Abstracts No.53: SGTSG Field Conference Halls Gap, 1999 by GSAustralia

The Specialist Group In Tectonics and Structural Geology Field Conference Halls Gap, Victoria, Australia February 14-19, 1999 Abstract Volume

Geological Society of Australia Abstracts Series #53 ISBN 0729-011X

The Specialist Group In Tectonics and Structural Geology Field Conference Halls Gap, Victoria, Australia February 14-19, 1999 Abstract Volume

Editor: M. Jessell Geological Society of Australia Abstracts Series #53 ISBN 0729-011X

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A G C R C

PASMINCO

Halls Gap SGTSG Conference Abstract Volume

LIST OF ABSTRACTS

OVERVIEW AND RECENT IDEAS OF THE STRUCTURAL EVOLUTION OF THE EASTERN SUCCESSIONS OF THE MOUNT ISA TERRANE DURING THE ISAN OROGENY, QUEENSLAND, AUSTRALIA. LAURENT AILLERES, DYLAN JEEFRIESS, DAVID GILES, PETER BETTS AND MEGAN HOUGH 1 CRUSTAL STRUCTURE OF NORTHERN TASMANIA BASED UPON A DEEP SEISMIC TRANSECT TIM J. BARTON 3 CORRELATION OF DEFORMATION FOR LARGE DISTANCES ALONG OROGENS AND K.A. HICKEY

T.H. BELL 5

INVERSION AND THE STYLE OF FAULTING IN WESTERN TASMANIA R.F. BERRY

EVIDENCE FOR A MID BASIN INVERSION IN THE PALAEO-PROTEROZOIC NORTHERN MOUNT ISA TERRANE PETER BETTS 8 GEOPHYSICAL MODELLING OF THE INVERTED FIERY CREEK FAULT SYSTEM, MOUNT ISA INLIER, AUSTRALIA. PETER BETTS & LAURENT AILLERES 11 SHEAR FOLD FROM ZAWAR, ARAVALLI MOUNTAINS, NW INDIA: COULD TRANSPRESSION BE CAUSE FOR DEVELOPMENT? T.K.BISWAL AND M. AN AND 13 STRUCTURAL CONTROLS OF ARCHAEAN GOLD IN THE NORTH PILBARA CRATON, WESTERN AUSTRALIA RICHARD S. BLEWETT 15 A CRUSTAL SECTION THROUGH THE MAWSON ESCARPMENT: PRELIMINARY STRUCTURAL RESULTS FROM THE SOUTHERN PRINCE CHARLES MOUNTAINS, EAST ANTARCTICA S. D. BOGER AND C. J. L. WILSON 17 NEOPROTEROZIOC DEFORMATION IN THE NORTHERN PRINCE CHARLES MOUNTAINS, EAST ANTARCTICA: EVIDENCE FOR A SINGLE PROTRACTED OROGENIC EVENT S. D. BOGER , C. CARSON, C. J. L. WILSON & C. M. FANNING 19 OZBLOCK, A CONTINENTAL-SCALE TECTONIC MODEL FOR THE AUSTRALIAN LITHOSPHERE IN THE MID- TO LATE PALEOZOIC JEAN BRAUN, RUSSELL SHAW, MIKE SANDIFORD, DAVID GRAY AND DAVID FOSTER 21 THE SYNERGETIC RELATION BETWEEN DEFORMATIONAL AND THERMAL PROCESSES IN CONVERGENT OROGENS, AND ITS ROLE IN GRANITE ASCENT MICHAEL BROWN AND GARY S. SOLAR 22 HIMALAYAN TECTONICS: FROM PLATE TECTONICS TO MECHANICAL INSTABILITIES JEAN-PIERRE BURG 24

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PROTEROZOIC ROCKS IN CENTRAL VICTORIA AND THEIR TECTONIC IMPLICATIONS ROSS A. CAYLEY, DAVID H. TAYLOR, DAVID H. MOORE, & ALFONS H.M.VANDENBERG 27 HISTORY OF MOVEMENT OF PORPHYROBLASTS AIGEN CHEN

THE

QINLING-DABIE

OROGEN

PRESERVED

IN 30

STRAIN PARTITIONING IN THE INTRACRATONIC PETERMANN OROGENY, CENTRAL AUSTRALIA DOROTHY CLOSE, IAN SCRIMGEOUR , MARTIN HAND, THOMAS FLOTTMANN AND CHRISTINE EDGOOSE 32 HEAT SOURCES AND TECTONIC MODELS FOR THE EAST BASALT STORY WJ. COLLINS

LACHLAN FOLD BELT: THE 33

3D MODELING OF THE NORSEMAN GOLD FIELD: INCORPORATING 100 YEARS OF HISTORY INTO CURRENT EXPLORATION MODELS KAREN A. CONNORS 35 PERCOLATION THEORY APPROACHES TO FLUID FLOW IN FRACTURE-CONTROLLED HYDROTHERMAL SYSTEMS S F COX, M A KNACKSTEDT AND J BRAUN 37 TECTONIC HISTORY OF NORTHERN PAPUA NEW GUINEA 40AR/39AR AND FISSION TRACK THERMOCHONOLOGY PETER CROWHURST, KEVIN HILL AND DAVID FOSTER 39 KINEMATIC VORTICITY AND TECTONIC SIGNIFICANCE OF THE ANITA SHEAR ZONE, FIORDLAND, NEW ZEALAND NATHAN R. DACZKO, KEITH A. KLEPEIS, GEOFFREY L. CLARKE AND RICHARD W. WHITE 41 THE CAUSES OF POLYPHASE DEFORMATION IN PLATE BOUNDARY ZONES JOHN F. DEWEY 44 PALAEOPROTEROZOIC OROGENIES IN NORTHERN AUSTRALIA AND THEIR TECTONIC IMPLICATION PUQUAN DING 46 RETHINKING THE PALAEOPROTEROZOIC GEOLOGICAL HISTORY OF BROKEN HILL, NSW, AUSTRALIA: SPECULATION TOWARDS A SYNTHESIS OF STRUCTURE, GEOCHRONOLOGY AND LITHOLOGICAL MAPPING. TONY DONAGHY, GEORGE GIBSON, ALLEN NUTMAN, MIKE HALL AND DAVID MAIDMENT 48 SHEAR ZONES FROM BROKEN HILL, AUSTRALIA: DEFORMATION, CONDITIONS FORMATION AND THE ROLE OF FLUIDS CHRIS DOYLE & IAN CARTWRIGHT

OF 50

PLATYPLUS PROJECT : A 3D RECONSTRUCTION TOOL CECILE DUBOZ, G.S. LISTER AND MARK JESSELL 53 CONCEPTS OF CLEAVAGE DEVELOPMENT D.W. DURNEY

STRUCTURAL GEOLOGY AND TECTONIC EVOLUTION OF THE NORTHERN PART OF THE EASTERN GOLDFIELDS PROVINCE, YILGARN CRATON T.R. FARRELL 57

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MICROSTRUCTURAL ANALYSIS USING ELECTRON BACKSCATTER DIFFRACTION TECHNIQUE JOHN D. HTZ GERALD, ULRICH H. PAUL, SHUQING ZHANG, AND MARTIN CMIRAL 60 FLUID ESCAPE FROM MIGMATITES—THE IMPORTANCE OF RETROGRADE DEFORMATION I.C.W. HTZSIMGNS 62 INFLUENCES OF BASIN/BASEMENT STRUCTURES ON THE DEVELOPMENT OF FOLDTHRUST BELTS THOMAS FLOTTMANN AND PAT JAMES 65 CRUSTAL WEDGES - A COMMON STYLE OF BASEMENT/COVER DEFORMATION AT THE MARGINS OF THE AMADEUS BASIN, CENTRAL AUSTRALIA THOMAS FLOTTMANN, MARTIN HAND, DOROTHY CLOSE, CHRISTINE EDGOOSE AND IAN SCRIMGEOUR 66 SHORTENING AND EXTENSIONAL STRUCTURES IN A COLLISIONAL SETTING, OTAGO SCHIST, NEW ZEALAND MARNIE FORSTER 67 PARTIAL MELTING, OROGENY, AND COLLAPSE OF THE CORDILLERAN OROGEN: RESULTS FROM THE NORTHERN IDAHO (BITTERROOT) BATHOLITH REGION, U.S.A DAVID A. FOSTER AND MARK FANNING 69 DEFORMATION RATES IN OROGENESIS: SOME GEOCHRONOLOGICAL EXAMPLES DAVID A. FOSTER AND DAVID R. GRAY 70 STRUCTURAL CONTROL OF THE BASALT VICTORIA M J GANE AND C J L WILSON

CONTACT

MINERALISATION,

STAWELL, 73

MODES OF EXTENSION, LOWER-CRUSTAL FLOW AND THE EVOLUTION OF THE NORTH WEST SHELF ANTHONY GARTRELL 76 EVIDENCE FOR AN EARLY "ISAN" TECTONO-THERMAL EVENT FROM THE SOUTHEASTERN MARGIN OF THE MOUNT ISA INLIER DAVID GILES, LAURENT AILLERES, PETER BETTS AND DYLAN JEFFRIESS 79 THE LACHLAN TRANSVERSE ZONE AND CONSTRAINTS ON THE DEVELOPMENT OF THE LACHLAN OROGEN R. A. GLEN, R. J. KORSCH D. M. FINLAYSON, D. W. JOHNSTONE AND J. L. WALSHE 82 BOUDINAGE CLASSIFICATION AND EVALUATION AS KINEMATIC INDICATORS GOSCOMBE AND CEES W. PASSCHIER

BEN 84

NUMERICAL MODELLING OF DEFORMATION AND FLUID-FLOW IN SHALLOW PLUTONIC COMPRESSIONAL ENVIRONMENTS PAUL GOW AND ALISON ORD 86 OMAN ANALOG FOR OTAGO SCHIST BELT, NEW ZEALAND: FOLD NAPPES, HIGH P METAMORPHISM, EXHUMATION AND CONVERGENT MARGIN SETTINGS DAVID R. GRAY, ROBERT T. GREGORY, AND JOHN MC. MILLER 88 ACCRETIONARY WEDGE-TYPE THRUST SYSTEMS DAVID R. GRAY AND DAVID A. FOSTER 91

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REGIONAL TIMING CONSTRAINTS ON THE ALICE SPRINGS OROGENY FROM SYNOROGENIC SEDIMENTATION AND COMPARISON WITH ISOTOPIC STUDIES PETER W. HAINES, MARTIN HAND AND MIKE SANDIFORD 93 INTRAPLATE DEFORMATION IN CENTRAL AUSTRALIA, THE LINK BETWEEN SUBSIDENCE AND FAULT REACTIVATION MARTIN HAND AND MIKE SANDIFORD 95 SUBMERGED AND DETACHED PORTIONS OF THE NEW FOLDBELTS EAST OF THE TASMAN SEA HJ. HARRINGTON TECTONICS OF C. J.L.WILSON

THE

NORTH-EASTERN

TIBETAN

ENGLAND

PLATEAU

AND

LACHLAN 96

M.HARROWHELD

AND 99

THE I-S LINE: A THRUST SHEET BOUNDARY IN EASTERN VICTORIA. MARC HENDRICKX .101 STRUCTURAL OF THE PAPUA NEW GUINEA AND IRIAN JAYA FOLD BELTS KEVIN C. HILL, R.D. KENDRICK, E. SUTRIYONO AND J. KEETLEY 103 FLUID TRANSPORT IN DEFORMING OVERPRESSURED ROCK MASSES B.E.HOBBS AND A. ORD 106 STRAIN CALCULATOR, GHOSHFLOW: PROGRAMS TO CALCULATE AND MODEL STRAIN, SHEAR, AND VORTICITY PARAMETERS R. J. HOLCOMBE 108 STRUCTURAL ANALYSIS OF THE SOLDIERS CAP TERRANE, SANDY CREEK AREA, MOUNT ISA INLIER, AUSTRALIA MEGAN HOUGH, BREGJE HULSCHER, LAURENT AILLERES, GORDON LISTER AND DAVID GILES 110 REASSESSMENT OF THE EVOLUTION OF THE MOUNT PAINTER INLIER, SOUTH AUSTRALIA CHRIS JANKA, PAUL BONS, MARLINA ELBURG, & JON DOUGHERTY-PAGE 112 THREE DIMENSIONAL ANALYSIS OF SUPERPOSED FOLD SEQUENCES, SNAKE CREEK ANTICLINE, EASTERN FOLD BELT, MOUNT ISA INLIER. DYLAN JEFFRIESS, LAURENT AILLERES, DAVID GILES AND MEGAN HOUGH 114 CONVERSATIONS BETWEEN ELLE, FLYNN, BASIL, SYBIL & MANUEL: THE FUTURE OF MICROSTRUCTURE SIMULATIONS? MARK JESSELL, LYNN EVANS, PAUL BONS, TERENCE BARR & KURT STUWE 117 DEFORMATION AND MAGMATISM ACROSS A TRANSITION FROM UPPER TO MIDDLE CRUST IN THE MESOZOIC PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO JOHNSON, S.E., TATE, M.C. & FANNING, C.M 119 INTEGRATED GEOLOGICAL, GEOCHEMICAL AND GEOCHRONOLOGICAL STUDIES ACROSS A CRUSTAL-SCALE "SUTURE" IN THE PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO JOHNSON, S.E., TATE, M.C., SCHMIDT, K.L., PATERSON, S.R., VERNON, R.H. & FANNING, C.M 121 THE MECHANICAL BEHAVIOUR OF QUARTZ GOUGE AT HYDROTHERMAL CONDITIONS K KANAGAWA, S F COX, S ZHANG 124

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DUCTILE THRUSTING NEAR THE RAYNER COMPLEX-NAPIER COMPLEX BOUNDARY, OYGARDEN ISLANDS, EAST ANTARCTICA NIGEL M. KELLY, GEOFFREY L. CLARKE, CHRISTOPHER J. CARSON, AND RICHARD W. WHITE 126 A SYNTHESIS OF DEFORMATION AND PARTIAL MELTING IN THE GLENELG RIVER COMPLEX: IMPLICATIONS FOR THE DELAMERIAN OROGENY TONY I.S.KEMP & C M. GRAY 128 SYNCHRONOUS TRANSPRESSION AND MAGMATISM IN THE HALLS CREEK MOBILE BELT (EAST KIMBERLEY, WA) TIM KENDRICK, PATRICE REY, SYLVIE COSTA, GARY MASUR, AND BEN FOLEY 131 THE CONTROLS ON DEFORMATION PATTERNS AND PLUTON EMPLACEMENT WITHIN AN OBLIQUELY CONVERGENT OROGEN KEITH A. KLEPEIS 134 A COMPARATIVE STUDY OF HELIUM DIFFUSION AND FISSION TRACK ANNEALING IN APATITES: BOREHOLE DATA FROM THE OTWAY BASIN, AUSTRALIA B. P. KOHN, M. A. HOUSE, K. A. FARLEY & A. RAZA 137 STRUCTURAL INTERPRETATION OF A DEEP SEISMIC REFLECTION TRANSECT IN THE VICINITY OF THE GRAMPIANS, VICTORIA RUSSELL J. KORSCH, TIM J. BARTON, DAVID R. GRAY, ANDREW J. OWEN AND DAVID FOSTER 139 THE BREAK-UP OF SUPERCONTINENTS AND OLDER OROGENS: WHY DID SOME OROGENS NOT RIFT? MAARTEN KRABBENDAM AND TERENCE D. BARR 142 A NEW STRUCTURAL MODEL AND CONTROLS ON QUARTZ VEIN HOSTED GOLD MINERALISATION: AN EXAMPLE FROM THE POVERTY LINE OF REEF, TARNAGULLA, VICTORIA J. KROKOWSKI DE VICKEROD 144 ASCENT OF MELT ALONG LOW- AND HIGH-ANGLE FAULTS DURING COLLISION JORN H. KRUHL, STEFFEN BUTTNER & BERND SPANNER

CONTINENT 146

MULTIPLE INVERSIONS IN INCLUSION TRAIL ASYMMETRY ON PORPHYROBLAST RIMS RESULTING FROM HETEROGENEOUS STRAIN HYUN WOO LEE 147 THE EMPLACEMENT AND DEFORMATION OF EASTERN LACHLAN FOLD BELT GRANITES ONE KEYTO TECTONIC MODELS LENNOX, P.G., TRZEBSKI, R. & PALMER, D 149 MICROSTRUCTURAL EVOLUTION AND FLOW OF ROCKS IN A ZONE OF ACTIVE TRANSPRESSIONAL DEFORMATION NEAR THE ALPINE FAULT, NEW ZEALAND T. A. LITTLE &R. J. HOLCOMBE 151 TOWARDS A NEW PROTEROZOIC TECTONIC SYNTHESIS - AN ALTERNATIVE VIEW OF THE PROTEROZOIC GEOLOGY OF AUSTRALIA SANDRAM CLAREN AND MIKE SANDIFORD 153 FINITE-ELEMENT MODELLING OF MATERIALS NEIL S. MANCKTELOW

SINGLE-LAYER

FOLDING

ELASTO-VISCOUS 154

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THE DEVELOPMENT OF BOUDINS IN A VISCO-PLASTIC SOLID AT HIGH METAMORPHIC GRADE. BRETT A. MARMO AND CHISTOPHER J.L. WILSON 157 GHOSTS OF MICROSTRUCTURAL GEOLOGY FROM 1850 TO 2050 W.D. MEANS

159

EXHUMATION OF HIGH-PRESSURE ROCKS AND THE TECTONIC EVOLUTION OF THE OMAN MARGIN J. MCL. MILLER, D. R. GRAY, R.T. GREGORY AND D.A> FOSTER 160 FLUID CHANNELLING AROUND HIGH-PRESSURE PILLOW LAVAS AND ITS IMPLICATIONS FOR LARGER-SCALE FLUID PATHWAYS IN SUBDUCTION ZONES JODIE A. MILLER AND IAN CARTWRIGHT 162 PLUTON EMPLACEMENT CONTROLLED BY STRIKE-SLIP FAULTS, EASTERN VICTORIA MORAND VJ., WILLMAN C.E., HENDRICKX M.A., HAYDON S.J. & VANDENBERG A.H.M 165 FROM CRUSTAL THICKENING TO EXHUMATION: THE STRUCTURAL DEVELOPMENT IN A COLLISIONAL OROGEN (WESTERN ERZGEBIRGE, EUROPEAN VARISCIDES) MATTHIAS NEGA, JORN H. KRUHL, OTTOMAR KRENTZ, DIETMAR LEONHARD 168 THE MYLONITE ZONE ON THE ALPINE FAULT: STRAIN ESTIMATES, DISPLACEMENTS AND IMPLICATIONS FOR THE DEEP STRUCTURE OF MAJOR FAULTS R. J. NORRIS AND A. F. COOPER 170 A THREE-PHASE EARLY CRETACEOUS RIFT HISTORY OF THE SOUTH ATLANTIC SALT BASINS AND ITS INFLUENCE ON LACUSTRINE SOURCE FACIES DISTRIBUTION. NORVICK, M. S. AND SCHALLER, H 172 THE PALAEOPROTEROZOIC TECTONIC EVOLUTION OF THE SOUTHERN MARGIN OF THE CAPRICORN OROGEN, WESTERN AUSTRALIA SANDRA OCCHIPINTI, STEVE SHEPPARD AND IAN TYLER 173 TIMING OF DEFORMATION, METAMORPHISM AND FAULT MOVEMENTS WITHIN THE ADELAIDE FOLD BELT: IMPLICATIONS FOR TASMANIDE OROGENESIS ROBIN OFFLER, DAVID R. GRAY, DAVID A. FOSTER AND THOMAS FLOTTMANN 175 ISOSTATIC CONSTRAINTS ON THE CENTRAL VICTORIAN LOWER CRUST; IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE LACHLAN FOLD BELT G. J. O'HALLORAN AND P. REY 177 SYNTECTONIC EE, SI AND O TRANSPORT DURING DEFORMATION OF HAMERSLEY PROVINCE BIFS: IMPLICATIONS FOR HEMATITE ORE GENESIS NICHOLAS H. S. OLIVER, LACHLAN K. STEWART & CHRISTOPHER MCA. POWELL 179 APPLICATION OF NUMERICAL CONTINUUM MODELLING TO FLUID FLOW IN SHEAR ZONES, VEINS AND ORES, WITH SOME AUSTRALIAN EXAMPLES OLIVER, N. H. S., ORD, A., AND HOBBS,B.E 181 NUMERICAL MODELLING OF THE GEOLOGICAL DEVELOPMENT OF THE WEST LACHLAN OROGEN A. ORD, B.E. HOBBS, D.R. GRAY AND D.A. FOSTER 183

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ZIRCON FISSION TRACK EVIDENCE FOR PRE-NEOGENE TECTONICS IN IRIAN JAVA PAUL B. O'SULLIVAN, KEVIN C. HILL, RICHARD D. KENDRICK 185 REGIONAL SEISMIC REFLECTION PROFILING, KALGOORLIE GOLDFIELD, WESTERN AUSTRALIA. A.J. OWEN, R. BAXEMAN, B.R. GOLEBY, AND B.J. DRUMMOND 188 READING SMALL-SCALE STRUCTURES IN ROCKS PASSCHIER, CEES

191

BASEMENT-INVOLVED FOLD THRUST BELT TERMINATIONS IN THE FLINDERS RANGES, SOUTH AUSTRALIA: REACTIVATED SYNDEPOSITIONAL FAULTS AND THERMAL PERTURBATIONS EIKE PAUL, THOMAS FLOTTMANN, MIKE SANDIFORD 192 GEODYNAMICAL EVOLUTION OF THE LACHLAN FOLD BELT: A NUMERICAL MODEL CRISTINA PAUSELLI, JEAN BRAUN, DAVID GRAY AND DAVID FOSTER 194 ORIGIN OF LAYER-PARALLEL GRANITIC SHEETS AS MAGMA-DOMINATED SHEAR ZONES MARK PAWLEY, BILL COLLINS & MARTIN VAN KRANENDONK 195 EXPERIMENTAL MODELLING OF STRETCHING LINEATIONS WITHIN SIMPLE SHEAR ZONES S. PIAZOLO AND C. W. PASSCHIER 198 RELATIVE DEFORMATION AGES AND THE SYSTEMATIC ASSESSMENT OF DEFORMATION HISTORIES GRAHAM POTTS & STEVE REDDY 199 SYNOROGENIC HYDROTHERMAL ORIGIN FOR GIANT HAMERSLEY IRON OXIDE OREBODIES CHRISTOPHER MCA. POWELL, ZHENGXIANG LI, DAVID MCB. MARTIN AND NICHOLAS H.S. OLIVER 200 STRUCTURAL AND TECTONIC EVOLUTION OF THE PALEOPROTEROZOIC OPHTHALMIA FOLD-AND-THRUST BELT CHRISTOPHER MCA. POWELL 202 TECTONIC EVOLUTION OF THE THRUST FRONT OF THE DELAMERIAN OROGEN NEAR CLARE, SOUTH AUSTRALIA W.V. PREISS 204 THE HIGH-PRESSURE SOLE OF THE NEW CALEDONIA OPHIOLITE BELT TIM RAWLING AND GORDON LISTER 206 CRUSTAL MODELS OF NORTHERN TASMANIA FROM WIDE-ANGLE SEISMIC DATA N. RAWLINSON, T.O. SEMENOVA, G.A. HOUSEMAN, C.D.N. COLLINS & B.J. DRUMMOND 208 FLUID-ROCK INTERACTION WITHIN SHEAR ZONES OF THE ARUNTA INLIER, CENTRAL AUSTRALIA: IMPLICATIONS FOR TECTONICS CAROLINE M. READ AND IAN CARTWRIGHT 211 THE ABSOLUTE DATING OF DEFORMATION: IMPLICATIONS FROM THE RELATIONSHIPS BETWEEN DEFORMATION MECHANISMS AND ISOTOPE SYSTEMATICS STEVE REDDY & GRAHAM POTTS 214 THE EFFECTS OF DEFORMATION-INDUCED MICROSTRUCTURES ON INTRAGRAIN ARGON ISOTOPE AGES STEVE REDDY, GRAHAM POTTS, SIMON KELLEY & NICK ARNAUD 216

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MIDDLE PALAEOZOIC DEFORMATION IN NORTHERN TASMANIA WITH IMPLICATIONS FOR THE EVOLUTION OF THE WESTERN LACHLAN FOLD BELT OF MAINLAND AUSTRALIA. ALISTAIR REED 218 ARCHAEAN REGIONAL STRAIN FIELD IN THE YILGARN CRATON (WA): FOLD SUPERIMPOSITION OR INCREMENTAL STRAIN FIELD INTERFERENCES P REY, S COSTA, O. VANDERHEAGUE, AND B FOLEY 221 BUOYANCY FORCES, ARGAND RATIO, AND THE MECHANICAL BEHAVIOR OF THE CONTINENTAL LITHOSPHERE THROUGH TIME PATRICE REY, GREG HOUSEMAN AND SYLVIE COSTA 223 MECHANISM AND TIMING OF GRANITE EMPLACEMENT FOR THE BEGA BATHOLITH, EASTERN LACHLAN FOLD BELT, N.S.W. S.W. RICHARDS AND WJ. COLLINS 226 STRIKE-SLIP FAULTING: A POSSIBLE EXPLANATION FOR THE METAMORPHIC AND CRUSTAL THICKNESS GRADIENTS WITHIN THE YILGARN CRATON W.A. JAMES RICHARDSON AND PATRICE REY 228 MICROSTRUCTURES, METAMORPHISM, AND TECTONIC SIGNIFICANCE OF METABASIC ROCKS, OCEAN-CONTINENT TRANSITION, IBERIA ABYSSAL PLAIN M. RUBENACH 231 PROGRESSIVE DEFORMATION AND CYCLIC FLUID-ROCK INTERACTION IN THE 33 SHEAR ZONE, VICTORY COMPLEX, ST IVES GOLDFIELD, W.A. K J RUMING AND S F COX 233 BASIN REACTIVATION ON A HOT CONTINENT MIKE SANDIFORD

235

STRUCTURAL ASPECTS OF GOLD MINERALISATION AT BENDIGO, VICTORIA: FIELD OBSERVATIONS AND NUMERICAL MODELLING PETER M. SCHAUBS 237 STRUCTURE AND MINERALISATION AT CONDOBOLIN, LACHLAN FOLD BELT, NEW SOUTH WALES MARTIN M. SCOTT 239 THE GRAMPIANS AND DERRIWONG GROUPS, A CONTRAST IN DEFORMATION STYLES LAWRENCE SHERWIN 241 SMALL SCALE LOCALISATION OF DEFORMATION HADI. M. SIM, PAUL. D. BONS & MARK. W. JESSELL 244 TOWARDS AN UNDERSTANDING OF GIANT GOLD SYSTEMS SORJONEN WARD, P., WALSHE J.L., HOBBS B.E., HALL, G., AND ORD, A 246 OCCURRENCES AND SIGNIFICANCE OF FRANCISCAN-LIKE MELANGE AND BLUESCHIST METAMORPHISM IN LACHLAN OROGEN FAULT ZONES CATHERINE V. SPAGGIARI, DAVID R. GRAY & DAVID A. FOSTER 248 INTEGRATING MICROSTRUCTURES AND METAMORPHISM RECORDED WITHIN GARNET PORPHYROBLASTS FROM THE FLEUR DE LYS SUPERGROUP, NEWFOUNDLAND, CANADA: IDENTIFYING INTRA- AND INTER-DEFORMATION HIATUSES IN GARNET GROWTH AARON STALLARD 250

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THE MR. CURLY GRANITE: AN INTERNALLY LAYERED AND LENTICULAR, MULTIPLY FOLDED LEUCOGRANITIC COMPLEX NICK STEBBING, PAUL BONS, MARLINA ELBURG & CATHERINE PARNELL 252 SUBECLOGITIC ROCKS AND THEIR IMPLICATIONS FOR CRUSTAL STRUCTURE IN THE WESTERN MUSGRAVE BLOCK, CENTRAL AUSTRALIA ALASTAIR STEWART 254 RECOGNITION, STRUCTURAL SIGNIFICANCE, AND PROSPECTIVITY OF EARLY {¥,) FOLDS IN THE MINERIE 1:100 000 SHEET AREA, EASTERN GOLDFIELDS, WESTERN AUSTRALIA ALASTAIR STEWART 257 BRITTLE-VISCOUS DEFORMATION OF BIPHENYL: INFERENCES ON MID-CRUSTAL SHEAR ZONE EVOLUTION JURGEN E. STREIT AND STEPHEN F. COX 259 THE GRAMPIANS ALLOCHTHON: AN OUTLIER OF THE WESTERN LACHLAN FOLD BELT DAVID H. TAYLOR & ROSS A. CAYLEY 261 STRUCTURE,STRATIGRAPHY AND PETROLOGY OF THE WESTERN WEEKEROO INLIER, OLARY PROVINCE GRANT TAYLOR AND ROBIN OLIVER 263 GRANITE EMPLACEMENT VS. FAULTING - CONTROL OR INTERPLAY? ROBERT TRZEBSKI & PAUL LENNOX 264 A STRUCTURAL REINTERPRETATION OF THE SOUTHERN EYRE PENINSULA, AUSTRALIA J J VASSALLO AND C J L WILSON

SOUTH 265

FOLD NAPPES AND THE PROBLEM OF MODEL DRIVEN INTERPRETATION PAUL F. WILLIAMS 267 DECONSTRUCTING THE BINDIAN DEFORMATION IN EASTERN VICTORIA WILLMAN C.E., MORAND VJ., HAYDON SJ., HENDRICKX M.A. & VANDENBERG A.H.M 269 FAULTING PROCESSES AT UPPER CRUSTAL LEVELS; AN EXAMPLE FROM THE NORTHERN SYDNEY BASIN, NSW T. WILSON, R. OFFLER AND S.F. COX 272 HIGH PRECISION RELATIVE THERMOBAROMETRY BRENTON WORLEY, ROGER POWELL & MIKE S ANDIFORD 274 THE ROLE OF DISSOLUTION-PRECIPITATION IN THE COMPACTION OF GRANITIC GOUGE MATERIALS SHUQING ZHANG AND STEPHEN F. COX 276 A CLARIFICATION OF AN ISSUE REGARDING THE INFLUENCE OF INITIAL IRREGULARITIES UPON FOLD SHAPE Y. ZHANG, B. E. HOBBS, A. ORD AND H. B. MUHLHAUS 278 3D COUPLED MECHANICAL/FLUID FLOW MODELLING OF THE YILGARN BLOCK, WESTERN AUSTRALIA Y. ZHANG A. ORD, B.E. HOBBS, J.L. WALSHE AND N. ARCHIBALD 280

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TECTONOTHERMAL EVOLUTION OF ARCHAEAN BASEMENT ROCKS FROM THE EASTERN ZONE OF THE NORTH CHINA CRATON: IMPLICATIONS FOR MANTLE PLUME TECTONICS GUGCHUN ZHAG, S. A. WILDE P. A. CAWGGD 282

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OVERVIEW AND RECENT IDEAS OF THE STRUCTURAL EVOLUTION OF THE EASTERN SUCCESSIONS OF THE MOUNT ISA TERRANE DURING THE ISAN OROGENY, QUEENSLAND, AUSTRALIA. Laurent Ailleres, Dylan Jeffriess, David Giles, Peter Betts and Megan Hough Australian Geodynamics CRC, Australian Crustal Research Centre, Monash University, laurent @ mail .earth .monash .edu. au The Mount Isa Tectonic Synthesis is an industry-funded Australian Geodynamics CRC project. Its aims are to provide (1) an understanding of the tectonic evolution of the Mount Isa inlier and (2) an understanding of the mineralisation within this tectonic framework. The project mainly involves geochronology (SHRIMP U/Pb; Page, 1998), interpretation of the reflection and refraction seismic data across the inlier (MacCready, 1998; MacCready et al., in press) and structural mapping (Betts et al., 1997a & b; O'Dea et al., 1997; Giles et al., this volume; Hough et al., this volume, Jeffriess et al., this volume). This paper presents the results of the project focusing on the tectonic history of the Eastern Successions of the Mount Isa inlier between -1590 and -1500 Ma during the Isan orogeny (Blake, 1987; Blake & Stewart, 1992). Geochronology Recent U/Pb SHRIMP dating on zircons (Page, 1988; Page et al., 1997; Page & Sun, 1998) refined the classical Uthostragraphic subdivisions of the Eastern Successions of Blake (1987) and Blake and Stewart (1992). Formations, previously attributed to Cover Sequence 2 have been dated to equivalent of the Mount Isa Group of the Western Fold Belt and consequently reclassified into the younger Cover Sequence 3. These include the Tommy Creek Block, Marimo-Staveley Block, Kuridala Formation and Soldiers Cap Group which yielded ages ranging from -1710 Ma to -1610 Ma. Sediments of this age host most of the major ore deposits in the inlier. Other formations, including the Mitakoodi quartzite and the Doherty Formation (the probable equivalent of the Corella Formation) have been assigned to Cover Sequence 2. Seismic Interpretation The recent acquisition of reflection and refraction seismic data and their interpretation has shown that the sub-surface geometry of the terrane is dominated by flat lying structures (MacCready et al., 1997; 1998). It is interpreted that the tectonic evolution of the Eastern Successions involved early thin-skinned tectonics evolving, not necessarily progressively, to thick-skinned tectonics with time (MacCready, 1997; 1998). This interpretation is supported by structural mapping in the Mitakoodi Culmination (O'Dea et al., 1997). Structural evolution of the Mitakoodi Culmination - Marimo Slates The Mitakoodi Culmination exposes greenschist facies rocks, conformably overlying the Argylla formation (U/Pb dated at 1783 Ma by Page [1988]). Recent mapping in the Mitakoodi Culmination area (O'Dea et al. 1997; Betts et al., 1997b) and Marimo-Staveley Block (Betts et al., 1997a) shows that their structural evolution involved processes similar to those encountered in modem tectonic environments. The area was subject to fold and thrust type tectonics evolving from thin-skinned to late thick-skinned tectonics. The associated

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deformation consists of the superposition of late upright, N-S trending folds on early recumbent folds associated with low-angle, west-directed thrusts. It is also suggested that the structure of the Marimo-Staveley block corresponds to the hanging wall of a duplex structure consequently refolded during the thick-skinned phase of deformation.

Conclusion - Potential Extension of this Tectonic Framework

Structural mapping in the Snake Creek area (Jeffriess et al., this volume) and the Sandy Creek area (Giles et al., this volume; Hough et al., this volume) suggests that this tectonic framework may also be valid although there minor variations. This is due, in part, to the progression of deformation. Consequently, correlating structures and deformations over these areas is dangerous and subject to controversy. We propose that the tectonic setting of the Eastern Successions during the Isan orogeny may correspond to an accretionary wedge on the edge of a west-dipping subduction zone. References BeUs, P.G., Ailleres, L. & Hough, M., 1997a. Structural elements in the hanging wall of the Overhang Shear: Evidence from the Marimo-Staveley Block and the Kuridala region. In Ailleres, L. & Betts, P.G., eds. Structural Elements of the Eastern Successions - A Field Guide Illustrating the Structural Geology of the Eastern Mount Isa Terrane, Austraha. Australian Crustal Research Centre, Technical Publication 63, 37-60. BeUs, P.G., Ailleres, L., O'Dea, M. & MacCready, T., 1997b. Poly-phase related deformation of the Mitakoodi Culmination: Evidence along the eastern Hmb of the Duck Creek Anticline. In Ailleres, L. & Betts, P.G., eds. Structural Elements of the Eastern Successions - A Field Guide Illustrating the Structural Geology of the Eastern Mount Isa Terrane, Australia. Austrahan Crustal Research Centre, Technical Publication 63, 17-36. Blake, D.H., 1987. Geology of the Mount Isa inher and environs, Queenslandand Northern Territory. BMR 225. Blake, D.H. & Stewart, A.J., 1992. Stratigraphic and tectonic framework. Mount Isa inlier. In Stewart, A.J. & Blake, D.H., eds. DEtailed studies of the Mount Isa inher. Austrahan Geological Survey Organisation 243, 111.

Giles, D., Ailleres, L., Betts, P. & Jeffriess, D., this volume. Evidence for an early "Isan" tectono-thermal event from the south-eastern margin of the Mount Isa inher. Hough, M., Hulscher, B., Lister, G., Ailleres, L. & Giles, D., this volume. Structural analysis of the Soldiers Cap terrane, Sandy Creek area. Mount Issa inlier, Australia. Jeffriess, D., Ailleres, L., Giles, D. & Hough, M., this volume. Three dimensional analysis of superposed fold sequences. Snake Creek antichne. Eastern Fold Belt, Mount Isa inlier. MacCready, 1998. Geologic Interpretation of the Mount Isa deep seismic transect. Unpublished PhD thesis, Monash University, Melbourne. MacCready, T., Goleby, B.R., Goncharov, A., Lister, G.S. & Drummond, B.J., 1997. An evolutionary framework for the Isan orogeny. Abstracts, Geodynamics and Ore Deposits conference, AGCRC, Ballarat, Victoria, 42-45. MacCready, T., Goleby, B.R., Goncharov, A., Drummond, B.J. & Lister, G.S., in press. The Mount Isa deep seismic transect. Economic Geology. O'Dea, M.G., Betts, P.G., MacCready, T. & Ailleres, L., 1997. Sequential development of a mid-crustal foldthrust complex in the eastern Mount Isa Inher, Austraha. Australian Crustal Research Centre, Technical Pubhcation 47. Page, R.W., 1983. Timing of supoerposed volcanism in the Proterozoic Mount Isa Inlier, Austraha. Precambrian Research 40/41, 1-19. Page, R.W. & Sun, S.S., 1998. Aspects of geochronology and crustal evolution in the Eastern Fold Belt, Mt Isa Inher. Austrahan Journal of Earth Sciences 45, 343-361. Page, P.W., Sun, S.S. & MacCready, T., 1997. New geochronological results in the central and eastern Mount Isa inher and implications for mineral exploration. Abstracts, Geodynamics and Ore Deposits conference, AGCRC, Ballarat, Victoria, 46-48. Acknowledgements: This contribution is released with the permission of the Director of the AGCRC. The authors would hke to thank BHP Minerals, North Exploration, Normandy Mining Ltd & Placer for their financial support.

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CRUSTAL STRUCTURE OF NORTHERN TASMANIA BASED UPON A DEEP SEISMIC TRANSECT Tim J. Barton (Tim.Barton@agso.gov.au) Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, Canberra, ACT 2601 A deep seismic reflection transect (Figure 1) in conjunction with satellite-derived gravity data and regional aeromagnetic data have been used to determine the crustal architecture along the north coast of Tasmania. At the western end of line 148/05, the Moho, interpreted as the base of the reflective lower crust, is at -30 km. It decreases to -21 km offshore of the Arthur Lineament and increases again to 1 km in the central north coast of Tasmania. This crustal thinning is consistent with crustal models based on refraction profiling (Rawlinson et al, 1998) undertaken in a coincident experiment with the seismic reflection acquisition. Along the eastern line, 148/04, the crust thins from -30 km in the west to -24 km at the eastern margin of the Boobyalla sub-basin. Further to the east, off the north-eastern tip of Tasmania, the Moho is underlain by a series of strong west dipping mantle reflections (A) down to -40 km. The middle and upper crust exhibits contrasting character between the Eastern and Western Tasmania Terranes. In the Western Terrane the mid crust is generally non-reflective with a few east-dipping reflectors, interpreted as thrusts down to the Moho.The boundary between the two terranes is defined by a 15 km wide zone of east-dipping reflectors (BZ) at the western end of line 148/04. The Eastern Terrane is more reflective. Immediately to the east, there is a ?mafic thrust stack - 6 km thick at a depth of 4 km which is beneath the western limit of the Mathinna Group. Underlying the Mathinna Group are two moderately reflective packages (B1 and B2) which are possible remnants of lower crustal extension beneath the Bass Basin. In the upper crust of the Western Tasmania Terrane a number of bodies are imaged, as shown in Figure 1, such as the Devonian Three Hummock Island Granite (DgrA) and the Arthur Lineament (AL), which has a west-directed thrust at its eastern boundary which continues down to the Moho. In the Eastern Tasmania Terrane the only significant structure in the upper crust is the Boobyalla Sub-basin, which is bounded on the west by an east dipping extensional fault. Gravity modelling undertaken on the structures interpreted from the seismic data provides constraints on body thicknesses and likely composition. Overall, the crustal architecture along the north coast of Tasmania is dominated by crustalscale planar thrust faults, superimposed on extensional structures associated with the Mesozoic breakup of Gondwana. REFERENCES Rawlinson, N., Collins, C. D. N., Semenova, T. O. and Houseman, G. A. (1998), Crustal architecture from seismic refraction data along the north and east coasts of Tasmania. In Finlayson, D. M. and Jones L. E. A. (eds.) Mineral systems and the crust-upper mande of southeast Australia. Australian Geological Survey Organisation, Record 1998/2. Acknowledgements: The seismic and aeromagnetic data were acquired by AGSO in 1995 as a part of a multidisciplinary study for the National Geoscience Mapping Accord (NGMA) TASGO project between the Commonwealth and the Tasmanian State Governments. TJB pubhshes with the permission of the Executive Director, AGSO and the Director, AGCRC.

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This paper is a component of MSc work being undertaken by the author at the Department of Earth Science, Monash University, Clayton, Vic. 3168 Figure 1. Northern Tasmania seismic transect location map and interpreted migrated stack sections. DgrA - Devonian Three Hummock Island Granite; DgrB, DgrC - ?Devonian granite, nPv - Neoproterozoic volcanics; ST - Offshore extension of the Smithton Trough; bRCE North-western bounding block of the Rocky Cape Element; AL - Arthur Lineament; BZ Boundary zone between the Western and Eastern Tasmania Terranes; TS - ?mafic thrust stack; Bsb - Boobyalla Sub-basin; A Sub Moho mantle reflections; Bl, B2 - Extended crust remnants.

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CORRELATION OROGENS

DEFORMATION

FOR

Volume

LARGE

DISTANCES

ALONG

T.H. Bell and K.A. Hickey School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia Successive sets of foliation inflexion/intersection axes in porphyroblasts (FIA) remain consistently oriented over large portions of orogens around folds, refolds and oroclines that postdate their development. Therefore, FIA provide a tool enabling correlation of deformation and metamorphism along and across orogens at a large range of scales and at a level of detail that previously was not possible. This increased level of detail results from preservation of multiple events within porphyroblasts that get destroyed in the matrix by younger deformation and metamorphism. However, because foliations anastomose, plate motions shift dramatically with time, and the direction of relative plate motion is rarely ever perpendicular to the length of an orogen, by what mechanism do successive FIA sets form that they can develop with such remarkably consistent trends from set to set? The relationship between foliation development on a porphyroblast margin versus in the matrix away from a rigid object holds the key to the consistency of FIA trends and successions around folds and refolds and along and across orogens. Foliations in the matrix are rotated and decrenulated by the effects of shear along bedding. They also anastomose around objects of different competency at a large range of scales. However, foliations form on the margins of porphyroblasts before they develop in the matrix, probably because the rigid porphyroblast localizes the strain normally distributed over its width to a narrow zone on its margins. Foliations form on the margins of porphyroblasts in sub-vertical or sub-horizontal orientations except where they wrap up against long, well developed, crystal faces. Because such foliations get preserved by further growth of the porphyoblast, their orientations reflect bulk motions better than matrix foliations, and they are much less affected by younger deformations. The consistency of FIAs along and across orogens has significance for how a fohation develops in relation to bulk shortening directions against the rim of a porphyroblast versus the matrix. The FIA trend is independent of the direction of motion on shallow foliations. Therefore, the FIA trend reflects only the bulk horizontal motions that produced the vertical foliations. Given the consistency in trend of FIA at scales as great as at least 700kms along an orogen, the FIA trend must be controlled by bulk motions operating at the plate scale. This would explain both dramatic and progressive switches in FIA trend as records of Euler Pole shifts preserved in the magnetic striping of the ocean floor indicate similar behaviour for relative motion between any two plates. Is there therefore, a direct relationship between FIA trend and the relative direction of plate motion?

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INVERSION AND THE STYLE OF FAULTING IN WESTERN TASMANIA R.F. Berry CODES SRC, University of Tasmania, Email Ron.Berry@utas.edu.au The structure in the Middle and Late Cambrian Dundas basin of western Tasmania reflects three deformational events: a middle Middle Cambrian extensional phase related to the postcollisional Mt Read volcanism and VHMS mineralisation. Late Cambrian N-S folding and basin inversion, and complex Devonian folding and faulting with reactivation of Cambrian structures. The structure of this area has been studied along seven sections spread over 70 km along the basin. In each of these sections the interpretation includes detailed consideration of the structural and stratigraphic relationships to differentiate the effects of each of these events. The basin is dominated by a deep central graben with thinner rift margin deposits where sedimentation was initiated in a late Middle Cambrian sag phase. The major normal faults were the Rosebery Tectonic Zone/Marinoak River Fault and the Henty Fault. The Rosebery Tectonic Zone was too steep for major reactivation and a hangingwall bypass thrust (Rosebery Fault) dominates the late structure whereas the Henty Fault was inverted along part of its length. The basin includes major transfer faults which strike at a high angle the basin bounding normal faults. The Rosebery Tectonic Zone steps far to the west on a transfer at Dundas (between 5363000mN and 5350000mN sections Fig. 1). The Henty Fault steps east on a transfer at the Mt Cripps Fault (between 5396000mN and 5392000mN sections). These faults have been reactivated in the Devonian and separate the basin into blocks with different structural styles. The 535000mN section is particularly useful for distinguishing the Late Cambrian inversion from the Devonian folding. On this section the Ordovician unconformity can be projected directiy onto half of the section and demonstrates the fold limb dips were 20° to 40° in the Middle Ordovician. The Devonian folding tightened these north south folds but the cleavage strikes 340° and transects the axial planes of the macroscopic folds in the area. This section lies south of the Rosebery Fault and the major shortening has been transferred partly to the west where the structure is lost under Permian cover. The distribution of the Mt Charter Group and correlates has a very restricted distribution in western Tasmania. However most of these limits are not due to basin bounding faults. The most common boundaries are late Cambrian and Ordovician unconformities. Thus the present distribution of the axial basin facies is related to the pattern of Late Cambrian inversion rather than to the Middle Cambrian extension. The Rosebery section (5374000mN) has the largest percentage shortening possibly because all the shortening is concentrated within a narrow zone. To the north and south the shortening spreads out over a wider zone. This change reflects the position of transfer structures inherited from the basin formation.

FIGURE

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Fig. 1 Structural cross-sections of the Dundas basin. 5396000mN , 5392000mN, 5374000mN, 5363000mN, 5350000mN, 5336000mN, 5326000mN

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EVIDENCE FOR A MID BASIN INVERSION IN THE PALAEO-PROTEROZOIC NORTHERN MOUNT ISA TERRANE Peter Betts AGCRQ Dept. Earth Sciences, Monash University, Clayton 3168, VIC email: pbetts @ mail, earth, monash. edu. au The Mellish Park Syncline (MPS) (Fig. la) in the northern Mount Isa terrane preserves evidence for a shortening event between basin phases of the Mount Isa terrane. The early tectonic history of the Mount Isa terrane involved intracontinental extension and the development of several unconformity bounded basins between -1800 Ma and -1600 Ma. In the Western Fold Belt these basins are termed the Leichhardt Super Basin and the Isa Super Basin (ISB) (Southgate et al., 1998). The MPS preserves the stratigraphy of both these basins and thus provides insight into the tectonic evolution of the terrane between periods of basin development. The fold geometry of the MPS differs significantly across the basin bounding unconformity, suggesting a period of shortening before the ISB evolved. The MPS can be divided into two components, the upper MPS and the lower MPS above and beneath the unconformity respectively. The lower MPS is a shallowly north plunging, inclined, cylindrical, N-S trending fold. The eastern limb dips shallowly to moderately to the W-NW and the western limb is vertical to slightly overturned (Fig. lb). Slightly oblique, north striking (350° to 360°), steeply dipping axial planar cleavages occur in the hinge zone and the eastern limb. The upper MPS is generally shallowly north plunging, and N to NNE trending and displays a variety of fold styles and geometries. Within the basal siliciclastic units of the ISB, the MPS is open and displays a 'similar' to 'chevron' fold style (Fig. 1). Ridges of chert within the overlying carbonate sequences define a 'box shaped' fold style. The fold axis is locally non-cylindrical and the hinge zone has an open dome and basin geometry within carbonate units. The fold axis trend changes from -N-S within the basal units, to NNE within the overlying carbonates (Fig. la). No cleavages occur in the upper MPS. On the western limb of the MPS there is a large angular discordance between the ISB, and the underlying Myally Subgroup and Quilalar Formation (Fig. 1). Bedding dips 10° to 30° northwards above the unconformity and 50° to 80° eastward below the unconformity. The degree of angular discordance decreases towards the hinge as the paleo-tilt of the Myally Subgroup decreases to -40°. The angular discordance is less pronounced on the eastern limb. Bedding dips -40° NNW and 15-30° NW above and beneath the unconformity respectively. The overall geometry of the MPS is interpreted to have evolved during three discrete deformation events: (1) initial folding event before the development of the ISB; (2) NW-SE directed extension; and (3) the Isan Orogeny. The ~N-S axial trace of the lower MPS suggests initial developed during a period of -E-W shortening. Reconstruction of the pre-Isan Orogeny geometry of the lower MPS was deduced by subtracting the plunge, and stereographically unfolding the limbs of the upper MPS. This revealed an open, N-plunging, inclined fold with a steep W-dipping axial surface.

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Following the initial regional shortening, the northern Mount Isa terrane underwent a period of NW-SE directed extension. Pre-MIRE sequences were tilted towards the SE as half graben evolved, dismembering and rotating (-20°) pre-existing folds. The MPS was rotated into an inclined geometry with the eastern limb becoming shallower and the western limb steeper. The high angle unconformity on the western limb developed as an uplifted tilt block was eroded. The entire Mount Isa terrane underwent regional E-W shortening during the Isan Orogeny. The lower MPS was tightened and amplified, and the upper MPS developed. The upper MPS was focussed above the pre-existing lower MPS. There may be an analogue to the MPS in the Mount Novit Ranges where there is an -90° angular unconformity between the Myally Subgroup and the Mount Isa Group (Connors et al., 1992). However, the consistent eastward paleo-tilt of pre-MIRE sequences throughout the Leichhardt Rift (O'Dea et al., 1997) suggests that pre-MIRE folds are localised. One explanation is that the pre-Isan folding is a consequence of a mid basin inversion whereby folding was localised in the hanging walls of normal faults. Figure 1. (a) Geological map of the Mellish Park Syncline. (b) Cross section through the Mellish Park Synchne showing the different fold geometry above and below a basin bounding unconformity. Connors, K.A., Proffett, J.M., Lister, G.S., Scott, R., Ohver, N.H.S. and Young, D J . 1992. Geology of the Mount Novit Ranges, southwest Mount Isa Mine. In Stewart, A J . and Blake, D.H. eds. Detailed studies of the Mount Isa Inlier. AGSO Bulletin 243, 137-160. O'Dea, M.G., Lister, G.S., Betts, P.G., Pound, K.S. 1997. A shortened intraplate rift system in the Proterozoic Mount Isa terrain, NW Queensland, Australia. Tectonics 16, 425-441. Southgate, P.N., Bradshaw, B.E., Idnurm, M., Jackson, M.J., Krassay, A.A., Lindsay, J.F., Page, R., Sami, T., and Scott, D. 1998. Mineral system prediction using chronostratigraphic basin framework: An essential dataset for reducing exploration risk in northern Australia. Geological Society of Austraha Abstracts 49, 418.

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Quiiaiar Formation I Myaiiy Subgroup Eastern Creek Volcanics

Fault

I Post-rift sequences Syn-rift sequences

bedding trace bedding dip Fiery C r e e k Fault

Jacqueline Fault

M P S : Mellish P a r k S y n c l i n e

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GEOPHYSICAL MODELLING OF THE INVERTED FIERY CREEK FAULT SYSTEM, MOUNT ISA INLIER, AUSTRALIA. Peter Betts & Laurent Ailleres Australian Geodynamics CRC, Australian Crustal Research Centre, Monash University, laurent @ mail .earth.monash .edu. au The tectonic history of the Mount Isa terrane spans the Paleo- to the Meso-Proterozoic. The oldest preserved rocks were poly deformed and metamorphosed during the Barramundi Orogeny at -1870 Ma and form the basement of thick supracrustal sequences which were deposited during four episodes of extension. These events are termed the Leichhardt Rift Event (E-W extension), the Myally Rift Event (N-S extension), the Wonga Event (N-S extension), and the Mount Isa Rift Event (MIRE, NW-SE extension) [O'Dea et al., 1997a &b; Betts, 1997]. Basin development was interrupted at -1590 Ma by the compressional Isan Orogeny (O'Dea et al., 1997b) which lasted until -1500 Ma. In this context, this paper presents a case study of an inverted MIRE aged Fiery Creek Fault System (FCFS) involving (1) detailed structural mapping and (2) 3D geophysical modelling.

The Fiery Creek Fault System

The FCFS is a large NE striking, steeply NW dipping fault system which is composed of several segments that offset the syn-MIRE stratigraphy. The northern FCFS is defined by a single fault. However, the fault system becomes increasingly wider to the SE where it is -3 kilometres wide and is defined by several subparallel segments. These segments are discontinuous and bounded by steeply dipping, NW striking transverse faults. Detailed mapping revealed that the stratal geometry and unconformity relations in the hanging wall of the fault, and thickness variations of syn-MIRE sequences across fault segments provides excellent evidence for extensional normal fault activity during the MIRE. Reverse offset and relatively high strain in the hanging wall of several fault segments indicates that the fault system was inverted during the Isan Orogeny. Along the FCFS and in the hanging wall of the Jacqueline Fault (Betts, 1997), steeply dipping, NW striking tranverse faults have been recognised. These faults facilitate large thickness changes of syn- and post-MIRE sequences and resulted in the development of smaller tilt-blocks and depositional compartments within larger half graben. These faults behaved as tear or scissor faults during differential displacement along normal faults.

Inversion of the Fiery Creek Fault System

The FCFS displays many of the characteristics documented in well studied inversion tectonic environments (Cooper SL Williams). These characteristics include: (1) reverse offset along normal faults; (2) increased hanging wall strain; (3) variation of the apparent offset along fault segments; and (4) the development of short-cut thrusts in the footwalls of normal faults. Much of the inversion along the FCFS is accomplished by the re-utilisation of normal faults.

Three Dimensional Geophysical Modelling

Rift and sag-phase basin filling sediments are non-magnetic. The volcanic suites within the pre-MIRE (Eastern Creek Volcanics) and syn-MIRE (Fiery Creek Volcanics) sequences are 11

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characterised by high and low to moderate magnetic responses respectively. This enables the distinction of a relatively simple magnetic stratigraphy and the magnetic response of the fault system is likely to reflect offset of these volcanic units. A 3D geometrical model of the FCFS including the volcanic suites was built in gOcad. The model is made of surfaces which match the geological information (geological map, crosssections). A 3D grid was divided into regions according to surface-bounded volumes. It was used to generate a rock property model, corresponding to average values for magnetic susceptibilities. New functionalities were developed within gOcad to export the property model towards Noddy (Jessell & Valenta, 1996) where the geophysical response of the model is calculated. The calculated aeromagnetics dataset shows a good correlation with the real aeromagnetics data. This result gives confidence to the interpreted 3D geometry of the area. This study has highlighted the usefulness of aeromagnetic data and three dimensional modelling in constraining subsurface geometries and identifying partially inverted normal faults in mildly deformed terranes. References Betts, P. G., 1997. The Mount Isa Rift Event: An example of Middle Proterozoic Intracontinental Extension. Unpubhshed PhD Thesis, Monash University, Melbourne. Cooper, M. A. & Williams, G. D., 1989. Inversion Tectonics. Geological Society Special Publication 44. Jessell, M.W. & Valenta, R.K., 1996. Structural Geophysics: Integrated stuctural and geophysical mapping. In : Structural Geology and Personal Computers, Ed. D. DePaor. Elsevier. O'Dea, M. G., Lister, G. S., Betts, P. G. and Pound, K. S, 1997a. A shortened intraplate rift system in the Proterozoic Mount Isa terrain, NW Qld, Australia. Tectonics. O'Dea, M. G., Lister, G. S, MacCready, T., Betts, P. G., Oliver, N. H. S., Pound, K. S., Huang, W., and Valenta, R. K., 1997b. Geodynamic evolution of the Proterozoic Mount Isa terrain. In Burg, J. P. and Ford, M. eds. Orogeny through time. Special Pubhcation of the Geological Society of London. Acknowledgments - This paper is published with the permission of the Director of the Australian Geodynamics CRC. The authors would also like to thank Dr Rick Valenta (MIM, Australia) for providing us with geophysical data.

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SHEAR FOLD FROM ZAWAR, ARAVALLI MOUNTAINS, NW INDIA: COULD TRANSPRESSION BE CAUSE FOR DEVELOPMENT?

T.K.Biswal and M. Anand Dept. of Earth Sciences, Indian Institute of Technology, Bombay, Powai, Mumbai, India, 400076 e.mail: tkbiswal@geos.iitb.emet.in The Precambrian Aravalli mountains of north-western India comprise rocks of three major supergroups such as the Archaean basement called Bhilwara Supergroup and Proterozoic cover rocks namely Aravalli and Delhi Supergroups (Heron, 1953; Geological Survey of India, 1980). In the central part of this mountain around Zawar, very low grade metamorphic rocks of the Aravalli Supergroup such as dolomite, quartzite, phyllite and phyllitic quartzite are exposed which bears the imprint of multiple deformational history. A set of isoclinal reclined folds, Fj has been developed over bedding which has been further refolded coaxially by open upright F2 fold producing type 3 interference pattern of Ramsay (1967). While pervasive axial planar slaty cleavage is characteristic of F^ fold, the F2 fold is marked by discrete crenulation cleavage and axial planar fracture which is often filled with quartz vein. The F^ and F2 folds and their cleavages are overprinted by WNW-ESE striking F3 shear fold which is open to tight and round-hinged to chevron in form. Apart from having thickened hinges, parallel dip isogons and constant axial plane thickness, the F3 fold is distinct in having close spaced shear fracture (S3) parallel to its axial plane. Further, the wavelength of the fold is not consistent with the thickness of the layer. Hence the F3 fold is classified as Class 2 fold (Ramsay, 1967) or similar fold. A shear fold has been described to develop by heterogeneous simple shear at high angle to a bed and it strictly adheres to the Class 2 or similar fold geometry (Ramsay, 1967). It has been compared with two juxtaposed shear zones having different sense of shearing (Ramsay and Huber, 1983). The occurrence of such folds is rare in nature, albeit folds closely resembling them, such as flattened parallel fold (Class IC) and Class 3 folds which are referred to as 'similar type' folds are common. Though mechanism of similar type folds is well understood, the shear fold is yet to be genetically explained. In this paper, based on field observation it has been suggested that the F3 fold in the study area is a shear fold and heterogeneous shear along S3 has contributed to its development. Various structural features subscribing to shearing along S3 are that thin N-S trending quartz veins emplaced along the axial plane of the F2 folds are offset in a step hke fashion by close spaced S3-planes both dextrally as well as sinistrally giving rise to F3 fold, S3 plane hosts sigmoidal en-echelon quartz veins marking simple shear strain along it, book-shelf ghding structure is noticed in the biotite porphyroblasts bounded by S3 shear planes which indicates sinistral sense of shearing on the left limb and dextral on the right limb of a F3 antiform consistent with shear fold (Ghosh, 1993) and mica-fish resulted from progressive shearing show reversal of shear sense across the axial plane of F3 fold. Extensive development of extensional crenulation cleavage parallel to S3 plane due to ductile shearing is noticed in the area (Biswal et al., 1997). This shearing is obviously heterogeneous as the sense of shearing varies across the axial plane of the fold.

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On the basis of these observations added with aerial photographic study (Roy, 1995), the study area has been described to have been transacted by a dextral ductile shear zone (DSZ) which has an overall N-S strike extension and has been curved to E-W direction in right hand en-echelon manner in the middle. The F3 shear folds are present in the E-W portion of the DSZ. The curvihnear nature (in two dimensions) of the shear zone has resulted in transpression along the E-W sector. Presence of horizontal as well as vertical stretching lineations on S3 plane provide unequivocal evidence of transpression (Tikoff and Greene,1997; Lin et al, 1998). Possibly in a transpression where shortening normal to shear zone acts simultaneous with strike slip shearing, the shortening component gives rise to folding whose flow direction would be guided by shearing along shear planes. The sense of shearing along the shear planes would be reversed across the axial plane of the fold to become compatible with shape of the folded layer. The axial plane thickness would remain constant as the above direction would be either line of no finite longitudinal strain or of equal strain. Hence it has been suggested that possibly transpression plays a role in developing shear folds.

References Biswal, T.K., Nayak Subhasree, Harish, T.R. and Unikrishnan, Smita (1997) Evidence of ductile shearing from the extensional crenulation cleavage: An example from Zawar area, the Aravalli mountain. Current Science 73, 701-703. Geological Survey of India. (1980). Lithostratigraphy map ofAravalli region, southern Rajasthan and northeast Gujarat. Ghosh, S. K. (1993) Structural Geology: Fundamentals and Modern Developments. Pergamon Press,Oxford. Heron, A.M. (1953). The geology of Central Rajputana. Memoir of Geological Survey of India 79. Lin,S; Jiang D and Williams, P.F. (1998) Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling.In : Holdsworth, R.E., Strachan, R.A. and Dewey, J.F. (eds) Continental Transpressional and Transtensional Tectonics. Geological Society, London, Special Publication, 135, 4157. Ramsay, J. G. (1967) Folding and Fracturing of Rocks. McGraw - Hill, NewYork. Ramsay, J. G. and Huber, M. 1.(1983J The Techniques of Modern Structural Geology. Strain Analysis 1, Academic Press, London. Roy, A. B. (1995) Geometry and evolution of superposed folding in the Zawar lead-zinc mineralised belt, Rajasthan. Proceeding of Indian Academy of Science (Earth and Planetary Science) 104, 349-371. Tikoff, B. and Greene, D. (1997) Stretching lineations in transpressional shear zones: an example from the Sierra Nevada Batholith, California. Journal of Structural Geology 19, 29-39.

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S T R U C T U R A L C O N T R O L S OF A R C H A E A N G O L D IN THE NORTH PILBARA CRATON, WESTERN AUSTRALIA Richard S. Blewett & David Huston Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601. (rblewett@agso.gov.au) This paper will consider major tectono-magmatic events and their relationship and control on contrasting styles of gold mineralisation in the Mallina Group, Pilgangoora Belt, Warrawoona Syncline and Mosquito Creek Group of the North Pilbara Craton (Western Australia). The following table and figure summarise the main tectono-magmatic and mineralising events of the North Pilbara Craton. Defm event D1

Timing

West Pilbara

East Pilbara

Comments

3515-3471

basement not exposed

tight folds and axial planar schistosity steep E-dip (pre refolding) in Coonterunnah Grp post Duffer Formation

Pre-Warrawoona u/c

Coppin Gap

Nijman 1998

Main fabric and folds in Warrawoona Grp variable trend envelops circular batholiths. Shear zones West Coongan. Thrusting Coppin Gap (-3300) Main fabric and folds NNE Wodgina, NE North Shaw (often crenulated) in Gorge Ck, Strelly & Abydos Grps, Tambourah Dome

Cut by 3270 Ma pegmatite dykes (3270). Zegers 1997 Cooper etal. 1981 Strelly Grp u/c above

hiatus

73465

extension

3445

34457-3340 possibly to <3270

basement not exposed

D3a

73200-3100 3240 Ar age metamorphi sm

meta to lower amp in Roeboume Grp Horizontal asymm folds, layer parallel faults, S-directed thrusts Whundo Grp & 3120 Ar/Ar date for Roeboume Grp meta Main GS to Amp fabric in Whundo Grp NE-plunging tight upright folds, ENE trending high-angle reverse faults E-W folds N-dip cleavage, Sholl (sinistral) shear zones, (Powereena folds)

Extension 7

3120

D3b

<31007 to > 3015

-3000-2950

<2950

2935-2900

Croydon anticline N-S folds, pencil cleavage w/ S4. NE to ENE folds tight, steeply inclined w/ SE dipping axial surface. Thrusting on Sholl SZ and dextral (40km) NW thrusting on steep faults (eg Loudens 7) post-tectonic granites

deformation in 7Cleaverville

E-W crenulations, N-S oriented shear zones (sinistral). Reworks main fabric to a cren or as shear bands. Mosquito Creek E-W folds, metamorphism to greenschist, thrusting N-S crenulations of shear zones (West Wodgina) and folds ENE-trending mod N-dipping crenulations

Davids 1997 (Ar age) Krapez & Eisenlohr 1998

Cleaverville u/c overlain by 3010 Whim Ck Grp. Fabric enveloping Karratha Grt cut by 3010 dyke

Last major deformation Mosquito Ck Au7 Assoc w/ Satirist Granite.

Weak E-W steep crens

Krapez & Eisenlohr 1998

post-tectonic granites

major supply of heat, extension major supply of heat, extension, high level Au/Ag/Sb Mallina Au (N-S extension) Opaline Well Intrusion Krapez & Eisenlohr 1998

<29007

Extension

2850

Extension

2780

Hamersley Basin initiated. Normal N-down Mallina P., Loudens F.

Hamersley Basin initiated

NW to N-trending folds, crens, kinks, gentle upright subhorizontal folds trend NW (Negri)

NW-trending folds and crens

Age Ma 3600

3500

3400

3300

3200

3100

3000

2900

2800

2700

2600

2500

2400

EAST PILBARA Halls Gap SGTSG Conference Abstract Volume

WEST PILBARA

KEY

HAMERSLEY BASIN

Archean events in the Pilbara Craton of Western Australia

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A CRUSTAL SECTION THROUGH THE MAWSON ESCARPMENT: PRELIMINARY STRUCTURAL RESULTS FROM THE SOUTHERN PRINCE CHARLES MOUNTAINS, EAST ANTARCTICA S. D. Boger and C. J. L. Wilson School of Earth Sciences, The University of Melbourne, Parkville, Vic. 3052, Australia email: s_sboger@eduserv.its.unimelb.edu.au (Steve Boger), cjlw@myriad.its.unimelb.edu.au (Chris Wilson) The southern Prince Charles Mountains (sPCMs), 600 km inland from the Antarctic coast, is one of the last unstudied geologic terrains on earth. It represents the most southerly exposures of the east Antarctic craton, and is unique in the preservation of Archaean granitic basement rocks, and low grade Palaeoproterozoic metasedimentary cover sequences. Further to the north, such rocks are pervasively overprinted by granulite facies Neoproterozoic and/or Palaeozoic orogenic belts, which dominate exposures along the more studied coastal margins of the east Antarctic craton.

The sPCMs is of considerable interest, both in terms of the age of the Archaean basement exposed, and the kinematics and age of the orogenic event(s) which deform the cover sequences. The age of the sPCMs basement is critical to the understanding of the preRodinian (c. 1000 Ma) cotinental confirguration, as these are the only basement blocks of the 17

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east Antarctic Shield exposed anywhere on the continent. Likewise the deformation and metamorphic history of the sPCMs is unconstrained by modem dating techneques, and subsequently remains as a large unknown in our understanding of the tectonic history of the east Antarctic. n this poster we present the preliminary results of our work along the 130 km long, 1 km high Mawson Escarpment. Of the 60 km studied and presented in cross-section, grade varies from greenschist to lower amphibolite facies ( garnet + stauralite ± kyanite bearing metapelitic assemblages) in the south to mid- to upper-amphibolite facies (garnet + cordierite -h sillimanite bearing metapelitic assemblages) in the north. Structurally the terrain preserves kilometre scale nappes, overprinted sequentially by upright folds and high strain zones, and low angle discrete reverse and normal mylonites. Deformation has resulted in the tectonic interleaving of Archaean basement and metasedimentary cover sequences, while occurring concurrently with deformation was the intrusion of syntectonic mafic dykes, partial melts and granites.

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NEOPROTEROZIOC DEFORMATION IN THE NORTHERN PRINCE CHARLES MOUNTAINS, EAST ANTARCTICA: EVIDENCE FOR A SINGLE PROTRACTED OROGENIC EVENT S. D. Roger a, C. Carson^, C. J. L. Wilson^ & C. M. Fanning^ ^ School of Earth Sciences, The University of Melbourne, Parkville, Vic. 3052, Australia ^ School of Geosciences, University of Sydney, NSW. 2006, Australia ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT. 0220, Australia

Email: s_sboger@eduserv.its.unimelb.edu.au (Steve Boger), chris@es.su.oz.au (Chris Carson), cjlw@myriad.its.unimelb.edu.au (Chris Wilson), Prise.Fanning@anu.edu.au (Mark Fanning) The northern Prince Charles Mountains (nPCMs) are exposed as a series of isolated inland ranges along the margin of the east Antarctic Shield. This margin has long been considered part of an extensive Neoproterozoic (c. 1000 Ma) orogenic belt, which has been correlated with metamorphic belts of equivalent age in India and parts of east Africa, Sri Lanka and Australia. It is thought to represent one of two major accretionary belts which lead to the formation of the Rodinian supercontinent. However, due to the paucity of geochronological data available from the nPCMs, this interpretation has recently been questioned. Particularly in light of the wholesale Palaeozoic (c. 550 Ma) overprint recognised in Prydz Bay, 200 km east of the nPCMs. Currently, neither the scale, nor the timeframe of the c. 1000 Ma orogenic event, or the extent of reworking by subsequent tectonic episodes within the nPCMs, is particularly well understood, an issue addressed by this paper.

Ion microprobe (SHRIMP) dating of structurally constrained felsic intrusives indicate that the rocks of northern Prince Charles Mountains (nPCMs), were deformed during a single long lived Neoproterozoic tectonic event. Deformation evolved through four progressively more discrete phases, in response to continuous north-south directed compression. U-Pb zircon ages suggest Dl-2 occurred at c. 990 Ma, D3 at c. 940 Ma, and D4 at c. 900 Ma. Neoproterozoic tectonism thus occurred over a period of approximately 90 Myr, during which time the terrain cooled, and was partially exhumed from granulite facies conditions (D1-D3) of about 800°C and 6-7 kbar, to crustal levels suitable to facilitate discrete mylonitisation and pseudotachylite formation (D4). The northern Prince Charles Mountains therefore represent an example of a protracted orogenic event, during which time both structural and mineral textural relationships evolved over a period of at least 70 Myr, but potentially well in excess of 100 Myr, when the uncertainties in the age data are considered. The results of this study also imply that all high grade fabric forming events observed in the northern Prince Charles Mountains occurred prior to discrete mylonite and pseudotachylite formation (D4). This occurred no later than c. 900 Ma, thus precluding the possibility of significant high grade overprinting of the terrain during the Palaeozoic.

Halls Gap SGTSG Conference Abstract Volume

S t m d u i r a l eiircili4icin o f t h e ncurttiorhi P r i n c e C h L ^ i i e i Mciitilainf •H

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OZBLOCK, A CONTINENTALSCALE TECTONIC MODEL FOR THE AUSTRALIAN LITHOSPHERE IN THE MID- TO LATE PALEOZOIC Jean Braun^ Russell Shaw^, Mike Sandiford^ David Gray^ and David Foster^ ^Research School of Earth Sciences, The Australian National University, Canberra, ACT, Jean. B raun @ anu. edu. au ^Australian Geological Survey Organisation, Canberra, ACT ^Department of Geology and Geophysics, University of Adelaide, Adelaide, SA ^Department of Earth Sciences, Monash University, Clayton, Vic ^Department of Earth Sciences, La Trobe University, Melbourne Victoria and Department of Geology, University of Florida, Gainesville Florida USA. The Australian continent has evolved from its Archaean nucleus to its present-day extent through a series of accretion events punctuated by periods of intense tectonic activity during which the sutures between the various accreted lithospheric blocks have been reactivated. Reactivation of the sutures may have been driven by subduction and/or rifting along the continent margins and/or by forces originating within the mantle beneath the continent such as the delamination of a gravitationally unstable continental root or the rise of a mantle plume. The most recent series of large-scale tectonic events to reactivate ancient sutures between the various Australian continent major lithospheric blocks took place in the mid- to Late Palaeozoic and culminated in the Alice Springs Orogeny of central Australia. In this paper, we attempt to analyse the large-scale deformation patterns observed within the Australian continent which formed structures in both the Amadeus Transverse Zone during the Alice Springs Orogeny and in the Tasman Fold Belt System during equivalent Late Ordovician to Carboniferous events To do this, we assume that the driving forces originated along the paleo- plate boundaries. To this end, we present the results of a two-dimensional thin-sheet finite element model of a large continent subjected to convergent and divergent velocity boundary conditions along its margins. Deformation within the continent interior is dictated by the presence of cratonic blocks of varying initial strength, thickness and thermal state. The strength of the lithospheric blocks is also determined by their thermal state and the thickness of the crust as both of which are continuously updated from the results of the thin sheet algorithm. The model predictions can be compared with observations of strain, paleo-topography, basin subsidence, crustal thickness, denudation, geochronology and metamorphic grade of exposed rocks, thereby providing support for the viability of the assumed tectonic model. We have also found that these models, although rather limited in their quantitative prediction capabilities, are useful tools with which tectonic scenarios may be tested, visualized and potentially improved.

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THE SYNERGETIC RELATION BETWEEN DEFORMATIONAL AND THERMAL PROCESSES IN CONVERGENT OROGENS, AND ITS ROLE IN GRANITE ASCENT Michael Brown and Gary S. Solar, Laboratory for Crustal Petrology, Department of Geology, University of Maryland, MD 20742, USA (mbrown/solar@geol.umd.edu) Observational data and geochronology show synchronous deformation, metamorphism and granite emplacement in convergent orogens. Multifarious microstructures suggest feedback relations between deformation and mineral growth, and show metamorphism typically is syntectonic; it varies from medium-P (kyanite-sillimanite) to low-P (andalusite-sillimanite) type. Clockwise P - r paths are characteristic, and result from the effects of heat conduction to the surface during syntectonic erosion of the thickening orogen, dissipation of mechanical energy generated during deformation, and material transfer within the orogen that advects hot crust to shallow depths. The initial distribution of heat production with depth, including the location of any anomalous heat-producing layers, is an important variable. These processes strongly perturb the geothermal gradient by displacing isotherms toward the surface, which creates a near-isothermal middle-lower crust in the orogen. Thus, many features of convergent orogens indicate a synergetic relation between deformational and thermal processes; we postulate it is this synergy in a dynamically evolving orogen that enables transfer of granite magma from the anatectic zone to a site of pluton construction. To illustrate features of the coupled mechanical and thermal evolution of convergent orogens, we use results from the Appalachian orogen of NE USA. In the Central Maine belt (CMB) of west central Maine, during Devonian (Acadian) dextral transpressive orogenesis, strain was accommodated heterogeneously. It was locaUzed into rheologically weaker strata and partitioned into zones of enhanced deformation that accommodated more displacement and record higher strain (higher strain zones — HSZs) than intervening zones, which are composed of rheologically stronger strata that record lower strain (lower strain zones — LSZs). These structures define the CMB shear zone system. Perturbations in ductile flow caused folding and thrusting, but different rheological behavior between stratigraphic units resulted in enhanced fold tightening, overturning and limb shear strain in HSZs that was not recorded in LSZs. In both zones, a regionally penetrative, moderately NE-plunging mineral elongation lineation is present. Oblique dextral-reverse kinematics is determined from asymmetric boudinage and strain shadows around porphyroblasts. In HSZ rocks, penetrative continuous mica and quartz-ribbon foliation and bladed muscovite and biotite mineral elongation lineations define the tectonic fabric (S > L tectonites — apparent flattening to plane strain). In contrast, in LSZ rocks, foliation is weakly developed or absent and bladed muscovite that forms the prominent penetrative mineral elongation lineation defines the fabric (L > S tectonites — apparent constrictional strain). In both types of zone, deformation and mineral growth were coeval, because the same minerals define the fabrics at the same grade; we interpret the low-P metamorphism to have been syntectonic. At metamorphic grades above the contemporary solidus, stromatic migmatite and concordant to weakly discordant irregular high-aspect ratio ('sheet-like') bodies of granite occur in the HSZs. These features suggest percolative flow of melt along the flattening fabric to form the migmatite leucosomes and viscous flow of melt channeled in 'sheet-like' bodies. We infer cyclic fluctuations of melt pressure caused instantaneous changes in the effective permeability of the flow network, which enabled pulsed flow of melt in batches.

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Inhomogeneous migmatite and irregular low-aspect ratio ('pipe-like') bodies of schlieric granite occur in intervening LSZs, which suggests migration of partially-molten material through these zones en masse by melt-assisted granular flow and in batches by pulsed flow of melt carrying entrained residue. Thus, in HSZs apparent flattening fabrics are associated with high-aspect ratio 'sheet-like' granite bodies, and in LSZs apparent constrictional fabrics are associated with low-aspect ratio 'pipe-like' granite bodies. This suggests the form of melt escape structures was deformation-controlled and governed by strain partitioning. Thus, melt transfer was syntectonic and melt flow was driven by pressure gradients generated by both buoyancy forces and tectonic stresses. Structures may trap melt to prevent further ascent. Alternatively, melt ascent may become inhibited with decreasing depth by thermal arrest and crystallization as the solidus is approached. This occurs near the britde-plastic transition during low-P metamorphism, where the balance of forces favors (sub-) horizontal fracture propagation. Stoping of wall rock may assist magma emplacement and lifting of the roof and/or sinking of the pluton floor, possibly coupled with plastic yield, may accommodate inflation. The resultant plutons have (sub-) horizontal tabular geometries with floors that slope down to the ascent conduit. Granite stuck in the ascent conduit is likely to have composite structure. Geochemical heterogeneity or its erasure by homogenization among successive melt batches will depend on many factors, including the interplay between rates of ascent and solidification. At the extremes, composite structure may develop if an individual batch of melt crystallizes before the arrival of a subsequent batch of melt, potentially preserving heterogeneity (e.g., in pluton roots), whereas successive arrivals of melt batches before extensive crystallization is conducive to mingling and mixing, potentially leading to homogenization (e.g., in large, laterally extensive tabular plutons). In the CMB case study, magmatic fabrics occur locally but commonly in granites; these fabrics are oriented conformably with the NE-striking, sub-vertical foliation in the country rocks. Precise U-Pb zircon/monazite crystallization ages of schlieric granite, granite in melt escape structures in migmatites, and kilometric plutons of granite are similar to each other and to U-Pb monazite ages from staurolite-grade metapelitic schists. These data support a feedback model of syntectonic melt flow through a crustal-scale shear-zone system, and suggest that a component of the heat to drive low-P metamorphism in the upper crust was derived from the crystallizing melt trapped in plutons. In one pluton, two different granites show heterogeneity in Nd isotope compositions, which is interpreted to reflect derivation from two isotopically-distinct sources, to preserve within-source heterogeneity and to imply efficient extraction and ascent of melt in batches. These data are consistent with the pluton being the ascent conduit for a once larger, tabular pluton now lost to erosion. Actively converging orogens are complex four-dimensional open systems that exhibit selforganization. The CMB shear zone system is an example of dissipative structure within a selforganized system. Here melt expulsion out of the anatectic zone was deformation-controlled in 'sheet-like' and 'pipe-like' melt escape structures that reflect strain partitioning, and pluton construction was syntectonic. It is the dynamic evolution of an orogenic system and the synergy between deformational and thermal processes that enables crustally derived melt to cross from the anatectic zone into the subsohdus crust and pond in plutons. Thus, at the crustal scale in convergent orogens, granite generation, extraction, ascent and emplacement are syntectonic processes, and expulsion of melt from the anatectic zone is a significant mass and energy dissipation mechanism in these orogens.

