Issuu

Geological Society of Australia

ABSTRACTS Number

AUSTRALASIAN TECTONICS

SGTSG CONFERENCE KANGAROO ISLAND 6-10 FEBRUARY 1989

fiobertson Australia Pty. Limited

Natural(Incorporated Resourcein N.S.W.) Consultants

With Comphments of

ph Floor, 77 Pacific Highv\/^, North Sydney, N.S.W. 2060 Australia Teleohone: (02) 957 3199 Facsimile: (02) 954 4011

CONTENTS

Title

Author

Boudinage Controlled Pb/Zn replacement at Rosebery, Mt Read Volcanics Tasmania

Domingo G.A.M.

Structural and Metasomatic History of the Eastem Creek Volcanics, East Moondarra Area, Mount Isa

John, H.C. et al

Archaean Tectonic Evolution of the Yilgam Block

John Baxter, et al

Porphyroblast Inclusion Trails: the key to Orogenesis

Timothy H. Bell

Microstructural evidence for a Westward Transport direction during Middle Cambrian obduction in Tasmania

Ron F. Berry

Mechanical understanding of the Brittle/Brittle-Ductile transition in experimentally deformed ploycrystalline naphthalene

P. Blumerfeld

Structural control on gold-quartz mineralisation, Inglewood Goldfield, Victoria

D.R. Byme

The Woodroffe thrust, Eastem Musgrave Block, N.T.: A problem of large Alfredo Camacho large scale melting during thrusting Portraiture of structure of the Eastem Papuan Fold Belt

George J. Carman

The Banda Arc - Australia Collision Zone

T,R. Chalton et al

Successive early Proterozoic deformation and metamorphic events in Anmatjira Range, Central Australia

Geoffrey Clarke et the al

Why Gold mineralisation at Kalgoorlie, Westem Australia, is not related to wrench tectonics

John Clout

Implications of an early fold/thrust event in the Westem Mount Isa Inlier

Karen Connors

The East Asian Collision zone: Himalayas to Banda Arc

M. Coward et al

Experimental fluid-rock interaction during deformation elevated temperatures and pressures - progress and prospects

S.R Cox et al

Numerical models of fault interaction and localization of deformation

SJ.D. Cox

Implications of the Finisterre Terrain collision fo development of the New Guinea Orogen

Keith Crook

Time - Balanced Sections

Delcan, G. De Paor

Late Proterozoic deformation and the evolution of the northem margin of the Amadeus Basin

P. Ding et al

Title

Author

The structural and metamorphic history of the Reynolds Range (N.T.)

Dirks P.H.

Upper Mantle deformation processes during lithosphere extension: The role and mechnisms of shear localisation

M.R. Drury et al

Stress measurements in the Bowen Basin and their relationships to the structural setting of the basin

Jim R. Enever et al

Tectonostratigrphic terranes and subduction complex melange, Northern C.L. Ferusson et al England Orogen, Central Queensland, Australia An Imbricate fauh system in the Lachlan fold belt near Goulbum, NSW

C.L. Fergusson et al

Grain Boundary structures in natural and synthetic carbonate rocks

J.D. Fitz Gerald et al

Geochemical and isotopic characteristics of igneous activity in the Adelaide fold belt: implications for mechanisms of crustal growth

Joh Foden et al

Doing it without porphyroblasts

Aidan Forde

Determining the location and geometry of early folds in some macroscopic refold problems: A technique andysing intersection lineation trend variations

T.J. Fowler

Deformation processes caused by offscraping at an ancient convergent margin

Annette D. George

Linked fualt familis in Basin and passive margin formation and deformation

Gibbs, A.D.

Metamorphism and deformation of the Glenelg river complex, Western Victoria and implications for correlations between SE Australia and Northem Victoria Land

Geroge M. Gibson

Basin inversion, thrusts and or deposits at Cobar, NSW

Glen, R,A.

Strike-slip and accretionary tectonics of the Lachlan fold belt

David R, Gray et al

Structural and metamorphic analysis of a deformed subduction accretion sequence - Nowendoc, Eastem NSW

Martin Hand

Conjugate faulting associated with orthogonal subduction in Lyal Harris Indonesia: Structural constraints for the timing of the rotation of Sumatra Dextral transpression in Central Victoria and its controls on gold mineralisation

Lyal Harris

The application of aeromagnetics in regional structural interpretation of the Norseman-Wiluna Belt, WA

Lyal Harris et al

A major Palaeozoic deformation event affecting precambrian terrians of Lyal Harris et al Australia and East Antartica: Implications for Continental scale crustal shortening The Structural evolution of the Albany Eraser Province and Leeuwin Block, WA Lyal Harris et al

Tide

Author

Extensional deformation of Fergusson and Goodenough Islands, PNG

EJ. Hill

Syntectionic precious and base metal mineralisation controlled by Mark, C. Hinman deformation partitioning during an early devonian orogeny at Peak Cobar, NSW The plumbing system for some Australasian ore-bodies

Bruce Hobbs et al

A finite stain and kinematic study across a major ductile thrust and shear zone, south of Normanville, Fleurieu Peninsula, SA

P.R. James J. Anderson

Microstructures and Rheology and Steady-state Flow

Mark. W. Jessell

The role of temperature in Determining Crystallographic Preferred Orientations

Mark W. Jessell

Deformation history of the Otago Schiss, New Zealand: Cyclic uplife collapse orogenesis and its implications

Scott E. Johnson

Preferred orientation pattems of Plagioclase, Mica and Quartztools for modelling complex strain histories

Jom H. Kruhl

Timing syntectonic mineralisation using porphyroblasts: The Bottie Creek gold deposit as an example

K.C. Lawrie

The Hastings Block - A key to understanding the tectonic development of the New England Orogen

Paul G. Lennox John Roberts

Amphibolites/metadolerites and their tectonic implications from the Mr Lofty Ranges metamorphic belt, SA

Songfa Liu Peter D. Fleming

Deformation of Synthetic quartz aggregates

Fuchun Luan

The influence of pre-existing irregularities on the development of deformation structures

Neil S. Manchtelow

Considerations for quantitative determination of the role of dislocation in selective dissolution

Annemarie Meike

Tectonics of the calliope volcanic arc Northem New England Fold belt

Vicent J. Morand

Tectonic subdivions of the Tasman Fold belt system in Queensland

Cecil G. Murray

Suspect terranes of the Gympie Province, New England Fold Belt

Cecil G. Murray et al

Structure fo the mertondale mine area and implications for regional geology B. W. Nisbet and mineralisation R. L. Hammond Ultramylonite zones transecting high-grade proterozoic rocks of the strangways orogenic belt The tectonic history of the strangways orogenic belt: a barometric response to late compression

A.R. Norman

A.R. Norman et al

Title

Author

Tectonics of the modem obliquely convergent plat boundary in the South Island, Richard J. Norris NZ, with implications for the interpretation of older terrane boundaries Peter O. Koons Kinematic indications in serpentinites the peel-manning fault system a test case

Robin Offleretal

Ancient earthquakes and adiabatic shear in the Musgrave Ranges

AOrd

Mafic Dyke swarms in relation to extensional tectonics: a proterozoic model

A. John Parker

Eastward thrusting associated with the Wyangala Batholith: Surge tectonics in the Lachlan fold belt?

Scott R. Paterson

Basins and oroclines formed by extensional collapse of collisional mountain belts

John Paul Piatt

Tectonostratigraphic terane constraints on Palaeomagnetic poles from the Tasman Fold Belt

G. Mc.A. Powell et al

Alpine fault tectonics and the redistribution of gold across the Australian Mark S. Rattenbury Pacific plat boundary, Wesdand, New Zealand Review of Tectonic Map concepts

M J. Rickard

Basement controlled monocline at the rock Nr Wagga Wagga, NSW

M J . Rickard et al

Structural constraints on metasomatic processes west of the Mount Isa fault Rubenach

Michael J.

Australasian tectonics: an introductory review

R W R Rutland

Pressure temperature strain histories in high grade metamorphic terrains: constraints on the strength of the continental lithosphere

Michael Sandiford

Tectonic map of the circum-pacific region, Southwest Quadrant, 1:10,000,000 scale - poster presentation Use of geophysical signature in definition of fold belts and terranes in Eastern Australia

Erwin Scheibner

Erwin Scheibner Cecil G. Murray

Extensional orogeny at Mount Isa, and its significance for mineralisation Alastair J. Stewart The fault history and emplacement of the Coolac Serpentinite, Lachlan fold belt, NSW

Peter G. Stuart-Smith

Porphyroblast inclusion trails - a novel approach to the kinematic interpretation of the Southem Adelaide geosyncline

Chris Steinhardt

Thrusting and the tectonic development of the Adelaide GeosyncUne

Chris Steinhardt

Interaction between deformation and chamockite emplacement during compression accompanied in the Bunder Hills, Antarctica

Kurt Stiiwe Chris J,L. Wilson

Title

Author

Characteristics of faults at Olympic Dam and theh* possible relationship to regional lineaments

Tim Sugden

The Grampian highlands of Scodand: an orogenic analogue to the Adelaide fold belt of South Australia

S. Temperley

White Range and Ruby Gap ductile duplexes: implications for Paleozoic deformation in central Australia

Christian teyssier et al

Strahan sub-basin, a wrench related basin on the west Tasmanian transform margin

B, Thomas

Tectonic Control of Early-Firt (Cretaceous) Sedimentation, Taranaki Basin New Zealand The Palaeomagnetism of late devonian red beds of the merimbula group Southeast NSW

Glenn P. Thrasher

G. A, Thrupp et al

Post delamerian magmatism: tectonic cotnrols on magma chemistry and Simon Turner et al evidence for post delamerian extension Tectonic evolution of the King Leopold orogen, Kimberley region, Western Australia

M. Tyler TJ. Griffin

Phanerozoic tectonic regimes of Australia, Gondwanaland, and the Globe J J Veevers The effect of chemical environment on the experimental deformation of Quartzite Jiannong Wang Mylonites and Problematical relationships along the Eastem Boundary of Peter Warner et al the Tumut Trough, NSW Structure and subdivision of the Tasman orogen, on the basis of the gravity and magnetic anomaly pattern

Peter Wellman

Structure and subdivision of the south west Yilgam and Albany Province using gravity and Magnetic anomalies and geology

Alan J. Whitaker

Orogenesis in the Yilgam: the critical role of porhyroblasts in Geometric analysis Colin Wilkins Fault zones and gold mineralisation, Stawell, Victoria, Australia

Thomas M. Will

Shear zones in the Leonora District, Eastem Goldfields: Style of Archaean tectonics

Peter R. Williams

Evidence for gold bearing quartz ein emplacement during flexural folding, Jacqueline Windh Hill End region, NSW The nature and significance of the Hyde-Macraes Shear Zone, in Central Otago, New Zealand

C.N. Winsor

Numerical Simulation of the Buckling Development of Elastic-Viscous Folds

Yanhyua Zhang

'.i

.I i \

GEOLOGICAL SOCIETY OF AUSTRALIA INC ABSTRACTS NUMBER 24

AUSTRALASIAN TECTONICS

Abstracts of a conference organised by the Specialist Group in Tectonics and Structural Geology. Held at Kingscote, Kangaroo Island, February 6-10, 1989 ISSN 0729 - OllX

LIST OF CONTENTS

Page

Alphabetical list of authors

Abstracts

i ALPHABETICAL LIST OF AUTHORS Page Aerden Bain et al. Baxter et al. Bell and Johnson Berry Blumenfeld and Wilson Byrne and Hy Camacho Carman Charlton et al. Clarke et al. Clout Coney Connors Coward and Audley-Charles Cox and Paterson Cox Crook De Paor Ding et al. Dirks Drury et al. Enever and Mallett Fergusson et al. Fergusson and Vandenberg Fitzgerald et al. Foden et al Forde Fowler George Gibbs Gibson Glen Gray et al. Hand Harris Harris Harris et al. Harris et al. Harris et al. Hill Hinman Hobbs et al. James and Anderson Jessell and Lees Jessell and Lister Jessell and Lister Johnson Kruhl

1 3 5 7 8 10 12 14 16 18 20 21 23 25 27 28 29 31 33 34 36 38 40 42 44 45 47 49 50 51 52 53 54 56 58 59 61 63 65 67 69 71 73 74 76 77 79 80 81

Lawrie Lennox and Roberts Li et al. Liu and Fleming Luan and Paterson Mancktelow Me ike Morand Murray Murray et al. Nisbet and Hammond Norman Norman et al. Norris and Koons Offler et al. Ord Parker Paterson et al. Piatt Powell et al. Rattenbury Rickard Rickard et al. Rubenach Rutland - abstract to be supplied. Sandiford Scheibner Scheibner and Murray Stewart Stuart - Smith Steinhardt Steinhardt Stuwe and Wilson Sugden Temperley Teyssier et al. Thomas Thrasher Thrupp et al. Turner et al. Tyler and Griffin Veevers and Powell Wang Warner et al. Wellman Whitaker Wilkins Will Williams and Etheridge Windh Winsor Zhang et al.

82 85 87 89 91 92 94 95 97 99 101 104 106 108 110 112 114 116 118 120 122 124 126 127 129 130 132 133 136 138 139 140 141 143 146 148 150 151 153 155 157 159 161 163 165 167 169 170 172 173 175 177

Addendum Drury et al

BOCJDINAGE CONTROLLED PB/ZN REPIACEMENT AT ROSEBERY, MT. READ VOLCftNICS, TASMANIA Domingo G.A.M. Aerden Geology Department, James Cook University, Townsville, Q 4811, Australia IXiring the Tabberabberan orogeny (Devonian) the mid Cambrian Mount Read volcanics were thrust westwards over older Cambrian units and a strong E-dipping cleavage was developed regionally. The Rosebery deposit is situated at the top of a strongly foliated zone of reverse shear movements, named the Rosebery fault zone. This zone is strongly cleaved and contains a downdip stretching lineation. Bedding foldaxes are doubly plunging. Foliation-boudinage structures formed late during the shearzone evolution with boudinage axes in two perpendicular directions (horizontal and downdip) caused by extension in the X and Y direction of the strain ellipsoid. Mineralizing fluids were focussed into down-dip plunging pipes caused by large scale foliation boudinage in the intermediate stretching direction. In these extensional sites, dilation occurred causing hydraulic fracturing and brecciation of the rock and thus allowing massive sulphide replacement of the wall rock (see diagram). This mineralization extended outwards from these structural traps along cleavage planes and secondary shears. By carparing the geometry of the orebody with the bedding structure in the country rock it is demonstrated that the orebody cross-cuts beddijig on ore body scale. This observation refutes earlier interi)retations of a tightly folded bedding parallel sulphide horizon and strongly supports a structural controlled origin. Orebody mineral-zoning also supports the "replacement" model. Cu/Fe versus Pb/Zn grades are used to discriminate between proxiinal and distal parts of the orebody relative to the source of mineralization. Several source areas can be defined in this way, characterised by anomalous high Cu grades. These areas match the extensional structures controlling the mineralization. A general trend of upward directed fluid flow is reflected by strong differentiation between Cu minerals at the base and downdip side of the orebody and Fb/Zn-rich ore updip in the mine-sequence. Thin section study on the timing of mineralization relative to the main cleavage confirms a synkinematic replacement origin for the bulk of the massive sulphide ore. Evidence is found in relic textures caused by partial overgrowth of a foliated rock by massive sulphides and small scale exairples of boudinage controlled mineralization. Sulphur isotope studies and the volcanic setting suggest a Cambrian volcanogenic origin of the massive sulphides. Therefore, the Rosebery orebody is best regarded as a Devonian metamorphogenic deposit which formed by dissolution of Cambrian sulphides, focussed fluid transport and reprecipitation in a structural trap.

-S-i

-eargT

This kinematic catloon illustrates the foliation boudinage control on massive sulphide replacement. The fluid path is reflected in the mineral zonation pattern of the ore body. Cu rich pyrite/chalcopyrite mineralization is located at the extensional ramp or pull-off contact (black). Away from this contact the mineralization becomes more sphalerite/galena rich (vertical stripes). At the tip of the ore shoots baritic mineralization occurs (dotted). Note that because of the 45 degree dip of the sequence a component of the mineral zonation is observed perpendicular to strike; a feature that has been used to favour a syngenetic origin of the orebody. On purely geometrical grounds, the orebody is easily mistaken as a folded massive sulphide horizon. Structural analysis however has pointed out that the orebody cross-cuts the bedding structure in the country rock. Bedding is indicated with broken lines; arrows represent plunges of foldaxes and the orebody axes.

STRUCTURAL AND METASOMATIC HISTORY OF THE EASTERN CREEK VOLCANICS, EAST MOONDARRA AREA, MOUNT ISA John H.C. Bain, G.Anthony M. Henderson and Christoph A. Heinrich Bureau of Mineral Resources, Geology Canberra, ACT 2601

and Geophysics,

G.P.O.Box

378

Wyborn (1987, Geol.Soc.LondonSpec.Publ, p.425) has indicated that distinctive regional geochemical and mineralogical changes in the Eastern Creek Volcanics (ECV) are possibly related to specific superimposed cleavage-forming structural and metamorphic events that were instrumental in the mobilisation of copper and some other elements involved in the formation of the Mount Isa copper ore bodies. To relate contemporary models of the Mount Isa copper deposits to processes that can be associated with geological features of regional extent it is necessary to determine the structural and metasomatic history of the possible host and source rocks, particularly the ECV. To this end our studies have been concentrated in a complexly deformed part of the Leichardt River Fault Trough 5-25km northeast of Mt Isa. Detailed chemical and mineralogical studies to determine the nature of fluids that passed through this area during deformation and .metamorphism, and geological mapping at 1:10 000 scale to elucidate its structural history, were aided by interpretation of Thematic Mapper Scanner (NSOOl) and Thermal Infrared Scanner (TIMS) imagery from the 1985 US-Australia Joint Scanner Project. Preliminary data analysis and interpretation permit a reconstruction of the early post D1 (pre-mineralisation) configuration of the ECV in the map area. This reveals major N-S repetitions of N facing sequences, possibly the result of D1 thrusting (parallel to SI cleavage). Fluid inclusions of CaC^-rich composition occur in weakly deformed metabasalts and in veins associated with massive quartz-epidote alteration. This early metasomatic event occurred during, and partly before, the local formation of penetrative cleavage (SI), which is subparallel to bedding. The brine inclusions have very similar microthermometric properties to the earliest fluid inclusions recognized in the dolomitic breccias associated with the Mount Isa copper deposit. Subsequent E-W shortening in two distinguishable but probably closely related episodes, resulted in folding along N and NNW trending axes, and development of conjugate NNW and NE trending strike slip and arcuate E-W normal faults. The latter control minor sequence repetitions resulting from local N-S extension. Foliation is confined to the vicinity of faults. The resultant mosaic of variously oriented fault blocks was subsequently deformed by well-spaced shatter zones along reactivated, generally N

trending faults, possibly resulting from minor extension phase of the earlier compressional event.

during

a rebound

The shatter zones are associated with calcite + magnetite + chlorite + hematite alteration and, locally, uranium mineralisation. Mass balance analysis indicates substantial de-silicification in some of the most calciteand chlorite-enriched rocks. Geochemical, mineralogical and fluid inclusion evidence is consistent with downward infiltration of oxidised NaCl- and/or C O 2 -rich fluids, which may have originated from the stratigraphically higher, locally evaporitic. Mount Isa Group sediments. It is possible that these brines represent the source of compositionally similar NaCl-rich fluids involved in the main stage of copper mineralisation at Mount Isa mine. If so, the structural and metasomatic events involved in the formation of the Mount Isa copper deposits produced recognisable geological features of regional extent, which by their nature and distribution may provide useful insights into the mechanisms, scale and timing of the mineralising process, and facilitate the development of improved methods of assessing the mineral resource potential of the region.

ARCHAEAN

TECTONIC

EVOLUTION

John Baxter^, Lyal Harris^ 1 2 3

and

THE

YILGARN

BLOCK

Richard Keele^

Hermitage Holdings, 9 Marie Way, Kalamunda 607 6 Geology Dept., University of Western Australia, Nedlands 6009 Kalgoorlie School Of Mines, Egan St, Kalgoorlie 6430

The Archaean Yilgarn Block of Western Australia has been divided into four tectonic provinces described by Gee et. al. (1981). Structural data collected from these provinces indicate that a consistent tectonic framework can be determined for Archaean tectonism. Western Gneiss Terrain ortho- and paragneisses show evidence for early (possibly 3.1by?) NW directed ductile thrusting identified in the Jimperding Metamorphic Belt. Within the Balingup Metamorphic Belt, N-S to NW~SE striking ductile structures showing dextral movement have been cross-cut and re-oriented by the NNW sinistral Donnybrook-Bridgetown shear zone, a 15-20km wide deformation zone truncated by the Albany Mobile Belt to the S and which appears to bend into parallelism with the W margin of the Yilgarn Block in the N. Criteria for sinistral movement comes from microstructural studies and the rotation of foliations discernable from regional satellite and aeromagnetic images. The Murchison Province of the Granite Greenstone Terrain has a consistent character in the southern and central portions, but has a unique structural regime in the N where, at Jillawarra, there is clear evidence of a northerly directed thrusting accompanied by conjugate brittle-ductile shear array implying N-S directed maximum compression. In the south and central portions three shear arrays are present. An early dextral transcurrent array is clearly evident in the Meekatharra district. The Meekatharra-Mt.Gibson Shear Zone has been reactivated by sinistral oblique movement which has induced much of the regional folding in the Province. The youngest event is a co-axial event implying E-W compression. Criteria for the shear arrays comes from field mapping, stereographic analysis and analysis of microstructural shear criteria. The Southern Cross Province contains four different shear arrays. The earliest NW trending ductile dextral shears are best seen in the Marvel Loch district. Reactivation with NW sinistral brittle-ductile oblique strike-slip movement can be seen in the Southern Cross district. Mylonites in orthogneiss along the 140° striking Koolyanobbing-Lake Johnson shear zone indicating dominantly sinistral transcurrent movement (Libby, 1988) are interpreted as being associated with this second event. Superimposed on these early events are ductile conjugate shears with shear planes oriented 140'' and 220"" indicating N-S compression. The Koolyanobbing-Lake Johnson shear zone has been locally dextrally offset by N-S structures (Libby, 1988) similar to that seen in the Eastern Goldfields Province. Finally an E-W compressive conjugate brittle shear array has controlled emplacement of the Widgiemooltha Dyke Suite. Criteria for the geometry derived for the Southern Cross Province include

geological mapping and stereographic analysis. In the Norseman-Wiluna Belt of the Eastern Goldfields Province, structures formed during local thrust repetition of the greenstone belt stratigraphy and early regional folding have been overprinted by two wrenching events. The consistent overprinting of structures fitting NW trending wrench geometries and microstructural field criteria indicate early sinistral and later dextral transcurrent movements (with variable oblique movement components) along the belt. Regional folds are overprinted by fabrics related to the shearing events, and foliations are rotated towards parallelism with regional shears in high strain zones. The superposition of both oblique wrenching events is apparent especially in the Kalgoorlie area (Mueller e t . a l 1 9 8 8 ) . Granitoid intrusion syntectonic with dextral wrenching such as observed in the Leonora area results in an increase in the normal shear component towards granitoid margins. Variations in the orientation of ductile dextral shear zones on the local scale fit D, R, and P orientations for NNW dextral wrenching. Conjugate 060® dextral and 120'' sinistral brittle-ductile shear zones cutting the above structures and N-S striking reverse faults indicate a phase of approximately E-W compression. Reactivation of N-S structures with reverse movement and sinistral secondary Riedel shears formed during dextral wrenching with dextral movement have locally taken place during this late Archaean event. The tectonic significance of early thrusting and folding in the Jimperding and Norseman - Wiluna belts and the early dextral and sinistral shearing in • the Southern Cross Province is not clear. Dextral northerly trending shear zones in the Murchison Province and the Western Gneiss Terrain are correlated with sinistral wrenching in the Eastern Goldfields Province implying a southerly movement of the central portion of the Yilgarn Craton. Sinistral displacements in the Murchison and Balingup Metamorphic Belt and dextral movement in the Eastern Goldfields implies a reversal of the movement in the central block. The ductile conjugate array and the northerly trending shears in the Southern Cross Province are correlated with this northward translation of the central block. This impingement structure has also resulted in thrusts in the Jillawarra district and at Jack Hills on the northern margin of the craton. The entire sequence has then been compressed in an easterly direction developing structures forming a conjugate shear arrray intruded by the 2.2-2.4by Widgiemooltha Dyke Suite. REFERENCES R. D., Baxter, J.L, Wilde, 3.A. & Williams, I.R., 1981. Crustal development in the Archaean Yilgarn block. Western Australia. In Glover, J.E. & Groves, D.I. (eds), Archaean Geology: Second International Symposium, Perth, 1980. Spec. Pubis. Geol. Soc. Aust. 7, 43-56. Libby, J.W.,1988. The nature and significance of the crustal scale Koolyanobb ing—Lake Johnson shear zone. BSc (Hons) Thesis Univ. Western Australia (unpub.). Mueller, A.G., Harris, L.B. & Lungan, A., 1988. Structural controls on greenstone-hosted gold mineralization by transcurrent shearing: a new interpretation of the Kalgoorlie Mining District, Western Australia. Ore Geology Reviews 3: 359-387. Gee,

PORPHYROBLAST INCLUSION TRAILS: THE KEY TO OROGENESIS Tinothy H. Bell and Scott E. Johnson Department of Geology, James Cook University Townsville Qld 4811 Detailed microstructural analysis of inclusion trails in hundreds of garnet porjdiyroblasts, from rocks vAiere spiral-shaped inclusion trails are ccmtoi, indicates that spiral-shaped trails did not form by rotation of the greying porj^yroblasts relative to geographic coordinates. They formed instead by progressive growth by porjiiyrcblasts over several sets of near-orthogonal foliations that successively overprint one another. The orientations of these near-orthogonal foliations are alternately near-vertical and near-horizontal in all proj^yroblasts examined, providing very strong evidence for lack of porjiiyroblast rotation. The deformation path recorded by these porj^yrc±>lasts indicates that the process of orogenesis involves a multiply repeated two-stage cycle of: (1) crustal shortening and thickening, associated with the development of a near-vertical foliatiai with a steep stretching lineation; followed by (2) gravitational instability and collapse of this uplifted pile associated with the development of a near-horizontal foliation, gravitational spreading, near-coaxial vertical shortening and ccnsequent thrusting ai the orogen margins. Correlatiai of inclusion trail overprinting relationships and asymnetry in porphyrc±>lasts with foliation overprinting relaticxiships observed in the field allows determination of vrfiere the rocks studied lie and have moved within an orogen. This information, combined with informatiai about chemical zoning in porphyroblasts, provides details about the structural/metamorphic P-T-t path those rocks have followed. The ductile deformation environment in v^ich a porphyroblast can rotate relative to geographic coordinates during orogenesis is very spatially restricted in continental crust to vertical, ductile tear/transcurrent faults across v^ich there is no component of bulk shortening or transpression.

MICROSTRUCTURAL EVIDENCE FOR A WESTWARD TRANSPORT DIRECTION DURING MIDDLE CAMBRIAN OBDUCTION IN TASMANIA Ron F. Berry Department of Geology, University of Tasmania, Hobart, Tas. The significance of widespread mafic/ultramafic complexes has been a major contentious issue in the tectonic interpretation of Western Tasmania. Berry & Crawford (1988) reassessed the evidence for these models and provided a new simpler interpretation of the origin of the mafic/ultramafic complexes. We interpreted these rocks as relicts of a middle Cambrian allochthonous sheet analogous to the Oman Ophiolite. This model predicts that the lenses of foliated amphibolite which have been reported from the margins of five of the mafic/ultramafic complexes were formed at the base of the allochthon during the early part of the overthrusting. In Oman and Newfoundland, mylonites within these amphibolites record the direction of movement of the thrust sheets (e.g Lippard et al. 1986). Oriented samples of the foliated amphibolites have been obtained from Anderson Creek (Beaconsfield), Heazlewood River, Wilson River, Serpentine Hill (West Coast) and Adamsfield (Southwest). Studies of the microfabrics indicate a mylonitic origin for four of these sites and give consistent westward transport directions (Fig.l). The amphibolite located above Nineteen Mile Creek on the margin of the Heazlewood River Complex was recognised by (Rubenach 1973) . This is largely composed of mylonite with a strong near vertical foliation striking 100^ and a hornblende lineation pitching S'^E. The foliation here is not parallel to the margin of the serpentinite body. It is separated from the margin by a thin sliver of foliated serpentinite suggesting there has been a body rotation away from the original orientation during emplacement. The sense of movement is now dextral which is consistent with a westward to southwestward transport depending on the assumptions made in reconstructing the original orientation. At the Argent Tunnel, mylonitic amphibolites have a strong foliation dipping moderately to the southeast. The hornblende lineation in these rocks plunges to the east. The extent of Devonian folding at this locality is unclear but the mylonites occupy an anticlinal position so no correction for folding was made in this analysis. The sense of shear indicated by shear bands and rotated porphyroclasts indicates transport to the northwest. Hornblende-quartz mylonites are exposed on the Strathgordon Road south of Adamsfield. They have a faulted contact with serpentinite. The gradation into less metamorphosed sedimentary rocks occurs over 30 m. The near vertical foliation strikes 330^ and after unfolding around the Devonian axis the hornblende lineation trends 065® with the sense of movement indicated by shear bands and rare deformed porphyroclasts indicating transport to the west. The situation at the Andersons Creek Complex is more difficult. There are metamorphic rocks as fault slices within the ultramafic complexes but while these rocks have a weak L/S fabric they are not mylonitic. The Simmonds Hill Metamorphics, on the western margin of the complex, contain gabbroic cataclasite and felsic mylonites. Shear bands in the mylonites suggest westward transport. There are two amphibolite bodies reported from the western margin of the Wilson River Complex (Rubenach 1978, Brown 1986). The amphibolite exposed on Harman Creek has a gneissic texture rather than a mylonitic fabric. The amphibolite on Ahearne Creek has a strong schistosity. The foliation is steeply dipping to the west and southwest so that after unfolding on the Devonian fold axis it is shallowly dipping to the northeast. A weak hornblende lineation, after unfolding, plunges to the ESE. No evidence of strong rotation has been found.

The amphibolites and other metamorphic rocks associated with the western margins of the mafic-ultramafic complexes in Tasmania are petrographically very similar to metamorphic rocks which underlie major allochthonous sheets in Oman and western Newfoundland. They consistently indicate a westward transport direction during emplacement. REFERENCES Berry R.F. & Crawford A.J. 1988. The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Aust. J. Earth Sc. 35. Brown A.V. 1986. Geology of the Dundas-Mount Lindsay-Mount Youngbuck region. Tasmanian Geological Survey Bulletin 62^ 221pp. Lippard S.J.^Shelton A.W. & Gass I.G. 1986. The ophiolite of northern Oman. Geol. Soc. London, Memoir 11 Rubenach M.J. 1973. The Tasmanian ultramafic-gabbro and ophiolite complexes. Ph.D thesis, Uni. Tas.

SO km..

Fig. 1. Map of Tasmania showing the distribution of mafic-ultramafic complexes from Brown (1986). Complexes discussed here are numbered 1: Andersons Creek, 2: Heazlewood River, 3: Wilson River, 4: Serpentine Hill, 5: Adamsfield. Transport directions are indicated by the arrows.

MECHANICAL UNDERSTANDING OF THE BRITTLE/BRITTLE-DUCTILE TRANSITION IN EXPERIMENTALLY DEFORMED POLYCRYSTALLINE NAPHTHALENE.

Philippe R. Blumenfeld and Christopher J.L.Wilson

Department of Geology, University of Melbourne, Parkville, Victoria 3052, Victoria

The ductility of a non-cubic polycrystalline aggregate is not only a function of general intrinsic parameters such as T, e , PH2O ....,but also of the statistical orientation of the grain lattice and boundaries.

The influence of these orientation parameters on the ductility is

governed by the ability of a deforming aggregate to overcome deformation incompatibiUties arising between the differently oriented crystals. Accommodation processes in polycrystals without mass transfer mainly involve 1) sliding along non-coherent boundaries that produces rotation and translation in the adjacent crystals, 2) migration of grain boundaries especially if they remain coherent, 3) shortening parallel to a slip direction by kinking of the crystals. If these processes do not occur, residual elastic strains and internal stresses accumulate until the crystal strength is exceeded and a crack is nucleated and propagates. The conditions that favour this type of brittle behaviour can be reproduced in plane strain see-through experiments, using strongly constrained elongate crystal aggregates of naphthalene ( T ^ = SO^'C) deformed in simple shear at strain rates of lO'^ s-^- ^ special texture characterised by a skeletal crystallization of melted naphthalene, after undercooling of lO^C, represents an array of two parallel crystal sets with alternating lattice orientations.

The

number of potential slip systems is restricted to 1 or 2 due to the low symmetry (monoclinic) of naphthalene and to the bidimensional type of experiment, as slip can only occur if the slip direction sub-parallels the plane of strain. Important incompatibilities of deformation arise therefore between the two sets of crystals and sliding along their boundaries is prevented as the contact between sample and apparatus is strictly rigid with the ends of the crystals locked. In such situations, brittle behaviour occurs up to 0.9 T ^ despite the normal plasticity of the crystals, followed at higher temperatures by a brittle-ductile behaviour. Depending on the orientation of the elongate crystals deformation concentrates after small strains in shear bands that nucleate either 1) in kink-bands, or 2) at new formed and "unlocked" grain boundaries. In the shortening field, crystals shorten their lengths by the development of kink-band boundaries (KBB) subperpendicular to the length of the crystals.

With progressive

deformation individual kink-bands (KB) become interconnected to create a continuous kink band across the sample and parallel to the displacement vector. This evolution of the kink-bands proceeds either by 1) nucleation of new misorientated KB in the rotating old KB, but at a small angle to the shear direction, or 2) direct rotation of the old KBB by accommodation migration. In the extensional field, an apparent boudinage of one set of crystals is achieved by migration of the grain boundary. Sliding is activated along the boundaries of the necked crystal and this local concentration of shear is progressively propagated by the necking of the adjacent crystal. The local stress and elastic strain anomaly at the tip of the sliding surface stimulate grain boundary migration creating new sites for sliding. Following the propagation of the shear band, recrystallization starts adjacent to the sliding surface by rotation of sub-grains. It appears that incompatibilities arising in the initial aggregate cannot be accommodated in the whole sample even at T « T^^ ^ Over T=0.9 T^^^ a local accommodation can occur by complex kinking and/or boundary migration, where internal stress and elastic strain decreases. Further shear is then concentrated into this zone and produces a softened and recrystallized band that transects the sample.

Hence the transition brittle/brittle-ductile

appears to be mainly controlled by the enhancement of the boundary migration by temperature.

STRUCTURAL CONTROL ON GOLD-QUARTZ MINERALISATION, INGLEWDOD GOLDFIELD, VICimiA. David R. Byme and Charlotte Hy Goldquest N,L., Inglewood, Victoria, 3517 The Maxwells Reef is one of the major gold producing structures of the Inglewood Goldfield. It occurs in tightly folded Early Ordovician turbidites of the Bendigo-Ballarat zone. In the Inglewood field, the folds are asymmetric, shallowly doubly plunging, tight with steeply east dipping axial surfaces. The gold-bearing structure is an east dipping reverse strike fault zone trending NNW-SSE, sub-parallel to that of the folds. It varies in dip from 650-750 where it is parallel to bedding, to 50O-60O where it crosses the relared syncline and anticline (Figure 1). The eastover-west transport implied by the orientation of the folds and major faults is in contrast to the general eastward transport in the thrust belt model proposed for the Bendigo-Ballarat Zone (Gray et al., 1988). w ,Hing« spurt

Figure 1 . Schematic Cross Section of the Maxwells Reef Various types of quartz forrnations are associated with the structure, all of which have been found to contain gold. Major types are (Fig.1): 1) laminated veins; 2) reef; 3) footwall spurs; 4) hinge zone spurs.

The laminated veins contain numerous sheets of slate parallel to their walls. Tectonic stylolites and boudinage are common. They are commonly cut by later veins. They are canposed of quartz with minor ankerite and pyrite^ and accessory arsenopyrite, chalcopyrite, sphalerite, galena and gold. The reef varies in thickness from Om to 5m. It is contained within a fault zone and is often bounded by laminated quartz. It is mainly composed of massive milky si±>-euhedral to euhedral quartz containing numerous vughs. Minor components include ankerite, chloritic quartz, pyrite, sphalerite, galena and gold. The spurs are located in two main positions relative to the fault and folds: in the hinges of most folds, and adjacent to the footvTall of the reef. They are also sometimes present instead of the reef when the latter is not developed. They are often sigmoidal veins formed during faulting and indicate reverse movement along the fault plane. Milky quartz dominates the mineralogy, with minor ankerite, chloritic quartz, pyrite and gold. The various generations of quartz within these formations suggest a complex and prolonged history of fluid injection and deformation associated with the major period of deformation. Historical records indicate that the gold is located in steeply northerly and shallowly southerly plunging elongate shoots, which have been attributed to intersections of the reef with favourable lithologies (e.g. graphitic slates). However, the plunge of the bedding-reef intersection does not always coincide with that of the gold shoots. Since geochemicallithological factors alone do not account for the position of the ore shoots, current research aims to further investigate and define the structural controls. Reference: Gray D,R., Allen R.L., Etheridge M.A., Ferguson C.L., Gibson G.M., Morand V.J., Vandenberg A.H.M., Watchom R.B. (1988): Structure and tectonics in Douglas J.G. and Ferguson J.A. (Eds): "Geology of Victoria". Victn. Div. Geol. Soc. Aust. pp 1-36.

THE WOODROFFE THRUST. EASTERN MUSGRAVE BLOCK, N.T.: A PROBLEM OF LARGE SCALE MELTING DURING THRUSTING. Alfredo Camacho Department of Mines and Energy, Alice Springs, N.T. The eastern Musgrave Block N.T. consists of two structurally and metamorphically distinct terrains of middle-late Proterozoic age. The Kulgera terrain is an area of acid volcanics and quartzo-feldspathic, calcareous and peraluminous gneisses. A U-Pb date on the zircons of the acid volcanic lithology yield an emplacement age of 1797± 40 Ma. Granulite/transitional granulite grade metamorphism (1200? Ma) occurred during two stages of deformation. D2 is isoclinal, produces a good layered foliation, has a N-S trend and overprints the isoclinal Di structures. Granite magmatism (approx 1200 Ma) was followed by extension and emplacement of a dolerite dyke swarm at 1054±13 Ma. Subsequent shortening during the Petermann Orogeny (600 Ma) superimposed the granulite Kulgera terrain onto the amphibolite grade Mulga Park Block. The latter contained porphyritic granites of unknown age which are unconformably overlain by the Dean Quartzite (800 Ma). Foliations present in this block are east-west and are interpreted to be related to the Petermann Orogeny. The two terrains are separated in the Mulga Park area by a zone, known as the Woodroffe Thrust, of mylonites and pseudotachylites dipping shallowly (30^) to the S and extending laterally for several hundred km. The thrust is 2-3 km thick and is divided into three different zones depending on the dominant rock type, from base to top these are: i/ Mylonite zone (1 km? thick): Coarse granitic mylonites at the base grade into finer grained mylonites and then to ultramylonites over a short distance (approx 40 m). Kinematic indicators suggest S over N movement. ii/ Ultramylonite zone (60 m thick): Narrow zone comprising a sequence of very fine grained, black and strongly foliated rocks. Kinematic indicators are consistent with the sense of movement observed in the mylonite zone. The upper section of this zone contains small (20 cm thick) pseudotachylite veinlets that have been deformed by ductile deformation. iii/ Pseudotachylite zone (1 km thick): This zone is only present in rocks that have attained granulite grade. Veins range in size from a few cm up to 3-4 m thick, are glassy and contain inclusions of unmelted country rock. Fault and injection vein relationships are similar to those described by Sibson (1975). There must be many generations of pseudotachylite generation but these cannot be characterised due to the lack of marker horizons present in the thrust zone. Further east in the Amata area, the corresponding zone in the Woodroffe Thrust occurs as a solid, 40 m thick slab of almost pure pseudotachylite.

The amount of melt generated in this zone in the Mulga Park area is conservatively estimated to be 510%. Assuming that the Thrust is a slab (in the mapped area only) 1 km thick, 30 km long and 5 km wide, this represents 7.5-15 km^ of melt. These figures are staggering and must therefore make us rethink our models of pseudotachylite generation. One of the major problems that arises is how conditions can remain suitable for melting over a significant, but unknown period of time. In striking contrast other thrust zones in Central Australia (e.g. Redbank deformed zone) with similar geologic settings show relatively little pseudotachylite development. References Sibson, R.H., 1975. Generation of pseudotachylite by ancient seismic faulting. Geophys. J. R. astr. Soc., 43: 775-794

PORTRAITURE OF STRUCTURE OF THE EASTERN PAPUAN FOLD BELT George J. Carman Department of Earth Sciences, Monash University, Clayton, Victoria, 3168. The Eastern Papuan Fold Belt is an 80-100km wide, southwest verging, fold-thrust belt extending about 400kms northwest of Port Moresby, Papua New Guinea (Fig. 1). Structural provinces within it are controlled by thickness and facies variations within the cover sequence and possibly basement morphology. Penetrative cleavage and fault bend folds are notably absent in the Eastern Papuan Fold Belt. Structural trends parallel lithostratigraphic units and major decollements occur both above and below a Palaeocene/Miocene carbonate sequence. Variations in structural style, as identified by surface mapping, interpretation of satellite and radar imagery and seismic data, result from thinning of the carbonate to the north and east. The Eastern Papuan Fold Belt is bordered to the west by the Papuan Thrust Front adjacent to the Austrahan cratonic shelf and to the east by the North New Guinea Thrust Belt. The latter also contains sediments of Miocene to Cretaceous age but is distinguished by the presence of cleaved Mesozoic metasediments and thrust slices of ophiolite.

STRUCTURE EASTERN

Map

PAPUAN

THE

FOLD

BELT

area

Figure

In the Port Moresby region three reverse-fault bounded zones expose over 1000 metres of Palaeocene and Eocene cherty carbonates in a southwest verging imbricate stack. Mesozoic calcarenites and limestones crop out sporadically at the leading edge of the major faults. Dips are homoclinally 30-60 degrees to the northeast in both monotonous sequences and mesoscale horse structures. Rare macroscale folds plunge northwest and soft-sediment deformation structures predominate in parts of the westem thrust slice. Chert nodules, commonly 80cm in diameter, are often concentrated adjacent to major fault zones. Towards the hinterland flat lying Astrolabe Agglomerate and Siro Conglomerate indicate tectonism had quiesced by the Early Pliocene (Fig. la). Three decollements are present offshore within 5 seconds two way reflection time (Fig. lb). Normal faults on the cratonic passive margin occur near the Papuan frontal thrust and listric normal faults occur at the present day shelf edge. Onshore vibroseis data display thrusted folds in a blind imbricate fan (Fig. Ic). Thrust faults affecting the unconformable overlap sequence and active mud diapirism in the frontal areas are indicative of continued deformation. At outcrop the overlap unconformity separates successor Middle and Upper Miocene reefal limestones and mudstone from thrusted Lower Miocene clastic and Eocene cherty formations. Fault propagation folds within the overlap sequence have amplitudes greater than 1km (Fig. Id). Frontal folds demonstrate considerable crestal erosion and are onlapped and overstepped by Pliocene molasse (Fig. le). Strong reflectors with high interval velocities are believed to originate from a thick, fold-supporting Neogene limestone. North of the overlap unconformity the Eastern Papuan Fold Belt is separated into two distinct provinces by the Aure Fault which is a major west verging thrust with detachment in the Mesozoic. It displaces Mio-Pliocene sediments of the Aure and Orubadi Formations (3000 metres thick), Palaeocene to Early Miocene limestones of the Mendi Group (1000m) and Mesozoic clastics at least 300m thick. Gravity data suggests there is no or little basement involvement. The marked change in structural strike of the Aure Fault is believed to be in response to a lateral ramp created by the subthrust Purari Fault. Structural strike to the south of the Aure Fault is generally towards 340 degrees. The frontal folds expose Plio-Pleistocene sediments whilst the hindward structures expose Middle Miocene strata. A southern sub-zone is characterised by imbricate slices and tighter, narrower thrusted anticlines whilst folds in the northern sub-zone are more open and separated by broad synclines. (Fig. If) Axial faults, overturned forelimbs and mesoscale footwall anticline/syncline couplets characteristic of fault propogation folds are common . North of the Aure Fault north-easterly dipping fault blocks expose a thick Miocene turbidite sequence. Fault propogation folds are again common farther west and towards the foreland where the Neogene carbonate is about 1000 metres thick (Fig. Ig). They trend northwest and occasionally expose Mesozoic strata in front of a broad syncline containing deformed Pliocene sediment. The Bevan Fault is a major thrust with about 2km vertical displacement at the leading edge of a hindward dipping duplex. This change in structural style is probably a result of northward thinning of the carbonate sequence to about 300 metres as measured at the Erun Anticline (Fig. Ih). In summary, predominantly Middle Miocene and younger sediments, and in the far north and west some Mesozoic strata, are exposed to the north of an overlap unconformity within the Eastem Papuan Fold Belt. Southwards the fold belt narrows and predominantly Pliocene strata occurs at the surface but Mesozoic to Eocene strata are again exposed in the Port Moresby area. Previously referenced structural provinces, from south to north, are the Port Moresby Sector, Oroi Sector, Lakekamu Embayment, the Tauri-Kapau Margin, Aure Tectonic Belt and the Pio-Purari Region. They are generally confined by the Papuan and North New Guinea Thrusts and fault propogation folds and/or imbricate stacks are common throughout. They are therefore all integral components of the single structural regime of the Eastern Papuan Fold Belt.

THE BANDA ARC - AUSTRALIA COLLISION ZONE T.R, Charltoni, A.J. Barber* & M.E.M. de Smet^ 1 Department of Geological Sciences, University College, Street, London WCIE 6BT, U.K.

Gower

2 Geology Department, Royal Holloway and Bedford New College, Egham, Surrey TW20 OEX, U.K. 3 Instituut voor Aardwetenschappen, Vrije Universiteit, Postbus 7161, 1007 MC Amsterdam, The Netherlands. The Banda Arc is a tightly curved island arc system forming the eastern subduction boundary between the Indo-Australian and Eurasian plates. As a result of Neogene collision, Australian continental crust is now entering the subduction system around the complete 180 degree bend of the Banda Arc. Rocks of Australian origin are exposed in the Banda forearc islands from Timor around the arc to Seram. The rocks exposed in these islands accumulated on the outermost edge of the Australian northwest passive margin, and have been accreted into the Asiatic active margin by subduction processes. Lithological units on Timor are either derived from the Australian passive margin (the parautochthonous units) or were located in the pre-collisional Banda forearc (the allochthonous units). The Maubisse Formation has been interpreted as allochthonous but it is increasingly clear that it is closely related to Australia and therefore may be parautochthonous. The Timor allochthon consists of the Mutis/Lolotoi Complex (ophiolitic metabasites, serpentinites and associated metasediments) , the Aileu 'Formation' (metasediments and metabasites in northern East Timor), the Palelo Group and Cablac Limestone Formation (sedimentary units resting unconformably on the Mutis/Lolotoi Complex), and the Manamas Formation (arc volcanic rocks). All these units could have originated in a pre-collisional forearc complex like that exposed in Sumba. In cross-section, Timor has the basic form of a subductionrelated forearc complex. In the northern (inner) slope of the Timor Trough and onshore into the Kolbano area of southern Timor, the dominant structural process was the accretion of Australian margin sediments into a frontally accreted imbricate stack. North of the Kolbano Mountains the Central Basin corresponds to a forearc basin in submarine forearcs. In the northern mountains, late Palaeozoic and early Mesozoic Australian margin sequences have been accreted into the Timor forearc complex by underplating. Australian sequences are structurally overlain by the allochthonous units. Locally the Maubisse Formation has been emplaced on top of the allochthonous units, possibly by out of sequence thrusting. The structure of Timor is complicated by an oblique component of convergence in the collision. Convergence in a northeasterly direction has resulted in NNE-SSW left-lateral wrench faulting.

19 Tectonic loading within the thrust stack has resulted in overpressuring of the shales within the Australian marginal sequence. Overpressuring is released along the wrench faults, with the expulsion of mud, water and hydrocarbon gas, producing mud volcanoes at the surface. At deeper levels, dewatering of the liquified shales produces scaly clays. We believe this mechanism to be the origin of the extensive Bobonaro Complex on Timor, which has alternatively been interpreted as a sedimentary olistostromal deposit or as a tectonic melange. A number of authors e.g. Audley-Charles and co-workers have inferred a Pliocene to Recent age for the deformation, while Berry, Grady and CO—workers have suggested a late Miocene inception of the collision based on age determinations from the Aileu Formation in East Timor. Late Miocene ages from the Aileu Formation may be the result of thermal events within the precollisional forearc complex. Deformation had begun in the allochthonous units during the late Miocene but arc-continent collision did not commence until the Pliocene. Interpretations of the Timor Trough suggest that tectonic activity at the deformation front slowed during the Pleistocene, and has only continued at a very low level to the present time. The Tanimbar islands can also be interpreted in terms of subduction processes. The inner slope of the Tanimbar Trough and the main island of Yamdena comprise an imbricate stack formed by frontal accretion. The decollement occurred at the base of the Miocene or near the top of the Palaeogene. Yamdena consists almost entirely of deepwater Miocene carbonates repeated by thrusting. Young out-of-sequence thrusting and left-lateral wrench faults bring Mesozoic rocks up to higher structural levels in the straits between Yamdena and the inner chain of islets. The trace of the thrust front and the line of the wrench faulting is marked by active mud volcanism. Fragments of rock types comparable to the allochthonous units in Timor are found in these mud volcanoes. Deformation in the Tanimbar islands, based on the age of the youngest imbricated unit (late Miocene), and the oldest post-deformation sediments (early Pleistocene), is of Pliocene age. Seismic profiles across the Tanimbar Trough show that the deformation front is overlain by sediments about 300m thick. This indicates that the deformation front is no longer active. The Kai islands, located at the apex of the 180 degree bend in the Banda Arc, consist of two distinct provinces: the eastern island, Kai Besar, consists of tilted normal fault blocks, downthrowing towards the Aru Trough. The Aru Trough has an extensional origin, in contrast to the Timor-Tanimbar Trough to the south. Extension continues westwards into Kai Besar. The central and western Kai islands are low-lying coral reef islets. Mud volcanoes break through reef in the central island groups (Kai Kecil and the Tayandu islands) indicating that these islands are part of the forearc, and that the presently inactive deformation front lies between Kai Kecil and Kai Besar. High grade metamorphic rocks in the eastern Kai islands are probably backthrust Australian basement.

