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Geological Society of Australia

ABSTRACTS Number ^ ^

9th INTERNATIONAL CONFERENCE on BASEMENT TECTONICS Canberra, Australia 2-6 Jul> 1990

9th

International Basement Tectonics Canberra July 2-6 1990

Organizing Committee:Mike Rickard - Chairman and Program (Australian National University); Larry Harrington - Field Excursions Peter Williams - Publications Peter Wellman - Finance (Bureau of Mineral Resources); Symposium Conveners: Marjorie Muir (CRA) Colin Simpson (BMR) Morrie Duggan (BMR) John Bain (BMR) Secretariat: Australian Convention and Travel Services. ACTS, GPO Box 2200, Canberra ACT 2601, Australia. We are grateful for the sponsorship of the following organizations for providing finance or personnel to assist with this Conference:International Basement Tectonics Incorporated. Geological Society of Australia and its Specialist Group in Tectonics and Structural Geology. The Department of Geology, ANU. The Bureau of Mineral Resources Geology and Geophysics. ARCO Oil and Gas Co, USA. BHP-UTAH Minerals International. Carpentaria Exploration Co Pty Ltd. CRA Exploration Pty Ltd. Western Mining Corporation Ltd. ANSETT Airlines. Qantas. Commonwealth Bank.

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INTERNATIONAL BASEMENT TECTONICS ASSOCIATION INC. 675 South 400 East, Salt Lake City, Utah 84111, U.S.A. Trustees 1987-1988:John J. Gallagher, Jr (Chairman) ARCO Oil and Gas Company 2300 West Piano Parkway Piano, TX 75075 Publications available:PROCEEDINGS OF THE FIRST-SIXTH INTERNATIONAL CONFERENCES ON BASEMENT TECTONICS. 1. 2. 3. 4. 5. 6.

Lineaments and fracture patterns of the earth. R.A. Hodgson, Editor, 636 p., 1976 U.S., Canada & Mexico $24.00 Overseas $25.50 Lineaments and fracture analysis; hydrocarbon accumulation M.H. Podwysocki & J.L. Earle, Editors, 595., 1979 U.S., Canada & Mexico $30.00 Overseas $31.75 Lineament analysis; basement controls of geological features D.W. O'Leary & J.L. Earle, Editors, 404 p., 1981 U.S., Canada & Mexico $33.00 Overseas $34.50 Basement faults and fractures (worldwide); lineament analysis R.H. Gabrielsen, I.B. Ramberg, D. Roberts and O.A. Steinlein, Editors, 382 p., 1984 U.S., Canada & Mexico $35.00 Overseas $37.50 Basement of Egypt and other areas S. Riad and D. Baars, Editors, 312 p., 1987 U.S., Canada & Mexico $37.50 Overseas $40.00 Basement faults and lineaments of the USA M.J. Aldrich, Jr & A.W. Laughlin Editors, 208 p., 1987 U.S., Canada & Mexico $25.00 Overseas $28.50 SET OF SIX $145.00 Overseas $160.00 U.S. CURRENCY ONLY! Utah orders add $6.25 sales tax.

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ilh

InternatiftPal

Basement

Tectonics

Prggraffi

Sunday

July

Arrive Canberra. 7.30pm

Monday

Jnly

Registration — Geology Department 8.00 am onwards, will be held in the Depi of Geology.

Technical

Session

09.00 . 09.15 09.15 - 10.00

1 Structure

Introduction — Prof Rutland (keynote)

the

Morning

10.30 - 11.00

Chair: R Korsch Williams & Duggan

11.00 - 11.30

Whitaker

White & Muir

12.00 - 12.30

Byrne &

Harris

and

Hall,

cover

ANU

All sessions

basins.

Tectonics of the Yilgarn Craton, Western Australia: Vertical versus horizontal models in the Eastern Goldfields. Magnetic lineament and deformation patterns in the southern Yilgarn Block. Role of Archean shear zones in the evolution of mobile zones surrounding the Yilgarn Craton. The structural elements of the Northampton Block. Western Australia.

Lunch

01.30 - 02.00

Chair: M Duggan Korsch et al

02.00 - 02.30

Zhou et al

02.30 - 03.00

Wellman

03.00 - 03.30

Little et al

03.30 - 04.00

Afternoon

04.00 - 04.30

10)

04.30 - 05.00

11)

05.00 - 05.30

craton

Bruce

Tea.

11.30 - 12.00

12.30 - 01.30

Australian

& Reception

MJ Rickard Basement Tectonics in Australia: introductory perspective

10.00 . 10.30

Registration

Geometry of Permian to Mesozoic sedimentary basins in eastern Australia and their relationship to the New England Orogen. The boundary between the Tasman Fold Belts and the Australian Craton: A reappraisal from studies of mafic rocks. Siting of sedimentary troughs along the boundaries between geophysical domains, Tasman Orogenic System. Late Palaeozoic tectonics of the North D'Aguilar Block, southeast Queensland.

Tea

Chair: L Harrington Krokowski & Olissoff The Pine Creek Shear Zone north of Pine Creek (Northern Territory) structural evolution and experimental studies. Shaw et al Thrust tectonics in Central Australia based on the Arunta—Amadeus Seismic Reflection Profile. Discussion

Tuesday

July

Technical Session

1 continued

09.00 - 09.30

12)

Chair: P Wellman Liu & Fleming

09.30 - 10.00

13)

Braun & McQueen

10.00 - 10.30

Morning

Tea

Technical Session 3 Basement Chair:

10.30 - 11.30 11.30 - 12.00

Itl 15) ic\

12.00 - 12.30

16)

12.30 - 01.30

structures

Continental

Regions

Wellman ^ keynote) Basement-cover relations in Central Europe. Thomas nnu ^ tectonic framework for the TransHudson Orogen. Canadian Shield, based on gravity and magnetic anomoly patterns. Haszeldine North-south British Isles linears; Proterozoic to present. Lunch

01.30 - 02.00

17)

Chair: R Korsch Alsinawi & Al-Banna

02.00 - 02.30

18)

Al-Saigh et al

02.30 - 03.00

19)

Sindi

03.00 - 03.30

20)

Edgell

03.30 - 04.00

04.00 - 04.30

21)

04.30 - 05.00

22)

05.00 - 05.30

Wednesday

Tectonic models for the Cambrian Kanmantoo Group in southeast Australia. 'Devon-shear' or basement tectonic evolution and basement development in Central and Western Australia in the late Palaeozioic.

35)

Afternoon

Tea

Chair:

An E-W transect section through central Iraq. Crustal structure along geotransect Baghdad-Dohuk, northern Iraq. Basement tectonism of the Arabian-Nubian Dome. Basement tectonics of Saudi Arabia as related to oil field structures.

Harrington Structural analysis of the basin by Virtual Basement Displacement (V.B.D.) method. Basement control of oil and gas traps: More Parker Gay common than we thought? Basement-linked syn-volcanic faulting and Adams & Henley high grade epithermal gold mineralization at Bimurra, north-east Queensland. Kodama

Tiilv

Field Excursion in Canberra Region.

Thursday

July

Technical Session 3 continued

09.00 - 09.30

23)

Chair: Carlson

09.30 -10.00

24)

Baars

10.00 - 10.30

M Muir

Morning

Genetic relationships between the Precambrian basement and Phanerozoic tectonics, Midcontinent region, North America. Conjugate basement rift zones in Kansas, Midcontinent, USA Tea

10.30 - 11.00

25)

Sears & Alt

11.00 - 11.30

26)

Gibson

11.30 - 12.00

27)

Katz

12.00 - 12.30

28)

Jons

12.30 - 01.30

A composite Proterozoic cratonic basin drawn from examples in North America and Australia. High grade metamorphic rocks of the Tuhua Orogen, Western New Zealand: Lower crustal analogues of the Lachlan Fold Bell, SE Australia. Tar pavement rift — transform tectonic model and some examples in nature. Basement tectonics on Mars.

Lunch

Technical Session 4 S t r u c t u r a l 01.30 - 02.30 02.30 - 03.00

29) 31)

03.00 - 03.30

32)

03.30 - 04.00

patterns

and

Mineral

Deposits

Chair: W Mayer O'Driscoll ( k e y n o t e ) Elusive trails in the basement labyrinth. Bassi The Sierra Alta de San Luis, Argentina, South America: A case history of regmagenic control of gold metallogeny. Muir Structural controls on the East Alligator Rivers uranium field. Northern Territory, Australia. Afternoon

Tea

04.00 - 04.30

33)

White et al

04.30 - 05.00

34)

Hodgson & Kvet

05.00 - 05.30

Discussion

07.30 -

Conference

Dinner

Basement reactivation and mineralization, Kimberley area. North western Australia. Global fracture systems: Mapping, analysis and economic utilization.

University

House

Friday (? J U I Y Technical Session 5 Techniques for analysing basement structures Chair: C Simpson 09.00 - 09.30 36) Tapley The detection and significance of geological structures mapped in the Canning Basin, Western Australia, using NOAA-AVHRR satellite imagery. 09.30 - 10.00 37) Nash Factors affecting the acquisition of structural data from remotely sensed images of eastern Australia. 10.00 - 10.30

Morning Tea

10.30 . 11.00

38) Brown et al

11.00 - 11.30 39) Dunlap 11.30 - 12.00 40) Creasy 12.00 - 12.30

Discussion

12.30 - 01.30

Lunch

Applying apatite fission track analysis to tectonics: Examples from southern Africa, south eastern Australia and Antarctica. Kinematic modeling and balancing of high strain thrust systems. Synthesis of multiscale remote sensing interpretations for defining basement/cover structures.

Technical Session 6 Structural patterns in Oceanic crust Chair: MJ Rickard 01.30 - 02.00 41) Palmer et al Seafloor spreading in the AustralianAntarctic Discordance. 02.00 - 02.30 42) Bostrom Relation of ocean-floor structures to the Australian continental margin: SEASAT images. 02.30 - 03.00 General Discussion 03.00 - 03.30 03.30 - 04.00 Afternoon Tea 04.00 - 05.00

Business Meeting IBT

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement tectonics in Australia: An introductory perspective

RW.R Rutland

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

As defined for this Conference, Basement Tectonics appears to be nearly synonymous with Intra-plate Tectonics i.e. with the tectonics of regions after a basement has been developed. This overview attempts to provide a general framework of the main themes of the conference with respect to Australia. The present topography of Australia is largely the consequence of Tertiary, and especially Neogene, tectonics. There is a broad relation to the Neogene geodynamic system, rather than to the orogenic structure of the basement rocks. This is illustrated by the passive margin development of the Great Dividing Range, and by the broad flexure of the Australian Shield on an E-W axis. However, the Neogene basement tectonics does show a dramatic change across the lines of the Torrens Hinge Zone and the Diamantina River, thus reflecting the boundary between the Precambrian craton and the Tasmanides. On the western edge of the Tasmanides, the Flinders Ranges are a remarkable example of reactivation of older structural trends within a broad NNW trending region of Tertiary subsidence. Within the Eromanga Basin the Tertiary reactivation follows northeasterly Permo-Triassic and older trends. The neotectonics, through its control on erosional and depositional history has exercised important controls on the distribution of mineral deposits, as well as on the quality of soils. In the Precambrian Shield of Australia it is clear that the main features of the basement blocks and of their cover basins are the consequence of rather well defined episodes of basement reactivation which are reflected in the pattern of dipole gravity anomalies and in aeromagnetic data. Thus the Archaean Pilbara and Y i l g a m provinces have had essentially cratonic character since 2400 Ma but much of the present boundaries are the results of reactivation during the period 1400-1000 Ma. This same period of reactivation has also largely obscured the primary relations between Proterozoic orogenic provinces in the Amadeus Transverse zone, where there has also been younger reactivation. The Rimberley region provides an example of repeated basement reactivation in nearly orthogonal fault zones which have controlled the emplacement of diamond pipes. Rimberlitic intrusions also occur in the region of the Delamerian fold belt xdiere the Cambro-Ordovician fold trends are controlled by reactivation of Proterozoic basement trends. Major faults are important in controlling mineralisation in the Archaean Y i l g a m province and in the Proterozoic provinces developed between 1800 and 1500 Ma. In both cases the faults affect sequences which have been subjected to extensional and compressional episodes and which apparently overlie on older basement. It is not clear that the faults reflect the structure of the older basement but they are important elements in discussions of lineament tectonics.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Tectonics of the Yilgarn Craton, Western Australia: Vertical versus fiorizontal models in the Eastern Goldfields