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Volume

HIMALAYAN TECTONICS: FROM PLATE TECTONICS TO MECHANICAL INSTABILITIES

Jean-Pierre BURG Geologisches Institut, ETH-Zentmm, Sonneggstrasse, 5, CH-8006 Switzerland Fax: 41 1 632 1030; e-mail : jpb@erdw.ethz.ch

The Himalaya and the Tibetan plateau are the dominant features of the geological history of Asia which, since the end of the Mesozoic Era, has been dominated by its convergence with India. The relative motion is well recorded by both oceanic magnetic anomalies and paleomagnetism. Therefore, the Himalayas were considered as a natural laboratory for understanding the relationship between plate tectonics and orogenic deformation. After the advent of plate tectonics, the basic philosophy was that the Himalaya - Southern Tibet ranges represent a young analogue for ancient continent-continent collision. The question of the past was: What can be learnt from the Himalaya to understand mechanisms of continental collision? We will document that: • (1) a suture zone is composed of several thrust sheets with distinct stratigraphy, state of deformation and metamorphic grade. In particular, it provides evidence for the previous existence of oceanic lithosphere (the ophiolites), a passive and an active continental margins. Yet, a few kilometres further upHft and the ophiolite klippen will be eroded, leaving little evidence of former oceanic material. • (2) The subduction - obduction dip can be identified from the persistent dip of axial planes, cleavages and facing direction of folds in nappes. The most significant thrusts are synthetic to the initial subduction zone, suggesting that the pre-existing subduction zone determines the basic polarity of thrust-driven collisional shortening. • (3) Intracontinental thrusts create upward decreasing deformation, foreland-ward migration of deformations and deformation belts different in age though having the same fabrics. The continent-continent orogen is shortened and grows vertically by the stacking of thrust sheets in the crust. Orogens are regions of accumulated strain in zones dominated by non-coaxial deformation. Regional stretching lineations depend on finite strain and do not represent the plate motions. • (4) Exposure of high-grade rocks is caused as much by tilting and movement along major thrusts as by uplift consequent on tectonic burial. • (5) A thickened crust experiences collapse, which is additional to the erosion process for the exhumation of deeper rocks. • (6) Intracontinental deformation may accommodate a large amount of shortening. Indeed, since the collision between 65 and 50 Ma., India has moved northward by nearly 2000 km with respect to Asia. This observation has led to the continental escape model, showing large portions of Southeast Asia moving along strike-slip faults to make room for India as it advances towards the north. However spectacular with striking similarities to the tectonic

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features in eastern Asia, this model does not explain the 70 km thickness of the Tibetan crust, or the complex deep structures revealed by seismic reflection profiling. The two latter points have focused attention on shortcomings of the present-day interpretation and the questions: How can the Tibetan continental lithosphere absorb large shortening and thicken over vast areas ? The answer heavily relies on geophysical information summarised as follows: • (1) Prominent reflections appear at depths of 15-18 km and seem to underhne a decoupling surface within the crust that separates the deep- from the upper-crust and straddles suture zones. • (2) Some of these reflections have unusually high amplitudes (bright spots) and coincident negative polarities implying that they represent a decrease in seismic velocity or density, or both. They are interpreted as fluids in the crust and may mark granitic magmas derived by partial melting of the tectonically thickened crust. • (3) Teleseismic magneto-telluric data suggests that the low-velocity zone lies within the crust. This would indicate that a partially molten mid-crustal layer of unconstrained thickness exists beneath southern Tibet. • (4) Consistently, earthquakes are generally shallow (generally less than 20 km deep). However, large earthquakes break through the entire upper crust, which calls for one important remark: there is a rheological layering. To bring some geological information in this disputed issue, the author has challenged classical concepts and investigated the deformation behaviour of large scale multilayer systems. Modelling results are that major imbricate thrust units of the upper crust are decoupled from lower crustal thrust units that may involve upper mantle rocks. Decoupling may exist in terms of directions (superposed thrusts across the lower and upper crustal layers may have opposite vergence) and style (different wavelength and amount of displacement on individual structures). Recent structural schemes suggest that the deep parts of the crust behave in a manner different from that of the upper parts. In addition, the upper crust can slide and/or fold independently of the lower crust. These processes are particularly well displayed in the Himalayan syntaxes. The eastern and western Himalayan syntaxes are large-scale antiforms situated at geodynamically similar locations and whose metamorphic evolution is coeval in the IndiaAsia collisional history. These large antiforms have grown within the last 4 Myr and are being strongly eroded, which resulted in exhumation of pre-folding 30km deep rocks in their cores. Their presence emphasises that lithospheric buckling is a basic response to large-scale continental shortening and an efficient mountain-building process. They are the unstable crustal response to forces among which plate tectonic forces play a subsidiary role. Less amplified lithospheric folds constitute the mountain ranges of western and northern Tibet, and are shown to be growing lithospheric buckles. What can be leamt from mechanisms of continental collision to understand the Himalaya? The efficient strain-regime in orogenic zones is shear, which is localised in networks of shear zones reported on maps and sections. Structural geologists must separate structures

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recognised within these shear zones from structures of the less deformed domains in between. Indeed, structures with similar shape and orientation may be juxtaposed, yet are chronologically and physically unrelated. Are we able and have we sorted out their individual significance? In the light of instabilities in orogens, one can argue that an identical string of local deformation modes results in an identical set of seemingly superposed structures (e.g. Dl, D2, D3) to reach an "equilibrium" total strain (the attractor in the mathematical sense). This would explain why many orogenic areas, whatever their age, display similar sets of main structures. The style of 'late phases of deformation' is controlled by mechanical instabilities in rocks that have acquired a strongly anisotropic fabric during previous periods of shear deformation. The timing of events and rates of development are an exciting challenge facing students of orogens. Within-crust decoupling is an important orogenic process. What are these decoupling horizons (are they migmatites?). Which is the deformation that characterises them? Far-field continental deformation is as important as plate boundary deformation. What do we know about far field structures? The style of deformation that is observed at least in some parts of the continent suggests that forces are acting today within Asia are not simply those due to the relative motion of the Indian and Eurasian plates. A balanced appreciation of the various components involved is necessary. If faulting is the dominant mode of deformation of the continental lithosphere, fault propagation would reconcile intracontinental deformation and plate tectonics. However, mechanical differences are responsible for differences in behaviour. Other mechanical instabilities are involved, such as large scale folding. The link between large scale and small scale structures is the enigma we are answering, slowly.

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PROTEROZOIC ROCKS IN CENTRAL VICTORIA AND THEIR TECTONIC IMPLICATIONS Ross A . CAYLEY, DAVID H . TAYLOR, DAVID H . MOORE, & ALFONS H.M.VANDENBBRG Geological Survey of Victoria, PC Box 500, East Melbourne, Vic 3002 e-mail address: Ross.Cayley@nre.vic.gov.au Isolated occurrences of tholeiitic metabasalt (greenstone) occur on the Victorian coast at Waratah Bay, Barrabool Hills, and Phillip Island. These exposures lie along the southern margin of the Melbourne Zone and are enigmatic w^ith respect to the tectonic setting of Victoria. These greenstones have previously been assumed to be Cambrian by lithological correlation with other greenstone belts in Victoria, but regional aeromagnetic data and field relationships, especially at Waratah Bay, suggest that in part they are Neoproterozoic greenstones with affinity to Tasmania and King Island. Using the aeromagnetic and other data, it is possible to trace these Victorian occurrences southwards beneath Bass Straight to King Island and the Arthur Lineament of western Tasmania. Within the Melbourne Zone the northward subsurface continuation of Neoproterozoic crust is reflected in the position and geometry of some overlying Palaeozoic to Recent structures, and therefore in topography also. The idea of Proterozoic crust underlying central Victoria is not new, its presence having been invoked to explain: a submarine plateau suggested by palaeogeographic reconstructions; the chemistry of S-type granites; the localized eastward vergence of the fold and thrust structures in the Victorian Lachlan Fold Belt (LFB); the deep seismic profile straddling the Heathcote Fault Zone, which images a complex lower crust beneath the Melbourne Zone that extends westward beneath part of the Bendigo Zone. Many other datasets, as listed below, further support the hypothesis for an older basement upon which the Melbourne Zone sediments were unconformably deposited. Many characteristics of the Melbourne Zone are notably different to the flanking Bendigo and Tabberabbera zones, although most correlate well with Tasmania. The chief differences between the Melbourne Zone and flanking zones in Victoria are: Cambrian Barkly River greenstones in the Melbourne Zone are • geochemically and structurally quite unlike the oceanic/forearc volcanics of the flanking zones but compare closely with the Mount Read Volcanics of western Tasmania—the Lower Ordovician sequence is condensed in the Melbourne Zone compared with the flanking zones—the Waratah Bay greenstones are unconformably overlain by the only Ordovician carbonate succession known in Victoria, more similar to the shallow marine carbonate sequences of central Tasmania than the deep marine turbidites typical of Victoria at that time—deformation in the Melbourne Zone (including the Ordovician sequence) did not begin until the Devonian whereas the flanking zones were first deformed in the Silurian; the Heathcote Fault Zone was deformed before the footwall was even deposited (440 Ma)—the Silurian to Devonian sediments of the Melbourne Zone sequence thin markedly towards the east—where developed, the thrust faulted margins with the flanking zones are both directed towards the Melbourne Zone, which paradoxically shows the least

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intense deformation—these overthrust margins preserve pods of blueschist facies rocks unlike anywhere else in the Victoria—^the structural shortening of the entire sequence in the Melbourne Zone (including the Ordovician) is much less intense than in the adjacent zones, and the fold style is different—granite enclaves in the Melbourne Zone are strongly deformed in contrast to the surface exposure and thus suggest a non-uniform crust—^Melbourne Zone (and some eastern Bendigo Zone) gold deposits are young, and show a distinct antimony (stibnite) association—the granites within the Melbourne Zone (and the easternmost parts of the Bendigo Zone) are dominated by cordierite-bearing Stypes which have been correlated with Tasmania. To explain all these differences a reinterpretation of the tectonic setting of the Melbourne Zone within the LFB is needed. When considered in the context of underlying Tasmanian Neoproterozoic crust, and a back arc setting for the western LFB, the anomalous history of the Melbourne Zone can be easily reconciled. The distribution and field relationships of the greenstones exposed along the Victorian coast suggests that some are Neoproterozoic rather than Cambrian. They form part of a continental crustal block of northwest Tasmania that was probably deformed in the Neoproterozoic "Penguin Orogeny". The trace of magnetic greenstone belts from Tasmania and King Island into Victoria makes it clear that the relative positions of Tasmania and the Melbourne Zone at least have remained essentially static since the Neoproterozoic. This conclusion reinforces the correlation of the Cambrian volcanics of the Melbourne Zone to the Mount Read Volcanics of western Tasmania, both highly prospective for base metals and gold. The long recognized similarity between the conformable Ordovician to Middle Devonian marine sequences of the Melbourne Zone and the Mathinna Group of eastern Tasmania suggests connection between these regions throughout Palaeozoic times. Recent AGCRC deep seismic suggests that the Mathinna Group overlies deformed Proterozoic crust. A lack of Ordovician to Silurian deformation characterizes both the Melbourne Zone and Tasmania, and sets these regions apart from the rest of the LFB of Southeast Australia. Rigid Proterozoic crust in Tasmania appears to have shielded overlying Palaeozoic sequences from the effects of widespread regional LFB deformation in the Ordovician to Silurian. The condensed Ordovician sequence of the Melbourne Zone was probably deposited on a continental shelf or submarine plateau developed on the Proterozoic crust. The overlying Silurian and Devonian sequences are derived largely from the flanking zones as they overthrust the Proterozoic basement during their regional deformation in the Ordovician to Silurian. The Late Silurian to Devonian sequences shed into the Melbourne zone form a thick clastic wedge (Darratweitguim Province) that thins markedly to the east (Mount Useful Province). This wedge geometry is indicative of deposition in a submarine 'foreland basin', as the Proterozoic basement was loaded and downwarped during substantial overthrusting of the Bendigo Zone. Like Tasmania, the Ordovician sediments of the Melbourne Zone were protected from deformation at this time by the buttress of Proterozoic basement. The unique style and late timing of the Melbourne Zone deformation during the Tabberrabberan Orogeny is consistent with thin-skinned shortening above rigid crust, and was probably driven by further overthrusting and strike28

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slip deformation during oblique convergence with the adjacent zones. Proterozoic crust at depth helps explain the present crustal thickness of the Melbourne Zone given the limited overall shortening of the exposed geology, and the limited post-orogenic uplift indicated by the low metamorphic grade. The geometry and vergence of flanking faults that bound the Melbourne Zone, the Heathcote and Governor faults, is strongly controlled by the rigid Proterozoic crust in their footwall. The underlying cold Proterozoic crust can explain the anomalous low temperature metamorphic facies of some rocks documented within these two fault zones. The convergence of these faults to the north probably delineates the extent of the underlying Proterozoic crust. Apart from extension during rifting as Australia, the Lord Howe Rise, and Antarctica separated, there seems little evidence of significant lateral displacements of Tasmania and the Australian mainland postulated by some authors. Acceptance of a basement of underlying Proterozoic crust for the Melbourne Zone easily and simply explains all the anomalous features previously observed for this region.

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HISTORY OF MOVEMENT OF THE QINLING-DABIE OROGEN PRESERVED IN PORPHYROBLASTS Aigen Chen School of Earth Sciences, James Cook University, Townsville, Qld 4811 (E-mail: aigen.chen@jcu.edu.au) The Dabie Region is located in central China and represents the easternmost part of the near E-W trending Qinling-Dabie Orogen. It separates the Sino-Korean Block in the north from the Yangtze Block in the south. The collision between the Sino-Korean and Yangtze Blocks is generally thought to have occurred during the early Mesozoic. Samples across three macroscopic folds in the Dabie Region of the Qinling Orogen in central China contain three foliations. Si, S2 and S3 preserved as inclusion trails within garnet porphyroblasts. Si, S2 and S3 predate the matrix foliation, S4, which was folded during a subsequent deformation that produced a crenulation cleavage, S5, which lies axial plane to the three macroscopic folds. A shallowly dipping crenulation cleavage (S6) overprints these fabrics. High pressure blue schist minerals have been found as inclusion trails. These high pressure minerals have been the focus of debate concerning orogenesis in the Dabie Region. Unfortunately, numerous tectonic models associated with either the exhumation of high pressure rocks and/or the plate colHsion process are still disputable. A significant problem is the lack of detailed documentation of microstructural history, particularly that preserved in porphyroblasts as some of these porphyroblasts preserve the relics of high pressure minerals! This study uses measurements of early intersection lineations preserved in garnet and albite porphyroblasts obtained from vertical thin-sections with various strikes and documents the switches of inclusion trail asymmetry for each sample. This three dimensional approach has big advantages over measurement on the 2-D inclusion trail geometries preserved in individual thin sections and the results obtained by using this method have great potential for solving problems surrounding the mechanism of spiral inclusion trail development. A total of 109 FIA (i.e. Foliation Inflexion Axes preserved in porphyroblasts) were recorded from 85 samples. Sixty samples contain one FIA, 24 samples have either two or three FIA and one sample (b5-9) contains 4 FIA sets. These FIA can be divided into 4 set based on their orientation and consistent relative timing. The mean vectors of these 4 set of FIA are 4.4° (25 FIAs), 56.8° (23 FIAs), 283.6°(46 FIAs) and 328.3° (15 FIAs) respectively. The relative timing of these FIA is based on truncations between inclusion trails, the relationship between matrix foliation and inclusion trails, different generations of porphyroblast growth (different phases or different stages of same mineral) and particularly the core-to-rim or core-medium-rim changes recorded within a sample. The FIA succession, from oldest to youngest is, NE-SW, ESE-WNW, N-S and SSE-NNW. FIA from Sets 1, 2 and 3, which formed prior to the development of the three macroscopic folds, lie oblique to the axial planes of these folds and do not change orientation across them, suggesting that garnet porphyroblasts did not rotate during the subsequent deformation. FIA from Set 4 are the youngest and are generally oriented parallel to the axial planes of macroscopic fold implying that they formed synchronous with the development of these macrostructures.

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Microstructures from intergrown, multicored and other porphyroblasts with complex inclusion trail geometries reveal critical relationships between the included foliations, that relate directly to the changes in FIA trend. For the intergrown porphyroblasts, it was found that the elongation resulting from the intergrowth of garnet porphyroblasts may be used as a reference frame to compare with other isolated porphyroblasts. The inclusion trails in the intergrown garnets are similarly oriented to those in isolated porphyroblasts. Furthermore, the consistent orientation of sigmoidal inclusion trails in porphyroblasts across a thin-section are clearly independent of deformation in the matrix even when important rotational components to the bulk deformation are involved. In the case of multicored porphyroblasts, the critical overprinting relationships between different generations of foliation are preserved because of the lack of crystal face control, which, otherwise, may be important for the formation of smoothly curving spiral shaped inclusion trails. Similarly, the relationship between FIA distribution and the other 4 types of porphyroblasts with complex inclusion trails can be determined. In each type, the FIA distribution is closely associated or interrelated with the porphyroblast microstructure. All these relationships require spiral-shaped inclusion trail geometries to have formed without rotation of the porphyroblasts during the multiple deformation events that predate the development of the three macroscopic folds. The FIA orientations and relative timing also suggest that a more protracted and complicated history of orogenesis has been preserved than envisaged by prevailing collisional models, which only deal with the formation of WNW-ESE trending fabrics. Correlation and comparison of FIA data in this study area with movement indicators and FIA data from other areas in the Qinling-Dabie Orogen (e.g., the Wudan Mountains, Mulanshan, and the mountains west of the Tan-Lu Fault) were made. The results, incorporated with available geochronologic and palaeomagnetic data, imply a possible connection between FIA and plate motion between North and South China Blocks in the period ranging from 240 Ma to 140 Ma. That means, during this period, the collision between North and South China blocks initiated in the easternmost section along a NE-SW trending Suture Zone, as indicated by the earliest NE-SW trending FIA set between 240-200 Ma. Further collision took place along SSE-NWW trending Suture Zone which preserved the second set of FIA with SSE-WNW orientation between 200-160 Ma. After that, a nearly E-W compressional deformation overprinted and formed nearly N-S trending FIA during the time of 160-140 Ma which was followed by the formation of still younger NW-SE to NNW-SSE trending, NE to ENE verging folds which account for the formation of Set 4 FIA in the study area.

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STRAIN PARTITIONING IN THE INTRACRATONIC PETERMANN OROGENY, CENTRAL AUSTRALIA Dorothy Closed Ian Scrimgeour Martin Hand^, Thomas Flottmann^ and Christine Edgoose^ 1 Northern Territory Geological Survey, Minerals House, AHce Springs dorothy.close@dme.nt.gov.au 2 Dept. Geology and Geophysics, The University of Adelaide, SA 5005 3 Montan-Universitat Leoben, Insitut fuer Geowissenschaften, Leoben, Austria

The 560-520 Ma Petermann Orogeny is a major intracratonic compressional event in central Australia that was associated with > 100 km of shortening. A south to north cross section through the Petermann Ranges orogenic belt, from the Mann Ranges in the Musgrave Inlier at the SA/NT border, to Bloods Range at the southern margin of the Amadeus Basin, represents a complete structural profile of the development of this orogen. The deepest crustal levels, (which are exposed south of the Woodroofe Thrust) are characterised by a pervasive mylonitic foliation that formed at ~13kbar 40 km depth) and 750°C. Strain partitioning within this lower crustal zone resulted in the formation of north-directed thrusts and dextral strike-slip zones that alternate on the km scale. In contrast, at mid-crustal levels, the transport direction of structures in the Petermann Ranges region are dominantiy north-vergent. The most spectacular of these north-directed structures is the Piltardi Detachment Zone, which comprises a 2-4 km thick zone of interleaved basement and cover. This detachment zone carries a basement slab of up to 5 km thick and can be traced for > 150 km parallel to the strike of the orogen. Continued development of the detachment resulted in internal stacking within the overriding slab, with thrust surfaces ramping laterally westwards to produce structural repetition of progressively younger stratigraphy towards the west. The original south-dipping detachments were then rotated, on a regional scale, into a north dipping orientation as the result of later, north-directed thrusting that domed the detachment zone. One interesting consequence of this doming effect is that the detachments north of the Petermann Ranges all have apparent normal movement. At the foreland limit of this internally stacked orogenic wedge, fault propagation folding of interleaved basal Amadeus Basin sequences has produced a regional-scale homoclinal flexure, reminiscent of the MacDonnell Homocline along the northern edge of the Amadeus Basin. The final stages of north vergent deformation retreated towards the hinterland with movement on the Woodroffe Thrust exposing the deep crustal fabrics. The exposure of a near complete crustal section in the Petermann orogen gives an insight into the way in which crustal strains were arranged spatially. In the mid-crust and toward the foreland, the north-directed transport along the major detachment horizons was effectively orthogonal to the strike of the orogen, implying the deformation was strongly convergent. However, in the deep crust that is exposed in the interior of the orogen south of the Woodroffe Thrust, dip-slip and strike-slip movement occurred simultaneously, indicating the orogen had an important component of strike-slip movement in its core. One implication of our observations is that without exposure of the deep crustal structures, the role that strike-slip movement apparently played in the development of the mountain belt would have been largely obscured.

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HEAT SOURCES AND TECTONIC MODELS FOR THE EAST LACHLAN FOLD BELT: THE BASALT STORY WJ. Collins Dept. of Geology, University of Newcastle, Newcastle, NSW, 2308. bcollins@geology.newcastle.edu.au An alternative approach to unravelling the tectonic evolution of the LFB, aside from the common structural and granite studies, is to examine basalt geochemistry. Here, a compilation of >500 published and unpublished mafic volcanic analyses, screened to remove altered samples and identify primary basalt magmas, is used to determine tectonic environment in the east LFB, throughout the Ordovician, Silurian and Devonian periods. Middle- to Late Ordovician volcanics from the east LFB (Rockley, Molong and JuneeNarromine volcanic belts) are subduction related (Glen Et aL, 1998), and their extremely primitive "mantle-like" isotopic signature (Wyborn and Sun, 1993) provides unequivocal evidence for an intra-oceanic arc setting, consistent with the lack of silicic granitoids in this period; dioritic and mafic monzonitic intrusives are the only major plutonic phases. The Early Ordovician (?) Jagungal volcaniocs in the Kiandra belt, which separate the east and central LFB, are unique. They have a very flat trace element pattern that closely resembles mid-ocean ridge basalts (MORB) and probably formed in an evolved (large) back-arc environment. The Middle/Late Ordovician Gooandra volcanics are transitional between Jagungal and the Late Ordovician Nine Mile volcanics, which closely resemble the Ordovician arc-rocks described above. This transition might reflect increasing proximity of the Kiandra "backarc" to the east LFB volcanic arc with time. Early-Mid Silurian extension in the east LFB heralded the onset of widespread silicic magmatism, and was coeval with the formation of the Cowra, Tumut and Hill End rift basins. The distinctive Early Devonian Cuga Burga volcanics of the Hill End basin closely resemble the Ordovician basalts, except for high HFSE (high-field strength elements) and Zr/Ti ratios, which are typical of a backarc, and distinct from a volcanic arc environment (Woodhead et al., 1993). This environment accords with that determined by Cas and Jones (1979), based on oaleogeographical considerations. In the Early Silurian Tumut Trough, the Honeysuckle Dasalts formed in an incipient backarc analogous to the Lau Basin, north of New Zealand (Dadd, 1998). The Early Silurian Nacka Nacka and Micalong Swamp igneous complexes, located west and east of the Tumut Trough, respectively, show extremely similar trace element patterns to the Honeysuckle basalts, and represent mafic plutonics intruded into the flanks of the backarc. Basalts of the the Middle-Late Silurian Ngunawal Basin, east of Canberra, also have backarc affinities.. The Early Silurian Gundary Basalt is shoshonitic in character (Jones et al., 1997) and resembles the Cuga Burga volcanics, whereas the more voluminous, lower-K Currawang basalts are Middle/Late Silurian and are more typical of back-arc basin basalts. Farther east, basaltic rocks of the Middle Late Devonian Eden-Comerong-Yalwal rift basin show significantly higher HFSE enrichment and the lowest "subduction-flux" component of the east LFB backarc basins. Dadd (1992) considered that the Comerong basalts erupted over a "hotspot", although it is also possible that slab breakoff allowed ingress of asthenospheric magmas into east LFB crust. The anomalous silicic magmatism of the LFB, extending over a -700 km-wide belt for -70 Ma during the Silurian-Devonian, is well recognised. Less appreciated are the coeval basalts, but these are the best indicators that the east LFB was an extensional backarc at this stage. What is the major heat source for the voluminous silicic magmatism in these extensional backarc environments? In normal arcs, mantle wedge melting is achieved by volatile fluxing and decompression melting (note that mantle heating is not involved). At -100 km depth.

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accumulated volatiles released from the subducting slab migrate into the base of an upwelling melting column, considerably reducing melting temperature. Volatile induced melting contributes up to 10% of arc magmatism, the remainder produced by decompression partial melting as the melt column rises, itself limited by crustal thickness. Melting is less if the overlying crust is thin, but the net crustal heat transfer is always high, causing substantial crustal interaction to produce silicic magmas. The most appropriate tectonic environment for generating widespread subduction-related magmatism is an extensional backarc, for two reasons: (1) decompression melting is enhanced because the crust is actively thinning, and (2) the rising melts track the migrating slab, which is moving oceanward relative to the overlying plate, so backarc magmatism is much more widespread. These effects are considerably enhanced if the overlying crust contains "fertile" rock-types, such as hydrated metabasalt (amphibolite) and mica-rich turbidites, which are probable source components for LFB silicic magmas. An optimum situation for generating voluminous silicic magmas is to pre-heat the crust, which appears to have been achieved in the east LFB because the most of the region underwent considerable and widespread shortening (and thickening) during the endOrdovician "Benambran" orogeny. Widespread thickening is most clearly indicated by the regional, virtually simultaneous uplift of the east LFB by the Early Silurian. Such dramatic crustal shortening would result in a large part of the east LFB being underlain by the EoOrdovician, W-dipping subducted slab (cf. Glen et al, 1998). The buried midcrustal Ordovician turbidites were subsequently heated by thermal relaxation. Subsequent slab retreat and the widespread, possibly rapid, onset of backarc extension during the Silurian injected vast amounts of hot, mantle-derived magma into the lower crust. Extension appears to be related to a significant change in east LFB plate motion at the endOrdovician (Li et al, 1990). Irrespective, the cause of the enigmatic Silurian-Devonian "flare-up" in the east LFB appears to relate to an unusual but plausible set of circumstances: a subduction-related setting, widespread fertile crustal sources, widespread end-Ordovician crustal thickening and heating, followed by equally widespread Silurian backarc extension. In this model, a profound thermal perturbation in the mantle (delamination/ convective mantle thinning) is not required. References Cas, R.A.F. & Jones, J.G., 1979. N.Z, J. Geol Geophys,, 2: 71-85. Dadd, K.A., 1992. Tectonophys,, 214: 277-291. Dadd, K.A., 1998. Aust. J. Earth ScL, 45: 109-121. Glen R.A., et aU 1998. Geology. 26: 751-754. Jones, J.A., et a/., 1995. Aust, J, Earth ScL, 42: 25- 34. Li, Z.X., etaL, 1990. J. Struct. GeoU 12: 567-575. Woodhead J. et al, 1993. EP.S,L,, 114: 491-504. Wybom, D. & Sun, S., \99?>. AGSO Res, Newsletter, 19: 13-14.

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3D MODELING OF THE NORSEMAN GOLD FIELD: INCORPORATING 100 YEARS OF HISTORY INTO CURRENT EXPLORATION MODELS Karen A. Connors Central Norseman Gold Corp., PC Box 56, Norseman WA 6443 email: karen.connors@wmc.com.au Robert Poulinet WMC Resources Ltd., PC Box 7001 Cloisters Square, Perth WA 6850 Anthony R. Gray Central Norseman Gold Corp., PC Box 56, Norseman WA 6443 3D modeling of orebodies, geologic units and structures in the Norseman gold field has resulted in reinterpretation of the controls on gold mineralisation and the development of new targets within a field that has been explored and mined for over 100 years. This modehng shows that the presence of cross cutting dykes and faults has been key to the development of high grade ore shoots within most of the quartz reefs, whereas previous interpretations suggested that they only played a role on one of the major reefs. In addition, this work has led to the hypothesis that the grade distribution on the E-dipping reefs can be used to predict the position of S-dipping "cross link" orebodies, thereby generating new exploration targets. In late 1997, CNGC undertook a project involving the digital capture of the historic mining data throughout the "main field" area. The work includes capture of mining openings, reef outiines from level plans and sections, and assay data from level plans, stopes and rises/winzes. The mines covered by the project area include the major ~E-dipping orebodies: Crown reef (~1 Moz), Mararoa reef (-1 Moz), and Norseman reef (-0.2 Moz), as well as the smaller S-dipping orebodies: Bluebird cross link, Bullen Mine (0.4 Moz) and OK/02 reefs (-0.3 Moz). In general, the Norseman-style orebodies comprise narrow (~lm) quartz veins with an average grade of 10-18 g/t. No significant gold values occur within the host rocks which typically include a narrow biotite altered selvedge. In order to put the orebodies of the main field area into their proper geological and structural setting, 3D models of the rock units, dykes (felsic, mafic & ultramafic) and structures have been constructed based on surface mapping, limited underground mapping (old level plans generally do not show host rock types), and drill hole data. Integration of these 3D models with those generated from the mine data, has provided key information regarding the controls on gold mineralisation and thereby aided in the generation of innovative exploration targets. The importance of dykes (felsic, mafic & ultramafic) of various orientations in the development of Norseman style orebodies has long been recognised. They are provide a good host for quartz reef development and/or cross cutting structures which localise ore shoots. The major reason why the dykes play such a key role, is that they occur at a high angle to the moderately to steeply W-dipping host stratigraphy which comprises a thick, monotonous sequence of mafic lavas. Due to its orientation, the host stratigraphy could not accommodate much strain during ~E-W shortening, however, the dykes are suitably oriented to have localised deformation and thereby localised fluid flow during mineralisation. The localisation of ore shoots around dykes was first recognised on the Crown reef in the late 1960's where SE-trending ore shoots occur around the intersection with NNW-trending, Edipping gabbro dykes and faults (Cameron 1968). This model, however, was never extrapolated to other N-trending, E-dipping orebodies such as the Mararoa reef. However, the recent 3D modeling shows that many SE-trending high grade ore shoots on the Mararoa reef are related to both NNW dykes and NNW faults. The importance of cross cutting dykes is

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also evident on the SE-dipping orebodies such as the Bluebird cross link, where subhorizontal ore shoots are related to the refraction of the reef through NE-trending, NW-dipping porphyry dykes. The localisation of high grade ore on the Mararoa reef has previously been attributed to changes in dip and strike (i.e. the "shear-link" model). During reverse dextral movement on the N-trending, E-dipping structures, the NE-trending and/or more gentiy dipping segments of the structure form the favourable sites for gold mineralisation. Although this model appears to be valid for much of the Mararoa reef, the reasons for the changes in dip and strike have remained unexplained such that the model cannot predict the location of these changes or the high grade ore trends that are related to them. The new 3D models show that at least some of the SE-trending high grade ore shoots on the Mararoa reef are localised around the intersection with NNW-trending gabbro dykes and faults. Therefore the changes in dip and strike can be related back to a fundamental cause. Thus the dykes can be used to target additional ore shoots on the old reefs, as well as provide a guidehne for assessing distribution of ore within new orebodies. More importantly, empirical observations suggest that the relationship between cross cutting dykes and high grade ore shoots can also be used to target for cross link orebodies such as the 0.4 Moz Bluebird cross link. The Bluebird cross link is hosted by a NE-trending, SE-dipping gabbro dyke which is bound to the east by the Butterfly dyke and the Mararoa reef. A SEtrending ore shoot on the Mararoa reef is spatially related to the intersection of the reef with the Bluebird cross Hnk and its host gabbro. It follows that other SE-trending ore shoots on the Mararoa reef, that lack obvious association with NNW-trending faults or dykes, could be related to NE-trending gabbros which may host quartz reefs similar to the Bluebird cross link. New cross link targets have been generated based on the SE-trending high grade ore shoots that fit these criteria. These targets are intersected by few drill holes, which support the hypothesis, including one target that is intersected by a drill hole which contains a 0.8 m quartz vein (NSA) within a 20m wide gabbro. This work is ongoing and there are several other key questions concerning gold distribution that are currendy being addressed. These include assessment of the controls on: 1) the NNW trend to the 5 major orebodies within the Mararoa reef, and their en echelon pattern (stepping down dip to the northeast); 2) the NE-plunging ore shoots on various reefs; and 3) the upper and lower limits to mineralisation on the various reefs. Reference: Cameron, E. 1968. The dyke pattern at Norseman and its relationship to ore on the Crown reef. Unpublished CNGC report K/1685.

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PERCOLATION THEORY APPROACHES TO FLUID FLOW IN FRACTURECONTROLLED HYDROTHERMAL SYSTEMS S F Cox\ M A Knackstedt' and J Braun^ ^ Research School of Earth Sciences and Department of Geology, The Australian National University, Canberra, ACT 0200, Australia, e-mail: sfcox@geology.anu.edu.au ^ School of Petroleum Engineering, The University of New South Wales, Kensington, NSW 2052, Australia ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Percolation theory provides powerful insights for understanding the architecture of fluid flow and the localisation of ore deposition where flow in hydrothermal systems is controlled by advection through networks of active faults, shear zones and associated fracture systems. The evolution of permeability during deformation is controlled by a dynamic competition between deformation-induced porosity-creation processes and porosity-destruction processes. Localisation of deformation in active faults and shear zones leads to flow localisation, with large-scale flow systems forming when active faults and shear zones link to create percolation networks. During metamorphic devolatilisation, for example, broad regions of fluid focusing can develop around the upstream (deeper) segments of active shear networks and fluid discharge regions develop in the downstream (shallower) parts of these systems. The length scales of fluid flow are controlled by connectivity among elements of an active fault/shear network. Progressive deformation promotes increasing connectivity among fractures, faults and shear zones. The onset of fluid flux at scales sufficient to link fluid sources and potential ore deposition sites occurs at a percolation threshold which is dependent on factors including strain, network geometries and relative rates of growth and nucleation of faults or shear zones. Critical strains required to reach the percolation threshold can be very low. The architecture of flow within fault/shear networks is influenced by the relative proportions of backbone, dangling and isolated structures in the network. Using numerical modelling, we illustrate the complexity of flow paths in fault/shear networks in both two and three dimensions. The relative proportions of isolated, dangling and backbone elements change as a function of the total strain and fault/shear density. For systems just above the percolation threshold, the backbone is a very small proportion of the total fault population and flow is localised on a few structures. Particularly in the mid- to upper-crustal seismogenic regime, rapid changes in permeabihty distribution in fault networks due to episodic fault slip, aftershock sequences and interseismic hydrothermal sealing can lead to sudden changes to the location and architecture of flow backbones. For hydrothermal systems well above the percolation threshold, fluid flow is distributed over a larger proportion of the fracture population. Near the percolation threshold in 3D networks, high fluid flux occurs along tortuous flow paths which have Hmited along strike continuity. We also illustrate flow patterns and the geometry of fluid focussing and discharge regions around permeable fault systems by the use of two-dimensional finite element models of

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Steady state fluid flow in and around simple, high-permeability fault networks that are embedded in a less permeable medium. These principles have applications to understanding ore localisation in epigenetic ore systems. For example, in Archaean shear-hosted gold systems, the clustering of gold deposits around a small fraction of the total population of faults and shear zones indicates that the hosting shear systems operated close to the percolation threshold (self-organised criticality?). Ore deposition involving fluid-rock reactions is favoured at fluid discharge sites in downstream segments of fault networks, especially in dead-end structures. Backbone elements are high flux structures which present limited opportunities for fluid-rock reaction except near their downstream termination. However, they are sites of potentially large fluid pressure fluctuations during seismogenic behaviour. Fluid mixing reactions are favoured at both the upstream and downstream terminations of dead-end elements where fluids focus into, or discharge from faults.

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TECTONIC HISTORY OF NORTHERN PAPUA NEW GUINEA, ' ' A W A R AND FISSION TRACK THERMOCHRONOLOGY Peter Crowhurst, Kevin Hill and David Foster Australian Geodynamics Cooperative Research Centre Earth Sciences, La Trobe University, Melbourne, Victoria 3083, Australia (P. Crowhurst @ latrobe. edu. au) A major metamorphic cooling event in northern Papua New Guinea has previously been dated at -20-27 Ma, using conventional K-Ar mineral dating. The timing of this metamorphic event has been included as an integral part of several models that attempt to explain the tectonic development of Papua New Guinea (Fig. 1). Six step-heating "^^Ar/^^Ar analyses on hornblende, muscovite and biotite samples from medium-grade metamorphic rocks in the western part of the New Guinea Mobile Belt yielded Middle to Early Miocene ages of -12-21 Ma with a mean of -17 Ma. All of the samples show disturbed spectra, which may indicate the presence of excess argon. The older apparent ages recorded from conventional K-Ar may be explained by excess contamination. The ages are consistent with Early Miocene zircon fission track ages in the Mobile Belt due to rapid exhumation and cooling. The new data imply that the major period of metamorphic cooling was at the end of the Early Miocene and was closely followed by the formation of the Maramuni Arc in the Middle Miocene. Apatite fission track ages from northern Papua New Guinea show that the area underwent rapid cooling below ~100°C due to uplift and denudation in the Late Miocene, mainly between 7-10 Ma in the east and slightly later in the west at 5-8 Ma. The culmination of the Papuan New Guinea Orogen occurred with uplift and denudation of the Papuan Fold Belt from 5-2 Ma. The thermochronological data has lead to a more constrained tectonic model for Papua New Guinea, summarised below and in the Table below. The model integrates the latest plate reconstructions produced for the SW Pacific (Hall, 1997). The Ontong Java Plateau juxtaposed a Paleogene arc along the southern margin of the Caroline Plate against Papua New Guinea in the Early Miocene, coeval with locking up of the west-dipping Solomon's subduction zone. These events initiated wrenching along the northern margin of Papua New Guinea and increased westward subduction of the Solomon Sea Plate beneath the eastern margin. The New Guinea Mobile Belt underwent extension above the downgoing slab with rapid cooling of metamorphic rocks at -17 Ma, immediately prior to emplacement of the Maramuni Arc from -17-12 Ma. A change in plate motion at -12-10 Ma terminated the arc and caused Papua New Guinea-Caroline plate convergence, creating the orogenic belt in New Guinea from 12-4 Ma. This resulted in -2-4 km of uplift, denudation and cooling of the entire Mobile Belt from -10-7 Ma, propagating westwards along the Mobile Belt at 8-5 Ma and southwards in the Fold Belt at 5-4 Ma. A further change in plate motion at 4-3 Ma returned the margin to transpression with local compression along strike-slip faults and ongoing collision of the Finisterre Arc terrane. The thermal histories demonstrate that arc-continent collision spanned 25 m.y. and is ongoing. It involved arc accretion, wrenching, extension, - 10 m.y. of oblique subduction beneath the continental margin generating a volcanic arc, - 8 m.y. of folding and thrusting, then transpression with ongoing convergence of the Finisterre Arc.

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ACKNOWLEDGEMENTS This abstract is published with the approval of the Director of the Austrahan Geodynamics Cooperative Research Centre. Neutron irradiations for fission track studies are supported by a grant from the Australian Institute of Nuclear Science and Engineering. The project was supported by BP Australia, Highlands Gold, LL&E and Mobil with logistical support from the GSPNG. EOCENE

PALEOCENE|

OLIGOCENE

PLIOCENE

MIOCENE

OMa

Fossils, pre and post mclamorphism Metamorphism in New Guinea Mobile Belt

Cooling age Bcna Bena gneiss, K/Ar on muscovite

Minimum age of metamorphism of Bena Bcna gneiss, Rb/Sr on muscovite

Whole rock, muse, bt and hbl Rb/Sr (14) and K/Ar (5) cooling ages on Ambunti and Kaindi met rocks

Two K/Ar ages on PUB granulile Met. rocks unconformably overlain by Eocene sediments

Age Constraints

Early Miocene subsidence and sedimentation (1-3 km) on the New Guinea margin

Finisterre - Adelbert arc volcanism, K/Ar

Maramuni Arc volcanism

Opening of N. PNG marginal basins, eg Marum Ophiolite

Deformation in the Mobile Belt ofNWPNG

Opening of the Coral Sea Basin

Metamorphism in New Guinea Mobile Belt

Caroline Plate spreading

Deformation in the Papuan Fold Belt

Uplift and cooling of metamorphic rocks

Metamorphic rocks stay buried Finisterre - Adelbert arc volcanism

Summary of tectonic events

Deformation in Papuan Fold Belt

Subsidence of PNG margin

Maramuni Arc volcanism in New Guinea Mobile Belt

EARLY MIOCENE

MID-MIOCENE

LATE MIOCENE

PLIOCENE • OMa

PREVIOUS WORK

Initial Finisterre collision

Previous orogeny

New Britain-Finisterre convergence caused compression throughout NG

Finisterre Range collision with NG

Finisterre collision

a z

•s

Extension in north NG, forming basins & metamorphic core complexes Arc collision

Thrusting -)rapid uplift

Ramu Basin inversion

Finisterre collision NG - Caroline Plate convergence causing orogeny

Wrenching along the NG margin

THIS PAPER Arc accretion

Wrenching along the NG margin Rapid tectonic denudation and cooling of metamorphic core complexes above subducting slab

Rapid oblique convergence between the Caroline Plate and NG causing orogeny Rapid uplift, denudation and cooling in the Mobile Belt

Subduction to the west beneath PNG margin building the onshore Maramuni Arc

Wrenchmg along the NG margm

Continued Finisterre convergence

Rapid uplift, denudation & Mobile Belt cooling in the Fold Belt pop-ups

Figure 1: Timing constraints on tectonic events in Papua New Guinea. REFERENCE Hall, R. 1997. Cenozoic tectonics of SE Asia and Australasia. In, Petroleum Systems of S.E. Asia and Australasia. Indonesian Petroleum Association conference, Jakarta, May 1997, p. 47-62.

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KINEMATIC VORTICITY AND TECTONIC SIGNIFICANCE OF THE ANITA SHEAR ZONE, FIORDLAND, NEW ZEALAND Nathan R. Daczko^ Keith A. Klepeis\ Geoffrey L. Clarke^ and Richard W. White'. ^School of Geosciences, Division of Geology and Geophysics, Building F05, University of Sydney, NSW, 2006, Australia, e-mail: nathan@es.su.oz.au 'Department of Geology, University of Melbourne, Parkville, Victoria, 3025, Australia. Interpreting the significance of superposed fabrics in ductile shear zones is an important tool for unravelling the evolution of multiply deformed regions. Using structural, metamorphic, finite strain and kinematic vorticity data, we examine the evolution and significance of superposed fabrics in the Anita Shear Zone located in northern Fiordland, New Zealand. This shear zone is 3.5-4 km-wide and comprises superposed foliations displaying mylonitic to cataclastic textures reflecting conditions of upper amphibolite to greenschist facies metamorphism. Shear zone structure is dominated by a steep, NNE-trending, mylonitic foliation that is superimposed on an older gendy dipping mylonitic fabric. Excellent exposure of structural relationships along two fiords that transect the Anita Shear Zone and its wall rocks allowed us to examine the kinematic development of superposed foliations in the shear zone and assess their overall significance using a variety of different techniques. Outcrop-scale variations in strain intensity allowed us to employ finite strain and kinematic analyses to areas affected by different degrees of deformation in identical hthologies. This enabled us to directiy compare different data sets without the complicating effects of changing physical properties between different lithologies and protolith composition. Clear crosscutting relationships, well-defined shear zone boundaries, and the spatial variations in strain intensity also allowed us to reconstruct the progressive development of different stages of deformation. Figure 1 shows foliation trajectories and outcrop-scale variations in finite strain intensity in an 80 to 85 metre wide outcrop within the Anita Shear Zone. This outcrop is composed of a relatively homogeneous metapsammite unit that contains well-preserved microstructures and rotated porphyroclast systems that are Strain Domains on Outcrop Scale High Interm.

High

Interm.

Low

Interm.

High

especially well suited for kinematic vorticity analyses. Figure 1: NW-SE cross-section showing foliation trajectories in an outcrop within the shear zone. The central regions of the shear zone contain a gently east- and southeast-dipping mylonitic foliation (Sj) and down dip, gently southeast-plunging mineral lineation (Lj). Within the shear zone, Lj-S^ is variably overprinted and transposed by a subvertical mylonitic

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foliation (S2) that contains subhorizontal, NE-plunging mineral lineations (L2). Low strain domains are dominated by Lj-Sj, which is weakly folded by upright to inclined, open folds (F2). The L2-S2 fabric dominates high strain domains. In these latter areas, L^-Sj is folded by upright, tight to isoclinal and rootless folds (F2). Intermediate strain domains show an intermediate degree of fold development where refolded folds and overprinting relationships are visible. The Lj-Sj fabric in low strain domains is defined by the metamorphic mineral assemblage garnet, biotite, plagioclase, quartz, amphibole, clinozoisite and rutile. Mineral chemistry data and thermobarometric analyses indicate this assemblage formed under lower crustal conditions equilibrating at 11.9±1.1 kbars and 581±34°C. L2-S2 in defined by the metamorphic mineral assemblage garnet, plagioclase, quartz, amphibole, biotite, clinozoisite and titanite. This mineral assemblage formed under mid-crustal conditions equilibrating at 8.7±1.2 kbars and 587±42T. We employed the methods of Lisle (1985), De Paor (1988) and Simpson & De Paor (1993) to facilitate a quantitative comparison of finite strain data in low and high strain domains using deformed cobbles in a conglomerate layer and deformed porphyroclasts in a metapsammite unit. Our results show that the best-fit three-dimensional strain ellipsoids for low and high strain domains are oriented at high angles to each other with X-axes that lie 70° apart, Y-axes that lie 20° apart and Z-axes that lie 107° apart. These results indicate that the stretching lineations parallel the X-axes of the strain ellipsoids in each domain respectively. In addition, the XY-plane of finite strain in high strain zones parallels the steep foliation and the boundaries of the shear zone. The results of our strain analyses indicate that deformation in both strain domains deviated only slightly from plane strain assuming minimal volume loss. Systems of rotated porphyroclasts with asymmetric tails composed of recrystallised matrix material were studied from identical lithologies in high and low strain domains to compare the kinematics of ductile flow that produced the superposed L^-Sj and L2-S2 fabrics. Over 40 grains were measured and distinguished on the basis of aspect ratio, orientation, rotation sense and type and geometry of asymmetric tail (a, 5, complex). The results of our analyses indicate that the L^-Sj fabric was produced by simple shear dominated deformation. Estimates of the kinematic vorticity number of ductile flow yielded Wj, = 0.84. The L2-S2 fabric was produced by pure dominated deformation with a kinematic vorticity number of ductile flow ofWk = 0.54. The results of our studies indicate that the superposed mylonitic fabrics (L^-S^ and L2S2) of the Anita Shear Zone were produced by kinematically distinctive ductile flow regimes, at different crustal depths and at different metamorphic conditions. We interpret deformation that produced L^-Sj as involving ductile normal faulting and crustal thinning leading to decompression and exhumation of lower crustal rocks. On the basis of a comparison with regional relationships and limited geochronological work, we suggest this phase of deformation occurred during the Cretaceous rifting of ancestral New Zealand from Australia. L2-S2 was produced by a mid-crustal, dextral transpressional event that may have resulted from late Mesozoic or Cainozoic oblique convergence. Late Tertiary brittie strike-slip faulting associated with the Australian-Pacific transform plate boundary reactivated the L2-S2 fabric under upper crustal greenschist facies conditions.

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De Paor, D.G. 1988. R/(t)f strain analysis using an orientation net. Journal of Structural Geology 10, 323-333. Lisle, R.J. 1985. Geological strain analysis: A manual for the /?/([) technique, Pergamon, Oxford. Simpson, C. & De Paor, D.G. 1993. Strain & kinematic analysis in general shear zones. Journal of Structural Geology 15, 1-20.

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THE CAUSES OF POLYPHASE DEFORMATION IN PLATE BOUNDARY ZONES JOHN F. DEWEY DEPT. EARTH SCIENCES, UNIVERSITY OF OXFORD, PARKS ROAD, OXFORD 0X13PR, ENGLAND, UK

Relative plate motion is converted, rarely homogeneously, into displacements, rotations and strains in continental plate boundary zones. Commonly, this occurs over large areas in contrast to oceanic plate boundary zones, and is partitioned, relayed and transferred both horizontally and vertically in complicated ways. Successive phases of deformation in a particular rock mass may have occurred at the same, progressively-changing or at substantially different PTt conditions, which helps to distinguish between kinematic continua and discontinua. Bulk biaxial (plane) and triaxial (in transtension and transpression) strains are common whereas uniaxial positive and negative strains are less common; symmetrically, monoclinic (simple shear or homogeneous transtension and transpression) and triclinic (inhomogeneous transpression and transtension) bulk strains are common, orthorhombic (orthogonal shortening and extension; pure shear) less common, tetragonal scarce and cubic (pure volume change) rare; few bulk strains do not include a vorticity component. There may be complex space and time relationships between plate-driven and buoyancy strains at a wide variety of scales from whole plate boundary zones to individual diapirs. Against these bulk strain backgrounds, there are many causes of polyphase deformation, some of which are: • Changing patterns of partitioning in a deforming zone. 12) Rotation of faults, veins and other high strain zones through changing instantaneous strain fields, particularly common in transpressional and transtensional zones with a large vorticity component (e.g. successive waves of rotated en echelon tension gashes. 13) Rock masses changing their position with respect to unstable plate triple junctions. 14) Rock masses changing their position along the seven kinds of transform faults. 15) Rock masses changing their position and rheology within part of a growing plate boundary zone such as a subduction accretion prism or other thrust complex. 16) Rock masses changing their rheology (dewatering, PTt) that can generate patterns of strain hardening and softening and strength changes (eg. albite/quartz-jadeite transition and strain rate). 17) The natural progression of bulk strains resulting from the changing relationship between plate boundary and buoyancy forces. 18) Progressive changes in slip vector at plate boundaries that must occur in mosaics consisting of more than two plates on a sphere. 19) Changes in relative plate motion that may result from plate boundary reorganization such as those related to major continental collision. 20) Changes in slip rate and high strain zone position orientation and partitioning that commonly occur in continental triple junctions such as the Maras/Adana junction between Anatolia, Arabia and Africa in southern Turkey.

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In natural plate boundary zones, it is likely that various combinations of these nine mechanisms occur to develop familiar polyphase deformation sequences.

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PALAEOPROTEROZOIC OROGENIES IN NORTHERN AUSTRALIA AND THEIR TECTONIC IMPLICATION Puquan Ding Normandy NFM Limited, 25 Greenhill Road, Wayville, SA 5034, Australia Orogeny Archaean gneiss and granite have been recognised in several places as the basement of Proterozoic orogenic provinces in northern Australia, where frequent major sequential orogenies have also been revealed from the Paleoproterozoic orogenic provinces. Each orogeny is represented by an angular unconformity that separates a new Group of stratigraphic sequence from the previously deformed basement (Table 1). The stratigraphic sequence of each orogenic cycle indicates a new orogen. The Granites - Tanami, the Halls Creek and the Pine Creek regions recorded only the five earlier orogenies, whereas the Arunta region appears to have been influenced by all the known orogenies listed in the Table 1. The Tennant Creek and the Mount Isa regions were obviously involved in the later orogenies but their early histories have not been fully recognised. Each younger orogeny caused ductile deformation in its direct basement and probably caused brittle to semi-brittle deformation in its adjacent area. Table 1. TYPE OROGENS AND STRATA OF EACH OROGE;NIC CYCLE CYCLE TYPE OROGEN ORIENTATION cycle 10 1700-1600 Mount Isa Orogen (Mount Isa Group) N-S ma cycle 9 -1740- ? Southern Arunta Orogen (? Group) E-W 1700 ma cycle 8 -1760- Harts Range Orogen (Harts Range Group) WNW 1740 ma cycle 7 -1780- Tomkinson Creek Orogen (Tomkinson Creek NE 1760 ma Group) cycle 6 -1810- Hatches Creek Orogen (Hatches Creek Group) WNW 1780 ma cycle 5 -1830- Birthday Creek Orogen (Birthday Creek NNW 1810ma Group: Mount Winnecke Formation and Supplejack Down Sandstone) cycle 4 -1855- Ord River Orogen (Whitewater Volcanics and E-W and ENE Panton River Formation) 1830 ma cycle 3 -1900- Lander River Orogen (Lander Rock beds), NE 1855 ma Halls Creek Orogen (Halls Creek Group) cycle 2 -1980- West Kimberly Orogen (Ding Dong Downs WNW 1900 ma Volcanics), Napperby Orogen (Woodford River Group) cycle 1 >2100- Tanami Orogen (Tanami Group) NE 1980 ma Pine Hill Orogen (Yundurbulu Range Group) Archaean basement (Billabong Complex) After each orogeny, part of the fold belt was subjected to uplifting and erosion, which often exposes part of the metamorphosed roots of the orogenic province at the surface. The eroded material must have been deposited in its adjacent basins, the orogens of a new cycle, and caused unconformity. The rocks below the unconformity would have subsided again to a deep

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position, while the sediments accumulated within the basin above the unconformity. After the following orogeny the rocks below the unconformity would have recorded one more deformational event than those above the unconformity. The cycle 1 Tanami Group recorded ductile deformations of D1 to D5, while the cycle 5 Mount Winnecke Formation recorded only one ductile deformation. Table 2 shows the correlation of deformations recorded in different orogenic cycles. Table 2. CORRELATION OF DEFORMATIONS RECORDED IN EACH TECTONIC CYCLE Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 C1D5 C2D4 C3D3 C4D2 C5D1 C1D4 C2D3 C3D2 C4D1 C2D2 C1D3 C3D1 C1D2 C2D1 ClDl Note: C - cycle; D - deformation Lineament Tectonism It is clear that the orogenies summarised above should not be interpreted as the results of subduction of the oceanic crust or collision between two continental plates. A revised model, the Lineament Tectonism is presented here. (1) At the beginning of each orogenic cycle, heat induced ascending plumes within the asthenosphere caused lateral elongation and vertical thinning of the lithosphere directly above the mantle plumes. The mechanism of the strain within the thinned upper mantle lithosphere is not really known, but the strain within the thinned continental crust can be interpreted as the result of ductile flow, detachment along very low angle shear zones at lower levels, or normal fault movement at higher levels. The extensional direction is usually at high angle to one or two of the geophysical lineaments. (2) Sediments were consequently deposited into the subsiding basins above the thinned lithosphere. (3) After a certain period of heat loss, the thermal plume within the asthenosphere weakened and the strained lithosphere would be restored to its original dimensions. (4) The restoration of the strained lithosphere means the orogeny within the continental crust, and, more apparently, within the sedimentary basins. The trend of folds usually parallel one or two of the geophysical lineaments. (5). Each orogeny induced simultaneous uplifting and erosion. The eroded material from the upward moving orogenic domain will be transported to other subsiding basins. (6). Later when the weakened thermal plume resumed or a new ascending thermal plume commenced , probably at a different site, a new subsiding basin would be formed over the thermal plume, possibly along a different orientation. (7) The new basin would usually be terminated by an orogeny and form a new orogenic domain. The rocks below the unconformity would have recorded one more deformational event than those above the unconformity. (8) When the thermal plume stopped or reduced to certain degree, the orogenic provinces were finally cratonised.

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RETHINKING THE PALAEOPROTEROZOIC GEOLOGICAL HISTORY OF BROKEN HILL, NSW, AUSTRALIA: SPECULATION TOWARDS A SYNTHESIS OF STRUCTURE, GEOCHRONOLOGY AND LITHOLOGICAL MAPPING. Tony Donaghy^ George Gibson^, Allen Nutman^ Mike Hall^ and David Maidment^ ^Australian Geodynamics Co-operative Research Centre, VIEPS Department of Earth Sciences, Monash University, Clayton VIC 3168 Australia. Email STdonag@earth.monash.edu.au ^Australian Geological Survey Organisation, Canberra ACT 2601 Australia. ^Research School of Earth Sciences, Australian National University, Canberra ACT 0200 Australia. The Palaeoproterozoic of Broken Hill is one of the premier polydeformed, high-metamorphic grade terranes in Australia and has been the subject of intense scrutiny and debate by geologists for over 100 years, largely because the region hosts the worlds' largest Pb-Zn-Ag ore-body. Although the lithologies of the Broken Hill region have been mapped at a highlydetailed scale by the Geological Survey of NSW, comparatively little is understood regarding the regional tectonothermal history and resulting structural geometry. Some of the key issues that need to be resolved are; (i) Does the Palaeoproterozoic Willyama Supergroup represent a continuous sequence or are there depositional and/or tectonic breaks in the package? (ii) What is the significance of synchronous ductile deformation, anatexis and intrusion at the base of the sequence with volcanosedimentary deposition at the top? (iii) What is the significance of the 1590-1600 Ma thermal event recorded in zircon and monazite SHRIMP U/Pb geochronology? Are we dating D2 or D3? (iv) What, if anything, do these factors mean for orebody formation? Recent research activities by the Broken Hill Exploration Initiative and the Australian Geodynamics CRC are intended to fill those gaps in our knowledge. This has created a wealth of structural and geochronological data to complement the existing lithological maps. The synthesis of those data with the mapped lithologies, the first speculative steps of which are reported here, are forcing us to rethink many of our preconceptions regarding the tectonic history, regional stratigraphy, palaeoenvironmental reconstructions and models for ore-body formation. The primary preconception of the local geology in Broken Hill that is being questioned is that the regional parallelism of outcrop strike is the result of conformable depositional contacts in a continuous supracrustal sequence at 1690 Ma. From this basic assumption has flowed a compounding series of assumptions and interpretations; starting with erecting a highly detailed regional stratigraphy, then using this as the basis for the regional structural interpretation assuming all repetitions of lithological packages are fold related, leading to palaeoenvironmental and tectonic reconstructions and finally orebody formation models. An example of this is the conspicuous, parallel-outcropping 1690 Ma meta-igneous mafic and quartzofeldspathic gneisses, which are more resistant to weathering in comparison to the surrounding pelitic metasediments. Consequendy, they, or their absence, were used extensively as supracrustal stratigraphic and structural markers with significant consequences for tectonic and palaeoenvironmental reconstructions.

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However, detailed structural mapping reveals that boundary relationships between the metaigneous suites and surrounding metasediments are generally ambiguous or intrusive. Also, indicators of high-strain such as boudinage, mylonitic fabrics, transposition, isoclinal folding and extreme attenuation are all common aspects of these outcrops and strongly suggest that the parallelism of outcrop is a tectonic rather than stratigraphic feature. Whether these are a result of syntectonic intrusion or subsequent deformation is an issue that remains to be resolved. The fact that these intrusions are completely lacking from units higher than the upper Broken Hill Group suggests some fundamental discontinuity at this point, either tectonic or disconformable. This is supported by detrital zirconology of metasediments where 1690 Ma detrital zircons first appear in the Sundown Group, overlying the Broken Hill Group, and --1640 Ma grains first appear in the Paragon Group, overlying the Sundown Group. The current model under evaluation is that the mafic and granite intrusions are syntectonic during a high-metamorphic grade event. This at first appears to be in direct conflict with a supracrustal sequence deposited at 1690 Ma. However, invoking a ductile extensional regime at the time of intrusion helps in the broad scale to explain many of the apparent "inconsistencies" between the stratigraphic interpretation and mapped structure. Early extensional structures during basin formation and sequence deposition were previously inherently assumed to be brittle in nature and to have affected the basement alone. The possibiUty of ductile extensional tectonics very early in the basin formation history has been largely ignored. Extensional tectonics allows; (i) elevated geotherms, raising the anatectic point to closer to the palaeosurface thus partially melting, intruding and imparting a ductile fabric to the lower portion of the sequence (Thackaringa Group) while depositing the Hores Gneiss and Sundown Group at the top of the sequence; (ii) volume creation and tectonic control on the intrusion of voluminous granite and mafic bodies that have resulted from partial melting of basement by an elevated geotherm; and (iii) removes the need for highangle unconformities between the different stratigraphic Groups that a pre-syn 1690 Ma compressional regime would require. The boundaries between the different groups could be subtle, shallow angle disconformities that would be difficult to recognise through subsequent high-strain events. Another contentious issue is the age of deformation events. The ubiquitous presence of 15901600 Ma metamorphic zircons and monazites has always been assumed to date the peak (D2) granulite metamorphic event. However, this dating is indirect and has never been conclusively linked to peak granulite deformation. Although D3 is post-peak granulite metamorphism, it is still a high-grade metamorphic event at upper amphibolite facies. Hence another, equally valid, interpretation is that the 1590-1600 Ma event dates D3 deformation and D2 lies much eariier in the deformation history, possibly pre-syn 1640-1660 Ma. This is supported by recent -1580-1590 Ma ages of "Mundi-Mundi"-style granite bodies that intrude syn-post S3 fabric formation. Particularly important in this reinterpretation are the D2 and D3 mylonite zones much of the microstructural fabric development in Broken Hill during the D2 and D3 events can best be described as submylonitic to ultramylonite. Previously neither the importance of the mylonites nor the regional isoclinal F3 folds had been recognised. This has resulted in widespread structural repetition, rotation and reorientation of lithological packages previously erected as separate stratigraphic markers. We also speculate that many of the key marker lithologies of the lode horizon, such as quartz-gahnites and garnet quartzites, as well as several of the ore horizons themselves, are structurally controlled in high-strain S3 fabrics.

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SHEAR ZONES FROM BROKEN HILL, AUSTRALIA: CONDITIONS OF FORMATION AND THE ROLE OF FLUIDS

DEFORMATION,

CHRIS DOYLE & IAN CARTWRIGHT Department of Earth Sciences, Monash University, Clayton, Vic 3168 email: ST-doyle@artemis.earth.monash.edu.au

Shear zones are structures which allow us to understand many aspects of the earth's geological history. They often form after the peak of regional metamorphism and are important in overall exhumation of the terrain. In addition, these structures often provide ideal sites for the channelling of fluid through the middle crust. The study of shear zones is especially important in ascertaining the conditions of regional retrograde metamorphism as retrograde activity is often focussed into these areas. In many terrains this may be the only record of events occurring after the peak of metamorphism. Much of the retrograde metamorphism and alteration which are characteristic of many shear zones can be attributed to the influx of a metasomatising fluid. The resultant changes in bulk chemistry and mineralogy allow the study of fluid/rock interaction and the intrinsic role that fluid plays in metamorphism and deformational processes. This study within the Broken Hill area. New South Wales has established the pressures and temperatures of formation, geochemical changes, and deformation for three distinct retrograde shears. In addition to providing a record of retrograde tectonic events for the area, an insight into the composition, sources, and volume of fluid present during retrograde activity in the Broken Hill region is also achieved. Mapping and sampling was conducted across three separate shear zones: the Stevens Creek Shear (approximately 6 km north of Broken Hill); the Thackaringa Pinnacles Schist Zone (approximately 35 km south-west of Broken Hill); and the Rockwell Shear (approximately 10 km south-east of Broken Hill). These shear zones cut granuHte facies meta-peUtic gneisses belonging to the Sundown Group (an upper stratigraphic unit of the Willyama Supergroup in the Broken Hill Block). All three shears contain amphibolite - greenschist facies retrograde micaceous schists and clearly show geochemical and mineralogical variation across strike. The high grade meta-pelites of the Sundown Group underwent at least two phases of deformation (D1 & D2) during prograde metamorphism (at approximately 1600 Ma; Page & Laing, 1992), which resulted in the formation of a regionally extensive NE trending planar fabric. This high grade schistosity is either overprinted or completely obliterated by the shear zone foliation within the shear zones. Formation of the retrograde shear zone fabric probably occurred at -1580 Ma during lower to middle amphibolite facies conditions and was contemporaneous with development of a regional retrograde fabric (D3), which is detailed in Marjoribanks et al.(1980). Shear zone fabrics are defined by an intense, steeply dipping foliation containing an abundance of aligned muscovite and chlorite, whilst the preserved (also steeply dipping), regional fabric in the meta-pelitic gneisses is defined by an alignment of biotite and sillimanite. Field observations and measured mineral lineations on aligned muscovite and

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relict sillimanite from within the shear zone suggest that shearing was sinistral, and dominantly dip-slip. This differs markedly from measured mineral lineations on aligned sillimanite and biotite from the gneissic host rock, suggesting a regime in which movement was predominandy strike-slip. Shear zone rocks are typically composed of: quartz-muscovite-chlorite-gamet-staurolite ± biotite ± plagioclase ± K-feldspar ± chloritoid, whilst their gneissic hosts contain: biotitesillimanite-quartz ± plagioclase ± garnet ± muscovite ± K-feldspar. This represents a transformation from a relatively "dry," high grade assemblage to one which is more hydrous and lower in grade. It is inferred that this retrogressive transformation is largely the result of infiltration by a metasomatising fluid. Evidence for fluid infiltration is provided by the degree of retrogression and metasomatism that has occurred within the sheared rocks, as well as the presence of numerous quartz veins. Notable modal increases in hydrous minerals (namely chlorite and muscovite), accompanied by a substantial increase in quartz towards the centre of the shear zone offer further evidence that fluid infiltration has played a significant role in the retrograde activity for shear zones in the Broken Hill area. Mineral compositions in the sheared rocks were used to estimate the pressure and temperature of shear zone formation for the Stevens Creek area. Using Thermocalc (Powell & Holland, 1989) average pressures and temperatures of 6.6 ± 1.3 kbars and 526 ± 40® C respectively, were calculated. These agree reasonably well with established pressures and temperatures of 5-5.5 kbars and 550-600® C (Corbett & Phillips, 1981), for retrograde shear zone formation (southern areas), and the onset of D3 regionally. Geochemical studies, employing the use of Gresens analysis to calculate metasomatic changes, show that metasomatism enriched the Stevens Creek Shear Zone in Si, Na, Fe, Mg and Ca, whilst sHghtly depleting it in K. These results are consistent with petrographic observations showing the crystalhsation of abundant quartz and muscovite, along with Ferich, K-poor components such as chlorite and staurolite. Following the methods outlined in Dipple and Ferry (1992), calculated time integrated fluid fluxes based upon silica addition are in the order of 1 x 10^ mVm^ to 7 xlO^ mVml Fluid fluxes of this magnitude are high enough to produce substantial quartz veining. This is evidenced in the field. A possible source for fluids is the large (10-20m width) pegmatites containing quartzplagioclase-K-feldspar ± muscovite. Although exact timing of their emplacement is still somewhat ambiguous, field observation suggests that this may have been synchronous to regional D3. This study has shown that shear zones at Broken hill were the sites of deformation, recrystallisation and fluid infiltration following granulite facies metamorphism. These processes would have been mutually re-inforcing, with recrystallisation enhancing permeabilities and thus allowing easier migration of fluids. These shear zones were probably active in the exhumation of the terrain and potentially provide a record of most of the tectonic activity at that stage.

REFERENCES Corbett, G.J., Phillips, G.N., 1981. Regional Retrograde Metamorphism of a High Grade

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Terrain; The Willyama Complex, Broken Hill, Australia. Lithos, 14, 59-73. Dipple, G.M., Ferry, J.M., 11992. Metasomatism and Fluid Flow in Ductile Fault Zones. Contrib. Min. Pet., 112, 149-164. Marjoribanks, R.W., Rutland, R.W.R., Glen, R.A., Laing, W.P., 1980. The Structure and Tectonic Evolution of the Broken Hill Region, Australia. Precambr. Res., 13, 209240. Page, R.W., Laing, W.P., 1992. Felsic Metavolcanic Rocks Related to the Broken Hill Pb-Zn-Ag Orebody, Austraha; Geology, Depositional Age, and Timing of High-Grade Metamorphism. Economic Geol, 87, 2138-2168.

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PLATYPLUS PROJECT : A 3D RECONSTRUCTION TOOL Cecile Duboz, G.S. Lister and Mark Jessell. AGCRC, Department of Earth Sciences, Monash University, VIC 3168, Clayton, Australia. cecile@mail.earth.monash.edu.au

Introduction The aim of this project is to develop a 3D reconstruction package allowing rigid plate motion as well as internal deformation to be tested. To do so, the typical approach (one plate considered as one object and able to undergo displacements defined by poles of rotation) has been replaced by a new approach based on points. The concept we opted for can be described as defining one object as one set of points, where each object is hierarchically linked to the others.

Methods The first step consists in generating a set of points over the surface of a spherical Earth. From geological or geophysical datasets, we can attribute properties to these points (e.g., topograpy, magnetics....).

Figure 1 : Attribution of the data to the points.

Regions of interest (corresponding to the objects we want to deal with) are defined by a boundary and all the points falling inside it. These objects are hierarchically organized in a way specified by the user, allowing one to define sub-regions of an object. The motions are applied either by just grabbing a selected object and moving it with mouse displacements or by reading a motion file. The motions are computed over the surface of a sphere and then projected back into the currently used projection to map the dataset.

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Results The initial experiment allow us to perform global scale interactive reconstructions (e.g., breakup of Gondwana) and provide animations with a view angle (Figure 2).

Figure 2 : Reconstruction of Gondwana (a) present time and (b) 200 Ma with a view angle

Next developements The first step will consist of using the PlatyPlus tool to visualize some already existing reconstructions in order to test the package and improve it. The second, and most important step will be the implementation of internal deformation. Because of its algorithm structure based on points, it allows the simulation of tectonic deformationl. A first way to do it would be to consider the crustal thickness as property of the points. Realising a "stretching" of the continental crust would then correspond to a decrease of the points density. Internal deformation would consist of allowing some point-point interactions, where the distance between points becomes a mirror of a stretching or compressing operation.

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CONCEPTS OF CLEAVAGE DEVELOPMENT D.W. Durney Department of Geology and Geophysics, Macquarie University, NSW 2109 email: ddumey@atlas.es.mq.edu.au The phenomenon of cleavage in deformed terrestrial rocks is, so far as known, unique in the physical world and has proved difficult both to reproduce in the laboratory and to gain a consensus as to its mode of formation. This presentation attempts to review concepts of morphology and development of cleavages, pointing out problems that require attention and suggesting solutions consistent with thermodynamic and kinetic theory of mass transfer. FABRIC TYPES The six fundamental cleavage fabric types proposed by Dumey & Kisch (AGSO Journal, 1994) will be used as a basis for considering cleavage development concepts below. The three basic types of penetrative cleavage fabric are: grain or schist type, clay type (both nondomainal) and slaty type (domainal). The three non-penetrative types are: spaced type, crenulation type and scaly type. The grain, slaty, spaced and crenulation cleavage fabric types may be concentrated in mesoscopic, differentiated "zones" , indicating local mass transfer at that scale as well as at the microscale, whereas the clay and scaly types do not display differentiation. These fabric types should be distinguished from "cleavage fissility" (oriented curviplanar cracks guided by the internal fabric, previously called " fracture cleavage" ), which is an effect of weathering and not an element or type of cleavage itself. Fissility can be useful as an index for cleavage intensity. OLDER MECHANISTIC CONCEPTS The older concepts of cleavage formation (mechanical rotation, mica crystallization, grainflattening, shear, fracturing) have either limited applicability or, in some cases, are inappropriate for explaining observed micro- and mesostructure of the differentiated cleavage fabric types. Rotation and shear are adequate only for the clay and scaly types, respectively. NEWER CONCEPTS INVOLVING MASS TRANSFER More recent concepts and models involving microscale mass transfer include: pressuresolution, stress-solution, shear solution, volume loss, microfracturing/dilatancy, nonassociated flow, Ortoleva-Merino fabric evolution, Carrio-Schaffhauser process zones, nonequilibrium theory and transient solution-transfer. Processes dependent on elastic or plastic strain energy and those that involve space generation at sites of dissolution do not appear to meet the requirements of identifiable source/sinks and energy dissipation consistent with imposed stresses.

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PERMEABILITY IMPLICATIONS Stress-driven mass transfer should constandy tend to eliminate open space (where thermodynamic potential will normally be lowest), especially in proximity to dissolution sites. Hence (a) the rock will tend to become tight, non-porous and impermeable, especially along cleavage domains, where dissolution is most active, and (b) continued action of the process will depend mainly on grain-boundary diffusion. These effects argue against large-scale fluid flow and major hydrothermal activity along cleavage planes. MECHANICAL BEHAVIOUR UNDER NON-COAXIAL FLOW The ability of cleavage-related fabrics to track components of progressive deformation, such as the XY plane, may depend on the scale. Grain-shape fabric may not track the full history if the grains divide or coalesce. Microscale domains, particularly in the slaty fabric type, may do so if cleavage domains relink during non-coaxial flow. In some cases this process may suppress development of multiple cleavages. Mesoscale cleavage zones are likely to behave more as material planes and thus not fully track a shifting XY plane.

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STRUCTURAL GEOLOGY AND TECTONIC EVOLUTION OF THE NORTHERN PART OF THE EASTERN GOLDFIELDS PROVINCE, YILGARN CRATON T.R. Farrell Geological Survey of Western Australia 100 Plain Street, East Perth WA 6004 t.farrell@dme.wa.gov.au The Eastern Goldfields Province is a complexly deformed and poorly exposed Archaean granite-greenstone terrane covering the eastern third of the Yilgam Craton, in Western Australia. Northern parts of the Province are characterised by large areas of monzogranite (c. 2680-2640 Ma), multiply deformed quartzofeldspathic gneiss (c. 2740-2700 Ma), and elongate, north to northwesterly trending greenstone belts (c. 2700-2630 Ma). Tectonic models for the development of the Province have been hampered by the lack of outcrop, and until recently, by a lack of detailed mapping, high quality aeromagnetic images, and geochronological data. Four phases of deformation (D1-D4) have been recognised in the region, on the basis of overprinting relationships of outcrop-scale structures, an interpretation of aeromagnetic images, and the correlation of structures between different parts of the area. First generation (Dj) structures have largely been overprinted during later events, but reUct Dj fabrics are preserved in gneisses, and possibly in parts of the greenstone sequence. The gneisses commonly contain a composite S-S2 fabric with remnant isochnal fold hooks of Sj leucosomes. There are rare, east-trending folds in some locations in the greenstone belts, and these may also be relict Dj structures. There may have been some strati graphic repetition of the greenstone sequences by low-angle thrusting, as argued for the Kalgoorlie-Kambalda area to the south (e.g. Martyn, 1987; Swager and Griffin, 1990), but such structures have not yet been documented in the northern part of the Province. A zircon date of 2697 Ma for a quartzofeldspathic gneiss on the western side of the Duketon greenstone belt (Nelson, 1997) constrains the timing of Dj to c. 2700 Ma or later. The second deformation event (D2) was associated with peak metamorphism and the onset of granitic magmatism. The effects of D2 are recognisable throughout the area, but are most pronounced in amphibolite facies, high-strain zones adjacent to granite-greenstone contacts. The characteristic D2 structure is a regionally extensive, steeply dipping, northnorthwesterly trending foliation (S2). However, second generation structures have been reoriented and overprinted during D3, and in many areas, the dominant foliation is probably a composite S2-S3 fabric. Metamorphosed layered cherts in high-grade zones within the Duketon and Yandal greenstone belts commonly contain a strong composite fabric (S0-S2) and numerous steeply plunging, tight to isoclinal, intrafolial folds (F2). The fold axes are typically subparallel to a prominent, combined intersection-mineral lineation (L2). The timing of granitoid magmatism relative to deformation has been inferred from the structural relationships of monzogranite intrusions in quartzofeldspathic gneisses. The gneisses typically contain F2 folds (of Sj leucosomes) that are cut by small monzogranite veins and pods with an S2 fabric. These veins are themselves cut by larger, weakly deformed c. 2660-2655 Ma monzogranite bodies, which are in turn cut by large bodies of undeformed monzogranite (generally c. 2660-2640 Ma; Nelson, 1997, 1998). These relationships suggest

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that granite magmatism commenced during D2, and probably peaked after the deformation ceased. The oldest monzogranites range up to c. 2685 Ma (e.g. monzogranite at Jones Creek; Nelson, 1997), indicating that D2 must have commenced at about this time. Taken together, these relationships constrain the age of D2 to c. 2685-2660 Ma. The second deformation event was followed by the uplift and erosion of monzogranite, and the deposition of granite-clast conglomerate in small, fault-bound basins (e.g. Jones Creek Conglomerate), marking a major change in the tectonics of the Eastern Goldfields Province. The Jones Creek Conglomerate contains zircons with a wide range of ages down to 2632±13 Ma (Nelson, 1997), defining a maximum age for the formation of these basins. The conglomerates are deformed in D3 shear zones, particularly along granite-greenstone contacts, indicating deposition prior to, or in the early stages of, D3. The last major penetrative deformation event (D3) was largely responsible for shaping the architecture of the greenstone belts. It resulted from strong east to northeast shortening, and led to the formation of large-scale, upright, shallow to moderate plunging folds (F3), and north to northwesterly trending shear zones (e.g. Perseverance, Ninnis, and Ockerburry Faults). It was a progressive deformation event during which the strain was increasingly partitioned into the developing shear zones. These shear zones are corridors of high strain, generally dominated by flattening but with locaUsed, intense constriction, within which preexisting structures are strongly overprinted by S3. Some of these zones also show a component of sinistral movement (e.g. De La Poer Fault), whereas others have S-C fabrics indicating dextral movement. Outcrop-scale F3 folds are typically upright, and open to tight, with shallow-plunging axes that are subparallel to a fine mineral lineation (L3). They refold D^ structures, and the L3 lineation overprints L2. In addition, many of the larger F3 folds, such as the Christmas Well anticline and the Lawlers anticline, are broadly symmetric and plunge to the north at about 20°. This suggests that S2 may have been gently dipping prior to D3. In high strain areas along F3 hinge zones (such as the central part of the Duketon greenstone belt), the overprinting of S2 by the axial planar foliation (S3) has resulted in the development of a prominent 'pencil cleavage' parallel to L3. A maximum age of c. 2640 Ma for D3 is indicated by two lines of evidence: a date of 2643±6 Ma for a monzogranite deformed along the Ninnis Fault (Mount MacDonald; Nelson, 1997), as well as the c. 2632 Ma date for the Jones Creek Conglomerate. The last recognised deformation event resulted in the formation of small-scale, variously oriented kink folds and crenulations. These structures are commonly associated with small-scale faults and quartz-filled tension gash arrays, and they occur in all rock types in the region. In some areas, there is a shallow-plunging crenulation Hneation parallel to L3 and the F3 fold axes. It is not clear whether this was coeval with L3, or if it represents a later structure formed during differential uplift of some parts of the granite-greenstone terrane. The timing of these structures is not clear, but they may be related to the formation of Early Proterozoic faults. References Martyn, J.E., 1987, Evidence for structural repetition in the greenstones of the Kalgoorlie district. Western Australia, Precambrian Research, 37, 1-18.

