Abstracts No.36: SGTSG Field Conference, 1994, Jindabyne

Geological Society of Australia

ABSTRACTS Number 36

SPECIALIST GROUP IN TECTONICS & STRUCTURAL GEOLOGY FIELD CONFERENCE DEFORMATION PROCESSES IN THE EARTH: FROM MICROCRACKS TO MOUNTAIN BELTS

JINDABYNE 6-12 FEBRUARY 1994

DEFORMATION PROCESSES IN THE EARTH FROM MICROCRACKS TO MOUNTAIN BELTS FIELD CONFERENCE AT THE STATION RESORT, JINDABYNE, NEW SOUTH WALES. FEB 6-11,1994 SPECIALIST GROUP IN TECTONICS AND STRUCTURAL GEOLOGY, Geological Society of Australia. Organizing Committee:(Canberra Branch) Richard Blewett (AGSO) Stephen Cox (ANU) John Fitz Gerald (ANU) Russell Korsch (AGSO) Russell Shaw (AGSO) Peter Stuart-Smith (AGSO) with past members Martyn Drury (ANU), Mark Rattenbury and Peter Williams (AGSO)

TABLE OF CONTENTS: Information for Registrants Scientific Programme List of Registrants Abstracts (Oral and Poster Papers) Author Index

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To Cooma 9(

MTSELWYN To Melbourne 540km

SNOWLINE

l v

LAKE JINDABYNE /

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VIC

•

To Canberra 170 km To Cooma 60 km To Sydney 470 km

GUTHEGA BLUE COW PERISHER

JINDABYNE

CHARLOTTE PASS

v

THE STATION RESORT

A

2228 m Mt Kosciusko

^

To Ingebyra

* During Winter Season chains are required inside National Park Entrance •

Park Entrance

/^ANAGRMENT HOUSES

SKI-HIRE

MAIM

STAFF

SttiulMfNlS-

YURTS

STAFF YURTS

iv TENNIS COURTS

CAR PARK

BAKJJO ON CONVEMTIOM ROOMS

STAF^ LAUNDRY

COTTAGES

SPORTS OVAL

INFORMATION FOR REGISTRANTS

SGTSG FIELD CONFERENCE • JINDABYNE FEB '94

SCIENTIFIC PROGRAMME:For this meeting, both oral and poster papers will be presented. This volume contains the full collection of abstracts for the conference, sorted alphabetically according to the first author, and correct at the time of printing. As can be seen in the programme of papers that follows, oral papers will be read mornings and afternoons of Mon, Tue, Thu and Friday. Poster papers will will be displayed Mon, Tue and Wed evenings.

ORAL PRESENTATIONS:The majority of oral papers have each been allocated 15 minutes plus 5 minutes for discussion. Session Chairs will enforce this time limit. Several keynote talks have been scheduled for 25 minutes plus 10 minutes discussion. Check your final timetable (issued at registration) for details.

The oral programme on Mon, Tue, Thu and Friday will commence at 08:30 sharp. All oral papers will be delivered in the RockSki Club in the main Resort building (see map). Session times are somewhat variable, but each session will consist of 3 to 5 papers. Each day is broken into four sessions by coffee breaks and lunches. Again, carefully check your timetables for details. Speakers - It is the responsibility of each speaker to ensure that their slides have been loaded in correct order and orientation. Speakers are requested to contact the projectionist at the back of the lecture hall at least 15 minutes before the start of the session in which they will speak to collect carousel(s) and load their own slides. Please ensure that a label (available from the projectionist) is affixed to your loaded carousels. Session Chairpeople should also be available at this time to acquaint speakers with the setup of lighting, projectors, etc.

POSTER PRESENTATIONSThe three Poster Sessions will be conducted in the evenings in Lawsons Lounge, adjoining the lecture theatre. Poster Boards are numbered 1 to 18. Poster presenters should check in the final programme received at registration for the number of the poster board that they have been allocated. Also, at 8:00pm on Mon and Tue and 08:30pm on Wed evening, each poster presenter has the opportunity to briefly speak to the key point of their poster. This part of the poster session will take place in the lecture theatre (RockSki Club) beginning at the times above. Each presenter has 2 minutes to speak, and the order of presentation will be according to number of allocated poster board. It is suggested that each speaker has one overhead transparency sheet displaying

v

INFORMATION

FOR

REGISTRANTS

the key aspect of their poster.

SGTSG FIELD CONFERENCE - J1NDABYNE FEB '94

Session Chairs will again enforce the 2-minute

limit. No time will be allowed for discussion. Poster presenters should be ready to stand and speak as soon as called upon by the chair of the session. At the end of the 2-minute speeches, the conference will move to view posters in Lawsons Lounge (where a cash bar will operate at subsidized prices). Each poster presenter will be expected to stand by their display until around 9:45pm to field questions and comments.

Each poster will only be erected for 24 hours. Posters are to be attached to the assigned board (velcro only) on the appropriate day, and taken down on the following day to make way for the next group of posters see s u m m a r y below.

The exception to this will be the Wed evening posters -

we expect the day's walk to Kosciusko on Wed might detract from the conference's enthusiasm for these posters. The Wed night session will proceed as scheduled, including the 2 minute speeches, with the normal viewing time and bar arrangements. However, Wed posters will remain up for longer - time has been set aside for further viewing of Wed posters during the day on Thursday. Note that computer posters (all scheduled for Wednesday) might be set up earlier in the week around tables in Lawsons. Summary of key times for Poster Presenters:Time to Dismantle

Poster Session

Time to Erect

Mon

Mon lunch break

Before lunch Tue

Tue

After lunch Tue

Before field-trip Wed

Wed

Late Wed afternoon

Fri lunch break

ADMINISTRATIVE DETAILS:CONFERENCE REGISTRATION:The registration desk will operate on Sun 6th from approx 3pm until 7pm. This desk will be found by the courtyard alongside Happy Jack's Bar, in the main Resort Building. All outstanding fees are payable at the time of registration. Credit card payments can be arranged, but cheques in $A are preferred. Receipts will be issued. ACCOMMODATION:SGTSG has a complex room allocation system to house people for this meeting, as you will all be aware through questions you answered on your registration forms.

Please bear with us during the small difficulties that will probably

develop here - this is all to keep your costs as low as possible.

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INFORMATION FOR REGISTRANTS

SGTSG FIELD CONFERENCE - J1NDABYNE FEB '94

Payment for all accommodation (including that for Fri 11th if required) must be made at registration on Sun 6th. After completing registration formalities, registrants should proceed to the resort Reception (see map) to pick up their room key. All rooms are serviced daily with full change of bedding and towels. Rearrangement of guests will be necessary on Friday morning. Guests leaving on Friday must clear their rooms after breakfast on Fri. Some of those remaining for Fri night will need to relocate in order to keep rooms fully occupied. Arrangements will be announced during the conference - please keep track of these requests and be prepared to finish all clearing and shifting before 08:30am (and this is on the morning after the conference dinner!!!). MID-CONFERENCE FIELD TRIP. On Wed 9th, a trip led by Lesley Wyborn (AGSO) has been arranged to explore the geology and the scenery of the Snowy Mountains High Country near Mt Kosciuscko, Thredbo and Charlotte's Pass (see map). An introduction to this trip will be given late in the afternoon of Tue 8th in the conference lecture theatre please be there to find out final details of arrangements. Conference buses will depart from the main Resort building at 08:30 sharp on Wed 9th and travel into the Kosciusko National Park. Note - leave your rock hammers at home! Packed lunches will be provided on entry to the buses and every tripper should be equipped to carry their lunch as the walk will be a full day affair. Buses return to the Resort around 5:30pm. Walking will be via well-graded tracks. It is likely that two alternatives will be offered - one ca. 20km walk and one ca. 10km walk. The High Country is at an altitude of around 2000m. In February, temperatures are likely to be around 25-30°C, but may be anywhere in the range 0 to 40°C. So, come prepared with hats and suncream for skin protection as UV radiation levels will be extreme if the day is clear. A drinking flask with your own water supply will be necessary. Please bring on the walk some warm and waterproof gear in case of sudden weather change during the day. CONFERENCE ICE BREAKER - SUNDAY 7:15PM:This function, to get the conference under way, will be held in the courtyard of the main resort building alongside Registration. This Spit-roast/smorgasboard meal (together with some beer, cask wines and juices) is included in your registration fee.

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INFORMATION FOR REGISTRANTS

SGTSG FIELD CONFERENCE - J1NPABYNE FEB '94

MEALS:Breakfasts and Lunches for Mon through Fri are all included in your registration fee.

These meals will be taken in the Stockwhip Dining Room in the main

Resort Building. Breakfast times 07:15 - 08:15 each morning.

Lunch times are slightly variable - see your final timetable for details. EVENING MEALS:Mon, Tue and Wed nights are the only times during the meeting you will need to fend for yourself and find some extra dollars. A modest restaurant operates in the Chargrill.

Other venues are available at a range of prices in Jindabyne

township. We request that all diners take note of the start time of the evening poster sessions in planning their arrangements.

To avoid congestion in the

Chargrill, please be seated early. The conference may be able to provide some assistance with transport on these evenings to carry people to the township and back for the start of the posters. CONFERENCE DINNER - THURSDAY 7PM:Dinner, for those registrants who have paid the fee, will be a casual event in the Stockwhip Dining Room at The Station. The fee includes a 3 course meal with table service and is accompanied by wines (2 whites and 2 reds) from the booming Canberra District Wineries. A limited quantity of other beverages will be available. A cash bar will operate if required. BREAKFAST - SAT MORNING:Those remaining at the Station for Friday night will find breakfast in the normal location, but will be required to pay cash for this meal. OTHER FACILITIES:Happy Jack's Bar at The Station is a public bar, and is available in the afternoons and evenings for use by conference registrants. Please respect the rights of other patrons at all times. Laundry and telephone facilities are shown on your Resort map. services, ask at reception. All keys must be handed in to reception before departure.

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For other

INFORMATION FOR REGISTRANTS

SGTSG FIELD CONFERENCE - J1NDABYNE FEB '94

MESSAGES AND ANNOUNCEMENTS:Any messages received for registrants during the meeting will be left on the noticeboard outside the lecture threatre. - please check here if you expect messages. Announcements will also be pinned to this board but this will be after they have been displayed on the overhead projector at the beginning and/or end of some of the oral sessions. SGTSG ANNUAL GENERAL MEETING - THU 10TH:This meeting will run 5:00 - 5:30pm. Two of the main items on the agenda are the venue for the next SGTSG meeting and the formation of the next executive. POST-CONFERENCE FIELD TRIPS:COOMA - Bus transport departs precisely at 08:00 on the morning of Sat 12th to commence this excursion. All requiring this service will meet the bus near the main Resort building. Those taking their own transport to Cooma need to depart shortly afterwards to be ready to board this bus at the Nebula Motel in Cooma at 09:15. Please check the noticeboard for any announcements. WAGGA-TUMUT - Bus transport will depart at 09:00 on Sat 12th for this excursion. Further details will be announced during the meeting - check the noticeboard. TRANSPORT TO CANBERRA - FRI 11TH:There is no bus transport on this evening due to insufficient demand. Check with the organizers and the noticeboard if you desperately need to get back to Canberra on Friday - several cars will undoubtedly be returning and may have spaces available. BUS TRANSPORT BACK TO CANBERRA - SAT 12TH:Bus transport departs at 09:30 bound for Canberra airport and Jolimont Centre. If you have not already requested this service, please do so at registration time, or with the organizers as early as possible during the week.

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PROGRAMME

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

Monday 7 th - Oral Presentations FLUIDS IN DEFORMATION PROCESSES KEYNOTE : The role of water in the evolution of large strike-slip faults J Byerlee Cataclasis of schist and fluid flow in an exhumed Cenozoic fault zone SR White Metasomatism and deformation of the Revenue Granite, Mary Kathleen fold belt, Mt Isa Inlier, NW Queensland T Aslund, N Oliver & I Cartwright Vein and fabric development within quartzo-feldspathic turbidite successions: implications for the role of fluid during deformation DR Gray, RW Gregory & DW Durney SHEAR PROCESSES KEYNOTE : Shear sense indicators in rocks: controversies and new possibilities CW Passchier Folding at very high shear strain in experimental shear zones. PD Bons & MW Jessell Evidence for a sub-horizontal shear zone formed at deep crustal levels in western Fiordland, New Zealand. EJ Hill The significance of the Alpine Fault for crustal deformation during oblique collision RJ Norris Comparision of microstructural indicators of flow kinematics in volcanic rocks and ductile shear zones JV Smith & B Marshall Dynamic microfracturing in sub-rigid feldspar during late stage mylonitisation in the Kashio shear zone, Japan K Michibayashi Strain localisation processes in granite S-C mylonites K Kanagawa & H Yamagishi Long lived shear zones and their relationship to deformation in surrounding country rocks VM Mares The Shacks mylonite zone, northern New England fold belt VJ Morand

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PROGRAMME

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

(Monday cont'd) FAULTS AND SHEAR ZONES Structure and strain analysis of the Wongwibinda Shear Zone, New England Orogen T Farrell Interpretation of shear sense from wrench striations in dolerite: a Tasmanian example. RF Berry Interaction of a regional compressive stress with a collapse caldera: Implications from a detailed fracture analysis of the Tavua volcano, Fiji GBegg

Monday. - Poster Prxswtatigns Low friction during sliding on simulated faults in porous quartz sandstone at hydrothermal conditions. SF Oox Blueschist assemblages controlled by fluid interaction with subduction-related shear zones. RJ Holcanbe, TA Little & CJ Stephens The geometry of fluid inclusions and their influence on deformation and dynamic recrystallization in quartz m/lonites of the Siirplon Fault zone, Swiss-Italian Alps. NS Mancktelcw, EL Johnson & D Grujic Fold development in shear zones: the origin of fold axes parallel to the extension direction in core complexes of southwestern U.S.A. RJ Scott & GS Lister Microstructural and stable oxygen isotopic evidence for externally derived vein forming fluids at Magdala gold mine, Stawell, Western Victoria. B Mapani & CJL Wilson Time relations between wide spread diapirism and the formation of dilational barite veins in the Bunkers Graben, central Flinders Range, South Australia. DPJ Mendis, PR James & RA Both The regional relationship between veining and mineralisation in the Kanmantoo Trough, South Australia. CN Winsor, RG Wiltshire, JK Janz & OG Gatehouse Structural controls on fluid movement and ore deposition at Porgera gold mine, Papua New Guinea. SM Mjnroe Evolution of a shear-hosted gold deposit from the Proterozoic Granites area, Tanami Desert, Northern Territory. G Adams, P Jarres & J Scott

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PROGRAMME

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

(Monday cont'd)

Nucleation and evolution of contrasted shear zone related golddeposits, example frcm MDuska gold mine, Abitibi, Canada. A Belkabir, C Hubert & AC Brown Stratigraphy, sedimentology and provenance studies: Powerful tools in determination of the relative timing of tectonic juxtapositioning and melange development. KS Pound Geometry of the Cambrian Kanmantoo Basin in South Australia and its relationship to different modes of convergence at the Australian and Antarctic paleo-pacific margin. T Flottman, J Foden, P James & S Marshak Upper crustal structure and deformation kinenatics, Tasminides, Australia. DR. Gray Compression and extension in mid-Proterozoic (c.1000 Ma) granulites: Bolingen Islands, east Antarctica. M Hand & P Dirks Extensional tectonics of the Sarrbagawa belt, Central Shikoku, Japan. K Okamoto Ductile deformation of the Ryoke metamorphic rocks along the Median Tectonic Line, central Japan. H Yamamoto & H Tatata Tectonic significance of a major porphyroblastic event during high pressure metamorphism in Alpine times in the Aegean Sea, Greece. A Raouzaios & GS Lister

Tuesday 8th - Oral Pl^serHations GEOCHRONOLOGY IN CRUSTAL PROCESSES KEYNOTE Indo-Asian Deformation Histories from Thermochronometry TM Harrison. A Yin, FJ Ryerson, PH Leloup & P Taponnier

K-feldspar

The 40Ar/39Ar isotopic dating method and its application to the study of the cooling and deformation history of orogenic terranes I McDougall Microstructural geochronology JKW Lee Microstructural processes and their effects on Argon diffusion GS Lister & S Baldwin The denudation of metamorphic rocks in the Basin and Range (USA) core complexes D Foster

•• XII

PROGRAMME

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

(Tuesday cont'd) Temporal and spatial controls of denudation within magmatic arcs: An example from the Peninsular Ranges Batholith, CA M Grove & TM Harrison Thermochronological constraints on the development of metamorphic core complexes in southwestern U.S.A. RJ Scott, GS Lister & DA Foster 40

A r / 3 9 A r and K/Ar thermochronology of the Paparoa Metamorphic Core Complex, South Island, New Zealand TL Spell, I McDougall & AJ Tulloch

Magmatic and tectonic evolution of the southern Gawler craton - New data on timing from U-Pb systematics CM Fanning & LR Rankin Structural, metamorphic and geochronologic evidence of shear zones in the Musgrave Block, Central Aust A Camacho GRANITE EMPLACEMENT Granites, magma migration and pluton growth: How it really works CK Mawer, JD Clemens & G Stephens Diapiric ascent of magmas RF Weinberg & Y Podladchikov Synchronous syntectonic granite emplacement across the South Palmer River region: The role of deformation intensity in the interpretation of timing of emplacement. BK Davis The relationship between anatectic leucogranites, compressional orogenies and extensional tectonics, Southern Omineca belt, Canadian Cordillera SD Carr Granite dykes, stocks and transitional forms: Magma pressure and tectonic stress controls on emplacement styles of the Davys Creek Granite. T Fowler Tectonism, deformation gradients and intrusion styles PG Lennox & TJ Fowler Preparation for Mid-Conference Field Trip Overview - Geology of the Kosciusko Region L Wyborn Thermo-tectonic evolution of the Snowy Mountains: an apatite fission-track study BP Kohn & AJW Gleadow

••• XIII

PROGRAMME

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

Tuesday. - Poster Presentations Analog modelling of crustal deformation along 2D and 3D listric normal faults. J Braun & G Batt Modelling fault block movement in the Snowy Mountains using thermochronology and a map algebra. SJD Cox, BP Kohn, AJ Gleadcw, P Bishop & G Goldrick Analogue models for transfer zones formed between offset rotational normal faults. AP Gartrell & LB Harris A new style of analogue modelling for the extensional reactivation of basement faults and resulting cover sequence defonration. RI Higgins & LB Harris Seismic velocity structure of the northern and southern margins of the Amadeus basin, Central Australia. HflS McQueen Differential stress control on the origin of flame perthite. LL Pryer & P-Y F Robin Gradational change of syn-tectonic recrystallisation from slaty cleavage to schistosity. S Suzuki & Y Nishidono The effect of rigid particles on the strength of synthetic marbles. G Dresen, M Eckhart, B Evans & D Olgaard Cooling rate histories from closure of garnet-biotite geothermcmetry: Evidence for transient events? K Ehlers, B Marmo, S Oussa & R Pcwell Structural and tectonic evolution of the Northern Australian Orogenic Province in N.T. Australia. P Ding Shear criteria in the evolution of the Walker Trough, McArthur KA Plumb Basin, Northern Territory. The geometry of the SE Ophthalmia Fold Belt, Pilbara Region, Western Australia - a combination of two regional fold sets. TM Johnson Extension of the WOodroffe Thrust, Musgrave Block, into Western Australia & Multiple folding in granulites of the Musgrave Block, Western Australia. A7 Stewart Structural and petrological study of a high-grade terrain, northwest corner of the Queen Elizabeth Pluton, Mt Isa Inlier, Australia. P Crcwhurst Deformational/metamorphic fabrics of the Judenan beds in the Molanite valley and tailings dam area, west of Mount Isa mines: Insights into the complex orogenic history of the Mount Isa inlier, Australia. W Huang Superposed folding in the Crystal Creek Block, Mt Isa Inlier, NW Queensland; Paradigm or Paradox. M O'Dea & GS Lister

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PROGRAMME

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

(Tuesday Cont'd) The structural evolution of the Lake Julius area. I Scxraia & GS Lister Thermal and deformational evolution of the Vortex, Mount Isa, Queensland. DJ Young

Wednesday.

Sih

MID-CONFERENCE HELD TRIP - KOSCIUSKO HIKE

Vjedriesd^y. - Poster

PieseritgMgns

Comparison between experiment and conputer modelling of deformed ice. CJL Wilson, Y Zhang & K Stiiwe Computer simulation of single layer buckling and its associated cleavage development. Y Zhang, BE Hokbs & A Ord Fracture mechanics and implications of peridotite-hosted leucocratic dykes, Vamrala mine, Finland. B Marshall & JV Smith Contact metamorphism around the Stawell Granite, Western Victoria. G Xu, R Powell & GJL Wilson Structure of the Anakie iretamorphics, Grass Tree Mountain Area, Clermont, central Queensland. TJ Green, CL Fergusson & IW Withnall Tectonostratigraphy of the central north D'Aguilar Block, southeast Queensland. R Sliwa, RJ Holcorbe & TA Little Three-diirensional geometry of the Bowen Basin north of 26°S. RJ Korsch & JM Totterdell Structural analysis of an area south of Queanbeyan, NSW. L Carson & M Richard Deformation history of flat-lying sedimentary sequences, southeastern Sydney Basin, NSW. H Mennrian & CL Fergusson Structure of the early Paleozoic Wagonga beds, northwest of Batarans Bay, NSW. M Mohajjel & CL Fergusson Structure in and surrounding northern extensions of the (?) Cumbrian Jindalee Group, Wallendbeen, NSW. IB Cooper Proto-Gondwana breakup and a lower-plate margin model as a solution to the Lachlan Qrogen Enigma. MJ Rickard Cn quartz c-axis analysis ky a new grey scale irrage processing technique. A Geiro, G Lister, M Jessell & E Papp

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PROGRAMME

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

(Wednesday cont'd) Grid sketching to aid teaching geological mapping. I Clark & P James GEOrient - an integrated structural plotting package for MSWmdcws. R.J. Holcarbe Desktop imltimedia presentations and simulations in structural geology. P James & I Clark GEOLMAP, A program to plot and manipulate data on maps and stereographic projections in the AutoCAD environment. R Sliwa

Thursday 10th - Oral Presentations DEFORMATION PROCESSES - MOUNTAIN BELTS TO MICROCRACKS Fold-fault relationships in low-angle detachment systems NS Mancktelow & TL Pavlis The evolution of folds and the development of crenulation cleavage BE Hobbs, Y Zhang, H-B Muhlhaus & A Ord Folding during bulk inhomogeneous shortening: BIS folding. TH Bell & A Forde Folds with axes parallel to the extension direction D Grujic & N Mancktelow Structural development during high-grade non-coaxial deformation: An example from the Rauer Group, East Antarctica J P Sims, CJL Wilson, PHGM Dirks & M Hand Mass transfer and microfracturing in gabbroic mylonites of the Guadalupe igneous complex, California B Lafrance & R Vernon Lattice preferred orientation and shape fabric analysis of deformed quartzites H Stiinitz Changes in quartz, calcite and dolomite fabrics in a metamorphic gradient: the Brenner area, Eastern Alps, Austria B Fugenschuh Transition from magma emplacement microstructures and magnetic fabrics. R Panozzo Heilbronner & A Berger

to

solid

state

deformation:

Experimental investigation of the effects of fluid heterogeniety upon the motion of a rigid porphyroblast analogue during bulk inhomogeneous shortening LK Stewart Three-dimensional mechanical models of crustal deformation Jean Braun xvi

PROGRAMME

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

(Thursday cont'd) Analysis and forward modelling of the structural history of the Mr Curly area, Ord River, East Kimberley, WA RK Valenta, NHS Oliver, J Tan & M Jessell The geological framework of northwestern and central Australia from geophysical mapping RD Shaw, CV Reeves, MP Morse & C Tarlowski Dynamic and rheological implications of crustal deformation in Central Australia: Numerical modelling results F Beekman, RA Stephenson & RJ Korsch Integrated structural and geophysical modelling MW Jessell & RK Valenta

Friday 11th - Oral Presentations CRUSTAL DEFORMATION - REGIONAL STUDIES. The evolution of an Archaean mountain belt. RL Hammond, BW Nisbet & CR Williams The Halls Creek Fault Zone: Repeated reactivation of a major North Australian tectonic boundary. IM Tyler, TJ Griffin, RW Page & RD Shaw Deformation and kinematics in the Pilbara granitoid-greenstone terrain, evidence of multiphase deformation events TE Zegers & SH White Structural geology of the Mt Whaleback Mine, Newman, WA J Ronaszeki Structural evolution and geometry of the Adelaide Foreland Fold Thrust Belt - A reassessment based on balanced cross-sections T Flottman & P James Detailed geometry and kinematic evolution of contraction fault complexes and shear zones in the Southern Adelaide Fold-Thrust Belt P James & T Flottman Tectonic development and mineralisation of the southern Gawler Craton, SA AJ Parker Tasmania: A continental ribbon in the Neoproterozoic Pacific Ocean. C McA Powell, PW Baillie & ZX Li A backstripping approach to deformation sequences in the east-central Lachlan Fold Belt I Ingpen

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PROGRAMME

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

(Friday cont'd) Structure of the early Palaeozoic Anakie Metamorphics, Clermont, central Queensland CL Fergusson, TL Green & IW Withnall Asymmetrical lithospheric rifting in L. Silurian: a cause for the Coen Orogeny and the development of the Hodgkinson basin in NE Australia? R Blewett & R Wellman Structure and metamorphism in the Koumac-Balade high pressure schist belt, northern New Caledonia C Streets, T Rawling & GS Lister Tectonic evolution of the Longmen Mountains and Western Sichuan Basin, China CJL Wilson, S Chen, B Worley, D Arne, Z Luo & S Liu REGIONAL SCALE DEFORMATION PROCESSES Earth deformation - the role for the Ocean Drilling Program (ODP) P Symonds Kinematics of the Main Central Thrust Zone, Nepal Himalaya SE Johnson Deformational styles and geometries in granulite terrains; constraints on deformation mechanisms P Dirks

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LIST OF REGISTRANTS

SGTSG FIELD CONFERENCE - JINPABYNE FEB 94

Garry Adams Department of Geology & Geophysics University of Adelaide P.O. Box 498 Adelaide SA 5005 Fax: 08 303 4347 e-mail: gadams@geology.adeiaide.edu.au

Tim Bell Department of Geology James Cook University Townsville QLD4811 Tel: 077 814 766 Fax: 077 251 501 e-mail: githb@jcu.edu.au

Nick Archibald Port Mineral and Mining Services 160 High St Fremantle W.A. 6160 Tel: 09 430 6144 Fax: 09 430 6160

Ron Berry Geology Department University of Tasmania GPO Box 252C, Hobart TAS 7001 Tel: 002 202 456 Fax: 002 232 547

Jo Arnold Department of Geology & Geophysics University of Adelaide Adelaide SA 5005 Tel: 08 303 5841 Fax: 08 303 4347 e-mail: jarnold ©jaeger.geology.adelaide.edu.au Tania Aslund Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 4878 Fax: 03 565 4903 e-mail: taslund @ artemis.earth .monash .edu .au Helena Basden 3 Norfolk Ave Collaroy Beach NSW 2097 Tel: 02 901 8262 Fax: 02 901 8246 Trevor Beardsmore Geology Department University of PNG PO Box 414 National Capital District Papua New Guinea Tel: 675 267 656 Fax: 675 260 369 Graham Begg Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 2083 Fax: 03 565 4903 e-mail: gbegg @ artemis.earth .monash .edu .au Abdelhay Belkabir Dept de Geologie Universite de Montreal Montreal Quebec H3C 3J7 Canada Tel: 514 343 6820 Fax: 514 343 5782 e-mail: belkabia®ere.umontreal.ca

Annette Bingemer Department of Geology University of Adelaide Adelaide SA 5005 Fax: 08 303 4347 e-mail: annetteb@jaeger.geology.adeiaide.edu.au Richard Blewett Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 Tel: 06 249 9713 Fax: 06 249 9983 e-mail: rblewett@agso.gov.au Jan Kees Blom TLL Delft Mijnbouwstr 120 Delft 2628 RX The Netherlands Tel: 015 785108 Fax: 015 784891 Paul Bons Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5763 Fax: 03 565 4903 e-mail: paul@artemis.earth.monash.edu.au Jean Braun Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 5515 Fax: 06 249 0756 e-mail: jean@rses.anu.edu.au Solange Brunet Dept de Geologie Universite de Montreal Montreal Quebec H3C 3J7 Canada Tel: 514-735-8336 Fax: 514-343-5782 e-mail: bmnetso@ere.umontreal.ca

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LIST OF REGISTRANTS Jim Byerlee US Geological Survey 345 Middlefield Rd. Menlo Park CA 94025 U.S.A. Tel: 415 329 4841 Fax: 415 329 5163 Alfredo Camacho Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 3959 Fax: 06 249 0738 e-mail: Alfredo.Camacho@anu.edu.au Sharon Carr Department of Earth Sciences Carleton University Ottawa Ontario K1S 5B6 Canada Tel: 613-788-2699 (ext 4417) Fax: 613-788-4490 e-mail: scarr@ccs.carieton.ca Leesa Carson Geology Department Australian National University Canberra ACT 0200 Tel: 06 249 4303 Fax: 06 249 5544 Shefa Chen School of Earth Sciences University of Melbourne Parkville VIC 3052 Tel: 03 344 7672 Fax: 03 344 7761 Ian Clark University of South Australia - Salisbury Smith Rd Salisbury East SA 5109 Tel: 08 302 5245 Fax: 08 302 5101 e-mail: CLARKI@Salisbury.UniSA.edu.au Bill Collins Department Geology University of Newcastle NSW 2308 Tel: 049 21 5410 Fax: 049 21 6925 Nick Cook Now at: Dept Geology and Geophysics University of New England Armidale NSW 2351 Tel: 067 73 2476 Fax: 067 71 2898 e-mail: ncook@metz.une.edu.au After 20 Feb 1994: School of Earth Sciences Macquarie University North Ryde NSW 2109 Tel: 02 805 7694 or 02-805 8418 Fax: 02-805 8428

SGTSG FIELD CONFERENCE - JINDABYNE FEB 94

Ian Cooper Department of Geology & Geophysics University of Sydney NSW 2006 Fax: 02 692 0184 Stephen Cox Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 4076 Fax: 06 249 0738 Simon Cox VIEPS, Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5762 Fax: 03 565 5062 e-mail: simon@artemis.earth.monash.edu.au W Crowe Department of Geology ANU Canberra ACT 0200 Tel: 06 249 2056 Fax: 06 249 5544 Peter Crowhurst Geology Department (c/- Dr Kevin Hill) La Trobe University Bundoora VIC 3083 Tel: 03 4791273 Fax: 03 479 1272 Brett Davies Aztec Mining Co Ltd P.O. Box 585, Victoria Park WA 6100 Tel: 09 470 1444 Fax: 09 470 1741 Brett Davis Department of Geology James Cook University Townsville QLD 4811 Tel: 077 815 205 Fax: 077 251 501 e-mail: glbkd@jcu.edu.au Gavin Day Department of Geology Southampton University Highfield Southampton S09 5NH U.K. Tel: 44 703 59 5000 Fax: 44 703 59 3052 Lisa Dell'Angelo Geologisches Institut ETH Zentrum Sonneggstrasse 5 Zurich CH-8092 Switzerland Tel: 41 1 56 3709 Fax: 41 1 252 7008 e-mail: lisa@erdw.ethz.ch

XX

LIST OF REGISTRANTS

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

Puquan Ding North Flinders Exploration 28 Greehill Rd Wayville SA 5034 Tel: 08 271 4355 Fax: 08 373 1213

Terry Farrell Department of Geology University of Newcastle University Drive Callaghan NSW 2308 Tel: 049 215 411 Fax: 049 216 925

Paul Dirks Department of Geology, Institute of Earth Sciences Utrecht University P.O. Box 80.021, Budapestlaan Utrecht 3508 TA The Netherlands Tel: 31 30 531 199 Fax: 31 30 535 030

Chris Fergusson Department of Geology University of Wollongong Locked Bag, 8844 South Coast Mail Centre NSW 2521 Tel: 042 213 860 Fax: 042 214 250

Georg Dresen Department 3.2 Geoforschungszentrum - Potsdam Telegrafenberg A26 14473 POTSDAM F.R.G. Tel: 49 331 310234 Fax: 49 331 310601 e-mail: dre@gfz-potsdam.de Dr Andrew Duncan School of Applied Geology Curtin University of Technology GPO Box U 1987 Perth WA 6001 Tel: 09 351 7972 Fax: 09 351 3153 e-mail: iduncana@info.curtin.edu.au Karin Ehlers School Earth Sciences Melbourne University Parkville VIC 3052 Tel: 03 344 7221 Fax: 03 344 7761 Martina Elburg Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5763 Fax: 03 565 4903 e-mail: marlina@artemis.earth.monash.edu.au David Esser Placer Expl. Ltd. GPO Box 558 Brisbane OLD 4001 Tel: 07 221 1944 Fax: 03 229 2482 Tim Evans 1171 Geelong Rd Mt Clear VIC 3350 Tel: 053 302655

John Fitz Gerald Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 4176 Fax: 06 249 0738 e-mail: jdf152@cscgpo.anu.edu.au Thomas Flottman Department of Geology The University of Adelaide Box 498 Adelaide SA 5001 Tel: 08 303 5493 Fax: 08 303 4347 David Foster VIEPS, Department of Geology La Trobe University Bundoora VIC 3083 Tel: 03 479 1516 Fax: 03 479 1272 e-mail: dfoster@mojave.latrobe.edu.au Tom Fowler P.O. Box 199 Bendigo VIC 3550 Tel: 054 447 375 Fax: 054 447 777 Bernhard Fugenschuh Geological Institute ETH-Zurich Sonneggstr 5 Zurich CH-8092 Switzerland Tel: 01 256 3637 Fax: 01 252 7008 A.P. Gartrell Dept Geology and Geophysics The University of Western Australia Nedlands WA 6009 Tel: 09 380 1922 Fax: 09 380 1037

Mark Fanning Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 5507 Fax: 06 249 0738

xxi

LIST OF REGISTRANTS

SGTSG FIELD CONFERENCE - JINDABYNE FEB '94

Alia Geiro Australian Crustal Research Centre Dept Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5769 Fax: 03 565 4903 e-mail: alia@artemis.earth.monash.edu.au

T Mark Harrison Dept Earth & Space Sciences UCLA Los Angeles CA 90024 U.S.A. Tel: 310 825 7970 Fax: 310 825 2779 e-mail: tmh@argon.ess.ucla.edu

R.A. Glen Geological Survey of NSW Box 536 St Leonards NSW 2065 Tel: 02 901 8346 Fax: 02 901 8256

Doug Haynes Exploration Division - Minerals Western Mining Corp PO Box 157 Preston VIC 3072 Tel: 03 480 4777 Fax: 03 416 8606

Dave Gray Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 4876 Fax: 03 565 4903 Tim Green Department of Geology University of Wollongong Locked Bag, 8844, South Coast Mail Centre NSW 2521 Tel: 042 213 841 Fax: 042 214 250 Marty Grove Dept Earth & Space Sciences UCLA Los Angeles CA 90024 U.S.A. Tel: 310 825 7970 Fax: 310 825 2779 e-mail: marty@argon.ess.ucla.edu Djordje Grujic ETH Zentrum, Geologisches Institut Sonneggstrasse 5 Zurich 8092 Switzerland Tel: 01 256 3691 Fax: 01 252 0819 Ron Hackney RSES ANU Canberra ACT 0200 Tel: 06 249 2056 Fax: 06 249 5544 Rod Hammond Orpheus Geoscience Pty Ltd 1st Floor, 681 Murray St West Perth WA 6005 Tel: 09 322 3318 Fax: 09 324 1195

Renee Panozzo Heilbronner Geological Institute Bernoullistr 32 Basel CH-4506 Switzerland Tel: 0041 61 267 2880 Fax: 0041 61 267 3613 e-mail: heilbronner@urz.unibas.ch Simon Hewson Department of Geology James Cook University Townsville QLD4811 Fax: 077 251 501 R.I. Higgins Dept Geology and Geophysics The University of Western Australia Nedlands WA 6009 June Hill School of Earth Sciences Macquarie University Sydney NSW 2109 Tel: 02 805 8370 Fax: 02 805 8428 e-mail: jhill@laurel.ocs.mq.edu.au Mark Hinman Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 Tel: 06 249 9578 Fax: 06 249 9983 e-mail: mhinman@agso.gov.au Bruce Hobbs CSIRO Division of Exploration and Mining Private Bag Wembley WA 6014 Tel: 09 387 0361 Fax: 09 387 1880 e-mail: shirley@per.geomechanics.csiro.au

Martin Hand GPO Box 498 Adelaide SA 5001

xxii

LIST OF REGISTRANTS

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Rod Holcombe Department of Earth Sciences University of Queensland St Lucia QLD 4072 Tel: 07 365 2178 Fax: 07 365 1277 e-mail: rodh@earthsciences.uq.edu.au

Sue Keay Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 3404 Fax: 06 249 0738 e-mail: sue.keay@anu.edu.au

Wanfu Huang Australian Crustal Research Centre Department Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5060 Fax: 03 565 5062 e-mail: whuang@artemis.earth.monash.edu.au

Barry Kohn VIEPS, Department of Geology La Trobe University Bundoora VIC 3083 Tel: 03 479 1516 Fax: 03 479 1272

Ian Ingpen Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 4878 Pat James Department of Geology & Geophysics University of Adelaide P.O. Box 498 Adelaide SA 5005 Tel: 08 303 5254 Fax: 08 303 4347 e-mail: pjames@geology.adelaide.edu.au Mark Jessell VIEPS, Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 4902 Fax: 03 565 4903 e-mail: mark@artemis.earth.monash.edu.au Scott Johnson School of Earth Sciences Macquarie University NSW 2109 Tel: 02 805 7694 Fax: 02 805 8428 e-mail: sjohnson @ laurel.ocs.mq.edu.au Tim Johnson c/- Geology Department The University of Western Australia Nedlands WA 6009 Tel: 09 380 2666 Fax: 09 380 1037 Kyu Kanagawa Dept Earth Sciences Faculty of Science Chiba University Chiba 263 Japan Tel: 81 43 290 2857 Fax: 81 43 290 2859 e-mail: kyu@science.s.chiba-u.ac.jp

Russell Korsch Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 Tel: 06 249 9495 Fax: 06 249 9972 email: rkorsch@agso.gov.au James Lally Department of Geology James Cook University Townsville QLD 4811 Fax: 077 251 501 email: gljl@jcu.edu.au James Lee Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 4176 Fax: 06 249 0738 e-mail: jkl152@cscgpo.anu.edu.au Paul Lennox Department of Applied Geology University of New South Wales P.O. Box 1 Kensington NSW 2033 Tel: 02 697 4809 Fax: 02 313 8883 Peter Lewis NSW Geological Survey G.P.O. Box 536 St Leonards NSW 2065 Tel: 02 901 8372 Fax: 02 901 8256 Gordon Lister Australian Crustal Research Centre Department Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5060 Fax: 03 565 5062 e-mail: gordon@artemis.earth.monash.edu.au A Lucas Department of Geology ANU Canberra ACT 0200 Tel: 06 249 2056 Fax: 06 249 5544 xxiii

LIST OF REGISTRANTS Neil Mancktelow Geologisches Institut ETH-Zentrum Zurich CH-8092 Switzerland Tel: 01 256 3671 Fax: 01 252 0819 e-mail: neil@erdw.ethz.ch Benjamin Mapani Department of Geology University of Melbourne Parkville VIC 3052 Tel: 03 344 7304 Fax: 03 344 7761 Vanadis Mares Department of Geology James Cook University Townsville QLD 4811 Fax: 077 251 501 Brian Marshall Department of Applied Geology University of Technology, Sydney PO Box 123 Broadway NSW 2007 Tel: 02 330 1775 Fax: 02 330 1755 Jo Mawby Department of Geology & Geophysics University of Adelaide P.O. Box 498 Adelaide SA 5005 Fax: 08 303 4347 Chris Mawer MIM Exploration GPO Box 1042, Brisbane QLD 4001 Tel: 07 214 9100 Fax: 07 214 9111 e-mail: ckm@sol.earthsciences.uq.edu.au Ian McDougall Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 4136 Fax: 06 249 0738 John Mclntyre Orpheus Geoscience Pty Ltd 1st Floor, 681 Murray St West Perth WA 6005 Tel: 09 322 3318 Fax: 09 324 1195 Herb McQueen RSES ANU Canberra ACT 0200 Tel: 06 249 5515 Fax: 06 249 0756 e-mail: herb@rses.anu.edu.au

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Hossein Memarian Geology Department Wollongong University P.O. Box 1144 Wollongong NSW 2500 Tel: 042 213 383 Fax: 042 214 250 Katsu Michibayashi Department of Geology James Cook University Townsville QLD4811 Tel: 077 815 052 Fax: 077 251 501 Greg Miles Department of Geology ANU Canberra ACT 0200 Tel: 06 249 2056 Fax: 06 249 5544 Mohammad (Kafshdouz) Mohajjel Geology Department Wollongong University P.O. Box 1144 Wollongong NSW 2500 Tel: 042 281 958 Fax: 042 213 262 Vince Morand Department of Geology Ballarat University College P.O. Box 663, Ballarat VIC 3353 Tel: 053 279 267 Fax: 053 279 144 Stuart Munroe Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 5591 Fax: 06 249 0738 Finbarr Murphy Pasminco Exploration Level 7, 380 St Kilda Road VIC 3004 Tel: 03 288 0473 Fax: 03 288 0211 Phung Nguyen Western Mining Corp P.O. Kambalda WA 6442 Tel: 090 27 6272 Fax: 090 27 6221 Bruce Nisbet Orpheus Geoscience Pty Ltd 1st Floor, 681 Murray St West Perth WA 6005 Tel: 09 322 3318 Fax: 09 324 1195

xxiv

LIST OF REGISTRANTS Richard Norris Geology Department University of Otago P.O. Box 56 Dunedin New Zealand Fax: 64 3 479 7527 e-mail: OUGEOLOGY@rivendell.otago.ac.nz Mark O'Dea Dept Earth Sciences Monash University Clayton VIC 3168 Tei: 03 565 5060 Fax: 03 565 5062 Robin Offler Dept Geology Univ Newcastle University Drive Callaghan NSW 2308 Tel: 049 21 7044 Fax: 049 216925 Kazuaki Okamoto Department of Geological Sciences Okayama University Okayama 700 Japan Fax: 81 86 252 4057

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Eva Papp VIEPS, Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 Fax: 03 565 5062 e-mail: evap@artemis.earth.monash.edu.au A.John Parker Geosurveys Aust PL 18 Highfield Ave St Georges SA 5064 Tel: 08 272 7597 Cees Passchier Inst Geowissenschaften Johannes Gutenberg - University Mainz Mainz 55099 Germany Tel: 49 61 31 39 3217 Fax: 49 61 31 39 4679 Mervyn Paterson Research School of Earth Sciences Australian National University Canberra ACT 0200 Tel: 06 249 2497 Fax: 06 249 0738 Paul Pearson Etheridge & Henley Geoscience

Dave Olgaard Geologisches Institut ETH Zentrum Sonneggstrasse 5 Zurich CH-8092 Switzerland Tel: 41 1 56 3709 Fax: 41 1 252 0819 e-mail: dave@erdw.ethz.ch

Ken Plumb Australian Geological Survey Organisation P.O. Box 378, Canberra ACT 2601 Tel: 06 249 9311 Fax: 06 249 9983

Nick Oliver Department of Earth Sciences Monash University Clayton, VIC 3168 Tel: 03 565 4879 Fax: 03 565 4903 e-mail: noliver@artemis.earth.monash.edu.au

Kate Pound Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5769 Fax: 03 565 4903 e-mail: kpound@artemis.earth.monash.edu.au

Alison Ord CSIRO Division of Exploration and Mining Private Bag Wembley WA 6014 e-mail: ao@per.dms.csiro.au

Chris McA Powell c/- Geology Department The University of Western Australia Nedlands WA 6009 Tel: 09 380 2666 Fax: 09 380 1037

Sharif Oussa School of Earth Sciences University of Melbourne Parkville VIC 3052 Fax: 03 344 7761

Roger Powell School of Earth Sciences University of Melbourne Parkville VIC 3052 Fax: 03 344 7761

XXV

LIST OF REGISTRANTS Bill Power CSIRO Division of Geomechanics e-mail: power@per.geomechanics.csiro.au

Graham Price CSIRO Division of Exploration and Mining Private Bag Wembley WA 6014 Tel: 09 389 8421 e-mail: g.price@dem.csiro.au Lynn Pryer 9-56 Trinculo PI. Queanbeyan NSW 2620 Tel: 06 251 7814 Fax: 06 249 0748 Adamandia Raouzaios Dept Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5060 Fax: 03 565 5062 e-mail: mandyr@artemis.earth.monash.edu.au Tim Rawling Dept Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5060 Fax: 03 565 5062 e-mail: timr@artemis.earth.monash.edu.au Mike Rickard Geology Department Australian National University Canberra ACT 0200 Tel: 06 249 2055 Fax: 06 249 5544 Janos Ronaszeki BHP Iron Ore Ltd P.O. Box 655 Newman WA 6753 Tel: 091 753 644 Fax: 091 752 919 Roye Rutland AGSO c/- RSES Australian National Univ Canberra ACT 0200 Tel: 06 249 3249 Fax: 06 295 0540 Martin Scott Geological Survey of NSW Box 536 St Leonards NSW 2065 Tel: 02 901 8356 Fax: 02 901 8256

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Robert Scott VIEPS, Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 4878 Fax: 03 565 4903 e-mail: rob@artemis.earth.monash.edu.au Russell Shaw Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 Tel: 06 249 9665 Fax: 06 249 9983 email: rshaw@agso.gov.au John Sims School of Earth Sciences University of Melbourne Parkville VIC 3052 Tel: 03 344 5994 Fax: 03 344 7761 e-mail: jsp@mullara.met.unimelb.edu.au Renate Sliwa Department of Earth Sciences University of Queensland St Lucia OLD 4072 Tel: 07 365 2166 Fax: 07 365 1277 John Smith P.O. Box 157 Lismore NSW 2480 Tel: 066 203 650 Fax: 066 212 669 Itta Somaia Dept Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5761 Fax: 03 565 5062 e-mail: sitta@artemis.earth.monash.edu.au Terry Spell Research School of Earth Sciences Australian National University GPO Box 4, Canberra ACT 2601 Tel: 06 249 3406 Fax: 06 249 0738 Randall Stephenson Netherlands Research School Sedimentary Geology Vrije Universiteit De Boelalaan 1085 Amsterdam 1081 HV Netherlands Tel: 31 20 548 4749 Fax: 31 20 646 2457 e-mail: ster@bronto.geo.vu.nl

xxvi

LIST OF REGISTRANTS

SGTSG FIELD CONFERENCE • JINDABYNE FEB '94

Alastair Stewart Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 Tel: 06 249 9666 Fax: 02 249 9983 e-mail: astewart@agso.gov.au

Rick Valenta VIEPS, Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5774 Fax: 03 565 4903 e-mail: rick@artemis.earth.monash.edu.au

Lachlan Stewart Department of Geology James Cook University Townsville OLD 4811 Fax: 077 251 501 e-mail: gllks@jcu.edu.au

A.(Fons) Vandenbeng Geological Survey of Victoria P.O. Box 98 East Melbourne VIC 3002 Tel: 03 412 7811 Fax: 03 412 7803

Caroline Streets Dept Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 5060 Fax: 03 565 5062

Ron Vernon School of Earth Sciences Macquarie University Sydney NSW 2109 Tel: 02 805 8413 Fax: 02 805 8428 e-mail: rhvernon@laurel.ocs.mq.edu.au

Peter Stuart-Smith Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 Tel: 06 249 9293 Fax: 06 249 9983 email: pstuarts@agso.gov.au

Roberto Weinberg Research School Earth Sciences Australian National University Canberra ACT 0200 Tel: 02 249 0318 Fax: 02 249 0738 e-mail: roberto@jason.anu.edu.au

Holger Stunitz Geological Institute Bernoullistr 32 Basel CH-4506 Switzerland Tel: 0041 61 267 3596 Fax: 0041 61 267 3613 e-mail: stuenitz@urz.unibas.ch

Simon Wetherley Dept Geology & Geophysics University of WA Nedlands WA 6009 Tel: 09 380 2666 Fax: 09 380 1037

Shigeyuki Suzuki Department of Earth Sciences Okayama University 3-1-1 Tsushimanaka Okayama 700 Japan Tel: 086 251 7882 Fax: 086 252 4057

Stephen White Geology Department University of Otago P.O. Box 56 Dunedin New Zealand Fax: 64 3 479 7527

Phil Symonds Australian Geological Survey Organisation GPO Box 378 Canberra ACT 2601

Colin Wilkins Department of Geology and Geophysics University of Sydney NSW 2006 Tel: 02 692 3244 Fax: 02 692 0184 e-mail: colin@es.su.oz.au

Jennie Totterdell Australian Geological Survey Organisation GPO Box 378 Canberra ACT 2601 Tel: 06 249 9407 Fax: 06 249 9972

Craig Williams Orpheus Geoscience Pty Ltd 1st Floor, 681 Murray St West Perth WA 6005 Tel: 09 322 3318 Fax: 09 324 1195

Ian M. Tyler Geological Survey of WA Mineral House, 100 Plain St, East Perth WA 6004 Tel: 09 222 3605 Fax: 09 222 3633

Chris Wilson School of Earth Sciences University of Melbourne Parkville VIC 3052 Tel: 03 344 6538 Fax: 03 344 7761

xxvii

LIST OF REGISTRANTS

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Robert Wiltshire Appl Geology - Gartrell School University of South Australia The Levels Pooraka SA 5095 Tel: 08 302 3178 Fax: 08 302 3378 Colin Winsor Appl Geology - Gartrell School University of South Australia The Levels Pooraka SA 5095 Tel: 08 302 3365 Fax: 08 302 3378 Bronwyn Witham 7 Weston St Yarralumla ACT 2600 Tel: 06 249 0593 Fax: 06 249 0738 Lesley Wybom Australian Geological Survey Organisation P.O. Box 378 Canberra ACT 2601 e-mail: lwybom@agso.gov.au Guowei Xu School Earth Sciences Melbourne University Parkville VIC 3052 Hiroshi Yamamoto Institute of Earth Sciences Kagoshima University 1-211-35 Korimoto Kagoshima 890 Japan Tel: 81 0992 858 130 Fax: 81 0992 594 720 David Young Department of Earth Sciences Monash University Clayton VIC 3168 Tel: 03 565 4878 Fax: 03 565 4903 e-mail: dyoung@artemis.earth.monash.edu.au Tanja Zegers Institute of Earth Sciences Utrecht University P.O. Box 80021 Utrecht 3508 TA The Netherlands Fax: 030 537 725 Y Zhang CSIRO Division of Exploration and Mining Private Bag Wembley WA 6014 Fax: 09 387 8642 e-mail: zhang@per.geomechanics.csiro.au

xxviii

ABSTRACTS OF PAPERS

E V O L U T I O N OF A S H E A R - H O S T E D G O L D D E P O S I T F R O M T H E PROTEROZOIC GRANITES AREA, TANAMI DESERT, NORTHERN TERRITORY. Garrv Adams, Pat James and John Scott Department of Geology and Geophysics The University of Adelaide The Granites gold field is located some 550 km northwest of Alice Springs, Fig 1. It consists of four open cut and two underground mines over a strike length of 9 km. The gold field was discovered in 1900 and minor production occurred until 1961. North Flinders Mines Limited commenced exploration in 1983 with production commencing in 1986.

FIGURE 1: Location and generalised geology of The Granites gold field. The deposits are hosted by the Early Proterozoic Mount Charles Beds which host all known gold mineralisation within the Tanami. At The Granites the Mount Charles Beds have been informally divided into three units by North Flinders Mines Limited, namely the 'Footwall Sequence', 'Main Host Unit' and the 'Hanging Wall Sequence'. The Footwall Sequence consists of pelitic schists and semi-pelitic schists. The Main Host Unit is composed of hornblende-cummingtonne and hornblende-almandine schists with carbonate and quartz rich veins interspersed throughout, and is the host to the majority of the known gold mineralisation at The Granites. The Hanging Wall Sequence consists of graphitic, andalusite and quartzbiotite schists. The Granites area has been subjected to amphibolite facies metamorphism and five periods of deformation. The earliest deformation event, Di, is enigmatic due to the strong overprinting relationships of subsequent events. Remnant early Di structures are isoclinal folds and rootless folds. Ding (1993) reported that an early shear fabric was also present. The second deformation event, D2, is not observed at The Granites, apart from possible interference of F3 on F2. Intense D3 deformation produced very tight to isoclinal, steep westerly plunging F3 folds, Fig. 2 (thus bringing bedding to subvertical), and an intense penetrative schistosity, S3. Cleavage vergence relationships of S3 on So show the presence of numerous minor F3 folds, while structural facing in relation to the cleavage vergence is dominantly east. Small scale reversals of the structural facing, as a result of interference of F3 on F2, indicate the presence of early F2 folds. Detachment zones formed at the boundaries of the Footwall Sequence - Main Host Unit and the Hanging Wall Sequence - Main Host Unit. Shearing within the Main Host Unit was intense during D3, while shearing within the two detachment zones intensified during D4. Subvertical, northwest plunging, open-close geniculate F4 folds, Fig. 3, were produced

1

during D4 with an axial planar schistosity, S4, formed only within the hinges of the F4 folds. Apart from minor shearing, the Footwall Sequence was essentially unaffected by D4 deformation as the Footwall Sequence - Main Host Unit detachment accommodated most of the strain associated with this event. D5 and D6 deformation events produced a gentle warp and reverse faulting respectively, and appear to have no role in the gold mineralisation.

FIGURE 2: Folded Hanging Wall Sequence at Shoe, showing F3 interference on earlier F2 folds.

FIGURE 3: Refold affected by brittle faulting, underground at bullakitchie in the Main Host Unit.

Mineralisation at The Granites consists of pyrrhotite, arsenopyrite and pyrite with minor chalcopyrite and gold. The mineralisation occurs as two different types, (1) gold associated with quartz and quartz-carbonate veining which appears to pre-date D4 deformation, and (2) gold associated with arsenopyrite and to a lesser extent pyrrhotite. Both types of gold mineralisation are found only within the low strain pockets between the anastomosing shears. DING, P. and GILES, C., 1993. Geological Setting of Gold Mineralisation in the Tanami Region, Northern Territory, Australia. Proceedings of the International Symposium on Gold Mining Technology, Beijing, June 1993, pp 15-17.

2

METASOMATISM AND DEFORMATION OF THE REVENUE GRANITE, MARY KATHLEEN FOLD BELT, MT ISA INLIER, NW QUEENSLAND.

Tania Asluncl Nick Oliver, Ian Cartwright.

Department of Earth Sciences, Monash University, Victoria 3168.

The Revenue Granite provides an ideal opportunity to study the dynamics of fluid flow in and around a syntectonic granite intrusion. An understanding of the interaction of fluids with granite intrusions is essential for the interpretation of many ore deposits. The Revenue Granite intruded syntectonically into Proterozoic evaporite-bearing calcareous sediments of the Mary Kathleen Fold Belt in the Mt Isa Inlier, NW Queensland. Scapolitisation of the granite and aureole rocks by high temperature, salt-rich fluids occurred shortly after emplacement. Actual fluid pathways can be recognised due to the corrosive nature of the metasomatising fluids, and thus the geometry and relative timing of fluid flow are readily apparent. The country rocks consist of calcsilicate rocks, marbles, siltstones and pelites of the Corella Formation. The sequence represents sediments deposited during the sag-phase of basinforming rifting between 1780-1760 Ma (Page 1983a,b). Calcsilicate rocks throughout the Corella Formation have a significant evaporitic component (Oliver et al 1992). The Revenue Granite intruded as a sill into lower-mid Corella Formation during Di extension (-1740 Ma), probably at depths near 5 km. The sill was subsequently tilted during the regional D 2 deformation (E-W compression, -1550 Ma). However, the Revenue Granite and aureole rocks are characterised by Di fabrics. The spatial distribution and orientation of D] fabrics within the granite suggest the early structural history of the sill was complex. E-W trending folds appear to both pre- and post-date granite emplacement. The earliest fabrics observed within the granite itself comprise a shallow broadly west-dipping foliation with a down-dip stretching lineation. Development of this fabric appears to be restricted to the upper portion of the sill, and includes zones of L-tectonite. At some time during Dj, this foliation became crenulated. Minor partial melting of granite coincided with the late stages of crenulation development. Within the lower and middle regions of the sill, a N-S fabric is apparent comprising a strong, subhorizontal stretching lineation and locally developed steep foliation. Discrete sinistral shear zones with subhorizontal stretch formed subparallel to and synchronously with this N-S fabric. Similar shear zones are observed to cross-cut the E-W trending L-tectonites mentioned above. Some shear zones are deformed by a) small E-W trending (Fj?) folds, and b) large scale D2 folds. The granite and aureole rocks were locally metasomatised during deformation. Metasomatism comprised both albitisation and scapolitisation, +/- pyroxene (both pervasive and as veins). This alteration is associated with L-tectonites in the roof zone of the granite, and with discrete shear zones in the central and lower portions of the sill. Above the sill, the metasomatising fluids preferentially altered dolerite, resulting in remarkably heterogeneous pyroxene-scapolite rocks. By contrast, below the sill, country rock within approximately 50 m of the contact has been pervasively scapolitised. Overprinting relations between metasomatism and structural fabrics suggest the granite and aureole rocks were deforming continuously during metasomatism. Trends obtained from isocon plots of whole-rock chemical data are consistent with scapolitisation of granite. Alteration resulted in depletion of K and Fe, and gains in Na and Ca. The chemical changes associated with metasomatism suggest the fluid was aqueous, saltrich and calcium-bearing. The occurrence of pyroxene in altered granite requires the addition of Fe and Mg. These components were probably obtained through metasomatism of the dolerites. Thus, mafic elements leached from dolerite during scapolitisation may have been

3

transported in the fluid phase back through the granite resulting in the formation of pyroxene. This implies significant fluid circulation. Carbon-isotopes from altered rocks of the Revenue granite and aureole give signatures indicative of a magmatic origin for the metasomatising fluids. 5 C-values obtained from scapolite range from -4.5 to -6.5%o and fall within the 'carbonatite box' for juvenile or magmatically derived fluids. By contrast, sediment-derived carbon characteristically gives a signature of around 0%o, and did not contribute significantly to the metasomatising fluids. Thus, despite the saline and calcic nature of the metasomatising fluids, it appears the fluids were predominantly magmatically-derived. There is absolutely no isotopic evidence for the presence of meteoric fluids. Metasomatism resulted in substantial shifts in the oxygen isotopic composition of all altered rock types within the granite-aureole system. In general, isotopic ratios of altered samples indicate interaction with a fluid with a 5 0 composition of around 9-1 l%o. This is compatible with a fluid of granite origin. Data obtained from the granite show evidence for two distinct trends. Altered granite from the upper part of the sill typically has 5 OWRvalues that are at least 1 %o lower than those of the adjacent unaltered granite. Samples from the middle and lower portions of the sill, however, exhibit trends of increasing rather than decreasing 8 OwR-values. The 8 OwR-values of the altered granite are 1 to 1.5%o higher than the adjacent unaltered granite. One explanation is that the lower values within the upper sill and aureole rocks reflect interaction of the fluids with the overlying dolerites. This also implies fluids were circulated, at least through the upper part of the granite-aureole system. In conclusion, the Revenue Granite intruded as a sill during Di extension. The granite and aureole rocks were locally altered by high temperature salt-rich fluids shortly after emplacement. Deformation was continuous and synchronous with metasomatism, resulting in the development of a wide range of variably oriented fabrics and overprinting relations. Significant fluid circulation and fluid-rock interaction occurred as evidenced by the formation of pyroxene in the granite, and local variations in the oxygen isotopic ratios. Stable isotope data suggest the metasomatising fluids were of granitic origin. It is likely the fluids were sourced from below the Revenue Granite, or possibly laterally. The fluids were preferentially channelled through shear zones in the granite and fractures in the overlying dolerites. Relatively impermeable calcsilicate rocks overlying the dolerites effectively acted as a cap, thus encouraging circulation of the fluids back through the underlying granite and dolerites. 13

9

1 8

18

18

18

References:

Page, R. W. 1983a - Timing of superposed volcanism in the Proterozoic Mt Isa Inlier, Australia. Precambrian Research 21,223-245. Page, R. W. 1983b - Chronology of magmatism, skarn formation and uranium mineralization, Mary Kathleen Queensland, Australia. Economic Geology 78, 838-853. Oliver, N. H. S.; Holcombe, R. J.; Hill, E. J. and Pearson, P. J. 1991 - Tectono-metamorphic evolution of the Mary Kathleen Fold Belt, northwest Queensland: A reflection of mantle-plume processes? Australian Journal of Earth Sciences 38, 425-455. Oliver, N.H.S: Cartwright, I.; Wall, V. J. 1992 - Internal control of fluid compositions in amphibolite-facies scapolitic calc-silicates, Mary Kathleen, Australia. Contributions to Mineralogy and Petrology 111, 94-112.

4

DYNAMIC AND RHEOLOGICAL IMPLICATIONS OF CRUSTAL STRUCTURE IN CENTRAL AUSTRALIA: NUMERICAL MODELLING RESULTS F. Beekman, R.A. Stephenson, S.A.P.L. Cloetingh, Institute of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands, and R.J. Korsch, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Deep seismic reflection profiling and other studies indicate that whole crustal faulting has significantly displaced the Moho in Central Australia. The resulting crustal structure, which is responsible for some of the largest gravity anomalies observed on the continents, has apparently been in place since around the end of the Paleozoic, when the last major orogenic event took place. There is little evidence for major tectonic re-adjustment thereafter. We have calculated, using the finite element method, the gravitational stresses associated with part of the crustal structure of Central Australia, in the area of the Redbank Deformed Zone, based on the results of Goleby et al. (1989; Nature, 337: 325-330). The finite element mesh and adopted crustal structure in the model are shown in the first figure. Dimensions are 200 km long by 60 km deep. Dark shading is mantle; intermediate is lower crust, and the lightest shading corresponds to upper crust. Loading is due to lateral variations in density resulting from displacements of these units in the Redbank Zone. Load cases incorporating a background tectonic stress, suggested in Central Australia by in-situ stress measurements, seismicity, and numerical modelling of the Australian plate, have also been examined. We used an elastic-plastic rheology with each element assigned a yield strength based on its lithology and depth or temperature (given a typical continental geotherm). Strain hardening of 10% was assumed. Y

The maximum (elastic) shear stresses associated with the density contrasts across the Redbank Zone are in the order of a few tens of MPa. These occur in the downthrust lower crustal and, symmetrically, the upthrust upper mantle segments, shown in the second figure, below.

5

The lower crustal layer is also the Theologically weakest part of the model. Although the stress magnitudes are smaller than have been estimated elsewhere (e.g. Lambeck, 1983; Geophys. J. R. astr. Soc., 74: 843-886), they are sufficient that much of the lower crust in the model is in a state of ductile failure, given commonly adopted rheological laws and parameters, with strain localizations in the downthrust segment. The total equivalent plastic strain arising from the gravitational stresses are illustrated in the third figure. The maximum value (corresponding to the darkest shading) is only 0.7%. Maximum vertical surface displacement associated with the plastic strain is less than 300 m subsidence, centred over the upthrust lower crust.

The results imply that the continental lithosphere, in the absence of thermo-tectonic rejuvenation, is strong enough to dynamically support geologically long-lived structures producing very large gravity anomalies such as observed in Central Australia. On the other hand, large segments of the crust in such areas are at their respective plastic failure limits and small perturbations in ambient stress or temperature could lead to further deformation.

