the australian stress map - australian school of...

Journal of the Geological Society, London, Vol. 157, 2000, pp. 915–921. Printed in Great Britain.

The Australian Stress Map

R. R. HILLIS1 & S. D. REYNOLDS1

1National Centre for Petroleum Geology and Geophysics, University of Adelaide, SA 5005, Australia(e-mail: [email protected])

Abstract: Knowledge of the in situ stress field of the Australian continent has increased greatly sincecompilation of the World Stress Map in 1992, principally by analysis of borehole breakouts anddrilling-induced tensile fractures in petroleum wells. Stress orientations are variable across the Australiancontinent as a whole. However, within 15 of 16 individual stress provinces defined in the Australiancontinent (of one to a few hundred kilometres scale), mean stress orientations are statistically significant.The stress provinces, and stress trajectory mapping, reveal that there are systematic, continental-scalerotations of stress orientation within Australia. Unlike many other continental areas, stress orientationsdo not parallel the direction of absolute plate motion. Nonetheless, the regional pattern of stressorientation is consistent with control by plate boundary forces, if the complex nature of the convergentnortheastern boundary of the Indo-Australian plate, and stress focusing by collisional segments of theboundary, is recognized.

Keywords: Australian Stress Map, in situ, stress fields, plate tectonics.

The in situ stress field of the Australian continent was poorlyconstrained at the time of compilation of the World StressMap (Zoback 1992). Furthermore, the limited data that didexist indicated complex and scattered stress orientations(Richardson 1992). This contrasted strongly with other conti-nental areas such as North and South America and WesternEurope, which are characterized by broad regions wheremaximum horizontal stress orientation is consistent and par-allel to the direction of absolute plate motion (Zoback et al.1989; Zoback 1992; Richardson 1992).

Since compilation of the World Stress Map, the number ofreliable (A–C quality ranked) in situ stress orientation data forthe Australian continent has increased from 95 to 319, princi-pally by analysis of borehole breakouts and drilling-inducedtensile fractures in petroleum wells. The new data reveal a clearpattern of stress orientations within the Australian continent,and confirm that, unlike other continental areas, regionalstress orientations do not parallel the direction of absoluteplate motion. This paper describes maximum horizontal stressorientations within the Australian continent, defining stressprovinces and mapping stress trajectories based on the newin situ stress data. It also argues that, despite the absence ofparallelism between stress orientations and plate motion, theforces acting on the Indo-Australian plate exert a first-ordercontrol on the pattern of stress within the Australiancontinent.

New in situ stress dataZoback (1992) reviewed the indicators from which horizontalstress orientation is inferred in the crust, namely: earthquakefocal mechanisms, borehole breakouts, engineering-typemeasurements (overcoring and hydraulic fracturing), andyoung geological data (fault slip and volcanic alignments).Maximum horizontal stress orientations in the AustralianStress Map are quality ranked ranging from A (highest qual-ity) to E (lowest quality) following Zoback’s (1992) scheme.A–C ranked data are considered to indicate stress orientationsreliably. Data ranked D-quality are associated with a high

standard deviation/low reliability, and E-quality data containno reliable information on stress orientations. The type,number and quality of stress indicators currently held in theAustralian Stress Map database are summarized in Table 1.New Guinea, the continental crust of which is contiguous withthat of Australia, is included in the Australian Stress Map.

Where Australian earthquakes are of sufficient magnitude toyield focal mechanism solutions (from which the orientation ofthe principal stresses can be inferred), these are routinelyundertaken by the Australian Geological Survey Organization(e.g. McCue 1996), and have been added to the database. Someadditional solutions published subsequent to the World StressMap compilation have also been added to the database(Greenhalgh et al. 1994). New engineering-type measurementshave been added to the database, especially for easternAustralia. Some of these measurements come from unusuallygreat depth for engineering-type measurements (in excess of1 km), and only those measurements considered to be unper-turbed by local site effects such as mining excavation wereincorporated (Hillis et al. 1999). No new geologically basedstress measurements have been added to the database.

Borehole breakouts and drilling-induced tensile fracturesin petroleum wells have been the primary source of new

Table 1. Stress indicators in the Australian Stress Map database

Quality

Type A B C D E Total

Focal mechanisms (FM) 1 29 66 22 10 128Breakouts (BO) 38 59 39 78 32 246DITF 11 10 3 4 0 28Hydraulic fracturing (HF) 5 27 25 18 1 76Overcoring (OC) 0 2 4 49 1 56Geological indicators (G) 0 0 0 0 2 2Total 55 127 137 171 46 536

DITF: drilling-induced tensile fracture. A–C quality data, of whichthere are 319, are considered to reliably indicate in situ maximumhorizontal stress orientation.