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Nelson, D.R., 1997, Compilation of SHRIMP U-Pb zircon geochronology data, 1996: Western Australia Geological Survey, Record 1997/2, 189p. Nelson, D.R., 1998, Compilation of SHRIMP U-Pb zircon geochronology data, 1997: Western Australia Geological Survey, Record 1998/2, 242p. Swager, C.P., & Griffin, T.J., 1990, An early thrust duplex in the Kalgoorlie-Kambalda greenstone belt, Goldfields Province, Western Australia, Precambrian Research, 48, 6373.

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MICROSTRUCTURAL ANALYSIS DIFFRACTION TECHNIQUE

USING

ELECTRON

BACKSCATTER

John D. Fitz Gerald^ Ulrich H. Faul, Shuqing Zhang, and Martin Cmiral (e-mail: John.Fitzgerald@ anu. edu.au) Research School of Earth Sciences The Australian National University Canberra, ACT 0200

Grain orientations of tectonites contain important information about deformation processes, rheological behaviour and strain-stress field as well as fluid distribution and transportation. The Electron Backscatter Diffraction (EBSD) technique provides an effective means for determining lattice orientations for individual grains or subgrains as small as 1-2 |im in size. The technique was initially developed for use in metals and ceramics and superseding the rocking beam/electron channelling method. In a scanning electron microscope, an EBSD pattern, with a spatial resolution of 1 |im or less, is formed when a stationary incident electron beam produces backscattered electrons that are Bragg diffracted. Good results are obtained from specimens tilted 70° to the incident electron beam. We use a working distance of 23 mm, an acceleration voltage of 20-25 kV and a probe current of 1 x lO'^ amps in a JEOL-6400 SEM with a tungsten filament.. The diffraction pattern is detected using a NORDIF system (a phosphor screen, a low light CCD camera, and a Hamamatsu image processor). The 'hkl' software system (Channel -f-) is used to detect bands and to provide solutions of lattice orientations for each EBSD pattern. The specimen surfaces were first polished with AI2O3 powder of 10-0.5 \im in size and finished by 0.05 |am colloidal silica on an vibration polisher. The specimen was then cleaned and coated with a thin layer of carbon. We have applied the EBSD technique to quartz, olivine, CaTiOs perovskite aggregates and olivine/basaltic melt system. We conducted microstructural analysis for experimentally deformed samples, ranging from determination of lattice orientations of finely recrystallized grains, inference of intracrystalline slip systems, to analysis of anisotropic melt distribution. (a) Determination of lattice preferred orientations for recrystallized olivine aggregates. The ultimate goal of this research has been to determine the relationship between the lattice preferred orientations and the plastic deformation geometry. A number of simple shear experiments were conducted under a confining pressure of 300 MPa and at temperatures of 1473K and 1573K (Zhang and Karato, 1995). Shear strains up to 150% were achieved. Measurements of lattice preferred orientations for large relic grains (>20|im) using a universal stage and light microscopy show that, at large strains, the orientations reflect flow geometry with [100] axes lying nearly parallel to the shear direction. Dynamic recrystallization with grain sizes of about 2-10 |im resulted from shear strains higher than 50%. Using EBSD technique, we determined that the orientations of these finely recrystallized grains have a bimodal pattern of [100] axes respectively parallel to the shear direction and perpendicular to the maximum principal compressive stress. Development of

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the stress-controlled orientations is closely associated with grain boundary migration processes during recrystallization and growth. (b) Influence of mica on deformation behaviour of quartzite. Quartz deformation fabrics are usually determined by bulk methods (such as X-ray diffraction) or by Hght microscope universal stage. While the former technique only yields information about the average fabric of an aggregate, the latter is restricted to grainsizes of - 5 0 |im or more. With EBSD fine grained mylonites can be examined and questions such as how the mica influences fabric development in quarzites addressed. Preliminary observations show variations in fabric strength and dominant orientation with varying mica content in a quartz mylonite from central Australia. (c) Inference of active slip systems. The slip systems active during deformation can be determined by measuring small misorientations (<5°) between subgrains, based on the assumption that lattice reorientation is controlled by crystal slip. If the total rotation angle across relic grains is larger than about 20°, slip plane and slip direction can be determined by identifying the common rotation axis. Using the subgrain orientations for a number of large relic grains in the sheared olivine aggregates, we determined the dominant slip systems to be (010)[100] and (100)[001]. (d) Influence of grain misorientations on the melt distribution in partially molten olivine aggregates. Bulk physical properties of partially molten rocks are substantially dependent on the melt inclusion geometry. In particular wetted two-grain boundaries exert a strong influence for example on permeability and deformation behaviour. In partially molten polycrystalline aggregates of olivine with mean grain sizes above 35 |im, both wetted and melt-free grain boundaries are observed after equilibration times at high pressures and temperatures of between 15 and 25 days . In order to asses a possible dependence of the wetting behaviour on the relative orientation of neighbouring grains, electron backscatter diffraction (EBSD), is used to determine grain orientations. From the (absolute) grain orientations relative orientations of neighbouring grains are calculated, which are expressed as misorientation axis/angle pairs. The distribution of misorientation angles and axes of melt-free grain boundaries differ significantly from a purely random distribution, whereas those of wetted grain boundaries are statistically indistinguishable from the random distribution. The relative orientation of two neighbouring grains therefore influences the character of their common grain boundary. However, no clustering towards special (coincident site lattice) misorientation axes is observed, with the inference that the energy differences between special and general misorientations are too small to lead to the development of preferred misorientations during grain growth.

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FLUID ESCAPE FROM MIGMATITES—THE IMPORTANCE OF RETROGRADE DEFORMATION LC.W. Fitzsimons Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6845, Western Australia. ianf@lithos.curtin.edu.au Hydrous fluids are effective means of heat and mass transfer in the crust, and the source of these fluids, the timing of their release, and the length scales over which they migrate have important implications for the thermal, chemical and rheological evolution of metamorphic belts. A high proportion of these fluids derive from the prograde breakdown of hydrous minerals, either by solid-state dehydration reactions or, at higher metamorphic grades, by dehydration melting reactions. Solid-state dehydration reactions are effectively irreversible, since the low viscosity and density of fluid coupled with the low porosity of the rock matrix means that the fluid is lost instantaneously (although they can be reversed by influx of fluids from elsewhere). In contrast, dehydration melting reactions do not release fluids immediately, but rather store these fluids as a dissolved component within the melt phase. Melting reactions are in principle reversible, since melt must crystallize on cooling, but in practice melt segregation means that the melt phase is isolated from the other products of the prograde melting reaction and thus cannot back-react to any great degree. Instead, the melt crystallizes as a closed system, causing a delayed burst of fluid release when it reaches water saturation. This retrograde volatile pulse will be focused both in time and space, and the behaviour of the released fluids will depend on a number of factors including the chemical and physical composition of the protolith, the metamorphic conditions, and in particular the degree of deformation during melt crystallization. This is illustrated by migmatites exposed in the Reynolds Range of central Austraha and Prydz Bay, East Antarctica. Both areas preserve evidence of partial melting in a pelitic protolith, but display quite different histories of melt crystallization, deformation and fluid release. The Pelite Unit of the Reynolds Range Group (Dirks & Wilson 1990; Dirks et al. 1991; Buick et al. 1998) comprises interbedded metapelite and quartzite with little pervasive deformation. Oblique shear bands are developed in the metapelite, but these rarely disrupt the competent quartzite layers. The quartzite has not melted, but metapelite has undergone up to 15-20% partial melting, producing a range of migmatitic textures from isolated leucocratic segregations and cordierite-quartz sweats (< 15 cm across), to diffuse veins along shear bands (1-5 cm across), discrete planar veins (5-50 cm across), and leucogneiss sheets (1-20 m across). This progression corresponds to increasing water contents in the melt. Cordierite in the sweats is relatively unaltered, with only limited retrogression in the adjacent gneiss. Cordierite in the discrete veins is commonly altered to fine-grained pinite, and a selvage of biotite-sillimanite, often of similar width to the vein itself, is developed within the adjacent metapeUte. This selvage reflects water release as the melt crystallized. Leucogneiss sheets are locally pegmatitic, contain rare pseudomorphs of cordierite completely altered to hydrous replacement minerals, and, unlike smaller quartz-dominated segregations, have sufficient feldspar to be minimum melts, implying that they are fluid-saturated melts extracted from the smaller veins. Mass balance calculations based on the width of retrograde selvages imply that 30-50% of the water extracted from the metapelites during melting was released back into

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these rocks as the melts crystallized. This is a minimum estimate as biotite-sillimanite-chlorite intergrowths dispersed throughout the metasediments also reflect water release from the melt bodies. The Brattstrand Paragneiss of Prydz Bay (Fitzsimons 1997; Fitzsimons et al. 1997) comprises interleaved metapelite, psammite and quartzite, with widespread evidence for pervasive deformation after peak metamorphism. Outcrop structure is dominated by a series of lowstrain zones preserving folded migmatites, separated by regions of intense ductile shear. The pelite has undergone 20-25% partial melting, with a textural progression from small quartz-cordierite leucosome veins (< 5 cm wide), to segregated leucogneiss sheets (1-20 m across) with significant feldspar contents and retrogressed cordierite. As in the Reynolds Range, the segregated leucogneiss sheets are interpreted as a hydrous melt component extracted from the smaller feeder veins. The ductile shear zones deform both the pelitic migmatites and segregated leucosome, indicating that at least the last stages of shearing postdated melt crystallization. Melt crystallization in the Brattstrand Paragneiss is associated with retrogression of peak assemblages to biotite-sillimanite-quartz, but, unlike the Reynolds Range, this retrogression is of very limited extent and is focused in the high-strain zones, which acted as conduits for fluids released from the crystallizing melts. There is very little evidence for retrogression outside of the high-strain zones, even adjacent to the larger leucogneiss sheets, and it seems likely that the bulk of the water dissolved in melt bodies (> 90%) was able to escape to higher crustal levels, thus allowing the widespread preservation of anhydrous granulite assemblages at depth. Pelitic migmatites in the Reynolds Range and Prydz Bay preserve similar peak mineral assemblages, but show marked differences in the degree of retrogression. Lower peak temperatures (800 rather than 860°C) and a more magnesian composition (bulk rock XMg = 0.45 rather than 0.30) mean that there was less melting in the Reynolds Range Group than in the Brattstrand Paragneiss, resulting in lower degrees of melt segregation. This could in part be responsible for the more efficient escape of fluids during melt crystallization in Prydz Bay since a higher degree of melt segregation will yield a more focused fluid release, but it seems likely that the major control was the retrograde deformation in Prydz Bay which provided pathways for fluid escape. Another important control on the high degree of retrogression in the Reynolds Range Group is the common occurrence of impermeable quartzite units which prevented many of the segregated melts from migrating up through the rock pile. In the past, an emphasis has been placed on the need to extract partial melts from their source rocks in order to stabilize and preserve granulite assemblages, and thus provide a link with granite plutons emplaced at higher crustal levels (Brown, 1994; Brown et al., 1995). This model has often been at odds with field evidence for only limited extraction of melt from migmatite terrains, but extraction is not required provided that a structural pathway is available to channel fluids away from the crystallizing melts. A number of recent studies have identified the importance of melt crystallization in determining the evolution of metamorphic belts (Stevens 1997; Kohn et al. 1997), and it is likely that transport of released fluids along ductile shear zones from crystallizing melts will have important implications, not only for the preservation of anhydrous assemblages in the source migmatites, but also for the evolution of rocks at higher crustal levels which will undergo widespread retrogression during the waning stages of metamorphism.

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REFERENCES Brown M., 1994. Earth-Science Reviews, 36, 83-130 Brown M., Averkin Y.A. & McLellan E.L., 1995. J. Geophys. Res. 100, 15,655-79 Buick I.S., Cartwright I., & Harley S.L., 1998. J. Metamorphic Geol. 16, 511-29. Dirks P.H.G.M. & Wilson C.J.L., 1990. J. Structural Geol. 12, 651-65. Dirks P.H.G.M., Hand M. & Powell R., 1991. J. Metamorphic Geol. 9, 641-61. Fitzsimons I.C.W., 1996. J. Petrol. 37, 395-414. Fitzsimons I.C.W., Kinny P.D. & Harley S.L., 1997. Terra Nova, 9,47-51. Kohn, M.J., Spear, F.S. & Valley, J.W., 1997. J. Petrol. 38, 1255-77. Stevens G., 1997. J. Metamorphic Geol. 15,141-54.

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INFLUENCES OF BASIN/BASEMENT STRUCTURES ON THE DEVELOPMENT OF FOLD-THRUST BELTS Thomas Flottmann and Pat James Dept. Geology and Geophysics, tflottma ©geology, adel aide. edu. au

The

University

Adelaide,

5005

During orogenic shortening the structural response to pre-existing basin forming structures can vary significantly, and pre-existing structures can significantly influence the structural style and kinematics of fold thrust belts. Conversely in many fold thrust belts contractional deformation is believed to dominate the final structural geometry of a fold thrust belt, regardless of pre-existing basinal structures which may be detached from the deforming wedge. Using balanced cross sections from different portions of the Adelaide Fold Thrust Belt, South Australia and the Lawn Hill Platform, Queensland we examine the influence of basin forming structures on the structural style of these fold belts. In both cases the primary basin architecture is formed by growth/relay faults which were active during sedimentation. The tectonic expressions and kinematic patterns developed during subsequent shortening are subtlely but recognizeably influenced by former growth and relay fault patterns which form distinct strain guides. The foreland regions of both orogens show distinct fold interference patterns are related to regional shortening, which is overprinted by a weak shortening. The latter shortening is presumably related to the taper of the orogenic wedge, the propagation of which appears pinned by the underlying basin structures and the wedge tapers at an angle oblique angle to the regional shortening direction in both fold belts. Only the more highly strained and thermally perturbed portions of the southern Adelaide Fold Thrust Belt may be interpreted as basement detached, however, whether unexposed basement is involved depends very on the individual interpretation. The outcomes of our work suggest that fold thrust belts, in particular those with significant structural compartmentalisation during sediment deposition, tend to be basement involved. In fact the structure and kinematics related to the contractional shortening of such fold thrust belts is very much controlled by the pre-existing basinal structures as revealed by balancing and restoration of cross sections. Both, the preexisting basement structures and the changing principal shortening directions and orientation of basin structures/fractures have significant implications on which structures can act as fluid conduits for mineralised brines or hydrocarbons through time.

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CRUSTAL WEDGES - A COMMON STYLE OF BASEMENT/COVER DEFORMATION AT THE MARGINS OF THE AMADEUS BASIN, CENTRAL AUSTRALIA Thomas Flottmann,! Martin Hand,l Dorothy Close^, Christine E d g o o s e ^ and Ian S c r i m g e o u r 2 1 Department of Geology and Geophysics, The University of Adelaide, SA 5005 2 Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, N.T. 0871 tflottma@geology.adelaide.edu.au Intracratonic deformation at the northern and southern margin of the Amadeus Basin during the Petermann 570 - 530 Ma) and the Alice Springs orogenys (400 - 300 Ma) respectively, is characterised by the formation of major crustal wedge systems which were inserted at the base of the Amadeus Basin. In both orogens the wedges were emplaced towards the basin, that is towards the north in the Petermann Orogen and towards the south in the Alice Springs Orogen. In both orogens, the evaporitic Bitter Springs Formation/Pinyinna beds form a detachment between the upper successions of the Amadeus Basin and lower units. The former were displaced along major backthrusts towards the south in the Petermann Orogen and towards the north in the Alice Springs Orogen. These foreland and hinterland propagating thrust systems form major tectonic wedges (or triangle zones) which are now exposed at different crustal levels in both orogens. The Petermann Orogen exposes mid-crustal kyanite-grade cover sequences with pervasive deformational fabrics that formed during intense crustal stacking. Despite the considerable metamorphic overprint and pervasive deformation, a basic stratigraphic template is clearly preserved, allowing a schematic restoration of the major thrust stacks. Results suggest over tenfold thickening of a stratigraphic package c. 1.5 km thick that includes: (1) basement, (2) a rift succession (Bloods Range beds), (3) the Dean Quartzite and (4) the evaporitic Pinyinna beds, occurred during the Petermann Orogen. Along the northern edge of Amadeus Basin in the vicinity of Ormiston Gorge, balanced sections allow a full restoration and forward modelling of the wedge/backthrust system. The results suggest an overall shortening of about 19 km occurred during the Alice Springs Orogeny, which is almost an order of magnitude less than the shortening in the Petermann Orogen. Ongoing shortening after the emplacement of the wedge/backthrust system in both orogens led to regional tilting of the wedge/backthrust system towards the Amadeus Basin. We suggest that although the wedge/backthrust systems along the Amadeus basin margins developed at different times they have several key features in common. Firstly, both are developed in the footwall of major crustal discontinuities consisting of the Woodroffe Thrust in the south, and the Redbank Shear Zone in the north. Both fault systems acted as crustal strain guides that prompted displacement along footwall shortcut thrust systems leading to the formation of crustal wedges. The partitioning of displacement into the wedge/backthrust system appears largely due to the presence of the mechanically weak horizon of the Bitter Springs Formation/Pinyinna beds which forms a suitable stratigraphically controlled detachment zone. Along this decollement, the upper successions were backthrust above the foreland propagating wedge systems. In all likelyhood the style of deformation is common where the stratigraphy of fold thrust belts contains suitable detachment horizons and thrust propagation is controlled by major pre-existing fault zones.

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SHORTENING AND EXTENSIONAL STRUCTURES SETTING, OTAGO SCHIST, NEW ZEALAND.

COLLISIONAL

Mamie Forster Monash University Clayton Victoria 3168 Australia. Email: mforster@earth.monash.edu.au The Otago Schist, South Island New Zealand is thought to represent a Jurassic collisional event (Bradshaw 1989). All the ductile features of this region, for example flat-lying foliations, transpositional layering, plus large-scale phenomena such as crustal thickening, thrusting, recumbent folds and high strain zones (shear zone) are attributed to this collisional event (Mortimer, 1993). Deformation is commonly accepted as progressive coaxial deformation. A major ductile shear zone occurs at the Caples/Torlesse boundary and is suggested to represent a thrusting event (Cox 1991). Other ductile shear zones recorded are in the western Otago and are spatially related to the large-scale recumbent folds (Craw 1985, Bishop et al 1976), and are described as the attenuated hmbs of fold nappes. A switch to extensional tectonics in the Cretaceous is suggested to have taken place, resulting in the exhumation of a welt of higher metamorphic grade rocks which trends parallel to the boundaries of the area. However, the large-scale extensional features in the Otago Schist that may have accompHshed this phenomenon are generally not explained, or defined. This study suggests that exhumation by extensional tectonics in the Central Otago Schist has occurred by the operation of large-scale ductile shear zones. These extensional phenomena overprint earlier compressional structures and are themselves overprinted by later compressional structures. Thus the history from compressional to extensional tectonics appears to be more comphcated for the Central Otago Schist region than has been previously suggested. This study has been undertaken so as to gain an understanding of the compressional and extensional tectonics of the Otago Schist region. The main focus has concentrated on the large-scale recumbent folds and shear zones. Work has shown that firstly, the large-scale recumbent folds and the shear zones are not always spatially, or temporally related. Secondly, many more shear zones exist in the Central Otago Schist than have been previously recorded and these shear zones differ in age and in kinematics. "^^ArP^Ar geochronology of these shear zones suggests that shearing occurred at three distinct periods in the Cretaceous. A detailed study of the Dunstan Range has shown that two largescale shear zones occur in this geomorphological dome. The structurally deeper shear zone has NS-directed movement, with meso-scale map interpretation suggesting extensional tectonics. The structurally higher shear zone displays EW-directed movement and is overprinted by the younger NS-directed extensional shear zone. Recumbent folds throughout the Central Otago are defined by a change in vergence, on the assumption that meso-scale folds are parasitic to the larger-scale recumbent folds (Means 1966). Hinges to these recumbent folds are generally not found, and folds are thus defined by their axial surface trace and determined by a change in vergence. Gray et al (1995) suggest

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that the sheath folding is the dominant fold-type in the region with the high strain zones occurring on the Hmbs of these folds. This study suggests that the lack of preservation of large-scale recumbent folds may be due to eariier pervasive shear zones and the formation of, or continuation of transpositional layering over the region. Vergence information can not alw^ays be related to recumbent folds as it may disrupted by younger overprinting shear zones. The Cretaceous shear zones, documented in this study, are not confined to the Hmbs of large folds, but could easily be interpreted as such due to the rarity of hinge zones. For example the Bendigo Fold has been transected obliquely by an NE/SW-directed shear zones, causing younger narrow zones of transpositional layering through the hinge. Multiple generations of large-scale recumbent folds have been detected where younger shear zones have themselves been folded. Both field observations and geochronology display distincdy different ages for these recumbent folds.

REFERENCES: Bishop, D.G., J.D. Bradshaw, C.A. Landis and I.M. Tumbull. 1976. Lithostratigraphy and structure of the Caples terrane of the Humboldt Mountains, New Zealand. NZ Journal of Geology and Geophysics 19. 827-848. Cox, S.C. 1991. The Capes/Aspiring terrane boundary - the translation surface of an early nappe structure in the Otago Schist. N Journal of Geology and Geophysics, 34. 73-82. Craw, D. 1985. Structure of schist in the Mt. Aspiring region, northwestern Otago, New Zealand. NZ Journal Geology and Geophysics, 28. 55-75. Bradshaw, J.D. 1989. Cretaceous geotectonic patterns in the New Zealand region. Tectonics, 8 (4). 803-820. Gray, DR, R.T. Gregory, R.J. Norris, & S.C. Cox. 1995. Regional scale sheath-folding and heterogeneously distributed shear strain in an evolving nappe pile, Otago Schist, New Zealand. Geol Soc. Aust. Abst. 40. 53. Means, W.D. 1966. A macroscopic recumbent fold in schist near Alexandra, Central Otago. NZ Journal Geology and Geophysics. 6. 801-816. Mortimer, N. 1993. Jurassic tectonic history of the Otago Schist, New Zealand. Tectonics, 12 (1). 237-244.

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PARTIAL MELTING, OROGENY, AND COLLAPSE OF THE CORDILLERAN OROGEN: RESULTS FROM THE NORTHERN IDAHO (BITTERROOT) BATHOLITH REGION, U.S.A. David A. Foster^ and Mark Fanning^ ^Department of Earth Sciences, La Trobe University, Melbourne, Australia, dfoster@geology.ufl.edu, also at Department of Geology, University of Florida, Gainesville, Florida 32611 U.S.A. ^Research School of Earth Sciences, Australian National University, Canberra, Australia The Idaho-Bitterroot batholith is an example of an exhumed, mid-crustal, plutonicmetamorphic complex that formed during maximum crustal thickening in the hinterland of the Cordilleran Orogen. The timing of metamorphism, partial melting, intrusion, and deformation provide an analogue for magmatic and deformation processes active at mid-crustal depths in orgenic belts like Tibet where geophysical data suggest large amounts of partial melt in the crust. Crustal thickening in this part of the northern Cordilleran belt began at about 100 Ma. SHRIMP U-Pb zircon dates indicate that high-grade metamorphism and partial melting in the Bitterroot complex was coincident with the intrusion of syntectonic quartz diorite plutons at ca. 75-80 Ma. Large scale melting of the middle crust occurred between ca. 65-53 Ma, leading to the intrusion of the voluminous "main-phase" granitic plutons as thick (3-4 km) sills. This was accompanied by further upper amphibolite facies metamorphism and partial melting forming migmatites at about 6.5 kb pressure. The youngest mid-crustal granitic intrusions are about the same age as initial crustal collapse at ca. 52-50 Ma. The youngest phase of high-grade metamorphism is also the same age as crustal collapse, and is typified by decompression textures and isothermal decompression. Collapse occurred mainly on the Bitterroot shear zone that deforms the younger intrusive rocks. Therefore, large-scale partial melting of the middle and lower crust followed crustal thickening, in this part of the Cordillera, by as much as 15-35 m.y., but pre-dated crustal collapse and exhumation by only 1-3 m.y. Collapse in this sector of the Cordilleran Orogen appears to have been focussed where partial melting and plutonism was most intense and long-lived. Exhumation is revealed by the transition from amphibolite facies mylonitization, to greenschist facies shearing, to brittle faulting, to inactivity of the shear zone that progressed from west to east from ca. 50-40 Ma, based on U-Pb and Ar-Ar results. Alkali feldspar granites were emplaced contemporaneous with the onset of exhumation, but were intruded only into the shallowest Eocene crustal levels. Their generation may have been linked to decompression of the lithospheric column during crustal thinning.

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DEFORMATION EXAMPLES

RATES IN OROGENESIS: SOME

GEOCHRONOLOGICAL

David A. Foster^ and David R. Gray^ Australian Geodynamics Cooperative Research Centre, ^Department of Earth Sciences, La Trobe University, Melbourne Victoria 3083; also at Department of Geology, University of Florida, Gainesville Florida 32611 USA. E-mail: dfoster@geology.ufl.edu. ^Department of Earth Sciences, Monash University, Melbourne Victoria 3168, Austraha Deformation within orogenic belts can be quantified as a strain rate (i.e.e = e/t, where e is the elongation, and t is the time in seconds) and/or displacement rate u (i.e. velocity of the displacement field). Strain rates in active tectonic environments have been recently determined using GPS monitoring systems. In ancient orogens determining rates of deformation has been problematical. Strain rates have been estimated assuming that deformation pulses have an average duration of < 5 Ma (e.g. Pfiffner & Ramsay, 1982). ArAr dating of slates and individual mica grains has been shown to be an effective way for dating metamorphism and cleavage formation in very low grade rocks. In most strongly cleaved slates the only significant potassium bearing phase is new metamorphic white mica. The metamorphic phengite closes to argon loss at about 350°±50°C, so that at low greenschist and prehnite-pumpellyite facies metamorphic conditions, they grow at or below their closure temperatures. Dating mica growth across an orogen or fold belt can therefore be used to constrain the timing and duration of deformation, which combined with either shortening estimates, measured finite strains and/or palinspastic restorations can be used to calculate both strain and displacement rates. Other thermochronometric methods such as fission track dating may also be effective in foreland fold and thrust belts. We include below examples using these methods as well as others utilising older K-Ar data. [Methodology assumes timeaveraged displacement over the given distance to produce the end geometry] • Example data from accretionary wedge-type thrust systems: • Robertson Bay terrane, Ross orogen, Antarctica (Dallmeyer and Wright, 1992): duration of 40 Ma (from 500 Ma to 460 Ma)/ -50% shortening/ exposed orogen width is 150 km (i.e. distance from Leap Year fault to the Antarctic coast) gives a fold propagation rate of - 4 mm/yr. • Lachlan orogen, eastern Australia (Bucher et al., 1996; Foster & Gray, in press): mica growth ages from major fault zones suggest a decollement propagation rate of mm/yr which translates to a strain rate of s'V Duration of deformation was -60 Ma (from 450 Ma to 390 Ma) with a palinspastic separation of ~ 1,000 km between Woomdoo-Moyston fault zone and the Mount Wellington fault zone ( i.e. restored width of the western Lachlan orogen) • Rheinisches Schiefergebirge of the Variscan orogen (Ahrendt et al., 1986): A crustal shortening of ~ 50% was achieved, with a duration of deformation -30 Ma (from -327 Ma to 305 Ma) operating over a crustal length segment of 150 km to give a "wave of folding and metamorphism" of - 5 mm/yr. • Example data from foreland fold- and-thrust type orogenic systems: • Appenines (Khgfield et al., 1986): duration of deformation was -17 Ma (from -27 Ma to -10 Ma). Strain rates were calculated to be in the order of to lO"^"^ s"^

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•Sevier orogen (Armstrong, 1968): palinspastic separation of ~ 90 km between Paris thrust (Ephrain conglomerate: fault active at 140 Ma) and Prospect thrust (Lookout Mountain conglomerate: fault active at --58 Ma) gives a displacement rate of mm/ yr. •Himalayan orogen • (Meigs et al., 1995): dates of unconformities and fission track cooling ages for the Main Boundary thrust in western Himalaya giving a displacement rate of -10 mm/yr for the Main Boundary thrust • (Harrison et al., 1992; Yin et al., 1995): Ar-Ar dates on the Gandise, Main Central and Main Boundary thrusts give a displacement rate of - 2 0 mm/yr • (Burbank et al., 1986): dates the initiation of conglomerate facies using magnetostratigraphy in the Jhelum re-entrant assuming that conglomerate initiation is in response to uplift of the range front (Pir Panjal Range) by thrusting. Data give a displacement rate of -30 mm/yr 140 120

100

1) Sevier orogen

slofs^' m^r^nng Sevbr orogen Imrwvn

roaion of tvpical displacgrnont " in ba\h Palaoazoic and Mescizoic orogen ic bisits - S20 mm/yr)

(90 niy)

1.03 mnVyr

2) Ross orogen (35 my) 4.50 mrrVyr (Robert^::in Bay terrane) 3) Variscan orogen {22 4.73mnVyr (Rheinisches Schiefergebirge) 4i Lachlan orcgan (40 my) 8.47 mnVyr (wester n subp rovi nee)

fast mayinq HimaJaYan oroqen II I I _J 20mm/vr) J— 50

1DD

15G

200

5) H i m a l a ^ orogen (23 niy) ^BB mm/yr

2EQ

Distance (km) Displacement rates within orogenic belts generally range from ~ 5-20 mm/yr (Fig. 1), and are probably dependent on plate movement rates, such that orogenic processes are externally controlled by plate motions, and climatic effects. Observed plate motion rates of --5-16 cm/yr must translate into orogenic deformation at convergent boundaries by partitioning into components of translation, rotation and distortion ('orogenic' strain). The distortional component is partitioned into strains at different scales within the orogen (e.g. fold-thrust shortening at the macro- scale to internal strain by cleavage formation at the meso-scale). Orogenic strain rates (e) can be approximated by dating mica growth at low grade (~280°C350°C) across the external parts of orogens, when appropriate rocks are present and metamorphic reactions are complete. The time-averaged velocity of the displacement field for these parts of orogens can be determined by dating mica growth in the basal decollement (gives the rate of fault propagation), or by dating micas defining cleavage growth (gives the rate of fold propagation). Results give maximum time-averaged displacement rates in the order of 10 mm/yr such that natural strain rates are in the order of 10'^^ to s'^ at the orogen scale. These values reflect the complex interactions between structural thickening (by folding and faulting) and concomitant metamorphism (recorded by mica growth). Values based on syn-tectonic sedimentation rates tend to be higher. REFERENCES Ahrendt, H., Clauer, N., Hunziker, J.C. & Weber, K., 1986 In: Intracontinental F.W. Eder, Springer-Verlag, Berlin, pp.323-338.

Fold Belts, Eds. H.Martin &

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Armstrong, R.L. 1968 GeoL Sac. Am. Bull 79, 429-458. Birbank, D.W., Raynolds, R.G.H. & Johnson, G.D., 1986 Spec. Pubis int. Ass. Sediment 8, 293-306. Bucher, M., Foster, D.A. & Gray, D.R. 1996 Geol. Soc. Am. Abs. Prog. 28, p.A-439. Dallmeyer, R. D. & Wright, T.O. 1991. Tectonics 11, 437-448. Foster, D.A. & Gray, D.R. in press Tectonics Harrison, T.M. and 6 others, 1992 Science 255, 1663-1670. Kligfield, R., Hunziker, J., Dallmeyer, R.D. & Schamel, S., 1986. Jour. Struct. Geol. 8, 781-798. Pfiffner, O. A. & Ramsay, J. G. 1982 Jour. Geophys. Res. 87, 311-321. Miegs, A.J., Burbank, D.W. & Beck, R.A., 1995 Geology 23, 423-426. Yin An, Harrison, T.M., Ryerson, FJ., Wenji, Chen, Kidd, W.S.F., & Copeland, P., 1994. Jour. Geophys. Res. 99,18175-18201.

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STRUCTURAL CONTROL OF THE BASALT CONTACT MINERALISATION, STAWELL, VICTORIA M J Gane^ and C J L Wilson' ^Stawell Gold Mines, PC Box 265, Stawell, VIC 3380 (mikeg@netconnect.com.au) 'School of Earth Sciences, The University of Melbourne, Parkville, VIC (cjlw@myriad.its.unimelb.edu.au)

3052

The Stawell Goldfield is located, within a package of deformed Cambrian turbidite sediments that are structurally juxtaposed against a deformed sequence of metabasalts within the Stawell Zone of the Lachlan Fold Belt. Gold mineralisation is hosted by deformed quartz veins produced as a result of the influx of gold-bearing hydrothermal fluids through large-scale shear zones and faults; formed during the Late Ordovician-Early Silurian Benambran Orogeny. Within the Magdala mine, deformation and fluid flow was focused within a 5km wide belt of Cambro-Ordovician basalts, volcanogenic sedimentary rocks and turbidites between the Pleasant Creek Fault and Coongee Fault, and north of the Stawell Granite (Fig. 1). The younger Scotchmans Fault Zone (Mapani and Wilson, 1994), a D5 deformation event, overprints the older Central Lode System, dominated by D4 deformation features, and a suite of Late Silurian quartz-feldspar porphyry dykes (Ame et al., 1998), with both systems having reverse movement sense. The Central Lode shear zone was formed adjacent to the western flank of a 15-30° northwestplunging basalt "antiformal thrust stack", producing a regime of low mean stress within the volcanogenic sedimentary rocks between the shear zone and the basalt. The influx of hydrothermal fluid through the Central Lode shear zone, under fault-valve conditions, led to the formation of several increments of fluid infiltration accompanied by sulphide-rich quartz veins. These lie in a shear zone foliation, defined by quartz augen, and bounded by faults that contain breccias, mylonites and sheared quartz+sulphide lodes. The development of complex fracture meshes within the underlying volcanogenics allowed the hydrothermal fluids to penetrate and react with the wallrocks. The resulting alteration assemblages include a goldbearing sulphidation of the chlorite-rich rocks, accompanied by vein infilling of the fractures. These are known as the Basalt Contact and Stockwork Lodes. The Basalt Contact Lodes occur at the contact between the Footwall basalts and the volcanogenic sedimentary rocks. The volcanogenic sequence is mafic in composition, consistent with the cessation of the submarine volcanism responsible for the basalts, but have undergone mid-greenschist-facies metamorphism. The chlorite-rich metasediments at the base of the volcanogenics have been sulphidised by the influx of the gold-bearing hydrothermal fluids in preference to the more chert-rich and carbonaceous units higher in the volcanogenic package. Chlorite-rich metasediments which occur higher in the volcanogenic package were similarly sulphidised during the formation of "stockwork" lodes, dominated by quartz-hcarbonate±chlorite±sulphide veins. Two conjugate ore-related vein orientations are observed within the Basalt Contact Lodes:- set 1, dipping 20-50° towards 300-340°; and set 2, dipping 40-50° towards 110-120°. Vein textures indicate that these are extensional vein sets, exhibiting crack-seal and free-face growth textures. Overprinting relationships suggest that the sulphidation of the metasediments and the accompanying veining represents an early phase of the fluid influx. The sulphidation of the metasediments and the extensional veins are overprinted by the 40-70° southwest-dipping Basalt Contact shear zones. These are located

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0.5 to 3 metres above the contact between the basalts and volcanogenic sedimentary rocks. This shear zone development allowed additional fluid flow through the shear zones, leading to the formation of quartz+sulphide lodes within the Central Lode and Basalt Contact shear zones. Palaeostress estimates for early movements in the Central Lode shear zone by Wilson et al. (1992) and Mapani (1995) have previously indicated that a^ was flat-moderately plunging towards the southwest, 02 plunged flatly to the northwest and a^ plunged moderately towards the east (Fig. 2). Palaeostress estimates from the extensional veins within the Basalt Contact lodes indicates a similar stress field (Fig. 2), with the northwest-dipping veins forming approximately perpendicular to Gg. The Basalt Contact shear zones possess an near-identical palaeostress to the Central Lode shear zone (Fig.2). The slight difference between the 02 and Gg orientations for the shear zones and veins (Fig. 2) is possibly due to rotation resulting from the overprinting shearing, or to variations in rheology close to the footwall basalts. References Ame, D C, Bierlein, F P, McNaughton, N, Wilson, C J L and Morand, V J, 1998. Timing of gold mineralisation in western and central Victoria: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks. Ore Geology Reviews 13, 251273. Fredericksen, D C and Gane, M, 1998. Stawell gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 535-542 (The Australasian Institute of Mining and Metallurgy: Melbourne). Gane, M J, 1998. Gold mineralisation within the Basalt Contact ore zones, Magdala mine, Stawell, Victoria. MSc. Thesis (unpublished), University of Melbourne (in prep.). Mapani, B S E and Wilson, C J L, 1994. Structural evolution and gold minerahsation in the Scotchmans Fault Zone, Magdala gold mine, Stawell, western Victoria, Austraha. Economic Geology 89:566-583. Mapani, B S E , 1995. Structural evolution and mineralisation at the Magdala gold mine, Stawell, western Victoria. PhD Thesis (unpublished). University of Melbourne. Wilson, C J L, Will, T M, Cayley, R A and Chen, S, 1992. Geologic framework and tectonic evolution in western Victoria, Australia. Tectonophysics 214:93-127.

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MODES OF EXTENSION, LOWER CRUSTAL FLOW AND THE EVOLUTION OF THE NORTH WEST SHELF Anthony Gartrell Tectonics Special Research Centre, The University of Western Australia, Nedlands WA 6907 agartrel@geol.uwa.edu.au Recent concepts for rheological controls on extensional basin evolution may have important implications for understanding the structure of the North West Shelf (NWS) of Australia. The rheology of the lithosphere is critical in determining extensional style (Buck, 1991; Bassi, 1992). More specifically, the width of a rift basin will be determined by the potential of the lithosphere to distribute deformation by ductile flow or to localise deformation by lithospheric necking. Narrow rifts are likely when the stress bearing layers in the lithosphere are predominantly plastic (i.e. at lower temperatures and for harder materials). Conversely, when viscous behaviour (ductile creep) dominates the lithosphere's rheology, deformation will be distributed over a broad zone. When this stable type of extension occurs, cooling during deformation (as deep rocks are brought closer to the surface) can strengthen the lithosphere due to the replacement of crust by stronger mantle. Strengthening of the extended lithosphere results in a migration of deformation to adjacent areas, producing a wide rift basin. Another important prediction arising from rheological studies is that certain layers of the middle- and lower-crust (depending on composition) will have low viscosity for all but the coolest of geothermal gradients. It has been suggested that such layers may behave as a ductile fluid, able to flow from areas of high lithostatic pressure to areas of low Hthostatic pressure at geological time scales. Extension is isostatically compensated within the crust, rather than in the asthenosphere. This suggestion was initially made to reconcile the relative uniformity in crustal thickness, gravity, Moho topography, and surface elevations across highly extended and less extended regions of the Basin and Range Province, U.S.A (Kruse et al., 1991). Lower-crustal flow may be either inwards, towards the basin axis, or outwards, towards the basin margins (Westaway, 1994). The direction of flow depends on the interplay between loading by sedimentation and unloading by extension and erosion. Furthermore, flow may also occur post-extension, as long as the ductile fluid layer is maintained. For example, the initial onset of thermal subsidence may enhance outward lower crustal flow, as subsidence and sediment loading in the basin is not accompanied by a decrease in upper- and middlecrustal thickness. If lower crustal flow has occurred in an extensional basin then the interpretation of subsidence becomes more complicated. Outward flow in a lower crustal fluid channel results in systematic overestimates in extension, due to subsidence from removal of lower-crust being attributed to thermal subsidence. Inward flow has the opposite effect and conventional analysis will underestimate extension as a result. The North West Shelf margin of Australia developed as a result of two main phases of extension associated with the disintegration of Gondwanaland. Late Carboniferous to Early Permian (Permo-Carboniferous) extension resulted in a very wide (> 500km) basin, the Westralian Superbasin. A thick Permian to Triassic sag sequence overhes the PermoCarboniferous rift section, usually interpreted as the result of thermal sag following break-up in the Early Permian. The second main phase of extension which occurred in the Mesozoic, included two break-up events (Argoland - Callovian; Greater India - Valanginian) as spreading ridges developed near the western margin of the Superbasin propagated south.

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Mesozoic extension also resulted in the development of narrow rift basins along the eastern margin of the Superbasin which remains attached to Austraha (e.g. Barrow, Dampier, Exmouth and Caswell Sub-basins; Vulcan, Mahta and Calder Graben). Various tectonic models have been proposed for different sub-divisions of the NWS, usually involving detachment systems. However, consistencies in structural styles occur over a vast area on the NWS, suggesting that a common basin forming process was responsible for its structural development. Primarily, detachment models are invoked in an attempt to account for the discrepancy between the observed strong thinning of the lower-crust without commensurate upper-crustal extension. However, these models require extremely long and laterally extensive detachments systems and do not account for the change in extensional style observed between the Permo-Carboniferous and Mesozoic extension episodes. The alternative model suggested here for the NWS combines the rheology based concepts for lithospheric extension in order to address the structural peculiarities of the region. Runaway lithospheric thinning during Permo-Carboniferous extension generated an extensive wide rift basin. Deformation eventually localised on the western margin of this basin, resulting initially in a narrow rift and then break-up with the Cimmerian block. If wide rift extension occurred, it suggests elevated lithospheric temperatures prior to and during extension. High temperatures may have also generated a relatively thick channel of weak ductile lower-crust. Following break-up in the Early Permian, the onset of thermal subsidence initiated flow of lower crustal material away from the centre of the basin to beneath the margins. This resulted in a thick and relatively undisturbed sag sequence, easily confused with a thermal decay sequence. The lower-crust was significantly thinned beneath the basin by flow, while the stronger middle- and upper-crust remained relatively undeformed. Thickening of the lowercrust is evident beneath the Superbasin margin. Deep lower-crustal reflectors represent boundaries between brittle deformation in middle- and upper-crust and viscous flow in the lower-crust. At some stage flow in the lower-crust will decrease or become insignificant due to cooling or thinning of the fluid channel (flow is most efficient in a thick, low viscosity channel). The remaining lithosphere will be relatively brittle (also enhanced by earlier crustal thinning) making it prone to narrow rift development and lithospheric rupture. Consequently, the change in rheology saw the generation of large fault blocks and the development of narrow rift basins on the NWS. Narrow rifts were localised at the eastern margin (large strength contrast) and close to the western margin which eventually generated a strip of continent (Argoland) in the Middle Jurassic. Break-up with Greater India followed as the spreading ridge propagated south. Previously unrecognised large intrusive structures are interpreted in deep seismic data from the eastern marginal rift basins (e.g. Barrow, Dampier & Exmouth Sub-basins). These structures are suggested to represent the initial development of failed spreading centres. References Bassi G., Charlotte E., Potter P. 1993. Contrasting styles of rifting: models and examples from the eastern Canadian Margin. Teconics 12, 639-655. Buck W. R. 1991. Modes of continental extension. Journal of Geophysical Research 96, 20161-20178. Kruse S., McNutt M., Phipps-Morgan J., Royden L. 1991. Lithospheric extension near Lake Mead, Nevada: a model for ductile flow in the lower crust. Journal of Geophysical Research 96, 4435-4456.

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Westaway R. 1994. Re-evaluation of extension across the Pearl River Mouth Basin, South China Sea: implications for continental lithosphere deformation mechanisms. Journal of Structural Geology 16, 823-838.

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EVIDENCE FOR AN EARLY "ISAN" TECTONO-THERMAL EVENT FROM THE SOUTH-EASTERN MARGIN OF THE MOUNT ISA INLIER David Giles, Laurent Ailleres, Peter Betts and Dylan Jeffriess Australian Geodynamics CRC, Department of Earth Sciences, Monash University ST-giles@earth.monash.edu.au A structural traverse at the south-eastern margin of the Mount Isa Inlier provides evidence of a period of thrusting and recumbent folding accompanied by upper amphibolite facies peak metamorphic conditions at c.l590-1580Ma. This is at odds with many tectonic models of this terrane which emphasise the correlation of a single metamorphic event (c.l550-1530Ma) synchronous with the regional "D2" deformation (upright N-S trending folds) throughout the inlier. We follow Beardsmore et al. (1988) in correlating the metasediments of the Kuridala Formation in the study area with those of the Soldiers Cap Group to the south-east of Cloncurry (Rybum et al., 1988). However, we differ from Beardsmore et al.(1988) in the specific location of stratigraphic boundaries and we have some misgivings regarding the identification of an additional stratigraphic unit, the New Hope Arkose, at the extreme east of the study area. Lithological variations in this area appear to be metamorphic rather than stratigraphic. Recent U-Pb zircon analyses from metasediments within the Kuridala Formation (Page, 1998) indicate deposition at <1670Ma and support correlations with the c.l700-1650Ma Soldiers Cap Group. This metasedimentary sequence was subjected to at least three regional fabric forming events during the Isan Orogeny (16]0-1500Ma). The eariiest of these is represented by a pervasive bedding parallel schistosity (s^). No folds related to this period of deformation have been observed. The second period of deformation resulted in tight to isoclinal, asymmetric and commonly non-cyhndrical folds with shallowly inclined to recumbent axial surfaces. These folds are spectacularly exposed in a south-easterly vergent fold train at the eastern margin of the study area but are rare elsewhere. A second generation structural fabric (^2) is axial planar to these folds. In micaceous lithologies 5-2 displays a spectrum of styles ranging from open crenulations of to penetrative mylonitic fabrics. ^2 crenulations are often highly asymmetric with mica rich long limbs and quartz rich short limbs. This period of deformation appears to have involved a significant component of simple shear. A third tectonic fabric (^3) is preserved as an open, 2 to 10mm, crenulation of the earher fabrics. 5-3 is axial planar to open to tight folds with N-S to NNE-SSW trending, upright to steep easterly inclined axial surfaces and variable plunges. The majority of the kilometre scale folds in the study area belong to this generation. Muscovite and biotite crenulations defining s^ consistently display decussate textures indicating metamorphic recrystallisation subsequent to deformation. We interpret the transition from a dominantly simple shear to a dominantly pure shear structural regime in the study area in terms of the Eastern Fold Belt tectonic framework introduced by MacCready et al. (1998) and O'Dea et al. (1997) whereby an early period of thin skinned deformation (low angle thrusts and inclined to recumbent folding) was overprinted by a later period of thick skinned deformation (basement piercing high angle thrusts and upright folding).

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Peak metamorphic conditions, ranging from middle to upper amphibolite facies at pressures <4kb, were reached during the early stages of the second deformation event. Recent geochronological studies in the south-eastern parts of the Eastern Fold Belt (Page and Sun, 1998; Perkins and Wyborn, 1998) have produced metamorphic ages of approximately 1590 to 1580Ma and provide the best constraint on the age of this metamorphism. The broad synchronism of peak metamorphism with a second phase of regional deformation is well documented in the Mount Isa Inlier, however it is generally assumed to have a consistent inlier-wide age c.l550-1530Ma (Connors and Page, 1995). In the Western Fold Belt peak metamorphic assemblages statically overprint fabrics developed during the second regional deformation event (Connors et al., 1992). This is more consistent with the sequence of events associated with the third deformation event of the study area. We propose that the third deformation event in the study area can be broadly correlated with "D2" of the Western Fold Belt and has an age of -1550 to 1530Ma. This event was preceded by at least two episodes of ductile deformation. The second of these was a period of recumbent folding and thrusting synchronous with a metamorphic event (c.l590-1580Ma) which is not well represented in the Western Fold Belt. The recognition of two distinct metamorphic events, with differing spatial distributions, challenges previously held concepts regarding the synchroneity of deformation and metamorphism throughout the Mount Isa Inlier. This abstract is released with permission of the Director of the Australian Geodynamics Cooperative Research Centre. The authors gratefully acknowledge financial assistance provided by the sponsors of the Mount Isa Tectonic Synthesis - BHP Minerals, Normandy Mining Limited, North Exploration and Placer Exploration Limited.

REFERENCES Beardsmore, T. J., Newbery, S. P. and Laing, W. P., 1988. The Maronan Supergroup: an inferred early volcanosedimentary rift sequence in the Mount Isa Inher, and its implications for ensialic rifting in the Middle Proterozoic of Northwest Queensland. Precambrian Research, 40/41; 487-507. Connors, K. A. and Page, R. W., 1995. Relationships between magmatism, metamorphism and deformation in the western Mount Isa Inlier, Australia. Precambrian Research, 71; 131-153. Connors, K. A., Proffett, J. M., Lister, G. S., Scott, R. J., Oliver, N. H. S. and Young, D. J., 1992. Geology of the Mount Novit Ranges, southwest of the Mount Isa mine. In: Detailed Studies of the Mount Isa Inher (eds. A.J. Stewart and D.H. Blake). Austrahan Geological Survey Organisation Bulletin 243; 137-160. Rybum, R. J., Wilson, I. H., Grimes, K. G. and Hill, R. M., 1988. 1:100 000 geological map commentary, Cloncurry, Queensland. Australian Government Publishing Service, Canberra. 30 pages. MacCready, T., Goleby, B. R., Goncharov, A., Lister, G. S. and Drummond, B. J., 1997. An evolutionary framework for the Isan Orogeny. Abstracts, Geodynamics and Ore Deposits Conference, Ballarat, Victoria. Australian Geodynamics Cooperative Research Centre; 42-45.

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O'Dea, M. G., Belts, P. G., MacCready, T. and Ailleres, L., 1997. Sequential development of a mid-crustal foldthrust complex in the eastern Mount Isa Inlier, Australia. Australian Crustral Research Centre Technical Publication No. 47. Page, R. W., 1998. Links between Eastern and Western Fold Belts in the Mount Isa Inher, based on SHRIMP U-Pb studies. Geological Society of Australia, Abstracts No. 49; 349. Page, R. W. and Sun, S.-s., 1998. Aspects of geochronology and crustal evolution in the Eastern Fold Belt, Mt Isa Inlier. Australian Journal of Earth Sciences (thematic issue. Eastern Succession, Mt Isa-Cloncurry district), 45(3); 343-361. Perkins, C. and Wybom, L. A. I., 1998. Age of Cu-Au minerahsation, Cloncurry district, eastern Mount Isa Inlier, Queensland, as determined by 40Ar/39Ar dating. Austrahan Journal of Earth Sciences (thematic issue, application of radiogenic isotopes to the study of Austrahan ore deposits), 45(2); 233-246.

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THE LACHLAN TRANSVERSE ZONE AND DEVELOPMENT OF THE LACHLAN OROGEN

Volume

CONSTRAINTS

THE

R. A. Glen^ ', R. J. Korsch^'^' D. M. Finlayson^ ^ D. W. Johnstone^ 'and J. L. Walshe^'" ^ Australian Geodynamics Cooperative Research Centre ^Geological Survey of New South Wales, Department of Mineral Resources, PO Box 536 St Leonards NSW 2065 ^Australian Geological Survey Organisation, GPO Box 378 Canberra ACT 2601 Division of Exploration and Mining, CSIRO, PO Box 437 Nedlands WA 6009 The Lachlan Transverse Zone (LTZ) is a major yet subtle WNW-trending structure that extends across the Lachlan Orogen. At its western end (north of Broken Hill), it links into the Olepoloko Fault that separates the Delamerian Orogen from the Thomson Orogen. At its eastern end it is represented by dykes and intrusions in the Sydney Basin. In this paper we explore the structural, intrusive and seismic character of the LTZ and implications for evolution of the Lachlan Orogen The LTZ is characterised by generally WNW-trending folds (visible through a weak meridional overprint) and WNW-trending faults. In the Eastern Belt of the Lachlan Orogen, the faults are largely accommodation structures that disrupt major Silurian to Carboniferous meridional folds and faults that define the regional grain of the Lachlan Orogen. The LTZ has also controlled the partitioning of upper crustal extensional and contractional deformation, localising changes in Silurian to Middle Devonian basin architecture. Mantle and crust-derived intrusions, from the Late Ordovician through to the Miocene, are also localised along the LTZ which is thus a fundamental tear in the lithosphere of the developing Australian plate. This tear is an extension of the southern boundary of the Proterozoic Amadeus Transverse Zone that cuts through cratonic Australia, and also of the WNW trending segment of the Tasman Line that controlled late Neoproterozoic break-up of cratonic Australia. The LTZ thus appears to have formed by the reactivation of these older Precambrian crustal structures that propagated into younger oceanic and continental crust. Seismic reflection studies and a major 350 km north-south, strike-parallel refraction line shot from Lake George in the south to east of Dubbo in the north by the Australian Geodynamics Cooperative Research Centre in 1997 provide some information on the middle and lower crustal architecture of the LTZ and similar cross structures to the north and south. These new data show that several subhorizontal boundaries defined by changes in P wave velocity in the upper to lower crust either terminate at, or change thickness across, boundaries that lie at high angles to the refraction line and (sub)parallel to the LTZ. These include: • a near-surface layer 1 that is generally 2-3 km thick, but down to 4 km thick below and north of the LTZ •a major thinning of layer 2 ( base from -15 km to ~5 km) from south to north. In the south, this layer corresponds to thickened quartz turbidites of the Ordovician Adaminaby Group and granites of the Wyangala Batholith finally emplaced by thrusts to depths of ~8-9 km, V. Morand, pers. comm. 1998). In the north, layer 2 corresponds to Ordovician volcanic rocks of the Macquarie Arc that can been tracked to depth using the seismic reflection data. The boundary between the two units is an inferred fault. Whereas most of the granites of the Wyangala Batholith are S type, those at the northern end are I type (Chappell et al. 1991), derived presumably from Ordovician volcanics at depth. •an impersistent layer 3 that extends from a depth of ~5-6 km at the top down to an irregular base at -10-15 km. This layer terminates south of Wyangala, where it is, separated from the

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thickened southern part of layer 2 by an inferred fault.. From reflection data, layer 3 is inferred to correspond to metamorphosed Ordovician volcanics. A high velocity bulge at the base of this layer occurs under the LTZ. Continuation of this layer to the east and west under younger basins would contain the source for Early Devonian volcanics (such as the Cuga Burga Volcanics) and intrusives that are chemically very similar to Ordovician volcanics (Watkins 1998a) and for Carboniferous granites of the Bathurst suite (e.g. Glen 1998, Watkins 1998b). •a deeper mid crustal layer that extends from an irregular upper surface at 10-16 km down to a depth of -30 km centred under the LTZ, but only km to farther north and -23 km farther south. This layer corresponds to a highly reflective layer in seismic reflection lines. The composition of this layer is not clear. Possibilities include metamorphosed Cambrian ocean crust (cf Glen et al. 1998) and/or mafic underplate during Palaeozoic igneous activity, or the Late Cretaceous opening of the Tasman Sea or during more recent (Middle Miocene) hot spot activity. The greater thickness of the middle crust under the LTZ may be a metamorphosed relic of a seamount subducted in the Middle-Late Ordovician (Glen et al. 1998). Boundaries of this thickened volume may correspond to cross structures in the upper crust. ACKNOWLEDGEMENTS Published with permission of the Director, AGCRC. RAG publishes with permission of Director-General, NSW Department of Mineral Resources. RJK, DMF and DWJ publish with permission of the Executive Director, AGSO. REFERENCES Chappell, B.W., Enghsh, P. M., King, P. L., White, A. J. R. & Wyborn, D., 1991. Granites and related rocks of the Lachlan Fold Belt (1:1 250 000 scale map). Bureau of Mineral Resources, Canberra. Glen, R. A. 1998. Tectonic development of the Lachlan Orogen: a framework for mineral exploration, in Lewis, P.C., Lachlan Fold Belt Conference '98. Australian Institute of Geoscientists Bulletin, 23, 1-6. Glen, R. A., Walshe, J. L., Barron, L. M. &. Watkins, J. J., 1998. Ordovician convergentmargin volcanism and tectonism in the Lachlan sector of east Gondwana. Geology, 26, 751754. Watkins, J. J., 1998a. Geochemistry of the Gregra Group, in Pogson, D. J. & Watkins, J. J., Bathurst 1:250 000 Geological Sheet SI/55-8: Explanatory Notes., Geological Survey of New South Wales, Sydney, 185-186. Watkins, J. J., 1998b. Geochemistry of the Carboniferous intrusions, in Pogson, D. J. & Watkins, J. J., Bathurst 1:250 000 Geological Sheet SI/55-8: Explanatory Notes., Geological Survey of New South Wales, Sydney, 262-264.

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BOUDINAGE INDICATORS.

CLASSIFICATION

AND

EVALUATION

KINEMATIC

Ben Goscombe and Gees W. Passchier*. Namibian Geological Survey, P.O. Box 2168, Windhoek, Namibia. beng@mme.gov.na *Institut fuer Geowissenschaften, Johannes Gutenberg Universitaet, Becherweg 21, Mainz, Germany, cpasschi @ mail .uni -mainz.de In monoclinic shear zones, a layer can be boudinaged either symmetrically, without slip on the inter-boudin planes, or asymmetrically with slip. If the orientation of the layer, the inter-boudin planes, the foliation and stretching lineations in the rock are in monoclinic symmetry, slip on the inter-boudin planes can be either synthetic (synthetic slip boudinageSSB) or antithetic (antithetic slip boudinage-ASB) with respect to sense of shear. In synthetic slip boudinage the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense. Asymmetric boudins have in the past been used as shear sense indicators by interpreting them as having developed by either SSB or ASB. However, SSB and ASB are mirror-image geometries, so some geometric criteria are needed to distinguish boudins operating by SSB and ASB. We have investigated the geometry of 1300 boudins from the literature and from the Kaoko Belt, Namibia, to try and develop criteria to recognise SSB and ASB-structures by their geometry alone. The Kaoko Belt is well suited to investigate the reliability of this attribution since nearly all types of boudin-geometry described in the literature are developed in profusion, and the Belt experienced a simple tectonic history of dominant sinistral transpression. Local sense of shear is independently constrained by sigma- and delta-type porphyroclasts, C'- and C-type shear band cleavage, and fringe folds. Comparing boudin geometry and bulk shear sense, we found that the geometry of boudin trains that formed by either SSB or ASB was in most cases indeed different, and we were able to recognise two geometric types: Shear band Type boudins that correspond to 100% of the observed SSB, and Domino Type boudins that correspond to 97% of ASB. These types have the following characteristics: Shear band Type boudins have rounded shapes, curved inter-boudin planes and sigmashaped boudins, with tapering wings, are typical. Dilation across the inter-boudin plane almost never occurs. These boudins typically form in the highest competency contrast scenarios, such as quartz veins within schists. Drag on the inter-boudin plane is almost always evident. The inter-boudin plane is at a low angle to both the boudin surface (averaging 39°) and the layer enveloping surface (averaging 25°). Aspect ratios of the boudin blocks are high (averaging 3.7). Lateral displacement along the inter-boudin plane is high, averaging 2.3 times the layer thickness. Though hard to recognise in the field, extension of the layer enveloping surface is high, averaging 158% and results in complete separation of boudin blocks in 62% of the investigated cases.

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Domino Type boudins have an angular, rhomb-shape and sharp, straight inter-boudin planes. Dilation across the inter-boudin plane and vein infill can potentially occur (in 30% of the investigated cases). Apparent "drag" on the inter-boudin plane, where present, is antithetic to the slip experienced, this being called "antithetic edge curl". The inter-boudin plane is at a high angle to both the boudin surface (averaging 70°) and the layer enveloping surface (averaging 53°). Aspect ratios of the boudin blocks are low (averaging 2.5). Lateral displacement along the inter-boudin plane is low, averaging 0.5 times the layer thickness. Extension of the layer enveloping surface is small averaging 123% and may even involve shortening (i.e. <100%) for high degrees of block rotation. Degree of apparent block rotation is typically higher than for Shear band Type, but this is not diagnostic with Domino Type averaging 18.4° and Shear band Type averaging 15.6°. Our conclusion is that asymmetrical boudinage structures are empirically reliable as shear sense indicators, provided that they can be attributed to Shear band or Domino Type. There is, obviously, a zone of overlap where structures cannot be reliably interpreted, and we are presently establishing its extend. The reason why layers develop either Shear band or Domino Type is presently under investigation by the authors, using analogue experiments. If this problem can be solved, asymmetric boudins could not only be used to determine shear sense, but also other kinematic parameters of flow in shear zones.

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NUMERICAL MODELLING OF DEFORMATION AND FLUID-FLOW SHALLOW PLUTONIC COMPRESSIONAL ENVIRONMENTS

Paul Gow and Alison Ord CSIRO Exploration and Mining, PC Box 437, Nedlands, WA 6009, Australia Australian Geodynamics Cooperative Research Centre, PO Box 437, Nedlands, WA 6009, Australia, Email: p.gow@ned.dem.csiro.au The formation of the Tertiary continental arcs of New Guinea and the Andes provide examples of large scale plutonism that occurred in broadly compressional, collisional, environments. Re-activation of transfer structures, and strike-slip tectonics, respectively, have been invoked to create dilation to host emplacement of the magmas in these areas (Hill, 1997; Mason & Ord, 1996). We examine at a more detailed scale how dilation at a shallow level within these settings occurs, and how this may influence both fluid flow (of exsolving magmatic fluids and formation waters), and possibly emplacement of late-stage porphyries or dykes. Many of the world's porphyry-style Cu-Au deposits are emplaced within these settings and understanding the nature of the fluid flow in these environments presents benefits for mineral exploration. The local stress field that develops within and around plutons in shallow plutonic environments appears to be controlled by several key factors. These are 1) the regional principal stress orientations, 2) the contrast in elastic moduli between the plutons and the host rocks, and 3) the geometry of the plutons. The contrast in mechanical behaviour is also dependent on the current stage within the cooling history being considered. The models presented here are assumed to be late in the emplacement history of the plutonic complexes, so that the main plutons have crystalhsed, and show material properties similar to fully crystallised plutons. Evidence from many magmatic-hydrothermal systems show that they are long-lived, and that fluids may still be exsolved and late-stage bodies still be emplaced well after emplacement of the first magmas (e.g. episodic alteration at the Chuquicamata deposit ranges from 35-31 Ma, Lindsay et al., 1995). The geometry of the plutons in the models has been constrained to a diameter of 8-15 km and a depth extent of approximately 5 km, similar to well-documented shallow crustal plutons (e.g. the Yerington Batholith, Dilles & Einaudi, 1992). In compressional environments a characteristic volumetric strain distribution develops around shallow-level plutons. Conjugate zones of volumetric dilation form around the pluton, with their distribution refracted at the pluton margins, so that they cut the margins of the pluton, but not the main body of the pluton. The intersection of the conjugate zones, which represents the most dilatant area in the section, is consequently located above the pluton. This has implications for fluid flow in the section. If horizontal compression is sufficiently intense it will result in volumetric dilation which can draw fluid up from the pluton and into the dilational conjugate zone intersections. Conversely fluid will be drawn down into the zone from above. Thus the dilational conjugate zone intersection may act as a locus for mixing of fluids sourced from the pluton, the host rocks or meteoric sources. Additionally the dilation may act to localise emplacement of late-stage porphyries or dykes. Deformation at different stages in the cooHng history has been simulated by assuming a simple increase in elastic moduli with increased cooling. The results indicate that the

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magnitude of the contrast in elastic moduli of the pluton and host rocks will alter the location of faults and shear zones that form in the vicinity of the pluton. Failure is more likely to occur within the pluton if it has significandy lower elastic moduli than the host rocks, and viceversa. This appears to be more pronounced at shallower levels where the vertical stress (a^) is decreased. The implication is that timing of local deformation within the pluton cooling history may be one of the major factors influencing the location of mineralised structures within porphyry systems. Fault systems, and fluid conduits, may form within plutons which are deformed early in their cooling history when they are less competent, whilst fault systems will form preferentially on the margins of plutons which were deformed later in the cooling history. The control on the location of fault formation exercised by the elastic moduli contrasts may also be useful for predicting broad fault distribution based on host rock type. In this case, faults are more likely to form within plutons if they are hosted by competent volcanic rocks, and on the margins of the pluton if the host rocks comprise a less competent sedimentary pile. As well as fluid flow along highly focussed permeable structures, evidence from documented magmatic hydrothermal systems indicates that fluid flow is also accommodated within broad zones of hydraulic brecciation, although the scope and magnitude of fluid flow in this manner is unclear for shallow systems. We incorporate yield-induced transient permeability increases into these models to allow for fluid flow hosted by hydraulic brecciation. The permeability is increased when the rock fails, and restored to the original value when undergoing only elastic deformation, equivalent to a vein closing after the supra-lithostatic fluid pressure is removed. The models indicate that the magnitude and distribution of flow by yield-induced permeability is strongly dependent on the volume of fluid entering the section. The spatial and temporal distribution of this flow may help explain the alteration patterns commonly observed in shallow-crustal magmatic-hydrothermal systems. References: Dilles, J.H., and Einaudi, M.T., 1992. Wall rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada - A 6-km vertical reconstruction, Economic Geology, 87, 1963-2001. Hill, K.C., 1997. Tectonics, timing and economic deposits in Papua New Guinea, Proceedings from Geology, Exploration and Mining Conference, Madang, PNG, 233-234. Lindsay, D.L., Zentilli, M., and Ossandon, G., 1995. Evolution of permeability in an active ductile to britde shear system controlling the mineralisation at the Chuquicamata porphyry copper deposit, Chile, in Clark, A.H. (ed.) Giant ore deposits - II : Proceedings of the second Giant Ore Deposits Workshop, Kingston, Ontario, Canada, April, 63-89. Mason, R. and Ord, A., 1996. ModelHng the effects of crustal structure during convergence. Abstracts from the Third International Symposium on Andean Geodynamics, St Malo, France, 17-19 September 1996.

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OMAN ANALOG FOR OTAGO SCHIST BELT, NEW ZEALAND: FOLD NAPPES, HIGH P METAMORPHISM, EXHUMATION AND CONVERGENT MARGIN SETTINGS David R. Gmy\ Robert T. Gregory^ and John Mc. Miller^ ^ VIEPS Department of Earth Sciences, Monash University, Clayton, Victoria 3168 ^ Stable Isotope Laboratory, Department of Geological Sciences, SMU, PO Box 750395, Dallas TX 75275-0395, USA Recent mapping and structural/metamorphic work in the Saih Hatat w^indov^ (Miller et al., 1998; Gregory et al., in press) of the Sultanate of Oman has provided a framew^ork to revisit the Otago Schist belt of New Zealand. Both terrains show fold nappes and relict high-P metamorphism in rocks which we argue were formed and exhumed during convergent margin tectonism. Common elements are regional scale sheath-folds associated with a widespread, flat-lying transposition layering containing a pronounced stretching lineation (L-S tectonites). In both regions a major shear zone separates lower pressure upper plate rocks from lower plate higher pressure rocks, a juxtaposition normally attributed to extensional exhumation. Exhumation however, was accomplished during crustal-scale compression with the attendant formation of fold-nappes during ascent. The regional fold-nappes evolve by folding and transposition of the high-pressure fabrics during plate-scale convergence while the overall lithostatic pressure is decreasing. The upper plate shows a marked strain gradient towards the upper plate-lower plate ductile high strain boundary, where fold hingelines rotate into parallelism with the stretching lineation accompanied by schistosity and transposition layering development, and high strains (X/Z commonly > 100:1, where X and Z are the maximum and minimum principal strains respectively). • Saih Hatat, Oman (Table 1): high P rocks (blueschist-to-eclogite facies metamorphism) occur within former continental shelf rocks below the obducted, allochthonous Samail ophiolite sheet. Intense deformation involving nappe development was associated with lower pressure blueschist metamorphism transitional into widespread greenschist facies metamorphism. The fold structures are cut by low-angle high strain zones which consistently show southwest-over-northeast sense of shear, opposite to the sense inferred for ophiolite emplacement. A major crustal discontinuity separates two zones of regional nappes with the upper plate hanging wall rocks at lower metamorphic grade than the footwall rocks (Fig. 1). As much as a 13 kbar pressure difference exists between the upper plate (6-10 kbar, carpholite-bearing assemblages) and lower plate glaucophane-bearing eclogites preserved within mafic mega-boudins at the structurally lowest levels exposed in the lower plate near As Sifah (>12 kbar, possibly >20 kbar).

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TABLE 1. SUMMARY OF STRUCTURAL AND METAMORPHIC DATA SAIH HATAT, OMAN Upper plate Lower plate Wadi Hulw-Wadi Mayh Structural • regional closures • regional sheathlike closures features • coaxial shear dominates • noncoaxial shear (top to northeast) • strain gradient (cylindrical to • large component of flattening sheathlike) Metamorphi c conditions

P = 6.8-9 kbar* 7 = 3 1 5 ^ 3 5 "C* Carpholite-Pyrophyllite assemblages P = 6-8 kbar^ T = 250 -350 Carpholite-Kaolinite assemblages P = 8-10 kbar^ T = 180 -250 "C^

Early P-7 conditions not known (7" <340 Metamorphism during nappe formation P = 4.5-5.5 kbar§ P = 7 ± 1 kbar^7>420

(Table from Miller et al., 1998)

^^Arrr^

quatzite SaJhAfist

LOW® PLATE

I ^

Stratigraphic younging

Lower plate As Sifah • regional sheathlike closures • noncoaxial shear (top to northeast) • large component of lattening P > 12 kbar^ 7= 500-580 P = 2 3 ± 2 . 5 kbar^P>20 kbar** Metamorphism during nappe formation P = 4.5-6 kbar^^ P = 6.8 ±1.6 kbar^ 7 = 5 5 0 ± 100

NrthaBBt

Fig.l. Structural profile along Wadi Meeh, NE Saih Hatat, Oman.

Fig.2. Structural block diagram of the Otago Schist belt showing the distribution of the upper and lower plates, the outcrop trace of the high strain zone (DDZ), the map pattern of stretching lineations, and the position of the Dun Mountain ophiolite belt bounded by the Livingstone Fault trace. (modified from Mortimer, 1992, fig. 5) • Otago Schist belt, New Zealand (Table 2): a major shear zone separating a less deformed and metamorphosed upper plate (Caples volcaniclastic association containing lawsonite and albite) from a strongly deformed metamorphosed lower plate, which constitutes the biotite grade, central part of the schist belt and the transposition zone II of Mortimer (1993) (Fig. 2). Evidence of a former high P metamorphism occurs as sporadic Na-amphibole relics within meta-cherts and greenschists of the lower plate, near the major upper plate-lower plate discontinuity. The blueschist metamorphism has almost been completely overprinted by later higher T greenschist facies metamorphism (Yardley, 1983). The development, structural evolution and exhumation of these rocks have been explained by a subduction-related colHsion between Caples (forearc) and Torlesse (accretionary wedge), with the Dun Mountain ophiolite representing the suture (after Coombs et al., 1976).

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TABLE 2. SUMMARY OF STRUCTURAL AND METAMORPHIC DATA , OTAGO SCHIST, NEW ZEALAND

Upper plate

Lower plate

Rock Association

Capias volcaniclastic association

Torlesse (quartzo-feldspathic greywacke association) with Aspiring (mafic schist-chert association)

Structural features

• thrust nappe- single "fold" with lower limb sheared out (i.e. transitional into the high strain zone or DDZ) • marked strain gradient towards base of thrust nappe and DDZ • change from bedded greywackes (upper limb) into intensely foliated transposition layering (lower limb/DDZ) • shear strains (g) range from ~4 to > 100 from the upper limb into the DDZ (Cox 1991, table 1)

• fold nappes with opposite facing closures • sheath-like form (some hinges parallel to the stretching lineation • lower plate closures fold the early transposition fabrics (and the high-P assemblages) • pronounced rodding lineation and L-tectonite development within fold hinges • intensely foliated transposition layering on the fold limbs (coincide with high strain zones?)

P = -4.6 kbar T= 200 -350 °C

Early P-T conditions not known crossite-epidote P= 6.4 Kbar, T< 350°C Metamorphism during nappe formation chlorite-epidote-albite calcite-actinolite-sphene-magnetite-phengite± stilpnomelane (Yardley, 1982)

Metamorphi c conditions

lawsonite-albite (Kawachi, 1975) crossite-magnesioriebeckite (Yardley, 1982)

P=4.6±0.6 kbar, T=~350-400°C (Yardley, 1982)

REFERENCES

Coombs, D.S., Landis, C.A., Norris, R.J. Sinton, J.M., Boms, D.J. & Craw, D. 1976 Am. Jour. .Sci. 276,561-603. Gregory, R.T., Gray, D.R. & Miller, J.Mc. in press. Tectonics Miller, J. Mc., Gray, D.R. & Gregory, R.T., 1997 Geology 26, 235-238. Mortimer, N., 1992 Tectonics 12, 237-244. Yardley, B.W.D. 1982 Contrib. Mineral. Petrol. 81, 317-327.

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ACCRETIONARY WEDGE-TYPE THRUST SYSTEMS David R. Gray^ and David A. Foster^ Australian Geodynamics Cooperative Research Centre, ^Department of Earth Sciences, Monash University, Melbourne Victoria 3168, Austraha dgray@earth.monash.edu.au ^Department of Earth Sciences, La Trobe University, Melbourne Victoria 3083; and Department of Geology, University of Florida, Gainesville Florida 32611 USA Accretionary wedge-type thrust systems occur within deepwater turbidites, chert and oceanic crust (basalt, gabbro, ultramafics) sequences, in contrast to the passive margin limestone, dolomite, shale ± sandstone sequences of the classic foreland fold- and thrust-belts (e.g. the Appalachians and the Canadian Rockies) (see Table 1). They have leading imbricate fan geometries involving tiered detachments, and duplexes within the upper part of the oceanic crust (Fig. 1). These thrust systems are part of subduction-accretion orogens, where continental growth occurs by deformation and structural thickening of very large, subcontinent size, accretionary complexes. Examples include the Tasmanides of eastern Australia and the Altides of Asia. Fault zones within such turbidite-dominated orogenic systems are characterised by higher than average strain and intense mica fabrics, transposition foliation and isoclinal folds, poly-deformation with overprinting crenulation cleavages, and steeply to moderately plunging meso- and micro-folds. They clearly have different character to fault zones in carbonate-shale sequences of typical foreland fold-and-thrust belts, which are largely dominated by brittle-plastic processes in both the internal and external fault zones. Within accretionary wedge-type thrust systems fault zones are wide (~5-10 km width), weak zones in the structurally thickening, sedimentary wedges. The LEADINGHMBRIC^E FAN, ACCRETIONARY WEDGE-TYPE THRUST SYSTEM listric faults and chovron-folding subverticalh/ dipping within tho turbidito mod^Q duploK'Zono in chort/ mgte-be^lt/melen^ orQsion reek association structural

sedimentation

mid-crustel duplox motebeseltic rocks (oceanic crust layer 1) (modified from Gray & Foster, in press, fig. 18) strong to intense cleavage and transposition layering, with multiple cleavages record continued foliation development by 'transposition cycling' in a non-coaxial, thrusting-related shear regime. The fault zones represent strain-softened zones in the basal parts of the major thrust sheets, and are dominated by mica growth and pressure solution. Major faults which expose oceanic crust duplexes are spaced at about 100-120 km, whereas major fault zones within the turbidites are spaced at -15-20 km, with these typical distances being related to the the thickness of the turbidite fan and oceanic layer. Metamorphism is greenschist to 91

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subgreenschist facies (epizone to anchizone) within the turbidite-dominated thrust sheets, but Franciscan-Hke 'knockers' in chaotic melanges contain rehct intermediate-high P transitional blueschist-greenschist assemblages in mafic/ultramafic blocks. Thrust-sheet style is typically chevron-folding with one dominant 'slaty' type cleavage showing a marked strain gradient towards major faults. Shortening within these thrust systems is from 50-70% over 800- 1000 km length scales. This is in marked contrast to the 30-50% (over 50-100 km length scales) in the classic foreland fold- and thrust-belts. Structural evolution of leading imbricate fan systems involves fault zone development in the upper part of the oceanic crust (basalt layer) and at the sediment-basaltic layer contact. A speculative evolutionary model requires 'peeHng' of the upper ~ 1 km of oceanic crust (basaltic layer) as a trenchward-propagating duplex during subduction of oceanic crust in an ocean-ocean subduction setting. At the same time the turbidite wedge undergoes structural thickening as part of a deformation front at the trench. Syntectonic sedimentation is ongoing as the deforming sedimentary wedge achieves a critical taper. TABLE 1: THRUST-BELT TYPES BASED ON ROCK ASOCIATION AND STRUCTURAL STYLE Thrust-Belt Type

Accretionary wedge type

Foreland fold-and-thrust belt

Rock Association

deep water turbidites, chert and oceanic crust (metabasalt, gabbro, ultramafics)

passive margin limestone, dolomite, shale sequence ± clean sandstone

Fault Geometry

• leading imbricate fan involving tiered detachments • oceanic crust duplexes

• imbricate fan with out-of-sequence thrusts • basement and basement-cover duplexes

Fault-zone Style

• poly-deformed high strain zone up to 4 km width • crenulation cleavages and transposition layering • marked non-coaxial deformation (fibres in pressure shadows)

• mylonitic shear zones (up to 1 km width) in basement (hinterland) to narrow brittle zones with cataciasites (foreland) (up to 500 m width)

Metamorphism

• greenschist to sub-greenschist (turbidite-dominated thrust-sheets) • greenschist (intra-zone faults) • intermediate-high P metamorphism: blueschistgreenschist transition (inter-zone faults)

dominated (foreland)

sub-greenschist

metamorphism

'CAI:T~ 150-300°C P ~ 2-5 kb 'CAI: conodont alteration indice

Thrust-sheet Style

• fold-dominated • chevron-folding • 1 dominant cleavage (slaty type) • marked strain gradient towards faults

fault-dominated • imbricate stack • fault-bend and faultbreak folds • 1 or more cleavages (stylolitic spaced types) commonly transecting folds

Shortening

50-70% (over 800-1000 km length scales)

30-50% (over 100-200 length scales)

Type Examples

• western Lachlan Orogen • Kodiak Accretionary complex

• Appalachian fold-and-thrust belt • Sevier (Canadian Rocky Mountain) fold-and-thrust belt

(Table from Gray & Foster, Jour Struct. Geo/., in press)

REFERENCES

Gray, D.R. & Foster, D.A. Character and kinematics of faults within the turbidite-dominated Lachlan orogen: implications for tectonic evolution of eastern Australia. Jour Struct. Geol In press.

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REGIONAL TIMING CONSTRAINTS ON THE ALICE SPRINGS OROGENY FROM SYN-OROGENIC SEDIMENTATION AND COMPARISON WITH ISOTOPIC STUDIES Peter W. Haines, Martin Hand and Mike Sandiford Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005. Email: phaines@geology.adelaide.edu.au The Devonian-Carboniferous Alice Springs Orogeny (ASO) was a major intraplate tectonic event in central and northern Australia. Similar aged orogenesis is also known from eastern Australia, but in this review we restrict our discussion to cratonic Australia west of the Tasman line. The effects of the ASO have been well documented in the northern Amadeus Basin and Ngalia Basin, and in the exhumed Arunta Inlier. However, the full regional extent of the event, as well as lateral variations in timing and intensity are less well known. Questions of timing and duration of intraplate orogenies are fundamental to understanding these events. Do intraplate orogenies evolve on similar timescales as plate margin orogenies? The timing of the orogenic activity can be investigated in two ways 1) by isotopic dating of shear zones, syn-orogenic metamorphism and intrusives; and 2) by biostratigraphic dating of syn-orogenic sediments preserved in the foreland of created topography. Sedimentation should be a sensitive indicator of the initiation of orogenic topography, while isotopic systems are better indicators of peak and waning phases of the orogeny. Of course preservation of sediments is dependant on the long term durability of accommodation space and the youngest detritus has generally been removed by subsequent erosion. Syn-orogenic sediments include fluvial and fan conglomerates and sandstones as well as some finer grained lacustrine deposits. There is no evidence of marine influence, except in the southern Bonaparte Basin where Late Devonian conglomerates and sandstones interfinger with a marine sequence. Biostratigraphigraphy thus relies mainly on fossil freshwater fish and palynology. From an isotopic perspective, the timing of the ASO is best constrained in the southern Arunta Inlier where it appears that convergent orogenic activity spanned most of the Devonian and Carboniferous, ie. from about 400-300 Ma, with a suggestion that rates of deformation increased in the mid-Carboniferous (-335-310 Ma). In contrast, the syn-orogenic Pertnjara Group in the northern Amadeus Basin suggests a somewhat different story. From biostratigraphic evidence, syn-orogenic deposition started in the late Early Devonian, with evidence of rapid erosive unroofing of the orogen in the Late Devonian (Brewer Conglomerate), after which the sedimentary record suddenly terminates near the end of the Devonian. However, from published fission track studies it is evident that a significant thickness of younger sediment has been subsequently eroded. This may partiy explain the lack of Carboniferous sediment, but it is also possible that sediment supply outpaced accommodation space and sediment was transported elsewhere eg. towards basins in eastern Australia. The age of early phases of syn-orogenic sedimentation in the Ngalia Basin is not well constrained, but palynological dating of the conglomeratic Mt Eclipse Sandstone indicates an age range of Late Devonian to mid-Carboniferous, representing the only Carboniferous proximal syn-orogenic sediments known from central Australia.