6

INTERACTION OF A REGIONAL COMPRESSIVE STRESS WITH A COLLAPSE CALDERA: IMPLICATIONS FROM A DETAILED FRACTURE ANALYSIS OF THE TAVUA VOLCANO, FIJI Graham Begg. Earth Science Dept., Monash University, Clayton, Victoria 3168, Australia Stress analysis using multiple indicators such as fault and dyke trajectories, kinematic slipline analysis of faults, geometric analysis of fracture sets, and seismicity studies, can be used in conjunction with a well defined chronological sequence of structural and magmatic events, to study the stress history of a region in great detail. At the Tavua Volcano, Fiji, a detailed fracture analysis of multiple datasets, including aeromagnetic data, regional mapping information at the volcano scale, and mapping from the Emperor Gold Mine, enables a reconstruction of the stress history of the volcano from the inception of volcanism, through a multiple-stage caldera collapse, to the immediate post-collapse history, and late overprinting. The earliest resolved principal compressive stress fluctuated between two separate orientations, WNW-NW and NNE-NE, with the former becoming dominant from the time of caldera collapse. This horizontal C\ then rotated through NNW to N with time (Table 1). Local reorientation of stress axes occurred around the collapse caldera. The development of heterogeneities such as convergent faults and tilted bedding, resulted in change in orientation of the local extension axes from purely horizontal to a combination of horizontal and vertical during continued horizontal compression subsequent to infill of the caldera. The 5.5 Ma Tavua Volcano is situated on the northern side of the island of Viti Levu, Fiji, and is one of a series of Upper Miocene to Pliocene shoshonitic volcanoes along the ENE trending Viti Levu Lineament. The lineament is believed to be associated with sinistral strike-slip motion along the ENE trending Fiji Fracture Zone to the north of Fiji, which forms part of a complex transform plate boundary between the Pacific and Australian plates. The Tavua Caldera forms the central portion of the volcano. Structural mapping, supported by aeromagnetic data, indicates the most prominent regional fault sets strike NW, WNW, NNE, and E-W. In the Emperor Mine, regional faults with these trends were exploited during oblique normal faulting associated with caldera collapse, segmenting the mine area into discrete tilt blocks. Dyke patterns across the volcano emphasise the prominence of a NW <j\ prior to and during caldera collapse. At the Emperor Mine, analysis of gently-dipping, areally extensive fractures known as flatmakes, that formed after collapse and infill of the caldera, indicates reverse and strikeslip (i.e. oblique slip) movement concurrent with epithermal mineralization. Formation of the flatmakes is consistent with a subhorizontal maximum principal stress. Flatmakes are generally subparallel to, and appear controlled by, bedding in the differently oriented tilt blocks. Pre-existing faults also locally effect flatmake orientation and distribution. The variation of <5\ from WNW through to N-S may be considered supportive for up to 60° anticlockwise rotation of Viti Levu (or conversely, rotation of the stress axes, or both) since the breakup of the outer Melanesian Arc about 5 Ma [3]. If so, then it is possible that the early WNW structures formed parallel to a NNW-N regional a i, and early NNE and NE structures formed parallel to a ENE-ESE regional C\. These two orientations correlate approximately with the movement vectors for the Australian and Pacific plates respectively at this time.

7

This study indicates that in near surface volcanic environments undergoing regional subhorizontal shortening, perpendicular subhorizontal extension is favoured in previously undeformed areas (resulting in steeply dipping faults and dykes), in areas of relatively simple steep fault patterns at low to moderate angles to the compression direction, and/or areas where local gravity driven (caldera) collapse occurs. Where both mechanisms are active, complex fault arrays can develop. The case of the Tavua Caldera demonstrates that following infill of the caldera (effectively removing the local extensional environment formerly represented by the steep-walled caldera) continued shortening may be accommodated by subhorizontal extension associated with existing steep faults and new faulting along anisotropics (dykes), concomitent with subvertical extension associated with initiation of gently dipping thrusts. TABLE 1. Compression History of the Tavua Volcano. Data Source basalt dykes augite andesite dykes biotite andesite dykes flatmake fractures syn/post flatmake faulting late fault slipline late flatmake slipline recent seismicity

Compression Direction 1. WNW - NW 2. NNE - NE NW NW NW - NNE NNW-N NW - NNW NNW-N 1. N (Fiji Platform) 2. NE (north of Fiji)

Age (Ma) TRefl 5.2- 4.6 [1] 5.2- 4.6 [1] 4.5 - 4.6 [1] 4.4- 4.5 [1] 3.7 [1] < 3.7 < 3.7 < 3.7 0 [2] 0 [2]

References 1. Setterfield T.N., Musset A.E., and Oglethorpe R.D.J., 1992. Magmatism and Associated Hydrothermal Activity during the Evolution of the Tavua Caldera: Ar - Ar Dating of the Volcanic, Intrusive, and Hydrothermal Events: Econ.Geol., v.187, 1130-1140. 2. Hamburger M.W., and Isacks, B.L., 1988. Diffuse back-arc deformation in the Southwestern Pacific: Nature v.332, 599-604. 3. Whelan P.M., Gill J.B., Kollman E., Duncan R.A., and Drake R.E., 1985. Radiometric dating of magmatic stages in Fiji: Earth Sci. Ser., Circum-Pacific Council Energy Mineral Resources, v.2, 415-440. 40

39

8

NUCLEATION AND EVOLUTION OF CONTRASTED SHEAR ZONE RELATED GOLD-DEPOSIT, EXAMPLES FROM MOUSKA GOLD MINE, ABITIBI, CANADA. Abdelhav Belkahir*. Claude Hubert * and Alex. C. Brown ** * Universite de Montreal, Dept. de Geologie, Montreal, Quebec, H3C 3J7, ** Ecole Polytechn. de Montreal, Dept. Genie Mineral, Montreal, Quebec,H3C 3A7, Canada Progressive deformation associated with anisotropic rocks occurs primarily in the ductile regime. In this case, shear zones display complex geometry, lateral variation of strain and weak lateral continuity. Economic geologists working within such context are interested in identifying sites of preferential planes of anisotropy that would have been activated during deformation as well as principal directions of opening or dilation areas. Many authors have noted the prevailing role of anisotropy on the localisation of auriferous shear zones. They have shown that an incompetent layer anisotropy hosted within a more competent one is often the loci of shear zone Here we discuss the role of lithological and geometrical factors on layer anisotropy related to three contrasted deformation corridors and their gold-related ore shoots. Because the shear zones commonly form narrow and extended corridors, it is expected that the geometry of lithological interfaces is as important as its strength or competency during the initiation of shear zone. We are also discussing the kinematic evolution of the shear zones. This evolution is strongly affected by local geological parameters (lithology, metamorphism and hydrothermal alteration), and may also reflects the particularity of anisotropic layer. Our work shows many similarities in the structural style, between the study area and the immediately adjacent gold deposit. These similarities suggest in both cases, that deformation associated with rheological domains of highly contrasted physical and geometrical properties, is mostly controlled by the anisotropy. They are weakly controlled by dynamic regime of deformation as predicted by the Andersonian fault model (Anderson 1951).

9

FOLDING DURING BULK INHOMOGENEOUS SHORTENING: BIS FOLDING T.H. Bell & A. Forde Department of Geology, James Cook University, Townsville, Qld 4811, Australia Measurement of spiral-shaped inclusion trail orientations in garnet porphyroblasts preserved around a kink show that it formed by bodily rotation of slices of rock in the short limb, in general agreement with previously published kinking models. In contrast, porphyroblasts distributed around four folds show a parallelism of inclusion trails that is not compatible with an origin involving any component of buckling and flexural flow. However, they are readily explained by a model of folding during bulk inhomogeneous shortening where movement takes place on planes at a high angle to the direction of shortening. Fold models of this type have rarely been proposed because of the perceived difficulty in explaining shearing on axial-plane foliations that are near normal to the shortening direction. A model of Bulk Inhomogeneous Shortening (BIS) folding is proposed where the vertical (or near horizontal) extensional response to horizontal (or vertical) shortening is noncoaxial due to the inhomogeneity of the rockmass being deformed. This results in folding due to locally greater vertical (or near horizontal) movement. To accommodate the shortening, differential movement takes place on zones parallel to the axial plane ranging in scale from cleavage seams to fold limbs. At a finer scale, within all of these zones, antithetic shearing, relative to the larger-scale motion, and congruent shortening occur oblique to the axial planes of developing folds.

10

INTERPRETATION OF SHEAR SENSE FROM WRENCH STRIATIONS IN DOLERITE: A TASMANIAN EXAMPLE R.F.Berrv- Geology Dept., University of Tasmania A survey of exposures of Jurassic dolerite was carried out across the whole of Tasmania. The dolerite is well exposed in fresh cuttings and quarries throughout Tasmania and fault striations were found in a high proportion of these exposures in the south and west of the state but they were not found in the north or northeast. These fault striation provide an unequivocal record of Mesozoic wrench faulting. Fault striations within the dolerite are dominantly of one type. These are chlorite-zeolite fibre veins. Several different zeolite associations also occur in sheltered areas on fault planes and undeformed fractures. The chlorite was not found in undeformed areas. Another major feature of the dolerite are clay zones 10-30 cm wide which are usually partly filled with massive quartz-zeolite veins. Only one example of these zones was found in which the original texture was recognisable. A 30 cm wide zone at Lake MacKenzie spillway contained cataclasite with wrench striations dominant. Middle Tertiary normal fault striations were recognised within the dolerite at two localities. In both cases these were grooves within clay gouges indicating very different conditions from those operating during the wrench faulting. The sense of movement of a third of the striations in this project were directly determined by the methods described by Petit et al. (1983) and Petit (1987). The sense of movement of the remainder of the fault striations was inferred from the sense of movement on faults of similar orientation. The most common fault plane structures are PO criteria. These structures occur on fractures making a high angle to c v mostly in the range 50-80°. These fractures have no discernible displacement. The next most common criteria in these rocks, for determining the sense of shear, is the presence of fibres crystallizing in the lee of asperities. These fibre veins grow on faults which are at 30-50° to with only a few exceptions at higher angles. R criteria are uncommon with only one site where they are the dominant type. RM and RO structures were found on faults with a wide range of orientations. T criteria were rare. Substantial numbers of striations were recorded from thirteen sites. The faults were dominated by wrench systems in which N-S striking faults were invariably sinistral in character and WNW-ESE striking faults were dextral in character. The fields of dextral and sinistral faults overlap at strikes of 140-170. Minor faults in this range are commonly very rough with many asperities in comparison with the smooth surfaces of other fracture orientations. Reverse faults were found at a few locations and are common at one the locality on the Southern Outlet, south of Hobart. At one location near Grove south of Hobart most of the faults have normal movements. Despite these minor variations the overall pattern across the state is very uniform. The association of wrench, reverse and normal faults matches the predicted pattern in wrench systems, and implies major compression from the NW. References Petit J.P. 1987. Criteria for the sense of movement on fault surfaces in brittle rocks. Journal of Structural Geology, 9, 597-608. Petit J.P., Proust F. & Tapponnier P. 1983. Criteres de sens de mouvement sur les miroirs de failles en roches non calcaires. Bulletin Societe Geologie France, 7 Ser XXV,589-608.

11

ASYMMETRICAL LITHOSPHERIC RIFTING IN LATE SILURIAN; A CAUSE FOR THE COEN OROGENY AND DEVELOPMENT OF THE HODGKINSON BASIN IN NE AUSTRALIA? Richard Blewett & Peter Wellman: Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT, Australia, 2601. The Coen Province of Cape York Peninsula, north Queensland comprises, Proterozoic sandstone, siltstone and dolerite which were metamorphosed to slate, phyllite, schist, gneiss and metadolerite/amphibolite and intruded by granite during the Siluro-Devonian Coen Orogeny. These rocks are exposed in the Coen and Yambo Inliers over an area of 300 km N-S by 50 km E-W immediately west of the Palmerville Fault Zone. These (Proterozoic ?) metamorphic rocks were most intensely deformed during the Coen Orogeny which began in the Siluro-Devonian. The intensity of D2 deformation and overall regional metamorphic grade in the metasediments both decrease in intensity westwards, away from the trace of the Palmerville Fault to the extent that they become relatively minor 100 km west of the fault. The Coen Orogeny can be viewed as a 100 km wide, N-striking zone of Siluro-Devonian deformation, metamorphism and granitoid intrusion situated immediately west of the Palmerville Fault. Immediately east of the Palmerville Fault is the Hodgkinson Province, a 150 km-wide basin filled mainly with marine sediments and minor tholeiitic basalts of late Ordovician to late Devonian age, with most of the basin fill represented by the Devonian Hodgkinson Formation. These basin sediments were probably folded and uplifted in the late Devonian, with W-directed thrusting in the west. This W-directed thrusting may be related to the D2 thrusting in the Coen Province adjacent to the west, during the Coen Orogeny There is no evidence for nearby subduction of oceanic lithosphere in the Silurian or Devonian. The granitoids of the Cape York Peninsula Batholith are intra-crustal (mainly S-types), they are not calc-alkaline. The Hodgkinson Basin sediments were probably not deposited and deformed in an accretionary prism (in the classical sense). C

It is proposed that the Coen Orogeny and the formation of the Hodgkinson Basin are causally linked by some form of detachment process. During the late Silurian the Australian continental lithosphere extended asymmetrically, on a W-dipping detachment fault which reached the surface at the Palmerville Fault. Lithosphere west of the fault formed the upper plate, and the heat of new mantle replacing the lower lithosphere provided the energy source for the metamorphism and granitoid generation during Siluro-Devonian Coen Orogeny. Lithosphere east of the detachment fault formed the lower plate; the slow cooling and consequent subsidence of this lithosphere provided the basin for the mainly Devonian sedimentation in the Hodgkinson Province. Importantly, this model provides a heat source for the Coen Orogeny, gives a mechanism for the subsidence of the adjacent Hodgkinson Province, and explains the time difference between the Siluro-Devonian Coen Orogeny, and the mainly early to late Devonian sedimentation of the Hodgkinson Formation.

12

Early Silurian

Chlllagoe & Mulgrave Fm basins Upper Crust

Upper Plat

Lower Plate

^

Lower Crust Lower Llthosphere

Asthenosphere

Late Silurian - after extension

Present - after compression PF

Coen Province

Laura Basin

\J\J\J\jCoen P. Metamorphlcs

WV\

AAA

& other upper crust (before/after compression)

Laura Basin

Lower Crust

Hodgkinson Fm

Lower Llthosphere

Pre Devonian seds F—T +

100 km

V/H = 1

i

—I

i—

1

+

PF 13

+

S/D granitoids Palmervllle Fault

FOLDING AT VERY HIGH SHEAR STRAIN IN EXPERIMENTAL SHEAR ZONES

Paul D. Bons & Mark W. Jessell

Department of earth Sciences, Monash University, Clayton VIC 3168 High to very high shear strains (>100) often occur in shear zones. Unfortunately it is very difficult to impossible to experimentally deform real rock samples in simple shear to such high shear strains. However, the combination of the use of a circular shear zone and soft rock analagues makes it possible to experimentally reproduce (micro-) structures that are often found in natural shear zones. We have used composites of the organic crystalline rock analogues camphor (C10H16O) and octachloropropane (OCP, C3CI8). These materials can be deformed easily at room temperature and deform by micromechanical processes that are similar to those operating in deforming minerals, such as dislocation creep processes and probably diffusional creep. Dynamic recrystallisation is important. Both materials were proven to be power-law creep materials with stress exponents of 3.3 (camphor) and 4.5 (OCP) at 28°C. Mixtures of OCP and camphor were deformed, using two different techniques which are both capable of achieving shear strains well over a hundred: 1) the transparent deformation cell, which allows microscopical observation of the deforming "thin-section-like" specimen throughout the experiment, but no accurate measurements of stress; 2) the hollow-cylinder deformation apparatus, which makes measurement of the strain rate history at an applied shear stress possible. However, observation of the specimen during deformation is limited. With both types of experiments, folding was observed at high shear strain in composites where the softer OCP formed a matrix, enveloping camphor inclusions (see figure below). Starting with an isotropic mixture of camphor inclusions in an OCP matrix , two stages in microstructural and rheological development could be distinguished : 1) y < 50-75: foliation development by stretching and rotation of the camphor inclusions into lenses and layers. Mechanical phase segregation leading to a compositional banding was sometimes observed. 2) y > 50-75: folding by buckling of the camphor layers and subsequent rotation and tightening of the folds. Two factors seem to play a role in initialising folding: thickness variations in individual layers and interactions between lenses and layers.

0.5 cm

^ ^

y=122

SPSS camphor

I

I OCP

Drawing of the microstructure in a sample of camphor in an OCP matrix, deformed in dextral simple shear to a finite shear strain of 122.

14

THREE-DIMENSIONAL MECHANICAL MODELS OF CRUSTAL DEFORMATION Tean Braun. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Recent advances in numerical methods and computing power have enabled us to solve numerically three-dimensional mechanical problems of application to the Earth's crust with a resolution that makes direct comparison of the results with laboratory and field observations possible. The main advantage of the numerical simulations is that they provide with a complete set of "observables": the full strain and stress tensors throughout the modeled piece of crust. This is quite contrary to field observations or laboratory experiments where the strain tensor is only approximately derived from measurements of the displacement field ana the stress tensor is unknown or extrapolated from the boundaries. . The numerical simulations therefore provide a new approach to "test" the predictions of conceptual models of the rheology of the crust. Of course, a numerical model can only reproduce what is fed into it. If the rheology assumed in a model is based, on the Mohr-Coulomb criterion, one must expect it to predict faults orientated at (n/4 ± 0/2) with respect to the most compressive principal stress. A more interesting question to be addressea is: under which conditions is the geometry of structures (faults/shear bands) "kinematically" controlled and uncier which conditions is it "dynamically" controlled? Kinematic structures are those that form according to the geometry of the boundary conditions as if the material was an inviscid fluid; dynamical structures are those that form according to the rheology of the material. In mathematical terms, kinematic structures obey: e = fityuo),

where e is the strain tensor and uo a set of imposed displacements along the model boundaries, whereas dynamical structures obey: where c is the stress tensor and R represents the rheology (angle of internal friction, tensile strength, etc...). In this paper we present the results of a series of numerical simulations ot the mechanical behaviour of the Earth's brittle upper crust under conditions representing different tectonic settings: transcurrent, convergent and transpressive plate interactions. The model assumes that the brittle crust is an elastic medium that fails according to Murrell's extension in three dimensions of Griffith's failure criterion. An associative plastic flow law is used to describe deformation beyond the brittle limit. Non-linear geometrical effects arising from finite deformations and rotations are included in the model. Gravity is included in the computations as a vertical body force proportional to the density of rocks. The model predicts the distribution ot deformation (strain) and stress within a region of the Earth's crust submitted to external forces or displacements by solving the equations of force and moment equilibrium in three dimensions. Wrenching the modeled crust leads to the formation of so-called "Riedel'-structures that are properly orientated with respect to the principal stress directions, tnat is according to Mohr's incipient fault hypothesis applied to Murrell's criterion. Seen in vertical cross-sections 15

the predicted Riedel-shears form flower structures that root into the base of the deforming layer; under uniform horizontal compression, the flower structures turn into palm-trees whereas under uniform horizontal tension, the flower structures turn into tulips. Under some circumstances, deformation is accommodated by so-called Y"-shears that are aligned with the direction of imposed wrenching. These are the "kinematic" structures and are the dominant mode of deformation if the zone of deformation is narrow or if the deforming region is under tension. Plate collision is modeled by imposing a discontinuous horizontal velocity field at the base of the crust that is meant to represent subduction by delamination of the mantle part of one of the two lithospheric plates. Subsequent shortening of the crust is accommodated by a set of shear bands dipping at 30°-40° to form a V-structure that roots into the base of the crust at the location of the velocity discontinuity. One of the shear band (called retro-thrust) is located on the "fixed" plate and accommodates most of the shortening by thrusting whereas the other shear band (the prothrust) accommodates low amplitude bending strains. The predicted geometry (dip) of the shear bands is in agreement with Monr's incipient faulting hypothesis. Across transpressive plate boundaries, that is where convergence is oblique to the strike of the boundary, the crust is shortened and sheared simultaneously. The model predicts the partitioning of deformation between thrust-like shallow-dipping structures and wrench-like nearvertical structures as a function of the parameter r defined as the ratio of the component of the imposed far-field displacement normal to the plate boundary to the total imposed far-field displacement. For values of r <35% (shear-dominated plate boundaries), two sets of structures develop and thrusting is decoupled from shearing; for values of r>35%, both components of deformation are accommodated by a single set of thrustlike shallow dipping structures. "Moving" Plate Plate Boundary

"Fixed" Plate

Geometry of an "idealized" transpressive plate boundary; uo, is the component of the imposed far-field displacement normal to the boundary; uo,t is the component parallel to the boundary. The continental crust is assumed to be "welded" to the underlying mantle but, due to its lighter density, it does not take part in the subduction process. Rather, it experiences deformation (shortening and wrenching). When wo,n=0/ the crust is sheared; when uo,t=0 the crust is shortened. n

16

ANALOG MODELLING OF CRUSTAL DEFORMATION ALONG 2D AND 3D LISTRIC NORMAL FAULTS Jean Braun and Geoffrey E. Batt, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Crustal extension is accommodated by movement along discrete, often listric, normal faults. In many instances, the exact fault geometry is poorly constrained and, therefore, the nature and amount of extension must be derived from the geometry of originally-flat marker horizons within the hangingwall or within the overlying sedimentary section. To be accurate, this approach must be based on a proper understanding of the internal deformation of the hangingwall and now it relates to the Fault geometry. In this paper, we present the results of a series of scaled laboratory experiments designed to address the problem of hangingwall deformation during crustal extension along two- and three-dimensional listric fault surfaces. The experiments were carried out in two specially designed perspex deformation rigs to simulate differing detachment surface orientations to the direction of extension. The detachment surfaces (or faults) are made of simple ramp structures shaped as exponential curves. The first apparatus is in fact two-dimensional whereas the second is composed of two symmetrical halves orientated at 45° to the direction of extension. Extension was achieved using a thin sheet of mylar overlying the fault surfaces and attached to a motor driven roller. The analog hangingwall consisted of multiple layers of homogeneous, cohesionless and isotropic sand with an angle of internal friction of 33°5. The broad deformation pattern observed in a first set of experiments using the 2D rig can be broken up into three regions: • the proximal region of the hangingwall, originally overlying the steep section of the fault, is characterized bv pure solid-body rotation; • the distal region of the hangingwall, originally overlying the flat section of the fault, has undergone pure solid-body translation; • the transitional region of the hangingwall (between the proximal and distal regions) has undergone horizontal extension and bending. The deformation pattern produced in the 3D modelling experiments is bilaterally symmetrical about the center line of the deformation rig. The surface of each of the mirror-image halves is marked by a similar tripartite division of features as seen in the 2D modelling experiments. Tne surface deformation is characterized by: • two very flat-surfaced proximal regions apparently deforming by solidbody rotation; • two distal regions deforming by solid body translation; • two transitional regions, characterized by normal fault scarps the strikes of which suggest that extension is aligned with the direction of maximum dip of each fault segment; • a central region, comprising the "beak" of the hangingwall, characterized by surface fault scarps aligned with the direction of imposed displacement, therefore indicating that extension is perpendicular to the direction of imposed displacement. The results of the experiments have been interpreted and used as the basis for a new kinematic model based on what we term the "rule of the normal". The model is based on the simple observation that, in the analog experiments, deformation of the hangingwall is driven by the

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"unfolding" of the mylar sheet as it is moved along the curved fault surface. It is therefore natural to assume, as in plate bending theory, that a line normal to the base of the hangingwall remains normal to the base of the hangingwall, even after finite deformation. An additional constrain built into the model is that of incompressibility. Using the rule of the normal, we computed the shape of the surface of the hangingwall following finite deformation along an exponentiallyshaped fault surface. We also computed the stretch and rotation tensors at a number of material points within the hangingwall. The results we obtained were remarkably similar to those of the 2D analog experiments: (1) the proximal region does not deform internally but undergoes a large amount of rotation, (2) the region of maximum extension is located in the transitional region of the hangingwall, and (3) extension rapidly decreases to become negligible in the distal region of the hangingwall. The rule of the normal was generalized to the three-dimensional case and used to compute the distribution of deformation of the hangingwall. Again, the kinematic model reproduced all the characteristics of the deformation field observed in the analog experiment: (1) symmetry about the center line, (2) a very flat surface over the proximal regions of the hangingwall undergoing substantial body rotation, C3) extension normal to tne fault trace in the transitional regions, (4) almost pure horizontal translation in the distal regions, and (5) extension in a direction perpendicular to the direction of imposed displacement in the central region. The rule of the normal has also been adapted to compute hangingwall deformation in cases where (a) the imposed displacement along the base of the hangingwall is not uniform and (b) the fault surface is not symmetrical about the direction of extension. 2im Interpretation of 2D experiment (side view) distal region

I transitional region |

| proximal I | region |

rZ.

translation

outline of central region

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THE ROLE OF WATER IN THE EVOLUTION OF LARGE STRIKE-SLIP FAULTS J. Byerlee U.S. Geological Survey, Menlo Park, CA

A model for the evolution of large strike-slip faults is developed in which, water that originally came from the country rock saturates the initially highly porous and permeable fault zone. During shearing, the fault zone compacts and water flows back into the country rock. However, the flow is eventually arrested by silica deposition that forms very low permeability seals between the fault zone and the country rock. Because of variations in the temperature and the mineralogical composition and the complex structure of the fault zone a three dimensional network of seals is formed in the fault zone, itself. Thus, the high-pressure fluid is not evenly distributed. As in deep oil reservoirs, the fluid will be confined to seal-bounded fluid compartments of various sizes and porosities that are hydraulically isolated from each other and from the hydrostatic regime in the country rock. When the seal between two of these compartments is ruptured, either by tectonic forces or transient forces generated by the passage of a seismic wave from a distant earthquake, water will diffuse from the high-pressure compartment to the low-pressure compartment. If the product of the width and porosity of the high-pressure compartment is greater than it is for the low-pressure compartment then the strength of the fault in this region will be reduced. If the tectonic stresses in the region are high enough, this reduction in strength will trigger an earthquake. During the earthquake, many of the remaining seals will be ruptured, and the width of the fault zone will increase by failure of the geometric irregularities on the fault. This newly created, highly porous and permeable fault zone will fill with water from the hydrostatically-pressured country rock, and the earthquake cycle described above will be repeated.

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STRUCTURAL, METAMORPHIC AND GEOCHRONOLOGIC EVOLUTION OF SHEAR ZONES IN THE MUSGRAVE BLOCK, CENTRAL AUSTRALIA. A. Camacho Research School of Earth Sciences, A.N.U., ACT 0200. The precise dating of mineral paragenesis during metamorphism has been difficult because the constituent minerals often contain unfavourable parent to daughter isotopic ratios, or have low closure temperatures so that measured ages commonly reflect the cooling cycle. Some recent studies have focused on the precise timing of formation and growth of garnet in metamorphic rocks (e.g. Mezger etal. 1989, Vance and O'Nions 1990) by using Sm-Nd and U-Pb techniques. It has been found that there is sufficient dispersion in Sm-Nd ratios between garnet and the total rock, and sufficient U in garnet to obtain quite precise isotopic ages using TIMS methods for analysis. These studies have shown that garnet is a particular useful mineral for dating because it is common in many metamorphic rocks, often forms during the prograde history and its growth can be placed along a given PT path. In addition, the diffusion rates of the major cations are low (Elphick et al. 1985). Thus, geochronology on the metamorphic minerals in a given mineral assemblage can provide better constraints on PTt paths. Using these and other approaches, the crustal evolution and exhumation history of the Proterozoic metamorphic rocks of the Musgrave Block is being investigated. The granulite facies rocks of the Musgrave Block contains shear zones with different metamorphic histories along which an exhumation history may be recorded. In addition, the rocks record the highest pressures in the Australian continent and provide an excellent opportunity to address some of the above issues on a section of the lower to middle continental crust. The east-west trending Proterozoic Musgrave Block consists of high-grade metamorphic rocks which have different structural histories. These rocks have been brought into juxtaposition by the east-west trending Woodroffe Thrust, N. Davenport, Davenport and Mann shear zones. The Woodroffe Thrust appears to represent the most prominent exposed discontinuity in the Musgrave Block and superimposes granulite facies on amphibolite facies rocks. Maboko et al. (1992) present 40Ar-39Ar ages of 550-530 Ma for muscovites growing in shear zones in amphibolite facies gneisses below the Woodroffe Thrust. The muscovite ages are identical (within analytical error) to muscovites growing within shear zones in the granulite facies rocks and Maboko et al. (1992) concluded that this event dated the juxtaposition of the two terranes along the Woodroffe Thrust during the Petermann Orogeny. The mylonites associated with the Woodroffe Thrust plunge shallowly to the south with a reverse component of movement. The lineation in the N. Davenport plunges to the south east whereas in the Davenport the lineation is strike-slip and movement sinistral. The Mann is less well understood as it outcrops poorly in the mapped area. Apart from the differences in the plunge of the lineations, there is also a large difference in metamorphic grade between the shear zones. The granulites above the Woodroffe Thrust preserve their original granulite facies assemblage. However, during the development of the Davenport and N. Davenport shear zones, the garnet bearing quartzofeldspathic gneisses developed coronas of new garnet and crystallised ilmenite, rutile and garnet in the pressure shadows. Orthopyroxene in the mafic gneisses developed coronas of simplectitic quartz and clinopyroxene. Clinopyroxene developed thin coronas of plagioclase. Garnet, ilmenite and rutile crystallised in the zones of high strain. Dolerites closely associated with the shear zones are the only rocks that are completely recrystallised and show a well equilibrated assemblage of garnet, clinopyroxene, albite, rutile, biotite, quartz and ilmenite. It is in these latter rocks that Ellis and Maboko (1992) estimated pressures of 12.5 ± 1 kb from the reaction anorthite = jadeite + quartz (Holland, 1980) and temperatures of 650 ± 50°C using

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the Ellis and Green (1979) garnet-clinopyroxene geothermometer. These estimates place these rocks in the eclogite facies. Sm-Nd internal isochrons from the well equilibrated assemblages in dolerites associated with the Davenport shear zone should constrain the timing of the high P event. However, completely recrystallised assemblages in the shear zones are not common. Therefore, estimates of the age formation of the shear zones are being carried out using the SHRIMP ion-microprobe on newly crystallised zircon, rutile and sphene. The above techniques, in conjunction with K-Ar and Ar- Ar dating of shear zones with K rich hydrous minerals, hopefully will provide a detailed history into the cooling and uplift trajectory of the Musgrave Block as these various isotopic systems have different closure temperatures. REFERENCES Ellis, D.J. and Green, D.H., 1979. An experimental study of the effect of Ca upon garnetclinopyroxene Fe-Mg exchange equilibria. Contributions to Mineralogy and Petrology, 27: 123-154. Ellis, D.J. and Maboko, 1992. Precambrian tectonics and physicochemical evolution of the continental crust. I. The gabbro-eclogite transition. Precambrian Research, 55: 491506. Elphick, S.C.. Ganguly, J. and Loomis, T.P., 1985. Experimental determination of cation diffusivities in aluminosilicate garnets. I. Experimental methods and interdiffusion data. Contributions to Mineralogy and Petrology, 90: 36-44. Holland, T.J.B., 1980. The reaction albite = jadeite + quartz determined experimentally in the range 600-1200°C. American Mineralogist, 65: 129-134. Maboko, M.A.H., McDougall, I., Zeitler, P.K. & Williams, I.S., 1992. Geochronological evidence for ~530-550 Ma juxtaposition of two Proterozoic metamorphic Terranes in the Musgrave Ranges, central Australia. Australian Journal of Earth Sciences, 39: 457-471. Mezger, K., Hanson, G.N. and Bohlen, S.R., 1989. U-Pb systematics of garnet: dating the growth of garnet in the Late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada. Contributions to Mineralogy and Petrology, 101: 136-148. Vance, D. and O'Nions, R.K., 1990. Isotopic chronometry of zoned garnets: growth kinetics and metamorphic histories. Earth and Planetary Science Letters, 97: 227-240. 40

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THE RELATIONSHIP BETWEEN ANATECTIC LEUCOGRANITES, COMPRESSIONAL OROGENIES AND EXTENSIONAL TECTONICS, SOUTHERN OMINECA BELT, CANADIAN CORDILLERA Sharon D. Carr, Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carle ton University, Ottawa, Ontario K1S 5B6 The present crustal architecture of the southern Omineca belt is a product of Eocene extension and crustal thinning superimposed on a crust that was thickened and deformed during Paleozoic and Jurassic to Late Paleocene compression. Amphibolite-facies metamorphic rocks exposed as gneiss complexes within the Shuswap core compex were buried during compression and were exhumed in the lower plates of low- to moderate-angle ductile-brittle Eocene extensional faults. Sheetlike complexes of variably deformed Late Cretaceous to Eocene leucogranites are ubiquitous in the gneiss complexes. The leucogranites are interpreted to have an anatectic origin, and are intimately related in space and time to both ductile thrust and normal faults. They generally occur in the hanging wall of the Monashee d6collement, a ductile thrust fault that roots to the west in the lower crust, and is interpreted to correlate with the basal decollement of the Rocky Mountain Foreland Belt to the east. Paleogene leucogranites were generated during the final stages of thrusting by decompression melting. They were hosted in hot upperamphibolite facies middle crustal rocks that were being carried to higher crustal levels up a southwest facing frontal ramp in the Monashee decollement, concurrent with denudation on the extensional fault systems that initiated during terminal stages of thrusting. Thermally softer granites and/or melts acted as transfer zones in normal fault systems and served to nucleate extensional shear zones. The extensional regime then facilitated the intrusion of late synkinematic to post-kinematic plutons. Older pulses of Late Cretaceous leucogranites in a similar structural setting in the hanging wall of the Monashee decollement suggest that there is a tectonic control on the initiation of leucogranite generation. If decompression melting due to extension is the controlling factor, then the presence of older suites of syn-kinematic leucogranites may indicate earlier periods of extensional tectonics.

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STRUCTURAL ANALYSIS OF AN AREA SOUTH OF QUEANBEYAN, NSW L. Carson and M. Rickard Department of Geology, Australian National University, ACT 0200 Palaeozoic units exposed south of Queanbeyan, New South Wales have undergone multiple deformation. Stauffer (1964) predicted the first generation folds to be recumbent from downward facing fold zones. Observed south of Queanbeyan is the first find of an early recumbent major fold The Palaeozoic units consists of Ordovician quartz rich turbidites, unconformably overlaid by Early Silurian proximal turbidites. Late Silurian sequence of shallow marine sediments and subaerial volcanics were deposited unconformably on the Early Silurian units. Intruding the Palaeozoic units are a number of Early Devonian granitoid bodies. Both the Silurian volcanics and the granitoid bodies are considered to be I - type. Of particular interest is a four kilometre continuous section of Ordovician turbidites in the east. The western portion of the sequence consists of interbedded sandstones, siltstones and black shales, eastwards the sequence is predominantly massive sandstones with minor shales beds. The intensity of deformation variesfromweak in the west to strong in the east. Detailed structural analysis has delineate two main generations of folds. First generation of mesoscopic folds are recumbent isoclinal folds (Fi) and have a "stripy" axial surface foliation (Si), which appears as a band of pelitic minerals. The S\ foliation is locally developed in the higher grade metasandstones. The dominant structural element is the second generation of folding, open to tight inclined folds (F2) and well developed spaced cleavage (S2). Locally well developed kink bands and crenulation cleavage overprint the folds and associated foliation. Within the section the overall metamorphic grade increasesfromupper greenschist facies in the west to lower amphibolite facies in the east. Metamorphic zones can be mapped by the minerals; chlorite, biotite and andalusite/cordierite (knotted schist). A metamorphic complex in the east consists of psammitic and pelitic schist with narrow belts of knotted schists. The weakly deformed Palaeozoic units in the west contain narrow bands of high strain and associated intense kink banding, disharmonic folding and small scale thrust faults. These narrow bands are isolated to the Ordovician black shale units, which occur within the Ordovician turbidites and as thin inliers within the Late Silurian volcanics. The relationship between deformation and metamorphism and the significance of narrow zones of high strain are important in the evolution of the Lachlan Fold Belt.

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GRID SKETCHING TO AID TEACHING GEOLOGICAL MAPPING. Ian Clark and Patrick James, Department of Geology and Geophysics, University of Adelaide, GPO Box 498, Adelaide, South Australia 5001 Fieldwork is commonly recognised as one of the most important contributing elements to a complete geological education (Kern and Carpenter, 1984; 1986; Locke, 1989). Field excursions used as an aid to geology teaching vary from fully guided instructional tours to individual mapping projects. Geological mapping is undoubtedly the most challenging, rigorous and stimulating field-based activity and consequently is often not included in introductory subjects. Its correct use as an adjunct to other modes of geological instruction can be a powerful method of enhancing the learning process (including most importantly the enjoyment and satisfaction of learning), especially at the introductory level. Few publications describe in detail a method of grid mapping or sketching, or especially how such a technique can be used as an aid to geology teaching. A method of rough grid mapping for particularly large and complex outcrops is described by Compton (1985 p.33), and Barnes (1981, p48) also briefly describes a method of producing a detailed sketch map of an exposure with field data plotted on squared (graph) paper. Although detailed grid mapping of well exposed terrains on a variety of scales is very well known and cited as a standard mapping tool and as an aid to more conventional geological mapping techniques (Hobbs et al, 1976; McClay, 1987), it has not received the thorough investigation and recognition it deserves as a basic geological learning method. In this poster we describe an extension of this technique which has been used to introduce mapping skills to beginning geology students and as an introduction to complex areas for more advanced students. Our method introduces field mapping at an early stage by using a simple technique of scaled sketching to teach fundamental mapping and geological principles. Using fully exposed shore platform outcrops of highly metamorphosed and polydeformed Proterozoic gneisses from Corny Point in South Australia, we have designed a simple exercise to get students to progressively transfer lm^ areas of rock platform, defined by a chalk grid, onto graph paper to quickly produce a geological map. Over many years of introducing students to geological mapping, we have found that: a) students invariably take a considerable time to gain sufficient confidence to "put (mapping) pen to paper" as they are introduced to the rigour of producing a geological map from a blank sheet of paper, topographic base map or aerial photograph, and b) once even the earliest mapping technique has been learned, students will tend to resist changing that technique even if it is not correct or sloppy ("bad habits die hard"). To ease the introduction of students to geological mapping we have developed a system whereby students can produce an accurate, detailed sketch map of a very small area of complex geology in a couple of hours. This, we find, is the ideal way to bring students to accept that they are able to succeed in the often difficult task of producing their first geological map. It also, as a complementary reward, teaches valuable introductory geological principles and develops in students an appreciation that fieldwork is the basis of most geological study. To make best use of the technique there are a number of requirements which appear to be important for success. The area chosen must be fully or almost fully exposed. As each eventual mapping area is no more than 3metres by 3 metres, it is usually not difficult to find appropriate areas Essentially flat, horizontal and clean rock sheets of outcrop are ideal for both marking out the grid and for mapping. The technique involves a combination of form surface (Hobbs et al, 1976, p.366) and lithostratigraphic mapping/sketching and has been used

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with undergraduate students from the Universities of South Australia and Adelaide (first and third year levels) and also with senior school students in their final year of secondary school. This technique was initially used with a large group of first year students from Adelaide University in the fifth or sixth week of their first geology course. It was their first opportunity to see "hard" rocks in the field, and certainly their first attempt at geological mapping. With four or five demonstrators, we supervised classes of 100 students who undertook this exercise over a 1-2 hour period. Follow-up laboratory and class exercises have also been incorporated into the program. Student evaluations to ascertain the value and productivity of the grid mapping method were generally very positive and students have noticeably retained and improved their attitude to, and competence, in field mapping through later subjects. The technique was also used with a group of third and fourth year undergraduate students as an introduction to a five day mapping exercise in a more complex terrain (middle Proterozoic metamorphics near Broken Hill in New South Wales). The students agreed that the technique is an instructive introduction which is exemplified by the higher than usual confidence the students had in the early stages of the project. As a result they were able to accomplish more in the five days than preceding groups. In the past we chose a small area of simply deformed sedimentary rocks for an introduction to mapping. Mapping was completed at a larger scale and it was impossible for students to see the results of their work after a short time. The advantage of the detail grid-mapping method is that students can see their map in a relatively short time and can compare it with what they see in the outcrop before them. Mapping onto graph paper allows more accurate transfer of field data and the necessity to choose areas with continuous outcrop means that there are no areas of uncertainty about which beginning students must 'guess'. The method also makes students observe the lithology and the rock relationships more closely. Perhaps the biggest advantage of this method is that it enables students to be introduced to the skills of geological mapping very early in their course and can be used in a variety of contexts to make students observe features closely. It is also inexpensive because aerial photographs are not required for the base onto which data is plotted. Graph paper, chalk, a pencil and fair weather are the only requirements for a most successful and envigorating learning exercise. References cited Barnes, J., 1981, Basic Geological Mapping: Geological Society of London Handbook Series, Open University Press, 112 p. Compton, R., 1985, Geology in the Field: John Wiley & Sons, 398 p. Kern, E.L. and Carpenter, J.R., 1984, Enhancement of student values, interests and attitudes in Earth Science through a field oriented approach: Journal of Geological Education, v. 32, p. 299-305. Kern, E.L. and Carpenter, J.R., 1984, Effect of field activities on student learning: Journal of Geological Education, v.34, p. 180-183. Locke, W. W.,1989, A nontraditional geology field trip: Journal of Geological Education. v.37(2), p. 107-109. McClay, K., 1987, The Mapping of Geological Structures: Geological Society of London Handbook Series, Open University Press 161 p.

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STRUCTURE IN AND SURROUNDING NORTHERN EXTENSIONS OF THE (?)CAMBRIAN JINDALEE GROUP, WALLENDBEEN, N.S.W.

Cooper. LB. Department of Geology and Geophysics, University of Sydney, Sydney, N.S.W., 2006. The (?)Cambrian Jindalee Group outcrops as a 230km long meridional belt between Grenfell and Tumut in southern N.S.W. and is considered to be one of the oldest groups within the Palaeozoic Lachlan Fold Belt. Units within the Jindalee Group include Bullawyarra Schist, Gundagai Serpentinite, Jones Creek Diorite, Wambidgee Serpentinite, and (?)Hoskins Formation. The Jindalee Group consists of metabasalt, metagabbro, quartz-mica schist, quartzite, phyllite, banded quartz - magnetite ± manganese metachert, massive and schistose serpentinite. The metamorphic assemblages are indicative of upper greenschist to amphibolite facies conditions. In the metabasalts the assemblage actinolite/hornblende - epidote - albite ± magnetite ± sphene ± quartz is preserved with some retrograde chlorite. The metagabbros preserve the assemblage actinolite/hornblende - labradorite - clinozoisite. Pelitic rocks contain quartz ± muscovite ± biotite ± garnet and metacherts contain quartz ± magnetite ± rhodonite ± pyrolusite. Contained as an 8km long fault bounded slice within serpentinite in the "Fontenoy" area is a sequence of andesitic to rhyolitic volcanics, phyllites, and quartz - hornblende magnetite ± pyrite ± chalcopyrite exhalites herein termed the Yandilla Volcanics. The Yandilla volcanics also contain an isoclinal folding event of similar orientation that affected the Jindalee Group. Based on geochemical and structural characteristics the Yandilla Volcanics are assigned an Ordovician age. At least three deformation events can be recognised in the area to the north and west of Wallendbeen. Firstly a ubiquitous high grade metamorphic foliation which is axial planar to shallowly south plunging folds with generally steep east dipping axial planes. Secondly, a widespread dextral shear event is represented by numerous lm to 150m wide serpentinite zones which are considered to relate to the opening of the Silurian Tumut Trough. The Silurian, (425 Ma), Young Granodiorite intrudes the Jindalee Group and while still plastic was affected by steeply east dipping dextral reverse slip faulting along the Mooney Mooney fault zone. This produced a 500m thick tectonic inlier of Jindalee Group serpentinites and metagabbros surrounded by Young Granodiorite that has been altered to gneissic and mylonitic varieties containing S - C mylonite fabrics.

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LOW FRICTION DURING SLIDING ON SIMULATED FAULTS IN POROUS QUARTZ SANDSTONE AT HYDROTHERMAL CONDITIONS Stephen F Cox. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200. Earthquake nucleation in the Earth's continental crust occurs on faults typically at depths between 5 and 20 km. A growing body of evidence indicates that several major, seismogenic fault zones are weak relative to the surrounding rock mass. Their strength is also much lower than predicted by laboratory determinations of the dry frictional strengths of appropriate geological materials. Field studies of exhumed, ancient fault zones are demonstrating that active faults are commonly sites of substantial fluid-rock interaction at seismogenic depths. There is a developing recognition that the presence of pore fluids can play a very active role in controlling the strength of faults, and is therefore a key factor controlling earthquake nucleation and recurrence. The potential importance of pore fluids is illustrated by the frictional failure criterion, Tf = C + |i(a - Pf). This relates the shear stress (if) at frictional failure to the fault cohesion (C), the coefficient of friction (|i), the imposed normal stress (a ) acting on the failure plane, and the fluid pressure (Pf). On the basis of this relationship, two classes of models have been employed recently to explain the apparendy low shear strengths of some major seismogenic faults. The first class of models invokes the development of high fluid pressures to reduce effective normal stress and thereby decrease the frictional resistance to sliding. The second class of models appeals to the possibility that faults may contain materials with intrinsically low friction coefficients. Our developing understanding of the factors controlling earthquake nucleation and recurrence is therefore being underpinned by laboratory studies of the processes which modify fault cohesion, friction coefficients and fluid pressures in fault zones. The particular aspect I wish to address relates to the role of chemically active pore fluids in controlling fault mechanics. A suite of high temperature experiments has been used to investigate frictional sliding behaviour in the presence of chemically active pore fluids. Experiments have been conducted on bare interfaces in porous quartz sandstone at 1200K, confining pressures of 300 MPa, and at pore fluid pressures of 200 MPa. Sliding interfaces were inclined at 30° to the axial shortening direction, and shear displacement rates ranged between 0.2 (im s and 1 [im s" . Two types of pore fluids have been used - argon and water. At the test conditions argon is inert. In contrast, the pore water promotes extremely rapid dissolution-precipitation reactions at the experimental conditions. In the presence of argon, and at an imposed displacement rate of 0.8 |im s" , simulated faults exhibit slip hardening behaviour. Slip commenced at shear stresses around 60 MPa and at friction coefficients around 0.5. The presence of pore water has a spectacular effect on the mechanical behaviour of sliding interfaces. At a displacement rate of 0.7 (im s" , the initiation of sliding at shear stresses around 50 MPa was accompanied by a rapid slip event and decrease in shear resistance. During ensuing stable sliding, the coefficient of friction is approximately 0.3. A transition to stick-slip behavior has been found at lower displacement rates. Microstructural observations indicate that dry sliding over distances up to 1mm produced only minor surface damage. However, during wet sliding, the sliding surfaces have been extensively modified by dissolution-precipitation processes. Wear damage that has developed early during slip has been etched by the pore fluid to form well-developed slickenlines over the entire slip surface, even after less than 1mm of displacement. The low friction coefficients measured during high-temperature wet sliding are substantially lower than values found previously for sliding in quartz-rich rocks at lower temperatures. The results suggest the possibility that therm ally-activated dissolutionprecipitation reactions and stress corrosion cracking can play a significant role in modifying the asperity structure of sliding interfaces, and thereby influence fault stability and decrease resistance to frictional sliding in faults containing chemically active pore fluids. n

n

_1

1

1

1

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MODELLING FAULT BLOCK MOVEMENT IN THE SNOWY MOUNTAINS USING THERMOCHRONOLOGY AND A MAP ALGEBRA S J D C o x l - B P Kohn2, A J W Gleadow2, P Bishop3 & G Goldrick3 Victorian Institute of Earth and Planetary Sciences, ]Dept of Earth Sciences, Monash University, Clayton, Vic 3168, 2Dept of Earth Sciences, Latrobe University, Bundoora, Vic 3083, ^Dept of Geography & Environmental Science, Monash University, Clayton, V7c 3768

Apatite fission track ages in uplifted crystalline rocks commonly show linear relationships between age and sample elevation A=(z+zo)/B for particular age ranges, where A is the observed apatite fission track age in Ma and z is the altitude in metres. The correlation is explained through simple models involving cooling of the rock column through particular isotherms. The slope B will be either related to the pre-uplift geothermal gradient for rocks from depths 1-3 km, or will correspond to the rate of ascent or denudation for rocks from greater depths, assuming that the geotherm was uniform and stable. These relationships potentially provide a unique post-cooling thermochronologic stratigraphic tool within basement rocks where use of conventional stratigraphic parameters may be limited. Fission track dates are particularly suitable for such an analysis because of the fine resolution of ages over relatively small elevation ranges. This tool may be applied most simply to a faulted pluton. If B is constant between blocks, then the difference between zo for adjacent blocks will give the relative vertical displacement of the blocks for the younger period. We have tested the method within a group of overlapping plutons in the Snowy Mountains and adjacent Monaro Plain of NSW. The rocks are cut by an extensive array of faults, which appear on geology sheets published by the NSW Geological Survey and AGSO (White et al., 1976, 1989; Wyborn et al., 1990). Very little is known about the fault displacements, though some of them (eg Jindabyne Thrust, Barney's Range Fault) coincide with topographic scarps, which suggests that there could be considerable, relatively young, vertical offsets. Relatively large relief in the Snowys potentially allows us to determine the gradient B through surface sampling only (Kohn & Gleadow, 1994). We use characteristic relationships between elevation and fission track age established from two fault blocks: a traverse from the Mt Kosciusko range down 830m to Thredbo yielded 12 ages showing a linear trend in the range 248 to 172 Ma; the results of a second traverse to the south along the Barry Way to Jacob's Ladder through an elevation range of 920m are being determined. The analysis was carried out using map algebra. Fault blocks were defined by digitising the major faults from the published maps. Where the mapped faults did not define closed polygons, minimal "virtual" faults were constructed to complete the map. The value of the ascent rate coefficient B was taken to be constant across the map, and the elevation datum zoi determined for each of the fault blocks by calibration with measured dates and sample site elevations within the block. The relationship for each fault block was then combined with altitude values from a gridded digital elevation model (DEM), purchased from AUSLIG. In areas where finer detail was required additional elevation points were filled in from the topographic maps. The output is maps showing predicted apatite fission track ages, and fault block offsets, for the entire area. GRASS4.1 (USACERL, 1993), a general purpose raster based GIS was used for most of the map analysis, and in particular the module r.mapcalc which allows complex boolean and algebraic manipulations involving multiple layers. We present a variety of graphical products, included shaded relief maps, pseudo perspective views, with various overlays of faults, and also 2D profiles showing the relative offsets of the various blocks. All of these are straightforward to produce using GRASS.

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The present model includes a number of simplifying assumptions: principally (i) the coefficient B is constant between blocks and (ii) the elevation datum zo and coefficient B are constant within blocks. The former restriction may be evaluated by comparing multiple master curves. The latter consideration may lead to (a) refinement of the map of fault blocks, or (b) the development of more complex age characteristics which allow tilting or warping to be incorporated into the model. This will apply where the blocks are not enclosed by a complete polygon of faults. However, as long as the relationship can be expressed in the form A=(z+zoi(x,y))/Bj(x,y) or similar, the map algebra capabilities of r.mapcalc may still be used for the model. We have found that the use of a GIS has been invaluable in visualising and modelling fault offsets using age/elevation data in this study of the Snowy Mountains.

References Kohn, B. P. & Gleadow, A. J. W., Thermotectonic evolution of the Kosciusko massif: an apatite fission track study. This volume. 1994 U S Army Construction Engineering Research Laboratory (USACERL), Geographic Resources Analysis Support System (GRASS) version 4.1, Champaign, 111. 1993 White A J R, Chappell B W, Williams I S, & Glen R A, Numbla 1:100 000 Geological Sheet 8624. Geol. Surv. NSW, Sydney. 1989 White A J R, Williams I S, & Chappell B W, Berridale 1:100 000 Geological Sheet 8625 Geol. Surv. NSW, Sydney. 1976 Wyborn D, Owen M, & Wyborn L, Geology of Kosciusko National Park (1:250 000 scale map). AGSO, Canberra. 1990

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STRUCTURAL AND PETROLOGICAL STUDY OF A HIGH GRADE TERRAIN, NORTHWEST CORNER OF THE QUEEN ELIZABETH PLUTON, MOUNT ISA INLIER, AUSTRALIA. Peter Crowhurst. School of Earth Sciences, La Trobe University, Melbourne, Vic 3083 The aim of the project was to provide observational constraints for models that attempt to explain the heat source for the regional low P/high T deformation and metamorphism that is widely recognized in the Mount Isa Inlier. The area of study lies within the high-grade Proterozoic terrain west of the Mount Isa Fault Zone in the Western Fold Belt of the Mount Isa Inlier. Three metasedimentary/metavolcanic and five meta-igneous phases of intrusion were distinguished within the study area. The oldest cover units are the metasediments and metavolcanics of the Haslingden Group, which overly Yaringa Metamorphic Basement and are intruded by the multiple igneous phases. The Haslingden Group was deposited during the last major period of extension and rifting recorded within the Mount Isa Inlier, and was terminated by widespread granitic plutonism (-1670 Ma). The Sybella Batholith was intruded at this time west of the Mount Isa Fault Zone. Within the study area, the oldest unit exposed is the May Downs Gneiss, which continues to be a source of debate over whether it is part of the basement or the overlying Haslingden Group. Field evidence indicates general conformity with the overlying units, which consist of metamorphosed quartz-sandstone of the Mount Guide Quartzite and the amphibolite-dominated Eastern Creek Volcanics. Multiple phases of igneous intrusions characterize the study area. These include medium-grained, light grey granodiorite, coarse-grained, pink Sybella Granite, pegmatite,aplite and dolerite. The granodiorite appears to be the oldest, as it is crosscut by the other intrusions, with the Sybella Granite and aplite appearing to be coeval and all are post-dated by the dolerite. The pegmatites intrude in multiple phases with wide ranging ages. After the granitic plutonism there is a considerable hiatus (50-130 Ma) before the onset of the regional compressional deformation and low P/high T metamorphism. Two generations of deformation are recognized in the study area, (defined as Di and D2), which formed during a progressive east-west shortening event. The first generation of folding developed tight to isoclinal, asymmetric, inclined Fi folds with shallow plunges and a strong axial planar foliation. The axial planar Si foliation is recognized as a very strong mineral alignment and is pervasive throughout the area. The second generation of folding is not as pervasive, forming generally open, upright and steeply plunging F2 folds with a subtle micaceous S2 crenulation cleavage. Metamorphism reached upper amphibolite facies and appears to have been generated late- to post-Di. Retrograde greenschist metamorphism was generally synchronous with the D2 event and associated multiple phases of metasomatism. Five types of metasomatism are recognized, which include silicification, epidote-alteration, quartz-tourmaline alteration, calcareous alteration and minor copper mineralization. The metasomatism is closely related to the late phase of pegmatites, which are an obvious source of fluids. The resultant altreation is very localised and does not appear to be part of any large regional fluid movements. Dolerite intrusions commonly crosscut and deviate the Si foliation, but are overprinted by amphibolite grade metamorphism, therefore implying they were emplaced late-syn to post-Di during the peak of metamorphism. The net-vein complex

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and porphyritic granite are also undeformed by the Di event and are intimately related to the dolerite emplacement. Recent models suggest large scale granitic or mafic magmas intruding at midcrustal levels as the source of the thermal pulse during low P/high T metamorphism, however, large granitic intrusives in the area, including the Sybella Granite have been dated as being considerably older. Crosscutting relationships indicate that the dolerite intrusions in the study area are synchronous with the timing of local high-grade metamorphism. This may imply that the widespread mafic intrusions are a major contributor to the heat source for the regional low P/high T metamorphic event.

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SYNCHRONOUS SYNTECTONIC GRANITE EMPLACEMENT ACROSS THE SOUTH PALMER RIVER REGION: THE ROLE OF DEFORMATION INTENSITY IN THE INTERPRETATION OF TIMING OF EMPLACEMENT Brett K. Davis. James Cook University ofNorth Queensland, Townsville, Queensland, 4811, Australia The marine siliciclastic rocks of the South Palmer River region have been intruded by several biotite-muscovite adamellite plutons, which have produced well developed metamorphic aureoles characterized by abundant andalusite. Outside the aureoles, the metamorphic grade has only risen to the chlorite zone. Around the Cannibal Creek Granite porphyroblast/matrix microstructures and inclusion trails within porphyroblasts are remarkably well developed. Around the Two Brothers Peak pluton, in the approximate centre of the area, all the andalusite porphyroblasts are altered to fine-grained mica. Inclusion trails are rare and restricted to those defined by graphite. Around the Kelly St George Granite pristine andalusite porphyroblasts are extremely common, but inclusion trails are exceptionally rare and have only been noted in a few samples. No inclusion trails have been noted in the porphyroblasts of the Desailley Granite aureole. Timing of emplacement of the granite bodies has been resolved relative to the regional deformation history through the integration of field and microstructural observations. Criteria used for determining the relative timing between pluton emplacement and deformation (e.g. pluton shape, solid state deformation in the pluton, truncation of wall-rock cleavages by plutons) are sometimes misleading. Porphyroblast-matrix relationships provide a powerful tool for resolving this problem and contain information on timing, kinematics and the nature and orientation of earlier foliations. However, deformation partitioning in pluton wall-rocks during granite intrusion that is synchronous with regional tectonism potentially creates structures suggesting different timing of emplacement due to the variation in development and preservation of deformation fabrics. This has an important bearing on the timing of pluton emplacement, particularly when using porphyroblast/matrix microstructural relationships. Consequently, structures within the pluton/wall-rock system must be carefully timed in order to avoid erroneous ages; for example, apparently different structural ages for plutons which were synchronously emplaced. In the South Palmer River region this problem has been overcome by mapping and making numerous spatially oriented thin sections from traverses away from the pluton aureoles across the transitions of structure orientation and style. These traverses have confirmed that the syn-emplacement crenulations, defined by their syn and post-porphyroblast development, are regionally developed. Six discrete tectonic events have affected the South Palmer River region. Microstructural relationships, particularly those between porphyroblasts and matrix, indicate that emplacement of the Cannibal Creek Granite and Kelly St George Granite occurred synchronous with the regional D deformation. Similar relationships indicate that emplacement of the Two Brothers Peak and the Desailley Granite post-date S . Consistent Permian isotopic ages, considered to reflect crystallization ages of granites belonging to the Whypalla Supersuite, indicate that all the plutons in the South Palmer River region are of the same age and therefore, were all emplaced during D . Variation in degree of foliation development within the plutons is interpreted as being due to partitioning of D deformation intensity from pluton to pluton. Granite isotopic ages, combined with structural timing evidence of porphyroblasts and deformation fabrics, indicates that D was not diachronous 4

3

4

4

4

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across the region. Accurate structural timing of aureole porphyroblasts, relative to regional deformation fabrics in the aureoles of different granites, adds confidence to isotopic dating.

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STRUCTURAL AND TECTONIC EVOLUTION OF THE NORTHERN AUSTRALIAN OROGENIC PROVINCE IN N.T. AUSTRALIA Puquan Ding North Flinders Mines Limited, 24-25 Greenhill Road, Wayville, SA. 5034.

Part of the Northern Australian Orogenic Province considered here includes the Tanami Complex in the Tanami Region, the Warrramanga Group in the Tennant Creek Inlier, the Pine Creek Geosynclinal sequences and the Arunta Complex. The stratigraphy and deformational history within the above four regions are surprisingly comparable. The stratigraphic sequence can be divided into three major groups: a lower pile of shallow water quartz sandstone and conglomerate which were originally deposited unconformably on the Archaean basement; a middle pile of pelitic and chemical sedimentary rocks deposited in a quiet enviroment and an upper pile of turbiditic greywacke which represents rapid erosion and deposition in an active enviroment. However the sequences are only fully exposed in the Tanami and Pine Creek regions. The area have been affected by five major orogenic deformational events and several phases of post-orogenic deformation. D1 has been used since 1990 (Ding 1990) by the author to describe an unusual deformational event occurred during an extensional and subsiding period of the Early Proterozoic basin and this contribution has been published at the first time in June 1993 (Ding and Giles, 1993a, 1993b). The D1 structures are mainly bedding-layer parallel to low angle extensional detachment shearing, include discrete shear zones, boudinage of competent lithologies, elongation lineation and intralayering recumbent folds (Fig. la). The layer parallel shear zone affected the whole Tanami region, tens of thousand squar kilometres, and has been observed in the Pine Creek regions. Low angle extensional detachment shear zones / faults usually have brought the upper stratigraphic unit against the lower stratigraphic unit with absent of some of the middle stratigraphic unit, as having been observed in the Tanami and Pine Creek regioms. Syn-Dl granitoids and dolerite intruded at a later stage of D1 extension and developed layer parallel schistosities. D2 is used to describe the first compressional event which caused the formation of NNE to NE trending fold belts. The F2 folds were upright and linea with gently plunging hinges (Fig.lb). Many F2 folds have been mapped in the Tanami region, however in the Pine Creek and the Tennant Creek regions F2 folds can only be recognised by structural analysis rather than direct observation because of the intense interference of later phases of folds. The D1 shear zones have been folded by D2 but also have facilitated the development of decollement. The syn-Dl granitoids and dolerite were also folded by D2. D3 is used to describe a regional reworking event in Northern Australian Orogenic Provinve. Intense WNW to NW trending folds and faults superimposed onto the NNE to NE trending D2 fold belts and formed complex interference pattern including tight but double plunging folds with steep hinges, dome-like, mushroom-shaped and lunaform folds (Fig.lc). D3 structures have been well observed in four regions mentioned above, including Arunta Block. Granitoids and porphyries intruded and cross cut the D3 structures. Some of the magma extruded and formed volcanic strata unconformably overlain the refolded strata in the Pine Creek area. D4 is used to described a regional north-south compressional event which caused major E-W trending faults and steep dipping schistosity in post-D3 intrusive rocks as well as in metamorphosed strata (S4). Schistosed or foliated granitoides and porphyries are extensively exposed in the whole orogenic province. Locallised E-W trending F4 folds caused dextral asymmetric kinking of F3 fold axial plains. D5 is used to described a regional E-W compressional events, which caused conjugate kink folds from E-W trending geological bodies in the Tennant Creek region, locally

34

developed NE, NNE and NNW trending folds with axial plane schistosity or crenulation cleavage in the Tanami region and the Tennant Creek region. The post-D3 volcanics have been intensely deformed by D4 and D5. Gold and base metal mineralisation mainly occurred during each of the orogenic deformational events. Post-orogenic cover sequence of Early Proterozoic age was deposited after D5. Many brittle and semiductile deformational events, which affected the post-orogenic cover sequence, occurred consequently. Post-orogenic granitoids intruded into the cover sequences as well as the orogenic basement REFERENCES: Puquan Ding and Chris Giles, 1993a. Geological setting of gold mineralisation in the Tanami Region, Northern Territory, Australia, Proceedings of the international symposium on gold mining technology, Betjing, 1993. Puquan Ding and Chris Giles, 1993b. Timing of deformational events with respect to gold mineralisation in the Tennant Creek Goldfield, Northern Territory, Australia, Proceedings of the international symposium on gold mining technology, Betjing, 1993.

Figoe. 1- Schematic

diagrams ^fxosomj siructural

/Vohffoehn Austivlt** Otogenic fhovtncz .

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e.uolution of th?