P.R. Williams and M.B. Duggan

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

The Extern Goldfields Province of the Yilgam Block in Western Australia comprises two major terranes, a western terrane dominated by basalt and ultramafic rocks, and an eastern terrane dominated by tholeiitic basalt, felsic volcanic and yolcaniclastic rocks and clastic sedimentary sequences. The western sequence is juxtaposed against the Southern Cross Province, which has a distinctive magnetic signature and different geological characteristics. These three "terranes", and the majority of the Yilgam Block, were intruded during a massive felsic magmatic event at 2.65-2.68 Ma. The nature of tectonic episodes which led to this gross geometry of the eastern Goldfields is not well constrained, despite continuing mapping and research over the past fifteen years. Following enthusiasm for plate tectomc continental margin models during the 1970's, in which the greenstones represented obducted oceanic crust, several lines of evidence showed that significant parts of the greenstone belts were underlain by continental crust, and ensialicriftingmodels were proposed to account for generation and intrusion of the mafic and ultramafic rocks. Later explanatior^ of the geometry in terms of "classical" plate tectonics suggest that the western basaltic and ultramafic belt formed in a large extensional back arc basin, with the extern terrane representing a remnant fore arc assemblage. More recently, mapping evidence has helped to define terrane boundaries more accurately, and an accretionary model, in which docking of continental fragments along largely transcurrent faults, has been proposed as an alternative model. A different class of model for Archaean tectonics invokes the emplacement of the vast granite batholiths as the tectonic driving force for orogeny, rather than ridge formation and subduction. Heat loss in the present-day version of this model is by conduction through the primitive crust (Campbell and Hill, 1988). The generation of basaltic and komatiitic rocks took place over large mantle plumes, and the short time interval between basalt magmatism and granite emplacement is a result of the time taken to transfer heat from the mantle plume to the crustal anatectic layer. This type of model is referred to as a vertical tectonic model. Structural evidence from throughout the Eastern Goldfields province has established a coherent three-phase deformation history: 1. Early thrust faults are well established in several areas, generally with large displacement, with north northwest - south southeast movement. 2. Upright north - south tight to open folds. 3. Horizontal faults and shear zones, commonly sinistral. The regional nature and consistent timing of deformation is best explained by the plate interaction models, rather than the vertical tectonic model. Of the plate interaction models, an accretionary model is highly attractive from the structural evidence, as it is the only model which can provide a ready explanation of the orogenparallel thrust movement within a short time of folding and transcurrent faulting. However, the vertical model provides a reasonable mechanism for granite generation across a large area of the Yilgarn Block, and the climactic tectono-magmatic event near the end of the Archaean may have ultimately been driven by plume activity in the mantle.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Magnetic lineament and deformation patterns in the southern Yilgarn Block

A Whitaker Bureau of Mineral Resources, GPO Box 378, Canberra, ACT, 2601, Australia The following study is based on analysis of regional BMR aeromagnetic surveys. The southern Y i l g a m Block is composed of two structurally distinct regions. In the west is the Western Gneiss Terrane which is older than 3200 Ma. The terrane is characterised by sparse, linear, short strike-length anomalies due to lithological banding. Numerous lon^ lineaments with common orientations of 310 to 335 ° and 045 to 055 crosscut the terrane. There is little evidence of ductile deformation associated with these lineaments. Dykes are little deformed and are thought to post-date the major ductile deformation events affecting the terrane. To the east are granite-greenstone terranes which are believed to have been formed during the interval 2900 to 2500 Ma. Compositional banding is relatively common and exhibits broadly sinuous patterns with strikes varying from north-iwrthwest to northerly. Common lineament orientations are 310 to 325 , 345 to 355 and 010 to 025^. Evidence of ductile deformation of the stratigraphy adjacent to and related to lineaments is abundant. Dyke intrusion has occurred both before and after major ductile deformational events in the region. The relationship between the bulk of the lineaments in the Western Gneiss Terrane and those of the granite-greenstone terranes is not known however most predate the widely distributed, post cratonic intrusion of the east-west dyke suite at about 2400 Ma. Post cratonic deformation of the Y i l g a m Block has been generally minor although at the margins it has been extreme. The southern/southeastern margin of the Block, 50 to 70 km wide, has been deformed and demagnetised adjacent to the thrust Proterozoic Albany-Fraser Province. Thrusting of the Albany-Fraser Province occurred during the period 1800 to 900 Ma. Abutting the Y i l g a m Block are mid? crustal granulites which were rigid prior to thrusting. The bulk of the Province to the south and south-east was ductily deformed, metamorphosed to amphibolite grade and cratonised in association with the thrusting event. The extent of deformation of the w e s t e m margin of the Y i l g a m Block is considerably less than of the southem margin. Structures generally only parallel the block boundary up to 5 km into the block. The southwest c o m e r of the Y i l g a m Block and w e s t e m Albany-Fraser Province have been more extensively deformed and folded southwards adjacent to the Darling Fault. The deformation is inferred to have occurred during emplacement of the Proterozoic basement to the Perth Basin. Phanerozoic deformation of the Y i l g a m Block has been relatively minor. Evidence for movement on faults includes preservation of down faulted Permian sediments and variations of regolith across faults.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Role of Archaean shear zones in the evolution of mobile zones surrounding the Yilgarn Craton S.K White^ and M.D. Muir^ ^Geology Department, University of Utrecht, Utrecht, Holland CRA Exploration Pty Ltd, 139 Canberra Avenue, Fyshwick, ACT, 2609, Australia The A r c h a e a n tectonics of the Y i l g a r n C r a t o n are c h a r a c t e r i s e d by sets of transcurrent shear zones with northwesterly, n o r t h e r l y , n o r t h - e a s t e r l y and e a s t e r l y trends. The last three of these trends are reflected in the P r o t e r o z o i c mobile zones that form the western, eastern, and southern b o r d e r s of the Y i l g a r n C r a t o n . Less, however, is known of the basement structures that influence the e v o l u t i o n of the C a p r i c o r n Mobile Zone between the Y i l g a r n and P i l b a r a Cratons, because of the extensive cover by late A r c h a e a n and P r o t e r o z o i c basins. The central zone of the C a p r i c o r n M o b i l e Zone is c h a r a c t e r i z e d by en echelon e a s t e r l y trending folds and can be traced into the bordering late A r c h a e a n - e a r l y P r o t e r o z o i c H a m e r s l e y Basin. Domes (anticlinal c u l m i n a t i o n s ) of A r c h a e a n basement in the H a m e r s l e y Basin exhibit e a s t e r l y trending shear zones w h i c h show two distinct p h a s e s of m o v e m e n t . The most evident is a p o s t - F o r t e s c u e G r o u p (?post H a m e r s l e y G r o u p ) dextral t r a n s p r e s s i o n which c o r r e l a t e s with the formation of the domes and other folds. T h e r e is also earlier sinistral t r a n s t e n s i o n which gives a n o r t h e r l y e x t e n s i o n , and which has been correlated with the d e v e l o p m e n t of the H a m e r s l e y Basin. That is, the A r c h a e a n basement under the H a m e r s l e y Basin was u n d e r g o i n g transcurrent shearing at the same time as the A r c h a e a n of the Y i l g a r n C r a t o n , and in a similar tectonic framework, The en echelon g e o m e t r y of basement s t r u c t u r e s in the C a p r i c o r n Mobile Belt, however, indicate that the basement shears are splays off a major s t r u c t u r e with an e a s t - s o u t h - e a s t e r l y trend, and which would correspond with the F o r t e s c u e lineament. The t r a n s t e n s i o n and t r a n s p r e s s i o n in the C a p r i c o r n Mobile Zone r e f l e c t s reversals in shear sense along the F o r t e s c u e lineament which may have acted as a c o n j u g a t e system to shearing in the Y i l g a r n C r a t o n .

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

The Structural elements of the Northampton Block, Western Australia

D.R. Byrne and LP. Harris Department of Geology, University of Western Australia, Nedlands, WA, 6009, AustraUa

The Proterozoic Northampton Block represents an outcropping portion of the Darling Mobile Belt (DMB), which extends along the western margin of the Yilgarn Block, and is basement to the Phanerozoic Perth Basin. It mainly consists of garnetiferous paragneiss and porphyritic granite, both of which are cut by a dolerite dyke swarm. Cu-Pb-Zn mineralisation post-dates the dykes and pre-dates the Silurian sandstones of the Perth Basin. The first deformation event involved folding of the gneisses into horizontal, shallowly inclined, tight to isoclinal folds with westerly fold vergence. This event resulted in E-W shortening across the DMB. The second deformation event (D2) produced open to gentle upright folds trending NW. The interference pattern of these two fold styles and topography form concentric elliptical patterns in the gneissosity observed on aerial photographs. The porphyritic granite intruded under a similar stress field, with the phenocrysts defining an S-foliation parallel to D2 fold axial surfaces, and C-shears involved in N-S dextral shearing. The dolerite dyke swarm, seen prominently on aerial photographs and aeromagnetic data, also intruded under a N-S dextral shearing stress field. While the larger dykes occupy T shears (trending NE), many small dykes and offshoots occur in D (trending N-S), R and R' positions. The Cu-Pb-Zn lodes also occupy the same shear orientations including P shears. Many of the deposits occur at intersections of these shears (eg T-R'), or where the shears curve from one orientation to another (eg T-R). These events reflect a long or repeated history of dextral shearing along the DMB. The aeromagnetic data and aerial photographs also show conjugate NNW and E-W trending shears which have displaced the dykes sinistrally and dextrally respectively. In the field the NNW shears are steeply dipping mylonite zones. These shears are possibly related to the Pan-African event which caused sinistral movement along the DMB.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Geometry of fermian to Mesozoic sedimentaty basins in eastern Australia and their relationship to the New England Orogen

RJ. Korsch, K.D. Wake-Dyster, P.E. O'Brien, DM. Finlayson andD.W. Johnstone Bureau of Mineral Resources, GPO Box 378, Canberra, ACT, 2601, Australia

Sedimentary basins peripheral to the New England Orogen in eastern Australia contain a record of events from the Early Permian to the Late Jurassic that provide information on the later history of the orogen. The geometries observed in BMR and company seismic data place constraints on the interpretation of the sedimentary basins. The Triassic Esk Trough shows a pronounced asymmetry in cross section, as expected during pure extension, but the bounding fault dips very steeply and no low-angle detachment has been observed on seismic sections. Hence strike-slip movements on the bounding fault are required to accommodate the observed geometry. The half-graben of the Permian Taroom Trough (southern, subsurface extension of the Bowen Basin) suggests an extensional origin, but the limited amount of extension that can be accommodated on the bounding fault implies oblique extension, with the fault having a significant strike-slip component. Thus the geometries of the southern Bowen and Esk basins appear to be controlled, to some extent, by strike-slip motion on the bounding faults. The Jurassic sediments are involved in structures such as localised thrusts, folds and positive flower structures, which we interpret to be the result of transpression associated with reactivation of the strikeslip faults. Some previous workers have postulated that the Bowen Basin and also the Sydney Basin initiated during a period of extension oriented ENE-WSW in the latest Carboniferous or earliest Permian, resulting in the formation of a series of half-grabens. In the Bowen Basin, the model proposes the opposite geometry to that observed on BMR and company seismic lines across the Taroom Trough, implying that a major transfer fault, across which the polarity of the basin changes, should exist in the subsurface between the two areas. Thus, the Bowen and Sydney basins were probably initiated by normal extension, and were separated by a zone in the southern Bowen and Gunnedah basins that was dominated by oblique extension and strike-slip. At the same time as the basins were initiating, movements on strike-slip faults also played a significant role in the adjacent basement in that they controlled the transport and accretion of displaced terranes such as the Gympie and Beenleigh terranes and the oroclinal bending of the accretionary wedge sequence. Hence the New England Orogen has been influenced by strike-slip faulting for a considerable part of its history.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

The boundary between the Tasman Fold Belts and the Australian Craton: A reappraisal from studies of mafic rooks e. Zhou^, K.J. Mills^ and S.F. Lii?

