtle records of tectonic regimes and can therefore be used to examine important aspects of lithospheric mechanics. This contribution focuses on the neotectonic record of SE Australia since the late Miocene when the Pacific and Australian Plate relative motions changed from transtensional to transpressional and the Southern Alps of New Zealand were born. Intraplate stress regimes are strongly influenced by plate boundary interactions (e.g., Coblentz et al., 1995). The question focussed on here concerns the record of tectonic displacement associated with the change from one stress regime to another, as is likely to have occurred in SE Australia following the birth of the Southern Alps. In particular, we pose the question of whether the displacement regime associated with the change in the stress regime is characterised by a smooth transition or by greatly enhanced displacement rates. In other words, is the tectonic response gradual or episodic. SE Australia is characterised by reverse to strike slip in-situ stress regimes with E-W to SE-NW S^max (e.g. Hillis & Reynolds, 2000). As has been previously noted, this is unusual in that unlike other plates characterised by intraplate compression it is almost orthogonal to plate motion vector, and thus cannot be easily ascribed to the main torques acting on the plate from lithospheric processes (eg, ridge formation, subduction, collision; Zoback, 1992). We associate this stress regime with torques associated with the Australian-Pacific plate interactions, because (1) platescale modelling of Coblentz et al., (1995) shows that this is a viable source of the in-situ stress field (Zhang et al., 1995, also show that lithospheric density structure contributes to the in-situ stress field) and (2) the tectonic record shows significant activity commencing in the latest Miocene. In SE Australia, seismic activity is most intense in the upland systems of the Mount Lofty and Flinders Ranges in South Australia and the Strezlecki Ranges in the Victoria. In both instances, some displacements

SGTSG abstracts. Febaiary 2001

on faults associated with prominent scarps can be demonstrated to be at least as young as Pleistocene, and in one instance less than 100 kyr. This suggests that much of the relief in the upland system is essentially neotectonic and not, as frequendy claimed, residual from the Palaeogene or earlier. In the vicinity of Adelaide, post-Miocene displacements on the faults that bound the western flanks of the Mount Lofty Ranges (Eden-Burnside and Para Faults) may be as much as 1 km. An apparently global regression in the terminal Miocene/earliest Pliocene has resulted in widespread disconformity between early-mid Miocene, carbonatedominated sequences of the southern Australian margin and Pliocene silici-clastics. In St Vincents Basin, western Victoria, and the Otway Basin, this disco nformity followed, or was associated with, significant tilting, frequently of up to 5° and locally up to 90® as well as folding. The available biostratigraphic constraints on this event place it between - 8 Ma and ^A Ma (Dickinson et al., in prep). Shoreface facies in western Victorian Pliocene sequences, and ongoing mafic volcanism, provide an exceptionally detailed insight into the subsequent tectonic activity associated with ongoing faulting. On the northern flanks of the Otway Ranges, post-Miocene faulting has produced - 100 m of differential relief, with consistent, SE up displacements revealed by the superposition of successive palaeo-drainage networks. The emerging picture is that the initial response to the modern stress regime, which can be traced back to 8-4 Ma, was characterised by substantial, regional-scale tilting, now best preserved by regional disconformities in offshore basins. Subsequent ongoing displacements at significantly lower rates continue to tilt and deform the landscape. These observations suggest that the Neotectonic record of SE Australia captures an "event" of substantial displacement at the terminal MiocenePliocene boundary, followed by ongoing, but less intense, displacement rates. In Victoria, Dickinson et al. (in prep) term this event, the "Moorabool Event". In South Australia, we have suggested the term "Spriggs Orogeny" to encapsulate the ongoing

processes that have built the upland systems. In a broader historical context, this event is almost certainly allied to the somewhat discredited, but nevertheless very real, "Kosciusko Uplift". If we are correct in attributing this record to Australian-Pacific plate interactions (i.e., a change fi:om one steady plate displacement regime to another), then it suggests that the most substantial strain accommodation in intraplate settings is allied to the change in intraplate stress regimes from one state to another. While this result may have been anticipated, we believe that it is non-trivial, in that it suggests that the episodic nature of the tectonic record in intraplate (and possibly plate margin) environments has as much to do with the nature of the mechanical response of the lithopshere to forcing as it does to the nature of the forcing.

161

References Coblentz, D.D., Sandiford, M., Richardson, R.M., Zhou, S. and Hillis, R.R., 1 9 9 5 . The origins of the intraplate stress field in continental Australia. Earth and Planetary Science Letters 133: 299-309. Dickinson, J., Wallacs, M., Holdgate, G., Gallagher, S.J., Thomas, L., A major neogene tectonic event in SE Australia, the Moorabool Uplift, submitted to Journal of Sedimentary Petrology. Hillis, R.R. and Reynolds, S.D., 2000. The Australian Stress Map. Journal of the Geological Society, London 157: 9 1 5 - 9 2 1 . Zhang, Y., Scheibner, E., Ord, A. and Hobbs, B.E., 1996. Numerical Modelling of Crustal Stresses in The Eastern Australian Passive Margin. Australian Journal of Earth Sciences 43: 161-175. Zoback, M.L., 1992. First- and second-order patterns of stress in the lithosphere: the world stress map project. Journal of Geophysical Research 97(B8): 1 1 7 0 3 - 1 1 7 2 8 .

SGTSG abstracts, February 2001

162 Chevron folding and fluid-flow in the Bendigo gold field, central Victoria: Constraints on gold mineralisation from numerical models p. Schaubs CSIRO Exploration and Mining, P.O. Box 437, Nedlands, WA, Australia 6009

Gold mineralisation in the Bendigo gold field of Victoria, Australia, is hosted in quartz veins associated with chevron folds, and with reverse faults which develop as a result of these folds locking up. Numerical models coupling deformation and fluid flow are aimed at determining what variables and processes control (1) the development of the shape of chevron folds and related gold-bearing quartz veins, and (2) the magnitude and direction of fluid flow during the formation of these folds. Both fold and quartz vein development and associated fluid flow are important factors affecting gold precipitation. Quartz veins associated with chevron folds such as saddle reefs and bedding parallel and extension veins themselves contain gold, and also acted as pre-existing structures and dilational sites, from which more auriferous reverse faults and associated veins later propagated. Specific questions which are addressed include: • What is the effect of varying sandstone to shale thickness ratios on the fold shape? • How early in the deformation sequence, are bedding-parallel veins likely to form, and of these veins, which are most suitable for reactivation by later faulting? These deposit scale models (100s of meters) are based on a generalised sequence of interlayered sandstone and shale (slate) and represent cross-sections of the Deborah Anticline, Bendigo, oriented at right angles to the fold plunge. Fluid-absent models show that flexural-slip must occur early in the fold history for chevron folds to develop and that thick sandstone layers are important for the growth of folds of this style. Models with welded contacts only reach shortening of 18% and form box folds. Models of this same package with, contacts which are allowed to slip freely, reach shortenings of up to 43% and form the shape of folds observed in the field. In this case flexural slip appears more important than flexural flow during the early stages of chevron fold development. Interfaces between sedimentary units do not have gradually increasing slip increments but rather exhibit a stick-slip behaviour. SGTSG abstracts, February 2001

Slip along many interfaces ceases by 40% shortening and may cause the fold to lock-up. Some interfaces, which have accelerated rates of slip towards the end of deformation, may be possible sites for reactivation of bedding-parallel veins and reverse fault formation; both which are sites of significant gold precipitation. Models are consistent with field observations for the location of fold-related veins. Dilational spaces and areas of yield in tension occur in the hinge area and represent saddle reefs. The rotation of the minimum principal stress to become parallel with bedding in the models is consistent with the orientation of steeplydipping tension veins in close proximity to the hinge of the fold. Zones of high differential stress propagating from thick sandstone layers may act as the locus for the formation of reverse faults after folds have lockedup Models which combine fluid and mechanical responses indicate that permeability and volume changes strongly control the direction and magnitude of fluid-flow within sandstone and shale layers. Sandstone layers are given higher permeability than shale layers in all models. Models in which the permeabilities are held constant during the model results in fluid-flow parallel to bedding, towards the hinge areas of folds undergoing positive volume strain. Fluid is directed away from high fluid pressure areas developed in the limb regions of the fold, which are contractant and undergo negative volume strain. Fluidflow is greatest along permeable sandstone layers whereas fluid- flow across contacts in the limb areas is less significant. Allowing permeability to change with volume strain enhances the results recorded in the other fluid models. The presence of negative volume strains in the limbs decreases the permeability and causes these areas to become high pressure zones, thereby forcing the fluid towards the high volume strain, high permeability and low fluid-pressure areas of the hinge. This is consistent with observations in the field, which show that the most of those gold bearing quartz veins developed during folding, occur in the hinge area,

The role of potential energy in the collapse of mountain belts

163

W.P. Schellart and M.W. Jessell Australian Crustal Research Centre, Department of Earth Sciences, PO Box 28E, Monash University, Melbourne, Victoria, Australia 3800

Three types of forces play a dominant role in the collapse of orogens. The first type is the gravitational force, which promotes collapse, the second is the mechanical strength of the lithosphere, which opposes collapse and the third are plate boundary forces, which can either promote or oppose collapse. Here, the gravitational force can be rewritten in terms of potential energy and the mechanical strength in terms of integrated strength of the lithosphere. The role of potential energy in the collapse of mountain belts and its role in the synchronous occurrence of extension in the interior and shortening along the edges of many mountain belts (e.g. Himalayas, Andes, Betic-Rif, Carpathians, Hellenics) has been the subject of considerable debate. Two different types of models have been proposed to explain this, the first related to plate interior body forces and the second to plate boundary forces.

Models In model 1, gravitational collapse of a mountain belt is driven by the potential energy difference between the mountain belt and the surrounding lowlands (Sonder et al., 1987; Dewey, 1988; Molnar & LyonCaen, 1988; Piatt & Vissers, 1989; Piatt & England, 1994). The difference in potential energy leads to the collapse and extension of the orogen and shortening at the edges of the orogen, resulting in a region with extensional structures in the centre, encircled by a radial pattern of shortening (Dewey, 1988; Piatt & Vissers, 1989; Piatt & England, 1994). Following this concept, then the potential energy difference is the driving force behind extension in the higher regions and shortening in the surrounding regions. The origin of this potential energy difference has been related to a thickened lithosphere, where the bottom of the lithospheric mantle is suddenly removed (Piatt & England, 1994). This removal occurs because the protrusion of a cold lithospheric root in the asthenosphere should create lateral temperature gradients that drive convection, which could remove at least the lower (weak) part of the thickened lithosphere. This sudden removal would lead to a rapid increase in potential energy, which would trigger the extensional collapse

of the lithosphere (Piatt & England, 1994). In model 2, the excess potential energy stored in mountain belts is not the driving force behind the collapse of these belts, but the potential energy is released only, when there is an external mechanism present, which allows the orogen to release its potential energy (Lonergan & White, 1997). This potential energy release results in extension of the thickened region. The external mechanism could be related to plate boundary forces such as rollback of subducting plates, which sink under their own negative buoyancy (Elsasser, 1971; Lonergan &: White, 1997) leading to the formation of empty space at the contact between the subducting and overriding plate. This results in the collapse of the overriding plate towards the retreating hinge of the subducting plate (Elsasser, 1971), because it is not strong enough to sustain an empty gap at the plate boundary (Shemenda, 1993). Along the subduction boundary shortening will be the dominant deformation type, related to friction at the contact between the upper plate and the subducting plate. Collapse is most likely to occur at places where the lithosphere has a relatively high potential energy and a relatively low integrated strength.

Calculations Some simple analytical calculations have been made in order to determine if potential energy difference alone can be the driving mechanism behind gravitational collapse of a mountainous region and shortening in the surrounding lowlands. Here, the potential energy difference between and the integrated strength of the mountainous region and surrounding lowlands have been calculated and have been brought together in a simple one-dimensional model, where collapse will only occur if the potential energy difference between the thickened and lowlands lithosphere is greater than the sum of their integrated strengths. In previous models (Sonder et al., 1987; Dewey, 1988; Piatt & Vissers, 1989; Mareschal, 1994; Piatt & England, 1994), the energy, needed to shorten the lowlands lithosphere to allow extension in the interior of a mountain belt, has never been incorporated as a source, which might prevent collapse.

SGTSG abstracts, February 2001

164

But if the lithosphere surrounding a mountain belt is relatively strong, it might be capable to prevent collapse of a mountain belt. Extension can also be accommodated if there is a moving boundary (as in rollback) surrounding the thickened region that provides the space into which the thickened lithosphere can extend. Therefore, it has been investigated if rollback of a subducting lithosphere is a capable mechanism to extract excess potential energy from a thickened lithosphere. Results Model 1 The results indicate that on a lithospheric scale gravitational collapse, as a mere consequence of the difference in potential energy between a mountainous region and its surrounding lowlands, is only possible for high potential energy differences (> 5 x N/m), initially high geothermal gradients (initial Moho temperature > 600°C), considerable thermal relaxation of the thickened lithosphere and thermal heating of the surrounding lithosphere. Thermal heating of the surrounding lithosphere can be realised by lateral heat flow from the thickened lithosphere towards the surrounding lithosphere, because the thickened column has a higher geothermal gradient, related to the heat production in the thickened crust. Without thermal heating of the surrounding lithosphere, it remains too strong to let the mountain range collapse, independent of the strength of the mountain lithosphere. On a crustal scale, where the crust is mechanically decoupled from the mande, collapse is more easily accomplished where collapse can occur with initially high geothermal gradients after thermal relaxation of the thickened crust without thermal heating of the surrounding crust. Localised crustal flow can also occur, which, for initially high geotherms, can take place without thermal relaxation of the thickened column. Here, middle to lower crustal material in the thickened crust can flow towards the lower pressure regions of the lower crust of the lowlands crust, but this will not result in deformation at the surface. Model 2 Form simple calculations, it can be shown that during rollback, the overriding plate will collapse into the space provided, no matter how strong the upper plate. The first of Byerlee's (1978) equations for the brittle strength of rocks (t = ms^; with m = 0.85) can be used

SGTSG abstracts, Febnjary 2001

to give an upper limit of the strength of a lithospheric column. This can be rewritten in terms of principle stresses which leads to S1-S3 = 0.79rgz(l-l) (for extension). When this equation is compared with the lithostatic pressure of the same column (s^ = rgz), then it is clear that the lithostatic pressure is always larger than the (extensional) strength of a plate and therefore its potential energy is always greater than its integrated (extensional) strength. Since it is empty space into which the overriding plate collapses, the entire potential energy of the overriding plate is available to extend itself. Therefore, the upper plate will always collapse into the space provided by the retreat of the subducting plate, no matter how strong the upper plate is, where the upper plate is most likely to collapse at places with a relative low strength and high potential energy, ^ ^ f f ^ References Bycrice, J.D., 1978. Friction of rocks. Pure Applied Gcophysics 1 1 6 : 615-626. Dewey, J.F., 1988. Extensional collapse of orogens. Tectonics 7: 1 1 2 3 1139. Elsasser, W.M., 1 9 7 1 . Sea-floor spreading as thermal convection. Journal of Geophysical Research 7 6 : 1 1 0 1 - 1 1 1 2 . Lonergan, L. & White, N., 1 9 9 7 . Origin of the Betic-Rif mountain belt. Tectonics 16: 5 0 4 - 5 2 2 . Mareschal, J.-C, 1994. Thermal regime and post-orogenic extension in collision belts. Tectonophysics 238: 4 7 1 - 4 8 4 . Molnar, P. & Lyon-Caen, H., 1 9 8 8 . Some simple physical aspects of the support, structure, and evolution of mountain belts. Geological Society of America, Special Papers 2 1 8 : 1 7 9 - 2 0 7 . Piatt, J.P, & Vissers, R.L.M., 1 9 8 9 . Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and the Gibraltar Arc. Geology 17: 5 4 0 - 5 4 3 . Piatt, J.P & England, PC., 1994. Convective removal of lithosphere beneath mountain belts: Thermal and mechanical consequences. American Journal of Science 2 9 4 : 3 0 7 - 3 3 6 . Shemenda, A.I., 1993- Subduction of the Lithosphere and Back-arc Dynamics: Insights From Physical Modeling. Journal of Geophysical Research 98: 1 6 1 6 7 - 1 6 1 8 5 . Sonder, L.J., England, PC., Wernicke B.P & Christiansen, R.L., 1 9 8 7 . A physical model for Cenozoic extension of western North America. In Coward,M.P, Dewey J.R & Hancock, PL. (eds). Continental extensional tectonics. Geological Society Special Publications 28: 1 8 7 - 2 0 1 .

structural data from oriented and unoriented drill core

165

R.J. Scott^ and R.F. Berry^ ^ Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001 2 School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001

In unoriented drill core, any regionally consistent fabric provides a reference from which the orientation of other structures can be potentially be determined (Fig. 1). Even if the orientation of a suitable reference fabric is not, or poorly, known at the time of core logging, relevant structural measurements can still be made, and converted to true orientations if constraints on the reference fabric orientation improve. To facilitate this task we have written a Microsoft Excelbased program for calculating the orientations of structures in drill core based their angular relation to the core axis and either a bottom-of-hole reference line (oriented core) or a reference fabric (either a plane or a line) of known orientation. The program can be used as a calculator, with data entered via a structured series of dialog boxes (explaining measurement conventions for the various input parameters) or by direct "on screen" entry. Alternately large data sets can be read from file, with results automatically stored in tabulated form, for subsequent analysis. Where a reference fabric is used to reorient the core, the "best fit" position is

determined by rotating the core about its axis until the angle between the known fabric and its reference orientation is at a minimum. Accordingly the accuracy of a solution is critically dependent on the chosen reference orientation at the point of interest, as well as the accuracy of the individual angular measurements from core. We conducted Monte Carlo simulations, in which computer-generated random numbers were used to represent the natural variation in the various input parameters, in order to determine: (1) how realistic uncertainties in the various input parameters, particularly orientation of the reference fabric, influence the calculated orientation of "unknown" fabrics; and (2) under what conditions (e.g. orientation of the known and unknown fabrics with respect to each other and the core axis) the solution procedure yields the best results. These simulations indicate the reliability of solutions is particularly sensitive to real variation in the orientation of the known (reference) fabric. Our analysis indicates that in general the standard deviation (s) in the orientation of the known

.drill hole angle b/w pole to planar fabric and core axis (not measured)

normal to "known" fabric (cleavage intersection) lineation on 'known" long-axis of elliptical section through "known" fabric

foliation angle "known" fabric

foliation angle angle between long axis of planar fabric and core axis (= 90° - a)

'known" planar ® fabric (e.g. bedding) bottom-of-hole line and the down-meters end of ellipse formed by planar fabric, measured anticlockwise from long-axis in plane perpendicular to core axis, looking down-meters.

angle b/w down-meters ends of the ellipses formed by a two planar fabrics, measured anticlockwise from the first or "known" fabric, in a plane perpendicular to core axis, looking down-meters.