Successive early Proterozoic deformation and metamorphic events in the Anmatjira Range, central Australia

Geoffrey L. Clarke^, William J. Collins^ & Ron H. Vemonl

1 School of Earth Sciences, Macquarie University, NSW. 2109 ^Department of Geology, University of Newcastle, NSW, 2108

Early Proterozoic rocks exposed in the Anmatjira Range, central Australia, preserve evidence of three major deformation events, D1.3, each event being synchronous with a temporallydistinct metamorphic event. The effects of Mi are preserved in the northwestern end of the range, where there is a sharp lateral transition from amphibolite to extremely low-pressure granulite facies; peak metamorphism is diachronous to a recumbent, layer-parallel Si foliation. Regionallyextensive megacrystic granitoids intrude post Si, and were deformed during a southwestwarddirected ductile thrusting event, D2. At the southeastern end of the range, D2 accompanied granulite facies metamorphism, which was possibly contempory with an amphibolite-facies overprint of Mi assemblages in the northwest. A further metamorphic event, M3, is evident in the southeast where migmatitic leucosomes that transect S2 are deformed by upright, southeast-trending F3 folds. Biotite+muscovite±kyanite-bearing mylonite zones transect all the high-grade foliations. Derived pressure-temperature paths suggest that cooling occurred at constant, or slighdy incresing, pressure subsequent to each metamorphic event. Events D1.3 did not result in an overthickened crust; the rocks were probably returned to the earth's surface by the much later shear zones.

WHY GOLD MINERALISATION AT KALGOORLIE, WESTERN AUSTRALIA, IS NOT RELATED TO WRENCH TECTONICS. John M.F. Clout Department of Earth Sciences,

Monash University,

Clayton,

VIC. 3168.

Wrench tectonics has recently been applied to the Kalgoorlie District to explain the localisation of gold mineralisation as well as the associated fracturing and dilation ( Mueller & Harris 1987). Application of the wrench tectonic model requires an assumed simple shear deformation in the area of interest ( Cloos, 1929 and Riedel, 192 9). The 3-D movement picture for faults in the district and the Golden Mile Deposit are inconsistent with deformation by simple shear. Dilation of the auriferous Golden Mile lodes is also not explained by simple shear. Kalgoorlie lies within the Norseman-Wiluna Greenstone Belt of the Archaean Yilgarn Block of Western Australia. This belt comprises elongate NNW trending greenstones cut by at least three sets of major (satellite and aeromagnetic) lineament features ( Groves et al. 1986 , O'Driscoll, 1985 ). Significant gold production (> 1800 tonnes Au) in the belt has come from a number of deposits, many of which are located adjacent to major north-north-west trending lineaments (Groves et al., 1986). In the Kalgoorlie-Kambalda area the NNW trending Boulder-Lefroy lineament is a major tectonic feature which incorporates the interpreted Boulder Fault at Kalgoorlie and the Lefroy Fault at Kambalda. This lineament has been widely interpreted as a single major strike-slip fault ( Mueller & Harris 1987, Eisenlohr 1987 ). Regionally extensive north-south striking oblique and strike slip faults have been interpreted as major splays which developed off the Boulder-Lefroy lineament under assumed regional simple shear ( Mueller & Harris 1987, Eisenlohr 1987, Groves 1988). A number of significant gold deposits lie on or around these interpreted splay faults. The Kalgoorlie District geology (Fig.l) is characterised by upright Di megascopic folds (eg. Kalgoorlie 3yncline ) associated with north-north-west trending Di faults (eg. Golden Mile Fault) which strike sub-parallel to stratigraphy. The movement sense on these faults is unknown except for some low angle thrust faults (eg. Flanagan Fault). The Boulder Fault is poorly exposed, the sense of movement (eg. Mueller & Harris 1987) is equivocal and its timing (Di?) is still unclear. Di folds and faults were cross-cut and offset by north-south trending oblique faults (obliques) during D2. These oblique faults have oblique net slips and tend to anastomose along strike (eg. Golden Pike and Adelaide Faults). Gold deposits within the district are either confined between sub-parallel obliques or located in the immediate footwall or hangingwall of an oblique. The relationship between the Boulder Fault and the obliques is unclear. At least some of the interpreted strike deflections of the Boulder Fault can be explained by obliques (eg. Golden Pike Fault) offsetting a possibly earlier unrelated Boulder Fault. The Golden Mile Deposit (>1200 tonnes Au) is a complex system of shear zones confined between the sub-parallel Adelaide and Golden Pike faults and developed largely within the Golden Mile Dolerite. The shear zone system has been superimposed onto Di folds and faults (Woodall, 1965). Away from the deposit the Golden Mile Dolerite is far less fractured despite similar oblique faulting. Woodall (1965) argued for a close spatial and temporal relationship between the obliques and gold mineralisation. The Golden Mile shear zone system is composed of many brittle zones (cataclasites) with 10-2500m lateral and 10-1200m vertical extents and widths of 0.01-1.5 m. The shear zones, particularly the mineralised ones, have been subdivided into main, caunter and cross lode orientation groups on the basis of their strikes (Finucane, 1941) . In detail there is a continuum of orientations between the four groups. Calculated net slips on numerous shear zones are in agreement with kinematic indicators such as asymmetrical tails on rotated porphyroclasts. Almost all the shear zones have oblique slips and reverse senses of movement with the exception of a small number that have exactly the same strike as either the average main (dip-slip) or cross lode (tensional, for any dip) (Fig.2). The slip and shear zone orientation data were compiled from within and outside of the deposit. The slip normals for this data were plotted using the method of Arthaud (1969 ) combined with later modifications by Aleksandrowski (1985 ). The slip normals plot along a single vertical plane along 330"" and indicates uniform shortening parallel to the

cross lode strike and extension within a vertical plane perpendicular to the cross lode strike. This constrains the stress field to an approximate axial compression with sigma-1 oriented along 060®and horizontal. The movement picture thus rules out a stress field involving simple shear. The auriferous lodes form a small subset of the total shear zone population within the Golden Mile. A lode comprises either a shear or a dilationally brecciated shear zone and a sericite-carbonate -pyrite alteration halo. The high dilational breccias comprise altered wall rock or gouge fragments cemented by crustiform textured euhedral quartz and carbonate vein fill. If the shear zones were moving under an applied sirrple shear^ the overall dextral oblique-slip sense of movement on the Adelaide and Golden Pike faults would allow the cross lode direction to be dilational and the main and caunter orientations to be non-dilational. Detailed orientation data indicate that only mains and caunter directions were dilational. Simple shear across the deposit is incortpatible with the observed dilation even if the sense of motion on the confining faults was reversed. REFERENCES Aleksandrowski^ P.^ 1985. Graphical determination of principal stress directions for slickenside lineation populations: an attempt to modify Arthaud's method. J. Struct. Geol. 1, 73-82. Arthaud, F.^ 1969. Methode de determination graphique des directions de raccourcissementr d'allongement et intermediaire d'une population de failles. Bull. Soc. geol. Fr., 7 Ser. 11, 729-737. Eisenlohr, B., 1987. University WA Geology Department and Extension Publication 11, 85-95. Finucane, K.J., 1941. East-dipping strike faults on the Boulder Belt, Kalgoorlie. Proc. Australas. Inst. Min. Metall., 124: 203-215. Groves, D.I., Ho, S.E., Houstoun, S.M. and Phillips, G.N., 1986.In Berkman, D.A. (Ed.), Geology and Exploration, 13th Comm. Min. Metall. Inst. Congress, Singapore 1986, 2, 243-250, Tien Wah Press, Singapore Groves, D.I., 1988.Gold mineralisation in the Yilgarn Block, Western Australia (Abs.). Bicentenial Gold 88. Geol. Soc. of Aust. Abstracts No. 22. Mueller, A. & Harris, L.B., 1987. ' University WA Geology Department and Extension Publication 11, 97-107. O'Driscoll, E.S., 1985. Observations of the lineament-ore relationship. Philos. Trans. Roy. Soc., London; Series A. Woodall, R., 1965. Structure of the Kalgoorlie Goldfield; in McAndrew, J. (Ed.), Geology of Australian Ore Deposits (2nd Ed.), 71-79. 8th Comm. Min Metall. Cong., Melbourne.

Magnetic North

MT CHARLOTTE OREBOOY

^- WSTERf* LODE (H3 F1»0 (BFB) CZ] OoW*!* MN« Dol«rlt« (O.M.O.) rn Paring* Baaatt (P.B.)

^BbULfiER

CZ3wHi.m.townOol.c.f.

f I 0«voo CO(MOI« BataH f^ H«in«M Laka Saroantmna EE3 «thi8 block up

Qaology aftar WooM <1«6S). Travta at H (1*7 II Fig.l Interpreted geology of the Kalgoorlie District. The Golden Mile is defined by the cluster of lodes confined between the Golden Pike and Adelaide faults.

Fig. 2 Shear zone spatial geometries and displacement senses. Main = dip slip. Cross = tensional, remainder = oblique slip

23 TECTONIC EVOLUTION OF THE TASMAN OROGENIC CONTEXT OF THE PACIFIC MARGIN OF GONDWANA Peter J. Coney, Department of Geosciences, Tucson, Arizona, 85721

The

University

SYSTEM

IN THE

Arizona,

From the perspective of most Phanerozoic continental margin mountain belts the Palaeozoic Tasman orogenic system is distinctively unique. For example, it has no transitional through-going precursor miogeocline or resulting foreland fold and thrust belt typical of North American mountain belts. Instead, what appear to be variable but mostly thick deep marine sedimentary and volcanic facies are abruptly juxtaposed against the ancient Precambrian basement of Australia across faults or narrow belts of poor exposure. Another way of putting this is that except for a very thin fringe along its western margin the entire Tasman orogenic system is "suspect" in the sense that its paleogeography is variably uncertain through much of Palaeozoic time. Since evidence for the presence of ancient cratonic Australian Precambrian crystalline crust below the Tasman is either lacking or not obvious this suggests the entire belt is accretionary and as a result the Australian craton must have increased in size by almost a third mostly during Palaeozoic time. The Tasman can be characterized as composed of two major sectors. A western belt of lower Palaeozoic terrains includes the Tasmania, Lachlan, Thomson and Hodkinson-Broken River terranes, and an outer, or eastern, belt of the greater New England terranes. At present levels of perception all or most of these terranes are best considered as composite. The western belt is characterized in the Lachlan by scattered submarine mafic volcanics of Cambrian age swamped in a flood of deep marine, mostly Ordovician, quartz-rich turbiditic sand and mud. Northward the western belt is mostly covered, metamorphosed, or poorly known in the Thomson sector, but certain similarities with the Lachlan are notable. In the Hodgkinson-Broken River sector smashed lower Palaeozoic greywacke, chert, and occasional mafic volcanics are abruptly juxtaposed against the Precambrian craton of the Georgetown block in north Queensland. The outer New England belt is mostly accretionary and made up of collapsed mid- to late Palaeozoic deep marine sedimentary and volcanic rocks juxtaposed against more proximal marine to shelfal sediments and volcanic piles. Most of the western belt suffered major orogenesis in mid-Palaeozoic time characterized by extreme telescoping and crustal thickening, variable extension and disruption, all accompanied by a voluminous magmatic "flareup" of extraordinary proportions. In contrast, the outer New England belt telescoped and collapsed in mostly Carboniferous time with continuing consolidation and widespread magmatism extending into late Palaeozoic-early Mesozoic time. In the Lachlan, at least, isotopic data and computational estimates based on shortening and present

crustal thickness suggest an attenuated or perhaps originally thin probably Late Proterozoic "sialic" crustal substrate must have existed below much of the evolving Ordovician "mud pile". If no continental basement lay below the "oceanic" parts of the New England belt, which seems likely, extreme telescoping and some combination of obduction and tectonic underplating and wedging seems necessary to explain present crustal thickness there. Most of the Tasman granites are probably recycled Tasman crust and resulted from crustal thickening from telescoping and melting perhaps aided by a thermal flux from delaminated overthickened mantle lithosphere. The Tasman shares several important characteristics with the rest of the Palaeozoic Pacific margin of Gondwana as exposed in South America and Antarctica. The lack of transitional miogeoclinal facies, the commonality of thick deep marine lower Palaeozoic rocks, and a persistent but variable mid-Palaeozoic orogenic episode over a distance of 20,000 km is striking as is also late Palaeozoic massive accretion of oceanic assemblages in southern Chile and New England. This suggests motions of Gondwana may have significantly influenced tectonic evolution from the northern Andes to Queensland. Recent proposed revisions in Gondwana's APW path yield a midPalaeozoic "advance" of Gondwana over the paleo-Pacific that seems to bracket much of the mid-Palaeozoic orogeny. Late Palaeozoic accretion in Chile and New England seems to correlate with an abrupt Early Carboniferous "elbow" followed by a clockwise rotation of Gondwana as it drove toward collision with North Amierica. Southern Chile and eastern Australia may have been leading edges sweeping up the oceanic accretions typical of both regions. Finally, the motion and collision of Gondwana with North America may have wrenched the entire Gondwana craton and North America as well. This perhaps produced two strikingly similar, but puzzling, coeval intraplate deep-crustal deformations: the Amadeus Basin-MacDonnell Ranges of central Australia and the Ancestral Rockies-Wichita belts of central North America. Both deformations reactivated Late Proterozoic intracratonic rifts.

25 Implications of an early fold/thrust event in the western Mount Isa Inlier Karen A. Connors Department of Earth Sciences, Monash University, Clayton, Victoria 3168 The ongoing controversy over an early thrusting event in the Proterozoic Mount Isa Inlier has prompted a detailed study. This has resulted in evidence for new interpretations of several aspects of the structural evolution. Regionally interpreted movement directions of the early thnists range from southerly directed in the northern Leichhardt River Fault Trough as well as the Mount Isa area (Bell, 1983, Bell, Perkins, and Swager, 1988), to westerly and northwesterly directed in areas east and south of the Deighton Pass (BMR, 1985), to easterly movement around Mount Isa (BMR, 1985). Previous workers (eg. Bell, 1983, Winsor, 1986, Bell, Perkins, and Swager, 1988) have defined three discrete folding events: Dj which produced E-W trending folds and the thrust faults discussed above, D2 involving E-W compression and vertical extension producing regional, upright, N-S oriented folds, and D3 involving NW-SE oriented structures. Evidence from this study suggests a structural history which is even more complex than that implied by the previous interpretations. Field evidence indicates the occurrence of five distinct fold generations Fi, F2, F3, F4, and F5. (A system of nomenclature specific to this area has been adopted in order to avoid unintentional correlation with the regionally interpreted events. The period of deformation labelled D4 in this study may correlate with the regional, N-S oriented D2 event of other workers, however it is difficult to confirm this possible correlation.) The overlapping ranges in orientation of structures resulting from Di, D2, D4, and D5 leads to a possible interpretation of a major period of orogenesis involving roughly E-W compression which incorporates all these events. No evidence of N-S oriented compression was observed, however the southerly directed thrusting interpreted by Bell (1983) may represent an earlier event elsewhere in the inlier. One scenario is that the Di, D2, and D3 of previous workers, in fact, represent periods of orogeny and not merely discrete folding events. In this case the five fold generations recognized in this area would all fall within the D2 orogeny. The area of study centers around the Haslingden and Mount Isa Group units of the Mount Novit Ranges approximately 20 km south of Mount Isa. In this region several different kinematic indicators exhibit a west over east sense of movement or a general W-E orientation of movernent possibly associated with thrust faults of the D2 folding phase. Two examples of small scale thrusts display a roughly west to east sense of movement. The prominent L2 mineral lineation is most intense in the units adjacent to the thrust where it plunges moderately to the west. Elsewhere L2 is weakly to moderately developed and plunges west to northwest on the western limbs of later folds. The consistent orientation of the mineral lineation across later folds suggests that the lineation is merely folded by subsequent events and has not been significantly reoriented. The mineral lineation is interpreted to have developed in association with the thrust faults and may represent the stretching direction during D2 deformation (ie. L2 is subparallel to the general direction of thrusting). The most significant structure found in the area is a large scale originally nearly recumbent F2 fold. The upper limb comprises much of the study area (roughly 5 km by 20 km present extent or greater), while the lower limb is exposed to the east as a result of refolding and is eventually cut-off by the Mount Isa Fault. The parasitic folds vary in orientaion from moderately west plunging with inclined axial planes to north plunging and upright due to a combination of original variation in plunge direction, non-cylindricity, and superimposition of younger structures. The thrust faults do not appear to be repeated to the east by the large scale F2 fold implying that thrusting occurred syn- to post-F2. An extensional crenulation cleavage (ECC) with a consistent west over east sense of movement has developed within the quartzites on the lower limb of the large scale F2 fold. The anisotropy in the quartzites which is affected by the ECC is interpreted as S2, suggesting that the ECC is syn- to post-S2. The ECC is interpreted to have developed in association with the thrust faulting. The D2 association of structures includes roughly N-S oriented thrust faults, non-cylindrical west to north plunging F2 folds, a W-E to NW-SE oriented mineral lineation, development of an ECC, and small scale thrusts (the two latter both with west over east senses of movement). This assemblage of structures may represent a period of roughly E-W compression in an orogenic belt involving easterly directed thrusting. Little evidence of the effects of the earliest recognized deformation, Di, can be observed in outcrop. The apparently recumbent, isoclinal nature and roughly N-S orientation of folds may be original or enhanced by subsequent events. The third event generated upright, NW-SE trending, doubly - plunging folds with an axial planar crenulation cleavage. D4 represents the most pervasive event, with the exception of D2, and the associated structures are prominent throughout the area. The characteristic upright, N-S oriented folds are non-cylindrical and commonly form doubly plunging structures which are similar to F3 folds. The S4 foliation forms a strong crenulation cleavage which is more

pervasive than S3. The younger D5 event resulted in NW-SE trending, upright folds with an associated fine crenulation (WNW-ESE to NW-SE) and restricted development of a crenulation cleavage. The consistent NW-SE to N-S orientation of structures from the five successive deformational events together with the similarity in style of deformation during the last three phases, support the concept of one major orogeny incorporating all these events. This E-W to SW-NE oriented compression produced a succession of structures beginning with two early stages of recumbent or nearly recumbent folding, thrust faulting, and concluding with three later phases of upright folding. The major conclusion of the work to date results from the recognition of the large scale F2 fold described above. A new theory for the structural relationship between the Haslingden Group rocks west of the Mount Isa Fault and the younger, lower grade Mount Isa Group units to the east of the fault can be proposed. Previous interpretations (eg. Lister, Etheridge, and Stewart, 1985) assumed the large scale N-S trending fold west of the Mount Isa Fault was a D2 antiform (ie. D4 this study) with an attenuated east limb and postulated an associated D2 synform in the Mount Isa Group units with the adjoining limb cut out by the fault. This implied west side up movement in a ductile shear zone prior to faulting. However, re-interpretation of the N-S antiform as an originally nearly recumbent F2 fold which has been reoriented to an upright position by D4 folding allows interpretation of large scale thrust sheets consisting of Haslingden Group rocks overlying the Mount Isa Group on the west side of the Mount Isa Fault. References: Bell, T.H., 1983, Thrusting and duplex formation at Mount Isa, Queensland, Australia: Nature, v. 304, pp. 493-497. Bell, T.H., Perkins, W.G., and Swager, C.P., 1988, Structural controls on development and localization of syntectonic copper mineralization at Mount Isa, Queensland: Economic Geology, v. 83, pp. 69-85. BMR, 1985, Nappe tectonics in the Mount Isa Inlier: Bureau of Mineral Resources Research Newsletter, v. 2, p. 11. Lister, G.S., Etheridge, M.A., and Stewart, AJ., 1985, Structure and tectonics of the Precambrian Mount Isa Inlier, northwest Queensland: Abstacts for the 14th BMR symposium, Canberra, October, 1985, BMR Record, 1985/33, p. 37. Winsor, C.N., 1986, Intermittant folding and faulting in the Lake Moondarra area, Mount Isa, Queensland: Australian Journal of Earth Sciences, v. 33, pp. 27-42.

27 THE EAST ASIAN COLLISION

ZONE:

HIMALAYAS TO BANDA ARC.

Coward, M. and Audley-Charles, M.J.

Geology Dept., Imperial College, London England.

Abstract to be supplied.

SW7

2BP,

EXPERIMENTAL FLUID-ROCK INTERACTION DURING DEFORMATION ELEVATED TEMPERATURES AND PRESSURES - PROGRESS AND PROSPECTS

S.F. Cox and M.S. Paterson Research School of Earth Canberra, A.C.T. 2601.

Sciences,

The

Australian

National

University,

P.O. Box 4,

Although the importance of fluid-rock interaction and mass transfer processes during crustal deformation is widely recognized, laboratory investigation of dissolution-precipitation deformation mechanisms so far has been limited for reasons of experimental difficulty. We report here on recent progress in the investigation of dissolution and mass transfer processes operating in conjunction with microcracking, grain translation, and crack healing and sealing processes. Deformation involving dissolution-precipitation processes has now been achieved at laboratory strain rates in quartz and quartz + mica grain aggregates at temperatures up to 900 ®C, confining pressures up to 300 MPa and with PH20/Pconf ^P ^^ 0.9. Both open-system and closed-system fluid-rock interaction has been simulated. The experimental design allows high fluid to rock ratios to be achieved at precisely controlled pore fluid pressures, and also facilitates measurement of dilatancy and permeability changes during deformation and fluid-rock interaction. We discuss microstructural development in experimentally deformed grain aggregates, the nature of grain interfaces, and pore-fluid distribution during dissolution-precipitation creep. The importance of high pore fluid pressures in enhancing permeability, fluid migration and the coupled operation of mass transfer and brittle deformation mechanisms is emphasised. The relative roles of diffusion and fluid migration as deformation rate-controlling mass transfer processes are evaluated and an approach to the derivation of constitutive relationships which describe dissolution-precipitation creep is outlined.

NUMERICAL MODELS DEFORMATION

OF FAULT INTERACTION

AND LOCALIZATION

S J D Cox CSIRO Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149 We have assessed tlie use of the Distinct Element Method in modelling the interaction of complex fracture arrays under compressive loading. Fracture networks with a complex geometry are a feature of immature fault systems as well as damage in samples broken in triaxial laboratory experiments. An important constraint on the stability of these is the progressive localization of deformation onto a dominant interconnected fault strand, and in particular, the relationship between localization and the peak stress. Tlie DEM is a versatile method particulaily well suited for modelling problems combining a complex geometry with large relative displacements. The body under consideration is represented by an assembly of discrete, defonnable continuum elements. Large displacements are easily attained since the discontinuities are formed simply by the contact of adjacent blocks, in contrast to Finite Element Models in which special joint elements have to be used. A diawback is that the geometry of the blocks must be specified at the start of the simulation, since no rupture of blocks is pennitted. The code used here (UDEC) has a fully explicit integration scheme to solve the equations of motion, so large siinulations can be run without exhausting core storage. However, this tends to lead to long convergence tiines. We ran numerical experiments to simulate microfracture growth and rupture localization in an elastic-brittle material. A random array of potential cracks was defined in advance within the sample for each realization. We generated a stochastic network, which was then geometrically transformed so that the stereological parameters matched reported experimental values. However, until a critical stress was exceeded for each segment the elastic properties of the ''candidate cracks" were stiff enough that they had no effect on the bulk properties. For subsequent deformation on these contacts, the shear tractions are determined by the Coulomb friction coefficient. For this series of simulations the material paiameters (elastic modulus, joint cohesion, friction coefficient etc) were held constant, since we were principally concerned to assess the effect of the fracture array geometry on the stress-strain behaviour. The simulation was designed to reproduce a triaxial fracture test in a very stiff loading frame. We monitored axial stresses at two points within each of the loading platens and the radial displacements at the mid-points of the sides. In addition, complete images of the model were saved at regular intervals and were inspected to see which joints had failed; i.e. which cracks had fonned. In this way the progress of nipture could be tied to the stress-strain history. Tlie stress-strain histories were veiy similai to experimental obsei-vations. Higher confining pressures tended to lead to more stable post-peak behaviour. We were able to reproduce the confining pressure dependence of strength for a suite of fracture networks generated using a particular method. Damage was initiated at approximately half the peak strength, but a significant observation was that the localization of defonnation generally occurred after the peak strength. However, this type of network (a Delauney triangulation of Poisson points selected over the area of the sample) failed to produce the amount of dilatancy observed in experiments. We also tested the Voronoi tesselation method of generating the initial fracture network. This was found to reproduce the dilatancy characteristics better, but was also considerably stronger, and took about five times as much computer time to converge.

A typical stress-strain history with the associated clainage patterns.

til 8trcse>8trQln

—U -x- U.UIK U -y- IQ^UB

Generating a suitably interconnected, but random, netwoiic is not a trivial task, and this appears to be the principal liinitation on using the DEM to model dainage growth. Furthermore, the microcomputer codes used are not realistically fast enough for an exhaustive suite of simulations. We conclude that a very detailed model of the fracture airay is needed to allow this method to be used successfully. Tlie DEM may, therefore, be useftil for modelling actual fault arrays where the geometry can be accurately defined.

IMPLICATIONS OF THE FINISTERRE TERRAIN COLLISION FOR DEVELOPMENT OF THE NEW GUINEA OROGEN. Keith A.W. Crook Department of Geology, Australian National University, GPO Box 4, Canberra, ACT 2601. The Finisterre terrane (Fig.1) occupies a key position in the New Guinea Orogen (NGO). It is the most recently accreted terrane on the PNG mainland, lying on the plate boundary that separates the dispersed island terranes of NE PNG from the already-amalgamated terranes that constitute the remainder of the NGO. It also adjoins the three principal structural units in the mainland NGO, recognised in a new synthesis of PNG geology prepared for the August 1988 Collision Zone Field Conference (Hilyard et al, 1988; Rogerson et al, 1988). These structural units (Fig.1) are: (a) the New Guinea Thrust Belt to the west, regarded by many workers as a region of sinistral terrane dismemberment, in which the sediments display slaty cleavage; (b) the 14,og

1440

1470

•

1530

I560

|57«30'E

Finisterre terrane

Plot*

boundary- eovergent

•'

Plate

boundary - divergent

Intro - plate foult Trend of fold ond minor foult (tructure*

Figure 1:

isQO

(x^

Palo«oioic crysfollin* bas«m«nt or ollochthoM Abeolute plote motion ond velocity' Velocitiee In . cm/year Relative plate motion

^ Boundory of

Popuon ploteou

PNG Tectonic & structural elements (after Rogerson et al, 1988)

Owen Stanley Thrust Belt to the south, including the Papuan Ophiolite and associated high-P,low-T metamorphics; and (c) the Papuan Fold Belt (PFB) to the southwest, a foreland fold and thrust belt in which sediments are uncleaved or display reticulate cleavage. Structural trends in the PFB are inflected opposite the Finisterre terrane (Fig.1). Not surprisingly, collision of the Finisterre terrane has been postulated as a mechanism for development of the observed structural pattern in the mainland NGO. However there are problems with this hypothesis. On the one hand, relationships between the Finisterre terrane, the Ramu infa-Basin (Miocene) and the North New Guinea Basin (NNGB) (a Pliocene-Recent overlap sequence) appear to support a mid-Miocence collision. Micaceous graywackes, which must have been sourced from the southern side of the orogen, have been noted north of Lae (where forams indicate a mid-Miocene to early Pliocene age) and elsewhere along the southern flank of the terrane. On the other hand, paleomagnetic data (Falvey and Pritchard, 1984) supports a Pleistocene collision, concomitant with opening of the Manus Basin during the past 4Ma. Furthermore, facies studies in the Leron Formation on the southern margin of the Finisterre terrane (Crook, 1988a) imply turbidite deposition at bathyal-abyssal depths during the early Pleistocene. Neotectonic data (Crook 1988b) yield uplift rates of 2.5-7.6m/102 a for this area, consistent with Chappell's (1974) data from the northern Huon Peninsula. The Leron Formation is interpreted as part of the NNGB overlap sequence by Hilyard et al. (1988). I regard it, together with the Mena and Movi Beds to the north and coeval units to the NW, as a subduction complex in the Finisterre terrane. Herein may lie the resolution of the enigma. All mapping is model-driven, consciously or unconsciously. Both models - overlap sequence and subduction complex - must be kept in view during field work and interpretation of structural and seismic reflection data. Chappell, J. (1974) Geol. Soc. Amer, Bull., 85, 553-570 Crook, K.A.W. (1988a) Sedim. Geol., (in press). Crook, K.A.W. (1988b) Tectonophysics, (in press). Falvey, D.A. & Pritchard T. (1984) Trans. 3rd. Circum-Pacific Energy & Mineral Resources Conf. 1982, CPCEMR, Tulsa, 593-599. Hilyard, D; Rogerson, R. & Francis, G. (1988) Geol. Surv. PNG. Rep. 88/9, 44p. Rogerson, R; Hilyard, D; & Silver E. (1988) Geol. Surv. PNG Rep. 88/17, 98p.

33 TIME - BALANCED SECTIONS Declan, G De Paor, Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland, 21218, USA

There are four ways to balance a geological cross-section; line balance, area balance, kink construction and construction of a strain grid (for a review, see De Paor 1988) . All contain assumptions about the initial and final states (eg. conservation of bedlength, fault cut-off angles etc) and none adequately accounts for complex displacement, deformation, and replacement phenomena that occur in real system. Time balancing a section involves computerized monitoring of displacement paths in XZT space where X and Z are section coordinates and T is time. All paths must be compatible with the grade of material in the final section and with its state of deformation. The temporal dimension of a balanced section is best illustrated using a temporal map - an XT projection of XZT space. This new type of mapping reveals section history with the same clarity as the spatial relations displayed on a traditional structural map. An application to the Palaeozoic Innuition fold belt is used to illustrate the method. De Paor, D.G. 1988 Balanced in thrust belts Part I: Construction. Amer. Assoc. Petrol. Geol. Bull. 72: 73-90.

Late Proterozoic deformation and the evolution of the northern margin of the Amadeus Basin. P. Ding, P.R. James, M. Sandiford Department of Geology and Geophysics, University of Adelaide, GPO Box 498, S.A. 5001. Folds, faults, thrusts and fold nappes along the northern margin of the Amadeus Basin involving the oldest part of the sedimentary sequence (Heavitree Quartzite and Bitter Springs Formation) and the underlying metamorphic basement of the Arunta Inlier have previously been ascribed to the Late Palaeozoic Alice Springs Orogeny. We demonstrate that at least some of these structures belong to an older. Late Proterozoic, deformation associated with the development of the angular unconformity above the Bitter Springs Formation. The Amadeus Basin sequences above the Bitter Springs Formation (Areyonga to Pertnjara Formations) show upright open folds, conjugate reverse faults, and numerous brittle normal faults (Figure 1). In contrast, the Bitter Springs Formation and underlying Heavitree quartzite display complex structures which are best interpreted as resulting from two deformation episodes. The earlier structures are characteristically "thin-skinned" and include asymmetric, tight to isoclinal, recumbent folds and thrust nappes which are cored by metamorphic and igneous basement (e.g. the Ormiston and Arltunga Nappe Complexes), in places truncated by the unconformity above the Bitter Springs Formation (Figure 1). This deformation event is named the Arltunga Orogeny. Subsequent upright folds and steep ("thick-skinned") brittle faults resemble the structures in the younger sequences and are considered to be Alice Springs in age. Outcrops of the unconformity are well preserved at many localities. Along the northwest limb of the Limbla Syncline steeply dipping Bitter Springs Formation (Loves Creek Member) is overlain by shallow Areyonga Formation. In the eastern closure of the Gaylad Syncline an isoclinally folded sequence of the Bitter Springs Formation (Gillen Member) is overlain by shallow dipping Pioneer Sandstone.

A TYPE CROSS SECTION AT THE NORTHEASTERN MARGIN OF THE AMADEUS BASIN NNE

Ruby Gap Nappe Complex lOKm

-J

Qaylad Synciine

Pzp €-Pzm

Pertnjara Group Cambrian to Mereenie Sandstone

Puf-Puj

Olympic Formation to Julie Formation

Pua-Puk Areyonga Formation to Aralka Formation putppub Heavitree Quartzite to Bitter Springs Formation PCa Metamorphic and igneous basin

Figure 1

Conglomerates and

Pioneer

in the Areyonga

Sandstone

contain

Formation,

fragments

Olympic the

Formation

Bitter

Springs

Formation, Heavitree Quartzite and their basement. Metamorphic clasts include mylonites resembling those found in shear zones in the Arunta Block. Thickening of the Areyonga Formation, Olympic Formation and Pioneer

sandstone

near the northern

margin

the

Amadeus

basin

together with the above cited evidence for convergent deformation in the

underlying

sequences

indicates

deposition

flexural

basin

developed to the south of a tectonic load emplaced along the present margin of the Amadeus basin. This model for the deformation of the northern margin of resembles

closely,

penecontemporaneous

the Amadeus Basin during the Arltunga although Petermann

clearly Ranges

less

Orogen

intense along

Orogeny

than, the

the

southern

margin of the Amadeus Basin, and invites a reconsideration of many of the structures previously ascribed to the Alice Springs Orogeny.

The structural Range, (N.T.)

and

metamorphic

history

the

Reynolds

Dirks P.H. Department of Geology, University of Melbourne, Vic. 3052 The Reynolds Range is a NW-SE. striking Mid-Proterozoic metamorphic fold belt located in the central part of the Arunta Block, approximately 150 km. NW. of Alice Springs. This fold belt is bounded to the south by the relatively undeformed sediments of the Late Proterozoic to Early Paleozoic (900Ma-400Ma) Ngalia Basin and to the north by a related metamorphic fold belt that crops out along the Anmitjara Range. Along the Reynolds Range a sequence of well differentiated quartzites, shales and carbonates can be recognized (the Reynolds Range Group sediments), that were probably deposited in a shallow marine barrier island type environment. They were laid down on a basement of folded poorly sorted sandstones and shales in the NW. and granitic gneisses and mafic and felsic granulites in the SE. All units overlying the basement, have been affected by one high temperature, low presure metamorphic event that was centered in the SE.part of the range. Metamorphic grades change along the range from greenschist facies in the NW. to low granulite facies in the SE. , with peak assemblages containing cordierite,- sillimaniteV k-spar, garnet, orthopyroxene and quartz indicating temperatures of around 750 C and pressures of up to 4-5 kb. The granulites are separated from the greenschists by a 10-15km. wide zone of near complete retrogresion. The basement units record a poly-metamorphic history and were subjected to at least one earlier cycle of high grade metamorphism. The structural development and the relative positioning of the metamorphic peak is very similar throughout the Reynolds Range, irrespective of peak metamorphic conditions. Five deformational events can be recognized within the Reynolds Range Group sediments. The earliest event (Dl), occurred pre peak metamorphism and is only recognised as straight inclusion trails (SI) in contact metamorphic andalusites. These have grown in aureoles around granites that intruded during the ensueing HT-LP.metamorphic event.In the high grade terrains this was accompanied by extensive partial melting of the more pelitic units. The second event (D2) occurred after peak metamorphic conditions were reached. It is characterised by tight to isoclinal folding along a near horizontal NW-SE trending axis together with the development of a penetrative subvertical axial planar cleavage (S2) and a steeply dipping mineral elongation lineation. The D2 fabrics were formed by an alignment of colourless micas in the NW. and sillimanite

and biotite in the SE in a NE-SW trending flattening field. Locally NW-ward thrusting on S2 took place causing extensive mylonitisation of the associated foldlimbs. During progressive deformation a variety of overprinting asymmetrical boudins, shearbands and kinkzones formed indicating a gradual anticlockwise shift of the principal stress direction within a horizontal plane. Partial melts that were present within the amphibolites and granulites were channeled and localised along extensional structures. These meltpockets are rich in garnet^ cordierite, k-spar and occasionally sillimanite. This progressive deformation culminates in the development of large scale (0.5-2km.) asymmetrical E~verging folds (D3) . They have subvertical ENE-WSW striking axial planes (S3), and steeply east plunging foldaxes. Such folds are related to extensive dextral strike-slip movement on several major shearzones that run along the axis of the range. Concomittant with the development of the D3 structures the rocks were extensively retrogressed on a regional scale. This was probably enhanced by fluid movement along the strike slip zones. By this stage high grade partial melts have evolved into granitic melts with a minimum melt composition. These in turn developed into pegmatites that intruded along the axial planes of the large scale folds. Related retrogressive assemblages range from muscovite, biotite, chlorite.epidote,albite,actinolite and quartz in the NW., to biotite, kyanite, corundum staurolite and quartz in the SE. This last assemblage is indicative of an anticlockwise cooling path during retrogression. All the above structures have been overprinted by large scale (0.2-2.5km.) open warps (D4) . They are south verging structures with E-W striking subvertical axial planes (S4), and subhorizontal foldaxes. Associated with the warps, numerous small ultramylonite and cataclasite zones, recording N-up movements, formed parallel to S4 within the basement gneisses. The warps are unrelated to the earlier structures and should be considered as ductile flexuring preceding the development of an anastomosing system of mostly steeply north dipping and NNW-SSE striking shearzones (D5) . The majority of these shears record a N-up sense of movement often in combination with a considerable dextral component. Many of these late shears were generated along already existing (D3), fault surfaces, thereby deforming the earlier retrogressive assemblages. During the renewed movement on the shears, fluids were introduced that caused a localised second phase of low grade retrogressive metamorphism. This event most likely occurred during the Alice Springs orogeny (360Ma) .

UPPER MANTLE EXTENSION: THE

DEFORMATION PROCESSES DURING LITHOSPHERE ROLE AND MECHANISMS OF SHEAR LOCALISATION

M.R. Druryl, E.H. Hoogerduijn Strating^, R.L.M. Vissers^ and D.van der Wal^. 1 Research School of Chemistry, Australian National University, Canberra ACT 2 601 2 Institute of Earth Sciences, State University of Utrecht, 3584 CD Utrecht, The Netherlands. Extensional structures in the lithosphere have so far been studied either by indirect geophysical methods or by investigations on the surface geology of exhumed crustal extensional terrains. These studies have tended to concentrate on crustal behaviour wheras deformation processes in the mantle have received less attention. Alpine type peridotites are mantle slices which have attained high structural levels during extensional tectonics and are later imbricated with crustal units during subduction and collision tectonics (Nicolas 1986). Petrological and rheological constraints indicate that the metamorphic assemblages and many deformation structures are preserved from the high temperature extensional phase and largely uneffected by later low temperature events. A detailed study has been made of the deformation history and development of microstructures in mantle peridotites from the Voltri Massif in the Ligurian Alps of NW Italy. Early high temperature ( T=1000-1200^ C), low stress, deformation in the spinel Iherzolite facies (P=10-20 kb) is inhomogeneous and localised into kilometre scale shear zones. Deformation occurred by dislocation creep with concurrent recrystallisation involving a combination of subgrain rotation and extensive grain boundary migration. Fabric studies indicate slip predominantly on the [a](okl) system. The high temperature microstucture and fabric is locally overprinted by 0.5-200 metre scale mylonitic shear zones. Mylonitic microstructures suggest that dynamic recrystallisation occurred by a mechanism involving grain boundary bulging related to nucleation of new strain free grains. Fabric studies indicate predominant slip on [c](hkO) systems. The original mineral assemblage is unstable during deformation and ortho- and clinopyroxene break down to an ultra-finegrained aggregate of pargasitic hornblende, indicating an influx of water rich fluid. The syntectonic growth of amphibole suggests an upper temperature limit of 1000^ C for the mylonitic deformation, while a lower temperature of 750^ C is indicated by the stability of spinel + orthopyroxene + olivine + water which at lower temperatures reacts to form chlorite. The peridotite mylonite zones are cross-cut by intrusive gabbroic dikes. Some peridotite mylonite zones have gradational contacts with serpentinite mylonitic shear zones which were active at temperatures lower than 600^ C, wheras other serpentinite mylonites cross-cut early structures and are clearly related to imbreciation of the peridotite unit with crustal units. By combining the inferred deformation and metamorphic history a model can be developed relating the evolution of the peridotites to the large scale tectonics. The high temperature spinel assemblage and microstructure indicate that the Voltri peridotites were derived from the asthenosphere by adiabatic uplift from initial depths of at least 75-100 km. The high temperature deformation can be correlated with asthenosphere flow during uplift. Preservation of

the spinel assemblage shows that the hot peridotites stopped rising along an adiabatic path at depths of about 35-45 km and were subsequently 'frozen' into the lithosphere. During cooling from initial conditions of about 1000^ C and while the lithosphere was still warm (700-1000^ C) continued extension was accommodated by the development of sharply localised mylonitic shear zones. Shear zone development was closely associated with fluid infiltration and hydration of the peridotites. These structures are cut by gabbroic dikes which demonstrates that the mylonite zones were active while the peridotites were located above melt producing uplifted asthenosphere. After further cooling to temperatures less than 600^ C the peridotite mylonites transformed to serpentine mylonites. This model indicates a sharp distinction between deformation processes in the lithosphere and asthenosphere. Deformation at temperatures less than 900-1000^ C in the lithosphere is sharply localised into hydrated mylonitic shear zones, wheras deformation at higher temperatures in the asthenosphere is more homogeneous but still localised on the kilometre scale. This transition in deformation behaviour can be related to the degree of strain softening which must increase sharply below about lOOO^C. The onset of drastic softening at lower temperatures may be related to two possible causes; 1) a change in olivine mechanical behaviour, with a switch of predominant slip from [a] to [c] systems and a change of recrystallisation mechanism; 2) the nucleation of shear zones on the soft hydrated wall rock assemblage of fluid filled fractures. There is experimental evidence for the occurrence of 'intrinsic' localisation in dunites at low temperatures (Post 1977), however there are problems with the extrapolation of this data to natural conditions (Chopra and Paterson 1984) . If intrinsic localisation is responsible for the development of sharply localised mylonitic shear zones then the concentration of fluid flow in the mylonite zones would be a consequence rather than a cause of localised deformation. Even so, the occurrence of concentrated fluid flow in the mylonite zones should have an important additional softening effect on the mylonite. This could occur because of the effect of water in enhancing dislocation creep and recrystallisation in olivine. In addition, the formation of new hydrous phases weaker than olivine can lead to a drastic softening if these phases become dispersed along foliation planes by deformation. The rheology of the mylonite zones and of the lithosphere could then be controlled by phases such as amphibole, phlogopite, chlorite, talc and serpentine depending on the metamorphic conditions during deformation. The limited data on overprinting relationships between the high temperature asthenosphere fabric and the lithosphere shear zones are not compatible with simple continuity of lithosphere detachments into the asthenosphere. This indicates that the asthenosphere flow and the lithosphere deformation are largely independent. The anhydrous nature of the asthenosphere assemblage and geochemical evidence for only minor melting indicates that the water rich fluids were probablty not derived from the mantle. This suggests a crustal source for the fluids which may have gained access to the mantle via downward propagation of crustal extensional faults, thus indicating continuity of crustal detachments with mantle shear zones.

STRESS MEASUREMENTS IN THE BOWEN BASIN AND THEIR RELATIONSHIP TO THE STRUCTURAL SETTING OF THE BASIN

Jim R. Enever and Cliff W. Mallett

CSIRO^ Division of Geomechanics, Syndal, Victoria

The results of a series of in situ stress measurements conducted throughout the Bowen Basin will be presented and their relationship to the structural setting of the Basin illustrated. The hydraulic fracture technique has been used to measure profiles of the horizontal stress field magnitude and orientation with depth at eight locations throughout the Basin. Measurements have been made mainly within the coal measures, close to outcrop zones, and near operating coal mines. Measurements have been conducted to depths of approximately 250 metres, covering the zone of primary concern for coal mining. This information is compiled in the form of a regional map showing the variation in orientation of the horizontal stress field across the Basin, along with a set of notes which includes profiles of the magnitude of the horizontal stress field with depth at each measurement site. Three very general trends are evident from the results of the stress measurement programme: A suggestion that the horizontal stress field magnitude (relative to depth of cover) is less toward the north of the Basin compared to the South. Evidence of two distinct horizontal stress fields; a relatively isotropic stress field oriented somewhat west of north (approximately paralleling the outcrop of the coal measures) and a more imbalanced stress field oriented somewhat east of north (probably reflecting the influence of regional compression) both of which occurred in some instances at the same site and even, in some cases, in the same test hole. A tendency for changes in stress field orientation, and accompanying ratio of horizontal stress component magnitudes, to occur at the level of coal seams in the sequences in which the measurements were made, suggesting that the coal seams constitute planes on which displacements have occurred, leading to adjustments to the stress field. The stress results can be interpreted within a revised structural framework for the Bowen Basin, revealed during a recent study. The study was mainly concerned with potential coal mining locations. Only relatively undisturbed areas were examined. The Bowen Basin initiated as a series of extensional troughs in the Early Permian, which were covered by more widespread sediments later in the Permian and the Triassic. It now consists of roughly North-South oriented troughs and

41 highs^ o n w h i c h t h e d o m i n a n t s t r u c t u r a l i m p r i n t is o v e r t h r u s t i n g d i r e c t e d f r o m t h e e a s t o r n o r t h e a s t . A t h r u s t a n d f o l d b e l t o c c u r s o n t h e e a s t e r n m a r g i n of t h e B a s i n . Its i n f l u e n c e d e c r e a s e s to t h e w e s t . T h e n o r t h - s o u t h t r e n d s are cut b y m a j o r b a s e m e n t t r a n s f e r s at 80 to 100 k m s p a c i n g , w h i c h s e p a r a t e a r e a s w i t h s i g n i f i c a n t c h a n g e s in t h e r e l a t i v e d i s p o s i t i o n of t r o u g h s a n d h i g h s a n d o t h e r b a s e m e n t s t r u c t u r e . T h e T r i a s s i c t h r u s t s t r u c t u r e s a l s o c h a n g e in s t y l e a n d l o c a t i o n in d i f f e r e n t a r e a s b e t w e e n t h e m a j o r t r a n s f e r s . F i v e of t h e s i t e s at w h i c h t h e s t r e s s e s w e r e m e a s u r e d lie o n t h e e a s t e r n e d g e of a b a s e m e n t h i g h d e s c r i b e d as t h e C o l l i n s v i l l e s h e l f o r t h e C o m e t p l a t f o r m in v a r i o u s p a r t s of t h e B a s i n . A t S a r a j i , t h e m e a s u r e m e n t s i t e w a s l o c a t e d t o t h e w e s t of t h e t h r u s t f r o n t . A t G e r m a n C r e e k a n d O a k y C r e e k , t h e m e a s u r e m e n t s i t e s w e r e l o c a t e d in a r e a s of low a n g l e e a s t e r l y d i p p i n g t h r u s t s . A t C u r r a g h and South Blackwater the measurement sites were associated with extensive t h r u s t s . W e s t of t h e t h r u s t f r o n t , t h e h o r i z o n t a l s t r e s s f i e l d a p p e a r s a p p r o x i m a t e l y i s o t r o p i c . W i t h i n t h e t h r u s t zone t h e r e is e v i d e n c e of a s u b s t a n t i a l r e s i d u a l c o m p o n e n t of h o r i z o n t a l s t r e s s o r i e n t e d e a s t of n o r t h . T h e m e a s u r e m e n t s i t e s l o c a t e d to t h e f a r n o r t h of t h e B a s i n (Goonyella a n d B u r t o n D o w n s ) w e r e a l s o i n f l u e n c e d b y t h r u s t i n g , r e l a t i v e l y m i n o r a n d low a n g l e at G o o n y e l l a . T h e m e a s u r e m e n t s i t e at B u r t o n D o w n s w a s l o c a t e d o n t h e o v e r r i d i n g b l o c k of a l a r g e s t e e p l y d i p p i n g t h r u s t . At M o u r a , t o w a r d t h e s o u t h of t h e B a s i n , t h e m e a s u r e m e n t s i t e w a s in a zone of major, almost horizontal thrusts, propagated from the E a s t .

TECTONOSTRATIGRAPHIC TERRANES AND SUBDUCTION ENGLAND OROGEN, CENTRAL QUEENSLAND, AUSTRALIA

COMPLEX

MELANGE,

NORTHERN

NEW

Christopher L. Fergusson^, Robert A. Henderson^ and Evan C. Leitch^ 1 Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, N.S.W. 2500 2 Department of Geology, James Cook University, Townsville, Qld. 4811 3 Department of Applied Geology, University of Technology, Sydney, P.O. Box 123, Broadway, N.S.W. 2007 From west to east, across strike, the northern New England Orogen in the Rockhampton-Gladstone region consists of: the Gogango Overfolded Zone, the Yarrol terrane, the Marlborough terrane, the Wandilla terrane and the Shoalwater terrane. Farther west the Permian to Early Triassic Bowen Basin developed as a retroarc foreland basin to the New England Orogen. The eastern Folded Zone of the Bowen Basin and the Gogango Overfolded Zone, which are separated by the 25 km wide Cainozoic Duaringa Basin, together comprise a fold-thrust belt developed at the outer edge of the foreland basin. The Yarrol terrane, and the Wandilla and Shoalwater terranes constitute a Devonian to Carboniferous forearc basin and subduction complex respectively. The Marlborough terrane consists of serpentinized ultramafics and undated metamorphics. The Connors and Auburn Arches lie north and south of the Gogango Overfolded Zone respectively and form the western part of the Yarrol terrane. Both arches consist of Carboniferous silicic plutonic rocks and poorly documented silicic volcanics that comprise a magmatic arc. Early Permian volcanics, described as andesites, overlap these Carboniferous rocks and form the lowest recognizeable stratigraphic unit in the Gogango Overfolded Zone. The Yarrol terrane east of the Gogango Overfolded Zone consists of Silurian to Permian sedimentary and volcanic successions which are intruded by mafic to silicic plutons of Late Palaeozoic and Early Mesozoic age. At the base is a thick succession of andesitic volcanics, volcaniclastic rocks and limestone of Silurian to Middle Devonian age that have been intruded by a Late Devonian intermediate pluton. The sequence is part of an active continental margin and/or island arc. The younger Palaeozoic strata in the Yarrol terrane consist of Late Devonian andesitic volcanics and volcaniclastics which overlie the underlying rocks conformably; a thick Early Carboniferous succession of shallow marine strata including oolitic limestones in the west and coeval deeper marine clastic rocks farther east; areally restricted Late Carboniferous marine clastics with a disconformity at their base and scattered Permian clastic units which unconformably overlie older successions. A substantial zone of Permian volcanics and volcaniclastics occupies the eastern sector of the Yarrol terrane. This zone is largely fault-bounded but locally rests unconformably on Middle Devonian volcanic strata.