^Department of Geology & Geophysics, University of Sydney, Sydney, NSW, 2006, Australia Department of Geology, La Trobe University, Bundoora, Victoria, 3083, Australia

The Tasman Line was originally defined as a stratigraphic boundary between the Precambrian and the Palaeozoic sequences in eastern Australia. Somerecentauthors have invoked the Tasman line as the western bound^ of the Tasman Fold Belts, and this has raised some problems, such as the tectonic position and significance of the "Kanmantoo Fold Belt". We propose a new "Tasman Transitional Zone" concept to describe this particular transition from cratonic basement to orogenic belts with the following main reasons: (1). The Early to Middle Cambrian Kanmantoo Group and similar metasediments are an integral extension of the stratigraphic succession of the Late Precambrian Adelaide Supergroup. As a consequence,tiie"Kanmantoo Fold Belt" should not be considered to be a separate entityfromthe "Adelaide Fold Belt". (2), The changefromcratonic basement blocks to orogenic belts in eastern Australia ti^sgresses tiie Precambrian - Cambrian boundary. Spatially, this transitional zone is not always coherent widi the general schematics of eastern Australia that sequence young eastwards. IntiieWonominta Block in western New South Wales,tiielocal reverse sequential order and various metamorphisms imply a complex deformation of the transition zone. (3). Within the transitional zone, mafic volcanisma display variable characters corresponding to different periods of tectonic activity oftiiebasement The initiation of tiie Phanerozoic orogenic belts occurred at differenttimesin different parts of tiie transitional zone. Some oftiielatest changes may be recorded intiieWonominta Block, wheretiieearly C^brian Mt Wright volcanism was likely emplaced in a continental environment, not in a volcanic arc as previously suggested. The mafic volcanism at ML Wright in westem New Soutii Walesreflectstiieearly stages of rifting oftiiecontinental crust astiieKanmantoo Trough formed andflyshdeposition became widespread on tiie eastem seaboard oftiiecontinent How far east the Kanmantoo sediments extend beneatii the younger Tasman Fold Belt units is not yet known, and thus the range of the transitional zone is yet to be defined.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Siting of sedimentary trouglis along the boundaries between geophysical domains, Tasman Orogenic System

P. Wellman Bureau of Mineral Resources, GPO Box 378, Canberra, ACT, 2601 The Tasman Orogenic System can be subdivided into 11 geophysical domains. The geophysical domains have subparallel dominant trends in gravity anomaly and short- and long-wavelength magnetic anomalies. The relative ages of the geophysical domains can be inferred, because at boundaries between domains, the younger domain has trends parallel to the boundary, and the older domain has trends oblique to the boundary. Deep sedimentary troughs are associated with about one half these domain boundaries, the axis of the trough commonly lying along the margins of the older domain. The cause and timing of the subsidence varies. The troughs are thought to be localized by weak lower-crustal structures associated with the boundaries of geophysical domains.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Late Palaeozoic tectonics of tlie Nortli D'Aguilar Block, southeast Queensland T.A Little^, R.J. Holcombe^, G.M. Gibson^, R. Silwa^ andS.K. Dobos^ Dept. of Geology and Mineralogy, University of Queensland, St Luda, Queensland, 4067, Australia University College of Southern Queensland, Toowoomba, Queensland, 4350, Australia Current studies, and analysis of previously published data, have lead to a new Interpretation of the North D'Aguilar block In the "Gympie Province" In southeast Queensland. The metamorphic rocks are part of a Late Devonian(?)-Early Carboniferous subductlon accretlonary complex and are unconfonnably overlain by volcanic and coarse-grained clastic rocks of probable Early or mid-Permian age. as well as by Eariy to Middle Triassic rocks of the Esk Rift Metamorphic fabrics In the basement rocks Include not only those that can be related to convergence, but also youngerfabricsthat may have developed during Late Carboniferous or Permian uplift and unroofing of the accretionary rocks. These Include a foliation in the amphibolitic aureoles of granodiorlte plutons that were intruded synkinematically at about 300Ma. Regional cooling through the K/Ar blocking temperature of white mica occun-ed at about 260Ma In the southem part of the block. This uplift and cooling rriay have been accomplished by extenslonal faulting during Pennlan Inception of the Eskrift,which flanks the block on the west A predominance of basic schists, some of which are blue amphlbole-bearlng. and associated fine-grained pelagic and hemipelagic sediments In the Northem D'Aguilar block contrast with the slllcldastlc-domlnated sediments of most of the accretionary complexes of the New England Qrogen. This has lead to a general acceptance In recent years that the North D'Aguilar block Is "different" - along with the rest of the "Gympie Province". However the similarity of stmctural style, and metamorphic and igneous history, with adjacent basement blocks, and with other blocks throughout the New England Qrogen. suggests that the North D'Aguilar block simply represents deeper, probably underplated. levels of the same subductlon complex.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

The Pine Creek Shear Zone north of Pine Creek (Nothern Territory) structural evolution and experimental studies

J. Krowkowski and S. Olissoff

School of Earth Sciences, Flinders University, Bedford Park, SA, 5042, Australia

The Early Proterozoic rocks in the Pine Creek Shear Zone (PCSZ), north of Pine Creek, show strong dominance of a subvertical and vertical foliation in the NNW-N direction. The foliation overprints the bedding. However, regarding the direction of the SI cleavage in the adjoining F1 structures (e.g.: the Spring Hill anticline and structures immediately to the east) the foliation is clockwise inclined about 15-30^. East of the PCSZ and the McKinlay Granite within rocks older than the Burrell Creek Formation the F1 folds are plunging to N and NNW and partially look like an en echelon. Meanwhile, to the west of the PCSZ the folds are trending NW-NNW and plunging to SE and SSE. These two regions have similarly trending, although contrary plunging the fold systems. The regions are separated by the differently oriented foliation of the PCSZ. Within the Zone the b edge lineation of the folds is mostly sub-horizontal. A series of experiments on clay, plastic clay and bread dough have been performed for an explanation of deformations in the Zone caused by these two contrary plunging fold systems. This special kind of folding was caused by complex deformation: - simple coaxial compression and folding, - rotational and longitudinal (perpendicular to the compression) fault in the basement causing opposite sides of the fault walls to plunge in contrary directions. Results of the experiments show that structures produced by simple compression have been reoriented and rebuilt when rotational movement associated with the fault is superimposed. The structures (folds, cleavage and faults) form a zone inclined from the direction of the maximum compression. The inclination is 15-40 and depends on the material as well as the experiment stage (amount of the deformation). Firstly folds, later cleavage and faults were formed. After 25 7. shortening and more than 15^ of fold axes contrary plunging in opposite direction, a phenomenon of refolding with vergence mostly outside of the deformation zone took place. The structures grew by both propagation and coalescence. The faults cut the older folds as well as faults and in later stages transformed themselves into a reverse type. The zone shows a sinistral transpression acted during the deformation. The transpression depends on the plunge angle of the contrary fold systems. The width of the deformational zone depends on the material used in the experiment as well as the deformation stage. The experiment can explain the inclination of the foliation in the PCSZ area as well as the horizontal attitude of the b lineation of the folds. The different plunging of the fold systems probably occured during the E-W stage of folding or a formation of the Pine Creek Embayment. However, it is very possible that the differences may have already taken place during a phase of major folding (Fl). The PCSZ represents a less competent crustal zone developed between two more rigid lobes of the Cullen Batholith. The zone was probably associated with deep crustal fracture. The sinistral transpression was probably later changed into a dextral strike-slip component of the wrench tectonics.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Thrust tectonics in Centra! Australia based on the Arunta-Amadeus Seismic Reflection Profile

R.D. Shaw, RJ. Korsch, C. Wright, B.R. Golebyand C.D.N. Collins

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

The integrated results of surface geology and deep seismic profiling experiments across the Eariy ProtCTOzoic Arunta basement and the Late Proterozoic to Late Palaeozoic Amadeus Basin in central Australia has led to a new understanding of thrust tectonics in central Australia. The seismic results show the thrust belt at the northern margin of the basin to be dominated by a 'thick-skinned', south-directed, basement thrust feature, the Redbank Thrust Zone, which has been imaged as a relative planar feature to mantle depths in two profiles. Other, more southerly, basement-cored thrusts appear to splay southwards from the Redbank Thrust Zone toward the basin forming a basement wedge. Based on various structural models, crustal shortening across the thrust belt has been variously estimated by usfromas low as 25-35 km (15-25%) to as high as 66 km (50%) by others. For the most part the structures imaged by the seismic reflection profiles datefiomthe Late-Devonian-Carboniferous (Alice Springs Orogeny). As thrusting associated with the Alice Springs Orogeny progressed, thick conglomeratic sediments accumulated in a narrow (foreland-like) footwall trough that forms the northern, thickest part of the Amadeus Basin. In the central and southern parts of the basin, the seismic results indicate north-directed, thin-skinned overthrusting on shallow detachments that were accompanied by Jura-style folding also during the Alice Springs Orogeny. The detachments sole out in a salt horizon near the b^e of the sequence. Some of these thrusts were also active in the Late Proterozoic. Duplication oftiiesedimentary section over a horizontal distance of at least 30 km has occurred at the leading northern edge of the thrust complex. A crust-cutting thrust is postulated attiiesoutiiem margin oftiiebasin to accommodate shortening on a crustal scale. Although both nortiiem and soutiiemtimistcomplexes were active at roughly the same time, the southern thrust complex continued longer and over-rode the footwall trough of the northem complex.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Tectonic models for the Cambrian Kanmantoo Group in southeast Australia

S.F. Liu and P.D. Fleming Department of Geology, La Trobe University, Bundoora, Victoria, 3083, Australia

Several models have been proposed regarding the tectonic setting of the Early-Middle C ^ b i ^ Kanmantoo Group in southeast Australia (von der Borch 1980; Rutland et al. 1981; Scheibner 1985; Brown et al. 1988). However, these models are either based on nanow categories of geological data or do not provide satisfactory explanations of some important geological features in this region. The K ^ a n t o o Group, the youngest unit in the Adelaide Foldbelt, is a marine shelf and a turbiditic sequence and has a conformable contact with the Early Cambrian Normanville Oroup. The thick sequence of Precambrian and Cambrian rocks was deformed and metamorphosed to various degrees during the Delamerian Orogeny (500 Ma). The Kanmantoo Group metasediments are in general structurally and metamorphically coherent with the older rocks including the Late Proterozoic Adelaide Supergroup. It is theref<TO mappropnate to separate the Kanmantoo Group tectonically ftom the rest of the A!! T^T allotting it to a different terrane or zone rather than including it in the Adel^de Fold^lt. The significance of defining a 'Kanmantoo Foldbelf and its west boundary is debatable. Qjmpositions of mafic rocks reflect the tectonic evolution of the Adelaide Foldbelt A^J^I^^F?''?^^ numerous mafic dykes, sills and plugs (500 Ma) in the southern AdelMde Foldbelt provides some key information on the nature of the upper mande and crast b e n ^ the Kanmantoo Group and hence the tectonic setting of die Group. Some of me m ^ c intrusions were emplaced at an early stage (pre- to syn-Dl) of the deforaation history, indicating that the mande had an important influence on the early t h e n ^ activity intiiesouthern Adelaide Foldbelt by supplying heatfromthe upwelling mande and the early intrusion of mafic magmas. The evidence for several varieties of w l y t h e ^ activity and for die early (pre- to syn-Dl) bimodal magmatism on Kangaroo Island mjcate a high heatflowat that stage. The MORB/BABB affinity of some mafic dyk« mdicates mel^g of a MORB/BABB type depleted mande soon after the deposition ot the Kanmantoo Group, which was therefore probably deposited in a well-advanced extensional basin. R^ent studies on granites of the region (Foden et al. 1990; Gray (in press)) suggest diat ftie^bnan rocks were not involved in the petrogenesis of granitic rocks emplaced in me Kanmantoo Group and Kanmantoo Group type metasediments in southeast South Austraha and western Victoria. The non-involvement of Precambrian materials for the w ^ s e d granites in this whole region means that either the graniticrocksderived fiom ftiecamtaan rocks were not emplaced at die present exposed cnistal leveL or most likely there is htde Precambrian crustal material beneath the Kanmantoo (jroiq). These considerations, together widi other available geological and geophysical data, allow a reassessment of die Cambrian tectonic development of the region. A general model involving extension and continental thinning and/or rifting to create new ocean in the region currentiy occupied by the Kanmantoo Group and its continuation under the Mupay Basin is favoured. More specific models perhaps involving a back-arc basin settmg, or possibly some odier variety of marginal sea are examined after consideration of earUer models of von der Borch (1980), Scheibner (1985) and Brown et al. (1988)

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

'Devon-shear' or basement tectonic evolution and basement development in Central and Western Australia in the late Palaeozoic

J. Braun and H. McQueen

Research School of Earth Sciences, Australian National University, Canberra, ACT, 2601, Australia

Areviewof the existing data on the stratigraphy and structure of the Amadeus Basin and neighbouring Canning Basin indicates that the episodes of most rapid basement subsidence in the two basins occurred during the same interval in the Late Devonian (Frasnian-Famennian) to mid-Carboniferous (Visean) periods. It is, however, hard to reconcile large-scale northsouth crustal shortening in the central Australian basins with nearly parallel (NNE-SSW) crustal extension in the Canning Basin located only a few hundred kilometers to the Ill west. We have been looking at a range of kinematic models of the tectonic evolution of the Australian plate that might resolve that difficulty. One of the few acceptable options is that the apparently inconsistent crustal movements result fiom a common inplane force acting along one of the Australian plate boundaries, but that a continental-scale shear zone was active in the late Devonian to early Carboniferous periods, pmially decoupling the central and western domains. In this picture, north-south compression was dominant to the east of the shear zone with overthrusting in the Amadeus, Officer, Ngalia and Wiso Basins while extension took place simultaneously to the west of the shear zone in the Fitzroy Graben and Bonaparte Gulf Basin to the north of it. While deformation proceeded on one side of the zone, shear was impeded by large and small scale irregularities along the shear zone and crustal stress was transmitted across the zone, driving a complementary deformation on the other side. It is difficult to determine whether the driving force for this overall tectonic evolution originated to the south or the north. Four different models can account for the observed crustal movements: compression in the south to the east of the shear zone; tension in the south to the west of the shear zone; compression in the north to the east of the shear zone; and tension in the north to the west of the shear zone (the model shown in the figure). One distinguishing feature is that the two latter models predict dextral shear which reaches a maximum along the northem part of the shear zone, while the former ones result in sinistral shear whose maximum is in the south. The location of the shear zone may be inferredfromsurface geology and seismic and gravity data, but the sense and variation in ma^tude of shear along the zone at a particular time in theremotepast are more difficult to estimate. It is a litde more plausible to attribute the inplane forces to the northem (Tethyan) margin of the Australian plate because Australia and Antarctica had not yet separated in the late Palaeozoic, although a southem source cannot be ruled out. Another means of distinguishing between the various models is the location of maximum deformation on either side of the shear zone. First-order calculations based on a plane-stress elast(>plastic finite element model allow us to determine the distribution of strain as a function of the location and sense of the driving force, and the shape of the shear zone. ArafuraSM^wiMt)

