CHAPTER 4 In situ stress field of Australia

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1 Geological Society of Australia Special Publication 22, CHAPTER 4 In situ stress field of Australia R. R. HILLIS* & S. D. REYNOLDS National Centre for Petroleum Geology and Geophysics, University of Adelaide, SA 5005, Australia. *Corresponding author: rhillis@ncpgg.adelaide.edu.au The Australian stress map comprises 331 reliable indicators of the orientation of horizontal, tectonic stresses in the Australian crust. In order to elucidate regional trends in stress orientation across the Australian continent, stress provinces have been defined and stress trajectories mapped, based on these indicators. Unlike most other continental areas, stress orientations in the Australian continent as a whole are variable and do not parallel the north to north-northeast absolute motion direction of the Indo-Australian Plate. The stress provinces and stress trajectories reveal systematic, continental-scale rotations in stress orientation. Maximum horizontal stress is oriented east west in western Australia. The east west orientation rotates to northeast southwest moving eastwards along the northern Australian margin and in central Australia. The east west maximum horizontal stress orientation rotates to northwest southeast moving eastwards along the southern Australian margin. The area of divergence between northeast southwest and northwest southeast maximum horizontal stress trajectories in central eastern Australia is characterised by east west or poorly defined (low horizontal stress anisotropy) trends. Regional stress orientations in the Australian continent are not affected to a first-order by either tectonic province, regional structural trends, geological age, or by the depth at which orientations are sampled. The regional pattern of stress orientation in the Australian continent is consistent with a first-order control being exerted by plate-boundary forces, if the complex nature of the northeastern boundary of the Indo-Australian Plate, and stress focusing by collisional segments of the boundary, is recognised. A number of locally anomalous stress orientations appear influenced by second-order sources of stress such as structure, topography and density heterogeneities. In situ stress orientations show a strong correlation with the direction of seismic anisotropy in the lithosphere. It is suggested that both datasets are predominantly controlled by present-day plate dynamics. KEY WORDS: Australia, in situ stress, plate dynamics, stress rotation. INTRODUCTION The Australian stress map is an ongoing project originally funded by the Australian Research Council ( ) in order to improve our limited understanding of the in situ field of Australia. The Australian stress map is one of the fundamental geophysical databases for the Australian continent. Our understanding of the evolution and dynamics of the Australian continent, and indeed the Australian Plate, relies in large part on databases such as those of in situ stress orientation, seismic velocity and anisotropy, magnetic and gravity anomalies, heat flow, and strain based on GPS observations, all of which are discussed in this volume. The Australian stress map primarily contains information on the orientation of the horizontal, tectonic stresses within the crust, but also contains significant information on stress magnitudes. The main applications of these in situ stress data are in the areas of geodynamics, hydrocarbon exploration and development, and in geotechnical studies, with specific uses including: modelling of plate driving/resisting forces; basin modelling; hydrocarbon reservoir management (e.g. subsurface fluid-flow directions, well placement and fracture stimulation); assessing fault-slip tendency and associated seismic hazard; and assessing the stability of mines, tunnels and boreholes. At the time of Zoback s (1992) compilation of the World stress map, the in situ stress field of the Australian continent was poorly constrained. Furthermore, the limited data that did exist indicated complex and scattered stress orientations (Denham et al. 1979; Lambeck et al. 1984; Richardson 1992). This contrasted strongly with other continental areas such as North and South America and western Europe, which are characterised by broad regions where maximum horizontal stress orientation is consistent and parallel to the direction of absolute plate motion (Zoback et al. 1989; Zoback 