The contact between the Yarrol and Wandilla terranes is a major fault marked by discontinuous serpentinite bodies. Farther north, widespread serpertinized ultramafics associated with undated metamorphics grouped as the Marlborough terrane separate the terranes. The Wandilla terrane consists of greywacke, mudstone, chert, tuff and greenstone, and is dominated by melange that formed in three deformations. The first deformation produced widespread lenticular melange and less abundant mud-seam melange. Lenticular melange shows progressive symmetrical bedding-parallel extension with pinch-and-swell and boudinage and rare asymmetrical boudinage. It formed by underthrusting of trench-floor successions with associated thickening of fault zones, perhaps with symmetrical layer-parallel extension partially induced by loading from overlying fault slices. Mud-seam melange occurs in originally thick-bedded greywacke units and shows abundant mud injection indicative of deformation of unconsolidated sediment. The second deformation has imposed a strong, northeasterly dipping slaty cleavage, with a prominent down-dip stretching lineation, on the pre-existing melange in a high-strain environment. The third deformation formed strike-slip faults at a high-angle to the older structures. Faulted against the Wandilla terrane is the Shoalwater terrane, consisting of a guartzose turbidite succession of probable Late Carboniferous age. In the Port Clinton sheet area the Shoalwater terrane consists of imbricated steeply dipping strata, mainly younging to the south-southeast and overprinted by a transecting cleavage induced by sinistral shear. This structural style is consistent with accretion to the subduction complex in a regime of higher sedimentation rates and lower strain rates than indicated for the Wandilla terrane. Much of the terrane shows a strong second deformation consisting of a shallow east-dipping cleavage and east-verging tight to closd folds associated with higher grades of metamorphism (greenschist to amphibolite facies). This deformation is equivalent to the second deformation in the Wandilla terrane and both are attributed to the Late Permian-Triassic Hunter-Bowen Orogeny. Weaker later deformations affect parts of the Shoalwater terrane. The tectonic development of the northern New England Orogen is divided into three phases: (1) a Silurian to Late Carboniferous convergent plate margin with a western magmatic arc, a forearc basin, and an eastern subduction complex; (2) an Early Permian uplift and basin formation event related to widespread extension in eastern Australia at this time; and (3) the Late Permian-Triassic Hunter-Bowen Orogeny involving major overthrusting to the west accompanying foreland basin deposition in the Bowen Basin and widespread ductile deformation in the Wandilla and Shoalwater terranes.

IMBRICATE FAULT SYSTEM IN THE LACHLAN FOLD BELT NEAR GOULBURN, NEW SOUTH WALES

Christopher L. Fergusson^ and A.H.M. VandenBerg^ 1 Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, N.S.W. 2500 2 Geological Survey of Victoria, P.O. Box 173, East Melbourne, Vic. 3002

Thrusting and buckling dominate the deformation style of the Wollondilly Tract in the Bungonia-Goulburn region of the northeastern Lachlan Fold Belt. This region has three main stratigraphic successions: Ordovician-Early Silurian quartz-rich turbidites and black shale. Late Silurian to Early Devonian quartz-rich turbidites, shallow marine clastics, limestone, volcaniclastics and volcanics, and Late Devonian quartz-rich clastics. A low-angle unconformity occurs between the two lower successions along the eastern margin of the Wollondilly Tract and records uplift and mild deformation in the Middle Silurian at the beginning of the thrusting event. The north-trending Yarralaw Fault divides the region into two domains. In the western domain major east-west shortening formed folds, axial planar cleavage and west-dipping contraction faults in all pre-Upper Devonian stratigraphic successions. This deformation formed an anticlinorium cored by a west-dipping thrust system with an overturned east-younging limb, which is bounded to the east by the Yarralaw Fault. In the eastern domain this deformation formed upright flattened chevron folds in Ordovician strata. Repetition of the succession occurs along steep contraction faults spaced at intervals of a kilometre or less. Zones of tectonic melange occur in the hanging walls of these faults and have a well-developed scaly fabric, with a steeply plunging striation, overprinting a bedding-parallel cleavage in mudstone. Asymmetrical boudinage and other shear sense criteria in these melanges show both dextral and sinistral lateral shear superimposed on the dominant dip-slip component. In the interbedded shales and limestones of the Bungonia Limestone a classical stair-step trajectory is recognized for the Frome Hill Fault which duplicates the succession and has undergone back-rotation to its present steep dip. Overall, thrusting progressed from east to west with cessation of deformation in the domain east of the Yarralaw Fault caused by the intrusion of the Early Devonian Marulan Batholith, whereas farther west deformation continued into the Middle Devonian. Deformation began in the Middle Silurian and was continuous until the end of the Middle Devonian. In the Early Carboniferous deformation the older north-south Yarralaw Fault was reactivated and several east-northeast trending high-level type drap folds were formed in the Late Devonian succession above reactivated basement structures. The imbricate fault system at Goulburn is part of a larger belt that extends along the eastern Lachlan Fold Belt as far south as eastern Victoria and is named herein the Yalmy-Bungonia fold-thrust zone. Tectonic development in the Silurian to Middle Devonian interval was in a complex evolving arc above a west-dipping subduction zone and the deformation possibly relates to underthrusting of an allochthonous terrane with major shortening in the hanging-wall block.

GRAIN BOUNDARY STRUCTURES

IN NATURAL AND SYNTHETIC CARBONATE ROCKS.

John D. Fitz Gerald^, David L. Olgaard^^ f ong Wong"^. 1 2 3 4

Janos L. Urai^ and Teng-

RSES, Aust.National Univ, GPO Box 4, Canberra, ACT 2601 Geologisches Inst, ETH-Zentrum, CH-8092 Zurich, SWITZERLAND. Inst Aardwetenschappen, PO Box 80021, 3508 TA Utrecht, NETHERLANDS Dept Earth Space Sciences, SUNY, Stony Brook, NY 11794-2100, USA.

The medium-grained marble from the mountain quarries near Carrara, Italy has been the material preferred for a range of experimental investigations in rock mechanics on the premise that this is a pure m a r b l e w h i c h has a c h a r a c t e r i s t i c a l l y h o m o g e n e o u s and i s o t r o p i c m i c r o - s t r u c t u r e . We have documented differences in grain boundary structures, mineralogy and chemistry of a set of visually-distinct C a r r a r a m a r b l e s . The g r a i n b o u n d a r y (GB) s t r u c t u r e s of t h e s e variants have been compared with those in a synthetic marble. Three Carrara m a r b l e s distinguished by subtle but clear colour differences were chosen; we label these CANU, CG and CW. All have grain sizes 220-250|im. The synthetic marble HP was hot isostatically-pressed from pure CaCOs powder (2-5^m) at 875K, 200Mpa for 3 hours to yield a densified rock of grainsize 30jim. In CW and HP, fracture surfaces studied in SEM were dominted by GB faces (Fig 1 and 2). In CANU and CG, both grain faces and c r y s t a l cleavages form the fracture surfaces. By comparing the two surfaces produced by a single fracture, excellent correspondence of structure can be observed down to a scale of IOMJTI. GB roughness varies; even neighbouring faces of the same grain can be quite dissimilar. At a finer scale in many GBs, sharp channels and rough pores revealed on one fracture surface were commonly absent from the opposite surface. GBs of CW and CANU were examined in more detail by SEM of p o l i s h e d t h e n - i o n - e t c h e d surfaces and by TEM. In addition to finding GB morphology as described above for each marble, distinct differences were recorded between the average aperture shown by the GBs in these two rocks (Fig 3). It appears that the marble CW, which fractures almost entirely along GBs, has much more open GBs than does CANU. CW and CANU are also distinguishable by differences in mineralogy of impurities. CG differs from both of these by having a significantly higher content of dolomitic grains, although no significant difference could be determined on the basis of major element chemistries. In HP, GB structures resemble those of partially healed cracks (Fig 4) with 'grooves' and pores. Three-grain corners are marked by continuous porosity in open channel structures. Using TEM and SEM, the conspicuous grooves on GBs have been identified as another component of porosity which has a flattened and elongated form and an extremely tortuous shape in the GB surfaces. Since many of the measured properties of rocks (such as p e r m e a b ility, strength and GB mobilities) can be extremely sensitive to impurity contents and structure of GBs, subtle differences found in some marbles from Carrara warrant further consideration. The strucures identified in HP are potentially important to the development of natural marble microstructures; they may constitute a new t r a n sient stage in the evolution of porosity in conjunction with grain growth. The structures also indicate that the processes involved when grains of complex and unmatched shapes 'heal' together cannot be satisfactorily described by analogy to those already known from healing of small-aperture, planar cracks.

Fig 1. A fracture surface consisting entirely of grain faces in the marble CW. Note the variations in surface roughness and the deep narrow channels and pores on some faces. [SEM image]

Fig 2. Cross section of a grain boundary in CANU. The gross boundary shape is shared by both grains but the fine structure is not. [TEM image].

20 urn 1 Fig 3. Polished-then-etched sections of CANU (left side) and CW (right side). Boundaries are significantly 'wider^ in CW. [SEM images using baclcscattered electrons].

Fig 4. A fracture surface in the synthetic marble HP. Pore volume is distributed between a)open channels at the edges of most grain faces, b) rounded pores, but also c)through a variety of complex structures on the grain faces. [SEM image].

GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS OF IGNEOUS ACTIVITY IN THE ADELAIDE FOLD BELT: IMPLICATIONS FOR MECHANISMS OF CRUSTAL GROWTH John Foden

1 , Simon Turner

and Annie Michard

1. Department of Geology and Geophysics, University of Adelaide, South Australia• 2. C.R.P.G., C.N.R.S. Vandoeuvre-les-Nancy, France. New Nd and Sr isotope data and some new U-Pb zircon geochronological data has been collected from a wide range of igneous rocks in the Adelaide Fold Belt. These data support a model for continental lithospheric growth in which the major addition of new material from the asthenosphere, takes place during phases of lithospheric extension. This material then provides the source region of magmas formed and moved to the upper crust during collisional orogenesis. The Adelaide Fold Belt provides an excellent setting in which to examine models of lithospheric evolution and growth. It has a long geological history, from the Late Middle Proterozoic to the Quaternary during which basaltic magmas sampled the mantle. The post Middle Proterozoic geological evolution of the Adelaide Fold Belt has been dominated by the development of one major extensional and largely sedimentary basin during the Adelaidean, followed by another period of extension and sedimentation in the Cambrian (the Kanmantoo Trough), an Early Ordovician, collisional orogeny (the Delamerian) and a possible third extensional episode in the late Early Ordovician. This latter event is only recorded by igneous rocks (with a bimodal silica distribution) which may imply that collapse of the continental crust after the Delamerian Orogeny did not occur to the extent that a marine basin was developed. The tectonic setting at this time may have been akin to that of the Basin-Range Province of the Rocky Moutains. Each of the three extensional phases produced mafic igneous activity: 1: The Callana sequence of shallow water sediments which commenced the development of the Adelaidean Basin, host the largest volume of mafic volcanics in the belt, these include the Wooltana and Depot Creek volcanics, which are also thought to be equivalent to the Beda Volcanics and the Gairdner Dyke swarms of the Stuart Shelf. Samples of this suite have been frequently exhumed by the diapirs in the Central Flinders Ranges. 2: The early sequences of the Cambrian (?), Heatherdale-Kanmantoo Trough, host relatively scarce mafic to trachytic volcanics as well as common, preF1, layer-parallel amphibolites thought to represent near syn-sedimentary flows or sills. These rocks have slightly incompatible element-enriched, tholeiitic chemistry somewhat like continental tholeiites or E-type MORB. 3: Following the last fabric-forming events of the Delamerian, posttectonic, silicic magmatism was contemporaneous with mafic activity. Some granitic intrusives are clearly cut by mafic dykes, but we also see evidence for mingling of mafic and felsic melts (as in the Mannum Granite) and for geochemical continuity between mafic and felsic magmas in individual magmatic systems as in the case of the Black Hill "norite" layered intrusion. This latter intrusion is the most spectacular manifestation of this phase of mafic igneous activity. The major silicic granitoid bodies were intruded during the Early

Ordovician Delamerian Orogeny which produced a Buchan-style metamorphic belt. The granites may be conveniently sub-divided into syn- and posttectonic suites. The syn-tectonic group have some deformational fabric and are I-type granites ranging from diorite to true granite. They are hydrous biotite and/or hornblende-bearing, are relatively calcic, Ba-, Sr-rich rocks with low Nb, Ti, Y, F and Ga contents. The post-tectonic granites already mentioned, are potassic, A-type magmas which form a series of granite and rhyolite outcrops in the southeast of the state. These are F-rich, relatively Fe-rich, often one feldspar magmas, with high LREE, Y, Nb and Ga contents and low Sr and Ba concentrations. The syn-tectonic granites have compositions unlike those expected from the melting of the underlying Proterozoic crustal basement. In some cases, they have relatively low Sr87/Sr86 ratios and those granites with higher Sr87/Sr86 ratios are very fractionated and as in the case of the Victor Harbor granite are probably contaminated by crustal assimilation. The post-tectonic granites have geochemical characteristics which are complimentary with respect to those of the syn-tectonic ones and this implies that they are the result of reprocessing of residues left during the production of the first granite group. The higher temperatures implied for this second event are then the likely responce to the last stage of lithospheric extension with the associated mafic magmas providing the heat source. - < o The Early Adelaidean mafic volcanics have epsilon Nd(0) values of -l.b to -3.7 and those from the Cambro-Ordovician range from -1.6 to The Early Ordovician granitic rocks range from -4.05 to -10.33. Interestingly, the Nd model ages of the whole suite of both the mafic and felsic rocks tend to cluster around IGa. As this is close to the probable age of initiation of Adelaidean sedimentation the interesting implication is that this was the main age of lithospheric growth beneath the Adelaide Fold Belt and that this lithosphere has provided much of the source of subsequent magmatic events through the Adelaide Fold Belt history.

DOING IT WTIHOOT PORTHYRDBIASTS

Aidan Forde.

Department of Geology, James Cook Univeirsity, Townsville, Q4811. Bell and Johnson (1988) have used porphyrdblast incliasion trails to determine the tectonic histories of mountain belts and to model orogenesis. Their method may also be applied to deformed rocks that do not contain porphyroblasts with inclusion trails. This application involves close examination of the overprinting relationships of the matrix foliations, especially those in 1CM strain areas. Two difficulties conplicate the interpretation of overprinting relationships in these rocks: (a) Rotation of early foliations by later deformations (this does not occur in porphyroblasts). Errors in the interpretation of horizontal versus vertical orientation for a particular foliation may occur unless the rotation of all foliations is qualitatively corrected. (b) Obliteration of early foliations by later, more intense deformations occurs easily in the matrix viiereas these foliations are commonly preserved in porphyroblasts. It is therefore necessary to find law strain areas formed during intense deformations in o r d ^ to examine earlier overprinting relationships. These lo^ strain areas are found in strain shadows formed against heterogeneities such as veins, pegmatites and plutons or are produced as a result of deformation partitioning. Preliminary work on overprinting relationships preserved in rocks from central Victoria indicates the presence of at least four successive foliations in two vertical/horizontal coi:plets. The geometry of these foliations shows that the core of the Tabberabberan orogeny must lie to the west. This finding is consistent with published cross-sections.

DETERMINING THE LOCATION AND GEOMETRY OF EARLY FOLDS IN SOME MACROSCOPIC REFOLD PROBLEMS : A TECHNIQUE ANALYSING INTERSECTION LINEATION TREND VARIATIONS. T.J. Fowler Geology Department, Bendigo College of Advanced Education P.O. Box 199, Bendigo, Victoria, 3550, Australia

A map and stereographic technique is devised for locating early gold inflection and hinge surface traces and determining early fold orientations for some cases of two-phase macroscopic folding of layers. The technique analyses bedding-cleavage intersection lineation trends developed by overprinting of early non-recumbent folds by a penetrative non-vertical cleavage which is axial plane to later macroscopic folds geometrically similar to slip folds. Lineation trends are graphed against distance along cleavage traces. The technique is specifically designed for areas where (1) early folds lack associated mesoscopic structures and (2) marker beds are absent and (3) young of strata is difficult to determine. The technique is therefore applicable to areas where standard structural analysis techniques fail (fold interference pattern analysis, homogeneous domain and early fold vergence mapping, and structural facing vector analysis). The technique is flexible in also being useful for purely qualitative inspection of lineation maps in areas where the later folds have small amplitude.

Deformation processes caused by offscraping at an ancient convergent margin. Annette D. George Geology Department, University of Melbourne, Parkville 3052. Early Cretaceous interbedded greywacke and argillite of the Torlesse accretionary prism terrane is exposed in the southernmost part of the North Island of New Zealand. The rocks are complexly deformed, and metamorphosed to prehnite-pumpellyite grade. Overprinting relationships indicate three phases of folding and multiple faulting events, which occurred during accretion and subsequently within the prism. Early deformation involved largescale Fi isoclinal folding and development of an anastomosing Si cleavage. Folding of the sediments promoted dewatering, and the subsequent disruption of strata, by shearing parallel to bedding and low-angle to bedding faulting, records the transition to more brittle responses to the deformation. The most widespread folding phase (D2), produced numerous upright, typically isoclinal folds, with local development of an axial planar S2 cleavage in macroscopic and some mesoscopic fold hinges. The variable plunge of the fold axes to the NNE and SSW within the axial surface, suggests they developed during a progressive deformation. Mesozoic strike-slip faulting most likely produced the open folds and warps of the third folding phase (D3), in bedding already rotated to moderate dips. The axial surfaces of these late folds strike E-W. The development of folds in response to the deformation suggests the Cretaceous margin was thickly sedimented and undergoing only slow to moderate rates of convergence. This style of deformation in the Aorangi Range, combined with the low grade of metamorphism, the frequent occurrence of sedimentary melange, and the existence of a trench slope basin deposit, suggests that these rocks were accreted by offscraping.

52 LINKED FAULT FAMILIES IN BASIN AND PASSIVE MARGIN FORMATION AND DEFORMATION

Gibbs, A.D. Midland Valley Exploration 14 Park Circus, Glascow, SCOTLAND, UK

Recent models of faulting tectonics in basin and passive margin development emphasise the linked nature of faults in a way which is analogous to that which was developed over the previous two decades for thrust tectonics in contractional regimes. This paper summarises the geometric constraints of such models and emphasises the role of each of the three critical components of a linked system. For large deformations which result in the formation of major extensional and strike-slip basins, the principal geometries can be understood by reference to regionally balanced fault models which involve detachment, ramp and sidewall or transfer elements. Tip strains and both vertical and lateral accommodation zones are included in the model but these probably account for less than 10% of the total upper crustal strain. The basic analysis of a linked basin system emphasises the transfer of displacement on and between the fault and shear zone components. The geometry of the stratigraphic units and the evolution of on and offlap margins can be used to deduce the active linkage on the fault system. In oblique slip systems and where the dominant extensional ramp is connected to oblique transfer and detachment systems lateral and sidewall sub-basins and intrabasinal uplifts occur as part of continuing slip on the fault system. These ramps control facies distribution and depocentre evolution. The existence of a basin forming linked system frequently controls later deformation of basins and margins and an appreciation of the basin-growth fault and stratigraphic geometry is essential in the interpretation of inverted basins and orogenic belts involving supracrustal formations. The paper concludes by summarising the key geometric elements in progressive re-deformation of a basin and highlighting discriminatory differences and similarities in extensional and contractional linked systems.

METAiyDRPHISM AND DEFORMATION OF THE GLENELG RIVER COMPLEX, WESTERN VICTORIA AND IMPLICATIONS FOR CORRELATIONS BETWEEN SE AUSTRALIA AND NORTHERN VICTORIA LAND George M. Gibson School of Applied Science, Darling Downs Institute, Toowoonba, Qld. 4350

Comparisons between the tectonostratigraphic teirranes of northern Victoria Land (NVL) and SE Australia provide irrportant constraints on reconstructions of Australia and Antarctica. In one such reconstruction (Stump et al., 1986) the Lower Palaeozoic Glenelg River Complex is combined with the Adelaidian Supergroup and Kanmantoo Group to form the Delamarian terrane and correlated with the eastern sector of the Wilson terrane. This part of NVL is primarily underlain by multiply-deformed amphibolite facies metasediments intruded by synmetamorphic Cambro-Qrdovician granitoids (Granite Harbour Intrusives) and metamorphosed at intermediate to (earlier) high pressures. VJhilst the Glenelg Complex is also intruded by Cambro-Ordovician granitoids and contains a similar range of metasedimentary lithologies, metamorphism is typically of low grade (subgreenschist facies) and produced regionally extensive slates, phyllites and metagreywackes. Higher grade schists, psamnites and calcsilicate rocks with amphibolite facies mineralogy are restricted to the immediate vicinity of the Wando Granodiorite and constitute a metamorphic aureole around this intrusion; mineral assenblages (andalusite ^ staurolite ^ garnet in pelitic rocks) are of the low-pressure type and were superimposed upon the earlier regional metamorphism. However, because intrusion occurred under syn-tectonic conditions, aureole rocks are commonly strongly foliated and share the same penetrative fabric (S2) as the marginal facies of the granodiorite. These relations cast doubts on correlations between the Glenelg Complex and Wilson terrane although rocks of similar metamorphic style and deformation occur in the Morozumi Range, a fault-bounded block of contactmetamorphosed turbidites presently included in the Wilson terrane but whose stratigraphic affinities are problenatic and more closely approximate those of the Robertson Bay terrane some 20-30 km further east. Alternatively, the Glenelg Complex and V^ilson terrane preserve different stnictural levels within what was once a single continuous metamorphic belt.

BASIN INVERSION, THRUSTS AND ORE DEPOSITS AT

COBAR, NEW SOUTH WALES

Glen, R.A. Geological Survey of New South Wales, G.P.O. Box 5288, Sydney, N.S.W., 2001.

The Cobar Basin is an intracratonic extensional marine basin which was initiated in the earliest Devonian. The basin was fault-bounded with an active scarp to the east (up to Howellella jaqueti (HJ) Pragian time) and less active scarps, to the south and probably also to the north. The western boundary is covered by post-rift facies and may also have been fault controlled. At the onset of HJ time, renewed extension migrated basinwards, with the formation of new normal faults in the hanging walls of the older eastern and northern bounding faults. In later HJ time, post rift facies rocks spread out over the western basin margin, topping old granite highs, old shelves and old deepwater basin rocks to define a half-steer head geometry. Most of the basin, except the western and southwestern parts, was inverted in a late Early Devonian event around 400 Ma (Glen et al. 1986, Dallmeyer et al. in prep.). The western and southwestern parts suffered only minor inversion then and were mainly inverted in the Carboniferous after a fluviatile basin had developed on top of the old marine basin. The most intense effects of the Early Devonian inversion are recognized in the eastern and northern parts of the basin, where they are reflected by high strain zones (a meridional zone 1 and a latitudinal subzone 2b, Fig. 1, Glen 1985). Recent mapping (Glen 1988) suggests zone 1 is a thrust zone (in part transpressional) and that subzone 2b may also be characterized by emergent or blind thrusts. The inference of thrusts is not easily made given poor outcrop, flat, topography and steep fault dips at the surface. Both high strain zones contain a regional SI subvertical slaty cleavage overprinting a bedding fissility (local other cleavages may also be present), a down-dip mineral lineation, variably plunging F1 folds and variably developed D2 structures. Zone 1 contains four major thrust plates, some of which are imbricated themselves (Fig. 1). The Queen Bee Plate in the SE corner is a complex pop-up zone bounded by the west-dipping sole fault (Rookery Fault) to the west. Mapping by D.J. Pogson and others has identified complex imbrication in this plate. Both splays and duplexes are present. NE and WNW tear faults cause major changes in geometry, especially at the subzone Ic/la boundary. The Rookery Plate is another pop-up, bounded by the Rookery Fault to the east and the Great Chesney Fault (east dipping backthrust) to the west and cut by WNW tear faults. The plate is shortened most at the latitude of Cobar, opposite a promontory or indenter in the shape of the underthrusting basement plate to the east. Major changes in orientation and geometry occur across this indenter. To the north, F1 fold and SI cleavage strains are strongly transpressive (left-lateral) and the sole fault lies within basement rocks. To the south, the sole lies at the basement/cover interface, F1 fold strains are orthogonal but SI cleavage strains are mildly transpressional. In subzone la, both strains are orthogonal to plate boundaries. The Chesney Plate defines a triangle zone between the Great Chesney Fault to the east and the west-dipping Cobar Fault to the west. Occupied mainly by slate, it is strongly imbricated. A significant imbricate of Chesney Formation is present. Indenter effects here are much less than to the east and shortening is more orthogonal and less transpressive, especially north of the indenter. The Cobar Plate is a meridional west-dipping plate, bounded to the west by the Myrt Fault which varies from emergent to blind beneath a F1 syncline cored by post-rift sediments. This plate is unaffected by the indenter and cleavage and fold shortenings are orthogonal to bounderies.

The Myrt Fault is also a strike-slip boundary with the lower strain subzone 2a to the west which is characterized by NW/SE F1 folds (NE/SW shortening) and NE/SW F2 folds (NW/SE shortening). Subzone 2b shares the same sets of structures with subzone 2a, but is a high strain zone. It is bounded to the north by the inferred south-dipping Little Tank Fault against shelf and to the south by a blind thrust (Bundella Fault) beneath a syncline cored by post-rift sediments. F1 folds are north vergent and folded by NE F2 folds. D1 north vergent emergent/blind thrusts are inferred. The following model is suggested. During inversion, oblique shortening was resolved into components normal to basin margins. The extensional faults (Rookery, Myrt, Little Tank, Bundella) were reactivated as thrusts. Shortening was greatest on the eastern side (zone 1), and reactivation was greatest on the Rookery Fault. As the Rookery and Myrt Faults steepened during inversion, strain was taken up by development of short cut faults, and both fore and back thrusts developed. Pop-ups formed as efficient ways of shortening the basin fill and removing strata by erosion. Thrusting was accompanied by variable amounts of strike-slip movement on some faults. Cleavage strain and fluid flows during the later parts of inversion led to faults being used as solution passageways. Major ore deposits in zone 1 lie on short cut faults, especially near cross faults which formed either as tears during thrusting or as dilational jogs or strike slip linkages during wrenching, The Elura deposit in subzone 2b probably lies above a blind thrust. References: Glen 1985. J. Struct. Geol. 7, 301-315. Glen 1988. Q. Notes GSNSW 73, 21-26. Glen, Dallmeyer, Black, 1986. Q. Notes GSNSW 64, 26-29. Published with permission, Director-General, NSW Department of Minerals & Energy.

STRIKE-SLIP AND ACCRETIONARY TECTONICS OF THE LACHLAN FOLD BELT David R. Grayl, Christopher L. Fergusson2 and Vincent L MorandS IDepartment of Earth Sciences, Monash University, Clayton, Vic. 3168 2Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, N.S.W. 2500 SDepartment of Geology, James Cook University, Townsville, Qld. 4811 The Lachlan Fold Belt of southeastern Australia consists of an amalgam of tectonostratigraphic terranes, which from west to east are: the Grampians-Ararat composite terrane, Ballarat terrane, Melboume terrane, Benambra terrane and the Parkes terrane. These terranes are described for the Victorian sector of the Lachlan Fold Belt with reference to eastem New South Wales for the two eastem-most terranes. The Grampians-Ararat composite terrane contains complexly deformed Cambrian? greenstones and Cambrian-Ordovician? quartzose turbidite successions that have arcuate structural trends deflected around the Broken Hill Block in far western New South Wales and the Adelaide Fold Belt (Glenelg Zone) in far westem Victoria. This deformation probably occurred in the Late Cambrian to Early Ordovician and is related to plate convergence that caused the Delamerian Orogeny. These rocks are unconformably overlain by a mildly deformed Late Silurian? volcanics and marine to fluvatile sedimentary succession. The eastem boundary of the terrane is the Avoca Fault which had a predominantly dextral strike-slip movement in the Silurian with minor thrusting towards the east. The Ballarat and Melboume terranes both contain a similar Cambrian-Ordovician succession with Cambrian greenstones and associated sediments developed along the eastern margins of both terranes. The Ordovician successions consist of thick monotonous quartz turbidites that are so characteristic of the Lachlan Fold Belt and reflect deposition in a passive continental margin setting. One difference between these Ordovician successions is that no thick succession of black shale occurs in the Late Ordovician of the Ballarat terrane as occurs in the eastem Melboume terrane. The Ballarat terrane was deformed in the Early to Middle Devonian by a diachronous deformation that migrated from west to east and developed by buckle-controlled chevron folding accompanied by inibricate thrust repetition. The Melboume terrane had continous sedimentation from the Ordovician into the Middle Devonian in a prograding and upwards shallowing continental margin prism that is unrecognized elsewhere in the Lachlan Fold Belt. The Melboume terrane was deformed in the Middle Devonian with major detachment from its underlying basement and overthrusting to the east along the Mount Wellington Fault Zone, which forms the eastem boundary of the terrane. The terrane boundary between the Melboume and Ballarat terranes is the near vertical Mt. Ida-Mclvor Fault Zone which had an early history of thrusting followed by dextral strike-slip motion. The southern extension of this fault actually dies out to the south amongst Upper Ordovician Riddell Sandstone which is consistent with relative minor displacements between the Melboume and Ballarat terranes. The Benambra terrane incorporates all of the Ordovician quartz turbidite succession in the remainder of the Lachlan Fold Belt outside the Melboume, Ballarat, and Grampians-Ararat terranes. Its subsequent Silurian to Middle Devonian history contrasts markedly with the former terranes during which it underwent significant disruption by dextral strike-slip faulting. In an earlier synthesis Fergusson et al. (1986) incorporated Ordovician basaltic-andesitic volcanics, volcaniclasticlastics, and limestone of an island arc succession in eastem New South Wales and Victoria within the Benambra terrane. However, the Ordovician continental margin sediment prism contains no evidence of contemporaneous volcanism and a faulted contact between theses two successions is likely. Within eastem New South Wales this fault must be a major overthrust that has been deformed by strong east-west folding. The Ordovician island arc terrane is now called the Parkes terrane and the Benambra terrane is thus reduced in extent. Overthrusting, and thus amalgamation, of the Parkes terrane onto the Benambra terrane occurred in the Early Silurian Benambran Orogeny which was associated with widespread east-west folding, thrusting and high temperature-low pressure metamorphism in the Benambra terrane. Subsequently these two terranes formed an amalgam although the dextral strike-slip motions dispersed fragments of the Parkes terrane into southeastern New South Wales and eastem Victoria.

The Benambra-Parkes composite terrane constitutes the bulk of the Lachlan Fold Belt and contains many diverse lithologic assemblages and patterns of deformation. In summary, the teirane assemblage is split into a westem domain dominated by several major fault zones, mainly with a dextral strike-slip history in the Middle Silurian to Middle Devonian (e.g. the Wonnangatta Line, Kiewa Fault, Ensay Fault and Gilmore Fault Zone), and an eastern domain dominated by a major fold-thrust belt in the south and several intra-arc rift basins in the north. Tectonic activity was controlled by a west dipping subduction zone m the Middle Silurian to Middle Devonian that probably had a steeper dip in the noi^ where major extension occurred in the Middle Silurian and deep-marine sedimentation persisted until the Middle Devonian. Farther south the subducting slab had a shallower dip and deformation was concentrated in the frontal arc region with the development of bucklechevron folds accompanying thrusting and the formation of the Yalmy-Bungonia fold-thrust zone throughout the entire Middle Silurian to Middle Devonian interval. Deformation was synchronous with sedimentation, volcanism and plutonism. The progression of the deformation was m^ked by out-of-sequence thrusting and normd faulting which has resulted in synorogenic unconformities and paradoxical stratigraphic-structural relationships in several areas. Oblique convergence along the subduction zone east of the Benambra terrane is partitioned into east-west shortening in the arc and dextral strike-slip motion in the back-arc region so that the Benambra terrane was transported southwards in the Middle Silurian to Middle Devonian and thus into juxtaposition with the Ballarat and Melbourne terranes farther to the west. Once these terranes became adjacent to the compression^ zone in the southem Benambra terrane they were selectively deformed due to their weaker oceanic basement and undeformed state in contrast to the relatively rigid southwestern Benambra terrane. Clearly the Ballarat terrane entered this region first and therefore records strong east-west shortening accompanied by dextral strike-slip movement along the boundary faults in the Early to Middle Devonian. The Melboume terrane entered the deforming belt in the latest Early Devonian and underwent overthrusting to the east and development of the Mount Wellington Fault Zone. By this time normal, as opposed to oblique, convergence was occurring along the subduction zone and as a result faults with older dextral motions were reactivated as sinistral strike-slip faults and a new conjugate strike-slip fault system overprinting the YalmyBungonia fold-thrust zone of the southeastem Benambra terrane. By the Late Devonian subduction had ceased and post-orogenic volcanism, plutonism and fluviatile sedimentation dominated the geologic record of the Victorian sector of the Lachlan Fold Belt. In the Early Carboniferous the whole orogen was affected by a major east-west shortening associated with events in the New England Orogen and had the most profound affects in the northeastem Lachlan Fold Belt (Powell, 1984). References FERGUSSON C.L., GRAY D.R. & CAS R.A.F. 1986. Overthrust terranes in the Lachlan fold belt, southeastem Australia. Geology 14, 519-522. POWELL C.McA. 1984. Uluru regime. In Veevers, J.J., Phanerozoic Earth history of Australia. Claredon Press, Oxford, 290-337.

STRUCTURAL AND METAMORPHIC ANALYSIS OF A DEFORMED SUBDUCTIONACCRETION SEQUENCE - NOWENDOC, EASTERN NSW. Martin Hand Geology Dept., University of Newcastle ABSTRACT The Oxley Metamorphics in the southern part of the Tia Complex consist of a sequence of oceanic sediments that preserve a complex structural and metamorphic history. Initial underthrusting within an accretionary prism produced a melange, and characteristic accretionary fabrics. Continued burial resulted in intermediate blueschist-greenschist facies metamorphism and the development of a rarely preserved penetrative foliation, S^. An apparent extensional episode of deformation during Dj produced a strong moderately southwesterly dipping S2 lenticular layering and localised mylonite zones. Kinematic indicators suggest that regional ductile shearing had a normal movement on Sj to the southwest. This was accompanied by a reduction of pressure from the initial deformational episode indicating the onset of uplift. The rocks were then refolded into a northeasterly verging Dj nappe structure that was thrust to the northeast along a zone of mylonites developed on its overturned limb. This thrusting caused the axes of mesoscopic Fj folds in the vicinity of the D3 stretching lineation. D, was accompanied by relatively high pressure, low temperature greenschist facies metamorphism. The entire Dj nappe structure was strongly folded into large southwesterly plunging structures during D^, producing the presently preserved structural trends in the area. Sub-biotite grade greenschist facies metamorphism during D^ overprints the earlier higher pressure assemblages. Subsequent to D^, two episodes of folding were associated with the final uplift of the rocks in the area. F5 and Fg folds are characterised by their consistent southwesterly and westerly verging direction which is believed to represent the tectonic transport direction during this time. It seems likely that this final uplift resulted in the unroofing of the Tia Complex by the early Permian. A P - T time path based on the changing compositions of minerals produced throughout the deformational history has a steep uplift trajectory suggesting that geotherms remain relatively depressed during much of the uplift. This has been interpreted to mean that underthrusting within the accretionary complex continued for at least some part of the uplift history. Foliation relationships within the serpentinites that border the Oxley Metamorphics to the south and lie within the Nowendoc Fault Zone indicate that an early episode of southwesterly directed normal movement on this fault was overprinted by northeasterly directed thrusting. Following these movements, right lateral and then left lateral strike slip movements occurred and may be related to similar Permian movements on other major serpentinite bearing faults in the Southern New England Fold Belt.

CONJUGATE FAULTING ASSOCIATED WITH IN INDONESIA: STRUCTURAL CONSTRAINTS ROTATION OF SUMATRA

ORTHOGONAL SUBDUCTION FOR THE TIMING OF THE

Lyal Harris Geology Department, University of Western Australia, Nedlands 6009. Sumatra and Java are often presented as characterising deformation styles relating to oblique and orthogonal subduction respectively. In Sumatra, the Central Barisan Fault Zone (BFZ) is a major dextral wrench system due to the oblique subduction of the northwards moving Indian plate beneath Sumatra. It has generally been assumed by the majority of previous authors that dextral wrenching, and hence oblique subduction, has taken place in an episodic manner since the early Tertiary up to the present day during which time Sumatra was in almost its present position. Almost orthogonal subduction in Java has resulted in the continued structuring under approximately N-S oriented maximum compressive stress since at least the Late Cretaceous: thrust faults and folds are present in an orientation sub-perpendicular to the current direction of plate convergence, along with normal faults sub-parallel to this direction and conjugate strike-slip faults. Faults show evidence for block rotation. Dextral shears rotated towards the regional thrust orientation show a subsequent increase in the component of reverse movement with increased angle of rotation. Other structures can be related to volcanic activity. There is no evidence in Central or Eastern Java for any major wrench system. The Lebong Tandai gold mine in SW Sumatra is dominated by structures which occur within an E-W sinistral wrench regime cross-cutting flat to gently SW dipping Miocene strata. Faults of the BFZ locally cross-cut the E-W sinistral wrench system. Veins in the area near Lebong Tandai are also dominantly in the D, R, R'& T, positions for E-W sinistral wrenching along with minor conjugate veins. Other veins are in the R, P, T and reverse positions for the younger BFZ dextral wrenching. In the Balimbing area, two main orientations of faults are present which cut volcanics of Miocene age: faults with sinistral movement striking 070° dominate; shorter faults developed between sinistral ones strike approximately 020°.These faults form a conjugate set of structures implying a NE-SW oriented maximum compressive stress. Some NNE striking faults have been reactivated as normal faults during subsequent dextral wrenching. Several other structural features in Sumatra are also inconsistent with dextral wrenching. The Takung River thrust zone near Bukittinggi and the Djambi thrust lie in the orientation for thrusting associated with the earlier NE-SW compression direction determined at Balimbing. E-W striking structures (which are cut by dextral faults) and NE-SW normal faults exert a major structural control on volcanic activity. A study of the drainage pattern of Sumatra also highlights the presence of conjugate

lineaments and a spatial volcanic centres.

association

of these

lineaments

with

This study has shown the existence of a coaxial deformation event implying NE-SW directed maximum compression affecting Miocene rocks and it appears that structures during the early part of the Tertiary described in Sumatra also fit this stress regime. It is therefore proposed that dextral wrenching in the BFZ commenced in the Late Miocene to Early Pliocene. The tectonic implications of the existence of a major phase of coaxial deformation prior to dextral wrenching in Sumatra are great. In island arc terrains, the existence of coaxial deformation regimes where the maximum compressive stress is perpendicular to a subduction zone has been considered to imply orthogonal subduction, whereas w r e n c h i n g p a r a l l e l to the subduction zone is a sign of oblique subduction. We may therefore conclude that there has been a period of orthogonal subduction under Sumatra post early Miocene. On structural grounds, in order to explain the change in deformation regime in Sumatra, it is therefore proposed that in the Late Miocene-Early Pleistocene, Sumatra has undergone a clockwise rotation of 20-30® whilst Java remained in approximately the same position. These conclusions agree with a study by Ninkovich (1976) who also points to there being a clockwise rotation of Sumatra post-Middle Miocene on the basis of a study of the geometry of the Indonesian volcanic arc, the distribution of volcanic activity and of spatial changes in the depth of the Benioff zone. There is also a marked difference in the evolution of conjugate faults due to orthogonal convergence in Sumatra compared with Java: block rotations as observed in Java have not been observed in Sumatra. This may either reflect the longer time span that Miocene rocks in Java have undergone compressional stress and coaxial deformation, or may reflect the different crustal structures between the two islands. REFERENCE Ninkovich, D.,1976. Late Cenozoic clockwise rotation of Sunnatra.

EPSL 29: 269-275.

DEXTRAL TRANSPRESSION IN ON GOLD MINERALIZATION

CENTRAL

VICTORIA

AND

ITS

CONTROLS

Lyal Harris Geology Dept., University of Western Australia, Nedlands 6009. Structuring of the Central Victorian section of the Lachlan Fold Belt is dominated by dextral t ranspression with an approximately N-S principal displacement orientation. Au mineralization in Central Victoria is situated within dilational sites within this wrench regime, with major transcurrent fault zones acting as conduits for mineralizing fluids.There is no evidence for a mid-crustal detachment fault acting as a sole thrust underneath the Ballarat and Melbourne terranes, nor for imbricate thrusting being the dominant deformation style along any of the major fault zones in Central Victoria. Two areas clearly show the validity of this model: the Heathcote - Costerfield Rushworth area and the Bendigo Goldfields. Structures associated with dextral wrenching have also been observed in the Maldon, Dunolly, Tarnagulla and Maryborough areas. In the Heathcote area, the geometry of faults within, and to the east of the Mt Ida-Mclvor Fault Zone, along with the orientation changes in fold axes east of this fault zone, points to dextral wrenching as being the dominant component of movement. Faults fit exactly the geometry for an approximately N-S oriented dextral wrench regime. Field observations in the Heathcote, Mt Camel and Wroo areas have verified that the movement along faults is consistent with this dextral wrench system. The Mclvor Fault is dominantly transcurrent with only a minor thrust component. This can be observed near Mt Camel, where elongation lineations and quartz fibres on fault planes (which dip 75®->286®) pitch south at 17° and en dchelon tension gashes and steps on the fault surface indicate dextral movement. In the Red Hill area, Heathcote, vertical brittle-ductile shears striking 080° (R' orientation) contained a horizontal elongation lineation with en Echelon tension gashes implying sinistral motion and a shear zone dipping 80°->155° with a lineation varying from down-dip to pitching 80° W showed normal movement. The Wroo mine south of Rushworth (an abandoned open cut into old underground drives) shows spectacular NW-SE striking reverse faults with flat-lying tension gashes stepping off them as well as sinistral shears (with a small reverse component) dipping 82°->360° (regional R' orientation) and dextral shears 255° (regional P orientation). The high grade areas mined at Wroo had an east-west orientation and steep dip and were therefore in the R' position; flat tension gash veins off reverse faults were also mineralized but ran a lesser grade. Fold patterns E of the Mt Ida-Mclvor Fault are also indicative of dextral wrenching. Fold axes are at 45° to the principal displacement orientation D (i.e. perpendicular to the implied NE-SW maximum compressive stress) away from zones of faulting and rotate into parallelism with the major faults as they are approached, a feature characteristic of folds developed in a wrench terrain. The Bendigo

Goldfield

is characterised

doubly

plunging

folds oriented approximately parallel to the two major NNW striking boundary faults, the Whitelaw and Sebastion Faults, which dip steeply W. Fold axes are generally parallel and persist for several km; however when they do die out, folds step in an en Echelon manner, with domal fold culminations (which carry the highest grades of Au found in saddle reefs) again stepping diagonally across the field. These fold styles again typify wrench zones where folds have rotated into parallelism with faults in areas of high strain. Field observations of the NNW striking faults in the Bendigo Goldfields again indicated dextral movement, eg.in shear zones along the Whitelaw fault, the asymmetry of pressure shadows around pyrite indicates dextral movement. Sinistral shears in the R' orientation and NE striking normal faults were also observed in several localities in the Bendigo area Bounding faults to the Bendigo goldfields are in the regional P orientation. Movement along P shears implies a volume problem in wrench zones without positive dilation on the regional scale; in the Bendigo area, this has been accommodated by the formation of limb thrusts and reverse faults during flexural slip folding. E dipping faults predominate over W dipping faults; a factor difficult to imagine in a detachement-thrust model. Although the Bendigo and Castlemaine goldfields are dominated by folding or fold associated faults controlling mineralization (in saddle reefs, spurs and leg reefs) faults or "cross-courses" which carried high grade Au mineralization are also present, eg. between the Sebastian and Whitelaw faults, Bendigo, lodes occur in the R, normal fault and reverse fault orientations. Dextral wrenching has controlled much of Central Victorian Au mineralization. In the Maryborough area, mineralization occurs along strike-slip faults and on the margins of lamprophyre dykes intruded along faults. Many of the Maldon Au reefs are also fault controlled, with cross-faulting disrupting the position of reefs. The Muckleford Fault Zone near Maldon shows dextral movement (with a slight normal component) and tension gashes stepping in a dextral manner in sandstones at a contact with hornfelsed black shales were seen near the Derby Hill mine at Maldon. Costerfield is situated in a prime dilational area based on a dextral wrench model as here the Costerfield Fault bends from a P orientation into an R orientation. The Phoenix area near Tarnagulla shows a combination of folding and faulting forming a laminated quartz veins 80°->256° up to 1 m wide. Faults in adjacent slates showed early, dominantly reverse movement followed by oblique slip (dextral/normal) movement. Extensions off the main vein occur in tension gash positions for both movements: this is in agreement with the early formation of folds followed by their rotation into parallelism with the regional principal displacement direction, reactivating reverse fault planes with dextral transcurrent movement. As in many other Au producing areas ranging from Archaean greenstone deposits to younger epithermal deposits, Au mineralization in Central Victoria is controlled by transcurrent faulting, a deformation regime which offers a range of dilational sites and provides the necessary conduits for fluid circulation. The application of wrench models is therefore paramount for Au exploration in Central Victoria.

THE APPLICATION OF AEROMAGNETICS IN REGIONAL INTERPRETATION OF THE NORSEMAN-WILUNA BELT,W.A.

STRUCTURAL

Lyal Harris^, Alasdair Cooke^' Dave Isles^ and Don Pridmore^. 1 Geology Dept., University of Western Australia, Nedlands 6009 2 Aerodata, 17 Emerald Terrace, West Perth 6005 3 World Geoscience Corporation, 17 Emerald Terrace, West Perth 6005 Digitally processed aeromagnetic data has enabled structural interpretation within a section of the Norseman Wiluna Belt of the Archaean Yilgarn Block granite-greenstone terrrain, W.A., where absence of outcrop has hampered previous regional structural synthesis. Lineament interpretations from aeromagnetic data have proved to be superior in this terrain to other remote sensing techniques as the offset of greenstone lithologies with distinct magnetic signatures is extremely clear, especially when artificial illumination or gradient techniques are used to enhance digitally processed images. The overprinting of mapped faults and aeromagnetic linears fitting a sinistral wrench system by structures comprising a dextral wrench system, along with detailed field studies, shows that dextral wrenching overprints and locally reactivates structures formed during early sinistral wrenching (Harris,1987; Mueller et al., 1988). These features are cross-cut by structures interpreted as being formed during approximately E-W regional compression: 060° dextral shears with minor 120® conjugate sinistral shears, E-W tension gashes and normal faults and N-S reverse faults dominate, however their may locally change slightly in orientation due to reactivation of pre-existing structures. The dominant sense of movement may be predicted for shear zones identified from aeromagnetics for which no offset is apparent (eg. shear zones parallel to the regional lithological strike). An example of structural interpretation of aeromagnetic data is given for the Menzies area. Linear features and the sense of offset along them fit the geometry of structures developed within a NW dextral wrench system. Conjugate strike-slip faults formed during the E-W maximum compression event cross-cut the area. Several orientations of both positively and negatively polarised mafic dykes are clearly recognisable. Dykes intrude tension gashes or normal faults for each deformation event or along earlier structures dilationally reactivated during subsequent deformation. From cross-cutting relationships seen on aeromagnetic images, the generations of dykes can be correlated with deformation events. The earliest generation of dykes is represented by the NNW striking Parkeston Dyke in the Kalgoorlie area. This dyke intrudes the Parkeston Fault which (along with the Boulder-Lefroy Fault) is one of the earliest faults formed in the Kalgoorlie region. Sinistral transcurrent shearing along this structure is indicated by the apparent drag and thinning of the Golden Mile Dolerite north of the Golden Mile. The intrusion of the Parkeston Dyke by granitoids also attests to its early age of formation. The next generation of dykes strike approximately 040°. Within the NNW dextral wrench system, normal faults or tensional openings strike 040®, i.e. at 45° to the principle displacement

orientation. The 040° trending dyke suite has intruded into these dilational positions. (Note that 040"" striking dykes have erroneously been attributed to movements along the Eraser Mobile Belt due to its nearly parallel strike.) Secondary Riedel shears (showing sinistral movement) striking approximately 060°-070° formed during dextral wrenching have a controlled subsequent dyke emplacement but do not appear to have been intruded by this dyke generation. An "E-W dyke suite" has intruded along the main E-W tensional position for the bulk coaxial shortening event. Dykes associated with this suite also strike at approximately 060°, i.e. along the dextral shears formed in this event. Dextral structures dominate their sinistral conjugates due to the existence of sinistral R' shears formed during dextral wrenching which have been reactivated or dilated during E-W compression, and minor fractures in this position may have also facilitated preferential failure even where no previous faults were present. Both sinistral and dextral displacements may therefore be seen across 060° striking dykes depending upon the age of formation of the structure or, if reactivated, on which event dominated. A close inspection of dykes shows that dykes striking E-W often bend over short segments into this 060° orientation, and less commonly into the 120° conjugate direction. Au mineralization may also be spatially associated with the intersection of 040° and 060° striking dykes and greenstone belts, especially where sinistral displacement is apparent across the 060° structures controlling dyke emplacement (i.e. R' shears for dextral wrenching). In highlighting the location of these major dilational structures via the presence of mafic dykes along them as well as mapping suitable host lithologies, aeromagnetics proves to be of great value in structural interpretation applied to Au exploration. Digital enhancement of aeromagnetics clearly highlights the shape of granitoid bodies. Where these occur within greenstone belts, lithological and shear trends in the host rocks can be used to indicate the relative timing of granitoids with respect to the transcurrent shearing events. Reverse and normal faults &/or tension gashes cutting the granitoids are apparent. Pre- and syn-tectonic granitoids are asymmetrically wrapped by strike-parallel shears in greenstones in the same manner as for pressure shadows around porphyroblasts. Granitoids are generally elliptical, with their long axes ranging from -30° to parallelism with major shears, implying a rotation towards the movement direction during progressive .deformation. The terminations of oblique granitoids are sometimes deformed, bending asymmetrically towards the regional movement direction and resemble km scale retort or en cornue structures. These features are excellent indicators of the bulk shear sense following granitoid emplacement, which in the area of study, is dominantly dextral. REFERENCES Harris, L.B., 1987. A Tectonic framework for the W . A . Shield and its significance to gold mineralization: a personal view. In Ho S.E. & Groves D.I. (eds), Recent advances in the understanding of Precambrian gold deposits. Geol. Dept. & Univ. Extension, Univ. W. A. Publ.ll: 11-27. Mueller, A.G., Harris, L.B. & Lungan, A., 1988. Structural controls on greenstone-hosted gold mineralization by transcurrent shearing: a new interpretation of the Kalgoorlie Mining District, Western Australia. Ore Geology Reviews 3: 359-387.