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information in the Australian Stress Map and their origin isbriefly discussed. The presence of an open wellbore disturbsthe far-field stresses within the subsurface. The circumferentialor hoop stress (��) acting at the wall of a vertical wellbore isgiven by:

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.where �H and �h are the maximum and minimum far-fieldhorizontal stresses respectively, � is the angle at the wellborewall measured from the azimuth of �H, Pw is the mud pressurein the wellbore, and Po the pore pressure of the formation (e.g.Moos & Zoback 1990, and references therein). Circumferentialstresses are maximized at right angles to the azimuth of �H

(i.e. at the azimuth of �h; Fig. 1). Where the maximumcircumferential stress exceeds the compressive strength of therocks forming the wellbore wall, compressional shear failuremay occur. Failure of intersecting, conjugate shear planesleads to pieces of rock spalling, or breaking off the wellborewall, and an elongation of the wellbore cross-section in thedirection of �h, which is known as borehole breakout, mayresult (Fig. 1). Circumferential stresses are minimized at theazimuth of �H. Tensile fractures form where the circumferen-tial stress is less than the tensile strength of the rocks formingthe wellbore wall, and are oriented at right angles to breakouts(Fig. 1). The ovalization of the cross-sectional shape of thewellbore associated with breakout can be recognized on wire-line logs with dual, orthogonal caliper readings such as dip-meter logs (Dart & Zoback 1989). Resistivity image logs (suchas the FMI of Schlumberger), which have evolved fromdipmeter logs, allow both breakouts and drilling-inducedtensile fractures to be recognized. Resistivity image logs werenot widely available at the time of compilation of the WorldStress Map and drilling-induced tensile fractures were notrecognized as a stress indicator at that time (Zoback 1992).However, they have become widely recognized since (Brudy &Zoback 1999), and they are quality ranked in the AustralianStress Map database using the same criteria as breakouts.

Breakouts and drilling-induced tensile fractures in wells thatare deviated from the vertical may not be reliable indicators of

horizontal stress orientation (Mastin 1988; Zajac & Stock1997). In deviated wells their orientations are controlled by theentire stress tensor, including the vertical stress magnitude,and, projected onto a horizontal plane, their orientations maydiffer from that in a vertical well. The vast majority ofbreakouts and drilling-induced tensile fractures analysedwithin the Australian Stress Map are from wells deviated lessthan 15� (most less than 5�) and are not thus affected. Datafrom a small number of wells with greater deviations have beenincorporated where knowledge of the full stress tensor indi-cates that horizontal stress orientation can be reliably inferred(eg. in a strike-slip faulting stress regime, boreholes mustdeviate at least 35� from the vertical before the horizontalprojection of a breakout differs by more than 10� from theminimum horizontal stress direction; Mastin 1988).

The composition of data in the Australian Stress Map issummarized in Table 1 and Fig. 2. Approximately half ofthe reliable, A–C quality data come from breakouts and

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Fig. 2. Distribution of reliable (A–C quality) data in the AustralianStress map database. (a) Distribution by stress indicator type.(b) Distribution by stress indicator type and depth.

916 R. R. HILLIS & S. D. REYNOLDS

drilling-induced tensile fractures in petroleum wells. Thirtypercent of the data come from earthquake focal mechanismsolutions, and the remaining 20% from engineering-typemeasurements. A large number of D-quality data are in thedatabase. In the case of breakouts, the D-quality data repre-sent few breakouts (<4) in a well and/or scattered orientations(standard deviation, 25�). In the case of overcoring data, mostof the D-qualities are assigned to data for which detailedinformation on the number of measurements at individual

locations was not available. These overcoring data representthe mean of multiple reliable observations (Enever pers. comm.1999) and the quality ranking assigned is thus conservative.Given the large amount of D-quality stress indicators, mapscomprising both A–C and A–D quality data are presented(Figs 3 and 4).