.second or "unknown" fabric (e.g. cleavage) bottom-of-hole line Figure 1.

angle b/w a lineation and the long-axis of the plane in which it lies. Measured in the plane, either anticlockwise (+ve) or clockwise (-ve) looking downmeters, from the down-meters end of the long-axis to either end of the line. The program accepts lineation angles > - 9 0 ° and ^ 6 0 ° .

Defininions and conventions for angles used in the core orientation program: SGTSG abstracts, February 2001

166

fabric should be <10® if the method is likely to yield meaningful results (Fig. 2). Regional variation in the orientation of a fairly planar fabric (e.g. an axial planar cleavage in an area of simple folding) will commonly exceed standard deviations of ±10°. However, in the relatively small volumes of rock sampled by individual drill holes, variation in the orientation of a suitable reference fabric is likely to be much smaller, suggesting useful results can be obtained in many situations. Even where standard deviation about the reference orientation of a known fabric is small (s <5^-10®), the reliability of orientation determinations for unknown

SGTSG abstracts. February 2001

fabrics depends on the angle between the reference and unknown planes and the core axis (i.e. "foliation angles", Fig. 1). Results are most reliable where the reference plane is at about 30° to the core axis (Fig. 2), and also increases with increasing foliation for the unknown plane. We also used Monte Carlo simulations to show our solution procedure produces comparable or better results than those obtained assuming a fixed strike for the known fabric. In particular, where the know fabric is >60° to the core axis, a fixed strike should only be used if it is nearly orthogonal to the trend of the drill hole, ^ f f ^

Megabreccias in the McArthur Basin, NT.: earthquaketriggered collapse of a carbonate-platform succession

167

R.J. Scott, S.W. Bull and D. Selley Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001

The genesis and emplacement of extensive intraformational breccias has been the subject of debate within the recent earth science literature. Arguments have focussed on (1) whether breccias were emplaced incrementally or during a single catastrophic event, and (2) the possible causes; e.g. failure of gravitationally unstable slopes, major earthquakes, catastrophic release of over-pressured pore fluid or gas, or even meteorite impacts. Large scale "slump breccias" are locally developed in Reward Dolomite (uppermost Umbolooga Subgroup) and Caranbirini Member of the Lynott Formation (Batten Subgroup) within the Palaeoproterozoic McArthur Group, McArthur Basin, N.T. (Brown et al., 1969; Jackson et al., 1987). These units overlie the economically important Barney Creek Formation - host to one of the world s largest base metal deposits, the 104 Mt (14.1% Zn, 6.4 % Pb, 64 g/t Ag) McArthur River stratiform Zn-Pb deposit (Hinman et al., 1994) - and reflect the ongoing stratigraphic development of the mineralised subbasins. Jackson et al. (1987) noted development of the breccias was restricted to a 30-40 km wide corridor west of the Emu Fault, a major NNW-trending fault that forms the eastern boundary of the Batten Trough in the southern McArthur Basin. The Emu Fault was at least intermittently active during deposition of the Umbolooga and Batten subgroups, suggesting seismic activity along the fault could have played an important role in breccia genesis. In order to better understand controls on sub-basin evolution as well as the origin and emplacement of megabreccias within the Reward and Caranbirini successions, we undertook detailed mapping of upper Umbolooga and lower Batten Subgroup strata in the Myrtle Basin area, 20-25 km SSW of the McArthur River Mine and 12-18 km west of the Emu Fault. The largest megabreccia body is a grossly wedge-shape sheet that tapers towards the east. Easternmost exposures of the breccia are less than 5-10m thick and occur entirely within the Reward Dolomite. Those to the west are up to 70 m thick and incorporate at least the lower part of the Caranbirini Member. Smaller breccia bodies occur both above and (more rarely) below the main

sheet. We have mapped the main body as a contiguous sheet over - 1 4 km^. However identical breccia at a similar stratigraphic positions elsewhere in the Myrde Basin, suggests a total extent in excess of 40 km^. The internal structure of the megabreccia is generally chaotic, although clasts are crudely imbricated in some areas. Discontinuous thin (to 10s of centimetres), roughly planar layers of finer grained breccia are also developed locally and probably represent internal shear planes. Contact relations indicate the breccias were not shed across the basin floor as a rock avalanche or debris flow (c.£ Brown et al., 1969; Jackson et al., 1987), but resulted from subsurface disaggregation of formerly coherent stratigraphy. The basal contact of the megabreccias ranges from a sharp fault- or scour-like surface to a gradational transition from coherent stratigraphy, through jigsaw-fit in situ breccia to completely disaggregated strata. The sole is always located in the Reward Dolomite and is subparallel or slightly discordant to the underlying strata. The upper contact of the main breccia body also ranges from sharp to gradational, and is generally sub-parallel to the overlying strata. However, the overlying coherent package is cut by lobes and "dykes" of breccia, which extend up to 100m above the main breccia sheet. No obvious angular unconformity is developed within the overlying package (upper Caranbirini and Hot Spring members), indicating that either brecciation did not result in significant disruption at the basin floor or that it post-dated deposition of the entire succession exposed in the map area. Given the nature and extent of the brecciation, we favour the latter interpretation, namely megabreccia formation occurred during or after deposition of the Hot Springs Member, and that the coherent package overlying the breccias was transported, along with the megabreccia, as one or more essentially coherent slab(s). This indicates the total thickness of the slide complex was in excess of 150 m. Although the total displacement of the megabreccia and overlying slabs is unknown, a range of internal structures including small-scale faults, rotated and imbricated clasts and "extensional tears" in the coherent SGTSG abstracts, February 2001

168

hanging wall indicate it was transported towards the west or southwest. An east to west transition from slope- to basinal- facies sediments in both the Caranbirini Member and the underlying Barney Creek Formation suggests movement (and thickening) of the megabreccia was towards deeper parts of the sub-basin. The sole of the megabreccia appears to have localised within an interval containing abundant medium to coarse grained grainstones (both dolarenites and arenites). These sediments exhibit evidence of extreme fluidization during emplacement of the megabreccia. A key exposure of the sole of the megabreccia in the south of the map area provides several important clues in regard to its genesis. Here the sole consists of two or three distinct quartz "sandstone" layers (30-50 cm combined thickness), which superficially resemble beds. Quartz grains within these layers are largely unstrained and do not exhibit evidence of cataclastic deformation. Small sandstone dykes that emanate from the lowermost layer are sheared out in the basal part of the megabreccia. The presence of the sandstone dykes and undeformed nature of the quartz grains suggests the megabreccia slid along a thin fluidized layer of unlithified granular sediment. Fluidization of granular material requires the granular pressure resulting from grain interactions (i.e. effective pore-pressure) equal to the weight of overburden. Anders et al. (2000) argued that beneath thick slide blocks this is only possible during rapid slip (i.e. catastrophic emplacement). Accordingly fluidization of unlithified grainstones at the base of the >150 m thick Myrtle Basin megabreccia is interpreted to reflect rapid emplacement. However, contrasts in the internal character of individual layers at the sole of the megabreccia are interpreted to reflect multiple episodes of rapid slip rather than a single catastrophic event. The entire McArthur Group succession exposed in the Myrtle Basin is likely to have been flat-lying or very gently dipping at the time of megabreccia formation; that is, below the maximum angle of repose (-6°) for submarine siliciclastic slopes and carbonate slopes <1000 m in height, (e.g. Schlager and Camber, 1986). Thus the gravitational potential for initiating the slide that produced the megabreccia is small. Accordingly, fluidization at the base of the megabreccia is likely to have been induced either by over-pressured pore fluids (e.g. Spence and Tucker, 1997) or dynamic fluidization during earthquakes (Melosh, 1983; Anders et al.,2000). High-porosity aquifers isolated from hydrostatic

SGTSG abstracts, February 2001

head by overlying impermeable horizons, may become over-pressured at times of relatively low sea level, due to the decreased weight of the overlying water column (Spence & Tucker, 1997). Regional facies characteristics of the Caranbirini and Hot Springs members record a progressive shallowing of depositional environment up sequence. This suggests deposition of the Hot Springs Member (and formation of the megabreccias) occurred at a time of low relative water depth in the basin. However, while the shallowing event was widespread in the southern McArthur Basin, megabreccias only occur within a 30 km corridor adjacent to the Emu Fault (Jackson et al., 1987). Collectively, the low relief of the Myrtle Basin, the regional distribution of the megabreccias, and the evidence for rapid emplacement all suggest seismic activity associated with slip on the Emu Fault (and related structures) was the fundamental trigger for megabreccia formation, . y f ^ f ^ Acknowledgements This research was funded by AMIRA/ARC Project P384A (1997-1998) and CODES SRC (2000). References Anders, M.H., Aharonov, E. & Walsh, J J . , 2000. Stratified granular media beneath large slide blocks: implications for mode of emplacement. Geology 28: 9 7 1 - 9 7 4 . Brown, M.C., Claxton, C.W. & Plumb, KJ\., 1969. The Proterozoic Barney Creek Formation and some associated units of the McArthur Group, Northern Territory. Bureau of Mineral Resources, Australia, Record 1969/145. Hinman, M., Wall, V.J. & Heinrich, 1994. The interplays between sedimentation, deformation and hydrothermal activity at the McArthur River Pb-Zn(-Cu) deposit. Geological Society of Australia Abstracts 37: 1 7 6 - 1 7 7 . Jackson, M.J., Muir, M.D. & Plumb, K A , 1987. Geology of the soudiern McArthur Basin, Northern Territory. Bureau of Mineral Resources, Geology and Geophysics, Australia, Bulletin 220: 173p. Melosh, H.J., 1983. Acoustic fluidization. American Scientist 71: 1 5 8 165. Schlager, W. & Camber, O., 1986. Submarine slope angles, drowning unconformities, and self-erosion of limestone escarpments. Geology 14: 762-765. Spence, G.H. & Tucker, M.E., 1997. Genesis of limestone megabreccias and their significance in carbonate sequence stratigraphic models: a review. Sedimentary Geology: 163-193.

constraints on north-vergent Palaeozoic intraplate deformation and exhumation in the northeastern Arunta Iniier I. Scrimgeour^

169

J.G. Raith^ and W. Frank^

1. Northern Territory Geological Survey, PO Box 2655 Alice Springs, NT, Australia 0871 2. Institute of Geological Sciences, University of Leoben, Peter-Tunner Stra3e 5, A8700 Leoben, Austria 3. Institute of Geology, University of Vienna, AlthanstraBe 14, A1090 Vienna, Austria,

The eastern Arunta Iniier of central Australia preserves a record of complex and long-lived Palaeozoic intraplate tectonics, which includes the late Ordovician to Carboniferous Alice Springs Orogeny. Most of the major structures relating to this event are S-vergent; however two major, steeply S-dipping shear zones near the northern margin of the eastern Arunta Iniier show the opposite sense of tectonic vergence. These structures, the Delny Mt Sainthill Shear Zone (DMSZ) and Entire Point Shear Zone (EPSZ) separate Ordovician granulites in the south from Palaeoproterozoic basement overlain unmetamorphosed Neoproterozoic sediments of the Georgina Basin to the north. This implies that exhumation of up to 20 km must have been accommodated along these shear zones during the Palaeozoic. To the south of the DMSZ and EPSZ is the Harts Range Group, which was metamorphosed at c. 10 kbar and SOO'^C during the Larapinta Event at 480-460 Ma, in a probable extensional environment (e.g. Mawby et al., 1999). These Ordovician granulites were juxtaposed against the Palaeoproterozoic Kanandra Granulite to the north along the EPSZ at 445 ± 5 Ma, with associated reworking of the Kanandra Granulite at 7 kbar and 700®C. The late Ordovician mylonites of the EPSZ by are overprinted by mylonites of the DMSZ that show a northwards decrease in grade from mid-amphibolite facies through to greenschist facies. The lowest grade mylonites in the DMSZ are muscovite-chlorite-quartz phyllites near the northern margin of the shear zone. "^Âr-^Âr step heating analysis of muscovite from two samples give ages of 362 ± 3 and 364 ± 3 Ma, interpreted to reflect the timing of deformation. "^Âr-^Âr ages of muscovites from higher grade mylonites in the DMSZ range between 366 ± 2 and 349 ± 2 Ma and are interpreted to reflect cooling through c. 350-420°C during exhumation. Hornblendes within the DMSZ and EPSZ typically preserve partially reset Proterozoic cooling ages, except where the hornblende recrystallised in the Palaeozoic. A garnet-hornblende amphibolite from the EPSZ, which has a Sm-Nd GrtHbl-WR isochron age of434 ± 6 Ma, has a hornblende 40Ar-39Ar cooling age of 403 ± 10 Ma. Hornblendes

from Ordovician granulites in the adjacent Harts Range Group give "^Âr-^Âr cooUng ages of 417 ± 10 and 390 ± 5 Ma, suggesting regional cooUng through c. SOO'^C at this time. The preservation of Proterozoic cooling ages for hornblende in boudins within the EPSZ is interesting given the fact that the surrounding Ordovician mylonites preserve upper amphibolite facies assemblies that formed at least 150®C above the closure temperature of the "^Âr-^Âr system in hornblende. Hornblende from a mafic amphibolite from the Palaeoproterozoic Cackleberry Metamorphics, immediately north of the DMSZ, has an 40Ar-39Ar cooling age of 1601 ± 10 Ma, which is similar to cooling ages preserved in boudins of Kanandra Granulite within the shear zones. This suggests that a thermal and/or exhumation event may have occurred in the northeastern Arunta Iniier at this time, coincident with high grade metamorphism in the Reynolds Range region. The muscovite ^Âr-^Âr data suggest that the final significant movement on the DMSZ occurred around 360 Ma, resulting in the exhumation and cooling of the terrain to the south. This was c. 90 Ma after the onset of compressional deformation in the region, and the combination of U-Pb, Sm-Nd and Ar-Ar data is consistent with a cooHng rate of 4°-5®C Ma"^ that remained relatively constant over this time. The DMSZ acted as a major retro-shear that accommodated exhumation at the rear of the south-vergent orogenic wedge during the Alice Springs Orogeny. The fact that deformation on this structure had largely terminated by 360 Ma implies that large-scale thick-skinned intraplate deformation in central Australia may have ceased by the late Devonian, and that Carboniferous deformation may have been largely restricted to thinskinned deformation with relatively limited shortening. This may account for the apparent lack of syn-orogenic sedimentation in the adjacent Georgina and Amadeus Basins during the Carboniferous. Reference Mawby, J., Hand, M. & Fodcn, J., 1999. Sm-Nd evidence for Ordovician granulite facies metamorphism in an intraplate setting in the Arunta Iniier, central Australia. Journal of Metamorphic Geology 17: 653-668.

SGTSG abstracts, February 2001

170 Origin and tectonic significance of broken formation, Dundas region, western Tasmania D. Selley CODES Special Research Centre, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001

The macroscopic geometry of the footwall to the Rosebery Fault in the Dundas region is characterised by non-cylindrical, upright folds with roughly meridional trends. A NNW-to NNE-striking cleavage transects folds in an anticlockwise sense, a geometry which is thought to have resulted from superposition of ENE-WSW directed Devonian shortening on preexisting gentle NNE- to NE-trending folds most probably developed during Late Cambrian basin inversion. Fold limbs and hinges are locally dismembered along subvertically-dipping high strain zones which in some cases juxtapose blocks with opposing stratigraphic facing. Broken formation is a conspicuous mesoscopic structural fabric developed in some of these high strain zones that was considered by earlier workers to record a period of CambroOrdovician accretionary tectonics, in part due its remarkable similarity to fabrics developed in modern and ancient accretionary complexes (Green, 1983; Corbett & Lees, 1987). This tectonic model was criticised by contemporary workers (Leaman et al., 1987; Berry and Crawford, 1988) and is now generally considered to be unviable within the broader early Palaeozoic tectonic framework of western Tasmania. However, the true origin and significance of the broken formation remained poorly understood. The broken formation involves pardy to completely fragmented sedimentary layers derived from originally well-bedded turbiditic packages of multilayered mudstone-siltstone and subordinate massive to parallel laminated quartz sandstone. No exotic material is contained within the formation. At both the mesoand microscopic scales it is characterised by a 'blockin-matrix fabric comprising tabular to lozenge shaped inclusions enclosed within a finer-grained, strongly cleaved matrix . Kinematic indicators including asymmetric boudinage, overprinting cleavage relationships and small-scale asymmetric folds reveal a complex strain history of dextral-reverse layer-parallel general shear. Intense flattening strains imposed during early stages of stratal disruption produced chocolatetablet boudinage with a subtle down-dip lineation defined by inclusion long axes, tentatively interpreted to have recorded a sub-horizontally directed maximum

SGTSG abstracts. February 2001

stretch. A flip to sub-vertically oriented maximum stretch later in the deformation history is indicated by a weak down-dip stretching lineation and subhorizontal microfractures and microveins. This strain history is interpreted in terms of a progressive bulk flattening deformation involving amplification, hinge parallel extension and anticlockwise rotation of Cambrian folds towards the NNW-striking XY plane of the bulk strain ellipsoid related to the Devonian fold phase. Relatively incompetent, but well-stratified packages of strata in sub-vertical limbs of tightened folds are considered to have become active as NNE striking, layer-parallel shear zones involving components of both dextral wrench and reverse movement. Textures of the broken formation resemble those described from accretionary complexes not only in the sense that they involve a 'block-in-matrix fabric but also in many small details. These include features which superficially resemble those attributable to prelithification deformation such as contorted, apparendy 'rolled' inclusions, ornate inclusion margins involving mudstone tongues and sandstone 'protrusions', progressive annihilation of internal sedimentary fabric of inclusions (suggestive of particulate flow in unconsolidated material), and the 'fluidal' form of many sandstone inclusions. Furthermore, the ubiquitous development of web structure' (dark cataclastic networks) in sandstones, a feature present at both the meso- and microscopic scales, commonly appears in the literature as a feature diagnostic of accretionary melange. However, detailed investigation of the grain-scale processes of stratal disruption, reveals disaggregation processes via cataclastic flow in consolidated sandstones and pressure solution within the enclosing mudstone matrix. Moreover, cataclasis and intimately associated diffusion mass transfer processes were active at the time of Devonian cleavage development. It is considered that elevated fluid pressures resulted from degradation of hydrous phases during cleavage development, thereby reducing the effective confining pressure and promoting brittle fracture and cataclastic flow in competent sandstone layers.