Bonaparte Gulf Basin

KknbeHeKBtock

Fitzroy Graben (Northern Canning)

Wiso Basin

Broome Arch ' j | l (Central Canning) ^ f AfunU Bk>ck (north) Ngaiia Basin

KidsonSub^in (Southem Canning)

Block (south)

Amadeus Basin

Ylgam Block

Musgrave Bk>ok

Eastern Officer Basin £ u d a Basin

AntarcMca

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement-cover relations in Central Europe

P. Bankwitz

Central Institute for Physics of the Earth, Potsdam, German Democratic Republic

Relation between the basement and platform cover in central Europe the basement is of Proterozoic and Palaeozoic age. Northern Europe collided with central Europe at the beginning of the Phanerozoic period and grew by subduction processes from the north to the south. The deeper crustal structures of central Europe are E-W trending and are obliquely overprinted by Hercynian tectonics. The post-Hercynian molasse formation had its maximum in the Lower Permian. The platform cover altogether reaches a thickness of more than 10 kilometres in the north German trough. The highest velocity of subsidence was not controlled by simultaneous loading of the crust. The Meso-Cenozoic development is characterized by tele-tectonic influences from the Alpidic belt in late Cretaceous and Lower Tertiary. In neotectonic times the reactivation of Hercynian fracture zones has influenced the platform sediments and even the recent morphology.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

A plate tectonic framework for the Trans-Hudson Orogen, Canadian Shield, based on gravity and magnetic anomoly patterns

M.D. Thomas

Continental Geoscience Division, Geological Survey of Canada, Ottawa, Ontario KIA 0Y3, Canada

The Proterozoic Trans-Hudson orogen follows a 2800 km long, tortuous path from South Dakota to northern Hudson Bay, but Is exposed for only 500 km of this distance, in the Canadian Shield. To the north and south it lays hidden beneath a Phanerozoic sedimentaiy carapace which attains a maximum thickness of about 4 km in the Williston Basin. Various images of gravity and magnetic data have been used to "penetrate" this cover and to delineate the boundaries and internal structures of the orogen. Subdivision of the orogen into 5 first order structural domains is proposed. The northernmost, characterized by N to NE geophysical trends, coincides with a major part of Hudson Bay. It is separated from the three southernmost domains, and apparently dextrally offset from them, by a narrow WNW-trending domain containing WNW trends. WNW trends are aberrant within the overall context of Trans-Hudson structures and may have an ancestry in transform faulting within a spreading Trans-Hudson ocean. The adjacent 2 structural domains to the south are subparallel, N-trending, dominated by internal N trends and share a boundary that coincides in part with the Tabbernor fault, immediately east of which E trends are noticeable. These domains are terminated to the south by a major SW-trending fault interpreted from geophysical patterns and forming the northern boundary of the southernmost domain. This fault apparently crosses into the Wyoming craton and runs parallel to the Cheyenne Belt, some 40 km to the southeast, which marks the southeastern margin of the craton. A striking feature of the orogen is the close similarity in the shapes of its western and eastern margins, which poses a dilemma for developing an evolutionary model. The similarity could be attributed to unilateral subduction associated with continent-continent or continent-island arc convergence. In the first scenario one margin would represent a suture zone and the opposite would correspond to the limit of reactivation induced by the underriding plate. Such a model requires the Trans-Hudson orogen to be formed largely of reactivated Archean crust. Isotopic evidence suggests that the orogen consists mainly of Proterozoic juvenile crust, and favours the second case, in which one margin would be a suture and the other would mark the limit of new crust. Modern plate analogues provide valuable comparative frameworks for Precambrian plate reconstructions. The structural pattern of the Trans-Hudson orogen and of its internal domains, as determined from geophysical anomaly patterns, mimics closely the plate tectonic mosaic in eastern Indonesia, specifically the area of the Banda Sea and the Philippines. This analogue is used to examine possible plate models for the Trans-Hudson orogen. Subduction in the Banda Sea has a counterpart in southern and eastern Hudson Bay; transform faults between Sulawesi and New Guinea have an analogue in the WNW domain of the orogen; and the Philippines region provides the analogue for the southern part of the orogen.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

North-south British Isles linears; Proterozoic to present

R.S. Haszeldine Department of Geology and Applied Geology, University of Glasgow, Glasgow, G12 8QQ, Scotland

The North Sea, between Britain and Norway is a currently subsiding marine rift basin, overlying 4km of Tertiary sediment. North-south submarine faults bound this basin; earthquakes occur on these faults at 20km. The famous Tertiary volcanic centres west of Britain align in a similar N-S rift zone west of Britain. The Cretaceous and Jurassic Viking Graben of the North Sea is a major petroleum province, with offshore depocentres 2km thick. Onshore, syndepositionally faulted sediments show activity along similar N-S trends. Carboniferous onshore basins 5km thick were controlled by an orthogonal network of steeply dipping N-S and E-W basement faults expressed as listric normal faults in the sedimentary sequence. N-S alignments of giant Zn-Pb sedimentary exhalative orebodies and Pb-Zn vein arrays in Ireland follow the same trends, as does Silurian gold mineralization in Scotland. Syndepositional N-S basement faults in Scotland controlled post-Caledonian Devonian molasse deposition, and are sparsely exposed. No major N-S features have yet been detected during the Caledonian Orogeny, or the sediments deposited in its Paleaozoic antecedent "lapetus Ocean". However, syndepositional N-S faults controlled mid-Proterozoic rift-fill red beds of the Stoer Group, and part of its marine equivalent, the Moine Supergroup. Field evidence demonstrates reactivation on at least one of these faults in the late Proterozoic, and again during the post-Cambrian. Does the matching of similar N-S trends before and after the opening and closure of the lapetus and Grenville Palaeozoic and Proterozoic "Oceans" deny their existence? Activity on these N-S features was most important when the British Isles was experiencing the breakup of large continental masses. Many rift basins and major dykes around the world show similar N-S palaeo-trends, suggesting a dominant E-W tension during continental rifting and rupture. This does not fit easily with conventional plate-tectonics. Does this stress field relate simply to the earth's rotation, or to its expansion?

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

An E-W transect section through central Iraq

S.A Alsinawi and AS. Al Banna

Department of Geology, University of Baghdad, Baghdad, Iraq

The geological and geophysical measurement available in Iraq, have been used to give an idea about the Crustal thickness beneath an EW Transect passing through central Iraq, with a length of 620 km. Due to the absence of direct information about the basement of Iraq, several studies in the neighbouring countries were compiled by Buday and Jassem (1987), they deduced that the basement of Iraq is metamorphosed and of Precambrian age. The basement depth determined for Aeromagnetic survey was used in this study to calculate the gravitational effect of the sedimentary column. The density contrast between the sedimentary and basement rocks is (0.12) gm/CC. Gravity model for the Crust was built to fit the smoothed anomaly. The Crustal thickness along the Transect ranges between (31-37) km, with an average of (34) km. The geologic and tectonic strip of the Transect was compared with the available Aeromagnetic and Bouguer anomalies and seismic reflection date. The result of the model was compared with that of the published empirical relations for Crustal thickness calculation, and close correlations were noted.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Cmstal structure along geotransect Baghdad-Dohuk, northern Iraq N.H. Al-Saigh^, I. Namik^ and I. Abdul HameecP

Saddam Dam Research Centre, Mosul University, Mosul, Iraq Department of Geology, University of Salahaddin, Salahaddin, Iraq Oil Prospecting Company, Iraq

Seismic reflection data correlated to five exploration wells along with geological, gravity and magnetic data are used to construct the subsurface structure along an approximately 100km wide corridor that traverses central Iraq in SE-NW direction for a distance of about 600 km from Baghdad in the central part of Iraq to Dohuk near the Turkish border. This transect is one of few such transects that represents the Iraqi contribution to the Global Geoscience Transits (GGT) project. The transect crosses the three major tectonic elements of Iraq from south to north; the Mesopotamian zone, the foothill zone and the high folded zone. A strong reflector has been identified at a depth of about 10-13 km. This reflector is believed to be the top surface of the crystalline basement. This seismic evidence finds support in the interpretation of gravity and magnetic data. The crystalline basement becomes shallower as we approach the Turkish border, until about 100 km from the border, and then becomes deeper again to subduct under the Anadolian plate. In this part the basement is heavily fractured. In central Iraq the basement also becomes deeper as we approach the Iranian border to subduct under the Iranian plate. It appeared that the thickness of the crust varies between 35.5 km in the central part of Iraq to about 30.5 km near the Turkish border.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement tectonism of the Arabian-Nubian Dome

H.O. Sindi Geology Department, King Saud University, P.O. Box 2455, Riyadh University, Saudi Arabia

Several geological studies have been carried out on segments of the Arabian-Nubian dome, but little detailed literature has been published. Geological and geochemical investigations including major, trace and rare earth elements have been carried out on selected areas from the Arabian Shield with other comparative studies from similar terrains. This study includes age determinations, types of tectonism and zones of metamorphism. non,^ vK eastern flank of the Arabian-Nubian Dome is the Khamis Mushayt fundamental gneiss in the southern region of the Arabian Shield which has a distinct REE pattern and a pre-Pan African age older than 1800 m.y. while the age of the oldest continental rock in this sub-plate is older than 2800 m.y. The major components in the Arabian Shield are plutonic rocks that composition with strongly alkaline or primitive tholeiitic series to mature calcic affinity, occurring in this vast region in a systematic trend defining at least five different subduction zones with a growth rate of about 20% of the I'hf®''^ ^^^ ^^ subduction zones are also indicated by the shallow crustal structure, type of lithology, poly-metallic mineralization oxygen isotope, heat flow and the geophysical and geochemical behaviour. These rocks are formed in ensimatic island arc and continental-marginal arc environments. Most of the granitic rocks in the Arabian sub-plate have variable geochemical compositions and genetic relationships which suggest that these rocks belong to the S-type and the I-type granites. With the exception of the northern Yemen and the Quaternary, Tertiary and Recent volcanic rocks in the Arabian Shield, the youngest postaccretion granitic bodies have an age of about 450 m.y. derived by massive crustal fusion modified by fractionation processes. Style and depth of these plutonic rocks have been recorded to be from mesozonal to epizonal emplacement. The crustal thickness of the Arabian sub-plate is estimated to be about 35 km.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement tectonics of Saudi Arabia as related to oil field structures

KS. Edgell King Fahd University of Petroleum and Minerals, KFUPM Box 940, Dhahran 31261, Saudi Arabia All the oil fields of Saudi Arabia are of the structural type and they all lie in the northeastern part of the country, including the Saudi offshore portion df the Persian Gulf. These oil field structures are mostly produced by extensional block faulting in the crystalline Precambrian basement along the predominantly north-south Arabian Trend, or the'old grain*of Arabia. These basement horsts,which have been periodically reactivated,underlie the world's largest oil field, Ghawar, and other major oil fields, such as Khurais and Mazalij. The basement horst beneath Ghawar Anticline, suggested by Aramco (1959), from a positive Eouguer gravity anomaly, practically mirrors the field, as shown by Barnes (1987). All Saudi Arabian offshore oil fields,and some near coastal fields, such as Abu Hadriyah and Dammam, are also produced by basement faulting which has cut the saliferous. Upper- Precambrian Hormuz Series, triggering deep-seated salt diapirism. Consequently, Saudi Arabian offshore and coastal fields are denoted by distinctive negative gravity anomalies. Some of these oil fields are circular, such as Dammam, Abu Hadriyah and Karan, while others are elongated, due to salt wall diapirism, as in the case of Khafji, Kura3m, Jana and Jurayd. The latter oil fields all follow a NE to MJE trend, believed to be due to left-lateral, strikeslip faulting in the basement, as seen in Kuh-e Namak on the Iranian side of the Gulf. One large offshore oil field at Manifa trends NW-SE, paralleling the Persian Gulf and is due to right-lateral, strike-slip faulting in the basement along the Erythraean Trend (von Wissman et al 1942). An undeveloped offshore oil field at Hasbah trends east-west, due to basement faulting in this direction. Some major, elongated offshore oil fields with negative gravity anomalies trend almost northsouth along the Arabian Trend, including Berri and Qatif, as well as the nearby Bahrain and Dukhan fields. All the known oil fields of Saudi Arabia and its offshore are thus related to four major directions of basement faulting, namely N-S, NE-SW, NW-SE and E-W. The major fault trend is the north-south Arabian Trend. In the Saudi offshore portion of the Perr^ian Gulf, northeast-southwest, left-lateral, strike-slip faulting of the basement has also been important in forming oil field structures. A major difference between Saudi Arabian onshore and offshore oil fields is that the former show strong positive gravity anomalies due to block uplift of basement, while the latter have pronounced negative anomalies due to deep-seated salt diapirism induced by faulted crystalline basement. Detailed analysis of the potential field of gravity in the oil field areas of Saudi Arabia,'both offshore and onshore, substantiates these major basement fault: directions and shows the marked coincidence of oil field outlines with large, basement-induced gravity anomalies. The regmatic shear pattern of basement faulting is also clearly shown by mapping the second derivative of the potential field of gravity.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Structural analysis of the basin by Virtual Basement Displacement (V.B.D.) method