The distribution of stress indicators with depth in theAustralian Stress Map is typical, with engineering-type measurements generally from less than 1 km depth,

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THE AUSTRALIAN STRESS MAP 917

petroleum-type measurements (borehole breakouts anddrilling-induced tensile fractures) generally from 1–4 kmdepth, and earthquake focal mechanism data from deeper(albeit usually poorly constrained), seismogenic depth. Ininterpreting the data it should be borne in mind that differentstress indicators sample different depths in the crust.

Stress provinces and stress trajectoriesStress orientations are highly variable across the Australiancontinent as a whole (Figs 3 and 4). However, within indi-vidual provinces (at the scale of one hundred to a few hundredkilometres), stress orientations are generally broadly consist-ent. Two techniques have been applied to clarify regionaltrends in stress orientation across the Australian continent:definition of regional stress provinces and mapping of stresstrajectories.

A minimum of four A–C quality stress orientation datawithin a distinct geographic region is defined to constitute astress province (Table 2 and Figs 5 and 6). The Rayleigh Testwas applied to the individual stress orientation data withineach stress province to investigate whether, and how stronglydeveloped any preferred stress orientation is within the prov-ince (Mardia 1972). A type 1 stress province indicates that thenull hypothesis that stress orientations in the province arerandom can be rejected at the 99% confidence level, type 2 atthe 97.5% level, type 3 at the 95% confidence level, and type 4at the 90% confidence level. A type 5 stress province indicatesthat the null hypothesis that stress orientations are random cannot be rejected at the 90% confidence level.

The categorization of stress provinces should not be con-fused with the quality ranking of individual stress orientations.The individual stress orientation data within a type 5 stressprovince are no less reliable per se than those in a type 1province, rather they display a more scattered orientation. Therange from type 1 to type 5 stress provinces is likely to reflectthe degree of horizontal stress anisotropy within the province,

with there being little or no significant regional horizontalstress anisotropy within a type 5 stress province, allowing localeffects such as those associated with faults to dominate. Thereis likely to be pronounced horizontal stress anisotropy in atype 1 province. Individual stress measurements of D- orE-quality may reflect relatively isotropic horizontal stresses(e.g. E-quality where there are clear breakouts in a well buttheir standard deviation is >40�). However, they may alsorepresent poor quality data (e.g. E-quality where no reliablebreakouts can be detected because there is no rotation of adipmeter tool in the well, hence no evidence of it having lockedinto the long axis of breakout).

The information on stress regimes in each of the stressprovinces has also been summarized (Table 2). Earthquakefocal mechanism solutions yield the orientations of the princi-pal stresses, and thus the associated stress regimes which are

Table 2. Stress provinces defined within the Australian Stress Map database

Type Quality Statistics

Province No. A–C FM BO HF OC DITF A B C Mean (�N) SD R Conf. Regime �

Amadeus Basin 11 0 11 0 0 0 2 6 3 013 27 0.632 97.5 —N. Bonaparte Basin 60 0 40 0 0 20 24 28 8 048 16 0.85 99 —S. Bonaparte Basin 4 0 4 0 0 0 3 1 0 057 4 0.990 97.5 —Bowen Basin 31 0 0 31 0 0 3 17 11 014 19 0.798 99 0.05Canning Basin 9 2 7 0 0 0 2 1 6 053 12 0.917 99 0.5Carnarvon Basin 27 0 27 0 0 0 7 13 7 101 35 0.465 99 —Cooper Basin 14 0 10 0 0 4 4 8 2 102 14 0.888 99 —Flinders Ranges 6 6 0 0 0 0 0 1 5 083 31 0.561 <90 0.17Gippsland Basin 7 0 7 0 0 0 2 2 3 130 20 0.787 99 —Irian Jaya 1 37 37 0 0 0 0 0 6 31 026 29 0.602 99 0.48Irian Jaya 2 13 13 0 0 0 0 0 8 5 044 27 0.641 99 0.31New Guinea 1 4 4 0 0 0 0 0 0 4 049 7 0.973 97.5 0.38New Guinea 2 8 8 0 0 0 0 0 4 4 016 24 0.698 97.5 0.06Otway Basin 5 0 5 0 0 0 2 0 3 136 15 0.873 97.5 —Perth 22 4 14 1 3 0 1 8 13 096 26 0.658 99 0.16Sydney Basin 28 5 1 22 0 0 3 10 15 054 41 0.356 95 0.04

SD: (circular) standard deviation. R: length of the mean resultant vector of maximum horizontal stress orientations within a province (Mardia 1972).If R exceeds a certain critical value dependent on the number of data, then the null hypothesis that stress orientations in the province are randomcan be rejected at the stated confidence level (Conf). <90% indicates that the null hypothesis can not be rejected at the 90% confidence level. Regimeis the average stress regime determined using Coblentz & Richardson’s (1995) parameter, �. For pure normal faulting �=1, for pure strike-slipfaulting �=0.5, and for pure thrust faulting �=0. See Table 1 for abbreviations of the stress data types.