1992; Richardson 1992). The Australian stress map now reveals first-order trends in stress orientations in Australia that are indeed more complex than those of the other continents, but which are clearly constrained by the available data. Horizontal stress orientation in the crust may be inferred from earthquake focal mechanisms, borehole breakouts, engineering-type measurements (overcoring and hydraulic fracturing), and young geological data (fault-slip and volcanic alignments): see Zoback (1992) and references therein for more details of these techniques. Maximum horizontal stress orientations in the Australian stress map are quality ranked ranging from A, highest quality to E, lowest quality (Zoback & Zoback 1991; Zoback 1992). A C ranked data are considered to indicate stress orientations reliably. Data ranked D-quality are associated with a high standard deviation/low reliability, and E-quality data contain no reliable information on stress orientations. This paper summarises the data held in the Australian

2 44 R. R. Hillis and S. D. Reynolds stress map, the pattern of stresses it reveals within the Australian continent, and compares and contrasts the stress map with other geophysical databases for the Australian continent, particularly that of seismic anisotropy. An accompanying paper discusses the implications of the in situ stress data for the dynamics of the Australian Plate (Reynolds et al. 2002). STRESS INDICATORS IN THE AUSTRALIAN STRESS MAP DATABASE Australia (especially the Bowen and Sydney Basins) undertaken in the course of civil engineering, coal mining and coal-bed methane projects. Some of these data come from unusually great depth for engineering-type measurements (in excess of 1 km), and only those measurements considered to be unperturbed by local site effects such as mining excavation have been incorporated. Hence the engineeringtype measurements in the Australian stress map are considered to largely sample the tectonic stress field and be relatively unperturbed by either near-surface effects (due The Australian stress map comprises 549 stress indicators of which 331 yield reliable A C quality information on the orientation of horizontal, tectonic stresses. The type, number and quality of stress indicators currently held in the Australian stress map database are summarised in Table 1 and Figure 1. New Guinea, the continental crust of which is contiguous with that of Australia, is included in the Australian stress map. Where Australian earthquakes are of sufficient magnitude to yield focal mechanism solutions (from which the orientation of the principal stresses can be inferred), these are routinely undertaken by Geoscience Australia (McCue 1996), and added to the database. Some additional solutions have also been incorporated in the database (Greenhalgh et al. 1994). However, focal-mechanism data comprise only 29% of the A C quality data in the Australian stress map database compared to 63% of the current World stress map database (Mueller et al. 2000). This reflects the relatively low levels of seismicity in Australia and the regional nature of the seismic network, which together preclude focal mechanism solutions being successfully determined for all but the few largest magnitude seismic events. Many of the focal-mechanism data in the Australian stress map are from New Guinea, which being located on the northeastern plate boundary is subject to much higher levels of seismicity than Australia. Engineering-type measurements comprise approximately 20% of the Australian stress map database compared to 9% of the World stress map database (Mueller et al. 2000). This higher proportion of engineering-type data in the Australian stress map is in part a consequence of the much lower proportion of focal-mechanism data. However, the majority of the engineering data are from an extensive program of overcoring and hydraulic fracturing in eastern Table 1 Stress indicators in the Australian stress map database. Type Quality Total A B C D E Focal mechanisms Breakouts DITF Hydraulic fracturing Overcoring Geological indicators Total A C quality data, of which there are 331, are considered to reliably indicate in situ maximum horizontal-stress orientation. DITF, drilling-induced tensile fractures. Figure 1 Distribution of reliable (A C-quality) data in the Australian stress map database. (a) Distribution by stress indicator type. (b) Distribution by stress indicator type and depth. DITF, drilling-induced tensile fracture.