A MAJOR PALAEOZOIC DEFORMATION EVENT AFFECTING PRECAMBRIAN TERRAINS OF AUSTRALIA AND EAST ANTARTICA: IMPLICATIONS FOR CONTINENTAL SCALE CRUSTAL SHORTENING Lyal Harris, Claude Delor, John Beeson and Jon Standing Geology Department, University of Western Australia, Nedlands 6009 The consistency in structural styles and of a 90° to 100® orientation of the implied maximum compressive stress have been recognised for contemporaneous deformation in widely spaced Precambrian terrains of Australia and part of the East Antartic Shield. The expressions of this event are summarised as follows: (i) Fraser Mobile Belt, The Fraser Mobile Belt has undergone intense structuring during this event. The Fraser Range Homestead area consists of basic and acid granulites which have been related to a mid-Proterozoic tectonothermal event. These rocks are deformed by 010® striking ductile shear zones presenting a down-dip mineral lineation marked by amphiboles, with shear criteria indicating E to W thrusting. Retrograde metamorphism of former Proterozoic granulite assemblages indicates amphibolite facies metamorphic conditions here for this event. Further evidence for a 100® compression event is given by the presence of amphibole+magnetite bearing pegmatites striking between 90® and 100® along with 060®-070® ductile dextral shear zones with minor 130® sinistral conjugates. On a regional scale, aeromagnetic data shows curvature of N trending foliations into parallelism with 060®-070® striking dextral shear zones several km wide. A 'backwards rotation' of domains between such shear zones results from the large displacements along them. In the S part of the Fraser Mobile Belt, dextral 060®-070® striking shear zones form the boundary with the Yilgarn Block and, in the southernmost extremity, with the Albany Mobile Belt. Splay faults cut the Archaean Ravensthorpe greenstone belt. Conjugate shears, 90® to 100® striking pegmatites and 010® trending folds and reverse faults are also present. (ii ) Albany Mobile Belt. Landsat and aeromagnetic interpretation of the Albany Mobile Belt shows major conjugate 070® and 130® shear zones cutting Proterozoic structures and structures related to ~650My movements along the Darling Mobile Belt. In outcrop, they appear as discrete brittle-ductile shears or zones within which structures on the m scale fit a wrench geometry. Granitoid and garnet bearing pegmatites have locally intruded these zones. Pegmatites occur within 100® striking tension gashes and along some Riedel shears within wrench zones. Minor 010® striking reverse shear zones formed during this event (iii) Darling Mobile B^^lt. Similar conjugate structures as seen in the Albany and Fraser mobile belts observed on regional aeromagnetics cutting the Leeuwin Block display km scale movements. Minor conjugate, dominantly brittle shears (the majority showing only small displacements) observed in the field cut ductile -GSOMy structures and structures related to a 045® compression event. Pegmatites formed during this event strike 090® to 100®. Shear zones along the W margin of the Yilgarn Block indicating reverse movement and folding of the Lower Proterozoic Cardup Group may also relate to this event. Although poorly constrained, ages of ~450My for the Mundaring granite and '-489My for the Donnybrook pegmatite may give the age of this event.

(iv) Mount Isa Block and Cooper Basin. Conjugate faults and approximately N-S folds implying an E-W to ESE-WNW maximum compression direction are present throughout the Mt Isa Block cutting and folding sediments, (including those of Cambrian age) and structures related to an earlier N-S compression event and NNE dextral wrenching. N-S to NNE-SSW striking dextral faults of the wrench event have been locally reactivated by reverse movement. Prominent E-W striking quartz veins also formed during this event. Seismic sections of the Cooper Basin S of the Mt Isa Block show NE-SW thrusts affecting Cambrian horizons, and that these have been reactivated, or that movement has continued, in the Permian with NW-SE sinistral and conjugate dextral shears. (v) Adelaide Geosyncline. The Delamerian orogeny of the Adelaide Geosyncline also forms part of this event. An approximately 90^-100° maximum compression direction is again implied here to form the dominant zone of structuring associated with sinistral wrenching along the NW-SE Crystal Brook Lineament and its conjugate dextral wrench zone in the N Flinders Zone. Metamorphism here is generally greenschist facies, with higher grades in the E and S Mount lofty ranges and Mount Painter inlier. (vi) Paterson Province. Conjugate faults similar to the above areas and en Schelon , quartz filled tension gash arrays are again indicative of a -100° compression event cutting structures formed during Proterozoic NW dextral wrenching. (vii)BrQken Hill Block. Interpretation of regional aeromagnetics of the Broken Hill Block again shows major conjugate structures (with clear strike-slip movements) and normal faults implying ESE-WNW maximum compression for their formation. These structures correspond to mapped retrograde shear zones in the Broken Hill area formed during the Delamerian orogeny. (viii) Eastern Arunta Block. Interpretation of NTGS published maps shows that the Gervois Fault, active in the Late Cambrian, may be a reverse fault associated with 140°-150'' sinistral faults (eg. Charlotte Fault), minor 060"" dextral faults, abundant 120°-130° quartz veins and NNE trending folds. This event is tentatively interpreted as evidence for ESE-WNW compression. (ix) Vestfold Hills and Rauer Islands. East Antarctic Shield. In both of these areas, Proterozoic granulites are dissected by major ductile OTO"" dextral and minor ISO"" sinistral shear zones up to several km in width. Biotite + K feldspar pegmatites striking 090° to 100° occur in the tension gash orientation for this event. Shear zones up to several m wide showing reverse movement also reactivate the margins of 010° striking pegmatites. Other thrust zones (eg. Platcha mylonite zone, Vestfold Hills), can reach up to a km in width. Within these structures, granulitic mineral assemblages have recrystallised to retrogressive biotite and/or amphibole bearing assemblages thus implying low amphibolite facies metamorphic conditions. Although precise ages are not available, it appears that Precambrian terrains of Australia and at least part of the East Antartic Shield have undergone a major early Palaeozoic orogenic episode reflecting continental-scale E-W to ESE-WNW crustal shortening contemporaneous with subduction and plate collision implied for this period in Eastern Australia.The remarkable agreement between structures in Australia and the studied part of the East Antartic Shield implies N-S separation of Antarctica from Australia without significant rotation.

THE STRUCTURAL EVOLUTION OF THE ALBANY FRASER PROVINCE AND LEEUWIN BLOCK, WESTERN AUSTRALIA Lyal Harris, Claude Delor, John Beeson and Jon Standing Geology Dept., University of Western Australia, Nedlands 6009. The Albany Fraser Province (AFP), can be divide into two Proterozoic mobile belts, the Albany Mobile Belt (AMB) and the Fraser mobile Belt (FMB) wrapping respectively the S and SE margins of the Archaean Yilgarn Block of Western Australia. Thre AFP comprises reworked Archaean gneisses and greenstones, Proterozoic meta-sediments, granitoids and mafic intrusives and, locally, a phase of Palaeozoic granitoids. The Leeuwin Block is located on the W margin of the Yilgarn craton within the Darling Mobile Belt and consists of ortho- and para-gneisses, mafic intrusives, syntectonic granitoids and younger aplite dykes and pegmatites. As the AFP is deformed by movements in the Leeuwin Block and the youngest events affect both terrains, the tectonic evolution of the two regions is considered together. Three E-W trending domains with distinct aeromagnetic, structural and metamorphic characteristics are present within the AMB. The Northern Domain comprises reworked Archaean gneisses, the Central Domain comprising pyroxene granulites with quartz-magnetite gneisses and a Southern Domain of amphibolite facies gneisses (with local remnant granulites) intruded by several generations of granitoids. Proterozoic sediments of the Stirling - Barren Group unconformably overly gneisses of the Northern Domain. The first deformation event (Dl) in gneisses is characterised by dextral transpression. In the Northern Domain, Dl intensity increases southwards from discrete ductile shear zones overprinting Archaean gneisses through to the development of a penetrative shear fabric, also present in granulites of the Central Domain and older gneisses of the Southern Domain. In the FMB Dl is a phase of NW directed thrusting accompanied by prograde granulite facies metamorphism of the reworked Archaean Granite Greenstone Terrain, Proterozoic sediments and basic intrusives. Dl therefore represents a NW directed compression, interpreted as implying a NW movement of a continental plate (which also is thought to incorporate the Antarctic craton) towards the Yilgarn craton. Thrusting in the FMB resulted where movement is orthogonal to the former Yilgarn margin, whereas dextral transpression in the AMB reflects oblique convergence along this margin. The second deformation event (D2) in the AMB has folded Dl foliations about dm to km scale folds overturned to the N and developed discrete deformation zones with both thrust and dextral transcurrent components (eg.the Northern Domain/Central Domain contact) contemporaneous with retrograde amphibolite facies metamorphism. Late tectonic granitoids intruded during the waning stages of D2 and show a local flattening foliation and discrete D2 shear zones. Low grade sediments of the Stirling Range have been deformed during oblique (dextral) thrusting along the Northern and Central Domain contacts. Cleavage development increases from N to S. The en Echelon stepping of regional folds overturned to the NW with NW directed limb thrusts, the change in the orientation and geometry of regional folds from N to S and the back- rotation of the folds in the N reflects a continued dextral

movement component. Late folds overturned to the SE in the S are interpreted as having developed over oblique antithetic basement thrusts thus forming a positive flower structure. In the Mount Barren Group, a lateral continuation of the Stirling Ranges further E, km scale overturned folds and thrusts formed during D2. In the Eraser Mobile Belt, D2 deformation produced discrete sinistral ductile shear zones with a small thrust component parallel to the 060° striking boundary with the Yilgarn Block, localised N-S striking shear zones with purely sinistral movement and folds. D2 structures imply a NNW oriented maximum compression direction indicating a clockwise rotation in the direction of plate convergence against the Yilgarn craton. The third deformation event (D3) in the AFP is the first recognisable event within the Leeuwin Block. In the W of the AMB, earlier structures have been rotated into a N-S orientation by sinistral movements along the Darling Mobile Belt. Discrete D3 shear zones in the D, R, R', normal and reverse orientations for N-S sinistral wrenching cut the AMB, with structures becoming more brittle further east. D3 structures are rare in the FMB: minor shear zones and NW striking lamprophyre dykes are thought to have been formed during this event.In the Leeuwin Block, N-S sinistral transcurrent ductile shearing parallel to the E margin of the Yilgarn block was accompanied by prograde upper amphibolite to granulite facies metamorphism. A penetrative shear fabric has been developed and folded during progressive deformation: early isoclinal folds parallel to the elongation direction have been coaxially refolded by tight to open folds and open folds with axial planes perpendicular to the implied NW directed compression. Bulk coaxial structuring due to a 045"^ oriented maximum compression direction constitutes D4 in the A M B (the second deformation event in the Leeuwin Block). In the Leeuwin Block, this event has produced m to km scale folds overturned to the SW, conjugate ductile and brittle-ductile shear zones and 045® striking pegmatites. D4 structures are less developed in the AMB and have not been recognised in the Eraser Mobile Belt. The next deformation event (D5) is a major phase corresponding to 090° to 100° regional compression recognised throughout the areas studied and is especially well developed in the Eraser Mobile Belt where ductile retrograde thrust zones and folds striking 000° to 010°, a major 060° wrench system and more localised conjugate shears have been formed. D5 conjugate shears, thrusts and pegmatite filled tension gashes occur within the AMB. The Leeuwin Block is cut by D5 conjugate and normal faults along with brittle conjugate joint , sets and pegmatite filled tension gashes. The last deformation event (D6), best developed in the Leeuwin Block, comprises conjugate brittle strike-slip and normal shears implying approximately E-W extension with reversals of the intermediate and maximum compressive stress. Dating of the Proterozoic D1 and D2 deformation events in the AMB is in progress. D3 is thought to be --GSOMy. The age of the Palaeozoic D4 and D5 events are unknown but a -450 My age is possible for D5 on the basis of metamorphic resetting and pegmatites dated within the Darling Mobile Belt. D6 is thought to be contemporaneous with break-up along the W margin of the Yilgarn block and movement on the Darling and associated faults from the early Palaeozoic.

EXTENSIONAL DEFORMATION ON FERGUSSON AND GOODENOUGH ISLANDS, PAPUA NEW GUINEA. EJ. Hill

Department of Earth Sciences, Monash University, Clayton, Victoria 3168 Fergusson and Goodenough Islands are located off the NE coast of the Papuan Peninsula, Papua New Guinea (figure 1). These islands have emerged as fault bounded domes of sialic continental crust at the point where the Woodlark Basin seafloor spreading system is propagating into the continental crust. Uplift occured during the Pliocene and Quaternary, and is associated with the intrusion of bodies of granodiorite. The basement rocks of Goodenough and Fergusson Islands were part of the subducted Australian continent. Deformation associated with this subduction resulted in the complex structures found in the core of the domes. This deformation was associated with peak metamorphism resulting in eclogites, granulites and migmatites. The domes have a 1 to 2 km thick skin of retrogressed mylonitic basement with a pervasive mineral lineation. The basement is separated from the overlying unmetamorphosed cover of ultramafic rocks by a shallowly dipping fault. The history of uplift of these high grade metamorphic rocks involves the development of a crustal scale ductile mylonitic shear zone in the basement, around 1 to 2km thick, and dipping at around 25° to 45° degrees (figure 2a). Normal movement along this shear zone resulted in the uplift of deep metamorphic rocks and the juxtaposition of non-metamorphosed cover rocks over these basement rocks. Succeeding extensional structures forming in the basement developed in progressively retrograde metamorphic conditions from granulite/eclogite fades to amphibolite facies to greenschist facies, indicating that this deformation was occuring during uplift. A transition from ductile to brittle behaviour occured late in the history when shear zones were brecciated and truncated by faults. Granodiorite intrusion continued after the juxtapostion of the ultramafic cover rocks on basement and may have caused movement on secondary, minor shear zones which resulted in the dome shape of the islands (figure 2b, c). Rifting and strike-slip faulting caused the breakup of Fergusson dome and offset the Goodenough dome. Epithermal gold mineralisation, hot spring activity and volcanic centres are located along many of the faults indicating that these faults were important pathways for the flow of magma and mineralising fluids to the earths surface. Hill (1987) and Davies and Warren (1988) compared the domes of these islands to the metamorphic core complexes of the north American Cordillera. These domes do show many similarities to metamorphic core complexes. They have a metamorphic basement with a mylonitic fabric and a pervasive mineral lineation. The basement is overlain by a detachment zone i.e. a low angle normal fault and shear. Also, the rocks of the hanging wall are completely distinct from the basement in that they are of entirely different composition, they are not regionally metamorphosed, and they show relatively weak deformation. Davies and Warren (op cit) have proposed a model of stacked

sub-parallel detachment faults which have later been folded, in which shearing and uplift is driven by extension. However, the present study has shown that many of these "detachment faults" are minor faults in which the movement is actually in the opposite direction to that of the major dome-bounding detachment fault, i.e. they are antithetic not synthetic. Oilier and Pain (1980) proposed a model of simple radial uplift and shearing driven by forceful granite intrusion. This study shows that the structures of the domes are not radial, however the intrusion of the granodiorite body does appear to play an important role in the formation of the domes. References Hill, E.J., 1987. Active extension in the D'Entrecasteaux Islands Papua New Guinea. BM,R. Res, Symp. 87 extended abstracts pp 51-57. Davies, H.L. and Warren, R.G., 1988. Origin of eclogite-bearing, domed, layered metamorphic complexes ("core complexes") in the D'Entrecasteaux Islands, Papua New Guinea. Tectonics 7: 1-21. Oilier, C.D. and Pain, C.F., 1980. Active rising surficial gneiss domes in Papua New Guinea. J,GeoL Soc. Aust 27: 33-44.

ultramafic rock dome-boundrig (autt dome-bounding shear

Figure 1. Simplified geology of Goodenough and Fergusson islands showing location of major faults basement (white) and cover rocks (stippled).

secondary shear zones

+ granodioffte ^

xs'v'V; ^^Vo

Figure 2. History of dome formation: (a) development of a l-2km thick shear zone; truncated above by a fault; (b) intrusion of granodiorite associated with development of antithetic secondary shear zones; (c) hypothetical cross-section across island dome.

SYNIECriONIC PRECIOUS AND BASE METAL MllBEKALIZATiaSI (XNTROT.T.EO BY DEPORMATia^ PARTITIONING DURING AN EARLY DEVONIAN OROGENY AT PEAK, OOBAR. N.S.W. Mark C. Hinman Department of Geology, James Cook University of North Queensland, Tc^msville, Australia. 4811. An undeveloped, geological resource of 4.5 million tonnes at 0.7% copper, 1.5% lead, 1.7% zinc, 21 g/t silver and 7 g/t gold at Peak is localised in a series of lenses with differing metal paragenic characteristics that lie in zones of highly heterogeneous, intense D2 strain. The high strain zones are formed by the partitioning of D2 shortening strain between the Cobar Group sediments, hosting the mineralization, and lensoidal slices of siliceous basement lithologies. Ihese slices were faulted into the base of the Cobar group sediment pile prior to the early Devoiian (Dallmeyer et al. 1988) D2 deformation. S2 is a steeply dipping, regionally developed, mesoscopically visible cleavage with a steeply north-plunging L22 stretching lineation that, at Peak, transects F1 folds that have a weak microscopically visible axicil planar cleavage. The zones of intense D2 strain are characterised by (i) the attenuaticxi of bedding and early veins, (ii) unfolding of F1 folds, (iii) anomalous bedding-cleavage intersection relationships (L02) with doubly plunging structures on all scales and (iv) the presence of syndeformational veins and replacenent syst&tis comprising the ore lenses. Prominent mappable shear zones and faults subparallel to S2 formed post D2 and range frcm shear zones reactivating S2, to D3 zones with crenulations, to very recent faults filled with pug or undeformed quartz. At Peak all these structures truncate mineralization and are genercdly only weakly mineralised. They have no significance in the genesis of the primary mineralization. Ihe slices of siliceous material around vAiich the high strain zones form during D2, corprise a group of lithologies exotic to the Chesney Formation and Great Cobar Slates. They include a glassy, vesicular, flow-banded acid volcanic with occasional feldspar phenocrysts and a very finely laminated, siliceous, crystal tuff. These lithologies are largely conformable with each other but grossly discordant with the enclosing Oiesney Formation and Great Cobar Slate sediments that host the mineralization at Peak. The siliceous lithologies are only mineralized in a distinctive hydrothermal breccia developed along the contacts with the Chesney and Cobar Slate sediments. The broad paragenetic sequence of mineralization events during D2 is: (1) D2 pyrrhotitization, sometimes associated with crack-seal silicifaction of the sediments. (2) Dissoninated silver-poor, sphalerite-galena mineralizaticn associated with strong silica replacenent and sediment silicification. (In the extrene, locally termed 'Elvan'.) Boudenaged quartz-sphaleritegalena veins parallel to S2 are tenporally, but not spacially associated, with this phase.

(3) Later fracture- and breccia- ccxitrolled silica-green chloritepyrrhotite-chalcc^yrite-gold mineralisation has formed in previously silicified lithologies and within the contact breccia. A tenporarily, but not spatially associated, vein system of the same mineralogy also exists that overprints stage (2) vein mineralization. (4) Very late in D2; massive to banded silver-rich, sphalerite-galenai r m sul^Aiide mineralizaticxi associated with black chlorite replacement overprints the stages (1)-(3). This banding is not coherent with D2 structures and is controlled by local late stage jostling between competent blocks. Very irregular, volumetrically minor, gold-rich, quartz-sjtoleritegalena-chalccpyrite-pyrrhotite-pyrite veins, representing ' renc±>ilized' mineralization, are post D2 and are sometimes localized along D3 structures. Dallmeyer, R.D., Glen, R.A., and Black, L.B., 1988. New isotopic Evidence for, and Implicaticxis of, an Early Devcaiian Deformatiai at Cdbar, in preparation.

THE PLUMBING SYSTEM FOR SOME AUSTRALASIAN ORE-BODIES BRUCE HOBBS (1), ALISON ORD (1), RICK VALENTA (2) and ERASER GARDINERd) (1) (2)

CSIRO Division of Geomechanics, P.O. Box 54, Mt Waverley, 3149 Earth Sciences Department, Monash University, Clayton, 3168

The mechanism of formation of any ore-body involves various aspects of the coupled thermal-mechanical-fluid flow-chemical transport and deposition problem. Various classes of ore-bodies involve emphasis on parts of this prob eni: for instance, vein-hosted gold or copper deposits involve emphasizing the mechanical-fluid flowchemical parts of the problem with little emphasis on the thermal aspect whereas epithermal gold mineralization involves emphasizing the thermal-fluid flow-chemical parts of the problem with little attention to the mechanical aspects. In this paper we describe various 2D and 3D computer codes that enable the mechanical-fluid flow-thermal aspects of the problem to be addressed; some chemical transport aspects are included in that transport and deposition of silica, controlled by pressure and temperature, can be modelled at this stage, in conjunction with the other codes. In this paper we confine our attention to the quartz-gold deposits of the Bendigo area, the vein-hosted and replacement copper deposits of Mt Isa and Hilton and the epithermal gold deposits of various deposits associated with the Pacific Rim. In the vein-hosted deposits the sites for mineralization are controlled by the partitioning of deformation between slip and separation on planar discontinuities on the one hand and ductile deformation of the host rocks, on the other hand. Slip and separation on planar discontinuities leads to the formation of massively dilatant zones which act as channel-ways for fluid migration; inhomogeneous ductile deformation of the host-rocks leads to an inhomogeneous distribution of effective stress which in turn leads to localized development of vein systems. The basic principles governing partitioning of deformation are discussed and illustrated with simple examples. It is shown that slight changes in mechanical properties can lead to contrasting modes of partitioning and sometimes to baffling slip and dilational geometries. In the epithermal gold deposits, the sites for mineralization are controlled by the detailed temperature distribution which in turn is influenced by the geometry of the permeability distribution and is coupled to the fluid flow geometry. The influence of boiling is discussed together with the influence of changes in the permeability distribution upon the fluid flow geometry and the temperature distribution.

A finite strain and kinematic study across a major ductile thrust and shear zone, south of Normanville, Fleurieu Peninsula, South Australia. P.R. James and J. Anderson Department of Geology and Geophysics, University of Adelaide, GPO Box 498, S.A. 5001 South of Normanville on the northwest coast of the Fleurieu Peninsula, a large near recumbent fold is delineated by the regional Adelaidean stratigraphy and is cored by basement of the Early Proterozoic Barossa Complex. The major fold axis lies perpendicular to the regional trend and plunges shallowly southeast parallel to the regional principal stretch. Its normal limb is moderately deformed compared to its overturned and highly attenuated inverted limb, which may be compared to a major zone of semi-ductile to ductile shearing. The principal stretch of the shear zone is delineated by a mineral and mineral aggregate elongation lineation within chloritic phyllonites of the basement and by the elongation of highly deformed pebbles and clastic grains in the stratigraphically overlying (but structurally underlying) basal Adelaidean sequence. Fold axes of highly flattened similar-style minor folds and associated intersection lineations and asymmetric boudin axes. from within layered Adelaidean metasediments are also near parallel to the regional principal stretch, attesting to the intensity of strain. A quantitative strain analysis of the area has been made using measurements of clastic grain shapes to estimate three-dimensional strain variations within and marginal to the shear zone. Shortening values across the foliation are in excess of 70% and minimum elongation parallel to

the stretching lineation are estimated as greater than 250%. Shortening has also been estimated from the degree of post-buckle flattening and buckle shortening deduced from small-scale quartz ptygma and other folds. These indicate comparable strains to those estimated from the deformed clastics. Deformation paths interpreted from the variation of finite strains suggest intense constrictional strains have developed from less significant earlier flattening strains. This model which is inconsistent with a single plane strain derived by simple shear in a shear zone, may represent the effects of strain superposition on an initial sedimentary fabric, or may be due to non-plane strain modification induced by volume change or the effects of inhomogeneous deformation during folding. Movement indicators in the shear zone fabrics include asymmetric shear bands, shear boudins, minor folds and asymmetric (rotated?) megacrysts. All indicate a consistent overthrust sense of displacement towards the northwest.

Microstructures and Rheology of Steady-state Flow

Mark W, Jessell and Glenn Lees Department of Earth Sciences, Monash University, Clayton, Victoria,3168

The inherent difficulties in deforming specimens under controlled conditions have in general limited the finite strains that can be achieved when studying natural or analogue samples in the laboratory. One solution that has been successfully applied in the past has been the use of torsional deformation rigs, so that high simple shear strains can be attained with constant imposed deformation geometries. We have designed and built a torsional analogue deformation rig which allows us to observe microstructural changes as the specimen reaches shear strains in excess of 50y» and achieves steady state flow. A new feature of this deformation rig is that the drive train includes a load cell which allows us to correlate microstructural evolution with rheological behaviour. One of the aims of these experiments is to try to define under which conditions shear-localisation occurs. The material, octachloropropane, passes through several different types of behaviour before reaching a steady state, deforming roughly homogeneously at first, before a localisation of the shear-zone occurs, followed by a gradual widening of this high strain-rate zone. The experiments described here consist of an initial period of constant bulk stram-rate deformation (at shear strain rates of 1.1 x 10"^ s"^) until steady-state flow is achieved. The strain-rate is then stepped up or down in approximately order of magnitude jumps, which quickly increases or decreases the observed flow stress, and the microstructural response is observed. The rheological response is found to be much faster than the rate of microstructural change.

77 Monash University Department of Earth Sciences The relative roles crustal deformation

pure

shear

and

simple

shear

during

Mark Jessell and Gordon Lister The single most important factor which determines the mechanical behaviour of rock is rheological softening. This phenomenon has many manifestations. In the middle to upper crust faulting is important. Ductile shear zones make their appearance with the onset of metamorphism, and crystal plastic behaviour. This variation can be described dependent rheological softening.

solely

terms

depth

Fault zones become weaker after faulting takes place. Rheological softening accompanying catastrophic failure is marked. Major fault zones have a long history of activity, and thus exert considerable geological influence. Ductile shear zones (to which many major faults may be linked) also apparently involve rheological (or strain) softening behaviour. Otherwise, as argued previously by other workers, shear zones would broaden with time, rather than becoming more localized. There is abundant evidence which suggests shear zones (at least under retrograde conditions) become progressively more localized as shear strain accumulates. The degree of rheological softening may be less than that involved for faulting. At depth, there is some suggestion that deformation is more penetrative and distributed through the rock mass. This would imply a lessening of the of strain softening behaviour. In fact we know very little about strain softening. Strain softening might be associated with dilatative behaviour, whether in the brittle field or the ductile field. Alternatively, several different basic mechanisms might lead to the same end result. In the ductile field, for example, strain softening may be limited to a particular field of recrystallization behaviour. Another difficulty is that coaxial shortening experiments have produced little evidence for the phenomenon, although there are a growing number of shear experiments which support the concept. Recent models for continental extension involve crustal scale shear zones (and/or lithospheric discontinuities) which may penetrate to great depth. We have no reason to argue that such phenomena are not physically unrealistic.

but such behaviour requires strain softening shear zones at depths beyond which they had been considered. We are left with fundamental questions concerning the mechanics (and the physics) of how large strains accumulate in polycrystalline aggregates. These questions need answers and so a program of research has been instigated. Basic mechanisms include:

proposed

date

for

strain

softening

(a)

shear heating - a zone deforming faster than its surroundings can become hotter and therefore softer, so that there is a tendency for a shear zone to narrow. This can be termed thermomechanical coupling.

(b)

fabric softening - a crystallographic fabric forms aligning the easy slip systems in a position favourable for continued deformation. This is a form of geometric softening.

(c)

grain-size softening - metamorphic reactions taking place may led to finer grain sizes, enhanced diffusion, and a variety of postulated softening mechanisms. Alternatively, reduction in grain size due to higher strain rates may lead to softening.

Preliminary results of our research are discussed. To date steady state microstructures and rheologies have been generated in simple shear with shear strains in excess of 100 using the ring shear apparatus. Recrystallization may enhance fabric softening because thereby "old grains" in an unfavourable orientation for deformation are removed, and grains in single slip orientations may grow. Preliminary results with the Jessell ring shear apparatus show interesting correlations between strain localization and strain rate.

The Role of Temperature in Determining Crystallographic Orientations

Preferred

Mark W. Jessell and Gordon S. Lister

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

Clayton,

The purpose of this paper is to evaluate a series of computer simulations of crystallographic preferred orientation (CPO) and grain shape fabric development in a quartzite shear-zone, with input parameters for the simulation chosen to reflect a systematic increase in temperature. The deformation processes considered by this model are the lattice rotations predicted by Taylor-Bishop-Hill theory, grain boundary migration driven by the work done by individual crystals and also by the grain boundary energy, and the recovery and hardening rates. The effects of a post-tectonic anneal can also be investigated. This model makes predictions of the CPO for all crystal axes, and also produces AVA diagrams. With the increased use by other workers of complete crystallographic fabric measurements we are also able to make detailed comparisons between the predictions of this simulation and fabrics measured in naturally deformed rocks. The simulations suggest that for quartzites, the interaction of lattice rotations and recrystallisation processes leads to steady state fabrics characterised by point maxima which reflect the activity of the weakest slip system available, even when they are constrained to deform by multiple slip. These comparisons suggest that for a given deformation geometry, much of the broad variation of CPO fabrics in quartzites can be correlated with changes in metamorphic grade. The effects of a post-tectonic anneal depend on the relative rates of recovery with respect to grain boundary mobilities, with higher grain boundary migration rates leading to a weakening of CPO's, and lower grain boundary migration rates leading to a strengthening of CPO's.

EEKBMftTICN HISTORY OF THE C m O O SCHISTS, NEW ZEALAND: C X C U C UPLHTCCUjAPSE CeOCTNESIS AND ITS IMPIiIC3ffiICNS. Scott E . Johnscn Department of Geology, James Cook University, Townsville, Qld, 4811. Foliation overprinting relaticaiships preserved in porphyrc±)lasts and the surrounding matrix in the Otago schists indicate developnent of up to 6 penetrative foliations during the late-Mesozoic Rangitata Orogeny. SI, S3 and S5 formed subvertical, and S2, S4 and S6 formed subhorizontal. OrientatiOTis within porE^TiyrdDlasts of fold axes associated with SI through S6 are all subparallel to one another, and to the length of the schist belt. A steeply-dipping foliation striking subparallel to the trace of the Alpine Fault overprints these earlier foliations in northwestern O t a ^ . This foliaticxi is labelled S7 for convenience, and is interpreted as postdating the Rangitata Orogeny, possibly forming during the late-Cenozoic Kaikoura Orogeny. SI and S2 are preserved only as inclusion trails in porphyrc±)lasts because of matrix-foliation reworking during progressive defoxmaticari. S3 through S6 are preserved in the matrix as well as in porphyroblasts, and are generally correlable from one to the other. S7 is found only in the matrix. Inclusiai-trail asyimietries define the location of the otogenic core during the Rangitata Orogeny vdien the porphyroblasts grew. Ihe locaticxi of this core is ccxisistent with macroscale geological and structural observations. Alternating subvertical and subhorizontal foliations are inferred to indicate alternating horizontal corpression and extensim. Otogenic models conpatible with continually alternating horizontal compression and extension are examined. Specific predictions from a model by Bell & Johnson (1989) are cotipared with observations at all scales, and the iwDdel is found to provide a particularly good framework within v^ich to understand the orogenic development of the schist belt. Ihe present structural state of the Otago schists is indicative of a collapse stage of the Bell & J d m s o i model of orogenesis. Some of the structviral features in the Otago schists indicative of a collapse stage are listed belcw. 1) Pervasive flat-lying foliations. 2) NaK>e structures resulting from folding of steep foliations about flat-lying axial planes. 3) Hi^-strain zcxies associated with gravity spreading occur on either side of the orogenic core. 4) Major thrust detachments occur on the lateral extremes of the orogen that displace material away from the orogenic core. Reconstruction of the gross geometry of the Otago schists, b ^ e d on the three-dimensional geonetry of porphyroblast inclusiOTi trails, is consistent with recent paleonagnetic reccxistructions of the New Zealand region. Reconstruction requires a northerly trend for the belt north of Otago during the Rangitata Orogeny. This suggests post-Rangitata inception of the Alpine Fault resulting in significant dextral rotation of the belt near the fault trace, acconpanied by 480 km of dextral displacement along the fault. Inclusion-trail orientation data indicates that porphyroblasts near the Alpine Fault did not rotate relative to geographic coordinates, suggesting that there has always been a component of progressive shortening deformation associated with Alpine Fault movanent.

Bell, T.H. & Johnson, S.E., 1989. Porphyroblast inclusion trails: the key to orogenesis. In press, Journal of Metamorphic Geology.

P R E F E R R E D O R I E N T A T I O N P A T T E R N S OF P L A G I O C L A S E , T O O L S FOR M O D E L L I N G C O M P L E X S T R A I N H I S T O R I E S

Jorn H .

MICA

AND Q U A R T Z --

Kruhl

Institut fiir Mineralogie 5 Kristallographie, D-1000 Berlin 12, F e d . R e p . G e r m a n y

Technische

Universitat

Berlin,

In regional metamorphic rocks, strain inhomogeneities which are observable on the meso- a n d micro-scale a p p e a r to be an extremely common phenomenon. For e x a m p l e , there are orientation differences between intersection lines of schistosities/ pencil structures a n d stretching directions. These differences are reflected b y the preferred orientation patterns of the different minerals. Micro textures of granitic gneisses from the Eastern a n d Western Alps h a v e been studied. Preferred crystallographic a n d dimensional orientations of plagioclase, micas a n d quartz h a v e been measured a n d related to the orientations of schistosity planes a n d lineations. Quartz c-axis patterns are a l w a y s related to the pencil structures which are mainly a geometric feature, indicating co-axial flattening or p l a n e strain as well as pure or simple shear in distinct pencil regions. However, plagioclase a n d mica orientation patterns are related to the stretching direction w h i ^ h is significant for kinematic analyses a n d may be oriented parallel, p e r p e n d i c u l a r or oblique to the pencil structure. Recrystallized plagioclase g r a i n s mostly develop [lOO] parallel to the stretching direction a n d (001) parallel to the schistosity p l a n e s . The gamma indicatrix axes of white micas are parallel to the stretching direction, as a result of crystallization processes d u r i n g greenschist facies as well as of s h e a r i n g a n d recrystallization d u r i n g amphibolite facies conditions. Preferred orientations of plagioclase/mica a n d q u a r t z , respectively, may generally reflect different deformation events as well as inhomogeneous strain distribution d u r i n g the same deformation event. Plagioclase a n d mica reflect, through the various crystal generations, better than quartz the various kinematic frameworks of rocks which suffered p o l y p h a s e deformation. Therefore, despite the measuring difficulties a n d the time consuming determinations of orientation patterns, it should be useful to shift preferred orientation studies a w a y from just quartz to potentially more informative minerals like plagioclase a n d m i c a .

TIMING SYNTECTONIC MINERALISATION USING PORPHYROBLASTS: BOTTLE CREEK GOLD DEPOSIT AS AN EXAMPLE

THE

K.C. Lawrie Department of Geology James Cook University Townsville QLD 4011

Timing syntectonic mineralisation relative to the deformation sequence is an essential step in identifying the key structural features and trapping geometries that localise mineralisation in structurally controlled orebodies formed in ductile and brittle-ductile regimes. This information provides the basis for predicting the occurrence of orebodies using a structural and microstructural analysis. If present, porphyroblasts with internal inclusion trails can help time events and provide information on the orientation of early structures, for which the matrix foliations have commonly been reactivated and reorientated (Bell, 1986). Although applicable to all syntectonic deposits, the structural and microstructural techniques outlined here apply where wallrock alteration or metamorphic conditions permit- porphyroblast growth. Alteration is frequently massive in areas adjacent to or hosting mineralisation, with pre-existing structural information in these areas usually obliterated through replacement, and later superimposed fabrics generally weak. It is only in transition zones on the margins of the alterations systems that the timing relationships between structural elements, such as cleavages and lineations and the alteration assemblages and vein systems, can be determined. In these marginal zones, alteration frequently occurs as porphyroblastic mineral growth associated with penetrative cleavage development and/or vein margins. Recent advances in understanding how porphyroblasts grow, and in the microstructural interpretation of their relationships with internal and external fabrics (Bell 1985; Bell et al, 1987) allow them to be used to time the mineralisation. Preliminary structural and microstructural studies at the Bottle Creek gold deposit in the Ularring greenstone belt of Western Australia demonstrate that primary mineralisation associated with a major NNW-SSE trending crustal lineament (Leggs et al, 1966) occurs as veins and semi-massive sulphide replacement within a steeply-dipping zone beneath the Tertiary weathering profile. Both mineralisation and associated alteration are discordant to host metavolcanic, metasedimentary and quartz-porphyry lithologies. Alteration includes silicification sulphidation, epidotisation and chloritisation, as well as the development of biotite and/or stilpnomelane and muscovite. Replacement textures are

predominant within ore-bearing veins massive sulphide zone.

and within the

semi-

Microstructural analysis of orientated samples reveals the presence of seven penetrative and sequentially overprinting cleavages (S1.7) in the mine vicinity. Garnet porphyroblasts are abundant in the mine vicinity, with growth initiated in D2 and continuing through to Dg. The garnets are typically poikiloblastic with successive growth zones indicated by change in garnet composition and by marked changes in the density, orientation and composition of the inclusion. The latter define internal foliations, and the consistency in inclusition orientations and geometrical relationships in the cores of the early-formed garnets is interpreted as indicating non-rotation of these prophyroblasts (Bell, 1985) despite extensive external foliation reactivation. Poikiloblastic plagioclase porphyroblasts are also common and grew principally during Dj. In the matrix, Si and S2 generally form a composite fabric expressed as a differentiated crenulation cleavage. Individual Si and S2 components are recognised both within prophyroblast strains shadows and internally within garnet cores where Sn is defined by a planar alignment of inclusions, but its original orientation is unknown. S21 is a subhorizontal differentiated crenulation fabric that lies in the axial plane of tight F2 microfolds. As few early syn-Dj porphyroblasts have been found, it has not been possible to determine the original orientation in S2. Most garnets grew syn-late D3 and early-syn to late-syn D5 and syn-Dg. Correlation of the internal inclusion fabrics in the garnets reveals that S4 and Sg are subhorizontal fabrics while S3 and S5 are subvertical. S5 is preserved in the matrix as a N-S trending subvertical differentiated crenulation that becomes more intense towards the ore zone where it is expressed as penetrative slaty cleavage associated with marked transposition and development of C-S fabrics in restricted shear bands. On a gross scale, mineralisation within the open pits is subparallel to this cleavage. Microstructurally, sulphidebearing veins and associated stilpnomelane-rich wallrock alteration assemblages truncate Si.4^ are overgrown by coalescing garnet poikiloblasts and are crenulated by Sg. S7 is a NE-trending subvertical crenulation that transects the mineralised zone, is expressed as a fracture cleavage in sulphide grains, and locally forms more intense shear offsets. Further structural and microstructural studies are required to determine precise controls on mineralisation within Sg zones in order to form a predictive m o d e l for blind deposits.

84 REFERENCES Bell, T.M. 1985. Deformation partitioning and porphyroblast rotation in metamorphic rocks: a radical reinterpretation. J. Metamorphic Geol.. 3, 109116. Bell,

T.K., Fleming, P.D & Rubenach, J.J., 1986. Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development. J. Metamorphic Geol. 4, 37-67

Ledge, P.G., Lawrie, K.C. & Monti, R., 1986. Bottle Creek discovery and research developments. In 'Research and developments for the minerals industry'. Curtin University, Kalgoolie School of Mines, Kalgoorlie.

THE HASTINGS BLOCK - A KEY TO UNDERSTANDING THE TECTONIC DEVELOPMENT OF THE NEW ENGLAND OROGEN Paul G. Lennox and John Roberts Department of N.S.W. 2033

Applied

Geology^

University

New

South

Wales^

Kensington^

The stable and little deformed Hastings Block is presently located in a tectonically anomalous position east of deep water sediments of a subduction complex of the New England Orogen and south of the multiply deformed and cleaved Nambucca Block. The Late Palaeozoic to Early Mesozoic development of the Yarrol-New England Orogen is interpreted as a west-dipping subduction zone^ modified during its latest stages of development by transpression and possibly large-scale transcurrent faulting. While Evans and Roberts (1980) invoked dextral followed by sinistral strike-slip movement^ Cawood (1982) and Cawood and Leitch^ (1985) considered sinistral strike-slip movement alone produced terrane dispersal in the southern New England Orogen. Murray and Whitaker (1982), Flood and Fergusson (1984) and Murray et al (1987) considered the Texas-Coffs Harbour Megafold to have been formed by 500 km of dextral strike-slip movement of the Yarrol Orogen along the north-southtrending Gogango-Baryulgil Fault Zone in the latest Carboniferous. An alternative model proposed by Korsch and Harrington (1987) and Wellman and Korsch (1988) proposes development of an Early Permian triple orocline involving the Texas-Coffs Harbour Orocline in the north and the Manning Orocline in the south. Both models for the development of the Yarrol-New England Orogen have deficiencies, and this study of the Hastings Block provides constraining evidence concerning the origin and mode of emplacement of the block. The Hastings Block contains Devonian to Carboniferous fore-arc basin sediments indicating it is out of place relative to elements of the orogen. This could be explained by: (1) transcurrent movement northwards from a southeasterly extension of the Tamworth Belt (Zone A) (Cawood 1982); (2) derivation from outside the New England Orogen; or (3) oroclinal folding of the southern part of the orogen (Korsch & Harrington 1987). The Carboniferous palaeoslope within the Hastings Block is towards the southwest, almost opposite that of the Tamworth Shelf (Lennox & Roberts in press). This reversal of the Carboniferous palaeoslope could be explained by either rotation of the Hastings Block or its derivation from a terrane separate from the New England Orogen. Early Permian sediments are shallow marine on the northern part of the Hastings Block, but change rapidly to deep water sediments northwards in the Nambucca Block. The beds on the northern part of the Hastings Block appear to be lithologically similar to the highly deformed and metamorphosed slate and phyllite in the Nambucca Block, suggesting juxtaposition of the blocks during the Early Permian. Emplacement of the Hastings Block relative to the Nambucca Block therefore must have taken place between the termination of Carboniferous deposition in the Westphalian?, and the Early Permian (Fauna II) marine transgression; i.e., during the latest Carboniferous or earliest Permian. Latest Carboniferous to earliest Permian emplacement coincides with the development of the Texas-Coffs Harbour Megafold and with Murray et ai's (1987) postulated southward (dextral) movement of the Yarrol Orogen relative to the New England Orogen, but differs from the timing of folding suggested by Korsch and Harrington (1987). The megafold formed after deposition of the Texas Beds (Early Carboniferous to possibly Namurian or Westphalian) and before Fauna II i.e., latest Carboniferous or earliest Permian.

Korsch and Harrington (1987) also postulated a southern megafold, the Manning Orocline^ within the New England Orogen^ inferring that rocks of both Zones A and B were refolded northwards^ within the southeastern part of the orogen, and that the pods of serpentinite scattered throughout the Manning and Hastings regions represent a refolded Peel Fault (H.J. Harrington personal communication 1988). Geological mapping by Roberts and Engel (1987) in the Hunter-Myall region of the Tamworth Belt indicates that there is no northward bending of strike directions within that belt necessary to sustain Korsch and Harrington's (1987) orocline hypothesis. Apart from the Port Macquarie Block (Leitch 1980), the greater Hastings Block (from the Manning Fault northwards) therefore appears to be a Late Devonian to Carboniferous (Westphalian?) terrane with a cover sequence of Early Permian marine sediments. The question is, was the Hastings Block derived from the New England Orogen, or is it allochthonous? The Late Devonian to Carboniferous succession has similarity with Tamworth Belt sediments in that most rocks were (1) derived from a volcanic arc (Lennox & Roberts in press); (2) the first influx of exotic lithologies, such as quartzite, slate and phyllite, did not take place until the Namurian to Westphalian (Roberts 1987); and (3) both have similar, though not identical, thermal histories (Offler & Hand in press). Faunal evidence is not indicative of a relationship with either the Tamworth or Yarrol Orogen because those Carboniferous zones in the Hastings Block are present in both areas. Derivation of the Hastings Block from the present eastern margin of the Tamworth Belt and translation northwards by sinistral faulting (Cawood 1982) is unlikely because Carboniferous rocks on the eastern margins of the HunterMyall region represent shelf-edge to slope deposits and would be unlikely to pass eastwards into the predominantly shelfal rocks. Hence, Cawood's (1982) proposed northward movement of the Hastings Block fits neither the geology of the eastern extremity of the Tamworth Belt nor facing direction of Carboniferous facies. Rotation of the Hastings Block, in a manner similar to that put forward by Crowell (1983) would correct the facing problem, but would still not be compatible with the geology of the present eastern margin of the Tamworth Belt (Fig. 1). The origin of the Hastings Block still remains obscure. Although there are similarities with the Tamworth Belt there are sufficient differences to slightly favour an allochthonous origin. Fig.1. Speculative latest Carboniferous to earlier Permian emplacement of the Hastings Block under dextral transpression. Modified from Crowell (1983). P.F. = Parrabel Fault, D.F. = Demon Fault, W.-S.C.F. =Wauchope-Sapling Creek Fault, K.F. = Kunderang Fault, Y.S. =Yarras serpentinite and T.S. = Taree serpentinite. References cited can be obtained from the authors.

T.S. 200

u km

HASTINGS BLOCK Deeper water ^ sedimentary fades

A Revised Apparent Polar Wander Path for Australia and Its Tectonic Implications Z.X, Li^, C.McA. Powell^, P.W. Schmidt^, G.A. Thrupp^ 1

Australian Plate Research Group, School of Earth Sciences, Macquarie University, NSW 2109

Rock Magnetism Laboratory, CSIRO Division of Exploration Geoscience, North Ryde, NSW 2113

New palaeomagnetic investigations have been carried out on the mid- to Late Palaeozoic rocks in central Australia, the Lachlan Fold Belt, and the New England Fold Belt, using modern techniques. Primary (or early diagenetic) remanent magnetizations have been revealed from the Mereenie Sandstone, the Parke Siltstone, and the Hermannsburg Sandstone in the Amadeus Basin (Li et al, 1988a) , from the Hervey Group sandstone and the Worange Point Formation in the Lachlan Fold Belt (Li et al, 1988b/ Thrupp et al, 1988), and from the Goonoo Goonoo Mudstone, the Merlewood Formation, the Currabubula Formation, and the Werrie Basalt in the New England Fold Belt (Schmidt, 1988) . A syn-folding remanent magnetization is revealed from the Mount Eclipse Sandstone in the Ngalia Basin (Li et al, 1988c) . Also,

,ME2

Fig.l Revised mid-Late Palaeozaic apparent polar wander path of Australia.

more Alice Springs Orogeny overprints were identified from the Amadeus Basin (Li et al^ 1988a). The mid-Late Palaeozoic apparent polar wander path is revised using the new and existing reliable data (Fig. 1)• These new results suggest that: (1) The palaeomagnetic data from cratonic Australia and the Lachlan Fold Belt agree very well since mid-Late Devonian^ and there is no palaeomagnetic evidence supporting a displaced terrane model for the Lachlan Fold Belt since as early as latest Silurian. (2) The Early Carboniferous-Permian data from the New England Fold Belt indicate palaeolatitudes similar to that of cratonic Australia^ but some relative rotations may have occurred during the Carboniferous. (3) Australia had stayed in moderate to low latitudes during the Early to mid-Palaeozoic^ but moved rapidly to a polar position during the Early to mid-Carboniferous. (4) The Alice Springs Orogeny in central Australia did not reach its peak until mid-Late Carboniferous. References Li^ Z. X.^ C. McA. Powell^ B. J. J. Embleton^ & P. W. Schmidt^ New palaeomagnetic results from the Amadeus Basin and their implications for stratigraphy and tectonics^ BMR J. Aus. Geol. & Geophys^ 1988a (submitted). Li^ Z. X., P. W. Schmidt & B. J. J. Embleton^ Palaeomagnetism of the Hervey Group^ central New South Wales and its tectonic implications. Tectonics, 7, 351-367, 1988b. Li, Z. X., C. McA. Powell & P. W. Schmidt, Syn-def ormational remanent magnetization of the Mount Eclipse Sandstone, central Australia, Geophys. J., 1988c (submitted). Schmidt, P. W., A rapid Carboniferous polar shift of New England from palaeomagnetism. Proceeding of the Symposium on the New England Orogen, University of New England, 1988. Thrupp, G. A., D. V. Kent & P. W. Schmidt, Preliminary palaeomagnetic results from Late Devonian redbeds of southeast Australia, abstract for the AGU fall meeting, 1988.

Amphibolites/metadolerites and their tectonic implications from the Mt Lofty Ranges metamorphic belt^ S.A. Songfa Liu and Peter D. Fleming Department of Geology, La Trobe University, Bundoora, Victoria, Australia 3083 The Mt Lofty Ranges metamorphic belt (MLRMB) and the rocks of the Cambrian Kanmantoo Group which occupy much of it are of tectonic importance because they lie at the boundary of the Precambrian block and the Palaeozoic Block of the Australian continent. In this regard, hitherto neglected features of the MLRMB are the numerous mafic dykes, sills and plugs that occur. Of great interest is their possible significance with respect to (i) the early onset of thermal activity (metamorphism, partial melting, and granitic intrusion, etc.), (ii) the fact that the Kanmantoo Group is the youngest unit of the Adelaide Foldbelt, yet the most extensively and highly metamorphosed, and (iii) the tectonic setting of the Kanmantoo Group. The mafic dykes and plugs are mainly confined to the Kanmantoo Group, although some do occur in the higher grade Precambrian metasediments just west of the Kanmantoo Group. They range in width from a few centimeters to 20-30 meters and from a few tens of centimeters to 1-2 kilometers in length. Detailed mapping in selected areas of the MLRMB reveals that there were intrusions of mafic rocks of various stages of the development of the MLRMB. These are: (i) Dyke and sill intrusions prior to the first deformation (Dl) in many parts of the MLRMB, e.g.. East of Springton, Cooke Hill, Tungkillo-Palmer, South of Williamstown, Blowhole Creek on the south coast, and west of Cape Hart on Kangaroo Island; (ii) Dyke intrusions post Dl and prior to D2 in some areas; most are now lineated and/or foliated; (iii) Dyke and plug intrusions post D2 in many parts of the metamorphic belt. These preserve doleritic textures and show little evidence of deformation. Major, trace element and REE c om.p o s i t i on s of amphibolites/metadolerites from the Tungkillo-Palmer-Cooke Hill area are similar to those of MORBs. Spidergrams of trace and minor elements show a similar pattern to E-MORBs. REE analyses for these rocks define very flat patterns which are parallel to those for EMORBs. On the Ti02-K20-P205 diagram of Pearce et al (1975), the analysed rocks fall into the oceanic field. On Pearce's (1980) ZrTi tectonomagmatic discriminating diagram they fall in the MORB field and show a typical MORB trend. On the Ti-Zr-Y diagram of Pearce & Cann (1973) they plot into the field for ocean floor, lowpotassium and calc-alkaline basalts, and on their Ti-Zr-Sr diagram they fall into the field for ocean floor basalts. On Meschede's (1986) Nb-Zr-Y diagram the plots also fall into the MORB field. The intrusion of some dykes/sills prior to penetrative Dl deformation indicates that the mantle and intrusions of uppermantle-derived materials made important contributions to the early (pre- to syn-Dl) thermal history of the MLRMB by providing heat for the early (pre- to syn-Dl) thermal activities. These early thermal

activities include: (1) metamorphism (e.g., crystallization of porphyroblasts, Fleming & Offler 1968 and this study); (2) partial melting in the migmatite zone (Fleming & White 1984); (3) igneous intrusion (Milnes et al 1977 and this study); and (4) fluid circulation for early sulphide mineralization (Seccombe et al 1985). At this early stage heating could have been due to an increase of geothermal gradient due to crustal thinning, bringing sediments and upper mantle into closer proximity, as well as the result of intrusion of mantle-derived materials. The peak of metamorphism was probably reached during D2 (Offler & Fleming 1968, Mancktelow 1979, and Sandiford et al 1988) as multiple heating sources took maximum effect. These sources eventually included intrusions of mafic dykes and intrusions of granites. The metamorphic peak may have also coincided with thickening of the metamorphic belt as the result of D1 and D2 deformation. Considering available geological and geophysical evidence, we suggest that the amphibolites/metadolerites in the MLRMB intruded in a continent-ocean boundary environment. Such an environment was implied by von der Borch's (1980) tectonic model involving accretion of oceanic crust in the southeastern part of the belt. The Cambrian Kanmantoo Group sediments may have been mostly deposited in a marginal basin where the crust was stretched and thinned to allow the deposition of the Kanmantoo Group and which soon become a region of the most abundant dyke intrusion, and consequently the most highly metamorphosed part of the Late Precambrian and Cambrian metasedimentary sequence in the MLRMB.