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classified in the World Stress Map scheme as thrust faulting(�H>�h>�v), thrust with strike-slip component, strike-slip(�H>�v>�h), normal with strike-slip component, or normal(�v>�H>�h), where �v, �H and �h are the vertical, maximumhorizontal and minimum horizontal stresses respectively(Zoback 1992). Hydraulic fracturing and overcoring tests yieldabsolute values of stress magnitudes (eg. Hillis et al. 1999) andthese have also been incorporated in the determinations ofaverage stress regime in each of the provinces (Table 2).Borehole breakouts and drilling-induced tensile fractures,which make up the majority of the Australian Stress Mapdatabase, do not yield information on stress magnitudes.

Previous attempts to characterize regional stress fields havedivided data both worldwide and within the Indo-Australianplate into 5� latitude �5� longitude bins and averagedstress orientations and stress regimes therein (Coblentz &Richardson 1995; Coblentz et al. 1998). Although broadlyreliable, this approach does lead to some anomalies. Forexample data from the Carnarvon Basin stress province (asdefined herein) would be split into four separate bins, thecentre point of which lies approximately in the middle of theCarnarvon Basin (Fig. 5). Similarly data from the NorthernBonaparte Basin stress province would be split into two bins,the eastern one of which would also include data from theSouthern Bonaparte Basin which forms a distinct data clustersome 500 km from the Northern Bonaparte Basin.

Stress trajectory determination provides a technique forsmoothing and interpolating unevenly distributed stress dataand thereby clarifying regional trends. A stress trajectory map(Fig. 7) has been calculated from the Australian Stress Mapdata following the technique of Hansen & Mount (1990). Thestress trajectories indicate the orientation of the maximumhorizontal stress at each point along the trajectory. However,they do not imply information about magnitudes (the spacingbetween the trajectories does not have any significance).

The technique applies a statistical smoothing algorithm tocreate an estimated stress field at each observed data location.In the technique, fidelity to the raw data must be balancedagainst the degree of smoothing. If the there is too muchfidelity to the raw data, the calculated stress trajectories simplyreflect the stress data, and if there is too much smoothing,variations are obscured.

In determining the estimated stress field, three weightingsystems were applied to the raw data. Firstly, data wereweighted according to their proximity to the observed datapoint being smoothed. Secondly, relative weightings of 4 weregiven to A-quality data, 3 to B-quality data, 2 to C-qualitydata and 1 to D-quality data. Thirdly, a robustness weight wasapplied to eliminate the presence of anomalous data thatdiffered substantially from other observed data in the region.The smoothed stress field, which was calculated at each of theobserved data points, was then used to calculate the stresstrajectories as outlined by Hansen & Mount (1990).

Regional stress regimes in the Australian continentThe stress regime data within the Australian Stress Mapdatabase suggest that horizontal stresses in the Australiancontinent are relatively high, as previously proposed byDenham et al. (1979) and Denham & Windsor (1991). Averagestress regimes range from strike-slip (�H>�v>�h) to thrust(�H>�h>�v, Table 2). No provinces display a normalfaulting stress regime. However, the stress regime data in theAustralian Stress Map database are based on engineeringmeasurements largely at depths of <1 km, and on focal mech-anism solutions that sample seismogenic depths. At the inter-mediate depths of petroleum exploration (1–4 km) a largenumber of leak-off pressures, which provide an approximationto the minimum horizontal stress magnitude (Hillis et al.1998), are available. These data indicate that, for example, inthe majority of the Bonaparte and Cooper–Eromanga Basins,the stress regime is either one of normal or strike-slip fault-ing (i.e. leak-off pressure is less than the vertical stress; Hilliset al. 1998). Leak-off pressures are not compiled within theAustralian Stress Map database because they do not yieldstress orientations. Nonetheless, comparison between leak-offtest data and that from engineering measurements and focalmechanism solutions suggests that there is variation in inferredstress regime with stress indicator type/depth, and thus thatinformation on stress regimes should not be extrapolated todifferent depths.