Conditions critical to the development of broken

171

formation are: (1) intense flattening strains, (2) marked competency contrast in the deforming medium, (3) elevated fluid pressures, (4) relaxation of along strike boundary constraints to allow stretching in two dimensions. These conditions are common in accretionary complexes but are not unique to them. Broken formation cannot be used in isolation to indicate deformation within an accretionary environment,

References

Berry, R.F. & Crawford, A.J., 1988. The tectonic significance of the Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Australian Journal of Earth Sciences 35: 523-533. Corbett, K.D. & Lees, T.C., 1987. Stratigraphic and structural relationships and evidence for Cambrian deformation at the western margin of the Mt Read Volcanics, Tasmania. Australian Journal of Earth Sciences 34: 45-67. Green, G.R., 1983. The geological setting and formation of the Rosebery volcanic-hosted massive sulphide orebody, Tasmania. Unpublished PhD thesis, University of Tasmania. Leaman, D.E., Brown, A.V. & WiUiams, E., 1987. Stratigraphic and structural relationships and evidence for Cambrian deformation at the western margin of the Mt Read Volcanics, Tasmania: discussion. Australian Journal of Earth Sciences 34: 531-532.

SGTSG abstracts, February 2001

172 Links between regional-scale deformation and granite generation: the Yarlarweelor gneiss complex, Western Australia S. Sheppard, S.A. Occhipinti, and I.M. Tyler Geological Survey of Western Australia, 100 Plain Street, East Perth, WA, Australia 6004

In this paper we discuss the origin of two I-type granite magmas from the Yarlarweelor gneiss complex in the Capricorn Orogen, Western Australia. Although the dates of intrusion of these magmas are only 10 m.y. apart, they were generated from separate source rocks, reflect contrasting amounts of melt generation and rates of melt extraction, and were formed in different tectonic regimes. The Palaeoproterozoic Capricorn Orogen formed as a result of (i) the convergence of the Archaean Yilgarn Craton and the Archaean to Palaeoproterozoic GlenburghTerrane of the southern Gascoyne Complex during the 2000-1960 Ma Glenburgh Orogeny, followed by (ii) collision of these combined terranes with the Archaean Pilbara Craton during the 1830— 1780 Ma Capricorn Orogeny. The southwestern part of the orogen includes the Yarlarweelor gneiss complex, the GlenburghTerrane of the Gascoyne Complex, and several deformed Palaeoproterozoic sedimentary and volcanosedimentary basins. During the Capricorn Orogeny, the northwestern edge of the Yilgarn Craton, which is marked by the Errabiddy Shear Zone, was intensely deformed and metamorphosed to form the Yarlarweelor gneiss complex (Occhipinti et al., 1998; Occhipinti & Myers, 1999; Sheppard & Swager, 1999). The complex consists mainly of early to late Archaean (3300-2640 Ma) granite and granitic gneiss, and Palaeoproterozoic (c. 1812-1800 Ma) granite and pegmatite. Intrusion of veins and sheets ofleucocratic, I-type, coarse-grained granite and pegmatite at c. 1812 Ma into the Yarlarweelor gneiss complex was concommitent with crustal thickening, upright folding, and upper amphibolite to granulite facies metamorphism. South of the Yarlarweelor gneiss complex, where the Yilgarn Craton was not reworked during the Capricorn Orogeny, coarse-grained granite and pegmatite are rare or absent. The c. 1812 Ma granite and pegmatite are characterised by low Rb, FeOVMgO, P2O5, Y, Zr, Th, U and SREE, and high Si02 and Na20. They therefore resemble leucosomes from stromatic migmatites (metatexites), which form

SGTSG abstracts, February 2001

by efficient separation of low melt fractions from restitic biotite, and accessory minerals included in the biotite, at, or near, the zone of melting. Extraction of small melt fractions was probably aided by deformation. Nd isotope data indicate that the granites and pegmatites formed by melting of late Archaean granites of the Yilgarn Craton. Evidence of incipient melting in granitic gneisses of the Yarlarweelor gneiss complex suggests that the current level of exposure of the coarsegrained granite and pegmatite is not far above the zone of partial melting. The precise melt-producing reaction is not known, but it probably involves either dehydration melting from breakdown of biotite in the source rocks, or water-present melting following introduction of water from an external source. At c. 1800 Ma, dextral and reverse movement on east-southeast striking faults and uplift of the Yarlarweelor gneiss complex, was accompanied by intrusion of dykes and sheet-like plutons of mediumgrained I-type granite, particularly adjacent to large faults. These granites have high K20/Na20 ratios, high Rb, Zr, SLREE, Y, Th and U contents, and low K/Rb ratios similar to high-K granite melts and diatexite migmatites world-wide derived by biotite dehydration melting of igneous rocks. Neodymium isotope data suggest that early Palaeoproterozoic igneous rocks of the Gascoyne Complex were the source of the granites. These source rocks had been overthrust by the northwestern margin of the Yilgarn Craton, probably during both the Glenburgh and Capricorn Orogenies. The source rocks may also have included low-grade metasedimentary rocks, which could have been a source of water for water-present melting in the overthrust Archaean crust at c. 1812 Ma. However, the lack of evidence for a sedimentary contribution to later granite melts suggests that the source was almost entirely igneous. Metamorphic studies suggest that the Yarlarweelor gneiss complex and adjacent areas remained in the lower crust (at P > 8 kbar) from the late Archaean until c. 1800 Ma (Muhling, 1990). Generation of

coarse-grained granite and pegmatite from melting of

173

Archaean crust at c. 1812 Ma was coeval with compression. At c. 1800 Ma components of dextral and reverse movement on east-southesterly trending faults suggest that uplift was accomplished by transpression. Decompression resulted in large degrees of partial melting of underthrust Palaeoproterozoic Gascoyne Complex, and intrusion of the derived medium-grained granite dykes and plutons into the overlying Archaean crust. The change in granite chemistry between c. 1812 Ma and c. 1800 Ma reflects a fundamental change in the nature of the source and in the type of melting, which was in turn controlled by the change in tectonic regime. The Capricorn Orogeny thus records early compression and overthrusting followed by uplift and decompression, which is consistent with previous suggestions of a clockwise P - T - t path (Muhling, 1990). Seismic studies indicate that crust underneath the Yarlarweelor gneiss complex is presently more than 40 km thick (Drummond, 1981), despite as much as 35 km having already been removed by erosion (Muhling, 1990). The coincidence of a clockwise P T-t path and double thickened crust implies that deformation and metamorphism in the Yarlarweelor gneiss complex during the Capricorn Orogeny marks continent-continent collision (Tyler & Thome, 1990). This interpretation suggests that the Palaeoproterozoic terrains that make up the Capricorn Orogen are prospective for a wide range of mineral commodities.

References

Drummond, BJ., 1981. Crustal structure of the Prccambrian terrains of northwest Australia from seismic refraction data. BMR Journal of Australian Geology and Geophysics 6: 1 2 3 - 1 3 5 . Muhling, J.R., 1990. The Narryer Gneiss Complex of the Yilgarn Block, "Western Australia - a segment of Archaean lower crust uplifted during Proterozoic orogeny. Journal of Metamorphic Geology 8: 4 7 - 6 4 . Occhipinti, S A . &Myers, J.S., 1999. Geology of the Moorarie 1:100 000 sheet. Western Australia Geological Survey, 1:100 000 Geological Series Explanatory Notes: 20 pp. Occhipinti, S A . , Sheppard, S., Nelson, D.R., Myers, J.S. & Tyler, I.M., 1 9 9 8 . Syntectonic granite in the southern margin of the Palaeoproterozoic Capricorn Orogen, Western Australia. Australian Journal of Earth Sciences 4$:. 5 0 9 - 5 1 2 . Sheppard, S. & Swager, C.R, 1999. Geology of the Marquis 1:100 000 sheet. Western Australia Geological Survey, 1:100 000 Geological Series Explanatory Notes: 21 pp. Tyler, I. M. & Thome, A.M., 1990. The northern margin of the Capricorn Orogen, Western Australia — an example of an early Proterozoic collision zone. Journal of Structural Geology 12: 685-701.

SGTSG abstracts, February 2001

174 Cambrian-aged (UHT) ultra-high temperature metamorphism in the Rauer Group, east Antarctica J.P. S i m s ^ M . Hand^'S C J . L Wilson^ and C.M. Fanning^ ^ School of Earth Sciences, The University of Melbourne, Victoria, Australia 3010 2 Geology and Geophysics Adelaide University, Adelaide, SA, Australia 5005 3 Research School of Earth Sciences, Australian National University, ACT, Australia 0200 ' Present address: Geoverde Pty Ltd, PO Box 479, Jamison Centre, ACT, Australia 2614

Metamorphic rocks of the Rauer Group in Prydz Bay, east Antarctica are one of the world s foremost examples of ultra-high temperature (UHT) metamorphism, with peak assemblages forming at temperatures > lOOO'^C and pressures around 10-12 kbar (e.g. Harley, 1998). Although the P-T conditions and evolution of the Rauer granulites have been the focus of numerous studies, there is still considerable ambiguity regarding the timing of the U H T metamorphism. Existing geochronological data is somewhat equivocal, with the possibility that UHT metamorphism is Archaean, mid-Neoproterozoic or even Cambrian in age (e.g. Harley, 1998). In this contribution we present SHRIMP U/Pb age data from pegmatites which indicates that UHT metamorphism in the Rauer Group is Cambrian in age, and forms part of the Pan African otogenic system that dominates large areas of east Antarctica. Three deformed pegmatite bodies were sampled from the southern part of the Rauer Group in the vicinity of Macey Peninsula. In this area, the steeply dipping regional gneissic layering contains a welldeveloped sub-vertical mineral lineation and has been locally overprinted by 1-8 m wide shear zones that also contain a steeply plunging lineation. Structural and metamorphic fabric relations suggest that many of these discrete shear zones are coeval with the interval over which the main gneissic layering developed. P-T calculations indicate that the peak metamorphic assemblages formed at > 1068 ± 83°C and 10.4 ± 1 kbar. Textural relationships indicate that the metamorphic peak occurred relatively late in the strain history, with peak assemblages overgrowing the regional gneissic layering and lineation. This textural relationship can also be observed in other parts of the Rauer Group (Sims & Wilson, 1998). In mafic rocks, the peak garnet-bearing assemblages have been overprinted by coarse-grained undeformed, delicate orthopyroxene-plagioclase ± hornblende-bearing symplectites (Figure la) that formed at around 900°C and 8.2 ±1.3 kbar at the expense of garnet. These reaction textures, and the derived P-T conditions are consistent with the well-established high-T decompressional evolution of the terrain (e.g. Harley, SGTSG abstracts. February 2001

1998). In places the principal gneissic fabric is crosscut by discrete shear zones that formed throughout the metamorphic evolution during constantly shifting patterns of strain partitioning. The progressive nature of shear zone development is demonstrated by the variety of mineral assemblages that are developed within them. In some instances, peak assemblages overprint the shear zone fabric, indicating they formed on the prograde path, whereas in other examples shear zones are retrograde and destroy textures that formed during the initial decompression of the terrain. One of the sampled pegmatites is a garnet-bearing body within the principal high-strain gneissic fabric in the Macey Peninsula region. The pegmatite is strongly foliated and contains the regional steeplyplunging lineation, which is defined by a gneissic quartz-ribbon fabric, indicating emplacement either before, or regional deformation. The bulk of the zircons within the pegmatite have recrystallised, and give a 206pb/238u age of 518 ± 10 Ma, which is interpreted to be either the emplacement age of the pegmatite, or the timing of high-temperature deformation and metamorphism that lead to zircon recrystallisation. The strongly deformed nature of the pegmatite contrasts with the low-strain character of both the peak assemblages, and the high-temperature retrograde symplectites in the surrounding rocks, and indicates that the UHT metamorphism in the southern Rauer Group is younger than 518 ± 10 Ma. The other two dated pegmatites, which give ages of 511 ± 6 Ma and 507 ± 7 Ma respectively for igneous populations, are both strongly foliated, and are located in shear zones (Figure lb) that give temperatures of around 860°C. The structural setting of the pegmatites suggests they were emplaced into developing shear zones, and were deformed during the continued shear fabric development at temperatures > 860®C. Following this, planar pegmatites were emplaced at 500 ± 1 2 Ma, when the terrain was at sub-solidus temperatures (Kinny et al., 1993). This implies that pressures were < 5 kbar at 500 ± 1 2 Ma (Harley & Fitzsimons, 1991), and suggests relatively rapid denudation of the terrain. Rapid denudation is also suggested by the high temperatures (up to lOOO^C)

obtained from Fe-Mg thermometry (Harley, 1998) and the near-isothermal retrograde trajectory. The Cambrian ages for the syn-metamorphic pegmatites in the Rauer Group confirms that ultrahigh temperature metamorphism in northern Prydz Bay developed synchronously with other high-grade areas along the Prydz Bay coast; specifically the Brattstrand Bluffs, Larsemann Hills, and Bolingen Islands. Correlation of structural histories, and geochronological data from several other studies (e.g. Carson et al., 1996; Fitzsimons et al., 1997), suggests that a continuous Cambrian collisional belt exposed along the Prydz Bay coast developed over a minimum time span of roughly 30 million years. This belt forms part of a continental-scale early Palaeozoic otogenic system that involved much of east Antarctica. In the Prydz Bay region, the development of the orogen was internally resolved by lower crustal convergent deformation (as presented by steep structures in the Rauer Group), accompanied by mid- to upper-crustal extension, which is recorded in places such as the Bolingen Islands in southern Prydz Bay (Dirks & Hand, 1995). This combination resulted in exhumation of lower crustal rocks, and pervasive development of metamorphic textures indicative of near-isothermal decompression, and suggests that exhumation may have occurred via extensional stripping of the mid-crust during either an on-going or periodic process, that was maintained by the convergent deformation. An implication of this scenario is that the extensional structures in Prydz Bay

do not represent the terminal collapse of the system. Within the Rauer Group, the Cambrian deformation was dominated by transpression and resulted in the accretion of two distinct terranes. The older of these two terranes consists mostly of Archaean, orthogneiss-dominated crust that is intruded by numerous Proterozoic mafic dykes. The younger terrane consists largely of metasediments that were deposited in the Mesoproterozoic and intruded by a suite of granite at approximately 1000 Ma. The collision of the two terranes at --520 Ma was accompanied by the emplacement of intermediate to mafic intrusives, predominantly into the younger rocks. This collisional episode may represent the final amalgamation of the Eastern Gondwana continent.

175

- r r References Carson, C J . , Fanning, C.M. & Wilson, C J . L , 1996. Australian Journal of Earth Sciences 43: 539-553. Dirb, P.H.G.M &Hand,M., 1995. Australian Journal of Earth Sciences 42: 157-172. Fitzsimons, I.C.W., Kinny, P.D. & Harley, S.L., 1997. Terra Nova 9 : 4 7 51. Harley, S.L. & Fitzsimons, I.C.W., 1 9 9 1 . Journal of Metamorphic Geology 9: 231-243. Harley, S.L.1998. Journal of Metamorphic Geology 16: 541-562. Kinny, P.D., Black, L.P. & Sheraton, J.W., 1993. Antarctic Science 5: 193-206. Sims, J.R & Wilson, C J . L , 1998. In Ricci, C A . (ed.). The Antarctic Region: Geological Evolution and Processes: 120-130.

F i g u r e 1 . (a) O r t h o p y r o x e n e - p l a g i o c l a s e - b e a r i n g s y m p l e c t i t e o v e r p r i n t i n g p e a k metamorphic garnet, (b) Foliated 5 0 7 ± 7 M a pegmatite in shear zone.

SGTSG abstracts. Febmary 2001

176

Ore inspiring structures - some perspectives from seismic surveys and numerical modelling p. Sorjonen-Ward\ P. Gow\ Y. Zhang\ B. Drummond^, B. Goleby^ ^CSIRO Exploration and Mining, PO Box 437, Nedlands, WA, Australia 6009 ^Australian Geological Survey Organisation, PO Box 378, Canberra City, ACT, Australia 2601

The importance of deformation and structural architecture in controlling fluid pathways and favourable depositional sites for hydrothermal ore deposits is now widely recognized and has become fundamental to exploration strategies, particularly with respect to gold. Structural controls are evident across a range of scales and crustal depths, from pluton-centered hydrothermal systems in the upper crust, to ductile shear zones active during high-grade regional metamorphism. Accordingly, we have embarked on a modelling odyssey to establish whether particular types of structures are favourable for generating effective mechanisms for transporting fluids through lowpermeability crust, and for focussing large volumes of fluid through effective depositional sites. Here we identify and model some otogenic processes and architectures common to compressional tectonic settings which may favour fluid migration and entrapment, while also ensuring preservation of mineralization. By analogy with deformed petroleum basins we emphasize examples that illustrate the potential for creating fault-bounded domains of differential uplift and overpressuring beneath relatively impermeable units that act as seals to contain hydrothermal systems. In particular, we concentrate on modelling fluid flow associated with large-scale tectonic wedging, the formation of popup structures, and the associated release of volatiles accompanying exhumation of the mid-upper crust.