DEFORMATIONAL STYLES AND GEOMETRIES IN GRANULITE TERRAINS; CONSTRAINTS ON DEFORMATION MECHANISMS Paul Dirks Institute of Earth Sciences, University of Utrecht, POBox 80021,3508TA, Utrecht, the Netherlands

Constraints on deformation mechanisms and flow behaviour in rocks are generally obtained from direct observations of microstructures in thin section in combination with rock analogue studies. These techniques are difficult to apply to granulites because pervasive syn- and post-kinematic annealing and grain growth generally obliterate all deformation features, whilst deformation in poly-crystalline materials with a large proportion of partial melt is difficult to simulate in analogue studies. Therefore deformation geometries become important as an indirect means of constraining the mechanical behaviour of granulites. In many granulite terrains the mechanical properties of the rocks are determined by the strength of quartzo-feldspathic matrix-minerals. These felsic granulites are characterized by a range of typical geometries that can be illustrated with rocks from Prydz Bay, Antarctica, where outcrops are superb and metamorphic conditions ranged from 14 to 5 kbar and 1000 to 700 °C. These conditions existed during at least two separate overprinting events in alternating compressional and extensional settings. The following structural features are important: a) Groups of structures that belong to one granulite-grade metamorphic episode are found to be colinear over very large areas. This means that all structural features, including all fold generations, the various truncating foliations that developed during one prograde event, high strain zones and melt/pegmatite veins contain one common lineation direction which parallels the mineral elongation lineation. Folds are generally disharmonic and high strain zones non-planar, whilst truncations of various generations of high strain zones or gneiss foliations, all containing the same mineral elongation lineation, are common. Fold sequences associated with each generation of gneissic foliation are similar, as are fold sequences associated with unrelated granulite events. Fold geometries or styles are therefore not useful to characterize a granulite event. What is useful, however, is the common lineation direction and the associated movement sense. Thus, in Prydz Bay various groups of structures with identical fold geometries can be distinguished because of their association with a unique lineation. This observation may serve as an important tool in mapping composite granulite terrains. b) If a granulite event associated with a gneissic layering and lineation direction overprints earlier fabrics, the early lineation, as defined by inclusion trails in coarse-grained phases and early lineation trails in low-strain pockets, generally remains directed in one constant orientation. In SE Prydz Bay, high strain zones with intense S-L fabrics separate technically distinct terrains and contain syn-kinematic decompression textures recording several kbars of uplift (at 750°C-53kbar). The shears clearly formed in response to non-coaxial flow, yet, garnet clasts within these zones preserve sillimanite inclusions that are aligned with each other and with lineations outside the high strain zones over a distance of 50 km, suggesting a non-rotational behaviour of the clasts. c) The survival of peak assemblages in zones of intense deformation during retrograde uplift is a common feature. In central Prydz Bay planar high-strain zones contain lineations defined by assemblages that record P-T conditions ranging from 12 kbar - 900°C to 5 kbar 700°C. The peak-metamorphic assemblages survived intense shear deformation enabling km's of uplift at high-grade conditions. d) Grain shape fabrics and dislocation substructures are lacking, and crystallographic preferred orientations are generally weak or absent in granulite grade shear zones. Exceptions to this were observed in quartz mylonites which deformed between 680°C-750°C and preserved good crystallographic fabrics. Likewise, classic kinematic indicators such as s-c fabrics or monoclinic shape asymmetries of feldspathic clasts do not occur, although garnet or orthopyroxene clasts in a quartz-feldspar matrix may preserve 8-clast asymmetries. Geometries 36

that have been observed include garnets with mantles of cordierite that preserve 5-clast asymmetries with stair stepping which formed at 750°C - 5-3 kbar, and garnet and orthopyroxene clasts with biotite-feldspar mantles that preserve 5-clast asymmetries without stair stepping which formed at ~850°C - 12-9 kbar. e) Partial melt plays an important role in granulite grade structures, since most prograde reactions in quartzo-feldspathic rocks involve the break-down of biotite and the production of a melt phase which will be dispersed on grain boundaries or segregated in pockets. In general, the high temperature granulites (T > 800°C) in Prydz Bay show few structurally arranged melt pockets. Instead, leucosome material is dispersed homogeneously along the foliation (stromatic and nebulitic migmatite). In contrast, lower temperature migmatites (T < 800°C) commonly contain leucosome pockets that occur along shear bands, boudin necks and fold hinges; all representing zones of increased finite strain. The dominance of a certain deformational mechanism at granulite grade appears to be strongly determined by rock type and metamorphic temperature, which, of coarse, is not surprising. The above observations indicate that for felsic granulite with a quartzo-feldspathic matrix, applied stress fields at granulite grade are uni-axial. In these rocks very large finite strains have been achieved without metamorphic reequilibration. This and a lack of grain shape and crystallographic fabrics suggest a dominance of grain boundary sliding processes in combination with grain boundary diffusion resulting in Newtonian flow. Only at lower temperatures (T < 800°C) do intra-crystalline dislocation-creep processes appear to play a role as suggested by the preferred occurrence of melt reactions in areas of increased strain and stair-stepping 8-clast geometries. The observations suggest that the operating deformational mechanisms in granulite inhibit rotation of larger clasts, even in zones with a clearly non-coaxial strain history. Explanations for this can only be guessed at, but a non-circular shape of the clasts, a major coaxial shortening component, as well as "lubricating" effects of grain boundary melts may have played a role.

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THE EFFECT OF RIGID PARTICLES ON THE STRENGTH OF SYNTHETIC MARBLES G. Dresen, GeoForschungsZentrum-Potsdam, FRG> M. Eckhart, B Evans, M.I.T., Cambridge, MA 02139, USA, and D. Olgaard, ETH-Ztirich, Switzerland. We synthesized pure and two-phase carbonate rocks using reagent-grade CaC03 powder and rigid second-phase particles ranging in size from 5 to 25 |im average diameter. The dilute dispersions contained up to 20% second phase and were hot isostatically pressed to form a finegrained carbonate rock of low porosity (4-5%). We also produced pure carbonates with porosities of up to 10%. A study of the brittle and semibrittle behaviour of this rock at room temperature revealed the importance of a crack-like porosity at the interface between matrix and second-phase particles on the strength and mode of failure. New tests at elevated temperatures (400°-600°C) and pressures (200-350 MPa) cover the transition from semibrittle to plastic flow in the aggregates. Samples deformed at 400°C still show cracking and pressure sensitivity indicating semibrittle deformation. At 600°C there is a substantial difference in strength between pure calcite and second-phase aggregates, which are stronger by a factor of 3 for 20% particles. Mixing models based on uniform stress predict a strengthening factor of only 1.07 for this material. Likewise particle content seems to control the transition from work hardening to steady-state flow. Deformation gradients between the rigid inclusions and the calcite matrix cause dislocation densities to be considerably higher close to the particle/matrix interface. At 600°C deformation is predominantly crystal plastic as there was no pressure sensitivity of this material.

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COOLING RATE HISTORIES FROM CLOSURE OF GARNET-BIOTITE GEOTHERMOMETRY: EVIDENCE FOR TRANSIENT EVENTS ? Karin Ehlers. Brett Marmo, Sharif Oussa, and Roger Powell, School of Earth Sciences, University of Melbourne, Parkville, Vic 3052, Australia A critical parameter in understanding the pressure-temperature-time history of metamorphic rocks, and thus the tectonic evolution of orogenic belts, is the cooling path following the metamorphic temperature peak. Such cooling paths may be directly inferred from geochronological dating or from the temperature-dependent element distributions between minerals. If a mineral assemblage such as garnet-biotite has re-equilibrated during cooling, then the thermometrically-inferred temperature-the closure temperature (T )-is a function of grain size, cooling rate (s) and diffusion parameters. These closure temperatures may be used in conjunction with the diffusion equation of Dodson (1986) to estimate cooling rates (Ehlers et al., in press). The cooling rate inferred from Dodson's equation is the cooling rate at the closure temperature. The primary control is grain size,with centres of large grains closing at higher temperatures than the centres of smaller ones. Consequently, changes of the cooling rate with temperature may be recorded in the minerals for a significant part of the cooling history. Using this, we have estimated cooling rate histories from three different terrains: the Koralm region (Eastern Alps, Austria) and two granulite facies terrains (the Bunger Hills, Antarctica, and the Gawler craton in South Australia). Cooling rate histories from all three terrains indicate extremely fast cooling rates at high temperatures and much slower cooling rates at lower temperature. Such fast cooling rates may be evidence for transient heating events in these different tectonic settings. c

y

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MAGMATIC AND TECTONIC EVOLUTION OF THE SOUTHERN GAWLER CRATON - NEW DATA ON TIMING FROM U-Pb ISOTOPICS CM. Fanning and L.R. Rankin*, Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, "Department of Mines & Energy, P.O. Box 151, Eastwood, SA, 5063. Three megacycles of orogenic development have been previously recognised in the southern Gawler Craton (Fanning er a/., 1988): 1) Archaean sedimentation and volcanism followed by plutonism and deformation during the Sleafordian Orogeny. 2) At least 3 phases of Palaeoproterozoic basin development with contemporaneous volcanism, accompanied by episodic granitic magmatism and deformation during the Kimban Orogeny. 3) Mesoproterozoic anorogenic acid magmatism resulting in voluminous and extensive high-level granite plutons (Hiltaba Suite), volcanics (Gawler Range Volcanics) and associated volcaniclastic and clastic sediments. Recent ion-probe U-Pb zircon geochronolgy of selected granitoids and gneisses from the Tumby Bay - Port Lincoln area, combined with detailed mapping has allowed more precise limits to be applied to the timing of several of the events within this history, but has defined a previously unrecognised late-stage magmatic event. Highly deformed garnetiferous quartzofeldspathic (+/-hypersthene) gneisses and mafic granulites exposed at Bratten Cairn represent a sliver of Archaean Sleaford Complex caught up within the Palaeoproterozoic Kalinjala Mylonite Zone. Zircons within the garnetiferous metaseimentary gneisses exhibit complex multi-stage Pb-loss, most likely associated with both the Sleafordian Orogeny, and possible multiple - stage movements along the Kalinjala Mylonite Zone. The core of one zoned zircon from the gneiss gives an age of ~3250Ma, representing some of the oldest material recognised within the Gawler Craton. This zircon is interpreted to represent a relic of the crystalline crustal precursor to the sediments represented by the Carnot Gneisses. Between the Sleafordian Orogeny of Megacycle 1, and the deposition of the Hutchison group metasediments prior to the onset of the Kimban Orogeny of Megacycle 2, a suite of migmatitic granitic gneisses were developed by intrusion og granitoids into the Archaean crust, followed by high grade metamorphism. Previously recognised in the Cowell - Plug Range area, recent mapping has shown the Miltalie Gneiss to be a major, extensive unit in the southern Eyre Peninsula. Earlier geochronology (Fanning et al., 1988) gave an age of -1964 for the high-grade metamorphism, and suggrsted thast a population of zircons at 2014+/-28 Ma represented the age of the gneiss precursor. Two samples from the Tumby Bay - Koppio area give precise ion-probe crystallisation ages for the Miltalie Gneiss of 1996+/-8 & 1991+/-7Ma. The intrusion and metamorphism of the Miltalie Gneiss is a major orogenic event that lies between the orogenies of Megacycles 1&2. The event is likely related to isothermal decompression of the Archaean crust during initial extesion at the onset of formation of the depositional basin for the Hutchison Group. In the Tumby Bay - Koppoi area, both the Miltalie Gneiss and the overlying Hutchison Group metasediments (~1960-1850Ma) are intruded by a multiphase suite of I-type

40

(+minor S-type) granitoids associated with D3 of the Kimban Orogeny. An hornblende granodiorite from the Moody Suite gave a crystallisation age of 1702+/-lOMa, which agrees with a previously determined Rb-Sr age of 1709+/-14Ma (Mortimer et al, 1986). This age is also within error of a Rb-Sr age of 1710 for a late-stage pegmatite which cross-cuts the principal mylonitic fabric of the KMZ, approximately 40kms east of the Moody Suite. The pegmatite date has been interpreted as a minimum age for movement during D3 along the KMZ (Parker et al., 1988), and the new age determination for the D3 Moody Suite provides further evidence that the KMZ was dominantly active during D3. The latest orogenic activity previously recognised in the southern Gawler Craton was the Wartakan Event, represented by minor kink and open folds and fractures within rocks of Megacycles 1,2&3. Parker (1993) has suggested that this event is contemporaneous with intrusion of the ca. 1630Ma St Peter Suite granitoids of the western Gawler Craton. Recent mapping and U-Pb dating of the Spilsby Suite granite exposed on islands east of Tumby Bay indicate an intrusive age of ~1500-1530Ma, with minor folding and foliation development within the granite. This intrusive age is unique for the Gawler Craton, although similar Rb-Sr ages (Webb et al., 1986) are common throughout the southern and western Gawler Craton. REFERENCES Fanning, C.M., Flint, R.B., Parker, A.J., Ludwig, K.R. & Blissett, A.H., 1988. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology. Precambrian Research,40/41:363-386. Mortimer, G.E., Cooper, J.A. & Oliver, R.L., 1986. The geochronological and geochemical evolution of the Proterozoic Lincoln Complex, Eyre Peninsula, South Australia. Geological Society of Australia, Abstracts series, 15:140-141. Parker, A.J., 1993. Palaeoproterozoic. In Drexel, J.F., Preiss, W.V. & Parker, A.J. (editors). The Geology of South Australia, Volume 1, the Precambrian. South Australia. Geological Survey. Bulletin, 54. Webb, A.W., Thomson, B.P., Blissett, A.H., Daly, S.J., Flint, R.B. & Parker, A.J., 1986. Geochronology of the Gawler Craton, South Australia. Australian Journal of Earth Sciences, 33: 119-143.

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STRUCTURE AND STRAIN ANALYSIS OF THE WONGWIBINDA SHEAR ZONE, NEW ENGLAND OROGEN Terry Farrell, Department of Geology, University of Newcastle, Callaghan, NSW 2308. The Wongwibinda Shear Zone, in the southern New England Orogen, is a major zone of uplift on the eastern margin of the Wongwibinda Complex. It is manifested by a wide, steeply dipping, NNW-trending zone of ductile deformation, which runs parallel to the Wongwibinda Fault through the Abroi Granodiorite, a member of the S-type Hillgrove Plutonic Suite. Shearing is first evident in the Granodiorite up to 2 km from the Fault with the development of composite planar (S-C) fabrics (Berthe et al. 1979, Lister & Snoke 1984). The S folia have the same orientation as the foliation in unsheared rocks, but are more closely spaced, and are deflected by a second, finer-grained foliation (C) which dips more shallowly to the west. The S and C folia are defined principally by the lattice preferred orientation of new mylonitic, and relict igneous biotite, and the dimensional preferred orientation of fine-grained recrystallised quartz and quartzo-feldspathic aggregates. Approaching the Wongwibinda Fault, the S and C folia become finer-grained, more pervasive, continuous and closely spaced, and the S-C angle decreases from approximately 30-35°, to 0-10° adjacent to the fault. In the most intensely deformed rocks, late shear bands (C') and thin ultramylonite zones are common, and more rarely, crosscutting pseudotachylite veins are developed. The S and C folia contain a well developed, down-dip mineral elongation lineation defined by the alignment of biotite and elongate aggregates of quartz, feldspar and biotite. This lineation is poorly developed in low strain rocks, but becomes better developed approaching the fault. A west-over-east, reverse sense of movement on the Wongwibinda Shear Zone is indicated by the angular relationships between S and C, the asymmetry of feldspar porphyroclasts, biotite fish, displaced broken grains, and asymmetric microfolds in quartz ribbons. The Abroi Granodiorite shows a systematic microstructural variation approaching the Wongwibinda Fault. Away from the fault (greater than -500 m), the finite strain is low and the microstructures are characteristic of shearing under amphibolite facies conditions. Quartz is recrystallised into finer-grained aggregates with low energy grain shapes (e.g., 120° triple junctions, relatively straight grain boundaries), suggesting that the temperature was still relatively high when deformation ceased. Plagioclase porphyroclasts are slightly elongate (parallel to S) and show limited development of recrystallised tails. The presence of new grains of oligoclase in the tails and in the neocrystallised matrix indicates that shearing took place under amphibolite facies conditions. Closer to the fault, the granitoids are finer-grained, more highly strained, contain a greater proportion of new grains, and show microstructures indicative of the amphibolite-greenschist facies transition. In particular, plagioclase shows greater development of recrystallised tails, but is also strained and commonly has fractures healed by K-feldspar ± quartz. Quartz occurs in polycrystalline aggregates containing varying proportions of relatively equant grains and very elongate, highly-strained grains, with irregular and sutured grain boundaries. A distinctive feature with increasing strain is the development of high aspect (X-Z) ratio quartz ribbons parallel to C. The constituent quartz grains are elongate and lie parallel to S and oblique to the long axis of the ribbon. The highest strain is developed adjacent to the Wongwibinda Fault (within ca. 100 m) and granitoids in these locations display microstructures dominated by deformation under middle-greenschist facies conditions; namely, quartz ribbons consisting of extremely elongate strained grains with irregular grain boundaries, highly strained and multiply fractured K-feldspar and plagioclase clasts, and ribbon structures in biotite. These microstructural variations are indicative of a gradual change in the dominant deformation mechanism in quartz, from recrystallisation-assisted dislocation creep to cataclastic 42

flow, and are consistent with a transition toward more brittle behaviour, and the partitioning of strain into a narrower zone close to the fault, with decreasing temperature. Minimum estimates of the displacement on the Wongwibinda Shear Zone were obtained by integration of the shear strain across the zone (cf. Ramsay & Graham 1970), assuming that: (i) simple shear was the dominant deformation mechanism, (ii) the C folia are parallel to the shear zone boundary, and (iii) the S planes are parallel to the X-Y plane of the finite strain ellipsoid. The shear strains in a series of samples on two traverses across the shear zone were calculated from S-C angles (0) and X-Z ratios of deformed quartz grains using the relationships (Ramsay & Graham 1970): X/Z = (y2 + 2 + y^y 2 + 4)1/2) / 2, a) y = 2 / tan 20. (2) The shear strain-distance profiles (Fig. 1) may be described by the general equations relating shear strain to distance from the fault: log(y) = log(a) + b.log(x), (3) Y = a . x\ (4 ) where a is the intercept on the shear strain axis, b is the slope, and x is the distance from the fault. Linear regression of the data was used to estimate the parameters a and b, and then the half-width displacement was calculated by integration of equation (4) over the width of the shear zone. Assuming that the strain is equally distributed on both sides of the Wongwibinda Fault, the average total displacement is -9.1 km. If the shear zone boundary is assumed to be planar and parallel to C (average dip 60°), then the total displacement can be resolved into a vertical component of -7.9 km, and a horizontal component of -4.6 km. These are minimum estimates because a significant proportion of the shear strain in very highly strained rocks may be accommodated in the formation of late shear bands and ultramylonite zones, as well as in slip along discrete planes (e.g., basal slip in biotite; slip along C), and therefore, the true displacement may be substantially greater. Berthe D., Choukroune P. & Jegouzo P. 1979. Struct. Geol. 1, 31-42. Lister G.S. & Snoke A.W. 1984. / . Struct. Geol. 6, 617-638. Ramsay J.G. & Graham R.H. 1970. Can. / . Earth Sci. 7, 786-813.

a uctf GO ?H a •r-t

<V

A

CO

Y = 39.99 x ° A 4 i ri.e

J 0 Y d x •» 4.4 km

10

100

1000

Distance from the fault (m) Fig. 1: Shear strain profiles across the WSZ. o = Yxyz> • = Ye- Approximate error limits shown for ye.

43

STRUCTURE OF THE EARLY PALAEOZOIC ANAKIE METAMORPHICS, CLERMONT, CENTRAL QUEENSLAND Christopher L. Fergusson , Timothy J. Green and Ian W. Withnall department of Geology, University of Wollongong, Wollongong NSW 2522, Australia Geological Survey of Queensland, GPO Box 194, Brisbane, QLD 4001, Australia The Anakie Metamorphics form much of the basement of the Anakie Inlier in central Queensland. We have undertaken a detailed investigation of the Anakie Metamorphics in a 300 km area to the north and south of the Clermont-Alpha road 20-40 km to the west of Clermont. This area is important because it contains all elements of the preliminary stratigraphy recognised by Ian Withnall for the metamorphics and also contains the best exposures within the inlier. Our mapping clearly demonstrates that the Anakie Metamorphics are a structurally complicated unit with lithological layering, and rare grading, the only preserved primary features. The major rock types include: mica schist, psammitic schist, quartzite, graphitic schist, greenstone (mafic schist), serpentinite and calc-silicate rocks. From identification of several associations of these rock types it is possible to map rock units across the inlier which form complex patterns related to the effects of several superposed phases of deformation. Three main deformations affect the Anakie Metamorphics. developed a strong foliation that is everywhere parallel to relict bedding where the latter is found. A relict lineation, probably an intersection lineation, is occasionally observed on S surfaces on the limbs of younger fold generations. Due to the affects of subsequent deformation we have no data on the orientation of D structural elements. A pervasive D event has affected the metamorphics with strong foliation development and intense folding of the previous S and S . D produced flat-lying structures with a widespread intersection lineation. Stretching directions are indicated by pressure fringes and mineral elongations and both trend east-northeast. F folds are pervasively developed at microscopic and mesoscopic scales with S forming a strong crenulation cleavage with widespread microlithon development. F and L^ have variable orientations spread along S planes with local concentrations in the stretching direction. In quartzites the foliation is mylonitic in character with strong folding and a strong lineation developed. D has caused significant reorientation of the D structures and formed the complex dome and basin patterns evident in lithological units throughout the inlier. The metamorphic grade throughout most of the Anakie Metamorphics is mid to lower greenschist facies with pervasive recrystallisation and foliation development and biotite muscovite - chlorite assemblages in schists and chlorite - epidote + actinolite in greenstones. A belt of middle amphibolite facies with andalusite - garnet + staurolite porphyroblasts occur in the far western part of the inlier. Metamorphism was synchronous with D and D and is of low pressure - high temperature (LPHT) type. The Anakie Inlier is only a relatively small region exposing the substrate to the Middle Palaeozoic to Mesozoic sedimentary basins that occur throughout much of central Queensland. In the past the Anakie Metamorphics have been regarded as part of the Thomson Fold Belt, essentially equivalent to the Lachlan Fold Belt to the south, but now thought to have more in common with Early Palaeozoic and Late Proterozoic units that occur along the western margin of the Tasmanides in north Queensland, western New South Wales and western Victoria. 1

1

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STRUCTURAL EVOLUTION AND GEOMETRY OF THE ADELAIDE FORELAND FOLD THRUST BELT - A REASSESSMENT BASED ON BALANCED CROSS SECTIONS Thomas Flotmann and Pat James Department of Geology and Geophysics, University of Adelaide, South Australia 5005 The Adelaide Fold Thrust Belt constitutes the foreland portion of the Delamerian orogen along the eastern margin of the Proterozoic Gawler craton in South Australia. The northern part of the Belt forms a large antitaxial curvature (Nackara Arc) of broadly folded Neoproterozoic strata of the Adelaidean basin ('geosyncline'). The internal structure of the southern Adelaide Fold Belt, which forms the syntaxial Fleurieu Arc of Fleurieu Peninsula and Kangaroo Island, is governed by emergent reverse faults and mylonitic shear zones. Additionally the eastern and southern parts of the belt are fringed by deposits of the Cambrian Kanmantoo basin. The western part of the fold thrust belt is lithologically characterized by the youngest Adelaidean strata. Structurally this domain displays fault propagation folds, and thrust ramps, and comparably low strains. To the east this zone is followed by an imbricate fan, the easternmost of which incorporate Mesoproterozoic basement in the core of major ramp anticlines. Balanced and restored cross sections reveal a remarkable change in structural style which spatially coincides broadly with the locus of Kanmantoo Group deposition. The Kanmantoo basin formed during a second phase of extension during the Cambrian. This is particular evident along principal growth faults across which the Cambrian strata show dramatic thickness changes, as revealed by restored section and by incorporating strain data into the balanced setions. Furthermore, the contractional evolution and the strain distribution within the Kanmantoo basin is largely controlled by the principal basin controlling growth faults, which were partly not fully inverted during the Delamerian orogeny (i.e. show normal stratigraphic offset, despite reverse slip along the faults). Owing to the steep easterly dips of these faults, in the western part of the basin lateral shortening of up to 55 % of the Kanmantoo basin is largely accommodated by intense folding and and fold axial-planar flattening strain. Further east strain magnitudes wane and the overall shortening of around 30% is predominantly accomodated by discrete thrust zones. In particular the Kanmantoo basin displays a remarkable swing from a north trending structural grain on Fleurieu Peninsula to partly (apparent) W-trends on Kangaroo Island. The geometry the Kanmantoo basin, which during the basin inversion underwent strong telescoping of metamorphic, as well as sedimentary facies is partly predetermined by the shape of the Gawler craton, and the oroclinal bending of the Fleurieu arc is probably less dramatic than previously assumed. Overall the structural geometry of the Adelaide Fold Thrust belt shows characteristic elements of fold and thrust belts elsewhere, it is however noteworthy that the overall shortening along major thrusts and reverse faults shows maximum displacements in the order of one to six kilometres only. Our results clearly indicate that the all rock units incorporated in the Adelaide Fold Thrust belt are of parautochthonous character.

45

GEOMETRY OF THE CAMBRIAN KANMANTOO BASIN IN SOUTH AUSTRALIA AND ITS RELATIONSHIP TO DIFFERENT MODES OF CONVERGENCE AT THE AUSTRALIAN AND ANTARCTIC PALAEOPACIFIC MARGIN Thomas Flottmann , John Foden , Pat James and Stephen Marshak 1

1

1

2

department of Geology and Geophysics, University of Adelaide, South Australia, 5005 department of Geology, University of Illinois, Urbana, IL 61801 The Kanmantoo Group forms a >6km thick sequence of flysch-like, mainly unfossiliferous sediments, which were rapidly deposited during Cambrian subsidence between 526 and 516 Ma. In South Australia the Kanmantoo basin displays a conspicuous curvature over a lateral distance of about 100 km. Whereas the basin trends N in the eastern Mt. Lofty Ranges, it trends ENE to the south on Kangaroo Island. Deposits equivalent to the Kanmantoo Group occur east of the Murray basin in the Glenelg River Complex of western Victoria, which forms the eastern inboard portion of the Delamerian Orogen of southeast Australia. In a Gondwana context the inboard portion of the southeast Australian Delamerian orogen (Adelaide Fold Thrust Belt, the Murray Basin province and the Glenelg River Complex) are traditionally explained as laterally continuous with the Wilson terrane of northern Victoria Land during Palaeozoic times. Both regions underwent contractional deformation during the early Palaeozoic Ross/Delamerian orogeny. On closer inspection, however, there appear to be characteristic differences between the Australian Delamerian and the Antarctic Ross Orogen. Sm-Nd isotope data from the Wilson terrane in northern Victoria Land, suggest that there is only no isotopic evidence for rocks of similar provenance to those of the Kanmantoo Group in southeast Australia. Rb-Sr dating of major contractional structures in northern Victoria Land furthermore shows that the Antarctic Palaeo-Pacific margin underwent protracted orogenic shortening, which is mainly subduction related. Contrastingly, convergence along the inboard Australian Palaeo-Pacific margin was only short-lived and was not preceded by subduction beneath the Glenelg River Complex, but is rather characterized by obduction/ collision processes. In the Murray Basin province of southeast Australia, there is abundant evidence for postorogenic Cambro-Ordovician magmatism, which is absent in northern Victoria Land. The orogenic record suggests that the Antarctic portion of the Palaeo-Pacific margin has undergone a greater amount of orogenic shortening over a longer time-span than the Australian portion of the same margin. The different magnitudes of shortening along Antarctic and Australian Palaeo-Pacific margin resulted in a zone of strike-slip accomodation, which is roughly located at the break-up suture between the two continents. Interaction of this strike-slip zone with the orogen-normal extensional component resulted in the apparent bending of the southern Kanmantoo basin in South Australia. During the Cambro-Ordovician Delamerian orogeny the Kanmantoo basin together with the underlying Neoproterozoic deposits were subsequently foreshortened and thrust westward in the Adelaide region and eastward in the Glenelg River Complex. The interpretation presented here suggests that the Antarctic Ross orogeny, and the Australian Delamerian orogeny, although laterally continuous, attest quite contrasting orogenic boundary conditions, i.e. contrasting rates of overall plate convergence in relationship to the rate of subduction.

46

THE DENUDATION OF METAMORPHIC ROCKS IN THE BASIN AND RANGE (USA) CORE COMPLEXES Foster, David A.. Victorian Institute of Earth and Planetary Sciences, School of Earth Science, La Trobe University, Bundoora, Victoria 3083 The recent literature abounds with complex and controversial models for the development and exhumation of metamorphic core complexes, most of which are based upon field relationships, geophysical data and physical models. The thermal histories of footwall and hanging wall rocks determined by thermochronological methods ( Ar/ Ar and fissiontrack) also provide valuable constraints on the nature of extension and evolution of metamorphic core complexes. In addition to dating the timing of extension in core complexes, thermochronologic data reveal information about rates of cooling, rates of exhumation, rates of fault displacement, the geometry of faulting, and paleogeothermal gradients. I will discuss data mainly from metamorphic core complexes and adjacent upper plate tilt blocks of the lower Colorado River area, California and Arizona. These include lower plate rocks exposed in the Chemehuevi, Sacramento, Whipple, Rawhide, Buckskin, Harcuvar, and Harquahala Mountains (Figure 1). Major extension in these mountain ranges appears to have occurred between -24 and -14 Ma, possibly along a linked series of detachment faults with the same displacement direction. Biotite and K-feldspar K-Ar and Ar/ Ar apparent ages, and zircon and apatite fission-track apparent ages from these core complexes consistently young in the displacement directions (southwest to northeast) of exposed detachment faults. "Breaks-in-slope" at 20-22 Ma on plots of apparent mineral age against distance in the extension direction for higher temperature thermochronometers (biotite and high temperature K-feldspar data) mark the onset of rapid core complex extension (Figure 2). Linear trends of age vs. distance with slopes of -3-8 km/Ma for lower temperature minerals (low temperature K-feldspar data, and fission-track) estimate displacement rates of detachment faults. Plots similar to Figure 2 for transects across tilted upper plate fault blocks, whose the degree of tilting is independently constrained by the dip of previously horizontal Proterozoic sills and Tertiary sedimentary rocks, reveal information about the temperature variation with depth before extension. These can be used to estimate pre- to syn-extensional geothermal gradients when more than one mineral curve yields the characteristic "break-in-slope" indicating the depth at which pre-extensional ages were zero. In the Mohave Mountains the paleodistance between the breaks-in-slope for biotite and K-feldspar (low temperature steps) apparent ages (a difference of ~100°C) is -4 km, suggesting a syn-extensional gradient of ~25°C/km. Apparent fission-track ages of apatite and zircon that mark the onset of extension in the Tortilla Mountains (southeast of the map area) are separated by -3.5 km for a difference in temperature of ~100°C, giving a gradient ~28°C/km. Finally, a paleogradient of ~30°40°C/km is estimated for the tilted Piute Mountains from apatite and K-feldspar apparent ages. The dip at which brittle detachment faults operate in the upper crust is still highly controversial. Argument centers on the question of whether low-angle normal faults were initiated and moved in their present low-angle configuration, or were active with relatively high dips, and have been rotated to a more gentle orientation through time. In the Chemehuevi Mountains the thermochronologic data reveal a trend increasing temperature in the slip direction from -200° to 350°-400°C over an exposed distance of 23 km, at the onset of extension. Assuming the geothermal gradients listed above, the relatively gradual increase in temperature with original depth, suggests only gentle southeast titling of the footwall and constrains the Chemehuevi detachment fault to have had a regional dip of ~15°-30°. 40

40

47

39

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500 km

Figure 1. Map showing the mountain ranges of the lower Colorado River region. The shaded area shows the region of greatest extension. Mountain ranges: BS, Buckskin Mountains; CH, Chemehuevi Mountains; HA, Harcuvar Mountains; MH, Mohave Mountains; OW, Old Woman Mountains; PM, Piute Mountains; SC, Sacramento Mountains; ST, Stepladder Mountains; TM, Turtle Mountains; WH, Whipple Mountains.

distance along section (km)

Figure 2. Example of a plot of mineral age against distance in the extension direction for data from the Chemehuevi Mountains, California (modified from John and Foster, 1993, GSAB).

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GRANITE DYKES, STOCKS AND TRANSITIONAL FORMS: MAGMA P R E S S U R E A N D T E C T O N I C STRESS C O N T R O L S O N E M P L A C E M E N T STYLES O F T H E D A V Y S C R E E K G R A N I T E

T.J. Fowler, Geology Department, LaTrobe University, Bendigo, Victoria, 3550. The Davys Creek Granite (DCG) of southeastern Australia is a high-level S-type microgranite (typically granophyric) pluton which was intruded late in the regional tectonic compression. The DCG is composed of two bulbous composite masses (the Spring Creek and Hawks Nest plutons) and numerous dykes and sills. The earliest intruded and most deformed DCG microgranite bodies are bedding- and cleavage-concordant sheets (dykes and sills) with aspect ratios (in the horizontal plane) of about 100. Some apparently former large dykes were inflated during intrusion probably due to increased magma pressure and have aspect ratios closer to 10. The inflated ends of these dykes are preserved as apophyses on the main plutons. Swelling at the dyke tip increased the tip radius of curvature which impeded dyke tip propagation and required distension of the dyke walls to accommodate the continued arrival of magma. The thermally and metasomatically softened wallrocks accepted dyke wall distension by yielding along ductile shears (defined by thermal metamorphic phases) and by incomplete transposition, minor folding and crenulation. The intrusion of magma as sheets appears to have given way to intrusion as inflated semi-concordant lenses for the largest individual intrusive masses. In both the inflated dykes and lenses, the original angular relations between wallrock cleavage and dyke/lens contact are preserved though grossly distorted. Latest styles of intrusion include piercing of the distended lens walls to produce discordant lobes. Cleavage-discordant undeformed dykes and small stocks have intruded near the end of magmatism. The changing style from sheet to lens to stock/lobe represents a progressive change from sheet-like through to pencil-like intrusion in accord with Emerman & Marrett's (1990) analysis of controls on the stable form of magma bodies in stress fields. Critical factors controlling the shape of the magma body include AG (stress difference in the horizontal plane), R (dimension of the magma body) and Tl (magma viscosity). Sheet-like intrusion is favoured by large A a, low R and low T|; pencil-like intrusion is favoured by low Aa, large R and large r|. Gradual change in one or more of these factors may be involved in the destabilisation of sheet mode of intrusion in the DCG. Declining tectonic stresses during late syn-tectonic intrusion may decrease Ac and at the same time relax constricted magma conduits allowing larger pulses of magma (larger R) to arrive. Alternatively decreased Aa may allow vapour loss from the magma (there is some evidence for this in the commonly developed quench textures) leading to an increase in T|. It is possible that the main control on the chosen shape of individual intrusive masses composing the DCG relates simply to the natural rise and fall in the rate of supply of magma (affecting R) during an intrusive event, reflecting increased efficiency in melt collection from the source region and decreased resistance to the ascent of magma along softened, lubricated conduits. The inevitable decline in magma supply may explain the return to late stage sheet-like behaviour.

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CHANGES IN QUARTZ, CALCITE AND DOLOMITE FABRICS IN A METAMORPHIC GRADIENT: THE BRENNER AREA, EASTERN ALPS, AUSTRIA Bernhard Fugenschuh. Geologisches Institut, ETH-Zurich, Switzerland The Brenner area is dominated by a major Miocene detachment (Brenner fault zone) that separates Penninic units (footwall) from Austroalpine units (hangingwall). Investigations were carried out 1) on quartz samples from within the Miocene detachment zone and 2) on quartz, calcite and dolomite samples from the Austroalpine hangingwall. 1) The Brenner fault zone is characterized by up to one kilometer thick mylonites (mainly calcareous shists, quartzites, quartzphyllites) dipping gently to the west. Shear sense indicators related to an east-west stretching lineation give a consequent top to the west sense of movement. Quartz c-axis show asymmetric single girdles as well as type I cross girdles. From north to south along strike of the mylonite zone a decreasing amount of basal glide and an increasing amount of rhomb- as well as prism glide could be demonstrated. This transition in glide mechanisms suggests an increase in temperature towards the south, which is in good agreement with the well established differential uplift history of the Tauern window that exposes deeper units in the south. 2) The hangingwall consists of polymetamorphic crystalline basement with its parautochtonous Mesozoic cover. These Mesozoic sediments (limestones, dolomites, slates, sandstones) were metamorphosed and deformed during the Cretaceous. With the exception of isolated, brittle fractures, the younger Brenner fault movements did not overprint these deformational features and samples were taken well outside its influence. Cretaceous deformation produced isoclinal folds of cm to m size with fold axes oriented WNW-ESE, parallel to a well developed stretching lineation. Associated with this stretching lineation is a top to ESE sense of movement. Within the investigated area various geothermometers have shown a temperature increase from 420°C in the north to 530°C in the south. Within this temperature range dolomite shows a transition in behaviour from brittle to crystalplastic. In the low temperature area, only faults and very gentle open, incoherent, fractured folds can be seen. With increasing temperature dolomite starts to flow; this process is strongly controlled by the calcite content of the dolomites. Only in the highest temperature region (>500°C) does pure dolomite behave plasticly, producing a preferred crytallographic orientation.

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Calcite is ductile throughout the whole area. Calcite textures show typical patterns for dominant twinning in combination with r- and f-glide. with increasing temperature calcite e-poles define an asymmetric single girdle and a-poles show one strong maximum at 90° to the girdle. This texture is similar to common quartz single girdles and shows no sign of twinning. Quartz textures developed during Cretaceous deformation of the hangingwall show similar behaviour to that described above for the Miocene Brenner fault. They define a trend with an increasing component of rhomb- and prism glide and a decreasing amount of basal glide with increasing temperature from north to south. Microstructures that can be investigated under the optical microscope (subgrains, deformation bands,...) do not show any consistent trend. Thus it is proposed that the preferred orientation pattern is more resistant to later overprinting that might have caused the observed microstructures. It has been possible in the Brenner area to calibrate the observed transitions in deformation mechanism and microstructure of dolomite, calcite and quartz relative to a well established temperature gradient.

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ANALOGUE MODELS FOR TRANSFER ZONES FORMED BETWEEN OFFSET ROTATIONAL NORMAL FAULTS A.P. Gartrell and L.B. Harris, Department of Geology and Geophysics, The University of Western Australia, Nedlands WA 6009, Australia In any given extensional system, offset normal faults may form which link via transfer zones. Presently, two end member theoretical models exist for the structure of transfer zones; the cross-fault model (e.g. Gibbs, 1984) and the relay transfer systems model (e.g. Ellis et al., 1990). The exact 3-D geometry and spacial variability of transfer zones is not well illustrated by these theoretical models. Furthermore, no consensus has been reached regarding which of these models is the most applicable. Sandbox analogue models were employed to investigate transfer zone formation, in particular the effects of along strike variations (curved steps, fig.l and 90° steps) in a listric master fault or detachment surface. Sections parallel to the extension direction, show the formation of typical roll-over anticline structures (cf Ellis et al., 1988). Figure 2 shows the surface expression of faulting after about 80% extension. In overall terms, three offset graben form out from each section of the detachment block, with the central graben stepped in towards the detachment block. Between offset graben, transfer zones form which link the graben faults via a series of interlinking en echelon stepping normal faults and relay structures including relay ramps. Transverse obliqueslip faults propagate out from the steps in the detachment and join into major synthetic faults in the outer graben. Section AA' (fig.3a) shows a flower like fault geometry typical of sections taken perpendicular to the transfer zone in the graben. The upper branches of the "flower" consist of normal faults that strike obliquely across the transfer zone (fig. 3b), whereas the stem of the "flower" consist of oblique-slip vertical to steep reverse faults. Section BB', taken across the transverse faults, shows the transfer zone to tighten up into a more discrete fault with large vertical displacement. The fault runs vertically upwards from the basement and shallows at the surface to give reverse offset of beds. Transfer zones formed in the analogue models display features of both the cross-fault model (flower structures, oblique-slip and reverse faulting) and the relay system model (relay ramps, interlinking minor faults etc.). They accommodate the effects of two basic processes resulting from an offset rotational master fault: 1) crestal collapse graben systems developed are offset in the same direction as the offset in the master fault and transfer structures form to transfer extension between the offset graben and; 2) horizontal offset in the master fault effectively places two horizontally offset rotating blocks next to each other. Horizontal and vertical displacement difference result which are accommodated on oblique-slip normal and reverse faults within the transfer zones. This project is jointly supported by Western Mining Corporation and the Australian Research Council. References Ellis, P. G., McClay, K. R. 1988. Listric extensional fault systems - results of analogue model experiments. Basin Research. 1, 55-70. Gibbs, A. D. 1984. Structural evolution of extensional basin margins. J. geoL Soc London . 141, 609-619. Morley, C. K., Nelson, R. A., Patton, T. L., Munn, S. G. 1990. Transfer Zones in the East African Rift System and Their Relevance to Hydrocarbon Exploration in Rifts. The American Association of Petroleum Geologists Bulletin. 74,1234-1252.

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Figure 1. Detachment Block

Figure 2. Surface expression after 80% ext.

Figure 3. a) Cross section AA' with interpretation. b)Schematic block diagram representaive of section AA'

53

ON QUARTZ C-AXIS ANALYSIS BY A NEW GREY SCALE IMAGE PROCESSING TECHNIQUE A.Geiro. G.Lister, M.Jessell, E.Papp,Victorian Institute of Earth and Planetary Sciences, Earth Sciences Department, Monash University, Clayton, Victoria, 57<5& We introduce a new technique for the complete determination of quartz c-axis orientation within mineral aggregates. The method uses standard mineral optics and a new grey scale image processing technique, which is based on the correlation between the optic orientation of quartz grains in an aggregate and the changes in the light intensities as a thin section of the aggregate is rotated on a microscope stage. The analysis consists first of taking the Fourier transform of the light intensity curve of the rotated & untilted (8 images) and rotated & tilted (8 images, 1 tilt operation) data at each of more than 200000 pixels. A novel numerical technique is then used to retrieve the c-axis orientation and the thickness of the thin section at each pixel from the transform (the first five coefficients sufficing). Upon visual inspection, the grey-scale image of the first of the Fourier coefficients appears to retain all of the information about grain and subgrain boundaries (fig. 1). The end result of the analysis consists of a) the azimuth, b) the inclination of the c-axis and c) the thickness of the section at each pixel plus three grey scale images of these quantities. The method has been developed for quartz but can be extended to the analysis of any uniaxial minerals. It's principal advantage being a capacity to analyse fabrics quickly.

Fig.l. a) Image of raw data, distribution of the Fourier coefficients: b) the first, d) ratio of the third and second and c) the azimuth .

54

UPPER CRUSTAL STRUCTURE AND DEFORMATION KINEMATICS, TASMANIDES, AUSTRALIA D.R. Gray Department of Earth Sciences, Monash University, Melbourne, Australia Fault-bounded structural zones showing differences in structural trends, timing and nature of deformation, and tectonic vergence characterise the Tasmanides of eastern Australia. These zones display no simple accretionary trends, no cratonwards verging thrust belts like other orogenic systems. Tight to open chevron folds cut by predominantly west-dipping, high-angle reverse faults are part of different thrust systems. Upper crustal shortening ranges from 50 to 70 % largely due to chevron folding. Mid-crustal detachments occur at the base of the Ordovician quartz-rich turbidite succession and within the Cambrian meta-volcanics. Thrust sheets show marked strain gradients; upper parts have low to moderate strains (XZ strains < 5.0:1) and sub vertical extension, whereas lower parts of thrust-sheets show high strains (XZ > 30:1) and noncoaxial deformation. Major faults exhibit brittle deformation features, but have high strain zones of varying widths with intense development of crenulation cleavages and variably but generally steeply plunging mesofolds and microfolds; overprinting cleavages within these zones indicate complex fault movements with early thrusting followed by wrenching. The apparently haphazard fold-belt wide deformation pattern and the combined effects of thrusting and wrenching reflect collisional micro-plate interactions along a zone of oblique convergence (dextral transform margin) during much of the fold belt history. Differences in crustal architecture relative to other orogens may be due to the size, nature, age and lithospheric-crustal densities of the colliding and accreting masses. Features of the Tasman Orogenic belt include the superposition of different age thrust-belts sharing a common mid-crustal detachment, linked contractional and strikeslip faults, and a marked pattern of alternating extensional (Late Silurian, Late Devonian and Early Carboniferous) and compressional (Late Ordovician-Early Silurian, Early Devonian, Middle Devonian, and middle Early Carboniferous) deformation over the history of the belt.

55

VEIN AND FABRIC DEVELOPMENT WITHIN QUARTZO FELDSPATHIC TURBIDITE SUCCESSIONS: IMPLICATIONS FOR THE ROLE OF FLUID DURING DEFORMATION D. R. Gray1. R.T. Gregory2 and D.W. Durney3 1

Department of Earth Sciences, Monash University, Melbourne, Australia; Department of Geological Sciences, Southern Methodist University, Dallas, USA. School of Earth Sciences, Macquarie University, Sydney, Australia.

2 3

Structural and oxygen isotopic studies of veins and deformed host-rocks within interbedded quartz greywackes and argillites from an upper level thrust belt (Lachlan Fold belt, Australia) and a mid- to upper crustal thrust-nappe complex (Otago Schist, New Zealand) suggest that in both cases deformation took place under conditions of a low integrated fluid flux (W/R <0.1). Vein formation in both tectonic environments did not necessarily require infiltration of massive amounts of externally derived fluids. The Lachlan succession is characterised by chevron folds cut by reverse faults. Quartz veins (up to 5% of rock volume) formed during the entire interval of deformation are uniform isotopically (±1 per mil) on local and regional scales. Vein growth is driven by diffusive processes coupled to local advection driven deformation. The mobility of the fluid phase is controlled by episodic dilatancy "pumping" in localised fracture networks. In this setting, the isotopic composition of the fluid averages inputs on the several hundred metre scales whereas the host rocks are not isotopically homogenised by the processes that produced the rock fabrics and veins. The Otago Schist is characterised by stacked nappes with a flat-lying schistosity/ transposition layering and spectacular development of quartz-albite veins (up to 30% of rock volume). Individual nappes are bounded by high strain zones with transposition foliation and sheath folds. Whole rock oxygen isotope values maintain a similar range and degree of heterogeneity independent of the degree of deformation. In contrast to the Lachlan Fold belt, Otago Schist veins reflect the same heterogeneous isotopic pattern as the coexisting host rocks. This suggests either a stronger local control on the isotopic composition of the veins or re-equilibration of the veins with the host rocks during post vein-growth dynamic recrystallisation and annealing.

56

STRUCTURE OF THE ANAKIE METAMORPHICS, GRASS TREE MOUNTAIN AREA, CLERMONT, CENTRAL QUEENSLAND Timothy J. Green1, Christopher L. Fergusson1, and Ian W. Withnall2 department of Geology, University of Wollongong, Wollongong NSW 2522, Australia Geological Survey of Queensland, GPO Box 194, Brisbane, QLD 4001, Australia

2

The Early Palaeozoic Anakie Metamorphics are exposed in a north-south trending inlier extending from Mt Coolon in the north, 500 Ion southwards to Anakie. The best exposure of the inlier occurs around Grass Tree Mountain, 30 km west of Clermont, in an area extending 300 km2 north and south of the Clermont-Alpha Road. Complex outcrop patterns, most notably of the greenstones, can be recognised on both the Landsat TM image and aerial photographs covering this area. The eastern and central parts of the area are structurally the most complex, comprising mica schist interlayered with both greenstones and quartzites. Serpentinite is also present locally, and appears to be structurally emplaced. The outcrop pattern to the west of the area is more uniform, comprising mica schist and a major quartzite unit, which is interlayered with graphitic and psammitic schists, gradually grading to a more psammitic schist-rich unit farther west. Granitic and gabbroic intrusions occur to the north of the area, while numerous dykes and sills are scattered throughout. The western edge of the area is bounded by the Silver Hills Volcanics. Deformation of the Anakie Metamorphics took place over three main periods, the most pre-dominant being D2. Little data is obtainable on Du with Sx generally rotated into parallelism with S2. S2 is the most pervasive foliation throughout the area, with most primary sedimentary features being destroyed during D2. A strong but variably oriented L2 lineation is common, often locally sub-parallel to the stretching lineation, which is defined either by pressure fringes or a mineral elongation. D3 is evident only on the macroscale, and along with D2, is responsible for the complex outcrop patterns observed within the Grass Tree Mountain area. Superimposition of D2 and D3 structures has resulted in the formation of a dome and basin interference pattern. The dominant feature of the area is a domal structure with a core of serpentinite and greenstone. North of the centre of the dome is a large relatively flat-lying body of greenstone which has been deformed by F3 folds, with fold axes plunging shallowly to the north. S2 in the schist and quartzite units in the western part of the area, dips west at approximately 45°, while in the interlayered schist and quartzite unit south of the dome's centre, S2 has a southerly dip of approximately 30°. Evidence of large F2 structures exists within this southern section, where a relatively large quartzite unit within the mica schist has been deformed by folds with east-west trending axial planes. The eastern flank of the dome has been strongly affected by F3 structures, and a smaller serpentinite body also occurs, situated along a northwesterly trending fault. Faulting throughout the area has contributed to the complex nature of the outcrop pattern, with northwesterly to north-northwesterly trending faults displaying both leftand right-lateral movement. An increase in metamorphic grade occurs across the Grass Tree Mountain area, grading from greenschist facies in the eastern and central parts, to middle amphibolite facies in the far west of the area. Here andalusite, garnet and rare staurolite porphyroblasts occur, while to the east biotite - muscovite - chlorite assemblages occur in the schists, and chlorite - epidote + actinolite assemblages occur in the greenstones. Metamorphism is of low pressure - high temperature type and occurred synchronously with and D2.

57

Temporal and Spatial Controls on Denudation Within Magnatic Arcs: An Example from the Peninsular Ranges Batholith, CA M. Grove. Universite Blaise Pascal & CNRS, URA 10, CNRS, Clermont Fd.; T. Mark Harrison, Dept. Earth & Space Sciences, UCLA, Los Angeles, CA, USA 90024 Terranes displaced along the western Cordillera of North America contain significant quantities of arc-derived sediment. While the origin of much of this erosional debris is widely debated, exhumed batholiths along the margin are likely source regions. Exactly when, where, and how much sediment was shed from these batholiths have become crucial questions in understanding crustal evolution along the margin. The deeply eroded, 120-90 Ma, northern Peninsular Ranges batholith (PRB) along SW North America (Fig. 1) represents a major potential source region. Less than 5% of the sediment inferred petrologically to have been eroded from it (>2xl0 km ) can be reliably accounted for in adjacent, post-batholithic deposits (clxlO km ). Within the northern batholith, depth of erosion increases northeastwards from hypabyssal levels along the SW margin (greenschist facies); to 7-10 km in the central region (amphibolite facies; And±Sil); and ultimately to 11-16 km in the NE (upper amphibolite facies; Sil only). Biotite K-Ar apparent ages reflect denudation patterns and indicate significant Late Cretaceous denudation (Fig. 1). Preserved batholith-derived sediment is Turonian(?) and Late Campanian-Maastrictian with intervening unconformities prevalent. Paleocene weathering surfaces and extra-regional depositional systems indicate that denudation was substantially complete by the early Tertiary. 5

4

3

3

Northern Peninsular Ranges Bathol'" Intrusive Framework

1

Synkinematic (120-105 Ma) ] Dominantly Lateto Post-Kinematic (100-90 Ma)

Canydn Fault

Underlying Crust T Oceanic [J Transitional ITTMarginal 1-11 Craton

Ensenada

Depth Zones Gastil (1979) Shallow HH Medium MM 100 km Deep Fill Biotite K-Ar Age Contours (Ma) "La Posta-l^pe" Plutons Krummenacher et al. (1975) Q

25 km

18 Eastern Limit of Plutonic 8 O < +8.5%* i I I I i i 85 Ma Biotite K-Ar Age Contour Western Limit of Miogeoclinal Rocks

Figure 1: Sketch map of the northern Peninsular Ranges batholith illustrating erosional levels; biotite K-Ar apparent age trends; and several of the youngest/largest intrusions ("La Posta-type" plutons). Density of intrusion exceeds 85%. Note that biotite apparent ages become progressively younger NE with age contours superposed at high angles across the boundaries of the -95 Ma "La Posta-type" plutons. Inset shows the studied area. Open boxes designate sample localities (including several from Krummenacher et al., 1975). Distribution of dominately "synkinematic" (120-105 Ma) and "late- to post-kinematic" (100-90 Ma) plutons after Todd et al. (1988); S 0 < +8.5 boundary (from Taylor and Silver, 1978; coincides approximately with Sr/ Sri < 0.704); Western boundary of Miogeoclinal rocks from various sources (coincides approximately with Sr/ Sri > 0.706). 18

87

86

87

58

86

Contrasting crustal types underlying the batholith may have influenced denudation patterns. Within the region studied, the central batholith is inferred to be underlain by oceanic crust while marginal craton apparently underlies rocks in the east (see inset in Fig. 1). Biotite and K-feldspar 40 Ar/ 39 Ar total gas ages decrease abruptly within the transition zone at the approximate position of the 85 Ma biotite age contour (Fig. 2). Apparent ages of 85 Ma mark a time 10 Ma after the last major intrusive phase within the batholith. The 85 Ma age contour appears, in part, to be a tectonic feature. A pronounced structural break, the Chariot Canyon fault (CCF), has formed where the 85 Ma contour is situated closest to the western region underlain primarily by oceanic crust (Fig. 1). The CCF juxtaposes 2-3 kbar rocks (west) against 4-5 kbar rocks (east) is characterized by greenschist facies, ductile shearing overprinted by prehnite-actinolite facies, brittle cataclasis. Changes in the tectonic setting (i.e.; onset of shallow subduction) may have sequentially destabilized overthickened crust outboard of the craton margin. Rapid cooling at 88-85 Ma and 76-72 Ma (Fig. 3) corresponds closely with the observed ages of batholith-derived sediment. A large volume of sediment with compositional characteristics of the east-central PRB was apparently stripped from the batholith during the Turonian and Late Campanian-Maastrictian) to ultimately become incorporated into one or several allochthonous terranes along western North America. Gastil, R. G., 1979, Geology, v. 7, p. 542-544. Krummenacher, D., Gastil, R. G., Bushee, J., and Doupont, J., 1975, G.S.A. Bull., v. 86, p. 760-768. Silver, L. T., and Chappell, 1988, Trans Royal Soc. of Edinburgh, Earth Sciences, v. 79, p. 105-121. Taylor, H. P., and Silver, L. T., 1978, U.S.G.S. Open File Report 78-701, p. 423-426. Todd, V. R., Erskine, B. G., and Morton, D. M., 1988, Rubey Volume VII, N.J., Prentice-Hall, p. 894-937. 130 -r

"ST QJ OA

120 110 -

Oceanic i Crust

Transitional Zone

a a

U-Pb Zircon & K-Ar Hornblende Apparent Ages

^Western ||||j

^

<3 100 cs 0) u d

H •i/ i/ i / i Eastern Rocks \ S \ S NS \ !:i:!f|:::::::Jll-15 km) s Western Rocks 400 . s (7-10 km)

| Craton I Margin

300-

U

111 Eastern ' •

H

200-

90 -

Gas Ages \ u d A ~ ' < 80 - Total UntitP rn V<?CbJ< O Biotite V 9m

%

A

mm

.

70 -

T

r

T

T

-30

-20

-10

0

10

ioo-;

£

• K-Feldspar

I 20

Turonian

O .

p

•

Late Campanian • Maastrictian

T—i • |' l—|—i I ' ' ' f ' 30

40

100

Relative Distance (km)

90

80

70

60

Time (Ma)

Figure 2: Biotite and K-feldspar Ar/^Ar (total gas) ages as a function of distance from the biotite 85 Ma apparent age contour (see Fig.l; includes several KAr results from Krummenacher et al., 1975). Apparent ages are relatively constant on either side of the 85 Ma age contour, but decrease eastwards in close proximity to it. Age spectra from western K-feldspars rise from 86 Ma to 95 Ma; those from eastern K-feldspars increase from <70 Ma to 78 Ma. K-feldspars in close proximity to the 85 Ma age contour typically exhibit more pronounced age gradients from <75 to >86 Ma.

Figure 3: Calculated K-feldspar cooling histories. Samples west of the 85 Ma age contour indicate slow cooling between 9388 Ma followed by 40°C/Ma cooling from ~300°C at 88-85 Ma. K-feldspars from more deeply eroded, eastern rocks indicate 40-80°C/Ma cooling from >375°C between 76-72. Specimens at the boundary between the two domains record both the 88-85 Ma and 76-72 Ma phases with slow cooling intervening at 85-76 Ma. Note (1) the close correspondence between episodes of rapid cooling and observed depositional ages; and (2) the transitional character of samples near the 85 Ma contour (dashed).

40

59

FOLDS WITH AXES PARALLEL TO THE EXTENSION DIRECTION Diordje Grujic. and Neil Mancktelow, Geologisches Institute ETH Zentrum, CH - 8092 Ziirich, Switzerland. Folds with axes parallel to the extension direction have generally been ascribed to passive rotation of fold axes during continued shear. Single- and multi-layer fold studies have generally considered shortening of layers oriented perpendicular to the maximum extension direction X (i.e. layers parallel to YZ), or in a more limited number of cases, oblique layers still containing the intermediate Y axis (e.g. Treagus 1973, Manz & Wickham 1978). Only one short note (Watkinson 1975) has considered the case where the extension direction X lies within the layer itself, although in nature folds with axes parallel to X are quite common. These folds have generally been ascribed to passive rotation of fold axes during continued shear (e.g. Sanderson 1973, Escher & Waterson 1974, Cobbold & Quinquis 1980), but as shown by Watkinson (1975) and confirmed recently in our laboratory, this need not necessarily be the case. The current experiments on folding have clearly demonstrated the possibility of buckling with axes parallel to the stretching direction of bulk imposed strain. Single-layer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate Y axis, and with the extension direction X lying within the layer itself. All experiments were conducted in plane strain — either in pure shear (Fig. 1) or simple shear. Two kinds of analogue materials were used: paraffin wax and polydimethylsiloxane (PDMS). Depending on the strain rate and viscosity contrast, the ratio of layer-parallel strain to bulk finite strain was 0.01-1 in the Z direction and 0.03-0.5 in the X direction, which implies that the matrix must have flowed around the layer. Folds only developed at high viscosity ratios (ca. 600:1). Initially the layer extended parallel to the X direction but there was effectively no further stretching parallel to the fold axes in the competent layer once folds had grown to an observable size. Buckling in itself cannot produce an extension parallel to the fold axis. The stretch in the X direction, therefore, must be taken up in the matrix rather than in the actively folding layer. As a result the measured layer-parallel strain was smaller than the bulk finite strain, confirming that the matrix must have flowed around the layer. Such a relative shear between the layer and matrix in the X direction is only possible if the layer is of finite length in this direction. In nature, this will only be possible if the layering is broken by boudinage or faulting on some scale. With a moderate viscosity ratio of ca. 30:1, introduced initial perturbations did not amplify to form folds, but were passively deformed according to the strain distribution in the layer. The presence of introduced perturbations does not markedly influence the development of structures in layers oriented parallel to the XZ plane. This is in clear contrast to similar experiments with layers oriented parallel to the YZ plane (cf. Mancktelow & Abbassi 1992). A variable original orientation of the layering or anisotropy results in a correspondingly variable orientation of fold axes — fold axes strictly parallel to Y or X axes of the bulk imposed strain are only end-members. The determining feature both for geometry and orientation is the shape of the 2-D strain ellipse parallel to the layer or plane of anisotropy (Ramsay 1967), which is itself a section through the bulk 3-D strain ellipsoid. As noted by Ramsay (1967), Field 2 strain geometries (i.e.2-D strain ellipses for which one principal axis is shortened while the other is extended) in layers are the most general and presumably most common situation. Folds, therefore, should often have a component of stretching parallel to their axes. The analogue models demonstrate that folding with axes parallel to the stretching direction in the layer can only occur if the layer is discontinuous, either originally or due to concomitant boudinage. It is tentatively proposed that this may be important on a wide range of scales from individual boudinaged mesoscopic folds to folds in low-angle detachment systems where the more competent "layer" is the strong middle crust.

60

References: Cobbold, P.R. & Quinquis, H. 1980. Development of sheath folds in shear regimes. Struct. Geol. 2, 119-126. Escher, A. & Watterson, J. 1974. Stretching fabrics, folds and crustal shortening. Tectonophysics 22,223-231. Mancktelow, N.S. & Abbassi, M.R. 1992. Single layer buckle folding in non-linear materials - II. Comparison between theory and experiment. J. Struct Geol 14, 105120. Manz, R. & Wickham, J. 1978. Experimental analysis of folding in simple shear. Tectonophysics 44, 79-90. Ramsay, J.G. 1967. Folding and fracturing of rocks. McGraw-Hill. Sanderson, D.J. 1973. The development of fold axes oblique to the regional trend. Tectonophysics 16, 55-70. Treagus, S.H. 1973. Buckling stability of a viscous single-layer system, oblique to the principal compression. Tectonophysics 19, 271-289. Watkinson, A.J. 1975. Multilayer folds initiated in bulk plane strain with the axis of no change perpendicular to the layering. Tectonophysics 28, T7-T11.

60 mm

130 mm / x.Y / '

/

•

/

290 mm

• /

Fig. 1. Schematic diagram of the model for pure shear experiments, showing the orientation of the initially flat, planar layer with respect to the XYZ bulk strain axes of the imposed deformation.

61

THE EVOLUTION OF AN ARCHAEAN MOUNTAIN BELT Rod Hammond. Bruce Nisbet and Craig Williams Orpehus Geoscience Pty Ltd 1st Floor 681 Murray Street West Perth WA 6005 Given that an Archaean mountain belt may not ever have attained the high elevations that can be seen in modern examples, the formation of mountain belts (or perhaps hill belts e.g. see Ridley 1992) in Archaean times nonetheless occurred. The results are the distictive map patterns and lithological associations that typify Archaean granite-greenstone terranes, such as those in southern Africa, Canada, and the Yilgarn Block of southwestern Australia. Despite the considerable volume of work that now exists for the Eastern Goldfields Province (EGP) alone (see Piatt et al. 1978; Archibald et al. 1981; Swager 1989), the origins of the distinctive regional map patterns which are a fundamental element of these terranes, have not generally been addressed. Greenstones in the EGP form sinuous and anastomosing, generally north-northwest trending belts which are characteristically grossly synformal in character. These synclinoria are also typically hingeless (though not universally), and are defined by both primary layering, and early penetrative foliations and cleavages. The axis and margins of most belts are marked by shear zones and more discrete discontinuities. In the southern EGP, however, where broad areas of low grade greenstone succession are exposed, map scale upright folding is recognised (e.g. Swager 1989), though still accompanied by hingeless synclines (e.g. Kalgoorlie Syncline). The form of intervening granitic complexes ranges from broadly parallel sided, through lozenge shaped blocks, to clearly intrusive complexes and rounded plutons. Other than these intrusives, granitic complexes in the EGP most commonly form lenticular massifs or elongate domes. Medium and high resolution aeromagnetic data now becoming widely available indicates that most are magnetically layered and clearly contain geological structure as significant as that within greenstone belts. These data also reveal the presence of anastomosing structural patterns and discrete complexes within the granitic and gneissic terranes between the major greenstone belts. A central issue in EGP regional geology, and the geology of many similar "granite greenstone" terranes, is the origin of this distinctive association of commonly domal granitic complexes and interspersed hingeless synclinoria of greenstones. Deformation history: Our work throughout the central and northern EGP leads us to suggest four main tectonic episodes as follows (Hammond & Nisbet 1992): Event Deformation De NNW-SSE extension. D1

NNW-SSE shortening.

D2

ENE-WSW shortening.

D3

N-S dextral wrench.

Style Shear zones on granitegreenstone contacts. North verging thrusting, stratigraphic repetition, penetrative foliation. Westward verging, very large-scale thrust imbrication, discrete, N-S fault and shear systems

62

Age pre- to syn- greenstone succession (pre-2.7 Ga). 2.69 Ga.

2.65 Ga.

Not known, but late Archaean.