Regional stress orientations in the Australian continentBoth the techniques used to clarify regional trends in stressorientation across the Australian continent show consistent

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patterns (Figs 6 & 7). The western part of the Australiancontinent is characterized by broadly east–west oriented maxi-mum horizontal stress (Carnarvon Basin and Perth provinces;Figs 3, 4, 6 & 7). Along the northern Australian margin theeast–west trend rotates to NE–SW (Canning Basin, Northernand Southern Bonaparte Basin, Irian Jaya and New Guineaprovinces). This swing in stress trajectories along the northernAustralian margin is broadly paralleled to the south whereeast–west oriented maximum horizontal stress in the Perthprovince rotates to NNE–SSW in the Amadeus Basin (centralAustralia). The Bowen Basin also exhibits NNE–SSW-oriented maximum horizontal stress. Moving west to east inthe southern part of the continent, maximum horizontal stressrotates from east–west in the Perth Basin to NW–SE insoutheastern Australia (Otway Basin and Gippsland Basinprovinces). The area of divergence between NNE–SSW andNW–SE maximum horizontal stress trajectories in centraleastern Australia is characterized by east–west or poorlydefined (low horizontal stress anisotropy) regional stress trends(Cooper Basin, Flinders Ranges and Sydney provinces).

It is immediately apparent that, unlike most other continen-tal areas, stress orientations in the Australian continent as awhole are variable and do not parallel the N–NNE absolutemotion direction of the Indo-Australian plate. North andSouth America and Western Europe are all characterized bybroad regions where maximum horizontal stress orientation isconsistent and parallel to the direction of plate motion(Zoback et al. 1989; Zoback 1992; Richardson 1992). Fromthis it is inferred that the forces driving and/or resistingplate motion are responsible for regional stress orientations inthose continental areas. The absence of correlation betweenstress orientation and absolute plate motion direction in theAustralian continent begs the question as to whether the in situstress field of the Australian continent is subject to differentcontrols than that of other continents. We first consider otherpossible controls, and then return to the hypothesis that plateboundary forces do indeed control stress orientation in theAustralian continent.

Regional stress orientations in the Australian continent donot appear to be affected either by tectonic province, regionalstructural trends, geological age or by the depth at which stressorientations are sampled. This is perhaps best witnessed by thePerth province. In the eastern part of the Perth province datafrom focal mechanisms, overcoring and hydraulic fracturingall indicate broadly east–west-oriented maximum horizontalstress (Fig. 5). These data come from the Precambrian YilgarnCraton which is separated by an approximately north–south-trending crustal-scale fault (Darling Fault) from thePhanerozoic Perth Basin to the west. Breakout data from thePerth Basin similarly indicate that maximum horizontal stressis broadly oriented east-west (although locally anomalousstress orientations are observed in the vicinity of knownfaults). Hence in the Perth province stress orientations areconsistent from near surface to several kilometres depth andacross a major tectonic boundary.

Again although some locally anomalous stress orientationsare observed, especially in the vicinity of faults, the NE–SWregional maximum horizontal stress orientation along much ofthe northern Australian margin is also unaffected by regionalstructural trends, geological age or by the depth at which stressorientations are sampled. The Southern Bonaparte Basin andonshore Canning Basin are Palaeozoic basins characterized byNW–SE structural trends. In the latter basin, focal mechanismand breakout data both indicate NE–SW-oriented maximum

horizontal stress. The northern Bonaparte Basin is dominatedby younger, Mesozoic NE–SW trends associated with theformation of the present passive margin. Structural trends inNew Guinea are NW–SE and associated with Cenozoic colli-sion in the area. All of these provinces, with different age andorientation of dominant structure, display a NE–SW regionalmaximum horizontal stress orientation.

Continental areas which exhibit consistent maximum hori-zontal stress orientations that are parallel to the direction ofabsolute plate motion are surrounded by relatively simple plateboundary configurations, compared to the Indo-AustralianPlate. For example NW-oriented maximum horizontal stress inwestern Europe is a consequence of compressional forcesgenerated orthogonal to the mid Atlantic Ridge and the Alpinecollisional belt (Golke & Coblentz 1996). In contrast,the northeastern boundary of the Indo-Australian plateexhibits uniquely complex and laterally varying convergenttectonic styles that impose a variety of torques on the plate.Continental collision is occurring along the Himalayan, NewGuinea and New Zealand segments of the plate boundary.Oceanic parts of the Indo-Australian plate are being subductedat the Sumatra–Java and Solomon–New Hebrides Trenches,and the Pacific plate is being subducted under the Indo-Australian plate at the Tonga–Kermadec Trench. With recog-nition of the heterogeneity of the northeastern boundary of theIndo-Australian plate, the variation of stress orientationwithin the Australian continent can be understood within thecontext of plate boundary forces.