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Beyond the southern and eastern Arunta Inlier there have been very few isotopic studies and the existence of ASO-aged uplift and deformation must be inferred from the preservation of biostratigraphically dated proximal syn-orogenic style sediments. Along the southern margins of the Georgina and Wiso Basins syn-orogenic sediments are confined to the centres of three large asymmetric synclinal structures, the Toko and Dulcie Synclines (Georgina Basin) and the Lander Trough (Wiso Basin). Each structure is bounded by major N or NE-directed thrusts on their southern sides. Biostratigraphy of the Dulcie Sandstone (Dulcie Syncline) and Cravens Peak beds (Toko Syncline) suggests a similar duration of sedimentation to that of the Amadeus Basin. The Lake Surprise Sandstone of the Lander Trough has no internal age constraints but its close similarity to the Dulcie Sandstone has been used to suggest approximate age equivalence. In the northeastern Officer Basin syn-orogenic sediments occur in the Munyari Trough situated to the south of a S-directed thrust system along the southern margin of the Musgrave Inlier. Late Early to Late Devonian ages have been obtained from this sequence. Syn-orogenic sedimentation is also preserved to the north of the eastern Musgrave Inlier as the Finke Group of the southeastern Amadeus Basin. Like the Pertnjara Group further north, the preserved sediment seems to be confined to the Devonian, but interestingly the succession is essentially reversed, with evidence for major uplift and denudation at the base and decreasing energy and depositional rate up-section. Several studies have suggested that these sediments were derived from the south. To the southeast, deformation of probable ASO-age is seen in the sub-surface Warburton Basin, which is also intruded by Carboniferous-aged granites. No locally derived syn-orogenic sediments are known, but the unconformably overlying conglomeratic basal Cooper Basin succession is Late Carboniferous in age, synchronous with the youngest isotopic evidence of exhumation from the Arunta Inlier. In northern Australia, the southern Bonaparte Basin contains syn-orogenic conglomerates and sandstones (Cockatoo Group) which intertongue with a biostratigraphically dated marine sequence indicating a major pulse of local uplift in the Late Devonian. An undated but otherwise identical succession in the adjacent Ord Basin (Mahony Group) is assumed to be of the same age. In the Bonaparte Basin a second major pulse of activity is recorded by the midCarboniferous conglomeratic Border Creek Formation. In contrast to the thrust dominated structural style of uplift in central Australia, the Ord and Bonaparte Basins are distributed along a major N-NE trending dextral wrench zone which effectively marks the western limit of ASO-aged deformation in Austraha. Although the effects of the ASO are most pronounced in the southern Arunta Inlier and adjacent Amadeus Basin, the total extent of deformation covered a significant proportion of the existing Australian continent. Syn-orogenic sedimentation indicates fairly synchronous initiation of local uplifts across central Australia in the late Early Devonian (Emsian: -390 Ma). In most areas the record is terminated towards the end of the Late Devonian (Famennian: -360 Ma), except in the Ngalia Basin. In the Ord and Bonaparte Basins there is evidence of two discrete pulses of transcurrent activity in the Late Devonian and midCarboniferous. The general lack of preserved Carboniferous sediments in central Australia may at least partly reflect subsequent erosion, as seems to be the case in the northern Amadeus Basin. However, the lack of isotopic data from basement involved areas other than the southern and central Arunta Inlier makes it unclear if Carboniferous exhumation and sedimentation should be expected in other areas. It is possible that the ASO was initially extensive but deformation became more locally focused during the Carboniferous. 94

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INTRAPLATE DEFORMATION IN CENTRAL AUSTRALIA, BETWEEN SUBSIDENCE AND FAULT REACTIVATION

THE

LINK

Martin Hand and Mike Sandiford Department of Geology and Geophysics, University of Adelaide, Adelaide SA, 5005. (mhand@geology.adelaide.edu.au) Central Australia has experienced two intraplate orogenic events that involved significant north-south shortening: the late Neoproterozoic to early Cambrian Petermann Orogeny and the Devonian to Carboniferous Alice Springs Orogeny. During each event, preexisting structures inherited from Mesoproterozoic terrain amalgamation were reactivated and basement rocks were exhumed from beneath thick sedimentary successions accumulated in the Centralian Superbasin, which was a broad intracratonic basin that covered much of the central Australian region. The pattern of fault reactivation during the Petermann and Alice Springs Orogeny's shows a striking similarity to the pattern of subsidence in the overlying Centralian Superbasin. Immediately prior to the Petermann Orogeny, the basin was thickest in the vicinity of the Musgrave Block, the region in which deformation was subsequently localised. At the same time, crustal-scale faults elsewhere in central Australia (e.g. the Redbank Shear Zone in the Arunta Block) that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate north-south shortening. Between the Petermann and Alice Springs Orogeny's, subsidence patterns shifted, such that fault systems in the Arunta Block and also those in the southern Musgrave Block were buried by significant thicknesses of sediment, whereas the major structures that were exhumed during the Petermann Orogeny were not significantly buried. During the Alice Springs Orogeny reactivation once again occurred along the most deeply buried faults, even in the instances where those faults had remained inactive during the earlier Petermann Orogeny. Importantly the major Petermann-aged structures that were not buried during renewed subsidence remained inactive during the Alice Springs Orogeny. The record of reactivation implies that the presence of pre-existing crustal-scale faults alone was insufficient to localise deformation. Rather, fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale. The correspondence between the distribution of basement fault reactivation and subsidence patterns during both the Petermann and Alice Springs Orogeny's implies a Hnk between relatively thick sedimentation and long-term lithospheric weakening. We show that this link is compatible with the thermal effects of a thick sedimentary blanket. In the context of central Australia, the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the Proterozoic basement.

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SUBMERGED AND DETACHED PORTIONS OF THE NEW ENGLAND AND LACHLAN FOLDBELTS EAST OF THE TASMAN SEA H J . Harrington Department of Geology, Australian National University (hjh@geology.anu.edu.au) Department of Geology and Geophysics, The University of Sydney Some comments on the map are:1.

It is a matter for reflection that the area of exposed Lachlan Foldbelt in southeast Australia and the Western Province of New Zealand is about the same as the unexposed area in the Lord Howe Rise (but the latter was extended by about 35% during Mesozoic opening of the Tasman Sea).

The New England Orogen possibly does not terminate in the south near Newcastle but continues along the eastern side of the Lachlan to somewhere west of North Island, New Zealand. It is truncated before reaching South Island.

The Brook Street, Murihiku and Maitai terranes or subprovinces of New Zealand are bent and telescoped by intense strain close to the southeast side of the Alpine Fault but it has been well established that they widen eastwards in Campbell Plateau. It is predicted that in the same way they widen northwestwards from the Alpine Fault. This can be tested by using the Stokes Magnetic Anomaly System.

The writer has maintained for 25 years that the Permian Highbury Volcanics and the Triassic sediments of the Gympie Province can be correlated with the Brook Street volcanics, and with the Triassic of the western flank of the Murihiku Synchne of New Zealand.

Similarly, 80 years of studies that started with Piroutet and Benson and continue with Black (1996), the Campbells, Grant-Mackie and others, have shown that the Western Province of New Caledonia correlates closely with central and easterly parts of the TriasJura Murihiku Syncline.

The Lachlan Fold Belt in the Western Province of New Zealand is separated from the Brook Street Volcanics by the telescoped Median Tectonic Zone (MTZ).

Geochemical studies have shown that the Brook Street and Highbury volcanics were parts of an intra-oceanic island arc.

The docking of the arc with Australia and New Zealand is thought to have occurred in the Mid Trias (Anisian - Ladinian) when the Sydney, Bowen, Cooper and Galilee basins were simultaneously deformed, and deposition started in the Murihiku Basin.

The MTZ (Extended) and the Brook Street Volcanic Arc cut obliquely across the Lachlan and New England fold belts. This continental-scale truncation means that parts of the fold belts have been removed. The break-up probably occurred in the latest

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Carboniferous and earliest Permian, and had effects to the west, causing the initial openings of the Sydney - Bowen Basin and other basins. 10. The search for the detached portion of Australia is pointing to North America west of the Roberts Thrust. REFERENCES Black, P., 1996. Mesozoic Evolution of the Norfolk Ridge System: Evidence from New Caledonia and Northern New Zealand. Geological Society of Australia Extended Abstracts 43: 90-93. Harrington, H.J., 1998. The Basement Geology of Lord Howe Rise and Norfolk Ridge Predicted by Projections from Australia and New Zealand. Pacific Exploration Technology Conference, Abstracts: 33-36. South Pacific Commission, Suva.

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TECTONICS OF THE NORTH-EASTERN TIBETAN PLATEAU M.Harrowfield and CJ.L.Wilson School of Earth Sciences, The University of Melbourne, Parkville, Vic.3052, Australia Email: m.harrowfield@student.unimelb.edu.au (Mathew Harrowfield). cjlw@myriad.its.unimelb.edu.au (Chris Wilson) North and east of the Himalayas, the Asian crust has been structured by several coUisional events. Amongst these events, the Indosinian or Cimmeride orogeny is thought to have affected a large region in southeast Asia, including northeastern Tibet, during the middle Triassic. The presently active, sinistral Xianshui He Fault is a lithospheric scale strike-slip fault in the eastern Himalaya, responsible for 4 earthquakes of magnitude >7 this century. In the past the Xianshui He Fault has been considered a Cenozoic feature resulting from the eastward extrusion of Asia due to northward penetration of the Indian plate. Our studies suggest that Himalayan activity on the Xianshui He Fault is a late reactivation of a major Triassic structure initiated c.200Ma.

Figure 1. Map of Sichuan Provence, People's Republic of China. The geometry of the Xianshui He Fault and Longmen Shan Thrust-Nappe Belt are illustrated, forming the transition from the Songpan Ganze Fold Belt to the Sichuan Foreland Basin.

The western margin of the Yangtze craton is marked by the Longmen Mountains Thrust-Nappe belt, a NE-trending zone of non-coaxial shear recording differential strain between the Yangtze platfrom's passive margin and what is now the neighbouring Songpan Garze Fold Belt (SGFB) : a Permo-Triassic marine basin which was closed and extruded onto the Yangtze platform during the late Triassic Indosinian orogeny (Fig.l). Subsequent tectonic loading of the cratonic margins has formed the Sichuan Foreland Basin (Chen et al., 1994). Our study concentrates on the structral and metamorphic history of an area of more than 50,000 square kilometres, surrounding the intersection of the Longmen Mountains Thrust-Nappe Belt and the Xianshui He Fault. Several earlier studies have documented the structural development of the Longmen Mountains Thrust-Nappe Belt (Chen and Wilson, 1996), all recording one dominant tectonothermal event in which foreland nappe development was synchoronous with transpressive strike-slip shear in the hinterland (Worley and Wilson, 1996). By correlating structural style.

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distribution, timing and metamorphism, the ductile deformation along the Xianshui He Fault is found to be synchronous with the development of the Longmen Mountains Thrust-Nappe Belt. Furthermore, transpressive shear parallel to the Xianshui He Fault is found to be conjugate to that observed on the Longmen Mountains Thrust-Nappe Belt, both of w^hich yield north-south palaeostress solutions (Fig.2).

Figure 2. Palaeostress solutions for the synchronous transcurrent shear observed on the Xianshui He Fault and the Longmen Shan Thrust Nappe Belt, both of which indicate North-South directed compression.

Chen, S., Wilson, CJ.L., Luo, Z.L. and Deng, Q. 1994. The evolution of the Western Sichuan Foreland Basin, S.W. China. J. Southeast Asian Earth Sciences , 10, 159-168 Chen, S.F. and Wilson, C.J.L. 1996 Emplacement of the Longmen Mountains Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau. J. Struct. Geol 18, 413-430. Worley, B.A. and Wilson, C.J.L. 1996 Deformation partitioning during transpressional orogenesis and the role of granite magmatism, an example from the Central Longmen Mountains, China. 7. Struct. Geol 18, 395-411.

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THE I-S LINE: A THRUST SHEET BOUNDARY IN EASTERN VICTORIA. Marc Hendrickx. Geological Survey of Victoria, P.O. Box 500, East Melbourne, Vic., 3002. Email: marc.hendrickx@dme.nt.gov.au A reappraisal of the distribution of I and S type granites east of the Governor Fault in eastern Victoria shows that S-type granites are confined to the Omeo-Wagga and Buchan Zones. They are absent in the Tabberabbera Zone and only occur in the Mallacoota Zone in association with high grade metamorphic complexes (e.g. Cooma and Kuark). The distribution suggests the presence of another I/S line, west of that proposed by Chappell and White (1974). It is proposed that these two lines link up and that this new I/S line coincides with the boundary of a major thrust sheet, emplaced into its current position by south directed movement during the Silurian. The thrust sheet includes the Omeo-Wagga and Buchan structural zones. The boundaries to the thrust sheet in Victoria are defined by the KancoonaKiewa-Cassilis-Ensay and Yalmy-McLauchlan Fault zones (Figure 1). These appear to be continuous under the Lower Devonian Buchan Rift. The I/S line represents the boundary between duplicated Ordovician-Silurian crust in the Omeo-Wagga and Buchan zones and thinner Ordovician-Silurian crust in the Tabberabbera and Mallacoota zones (Figure 2). Recognition that the I-S line represents an upper crustal feature defining the edge of duplicated Ordovician crust has important implications for granite genesis models in the Lachlan Fold Belt. This model does not allow for the presence of Proterozoic crust underneath the eastern Lachlan Fold Belt and hence does not provide support for the restite model for granite genesis (eg. Chappell et al., 1988). However it does provide a method for burying large amounts of Ordovician metasediments to depths in excess of 20 km required by the three component mixing model of Collins, 1996. References: Chappell, B.W. & White, A.J.R., 1974. Two contrasting Granite types. Pacific Geology 8, pp.173-174. Chappell, B.W., White, A.J.R. & Hine, R., 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 35, pp. 505-521. Collins W.J., 1996. Lachlan Fold Belt granitoids: products of three-component mixing. Transactions of the Royal Society of Edinburgh: Earth Sciences. Vol.87, pp.171-181.

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Figure I. Geological sketch m a p of eastern Victoria showing the proposed position of the 1-8 line: coinciding with the Karicoona-Kiewa-Cassilis-Ensay fimlts on the west side of the JJuehan Rift and the Yaimv-McLaiichlin faults on the east side. Note the absence of S~t\'pe granites in the Tabberabbera Zone. y^^ Thrust Faufe

•

Structural trend lines

Vblcanics, Limestone

Lower Devonian ZH

gmH \\

l/S line

Upper Silurian (Ludlow-Prldoll)

Schematic Cross Section Line

:i[rii

Uniestone Creek Graben. Wombat Creek Graben. Mount Tambo Group Sanrfine Creek Gratjen S Type granite (eg. Bulient)afong Suite)

Lower Silurian (Wenlock) Lower Silurian (Llandovery)

Yafmy Group S-Type granite, {Cooma Suite)

(earty Llandovery) Ordovician

f-Type granites Boggy Plains Supersuite

Low metamorphic grade

m-m

^ Deep marine High metamorphic grade j ^ sediments Volcanics ?? Imbricated ocean crust

CSZ: Cassiiis Shear Zone B.F: Banmouth Fault Lucas Point Fault

Omeo Zone

l/S line

TabberabberaMallacoota Zone

Simple deformatior. ^

-30 Km?

Figme 2. Schematic NW-SE cross section through eastern Victona showing crustal architecture ai the end of the Benambran Orogem m the Early Silunan. I h e area of thickest cmst coincides with a stack of tlirust slices of high and low grade Ordovician nieiasediments underneath the Yaimy Fold and Thrust Belt.

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STRUCTURAL OF THE PAPUA NEW GUINEA AND IRIAN JAYA FOLD BELTS Kevin C. Hill, R.D. Kendrick, E. Sutriyono and J. Keetley. Australian Geodynamics Cooperative Research Centre, Earth Sciences, La Trobe University, Melbourne, Australia 3083 Both structural and tectonic models for the evolution of PNG have evolved through time, using the popular model of the day. This is largely because of the lack of data in the karstic jungle-covered terrane of PNG, requiring the use of external models. In the 60's, interpretations v^ere dominated by geosynclinal models involving vertical uplift of a hinterland basement complex and gravity sliding of the cover rocks to create the fold and thrust belts. The individual structures were interpreted in terms of the Ramsay-type structures prevalent then, ie. mainly asymmetrical folding with occasional break-through thrust-faults. In the 70's, interpretations evolved to incorporate the new plate tectonic models, ie. arccontinent collision creating an uplifted basement complex with subsequent gravity spreading to build the fold and thrust belt. However, using the well known mountain belts of North America as an analogue, the individual structures were interpreted as imbricate thrusts with relatively little folding. In the 80's, rigour was seemingly applied to the geometrical analysis of fold and thrust structures, assuming that the fundamental structure was the through-going detachment and associated thrust ramps and that the folds formed passively. Regional interpretations involved Neogene and Mesozoic duplexes detached above basement, but also suggested some basement inversion. Most models for the tectonic evolution of New Guinea involved Early and Late Miocene arc-continent collisions, creating an orogenic belt. More rapid progress was made in the 90's following the acquisition of much more field and borehole data due to the success of exploration and new geophysical techniques. However, interpretations were still model driven. As elsewhere in the world inversion structures became popular and, supported by magneto-telluric data, the fold belt was interpreted to have minimal shortening and to be due to compressional reactivation of previously extensional faults in basement. However, drilling of the Kutubu Field wells demonstrated more complex structures with considerable asymmetrical folding and breakthrough thrust-faults (as inferred in the 60's). Field analogues from Cape Liptrap, near Melbourne and structural experiments indicated considerable thickness changes in the Jurassic and Cretaceous shales and that the competent sandstones and limestones may have deformed independentiy. The Irian Jaya Fold Belt and Lengguru Fold Belt are less well known than the Papuan Fold Belt along strike and it was found that the Papuan models were not generally applicable, particularly to the Irian Jaya Fold Belt. There are substantial and important differences between the areas, which affect the structure and prospectivity:1. The Irian Jaya Fold Belt is nearly twice the length of the Papuan Fold Belt, which is twice the size of the Lengguru Fold Belt. 2. The Irian Jaya Fold Belt, reaching 5 km asl, is much higher than the Papuan Fold Belt, generally at <3 km asl, whilst the Lengguru Fold Belt is low-lying at km asl. This suggests substantially more shortening recently and/or deeper detachments in the Irian Jaya Fold Belt.

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3. The Irian Jaya Fold Belt is generally narrower than the Papuan Fold Belt, whilst the Lengguru Fold Belt is of comparable width in the south, but tapers to nothing in the north. Coupled with the elevation data, this indicates that the Irian Jaya Fold Belt has undergone comparable shortening but focussed in one locality. In contrast the Lengguru Fold Belt has spread the shortening over a wide area. This indicates that the strength and discontinuities in the underlying lithosphere had a major influence on structural evolution. 4. The Irian Jaya Fold Belt trends uniformly E-W, only turning SE near the Papuan border. The Papua Fold Belt trends ESE, whilst the Lengguru Fold Belt is highly oblique, trending NNW. Again theis indicates an underlying structural control in the Irian Jaya Fold Belt. 5. Irian Jaya was influenced by Late Proterozoic-Paleozoic NW Shelf rifting, resulting in up to 8 km of pre-Mesozoic sediments, including Late Proterozoic, Ordovician and Permian detachment horizons. In contrast, PNG basement comprises Permo-Triassic intrusives into low-grade Paleozoic metamorphics, indicating that more complex structures can be expected in Irian Jaya. 6. Due to NW Shelf rifting the Irian Jaya Fold Belt is underlain by oblique graben structures, such that the graben-bounding faults are preferentially reactivated during inversion of the marginal basins leading to the development of regional cross-cutting structures. 7. Paleozoic strata commonly occur at surface along major fronal thrusts in the Irian Jaya Fold Belt, confirming the deep detachments, but basement exposures are rare in the Papuan Fold Belt and occur only in the hinterland of the Lengguru Fold Belt. 8. The Irian Jaya Fold Belt contains common ESE trending oblique thrusts within the E-W belt, suggesting a commong compression direction to PNG, but an underlying E-W crustal fabric. 9. Unlike the other two belts, the Lengguru Fold Belt has common normal faults cutting across the belt and substantial normal faults in the hinterland, throwing down to Cenderawasih Bay. This area has been subject to post-orogenic collapse, perhaps due to extension and formation of oceanic crust in Cenderawasih Bay. 10. Like the Papuan Fold Belt, the Lengguru Fold Belt can be divided into structural provinces with different structural styles, including large anticlinal structures similar to the oilfields in the Kutubu and Gobe areas and the gas-condensate field at Hides. The dominant structure evident in the Irian Jaya fold belt is the Mapenduma Thrust, a crustal-scale fault across which changes in facies and >8km thickening of the pre-Tertiary section occur. The Mapenduma Thrust is interpreted as a reactivated shelf-edge extensional fault which accommodates -52 km of shortening divided between detachments at the base of the Nerewip Formation and a much deeper mid- to lower crustal level. An older, structurally higher thrust system is preserved in the hangingwall of the Mapenduma thrust. Thin-skinned thrusting within the Kembelangan and New Guinea Limestone Groups is largely the result of slip along detachments in the lower Piniya and Kopai Formation shales. Total shortening recorded in this system is approximately 17 km. Complex faulting and folding in the overlying limestones is depicted on an enlarged section which details backthrusting and interbed slip in the New Guinea Limestone. The Lengguru Fold Belt is highly arcuate and divisible into structural Belts, just like the eastern Papuan Fold Belt, which contains all of PNG's currently discovered oil reserves. The two Fold Belts are similar, but a key difference is the Pleistocene extensional faults in the Lengguru Fold Belt, that both cross-cut the belt and down-faulted the northeastern part of the 104

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Belt towards Cenderawasih Bay. This is here interpreted to be due to Pleistocene orogenic collapse associated with the opening of oceanic crust in Cenderawasih Bay. A second difference is the elevation of the fold belts and adjacent foreland basins. Seismic data from the offshore portion of the Lengguru Fold Belt (Fig. 3.22) clearly show that the frontal anticline was >1 km higher than at present as it was denuded, but has since subsided below sea level to be onlapped by Pleistocene sediments. This is consistent with a regional subsidence of the Lengguru Fold Belt to its present low elevation, probably associated with Pleistocene extension. A fundamental difference in lithospheric strength must be the basis of some of the variation noted between the fold belts. In the Irian Jaya Fold Belt a thrust sheet consisting of Late Proterozoic through Tertiary strata was transported up an inverted extensional fault to the surface reaching elevations of 5+ km. Virtually no deformation is evident in the upper portion of the footwall with a foothills belt only a few kilometres wide and minor deep shortening accommodated by folding within basement. A similar situation is found in the Digul Uplift in the easternmost Irian Jaya Fold Belt where a Lower Paleozoic through Tertiary sequence is thrust over Tertiary limestones and molasse sediments in the form of a large monocline. Here the belt of deformed footwall rocks is again only a few kilometres in width. The Muller Anticline in western PNG also shows large basement thrusting and a narrow fold belt. This structural style is in contrast to the wide, arcuate and more shallowly detached Lengguru Fold Belt and the Papuan Fold Belt east of the Bosavi Lineament. Hill (1991) interpreted the change in structural style in the Papuan Fold across the Bosavi Lineament to be due to stronger lithosphere to the west, beneath the Muller Anticline. It seems likely that this strong lithosphere continues to the east beneath the Irian Jaya Fold Belt, controlling the structural style. It probably also continues north to the original shelf-edge fault, which was reactivated as the Mapenduma thrust. The strong lithosphere focussed the deformation along the Mapenduma thrust, supporting the very high mountains that resulted and flexed down in the foreland to create a foreland basin. In contrast, the Lengguru Fold Belt, like the eastern Papuan Fold Belt, is probably underlain by young, weak, hot and broken lithosphere, perhaps in part oceanic?, and hence was more prone to subsidence within the Fold Belt, resulting in lower broader mountains. Acknowledgements This abstract is published with the permission of the Director of the AGCRC. The sections were constructed using Geosec, made available by Paradigm Geophysical Ltd. Structural analysis of New Guinea was made possible by collaboration with the Indonesian Geological Research and Development Centre and the GSPNG and by support from:- ARCO, BP British Gas, Caltex, Chevron, Conoco, Esso, Highlands Gold, Lasmo, LL&E, Mobil, Oil Search JV, PT Freeport and Union Texas amongst others.

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FLUID TRANSPORT IN DEFORMING OVERPRESSURED ROCK MASSES B.E.Hobbs and A. Ord Australian Geodynamics Cooperative Research Centre, CSIRO Exploration & Mining, Private Bag, Wembley, Western Australia, 6014. email: b.hobbs@per.dem.csiro.au In the absence of a driving force which maintains a lithostatic fluid pressure gradient, a column of overpressured fluid in a rock mass with interconnected porosity will flow until a hydrostatic fluid pressure gradient develops. The time scale for this to occur is short compared to geological time scales. This means that the top of the column becomes overpressured, and the base of the column underpressured, with respect to lithostatic pressure. Given a high enough column of interconnected fluid, the top will ultimately deform to relieve the overpressure, by generating new porosity, and the base will deform to close up porosity. The density difference between the fluid and the country rock can drive this fluid compartment upwards once the compartment exceeds a critical height. Connolly has demonstrated that waves of these higher porosity packages can propagate upward, in a rock mass of intrinsically low permeability, from a devolatilization front. The situation examined by Connolly arises in a rock mass which is isotropically stressed, the process being driven by buoyancy induced stresses which in turn produce porosity ahead of the rising porosity wave. The propagation of these waves is controlled by the rheological distribution in the rock mass. In a low permeability rock mass which is deforming under conditions where the deformation induces new porosity (and related permeability) an additional (but related) process arises in addition to these porosity waves. In regions where the effective stress induces deformation, higher permeability regions are developed which can themselves behave as porosity packages. If these exceed a critical height they move upwards under the influence of buoyancy. These porosity packages depend only on deformation for their development and not upon porosity generated at a devolatilization front. The structure within these deformation induced porosity packages depends upon the stress field in the package. We propose here that the vein systems common in many overpressured metamorphic terrains constitute the structure within the deformation induced porosity packages. We contrast here the following two situations: * Darcy flow in a rock mass which is deforming by a shortening deformation in which folding takes place together with the development of shear zones. The constitutive behaviour is elasto-plastic with deformation induced dilatancy (including the ability to hydrofracture once the effective stress induces yield). Fluid flow is focussed, the focussing being controlled essentially by deformation induced changes in pore-pressure and permeability rather than by the rheology of the material. * Flow which takes place through the propagation of relatively high permeability packets, the permeability distribution being controlled by deformation induced permeability increases and decreases once the material yields. The distribution of yielding regions is controlled by the externally imposed deformation and the (heterogeneous) distribution of effectives stress. Fluid flow may or may not be focussed but the overall control on fluid flow is now the

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Theology and stress state of the material rather than the initial porosity and permeability distribution. The geological consequences of these two contrasted modes of fluid transport is that under conditions of Darcy flow, fluid is expected to be focussed into rocks of high intrinsic permeability such as sandstones which are commonly strong relative to shales. However, under conditions where fluid flow occurs by porosity package migration, fluid is expected to be focussed into rocks of low strength such as shales or schists which have low intrinsic permeability. We apply these concepts to several overpressured regions such as the Yilgam, Broken Hill and the Ballarat-Bendigo region in an attempt to relate the observed structures with fluid transport mechanisms.

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STRAIN CALCULATOR, GHOSHFLOW: PROGRAMS TO CALCULATE AND MODEL STRAIN, SHEAR, AND VORTICITY PARAMETERS R. J. Holcombe (University of Queensland, Queensland 4072, Australia; e-mail: rodh @ earthsciences. uq. edu. au) STRAIN CALCULATOR Strain Calculator provides simple, fast calculation and display of various strain and shear strain Calculations Bint Strain parajnelefs Shear X/Y

X/2 ;

Volume change Sirmn Pammeters

At constant volunrte: X;Y:Z - 3.34:1.e9;0,27 FImn Rot X/y - 3M lY/Z '

k - 0,69 r-6.06

Ramsay log plat: togX/Y - 0.49 LogZ/Y - -a.B Nadai stfoin magnHyde - 1.77 Lade's parameter ^ -0.08

5.5

fTT

LpqXft' t^qZ/Y i jo |o / i

^Dsil 1;||i||| Zoom Log X/Y y

5 pts

Log Z/Y

i Strain Calculations S i - Rjnt parameters. The displays can be Strain pammefers copied to the clipboard for j' Kin Vartidty No ' Rotated Lmo inclusion in other documents. B e 3-D finite strain parameters \?7 hi 3? SOX ' I8 1 • Pvm (principal stretches, strain Vartafate^ T C O C C ,: Shear Shtear Shear 0 magnitude, strain type, etc) are Simple shear param^ anTRu^er, W f f ; a n d 2,12) calculated and displayed given input either as pairs of 2-D strain ratios or as arbitrary X:Y:Z ratios. Shear parameters for both simple shear and general shear flow, (including ellipticity, rotation component, orientation of initial and final strain axes, initial and final orientations of rotated markers) are displayed with any parameter being used as an input variable.

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GHOSHFLOW Rotation of rigid and passive objects in plane strain general shear

continuous.,.

1.5 ^

gamma (shear strain) K orient^^ 88; R 16:3 Show: r Long axe-. Wk (kinematic vortrcity number) C Si ^ Long axe^ Shear pletie: 90 (Sense n/g)

H&lt cpntinyous,..

^^n m m sn mfi if:n Long axis orientation vs aspect ratio

100 120

160 180

Long axis vs Si orientations

5 Smallest stable asped ratio: 1 7 with stable oiientation of: 120.

GhoshFlow models the orientation distributions produced by the rotation of rigid bodies of different aspect ratios undergoing general plane strain shear. It is based on the mathematical relationships developed by Ghosh and Ramberg (1976) and Ghosh (1993). In addition to various plots of the orientation distribution of the long axes of bodies that originally had a uniform , the program also calculates the hypothetical orientation distributions of internal foliations (Si) of rotated porphyroblasts. The assumption is that the original unrotated internal foliation is parallel to the shear/flattening plane. In particular, plots of the orientation of Long Axes vs Si orientation have been found to produce a particularly sensitive vorticity and shear strain gauge (for situations fitting the model constraints). In such plots, the pattern of distribution paths for a range of different aspect ratio objects is very distinctive under any given combination of shear strain and vorticity number. The shear strain component is given by the final orientation of internal foliation within equant objects (whose rotation rate is solely a function of the shear strain rate and independent of the vorticity number). For any given shear strain the distribution pattern changes greatly with change in vorticity number. The strength of using such plots as a vorticity gauge is that only the pattern for different aspect ratios need be recognised, not the complete distribution, hence the gauge is not dependent on the initially orientation distribution. Strain Calculator and GhoshFlow are programs written for running on PCs under Windows95 or Windows NT. They are available free for downloading from the UQ Earth Sciences Software Development site at: http//www.earthsciences.uq.edu.au/~rodhysoftware/ References: Ghosh, S.K. and Ramberg, H. 1976. Reorientation of inclusions by combination of pure shear and simple shear. Tectonophysics, 34, 1-70. Ghosh, S.K. 1993. Structural Geology: fundamentals and modem developments. Pergamon, 598p.

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STRUCTURAL ANALYSIS OF THE SOLDIERS CAP TERRANE, SANDY CREEK AREA, MOUNT ISA INLIER, AUSTRALIA. Megan Hough, Bregje Hulscher, Laurent Ailleres, Gordon Lister and David Giles Australian Geodynamics CRC, Department of Earth Sciences, Monash University mhough@mail.earth.monash.edu.au Detailed structural mapping of the Sandy Creek area (southern Eastern Successions) provides an understanding of its structural evolution during the Isan Orogeny. Three fabric generating events are evident from the overprinting relations found throughout the area. They are associated with dominant recumbent D2 and upright D3 mesoscopic to macroscopic folds. Stratigraphy The lithological units of the "Soldiers Cap Terrane" were originally assigned to the Kuridala formation, a unit of Cover Sequence 2 (Blake, 1987; Blake & Stewart, 1992). Later detailed studies in the Eastern Successions correlated the upper three units of the Kuridala Formation with the Soldiers Cap Group (Derrick et al., 1976), although both units were considered to be of Cover Sequence 1 age (Beardsmore, 1988). Recent age dating (Page, 1998) indicates that the units of the Soldiers Cap Terrane are equivalent to Cover Sequence 3 elsewhere in the Inlier. Structural Elements Bedding on the outcrop scale is best observed in alternating psammitic and pelitic units. However, due to the intensity of deformation and grade of metamorphism throughout the area, bedding was sometimes difficult to discern from the dominant regional fabric, S2. Younging criteria is poorly preserved. Unequivocal F^ folds were not observed in the area. Where preserved, the Sj fabric is defined by the preferred alignment of metamorphic recrystallized micas in the limbs of S2 crenulations. S2 is the dominant continuous cleavage in the area and is axial planar ro F2 folds. F2 folds are tight to isoclinal with recumbent to gendy inchned axial planes. Bedding and Sj are often transposed into the dominant regional S2 fabric, especially in high strain zones. F3 folds are upright, open to tight, moderately north plunging and vary in amplitude from outcrop to kilometre scale. The S3 axial plane foliation of F3 folds is non-penetrative and forms open to tight crenulations of the S2 fabric. The F3 folds, concentrate strain in the hinges where the S3 becomes penetrative. F3 folds can be non-cylindrical with assimilating and en echelon crenulation fold axes. The plunge of F3 varies across the study area, becoming shallower to the east. In micaceous units, there is evidence for the development of two later non-penetrative, postD3 fabrics. The fabrics seem to result from progressive deformation related to localised high strain zones. Post D3 deformation is also characterised by strike-slip faults associated with metasomatism and brecciation. Summary

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The present geometry of the Sandy Creek area is dominated by upright F3 folds in the west and by recumbent F2 folds to the east. The significance of the early Sj fabric is uncertain. D2 is interpreted to be associated with peak metamorphism, and may have involved folding and thrusting resulting in shallowly north plunging, isoclinal, recumbant F2 folds. Continuing progressive east-west compression during D3 resulted in upright, N to NNE trending, close F3 folds. The Sandy Creek area seems to comply with the recent tectonic models developed within the Eastern Successions (O'Dea et al., 1997; MacCready, 1998). These models, based on structural mapping and seismic interpretation, suggest an early period of thin-skinned tectonics evolving to thick-skinned tectonics over time. Acknowledgments: Work reported here was conducted as part of the Australian Geodynamic Cooperative Research Centre and this abstract is published with the permission of the Director, AGCRC. The authors would like to thank BHP Minerals, North Exploration, Normandy Mining Ltd and Placer for their financial support. References Blake, D.H. 1987. Geology of the Mount Isa inlier and environs, Queensland and Northern Territory. Bureau of Mineral Resources, Bulletin 225, Canberra. Blake, D.H & Stewart, A.J. 1992. Stratigraphic and tectonic framework. Mount Isa Inher. In: Stewart, A.J. and Blake, D.H., eds. Detailed studies of the Mount Isa inlier. Australian Geological Survey Organisation, Bulletin 243, 1-11. Beardsmore, T.J., Newbery, S.P. & Laing, W.P. 1988. The Maronan Supergroup: An inferred early volcanosedimentary rift sequence in the Mount Isa Inher, and its implications for ensialic rifting in the Middle Proterozoic of north-west Queensland. Precambrian Research 40/41, 487-507. Derrick, G.M., Wilson, I.H. & Hill, R.M. 1976. Revision of the stratigraphic nomenclature in the Precambrian of northwestern Queensland: V. Soldiers Cap Group. Queensland Government Mining Journal, 77, 601-604. MacCready, T. 1998. Geologic Interpretation of the Mount Isa Deep seismic transect. Unpublished PhD thesis, Monash University, Melbourne. O'Dea, M.G., Betts, P.G., MacCready, T. & Ailleres, L., 1997. Sequential development of a mid-crustal fold thrust complex in the eastern Munt Isa Inlier, Australia. Australian Crustal Research Centre, Technical Pubhcation 47. Page R.W. 1998. Aspects of geochronology and crustal evolution in the Eastern Fold Belt, Mt Isa Inlier. AustraHan Journal of Earth Sciences, 45, 343-361.

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REASSESSMENT OF THE EVOLUTION OF THE MOUNT PAINTER INLIER, SOUTH AUSTRALIA Chris Janka, Paul Bons, Marlina Elburg, & Jon Dougherty-Page Dept of Earth Sciences, Monash University, Clayton, Victoria, 3168, Australia The Mount Painter Province is a polydeformed Proterozoic Inlier situated on the north west comer of the Curnamona Craton, northern Flinders Ranges, South Australia. The basement is a complex sequence of meta-igneous and meta-sedimentary units, assumed to be Palaeoproterozoic, that are overlain by the younger Adelaidean units which constitute the bulk of the Flinders Ranges. The region was mapped by Coats and Blissett in the early 1970's and they recognised the upper basement units as the "unnamed member of the Freeling Heights Quartzite" that was overlain by the "Mt Neill Granite Porphyry". In the south west comer of the Inlier near Nooldoonooldoona Waterhole, Teale (1993), recognised a Mesoproterozic Trondhjemitic intrusive. This led to a revision of the stratigraphy sequence for the basement and nomenclature change to amend the stratigraphy after Coats and Blissett. The Mt Painter Inlier is assumed to be Palaeoproterozoic due to strong correlations in the lower stratigraphy with the neighbouring Clary and Broken Hill Blocks (Coats & Blissett, 1971). The assumption suggests similarities in genesis to Broken Hill and similar palaeoenvironment of deposition and emplacement (Teale, 1993). The amended stratigraphic sequence was based in U-Pb isotope ratios of the Trondhjemite, in the Nooldoonooldoona Waterhole area, that revealed a mesoproterozoic age (Sheard et al,1992). The assumption that the Tronhjemite intruded the existing granite was based on field observations by Teale (1993). This region has localised concentrated shear bands that display excellent S-C fabric and these fabrics are intensified closer to the Quartzite contact below. Sections of the region showing litde deformation (Trondhjemite) appear juxtaposed along highly deformed quartz augen and quartzo-feldspathic gneisses and schists that are derivatives of the Mt Neill Granite Porphyry. The sheared samples show evidence of two well developed cleavages with the presence of a third poorly developed fabric. A structural analysis of the Nooldoonooldoona Waterhole region showed httie or no evidence of a later stage intrusive event. Further investigation led to a complete geochemical analysis of all the undeformed and deformed samples in and around the waterhole to clarify the lithological variation seen on the metre scale in the area. Results indicate that the geochemical variation within the Mt Neill Granite Porphyry and the Trondhjemite correspond to metasomatism, replacement and alteration. Data plots indicate the removal of potassium, calcium and sodium from the system and the remobilisation of Iron throughout the samples and the Ti/Zr plot Vs Si02 for all samples indicate that this is originally a single lithological unit. Coupled localised shearing, fluid flow and metasomatism produced the lithological variation now found in the area. Therefore there is no distinct trondhjemitic Mesoproterozoic intmsive into the Mt Neill Granite Porphyry. Following the geochemical analysis several samples from the region were analysed in the Thermal Ionic Mass Spectrometer, (using Pb / Pb isotope ratios) for an accurate age constraint on both the deformed gneiss and schist samples as well as the Trondhjemite and Mt Neill Granite Porphry. Results indicate that the age range for all the samples plotted constantly, regardless of sample from the region, and these correlate to a Meoproterozoic age of 1576+_

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2.4 Ma. Therefore the large lithological variation on a localised scale is product of metasomatism, alteration, and deformation and the trondhjemite is an alteration product of the protolith which is the Mt Neill Granite Porphyry. References: Coats,R.P., and Blissett, A.H., 1971 Regional and economic geology of the Mount Painter Province. Geological Survey of South Australia Bulletin 43 pp 360-430 Sheard, MJ., Fanning, C.M., and Flimt, R.B., 1992 Geochronology and definition of Mesoproterozoic volcanics and granitiodss of the Mount Babbage InHer, northern Flinders Ranges. South Australia Geological Survey, Quarterly Geological notes, 123 pp 18-30 Teale, G.S., 1993 The Nooldoonooldoona Trondhjemiteand other newlv recognised Mesoproterozoic intrusives of the Mount Painter Province. Geological Survey of South Australia, Quarterly Geological notes, 125 pp 20-31 Teale, G.S., 1993 Mount Painter and Mount Babbage Inliers. In Drexel, J.F., Preiss, W.V., and Parker, A.J., (Eds), Geological Survey of South Australia, Bulletin 54, 1, pp 93-100

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THREE DIMENSIONAL ANALYSIS OF SUPERPOSED FOLD SEQUENCES, SNAKE CREEK ANTICLINE, EASTERN FOLD BELT, MOUNT ISA INLIER. Dylan Jeffriess, Laurent Ailleres, David Giles and Megan Hough Australian Geodynamics CRC, Department of Earth Sciences, Monash University dy Ian @ earth .monash. edu. au The Snake Creek - Cloncurry region records a set of fold styles and orientations which are out of character with the typical N-S trend developed within the rest of the Mount Isa Terrane. Fold orientations approximate either a north-south or east-west orientation. Previous authors have attributed these variation in fold orientations to multiple variations in the local geological deformation history however, controversy surrounds the relative timing of individual fold elements (Glikson and Derrick 1970, Ryburn et al., 1988; Loosveld, 1992; Lewthwaite, 1998). Detailed structural mapping completed by the Mount Isa Tectonic Synthesis team has revealed that fold structures present within the core of the Snake Creek anticline are related to the superposition of two variably aged and orientated fold sets (figure 1). First generation fold sets (Fj) identified are angular and isoclinal in style. They are associated with a well developed axial-plane foliation and mineral lineation, defined by the alignment of biotite. Second generation fold sets (F2), are dominantly north-northwest trending, tight to isoclinal, non-cylindrical and inclined to reclined in style. A well developed axial plane foliation and foliation intersection Uneation are associated with F2 folds. Measured F2 fold axes record a shallow to the south plunge. The superposition of F2 folds and their associated fabrics has resulted in the overprinting/refolding of F^ folds and their associated fabrics (Figure 1). S2 foliations either tightly crenulate pre-existing Sj fabrics or couple with them resulting in the formation of a composite S1/S2 fabric. The evolution in orientation of Sj with respect to F2 folding is interpreted to have controlled the resultant effects of S2 on S^. Tracing of regional S2 foliation trends and overprinting relationships observed within the hinge region of the Snake Creek anticline indicates that this structure is F2in age. Removal of the effects of F2 folding shows that F^ folds originally trended in a obhque direction. Similar trending west-northwest Fj folds have been documented within the Middle Creek anticline which is also interpreted by Loosveld (1992) as F2 in age. The large scale implications of Fj structures have not been fully reconciled however, preliminary findings suggest that the tectonic evolution of the Cloncurry area has involved early thrusting followed by subsequent folding. This is interpreted to be similar at a crustal scale to the tectonic evolution of the Mitakoodi Culmination (O'Dea et al., 1997), which involved initial thin skinned tectonics followed by later thick skinned tectonics. This interpretation is consistent with the present day crustal structure imaged by the Mount Isa Seismic transect (MacCready, 1998). Glikson, A. Y. and Derrick, G. M., 1970. The Proterozoic metamorphic rocks of the Cloncurry 1:100 000 Sheet area (Soldiers Cap Belt), northwest Queensland. B.M.R. 24. Lewthwaite, K. J., 1998. Multiple orogenesis and metamorphism of the mid-Proterozoic Soldiers Cap Group, Eastern Fold Belt, Mount Isa Inher. Geological Society of Australia, 49, p271.

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Loosveld, R. J. H., 1992. The geology of the central Soldiers Cap Belt, Mount Isa Inher. Australia. In: Stewart, A. J. and Blake, D. H (editors). Detailed studies of the Mount Isa Inlier. B.M.R., Bulletin 243, pp349-359. MacCready, T., Goleby, B. R., Goncharov, A., Drummond, B. J. and Lister, G. S. (in press). The Mount Isa deep seismic transect. Economic Geology. O'Dea, M. G., Betts, P. G., MacCready, T. and Ailleres, L., 1997. Sequential development of a mid - crustal fold - thrust complex in the eastern Mount Isa Inlier, Australia. Australian Crustal Research Centre, Technical PubHcation No. 47. Ryburn, R., Wilson, I. H., Grimes, K. G. and Hill, R. M., 1988. Cloncurry, Queensland. B.M.R., Australia, 1:100 000 Geological Map Commentary. Acknowledgments: This abstract is released with the permission of the Director of the Australian Geodynamics Cooperative Research Centre. The authors gratefully acknowledge the financial assistance provided by the sponsors of the Mount Isa Tectonic Synthesis - BHP Minerals Exploration Pty. Ltd., Normandy Mining Limited and North Exploration Pty. Ltd. Figure 1. Structural geology of the Snake Creek area including equal-area projections for all collected structural data.

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Volume

N =94

8 o

CX) CD

Soldiers Cap Group Dolerite Dyke

Granite

First Generation Foliation

Mesa Capping

Second Generation Foliation

Intersection Lineation

0—

Vergence

Creeks

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Bedding Attitude Bedding Form Surface First Generation Fold Second Generation Fold

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CONVERSATIONS BETWEEN ELLE, FLYNN, BASIL, SYBIL & MANUEL: THE FUTURE OF MICROSTRUCTURE SIMULATIONS? Mark Jessell, Lynn Evans, Paul Bons, Terence Barr & Kurt Stuwe* Dept of Earth Sciences, Monash University, Clayton, Vic, 3168. *Now at: Institut fiir Geologic, Universitat Graz, Heinrichstr 26., A-8010 Graz, Austria mark@earth.monash.edu.au

The microstructure of a rock is the spatial and geometric configuration of all those components that make up a rock and reflects part or all of the geological history of that rock. Microstructures are the product of a wide range of competing processes, and rocks from most tectonic settings have undergone simultaneous deformation and metamorphism. The aim of this project is to solve a class of problems in structural geology and metamorphic petrology where the kinetics of the deformation processes and metamorphic reactions involve similar time scales, and thus do not lend themselves to solution using currently available physical or numerical modelling systems. The investigation of deformation microstructures generally assumes the dominance of a single deformation process, and this is no doubt in part because the theoretical framework for multiprocess systems is still in its infancy. The interpretation of metamorphic assemblages generally takes account of the impact of deformation to distinguish different generations of minerals, however the role of deformation on the distribution of minerals in a single assemblage has not been studied in detail. This project has concentrated on building the basis for a numerical model that describes all the fundamental grain scale deformation and metamorphic processes. This is in contrast to, but will draw upon, the many studies that are concerned with the details of a single process. While these studies are of enormous value, we believe that at least the first order behaviour of these processes is understood, and that we can learn much more by taking this understanding and applying it to the interplay of the processes than by continuing to study individual ones. In order to develop a microstructural modelling system based on individual micro-process simulations we have divided up the tasks involved. We define a universal data structure that describes the geometry and chemistry of the microstructure at any instant, process algorithms that modify this data structure, and a central control program that defines which processes are active (known as Elle). Data structure The basis of the model is a 2-dimensional finite element map of a grain aggregate (a). Each grain is represented by a polygon defined by a number of straight boundary segments and possesses a number of attributes including the mineralogy and a compliance tensor. Each grain can be subdivided into a number of polygonal sub-grains or Flynns, each of which in turn has a number of properties assigned to them, such as crystal lattice orientation (b).

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Finally, all polygons are triangulated, using a Delauny triangulation routine, to the smallest elements (c). These elements also have individual properties, including trace element concentration and, most importantly, the stress and strain rate state during deformation. This hierarchical data model allows us to efficiendy describe a grain aggregate consisting of different minerals, which may have sub-grains with slight variations in lattice orientation and gradients in stress state and trace element composition. Grain and sub-grain boundaries can have properties assigned to them as well, such as width and the presence and composition of a grain boundary fluid (d).

Process algorithm Each process algorithm describes and enacts the effects of one grain-scale process, and is written to be independent of any particular driving force, so that processes which can have several independent driving forces can be modelled (such as grain boundary migration), and so that as new concepts evolve for the driving forces for these processes, they can be included. These process algorithm can be invoked in any combination by the Elle program and it is by combining different process algorithms together that we can investigate the interaction between processes at the grain scale. Central control program This program, which is currently simply a shell script, and has been named Elle, defines the overall behaviour of any one simulation run. This script controls the order and rate of execution of the individual process algorithms, and thus determines which micro-processes will be involved in the microstructural evolution of the experiment, and also defines the evolution of the external parameters (including pressure, temperature, boundary conditions and fluid state) as the experiment progresses. At this stage process algorithms simulating homogeneous deformation (Manuel), non-linear viscous flow (Using the finite element code BASIL), grain boundary migration, sub-grain formation, grain boundary diffusion, intra-grain diffusion and lattice rotations have all been incorporated into the Elle framework, and we will show examples of the types of investigations that may be carried out using combinations of these processes.

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DEFORMATION AND MAGMATISM ACROSS A TRANSITION FROM UPPER TO MIDDLE CRUST IN THE MESOZOIC PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO Johnson^ S.E., Tate', M.C. & Fanning^ C.M. ^Department of Geology and Geophysics, Macquarie University, Sydney, NSW 2109 ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 email for corresponding author: sjohnson@laureLocs.mq.edu.au The northern half of the Baja California peninsula comprises a magnificently exposed section through the upper- and middle-crustal levels of the Peninsular Ranges batholith (PRb), which is a classic circum-Pacific magmatic arc (Fig. 1). The PRb is actually composed of two separate arcs that were juxtaposed between -133 and 108 Ma. The metaluminous western arc has petrological and geochemical characteristics of an island arc formed on oceanic crust, whereas the mainly peraluminous eastern arc formed on the continental margin of North America (see Johnson et al., this volume, for references). The "suture" between the two arcs is particularly well exposed in the Sierra San Pedro Martir region, and Fig. 1 shows the results of an extensive mapping program in the area that began in 1996. This poster will present a final colour version of Fig. 1, as well as detailed maps of several areas in Fig. 1 that illustrate the following points. • A previously unrecognized province of magmatic ring complexes occurs in the western arc, as exempHfied by the 115 Ma Zarza Intrusive Complex (Fig. 1; Johnson et al., 1999; Johnson & Tate, 1999; Tate et al., 1999a). This complex hosts three nested, southwardmigrating intrusive centers, and the northern and central centers show the same evolutionary sequence of: (a) intrusion of concentric, gabbroic cone sheets; (b) intrusion of massive gabbro cores; and (c) collapse along subvertical, ductile ring faults. The southern center contains equal proportions of gabbro and tonalite, and lacks cone sheets. This is the best preserved, cone-sheet-bearing ring complex described in North America, and has provided information about emplacement processes at shallow crustal levels in arcs, magma transfer between ring complexes and overlying volcanoes/calderas, and tonalite generation by lowpressure fractionation. • The contact, or suture, between the two arcs is marked by a major structural, metamorphic, lithological and geochronological discontinuity. Aptian-Albian (Cretaceous), basinal volcano-sedimentary rocks west of the suture experienced greenschist facies metamorphism, and show progressively more intense deformation towards the suture. East of the suture, metasdimentary country rocks experienced upper amphibolite facies metamorphism and our preliminary SHRIMP U-Pb zircon work has identified highly deformed, -133 Ma sheeted orthogneisses and plutons. The zone of intense deformation either side of the suture, and the suture itself, are intruded by metaluminous tonalites that range in age from ~ 108-102 Ma (not including the SSPM pluton-see below), and formed at ca. 10 kbar, consistent with the current moho depth in the western arc (-40 km, Tate et al., 1999b). The occurrence of these tonalites east of the suture suggests that it dips east, consistent with structural mapping (Fig. 1). Very intense deformation occurred well east of the suture, indicating that juxtaposition of oceanic and continental crusts created an important rheological boundary, which may have been accentuated by syn-deformational magmatism in the continental crust.

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•

Voluminous -133 Ma orthogneisses and granitoid plutons east of the suture suggest that PRb magmatism on the continental margin started no later than -133 Ma. Magmatism in the western arc apparently began -130-140 Ma (Walawender et al., 1990), the later phase of which (-108-102 Ma) stitched the suture in the area of Fig. 1. Mesozoic magmatism then culminated in the very large La-Posta type plutons, exemplified by the -97 Ma San Pedro Martir pluton (Fig. 1). These voluminous late plutons have chemical and isotopic affinities with both the western and eastern source regions (Walawender et al., 1990), and stitched the boundary between the two arc over a distance of more than 800 km.

REFERENCES

Johnson, S.E. et al., 1999, Bull. Geol. Soc. Am., in press; Johnson & Tate, M.C., 1999, Memoir Geol. Soc. Am, in press; Tate, M.C. et al., 1999a, J. Pet., in press; Tate, M.C. et al., 1999b, Geology, in review. Walawender et al., 1990, Geol. Soc. Am. Memoir 174, 1-18.

WVsleni Belt ({50-102 Ma) Plulon ..iianicters: smaller plulons (<l".'i km) lithoUigiev. gabbro ( - • < t o n a i i t e CiTisfal. aej>di. sijhivykauu- ( •i.O- 3.()kbar) Opaque oxides: maj^iu'tite and liiiienilo Whole rock convposibon: mot;?}ummfrus FE£ and isotopes: '^'Sr/^Sri (0.703 lo 0.705):

Tîstern (lO'vSO \ f a ) Phiton ihanioter;-: Idr.e.er {•lutons ( -lO km bth.^'loîes: sâbbro -ust;il depth: piutonu (<-.>.:> kbar) t-.^pavpe o.-<jdos: dTTienUt oulv Uhrtic- Vvick evjitipoMlioTi pM Mhiminous;- >nit>tahm»ino»is and isotopes.

^'Si ^sijiO^Oot 0

]«•)

'PLOS

rsi ; Angelas.; Tijuana Ensenada/# San Quintin f^ 'San Felipe

Southern map area

low-P tonaliles kbar) No exaggeration

120

Moho

higher-P (ca. 10 kbar)

tohalitcs

00'

~133Maplytor^s and orthogneiss in age oending. metasediments. * i "undelormed" strongly deformed A TTTX'^' ' ^PTT- 1 ! \ \

North American continental crust

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INTEGRATED GEOLOGICAL, GEOCHEMICAL AND GEOCHRONOLOGICAL STUDIES ACROSS A CRUSTAL-SCALE "SUTURE" IN THE PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO Johnson\ S.E., Tate\ M.C., Schmidt', K.L., Paterson', S.R., Vernon^ R.H. & Fanning', C.M. ^Department of Geology and Geophysics^ Macquarie University, Sydney, NSW 2109 ^Dept. of Earth Sciences, Univ. of Southern California, Los Angeles, CA 90089-0740, USA ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 email for corresponding author: sjohnson@laureLocs.mq.edu.au The Jurassic(?) to Cretaceous Peninsular Ranges batholith (PRb) is one of the great Mesozoic bathohths of western North America, extending for >1600 km from southern California, USA to the southern tip of Baja California, Mexico (Fig. 1). In northern Baja Cahfomia, the batholith intruded Mesozoic arc and basinal volcano-sedimentary assemblages, and Proterozoic(?)Paleozoic passive margin sediments. The arc can be subdivided into distinct western metaluminous and eastern peraluminous belts (Gastil et al., 1975; Silver & Chappell, 1988); major geological/geochemical differences between them are listed in Fig. 1. The enigmatic boundary between the two belts has been interpretated as: (1) a Late Jurassic (Shaw & Todd, 1995) or Early Cretaceous (Gastil et al,. 1981; Todd et al., 1988) suture between an island arc and cratonal North America; (2) a mid-Cretaceous (105-97 Ma) suture between a fringing arc and the craton, which were separated by a narrow back-arc basin (Busby et al., 1998); and (3) a pre-Triassic boundary formed along the Late-Proterozoic/Paleozoic passive margin transition between oceanic and continental crust (Thompson & Girty, 1994). This boundary, or "suture", is well exposed in northern Baja California, and our research has focussed partly on understanding its origin and timing, and how it influenced the tectonic development of the arc. Our approach has involved detailed mapping, structural, metamorphic, geochemical and SHRIMP U-Pb zircon studies in the two areas shown in Fig. 1, and our work to date indicates the following from west to east across the batholith. 1. In the western third of the western belt, volcanogenic country rocks are gently folded, tectonic foliations are rare and metamorphic conditions appear to be sub-greenschist facies. 2. In the SW comer of the northern map area (Fig. 1), we have discovered a group of magmatic ring complexes (~113-115 Ma; Johnson & Tate, 1999) that show evidence for multiple intrusive centers (Johnson et al., 1999; Tate et al., 1999a). The volcanogenic country rocks are generally strongly deformed at the margins of the complexes, which appear to have intruded late in the main phase of regional deformation. 3. From the ring complexes east towards the suture, along A-A': (a) the rocks change from dominantly volcanic flows and tuffs to an assemblage of volcano-sedimentary rocks; (b) a strong deformation gradient leads to isoclinal folds with down-dip fold axes and a strong axial-surface foliation; (c) a transition occurs from metaluminous tonalites formed by lowpressure fractionation to younger metaluminous tonalites formed at pressures corresponding to current moho depths (-40 km depth; Tate et al., 1999b). The younger tonalites (-108-97 Ma) appear to largely post-date the main phase of regional deformation. 4. At the apparent suture, strongly deformed mid-early Cretaceous (Aptian-Albian) rocks to the west are juxtaposed against intensely deformed orthogneiss that our preliminary SHRIMP work indicates are of early Cretaceous age (-133 Ma) in the northern map area. The discontinuity is marked by a zone of intense ductile deformation, which is locally overprinted by a distinct fault in the southern map area. Country rocks west of the suture

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experienced regional greenschist-facies metamorphism, whereas those to the east commonly contain sillimanite and are locally migmatitic. 5. The entire area between the suture and the Sierra San Pedro Martir Fault comprises a fanshaped geometry of solid-state and magmatic foliations, metamorphic layering and plutonic sheets that dip inward on either side of the structure, and steeply in the center. Only the western half of the fan is shown in the cross section below, but the entire fan is well preserved in the southern map area, south of the Sierra San Pedro Martir Pluton, where it is approximately 20 km wide and has been mapped in detail by Schmidt (1996). Our work indicates that the boundary between the western and eastern belts in the PRb is a major tectonic feature that reflects the accretion of an island arc that was long-travelled, or perhaps a fringing arc separated from the continent by a marginal basin. The marked rheological contrast between juxtaposed oceanic and continental crusts, combined with voluminous syn-deformational plutonism in the continental crust, caused strong partitioning of deformation in the arc leading to the structural geometry observed today (Fig. 1). On the basis of SHRIMP U-Pb zircon ages and the ages of sediments involved in the northern map area, the main phase of deformation related to convergence of the two arcs occurred between -133 and -108 Ma, and minor, waning deformation may have continued up to -87 Ma. Intrusion of -108-87 Ma plutons during this waning phase stitched the suture. References

Busby, C. et al., 1998, Geology, 26, 227-230; Gastil, R.G. et al., 1975, Memoir Geol. Soc. Am., 140, 170 pp; Gastil, R.G. et al., 1981, In Ernst, W.G. (ed.), Rubey Volume I, 284-305; Johnson, S.E. et al., 1999, Bull. Geol. Soc. Am., in press; Johnson & Tate, MC., 1999, Memoir Geol. Soc. Am, in press; Schmidt, K.L., 1996, Geol. Soc. Am. Abs (Cord. Sect.), 28, 109; Shaw, S. & Todd, V.R., 1996, third Hutton Symposium abstracts, USGS circular 1129, 139; Silver, L.T. & Chappell, B.W., 1988, Trans. R. Soc. Edin., v. 79, 105-121; Tate, M.C. et al., 1999a, Geology, in press; Tate, M.C. et al., 1999b, Geology, in review; Thompson, C.N., & Girty, G.H., 1994, Tectonics, 13, 1108-1119; Todd, V.R. et al., 1988, In Ernst, W.G. (ed.), Rubey Volume VII, 894-937.

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Volume

3r05'

— 31°00'

30°55'

SO-SO' Misc. intrusions; those west of the suture contain gabbro and tonalite, whereas those east of the suture are granitoids lacking gabbro

\>o

115.0±1.0 Ma, "undeformed"

low-P tonalites <<(<2 kbar)

Trend lines follow bedding A west of the suture, and \ foliation east of the suture

108.0±2.0 Ma, "undeformed"

Important lithological boundaries in the Alisitos Fm of the western arc

-133 Ma plutons and orthogneiss in age pending, metasediments, » | "undeformed" strongly deformed A

higher-P tonalites (ca. 10 kbar)

Moho No exaggeration

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THE MECHANICAL BEHAVIOUR OF QUARTZ GOUGE AT HYDROTHERMAL CONDITIONS K Kanagawa\ S F Coxl S Zhang^ ^Department of Earth Sciences, Chiba University, Chiba 263-8522, Japan ^Research School of Earth Sciences and Department of Geology, The Australian National University, Canberra, ACT 0200, Australia, e-mail: sfcox@geology.anu.edu.au ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

The strength and mechanical behaviour of simulated quartz gouge has been examined at high temperature hydrothermal conditions {T = 1200 K, P^ = 300 MPa, and P^^o = 200 MPa) and at nominally constant axial displacement rates between 0.13 -6.68 Maximum displacements were up to 1.8 mm on gouge layers up to 1 mm thick. Under these conditions simulated quartz gouge exhibits two types of mechanical behavior which are influenced by grain size and displacement rate. Continuous slip-hardening with coefficients of friction as high as 0.7 occurs in fine gouge {d < 5 jam), and in coarse gouge (d < 250 |im) at slow displacement rates (<0.36 [ims ^- For coarse gouge, and at displacement rates greater than 0.6 |Lims ^ initial slip-hardening is followed by slip-softening and subsequent quasi-stable sliding with coefficients of friction as low as 0.45. In both regimes dissolution-precipitation processes have promoted rapid compaction of gouge layers. Diagnostic microstructures include grain interpenetration, truncation of grain shapes and the development of faceted overgrowths in pores. In the sHp-hardening regime, shear strain is distributed across the gouge layer, although some localisation occurs on dilatant extensional shear bands at the higher displacements. Although microstructural evolution is strongly influenced by dissolution-precipitation processes, relative insensitivity of shear strength to displacement rate in this regime indicates that frictional mechanisms such as particulate flow and grain rotation could be rate-controlling. Progressive slip hardening is attributed to time-dependent increase in load-bearing grain contact area during compaction by dissolution-precipitation creep. In the slip-softening regime, the initial behavior is similar to that of the sHp hardening regime. Maximum shear strength is attained at displacements of 0.3 to 0.5 mm and at maximum apparent friction coefficients of 0.65. Slip-softening is associated with a transition from distributed shear in the gouge layer to localised slip at the interface between the gouge layer and the top forcing block. Microstructures on the slip interface provide spectacular evidence for the operation of both frictional and dissolution-precipitation processes during slip. The interface is covered by wear tracks parallel to the displacement direction. Etching of wear tracks, together with growth of euhedral quartz in pores at the interface, indicate that dissolution-precipitation reactions were active during interface sliding. The sensitivity of shear strength to effective normal stress in this regime indicates that it is still dominandy frictional, although the rate-dependence of shear strength is likely influenced by the effects of dissolution-precipitation reactions in modifying asperity structures, mechanisms of asperity interaction, or to the effects of lubricating fluid boundary layers. The transition from slip hardening to slip softening behaviour with increasing displacement rate is interpreted in terms

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of time-, strainrate- and grainsize-dependant changes in the relative strength of the simulated gouge and the gouge layer boundaries. The experiments highlight the importance of reactive pore fluids in influencing the mechanical behavior of fault rocks. In particular, reactive pore fluids actively promote interseismic compaction and healing of gouge, and lead to time-dependent effects on fault strength. Additionally, in hydrous regimes at seismogenic depths, the mechanical behaviour of faults is unhkely to be governed purely by frictional rate laws. Grainsize and ratedependent transitions from high strength behavior (involving distributed deformation) to low strength behavior (involving displacement localisation to slip interfaces) in quartz gouges is expected to be a factor influencing both the strength and stability of faults containing reactive pore fluids. Significantly, rate-, depth- and temperature-dependent transitions between velocity weakening and velocity strengthening behavior are unlikely to be as simple as envisaged by earlier studies.

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DUCTILE THRUSTING NEAR THE RAYNER COMPLEX NAPIER COMPLEX BOUNDARY, OYGARDEN ISLANDS, EAST ANTARCTICA. Nigel M. Kelly\ Geoffrey L. Clarke^ Christopher J. Carson\ and Richard W. White^ ^Dept. of Geology and Geophysics, University of Sydney, NSW 2006. ^School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052.

The Oygarden Group of islands lie within the Rayner Complex, immediately east of the Archaean Napier Complex, in east Antarctica. The Rayner Complex is interpreted to represent a mobile belt created during continent-continent colHsion involving the Napier Complex with another crustal fragment at c.lOOOMa. The Oygarden islands record evidence of a high strain, ductile thrusting event related to this collision and involved the reworking of Archaean crust. The Oygarden islands are predominantly layered felsic, intermediate and mafic orthogneisses, with subordinate pelitic, sub-pelitic and calc-silicate gneisses. These gneisses have been multiply deformed and have been metamorphosed at granulite facies conditions. The earliest structural feature, Sj, is parallel to compositional layering in layered orthogneiss that has been deformed by F2 isochnal folds. D2 produced a pervasive S2 gneissosity in post-D^ felsic orthogneiss and the transposition of S^ into parallelism with S2. Dj and D2 are inferred to predate the first episode of Rayner deformation and may be Archaean in age. D2 was followed by the intrusion of a suite of mafic dykes that may be the metamorphosed equivalent of the C.1200 Ma Amundsen dykes found in the Napier Complex. The first stage of Rayner deformation, D33, involved very high strain, subhorizontal ductile thrusting with an E-W transport axis. Strain in this event was strongly partitioned, with highest strain zones showing the complete transposition of earlier structures and lithologies (including mafic dykes) into a subhorizontal gneissic layering. Meso- to macro-scale isoclinal-recumbent folds and sheath folds have subhorizontal, east trending axes that are colinear with a pervasive Lg^ mineral and rodding lineation. Rare nappe structures indicate an east over west thrust movement sense. In areas of lower strain, the transposition of earlier structures was less pervasive, with dykes still transgressive to S1/2 layering and some refolded F2 folds having variably oriented fold axes. In these areas of lower strain a 83^ gneissic foliation is often observed in mafic dykes and the strong L3 mineral and rodding lineation may be present in felsic lithologies. 03^, the second stage of what is probably a progressive episode of deformation, also shows extensive strain partitioning with the highest strain concentrated in E-W trending, steeply south-dipping shear zones up to 10 metres in width. In zones of lower strain, open to tight, upright-inclined folds occur, folding S1/2, 83^ and some F^^ isoclinal folds. With increasing strain the intensity of folding increases, with folds ultimately being attenuated into a gneissic layering in high strain zones. F^^ fold axes and L^^ mineral and rodding lineation in the high strain zones are colinear to F^^ and L^^. The geometry of 03^ structures suggests that during D3 deformation there was a rotation of the axis of compression while maintaining a constant stretching direction. An east trending, steeply south-dipping D4 high strain shear zone >lkm across cuts D3 structures in the south of the Oygarden islands. This event has caused the progressive rotation of L3 and F3 fold axes from being subhorizontal and east trending features to steeply southeast-plunging. The last deformation event, D5, produced mylonites and ultramylonites (with minor pseudotachylite development) that are often marginal to post-D4 pegmatites or developed along other zones of weakness.

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Ultramylonites may form mutually crosscutting sets but no regular trend of the mylonites can be inferred. Preliminary thermobarometry implies conditions during D3 involved P=7-10 kbars at T>800°C. Reaction textures in metapelites and mafic gneiss that overprint both S3 and S4 gneissosities indicate that a period of decompression occurred either late in, or after, D4. The development of the large D4 high strain zone may have provided a mechanism to assist in the decompression of the terrain.

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A SYNTHESIS OF DEFORMATION AND PARTIAL MELTING IN THE GLENELG RIVER COMPLEX: IMPLICATIONS FOR THE DELAMERIAN OROGENY Tony I.S.Kemp^ & C.M. Gray^ ^ Department of Geology, Australian National University, Canberra, A.C.T. 0200 ^Department of Earth Sciences, La Trobe University, Bundoora, Victoria, 3083

The Glenelg River Complex (GRC) of far western Victoria, is an eastern element of the Late Proterozoic to Early Ordovician Adelaide Orogenic Belt. It comprises a turbiditic metasedimentary sequence, of inferred Cambrian depositional age, that was regionally metamorphosed, multiply deformed and intruded by igneous phases during the CambroOrdovician Delamerian Orogeny (-SOOMa). Metamorphism is ascribed low P-high T, or andalusite-sillimanite facies type (Anderson & Gray 1994), and five deformational episodes are resolved. Metasedimentary rocks exhibit a coherent regional zonation, where low grade rocks, outcropping in the southwest and northeast of the complex, prograde inwards towards a high grade axis dominated by migmatitic hthologies, in turn enveloping a batholith of biotite granodiorite. It is convenient to consider the metamorphic succession either side of this central batholith separately. In the southwest, lower greenschist facies metagreywackes, metasiltstones and calcareous slates, pass into amphibolite facies calc-silicates and schists in which andalusite, staurolite, and garnet metamorphic zones are delineated (Anderson & Gray 1994). These rocks are ultimately superseded by upper amphibolite facies sillimanite schists and stromatic migmatites, the latter defining the onset of a thin migmatite zone. Metabasites are interspersed throughout the lower grade sequence. Conversely, in the far northeast of the complex, amphibolite facies quartzofeldspathic schists undergo a rapid transition southwestwards into a wide migmatite zone, where rocks are intimately enmeshed with muscovite adamellites and granodiorites. Structures of five deformations (D1-D5) are recognised. D^ features are overprinted by intense later deformation, and are preserved primarily as a remnant Si foliation in S2 microlithon or a fine layering being rotated around F2 closures. D2 imposed the pre-eminent structural imprint, with S2 being the penetrative northwesterly trending regional fabric of the GRC. Isoclinal F2 folds, with thickened apices and attenuated limbs, are ubiquitous, plunge NW or SE, with a pervasive axial planar foliation or crenulation cleavage being diagnostic. In high strain zones, F2 closures are often rootless or intrafolial and contained within strongly transposed layering. A mica stretching lineation and quartz rodding in schists parallels boudinaged F2 hinge lines, attesting to extreme D2 extension collinear with F2 axes. Concordant pegmatite sheets and thick quartz veins are also strongly boudinaged and have undergone layer-parallel shearing during D2, such that asymmetric boudin axes now also plunge subparallel to F2. Third generation folds coaxially rotating S2 are widely developed, but most prolific in amphibolite facies rocks. F3 generally forms tight, upright-inclined similar folds with slightly thickened hinges and rounded V-shaped profiles. F3 in quartzofeldspathic schist or thick pegmatite dykes are close-open parallel folds. The S3 fabric is essentially localised to metapehtes or micaceous melanosomes in cuspate fold cores, and an incipient S3 zonal crenulation cleavage occasionally.

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The mild fourth deformational event propagated close to open crenulations in Sj or S3, with planar limbs and angular hinge zones. Coaxial to Fj and F3, F4 folds are never as tight as F3 and produce no axial fabric. Further, they are only conspicuous is the more ductile migmatitic rocks of the northeastern zonation, with a more subtle expression in the southwestern GRC. In contrast D5 structures are pervasive in all metasedimentary rocks as open parallel folds that warp S2 without production of an axial foliation; metapelitic schist occasionally supports monoclinal kink bands. F5 structures are geometrically distinct, plunging NE-SW, steeply perpendicular to earlier coaxial generations. Late syn-tectonic adamellites in the northeastern zonation are also deformed by D5, with rotation of micaceous schlieren into broad asymmetrical warps coplanar with F5 in schists. Significantly, macroscopic D5 folds occur across the GRC, most explicit at Wando Vale, where an F5 antiform is responsible for the arcuate geometry of metamorphic isograds and broad curvature of the lithological layering (Anderson & Gray 1994). This implies that the macrostructural and metamorphic attributes of the GRC are governed by F5 folding, and that D5 is a fundamental event within the tectonic evolution of the eastern Adelaide Orogen. Although rocks of both the southwestern and northeastern metamorphic zonations share a common deformational history, each records a different metamorphic and anatectic history. The highest metamorphic grades in the southwestern zonation were clearly attained pre- to syn-D2, followed by a rapid decline to biotite grade by D3 (Anderson & Gray 1994). Migmatite leucosomes are deformed and recrystallised by D2, indicating in situ anatexis preto syn-D^ (MJ. In contrast, the northeastern zonation experienced more protracted high-grade metamorphism. Isoclinally folded leucosomes are consistent with pre- to syn-D2 partial melting, synchronous with Ml in the southwest. However, field relations indicate that anatexis recommenced by syn-D3, with new leucosomes overprinting D2 structures and M^ leucosomes. This second partial melting episode (M2) was far more profound and culminated with the generation of nebulitic migmatites syn-D4 to pre-Ds. Deformation of melt-saturated migmatites during D3 and D4 engendered widespread disharmonic fold structures and stimulated efficient segregation of partial melts into dilatant structures or dykes, and ultimately ponding into plutons (Kemp et al 1998). Structurally coherent nebulitic migmatites lacking melt segregation features intimate that anatexis outlasted D4 but had ceased by D5, as no leucosomes crosscut F5 features. The absence of sillimanite, survival of prograde Mj muscovite, and crystallisation of primary muscovite by migmatites accords with fluid-rich conditions for M2 in the northeastern zonation, estimated at 650-700°C, 550-800 MPa (Kemp 1995). However, M2 appears to be significantiy lower temperature in the southwest, retrogressing amphibolite facies M^ assemblages. It is probably responsible for crystallisation of retrograde muscovite porphyroblasts in stromatic migmatites, epidote in hornblendic tonalites, actinolitic amphibole in metabasites, and the formation of retrograde shear zones. The restriction of M2 anatexis to the northeastern zonation has ramifications for the tectonothermal evolution of the GRC, and the Delamerian Orogeny. The latter is postulated to be a coUisional event affecting the Gondwana continental margin involving closure of a marginal sea, where GRC sediments were accumulated, by subduction, and subsequent collision with an outboard volcanic arc (Gray & Webb 1995). M^ appears to be early syncollisional, perhaps caused by attendant crustal thickening during orogenic convergence. M2 129

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is plausibly related to liberation of fluids from underthrust hydrated sediments during continued collision. This may have triggered melting in the overlying metasedimentary rocks of the northeastern zonation, more proximal to the collisional zone, but caused only retrogression in the more distant, cooler southeastern zonation sequence.

REFERENCES Anderson J. A. C. & Gray C. M. 1994. Geological affinities of the Glenelg River Complex, western Victoria. Australian Journal of Earth Sciences 41, 141-155. Gray C.M. & Webb J.A. 1995. Provenance of Palaeozoic turbidites in the Lachlan Orogenic Belt: strontium isotope evidence. Australian Journal of Earth Sciences 42, 92-105. Kemp A. I. S. 1995. The geology of the Glenelg River Complex in the Harrow district of western Victoria. BSc (Hons) thesis. La Trobe University, Melbourne (unpubl.). Kemp A.I.S., Ellis D.J. & Gray C.M. 1998. Generation of compositional diversity within granitic rocks by fractional melting: an example from the Glenelg River Complex, western Victoria. Geological Society of Australia Abstracts 49, 241.

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SYNCHRONOUS TRANSPRESSION AND MAGMATISM IN THE HALLS CREEK MOBILE BELT (EAST KIMBERLEY, WA). Tim Kendrick, Patrice Rey, Sylvie Costa, Gary Masur, and Ben Foley Department of Earth Science, VIEPS, Monash University, Clayton, Victoria, 3168. e-Mail: Timk@earth.monash.edu.au Introduction The Halls Creek Mobile Belt (HCMB), is a NE trending, linear belt, approximately 450 km long and 50 km wide. It is part of the Barramundi orogen which developed in the Northern part of the Austrahan continent in the Lower Proterozoic (1880-1820 Ma) (Etheridge et al., 1987). Griffin and Tyler (1992) have divided the HCMB into three zones. The central zone, which is the subject of this study, is characterised by the Bow Batholith, the Tickalara Metamorphics, and a number of mafic intrusions. Two low metamorphic grade domains flank this central zone. The eastern zone is dominated by low- to medium-grade metamorphic rocks, and volcano-sedimentary sequences. The Halls Creek Fault marks the boundary between the central and eastern zones. The western zone is dominated by the Whitewater Volcanics and the low-grade volcano-sedimentary series of the Marboo Formation (Griffin and Tyler, 1992). The quartz rich sandstone of the Speewah Group (1835-1830 Ma) overlays these units. Structural geology We focus here on the structural and kinematic analysis of some of the shear and fault zones that control the overall structure of this belt. Our observations were gathered from an area located beween the Mt Violet station, north of the Ord River, and the Springvale station. Three types of shear zones have been identified: magmatic, ductile and brittle fault zones. • An example of a magmatic shear zone outcrops in Spring creek (Bungle Bungle track) where it affected both felsic and mafic magmas. This shear zone strikes N015-025 and dips 70-80°NW. In the felsic rocks, the magmatic fabric is characterised by shape-preferred orientation of plagioclases and hornblendes. Quartz aggregates do not show any ductile stretching, and neither the plagioclase nor the hornblende show any recrystaUisation. Mafic dykes intruded the felsic magma. Because of the temperature contrast with the felsic magma, mafic dykes crystallised relatively quickly, and registered subsequent stages of ductile deformation, while the felsic magma still behaved as a viscous suspension. Some mafic dykes are isoclinally folded, sheared or boudinaged. Some were progressively dismembered into enclaves. The mechanical processes responsible for the formation of mafic enclaves include boudinage of thin dykes (10-20 cm), and the fracturation of thicker dykes (>50 cm). Thick dykes were fractured by anastomosed veins of felsic magma. This vein network isolated lenticular pockets of mafic magma that were progressively dispersed in the felsic magma, with the felsic veins acting as gliding planes. The strain and the proportion of mafic rocks increase toward the SE and the contact with the Tickalara Metamorphics. To the NW, the magmatic shear zone is affected by greenschist facies shears oriented N045 and dipping 75° NW. At the regional scale, this magmatic shear zone accommodated the relative displacement of the Mable Down Granodiorite (hangingwall) over the Sally Down tonalite and Tickalara Metamorphics (footwall) further south. Fabrics in the Sally Downs and Mable Downs are magmatic fabrics and not related to ductile deformation as described by previous authors. • West of the Northern Highway, a kilometer thick shear zone striking N040-055 and dipping 45-65°NW, affects the Tickalara Metamorphics. A strong foliation plane carries a

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down-dip stretching lineation. The shear zone is an S>L tectonite. Despite the strong flattening, S-C fabric and asymmetric pressure shadows indicate a top to the SE sense of shear. Pseudotachylites up to 20 cm thick occur throughout the shear zone. The pseudotachylites cut through the ductile fabric, but some show ductile shearing. This may suggest that they were contemporaneous with ductile shearing. 30 km further south a similar shear zone with the same orientation runs parallel to the Three Sister Hills ridge. This shear zone (previously named Springvale Fault) outcrops in the Panton River and kinematic indicators are globally consistent with those found in the Ord River. Here again, it is an S>L tectonite indicating a strong flattening. •The Springvale-Panton fault zone is a vertical britde/ductile fault oriented NS that runs through the ductile shear zones described above. Lineations and slickensides dip a few degrees to the north. Riedels and T fractures, shear bands oblique to foliation, and sheared quartz veins, are all consistent with a sinistral sense of shear. Discussion We propose that the Ord River-Three Sister Hills (OR) shear zone acted as a thrust during the Barramundi Orogeny, and was later offset (-25 km) by the sinistral Springvale-Panton strikeslip fault (figure). The Ord River shear zone exhumed the granulite facies Tickalara metamorphics (Kinzigite) over lower grade metamorphic rocks, and now underlines the boundary between Ni-Cu-Co and Cr-PGE-Ni-Cu bearing mafic intrusions. The large amount of flattening and the strong dip (40 to 90°) of the fold axis on both sides of the shear zone, indicate that the HCMB developed in a transpressional setting during which deformation and magmatism where contemporaneous. The long axis of the Sally Down and Mcintosh intrusions (1821 and 1830 Ma respectively. Page), and the magmatic shear zone that underlines the base of the Mabel Down intrusion (1830 Ma, Page) are parallel to the strike of the thrust. Because of the mainly magmatic nature of their fabrics, the emplacement of these felsic and mafic intrusions was likely contemporaneous with deformation, and provides an upper limit for the age of the Ord River thrust.