Examples of contrasting orogenic architecture and mineralization — Papua-New Guinea and the Yilgarn Two intensely mineralised systems, at opposite ends of the Australian continent in space and time, illustrate this diversity of structural controls particularly well, namely the currently active Papua New Guinea Fold Belt, and the late Archean Yilgarn craton. The Cu and Au deposits in PNG occur in areas that have experienced rapid uplift following initial collision, possibly related to a shift in plate convergence vectors, and show a strong spatial relationship to distinctive SGTSG abstracts, February 2001

felsic intrusions and high-angle transfer zones, which have been inherited from an earlier passive margin rifting event within the underthrust Australian foreland. Despite the lack of post-Archean reworking, the tectonic interpretation of the Yilgarn is more contentious, as there is no clearly identifiable foreland and the kinematics of early deformation events are poorly understood. The existence of a continental basement beneath the Eastern Goldfields Province has however, been inferred from isotopic inheritance and modelling, and areas of older crust are indeed exposed in the south and west of the craton. It is nevertheless appears from the regional context of mineralization that most Yilgarn gold deposits formed at a relatively late stage in the structural and thermal evolution of the craton, perhaps 30 Ma or more after the earliest deformation events. W h e r e P-T conditions of mineralization are documented, late peak to retrograde conditions are indicated, particularly in domains of lower metamorphic grade, while studies of mineral parageneses also demonstrate that gold precipitation was effective over a broad range of temperatures and pressures. Moreover, structural studies of vein systems favour a compressive rather than extensional deformation regime during mineralization. It is therefore reasonable to infer that mineralization occurred during active uplift and exhumation during ongoing compression. Unless we attribute these contrasts to an entirely different thermal regime for Archean orogeny, we are presented with several interesting questions: • If rapid uplift rates are important in generating large mineral deposits - for reasons such as promoting f l u i d release by decompression melting, or enhancing lateral as well as vertical thermal and hydraulic gradients, then what is the long-term potential for preservation of deposits formed at high levels in elevated terrains? • Should we therefore be looking for prospective terrains that record rapid uplift but nevertheless remained below sea level before incorporation into an orogenic wedge, and what might the lithic,

structural and thermal characteristics of such terrains be? • On the other hand, does the evidence from the Yilgarn for effective gold deposition across a continuum of crustal depths imply that epithermal systems might once have been present there, or conversely, that gold deposits are currently forming at greenschist and higher metamorphic grades deep beneath PNG? • Alternatively, are there distinctive aspects of the architecture, thermal structure or deep crustal and lithospheric composition of the present day PNG collisional zone which might predispose the region to forming mesothermal deposits over the next 20 Ma? Numerical modelling of deformation and fluid flow The 2D and 3D models presented here have been constructed with the finite-difference codes FLAG, and FLAG3D, which enable the interaction between deformation and fluid flow to be simulated, such that changing pore fluid pressures can promote rock failure and influence permeability, while changing rock properties conversely influences fluid pressure and migration. In the case of PNG models, zones of rapid uplift:, as a consequence of prescribed geometries and crustal properties, correlate with maximum dilation induced by strain and effectively focus fluids into sites of enhanced vertical extension. Where uplift: leads to erosional breaching of relatively impermeable layers, transient high pressure gradients combined with the stratified permeability contrasts in the passive margin sedimentary pile produce zones of localized high fluid flux. Models for the Yilgarn have been based on a structural framework derived to a large extent from deep seismic data. In particular, the models emphasize a crustal architecture involving opposing — though not necessarily overprinting - imbricate thrust systems. Results illustrate the potential for fluid focusing and mixing in shear zones, including downflow of meteoric water, lateral fluid flow driven by topographic elevation and upward flow of fluids derived from melting and metamorphism in the deep crust. They also provide limited support for the concept that backthrusting or tectonic wedging may generate effective permeable seals during otogenic deformation; this may then enhance focussing, circulation and retention of fluids within a middle crustal environment, compatible with greenschist facies conditions during mineralization. Results also suggest that "pop-up" wedges facilitate

effective fluid upflow and downflow during uplift, while topographic elevation related to asymmetric thrust migration and loading tends to promote lateral fluid flow. Moreover, the effect of topography and the ensuing hydraulic head appears more important than the precise depth or location of the site of fluid production and overpressuring in the deep crust.

177

Some critical architectures and comparison of deep seismic data from various orogenic belts. The backthrusting or tectonic wedging geometry identified in the Yilgarn is discernible in a number of deep seismic sections through orogenic belts of varying age, including highly mineralized terrains such as the Paleozoic Lachlan fold belt in Western Victoria, the Skellefte district of northern Sweden and the Mesozoic Great Valley sequence of California. Moreover, at larger scales, this "bivergent" or tectonic wedging geometry and associated backthrusting may also contribute to the formation and preservation of greenschist facies deposits, in contrast to the lower long term preservation potential for deposits formed in elevated foreland fold and thrust belts. Whether this is a critical and ubiquitous ingredient in promoting fluid flow in the right place at the right time for forming ore deposits remains to be established. More critical perhaps, is to appreciate that these results implicitly assume that appropriate amounts of fluid are available within the crust, and this requires that magmatic and metamorphic fluid generation is precisely timed with respect to deformation. This ftirther reinforces the need to understand the dynamic feedback between deformation, magmatism and fluid production and migration, as part of an integrated and ongoing structural odyssey ^ f ^ f ^

SGTSG abstracts. February 2001

178 The Governor Fault: A structural and geophysical perspective of the boundary between the western and central Lachlan orogen C.V. Spaggiari\ D.R. Gray\ D.A. Foster^and E. Harris^ ^ Department of Earth Sciences, Monash University, Melbourne, Victoria, Australia 3168 2 Departnnent of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA 3 AngloGold Australasia Ltd, Level 11, 60 City Rd Melbourne, Victoria, Australia 3006

The Governor Fault is a major structural boundary that separates the Western (Melbourne zone) and Central (Tabberabbera zone) Lachlan Orogen subprovinces, and is coincident with the Mount Wellington greenstone belt. Major, regional scale, imbricate-fan thrust systems occur on either side of this boundary — with the Mount Wellington Fault Zone and Governor Fault Zone as the respective frontal fault zones. Differences in tectonic vergence, lithological associations, structural development, and metamorphism, indicate that the two thrust systems evolved separately prior to their collision and amalgamation. At this time folding of the Governor Fault, coupled with local reactivation in the latest stages of its evolution produced a complex structure with along strike variations. By definition, the Governor Fault is a major structural feature, however its location, structural geometry, and kinematic evolution have remained enigmatic. This is partly due to poor outcrop and cover by younger sequences. In this study we combine detailed structural and lithological mapping of key areas with interpretation of aeromagnetic data to address these problems. The nature of the Governor Fault, and its relationship to the Mount Wellington greenstone belt has implications for correlations with "greenstones" in Tasmania. The Governor Fault separates two, lithologically distinct. Middle to Late Cambrian (-510-490 Ma) igneous associations that together comprise the Mount Wellington greenstone belt. The two igneous associations are calc-alkaline, andesitic lavas and volcaniclastics, and minor rhyolite with volcanic-arc geochemical signatures preserved as fault slices within the Mount Wellington Fault Zone, and tholeiitic basalts and dolerites, boninites, and ultramafics within the Governor Fault zone. Based on similarities in age and geochemistry, some workers have proposed that the calc-alkaline association (Jamieson and Licola volcanics) is related to the Mt. Read volcanics of Tasmania. The interpretation of such correlations, and the ability to trace major structures SGTSG abstracts, February 2001

are important as they have implications for the tectonic evolution of the Lachlan Orogen, and provide insight into long-standing questions regarding the nature of the lower crust in Victoria, particularly beneath the Melbourne zone. .^y-JFi^

Can Zand A/help us to understand fold mechanisms in schistose rocks?

179

A.R. Stallard' and K.H. Hickey' ^ Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan 2 Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada

Folding of natural rocks is generally described in terms of four simple kinematic end-member models. They are tangential-longitudinal strain folding (TLS), pure shear folding (PS), flexural flow folding (FF) and slip folding (SF). Determining the relative importance of these models during folding is of great interest to structural geologists, and has been the subject of numerous theoretical and field-based studies that seek to find criteria that discriminate between them (e.g., Hudleston & Lan,1993; Hudleston et al., 1996). The geometry of inclusion trails and folds offer a means of distinguishing between the different models, as each of the four fold mechanisms produce different amounts of porphyroblast rotation relative to fold limbs and axial planes for a given limb rotation during folding (e.g., Williams & Jiang, 1999). This results in different geometric relationships between limb rotation and inclusion trail curvature in sub-spherical porphyroblasts for each of the four fold mechanisms. In real rocks, the amount of syn-folding porphyroblast rotation relative to fold limbs can be estimated from inclusion trail curvature. The relationships between key parameters that define porphyroblast rotation during folding have been determined for deformation that comprises a combination of FF and PS folding (Fig. 1). TLS and SF are considered to be unsuitable mechanisms to explain inclusion trail curvature, as TLS folding doesn't generate vorticity around porphyroblasts, and SF doesn't accommodate shortening normal to axial planes, requires extreme anisotropy or isoclinal folds to generate modest amounts o f inclusion trail curvature, and produces large amounts o f porphyroblast rotation relative to geographic coordinates, which is inconsistent with previous theoretical and field-based studies (e.g. Jung et al., 1999; Williams & Jiang, 1999). The parameters considered are limb rotation ( O ) , variation in vorticity between different layers within the rock mass (N; Jiang, 1994), rotation relative to fold limbs (Hf), rotation relative to the fold axial plane ( ) > ^he FF component of limb rotation (FF O ) . The limiting factor on the maximum possible FF Q f

during each fold event proved to be the ratio of • to O (this ratio is herein termed the parameter Zand represents the ratio between the total porphyroblast rotation relative to fold limbs, and the total limb rotation). The relationship between Z, maximum FF O and A'' is displayed graphically in Fig. la. These relationships were determined by solving F F O in terms of Z for different N. From these calculations, the following relationship was determined: FFO

N The relationships of F F i ^ f (inclusion trail curvature resulting from vorticity-induced rotation of porphyroblast during FF) and • Q,2 (total rotation of porphyroblasts relative to fold axial planes) to Z, N and FF O have been determined in a similar way and yield the following relationships: FFa 1 _

FFO 2XO-FFO

This relationship is shown in Fig. lb.

This relationship is shown in Fig. Ic. From Fig. 1 it can be seen that Zand A^are the two key parameters that describe the relationships between fold mechanisms and inclusion trail curvature. Once Zand A^have been constrained for a given fold or fold event, it is then possible to determine (i)

the proportions of limb rotation resulting from FF and from PS (Fig. la), (ii) the amounts of porphyroblast rotation relative to fold limbs produced by FF and by PS (Fig. lb), and (iii) the total amount of porphyroblast rotation relative to axial planes (Fig. Ic). Note that the amount of rotation relative to the fold axial plane is constant for a value of Z independent of N (Fig. Ic). This last observation means that rotation relative

SGTSG abstracts, February 2001

180

References

to axial planes can be determined once the value of Z is known, and remains valid independent of the value of N and the proportions of FF and PS folding. The maximum possible FF component of folding, however, is greater with increasing N. Thus, the value of N in sampled layers must be determined before the components of FF and PS folding can be calculated. The above relationships offer a means of determining fold mechanisms and net porphyroblast rotation in folded terrains where the parameters Z and N can be determined.

1 •

Huddleston, P.]., & Lan, L., 1991. Information from fold shapes. Journal of Structural Geology 15: 253-264. Hudleston, P.J., Treagus, S.H., & Lan, L., 1996. Flexural flow folding: does it occur in nature? Geology 24: 203-206. Jiang, D., 1994. Flow variation in layered rocks subjected to bulk flow of various kinematic vorticities: theory and geological implications. Journal of Structural Geology 16: 1159-1172. Jung, W.-S., Ree, J.-H. & Park, Y., 1999. Non-rotation of garnet porphyroblasts and 3-D inclusion trail data: an example from the Imjingang belt, South Korea. Tectonophysics 307: 381-395. Williams, P.F, & Jiang, D., 1999. Rotating garnets. Journal of Metamorphic Geology 17: 367-378.

\\ \\\

l<t> 0.5 -

\ \

IN^O.S o 0.5

\A^1.5 Wl \

\ \

\ 1.5

(=Z)

Distribution of F F Q °

Distribution of Z

-I<I>

values o f Z <I>

\\ \\\ \ \ \

Z<I>

Zcl>

•

-30

0.5

1.5

V-2

Nm).5

\NL \ \ AJUO.5 0.5

-60'

FF<t>

0.5 ^

Distribution o f I tfj when

-60

Distribution of FFQ^ when I Q ^ - e O *

0.5

\ 1

1.5

Figure 1. Graphs showing the relationships between key parameters that describe porphyroblast rotation during folding. The relationships shown in these graphs are true for any fold event that involves components of FF and PS. (a) Relationship between Z, FF<I> and N. The relationships of these parameters to total inclusion trail curvature generated by rotation in FF (FFQ?(%)) is shown in (b). The amount of total porphyroblast rotation relative to the axial planes of the developing folds (2 ) is shown in (c). The values of FF Q?(b) and SQ? (c) vary with total limb dip, and are thus shown as % and ratio of ^ respectively. Samples of these relationships with real values of total limb dip are shown in (d) and (e) respectively. SGTSG abstracts. February 2001

The Dimboola Igneous Complex in western Victoria: The crash test dummy for a Delamerian arc-continent collision?

181

D.H. Taylor Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria, Australia 3002

The cause of the Cambrian Delamerian Orogeny in mainland Australia has long been enigmatic. The well exposed inboard portion of the Delamerian Fold Belt in South Australia shows the platform to deep marine passive margin sequence of the Adelaide Geosyncline thrust westward back onto the older craton as a foreland fold and thrust belt (Jenkins, 1989). A recent review of the Orogen noted "the need for an outboard collider in some sort convergent setting has long been recognised, but the fundamental details such as the location and nature of the colliding mass have remained speculative" (Haines & Flottman, 1998). In the late 1980s geophysical images of the more outboard portion of the Orogen in western Victoria revealed a major belt of magnetic rocks concealed beneath the Murray Basin that was named the Stavely Belt (Brown et al., 1988). This belt was inferred to be the continuation of the calc-alkaline Mount Stavely Volcanics which outcrop along strike farther south and which had already be speculated as a possible Andeantype arc hosted in a (micro)continental fragment. It was collision of the Adelaide Geosyncline on a downgoing lower plate beneath this continental fragment that was interpreted as the cause of the Delamerian Orogeny (Scheibner, 1985). Whilst the fundamental concepts and geometry of that scenario are correct, two pieces of more recent information force a revised interpretation. (1) dating of the Stavely Volcanics in the mid 1990s shows they erupted at 500 Ma at the end of the Delamerian Orogeny so that they are a consequence of the 520500 Ma deformation rather than part of the cause (Crawford et al., 1996). (2) drilling of the magnetic belt by North Ltd in the mid 1990s shows the rocks to be a mafic-ultramafic "ophiolite" sequence rather than an Andean-Type calc-alkaline arc. Named the Dimboola Igneous Complex, these rocks can be interpreted as remnants of an intraoceanic forearc-arc and this suggests that arc-continent, rather than continent-continent collision drove the deformation (Taylor et al., 2000; Taylor et al., in prep). Samples from the complex have a boninitic geochemistry that demands formation above an intraoceanic subduction zone. The rocks are predominantly mafic-ultramafic

cumulates of gabbro and pyroxenite with lesser amounts of sub-volcanic equivalents and basaltic lava. The rocks are all intensely altered to talc, serpentine and chlorite at greenschist metamorphic conditions yet lack any structural fabric. This is indicative of high fluid flux under static conditions such as hydrothermal sea-floor alteration which is common in forearc regions above the dehydrating subducting slab. Located at the most outboard margin of the Delamerian Orogen, the Dimboola Igneous Complex is taken to represent an ophiolitic arc-forearc package accreted onto the passive margin of the Adelaide Geosyncline as that sequence and its rifted continental basement was progressively drawn into an east-dipping subduction zone. The early collisional contactsethe suture in the classic sense of the word between the upper and lower platesaeis possibly preserved along the deeply buried western side of the northern portion of the Dimboola Igneous Complex. This suture is herein named in concept as the Yanac Suture (after a small town in this northern region). In many places, particularly farther south into the area of outcrop, the suture appears to have been disrupted and/or reactivated by later faults such as the Escondida and Yarramyljup faults. Discrete blobs of the Hummocks Serpentinite occurring in a strange context a little farther to the west (see Morand et al., this volume) may be remnants of this ophiolite sheet originally emplaced along the flat-lying Yanac Suture. In Tasmania similar scattered remnants of maficu l t r a m a f i c rocks (the M U C s ) have also been interpreted as fore-arc ophiolites emplaced onto an older continental basement during eastward dipping subduction at this time, (Berry & Crawford, 1988) although a thick passive margin sequence upon the basement was lacking there. This initiation of convergent tectonics along the Gondwana margin in the early Palaeozoic represents the second half of the Wilson cycle—the closure of the what must have been a huge PalaeoPacific Ocean generated during the Neoproterozoic break-up of Rodinia (Powell et al., 1994). As the expanse of oceanic SGTSG abstracts, February 2001

182

crust became extremely large and unstable is it likely that intraoceanic subduction began in a setting much like present Marianas. This generated the unusual Early Cambrian boninite and oceanic island arc lithologies on the upper plate that characterise the Cambrian greenstones of southeast Australia (Crawford et al., 1984).