3 In situ stress field of Australia 45 for example to topography, jointing or thermal effects) or by site effects. There are a large number of overcoring stress measurements ranked D-quality in the Australian stress map (Table 1), because detailed information on the number of measurements and their standard deviation at individual locations was not available. Standard deviations are required to quality rank the mean stress direction from multiple overcoring tests at an individual location. Although details of the number of measurements and their standard deviation are not available, these overcoring data do represent the mean of multiple reliable observations (J. R. Enever pers. comm. 1999) and their quality ranking is thus conservative. The Australian stress map comprises no reliable (A C quality) indicators based on young geological data. Only two E-quality indicators based on volcanic vent alignments in Queensland (Nulla Basalt Province and McBride Volcanic Province) are included in the database, and these have been ranked of poor quality because the intrusions may be pre-quaternary, i.e. their orientation may reflect palaeo-stress rather than present-day stresses. The absence of data based on young geological data in the Australian stress map results from the common, but in our view erroneous, perception of Australia as a stable continent with little neotectonic activity. Recently there has been increasing recognition of neotectonic activity in Australia (Love et al. 1995). We believe that analysis of neotectonic activity will yield significant input to the Australian stress map over the next few years, to a level where its representation in the Australian stress map is comparable with that in World stress map, where young geological data constitute 5% of the database (Mueller et al. 2000). Importantly, stress orientations inferred from neotectonic activity may provide information in some of the areas of the Australian stress map that are to-date relatively poorly sampled. Breakouts and drilling-induced tensile fractures comprise 50% of the Australian stress map database compared to only 23% of the World stress map database (Mueller et al. 2000). The relative overrepresentation of breakouts and drilling-induced tensile fractures in the Australian stress map is in part a consequence of the much lower proportion of focal-mechanism data. However, the group compiling the Australian stress map has been particularly focused on petroleum exploration and development-related problems and hence petroleum-related stress data are relatively overrepresented (Hillis et al. 1995; Hillis 1998). Borehole breakouts and drilling-induced tensile fractures provide reliable indicators of stress orientation in petroleum-exploration wells. Where the maximum stress acting around a wellbore exceeds the compressive strength of the rocks forming the wellbore wall, compressional shear failure of the wellbore wall may occur. Failure of intersecting, conjugate shear planes leads to pieces of rock spalling, or breaking off the wellbore wall, and an elongation of the wellbore cross-section in the direction of minimum horizontal stress, which is known as borehole breakout, may result. Drilling-induced tensile fractures form where the minimum stress acting around a wellbore is less than the tensile strength of the rocks forming the wellbore wall, and are oriented at right angles to breakouts, i.e. in the maximum horizontal stress direction. Breakouts can be recognised on dipmeter logs. However, drilling-induced tensile fractures can only be recognised on the resistivity image logs that have superseded dipmeter logs. Resistivity image logs were not widely available at the time of Zoback s (1992) compilation of the World stress map, and drilling-induced tensile fractures were not recognised as a stress indicator at that time. However, they have become widely recognised since (Brudy & Zoback 1999) and they are quality ranked in the Australian stress map using the same criteria as breakouts. Breakouts and drilling-induced tensile fractures in wells that are deviated from the vertical may not be reliable indicators of horizontal stress orientation (Mastin 1988; Zajac & Stock 1997). In deviated wells their orientations are controlled by the entire stress tensor, including the vertical stress magnitude, and, projected onto a horizontal plane, their orientations may differ from that in a vertical well. The vast majority of breakouts and drilling-induced tensile fractures analysed in the Australian stress map are from wells deviated less than 15 (most less than 5 ) and thus are not affected. Data from a small number of wells with greater deviations have been incorporated where knowledge of the full stress tensor indicates that horizontal stress orientation can be reliably inferred (e.g. in a strike-slip faulting stress regime, boreholes must deviate at least 35 from the vertical before the horizontal projection of a breakout differs by more than 10 from the minimum horizontal stress