Considering the Indo-Australian plate as a whole, there is anobserved anticlockwise rotation of stresses from broadlynorth–south in India to NW–SE in the vicinity of the NinetyEast Ridge to east–west in the Carnarvon and Perth provinces,and finally to NE–SW along the northern Australian marginand in New Guinea. This broad rotation can be accounted forby focusing of stresses orthogonal to the Himalayan and NewGuinea continental collision segments of the northeasternboundary of the Indo-Australian plate, as has been demon-strated in greater detail by the modelling of Coblentz et al.(1998; Fig. 8). Similarly, we interpret the swing in maximumhorizontal stress from east–west to NW–SE along the southernpart of Australian continent to reflect control by the NewZealand segment of the plate boundary, to which the NW–SEstress orientation of southeastern Australia is orthogonal. Theso-called area of divergence (Sydney Basin and FlindersRanges) is interpreted to be an area where the force balanceacting along the plate boundary results in relatively lowhorizontal stress anisotropy.

Summary and conclusions(1) Since compilation of the World Stress Map in 1992,the number of reliable (A–C quality ranked) in situ stressorientation data for the Australian continent has increasedfrom 95 to 319, principally by analysis of borehole breakoutsand drilling-induced tensile fractures in petroleum wells.

(2) Sixteen stress provinces have been defined within theAustralian continent. The provinces comprise a minimum offour A–C quality stress orientation data within a distinctgeographic region and they are typically of one hundred to afew hundred kilometres scale. Fifteen of the 16 provinces showstatistically significant mean stress orientations at the 95%, orgreater, confidence level.

(3) Both the stress provinces and stress trajectory mapping(Figs 6 and 7) reveal systematic, continental-scale rotations in


stress orientation. Unlike most other continental areas, stressorientations in the Australian continent are variable and donot parallel the N–NNE absolute motion direction of theIndo-Australian plate.

(4) Regional stress orientations in the Australian continentare not significantly affected either by tectonic province,regional structural trends, geological age or by the depth atwhich orientations are sampled.

(5) Despite the absence of parallelism between absolute platemotion and stress orientations, the regional pattern of stressorientation in the Australian continent is consistent withcontrol by plate boundary forces, if the complex nature ofthe northeastern boundary of the Indo-Australian plate, andstress focusing by collisional segments of the boundary, isrecognized.

The Australian Stress Map is an ongoing project originally funded bythe Australian Research Council (1996–1998). For updates on theAustralian Stress Map see: www.ncpgg.adelaide.edu.au/asm. D.Denham and C. Windsor are thanked for their support of the project.J. Enever is thanked for providing an extensive engineering-basedstress database from eastern Australia, and for collaborative work on

that data. Much of the data on the Australian North West Shelf wascompiled as part of a PhD project undertaken by S. Mildren andfunded by AGSO. CSIRO’s Division of Petroleum Resources arethanked for their collaboration with the project. The following com-panies are thanked for providing data: Ampol, Apache, BHPP, Boral,BP, Cultus, Magellan, MIM, Norcen, Oil Company of Australia,Petroz, Phillips, Santos, TCPL, WMC and Woodside. We arealso grateful for data provided by the South Australian, WesternAustralian and Northern Territory Departments of Mines and Energyand the Australian Bureau of Resource Sciences. J. Stock is thankedfor her constructive review.

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Received 7 February 2000; revised typescript accepted 2 May 2000.Scientific editing by Nick Rogers.

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Fig. 8. Stresses predicted by elastic finite element modelling of theforces acting on the Indo-Australian plate. In this model, ridge pushtorques were balanced by fixing the collisional segments of theconvergent northeastern boundary of the Indo-Australian plate (i.e.Himalayas, New Guinea and New Zealand). Subduction zones weremodelled as free boundaries and no basal drag force was applied tothe lithosphere. Bars indicate the orientation and nonlithostaticmagnitude of the maximum and minimum horizontal stresses (solidbars indicate compression and open bars tension). From Coblentzet al. (1998).


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