Eventually the Australian

rifted margin of Rodinia on the lower plate (which on the mainland had the Adelaide Geosyncline upon it) arrived into this setting to begin the series of Palaeozoic convergent deformations that built up southeastern Australia (VandenBerg et al., 2000). After this initial Delamerian arc-continent collision major plate reorganisation is evident, with convergence being locally "stoppered up" and transferred elsewhere to leave much of Victoria as a quiescent ocean basin receiving the Lachlan mud pile until the next convergent onslaught.

References Berry, R.F. & Crawford, A.R., 1988. The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Australian Journal of Earth Sciences 35: 523-533. Brown, C.M., Tucker, D.H. & Anfiloff, V.L, 1988: An interpretation of the tectonostratigraphic framework of the Murray Basin region of southeastern Australia—based on an examination of airborne magnetic patterns. Tectonophysics 154: 309-333.

Crawford, A.J., Cameron, W.E. & Keays, R.R., 1984. The association boninite-low-Ti andesite-tholeiite in the Heathcote Greenstone Belt, Victoria: ensimatic setting for the Early Lachlan Fold belt. Australian Journal of Earth Sciences 31: 161-177. Crawford, A.J., Donaghy, A.G., Black, L.R & Stuart-Smith, P.G., 1996. Mount Read Volcanics correlatives in western Victoria: a new exploration opportunity. Australian Institute of Geoscience Bulletin 20: 97-102. Jenkins, R.F.J., 1989. The Adelaide Fold Belt: Tectonic reappraisal. The Evolution of a Late Precambrian - Early Palaeozoic Rift Complex: The Adelaide Geosyncline, Geological Society of Australia, Special Publication 16: 396-420. Haines, PW. & Flottman, T. 1998. Delamerian Orogeny and potential foreland sedimentation: a review of age and stratigraphic constraints. Australian Journal of Earth Sciences 45: 559-570. Powell, C.McA., Preiss, W.V., Gatehouse C.G., Krapez B, and Li Z..X. 1994. South Australian record of a Rodinian epicontinental basin and its mid-Proterozoic break-up (-700 Ma) to form the PalaeoPacific Ocean. Tectonophysics 237: 113-140. Scheibner, E., 1985. Suspect terranes in the Tasman Fold Belt System, eastern Australia. In D.G. Howell (ed) Tectonostratigraphic terranes of the Circum-Pacific region. Circum Pacific Council of Energy and Mineral Resources, Earth Science Serial 1: 493514. TAYLOR, D . H . , CAYLEY, R . A . , M O R A N D , V . J . , W O H L T , K . E . a n d M O O R E ,

D.H. 2000. The Delamerian Orogeny in western Victoria: consequence of arc-continent collision. Geological Society of Australia, Abstracts: 59, 492. VandenBerg, A.H.M., Willman, C.E., Maher, S., Simons, BA., Cayley, R.A., Taylor, D.H., Morand, V.J., Moore, D.H. & Radojkovic, A., 2000. The Tasman Fold Belt System in Victoria. Geology and mineralisation of Proterozoic to Carboniferous rocks. Geological Survey of Victoria Special Publication. Department of Natural Resources and Environment.

Early Cambrian 520 Ma; approach of passive margin

Middle Cambrian 515-505 Ma; arc-continent collision

Figure 1. Cambrian arc-continent responsible for Delamerian Orogeny.

SGTSG abstracts, February 2001

Tectonic significance of the P-T-D path of the Sleaford Complex granulites at Fishery Bay, Eyre Peninsula, South Australia

183

L Tong, J.J. Vassallo and C.J.L Wilson School of Earth Sciences, The University of Melbourne, Victoria, Australia 3010

The reworked Sleaford Complex is located west of Sleaford Bay in the southern Eyre Peninsula, South Australia and consists of a series of Archaean to Palaeoproterozoic granulite grade basement orthogneisses and paragneisses dated at 2600-2400 Ma (Fanning et al., 1988). They are pervasively intruded by Proterozoic mafic dykes, and subjected to the Kimban Orogeny (Vassallo & Wilson, 1999). Recent detailed structural surveys have shown that the Kimban Orogeny (1750-1700 Ma) controlled the major structural development in this region, characterized by first SW-NE stretching deformation and second bulk flattening deformation (Vassallo and Wilson, 1999; in press), however, the P-T-D evolution during the Kimban Orogeny is not clear. Field observations for the Sleaford Complex at Fishery Bay suggest that some refolded upright tight KDi fold and KSj fabrics are still preserved in KD2 high-strain zones in the psammitic gneisses. The microstructural evidence from the high-grade assemblages in these mafic granulite boudins indicated an anticlockwise evolutionary P-T path with first near isobaric cooling and subsequent near isothermal decompression. The thin garnet corona textures with clinopyroxenes between orthopyroxenes and plagioclases imply a cooling from the medium pressure granulite field (Opx+Pl) into the high-pressure granulite field (Grt+Cpx+Qtz), whilst the orthopyroxene and plagioclase corona textures between garnets and quartzs indicate a subsequent decompression from peak assemblage (Grt+Qtz) to typical decompression assemblage (Opx+Pl). The P-T estimates from Grt-Opx thermobarometers (Harley and Green, 1982; Lee and Ganguly, 1988) imply that the peak conditions of the KD^ high-grade deformation probably reached ^-900^0 and -12 kb. However, the granulite facies pelitic migmatites preserve some typical S-C fabrics defined by biotites parallel to and oblique to the gneissic layering. They experienced major KD^ high-grade thrust deformation, and were strongly overprinted by KD2 highgrade transpressional ductile shear zone deformation fabrics, which indicate a dextral sense of movement.

Furthermore, there is no evidence for the development of plagioclase decompression corona texture around the garnet porphyroblasts, in psammitic gneisses and semipelitic migmatites from KD2 low-strain zones, these commonly develop plagioclase decompression corona textures. By using the average P-T method (Powell & Holland, 1994) and Grt-Opx thermobarometers (Harley & Green, 1982; Lee & Ganguly, 1988), P-T calculation results from the core-rim compositions in the garnet porphyroblasts clearly indicate a clockwise decompression P-T path, suggesting that KD^ peak P-T conditions reached 826°-864°C and 9.2-9.3 kb, and post-peak conditions reached 684°-745°C and 4.0-5.3 kb. These indicate a post-peak decompression of 3.4-5.0 kb accompanied by a rapid cooling of >100°C. The post-peak decompression probably follows the KD^ thrust deformation and the KD2 transpressional shearing. The inferred anticlockwise P-T path with near isobaric cooling followed by near isothermal decompression from the mafic granulite boudins in the psammitic gneiss contrasts with the clockwise near isothermal decompression P-T path inferred from the pelitic migmatites. This is probably because the pelitic migmatites were strongly overprinted by KD2 highgrade ductile shear deformation. Nevertheless, these contrasting P-T paths are not inconsistent with the tectonic evolution of the Kimban Orogeny in this region. The anticlockwise near isobaric cooling P-T paths are ofi:en produced in an extending tectonic setting, thus the inferred KD^ anticlockwise near isobaric cooling P-T path is compatible with the first SW-NE stretching tectonism (Vassallo and Wilson, in press). However, the clockwise near isothermal decompression P-T paths are generally ascribed to the later stage of the thermal evolution of overthickening continental crust, with or without the additional effects of mantle-derived magmas, and tectonic thickening is considered to be the principle setting for the genesis of this decompression P-T path. Therefore, the inferred clockwise decompression P-T path from the garnet porphyroblasts in the pelitic migmatites in this area is mainly associated with and reflects the post-peak KD2 SGTSG abstracts. February 2001

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evolution, which is characterized by an accompanied important cooling, suggesting the consistence with the second high-grade tectonism of bulk

flattening

(Vassallo & Wilson, in press). Whereas, the coevallyaccompanied significant cooling is probably intimately associated with the partial crust thickening resulted from the second bulk flattening deformation,

^'f^

References Fanning C.M., Flint R.B., Parker A.]., LudwigK.R. & BlissetA.H., 1988. Refined Procerozoic evolution of the Gawler Craton, South Australia, through U-Pb Zircon chronology. Precambrian Research 40/41: 363-386. Harley S.L & Green D.H., 1982. Garnet-orthopyroxene barometry for granulites and peridotites. Nature 300: 697-701. Lee H.Y. & Ganguly J., 1988. Equilibrium compositions of coexisting garnet and orthopyroxene: experimental determinations in the system F e 0 - M g 0 - A l 2 0 3 - S i 0 2 and applications. Journal of Petrology 29: 93-114. Powell R. & Holland T.J.B., 1994. Optimal geothermometry and geobarometry. American Mineralogist 79: 120-133. Vassallo J.J & Wilson C.J.L., 1999. Palaeoproterozoic geology of southeastern Eyre Peninsula, south Australia. In: Wilson, C.J.L. (ed.). The Great Southern Transect II: a geological section incorporating the Lachlan Fold Belt, Adelaide Fold Belt and Gawler Craton, Halls Gap (Victoria) to Port Lincoln (SA). Specialist Group in Tectonics and StructuralGeology, Field Guide 6, Geological Society of Australia. Vassallo J.J & Wilson C.J.L., in press. Palaeoproterozoic regional-scale non-coaxial deformation: an example from the eastern Eyre Peninsula, South Australia. Journal of Structural Geology.

SGTSG abstracts, February 2001

A new gravity traverse across the northern Wyangala Batholith (Eastern Lachlan Fold Belt)

185

R. Trzebski, P.G. Lennox and D. Palmer School of Geology, University of New South Wales, Sydney, Australia 2052

The Wyangala Batholith is located in the MolongWyangala Zone, Eastern Lachlan Fold Belt and consists of numerous small mainly S-type plutons forming overall a north-south striking complex (Fig. la). The Wyangala Batholith intruded deformed Ordovician turbidites and volcaniclastic rocks, and late Early Silurian volcanics and shales west of the Copperhannia Thrust during regional dextral shearing probably in the latest Early Silurian. Subsequent deformation around 380 Ma and 340 Ma caused foliation development in the granites (Glen et al., 1999), large ductile shear zones and kinking within and along the eastern margins of individual plutons. Paterson et al. (1990) associated the emplacement of the Wyangala Batholith with pre-existing, west-dipping reverse faults caused by east-west shortening in the Silurian. The geometrical and spatial relationship between these faults and the batholith is not clear due to the poor exposure of these faults and the consequent lack of information about the three dimensional geometry of these structures. A new gravity line (Cowra-Neville) consisting of 113 stations with an average spacing of 500 m over a total distance of 53 km has been undertaken across the northern plutons of the Wyangala Batholith (Fig. lb). The traverse crosses the regional gravity low, which shows a minimum along the Carcoar Fault east of the batholith. It crosscuts mainly granodiorites, tonalites, leucogranites, metasediments, volcanics and several regional faults (Fig. lb). Three lows with a maximum magnitude of 18 mGal separated by two relative highs dominate the Cowra-Neville gravity field. The westernmost low is caused by the Grants Corner Granodiorite, whereas the central low is produced by the composite intrusive body of the Swan Ponds Tonalite, Lucan Complex and Garland Granodiorite all outcropping adjacent to and west of the Carcoar Fault (Fig. 2). The easternmost low coincides with the combined gravity effect of the Bugs Ridge and southern Sunset Hills granites west of the Copperhannia Thrust. The relative high on either side of the Wyangala Fault is due to the exposure of the dense Middle Ordovician Walli Volcanics and Middle Ordovician Adaminaby Group (Fig. 2)..

Gravity models show that the Grants Corner Granodiorite and the Swan Ponds-Lucan-Garland complex both have asymmetrical shapes with the steeper flanks at their eastern margins and flat to gende flanks at their western margins. Both plutons show a maximum thickness of 5 km along the 'root zones. The Grants Corner Granodiorite and the Swan PondsLucas-Garland granites are separated by the Wyangala Fault which is interpreted to dip gently to the west. The Woodstock Fault west of the Grants Corner Granodiorite does not produce a significant effect on the gravity curve and therefore its dip direction cannot be clearly determined. The gravity effect of the Carcoar Fault is clearly reflected by a steep gradient of 5 mGal east of the Swan Pond-Lucas-Garland complex and this suggests that it dips steeply (-70°) to the west along the east margin of the Garland Granodiorite. Geological evidence indicates that the Garland Granodiorite may have been uplifted about 10 km along this fault with apparent dextral translation of the Coombing Formation-Adaminaby Group boundary (see Lennox et al., this volume). The Bugs Ridge and southern Sunset Hills granites both form sheet-like, westerly-dipping plutons with a maximum thickness of 3 km. Although both granites show significant differences in geochemistry, but they may merge to a single pluton at depth. The Copperhannia Thrust is reflected by a smooth gradient in the gravity curve which indicates that it is gentle west-dipping (Trzebski ^Lennox 1999). . ^ f f ^

Acknowledgements The Deutsche Forschungsgemeinschaft and ARC Small ARC Grant (RMS3203) supported our research in the northern part of the Eastern Lachlan Fold Belt.

References Glcn,R.A., Lennox, P.G. & Foster.D.A., 1999. 40Ar-39Ar dating of deformations west of the Hill End Trough, Lachlan Orogen, New South Wales. Quarterly Notes of the Geological Survey of New South Wales 110: 13-22. Paterson, S.R., Tobisch, O.T. & Morand, V.J., 1990. The influence of large ductile shear zones on the emplacement and deformation of the Wyangala Batholith, SE Australia. Journal of Structural Geology 12: 639-650. Trzebski, R. & Lennox, PG., 1999. Contrasts in morphogenesis and tectonic setting during contemporaneous emplacement of S-

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Igg

and I-type granitoids in the Eastern Lachlan Fold Belt, southeastern Australia. In Vigneresse, J-L. (ed.) Understanding granites: Integrating New and Classical Techniques. Geological Society Special Publications 158: 123-140.

Figure 1. Simplified geology of the study area: (a) Regional geology (Ba: Bathurst Granite, Wy: Wyangala Batholith); (b) Local geology around the gravity traverse Cowra-Neville. GCGd: Grants Corner Granodiorite, SPT/LC/GGd: Swan Ponds Tonalite-Lucas Complex-Garland Granodiorite complex (gray shaded); RBG: Rocky Bridge Granite; SHG: Sunset Hills Granite; WSF: Woodstock Fault; WF: Wyangala Fault, CF: Carcoar Fault; CT: Copperhannia Thrust.

Figure 2. Gravity line Cowra-Neville (top) and 2.5D gravity model (bottom).

SGTSG abstracts. February 2001

Collision orogeny during the Palaeoproterozoic in Western Australia

187

I.M. Tyler Geological Survey of Western Australia, 100 Plain Street, East Perth, WA, Australia 6004

The Australian crust had been assumed to have formed a single entity by the late Archaean, with most Palaeoproterozoic tectonic and magmatic activity occurring in intracratonic settings between 1900 and 1800 Ma (e.g. Etheridge et al., 1987). However, complex Palaeoproterozoic tectonic histories are indicated by more recent geochronological data. Remapping of the Capricorn Orogen, the Paterson Orogen, and the King Leopold and Halls Creek Orogens in Western Australia, combined with geophysical and geochemical data, indicate that their Palaeoproterozoic tectonic evolution can be interpreted in terms of continental break-up, terrane accretion, and plate collision (Myers et al. 1996, Tyler et al., 1998). The Capricorn Orogen formed between the Archaean Yilgarn and Pilbara Cratons. At its northern margin rifting of the Archaean Pilbara Craton initiated the Hamersley Basin at c. 2770 Ma, followed by breakup at 2690 Ma. The upper part of the Hamersley Group may have been deposited in a collisional setting between 2470 and 2440 Ma. TheTuree Creek Group and lower Wyloo Group were deposited in the McGrath Trough, a possible foreland basin developed in front of a northward verging fold belt during an Ophthalmian otogenic event at c. 2200 Ma (Martin et al., 1998). At the southern margin of the orogen the Yerrida Basin formed at c. 2175 Ma initially as a sag basin on the Archaean Yilgarn Craton, followed by an abrupt change to a rift-fill setting. To the west, in the southern part of the Gascoyne Complex collision of a late Archaean to early Palaeoproterozoic micro-continent with the northwestern margin of the Yilgarn Craton produced extensive deformation, metamorphism and felsic magmatism during the 2000 to I960 Ma Glenburgh Orogeny (Sheppard et al., 1999). The Bryah Group may have been deposited in a back-arc basin setting at this time. The 1840 to 1790 Ma Capricorn Orogeny involved extensive deformation, metamorphism and felsic magmatism (Occhipinti et al., 1998; Krapez & McNaughton, 1999; Sheppard et al., this volume) during the oblique collision and suturing of the

geologically distinct Archaean Pilbara and Yilgarn Cratons to form the West Australian Craton. The King Leopold and Halls Creek Orogens are part of the North Australian Craton and have a distinctly different Palaeoproterozoic tectonic history than the Capricorn Orogen. In the Halls Creek Orogen, rifting of a continental margin began in the Eastern zone of the Lamboo Complex at c. 1910 Ma and continued with deposition of the lower Halls Creek Group at c. 1880 Ma. In the Western zone of the Lamboo Complex and in the Hooper Complex of the King Leopold Orogen accretion of continental fragments to the eastern edge of the Kimberley Craton occurred before c. 1900 Ma. Turbidites derived from late Archaean and early Palaeoproterozoic crust forming the Kimberley Craton were deposited at c. 1870 Ma (Tyler et al., 1999). Deformation, metamorphism and extensive felsic and mafic magmatism occurrred during the 1865 to 1850 Ma Hooper Orogeny (Griffin et al., 2000) The Central zone of the Lamboo Complex formed at c. 1865 Ma either as an island arc (subduction to the southeast) or an ensialic basin marginal to the Kimberley Craton (subduction to the northwest) (Sheppard et al., 1999). Deformation and metamorphism to high-grade at c. 1845 Ma (Bodorkos et al., 1999), followed intrusion of numerous felsic, and basic to intermediate sheet-like bodies during convergence and collision of the Central zone with the Western zone. Layered mafic-ultramafic intrusions were emplaced into both the Western and Central zones of the Lamboo Complex at c.l855Ma. Further rifting along the eastern continental margin was marked by alkaline volcanism in the Eastern zone of the Lamboo Complex between 1870 and 1850 Ma. Turbiditic rocks of the upper Halls Creek Group were deposited parallel to the continental margin by a submarine fan. Eruption of felsic and mafic volcanic rocks during rifting of the Central zone at c. 1840 Ma was accompanied by the emplacement of layered maficultramafic intrusions. Continued subduction of oceanic crust to the northwest led to collision and suturing of the Kimberley Craton with the rest of the

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North Australian Craton by c. 1820 Ma during the Halls Creek Orogeny. Folding and thrusting accompanied metamorphism in the Central zone of the Lamboo Complex. During and immediately following collision, plutons of granite and gabbro were intruded to form the Sally Downs supersuite at the same time as the intrusion of large layered maficultramafic bodies. As the Sally Downs supersuite was being intruded into the Lamboo Complex the Speewah Group was deposited on the Kimberley Craton at c. 1835 Ma,. The Kimberley Group oversteps the Speewah Group onto the Lamboo Complex, and was derived from the north. The intrusion of the Hart Dolerite at c. 1800 Ma may be related to continental break-up centred to the north, at the same time as granite intruded into the southern part of the Lamboo Complex, and into the Granites-Tanami Complex further to the south. In the Rudall Complex of the Paterson Orogen a foreland basin may have developed prior to c. 1800 Ma. Granite intrusion took place at c. 1800 Ma, followed by deformation, metamorphism and further granite intrusion between 1790 and 1760 Ma during the Yapungku Orogeny. West-verging thrusting and high-P metamorphism may have accompanied collision of the West Australian Craton with the North Australian Craton, which recent palaeomagnetic evidence indicates were unified by c. 1700 Ma (Li, 2000). This event may be reflected in the Strangways Orogeny in the Arunta Orogen (Bagas & Smithies, 1997).