direction: Mastin 1988). The distribution of stress indicators with depth in the Australian stress map is consistent with that in the World stress map (Zoback 1992), with engineering-type measurements generally from less than 1 km depth, petroleum-type measurements (borehole breakouts and drilling-induced tensile fractures) generally from 1 to 4 km depth, and earthquake focal mechanism data from greater (albeit usually poorly constrained), seismogenic depths (Figure 1b). When interpreting these data it should be borne in mind that different stress indicators sample different depths in the crust. ELUCIDATING REGIONAL IN SITU STRESS ORIENTATIONS IN THE AUSTRALIAN CONTINENT Stress orientations are variable across the Australian continent as a whole (Figures 2, 3). However, within individual provinces (at the scale of one to a few hundred kilometres), stress orientations are generally broadly consistent. The consistency of stress orientations within individual provinces indicates that statistically significant regional orientations are being resolved. Nevertheless, the variation of stress orientation between individual provinces suggests that the first-order controls on stress orientations in the Australian continent are more complex than those in other continental areas. Two techniques have been applied in order to clarify regional trends in stress orientation across the Australian continent and thereby facilitate analysis of the first-order controls on the stress field of Australia: definition of regional stress provinces and mapping of stress trajectories. Stress provinces A minimum of four A C-quality stress-orientation data in a distinct geographic region is defined to constitute a stress

4 46 R. R. Hillis and S. D. Reynolds Figure 2 Australian stress map: A C-quality data. Figure 3 Australian stress map: A D-quality data. province (Table 2; Figure 4). The Rayleigh Test was applied to the individual stress-orientation data within each stress province to investigate whether, and how strongly developed, any preferred stress orientation is in the province (Mardia 1972) (Figure 5). A type 1 stress province indicates that the null hypothesis that stress orientations in the province are random can be rejected at the 99.9% confidence level, type 2 at the 99% level, type 3 at the 97.5% level, type 4 at the 95% level, and type 5 at the 90% level. A type 6 stress province indicates that the null hypothesis that stress orientations are random cannot be rejected at the 90% confidence level. The categorisation of stress provinces should not be confused with the quality ranking of individual stress orientations. The individual stress-orientation data within a type 6 stress province are no less reliable per se than those in a type 1 province, rather they display a more scattered orientation. The range from type 1 to type 6 stress provinces is likely to reflect the degree of far-field, possibly plate-boundary-controlled horizontal stress anisotropy in

5 In situ stress field of Australia 47 Table 2 Stress provinces defined in the Australian stress map database. Province No Type Quality Statistics Regime A C FM BO HF OC DITF A B C Mean SD R Conf. a Amadeus Basin Northern Bonaparte Basin Southern Bonaparte Basin Bowen Basin Canning Basin Carnarvon Basin Cooper Basin Flinders Ranges < Gippsland Basin Irian Jaya Irian Jaya New Guinea New Guinea Otway Basin Perth Sydney Basin No, number; 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 random can be rejected at the stated confidence level (Conf). <90% indicates that the null hypothesis can not be rejected at the 90% confidence level. Regime is the average stress regime determined using Coblentz & Richardson s (1995) parameter, a. For pure normal faulting a=1, for pure strike-slip faulting a=0.5, and for pure thrust faulting a=0. See text for further discussion of the determination of average stress regime. Type, stress data types: FM, focal mechanisms; BO, breakouts; HF, hydraulic fracturing; OC, overcoring; DITF, drilling-induced tensile fractures. Figure 4 Mean stress orientations within Australian stress provinces. Isolated A- and B-quality data that do not lie within the defined stress provinces are also shown. the province, with there being little or no significant regional horizontal stress anisotropy in a type 6 stress province, allowing local effects such as those associated with faults to dominate. There is likely to be pronounced far-field horizontal stress anisotropy in a type 1 province where local effects have little influence on horizontal stress direction. Individual stress measurements of D- or E-quality may reflect relatively isotropic horizontal stresses (e.g. E-quality where there are clear breakouts in a well but their standard deviation is >40 ). However, they may also represent poor quality data (e.g. E-quality where no reliable breakouts can be detected because there is no rotation of a dipmeter tool in the well and hence no evidence of it having locked into the long axis of breakout).