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References Hancock, S. L., and R. W. R., Rutland, 1984. Journal of Geodynamics, 1, pp. 387-432. Griffin, T.J., I.M.,Tyler, 1992. Geological Survey of Western Australia. Record 1992/17, 28p.. Etheridge, M. A., R. W. R. Rutland, and L. A. I. Wybom, 1987. A.G. U., Geodyn. Series, pp. 131-147.

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THE CONTROLS ON DEFORMATION PATTERNS AND EMPLACEMENT WITHIN AN OBLIQUELY CONVERGENT OROGEN

PLUTON

Keith A. Klepeis School of Geosciences, Division of Geology and Geophysics, The University of Sydney, Building F05, NSW, 2006 Australia, Tel: (02) 9351-4940, e-mail: keith@es.su.oz.au Interpretations of deformation patterns and pluton emplacement mechanisms at obliquely convergent margins commonly are hindered by problems such as the inadequate memory of rock fabrics, changing kinematic regimes, and limited exposure of structural relationships in three-dimensions. These problems have led to considerable debate on the following issues: (1) the significance of spatially complex, rapidly changing deformation patterns within obliquely convergent margins; (2) how arc-parallel and arc-normal components of oblique convergence are partitioned at deep crustal levels during orogenesis; and (3) the interplay among arc-parallel, vertical, and arc-normal horizontal displacements and the emplacement of arc-related plutons. I address these problems using new structural, kinematic and U-Pb isotopic data from a well-exposed section of the Coast Mountains batholith located in northern coastal British Columbia, Canada (54.5° N latitude). U-Pb isotopic analyses on zircons show that the central Coast Mountains batholith was emplaced between -75 and 50 Ma. During this interval relative motion between the Kula and North American plates was changing from near-orthogonal convergence to obliquely convergent, dextral strike-slip motion. I document the structural and kinematic evolution of two coeval, crustal-scale shear zone systems that developed within this obliquely convergent regime and that evolved concomitantly with batholith emplacement. The two shear zone systems formed in different localities within the batholith and display widely different orientations and kinematic relationships. At the western boundary of the Coast Mountains batholith, the Coast shear zone formed between -65 and 55 Ma during emplacement of tabular arc-related plutons. Subvertical high strain areas of the Coast shear zone parallel the north-northwest strike of the orogen for up to 800 kms. Two phases of high-temperature (600-700° C) arc-normal displacements occurred within this shear zone. The first phase (~6557 Ma) involved east-side-up reverse displacements, the second phase (-57-55 Ma) involved east-side-down, normal displacements. At the same time arc-normal displacements occurred within the Coast shear zone at the western margin of the batholith, major arc-parallel displacements occurred inside the bathoHth between -67 Ma and 51 Ma. These arc-parallel displacements took place within a network of curved shear zones that formed at the margins of tabular plutons and sheeted sill complexes. At the roof and sides of one well-exposed, 8-km-thick sill complex, is a series of subparallel ductile normal faults. Toward the sides of the sill complex, these ductile normal faults steepen and merge into a series of subvertical shear zones that record arc-parallel sinistral displacements. Despite the variabiUty in shear zone surface orientations, stretching lineations within all segments of the shear zones are parallel. This parallelism indicates that arc-parallel normal displacements at the roof of the sill complex were kinematically compatible with arc-parallel sinistral motion at the sides of the sill complex. U-Pb zircon ages from sills both affected by and crosscutting the shear zones indicate that arc-parallel

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displacements in these shear zones occurred simultaneously with pluton emplacement between -67 and 51 Ma. Simultaneous arc-parallel displacements in curved, kinematically linked shear zone systems explains the three-dimensional complexity of deformation within the batholith. One major consequence of this coordinated deformation was the tectonic denudation and rapid exhumation of the deep roots of the batholith prior to 50 Ma. The dominance of intra-arc normal faulting and sinistral transtension within an overall dextral transpressional plate boundary setting contrasts with style and kinematics of deformation described in other continental arcs affected by dextral transpression such as in the Andes and the Sierra Nevada batholith. In these other orogens, arc-parallel displacements during and after batholith construction were accommodated along steep, arc-parallel to oblique dextral strike-slip faults, features that were not observe in the central Coast Mountains. My results from the Coast Mountains suggest that intra-arc strain patterns do not necessarily reflect a direct coupling between deformation within the arc and associated plate motions. My results suggests that a partitioning of arc-parallel displacements inside the batholith (A in Fig. 1) were controlled by the following: (1) pre-existing structural anisotropics, (2) kinematic compatibility requirements created by simultaneous motion on curved, pluton-bounding shear zones and (3) volume adjustments that accompanied the inflation of the hanging walls of shear zones with magma during batholith emplacement. In contrast, a partitioning of arc-normal displacements in the Coast shear zone at the western boundary of the batholith appears to have occurred because this shear zone was not physically or kinematically linked to intra-arc normal faults or curved sinistral shear zones. Normal-type displacements in the Coast shear zone and in the Shames mylonite zone on the eastern side of the batholith (B in Fig. 1) reflect a period when normal faulting dominated the central Coast Mountains batholith during the final stages of its denudation and exhumation. These contrasting styles of deformation in the Coast Mountains imply that factors such as kinematic compatibility and the reactivation of preexisting anisotropics superseded a partitioning of the arc-parallel and arc-normal components of oblique plate convergence into strike-slip and thrust faults, respectively.

Figure 1. Two-phase model of deformation accompanying emplacement and denudation of the Coast Mountains batholith. Bold surfaces are major pluton-bounding shear zones; dashed lines show displacement directions; shaded areas are plutons. A: Deformation accompanying batholith emplacement; CSZ^ is first phase (reverse) of Coast shear-zone deformation. B:

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Extensional collapse of orogen; CSZ2 is second phase (normal) of Coast shear-zone deformation, and SMZ is Shames mylonite zone.

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A COMPARATIVE STUDY OF HELIUM DIFFUSION AND FISSION TRACK ANNEALING IN APATITES: BOREHOLE DATA FROM THE OTWAY BASIN, AUSTRALIA B. P. Kohn, Australian Geodynamics Cooperative Research Centre, Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia, e-mail: b.kohn@latrobe.edu.au M. A. House, Saint Louis University, Department of Earth & Atmospheric Sciences, St. Louis, MO 63103, USA. K. A. Farley, California Institute of Technology, Division of Geological & Planetary Sciences, Pasadena, CA 91125, USA. A. Raza, Australian Geodynamics Cooperative Research Centre, Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia. The development of the (U-Th)/He thermochronometer has opened a variety of new research avenues in geodynamics by providing a tool with which to document the thermal history of the upper crust to very low temperatures (e.g. Zeitler et al. 1987; Wolf et al. 1996). The value of helium thermochronometry arises from the fact that its closure temperature is lower than that available from any other technique, including apatite fission track thermochronology. Previous work by Wolf et al. (1996) has suggested a nearly invariant closure temperature of 75 ± 5 °C (assuming a cooling rate of 10 °C/km) in a range of coarse-grained apatites, (>100 _m in minimum dimension), whereas other workers have reported both higher and lower values, possibly correlated with chemical composition (e.g. Zeitler et al. 1987; Wamock et al. 1997). Beyond the uncertainties in the laboratory calibration, a second key issue for successful application of apatite helium thermochronometry is the validity of the extrapolation of laboratory data to natural settings. The confidence with which such extrapolations can be made depends on the precision and accuracy of the diffusion data, the magnitude of the requisite extrapolation, and whether the mechanism of diffusion investigated by vacuum experiments is the same that applies in nature. Thus, the application of laboratory diffusion data to thermochronological studies in natural settings involves substantial extrapolations across time and temperature, as well as presently untested assumptions regarding the mechanism of diffusion in two very different environments. Here we evaluate helium diffusion from apatites under natural conditions using helium age data from a suite of borehole and surface samples from the Otway Basin, Australia. The Otway Basin has historically played an important role in the development of fission track thermochronometry as well as the refinement of fission track-length modelling techniques. Studies from the basin also demonstrated the effect of apatite chemical composition on annealing temperatures (e.g. Green et al. 1989). The Otway Basin studies have produced an abundance of apatite fission track data and thermal models for the basin that are complemented by vitrinite reflectance data and subsidence models generated as part of ongoing hydrocarbon exploration in the region. The basin contains a widespread unit (the Lower Cretaceous Otway Supergroup) which is rich in volcanogenic detritus, including apatites. Following initial subsidence and deposition of the Otway Supergroup, the burial and thermal histories of the eastern and western regions of 137

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the Otway Basin diverged. While the tectonic history of the western basin is, to first order, characterised by monotonic burial and heating, that of the eastern basin is marked by at least two periods of structural inversion and uplift (mid-Cretaceous and Late Tertiary). In spite of the differences in the burial histories, apatite fission track models and subsidence studies from both regions indicate that, as a whole, the Otway Supergroup arrived at its present depths and temperatures sometime during the last 1 0 - 4 0 Ma. Helium ages were obtained from four boreholes (Anglesea-1, Fergusons Hill-1, Eumeralla-1 and Heathfield-1) as well as two surface localities in the Otway Ranges. Downhole apatite helium ages define a broad band of values from 58 - 65 Ma at the surface (12.5 °C) to zero at depths corresponding to ambient temperatures of ~ 67 - 83 °C (House et al. 1999). This swath results from at least three effects: (1) uncertainties in corrected borehole temperatures, (2) differences in the thermal history experienced by the various boreholes, and (3) slightly different helium diffusivities among the detrital apatite samples. Despite these complicating effects, the shape of the helium age profile is consistent with predictions based on the extrapolation of laboratory diffusivity data for Durango apatite coupled with published thermal histories for the Otway Basin, providing strong support for an apatite helium closure temperature of ~ 75°C. We have also carried out detailed apatite fission track measurements and chlorine analyses on the same aliquots from which the helium measurements were made. The apatite fission track data for individual samples show a wide spread of apparent ages and compositions, whereas helium measurements on coexisting grains usually fall within a restricted age range. Although not conclusive, this finding supports previous suggestions that composition does not appear to affect the sensitivity of the He closure temperature.

References P.F. Green, I.R. Duddy, A.J.W. Gleadow and J.F. Lovering, 1989. Apatite fission track analysis as a paleotemperature indicator for hydrocarbon exploration. In Thermal history analysis in sedimentary basins, N.D. Naeser and T. McCulloh, eds., pp. 181-195, Springer-Verlag, Berlin. M.A. House, K.A. Farley and B.P. Kohn, 1999. An empirical test of helium diffusion in apatite: borehole data from the Otway Basin, Australia. Earth & Planet. Sci. Letts, (in press) H.J. Lippolt, M. Leitz, R.S. Wemicke and B. Hagedom, 1994. (U-hTh)/He dating of apatite: experience with samples from different geochemical environments. Chem. Geol. 112, 179-191. A.C. Wamock, P.K. Zeitler, R.A. Wolf and S.C. Bergman, 1997. An evaluation of low-temperature apatite U-Th/He thermochronometry. Geochim. Cosmochim. Acta 61, 5371-5377. R.A. Wolf, K.A. Farley and L.T. Silver, 1996. Helium diffusion and low thermochronometry of apatite. Geochim. Cosmochim. Acta 60, 4231-4240.

temperature

P.K. Zeitler, A.L. Herczig, I. McDougall and M. Honda, 1987. U-Th-He dating of apatite: a potential thermochronometer. Geochim. Cosmochim. Acta 51, 2865-2868.

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STRUCTURAL INTERPRETATION OF A DEEP SEISMIC REFLECTION TRANSECT IN THE VICINITY OF THE GRAMPIANS, VICTORIA Russell J. Korsch^ (Russell.Korsch@agso.gov.au), Tim J. Barton^ David R. Gray^ Andrew J. Owen^ and David Foster^ ^Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, Canberra, ACT 2601 Australia ^Australian Geodynamics Cooperative Research Centre, VIEPS Department of Earth Sciences, Monash University, Melbourne, Vic. 3168 Australia ^^ Australian Geodynamics Cooperative Research Centre, Department of Earth Sciences, Latrobe University, Bundoora, Vic. 3083 Australia A deep seismic reflection transect in western Victoria, undertaken by the AGCRC as part of a multidisciplinary study into the geodynamic factors controlling gold mineralisation in the region, was designed to provide insights into the structural relationship between the Lachlan and the Delamerian Orogens. Three seismic lines were acquired to provide images of the subsurface from west of the Grampians Range to east of the Stawell-Ararat Fault Zone (Figure 1). I

I Post Devonian

cover

Devonian

granites

Grampians

Group

Undifferentiated

turbidites

Sandstone and slate in the Pyrenees StArnaudBeds L JrJ r-~~-l

Schist and roci<s

volcanogenic

r • " • "I l^etamorphosed of the Adelaide [XVv[

Mt Stavely

turbidites Fold Belt

volcanics

1l> \ ij Jallukar and Black 1 " r .1 volcanics —*—

Range

Thrust Strike slip fault Inferred Strike slip fault Geological boundarydefinite, inferred

T A

Anticline,

Syncline

Figure 1. Geological sketch map of western Victoria showing the locations of the three AGCRC deep seismic lines. Based on regional field mapping, Cayley & Taylor (1996) recently defined the boundary between the Delamerian and Lachlan orogens as the Moyston Fault Zone (MFZ). In the vicinity of the seismic survey, this fault is intruded by granite at the surface, but at depth the east dip to the fault proposed by Cayley & Taylor is confirmed. East of the MFZ, the uppermost crust is very poorly reflective. These weak reflections represent rocks of the Lachlan Orogen, and are typical of the reflective character seen on other seismic images from elsewhere in the Lachlan Orogen. The low reflectivity is attributed 139

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to the presence of granitic and strongly folded turbiditic rocks. Within the Lachlan Orogen, the Pleasant Creek Fault (PCF on Figure 2) is also east-dipping and essentially parallels the MFZ in the plane of the seismic section. Rocks of the Delamerian Orogen occupy the region below the surficial cover to the west of the MFZ (Figure 2). Generally, the upper crust is only weakly reflective, but sub-horizontal reflections at very shallow depths (less than 250 ms in Figure 2) in the area labelled GG represent the allochthonous Grampians Group (Cayley & Taylor, 1996). The base of this reflective package represents the underlying decollement surface. The relationship of this decollement to the Escondida Fault (EF on Figure 2) is uncertain, but to the east the EF has an apparent dip of gently to the east and it appears to link into a possible detachment surface at the top of a highly reflective package at about 2 sec TWT (6 km depth). Further east, the Golton Fault (GF) also dips to the east. The middle to lower crust below the Delamerian Orogen is strongly reflective, but is quite variable in dip and in the character of the reflections, suggesting possibly different crustal types and evolutionary histories. On the western side of line 1, the lower crust has a dominantly east dipping fabric, whereas on the eastern side of line 1 the reflections dip dominantly to the west. On line 2 the reflections are subhorizontal to east dipping. The overall character is suggestive of at least two major antiformal structures in the middle crust. On both deep seismic stack sections (Figure 2) the Moho is interpreted as a slightly undulating horizon at the base of the highly reflective middle to lower crust at 11 -12 s TWT (about 33-36 km depth). REFERENCE Cayley, R.A. & Taylor, D.H. (1996). Geological evolution and economic potential of the Grampians area, Victoria. Australian Institute Geoscientists Bulletin. 20: 43-48.

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25 km along strike

E SW

Station number 1300

1500

: •

MOHO

••- •

20 km

Figure 2. Preliminary stack 20 s sections of lines 97AGS-V1 and 97AGS-V2. Display is 1:1 assuming an average crustal velocity of 6 km s '. GG = Grampians Group, EF = Escondida Fault, GF = Golton Fault, MFZ = Moyston Fault Zone, PCF = Pleasant Creek Fault.

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THE BREAK-UP OF SUPERCONTINENTS AND OLDER OROGENS: WHY DID SOME OROGENS NOT RIFT? Maarten KRABBENDAM and Terence D. BARR Department of Earth Sciences, Monash University, Clayton VIC 3168, Australia email: maarten @ earth .monash .edu. au It has been recognised for some time that ancient orogens appear to localise rifting of (super)continents and many examples of alignment between old orogens and rifts of Gondwana break-up have been described. In this contribution these coincidences are critically assessed. Of the -27000 km of rifted Gondwana margin, -14500 km lies parallel to old orogens, -8700 km cuts across previous structures or cratons and -3700 km is uncertain. Some 5400 km of rifting may be explained by close (-1000 km) proximity to a hot-spot. In total, however, some 7100 km of rifting can not be clearly explained by either hot-spot activity or old orogens. Furthermore, more than 10 000 km of pre-existing structures did not localise rifting, notably the Trans-Saharan belt (despite its favourable orientation with respect to the opening of the North Atlantic) and the Damara Orogen (despite the proximity of the Tristan da Cunha hot-spot on the western side and presence of the Botswana dyke swarm on eastern side) and the NW part of the Albany-Fraser belt, despite the fact that the SE part has rifted. To pose the problem in a different way: If Gondwana would have rifted along all Meso-Proterozoic to Paleozoic orogens, the dispersal would have looked something like Fig. 1. This begs the question why some old orogens localised rifting and others did not. Convective removal of thickened mantle lithosphere, or TBL removal' (Houseman et al 1981) and concomitant orogenic collapse (Dewey 1988) is generally regarded as a very efficient way of weakening the lithosphere (Dunbar & Sawyer 1989, but see also Vauchez 1997 and Ryan & Dewey 1997 for other mechanisms). Orogenic collapse has been proposed for many orogens (eg. Himalayas, Basin & Range, Variscan, Caledonides, eg. Dewey 1988) but need not be inevitable, as exemplified by the Urals. The Urals still have a high crustal thickness (50-55 km) but a low topography (-1000 m), a subdued Bouguer anomaly (-40 mgal) and low heat flow (20-30 mW/m ) (eg. Berzin et al 1996), features best explained if orogenic collapse did not take place, suggesting that over-thickened mantle lithosphere is still present. This probably means that the Uralian lithosphere is stronger than adjacent lithosphere, explaining why Laurussia rifted along the Caledonides rather than along the Urals. Thermal modelling suggests that large, thick orogens may result in lithospheric zones of weakness, whether TBL removal occurs or not. Small orogens (thickening factor < 1.5) that do not suffer TBL removal, however, may remain lithospheric zones of strength. In such orogens there is also less inherent reason for TBL removal to occur. It is suggested that some ancient orogens form lithospheric weak zones; others, however, form strong zones, having an integrated shear strength higher than that of normal lithosphere. There are few reports of extensional detachments and other manifestations of orogenic collapse of the internal orogens in Africa: It is possible because they are not there. Orogenic collapse is not an inevitable process and if it does not occur it may leave an orogen as a lithospheric strong zone, unlikely to rift. References:

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Berzin, R., Oncken, O., Knapp, J.H., Perez-Estaun, A., Hismatulin, T., Yunusov, N. & Lipilin, A. 1996. Orogenic evolution of the Ural Mountains: Results from an integrated seismic experiment. Science, 274, 220-221. Dewey, J. F. 1988. Extensional collapse of orogens. Tectonics, 7, 1123-1139. Dunbar, J. A. & Sawyer, D. S., 1989. How pre-existing weaknesses control the style of continental break up. Journal of Geophysical Research, 94, B6, 7278-7292. Houseman, G. A., McKenzie, D. P. & Molnar, P. 1981. Convective instability of a thickened boundary layer and its relevance for the thermal evolution of continental convergent belts. Journal of Geophysical Research, 86, B7, 6115-6132. Ryan, P. D. & Dewey, J. F. 1997. Continental eclogites and the Wilson Cycle. Journal of the Geological Society of London, 154, 437-442. Vauchez, A., Barruol, G. & Tomassi, A. 1997. Why do continents break-up parallel to ancient orogenic belts? Terra Nova, 9, 62-66.

\ Alii

kuK

r.;

Fig. L m G m d m n a in a ^terefflmphie pt:ajection. Ixl m pretend lor a while that do not know whanhe plole mnfiginiition of tcxlay like ... Given Ihut îtes of rifling ore expeclBd along pre-eciitlng jsimclumH. and centered around hot-^pols, m it p^îhle to prediei theposîtion of GvMidwana rifls? b) Ixi'^ have a bit of fun! Fo^îMe GondTOoa break-up eonfiguration it riflis initialed on all niantle plurae^. and follciwed all pre-eiistioa orogeni Ibugli luck for Mefbciurne and Adeliiide!

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A NEW STRUCTURAL MODEL AND CONTROLS ON QUARTZ VEIN HOSTED GOLD MINERALISATION: AN EXAMPLE FROM THE POVERTY LINE OF REEF, TARNAGULLA, VICTORIA J. Krokowski de Vickerod, E-mail: Pkrok@hyper.net.au The Poverty reef (PR) is the major gold producing reef of the Tamagulla Goldfields, which is set within the western part of the Bendigo-Ballarat zone of the Lachlan Fold Belt. At the Poverty Reef, the Bonanza Shoot mined from 1853 to 1866 produced 360 000 oz of gold from 120 000 t of ore with an average grade of 92 g/t. Since the discovery of the high grade Nick O'Time shoot in 1994, 25 000 tonnes of ore have been produced averaging 28g/t. The Poverty-Line-of-Reef (PLR) consists of sinistral, en echelon, quartz segments within a regional framework of complex anticlinoria and synclinoria, which form the main regional deformation, characterised by NNW-trending, tight, chevron in style F1 folds and low greenschist facies metamorphism. The folds exhibit amplitudes and wavelengths of a few to lOO's m with predominant western vergence and associated SI axial planar cleavage. In the Tamagulla synclinorium host, the PR segment occurs in the top of a regional 'structural high' of a diagonal, crossfold type saddle between opposite plunging, adjoining, north-south anticlinoria. An opposite sense of plunge in the synclinorium created different strike-slip components of the F1 flexural-slip either side of the 'structural high's' flanks. This earlier movement combined with later sinistral, north directed torspression (torsional equivalent of transpression - Krokowski de Vickerod & Olissoff; 1992) formed the PLR whilst the 'structural high' focussed and acted as a trap for gold bearing fluids. A new, polyphase deformational model of the PLR structural evolution, exhibits several (D1-D7) stages characterised by different stress fields. The reef related fault system commenced late in Dj , after 'lock-up' of F^ folds, and progressively developed during D2-D3D4 as oblique, reverse shears with substantial sinistral strike-slip and secondary, torsional, dextral and sinistral components grading to dextral transpressional D5 shears, which are syngranite intrusion. The Tamagulla pluton and dyke association intruded into the metasediments during the D5 dextral, transpressional regime, which formed the transposition S5 foliation and F5 folds in the cordierite zone, and also modified some Fj fold axes towards a NNE direction. The late, post-mineralisation and post-granitic, D^ deformation exhibits the brittle characteristics of conjugated faults, which relate to E-directed far-field compression. This transformed into a strong dextral D7 transpression producing D7 low-angle, reverse dextral faults. Thus, the PLR structure exhibits a reverse, en echelon sinistral pattem additionally broken up into segments by post-ore faults. The PR quartz segment is 650 metres in strike length and 200 metres in down-dip extent with a maximum 'reef channel' thickness of 10 metres. The PR segment may be divided into three, structurally distinctive parts: the Bonanza shoot (BS) at its northem end, the Nick O'Time shoot (NO'TS) at its southem end, and a zone of low-grade massive quartz/hydraulic breccia forming the centre of the reef. In the PR segment, stress-strain trajectories vary from its competent quartz centre to each end. The initial trajectory of the intermediate stress/strain was in sympathy with the moderate southem plunge of the reef segment, and Poverty Syncline host. In the course of deformation, marginal parts of the reed behaved differently exhibiting a predominant torsional component with an opposite sense at

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both ends. Stress trajectories in either end used variations in the bedding-cleavage geometry, which are distinctive guides to structurally favourable sites for oreshoot formation. L^i and lineations change their geometry within the hangingwall side of the reef rotating in the plane subparallel to the reef The L^j and lineations in the central part of the quartz segment are subparallel, both showing the plunge of the primary L^j lineation. However, the V^lV^ lineations in the N O T S are steeper south) than the primary L"0 lineation (15-20° south) reflecting the overall plunge of the host quartz reef segment. Thus, the pitch of the N O T S is steep (75'-80" south). In contrast, the northern end of the PR segment, the BS, shows L^^ lineation is horizontal or shallowly north plunging. The difference in value and sense of plunge between the L^j and lineations in the hangingwall, may be treated as an indicative structural feature of the distance towards its marginal parts and, as a consequence, towards the favourable position for mineralisation. This distance can be estimated by extrapolating L^j lineations determined from drillcore and plotted on a longitudinal section. The extrapolated lineations should intersect at the mid-point of the marginal boundary of the ellipsoidal quartz reef segment (on its long axis). To estimate this distance at least two holes with different L^j and lineation geometry should have been drilled within the same reef segment. During F^ folding, and early thrusting (D2), minimal principal stress (03) was subvertical and/or steep. The rapid stress transformation in the NO'TS, produced the local transpressional-transtensional regime, with oblique, gently N-plunging to sub-horizontal positions of the local o^ stress/strain. The local extension created openings for incoming fluids, whilst the steep attitude of the quartz subparallel to the local O2 stress, channelled the fluids upward to form the NO'TS. Longitudinal projection of these changes, together with longitudinal projections of the hangingwall L^j and L®2 lineations form a powerful tool in helping to predict sites of gold mineraUsation. During the formation of the reverse, reef fault system, because of the overall dip-slip stress/strain component, and displacement of its hangingwall, the apparent distance between its northern and southern ends, has been shortened. Contrary to the hangingwall, the footwall side predominantly adjusts by shortening with steep folding and rotation of the L^j lineation in a subhorizontal plane. Contrasting mechanisms of fluid transport were active in the reef segment. Whilst the main mechanism of fluid flow in the centre of the segment was advection, stagnation of fluids predominated its each end. Once the fluid stagnated, reactions were driven by diffusive gradients. The operative mechanism was fault-valve cycling (Cox, 1984, 1995, Cox et al, 1991a,b, 1995) via stagnation (but with cycles of fresh influxes) of fluids and predominandy diffusion driven transport into permanendy active en echelon, torsional traps. The economically important gold mineralisation at the PR was pre-peak contact metamorphism and it corresponds with the first phase of gold mineralisation in the models suggested by Ame et al (1998) and Foster et al (1998) In the NO'TS, the mineralisation shows two phases related to the D^^ and D4 deformations. The presence of gold in quartzfeldspar D5 veins suggests that magmatic fluids scavenged and deposited gold during the Tamagulla pluton intrusion. This forms the phase which corresponds with the second gold mineralisation window proposed by Ame et al (1998) and Foster et al (1998).

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ASCENT OF MELT ALONG LOW- AND HIGH-ANGLE FAULTS DURING CONTINENT COLLISION Jorn H.

Steffen Buttner^'^ & Bernd Spanner^'^

^^^Institul fur Angewandlc Geologic und Mineralogie, TU Munchen, D-85747 Garching, Germany, e-mail: Joem.Kruhl@geo.tum.de ^^^Institut fiir Angewandte Geowissenschaften I, TU Berlin, D-10623 Berlin, Germany ^^^Camino de Huebro 10, E-04100 Nijar/Almeria, Spain

During continent collision the ascent and emplacement of granitoid magmas may be governed by deep-seated crustal-scale shear zones and may influence the birth and growth of these shear zones. The Ribeira Fold Belt in SE-Brazil and the Central European Variscides (Southern Bohemian Massif) provide good examples for such relationships. During the early stage of collision and nappe piling melt may ascent along low-angle dip-sUp shear zones. In the geological map this situation is reflected by long and partly diffuse stripes and lenses of granitoids parallel to the main foliation of the high-grade country rocks. These granitoids often show high-temperature deformation structures in quartz and feldspars as a result of syntectonic crystallization. During the late stage of collision (and crustal thickening) steep and often conjugate strike-slip zones develop, along which ~ partly in pull-apart basins - additional pulses of melt ascend. In the geological map this situation is reflected by granitoid lenses which curve into the shear zones. The close kinematic relationship between low- and high-angle shear zones is documented by (i) the coincidence of the main compression directions and (ii) the transition from high-grade metamorphism during dip-slip shearing to high- (and subsequent low-) grade metamorphism during strike-slip shearing. The development from low- to high-angle shear zones in combination with the ascent of melt appears to be a general process during continent collision.

Buttner,S. (1996): Die spatvariszische Krustenentwicklung in der siidlichen Bohmischen Masse: Metamorphose, Krustenkinematik und Plutonismus.- Frankfurter Geowiss. Arb., Ser.A., 16, pp.208. Spanner,B. & Kruhl,J.H.: Varying mechanisms of pluton emplacement due to a changing deformation regime during the Brasiliano orogeny: the Carmo and Cindacta plutons (SE Brazil).- Precambrian Research (subm.)

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MULTIPLE INVERSIONS IN INCLUSION TRAIL ASYMMETRY PORPHYROBLAST RIMS RESULTING FROM HETEROGENEOUS STRAIN

Hyun-Woo Lee (hyun.lee@jcu.edu.au) Structure and Metamorphic Research Institute, James Cook University, Austraha When dealing with thousands of petrographic sections of deformed rocks, we may confront local geometries additional to those generally preserved. As microstructural geologists interested in foliation development we try to determine the significance of every single deflection of a foliation preserved in porphyroblasts and the matrix. It is only when we can explain with confidence all microstructures present that we can be sure we have gained an understanding of the structural development of the rocks observed and the tectonic histories preserved. Recent researchers (e.g., Treagus and Lisle, 1997) have suggested heterogeneities of stress and strain in three dimensions can cause traces of foliations projected onto a plane, such as a map, to not represent the true pattern or nature of the deformation. This should also apply in section and potentially could result in fabrics that are overgrown synkinematically in strain shadows of porphyroblasts may not reflect real structures. Up to 10 radially oriented vertical petrographic thin sections and a horizontal section were made from each rock sample from the Ogcheon Orogenic Belt, in South Korea, which has undergone four ductile deformation events. Most of the garnet porphyroblasts in mica schists along the orogen preserve inclusion trails of the core continuous with the matrix fabrics in three dimensions. This made it easy to follow the three dimensional trajectories of the structural fabrics. However, some garnets contain multiple shear sense inversions, staircase geometries and structural truncations, in certain orientations of the thin sections while only one pair of spiral trails is observed in other thin sections from the same rock. These geometrise and their larger scale shapes in 3-D are used to interpret the deformation history of the rock and are explained in terms of heterogeneous strain posed upon the existing matrix fabrics around a porphyroblast. The closer the matrix to the porphyroblast, the greater protection from subsequent shear strain provided. Consequently, the foliation, or micro fold axes are progressively rotated away from the porphyroblast margin towards the nearest microshear plane - this region is called a "strain transition zone". This can cause more than one pair of shear senses to be revealed in inclusion trail geometries in some cross sections for one deformational event, even though there has only been one shear sense. Similarly, local apparent truncational microstructures of fabrics can develop where the shear sense reverses. However, these structures, where the shear senses abruptiy switch, are the result of a cut effect rather than an additional deformation event. Comparable microstructures can also result from the differential growth of some porphyroblast crystal faces relative to the developing matrix foliation: for example, porphyrobast tend to grow more into their strain shadows. The fabrics outside the porphyroblast are exposed to subsequent deformation and may be incorporated at a later stage. Consequently, we can expect primary and secondary deflections of fohation in the porphyroblasts. Seeming millipede inclusion trail geometries in some samples actually developed as secondary deflections rather than as primary structures.

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Criteria for recognition of these phenomena are essential for inclusion trail studies and are presented to aid future workers in this field. REFERENCE Treagus, S.H. and Lisle, RJ., 1997. Do principal surfaces of stress and strain always exist? Journal of Structural Geology, 19, No 7, 997-1010

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THE EMPLACEMENT AND DEFORMATION OF EASTERN LACHLAN FOLD BELT GRANITES - ONE KEYTO TECTONIC MODELS Lennox, P.G., Trzebski, R. & Palmer, D., School of Geology,UNSW, Sydney 2052; P.Lennox @ unsw.edu.au This study will focus on the Siluro-Devonian CarcoarGranodiorite, Barry Granodiorite and Sunset Hills Granitewithin the Eastern Lachlan Fold Belt (ELFB). The three granites lie acrossthe southern margin of the Lachlan Transverse Zone. The Carcoar and BarryGranodiorites are weakly deformed, enclave-containing, I-type granitesnorth of the moderately deformed, metasedimentary xenolith-containing, partly S-type Sunset Hills Granite. Thenorthern most Carcoar Granodiorite is a blocky body of the approximately lOby 10km bounded by the Carcoar Fault on the west and the Quarry Fault on the east, possibly acontinuation of the Ammerdown Fault. The hour-glass shaped, north-southelongated Barry and Sunset Hills granites both lie west and parallel to theconcave-shaped Quarry Fault and nearby Copperhannia Thrust. These granites have been emplaced at shallow crustal levels(Carcoar & Barry granites: 6 ± 2.6, A. Muller pers comm. 1998) into anOrdovician greywacke and volcaniclastic sequence deformed atsubgreenschist facies. The increase of more ductile features in the country rocks to the south indicates that thesegranites at the present level of exposure are surrounded by country rockswhich were slightiy deeper levels in the south around the Sunset HillsGranite compared with those to the north around the Carcoar and Barry Granodiorites. This may reflect eithertilting of the sequence, thrusting to the north along an east-weststriking, low angle detachment(s) which have brought deeper rocks to thesurface in the south or differential erosionto the south. Recent gravity studies have demonstrated a difference in the shape of thesegranites from north to south. The Carcoar Granodiorite has two north-southtrending, keel-like roots up to 5km deep either side and is bounded on itswestern side by the meridional Carcoar Fault. Studies on the Carcoar Fault show it dips 60-70^ east, has a complex movementhistory and its continuation further south shows up to 8km of dextral displacement.A precursor of this fault acted as a crustal weakness into which the granite intruded during possible sinistral movement of meridionalfaults during extension connected with the oblique northeast-southwestopening of the nearby Hill End Trough. The Carcoar Granodiorite is boundedon its east by the Quarry fault dipping 60-70^ west. The Quarry Fault shows down-dip movement overprinted bysinistral-strike-slip movement. The Carcoar Granodiorite forms awedge-shaped body increasing in thickness from less than SOOmetres to 5-7km thick to the south. Analysis of the Linsser filtered gravity data and field studies show the presence of two zoneswhich disrupted the general meridional-trending foliations and uppercrustal faults; one trending east-northeast across the southern half ofthe granite and another trending northwest from near the Browns Creek Mine to the southeast. These structures arepossibly related to the late Kanimblan reactivation on conjugatenorthwest- and east-northeast trending structures in the basement becauseof north-south shortening at this time. The north-south elongated Barry Granodiorite at the surface formswedge-shaped body looking north; at depth increasing in thickness to 4-5kmin the east and bounded by a fault on its eastern-side. Field studies donot show a fault in this area. Dating of hornblende in the

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Barry Granite and biotite in the Carcoar Granodiorites bya variety of methods produces similar radiometric ages consistent withtheir intrusion and cooling during the Bowning. The north-south elongated Sunset Hills Granite intrudes mainly Adaminaby Group greywackes which show evidence of pre-granitedeformation fabrics, an increase in the development of local foliations tothe south, only a narrow metamorphic aureole and a distinct change insubsurface shape to the south. In the north the granite forms a sheet with a root zone up to 3km deep on its eastemside, whereas to the south the gravity studies are consistent with ashallowly, west-dipping 3-4km sheet with bounding 40^ west-dipping faults on both its eastern and a possibly steeper-dipping fault on its western margin. It is possible although untested thatthis granite connects at depth to the nearby Wyangala Batholith (WB) . Thegeochemistry of the Sunset Hill Granite and plutons which make up thenearby WB are similar and it has been argued elsewhere that these plutons have been subjected to similardeformation histories. The extensive fault zone which includes theCopperhannia Thrust east of the Sunset Hills Granite containing meridionalslivers of Ordovician and Silurian rocks. These faults represent the re-activated boundary between the Molong High tothe west and Hill End Trough to the east. Recent dating of sericite withinfoliations in the Kalangaloolah Volcanics within the Copperhannia Faultexhibits an Ar-Ar spectra with evidence of two steps, one related to the Bindian and another related to theKanimblan (D. Foster pers. comm. 1998). Studies of the Copperhannia Thrustat Newbridge demonstrate it dips east and has a two stage movement history;a dip-slip east-down movement (? related to Silurian opening of the Hill End Trough) and a later dextralstrike-shp displacement (? related to east-west shortening). Dating ofbiotites within the Sunset Hills Granite yields a younger radiometric agecompared with the Barry and CarcoarGranodiorite, consistent with resetting of biotite in the Reedy CreekShear Zone (?during the Tabberabberan) for one separate and partialresetting during the Kanimblan for the other sample which comes from a wellfoliated section . The Benambran deformation of the Ordovician sequence was followed by oblique extensionconnected with the Middle Silurian opening of the Hill End Trough and LateSilurian granite emplacement into the upper crust at around 4-6km depth.These granites appear to have utilisedeither existing meridional faults possibly in a pull-apart structure oraided fault development. Deformation during the ?Tabberabberan caused development of the dominant meridional foliation within thegranites and the country rocks and Kanimblan deformation caused kinking or bending on mainly northwest-oriented shear zones.Dating and the bending of the foliation within the Barry Granite would beconsistent with translation of the Barry Granite eastward along the ENE-WSWtrending Reedy Creek Shear Zone in the Tabberabberan. Acknowledgements: Hargraves Resources NL and the DeutscheForschungsgemeinschaft (Tr 402/2-1) are thanked for supporting our researchin the northern Lachlan Fold Belt.

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MICROSTRUCTURAL EVOLUTION AND FLOW OF ROCKS IN A ZONE OF ACTIVE TRANSPRESSIONAL DEFORMATION NEAR THE ALPINE FAULT, NEW ZEALAND T. A. Little (Victoria University of Wellington, timothy, little @ vuw. ac. nz) R. J. Holcombe (University of Queensland,

Wellington Queensland

6000, New Zealand; 4072,

Australia;

e-mail: e-mail:

rodh@earthsciences.uq.edu.au J The Alpine Schist adjacent to the active Alpine Fault in New Zealand consists, in the Franz Josef and Fox glaciers area, of a monotonous sequence of graphitic psammopelitic schist, with minor competent psammitic and metavolcanic units. Within -10 km of the fault, metamorphic grades are upper greenschist to lower amphibolite facies, with metamorphic zones subparallel to the fault and apparently attenuated. This generally east-tilted sequence of midcrustal rocks has been ramped to the surface in the past m.y. in the hanging wall of the oblique-slip Alpine Fault. Within about 1.5 km of the present surface trace of the fault, the youngest fabrics in the schist are protomylonitic to mylonitic, with fabrics that broadly reflect the known kinematics of the SE-dipping fault (dextral with a slight reverse component). Grain shape fabrics, poorly developed stretching lineation, and abundance of conjugate shear bands reflect a strongly transpressional flow component that has developed oblate finite strains. Further away from the fault, this youngest ductile fabric is no longer pervasive and is replaced by multiple and composite overprinting fabrics that strike obliquely to the fault. Zones of strong folding (Fb) of an older schistosity (S2) separate a 2-3 km-wide zone of very planar, strongly lineated, high strain fabrics. Fabric orientations vary systematically away from the Alpine Fault; dihedral angles between the fault and the dominant foliation, as well as the southwesterly pitch of the prominent lineation within the foliation, increase with increasing distance from the fault. Abundant coarse porphyroblasts of biotite and garnet, with well-preserved internal foliations, occur in the graphitic schist and have grown syn-kinematically with Fs folds and subsequent fabrics. Both porphyroblast species are coarse enough that individual porphyroblasts generally overgrow several graphitic laminae, and excellent preservation allows each internal lamination to be correlated with its external parent. Thus, we have a fine-scale displacement gauge in these rocks which we have used (together with strain shadow shape data from garnet and ilmenite porphyroblasts) to estimate post-growth stretch components of the finite strain. Both of the main porphyroblast species show evidence of rotation associated with the shortening across the layers; in biotite this is associated with extensive bookshelf faulting, and in both there is an element of shear asymmetry to strain shadows. Estimates of the vorticity of the flow leading to these fabrics has been obtained from widespread oblique quartz grainshape fabrics, and semi-quantitatively from both the orientation distribution of microboudinage fractures in garnet porphyroblasts and the orientation distribution of rotated biotite laths (see Holcombe, this volume). The latter technique has also yielded a quantitative assessment of the shear strain component. The strain data show that both the folded and planar zones have resulted in only a modest bulk shortening (minimum strain ratios of -1.4 : 1.4 : 0.5 since peak mineral growth). The planar zones are simply attenuated macroscopic F3 limbs in which the strongly lineated fabrics are largely inherited from the earlier D2 deformation. The hinges of the Fs folds are almost perfectly colinear with the intense inherited lineation in the folded older fabric. In contrast to the strong LS characteristics of the inherited fabric, our data show that the finite strain

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associated with the folding and subsequent deformation is very oblate, with a slight down-dip maximum stretch. We attribute the Fs fabric to be the result of late Cenozoic shortening of a pre-existing LStectonite, with the inherited D2 fabric probably being Mesozoic in age. East of the Alpine Fault, the schists are inferred to be delaminated along a decollement in the mid-crust, before being translated through the 40-50° dipping footwall ramp of the Alpine Fault. The macroscopic Fs crenulation fabric was probably imprinted on the rocks as they converged obliquely against the toe of the ramp. Alternatively, late Cenozoic deformation at the ramp may have reinforced a pre-existing, near-vertical crenulation fabric. The flow regime producing shortening across the Fs limbs was characterized by sub-simple shear. We estimate a kinematic vorticity number of -0.2, with a bulk shear strain of about 0.6 (this vorticity number estimate is blurred by 3-D considerations). The shear sense is consistentiy eastblock-down (normal relative to the current steeply SE-dipping orientation of fohation). This shear sense is independent of position on the F3 folds and apparently overprints those structures, resulting in widespread oblique quartz grain-shape fabrics of uniform shear sense. As the schists were transported up the ramp towards the surface, bulk shear of the strongly anisotropic rocks was partitioned into rotation of schist packets, oblate coaxial stretching of packets, and normal-shear between them.

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TOWARDS A NEW PROTEROZOIC TECTONIC SYNTHESIS - AN ALTERNATIVE VIEW OF THE PROTEROZOIC GEOLOGY OF AUSTRALIA SandraMcLaren and Mike Sandiford Department of Geology and Geophysics, University of Adelaide, SA 5005 (smclaren@geology.adelaide.edu.au) While much effort has been expended in the study of Proterozoic geology, the challenge remains to integrate these observations in the development of a physical, tectonic and thermal model for Proterozoic crustal behaviour (eg., Etheridge et al., 1987). Many (but by no means all) studies have sought to explain Proterozoic geology using models inferred from the Phanerozoic. Such an approach w^ould prove to be deeply flawed if fundamental differences exist between Proterozioc nad Phanerozoic crust, and consequently we believe that is important that any new attempt to synthesize Proterozoic process draws fundamentally on what we can observe at a primary level in these terranes. Based on the fundamental principal that crustal evolution is sensitive to the thermal structure of the lithosphere, we are principally interested in factors that control lithospheric thermal regimes. Primary observables such as heat flow, heat production and thermal conductivity influence crustal thermal regimes and we seek to use these to make some generalizations about crustal process in the context of various Australian Proterozoic terranes. Of particular relevance to this program is the recognition that Australian Proterozoic crust contains approximately twice the normal complement of heat producing elements, and that magmatic Proterozoic processes involved unusually large-scale fluxes of heat producing elements. For example, the heat-production in the Sybella Batholith in the western Mt Isa Inlier is estimated to contribute -20 mWm"^ to the surface heat flow in this region (an amount that by conventional wisdom would constitute almost the total complement of heat producing elements for the Palaeoproterozoic crust). The generation, segregation and emplacement of such intrusives, and their subsequent vertical motion within the crust during deformation and denudation, has far-reaching implications for the long-term thermal and mechanical structure of the lithosphere. In this contribution we discuss some thermal and mechanical consequences of such large-scale intra-crustal redistribution of heat producing elements. The potential implications are illustrated with examples relating to basin formation and metamorphism in Australian Proterozoic crust. References: Etheridge M.A., Rutland R.W.R. & Wybom L.A.I. 1987. Orogenesis and Tectonic process in the Early to Middle Proterozoic ofNorthem Australia. In, Kroner A. (ed.) Precambrian Lithospheric Evolution.Am^ncfln Geophysical Union, Geodynamic Series, 17, p.131-147.

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FINITE-ELEMENT MODELLING OF SINGLE-LAYER FOLDING IN ELASTOVISCOUS MATERIALS Neil S. Mancktelow Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland e-mail: neil@erdw.ethz.ch Although rocks, in common with all materials, show both short-term elasticity and longerterm viscous creep, theoretical and numerical studies of folding have generally considered only hmited combinations of the full behaviour. The assumption has generally been that, for slow natural deformation rates on the order of s"^ viscous behaviour dominates and that the elastic component exerts little influence on fold development. The correspondence between purely viscous theoretical and numerical models and slowly deformed analogue scale-models, in which the materials are truly elasto-viscous, would support this assumption. However, the proposal has certainly not been fully tested, and the possible importance of elastic behaviour in the initial stages of folding for introducing both shorter wavelength components and greater spatial variability has been emphasized by several authors. The influence of periodic, isolated bell-shaped and random initial perturbations on single-layer fold amplification was numerically modelled for a wide range of elasto-viscous material properties. The results from this finite-element modelling (FEM) are markedly different from previous finite-difference (FLAG) models, but similar to analogue scale-models. For periodic perturbations, only the introduced waveform is amplified into folds, even for initial wavelength much shorter or longer than the fastest growing 'dominant' wavelength. Hinge and inflection points remain fixed to the same material points and there is no hinge migration to allow development of the dominant wavelength. Increased elastic behaviour increases the growth rate of shorter wavelength components and hence modifies the final fold shape, but hinge or inflection points still remain fixed. For initial isolated bell-shaped perturbations, a slow serial sideways propagation of folding along the layer leads to the eventual development of an internally periodic fold packet of near constant amplitude at high values of shortening (Fig. 1). The serial development of the fold train is only recorded by the variation in layer thickness symmetric about the initial isolated perturbation. Increased elastic or non-linear power law viscous behaviour promotes localization about the initial isolated perturbation. Even for 'random' initial irregularities, the final high-amplitude fold shape is only quasi-periodic and still shows the influence of the initial perturbation geometry. The maximum amplitude of these initial irregularities also influences the final fold shape, especially when the growth rate of the folds is low. For the same viscosity contrast, smaller initial amplitude promotes growth of long wavelength components producing final shapes similar to those developed in higher viscosity ratio experiments. Increased elastic behaviour promotes shorter wavelengths, faster growth rates and greater wavelength selectivity, resulting in more regular periodic forms that are less influenced by initial perturbation amplitude (Fig. 2). However, in all cases investigated, the initial perturbation geometry still exerts an influence on the finite fold shape and the irregular, only quasi-periodic form of many natural folds strongly reflects this initial irregularity control. The mean stress ('pressure') distribution around both periodic and irregular folds was also analysed in a series of numerical models. Pressure varies continuously along and across the

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layer and discontinuously across the layer interface. Initial pressure variation in the matrix is small but becomes strongly accentuated on the inner arc of the fold at high amplitudes as the fold approaches an isoclinal form. The magnitude of these tectonic pressures is not insignificant and can reach lOO's of MPa. For strong elasto-viscous layers, these effects may be discernable in the metamorphic paragenesis. However, the local pressure gradients may be even more important in promoting material redistribution by diffusion and fluid flow. The pressure variation established from the numerical models of single- and multi-layer folding provides a pattern that can be compared with mineralogical and compositional variation around natural folds.

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R = 50 linear viscous Pert. B

v/wv;!" \AAA/ vAAA/ initial amplitude = 1/20 layer thickness Figure 1 Random Distribution 1 Linear Viscosity Viscosity Contrast 100:1 Logarithmic Strain = -0.6 a) Dei = 0.25

•'WVAAAAA/V/^ b) Dei = 0.05

c) Dei = 0.025

Figure 2

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THE DEVELOPMENT OF BOUDINS IN A VISCO-PLASTIC SOLID AT HIGH METAMORPHIC GRADE. Brett A. Marmo and Chistopher J.L. Wilson School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3052, Australia b.marmo@pgrad.unimelb.edu.au(Brett Wilson)

Marmo),

cjlw@myriad.its.unimelb.edu.au(Chris

Boudins and pinch-and-swell structures have been observed in the outlets glaciers of the Framnes Mountains, east Antarctica (Marmo and Wilson, 1996). Fracture traces are preserved when crevasses, filled w^ith water and snow, freeze to form coarse-grained columnar ice. The ice in fracture traces is rheologically more competent that the bubbly blue glacial ice that host them. Fracture traces are passively transported into regions of the glacier were extension occurs parallel to their length, resulting in boudinage. The evolution from crevasse, to fracture trace, pinch-and-swell structure and finally boudin development, can be observed simply by walking down a flow-line from a crevasse field in an ablation zone of a polar glacier. A 1.0 km flow-parallel transect is presented to illustrate the boudinage of fracture traces with a mean width of 0.30 m. The structural analysis has been integrated with surface flow rate monitoring to providing a constraint on the strain rate. The surface flow is -20 ma'^ , so the transect represents 50 years of deformation history over which time the traces have been elongated by 60% and sheared 0.35 Radians. Constraints on geometry and strain rate, combined with considerable knowledge on the rheology of ice close to its melting point, provides a unique opportunity to numerically model the deformation history associated with necking in pinch-and swell and boudinage structures. Previous attempts to numerically model boudinage have dealt with plastic flow (Lloyd and Ferguson, 1981, Lloyd et al., 1982) or linear viscous ow (Smith, 1975). However, ice like high grade metamorphic rocks, deforms as a a visco-plastic solid according to a non-linear contituitive ow relation of the form:

where 6 is the strain rate, a is stress and n and A are flow parameters. This constituitive relation has been used to model the boudin formation with the explicit finite diffierence code, FLAG (Fast Lagangian Analysis of Continua). The deformation history of boudinaged fracture traces are discussed with particular reference to the di fierential stresses in the necking region. References Lloyd, G.E, and Ferguson, C.C. 1981. Boudinage structure: some new interpretations based on elastic-plastic finite element simulations. Journal of Structural ,117128. Lloyd, G.E, Ferguson, C.C. and Reading, K. 1982. A stress-transfer model for the development of extension fracture boudinage. Journal of Structural, 335-372.

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Marmo, B.A. and Wilson, CJ.L. 1996. Strain localisation and incremental deformation within ice masses, Framnes Mountains, east Antarctica. Journal of Structural, 149-162. Smith, R.B. 1975. Unified theory of the onset of folding, boudinage and mullion structures. Geological Society of America Bulletin , 1601-1609.

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GHOSTS OF MICROSTRUCTURAL GEOLOGY FROM 1850 TO 2050 W.D. Means, Dept. of Earth and Atmospheric Sciences, University at Albany, Albany, NY 12222, USA In microstructural geology (or any science), papers are dead-on-arrival. After that they exist simply as ghosts, hovering around new work and influencing it more or less. Eventually they vanish. Sorby created a ghost around 1850 when he looked at a rock in thin-section and wrote down what he saw. This ghost is still among us, thanks to Sander's work early this century and the work of non-geologists who invented more powerful microscopes and even chemical microscopes. The ghost of Sander's AVA mapping has shown new vigor lately, but much of his terminology is a ghost not expected to be sighted again. Ghosts haunting microstructural geology at present include methods for measuring finite strain in thin-section and whole-rock crystallographic orientation diagrams. Both have uncertain futures, as interest in finite strain wanes and interest in heterogeneous deformation increases. But Dumey and Ramsay's work on incremental strains is a ghost with a still-bright future, as are the ghosts of using microstructure to recognize deformation, recovery, and reaction mechanisms, and computer modelling of these processes. The ghost of using microstructure to discriminate between the effects of Dl, D2, and D3 and the sense-of-shear ghost hover around, but seem enfeebled. The ghost that Griggs reinvigorated when he started deforming rocks indoors seems to have at least one permanent handicap, yet a future. By 2050 our ghosts will include the idea that understanding microprocess is the key to accounting for macrobehavior. Savants of the time will still stubbornly adhere to this dictum. They will also insist that full chemical and age-mapping of thin-sections should accompany geometrical mapping for respectable process and historical reconstructions. A ghost on its last legs by 2050 will be experiments on non-minerals in see-through apparatus, following the spectacular development of a see-through apparatus for use with natural silicates by Paterson, Tullis, and Urai in 2040.

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EXHUMATION OF HIGH PRESSURE ROCKS AND THE TECTONIC EVOLUTION OF THE OMAN MARGIN J. McL Miller and D. R. Gray Dept. of Earth Sciences, Monash University, Melbourne, Australia, 3168 R. T. Gregory Dept. of Geological Sciences, Southern Methodist University, Dallas, U.S.A., 25275 D. A. Foster Dept. of Earth Sciences, La Trobe University, Melbourne, Australia, 3083 Continental crustal rocks, now structurally beneath the allochthonous Samail Ophiolite and Hawasina Complex (oceanic sediments), underwent blueschist to eclogite facies metamorphism prior to the emplacement of the Oman ophiolite onto the Arabian margin. These rocks now occur as stacked fold-nappes within a less deformed upper-plate and a strong-to-intensely deformed lower-plate separated by a major crustal discontinuity. Biostratigraphic data suggests the allochthonous Hawasina Complex was emplaced between 86 and 83 Ma (Warbuton et al., 1990). Whilst high-pressure minerals occur in both plates, the lower-plate rocks have higher peak pressure and temperature assemblages that locally reach eclogite facies. Cooling "^Âr-^Âr ages on high-pressure fabric elements in the lower-plate range from 131 Ma to 82 Ma. The majority of these ages are inferred to represent cooling by the progressive emplacement of colder units during convergent margin tectonism, prior to emplacement to higher structural levels. Some of the ages are older than the crystallisation of the Samail Ophiolite (-95 Ma, Tilton et al., 1980). "^Âr-^Âr correlation plots for the older ages (114-116 Ma) provide good fits with "^Âr-^Âr ratios close to atmospheric. Field based observations and "^Âr-^Âr age data support a two stage exhumation process for the high-pressure rocks. Partial exhumation of the lower-plate sequences occurred prior to juxtaposition with the upper-plate during an intense top-to-the-northeast shearing event that pervasively affects the lower-plate. This resulted in the formation of regional closures by the folding and transposition of earlier high-pressure fabrics. The lower-plate closures are truncated by the structural break separating the two plates. 82-79 Ma "^Âr-^Âr ages are associated with lower-grade assemblages in the lower-plate that define transposition which overprint the high-pressure mineralogy. The lower-plate ^Âr-^Âr cooling (?) ages related to exhumation of the lower-plate sequences overlap with, or are older than, ^Âr-^Âr ages from the upper-plate. Emplacement of the lower plate units to shallower crustal levels may have occurred prior to, or synchronously with, high-pressure metamorphism of upper-plate units, and also the emplacement of the allochthonous units overlying the continental rocks. The carpholite-bearing upper-plate rocks have been folded into regional nappe structures which exhibit a marked increase in deformation intensity towards the boundary with the lower-plate. These nappes formed during the exhumation of the upper-plate units during northeast-directed transport over the higher-grade lower-plate units. The emplacement of the upper-plate deformed the lower-plate with the formation of extensive northeast-vergent crenulation cleavages associated with new mica growth. Juxtaposition of the two plates, and the formation of the upper-plate closures, is constrained by 76 Ma to 70 Ma crystallisation

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ages on mica from axial surface fabrics to upper-plate nappes. The younger mica crystallisation ages are very close to the deposition age of -68 for Maastrichtian autochthonous units (e.g. Warbuton et al., 1990) that unconformably overly all of the sequences. This suggests the upper-plate nappe forming event may have been synchronous with structurally higher low-angle normal faulting and/or rapid erosion of the nappe pile and the south-directed movement of the overlying Samail Ophiolite and Hawasina Complex. REFERENCES Tilton, G. R., Hopson, C. A. & Wright, J. E. 1981. Uranium-lead isotopic ages of the Semail Ophiolite, Oman, with applications to Tethyan ridge tectonics. Journal of Geophysical research, 86, 2763-2775. Warbuton, J., Bumhill, T. J., Graham, R. H. & Isaac, K. P. 1990. The evolution of the Oman Mountains Foreland basin. In: Robertson, A. H., Searle, M. P., and Ries, A. C. (eds). The Geology and Tectonics of the Oman Region. Geological Society of London Special Publication 49, 419-427.

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FLUID CHANNELLING AROUND HIGH-PRESSURE PILLOW LAVAS AND ITS IMPLICATIONS FOR LARGER-SCALE FLUID PATHWAYS IN SUBDUCTION ZONES Jodie A. Miller and Ian Cartwright Department of Earth Sciences, Monash University, Wellington Rd, Clayton, VIC, 3168, Australia. jmiller@earth.monash.edu.au Numerous petrological and geochemical studies in convergent orogenic settings have used the unusual geochemistry of island arc basalts when compared to MORB to infer that large-scale fluid flow must occur within subduction zones. This has been supported by theoretical models that indicate large volumes of fluids should be generated during dehydration of the hydrated subducting slab (e.g. Peacock, 1993). The generation of fluids in subduction zones may facilitate deformation on faults and shear zones within the subducting slab, and also promote the break-up of the slab in the subduction zone. Thus, identifying the fate of fluids generated in subduction zones is critical to our understanding of metamorphic, tectonic and igneous processes in subduction zones. However, studies that have documented evidence for fluidrock interaction in exhumed subduction-zone rocks from the European Alps have generally found limited evidence for fluid flow (e.g. Getty & Selverstone, 1994; Bamicoat & Cartwright, 1995) These studies suggested that the lack of evidence for pervasive fluid flow in high-pressure rocks indicates the migration of fluids out of the subduction zone is likely to be strongly channelled or fracture-controlled. The pathways along which fluids may migrate out of subduction zones have typically been much easier to identify in the shallow parts of subduction complexes. For example, Bebout (1997) showed that melange zones in the Catalina Schist (California, USA) represented sites of intense fluid-rock interaction, but that outside these zones, there was little evidence for large-scale fluid flow. While there seems littie argument that large amounts of fluid must be generated at deep levels within subduction zones, the role and fate of fluids generated at deep levels in subduction zones is not well understood, and hence fluid pathways in deep subduction complexes are poorly constrained. Oxygen isotope values of pillow lavas, especially those that occur in high-pressure terrains, have typically been reported as a single value. However, fluid flow may be strongly channelled around the pillows because of structural and/or permeability differences between the pillow basalts and the interstitial sediments. If this is the case, the cores and rims of the pillows may record different oxygen isotope values, and thus provide some clues as to how fluids move through and interact with the high-level parts of the subducting slab. In this study we have analysed oxygen isotope ratios in the cores and rims of 58 pillows from the unmetamorphosed Troodos and Ligurian ophiolites and from the Balanga Nappe on Corsica, as well as from the high-pressure Corsican ophiolite and the Zermatt-Saas ophiolite (Switzerland). We found that the majority of pillows show a clear difference in oxygen isotope ratios between their cores and rims. Ocean floor pillow basalts and ophiolitic pillows that have not undergone high-pressure metamorphism have rims with statistically-higher values than their cores whereas the reverse is true for metamorphosed pillow lavas. This reversal indicates that the S^^O values of the pillow lavas rims at least have been modification high-pressure metamorphism. The two principle mechanisms by which the values of the pillows could have been reset are through devolatilisation and fluid-rock interaction. Whole rock geochemistry indicates that there is little correlation between and H20^ in the pillow 162

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basalts suggesting that the variations between pillow cores and rims are not the result of devolatilisation. It therefore seems most likely that the variation in between the pillow cores and rims is the result of fluid flow occurring during subduction and high-pressure metamorphism. Fluids associated with high-pressure metamorphism need to be derived from rocks already within the subduction zone. This is because fluids, which are buoyant compared to the surrounding host rocks, will migrate upwards and out of the subduction zone rather than downwards into the subduction zone. The oxygen isotope data presented above, indicates that fluids involved in high-pressure metamorphism were most likely derived from the ophiolitic units, since fluids derived from the overlying metasedimentary units would have values that were too high to lower the values of the pillow lava rims. The fluids were most likely derived from devolatilisation of units lower in the ophiolite pile, such as the sheeted dykes, as these would have lower values than the pillows. The fluids responsible for lowering the value of the pillow rims relative to the cores, may have travelled through the pillow lava pile in two principle ways: (1) through the whole of the pillow; or (2) around the pillow margins. In the first case, flushing the whole of the pillow should homogenise the values in the whole pillow and would not generate variations between the cores and the rims. In the case where the fluids migrate around the margins of the pillow lavas, diffusive exchange between the pillow lava and the fluid would lead to a lowering of values in the pillow rims but not necessarily in the cores. If the fluids flowed around the margins of the pillow, this should also result in equilibration of values between the interstitial sediments and the pillow lavas, something that is supported by the oxygen isotope data. We therefore feel that the fluids were most likely strongly channelled around the pillow margins, suggesting that while fluid flow was not pervasive, fluid pathways were evenly distributed throughout the pillow lava pile. Assuming that isotopic exchange occurs via diffusion, the duration of isotopic exchange can be estimated using the spherical diffusion model of Crank (1975). This model generates a series of geochemical profiles of the form t' = where t' is dimensionless time, D^ is the effective diffusion coefficient, t is time, K^ is the effective partition coefficient and a is the pillow lava radius. Since fluid-hosted diffusion is relatively rapid compared with diffusion within minerals, D^ ~ D f , where f is the porosity, and D^ is the diffusion coefficient for O in a fluid ~ 10'^ mVsec. For small porosities, K^is given by K^ = fp.A'J/p^ where K^ is the relative amount of oxygen in the rock and the fluid and r, and r^ are the densities of the fluid and solid (e.g. Bickle and Baker, 1990). Comparison of the variations recorded in the Corsican pillow lava cores and rims with these geochemical profiles indicates that the differences in between the pillow cores and rims would develop on timescales of less than 0.5 Ma, which is most likely shorter than the duration of metamorphism, even for low porosities (/*= 10"^) and large pillow sizes (>lm diammeter). Thus, the preservation of variations between the cores and the rims of high-pressure metamorphosed pillow lavas implies that fluid flow must have taken place on relatively short timescales or been very strongly channeled such that individual pillows did not interact with fluids for the entire duration of the fluid flow event.

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Identifying the patterns and effects of fluid flow during high-pressure metamorphism has been difficult because of the complex alteration history of the rocks being subducted. In this study we have demonstrated that one way of identifying high-pressure fluid flow is by looking at oxygen isotope variations in the cores and rims of pillow lavas. Our results indicate that unmetamorphosed pillow lavas tend to have higher values in the rims than the cores, whereas metamorphosed pillow lavas record the reverse pattern. We suggest that this reversal ocurred as a result of fluid-rock interaction. However, the scale of isotopic resetting is relatively small, suggesting that the fluids were channelled around the margins of the pillow lavas and that fluid flow occurred over short timescales. Previous studies looking at fluid flow at deep levels in suduction zones have suggested that at high-pressures fluid migration may become limited to grain-scale tranport (e.g. Phillipot & Selverstone, 1991). This is in contrast to fluid flow studies in the shallow parts of subduction zones where fluid flow is thought to be channelled along structural pathways, such as fracture systems and shear or melange zones (e.g. Bebout & Barton, 1993). The results of this study suggest that fluid flow may also be structurally controlled at deeper levels but that the paths along which the fluids are channelled are less well defined. Away from these sites, high-pressure rocks are unlikely to show evidence for fluid flow during high-pressure metamorphism. We suggest therefore, that the identification of fluid flow pathways in high-pressure terrains is the most important step in assesing the role and fate of fluids liberated during devolatilisation of the subducting slab. Barnicoat, A.C. and Cartwright, I., 1995. Earth and Planetary Science Letters, 132, 53-61. Bebout, G.E., 1997. American Geophysical Union Monograph, 96, 179- 194. Bebout, G.E. and Barton, M.D., 1993. Chemical Geology, 108, 61-92. Bickle, M.J. and Baker, J., 1990. Earth and Planetary Science Letters, 97, 79-93. Crank, J., 1975. The mathematics of diffusion. Oxford University Press, Oxford, U.K., 414p. Getty, S.R. & Selverstone, J., 1994. Journal of Metamorphic Geology, 12, 747-760. Peacock, S.M., 1993. Geological Society of America Bulletin, 105, 684-694. Phillipot, P. & Selverstone, J., 1991. Contributions to Mineralogy and Petrology, 106, 417430.

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PLUTON EMPLACEMENT CONTROLLED BY STRIKE-SLIP FAULTS, EASTERN VICTORIA MORAND V J . , WILLMAN C . E . , HENDRICKX M . A . , HAYDON S J . & VANDENBERG A . H . M .

Geological Survey of Victoria, P.O. Box 500, East Melbourne, Vic. 3002. E-mail: Vincent.Morand® nre. vic.gov.au In eastern Victoria the Siluro-Devonian Bindian Deformation resulted in substantial translation of structural zones along strike-slip and thrust faults, accompanied by emplacement of numerous I-type plutons. Prior to this the low-P high-T Omeo Metamorphic Complex was generated in the Early Silurian Benambran Deformation, accompanied by intrusion of S-type granites. After a period of Late Silurian extension the Bindian Deformation occurred at about the Siluro-Devonian boundary. Large faults formed between basement blocks which became the present structural zones: Tabberabbera, Omeo, Buchan and Mallacoota. The main effect was the southward translation of the Omeo Zone by up to 130 km. Prominent faults separating and cutting through the zones include the Cassihs Shear Zone and Ensay Shear Zone, both dextral strike-slip faults running along the SW edge of the Omeo Metamorphic Complex. In the Omeo region these faults have been intruded by several elongate I-type plutons (mainly tonalite and granodiorite) which display a range of deformation styles and intensities, all implying syn-faulting emplacement. The Cassilis Shear Zone, comprising schistose mylonite, runs approximately E-W with subvertical foliations, slightly wrapping around the northern side of the Siluro-Devonian Swifts Creek Igneous Complex. One of the components of this intrusive complex, the Rileys Creek Granodiorite, has been dragged out to the east and southeast along the Cassilis Shear Zone to form a remarkable tail which progressively thins from about 1000 m wide at its western end to less than 20 m wide at its southeastern end, where it is apparently intruded by the Doctors Flat Tonalite. The granodiorite within the tail has a strong mylonitic foliation, but shear sense indicators and stretching lineations are rare. A dextral shear sense is given by the map pattern of the tail, which also stands out clearly on the aeromagnetic image. Abundant dark enclaves in the shear zone are stretched out, in places forming a layered gneissic rock. However, the degree of flattening of enclaves is commonly more than would be expected from the intensity of the foliation in the host granodiorite, so they appear to have undergone significant deformation in the magmatic state. This indicates syn-plutonic shearing. Where the tail thins, rocks on either side show little evidence of shearing - they are cordierite schist in which bedding and the regional S2 schistosity are clearly visible. Strong shearing was confined to the granodiorite, presumably because the magma has localised the shear zone. Although only the northern part of the Swifts Creek Igneous Complex has been sheared, the whole complex is wrapped by a Bindian schistosity which forms a schistose aureole. This was previously regarded as part of the Benambran Omeo Metamorphic Complex. The Ensay Shear Zone strikes SE, cutting across Early Silurian structures and isograds. It is a steep, dextral strike-slip shear zone with abundant S-C mylonite fabrics as well as cataclasite. For 35 km along its SW side the Doctors Flat Tonalite forms an elongate pluton in which deformation varies from weak to strong. Next to the shear zone the pluton is variously

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mylonitic and cataclastic. Strong alignment of biotite and hornblende defines a foliation parallel to the shear zone that is best developed at the margins of the pluton. Ductile deformation of plagioclase and hornblende indicate reasonably high-T deformation in places. A strongly schistose aureole of cordierite schist attests to syntectonic emplacement. Two KAr determinations on biotite from the Doctors Flat Tonalite give an age of about 412 Ma, the age of the Bindian Deformation. The pluton appears to have intruded along a releasing bend in the shear zone. Cataclasite may have developed during reactivation of the shear zone in the Middle Devonian Tabberabberan Deformation. Aeromagnetics have revealed four previously undetected I-type plutons (Polar Star Suite) in the Ensay Shear Zone SW of Omeo. These are thin, elongate bodies running along the shear zone but forming a slightly en echelon array, suggesting syn-shearing emplacement. They are mostly foliated but some contain massive portions. Two of the plutons contain mylonite with dextral S-C fabrics, parallel to foHations in the enclosing shear zone. Contact aureoles are not developed around these bodies, even the unfoliated parts. This suggests that any aureole has been sheared out during fault movement. One of the plutons, the Polar Star Tonalite, has a UPb zircon age of 418 ± 4 Ma, i.e. Late Silurian. This is slighdy older than the Bindian Deformation, which on strati graphic grounds cannot be older than about 410 Ma. Perhaps dextral movement on the Ensay Shear Zone started during the Late Silurian extensional event, which occurred during southward movement of the western part of the Omeo Zone relative to the eastern part. During the Bindian Deformation the Omeo Zone moved southeastward (relative to the Tabberabbera Zone) as two complex thrust sheets, with the Cassilis and Ensay shear zones and the Kiewa and Kancoona Faults taking up most of the dextral displacement along the SW side of the western sheet. The Indi and Yalmy Faults, with SE transport, were the leading edges of these thrust sheets. I-type magmas moving up through the crust at the time were captured by and channeled into the strike-slip shear zones, possibly enhancing the movement of the thrust sheet by surge tectonics. In summary: This is a tale of plutons and faults, I hope it will not bore ya. It starts in Late Silurian times, in a place called eastern Victoria. Crustal blocks were jostling around, with strike-slip faults between, I-type magmas did abound, not an S-type to be seen. This magmatic event reached far and wide, with granites in every quarter. But several wayward plutons did not behave like they oughta. These magmas entered fault zones to form plutons stretched and sheared. Their bodies are thin, just skin and bones, with shapes that are truly weird. The Rileys Creek Granodiorite resembles a mythical beast. With bulbous body and wonderous tail that thins from west to east. Dextral shear is plain to see, though not on the mesoscale. But peer at the aeromag and you'll agree, this is not a tall tale. The Doctors Flat has a broad curved shape, and Morand (1992) has shown That it filled the void of a dextral gape in the Ensay Shear Zone. On the aeromag the pluton appears bright red, nothing could be bolder.

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It's dated at 412 million years, but could be slightly older. In the very same fault lie the Polar Star plutons (there are four of them you know). They're long and thin and en echelon, except for one that doesn't quite fit. The outcrop is poor, but there's enough to be sure that they intruded while the fault was in motion, Shear sense was right-handed, S-C fabrics demand it. Now who could argue with such a notion?

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FROM CRUSTAL THICKENING TO EXHUMATION: THE STRUCTURAL DEVELOPMENT IN A COLLISIONAL OROGEN (WESTERN ERZGEBIRGE, EUROPEAN VARISCIDES) Matthias Nega, Jorn H.

Ottomar Krentz, Dietmar Leonhard^^^

(1) Institut fur Angewandte Geologic und Mineralogie, TU Miinchen, D-85747 Garching, Germany (2) Sachsisches Landesaml fiir Umwelt und Geologie, D-09599 Freiberg, Germany

Many orogenic belts are the result of the collision of continental crust. During this process different small- and large-scale tectonic structures develop. Most prominent amongst them are (i) nappes and nappe piles which may show inversion of metamorphism, (ii) large-scale monoclinal or orthorhombic folds ~ accompanied by a variety of small-scale structures ~ as the result of pure and simple shear conditions, (iii) shear zones, up to crustal scale, of variable orientation and displacement. These structures and their chronology reflect different stages of subduction, imbrication and crustal thickening, uplift and exhumation. Although the variety and complexity of the related geologic processes may lead to a specific nature of each orogenesis, there are common characteristic structures and developments of structures in orogens, which point to similarities in the general development of an orogenic continental crust. In addition to the variety of low- to high-temperature metamorphic rocks, such a crust frequently contains relics of high-pressure rocks which have been lifted from depths of more than 40 km. The structures in these rocks provide information about the processes which lead to subduction to and obduction from deep crustal levels and may give insight into general processes of the large-scale crustal reorganization during the orogenesis. The Erzgebirge is located within the Saxothuringian Zone of the Central European Variscides. This zone is considered as part of the Armorican terrane assemblage which was separated from Gondwana during the Ordovician and colHded - together with Avalonia - with Laurentia/Baltica during the Devonian (Tait et al. 1997). The collision led to large-scale horizontal movements which are also reported from other parts of the European Variscides (e.g. Franke, 1989). The western Erzgebirge exemplifies how the tectonic development creates a complex pattern and sequence of microstructures and, vice versa, how these microstructures can serve to unravel the tectonic history of a crustal segment. The development of micro- and macrostructures in the western Erzgebirge is best explained as the result of a ESE-WNW oriented continent coUision during which the continental crust was doubled and isostatically uplifted. However, crustal doubhng and isostatic uplift should not be envisaged as subsequent but as coeval processes with crustal thickening more effective in the beginning and isostatic uplift more effective at the end. The early stage of collision had various effects. (i) During crustal doubling large slices of both continental crusts were produced, together with an intensive foliation. (ii) The crustal slices derive from the upper part of the lower plate as well as from the lower part of the upper plate. (iii) As indicated by the different conditions of maximum metamorphism, crustal slices were first brought to different levels by the subducting plate and then brought together at a higher level by the advancing upper plate. This process should be envisaged as complex and continuous over a long period.

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(iv) The slices came from the lower as well as from the upper plate. Therefore, the boundary between the two plates is not a single thrust plane but is represented by a broad lithologically variable zone which also contains a variety of eclogite, high-pressure-granulite, and metapelite lenses. (v) As is indicated by the relics of high-pressure rocks, slices of deeply buried oceanic and continental crust have been first lifted by a still unknown process and then incorporated in the variable zone between the two plates during the continent collision, as described above. (vi) Uplift of the upper plate was probably due to both, immediate isostatic uplift and upwards thrusting. (vii) The pihng and imbrication of the slices together with the formation of foliations led to a large- as well as small-scale planar anisotropy. During the later stage of collision, supported by the large-scale planar anisotropy, large monoclinal folds developed within the same kinematic framework. This is also characteristic of other collisional orogens or parts of collisional orogens, e.g. the eastern Alps (Kruhl, 1993). The open and upright folding during the final stage of collision indicates the switch from dominantly simple to dominantly pure shear conditions in a highly thickened continental crust. Subsequent to this folding the first signs of crustal extension appear. Extension features occur in the already uplifted and cooled upper parts of the doubled crust. But they do not shape any large-scale structures. Three general conclusions related to collisional orogens may be obtained from the lateVariscan history of the western Erzgebirge. (i) The large-scale and most dominant structures, Hke nappe piles, large monoclinal folds and dome structures, are produced in an overall compressional regime, (ii) Extension on a larger scale only occurs during uplift in upper parts of the thickened crust, (iii) Crustal compression and extension act as continuous coeval processes, the first in the lower and middle, the second in the upper crust. Consequently, during uphft and cooling of a thickened crust, the rocks change from a compressional to an extensional deformation regime. Franke,W. (1989): Tectonostraligraphic units in the Variscan Belt of Central Europe.- Geol.Soc.Am.Spec. Paper, 230, 67-90. Kruhl,J.H. (1993): The P-T-d development at the basement-cover boundary in the north-eastern Tauem Window (Eastern Alps): Alpine continental collision.- Journal of metamorphic Geology 11, 31-47. TaitJ.A., Bachtadse,V., Franke,W. & Soffel,H.C. (1997): Geodynamic evolution of the European Variscian Fold Beld: Paleomagmatic and geological constraints.- Geol.Rdsch., 86, 585-598.