K. Kodama Geological Survey of Japan, Tsukuba, Japan

In Japan, several oil and gas deposits are found in fracture type reservoirs of Miocene volcanics at depths greater than AOOOm. There are large scale faults and complex horst and graben are formed below 4000m. These structures are very different from the shallow parts with gentle folded structures. Therefore the conventional method of extrapolating the shallow geologic structures is not applicable to the exploration of the deep-seated reservoirs. A new system of basin analysis by tectonic simulation was developed. Here we have introduced the concept of inversion problem and devised a method of assuming a hypothetical deformation of the basement. We call this Virtual Basement Displacement(V.B.D.) method. The analytical method is as follows. First, provide a V.B.D. to an arbitarily set basement.. Reconstruct the deformation within and on the surface of the geologic units by elastic-plastic finite element method under the above conditions. Compare the calculated deformation of the surface with the known data, and modify the V.B.D. until the error becomes sufficiently small. The V.B.D. at this point is the optimum basement displacement, and incremental deformation for this tectonic stage is obtained. The cumulative deformation, from a certain geological period co present, is provided by overlapping these processes. Figure shows the distribution of faults formed by the incremental deformation during some tectonic stage in Niigata oil and gas basin, central Japan. The cross lines indicate the conjugate fault planes, and the lengths and number of the lines visually express the scale of the faults coresponding to the magnitude of the strain. These faults are considered to be effective for migration and accumulation of oil and gas. The geologic structures of shallow and deep zones are constructed disharmoniously by the tectonic simulation. Some oil company applied this method to actual exploration in the natural gas field, and others to another type of deposits such as vein type ore deposits or geothermal reservoirs.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement control of oil and gas traps: More common than we thought?

S. Parker Gay, Jr. Applied Geophysics Inc., Salt Lake City, Utah, USA

Applied Geophysics, Inc., has been mapping basement structure in sedimentary basins for petroleum companies since 1982 using high resolution aeromagnetic methods and a newly developed processing technique called NewMag®.

In that time we have surveyed a combined

600,000 sq. kms in 24 petroleum basins scattered throughout the United

States.

Careful

comparison

of the basement

data

with

literally hundreds of detailed subsurface and seismic maps, coupled with studies of outcropping basement rocks, has led to the concept of the "basement fault block pattern".

This is not a new concept,

however, having been espoused in the 1930's and 1940's by European geologists, most notably Hans Cloos. For this presentation we have classified oil and gas traps controlled by basement into 19 categories, and will show actual field examples of most of them.

Of these 19 categories of traps,

11 result from fault movement along basement block boundaries and 8 from gravitational compaction of the" sedimentary section over basement topography. considered

Additionally, 11 of the 19 categories are

"purely

stratigraphic"

origin>

relationship to basement is easily demonstrated. different

classification

scheme developed

by Hyne

but a

their

somewhat

in 1984

and

published by Pennwell (Tulsa) as a wall chart, 20 of the 29 types of generalized

traps can be due to basement control under the

proper

geological

control

can manifest

circumstances. itself

scheme,

basement

in 73% of the different

Hyne's

types of

structural traps, 58% of the stratigraphic traps, and 83% of the "combined" traps, for an overall total of 69%. Our work indicates that basement control of oil and gas is much more common than has heretofore been realized.

Consequently,

petroleum companies using basement concepts and basement mapping techniques will be more successful in finding hydrocarbons, and have lower finding costs than those that do not.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Genetic relationships between the Precambrian basement and Phanerozoic tectonics, Midcontinent region, North America

M.P. Carlson Nebraska Geological Survey, University of Nebraska, Lincoln, Nebraska, 68588, USA Crustal influence on geologic history of the midcontinent region of North America is illustrated by the Precambrian/Phanerozoic relationships of two major tectonic features: the Transcontinental Platform (Arch) and the Midcontinent Rift System (Central North American Rift). Although historically referred to as the Transcontinental Arch, this feature is a platform consisting of a series of structures oblique to the apparent northeast-southwest trend of the platform. Thickness maps of Phanerozoic units illustrate repetitive movement on these secondary structures along tectonic axis and/or lithogenetic junctures in the Precambrian basement. The Platform was accentuated by the Phanerozoic development of the Williston Basin to the north and the Oklahoma/ Anadarko Basin to the south.

TRANSCONTINENTAL PLATFORM

iOOMMil SOO KHOMIfliS

Distribution of Paleozoic rocks over the Midcontinent Rift System suggests that both thermal events and wrenching along transforms have occurred. In Nebraska, these rift transforms were reactivated by the Middle Carboniferous Ouachita Orogeny. Wrench faults were propagated into the adjacent basement rocks creating secondary anticlines and faults in the overlying Paleozoic rocks. The geometry illustrated for these two major tectonic features is an example of the interactive interpretation made possible by combining knowledge of Phanerozoic and Precambrian rocks and tectonic history.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Conjugate basement rift zones in Kansas, Midcontinent, USA

D.L Baars

Kansas Geological Survey, University of Kansas, Campus West, Lawrence, Kansas 66047, USA

The structure of the Precambrian basement of Kansas, Midcontinent USA, is dominated by conjugate NNE- and NWtrending wrench fault zones. NNE-trending faults of the Midcontinent Rift System (MRS) extend from Lake Superior across Kansas and into north-central Oklahoma. The fault zone widens from about 100 km in NE Kansas to more than 160 km in south-central Kansas in a series of horsetail splays. NNEtrending structures of the MRS are displaced by about 80 km of dextral offset by the NW-trending strike-slip fault zone. Apparently penecontemporaneous NW-trending wrench faults of the Bourbon Arch-Central Kansas uplifts cross the state from soutiieast to northwest, offsetting MRS structures. The two conjugate wrench fault zones are complexly interrelated in central Kansas, where internal antitiietic shears complicate axial horsts and grabens of the MRS. The Bourbon Arch is ofifeet about 100 km by sinistral slip from the Central Kansas uplift along the MRS. The Himiboldt fault zone at the eastern margin of the MRS w ^ not offset significantly by NW-trending faults, suggestmg that the present-day expression of the southwardweakening fault zone was created during Pennsylvanian (Upper Carboniferous) rejuvenation of the basement fabric. Stratigraphic relationships record a history of repeated reactivation in Paleozoic time that strongly affected petroleum entrapment, with an especially strong pulse of uplift during Pennsylvanian time. These rift zones are segments of continental-scale basement lineaments that are fundamental to the structural fabric of the North American basement. The Bourbon Arch-Central Kansas structural lane lies sub-parallel to the Olympic-Wichita Lane that extends from southern (Oklahoma to the norfiiwest through the Paradox basin of eastern Utah, and the MRS lies sub-parallel to the Colorado Lineament which extends from the Grand Canyon in Arizona to the Lake Superior region. Thus, the basement of the western Midcontinent and Southern Rocky Mountains consists of large-scale fault zones that delineate sub-orthogonal basement blocks.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

A composite Proterozoic cratonic basin drawn from examples in North America and Australia

J.W. Sears and D. Alt

University of Montana, Missoula, Montana 59812, USA

Proterozoic basins of the North American and Australian cratons have consistent sets of rock assemblages and structure. We combine data from about two dozen separate basins exposed and imaged at differing levels to construct a composite Proterozoic basin. Phanerozoic platformal sediments mantle the upper levels of large basins which have not been deeply eroded. Middle levels contain Proterozoic fluvial/alluvial and lacustrine/shallow marine sedimentary sequences. These are interlayered with flood basalt flows and/or massive diabase sills up to a few km in aggregate thickness. Deep levels of large basins are rarely exposed, but a few examples reveal gabbro/granophyre lopoliths up to several km thick underlying the sediments or flood basalts, and mixed with breccia zones. Geophysical data shows that the fill in some large basins is as great as 20 to 30 km, constituting a significant part of the cratonic crust. In plan view, the basins are crudely circular and commonly have radiating rifts and diabase dike swarms. We emphasize the significance of the mafic igneous rocks in the basin dynamics and suggest that Proterozoic cratonic basins are analogous to Phanerozoic flood basalt provinces which erupt voluminous basalt into basins prior to continental rifting. We suggest that such basalt outpourings may be triggered by pressure-relief on the mantle following large impacts. The impact crater then fills with basalt to form a lava lake. Eruptions from the lake form flood basalts or basin-wide sills, while deeper parts crystallize into lopoliths. The dense mafic rocks sink as sediments accumulate in the basins over periods as great as a billion years. Differences between Proterozoic and Phanerozoic examples may be due to secular changes in sedimentation rates, lithospheric mobility, geothermal gradient and depth to the asthenospheric magma source.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

High grade metamorphic rocks of the Tuhua Orogen, Western New Zealand: Lower crustal analogues of the LachlanTold Belt, SE Australia G.M. Gibson University College of Southern Queensland, Toowoomba, Queensland, 4350, Australia

Late Mesozoic continental extension together with more recent transpression along the mid-Tertiary Alpine Fault have combined in Fiordland, southwest New Zealand to uplift and expose the deeper structural levels of a mid-Cretaceous metamorphic core complex. Among other rock types exhumed in this complex are 350-380 Ma amphibolite facies metasediments whose protoliths include Late Proterozoic-Lower Palaeozoic fossiliferous sequences comparable to those developed in Nelson and the Lachlan Fold Belt. These metasediments constitute an important component of the mid-Palaeozoic Tuhua Orogen and have been uplifted from depths of 25-30 km; they are interstratified with subsidiary amounts of granitic orthogneiss, metagabbro, metamorphosed anorthosite and tonalitic gneiss, which collectively offer a useful insight into the nature and composition of the continental crust beneath what was once part of eastern Gondwana. Metamorphism of these rocks occurred under pressures of 7-9 kbar and led to the widespread development of kyanite + garnet + K feldspar assemblages in pelitic lithologies throughout western Fiordland whilst at higher structural levels in central Fiordland lower pressure (3-4 kbar) sillimanite ± K feldspar ± cordierite assemblages more in keeping with the low P/T style of metamorphism (andalusite-sillimanite facies series) in the Lachlan Fold Belt were produced. Assuming palaeogeographic reconstructions for eastern Gondwana during the Palaeozoic are valid, the middle to lower crustal rocks of Fiordland may give some indication as to what lies at depth beneath SE Australia.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Tar pavement rift - transform tectonic model and some examples in nature

M.e. Katz

Dept. of Applied Geology, University of New South Wales, PO Box 1, Kensington, NSW, 2033, Australia

Tar pavement structures have been observed, on an Australian street in Sydney, to geometrically resemble rift - transform fault systems. These consist of a series of spaced, dilatant fractures or rifts connected up by linear arrays of en echelon cracks or transform fault lines (Fig. 1). In the tar pavement structures the shear transform fault line zones are characterised by low angle (10^-20^), to higher angle (40^-50^), often sigmoidal, en echelon cracks. Most cracks are dilatant and this would indicate that both Riedel shear and tensional gashes are developing. The geometrical relationships in these tar pavement structures resemble tectonic rift-rift offsets and transform faults described in nature and experiment. These structural models are applied to structural stages in the tectonic evolution of the East African rift and Rhine graben, as well as the southern portion of the Australian New England Orogen.

Figure 1.