Fleming PD & Offler R 1968, Pretectonic metamorphic crystallization in the Mt Lofty Ranges, South Australia. Geol. Mag., 105: 356-359. Fleming PD & White AJR 1984, Relationship between deformation and partial melting in the Palmer migmatites. South Australia. Aust. J. Earth Sci.f 31: 351-360. Mancktelow NS 1979, The structure and metamorphism of the southern Adelaide Fold Belt. Ph.D. thesis, Univ. Adelaide (unpubl.). Mechede M 198 6, A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb-Zr-Y diagram. Chem. Geol., 56: 207-218. Milnes AR, Compston W & Daily B 1977, Pre- to syn-tectonic emplacement of Early Palaeozoic granites in southeastern South Australia. J. Geol. SQC . hlA^l^f 24: 87-106. Pearce JA 1980, Geochemical evidence for the genesis and eruptive setting of lavas from Tethyan ophiolite. Proceedings of International Ophiolite Symposium, Cyprus, 1979: 261-272. Pearce JA & Cann JR 1973, Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett.. 19: 290-300. Pearce TH, Gorman BE & Birket TC 1975, The Ti02-K20-P205 diagram: a method of discriminating between oceanic and non-oceanic basalts. Earth Planet. Sci. Lett,. 36: 121-132. Sandiford M, Oliver RL, Mills KJ & Allen RV 1988, A cordierite-staurolotemuscovite association, east of Springton, Mt Lofty Ranges: implications for the metamorphic evolution of the Kanmantoo Group. Brian Daily Memorial Volume, Geol. Soc. Aus, Spec. Publ. (in press). Seccombe PK, Spry PG, Both, RA, Jones MT & Schiller JC 1985, Base metal mineralization in the Kanmantoo Group, South Australia: a regional sulfur isotope study. Econ. Geol., 80: 1824-1841. von der Borch CC 1980, Evolution of Late Proterozoic to Early Palaeozoic Adelaide Foldbelt, Australia: comparison with post-Permian rifts and passive margins. Tectono., 70: 115-134.

DEFORMATION OF SYNTHETIC QUARTZ AGGREGATES Fuchun Luan and Mervyn S. Paterson Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra 2601 Constant strain rate deformation experiments have been carried out at 300MPa and BOOK on quartz aggregates that were fabricated by isostatic hot-pressing from natural quartz powders, from silica gel, and from silicic acid. The quartz-origin specimens are comparable in strength to similarly-deformed natural quartzites, behaving in a britde-ductile transition manner typified by a turn-over in the stress-strain curve and the development of fine-grained shear zones at approximately

to the loading direction. The gel- and silicic acid-origin specimens are similar

in strength to wet synthetic single crystals and deform in a rather ductile manner, as shown by apparently steady-state flow and characteristic plastic deformation microstructures.

measurements reveal that the water content in the gel-origin specimens (6000-10000 H/lO^Si) is higher than that the silicic acid-origin and quartz-origin specimens, which each contain about 1000-2000 H/lO^Si. The observation that the quartz- and silicic acid-origin specimens are similar in water content but different in strength, and that the gel- and silicic acid-origin specimens are different in water content but similar in strength indicates that it is not the bulk concentration that is the important factor in the water-weakening of quartz. Both the gel- and silicic acid-origin specimens are crystallized from an amorphous phase and water can therefore be incorporated into the newly-formed and growing crystals with a different state of dispersion and chemical potential than for the water in the quartz-origin specimens in which the grains have not crystallized during the experiment. It is hoped that this approach will throw new light on kinetic factors involved in laboratory deformation studies on quartz aggregates and permit the effects of prior history as well as of various grain size and water contents to be elucidated.

THE INFLUENCE OF PRE-EXISTING IRREGULARITIES ON THE DEVELOPMENT OF DEFORMATIONAL STRUCTURES Neil S. Mancktelow Geologisches Institute ETH-Zentrum, CH-8092 Zurich, Switzerland

The initiation of mechanical instability in rocks, which leads to the development of structures such as folds, boudinage and shear zones, depends on two basic requirements: firstly, an overall strain softening mechanism, which may be geometrical (e.g. folding or boudinage of relatively thin, more competent layers) or due to material properties (strain softening by the development of crystallographic preferred orientation, grainsize reduction, reaction to weaker mineral assemblages etc.); and secondly, a preexisting irregularity or perturbation on which the mechanical instability can nucleate. The importance of this second factor to the development of deformation structures has been underestimated. Theoretical studies consider initial perturbations which have either a simple regular form (e.g. a sine wave) or in which all possible perturbations are present (''white noise") to arrive at a tractable model. In nature, systems are seldom so simple and pre-existing perturbations may be both common and irregular in form (e.g. sedimentary structures in bedding such as ripple marks, climbing ripples, load casts, flute casts, and channels or on a larger scale graben and half-graben structures) . These irregularities will be amplified during deformation and may exert a major control on the distribution and geometry of deformation structures. This important effect has been investigated in two different series of analogue modelling experiments, concerned with: 1) the effect of initial perturbation symmetry on the symmetry of developed folds. Here it can be shown that large amplitude, asymmetric folds may be developed by amplification of a very small asymmetric initial irregularity, even though the overall boundary conditions are symmetric and non-rotational. Field examples showing characteristics of such asymmetric folds may be quite common. 2) the initiation of shear zones. Shear zones nucleate at the interface between materials with different properties, as a result of the strain gradient established across this interface. This is very commonly observed in the field, where shear zones are found at the contacts of dykes, of major intrusive bodies and of ore bodies.

These results may be extrapolated to all scales, although for larger dimensions (> ca.lOOm) the effect of gravity can no longer be ignored. In particular, fairly tight, geosynclinal structures, such as the '^Adelaide G e o s y n c l i n e " and '^Kanmantoo Trough'', presumably represent an amplification during bulk shortening (the regional slaty cleavage forming event during the Cambro-Ordovician Delamerian Orogeny) of the original, but less pronounced, synclinal form of the sedimentary basin. This regional structure has then been partially modified by isostatic recovery of the overthickened crust along steep shear zones, marked by broad zones of later crenulation cleavage development.

94 Considerations for quantitative determination of the role of dislocations in selective dissolution Annemarie Meike Material and Chemical Sciences Division Lawrence Berkeley Laboratory Berkeley, California 94720 Deformation assisted diffusive mass transfer can produce effects that can not be predicted by either chemistry or mechanics alone. However, although the complexity of the chemical aspects is well recognized, the equally complex mechanical aspects are often ignored. The omission of mechanical considerations in theoretical and experimental attempts to determine the energetic significance of dislocations on dissolution may be responsible for their frequently counterintuitive and conflicting results. This paper presents micromechanical aspects of dislocation enhanced selective dissolution (DESD). The effect of dislocation microstructure on strain energy is examined in the context of well known dislocation strain energy models. Based on these energetic considerations, as well as transmission electron microscope evidence from naturally and experimentally deformed specimens particularly susceptible DESD micrestructures are identified. The conditions for a regime that would favor the DESD microstructures are defined. In contrast to standard thought, it appears that homologous temperatures and strain rates should be relatively low. An attempt to define threshold conditions for the DESD regime suggests that standard critical resolved shear stress measurements are inadequate. A measurement of greater resolution, dislocation activation stress, is required, but is not yet known for any rock-forming mineral. Experimental procedures and preliminary results intended to determine dislocation activation stresses are outlined. Subsequently, the DESD regime is located on a general deformation mechanism map at the transition between the dislocation glide and climb fields. Comparison of deformation mechanism maps for a variety of minerals suggests that the DESD mechanism can operate through a wide range of crustal conditions and that those conditions depend on the mineral assemblage.

TECTONICS OF THE CALLIOPE VOLCANIC ARC NORTHERN NEW ENGLAND FOLD BELT. Vincent J, Morand Department of Geology, James Cod^ University, Townsville Qld 4811 The Gallic^ Volcanic Arc is ccnposed of Late Silurian to Mic3dle Devonian volcanic assemblages preserved in isolated fault blocks extending from north of Rockhairptai to southwest of Brisbane (Day et al., 1978). These are the oldest dated rocks in the northern New England Fold Belt. In the main area of outcrc^, near Rockhanpton, the arc assemblage consists of four main units : Ccillic^ Beds, Mt Ifolly Beds, Barmundoo Beds and Capella Creek Beds. Primary volcanics are not ccmmon in these units, most rocks being shallow marine volcanogenic sediments, ranging from ccxiglcmerate to mudstone, and limestcxie. Volcanic rocks include rhyolite, dacite, andesite, basalt and silicic tuff. Silicic and intermediate volcanics and volcaniclastics occur in about equal proportions, vdiereas mafic rocks are less comoi. The Barmundoo Beds consist of thinly bedded sandstone and siltstaie, and are possibly a fore-arc sequence. Whether the volcanic arc was related to an oceanic or ccxitinental margin setting is yet to be determined, but the silicic nature of much of the volcanics and the presence of abundant detrital quartz in many of the sediments, favours the latter. Rocks in the four units are metamorpiiosed to greenschist facies and are folded into largely upright, suldx)rizontal, north to northwest-trending folds. Slaty cleavage, axial planar to the folds and coeval with metamorj^ism, is develc:^>ed in all units except the Capella Creek Beds. Folds are tight in the Gallic^ Beds and Mt Holly Beds, tight to open in the Barmundoo Beds, and open in the Capella Creek Beds. Ihe Middle Devonian Mt Morgan Tonalite intrudes the Capella Creek Beds. This high level intrusicxi is the oldest dated pluton in the New England Fold Belt. Kirkegaard et al. (1970) and Dear et al. (1971) cited intrusion of this pluton, and unconformities between Calliope Volcanic Arc rocks and Late Devonian to Early Carboniferous rocks, as evidence for a late Middle Devonian orogeny, with cleavage forming at this time. However, some of the alleged uncaiformities have recently been disproved (Fergusson, Henderson & Morand, unpublished data), and the Mt Morgan Tonalite is simply one of many plutons etplaced into the volcanic arc over an enduring time period. Therefore the Mt Morgan Tonalite is not considered to mark a regional orogenic event. Strain in rocks of the Calliope Volcanic Arc is quite heterogeneous, but it generally increases to the east, vAiere the Calliope Beds occupy a highly deformed zone alaig the Yarrol Fault, to the east of v^ich is a deformed subduction complex (Wandilla Terrane). Cleavage in the Mt Holly Beds and the Barmundoo Beds is vertical or dips steeply east, and several east-dipping faults cut these rocks. Such faults appear to be thrusts, and it is suggested that the structure in the Callicpe Volcanic Arc is that of a fold-thrust belt, with westward transport on the thrusts. Folds and cleavage in the Calliope Volcanic Arc have similar orientatiais to those in the Carboniferous rocks of the Yarrol Shelf,

vdiich are generally in fault cxxitact with the arc assemblage. It appears that the deforroation and metamorE^sm vdiich affected the Calliope Volcanic Arc was of the same age as that vdiich affected the Yarrol Shelf: mid to Late Permian. References: Day, R.W., Murray, C.G. & Whitaker, W.G. 1978. The eastern part of the Tasman Orogenic Zone. Tectorx^ysics 48: 327-364. Dear, J.F., McKellar, R.G. & Tucker, R.M. 1971. Geology of the Moito 1:250 000 Sheet area. Geol. Surv. Qld Report 46. Kirkegaard, A.G., Shaw, R.D. & Murray, C.G. 1970. Geology of the Rockhamptcxi and Port Clinton 1:250 000 Sheet areas, Geol. Surv. Qld Report 38.

TECTONIC SUBDIVISIONS OF THE TASMAN FOLD BELT SYSTEM IN QUEENSLAND Cecil G, Murray Queensland Department of Mines, Brisbane, Qld WOO The Tasman Fold Belt System in Queensland comprises the Hodgkinson-Broken River Fold Belt (H-BRFB) in the N, the Thomson Fold Belt (TFB) in the SW, and the New England Fold Belt (NEFB) in the SE (Fig. 1). The evolution of each segment can be traced through orogenic (pre-cratonic), transitional, and cratonic stages. The different timing of these stages within each fold belt indicates differing tectonic histories up until Late Devonian time at least. The TFB is largely concealed and poorly known. Rocks of its orogenic stage, as seen in basement cores, are quartzose turbidites which have been folded more than once and are now steeply dipping and strongly cleaved. These rocks are considered to be of early Palaeozoic age, and to have been deposited on thinned Proterozoic crust formed by rifting and extension SE of the Diamantina River Lineament (Fig. 1). However, in the Anakie Inlier, similar quartzose turbidites appear to overlie an oceanic sequence of mafic volcanics, calcsilicates and serpentinites. The early Palaeozoic sequence was probably folded in late Ordovician time, prior to the emplacement of Silurian-Devonian granitoids. The early Palaeozoic history of the Lolworth-Ravenswood Block (Fig. 1) differs from that of the remainder of the TFB, and perhaps it should be treated as a separate tectonic unit. It consists of Cambrian-Early Ordovician volcaniclastic sediments and calc-alkaline volcanics (also occurring in the SE Georgetown Inlier), and Precambrian or early Palaeozoic metamorphics intruded by a large composite batholith of Ordovician to Devonian age. By the end of the Silurian, the TFB had been raised to near sea level, and in Devonian-Early Carboniferous time transitional basins developed over large areas (Drummond and Adavale Basins and Warrabin Trough; Fig. 1). Initial deposits in these basins were extensive, mainly silicic continental volcanics, suggesting an extensional, rift-related origin. E-W compression ended deposition in mid-Carboniferous time, and cratonised the TFB. The main pre-cratonic development of the H-BRFB in Silurian-Devonian time was deposition of quartz intermediate flysch which now forms most of the fold belt. The flysch was derived mainly from the Precambrian craton to the W; volcanic detritus is insignificant. Carbonate rich marine shelf deposits accumulated along the western margin of the fold belt, but are preserved only in the SW of the Broken River Province. Limestones along the western side of the Hodgkinson Province mostly slid to their present position from a source to the W. The flysch sequences are multiply deformed and are structurally complex. Imbricate thrust slices and melange are well developed. Two tectonic models have been proposed for the H-BRFB. One envisages the fold belt as an arc-trench gap assemblage (forearc basin and accretionary wedge) associated with a continental margin volcanic arc (now completely eroded away) on the Precambrian craton to the W. The second sees the fold belt as the fill of a marginal basin formed by rifting and crustal extension of the Precambrian craton behind a continental margin arc now lost to the E. The tectonic history of the NEFB is believed to be relatively well understood. From Late Silurian to Early Permian time it was the site of extensive episodic calc-alkaline magmatism related to W-dipping subduction. The oldest rocks may have formed at least partly in an island arc, but from the Late Devonian the fold belt developed as a convergent Andean-type continental margin. Through Late Devonian-Carboniferous time, parallel belts representing continental margin volcanic arc, forearc basin, and accretionary wedge assemblages can be recognised. Some structural blocks which are out of place in the overall

palaeogeographic setting, particularly the Gympie Province (Fig. 1), are currently regarded as suspect terranes. Widespread magmatism, including both eruption of continental silicic volcanics and intrusion of granitoids, continued in the NEFB through most of Mesozoic tinne. Opinion is divided whether this post-orogenic magmatism was related to W-dipping subduction or not. Boundary of Mesozoic intracratonic Great Artesian Basin Subsurface boundary of Late Carboniferous Middle Triassic basins Subsurface boundary of Devonian Early Carboniferous basins Late Carboniferous - Middle Triassic basins

Cambrian - Early Carboniferous Thomson Fold Belt

Precambrian inliers

GYMPIE PROVINCE

154° MP 3 4 . 8 9 . 0 4

Drawn by Mapping Services, Department of Mines, Old.

Figure 1. Subdivisions of the Tasman Fold Belt System in Queensland

SUSPECT TERRANES OF THE GYMPIE PROVINCE, NEW ENGLAND FOLD BELT Cecil G. Murray^, Rod 3. Holcombe^ and Hiroaki Ishiga^ ^ Queensland Department of Mines, Brisbane, Qld ^000 ^ University of Queensland, St Lucia, Qld ^067 ^ Shimane University, Matsue 690, 3apan The Gympie Province N of Brisbane (Fig, 1) is a unique stratotectonic unit which does not fit into the overall palaeogeographic pattern of the New England Fold Belt (NEFB), It lies E of the accretionary wedge-forearc basin-continental margin magmatic arc system which characterised the entire fold belt in Late Devonian-Carboniferous time (see Fig. 1). The Gympie Province has been regarded as a suspect terrane, but in fact contains at least three distinct sequences which may be exotic to the remainder of the NEFB and also to each other. The Rocksberg Greenstone, Kurwongbah beds and correlatives comprise tholeiitic mafic metavolcanics, possibly representing an early stage of island arc development (Rocksberg Greenstone), and fine grained metasediments (Kurwongbah beds) (see Fig. 1). Stratigraphic relationships are disrupted by at least two major and some minor generations of structures. The dominant lithologic banding in the metasediments is an intense first generation tectonic layering, and is possibly a strong shear fabric. Correlatives of the Rocksberg Greenstone and Kurwongbah beds appear to be unconformably overlain by the Early Carboniferous-Early Permian Marumba beds, and are intruded by granitoids dated at 298 and 320 Ma, giving a pre-Middle Carboniferous age. The Good Night beds and correlatives consist of a rather monotonous sequence of slate and phyllite with sparse lenses of sandstone, chert, limestone and mafic volcanics. The dominant S^ foliation is axial plane to rare mesoscopic isoclinal folds. Apart from the occurrence of lenses of interbedded limestone and mafic volcanics, the lithologies and structures are similar to those in the accretionary wedge to the W. The limestones have yielded varied faunas, including conodonts of Middle Carboniferous age, which indicate warm water. Contemporaneous Levipustula levis faunas of forearc basin strata of the NEFB to the W are regarded as cold water, and on this basis it has been suggested that the Good Night beds and their correlatives W of the Esk Trough (Fig. 1) were an exotic terrane. The Permian Gympie Group and the Early Triassic Kin Kin beds and Brooweena Formation form the easternmost part of the Gympie Province. The similarity of these rocks to sequences in New Zealand has been used as an argument that they are an exotic terrane. They consist of mafic volcanics with intra-oceanic arc chemistry overlain by volcaniclastic sediments and a prominent limestone horizon. The structure is superficially simple, with broad, open folds and preservation of stratigraphy, although variably developed cleavage relationships hint at a more complicated history. To the W, the Amamoor beds comprise a varied sequence of sediments, including cherts, and basaltic volcanics. Their relationship to the Gympie Group is uncertain. The two units at least partly overlap in age, because the Amamoor beds contain Early Permian faunas at a few localities. The Amamoor beds are more complexly folded than the Gympie Group; two generations of folding are common, and tight folding of a pre-existing foliation can be seen locally. Either the deformation increases westward, or tectonic slices of more deformed rocks make up a greater proportion of the sequence to the W. In fact there is some evidence that the Amamoor beds should be regarded as a composite terrane themselves, and not just as facies equivalents of the Gympie Group. This evidence includes: (1) preliminary analyses of basalts from the Amamoor beds indicate the existence of at least two

100

geochemically distinct groups both of which differ from the volcanics of the Gympie Group; and (2) well preserved radiolarians from one chert lens are Early Carboniferous in age. Contacts of the Amamoor beds - Gympie Group with probable older suspect terranes to the W are not well defined. The boundary between the Gympie Group and Good Night beds in the northern part of the Gympie Province (Fig. 1) is lithological; its structural significance is unknown. The contact between the Amamoor beds and equivalents of the Rocksberg Greenstone - Kurwongbah beds is well established only where it coincides with two large serpentinite masses (Fig. 1). Further S, the precise position and nature of the boundary are uncertain. " T 154°

/ Latest Triassic — Early Cretaceous

Late Triassic votcanics and

sedimer}ts

sedimertts

Middle Triassic votcanics and sediments

of Esk

Trough

1++ ++ ++ ++ Permian — Late Triassic granitoids •»• -t- 4- +

Permian Northbrook

< <1^ V

• a

beds and Cressbrook

Creek

Group

Permian — Early Triassic Amamoor beds, Gympie Group. Kin Kin beds and Brooweena Formatkjn

Early Carboniferous

— Early Permian Marumba

Middle-Late Carboniferous and correlatives

Good Nigfit

beds

Serpentinite

Late Carboniferous

S-type

granitokis

Pre-Mkldle Carboniferous Rocksberg Kurwongbah beds and correlatives

Greenstone.

Late Devonian — Carboniferous

accretkynary

Late Devonian — Carboniferous Forearc Basin

sediments

Early-Middle

Devonian sediments

wedge

of Yarrol

and volcanics

Calliope Island Arc

50 I

10I 0

154° I MP 34.88.02

Drawn by Mapping Services, Department of Mines, Old.

Figure 1. Simplified geological map of the Gympie Province (Cainozoic deposits omitted)

101

STRUCTURE OF THE MERTONDALE MINE AREA AND IMPLICATIONS FOR REGIONAL GEOLOGY A N D MINERALISATION B. W. Nisbet St R. L. Hammond

HUNTER RESOURCES LIMITED, 681 Murray St., West Perth, W.A. 6005.

The Mertondale Deposits (Merton's Reward discovered 1899, Mertondale 3 and 4 discovered 1985) occur 30 km to the northeast of Leonora in the Archaean Yilgarn Block of Western Australia. The Mertondale Au deposits occur in and marginal to an approximately north trending, diffuse and anastomosing, but commonly strong shear zone, the Mertondale Fault Zone (MFZ), that has a complex movement and mineralisation history. The MFZ locally separates largely acid metavolcanics to the west from largely mafic metavolcanics in the mineralised zone and to the east. Carbonate, silica and sericite alteration have variously affected rocks within and enclosing the mineralised zones. Lensoidal masses of quartz microporphyry, and highly deformed and disrupted carbonaceous metasediments and tightly folded metaturbidites? also occur in the sheared mafic mine sequence. Mineralisation Nisbet and Williams (1988) categorised mineralisation in the Mertondale deposits into two types: shear lode type mineralisation occurring in all deposits, and intershear lodes found only at Merton's Reward. Merton's Reward shear lodes are normally continuous along strike for 50 to 100 m, contain numerous quartz-carbonate veinlets parallel to the foliation in a carbonated and sericitised sheared metabasalt host, and normally assay greater that 30 g/t Au. Au is associated with abundant (5 to 10%) finely disseminated pyrite and arsenopyrite. Intershear lodes at Merton's Reward are narrow (up to ^Ocm wide), shallowly to moderately northeasterly dipping layered quartz veins. They are enveloped by carbonated and pyritic alteration halos up to 12 m thick which are largely composed of ankerite, silica, sericite, chlorite, pyrite (up to 20%) and arsenopyrite (up to 5%). Au typically occurs in the axial veinlet of this package (normally >30 g/t), with lower grades in the vein selvedges (up to 8 g/t). Intershear lodes have strike lengths of up to 40 m, and normally terminate against shear lodes. Some examples of intershear lodes cross-cutting shear lodes have been observed {e.g. Nisbet and Williams 1988) and it is thought that the two developed essentially synchronously. The much larger scale shear lodes of the Mertondale 3 and 4 (M3 and m ) deposits differ in being much more strongly and pervasively silicified, having a lesser development of sericitisation, and more intensely developed foliations and lineations. M3 and m shear lodes have strike lengths of up to 300 m, are up to 30 m thick, and extend down dip for up to 75 m. They are parallel to the foliation, and occur at or near the contact of quartz microporphyry lenses and sheared metabasalt within the shear zone. Geological structure At the Merton's Reward deposits, geometries defined by north to north-northwest trending, sub-vertical to steeply east dipping shear lodes and the generally northwest striking and moderately northeast dipping intershear quartz lodes (i.e. tensional quartz vein system) implies a sinistral sense of shear, with

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some reverse vertical component of movement. Mesostructural verification of this shear sense has also been obtained (N. Swager, unpublished data). The shear zone foliation at m dips steeply to the east with a very strong mineral lineation pitching 10 to 20 degrees to the south in the foliation plane. The larger porphyry bodies are irregular, but flattened and strongly elongate with a sub-horizontal plunge, and dip less steeply to the east than the shear zone foliation. S and C plane geometries from highly deformed quartz porphyry lenses, shear bands (or Riedel shears) in basalt derived phyllitic schists, and the asymmetry of tails on quartz aggregates (relict quartz veins) generally indicate a dextral shear sense in the main, north trending part of the shear zone containing M4. The Mertondale deposits arguably represent two phases of Au mineralisation occurring during separate episodes of the Mertondale Fault Zone's (MFZ) evolution. A complicating factor is the likelihood of post-mineralisation movements along a more discrete, fault-like structure occurring immediately to the west of M4 that now marks the contact between sheared acid metavolcanics and mineralised sheared mafic metavolcanics and is the trace of the Mertondale Fault proper. South of m the trace of this fault swings further to the west around the Merton's Reward mineralisation before resuming its approximate north-south trend. It appears to truncate the northern continuations of the narrow shear zones that are mineralised to form shear lodes at Merton's Reward. This discrete structure also arguably truncates the southern extension of the M3 - M4 mineralised zone as the letter's strike swings into the structure. In summary, early sinistral shear zones, probably in the periphery of a major shear zone immediately to the west, localised Ml and M2 "ineralisation. Subsequent dextral reactivation of the zone resulted in M3 and m mineralisation (possibly remobilised Au from the earlier phase). A late episode of fault movement localised along the zone truncated and reoriented mineralised structures. Regional structures . The gross trend of the regional foliation to the east and west of the MFZ is generally north-northwest. This foliation does not overprint the MFZ foliation, but its geometric relationship with the latter is consistent with a dextral shear sense. The southern extension of the MFZ encounters the Keith-Kilkenny Tectonic Zone (KKTZ, Hallberg 1985) approximately 10 km south of the mines. The KKTZ is a north-northwest trending zone which overprints earlier, originally flat lying foliations. The KKTZ is marked by younger, relatively narrow, steeply dipping intense shear zones with shallowly plunging stretching lineations (e.g. Mount Germatong East area, 10 km east of Leonora), and is essentially bounded by such shear zones. In the Leonora area, the Mount George Shear Zone approximates the western boundary of the KKTZ and has a sinistral shear sense (Williams, Nisbet & Etheridge in prep.). Sinistral shear sense indicators have been observed elsewhere in the KKTZ. North to northwest trending, regional scale, sinistral shear zones appear to be common within the greenstone belts of the Laverton - Leonora region. The major zones are: 1/. The KKTZ . . ... 2/. A very wide (10 km) zone encompassing the Patricia and Edjudina mining areas, and passing north through the Mount Korong area (Mount Hornet Shear Zone) . . . . .-i, 3/. A wide zone wrapping around the Granny Smith deposit, continuing north to the east of Laverton, and encompassing numerous less significant Au occurrences, including the Duketon mines.

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These shear zones are notable for the lack of major Au deposits within the main part of the shear zone, though major deposits such as Lancefield, Granny Smith and Mount Morgans commonly occur peripheral to the zones. In each example it can be argued that the above shear zones are relatively young features in the regional deformation history {e.g. Williams et al., op. cit.), and field relationships suggest that they post date most of the major, "internalgranitoid bodies. Another feature of these zones is that they appear to terminate along strike, either by dispersing into numerous, much less significant shear zones {e.g. southern continuation of the KKTZ), or fading out, probably into broader zones of lower strain. Conclusion ,. No clear relationships exist between the MFZ and the major, sinistral KKTZ. However, the earlier phases of movement on the MFZ may relate to earlier deformation events than the KKTZ, and the Mertondale deposits, and perhaps other Au occurrences, may predate many of the major regional shear and fault zones that now dominate the gross distribution of greenstones. Sons of Gwalia, Harbour Lights and King of the Hills deposits are examples of mineralisation occurring in, and genetically related to the earliest recognisable structures. Conversely King of Creation, Cox's Find and possibly Corktree well and Mount Morgans are examples of mineralisation associated with the late, major sinistral shear systems. The Au occurrences at Mertondale may be intermediate in age between these two. Mineralisation in the Leonora - Laverton region was deposited during a number of temporally separate deformation/metamorphic events. This conclusion contrasts with the proposition that Au mineralisation is universally a late feature in greenstone belt evolution (Groves et al. 1988) . RG foTGncos Groves, D. I., Ho, S. E., McNaughton, N. J., Mueller, A. G., Perring, C. S., Rock N, M. S. and Skwarnecki, M. S., 1988, Genetic models for Archaean lode-gold deposits in Western Australia. In Ho, S. E. and Groves, D. I. (eds). Advances in understanding Precambrian gold deposits, volume II, The Geology Department & University Extension, University of Western Australia, Publication No. 12, p. 1-22. Hallberg, J. A., 1985, Geology and mineral deposits of the Leonora-Laverton area, northeastern Yilgarn Block, Western Australia. Hesperian Press, Perth, 1^0 pp.. Nisbet, B. W. and Williams, C. R., 1988, Gold Mineralisation at the Mertondale Mine, Leonora area. Western Australia. Geol. Soc. Aust.. Abs.. No. 23, p. 98-102. Williams, P., Nisbet, B. W. and Etheridge, M. A., in prep.. Shear zones, gold mineralisation and structural history in the Leonora district. Eastern Goldfields Province, W.A., submitted to Aust. J. Earth Sci.•

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ULTRAMYLONITE

ZONES

TRANSECTING

HIGH-GRADE

PROTEROZOIC

ROCKS

THE

STRANGWAYS OROGENIC BELT A. R. NORMAN School of Earth S c i e n c e s , Macquarie U n i v e r s i t y , N.S.W. 2109 ESE t o SE trending u l t r a m y l o n i t e zones t r a n s e c t a m u l t i p l y deformed (F2-F^) granulite f a d e s terrain (Strangways Orogenic B e l t ) south o f the Harts Ranges. The u l t r a m y l o n i t e zones are up t o 4 metres wide, d i p p i n g a t high t o moderate angles t o the north and contain a s t r e t c h i n g l i n e a t i o n trending predominantly 055. Ultramylonite zones are of v a r i a b l e thickness most occur on the mm t o cm scale. Width v a r i e s dovm-dip and along s t r i k e within the same zone and the zones commonly b i f u r c a t e . Wider zones tend t o o c c u r i n the more felsic rocks, whereas zones i n the mafic rocks tend t o be narrower and more c l o s e l y spaced. The w a l l s o f the u l t r a m y l o n i t e zones are generally d i s t i n c t , although a m y l o n i t i c f o l i a t i o n may p e r s i s t i n t o the country r o c k . The high-grade S 2 f o l i a t i o n i n the country rock o f t e n c o n t a i n s a q u a r t z , garnet, hornblende o r hypersthene s t r e t c h i n g l i n e a t i o n ( L g ) , which becomes more p r e v a l e n t i n c l o s e proximity t o the shear zones, s u g g e s t i n g an o r i g i n during m y l o n i t i s a t i o n . Most u l t r a m y l o n i t e s p r e s e r v e a n o r t h - s i d e down sense o f shear. In the f i e l d the sense o f shear i s determined using asymmetric and r o t a t e d augen, o b l i q u e f o l i a t i o n s (S p l a n e s ) , asymmetric f o l d s and the bending o f S2 i n t o the shear zone. 'A c o n f i d e n c e l e v e l from A t o E i s given t o each interpretation, which i s u s u a l l y up-graded a f t e r s l a b b i n g and t h i n sectioning. In t h i n - s e c t i o n , o b l i q u e e x t i n c t i o n in q u a r t z , an o b l i q u e f i n e - g r a i n e d b i o t i t e f o l i a t i o n and rare "mica f i s h " are used as a d d i t i o n a l sense of shear criteria. Fine-grained biotite together with r e c r y s t a l l i z e d quartz and f e l d s p a r form good f o l i a t i o n (C p l a n e ) bands up t o 3 mm wide. R e c r y s t a l l i z e d f i n e - g r a i n e d aggregates o f hypersthene, g r e e n brown t o b l u e - g r e e n hornblende and almandine-rich garnet may a l s o define the m y l o n i t i c f o l i a t i o n . Porphyroclasts o f q u a r t z , plagioclase, K feldspar, almandine, hypersthene, hornblende, s c a p o l i t e , s i l l i m a n i t e and biotite occur. Coarse-grained sillimanite porphyroclasts are not r e t r o g r e s s e d and have a random o r i e n t a t i o n . Hypersthene c l a s t s g e n e r a l l y r e c r y s t a l l i z e t o orthopyroxene (and p o s s i b l y c l i n o p y r o x e n e ) o r green/brown hornblende. Hornblende r e c r y s t a l l i z e s t o f i n e r - g r a i n e d hornblende and almandine r e c r y s t a l l i z e s t o f i n e - g r a i n e d granular garnet and biotite. Neocrystallization of f i n e - g r a i n e d s i l l i m a n i t e e l o n g a t e in the mylonitic f o l i a t i o n and s u b p a r a l l e l t o the s t r e t c h i n g l i n e a t i o n o c c u r s where sillimanite g n e i s s e s have been m y l o n i t i z e d . The n e o c r y s t a l l i z a t i o n of s i l l i m a n i t e and the r e c r y s t a l l i z a t i o n of opx, almandine-rich garnet and hornblende indicate that hot hydrous conditions existed during mylonitization. The movement along most shear zones i s down-dip. However, two ESE t r e n d i n g master shear zones have been r e c o g n i z e d , along which there has been significant (> 5 km) l a t e r a l d e x t r a l displacement. Similar master shear zones may have e x i s t e d a l o n g the Gough Dam S c h i s t Zone t o the north and i n the Hale River P l a i n s t o the south, but they have been o b l i t e r a t e d by subsequent south d i r e c t e d t h r u s t i n g e v e n t s . Most shear zones branch from

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the master s h e a r zones o r b i f u r c a t e from each o t h e r towards the SE. This b r a n c h i n g may be a n a l o g o u s t o r e i d e l s h e a r s , which a l s o i n d i c a t e that a dextral shear component existed during mylonitization. Lateral d i s p l a c e m e n t d e c r e a s e s t o t h e SE and many shear zones e i t h e r d i s a p p e a r or form f r a c t u r e zones t o t h e SE. Between the s h e a r z o n e s the S2 f a b r i c i s f o l d e d ( F ^ - F ^ ) ; the f o l d axes may be bent towards the m y l o n i t i c e x t e n s i o n d i r e c t i o n ( 0 5 5 - 0 8 0 ) and t h e limbs can be bent a n d / o r o v e r t u r n e d . In p l a c e s , F^ s y n c l i n e s have been s h o r t e n e d i n t o b a s i n s and F^ a n t i c l i n e s s h o r t e n e d i n t o domes, suggesting that the shear zones a r e l i s t r i c with d e p t h . The d e s t r u c t i o n and m o d i f i c a t i o n o f F4 folds is not common and i s r e s t r i c t e d t o a r e a s between c l o s e l y spaced prominent s h e a r z o n e s . No m e s o s c o p i c f o l d s a s s o c i a t e d w i t h shortening between the s h e a r zones a r e o b s e r v e d and the u l t r a m y l o n i t e zones are not deformed. The r e s u l t o f h a v i n g c l o s e - s p a c e d l i s t r i c normal s h e a r z o n e s i s a space problem towards the base o f t h e zone where i t j o i n s a n o t h e r zone. This problem i s accommodated by s h o r t e n i n g between the ESE zones and the development o f a ENE s i n i s t r a l s h e a r z o n e , which d i s p l a c e s the easterly b l o c k towards the SE. The s e n s e o f shear on t h e s e ENE zones i s a n t i t h e t i c t o the o v e r a l l system. Where i n t e r f e r e n c e between shear zones o c c u r s and antithetic zones have d e v e l o p e d , t h e s t r e t c h i n g l i n e a t i o n t r e n d s more to the e a s t . Further evidence for t h i s volume accommodation is that a b r e c c i a t e d marble has accumulated a l o n g one master shear z o n e , p r o b a b l y as a result of 'squeezing' of interbedded c a l c - s i l i c a t e s . This marble contains isoclinally f o l d e d fragments of ultramylonite petrographicallj^ similar to ultramylonite i n o t h e r shear z o n e s . Fragments o f quartz, d i o p s i d e and g a r n e t a l s o e x i s t i n t h i s m a r b l e . The i s o l a t e d u l t r a m y l o n i t e fragments a r e f r a c t u r e d and p u l l e d a p a r t and c a l c i t e has f l o w e d and r e c r y s t a l l i z e d around t h e f r a g m e n t s . The r e c r y s t a l l i s a t i o n o f c a l c i t e was a s s o c i a t e d w i t h the development o f a garnet c o r o n a around d i o p s i d e clasts. Subsequent t o u l t r a m y l o n i t i z a t i o n , t h r u s t i n g from the NNE o c c u r r e d along the n o r t h e r n and s o u t h e r n margins o f the g r a n u l i t e f a c i e s terrain. This granulite t e r r a i n was undeformed d u r i n g t h i s t h r u s t i n g , but was probably brou^t t o h i g h e r c r u s t a l l e v e l s through b l o c k movement. Therefore the p r e s e n t a t t i t u d e o f F4 f o l d axes and u l t r a m y l o n i t e zones d o e s not reflect their initial orientation. I f the F4 f o l d axes were i n i t i a l l y h o r i z o n t a l and the shear zones were n o r t h - d i p p i n g normal s h e a r zones the present a t t i t u d e o f f o l d axes and t h e s h e a r zones c o u l d be e x p l a i n e d by a 50 d e g r e e r o t a t i o n of the g r a n u l i t e b l o c k . I f the shear zones were i n i t i a l l y p a r t o f a NE d i r e c t e d s o u t h d i p p i n g t h r u s t s y s t e m , a g r e a t e r r o t a t i o n o f t h e whole b l o c k would be r e q u i r e d . The p r e s e n t geometry o f the s h e a r zones i s b e s t e x p l a i n e d by a moderatelydipping e x t e n s i o n a l system. Marble b r e c c i a , the l a c k o f m e s o s c o p i c SW-NE s h o r t e n i n g f o l d s , t h e i r r e g u l a r i n t e r f e r e n c e o f Fg^^ f o l d s between the s h e a r zones, and SE-branching r a t h e r than an anastamosing p a t t e r n o f t h e z o n e s , also s u g g e s t an e x t e n s i o n a l s e t t i n g . The metamorphic grade o f t h e shear zones seems t o d e c r e a s e n o r t h w a r d s , as e v i d e n c e d by a change from greenbrown h o r n b l e n d e t o a b l u e - g r e e n a m p h i b o l e . Although more work i s r e q u i r e d to verify this grade c h a n g e , i t would a l s o s u g g e s t a n o r t h - s i d e down extensional regime. The d e x t r a l s h e a r on some z o n e s , the d e c r e a s e in displacement t o the SE and t h e e a s t e r l y e x t e n s i o n a s s o c i a t e d with antithetic ENE s h e a r zones s u g g e s t t h a t e x t e n s i o n i n i t i a t e d f r o m t h e w e s t .

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THE

TECTONIC HISTORY OF THE STRANGWAYS OROGENIC A BAROMETRIC RESPONSE TO LATE COMPRESSION

BELT:

A. R. NORMAN, G- L- CLARKE and R. H. VERNON School of Earth S c i e n c e s , Macquarie U n i v e r s i t y , N.S.W. 2109 The Strangways Orogenic B e l t (James and Ding, 1987) c o n s i s t s p r e d o m i n a n t l y of a l a y e r e d sequence of q u a r t z o - f e l d s p a t h i c g n e i s s e s and 2-pyroxenehornblende m a f i c r o c k s m u l t i p l y deformed and metamorphosed d u r i n g the e a r l y Proterozoic. These rocks a r e c u t by zones of u l t r a m y l o n i t e (Norman 1989 t h i s volume). L a t e r major zones o f south d i r e c t e d t h r u s t i n g bound the Strangways Orogenic B e l t t o t h e south and n o r t h . South-west of t h e Harts Range the Strangways Orogenic B e l t can be divided i n t o two t e r r a i n s : a g r a n u l i t e f a c i e s t e r r a i n t o t h e e a s t and a lower grade t e r r a i n to the west. The g r a n u l i t e t e r r a i n e x p e r i e n c e d 3 m a j o r folding episodes (F2-F4) subsequent t o a high temperature-mo d e r a t e pressure g r a n u l i t e f a c i e s metamorphism (M-,). M^ i s c h a r a c t e r i s e d by a g r a n o b l a s t i c , two p y r o x e n e - h o r n b l e n d e - p l a g p a r a g e n e s i s i n m a f i c r o c k s and by a p r o b a b l e primary assemblage of s p i n e l - i l m e n i t e - q t z - K f s - c o r d i e r i t e - g a r n e t (almandiner i c h ) i n the m e t a p e l i t e s . A n a t e x i s accompanied t h i s metamorphic p e a k . Any Si fabric t h a t accompanied g r a n u l i t e f a c i e s metamorphism has been o b l i t e r a t e d by a n o n - c o a x i a l d u c t i l e deformation (Dj) t h a t produced the pervasive S2 f o l i a t i o n . Sg i s d e f i n e d by b i o t i t e alignment, coarse hornblende e l o n g a t i o n and abiindant medium g r a i n e d q t z - K f s leucosome that are a x i a l planar to r o o t l e s s i n t r a f o l i a l i s o c l i n a l f o l d s . Boundinage is common i n t h i s f o l i a t i o n . Both F j f o l d axes and boudin necks appear s u b parallel to a steep, S E - t r e n d i n g L2 hornblende e l o n g a t i o n , which is i n t e r p r e t e d as the t r a n s p o r t d i r e c t i o n d u r i n g D2"- The c r y s t a l l i z a t i o n of leucosome a f t e r melt d u r i n g D2 s u g g e s t s t h a t the t e r r a i n was c o o l i n g from

Coarse-grained s i l l i m a n i t e - b i o t i t e f o l i a cut q t z - K f s l e u c o c r a t i c l a y e r i n g , but a r e s u b p a r a l l e l t o t h e o v e r a l l S2 f o l i a t i o n . Sillimanite is poorly l i n e a t e d p r o b a b l y r e f l e c t i n g a f l a t t e n i n g s t r a i n l a t e i n D2 • In the metapelites, the s i l l i m a n i t e f o l i a a r e c u t by c o a r s e - g r a i n e d irregularlyshaped K f s - g a r n e t - q t z pods t h a t c o n t a i n a p o o r l y d e v e l o p e d S2 foliation. Pods of c h a r n o c k i t e i n f e l s i c g r a n u l i t e s and i r r e g u l a r networks o f plaghornblende i n m a f i c g r a n u l i t e s a l s o c r y s t a l l i z e d l a t e i n D2The S2 f o l i a t i o n , i n c l u d i n g leucosome a f t e r m e l t , i s f o l d e d by NE-plunging discontinuous, t i g h t t o i s o c l i n a l F3 f o l d s . F3 f o l d s a r e r e f o l d e d by open p l u n g i n g asymmetric F4 f o l d s and were p r o b a b l y recumbent p r i o r t o F4. F3 and F4 f o l d s a r e c o l i n e a r and both d i s p l a y a NW v e r g e n c e . Deformation producing F3 and F4 f o l d s i s here d e s i g n a t e d D3. Many F3 f o l d s c o n t a i n an a x i a l planar quartz mylonitic f o l i a t i o n , and some have q t z - k f s leucosome and s i l l i m a n i t e i n the a x i a l p l a n e . In f e l s i c granulites pargasitic amphibole a f t e r M^ hypersthene and b i o t i t e form an S3 a x i a l planar foliation. S3 i s c o p l a n a r t o S j . An a x i a l p l a n a r f o l i a t i o n i s not w e l l developed i n the open F4 f o l d s , although a poorly-developed quartz elongation (L4 ) i s p a r a l l e l t o t h e f o l d a x i s . Some q t z - K f s leucosome crystallized i n c o n j u g a t e e x t e n s i o n a l c l e a v a g e d i r e c t i o n s on the limbs of F4 f o l d s . Q t z - K f s - b i o t i t e p e g m a t i t e c u t s F4 f o l d s and has been s h e a r e d by D4 m y l o n i t i z a t i o n . In m e t a p e l i t e s , a s y m p l e c t i c i n t e r g r o w t h of aluminous

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orthopyroxene/ fine-grained sillimanite and m a g n e t i t e replaces M-| cordierite. This ' u p - p r e s s u r e * r e t r o g r a d e e v e n t i s r e f e r r e d t o as M2 and o c c u r r e d a f t e r t h e development o f c o a r s e s i l l i m a n i t e S2 f o l i a but, before D4 m y l o n i t i s a t i o n . The l o w e r grade t e r r a i n t o t h e w e s t c o n s i s t s o f b l o c k s o f m u l t i p l y deformed cordierite gneisses, a m p h i b o l i t e s and q u a r t z - r i c h g n e i s s e s e n c l o s e d by a phyllonitic shear f o l i a t i o n . Two shear f a b r i c s p r e d o m i n a t e : a NNE-NE t r e n d i n g r e t r o g r a d e s h e a r f o l i a t i o n c o n t a i n i n g a ESE-SE t r e n d i n g biotite elongation lineation and a ESE t r e n d i n g shear f o l i a t i o n w i t h a NNE-ENE trending b i o t i t e , quartz or anthophyllite elongation lineation. A megacrystic g r a n i t e and numerous narrow g r a n i t i c layers intrude these blocks but have been deformed by subsequent s h e a r f o l i a t i o n s . Obvious intrusive granites do n o t e x i s t i n the g r a n u l i t e f a c i e s t e r r a i n to the east. 3 m e s o s c o p i c f o l d i n g phases have been r e c o g n i z e d i n t h e cordierite g n e i s s e s and a r e s i m i l a r t o F 2 - F4 i n the g r a n u l i t e terrain. These gneisses contain a granoblastic cordierite-garnet-Kfs-qtz-spinel-ilmenite primary a s s e m b l a g e . C o r d i e r i t e Si f o l i a a r e boundinaged and isoclinally f o l d e d by D2 w i t h K f s - q t z + c o r d i e r i t e leucosome a f t e r m e l t f o r m i n g S2 • A c o a r s e - g r a i n e d s i l l i m a n i t e f o l i a t i o n c u t s the c o r d i e r i t e b e a r i n g leucosome and both a r e f o l d e d by t i g h t F3 f o l d s and more open F4 f o l d s . The cordierite is p a r t i a l l y retrogressed to a symplectic intergrowth of opxs i l l - m a g n e t i t e as i n the g r a n u l i t e t e r r a i n . I t seems l i k e l y t h a t t h e ' u p p r e s s u r e ' metamorphism (M2) was synchronous i n b o t h t e r r a i n s . Qtz-Kfs-biotite gneisses are very i r r e g u l a r l y interlayered with metaigneous amphibolites . E s p e c i a l l y near NNE shear zones the f e l s i c g n e i s s e s form an i r r e g u l a r network i n v a d i n g large amphibolite pods. Pervasive melting of the f e l s i c r o c k s ' f o r m i n g the i r r e g u l a r o u t c r o p p a t t e r n appears t o have been a s s o c i a t e d w i t h .the development o f t h e NNE-NE s h e a r f o l i a t i o n . Lineations and asymmetric augen i n t h i s shear f o l i a t i o n i n d i c a t e a SE s i d e up sense o f shear. Megacrystic g r a n i t e w i t h the K f s megacrysts horizontally aligned parallel t o the shear f a b r i c p r o b a b l y began to crystallize during t h i s deformation. The NNE-NE shear fabric and lineations bend i n t o and a r e c r o s s - c u t by a ESE s h e a r f a b r i c w i t h a NE-ENE lineation. T h i s ESE s h e a r fabric i s in places continuous with the u l t r a m y l o n i t e zones i n t h e g r a n u l i t e t e r r a i n . Di and D2 show a t y p i c a l P r o t e r o z o i c a n t i - c l o c k w i s e P - T - t p a t h ; c o m p r e s s i o n accompanied by c o o l i n g . D3 and D4 may r e f l e c t a c l o c k w i s e P-T-t path typical of collisional-type tectonics. I t i s suggested that F3 and F4 folding {D3) was a s s o c i a t e d w i t h a SE-NW s h o r t e n i n g which a l s o t h r u s t the granulite t e r r a i n o v e r a l o w e r grade t e r r a i n p r o d u c i n g a NNE-NE t r e n d i n g orogenic belt. This d e f o r m a t i o n was r e s p o n s i b l e for 'up-pressure' r e t r o g r e s s i o n (M2) • M e l t i n g i n the l o w e r p l a t e and o v e r t h i c k e n i n g may have initiated extension (D4) a t a high a n g l e t o the o r o g e n i c belt. Fluids derived from t h e s t r u c t u r a l l y lower p l a t e may e x p l a i n the hydrous r e t r o g r e s s i o n i n the u l t r a m y l o n i t e zones t r a n s e c t i n g t h e g r a n u l i t e t e r r a i n .

James, P. R. and D i n g , P.

1987. p.55.

Abs.