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THE MYLONITE ZONE ON THE ALPINE FAULT: STRAIN ESTIMATES, DISPLACEMENTS AND IMPLICATIONS FOR THE DEEP STRUCTURE OF MAJOR FAULTS

R. J. Norris and A. F. Cooper Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand richard.norris@stonebow.otago.ac.nz The Alpine Fault in the South Island of New Zealand is one of the world's major active transpressional fault zones. Mylonites form a 0.5-1 km wide zone parallel to and east of the Alpine Fault trace in South Wesdand. They represent a zone of intense ductile deformation along the fault at depth which has been uplifted and exhumed by dip-slip displacement. Initial deformation of the mylonites occurred under amphibolite facies conditions at depths of at least 20-25 km. Their uplift has been very rapid (up to 10 mm/yr) and recent (K/Ar ages < 1 Ma). Whereas mylonites are common in exhumed fault zones worldwide, the Alpine Fault mylonites are one of the youngest examples and one of the few associated with a currently active fault. Their exposure allows reconstruction of the fault zone structure from the surface to depth. A question common to many major faults is whether the slip on the fault at the surface (25-30 mm/yr in the case of the Alpine Fault) is largely accommodated at depth by ductile creep within a narrow mylonite zone, or whether a high proportion is distributed over a broad area of deforming crust. A zone of pegmatites within Alpine Schist on the Mataketake Range north of Haast has been dated at 68 Ma. The pegmatites contain coarse muscovite, biotite and feldspars, are commonly discordant to foliation, and range from 0.5 to 48 m in thickness. Mineral parageneses and compositions indicate intrusion under amphibolite facies conditions at depths of at least 25 km. The pegmatites extend into the Alpine Fault zone south of the Paringa River and may be traced continuously northwards within the mylonite zone to Waikukupa River south of Franz Josef, a distance of approximately 100 km. Within the mylonites, the pegmatites are highly deformed and attenuated. Measurements of thicknesses of pegmatite veins show a reasonable fit to a log-normal distribution. The mean thickness decreases between the Mataketake Range examples and those in the protomylonites whereas measurements from the mylonites and ultramylonites have a mean thickness two orders of magnitude less. We make the following assumptions: • The Mataketake Range log-normal distribution represents the thickness range of the pegmatites before mylonitisation and was used as a starting point for numerical modehng. • The pegmatites can be considered as a randomly oriented array prior to mylonitisation. • Shear within the mylonite zone may be represented as simple shear. An array of randomly orientated veins with a thickness distribution as in the Mataketake Range was deformed by various amounts of simple shear and the resulting thickness distribution calculated. Good fits were obtained with the measured data for shear strain values of 20 for the protomylonites and 200-300 for the mylonites and ultramylonites. A shear strain of 250 over a mylonite zone 1 km wide impHes a shear displacement of 250 km. If we assume that the pegmatites were incorporated into the mylonites near the base of the crust and were smeared out during oblique shear on the fault over the last 5 Ma, the strain 170

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values and the distribution of pegmatites within the mylonite zone are compatible with the near surface displacement rate on the fault being accommodated at depth by ductile creep

4rs

ALPINE FAULT Franz Josef

HOPE FAULT

Mataketake Range

Haast

39 mm/yr Christchurch

Milford

45°S

Dunedin Permian Ophiolite Belt

175°E

Western Province

greywacke

schist

Figure 1: Locality map of the South Island showing Alpine Fault, regional geology, and places referred to in abstract. within a narrow zone of strain-weakened mylonites less than 2 km wide.

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A THREE-PHASE EARLY CRETACEOUS RIFT HISTORY OF THE SOUTH ATLANTIC SALT BASINS AND ITS INFLUENCE ON LACUSTRINE SOURCE FACIES DISTRIBUTION. NORVICK, Martin S., Melbourne and SCHALLER, Hannfried, BHP Petroleum, Rio de Janeiro Detailed maps are presented illustrating three phases of rifdng that occurred during the Early Cretaceous in the South and Equatorial Atlantic. The first phase began in the late Tithonian or Neocomian (Rio da Serra Brazilian stage) and resulted in the formation of a rift valley complex, which ran obliquely across the future Atlantic from the Tucano-Reconcavo basins to northern Angola. A transcurrent fault, the Trans-Brazilian Shear, controlled the northwestern limit of the rift valley. In northeast Brazil, a second group of rift basins, the Potiguar, Araripe and Jatoba basins, formed parallel to this transcurrent fault, which may have extended as far north as the Hoggar Mountains in Algeria. Meanwhile, flood basalts were extruded in the Parana and Etendeka regions and these may have been continuous with areas of basic vulcanism in the Espirito Santo, Campos, Santos and southern Kwanza basins. Kerogen-rich lacustrine shales accumulated in the central, shielded part of the rift valley complex, especially during the Valanginian and Barremian. These source rocks were probably responsible for the generation of the large oil fields in Reconcavo and Potiguar, as well as some of the oil in southem onshore Gabon, Congo and Cabinda. Oxidised, coarse clastic sediments without source potential dominated the northern extremities of the rift complex (eg northem Tucano and Jatoba) and some of the lateral branches. The rifting direction rotated about 20 degrees clockwise during the Late Barremian (Buracica and Jiquia stages), resulting in the abandonment of the Trans-Brazilian Shear and the development of a new, second set of broader rift basins parallel to the future Atlantic margin from Cameroon to Santos. The onshore basins in northeast Brazil were uplifted and became inactive. The second rift phase lasted until the Early Aptian. Extensive organic-rich shales accumulated under freshwater lacustrine conditions in the north of the rift system and in slightly saline lakes further south. Much of the oil in the Campos basin, southem onshore Gabon, Congo and Cabinda are probably derived from these source rocks. A third phase of extension occurred during the Early Aptian to earliest Albian (Alagoas stage), characterised by initial emplacement of oceanic crust and thermal sagging, followed by evaporite accumulation. Thick salt was deposited, from Gabon and Sergipe-Alagoas in the north to Kwanza and Santos in the south, in a basin approximately similar in size, shape and climatic position to the modem Red Sea. There may have been a contribution of sulphur-bearing oils from hypersaline kerogen-rich shale underneath the salt. A continuing debate exists whether the salt was formed in the final stages of rifting or by desiccation of a small ocean basin. At the same time as salt deposition, rifting and transcurrent faulting propagated eastwards and westwards through the Equatorial Atlantic basins, linking the Jurassic oceanic crust off Senegal with the new oceanic crust in the South Atlantic. Additional strike-slip faults, including the Romanche Fracture Zone, became active between northeast Brazil and the Central Airican basins. A string of small trans-tensional rift basins formed at this time, including the Barreirinhas, Mundau and outer Potiguar basins in Brazil, and the Benue, Bomu and Doba basins in Africa. There was minor kerogen-rich lacustrine sedimentation in the Mundau and Potiguar basins. This final rifting phase was completed when oceanic crust began to be emplaced in small ocean basins between Nigeria and Senegal during the Late Albian.

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THE PALAEOPROTEROZOIC TECTONIC EVOLUTION OF THE SOUTHERN MARGIN OF THE CAPRICORN OROGEN, WESTERN AUSTRALIA Sandra Occhipinti, Steve Sheppard and Ian Tyler Geological Survey of Western Australia s.occhipinti@dme.wa.gov.au The Capricorn Orogen is a major Proterozoic tectonic zone that developed between the Archaean Yilgam and Pilbara Cratons. Recent geological mapping in the southern part of the Orogen, combined with geochemical data and new geochronological results (Nelson, 1998; in press) has defined two major Palaeoproterozoic tectonic events: one at 2000 to 1970 Ma and a second at 1820 to 1800 Ma. The southern part of the Capricorn Orogen includes the volcano-sedimentary Bryah and Padbury Basins, medium to high-grade meta-igneous and metasedimentary rocks of the Gascoyne Complex, and the Yarlarweelor gneiss complex that consists of re-worked Archaean crust of the Yilgarn Craton. The boundary between the Yilgam Craton and the Gascoyne Complex is marked by the Errabiddy Shear Zone. The oldest rocks in the southern Gascoyne Complex are tonalites and monzogranites that were intruded at c. 2000 Ma. There is no evidence of Archaean rocks. Dominantly pelitic sedimentary rocks were also deposited at c. 2000 Ma. At the same time the Bryah Basin developed as a back-arc basin to the east of the Gascoyne Complex and records a history of mafic to ultramafic volcanic and intrusive activity followed by the deposition of sedimentary rocks (Pirajno et al., 1998). In the southern Gascoyne Complex the tonalites and monzogranites were heterogeneously deformed before being intruded by voluminous granodiorite and monzogranite at c. 1970 Ma. Magmatism accompanied high-grade metamorphism, with the local migmatisation of pelitic rocks taking place at the same time as the formation of a layer parallel tectonic fabric. The 2000 to 1970 Ma granites do not intrude the Archaean Yilgam Craton and are interpreted as a separate Palaeoproterozoic terrane which may represent an island arc that developed over a north-dipping subduction zone. Deformation and metamorphism at c. 1970 Ma may reflect collision of this arc and the Yilgam Craton. To the east the < 2000 Ma Padbury Basin probably developed as a foreland basin (Martin, 1994) to this collision. This event is significantly older than the Capricom Orogeny of Tyler and Thome (1990, 1994) and is here named the Glenburgh Orogeny. At 1820 to 1800 Ma the southem part of the Capricom Orogen was affected by deformation, metamorphism and extensive felsic magmatism, during which the Yarlarweelor gneiss complex developed. Archaean granitic gneiss was intruded at c. 1810 Ma by sheets and veins of leucocratic granite. Intmsion took place parallel to an Archaean gneissosity that is reorientated by Palaeoproterozoic east to northeast trending tight to isoclinal upright folds. Locally the granites cut the gneissosity but are also seen to be folded with it. The c. 1810 Ma granites were emplaced during upper amphibolite facies metamorphism and locally show incipient partial melting. In contrast, c. 1800 Ma granites form dykes and large sheet-like bodies intruded into major east-southeast trending fault zones that cut the c. 1810 Ma granite. The younger granites contain sericitised feldspar and chlorite (after biotite) indicating a greenschist facies metamorphic overprint.

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The change in deformation regime from ductile to britde, and the coincident drop in metamorphic grade from upper amphibolite to greenschist facies implies that tectonic uplift took place between 1810 and 1800 Ma in the southern part of the Capricorn Orogen. This involved thrusting of the Yarlarv^eelor gneiss complex over the Yilgarn Craton, and reactivation of the Errabiddy Shear Zone. The low-grade metasedimentary and meta-igneous rocks of the Bryah and Padbury Basins were tectonically interleaved with the Yarlarweelor gneiss complex during this event. The voluminous biotite and muscovite-bearing monzogranites and syenogranites that intruded into the Yarlarweelor gneiss complex and Gascoyne Complex are consistent with syn- to postcollisional magmatism. The c. 1820 to 1800 Ma event is equated with the final stages of the Capricorn Orogeny (Tyler and Thome 1990, 1994), which reflects the final collision between the Archaean Yilgarn and Pilbara Cratons. References Martin, D. M., 1994. Sedimentology, sequence stratigraphy, and tectonic setting of a Palaeoproterozoic turbidite complex. Lower Padbury Group, Western Australia. Ph.D. thesis, University of Western Australia, Western Australia. Nelson, D. R. 1998. Compilation of SHRIMP U-Pb zircon geochronology data, 1997. Geological Survey of Western Australia Record 1998/2. Nelson, D. R. in press. Compilation of SHRIMP U-Pb zircon geochronology data, 1998. Geological Survey of Western Australia Record. Pirajno, F., Occhipinti, S. A., Swager, C. P., 1998. Geology and tectonic evolution of the Palaeoproterozoic Bryah, Padbury and Yerrida Basins (formerly Glengarry Basin), Western Australia: implications for the history of the south-central Capricorn Orogen. Precambrian Research 90, 119-140. Tyler, I. M. and 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. Tyler, I. M. and Thome, A. M., 1994. The role of structural geology in the search for highgrade iron ore bodies in the Hamersley Basin. Geological Society of Australia Abstracts 37, 437.

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TIMING OF DEFORMATION, METAMORPHISM AND FAULT MOVEMENTS WITHIN THE ADELAIDE FOLD BELT: IMPLICATIONS FOR TASMANIDE OROGENESIS Robin Offler \ David R. Gray David A. Foster ^ and Thomas Flottmann^ ^Department of Geology,University of Newcastle, Callaghan, NSW 2308 roffler@geology.newcastle.edu.au ^Australian Geodynamics Cooperative Research Centre, VIEPS Department of Earth Sciences, Monash University, Melbourne, Victoria 3168 ^Australian Geodynamics Cooperative Research Centre, VIEPS School of Earth Sciences, La Trobe University, Melbourne, Victoria 3083 "^Department of Geology and Geophysics, University of Adelaide, Adelaide, S.A. 2308 Deformation within the Adelaide Fold belt has previously been defined by U-Pb zircon ages from syntectonic granites in the high T/low P (HTLP) part of the fold belt in the southern Mt Lofty Ranges, such that Delamerian orogenesis has been bracketed between 523-486 Ma (Turner et al 1996). Ar-Ar spectra obtained from slates in the fold and thrust belt of the southern Flinders Ranges, north of the HTLP belt in the Mt. Lofty Ranges, indicate however that deformation commenced at -550-540 Ma ago. Resetting of the system appears to have occurred at 510-505 Ma along the eastern margin due to the intrusion of the syn- to posttectonic, Cambro-Ordovician granitoids in the interior of the fold belt (Fig. 1). Exhumation of the fold-thrust belt by movement on detachment zones along the western margin of the fold belt was still ongoing at -450-430 Ma; as recorded by cooling ages from biotite and muscovite in schistose rocks from these detachment zones. These relatively younger dates presumably record a phase of west-directed thrusting caused by major deformation and accretion of the western Lachlan Fold Belt. The results suggest that deformation is diachronous and is transitional into events that were taking place outboard in the developing Lachlan orogen.. Marked reactivation took place along the leading faults at the western margin of the fold-belt suggesting craton-directed thrusting at this time. The marked diachroneity from north to south is also reflected by marked changes in structural geometry. The width and curvature of the fold belt changes dramatically along strike. It changes from a narrow (-50 km width), southern Appalachian style fold-thrust belt consisting of imbricated thrust sheets in the south (Fleurieu Arc), to a wide (-200 km width) Jura-style fold belt in the north (Nackara Arc) where Neoproterozoic strata are concentrically folded above a regional decollement. The decollement zone consists of highly strained, L-S tectonite schistose rocks with transposition layering, isoclinal folds, shear bands and asymmetric shear lenses which indicate westward transport with an east over west sense (Flottmann et al., 1994). In the north anticlinal structures and domal culminations are cored by Callana Beds, present as diaprically intruded carbonate breccia and broken formation. This breccia is similar to the Max Meadows breccia of the decollement zone of the Southern Appalachian fold-and-thrust belt (cf. Diegel, 1988). Folds in the northern part are upright, open to close (interlmib angles are 120°-90°) with wavelentghs on the order of 20 km. Folds become tighter to the E, and show a change in wavelength from -20 km to -14 km, and are cut by both E- and W-dipping reverse faults (spaced at 20-30 km) on the E side of the belt. Basement involvement in the southern part is indicated by a number of fault-bounded slices (horses) of Proterozoic basement, including the Houghton, Aldgate and Nommanville inliers (Fig. 1). In a transitional zone (the Burra subdomain of Marshak & Flottmann, 1996) between the northern folded and southern imbricated domains, the fold style changes and is accompanied by an increase in penetrative strain, such 175

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that the belt here has similarities to the Welsh slate belt (cf. Wood, 1974). Folds show a wavelength change from -16 km to ~21 km from W to E, interlimb angles between 50°-90°, and are upright to inchned to the E. Studies on white micas (Offler et al., 1998) suggest that most of the exposed fold belt has undergone two distinct metamorphic events. Illite crystallinity determined from K-white micas in slates throughout the southern Flinders Ranges, indicate that grade increases from diagenetic (zeolite facies) at the western margin of the belt to epizonal (greenschist facies) in the hinge of the Nackara arc, reaching biotite grade at the eastern margin. Biotite is developed sporadically in the Clare region. The b^ cell parameters of these micas (x=9.040; an-l=0.012; n=70) reveal that the rocks in the southern Flinders Ranges have been deformed under much cooler, intermediate pressure type conditions (~23°C/km) than those in the Mt Lofty Ranges (~35°C/km; Dymoke & Sandiford, 1992). Significantly the b^ cell parameters decrease dramatically in the Sedan area just north of the HTLP belt, suggesting that the cooler conditions prevailing earher in the fold belt, have been overprinted by the younger HTLP metamorphism. Implications The new Ar-Ar data indicates that the "Delamerian orogeny" is a much longer lived and more complex event than was previously thought. The time constraints required by the high T/low P metamorphism that dominates the southeastern part of the fold belt do not really delimit the time span of orogenesis. It is now clear that this metamorphism is only part of the structural and metamorphic evolution. Deformation was initiated at -550 Ma and involved diachronous E- to W migration of a deformation front as a wave of folding and cleavage development under low geothermal conditions. The main effects of this are preserved in the north, within the Nackara Arc. After a phase of extension related to the deposition of the Kanmantoo Group in the south (Fleurieu Arc), an event associated with high metamorphic grade which is mainly seen in Cambrian Kanmantoo Group post-dates and overprints this low T/higher P deformation/metamorphism by -20-30 my, and was ongoing for -40 my (from 523-486 Ma; Turner et al., 1996). Deformation and exhumation of the fold belt was however still ongoing at 480, 440 and 400 Ma, as shown by discordant whole rock age spectra and cooling ages from muscovite and biotite from schist of the frontal fault zones. These faults were clearly moving at these times. Exhumation is also reflected by the influx of sediment into the deforming and prograding sediment wedges of the developing Lachlan Orogen outboard to the east (see detrital mica ages from Turner et al., 1996 and Foster et al., 1998, fig.5). The geological evolution of eastern Australia is not linked to a simple set of short-lived deformational pulses that occupied specific niches in time. Deformation was progressive, albeit sporadically with linkages in both time and space between the earlier formed fold belts/orogens and those developed subsequently. REFERENCES Diegel, F.A. 1988. In: Geometries and mechanisms of thrusting, with special reference to the Appalachians, G. Mitra & S. Wojtal eds., Geol Soc. Am. Spec. Paper 222, 137-164. Dymoke, P. & Sandiford, M. 1992. Contrib. Mineral Petrol. 110, 121-132. Flottmann, T., James, P. R., Rogers, J. & Johnson, T. 1994. Tectonophysics 234, 95-116. Foster, D.A., Gray, D.R., Kwak, T.A.P. & Bucher, M. 1998. Ore Geol. Reviews 13, 229-250 Offler, R., Foster, D.A., Gray, D.R. 1998. Geol. Soc. Aust. Abs. 49, p.338. Marshak, S. & Flottmann, T. 1996. J. Struct. Geol. 18, 891-908. Turner,S.P., Kelley, S.P., Vandenberg, A.H.M., Foden, J.D., Sandiford, M. & Flottmann, T. 1996. Geology 24, 941-944.

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Isostatic Constraints on the Central Victorian Lower Crust; Implications for the Tectonic Evolution of the Lachlan Fold Belt G. J. O'Halloran^ and P. Rey^ ^ Esse Australia Ltd. 12 Riverside Quay Southbank VIC

^ Department of Earth Sciences Monash University Wellington Rd Clayton VIC 3168

ABSTRACT Middle Devonian (Tabberabberan) deformation in the central parts of the southern Lachlan Fold Belt resulted in > 50% shortening in the upper crust (Gray 1988), and a complete cessation of marine sedimentary environments thereafter (Cas 1983). In this paper some isostatic consequences of Middle Devonian deformation in southeastern Australia are examined via geodynamic modelling. Constraints are placed on the density of the lower crust of the Lachlan Fold Belt, and insights gained into the Palaeozoic tectonic evolution of this unusual orogenic belt. The approach used in this study follows that of Sandiford and Powell (1990), whereby the evolution of strain within an orogen can be quantified by crustal and (whole) lithospheric thickening factors fc and fl. Relative changes in the thickness of the crust and lithospheric mantle generated during orogenesis, and associated physical responses in the deforming lithosphere (such as changes in isostatically supported elevation) can be portrayed in fc-fl space. A model for the strain history of central Victoria has been proposed by taking into account known present-day characteristics of this part of the southeastern Australian crust, as well as metamorphic and structural evidence. The model assumes an initial (ie. PreTabberabberan) crustal thickness of 23km. A two-layered model has allowed for a range of density contrasts between the upper and lower crust to be incorporated into the calculations (ie. "dense", "intermediate" and "less dense" crustal substrates). Models in which a "dense" crustal substrate is assumed (p = 3.05 gcc"^) yield significantly different isostatic responses to those in which relatively lower densities are assumed ( p = 2.85 gcc ^ and p = 2.68 g c c ' V For the purposes of this study. Middle Devonian deformation is considered to have involved up to 50% shortening with a doubling in the pre-Devonian crustal thickness. Predicted isostatically-supported elevations generated during Tabberabberan deformation are in the order of 1650m-2550m (before thermal re-equilibration), for the "dense" substrate model. These values are argued to be geologically realistic, if stratigraphic and sedimentological evidence from the Lachlan Fold Belt is taken into account. This contrasts with the "intermediate" and "low" density substrate models, where isostaticallysupported elevations predicted for the "Tabberabberan Highlands" are in the order of 30004800m, which are considered to be unrealistically high. In addition, the modelling provides some insights as to the nature of crustal deformation within the Palaeozoic Lachlan Fold Belt. The effects of a partitioning of strain within the crust are assessed, by the use of different fc values for the upper and lower crust. Isostatic responses for two particular tectonic scenarios are evaluated; i) subduction of the lower crust and associated imbrication and thickening of the overlying sedimentary pile, and ii) homogeneous thickening of both the lower crust and the overlying sedimentary pile (eg. via imbrication).

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From the results of the isostatic modelling it is suggested that i) the central southern LFB is largely floored by dense (?oceanic) crust, and that ii) the entire crustal section (both upper and lower crust) has been homogeneously thickened via imbrication, as opposed to a subductionstyle model in which only the overlying sedimentary pile would be expected to be thickened.

REFERENCES

Sandiford M. and Powell R. 1990. Some isostatic and thermal consequences of orogenesis. Earth and Planetary Science Letters 98, 154-165.

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SYNTECTONIC FE, SI AND O TRANSPORT DURING DEFORMATION OF HAMERSLEY PROVINCE BIFS: IMPLICATIONS FOR HEMATITE ORE GENESIS 'Nicholas H. S. Oliver 'Lachlan K. Stewart ^ Christopher McA. Powell 'School of Earth Sciences, James Cook University, Townsville, Australia 4811 ^Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA. 6907 Axial planar replacement seams, mineralized thrust surfaces, and quartz veins with iron oxide selvages are evidence of cm- to m-scale mass transfer during the Opthalmian orogeny (c. 2200 Ma) around the giant hematite ore bodies of the southern Pilbara Block. Although the giant orebodies were formed after the peak of folding in the lower greenschist facies Opthalmian Orogeny, it is possible they were related to the waning stages of the deformation, in particular the collapse of the fold belt (Powell et al this volume). In this contribution, we document evidence for mass transfer during and soon after folding in the Opthalmian Orogeny. This mass transfer possibly represents the initiation of the fluid flow system that culminated in genesis of some of the hematite orebodies nearby. Microstructural evidence clearly shows some magnetite and hematite was concentrated or emplaced during and/or soon after the D2 fold/thrust deformation, and silica was also transported, as follows: • Magnetite-rich seams axial planar to F2 cm-dm-scale folds formed during the foliation forming event, by a combination of mechanical and chemical segregation very similar to crenulation cleavage formation in schists. This produced 3mm - 10mm thick iron-rich foliation seams, along with inter-foliation domains rich in fibrous vein quartz, with fibres aligned in the sub-vertical extension direction. Locally, this process produced martite/hematite rather than magnetite, and replacement of magnetite by martite is common in the very iron-rich ("mini-ore") seams. This pattern occurs in all of the iron formations examined in detail (Dales Gorge, Joffre, Weeli Wolli). Quartz-rich strain shadows around pre- to early syn-D2 magnetite/martite porphyroblasts are also common. Scales of mass transfer involved were between mm and cm. Calculations from scanned and enhanced rock slab images suggest local mass balance was attained for Fe and Si, with the deformation mechanism in some layers being accomodated primarily by mass transfer, adjacent to layers in which deformation was accomodated primarily by buckling. • Zones of replacive magnetite and hematite are observed around thin axial planar quartz veinlets. These are similar to the above, but show a diffuse boundary between the outer edges of the alteration zone and the host rock. Oxygen isotope data (below) suggests a fluid from outside the scale of the outcrops (>200m) was involved in the formation of these zones. • Cm-wide quartz veins with hematite or magnetite selvages (rims of altered wallrock) are aligned within the S2 foliation but are not strongly deformed by it, suggesting they formed 179

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syn- to late-D2. petrographically.

Direct hematite replacement

of magnetite can be observed

• hematite "ore" stringers, up to 3 - 4 cm wide, he in "thrust"-like geometries on north verging D2 folds. The hematite replaces previous magnetite-rich BIF, and is associated thin quartz veinlets. For some of the samples, "closed system" mm- to cm-scale mass transfer is supported by oxygen isotope analysis of separated quartz and iron oxides. Values for minerals in the segregation seams, and some veins, v^ere sufficiently close to those from the immediately adjacent wallrocks that external fluid sources are not required. However, in some of the axial 18

planar seams (with diffuse boundaries) and larger veins (e.g. in thrust-like geometries), 5 O values for oxides between -h3%o and -2%o, in rocks that otherwise contain magnetite of 5 to 7%o, suggest an externally derived fluid. These relationships indicate that fluid was transported, at least over >100m scales, during and soon after the Opthalmian (D2) orogeny. 18

The giant ore deposits have 5 Oj^ematite values between -1 and -4 %c. The similarity of these values with some of the syn-D2 alteration leads us to speculate upon a genetic link between the various processes. Local mass transfer was initiated by fluid-assisted mass transfer accompanying folding. Subsequently, relaxation (?elastic) or dilation of these syn-folding ]8 structures allowed ingress of O-depleted, oxidised fluids into the rocks, forming a variety of veins and reaction rims from fluid that was initially out of equilibrium with the BIFs. Finally, as the belt collapsed, wholesale introduction of similar oxidising fluid allowed formation of the giant hematite deposits. Acknowledgements, The work was supported by the ARC, MERIWA, BHP Iron Ore, and Robe River Iron. Particular thanks to Chris Powell (UWA), Dave Mason and Tim James (RRI), Janos Ronaszeki (BHPIO). Ian Cartwright provided stable isotope results.

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APPLICATION OF NUMERICAL CONTINUUM MODELLING TO FLUID FLOW IN SHEAR ZONES, VEINS AND ORES, WITH SOME AUSTRALIAN EXAMPLES 'Oliver, N. H. S., 'Ord, A., and 'Hobbs, B. E, ^Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Qld, 4811 Australia 'AGCRC, CSIRO Exploration & Mining, PO Box 437, Nedlands WA 6009; email Nick.Ohver@jcu.edu.au Numerical modelling of fluid flow during deformation has two purposes: 1) to simulate geologically realistic scenarios to develop predictive models; and 2) to assist in thought processes that greatly improve understanding of geological processes. All-too-often, the latter is considered as a time-wasting exercise relative to the "real" needs (the former). However, rapid advances in numerical modelling in recent years have been made by a growing appreciation of the "thought-enhancing" capacity of the simulations. Speeds of modem computers are sufficient that simulation of a series of first order, geologically realistic deformation and fluid flow scenarios is more rapid than most people's ability to absorb the implications of subtle changes to each model in the series. Using a continuum code (FLAC), we here explore the steps commonly taken in simulation of both conceptual and ore deposit-specific models of coupled deformation and fluid flow. Surprising results often ensue, and examples are given whereby the modelling opens up new possibilities for ore genesis or regional fluid flow, not previously considered by the soft input models. For the vein-style, skam-hosted Mary Kathleen U-REE orebody, for example, the initial geological model proposes ore genesis by interaction between a ductile shear zone and the adjacent skam, but uncertainties remained about the time sequence, mechanisms and fluid and mass sources. Early numerical models show how it is difficult for fluid to simultaneously flow through the shear zone and the orebody host with constant boundary conditions. By changing the relative ratios of the amounts of simple and pure shear (with time) on the model boundaries, simulated fluid flow fluctuates between the the Mary Kathleen Shear Zone and the adjacent hard skam, and fluid is accessed periodically from both nearby rocks and deeper seated sources. Numerical simulation of the coupling of shear zone deformation with an adjacent hard block provides a geometrically realistic answer for ore genesis, but the process of generating this answer through a series of complicated modelling steps provides a huge leap in understanding not possible (at least for us) by simple inspection of the geology. Giant calcite-filled veins elsewhere in this same district provoke awe and confusion upon first inspection. Veins up to 5 m across or more are common, as first reported at the 1987 SGTSG meeting at Mt Buffalo. Existing models for the distribution of stress and fluid pressure in the mid crust (in this case rock P and T of 400 Mpa and 600°C) do not readily allow for the presence of 5 m wide fluid-filled cracks, yet the geometry and grainsize of some of these veins would seem to require that this was the case. Amphibole crystals up to 1 m long, perfecdy euhedral clinopyroxenes of similar sizes, and calcite crystals up to a cubic metre in size, cannot easily be explained by progressive growth of a crack with precipitation proceeding at a similar rate. Prismatic crystals growing into cavities, subsequently filled with

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late precipitates, are common at cm-scales in many amphibolite facies rocks worldwide. The presence of such vein minerals with moderate P-T stabilities also commonly suggests formation at depth, possibly even close to lithostatic fluid pressures. The key requirement to grow such large cracks in a short space of time is that the fluid underpressuring soon after crack opening does not cause catastrophic brecciation - interestingly such brecciation has occurred to some (but not all) of these veins, or to their immediate roofs. To prevent drastic underpressuring requires that the commonly assumed situation of average lithostatic rock pressures (and gradients) and near-lithostatic fluid pressures (and gradients) does not apply. Here, FLAG is used to show that scenarios are possible whereby the partitioning of stress and hydraulic gradient in saturated rock masses with large rheological contrasts can produce strong perturbations of fluid and rock pressure gradients sufficient to explain these giant veins. Immediately above and below flat, tabular, hard bodies (e.g. dolerite) subjected to subhorizontal shortening by pure shear in a matrix of soft, weak material (e.g. banded calcsilicates), zones of hydrostatic or sub-hydrostatic fluid pressure gradient may develop. However, the sub-lithostatic pressure gradients can be developed at near lithostatic pressures, thus explaining the presence of "normal" amphibolite facies mineral assemblages in the rocks and veins. Cycling of the locii of intense deformation with time explains why some veins are brecciated, why some are subjected to ductile deformation on their margins, and why many have brecciated roofs. Some of the spectacular high temperature chalcopyrite-calcite deposits of the Mt Isa Eastern Succession show similar features, vein dimensions and grainsizes, also suggesting that the giant cracks were non-unique and were connected to giant hydrothermal systems. In both examples, the numerical modelling has not acted as a proxy for the collection of detailed geometric and geologic information, but it has acted as a critical neural stimulant that has helped us develop and refine working, process-oriented models. Geologic data and preliminary soft models for fluid flow in the Hamersley Province are now being conducted to test the relative contribution of compaction, topography and deformation in driving basinal fluids, and, potentially, contributing to the genesis of giant hematite ores.

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NUMERICAL MODELLING OF THE GEOLOGICAL DEVELOPMENT OF THE WEST LACHLAN OROGEN A. Ord\ B.E. Hobbs\ D.R. Gray^ and D.A. Foster^ Australian Geodynamics Cooperative Research Centre 'CSIRO Exploration & Mining,, PO Box 437, Nedlands, WA 6009. ViEPS Department of Earth Sciences, Monash University, Melbourne, Victoria, 3168 ^VIEPS Department of Earth Sciences, La Trobe University, Melbourne Victoria 3083; and Department of Geology, University of Florida, Gainesville Florida 32611 USA The modelling presented here is based on the extensive structural geological and geochronological studies conducted in the West Lachlan Orogen over the past decade (see Gray, 1997; Foster et al., 1998 for reviews). The goal of this work is to present a detailed quantitative analysis of the deformation, fluid flow and thermal transport associated with the evolution of this orogen from the late Cambrian (approximately 510 Ma) to late-Devonian (approximately 376 Ma), a period of ca. 130 million years. The ultimate purpose of the work is to gain a quantitative understanding of the localization of the Stawell-Ballarat-Bendigo gold mineralization. Modem seismic studies indicate a crustal structure consisting of a lower crust extending from ca. 18 km to ca. 36 km of unknown composition but here assumed to be comprised of layered mafic/felsic granulites. A detachment at ca. 17 km is proposed as the base of an approximately 5-10 km thick imbricate stack of mafic volcanics. Overlying this imbricate stack is an approximately 8 km thick package of folded Ordovician sediments. During the peak of metamorphism this package of folded Ordovician sediments could have reached a thickness of 25 km prior to subsequent erosion. The total crustal thickness now is ca. 36 km. Sedimentation, deformation, metamorphism and plutonism evolves from West to East across the presently exposed orogen extending from the edge of the Stawell Zone in the West to the edge of the Melbourne Zone in the East; a distance of ca. 325 km. Deformation begins at ca. 450 Ma in the West and extends through to ca. 380 Ma in the East. Plutonism occurs in the period 410-390 Ma predominantly in the Stawell Zone and west part of the Bendigo-Ballarat Zone and extends to 380-360 Ma in the Bendigo-Ballarat Zone and in the Melbourne Zone. The starting point for modelling involves placing constraints on the thermal/plutonic history. We assume, as a base level scenario, that the Ordovician sediments have a composition similar to those of the Bega-Kosciusko region (B. Chappell, private communication), namely, 3.54% K2O, 16.9 ppm Th, and 3.5 ppm U. In the Cambrian/Ordovician this composition is equivalent to a heat production rate of ca. 3 microWatts/cubic metre. We assume the Ordovician/Silurian sediments to have had this internal heat production rate uniformly with depth. In order to be compatible with modem heat flow measurements, this then constrains the internal heat production rate of the mafic volcanics and the lower crust to be, on average, ca. 1 microWatt/cubic metre (assuming a modem heat flux at the Moho of 20 milliWatts/square metre). These assumptions lead to temperatures at the Moho in the Cambrian /Ordovician of ca. lOOO^C for a Moho heat flux of 30 milliWatts/square metre and ca. 820^C for a Moho heat flux of 20 milliWatts/square metre. Thus, it seems that the granites typical of the West

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Lachlan Orogen can be generated simply by thickening of the crust by folding and imbricate stacking without calling upon ad hoc processes (accompanied by high heat flow) in the mantle. We also explore the time lag between deformation and plutonism (ca. 50 million years) and show that this is compatible with melting arising from the progressive Eastward evolution of tectonic thickening. The orogen is modelled as an Eastward thinning wedge of Mohr-Coulomb, dilating material underlain by a lower-crust and mande which has a non-linear viscous response. The material erodes as it thickens due to deformation and the resultant material propagates the wedge eastwards. Fluid flow in the wedge is coupled to the deformation through deformation induced porosity (and hence, permeability). The resultant fluid flow takes place via the porosity wave mechanisms proposed in Hobbs et al. (this volume). Several boundary conditions are explored including a basal shear stress condition that simulates the movement of a subduction zone. In each scenario, dilatant shear zones develop which control the localization of fluid flow. We explore the controls on the timing and spacing of these zones and the conditions that result in focussing of fluid flow into regions such as Stawell and Bendigo-Ballarat. The linking of thermal history and fluid flow enables regions of devolatilization to be delineated in the lower crust together with the plumbing systems that enable these fluids to be transferred to the upper crust. This modelling enables an holistic, quantitative model for the sedimentation/deformation/ plutonism/fluid flow/mineralization process to be developed. References Foster, D.A., Gray, D.R., Kwak, T.A.P. and Bucher, M. 1998. Chronology and tectonic framework of turbidite-hosted gold deposits in the Western Lachlan Fold Belt, Victoria. ^^Ar results. Ore Geology Reviews, 13, 229-250. Gray, D.R. 1997. Tectonics of the southeastern Australian Lachlan Fold Belt: structural and thermal aspects. In: Orogeny Through Time. Burg, J.-P. and Ford, M. (eds). Geological Society Special Publication, 121, 149-177.

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ZIRCON FISSION TRACK EVIDENCE FOR PRE NEOGENE TECTONICS IN IRIAN JAYA Paul B.O'SullivanKevin C. Hilll, Richard D. Kendrickl, Edy Sutriyono^, & Kaspar Lumbanbatu^ ^Australian Geodynamics Cooperative Research Centre Earth Sciences, La Trobe University, Melbourne, Victoria 3083, Australia (p. osullivan @ latrobe. edu. au) ^Geological Research and Development Centre, Jalan Diponegoro, Bandung, Indonesia As a part of a major project designed to investigate the effects of the ongoing tectonism in the Irian Jaya Fold Belt and the Bird's Head (Fig. 1), over 200 samples were processed for either apatite or zircon fission track analysis. Fission track studies are ideal in constraining the timing of tectonic events within mountain belts as well as determining the time-temperature history of basin sedimentary rocks. The zircon fission track (ZFT) age effectively records the timing of cooling below ~200-240°C so usually records the provenance age of detrital grains which may correspond to volcanism, cooling of plutons or major uplift and denudation events in the hinterland. Alternatively, the ages can be reset by metamorphism.

Figure 1. Tectonic map of Irian Jaya and western Papua New Guinea showing the regional location of the outcrops and wells sampled for this study. Key: R-1 Roabiba-1; K-1, Kalitami-1; S-1, Sebyar-1; A-2, Ayot-2; T-2, Tarof-2; TBJ, TBJ-IX; TBE, TBE-IX; BB, Bintuni Basin; SB, Sulawati Basin; LFB, Lengguru Fold Belt; KOM, Kumawa-Onin-Misool; K, Kemum terrane DFZ, Derewo Fault Zone; WO, Weyland Overthrust; E, Enarotali; MTB, Mamberamo Thrust Belt; IJO, Irian Jaya Ophiolite Domain; SC, Sepik Complex; TB, Teluk Basin; W, Wandamen Peninsula. Active subduction zones and spreading axes are shown by heavy lines, inactive zones are shown by dashed lines.

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ZFT analyses were completed for 52 samples collected from 10 stratigraphic levels from Plio-Pleistocene to Precambrian. From the ZFT analyses, seven major zircon provenance/reset populations were determined: 1) Pliocene-Late Miocene (<10 Ma); 2) Early Miocene-Oligocene (15-35 Ma); 3) Early Eocene-Paleocene (50-65 Ma); 4) Middle Cretaceous (80-120 Ma); 5) Early Jurassic-Late Triassic (180-220 Ma); 6) Early Permian-Late Carboniferous (280-320 Ma); and 7) Early Paleozoic or older (>450 Ma). Each of these can be correlated to regional tectonics or the occurrence of volcanics. LATE MIOCENE-PLEISTOCENE AND EARLY MIOCENE-OLIGOCENE ZIRCONS Most samples containing Upper Miocene-Pliocene-aged zircons were collected from the Pleistocene Buru Formation located throughout the Bird's Head and the western Irian Jaya Fold Belt. In general, the Buru from the central and western parts of the Fold Belt does not contain contemporaneous zircons, a result that is consistent with the absence of volcanism during the peak of compressional deformation there. Within the Bintuni Basin and western fold belt, however, there was Miocene volcanism, consistent with collision of an arc with the eastern Birds Head to form the Lengguru Fold Belt. EARLY EOCENE-PALEOCENE ZIRCONS Zircons with Early Eocene to Paleocene ages were probably derived from magmatic bodies which formed in response to the Paleocene formation of oceanic marginal basins along the north coast of PNG and possibly Irian Jaya. Within the Irian Fold Belt, most samples that record Paleocene provenance ages occur in the eastern or central parts of the fold belt, suggesting that Paleocene tectonism and magmatism were focused at the eastern end of the fold belt. This is consistent with Paleocene opening of the Coral Sea between PNG and Australia and also Paleocene formation of the oceanic marginal basins along the north coast of PNG. Mid-Cretaceous

Zircons

Zircons with middle Cretaceous ages were probably derived from rocks that experienced significant denudation of middle or Lower Cretaceous strata during the Late Cretaceous to Paleogene and in the Mio-Pliocene, resulting in deposition of sediments in the western Fold Belt and Bintuni Basin. The preferred interpretation is that the zircons originated from midCretaceous volcanics predating the early stages of the Coral Sea breakup of the continental margin (Fig. 1). Alternatively, the volcanics may have been derived from an arc along the northern margin of New Guinea, consistent with the Mount Victor granodiorite in PNG. Furthermore, the presence of middle Cretaceous-aged zircons in the Woniwogi Formation in the central part of the Irian Fold Belt suggests that the unit is probably Albian in age rather than Neocomian. Late Triassic-Early

Jurassic

Zircons

Late Triassic-Early Jurassic zircons probably originated from contemporaneous volcanism in the west and central fold belt and Bintuni Basin associated with breakup of the northwestern continental margin of Australia, and possibly from Triassic plutonism in PNG. Furthermore, the data suggest significant denudation of Triassic-Lower Jurassic strata in the Late Cretaceous-Paleogene and in the Mio-Pliocene, depositing sediment in the west-central fold belt and Bintuni Basin.

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Early Permian-Late Carboniferous Zircons The presence of Early Permian to Late Carboniferous zircons suggests that significant denudation of Lower Permian to Upper Carboniferous strata occurred during the Cenozoic, depositing sediment throughout the Bird's Head and the Irian fold Belt. The preferred interpretation is that the zircons originated from Lower Permian to Upper Carboniferous volcanics, the exact location of which is unknown at this time. Proterozoic to Early Palaeozoic Zircons The presence Early Palaeozoic zircons and possibly Proterozoic suggests significant denudation of an Early Palaeozoic hinterland, particularly in the Cretaceous, supplying sediment to the western and central Irian Fold Belt and the Bintuni Basin regions. These zircons probably represent reworked grains originating from the Australian craton.

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REGIONAL SEISMIC REFLECTION PROFILING, KALGOORLIE GOLDFIELD, WESTERN AUSTRALIA. A J . Owenl, R. Bateman^, B.R. Golebyl, and B J . Drummondl 1 Australian Geodynamics CRC, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601. email aowen@agso.gov.au 2Kalgoorlie Consolidated Gold Mines (KCGM), PMB 27, Kalgoorlie, WA, 6430. Seismic reflection profiling recorded in the Kalgoorlie region of Western Australia has imaged the structure of the Archaean supracrustal greenstones, the detachment between these supracrustal greenstones and the underlying strongly reflective felsic crust and the geometric relationships of the various mapped fault systems. The interpreted seismic data show two levels of structural detail. The first shows the regional large open D2 structures formed by large scale thrusting along a regional detachment surface. The second shows evidence of an earlier episode of complex thrust stacking and repetition within each of these larger structures, suggesting these D2 domes to be zones of stacked imbricated greenstone lozenges. The earliest deformation event identified was north-south extension that formed the basin, into which the volcaniclastics were deposited. In terms of the seismic data, the D^ thrusting is effectively indistinguishable from the earlier extensional event. D2 deformation involved the ENE-WSW shortening that resulted in the general NNW-SSE trends seen as the broad synformal and antiformal structures (eg Mount Pleasant and Scotia Kanowna AnticUnes). At the mine scale, D2 deformation consists of SW over NE reverse faulting and folding. We suggest the Au mineralisation began at this time. D3 deformation occurred in a transpressional regime with partitioning of shortening and simple shear deformation and associated with N-S orientated strike-slip faults throughout the region. Some of these faults (eg Zuleika Shear, Emu and Mt Monger Faults) sole into the basal greenstone detachment. Other D3 faults and shear zones cut through the basal detachment and penetrate the upper crust (eg Bardoc Shear) or middle to lower crust (Ida Fault). By penetrating and cutting the basal detachment, the Bardoc Shear likely acted as a major fluid conduit from depth to the surface. Previous interpretations of the 1991 deep seismic data show the greenstones to be 5-7 km thick, separated from basement by a regional basal detachment surface and, at the regional scale, matched the regional geology extremely well. Our re-interpretation of the seismic data demonstrates it is possible to further unravel the results of the successive deformation events. Each deformation event can be identified in the data, with the interpretation supporting a thrust stack model of crustal formation. The unravelling of the deformation history is dependent on the identification of kinematic indicators associated with a given deformation surface. The east side of the Scotia Kanowna Dome is imaged as a series of west-directed greenstone thrust slices which sole into the basal detachment, while the western side consists of similar greenstone slices but thrusted towards the east, and lying structurally below the west-directed thrust wedges. Kinematic indicators on the detachment show opposing directions of tectonic transport. The Scotia Kanowna Dome is therefore a number of thrust duplexes, thrust from both the east and west, rather than a broad folding of coherent stratigraphy across the whole of the dome. This implies that the greenstone stratigraphy may be repeated a number of times within the dome. A similar interpretation is proposed for the Mount Pleasant Dome, where the dome consists of a series of greenstone thrust wedges.

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although the eastern part of the Mount Pleasant Dome is lost as it is truncated against the younger D3 Bardoc Shear Zone. In the Kalgoorlie region, similar geometries are imaged. The basal greenstone detachment is irregular with 5-9 km variation in greenstone thickness. The seismic data show the same two levels of structural detail. Regional large scale structures are imaged that show each consists of a series of greenstone packages exhibiting an overall lozenge-shape geometry. The termination of strong reflectors and enveloping surfaces of the lozenge-shape geometry determines the sub-surface image of fault systems. Mapped fault systems are interpreted by geometric relationships. Some of the strong reflectors may also be relic bedding, associated with deposition into the original basin. The lozenge-shape greenstone geometries are believed fractal as this geometry also occurs on mine scale detailed seismic data, and is confirmed by mapping in the Eastern and Western Lodes of the Kalgoorlie Gold Field, where the faults and lodes make an anastomosing pattern, enclosing lozenge-shaped blocks. 4.5 km

w 4070 J

Fab ,

4120 I

Fuk ,

97KAL-4

97-KAL-3

Station number

Station number Fbo 3150 3100

4170 L

3050

3000

The figure shows details from new seismic data in Kalgoorlie's Golden Mile region. The large open D2 duplex structures consist of thrust wedges dipping west into the detachment surface. The foot of the duplex is the detachment surface. The duplex appears to be thrust towards the east, although the data are two dimensional and this should be viewed as an apparent movement. This duplex is truncated up section by an anastomosing fault system (1.2 sec TWT -- 3.6 km) which in turn has deformed higher level duplexes. Above this region both east and west dipping reflectors indicate brittle deformation and potential fluid migration pathways. The antiformal domes are evidence of earlier complex thrust stacking and repetition.

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Acknowledgments AO, BRG and BJD publish with the permission of the Executive Director of AGSO and Director AGCRC. The transects were part funded by Kalgoorlie Consolidated Gold Mines.

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READING SMALL-SCALE STRUCTURES IN ROCKS PASSCHIER, Cees, Institut fiir Geowissenschaften, Gutenberg Universitat, 55099 Mainz, Germany, cpasschi@mail.uni-mainz.de "Lapidum natura restaU hoc est praecipua morum

insania..."

The nature of rocks remains to be considered, a particular source of irresponsible behaviour in men... (C. Plinius Secundus Maior, Naturalis Historiae Liber XXXVI)

One of the cornerstones of structural geology is the interpretation of geometries in rocks in terms of deformation processes and series of events. For this purpose, small-scale structures have been studied for a long time and important steps in the second half of this century were the recognition of the potential of inclusion patterns in porphyroblasts to determine sequences of events, and of structures with monoclinic shape symmetry ("asymmetry") as shear sense indicators in mylonite zones. In the last decades of this millennium, geologists have started to use small-scale structures to retrieve quantitative data on deformation and flow parameters in rocks. Small-scale structures in rocks carry a lot of information on deformation that we cannot obtain otherwise, but it is difficult to assess which parts of their complex shapes are relevant for the purpose of retrieving particular data. Useful structures where this problem has partly been cleared are porphyroblasts, manded porphyroclasts, shear bands, asymmetric boudins, veins and fringe structures. The shape of these structures is influenced by the geometry of flow in the surrounding rock, its change with time, and finite strain. Detailed field observations, analogue experiments and computer modelling are being used to analyse such structures and it is now in many cases possible to make estimates of parameters such as kinematic vorticity and dilatancy numbers of progressive deformation using object geometry. Although small scale structures have a great potential, they can only be used to a limited extend now. More and better experiments are needed for their "calibration". However, many of our present modelling attempts are rather simplistic: they focus on homogeneous flow and time-independent progressive deformation of isotropic materials. Also, present models are usually two-dimensional sections assuming deformation to have monoclinic symmetry. Obviously, very few natural rocks will follow these assumptions and it is uncertain how serious the error is using ideahsed models. Experimental simplification is not only due to technical, economic or human limitations but is mainly caused by a necessity to investigate end-member situations, given the myriad of theoretically possible paths and geometries. One way to build better, more complex, time dependent three dimensional models of progressive deformations, will be to improve the accumulation and storage of three-dimensional data on deformation structures in the field and in outcrop. Some structures such as manded porphyroclasts contain mainly information on flow geometry at late stages of progressive deformation while others, such as fibrous veins and porphyroblasts, store more information on deformation paths. A combined three-dimensional study of both types of structures can help to determine common deformation geometries and deformation paths in nature as a basis for more complex modelling studies.

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BASEMENT-INVOLVED FOLD THRUST BELT TERMINATIONS IN THE FLINDERS RANGES, SOUTH AUSTRALIA: REACTIVATED SYNDEPOSITIONAL FAULTS AND THERMAL PERTURBATIONS Eike Paul, Thomas Flottmann, Mike Sandiford Dept. Geology and Geophysics, the University of Adelaide, SA 5005 (tflottma @ geology.adelaide.edu.au) The northern Flinders Ranges and the eastern termination of the Nackarra Arc (Adelaide Geosyncline) are characterised by basement-involved deformation in a comparatively low strain environment (shortening « 20%). In the northern Flinders Ranges, basement is predominantly exhumed along two major faults that were active during deposition: the Norwest and Paralana Faults, which formed a major growth fault and relay structure, respectively during the deposition of Neoproterozoic sediments. The structural style is best interpreted to be thick-skinned involving basement with only a minor proportion of the overall shortening accommodated along stratigraphically-controlled detachments. Much of the contractional deformation was localised by the inversion of former extensional faults such as the Norwest and Paralana Faults, which both controlled the deposition of Neoproterozoic cover successions. As such, both faults represent major, long-lived structures which effectively define the present boundaries of the northern Flinders Ranges with the Gawler Craton to the west, and the Cumamona Craton to the east. The Willyama Inliers form the interface between the Cumamona craton and the main depocenter of the Adelaide Geosyncline in the central Nackarra Arc. Detailed examination of sediment thicknesses and structures and kinematics during Delamerian deformation suggest that the Willyama Inliers represent major upthrust segments of former growth faults associated with the regional MacDonald Fault zone. The implication is that reactivated growth faults form major strain guides with a complicated kinematic history. In the Willyama Inliers, Delamerian structural evolution comprises a two phase deformation path where initial northwest directed contraction is accommodated by a first phase of west directed shortening and a later phase of northwest and north directed shortening. The early phase is interpreted to be due to strike slip deformation along major relay structures that accommodated Neoproterozoic extension (eg. Darling River lineament) and evidenced in the formation of north-south trending folds in the Neoproterozoic cover. During progressive shortening the overall regime modifies into a transpressional regime, with the tectonic transport direction rotates into a northwesterly to northerly direction. This phase is interpreted to be coeval with the onset of the main phase of thin-skinned fold deformation in the central Nackara Arc and with the re-activation of the Willyama Inliers along former north-south as well as east-west trending growth faults. Although the regional geology can be explained in terms of the reactivation of major growth faults, there are several observations that suggest that the style of deformation was sigificantly influenced by the attendant thermal regime. The Flinders Ranges are characterised by anomalous heat flows related to exceptional heat-production values in basement rocks. For example, the most intense basement-involved deformation, which resulted in exhumation of the basement along the Paralana Fault to form the Mount Painter and Babbage Inliers, coincides with the most extreme heat flows (-120 mWm'^) related to extraordinarily high heat

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production rates in the basement rocks. The Willyama Inliers are characteriosed by intermediate heat flows (-70-75 mWm'^), while the lowest measured heat flows occur in the central Nackarra Arc (-60 mWm'^) where basement-involved deformation is least conspicuous.

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GEODYNAMICAL EVOLUTION OF THE LACHLAN NUMERICAL MODEL Cristina Pauselli^'^, Jean Braun^, David Gray^ and David Foster"^

FOLD

BELT:

^Department of Earth Sciences, University of Perugia, Perugia, Italy cristina@rses.anu.edu.au ^Research School of Earth Sciences, The Australian National University, Canberra, ACT ^Department of Earth Sciences, Monash University, Melbourne, Vic "^Department of Geology, University of Florida, Gainesville, Fl, USA. The Lachlan Fold Belt is part of the Tasman Orogen (eastern Australia) that is a fragment of an originally much longer system formed along the Pacific margin of Gondwanaland during Paleozoic time. The structural style of the Lachlan Fold Belt is quite uniform and generally characterized by chevron folds cut by high-angle reverse faults but, across the 700 km wide belt, it is possible to recognize marked differences in lithology, magmatic rocks, metamorphic grade, structural trends and isotopic ages that have led to a division of the belt in three separate and distinct sub provinces (the western, the central and the eastern). Lithologically, the Lachlan Fold Belt consists in three main units: (1) Cambrian mafic volcanics and sedimentary rocks of oceanic affinity (the oldest rocks localized in the western sub province); (2) a large unit made of a deformed deep-marine succession dominated by Ordovician quartz rich turbidites; and (3) a Middle Silurian to lower Carboniferous complex of clastic rocks, limestones and volcanic and plutonic rocks. The Lachlan Fold Belt is dominated by a greenschist to sub-greenschist facies metamorphism, with high T/low P metamorphic rocks found in the central and eastern parts of the belt. New Ar-Ar data from white mica in slates and phyllites from a series of fault zones have given the possibility to define the timing of deformation, metamorphism and exhumation in the Lachlan Fold Belt. This wealth of data on the Lachlan Fold has given rise to several tectonic models that attempt to explain the complexity of the observed deformation, including the multiple thrust wedge, synchronous imbrication in opposite directions, subduction-related magmatism and the location of melange zones. Among these models, one involves the presence of three subduction zones active during the Silurian period, relating the differences between the three sub provinces to their different geographical and structural locations during the geodynamical evolution of the system. The final closure of the two oceanic basins in the DevonoCarboniferous period led to the final accretion of the fold belt and to its present-day morphology. In this work, we attempt to reconstruct the geodynamical evolution of the Lachlan Fold Belt through a fully-coupled thermo-mechanical finite-element model in which lithospheric deformation is driven by subduction. The model predicts deformation patterns at the crustlithosphere scale, erosion/sedimentation patterns, exhumation and isotopic ages for rocks reaching the surface at the end of the computations. By comparing these predictions to the available data we are able to test, in a quantitative manner, the applicability of the triple subduction model to the geodynamic evolution of the Lachlan Fold Belt.

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ORIGIN OF LAYER.PARALLEL GRANITIC SHEETS AS MAGMA-DOMINATED SHEAR ZONES Mark Pawley^, Bill Collins ^ & Martin Van Kranendonk^ ^Department of Geology, University of Newcastle, Callaghan, NSW, 2308, Australia (tassie @ geology, newcastle. edu. au) ^Geological Survey of Western Australia, Mineral House, 100 Plain St, East Perth, WA 6004, Australia Large tracts of high-grade Precambrian terranes consist of heterogeneous granitic rocks that show cm- to m-scale, compositional layering, which is commonly concordant to a tectonic foliation. The rocks are often described as "migmatitic or banded gneisses" and many workers consider these to be metamorphic rocks, but we concur with Lucas & St-Onge (1995) that many are granitic bodies, comprising alternations of syn-kinematic, layer-parallel, granitic veins and sheets. Based on examples from the Late Archaean granitoids of Pilbara Craton, Western Australia, we consider that many form in ductile shear zones during non-coaxial, compressive deformation, and at high magma pressure, they may become the shear zone, for reasons given below. The Archaean Pilbara Craton of Western Australia is a typical granite-greenstone terrain dominated by large granitoid domes and structurally complex greenstone synclines, which formed before 3000 Ma. At -2950 Ma ago, regional WNW-ESE compression produced a penetrative 020-trending foliation throughout the central Pilbara, and associated numerous NNW-trending sinistral faults and smaller NE-trending conjugate dextral faults. Deformation was accompanied by abundant 2950-2930 Ma intrusions, less commonly as layer-parallel, sheeted granitoids. Regardless of the cm- to m-scale range in thickness, the sheets rarely show internal variation and locally possess a magmatic flow fabric defined by euhedral feldspar phenocrysts, with or without a concordant solid-state foliation. At Mulgandinnah Hill in the Shaw BathoHth, folded migmatitic gneisses are cut by a metrescale, ENE-striking, dextral shear zone. The shear zone is defined by cm-scale sheets of 2930 Ma old granite. Euhedral feldspars are aligned parallel to the shear boundary and have recrystallised asymmetric tails, indicating that an original magmatic flow has been overprinted by a concordant, mylonitic shear fabric. Apophyses of granite extend south westward from the shear zone into hinges of adjacent drag folds, suggesting that magma flow was to the southwest. These features suggest that the fine (cm)-scale granite sheets formed by intrusion into an active dextral shear zone, with deformation continuing after solidification. In a large (200 x 80m) rock platform in the Yule Batholith, m-scale gneissic xenoliths are entrained within granitic sheets. The blocks are imbricated, exhibiting consistent clockwise rotation, relative to the adjacent granite sheets. However, even though the enveloping granite sheets are strongly attenuated and locally truncated around the blocks, a lack of dynamically recrystallised textures indicate an absence of strain during and after solidification. This indicates that the granite sheets were magmatic during clockwise block rotation, suggesting sheeted intrusion into an active dextral shear zone.

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Other gneissic blocks on the platform are iwrapped" within a spiral of granitic sheets. The innermost sheets are often isoclinally folded and truncated by the younger, outer sheets. Outward from the core, the sheets and truncation surfaces are progressively less convoluted, with the outermost sheets aligned in the regional ENE direction. The feature has strong similarities with spiral porphyroblasts, with the "wrapped granitic sheets" analogous to curved inclusion trails. Unwrapping the sheets and truncation surfaces suggests that the gneissic block has undergone at least 245^ dextral rotation during shearing. An important feature is the absence of dynamic recrystallisation in the iwrappedi granite sheets, indicating that the sheets were magmatic during spiral formation and dextral shearing. In the eastern Shaw Batholith, granitic dykes of variable composition form a network in a rigid granite host, but are dextrally realigned to form a series of fine-scale, layer-parallel sheeted intrusions concordant with a shear zone boundary. However, no solid-state fabric is observable in the sheets, so realignment was not associated with tectonism. Rather, the field relations suggest that it resulted from magma encountering an active shear zone and flowing along it, after percolating through fractures in the rigid granitoid host. Magma overwhelmed the shear zone and migrated by laminar flow, resulting in the juxtaposition of fine-scale, layer-parallel sheets and an absence of protolith septa. Therefore, such layer-parallel, sheeted intrusions are considered to represent magma-rich shear zones. Foliation/layer-parallel sheets or veins may develop by (1) opening at dilational jogs (Sawyer & Robin, 1986); (2) chemical segregation at compositionally distinct lithological boundaries (Sawyer & Robin, 1986); (3) bulk shear deformation of oblique extensional veins (Myers, 1978); (4) syn-kinematic emplacement along extension fractures parallel to layering, but perpendicular to j (Lucas & St-Onge, 1995); (5) diffusion-controlled melt migration along axial planes (Hand & Dirks, 1992); (6) multiple intrusion in active extensional shear zones (Hutton et al, 1990); or (7) multiple intrusion in active compressional shear zones (Hutton, 1992; Collins & Sawyer, 1996). At the Pilbara locations described above, sheeting is demonstrably confined to shear zones and is subconcordant with zonal boundaries. The 8-km wide, N-trending, sinistral Mulgandinnah Shear Zone is orientated consistendy -70^ oblique to the smaller ENEtrending, dextral shear zones described above. This major zone also contains numerous concordant, syn-kinematic, sheeted granitoids (Van Kranendonk & Collins, 1998) of the same 2950-30 Ma age as those in the dextral zones. The persistent 020-trending solid-state foliation in the Shaw and Yule batholiths bisects the dextral and sinistral shear zones, suggesting that all structures are related, forming in response to ESE-WNW compression. The sheets appear to have exploited favourable zones of weakness where shear strain was greatest and sheet-tip propagation stresses could be most effective (cf. Hutton, 1992). Therefore, we conclude that model (7) is the most appropriate: the layer-parallel, sheeted granite systems intruded into active shear zones that formed during compressive deformation at~30-50Otoai. Field relations suggest that cm- to m-scale sheeting occurred by percolation of compositionally variable granitic magmas through a fracture network into active shear zones, the magmas reorientating subconcordandy with the shear zone boundaries. This is consistent

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with the dominance of igneous textures and localised flow foliations within the shear zones, which indicate crystallisation from a magma. The general lack of layer-parallel, solid-state foliation within the sheets implies that active shearing ceased before complete solidification of the magma. If magma pressure is high, it dominates the shear zone and may exceed the effective confining pressure, creating dilational space. Magma migrates by laminar flow to produce a magmarich shear domain characterised by subparallel alignment of granite sheets. In these cases, the granite sheets become the shear zone. Some of these zones are kilometres thick and resemble the large-scale, sheeted plutonic complexes described by Lucas & St Onge (1995), but they are aligned oblique to O], not perpendicular to it. References Collins, W.J. & Sawyer, E.W., 1996. 7. Met. GeoL, 14, 565-579. Hand, M. & Dirks, P.H.G.M., 1992. J. Struct GeoL, 14, 591-604. Hutton, D.H.W., 1992. Trans. Roy. Soc. Edin., 83, 377-382. Hutton, D.H.W., Dempster, T.J., Brown, P.E. & Becker, S.M., 1990. Nature, 343, 452-5. Lucas, S.B. & St. Onge, M.R., 1995. J. Struct. GeoL, 17, 475-491. Myers, J.S., 1978. Precamb. Res., 6, 43-64. Sawyer, E.W. & Robin, P.-Y.F., 1986. J. Met. GeoL, 4, 237-260. Van Kranendonk, M.J. & Collins, W.J. 1998. Precamb. Res., 88, 207-232.

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EXPERIMENTAL MODELLING OF STRETCHING LINEATIONS WITHIN SIMPLE SHEAR ZONES S. PIAZOLO and C. W. PASSCHIER Institut fiir Geowissenschaften, Gutenberg Universitat, 55099 Mainz, Germany The shape of deformed and recrystallised mineral aggregates in ductile shear zones is commonly thought to build stretching lineations and planar shape fabrics, and to reflect the shape of the finite strain ellipsoid. Theoretically, however, many factors such as initial grainsize and composition of the rock, and the rheology of different recrystallised mineral phases in the rock play an important role to determine the shape of the aggregates. We present first results of analogue experiments in progress, carried out to model the effect of preexisting fabrics and viscosity contrast between phases within a shear zone on the final shape fabric. The experimental setup is a modified version of the coaxial cylinder viscometer used for analogue modelling of mantied porphyroclasts by Passchier and Sokoutis (1993). Two cylinders of different diameter are placed within eachother, and the inner cylinder is rotated resulting in a non-coaxial (circular Couette) flow between the two cylinders. Spheres and preformed prolate or oblate objects of a viscous Plastilina-Rhodorsil gomme mixture with power law behaviour are inserted in a optically transparent Newtonian polymer matrix (PDMS). The development of lineations formed due to the deformation of the suspended objects can be observed at all angles during the entire experiment in three dimensions, since the machine is equipped with a transparent floor to the specimen chamber. Present results show that the final 3D-geometry of an initial sphere is strongly dependent on the viscosity contrast of the object and the matrix especially at high strain rates. At low strain rates and high n values of power law flow, a viscous sphere can remain undeformed. Pulsating ellipsoids may also develop continuously shortening and stretching while rotating towards, into and out of the shear plane. Beyond a certain strain rate and depending on n values, a viscous inclusion may start to deform permanentiy. These results confirm findings by Passchier and Sokoutis (1993), but allow a full quantification of the phenomena. When deforming objects with predeformation shape and a specific angle to the model shear zone, former lineations, preexisting to the shear zone development, may be shortened along their principle elongation axis and form near spherical objects at certain values of strain rate and angle to the shear zone. At certain circumstances predeformational lineations may be destroyed due to the deformation within a newly developing shear zone, while new shear zone induced lineations have not yet formed. Further experiments are currently being undertaken to clarify the quantitative relationships of the different factors of the mentioned issues. References: C. W. Passchier & D. Sokoutis (1993) Experimental modelling of mantled porphyroclasts; Journal of Structural Geology, 25, 895-909

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RELATIVE DEFORMATION AGES AND THE SYSTEMATIC ASSESSMENT OF DEFORMATION HISTORIES Graham Potts (gpotts@liv.ac.uk) Dept. of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK Steve Reddy (sreddy@lithos.curtin.edu.au) Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, Perth, WA 6102, Australia. The systematics of deformation histories have been investigated using basic combinatorial mathematics. This approach enables the number of possible relationships associated wîth a given number of structures and the range of possible deformation histories to be calculated. Tvô different cases are considered: non-cyclic and cyclic deformation histories. The former considers the case when structures develop only once through time. Cyclic deformation is the repeated development of a particular structure during deformation. Our models show that the assessment of deformation histories based on the use of relative deformation relationships is generally ambiguous. This ambiguity arises from three areas: 1) The difficulty in observing all structural relationships wîthin an area, 2) The fact that structural relationships in non-cyclic deformation histories are subsets of more complex deformation histories involving cyclic or repeated structural histories 3) The multiple occurrence and/or overlap of different structures developed over different time scales. We have developed a procedure for the construction of deformation histories that is systematic, repeatable and rapid and also allow^s easy recognition of ambiguities in the structural data set. This approach allows the impact of unobserved relationships on the confidence that can be placed on any particular deformation history to be qualitatively assessed. The number of possible deformation histories that are compatible with a collection of age relationships can also be used as a crude measure of confidence in those histories.

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SYNOROGENIC HYDROTHERMAL ORIGIN FOR GIANT HAMERSLEY IRON OXIDE OREBODIES Christopher McA. Powell^ Zhengxiang Li\ David McB. Martin^ and Nicholas H.S. OHver^ E-mail: cpowell@geol.uwa.edu.au ^ Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA. 6907 ^ Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland 4811 Geological mapping and basin analysis over the past eight years has established that the McGrath Trough on the southern margin of the Hamersley Province was a foreland basin in front of the northwards-advancing Ophthalmia fold-and-thrust belt. East-trending folds, inclined to locally overturned to the north, multiple refolded unconformities, and sediments increasing in grain size upwards, comprise the foreland basin, which was yoked to the Ophthalmia fold-and-thrust belt. The age of deformation is constrained to be younger than -2.45 Ga, the age of the underlying Woongarra Rhyolite, and mostly older than -2.2 Ga, the age of the Cheela Springs Basalt inferred from the SHRIMP-dated zircons recovered from an epiclastic unit high in the formation (Martin et al., 1999). Some east-trending folding continued after deposition of the Cheela Springs Basalt, which is erosionally truncated by the younger sediments of the Ashburton Basin. Clasts bearing microplaty hematite grains are known from the Barrett-Lennard placer deposit at the base of the Ashburton Trough, and have recently been discovered in conglomerate at the base of the Beasley River Quartzite and several horizons above, demonstrating that oreforming processes were operating during formation of the fold-and-thrust belt. We postulate that it is possible that microplaty hematite and possibly martite-goethite ore bodies formed during the same time interval as the fold-and-thrust belt. Previous models for the genesis of the Hamersley iron ores have included supergene enrichment followed by burial metamorphism (Morris, 1985) and enrichment by leaching of silica and/or precipitation of iron by tectonically and/or thermally driven fluids (Li et al., 1993). The evidence now available points towards involvement of oxidising fluids at temperatures above 100°C, in places over 200°C, during deformation. Individual ore bodies show evidence of having formed by precipitation either above or below fluid chemical interfaces before and after folding at particular locahties. Structural associations of the microplaty hematite orebodies are that they are commonly located in or near the hinges of large F2 synchnes, commonly in locations where there are also post-F2 low-angle extensional faults. NW-trending dolerite dykes, thought to be the feeders of the Cheela Springs Basalt, limit the extent of the ore in some of the deposits, implying that the dykes were present before ore formation. Moreover, there is no metamorphism of the microplaty hematite at the dyke walls. Elsewhere, NW-trending dolerite dykes enclose fragments of microplaty hematite ore, implying that the dykes there are younger than the ore. The model we propose is that microplaty-hematite, and possibly martite-goethite, orebodies could have formed by a combination of orographically-driven groundwater mixed with deeper-seated orogenic fluids squeezed out of the orogenic pile ahead of the growing

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mountain belt (Powell et al., 1999). In this model, the fluid pressure rises in the shaly zones, which become the detachment zones along which the fold-and-thrust belt moves to the north. Likely detachment zones are in the upper Fortescue Group (Jeerinah Formation) and the Wittenoom Formation. The BIFs are enclosed by shale, and thus initially sealed from the hot oxygenated formation waters travelling along more permeable horizons (e.g., Wittenoom Dolomite). Break-thrusts formed during the F2 folding and normal faults formed during the subsequent orogenic collapse bring the detachment zones into contact with the BIFs, especially the Marra Mamba Formation which overlies the Jeerinah Formation and the Dales Gorge Member of the Brockman Iron Formation which overlies the Wittenoom Formation, thereby enabling the hot oxygenated formation waters to interact with the BIF. Rising temperatures during fluid flow enhances stripping of silica and other elements from the BIFs, leaving the iron oxide as a largely residual in-situ deposit. If correct, this model for the formation of the giant Hamersley iron-ore bodies is an example of tectonically-driven lateral fluid flow proposed speculatively by Oliver (1986) as the possible origin of oil and gas fields and base-metal deposits in North America. The principle involved is that hot, variably oxidised fluids moving laterally come in contact with indigenous bodies of formation fluid of different composition, and, if the chemistry is right, form an ore body. Orographic fluids could also have formed orebodies during late-stage extension accompanying orogenic collapse. Various palaeomagnetic, isotope and fluid inclusion tests are in progress to refine the model, and we recognise that not all the giant Hamersley iron ore bodies may have formed in this way. The model overcomes some of the major deficiencies in the existing models, and opens up new horizons for iron-ore prospectivity.

REFERENCES CITED Li Z.X., Powell, C.McA. & Bowman, R. 1993. Timing and genesis of Hamersley iron-ore deposits. Exploration Geophysics 24, 631-636. Martin, D. McB., Li, Z. X., Nemchin, A.A. and Powell, C.McA., 1999. A pre-2.2 Ga age for giant hematite ores of the Hamersley Province, Australia? Economic Geologyim press). Morris R.C. 1985. Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes - a conceptual model. In: Wolf K.H. (ed.) Handbook of strata-bound and stratiform ore deposits Elsevier 13: 73-235. Oliver J. 1986. Fluids expelled tectonically from orogenic belts: Their role in hydrocarbon migration and other geologic phenomena. Geology 14, 99-102. Powell, C.McA., Oliver, N.H.S., Li, Z.X., Martin, D. McB., and Ronaszeki, J., 1999. Synorogenic hydrothermal origin for giant Hamersley iron oxide orebodies. Geology (in press). Acknowledgements: We acknowledge the generous support for this work over many years by BHP Iron Ore, Hamersley Iron Pty Ltd, MERIWA and Robe River Mining Associates.

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STRUCTURAL AND TECTONIC EVOLUTION OF THE PALEOPROTEROZOIC OPHTHALMIA FOLD-AND-THRUST BELT Christopher McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Austraha, Nedlands, WA 6907. E-mail: cpowell@geol.uwa.edu.au The Ophthalmia fold-and-thrust belt, along the southern margin of the Hamersley Province in the Pilbara Craton, is part of the broader latitudinally-trending Proterozoic Capricorn Orogen, which separates the Archaean Pilbara and Yilgam cratons and preserves their amalgamation history. Work over the past nine years has refined the structural and tectonic history of this orogen by recognising sets of superimposed deformations and relating them to the stratigraphic evolution of the basins (Powell & Horwitz, 1994; Martin et al. 1999). The oldest deformation is an extensively-developed NE-trending set of rotational boudins, best developed in the late Archaean Marra Mamba Iron Formation but also present in iron formations as high as the Joffre member of the Brockman Iron Formation. There are also rare intraformational recumbent folds (Fj) at a number of stratigraphic levels from the Fortescue Group to the lowermost Boolgeeda Iron Formation. The rotational boudins and Fj are not erosionally truncated, implying that they formed below a cover of sedimentary rock. They are interpreted to have formed during the deposition of overlying higher units of the Hamersley Group, possibly as a result of seismogenically-triggered deformation accompanying emplacement of the 2.45 Ga Woongarra Rhyolite. Imbrication and rotation towards the northwest of the sedimentary layering within the boudins implies movement towards the southeast. The second deformation produced the east-trending set of folds (F2), which are part of the Ophthalmia fold-and-thrust belt that involved south-to-north transport of the Hamersley Province cover above the older Archaean (>2.9 Ga) granite-greenstone basement. F2 are most intensely developed along the southern margin of the Hamersley Province, where they form overturned to locally recumbent folds verging to the north. In the southern part of the Province, slivers of the granite-greenstone basement have been incorporated in the folding, implying that the thrust detachment surfaces cut progressively deeper into the Earth's crust towards the south. The style of deformation is dominated by folding, with only minor thrusts cutting up through the banded iron formations. Detachments are inferred at deeper stratigraphic levels in the belt. The second deformation, which in places is associated with pervasively developed crenulation-style cleavage, was active during deposition of the Turee Creek and lower Wyloo Groups in the McGrath Trough, a foreland basin in front of the advancing fold-andthrust belt. Folded unconformities, coarse terrestrial clastics derived from the south and erosionally-truncated early east-trending F2 folds are found along the southern margin of the McGrath Trough. Geological relationships combined with SHRIMP dating of zircons show that the fold-and-thrust belt was active between <2.45 Ga and 2.2 Ga, considerably older than previously thought, with some deformation continuing after 2.2 Ga. Synorogenic extension in the internal part of the Ophthalamian orogen could have overlapped the northward-directed folding and thrusting in the external parts of the foldand-thrust belt. ESE- to SE-trending dolerite dykes were emplaced during this extension. 202

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possibly as feeders to the -2.2 Ga Cheela Springs Basalt (Martin et al., 1999). Low-angle normal faults, recognised in the major iron-ore mines at Mt Whaleback, Mt Tom Price and Paraburdoo, also belong to this interval of extension which cuts the F2 folds in those places (Powell et al., 1999). Following the Ophthalmian deformation, the southern margin of the Pilbara Craton was rifted and the deep-water turbidites of the Ashburton Formation were deposited in a region that previously had been the uplifted source of terrestrial clastics in the McGrath Trough. The June Hill Volcanics, in the lower part of the Ashburton Formation, are related to this extension and have been dated at -1843 Ma. The second major shortening deformation involved East- to SE-trending folds and high-angle reverse faults that deform the Ashburton Formation but pre-date the Mesoproterozoic Bangemall Basin cover rocks. Best estimates of the age of this contractional deformation are between 1800 and 1650 Ma. Deformation in the western Hamersley Province after formation of these SE-trending folds include ENE-trending folds and steep ESE-trending faults that formed in a dextral strikeslip regime. The age of these dextral faults and folds is not certain; they could be the late stages of the end-Paleoproterozoic deformation but they could also be considerably younger, possibly even Neoproterozoic. This revised structural and stratigraphic analysis shows that the structure of the Hamersley Province involves two major shortening events, one of which is early Paleoproterozoic (2.4 to 2.2 Ga or a little younger) and the other end-Paleoproterozoic (1.8 to 1.65 Ga). The younger deformation could be related to the collision of the Pilbara and Yilgam Cratons, but the broader tectonic setting of the older fold belt is as yet undetermined. REFERENCES Martin, D.McB., Li, Z.X., Nemchin, A.A. and Powell, C.McA., 1999. A pre-2.2 Ga age for giant hematite ores of the Hamersley Province, Australia? Economic Geology {in press). Powell, C.McA. and Horwitz, R.C. Late Archaean and Early Proterozoic tectonics and basin formation of the Hamersley Ranges. Geol Soc. Aust (WA Division) Excursion Guidebook, 4: 57 pp. Powell, C.McA., Oliver, N.H.S., Li, Z.X., Martin, D.McB., and Ronaszeki, J., 1999. Synorogenic hydrothermal origin for giant Hamersley iron oxide orebodies. Geology (in press).

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TECTONIC EVOLUTION OF THE THRUST FRONT OF THE DELAMERIAN OROGEN NEAR CLARE, SOUTH AUSTRALIA W.V. Preiss Primary Industries and Resources South Australia E-mail: wpreiss@msgate.mesa.sa.gov.au The Delamerian Orogen in South Austraha was imposed on Neoproterozoic (Adelaidean) to Cambrian sediments of the Adelaide Geosyncline and their PalaeoproterozoicMesoproterozoic basement at -500 Ma. The structural style and amount of shortening change markedly along the length of the orogen. The least deformed domain is the Central Flinders Zone, characterised by open, dome and basin interference folds. To the south is the Nackara Arc, consisting of long, arcuate, generally upright folds that swing from NNW-trending in the south near Burra to ENE-trending in the NE near Clary. The Nackara Arc passes southward across an ill-defined NNW-trending boundary (approximately the "G2 structural corridor") into the Fleurieu Arc, which is convex to the SE. The inner (western) part of the Fleurieu Arc is probably the most highly shortened part of the orogen, with NW-directed tectonic transport on shallowly SE-dipping shear zones. The strike of the dominant structures curves from northeasterly near Adelaide to northerly and north-northwesterly in the southern portion of the Nackara Arc. This poster presents preliminary results of structural mapping of a portion of the Delamerian Orogen between the Alma Scarp and Clare in the transition between the Nackara and Fleurieu Arcs, as part of regional mapping of the BURRA 1:250 000 map sheet (lat. 33°-34°, long. 138°-139°30'). Impediments to mapping (generally poor outcrop in agricultural and viticultural country) are balanced by relatively well understood lithostratigraphy (derived from structurally simpler sections -30 km to the north), good lithological differentiation, and the numerous small disused quarries and deeply eroded creek sections where reliable structural measurements can be made. The Alma Scarp (a Cainozoic reactivated fault-line scarp) west of Clare exposes the most northerly occurrence of shallow-dipping shears in the zone of transition between the Nackara and Fleurieu Arcs. A major steep, north-south fault (the Clare-Spalding Fault) separates openly folded lower and middle Adelaidean strata (Burra and Umberatana Groups) of the Nackara Arc in the east from a complex belt of tightly folded and sheared lower Adelaidean (lower Burra Group) to the west. Within the complex belt, foliations and ductile shears are sub-horizontal to gently east-dipping in a western "flat zone", but gradually steepen to the east into a "steep zone" adjacent to the Clare-Spalding Fault. Differences in thickness and facies in the Adelaidean strata across the Clare-Spalding Fault suggest a fault, in a similar position, active during deposition of the lower Adelaidean and clearly reactivated in the Delamerian Orogeny. The relationships are best observed at Spalding, 30 km north of Clare, where the fault separates a thicker section of Burra Group (in particular the basal Rhynie Sandstone) to the west from Callanna Group rocks of the Spalding Inlier to the east. The inlier is overlain by very thin Rhynie Sandstone (only the uppermost beds are present, and appear to have onlapped the Callanna Group), followed by marble of the Skillogalee Dolomite (partly in sheared contact) and the remainder of the Burra and Umberatana Groups in a regional, open, slightly eastward-overturned syncline (Hill River

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Syncline). The Clare-Spalding Fault precursor is interpreted as a west-dipping extensional fault during deposition of the Burra Group, forming the eastern boundary of a graben, the western boundary of which were normal faults in the Torrens Hinge Zone. In Skillogalee time in particular, this graben was filled with coarse-grained, craton-derived feldspathic sands with only occasional thin dolomite intercalations; in contrast the zone to the east was a less subsident, eastward-tilted fault block acting as a barrier to clastic supply, and pure dolomite (now recrystallised to marble) was deposited at the same time. During Delamerian deformation, pre-existing faults within the Nackara Arc such as the ClareSpalding Fault and probably the Kingston Fault at Burra, 30 km to the east, controlled folding. Typically these faults are steep and separate regional synclines without intervening anticlines. The faults do not have large stratigraphic separations yet delineate belts of different sedimentary characteristics, and are likely to have involved significant Delamerian strike-slip movement. To the east of the Clare-Spalding Fault, the Hill River Syncline has a steep west-dipping axial plane and slaty cleavage, and a slightly overturned (west-dipping, eastward-younging) west limb. The complex zone of the Alma Scarp has a totally different structural style, involving westdirected thrusting on sub-horizontal shears. Three phases of deformation can be recognised: an early shallow-dipping foliation (S|) at a low angle to bedding, varying from simple slaty cleavage to strongly developed schistosity with local E-W lineation) is folded on generally gendy north-plunging, open to very tight folds and crenulations (F2) with steeply to moderately east-dipping axial planes and local crenulation cleavage (S2). Another set of folds deforming S], with axial planes dipping steeply to moderately SW, can locally be demonstrated to fold S2 also and are designated F3. Shears range from minor bedding-parallel detachments to zones of highly schistose tectonites with totally transposed bedding up to hundreds of metres wide. The orientation of cleavage, shear foliation and axial planes of tight folds steepens gradually eastward as the Clare-Spalding Fault is approached. The timing relationships between structures in the "flat zone" and "steep zone" are uncertain. The greater part of the Nackara Arc to the east shows evidence of only one phase of deformation with a single, simple axial plane slaty cleavage. Sub-horizontal, west-directed thrusts are not seen in the Nackara Arc where indeed, many of the folds and associated local reverse faults verge slighdy to the east. Two working hypotheses may be proposed to explain the relationships, but unambiguous interpretation of the structural sequence has not yet been possible: 1. West-directed thrusting on the Alma Scarp is related to northwest-directed thrusting in the Fleurieu Arc (Dj) but did not affect the Nackara Arc; the thrusts were folded by a regional fold event (D2, with north-south axes) that also produced the regional folds of the Nackara Arc. 2. All the structures in the Nackara Arc and on the Alma Scarp were formed during a single, continuous deformation in a sinistral transpressive regime with opposing vergences across one or more master NNW-striking wrench faults.