Very large-scale imbrication: We ascribe the regional strike, and the gross distribution of granitic complex versus greenstone largely to the D2 event. It is proposed here that the main granitic complexes are thrust imbricated slabs of the upper crustal section (10 to 15 km), and that many have a ramp anticline character. Greenstones are fortuitously preserved (in some instances extensively, elsewhere not at all) either being carried on the granitic complexes or over-ridden by them, giving rise to the hingeless synclinorium morphology. The tendency for domal complexes to be missing their western flank (e.g. Lawlers Anticline), and a commonly observed east block up movement sense in major fault zones indicates a west-southwest transport direction and westward verging imbrication. An early effect with the initiation of each new granitic thrust sheet as it began to over-ride basement adjacent to its west would have been to push overlying greenstone succession (already D! thrust effected) ahead of the over-riding thrust sheet. The earliest local D2 structures in the greenstones might therefore be low angle and thin-skinned in style. However, before very large displacements could be accommodated, a newer thrust would initiate (due to the weakness of granitic crust in the Archaean geothermal regime), and recently developed D2 structures would be steepened by riding piggy-back on the new underlying thrust sheet. The style of D2 thrusts developing locally would thus progress from essentially low angle thrusts, to high angle, out of sequence reverse faults. The scenario outlined generally holds for each greenstone belt, such that each belt can be regarded as a separate system of D2 thrusts emanating from a main thrust. These main thrusts separate granitic complexes at depth, ultimately flattening to join a major detachment. The sequence of development of greenstone belts can be inferred, from this model, to have occurred from east to west, and the steepness of dips and depth of exhumation of greenstone belts will tend to increase eastwards, although this is unlikely to be a well preserved or systematic relationship. The style of D2 thus has the potential to readily explain the distinctive form of greenstone belts, and is significant for the extent to which basement is involved in the deformation. It follows that this style of tectonism may be central to the development of Archaean-style terranes, and is perhaps related to Archaean geothermal regimes. AGSO's seismic traverse across parts of the southern EGP has provided an equivocal verification of the model, and the initial GSWA and AGSO interpretations provide for new dimensions to the concepts. Archibald N.J., Bettenay L.F., Bickle M.J. & Groves D.I. 1981, Evolution of Archaean crust in the Eastern Goldfields Province of the Yilgam Block, Western Australia, in Archaean Geology, eds Glover J.E. & Groves D.I. Special Publications of the Geological Society of Australia, 7, pp 491-504. Hammond, R.L., & Nisbet, B.W. 1992, Towards a Structural and Tectonic Framework for the Central NorsemanWiluna Greenstone Belt, Western Australia, in The Archaean: Terrains, Processes and Metallogeny, eds Glover, J.E. and Ho, S.E., Geology Department (Key Centre) & University Extension, The University of Western Australia, Publication No. 22, pp 39-49. Piatt J.P., Allchurch P.D. & Rutland R.W.R. 1978, Archaean tectonics in the Agnew supracrustal belt, Western Australia. Precambrian Research, 7, pp 3-30. Ridley, J.R. 1992, The Thermal Causes and Effects of Voluminous, Late Archaean Monzogranite Plutonism, in The Archaean: Terrains, Processes and Metallogeny, eds Glover, J.E. and Ho, S.E., Geology Department (Key Centre) & University Extension, The University of Western Australia, Publication No. 22, pp 275-285. Swager C.P. 1989, Structure of Kalgoorlie greenstones - regional deformation history and implications for the structural setting of the Golden Mile gold deposits. Geological Survey of Western Australia, Report 25. pp 59-84.

63

COMPRESSION AND EXTENSION IN MID-PROTEROZOIC (c.1000 Ma) GRANULITES: BOLINGEN ISLANDS, EAST ANTARCTICA Martin Hand * Paul Dirks 1

2

School of Earth Sciences, University of Melbourne, Parkville Vic. 3052. Department of Earth Sciences, University of Utrecht, P.O. 80.021, 3508TA the Netherlands 1

2

In mid-Proterozoic granulite facies rocks in the Bolingen Islands, east Antarctica (Fig. 1), structures associated with crustal shortening and peak-metamorphic conditions are overprinted by a series of extensional structures that formed during terrain decompression. Important similarities between structures in the Bolingen Islands, and those developed in the Larsemann Hills, Brattstrand Bluffs and Rauer Group suggest the structural history in the Bolingen Islands is a relevant framework in which to consider the tectonic evolution of mid-Proterozoic rocks in the Prydz Bay region. Two structural domains can be Figure 1. Location of the Bolingen Islands distinguished in the Bolingen Islands on the basis of dominant lithology, general structural sequence and the relative age, geometry and kinematic signature of the dominant gneissic foliation. These are: (i) the southern Bolingen Islands (including S0strene Island) which are dominated by mafic gneiss, (ii) the northern Bolingen Islands which consists of a metasedimentary succession thought to) overly the mafic rocks in the southern Bolingen Islands. Southern Bolingen Islands (SBI) In the SBI, S2 is the dominant gneissic layering. This layering typically dips to the south and contains an east-pitching mineral elongation lineation (L2). Peak metamorphic garnet-bearing assemblages in mafic gneiss are elongate in L2. Estimates of peak conditions are ~ 10 kbar at 980°C (Thost et al 1991., J. Met. Geol. 9, 245-256). D2 kinematic information is sparse, but fractured garnets with fracture orientations at constant angles to S2 and trains of asymmetrically arranged calc-silicate lenses indicate a south-up sense of shear. S2 is folded by up to three generations of L2-parallel plunging F2 folds. S2 is locally reactivated, overprinted, transposed and/or replaced by an S3 gneissic foliation that parallels S2, and contains a southwest-plunging mineral and elongation L3 lineation. Decompression textures formed either during, or before D3. In mafic gneiss, decompression is indicated by the formation of orthopyroxene, plagioclase and hornblende at the expense of garnet, and in pelitic gneiss, garnet and sillimanite have reacted to form cordierite and spinel. Three types of S3 foliation occur: (1) Zones up to several hundred meters wide in which L2 is absent; (2) Discrete shear zones (<lm wide) superimposed on pegmatites axial surface to F2 folds; (3) Composite S2-S3 gneissic layering that preserves both L2 and L3 lineations. D2 and D3 structures are transected by 5-15m wide D4 zonesrichin recrystallised pseudotachylite. Related offsets suggest a southwest-down sense of shear. The pseudotachylite-rich zones are generally reactivated along narrow (< 50 cm) variably oriented, mylonite and ultramylonite zones. The movement sense on these shears is mostly normal, with a south-down sense. Two-pyroxene-hornblende assemblages in metabasic rocks, garnetclinopyroxene assemblages in calc-silicate and sillimanite-bearing assemblages in metapelite suggest the mylonites and ultramylonites formed at upper-amphibolite facies conditions. y

* Present address, Department of Geology and Geophysics, Adelaide University, Adelaide 5005

64

Locally D4 structures are overprinted by an S5 gneissic layering that developed as a coarsegrained foliation in zones up to 30m wide. These D5 gneiss zones trend roughly east-west, and L5 generally plunges southeast or southwest. Kinematic indicators are not well developed within the D5 zones, however the sense of drag of the adjacent foliation into the zones suggests a normal (south-down) movement sense. Northern Bolingen Islands (NBI) Almost all the gneissic layering in the NBI consists of S3 surfaces that are associated with a strongly developed SW-plunging L3 mineral lineation and SW-plunging folds. Rarely, pre-D3 garnet-bearing mafic gneiss is preserved as boudins within the S3 fabric. Toward the margins of the boudins, garnet has reacted to orthopyroxene and plagioclase. Metamorphic assemblages developed in felsic, mafic, calc-silicate and pelitic gneiss indicate that P-T conditions during D3 evolved from 6 ± 1 kbar at 800 ± 50°C to 4 ± 1 kbar at 700 ± 50°C (Thost et al., 1992., SGTSG abstracts, Melbourne University). D3 high-strain zones in the NBI are characterised by intense foliations and lineations that are relatively planar over strike distances of 100 m's. At scales greater than this, zones curve through angles of up to 120° without any deflection of the lineation. Much of this curvature is due either to bifurcation of the high strain zones, or the presence of discontinuous internal truncations. This implies that the high-strain fabrics developed in non-planar zones. A consequence of the variation in D3 high-strain zone orientation is that it is almost impossible to distinguish whether the gneissic form surface is the same fabric from place to place. Observations suggesting that the D3 fabric is not uniquely defined in space or time include: 1) S3 truncated at high angles by a second, similar looking foliation with an identical lineation. The truncating foliation commonly curves back into the truncated foliation; 2) Gradual transposition into a new similar looking foliation with an identical lineation; 3) The identical gneissic layering lineation.gradually merging with an earlier similar looking foliation with an The sense of shear in D3 high-strain zones is generally southwest-down normal. However, shear indicators in some S3 foliations locally suggest south-up, suggesting that D3 structures are progressive from D2. Numerous generations of F3 shear-folds occur, which fold S3. All such folds are generally colinear, have sinistral asymmetry's when viewed down plunge, and increase in tightness with relative age as their fold axial plane rotates into parallelism with the gneissic layering. These folds are typically disharmonic with amplitudes decreasing instantaneously to zero across discrete planar zones. In contrast to the high-strain zones, relatively low-strain D3 regions between the high strain zones are characterised by complex fold interference patterns, and markedly non-planar foliations. A common fold sequence is present in which at least four generations of southwestplunging, more or less regionally consistent fold geometries occur. These include: (1) Two generations of relatively early tight to isoclinal folds with no definite asymmetry; (2) One generation of mostly open to closed folds with sinistral asymmetry and similar orientation to the folds in the high-strain zones; (3) A final generation of upright folds with no axial planar foliation and an E-W trending axial planar trace. The fourth fold generation is disharmonic on a very large scale (up to 1km): such folds first appear directly adjacent to high-strain zones, but are absent within the high-strain zones. Taken with kinematic information, the L2 and L3 lineation directions in the Bolingen Islands indicate that deformation occurred in a large-scale dextral shear setting. The similarity in horizontal transport direction between D2 and D3, the association of early compressional fabrics with peak-pressures, and later extensional structures with lower pressure conditions is suggestive of a single tectonothermal event in which exhumation of medium pressure granulite facies rocks occurred during thinning of overthickened crust. 65

DMDO-ASIAN DEFORMATION HISTORIES FROM K-FELDSPAR THERMOCHRONOMETRY T. Mark Harrison. An Yin, Department of Earth & Space Sciences andlGPP, UCLA, Los Angeles, CA 90024, USA., F J. Ryerson, Institute of Geophysics & Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA., P.H. Leloup and P. Tapponnier, Institut de Physique du Globe de Paris, 4, place Jussieu, 75252 Paris Cedex 05, FRANCE. The natural radioactivity of potassium has long been used to constrain the timing of tectonic events, usually by determining rock-forming ages of igneous intrusions that cross-cut deformation structures. This application is, however, restricted to shallow levels in the crust as loss of daughter argon occurs from minerals at elevated temperatures. Thus it might seem paradoxical that a second approach to establishing deformation histories takes advantage of the fact that the chemically inert daughter product is incompletely retained in minerals at mid to deep crustal levels. During cooling in the crust, a thermal chronology is recorded by the radiogenic argon distribution developed within a potassium-bearing phase. Information in the temperature range 400-150°C can be obtained from K-feldspars using the Ar/ Ar age spectrum technique. Because virtually all deformation events in the lithosphere involve discontinuities in heat flow, we can use these cooling-related isotopic variations preserved within minerals, or thermochronometry, to reveal evidence of past tectonic activity. The potential of this technique is well-demonstrated by two interrelated studies of the evolution of the Indo-Asia system since collision began at -50 Ma. 40

39

The Red River fault zone, an impressive geological feature that can be traced for over 1000 km from Tibet through Yunnan to the South China Sea, preserves a structural, penological and thermal record of two distinct phases of tectonic activity; a left-lateral ductile shearing that terminated during the early Miocene, and a still-active phase of ductile-to-brittle extension which began between 23-17 Ma in the Ailao Shan massif, and at 4.7 Ma in the Diancang Shan massif. Mylonitic rocks in the core of the massifs contain an early, steep, high temperature foliation and a horizontal stretching lineation that are both parallel to the trend of the belt. Kinematics indicators indicate that shear was left-lateral. The complex shape of the high temperature schistosity at the southern termination of the Diancang Shan likely corresponds to a large scale, oblique, left-lateral shear plane that dismembered the shear zone separating the Ailao Shan and Diancang Shan at the end of the left-lateral deformation. Ar/ Ar K-feldspar results from the Ailao Shan reveal a pattern of ages indicating that oblique extensional unroofing, probably due to misfit of the shear zone with the rotations of the Yangtze and Indochina blocks, was underway by 23 Ma. The inference of oblique extensional uplift has subsequently been confirmed petrologically. Rapid cooling associated with onset of the normal/right-lateral faulting along the Diancang Shan, and probably the Range Front fault further south along the Ailao Shan as well, began at 4.7±0.1 Ma. Results of this study tend to support the view that extrusion of Indochina occurred along the left-lateral Red River shear zone between about 35 and 17 Ma. Initiation of normal /right-lateral slip during the late Miocene likely relates to initiation of a second phase of crustal extrusion. 40

39

In southern Tibet, inferences from Ar/ Ar thermochronology led to discovery of two previously unrecognized thrust systems: the north-dipping Gangdese system (GTS) and the 40

39

66

younger, south-dipping, and probably still-active Renbu-Zedong system (RZT). West of Lhasa, the Gangdese thrust juxtaposes Late Cretaceous forearc-basin deposits of the Lhasa Block (Xigaze Group) over Tethyan sedimentary rocks of the Indian plate, whereas east of Lhasa, the fault juxtaposes the Late Cretaceous-Eocene Andean-type arc (Gangdese batholith) over Tethyan sedimentary rocks. North of Xigaze and west of Lhasa, a major south-dipping backthrust in the hanging wall of the Gangdese thrust puts the Xigaze Group over Tertiary conglomerates and the Gangdese plutonics. Cross-cutting relationships constrain movement on this structure to be between 27-18 Ma. In most places, the apparently still-active RZT is thrust over the trace of the GT, obscuring its exposure. Near Zedong, 150 km southeast of Lhasa, the Gangdese thrust is marked by a >200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks. The age of the GT is constrained to between 27-23 Ma based on Ar/ Ar thermochronology. Rocks close to the GT were rapidly cooled due to the refrigerating effect of the thrust beginning at 27 Ma. Along the northern margin of the Gangdese belt, well away from the thermal effect of the lower plate of the GT, rapid cooling due to crustal thickening-related uplift and denudation did not begin until 21 Ma. This lag is due to both the relatively slow advance of the thickening front and to the slow thermal response of the mid-crust to rapid denudation at the surface. Thrust displacement of 80 km, corresponding to an average slip rate of 20±10 mm/yr, is inferred from the length scale of rocks uplifted during the early Miocene. The age of the Gangdese thrust provides an upper age limit of -24 Ma for the initiation of movement on the Main Central thrust (MCT) because the sequence of thrust development on the southern edge of the collision zone has been younging southward. 40

39

We interpret the results of these studies to indicate that a significant fraction of the convergence of India with Eurasia during the Oligocene was taken up by extrusion along the Red River fault. As a consequence, significant crustal shortening (and consequent uplift and erosion) was forestalled until motion on this fault terminated during the early Miocene. After cessation of strike-slip motion, convergence accommodated by the GTS (and then the MCT) was immediately translated into uplift and exhumation of southern Tibet, followed shortly thereafter by the Himalaya. Subsequently, thickening continued in the Himalaya and Tibet until the early Miocene when, possibly due to delamination of the mantle lithosphere, the plateau achieved something akin to its present extent and elevation. One of the best estimates of the timing of this event comes from thermochronometry in the footwall of a detachment fault thought to represent tectonic collapse following attainment of maximum elevation. Ar/ Ar K-feldspar results from the Nyainqentanghla range indicate that the rapid cooling associated with tectonic denudation began at 7±2 Ma. 40

39

Thermochronological measurements, both distal and proximal to faults, can play an important role in elucidating deformation histories. In those cases where structures are unexposed or eroded away the thermal imprint on the adjacent rock may be the only preserved signal of a tectonic event. For example, both the Early Miocene normal faulting in the Red River fault zone and discovery of the Gangdese thrust system followed directly from detection of thermal histories characteristic of those environments. Recent advances in interpretive theory (the multi-diffusion domain model) and cleansing of Cl-correlated excess Ar (MR CLEAN) have vastly improved the accuracy of calculated thermal histories and extended routine recovery of K-feldspar thermochronometry to temperatures as low as 120°C. 40

67

A NEW STYLE OF ANALOGUE MODELLING FOR THE EXTENSIONAL REACTIVATION OF BASEMENT FAULTS AND RESULTING COVER SEQUENCE DEFORMATION R.I. Higgins and L.B. Harris, Department of Geology and Geophysics, The University of Western Australia, Nedlands WA 6009, Australia There is a poor understanding of the deformation of sedimentary cover sequences above reactivated basement faults. In particular, what effect do the geometry of basement faults and lithologies of cover sequences have? A new style of analogue modelling has been developed to simulate the extensional reactivation of basement faults and the resulting cover sequence deformation. This new style is able to test effects of variations in basement fault geometry and cover sequence lithology. The deformation apparatus consists of a rubber sheet above a wooden platform. The rubber sheet is clamped to one end of the platform and connected to the base of a movable end-wall at the other. The experimental models sit upon the deformation apparatus and consist of a basal silicone putty layer, "pre-faulted" clay and alternating layers of contrasting coloured sand (fig. la). The clay represents a faulted crystalline basement with the silicone putty accommodating any movement or rotation of the basement during extension, in a manner similar to the lower ductile crust. The sand layers represent a pre-rift sedimentary sequence above faulted basement with offsets of these layers defining the positions of faults. Other experiments employ a layer of silicone putty of lower viscosity than the basal putty between the clay and sand (fig. lb). This layer represents a basal evaporite or shale, whereas the experiments without this layer represent clastic sediments directly on basement. During extension of the models, layers of sand which contrast in colour to the pre-rift sediments are added, simulating syn-rift sedimentation. After each experiment, the models are impregnated with a water-gelatine mixture and allowed to set. They are then sectioned in the desired orientation. Several experiments were performed to test the effect of varying the cover sequence lithology and basement fault geometry. Two main types of models as previously described (fig. 1) were employed to achieve this. Experiments without the upper ductile layer result in the development and restriction of cover faults to areas directly above basement faults, with areas in between essentially undeformed. In section it is evident that some synthetic faults are curved and although all faults are normal, reverse offset is often observed in the syn-rift where such faults change dip direction (fig. 2a). This curved nature results from the displacement and rotation of individual basement blocks, with the development of the fault about some point of block rotation (fig. 2a). Experiments involving an upper ductile layer do not restrict the deformation to areas directly above basement faults. Graben develop in areas above unfaulted clay and oblique to the bulk extension direction (fig. 3) and deformation above basement faults is actually reduced. The ductile layer distributes stress into the overburden by "flowing" during block rotation. As it thickens over the fault, it reduces the amount of vertical displacement in the cover, thereby decreasing deformation above basement faults (fig. 2b). To accomodate this thickening across the fault, the putty is thinned away from it, creating a localised extension in a direction dependent on the movement and rotation of the individual basement blocks (fig. 2b). This work is jointly supported by WesternMinirtg. Corporation and the Australian Research Council

68

(a)

(b)

=

— SAND:

-SAND-

++++++++++4 + + + + + + + + UPPER SILICON :_PUTTY+ + - ' ' F + FAULT+ + + 4 ++++++4 •++++TTT++++ RUBBER

mmm:

BASAL SILICONE PUTTY

Figure 1. Arrangement of models, (a) without an upper ductile layer; (b) with an upper ductile layer.

Sand

Figure 2. Cross section showing (a) curved synthetic fault and associated reverse offset due to basement block rotation and movement, (b) effect of upper ductile layer (ductile layer was horizontal and of uniform thickness prior to extension).

Cover faults

^Extension

10 cm

Figure 3. Plan view schematic of the development of cover faults above basement blocks where a ductile layer is present.

69

EVIDENCE FOR A SUB-HORIZONTAL SHEAR ZONE FORMED AT DEEP CRUSTAL LEVELS IN WESTERN FIORDLAND, NEW ZEALAND E J. Hill School of Earth Sciences, Macquarie University, NSW 2109, Australia Multiply deformed, high-grade metamorphic rocks (granulite and amphibolite facies) are exposed in the Western Fiordland block, New Zealand. A NE-trending antiformal structure runs the length of Western Fiordland, the core of which is composed of early Cretaceous Western Fiordland Orthogneiss (WFO). The WFO is largely overlain by Palaeozoic metasediments and intrusive rocks of the Tuhua Sequence. The Tuhua Sequence records an early phase of low-pressure high-temperature metamorphism. Both the Tuhua Sequence and the WFO subsequently suffered higher pressure, amphibolite facies metamorphism (11-13 kbar). A recent detailed study of the structure of northern Western Fiordland revealed the presence of a 3-4 km wide mylonitic shear zone (the Anita Shear Zone), which trends NE parallel to the coastline and separates rocks of the Tuhua Sequence from the WFO and gneisses of the Arthur River Complex (meta-intrusive rocks of unknown age). The shear zone is now steeply dipping, because it has been folded by tight to isoclinal post-shear zone folds and lies on the western limb of a major antiform. However, fold geometries indicate that the original dip of the shear zone was much more shallow, possibly subhorizontal. Mineral lineations and sense of shear indicators (determined from asymmetric structures in the shear zone) indicate a consistently dextral sense of shear. When the effects of post-shear zone folding are removed, the sense of shear is top-to-the-northeast. Shear zone fabrics are defined by amphibolite facies minerals. Pressure estimates by Bradshaw (1989) for a shear zone sample are -12 kbar (at ~680°C), indicating that the shear zone was active at depths of approximately 40 km. The shear zone fabrics overprint a gneissic LS tectonite fabric in the WFO and are in turn overprinted by sub-vertical retrograde shear zones and axial plane fabrics of associated upright folds. The same relative timing relationships have been observed for a folded shallowly dipping shear zone in southern Western Fiordland, namely, the Doubtful Sound "Thrust" (Oliver, 1980, Gibson, 1990) suggesting that both shear zones formed during the same time interval. Kinematic indicators reported for the Doubtful Sound shear zone are top-to-the-northeast, as for the Anita Shear Zone. Furthermore, both shear zones occur along the same laterally continuous lithological boundary, i.e., the boundary between the WFO and the Tuhua Sequence. It is concluded that the Anita Shear Zone not only formed at the same time as the Doubtful Sound shear zone, but is 70

most likely part of the same shear zone. Together they form a major shallowly dipping or sub-horizontal tectonic boundary that runs the entire 150 km length of Western Fiordland. In northern Western Fiordland the WFO is reported to intrude the Arthur River Complex (Bradshaw, 1990). In southern Western Fiordland the boundary between the WFO and the Tuhua Sequence is the Doubtful Sound shear zone. These observations led workers in the north to propose that the WFO was intruded into the country rocks, including the Tuhua Sequence, and workers in the south to propose that the WFO was emplaced beneath the Tuhua rocks by movement along a shear zone. These conflicting ideas led to different interpretations of the metamorphic history of Western Fiordland and hence the following two contrasting hypotheses regarding the mid-Cretaceous tectonic evolution of Western Fiordland. (1) During the mid-Cretaceous, Western Fiordland was undergoing plate collision and crustal thickening. The Tuhua Sequence was intruded by the WFO causing lowpressure high-temperature contact metamorphism. These units were subsequently buried to deeper crustal levels during crustal thickening (McCulloch et al., 1987; Bradshaw, 1989). (2) During the mid-Cretaceous, Western Fiordland was undergoing continental extension, during which time displacement along a deep-crustal low-angle extensional shear zone brought the WFO into juxtaposition with the Tuhua Sequence. Low pressure metamorphism occurred in the mid-Palaeozoic and is unrelated to Cretaceous events (Gibson et al, 1988). The results of this study are inconsistent with the first hypothesis. The study has shown that the Anita Shear Zone and Doubtful Sound shear zone are part of a regionally extensive shear zone that separates the WFO from the overlying Tuhua Sequence. The intrusive contact between the WFO and the country rocks (i.e. the Arthur River Complex) lies below this tectonic boundary. Therefore the low-pressure metamorphism recorded in the rocks of the Tuhua Sequence (i.e. above the shear zone) cannot be related to intrusion of the WFO, and hence there is no evidence to support burial of the Tuhua rocks during the mid-Cretaceous. REFERENCES: Bradshaw, JY, 1989. Contrib. Mineral. Petrol., 103, 246-360. Bradshaw, JY, 1990. NZ J. Geol. Geophys., 33,465-484. Gibson, GM, McDougall, I & Ireland, TR, 1988. Geology, 16, 405-408. Gibson, GM, 1990. in Exposed Sections of the Continental Crust (Salisbury & Fountain), 71-101. McCulloch, MT; Bradshaw, JY & Taylor, SR, 1987. Contrib. Mineral. Petrol., 97, 183-195. Oliver, GJH, 1980. NZ J. Geol. Geophys., 23, 27-41.

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THE EVOLUTION OF FOLDS AND THE DEVELOPMENT OF CRENULATION CLEAVAGE B.E. Hobbs. Y. Zhang, H-B. Muhlhaus, and A. Ord, CSIRO Division of Exploration and Mining, Private Bag, P.O. Wembley, WA. 6014. In this paper we summarize the results of recent analytical treatments and numerical simulations relevant to the development of folds and to the evolution of crenulation cleavage. The emphasis is on non-linear behaviour in contrast to early workers such as Biot. The influence of axial constraints on the evolution of buckling, instability driven folding of single layers is investigated. The layer is modelled as a think, elastic or viscous plate (both are considered) which is embedded in a Newtonian fluid. Two types of constraint are considered. For the elastic layer the axial displacement is kept constant at the plate edges after load application, for the viscous layer the axial displacement rate is kept constant. In both cases the axial stress is not constant. The change in the axial stress is determined by a geometrical nonlinearity in the constitutive relations. This type of nonlinearity does not occur in Biot-type (1965) models with free plate edges. In the case of elastic layer it is shown that there exists a countable set of nontrivial equilibrium states. All these equilibrium states are unstable except for one, the wavelength of which is equal to the plate length. For the viscous layer an analytic solution is derived, assuming that the initial displacement contains a single harmonic only. The dynamics of the fold evolution, and the effect of various types of initial conditions, are illustrated by numerical analyses. Many of the features of natural folds are duplicated by these analyses, including the amplification of more than one wave-length. Numerical analyses show that the formation of crenulation cleavage in the folded layer is controlled by micro-shearing mechanisms associated with folding. In the very early stage of deformation, strain is basically homogeneously distributed in the layer. After a certain bulk deformation, however, plastic yielding occurs and plastic strain starts to localise along the direction of micro-shearing. This leads to the formation of a series of shear bands which represent the nucleation of crenulation cleavage. The shear strain of these nucleated cleavage bands is in the range of 0.4-0.8. With further deformation/folding, strain localisation becomes further intensified and these crenulation cleavage bands become amplified. In the late stage of folding, the formation of layer-parallel shear bands becomes significant. These layer-parallel shear bands are usually at the places where crenulation cleavage bands terminate, suggesting they play an important role in the accommodation of crenulation cleavage. For layers with large mechanical contrasts these layer parallel shear bands dominate the formation. The orientation of the crenulation cleavage developed in this model is determined by the orientation of plastic yielding corresponding to the micro-shearing and corresponds to well developed continuum mechanics principles. The crenulation cleavage bands are not parallel to the fold axial plane or normal to the shortening axis. They generally show a clear angle to the both directions and the values of the angles are influenced by the constitutive parameters adopted in the model. The orientation of crenulation cleavage also changes following the development of folding. The cleavage tends towards parallelism with the axial plane as the fold amplifies.

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GEOrient - AN INTEGRATED STRUCTURAL PLOTTING PACKAGE FOR MSWINDOWS. R J Holcombe. Department of Earth Sciences, The University of Queensland\ Qldf 4072, Australia GEOrient1 is a Windows 3.1-based structural plotting package developed as an upgrade of the previous DOS-based ORIENT. • GEOrient plots geological structural orientation diagrams (equal area or equal angle stereographic projections, and rose diagrams, from orientation data held in ASCII text files in a wide range of file formats, and using a range of orientation conventions. im ||i file

JBataType

ElotType

Edit

fiptfons

window

Help

• Data from multiple files can be merged as a single dataset, or can be overlayed using different symbols. Multiple plot windows can be opened simultaneously and can be arranged on a page prior to printing. • Data can be presented either as coloured symbols representing point densities, as great circles, as contours of gridded point density, or as rose diagrams of polar or non-polar data. Best-fit great circles and mean and variance statistics are automatically calculated. • Data can be rotated in various ways, and appropriate polar line data (such as sedimentary flow directions) can be rotated to the horizontal by rotating associated bedding planes and fold hinges. • Plots can be copied to the clipboard for transfer to other packages. • On-line help with hypertext links provides a complete manual for the package. 1

GEOrient is available from the author at a nominal cost of $25.

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Examples of output arranged on-screen and printed to a laser printer: L21

L21 Mt Mee

Mt Mee

1 pt. (0.52%) - 1 % - 2 %

-4% (Max.=8.33%) Spherical Mean= 46-086 Spherical Variance= 0.4 Calculated girdle: 38-070

192 Pis

Circular Mean= 085 Circular Variance= 0.5 Mean Resultant= 0.5 Circular Std.Dev.= 67D Maximum =9.4%

192 Pts

S2 & L21 Mt Mee

Mt Mee

1 pt. (0.52%) - 1 % - 2 %

-4% (Max.=8.33%) Spherical Mean= 44-094 Spherical Variance= 0.4 Calculated girdle: 38-070 Calculated beta axis: 52-250

192 Pts

Spherical Mean= 33--178 Spherical Variance= 0.6 Calculated girdle: 20-036 Calculated beta axis: 70-216

46 Pts

L10 arid S1

S4

Circular Std.Dev.= 38D Maximum =15.2%

74

BLUESCfflST ASSEMBLAGES CONTROLLED BY FLUID INTERACTION WITH SUBDUCTION-RELATED SHEAR ZONES

R J Holcombe. T A Little, and C J Stephens l Department of Earth Sciences, The University of Queensland, Qld 4072, Australia 2 Now at: Department of Geology, Victoria University of Wellington, New Zealand Epidote-blueschist facies mineral assemblages in the Rocksberg Greenstone at Mt Mee, just north of Brisbane, occur in at least three mappable panels of schistose rocks interleaved within much less deformed, and superficially lower grade, mafic metavolcaniclastic rocks (Holcombe & Little, 1994). The thesis presented here is that the blueschist panels simply represent shear zones within the pile of metavolcaniclastic rocks, and that the development of equilibrium epidote-blueschist assemblages is deformation-controlled. We have previously interpreted the Rocksberg Greenstone, and associated phyllite and serpentinite units, as underplated oceanic components of the mid-Carboniferous New England Orogen accretionary complex that were exhumed during Late Carboniferous crustal extension (Little et al., 1993). Greenschist facies M2 fabrics, associated with D2 extensional structures, occur abundantly in all rock types and are well-constrained to -300 Ma (Little et al., 1991). D2 fabrics overprint variably developed D1 fabrics and structures that are interpreted as subduction-related. Epidote-blueschist facies assemblages in the blueschist panels are clearly associated with very strong D1 fabrics, but in the adjacent metavolcaniclastic rocks D1 fabrics are poorly developed, and generally represented by actinolite-rich assemblages that are difficult to distinguish from the M2 overprint. Despite the stark contrast in D1 deformation intensity and apparent Ml metamorphic grade between the blueschist panels and the adjacent greenstone units, there is considerable evidence that the greenstones are the protolith for the blueschists. The volcaniclastic units are characterized by being coarsely fragmental and containing abundant primary augite crystals 12 cm across, occurring both as matrix grains and as phenocrysts in clasts. The blueschists have a mafic protolith which, although poorly defined, are also mainly fragmental. Relict augite crystals are preserved locally within one of the blueschist panels, and pseudomorphs after augite are very common in all the areas of blueschist. These pseudomorphs are most commonly 1-2 cm euhedral aggregates of blue amphibole and white mica, bu include single barroisite grains perfectly mimicking augite crystals, including the distinctive twinning. Although the two rock types appear to have the same parent, the presence of tourmaline-rich laminations in the blueschists suggests a supply of extraneous fluids to these rocks. Pressures of >5kb (at T>350°C) have been estimated for the epidote blueschists Ml event (Dobos et al., 1993 ) and, despite the lack of blueschist assemblages in the relatively massive 75

volcaniclastic greenstone units, it is probable that they have been subjected to the same P-T history. Clearly the massive volcaniclastic greenstone units are the protolith for the blueschists panels, and the two must have remained in close physical proximity throughout their subduction and exhumation history. That is, it is highly unlikely that the blueschist panels have been independently subducted to depths in excess of 18km and then, during exhumation, been re-united with their unsubducted parent rock. The suggestion that the greenstones have at least seen epidote-blueschist conditions is supported by rare occurrences of very small blue amphiboles in fractures in primary augite grains, and by the occurrence of submicroscopic rims of slightly sodic clinopyroxene around primary augite grains. The major differences between the blueschist panels and the surrounding greenstone units are the degree of development of the S\ fabric, the degree of neocrystallization and recrystallization accompanying this fabric, and minor bulk compositional changes implied by such features as the tourmaline laminations. We infer that the blueschist-greenschist units represent high strain D\ shear zones within the mafic pile, and that these zones have controlled fluid movement (perhaps derived from dehydration reactions at lower levels). Such fluids would not only generate the subtle compositional changes (such as a variation in oxygen fugacity observed as lower Fe^/Fe^" ") required to locally produce blue amphibolerich assemblages but would also promote strain softening or widening of such high strain zones that in turn could control reaction kinetics and the supply of components. Equilibrium epidote-blueschist facies assemblages only developed within the shear zones where straininduced dilation was highest, leaving the primary composition and mineralogy of the country rock virtually unchanged. 1

DOBOS, S.K., STEPHENS, C.J., & HOLCOMBE, R.J. 1993. Greenschists and blueschists of the Rocksberg Greenstone at Mt Mee, southeast Queensland - mineralogy and composition of a higher-pressure assemblage, in: Flood, P.G. and Aitcheson, J.C., eds. New England Orogen, eastern Australia, NEO '93 Conference, Dept of Geology and Geophysics, University ofNew England, Armidale, 545-548. HOLCOMBE R.J. & LITTLE T.A. 1994. Blueschists of the New England Orogen: Structural development of the Rocksberg Greenstone and associated units near Mt Mee, southeastern Queensland. Australian Journal ofEarth Sciences, 41, in press. LITTLE T.A., HOLCOMBE R.J., GIBSON G.M., OFFLER R., GANS P.B., & McWILLIAMS M.O. 1992. Exhumation of Late Paleozoic blueschists in Queensland, Australia, by extensional faulting. Geology; 20, 231-234. LITTLE T.A., HOLCOMBE R.J., & SLIWA R. (1993). Extensional exhumation of blueschist-bearing serpentinite-matrix melange in the New England Orogen of southeastern Queensland, Australia. Tectonics.

76

DEFORMATIONAL/METAMORPHIC FABRICS OF THE JUDENAN BEDS IN THE MOLANITE VALLEY AND TAILINGS DAM AREA, WEST OF MOUNT ISA MINES: INSIGHTS INTO THE COMPLEX OROGENIC HISTORY OF THE MOUNT ISA INLIER, AUSTRALIA Wanfu Huang Department of Geology, James Cook University Townsville, Australia ABSTRACT Resolution of structural history and progressive development of the "Judenan Beds" west of the Mount Isa Fault Zone is essential for development of a correct tectonic model for the Mount Isa Inlier as a whole. Geological mapping (1:5000 scale) and detailed microstructural analysis in the Molanite Valley and Tailings Dam area has resolved several critical structural relationships. Six domains defined on the basis of deformation and metamorphism, and bounded by major faults, contain five correctable generations of structures. Evidence for the earliest deformation is only preserved within porphyroblasts as inclusion-trails, but the significance of this event was not resolvable. Microstructures, particularly relationships between the deformation fabrics and metamorphism/metasomatism, strongly suggest that the second and third generation structures formed during a progressive east-west shortening event corresponding with regional metamorphism/metasomatism. The metamorphic peak overlapped the second deformation (D2) and early stages of the third deformation (D3). D3 was synchronous with large-scale fluid migration and metasomatism as well as hydrothermal alteration-mineralisation in the Mount Isa Mines to the east of the Mount Isa Fault Zone. The fourth generation of structures formed at shallower tectonic level, and may represent the end of this orogenic event. Fifth generation folds (D5) resulted from local deformation. Field mapping and microstructural data used for timing movement along the Mount Isa Fault Zone, the Holly Fault, the Settling Tanks Fault, the Meerenurker Fault and the Mount Gordon Fault indicates that they were predominantly active late in this east-west shortening event. Consequently, abrupt changes in metamorphic grade occur across them. However, multiple deformation fabrics have been identified within these faults, suggesting multi-stage displacement. For instance, the Mount Isa Fault Zone moved twice: 1) Earlier west-side up movement associated with ductile deformation and; 2) later west-side up movement with a strike-slip component and predominantly brittle deformation. The Mount Gordon Fault truncates the western limb of the regional D2 syncline and has several kilometres of displacement.

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A BACKSTRIPPING APPROACH TO DEFORMATION SEQUENCES IN THE EAST-CENTRAL LACHLAN FOLD BELT Ian Ingpen Department of Earth Sciences, Monash University Clayton, Victoria, 3168 ABSTRACT A backstripping approach has been adopted to define and understand the structural history of the sparsely outcropping, deformed sequences in the central west of New South Wales. This region incorporates the eastern limits of the Wagga Metamorphic Belt and rocks previously referred to as constituting parts of the Girilambone Anticlinorial and Tumut Synclinorial Zones of the Lachlan Fold Belt. The study area also straddles part of the extensive and tectonically significant Gilmore Fault Zone. The backstripping approach considered three geological "time slices" and the varying influence upon these of a number of deformational events. By identifying the effects of the younger deformational events and removing the influence of these upon the older sequences, it is possible to identify those deformational events that affected only the older sequences. The three, major "time slices" or deformed sequences are the Late Ordovician to very earliest Silurian, Late Silurian to Early Devonian, and Late Devonian to earliest Carboniferous (Fig. 1). Each "time slice" includes sedimentary, volcanic and intrusive rocks. Early Carboniferous

AGE m.y.

WEST

350

Late Devonian - 3 6 0 370 Mid Devonian Early Devonian Late Silurian Early Silurian

m

GILMORE FAULT I I ZONE ITxxiXIX XIX |XDC

330 390 400 410" 420

/i+I+i+I+L+i'i PtWl

EAST

I I I I I 4—

(T v ' v v l ' v l k

430 440"

Late Ordovician

450 460

X

Peralkaline and I-type granites, e.g. Narraburra and Barmedman Granites respectively.

^ ^

Dolerite intrusions.

| ] ] Freshwater sedimentary successions of Hervey Group, e.g. within the Tullamore Syncline. I^* J Acid volcanic rocks, e.g. Milpose Volcanics. 1

r + a S-type granites, e.g. Wyalong Granodiorite and Kikoira Granite. Locally turbiditic and shallow marine sedimentary rocks with basal conglomeratic and minor interbedded volcanic units, e.g. Derriwong Group and Yiddah Formation. ^yY Intermediate (shoshonitic?) composition volcanic rocks, e.g. Gidginbung Volcanics and ^ VI Lake Cowal Volcanics. Intermediate composition intrusions, e.g. Bland Diorite and Mother Shipton Monzodiorite. Deep water flyschoid sedimentary rocks with local calcareous units, e.g. Wagga Metamorphics and Cotton Siltstone.

Figure 1 Time-space plot of the east-central region of the Lachlan Fold Belt

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Table 1 summarises the structural elements of each deformational event affecting the "time slices". AGE OF DESCRIPTION OF STRUCTURES GEOLOGICAL DEFORMATIONAI EVENTS "TIME SLICE" AFFECTING Orientation Dip of Plunge Fold "TIME SLICE"

LATE DEVONIAN

of Axial Surface

Axial of Fold Wavelength Surface Axis

Profile

NtoNE

Upright Shall owly doubly plunging

Open or tight

- 40km or 10-15km

Orientation Dip of Orientation of Foliation Foliation of Plunging within Shear Pebble within Zone Shear Zone Elongation LATER

NNWtoN

Steep towards EorW

NtoNNE

SILURODEVONIAN

EARLY

LATER LATE ORDOVICIAN EARLY

Cleavage Developed No

Evidence of Movement on Shear a) Shear bands indicate E over W thrusting in south. b) Shear bands indicate W over E thrusting in central region. c) Box folds/kinks indicate dextral strike-slip with steeply oriented compression in central region.

Orientation Dip of Plunge of Axial Axial of Fold Wavelength Surface Surface Axis

Fold Profile

Cleavage Developed

NWtoNNW

Steeply Variably inclined plunging to upright

5-20km or 10-20m

Open to tight

Yes

ENEtoESE

Steeply inclined to upright ?

Small scale (m)

Tight

Yes + slylolitic cleavage ?

5-50m ?

Tight to close

Yes

?

NEtoNNW Upright to Doubly inclined or plunging recumbent ?

Table 1 Description of the structural elements of each of the deformational events The backstripping approach suggests that the structural history of the deformed sequences in the central west of New South Wales involved five deformational events. The two earliest events affected the Late Ordovician succession. An approximately east-west oriented compressional or related event was followed by a nearly north-south oriented compressional event. These two events are most recognisable in the eastern part of the study area. The SiluroDevonian "time-slice" was subjected to a large scale, northeast-southwest compressional deformational event followed by multiple movements or re-activations of the major shear zones within the the Gilmore Fault Zone. Finally, all three "time slices" were affected by a regional scale, approximately southeast-northwest oriented, compressional or related event. This event is most prominent in the Late Devonian sequences and produced structures including the Tullamore, Parkes and Hervey Synclines. The deformational events affecting the SiluroDevonian and Late Devonian "slices" produced structures of larger scale and somewhat different orientation than those evident within the Late Ordovician successions.

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DESKTOP MULTIMEDIA PRESENTATIONS AND SIMULATIONS IN STRUCTURAL GEOLOGY Pat James & Ian Clark 1.Department of Geology and Geophysics, University of Adelaide, South Australia 5005 2.Department of Geology, Salisbury Campus, University of South Australia, S.A. Desktop microcomputer systems are powerful and cheap enough to be used as multimedia presentation systems in the lecture room and laboratory. Presentation software is also commercially available and allows development of visual/audio aids including text, graphics, video, animation, and audio to form integrated multimedia delivery packages. What is not readily or commercially available is the authored software (courseware) developed in a structured curriculum, to form the basis for lectures, workshops, practicals, seminars, tutorials or other teaching and learning courseware modules. Over the last few years we have developed a range of courseware using commercially available software including Microsoft Word™ and Powerpoint™, Aldus Freehand™, Macromind Director™, Authoreware Professional™, Stereoplot™, Instrain™, Fault n™. The courseware is presented using the packaged/viewer facilities of Authoreware™ or Powerpoint™ and may be demonstrated by the teacher/lecturer in place of black/whiteboard, overhead transparencies, 35mm slides and/or video. It may also be viewed by students on computer teaching suites or interogated as interactive tutorial material. Topics available so far cover introductory and advanced structural geology, including, stress & strain theory and estimation, rheological principles, folding, fracturing (fault, joint and vein formation), foliation and lineation fabric formation, stereographic projection, section balancing and forward modelling. The courseware is ideal for the demonstration and display of the geometric and kinematic principles of earth structures and their formation. Real-time manipulation of simple figures has allowed the illusion of sophisticated motion (animation) for the development of progressive models or graphs. More sophisticated three dimensional geometries have also been produced as graphic images and imported to the presentations. Recently, the availability of Apple's Quicktime™ software has allowed the inclusion of real time full motion colour video and audio (sound effect) material. Further advancements of the system under development include, the importation of scanned images including maps, figures, diagrams, graphs and colour slides. The system has a number of advantages in teaching and learning making it a useful (though not exclusive) adjunct to standard lecturing and workshop techniques. These advantages include, very simple updating of courseware and slide manipulation, professional visual presentations, readily available handout material and finally availability and rapid review of the courseware material by the students via floppy disc. Our system has been exclusively developed on the Apple Macintosh system. Recent multiplatform software developments now make the courseware available in both Apple and IBM PC environments. 1

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2

DETAILED GEOMETRY AND KINEMATIC EVOLUTION OF CONTRACTION FAULT COMPLEXES AND SHEAR ZONES IN THE SOUTHERN ADELAIDE FOLD-THRUST BELT Pat James and Thomas Flottmann Department of Geology and Geophysics, University of Adelaide, South Australia 5005 Delamerian shortening of the southern Adelaide Fold-Thrust belt was strongly partitioned into high and low strain zones displaying characteristic structures both of typical foreland fold-thrust belts and intense ductile shear zones. Other than in the FoldThrust belt-parallel shear zones, deformation is concentrated in the hanging wall regions of both the shear zones and of discrete mappable thrusts. Low strain folded and thrust areas dominate spatially but represent only a small percentage of the overall shortening of the belt. High strain shear zones concentrate on either side of thrust-wedge culminations which expose crystalline basement and also in areas of extensional fault reactivation (similarly located close to basement involved structures). Both basement and Adelaidean and Kanmantoo Group cover are sheared to fine grained mylonites and phyllonites with composite (shear band) foliations, asymmetric porphyroclasts, sheath folds, parallel stretching, mineral, intersection and fold axis lineations and complex multiple vein injections. The Adelaide Hills, Clarendon, and Mt Bold ductile shear zones are thick (100fs metres), north south trending, shallow east dipping zones of intense west directed overthrust displacement. Like the NE-SW trending Normanville shear zone these zones lie in the footwall to exposed crystalline basement thrust sheets. Within the shear zones strain is intensly non-coaxial and further partitioned by lithology. Stiffer quartzites, gneiss and metasandstone buckle, boudinage and deform discontinuously, while more plastic mudstones, shales and carbonates, thin and provide glide horizons. The Aaron Creek shear zone is one of a number of structurally higher level intense non coaxial mostly layer parallel ductile shear zones, which are lithologically controlled and effected the inversion of the Kanmantoo basin thrusting it over the Adelaidean strata and its crystalline basement substrate. The zone comprises linear bands of highly deformed, intensly veined, and intrafolially folded recrystallized marble. Complex low angle and layer parallel folded and boudinaged veins indicate high fluid pressure during shear displacement. Down dip elongation lineations and normal- and reversed- sense kinematic indicators record (early?) extensional and (later?) shortening displacements. On Kangaroo Island, the E-W trending Kangaroo Gulley - American River shear zone separates weakly deformed platformal sediments sitting on shallow basement from thicker, overthrust and more intensley deformed Kanmantoo Group metasediments. The shear zone cuts across Delamerian structures in the Kanmantoo Group and may have accentuated the apparent oroclinal bend of the Fleurieu Arc. Deformation in this shear zone is consistent with intense flattening and north directed non coaxial shear. Layer parallel ductile shear fabrics include intensly flattened veins, overturned isoclinal folds, assymetric foliation boudinage and repeated listric reverse imbricate faults. Reorientation of local fold axes towards the stretching direction and tightening of folds shows the transition into the shear zone and suggests dominant heterogeneous simple shear. These high strain ductile shear zones of the Southern Adelaide Fold-Thrust Belt separate areas of low strain upright and overturned folding associated with emergent imbricate fans, low angle discrete thrusts and local duplexes. The belt is now recognised as a complex inverted passive basin/platform and local deep basin belt.

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INTEGRATED STRUCTURAL AND GEOPHYSICAL MODELLING Mark W.Jessell and Rick K.Valenta Victorian Institute of Earth and Planetary Sciences, Monash University, Clayton, Victoria, 3168, Australia In structural mapping, the primary goal is the determination of the three dimensional geometry of the earth and the distribution of rock types. Inferences with respect to kinematics and dynamics can then be made, however it is the geometry which we must first correctly establish. Given the improving spatial resolution of gravity and magnetic surveys, which can now resolve quite narrow lithological units, it is perhaps surprising how few tools are available to interpret the observed structural and geophysical data in an integrated fashion. We present a technique for the integrated forward modelling of the structure and geophysical response of multiply deformed terrains. This technique allows information collected by field geologists and geophysicist to be reconciled by the development of a simplified structural history of the area. The structural modelling is based on the deformation history of the area, in terms of a succession of structural events, such as folds, shear zones and intrusions. The interaction of these events with a starting stratigraphy then results in the prediction of the geometry of the structures. By specifying rock properties for the units in the initial stratigraphy we can also make predictions as to the potential field anomalies for gravity and magnetics. The accuracy of the model can be gauged by comparing the predictions with the dual constraints provided by the observed structural and geophysical data. This approach points to a new methodology for the reconstruction of the geometry of structures in the Earth's crust, and has potential as a tool for both research and training.

82

KINEMATICS OF THE MAIN CENTRAL THRUST ZONE, NEPAL HIMALAYA Scott E. Johnson, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia The Main Central Thrust Zone (MCTZ) is a major ductile shear zone along the southern base of the Himalaya Mountains. The MCTZ: (1) is defined by a zone, commonly exceeding 10 km in thickness, of highly-strained schists and gneisses with a gentlydipping foliation; (2) is traceable for more than 2000 km along strike; (3) displays a throw that is consistently around 100 km; and (4) has a strong mineral elongation lineation along its entire length. For these four reasons, the MCTZ is one of the most impressive geological structures exposed at Earth's surface. An on-going project investigating deformational and metamorphic evolution of the MCTZ in the Annapurna-Manaslu-Ganesh region of the Nepal Himalaya shows how little is actually known about kinematics and deformation paths in the MCTZ. In the Annapurna region, the upper portion of the MCTZ is composed of kyanite and sillimanite gneisses of the Tibetan Slab, whereas the lower portion is composed of chlorite, biotite and garnet schists of the Lesser Himalayas. The MCTZ is generally considered to have accommodated north-over-south displacement, bringing the higher grade gneisses over the lower grade schists. A "thrust plane" lying at the contact between schists and gneisses has been accepted by many workers, and is known as the Main Central Thrust. The MCTZ might be expected to have a relatively simple deformation history involving a significant component of simple shear along one main foliation, possibly with development of shear bands or S & C planes. Although such features are present, the MCTZ has a more complex deformational history involving the development of crenulation cleavages at both high and low angles to the main, gently-dipping foliation. In fact, in most samples collected from the schists within the MCTZ, the main, gently-dipping foliation is a differentiated crenulation cleavage at various stages of development. This gently-dipping crenulation cleavage is continuous with sigmoidal inclusion trails in some chloritoid, garnet and staurolite porphyroblasts, and therefore formed during prograde metamorphism. The MCTZ might also be expected to contain kinematic indicators showing unambiguous evidence for north-over-south displacement. Although many indicators are consistent with such displacement, there are complexities and contradictions in the kinematic history. In the Annapurna region, flat-lying shear bands overprint the main, gently north-dipping foliation, and in 85% of rocks containing shear bands they indicate north-over-south displacement. The other 15% contain conflicting shear bands that generally form conjugate sets, indicating local coaxial deformation. There is no systematic increase in development of shear bands as the MCTZ is approached from the south. The best development of the bands is at the base of the gneisses, just above the thrust plane, and in local horizons in the schists below. Thus, deformation and shear-band development were heterogeneous throughout the MCTZ. The shear bands formed during retrograde metamorphism, as shown by their close association with chlorite that replaces garnet porphyroblasts, and muscovite that replaces kyanite porphyroblasts. Thus, the shear bands significantly postdate development of the prograde mineral assemblage and the main, gently-dipping foliation. Some previous workers (e.g. Brunei, 1986, Tectonics, 5, 247-265) have postulated that the late-stage movement forming the shear bands was probably responsible for most of the nappe transport, but little is known about the kinematics of the MCTZ during the earlier prograde metamorphism. Past workers have referred almost exclusively to garnet porphyroblasts with spiral-shaped inclusion trails to support the idea that the prograde kinematics involved north-over-south

83

movement (e.g. Brunei, 1986). Apart from the fact that the mechanism of spiral-shaped inclusion-trail formation is controversial (Johnson, 1993, J. Met. Geol., 11, 635-659), use of these porphyroblasts as shear sense indicators assumes that they grew during formation of the MCTZ. This assumption can be tested by determining the 3-D orientations of spiral axes in garnet porphyroblasts (Johnson, 1993, J. Met. Geol., 11, 621-634). If the following three criteria can be met, it may be reasonably argued that the porphyroblasts are contemporaneous with the MCTZ: (1) the matrix foliation is continuous with the inclusion trails in the porphyroblasts, (2) the spiral axes lie in the matrix foliation, and (3) the spiral axes are perpendicular to the mineral elongation lineation. If these three criteria cannot be met and there is no evidence for more than one rotational axis within the porphyroblasts, it is likely that the porphyroblasts are not contemporaneous with the MCTZ. Garnet porphyroblasts in most samples so far tested are probably not contemporaneous with the MCTZ, and are therefore not useful as indicators of shear sense or deformation paths within the MCTZ. Numerous samples still need to be tested. Other prograde microstructures include symmetrical strain shadows around opaque (magnetite or ilmenite) grains, and foliation-parallel boudinage of metamorphic minerals where the two halves of bouninaged minerals have not rotated relative to one another. These observations may suggest that, at least locally, the deformation was coaxial, but they do not elliminate the possibility of an overall non-coaxial deformation path. Bouchez & Pecher (1981, Tectonophysics, 78, 23-50) conducted a strain analysis on quartz porphyroclasts in the lower part of the MCTZ in the Annapurna region. Very low K values (0.12-0.35) place the rocks in the flattening field on a Flinn diagram, and may provide further evidence that, at least locally, pure shear was important. These authors argued that shear strains probably reached higher values towards the thrust plane, but the strain profile remains unknown, owing to lack of suitable markers. Bouchez & Pecher (1981) have also used quartz c-axis fabrics to study MCTZ kinematics. They noted that 80% of their samples indicated north-over-south displacement, and suggested the other 20% indicated local heterogenieties in either the strain gradient or strain path. It is now generally accepted that c-axis fabrics within the MCTZ probably largely reflect the later deformation associated with shear bands and retrograde metamorphism (Brunei, 1986). Thus, quartz c-axis fabrics appear to be of little use in unravelling the prograde kinematics. Given the above information regarding the MCTZ, some questions that need to be answered include the following. (1) Are the main, gently-dipping foliation and later shear bands products of a deformational continuum that spanned different metamorphic conditions, or products of separate deformation events? (2) Why is the main, gently-dipping foliation most commonly a crenulation cleavage in the schists below the thrust plane? Was the deformation that formed this foliation highly noncoaxial on the scale of the MCTZ, as is generally assumed? (3) If most of the nappe transport occurred during the late-stage deformation that produced the shear bands, what was the role of the MCTZ during its earlier history? (4) If spiral-shaped inclusion trails in garnet porphyroblasts are not contemporaneous with the MCTZ, they provide no evidence for non-coxial deformation within the zone. When did these garnet porphyroblasts grow? Radiometric dating may help solve this mystery.

84

THE GEOMETRY OF THE SE OPHTHALMIA FOLD BELT, PILBARA REGION, WESTERN AUSTRALIA. - A COMBINATION OF TWO REGIONAL FOLD SETS. T. M. JOHNSON

(Department of Geology, University of Western Australia, Nedlands 6009, W.A.) Abstract - The SE Ophthalmia Fold Belt deforms the late Archaean to early Proterozoic Fortescue and Hamersley Groups. The SE Ophthalmia Fold Belt displays a complex polyphase deformation history which includes multiple phases of folding and faulting. Four separate fold sets have been recognised and are considered to result from separate deformation events. F! is a small scale fold set that was recognised and described as D i c by Tyler (1991). F 2 and F 3 are the main regional fold sets and are present on all scales. F4 is a late NE/SW trending set of folds that produces dome-and-basin interference patterns where they coincide with minor F2 or F3 folds. Owing to differences in style and orientation, F4 as described at Mt. Whaleback mine (Ronaszeki, 1992) could belong to a different fold set from F 4 elsewhere. Shallow normal faults at Mt Whaleback are folded by F 4 (Ronaszeki, 1992). F 2 was recognised and described by Tyler (1991) as the main expression of the D 2c deformation and as the dominant regional fold set. F2 folds are characterised by tight to close interlimb angles, moderate to shallowly dipping axial planes, northward vergence, and overturned northern limbs. F 2 axial trends vary from SW/NE to NW/SE. Plunge angles are generally shallow and non-cylindrical folding on all scales results in abundant plunge reversals. F 3 folds are open and upright with a NW/SE to WNW/ESE axial trend. This orientation does not change significantly across the study area and cross-folding angles are generally dependant on changes in the orientation of F2. Large-scale F3 folding is best recognised on shallowdipping F2 normal limbs where they produce zones of bedding that dip shallowly to the north. F 2 folds with shallow axial planes are observed to be refolded to recumbency by F 3 at numerous localities and at various scales. The implications of F3 as a significant regional-scale fold event have not been recognised during prior investigation of this region. Previously, F3 has not been distinguished from F2 as a significant and separate regional fold event. Tyler (1991) considered the SE Ophthalmia Fold Belt to be a foreland fold-and-thrust belt which can be directly related to deeper structural and metamorphic levels represented by ductile shear zones along the southern margin of Sylvania Inlier. This thrust and fold terrain may be divided into three structural domains, termed zones I, II and HI. Zone I represents the ductile shear zones and southward dipping thrusts and foliations of the Sylvania Inlier. The division between Zones II and III within the SE Ophthalmia Fold Belt is based on regional fold geometry; - Zone II is dominated by tight to close, overturned folds whilst Zone III is dominated by open, upright to gently inclined folds. The transition from zones II to III is abrupt; fold geometry changes directly from tight, overturned to open, upright over a few hundred metres. A transition zone of strongly asymmetric folds with moderate axial planar dips is not present. This abrupt transition suggests a change in fold generation rather than in fold style; hence the tight to close, overturned folds are considered to belong to the F 2 set whilst the upright, open folds correspond to F3. Refolding of F2 by F3 at the transition zone confirms this relationship. In general, zone II is dominated by F 2 folds which are refolded by F 3 , resulting in variable F2 axial planar angles. The presence of F3 in Zone II is masked to some extent by abundant F2 folding. Isolated close, overturned F 2 folds are present in Zone i n and are refolded by F3. The majority of the folds in Zone in are considered to be F3.

85

The regional fold geometry of the SE Ophthalmia Fold Belt consists of a combination of F folds defining an approximately arcuate fold belt centred on the Sylvania Inlier,.and cross cutting F folds produced during a later buckling event. F folding occurred during a NE directed shortening event that may be termed D in the study area. The majority of the D orogen is located to the southwest or west of the Sylvania Inlier and may correspond to the Ashburton fold belt. Timing relationships between F , F and iron orebodies in the study area are not yet fully understood; however it is clear that the following relationships are important; - Distribution of large iron orebodies appears to correspond to areas characterised by tight, overturned F folding. The majority of large, economically viable orebodies are located in Zone II (Mt Whaleback, Orebody 29, etc.) or in areas of tight F folding within zone in (e.g., Mining Area C). - Iron orebodies appear to be preferentially located within synclinal keels. A fault, crossfold or other cross cutting feature is a necessary additional feature required to localise an orebody (Morris, 1987). - The Mesozoic, Tertiary MG (martite-goethite) orebodies appear to postdate both F and F . At least one orebody of this classification is structurally controlled by a syncline, the geometry and orientation of which is consistent with that associated with F (Orebody 29). - Proterozoic microplaty orebodies (e.g., Mt Whaleback) postdate F . The timing of ore formation with respect to F is not yet known. If microplaty orebodies predate F then no microplaty ore should exist within F synclinal keels (If F is not also present). Consequently, the lack of microplaty ore in structurally favourable areas such as faulted synclines may be explained by associating the fold structure with F . Alternately, if microplaty orebodies were formed after F then interference (cross-folding) between F and F may be a structural control localising ore formation.

2

3

3

3

2

3

3

2

2

2

3

3

2

3

3t

3

2

3

3

2

References:

3

Tyler, I.M. 1991. The Geology of the Sylvania Inlier and southeastern Hamersley Basin, West. Aust. Geol Surv. Bull 138. Morris, R.C. 1987. Iron ores derived by the enrichment of banded iron-formation, in The Genesis of Ores and Petroleum Associated with Sedimentary Siliceous Deposits (Ed.J.R. Hein), pp. 231-267 (Van Nostrand Reinhold Company: New York). Ronaszeki, J. 1992. Structural geology and its controls on iron ore mineralisation at the Mt. Whaleback mine, Newman, Western Australia. A field guide for the Mt. Whaleback stops of the 'Excursion to the Southern Margin of the Pilbara Craton', Specialist Group in Tectonics and Structural Geology of the Geological Society of Australia, (unpublished)

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STRAIN LOCALIZATION PROCESSES IN GRANITE S-C MYLONITES Kvuichi Kanagawa and Hiraku Yamagishi, Geology Dept., Univ. Tokyo, Tokyo 113, Japan S-C mylonites have two distinctive foliations; S-surfaces subnormal to the finite shortening direction and C-surfaces subparallel to the shearing direction, the latter being narrow zones of localized shear strain. Since granite is a major constituent in the earth's crust and strain localization occurs on sample scale in S-C mylonites, granite S-C mylonites would provide useful information on strain localization processes in the crust. We are currently investigating how C surfaces develop in granite S-C mylonites from different shear zones formed under epidote-amphibolite to lower amphibolite facies conditions, based on microstructures, quartz crystallographic fabrics and feldspar chemistry. In these granite SC mylonites, 5-surfaces are characterized by quartz ribbons and elongate K-feldspar porphyroclasts, while C-surfaces are characterized by thin fine-grained (d < 40 /J,m) bands composed of K-feldspar, plagioclase and quartz, and planar concentration offine-grainedmica. Infine-grainedbands plagioclase and quartz grains are interstitiallyfilledby K-feldspar. Finegrained K-feldspar with higher K contents than porphyroclast K-feldspar also dominates pressure shadows and fracture-fillings. In all the granite S-C mylonites examined myrmekite lobes of plagioclase and vermicular quartz are found around K-feldspar porphyroclasts on sides subparallel to S-surfaces, and laterally connected to fine-grained K-feldspar at pressure shadows, where they mix up to formfine-grainedbands. The above microstructural features as well as chemical compositions of feldspars can be well explained by the following deformation-induced myrmekite-forming reaction as previously proposed: (l+x) Ki_ Na AlSi 0 + (1-x-y-xy) Na + *Ca = Nai_ Ca* A\ Si . O + 4jcSi0 + (l+x)(l-y) K where x and y are variables dependent on compositions of myrmekite plagioclase and porphyroclast K-feldspar, respectively. Since this reaction is accompanied by a volume loss, myrmekite is preferentially formed at high-stress interfaces of porphyroclast K-feldspar. Myrmekite lobes around K-feldspar porphyroclasts most frequently occur along porphyroclast boundaries subparallel to 5-surfaces, but many of them also occur along boundaries which make larger angles with C-surfaces. This is probably because myrmekite continuously forms along porphyroclast boundaries normal to the incremental shortening direction while porphyroclasts rotate toward the shear plane as deformation proceeds. The reaction requires Na and Ca to produce plagioclase in myrmekite. The presence offluidduring the reaction is evidenced by abundant fluid inclusions in myrmekite. Disappearance of perthite lamellae as well as depletion in Na contents at K-feldspar porphyroclast rims suggest that Na is mainly derived from K-feldspar porphyroclast rims possibly via solid-state diffusion. Depletion in Ca contents from plagioclase porphyroclasts probably through their fracturing and subsequent fluid access to grain interiors must be the source of aqueous Ca . K released by the reaction will precipitate from solution as K-rich K-feldspar in such low stress regions as pressure shadows and fracture-fillings. This also explains the interstitial appearance of K-feldspar in fine-grained bands. Thus the deformation-induced myrmekite-forming reaction is an incongruent pressure solution. In weakly deformed granite S-C mylonites, C-surfaces exclusively develop around K-feldspar porphyroclasts where myrmekite lobes are connected with fine-grained bands, and do not develop in other areas. This occurrence of C-surfaces indicates that strain becomes localized into fine-grained bands originated from myrmekite and K-feldspar precipitated from solution, both of which are the products of the myrmekite-forming reaction. This incongruent pressure solution may therefore significantly contribute to strain localization and the development of ductile shear zones in the middle crust y

y

3

8

+

2+

x

Ux

3 x

g

+

2

+

2+

+

2+

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THERMO-TECTONIC EVOLUTION OF THE SNOWY MOUNTAINS: AN APATITE FISSION-TRACK STUDY B.P. Kohn and AJ.W. Gleadow Victoria Institute of Earth and Planetary Sciences, Department of Geology, La Trobe University, Bundoora, Victoria 3083 Apatite fission track (AFT) ages in mountain belts typically increase with increasing elevation. Analysis of this relationship has been used with remarkable success to elucidate tectonic histories of such belts in terms of constraining timing, magnitude and rate of uplift. The observed pattern has generally been interpreted in terms of uplift through an effective closure temperature. The total amount of uplift and erosion may be considerable, usually more than a few kilometres and the rocks can then be considered as moving upwards, more or less continuously through the crustal isotherms. The above situation contrasts With that found in many extensional terrains, such as rifted continental margins, where block uplifts of generally less than 1-3 km are often observed. Such settingsfrequentlypreserve evidence in the form of uplifted plateau surfaces of an earlier cycle of stability and peneplanation which pre-dates the tectonic reactivation of the area during extension. Erosion within such uplifts will expose rocks which may also exhibit gradients of apparent AFT age, but which do not necessarily reflect timing and rate of uplift Long periods of tectonic quiescence should lead to the establishment of a stable thermal regime in which a well-defined partial annealing gradient is established within the upper -2-5 km of the crust. The shape of the fission track age profile with depth will depend on the particular thermal history experienced by the rock column and can be simulated using numerical forward models. Geomorphologically such an area may be expected to be characterised by a mature peneplain representing the end point of the previous erosion cycle. Areas of differential erosion up to this time will be characterised by contrasting patterns of apparent age on the erosion surface. Where such an erosion surface is later disrupted by neotectonic reactivation, block faulting and erosional rejuvenation, the AFT ages, characterised by relatively rapid variations with elevation in the uplifted annealing zone, will serve as paleo-depth markers in the pre-uplift crust. Mapping the vertical and horizontal variations in AFT age can thus be used to reconstruct the structural and tectonic evolution of an area particularly where the former erosion surface has been removed and the regional architecture is not clear from the geomorphology. This method is particularly powerful in crystalline basement settings where the use of conventional stratigraphic and structural parameters is not possible. In this study AFT data were measured on 75 basement samples from the Snowy Mountains and low relief terrain of the Monaro tableland to the east. In the area of the Kosciusko massif samples were taken along five different vertical profiles, covering an elevation difference of -1700 m. AFT ages vary from -290 to -90 Ma, a range that is only presently revealed in the vicinity of the SE Australian margin laterally over distances from -200 km inland to the present coast. The strong positive correlation between age and elevation describes a master reference profile for the region which mainly preserves a pre-uplift partial fission track annealing zone. Consideration of the AFT ages in the context of regional trends, together with the slope of the profile indicates that the Kosciusko massif records crustal temperature differences of some 40°-50°C. The difference translates into a pre-uplift geothermal gradient of ~25-30°C/km. A possible inflection point in the lowest part of the profile at 90 ± 10 Ma is interpreted as approximating the time of onset of intial uplift of the Kosciuko massif. This timing unequivocally confirms that evolution of the massif and the Great Dividing Range is temporally related to the formation of the SE Australian margin during Cretaceous rifting. Reference to the observed master reference profile permits determination of relative offsets and differential erosion between fault bound blocks of die massif and the Monaro tableland

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THREE-DIMENSIONAL GEOMETRY OF THE BOWEN BASIN NORTH OF 26°S R J. Korsch & J.M. Totterdell, Australian Geological Survey Organisation, Canberra The Bowen Basin forms the northern part of the Permian-Triassic Bowen-Gunnedah-Sydney basin system in eastern Australia, and consists of up to 10 km of shallow marine to terrestrial clastic sediments which contain important coal and petroleum resources. The basin was initiated by an extensional event in the Early Permian and was converted to a foreland basin in the Late Permian due to thrust loading and crustal thickening in the New England Orogen to the east of the basin. A regional grid of industry and AGSO seismic reflection data covering an area approximately 275 km x 250 km has been interpreted to the north of 26°S; this has involved over 5000 line km on 260 seismic sections. A similar sized area south to the NSW border is currently being interpreted. The interpretation has involved mapping seismic sequence boundaries and determining the geometry of major faults in the basin, leading to a better picture of the overall geometry of the basin. The structural styles dictating the basin geometry differ markedly between the Denison Trough in the west, the Taroom Trough in the east and the Blackwater-Shotover area in the north. Denison Trough: Early Permian extensional faults bound several discrete half graben, which contain up to 8 km of sediment fill. Later reactivation of some of these faults as thrusts has led to partial basin inversion and formation of fault-propagation anticlines. Some of the anticlines formed traps for significant gas reservoirs. Vertical displacements of up to 900 ms (about 1.7 km) occur on the thrusts. New thrust faults have also formed, often as backthrusts to the reactivated extensional faults. The orientations of the early faults played an important role during the reactivation: north-south striking faults show strong reactivation whereas NWSE oriented faults show virtually no reactivation. In places, the faults have propagated to the top of the Bowen Basin succession, indicating that the majority of the movement is MiddleLate Triassic in age. Taroom Trough: There is only minor faulting within basin succession, principally on the Burunga Fault near the eastern margin. However, there has been strong tilting of the eastern margin of the succession, with over 3 km of uplift. This probably occurred above an eastdirected, bedding-parallel backthrust forming the top of a duplex consisting of west-directed thrusts that occurred mainly east of basin in the volcanic basement. Blackwater-Shotover area: This area is structurally very different to that of the main Taroom and Denison depocentres of the basin, having a very different thrust geometry that is dominated by very low-angle, east-dipping thrust faults. Here, the thrustfronthas propagated a long way into the basin from the orogen in the east Deep seismic data acquired by AGSO in the Blackwater area show that the thrust faults form an imbricate thrust fan and sole into a major detachment dipping shallowly to the east. Farther south in southernmost Queensland and in the Gunnedah Basin in NSW, deep seismic data show very different upper crustal geometries within the basin system and adjacent New England Orogen. That is, the upper crust is partitioned from north to south into zones of different deformational style controlled by fundamentally different crustal architectures.