^ff^

References Bagas, L. & Smithies, R.H., 1997. Palaeoproterozoic tectonic evolution of the Rudall Complex, and comparison with the Arunter Inlier and Capricorn Orogen. Western Australia Geological Survey, Annual Review 1996-97: 110-115. Bodorkos, S., Oliver, N.H.S. and Cawood, P A., 1999. Thermal evolution of the central Halls Creek Orogen, northern Australia. Australian Journal of Earth Sciences 46: 453-465. Etheridge, M A . , Rutland, R.WR. & Wyborn, L.A.I., 1987. Orogenesis and tectonic processes in the early to middle Proterozoic of northern Australia. In Kroner, A., (ed.), Proterozoic Lithosperic Evolution. American Geophysical Union, Geodynamics Series 17: 131-147. Griffin, T.J., Page, R.W., Sheppard, S. & Tyler, I.M., 2000. Tectonic implications ofPalaeoproterozoic post-collisional, high-K felsic igneous rocks from the Kimberley region of northwestern Australia. Precambrian Research 101: 1-23. Krapez, B. & McNaughton, N.J„ 1999. SHRIMP zircon U-Pb age and tectonic significance of the Palaeoproterozoic Boolaloo Granodiorite in the Ashburton Province, Western Australia. Australian Journal of Earth Sciences 46: 283-287. Li, Z. X., 2000. Palaeomagnetic evidence for the unification of the North and West Australian cratons by ca. 1.7 Ga: new results from the Kimberley Basin of northwest Australia. Geophysical Journal International 142: 173-180. Myers, J.S., Shaw, R.D. & Tyler, I.M., 1996. Tectonic evolution of Proterozoic Australia. Tectonics 15: 1431-1446.

SGTSG abstracts, February 2001

Martin, D.McB., Li, Z.X., Nemchin, A.A. & Powell, C.McA., 1998. A prc-2.2 Ga age for giant hematite ores of the Hamersley Province, Western Australia. Economic Geology 93: 1084-1090. Occhipinti, SA., Sheppard, S., Nelson, D.R., Myers, J.S. & Tyler, I. M., 1998. Syntectonic granite in the southern margin of the Palaeoproterozoic Capricorn Orogen, Western Australia. Australian Journal of Earth Sciences 45: 509-512. Sheppard, S., Tyler, I. M., Griffin, T J. & Taylor, W. R., 1999. Paleaoproterozoic subduction-related and passive margin basalts in the Halls Creek Orogen, northwest Australia. Australian Journal of Earth Sciences 46: 679-690. Sheppard, S., Occhipinti, S. A., Tyler, I. M. & Nelson, D. R., 1999. The nature of c. 2.0 Ga crust along the southern margin of the Gascoyne Complex. Western Australia Geological Survey, Annual Review for 1998-99: 56-61. Tyler, I. M., Page, R. W & Griffin, T. J., 1999. Depositional age and provenance of the Marboo Formation from SHRIMP U-Pb zircon geochronology: Implications for the early Palaeoproterozoic tectonic evolution of the Kimberley region. Western Australia. Precambrian Research 95: 225-243. Tyler, I. M., Pirajno, R, Bagas, L., Myers, J. S. & Preston, W. A., 1998. The geology and mineral deposits of the Proterozoic in Western Australia. AGSO Journal of Australian Geology and Geophysics, 17: 223-244.

Unravelling polydeformed Palaeoproterozolc basement structure of the Tanami gold province, NT: 5% outcrop, 100% geophysics

189

LC. Vandenberg, M.A. Hendrickx, A.J. Crispe, K.R. Slater & A.A. Dean Northern Territory Department of Mines and Energy, Northern Territory Geological Survey, Minerals House, 58 Hartley Street, PO Box 2655 Alice Springs, NT, Australia 0871

The Tanami Region, a major gold province in the Northern Territory, lies 600 km northwest of Alice Springs. Extremely poor exposure has frustrated previous attempts to constrain the structural development of the region (e.g. Blake et al., 1979). This paper presents preliminary results of current NTGS investigations that integrate detailed structural and stratigraphic mapping, recently acquired geophysical data, more precise radiometric age determinations and NTGS and exploration company drilling. Preliminary results indicate Archaean and Palaeoproterozoic basement stratigraphy of the Tanami Region has been subjected to at least seven regional deformation events (D*i, Dj-D^^; Vandenberg et al., 2000). An initial basin forming event ( 0 * ^ ) is stratigraphically inferfed and suggested to have involved extension and/or subsidence of Archaean basement (Hendrickx et al., 2000). D1-D2, together v^ith early M^ regional greenschist to amphibolite facies metamorphism, characterise the Barramundi Orogeny in this region (-1845 Ma). Dj structures are variably orientated due to later deformation and are characterised by asymmetric, disharmonic F^ fold couples. Within siliceous beds the Sj axial planar fabric is a discontinuous, anastomosing dissolution-style cleavage. Within siltstone beds the Sj fabric is a slaty cleavage. Fabric-porphyroblast overprinting relationships indicate peak M^ was syn-to post-D^. Peak M^ metamorphic assemblages are: biotite/andalusite/ amphibole (± garnet, cordierite) in pelitic units, and quartz/ garnet/bio tite/ epidote ± amphibole ± cordierite in semipelitic units. These assemblages indicate T - 6 0 0 ° C and P - 3.5 kbar (10-14 km depth; Scrimgeour & Sandiford, 1993), suggesting shallow-to-mid crustal burial during Dp D| and M^ fabrics are deformed by more open D2 fold structures, oriented mainly north-south. Within pelites and lower grade equivalents the S2 fabric is a

spaced crenulation and/or crenulation cleavage (0.1 1 cm). Rare oblique Fj and F2 mushroom style fold interference structures are also observed. First generation structures within Pargee Sandstone (a late syn-Di molasse deposit) are regional D2 structures. D3 structures are characterised by northeast-to-east striking chevron folds and kinkbands. S3 fabrics are generally fracture style cleavages within siliceous units. Mineralogically differentiated S3 cleavages within less silicious lithologies are rare. These observations suggest that basement rocks formerly at shallow-to-mid crustal depths during D^ and D2 were by D3 times at higher crustal levels. Deposition of Mount Charles Formation, later intrusion of Coomarie Suite granite (SHRIMP zircon 1815 ± 4 Ma; Smith, 2000) and extrusion of associated felsic volcanics appear consistent with extensional basin formation (D4/E2). D1-D3 structures are not observed at this stratigraphic level. D5 shear zones, thrusts and oblique slip thrusts dissect the region and constitute major bounding structures. D5 structures operated after emplacement of Coomarie Suite granites, and are spatially coincident with significant gold deposits (e.g. Bonsai, Jims Find, Tanami mine corridor). In the Tanami mine corridor numerous D5 thrusts, backthrusts, layer-parallel decollemont and breccia horizons point to strong theological controls in the formation of a significant network of structures at upper crustal levels. Gold mineralisation at The Granites and Dead Bullock Soak areas is coincident with localised post-Barramundi shear zones and faults within suitable lithologies and fold structures and formed at deeper crustal levels than the Tanami corridor. D^^ thrusts and oblique slip faults cut all earlier structures and have affected fault-propagated folding within overlying Birrindudu Group (post 1770-1660 Ma). Early regional stress (D1-D2) involved east-west directed shortening related to collisional processes in the Halls Creek Otogenic zone to the northwest. SGTSG abstracts, February 2001

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Subsequent deformation events involved northwestsoutheast to north-south regional shortening. These later events and localised formation of extensional sedimentary basins probably reflect the influence of regional transpression focussed along the southern margin of theTanami Region. The late stage structural evolution of the Tanami Region (D5/D5^) involved the interaction of several granite-cored structural domains.

Acknowledgements Abstract released with permission of the Director of the Northern Territory Geological Survey, Dr. R Dennis Gee. The cooperation and assistance of Normandy N F M Ltd., Otter Gold NL., and Anglogold Australasia Ltd., is also greatly appreciated.

References Blake D.H., Hodgson LM. & Muhling P.C, 1979. Geology of The Granites-Tanami Region, Northern Territory and Western Australia. Bureau of Mineral Resources, Geology and Geophysics Bulletin 197. Hendrickx M.A., Slater K.R., Crispe A J . , Dean A.A., Vandenberg L.C. & Smith J . B . , 2000. Palaeoproterozoic stratigraphy of the Tanami Region: regional correlations and relation to mineralisation -preliminary results. Northern Territory Geological Survey Record 2000/13. Scrimgeour, I. & Sandiford, M., 1993. Early Proterozoic metamorphism at the Granites gold mine, Northern territory: Implications for the timing of fluid production in high-temperature, low-pressure terranes. Economic Geology 88/5: 1099-1113. Smith, J . B . , 2000. N T G S - A G S O Geochronology Project Report 3. Australian Geological Survey Organisation Professional Opinion 2000/27 Vandenberg, L.C., Hendrickx, M . & Crispe, A., 2 0 0 0 . Preliminary structural analysis of the Tanami region. In Annual Geoscience Exploration Seminar (AGES), Darwin, 21 March 2 0 0 0 : abstracts. Northern Territory Department of Mines and Energy, Open File Geological Survey Record GS 2000-0010. Vandenberg L.C., Hendrickx M., Crispe, A., Slater K.R., Crispe A.J. & Dean A.A., in prep. Electronic pre-release: Structural geology of the Tanami region- preliminary results. Northern Territory Geological Survey Record. For further information See D M E website hrtp://www.dme.nt.gov.au for electronic pre-releases available in Pdf format: Hendrickx et al. (2000) and Vandenberg et al. (in prep).

SGTSG abstracts, February 2001

Regional non-coaxial deformation of ArchaeanPalaeoproterozoic basement in Eyre Peninsula, South Australia

191

J.J. Vassallo and C.J. L. Wilson School of Earth Sciences, The University of Melbourne, Victoria, Australia 3010

Parallel and across strike sections of the Kalinjala Shear Zone along the coast of Eyre Peninsula provides us with an insight into the architecture of a granulite basement terrain. Three main rock packages were deformed in the period 1750-1700 Ma: the Archaean Sleaford Complex and the Palaeoproterozoic Lincoln Batholith and the Hutchison Group metasediments. Structures formed at that time are attributed to the Kimban Orogeny. Structures previously attributed to the 2600 Ma Sleafordian Orogeny at Whalers Way have been completely transposed during the Kimban and are no longer recognisable. KDj structures are dominated by mesoscopic sheath and tubular folds that evolved from a series of interconnected steep and flat-lying shear zones during northward transport. Feldspar clast [X:0.2, Y:2.4, Z:25] dimensional ratios lie in the field of constriction and define a very strong linear fabric in the field. KS^ is the strongest planar fabric in the region and lies parallel to lithological layering. Pre- to syn-Dl leucosomes lie along the KS^ plane. KF^ fold plunges vary from steep to gentle, dominantly plunging to the south.

southern Eyre Peninsula suggest that the mesoscopic structures described above are secondary and tertiary culminations on much larger structures tens of kilometres long. These primary structures lie with their axes parallel to the KSZ. Both small-scale and largescale structures suggest that deformation was driven by dextral transpression. Re-examination of structures in the central Eyre Peninsula both in the field and on large-scale maps suggest that macroscopic-scale folded sheath folds are also common in the Hutchison Group. This suggests that structural repetition of stratigraphic units is to be expected based on rock interference patterns. One example is the apparent occurrence of chemically and petrologically identical Upper and Lower Middleback Jaspilites in the Middleback Ranges. When recast in the form of a refolded F1 sheath fold, rather than the previously suggested folded cylindrical fold with Type 2 interference, both Jaspilites would be the same unit. Therefore, on a broad scale the stratigraphy is likely to be simpler than originally thought. Original estimates of stratigraphic thickness maybe reduced by one third,

KD2 deformation is of a fold-thrust type commonly documented in low-grade upper crustal regions. KF2 folds lie on a north-trending axis formed by the dominantly eastward migration of curved thrust surfaces. For example, units were stacked sub-vertically via listric thrusts and duplex systems that dip towards the hinterland. KF1/KF2 fold interference produced varying types of interference patterns that are truncated by KD2 shear zones. In the YZ plane Type 2 closed boomerang shapes occur. In the XZ plane Type 0 to Type 3 patterns are prevalent. On a regional scale the Lincoln Batholith forms part of a fan-type termination of the KSZ, with alternating domains of high- and low-strain. The Sleaford Complex lies in the heart of the shear zone, recording high strains throughout. The Hutchison Group records the highest strains along the KSZ. New (1999) aeromagnetic images (courtesy of Primary Industries and Resources South Australia) of the SGTSG abstracts, February 2001

192 Understanding an evolving orogen C. Wijns, L. Moresi, B. Davies and A. Ord CSIRO Exploration And Mining, PO Box 437, Nedlands, WA, Australia 6008

In studying large-scale ore systems, we are interested in modelling the evolution of structures that control the fluid pathways responsible for mineralisation. The link between tectonism and fluid migration over large length and time scales is the focus of our investigations of the Yilgarn and Olary/Broken Hill regions. Our conceptual models, developed through field studies, are highly time and space dependent, with a key feature being the migration of deformation 'fronts' outwards from the axis of an orogen as it evolves. The basis of the conceptual models is that fluid flow follows the outward migration of these strain 'fronts, allowing progressive access to increasingly distal positions. This would be facilitated by, for example, an outwardly propagating thrust system. The implication is that mineralised domains may not represent discrete events, but form part of a continuum that sees early domains overtaken and reworked by subsequent deformation as the orogen expands. No pre-existing structures or zones of weakness are imposed as initial conditions, in contrast to many modelling approaches, which rely on initial geometries. We are able to simulate the formation of structures

SGTSG abstracts, February 2001

arising naturally out of the stress regime in the lithosphere. This leads to the possibility of modelling the development of fluid reservoirs and timedependent mixing of pore fluids. The code we use has been developed explicitly to deal with high strains, and is well suited to problems of continental collisions, crustal extension or subduction, and intrusions. Pore fluid flow is controlled by Darcy s law and contributes to crustal deformation through the effective stress. Different rheologies and yield criteria may be prescribed for the rock matrix. The feedback between pore pressure and crustal stress is important in dictating where regions of high strain will develop, and how they evolve. The eventual vision of our modelling is to fully couple deformation, fluid flow, thermal and chemical effects into a unified approach for simulating the genesis of mineralising systems in orogenic settings.