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THE HIGH-PRESSURE SOLE OF THE NEW CALEDONIA OPHIOLITE BELT Tim Rawling and Gordon Lister

Australian Crustal Research Centre, Monash University timr@earth.monash.edu.au Limited understanding of large-scale geometry in the poorly exposed Pam-Panie region has always been a stumbling block to geometric and tectonic reconstructions of New Caledonia. However this can now be constrained, based on structural traverses around Pointe Nandiarane on the Pam Peninsula, across the Amoss ridge, east from Col d'Amoss toward Bonhomme, along the old road from Balade toward Ouegoa, from the coast to the peaks of both Mt Ignambe and Mt Panie and finally along the coast between Bouanavio and Pindache. This work supports the interpretation of the high pressure belt as a metamorphic sole generated during ophiolite obduction (Avias 1967). There are many different models that have been proposed for the origin of the high-pressure belt of New Caledonia. For example, exhumation of the high-pressure rocks has been considered as the result of wrench faulting (Black et al. 1993; Ghent et al. 1994). However as noted by Cluzel et al. (1995) kinematic data are not consistent with this interpretation. The geometry of southwest dipping foliations and arcuate faults, southwest of the high-pressure zone, and the pattern of metamorphic grade increasing towards the northeast is consistent with the effects of late stage extensional tectonism superimposed upon the geometry of an earlier formed collisional orogen (Cluzel et al. 1995, Aitchison et al. 1995). The origin of the highpressure schist belt of northeast New Caledonia was thus be interpreted as the result of continental subduction followed by exhumation during extensional tectonism (Cluzel et al. 1995). However, the data does not provide explicit support for the model of subduction then "crustal thinning and extensional detachment during the diapir-like uplift and unroofing of the eclogite core" (Cluzel et al. 1995). Neither does it support the division of the high-pressure schist belt into multitude of different terranes as proposed by these authors. Cluzel et al. (1995) and Clarke et al. (1997) suggest that the highest grade metamorphic rocks are found within the core of a regional "foliation antiform" in which the folded shear fabrics were interpreted to represent the domed mylonite front of a metamorphic core complex. This study shows that the regional antiform is a relatively young structure, formed during crustal shortening subsequent to a an early epoch of extensional tectonism. The regional antiform is then subject to blockfaulting, and young normal faults upthrow the regolith in a fault-bounded coastal range. The regional anticline then appears to have little or nothing to do with the exhumation of the New Caledonia eclogites. The highest pressure rocks are found immediately beneath (and incorporated within) a folded serpentinite sheet, which is draped across the youthful regional antiform. We suggest this sheet represents the high pressure sole of the New Caledonia ophiolite. What remains of the serpentinite sheet is today exposed as a series of "abandoned" sheets of serpentinite containing "knockers" of various composition and levels of structural complexity. These

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knockers represent boulders that were "plucked" from the lower plate and entrained within the serpentinite sheet. Remnants of the serpentinite sheet are also exposed as tight upright infolds (some of which may have faulted out limbs or occasionally as discrete blocks completely bound by steep brittle block faults. Previous workers have not identified these serpentinite remnants as being part of a single sheet that originally overlay the rocks of the high-pressure zone. Rather the eclogite-bearing serpentinite has been interpreted to have been tectonically injected from depth along steep thrust surfaces (Lillie, 1970; Lillie, 1975; Brothers and Blake, 1973; Black and Brothers, 1977). However, when the effects of middle-stage upright folding are removed the dominant fabric within the serpentinite sheet is restored to a flat-lying orientation. This fabric formed during emplacement of the serpentinite melange and this geometry is difficult to reconcile with the concept of injection along vertical faults. Clarke et al. (1997) recognised two different types of eclogite in the region (Type I and Type II eclogites) based on petrological differences. However, these eclogites are from the gneissic core and few if any of the high grade knockers from within the serpentinite sheet were analysed. In cross sections through the Pam Peninsula region these authors did not include a serpentinite sheet and in fact proposed that the lower grade rocks of the Diahot Valley region overlay the high-pressure schist belt in the manner of Yokoyama et al. (1986). The overthrusting of this ophiolitic sheet is interpreted to have resulted in significant crustal thickening and been critical to high-pressure metamorphism in the region. The identification of undeformed and fresh (non-retrogressed) eclogitic assemblages in knockers within the sheet, as well as more strongly deformed and locally retrogressed coherent eclogites in the lower plate indicates that both the serpentinite sheet and the gneissic core were subjected to high-pressure metamorphism. The thrust surfaces at the base of these units subsequently became a locus for later extensional shearing during which time S2 fabrics were completely overprinted by younger shear fabrics and often retrograde metamorphic assemblages. Paper presented courtesy of Australian Geodynamics CRC, with permission of the Director.

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CRUSTAL MODELS OF NORTHERN TASMANIA FROM WIDE-ANGLE SEISMIC DATA N. Rawlinson^T.O. Semenova' ', G.A. Houseman', C.D.N. Collins''^ & B.J. Drummond' ^Australian Geodynamics Cooperative Research Centre 'Department of Earth Sciences, Monash University, Clayton VIC 3168 ^Australian Geological Survey Organisation, Symonston ACT 2609 email: nick@earth.monash.edu.au In 1995, AGSO's research vessel Rig Seismic performed a circumnavigation of Tasmania to acquire data for the TASGO project. Approximately 36,000 shots were fired from its airguns during the cruise with an average shot spacing of 50m. A network of 42 recorders distributed throughout Tasmania recorded seismic energy from these shots. The resultant 3-D refraction and wide-angle reflection dataset provides us with an opportunity to analyse the deep crustal structure of Tasmania much more comprehensively than has previously been possible. Below, we describe some of the 1-D refraction interpretations and 2-D and 3-D forward modelling of northern Tasmania using the TASGO wide-angle seismic dataset. Our 1-D refraction interpretations assume that the first arrivals are head waves which travel along horizontal planar interfaces between constant velocity layers. The models that are produced have layer thicknesses and velocities that best satisfy, in a least-squares sense, the observed traveltimes. Thus, each model represents the mean vertical seismic structure of the region spanned by the data used to constrain it. The following description of northern Tasmania's crustal structure is based on these interpretations. Along the north coast, from just south of Three Hummock Island to Forth River, reversed profiles show the crustal thickness to be relatively uniform at about 31 km with an average crustal P-wave velocity of 6.0 km/s and a P^, velocity (the P-wave velocity of the mantle beneath the Moho) of about 7.9 km/s. Two crustal layers that are separated by an interface at approximately 8 km depth are discemable from the data; the upper crustal layer has a P-wave velocity of 5.6 km/s while the lower crustal layer P-wave velocity is approximately 6.3 km/s. In addition, a 1-3 km thick layer with a P-wave velocity of about 4.5 km/s overlies the two main crustal layers; this corresponds to the clastic carbonate sequence known as the Rocky Cape Group (Williams, 1989). Further east, across the so-called Tamar Fracture System, the crustal thickness decreases to 27 km and remains at this value as far as Banks Strait. Inboard from both the eastern and western north coasts, the crust appears to thicken by several km towards the centre of the island. A 2-D crustal model of the western north coast of Tasmania has been constructed using both refraction and wide-angle reflection traveltimes from a shot line that runs between Three Hummock Island and Forth River to a receiver located near each end of the line. The model is parameterized in terms of uniform cubic B-spline interfaces and layer velocities that vary linearly with depth so that ray paths consist of circular arc segments. The most prominent lateral feature of the model is an upward deflection of both the Moho and the mid-crustal interface in the vicinity of the Arthur Lineament. The model is supported by free-air gravity data, in which the Arthur Lineament is expressed as a gravity high.

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The raytracing program used to construct the 2-D model described above has been extended to 3-D. Now, each interface is a surface described by a mosaic of uniform bi-cubic B-spline surface patches which are C2 continuous across suture. A model of the Rocky Cape Element (NW Tasmania) has been constructed using the 3-D raytracing program in a forward modelling capacity. The starting model (described by 172 surface patches) was based on the 2-D model which runs across the northern section of the crustal element. The final model, constrained by over 600 reflection and refraction raypaths, shows that the Moho surface beneath the Rocky Cape Element dips approximately SE, with the crustal thickness increasing from about 25 km south of Three Hummock Island to over 30 km just beyond the eastern margin of the element. This dip is much less pronounced along the northern edge of the element, where the 2-D model remains valid. The main non-linear feature of the final model is a NE-SW trending upward deflection of the lower crustal layer which approximately coincides with the surface expression of the Arthur Lineament. While some regions of the model are not well constrained by the data, we believe that the basic features described above are robust. According to Christensen & Mooney (1995), the global average P-wave velocity of continental crust is 6.45 ± 0.21 km/s while the global average crustal thickness is 41.0 ± 6.2 km. The corresponding values we have determined for northern Tasmania are 6.10 ± 0.11 km/s and 29.7 ± 2.0 km, both well below the global average values. This discrepancy can be explained if we consider the global average values of different crustal types. Christensen and Mooney (1995) divide continental crust into five tectonic provinces: Orogens, shields and platforms, continental arcs, rifts and extended crust (e.g.: Basin and Range, much of western Europe). For extended crust tectonic provinces, the global average P-wave velocity is 6.21 ± 0.22 km/s and the global average crustal thickness is 30.5 ± 5 . 3 km. Of all the tectonic provinces, northern Tasmania fits best, in terms of both velocity and thickness, into the classification of extended crust. Hence, according to this comparison, the present day crustal velocity structure is consistent with the idea that northern Tasmania (and eastern Tasmania see Rawlinson et al, 1998) has undergone significant extensional deformation. The Moho geometry we have obtained along the north coast from our modelling is broadly consistent with independent interpretations of marine deep reflection profiles (Barton, 1998) that were collected concurrently with the wide-angle data. Most notably, an upward deflection of the Moho in the vicinity of the Arthur Lineament has also been interpreted from the reflection sections. Our long term goal with this very large dataset is to invert traveltimes for 3-D crustal structure and velocity to produce a high resolution tomographic image of the whole island to at least Moho depth. Such an image will undoubtedly enhance our current understanding of Tasmania's deep geology and tectonic evolution. REFERENCES Barton, T.J., 1998. A geophysical transect across northern Tasmania. Australian Geological Convention, Townsville. Geological Society of Australia, Abstracts 49, 24. Christensen, N.L & Mooney, W.D., 1995. Seismic velocity structure and composition of the continental crust: A global view. J.geophys.Res., 100, 9761-9788. Rawlinson, N., Semenova, T.O., Collins, C.D.N. & Houseman, G.A., 1998. Crustal structure beneath the north and east coasts of Tasmania from seismic refraction data. Australian Geological Convention, Townsville. Geological Society of Australia, Abstracts 49, 372.

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Williams, E., 1989. Summary and Synthesis. In: Burrett, C.F. & Martin, E.L. eds. Geology and mineral resources of Tasmania, Geological Society of Australia, Special publication 15,468-499. Further information at http://www.earth.monash.edu.au/-nick

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FLUID-ROCK INTERACTION WITHIN SHEAR ZONES OF THE ARUNTA INLIER, CENTRAL AUSTRALIA: IMPLICATIONS FOR TECTONICS Caroline M. Read and Ian Cartwright Department of Earth Sciences and VIEPS, Monash University, Clayton Campus, VIC 3168 Caroline.Read@mail.earth.monash.edu.au. Shear zones represent sites of intense fluid-rock interaction and commonly form after the peak of metamorphism. Therefore, they potentially yield important information about fluid flow during the exhumation of metamorphic terrains. Fluids are integral in driving metamorphic and tectonic processes and may facilitate movement on shear zones. Hence documenting fluid flow is important to our overall understanding of tectonic processes. The Arunta Inlier is a 200,000 km^ multiply deformed and metamorphosed Proterozoic terrain in central Austraha. Major faults and shear zones that dissect the Arunta Inlier (Figure 1) have a regional NW-SE to W-E orientation and range in size from micro-shears to crustalscale zones, such as the Redbank High Strain Zone. Rocks within the shear zones have a mylonitic fabric and amphibolite to greenschist facies mineralogy. In the Northern Province there are steeply north- and south-dipping shear zones with reverse movement, that Collins and Teyssier (1989) suggested formed a pop-up structure related to the exhumation of the Northern Province rocks. While at the contact between the Central and Southern Province is the Redbank High Strain Zone which is a large north-dipping thrust structure. These structures facilitated the exhumation of the Arunta Inlier during the 300-400 Ma Alice Springs Orogeny, as indicated by dating of Shaw and Black (1991) and Cartwright et al. (1998).

^ Alice Springs Redbank High Strain Zone Ormiston Gorge ' Shear

zones

and

24° -

faults

[jjlllll Northern Province

\Central Province

| J Southern Province

Figure 1. The Arunta Inlier, central Australia There is abundant evidence that significant fluid flow occurred within the shear zones. This includes: the general retrogression of high grade assemblages; silicification and epidotisation of sheared augen gneisses within the Redbank High Strain Zone and Aileron Shear Zone; kyanite and quartz veining at Blue Bush Swamp south of the Reynolds Range; and resetting of oxygen isotope ratios in the Mt Weldon Shear Zone. The Redbank High Strain Zone forms the boundary between Central and Southern Province rocks across a large portion of the Arunta Inlier. Deep seismic, teleseismic and gravity

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analyses suggest the Redbank High Strain Zone penetrates the deep crust (Lambeck et al. 1988). The zone comprises moderate to steeply north dipping anastomosing zones of intense shearing, separated by zones of unsheared granitic augen gneisses The sheared rocks are probably retrogressed micaceous equivalents of the country rock and in places are schistose. Within several shear zones areas of silicification and epidotisation occur parallel to the shear fabric. Although these rocks are intensely altered, relic gneissic textures indicate that they are the metasomatised equivalents of the surrounding rocks. The 5-15 m wide metasomatic zones suggests that large quantities of fluid was channelled within the shear zones. Sillimanite-bearing Mt Freeling pelitic schists at Blue Bush Swamp are sheared within the Aileron Shear Zone, which anastomoses SE-NW along the south of the Reynolds Range. Within the shear zone, boudins of quartz occur, some containing large kyanite crystals. While the quartz boudins are not restricted to the shear zone, no kyanite was found outside the shear zone. The abundance of quartz boudins and kyanite, along with the retrogression of the pelitic schists, suggests that fluids were present during deformation and metamorphism. Silicification of 5-10 metre wide shear-parallel zones of Napperby Gneiss along strike of the Aileron Shear Zone suggests, as with the Redbank rocks, that locally high volumes of fluid were channelled through the shear zone. The Mt Weldon shear zone is situated in the south eastern Anmatjira Ranges, Northern Province. It forms a NW-SE oriented, 300m wide, steeply north dipping network of shears that dissect granulite facies para and orthogneiss. An oxygen isotope transect across this shear zone indicates that values of both sheared and unsheared rocks range from 9.7-11.7%o for the Weldon Metamorphic Group paragneiss and 7.3-8.6%o for the Mt Airy Orthogneiss. Fault rocks within the Orthogneiss at the SW edge of the lithological contact locally have 51^0 values as low as -1.7%o. Rocks that equihbrated with normal crustal fluids typically have 51^0 values greater than 5%c (Sheppard, 1986). Closed system isotopic resetting by devolatihsation reactions and partial melting typically lowers the values by less than 2%c (Kohn, 1993). This resetting therefore indicates an open system where external fluids of surface origin have infiltrated the crustal rocks. The meteoric fluid flow is highly focused within the shear zones. Elsewhere in the Reynolds Range there is further evidence of meteoric fluid penetrating the ductile crust through Alice Springs age shear zones (Cartwright et al. 1998). Information on flow pathways and sources of the fluids are important in deciphering the fluid histories during retrogression of polydeformed metamorphic terrains. Within the Arunta Inlier retrogression of high grade assemblages, veining, metasomatism, and isotope exchange indicate substantial fluid flow occurred within shear zones during the Alice Springs Orogeny (300-400 Ma). The presence of large volumes of fluids may have facilitated movement on these shear zones and controlled the exhumation of the terrain. REFERENCES Cartwright, I., Buick, I. S. and Lambert, D. D. 1998. Infiltration of meteoric water through Alice Springs age shear zones in the Reynolds Range, central Australia: Implications for tectonics. In: Geofluids II (eds) Hendry, J., Carey, P., Pamell, A., Ruffell, A. and Worden, R. Antony Rowe Ltd, Chippenham, UK, 77-80. Collins, W. J. and Teyssier, C. 1989. Crustal scale ductile fault systems in the Arunta Inlier, central Australia. Tectonophysics 158, 40-66. Kohn, M. J. 1993. Modelling of prograde mineral 51 ^O changes in metamorphic systems. Contributions to Mineralogy and Petrology 113, 249-261. Lambeck, K., Bergess, G. and Shaw, R. D. 1988. Teleseismic travel-time anomahes and deep crustal structure in central Australia. Geophysical Journal of the Royal Astronomical Society 94, 105-124. Shaw, R. D. and Black, L. P. (1991) The history and tectonic implications of the Redbank Thrust Zone, central Australia, based on structural, metamorphic and Rb-Sr isotopic evidence. Australian Journal of Earth Sciences 38, 307-332.

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Sheppard, S. M. F. 1986. Characterisation and isotopic variations in natural waters. In: Valley, J. W., Taylor, H. P. T. and O'Neil, J. R. (eds) Stable isotopes in high temperature geological processes. Mineralogical Society of America, Reviews in Mineralogy. 16, 165184.

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THE ABSOLUTE DATING OF DEFORMATION: IMPLICATIONS FROM THE RELATIONSHIPS BETWEEN DEFORMATION MECHANISMS AND ISOTOPE SYSTEMATICS Steve Reddy (sreddy@lithos.curtin.edu.au) Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, Perth, WA 6102, Australia. Graham Potts (gpotts@liv.ac.uk) Dept. of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK

An assessment of the rates of deformation and tectonic processes requires knowledge of the absolute ages of individual deformation events. In metamorphic rocks where palaeontological data is unavailable, radiometric dating techniques must be used to constrain the absolute timing of deformation. Unfortunately in deformed minerals, isotope systematics are usually difficult to interpret because the migration and consequential resetting of isotope systems may be a function of both deformation and temperature. In this paper we look at the ways in which different deformation mechanisms may modify grain size and thereby effect isotope systematics. The migration of isotopic species by diffusion is a strongly temperature-dependent process governed by an Arrhenius relationship. The extent of diffusive equilibration in a mineral is also related to the grain size. Deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of changing grain size and therefore affecting isotope systematics. Relationships between diffusion, temperature and grain size in geological materials may be considered in terms of the mathematical approximation known as the "closure temperature" {T^. T^ is the mean age of the age profile across a grain in a simple cooling system and corresponds to a temperature below which significant daughter isotopes accumulate in the sample. We consider three different thermal scenarios with deformation taking place during a) the prograde heating path, b) at temperatures close to the closure temperature of the deformed mineral during cooling and c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. We restrict our discussion to the ^Âr/^Âr dating technique and have placed emphasis on the behaviour of argon in white mica. Although we have restricted our analysis to a specific isotope system and mineral, our conclusions are general and will be equally applicable to other isotopic and mineral systems. The range of possible deformation mechanisms in white micas has significant implication for grain size modification and the resetting of isotope systems during deformation. Different deformation mechanisms may lead to increasing or decreasing grain size and may or may not lead to syn-deformational isotopic resetting. Simple modelling illustrates that in deforming rocks in which there is no change in grain size, it is difficult to establish deformation ages. In the cases where deformation causes a change in grain size, it is important to characterise the temperature of deformation and the closure temperature of grains formed during the deformation. Without this information, distinction between cooling and deformation ages is equivocal. The development of grains with T^ greater than the deformation temperature may record a deformation age. This situation will arise in the following cases; i) neocrystalHsation, ii) when grain size reduction occurs at temperatures below Tôf the reduced grain size and the deformation mechanism has reset the grains; or iii) when deformation-induced grain

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coarsening has occurred. Since closure temperatures are also dependent upon cooling rate and diffusion parameters, these must also be known for a correct interpretation of the radiometric data.

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THE EFFECTS OF DEFORMATION-INDUCED INTRAGRAIN ARGON ISOTOPE AGES

Volume

MICROSTRUCTURES

Steve Reddy (sreddy@lithos.curtin.edu.au) Tectonics Special Research Centre, Curtin University, Perth, WA 6102, Australia. Graham Potts Department of Earth Sciences, University of Liverpool, Liverpool, UK, L69 3BX, UK. Simon Kelley Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, UK. Nick Arnaudde Geologic, Univesite Blaise Pascal, Clermont Ferrand, 63038 France. Laboratoire Constraining the timing of deformation requires an understanding of how deformation processes affect grain-scale isotope mobility. However, quantifying the relationship between apparent age and microstructure is a difficult task. A combination of forescatter scanning electron microscopy and both ultra-violet (UV) laser and cycle stepped-heating dating techniques have been applied to individual deformed grains of potassium feldspar to investigate the relationship between microstructure and ^Âr/^Âr isotope systematics. The combination of newly developed techniques in scanning electron microscopy (SEM) and ultraviolet (UV) laser ^Âr/^Âr dating enables internal variations in crystallographic orientation to be quantified and compared with precise ages from the same grain. Orientation variations are related to strain and thus we can directly link intragrain domains with age data. We have applied these techniques to a sample of coarse-grained arkose from the Proterozoic Torridonian Supergroup of NW Scodand. The sample comprises K-feldspar clasts proximally derived from Archaean (Lewisian) basement. The depositional age of the sediment is c.800 Ma while the feldspar population is derived from rocks metamorphosed at amphibolite-facies conditions c. 1900 Ma ago. The arkose has a well- developed bedding-parallel foliation defined by shape alignment of quartz and feldspar grains and matrix phyllosilicate phases. The sample comes from the overturned limb of a major fold structure, the Lochalsh Syncline. This fold is cut by thrusts of the Moine Thrust belt. The sequence of structures is thought to form part of a progressive deformation constrained (by numerous earlier studies) to have taken place at c.420 Ma. We interpret this date to be the time of the arkose deformation. At the grain-scale, deformation within the arkose is heterogeneous with localisation of higher strain at the contacts between coarser feldspar and quartz clasts. This strain is clearly post-depositional and is inferred to have taken place during the 420 Ma deformation event. Several grains have been studied in detail and the form and distribution of deformationinduced microstructures has been mapped using intragrain variations in lattice orientation. Internal variations in crystallographic orientation within the K-feldspars define two microstructurally distinct deformation domains. Relationships between these domains and the detrital grain shape of the feldspars enable the domains to be correlated to the pre- and postdepositional deformation histories of the feldspars. These domains are separated by orientation domain boundaries (ODE) - boundaries between parts of the feldspar grain with different crystallographic orientations.

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Over one hundred in situ ^Âr/^Âr analyses were measured from the three grains using a UV laser. Ages vary from 1030 to 317 Ma. Variations in argon apparent ages directly reflect the internal microstructure of individual feldspars. The grain showing the least amount of strain records a range of ages from 1030 to 420 Ma. The oldest ages are found at the centre of the grain with ages generally decreasing concentrically towards the grain rim. This distribution is inferred to reflect partial resetting due to heating around the time of deformation. This simple pattern is truncated by younger ages spatially associated with fractures and (weakly) internally deformed areas. The relationship between deformation and age variations is best seen in the most deformed grain where there is no spatial relationship between age and grain boundaries. Ages preserved in this grain range from 452 to 317 Ma. These ages are generally younger than the inferred age of deformation and slightly overlap with ages obtained from the least deformed grain. The oldest ages from this grain are recorded from ODBs. This requires a component of excess argon in the vicinity of OBDs. Cyclically-heated data from the remainder of the same deformed grain (in the form of age profiles, ^Âr/^Âr correlations and log (r/ro) plots) indicate the presence of three distinct argon domains. These domains are interpreted to be (1) excess argon within fluid inclusions within the fractures cutting the feldspar grain, (2) excess argon released from the lattice adjacent to the ODBs and (3) radiogenic argon from the feldspar lattice. The absence of ages younger than 420 Ma in the low strain feldspar indicates that diffusion in the deformed feldspar continued to significandy lower temperatures. This is interpreted to reflect the presence of a deformation-induced population of small diffusion domains in combination with 'short-circuit' diffusion along deformation-induced defects. This interpretation is consistent with the localised presence of excess argon in defect concentrations (ODBs) in the deformed grain. These results provide the first direct comparison between intragrain microstructural data obtained using forescatter SEM and ^Âr/^Âr ages from a naturally deformed sample. This combination of techniques yields results that show a clear relationship between intragrain deformation-induced microstructures, the formation of fast diffusion pathways and the development of a range of diffusion domain sizes throughout grains of K-feldspar. In the past, variations in domain sizes within K-feldspar have been used to reveal thermal histories by sophisticated cycle heating and modelling techniques. However, there has been a great deal of discussion as to the actual structures which give rise to the age variations within samples. Our results demonstrate one possible source of domain size variations, that is, intragrain deformation. Deformation appears to play a significant role in modifying the diffusion characteristics of individual mineral grains even when the structural history of the rock is relatively simple. In K-feldspars and in other minerals commonly used to constrain the cooling history of metamorphic rocks, the evolution of intragrain microstructure during late stage deformation must play an important role in the closure of grains to argon diffusion and therefore to the age recorded.

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MIDDLE PALAEOZOIC DEFORMATION IN NORTHERN TASMANIA WITH IMPLICATIONS FOR THE EVOLUTION OF THE WESTERN LACHLAN FOLD BELT OF MAINLAND AUSTRALIA. Alistair Reed, Mineral Resources Tasmania, P.O.Box 56, Rosny Park, Tasmania 7018. email: areed @ mrt. tas. go v. au The results of recent work in northern Tasmania have shown that deformation previously considered Middle Cambrian has affected Ordovician to Devonian rocks. This revelation has facilitated reinterpretation of the relationship between the Eastern and Western Tasmanian Terranes and has contributed to defining a new model incorporating Tasmania into Silurian to Late Devonian Lachlan deformation on the Australian mainland. The Badger Head block is a 30 km wide NW-trending strip of deformed turbidites located in the central north of Tasmania. These rocks were previously considered Proterozoic but show an almost identical sedimentology and structural history to Middle Palaeozoic rocks in NE Tasmania. East-facing recumbent folds and thrusts in the Badger Head block, previously attributed to Middle Cambrian deformation (Elliot et aL, 1993), are identical to structures developed in the hanging wall to a sub-horizontal thrust in Middle Palaeozoic rocks in NE Tasmania. The recumbent geometry and elevated strain state in the Badger Head block reflects rotation of the principal stress tensor proximal to an underlying detachment. This detachment separates the Badger Head rocks from underlying Proterozoic and Cambrian metasedimentary and igneous rocks further west and is inferred to be the same structure as that separating Middle Palaeozoic from underlying Cambrian rocks in the Western Lachlan Fold Belt (Fig. 1). Southwest-directed thrusting and folding has deformed the detachment in Tasmania. This SW-directed event is not clearly evident on the Australian mainland and has proven problematic when attempting to place Tasmania into context with Middle Palaeozoic deformation throughout SE Australia. However, the composition and ages derived from inherited detrital zircons from granites in the Lachlan Fold Belt (Collins, 1998) suggest the crust underlying Middle Palaeozoic rocks on the mainland lack rocks equivalent to Proterozoic quartzite-pelite sequences of the Tyennan and Rocky Cape regions in Tasmania. Regional uplift and folding of the detachment above which Middle Palaeozoic rocks were thrust NE is inferred to have been caused by collision of buoyant Tyennan and Rocky Cape rocks with Delamerian continental crust west of Tasmania. Continued shortening was accommodated by thrusting of Proterozoic and Palaeozoic rocks back toward the SW, up against and over the Tyennan region. East-directed thrusting of Middle Palaeozoic rocks continued on the mainland. The resultant scissoring of the crust preceded an end to Lachlan deformation in the Western Lachlan Fold Belt. Erosion has since exposed the NE limb of the folded detachment west of the Badger Head Block. The detachment dips NE beneath Mathinna Group turbidites in NE Tasmania. A pronounced aeromagnetic lineament paralleling the west coast of Tasmania and truncating the Arthur Lineament is inferred to be the western limb of the folded detachment. The surface trace of the detachment defines a window through Middle Palaeozoic rocks to underlying Proterozoic and Cambrian strata. The detachment plunges NW and is not exposed on the mainland.

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The real terrane boundary between eastern and western Tasmania cannot be drawn as a simple line on a map. The boundary corresponds to the basal detachment above which Palaeozoic rocks were thrust NE. This is the same structure separating Lower Palaeozoic rocks from Middle Palaeozoic rocks on the Australian mainland. The difference between the mainland and Tasmania is that the detachment in Tasmania was refolded during subsequent SW-directed thrusting. It is now of a variable orientation and emergent in more than one location. This places Tasmania in the deformation history of Lachlan Fold Belt. The added complexity to Tasmanian geology is likely a function of the key part played by Tasmanian rocks in the tectonic evolution of SE Australia. Collins, W. J. (1998) Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: implications for crustal architecture and tectonic models. Australian Journal of Earth Sciences 45, 483-500. Elliot, C. G., Woodward, N. B. and Gray, D. R. (1993) Complex regional fault history of the Badger Head region, northern Tasmania. Australian Journal of Earth Sciences 40, 155-168.

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(A) Continent-continent collision in Tasmania. A' Delamarian Rocks

Mainland Australia V Cambrian igneous and sedimentary rocks ?

Tasmania

Continent-microcontinent collision

(B) Southwest-directed thrusting. Mainland Australia

t UPLIFT

Possible eastward migration of subduction zone.

Tasmania

exposure.

^ ^

^ ^ ^ ^ Moyston^ultZone^

^ ^^ Heathcote Fault Zone ^ ^

Mt Wellington Fault ^ n e

Mainland Australia

Rocky Cape Element

Badger Head Block Beaconsfield Block Northeastern Tasmania

Tasmania

Fig. 1. Evolution of Middle Palaeozoic deformation in the Tasmania and the Western Lachlan Fold Belt.

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ARCHAEAN REGIONAL STRAIN FIELD IN THE YILGARN CRATON (WA): FOLD SUPERPOSITION OR INCREMENTAL STRAIN FIELD INTERFERENCES? P Rev*, S Costa*, O. Vanderheague**, and B Foley* *Dept of Earth Sciences, Monash University, Clayton VIC 3168: prey@earth.monash.edu.au; ** Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1 Canada Introduction In this paper we present a re-assessment of the geology of the Murchison Province of the Archaean Yilgam craton. We show that the regional strain field is consistent with regional Incremental Strain Field Interferences (ISFI) (1) in between multiple local strain fields related to the emplacement of granitic domes, and (2) in between these local strain fields and the regional strain field. As we define it, the concept of ''interferences'' includes an overlap in both space and time. Therefore, ISFI should not be confused with that of Finite Strain Field Superposition (FSFS). Patterns related to fold superposition (improperly named Fold Interferences) are the results of the overlap in space of successive, therefore diachronous, folding events. Regional Finite Strain Field in the Murchison Province The Murchison Province displays a classical granite-greenstone association, with N-S elongated synformal keels of greenstone sequences (3.0 to 2.8 Ga old) separated by granitoid domes (2.7 to 2.6 Ga old). The fold superposition model proposed by Myers and Watkins (Geology, 1985 v. 13, pp. 778-780) to explain the dome-and-basin patterns, was based on cartographic patterns (in particular the presence of NS and EW oriented synformal hinges), and the assumption that the NS-trending regional foliation cross cuts an older EW-trending foliation. Alternatively, our study emphasizes that the main features of the regional strain field observed in the Murchison Province was likely produced during a single event of progressive deformation, rather than during successive regional folding events. In particular, we found a single, steeply dipping, regional foHation that shows a large variation in strike, for a dip varying in the range of 50 to 90°. In the greenstones, the foliation wraps around hinges of synforms and trends parallel to the granitoid/greenstone contacts. In the granitoids, the foliation underlines the shape of granitoid/greenstone contacts, and the fabric varies from magmatic to solid state towards the contact with the greenstones. In the granitoids, away from the contact with the greenstones, stretching and mineral lineations show a large scattering. This distribution progressively becomes more coherent toward granitoid/greenstone contacts, where lineations dip toward the greenstones. Kinematic criteria indicate downward displacement of the greenstones relative to the granitoids. "Fold interference" patterns, used by Myers & Watkins as evidence for multiple events of regional deformation, occur only at foliation triple points within the greenstone synforms, or in banded migmatitic gneisses located in the core of the domes. Interpretation Robust field observations consistent with the superposition of EW, then NS folding include: (1) regional unconformity between the two folding events, (2) contrasted metamorphic grades and/or microstructures between the axial planar fabrics (for example biotite and ductile feldspars in in contrast to chlorite and brittle feldspar in and (3) regionally coherent

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cross cutting relationships between S^ and S^^j. None of them has been observed in the Murchison Province. Robust field observations consistent with progressive Incremental Strain Interferences include: (1) fabrics which show similar metamorphic grades and similar microstructures within a given locality, (2) when they exist, interference patterns develop only locally where independent strain fields interact (typically at foliation triple point), and (3) contradicting cross-cutting relationships (S^+i cuts S^, S^cuts S^^i). These features characterise the regional finite strain field in the Late Archaean. Consequently, we propose that the regional finite strain field observed in the Murchison Province reflects the complex interaction of multiple granitic domes with the regional strain field dominated by EW shortening and NS extension. This interpretation is consistent with the distribution of metamorphic isogrades in greenstone synforms, which indicates a decrease in temperature from the greenstones/granite contact toward the core of the greenstones. At a larger scale, it is worth noting that strain patterns, metamorphic features, and the timing of granitoid emplacement in the Murchison Province are similar to what is described in the Southern Cross Province further South (Bloem et al., Precb. Res, 1997 v.85, pp. 147-171). Implication for Archaean Geodynamics The Fold Superposition model proposed by Myers and Watkins (1985), was used to support plate boundary forces and therefore plate tectonics activity at that time. Although our conclusions go against fold superposition, it does not rule out some form of Plate Tectonics, since at the regional scale the anisotropy of the finite strain field suggests the existence of plate boundary forces. In the other hand, the relative homogeneity of the strain field (domes and basins) across the whole craton, and the fact that no significant age gradient exists over 800 km across the Yilgam craton, suggest that magmatism, metamorphism, and deformation occurred synchronously across a large continental area at 2.7-2.6 Ga. This is not representative of modem intra-continental deformation, but would be more compatible with a model of mantle plume underplated beneath a 3.2 to 2.8 Ga old protocraton.

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BUOYANCY FORCES, ARGAND RATIO, AND THE MECHANICAL BEHAVIOR OF THE CONTINENTAL LITHOSPHERE THROUGH TIME Patrice REY; Greg HOUSEMAN; Sylvie COSTA Department of Earth Sciences, Monash University, 3168 Clayton, Australia prey @ earth. monash. edu. au Introduction In the past fifteen years, internal buoyancy forces, related to horizontal heterogeneity in density, have been proposed to explain deformation in some large intra-continental areas, sometimes hundreds of kilometers away from any active margin or collisional zone. In this contribution we propose that buoyancy forces played a major role in Archaean tectonics, a role which has decreased since the early stages of the Earth's history. The Argand Ratio The ability of buoyancy forces to affect regional tectonics is proportional to the Argand Ratio (AR): the ratio of the buoyancy forces due to gravitational potential energy to the tectonic force necessary to deform the lithosphere at a given strain rate (in other terms the strength of the lithosphere). AR should not be confused with the Argand Number defined by England and McKenzie (1982), which is a measure of the Argand Ratio for specific conditions. When AR>1 (strong buoyancy forces and/or weak lithosphere), buoyancy forces play a significant role in the regional tectonic history. Because extensional and contractional buoyancy stresses are considered positive and negative respectively, lithospheres will enjoy extensional collapse when AR>1, and contractional collapse when AR<-1. Argand Ratio in Phanaerozoic time Figure 1 shows the fc-fl plane (Sandiford & Powell, 1990. E.P.S.L., 98) mapped for AR<1 assuming isothermal deformation, and "standard" lithospheric rheological model. For a large range of strain paths, AR remains < 1, and the buoyancy forces do not affect the deformation history during the protracted history. During thermal relaxation however, AR for a thickened Hthosphere progressively increases. Figure 1 shows the situation 25 Ma after the end of thickening. Most lithospheres with a crustal thickening factor >1.5 are in the domain of extensional collapse. High Moho temperatures can be achieved through thermal relaxation of a thickened crust, but it is more likely to occur on a fast time scale by gravitational instability leading to convective thinning of the hthosphere. In either case, buoyancy-driven deformation is the consequence of significant hthospheric thickening. Argand Ratio in Archaean time High-temperature geotherms that correspond to transient situations in Phanaerozoic times could have corresponded to equilibrium conditions in the Archaean. Figure 2 displays the fcfl plane mapped for -1<AR<1 of three Archaean lithospheres, assuming isothermal deformation. Phanerozoic lithospheres are also shown for comparison. The striking feature is that, the domain for which -1<AR<1 is much smaller for Archaean lithospheres than for Phanaerozoic lithospheres. This suggests that buoyancy forces played a more important role in the Archaean than in the Phanaerozoic. For example AR becomes >1 for very small amounts of thickening along the strain path for which only the crust is thickened. Alternatively, the domain of contractional collapse (in which AR<-1 and continued convergence drives the AR increasingly negative) is reached for relatively small amount of thickening when the deformation is homogeneous. During homogeneous deformation however, the lithospheric mande root which causes a negative AR may be gravitationally 223

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unstable. If the root is removed by delamination or convective thinning, AR rapidly increases, and the lithosphere enters the domain of extensional collapse.

Discussion and conclusions F^ijsf e 1: ft^H pilose mjf^std im AR<1 tlf

a m^m tMij dC

crhmrn 545T\ Hm^mft^Mmhm^t^m^ sIHIs^ ddm^l ^ndi m^ mm^m^ mK^mmd F a: iai p taa^p cif pitii mkk ii^sî lii^^)^ AR tmmm^ <\.. kdit^lasg ihM tki^ fmcm mm mmirn^ is ih^ k im Âm f Îd A^ rhmiml idmMm^

IMdm m^lmm f AR ma

# «

sflfo^^fr2ji md

to 1 M

hMMMm ^m^

I -

' ^ t^b^ d^^mm^d rnm^m m fm w^sid^ AR<1 - 25 Mm at^ md . rndkm^m^ AM ddmm^d wills ^ cri^j^aJ

a V.

fmrn mrn;^ 1hmrn^immi,

Fipre 2: fc-fl fiair^e js^jfipeci for -I <AR<!, fcr siiM im

ddm^mm.

1.2 fm iMum^ d^mrnMim tm^v haViS AR>I. l^m tkm&

3^1

m mm

Tw^smi

refssresn^ee me msl

mmmmg a li^m^m

iemp:0rmm>2, m^ cof^ dared fcsr ead^ lAo^kere (see texi), It seems that buoyancy forces may have had a more important role in the Archaean than in the Phanaerozoic, as large Argand Ratio were reached for smaller amounts of thickening. Two main consequences can be envisioned for the Archaean: (1) constrasts in crustal thickening could have been limited, and (2) deformation could have been distributed over extended areas, as the maximum crustal thickness was reached for a small amount of thickening. In the Yilgam craton of Western Australia for example, the limited amount of crustal thickening is consistent with the sub-aerial nature of volcanogenic materials of the Greenstones sequences, the rare occurrence of detrital sediments, and the relative homogeneity of the erosional level now exposed at the surface. In the Yilgam craton, the broad distribution of the deformation is consistent with the absence of significant age gradients in the tectonic, metamorphic, and magmatic activities across the whole craton.

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Fundamental differences exist between Phanaerozoic (Plate Tectonic related) regional strain fields, and Archaean regional strain fields. These differences may, reflect either contrasted mechanical properties, contrasted geodynamic processes, or both.

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MECHANISM AND TIMING OF GRANITE EMPLACEMENT FOR THE BEGA BATHOLITH, EASTERN LACHLAN FOLD BELT, N.S.W. 5.W. Richards' and WJ. Collins' Department of Geology, The University of Newcastle, Callaghan, N.S.W., 2308. 6. srichards@geology.newcastle.edu.au 7. bcollins@geology.newcastle.edu.au The central Bega Batholith, Lachlan Fold Belt (LFB), New South Wales, is characterised by a number of elongate, N-S trending I-type plutons which are separated in sections by high-grade, meta-sedimentary screens composed predominantly of stronglyfoliated schlieren migmatites. Within the high-grade screens, regional S2 is the first identifiable cleavage which is dominantly preserved within the meta-psammites as a subvertical differentiated cleavage that resembles the cordierite-bearing stripy gneisses of the Cooma Complex. It is associated with upright, N-S trending, km-scale folds that developed prior to the intrusion of the Bega BathoHth granitoids. D3 within the screens is recognised as an intense mylonitic flow-foliation within schlieren migmatites, which contain melt proportions of up to 80%. The migmatites show hybridization with the granites indicating that D3 is synchronous with pluton emplacement. D4 folds are open to tight with axial planes aligned sub-parallel to pluton boundaries. D4is most intense close to pluton boundaries where magma emplacement into thermally softened crust caused the reorientation of D2 and D3 structures. Compressive deformation (D5) during the late stages of emplacement caused broad asymmetric folding of the initially east-dipping granite sheet that is the Kameruka - Pericoe pluton. Folding of the pluton is syn-magmatic, indicated by the presence of leucosomes within F4 axial planes within the migmatites. D^ is a weak cleavage developed within rocks close to the Burragate Fault and is associated with the transposition of the flow-foliation within the Early Devonian Kameruka Granodiorite into parallelism with the Burragate Fault. This suggests that deformation is late syn-plutonic and occurred during the early to mid-Devonian. These screens preserve structures that formed independently of the regional deformation, outhned within the low-grade coastal zone to the east. Within the low-grade zone, the earliest developed fabric is identified as a microscopic, bedding parallel foliation (Sj), that formed during imbrication in an arc-frontal setting (Powell, 1983; Miller and Gray, 1997) and has been dated at 445 +/- 2 Ma (Offler et a/., 1998). D2 produced regional, tight, upright, N-S trending folds associated with a differentiated or stripy axial plane cleavage and correlates with S2 within the high-grade screens. Open folding of the S2 fabric produced a variably developed, widely spaced, 1-2 cm crenulation cleavage which corresponds with midDevonian, D3 deformation of Powell (1983) and correlates with D^ within the high-grade screens. The cause of this inconsistency in deformation events and the highly variable structural trend between high- and low-grade zones is attributed to the preferential partitioning of strain into rocks that were thermally softened during migmatization, combined with deformation caused by pluton emplacement. The schlieren migmatites form thick, pluton-margin-parallel sheets or screens reaching several kilometers in width. They are intersheeted with granites along the western margin of the Candelo, Yurammie, the Kameruka and the Pericoe plutons and are interpreted as representing large-scale, melt-lubricated shear zones along which granite magma was emplaced. The granite initially intruded as sheets parallel to, and synchronous with, the development of the mylonitic flow-foliation preserved as the schlieren migmatites.

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Leucosomes produced during migmatization migrate along the shear which acts to lubricate the shear zone and enhances the rate of granite emplacement. Shear-controlled, sheeted emplacement within the deeper-crust is succeeded at higher crustal levels, and during the late stages of emplacement, by sheet inflation and ballooning under increasing magma pressure resulting in the final 'elliptical' shape exhibited by the plutons. Evidence from the central Bega Batholith suggests that regions subject to syn-tectonic plutonism will undergo a more complex structural history of deformation, that may only be recorded in high-grade migmatitic screens, which appear to be the remnants of melt-lubricated shear zones. References Browne, W.R., 1914. The geology of the Cooma district. Journal of the Proceedings of the Royal Society of New South Wales, 48, 172-222. Joplin, G.A., 1942. Petrological studies in the Ordovician of New South Wales. 1. The Cooma Complex. Proceedings of the Linnean Society of New South Wales, 67, 156-196. Miller, J.M. and Gray, D.R., 1997. Subduction-related deformation and the Narooma Anticlinorium, eastern Lachlan Fold Belt. Australian Journal of Earth Science, 44, 237 - 251. Offler, R., Miller, J.McL., Gray, D.R., Foster, D.A., Bale, R., (1998). Crystallinity and b, spacing of K-white micas in a Palaeozoic accretionary complex, eastern Australia: metamorphism, palaeogeotherms and structural style of an underplated sequence. The journal of Geology, 106, 495-509. Powell, C.McA., 1983. Geology of the NS.W. South Coast and adjacent Victoria with emphasis on the Pre-Permian structural history. S.G.T.S.G. Field Guide 1, 118 pp.

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STRIKE-SLIP FAULTING: A POSSIBLE EXPLANATION FOR THE METAMORPHIC AND CRUSTAL THICKNESS GRADIENTS WITHIN THE YILGARN CRATON, WA. James Richardson and Patrice Rey Department Of Earth Science, VIEPS, Monash University, Clayton, Victoria 3168 e-Mail: jamesar@earth.monash.edu.au At the scale of the Archaean Yilgam Craton, there is a regional gradient of increasing metamorphic grade and a decrease in crustal thickness from the northeast to the southwest (Gee et al, 1981; Drummond, 1988). The crust is 37 to 38 km thick within the Eastern Goldfields Province and 33 km thick within the Southern Cross Province (Swager et al, 1997). Metamorphic domains within the Yilgam range from low grade Prehnite-Pumpellyite facies (3-6 kbar, 150-200°C) in the northeast, through to granulite facies (5-10 kbar, 700800°C) in the southwest (Gee et al, 1981). A number of shear zones cut across the Yilgam Craton. Some of them show a north northwest trend and sinistral kinematic sense of movement. We have looked in detail at one of these faults, the Koolyanobbing Shear Zone (KSZ) to assess the possible role of this fault in the development of metamorphic and crustal thickness gradients. The KSZ is a northwest trending, crustal scale ductile shear zone within the Archaean granitoid-greenstone terrane of the Southem Cross Province. The shear zone is 6 to 15 km in width and extends for a distance in excess of 300 km (Libby et al, 1991). Myers and Swagers (1997) suggested that the KSZ divides the Southern Cross Province into the Barlee and Yellowdine Terranes. These two domains contain similar lithologies with contrasting metamorphic grades to either side of the shear zone. The Barlee terrane shows Prenhite - Pumpellyite facies metamorphism (3-6 kbar, 150 - 200°C), while the Yellowdine terrane shows Upper Greenschist to Amphibolite facies metamorphism (5-9 kbar, 350-450°C). It has been suggested that the Yellowdine terrane is a deeper crustal equivalent of the Barlee terrane (Myers and Swagers, 1997) and as such the application of the concept of terranes is not justified.

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The KSZ outcrops along the northern shore of Lake Deborah East and is continuous for approximately 6 km. All structures within the granite are steeply dipping with a similar strike of around 320°, while the lineation along the shear zone plunges shallowly (5-10°) to the north. The KSZ affects both monzogranitic and granodiorite rocks. The structures range from magmatic to sub-magmatic flow in the west, giving way to solid state ductile deformation features to the east. S-C fabrics developed during solid state deformation. From west to east the angle between the S and C planes progressively decreases from -20° to less than 5°, indicating a corresponding increase in shear strain. The KSZ developed during heterogeneous progressive deformation. Kinematic analysis indicates a sinistral sense of movement in accordance with the study of Libby et al (1991). The coaxiality of magmatic and solid state structures and microstructures are compatible with syn to post emplacement shearing. The shear zone is likely to have accommodated the emplacement of the Ghooli Dome. Shallow dipping lineations in strike slip faults can accommodate significant vertical displacement. Previous work by Hull (1988) on width to length ratio of faults is used to approximate the horizontal offset along the KSZ. The highly strained core of the shear zone is likely to be 1.5-2km in width, which gives a horizontal offset range of 94.5-126 km from Hull's methods. Assuming an average horizontal offset of 100 km, a 5° dipping lineation amounts to a vertical displacement in the order of 8 km. Such an amount of vertical displacement is equivalent to a AP of 2.0-2.5 kbar, and a AT of 120°-160°C, compatible with the differences in metamorphic grade observed to either side of the KSZ. A small number of sinistral strike slip faults, with a shallow north dipping lineation could then explain the metamorphic gradient, and the corresponding thinning of the crust towards the southwest of the Yilgam Craton. Such sinistral strike slip faults exist in the Yilgam Craton, they include: the Boorara-Menzies Shear, Moriarty Shear, Boulder-Lefroy Fault, and the Zuleika Shear (Witt, 1993).

REFERENCES Drummond, B. J. (1988) A review of crust/upper mantle structure in the Precambrian areas of Australia and implications for Precambrian crustal evolution. Precamb. Res. 40/41, pp 101 to 116. Gee, R.D., Baxter, J.L., Wilde, S.A., and Williams, I.R. (1981) Crustal development in the Archaean Yilgam Block, Western Austraha. In, Archaean Geology International Symposium, Perth, 1980. Glover, J.E., and Groves, D.I. (eds), Geol. Soc. Austraha Special Pubhcation N°7. Hull, J. (1988) Thickness-displacement relationships for deformation zones. J.Struct.Geol. 10, pp 431 to 435. Libby, J., Groves, D.I., and Veamcombe, J.R. (1991). The nature and tectonic significance of the crustal-scale Koolyanobbing Shear Zone, Yilgam Craton, Westem Austraha. Aust. J. Earth Sciences. Vol. 38, pp 229 to 245. Myers, J.D. and Swagers, C. (1997) The Yilgam Craton, In, Greenstone Belts. DeWit, M. and Ashwal, L.D. (eds), Oxford University Press, New York, 1997.

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Swager, C.P., Goleby, B.R., Drummond, BJ., Rattenbury, M.S., and Williams, P.R. (1997) Crustal structure of granite-greenstone terranes in the Eastern Goldfields, Yilgam Craton, as revealed by seismic reflection profiling. Precamb. Res. 83, pp 43-56. Witt, W.K. (1993). Lithological and structural controls on gold mineralization in the Archaean MenziesKambalda area, Western Australia. Aust. J. Earth Sci. 40, pp 65 to 86.

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MICROSTRUCTURES, METAMORPHISM, AND TECTONIC SIGNIFICANCE OF METABASIC ROCKS, OCEAN-CONTINENT TRANSITION, IBERIA ABYSSAL PLAIN M. Rubenach School of Earth Sciences, James Cook University, Townsville, Qld. 4811 The Iberia Abyssal Plain has been recognized as an excellent example of an ocean-continent transition that formed at a passive margin (Beslier et al., 1993; Whitmarsh & Sawyer, 1996). In the earlier stages of the opening of the Atlantic Ocean in the Cretaceous, the continent was thinned and broken into fault blocks, which were transported westwards on detachment faults. The blocks now overly variably serpentinized mande, which was exhumed and exposed on the ocean floor as the "peridotite ridge". The mid-Atlantic ridge, with its associated ocean floor basalts, did not commence development until around 280 km from the Portugese coasthne. In 1997, Leg 173 of the Ocean Drilling Program continued with research on the Iberian ocean-continent transition commenced with Legs 103 and 149. Basement rocks penetrated during Leg 173 included the following: 1. Holel067. Amphibolite facies metagabbros, variably foliated, with metatonaUte dykes, intervals of brecciation (chlorite, epidote veining and matrix), thin horizons of fault gouge, and minor pseudotachylite. 2. Hole 1068A. 43 metres of breccia (? talus origin), comprised of clasts of gabbro, anorthosite, amphibolite to granulite facies metagabbro, mafic mylonite and minor metatonalite in a carbonate matrix (in part sedimentary, with Early Cretaceous microfossils). Hydrothermal overprint and zones of fault gouge occur at the base of the breccias, which are underlain by a serpentinite shear zone (? detachment fault), followed by serpentinized mande peridotite. 3. Hole 1069 recovered only fragments of greenschist facies arkosic wackes, thought to be clasts in a sedimentary breccia with a mud matrix. 4. Hole 1070. 18 metres of breccia consisting of clasts of serpentinized peridotite and minor gabbro in a carbonate matrix. The breccia overlies serpentinized peridotite, and contains dykes of homblende-clinopyroxene gabbro which represents early syn-rift melts. The metabasic rocks encountered in Holes 1067 and 1068 were interpreted as being early synrift, probably having intruded lower (thinned) crust and deformed along syn-rift ductile shear zones (ODP Leg 173 Shipboard Scientific Party, 1998). This was in accordance with interpretations on previous legs (e.g. Brun & Beslier, 1993), and was supported by shipboard major and trace element analytical results which indicated that the mafics were E-type MORB's. However, SHRIMP dating of zircons from a metatonalite revealed a magmatic rim of 340Ma and a core of mainly 595 Ma (Rubenach & Wysoczynski, unpublished data). These dates correspond respectively to Hercynian and Cadomian ages commonly obtained on the Iberian Peninsular and elsewhere in western Europe. The metabasic blocks therefore do not represent syn-rift gabbros but instead are fragments of continental basement transported along the detachment faults during rifting. Ductile deformation of the metabasic rocks, including the development of mylonites, is therefore probably Hercynian rather than the result of syn-rift faulting, the fault gouge of brittie derivation representing the only relicts of syn-rift deformation. 231

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Hornblende geobarometry on igneous amphiboles preserved as inclusions in plagioclase porphyroclasts in a metatonalite from Hole 1067A indicate a crystallization depth of 6 kbar, corresponding to mid-continental crust. Amphibole compositions in metabasic rocks suggest metamorphism at medium pressures, while gabbro coronas (olivine + plagioclase = tremolite + spinel + clinopyroxene) indicate metamorphism at less than 7 kbar. The compositions of clinopyroxene neoblasts in metagabbro clasts from Hole 1068 indicate granulite facies conditions. Under the conditions of recrystallization, plagioclase was less ductile than clinopyroxene or hornblende. Although some ages consistent with syn-rift igneous activity and metamorphism have been obtained, it is believed that syn-rift igneous activity and ductile deformation/metamorphism was relatively minor during the development of the continent-ocean transition. Indeed, if these processes were significant, a mid-ocean riudge would have probably formed much earlier. The processes involved in the the formation of the Iberian ocean-continent transition were probably similar to those described by Froitzheim & Manatschal (1996) for pre-Alpine basin development now preserved in the Swiss Alps. BesHer, M. -O., Ask, M., & Boillot, 1993. Ocean-continent boundary in the Iberia Abyssal Plain from multichannel seismic data. Tectonophysics 218, 383-393. Brun, J. P., & Besher, M. -O., 1996. Mantle exhumation at passive margins. Earth and Planet. Sci. Lett. 142, 161-173. Froitzhein, N., & Manatschal, G., 1996. Kinematics of Jurassic rifting, mantle exhumation, and passive-margin formation in the Austroalpine and Penninic nappes (eastern Switzerland). Geol. Soc. Amer. Bull. 108/9, 1120-1133. ODP Leg 173 Shipboard Scientific Party, 1998. Drilling reveals transition from continental breakup to early magmatic crust. Eos 79, 173-181. Whitmarsh, R. B., & Sawyer, D. S., 1996. The ocean/continent transition beneath the Iberia Abyssal Plain and continental rifting to seafloor-spreading processes. Proc. Ocean Drill. Sci. Results. Ocean Drilling Program, College Station, Texas, pp. 713-736.

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PROGRESSIVE DEFORMATION AND CYCLIC FLUID-ROCK INTERACTION IN THE 33 SHEAR ZONE, VICTORY COMPLEX, ST IVES GOLDFIELD, W.A. K J Ruming' and S F Cox' ^Department of Geology, University of Newcastle, University Drive , Callaghan, NSW 2308; e-mail: kruming@geology.newcastle.edu.au 'Department of Geology and Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200. The 33 shear zone is a mixed brittle-ductile shear zone that forms part of a large, gold-hosting system of stacked thrusts (Victory Complex) developed adjacent to the regional Playa/Boulder-Lefroy shear system near Kambalda in WA. The local Archaean greenstone stratigraphy contains a sequence of sub-aqueously extruded mafic to ultramafic lavas and minor intercalated sediments. These rocks are intruded by numerous small felsic and intermediate plugs and dykes, and mafic sills. The sequence has undergone early thrusting, and upright folding prior to the attainment of peak metamorphic conditions of lower amphibolite facies. The Victory Complex developed early in the retrograde metamorphic history. The 33 shear zone dips approximately 30 degrees to the south-east, varies in thickness from 10 - 50 m, and contains gold mineralisation over an area of approximately 0.8 km^. It is hosted by the Defiance Dolerite and the Paringa Basalt (tholeiitic affinity). In cross-section the shear zone consists of three main zones: (1) a central brecciated/vein complex hosted by weakly to moderately foliated basalt or dolerite; (2) an intermediate biotite-rich, schistose zone, which bounds the central zone in both the foot-wall and hanging-wall and (3) an outer, chlorite-rich zone that merges with the intermediate zone and becomes progressively less strongly foliated towards the margin of the 33 shear zone. The core of the 33 shear zone records a complex history of foliation development, faulting, brecciation, extension veining and progressive hydrothermal alteration. Eight distinct phases in the evolution of the 33 shear zone are recognised on the basis of deformation styles, accompanying hydrothermal alteration, and overprinting relationships. The first and second phases are not associated with gold mineralisation. An early phase of ductile creep in the 33 shear zone is preserved as a weak to moderate, SW dipping foliation within the core of the shear zone. This foliation is weakly domainal and is defined by layers of randomly oriented biotite and chlorite that anastomose around aggregates of quartz, carbonate and partially recrystallised plagioclase. The second phase produced a series of N/NE striking, moderately to steeply, SE dipping, discrete shear zones ranging in thickness from several centimetres to a maximum width of 5 m. Quartz and carbonate veins, up to 2 cm thick and 1-2 m in length, occur parallel to the foliation defined by layers of carbonate, quartz and poorly to highly oriented biotite/chlorite. The onset of brittle deformation is manifested by SW dipping faults, characterised by the sporadic development, along them, of brecciated zones that have been repeatedly sealed and re-fractured. The breccia matrix consists of fine grained albite and carbonate that encloses angular, foliated fragments of wall-rock that range in size from sub-millimetre to 20-30 cm. Some clasts may be completely by replaced by albite and carbonate resulting in offwhite/fawn colouration and others may retain their original black colour however, the wall233

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rock adjacent to these breccias appears unaltered. Minor gold mineralisation is associated with these structures. Planar to sigmoidal extension veins and associated breccias overprint the albite/carbonate breccias and the early, penetrative foliations in the core of the 33 shear zone. These veins are gently to moderately S/SW dipping and spatially associated with orange/red to light grey hydrothermal alteration haloes dominated by albite-carbonate alteration assemblages containing minor biotite, chlorite, pyrite (a primary indicator of gold) and quartz. These haloes extend up to 10 cm into the adjacent wall-rock. Major gold mineralisation, within the core zone of the 33 shear zone, is associated with the development of a second generation of gendy to moderately, S/SW dipping extension veins and associated dilational breccias. These veins overprint similarly oriented albite-carbonate veins and are sealed by quartz and minor carbonate, biotite, chlorite and pyrite. This event is characterised by hydrothermal alteration haloes up to several centimetres wide, of randomly oriented biotite and disseminated coarse-grained (2-5 mm) euhedral pyrite. This veining and alteration phase is kinematically related to the previous veining and associated alteration as both are a result of W/NW directed shortening. After the main gold mineralising events within the core of the 33 shear zone, the moderately SE dipping foot-wall and hanging-wall schistose zones record an episode of vertical flattening. The earlier, compressional related structures, such as the gently to moderately dipping extension veins, have been folded and boudinaged immediately adjacent to the footwall and hanging-wall. NE trending, sub-vertical joints post-date the extensional phase and cross-cut both the core zone and schistose foot-wall and hanging-wall zones. These joints contain quartz and have characteristic, millimetre scale, red coloured hydrothermal alteration haloes which comprise carbonate and albite with minor haematite and pyrite. Minor gold mineralisation is associated with these joints. In contrast, the last phase recognised in the evolution of the 33 shear zone is associated with narrow (centimetre scale) thrust faults which record multiple sealing and refracturing episodes in dilational jogs. Red/orange and green coloured alteration haloes are common along these faults, but are not uniformly developed. The alteration mineralogy is composed of carbonate with minor chlorite, muscovite, haematite and sporadic patches of gypsum. The 33 shear zone initiated as a ductile thrust, but evolved with progressive hydrothermal alteration to become a predominantly brittle structure in which deformation was accommodated by the formation of extension vein systems and dilational breccias during gold mineralisation. The transition from predominantly ductile to brittle deformation is interpreted in terms of embrittlement associated with increased fluid pressures during high fluid flux hydrothermal alteration and gold mineralisation. Subsequent ductile extensional deformation may have occurred at lower fluid pressures. Late brittle deformation occurred at lower temperatures. Variations in hydrothermal alteration styles indicate cyclic potassic and sodic alteration during thrusting and subsequent evolution of the Victory thrust complex. These variations may be interpreted in terms of the active and permeable thrust complex localising fluid flow and mixing between fluids derived from different reservoirs. 234

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BASIN REACTIVATION ON A HOT CONTINENT Mike Sandiford Department of Geology and Geophysics, University of Adelaide, SA 5005 (msandifo ©geology. adel aide .edu. au)

Basin development results in subtle, long-term changes to the compositional and thermal structure of the lithosphere, and therefore should impact on its long-term mechanical behaviour. Such changes may make basins more or less susceptible to subsequent deformation, and thus the subsequent response of intracratonic basins to intraplate deformation events provides a wonderful 'natural' experiment that can be used to help unravel the factors controlling lithospheric rheology. This contribution will outline some of the important factors that control the long-term mechanical evolution of continental lithosphere following rift-basin development. It will be shown that depending on the initial configuration and nature of the basin-fill, basin formation will normally lead to the significant long-term lithospheric weakening, with the locus of maximum weakening either in the centre of the basin or at the basin margin. Only when the initial lithospheric configuration is characterized by low total concentrations of heat producing elements and the basin-fill is much less dense than the pre-existing crust, will basin formation lead to long-term lithospheric strengthening. This analysis will be used to highlight aspects of basin reactivation (and inversion) and intraplate deformation in central and South Australia; regions which are characterized by unusually high heat flows due to exceptional enrichments in heat-producing elements. Examples will include the localization of the Petermann and Alice Springs Orogenies, Cambro-Ordovician basin inversion in the northern Flinders Ranges and the active deformation in the Mount Lofty-Flinders Ranges.

(a, b & c) basin formation leads to changes in the distribution and amount of heat production in the pre-existing crust and therefore to long term changes in the geotherm. These changes are likely to lead to increases in temperature at any given depth (below the heat producing parts of the lithosphere). Depending on initial configuration and density of the basin-fill

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(which controls the long-term change in Moho depth) the Moho temperature may increase ( c ) or decrease. Even in instances where Moho temperatures are reduced beneath the basin, basin margins will normally be characterised by slight (~10°C) increases in Moho temperature (d & e).

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STRUCTURAL ASPECTS OF GOLD MINERALISATION AT BENDIGO, VICTORIA: FIELD OBSERVATIONS AND NUMERICAL MODELLING Peter M. Schaubs, School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3052 e-mail: p.schaubs@pgrad.unimelb.edu.au The relative temporal and spatial relationships between gold mineralisation and deformation and its associated structures in the goldfields of central Victoria has been the focus of active study by many researchers over the past century. The general consensus is that quartz vein emplacement and associated gold mineralisation are synchronous with respect to the deformation. The Bendigo goldfield is dominated by Ordovician turbidites made up of metasandstone, slate and meta-siltstone that have been deformed into upright chevron folds and metamorphosed to lower greenschist facies. Gold is found in narrow quartz veins, which are most prolific in the domes of gently plunging anticlines, where they have been deformed by later reverse faults. Along the Deborah Anticline gold is identified in both fold related veins such as bedding-parallel, cleavage-parallel and steeply dipping tension veins and in greater amounts and in later generally discordant, fault-related veins, including fault-parallel veins, sub-horizontally dipping tension veins and en echelon vein arrays. Early bedding-parallel veins are interpreted to have formed during folding in part via flexural-slip, while faults and related veins formed after chevron folds had locked up. The majority of gold is associated with fault-related structures such as west-dipping faults and disrupted saddle reefs; however, the fold-related structures are important in determining where these later brittle-ductile structures have propagated. In many cases the major gold-bearing fault structures have reactivated earlier formed bedding-parallel structures. In the majority of the literature pertaining to Bendigo, the goldfield is characterised as being the premier site for the formation of spectacular saddle reefs, and therefore there is the misconception that these are the major gold-bearing structures. Saddle reefs are areas of massive quartz in the hinges of anticlines, which form as a result of dilation during chevron folding of multilayers of high competency contrast. Although saddle reef type structures are quite common in the Bendigo goldfield (more so than in other Victorian goldfields), they are not the major gold-bearing structures. Saddle reefs are most auriferous where they have been modified by reverse faults to become neck reefs and therefore the faults and their associated quartz veins are the source of the larger proportion of gold mined in Bendigo. The effect of imposing a fault and fracture network upon a pre-existing folded area increases the overall permeabihty and allows fluids to reach a larger area. During folding, fluid flow was generally restricted to bedding planes. Slate units acted as impermeable barriers and caused gold to precipitate near them. Later discordant faults transected bedding planes and allowed fluids to travel into areas previously inaccessible during folding, by creating a suitable fracture network (eg. Sibson 1996).

The finite-difference computer program FLAG, has been used to help in determining the combined evolution of fold, vein and fault geometries within a multilayered sequence. The model is based on a generalised sequence of interlayered sandstone and slate in the Deborah

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Anticline. The model shows how structures developed as the multilayer was progressively shortened and these data are in turn compared with field observations. Specific questions which are addressed, include: How early in the deformation sequence and in which portions of the sedimentary pile do bedding parallel veins begin to form? Of these bedding parallel veins which are most suitable for reactivation by later faulting? What controls whether a major east or west-dipping fault will be the major quartz-gold bearing structure along a particular anticline? What ratio of sandstone to slate and relative thicknesses of these units controls where the major structures will occur? Fluids may also be incorporated into the model to determine the fluid pathways with respect to deformation. REFERENCES Sibson, R.H. 1996. Structural permeabihty of fluid-driven fault fracture meshes. Journal of Structural Geology, 18: 1031-1042.

Neck reef (disrupted crescent vein)

Sub-horizontal dipping tension veins

Crescent vein

Bedding parallel vein

Reverse Fault

A-C vein Steeply dipping tension vein (B-C)

Cleavage parallel vein

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Martin M. Scott Geological Survey of New South Wales, Department of Mineral Resources, PO Box 53 Orange, NSW 2800. scottm@minerals.nsw.gov.au The Condobolin 1:100 000 map sheet has recently been mapped as part of the National Geoscience Mapping Accord between the Geological Survey of NSW and the Australian Geological Survey Organisation. In the western part of the map sheet, a major NNW-trending fault zone, either the Gilmore Fault Zone or one of its parallel splays, includes fault slivers of ?Ordovician ultramafics and bounds two areas with separate geological histories. Within and north of the fault zone, the ?Ordovician Girilambone Group crops out with only rarely preserved bedding (SQ) and a weak Sj cleavage at the mesoscopic scale. Bedding is folded by recumbent F2, with the main foliation (S2), a quartz-muscovite segregation cleavage, axial plane to the folding. S2 is folded by S3 crenulations, mesoscopic kinks and mesoscopic F3 folds. Two structural domains in the Girilambone Group are recognised by consistentiy orientated structural elements, and are divided by the NW-trending Condobolin Fault. North of this fault. So strikes NW and S2 dips moderately to the ENE. South of this fault, S2 commonly dips moderately to steeply to the south and SE, with SSW-plunging F3 reorientating S2. The main deformation (Dj, D2, D3) and metamorphism of the Girilambone Group occurred during the Late Ordovician-Early Silurian (see also Pogson & Felton 1978). Epithermal Au-Ag-Pb-Cu quartz veins in the Condobolin base metal district, dip steeply SE in the Girilambone Group, and have an associated arcuate high K radiometric anomaly. Epithermal veining and mineralisation cuts across S^, S2, S3 in both domains of the Girilambone Group, and so postdates the Condobolin Fault and the main deformation and metamorphism of the Girilambone Group. Epithermal veining and mineralisation is constrained to the Middle-Late Silurian, as locally-derived clasts of epithermal veins, and cleaved (S2, S3) and metamorphic-quartz veined Girilambone Group, are found in the unconformably overlying Pridoli-Lockhovian Edols Conglomerate of the Derriwong Group. Constraint on epithermal veining at Condobolin may have implications for the timing of similar mineralisation along the Gilmore Fault Zone. The Edols Conglomerate is relatively unmetamorphosed, and crops out in shallow NNWplunging F4 folds unconformably above the Girilambone Group. F4 also folds the Condobolin Fault and tilts the underlying Girilambone Group. S4 cleavage is not evident at the mesoscopic scale, although a 1-lOmm spaced rough anastomosing cleavage occurs in Edols Conglomerate within the NNW trending fault zone. F4 folds in the Derriwong Group are unconformably overlain by the Lockhovian-Pragian Yarra Yarra Creek Group (Sherwin 1997), indicating the folds formed during a Lockhovian D4 deformation. The similarly trend of S4 in the major fault zone and F4 folds, suggests they formed in a single D4 compression with & orientated ENEWSW, with greatest strain in the major fault zone. South of the fault zone, the Early Silurian Ugalong Dacite is overlain by the ?Silurian Euglo Formation, in which younging defines a north-plunging anticline with an associated steep cleavage. The Euglo Formation consists of quartzose and lithic sandstone, siltstone, chert, and 239

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felsic volcanics which host Au at the Gnarly and Laurieston prospects. Quartz-muscovite development and mesoscopic isochnal folds in the Euglo Formation, indicate higher metamorphic grade and intensity of deformation adjacent to the fault zone. Asymmetry of steeply-plunging isoclinal folds indicate sinistral movement along the major fault zone. Offset of WSW-trending cross cutting mafic dykes recognised on total magnetic intensity geophysical images, indicate late sinistral strike slip reactivation of the fault zone probably during the Middle Devonian-Carboniferous. Extensional reactivation in the Tertiary, caused the fault zone to act as a dilational conduit for olivine leucite basalt at the Weebar Hill intrusive centre. References Pogson D.J. & Felton E.A. 1978. Reappraisal of Geology, Cobar-Canbelego-Mineral Hill Region, Central Western New South Wales. Quaterly Notes of the Geological Survey of New South Wales 33, 1-14. Sherwin L. 1997. Narromine 1:250 000 Geological Sheet SI/55-3, and Explanatory Notes, viii -h 104pp. Geological Survey of New South Wales, Sydney. Acknowledgements: Published with permission of the Director General, New South Wales Department of Mineral Resources.

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THE GRAMPIANS AND DEFORMATION STYLES

DERRIWONG

Volume

GROUPS,

CONTRAST

Lawrence Sherwin Geological Survey of New South Wales, P.O. Box 53, Orange, New South Wales 2800. (Published with permission of the Director, Geological Survey of New South Wales) Despite its isolated geographic position, the Grampians Group has much in common with similarly dated sandstone dominated units in central New South Wales such as the Derriwong Group. The latter represents a variety of environments ranging from marginal, verging on fluviatile, to deep water facies and is fact a south-eastern extension of the Cobar Group, the separation of the two being somewhat arbitrary. Other groups in central New South Wales that are essentially synonyms of these are the Kopyje and Ootha Groups. A close correlative, both in time and sedimentation, is the Mt Daubney Formation to the north east of Broken Hill in far western New South Wales. The Derriwong Group ranges in age from Late Silurian, possibly as low as late Ludlow, to Early Devonian (Lochkovian), these ages being well based on invertebrate (Sherwin 1992) and micro vertebrate (Turner 1993) fossils from several horizons. The age of the Grampians Group is less constrained because of the limited and poorly preserved micro vertebrate fauna of acanthodians and thelodonts (Burrow 1997, Turner 1986) from what is probably a single horizon in the Silverband Formation. Notwithstanding the state of preservation, thelodonts are not known from pre Silurian strata (S. Turner, pers. comm.), ruling out an inferred Ordovician age for the Grampians Group. Radiometric ages of intrusions into, or volcanics overlying, the Grampians Group are all well within the Early Devonian, using the revised dating of Tucker et al. (1998). Because there is only one datable horizon within the Grampians Group it is possible only to demonstrate time equivalence with the lower part of the Derriwong Group, ie., a Ludlow-Pridolian age. Burrow (1997), on the basis of acanthodian scales in the Silverband Formation, has determined an age within the Late Silurian crispa-eosteinhornensis conodont zones, an horizon dated by Tucker et al. at 420.2+/-3.9 Ma. The Derriwong Group is widely distributed in central New South Wales to the west of Forbes. The basal unit, the Edols Conglomerate, which varies considerably in thickness and distribution, forms very prominent ridges between Bogan Gate, Wyalong and Condobolin, comparable with the Grampians Range. Unlike the Grampians Group, the Derriwong Group has a recognisable erosional contact with older formations, although a thick layer of scree around all but the lower outcrops obscures contacts with older rocks. Between Bogan Gate and Condobolin the sediments overlying the Edols Conglomerate show structures more typical of deep water sedimentation, including graded bedding. These deeper water deposits are much less fossiliferous and in the past have been assigned to the Ootha Group. The juxtapositioning of shallow and deep water sedimentation is interpreted as marginal to shallow shelf bordering half to full grabens. The bounding faults have sub meridional trends in central New South Wales. The depth of some grabens is believed to have been sufficient for deposition of tubidites and the rate of erosion of the flanks such as to produce the very coarse boulder basal Edols Conglomerate, the well rounded clasts clearly derived from the local basement. The upper half or more of the Derriwong Group consists of fine muds and silts with medium grained sandy beds a minor component.

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In the vicinity of Forbes, the Derriwong Group overlies the mid Silurian Forbes Group v^ith a slight angular unconformity and further north near Bogan Gate the unconformity is on Late Ordovician Goonumbla Volcanics. Just north of Condobolin, outliers of the Derriwong Group form spectacular cappings on the tightiy folded Early to Mid Ordovician Girilambone Group. In these areas the Derriwong Group is moderately dipping to sub horizontal in broad open folds with NW-SE fold axes. The deformation can be dated as mid Early Devonian (late Lochkovian) because the Derriwong Group in this area is overlain unconformably by the late Early Devonian Yarra Yarra Creek and Trundle Groups (Sherwin 1996). The deformation timing of these younger groups is less well constrained. There was probably a minor disturbance in mid Devonian time, there being very little discordance with the overlying Late Devonian Hervey Group where the latter is present. However, the submeridional thrusting which disrupts the Yarra Yarra Creek and Trundle Groups also truncates the Hervey Group, indicating that the last major deformation is Early Carboniferous or even younger. In the Ootha district, the fine grained sediments of the Derriwong/Ootha Group have a variably developed SI cleavage. To the north, where the contact with the Girilambone Group basement is exposed, this SI parallels the basement S3. Some of the rhyolites in the vicinity of Ootha, previously regarded as flows within the Derriwong/Ootha Group, show trends discrepant with the folding of the sediments, suggesting that they are dykes. The outcrop of these fine sediments is inadequate to determine if these probable dykes have intruded along fault lines. To the south of Condobolin and Ootha, south of the Lachlan River alluvial plain and closer to West Wyalong, the Derriwong Group in many areas is strongly cleaved and steep dipping, where bedding can be recognised at all. Much of the outcrop consists of the Edols Conglomerate alone, which to the east of Wyalong is almost certainly fault bounded, based upon outcrop and aeromagnetic anomalies. The trends of the faults, part of the Gilmore Fault system, suggest that they are very steep, at least near the surface. The shearing of the conglomerate is sufficiently severe to obliterate even very coarse (30 cm diameter) quartzite boulders to produce cataclastic textures. Where deformation is less severe the cobbles are quite distinctly "stretched". Although the Derriwong Group is locally detached from basement by faulting, there is no evidence of large scale stacking or dislocation as has been described for the Grampians Group (Cayley & Taylor 1997). The submeridional trend of these faults near Wyalong suggests that the deformation is related to the ?Early Carboniferous event further north. References Burrow, C., 1997. A poracanthodid acanthodian from the Silverband Formation, the Grampians, Victoria, and its palaeobiogeographical significance. Geological Society of Australia Abstracts 44, 95. Cayley, R.A. & Taylor, D.H., 1997. Grampians special map area geological report. Geological Survey of Victoria Report 107. Sherwin, L., 1992. Siluro-Devonian biostratigraphy of central New South Wales. Geological Survey of New South Wales, Quarterly Notes 86, 1-12.

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Sherwin, L., 1996. Narromine 1:250 000 Geological Sheet SI/55-3: Explanatory Notes, viii+104 pp. Geological Survey of New South Wales, Sydney. Turner, S., 1986. Vertebrate fauna of the Silverband Formation, Grampians, western Victoria. Royal Society of Victoria, Proceedings 53-62. Turner, S., 1993. Palaeozoic microvertebrate biostratigraphy of Eastern Gondwana. In Long, J.A. (Ed.), Palaeozoic Vertebrate Biostratigraphy and Biogeography, Bellhaven Press, London, pp. 174-207. Tucker, R.D., Bradley, D.C., Ver Straeten, C.A., Harris. A.G., Ebert, J.R. & McCutcheon, S.R., 1998. New U-Pb zircon ages and the mduration and division of Devonian time. Earth and Planetary Science Letters 158, 175-186.