Tar Pavement Structures. Arrows indicate general sense of movement. Twenty cent piece for scale.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement tectonics on Mars

H.-P. Jons Tedmische Universitat Clausthal-Zellerfeld, Leibnizstrasse 10,3392 Clausthal-Zellerfeld, F. R. Germany

An investigation of appr. 90 000 linear features obtained from Mariner 9 and Viking I and II maps of the martian surface has revealed numerous tectonic details; especially of the ancient ("continental basement") martian uplands of the southern hemisphere of Mars and of its updomed areas. A histogram of all linear features of the ancient uplands shows surprising similarities between the main tectonic trends of that area and those of terrestrial cratons. The tectonic trends in a 0° - 180 - histogram show maxima around north, 45° east and east, 45 south with a prominent minimum around the east - west direction (90° striking angle). It is noteworthy that the main tectonic feature of Mars, the Valles Marineris -a system of giant Red Sea - like grabens- which trend nearly exactly east - west show along their border again the typical basement pattern with the trends which have been described before. A special feature of the martian upland tectonics is indicated by a prominent maximum around the north - south direction (0° striking angle). That tectonic trend occurs especially in the martian relief along the transition zone between the ancient uplands and the young lowlands. This fact is of considerable importance with respect to the still unanswered question of the origin of the martian lowlands which show nearly no linear features - obviously due to a thick coverage of young sediments. Finally, it is important that in some cases very old unclassified upland volcanoes occur along prominent graben — like linear features. That type of volcano - tectonic relation probably can be interpreted as a very old embryonic stage of block tectonics in the area of the ancient martian uplands. Such a stage of endogenic dynamics probably happened on Earth as well in archaic/proterozoic times — probably with similar (identical?) results with respect to the terrestrial relief of that time.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Elusive trails in the basement labyrinth

EST. O'Driscoll

14 Renwick Street, West Beach, SA, 5024, Australia

The Australian continental crust is characterised by subtle pattern breaks corresponding to what E. Sherbon Hills called "zones of yielding'^ in the basement. These breaks can be revealed in plots of standard geological and geophysical data by the application of appropriate techniques. Pattern breaks appear both as linear and arcuate (ring) discontinuities, and generally correspond to thresholds of change in geological structures and/or material compositions. Intersecting linear discontinuities form the systematic floor plan of the continental basement which controls the nature and distributions of surface geological patterns. They localise the propagation of crustal energies ultimately expressed as crustal disjunctions and geological interruptions of one kind or another. Such disjunctions generally correspond to fault and shear zones (including rifts), and to zones of anomalous conditions, in terms of deformation, metamorphism and mineralisation. GlOA

The Bouguer gravity contour map of Australia (from B.M.R. sources) is a convenient vehicle for displaying a number of continental megalineaments, some crossing from coast to coast. The accompanying figure (adapted from Phil. Trans. Roy. Soc. Lond., 317 (1986)) shows part of the map covering southwestern Australia. The internal trails of selected lineaments are indicated by the directed marginal arrows pointing into the map area. The lineaments can be shown to be related to singular geological and physiographic features, and are characteristically associated with mineralised centres, especially at their intersections. In the western part of the figure, lineament GlOA is known as the "gold line", but is also a maximum nickel trend. Two major ore centres are circled at its intersection with lineament G4, the northernmost circle containing Australia's greatest gold field (Kalgoorlie), and the southernmost circle containing Australia's greatest nickel centres (Kambalda & St. Ives). In a similar context 1400 km to the east, on lineament G2, is Australia's greatest copper and uranium deposit (Olympic Dam), circled where G2 is crossed by lineament G9C. Additional

regional gravity lineaments are visible in the figure and will be demonstrated on a full-size continental gravity map. This shows their association with other major Australian ore deposits, including Broken Hill (lead-zinc) and Mount Isa (copper-lead). Evidence continues to point to Sherbon Hills' conclusion that continental lineaments represent crustal trends of great antiquity. Their known association with ore deposits of Paleozoic and Precambrian age supports this conclusion, and points to their common and fundamental deep crustal origin as suppliers of source materials for both syngenetic and epigenetic ores. In both these circumstances the lineament must predate ore deposition. In a sense, the modern sea-floor "smokers" demonstrate the two ore-forming processes. Intermittent reactivation oyer a period of time can produce the familiar pattern of stacked ore deposits. The ancient historical role of lineaments becomes evident from their applications to paleogeologic maps such as those compiled for the Adelaide Geosyncline by Preiss (1987). These maps provide evidence to show that previously recognised lineaments have been powerful controllers of patterns of early stratigraphy reaching back to Precambrian times, and, in this instance, have exercised active control over Precambrian (Torrensian-Marinoan) sedimentation and structures at the time of deposition. There they are clearly foundation designers of the local floor-plan of crustal stratigraphy and geology. A challenge to interpreters of historical global tectonics is the observation that systematic contmental lineaments are seen to be replicated by parallel linear counterparts in adjacent ocean floors. In some instances, as in eastern and western Australia, an on-shore lineament is collmear with a matching sea-floor lineament, suggesting they are registering the same feature. Others appear to be displaced at the continental margin, and their combined geometry offers a commentary on global expansion assuming this to have been achieved by expansion of oceanic crust rather than of the continents. Many partial or complete ring trends of various sizes are visible in geo-data plots of the Australian continent. In South Australia, the large Woomera Ring (rad. 160km) is discernible in data from aeromagnetics, geological ingredients, gravity, satellite imagery, topography and drainage patterns. It is slightly overlapped by the Lake Acraman Ring (rad. 50km) already described in literature as an impact structure. Their respective centres are aligned on an ENE trend matching the ENE axis of a succession of arcuate lakes, convex to the east, which, if due to impact repercussions, may mark a line of infall in that direction, like bow waves before speeding boats. The lake-arcs are centrally orthogonal to this "line of flight", as are some prominent structural features such as the Lake Blanche Fault and the Northwest Fault, as well as the Gawler Range mafic dyke swarm which the Ring neatly and symmetrically encompasses. Two larger but similar rings worthy of mention are the Georgetown Ring (rad. 280km) in north Queensland, and the Kookynie Ring (rad. 290km) in Western Australia. All three rings are perceptibly boxed by prevailing regional lineament systems, the Woomera Ring having a hexagonal boundary in geological ingredient data. Since the lithosphere and atmosphere share the same system of terrestrial rotational dynamics, some comparisons may be drawn between their structure patterns. A geologist's cross-section through a lithospheric subduction zone bears an uncanny resemblance to the cross-section a meteorologist draws through an atmospheric cold front. In plan, the typical atmospheric trough of the southern hemisphere, with its familiar cold-front/warm-front "y-structure", containing the warm uplifted sector inside the fork of the "y", has counterparts in lithospheric troughs of which the Adelaide Geosyncline may provide an example. Here the "cold-front" Torrens Hinge Zone, and the "warm-front" FleurieuNackara Arc form the arms of the "y", and converge southwesterly to its foot at Kangaroo Island. The diapiric structures uplifted in the axial belt of the Geosyncline may then be analogous to the thermal upwellings hoist by a cold front in an atmospheric system. The Western Australian shield has a similar but broader framework, with the Darling Fault and the ENE Bremer-Fraser Fault forming the arms of the "y", and converging southwesterly to the Naturaliste Plateau. In this instance the shield occupying the enclosed warm sector (diapiric granites) has been worn down to its Archaean roots. Such patterns may be seen as foundation inheritances from the fluid dynamics of a rotating globe with a once-molten crust in which continental lineaments were developed during subsequent consolidation. Oscillatory reactivation could then persist intermittently throughout time. For our ingenuity the Basement is as much of a challenge as ever.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

The Sierra Alta de San Luis, Argentina, South America: A case history of regmagenic control ot gold metallogeny

H.G.L Bassi Cent. Investigaciones en Recursos Geologicus, Ramirez de Velasco 847, (1414 Buenos Aires, Argentina The metallogeny of the Sierra Alta de San Luis is controlled by a net of pre-intrusive and pre-volcanic fractures striking NNE - WNW supporjt ing the hypothesis of a regmagenic network in the South American Cone. The gold district is located within the Crystalline Complex of the Sierras Pampeanas, of Early Paleozoic metamorphites intruded by Pale£ zoic granitoids and pegmatitic occurrences related to W and Be deposits. The Pretertiary peneplain was during the Andean Orogeny uplifted and broked into blocks. A volcanic event took place, yielding trachyand^ sitic rocks associated with an Au-Ag metallogeny also related to Pb, Zn, Cu and As. The district shows an extensive hydrothermal alteration which is both linear and disseminated. The linear type produces very poorly defi ned vein-like occurrences. The disseminated alteration spreads ov¥r large areas and is the source of important gold detritic deposits. As a whole the alteration conforms a NNE - WNW metallogenic network reflecting the cortical fracturing system.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Structural controls on the East Alligator Rivers uranium field, Northern Territory, Australia

MD. Muir CRA Exploration PTY. Ltd., 139 Canberra Avenue, Fyshwick, ACT, 2609, AustraUa T h i s u r a n i u m field c o n t a i n s the m a j o r m i n e , R a n g e r , two large u n m i n e d d e p o s i t s , J a b i l u k a and K o o n g a r r a , and the small w o r k e d - o u t mine, N a b a r l e k , plus n u m e r o u s p r o s p e c t s of v a r i a b l e size. The first three d e p o s i t s are h o s t e d in the L. P r o t e r o z o i c C a h i l l F o r m a t i o n , near its contact w i t h the late A r c h a e a n N a n a m b u C o m p l e x , and are r e g a r d e d as u n c o n f o r m i t y - t y p e d e p o s i t s . N a b a r l e k is h o s t e d in a crush zone, and is c h e m i c a l l y d i s t i n c t from the o t h e r three. In g e o l o g i c a l maps, the NW t r e n d i n g s i n i s t r a l s t r i k e - s l i p B u l m a n Fault and a set of e a s t - w e s t n o r m a l faults are the dominant structures. Image p r o c e s s e d r e g i o n a l a e r o m a g n e t i c data show that the B u l m a n is a m a j o r fault zone ca.l5 km wide. T h e EW faults have no m a g n e t i c e x p r e s s i o n . Regional g r a v i t y d a t a show a n o r t h - t r e n d i n g g r a v i t y h i g h to the east of the m a j o r d e p o s i t s . R e g i o n a l r a d i o m e t r i c d a t a do not c o n t r i b u t e to a s t r u c t u r a l u n d e r s t a n d i n g . Riedel a n a l y s i s i n d i c a t e s that the B u l m a n Fault zone is the c o n t r o l l i n g regional s t r e s s d i r e c t i o n . The NS g r a v i t y high is in the p o s i t i o n w h e r e r e v e r s e faults are e x p e c t e d , and i n d i c a t e s dense g r a n u l i t e facies rocks near s u r f a c e , p e r h a p s related to the P r o t e r o z o i c N i m b u w a h C o m p l e x to the east. T h e n o r m a l EW faults of the g e o l o g i c a l maps, are in the correct p o s i t i o n for the R i e d e l a r r a y . T h e r e v e r s e fault implied by the g r a v i t y h i g h is i n t e r p r e t e d as i n d i c a t i n g an i n v e r s i o n phase of a f o r m e r l y e x t e n s i o n a l terrain, w i t h the B u l m a n Fault system as the t r a n s f e r s . The NW t r e n d i n g N a b a r l e k ore b o d y m a y have a c c u m u l a t e d in a pullapart in a t r a n s f e r fault. T h e u n c o n f o r m i t y contact of the other d e p o s i t s is sheared and may have b e e n an o r i g i n a l listric fault, and the N a n a m b u C o m p l e x , an o r i g i n a l core complex. T h i s d i f f e r e n c e in s t r u c t u r a l s e t t i n g may h e l p to e x p l a i n the d i f f e r e n c e s b e t w e e n N a b a r l e k and the other m a j o r deposits.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement reactivation and mineralization, Kimberley area, North western Australia S.H. White^, M.D. Muir^ and C.B. SmiW

^Department of Earth Sciences, Utrecht University, Utrecht, Holland ^CRA Exploration Pty Ud, 139 Canberra Avenue, Fyshwick, ACT, 2609 ^C.B. Smith, CRA Ej^loration Pty Ud, 21 Wynyard Street, Belmont, WA, 6014, Australia

The Northern Australian Craton is characterised by two dominant tectonic trends, one is a WNW direction and the other in a N to NNE direction. Both appear to have developed at about 1850 my ago, and may even have originated in the Archaean. They have dominated the tectonic evolution of the margins of the Kimberley Block, The King Leopold Mobile Zone belongs to the WNW set and the Halls Creek Mobile zone to the NNE set. The two zones constitute orthogonal planes of crustal weakness and have acted as coupled structures for much of their tectonic histories. It is now known that at least one of the constituent faults in the King Leopold Mobile Zone, the Oscar Fault, cuts through the crust into the upjper mantle, and had influenced the siting of the diamondiferous lamproites of the West Kimberley field and base metal mineralization. The occurrence of the Argyle Pipe within a dilatant area of the Halls Creek Mobile Zone suggests that the latter also enters the upper mantle.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Global fracture systems: Mapping, analysis and economic utilization

RA. Hodgson^ and R. Kvef

^RAH Geological Consulting Services, PO Box 531, Jamestown, Pennsylvania 16134, USA Czechoslovak Academy of Sciences, Geographical Inst., Mendlovo nam. 1,662 82, Brno, Czechoslovakia Elements of the Earth's global fracture networks range in length from a few centimeters to hundreds of kilometers. All elements of the fracture networks occur showing an areal disposition which is systematic in nature and consistent over large regions. The remarkably constant angular and azimuthal relations of linear elements in the networks to each other and to the Earth's rotation axis suggest, from theoretical considerations, that the fracture systems have a genetic relation to the forces which cause the Earth to change shape in a systematic fashion. Evidence suggests these relations may change periodically as reflected also by major orogenic episodes. Separating pattern elements of the observed complex fracture networks on a theoretical basis and comparing the results to the areal distribution of dated geologic events has shown a positive correlation as demonstrated in the example of Czechoslovakia. Such studies allow evaulation of the relative tectonic and economic significance of individual elements and anomalous patterns within the networks. Selected examples are given to illustrate the methods of fracture mapping and analysis as applied in evaluating various types of structural and stratigraphical economic deposits associated with lineaments and other fracture types. Fracture elements at all scales provide avenues for the movement of fluids and gases across otherwise impermeable lithologic boundaries throughout the stratigraphic section. The environmental implications are profound. This condition can be utilized in testing the economic potential of areas prospective for hydrocarbons and minerals through geochemical investigations. Fracture mapping and analysis can contribute significantly to the success of any economic or environmental project, particularly where used in conjunction with other geological, geochemical and geophysical methods.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basempnt-Iinked syn-volcanic faulting and high grade epithermal gold mineralisation at Bimurra, north-east Queensland D.P.M. Adams^ and R.W. Henley^