6 48 R. R. Hillis and S. D. Reynolds Figure 5 Rayleigh test plot for the 16 stress provinces. Provinces plotting above and to the right of a confidence percentage line pass the Rayleigh test at that confidence level. The R value is the length of the mean resultant vector of maximum horizontal stress orientations within a province (Mardia 1972). Figure 6 Stress trajectories based on the Australian stress map data following the technique of Hansen & Mount (1990). The information on stress regimes in each of the stress provinces has also been summarised (Table 2). Earthquake focal mechanism solutions yield the orientations of the principal stresses, and thus the associated stress regimes which are classified in the World stress map scheme as thrust faulting (s H > s h > s v ), thrust with strike-slip component, strike-slip (s H > s v > s h ), normal with strike-slip component, or normal (s v > s H > s h ), where s v, s H and s h are the vertical, maximum horizontal and minimum horizontal stresses respectively (Zoback 1992). Hydraulic fracturing and overcoring tests yield absolute values of stress magnitudes (Hillis et al. 1999) and these have also been incorporated in the determinations of average stress regime in each of the provinces (Table 2). Borehole breakouts and drillinginduced tensile fractures, which make up the 50% of the Australian stress map database, do not yield information on stress magnitudes. Stress trajectories Stress-trajectory determination provides a technique for smoothing and interpolating unevenly distributed stress data and thereby clarifying regional trends. A stress-trajectory map (Figure 6) has been calculated from the Australian stress map database following the technique of Hansen and Mount (1990). Stress trajectories determined following this technique indicate the orientation of the maximum horizontal stress at each point along the trajectory. However, they do not imply information about magnitudes (the spacing between the trajectories does not have any significance).

7 In situ stress field of Australia 49 The technique applies a statistical smoothing algorithm to create an estimated stress field at each observed data location. In the technique, fidelity to the raw data must be balanced against the degree of smoothing. If there is too much fidelity to the raw data, the calculated stress trajectories simply reflect the stress data, and if there is too much smoothing, variations are obscured. In determining the estimated stress field, three weighting systems were applied to the raw data. First, data were weighted according to their proximity to the observed data point being smoothed. Second, relative weightings of 4 were given to A-quality data, 3 to B-quality data, 2 to C- quality data and 1 to D-quality data. Third, a robustness weight was applied to eliminate the presence of anomalous data that differed substantially from other observed data in the region. The smoothed stress field, which was calculated at each of the observed data points, was then used to calculate the stress trajectories as outlined by Hansen and Mount (1990). PATTERN OF REGIONAL STRESS ORIENTATION IN THE AUSTRALIAN CONTINENT Both techniques used to clarify regional trends in stress orientation across the Australian continent show consistent patterns (Figures 4, 6). The western part of the Australian continent is characterised by broadly east west-oriented maximum horizontal stress (Carnarvon Basin and Perth provinces). The east west maximum horizontal stress orientation in the western part of the Australian continent rotates to northeast southwest along the northern Australian margin (Canning Basin, Northern and Southern Bonaparte Basin, Irian Jaya and New Guinea provinces). This swing in stress trajectories along the northern Australian margin is broadly paralleled to the south where east west-oriented maximum horizontal stress in the Perth province rotates to north-northeast south-southwest in the Amadeus Basin (central Australia). The Bowen Basin also exhibits north-northeast south-southwest-oriented maximum horizontal stress. Moving west to east in the southern part of the continent, maximum horizontal stress rotates from east west in the Perth Basin to northwest southeast in southeastern Australia (Otway Basin and Gippsland Basin provinces). The area of divergence between northnortheast south-southwest and northwest southeast maximum horizontal stress trajectories in central eastern Australia is characterised by east west or poorly defined (low horizontal stress anisotropy) regional stress trends (Cooper Basin, Flinders Ranges and Sydney Basin provinces). Both the stress trajectories and stress provinces identified a regional northwest southeast stress trend throughout southeastern Australia, although sections of onshore southeastern Australia are dominated by scattered focal mechanisms and engineering-type measurements indicating an approximate east west stress direction. The scattered nature of these stress indicators suggest that local stress sources strongly influence the stress field and hence we believe these indicators are not a reliable guide to the nature of the regional stress field. It is immediately apparent that, unlike most other continental areas, stress orientations in the Australian continent as a