19. G e o l o g i c a l S o c i e t y o f

Australia

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TECTONICS OF THE M O D E R N OBLIQUELY CONVERGENT PLATE BOUNDARY IN THE SOUTH ISLAND, N . Z . , WITH IMPLICATIONS FOR THE INTERPRETATION OF OLDER TERRANE BOUNDARIES. Richard J . Norris and Peter 0 . Koons D e p t . o f G e o l o g y , U n i v e r s i t y of O t a g o , D u n e d i n , N . Z . The Alpine Fault in the South Island of New Zealand forms the m o s t obvious s u r f a c e m a n i f e s t a t i o n of t h e o b l i q u e l y c o n v e r g e n t p l a t e b o u n d a r y b e t w e e n t h e A u s t r a l i a n a n d P a c i f i c p l a t e s . It s e p a r a t e s u p l i f t e d M e s o z o i c A l p i n e S c h i s t of t h e P a c i f i c p l a t e f r o m g r a n i t e s , g n e i s s e s a n d L o w e r P a l e o z o i c s e d i m e n t s o n t h e A u s t r a l i a n p l a t e . C u r r e n t r a t e s of r e l a t i v e p l a t e d i s p l a c e m e n t a r e a r o u n d 45 m m / a in t h e c e n t r a l S o u t h I s l a n d , w i t h t h e d i s p l a c e m e n t v e c t o r o r i e n t e d a p p r o x i m a t e l y e a s t - w e s t ; t h i s l e a d s t o a s t r i k e - s l i p c o m p o n e n t of a r o u n d 40 m m / a a n d a c o n v e r g e n c e c o m p o n e n t o f a b o u t 22 m m / a . T h e t o t a l r a t e and the component of convergence have gradually increased since the i n c e p t i o n o f m o v e m e n t o n t h e A l p i n e F a u l t in l a t e O l i g o c e n e t i m e , d u e t o t h e s o u t h w a r d m i g r a t i o n of t h e p o l e of r e l a t i v e r o t a t i o n . B e t w e e n l a t e E o c e n e and late Oligocene, movement on the plate boundary, which was probably not c o i n c i d e n t w i t h t h e p r e s e n t A l p i n e F a u l t , i n v o l v e d a c o m p o n e n t of d i v e r g e n c e , p a r t i c u l a r l y in s o u t h w e s t N e w Z e a l a n d . S i g n i f i c a n t c o n v e r g e n c e began around 10-15 Ma with a larger amount occurring in the last 5 M a . D u r i n g t h i s t i m e , a p p r o x i m a t e l y 70 k m o f c o n v e r g e n c e n o r m a l t o t h e p l a t e boundary has b e e n taken up in the c e n t r a l South Island. The orientation a n d distribution of strain along the plate b o u n d a r y v a r i e s . In t h e c e n t r a l S o u t h I s l a n d , t h e A l p i n e F a u l t is a m o d e r a t e t o s t e e p l y dipping oblique-slip zone which forms the western b o u n d a r y of a wide zone of d i s t r i b u t e d d e f o r m a t i o n . The present deformation a n d g e o l o g i c a l pattern of the Southern Alps m a y be described in context of a sub-aerial, critically d e f o r m i n g w e d g e u n d e r g o i n g o b l i q u e c o m p r e s s i o n . T h e t o p 2 0 - 2 5 k m o f c r u s t is b e i n g d e l a m i n a t e d a n d thrust u p at the Alpine Fault at rates of a r o u n d l O m m / a , leading to high heat flow and a shallow brittle/ ductile transition a d j a c e n t t o t h e f a u l t . It is s u g g e s t e d t h a t t h e l o w e r c r u s t is i m b r i c a t e d b e n e a t h t h e A l p s t o f o r m a c r u s t a l r o o t w h e r e a s t h e m a n t l e p a r t of t h e l i t h o s p h e r e is s u b d u c t e d w e s t w a r d . T h i s m o d e l is s u p p o r t e d b y micro-earthquake studies. T h e c o m p o n e n t of c o n v e r g e n c e n o r m a l t o t h e A l p i n e F a u l t h a s r e s u l t e d in t h e formation of an eastern fold-thrust b e l t , extending from the Alpine divide eastwards to the present coastline, and a western region of high-grade schist e x p o s e d between the divide and the f a u l t . Coupling of m e c h a n i c a l and e r o s i o n a l p r o c e s s e s h a v e r e s u l t e d in h i g h u p l i f t r a t e s (c. 10 m m / a ) inmediately east of the Alpine F a u l t . N u m e r i c a l modelling shows that the e a s t e r n f o l d - t h r u s t b e l t of u p p e r c r u s t a l m a t e r i a l a n d t h e w e s t e r n h i g h u p l i f t r e g i o n a r e i n t r i n s i c a l l y r e l a t e d t h r o u g h t h e o v e r a l l m e c h a n i s m s of a d e f o r m i n g r e g i o n . T h i s c o u p l i n g y i e l d s d i s t i n c t i v e s p a t i a l p a t t e r n s in t h e p r e d i c t e d g e o p h y s i c a l , g e o l o g i c a l , s t r u c t u r a l a n d i s o t o p i c s i g n a t u r e s of c o l l i s i o n a l t e r r a i n s . These include the exposure of deep crust with very v o u n g a g e s o n t h e w i n d w a r d s i d e of t h e o r o g e n a d j a c e n t t o o l d e r , h i g h e r l e v e l c r u s t . T h e s t r u c t u r e of t h e t w o b e l t s d i f f e r s w i t h t h e d i r e c t i o n of r e l a t i v e t r a n s p o r t b e i n g o p p o s i t e in s e n s e .

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The t h r e e - d i m e n s i o n a l nature of the collision zone is e x e m p l i f i e d in north-west Otago^ where d e f o r m a t i o n east of the A l p i n e fault is m u c h m o r e b r o a d l y d i s t r i b u t e d . Imbrication on n o r t h - t r e n d i n g f a u l t s , w i t h a s s o c i a t e d f o l d i n g , is w e l l - d e v e l o p e d ; the d e f o r m a t i o n extends as f a r east as the coast with o b l i q u e - s l i p faulting on north-west a n d north-east f a u l t s . The result is a b e n d i n g a n d shortening east of the fault o v e r an area 200 k m w i d e . Uplift at the A l p i n e Fault in the south is less t h a n f u r t h e r n o r t h but is d i s t r i b u t e d o v e r a w i d e r a r e a . The c r u s t a l r o o t , as e v i d e n c e d from the g r a v i t y a n o m a l y , is l a r g e r , w i d e r , a n d extends f u r t h e r to the e a s t , as e x p e c t e d from the d i s t r i b u t i o n of surface d e f o r m a t i o n . M u c h of the d e f o r m a t i o n has b e e n c o n t r o l l e d b y reactivation of e a r l i e r f e a t u r e s .

In d e t a i l , structures in the schist adjacent to the A l p i n e F a u l t , f o r m e d d u r i n g u p l i f t , indicate m o r e of a c o a x i a l s t r a i n , representing shortening s u b n o r m a l to the f a u l t , c o m p a r e d with structures in the m y l o n i t e zone i t s e l f . S i m i l a r l y , imbrication a n d folding of the m y l o n i t e s d u r i n g the development of h i g h - l e v e l overthrusts along the fault also indicate shortening n o r m a l to the fault trace although slickenside lineations on the base of the thrust complexes are p a r a l l e l to the the p l a t e displacement v e c t o r . D i s p l a c e m e n t directions on faults f u r t h e r east are also v a r i a b l e , with f o r i n s t a n c e , d o m i n a n t l y reverse m o v e m e n t on n o r t h e a s t striking faults (parallel to the A l p i n e F a u l t ) . It is c l e a r t h a t , away from the a c t u a l slip p l a n e of the Alpine F a u l t , a considerable amount of strain p a r t i t i o n i n g among the various structures is o c c u r r i n g , so that d e t a i l e d study of s m a l l scale features within the plate b o u n d a r y d e f o r m a t i o n zone w i l l not be d i r e c t l y indicative of plate m o t i o n s . O n l y c o n s i d e r a t i o n of the o v e r a l l zone w i l l l e a d to a correct i n t e r p r e t a t i o n . Because of d e c o u p l i n g a n d uplift of the u p p e r crust along the A l p i n e F a u l t , r a p i d cooling a n d h i g h t h e r m a l gradients result in the t o p few k i l o m e t r e s . Below t h i s , the t h e r m a l gradients w i l l be r e d u c e d , while in the u n d e r p l a t e d l o w e r c r u s t , d e p r e s s e d gradients a n d slow h e a t i n g w i l l o c c u r . Thus r a p i d cooling n e a r the surface is contemporaneous with slow w a r m i n g at d e p t h . R a d i o m e t r i c ages from the rapidly u p l i f t e d schists are v e r y y o u n g , recording the tectonic uplift along the f a u l t . H o w e v e r , these rocks are c o n s t a n t l y b e i n g e r o d e d as f u r t h e r uplift o c c u r s , so t h a t , in e f f e c t , the rocks pass through the l a n d s u r f a c e . The root on the o t h e r h a n d is cumulative with time a n d is w a r m i n g u p . O n cessation of tectonic c o n v e r g e n c e , the rapidly c o o l e d rocks w i l l m a i n l y d i s a p p e a r b y erosion a n d s l o w , e r o s i o n - c o n t r o l l e d , isostatic uplift of the root w i l l o c c u r . The radiometric ages a c q u i r e d b y this isostatically u p l i f t i n g crust m a y p o s t - d a t e the tectonic convergence b y tens of m i l l i o n s of y e a r s . Rates of uplift c a l c u l a t e d from differences in ages on c o e x i s t i n g m i n e r a l s w i l l reflect slow isostatic uplift (and t h e r e b y e r o s i o n rates) rather than t e c t o n i c a l l y d r i v e n u p l i f t . E v e n where the u p p e r p l a t e is p r e s e r v e d , slow cooling following cessation of r a p i d uplift w i l l result in an age p a t t e r n having little m e a n i n g w i t h respect to p l a t e convergence.

110 KINEMATIC INDICATORS IN SERPENTINITES - THE PEEL-MANNING FAULT SYSTEM, A TEST CASE Robin Offler^, David S. O'Hanley^ and Paul G. Lennox^. 1 Department of Geology, University of Newcastle, N.S.W. 2308. 2 Department of Mineralogy & Geology, Royal Ontario Museum Toronto, Ontario, Canada. 3 Department of Applied Geology, University of NSW, Kensington, N.S.W. 2033. Serpentinite-matrix melanges commonly occur in fault zones bordering crustal blocks. Generally, little attention is paid to the mesoscopic and microscopic structures that these rocks contain. Close examination shows that they, like other tectonic melanges and mylonites, contain kinematic indicators which demonstrate sense and direction of movement. The most common and Possibly the most useful indicator is a structure referred to as "phacoidal or scaly cleavage which closely resembles s-c fabrics in mylonites. Other indicators which are commonly present include retort-shaped phacoids of massive serpentinite, slip-chrysotile fibre lineations and slickenlines. Microscopically, serpentine "fish" and antithetic faults in bastites declare the sense of movement. Such structures have been observed and analysed in serpentinite melanges associated with the Peel-Manning Fault System (PMFS), a major geosuture extending over 270km in the Southern New England Fold Belt (SNEFB). Earlier studies of scaly cleavage and fibre lineations in serpentinites from a small area encompassing part of the PMFS, indicated that movement was dominantly, sinistral, strike slip (Offler & Williams, 1987). Subsequent investigations in other areas have verified this movement pattern, but in some outcrops there Is evidence for earlier phases of shearing. Serpentine filled extension veins that occur in partially serpentinized peridotite are considered to be another kinematic indicator. The possibility of usine these structures to determine movement on shear zones arose out of the T t i S L ^ o f asbestos deposits in Canada by O'Hanley (1987, 1988) who noted that asbestos-bearing veins were spatially associated with shear zones. He showed that different generations of veins were related to several stages of movement on the shear zones and that motion on them could be determined from the orientation of the veins and the shear zones. Features similar to those recognised by O'Hanley (1987, 1988) occur in the Woodsreef asbestos deposit, east of Barraba, adjacent to the PMFS Glen & Butt, 1981). In this deposit, several generations of fractures filled with asbestos or other forms of fibrous serpentine, have formed. An analysis has indicated that these fractures are not '^^l-ted to the NNW-trending as originally thought, but to shear zones trending 310, 330-340 and 020-030 occurring to the east of this structure. Fracture and ^^ear zone orientations suggest that movement on these shear zones is sinistral, strike slip (310) and dextral, reverse, oblique slip (330-340; 020-030). These studies show that serpentinites are potentially as useful as mylonites in unravelling the kinematic history of shear zones and should not be ignored as they have been in the past.

References

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Glen, R.A. & Butt, B.C., 1981. Chrysotile asbestos at Woodsreef, New South Wales, Econ. Geol. Tb: 1153-1169. Offlet, R . & Williams, A . 1987. Evidence for sinistral movement on the Peel Fault System in serpentinites, Glenrock Station, NSW. In: Terrane Accretion and Orogenic Belts. E.G. Leitch & E . Scheibner (eds.) American Geophysical Union, 141-151. O'Hanley, D.S. 1987. eastern Quebec.

The origin of the chrysotile asbestos veins in south Ganad. J . Earth Sci. T M 1 - 9 .

O'Hanley, D.S. 1988. The origin of alpine peridotite hosted cross-fibre, chrysotile-asbestos deposits. Econ. G e o l . 83: 256-265.

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ANCIENT EARTHQUAKES AND ADIABATIC SHEAR IN THE MUSGRAVE RANGES A ORD CSIRO Division of Geomechanics, PO Box 54 Mt Waverley, Victoria, Australia

A spectacular and massive zone of fractured, brecciated and veined rock, possibly including melted material, lies within the Kulgera section of the Woodroffe Thrust. This pseudotachylite zone lies only within the granulite Kulgera terrain (A Camacho, this volume), and is underlain by ultramylonites, mylonites, and the amphibolite grade Mulga Park Block. The zone is at least 1 km thick within this region, and continues at this thickness for at least 30 km along strike. Within the Kulgera area, the zone has a minimum areal extent of 60 km^. There is no strong penetrative fabric within the zone. Instead, a variety of overprinting and overprinted structures are observed. The overall impression is of many small events occurring at different times and at different places within the one zone. Pods 10 m or so across may be formed of mainly felsic granulites which are finely veined with pseudotachylite, or they may be formed mainly of pseudotachylite, with minor small (1 cm or less) clasts of the felsic granulite. In between these two extremes are varying proportions of felsic granulite to pseudotachylite with much veining, brecciation, and rounding of clasts. These structures are similar to these described by Sibson (1975), and interpreted by him to be the result of melting caused by frictional sliding on brittle fault planes. Obviously, if a deformation process produces melt, then the structures associated with the melt will always be the same, regardless of the form of the original process. The structures observed in the Kulgera region are consistent with the effects of hydrofracturing events which might be expected to result from the formation of a low viscosity fluid. It therefore becomes important to consider whether or not the very large, highly overprinted zone in the Kulgera region could be explained as the results of brittle fracture (Sibson, 1975) or as a plastic instability (Hobbs et al., 1986). The latter case is considered in this paper. Formation of an adiabatic shear band during plastic deformation results in an increase of temperature within the shear band as some proportion of the plastic work is converted to heat. We consider here whether or not this increase in temperature would be sufficient to melt the granulites. During adiabatic shear of a structural steel, the maximuun temperature attained occurs during a stress-drop period (Marchand and Duffy, 1988) and we also consider the implications of this timing with respect to the relative timings for formation of a shear band, an earthquake, production of melt, and hydrofracturing of the surrounding rocks. The local stress field and the mechanical properties of the rock change as a result of such an event so that throughout the

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zone represented in the Kulgera region, numerous small events occur at different times and places as the local conditions of stress, strainrate and mechanical behaviour favour the formation of adiabatic shear bands. Large quantities of melt may then be built up through production of numerous small pockets of melt.

References Camacho, A., 1988. The Woodroffe Thrust, Eastern Musgrave Block, N.T.: A problem of large scale melting during thrusting. This volume. Hobbs, B.E., Ord, A. and Teyssier, C., 1988. regime? Pageoph, 124, 309-336.

Earthquakes in the ductile

Marchand, A. and Duffy, 1988. An experimental study of the formation process of adiabatic shear bands in a structural steel. J. Mech. Phys. Solids, M , 251-283. Sibson, R.H., 1975. Generation of pseudotachylyte by ancient seismic faulting. Geophys. J.R. astr. Soc., 775-794.

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MAFIC DYKE SWARMS IN RELATION TO EXTENSIONAL TECTONICS A PROTEROZOIC MODEL. A. John Parker South Australian Department of Mines and Energy, P.O. Box 151, Eastwood, South Australia 5063

In recent years there have been a number of models proposing various forms of extension during the Precambrain. Some of these are based on modern extensional tectonic environments such as the Red Sea or East African rift system where mafica volcanism and intrusion play an important role not only in filling rift basins but also in accommodating at least some of the extension within the upper and lower plates (eg Voggenreiter et al.. 1988). Mafic dykes are common in many Precambrian provinces (eg Mount Isa, Gawler Craton, Yilgarn Block: Parker et al.. 1987) so the question becomes, into what tectonic setting were such dyke swarms emplaced, are they related to Precambrian extension in the same way as they are to modern extension and can they tell us something about Precambrian crust/mantle processes? This paper will address these questions by looking at Early Proterozoic mafic dyke swarms on Eyre Peninsula in the southern Gawler Craton. Late Archaean and Early Proterozoic metasedimentary sequences of the southern Gawler Craton are intruded by at least three suites of mafic dykes : a suite of multiply-deformed amphibolite layers in layered orthogneisses and orthopyroxene-bearing granitoids; a suite of relatively-undeformed, north-south oriented, igneoustextured dykes (Mortimer et al., 1988); and a suite of undeformed, northwest-oriented dykes (Parker et al., 1987). All dyke swarms occur within intraplate environments and can be linked in very broad terms with other magmatic features related to rifting or particular stages of crustal extension and/or deformation. The multiply-deformed amphibolite intrusives were emplaced during the period 1845 - 1750 Ma (probably earlier rather than later), the north-south dykes were emplaced shortly thereafter, but the northwest-trending dykes, the Gairdner Dyke Swarm, were emplaced ca 1100 Ma at about the same time as rifting and formation of an extensive basic volcanic suite immediately adjacent to the Adelaide Geosyncline. Considering only the first two swarms directly related to formation of the Early Proterozoic, there are several features which are considered to be important: although there were multiple intrusions according to structural fabrics, all dykes have similar geochemical signatures indicating a similar or related source;

115

dykes were emplaced mainly within an I-type orthopyroxenebearing granitoid complex, with similar geochemical signatures, east of a major mylonite zone on southeastern Eyre Peninsula; dykes of the southeastern region are geochemically similar to conformable amphibolites in the adjoining supracrustal sequence west of the main mylonite zone; dykes are sequence;

rare

non-existant

the

supracrustal

deformation of the dykes closely followed emplacement suggesting rapid transition from an extensional to compressional environment; and although deformed dykes may have been rotated, they are generally subparallel to or within 30^ of the gneissic foliation within host lithologies. It is concluded that the various dyke swarms all formed in a narrow zone of extension above a linear convection cell. The convection cell was the driving mechanism behind the extension but achieved this by pushing the plates apart rather than just dragging them. Therefore as extension proceeded, zones of initial extension rapidly moved into zones of compression, compression that was oriented essentially parallel to the original extensional direction. This occurs mainly in the lower plate of modern extensional models and accounts for many of the observed features not only in the Early Proterozoic development of the Gawler Craton but also in many other Early Proterozoic provinces.

References Mortimer, G.E., Cooper, J.A. and Oliver, R.L., 1988. Proterozoic mafic dykes near Port Lincoln, South Australia: Composition, age and origin. Australian Journal of Earth Sciences, 35: 93-110. Parker, A.J., Rickwood, P.C., Baillie, P.W., Boyd, D.M., Freeman, M.J., McClenaghan, M.P., Murray, C.G., Myers, J.S., and Pietsch, B.A., 1987. Mafic dykes swarms of Australia. In Mafic dyke swarms. Editors, Halls, H.C. and Fahrig, W.F., Geological Association of Canada Special Paper 34 : 401-417. Low-angle Voggenreiter, W . . , Hotzl, H . and Mechie, J., 1988. detachment origin for the Red Sea Rift System? In: E. Bonatti (Editor), Zabargad Island and the Red Sea Rift. Tectonophysics, 150: 51-75.

116 EASTWARD THRUSTING ASSOCIATED WITH THE WYANGALA BATHOLITH: SURGE TECTONICS IN THE LACHLAN FOLD BELT? Scott R. Paterson^, Othmar T. Tobisch^, and Vince J. Morand^ ^Earth Science Board, University of California, Santa Cruz, CA 95064

U.S.A.

2 Dept. Geology, James Cook University, Townsville, Queensland. Australia The Wyangala batholith consists of many small plutons, elongate in a north-south direction. Our preliminary studies of this batholith suggest that north-south striking magmatic foliations exist in some of the individual plutons but are not strongly developed. A few individual plutons show evidence of forceful emplacement, but in general most do not. Weakly to intensely developed solid-state foliations are widespread, particularly along the eastern margin of individual plutons and the bathol ith as a whole (Fig. 1). Zones of mylonite and S-C structure occur in these fol ated regions, with C-surfaces usually dipping 0 to 50 degrees to the west, ."^ere examined the sense of shear is consistently reverse (west-over-east). We interpret these zones of intense solid-state deformation as east-vergent thrust faults. At least 4 generations of structures are developed in wall rocks near the batholith, although the fourth, an eastwest trending crenulation is only locally developed. Where we have examined timing relationships (e.g. Fig. 2), plutons were emplaced after some cleavage development, Sl(?), but prior to the widespread development of a transposed, composite foliation, Sy. Sj strikes north-south and dips moderately to steeply west, and is overprinted by a northsouth striking crenulation cleavage. See, that dips steeply to the west. Asymmetries of folds associated with both Sj and See consistently indicate a west-over-east sense of motion. We suggest that these structures formed during movement along the large Kilometers ductile shear zones outlined in Figure 1. We tentatively propose that most of the plutons in the Wyangala batholith were p l u t o n (Fig. 2). initially emplaced as northsouth trending sill-like bodies (e.g. Fig. 2) parallel

117 to a pre-existing cleavage and/or ductile shear zones. During continued west-over-east thrusting, the plutons formed part of the thrusrt sheets and were either deformed if located at the basal thrust, or road "piggyback" and remained largely undeformed if higher up in the sheet. Scant values of total strain suggest that individual thrust sheets more than doubled in thickness during the development of Sy, and rotated to steeper dips during the shortening associated with See. An examination of published studies in areas well away from the batholith indicates that the thrusting, complex deformation, and upper greenschist facies metamorphism occur only near or in the batholith. However, it remains unclear whether the emplacement of the batholith and accompanying heat are genetically related to the thrusting and complex deformation, or whether the presence of the thrust faults provided a conduit for pluton emplacement. If the former is case, the Wyangala batholith and associated deformation may represent a shallow level expression of Hoi lister and Crawford's "surge tectonics." If true, at shallow crustal levels plastic flow may be ^ Y a r r a Pluton triggered (enhanced?) by the presence of heat and fluids, VVyangala Batholith rather than a melt phase as suggested for deeper Cleavage Trends Kilometers structural levels. Figure 2 Sj cleavage trends in and around the Yarra pluton and eastern edge of the W y a n g a l a batholith.

118

BASINS AND COLLISIONAL John

Paul

OROCLINES FORMED MOUNTAIN BELTS

EXTENSIONAL

COLLAPSE

Piatt

Department of Earth Sciences, Parks Road, Oxford 0X1 3PR, England. Extensional basins that have formed on the site of previous continental convergence and crustal thickening are a common phenomenon throughout the system of mountain belts and island arcs that extends from Gibraltar to the Western Pacific. Many of them have formed during continuing convergence of the bounding plates, and many are surrounded by arcuate thrust belts. Rates of extension in the basins are comparable to the rates of contemporaneous shortening in the surrounding thrust belts. Examples in the Mediterranean region are the Pannonian, Aegean, Tyrrhenian, and Albor^n basins, all of which have formed during Neogene time on the sites of Late Cretaceous to Palaeogene continental convergence and thickening. Extension directions vary from N-S (Aegean, Albor^n) to E-W (Pannonian). The basins occupy a zone of distributed deformation up to 2000 km wide between the African and Eurasian plates, which have been converging in a roughly N-S direction throughout Tertiary time. The Aegean and Tyrrhenian Seas are flanked on one side by subduction zones, and can be explained in part by the "rollback" of sinking African lithosphere. The thrust belts around other basins (Pannonian, Albor^n, north Tyrrhenian) have advanced onto continental crust, and cannot be explained in these terms. The formation of these basins may best be explained in terms of the extensional collapse of thickened continental lithosphere. If boundary conditions remain constant, the crustal thickness and surface elevation in a zone of continental convergence tend to approach equilibrium values, such that the gravitational forces produced by the high elevation balance those produced by compression. Once this state has been reached, the crust gets no thicker, and convergence is taken up in the regions surrounding the elevated plateau. Should this equilibrium be disturbed, the crust may start to thicken or thin, depending on the sense of the disturbance. One likely cause of disturbance is a change in the rate of convergence. Another is the detachment of part of the thickened root of cold lithospheric mantle beneath the zone of convergence. Lithospheric mantle is in any event convectively unstable, and if it has been thickened by convergence the likelihood that it will be removed by convection, and the rate at which this will occur, are greatly increased. The effect of the removal of part of the underlying lithosphere is to increase surface elevation by perhaps 1-3 km. This disturbs the dynamic equilibrium: body forces outweigh compression, and the region starts to extend. Extension is facilitated by the increase in temperature resulting from the detachment of the lithospheric root. If significant extension occurs, it may become self-driving: the remaining crust and lithosphere stretch and thin, and are replaced by asthenospheric mantle that maintains an extensional driving force, equivalent to the

119

force that acts on m i d - o c e a n r i d g e s . The crust m a y eventually be g r e a t l y t h i n n e d , o r r u p t u r e a n d be r e p l a c e d b y o c e a n i c c r u s t . E x t e n s i o n in the c o l l a p s i n g region is t a k e n up b y s h o r t e n i n g in the s u r r o u n d i n g t h r u s t b e l t s . The b a s i n p l u s t h e s u r r o u n d i n g t h r u s t b e l t s t h u s form a d y n a m i c a l l y i n d e p e n d e n t s y s t e m w h o s e behaviour is largely unconstrained by the motions of the surrounding p l a t e s . The A l b o r ^ n Sea (in the westernmost Mediterranean between Spain a n d Morocco) may be a type-example of this p r o c e s s . 1) The b a s i n has formed on a zone of P a l a e o g e n e c o n t i n e n t a l c o n v e r g e n c e w h e r e the crust was at least 50 km t h i c k , as indicated b y h i g h - p r e s s u r e metamorphic assemblages. 2) Africa converged with Europe by about 130 km in the region while the basin extended N-S by about 100 k m . 3) The basin is now underlain by thin (13-20 km) crust and anomalously low v e l o c i t y (Vp = 7.6-7.9 km/sec) u p p e r m a n t l e ; it has an E-W-trending horst and graben morphology; was the locus of extensive N e o g e n e v u l c a n i s m ; a n d has s u b s i d e d up to 5 km since the middle Miocene. 4) E x t e n s i o n a n d s u b s i d e n c e c o i n c i d e d in t i m e w i t h t h r u s t i n g to the N , W , a n d S in t h e s u r r o u n d i n g m o u n t a i n chains (Betics, R i f , and T e l l ) . 5) P o s t - t h r u s t i n g structures onshore include e x t e n s i o n a l detachments w i t h mylonitic footwalls and cataclasite in the hangingwalls, that separate high-pressure metam o r p h i c rocks b e l o w from low-grade rocks a b o v e . The d e t a c h m e n t s are cut b y y o u n g e r s t e e p e r n o r m a l f a u l t s . 6) S o l i d b o d i e s of m a n t l e p e r i d o t i t e w e r e e m p l a c e d at a s t h e n o s p h e r i c t e m p e r a t u r e s into t h e l o w e r crust at the start of e x t e n s i o n , a n d w e r e subsequently rapidly exhumed. 7) L o c a l i z e d i n t e r m e d i a t e to lowpressure metamorphism accompanied exhumation and decompression, and is particularly associated with the peridotite m a s s i f s .

120

TECTOHOSTRATIGRAPHIC TKRKANE COHSTRAUTTS OH FALAEOHAGNETIC POLES FROM THE TASMAN FOLD BELT C. McA. Powell^, Z.X.Li^' P.¥. Schmidt^ and G.A. Thrupp^

^ Australian Plate Research Group School of Earth Sciences, Macquarie University, NSW 2109, Australia 2 Rock Magnetism Laboratory, CSIRO Division of Exploration Geoscience, North Ryde, NSW 2113, Australia The Tasman Fold Belt in Eastern Australia was part of the Pacific facing Gondwanan margin for nearly 500 Ma, from latest Precambrian to mid-Cretaceous. It is notable because of its preserved breadth, in places over 1000 km wide, and because of its distinctively Gondwanan characteristics. Geologically, the Tasman Fold Belt can be subdivided into three meridional tectonic realms, each of which has a history overlapping the one adjacent, but with varying events in each at common times. The westernmost realm, the Kanmantoo Orogen, is represented by rocks outcropping in eastern South Australia, western Victoria and NSW. It had a tectonically active history from the late Precambrian (-650-600 Ma) until the Early Ordovician (--500 Ma). The central realm, the Lachlan-Tho^son Orogen, is the widest, and was tectonically active from some time in the Cambrian (--550 Ma) until the mid-Carboniferous (--330 Ma). The eastern realm, the Neir England Orogen, has an uncertain Early Palaeozoic beginning, but was known to be active by the Silurian (--430 Ma) and continued to be tectonically active until the mid-Cretaceous (-^100 Ma). The Kanmantoo Orogen comprises late Precambrian to mid-Cambrian quartzose clastics deposited mainly in turbiditic facies. It was uplifted, deformed, and intruded by granites in the Middle Cambrian to Early Ordovician interval (530 to 490 Ma). It could well have been a marginal sea formed adjacent to Australia during the late Precambrian continental breakup. The event which terminated the orogenic history of the Kanmantoo realm, the Delamerian Orogeny, was felt in many parts of the Tasman Fold Belt and its extension in the Antarctic and South American sectors of Gondwanaland. The Lachlan-Thompson Orogen was a deep-water oceanic realm in the Cambrian, its history imperfectly known from the few, scattered outcrops available. Mafic volcanics, cherts and distal turbidites suggest marginal seas or back-arc basins of the west Pacific type. The Ordovician history is far better known. Widespread quartzose clastics of turbidite facies have remarkably consistent palaeocurrents indicating provenance to the west and south. The Early to Middle Ordovician appears dominated by a turbidite apron marginal to the uplifted Delamerian Highlands to the west, but by the Late Ordovician a mafic volcanic arc had formed in the east. The onset of widespread silicic magmatism in the mid-Silurian (~430 Ma) followed deformation known locally as the Benambran and Quidong Orogenies, and accompanied a change to an extensional, horst-and-graben tectonic phase. Late Silurian and Early Devonian volcanics occur throughout the Lachlan-Thompson Orogen. Earth movements varying from intense (central and eastern Victoria and Anakie Inlier, Queensland) to mild (central New South

121

Wales) terminated the silicic volcanic interval and allowed the mineralogically mature, quartzose clastics of continental facies to prograde eastwards. The entire orogen, as w e l l as the cratonic basins of central Australia, was deformed in the mid-Carboniferous (-^330 M a ) by a compressive event that terminated the orogenic history of Eastern Australia west of the New England realm. The N e w England Orogen has a long and complex history that provides some evidence for accretion of exotic terranes, and also for large-scale longitudinal movement forming megafolds or oroclines. The pre-Devonian history is poorly known, but consistent w i t h having lain oceanward of coeval environments in the Lachlan-Thompson Orogen. Further south, in N S W , there is evidence of a subduction-related terrane lying east of the inferred fore-arc Tamworth Basin. Evidence for connections between the Lachlan and N e w England Orogens prior to the Late Carboniferous is tenuous, and several possible tectonic reconstructions exist. By Late Carboniferous, the N e w England Orogen was dominated by a continental, Andean arc, at the edge of the now^extended Gondwanan continent, facing east into a subduction complex. Major dextral shear along the New England Orogen could have formed the Yarrol-New England megafold in the Late Carboniferous or earliest Permian, prior to the massive Permian intrusion of granitoids throughout northeastern N S W and adjacent Queensland. The Sydney-Bowen Basin developed initially o n a divergent continental margin from Late Carboniferous u n t i l mid-Early Permian, and from then u n t i l Middle Triassic as a foreland basin to the emerging N e w England Orogen. The Sydney-Bowen Basin forms the overlap assemblage between the N e w England and Lachlan-Thompson Orogens. Implications of this tectonic history for palaeomagnetic studies are: 1. The Kanmantoo Orogen has probably been welded to the Australian craton since the Early Ordovician (500 M a ) . 2. The connection between the Lachlan-Thomposon Orogen and the Australian craton prior to mid-Devonian is speculative and uncertain. 3. The Lachlan-Thompson Orogen has been craton since the Middle Devonian (370 M a ) .

connected

the

Australian

4. The relationship between the New England orogen and the rest of the Tasman Fold Belt is speculative prior to the Late Carboniferous (330 M a ) . 5.

There could be some exotic terranes in the N e w England Orogen.

6. There is good evidence for a Late Carboniferous Permian megafold (orocline) in the New England Orogen.

earliest

Late

122

ALPINE FAULT TECTONICS AND THE REDISTRIBUTION OF GOLD ACROSS THE AUSTRALIAN-PACIFIC PLATE BOUNDARY, WESTLAND, NEW ZEALAND. Mark S. Rattenbury

Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001. The geological record of Alpine Fault movement is well preserved in central Westland, where a number of contrasting fault and fault-rock types occur, reflecting the strike-slip and oblique-slip tectonic processes along and across the AustralianPacific plate boundary during the Cenozoic. Mylonitic deformation occurs immediately west of the present Alpine Fault trace within a structural complex of granitoids and high-grade gneisses, collectively known as the Fraser Formation. The Eraser Formation occurs along an 75 km length parallel to the present Alpine Fault trace, and has a maximum width of 5 km. Within the Fraser Formation, ductile mylonite zones anastomose around regions of less strained gneiss and granitoid, as well as deforming mid Cretaceous (?) lamprophyre dykes. Mylonitic foliation is predominantly parallel to the Alpine Fault trend (055), and a generally shallow plunging stretching lineation is commonly developed. Sense of shear obtained from various microstructural criteria indicates predominantly dextral strike-slip, although deformation was clearly

Fraser Formation

^ ^

Au —' ~ Au " Mt Cook

restricted Au-quartzcarbonate veining 17VE

50 km 172'E

123 inhomogeneous. Foliation^ lineation and sense of shear in a ductile fault zone are all compatible with Alpine Fault strike-slip in the mid-late Tertiary. The Fraser Formation is bounded to the west by the Fraser Fault which is characterised by a steeply dipping zone of cataclasites. Uplift of the Fraser Formation along the Fraser Fault commenced in the late Miocene^ and was completed before deposition of Pliocene conglomerates. The timing of uplift is consistent with the onset of convergent tectonics in Westland. Rapid uplift of the Fraser Formation was initiated in the late Pleistocene along the Bald Hill Range Thrust, the trace of which occurs 3-5 km west of the most recent Alpine Fault trace. The Bald Hill Range Thrust is the earliest known splinter thrust of the present Alpine Fault system. A number of other splinter thrusts have been recognised recording an general southeast migration of the Alpine Fault trace since the late Pleistocene. The present Alpine Fault trace is marked by alluvial and scree gravels which have been successively overthrust several metres by mylonitic schists and cataclasites. Migration of the active fault trace has occurred as recently as 2600 yrs B.P. Detrital gold mining in Westland has yielded approximately 237000 kg of gold since around 1860. Virtually all of the gold has been mined from late Pleistocene glacial deposits, and younger river deposits and beachsands. A nearly continuous Tertiary sedimentary record in Westland has no economic occurrences of alluvial gold. Two main sources of the gold have been described; the gold-bearing quartz reef systems within the Ordovician Greenland Group metasediments, and young quartz-carbonate veins within the rapidly uplifting and eroding Alpine schists east of the Alpine Fault. The gold-bearing veins within the Alpine schists are rare and volumetrically minor and small alluvial gold deposits occur downriver of these vein systems. The bulk of alluvial gold in Westland is considerably further from known occurrences of in situ Alpine schist gold than the auriferous Greenland Group quartz reefs. The Bald Hill Range Thrust and related Alpine Fault splinter thrusts have also locally elevated the Greenland Group metasediments, and on-going erosion will have released gold into some catchments. Most of the alluvial gold in Westland is therefore believed to have been derived from the Greenland Group quartz reef systems. The uplift of the Southern Alps has, in conjunction with cool climates through the Pleistocene, resulted in extensive glaciation and large erosive river systems. The glaciers and rivers have reworked existing detrital gold deposits and scoured the Greenland Group bedrock and gold-bearing reef systems to produce the rich detrital deposits of Westland. The effects of several hundred kilometres of strike-slip across the Australian-Pacific plate boundary during the Tertiary have not resulted in any known economic gold concentrations in Westland. With the initiation of convergent tectonics across the plate boundary during the late Miocene, however, the rapid sustained uplift of the Southern Alps and generally cool climatic conditions have resulted in extensive glacial and fluvial redistribution and concentration of gold deposits in Westland. Early Alpine Fault-related thrusting also elevated potentially auriferous lithologies. The erosion of huge volumes of the Alpine schists is not considered to have added significantly to the detrital gold deposits of Westland, but in a general case, with more favourable host rocks, the contribution of gold from rapidly uplifting and eroding terrains at convergent plate boundaries is potentially very important.

124

REVIEW OF TECTONIC MAP CONCEPTS M.J. Rickard Department of Geology, Australian National University, Canberra 2601 Tectonic maps seek to represent the history of crustal deformation. Thus maps should display not only the areal distribution of different tectonic domains but also the Vertical' time sequence of various deformational stages. Early tectonic maps merely added structural symbols to stratigraphic maps, later the concept of tectonic regime allowed the represention of rocks of the same age as different strato-tectonic units generally following the geosynclinal model. Work in all continents for the International Tectonic Map Project initially followed the Russian legend in which orogenic belts were characterized by the age of climactic deformation, and platform covers were treated as transparent and characterized according to the age of their basements. Some of the submissions to this project departed from the general legend principles; eg. King's map of the U.S.A. emphasized numerous separate (older and younger) domains, of ortho- and mio-geosynclinal character, and successor basins, whereas the Australian map introduced the concept of 'transitional tectonism' marking the change from oceanic or continental margin jnfi.cratonic to ensialic cratonic conditions. The representation of platform basins also differed from the "Russian model" in that some attempt was made to represent their stages of development separately from the orogenic domains. The African and South American maps also attempted this with a more stratigraphic treatment of basins. A variety of contour and isopach treatments of platform cover allow the configuration of basins and sub basins where drill hole or seismic data are sufficient. The valuable concept of transitional tectonism leading to cratonization was further developed in the Tectonic Map of New South Wales, and the principle that 'tectonic stages' could apply to platform cover as well as fold belts was developed for the SE Asian Tectonic Map. Some interpretative maps based on plate tectonics, eg. Indonesia, appeared in which emphasis was given to inferred strato-tectonic elements related to plate margins, and all these principles have been embodied in the Circum Pacific Map especially for the Cainozoic tectonics. The recognition of strato-tectonic domains - packets of rock with a common sedimentational and deformational history - forms the basis for the map of Tectonic Lithofacies of the Appalachian Orogen and the current map of the Tasman Fold Belt. Such a representation tends to emphasize differences rather than similarities between domains and may result in a patchwork-quilt effect. As with most other maps, domain boundaries are difficult to define unless marked by faults. The new developments in 'terrane analysis' place increased emphasis on domain boundaries and on the possibility that large-scale allochthoneity has interupted the paleogeographic continuity of adjacent tectonic domains.

125

In developing new maps there are conceptual difficulties in establishing an objective base within the framework of any chosen tectonic model. Ideally, interpretation should be kept to a minimum, and the map should be capable of yielding different user interpretations from its factual base. The separate, and equal, representation of strato-tectonic, tectonic, and morpho-tectonic elements still provides the ideal basis for a map legend. Strato-tectonic and deformational domains may not be coincident and the difficulties of dating structures and the problem of representing overlapping and polychronous structural elements in fold belts and platforms, best treated on the French map, presents the greatest challenge especially on small-scale maps. The principles discussed will be illustrated with examples from the tectonic maps of Australia, New South Wales, South Australia, USA, Pacific, Indonesia, Africa, South America, Europe and France.

126

BASEMENT CONTROLLED MONOCLINE AT THE ROCK Nr WAGGA WAGGA N.S.W. M J . Rickard, P. Price and B.A. Duff Department of Geology, Australian National University, Canberra 2601 Open N-S folds in the outlier of upper Devonian molassic sandstones southwest of Wagga Wagga plunge gently northward and the outcrop is terminated on its northern side by the Pulletop Creek valley. To the north only Ordovician rocks outcrop and a major fault is inferred trending southeastwards. The fault line is marked by a strong linement on air photos and geophysical maps and truncates several plutons of the Wagga batholith to the southeast. It aligns with the Berridale fault although it does not cut across the intervening Wagga metamorphic belt; the fault movement is unknown. In the Devonian sandstones, a large monoclinal flexure with kinked-limb width of 100 m forms the prominent hill of The Rock' State Park and trends parallel to the inferred fault line, cutting across the N-S open folds. Strong solution-transfer type axial planar spaced cleavage is developed in the sandstones parallel to the open folds and the monocline. The monocline is interpreted as a drape fold developed over the basement fault. The age of the faulting must be post granite that is Mid Devonian, but part at least of the movement must be a post Late Devonian reactivation to develop the drape monocline.

127

STOUCIURAL GONSTRAINrS ON METASOMATIC PROCESSES WEST OF THE MOUNT ISA FAULT. Michael J. Rubenach Geology Department, James Cook University; Townsville, Qld

4811.

West of the Mount Isa Fault at Mt Isa, the essential structural history is as follows: D-| event, thrusting from the north, dominantly shallow foliation, shallow mineral elongaticxi, age 1610 m.y.; D2 event, strcxig NS-oriented steep foliation, steeply plunging minersil elongation, greenschist to ainAiibolite facies, age 1544 m.y.; D3 event, NNW-oriented steep crenulations or foliaticxi, steep mineral elcxigation, retrograde metamorf^sm, age ISIQm.y. (Wilscxi, 1970; Page and Bell, 1986). D2related isograds trend NS, with the biotite zone immediately west of the fault, followed by the cordierite zone, sillimanite zc»ie and sillimanite - Kfeldspar zone. Pegmatites are abundant in the sillimanite zone, can be timed as syn - D2 (Downing, 1986), and are probably significant in the heat budget during the peak of metamorphism. Normal metabasic rocks (originally both flows and dykes) coisist of hornblende and plagioclase throu^iout the zcxies. However, large patches have been metasomatized, and are now schists coisisting mainly of chlorite and quartz, involving removal of most Na and Ca and enrichment in Mg relative to Fe. This occurred either syn, or, more likely, preD-| since relict L-j mineral elongation as well as crenulation hinges are preserved. The alteratiai may have been produced by circulation of ScLLine diagenetic/metamorpiiic fluids, analogous to the formaticai of similar ccnpositiais in ocean floor basalts. In the middle of the cordierite zone, the chlorite + quartz assemblage is joined in some rocks by talc and anthc^yllite, both timed as syn-D2, Less altered metabasic rocks show assenblages with various cxxnbinations of curnningtonite, hornblende, and anthcphyllite - gedrite with plagioclase. An unusual metasomatic type consists of cordierite, quartz, with or without biotite, andalusite, and plagioclase (ie muscovite - free). This is restricted to strain shadow regions vAiere muscovite + cordierite + quartz schists wrap around boudinaged lenses of amfiiibolites. This type has replaced the muscovite-cordierite schists, rather than being metasomatized basic rocks or high-Mg metasediments in an ev^poritedolomite associatiOT (cf. Reinhardt, 1987). Porfdiyroblast-matrix relationships show that cordierite grew relatively early in the D2 event in the muscovite schists, but additicxial cordierite and andalusite grew relatively late in D2 in the metasomatized rocks. Fracturing in the strain shadow regicxis is postulated to provide better access to metasomatic fluids, and it is believed that the metasomatic cordieriterich rocks formed by exchange of compoients with the contiguous more highly strained anphibolite lenses. D3 related retrogression of some of these metasomatic rocks produced assemblages including chlorite and anthc^iiyllite. Partial retrogressioi of unknown timing in some samples produced chlorite + kyanite, suggestive of an anticlock wise P-T path. Close to the Mt. Isa Fault, pre~D2 metasomatism produced localized dolomite, dolomite + tremolite, talc + tremolite and talc + chlorite assemblages.

128

References: Downing, J.G., 1986. Structural and metamorEMc geology of the Mica Creek area, Mount Isa, Northwest Queensland. Unpub. Hcxis. Thesis, James O x k University. Page, R.W., Bell, T.H., 1986. Isotopic and structural responses of granite to successive deformation and metamor^^ism. J. Geol. 94, 365-79. Reinhardt, J., 1987. Cordierite - anthc^yllite rocks from north-west Queensland, Australia: roetamoritosed magnesian pelites. i. Metamorchic Geol. 5, 451-472. Wilson, C.J.L., 1975. Structural features west of Mount Isa. Soc. Australia, 22, 457-476.

J. Geol.

129 AUSTRALASIAN TECTONICS: AN INTRODUCTORY REVIEW R W R Rutland.

Current understanding of Australasian Tectonics is a function of the special characteristics of the region and of the distinctive characteristics of the scientists who have contributed to that understanding. Only in recent years has there been a wide international interest in the tectonics of the region, as it has emerged as a key region for the understanding of global tectonics. In this paper, the special configurational features of the Australian region are summarised and current concepts of Phanerozoic and Precambrian evolution are reviewed from an Australian perspective. The importance of these concepts for the understanding and assessment of petroleum and mineral potential is emphasised.

130

PRESSURE-TEMPERATURE-STRAIN HISTORffiS IN HIGH GRADE METAMORPHIC TERRAINS: CONSTRAINTS ON THE STRENGTH OF THE CONTINENTAL LITHOSPHERE Michael Sandiford Department of Geology and Geophysics, University of Adelaide The thermal and structural evolution of granulite terrains provides important insights into the rheology of the lower crustal regions of orogenic domains. Pressure-temperature-strain

(P-T-e) histories of several

regional scale granulite terrains are now well constrained enabling the distinction of two distinct types of granulite terrain. In granulite terrains metamorphosed at temperatures in excess of 800-850°C, the retrograde PT history typically involves near-isobaric cooling. In contrast, the retrograde P-T history of granulite terrains metamorphosed at temperatures less than 750-850°C frequently involves significant near isothermal decompression over depth intervals of up to 15 kilometres. Decompression in these terrains must have been rapid, at least in comparison to the time scale for thermal re-equilibration of the lithosphere

50 Ma). In both types of terrain the peak metamorphism is

typically synchronous with intense deformation resulting in the development of recumbent fabrics, and cooling occurs in a static environment. Baric changes immediately following deformation are sensitive indicators of finite crustal thickness because they record the amount of denudation, while the rate of change of pressure and temperature in the static episode following a deformation reflect the rate of denudation. For isobarically cooled terrains the crustal thickness at the end of deformation must have been essentially that of normal thickness

131

crust (ie. 30-40 km). In the lower temperature terrains which suffered rapid post peak decompression the crustal thickness at the end of deformation is likely to have been significantly overthick, possibly as thick as 60-70 km. The apparent upper temperature limit of 800-850°C for the rapidly decompressed granulite terrains is therefore significant as it reflects the limiting temperatures for which the lower crust can sustain the deviatoric stresses associated with significant overthickening. The origin of the recumbent deformation in the two types of granulite terrains is also likely to differ. In the isobarically cooled terrains deformation probably reflects the extensional collapse of formerly overthickened crust, while in the isothermally decompressed terrains deformation may reflect crustal

thickening.

132

TECTONIC MAP OF THE CIRCUM-PACIFIC REGION, SOUTHWEST QUADRANT, 1:10,000,000 SCALE - POSTER PRESENTATION* Erwin Scheibner Geological Survey of New South Wales, P.O. Box 5288, Sydney, Australia

N.S.W. 2001,

ABSTRACT The Circum-Pacific Tectonic Maps are part of an integrated series of maps showing the relationship of known hydrocarbon and mineral resources to the geology, tectonics, and crustal dynamics of the Pacific region. The tectonic maps distinguish areas of oceanic and continental crust and active plate margins. With the search for hydrocarbons in mind, further subdivision is into basement complexes, deemed to be too deformed to contain hydrocarbons, and cover sequences. Symbols in red mark active plate boundaries and colored patterns show tectonic units at active plate margins. Well documented inactive plate boundaries are shown by symbols in black. The tectonic development of oceanic crust is shown by episodes of seafloor spreading which correlate with the rift and drift sequences at passive continental margins and episodes of tectonic activity at active plate margins. Oceanic plateaus and other prominences of greater than normal oceanic crustal thickness such as hot spot traces are also shown. Colour codings show the age of deformation of basement complexes, transitional tectonic (molassic) and reactivation sequences, and inception of platform basins. Coloured patterns show the age of inception of continental-margin (rift and drift) sequences. Isopachs in metres indicate thickness of platform strata on continental crust or continental margin sequences, and in two way time thickness of cover on oceanic crust. Symbols mark folds and faults, some of the sutures, and special symbols show volcanoes and other structural features. As the tectonic setting is one of the determining factors in the formation of deposits of nonrenewable resources, the Circum-Pacific Tectonic Maps should be important in energy and mineral exploration. This map including explanatory notes will be published in early 1989 and available from AAPG. * Published with the permission of Director-General, N.S.W. Department of Minerals and Energy.