^Aberfoyle Resources Ltd, Garbutt, Townsville, Queensland, 4814, Australia Consultant, Epithermex International Pty Ltd, Aranda, ACT, 2614, Australia

Bimurra is a large Upper Devonian-Lower Carboniferous epithermal system containing a number of quartz-adularia-calcite veins within a clay-altered sequence of felsic pyroclastics and epiclastics. A number of well preserved silica sinters were recently recognised in the district. Gold was mined in the early 1930's and the area extensively explored since 1980 for bulk tonnage, open pittable resources. Field and high resolution aeromagnetic data suggest that gold mineralisation at Bimurra and Wirralie is hosted by volcaniclastics and sediments whose distribution is controlled by a strong northeasterly fault array. Remapping of the Bimurra district has supported this analysis and identified a major 'growth fault' style structure which locally controls the distribution of lacustrine sediments and epiclastics as well as the distribution of alteration and of indicator elements. Diamond drilling in 1989 confirmed syn-tectonic volcanism and hydrothermal activity through intersection of a sequence of hydrothermal eruption breccias and their reworked equivalents within a 200 meter thick basin sequence. High grade mineralisation (0.92 m at 71 g/t gold) was encountered in a banded quartz vein in the basin sequence. The presence of adularia, calcite and scolecite provides strong evidence for boiling at shallow levels within the system. Alteration data indicate that only a few tens of metres have been eroded from the paleosurface. The sediment-volcaniclastic basin at Bimurra is interpreted as a small extensional sub-basin whose geometry was controlled by reactivation of north-easterly basement structures. The strong association of gold mineralisation with contemporaneous extensional faulting and volcanism is similar to that observed in active volcanic terranes. Recognition of this association through integrated geological and geophysical analysis of aeromagnetic data may be important in exploration for similar targets both in the Drummond Basin and elsewhere.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

The detection and significance of geological structures mapped in the Canning Basin, Western Australia, using NOAA-AVHRR satellite imagery

IJ. Tapley

CSIRO Division of Exploration Geoscience, Private Bag, Wembley, WA, 6014, Australia

Enhancements of NOAA-AVHRR night-thermal imagery of the Canning Basin have been mterpreted to compile a map of regional and local lineaments. A total of 233 lineaments, ranging in length from 37 to 700 km and oriented in four major and two nunor directions, were interpreted from a night-thermal composite image and direction^y-filtered images. Most lineaments are manifested on the imageiy as combmations of surface and sub-surface geological structures, palaeovalleys, and linear changes m tonal contrasts marking differences in Uthology, soil moisture and vegetation. &cept where a lineament coincides with straight segments of a palaeochannel, the bneaments are often of dimensions which make them not apparent to the ground observer. They often cross several geomorphological and geological provinces, with little evidence of dislocation or change The close coincidence between several of the NOAA-derived lineaments and previously mapped faults that define the boundaries of the Canning Basin's main structural units is widence that these NOAA lineaments represent faults and fractures, and just as important, that considerable reliance can be placed on the other NOAA lineaments. It is significant that most of the mapped faults within the Basin have minimal surface egression and are primarily revealed seismic survey, yet they each exhibit a thermal signature. The parallelism of the lineament assemblages with mapped faults defining the structural gram of adjacent provinces is also strong evidence for their existence. A prominent, previously undefined, 285-290 • group may be unique to the scale, resolution and thermal aspects of the NOAA imageiy. Significantiy, several lineaments coincide with mapped faults m the Granites-Tanami Block, whilst others paraUel the fault-bounded, northeastern margin of the Pflbara Block. One prominent lineament appears to control the emplacement of a 370 km length section of the Wallal Palaeoriver. This group of hneaments may be a conjugate set to a 315-330* group, which paraUel the northern rift^ge of the Canmng Basin. A 20-30 • group matches the south-eastern edge of the Halls Creek Mobile Zone. The largest lineament matches the Halls Creek Fault and can be trawd for 700 km from Lake Argyle to the centre of the Canning Basin where it coincides with the alignment of a prominent palaeochannel. The parallel relationship of the lineaments from these three groups with the Canning Basin's faulted margins suggests that they are surface expressions of deep-seated faults and zones of weakness associated with tertonics and development of the Basin. Several lineaments coincide with breaks or trends in contoured gravity and aeromagnetic data. TTiis suggests that the basement tectonics, through later reactivation, may have printed through the thick sedimentary sequence. Circular and arcuate features are also apparent on the night-infrared imagery. It is not known if they are controlled subsurface intrusion or hy basement folding; however, their presence is geomorphically manifested by the effect on local and regional drainage patterns. ®

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Factors affecting the acquisition of structural data from remotely sensed images of eastern Australia

C.R. Nash

Australian Photogeological Consultants, 48 Jacka Crescent, Campbell, ACT, 2601, Australia

Systematic structural analysis based upon the spatial information contained in remotely sensed images depends from the outset upon accurate interpretation and annotation of the attitudes and surface traces of bedding and foliation planes, faults and joints. In regions of fluviatile dissection these fundamental elements are mainly expressed as stream alignments and interfluve ridges; their interpretatxon may however be compromised by several inter-related factors associated with image scale, solar illumination, terrain roughness and erosional history. The effects of these factors are illustrated from the results of research in eastern Australia. The selection of appropriate image scale is of importance since a proportional relationship exists between scale on the one hand and size and number of interpreted structures on the other. The well documented effects of solar azimuth and elevation at the time of image acquisition are critical in eastern Australia since the identification of structure in dissected terrains is largely dependent upon shadow enhancement. Structures with trends parallel to the direction of solar Illumination will consequently be under-represented. In regions of moderate to high latitude the effects of low winter solar elevation provide considerable enhancement of structure. Variations in terrain roughness may be caused by lithologic inhomogeneity or by local tectonic movements resulting in accelerated erosion. In southeastern Queensland it has been demonstrated that the density of Interpreted structural information is proportional to terrain roughness, •nils effect is responsible for the irregular distribution of structural information depicted upon most photogeological maps and requires careful consideration. The geomorphic history of regions of fluviatile dissection should also be carefully considered when undertaking structural interpretation. A cited example from far northern Queensland illustrates the manner in which linear drainage systems, inherited from overlying jointed sedimentary cover, have propagated downward and now flow across the structural grain of exhumed Palaeozoic rocks, in which they suggest a spurious set of structural lineaments.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Applying apatite fission track analysis to tectonics: Examples from southern Africa, south eastern Australia and Antarctica R. Brown^, P. FitzgeralcP and A. Gleadow^

Fission Track Res. Group, Dept. of Geology, La Trobe University, Bundoora, Victoria, 3083, Australia Geology Department, Arizona State University, Tempe, Arizona 85287, USA

"^e analysis of fission tracks in apatite provides a unique methodology for reconstructing the themd, temporal and spacial evolution of the upper few kilometers of the Earth's crust. The technique makes use of the thermal sensitivity of the radiation damage produced by the spontaneous fission of trace amounts of 238u present in the apatite crystals. Temperatures charactenstic of the upper crust (~60®-110®C) are sufficient to annealfissiontracks in apatite over geologic^ time scales with the effect of anneaUng being to shorten the lengths of the fission tracks Md consequenUy to reduce the apparent ages measured in the apatite crystals. Therateof theimal a n n ^ g and therefore the degree of track length shortening and agereductionis direcdy proportion^ to temperature witii the most noticeable effect occuring between -60° and ~110°C (for geological heating times). T^e effect of thermal annealing offissiontracks in apatite produces a systematic decrease in track length and apparent age with temperature (and hence depth) within the crust, reaching zero at temperatures ~110®C. The form of the apparent age and track length profiles within the crust wm thwfore be diagnostic of the thermal stracture of the crast as well as preserving a record of Its earlier themal evolution. Detailed analysis of apatite fission track crustal profiles, in conjunction with a modeling procedure based on a mathematical description of the annealing behaviour offissiontracks in apatite, can thus provide quantitative consttaints on thetimingand style of tectomsm that affects thetiiermalstracture of the crust. "Die diagraim below demonsti^te the principles of interpreting the results of apatite fission track malysK for a series of samplesft-omeither a bore-hole or from arangeof topographic elevations. The modelresultsare based on datafromthe Transantarctic Mountains and clearly show the systemtic variation between the meanfissiontrack length and the shape of the track length distiibution (standard deviation) and the relationship between the mean track length and apparent age. The distinctive inflextion points on the apatitefissiontrack profiles are diagnostic of a discreet episode of cooling. The magnimde and time of cooling can be calculated from the posmoM of these inflextion pointsrelativeto thetiiermalfirameofreferenceoftiiecrust. This profile ^ m Mt Doorly indicates that a relativelyrapidepisode of coolingresultingfrom~4 km of denudation occured at ~ 50 Ma, which is coeval witiitiieUtiiospheric extensiontiiatis tiiought to have produced the Transanarctic Mountains andtiieadjacent Ross Sea basin Similar styles of apatitefissiontrack profiles have been identified ontiieAtiantic margin of southern Africa and in soutii eastern Austi-aUa. The sampling approach and interpretive metiiodology outiined here has also proved to be useful in several otfier tectonic settings such as tiie New Zealand and European Alps,tiieBrooks Range in nortiiem Alaska, the Papuan Fold Belt and mtiieBasin and Range Province intiieUSA. Mean Track Length (jim) 4

/+ ^ 50

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Apatite Fission Track Age (Ma)

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Apatite Age (Ma)

Mean Fission Track Length (>im)

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Kinematic modeling and balancing of high strain thrust systems

WJ. Dunlap

Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, 55455, USA

Balancing cross-sections of defonned rocks from the mid/lower crust has been hindered by the lack of reliable strain and kinematic data. The Ruby Gap high strain duplex exposed in the Arltunga Nappe Complex, central Australia, is a 5 km thick antiformal stack composed of five sheets, each containing Heavitree quartzite. Both thefinitestrain and the degree of recrystallization increase upward through the duplex from a poorly defc^ed basal sheet 1 to the highly mylonitized sheet 5. Strain estimates, which were obtainedfinomdeformed pebbles, indicate a plane strain history. There is a good COTrelation between the magnitude of strain and the percentage of reoystallized grains, such that each sheet of the duplex can be assigned a stretch. Kinematic indicators as well as qi^C-axis fabric patterns suggest that the deformation history was dominated by smiple shear although a component of pure shear is evidenced. It is generally agreed that the development of crustal duplexes involves piggy-back thrusting, as opposed to the reverse sequence of imbrication. In a high strain thrust system, piggy-back thrusting wiU result in accumulation of the highest strain in the uppermost sheet of the duplex. The lowest sheet, being the last to be imbricated into the duplex, will exhibit the lowestfinitestrains. It is suggested that piggy-back thrusting dunng imbncation may likely be followed by a reverse sequence of thrust cessation during the waning stages of movement Using strain and kinematic data from the Ruby Gap duplex, several kinematic models of piggy-back thrusting arc examined in oider to Illustrate the large range of shortening valuesrcsultingfrom the various models. Even if therelativeimportance of pure and simple shear are known, the kinematic evolution of a high strain duplex is largely dependent on therelativeorder of (1) strain developed within sheets and (2) displacement on major thrusts. Two extreme endinember cases are possible, where deformation occurs either entirely before or entirely a f ^ thrust displacement Intermediate cases involve simultaneous incremental thrusting and straining and are probably mostrepresentativeof duplex evolution. Displacement estimates fortiieRuby Gap duplex are minimized when thrusting dommates early (TS) in the deformation history and maximized when thrusting dominates later (ST) in the defonnation histoiy regardless of tlierelativecontribution of pure and simple shear. Therefore, in order to assess the magnitude of displacement in high strain duplexes accurately it is important to address the temporal dominance of tlOTsting and straining. Displacement estimates are also extremely sensitive totiiechoice of strain history. For example, shortening estimates rangefix)m34 km to 64 km, depending only on therelativecontribution oftiieend-members pure and simple shear. In order to obtain even rough estimates of shortening in ductilely deformed crust tiie strain history must be shown to be relatively simple and the relative order of straining andtiinistingmust be known.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Synthesis of multiscale remote sensing interpretations for defining basement/cover structures