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MASS TRANSFER AND MICROFRACTURING IN GABBROIC MYLONITES OF THE GUADALUPE IGNEOUS COMPLEX, CALIFORNIA, USA Bruno Lafrance and Ron H. Vernon. School of Earth Sciences, Macquarie University, Sydney, NSW 2109 Gabbroic rocks of the Guadalupe Igneous Complex (GIC), California, USA, were deformed in the Bear Mountains Fault Zone shortly after emplacement of the complex, producing protomylonites, mylonites and ultramylonites. Temperatures during the deformation were high, neocrystallized mineral assemblages suggesting upper to lower amphibolite facies conditions. The deformation occurred essentially in the solid state, but fluid flow, microfracturing and mass transfer also played important roles in the development of the shear foliation. For example, microfracturing is evidenced by broken porphyroclasts of plagioclase and pyroxene (with tension fractures at high angles to the inferred shear plane) that are elongate parallel to the mylonitic foliation. Furthermore, no subgrains or any other evidence of internal plastic deformation have been observed in pyroxene or amphibole porphyroclasts. Plagioclase-rich layers parallel to the mylonitic foliation are the products of plastic deformation by dislocation creep accommodated by dynamic recrystallization involving grainboundary migration. In contrast, amphibole-rich layers were formed by fluid flow, microfracturing and mass transfer. Variations in pore fluid pressures, strain rates and/or stresses caused microfracturing, opening of dilatant sites in the "pressure-shadows" adjacent to pyroxene and amphibole porphyroclasts, and the crystallization in these sites of new amphibole grains to form "tails" of smaller, elongate amphibole grains. Coalescence of the "pressure-shadows" and further crystallization of new amphibole grains in gaps formed during grain-boundary translation produced amphibole-rich layers. Very elongate grains of scapolite and sphene appear to have crystallized as veins in foliationparallel fractures. The sphene veins, consisting of one long narrow grain per thin section, are both parallel and transgressive to the foliation. They indicate mobility of titanium under these conditions, as do planar concentrations of ilmenite occurring as layers in recrystallized plagioclase and rare transgressive veins. Quartz plates parallel to the foliation also appear to be veins with sharp, planar boundaries, and are composed of a few large grains that generally span the width of the veins. In rocks undergoing deformation at high temperatures, some microstructures formed by coupled mass transfer and microfracturing resemble microstructures formed by recrystallization and plastic deformation. For example, new elongate amphibole grains extending into amphibole and pyroxene porphyroclasts, which previously have been interpreted as the products of chemically-assisted dynamic crystallization, are neocrystallized grains that grew initially in a fluid and then into the porphyroclasts in the GIC mylonites. Furthermore, polygonal grains of pyroxene, which are usually taken as evidence of dynamic recrystallization, were probably formed by cementing or sintering of fragments produced by cataclasis in the GIC rocks. The GIC mylonites highlight the major role of fracturecontrolled processes in mylonitic deformation, even at high temperatures.

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MICROSTRUCTURAL GEOCHRONOLOGY James K.W. Lee. Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 0200, Australia [phone: (06) 249 4176; fax: (06) 249 0738; e-mail: jkll 52 @ huxley. anu. edu. au] Recent advances in the resolution, precision and accuracy of the radiometric dating techniques has led (not surprisingly) to increased complexity in the interpretation of geochronological data from a variety of minerals. Moreover, the widespread application of geochronological techniques in such diverse fields as volcanism, metamorphism, tectonics, paleomagnetism, sedimentary provenance and basin analysis, the isotopic evolution of the Earth, and diffusion has meant that an increasing number of geological/geochemical/geophysical studies depend upon an accurate and reliable evaluation of the significance of geochronological results. Traditionally, the interpretation of geochronological data has involved, directly or indirectly, the application of solid-state diffusion theory. Solid-state (also known as lattice or volume) diffusion is described by Fick's well-known Second Law: ^ = V-DVC dt where C is the concentration of the diffusing substance, D is the diffusion coefficient and t is the time. Because Fick's Second Law describes the migration of atoms or ions through a homogeneous crystal structure, this is a critical and necessary condition for any valid application of volume-diffusion theory. Crystals in geological materials, however, are far from perfect and commonly contain extended defects such as dislocations, micropores, fractures, subgrain boundaries (e.g. exsolution lamellae), radiation damage (fission, recoil tracks), etc. all of which may act as pathways for much more rapid transport through the crystal. In the past, the influence of microstructures on mineral ages has largely been ignored. Analyses of bulk (i.e. lO's of mg) mineral separates tended to homogenize and subsequently obfuscate such effects. However, improvements in the sensitivity and resolution of mass spectrometers coupled with technological advances in microanalytical techniques [e.g. the 40Ar/39Ar laser microprobe, the ion microprobe (SHRIMP)] have meant that ages can be obtained within individual mineral grains with a spatial resolution of -10-30 |nm and a sample size on the order of micrograms to nanograms. More significantly, the spatial resolution of such "age" microprobes allows us to directly assess the effect of extended defects on the transport of geochronologically important species (e.g. Ar, Pb) for the very first time. Examples of the importance of microstructures in the interpretation of ages are given from two of the most common radiometric methods currently in use: 40Ar/39Ar and U-Pb. 40

Arfi9Ar

It has been commonly assumed that the incremental-heating (or step-heating) technique can accurately reflect the spatial distribution of argon in hornblende. In a combined incrementalheating and laser microprobe study, however, Lee et al. (1990) reported an apparent plateau age spectrum for a hornblende containing significant age gradients. Laser microprobe traverses across two hornblende grains showed age gradients extending from -2.3 Ga in the grain cores to -2.7 Ga in the grain rims. Laser step-heating of another hornblende grain from the same rock sample yielded a plateau of intermediate age (-2.45 Ga), however, demonstrating that the true spatial distribution of Ar was not revealed by the incremental-heating technique. Subsequent XRD and SEM studies by Wartho et al. (1991) and Lee et al. (1991) of hornblende heated in vacuo (under conditions identical to those during an 40Ar/39Ar experiment) showed that Ar release was directly correlated with the structural decomposition (dehydroxylation, incongruent melting) of the mineral. Moreover, because decomposition took place preferentially along extended defects such as lamellar boundaries, cleavage and microcracks everywhere throughout the mineral grains, any Ar released from these regions during step-

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healing would be subsequently homogenized, making it highly unlikely that an original spatial gradient of Ar would be preserved in the resultant age spectrum. More importantly, these results also suggest that independent criteria (e.g. field relationships, other geochronological techniques) may be required to decide whether a plateau age is geologically significant. U-Pb Despite a general consensus that U and Pb diffusion in zircon (ZrSiCU) must be very slow, experimental estimates from the published literature of Pb diffusion parameters in zircon vary over -20 orders of magnitude. This disparity in the diffusion data is the result of a number of causes - the most important of which are sample inhomogeneities, intragrain variations in zircon crystallinity and the correspondingly poor or non-existent physical characterization of most of the zircon samples. It is well-known that zircon is commonly zoned with different concentrations of radioactive elements such as U and Th. As these elements decay, metamictization resulting from the associated radiation damage to the crystal structure produces a corresponding volume expansion. Such an expansion also produces corresponding internal stresses, and elastic models predict that such stresses will generate radial and/or concentric fractures in the crystal depending on which part of the zircon becomes metamict. A suite of zircons from the Petersburg Granite, U.S.A. containing metamictization-induced radial and concentric fractures has been examined by SEM and analysed by ion microprobe (Lee and Williams, 1993). Regions of zircon probed by SHRIMP show that fractured but unaltered regions of zircon yield essentially identical Pb/U ages to the concordant ages from unfractured zones. Spot analyses of fractured and altered zones, however, yield ages significantly less than the granite age, suggesting substantial Pb loss. Thus, it appears that penetration of the interiors of the zircon crystals by hydrothermal fluids via the large-scale (10 s of micrometres) metamictization-induced radial and concentric fractures can result in chemical alteration and subsequent Pb loss. Consequently, Pb loss here is not correlated directly with a simple diffusional process, but rather with fluid alteration of the fractures, implying that U and Pb mobility is enhanced by radiation damage through extended defects which provide a network of pathways for fast transport. Crystal imperfections can arise not only from laboratory-induced phenomena (e.g. mechanical crushing during sample preparation, mineral decomposition during analysis) or natural mineralogical constraints (e.g. crystallization, metamictization) but also from geological processes (e.g. deformation). Thus, it may be essential to carefully characterize (using, for instance, XRD, SEM, TEM, cathodoluminescence, etc.) mineral samples to be dated, since microstructurally-dependent mechanisms may serve as the dominant control on the migration of radioactive/radiogenic species (Ar, U, Pb, etc.) in geochronologically important minerals. Moreover, this short-circuit (SC) diffusion process has important implications for thermochronology. Because overall transport rates in a crystal will be enhanced through SC diffusion, the effective diffusion coefficient will be greater than the volume-diffusion coefficient, leading to potentially significant decreases in both closure (cooling) ages and closure temperatures. References f

Lee JKW, Onstott TC and Hanes JA (1990) An Ar/ Ar investigation of the contact effects of a dyke intrusion, Kapuskasing Structural Zone, Ontario: A comparison of laser microprobe and furnace extraction techniques. Contrib. Mineral. Petrol. 105: 87-105. Lee JKW, Onstott TC, Cashman KV. Cumbest RJ and Johnson D (1991) Incremental heating of hornblende in vacuo: Implications for Ar/ Ar geochronology and the interpretation of thermal histories. Geology 19: 872-876. Lee JKW and Williams IS (1993) Microstructural controls on U-Pb mobility in zircons. EOS Trans. Am. Geophys. U. (Fall Meeting), in press. Wartho J, Dodson MH, Rex DC, Guise PG and Knipe RJ (1991) Mechanisms of Ar release from Himalayan metamorphic hornblende. Am. Mineral. 76:1446-1448. 40

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TECTONISM, DEFORMATION GRADIENTS AND INTRUSION STYLES

P.G. Lennox and T.J. Fowler, Department of Applied Geology, UNSW, PO Box 1, Kensington 2033; Department of Geology, LaTrobe University College of Northern Victoria, PO Box 199, Bendigo 3550 The Carcoar, Barry and Davys Creek Granites in the northern Lachlan Fold Belt provide an ideal natural laboratory to investigate the nature of high-level, weakly to moderately deformed Devonian to Carboniferous granites in weakly to moderately deformed Ordovician metasediments, mafic and silicic metavolcanics and greywackes (Fig. 1). The Carcoar Granite (CG) and northern Barry Granite (NBG) are I-type granites with localised concentrations of microgranitoid enclaves. The southern Barry Granite (SBG) and Davys Creek Granite (DCG) are S-type granites with minor peripheral albite dykes and metasedimentary xenoliths. These small plutons are petrographically similar to the Wologorong and Wyangala granties. Whereas the DCG is not related to regional-scale ductile shear zones, the Carcoar and more so the Barry Granites are marginal to a dextral, ductile shear zone which caused significant west-overeast thrusting of the nearby Wyanagala batholith. In all these plutons the strain is strongly heterogeneous from the microscopic to regional scales.

Figure 1: Locality Map Figure 2: Speculative development of the Copperhannia Fault associatedwith Carcoar and Barry Granite emplacement.

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In all cases the granites exhibit a foliation varying from poorly developed (and grading into joints) to penetrative. The spaced, crack-like cleavage in the DCG was produced by brittle-ductile transitional behaviour involving simultaneous plastic deformation of quartz and groundmass intergrowths and subcritical intergranular crack growth. Whereas the CG/NBG are cut by major faults with mylonitic fabrics they also display brittle structures which superfically resemble mylonites. Mylonites are present in the SBG and the initially finest-grained microgranites in the DCG. The bulbous to angular shapes of the CG & DCG constrasts with the elongated Barry Granite and the nearby, probably older gneissic, elongated Wyangala and Wologorong batholiths (Fig. 1). The CG and BG intrude across the northeasterly-trending metamorphic fades boundaries within the country rocks which grade from prehnite-pumpellyite in the northwest to biotite in the southeast. Both the DCG and BG appear to have intruded as elongate sacklike bodies with orientation pre-determined by regional anisotropy (bedding-cleavage trends), rather than the prevailing tectonic stress orientation. The DCG became enlarged by magma pressure, whereas the BG passively intruded with limited aureole deformation. The CG/NBG are weakly deformed whereas the SBG is moderately deformed. The SBG contains a dominant foliation, mylonites and rare S-C structures. From north to south there is a progressive increase in the apparent depth of crust being exposed which is reflected in the increasing metamorphic grade of the country rocks and the transition from mainly brittle to brittle-ductile structures in the granites from the northern Carcoar Granite to the southern Barry Granite. The mechanisms of emplacement are quite different for each of these small plutons, depending upon their local tectonic environment. The DCG had to make its own room by forcing aside the wall rocks. The BG may have had its site prepared by extensional events associated with a pull-apart basin connecting dissimilar trending segments of the Copperhannia Fault (Fig. 2), or the fault propagated around buried granites before the high level emplacement of the Barry and Carcoar Granites. The section of the Copperhannia Fault adjacent to the NBG may have been pinned in the Devonian by a buried granite (at approximately 300- 500m depth for its upper surface) which has been identified from regional magnetic studies. The DCG was deformed by the same tectonic forces which resisted intrusion as it is a syntectonic granite. In contrast the Barry Granite then underwent dextral ductile shearing because of pinning by the buried granite and the fact it lay adjacent to a northeast-southwest oriented segment of the Copperhannia Fault being subjected to east-west compression. It is possible the segment of crust adjacent to the Copperhannia Fault was tilted during movements associated with the tectonic development of this rifted Palaeozoic crust resulting in the observed increasing metamoprhic facies gradient, changes in structural style and increasing deformation gradient from north to south. Further dating studies are proceeding to establish a well constrained chronology for events in this section of the Lachlan Fold Belt.

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DIFFUSION OF ARGON IN MINERALS FROM METAMORPHIC TECTONITES AND THE ROLE OF MICROSTRUCTURAL PROCESS Gordon Lister and Suzanne Baldwin .Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Victoria, 3168 Australia In this abstract we review some of the models for argon diffusion in mineral grains in metamorphic tectonites, in the context of the types of P-T-t histories to be expected in complexly deformed and/or multiply metamorphosed terranes. The effect of specific P-T-t histories can be modelled using finite difference equations to predict the time history of 40Ar concentration profiles across individual diffusion domains, and the MacArgon program has been written to allow this to be accomplished using an Apple Macintosh computer. By using the MacSpectrometer to analyze the results we can assess concepts commonly used to facilitate the interpretation of 40Ar / 39Ar apparent age spectra. Considerable caution needs to be exerted in the use of the concept of closure temperature, and/or in the interpretation of the significance of plateaux observed in many apparent age spectra. There is a need to reassess the analysis of 40 Ar / 39Ar apparent age spectra from real rocks in circumstances where episodes of argon loss have taken place during complex P-T-t histories. The concept of closure temperature is relevant only to a P-T-t history which involves cooling from temperatures at which the mineral is unable to accumulate a significant concentration of radiogenic argon. The concept is made more difficult in that in an uplifting metamorphic terrane there can be a significant pressure drop which accompanies such cooling, and this produces a competing effect which can lead to "reopening" of the system to diffusion. Plateau in apparent age spectra also need to be treated with caution, particularly when the shape of the argon spectra suggest partial loss. Argon loss can be caused by a thermal pulse, depressurization, and/or slow cooling, but also as the result of crystallization at temperatures high enough to cause loss at the rims of diffusion domains, but sufficiently low so as to enable argon cumulation in the domain cores. In all cases, once the amount of argon lost is sufficient to allow the establishment of a quasi-steadv state concentration profile across an individual diffusion domain, similarly shaped 40 Ar / 59 Ar apparent age spectra result. Well-defined plateau appear in the 40 Ar / 3^Ar apparent age spectra, but these have no fundamental significance. Thus plateaux ages inferred from apparent age spectra which display the effects of partial loss may also have little or no significance. There are other factors which affect the development of argon spectra, and in metamorphic tectonites the results of crystal plastic behaviour and recrystallization seem the most obvious culprits. The potential effect of migration recrystallization is discussed in the context of decussate fabrics, and the potential role of substructure induced during deformation in shear zones. This is of importance because recent work in the metamorphic core complexes of the Colorado River extensional corridor in the western United States has argued for "cold" mylonites, formed at temperatures in the range 200-300°C. The rocks immediately adjacent (meters to tens of meters distant) do not have mylonitic fabrics, and the temperatures inferred from these rocks are up to 150°C cooler. There is no need to argue that these "mylonites aren't so hot"! We postulate that ductile shear zones formed in the thermal aureoles of intruded igneous bodies, and that deformation did not go on for very long. There is clear evidence of a superficial crustal level at the time of mylonitization, and clear evidence for the existence of thermal aureoles. It is regrettable that so many authors have been overcome by the persuasive argument that temperature is related to "ambient" conditions (and thus determined by the average geotherm, and the depth in the crust), whereas the weight of evidence in fact points to "transient" conditions. It is quite incorrect to argue (as have so many recent authors) that rapid cooling is the direct result of rapid uplift!

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The feature of apparent age spectra that most requires explanation is the survival of old apparent ages in minerals which have been subjected to later deformation at temperatures in excess of 300-550°C. By all indications, the minerals have been held above their so-called "closure temperatures" for considerable periods of time. Equally important is the significance of the plateau in these apparent age spectra and the observed rise in apparent ages during step-heating experiments. These features can be interpreted as indicating partial argon loss from the rim zone of diffusion domains while the core zone remained retentive. Such circumstances can apply during a short thermal pulse. For example, to explain our data from Ios (Cyclades, Greece) we propose that the protolith of the central granite gneiss terrane consisted of a complex of igneous and metamorphic rocks of different ages. These rocks occupied a shallow crustal setting, with low ambient temperatures. They were subjected to the alpine orogeny, but temperatures remained low (although pressures may have been very large, attaining those necessary to form jadeite). During the Miocene the temperature briefly rose, and the entire complex was stretched in a Miocene ductile shear zone. The geochronological data supports the inferred Miocene age for the D shear zones. However to explain the markedly different argon release spectra have been obtained from adjacent samples it is necessary to postulate the existence of a short lived thermal pulse, and that the D mylonites formed during this event. There must have been substantial lateral thermal gradients in the mylonite pile at the time D^ took place, and different samples would have been exposed to radically different thermal histories. Rapid cooling in this case is not due entirely to rapid unroofing of the metamorphic core complexes. Shallow intrusion of hot igneous bodies produces the same result. Such a thermal history as we discussed could be produced only by episodic and intermittent intrusion of granitoid sills during D . Argon would be rapidly lost from minerals in rock immediately adjacent to such bodies. Samples a little further away would retain radiogenic argon. Recrystallization and grain growth during deformation would result in new white micas, analysis of which would tightly constrain the age of the deformation event Intense zones of ductile shear can form during the cooling of sills and/or plutons. Thermal modelling shows that such cooling is relatively rapid and thermal decay constants of 10,000100,000 years are not difficult to explain. Our data suggest the rapid operation of ductile shear zones during cooling of granite batholiths. It is interesting to note that the wall rocks of a 100 m thick granitoid sill intruded at ~800°C would not cause wall rocks to rise much above 600°C. The sill would have solidified after <200 Yr, with the thermal decay constant for subsequent cooling -1,000 Yr. A sill of 400 m thickness might take 200 Yr to solidify, with -10,000 Yr as the thermal decay constant. There is no doubt that aspects of this model may apply to metamorphic core complexes in the Basin and Range province of the western United States where similarly enigmatic argon data suggests rapid cooling after thermal pulses, with ambient temperatures before deformation apparently too low to allow explanation of greenschist and/or amphibolite facies mylonites. Alternative explanations for old apparent ages include the possibility that (biotites in particular) have accumulated ancient radiogenic argon released from other minerals, or that the quoted closure temperatures are inappropriate and specific minerals have retained argon accumulated during a much longer period. For example phengite may close at a higher temperatures (?) than either white mica or phlogopite, and thus readily retain old apparent ages. Another possible explanation is that temperatures exceeded the blocking temperature for sufficiently short periods of time so as to preempt complete resetting of the argon clocks. The effect of excess argon must also be considered. It might be possible that some of the minerals (e.g. biotite) have incorporated radiogenic argon produced elsewhere, and released as the result of later metamorphism. If the minerals are crystallizing in a later orogenic event in a shear zone which acts as a conduit for the flushing of excess argon towards the surface, perhaps it is possible for the growing minerals to incorporate the argon from the fluid as they grow. This seems unlikely given the difference in partial pressure that must apply between argon within the crystal structure, and argon in a fluid system, even at the same mean stress. 3

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FOLD-FAULT RELATIONSHIPS IN LOW-ANGLE DETACHMENT SYSTEMS Neil S. Mancktelow. Geologisches Institute ETH-Zentrum, CH-8092 Zurich, Switzerland and Terry L. Pavlis, Department of Geology and Geophysics, University of New Orleans, Atew Orleans LA 70148, USA. A close kinematic and timing relationship between low-angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well-exposed Neogene detachment systems — the Simplonpass region of the Alps and the Death Valley region of California. The Simplon Fault Zone is a well-documented example of late low-angle extensional faulting in a convergent orogen, whereas the domed turtleback detachments of Death Valley have often been considered as classic examples of faults developed in an extending orogen. This study highlights similarities in the geometry and kinematics of these two areas which points to more general affinities in their tectonic environment. The Simplon Fault Zone is a major normal fault bounding the western end of the Lepontine metamorphic dome in the Central Alps of Switzerland. Its continuation to the west and southwest can be linked to a broad zone of dextral transpression which runs along the Rhone Valley, through the zone of external basement massifs in the French Alps (Mt. BlancAiguilles Rouges, Belledonne and Pelvoux) to transfer into coeval thrusting to the southwest in the Haute Provence. The whole kinematic system of linked faults accommodates a component of southwest directed, orogen-parallel displacement of the more internal upper Pennine nappes relative to more external units (i.e. the lower Pennine and Helvetic nappes, Jura fold and thrust belt, autocthonous European foreland units etc.) during Neogene oblique convergence. The exhumation history of the footwall during fault activity is recorded as a transition from initially distributed deformation in a broad ductile shear zone to more concentrated deformation in a narrowing zone and eventual transition to cataclastic deformation with the development of a discrete detachment fault. The total relative vertical displacement is on the order of 15 km (i.e. -36 km parallel to the fault zone with 25° dip). In the area around the Simplonpass, late southeast vergent folding produced a regional antiformsynform pair in the footwall of the Simplon Fault Zone — the Glishorn Antiform and Berisal Synform. The backfolds have folded a retrograde greenschist-facies mylonitic foliation associated with movement on the Simplon Fault Zone. This can clearly be demonstrated by direct observation of small scale folds of the mylonitic foliation in the field and from the rotation of quartz preferred orientation fabrics around the regional folds. Both the measured mesoscopic fold axes and constructed regional fold axes are statistically parallel to the ubiquitous stretching lineation in the mylonites, plunging around 25° towards 240-250°. The axial surfaces to the regional backfolds dip moderately to the northwest and appear to shallow upwards. These axial surfaces are truncated against the discrete cataclastic detachment fault (the so-called "Simplon Line") of the Simplon Fault Zone. In a narrow zone of a few hundred metres width below the detachment, the mylonitic foliation that is folded around the backfolds is itself overprinted by a younger greenschist facies mylonitic foliation, of slightly colder microstructural aspect, concordant with the detachment plane and thus transecting the earlier backfolds. Within the steep limbs of the earlier backfolds, new fairly open folds with the younger shallowly-dipping mylonitic foliation as axial plane develop. The fold axes of these younger folds are coaxial with the earlier backfolds and with the stretching lineation common to all the retrograde fabrics. Sense-of-shear criteria (shear bands, quartz crystallographic preferred orientation etc.) consistently indicate the same kinematics in all the mylonite fabrics, namely upper block down to the SW parallel to the stretching lineation. The Death Valley region of southeastern California contains one of the youngest exhumed low-angle normal fault systems in the Basin and Range province of western North America. This extensional system is the product of both the Plio-Pleistocene opening of the Death Valley pull-apart basin, and an older, Late Miocene extension that may also record strike-slip related pull-apart, though this remains controversial. Along the eastern side of the Death Valley basin, the Black Mountains expose a complex, Late Miocene to Recent normal fault

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system. The structural exhumation of the Black Mountains has typically been characterized as a predominantly two-dimensional process with the Amargosa and Black Mountains frontal fault systems representing a single detachment system that developed through northwestward migration of a "rolling hinge" across what is now the Black Mountains. However, this is an incomplete description of the kinematics of a system that is highly three-dimensional. The most significant three-dimensional complication is the development of large-scale corrugations in the detachment fault system of the Black Mountains that are spatially related to systems of folds. Folds were described by early workers in the Death Valley region but the interpretation of many of these structures as folds has been largely lost in the context of extensional tectonics, where they were generally viewed as primary corrugations in the fault surfaces. The Death Valley turtlebacks represent the deepest structural levels within the Black Mountains. On a large scale, the turtlebacks are doubly plunging antiforms that expose Precambrian basement and metamorphosed miogeoclinal rocks in their cores with a surrounding "cover" of Miocene plutonic rock. Fabric asymmetries associated with the main foliation (SI) in the turtlebacks suggest that the deformation occurred in a shear zone with a shear sense of top to the northwest. This foliation is warped into a series of conspicuous folds. These folds (F2) range in scale from wavelengths of a few meters to ~1 km and vary in style from open warps of SI to sub-isoclinal folds associated with a second foliation (S2) axial planar to the F2 folds. F2 axes parallel the prominent mineral and elongation lineation developed on the SI foliation and systems of F2 folds produce the anticlinoria which define the turtlebacks. F2 folds in the turtlebacks are transected by a system of brittle faults and retrograde shear zones which are themselves curviplanar. The curviplanar form of the brittle faults in the turtlebacks may be at least in part a result of fault systems exploiting metamorphic foliation as a reactivation surface. However, most of this curvature may reflect a continuation of F2 folding during evolution of the detachment system. A temporal, geometric and kinematic relationship between shearing and folding can thus be established in both the Simplon Alps and the Death Valley region. Fold amplitude is largest in the oldest mylonitic foliation of the footwall. These folds are transected by younger mylonitic foliation(s) and cataclastic detachment faults which are themselves folded about similar axes parallel to the movement direction within the shear zones. In general, the amplitude decreases and the wavelength increases for progressively younger fold structures. The folds are not passive sheath folds but mechanically active buckle folds in layering, foliation and the detachment surfaces themselves and generally have moderately to steeply dipping axial surfaces. They therefore represent an overall bulk shortening in a near horizontal direction perpendicular to the extension direction of the associated normal faulting. Changes in length in the vertical direction will depend on the relative contribution of horizontal contraction/folding and horizontal extension/faulting to result in either bulk crustal thickening (with an overall flattening or oblate strain), constant thickness (plane strain) or thinning (with an overall constrictional or prolate strain). Major detachment fault systems involving dramatic and rapid exhumation of the footwall block may not necessarily involve significant overall crustal thinning, but could also develop during transpression/transtension or during lateral extrusion without dramatic changes in overall crustal thickness.

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THE GEOMETRY OF FLUID INCLUSIONS AND THEIR INFLUENCE ON DEFORMATION AND DYNAMIC RECRYSTALLIZATION IN QUARTZ MYLONITES OF THE SIMPLON FAULT ZONE, SWISS-ITALIAN ALPS. Neil S. Mancktelow. Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland, Eric L. Johnson, Department of Geology, Central Michigan University, Mount Pleasant, M/ 48858, C/SA and Djordje Grujic, Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland. The Simplon Fault Zone, a low-angle normal fault in the Simplon Alps of Switzerland and Italy with around 15 km of vertical throw, displays microstructural transitions related to increasing structural depth in both a NW-SE direction parallel to the fault trend and with distance into the footwall. Vein quartz samples transitional from fracturing to low-temperature plasticity were deformed below the temperature for unmixing of C02-H20-NaCl fluids and show healed microfractures and repeated deformation lamellae decorated by aqueous and mixed aqueouscarbonic (H20-rich) inclusions, whereas C02-rich inclusions are restricted to grain and subgrain boundaries. With increased temperature, the vein quartz is more pervasively (mylonitically) deformed, with advanced subgrain rotation recrystallization producing a marked grainsize reduction. The common fluid inclusions are too small to analyse optically. SEM backscatter orientation contrast studies on highly polished surfaces and secondary electron images of broken surfaces show that inclusions are concentrated on subgrain and grain boundaries, with a preference for boundaries perpendicular to the extension direction. Intracrystalline inclusions along healed fractures are rare. Inclusions are unconnected, with regular planar boundaries suggestive of inverse crystal shapes typical of carbonic inclusions. The total fluid inclusion content of different samples can vary dramatically — this difference in inclusion density is not reflected in any distinctive difference in the measured crystallographic preferred orientation suggesting that fluid inclusion density exerts little control on the dislocation glide processes which produce the preferred orientation. With a further increase in temperature, there is a transition from progressive subgrain rotation (PSR) recrystallization to grain boundary migration (GBM) recrystallization and consequent grain growth. This transition is quite irregular, and vein quartz samples from adjacent outcrops can show a wide range in grain size and in predominant recrystallization mechanism. Traditional models for quartz recrystallization stress that the transition from PSR to GBM is a function of temperature. If temperature is the only variable, however, one would not expect to find kilometre wide PSRGBM transition zones with outcrop scale variations in recrystallization mechanism. In the Simplon Fault Zone samples, fluid inclusions in the southern zones (GBM recrystallization mechanism) contain abundant C02-H20-NaCl fluid inclusions (XC02=0.3; relative salinity = 5-7%). In the northern section of the SFZ (i.e. across the GBM - PSR transition) the inclusion populations consist of a bimodal distribution of C02-rich and H20-rich inclusions. We attribute this change to the onset of H2O-CO2 fluid immiscibility. The onset of fluid immiscibility for H 0-C0 -NaCl fluids with XCO2 =0.3 will occur at ca.450-300°C. This corresponds to the temperature range estimated from the associated metamorphic mineral parageneses for the GBM-PSR transition along the Simplon Fault Zone. The position of the miscibility gap in pressure-temperature space, however, is sensitive to small changes in fluid composition, such that the transition may occur at significantly different temperatures in adjacent samples with only slightly different fluid compositions. Fluid unmixing will produce high surface energy CO2 fluid pockets which are observed to decorate grain boundaries and may significantly reduce the rate of grain boundary migration, retarding the GBM recrystallization mechanism relative to the competing SBR mechanism. From these considerations, it is proposed that the observed irregular transition in recrystallization mechanism with decreasing temperature from SE to NW along the Simplon Fault Zone may be strongly influenced, and perhaps even controlled, by the coincident unmixing of the ubiquitously present H20-CC>2-NaCl fluids and the development of C02-richfluidinclusions decorating grain boundaries. 2

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MICROSTRUCTURAL AND STABLE OXYGEN ISOTOPIC EVIDENCE FOR EXTERNALLY DERIVED VEIN FORMING FLUIDS AT MAGDALA GOLD MINE STAWELL, WESTERN VICTORIA. Benjamin S.E. Mapani and Christopher J.L. Wilson. School of Earth Sciences, University of Melbourne, Parkville 3052, Victoria. The mineralized quartz reefs at Magdala mine Stawell, are located in two shear zone systems, the Central lode system and the Scotchmans Fault Zone (Fig. 1). The Central lode is controlled by reverse 40° to 70° SW dipping faults that vary in width from 1-4 m, and contains footwall and hanging wall quartz lodes and a related set of flat quartz veins (No. 1 and No. 2 flat vein systems, Fig. 1). The Central lode crosscuts the mine schist and vocanogenic sediments, such that portions of these lithologies are present as included selvedges in the quartz veins. The central lode structure possess three generations of S-C fabrics. The largest are 0.3-1 m wide, defined by deformed quartz with mylonitic textures that possess a 0.1-0.15 m thick layers of graphite. In areas where layer silicates are present between the ribbon quartz, phyllosilicates exhibit grain-size reduction features and occur as 30-40 micron shear bands. These textures together with refolded quartz veins show that the Central lode system has experienced up to 75-85% shortening. Two generations of sulphides are present along deformed quartz vein boundaries and make up about 5%-10% of the vein mineralogy. The first assemblage is pyritepyrrhotite-sphalerite-chalcocite and gold is present as fine-grained inclusions (>3 microns) in pyrite. This assemblage is deformed along vein margins forming elongate lensoidal trails that parallel C-planes, but is absent in S-planes. Superimposed upon the early sulphide assemblage of the Central lode system, is a later assemblage of pyrite-arsenopyrite-chalcopyrite ±pyrrhotite, in which gold is associated with all the sulphides occurring as inclusions and varying in size from sub-microscopic to 30-40 microns. The pyrite-arsenopyrite-chalcopyrite-pyrrhotite assemblage characteristically forms euhedral crystals. Associated with the later sulphide assemblage is strain free quartz occurring in pre-existing S-C planes. Barren late veins composed of strain free euhedral calcite, dolomite and minor quartz overprint both sulphide assemblages. The Scotchmans Fault Zone cross-cuts the Central lode and is defined by 25°-60° NW dipping curved thrusts that generally follow the fold hinge line of the Magdala anticline. Laminated veins of the Scotchmans Fault Zone possess comb and fibrous quartz textures. Comb textures are generally characterised by composite vein types with individual veins having experienced different amounts of strain. The strain features present in these comb textures are serrate grain boundaries, undulose extinction, deformation bands and to a lesser extent deformation lamellae. Quartz deposited in a low strain regime is accompanied by sulphide phases pyrite-arsenopyritechalcopyrite and the gold is hosted in pyrite and arsenopyrite. Upon these textures are superimposed quartzfibresthat are straight with a high length to width ratio. Inclusion trails in these fibres suggest a crack seal mechanism for their formation. The contact between the comb textures and quartz fibres (partition surface) possess a stylolitic morphology implying that dissolution has taken place. The host rocks to the Central lode system are pelitic schists with a quartzmuscovite-chlorite-rutile-graphite mineralogy (Fig. 1). The pelitic schists possess a strongly foliated fabric. In the 0.5 -1 m contact zone between the pelitic schists and the Central lode system, the schistose fabric is overprinted by a calcite-chlorite-quartzpyrite-arsenopyrite mineralogy with characteristic decussate texture. The alteration halo between the Central lode system and the volcanogenic rocks is wider than that between the Central lode and the pelitic schists, measuring up to 2 m in thickness. This alteration halo overprints the regional metamorphic schistosity defined by layer silicates, chlorite, muscovite and biotite wrapping around lenticular actinolite and

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quartz. Minor calcite and albite are present in the volcanogenic foliation. The mineralogy of the alteration halo is dominated by calcite porphyroblasts rimmed by randomly oriented 60-100 micron stilpnomelane flakes surrounded by chlorite. The groundmass in which these textures occur, is composed of decussate quartz and chlorite. Minor shears cross-cut these textures, and within the shears is developed quartz-pyrite and arsenopyrite. Stable oxygen isotope values in the quartz reefs range from 15.852 to 17.292 per mil whereas the host rock values differ depending on the lithology. The highest oxygen isotope values in the host rocks were recorded in the volcanogenics with values in the range 12.162-14.128 per mil, followed by the mine schists with values of 12.2-13.45 per mil. The lowest values occur in the metabasalt with values of 9.751-11.946. For the volcanogenics and mine schist, elevated 5 0 values were recorded in areas close to the quartz veins (approximately 5-10 m). The observed narrow range of 15.85217.292 per mil for 5 0 in quartz veins is independent of host rock lithology, whereas 8 O values in host rocks are spread over the interval 9.751-14.128. In the alteration zones, carbonates are ubiquitous, implying that the fluid reservoir carried significant CC>2. These observations suggest isotopic disequilibrium between host rocks and quartz veins. From these observations it is clear that lateral diffusion of chemical components such as Au, and CO2, would be inconsistent with mesoscopic, microstructural, isotopic and structural data. The dissimilarity in 8 O of the host rocks and quartz veins suggest a high volume of channelized fluid flow as opposed to lateral diffusion. The presence of hydrothermal alteration halos indicates that the vein forming fluids were not in equilibrium with the host rocks. The structural setting of the veins in fault zones, the textural differences observed in the form of deccusate textures overprinting preferred oriented metamorphic textures in quartz vein-host rock contacts, together with stable isotopic evidence, suggest that the fluid responsible for vein formation was externally derived. 18

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LEGEND EASTERN SCHISTS MINE SCHISTS VOLCANOGENICS METABASALTS (FOOTWALL VOCANICS) FAULT PARALLEL QUARTZ VEINS SL = SCOTCHMANS LOWER FLAT SU = SCOTCHMANS UPPER FLAT SV = SCOTCHMANS VERTICAL SFZ = SCOTCHMANS FAULT ZONE

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LONG-LIVED SHEAR ZONES AND THEIR RELATIONSHIP TO DEFORMATION IN SURROUNDING COUNTRY ROCKS V.M. Mares Department of Geology, James Cook University, Townsville Q., Australia 4811 Ancient crustal-scale shear zones faithfully record evidence of the deformational events that they and the country rocks surrounding them have undergone. The Black Rock anticline of the Halls Creek Mobile Belt in Western Australia and the two shear zones that border it (the Alice Downs shear zone on the west and the Halls Creek shear zone on the east - previously referred to as the western and eastern splays of the Halls Creek fault, respectively) record six deformational events, five ductile and one brittle. The structures due to five of these events can be correlated in all rock types over the entire region and are all consistent with the events that created them. An initial compressive event, D produced bedding-parallel foliation, regionally affecting all rocks, and discrete layer-parallel shear zones locally, l-2m in width or less, within the rocks of the Black Rock anticline. D , the strongest event, is characterised by tight to isoclinal upright folds with steeply dipping axial surfaces that initially trended ENEWSW. A well-developed pervasive axial-plane cleavage exists in all rocks and differentiated cleavage is present in granodiorite layers of the Alice Downs shear zone as well as the lower grade melanges of the Halls Creek shear zone. Schists contain a tight crenulation cleavage of approximately the same orientation throughout the eastern half of the anticline. u

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Another strong event, D , produced a flat-lying (25°-35° dip) cleavage everywhere. D folds are recumbent, axial surfaces dip shallowly to the NE, and vergence is to the east. This event is responsible for rotating the initially subvertical D structures into a much shallower orientation. A mostly left-lateral shear sense for D can be traced across the anticline. Locally, D and D cleavage traces are preserved within the melange of the Halls Creek shear zone and quartzite pods enclosed by melange of the Alice Downs shear zone reveal the same overprinting relationships and orientations. 3

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D (previously recognised as D ) resulted in open upright NE - plunging folds of large wavelength and amplitude. This event caused a Type 2 interference with "arrowhead" refolded fold patterns. The prominent direction of shear during D was vertical and this is reflected in a narrow band of vertical mylonites within the Alice Downs shear zone and a vertical cleavage overprinting S in the Halls Creek shear zone. D , the weakest of all the ductile events affecting this area, is not represented in the shear zone rocks. It can only be observed within the schists of the Black Rock anticline. Brittle deformation during D has produced two north/south - trending, left-lateral strikeslip faults, transecting the Black Rock anticline along a D fold limb, as well as faults and fractures at all scales. Within the Alice Downs shear zone faulting has produced extensional fractures along what may have been a D or D cleavage. 4

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FRACTURE MECHANICS AND IMPLICATIONS OF PERIDOTITE-HOSTED LEUCOCRATIC DYKES, VAMMALA MINE, FINLAND Brian Marshall! and John V. Smith^, ^Department of Applied Geology, University of Technology, PO Box 123, Broadway, NSW 2007. ^Faculty of Resource Science and Management, University of New England - Northern Rivers, PO Box 157, Lismore, NSW 2480. Vammala Ni-Cu-Fe sulphide mine is located in the SW of Finland, 175 km NW of Helsinki. The mineralization is hosted by the lowermost layer of the Stormi ultramafic complex (Fig. 1), which has intruded Svecokarelian migmatitic gneisses (Hakli & Vormisto 1985). The ultramafic body has a radiometric age of 1.89 Ga, which suggests that it is synorogenic (Papunen & Vorma 1985). Despite this, uncertainty exists over the time (relative to the deformation sequence) and level of emplacement of the ultramafite. Some (Hakli & Vormisto 1985) advocated shallow emplacement close to the sea-floor prior to folding and metamorphism, whereas others (Peltonen 1990; Peltonen 1993; Marshall & Mancini 1993) implied emplacement at a depth compatible with medium to high grade metamorphism. Mineralogical assemblages in the gneisses, and geothermometry on the gneisses and ultramafite, suggest metamorphism attained conditions of the upper amphibolite/lower granulite facies (Peltonen 1990, 1993; Kilpelainen & Rastas 1992). The gneisses record three deformation events. D2 formed the migmatitic gneissosity (S2), which then acted as form-surface for the regionally significant D3 event and the minor kink folds of D4. The D3 folds segregated leucosome along their axial surfaces and did not retrograde the S2 mineralogy; metamorphism therefore remained at or near peak conditions throughout the D2-D3 period (Kilpelainen & Rastas 1992). In contrast with the foliated gneisses, the peridotitic ultramafite has a well-preserved cumulate texture. Cores of clinopyroxene remain within poikiloblastic tremolitic hornblende, in turn overgrown by of magnesio-cummingtonite at the interface with partly serpentinized olivine (Marshall & Mancini 1993). Where Ni-Cu-Fe sulphides abound, the ore displays primary net-texture. A system of thin dykes paralleled by narrow zones of wallrock alteration cut the ore and sub-economic peridotite. The leucocratic dykes comprise plagioclase (Na20/Ca0 « 2 to 5), magnesio-hornblende to magnesio-cummingtonite, minor phlogopite and interleaved chlorite, coarse apatite, and variable amounts of quartz and K-feldspar; the alteration zones are dominated by magnesio-cummingtonite and phlogopite/chlorite (Marshall & Mancini 1993). Both assemblages are compatible with amphibolite facies metamorphism, but neither exhibits secondary tectonism. Although commonly 5-10cm thick, the abundant dykes have resulted in a 5-15% volume increase relative to the original peridotite (Marshall & Mancini 1993). "T", "X" and "K dyke-intersections are consistent with a dilational opening mode. Minor offsets are compatible with the oblique extension that inevitably accompanies divergence of blocks in three dimensions. More substantial offsets could reflect D4 activation, but most were synchronous with late serpentinization. M

Orientation data for the dykes were obtained from the Sotka and Iivari orebodies (Fig. 1). The main orientations from within and between levels of the same orebody are consistent, but differences exist between orebodies located in different parts of the ultramafite (Fig. 2). The Sotka system resolves into a conjugate set with its large acute dihedral angle bracketing a N-S set (Fig. 2a); a shallow, WSW-dipping subordinate set is also present. The Iivari system yields a less well-defined conjugate set, but the acute dihedral angle is E-W rather than N-S (Fig. 2b). A vertical N-S set and shallow E- to NE-dipping sets are also developed. The Sotka geometry could be interpreted in terms of the N-S direction of contraction proposed for D3 (Kilpelainen & Rastas 1992), but this would disregard the shallowly dipping set (Fig. 2a) and the pattern of the Iivari system (Fig. 2b). A better interpretation of both patterns is that the fracture systems constitute cooling joints controlled by the shape of the peridotite intrusion. Thus, WNW- to NNW-trending sets parallel the long axis, NE trends form a high angle with this axis, and the shallowly dipping sets approximate the orientation of the local upper or lower contacts. Nevertheless, fracture development under the influence of the local orientation of the regional D3 stress system (or residual system) cannot be discounted. In order to dilate and possibly aid propagation of the fracture system, the migmatite-derived melt must have exerted pressure in excess of the local confining pressure or, for concurrent propagation, a3 plus the tensile strength of the peridotite. Emplacement of the peridotite and the contained dykes pre- or syn-D2 is incompatible with the igneous texture of the peridotite, and the unfoliated dykes and wallrock alteration zones. Emplacement post-D2 and pre-D3 is opposed (but not negated) by lack of evidence of a D3 overprint, and by the largely consistent fracture patterns; this similarly applies to syn-D3 emplacement. Syn- to post-D4 emplacement is rejected because the metamorphic grade was too low

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to produce granitic melt. Thus, emplacement of the peridotite, development of the fracture systems and intrusion of the leucocratic melt, most probably spanned the late- to post-Dj period. Hakli, T.A. & Vormisto, K. 1985. The Vammala nickel deposit. Geol. Surv. Finland, Bull. 333:123-143. Kilpeiainen, T. & Rastas, J. 1992. Vammalan Stormin Ni-malmin ymparistGn metamorfisista ja rakennegeologista tutkimuksista. Institute of Geology and Mineralogy, University of Turku, Finland, Publication No. 30:1-18. Marshall, B. & Mancini, F. 1993. Ni, Cu and major element remobilization, Vammala Mine, Finland. In: Current Research in Geology Applied to Ore Deposits. Univ. of Granada, Spain, pp. 163-166. Papunen, H. & Vorma, A. 1985. Nickel deposits in Finland, a review. Geol. Surv. Finland, Bull. 333: 123-143. Peltonen, P. 1990. Metamorphic olivine in picritic metavolcanics from southern Finland. Geol. Soc. Finland, Bull. 362: 99-114. Peltonen, P. 1993. Magma-country rock interaction and the genesis of Ni-Cu deposits in the Vammala Ni-belt, SW Finland. Terra abstracts, Abstract supplement 3, Terra Nova 5: 42. Fig. 1(a) Surface and selected level plans of the Stormi ultramafite, showing the Sotka and Iivari orebodies. UL = upper layer, IL = intermediate layer, LL = lower layer, OB = orebody and GH = gneissic hostrocks. Section lines for Fig. 2b are AA' and CC\ (b) Vertical sections through the Stormi ultramafite and the main orebodies. Legend and section lines as on Fig. 2a. Fig. 2. Equal-area stereoplots of poles to the dyke and alteration assemblages on (a) the 200m level of the Sotka orebody - contours at 1, 3, 5 and 7% per 1% area - n = 573 poles; and (b) the 250, 270 and 300m level of the Iivari orebody - contours at 1,3,4 and 6% per 1% area - n = 531 poles.

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Granite Magma Migration and Pluton Growth: How It Really Works. C.Kt Mawer, Project Generation, MIM Exploration, Brisbane, Australia J.D. Clemens, Department of Geology, The University, Manchester, England G. Stephens, Department of Geology, The University, Manchester, England Voluminous generation of granite magmas, and thus effective crustal differentiation, seems viable only during high-temperature,fluid-absentmetamorphism. Temperatures in excess of 850°C are necessary for generation of significant percentages of melt from fertile source rocks. Production of these melts will be highly non-linear, and will involve positive volume changes. This will lead to melt fracturing, and transport of melt and magma from generation sites through the resultant fractures. The highly water-undersaturated melts/magmas are effective transporters of heat, water and chemical components upwards through the crust. Granite magmas commonly ascend from fertile source regions to upper crustal emplacement sites through tens of kilometres of otherwise unremarkable crust Passage of these granite magmas is rapid and thermally efficient, and voluminous plutons can be grown in geologically short periods of time. Granitic magmas ascend, in general, through propagating fractures as dykes; diapiric transport of such magmas through crustal sections is not viable on thermal or mechanical grounds, and there is an absence of geologic evidence for diapiric transport even in supposed 'type' localities. These facts are geodynamically significant. Granite magmas are trapped in dilatant sites. These sites are either within shear zones of apparently any kinematics (though zones with extensional or strike-slip kinematics, or their combinations, are preferred), or as laccolith-style plutons in strata with roughly horizontal layering, anisotropics and/or discontinuities. It is unclear which of the settings (shear zones, laccolith-promoting) is more common. When trapped, magmas will cool rapidly, exsolving late-crystallization pegmatitic and hydrothermalfluids.These will in turn fracture their way upwards, and to a lesser extent sideways, from the pluton. This has important consequences for patterns, vectors and magnitudes of hydrothermalfluidconvection and heat transfer associated with plutons.

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THE ^Ar/^Ar ISOTOPIC DATING METHOD AND ITS APPLICATION TO THE STUDY OF THE COOLING AND DEFORMATION HISTORY OF OROGENIC TERRANES Ian McDottgall, Research School of Earth Sciences, The Australian National University, Canberra ACT The decay of the naturally occurring radioactive isotope of potassium, ^K, to^Ar provides the basis for the potassium-argon (K-Ar) and ArP Ar methods for dating rocks. These techniques can provide precise eruption ages for rapidly cooled igneous rocks, not subsequently thermally disturbed. For slowly cooled rocks, whether igneous or metamorphic, these methods commonly yield information concerning the time since last cooling below the closure temperature for argon retention. Closure temperatures range from ~500°C for hornblende, through about 350°C for muscovite, ~300°C for biotite, to as low as 150°C-200°C in some cases for alkali feldspar. By combining Ar/ Ar step heating experiments on appropriate minerals with thermobarometric studies, post-metamorphic P-T-t paths of metamorphic terranes can be reconstructed, providing constraints on cooling and uplift histories as well as on tectonic models. The step heating experiments themselves, especially those done on feldspar, can yield detailed thermochronological information, including information related to closure temperatures and diffusion domain size distributions (Lovera et al., 1989). Obviously, geochronological and thermochronological studies of orogenic terranes are most effective when done following appropriate geological mapping and structural investigations. 40

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Reconnaissance Ar/ Ar dating of minerals from the metamorphic basement rocks of the Musgrave Ranges and the Arunta Block, Central Australia, south and north of the Amadeus Basin, respectively, have been undertaken in recent years. Examples will be drawn from these studies to illustrate the usefulness of this approach in elucidating the cooling and deformation history of this major intracratonic orogenic terrane. Cooling to <350°C subsequent to the high grade metamorphism recorded in the basement rocks, occurred by -1000 Ma in both the Musgrave and Arunta Blocks, based upon measurements on hornblende and muscovite. In the Musgrave Ranges, ages of 530 to 550 Ma on muscovite that crystallized during movement on the Woodroffe Thrust suggest that the juxtaposition of the granulite facies terrane structurally above the amphibolite facies terrane in that region took place at this time, possibly correlated with the Petermann Ranges Orogeny (Maboko et al., 1992). Overall cooling of these terranes to <200°C only occurred about 350-400 Ma ago, as indicated by Ar/ Ar age spectra on alkali feldspar. Similarly, alkali feldspar age spectra from basement metamorphic rocks of the Arunta Block show unequivocal evidence for final uplift and cooling to<200°C at about 300320 Ma, associated with the latter stages of the Alice Springs Orogeny (Shaw et al., 1992). That this occurred at the same time as the formation of the Arltunga Nappe Complex, involving both basement gneisses and sediments of the Amadeus Basin, recently has been demonstrated by measurement of ages of 310-330 Ma on phengitic micas that grew during the deformation (Dunlop et al., 1991). Thus, both thermal and deformation histories effectively can be elucidated using these dating techniques. In conjunction with other geochronological and geological information, it is becoming possible to construct increasingly detailed histories of orogenic terranes. 40

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Dunlop, W.J., Teyssier, C, McDougall, I. and Baldwin, S. (1991). Ages of deformation from K/Ar and Ar/ Ar dating of white micas. Geology, 19,1213-1216. Lovera, O.M., Richter, F.M. and Harrison, T.M. (1989). The Ar/ Ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes. 7. Geophys. Res., 94,17,917-17,935. Maboko, M.A.H., McDougall, I., Zeitler, P.K. and Williams, I.S. (1992). Geochronological evidence for -530550 Ma juxtaposition of two Proterozoic metamorphic terranes in the Musgrave Ranges, central Australia. Austr. J. Earth Sciences, 39,457-471. Shaw, R.D., Zeitler, P.K., McDougall, I. and Tingate, P.R. (1992). The Palaeozoic history of an unusual intracratonic thrust belt in central Australia based on Ar/ Ar, K/Ar and fission track dating. J. Geol Soc. London, 149,937-954. 40

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SEISMIC VELOCITY STRUCTURE OF THE NORTHERN AND SOUTHERN MARGINS OF THE AMADEUS BASIN, CENTRAL AUSTRALIA

H.W.S. McQueen. Research School of Earth Sciences, Australian National University, Acton, ACT 0200 Lateral changes in crustal structure and composition are reflected in variations of seismic velocities. Velocity anomaly images are presented for several cross sections across the northern and southern margins of the Amadeus Basin and various tests of the robustness of specific features are applied. The images are constructed from information on the arrival times of teleseismic signals collected on portable seismograph arrays and numerically inverted. The inversion algorithm iteratively redistributes travel time anomalies along incoming raypaths, subject to regularization constraints. In this way, direct estimates of the lithospheric velocity field in the crust and lithosphere through which the signals pass can be obtained at a scale of the order of 5-10km. Based on the strong east-west strike of surface geology and gravity structure in the area, we assume the model structure to be two dimensional, although ray tracing is performed through a fully three dimensional grid. Spurious or ill constrained structures are suppressed using a filter based on the density of ray crossings, near surface structure is preferentially weighted to test the robustness of inference of deep structures. The most prominent feature of the solutions for all the lines is a sharp interface between slow and fast regions, dipping away from the basin and beneath the adjacent basement blocks. The fastest region, on the upper side of the interface, corresponds to a belt of high grade metamorphic rocks where they outcrop at the surface. The interface between slow and fast regions extends to at least 50km depth in all cases, dipping at about 50-60° on the northern lines and 60-80°on the southern line. The results are consistent with thick skinned thrusting beneath each of the profiles. Predicted gravity profiles derived from the velocity sections using empirical velocity-density relations resemble observed gravity in both style and approximate magnitude, providing support for the general features of the models. On the northern lines the dipping interface can be correlated with the Redbank Thrust identified in deep seismic reflection data, but the velocity interface seems to dip more steeply. Several possible reasons for this are discussed. Secondary features on the profiles include a possible knee on the dipping interface on the southern line and a more complex structure on one of the northern lines suggesting that one or more steeper faults cut through the hanging wall of the thrust there.

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DEFORMATION HISTORY OF FLAT-LYING SEDIMENTARY SEQUENCES, SOUTHEASTERN SYDNEY BASIN, NSW. H. Memarian and C.L. Fergusson Department of Geology, Wollongong University, Wollongong, NSW 2522 ABSTRACT In the southeastern part of the Sydney Basin several gentle northwest-trending folds are developed on the eastern limb of the major northerly plunging Camden Syncline. Northwesterly trending syn-depositional normal faults, with net slips typically about 6070 m, are developed subparallel to these folds. Other notable features are numerous NNE-trending normal faults with net slips of less than 9 metres. Fracture mapping of Illawarra Coal Measures, between Coalcliff and Wollongong, shows that most of the joints developed with three preferred orientations, namely N-NNE, NE and NW. All the joints developed originally in extension (mode I), but most of them were reworked and slipped laterally in subsequent events. Cross-cutting joint terminations and other joint relationships indicate that all the joints, faults, and dykes, aligned in one direction, all through the study area, did not necessarily propagate during a limited time or in a single deformational event. Relationships between joints, faults, and folds with Late Permian depositional features and dykes constrain the deformational history of the southeastern Sydney Basin. Some of the northwesterly trending normal faults were active during the deposition of Illawarra Coal Measures. Slip along these faults contributed to the formation of northwesterly trending gentle folds. Jointing was initiated in the succession after lithification enabled brittle deformation. Continuing joint development involving formation of joints in the three major directions. Fracturing occurred during extension which also propagated more NW normal faults, in addition to a set of NNE-trending normal faults and dykes in the area. It is considered that the later part of this extensional phase may have been related to rifting predating opening of the Tasman Sea and subsequent extension into the Early Tertiary. The next deformational events were compressional, causing recracking, enlargement, and lateral slips of previous joints, in at least four different orientations. The mostly horizontal compression also reworked the previous faults and dykes. A NNE-oriented c l9 was responsible for the dextral and sinistral movements of NNE and NE joints respectively. This was presumably the largest compressional deviatoric stress experienced by the rocks of this area. The effect of another compressional stress, acting in an E-W direction, also produced lateral movements along pre-existing fractures. Most of the existing cross joints developed in a younger phase during unloading and other more recent phenomena.

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TIME RELATIONS BETWEEN WIDE SPREAD DIAPIRISM AND THE FORMATION OF DILATIONAL BARITE VEINS IN THE BUNKERS GRABEN, CENTRAL FLINDERS RANGES, SOUTH AUSTRALIA D.P.J.Mendis. P.R James, and R.A.Both, Department of Geology & Geophysics, University of Adelaide, SA 5005, Australia. The Bunkers graben, located in the Central Flinders Ranges of South Australia is rich in hydrothermal, dilational vein barite deposits cutting across Mid-Marinoan to Cambrian sequences (Fig. 1). The largest barite mine currently operating in South Australia is situated within the graben, in Adelaidean Brachina Formation. A detailed structural study of the vein deposits in the Mine area (Fig. 2), as well as a general study of major veins in the Central and Northern Flinders Ranges (Fig. 3) has been carried out, showing the veins to be complexly and multiply generated during deformation which included tectonic shortening. Minor structures in the Mine area were also studied, and the following sequential structural history for barite veins has been constructed as a working model. (1) Formation of 1A and ID type, NE-SW trending veins, vertical or steep dipping to NW or SE, along tensional fractures due to a probable N-S dextral shear couple (Fig. 4). (2) Continued shear resulted in a riedel and anti-riedel conjugate shear fracture system which initiated the major Bunkers graben structure including minor grabens within it (3) Continued N-S shear couple on the graben, resulted in moving the graben-fill downwards, as well as towards the SW along decollement(s) where the Oraparinna diapir occurs. This resulted in a series of NE-SW propagation structures such as bedding-slip faults of the host and fault-bend folds with steeper leading (SW) dips than gentle trailing (NE) dips. The axial planes of fault-bend folds also led to some limited dilational fracturing and formed Longitudinal Lodes (Fig.2). Further it formed groove lineations along the boundary of 1A and ID Type barite vein systems. (4) NE-SW subhorizontal movement of the graben led to a lateral space problem. This was accommodated by a series of NW-SE propagating shortening structures including gentle folds, fault propagation folds and crenulation folds, increasingly concentrated closer to thicker competent barite veins. At this time, faulting occurred along an array of shallow conjugate reverse-thrusts, along significant number of northerly dipping bedding planes forming minor Flat Lodes. Once the system was locked, the second faulting system appeared at a shallow conjugate angle to bedding, leading to dilational fractures (Link Lodes), which further propagated towards prominent kink planes. Diapirs in the Central Flinders Ranges occur in association with anticlines and some major faults. They further seem to be associated with NW-SE trending Di folds which were refolded by NE-SW trending D2 folds, probably due to a N-S shear couple which was responsible for graben formations. Figure Captions Fig. 1: Central Flinders Ranges including the major fold axial traces (Di), diapirs, grabens and major fault patterns. The inset shows the barite lode nomenclature used in the Oraparinna Barite Mine. Fig. 2: Equal area stereonet showing the poles to barite vein types in the Oraparinna Barite Mine. ID, Link, Flat and 1A Lode types were formed due to NW->SE shortening while Longitudinal Lode type was formed due to NE->SW shortening. Fig. 3: Equal area stereonet showing poles to all major barite veins assessed to have >2000

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tons resource within Copley and Parachilna 1:250,000 sheets covering the Central and Northern Flinders Ranges. Major veins trend NE-SW and are nearly vertical. Fig. 4: Geometry of dilational tension fractures (T), riedel (R) and anti-riedel (R') conjugate shear fractures in a shear zone. ,, ^

DYNAMIC MICROFRACTURING IN SUB-RIGID FELDSPAR DURING LATE STAGE MYLONITIZATION IN THE KASHIO SHEAR ZONE, JAPAN Katsuyoshi Michibavashi Department of Geology, James Cook University, Townsville, QLD. 4811 Australia A statistical analyis of fractured feldspar grain shapes and sizes in a granitoid mylonite within the Kashio Shear Zone of the Ryoke metamorphic belt, central Honshu, Japan revealed that dynamic microfracturing occurred in a systematic manner during the late stage of mylonitization. Porphyroclastic feldspar grains have a strong shape-preferred orientation parallel to the maximum elongation directions and approximately 16% of these grains contain microfractures that are transgranular into the surrounding dynamically recrystallized matrix. The development of microfracturing is highly dependent on the grain size. A critical grain size, below which few microfractures develop, occurs at around 700/xm. The aspect ratio, between one and two, is relatively stable independent of the grain size, and plays a specific role in microfracturing. Microstructurally, these fractures are sub-tensile in type and form preferentially along the feldspar cleavages. They are statistically dominantly located near the center of each grain, suggesting that they developed through afibre-loadingmechanism even though they were not truly rigid at these conditions of mylonitization. Four types of microfracture systems formed during mylonitization; plastic, sub-plastic, sub-rigid and rigid microfractures. Each fracture type can have formed in a similar manner to the fibre loading mechanism, but different textures may be developed before and after microfracturing, dependent on the internal ductility associated with the elasticity of the feldspar. The proportion of broken grains with respect to aspect ratio was used to measure a relative differential stress for the late stage of mylonitization. This, combined with the estimated applied stress from a grain size paleopiezometer, enabled the mean fracture strength of feldspar to be calculated at greater than two times the applied stress, depending on the ratio of elasticity between feldspar and matrix quartz. If the applied stress was 100 MPa and the elastic ratio was as small as 1.1, the mean fracture strength of feldspar was 262 MPa.