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USE OF GEOPHYSICAL SIGNATURE IN DEFINITION OF FOLD BELTS AND TERRANES IN EASTERN AUSTRALIA* Erwin Scheibner^ and Cecil G. Murray^ ^New South Wales Geological Survey, PO Box 5288, Sydney, NSW 2001. Geological Survey of Queensland, GPO Box 194, Brisbane, QLD, 4001. In many instances adjoining fold belts and terranes in eastern Australia are comprised of rock complexes which have differing physical properties and hence the widely available geophysical data can help in definition of these features (Scheibner, 1988). Most useful are the aeromagnetic data (e.g. Brown et al., in press), Bouguer gravity (e.g. Wellman, 1988), especially the Bouguer gravity data filtered to eliminate long over 250 km wavelength anomalies (these filtered data reflect density differences within the crust and give clear indication of source shape) (e.g. Murray et al., in press). All remotely sensed data are of considerable value especially in eastern Australia, as large areas are concealed beneath L. Palaeozoic to Cainozoic cover rocks. A residual Bouguer gravity image (wavelength cut-off 250 km) enables revision of the Tasman Line (cf. Murray et al., in press), which separates the orthotectonic Tasman Fold Belt System (TFBS) or Tasmanides in the E from the contemporaneous paratectonic belts and the Proterozoic Australian Craton in the W. However, latest structural data from the S part of the paratectonic Adelaide Fold Belt (AFB) (Jenkins, 1988) indicate that even this belt locally experienced more mobility than previously thought, and that foreland-type fold and thrust belt deformation (including basement in some instances) developed during the Delamerian Orogeny. Theoretically, L. Proterozoic to E. Mesozoic tectonostratigraphic terranes can occur E of the Tasman Line (Scheibner, 1987, in prep.). The individual orthotectonic fold belts: Kanmantoo (FKB), Lachlan (LFB), Thomson (TFB), Hodgkinson-Broken River (H-BRFB) and New England (NEFB) (see definitions in Scheibner, 1987), of the TFBS can be viewed as 'super-terranes' (Leitch and Scheibner, 1987). The NEFB, has the best chance of containing allochthonous terranes, and all authors agree that the Gympie Terrane is the most probable candidate for an exotic terrane in E. Australia. The KFB occupies Tasmania (TAS) W of the Tamar Fracture System, W Victoria (VIC) W of Woorndoo Fault, the adjacent SE part of South Australia (SA) E of the paratectonic AFB, and an area in NW New South Wales (NSW), NW of the Darling Depression. Large tracts of KFB are concealed beneath the Murray Basin and older infrabasins. However, the geophysical data (Brown et al., in press, Murray et al., in press, Wellman, 1988) help to define smaller entities. Of these, the Mt Wright-Padthaway Terrane (Scheibner, in prep.) appears to represent remnants of the most westerly E. Palaeozoic volcanic arc in the TFBS and the post-kinematic A-type Ordovician igneous rocks (Foden et al., 1988) contribute to distinct geophysical signature. The Stavely-Lake Wintlow gravity-aeromagnetic ridge (Brown et al., in press) represents either the same arc doubled-up by strike-slip, or the arc split to form an inter-arc basin and consequently the correlation of Kanmantoo and Glenelg complexes needs revision. The LFB occupies most of NSW and VIC, as well as E TAS. It is a very complex region and opinions on the tectonic development and terrane accretion vary widely. Geophysical signature helps in defining smaller entities (e.g. Scheibner, 1988; Wellman, 1988). The Gilmore Suture bounding the Wagga-Omeo Terrane against other terranes in the E has spectacular geophysical expressions. The geophysical signature of the area W of the Gilmore Suture is dominated by earliest Devonian terrane dispersal and basin formation. The explanation to a gravity paradox, i.e. Bouguer gravity positive ridges in areas with largest

134

thickness of sediments, is probably due to addition of dense igneous material into the crust and at the base of rift sequences (in the present Darling Depression). Similar E. Devonian rifting occurred N of the Darling River Lineament in the Louth Block (Scheibner, 1987) and perhaps also in the Timbury Hills Basin of Murray (1986) which usually is included into the TFB. Hence this area of E. Devonian rifting might link and/or divide the LFB and TFB. The TFB (Murray, 1986) occupies major part of Queensland (QLD), and the NE corner of SA and as not yet precisely determined, part of N NSW. The TFB is mostly concealed beneath cover rocks, and appears to be equivalent of the combined KFB and LFB, but scarcity of data prevents separation of these components. The strong gravity ridge just W of the Diamantina River Lineament probably represents a L. Proterozoic to E. Cambrian passive margin rift complex (Murray et al., in press). The structural grain of the TFB expressed in geophysical signature reflects conformity with the old passive margin and also structures imparted during various deformations. The H-BRFB occupies NE QLD (Murray, 1986). Some of the structural units and suggested terranes have good geophysical expression especially on the modern aeromagnetic maps. The NEFB occupies NE NSW and E QLD, east of the L. Palaeozoic-Triassic Bowen-Sydney Basin. The results of modern radiolarian studies, geological mapping and study of residual filtered Bouguer gravity data require some revision of earlier ideas. The Peel and Yarrol Fault Zones are late, W directed thrust faults, and not terrane boundaries. The main terrane boundary is the trace of the former W verging Benioff-Wadati Zone, now refolded, at the sole of ophiolitic basement of the fore-arc belt, and at the top of the accretionary complexes (Blake and Murchey, 1988). The Tamworth Belt maybe completely allochthonous and thrust westwards over its own arc in NSW. Geophysical data confirm the doubling of the arc and its southwards displacement (cf. Murray, 1986). The Bowen-Sydney Basin originated as a rift basin and Meandarra Gravity Ridge is an expression of the original rift and not of the burried Devonian-Carboniferous volcanic arc as has been often suggested. References; BLAKE, M.R. Jnr., and MURCHEY, B.L., 1988. A California model for the New England fold Belt. N.S.W. G.S., Q. Notes, 72, 1-9. BROWN, C.M., TUCKER, D.H., and ANFILOFF, B., in press. An interpretation of tectonostratigraphic frame-work of the Murray Basin region of southeastern Australia - based on an examination of airborn magnetic patterns. Tectonophys. FODEN, J.D., TURNER, S., and MORRISON, R.S., 1988. Tectonic implications of Delamerian magmatism in South Australia and west Victoria. G.S.A., S.A. Div., B. Daily Mem. Vol. JENKINS, R.J.F., 1988. The Adelaide Fold Belt: tectonic reappraisal, ibid. LEITCH, E.C., and SCHEIBNER, E., 1987. Stratotectonic terranes of the eastern Australian Tasmanides. AGU, Geodyn. Ser., 19, 1-19. MURRAY, C.G., 1986. Metallogeny and tectonic development of the Tasman Fold Belt System in Queensland. Ore Geol. Rev. 1, 315-400. MURRAY, C.G., SCHEIBNER, E., and WALKER, R.N., in press. Geologic interpretation of a digital coloured residual Bouguer gravity image of eastern Australia with a wavelength cut-off of 250 km. Tectonics. SCHEIBNER, E., 1987. Paleozoic tectonic development of eastern Australia in relation to the Pacific region. AGU, Geodyn. Ser. 18, 133-165. SCHEIBNER, E., 1988. Geophysical signature of terranes in eastern Proc. 4th Circum-Pacif. Terr. Conf. Nanjing, 82-88. SCHEIBNER, Australia. E., in prep. The status of terrane analysis in the Tasman Fold Belt System. In: Expl. Notes Tect. Map of the T.F.B.S., BMR. WELLMAN, P., 1988. Growth of the Lachlan Orogeny by eastwards accretion. BMR Res. Newslett., No. 8, 14p. •Published with the permission of the Director-General, NSW Department of Minerals & Energy and the Chief Government Geologist of Queensland.

135

•NSONEN RfVER

" BELT

TERRAW}

WOOLOMIN-TEXAS

ORT MACQUARIE) :COCKBURN-KEERA) XWOOLOMIN) (WOODSREEF) (COPES CREEK)

7Ta«man Line

—s-

Schematic structural map showing terranes and fold belts in eastern Australia, adopted from Scheibner (1988)

160* E E. TASMANIA (MATHINNA)

JUBILEE

136

EXTENSIONAL OROGENY AT MOUNT ISA, AND ITS SIGNIFICANCE FOR MINERALIZATION Alastair J. Stewart Division of Petrology & Geochemistry, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T., 2601 The Mount Isa Inlier in northwestern Queensland (Blake 1987, Byir, Min^r, Aust. Bull. 225) is a region of some 50 000 km of Proterozoic crystalline rocks surrounded by younger sediments. It comprises a basement and three overlying cover sequences. The basement was folded and metamorphosed before about 1875 Ma. Sequences 1 and 2 underwent extensional faulting at about 1740 Ma, and this is thought to have initiated half-graben and basins in which sequence 3 was deposited. Compressive deformation began at about 1610 Ma, and was accompanied by low to medium-grade regional metamorphism. Granite batholiths were emplaced between 1860 and 1500 Ma. The inlier is longitudinally divided into a Western Fold Belt, which consists mostly of sequences 2 and 3, and minor granite and sequence 1; the central Kalkadoon-Leichhardt Belt (basement, sequences 1 and 2, and abundant granite); and the Eastern Fold Belt (predominantly sequence 2 and granite). The Kalkadoon-Leichhardt Belt is partly overlain by, and partly faulted against sequence 2 of the Western Fold Belt, and is separated from the Eastern Fold Belt by a major deformed zone, the Wonga belt. The basement consists of metamorphosed sediments, volcanics, and felsic and mafic intrusions. The overlying sequences comprise shallow-water quartz and feldspathic sandstones and calcareous rock, and largely subaerial felsic and mafic volcanics, all occurring at different stratigraphic levels and metamorphic grades. An unconformity separates basement and sequence 1, and also sequences 2 and 3, but generally no marked angular discordance separates sequences 1 and 2, except in a central tectonic ridge comprising these sequences and basement. Regional post-basement shortening occurred after deposition of sequence 3, and involved three main episodes (Winsor 1983, Tectonophvsics 92, 195). The first (D^) at about 1610 Ma (Page & Bell 1986, JL. Geol. 94, 365) produced mainly east-west folds and low-angle thrusts; several different transport directions have been proposed. The second deformation (D2; about 1544 Ma) produced northsouth upright folds, commonly with gently plunging axes and steep axial plane foliation, and also reverse faults. The third deformation (D^; about 1510 Ma) produced local folds and crenulations near Mount Isa. Major strike-slip faulting occurred at about 1500 Ma; the major faults are dextral, strike northeast, and have local folds, cleavage, and transpressional structures associated. Low-angle extensional faulting was first proposed by Dunnet (1976, Phil. Trans. Rov. Soc. Lond. A283, 333) north of Mount Isa, but that area has been reinterpreted as a contractional duplex by Bell (1983, Nature. 304, 493). The first evidence for low-angle extensional faulting was published by Passchier (1986, Geology 14, 1008), who described imbricate fault slabs overprinted by and therefore earlier than the regional shortening. The fault imbricates are sandstone lenses up to 200 m thick and 1 km long in sharp contact with metadolerite, felsic porphyry, calc-silicate rock, or sandstone of different composition or bedding orientation. Bedding in the lenses is at 40-60 to their margins, and ramps are present in many, where bedding orientation changes from oblique to subparallel to the margin. Parts of the stratigraphic sequence are incomplete or missing. The evidence indicates a system of normal faults which cut originally horizontal bedding at a large angle, and then underwent extension. Passchier & Williams (in press, Geol. Maa.) describe domino-shaped normal imbricate fault blocks; displacement between the blocks indicates about 75% extension. The blocks are surrounded by tourmaline-rich breccia and metadolerite which has been deformed and metamorphosed during, and therefore predates, D^. Movement of the hanging wall and its imbricate slabs was

137

southward. The extension is dated by synkinematic granite which was emplaced into the extending crust at a deeper level than the fault imbricates^ and gives U-Pb zircon dates around 1740 Ma (Page & others 1984, Proc. 27th Int. Geol. Conar. 5, 25). The deeper-level extension has been studied in the Wonga Belt 50 km east of Mount Isa township by Holcombe & others (1987, Geol. Soc. Aust. Abstr. 19, 35) and Pearson & others (1987, ibid.t 37). Here, metasediments, metavolcanics, and gneissic granitoids form an intensely deformed north/south belt about 100 km long and 5-10 km wide. The belt is characterized by a strong subvertical gneissic foliation which was originally subhorizontal, but was rotated during D2, reaching a largely vertical attitude except where it wraps around hinges of D2 folds. The foliation has two lineations: a relict subhorizontal pre-D2 lineation (combined extension and intersection), and a subvertical D2 extension lineation. Shear sense indicators in the pre-D2 foliation suggest that the upper plate moved north, but are not wholly unambiguous. The pre-D2 movement was accompanied by synkinematic intrusion of 1740 Ma-old granite. The belt is interpreted as an originally gently dipping extensional mylonite zone which straddled a mid-crustal detachment surface, and was subsequently folded. About 10 km west of the Wonga Belt, three outliers of sequence 2 are bounded by pre- or early D2 normal or vertical faults on their eastern sides, and are conformable with underlying beds on their western sides (Stewart, in press, Aust. Jour. Earth. Sci.). Two other outliers are in largely beddingparallel, pre-D^ fault contact with the underlying beds. Stratigraphic omissions along the eastern sides of the outliers place younger rocks on older, indicating extension in a southwesterly direction. Extension was followed by D^ and D2 shortening, which steepened the early normal faults. The 1740 Ma date for the extensional faulting predates deposition at about 1670 Ma (Page 1981, Econ. Geol. 76, 648) of sequence 3, a time-lag sufficient for cooling and sagging to take place after extension. The faulting could have initiated basins and half-graben in which fanglomerate succeeded by basemetalbearing saline lake sediments of the Mount Isa Group (sequence 3) accumulated (Neudert 1986, 12th Int. Sed. Conar. Abstr. 229) and also provided channels for metal-bearing brine to flow into the basins during and/or after deposition (Neudert 1986, ibid. 228) and form the ores.

138

THE FAULT HISTORY AND EMPLACEMENT OF THE COOLAC SERPENTINITE, LACHLAN FOLD BELT, N.S.W. Peter G. Stuart-Smith Bureau of Mineral Resources, PO Box 378, Canberra City, ACT 2601 Australian National University, GPO Box 4, Canberra, ACT 2601 The Tumut Trough, located in the southeastern part of the Lachlan Fold Belt, forms a fault-bounded belt of Silurian volcanics and flyschoid metasediments, east of the Ordovician Wagga Metamorphic Belt. The Mooney Mooney Fault System, containing an extensive ultramafic belt known as the Coolac Serpentinite, forms the eastern margin of the trough. The ultramafics together with mafic and volcanic and intrusive gabbroic rocks are thought to represent early Palaeozoic oceanic crust, dismembered and obducted into its present position during deformation of the Silurian trough sequence. A structural study of the Mooney Mooney periods of movement:

Fault

System

indicates

three

distinct

1. Reverse faulting. The earliest (pre-early Devonian granite intrusion) preserved structures are associated with the development of the Jugiong Shear Zone; a major mylonite zone up to 1 km wide which merges with the Mooney Mooney Fault System in the south. S-C fabrics are well-developed in the mylonite, with a steeply pitching mineral-elongation lineation. A reverse (east-side up) movement is indicated. The ultramafics were not involved in this tectonism to any large degree and where the Jugiong Shear Zone diverges from the margin of the u l t r a m a f i c s i n t r u s i v e r e l a t i o n s h i p s b e t w e e n the U p p e r S i l u r i a n Y o u n g Granodiorite and the Coolac Serpentinite are locally preserved. 2. Early Devonian sinistral strike-slip faulting. The ultramafic belt has a serpentinised western margin characterised by an S-C fabric formed by a steeply E-dipping foliation and a subvertical ENE-dipping shear plane. The eastern margin with the Young Granodiorite is typified by normal faults orientated orthogonal to the foliation. The structural geometries indicate oblique-slip motion with a predominantly sinistral strike-slip displacement. Matching of s t r a t i g r a p h i c u n i t s a c r o s s the fault s y s t e m s u g g e s t s a t o t a l h o r i z o n t a l displacement of about 20 km. 3. Early Devonian dextral strike-slip faulting. Dextral strike-slip faults and shear zones represent the youngest structures in the Mooney Mooney Fault System as they displace both earlier structures. Within the ultramafics older fabrics are obliterated with a new S-C fabric developed. The faults probably formed in localised transpressive environments at the waning stages of movement of sinistral strike-slip movement on the zone. As the ultramafic rocks were present in the upper crust prior to formation of the Jugiong Shear Zone and deformation of the Silurian trough sequence, they cannot represent oceanic crust obducted at that time. They probably represent older mantle-derived material emplaced within a strike-slip fault zone either during an earlier ?Ordovician orogeny or during Silurian extension.

139

PORPHYROBLAST INCLUSION TRAILS - A NOVEL APPROACH TO THE KINEMATIC INTERPRETATION OF THE SOUTHERN ADELAIDE GEOSYNCLINE Chris Steinhardt Department of Geology, James Cook University of N . Queensland, Townsville, Q . 4811, Australia ABSTRACT The structural history of the Adelaide Geosyncline has been reexamined using a n o v e l microstructural approach. This approach is based on the fact that most porphyroblasts do not rotate with respect to geographic co-ordinates and has been used in combination with structural observations on the meso- and macroscales. Foliation overprinting relationships preserved in porphyroblasts from metapelites of the Kanmantoo Group indicate a sequence of up to six foliation producing deformation stages during the (Cambro-Ordovician) Delamerian orogeny. These overprint each other with alternating steep and shallow orientations. However, the macroscopic geometry of the foldbelt is superficially simple: This occurs partly because the fold forming steep deformation events were oriented subparallel to each other, and partly because the shallow deformation events have the effect of unfolding pre-existing foliations. The deformation history determined from porphyroblast inclusion trail geometries at three widely separated localities is found to agree with a m o d e l of orogenesis calling for cyclic switching of deformation between lateral crustal-scale compression causing uplift and folding, and gravity driven collapse and spreading. Microstructural and petrographic observations indicate that the peak of (high T/low P) metamorphism occurred late during the Delamerian orogeny. This explains the marked obliquity between the metamorphic isograds and the major structures. The problem of long term high heat flow in low P metamorphism is discussed. The present-day structural asymmetry of the Adelaide Geosyncline is interpreted as a late stage feature of brittle deformation affecting an initially symmetrical orogen.

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THRUSTING AND THE TECTONIC DEVELOPMENT OF THE ADELAIDE GEOSYNCLINE Chris Steinhardt, Department of Geology, James Cook University, Townsville, Qld, 4811 AUSTRALIA Re-examination in the field of key structural relationships leads to a new interpretation of the Southern Adelaide Geosyncline as a fold-and-thrust b e l t . The type section of the overly thick Kanmantoo Group contains at least one major thrust which has not previously been recognized and this explains the impossible stratigraphic thickness arrived at by earlier workers. A basement inlier on Fleurieu Peninsula is considered to be thrust onto parautochthonous Adelaidean sediments and roofed by a thrust contact with overlying Kanmantoo Group. Away from the amphibolite grade metamorphics in the core of the orogen the foliation shallows and acquires a mylonitic character. In the Adelaide region, fold axial planes become shallower westward from the core. The structural development of the Adelaide Geosyncline involved uplift due to crustal shortening. This resulted in thrusting to the northwest (on Fleurieu Peninsula) and west (in the M t . Lofty Ranges). Of several tectonic models discussed, one by Bell and Johnson (1988) appears to best fit the data. It involves cyclic switching of deformation regimes between one dominated by folding about steep foliations and uplift and one formed by gravitational forces producing shallow foliations and thrusts. The orogen is structurally symmetrical about its core but most of its eastern half lies concealed beneath Tertiary cover. The double curvature of the Adelaide Geosyncline resulted from differential shear during or shortly after the establishment of the macroscopic fold geometry. This caused rotation of the central portion of the belt towards the NW-SE bulk stretching direction that is near constant in orientation throughout the southern Adelaide Geosyncline. The Kanmantoo Group is largely allochthonous and seismic profiling would reveal one or several shallowly east-dipping reflectors at depth. Its age is considered to be similar to the Adelaidean succession it now overlies and its sedimentologic properties fit that of a turbidite sequence deposited off a continental shelf consisting of Adelaidean rocks in Late Proterozoic time.

141

INTERACTION BETWEEN DEFORMATION AND CHARNOCKITE EMPLACEMENT DURING COMPRESSION ACCOMPANIED COOLING IN THE BUNGER HILLS, ANTARCTICA KurtStiiwe and Chris J.L. Wilson Department of Geology, University of Melbourne, Parkville, Victoria, 3052, Australia The Bunger Hills, eastern Antarctica are an area of low pressure granulite facies metamorphic rocks located along the coast-line of the East Antarctic Shield at about lOC'E and 60°S. The area is isolated from other terrains against which its geological history could be evaluated but the area displays multiple igneous emplacement and characteristic microstructures which can be used to establish the relative timing of deformation and metamorphism events. In particular the regional structural trends around the two chamockite bodies delineate different emplacement times relative to the metamorphic and structural events. The pressure-temperature-deformation (PTD) history arising from this is very different from other low pressure granulite facies terrains and invites a new tectonic model to account for its features. The first deformation (Dl) was accompanied by the intrusion of a major chamockite body in the northem part of the area. The contacts of the body are largely layerparallel and displacement of the gneisses around the body caused an elliptical arrangement of the lithological layering around the body. The Dl-synkinematic timing of the intrusion is indicated by the undisturbed continuation of the contact-parallel SI foliation into the margins of the body. Towards the centre, the body becomes unfoliated, suggesting a competence contrast with the surrounding gneisses. The peak metamorphic (Ml) assemblages in pelitic gneisses consist of cordierite-gametspinel-ilmenite-two feldspars-quartz or sillimanite-gamet-spinel-ilmenite-rutile-two feldsparsquartz. The presence of hercynitic spinel with quartz in both assemblages and the absence of sillimanite-cordierite indicates peak pressures below the reaction spinel-rutile = sillimanitecordierite-gamet-ilmenite with quartz, melt and feldspar in excess. Peak conditions around 750°C and 4±1 kbar are indicated. Coarse grains of the ferromagnesian peak assemblages indicating these low pressures are strongly aligned within an SI foliation and individual grains of these assemblages may be strongly elongated along an LI stretching lineation. Outside these strongly oriented SI bands, the gneisses consist of little deformed coarse quartz and feldspars. The coarse undeformed nature of individual grains is the consequence of late textural equilibration during M2 and erases many of the earlier micro-textures; however, the strong preferential alignment of the ferro-magnesian phases parallel to the dominant foliation evidences the pervasive early Dl deformation. Boudinage of incompetent early dykes that may cause sparation of boudins up to hundreds of metres suggests that this deformation may have been extensional. The early Dl deformation, low pressure Ml metamorphism and intrusion of the northem chamockite body were followed by a period of strong compression during the initial cooling of the terrain. Massive growth of gamet cmsts that circumscribe mafic boudins, partial melting in the northem chamockite body and, on microscopic scale, substantial sillimanite+cordierite+gamet growth from the SI-aligned spinel grains evidence this pressure increase. The pressure maximum was reached during M2 around 6.5±1.2 kbar at temperatures only little below the peak temperatures (no less than 720°C). This compression period was accompanied by the major shortening deformation D2. It is therefore reasonable to suggest that the D2 deformation caused the crustal thickening evidenced by the pressure increase. The coarse, textural equilibration on all

142

scales occurred late during D2 near the pressure maximum M2 and there is consequently few microstructures associated with the D2 event. The regional orientation of the F2 axes is inconsistent over the extent of the Bunger Hills. East of the northern chamockite body the axes dip steeply towards the south, south and southeast of the body they dip intermediately towards the west and west of the body the axial traces strike north south. This conspicuous pattern of the fold-axes around the chamockite body may indicate a competence contrast between the chamockite body and the surrounding gneisses which may have caused the folding of the gneiss sequence around the body. The fact that this shortening deformation was passive and not due to active lateral compressional forces aids to explain the inconsistent distribution of the F2 axes. A second, large chamockite body intmded in the southern part of the Bunger Hills at the end of D2, probably near the pressure maximum. This body is coarse grained throughout, with typical igneous textures, no foliation and a narrow contact metamorphic halo in the surrounding gneisses. Pyroxene thermometry indicates intmsion temperatures near 1000°C. Dykes with similar textures, grain size and pyroxene compositions intrude the gneissic sequence throughout the Bunger Hills, including the northern chamockite body. After the intrusion of the body the Bunger Hills cooled, probably to the stable geotherm. A strong, late doming event, D3, accompanies this cooling and causes strong reorientation (mainly steepening) of the earlier stmctural features. The association of the major shortening deformation D2 with the crustal thickening evidenced by the metamorphic texnares cannot be the consequence of continental collision for two important reasons, (i) the metamorphic peak Ml lies at too low a pressure to be attained by thermal relaxation of tectonically overthickened crust; (ii) the metamorphic maximum is followed by a period of compression and not decompression. We therefore suggest that peak metamorphism occurred in an environment of thin continental crust (possibly thinned during Dl) overlying a thermal anomaly in the asthenosphere. Thermal weakening of the cmst caused the gravitational instability of the thin cmst between the adjacent areas of thick continental cmst to equilibrate in a period of strong shortening deformation (D2). This deformation is therefore the consequence of passive, rather than active cmstal shortening. The timing and temperatures of the southern chamockite body are consistent with its generation at the bottom of the cmst in the late stages of this compression period. Cooling of the Bunger Hills occurred at depth and excavation of the terrain to the surface must be accounted for by extemal and probably much later processes. The greenschist and amphibolite facies shear zones that occur throughout the Bunger Hills may have formed during this excavation event.

143

CHARACTERISTICS OF FAULTS AT OLYMPIC DAM AND THEIR POSSIBLE RELATIONSHIP TO REGIONAL LINEAMENTS Tim Sugden Department of Geology Olympic Dam Project Olympic Dam 5725 The Olympic Dam Copper-Uranium-Gold-Silver Deposit (an inferred resource of 2000 million tonnes ) is located approx 520 km north-northwest of Adelaide, South Australia and has recently been brought into production as a joint venture between Western Mining Corp Ltd and the British Petroleum Group. The deposit is situated within the Stuart Shelf geological province where more than 260 m of flat-lying Upper Proterozoic sediments of the Gawler Craton. Undeformed 1595 m.y. A-type granitic rocks host the mineralised hydrothermal breccias. Since 1982, over 30 km of underground development has been mapped by mine geologists, enabling fault categorisation and, recently, a program of kinematic analysis. The mine area (approx 4 x 3 km) is criss-crossed by a number of generations of fault and joint sets. In this contribution, the structural history of areas in the northwest and southeast of the mine are considered in detail and related to a generalised tectonic model. Hematite-rich breccia zones (+ disseminated sulphides) trend approximately NW to NNW and form an irregular array with an overall WNW trend (AMG) . The boundaries of these multiply brecciated, heterolithic breccia zones generally have quite sharp, steep contacts with the surrounding brecciated granite protolith, but in some areas grade into zones of discontinuous black hematite veins. The breccias appear to have been generated by a variety of hydrothermal, phreatomagmatic and tectonic processes. Orthogonal extension joints occur on the margins of some breccia zones and may represent an incipient stage of brecciation. Grainscale disaggregation of the granite is widespread. In certain discrete, irregular zones this appears to have been a product of chemical corrosion combined with cataclasis. These processes apparently reduced the granite to a noncohesive aggregate which, in some instances, developed flowfabrics in zones of faulting or volatile streaming. Faults are ubiquitous at a mesoscopic scale. They are typically irregular and discontinuous with curving and superimposed slickensides. Many are observed to bend around breccia blocks and refract or die out at lithological contacts. Deformation of the breccia complex appears to have been accommodated by ^block jostling' resulting in complex and episodic movement histories for individual

144

faults. Therefore, it is not possible to construct a simple chronological classification. However, larger faults (having strike lengths exceeding 50 m) which presumably formed through linkage of individual discontinuities, can be grouped according to general orientation. Many of these faults have distinctive gouge characteristics and w a l l rock relationships which may enable further classification. N to NNW trending strike-slip faults formed conduits for many mafic intrusives and some terminated in zones of phreatomagmatism (peperites) and, in at least one location, a diatreme and maar evolved. Some intrusives are relatively fresh and undeformed but most are highly sheared and sericitised indicating that these faults were active during the principal breccia-forming hydrothermal episodes. W e l l developed shear bands in wall rock breccias, as well as steps in deformed fluorite-chalcopyrite veins indicate dextral shear sense. However, other less well defined shear criteria suggest a history of antithetic movements. WNW trending strike-slip faults can be traced across much of the deposit. These are associated with metrewide zones of complex subsidiary faulting (including sub-horizontal flats) and localised cataclasite, but displacements are thought not to exceed 100 m. Mineralised dilational fault jogs indicate dextral shear sense. Undevitrified psuedotachylite is preserved in at least one of those faults. Complex small-scale extensional fault systems observed in stratified pyroclastic deposits may be related to localised maar collapse. Reverse faults which clearly post-date the breccias and the early strike-slip faults are predominant in NW part of the deposit. Although having an overall NW-SE strike, these faults are discontinuous and display curvilinear and scalloped morphologies. Individual segments may be linked by splays, fault bends or complex subsidiary fault zones. Due to the complex nature of the host breccias, unequivocal displacements are only rarely recognisable. Evidence for reverse faulting is gained from a number of criteria including Riedel shears, foliation in cataclasites, mineralised fault jogs and en echelon veins. Dilatant jogs are commonly filled with barite-fluorite-siderite assemblages, some of which have undergone extreme deformation during subsequent shearing over asperities. Actual displacements are believed not to exceed 20m and appear to bear little relationship to gouge thickness. At least three other fault sets seem to have operated with the principal reverse faults to achieve bulk three dimensional strain. Many of these faults have clearly exploited pre-existing anisotropies such as the

145

contacts between heterolithic breccias and the adjacent granitic breccias; sub-horizontal slickenlines are commonly seen to overprint steep corrugations. In other instances individual fault segments have exploited pre-existing extension joints. Movement plane analyses of the multiple fault sets indicate a NE-SW bulk shortening axis. This conclusion is supported by the occurrence of late stage, apparently en echelon barite-fluorite veins which trend approx NEN-WSW. Conjugate ? fracture sets (trending approx 285° and 225°) are observed in the overlying upper Proterozoic to Cambrian cover rocks. Attempts to relate mine-scale structures to a regional m o d e l are impeded by a paucity of data on basement structure outside the mine area. Moreover, the breccia forming processes of hydraulic fracturing, chemical corrosion and phreatomagmatism have obliterated many original structural relationships. A simple kinematic model involving sinistral movement on the WNW trending faults can explain the post-brecciation NE-SW bulk shortening and en-echelon barite-fluorite vein array. Conjugate fracture sets in cover rocks have a Riedel geometry compatible with sinistral movement on these faults. The occurrence of fresh pseudotachylite suggests that such reactivation may have been relatively recent. The surface expression of this movement may correspond with a regional scale WNW trending photolineament, recognised prior to this study. The array of breccia zones and N to NNW trending strike-slip faults may be explained in terms of a ^mesh' model of linked extensional and shear fractures, but as yet, this can not be confidently related to any known regional lineament. Thus the possibility that pre-existing faults influenced the initiation of hydrothermal activity and brecciation remains unconfirmed. However, the complex history of postmineralisation faulting and its hypothesised relationship to fracturing in the cover sequence indicates that the deposit was a significant zone of anisotropy and formed a long-lived focus for tectonic activity.

146 THE GRAMPIAN HIGHLANDS OF SCOTLAND: AN OROGENIC ANALOGUE TO THE ADELAIDE FOLD BELT OF SOUTH AUSTRALIA S . Temperley, University of Hull, HU6,1 7RX, England The Grampian Highlands of Scotland lie within the orthotectonic zone of the British Caledonides. Results from the LISPB deep seismic refraction profile (Bamford et.al. 1977) suggests the presence of two basement units in this region. The lower, unexposed unit is interpreted as a direct southward extension of the Archaen to Lower Proterozoic cratonic Lewisian assemblage which constitutes a foreland of depleted granulite facies orthogneisses and which is exposed in the form of structural inliers within the Moine rocks of the Northern Highlands of Scotland. The Moine rocks exposed in the Grampian Highlands comprise a younger. Upper Proterozoic basement assemblage of metasediments (Piasecki & Van Breemen 1983; Piasecki & Temperley 1988), the first deformed during the Grenville Orogeny (1 000-1 100 Ma) and subsequently reworked during the Knoydartian event' (750-800 Ma) . During the late Riphean, somewhere around 750 Ma ago, the basement began to undergo NW-SE extension as a result of lithospheric stretching. Initial thinning of the basement led to the formation of a broad, shallow basin. Continued extension resulted in the development of normal faults, which resulted in the breakup of the shallow basin into a series of deeper, fault-bounded ensialic rift basins, with intervening basement highs, or horsts (Anderton 1982) . The lithologically diverse sedimentary rocks which constitute the Dalradian Supergroup represent the products of deposition in this complex of ensialic basins. Sedimentological evidence suggests that basin evolution was marked by increasing instability with time, with deeper water, turbidite sedimentation superceding the deposition of shallow water and estuarine clastic and calcareous sediments. The Dalradian Supergroup is subdivided on a lithological basis into four groups which range in age from about 750 Ma until the middle Cambrian (Harris et.al. 1975). They therefore comprise a sequence of Riphean, Vendian and Lower Palaeozoic rocks which is cumulatively some 25 km thick, although this thickness is never attained in any one locality. Deposition of the uppermost Southern Highland group was associated with mafic volcanism, and although the Dalradian sequence is nowhere underlain by oceanic crustal material, such volcanism may represent incipient, albeit failed, continental break up in this area. Actual continental break u p , with the initiation and growth of lapetus Ocean floored by oceanic

147

crust/ was onwards.

achieved

further

the

from

about

600

Initial shortening of the Dalradian tract with its associated basement, during Late Cambrian orogenic contraction, resulted in major crustal thickening and the development of large scale upright Di folds. A combination of gravitational collapse and continued contraction during the Ordovician resulted in the modification of these folds into both NW and SE facing recumbent nappe structures during D2, followed by peak metamorphism, rapid uplift and erosion. These tectonothermal events together constitute the Grampian Orogeny. Further shortening during the Silurian (the Caledonian Orogeny sensu stricto) was markedly heterogeneous and resulted in open to isoclinal upright refolding of the recumbent nappes of the Grampian Highlands. One NE-SE trending linear zone of intense upright, sub-isoclinal folding, known as the Central Highland steep belt, can be traced for almost 200 km and for most of its length coincides withe the boundary zone between NW facing and SE facing Dalradian nappe structures. The steep belt has been interpreted as a primary "root zone" of the diverging nappes (Thomas 1979), but recent structural and isotopic evidence presented here which would appear to demonstrate that belt formed late in the tectonic history of the Grampian Highlands. There are striking parallels between the geological history of the Grampian Highlands of Scotland and that of the Adelaide region of South Australia (von der Borch 1980). Both comprise basement sequences deformed during Precambrian events of comparable age. In both areas, cover sediments were deposited in a complex of ensialic basins as a result of basement rifting associated with lithospheric extension during the late Precambrian. Moreover, both areas were subsequently shortened during orogenic contraction initiated during the late Cambrian with a resulting dominance of upright fold structures. In the light of such striking analogies, the disclosure of data and ideas related to the geological history of each area can only be to the mutual benefit of all concerned. Anderton, R. 1982.

J. GEOL. SOC. LONDON, v. 139, pp 421-431.

Bamford,

1977.

D. et.al. 481-488

Harris, A.L. et.al. 1975.

GEOL.

SOC.

LONDON,

133,

GEOL. SOC. SPECIAL REPORT. No 6.

Piasecki, M.A.J. & van Breemen 0. 1983. V. 73, pp 119-134.

TRANS. R. SOC. EDIN.

Piasecki, M.A.J. & Temperley S. 1988. in Winchester J. (ed) . LATER PROTEROZOIC STRATIGRAPHY OF THE NORTHERN ATLANTIC REGIONS. Thomas, P.R. 205-211,

1979.

GEOL. SOC. SPECIAL PUBLICATION. No 8, pp

von der Borch, C.C. 1980.

TECTONOPHYSICS. v.70, pp 115-134.

148

WHITE RANGE AND RUBY GAP DUCTILE DUPLEXES: IMPLICATIONS FOR PALEOZOIC DEFORMATION IN CENTRAL AUSTRALIA Christian Teyssier, W. James Dunlap and David Kirschner University of Minnesota, Department of Geology and Geophysics, Minneapolis, Minnesota, 55455 USA The relative importance of Paleozoic versus early Proterozoic deformation in the metamorphic basement involved in the Alice Springs orogen, central Australia, has been the object of considerable recent discussion. Arguments can be partly alleviated by examining the deformation in thrust sheets that contain the Late Proterozoic sedimentary sequences (Heavitree Quartzite and Bitter Springs dolomite) best exposed in the Arltunga Nappe Complex (Figure #1). Two such critical structures, the White Range and Ruby Gap duplexes, allow finite strain and bulk Paleozoic displacement to be estimated. The one kilometer thick White Range duplex is composed entirely of Heavitree Quartzite which unconformably overlies granitic and amphibolitic gneisses to the west and south and is in turn overlain by a granitic gneiss sheet to the east and north. Isoclinal folds are ubiquitous throughout the White Range on scales of centimeters to hundreds of meters in limb to limb distance. Axial planes are almost everywhere coplanar with gently north dipping foliation and hinge lines are colinear with a north oriented stretching lineation. North dipping thrusts dissect the isoclinal folds and overthrust seven sheets of quartzite from north to south. Finite strain data and crystallographic fabrics suggest that the bulk strain of the White Range approximated plane strain conditions with minor fluctuations into both the apparent constrictional and flattening fields. The basal conglomerate unit in the Heavitree records the highest strain values in the Range (approximately 21:4:1). Strain determined by axial ratios of quartz porphyroclasts in a recrystallized quartz-muscovite matrix record strains of 5:2.5:1. Microstructural observations show that the entire range is ductilely deformed and in places is entirely recrystallized. Based solely upon the strain values and assuming a simple shear history, minimum displacement accommodated by the White Range is on the order of 3 km. However, based upon field and microstructural observation of both the roof and floor thrust sheets, we propose a conservative estimate of 10 km displacement accommodated in this 3km thick shear zone package. Ruby Gap duplex is a west plunging, ductile, antiformal stack of thrust sheets that was involved in a southward overthrust. Sheet 1 represents the parautochthonous footwall and sheets 2-5 the duplex itself. The duplex roof thrust extends from the Illogwa Shear Zone to the White Range duplex (Figure #1). Sheets 2-5 are composed of highly deformed horses of basement gneiss, Heavitree quartzite and Bitter Springs dolomite.

149 These sheets vary in thickness laterally, display smaller scale imbricate thrusts, and contain isoclinal folds with generally north dipping axial planes. Shallowly dipping foliation surfaces define the broad antiformal structure. Stretching lineations lie east-west in the sheet 1, but are north oriented in sheets 2-6. Kinematic criteria indicate that the 3 km thick stack formed during southward shear. Mylonitic fabric, strain, and displacement all increase upward through the duplex. The proportion of recrystallized quartz grains increases progressively from sheet 1 (5% by volume) to sheets 3-5 (70% to 100% by volume). Correspondingly, weak quartz c-axis preferred orientation in sheet 1 becomes stronger in the overlying sheets. Finite strain in sheet 1 gradually increases from south (1.7:1.4:1) to north (5.3:2.1:1) near the thrust where cobble metaconglomerates locally record high strain (20:6:1). Average strain values in sheet 2 vary from 4:2:1 to 8:3:1. Given the degree of recrystallization in the overriding sheets, finite strain measurements become increasingly tenuous. Based solely upon the above strains and assuming a simple shear history, displacements would be on the order of kilometers; however, restored cross-sections suggest a minimum displacement for the entire duplex of 40km. The important result of this study is that Paleozoic deformation is substantial and affects not only the Heavitree Quartzite and Bitter Springs dolomite but also the metamorphic basement. Preliminary mapping and kinematic analysis in the Paradise sheets (Figure #1) overlying the Ruby Gap duplex indicates that Paleozoic deformation is localized within a system of thick ductile-shear zones which separate unaffected blocks. These preserved domains contain older fabrics that are metamorphically distinct from those of the Paleozoic deformation.

WHITE RANGE DUPLEX

ILLOGWA

150 SERAHBN SUB-BASIN, TRANSFORM MARGIN.

A WRENCH HELMED BASIN (W THE WEST T A a « N I » N

B.Thcmas School of Earth Sciences, Flinders University of S.A., S.A. 5042.

The Strahan Sub-Basin is the southernmost element of three enechelon sub-basins underlying the West Tasmanian continental shelffrcm North to South : King Island, Sandy Cape, Strahan sub-basins These basins are contemporaneous of the Otway and Great Australian Bight basins and were originated in earliest Cretaceous in respcxise to the rifting between Australia and Antcirctica. The Strahan basin is a 50 km long - 25 km wide half-graben bounded by NNW-SSE and E-W faults. It accumulated thick sedimentary series (6 km) corparable to the infill of the Otway Basin : a thick Early Cretaceous monotonous syn-rift continental sequence (Otway Group), unconformably overlaid by post-rift shallcw marine deposits. This midCretaceous break-up unconformity is related to the initiation of oceanic spreading between Australia and Antarctica, and corresponds to the beginning of the sag phase of the southern margin. On the West Tasmanian margin, the development of N-S wrench structures (flower structures) and normal activity on the Strahan Basin bounding faults prevailed until the Oligocene. The major unccxiformity marking the beginning of the thermal sag phase occurred in Oligocene and clearly postdates the mid-Cretaceous break-up unconformity of the southern margin. This Oligocene unconformity can be related to the final clearance of the Antarctic and Australian plates. Therefore, the prolongated continent-continent contact during the strike-slip motion of the Antarctic plate along the West Tasmanian margin accounts for the time lag between the Mid-Cretaceous post-break-up and Oligocene "postcontinental clearance" unconformities. Formation and evolution of the Strahan Sub-Basin appear structurally controlled by N-S (or NNW-SSE) wrench movements related to the relative movement of the Australian and Antarctic plates. Subsidence of depocentres along strike-slip and normal faults are cotmon features of transform margins shelves, (i.e. Southern Grand Bank of Newfoundland, Svalbard-Spitsbergen margin, Gulf of Guinea and Northern Brazil margins...) and the geological evolution of the Strahan graben is consistent with the interpretation of the West Tasmanian Margin as a transform margin.

151

Tectonic Control of Early-Rift (Cretaceous) Sedimentation, Taranaki Basin, New Zealand. Glenn P. Thrasher New Zealand Geological Survey, Lower Hutt Taranaki Basin, New Zealand's only hydrocarbon producing basin, is located mostly offshore west of the North Island. The basin was first formed as an early-Cretaceous failed rift along the Gondwana margin. Following this initial rifting, subsidence and sedimentation continued throughout the Cretaceous. Since the early-rift phase, the basin has undergone a complex history of subsidence, compression and additional rifting. The present basin is open-ended to the northwest, where it continues as the oceanic New Caledonia Basin. A major geophysical interpretation of the basin, utilizing petroleumexploration seismic reflection data, has allowed an understanding of the rifting geometry of the basin and the tectonic control on sedimentation. Although there is no hydrocarbon production from Cretaceous sediments within Taranaki Basin, these early-rift rocks are very likely source rocks for the hydrocarbons presently being exploited, and may be possible reservoir rocks. An isopach map of the sedimentary unit between basement (mainly Gondwana continental rocks) and the top-Cretaceous reflector (determined from well control) reveals a system of Cretaceous faults with two distinct orientations: north-south and northeast-southwest. These two sets of faults control several sub-basins which contain Cretaceous sediments ranging in thickness from 2000 to 4000 metres. Many of these sub-basins lie in a zone about 50 km wide and 400 km long aligned NE-SW between Albatross Point on the North Island (the Te Ranga sub-basin) and the Northwest corner of the South Island (the Pakawau sub-basin). Another group of sub-basins is concentrated in the northern part of Taranaki Basin, between the Moa basement high and the Tangaroa basement high. These sub-basins are controlled by major north-south, down-to-theeast, normal faults, and contain up to 4000 metres of Cretaceous sedimentary section. To the west of the two rift systems mentioned above thin (generally less than 1000 m) Cretaceous sediments overlie a peneplaned basement surface and pinch out against the high standing Challenger Plateau. To the east lies the Taranaki Boundary Fault, along which basement rocks were thrust to the surface during the Miocene. Although extensive erosion has occurred in this region, there is no evidence that Cretaceous sediments were ever deposited to the east of this fault. Only 11 petroleum exploration wells have penetrated Cretaceous rocks in Taranaki Basin. Most of these wells have found only Haumurian (latest Cretaceous) sediments, but wells Te Ranga-1 and Cape Farewell-1, located at both ends of the major NE-SW lineation of sub-basins, have interesected sediments of Clarence (late early-Cretaceous) age. Neither well reached basement. Although most drilled sequences have been terrestrial coal measures, a few wells, and seismic stratigraphic interpretation, indicate that marine rocks are present in many of the sub-basins, especially along the NE-SW trend. Marine sediments are also present as a latest Cretaceous transgression from the New Caledonia Basin.

152

Although younger tectonic events have confused the early-rift picture, one possible interpretation of the Cretaceous rifting history is that the basin initially formed as a continental transform boundary during the opening of the New Caledonia Basin in the early Cretaceous. This transform was obliquely extensional and extended from the head of the New Caledonia rift, near the present coastline of the North Island north of the Taranaki Peninsula, through Taranaki Basin, and along the west coast of the South Island. The oblique extension gave rise to the series of sub-basins oriented NE-SW, and perhaps to those along the west coast of the South Island. This is a situation very similar to the Gulf of Aqaba - Dead Sea rift today. By the late Cretaceous the New Caledonia rift failed, perhaps due to the opening of the Tasman Sea about 80 Ma. This caused motion on the continental transform to cease, and allowed the region to thermally subside. Several conclusions can be drawn from this model, and possibly verified by seismic reflection data. The NE - SW fault trend should represent transfer faults along which sinistral strikeslip motion was accommodated. The N-S trend is probably aligned along the pre-existing basement grain, and may have accommodated much of the extension. As the New Caledonia rift opened, a Cretaceous seaway could have existed through the zone of oblique extension, flooding many of the sub-basins which presently underlie the Taranaki Basin.

New Caledonia Basin

e Ranga-1

NORTH

Tasman / \ Basin / /

SOUTH

ISLAND

153

THE PAUVEOMAGNETISM OF LATE DEVONIAN RED BEDS OF THE MERIMBULA GROUP^ SOUTHEAST NEW SOUTH WALES

G. A. Thrupp^^ D. V. Kent^, P. W. Schmidt^, and C. McA. Powell^ 1 Earth Sciences^ Macquarie University^ NSW 2109^ Australia 2 Lamont-Doherty Geological ObservatoryPalisades^- NY 10964^ USA 3 CSIRO Division of Exploration Geosciences, PC Box 136, North Ryde, NSW 2113, Australia

Gently folded strata of the Late Devonian Merimbula Group are well exposed in the vicinity of 37^ S latitude along the coast and in road cuttings in the Yurairanie State Forest of southeast New South Wales. The Merimbula Group sediments are similar to numerous Late Devonian - Early Carboniferous quartzose sandstone deposits, the Lambie Facies, that occur in southeast Australia. Because these sands overlap the early Palaeozoic tectonic elements of the Lachlan Fold Belt, large displacement since Late Devonian of any southeast Australian terranes, with respect to central Australia, is precluded. We collected oriented core samples from 36 sites in the reddish, quartzose litharenites, primarily of the Worange Point Formation, with the aim of obtaining a Late Devonian pole position with respect to Australia. Incremental thermal demagnetisation reveals that the magnetisation of the red sandstones is dominated by a consistent, well-defined, steepupward-north component that postdates the mid-Carboniferous folding. This secondary magnetisation is probably related to mid-Cretaceous rifting of the southeast Australian margin that led to opening of the Tasman Sea. A mid-Cretaceous thermal disturbance of the Tasman coastline is recorded by remagnetisation in the Sydney Basin (Schmidt and Embleton, 1981), and reset fission tracks in apatites from basement Plutonic rocks (Moore et al., 1986). The exclusive normal polarity of the overprint, and its pole postion, are consistent with a midCretaceous acquisition. A characteristic component of magnetisation is isolated between ^-660 and '-680 in a small minority of the samples. The concentration of mean directions from four sites in which the characteristic component is best defined improves with correction for tilt of bedding. Both polarities are represented, and they are roughly antipodal. For many samples the path of directions defined by the high temperature demagnetisation steps trends toward the characteristic component. The intersection of planes defined by high temperature demagnetisation steps is close to the characteristic direction determined for the four sites. The characteristic component is particularly well-defined in a few mudstone samples. Work is in progress on additional samples from sites with fine-grained strata and varying attitudes to elucidate the nature of the characteristic component.

154

The Late Devonian pole postion suggested by the preliminary result lies between southernmost Africa and Antarctica^ close to two other Late Devonian - Early Carboniferous pole positions derived from Australian rocks: (1) Hervey Group (Li et al., 1988) of the Parkes-Cowra region of southeast Australia; (2) Canning Basin Limestone (Hurley and Van der Voo^ 1987) of northwest Australia. These results indicate a low latitude position for Australia. The consistency of the three palaeomagnetic poles supports the interpretation based on the Lambie Facies overlap assemblage and consistency of Gondwanaland palaeomagnetic data (Schmidt et al., 1986, 1987): the Lachlan Fold Belt was closely tied to interior Australia at least since the Devonian. The progression and large spread of the pole postions derived from (1) the Late Silurian - Early Devonian Snowy River Volcanics (Schmidt et al., 1987), (2) the Middle - Late Devonian Comerong Volcanics (Schmidt et al., 1986), and (3) the Late Devonian - Early Carboniferous Lambie Facies (Li et al., 1988, and this study) indicate angular apparent polar wander rates exceeding 1^/Ma during the Devonian.

References Cited Hurley, N.F., R. Van der Voo, 1987, Paleomagnetism of Upper Devonian reefal limestones. Canning Basin, Western Australia: Geol. Soc. Am. Bull., V. 98, p. 138-146. Li, Z.X., P.W. Schmidt, B.J.J. Embleton, 1988, Paleomagnetism of the Hervey Group, central New South Wales and its tectonic implications: Tectonics, v. 7, p. 351-367. Moore, M.E., A.J.W. Gleadow, J.F. Lovering, 1986, Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatites from south-eastern Australia: Earth Planet. Sci. Lett., V. 78, p. 255-270. Schmidt P.W., B.J.J. Embleton, 1981, Magnetic overprinting in southeastern Australia and the thermal history of its rifted margin: Jour. Geophys. Res., v. 86, p. 3998-4008. Schmidt P.W., B.J.J. Embleton, T.J. Cudahy, C. McA. Powell, 1986, Prefolding and premegakinking magnetizations from the Devonian Comerong Volcanics, New South Wales, Australia, and their bearing on the Gondwana Pole Path: Tectonics, v. 5, p. 135-150. Schmidt P.W., B.J.J. Embleton, H.C. Palmer, 1987, Pre- and post-folding magnetisations from the early Devonian Snowy River Volcanics and Buchan Caves Limestone, Victoria, Geophys. J.R. astr. Soc., v. 91, p. 155-170.

155

POST DELAMERIAN MAGMATISM: TECTONIC

CONTROLS ON MAGMA

AND EVIDENCE FOR POST D E L A M E R I A N

Simon Turner, John Foden and

CHEMISTRY

EXTENSION

John

Cooper

Department of Geology and G e o p h y s i c s , University of A d e l a i d e , South Australia

The Cambro-Ordovician Delamerian Orogeny brought the sedimentological history of the Adelaide Geosyncline to a close producing a Buchanstyle metamorphic fold belt. Outrop on the eastern edge of the fold belt is composed of the Cambrian Kanmantoo Group in which deformation is often quite intense and metamorphic grades reach middle to upper amphibolite facies. This high grade region is associated with various, predominantly granitic intrusives which can be divided on structural, petrographic and geochemical grounds into syn- and post tectonic suites. The Early Ordovician post tectonic magmatism was bimodal producing both mafic dykes and plutons as well as high silica granitic plutons and rhyolitic volcanics. The post tectonic granites and associated volcanics extend across the south east of South Australia and into western Victoria where a similar syn- and post tectonic magmatic history in the Glenelg River Complex supports other evidence that this area is part of the Kanmantoo terrain. The post tectonic granites evolved as relatively dry, high temperature magmas as indicated by their low modal content of hydrous phases, the occurrence of pyroxene and even olivine and a tendency to be one-feldspar (hypersolvus) rocks. Geochemistry shows that these rocks evolved via extensive plagioclase fractionation whilst low initial Sr ratios suggest a relatively primary source. They show low A1 and Ca contents and are enriched in Nb, Y, Zr, Ga and F. These features are characteristic of A-type granites which are typically associated with anorogenic or extensional tectonic settings. The presence of granophyric intergrowths, beta quartz and the association with extrusives indicates that the post tectonic granites were intruded fairly rapidly to high crustal levels which would be in accord with an extensional environment. The mafic component of this post tectonic suite provides additional evidence for post Delamerian extension and is manifested by the Black Hill intrusive complex and in basaltic dykes. At Mannum for example one of the post tectonic granites is cut by a basaltic dyke and there is also evidence for mingling between mafic and silicic magmas here. The non-deformed Early Ordovician Black Hill intrusive complex, just east of the outcropping Kanmantoo of the Mt. Lofty Ranges, is evidence of major mantle derived mafic activity following the D e l a m e r i a n Orogeny. The complex comprises at least four large (>6x6km), layered plutons with a continental tholeiitic nature which are not unlike those of the extensional Tertiary igneous province of the north-east Atlantic. At Black Hill the range of lithologies extends from peridotites and troctolites through olivine gabbros and norites to gabbronorites and pyroxene monzonites as well as late dyke like equivalents of the post tectonic g r a n i t e s . Geochemistry indicates that the Black Hill gabbros resulted from a mantle derived tholeiitic magma which evolved via p l a g i o c l a s e + o 1 i v i n e and plagioclase+pyroxene fractionation combined with c o n c o m m i t a n t crustal assimilation. Mineralogical trends such as the presence of an olivine gap and a pigeonite zone are reminiscent of the Skaergaard intrusion. Some foliated tonalltic to granod1 or11ic rocks are thought to be xenoliths of

156

syn-tectonic granitic country rock. Ongoing research is directed towards evaluating whether this extensional phase may be related to underplating and remelting of lower crustal rocks which earlier sourced the syn-tectonlc magmas and the precise relationship between mantle input and the post tectonic granites. Tectonic conditions during the deposition of the Kanmantoo Group and the subsequent Delamerian Orogeny are currently the centre of considerable debate. Clearly the evolution of the magma types reflects changes in the thermal regime and mantle component of the deep crust which may be intimately related to the structural evolution of the mountain belt. Thus the magmatic history of the terrain gives an indication of tectonic environment as well as providing useful clues about the nature of the lower crust and allowing recognition of distant parts of the same terrain (e.g. Glenelg River Complex). The bimodal post tectonic igneous suite suggests a period of extensional tectonics very shortly after the close of the Delamerian Orogeny. As yet substantial structural evidence for such an event has not been documented. Concievably this period of extension may have been a response to thermal weakening of the lithosphere associated with crustal overthickening during the Delamerian.