Geodynamic modelling of Irish-type base metal deposits of 193 the central Irish midlands, and genetic comparisons with the Rosebery lead-zinc deposit, Tasmania G. Willetts, B. Hobbs and A. Ord C S I R O Exploration and Mining, P O Box 437, Nedlands, WA, Australia 6009

The aim of this work is to explore and evaluate

scenario provides a template for subsequent coupled

topography-driven flow (Hitzman & Beaty, 1996) and

thermal and chemical reactions. A constant thermal

extension-convection (Russel, 1986) models, through

gradient of 35°C/km is imposed; H2S is injected along

geodynamic modelling, and to assess their feasibility

the base of the model and S04^' is injected along the

for the formation of the Rosebery Zn-Pb (-Cu-Au)

top, at a constant rate. This model shows convection

deposit and the Zn-Pb (-Ba-Ag) deposits of the Central

cells that partially agree with Russels model but the

Irish Midlands. Geodynamic modelling is undertaken

resultant mineralisation patterns and precipitation rates

to simulate and understand fluid

and

do not reflect the mineralisation seen in the Irish

mineralisation patterns associated with Irish-type Zn-

Midlands. A revised model involving lower perme-

Pb deposits and to provide constraints on both models

abilities and lower initial concentrations of sulphur

proposed. In this context, the term "geodynamic

species overcomes this problem and does produce

modelling" refers to mathematical coupling o f

representative mineralisation patterns.

flow

deformation with physical and chemical processes

When considering ore genesis, topography-driven

operating at various scales throughout the Earths crust.

flow and extension-convection models are feasible

The computer modelling highlights the behaviour of

mechanisms for the mineralisation at Rosebery. Similar

the thermal and fluid flow systems operating under

tectonic histories and structural relationships with

specified geometries and boundary conditions, which

orebodies suggest comparable processes were in

simulate the architecture for both of the proposed Irish-

operation during ore genesis.

type genetic models. The topography-driven flow scenario involves mechanical and fluid flow coupled models promoting continuous northward flow through a unit simulating the Old Red Sandstone. Several different variations of the initial scenario are investigated in order to evaluate

References Russel, M.J., 1986. Extension and Convection: a genetic model for the Irish carboniferous base metal deposits. In Andrew, C.J., Crowe, R.WA., Finlay, S., Pennell, W.M. &. Pyne, J.E (eds). Geology and Genesis of Mineral Deposits in Ireland: 545- 555. Hitzman, M.W & Beaty, D.W., 1996. The Irish Zn-Pb-(Ba) orefield. Society of Economic Geologists Special Pub. 4: 112-143.

the effectiveness o f the Old Red Sandstone as a mineralising agent in the Irish Midlands. The only scenario that produces continuous northward topography-driven flow involves an unconfined aquifer, which has been partially inverted as a result of compression, to a geometry with five kilometres of relief In this model, fluids migrate across a 90 km basin with 100 m of water overlying the recharge zone. Further modelling simulates a more realistic length of the Munster Basin and assesses heterogeneous permeabilities, which decrease with depth. The effect of topography-driven flow is also assessed in the Midlands Basin. A similar strategy involving models simulating the early Carboniferous Irish Midlands Basin illustrates the favourable effect extension has on meteoric fluid flow descending down pre-existing faults. A combination of these models and the Russel (1986) convection SGTSG abstracts, February 2001

194 Structure of the Tabberabbera Zone in Victoria C.E. Willman, A.H.M. VandenBerg, B.A. Simons, C. Quinn^ and V.J. Morand Geological Survey Of Victoria, PO Box 3100, Bendigo, Victoria, Australia 3554 ^ University of Melbourne, Victoria, Australia 3010

The Tabberabbera Zone in eastern Victoria forms the western margin of the BenambraTerrane, a distinctive part of the Lachlan Fold Belt (VandenBerg et al., 2000). Until recently, the internal geology of the zone was poorly known. Fergusson (1987, 1998) mapped a major fault (Wonnangatta Fault) in the south of the Tabberabbera Zone separating Silurian and Ordovician turbidites. Fergusson (1998) showed that Cambrian oceanic volcanics form the leading edge of a southwestvergent fold and thrust zone along the western margin of the Tabberabbera Zone and suggested they probably underlie much of the zone. Spaggiari (2000) suggested that blueschists and associated melange along the western margin indicate the Tabberabbera Zone was an Ordovician/Silurian accretionary prism. The Geological Survey of Victoria has flown 200 m spaced airborne geophysical surveys over the whole zone which has been followed up by detailed and reconnaissance mapping throughout the parts of the zone not mapped by Fergusson or Spaggiari. We have traced the Wonnangatta Fault northwards to the Mount Buffalo area where it links with the Bread and Butter Shear Zone. Together, these major fault zones divide the Tabberabbera Zone into two subzonesaethe Eastern Tabberabbera Subzone (ETS) and the Western Tabberabbera Subzone (WTS). The ETS consists mainly of Early to Middle Ordovician Pinnak Sandstone with sparse outcrop of Late Ordovician Bendoc Group whereas the WTS largely consists of Early Silurian Cobbannah Group interrupted by fault slices of Bendoc Group. The main folding and faulting appears to be Early Silurian (Benambran Orogeny) but few timing constraints are available. Early Devonian (?) granites provide a younger constraint as these stitch the main structures (e.g. Mount Buffalo Granite). In the Mount Buffalo area the W T S forms a complex west-vergent fold and thrust zone dominated by Cobbannah Group (mudstone with minor sandstone and quartzite) and fault slices of Bendoc Group (black shale, mudstone and sandstone). Bedding consistendy trends towards about 340° which compares with more variable trends found in the adjoining part of the Eastern Tabberabbera Subzone

SGTSG abstracts. February 2001

(300° to 330°). The ETS differs from the WTS by its greater structural complexity and by the absence of Cobbannah Group. The ETS has more variable structural trends and second and third generation cleavages are more extensively developed. Ordovician Pinnak Sandstone forms most of the ETS except for a belt of Bendoc Group along the western margin in the hanging wall of the Wonnangatta Fault Zone. Overall structural vergence in the ETS is probably to the southwest although some significant east-vergent areas have been mapped in the Mount Buffalo area and south of Mount Baldhead. Regional structural trends in the ETS show that strike of bedding steps westward from south to north in several sigmoidal segments. One of these occurs near the linkage of the Wonnangatta Fault with the Bread and Butter Shear Zone. A strike-slip component is indicated for some faults in the Mount Buffalo area and for some narrow northwest-trending zones in the WTS where folds have been rotated to nearly vertical. The significance of the strike-slip component remains uncertain but it may indicate that regional transpression was partitioned to narrow weak zones during the main Silurian deformation. In the far southeastern part of the Tabberabbera Zone, structural trends swing from northwest to east-west on approaching the Buchan Rift but the eastern margin of the zone lies under Early Devonian volcanics. The zone may extend to the Lucas Point Fault or to a buried parallel structure. The character of the Wonnangatta Fault changes markedly along strike. In the south, the fault is associated with a 1-2-km wide fault zone with a very large dip-slip displacement—the thickness of the Pinnak Sandstone is not known but a displacement of at least several kilometres is likely. Here it is a major northeast-dipping fault zone separating lowermost Pinnak Sandstone, and in places the CambroOrdovician Howqua Chert, from Bendoc and Cobbannah groups. The hanging wall of one of the main faults exposes talcose bedded rocks that may be Cambrian volcaniclastic rocks. In the north the Wonnangatta Fault Zone and Bread and Butter Shear Zone are more complex and appear to branch into a number of faults with smaller displacements straddling

the subzone boundary, covering a wide area. However, this apparent greater complexity in the north may simply be a function of the more detailed mapping carried out in the BuifFalo area. Broken formation forms a small component of some fault zones but unlike some well known accretionary prisms, they form a tiny percentage of the total rock volume. In the WTS, small packages (< tens of metres wide) of broken formation occur in the Cobbannah Group or in fault sUces of Bendoc Group. Larger packages up to hundreds of metres wide are found in the ETS either in Pinnak Sandstone or Bendoc Group (e.g. Nug Nug Shear Zone). In some cases the broken formation is overprinted by the main Si cleavage suggesting an early syn-sedimentary origin. Another major fault in the southeastern part of the zone is the Barmouth Fault. This northwest-trending structure bounds a narrow slice of the Barmouth Group (?Late Silurian). It consists of conglomerates with andesite or rhyolite clasts and quartzose sandstone with occasional large clear quartz grains, probably derived from rhyolite. The fault was active in the Siluro-Devonian Bindian Orogeny as shown by an extensively developed $2 crenulation cleavage that is confined to a wedge-shaped belt between the Barmouth and Kiewa faults. The S2 cleavage is also sporadically well developed throughout other parts of the ETS but its timing is less certain. In the northern ETS the $2 may be a late Benambran fabric. Thirdgeneration structures are well-developed in the southern ETS. Generally regarded as Tabberabberan they consist of a north-trending crenulation cleavage and open to tight, low-amplitude minor folds with wavelengths up to a few metres. ^ f ^

195

References FERGUSSON, C . L , 1987a. Early Palaeozoic back-arc deformation in the Lachlan Fold Belt, southeastern Australia: implications for terrane translations in eastern Gondwanaland. In Leitch, E.C & Scheibner, E. (eds.), Terrane Accretion and Otogenic Belts. American Geophysical Union, Geodynamics Series 19: 39-56. Fergusson, C . L , 1998. Cambrian-Silurian oceanic rocks, upper Howqua River, eastern Victoria: tectonic implications. Australian Journal of Earth Sciences 45: 633-644. Spaggiari, C.V., Gray, D.R., Foster, D A . & Fanning, M., 2000. Oceanic setting and subduction-related tectonics for the Central Lachlan Orogen, Southeastern Australia. In Skilbeck, C.G. & Hubble, T.C.T. (eds). Understanding Planet Earth: Searching for a sustainable future. Abstracts of the 15^^^ Australian Geological Convention, University ofTechnology, Sydney, NSW, Australia. July 3-7, 59: 468. VandenBerg, A.H.M., Willman, C.E., Maher, S., Simons, B.A., Cayley, R A , Taylor, D.H., Morand, V.J., Moore, D.H. & Radojkovic, A , 2000. The Tasman Fold Belt System in Victoria. Geology and mineralisation of Proterozoic to Carboniferous rocks. Geological Survey of Victoria Special Publication. Department of Natural Resources and Environment.

SGTSG abstracts, Febaiary 2001

196 Influence of hydrothermal activity on the Federal-Albion area of the Magdala Mine, Stawell, Victoria B. Witham, J.McL. Miller, C.J.L Wilson and G. Phillips School of Earth Sciences, The University of Melbourne, Victoria, Australia 3010

The Stawell Zone hosts some of the oldest stratigraphic sequences and highest grade metamorphism known in the Victorian Lachlan Fold Belt. Regional folding is widespread across the zone early on in its tectonic history. This has provided the fundamental framework upon which the Magdala gold deposit at Stawell has been superimposed. The stratigraphy of the Magdala Mine is essentially a sequence of tholeiitic basalts with associated volcanogenic sediments overlain by a thick package of turbidites. Early regional deformation has created a series of folds that are cored by basalt and draped by the sedimentary packages. Subsequent deformation is typically localised where there are strong competency contrasts between adjacent rock types. The dominant form of deformation structures within the mine area are large shear zones (5-20 m wide, >1 km deep and 1 - >5 km long) that overprint the regional metamorphism. It is well established from previous work (Mapani, 1995; Gane, 1998) that the distribution and concentration of gold in the Magdala Mine is primarily structurally controlled. It has also become apparent from current work that the gold deposition has occurred within particular windows of time, principally 455-440 Ma. The Federal-Albion area of the Magdala Mine is generally a lower grade ore zone in comparison to other areas of the mine. This area was chosen for systematic mapping and sampling so that comparisons could be made between the structural styles, intensity of deformation and the nature of alteration and mineralisation phases between a weakly mineralised area and strongly mineralised areas in order to further identify the controlling factors on gold mineralisation. Major ore lodes that occur throughout the mine, such as the Basalt Contact Lode and the Central Lode System also pass through the Federal-Albion area. The area has a long history of being affected by hydrothermal fluids. Volcanogenic sediments, which contain early cleavage parallel veins, have metamorphic biotite dated at 480 Ma (Mapani, 1995) while quartz porphyry dykes which intruded after the establishment of the major shear zones, and which themselves are quartz veined and hydro thermally altered, are dated at 413+3 Ma (Arne et al., 1998). The expression of SGTSG abstracts, February 2001

different generations of quartz veins is typically strongly influenced by the rheology of the individual lithological units, but especially by the competency contrasts between adjacent lithological units. This implies that not all areas or stratigraphic units have experienced the same fluid flow mechanisms or fluid flow intensity over the lifetime of the hydrothermal system(s). Their quite different expressions in the different lithological units accentuated the already high competency contrasts between adjacent lithological units. These contrasts eventually focus deformation to narrow zones: 1. In all stratigraphic units there is an early phase of quartz veining associated with regional folding that is cleavage parallel (steep NE and SWdipping). This is concentrated in the sedimentary units and is associated with very limited chlorite-sericite alteration that has a pervasive style of distribution rather than zoning around the veins. 2. This is followed by further veining, in much the same orientation, characterised by the development of a crenulation cleavage, folding of the pre-existing veins (fold hinges of both plunge shallowly to steeply N W ) . More extensive and intense, pervasive chlorite-sericite-silica alteration and associated pyrite overprinting sedimentary-derived Fe sulphides now in the form of pyrrhotite. A bleaching carbonate alteration of the basalt appears to be confined to this stage. 3. Postdating this stage the sedimentary packages change their response to the imposed stress regime. Large shear hosted quartz veins (0.5-5 m wide) develop in positions that are now ore lodes. Deformation becomes more localised and starts to show significant brittle characteristics, such as tension gash veins, breccias and spaced cleavages within the shear zones. This is probably due to a combination of factors, such as a rotation in the stress field and the sedimentary packages behaving in a more competent manner because of the large amount of quartz veining being developed within them. From this point onwards the alteration of the host rocks also becomes much more localised, typically around quartz veins (laminated or massive)

and shear zones. Haloes o f arsenopyritechalcopyrite+pyrite are particularly common around massive tension gash veins with massive quartzchlorite (often vermicular)-Fe rich carbonatesstilpnomelane. 4. Once the deformation had become confined to narrow zones, the brittle structures become more and more c o m m o n and intense. Ultimately alteration, veining and mineralisation became enclosed within the margins of the shear zones or veins.

197

It should be noted that the mineralogy of alteration and mineralisation assemblages does not essentially change over time. Alteration and gold mineralisation phases show repetitive overprinting relationships, implying that the mineralisation processes occurred in cycles over more than one of the broadly defined deformation episodes. As the deformation and alteration became more localised chlorite and sericite were commonly replaced by carbonate and silica. The veining and hydrothermal alteration phases indicate that fluid flow mechanisms also changed from being fairly pervasive to becoming very localised, ^ ' f ^ f ^

Acknowledgments Stawell Gold Mines, MPI for financial and logistical assistance to this project.

References Arne, D.C., Bierlein, RR, McNaughton, N., Wilson, C J . L . & Morand, VJ., 1998. Timing of gold mineralisation in western and central Victoria, Australia: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks. Ore Geol. Rev. 13: 251-273 Gane, M., 1998. Gold mineralisation within the Basalt Contact Ore Zones, Magdala Mine, Stawell Victoria. Unpub. MSc thesis. School of Earth Sciences, University of Melbourne (Victoria): 250 pp. Mapani, B.S.E., 1995. Structural evolution and mineralisation at the Magdala Gold Mine, Stawell, Western Victoria. Unpub. PhD thesis. School of Earth Sciences, University of Melbourne (Victoria): 303 pp.

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198

The ophiolites in Mian-Lue zone and Mian-Lue-Ning Block in South Qinling, China and their tectonic significance C. Zhang, K. Guo and Y. Dong Institute of Geology and Mineral Resources, Nanjing , PRC 210093

The Mian-Lue zone is one of the key areas in the study of the Qinling orogen. Many geologists believe that the Mian-Lue tectonic belt is the subduction and collisional suture between Yangtze plate and the south Qinling orogen (Fig. 1). In this paper, on the basis of the detailed study of the mafic-ultramafics, radiolaria chert and geological and geochemical characteristics of the metavolcanic rocks in this zone, the authors suggest that they are independent geologic bodies. The mafic-ultramafics and the basic volcanic rocks are Proterozoic rather than Late Paleozoic-Mesozoic, as they have recorded the plate tectonic evolution from Meso-Proterozoic to Late Proterozoic. This

corresponds with the global Rodinian super-continent formation event (about 1000 Ma). The DevonianCarboniferous radiolaria chert intimately coexists with immature proximal-provenance clastics, and their geological and geochemical characteristics show that they developed on a continental base rather than on oceanic crust. Further to this, there are no LatePaleozoic ophiolites in the Qinling orogen. Combining this study with recent work in north Qinling, we suggest a new evolutionary model of the Qinling orogen from the Meso- to Neoproterozoic.

Figure 1. Sketch geologic map of the Mian-Lue zone and Mian-Lue-Ning block, Southern Qinling. LArchean, 2.Proterozoic, 3.Sinian, 4.Sinian-Paleozoic, 5.Lower Paleozoic, 6.Devonian, 7.Carboniferous, S.Mesozoic, 9.Diabase, 10.Fault, 11 .Mafic-ultramafic rocks, 12.Mixtite and mylonite, 13.Granite. KX: Kangxian; GZ, Guozhen; XJP, Xujiapin; ZYB, Zhuangyuanbei; JJH, Jinjiahe; LY, Lueyang; ZK, Zhuangke; JGT, Jieguantin; LHK, Lianghekou; HHB, Heiheba; W J H , Wangjiahe; YTB, Yangtianba; CD, Chadian; SJB, Sijiba; CC, Tongchang; LSH, Lesuhe; MX: Mianxian, YDZ: Yudongzi

SGTSG abstracts, February 2001

199

Faulting related deformation and fluid flow Y. Zhang\ B.E. Hobbs\ A. BarnicoatS A. Ord^ and J.L. Walshe^ ^ CSIRO Exploration & Mining, Nedlands, WA, Australia 6009 2 Rock Deformation Research, School of Earth Sciences, University of Leeds, Leeds, UK

Veins, faults, and deformed rock are spatially related. We explore the coupled faulting-deformation-fluid flow process resulting in this association. More specifically, we explore the geodynamics of faulting and fluid flow numerically, with special attention to displacement distribution, stress and deformation patterns, fault growth, permeability development, and fluid focusing and mixing. The results are of interest in understanding the relationship of ore deposits to faults. Conceptual finite-difference models have been constructed to simulate a rock containing a single fault or multi-fault structure. A strength contrast between the fault and the rock is incorporated so that displacement along a fault will take place when the resolved shear stress along the fault reaches a critical value, generally before the rock starts to accumulate permanent deformation. The model is loaded by compressive stress with the maximum compressive stress oblique to the fault strike. To explore permeability development during the faulting process, a mechanism that enables permeability to increase when tensile failure occurs (reflecting the development of tensile cracks or hydro-fractures) is incorporated in the models. Darcy s law is coupled with an elasticplastic rheology to explore the feedback between deformation and fluid flow. Displacement distribution along a single fault, predicted by this model, is consistent with several reports based on field evidence. Displacement is generally maximized at the central part of a fault and gradually decreases towards the ends of the fault. This pattern is correlated with low stresses at the central segment of the fault and high stresses at the end (displacement releases stresses). Stress localisation near fault tips shows strong asymmetry. This represents two contrasting stress regimes on both sides of the fault tips, that is, compressive domain and tensile domain. Such asymmetry is also reflected in deformation features including shear strain distributions, failure and dilation patterns; also a tensile fracture and dilation zone array develops in the tensile domain. These patterns are compared with some observed vein structures. The displacement, stress, deformation and

failure patterns described above become more complex when multi-faults are involved. Fluid flow is strongly influenced by bulk deformation associated with faulting. While faults act as fluid transport channels, new permeability structures also develop as a result of tensile failure and associated fracturing/hydro-fracturing; either instantaneously or persistent with time. Highly permeable faults and new permeability structures form an efficient flow network, enabling fluids to migrate, localise and mix at structurally favourable locations. The present results show that dilations zones associated with faulting provide ideal fluid focusing and mixing locations (Figs l a and lb), ^ ^ f f ^

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200

1 Djisttion zones

Figure 1. a) Dilation zones (darker grey areas) developed around two parallel strike-slip faults. Arrows indicate shear sense. The maximum compressive stress is vertical, b) Instantaneous Darcy fluid flow vectors show that fluids focus into and mix in the dilation zones between the two faults, where a higher permeability is developed. The white zones indicate the faults.