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STRUCTURE OF THE EARLY PALAEOZOIC WAGONGA BEDS, NORTHEAST OF BATEMANS BAY, NEW SOUTH WALES M. Mohajjel and C.L. Fergusson Department of Geology, University of Wollongong, Wollongong NSW 2522, Australia The ?Late Cambrian-?Early Ordovician Wagonga beds occur along the South Coast of New South Wales and are the oldest unit exposed in the eastern Lachlan Fold Belt. In the well exposed coastal section between the southern end of Myrtle Beach and Oaky Beach (about 4 km) they consist of three units: black shales, bedded cherts and mudstone-dominant melange. The black shale unit is most common and consists of thin-bedded shale with local siltstone and sandstone turbidite layers. One mafic volcaniclastic sandstone bed is up to 1 m thick and forms a local marker horizon. The bedded chert consists of alternating thin layers of black pelite and chert. The melange occurs in a fault-bounded slice about 50 m across and is similar to melanges farther south in the Batemans Bay district. Three episodes of deformation have been mapped. Di formed local easterly trending upright folds that are close to tight. Fx have axial planes dipping 50-80° to the north and plunge 50° to the northwest. They have no associated axial plane cleavage. Fr are overprinted by the more intensely developed D2 deformation which is dominated by northerly trending F2 with steeply to moderately pitching fold axes within S2. S2 commonly dips at about 50° to the west. F2 are tight to isoclinal and are commonly strongly appressed. They are asymmetrical with attenuated and broken overturned limbs which indicates tectonic transport to the east. An axial planar S2 slaty cleavage is well developed throughout the area. Layering is dismembered by abundant contraction faults that occur along S2 and are associated with D2. All contacts between the rock units are faulted. The map-scale structure consists of an east-vergent overturned F2 antiform. Local east-west steeply dipping dextral kink bands and post-D2 strike-slip faults indicate a third deformation (D3). D2 is considered part of the regional Late Silurian-Early Devonian deformation that formed the strong north-south structural grain of the South Coast of New South Wales. The intensity of deformation, that farther south has formed extensive melange, and its strong easterly vergence is consistent with westerly-directed subduction occurring to the east of the area, perhaps in a southern continuation of the New England Fold Belt.

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THE SHACKS MYLONITE ZONE, NORTHERN NEW ENGLAND FOLD BELT Vincent J. Morandr Department of Geology, Ballarat University College, P.O. Box 663, Ballarat Vic. 3353. The Shacks Mylonite Zone is part of the Stanage Fault Zone, a NNE trending fault in the central Queensland coastal region. This fault zone juxtaposes the Yarrol terrane (arc and fore-arc basin) against the Marlborough terrane (ophiolite) and the Shoalwater terrane (accretionary prism). Adjacent to the Stanage Fault Zone structural trends in the Yarrol terrane are N to NE, in contrast to the dominant NW trends elsewhere in the northern New England Fold Belt. The Gogango Overfolded Zone, a NW striking fold-thrust belt, curves around the edge of the Marlborough terrane to become parallel to the Stanage Fault Zone near Marlborough. Between Shoalwater Bay and Broad Sound the Stanage Fault Zone outcrops as a two kilometre wide mylonite zone - the Shacks Mylonite Zone -which juxtaposes high grade metamorphosed Shoalwater terrane (the Broome Head Metamorphics) against low grade Yarrol terrane rocks. It is cut by later faults, and to the south it is cut by post tectonic granite and diorite, while to the north it is covered by sea. On the Shoalwater Bay coast the Broome Head Metamorphics consist of garnet-sillimanite gneiss, amphibolite and quartzite. These rocks are polydeformed and were metamorphosed at about 700°C and about 5 kbar pressure. They are faulted against Yarrol terrane rocks consisting of greenschist facies volcaniclastics and marble of the Early Devonian Mount Holly Formation which have one strong cleavage. Here the Shacks Mylonite Zone strikes NE and dips moderately SE. Rocks derived from the Broome Head Metamorphics (mylonitic quartz-mica-garnet schist and amphibolite) occur in the SE part of the mylonite zone, and rocks derived from the Mount Holly Formation (strongly cleaved low grade marble and volcaniclastics) occur on the NW. Metamorphic grade decreases across the shear zone from amphibolite to greenschist facies. Inland from Shoalwater Bay the Shacks Mylonite Zone is cut by a N-striking fault with apparent dextral displacement of about 5 km. On the W side of this late fault a fragment of Shoalwater terrane consisting of low grade quartz-rich sandstones occurs on the NW side of the Shacks Mylonite Zone, and the SE side is a hornblende-bearing granite of unknown age and affinities. Here rock types in the shear zone are foliated and mylonitic granite on the SE side and quartzgraphite mylonite on the NW side. Most rocks in the mylonite zone have one strong foliation S m , containing a well developed lineation L m . S m strikes N to NE and dips moderately east to SE, while L m plunges shallowly to the E and NE. Some mylonites have an S-C fabric, with the intersection of S and C perpendicular to L m , thus supporting

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the interpretation of this lineation as the extension direction. Shear sense indicators such as S-C fabrics, oblique quartz fabrics and mica fish, are not common, but all indicate that the Shacks Mylonite Zone is an oblique thrust with a large component of dextral strike-slip. The transport direction was to the WSW. Late N trending open folds with steep axial planes deform S^ and are correlated with the latest folds in the Broome Head Metamorphics. These may be responsible for bending the western part of the mylonite zone into a N-S strike. In some outcrops the axial plane is marked by a crenulation cleavage. Fault movement occurred during metamorphism, as indicated by amphibolite facies assemblages in mylonites on the high grade side and the gradation into greenschist facies mylonites on the low grade side of the shear zone. This metamorphism is Late Permian age, coincident with the Hunter-Bowen Orogeny. The displacement along the Shacks Mylonite Zone may be as much as 30 km, bringing mid-crustal metamorphics from the vicinity of Townshend Island southwestwards to over-ride low grade rocks of the Yarrol Terrane. The dextral component of movement is consistent with the Stanage Fault Zone being a dominantly strike-slip fault that has ruptured the Marlborough terrane into two displaced portions: one on the mainland at Marlborough and the other outcropping on the Percy Isles to the NE. This fault may be a large transfer fault linking the NW striking thrusts of the Gogango Overfolded Zone with a similar NW trending thrust zone that is concealed off the coast north of Broad Sound. This region of the New England Fold Belt is dominated by major thrust structures, including the Gogango Overfolded Zone, the Marlborough terrane and the Shacks Mylonite Zone, all with transport directions to the SW and all active during the Hunter-Bowen Orogeny. A possible explanation for these structures is that a rigid indentor (now hidden under the sediments of the continental shelf) impinged on the fold belt in this area during collisional orogenesis, causing major SW thrusting and tearing the fabric of the fold belt along the Stanage Fault Zone.

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STRUCTURAL CONTROLS ON FLUID MOVEMENT AND ORE DEPOSITION AT PORGERA GOLD MINE, PAPUA NEW GUINEA S.M.Munroe. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. The world-class Porgera Gold Mine in the highlands of Papua New Guinea has produced over 100 tonnes of gold since opening in late 1990. Mineralisation is associated with Late Miocene (6 Ma) stocks and dykes of the Porgera Intrusive Complex which were emplaced into sediments at shallow crustal levels (2-4 km). The sediments include Late Cretaceous, carbonaceous black shales and Cainozoic limestones, deposited in a shelf environment at the northern edge of the Australian Craton. Folding and thrusting occurred prior to mineralisation at Porgera. A period of rapid uplift to form the New Guinea Highlands occurred during or shortly after emplacement and mineralisation of the Porgera Intrusive Complex. Early auriferous pyrite+galena+sphalerite veins (A-type) are up to 50 cm in width and up to 100 m in length. These A-type veins cut all rock types including the stocks and dykes of the Porgera Intrusive Complex. The A-type veins are compositionally banded and undeformed. Later quartz+roscoelite+pyrite+native gold (D-type) mineralisation is related to the E-W trending Roamane Fault Zone which cuts a number of stocks and dykes in the southern part of the intrusive complex. A-veins are cut by D-type mineralisation and the Roamane Fault Zone. Fracture permeability within the Roamane Fault Zone has acted as the principal pathway for the D-type fluids. The Roamane Fault Zone consists of a 2 m wide principal displacement zone and numerous subsidiary faults which splay into the intrusive stocks and carbonaceous black sediment wallrock. Subsidiary faults have developed in the wall rock up to 50 m away from the principal displacement zone. Movement on the Roamane Fault was initially normal and later evolved to dextral strike-slip, indicating a change in the tectonic environment from extensional to transcurrent movement Repeated seismic activity in the Roamane Fault Zone has resulted in implosion brecciation which was accompanied by significant fracture permeability during the development of the subsidiary faults. Brittle failure of the principal fault has resulted in cataclasis in the immediate footwall. D-type mineralisation precipitated from hydrothermal fluids into the matrix of the breccias and cataclasites. The A-type veins dip steeply NW and steeply SE suggesting an horizontal NW-SE minimum principal stress (<73). This direction is very different from that inferred for the Roamane Fault Zone (03 horizontal N-S for normal movement and horizontal NE-SW for dextral strike slip movement). It is suggested that the Roamane Fault Zone was not active until the latest stages of A-type vein deposition. Initial D-type mineralisation was deposited parallel and adjacent to the A-type veins but as the Roamane Fault Zone evolved the circluation of D-type fluids was controlled by fracture permeability.

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THE SIGNIFICANCE OF THE ALPINE FAULT FOR CRUSTAL DEFORMATION DURING OBLIQUE COLLISION Richard J Norris. Dept of Geology, University of Otago, PO Box 56, Dunedin, New Zealand The Alpine Fault runs down the west coast of the South Island of New Zealand and forms the major structural manifestation of this section of the Australia - Pacific plate boundary (fig. 1). The role played by the Alpine Fault in accommodating the total interplate motion of 39 mm/a (Nouvel-1 solution) has many ramifications for continental deformation. Although forming a remarkably straight trace on satellite photographs, in the field the fault consists of short, more northerly trending segments showing oblique thrust displacement, linked by more easterly trending segments displaying dominantly right lateral slip. The slip direction on both segments is similar and parallel to their line of intersection, which is also subparallel to the overall plate vector (070°-080°). Typically, a zone of mylonite up to 1 km thick is developed from the Alpine Schists in the hanging wall of thrust segments and is bordered by cataclasites up to 50 m thick (fig 2). The mylonite zone is always substantially wider than the zone of cataclasis supporting the view that the fault zone widens with depth. Strain in the mylonite increases towards the fault, with ultramylonite commonly adjacent to the cataclasite. If the strain distribution within the mylonite was originally broadly symmetrical, an equal width of mylonite may remain unexposed at depth on the footwall. The strain within the mylonites is very high - estimates from stretched pegmatite veins, separated from their undeformed counterparts outside the mylonite zone by 80 km, suggest shear strains of at least 100. Mylonite lineations show shear subparallel to the plate velocity vector. The mylonites were formed under conditions of at least the garnet zone and have been exhumed during uplift of the hangingwall. The metamorphism in the Alpine schists decreases to the east from amphibolite facies adjacent to the mylonite to piehnite-pumpellyite facies on the Main Divide, indicating that most of the uplift and exhumation is concentrated close to the fault. Emplacement of the hangingwall over late Quaternary gravels indicates minimum displacement rates on the fault itself of approximately 20-30 mm/a, half to threequarters of the total plate motion. Uplift rates of 5-8 mm/a are also consistent with estimated convergence rates on the fault. Offset of Fiordland-derived Pliocene conglomerates in south Westland indicate minimum average horizontal slip rates on the Alpine Fault over the last 3-4 Ma of 27-34 mm/yr, close to the Nouvel-1 rate. The following conclusions are drawn: 1) The Alpine Fault has accommodated the bulk of the interplate displacement over the last 5 Ma, during which the Southern Alps have developed. 2) The fault zone widens with depth but remains a strain softened zone of localised shear to depths of at least 20 km. 3) The fault accommodates the oblique shear displacement of the two plates without largescale partitioning into separate convergent and strike-slip components in discrete parallel zones. 4) The fault dips moderately eastwards and flattens at depth with detachment of the upper 2025 km of crust as an upthrust wedge. 5) The localisation of deformation suggests a crustal deformation model incorporating either the Australian plate as a rigid indentor, or else a strong mechanical linkage between the crust and subducting mantle lithosphere beneath. 6) These characteristics of the plate boundary change along strike as the crust on the Pacific plate to the north and the Australian plate to the south becomes oceanic in nature.

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Fig. 1: map of South Island showing Alpine Fault

imbricated mylonites/ duplex structure iocai normal faults

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SUPERPOSED FOLDING IN THE CRYSTAL CREEK BLOCK, MOUNT ISA INLIER, NW QUEENSLAND: PARADIGM OR PARADOX Mark OfDea and Gordon Lister Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Victoria, 3168 Australia The map pattern of the Leichhardt River Fault Trough has perplexed geologists for many years, and structural syntheses of the region have been plagued with inconsistencies. Any geometrical interpretation must account for the following structural features (a) the origin of the complex distribution of fault blocks; (b) the cause of fault repetition of the central anticline stratigraphy by EW trending faults; (c) the origin of EW synclines associated with these EW faults; and finally (e) the reason that upright D2 synclines with sub-horizontal fold axes are found on either side of a regional antiform in which bedding plunges consistently to the north, with moderate to steep dips. These features led Bell (1983) inexorably towards a hypothesis which is difficult to circumvent in the absence of detailed mapping of the structures in question. The Bell (1983,1992) model was all encapsulating, self-consistent and synthesised remarkably well the geological database of the Leichhardt River Fault Trough. However it lacked detailed documentation and the support of field mapping. Furthermore, at the core of the hypothesis was the assumption that orientation alone could be used as a means of correlating deformational events across tens of kilometres. Detailed work in several localities has now attested to the wide variability in orientation achieved as the result of the impact of later deformation on earlier formed structures. Nevertheless, regardless of the significance of the (mythical ?) giant roof thrust, there are elements of the Bell hypothesis that have been supported by ongoing research (e.g., the existence of early formed thrusts). The difficulty with the Leichhardt River Fault Trough (LRFT) is that along with much of the geology of the Mount Isa terrane, it is difficult to ascribe its features to any one simple model. For example, in the LRFT there are features that may have been caused by an early history of rifting. Similarly, there many are features that can be explained by early thrusting. Certainly the geology does not fit well into the family of structures that are commonly described in welldocumented thrust terrains. Geometries that are the result of shortening of a previously extended crust are liable to be variable and complex due to the influence of a pre-existing extensional architecture. Equally so many features of Mount Isa geology can be explained as the result of large-scale wrenching (e.g., Lister 1990). However the Mount Isa area also contains regions of complex superimposed folding, and it contains fault bounded blocks of rock whose structural trends change abruptly from one domain to another. These features cannot be explained by wrenching alone (as we currently understand it!). The sequential fold history of the pelites, siltstones and quartzites of the Crystal Creek block is quite distinct from that of its surroundings (which are defined largely by competent fault bounded blocks of Haslingden Group continental tholeites, volcanogenic sediments, and thick arenites). Superimposed folding has resulted in a large scale interference pattern wherein north-trending F2 folds overprint east-west trending Fx folds. Such structures do not occur in the adjacent mechanically stronger lithologies. This disparity has been a major stumbling block for geologists interesting in correlating structures within the LRFT.

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The difficulty in solving this problem can be attributed to the fact that the following questions have not been answered satisfactorily. Do the Mount Isa Group sediments within the Crystal Creek block belong to a stylistically different structural level and thus offer a rare glimpse into a now widely eroded crustal level? If so, can the history of the Crystal Creek block be used as a paradigm for the entire LRFT? Conversely, did the rocks of the Crystal Creek block evolve under a unique set of mechanical and geometrical boundary conditions imposed by the relatively stiff rocks which presently surround and presumably underlie it? Recent mapping indicates that rocks in the Crystal Creek block were involved in at least two deformation episodes. The earliest deformation produced a locally developed S slaty cleavage, which is axial planar to EW-trending, tight F folds. This cleavage intensifies to the south, towards the Mount Robert Fault, where the frequency and tightness of mesoscopic asymmetric Fj folds increases, indicating a strain gradient and thus a causal link between movement on the Mount Robert Fault and the development of S cleavage. In the northern half of the Crystal Creek block, away from the Mount Robert Fault, F folds have more open profiles and do not contain an S cleavage indicating lower strain. Shear sense indicators, examined along a well-exposed section of the Mount Robert Fault, contain a steep to moderately plunging stretching lineation and show a top to the northwest sense of shear. Also, within the central part of the Crystal Creek block, stratigraphy is repeated by a now folded, steeply north-dipping south-directed D thrust. Indeed, thrusting appears to have played a significant role in the early history of the Crystal Creek block. Roughly NS-trending F folds overprint F folds and T> faults. S is very well developed, particularly within F axial zones. Unlike Sv however, it is not restricted to the pelitic units but occurs in all lithologies including massive dolomite and dolomitic siltstone. A 2km wide corridor consisting of intensely developed S cleavage, winds down the eastern edge of the Crystal Creek block defining a D high strain zone. To the south, adjacent to the Lake Julius Fault, this cleavage is folded by and therefore pre-dates movement on the Lake Julius Fault. The Lake Julius Fault and the Mount Robert Fault form part of the southern bounding structure of the Crystal Creek block. This bounding structure, however, consists of several generations of fault segments which can be distinguished by their spatial and temporal relationship to the Sj and S cleavage. Some fault segments are overprinted by Sv Others are synchronous with Sv Other fault segments post-date S . While this array of faults appears related, based on their orientations, the actual kinematics may be significantly more complicated due to a multiple movement history. The Crystal Creek block has recorded a complicated deformational history with multiple generations of faulting and folding. Thrusting appears to have been operative during Dv and may in fact be largely responsible for the present day stratigraphic pattern of the LRFT. Taken collectively, however, the preserved structures of the Crystal Creek block do notfitneatly into any "type" terrane. Gaining a clear understanding of the evolution of the Crystal Creek block may require that one consider more closely the role that rheological contrasts have played in the development of various deformational styles and generations of structures. References cited Bell, T. H. 1983. Thrusting and duplex formation at Mount Isa, Queensland, Australia. Nature 304,493 - 497. Bell, T.H. 1992. The role of thrusting in the structural development of the Mount Isa mine and its relevance to exploration in the surrounding region. Econ. Geol. 86, 1602-1625. Lister, G.S., Origin of the Lake Julius "pop-out" structure. Abstract volume, Mount Isa conference, V.I.E.P.S. Monash, 1990. x

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EXTENSIONAL TECTONICS OF THE SAMBAGAWA BELT, CENTRAL SHIKOKU, JAPAN Kazuaki OKAMOTO

Department of Geological Sciences, College of Liberal Arts, Okayama University 2-1-1, Tsvshima-Naka, Okayama 700, Japan Metamorphiczonationof the high P/T metamorphic, Sambagawa belt has been done extensively in Central Shikoku, based on phaserule constrained mineral parageneses of pelitic schists. In increasing order of grade these zones are chlorite, garnet, albitebiotite and oligoclase-biotite (e.g. Banno& Sakai, 1989). Higher grade (garnet and biotite) zones include the peridotites and the metagabbros which belonged to the eclogite and granulite fades. The geological structure of the Sambagawa belt is characterized by the dome structure of the lower grade (chlorite) zone and overlying the higher grade (garnet and biotite) zones on the north side of the dome (Fig. 1). In the south, the Jurassic accretionary complex, Chichibu and Mikabu belts, overlie the lower grade zone (Kawato et al., 1991). Lithologies are folded in two phases (D1=D2 fold and D3 fold) (Haraet al., 1977; Faure, 1985; Wallis, 1990). Duringthe D1=D2 phase, overthrusting of the higher grade (high P, T) zones on the lower grade (low P,T) zone, associated with S-vergent folds caused the inverted thermal gradient in the lower grade zone. Duringthe D3 phase, doming caused the N-ward transpressional zone on the north side of the dome (Fig. 2). The transition from upright folds in the dome of the lower grade zone to N-ward recumbent folds in the higher grade zones on the north side of the dome implies the upward increase of the shear strain gradient (Fig. 3) (c.f. Sanderson, 1982). Inverted thermal structure might control the rheology (c.f. Collins & Vernon, 1992). The sense of shear inferred from S-C mylonites and N-ward folds suggest that the MTL, known as the typical strike slip fault, could be the gently N-dipping normal fault located structurally above the Sambagawa belt. Geophysical studies recently have found the MTL is a gently dipping fault (Ito et al., 1993). References BannoS. & Sakai S. 1989. Geol.Soc. Spec. Pub. No.43, 519-532. CollinsW.J. & Vernon R.H. 1993. Abstracts IGC, KyotQ 2,449. Faure M. 1985. Jour.Struct. Geol.l, 175-186. 120

South

a

b

Fig.

1.

zonation

Cross

sections

(b)

Central

in

of

geological

Shikoku.

Open

map

(a)

box

indicates

and

metamorphic loction

121

Hara I. et al.1977. The Sambagawa Belt. Hiroshima Univ. Press. Ito et al. 1993. Abstracts 100th ann. meet. Geol.Soc. Japan 732. Kawato T. et al. 1991. Jour. Geol.Soc. Japan97, 959-979. Sanderson D.J.1982. Tectpophysics,&8, 201-233.

N o i - -fc h

of

Fig. 2.

Fig. 2. Cross section shows the N-ward recumbent folds develop in the higher

(biotite and garnet) grade zones on the

side of the dome.

north

Ito et al. (1993) revealed the Median Tectonic

Line is the gently N-dipping fault.

F i g . 3. P a t t e r n s of s t r e t c h t r a j e c t o r i e s f o r nappe p i l e s of t h e Sambagawa b e l t , w i t h l a y e r p a r a l l e l s h o r t e n i n g ( s e e S a n d e r s o n , 1982) show N-ward and upward s h e a r s t r a i n g r a d i e n t i n t h e n o r t h s i d e of t h e dome.

TRANSITION FROM MAGMA EMPLACEMENT TO SOLID STATE DEFORMATION: MICROSTRUCTURES AND MAGNETIC FABRICS Renee Panozzo Heilbronner and Alfons Berger, Geological Institute, Bernoullistr. 32, CH-4056 Basel, Switzerland In many intrusives it is quite difficult to identify a magmatic lineation. In the Bergell, the problem is compounded by the fact that the magmatic fabric is weak, and overprinted by a second weak fabric which is caused by solid state deformation. The solid state deformation appears to be coeval with the magmatic intrusion. However, depending on the structural site, the onset may be early or late during the emplacement. As a consequence, the proportions of the respective contributions of magmatic and solid state deformation to the find fabric vary from place to place. In an intruding and cooling magma, the active deformation mechanisms range from fluid flow to recrystallization. At depth, the percentage of solid phases is small, and the magma is assumed to deform as a viscous fluid, by deformation mechanisms that leave no microstructural trace. At intermediate levels, hornblende and plagioclase start to crystallize, transforming the fluid magma into a material that may best be described as "crystal mush". The phenocrysts of both phases rotate freely within the flowing magma. As their number increases, they begin to interact. Finally, when the melt percentage has decreased below a few percent, the phenocrysts are interlocked, and a magmatic lineation is formed. From then on, the mechanical behaviour of the magma is that of a weak solid (since it is still hot), and the material deforms by solid state deformation mechanisms only. The aim of the present study is to analyse and correlate the development of the microstructural and magnetic fabric in the context of a cooling and intruding magma. This should help us to understand the physical basis for the correlation between the magnetic susceptibility anisotropy, the shape fabrics and the crystallographic preferred orientations (CPO's) of the phases. The magnetic fabric is described in terms of the AMS (anisotropy of magnetic susceptibility), the shape fabric in terms of the ACF (autocorrelation function), and the CPO's are measured on the universal stage. Five tonalite samples were selected from the Southern margin of the Bergell intrusion. On the basis of field evidence, they represent a traverse from purely magmatic fabrics to relatively strong solid state overprint. The corresponding sequence of specimens is the following: 2C151 -> 3A4 -> 2A178 -> 3A5 -> 1A6 The least overprinted tonalites are composed of 10% quartz, 50% feldspar, 20% hornblende and 17% biotite. With increasing solid state deformation, the biotite content increases at the expense of feldspar and hornblende (see figure 1). The main (para-)magnetic minerals are hornblende and biotite, with hornblende having a prolate, biotite an oblate susceptibility ellipsoid. Ferromagnetic minerals such as magnetite (which has a high bulk susceptibility, Km) are generally absent. Ilmenite/hematite and epidote do not contribute significantly on account of their low percentages and low Km - values. FIG. 1

Mineral composition of Bergell tonalites

FIG. 2 Average grain size from ACF contours

60

O

<

<

<

Sample

On three orthogonal polished surfaces of each specimen, 31-31 mm2 areas were digitized and

122

the ACFs were calculated. The macroscopic z-direction is perpendicular to the foliation plane, the x-direction parallel and and the y-direction normal to the macroscopic lineation. From the cross-sectional areas of the ACFs, the grainsize was determined (figure 2). Grain size increases from 2C151 to 3A4, and then decreases again. For each sample, it is largest in the x-y plane, corresponding to a general oblate shape, with the flattening plane parallel to the foliation. The same is indicated by the axial ratios, b/a (figure 3). With exception of the first sample (2C151), they are highest on the x-y plane. On the x-z plane (i.e., parallel to the lineation), b/a increases, first slowly then rapidly, indicating an overall continuously decreasing anisotropy, while on the y-z plane (i.e.normal to the lineation), b/a decreases and increases again, indicating a maximum anisotropy for the intermediate sample (2A178). FIG. 3

Average b/a ratios of ACF contours

FIG. 4

Anisotropy of Magnetic Susceptibility

For the derivation of the AMS, small cylinders (of 25 mm diameter and 10 cm3 volume) were cored and the samples were analyzed using the Kappabridge KLY-2 equipment (of Geofyzika, Brno), located at the Laboratoire de Petrophysique et Tectonique of the Universite PaulSabatier in Toulouse, France. The results are shown in figure 4. The anisotropy intensity, P, is smallest for the first sample, and then remains approximately constant (at values corresponding to 20 to 23 %). The foliation parameter, F, increases steadily. The lineation parameter, L, and the discriminator for prolate/oblate fabrics, T, both display a noticeable excursion for the sample (3A4). There, L reaches a maximum of 10%, T a minimum of 0.2, indicating a strongly linear and prolate fabric. This behaviour cannot be explained in terms of increasing strain. Rather, the fabrics have to interpreted using microstructural and textural information and numerical modelling of the deformation mechanisms. For example: In sample (2C151), the hornblende content is high, the grains are elongated and oriented parallel to the lineation. This would suggest that the magnetic lineation, L, should be highest in this sample, which it is not (see figure 4). Why ? The biotite platelets are aligned on magmatic shear zones. These are oriented parallel to a zone axis, which lies in the plane of foliation, normal to the lineation direction. A zonal arrangement of oblate magnetic ellipsoids yields a prolate signal, and since this is normal to the lineation of the hornblendes, it tends to reduce L. In sample (3A4), the hornblende and biotite contents are approximately the same as in the previous case. However, since the magmatic shear bands are absent, the only contribution to the AMS is the linear signal of the hornblende. Lineation, L, assumes the highest, shape factor, T, the lowest value of the whole series. In sample (2A1787), the biotite content is increased at the expense of the hornblende. The biotite platelets begin to align themselves in the foliation plane. The foliation, F, increases further, the lineation, L decreases again, and the shape factor, T, increases again. This trend is continued in the samples (3A5) and (1A6). etc. It is obvious that the bulk fabrics (microstructures and magnetic fabrics) depend very sensitively on the percentage and the crystallographic preferred orientation of the main magnetic minerals. The rather complicated development of the AMS and the ACF of the Bergell tonalite can be explained quite simply in terms of the volumetric and microstructural changes of the hornblende and the biotite. The reader is warned against using AMS or ACF measurements for strain analysis...

123

TECTONIC DEVELOPMENT AND MINERALISATION OF THE SOUTHERN GAWLER CRATON, SOUTH AUSTRALIA A. John Parker Geosurveys Australia PtyLtd, 18 HighSeld Ave, St Georges SA 5064 The Gawler Craton is a major Archaean to Mesoproterozoic tectonic province comprising a broad spectrum of structural domains and stratigraphic successions ranging from highly deformed and metamorphosed gneisses to relatively undeformed acid volcanics and clastic sediments. This paper will concentrate on the tectonic development of the southern Gawler Craton between 1950 and 1700 Ma and relate that development to the spatial distribution of rock units and mineralisation prior to formation of major shear zones ca. 1720-1710 Ma. Within the Cleve Subdomain, there are two principal rock groups: the metasedimentary Hutchison Group which crops out extensively inland on east-central Eyre Peninsula, and the granitic Lincoln Complex which crops out along the southeastern coastline and immediately west of the Middleback Range. The Hutchison Group comprises a basal quartzite unit overlain by mixed chemical and semipelitic sediments which host several iron ore and base-metal mines and prospects. Carbonates consistently containing anomalous Zn-Pb-Ag-Cu are prominent in an elongate but disjointed sinusoidal belt from Menninnie Dam in the north to Sleaford Bay in the south whereas banded iron formations are more prominent to the east. At the top of the Hutchison Group is a semipelitic schist unit which locally contains acid volcanics dated at ca. 1845 Ma. The volcanics are temporally associated with deformed I-type granitoids near Port Lincoln, the Lincoln Complex, and together they constitute the end of a major stratotectonic cycle comprising early extension/rifting (represented by coarse clastic sediments) followed by sagging (mixed chemical and semipelitic sediments) and intrusion of complex bimodal granitoids above a mantle underplate. On southern Eyre Peninsula, the Hutchison Group and Lincoln Complex are separated by a major mylonite zone(s). At Port Neill, strain markers within ultramylonite infer dextral strike-slip shear but sheaf folds in protomylonitic augen and granite gneiss to the east infer verticle movement. Furthermore, although the Hutchison Group was metamorphosed to upper amphibolite facies, it was not likely buried to more than 6-7km whereas the Lincoln Complex was emplaced at depths greater than 10km. Therefore, there has been uplift of the eastern domain relative to the western domain. Mylonites also separate Hutchison GroupfromLincoln Complex on northeastern Eyre Peninsula but the granites were intruded to slightly higher crustal levels and enclose large rafts of metasediment. Based on the recognition of at least three phases of deformation culminating in formation ca. 17201710 Ma of ultramylonites, it is concluded that southern Eyre Peninsula was subjected to E-W compression over a long period of time resulting initially in folding and local thrusting with eastover-west vergence, followed by formation of a major NE-SW trending dextral shear zone to produce the sinusoidal fold belt as it is now preserved. When the effects of that shearing event are removed, it is possible to restore the Lincoln Complex gneiss domains of southern and northeastern Eyre Peninsula into juxtaposition. It then becomes clear that the carbonate sequences of Menninnie Dam and Sleaford Bay were originally spatially much closer to and likely NNW along strike from each other. This has important ramifications for mineral exploration on Eyre Peninsula.

124

SHEAR SENSE INDICATORS IN ROCKS; CONTROVERSIES AND NEW POSSIBILITIES C. W. Passchier. Department of Geology, University of Mainz, 55099 Mainz, Germany The fabric of deformed rocks is an important direct source of information for tectonic reconstructions. Shear zones provide much information on large scale tectonics, mainly through determination of tectonic transport direction, and sense of shear. A large number of kinematic indicators are now available for this purpose but many have been recognised empirically and their development is incompletely understood. This talk summarises data on available kinematic indicators, and outlines some of the major problems and untapped posibilities. Stretching lineations are usually taken to indicate tectonic transport direction in shear zones. Intersection lineations that are oblique to tectonic transport direction can be indistinguishable from stretching lineations in some cases. Mineral lineations may be a better indicator of tectonic transport direction, especially in high-grade rocks. Shear sense is usually determined on faces parallel to the stretching lineation. However, many mylonites show asymmetric structures on faces normal to the lineation. This may be due to a non-oblique orientation (or even parallelism) of the vorticity vector and the developing stretching lineation. Many shear sense markers rely on the monoclinic shape symmetry of flow inhomogeneities, or flow partitioning. Such kinematic indicators may only be used to determine shear sense in flow on a scale significantly exceeding that of the inhomogeneities; on a smaller scale they may give erroneous results. In some mylonites with strong stretching lineations, especially those formed at high metamorphic grade, there are no asymmetric structures at all that could be used as shear sense markers. This symmetry may be due to a general lack of inhomogeneities that are crucial for the development of macroscopically visible shear sense indicators. Also, symmetric porphyroclasts or 'augen' may have wide mantles of recrystallised material that inhibit the development of asymmetric 'tails'. Finally there are some new possibilities to expand the use of kinematic indicators beyond the determination of shear sense. There is a theoretical possibility to use them to recognise shear zones that did not deform by simple shear, e.g. shortening and extensing shear zones - especially vein sets and shear band cleavage are important potential markers in this respect. Also, some structure such as mantled porphyroclasts and mica fish may be used to distinguish between a Newtonian or non-Newtonian rheology of mylonites.

125

SHEAR CRITERIA IN THE EVOLUTION OF THE WALKER TROUGH, McARTHUR BASIN, NORTHERN TERRITORY. K.A. Plumb, Australian Geological Survey Organisation, GPO Box 378, Canberra 2601. Models for the evolution of the mid-Proterozoic McArthur Basin involve multiple reactivation of major faults, northerly-trending asymmetric rifts, sag basins, and inversion and uplift (Plumb, K.A., Ahmad, M., and Wygralak, A . S . , MID-PROTEROZOIC BASINS OF THE NORTH AUSTRALIAN CRATON, Australas.

Inst. Mm. Metall Monogr. 14, 881-902, 1990). The published stratigraphy of one of these rifts, the Walker Trough in central Arnhem Land, proposed four main units, from bottom to top: Ritarango beds, Fagan Volcanics, Parsons Range Group, and McArthur/Habgood Groups (Fig.l). The Ritarango beds, characterised by relatively intense folding, shearing and cleavage development, and the unconformablyoverlying, moderately-deformed Fagan Volcanics, were both considered to be 'basement'; part of the late Barramundi Orogeny magmatism. These were unconformably overlain by the structurallyconformable Parsons Range and McArthur/Habgood Groups. As part of the new National Geological Mapping Accord project in Arnhem Land, by AGSO and NTGS, the Parsons Range Group, Fagan Volcanics, and Ritarango beds were targeted for outcrop scale observations of the attitudes, styles, and chronologies of movements of fault and shear zones, in the expectation that discrete structural events related to pre-McArthur Basin 'basement', to McArthur Basin extension, and to later inversion could be documented. This paper summarises the structural criteria used and the results obtained from this study.

REFERENCE ZHH ]

Slaty cleavage present Roper-Malay Rd Gps

liil&vil P a r s o n s Range Group u ^ i Fagan V. - Ritarango beds Irt/I & equiv f Mirarrmina Complex,

The first new result is that the Ritarango beds, [ | "McArthur -Habgood Groups" Fagan Volcanics, and Parsons Range Group are conformable; all are, in fact, part of the Figure 1. Structural sketch map, Walker Trough area, eastern Arnhem Land McArthur Basin sequence. Much of the original 'Ritarango beds' comprise imbricately interlayered with Ritarango beds and recessive Fagan Volcanics (stratigraphic nomenclature is being revised). The intense folding and cleavage development is confined to a specific NNEtrending belt, rather than being stratigraphically constrained, and locally affects the Ritarango beds, Fagan Volcanics, a small area of Parsons Range Group, and all of the Habgood Group (Fig. 1). Therefore, most of the structures observed are due to relatively young inversion. | V j 'Nathan Group"

Major faults in the Walker Trough are northerly-trending and steep to vertical in outcrop. Many dip steeply to both east and west. Larger faults separate major domains of differing geology, and so strike-slip displacements can only rarely be determined from offsets at map scale.

126

Mesoscopic criteria for sense of shear include Reidel shear sets, en-echelon quartz tension gashes, quartz fibre orientations, sheared porphyroclasts, and C-S fabrics. Different ductilities, which these features infer, may be correlated with stratigraphy, structural events, and rock type. Major faults have been reactivated several times. Overprint criteria are rarely preserved but, when present, are consistent. At least three principal events may be identified (Fig. 2).

1. NNE-trending dextral faulting.

Faults strike 015°±20°. Brittle to semi-brittle. Shear sense indicated by Reidel (Rl) fractures, and small R1/R2 shear sets. Most apparent within Parsons Range Group, where dextral offsets common at map scale. Along the major Bath Range Fault, small listric extensional faults are preserved in a block where intrarift, syndepositional uplift may be inferred from the stratigraphy. Some may therefore have originated from syndepositional, oblique extension and rifting. However, other areas demonstrate dextral transpressional deformation, probably due to reactivation during later inversion (Phase 3).

2. Inversion.

2a. Folding, cleavage development, thrusting, and uplift, about a cleavage and shallowly-plunging fold axes trending 015°±10°. Developed in a narrow NNE-trending belt, from the northwest edge of the Parsons Range, through the middle Mitchell Range, to Flinders Peninsula. Affects Ritarango beds, Fagan Volcanics, Parsons Range Group, and Habgood Group (Fig. 1). At least post-Habgood Group in age, and possibly post-Roper Group, but no exposed relationship between the Roper Group and cleaved rocks. Within the Mitchell Range, a sinistral component of shear is indicated by deformed porphyroclasts and C-S fabrics (Fig. 2/2a) but, on Flinders Peninsula, ai is normal to cleavage or there may even be a slight dextral component of shear. 2b Semi-ductile sinistral displacements on faults trending 360°+25°. Movement patterns indicated by Reidel fractures, en-echelon quartz tension gashes, and quartz fibre orientations. Best developed in the Mitchell Range, where it overprints dextral shearing, but is also apparent elsewhere at map scale. Late-penecontemporaneous to younger than folding and cleavage development (2a). Associated mesoscopic conjugate shear sets consistently indicate a SE to ESE orientation of ai #

3. Compression.

•

t \ji[/

2b. M \ 1 J I /

Figure 2. Diagrammatic structural stages

Semi-brittle, conjugate shears, at outcrop scale, consistently indicate o\ from 080°±15°. Displacement directions indicated principally by en-echelon quartz tension gashes, quartz fibre orientations, and Reidel fractures. Offsets Phase 2 cleavage. Cross faults offset major northerly-trending faults at map scale. Further folding and uplift probably involved. Dextral reactivation of earlier NNE-trending faults. The conjugate faults extend regionally, far beyond the Walker Trough, and are definitely post- Roper Group in age. The cause of the distinct NNE-trending shear belt (Phase 2) remains obscure; adjacent areas are poorly exposed. The 'buttressing' effect of an adjacent rigid metamorphic core complex (Mirarrmina Complex, Fig. 1) might explain localised deformation of the Mitchell Range, although a Phase 2 overprint is apparent in the complex itself. Extension of cleavage and intense shear onto Flinders Peninsula, but nowhere else in the Walker Trough, is more problematic. Northward continuation of the Mirarrmina Complex, beneath the Cambrian Arafura Basin to the west of Flinders Peninsula, would seem to be implied. 127

STRATIGRAPHY, SEDIMENTOLOGY AND PROVENANCE STUDIES : POWERFUL TOOLS IN DETERMINATION OF THE RELATIVE TIMING OF TECTONIC JUXTAPOSITIONING AND MELANGE DEVELOPMENT K. S. Pound. Earth Sciences Department; Monash University, Clayton, VIC 3168 Interpretations of the relative timing of deformational events, tectonic juxtapositioning and melange development within structurally disrupted sub-amphibolite facies sedimentary rocks is often difficult. Structural and stratigraphic problems can often be resolved with assistance from provenance studies. Provenance studies incorporate qualitative and/or quantitative assessments of the composition of a sediment to provide information on its source rocks. Provenance studies can identify 1) the source-rock type(s) supplying material; 2) sedimentary sources shared by sedimentary units; 3) new sources that may be unique to one or more of the sedimentary units and 4) recycling or cannibalism of material from older sedimentary units into younger ones. In addition, the composition of detrital heavy minerals (e.g. clinopyroxene, chromite, garnet and amphibole-group minerals) can be used for interpreting the tectonic setting of the source rocks and providing evidence for provenance linkage between different sedimentary units, which allows for interpretation of the terrane history and the development of tectonosedimentary models. The volcanic and sedimentary Middle to Late Cambrian Haupiri Group rocks of NW Nelson, New Zealand are structurally disrupted and metamorphosed to mid greenschist grade. The use of provenance studies together with detailed structural and stratigraphic work has provided a basis for a revised stratigraphic nomenclature, and development of a tectonosedimentary model. Haupiri Group stratigraphy (Figure 1) is broken into at least four main fault-bounded units (FBU's) by north-south striking faults (Junction FBU, Salisbury-Heath FBU, Balloon Melange FBU and Benson-Tasman Lockett FBU). Intrusive, extrusive and epiclastic volcanic rocks (Heath, Salisbury and Benson Volcanics) within the FBU's have compositional and geochemical (Stewart, 1988) signatures that indicate they represent at least two and possibly three separate 'basement terranes'. These basement terranes probably formed in a spatially- and technically- linked setting, which was subsequently modified significantly by strike-slip movement. Petrographic work on the sedimentary rocks overlying the basement terranes has shown that they (Salisbury Conglomerate and Sandstone and Tasman Formation - Figure 1) are linked to each other because they share a source dominated by continental basement and (meta)sedimentary rocks, yet they also each contain clasts derived from their immediate 'basement'. Salisbury Volcanics clasts occur only within the overlying Salisbury Conglomerate and Sandstone, and Benson volcanics clasts occur only within the overlying Tasman Formation. Provenance studies have also allowed determination of the relative age of each sedimentary unit, via observation of new and/or recycled detrital material; lithified Junction Formation supplied material to the Salisbury Conglomerate and Sandstone, Tasman Formation and Lockett Conglomerate, and is thus the oldest stratigraphic unit. Lockett Conglomerate records the influx of a wide range of clast types including amphibole-bearing metavolcanics, gabbroic to dioritic rocks, altered ultramafic clasts and chromite (some of which are also present in very limited amounts in the Tasman Formation). Detrital amphibole-group grains in the Lockett Conglomerate suggest derivation from the same source as the amphibole-bearing clasts (comprised of amphibolitic metavolcanics, altered ultramafics and altered amphibole-bearing gabbroic to dioritic rocks). Detrital garnet composition is indicative of both a Barrovian-type metamorphic and a calc-alkaline volcanic source, and of gabbroic rocks metamorphosed to epidote-amphibolite facies.

128

The composition of detrital chromite within the Tasman Formation and Lockett Conglomerate indicates derivation from a stratiform igneous intrusion or a hydrothermally altered and metamorphosed type III peridotite, probably now represented by the Cobb Igneous Complex. These detailed observations have led to a new model for the development of the Haupiri Group (Pound, 1993). Sedimentation within the Haupiri Group commenced with deposition of arkosic turbidites at a passive continental margin (Junction Formation). The Junction Formation was subsequently disrupted by the development of an accretionary prism. The accretionary prism complex is represented by disrupted Junction Formation sediments, but also includes representatives of an acid to intermediate calc-alkaline volcanic succession (Heath volcanics) and a probable back-arc or marginal basin sequence (Salisbury volcanics), which was overlain by polymict conglomerates and sandstones (Salisbury conglomerate and sandstone). The accretionary prism complex developed into a 1 - 5 km+ wide melange (Balloon Melange) marking the fossil suture zone between the aforementioned units and an oceanic island arc (the basaltic-andesitic Devil River Volcanics / Benson volcanics) and adjacent marine basin (Tasman Formation). The margin of the marine basin (Tasman Formation) and the deforming accretionary prism (Balloon Melange) was overlain by a fan-delta deposit (the Lockett Conglomerate). Fault-bounded blocks of Lockett Conglomerate within the Balloon Melange indicate continued development of the Balloon Melange following deposition of the Lockett Conglomerate. The Balloon Melange includes sedimentary and igneous rocks derived from all the other Haupiri Group lithologies. The use of detailed petrographic work and detrital heavy mineral composition within Haupiri Group sedimentary rocks, together with detailed stratigraphic work has thus proved crucial to the development of the new tectonosedimentary model (Pound, 1993), and illustrates the potential use of such tools in structurally and/or technically complex areas. 510 Payntonlan

BENSON-TASMAN -LOCKETT FBU

/

BALLOON FBU SALISBURY„ HEATH 5 FBU 33

v

prePayntonlan to Idamean

Mindyallen

5 17- Boomer angian

530

Key *

o•

* J^=£VTasman. Fm.

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age - diagnostic fossils age and affiliation of Cobb Igneous Complex uncertain

ivolcanicsS

^

" " " J U N C T I O N FBU

Junction Formation Associated Sediments within the Balloon Melange in possible sedimentary 00,1,801 with H e a t h stratigraphic position of Heath volcanics Is uncertain volcanics

provenance linkage / • V . / - ^ unconformable or eroslonal sedimentary contact

Figure 1 - Tectonostratigraphic columns for Haupiri Group rocks, NW Nelson, New Zealand Pound, K. S. 1993 : Geology of the Lower Paleozoic Haupiri Group rocks, Cobb Valley area, NW Nelson, New Zealand (with a special emphasis on the provenance and sedimentology of the sedimentary rocks). Unpublished Ph. D. thesis submitted to University of Otago, Dunedin, New Zealand 326p. Stewart, M. 1988 : Geology of the Cobb Reservoir Area, North West Nelson. Unpublished MSc. thesis lodged at Canterbury University, Christchurch, New Zealand, 107p.

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TASMANIA: A CONTINENTAL RIBBON IN THE NEOPROTEROZOIC PACIFIC OCEAN C. McA. Powell. P.W. Baillie, and Z.X. Li, Department of Geology, The University of Western Australia, Nedlands, WA 6009. Palaeomagnetic data support a late Mesoproterozoic (1050 Ma) to mid-Neoproterozoic (725 Ma) reconstruction in which Laurentia (North America) was juxtaposed against the eastern margin of Australia and Antarctica in a supercontinent known as Rodinia. Neoproterozoic apparent polar wander paths (APWPs) for Laurentia and East Gondwanaland suggest that by the late Vendian (late Neoproterozoic HI) the two continents were separated latitudinally by up to 50°, and by an unknown amount longitudinally (Powell et al., 1993a). The breakup of Rodinia, arguably around 700 Ma, formed the Pacific Ocean, and implies that any ocean crust flooring the Tasman Fold Belt should date from that time (Powell et al., 1993b). Tasmania, the only exposed Precambrian continental fragment in the Tasman Fold Belt, could thus have developed as part of a continental ribbon stranded in the developing Pacific Ocean. The pre-Carboniferous geology of Tasmania can be considered in terms of six major tectonic zones, which from west to east are: 1: Rocky Cape and King Island platform, 2: Arthur lineament, 3: Dundas "trough", 4: Mt Read Volcanic belt, 5: Tyennan metamorphic complex, and 6: Mathinna Basin. The first five of these tectonic elements comprise the Western Tasmania Terrane and the sixth is the Eastern Tasmania Terrane (Banks and Baillie, 1989). The oldest known rocks occur in the Rocky Cape platform and in the Tyennan metamorphic complex, where high T - low P metamorphic rocks formed around 780 Ma (Turner, 1989). The protoliths are quartzite, carbonate and shale, very similar to the platform-style carbonate, shale and quartz arenite preserved in the Rocky Cape region. Rb/Sr dates on detrital micas suggest a sedimentation age around 1100 Ma (Raheim and Compston, 1977). Widespread deformation, mafic volcanism and granitoid intrusion occurs in the Rocky Cape and King Island region and in the Arthur lineament between 750 and 720 Ma, and is succeeded by deposition of platform carbonates in both the Rocky Cape and Tyennan regions. Renewed tectonic activity between 600 and 550 Ma in the Western Tasmania Terrane is accompanied by widespread intrusion and extrusion of mafic igneous rocks. The Dundas "trough" probably formed at this time and persisted into the Cambrian, with the Mt Read Volcanic belt representing a relatively brief late Middle to Late Cambrian episode of calc-alkaline magmatism. Coarse terrestrial to shallow-marine siliciclastics in the Late Cambrian persisted into the Ordovician and passed upward into platform carbonates. In contrast, in the Eastern Tasmania Terrane the oldest known rocks are deep-marine Ordovician turbidites. This varied geology records five tectonic stages associated with the Neoproterozoic development of the Pacific Ocean, and the subsequent closure of part of that ocean in the Middle and Late Cambrian. The first stage, from -1100 to -800 Ma, predates extension between Laurentia and Eastern Gondwanaland, and is part of the broad epi-continental sag basin phase which is preserved in Central and South Australia, as well as in western North America (Powell et al., 1993c). The second stage, from -800 Ma until -700 Ma, reflects continental extension leading to the eventual successful breakup of Laurentia from East Gondwanaland at -700 Ma. Extrusion of mafic igneous rocks, formation of the deep-marine basin containing the turbiditic Burnie Formation, intrusion of the King Island granitoids, as well as the high T - low P metamorphism of quartzites and carbonates of the Tyennan region, all occurred in this interval. The Penguin Orogeny is thus interpreted as an extensional orogen, with the Tyennan region being the upper plate and the Rocky Cape region the lower. Subhorizontal, west-over-east early fabrics in the Tyennan block (Boulter in Turner, 1989) are interpreted as having formed in the distributed ductile shear between the upper and lower blocks, along which mafic rocks emplaced at the base of the crust (Collingwood River - Lyell Highway eclogites) were brought into juxtaposition with greenschists.

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The third tectonic stage, from -700 Ma to -600 Ma reflects basin sag. Platform carbonates deposited in central and western Tasmania record the tectonic quiescence which occurs after successful continental breakup. The Palaeo-Pacific would have been widening during this interval, and it is not known how far Tasmania was separated from Australia. The absence of well-defined glacial deposits, so characteristic of much of the rest of late Neoproterozoic Australia and its conjugate Laurentian margin (Powell et al., 1993c) can be explained if Tasmania were located in the growing Pacific Ocean well away from the major landmasses. Renewed extension around 600 Ma marks the onset of the fourth tectonic stage which lasted for 70 million years into the Early Cambrian. Mafic volcanics intercalated with carbonates accumulated in the Smithton Basin in the Rocky Cape region. The fifth tectonic stage commenced in the Middle Cambrian with formation of a succession of contractional structures and the possible overthrust of an ophiolitic sheet in the middle Middle Cambrian (Crawford and Berry, 1992). After the emplacement of the ophiolite, calc-alkaline magmatism along the western margin of the Tyennan region formed the Mt Read Volcanic belt. Contractional deformation continued with uplift of the Tyennan region and development of a foreland basin in the Rocky Cape region. These events can all be related to the closure of part of the Pacific Ocean along its Australia - Antarctic margin. The events appear to record first westward obduction of an intra-oceanic island arc leading to emplacement of the ophiolite sheet, then a brief interval of westward oceanic subduction beneath the eastern margin of the Tyennan region, followed by continued contraction as the Tasmanian region was closed westward onto the Australia - Antarctic margin during the Middle to Late Cambrian Ross Delamerian Orogeny. The 700 Ma to 500 Ma interval thus records one complete cycle of opening and closing in the Palaeo-Pacific ocean. Initial continental breakup around 700 Ma brought to an end a 100million year interval of intra-continental extension that led to the birth of the Pacific Ocean. There appears to have been a second phase of extension commencing around 600 Ma. Closure of part, but not all the Pacific Ocean, appears to date from -525 Ma and was completed in 20 million years by the beginning of the Ordovician. The Mathinna Basin, and much of the Lachlan Fold Belt to the north, formed in this younger cycle of basin extension along the Pacific margin of Gondwanaland. Palaeomagnetism from early Late Cambrian sediments in the Smithton Basin shows that northwestern Tasmania became part of Gondwanaland by that time. References Cited: Banks, M.R. and Baillie, P.W. (with contributions from 10 others), 1989. Late Cambrian to Devonian, in: Burrett, C.F. and Martin, E.L.(eds), Geology and Mineral Resources of Tasmania. Spec. Publ. Geol. Soc. Tasm. 15: 182-237. Crawford, A.J. and Berry, R.F., 1992. Tectonic implications of Late Proterozoic—Early Palaeozoic igneous rock associations in western Tasmania. Tectonophysics 214: 37-56. Powell, C. McA., Li, Z. X., McElhinny, M. W., Meert, J. G and Park, J. K., 1993a. Paleomagnetic constraints on the Neoproterozoic breakup of Rodinia and the mid-Cambrian formation of Gondwana. Geology (in press). Powell, C. McA., Li, Z. X. and McElhinny, M. W., 1993b. Palaeomagnetic tests of tectonic models of the Tasman Fold Belt during the Neoproterozoic and Palaeozoic. Explor. Geophys. (in press). Powell, C.McA., Preiss, W.V., Li, Z.X., Krapez, B. and Gatehouse, C.G., 1993c. South Australian record of a Rodinian epicontinental basin and its mid-Neoproterozoic breakup to form the Palaeo-Pacific Ocean. Tectonophysics (in review). Raheim, A. and Compston, W., 1977. Correlations between metamorphic events and Rb-Sr ages in metasediments and eclogite from western Tasmania. Lithos 10: 271-289. Turner, N.J. (with contributions from 7 others), 1989. Precambrian in: Burrett, C.F. and Martin, E.L.(eds), Geology and Mineral Resources of Tasmania. Spec. Publ. Geol. Soc. Tasm. 15: 5-46.

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DIFFERENTIAL STRESS CONTROL ON THE ORIGIN OF FLAME PERTHITE PRYER, Lynn L.* and ROBIN, Pierre-Yves F., Dept. of Geology, Erindale College, University of Toronto, Mississauga, ON L5L 1C6, Canada. Flame perthites are observed in quartzo-feldspathic rocks that have been subjected to a differential stress during retrograde metamorphism to greenschist conditions. Albite flames are produced by the replacement in K-feldspars of K+ by NaV The Na+ is liberated in the breakdown of oligoclase to albite and epidote during greenschist facies metamorphism. The K+ liberated from K-feldspar as a result of replacement combines with the excess Si and A1 in the altered plagioclase to form muscovite. In both the Kfeldspar replacement and the oligoclase alteration, Si and A1 remain in situ such that only the K and Na ions migrate from one grain to the next. Albite flames have a preferred orientation subparallel to the maximum compression direction inferred from the asymmetry of microstructures in the host rock. Flames are also developed at high-stress points such as grain-to-grain contacts. Replacement within K-feldspar generally occurs along a 'normal' perthite crystallographic direction which is the orientation of the plane of minimum lattice misfit between the two phases. An exact (coherent) fit across the phase boundary requires significant lattice elastic strain in one or both phases as is the case in cryptoperthites. The application of a high differential stress on the K-feldspar crystal parallel to this plane of best fit, can reduce the lattice spacing along the b-axis ii K-feldspar which (1) increases the Helmholtz energy of the K-feldspar and (2) reduces that of the albite lamellae or of the phase boundary. Thus, albite replacement allows a decrease in free energy, affected by a volume reduction without change to the alumino-silicate framework and is therefore favoured under conditions of high differential stress. * Present address: 59 Biffin St., Cook, ACT 2614

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THE TECTONIC SIGNIFICANCE OF A MAJOR PORPHYROBLASTIC EVENT DURING HIGH PRESSURE METAMORPHISM IN ALPINE TIMES IN THE AEGEAN SEA, GREECE

Adamandia Raouzaios and Gordon Lister Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Victoria, 3168 Australia

A detailed structural and microstructural study on the island of Sifnos in the western Cyclades, Aegean Sea, Greece, has revealed a complex history of deformation and metamorphism. At least four major periods of deformation have taken place, and several episodes of mineral growth. Our interest in this island stems from the fact that it is one of a number of islands on which outcrop spectacular exposures of high pressure metamorphic rocks that are relatively unaffected by Miocene extension of the Aegean continental crust. A study of Sifnos should therefore reveal information relevant to the early history of the formation of the Miocene metamorphic core complexes which are found in the central Cyclades. Sifnos (in the north west Cyclades) contains a coherent, well preserved suite of eclogiteblueschist facies rocks. These overlie a sequence of schists overprinted by a Miocene greenschist facies retrogression. The two schist horizons are separated by thick marble bands and form distinct structural and metamorphic domains. The eclogite-blueschist domain (EBD) outcrops in the north whereas the greenschist domain (GSD) outcrops in the central and south of the island. There is little information available about D t on Sifnos, except that in the relatively few places where Sx can be identified (e.g. in microlithons), it is defined by alignment of fine grained high pressure minerals such as jadeite, omphacite, and/or glaucophane. D2 developed strong fabrics, and S2 is penetratively developed across the island. D3 was also an intense deformation, particularly at lower structural levels. In the north, in the eclogite-blueschist domain, tight kilometer scale recumbent folds have been identified (e.g., at Vroulidia Bay). In the south, particularly in the upper structural levels of the greenschist domain, D3 is associated with intense zones of mesoscopic recumbent folding. In the lowermost structural levels of the greenschist domain (in the region north of Faros), there is a 500-1000m thick D3 ductile shear zone in which complete transposition of pre-existing fabrics has taken place. D3 is associated with NE-trending mineral and/or extension lineations, and the sense-of-shear is consistently NE-vergent. The tectonic significance of S3 is obviously of interest, and will be discussed in this contribution. Deformation continued after D3, in both structural domains on Sifnos, with formation of NWtrending D4a mineral lineations, and subsequent D4b recumbent folds (which are also NW trending). D4 may be restricted to local zones, but insufficient information is available to the authors at the time of submission of this abstract. Avigad et al. (1992) suggest that D4 folding is associated with the exhumation of the greenschist domain during Miocene continental extension. The last significant event to affect these rocks involved kilometerscale open upright folds, and the formation of an island-scale north-plunging open antiform.

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Avigad (in press) propose that the eclogite-blueschist domain is separated from the greenschist domain by a low angle normal fault Later brittle normal faults have dissected the island and these faults strongly influence the present geomorphology. Both the eclogite-blueschist domain and the greenschist domain have undergone peak metamorphism (M ) at eclogite facies conditions as a result of the Alpine collision between the Apulian and European plates, during the Eocene. A differentiated cleavage (S ) defined by the alignment of jadeite, omphacite, glaucophane and white-mica developed at these high pressure conditions. This was followed by a major porphyroblastic event (M ) during which large porphyroblasts (up to 5cm) overgrew the S cleavage. In the EBD, M involved the growth of garnet, glaucophane, epidote and white-mica at P-T conditions within the jadeite + quartz stability field. In contrast jadeite is absent from the assemblages of the GSD and the growth of albite porphyroblasts appears to predominate the M episode. We intend to focus now on the significance of the M porphyroblastic event, and the subsequent deformation which took place during D . The simplest explanation for this sequence (blastesis followed by a major deformation) is that the rocks were affected by heat and/or fluids leading to a period of crystal growth, and the weakening which thereby ensued provided circumstances favourable to a major episode of continental deformation. Argon thermochronology (Wijbrans et a/., 1990) has been carried out on single white mica crystals which grew in the eclogite-blueschist domain (in the north) during M . Forward modelling the effect of arbitrary P-T-t histories to constrain the significance of these results seems to imply rapid cooling at ~42Ma. This in turn seems to imply a constraint on the timing of M and the onset of D to have occurred during Eocene times. We propose the hypothesis that the eclogite-blueschist domain forms an allochthonous unit which was thrust to shallow crustal levels (15-20km), using major crustal shear zones such as the one that outcrops near Faros in the southern structural domain. The following data is consistent with this hypothesis: (a) rapid cooling in the EBD subsequent to M ; (b) possible higher pressures for the M event (pre D ) in the eclogite-blueschist domain compared with the greenschist domain; and (c) the fact that little post-D deformation and metamorphism occurs in the northern domain, whereas the greenschist domain has had a considerably more extensive history of deformation and mineral growth. x

2

2

2

2

2

2

3

2

2

3

2

2

3

3

References Cited Avigad, D., Matthews, A., Evans, B. W. & Garfunkel, Z 1992. Cooling during the exhumation of a blueschist terrane: Sifnos (Cyclades), Greece. European Journal of Mineralogy, 4, 619-634. Avigad, D., in press. Tectonic juxtaposition of blueschists and greenschists in Siphnos Island (Aegean Sea)-implications for the structure of the Cycladic blueschist belt. Journal of Structural Geology. Wijbrans, J. R., Schliestedt, M. & York, D 1990. Single grain argon laser probe dating of phengites from the blueschist to greenschist transition on Sifnos (Cyclades, Greece). Contributions to Mineralogy and Petrology, 104, 582-593.

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PROTO-GONDWANA BREAKUP AND A LOWER-PLATE MARGIN MODEL AS A SOLUTION TO THE LACHLAN OROGEN ENIGMA M J. Rickard, Geology Department, Australian National University, Canberra ACT 0200. The Lachlan foldbelt is unusual in that, in spite of its width (800km) there is no miogeocline, no foreland fold belt, no large-scale overthrusting, no clear arc-forearc-trench arrangement, no age polarity across the abundant granites or across folded zones. Moreover there is no molasse foredeep or evidence for uplift to mountain chain as is common with crustal thickening. Two enigmas have hindered tectonic interpretation: i) The deposition of Siluro-Devonian terrestrial-shallow water felsic volcanics on top of thick Ordovician oceanic turbidites and ii) The lack of an acceptable subduction scenario. Petrological and geochemical evidence does not support subduction; Ordovician volcanics are of intra-plate origin, and the lack of tonalites and gabbros, the K-rich nature of the granodiorites and the abundance of S-types are features unlike subduction-derived granitoids. Geochronology and geochemistry demand variable sedimentary and igneous sources of Proterozoic age. A solution is presented, that the Lachlan developed as a complex lower-plate margin after break up of Proterozoic Gondwana.As western North America moved away from Australia in the Cambrian, the crust was sliced by an extensional fault system that cut out sequentially Proterozoic sediments then the lower (igneous underplated) crust; this transected junction explains the present S-I line. The thinned and listrically block-faulted Proterozoic crust was covered first by fault-located ophiolitic volcanic edifices, then in the Ordovician by a wide thermal sag basin that was filled with extensive turbidite sequences prograding over the margin as a continental-rise prism and with some local intraplate volcanic extrusions. This 4-5 km thick flysch sheet returned the crust to about normal thickness over the upwarped lithosphere resulting from the lower-plate geometry. Compression, perhaps caused by the begining of a sea-floor spreading reversal, began to thicken the deposits so that a low-pressure facies metamorphism accompanied folding in the Early Silurian. Melting of the buried Proterozoic crust allowed formation of S-type granites and their intrusion in Mid to Late Silurian and by Mid Devonian Itype granites from a deeper source. Because there was no addition of melt from the mantle there was no net increase in volume and therefore no uplifted mountain chain developed. The Silurian and Devonian volcanics and even the Late Devonian molasse was deposited at or close to sea level. Thus the Lachlan orogenic belt developed on a passive margin, essentially without subduction. Subduction commenced in the Late Palaeozoic to develop the New England-Yarrol orogen. This explains why the Lachlan orogen has so many anomalous features, more similar to the Australian Proterozoic ensialic orogenic belts than Cordilleran orogens.