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SMALL SCALE LOCALISATION OF DEFORMATION Hadi. M. Sinf\ Paul. D. Bons' & Mark. W. Jessell' "" School of Earth Sciences, The University of Melbourne, Parkville, Vic 3052, Australia ^Department of Earth Sciences, Monash University, Clayton, Vic 3168, Australia Email: hmsim@hotmail.com (Hadi Sim), paul@artemis.earth.monash.edu.au (Paul Bons), mark@orion.earth.monash.edu.au (Mark Jessell) Localisation of deformation is a process in which strain becomes concentrated into a relatively smaller volume of the deforming rock. It is generally associated with pre-existing heterogeneities in the rock, which can be either lithological, structural or compositional heterogeneities, and syn-deformational heterogeneities, which are developed during the deformation processes. An understanding of the origins of the processes related to localization of deformation in high-strain zones, such as mylonite zones and shear zones, are important in interpreting the kinematics of the deformation. Small scale localisation of deformation is localisation in centimeter to micrometer scale. This kind of localisation of deformation is very difficult to identify, because the associated microstructures are easily modified or even obliterated by the ongoing deformation processes such as dynamic recrystallisation. This poster describes and compares the small scale localisation of deformation in deformation experiments and from field investigation. Syn-kinematic microscopy techniques were used to observe organic rock analogue, polycrystalline norcamphor, deformed in simple shear. In this study we used a Urai deformation apparatus. Microphotographs and computer images of the deforming sample were taken every five minutes. These computer images were then analyzed using a Pattern Matching Program (Bons & Jessell 1995) to identify where and why small scale localisation of deformation occurs. In the experiments, small scale localisation of deformation occurred as highly deformed zones and micro-shears. Micro-shear zones occurred as small and short lived anastomosing bands which were passively moved along with the deforming sample. They continuously appeared and disappeared, in a process involving the homogenization of the finite strain. Localisation of deformation can only be identified using deformation grids because it does not leave any visible physical microstructure. The shearing of the anastomosing bands formed these microshear zones. Material transfer via diffusion occurred because the sample is restricted so that it cannot increase in thickness. These experimental results were compared with field observations from granitic gneisses and schists in the southwest corner of the Mt. Painter Inlier, South Australia. The natural rocks show heterogeneous finite strain and centimeter to micrometer scale localisation as shear bands, schistosity bands and foliation. Thin section analysis revealed that they have an anastomosing structure, which evolves into S-C fabric and finally into a single foliation fabric as deformation progressed. Comparison between the experimental studies and the field study suggests that there are similarities as well as differences in the style and trend of the strain. In 244

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both cases, small scale localisation of deformation occurs and displays anastomosing structure. The differences are on the finite strain and the forms and life span of the small scale localisation. Reference: Bons, P. D. & Jessell, M. W. 1995. Strain analysis in deformation experiments with pattern matching or a stereoscope. J. Struct. Geol Vol. 17. No. 6, 917-921.

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TOWARDS AN UNDERSTANDING OF GIANT GOLD SYSTEMS Sorjonen-Ward\ P., Walshe^ J.L., Hobbs^ B.E., Hall', G., and Ord\ A. ^Australian Geodynamics Cooperative Research Centre, CSIRO Exploration & Mining, PO Box 437, Nedlands, WA 6009. 'Placer Dome Exploration Limited email: p.ward@dem.csiro.au The giant gold deposits are considered products of crustal-scale fluid-flow systems in which reduced fluids stored in mid to lower crustal reservoirs were discharged into mid to upper crustal sites of deposition. Essential elements of the architecture of these systems include: • structures that transect the crust, • reduced packages of rock to maintain the integrity of ore fluids, • regional seals to permit reservoir development in the mid to lower crust and to maintain heat and gas content of fluids in the upper crust The common Au bearing fluids were C02-H2S-rich aqueous fluids but non-aqueous fluids containing C02 , CH4, N2 , H2 and H2S may have been significant in the deeper parts of the crust. High fluid fluxes in over pressured domains in the system were maintained by porosity waves. Geodynamic modelling suggests abnormal heat input from the mantle may not be required once the system is perturbed by thrust stacking or by loading with volcanosedimentary sequences. Mixing of deep crustal fluids with upper crustal fluids of contrasting chemistry and/or hypogene upgrading of proto-ore, driven by pressure fluctuations within over-pressured compartments, were probably the most significant precipitation processes at the sites of gold deposition. The challenge of the giant systems The challenge for ore geneticists in the modem era is to develop robust models for the formation of high quality resources (large tonnage, high grade, suitable metallurgical properties) that will impact significantly on global exploration strategies. Regional-scale data, particularly geophysical and isotopic data, are fundamentally modifying perceptions of the physical dimensions of hydrothermal systems. It will be increasingly possible to formulate models at the regional to crustal scale. This will drive a shift in perspective away from the traditional deposit-oriented and class-oriented approaches to ore deposit research towards understanding the basic and common elements of hydrothermal systems. The role of geodynamic modelling Geodynamic modelling provides a framework to interrogate geological, geophysical and geochemical data at all scales and build thoroughly integrated and quantitative models of ore formation. This holistic view of the ore system is the key to a powerful new tool for understanding the formation of the giant deposits and in designing and evaluating exploration programs. A soft model - hard model approach is being developed. Available data from the regional, deposit and microscales is being integrated into "soft" conceptual models of the

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hydrothermal systems which formed the outsized deposits. The modeUing begins by asking five questions about the system 1. 2. 3. 4. 5.

What is the size and structural-lithological architecture of the system? What is the P-T and geodynamic history? What is the nature of fluid reservoirs in the system? What mechanisms advect/convect/focus fluids? What are the metal transport/depositional mechanisms of ore formation?

The answers to those five questions generate the soft models that in turn provide a framework for the hard models. The quantitative modelling (the hard modelling) involves complete coupling between fluid flow, heat transport, rock deformation and chemical reaction The hard modelling is a tool to answer the "what-if questions. It is potentially a new tool in designing and evaluating exploration programs: the essence of the new paradigm in predictive mineral exploration.

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OCCURRENCES AND SIGNIFICANCE OF FRANCISCAN-LIKE MELANGE AND BLUESCHIST METAMORPHISM IN LACHLAN OROGEN FAULT ZONES Catherine V. Spaggiari^ David R. Gray^ & David A. Foster^ Australian Geodynamics Cooperative Research Centre ^Department. Earth Sciences, Monash University, Melbourne VIC. 3168, Australia, ST-cath@earth.monash.edu.au ^Department of Earth Sciences, La Trobe University, Melbourne VIC. 3083, Australia. Isolated pockets of chaotic serpentinite matrix and mud-matrix melanges containing mafic to ultramafic blocks of blueschist meta-igneous rocks are associated with some of the faultbounded Cambrian "greenstone" belts of the western and central Lachlan orogen. These represent the type-B blueschist association (Maruyama et al., 1996), typical of ocean-ocean and/or ocean arc collision, where 1) protoliths consist of rock assemblages which form in oceanic settings (e.g. turbidite-chert-black shale-basalt-gabbro-peridotite), and 2) which have documented maximum pressures of metamorphism of up to 12 kbars. The fault zones which host the "blueschists" in the Lachlan orogen occur as long (up to 200 km exposed lengths), linear, generally N-S trending belts which commonly show changing displacement and geometry along their lengths. These faults form leading-imbricate fan 'thrust' systems, with tiered detachments within the turbidite-basalt stratigraphy. Slivers of "oceanic" crust within these faults are considered to result from 'peeling' of the upper oceanic layers by a duplex mechanism during subduction (Fig. 1). Metamorphic grade of the overlying turbidites is epizonal (greenschist facies) or lower, with b^ lattice parameters of white micas in the slates indicating intermediate pressure metamorphism (c. 4 kbars) and a low geothermal gradient existing between 450-430 Ma (Offler et al., 1998). The tectonic melanges, which include blocks of chert, turbidite and variably metamorphosed meta-igneous ("greenstone") rocks that range in grade from prehnite/pumpellyite, pumpellyite/actinolite, greenschist, and blueschist facies, have been located within both the Heathcote fault zone and the Howqua River area of the Governor fault zone. The lithological and structural nature of these melange zones is not unlike those of the Franciscan complex in California, where blocks of high and intermediate pressure rocks are found in either serpentinite or mud-matrix melanges. Melange localities 1). Heathcote fault zone: The Heathcote fault zone, a major fault within the western Lachlan orogen, is the leading fault for the Bendigo-Ballarat Zone. The eastern boundary is characterised by a steeply, west-dipping reverse fault system. It changes from a fault-bounded zone of essentially homoclinally dipping, Cambrian metabasalts and clastics in the north, to a structurally complex zone in the central portion near Heathcote. Here, the strike of the fault zone changes to northwest-trending, and to a single fault in the south where the fault system loses stratigraphic displacement. The fault-bounded portions of the Cambrian meta-igneous rocks are strongly foliated adjacent to fault boundaries, and less foliated towards the interiors of the 'fault slices', where igneous and metamorphic textures are well preserved. At Heathcote, intermediate pressure rocks consist of the assemblage winchite (sodic-calcic amphibole transitional between actinolite and glaucophane), albite, Mg-Cr spinel, chlorite, stilpnomelane, quartz, ± pumpellyite, with geobarometry (after Brown, 1977) indicating pressures between 5 and 7 kbars.

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2). Governor fault zone: The Governor fault zone is the leading fault for the Tabberabbera Zone, which is part of the central Lachlan orogen. At Howqua, a sequence of pillowed, actinolite/pumpellyite facies metabasalts, bedded chert and siliceous black shale is overlain by turbidites. This is interpreted as a conformable sequence of upper oceanic stratigraphy. The base of the sequence consists of sheared, serpentinite-matrix melange containing pods of meta-igneous blueschist rocks. Structurally below this is a heavily veined, slaty, black mud-matrix melange which incorporates blocks of sandstone, siltstone, greenstone, and chert. Larger blocks (approximately 2-3 m) of winchite and glaucophane-bearing blueschists occur as isolated 'tors' to the southwest at Tobacco Flat. Any matrix the blocks may have been "wrapped" in is now weathered out, suggesting it was probably mud. This is typical of the Franciscan, where blueschist and sandstone blocks outcrop as isolated "tors" on grassy hillsides (termed "knockers"). They are interpreted to have been immersed in mudmatrix which has weathered away, and only the more resistant blocks remain. Geobarometry (after Brown, 1977) indicates pressure of metamorphism of the blueschist blocks at Howqua was about 7 kbars, translating to depths of approximately 21 km. Blueschist metamorphism is indicative of subduction, however the host fault zones described above do not mark individual subduction zones. The Heathcote and Governor fault zones represent a splay and leading fault, respectively, formed by a subduction 'peeling' mechanism (see Fig. 1) in the western and central Lachlan orogen, as part of an inferred double-divergent subduction zone (Soesoo et al., 1997). REFERENCES Brown E.H. (1977). The crossite content of Ca-amphibole as a guide to pressure of metamorphism. Journal of Petrology, v. 18, part 1, 53-72. Maruyama S., Liou J.G., Terabayashi. 1996. Blueschists and eclogites of the world. International Geology Review, 38, 485-594. Offler R., McKnight S., Morand V. (1998). Tectonothermal history of the western Lachlan Fold Belt, AustraUa: insights from white mica studies. Journal of Metamorphic Geology, 16, 1-11. Soesoo A., Bons P.D., Gray D.R., & Foster D.A. 1997. Divergent double subduction: tectonic and petrologic consequences. Geology, 25, 755-758.

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INTEGRATING MICROSTRUCTURES AND METAMORPHISM RECORDED WITHIN GARNET PORPHYROBLASTS FROM THE FLEUR DE LYS SUPERGROUP, NEWFOUNDLAND, CANADA: IDENTIFYING INTRA- AND INTER-DEFORMATION HIATUSES IN GARNET GROWTH AARON STALLARD School of Earth Sciences, James Cook University, Townsville 4811, Australia email: aaron. stallard®jcu. edu. au

Inclusion trails in garnet porphyroblasts from the Fleur de Lys Supergroup, Newfoundland, preserve successive generations of microstructures, some of which correlate with microstructures in the matrix. Microstructure-porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1-S7) and four stages of garnet growth. S1-S5 are recognised within garnet porphyroblasts, 85-85 within albite porphyroblasts, and 84-87 within the rock matrix. Significant alteration and destruction of early foliations has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1-G4). Garnet growth during each of the four stages did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories for individual garnet porphyroblasts from the same outcrop. The inclusion trails within garnet porphyroblasts record a succession of crenulation cleavages without rotation of the porphyroblasts relative to other porphyroblasts in the population. Compositional zoning profiles across the garnet porphyroblasts document significant anomalies, including zoning reversals, second-order oscillations, steepened compositional gradients and compositional discontinuities. The intervals of reversed zoning represent a hiatus in garnet growth, and result from compositional modification of the garnet rim by intracrystalline diffusion during re-equilibration with changing cation concentrations in the surrounding matrix. The steepened compositional gradients are related to changes in the reaction history across boundaries separating different stages of garnet growth, and coincide with microstructural discontinuities outlined by inclusion trails within the porphyroblasts. The compositional data supports the four-stage model of porphyroblast growth identified using microstructural criteria. Estimates of changes in the reaction history, metamorphic conditions, and deformation kinematics between successive garnet growth stages can be made from measured changes in garnet compositional zoning, inclusion trail mineralogy and inclusion trail microstructures across the boundaries between growth stages. Two of the three identified boundaries between successive growth stages within the garnet porphyroblasts (G1-G2 and G2G3) are tentatively characterised in this way. Significant differences exist in the compositional and microstructural changes across the G1-G2 boundary compared with the changes observed across the G2-G3 boundary. The data suggests the G1-G2 boundary represents an extended hiatus in growth between distinct deformation events of fundamentally different kinematic and metamorphic conditions. By contrast, the G2-G3 boundary represents a relatively minor pause in growth during a continuous orogenic event. This pause in growth is coincident with a transition between successive crenulation cleavages, with no discemable accompanying change in P-T conditions, reaction history or included mineral phases. This may be the first 250

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characterisation of both inter-orogenic (G1-G2) and intra-orogenic (G2-G3) hiatuses in garnet growth within the same rock mass.

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THE MR. CURLY GRANITE: AN INTERNALLY LAYERED AND LENTICULAR, MULTIPLY FOLDED LEUCOGRANITIC COMPLEX Nick Stebbing, Paul Bons, Marlina Elburg & Catherine Parnell Department of Earth Sciences, Monash University, Clayton 3168 The Mr. Curly Granite is a Proterozoic leucogranitic complex with an unusual internal structure, which may give insight in the transport and emplacement mechanisms of granites. It forms part of the Dougall Suite (Sheppard et al. 1995) of granites in the NE part of the central zone of the Halls Creek Mobile Belt, Kimberley region, West Australia. The SHRIMP U-Pb age obtained by Page (1995) is 1850±2 Ma, which is about 30 Ma older than the adjacent Sally Downs Tonalite, dated at 1821 ±4 Ma (Sheppard et al. 1995). The leucogranite intruded predominantly amphibolites and minor limestones, metapelites and metapsammites of the Tickalara Metamorphics. Mafic intrusives are abundant, but are often difficult to distinguish from older metabasaltic amphibolites. Small granitoid intrusions and veins are ubiquitous in the area. The Mr. Curly complex has been folded at least twice on the 0 . 1 - 1 km scale. A 3 x 5 km area was studied and mapped in detail. Here the multiple folding produced the characteristic Mr. Curly shape. The leucogranite extends further south and probably resurfaces further east again (P. Rey, pers. comm.). Folding of the granite formed weak to moderately strong axial planar fabrics inside the granite, with stretching of quartz and some alignment of other minerals. Undulose extinction in quartz and dynamic recrystallisation are commonly found in thin sections. The fold structure can also be found in partially skamed marble layers, which can be traced for kilometres and that form excellent structural marker horizons in the otherwise rather featureless host rock. The marbles are internally very strongly deformed with sheet folds as the most distinct indicator of the high strain. The granites intruded as parallel lenses and layers, each metres to tens of metres thick and extending for up to hundreds of metres and concentrated in an originally planar zone of up to a few hundred metres thickness. The lenticular and layered nature is clearly visible on aerial photographs, but is often difficult to discern in the field where the granite can have a massive character, especially where amalgamation of lenses and layers formed large irregular 'blobs'. Internally, the granite often has a layer-parallel cryptic layering on the mm - cm scale. This layering often has a stromatic migmatitic character: wavy and with small extensional shear zones, often with more leucocratic veins than the surrounding granite. The cryptic migmatitic layering is interpreted as the result of deformation of the crystallising granite melt, causing segregation of remaining melt from the crystal mush. The granite thus intruded during active tectonic activity. Granite transport was focussed in the planar zone that now constitutes the Mr. Curly complex. Granite transport was by many individual lenticular batches which were arrested to crystallise as separate lenses and sheets, instead of draining into one or a few large batholiths at a higher crustal level. Contemporaneous folding of the focussing zone may have trapped melts in "granitic saddle reefs" and/or perhaps the granite formed from water-present melting of deeper sediments

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which caused rapid crystallisation on decompression with ascent. The apparent "premature arrest" of the ascending melts at Mr. Curly may give us an unusual glimpse of the transport mechanisms of granite through the crust. Page, R.W. et al. (1995). Abstract Austral. Conf. on Geochron., Curtin Univ., Perth. Sheppard, S., Griffin, T.J. & Tyler, I.M. (1995). Geochemistry of felsic igneous rocks from the southern Halls Creek Orogen. Geol. Surv. W.A. Record 1995/4, 81 pp.

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SUBECLOGITIC ROCKS AND THEIR IMPLICATIONS FOR CRUSTAL STRUCTURE IN THE WESTERN MUSGRAVE BLOCK, CENTRAL AUSTRALIA Alastair Stewart Minerals Division, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601; astewart@agso.gov.au Subeclogitic rocks — rocks metamorphosed under conditions transitional from granulite to eclogite facies — are known in Australia only in the Musgrave Block (Clarke 1993: AGSO Research Newsletter, 18, 6-7; Clarke et al. 1995a: AGSO Journal of Austrahan Geology & Geophysics, 16, 127-146; Scrimgeour & Close in press. Journal of Metamorphic Geology; Fig. 1). In the Bates 1:100 000 Sheet area, subeclogitic rocks that formed at ~40-km depth crop out over an area of 2000 km^. They are characterised by regionally developed garnetbearing coronas around mafic grains in Meso- to Neoproterozoic granulite, granite, and mafic dykes. This abstract discusses the crustal structure of the western Musgrave Block, and presents two competing schemes for explaining the present crustal structure. The Musgrave Block consists of metamorphic rocks, granites (some metamorphosed), layered mafic-ultramafic intrusions (Giles Complex), and mafic dykes. Regional metamorphic facies ranges from greenschist to subeclogite. Major east-striking low to high-angle faults cut the block and penetrate the crust. The largest is the Woodroffe Thrust. North of it, felsic gneisses and deformed granite have amphibolite-facies mineral assemblages dated at 1600-1550 Ma. South of it, felsic and subordinate mafic volcanic and shallow-water sedimentary rocks accumulated between --1580 and 1300 Ma, and were metamorphosed to granulite facies at about 1200 Ma. Voluminous granite masses dated at about 1190 Ma, outliers of the Giles Complex, and three generations of mafic dykes succeeded the granulites. The relationship of the two regions before they were juxtaposed is not known in the Bates area.

The Woodroffe Thrust dips genriy south, and formed during the Petermann Ranges Orogeny at 550-530 Ma in response to north-south compression of the Australian plate (Lambeck & Burgess 1992: Austrahan Journal of Earth Sciences, 39, 1-19). The Mount Aloysius Fault crosses the Bates area in the south, is steeply south-dipping and normal, and has granulitefacies rocks on both sides. Its existence is inferred from coincident magnetic and topographic lineaments along the northern edge of the Mount Aloysius massif, and from pressure estimates by Clarke et al. (1995a: op. cit.) of 1000-1400 MPa (equating to a depth of formation of 40 km) to the north of the fault and 300-500 MPa to its south. The subeclogitic rocks, products of regional metamorphism during the Petermann Ranges Orogeny, display ubiquitous and spectacular garnet-bearing coronas around mafic grains in the hanging-wall rocks of the Woodroffe Thrust. Mineral assemblages in the intensely deformed thrust zone range from subeclogitic to greenschist, indicating changing metamorphic conditions as the overriding rocks travelled up-dip. Where they are unaffected by subeclogitic metamorphism, garnet-bearing assemblages in intermediate and mafic granulites from the area south of Bates have yielded temperature and pressure estimates of 750°C and -500 MPa for D2. D3 pressure estimates are 400-600 MPa for

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a temperature of 700°C (Clarke et al. 1995: op. cit., p. 130). Gray (1978: Journal of the Geological Society of Australia, 25, 403-414) dated the granulite metamorphism as 1222 ± 39 Ma (Rb-Sr whole-rock isochron), which Sun & Sheraton (1992: AGSO Research Newsletter, 17, 9-10) confirmed with a SHRIMP U-Pb zircon age of 1200 Ma for synmetamorphic augen gneiss. Subeclogite-facies coronas around orthopyroxene, clinopyroxene, hornblende, and opaque grains in rocks south of the Woodroffe Thrust comprise concentric shells of garnet ± plagioclase ± biotite ± clinopyroxene ± hornblende ± rutile. Pressure estimates are consistently 1000-1400 MPa; temperature estimates range from 700-875°C (Clarke et al. 1995: op. cit., p. 141; White & Clarke 1997: op. cit.). The subeclogitic metamorphism was dated by Clarke et al. (1995b: AGSO Journal of Australian Geology & Geophysics, 16, 25-39) with Sm-Nd mineral-pair ages of 536 ± 16 and 533 ± 16 Ma for a metagabbro dyke. This agrees with Rb-Sr and Ar-Ar ages of 550-530 Ma for the Petermann Ranges Orogeny 250 km to the east (Maboko et al. 1992: Australian Journal of Earth Sciences, 39, 457-471; Camacho & Fanning 1995: Precambrian Research, 71, 155-181), and with an Sm-Nd gamet-homblende-whole-rock-mineral isochron age of 494 ± 59 Ma in the adjoining Petermann Ranges 1:250 000 Sheet area (Scrimgeour & Close in press: op. cit.). The present-day crustal structure of the Musgrave Block dates from the Petermann Ranges Orogeny. It has been determined by Lambeck & Burgess (1992: op. cit.) from teleseismic travel-time studies. Overthrusting along the Woodroffe Thrust accounts for the upward movement of the subeclogitic rocks to their present position in Bates, but the mechanism of their descent to 40 km is unclear. Following Lambeck & Burgess, I previously depicted the Woodroffe Thrust as steepening at depth, and invoked underthrusting of the footwall block (Stewart 1997: AGSO Record 1997/5, fig. 17). This left the mid-crustal granulites of the hanging wall stranded well above 40 km. Scrimgeour & Close (in press: op. cit.) presented a possible solution when they recognised the regional extent of subeclogitic rocks immediately east of Bates, and concluded that the Petermann Ranges Orogeny involved substantial crustal thickening. According to one possible sequence of events, crustal compression and thickening depressed the 1200-Ma-old mid-crustal granulites to subeclogitic-facies depths of 40 km at about 550 Ma. East-northeast overthrusting along the Woodroffe Thrust transported the subeclogitic rocks from the lower crust onto upper-crustal amphibolite-facies rocks, and accompanied high-angle reverse faulting along equivalents of the Lindsay and Wintiginna Lineaments south of the Bates area. Movement directions on the Mann Fault have long been problematical. Lambeck & Burgess (1992: op. cit., p. 17) considered it to be a thrust. However, as noted above, pressure estimates north and south of the Mount Aloysius Fault, which is a splay or en echelon offshoot of the Mann Fault to the east, indicate that lower crust south of the Mann Fault is displaced downwards relative to the hanging-wall block of the Woodroffe Thrust — i.e., the nett movement on the Mann Fault was normal. Normal (or transtensional) faulting along the Mount Aloysius-Mann Fault left a crustal wedge as the highest part of the region. The subeclogitic lower-crustal rocks in the lower portion of this wedge are preserved as the hanging-wall block of the Woodroffe Thrust in Bates.

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An alternative scheme, based on modelling of the Alps and Himalayas (Butler 1986: Journal of the Geological Society of London, 143, 857-873), involves two episodes of thrusting. Underthrusting along the proto-Mann Fault (low-angle) depresses a slab of crust to subeclogitic-facies depths. Subsequent initiation of the Woodroffe Thrust and renewed contraction elevated the subeclogitic rocks to their present level, and steepened the Mann Fault to its present attitude. The two models differ significantly in their sense of movement — normal or reverse — on the Mann Fault, and in their depiction of the Moho just south of the Bates region. The differences could be tested by detailed structural study of the Mann Fault and its adjoining rocks to determine the sense of shear, and by a deep seismic survey across this part of the Musgrave Block. Acknowledgments AGSO colleagues David Blake, Shen-Su Sun, and Peter Wellman reviewed the paper.

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RECOGNITION, STRUCTURAL SIGNIFICANCE, AND PROSPECTIVITY OF EARLY (Fj) FOLDS IN THE MINERIE 1:100 000 SHEET AREA, EASTERN GOLDFIELDS, WESTERN AUSTRALIA Alastair Stewart Minerals Division, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601; astewart@agso.gov.au In the Eastern Goldfields, shear zones and folds near major faults controlled gold mineralisation, so an understanding of their configuration is essential in the search for ore. This abstract describes large Fj folds extending over 1000 km^ in the southern part of the Minerie Sheet area. Dj faults associated with one of the folds may be prospective for gold, as in the neighbouring Leonora area. The Minerie region is located in Western Australia's Yilgam Craton ,and was mapped by AGSO from 1989 to 1995 as part of the National Geoscience Mapping Accord. It comprises a sequence of Archaean greenstones — metamorphosed felsic volcanic and volcaniclastic rocks, mafic flows and sills, ultramafic rock, and minor sedimentary rocks — intruded by granite^ rare syenite, and Proterozoic mafic dykes. In the southern part of the Minerie Sheet area, major folds (F1 and F2) and foliation trends (SI and S2; Fig. 1) are products of D^ and D2 deformation. Two key areas are critical to the interpretation. Firstly, in the east, a large north-northeast to northeast-trending open to close upright syncline is cut at right-angles in its hinge zone and west limb by a steep northwest-striking foliation. The foliation is assigned to S2 because it is a prominent schistosity that parallels the general direction of S2 foliation throughout the Eastern Goldfields. Hence, the syncline is Fj. A steep north-northeast-striking foliation, also in the hinge zone of the syncline, parallels the fold's axial plane, and is assigned to S^. The recognition of this upright Fj fold raises the possibility of others being present, and three north-northeast-trending open folds in the south may be such. Secondly, in the centre of the area, the outcrop pattern of volcaniclastic conglomerate, sandstone, and tuff of the Welcome Well Complex and its lateral felsic volcanic equivalents to the west is strongly suggestive of a gently south-dipping isoclinal Fj fold subsequently folded by a south-plunging upright F2 anticline. The strata face north on the northern limb and south on the southern limb. The F^ fold is therefore a tight to isoclinal anticline, possibly originally recumbent. Its limbs form a south-plunging arc, and its hinge zone has been disrupted and stoped by dolerite. Hence, the original attitudes of the Fj axis and axial plane are indeterminable. Symmetrically unfolding the F2 anticline about its north-south axial plane causes the Fj fold to take up an east-west trend. The north-northeast to northeast trend of the eastern F^ fold is unusual for the Eastern Goldfields. Large Dj structures generally trend easterly where they are not reoriented by D2 (Archibald et al. 1978: Precambrian Research 6, 103-131), or are parallel to D2 structures ^ 'Granite' includes syenogranite, monzogranite, granodiorite, and tonalite.

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where they have been reoriented (Swager & Griffin 1990: Precambrian Research 48, 63-73; other references in Swager 1997: Precambrian Research 83, 18). Witt (1994: Geological Survey of Western Austraha, Mehta 1:100 000 Explanatory Notes) recognised north-, east-, and northeast-trending large Fj folds reoriented by D3 in the northeast of the Melita Sheet area, immediately southwest of Minerie. Liu & Chen (AGSO Research Newsletter, November, 1998) suggest that sinistral movement along the Kilkenny Fault could have rotated originally east-west F^ folds to their present north-northeast to northeast trend during D2 and perhaps D3. Chen et al. (1998: Geological Society of Australia, Abstracts 49, 79; Western Australia Geological Survey, Annual Review 1997-98, in press) and Liu & Chen (1998: ibid., 278) accounted for the north-northeast-trending folds (Fj of this report) by local compression during east-west D3 transpression. The folds formed due to local compression induced by sinistral strike-slip on north-northwest-striking faults. The northwest-striking S2 foliation that cuts the eastern F^ fold (Fig. 2) contradicts their interpretation. Two layer-parallel faults on the western limb of the folded F^ anticline in the west may be Dj faults. A folded backthrust cutting the core of this fold is also D^. Dj shears in the Leonora area carry gold (Williams et al. 1989: Australian Journal of Earth Sciences, 36, 383-403; Williams 1998: AGSO Record 1998/9), and so the interpreted Dj faults in the Minerie area may be similarly prospective. Several small gold mines are located on or near the eastern layer-parallel fault. Acknowledgments AGSO colleagues Songfa Liu, Alan Whitaker, and David Denham reviewed and improved the manuscript.

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BRITTLE-VISCOUS DEFORMATION OF BIPHENYL: CRUSTAL SHEAR ZONE EVOLUTION

INFERENCES ON MID-

Jurgen E. Streit^ and Stephen F. Cox^ ^ Department of Geology, The Australian National University, Canberra, ACT, 0200; e-mail: j streit @ geology. anu.edu. au 2 Department of Geology and Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200 Shearing of poly crystalline biphenyl (C5H5C5H5) samples by both viscous and brittle processes was induced using a Urai-Means see-through deformation apparatus. The isostatically hot pressed samples 200 |im thick) contain monoclinic, prismatic and platy grains that resemble feldspar and epidote group minerals in shape. One sample v^as prepared dry, all others were prepared wet with fluids comprised of water-alcohol mixtures. Simple shear deformation was confined to 2 mm wide zones. Despite the high homologous temperatures of 0.94 - 0.97, fractures formed at all applied strain rates (lO"^ to 10"^ s"^). Fracture arrays with similar orientations developed in all experiments during initial shearing, whereas the microstructural development in the intervening matrix was variable, depending on the strain rate during progressive shearing. Initial fracture orientations are predominantly oblique to the shear zone boundaries and enclose angles of 10 to 20° with the latter. These fractures are up to 2 mm in length. Some fractures form angles that are larger or smaller than 10 - 20°. Only limited fracturing occurred in the dry sample, which deformed predominantly by viscous flow at the slowest strain rate. During progressive shearing, two types of strain localization occur. (1) At the fastest strain rate (10'^ s"l), a single, continuous fault zone forms along one shear zone boundary. In contrast, (2) at the intermediate (10"^ s"^) and the slowest (10"^ s'^) strain rates, a fault zone system consisting of fractures linked by microshear zones is distributed over the entire shear zone width. The microshear zones are subparallel to the shear zone boundary and are either shear fractures or zones of intense viscous flow. (1) The narrow 0.4 mm wide) fault zone that forms during progressive shearing at the fastest strain rate consists of anastomosing shear planes and dilational jogs. Progressive shearing (y < 2.5) leads to the development of principal fault planes. In compressional stepovers, shear strain is partially accommodated by viscous flow in fine-grained aggregates. (2) At intermediate and slow strain rates, fractures are usually embedded in zones of grainsize reduction. Shear fractures may still lengthen during progressive shearing, but usually rotate to enclose higher angles (15 to 30°) with the shear zone boundary. These fractures develop sigmoidal shapes, as fracture tips appear to remain at low angles to the bulk shear zone boundary. These oblique fractures become interconnected along microshear zones with orientations subparallel to the shear zone boundary. Further shearing occurs predominantly along these microshear zones and causes the obHquely oriented shear fractures to open as dilational jogs and veins. Rapid crystal growth in liquid and gas-filled jogs or veins counteracts ongoing dilation. Progressive shearing leads to the development of one dominant

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system of stepwise linked jogs at y < 2. This mixed brittle-viscous shear system extends across the entire 2 mm wide sample shear zone. In the dry sample, only one linked jog system developed. The development of a pronounced shape-preferred orientation during progressive shearing in this sample demonstrates that dislocation flow is an important deformation mechanism in the absence of fluid. The microstructures produced during hot shearing of biphenyl are similar in shape and geometrical distribution to those found at a range of scales in mid-crustal shear zones that exhibit mixed brittle-viscous behaviour. Our experiments provide insights about coupling between fracture growth, dislocation flow and mass transfer in shear zones and the progressive evolution of fracture geometries at low to moderate shear strains. In particular, the geometry of dilatant fractures, linkages with microshear zones and shear fractures, as well as styles of strain locahsation at y ^ 2 are rate-dependent. Microscale linked vein and jog structures provide useful shear sense information. The experiments also indicate that the presence of fluids in shear zones may enhance the formation of shear fractures and creates conditions favouring strain localisation over bulk viscous flow.

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THE GRAMPIANS ALLOCHTHON: AN OUTLIER OF THE WESTERN LACHLAN FOLD BELT David H. Taylor & Ross A. Cayley Geological Survey of Victoria, PO Box 500, East Melbourne, Victoria 3002 e-mail: David.Taylor@nre.vic.gov.au The Grampians provide the only fossil age control on the deformation of the bedrock in western Victoria. The Grampians lie just to the west of the terrane boundary (the Moyston Fault) between the Lachlan Fold Belt (LFB) and the Adelaide Fold Belt (AFB). The sedimentology, provenance, palaeocurrent data and deformation history the Grampians indicates that it is a structural outlier of the LFB emplaced onto a basement of AFB. The Grampians Group was previously considered to be a passive basin deposit comprising approximately 7000 m of stratigraphy, gently deformed by folds and cut by a few steep faults. New mapping (Cayley & Taylor, 1997) interprets the Grampians as a duplexed thrust stack truncated at shallow depths by a later low angle extensional decollement which gives the Grampians an allochthonous relationship to the underlying basement of AFB rocks. When the effects attributed to thrust-stacking are removed there is approximately 3700 m of stratigraphy, consisting of a lower quartzo-felspathic to micaceous sandstone package (Red Man Bluff Subgroup; 1900 m thick), an intervening micaceous mudstone dominated package (Silverband Formation; 750 m thick) and an upper, quartzose sandstone package (Mount Difficult Subgroup; 1050 m thick). The Grampians Group comprise fluvial, shallow marine, and aeolian deposits which probably accumulated along the margin of the AFB, transitional to the deep marine LFB turbidite sequence accumulating to the east. A poorly preserved fossil assemblage has been previously documented from the top of the Silverband Formation, and contains fragments of jawless (agnathan thelodont) and spinyfmned (acanthodian) fish of likely upper Silurian to Lower Devonian age with the SiluroDevonian boundary most recently placed at 418 Ma (Tucker et al, 1998). About 2500 m of sediment exist below the fossiUferous horizon and until sedimentation rates are investigated it is possible that parts of the Grampians Group may extend back into the Ordovician at 434 Ma. An upper age constraint on Grampians deformation is provided by post-tectonic Early Devonian granites (-400 Ma) their related volcanics (-410 Ma) which intrude or overlie eroded remnants of the Grampians. Therefore the Grampians must have been finally deposited, thrust stacked, extensionally segmented, and substantially eroded before the intrusion of the granites within a relatively short time period around -425-415 Ma. This estimate just starts to overlap with recent argon-argon dating of micas formed during the regional deformation of the western Lachlan (Foster, et al., 1998) which gives a range of ages clustering at -450-425 Ma. Crustal shortening at the time of this Grampians deformation is not expressed in the AFB basement. Therefore Grampians deformation is inferred to be related to the accreting LFB to the east. A marked absence of internal sediment reworking, lateral facies changes or high grade metamorphic detritus argues against the development of the Grampians Group as a foreland basin to the LFB as it was accreted against the AFB during the Silurian. Instead the Grampians appear to be the littoral equivalents of the deep marine sequences further east. Extensive post-orogenic denudation of the western LFB has probably removed much of the direct Ordovician to Silurian age deep marine equivalents to the Grampians, with only Cambro-Ordovician turbidites now exposed. Emplacement of the LFB turbidite wedge upon the AFB along the crustal ramp of the Moyston Fault caused

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shortening in the Grampians accommodated by the thrust stacking and open folding. The duplexed thrust stack is cut by a low angle decollement-the Marathon Fault-which truncates thrust faults and separates the Grampians from the underlying AFB. The fault is poorly exposed but Grampians outcrop distribution, company drilling and gravity data support a thinskinned allochthonous setting—more than 3000 m of thrust-stacked and folded sequence may be missing in places. The Marathon Fault omits stratigraphy in contrast to the earlier thrust faults, and geometrical relationships suggest that it was initiated as a listric extensional structure, segmenting the earlier thrust-and-fold stack. This fault may be one expression of post-orogenic extensional relaxation of the LFB/AFB suture. While the depositional and deformational history of the Grampians provides some field and timing constraints on the deformation of the western Lachlan the palaeogeographic relationships between the LFB and the AFB remain poorly understood. Today the eastdipping Moyston Fault is the obvious crustal suture between the two fold belts, which have clearly different structural histories. Both fold belts share the same distinctive tholeiiteboninite oceanic crust however, which suggests a linked geological history prior to the onset of the Delamerian Orogeny in the Middle Cambrian. Part of this crust was incorporated into the Tasmanian and AFB cratons during the Delamerian Orogeny, while at the same time, other parts of this crust in west-central Victoria were still in a deep marine setting and received the flush of terrigenous sediment derived from the uplifted Delamerian Highlands before cratonisation in the Silurian. The question of how different parts of the same oceanic crust were cratonised at such different times remains. REFERENCES Cayley, R.A. & Taylor, D.H., 1997 Grampians special map area geological report. Geological Survey of Victoria Report 107. Foster, D.A., Gray, D.R., Kwak, T.A.P., Bucher, M., 1998: Chronology and tectonic framework of turbidite hosted gold deposits in the western Lachlan Fold Belt, Victoria: "^^Ar/^^Ar results. In Mesothermal gold mineralisation in space and time. Ore Geology Reviews 13, issues 1-5. Tucker, R.D., Bradley, D.C., Ver Straeten, C.A., Harris, A.G., Ebert, J.R. & McCutcheon, S.R., 1998. New U-Pb zircon ages and the duration and division of Devonian time. Earth and Planetary Science Letters 158, pp. 175-186.

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STRUCTURE,STRATIGRAPHY AND PETROLOGY OF THE WESTERN WEEKEROO INLIER, OLARY PROVINCE Grant Taylor^ and Robin Oliver^ ^Aurora Gold Ltd, 24 Outram Street, Perth, ^Dept. Geology & Geophys., Uni. Adelaide, SA 5005 The western Weekeroo Inlier, Olary province, consists of Basement Palaeoproterozoic Willyama Supergroup metasediments and amphibolites overlain by Neoproterozoic Cover metasediments of the Adelaide Supergroup. The basement rocks of the area are dominated by structures of the third Olarian event. Macroscopic anticlines and synclines are open to tight, easterly plunging, with a southerly dipping axial surface. The third generation penetrative schistosity cross-cuts a former layer parallel schistosity (SI or S2) in fold closures. Abundant crenulations and kinkbands are likely to belong to the first Delamerian folding event (D4) which reactivated many basement structures of the Weekeroo Inlier. Axial surface traces of D4 macroscopic folds trend broadly N-S; the latter are overprinted in places by WNW-ESE trending D5 stuctures. A stratigraphic sequence is recognised whereby pelites ("Mica Schists") overlie psammopelites and quartz-albite rocks ("Bedded Schists"). A very broadly conformable sequence of massive, brecciated and layered amphibolite is thought to be "stratigraphically positioned" at the top of the Bedded Schists. From a consideration of abundant sedimentary structures, together with facies changes and overall stratigraphic relations, likely depositional models include a very shallow marine shelf, a broad shallow inland lake-alluvial fan toe complex, and a river-dominated, regressive deltaic-sabkha situation. Olarian metamorphic conditions ranged from those characteristic of the upper Greenschist facies to those typical of the mid-Amphibolite facies, manifested by actinolite, hornblende, epidote, albite, opaques and sphene in amphibolites, and fibrolite, andalusite, chloritoid, almandine, biotite, muscovite, sericite, quartz, minor staurolite and minor chlorite in pelites. Subsequent retrogressive metamorphism (lower Greenschist facies), exemplified particularly by complete sericitisation of formerly abundant andalusite,is considered to represent Cover deformation events of the Delamerian Orogeny. Closely associated with the amphibolite bodies of the Weekeroo Inlier are albitites and calcalbitites. Previously, a metasomatic origin was proposed for these albite rich rocks. Origin as an evaporative sediment with a possible tuffaceous component is now considered more likely. The Weekeroo amphibolites are chemically similar to ferro-tholeiites of ocean floor/mid oceanic ridge, transitional to those of a continental origin. REFERENCES Taylor, G. J. 1985. The amphibolite and metasediments of the northwest Weekeroo inlier, Olary Province. Hons thesis. Unpublished.

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GRANITE EMPLACEMENT VS. FAULTING - CONTROL OR INTERPLAY? Robert Trzebski & Paul Lennox School of Geology, UNSW, Sydney NSW 2052, Australia R.Trzebski@unsw.edu.au The mechanisms of ascent, accumulation and final emplacement of granitic magma in the mid- and upper crust are still poorly understood and remain the subject of controversy. Although various modes of magma emplacement were proposed ranging from ascent along faults, diking, "hot Stokes" diapirism and ascent during heterogeneous flow, none of these means of magma transport is fully constrained by the field evidence. Furthermore, as these models are mainly restricted to observations at outcrop level, they lack the spatial control at depth. In recent years structural and gravity studies, however, increasingly demonstrated a spatial relationship between granite plutons and faults suggesting a genetic link between granite emplacement and coeval faulting. In this study we compare four different crustal environments of pluton emplacement and regional deformation, intrusion level and timing, geometry of the plutons and their proximity to adjacent faults, I- and S-type granites in terms of their deformation style and response to deformation. We use examples from the Eastern Lachlan Fold Belt and the Bohemian Massif (Germany/Czech Republic) and compare these with the results of similar studies in the Armorican Massif (France) and the Southern Appalachians (USA) by Paterson and Schmidt (1998). These authors suggested that the magma ascent and emplacement is a relatively unfocused process within orogenic belts and that faults do not preferentially facilitate magma transfer in the crust. In contrast, the work in the Eastern Lachlan Fold Belt (Trzebski et al. 1998) and the Bohemian Massif (Trzebski et al. 1997) shows a close spatial relationship between plutons and major faults implying a relationship between magmatism and regional deformation. However, these studies were mainly restricted to surface observations and further detailed evaluations of the relationships between populations of igneous bodies and structures in a wide variety tectonic settings and crustal depths are therefore required for an adequate analysis. We will present a comparison between magma emplacement in the mid- to upper-crust during brittie and/or ductile deformation. Distinctive shapes of plutons result during different deformations styles, i.e. compression, extension and transcurrent faulting which on the other hand is related to the timing between granite emplacement and the "peak" of the tectonometamorphism. The comparison between S- and I-type granites showed a clear difference in their geometry which is due mainly to the difference in their rheology and their response to deformation (e.g. Vernon & Flood 1984). The exposure level, i.e. the erosion rate in the relevant region, is an important criteria for a valid comparison. Large plutons are commonly heterogeneous bodies with structures which considerably vary, for instance, between the roof zone and the base. Further essential aspects refer to the dating of granite emplacement and fault propagation, i.e. the question arises whether the magma used the fault as a channel for emplacement or the pluton and the host-rocks impose a rheological anisotropy for fault propagation.

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A STRUCTURAL REINTERPRETATION OF THE SOUTHERN EYRE PENINSULA, SOUTH AUSTRALIA J J Vassallo and C J L Wilson (j.vassallo@pgrad.unimelb.edu.au) School of Earth Sciences, The University of Melbourne, Parkville VIC 3052 Four orogenic events have previously been recognized in the Gawler Craton, southern Eyre Peninsula, South AustraHa, spanning the period from late-Archean to mid-Proterozoic. The true structural significance of these events, the -2440 Ma Sleafordian Orogeny, -2000 Ma MiltaHe Event, -1850-1710 Ma Kimban Orogeny and the -1500 Ma Wartaken Event, remained unclear from previous investigations. To test this geologic framework and unravel the structures of this region a 50 km section of well-exposed coastal outcrop was examined and key areas were mapped in detail (1:2000 scale and less). Three main rock packages were confirmed: a) the -2440 Ma para-orthogneissic Sleaford Complex, b) the -1950-1850 Ma Hutchison Group metasediments, and c) the -1850 Ma Lincoln Batholith. At least two sub vertical mafic dykes sets, the -1850 Ma synplutonic Jussieu dykes and -1800 Ma Toumefort dykes, were found to intrude these packages. The effects of the Kimban Orogeny controls the gross structure of the region. It has overprinted the earlier events to such an extent that the Sleafordian and Miltalie Events are preserved only as gneissic fabrics. However, Kimban low strain zones preserved in the Lincoln BathoHth, show that there is evidence of syn-Lincoln deformation, that we define as the Lincoln Orogeny. The structures of the Lincoln Orogeny are preserved as high-grade reverse shear zones, pinch and swell of Jussieu dykes and folded parallel magmatic/solid state fabrics indicative of an east-west compressional event. The Lincoln Orogeny is post-dated by the intrusion of a voluminous set of tholeiitic, mutually cross-cutting mafic dykes consisting of at least six separate swarms, the Toumefort dykes. Where preserved Toumefort dykes form a series of left and right-stepping en-echelon arrays indicating intmsion perpendicular to the least compressive stress, Gj. The similar intrusion style of all the Toumefort dykes and their bulk volume throughout the region suggests they penetrated the crust during a regime dominated by tensional forces, -1800 Ma. To some extent all the Toumefort dykes have been affected by the Kimban deformation, hence restricting the boundaries of the Kimban Orogeny to a period between -1800-1710 Ma. The Kimban Orogeny consists of two amphibolite to granulite facies deformational events (KD|, KD2) of identical structural style, yet with opposing senses of vergence. Both KDj and KD2 are controlled by the production of reactivated overthrust shears. KDj structures indicate transport to the west with the formation of a high-grade gneissosity, reverse east-up shears with steep (>80°) and gently plunging (0-30°) amphibole/feldspar elongation lineations and localization of the strain into these discrete zones. Offsets range from 2 m to several hundred metres as transpressional zones with a large strike-slip movement component. KD2 reoriented and completely transposed KDj structures as it was the highest strain event in the region. KD2 shear zones are both steep and flat-lying with many feldspar/amphibole elongation lineations dipping to the south indicating bulk rock movement from the southwest to the northeast. In both events crustal stacking occurred in the form of nappe and duplex development controlled by major reverse shears with smaller shear splays forming a listric pattern. The shears alter from steep (>70°) to gentle dips (020°) controlling the local fold style of an area up to 5 km across strike. Where they gently dip

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Strain was focussed in a constrictional manner and nappes formed. The continuance of rock movement after the shear zones seized then caused upright open folds to develop, hence deforming the shear zone during the same deformation. Where shear zones are steep strain is predominantly of a flattenning type with tight upright folds developed and the shear itself is not folded. The structural development of the region was controlled by changes in metamorphic grade/rock fluid content thereby enhancing and reducing rheological contrasts. During cooler metamorphic conditions mafic dykes behaved in a brittle manner with respect to surrounding metagranitoids causing the development of boudins. An increase in grade caused both the mafic dykes and metagranitoids to behave in a rheologically similar fashion, hence forming pinch and swells. This apparent dyke weakness during high-grade conditions encouraged shear zones to nucleate within the mafic lithologies while the surrounding metagranitoids remained relatively undeformed. The Wartaken Event is a compressional event of minor structural significance. It produced mainly weak open warps folding previously-formed lineations and strike-slip shear zones offsetting the earlier structures. Compression was from the east-southeast. The end of the Wartaken Event marked the cratonization of the Gawler Craton.

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FOLD NAPPES AND THE PROBLEM OF MODEL DRIVEN INTERPRETATION Paul F. Williams Dept of Geology, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada In the last century fold nappes were recognised for the first time in Scandinavia and the Alps. At that time they were recognised as ductile structures and the emphasis was on observation and documentation. The problem of how folds of such large amplitude develop was recognised, but not solved. The details of these structures vary, but commonly they have an amplitude measurable in kilometres to tens of kilometres, there is evidence of intense strain at all scales and there are multiple generations of folds. For example, in the Pennine and Austro Alpine Nappes of the Alps and the Caledonian Nappes of northern Norway and Sweden, packages of rock of very different age and origin are juxtaposed and layering is multiply folded and transposed into parallelism with the nappe boundaries. Folds are rotated into parallelism with a stretching lineation, commonly interpreted as the transport direction, and they may or may not have sheath fold geometry. In the present century foreland fold and thrust belt type tectonics were investigated and thoroughly described in the Rocky Mountains. The deformation processes involved were modeled extensively and it is probably fair to say that this is a well understood deformation environment. As geologists working in the Canadian Cordillera extended their sections further west into rocks deformed under middle crustal conditions they applied the Rocky Mountain model to their interpretation, concentrating their attention on boundaries between tectonic units and largely ignoring the significance of the internal deformation. Thus for example the Monashee Complex which forms a structural dome is interpreted as a thrust duplex, separated from overlying rocks by a roof thrust. This model-driven approach has been applied elsewhere so that we see a general reinterpretation of nappes from Scandinavia to the Moine Thrust to the Himalayas, in terms of the foreland fold and thrust belt model. Concurrent with this trend has been an increasing disenchantment with studies of "postage stamp areas", i.e. with detailed studies of deformed rocks. A problem with such models as applied to deep crustal rocks is that they offer no explanation for the intensity of ductile strain nor do they allow explanation of the large recumbent folds. If we continue trying to force all data into such models into the next century I think the prospect is going to be as intellectually bleak as Scrooge's prospects were personally bleak. Detailed structural analysis is slow and our input into tectonics, as structural geologists, is consequendy small. It is commonly argued that it is the big picture that matters in tectonics and not the details which are too complex to worry about. However, a model that does not offer an explanation for the details is inadequate. It was reasonable to test the foreland fold and thrust belt model in deep crustal settings, but if we are to progress it is time to look for and to test new models. A possible explanation of fold nappes is that they start as upright folds and are modified by horizontal flow. This flow takes place under middle crustal conditions and involves large shear strains. The deformation is what might be expected in a subduction zone but the metamorphism is not; so that some other large scale, middle crustal, high shear environment

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is required. Boundaries between nappes may be coeval shear zones or they may be earlier structures, such as, for example, transcurrent faults, rotated and modified by shear. If such a model is applicable many of our interpretations of fold nappes will require considerable modification. This alternative model is discussed using the Monashee Complex of the Canadian Cordillera as an example.

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DECONSTRUCTING THE BINDIAN DEFORMATION IN EASTERN VICTORIA Willman C.E., Morand VJ., Haydon SJ., Hendrickx M.A. & VandenBerg A.H.M. Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002 E-mail: Clive. Willman@nre.vic.gov.au The Omeo region lies near the junction of four structural zones of the Lachlan Fold Belt: the Omeo, Tabberabbera, Buchan and Mallacoota zones. A complex structural history resulted because three major periods of deformation occurred over a relatively short interval of about 45 Ma (Early Silurian to Middle Devonian). The overall effect was the wholesale southeastern transport of the Omeo and Tabberabbera zones which led to collision with the Buchan and Mallacoota zones in the Late Silurian. This was the Bindian Deformation. The Early Silurian Benambran Deformation deformed the Omeo and Tabberabbera zones which were then parts of a single large zone subjected to low-pressure metamorphism. During the Silurian the newly formed crustal blocks were subjected to localised extension and substantial southeast tectonic transport. The Tabberabbera-Omeo zones probably started to move in the mid-Silurian, culminating in the Bindian Deformation at about the Silurian-Devonian boundary. This movement was concentrated along a number of large marginal strike-slip faults which link with thrust faults at the leading edges of thrust sheets. The Tallangatta Creek Fault Zone (TCFZ) accommodated large scale internal dislocation within the Omeo Zone causing differential movement between two subzones, the High Plains Subzone in the west and Corryong Subzone in the east. Initial sinistral strike-slip movement along the TCFZ led to the deposition of the Mitta Mitta Rhyolite, Wombat Creek Group and Mount Tambo Group (MTG) in a pull-apart basin. At the same time the Limestone Creek Graben (LCG) formed north of the Nunniong Domain of the Buchan Zone. Deposition of the Wombat Creek and Mount Tambo Groups, and of rocks in the LCG, was terminated near the end of the Silurian. Their contents were deformed by the continued southeastward movement of the Corryong Subzone which was thrust over the LCG along the Indi Fault. The TCFZ links with the Indi Fault which probably underlies the subzone at depth as the sole thrust. Further northeast the Indi Fault links with the sinistral strike-slip Gilmore Fault. The Corryong Subzone therefore forms a sheet bounded to the west by the dextral strike-slip TCFZ and to the east by the sinistral Gilmore Fault. As the Corryong Subzone thrust over the Buchan Zone it became tilted, exposing progressively lower stratigraphic units (and higher metamorphic grade) towards the Indi Fault. Another Late Silurian (Bindian) linked fault system occurs along the boundary of the Tabberabbera and Omeo Zones. During southeastward transport, the zones were separated by dextral strike-slip movement along the Cassilis Shear Zone which links the Kiewa Fault Zone with the Ensay Shear Zone, both dextral strike-slip faults. This caused collision of the Tabberabbera Zone and High Plains Subzone with the Buchan and Mallacoota Zones. The Ensay Shear Zone and the Yalmy Fault Zone may be hnked under the Buchan Rift. The Yalmy Fault Zone is inferred to be a major northwest-dipping thrust fault which forms part of the leading edge of this thrust sheet, cutting earlier east-west Benambran thrusts. Interpretation of aeromagnetics suggests the boundary between the Tabberabbera and Mallacoota zones runs north-south and is cut by the Ensay-Yalmy fault system. This in turn indicates the amalgamation of the Tabberabbera and Mallacoota zones occurred either early in the Bindian Deformation or during the Benambran Deformation.

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A thorny problem is the MTG, lying at the junction of the TCFZ and the Indi Fault. It is now known to be Late Silurian, and its structure trends at about 90° to Bindian trends in the adjacent LCG. However, mid-Devonian reactivation of the Indi Fault and the TCFZ has probably caused the present juxtaposition of these fault blocks, which may have originated some distance apart. Structural trends of the MTG are parallel to Bindian trends in the Wombat Creek Graben sited further north along the TCFZ. For this reason the MTG is thought to have been linked to the Wombat Creek Group in a pull-apart basin within the TCFZ.

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Fig. 1. Development of Omeo region during the Late Silurian to Eariy Devonian, showing movement of various zones and subzones (arrows). A. Early Bindian Deformation (latest Silurian). B. Middle Bindian Deformation. C. Late Bindian Deformation (Earhest Devonian). D. Early Devonian (post-Bindian). Oudines of Omeo and Benambra 1:100 000 sheets shown.

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FAULTING PROCESSES AT UPPER CRUSTAL LEVELS; AN EXAMPLE FROM THE NORTHERN SYDNEY BASIN, NSW. T. Wilson^ R. Offler^ and S.F. Cox' 1. twilson@geology.newcastle.edu.au, Department of Geology, The University of Newcastle, CALLAGHAN, NSW, 2308 2. Department of Geology and Research School of Earth Sciences, The Australian National University, CANBERRA, ACT 0200 Normal, reverse and strike-slip faults are developed throughout the Permian coal measures of the northern part of the Sydney Basin. Awây from the thrust-controlled north-east margin of the Sydney Basin, steeply-dipping normal faults are more prevalent and predominandy trend NW, N and NE. This study focuses upon twô subparallel, N-S trending, steeply E dipping, normal faults which crop out in coastal exposure approximately 6km SW of Newcastle. The two principal slip surfaces (PSS's) define a complex fault bounded block approximately 1.5 m wide. A total displacement of 10m is observed on the west bounding fault. Stratigraphic separation between the fault bounded and hangingwall blocks indicate a vertical displacement of 3m along the east bounding fault. Depth of burial during faulting is estimated at 3-6km (ie. within the continental seismogenic regime), based on temperatures obtained from vitrinite reflectance (Ro=0.732, T=110°C, burial regression curve of Barker 1993), and assuming geothermal gradients ranging from 18°-30°C/km. The effects of brittle deformation processes along PSS's, within footwall and hangingwall damage zones and the fault bounded block, vary in their intensity and extent. The footwall damage zone extends up to 5m within interbedded siltstone and coal. Subsidiary normal faults, exhibiting a maximum of 15cm displacement, are developed within this zone. In addition, a subvertical, extensional joint set is locally developed and increases in density with decreasing distance from the PSS. Within competent tuff units of the footwall, minimal damage is observed. Damage zone structures observed within the hangingwall extend over 1.5m within tuff units. Minor subsidiary normal faults, with maximum displacements of 20cm, are also developed. Narrow zones of closely spaced, variably oriented fractures are observed within the hangingwâll immediately adjacent to the PSS.

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Spatial accommodation processes occurring within the fault bounded block have generated localised slip and associated rigid block rotation along synthetic and antithetic normal faults and reverse faults. Bedding parallel slip is also observed along incompetent - competent rock interfaces, resulting in displacement of subsidiary reverse faults and localised development of a coarse millimetre scale 'foliation'. Competent material wîthin the fault bounded block is characterised by intense, closely spaced fractures. Abrasive wêar along PSS's has produced slickenlines and incohesive attrition breccias. A footwall horse also occurs wîthin a 20cm wîde attrition breccia zone. In contrast, increases in fracture density has produced cohesive dilational breccias within fault block material adjacent to subsidiary slip surfaces. Clay-rich gouge zones, with maximum widths of 1cm, are developed along parts of both PSS's. Development of gouge is typically associated with both footwalls, however, a stepover from footwall gouge to hangingwall gouge occurs along the east bounding PSS. Anastomosing subsidiary slip surfaces, with associated layered gouge development, encompass large, internally brecciated coal fragments within the fault bounded block. Structures associated with PSS's are interpreted as the possible result of co-seismic wear and damage processes. Some subsidiary slip surfaces within the fault bounded block and damage zone may have developed during aftershock sequences and interseismic creep.

Reference: BARKER C.K. 1993. Calibration of a vitrinite reflectance geothermometer using peak temperature from fluid inclusions. American Chemical Society, Division of Geochemistry. National Meeting, Chicago. Abstract 19.

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HIGH PRECISION RELATIVE THERMOBAROMETRY Brenton Worley*, Roger Powell School of Earth Sciences, University of Melbourne, Parkville, Vic 3052, Australia and Mike Sandiford Department of Geology and Geophysics, University of Adelaide, S.A., 5005, Australia *b.worley@earthsci.unimelb.edu.au Thermobarometry is now a routine technique which is applied by metamorphic and even structural geologists the world over. It has been revolutionised during the \computer" decades of the 80's and 90's, primarily in response to the advent of internally consistent thermodynamic data sets and software for performing multi-equilibria thermobarometric calculations. The accuracy and reliability of thermobarometry is constantly being refined by the incorporation of new experimental data, and more realistic activity-composition {a-x ) models. However, with realistic (2o) uncertainties on the order of ±1.5-4 kbar and ±40120°C, the absolute precision of thermobarometry remains relatively poor, and consequently our present ability to constrain geodynamic processes via thermobarometric calculations is limited. In general, precise knowledge of the absolute PT conditions at which a given assemblage equilibrated is not particularly critical. In fact, subde spatial variations in pressure and temperature are potentially much more useful in terms of constraining metamorphic and structural processes. Thermal and baric field gradients, for example, may provide important constraints on exhumation rates and relative contributions of advective and conductive heat transfer during orogenesis (Day 1987, Chamberlain & Sonder 1990, Sonder & Chamberlain 1992), while mesoscopic pressure gradients may hold the key to constraining the magnitude of the deviatoric stresses which drive deformation. Recognition of the nature of different sources of input uncertainties in thermobarometry has enabled the development of a high-precision thermo-barometric procedure for the calculation of PT differences, the APr approach. For example, if a thermobarometer is applied to two rocks with the same assemblage, then the contribution from the uncertainties associated with a-x models and thermodynamic data is almost identical in each case, leading to strongly correlated PT results. Such uncertainties are systematic, in contrast to uncertainties related to analytical imprecision, which are sample specific and hence random (e.g. Powell & Holland 1988). The APT method involves the rigorous propagation of input uncertainties to give uncertainties on pressure and temperature differences, AP and AT . The correlations which arise from the systematic uncertainties mean that the effect of these uncertainties is dramatically reduced. Such improvements should enable the calculation of subde variations in pressure and temperature, so that tectonic processes may be quantitatively constrained. References Chamberlain, C. & Sonder, L. 1990. Heat-Producing elements and the thermal and baric patterns of metamorphic belts. Science 250 : 763-769. Day, H. 1987. Controls on the apparent thermal and baric structure of mountain belts. Journal of Geology 95 : 807-824.

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Powell, R. Holland, T. 1988. An internally consistent thermodynamic data set with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program, Journal of Metamorphic Geology 6 : 173-204. Sonder, L. & Chamberlain, C. 1992. Tectonic controls of metamorphic field gradients, Earth and Planetary Science Letters 111: 517-535.

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THE ROLE OF DISSOLUTION-PRECIPITATION IN THE COMPACTION OF GRANITIC GOUGE MATERIALS Shuqing Zhang and Stephen F. Cox (e-mail: Shuqing.Zhang@anu.edu.au) Research School of Earth Sciences The Australian National University Canberra, ACT 0200

Recent seismological, heat flow, and stress measurements and structural analysis of fault zones suggest that fluid pressures well in excess of hydrostatic may be prevalent near the base of the seismogenic zone. The existence of suprahydrostatic fluid pressures within an active fault zone depends on the development of mineral seals within faults or between fault zones and their country rocks in the intervals between earthquakes. Whether a mineral seal will be formed depends on the evolution of porosity and permeability in the porous gouge materials of an active fault zone relative to its surrounding country rocks. We have conducted laboratory hydrothermal hot-pressing on synthetic granitic gouge materials to determine the kinetics of dissolution-precipitation creep and thus to evaluate its role in the development of overpressurized fluid pressure within crustal fault zones. We have examined the effects of temperature, pore fluid pressure, and deviatoric stress on the kinetics of dissolution-precipitation of granitic gouges. The granitic powder was prepared by mechanically crushing Westerly granite and passing through a sieve of # 170 mesh (<87 |Lim). Westerly granite has a modal mineral composition of 35% microcHne, 32% plagioclase, 28% quartz, 5% mica and < 1% magnetite. The granitic powder was first cold-pressed and then hydrothermally cooked up to 100 hours at temperatures of 400600°C, confining pressures of 200-300 MPa, and pore water pressures of 50-200 MPa. We measured both porosity and permeability in situ as a function of time. From the data of porosity reduction rates at similar total porosities but at different temperatures, the activation energy for compaction of granitic gouge is derived to be about 6 6 - 1 1 4 kJ/mole. Porosity reduction rate increases with increasing effective confining pressure (confining pressure minus pore water pressure). At the same effective pressure, the porosity reduction rate also increases with increasing pore fluid pressures. Applications of differential stress at the same effective mean stress significantly increase the porosity reduction rate in the direction parallel to the maximum principal stress direction. Creep tests on the hydrothermally hot-pressed specimens at constant effective pressures give a stress exponent of about 1.5. Observation of fractured surfaces of hot-pressed granitic gouge under SEM reveals many features indicative of dissolution-precipitation process. In specimens that went through long duration of hydrothermal hot-pressing at 400°C, quartzfeldspar particles larger than micron sizes remain their angular shapes and point contacts between particles are common. However, dissolution pits exist on many large particle surfaces and submicron-sized particles are euhedral-shaped. At 600°C, micron-sized, euhedral-shaped grains of quartz, potassium feldspar and plagioclase are widespread. Flat surface contacts are common. Ridge-channel surface structures are present on many large grain surfaces. Growth of euhedral biotite grains occurred within pore spaces between large granular grains.

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The strong effects of pore fluid pressure and also effective pressure on porosity reduction rates are consistent with fluid diffusion-controlled solution transfer compaction. Pore fluid pressure and the effective pressure influenced the compaction rates via their effects on silica concentration. With a knowledge of the effects of physical parameters on compaction rates, we have re-analysed a number of previous sets of hydrothermal compaction data and attempted to extrapolate them to crustal fault zones. The extrapolation sheds light on the pore fluid pressure evolution within crustal fault zones.

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A CLARIFICATION OF AN ISSUE REGARDING THE INFLUENCE OF INITIAL IRREGULARITIES UPON FOLD SHAPE. Y. Zhang, B. E. Hobbs, A. Ord and H. B. Miihlhaus AGCRC, CSIRO Exploration & Mining, Nedlands, WA 6009 Zhang et al. (1995) shows that for a single layer with elastic-viscous (Maxwell) constitutive behaviour, embedded in a Maxwell solid of lower viscosity and elastic moduli, the Biot wavelength is generated independently of the distribution of initial periodic small geometrical perturbations. On the other hand, Mancktelow (1998) shows that for exactly the same mechanical properties, initial geometrical perturbations have a strong influence on the resulting wave form, and that the Biot wavelength is not generated. The difference between these two studies lies solely in the imposed strain rate. For Zhang et al. (1995), this was E-6 /s, while for Mancktelow (1998), this was E-14 /s. Mancktelow's (finite element) results can be exactly reproduced using Zhang et al's (finite difference) computer code, so the difference in results does not reside in the computer code used. The implication of Mancktelow's work is that the Biot wavelength is never developed under geological conditions and that initial irregularities always play a fundamental role in controlling the shapes of natural folds whereas the impUcation of Zhang et al's work is that initial small irregularities never play a role in controlling fold shape and the Biot wavelength is always developed. This paper shows that both Zhang et al's results and Mancktelow's results are consistent with each other; they simply represent mechanical responses in different parts of strain rate-viscosity -elastic moduli space. The essential difference in the mechanical responses between the Zhang et al. and Mancktelow studies is that in the Mancktelow study, almost all (greater than 90%) of the deformation consists of a homogeneous shortening with very small contribution due to buckling, whereas in the Zhang et al. study, the reverse is true, and most of the deformation arises through buckling after an initial, relatively small, homogeneous shortening deformation. Hence, in the Mancktelow study, initial perturbations are (mostly) passively amplified during homogeneous deformation, whereas in the Zhang et al. study, there is dynamic amplification of selected perturbations which ultimately grow to produce the Biot dominant wavelength. A closer inspection of the relevant time scales reveals that in Mancktelow's study, the Deborah number (time scale of interest/relaxation time = l/(growth coefficient of dominant mode * relaxation time) ) is larger than unity, which explains why unstable structures are not amplified in that study. In this paper, we explore not only differences in viscosity contrast and elastic contrast, but also the influence of strain rate and of changes in the absolute values of viscosity and elastic moduli, upon the subsequent mechanical response. The result is that for geologically realistic strain rates (E-14 /s through to E-10 /s) and for geologically realistic values of the viscosity and of the elastic moduli, the mechanical response can be either homogeneous shortening or Biot amplification, depending on the strain rate and on the absolute magnitude of the constitutive parameters. We demonstrate the complete transition between Biot amplification and homogeneous shortening depending on these various parameters and on the strain rate. If there is a strong control of fold shape by initial perturbations, this tells us that homogeneous shortening was a dominant mode of deformation. On the other hand, if the

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wave form is what one would expect from Biot theory, then buckling is the dominant mode. This enables us to say a few important things about where we are in strain rate-viscosityelastic moduli space. Thus, we believe that fold shape can be a sensitive indicator of where one is in strain rate-viscosity-elastic moduli space. References Zhang, Y., Hobbs, B.E., Ord, A. & Muhlhaus, H.-B. 1996. Computer simulation of single layer buckling. J. Struct. Geol. 18, 643-655. Mancktelow, N. S. 1998. Finite-element modelling of single-layer folding in elasto-viscous materials: the effect of initial perturbation geometry. J. Struct. Geol. (in press).

Figure 1. a) Initial geometry of the model; note periodic small geometrical perturbations exist in the central layer and only part of the matrix is plotted, b) The final geometry of an example showing the domination of homogenous shortening and passive fold amplification at 30% bulk shortening; strain rate is le-14 /s, viscosity, bulk modulus and shear modulus are 2e21Pa s, 4.666ell Pa and 2.8el 1 Pa for the layer respectively, and lel9 Pa s, 2.333e9 Pa and 1.4e9 Pa for the matrix respectively, c) The final geometry of an example showing the domination of Biot dynamic fold amplification at 30% bulk shortening; strain rate is le-14 /s, viscosity, bulk modulus and shear modulus are 2e23Pa s, 2.333elO Pa and 1.4el0 Pa for the layer respectively, and le21 Pa s, 1.1665e8 Pa and 0.7e8 Pa for the matrix respectively. Note that the value of the ratio of viscosities and of elastic moduli for layer and matrix for both (a) and (c) is 200.

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3D COUPLED MECHANICAL/FLUID FLOW MODELLING OF THE YILGARN BLOCK, WESTERN AUSTRALIA Y. ZhangS A. Ord^ B.E. Hobbs\ J.L. Walshe^ and N. Archibald' ^AGCRC, CSIRO Exploration & Mining, PC Box 437, Nedlands WA 6009; zhang@ned.dem.csiro.au ^Fractal Graphics, Nedlands WA 6009 The interaction between deformation and fluid flow in the Eastern Goldfields Province, Yilgam Block has been investigated by Upton and coworkers (1997; also see Hobbs et al 1998) in their 2D models based on the Yilgam deep seismic transect (Drummond and Goleby 1993). One implication of their results is that fluids focus into shear zones in greenstones, with two major faults (Ida fault and Bardoc shear) providing crustal scale channels. In this study we expand the model into a 3D space and aim to explore the controls of fault structures on fluid flow, in particular the effect of ENE fault structures (Archibald 1998). This paper reflects the preliminary results of the study. Our model simulates a lithospheric block of 140 x 120 x 53 (depth) kms. The block consists of the upper crust, lower crust, lithospheric upper mantle and a greenstone layer (within the upper crust). Four faults are simulated in the model. They are the Ida fault, the Bardoc shear and the Zuleika fault, all the three being parallel to the NNW-SSE regional structural trend, and a ENE fault which cross-cuts the three other faults. This lithospheric architecture is basically consistent with the interpretation of the Yilgam deep seismic transect (Drummond and Goleby 1993). Initial pore pressure is an important starting hydrological condition in the model. Both hydrostatic and lithostatic fluid pressure gradients are considered here. A shortening deformation is achieved for the modelled block by applying horizontal velocity boundary conditions. The results confirm that an overpressured system (i.e. lithostatic pore pressure gradient), on deformation, is dominated by significant fluid vertical upflow. Under these conditions, mineralised fluids may migrate long distances from deep sources to shallower deposition locations. In contrast, fluid flow in an underpressured system (hydrostatic) is much weaker and fluid down flow is possible. This provides an easy way for meteoric water to reach deeper locations in the crust. We speculate that the spatial and/or temporal combinations of high and low fluid pressure gradients may be sufficient to result in fluid mixing between meteoric water and fluids from deep sources. The faults are shown to be major channels for fluids to migrate upwards. The Ida fault is particularly important because it accesses great depths in the crust, possibly to the Moho. This fault efficiendy extracts fluids from the lower crust or fluids generated from fluid sources near the Moho, and transports them into the upper crust where other faults can possibly take a role. The model also shows that the ENE fault appears to be even more efficient in transporting fluids if it does reach the lower part of the upper crust or even the lower crust, probably because of the orientation of this fault (at small angle to the shortening axis). In the shallower levels of the crust (e.g. the greenstone layer), such a fault is more likely to play a dominant role in focusing fluids into the greenstone layer and may itself provide space for the formation of the ore body.

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References Archibald, N. 1998. 3D geology and tectonic synthesis of the Kalgoorlie Terrane. In: abstract volume of AGCRC workshop on Geodynamics and Gold Exporation in the Yilgarn, pp 17-21 (August 1998, Perth). Hobbs, B.E., Ord, A. and Walshe, J.L. 1998. The concept of coupled geodynamic modelling with special reference to the Yilgarn. In: abstract volume of AGCRC workshop on Geodynamics and Gold Exporation in the Yilgarn, pp 36-39 (August 1998, Perth). Upton, P., Hobbs, B.E., Ord, A., Zhang, Y., Zhao, C., Drummond, B. & Archibald, N. 1997. Thermal and Deformation Modelling of the Yilgarn Deep Seismic Transect. In: abstract volume of AGCRC Geodynamics and Ore Deposits conference, pp 22-25 (19-21 Feb. 1997, Ballarat, Victoria). Drummond, B. and Goleby, B.R. 1993. Seismic reflection images of the major ore-controUing structures in the Eastern Goldfields Province, Western Australia. Exploration Geophysics 24, 473-478.

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TECTONOTHERMAL EVOLUTION OF ARCHAEAN BASEMENT ROCKS FROM THE EASTERN ZONE OF THE NORTH CHINA CRATON: IMPLICATIONS FOR MANTLE PLUME TECTONICS GUGCHUN ZHAG, S. A. WILDE & P. A. CAWGGD Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPG Box U1987, Perth, WA 6001, Australia (Zhao@lithos.curtin.edu.au) The eastern zone of the North China Craton consists mainly of pre-tectonic high- and low-grade gneisses, syn-tectonic granitoids and minor amounts of ultramafic (komatiitic) to mafic volcanic and sedimentary supracrustal rocks. High-grade gneisses dominate the Eastern Hebei, Miyun-Chengde, Eastern Shandong, Northern Liaoning and Southern Jilin domains, and low^-grade gneisses (granite-greenstone belts) occupy the Western Shandong, Southern Liaoning and Anshan-Benxi domains. In both high- and low-grade domains, gneisses and granitoids make up more than 80% of the total exposure. Supracrustal rocks, although volumetrically subordinate, are present throughout the domains. Basement rocks from all domains in the eastern zone of the North China Craton, regardless of their ages, metamorphic grades and compositions, show substantially similar metamorphic characteristics. They are all characterized by anticlockwise P-T paths. The similarity in evolution of metamorphic rocks from these domains is clearly not an artefact of thermobarometry, since the inferred P-T paths are well constrained by metamorphic reaction textures, as well as by the P-T estimates. Most mafic granuhtes, amphibolites and some pelitic rocks in the eastern zone preserve the prograde, peak and post-peak nearly isobaric cooling textures. The prograde metamorphic textures are indicated by inclusions within minerals developed at the peak stage, and are represented by assemblages of hornblende + plagioclase -h quartz ± biotite in mafic granulites, chlorite + actinohte -f- epidote -h plagioclase + quartz in amphiboHtes and biotite -h plagioclase + quartz in pelitic gneisses. The peak stage is shown by assemblages of orthopyroxene -h clinopyroxene + garnet + plagioclase -h quartz from the mafic granulites, hornblende -f- plagioclase -h quartz -h garnet from gametiferous amphibolites and garnet -h sillimanite -h plagioclase + quartz -h biotite from peUtic gneisses. Near-isobaric cooling textures are exhibited by garnet -h quartz symplectic coronas in mafic granulites, actinolite -h garnet (rim) retrogressive rims around garnet or hornblende grains in amphibolites, and kyanite replacing sillimanite or staurolite replacing sillimanite -h garnet in pelitic gneisses. These textural relations and their P-T estimates define anticlockwise P-T paths for the basement rocks in the eastern zone. The calculated anticlockwise P-T-t paths reflect an origin related to the intrusion and underplating of large amounts of mantle-derived magmas that not only provide heat for the metamorphism but also add a large volume of mostly mafic material to the base of the crust. Large volumes of underplating magma leading to metamorphism with an anticlockwise P-T path may occur in continental magmatic arc regions (Wells, 1980; Bohlen, 1991), mantle plume tectonic regimes (hot-spots) (Bohlen, 1991) and continental rift environments (Sandiford and Powell, 1986). The continental rifting and continental magmatic arc models are inappropriate for the origin of the basement rocks in the eastern zone as they cannot explain the widths of exposed basement rocks, dominant oval structures and lack of intrusion

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of abundant mafic dykes which are commonly associated with rifting and continental magmatic arc regions. And also, the continental rifting and continental arc models cannot reasonably explain the occurrence of anomalously high-temperature komatiitic rocks in the basement of the North China Craton. Alternatively, we favor a mantle plume (hot-spot) model to explain the origin of the basement rocks of the eastern zone of the North China Craton. Combining the mantie plume thermal structure with the available lithological, structural, metamorphic and geochronological data, we propose the following tectonic scenario for the formation of the basement rocks in the eastern zone of the North China Craton: (1) In the Early Archaean there was a primitive sialic continental crust which is represented by the granitic gneiss of c. 3800 Ma in the Anshan domain and fuchsite quartzite of c. 3800 Ma in the eastern Hebei domain. The source and initial location of their material is unknown. (2) In the Mid-Archaean, enormous volumes of ultramafic to mafic volcanic rocks and pre-tectonic TTG plutons were produced owing to the interaction of upwelling mantle plumes and lithosphere. The huge plume head uplifted the mantle lithosphere and overlying continental crust and caused lithospheric stretching, leading to extensive eruption of ultramafic to mafic volcanism (komatiites and basalts). At the same time, the heat transfer from the plumes to the upper mantle or lower crust resulted in extensive partial melting of basaltic or amphibolitic rocks to form large volumes of light TTG magma which becomes unstable at the base of the crust and rises diapirically into the lower and upper crust, forming the domes. The relief associated with the diapiric intrusion of the TTG plutons led to local erosion and sedimentation, which is represented by minor amounts of sedimentary supracrustal rocks in the zone. (3) In the Late Archaean, another episode of mantie plumes moved to the base of the crust, causing local volcanism, but did not result in large volumes of additional TTG magma, since the low-melting component had been extracted from this section of the crust during the last plume event (see Hill et al., 1992). Significant geological consequences of this plume event are extensive regional metamorphism and the production of syn-tectonic granitoids. At first, the relatively cooler plume head heated the crust, causing prograde metamorphism (Ml). The crust close to the plume head underwent amphibolite-facies metamorphism, as in the high-grade domains, while the crust further away from the head underwent greenschist-facies metamorphism, as in the low-grade domains. At the same time, large volumes of magma were added to the base of the crust and may intruded to higher levels, resulting in crustal thickening. Subsequentiy, the hot plume "tail" heated the crust, causing peak metamorphism (M2) at amphibolite to granulite facies, depending on the distance to the plume, and widespread anatexis of TTG gneisses, which resulted in the production of syn-tectonic granitoids. Finally, the heated crust experienced near-isobaric cooling (M3) when the effect of heating ceased through the termination of plume activity. This tectonothermal process is consistent with the anticlockwise P-T paths estimated from the basement rocks in the eastern zone of the North China Craton.

References

Bohlen, S.R., 1991, On the formation of granulites: Journal of Metamorphic Geology, v. 9, p. 223-229.

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Hill, R.I., Campbell, I.R., Davis, G.F., and Griffiths, R.W., 1992, Mantle plumes and continental tectonics: Science, v. 256, p. 186-193. Sandiford, M., and Powell, R., 1986, Deep crustal metamorphism during continental extension: ancient and modern examples: Earth Planetary Science Letter, v. 79, p. 151-158. Wells, P.R.A., 1980, Thermal models for magmatic accretion and subsequent metamorphism of continental crust: Earth Planetary Science Letter, v. 46, p. 253-265.

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