J. Creasy

CSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, NSW, 2113, Australia

Remote sensing has been used extensively for studying basement/cover structural relationships. In particular, the interpretation of fracture and lineament patterns in sedimentary basins can be used to define regional structures inherited from the underlying basement. The difficulty with brittle structures is that they can range in size from centimetres to kilometres. Furthermore the interpretation of fracture and lineament patterns from remotely sensed imagery is influenced by various parameters including scale, ground resolution, sun angle, and interpreter bias. Remote sensing data setsvary in resolution and scale from AVHRR (1.1 kilometre pixels; 1:2,500,000 scale), Landsat MSS and TM (80 to 30 metre pixels; scales up to 1:250,000), SPOT (10 metre pixels; 1:250,000 up to 1:25,000) and aerial photography (1:80,000 to 1:10,000). These resolution/scale constraints effectivelyfilterstructural features, with higl frequency smaller fractures only resolved at large scales. Conversely, regionally dominant low frequency lineaments identifiable at small scales are usually diffuse and difficult to clearly identify on larger scale imagery. Variations in the trends of structures can be derived from different scales of interpretation. Thus interpretation made at one scale enables only one level of structures to be defined and ultimately results in poor correlation when compared to mapped fracture patterns at outcrop scale. Multiscale fracture trace interpretations were made from Landsat MSS and 1:80,000 aerial photography over the Sydney Basin. The interpretations were compiled onto 1:100,000 base maps, then overlayed with geology and topographic maps. A synthesis map showing the lineament pattern for each map sheet was then compiled from the different scales of interpretations. A 1:500,000 basin-wide lineament pattern was then derived from a synthesis of all 1:100,000 scale interpretations. The resulting lineament pattern, defined through synthesis of different scales of imagery, is more detailed than a single small sc^e interpretation. Lineaments can be further categorised according to their expression, for example, as fracture lineaments or lineament zones. In the Sydney Basin the synthesis-derived lineament pattern can be related spatially and geometrically to basement structural trends. The Permo-Triassic Sydney Basin unconformably overlies the Palaeozoic Lachlan Foldbelt. The fold belt comprises grossly meridionally trending folded sediments that have sharp changes in strike across narrow ENE and BSE trending megakink planes. These transverse zones arespatially related to lineaments identified previously on Lan^at MSS imagery. Other basement anisotropics occur at different scales and include variations in lithology and intensities of meridional folding. The Lower Permian sandstones of the southern Sydney Basin display different lineament styles that reflect the type of basement structure. NNE trending fold axes marking the silicic volcanic strike ridges of the Devonian Budawang Synclinorium coincide with singular long lineaments in the overlying cover. Zones of intense mesoscale kinks relate to broader and shorter lineament zones that trend E to ESE. Other SE trending lineaments are related to major basement transverse faults. The lineament pattern developed in the extensive plateau-forming Triassic sandstones over the basin consists of four major trends. The WNW trend parallels transverse structural zones in the fold belt, including the Lachlan River Lineament Zone (LRLZ). The NNE trending lineaments parallel basement strike south of the LRLZ and northwards they form complex zones indicative of wrench style deformation. ENE trendind zonal lineaments parallel fold belt transverse structural zones and are coincident with transform fracture zones associated with Tasman Sea rifting. The widely distributed NW trending lineaments relate to basin-^de extension.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Seafloor spreading in the Australian-Antarctic Discordance J. Palmer^, J.-C. Sempere^, D. Christie^, J. Phipps Morgan^ and A. Shoi^ ^School of Oceanography, University, of Washington, Seattle, Washington 98195, USA College of Oceanography, Oregon State University, Corvallis, Oregon 97331, USA y e p t . of Earth, Atmospheric and Planetary Sciences, M.IT., Cambridge, Massachusetts 02139, USA Hawaii Institute of Geophysics, Honolulu, Hawaii 96822, USA In 1988 we conducted an integrated survey of the Southeast Indian Ridge (SEIR) between 123"E and 132«30'E usmg the SeaMARC II sidescan sonar and swath bathymetry system, magnetics, gravity, and dredgmg. From 120»E to 128»E, the SEIR crosses the Australian-Antarctic Discordance (AAD) a zone of rugged, anomalously deep bathymetry stretching from Australia to Antarctica. Geochemical data suggest that the AAD marks the boundary between two global mantle isotopic provinces and models of global manUe flow predict that such a boundary may be manifested as a zone of as'thenosphenc downwelling. Geophysical data examined by other workers support the hypothesis that anomalous mantle underlies the AAD. Within the AAD. the SEIR is divided into five transform-bounded sections (B1-B5), of which the eastern three (B3-B5) were surveyed during this study. Initial observations reveal that each of these major secuons is further segmented by non-transform offsets of the spreading axis. Even though the s p ^ g is arelativelyconstant 74 mm/yr (total rate) in this region, pronounced contrasts in axis Md ndge offset morphologies are noted among the AAD ridge segments and proximal sections of the SEK. arguing against the simple spreading rate controls on axial morphotectonics that have previously ^ n In general, the SEIR within the Discordance is characterized by a deep axial rift valley, while the spreading center to the east generally corresponds to an axial peak. In addition the spreadmg axis within the AAD lies at an average depth of 4500 m. nearly 2 km greater than the' worldwide average. Ridge-transform intersection morphologies also appear to reflect local magma supplyratherthan simple spreading rate controls, with well-defined nodal basins located adjacent to deep nft valleys and absent elsewhere. TTie wide range of spreading center moiphologies and axial depths su^ests a maiked variation in magma supply between adjacent ridge segments in the AAD region. with transform and non-transform offsets demarcating transitions in magmatic budget The non-transform offsets within the AAD are relatively small (less than a few km of offset) and subUe in structural expression, similar to offsets observed on the slow-spreading Mid Atlantic Ridge but our data suggest that theyrepresentfundamental transitions in crustal accretion along the SEIR. BetweCT I24«40'E and 125<'30'E. the SEIR lies withui a zone of chaotic tectonic fabric and poorly organized seafloor spreading which we believe corresponds to a region of particularly low or erratic magma supply. Outside of this zone, axial morphologies are weU-defined and tectonic febric is lineated strongly paraUel to the trend of the plate boundary. We note that this portion of the spreading center roughly corresponds to the location of the AAD isotopic boundary and to the intersection of the spreading axis with the trend of maximum residual depth anomalies within the Discordance These observations suggest a correUtion between the low and/or erratic magma supply to this portion of the SEIR and the anomalous manUe processes beneath the AAD. The SEIR has long been known to be a region of markedly asymmetric seafloor spreading. The integration of bathymetric data with structural information gleaned from SeaMARC n sidescan sonar coverage reveals a distinct pattern of asymmetric spreading that has persisted within at least one section (B5) of the AAD during the last 1 m.y. This asymmetry is characterized by the formation of an unbalanced moiphotectonic fabric that is preserved off-axis, with wider and shallower abyssal hills found on the faster-spreading flank of the spreading center. We interpret this asymmetric accretion to be generated by incremental migration of the spreading axis away fi-om the faster-spreading south flank of the ridge section. This result is consistent with the overall northward migration of the SEIR with respect to the underlying mantle.

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Relation of ocean-floor structures to the Australian continental margin: Seasat images

R.C. Bostrom University of Washington AJ-20, Seattle, Washington 98195, USA T h e e f f e c t h a s b e e n e x a m i n e d of s e a - f l o o r t e c t o n i c f e a t u r e s s u c h as f r a c t u r e z o n e s on the c o n t i n e n t a l m a r g i n of S W A u s t r a l i a (Perth r e g i o n ) , N W A u s t r a l i a ( R a n k i n r e g i o n ) , N . A u s t r a l i a ( r e l a t i o n to a d j a c e n t s u b d u c t l o n ) , a n d e a s t e r n A u s t r a l i a ( r e l a t i o n to a trough p a r a l l e l i n g the e a s t e r n s h o r e ) . P r i n c i p a l l y , w e h a v e used a l t i m e t r i c s a t e l l i t e d a t a ( S e a s a t and G e o s a t ) . In the P e r t h r e g i o n , a " m a s t e r F Z " a p p r o a c h e s the c o n t i n e n t a l m a r g i n from the N W . T h e i n t e r s e c t i o n s e e m s to be c o n n e c t e d w i t h the d e v e l o p m e n t of an E W f e a t u r e c u t t i n g a c r o s s the c o n t i n e n t a l s h e l f , in the d i r e c t i o n of the s e i s m i c locality on the D a r l i n g F a u l t w e s t b o u n d i n g the Y i l g a r n B l o c k . In the R a n k i n r e g i o n g r a v i t y d a t a s u g g e s t the e x i s t e n c e of s e d i m e n t s b e l o w s e i s m i c r e a c h . In the n o r t h , a m a j o r N W - S E d i s l o c a t i o n , on p o o r e v i d e n c e h a v i n g s i n i s t r a l d i s p l a c e m e n t , n o r t h - b o u n d s the J a b i r u p e t r o l e u m p r o v i n c e and c u t s in two the b a s e m e n t of the B o n a p a r t e b a s i n . Small b a s i n s h a v e b e e n s o u g h t on the narrow shelf offlying eastern Australia, possibly c o n n e c t i n g w i t h the s e d i m e n t a r y a c c u m u l a t i o n in a t r o u g h p a r a l l e l i n g the c o n t i n e n t a l m a r g i n . T h e d e v e l o p m e n t of small b a s i n s o f f l y i n g B r i s b a n e and N e w c a s t l e m a y be a s s o c i a t e d w i t h the i n t e r s e c t i o n of a d j a c e n t f r a c t u r e z o n e s and the c o n t i n e n t a l m a r g i n . In s o u t h e r n A u s t r a l i a w e h a v e s o u g h t b a s i n s c o n n e c t i v e w i t h the m a s s i v e o f f s h o r e s e d i m e n t a c c u m u l a t i o n s . In s o m e l o c a l i t i e s as at E s p e r a n c e f r a c t u r e z o n e s t r u c t u r e s s e e m to e x t e n d to the s h o r e - l i n e .

9TH INTERNATIONAL BASEMENT TECTONICS SYMPOSIUM

Basement-cover relationships in orogenic belts

MJ. Rickard Department of Geology, Australian National University, Canberra, ACT, 2601, Australia

Traditionally basement-cover effects are thought of as cratonic structures — basement faults and movements affecting cover basins etc. However, as most orogenic belts are located at craton margins, the craton edge itself could be expected to have an important tectonic effect during the subduction and or collision processes accompanying plate movements that are thought to cause orogenesis. The structural complexities resulting from collision impingement i.e. Indian with Asia, and Iberia are well documented. Other important structural features could well be ascribed to basement influence. For example the location of major thrust fronts, common in many orogenic belts, are probably localized by the strut effect of the continental edge. Basement slices in thrust belts may well result from reversal of movement on original continent-edge basin margin (hstric) faults. This overcomes the difficult mechanical process of slicing consolidated crustal basements otherwise needed to explain such phenomena. Continent-edge upwards are likely to influence sediment deposition and subsequent deformation. Another type of basement control, that of the plan geometry of orogenic belts is caused by the influence of salients and recesses in the original rifted craton margin. These not only control the sedimentary basin development but also the intensity of deformation during closure, with strong deformation at the salients and weaker effects in the recesses. In fact subsequent rifting and sedimentary basins of a new tectonic (Wilson) cycle may be controlled by the original irregularities so that major reflection effects may be deduced. Cratonic edge effects may also influence fold-belt trends and oroclinal bends. Some examples illustrating these concepts will be presented.

AUTHOR INDEX Abdul Hameed, 1. Adams, D.P.M. Banna, A-S. Alsinawi, S.A. Al-Saigh, N.H. AIt,D. Baars, D.L. Bassi, H.G.L. Bostrom, R.C. Braun, J. Brown, R. Byrne, D.R. Carlson, M.P. Christie, D. Collins, C.D.N. Creasy, J. Dobos, S.K. Duggan, M.B Dunlap, WJ. Edgell, H.S. Finlayson, D.M.6 Fitzgerald, P. Fleming, P.D. Gibson, G.M. Gleadow, A. Goleby, B.R. Harris, L.P. Haszeldine, R.S. Henley, R.W. Hodgson, R.A. Holcombe, R J. Johnstone, D.W. Jons, H.-P. Katz, M.B. Kodama, K. Korsch, R J .

18 35 17 17 18 25 24 31 42 13 38 5 23 41 11 40 9 2 39 20 38 12 9,26 38 11 5 16 35 34 9 6 28 27 21 6,11

Krowkowski, J. 10 34 Kvet, R. Little, T.A. 9 Liu, S.F. 7,12 McQueen, H. 13 Mills, KJ. 7 Muir, M.D. 4,32,33 Namik, 1. 18 Nash, CR. 37 O'Brien. P.E. 6 O'Driscoll, E.S.T. 29 Olissoff, S. 10 Palmer, J. 41 Parker Gay, S. Jr 22 Phipps Morgan, J. 41 Rickard, MJ. 43 Rutland, R.W.R. 1 Sears, J.W. 25 Semp6r6, J.-C. 41 Shaw, R.D. 11 Shor, A. 41 Silwa, R. 9 Sindi, HO. 19 Smith, C.B. 33 Tapley, IJ. 36 Thomas, M.D. 15 Wake-Dyster, K.D. 6 Wellman, P. 8 Whitaker, A. 3 White, S.H. 4,33 Williams, P.R 2 Wright, C 11 Zhou, B. 7