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STRUCTURAL GEOLOGY OF THE MT. WHALEBACK MINE, NEWMAN, WA. J. Ronaszeki BHP Iron Ore Ltd., P.O. Box 655, Newman 6753, W.A. The Mt. Whaleback Mine is located in the Hamersley Basin of the Pilbara Craton in Western Australia. It is the largest single-pit iron ore mine in the world with an original iron ore reserve of approximately 1600 million tonnes. The structure of the Mt. Whaleback orebody is highly complex and has a significant influence on iron ore genesis. The majority of the ore is part of the Precambrian microplaty hematite-martite orebody and is formed in the Dales Gorge and Joffre Members of the Brockman Iron Formation of the Hamersley Group. A small Tertiary martite-goethite orebody is also located near the present surface. The iron ore is derived from the supergene enrichment of Banded Iron Formation (BIF) (Morris, 1985) which was originally deposited at approximately 2.5 Ga. 43% of the original thickness of BIF was lost during the formation of the iron ore at Mt. Whaleback (Ronaszeki, 1992). This corresponds to a volume reduction of 300-400 million m3. The volume loss has caused significant space problems and resulted in additional, local, non-tectonic, ductile deformations during ore genesis. The local deformation history of the mine area can be separated into six tectonic phases in the following order: The first deformation (F^ of the Hamersley Group in the mine area comprises of early small scale, layer-parallel, isoclinal folds that have been indicated in the region by Tyler (1990). These folds have not yet been observed in the Mt. Whaleback Mine. The main folding phase (F2) includes large to small scale, ductile, open to layer-parallel folds. The folds were produced by the Proterozoic Capricorn Orogen (D2c deformation of Tyler, 1991). F2 folds verge to the north and are typically overturned or recumbent, some folds are downwards facing. The plunge is to the west but there is a plunge reversal in the western part of the mine. Two large, low angle, normal faults, the East Footwall Fault Zone (EFFZ) and the younger Central Fault (CF), cut the F2 fold set. Movement along them is over 100 m. The CF is located 50-200 m above the EFFZ. The EFFZ is a complex zone of sub-parallel splays whilst the CF is represented by a single fault or a very narrow fault zone. The orientations of the large, low angle faults are similar but not identical to the axial planes of the large scale F2 folds and are not considered to be F2 axial plane faults. Tyler (pers. comm. 1993) considers them to be collapse structures. 5 km south of the Mt. Whaleback Mine a large NW trending F3 syncline has been identified (Orebody 29 area). In the Mt. Whaleback Mine F2 plunge variations could be attributed to this deformation phase. The F3 phase has been recognised over a wider area by Johnson (this volume).

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Large, high angle normal faults clearly cut the main F fold set. The largest high angle fault is the ENE trending Whaleback Fault (WF) which forms a fault zone (WFZ) in the mine and has a throw of more than 1000 m. The WFZ also cuts the low angle normal faults (Hobbs et al. 1987). The Whaleback Fault has been correlated with the opening of the Breshnahan Basin by Tyler (1991) and given an approximate age of 1.6 Ga. 2

Small to medium scale WSW trending F folds deform the low angle EFFZ and CF. In one area the high angle WFZ is also deformed by F . These folds, characterised by a brittle deformation style (often with box fold geometry) are considered to be the product of the last significant, large scale, tectonic phase in the mine. 4

4

Three phases of iron ore formation have been identified within the mine (Ronaszeki, 1992). The majority of the ore formed during the first two phases (0 and 0 ). The first phase started after the formation of the EFFZ and progressed from the bottom of the Brockman Iron Formation towards the top of the formation and down-plunge from east to west. The end of the second phase postdates the WFZ. The third phase (0 ) of ore genesis consists of a minor Mesozoic or Tertiary martite-goethite supergene orebody in the same stratigraphic units. The lower age limit of iron ore genesis is constrained by F (ore post dates F ). There is no constraint for the upper limit of Proterozoic ore genesis phases as ore genesis finished after the Whaleback Fault formed. References: Hobbs, B., Mason, R., Davidson, D., and Preston, K. 1987. Structural Geology of the North Wall of the Mt. Whaleback East Pit and the Implications for Slope Stability. 1987. CSIRO Division of Geomechanics, p. 48, (unpublished). Johnson, T.M. (in preparation). The geometry of the SE Ophthalmia Fold Belt, Pilbara Region, Western Australia. - A combination of two regional fold sets. (Abstract). SGTSG Field Conference, Jindabyne, February 1994. Morris, R. C. 1985. Genesis of iron ore in banded iron formation by supergene and supergenemetamorphic process - a conceptual model. In: Handbook of Strata-Bound and Stratiform Ore Deposits, Volume 13 (Ed. K.H. Wolf). Elsevier, Amsterdam, p. 73-235. Powell, C. McA. and Z.X. Li, 1991. (abstract) New evidence for the age of deformation along the southern margin of the Hamersley Province: relevance to the palaeogeographic evolution and time of iron-ore formation, pp. 76-77. In: Excursion Guide to the Southern Margin of the Pilbara Craton. Ed.: Tyler, I.M., Horwitz, R.C. and Powell, C. McA, Geological Society of Australia, Specialist Group in Tectonics and Structural Geology. Ronaszeki, J. 1992. Structural geology and its controls on iron ore mineralisation at the Mt. Whaleback Mine, Newman, Western Australia. A guide for the Mt. Whaleback stops of the 'Excursion to the Southern Margin of the Pilbara Craton', run under the auspices of the Specialist Group in Tectonics and Structural Geology of the Geological Society of Australia. 41p (unpublished). Tyler, I.M. 1991. The geology of the Sylvania Inlier and southeastern Hamersley Basin. Bui. West. Aust. Geol. Surv. 138. pp. 108. lw

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FOLD DEVELOPMENT IN SHEAR ZONES: THE ORIGIN OF FOLD AXES PARALLEL TO THE EXTENSION DIRECTION IN CORE COMPLEXES OF SOUTHWESTERN U.S.A, R. J. Scott and G. S. Lister, Dept. of Earth Sciences, Monash University, Victoria, 3168. Fold axes parallel, or sub-parallel, to the extension lineation are commonly developed in shear zones. Often this is due to re-orientation during shearing, with fold axes being rotated towards the extension direction with increasing strain. The original folds may have been unrelated structures that were incorporated into the shear zone, or have formed during shearing, where layering was locally rotated into the shortening field. Typically such folds are highly noncylindrical and fold axes are deflected by up to 180° in the axial plane of the fold (i.e. sheath folds). Folds with axes parallel to the extension lineation commonly overprint pre-existing fabrics within the mylonitic successions that dominate the lower plates of metamorphic core complexes in southwestern U.S.A. The folds and mylonitic fabric were both developed during regional extension that occurred in the Late Oligocene to Early-Mid Miocene. The orientation of the majority of these folds cannot be explained by re-orientation of earlier structures in highstrain zones, as the folds are similarly oriented, and often better developed, in low-strain domains between the mylonitic zones. Detailed structural analysis near Planet Peak, in the lower plate of the Buckskin-Rawhide Mountains core complex, western Arizona, indicates the majority of folds formed with fold axes at a low-angle to the SW-trending extension lineation. The orientation of the folds appears to have been largely controlled by the NE-trend of the pre-existing, steeply-dipping Proterozoic gneissic fabric in the lower plate. Folds are largely restricted to lenticular domains where the gneissic fabric or modified gneissic fabric is preserved. The folds are attenuated or obliterated in areas where the mylonitic fabric is strongly developed, however, the orientation of the folds does not vary significantly with increasing strain. The mylonites generally dip gently to moderately southwest and kinematic indicators suggest they were formed within a zone of predominantly top-to-the-NE shear. At Planet Peak, as elsewhere in the Buckskin-Rawhide Mountains, three major rock-units form the bulk of the lower plate. The oldest and most extensive are the Proterozoic gneisses (intruded by pegmatites and granitoids of Proterozoic to (?)Mesozoic age). Second, are discontinuous layers of Palaeozoic-Mesozoic metasedimentary rocks, dominated by marbles, calc-silicates and quartzites. The metasedimentary units are widespread, but generally less than 10 m thick, and not volumetrically significant. Incorporation of these rocks into Proterozoic basement is thought to be a result of Late Cretaceous to Early Tertiary thrust faulting, during the Laramide Orogeny. The third group are sheet-like, felsic-intermediate to mafic intrusions, emplaced both prior to, and during the Oligo-Miocene mylonitization. The different structural style in each of the units provides a clue to the origin of the folds in the lower plate. The distribution of the metasedimentary units indicates they were oriented subparallel to the mylonitic flow plane (-orthogonal to the Proterozoic gneisses) prior to the OligoMiocene extension. Layering in the metasediments is often intensely and irregularly folded. The axial planes are sub-parallel to the mylonitic fabric and the fold hinges are typically highly non-cylindrical (Fig. la). In contrast, open to isoclinal folds in the Proterozoic gneisses are remarkably cylindrical (Fig. lb) and generally have SE-over-NW asymmetry (Fig. la). The axial planes are also parallel or sub-parallel to the mylonitic foliation. On average, the trend of the fold axes (parallel to the intersection between the mylonitic and gneissic fabrics) is about 10° more westerly than the mylonitic extension lineation (Fig. la). Within weakly mylonitized domains, even small open folds or warps in the gneissic fabric (amplitudes and wavelengths of only a few centimetres) can be sub-parallel to the extension lineation for several metres. The cylindricity of the folds within the low-strain domains, indicates the fold axes have not been significantly re-oriented. Adjacent to the mylonitic zones the gneissic fabric is generally attenuated and rotated into parallelism with the mylonitic fabric. In these areas the folds are more commonly tight to isoclinal, and the gneissic fabric transposed to form the mylonitic fabric. Owing to the small angle between the fold axes and the extension lineation, the fold axes are similarly oriented regardless of the intensity of the mylonitic fabric.

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However, rare non-cylindrical folds with axes at a high-angle to the extension lineation, also occur within the high strains zones. The Tertiary intrusions generally have a well developed planar mylonitic fabric parallel to their contacts, and parallel or sub-parallel to the mylonitic fabric in the wall rocks. Folds in the mylonitic fabric within the intrusions are extremely rare. The lack of folds in the intrusions suggests folds were only developed within rocks that contained a pre-existing layering. The difference between the structures in the gneisses and the metasedimentary rocks must reflect the initial orientation of the layering in these units. This is supported by the fact that highly noncylindrical folds also occur in the Proterozoic rocks where the gneissic fabric is re-oriented subparallel to the mylonitic fabric. Non-coaxial folds may form at any stage during mylonitization, if slight perturbations in the flow plane rotate layering into the shortening field. On the other hand, the cylindrical folds overprinting the gneisses, reflect the orientation of the layering prior to the Tertiary deformation. While shortening perpendicular to the mylonitic foliation and extension parallel to the stretching lineation would merely attenuate the mylonites and metasedimentary layers, the steeply-dipping gneissic fabric would be folded. The sense of asymmetry of the folds is consistent with shortening perpendicular to the mylonitic foliation and the steep NW-dip of the gneisses (suggested by their orientation in the areas least affected by Tertiary deformation, Fig. lc). The slight obliquity between the fold axes and the extension lineation is appropriately oriented to attenuate folds with SE-over-NW asymmetry during subsequent top-to-the-northeast non-coaxial deformation. Thus, folds in the gneisses appear to be the result of both a strong pre-existing anisotropy and a combination of coaxial and non-coaxial deformation during the Tertiary extension.

Figure 1

a) Block diagram illustrating structural relationships in lower plate rocks near Planet Peak. b) Equal area stereographic projection of fold axes and poles to gneissic or relic gneissic (partially mylonitized) layering. The great circle distribution of the gneissic layering and the cluster fold axes about the pole to this great circle (23°-> 234°) emphasises the cylindrical nature of the folds. Contour interval: 1%/I%area. c) Poles to the gneissic layering from the areas least affected by Oligo-Miocene deformation. The predominance of shallowly southeast plunging poles indicates the gneisses probably dipped steeply northwest prior to extension. Contour interval: 2%/l% area.

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THERMOCHRONOLOGICAL CONSTRAINTS ON THE DEVELOPMENT OF METAMORPHIC CORE COMPLEXES IN SOUTHWESTERN U.S.A. R. J. Scott . G. S. Lister and D. A. Foster , Dept. of Earth Sciences, Monash University, Victoria, 3168, Dept. of Geology, La Trobe University, Bundoora, Victoria, 3083. Mylonites in the lower plates of metamorphic core complexes probably originated in shear zones that formed the down-dip continuation of the detachment faults that now overlie them. Detachment systems are generally thought to be deeply-rooted in the crust and to have developed in response to major intraplate stresses. The presence of mylonites in the lower plate suggests these rocks were uplifted from depths where the ambient temperatures were sufficient for ductile deformation (i.e. 12-15 km, e.g. Sibson 1986). However, based on field relations in the lower plate of the Buckskin-Rawhide Mountains metamorphic core complex (western Arizona), we interpret existing thermochronological data from southwestern U.S.A. to indicate the mylonites formed at much shallower depths in the crust. Accordingly, metamorphic core complexes in this region may not be as deep-seated as previously thought. Rehrig & Reynolds (1980) considered Tertiary intrusions to be characteristic of the lower plates of metamorphic core complexes in southwestern U.S.A., suggesting anomalously high geothermal gradients may have existed during extension, facilitating mylonitization at shallower levels. Indeed, the correlation between magmatism and extension in the Basin and Range province has long been recognised (Gans et al. 1989 and references therein). The significance of plutonism, however, is controversial as mylonites generally appear more widely distributed than igneous bodies of the same age (e.g. Spencer & Reynolds 1991). At Planet Peak, in the southwestern Buckskin Mountains there is a spatial correlation between areas of mylonitic fabric development and syn-extensional intrusions. The mylonites are best developed in or adjacent to the larger Tertiary intrusions or within zones containing numerous gently-dipping, thin hypabyssal sheets. In many cases the intensity of the mylonitic fabrics overprinting the original gneissic country rocks increases abruptly adjacent to the intrusions. Preferential strain localisation (mylonite development) in and around intrusions, suggests mylonitization occurred at shallow levels ( « 1 0 km) in the crust, because only those rocks within the thermal aureoles of the intrusions were hot enough to deform ductilely. Elsewhere, mylonites not immediately adjacent to Tertiary intrusions may have formed within the thermal aureoles of unexposed intrusions. A zone of high gravity coinciding with the belt of core complexes in the southern Basin and Range, has been interpreted to reflect the presence of Tertiary mafic intrusions at depth (Simpson et al. in Parsons & Thompson 1993). Existing thermochronological data from core complexes in the southern Basin and Range can be used to evaluate this hypothesis, and thereby, the role of magmatism in core complex formation. If the mylonites were formed at relatively shallow levels, within the thermal areoles of (unexposed) igneous bodies, Ar/ Ar spectra from previously cooled lower plate rocks should exhibit variable amounts of degassing, depending on their proximity to the igneous bodies. Thus the distribution of apparent ages may be quite heterogeneous, and not reflect structural position in the lower plate. Lower plate rocks in most core complexes of the southern Basin and Range record a mixture of Early Tertiary and Oligo-Miocene (syn-extensional) cooling. Major variations in Ar/ Ar apparent ages occur over small areas. In the Whipple, Harcuvar and South Mountains core complexes, variations in apparent age indicate that mylonitic rocks were up to 150°C hotter than non-mylonitic country rocks a few tens of metres away (DeWitt et al. 1990). Preservation of old (i.e. pre-extension) ages in less argon-retentive minerals (K-feldspar and biotite) adjacent to mylonite zones indicates these rocks were in the upper crust and remained cool (T<200-300°C) during extension. DeWitt et al. (1990) argue that, in some instances, mylonites were formed at levels in the crust where ambient temperatures were below 200°C. Rapid Miocene cooling recorded throughout much of the southern Basin and Range has been interpreted to reflect tectonic denudation during extension (e.g. Davis 1988). However, rapid 1

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cooling could be due to either rapid uplift of deeper level "hot" rocks, or resetting due to shallow level intrusion. The partial degassing of previously cooled rocks during the OligoMiocene (DeWitt et aL 1986), and the irregular distribution of apparent ages, are seemingly more consistent with renewed thermal input during extension (e.g. Knapp & Heizler 1990). 40

Ar/ 39 Ar age spectra for K-feldspar and biotite from the foot wall of the Buckskin-Rawhide fault system generally indicate Miocene minimum apparent ages or cooling: 13-20 Ma in the Buckskin Mountains (Richard et al. 1990) and 22-28 Ma in ranges to the south (structurally higher levels of the lower plate) (Knapp & Heizler 1990). This suggests that the ambient temperatures of presently exposed lower plate rocks in the Buckskin Mountains were above the closure temperature for biotite (>300°C) during mylonitization (Richard et al. 1990). However, Shackelford (1980) reported nearly concordant hornblende and biotite K-Ar ages (57.4 and 52.3 Ma, respectively) from the Rawhide Mountains. If these apparent ages reflect original cooling ages or partial degassing during the Oligo-Miocene, rather than accumulated extraneous (excess) argon, the deepest exposed levels of the Buckskin-Rawhide fault system were not uniformly hot enough to reset biotite everywhere. This suggests that locally temperatures could not have exceed 350°C for longer than ~1 m.y., or 400°C for longer than 0.1 m.y. (modelling by GSL). DeWitt et al. (1990) interpreted the heterogeneity in their 40 Ar/ 39 Ar data to reflect argon loss due to grain-lattice deformation and/or metamorphic reactions, during mylonitization at relatively shallow levels. However, the metamorphic grade of mineral assemblages defining mylonitic fabrics also indicates the mylonites were formed at high temperatures. We suggest the development of core complexes in the southwestern U.S.A. may be related to relatively shallow-level plutonism. The crustal weakening caused by addition of magma could facilitate rapid extensional deformation, given a favourable regional stress field. Detailed investigations of metamorphic core complexes in the Aegean (Lister et al. unpubl. data) and Solomon (Hill et al. 1992) seas also reveal a correlation between pulses of intrusion and extremely rapid operation of the shear zones. Strain softening and/or shear heating following the initial localisation of strain may have been sufficient to prolong ductile deformation after the initial heat input from the intrusions dissipated.

REFERENCES Davis, G.A., 1988: Geol Runds.,11 191-209. DeWitt, E., Sutter, J.F., Davis, G.A. & Anderson, J.L., 1986: Geol. Soc. Am. Abs. w. Prog. 18, 584. DeWitt, E., Sutter, J.F., & Reynolds, S.J., 1990: Geol Soc. Am. Abs. w. Prog. 22, 18. Gans, P.B., Mahood, G.A. & Schermer, E., 1989: Spec. Pap. geol Soc. Am. 233, 53 p. Hill, E.J., Baldwin, S.L. & Lister, G.S., 1992: Geology 20, 907-910. Knapp, J.H. & Heizler, M.T. 1990: J. geophys. Res. 95, 20049-20073. Parsons, T. & Thompson, G.A., 1993: Geology 21, 247-250. Rehrig, W.A. & Reynolds, S.J., 1980, in Crittenden, M.D. Jr. et al., eds.: Mem. geol Soc. Am. 153, 131158. Richard, S.M. Fryxell, J.E. & Sutter, J.F., 1990: J. geophys. Res. 95, 19973-19987. Sibson, R.H., 1986: Ann. Rev. Earth Planet. Sci. 14, 149-175. Shackelford, T.J., 1980: Geology 8, 190-194. Spencer, J.E. & Reynolds, S.J., 1991: Tectonics 10, 1204-1221.

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The geological framework of northwestern Australia from geophysical mapping R.D. Shaw, M.P. Morse and C. Tarlowski, Australian Geological Survey Organisation Greater understanding of the basement geology of northwestern and central Australia is made possible as a result of an ongoing AGSO project instigated in the Geophysical Mapping Program to produce a 'Basement Tectonic Elements Map of Australia' using magnetic and gravity anomalies to map the major tectonic domains forming the basement throughout the Australian continent. In construction of the map, the main materials used were; (i) images at 1:5 M scale of gridded gravity and magnetic data for the Australian continent; (ii) a 1:2.5 M grey-scale image of magnetic features having wavelengths of less than 50 km ; and (iii) 1:2.5 M contour maps of Bouguer gravity and magnetic data. In some problem areas, the magnetic data was studied as image and in contour form at larger scales.In our interpretation, magnetic and gravity anomalies were treated separately and grouped into domains. In some cases, known geology used to differentiate between first and second order geophysical domain boundaries and to further constrain the final tectonic model. This process allows for a consistent and coherent model of the tectonic elements to be generated. Two types of large domains boundary were recognised: (i) those involving a change in trend or pattern indicating a structural domain boundary, and (ii) those involving a major change in field magnitude indicating a change in upper crustal material. Such compositional domain boundaries may or may not be structural. Many of the larger domains may be further subdivided into subdomains using features such as trends, boundaries reflecting major rock bodies, and faults/lineations. In most cases, exposed geological boundaries correlate well with the geophysical tectonic element boundaries. Exceptions arise in cases where basin margins, like that of the Ngalia Basin, represent remnants of a much larger, complex and more extensive basin, or where basin formation was dominated by extreme extension, leading to structures showing few signs of basement inheritance, such as parts of the Canning Basin. Three sets of domains were digitised: (i) those derived from magnetic data, (ii) those derived from gravity data, and (iii) the estimated positions of the boundaries for the interpreted tectonic domains at the basement surface. In some cases, is was necessary to extrapolate these boundaries in order to complete individual polygons. Based on the experience in North America and eastern Australia, zones of 'reworking' are recognised along some of these structural domain boundaries, such as that bordering the western margin of the Mount Isa Block. Reworked zones are recognised by features such as a change, generally a drop, in magnetic intensity and a change in the nature and trend of shortto intermediate-wavelength anomalies (< 50 km). Such reworked zones mark where the older provinces have undergone the orogenesis (intrusion, metamorphism, deformation) characterising the adjacent and younger, orogenic province. In places, similar zones of 'reworking' are apparent in the gravity data as is the case of the western margin of the Mount Isa Block. Relative age of magnetisation and 'cratonization' of crustal blocks can also be recognised from cross-cutting trends at domain boundaries, although multiple deformations can cause problems with such simple interpretations, as is the case for the domain feature separating the Canning and Amadeus basins. Older 'nuclei-style' domains are also recognised that are eqidimensional in shape and show internal structures that are partly discordant to sets of markedly elongate domains, that surround them. Some of these elongate domains are characterised by positive or negative gravity anomalies and correspond to tectonic mobile belts. An example is the King Leopold and Halls Creek orogens, surrounding the Kimberley Basin, the central region of which has a 'nuclei-styled' domain character. Other domains with

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2 a 'nuclei-style' include those aligning with the Pine Creek Geosyncline, the Kidson SubBasin of the Canning Basin, possibly the Granites-Tanami Block, and possibly the southeasternmost Geogina Basin. The latter is a more complex and disturbed domain that is very similar to domains corresponding to much of the central province of the Arunta Block. The basement elements mapped underlying the Officer Basin both differ in geophysical character and are discordant to those of the 1100-1200 Ma or older Musgrave Block, suggesting they are older. A second, less well-defined 'older' domain north of those corresponding to the Musgrave Block, relate to regions affected by the 530-550 Ma Petermann Ranges Orogeny. Neither these domains , nor those associated with the Musgrave Block can be linked to the domains corresponding to the 550-750 Ma Paterson Province to the west. Those domains associated with the Musgrave Block are more like those associated with the 1100-1300 Ma Fraser Block. The magnitude of gravity gradients corresponding to fault systems located along or near province boundaries bordering the Musgrave and Arunta Blocks indicate they reach the crust-mantle boundary. Such fault systems include: (i) the Redbank Thrust Zone and the linked Ormiston and Charles Creek Thrust Zones at the northern margin of the Amadeus Basin, active in a series of pulses from 400 Ma to 300 Ma and (ii) the Woodroffe-Mann Fault Zone and the related Petermann Ranges Thrust Zone, active at 530-550 Ma at the southern margin of the Amadeus Basin. A similar, but more complex, feature at the northern margin of the Officer Basin may record interaction along what could be a 'suture zone' separating the Musgrave region from basement elements to the south. The scalloped-shape of this magnetic boundary may record a series of overthrust segments, as documented in one case from the northeastern margin using seismic and gravity data. This 'suture zone' does not appear to continue eastward to link with the southern margin of the Mount Isa Block, as shown in earlier interpretations. A discordant boundary near the northern margin of the Amadeus Basin, more evident in intermediate wavelength anomalies (25-55 km), corresponds in part to the northern limit of the 'central ridge' and separates the Amadeus Basin into two components. A more important crustal boundary probably corresponds to a major gravity gradient aligning with the Redbank Thrust Zone, separating the central and southern provinces of the Arunta Block. There is an indication in the magnetic data that a new, complex, east-west fault zone, showing marked reworking in magnetic images on both its northern and southern margins, may link features at the southern margin of The Granites-Tanami Block, through the northernmost part of the Arunta province as a newly recognised series of lineaments, to faults outlining the western embayment of the Georgina Basin. This zone separates off the northernmost and northeasternmost segment of the Arunta Block from the main body of the Arunta Block to the southwest. The region of the Arunta Block can be further subdivided into several domains and subdomains. One important new domain boundary appears to correspond to the Florence, Gough Dam and Wallaby Knob shear zones, jointly referred to as the Cadney Fault Zone. Domains in the eastern part of the Arunta region are disrupted by several cross-cutting features including a complex feature in the Arltunga-Harts Range region. A major domain, centred on the McArthur Basin, may extend as far south as the Murphy Ridge and be separate from another domain centred on the Gulf of Carpentaria. Extrusives of the Tawallah and Katherine River groups correspond to two very large subdomains, making up most of the McArthur domain. The Walker Fault Trough corresponds to a well-defined geophysical feature, unlike its continuation as the Batten Fault Trough. Much of remaining central-northern part of the Northern Territory appears to form a single domain with important subdomain boundary centred on the Davenport Province.

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STRUCTURAL DEVELOPMENT DURING HIGH GRADE NONCOAXIAL DEFORMATION: AN EXAMPLE FROM THE RAUER GROUP, EAST ANTARCTICA John. P. Sims1, Christopher. J. L. Wilson1, Paul H. G. M. Dirks2 & Martin Hand1 ^School of Earth Sciences, University of Melbourne, Parkville Vic. 3052. 2 Department of Earth Sciences, University of Utrecht, PO Box 80.021,3508TA,Utrecht, the Netherlands Gneissic lithologies in the Rauer Group can be classified into four main categories with a generally east-west transition from oldest to youngest (Fig. 1). A high-grade, noncoaxial, progressive deformation event at approximately 1000 Ma, recording NW-directed sinistral transpression, has resulted in the interleaving of these lithologies and the development of a complex range of structures. The identified lithologies are: 1) composite Archaean gneisses (3300 - 2800 Ma) probably deformed in the Archaean and repeatedly deformed in. the midProterozoic, interspersed with multiple generations of early-mid Proterozoic mafic dyke suites; 2) a mid-Proterozoic orthogneiss (1750 - 1250 Ma) intruded by multiple generations of midProterozoic mafic dyke suites; 3) mid-Proterozoic metasediments (1240 - 1050 Ma) that contain few mafic dykes; and, 4) a mid-Proterozoic (approx. 1000 Ma) syntectonic granite-suite. Peak metamorphic conditions during the c. 1000 Ma deformation event were extreme (>10 kbars at approximately 1000°C); peak assemblages include linear aggregates of orthopyroxene sillimanite and quartz in magnesian metapelites, and garnet - clinopyroxene - orthopyroxene developed as the result of hornblende dehydration-melting reactions in metabasics. During progressive deformation spectacular decompression textures developed in most lithologies, with the lower pressure assemblages generally aligned in the ubiquitous steep SE-plunging lineation. For example, the orthopyroxene - sillimanite assemblage reacted to sapphirine cordierite, producing textures whereby moats of recrystallised cordierite separate aligned orthopyroxene from domains in which the same lineation is defined by sillimanite and sapphirine needles. Continuous decompression is also evidenced by mineral textures being repeatedly cross-cut by structures that also developed decompression textures. For example, late recrystallised mylonites, that cross-cut gneissic foliations and are in turn cross-cut by gneissic foliations, preserve garnet bearing assemblages in which the garnets are partially to completely replaced by symplectites of orthopyroxene and plagioclase. That these mylonites are recrystallised suggests that the cooling path was not smooth but that heat pulses may have affected the terrain during the deformation history. Mineral assemblages developed in the latest 1000 Ma structures record upper-amphibolite facies metamorphic conditions. Strain was increasingly partitioned and localised with progressive deformation. Zones of strain localisation are characterised by the total transposition of earlier foliations and lithological boundaries to produce "simple" gneissic foliations. In areas dominated by lithologies 1 & 2, deformation features are relatively homogeneous and foliations may be described as either continuous, planar high strain zones for the duration of the deformation, or, as a sequence of increasingly localised planar foliations between which earlier foliations are re-oriented. In contrast, in areas where lithologies 3 & 4 dominate, early developed foliations in paragneisses are intruded by numerous syntectonic orthogneisses, and are repeatedly re-oriented about a progression of later cross-cutting and anastomosing foliations, all of which may have been reactivated at different stages. With progressive deformation, strain localisation in any bulk rock may have occurred in response to strain softening, due to: metamorphic reactions; thermal pulses (e.g. granite emplacement); changing fluid activity (e.g. rehydration in response to granite crystallisation); or, changing deformation mechanisms. The transposition and extreme attenuation of marker lithologies such as mafic dykes, within these high-strain zones, suggests that strain localisation may have resulted in strain-rate variations across the terrain.

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Figure 1. Distribution and geometry of 1000 Ma gneissic foliations in the Rauer Group. The demarcation between lithologies is based on Kinny et al. (1993, Antarctic Science 5, 193-206). Geometrical data is contoured for poles to 1000 Ma gneissic foliations, symbols represent: (ti)fold axes, (*)mineral lineations and, (^elongation lineations. Foliations on some central western islands are not represented.

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GEOLMAP: A PROGRAM TO PLOT AND MANIPULATE DATA ON MAPS AND STEREOGRAPHIC PROJECTIONS IN THE AutoCAD ENVIRONMENT R.Sliwa. Department of Earth Sciences, University of Queensland Geolmap is a set of AutoLISP programs for use with AutoCAD. Its main application is to plot and manipulate geological field-mapping data, including the ability to define structural or lithological map domains. It includes routines to import data from ASCII files and plot them either as symbols on a map, as simple stereographic projections, or as rose diagrams. Optionally the data can be imported as both map symbols and stereographic projections that are linked so that each can later be manipulated with respect to the other. Other routines facilitate digitising and exporting field locations for use in database packages such as dBaseIII+ or Microsoft Access. Combining Geolmap with an appropriate textual database, and the power of AutoCAD for digitising data from various sources at different scales, creates an effective tool for processing field mapping data, from the stage of compiling the data from air photos or base maps, to plotting publication-quality maps, and illustrations. A typical sequence of tasks would begin by digitising fieldlocations, and other geological mapping data, directly from air photos or fieldmaps to an AutoCAD drawing. The geographical coordinates and fieldlocation numbers are then transferred to the database via an ASCII file, to form new records to which the textural part of the field mapping data is added. It is now at the liberty of the user to create any number of ASCII files containing sets of grid coordinates and associated fabric data. These files can then be imported into AutoCAD to produce maps, stereographic projections or rose diagrams.

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TECTONOSTRATIGRAPHY OF THE CENTRAL NORTH Df AGUILAR BLOCK, SOUTHEASTERN QUEENSLAND. R.Sliwa1. R.J.Holcombe1 and T.A.Little2 1 Department of Earth Sciences, The University of Queensland 2 Department of Geology, Victoria University of Wellington, New Zealand The North D'Aguilar Block (NDB) is part of the New England Orogen accretionary complex in southeastern Queensland (Fig. 1). It is a collage of fault blocks in which deeply subducted elements of the complex are locally juxtaposed against higher level accretionary rocks. The major structural controls on this juxtaposition are a gently arched normal fault, the Mt Mia Fault, and a low angle thrust fault, the Claddagh Thrust (Little et al, 1993). We interpret the Mt Mia Fault as a detachment surface exhuming polymetamorphic rocks in its lower plate, some containing epidote blueschist facies assemblages. The Claddagh Thrust further imbricates the structure, and exposes Late Carboniferous granitoids that contain fabrics syntectonic with the detachment deformation (Little et al, 1992). Both of these faults are defined in the northern NDB where footwall-hangingwall relationships are well exposed and metamorphic discontinuities are clearly defined. Both faults can be traced southward into the central part of the NDB where their traces are interrupted by Triassic plutons, and where the structure is complicated by several small fault-bounded basins containing Early Permian marine diamictites. This strip of Permian and Triassic rocks divides the metamorphic basement units of the central NDB into two groups: units to the north are continuous with well-defined units in the northern NDB; while those to the south are more ambiguous in their regional correlation. The northern group includes units from both the lower and upper plates of the Mt Mia Fault, as well as the southerly extension of foliated Late Carboniferous granitoids exposed in the allochthonous sheet of the Claddagh Thrust. The basement units to the south of the median strip consist of the simply deformed Booloumba Beds, the newly defined polydeformed Peters Ck Greenstone and the Jimna Phyllite, a new name given to a 5km thick package of low grade polydeformed phyllite. The interpretation of these southern units is the focus of this paper. Booloumba Beds and Peters Ck Greenstone. The Booloumba Beds consist of a series of simply deformed, interbedded slate/ metasandstone, slate, quartzite and massive pillow basalt. The dominant lithology is a rhythmically interbedded sequence of discontinuous graded beds of metasandstone to argillite, resembling broken formation. The unit is simply deformed, containing a steep, northnorthwest trending pervasive slaty cleavage. This fabric is similar to that in other upper plate rocks, such as the Amamoor Beds. The Booloumba Beds as defined by Murray et al (1979) contain multiply deformed, map-scale lenses of mafic greenschist, containing relict augite. The slightly higher metamorphic grade and the polydeformational fabric of the greenstone lenses contrasts with the simply deformed, adjacent pelitic rocks, indicating a more complex deformational history. We therefore define a new unit, the Peters Ck Greenstone. The deformational fabrics and Fig. 1 Location of major blocks and structural mineral assemblages are very similar to the Rocksberg Greenstone in the southern NDB assemblages in SE Queensland.

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(Holcombe and Little, 1993), and the unit is correlated with this lower plate unit. Jimna Phyllite. The Jimna Phyllite is a thick fault-bounded package, of polydeformed slate, phyllite, metachert and mafic greenschist. The pelitic lithologies contain a subhorizontal, pervasive slaty cleavage with a strong combined intersection/stretching lineation. This cleavage is locally a crenulation fabric. Mafic greenschists occur in narrow lenses (<lkm wide) parallel to the foliation. The polydeformed fabrics of the Jimna Phyllite resemble those observed in lower plate phyllitic units but its lithologies and low metamorphic grade are typical of upper plate units. An unresolved problem is its great thickness of more than 5km, probably resulting from thrust imbrication concurrent with the Claddagh Thrust. Nature of boundaries. The low angle Mt Mia Fault juxtaposes upper and lower plate lithologies of contrasting metamorphic grade and deformational fabrics. It can be discontinuously traced from the northern NDB, and is gently folded into an anticlinorium that plunges to the southeast beneath the median strip of post-Carboniferous rocks. South of the median strip, steeply dipping shear zones separate the presumed "lower plate" Peters Ck Greenstone units from the presumed "upper plate" Booloumba Beds. There is some support for the argument that these steep faults are limb segments of a folded low angle fault, such as the Mt Mia Fault. Mesoscopic fold geometry in the Booloumba Beds and the orientation of the dominant cleavage in both the Booloumba Beds and Peters Ck Greenstone are consistent such folding. However there are major problems with this model. The cleavage in the Booloumba Beds is a single generation slaty cleavage with no evidence of a refolding history. Unfolding such a low angle fault would produce subhorizontal bedding orientations in the upper plate of the fault, in contrast to all other known exposures above the Mt Mia Fault. We therefore regard the boundaries of the Peters Ck Greenstone as steeply dipping faults, perhaps extensional normal faults subsidiary to the Mt Mia Fault. The Claddagh Thrust forms the eastern margin of the foliated granodiorite bodies and remnants of their amphibolite facies aureole. It mainly dips shallowly to the west, although younger high-angle fault segments also occur. The fault boundary against the foliated granodiorite can be traced south to the median strip, although interrupted by a number of Late Triassic plutons. The lower boundary of Permian sediments with the underlying foliated granodiorite and Jimna Phyllite is subhorizontal. Bedding in the sediments is generally moderately steep, but poorly defined close to this boundary, so that the nature of the contact is unknown. These sediments are either part of the allochthonous sheet of the Claddagh thrust, or they form a separate imbricated sheet. The Booloumba Beds are in steep fault contact with strongly cleaved Permian rocks to the east, but show no overprinting crenulation fabric that could be correlated with this younger cleavage. This suggests that the fault separating the two units has considerable displacement that must have occurred after cleavage formation in the Permian rocks, but before intrusion of the Late Triassic granitoids. We correlate this fault with the Bracalba Fault, that forms the western boundary of the NDB further south. The eastern boundary of the serpentinite-matrix melange north of the median strip is a steep fault which is a likely further extension of the Bracalba Fault. REFERENCES HOLCOMBE, R.J. and LITTLE, T.A., 1993. Blueschists of the Rocksberg Greenstone and Kurwongbah Beds near Mt Mee, southeastern Qld: Protolith and subduction related structures. In FLOOD, P.G. and AITCHINSON, J.C., New England Orogen, eastern Australia, 537-544. LITTLE, T.A., HOLCOMBE, R.J, GIBSON, G.M., OFFLER, R , GANS, P.B. and McWILLIAMS, M.O, 1992. Exhumation of late Palaeozoic blueschists in Qld, Australia, by extensional faulting. Geology, 20: 231-234. LITTLE, T.A, HOLCOMBE, R.J. and SLIWA, R , 1993. Extensional exhumation of blueschist bearing serpentinite-matrix melange in the New England Orogen of southeastern Qld, Australia. Tectonics, 12: 536-549. MURRAY, C.G., McCLUNG, G.R. and WHITTAKER, W.G., 1979. Early Permian fossils from the Amamoor Beds and the age of the Booloumba Beds - a new stratigraphic unit of the North D'Aguilar Block. Queensland Government Mining Journal, 80: 71-78.

149

COMPARISON OF MICROSTRUCTURAL INDICATORS OF FLOW KINEMATICS IN VOLCANIC ROCKS AND DUCTILE SHEAR ZONES John V. Smith1 and Brian Marshall2,1Faculty of Resource Science and Management, University of New England-Northern Rivers, P.O. Box 157, Lismore N.S.W., 2480, 2 Department of Applied Geology, University of Technology, P.O. Box 123, Broadway N.S.W. 2007. Kinematic indicators in volcanic rocks have primarily been used to determine the flow direction. The techniques of analysis are similar to those for determining sense of shear in ductile shear zones (Vernon, 1987). Awareness of the limitations of simple shear as a model for shear zone kinematics has led to studies of a continuum of kinematic types including coaxial and non-coaxial flows. The results of these studies are only beginning to be applied to volcanic rocks. Interpretation of structures in volcanic rocks is assisted by knowledge of modern volcanic flow processes in contrast to metamorphic processes which can rarely be observed directly. On the basis of the geometry of volcanic rock bodies coaxial flow could be expected in a dome but other types of flows have been assumed to occur by simple shear or other highly non-coaxial flow kinematics. However, the tops of flows are characterized by steep to vertical foliations and upright folds attesting to longitudinal shortening during flow. Nearsymmetrical recumbent folds above the vent of one banded rhyolite flow indicate a gravityspreading process similar to that observed in some mountain belts. These coaxial flow features give way to gently dipping foliations and asymmetric folds deeper in the flows which indicate the role of non-coaxial flow in distributing lava away from a vent. Rock textures also provide evidence of a variety of kinematic types in volcanic flows. As for metamorphic rocks, a distinction can be made between spaced foliations and penetrative foliations. The most important of each type being glassy banding and crystal alignment respectively. These volcanic foliations are commonly described as flow banding or flow texture and the assumption often follows that the foliation represents the !flow plane'. However, a flow plane (in the sense of parallel displacement vectors) is only found in simple shear. A more appropriate approach is to attemt to relate volcanic textures to the strains which result from the displacement fields of volcanic flows. Banding forms by the stretching out of semi-solidified parts of the flow and is generally considered to form in the conduit or vent. Banding represents a very high strain in which the stretching direction has become almost parallel to the shearing direction, such that boudinage ^

. ^

^

^ 400Mm

_

.

I

150

^

P

lOOMm

^

^

of bands, rootless folds, and microfolds adjacent to phenocrysts and clasts are common features. There is a strong similarity in appearance and microstructure between mylonites and banded volcanic rocks. Determining the kinematics involved in the initiation of banding will require studies of incipient banding. In contrast, evidence of the kinematics involved in the initiation of crystal alignment is more readily available. For example, Vernon (1987) demonstrated that in a sample of obsidian, banding showed evidence of foliation-parallel shear that pre-dated folding but that crystals and vesicles become aligned in the axial planes of folds. This indicates that crystal alignment foliations can initiate in a plane of principal strain. None-the-less, as for metamorphic rocks, contention exists over the relative influence of flattening and shearing in the initiation and development of volcanic foliations. Further evidence of the formation of crystal alignment in a plane of principal strain comes from conjugate microshear zones imposed on planes of crystal alignment before solidification of the rocks. Such textures have been found in an andesite dyke (Smith et al., 1993), a dacite dome (Fig. a & b; Smith et al., in press a) and a basalt flow (Smith et al., in press b). In these rocks the plane of crystal alignment bisects (horizontal in Fig. a & b) the equally developed conjugate microshear zones (diagonal in Fig. a & b). The symmetry of this texture indicates that the plane of crystal alignment was parallel to principal strains, in the final stage of solidification. More significantly, the symmetry of the texture indicates that coaxial flow accompanied the last phase of solidification. Although coaxial flow had not previously been considered in the interpretation of volcanic textures, geometrical considerations discussed above, suggest that coaxial and non-coaxial domains coexist in natural volcanic flows. Study of the interactions of flow types requires evidence of timing such as syn-kinematic crystallization and/or overprinting of structures. One example of syn-kinematic crystallization in volcanic rocks comes from studies of crystal length and orientation of microlites related to microshear zones. In the texture of a dacite dome it was found that the shortest crystals were irregularly oriented but alignment of crystals became better with length and the longest crystals had a well defined parallel alignment. The swallow-tail terminations and box-sections indicate that these were quench crystals. If each crystal grew at a similar rate then the texture records syn-kinematic crystallization involving the physical rotation of crystals into alignment. This is consistent with the 'Marchian' model of foliation development in a plane of principal strain. Microlites within the microshear zones indicate an asymmetric relationship between crystal length and orientation consistent with the rotation of the plane of finite strain in a non-coaxial flow. In contrast, within the groundmass between microshear zones the relationship between length and orientation of crystals is symmetrical indicating coaxial flow. Detailed structural analysis of volcanic flow textures not only provides a better understanding of eruptive processes but can also enhance understanding of the significance of microstructures of ductile shear zones and other metamorphic rocks. References Smith J.V.,^Miyake Y. and Yamauchi S. 1993a. Flow direction and groundmass shear zones in dykes, Shimane Peninsula, Japan. Geol. Mag., 130, 117-120. Smith, J.V., Yamauchi, S. and Miyake, Y. (in press a). Microshear zones in a submarine dacite dome of southwest Japan. Bull. Vole. 55. Smith, J.V., Yamauchi, S. and Miyake, Y. (in press b). Coaxial progressive deformation textures in extrusive and shallow intrusive rocks, southwest Japan. J. Struct. Geol. Vernon. R.H. 1987. A microstructural indicator of shear sense in volcanic rocks and its relationship to porphyroblast rotation in metamorphic rocks. J. Geol., 95, 127-133.

151

THE STRUCTURAL EVOLUTION OF THE LAKE JULIUS AREA.

Itta Somaia and Gordon Lister Victorian Institute of Earth Sciences, Department of Earth Sciences, Monash University, Clayton 3168, Australia To the immediate west of Lake Julius, 80 kilometres north of Mount Isa in the Leichhardt River Fault Trough (LRFT), lies a pattern of north-east to north-west trending folds. The folds are disrupted by an array of faults resulting in a complex puzzle of reoriented folds and fault blocks in an area that has a typical north-south grain. The structural history of the field area is controversial since it has been previously interpreted in terms of thrusting by Bell (1983, 1992), and wrenching by Lister (1986). This study provides new maps of fault, bedding and cleavage form surfaces of the Lake Julius area (at 1:5 000 and 1:10 000 scales, Somaia 1993 unpublished). The detailed mapping has advanced the understanding of the structural history of the Lake Julius area. The stratigraphy of the Lake Julius area consists of low grade mid-Proterozoic, mid-crustal pelites, sandstones, dolomites, quartzites, and various types of conglomerates. The rock types are divisible into Myally subgroup, Bigie Formation, Quilalar Formation, Surprise Creek Formation and the Mount Isa Group. Stratigraphic relations between these units suggest that an open phase of folding or tilting may have occurred during the sedimentation prior to the onset of the Isan orogeny. Pre-main phase folding (Dj) is characterised by an east-west trending synform of a noncylindrical, open nature, associated axial planar slaty cleavage or weak intersection lineation. No corresponding antiform exists but this is a common characteristic of the LRFT (e.g., the Somaia Synform, Horses Head, Crystal Creek Synform; see Lister et al. 1992). The missing antiforms have been explained in terms of an imbricate thrust stack (Bell 1983). If this is the explanation subsequent overprinting has destroyed any direct evidence for thrust faults in the Lake Julius area. Main phase folds (D2) have a variety of trends; north-west, north-south, north-east. Within the Fj fold, Sj cleavage has been overprinted by a north-south crenulation cleavage associated with an open north-south trending synform. This has resulted in a basin-like structure attributed to D J - D J interference. Other D2 folds ranging from open, tight, isoclinal to disharmonic, have axial planar slaty, spaced, or fracture cleavages, or fold transecting cleavage.

152

Cleavage nature (i.e., slaty, spaced or crenulation cleavage) is not necessarily diagnostic of structural generations in these low grade rocks but is a useful characteristic for correlation. Crenulation cleavages are present in fold hinges of Surprise Creek pelites, traced into spaced cleavages in Quilalar sandstones and further into slaty cleavages in Myally rocks. The cleavage and fold natures are therefore dependent on the rock type, competency, grade and the amount of strain subjected to the area. The latest deformation is caused by a network of faults. These strike slip faults are mostly D3, however a few trend parallel to the fold (syn-D2) and later truncate the fold at a different location (post D2). The faults have either an easterly, north-west or north-east orientation. North-west faults tend to have a left lateral displacement, and north-east faults a right lateral motion. The fault zones are characterised by brecciated quartz and rock gouge. Dips on these faults are steep due to the steep plunges of the drag folds (syn D2-D3) which lie adjacent and within the fault zone. The network of slave strike slip faults originate from two master faults; the Quilalar and Paroo Faults. The strike slip network has distorted a series of originally north trending F2 folds causing them to rotate into adjacent structures, break and push these structures above and below the field area (June Hill Synform & Missing antiform. This results in a variety of F2 orientations. The Lake Julius Fault, a main slave off the Quilalar Fault may act as a transfer fault, accommodating movements along the Quilalar fault, Lake Julius area and the Crystal Creek block. Again transpressional forces at this end of the field area have caused syn- to post- D2 block rotations. The strike slip complex is thought to have been generated by easterly movement of the Lake Julius indenter towards the Quilalar Fault (Lister et al 1992) providing a mid crustal analogue to modern wrench terranes. Cited References Bell, T.H., 1983. Thrusting and duplex formation at Mount Isa, Queensland, Australia. Nature 304, 493 - 497. Lister, G.S., 1986. Transpressional strike slip faulting in the Mount Isa Inlier. BMR Research Newsletter, 1986. Lister, G.S, O'Dea, M, Somaia, I., 1992. The implications of large scale reorientation during wrenching in the Proterozoic Mount Isa terrane, Australia. Technical publication #2, Australian Crustal Research Centre (restricted circulation). Somaia, I., 1993. The Structural Evolution Of The Lake Julius Area. Unpublished thesis, Monash University

153

40Ar/39Ar AND K/Ar THERMOCHRONOLOGY OF THE PAPAROA METAMORPHIC CORE COMPLEX, SOUTH ISLAND, NEW ZEALAND T L Spell, I McDougall (Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia), and A J Tulloch (Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand) The Paparoa Range (South Island, New Zealand) is a metamorphic core complex formed during breakup of the Pacific margin of Gondwana in the late Cretaceous. Amphibolite facies para- and orthogneisses (Charleston Metamorphic Group), granitoids and anatectic granites comprise the lower plate or core of the complex. These lithologies are separated from lower greenschist facies upper plate greywackes and argillites (Greenland Group) and intrusive granites by shallow dipping mylonite zones (detachments) which indicate extensional deformation. At the southern (Pike) detachment the transition from lower plate (sillimanitebearing paragneisses) to upper plate rocks (mildly metamorphosed sedimentary rocks) occurs across <5 metres distance. K/Ar ages for muscovite, biotite and K-feldspar (recording cooling through the -400180 °C interval) from lower plate rocks (gneisses and Cretaceous granites) are generally in the range -95-105 Ma, although some variability exists between areas. Similar K/Ar ages were obtained from gneisses -20 km north of the Paparoa Range where there is a possible resurfacing of the detachment fault. In contrast to results from core rocks, Paleozoic granites which intrude the upper plate metasedimentary rocks (Barrytown and Meybille granites) indicate extended cooling histories (>100-150 Ma) beginning in the early Carboniferous to Penman and ending in the Cretaceous. 40Ar/39Ar dating of muscovite, biotite and K-feldspar from lower plate rocks yields relatively flat, nearly concordant age spectra with ages of -90105 Ma. The 40Ar/39Ar spectra from upper plate rocks are more discordant both internally and between minerals with differing closure temperatures. Multidomain modelling of argon diffusion from K-feldspars also indicates that rocks from the lower plate experienced fairly rapid cooling as diffusion domains with substantially differing closure temperatures yield similar ages. This is in contrast to a K-feldspar age spectrum from an upper plate granite (Barrytown granite) which indicates closure of different diffusion domains over an -100 Ma interval. All data indicate rapid cooling of core rocks relative to upper plate rocks. The Pororari Group is a subaerial succession of fanglomerates shed from the unroofing core complex over an -15 Ma interval beginning at -113 Ma. Granite clasts in the Pororari Group thus preserve information on the cooling history of the core complex for a substantial interval of time prior to that obtainable from presently exposed lower plate rocks. Initial K/Ar and 40Ar/39Ar dating of samples from the Pororari Group indicate cooling rates similar to those of presently exposed lower plate rocks, and suggest that relatively rapid unroofing of the core complex was continuous over at least a 5 Ma interval. Estimated cooling rates for lower plate rocks range from 15 to 75 °C Ma-1 and average -35 °C Ma-1 whereas those for upper plate rocks are in the range 5 to 0.5 °C Ma-1. Lower plate rocks cooled rapidly due to thermal relaxation following intrusion of granites and/or uplift and denudation during an interval around 100 Ma. The thick sequence (up to 5 km) of coarse subaerial sediments preserved in the Pororari Group suggests a substantial amount of uplift and erosion occurred over a period of several million years. These data provide a temporal and spatial link between continental extension as recorded in the Paparoa core complex and inception of seafloor spreading in the Tasman Sea (oldest crust -85 Ma) and subsequent rifting of New Zealand from Australia and Antarctica.

154

EXTENSION OF THE WOODROFFE THRUST, MUSGRAVE BLOCK, INTO WESTERN AUSTRALIA AJ.Stewart, Australian Geological Survey Organisation, G.P.O.Box 378, Canberra, ACT, 2601 The Woodroffe Thrust in the Musgrave Block, central Australia, is part of a major system of south-dipping thrust faults that penetrate the continental crust on the southern margin of the Amadeus Basin (Fig. 1). The thrust dips south at 20-30 and separates high-grade metamorphic rocks in the south from moderate-grade metamorphics in the north. It has been interpreted as the root zone of the Petermann Ranges Nappe at the southwest margin of the Amadeus Basin (Fig. 1). In the Amata and Kulgera areas, northward movement on the thrust is well documented (Collerson & others, 1972: Journal of the Geological Society of Australia, 18, 379-393; Edgoose & others, 1993: Kulgera 1:250 000 Geological Map and Explanatory Notes SG/53-5 [second edition], Northern Territory Department of Mines & Energy). Before the mapping described here, the most westerly known exposure of the Woodroffe Thrust had been about 30 km west of Amata (Fig. 1). The thrust was inferred from aeromagnetic contour data to exist farther west (Forman, 1972: Petermann Ranges 1:250 000 Geological Sheet and Explanatory Notes, SG/52-7, BMR; Forman & Shaw, 1973: BMR Bulletin 144; D'Addario & others, 1976: Geology of the Northern Territory, 1:2 500 000 geological map, BMR). Pharaoh (1990: BMR Record 1990/5, PI. 2) interpreted from reprocessed aeromagnetic data an extension of the thrust into Western Australia, but no field exposures of it were known there. The considerable tectonic significance of the Woodroffe Thrust, and the question of its existence in Western Australia, prompted the Australian Geological Survey Organisation to conduct detailed mapping in the Bates 1:100 000 Sheet area (Fig. 1) as part of the Musgrave National Geoscience Mapping Accord project. The results included the discovery and delineation of a major east-striking mylonite zone precisely at the predicted position of the Woodroffe Thrust. The mylonite zone is about 1 km wide. It separates granulite-facies garnet-hornblende granite, patchily recrystallized to eclogite facies (Clarke & others, 1993: AGSO Research Newsletter, 18, 6-7), thrust over amphibolite-facies schistose biotite granite to the north. The mylonite zone is derived from the schistose granite, and displays a transition in rock type, as follows: • along the northern and southern margins of the mylonite zone, the schistose granite is sliced by numerous anastomosing mylonite shear bands; these are a few centimetres thick and 10-20 cm apart in the north, a few metres thick and several metres apart in the south; • near the centre of the zone, fine-grained schistose friable mylonite with small feldspar augen is intensely foliated and lineated; and • in the centre of the mylonite zone, .the most intensely deformed rock comprises either thin gently south-dipping alternating layers of black aphanitic ultramylonite and pale mylonitic schistose granite, or gently folded clasts of mylonitic granite in ultramylonite; steeply dipping extensional shear bands of mylonite cross-cut the gently-dipping mylonite and schistose granite.

155

The mylonite zone is cut by a north-striking cross or tear-fault which also has mylonite along it. The mylonite is similar in grade and thickness to that along the east-west fault, implying that the two faults formed at the same time. Foliation trends throughout the southern and northern terranes are at large angles, and hence are unrelated to, the east-west mylonite zone. The lineation in the mylonite zone is almost everywhere subhorizontal to gently west-plunging, and indicates an east-west movement direction. Kinematic indicators were observed at only four locations; two indicate movement of the top block to the west, and two indicate movement of the top block to the northeast. The scarcity of shear indicators suggests that the fabric is not markedly asymmetrical, because large strain produced almost parallel shearing and flattening planes. The east-west lineation and hence movement direction in the mylonite zone are nearly parallel to the general strike of the mylonite zone, and differ from the northward overthrusting recorded on the Woodroffe Thrust in the Amata and Kulgera areas. Neverthless, the low dip of the Bates mylonite zone and the thickness of the mylonite exposures resemble features of the Woodroffe Thrust, and support the interpretation that this thrust extends into Western Australia. Mapping of the hitherto unmapped isolated exposures north of the Mann Range in the southwest Petermann Ranges 1:250 000 Sheet area should locate the Woodroffe Thrust there also.

AMADEUS BASIN

oL

100 km

— 26'

/

,

- t

Geological boundary, mapped —?— Thrust fault, aeromagnetic (queried where inferred) interpretation (5) Fault, aeromagnetic Geological boundary, interpretation (1) aeromagnetic interpretation — y — Thrust fault, aeromagnetic Fault, mapped interpretation (6) f Thrust fault, mapped l 1:100 000 sheet area I PR —f-- Thrust fault, inferred (2) Petermann Ranges 1:250 000 sheet area —r - Thrust fault, inferred (3) Fault, aeromagnetic interpretation (4) Bates

1

Fig. 1. Structure of the Musgrave Block lAJ. Stewart (AGSO: unpublished data); 2Forman (1972); 3D'Addario & others (1976); 4>5Pharaoh (1990); 6Edgoose & others (1993); see text for abbreviated bibliographic references.

156

MULTIPLE FOLDING IN GRANULITES OF THE MUSGRAVE BLOCK, WESTERN AUSTRALIA A.J. Stewart, Australian Geological Survey, GPOBox378, Canberra, ACT\ 2601 The Mount Aloysius massif (Fig. 1) comprises a sequence of felsic, intermediate, and mafic granulites interlayered on scales ranging from centimetres to hundreds of metres and intruded by small bodies of granite, syenite, and innumerable mafic sills and dykes. The massif preserves four episodes of folding, the major one forming an upright steeply plunging F 2 antiform later bent by F3 to an arcuate shape. Thin SI layering is imparted to the rocks by changes in mineral proportions and grain size, and is of metamorphic differentiation origin. On the other hand, compositional variations on the hundreds of metres scale are probably original sedimentary or volcanic rock bodies. The metamorphic layering is broadly parallel to these macroscopic layers, and may have been bedding in rocks of sedimentary composition.

Fig. 1. Geological map of Mount Aloysius massif. Clarke & others (1992: AGSO Research Newsletter 17, 6-8 and 18, 15) recognized seven episodes of deformation D 1-7 in the Champ de Mars - Hinckley Range area, 30 km east of Mount Aloysius. The events recognized in the Mount Aloysius massif include an additional episode of mesoscopic folding between D 3 and D 4 of Clarke & others (1992), denoted as D 3 5 . Deformations D 4 . 7 have not been recognized at Mount Aloysius. D x formed the granulitic S x layering, which normally has a massive granuloblasts texture. Scattered F x folds affect S1 layering and so formed after D2 began, and finished after S x layering had been completed. The folds lack axial-plane foliation, and are preserved in the hinge regions of large-scale F 2 folds. In places, F x folds are transected by S 2 . D 2 formed large upright F 2 folds in the east, and, to the west, a reclined antiform-synform pair. The closed map pattern of the lower granulite units outlines the antiform (Fig. 1).

157

Axial-plane foliation S 2 is everywhere steep. S 2 dips at a lower angle than S1 layering on the eastern side of the closure, and a parasitic F 2 fold on the western side verges east; these observations indicate that the closure is an antiform. The axial planes of the reclined folds dip southwest in the south of the massif, and are bent around large-scale F 3 axes to a northwest and eventual north dip in the north of the massif (Fig. 1). Mesoscopic F 2 folds are parasitic on the limbs of large-scale F 2 folds. In places, F 2 folds are transected by S 3 foliation, which cuts undeviated through both F 2 limbs. L^ also is folded by F 3 D 3 formed the major upright F 3 folds in the centre and northwest (Fig. 1). Interference with the reclined F 2 antiform bent it into its present arcuate shape. Mesoscopic F 3 folds are found mostly in the northwest. Plunges are steep, reflecting the pre-existing steep attitude of layering resulting from D 2 . S 3 foliation is steep, cross-cuts Sx layering, and in places transects ¥ 1 and F 2 folds. D 3 5 folds are mesoscopic only, and of restricted extent. Minor mylonites are present. They are high-strain zones a few metres wide of granulitefacies fine-grained gneiss or augen gneiss with extreme flattening and elongation of quartz to ribbons, and formed syn-D2, syn-D3, and post-D3. The overall sequence of structural events in the Mount Aloysius massif is set out below: Event

Age (Ma)

Mylonitization

Mafic intrusion

Post-D35 mylonite Type C dykes clOOCP

D35 Mafic intrusion

10542

D3 Mafic intrusion

D2

11891

Felsic intrusion Dl

Volcanism, sedimentation

This report

15641

F3.5 upright, gentle to open folds, NNE trend Type B dykes near end of D3 F3 upright, gentle to tight folds, NW to WNW plunge; syn-D3 mylonites Type A sills

Aphanitic to porphyritic, unmetamorphosed, NE to NNE strike NNE-striking S3.5 axial plane foliation Moderately metamorphosed, NW strike S3 axial-plane foliation strikes SE to E, cataclastic to mylonitic, partial melt Strongly metamorphosed, subparallel to and folded with Si S2 axial-plane foliation, granoblastic

F2 upright to reclined, close to isoclinal, NNW-trending folds; syn/post-D2 mylonites Granite and syenite bosses Fl reclined, close to Si layering, granuloblasts, isoclinal folds, no axialwith parallel foliation in south plane foliation Protolith of felsic, mafic, calcareous granulites, quartzite

1

Clarke & others (1992)

Type C dykes

Type B dykes D3

Type A dykes

D2

Charnockite Dl

Protolith of paragneisses

Rb-Sr whole rock, Gray (1978: Journal of the Geological Society of Australia, 25, 403-414); 2 Rb-Sr whole rock, A. Camacho, personal communication in Clarke & others (1992); 3 U-Pb on zircons, S-S. Sun, personal communication in Clarke & others (1992).

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EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF FLUID HETEROGENEITY UPON THE MOTION OF A RIGID PORPHYROBLAST ANALOGUE DURING BULK INHOMOGENEOUS SHORTENING. Lachlan K. Stewart Department of Geology, James Cook University, Townsville, Qld 4811, Australia The motion of porphyroblasts and porphyroclasts contained within deforming rock masses is currently an issue of interest. Inclusion trails preserved within porphyroblasts may provide valuable information regarding the deformation history of the rock they are contained within. There are two principal models of the deformation process. One model proposes that deforming rocks behave as homogeneous fluids, the vorticity within the fluid due to applied shear stresses causing rigid objects to rotate. The other model proposes that heterogeneities within the rock, at the porphyroblast/porphyroclast scale, result in deformation partitioning and little or no rotation of the rigid objects. These two models give rise to conflicting interpretations of the geometry of inclusion trails contained within porphyroblasts. Several experiments were performed in an apparatus that allowed the real time observation of porphyroblast analogue motion during deformation. The method of deformation of the fluid was by bulk inhomogeneous shortening. In one group of experiments the time behaviour of rigid, ellipsoidal, porphyroblast analogues embedded within a homogeneous fluid were recorded. Their behaviour was compared with the motion displayed when a less viscous fluid heterogeneity was introduced into the fluid. Surrounding the porphyroblast analogues with less viscous fluid resulted in dramatically altered behaviour, despite computerised analysis of the deformations which showed similar bulk and regional deformation characteristics for the two fluid systems. Initially aligned axes of rigid ellipsoids were rotated in a homogeneous fluid with the same sense as the regional average vorticity and the adjacent strain marker. However, three different rigid body behaviours were observed for initially aligned ellipsoids in the system containing fluid heterogeneities: 1) Opposite-sense rotation to that of the regional average vorticity and strain marker; 2) Negligible rotation of the objects; 3) Rotation in the same sense as regional bulk flow and strain marker. These three behaviours are descriptively the same as the full range of behaviours possible for ellipsoidal rigid objects contained within a homogeneous fluid undergoing combinations of pure and simple shear. However, two of these behaviours (non-rotation and back-rotation) are produced by different processes to those affecting such behaviour in a homogeneous fluid. The experiments displayed that significant differences can occur in rigid body behaviour between similarly deformed homogeneous and heterogeneous systems. The results provide evidence which suggests that homogeneous fluid flow theory is not suitable for establishing the past behaviour of porphyroblasts/porphyroclasts in systems where fluid anisotropics were present at the rigid body scale.