157

TECTONIC EVOLUTION OF THE KING LEOPOLD OROGEN, KIMBERLEY REGION, WESTERN AUSTRALIA I. M. Tyler and T. J. Griffin

Geological Survey of Western Australia, Mineral House, 100 Plain Street, Perth 6000 The King Leopold Orogen is located in the Kimberley region of northern Western Australia. It forms a linear belt at the southwest margin of the Kimberley Basin and is similar to the better known Halls Creek Orogen at the basin's southeast margin. The King Leopold Orogen includes the Hooper Terrane which consists of deformed and metamorphosed igneous (intrusive and extrusive) and sedimentary rocks; together with the tightly folded Kimberley Basin succession. To the southwest the orogen is unconformably overlain by Palaeozoic rocks of the Canning Basin. The orogen is the product of a complex tectonic history during the Proterozoic and probably the early Phanerozoic. The Hooper Terrane has four components. The oldest is a monotonous sequence of turbiditic greywacke sandstones (?equivalent to the Halls Creek Group in the Halls Creek Orogen) that have been intruded by mafic sills (?equivalent to the Woodward Dolerite). The metasediments and the mafic sills are unconformably overlain by felsic volcanic rocks which are equivalent to the cl850 Ma Whitewater Volcanics. All three components were intruded by extensive linear granitoid plutons (cl840 Ma). The earliest deformation (Dl) affected the metasediments, and caused tilting of the rocks prior to deposition of the felsic volcanics. A layer-parallel foliation is present and tight to isoclinal small-scale folds are infrequently found associated with it. A later set of folds (D2) affect both the metasediments and the volcanics. These folds are open to tight and have upright, west-northwest trending axial surfaces and plunge moderately to steeply to the west-northwest or eastsoutheast. A crenulation cleavage is developed axial planar to these folds. Deformation was accompanied by granitoid intrusion with a foliation being imparted prior to the rocks being fully solidified. Metamorphic grade ranged from greenschist facies to granulite facies, with peak metamorphism occuring between Dl and D2. The two tectonic events are thought to correspond to early Proterozoic rifting and collision recognised throughout northern Australia. The Hooper Terrane is overlain by the Kimberley Basin succession, a sequence of quartzose and feldspathic arenites, mature quartz sandstones, hematitic sandstone and conglomerate, and siltstone together with tholeiitic basalt and minor dolomite. The basin extended to the southwest across the Hooper Terrane with clastic sediment transported from the northeast (Speewah Group) and north-northwest (Kimberley Group). The

158

original extent of the basin is not known. The Speewah and Slmbe^^tiy S - u p s are intruded by mafic sills collectively known as the Hart Dolerite

(1760 Ma).

The next tectonic event (D3) affected both the Hooper Terrane Ind ^ h f K i m L r l e y Basin succession. In the Hooper Terrane defo^m^tion took the form of large, - ^ i - ^ ^ r r ^ i t n t t ^ L i r " which have a west-northwest trend and generally dip ^êeply to tE^south-southwest. Shear criteria 'rseô^^ UP or dextral movements, or a combination of both. A second c L n u l a U o n cleavage affects rocks adjacent to the shear zones. Shearing was accompanied by amphibolite facies metamorphism. Two phases of folding affected the most westerly part of^the S b L l e v Basin succession. The first (D3a) produced largescale, ^ L r isoclinal folds with gently inclined to recumbent axta^surfaces that dip to the south-southwest. In Cascade Bay . ^^^ t ^ ^ c o n t a c t between Kimberley Group rocks granitoid is a thrust with shear criteria transport of the basin sediments. The second told phase D.5b) c ^ n l i f t r o f large-scale, north-facing folds with steeply south-southwest dipping axial surfaces. Folding becomes l e s s intense to the north dying out beyond Koolan Island. D3a struciCres are only found to the south of ône Bay where they are refolded bv D3b structures. Shear zones in the Hooper ?eJrIne are seen to cut into the Kimberley Basin succession w h e r ^ t h e y are orientated parallel to the surfaces of D3b folds The age and cause of this tectonic event is uncertain I ^ t h o U t L open warping of rocks in the Kimberley Basin which occurred prior to deposition of late Proterozoic glacigene rocks, may be related to it. a

T.rrane, the t h r H a U , CreerSrogen.

^lir'^ar^o:^^

th Lr^r'^triKe-.lip fault .ovejents In The contact between the Hooper Terrane

underlying Hooper Terrane.

rigid cratonic basement presumed to underlie ^he Kimberley fiifin. Each event was controlled by a tectonic framework established in the early Proterozoic.

159

PHANEROZOIC TECTONIC REGIMES OF AUSTRALIA, AND THE GLOBE

GONDWANALAND,

J.J. Veevers and C.McA. Powell Australian Plate Research Group, School of Earth Macquarie University, N.S.W. 2109, Australia

Sciences,

Following the Late Proterozoic Adelaidean regime, Phanerozoic Australia was marked by three regimes: Regime

M a ago

Potoroo

95 toO

Innamincka

320 to 90

Uluru

575 to 320

Adelaidean

c. 850 to 575

Stage

M a ago

c Cenomanian Interregnum Late Middle Early Devonian/Carboniferous Silurian/Devonian Ordovician Cambrian Ediacaran

95 to 90 190 to 90 225 to 190 320 to 225 370 to 320 425 to 370 500 to 425 575 to 500 c. 650 to 575

From Phanerozoic earth history of Australia Clarendon, Oxford, 1984

(ed. J.J. Veevers),

The breaks between regimes are at Ma

Ase.

mid-Cretaceous

320

mid-Carboniferous

575

Proterozoic/Phanerozic (Ediacaran/Cambrian)

Regime POTOROO INNAMINCKA ULURU

ADELAIDEAN Each regime is a complex of uniform plate-tectonic and palaeoclimatic events at a similar or a uniformly changing latitude. The Uluru regime is characterized by convergence at the eastern or Paleo-Pacific margin, divergence at the western or PaleoTethyan margin, and by shallow marine (including COo and evaporitic) deposition on the interior platform, all at low latitudes; intense compressive deformation in Eastern and Central Australia during the Early Carboniferous was followed in the Innamincka regime by an initial stratigraphic gap across the continental platform until deposition resumed with widespread subsidence in the Permian in a high-latitude glacial climate (with no significant CO^ or evaporite), again with convergence on the east and divergence on the west but now with non-marine sediment on the interior platform. The Potoroo regime encompassed a change from high to decreasing latitude; the Pacific margin changed from mainly Chilean-type to wholly Mariana-type subduction (by the opening of the marginal basins of the

160

Southwest Pacific), the pattern of seafloor spreading in the Indian Ocean changed, including the inception of spreading between Australia and Antarctica, and in the interior CO3 deposition resumed after mainly detrital deposition. The Australian tectonic regimes reflect those of the other Gondwanaland fragments as expressed along the various generations of the convergent Pacific province, divergent Tethyan-Indian province, and interior or Gondwanan province. The most vivid example is the Gondwana sequence of peninsular India and its equivalents in Australia (Innamincka), Antarctica (Victoria), southern Africa (Karoo), and South America (Santa Catarina) that follow the Late Carboniferous stratigraphic gap. Covering one third of the earth's circumference, Gondwanaland had many latitude zones; e.g., at the other end of Gondwanaland from Australia, northern Brazil was frigid (higher latitude) in much of the middle Palaeozoic and warm (CO3 and evaporites in lower latitudes) in the Permian. Apart from the local effect of latitude, the general cycle is the same throughout Gondwanaland. The Gondwanaland regimes, in turn, reflect those of the entire globe, as part of Fischer's (1984) two Phanerozoic supercycles, each c^ 300 Ma long, one Palaeozoic, the other MesozoicTertiary. Each reflects a cycle of mantle convection expressed through time variation in plutonism and hence CO2 atmospheric concentration and greenhouse/icehouse effects, continental dispersion and aggregation (Pangaea) and eustatic sea level, and sediment type (marine platform sediment during high sea level during dispersion, non-marine during low sea level during aggregation). In this scheme, the Gondwana cycle corresponds to slow mantle convection reflected in lowered plutonism, continental aggregation (in Pangaea), lowered sea levels, icehouse state, and is bracketed by the early-middle Palaeozoic and late Mesozoic-Tertiary cycles of fast mantle convection reflected in higher plutonism, continental dispersion, higher sea level, and greenhouse state. In the Quaternary, the declining plutonism, increasing continental aggregation into a single supercontinent (except Antarctica), lowered sea level, and an icehouse state mark the start of another cycle. Reference Fischer, A.G., 1984, The two Phanerozoic supercycles, in Berggren, W.A. and van Couvering, J.A. (eds). Catastrophes and earth history. 129-150. Princeton University Press, Princeton.

161

The Effect of Chemical Environment on the Experimental Deformation of Quartzite Jiannong Wang Dept. of Earth Sciences, Monash University, Clayton, Vic. 3168 Alison Ord, Bruce Hobbs C.S.I.R.O, Division of Geomechanics, Mt. Waverley, Vic. 3149 ABSTRACT The effect of changing oxygen fugacity (and hence water fugacity and hydrogen fugacity) on strength, rheological behaviour, microstructural development, and development of crystallographic preferred orientations has been investigated by deforming Heavitree quartzite in creep at a temperature of SOO^C, 1.5GPa confining pressure, and through a range of differential stresses from 0.1 to 0.7 GPa. Specimens are contained in silver capsules together with excess water and a solid oxygen buffer. The buffers are Ta/Ta205 and Mn304/Mn203, which produce oxygen fugacities (at SOO^C and 1.5GPa confining pressure) of 1.56 x 10"3l and 2.587 x lO'^ MPa respectively (corresponding to water fugacities of 3.845 x 10'^ and 5.01 X 103 MPa, and hydrogen fugacities of 1.82 x 10^ and 2.00 xlO'^MPa). Specimens deformed under high oxygen fugacities(Mn buffer) are about four to seven times weaker than those deformed under very low oxygen fugacities(Ta buffer)(Fig.l). No change in the stress exponent(n) in

the power law form of flow law with changing environment is observed.

However there does appear to be an indication of a decrease in n from about 3 in the high differential stress range(greater than 0.3GPa) to about 1 in the low differential stress range for specimens deformed under both high and low oxygen fugacities(Fig.2). Subgrains and intragranular recrystallized grains are well developed in the specimens deformed under high oxygen fugacities. There is little recrystallization but good development of deformation lamellae in specimens deformed under very low oxygen fugacities. Deformation lamellae are mainly subbasal in the specimens deformed under very low oxygen fugacities, but mainly prismatic in the specimens deformed under high oxygen fugacities. All specimens deformed under very low oxygen fugacities develop a single maximum c-axis fabrics, parallel to the shortening direction. Under high oxygen fugacities, specimens deformed at 0.1 to 0.5 differential streses develop small circle girdle c-axis fabrics symmetrical about the shortening direction, but the specimen deformed at the highest differential

162

stress of 0.7GPa exhibits a maximum parallel to the shortening direction (Fig.3). The contrasts in rheology, microstructures, and c-axis preferred orientations observed in these specimens deformed in creep indicate that the chemical environment (in this case, contrasting oxygen, water, and hydrogen fugacities) has a very important influence on the deformation of quartzite, analogous to changing temperature and/or strain rate. — 4 5

— 3 0

—15

80 1

TIME

120 1

( l O E + 3 ) SECS)

Fig.l Comparison of strain rate at 0.1 GPa differential stress 0.3

0.1 -3-p

0.5

0.7 (GPa)

i 1 i

-4 n1(Mnl^1.0+/-0.1

n2(Ta)=2.5+/-0.4

-6-1

Fig. 2 Flow behaviour for specimens deformed under high(Mn) and low(Ta) oxygen fugacities. Change in the stress exponent(n) from high differential stresses to low differential stresses, but no change in n with changing oxygen{water) fugacity are indicated.

0 log

(stress)

(GPa)

Fig.3 Comparison of c-axis preferred orientations of specimens deformed at 0.1 GPa differential stress, compression direction vertical, equal area, contours( /1% area): >= 4% >=2% >= 1% Mn buffer 50% strain, 158 grains

Ta buffer 30% strain, 205 grains

163

MYLONITES AND PROBLEMATICAL RELATIONSHIPS ALONG THE EASTERN BOUNDARY OF THE TUMUT TROUGH, N.S.W. Peter Warner, Brian Marshall and Brenda Franklin Department of Applied Geology, University of Technology, Sydney P.O. Box 123, Broadway, N.S.W., 2007. The Tumut Trough occupies the southern portion of the Tumut Synclinorial Zone (TSZ), and consists of variably deformed and metamorphosed remnants of a marine sedimentary trough sequence. The geological development of the Tumut region of the Lachlan Fold Belt has traditionally been interpreted in terms of intra-arc rifting followed by basin inversion to yield the TSZ. This is an autochthonous tectonic model reflecting the notion that the relative position of all rock bodies, other than ophiolite slivers, was the same in the Late Silurian as it is today. Alternative models have included both allochthonous strike-slip and allochthonous thrust tectonic models of terrane amalgamation and accretion. Allochthonous tectonic models have involved the recognition of five suspect terranes. The Mooney Mooney Fault System (MMFS) separates the three terranes identified within the Tumut Zone from the dominantly S-type silicic magmatic Young Terrane to the east. Ophiolitic components isolated within the MMFS are grouped within the Mooney Mooney Terrane. The ophiolite has traditionally been interpreted as a west younging sliver of oceanic crust obducted during basin inversion. It comprises the Coolac Serpentinite, the intrusive ultramafics of the North Mooney Complex and equivalents, and the extrusive metabasics and sediments of the Honeysuckle Beds. The eastern strand of the MMFS is marked by the intermittent development of mylonitic rocks (ranging from protomylonite to ultramylonite) within the margin of the Young Granodiorite, and its associated volcanics. Similar mylonitic rocks occur within the Young Granodiorite along the NS trending Jugiong Shear Zone which branches off the MMFS. Depending on which tectonic model is favoured, the mylonite series rocks are envisaged as having formed either when the Young Terrane was thrust over the TSZ or during strike-slip faulting. Obduction of the Coolac Serpentinite may have also influenced mylonitization along the MMFS. The most intense mylonitization (ultramylonite) does not always occur at the contact between the granodiorite and the ophiolite. Similarly, deformation in the ophiolite is not always most intense at this contact. In the case of the ophiolite, this may be explained in terms of the differing mechanical properties of various components of the ophiolite sequence but no such ready explanation exists in relation to the granodiorite.

164

The elongation lineation in the mylonites is also variably developed. In some mylonites it is an intense structure whereas in some ultramylonites it is only weakly developed. Possible explanations range from differential partitioning of deformation in a single event to overprinting of discrete events. Kinematic indicators provide data on the movement sense at selected locations. The results are integrated with information on deformation intensity and interpreted within the context of the Late Silurian - Early Devonian tectonic development of the Lachlan Fold Belt. Various conformable and intrusive relationships are documented: (i) Incorporation and intrusion of gabbroic rocks from the North Mooney Complex by the Young Granodiorite. (ii) Intrusion of the Young Granodiorite into the Coolac Serpentinite. (iii) Interbedding of trough sequence dacite (Blowering Beds) with eastward younging pillow basalts of the Honeysuckle Beds. (iv) Intrusion of the Young Granodiorite into the Blowering Beds (west of the MMFS). Post-Late Silurian large scale displacements along the MMFS are incompatible with these relationships. Concepts of ophiolite development during the waning of a continental extensional environment are considered to account for: (i) The transition from voluminous acid magmatism to bi-modal volcanism. (ii) The dichotomy of eastward younging bimodal volcanics bounded by intrusive ultramafics and serpentinite to the east. The implications of these relationships for models of the tectonic development of the Tumut region are substantial.

165

STRUCTURE AND SUBDIVISION OF THE TASMAN OROGEN, ON THE BASIS OF THE GRAVITY AND MAGNETIC ANOMALY PATTERN Peter Wellman Bureau of Mineral Resources, P.O. Box 378, Canberra ACT 2601. There has been considerable debate on the positions and significance of the structural subdivisions of the Tasman Orogen, on the extent of these subdivisions under the cover sediments, and whether Precambrian basement is present as a lower crust, present as small remnants, or is absent. These topics can be studied using the pattern of gravity and magnetic anomalies. The gravity and magnetic anomalies have been observed over the whole area. They give three data sets reflecting different depths below the surface. In order of increasing depth they are short-wavelength magnetic anomalies reflecting bodies at the formation level at the surface of the basement, medium wavelength (>20 km) magnetic anomalies, and gravity anomalies. Subdivision of the orogen into terranes has been done on the basis of the dominant trends. Within a terrane the gravity and magnetic anomaly trends are subparallel. The relative age of cratonization of the terranes has been inferred from the direction of the trends within them; a younger terrane has trends parallel with its margin with the older one, whereas the older terrane has trends oblique to the boundary. In detail the younger terrane impresses its trends on the margin of the adjacent older terranes, truncating the older trends. Within the area of the Tasman Orogen the major boundaries are of at least three types. At type one boundaries, the trends of the older terrane are at a large angle to the boundary, but they die out about 20 km from the boundary, because of reworking. There are prominent magnetic and gravity lows on the margin of the older terrane, and prominent highs on the margin of the younger terrane. At these boundaries part of the existing cratonized crust must have been displaced or incorporated in younger terranes, and the process of continental rifting or later accretion has reworked the edge of the existing continent in to a distance of about 100 km. The boundaries of this type include a major boundary along a line between the towns Tumut, Bourke, Roma, to Hughenden, and the boundary between the Thompson Orogen and the Mount Isa and Arunta blocks. At type two boundaries, trends in the new terrane are only at a small angle to the adjacent older terrane, and the width of the anomalies is narrower - in particular the width of the magnetic low is only about 15 km. Again there are prominent magnetic and gravity lows on the older terrane, and highs on the younger terrane, but the gravity high overlaps the edge of of older terrane. A major boundary of this type is the western boundary of the New England Orogen. At this boundary there is apparently little crust removed before accretion, and little evidence for extensive thermal and or mechanical reworking of the older terrane. At type three boundaries there is an angular discordance between the trends of the two terranes, but there are no high-amplitude gravity or magnetic anomalies along the boundary. Boundaries of this type occur in central Victoria. In areas where they outcrop, the boundaries of terranes defined on gravity and magnetic anomalies correspond with geologically defined terrane boundaries. At least at type one and three boundaries, there is a sudden change in basement metamorphic facies at the terrane boundaries, generally with a 70-150 km wide band of

166

greenschist to amphibolite facies on the older terrane, and basement of facies lower than greenschist on the younger terrane. The Tasman Geosyncline can be divided into five major blocks. The oldest may be the Thompson Orogen with northwest trends. This block is not exposed. The next oldest block comprises the terranes in the Murray Basin area. Most of these terranes have north trends in Victoria, curving to east trends in the northeast. Between this block and the Thompson Orogen is an area of high-amplitude anomalies that may be another relatively old terrane, and may be an area of later intracratonic deformation. The next oldest major block has north trends and a western boundary between the towns Tumut, Bourke, Roma and Hughenden. In NSW this block has been fragmented by displacement on east trending transcurrent faults; the style of faulting is consistent with this block being cool at the time of emplacement and brittlely fracturing against a promontory in the Australian Continent. In southern NSW this block is truncated by a NNW trending coastal block that has a boundary near the S-I granite-type line. Both these blocks are adjacent to the New England Orogen. Within this orogen the tectono-stratigraphic terranes are bent at the Texas and Coffs Harbour Oroclines. Trends in the western part of the Tasman Fold Belt are parallel to the margin of the adjacent older Australia. This is consistent with the crust being formed either on extended older crust, or solely by young accreted crust. Trends in the eastern part of the Tasman Fold Belt are not parallel to these early trends. Either this crust was formed after a second extensional phase, or it is new, accreted material. The NSW part of the belt was brittlely deformed along east striking transcurrent faults when it was first deformed against existing Australia. This is consistent with it being cratonized before its accretion onto the Australian Continent. The terrane east of the S-I line and the New England Orogen are now generally accepted as being totally accreted.

167

STRUCTURE AND SUBDIVISION OF THE SOUTH WEST YILGARN AND ALBANY PROVINCE USING GRAVITY AND MAGNETIC ANOMALIES AND GEOLOGY Alan J. Whitaker Bureau of Mineral Resources, P.O. Box 378, Canberra ACT 2601 Extensive regolith and deep weathering have severely hindered the establishment of regional stratigraphy and tectonic understanding of the south west Yilgarn and Albany Province. In the Albany 1:IM sheet area the 10 - 50 km wavelength gravity and aeromagnetic anomalies map the major basement subdivisions. The short wavelength magnetic anomalies map that lithological banding with a strike length over 1.5 km; this banding gives the internal structure of a province and the relationship of the province with its boundaries. The region can be divided into a NNW trending Yilgarn Block, a younger east trending Albany Province to the south and a younger again north trending Perth Basin and its Proterozoic basement to the west. The south west Yilgarn is subdivided into several bands previously grouped in the Southern Cross Province in the east and the Western Gneiss Terrane to the west. The Southern Cross Province contains strike-extensive sinuous magnetic fabric. In the east, magnetic anomalies with high amplitude are known from outcrop to be due to banded iron formations and ultramafic rocks in greenstone belts. Associated trends of low magnetic amplitude are due to sediments and basalts while broader areas of low to weakly magnetic granite separate the belts. Further west, similar sinuous bands of low to medium magnetic amplitude in areas of intermittent granite outcrop are thought to be due to similar crust eroded to a lower structural level. Hence the western boundary of the Southern Cross Province is interpreted here to be at the eastern margin of a NNW magnetic granulite belt (-117 30') and not the western margin of outcropping greenstones located lOOkm further east. The Western Gneiss Terrane is dominantly composed of sparse, short length magnetic fabric, consistent with mapped widespread, poorly laminated quartzo-feldspathic gneiss and granite. Sub-province bands from east to west are, a magnetic granulite belt, a deformed gneiss belt, a zone of quartzo-feldspathic gneiss with little structure and a complex zone with banded gneiss and greenstones. The magnetic granulite belt has strong NNW to N foliation/deformation. In the Archaean, it was part of a broader zone of strong tectonic activity at the boundary between the Southern Cross and Western Gneiss Terranes. A step in the gravity gradient occurs at the western margin of the deformed gneiss belt. Seismic refraction studies show a major change in crustal structure at this line, with higher crustal densities to the south-west. The Albany Province to the south of the Yilgarn is dominated by strong easterly trends. The southern part of the province has weak to moderate magnetic amplitude, weakly sinuous magnetic fabric and is cut by relatively few linears. It is largely composed of granites and quartzo-feldspathic gneiss. The northern part of the province consists of a band of highly magnetic granulite facies rocks. The northern boundary of the magnetic band transgresses the strongly linear internal structure. The band is cut by numerous faults and thrusts. The transcurrent component of movement on E to ENE thrusts and NW trending faults is consistently dextral. In the east of the Albany 1:1M sheet, the easterly trending magnetic belt cuts off ENE trends of

168

the Fraser Province; indicating that thrusting of the Albany Province post-dates that of the Fraser Province. The southern Yilgarn has been extensively deformed along its boundary with the Albany Province. Sub-parallel -east trending linears overprint the Yilgarn's NNW to N trending packages and lithological banding for up to 50 km from the boundary. Demagnetization and total destruction of the margin's Archaean lithological structure extend up to 20 km from the boundary. The gravity boundary between the Yilgarn Block and Albany Province is not simple. In the west, the Western Gneiss Terrane and northern magnetic granulites of the Albany Province are both dense, so the gradient is to the south, situated over the boundary between the dense and light parts of Albany upper crust. In the east, the crust of the Southern Cross Province is light relative to the granulites of the Albany Province so the gravity gradient is to the north and situated over the boundary between the Yilgarn Block and Albany Province. The western boundaries of the Yilgarn and Albany Province are mapped by a change from short wavelength magnetic fabric to very smooth fabric over the Perth Basin. The boundary corresponds with the position of the Darling Fault. The northern magnetic belt of the Albany Province is folded southward parallel to this fault. A major gravity gradient is coincident with the magnetic boundary and strong gravity lows occur over relatively low density Perth Basin sediments. Numerous linears cut the Yilgarn and to a lesser extent the Albany Province. They are due to dykes and demagnetized fault zones; the dykes are variably magnetized with normal or reverse polarity. Movement on faults may only be inferred where they dislocate well defined compositional banding such as in the greenstone and metamorphic belts. Granite bodies occurring in both the Yilgarn Block and Albany Province can be accurately mapped using magnetics and they generally cause local gravity lows. The granites are of a wide range of types, varying in their magnetization, in the presence of a highly magnetic rim, in their shape, and their radioactivity. The magnetics are particularly useful in mapping the extent of migmatite (mixed gneiss and granite). Integrated magnetic, gravity and geological interpretation provides a significant advance in understanding the distribution of basement lithologies and past tectonic interaction.

169

(OOGENESIS IN THE YILGARN: THE CRITICAL ROLE OF PO^HYROBLASTS IN GBOMEIRIC ANALYSIS Colin Wilkins Department of Geology, James Code University To/nsville Qld 4811 A detailed structural analysis of the Big Bell Greenstone Belt was conducted to ccmpare the orogenic development within the Murchison Province with the more extensively studied Eastern Goldfields Province of the Archaean Yilgam Block, Western Australia. A traditional gecmetric analysis was combined with a microstructural analysis using inclusion trails in prc^hyrcblastic minercils and their relationship to external foliations, in order to evailuate the sequence of structural events that affected the greenstcxie belt and to compare and contrast the information gained from each approach. In principal, the geometric analysis should have provided all the information required to determine the sequence of structural events affecting the rocks. However examination of foliatioi overprinting relationships indicated that the matrix has been affected by extensive recovery and recrystallization and that evidence for some of the deformaticxi history has been obliterated. This microstructural approach enabled foliatiois be correlated consistently from porphyrcblast to porj^yrcblast and between thin sections cut from rocks collected many kilometres apart. Six stages of deformation are recognized but oily three have associated meso- or macroscopic folds. Furthermore, one of these three fold jdiases, D4, could only be allocated a positioi within this sequence after a careful inspection of inclusion trail textures and matrix foliations in oriented thin sections frxDm porphyroblastic rocks throughout the belt. Shallowly-dipping D4 structures were folded and rotated by an upright macroscopic D5 event that caused intense reactivatioi of lithological layering. This tended to destroy the mesoscopic relationship between stages D4 and D5. Ibis study has shown that the earliest stages of deformaticxi are poorly represented as meso- and macrosccpic structures, as well as foliations in the matrix of thin secticxis. However, foliations due to stages D1 to D5 and their overprinting relaticxiships are all preserved as inclusion trails in porphyrc±>lastic minerals. Transcurrent shear zones alcxig the granite-greenstone contact form the final deformation stage in the belt. The structural history of the Big Bell Greenstcne Belt is discussed in terms of the orogenic model proposed by Bell and Jdinson (1989), and compared with traditional Phanerozoic orogenic belts (subductioi zone, continent-continent collision suture zcxie and ensialic marginal basin). Ihe attitude of SI and S2, which are preserved as inclusion trails in porj^yrdDlasts, can oily be inferred. S3 and S5 are related to upright fold phases caused by E-W compression. Whereas S4, vAien unfolded, is related to N-directed thrusting. The early history documented in the Big Bell Greenstone Belt has not previously been recorded from other areas in the Yilgam Block and thus has considerable significance to the development of Archaean granite-greenstone belts. References: Bell T.H. & Johnson S.E. (1989), Porphyroblast Inclusion Trails: The Key To Orogenesis. In Press, J. MCTAMORPHIC GEDL.

170

Fault Zones and Gold Mineralisation^ Stawell, Victoria^ Australia.

Thomas M. Will Department of Geology, The University of Melbourne, Parkville, Victoria 3052, Australia.

The Stawell goldfield in which the Magdala Mine is situated is located in the western portion of the Palaeozoic Lachlan Fold and Thrust Belt. In the Magdala Mine, lower to middle greenschist facies Cambro-Ordovician turbiditic sequences are in nonconformable and tectonic contact with late Proterozoic to early Cambrian Footwall metavolcanics. The Footwall sequence consists of basalt flow units, pillow basalt layers, interlayered interflow sediments and flow top breccias. Geochemical data from the tholeiitic metavolcanics indicate a composition transitional between island-arc tholeiites and mid-ocean ridge basalts. Thus, the metavolcanics are interpreted as having been erupted in a back-arc basin setting. The structures in the Stawell goldfield display a transition from early ductile to brittle-ductile and late brittle deformation. North-easterly dipping zones of volcaniclastic and turbiditic material in the Footwall sequence (Waterloo structures) display a strongly sheared fabric and occur mainly in fault contact with the metavolcanics. These zones are sites of high shear strain concentrations as indicated by the S-foliation defining layer silicates which have often been rotated into a sub-parallel position with respect to the C-planes. Strain estimates by use of the Rf/O technique give a tectonic strain of 4.9:3.2:1 and define a flattening strain with a k-factor of 0.25. Gold mineralisation coincides with the brittle-ductile transition and is confined to the complexly deformed western side of the "Magdala Anticline" and occurs either in reverse brittleductile shear zones ("ramps") following the contact between the Hangingwall turbidites and the Footwall metavolcanics and their associated strain accommodation features or in subhorizontal dilational openings which connect individual "ramp" segments. Continuing deformation leads to the formation of brittle reverse, and due to a progressive change in the orientation of the paleostress field to the formation of both, sinistral and dextral, oblique strike-slip faults which complicate the ore-body geometry. The moderate to steep (40^-70^) south-westerly dipping "ramps" exhibit high concentrations of strain and are connected by subhorizontal "flats" or fault jogs with little subhorizontal displacement on them. These fault jogs are sites of low stress concentration and the loci for dilational openings and repeated

171

cycles of hydraulic fracturing. Some of the "flats" are tens of metres wide and reveal numerous generations of syntaxial veins. Local folding of the veins associated with these faults suggests strongly that their deposition occurred when the brittle-ductile shear zone was active. The different generations of subhorizontal veins indicate repeated stages of precipitation of vein material into the dilational zones. This suggests a changing pore fluid pressure with time. The variations in pore fluid pressure are probably due to stress cycling which in turn led to multi-stage seismic events. Using thermodynamic data the peak-metamorphic conditions have been calculated to be 1.7 + 0.7 kbar (2a) and 450±20 ^C. The average pressures calculated are not very sensitive to the fluid composition. Between X C 0 2 = 0.35-0.90 the average pressures are extremly uniform and show the smallest errors at about 0.5-0.7 XCO2. The origin of the ore-bearing fluids is probably of a metamorphic nature. Prograde devolatilization reactions at depth release fluids which intrude cooler country rocks at higher stratigraphic levels. During upward migration the fluid dissolves gold from the host rocks^ therefore becomes more auriferous and is channelised into the structural discontinuites such as the shear zones where the gold finally precipitates in the sites of low confining pressure. Hence^ precipitation and deposition of gold at its current stratigraphic level post-dates the peak metamorphic conditions at this given level. It is important to emphasize that the structural constraints control the locus of mineralisation but it is the physicochemical conditions of the fluid phase and the proper P-T window that controls the deposition of any metaliferous phase.

172

Shear Zones tectonics.

the

Leonora

District,

Eastern

Goldfields:

Style

Archaean

Peter R. Williams and Michael A. Etheridge Bureau of Mineral Resources, GPO Box 378, Canberra, ACT. 2601 Shear zones are the locus of movement between different structural units in the Yilgarn Craton, and most gold mines are located in or near these zones. The nature of several zones of non-coaxial deformation in the Leonora area of the Eastern Goldfields Province, and the regional distribution of these zones shows that two phases of deformation, separated by a phase of upright folding, were responsible for the shear zones. The first phase of deformation resulted in regional east-west oriented shear zones: deformation is localized in narrow quartz mylonite and silicified shale horizons. This phase is nappe style, originally gently dipping movement zones predominating. The second phase of d e f o r m a t i o n r e s u l t e d in u p r i g h t , n o r t h t r e n d i n g s h a l l o w l y p l u n g i n g long wavelength folds. These were re-activated, and a new set of folds and faults formed during the third, regional northwest trending sinistral strike-slip deformation which now dominates the rock distribution in the Leonora district. Movement directions on the early shear surfaces indicate top-side-south motion to the west of the major late shear zone (Mourit George shear zone) , but a tentative top-side-north sense is indicated in the east. The change of movement pattern, scale of the shear zone, and intensive silicification in the zones suggest that the late zones are very large, possibly crustal scale structures. The timing of granite intrusions provides a constraint on models for the e v o l u t i o n of the P r o v i n c e . Granite intruded earlier than nappe-related s t r u c t u r e s in the w e s t e r n part of the d i s t r i c t . Mylonitic orthogneiss consisting of low-grade (upper greenschist facies) assemblages, was formed during this event. The lack of melt p h a s e s t r u c t u r e s , t o g e t h e r with interleaving of granite and amphibolite in these mylonitic zones precludes synchronous deformation and intrusion. In the east, large circular plutons of hornblende biotite granite have a marginal foliation parallel to the pluton outline, and the regional upright cleavage is warped around the plutons. The granite foliation is a sub-solidus structure, defined mainly by recrystallized quartz grains, and the plutons are thus inferred to have intruded prior to the late cleavage. They show no earlier fabric, and therefore postdate the nappe structures. No unequivocal syn-wrenching granites have been mapped, although dolerite magmatism may have occurred during this event. Granite and porphyry dykes formed on early nappe surfaces by in-situ melting, and also as late tectonic intrusions. A suite of syenite and porphyry is undeformed and probably postdates the wrench tectonics. This scheme of granite/tectonic relationships is forming the basis of U-Pb zircon geochronological program jointly with the RSES. The pattern of deformation is repeated elsewhere in the Eastern Goldfields. In particular, features mapped in the Leonora District can be seen as far east as Laverton (approx. 160km), and we infer that the northwesterly tectonic grain to the g r e e n s t o n e s is the result of r e g i o n a l sinistral wrench tectonics superimposed on the earlier nappe-style deformation, previously folded into upright north-trending folds. The pattern of folds and faults between late shear zones may be the result regional compression and rotation of earlier structures during the late wrenching event.

173

EVIDENCE FOR GOLD-BEARING QUARTZ VEIN EMPLACEMENT DURING FLEXURAL-SLIP FOLDING, HILL END REGION, NEW SOUTH WALES Jacqueline Windh, School of Earth Sciences, Macquarie North Ryde, New South Wales, 2109.

University

Gold-bearing quartz veins, and associated alluvial gold shed from these veins, have accounted for about 15 t (500 000 oz) of recorded gold production at Hill End. Further unrecorded gold production, as well as gold production from numerous smaller producers in the area would significantly raise this figure for the Hill End region. The main gold-bearing veins are parallel to bedding, in typical saddle reef-type geometry. Structural characteristics of the veins, observed both in the field and in thin section, indicate that vein emplacement occurred over a protracted period during folding. Bedding-parallel veins have been the main gold-producing veins in the Hill End region, with only minor occurrences of known cross-cutting or fault-associated auriferous veins. The veins are continuous along single beds for hundreds of meters vertically and thousands of meters horizontally. They occur most frequently in well-bedded slate and greywacke units within the "5 km thick Silurian-Devonian sedimentary succession of the Hill End Trough. Associated with the bedding-parallel veins are thin auriferous "leader" veins. Both bedding-parallel veins and leaders were emplaced during the Carboniferous .Kanimblan deformation, which deformed the Hill End succession into tight upright folds with wavelengths of hundreds of meters and a regional shallowly northward plunge, and produced a strong axial planar slaty cleavage. Bedding-parallel veins occur as thick quartz veins at anticlinal crests ("cap veins") that thin downwards along fold limbs (to "leg veins") and generally pinch out before reaching the adjacent synclines. Leg veins are up to 15 centimetres thick, and consist of quartz laminations oriented parallel to the vein walls that contain thin phyllosilicate laminae and steeplyplunging quartz fibres or so-called "slickensides". Leg vein mineralogy consists dominantly of quartz, with significant carbonate (locally exceeding 30% calcite and minor ankerite), chlorite, minor pyrite and gold, as well as local minor concentrations of pyrrhotite, sphalerite, galena and chalcopyrite. Cap veins may be up to 2 metres thick, and consist of white vuggy quartz, with only minor chlorite, carbonate, pyrite and sub-economic gold. Some cap veins contain coarsely spaced laminations similar to those in the leg veins, separated by white vuggy quartz; quartz fibres along these cap laminations may be oriented in any direction. The "slickensides" in the leg veins are actually quartz fibres, oriented approximately perpendicular to the regional fold plunge; these quartz fibres suggest that the veins were emplaced during regional flexural-slip folding. The leg veins may be very planar, with straight, linear quartz fibres oriented parallel to one another in all the laminations of a particular vein. More commonly, however, leg veins show evidence of varying degrees of deformation. They are commonly folded into parasitic folds with

174

wavelengths of several decimetres, with parasitic fold plunges generally close to the regional shallow northward plunge. Folded quartz fibres in veins show a small-circle distribution about, parasitic fold axes. Leg veins are commonly boudinaged, and the boudin axes are parallel to the regional fold plunge. Boudin necks are infilled with coarse-grained white quartz. The coaxial nature of the parasitic folds and boudins to the regional folds suggests that these veins were deformed by the main Kanimblan deformation; these veins were therefore emplaced before the end of the deformation. The planar, undeformed nature of some of the veins suggests that they were emplaced towards the end of the folding event. Bedding-parallel vein emplacement therefore occurred both during the Kanimblan deformation and towards the end of the deformation. The hangingwall laminations of the bedding-parallel veins are commonly more folded than the footwall portions of the veins; this suggests that the hangingwall of the vein is older than the footwall, and that the vein grew by incremental accretion at the footwall side as deformation progressed. Quartz fibres within the leg veins plunge steeply south, but vary systematically from steeply south-plunging on hangingwall laminations to very steeply south-plunging to sub-vertical on footwall laminations. The hangingwall (older) fibres are approximately perpendicular to the regional fold plunge, whereas the footwall (younger) fibres are steeper and at an angle to the regional fold plunge. I suggest that the fibres record a highly oblique vein opening direction, or slip direction. During the early stages of folding this slip direction was perpendicular to a sub-horizontal regional fold axis, and the early-formed fibres pitched sub-vertically in the plane of the vein. As folding progressed, the folds rotated into their present gently north-plunging attitude, and the early fibres rotated into a steeply south-plunging attitude. Flexuralslip folding continued as the fold plunge steepened, resulting in a sub-vertical bedding-plane slip direction oriented at an acute angle to the regional fold axis; later quartz fibres thus plunge more steeply than earlier fibres, and form an acute angle with the regional fold axis. This model adequately explains the observed variation in quartz fibre attitude, and is consistent with the interpretation that the veins accreted at the footwall side. Leader veins are thin (generally <1 to 20 millimetres) quartz veinlets that may contain accessory chlorite, carbonate, pyrite, pyrrhotite, chalcopyrite, sphalerite and gold. They cut cleavage, but are themselves folded and boudinaged, and were therefore emplaced during the Kanimblan folding event. They occur at the hangingwall side of the bedding-parallel veins, and appear to have formed by hydraulic fracturing resulting from high fluid pressures within the bedding-parallel veins. The structural characteristics of both bedding-parallel veins and leader veins in the Hill End region, therefore, indicate (1) that these veins were emplaced during flexural-slip folding, (2) that vein emplacement occurred both during and towards the end of folding, and (3) that emplacement of individual bedding-parallel veins took place over a protracted period of time during folding.

175

The nature and significance of the Hyde - Macraes Shear Zone, in Central Otago, New Zealand. C.N. Winsor Geology Department, University of Otago, PO 56, Dunedin, New Zealand The Hyde-Macraes Shear Zone is a regional structural feature in the Mesozoic Otago Schist, South Island, New Zealand. The shear zone has a strike length of about twenty five kilometres, varying considerably in thickness from 5-130 metres and interpreted (Teagle & Norris in prep.) as a thrust duplex with mesoscopic ramping evident. The zone is oriented subparallel to a penetrative schistosity (S2)/ generally shallowly northeast to north dipping. A third deformation event, correlated with the Manorburn Generation (Means 1966), resulted in a coarse crenulation cleavage and asymmetrical, northwest plunging folds. F3 vergence changes across the shear zone indicate the shear is locally situated on the lower limb of an inclined macroscopic Manorburn anticline, although transecting this structure along strike. The shear zone is interpreted to have originated as a thrust within the brittle-ductile transition and had a subsequent history within the brittle field during uplift of the Otago Schist. An initial direction of movement may be inferred from the vergence and axial plane geometry of minor folds associated with shear ramps (Teagle & Norris in prep.). This direction varies considerably along the shear (through about 60-70^), implying either deformation of the shear or more than one movement phase. A regional warp in the pervasive stretching direction, in the schist to the southwest of the shear, is considered to have deformed the shear. Recovering the effect of this warp restores the shear to a near constant orientation over its length, although the movement direction still varies markedly. The inference is made that shear zone segments underwent thrusting after the regional warping with different movement directions. Minor shears, generally moderately to steeply north dipping within the shear zone may be related to the initial thrust movement, although they have generally remained undilated. They can alternatively be associated with later normal movement as secondary riedel shears. Many minor shears display a reverse movement sense, however others show normal displacement. Three movement episodes are believed to have occurred on the shear: 1) thrusting followed by regional warping, possibly associated with a northeast trending fine crenulation and minor kinks; 2) further thrusting and 3) normal movement. Slickensides trending N-S at Round Hill are

176

interpreted to be related to late stage normal faulting. The Hyde-Macraes Shear Zone is an important zone into which gold mineralization has been structurally focussed. Dilation sites within the shear as recognised by Teagle & Norris (in prep) are related to thrusting and the later structural history. Mineralized quartz veins transect F3 macro- and mesofolds indicating mineralization post F3, possibly late-D3^ This timimg may be inferred from the association between veins and F3 folds, plus the strong alignment and sheared nature of some mineralized quartz. An ore shoot at a high angle to the shear within the Round Hill Deposit (McKeag 1987) may represent a ramp, anticlinal stack or enhanced dilation subnormal to F3 axial planes. The recognition of other Hyde-Macraes type shear zones in eastern Otago is in an inital stage, however preliminary observations, primarily from other mineralization sites, suggest mesoscopic low angle shears subparallel to the schistosity are more common in the region than previously recognised. They may not, however, always be related to mineralized veins and sometimes are undilated, although they often have associated structural features supporting reverse movement. Other gold lodes in eastern Otago may be considered (Craw & Norris 1988) as up-dip equivalants of the Hyde-Macraes Shear Zone and as such could have been through the thrust episode evident in that zone. Alternatively they may have only been affected by the final movement stages, i.e. late stage normal displacement during uplift of the Otago Schist (McKeag & Craw in press).

References Craw, D., and Norris, R.J., 1988. Metallic mineralisation in low-grade metasedimentary rocks in a collisional renvironment: Otago & Alpine Schist, New Zealand. Proc. of conference: New England Orogen Tectonics and Metallogenesis. The University of New England, Armidale N.S.W. Mc Keag, S.A., 1987 Round Hill gold and scheelite mine Otago, New Zealand. Unpubl. MSc thesis University of Otago. McKeag, S.A., and Craw, D., in press. Contrasting fluids in goldquartz vein systems formed progressively in a rising metamorphic Otago Schist, New Zealand. Econ. Geol. Means, W.D. 1962. Mesoscopic structures and multiple deformation in the Otago Schist. NZ J. geol. & geophy. 6, 801-816. Teagle, D.A.H., and Norris, R.J. in prep. The structural controls of gold mineralization in the Hyde-Macreas Shear Zone, East Otago, New Zealand.

177

Numerical Simulation of the Buckling Development of Elastic-Viscous Folds Yanhua Zhang Dept. of Earth Sciences, Monash University, Clayton,Vic.3168 Bruce Hobbs, Alison Ord CSIRO, Division of Geomechanics, Mt. Waverley, Vic.3149

ABSTRACT A finite-difference simulation code FLAC ( Cundall and Board, 1988 ) has been employed to investigate the buckling process for elastic-viscous layers. The model ( 120 metres thick by 205 metres long ) consists of a single layer ( 4 metres thick ) with high viscosity embedded in a matrix of lower viscosity. The high viscosity layer is seeded with initial periodic irregularities with wavelengths corresponding to those predicted by

the wavelength

selection equation of Biot ( 1957, 1961 ) ( Fig.l ). The model is subjected to layer-parallel compression. The results of the modelling for a viscosity contrast of 10:1 show that wavelength ( Fig.2 ), limb dip angle, amplitude ( Fig.3 ), layer thickness and number of buckles within the high viscosity layer vary with progressive strain. Up to about 22 percent overall shortening, the growth rates of amplitude and dip angle are very slow; wavelength decreases and thickness increases with increasing shortening. Between 22 and 25 percent overall shortening, the growth rates of both amplitude and dip angle markedly increase, wavelength stays constant and thickness continues to increase. At 25 percent overall shortening, a sharp and rapid change occurs within a small strain increment. The wavelength is almost doubled, amplitude and dip angle increase from 1 to 5 meters and from 7 to 35 degrees respectively, average thickness is decreased ( due to late stage extension in the high viscosity layer ),and the buckle number is decreased from initially 6.5 to a final value of of 3.5 full waveforms consistent with wavelength change ( Fig.4 ). It is concluded that the wavelength selection theory is only effective for strains below

25 percent. At higher strains, wavelength is

highly sensitive to changes in strain. The results of more

178 complicated models and models dealing with the Mohr-Coulomb and power law materials are also presented.

STRAIN ( % ) FIGURE 1. INITIAL FINITE DIFFERENCE MESH OF THE MODEL.

FIGURE 2. PLOT OF WAVELENGTH AGAINST STRAIN.

20 STRAIN ( % )

FIGURE 3. PLOT OF AMPLITUDE AGAINST STRAIN.

FIGURE

FINAL CINFIGURATION OF THE MODEL.

References Biot, M.A., 1957. Folding instability of a layered viscoelastic medium under compression. Proc. R. Soc. London, Ser. A, 242: 444-454. Biot, M.A., 1961. Theory of folding of stratified viscoelastic media and its implications in tectonics and orogenesis. Geol. Soc. Am. Bull., 72:1595-1620. Cundall, P.A. and Board, M., 1988. 6th Int. Conf. on Numerical Methods in Geomechanics. Innsbruck, Austria., 11-15 April.

ADDENDUM

STUDIES OF CRYSTALLOGRAPHIC ELECTRON MICROSCOPY

FABRICS

QUARTZITEK

TRANSMISSION

Martyn R. Drury^, John F. Savage^ and Lucie E,P, Wenmakers^ 1. Research School Canberra ACT 2 601»

Chemistry,

Australian

National

2.1nstituut voor Aardwetenschappen, Rijksuniversiteit Budapestlaan 4, 3584 CD Utrecht, The Netherlands

University^

Utrecht,

Transmission electron microscopy is seldom used in studies of crystallographic fabric development mainly because of the small volumes that can be measured. Despite of this drawback TEM can provide unique information on the correlation between individual grain structure, orientation and defect content* In addition discriminatory fabric measurements on ultra-finegrained rocks can only be made by TEM. In this study the practical problems and feasibilty of fabric analysis by TEM have been re-examined and a preliminary study made of defect substructures associated with particular quartz fabrics. The microscopy techniques used and preliminary results have been described elsewhere (Drury 1987, ultramicrocopy 21,189; Wenmakers & Drury 1988, Ultramicroscopy) * Two quartzites have been studied; an ultra-fine-grained highly deformed chert from Portugal and a quartz myionite from the Pyrennes. Orientation measurements of 40 grains in the deformed chert show a uniform fabric that is statisically indistinguisable from a random distribution of [c] axes. Simple allignments of slip planes or directions across grain boundaries do not occur. About 25% of the intergrain orientation relationships across grain boundaries correspond to the "special orientation" Coincident Site Lattice (CSL) rotations determined by McLaren (1986) for quartz* The microstrcture, grainsize (1-5 ^m) and lack of strong fabric are indications that deformation of the chert involved substantial grain boundary sliding with a significant component of dislocation creep. The occurrence of distinct intergrain orientation relationships in a rock with a uniform fabric suggests that some form of orientation c o m p a t a b i l i t y b e t w e e n grains plays a role in c o n t r o l l i n g local grainscale fabrics during '^superplastic" deformation. The quartz myionite has a strong asymmetric cross girdle [c] axis fabric, which is reproduced by the measurement of only a small sample of grains by TEM, TEM measurements of <a> axes orientations shows a great circle distribution with a maximum in the XZ plane from the lineation (X)Orientation relationships across grain boundaries were measured to determine the degree of slip system allignment between adjacent grains• Close allignment of the <a> slip directions occurs for only 7 out of 23 grain pairs measured. Three allignments were sub-parallel to Y with the

rest sub-parallel to the main <a> maximum. Despite the average misallignment of slip directions, the occurrence of some dislocations apparently crossing grain boundaries suggests that slip transfer may be possible across general grain boundaries in quartz. Measurement of dislocation ciensities (p) showed no strong dependence on orientation. All grains had p in the range 1.4 - 5,6 x 1012,^1-2. There was a tendency for old unrecrystallised grains and new recrystallised grains orientated for rhomb and prism slip to have a higher p « 2.5-5,6 X 10^2.jn-2 than new grains orientated for basal slip p = 2 . 2 - l , 4 x 10^2,^-2_ Analysis of dislocation Burgers vectors showed that multiple slip in all three <a> directions occurred in all grains analysed, including those orientated for easy single slip. Within old ribbon grains a few intragranular dislocation free grains were present, formed by static recrystallisation, which involved orientated nucleation, with the [c] axis of the new grain parallel to an <a> axis of the old grain (Japan twin law^ Molaren 1986) . The TEM observations on the quartz mylonite are consistent with multi-slip models of fabric development where grain boundaries generally act as obstacles to slip. The weak dependence of dislocation density on grain orientation indicates a limited role for dynamic migrationrecrystallisation in fabric development. The occurrence of orientated nucleation during static recrystallisation can have an important influence in modifying the deformation induced fabric.