SGTSG abstracts, February 2001

K-Ar dating of clay-rich fault gouge, northern Sydney Basin H. Zwingmann^

201

T. Wilson^ and R. Offler^

^ CSIRO - Division of Petroleum Resources, PO Box 136, North Ryde, NSW, Australia 1670 2 present address : Centre of Excellence in Mass Spectrometry, Curtin University, WA, Australia 6102 3 Department of Geology, The University of Newcastle, Callaghan, NSW, Australia 2308

The clay minerals illite and illite-smectite ( I I S ) are commonly present in fault gouges. However, until recently, few attempts have been made to use these minerals to date fault movements. We present K-Ar data obtained from 1/S formed during fault movement, from fault zones located in the northern Sydney Basin. This study is the first of its type carried out in Australia. In the first part of the study, eleven samples were selected from two fault zones and host rocks located at Burwood Beach, SSW of Newcasde. The faults occur in siltstones and tuffs of the Lambton Subgroup of the Newcastle Coal Measures. They are subparallel, approximately 1.5 m apart, trend N-S and have steep E dips. A total displacement of 10 m occurred on the western fault and 3 m of vertical displacement on the eastern fault. 1cm wide, foliated gouge zones are developed along both fault surfaces representing the regions in which principal displacement has occurred. Samples were collected from the gouge zones and host siltstones and tuffs within and outside the damage zone. They were disaggregated using a repetitive freezethaw technique to separate <2 and 2-6 [im fractions. The mineralogy of the fractions was determined by X-ray diffraction (XRD) on air dried, glycolated and heated (550®C) samples. K content was determined by atomic absorption. Argon was extracted from the separated mineral fractions by fusing samples within a vacuum line and analysed for isotopic composition by mass spectrometry afi:er addition of an ^^Ar spike. XRD analyses of samples indicate that I/S and kaolinite in varying proportions are present in the fractions from the fault gouges and host rocks in the damage zone. In most samples, the I/S contains 7090% iUite, suggesting temperatures <100®C during faulting. Twenty two K-Ar dates have been determined. The ages of samples in the gouge and damage zones range from 126.8 to 164.5 (n=9; x=l48.3 Ma; sn=10.5) for the 2-6mm fraction and 122.2 to 150.9 Ma (n=10; x= 137.2 Ma; sn=7.8) for the <2 |Llm fraction. Older ages of 272 -281.8 Ma and 237-244.9 Ma for the 26 and <2 [im fractions, have been obtained from undeformed host rock. Radiogenic ^^Ar ranges for

these samples range between 59.28 and 98.1% indicating negligible atmospheric Ar contamination. The reliability of the ages is also confirmed by the agreement within 2s analytical limits for the duplicate analyses of the <2 |im fraction of some samples. The 272-281 Ma K-Ar dates obtained from the 26 \Xm fractions of the host rocks are older than the age of the sequence in which the samples occur (245252 Ma; Roberts et al., 1996). This is due to the presence of detrital muscovite which is a common phase in these rocks. The younger ages of 237244.9 Ma are thought to reflect the time at which diagenesis occurred. The <2 \lm ages (122-150 Ma) obtained from the fault gouge date the last slip event occurring on the faults, which is possibly related to underplating and uplift of the Australian continent. Samples for the second part of the study were collected from drill core intersecting the Hunter Thrust, a major NW trending, shallow NE dipping fault zone separating the Permian sediments and coal measures of the Sydney basin from the Carboniferous, forearc basin sediments and volcanics of the southern New England Fold Belt. Zones in drill core from two drill holes (DDH BMP 106A and 127), showing litde fracturing to intense comminution, were sampled for analysis. XRD studies indicate that I/S and kaolinite to be the dominant phases in the fractions separated from the highly comminuted samples; chlorite-smectite is an additional phase in the host rocks (siltstone, tuff). Preliminary results obtained from the highly comminuted samples reveal a wide variation in ages in the different fractions. In 106A, the ages range from 227.0 Ma (<2 \im) to 189.3Ma (1-2 |im) to 174.3 Ma (<0.5 |im) and in 127 from 226.7 Ma (2-6 |im) to l60.2-176.4Ma (<2 |LLm). The older age obtained from the 2-6 |Llm fraction in 127 is attributed to die presence of detrital, 2M polymorph which XRD analysis showed to be present. However, this explanation is not applicable to 106A which contains <5% of the 2M polymorph. Particle size analyses are required to ascertain whether <2 |Hm fraction contains a higher percentage of coarser particles than the 1-2 |Xm

SGTSG abstracts, February 2001

202

fraction. Further, K-Ar analyses of the finer fractions in other samples are needed to constrain the timing of movements on the Hunter Thrust. This study highlights the potential and value of isotopic dating of synkinematic, illite to determine upper crustal deformation events.

Reference

Roberts, ]., Claoue-Long J.C. & Foster, C.B., 1996. SHRIMP zircon dating of the Permian System of eastern Australia. Australian Journal of Earth Sciences 43: 4 0 1 - 4 2 1 .

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203

Email addresses Laurent John Ron Peter Richard Ingrid Peter She Fa Moonsup Dorothy Alan William Stephen Jane Nathan Pradipta Garry Puquan Nicholas Cecile Christopher Ian Caroline David David Rick Ben David Kunyi Peter Michael Martin Dorte Anthony Quinton Bruce Rod David Oliver Bregje Patrick Sarah Anthony Sarah Myra Yoonsup Russell Paul Zheng-Xiang Gordon Timothy David Lucas Sebastien John Vince Brendan

Ailleres Baxter Berry Betts Blewett Campbell Cawood Chen Cho Close Collins Collins Cox Cunneen Daczko Das Davidson Ding Direen Duboz Fergusson Fitzsimons Forbes Foster Giles Gordon Goscombe Gray Guo Haines Hall Hand Hansen Harris Hills Hobbs Holcombe Hollingsworth Holm Hulscher James Johnson Johnston Jones Keep Kim Korsch Lennox Li Lister Litde Maidment Marshall Meffre Miller Mo rand Murphy

lauren t @mail. earth. monash.edu. au traycee@cdesign.com.au berry@geo.geol.utas.edu.au giles@mail.earth.monash.edu.au richard.blewett@agso.gov.au campbell@sub.net.au p.cawood@info.curtin.edu.au s.chen@dme.wa.gov.au mcho@tsrc@uwa. edu.au dorothy.close@nt.gov.au alanc@lithos.curtin.edu.au bcollins@geology.newcastle.edu.au sfcox@geology.anu.edu.au jcunneen@tsrc.uwa.edu.au ndaczko@mail.usyd.edu.au pdas@ncpgg.adelaide.edu.au Garry.Davidson@utas.edu.au xugu@chariot.net.au Nick.Direen@agso.gov.au cecile@mail. earth. monash. edu. au Chris_Fergusson@uow.edu.au ianf@lithos.curtin.edu.au caroline@mail.earth.monash.edu.au dfoster@geology.ufl.edu giles@mail.earth.monash.edu.au r.gordon@earth.uq.edu.au bengos@poboxes.com dgray@mail.earth.monash.edu.au kunyi-g@publicLptt.js.cn Peter.Haines@utas.edu.au mall@mail.earth.monash.edu.au martin.hand@adelaide.edu.au dorte@mail.earth.monash.edu.au a.harris@earthsciences.uq.edu.au ST-hills@mail.earth.monash.edu.au b.hobbs@per.dem.csiro.au rodh@earthsciences.uq.edu.au hollingd@lithos.curtin.edu.au oholm@postoffice.utas.edu.au bhulscher@tsrc.uwa.edu.au patrick.james@adelaide.edu.au sjohnson@tsrc.uwa.edu.au johnstoa@minerals.nsw.gov.au Jonessa@postoffice.utas.edu.au mkeep@tsrc.uwa.edu.au yskim2@snu.ac.kr Russell. Korsch@agso.gov. au P.Lennox@unsw.edu.au zli@tsrc.uwa.edu.au gordon@mail.earth.monash.edu.au timothy.litde@vuw.ac.nz David.Maidment@agso.gov.au lucas.marshall@jcu.edu.au smeffre@utas.edu.au j.miller@earthsci.unimelb.edu.au Vincent.Morand@nre.vic.gov.au bmurphy@stfx.ca

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204

Phung Matthew Richard Tim Sandra Robin Alison Mervyn Mark Sergei Chris Wolfgang Steven Alistair Patrice James Mike Gideon Michael TonyKevin Roye Mike Wouter Ian David David Peter Renate Peter Catherine Aaron David Laixi Ian Alfons Leon Jeff Ron Christopher Clive Chris Bronwyn Chuanlin Yanhua

Nguyen Noble Norris O'Driscoll Occhipinti Offler Ord Paterson Peternell Pisarevsky Powell Preiss Reddy Reed Rey Richardson Rickard Rosenbaum Rubenach Rudge Ruming Rutland Sandiford Schellart Scrimgeour Selley Seymour Shaubs Sliwa Sorjonen-Ward Spaggiari Stallard Taylor Tong Tyler VandenBerg Vandenberg Vassallo Vernon Wijns Willman Wilson Witham Zhang Zhang

SGTSG abstracts, February 2001

ST-noble@mail.earth.monash.edu.au richard.norris@stonebow.otago.ac.nz mnwhite@senet.com.au s. occhipinti@dme.wa.gov.au robin.offler@newcastle.edu.au a.ord@ned.dem.csiro.au mervyn.paterson@anu.edu.au peternellmark@hotmail.com spisarev@tsrc.uwa.edu.au cpowell@tsrc.uwa.edu.au preiss.wolfgang@saugov.sa.gov.au sreddy@lithos.curtin.edu.au areed@mrt.tas.gov.au prey@es.usyd.edu.au jamesar@mail.earth.monash.edu.au gideon@mail.earth.monash.edu.au michael.rubenach@jcu.edu.au trudge@mail.earth.monash.edu.au kruming@geology.newcastle.edu.au roye.rutland@anu.edu.au m.sandiford@earthsci.unimelb.edu.au wouter@mail.earth.monash.edu.au Ian.Scrimgeour@nt.gov.au D.Selley@utas.edu.au dseymour@mrt.tas.gov.au peters@ned.dem.csiro.au r.sliwa@cat.csiro.au peter.sorjonen-ward@dem.csiro.au catherine@mail.earth.monash.edu.au aaron@se-geomail.sci.shizuoka.ac.jp david.taylor@nre.vic.gov.au 1. tong@pgrad.unimelb.edu. au i.tyler@dme.wa.gov.au Fons.VandenBerg@nre.vic.gov.au Leon.Vandenberg@nt.gov.au j.vassallo@pgrad.unimelb.edu.au rhvernon@laurel.ocs.mq.edu.au chris@ned.dem. csiro.au clive.willman@nre.vic.gov.au cjlw@unimelb.edu.au b.witham@pgrad.unimelb.edu.au kunyi-g@publicl .ptt.js.cn zhang@ned.dem.csiro.au

205

Index of authors

Ailleres, L. 1, 93 Archibald, N.J. 1 Armstrong, R. 108 Ashwal,L.D 135 B Barnicoat, A. 199 Baxter, J.L. 3 Bendall.B. 72 Berry, R.F. 5, 89, 101, 118, 165 Betts,P. 7, 55, 150 Blewett, R.S. 9 Bodorkos, S. 11 Boiiden Geological Staff 101 Brown, R. 157 Buick, I.S. 13 Bull,S.W. 167

Carder, M. 85 Cawood.EA. 11, 15, 21, 44, 53, 87 Cayley, R.A. 124 Chen,S.F. 17 Cho,M. 19 Clarke, G.L. 31 Cobb, M.M. 21, 44 Coleborn, D. 83 Collins, A.S. 23,25, 91, 137 Collins, W.J. 27 Conor, C. 95 Cox,S.F. 28, 158 Coyner, S. 50 Crawford, A.J. 38 Crispe,A.J. 189 Cummings, A. 32 Cunneen, J. 29 D Daczko, N.R. 31 Dahl,K.L. 91, 137 Das.P.K. 32

Davies, B. 192 Dean,A.A. 189 Ding, P. 34, 36 Direen,N.G. 38 Dong,Y. 40, 67, 198 Drummond, B. 176 Duboz, C. 41, 155, 157

Everard, J. 57

Fanning, C M . 5 0 , 8 3 , 1 7 4 Fergusson, C.L. 42, 123 Fielding, C.R. 77 Fitzgerald, D. 157 Fitzsimons, I.C.W. 21, 23, 44, 91, 135, 137 Forbes, C. 81 Forster,M.A. 46, 112 Foster, D.A. 48, 50, 178 Fowler, A. 51 Fowler, M.B. 126 Frank, W. 169

Gallen,M.J. 53 Giles, D. 7, 55, 75, 81 Gleadow,A. 157, 160 Goleby,B. 176 Gordon, R. 85 Goscombe, B. 57, 59, 61, 63 Gow,P 176 Gray,D.R. 48, 65, 66, 178 Gregory, R.T. 65, 66 Guo,K. 40, 67, 198 H Haines, P.W. 68 Hall,M. 118 Hand,M. 13, 59, 61, 70, 72, 174 Hansen, D. 75

Harbort,T. 77 Harris, A.C. 79 Harris, E. 178 Hendrickx, M.A. 189 Hickey,K.H. 179 Hills, Q.G. 75, 81 Hoadley.E. 83 Hobbs,B.E. 84, 131, 193, 199 Holcombe, R.J. 11, 85, 114 Hollingsworth, D.A. 87 H o l m , 0 . 89 Huischer, B. 23, 91, 137 Huynh,T.H. 93 I lig, B.R. 114

James, E 95, 107, 147 Jessell,M.W. 163 Johnson, S.L. 97 Johnson, S.P 91, 99, 137 Jones, S.A. 101 Jonsson,M.K. 91, 137 K Keep, M. 103 KelleyS.P 68, 146 Kinny.RD. 21, 44 Klepeis, K.A. 31 Kohn,B. 157 Koons, PO. 129 Krabbendam, M. 75 Kruhl,J.H. 105

Lau, I. 107 Laubsch,N.C. 3 Lee,S.R. 19 Leitch,E.C. 15 Lemon, N. 32 Lennox, PG. 108, 185 Li,Z.X. 110

SGTSG abstracts, February 2001

206

Lister, G.S. 1, 41, 46, 55, 81, 112, 127,155,157 Little, T. A. 114 M Marshall, L J . 116 Mawby,J. 13, 70, 72 McNeill, A. 118 MefFre,S. 118 Miller, J.A. 13 Miller, J.McL 66, 121, 196 Mohajjel, M. 123 Monek, B. 87 Morancl,V.J. 124, 194 Moresi.L. 84, 131, 192 Muhlhaus, H-B. 84 Muller,A. 108 Murphy, J.B. 126 N Nance, R.D. 126 Nguyen, P.T. 28 Noble, M.P. 127 Norris,R.J. 129 Norvick, M. 157 O Occhipinti, SA. 144, 172 ODP Leg 190 Shipboard Scientific Party 42 Offler,R. 201 Oliver, N.H.S. 11,116 Ord,A. 84, 131, 192, 193, 199

Palmer, D. 185 Parker, K.D. 126 Passchier, C. 63 Passmore, A.R. 91, 132, 137 Peternell,M. 105 PhiUips.G. 196 Pisarevsky, S.A. 41, 133, 139 Potts, G J . 134, 146 Powell, C.McA. 41, 91, 132, 133, 135, 137, 139, 140 Pratt, C. 85 Preiss,W. 142 Q Quinn, C. 194

SGTSG abstracts, February 2001

R RaiA,J.G. 169 Rawling,TJ. 112 Raza,A. 50 Reddy,S.M. 53, 134, 144, 146 Reid,A. 147 Rey,P.F. 148 Richardson, J. 150 Rickard,M.J. 152, 154 Rosenbaum, G. 155 Rosendam, G. 81 Rubenach, M. 83 Rudge,T. 157 Ruming, K. 28, 158 RusseU-Head, D. 159

W Walshe,J.L. 199 Wartho,J. 68 Wijns,C. 192 Willetts, G. 193 Williams, LS. 13 Willman,C.E. 194 Wilson, C.J.L. 121,159, 174, 183, 191,196 Wilson, T 201 WindleyB. 23 Wingate, M.T.D. 135 Witham,B.A. 121,196 Wohlt, K.E. 124 Y Yi,K. 19

Sahandi, M.R. 123 Sandiford,M. 11, 160 Sauter, PC. 3 Schaubs, P 162 Schellart, W.P 163 Scott, R J . 165, 167 Scrimgeour, L 169 SeUey,D. 167, 170 Sheppard, S. 172 Siebel,W. 108 Simons, B.A. 194 Sims,J.P 174 Sisois, 1. 83 Slater, K.R. 189 Sorjonen-Ward, P 176 Spaggiari, C.V. 48, 178 Stallard, A.R. 179 Steele, D. 89 Stone, W.E. 28 Strachan, R.A. 126 T Taylor, D.H. 124, 181 Tong, L. 183 Trzebski, R. 108 185 Tucker, R.D. If Turner, S.P 68 Tyler, I.M. 172, 187 V VandenBerg, A.H.M. 194 Vandenberg, L.C. 189 Vasconcelos, P 77 Vassallo, J J . 183, 191

Zhang, C. 40, 67, 198 Zhang, Y 176, 199 Zwingmann, H. 201