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STRUCTURE AND METAMORPHISM IN THE KOUMAC-BALADE HIGH PRESSURE SCHIST BELT, NORTHERN NEW CALEDONIA Caroline Streets, Tim Rawling and Gordon Lister Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Victoria, 3168 Australia This study was initiated in order to determine whether or not the high pressure rocks now exposed in northeast New Caledonia have been formed during a history which is solely the result of collisional orogenesis. The existing literature suggests that extensional tectonics is not involved in the uplift and exposure of these rocks, nor is extension cited as indirectly responsible for their preservation because of the rapidity of the processes involved. New Caledonia is located in the southwest Pacific Ocean on the western boundary of a series of opposite facing subduction zones. In the late Jurassic to early Cretaceous the island was believed to be a non volcanic outer arc of an island arc setting. Today it is part of a complex tectonic zone in the SW Pacific. The Tertiary high pressure schist belt of northeastern New Caledonia comprises sedimentary and igneous rocks which have been metamorphosed to eclogite and blueschist grade. The high grade metamorphics in the schist belt are thought to have formed in an obduction setting, where oceanic lithosphere has been obducted onto older crust. The presence of the overlying slab could easily result in the necessary increase in pressure and temperature to approximately 15 kbars and 550° C, initiating eclogite metamorphism. However, previous workers have described the belt as involving a sequence of SW-dipping thrusts. Metamorphic grade increases across a sequence of isograds towards the NE, with eclogite facies achieved at the deepest structural levels. We have demonstrated that high pressure metamorphism (related specifically to the formation of eclogites) occurred early in the tectonic history. Eclogite boudins are preserved as relicts in gneisses that have undergone thorough and pervasive transposition. One problem with the previous interpretation of the belt as described above is that it implies a normal metamorphic gradient, whereas a crustal section through an orogenic belt in which high pressure rocks resulted solely as the result of collision would involve deeper level rocks thrust up and over shallower level rocks. A general increase in metamorphic grade is to be expected as one moves deeper into the orogen, but a crustal section with the structure as previously described for New Caledonia is more easily reconciled with extension tectonics than collisional processes. This study comprises a preliminary synthesis of results obtained in a number of detailed studies of "key" localities, and as the result of a transect across northern New Caledonia. In general the line of transect follows the road from Koumac to Ouegoa, and then in the gorge of the Amoss river, with additional results from "key" localities in the Mayavetch River. The identification of fault controlled grade changes and major post-peak metamorphic shear zones indicates the region is more structurally complex than previously interpreted. Isograds that have been mapped as describing the variation of the grade of peak metamorphism reflect more the pattern of the later tectonic evolution of belt (e.g., jumps in grade occur across major structures formed late in the tectonic history). Previous workers have located a series of isograds in which the rocks increase in grade across the schist belt from southwest to northeast with the highest grade rocks outcropping on the northeast coast. This study has identified the same metamorphic isograds. However detailed

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2 structural mapping indicates that these isograds are commonly bound by north trending faults in the region between Koumac and Ouegoa. Thus the identification of these changes in grade as true metamorphic isograds must be re-examined and the region reconstructed to assess the tectonic relevance of the metamorphic isograds. The previous interpretation of the existence of shallow dipping isograds across the belt has not been confirmed. It is very difficult to imagine a pattern of isograds which are supposed to be related solely to the variation of palaeo "pressure", in particular since this interpretation has not taken account of the later effects of the major deformations which are implied by the existence of intensely developed later fabrics. The original metamorphic assemblages may once have been related to shallow-dipping isograds, but later tectonism defined the dominant features now visible in the New Caledonia orogenic belt. It seems remarkable to assume that the sporadic preservation of original parageneses throughout such a gneiss terrain would allow the conclusion that the isograds are shallow lying, whereas the gneissic foliations in which relicts of the high pressure assemblages are found are generally steep. Previous workers have described a series of imbricate thrusts at the southern boundary to the belt. This thrusting is thought to have resulted in the development of a melange zone consisting of ophiolites and metamorphic rocks. There is some confusion as to the origin of the melange zone, however. This study suggests that the presence of melange throughout the schist belt is not clearly the result of thrusting, but may be related to a more complicated mechanism involving extensional processes. Evidence for four distinct deformation events has been identified in the high grade rocks of the high pressure schist belt (eclogite-blueschist grade); D D . Structurally overlying these is an intensely deformed region of lower grade metamorphics (lower blueschist-greenschist grade) which show evidence for at least six deformation events. D D in the high pressure rocks appear to correlate with D -D in the lower grade rocks. Peak metamorphic conditions resulting in eclogite metamorphism occurred early in the deformation history, during D D was accompanied by retrogression to blueschist facies and development of a regionally extensive, strongly differentiated cleavage and a mineral lineation. Evidence indicates that S was steep before the onset of D , during which it was transposed and horizontally reoriented. D is associated with folding in both the high and lower grade rocks. However zones of intense shearing and the tectonic emplacement of ultramafics is only seen within the high grade rocks. Retrogression is also associated with this deformation in which there is a transition between blueschist and greenschist pressures and temperatures and late porphyroblastic mineral growth in the higher grade rocks. D is associated with localised shearing, folding and the development of an S crenulation cleavage in both the high grade and low grade regions. D is a folding event which produced a localised S crenulation cleavage and clearly overprints S . D is characterised by the formation of kinkbands which intersect the S surface forming L^ intersection lineations. r

4

r

x

v

4

4

2

2

3

3

4

4

5

5

4

6

2

It is supposed that the lower grade rocks were significandy higher in the obducted slab and did not undergo eclogite metamorphism. Altered ultramafics from the obducted slab were juxtaposed against the high pressure rocks as the result of the operation of normal sense ductile shear zones and brittle faults formed during D . This was a period of horizontal extension of the New Caledonia crust. Injection of serpentinite from the overlying slab into boudin necks and faults in the underlying metamorphic rocks took place during this event. This period of intense shearing resulted in fluid infiltration and subsequent retrogression to lower blueschist and greenschist facies. Porphyroblastic growth of late minerals such as euhedral garnets and decussate mica, glaucophane and actinolite took place early in this event, but intense deformation and retrogression continued in localized zones thereafter. 3

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Lattice preferred orientation and shape fabric analysis of deformed quartzites. Holger Stunitz. Geologisches Institut, Univ. Basel, Bernoullistr.32, 4056 Basel, Switzerland The relationships between the shape fabrics and lattice preferred orientation (l.p.o.) of dynamically recrystallized materials may provide additional information about the deformation history, kinematics and type of deformation of a deformed rock body. The Suretta Nappe (the uppermost nappe of the penninic nappe pile in the Central Alps, Switzerland) was chosen as a suitable example for such a study. The Suretta Nappe consists of a basement gneiss core and an autochthonous cover, consisting of conglomerate gneisses, marbles and a single, almost continuous layer of quartzites. The whole nappe underwent two phases of folding, a first phase, which is related to the nappe emplacement, and a second backfolding event, which affects only the frontal and central parts of the nappe. The axial planes of both fold phases are gently dipping approximately to the NE. The regionally limited overprint of the first phase structures by the D2 folding allows the separation of l.p.o. and shape fabric development during the two phases of folding. Deformation took place under lower greenschist facies conditions. The l.p.o. of the quartzites were measured with the texture goniometer, using the a-, mand r+z-reflections. From these measurements, the ODF was calculated with the program package MENTEX by H. Schaeben. From the ODF, c-axis pole figures were recalculated. The shape fabrics were obtained with the program packages SURFOR and PAROR by R. Heilbronner from digitized images taken from polished thin sections in reflected light using white light interference phase contrast to enhance the grain boundaries. The F2-folds have their fold axes parallel to the stretching lineation. The stretching is indicated by elongation of conglomerate pebbles in the conglomerate gneisses immediately underlying the quartzites. The distribution of l.p.o. is homogeneous throughout the folds, indicating that the development of l.p.o. is later than the active folding of the layers. All pole figures of a- and c-axes are consistent with a constrictional finite strain. The shape fabrics determined on two mutually perpendicular sections normal and parallel to the fold axis (normal to the axial plane) indicate long axes of grains inclined to the axial plane and internally asymmetric patterns in both sections. Such asymmetric patterns yield a triclinic symmetry for the average shape of the quartz grains. The ratio of long and short axes of grains plot in the flattening field of a Flinn-type fabric diagramm, contrasting with constrictional l.p.o. patterns. However, shape fabrics are known to be more sensitive to late increments of deformation than l.p.o. Thus, it appears that late stages of deformation are of a flattening-type and have not reset the l.p.o. The axial ratios of pebbles in the conglomerate gneisses below the fold samples plot in the constrictional field of the Flinn diagram, so that the finite strain is indeed of a constrictional type. The discrepancy between quartz grain shapes and pebble grain shapes may be explained by the fact that shape fabrics in quartzites only record last stages of deformation. The fact that 162

l.p.o. and pebble shapes yield similar results could point to fact that the memory of quartz l.p.o. might be longer than usually assumed. The l.p.o. patterns are quite different in different parts of the nappe. This is explained by a heterogeneous overprint during D2 deformation. However, the quartz l.p.o. and shape fabrics are a very sensitive and reliable tool for the detection of D2 deformation, even where mesoscopic features of D2 are absent. D2 deformation is mostly of a coaxial type, indicated by the pole figures, which usually do not show any asymmetry with respect to the axial planes of the folds. The regional pattern of D2 deformation is interpreted as late, heterogeneous extension of the folds in a northeasterly - southwesterly direction.

Upper Austroalpine sediments Lower Austroalpine sediments Austroalpine basement

ophiolites

IH'ilil Avers Bundnerschiefer

V^Tm allochthonous Middle-Penninic V'dvA sediments

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j | f | f | Mesozoic cover of basement nappes ~J N-Penninic Flysch (Tertiary) I/!'!;'/1 Insubric mylonite belt

Gradational change of syn-tectonic recrystallization from slaty cleavage to schistosity Shigeyuki Suzuki and Yasuko Nishidono Department of Earth Sciences, Faculty of Science, Okayama 700,Okayama Japan University; 3-1-1 Tsushimanaka, Gradational change from slaty cleavages to schistosities is observed in the Sotiel-Calanas area, the Iberian Pyrite Belt (South Portuguese Zone), southwestern Iberian Peninsular. The area is composed of Upper Devonian to Lower Carboniferous successions which are lithologically divided into the Phyllite-Quartzite Group (PQ), the Volcanic-Siliceous Complex (VS) and the Culm Group (Culm) in ascending order. They form one anticlinolium, and slaty cleavages (Si) and schistosities (Si) are subparallel to the axial plane. The specimens investigated were systematically collected from each horizon in the same anticlinolium. Microscopic observation (modal analysis of Si texture) and X-ray analysis (illite "crystallinity" and polymorphism) were carried out.

a b e Fig.l Horizontal change of Si texture and illite "crystallinity". All specimens are fine grained black pelitic rocks, containing detrital grains around 15% of whole rocks. a ; Components of Si texture. Matrices consist of clay and recrystallized quartz, b ; Thickness of zonal layers of secondary platy minerals, c ; "Crystallinity" of illite (*) and chlorite (x).

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Pelitic rocks investigated are composed of detrital grains, matrices, dusty seams and secondary platy minerals. Si texture is defined by parallel alignment of dusty seams and secondary platy minerals. The secondary platy minerals tend to grow larger and together, to make zonal domains toward the PQ. The clay matrices are changed to recrystallized quartzs towards the PQ. The grain sizes of the recrystallized quartz are less than

in the Culm, around

in

the VS and about 5 to lO^im in the PQ. The domain of matrixes (the microlithon area) has a tendency to decrease in lower horizon. The dusty seams also decrease in lower horizon and tend to wispy and thin in the PQ. Pressure solution contacts of detrital grains are observed in each horizon. The dusty seams are in contact with the pressure solution contacts. Therefore "solution ways" develop into the dusty seams, and then the seams decreasing the "solution ways" develop into the zonal layers of phyllosilicates. The change from the slaty cleavage texture to the schistosity texture is gradational as shown in Fig. la,b and Fig. 2. Illite and chlorite "crystallinity" (Kubler index) was measured and almost all values were found to be below 0,25, supporting that they belong to the Prehnite-Pumpellyite and Lower Greenschist facies (Munha, 1983). Kisch (1981) obtained similar illite "crystallinity" values from the Prehnite-Pumpellyite and Pumpellyite-Actinolite facies. There is a tendency for illite "crystallinity" values to decrease from the Culm to the VS, but the values are disturbed in the upper part of the PQ (Fig. lc). Results of the ratio of illite polymorphs 2Mi/lMd measurements show that the illites of the VS and the PQ went to completion in the lMd->2Mi transformation. The above results leads to the following discussion and conclusions. The development of Si texture which reflects deformation are contemporaneous to the metamorphism. Pressure solution - diffusion transfer - syntectonic recrystallization occur during the development of Si texture. lMd->2Mi transformation occurs during the recrystallization process. The horizontal change of the S l texture may be mainly caused by differences of confining pressure. In the upper horizon , deformation started from shallower position. On the other hand, lower successions underwent diagenesis for a longer time than the upper successions. This study provides the example of the gradational change from slaty cleavage to schistosity.

Fig.2 Schematic development of SI texture. d ; the Culm, e ; the VS, f ; the PQ, g ; the Pulo do Lobo Group. This Group is fault contacted with the PQ, is estimated to be lower Devonian in age (Oliveira, 1990). 1 ; detrital grain, 2 ; clay matrix, 3 ; dusty seam, 4 ; secondary platy mineral, 5 ; pressure fringe, 6 ; recrystallized quartz.

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EARTH DEFORMATION - THE ROLE FOR THE OCEAN DRILLING PROGRAM (ODP) P. A. Symonds, Australian Geological Survey Organisation, Canberra The Ocean Drilling Program (ODP) is arguably the world's premier Earth Science research program and one of the most successful multi-national science research efforts ever conducted Along with its predecessor the Deep Sea Drilling Program (DSDP), which commenced in the mid-1960fs, it is also one of the longest lasting scientific research endeavours- ODP explores the structure and history of the Earth's ocean floor, and the evolution of the ocean's chemistry and the Earth's climate. Drilling within ODP is conducted in all but the shallowest water depths (> 150 m) and includes the deep ocean basins, as well as continental margins and submerged continental fragments. One of ODPs main themes concerns large-scale structural features and the processes of deformation that cause them, including those currently active at plate boundaries and those recorded in structures and sediments of former plate boundaries. These diverse topics are united by the nature and mechanisms of deformation, the architecture of structures produced by deformation, the kinematic history and, ultimately, the driving forces that have produced this deformation. Tectonic processes transgress the boundary between land and marine domains. Processes forming volcanic arcs or collisional systems commonly begin below water and end up on land. Others, such as continental rifting and passive margin formation begin subaerially and normally end up in the marine realm. Accordingly, ODP has an important role to play in comparison of land and marine geology, and provides the means of studying the full width of deformation systems that extend offshore. The drill can give us a knowledge of active processes and the conditions of the materials undergoing these processes. It can also give us a one-dimensional "ground-truth" of the rocks involved in these processes, or in inactive regions, of the vertical sequence of rock types. The drill is unlikely itself, however, to contribute much to our understanding of a tectonic process without a firm placement of the drill results in a regional three-dimensional context. Clearly, in the study of earth deformation, drillingrepresentsonly one tool in an arsenal of geological and geophysical techniques that can be brought to bear upon a problem, rather than the end in itself. ODP has done much to improve our knowledge of convergent margins. In particular forearc drilling has led to significant progress in understanding the processes associated with accretion and subduction of oceanic basement and overlying sediments. A substantial effort will be needed to bring our knowledge of divergent and transform margins to the same level. In the last year ODP has tackled some of the problems of so-called volcanic and non-volcanic rifted margins in the north Atlantic. Future legs will extend this work, but will also examine collisional systems in the Mediterranean, and transform margins off Africa along the Equatorial Atlanticfracturezone. There have already been six ODP drilling legs off Australia and the Australian Antarctic Territory, and thirty six Australian scientists have been directly involved, as well as about two hundred andfiftyinternational scientists. There are currently highly ranked proposals in the ODP system to examine deformation processes in the Australian region - collision and foreland basin development along the northern Australian margin and rift processes off southern Australia. It is possible for Australian, land-based structural geologists to be involved in any of the future drilling legs through Australia's membership of ODP via a consortium arrangement with Canada.

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THE HALLS CREEK FAULT ZONE: REPEATED REACTIVATION OF A MAJOR NORTH AUSTRALIAN TECTONIC BOUNDARY I. M. Tyler 1. T. J. Griffinl, R. W. Page2 and R. D. Shaw2, 1 Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth 6004, Western Australia; 2Australian Geological Survey Organisation, GPO Box 378, Canberra 2601, ACT. The Halls Creek Fault Zone forms a major tectonic feature in northern Australia, consisting of a zone, 850 km long and 120 km wide, of north-northeasterly trending, sub-parallel, partly linked faults. The fault zone is bounded by the Greenvale Fault to the west and the Halls Creek Fault itself to the east. Its continuation to the north into the Northern Territory is marked by several faults including the Victoria River, Cockatoo, Chalanyi and Turner faults, and possibly the Giants Reef Fault. Faulting is exposed mainly within Early Proterozoic (c. 1910 Ma - 1810 Ma) low- to highgrade metasedimentary rocks, and mafic and felsic intrusive and extrusive igneous rocks of the Lamboo Complex and the Litchfield Province. It also affects the overlying Early Proterozoic Kimberley Basin, the Middle Proterozoic Birrindudu and Victoria River basins, Late Proterozoic glacigene rocks, and the Palaeozoic Ord Basin. The faults divide the Lamboo Complex into three distinct zones. The eastern zone is characterized by the low- to medium-grade Halls Creek Group (c. 1870 Ma). It is separated from its medium- to high-grade equivalents in the central zone, the Tickalara Metamorphics, together with felsic igneous rocks of the c. 1840 Ma Koongie Park Formation, by a combination of the Osmond Fault, a segment of the Halls Creek Fault, and by the Angelo Fault. The western zone is separated from the central zone by the Ramsay Range Fault, the Springvale Fault, and by the northern part of the Halls Creek Fault. The western zone is made up of low- to high-grade metasedimentary rocks of the Marboo Formation (> 1880 Ma), and a major felsic igneous suite consisting of the c. 1860 - 1850 Ma Whitewater Volcanics, and the associated granitoid rocks of the Bow batholith. As such it is similar to, and may correlate with, the Hooper Complex in the King Leopold Orogen. It is difficult to relate geological events across the boundary between the central and western zones, and the boundary can be regarded as separating two distinct tectonostratigraphic terranes that are "stitched" by the intrusion of c. 1810 Ma granitoids. The original nature of this terrane boundary is uncertain, being obscured by later igneous intrusion and deformation. A complex history of deformation, metamorphism, and igneous intrusion, ranging in age from c. 1870 Ma to c. 1780 Ma, can be recognized within the Lamboo Complex that can be related to collisional events involving the Kimberley "Craton", the Sturt Block and continental crust that now underlies the Canning Basin. Subsequent reactivations of the terrane boundary appear to have been intracratonic, as a response to events at remote plate boundaries. The first reactivation was as part of an extensional fault system during initiation of the c. 1800 Ma Kimberley Basin. The fault system became inactive, and was buried during deposition of the upper part of the Kimberley Basin sequence. A striking feature of the Halls Creek Orogen is the pattern of steeply dipping, northnortheasterly trending sinistral faults and east-northeasterly trending dextral faults. This pattern can be interpreted as synthetic and antithetic faults developed during an overall sinistral strikeslip movement controlled at depth in the crust by the pre-existing, northeasterly trending terrane boundary. Both zone boundaries in the Lamboo Complex are offset sinistrally, and movement took place after the deposition of the Kimberley Basin and the intrusion of pegmatite

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dykes at c. 1740 Ma, and may have occurred synchronously with the deposition of the Carr Boyd Group, currently inferred to have taken place between 1250 Ma and 1100 Ma. This deformation (regionally D4/5) was initially ductile and took place under low- to mediumgrade metamorphic conditions. Mylonites were developed in both Halls Creek Group and Kimberley Group rocks along the Halls Creek Fault and its associated structures, with oblique upright, tight folds and an axial planar crenulation cleavage developed in adjacent rocks. Close to the faults the folds become subparallel to the them, suggesting a transpressive regime, possibly developed synchronously with the intracratonic, c. 1.3 to 1.0 Ga Yampi Orogeny in the King Leopold Orogen. Gold mineralization in the Halls Creek Orogen appears to be controlled initially by this fault system. Younger, brittle reactivations of the fault zones, again as sinistral strike-slip structures, this time at low- to very low-grades (D6 and D7), affect Late Proterozoic and Cambrian rocks, and are characterized by extensive, locally gold-bearing quartz veins, and the development of kinkbands and associated crenulation cleavage. K - Ar ages of c. 560 Ma and c. 500 Ma from the King Leopold Orogen may date these deformations, which bracket a period of extensional tectonism during which the Antrim Plateau Volcanics were erupted, and the Ord Basin was initiated. Faulting and large-scale open folding of Devonian rocks may represent further reactivation due to the effects of the Late Devonian to Carboniferous (c. 300 - 370 Ma) Ma Alice Springs Orogeny.

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ANALYSIS AND FORWARD MODELLING OF THE STRUCTURAL HISTORY OF THE MR CURLY AREA, ORD RIVER, EAST KIMBERLEY, WA Rick Valenta, Nick Oliver, Jim Tan, and Mark Jessell, V.I.E.P.S .Department of Earth Sciences, Monash University, Clayton, Victoria 3168 The aims of structural analysis are to determine the geometry of rock units, the structural history which produced this geometry, and ultimately to develop an understanding of the geological processes responsible for this geometry and history. While it is often possible to formulate detailed geometric interpretations of a given area, hypothesized structural histories often take the form of schematic diagrams or frustratingly crumpled butcher's paper, which cannot be tested in detail against field observations. In this paper we outline the application of a structural forward modelling package to analysis of the structural history of an area of polydeformation. In this package (see Jessell and Valenta, this volume) an initial stratigraphy is subjected to a series of structural events in an effort to match observed geometries and overprinting relationships. The structure of the Mr Curly area has been modelled using a series of fold and fault events, with geological boundaries and foliations acting as markers of successive deformation stages. At all stages of the modelling it is possible to extract the orientations of bedding, foliations, and intersection lineations from any point in the three dimensional volume under consideration. Using this approach, the hypothesized history can be rigorously tested against the positions and orientations of mapped geological boundaries, the orientations of structural elements, and local timing relationships. This type of exercise often reveals important complications which do not become apparent in less rigorous analyses.

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DIAPIRIC ASCENT OF MAGMAS Roberto F. Weinberg and Yuri Podladchikov - Institute of Earth Sciences, Uppsala University, S-752 36 Uppsala, Sweden 1

2

1 now at: Research School of Earth Sciences, ANU, Canberra, ACT0200, Australia 2 now at: Institute of Earth Sciences, Vrije Universiteit, Amsterdam, The Netherlands The equation governing the velocity of viscous spheres or cylinders rising slowly through power-law fluids is written here in the form of the Hadamard-Rybczynski equation. This equation (called here the extended Hadamard-Rybczynski equation) allows calculation of the ascent rates of diapirs by direct application of rheological parameters of the wall-rocks, and does not require fixing the value of the strain rate in order to calculate the effective viscosity of the wall-rock. Instead, effective viscosity (defined as the viscosity of the Newtonianfluidthat substitutes the power-lawfluidwithout changing the sphere's velocity) is calculated using the buoyancy stress of the diapir. This equation has three correction factors dependent on the value of the power-law exponent n, and these factors were determined from data reported in the literature offluidmechanics and chemical engineering on the motion of solid spheres and drops (e.g. Nakano and Tien 1968, and Crochet et al. 1984), and from 2D finite-difference calculations for buoyant viscous circular cylinders. The derived correction factors for both cylinders and spheres are all close to unity, indicating that our definition of effective viscosity is appropriate. The velocity equation is numerically integrated for the ascent of the Tara granodiorite in Australia, studied by Miller et al. (1988). The spherical diapir rises through granitic rocks with temperature and strain-rate dependent viscosity and the rheology of Westerly granite (as determined by Hansen and Carter 1982). Using the diapir buoyancy and initial depth (from Miller et al., 1988), the diapir starts rising at a calculated velocity of 40 m/y, one order of magnitude faster than previously predicted. As it rises and cools, velocity decreases due to gradually colder and more viscous crust. Two calculations are carried out, one where thermal softening of the wall-rock is considered (following the procedure developed by Daly and Raefsky 1985) and one without such effects. Thefinaldepth of emplacement of the diapirs in the two calculations, defined as the depth where the diapir's temperature reaches the magma solidus, differ by only 300m and are both around 15km from the surface. A systematic study of the depth of solidification of the Tara diapir as a function of the controling parameters shows that theflow-lawparameters of the wall-rock are by far the most important. If a diapir is still buoyant after magma solidification is completed, ascent continues, but at slower rates than before, due to increased density and possibly due to increased viscosity. Calculations were made for solid-state power-law diapir of two extreme cases: that of a diapir stiffer than the wall rock and that of a diapir softer than the wall rock. The results suggest that a rise of a couple of kilometers take place in 10 Ma. This solid-state ascent would cause several of the observed features around natural plutons such as the superposition of solid-state foliation on magmatic foliation and the superposition of late cold foliation on early hot metamorphic aureole. In conclusion, the difficulties previous authors had in demonstrating that diapirs can rise through the crust was due to the absence of a definition of the effective viscosity of power-law rocks around a buoyant body. When using effective viscosity calculated based on the diapir's buoyancy stress, diapiric velocities deep in the crust may be considerably higher than previously predicted (10 to 100 m/year), allowing diapirs to ascend to middle or upper crust before solidification. Relatively small

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variations in the values of the flow law parameters of the wall-rocks, cause large variations in the depth of diapir solidification. Improved prediction of the diapiric ascent of magmas relies upon increased understanding and more precise determination of the values of theological parameters of crustal rocks. References Crochet, M. J., A. R. Davies, and K. Walters, Numerical simulation of Non-Newtonian Flow, Rheology Series, pp. 352, Elsevier, New York, 1984. Daly, S. F., and A. Raefsky, On the penetration of a hot diapir through a strongly temperaturedependent viscosity medium, Geophys. J. R. astr. Soc., 83, 657-681,1985. Hansen, F. D., and N. L. Carter, Creep of selected crustal rocks at a 1000 MPa, EOS, Trans. Am. Geophys. Union, 63,437,1982. Miller, C. F., M. E. Watson, and T. M. Harrison, Perspective on the source, segregation and transport of granitoid magmas, Trans. Royal Soc. Edinburgh, 79y 135-156,1988. Nakano, Y., and C. Tien, Creeping flow of power-law fluid over Newtonian fluid sphere, AJ. Ch.E.J., 74,145-151,1968.

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CATACLASIS OF SCHIST AND FLUID FLOW IN AN EXHUMED CENOZOIC FAULT ZONE S. R. White, University ofOtago, P.O. Box 56, Dunedin, New Zealand The Moonlight Fault (MF) in the Haast Pass area of northwest Otago, southern New Zealand (Figure 1), is the northern-most of three semi-continuous structures which together define the Moonlight Tectonic Zone (MTZ). The area is underlain by the Haast Schist Group which comprises psammitic, pelitic and metabasic rocks which were metamorphosed in the mid to late Mesozoic and subsequently deformed by regional scale Cenozoic folding and faulting related to the inception of the present-day plate boundary (the Alpine Fault) through the South Island of New Zealand. The section of the MF described here is subvertical to steeply NW dipping and strikes 050°060° parallel to, and approximately 25 km SE of, the Alpine Fault in an area of greenschist facies to garnet grade schist. Amphibolite facies schist is exposed closer to the Alpine Fault where late Cenozoic to Recent uplift during the Kaikoura orogeny is estimated at 10-21 km in the last 3-7 Ma. Models of total uplift and uplift rate profiles predict uplift rates of 8-10 mm per year for the Mt Cook area north of Haast Pass. The area around the MF has probably experienced uplift rates of 2-4 mm per year over the last 3 Ma bringing mid to upper greenschist facies rocks to their present surface exposure.

The fault rocks developed along the MF reflect the textural character and metamorphic grade of their parent rocks and several lines of evidence point to differential displacement in the order of a few kilometres. Segregated biotite and garnet grade quartzofeldspathic (QF) schist exposed along most of the SE side of the structure is, in places, faulted against chlorite to biotite grade QF schist and (?)garnet grade pelitic schist to the NW. A 2-5 m thick, hard, blocky cataclasite forms the SE wall of the fault along most of its length and grades over a 1-3 m interval of

172

protocataclasite into largely intact schist. Although crushing and fracturing of the quartz and feldspar components attest to the predominantly brittle mode of deformation, the cataclasite is strongly foliated in contrast to cataclasites developed from more-or-less isotropic crystalline rocks elsewhere which typically have a random cataclastic fabric. It has been shown experimentally that mica-rich schists (of which the Haast Schist is an example) are susceptible to deformation by dislocation glide and kinking of the micas. Brittle faulting of strongly foliated mica schist may, therefore, result in a foliated cataclasite as the micas are rotated and preferentially accommodate slip. Lower grade schist, and pelitic schist with greater than 70% micas, are exposed on the NW side of the fault and were deformed at higher crustal levels. These rocks are significantly weaker than the segregated QF schist on the SE side of the fault and deformation extends over a wider zone of up to 20 m. Intense shearing has been accommodated by slip on the schistosity planes, and tight kink folding close to the fault. Kink folds become increasingly open further from the fault and intact schist resumes at distances greater than 20 m. CC>2-rich fluids related to the carbonatitic Alpine dike swarm, which this section of the MF intersects, have infiltrated the fault zone at depths of 6-8 km. Fluid flow appears to have been focussed about a left stepping jog in the fault and fault rocks over a 10 km along-strike interval have been carbonated and albitised with concomitant removal of micas and quartz. Stable isotope and trace element data confirm the carbonatitic affiliation of the fluids and the alteration closely resembles fenitisation of country rock observed from the area immediately to the north. The alteration is almost exclusively confined to cataclasites in the SE wall of the fault, except where it is most intense in the area of the jog, and is in part responsible for the hard, flinty character of the cataclasite. Close to the fault, much of the original cataclastic texture has been overprinted by the fenitisation. Beyond 1-2 m from the fault surface, alteration intensity decreases becoming patchy and preserving remnants of the cataclastic texture. The fenitisation nowhere extends more than 10 m from the fault and in most cases its effects are not noticeable beyond 3-5 m. Individual lamprophyres of the swarm are cut and deformed by the MF at at least two localities and elsewhere are hosted in E-W trending faults approximately coeval with the MF. These observations, combined with the findings above, suggest a two-phase movement history followed by exhumation of the MF as summarised in the following table.

Timing

Process

Product/Effect

pre-25 Ma

Moonlight Fault inception

generation of cataclasite

24.6-20.02 Ma

intrusion of dyke swarm and carbonatitic fenitising fluids

fenitisation of cataclasites at depths of 6-8 km

post 20 Ma

reactivation of MF

uplift of the SE side of the fault

beginning at 3-7 Ma and continuing to present-day

uplift on the Alpine Fault

exhumation of the MF to its present level of exposure

173

TECTONIC EVOLUTION OF THE LONGMEN MOUNTAINS AND WESTERN SICHUAN BASIN, CHINA Christopher J.L. Wilson , Shefa Chen , Brenton Worley , Dennis Arne , Zhili Luo , Shugen Liu School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia. Department of Earth Sciences, Dalhousie University, Halifax B3H 315, Afova Scotia Chengdu Institute of Technology, Chengdu, Sichuan 610059, Peoples Republic of China The Longmen Mountains, China, form the NE margin of the Tibetan Plateau within which four tectonic units can be distinguished (Fig. 1): (1) the Songpan-Ganzi Fold Belt, a turbidite dominated sedimentary basin that closed at the end of the T3 and overlies Proterozoic basement; (2) a Thrust-Nappe Belt composed of several SE-directed thrusts and associated nappes; (3) the Western Sichuan Foreland Basin, initiated at the beginning of the T3 , with its NW portion being incorporated into the over-riding Thrust-Nappe Belt; and (4) the Yangtze Craton, with its NW part occupied by the Sichuan Basin. The Songpan-Ganzi Fold Belt, a region of considerable topographic relief, contains evidence for appreciable crustal thickening of a Cambrian to Late Triassic sequence, with a minimum of 50% shortening developed during the first of three phases of the Indosinian Orogeny that is at the end of the Late Triassic T3 ), about 200 Ma ago. The large-scale, Indosinian structural geometry of the SongpanGanzi Fold Belt is dominated by refolded isoclinal folds, the intrusion of granites, and an en echelon arrangement of major zones of higher D2/D3 shear strain. Barrovian-type medium-pressure greenschist to amphibolite-grade metamorphic complexes are associated with the high strain zones which also uplift Proterozoic basement complexes. Detailed observations in one of these high strain zones, the Wenchuan-Mouwen Shear Zone, has revealed the following structural-metamorphic relationships: (i) Macroscopic, isoclinal Fi folds developed during Di as a result of NE-SW directed compression, (ii) Continued NE-SW compression during D2 resulted in the build-up of differential strain between the SongpanGanzi Fold Belt and the Yangtze Craton. This was accommodated by the development of the non-coaxial shear zones along earlier syn-sedimentary faults. Kyanite-grade peak metamorphic conditions were reached syn-post D2. (iii) D3 strain is localised into mylonitic reverse shear bands which are associated with retrogressed metamorphic mineral assemblages and indicate appreciable uplift of the metamorphic complexes during D3. A second phase of exhumation of the Songpan-Ganzi Fold Belt is seen as the development of brittle faults that are superimposed on the metamorphic sequence. The Thrust-Nappe Belt is associated with four regionally significant faults that are all NWdipping listric reverse faults, and each fault is usually accompanied by a splay of several second-order faults and duplex structures. The Thrust-Nappe Belt is divided into three major nappes, named after the faults along which they move: the Yingxiu-Beichuan, GuanxianAnxian and SE Marginal Nappes. The Yingxiu-Beichuan Nappe is composed of the Jiudingshan, Tangwangzhai and "Pengxian-Guanxian Basement Complex" Sub-Nappes. Deformation intensity of the nappes decreases from NW to SE, as thrusting movements propagated eastwards, that is, from hinterland to foreland in a "Piggy-Back" style. The Thrustnappe Belt was developed by the SE-directed compression from the Songpan-Ganzi Fold Belt and can be related to three evolutionary stages: an extensional stage (Devonian-Late Triassic T3 ), a structural inversion stage (Late Triassic T3 ), and a continued thrusting stage (JurassicQuaternary). The Western Foreland Basin is situated southeast of the Yingxiu-Beichuan and the Xiangshui Faults and was initiated by interaction of the Songpan-Ganzi Fold Belt and the Yangtze Craton at the beginning of the T3 . This initial Indosinian movement on the major thrusts produced crustal loading and erosion of the thrust sheets with deposition in the basin and a redistribution of load. As the neighbouring thrust sheets in the Longmen Mountains were gradually propagated from NW to SE, the foreland basinfill was incorporated into the Thrust-Nappe Belt, 1

1

2

3

1

3

1

2

3

2

3

2

2

3

3

174

and the depocentres migrated eastwards. The tectono-depositional history of the foreland basin can be divided into three stages: (1) an initial stage (T^-T^x ) characterised by fine-grained sediments deposited in a transitional marine-terrestrial environment, responding to SE-directed rapid thrusting on the Yingxiu-Beichuan Fault; (2) a fault-dominated stage (T3 -Kij) marked by rhythmic thrusting on the Guanxian-Anxian Fault and deposition of terrestrial molasse-type sediments; and (3) a fold-dominated stage (K2j-Q) during Himalayan movements, characterised by a decrease of the basin extent and deposition rate. Preliminary data from fission track thermochronology of detrital apatite grains in outcropping Triassic and Jurassic sedimentary rocks from the Thrust-Nappe Belt and the Western Sichuan Foreland Basin indicate a regional cooling episode at -50 Ma that is tentatively related to the collision of India with Asia and the initiation of Himalayan uplift. Apatite fission track data from Indosinian granite samples records rapid cooling during the late Tertiary in response to more recent phases of Himalayan exhumation and information regarding early Tertiary cooling has been lost. Two major uplift events occur at 50 and 25 Ma along faults in the Thrust-Nappe Belt and length and single grain ages suggest that between 1.5 and 10 km of the overlying section was removed from the highest parts of the mountains in the last 50 million years. However, estimated maximum palaeotemperatures and uplift rates vary between the tectonic zones and can be attributed to differential reactivation along the faults. 2

x

QLFB

3

Fig. 1: Locality diagram: SGFB - the SongpanGanzi Fold Belt; YZC - the Yangtze Craton; NCB - the North China Block, (b) 1. ophiolite melange zones; 2. reverse fault; 3. strike-slip fault; 4. the Songpan-Ganzi Fold Belt (SGFB); 5. the Thrust-Nappe Belt (TNB) in the Longmen Mountains; 6. the Western Sichuan Foreland Basin (WSFB); 7. the Sichuan Basin (SB);. CB - the Changdu Block; QLFB - the Qinling Fold Belt.

, 300 km ,

175

COMPARISON BETWEEN EXPERIMENT AND COMPUTER MODELLING OF DEFORMED ICE C.J.L. Wilson 1. Y. Zhang2 and K. Stliwe3 1 School of Earth Sciences, University of Melbourne, Victoria, Australia. ^ CSIRO, Division of Exploration and Mining, P.O. Box 3000, Glen Waverley, Victoria. 3 Department of Geology and Geophysics, University ofAdelaide, South Australia, Australia. Using polycrystalline ice in see-through microscope experiments and computer models, it is possible to model some of the processes that may be inferred in deformed and partially melted rocks. The model has correctly reproduced the deformational and microstructural features caused by glide on (0001) in aggregates of polycrystalline ice. This success is particularly prominent for those ice grains with a lattice orientation suitable for hard or easy glide or kinking, and where there is a sub-horizontal c-axis and a larger grain size. A limitation may be that the model cannot explicitly simulate recrystallization and grain boundary migration, which are two other important processes operating jointly with glide in experimentally deformed ice. However, through the use of the models, it is possible to show how kinematic factors can control the processes of recrystallization. The localization of recrystallization in the polycrystalline ice aggregate is determined by the stress and strain variations between neighbouring grains. Similarly, by deforming polycrystalline ice in a pure shear and plane strain environment at -1°C, it was possible to identify melting that was initiated at local sites subparallel to a set of slip-lines. Three processes may be responsible for this phenomenon: (1) dramatic deviatoric pressure increase in the partially melted portions of the sample. (2) heterogeneous temperature distribution in the sample, caused by shear heating. (3) lowering of the melting point, as a consequence of the Gibbs energy changes, associated with increases in the lattice defect density. We evaluate these possible reasons for the melting phenomenon in the ice by comparing detailed microstructural observations on the distribution of melt in space and time with the distribution of shear strain in space and time in a numerical analogue and the experiment. The localized strain variation across grain boundaries, particularly at triple point junctions, is bound to create deviatoric stress increases in the vicinity of these boundaries, which may lead to heating during the deformation and consequently melting. It is shown that these deviatoric stresses are unlikely to be sufficient for pressure melting to occur, but that both shear heating and melt point lowering due to internal storage of strain energy are important processes in the dynamic evolution of the sample. By observing the microstructure of crystal aggregates, crystal habit, and internal heterogeneity, it is possible to analyse the growth or post-growth histories of single and multiple stages in the crystallization of the melt, and relate this to the kinematic environment. The texture of the rock that develops through crystallization is dependent on the presence or absence of a deviatoric stress and can vary from (1) coarse crystal aggregates involving static growth where there is no significant macroscopic deviatoric stress at the time of crystallization, the free-face growth model to (2) polycrystalline aggregates involving static growth which develop from a mixture of skeletal crystals and fluid in a stressed environment. In the latter situation, the crystals and fluid coexist until the macroscopic stress can be transmitted between impinging crystals and, at this stage, fast dynamic recrystallization and grain boundary migration occurs. This recrystallization has been termed the contact growth model.

176

THE REGIONAL RELATIONSHIP BETWEEN VEINING AND MINERALISATION IN THE KANMANTOO TROUGH, SOUTH AUSTRALIA

Colin N Winson Robert G Wiltshire, Joanne K Janz* and Colin G Gatehouse* Gartrell School of Mining, Metallurgy and Applied Geology, University of South Australia THE LEVELS, SA, 5109 The South Australian Department ofMines and Energy, Geological Survey of South Australia, PO Box 151, EASTWOOD, SA, 5063 The regional relationship between vein systems, folds and mineralisation in the Kanmantoo Trough - Foldbelt (figure 1) is under investigation by researchers at the University of SA, the SADME and Aberfoyle Resources. Structural relations are being studied across ten traverses through the Kanmanntoo Trough (figure 1). The Trough is a narrow arcuate belt of multiply deformed, Cambrian metasediments on the eastern margin of the Adelaide Geosyncline. The Kanmantoo Group comprise immature flysch like clastic sediments and minor carbonates, intruded locally by granites. These periods of deformation have been recognised (e.g. Offlier & Fleming 1968, Manktelow 1979). Di resulted in tight folds and a slaty cleavage. F2 folds exhibit a similar geometry to Fi folds. F3 folds plunge mainly to the northwest. Vein sets in the Kanmantoo exhibit a varitey of orientations and forms, however most quartz veins strike NW or NNW, dip steeply west, transect Si - F2 folds, and are subparallel to F3 axial planes. Veins generally comprise white recrystallised massive quartz, of widths from 0.5 cm to 1 metre but often less than 10 cm. They commonly maintain strike lengths of 2 to 5 metres. Locally fibrous vein material is preserved. A progressive sequence of dilation is suggested from syn S2 to post F3, as veins exhibit variable geometrical relations to bedding and the dominant foliation. Little country rock is incorporated into veins. Locally on vein margins wall rock alteration has occurred. In addition to veins subparallel to F3 axial planes two sets of pre - F3 veins comprising a conjugate set are strongly developed across traverses 4-7, possibly related to local extension due to the curvature of the trough. Spatially areas of lead, zinc, copper and pyrite-pyrrholite mineralisation in the trough, occur close to traverses 6-8 (Both 1990). This may be of structural significance in that it is a region where curvature of the Kanmantoo Trough is at its greatest. Late stage mineralisation may postdate the curvature or be enhanced by dilation in the region. Earlier syngenetic mineralisation exhibits a stratigraphic control. f

REFERENCES Both, R.A., 1990. Kanmantoo Trough - geology and mineral deposits. IN Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed. F.E. Hughes), pp. 11951203. AIMM. Manktelow, N.S., 1979. The structure and metamorphism of the southern Adelaide fold belt. PhD thesis (unpublished). The University of Adelaide. Offlier, R. and Fleming, P.D., 1968. A synthesis of folding and metamorphism in the Mt. Lofty Ranges, South Australia. J. Geol. Soc. Aust, 15: 245-266.

177

Figure 1 Location of the Kanmantoo Trough, displaying bedding and foliation trends and traverses 1-10

Traverse 1

Angaston Traverse 2 Trend lines

ADELAIDE

178

CONTACT METAMORPHISM AROUND THE ST AWELL GRANITE, WESTERN VICTORIA GuQwei Xp, Roger Powell, Christopher J. L. Wilson, The School of Earth Sciences, Melbourne University, Parkville, Victoria, 3052, Australia The 20 km long (east-west) and 13 km wide (north-south) Early Devonian (396±5 Ma) Stawell granite intrudes the adjoining portions of the Lachlan Fold Belt and Adelaide Fold Belt. The local stratigraphy of the Stawell area includes a sequence of deformed Late Proterozoic basic volcanics and the Cambrian sandstone and shale of the St Arnaud Beds, which are regionally metamorphosed to greenschist facies. The contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite are divided into three zones: the low-grade, the medium-grade and the highgrade zones. The markers dividing the three zones are the formation of, contact metamorphosed cordierite porphyroblasts, K-feldspar, and corundum- and spinel-bearing assemblages, respectively. Detailed penological study shows consistency of element distributions implying that equilibrium is widely maintained in the rocks, although equilibrium volumes are generally small (mm-scale) and considerable mineral chemical variations exist between adjacent domains. Re-equilibration between biotite inclusions in cordierite porphyroblasts occurs where the volume difference of the two minerals is large. The metamorphic mineral assemblages developed in the rocks are generally of high variance (KFMASH variance > 2). Consequently, the mineral chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the role of the former factor being more important in most cases. The chemographic relations of mineral assemblages in low- and medium- to highgrade zones are modelled in the system KFMASH, and are represented in compatibility diagrams projected from biotite, quartz and H2O, and biotite, K-feldspar and H2O, respectively. The compatibility diagrams projected from biotite, K-feldspar and H2O have the advantage of showing the chemographic relations of both quartz-bearing and quartzabsent assemblages and the breakdown of muscovite-bearing assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid which combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraint from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks: chlorite + muscovite + quartz = biotite + cordierite + fluid chlorite + cordierite + quartz = garnet + fluid biotite + muscovite + quartz = cordierite + K-feldspar + fluid muscovite + quartz = cordierite + K-feldspar + andalusite + fluid (or KASH muscovite + quartz = K-feldspar + andalusite + fluid) muscovite + cordierite = biotite + K-feldspar + andalusite + fluid biotite + muscovite + andalusite = K-feldspar + spinel + fluid biotite + andalusite = cordierite + K-feldspar + spinel + fluid biotite + muscovite = K-feldspar + spinel + corundum + fluid muscovite = K-feldspar + andalusite + spinel + corundum + fluid (or KASH muscovite = K-feldspar + corundum + fluid) biotite + cordierite + quartz = garnet + K-feldspar + fluid The combined KFASH and KMASH grid can provide constraints on reaction coefficients. The calculated grid gives an estimate of pressure at below 2 kbar, and temperature in the range of 500°C to 650°C for the contact metamorphism around the Stawell granite.

179

DUCTILE DEFORMATION OF THE RYOKE METAMORPHIC ROCKS ALONG THE MEDIAN TECTONIC LINE, CENTRAL JAPAN Hiroshi YAMAMOTO. Institute of Earth Sciences, Faculty of Science, Kagoshima University, 1-21-35 Korimoto, 890 Kagoshima, Japan Hirokazu TAB ATA, Geological Institute, Faculty of Science, University of Tokyo, 7-3-1 Hongo, 113 Tokyo, Japan Metasedimentary rocks of the Ryoke Belt occur in the southwestern part of the Akaishi Range near the Median Tectonic Line (MTL). The rocks suffered amphibolite facies metamorphism and variable degrees of ductile shear deformation. A mineral assemblage observed in the pelitic rocks (andalusite, sillimanite, cordierite, K-feldspar, biotite, muscovite and quartz) indicates low-pressure/high-temperature metamorphic conditions of 600-700°C at 2-4 kbar. Deformation microstructures of banded quartz schist (metachert), which consists of quartzose and micaceous layers, are analyzed. Garnet appears in metachert as an accessory mineral. Some of the garnet grains with elongated shapes are fractured and separated into several fragments which are aligned parallel to the stretching lineation. The fractures do not penetrate the quartzose matrix. The mineralogy of the fracture fillings (mostly quartz, rarely K-feldspar and biotite) is similar to that of the rock matrix. These observations suggests that the garnet was cracked by stress caused by ductile flow of the matrix. Compositional zoning of a fractured garnet, together with the geothermometry of garnet fragments and biotite fillings, indicates that the fracturing occurred during cooling at a temperature of 550°C or lower. The metachert has shape fabric defined by elongated quartz grains and crystallographic fabric defined by the alingnment of c-axis of recrystallized quartz grains. These fabrics found from the rim of the tightly folded metachert (ca. 10 cm half wavelength) are similar to those from the hinge. Despite that these quartz fabrics indicate homogeneous deformation of the quartzose layer throughout the fold, mica flakes in the hinges show disharmonic kink and folds whereas those in the rims form trains of lens-shaped mica 'fish'. Thus, it is suggested that the orientation of mica flakes before deformation is a significant factor in the microstructural development of quartz-mica rocks.

180

THE THERMAL AND DEFORMATIONAL EVOLUTION OF THE VORTEX, MOUNT ISA, QUEENSLAND. David Young: Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia. Significant compressional deformation occurred in the Mount Isa Inlier in the Middle Proterozoic, between 1620 and 1520 Ma. Shortening was accompanied by low pressure metamorphism, varying from lower greenschist to upper amphibolite facies throughout the Inlier. Between one and three generations of macroscopic structures are present in many areas, indicating deformation was heterogeneous. Debate concerns the question of whether these were distinct events separated by considerable time breaks, or whether they were increments of a progressive shortening event. The field area under examination, informally referred to as the "Vortex is located approximately 15 km southwest of Mount Isa. The main lithologies present are quartz-rich rocks (quartzites, feldspathic quartzites, quartzofeldspathic gneisses and quartz/feldspar/mica schists) of the Mount Guide Quartzite and the May Downs Gneiss (lower Haslingden Group). Metagranite of the Sybella Batholith surrounds the metasedimentary units exposed in the centre of the area (Figure 1). Two fold and fabric associations have been identified in the study area. A large-scale, domal Fi antiform forms the dominant structure in the area, and plunges to the south and northeast (Figure 1). The axial planar foliation to Fj folds (Si) is characterized by a preferred orientation of mica. The Sybella Granite is distinctly deformed by this event. A second generation of structures near-coaxially overprints the Fi antiform, forming complex fold superposition patterns on the western limb. S\ mica fabrics are crenulated in this zone and a coarse S2 fracture cleavage occurs in quartz-rich rocks. The textures at the micro-scale show the evolution of heat and deformation. Sj fabrics were subject to static recrystallization after their development, and little of their syndeformational morphology is preserved. This recrystallization was associated with syn- to post-S 1 upper amphibolite facies metamorphism, which is also manifest in localized partial melting in quartzo-feldspathic rocks, and the growth of rare sillimanite. In areas where overprinting S2 fabrics are developed, dynamic recovery and recrystallization microstructures are superimposed upon the earlier, statically recrystallized textures. Temperatures were therefore also high enough during the D2 deformation for these processes to operate efficiently. The data indicate the two structural generations developed under elevated temperature conditions. This suggests either a polyphase deformation event during a single metamorphic pulse, or if the two deformations were separated by significant time breaks, multiple metamorphic episodes.

181

|v%]

Eastern Creek Volcanics

V dolerite / amphibolite

| • ] Mount Guide Quartzite

[+ ]

fine-grained granodiorite

May Downs Gneiss

| • |

Sybella Granite

S 0 ; S 0 form surface

f

f

F

f

S, ; S, form surfacc (interpreted)

; S2 form surface

I told axis ; F 2 fold axis

Li mineral lineation

creeks

182

track

Deformation and kinematics in the Pilbara granitoid-greenstone terrain, evidence of multiphase deformation events. Tanja E. Zegers & Stan H. White (Department of Geology, Utrecht University, The Netherlands) The Pilbara craton in NW Australia is a mid to early Archaean (2.8- 3.6 Ga) granitoidgreenstone terrain. It is characterised by bulbous granitoids surrounded by greenstone belts (see figure 1). The Archaean supracrustal stratigraphy can roughly be divided in the Warrawoona Group (3470-3450 Ma), consisting of volcanics and minor sediments, unconformably overlain by the Gorge Creek Group and Whim Creek Group. In previous years contrasting models have been developed for the pre-Gorge Creek group structural and tectonic history of the central part of the Pilbara craton. These models include solid state diapiric rise of granitoids into greenstones (Hickman 1983,Theyssier & Collins 1990), based on the Warrawoona Belt, and horizontal compressional tectonics (Boulter et.al. 1987), based on part of the Western Shaw Belt. A regional kinematic study,which also includes Msc and undergraduate mapping projects, is currently being undertaken, concentrating on the Eastern Pilbara. The initial field results and their consequences for kinematic, structural and tectonic models will be discussed. On structural grounds the Pilbara granite-greenstone terrain can be divided in 3 domains separated by NNE trending structures (figure 1). In the Central domain early structures are best preserved. The Western and to a lesser extent the Eastern Domain show more pronounced reworking by later structures. The structural evolution of the Pilbara is the result of a complex interplay of extensional, compressional and strike slip deformation, generally inhomogeneously concentrated in shearzones. The deformation varies both temporaly and spacially from belt to belt. Temporal correlation of specific events is difficult because of stratigraphic variations,, especially in the Warrawoona Group. However coarse temporal subdivisions can be made by relating structural events to the intrusion of the granitoids and to deposition of the upper parts of the greenstone sequence, chiefly the Gorge Creek and Whim Creek group. There is evidence for repeated episodes of crustal thickening and extension intersperced with granitic intrusions. In some instances strike-slip tectonics can generate local extension or compression; further complicating the overal structural scenario.

The assistance of the following Utrecht students and staff is acknowledged: Corine Davids, Maike Knoop, Armelle Kloppenburg, Martin de Keijzer, Bart Willegers, Paul Dirks and Wout Nijman. In addition the Schiirmannfund is thanked for financial support.

183

Pilbara granite greenstone terrain

LEGEND

GREENSTONE BELTS granitoids greenstones

00

interleaved granite & greenstone proterozoic phanerozoic

Figure 1

1. Marble Bar 2. Gorge Creek 3. Warrawoona 4. Lalla Rookh 5. Western Shaw 6. Coongan

COMPUTER SIMULATION OF SINGLE LAYER BUCKLING AND ITS ASSOCIATED CLEAVAGE DEVELOPMENT

Y. ZHANG,

B.E. HOBBS

and A. ORD

CSIRO Division of Exploration & Mining, Private Bag, Wembley, WA 6014

Single layer buckling has been numerically modelled for both elastic-viscous and elasticplastic materials using an explicit finite difference computer code FLAC. The model considers two types of starting geometry of single-layer assemblies (a competent layer embedded in less competent matrix), which involve single layers initially containing a series of periodic small perturbations and a single isolated perturbation. The assemblies was deformed under pure shearing and the buckling development of the layers was examined throughout whole deformation histories. The results show that the buckling behaviour of the layer/matrix assemblies is dominantly dependent upon the distribution of initial perturbations in the layers, and the competency contrast between the layer and the matrix and the geometry of initial perturbations are two important factors which co-control the final geometry of folds. For the single layers initially with a series of periodic small perturbations, the current buckling result generally agrees well with the classic dominant wavelength selection theory (e.g. Biot 1959, Ramberg 1961, Sherwin & Chappie 1968). The wavelength of folds is controlled by the competency contrast when the layer thickness is fixed. The current results also explicitly demonstrate the "explosive" manner of fold amplification. In particular, this study reveals that the timing of "explosive" amplification stages during buckling development is related to the competency contrast (the larger the contrast, the earlier the "explosive" amplification). Furthermore, it is shown that layer-parallel shortening in earlier stages could be followed by some later layer-parallel lengthening. The timing of the change-over points between the two stages also depends on competency contrast. For the layers containing single isolated initial perturbations with a finite amplitude, the geometry of the initial perturbation and the competency contrast are both important in influencing buckling. When the scale (wavelength) of the initial perturbation is roughly within a range sustainable for the competency contrast, the initial perturbation can simply grow, subjected to a certain influence of the competency contrast, and the geometry (e.g. symmetry) of the initial perturbations can control the geometry of the final folds. This is basically consistent with the prediction of the previous models focusing on the buckling development of single isolated initial perturbations (e.g. Abbassi & Mancktelow 1990,1992). If the scale of the initial

185

perturbation is far over the buckling ability allowed by the competency contrast, however, the simple growth of the initial perturbation is only possible for early deformation stages. Then the perturbation splits into two or more secondary perturbations, dominantly determined by the competency contrast. These secondary perturbations could all grow into finite folds or, in the situation of strain localisation, only some of them can do so. The perturbation generation and fold propagation in the rest part of the layer are totally controlled by the competency contrast. This buckling-amplification feature of single isolated perturbations predicts an important style of natural folding and probably explains why natural fold trains are usually very irregular. With the assumption that cleavage develops normal to the shortening axis, the cleavage traces in the matrix are analysed. Their orientations show a regular variation with respect to the fold axial plane in a single fold system, ranging from parallel to normal and depending on the location relative to the buckled layer. This feature may tip that we should take some real care when identifying tectonic events according to cleavage orientations in folded regions.

References Abbassi, M. R. & Mancktelow, N. S. 1990. The effect of initial perturbation shape and symmetry on fold development. /. Struct. Geol. 12, 273-282. Abbassi, M. R. & Mancktelow, N. S. 1992. Single layer folding in non-liner materials—I. Experimental study of fold development from an isolated initial perturbation. J. Struct. Geol. 14, 85-104. Biot, M. A. 1959. On the instability of folding deformation of a layered viscoelastic medium in compression. J. Appl. Mech. 26, 393-400. Ramberg, H. 1961. Contact strain and folding instability of a multilayered body under compression. Geol. Rundsch. 51, 405-439. Sherwin, J. A. & Chappie, W. M. 1968. Wavelengths of single layer folds: a comparison between theory and observation. Am. J. Sci. 266, 167-179.

186

INDEX OF AUTHORS In this index, each paper has a 2-part code such as:- Korsch RJ_ -5(4b). The first part of this code is the page number where the abstract is reproduced. A second part, the 2-character symbol in brackets, shows where the paper appears in the conference programme. The number indicates the day (1 = Mon through to 5 = Fri); the letter 'a' indicates an oral presentation in the morning, 'b' an oral in the afternoon, and 'p' a poster presentation. Adams G^, AmeD AslundT Baillie PW Baldwin S Batt GE Beekman F BeggG Belkabir A BellTH Berger A„ Berry RF„ Bishop P_ Blewett R_ Bons PD„ BothRA_„ BraunJ Brown AC Byerlee J Camacho A„ Carr SD Carson L Cartwright I„ Chen S Clark IF Clemens JD„

Cloetingh SAPL„ .5(4b) „26(3p) Cooper IB 27(lp) Cox SF 28(2p) Cox SJD ,30(2p) Crowhurst P_ „32(2b) Davis BK »34(2p) Ding P Dirks PHGM. „36(5b), 64(lp), 144(4a) Dresen G 38(2p) Durney DW_ 56(1 a) „38(2p) Eckhart M_ „39(2p) Ehlers K„ „38(2p) Evans B „40(2b) Fanning CM_ „42(lb) Farrell T „44(5a), 57(3p), Fergusson CL. 108(3p),112(3p) „45(5a), 46(1 p), 81 (5a) Flottman T_ Foden J 46(1 p) Forde A 10(4a) „47(2a), 140(2a) Foster DA_ _49(2b), 93 (2b) Fowler TJ 50(4a) Fiigenschuh B„ „52(2p) Gartrell AP„ ,177(lp) Gatehouse CG_ _54(3p) Geiro A _28(2p), 88(2b) Gleadow AJW

.Kip) „174(5b) 3(1 a) 130(5a) 95(2a) 17(2p) 5(4b) 7(lb) 9(lp) .10(4a) .122(4a) 1l(lb) 28(2p) 12(5b) „14(la) ,109(lp) 15(4b), 17(2p) 9(lp) 19(la) _20(2b) „22(2b) _23(3p) 3(la) .174(5b) _24(3p), 80(3p) 105(2b)

187

AUTHOR INDEX

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Gray DR_

„55(lp), 56(1 a)

Green TJ

44(5a), 57(3p)

Gregory RT„

56(1 a)

Griffin TJ

,167(5a)

Grove M

„58(2a)

Grujic

„60(4a), 99(1 p)

Hammond RL_

64(lp), 144(4a)

Harris LB

52(2p), 68(2p)

Harrison TM

58(2a), 66(2a)

Heilbronner R

122(4a)

Higgins RI

68(2d)

Hill EJ

70(1 a)

Hobbs BE

75(lp), 148(3p)

Liu S

174(5b)

Luo Z

174(5b)

Mapani BSE

_ _ _ _ 1 0 0 ( l p )

39(2p)

Marshak S

46(1 p)

Marshall B

„103(3p), 150(1 a)

McDougall I„ McQueen HWS„

77(2p)

102(lb)

MarmoB

MawerCK

_73(3p), 75(lp), 148(3p)

Huang W

Little TA

Mares VM

72(4a), 185(3p)

Holcombe RJ_

54(3p), 95(2a), 118(2p), 133(lp), 138(lp), 140(2a), 152(2p), 160(5b)

Mancktelow NS„ „60(4a), 97(4a), 99(lp)

62(5a)

Hand M

Lister GS

105(2b) .106(2a), 154(2a) 107(2p)

Memarian H

108(3p)

Mendis DPJ

109(lp)

Michibayashi K_

lll(lb)

Mohajjel M

H2(3p)

Hubert C

9(lp)

Ingpen I

J78(5a)

James PR

l(lp), 24(3p), 45(5a), 46(lp), 80(3p), 81 (5a), 109(lp)

Morand VJ

113(lb)

„177(lp)

Morse MP

142(4b)

Janz JK

Jessell M„14(la), 54(3p), 82(4b), 169(4b) Johnson EL

99(1 p)

Johnson SE

83(5b)

Johnson TM

85(2p)

Kanagawa K

87(lb)

Kohn BP

28(2p), 88(2b)

Korsch RJ

5(4b), 89(3p)

Lafrance B

Muhlhaus H-B

72(4a)

Munroe SM

115(lp)

NisbetBW

62(5a)

Nishidono Y„

,164(2p)

Norris RJ

_116(la)

ODeaM

„118(2p)

Okamoto K_

.120(lp)

Olgaard

90(4a)

„38(2p)

Oliver NHS

3(la), 169(4b) 72(4a), 185(3p)

LeeJKW

91 (2a)

Leloup PH

66(2a)

OrdA

Lennox PG

93(2b)

Oussa S

39(2p)

Page RW

167(5a)

Panozzo Heilbronner R

122(4a)

Li Z X

1

130(5a)

188

AUTHOR INDEX

SGTSG FIELD CONFERENCE - JINPABYNE FEB '94

Papp E__

54(3p)

Sttiwe K

__176(3p)

Parker AJ

124(5a)

Suzuki S

164(2p)

Passchier CW

125(1 a)

Symonds PA..,

PavlisTL

97f4a>

Tabata H

180(lp)

Plumb KA

126(2p)

Tan J

169(4b)

Podladchikov Y

170(2b)

Taponnier P

„66(2a)

Pound KS

128(lp)

Tarlowski C

__142(4b)

Powell C McA

130(5a)

Totterdell JM

89(3D)

Tulloch AJ

154(2a)

Powell R

39(2p). 179(3p)

Pryer LL_

132(2p)

Tyler IM

_166(5b)

_167(5a)

Rankin LR

40(2b)

Raouzaios A

133(lp)

Vernon RH

90(4a)

Rawling T

160(5b)

Weinberg RF

170(2b)

Reeves CV

142(4b)

Wellman R

12(5b)

Rickard MJ

23(3p), 135(3p)

White SH

183(5a)

Robin P-Y F

„132(2p)

White SR

_172(la)

Ronaszeki J

„136(5a)

Williams CR

62(5a)

Ryerson FJ

_66(2a)

Wilson CJL „100(lp), 144(4a), 174(5b), 176(3p), 179(3p)

Scott J

Kip)

Scott RJ

_ 1 3 8 ( l p ) , 140(2a)

Shaw RD Sims J P

144(4a) 146(3p), 148(3p) 103(3p), 150(1 a)

Somaia I

152(2p)

Spell TL

154(2a)

Stephens CJ Stephens G Stephenson RA Stewart AJ

Wiltshire RG

142(4b), 167 (5a)

SliwaR Smith JV

Valenta RK

75(lp) 105 (2b) 5 (4b) 155(2p), 157(2p)

Stewart LK

159(4b)

Streets C

__160(5b)

StunitzH

m162(4a)

189

82(4b), 169(4b)

177(lp)

Winsor CN

177(lp)

Withnall IW

44(5a). 57(3p)

Worley B

174(5b)

XuG

179(3D)

Yamagishi H

87(lb)

Yamamoto H„.

180(lp)

Yin A

66(2a)

Young DJ

181(2p)

Zegers TE

183(5a)

Zhang Y

72(4a), 176(3p), 185(3p)