whole are variable and do not parallel the north to north-northeast absolute motion direction of the Indo- Australian Plate. South America, western Europe and midplate North America are all characterised by broad regions where maximum horizontal stress orientation is consistent and parallel to the direction of plate motion (Zoback et al. 1989; Zoback 1992; Richardson 1992). From this it is inferred that the forces driving and/or resisting plate motion are responsible for regional stress orientations in those continental areas. The absence of correlation between stress orientation and absolute plate motion direction in the Australian continent begs the question as to whether the in situ stress field of the Australian continent is subject to different controls than that of other continents. Regional stress orientations in the Australian continent are not obviously affected either by tectonic province, regional structural trends, geological age or by the depth at which stress orientations are sampled. This is perhaps best witnessed by the Perth province. In the eastern part of the Perth province data from focal mechanisms, overcoring and hydraulic fracturing all indicate broadly east west oriented maximum horizontal stress (Figure 3). These data come from the Precambrian Yilgarn Craton which is separated from the Phanerozoic Perth Basin to the west by an approximately north south-trending crustal-scale fault (Darling Fault). Breakout data from the Perth Basin similarly indicate that maximum horizontal stress is broadly oriented east west (although locally anomalous stress orientations are observed in the vicinity of known faults). Hence in the Perth province stress orientations are consistent from nearsurface to several kilometres depth and across a major tectonic boundary. Again although some locally anomalous stress orientations are observed, especially in the vicinity of faults, the northeast southwest regional maximum horizontal stress orientation along much of the northern Australian margin is also unaffected by regional structural trends, geological age or by the depth at which stress orientations are sampled. The Southern Bonaparte Basin and onshore Canning Basin are Palaeozoic basins characterised by northwest southeast structural trends. In the latter basin, focal mechanism and breakout data both indicate northeast southwest-oriented maximum horizontal stress. The northern Bonaparte Basin is dominated by younger, Mesozoic northeast southwest trends associated with the formation of the present passive margin. Structural trends in New Guinea are northwest southeast and associated with Cenozoic collision in the area. All of these provinces, with different age and orientation of dominant structure, display a northeast southwest regional maximum horizontal stress orientation. The swing in maximum horizontal stress orientation from east west in western Australia to northwest southeast along the northern Australian margin is part of a plate-wide anticlockwise rotation in stress orientation from broadly north south in India to northwest southeast in the vicinity of the Ninety East Ridge to east west in western Australia. This broad rotation can be accounted for by focusing of stresses orthogonal to the Himalayan and New Guinea continental collision segments of the northeastern boundary of the Indo-Australian Plate. We believe that the first-order pattern of stresses in continental Australia can indeed be accounted for by plate-boundary forces, if the complex

8 50 R. R. Hillis and S. D. Reynolds Figure 7 Comparison of mean maximum horizontal stress directions in the 16 stress provinces defined herein with SV-wave azimuthal anisotropy from Rayleigh wave inversion (waveforms analysed for the frequency range s). SV-wave azimuthal anisotropy is from Debayle & Kennett (2002 figure 4a). SV-wave: shear wave polarised such that motion is in the vertical plane which also contains direction of wave propagation. nature of the northeastern boundary of the Indo-Australian Plate is recognised. In an accompanying paper, Reynolds et al. (2002) have considerably improved the fit of plateboundary force-induced stresses to the observed stress data from that of previous modelling studies (Coblentz et al. 1998), especially in eastern Australia, by considering a very large number of permutations of plate-boundary forces. Stress orientations in Australia contrast with those of other continents in not being parallel to the direction of absolute plate motion, not because the first-order control on stress orientations in Australia differs, but because its plateboundary configuration is more complex than that of most other continents. Continental areas that exhibit consistent maximum horizontal stress orientations that are parallel to the direction of absolute plate motion, such as western Europe, South America and mid-plate North America, are surrounded by much simpler plate-boundary configurations. For example, northwest-oriented maximum horizontal stress in western Europe is a consequence of compressional forces generated orthogonal to the Mid-Atlantic Ridge and the Alpine collisional belt (Gölke & Coblentz 1996). Throughout continental Australia locally anomalous second-order stress variations occur which are not controlled by plate-boundary forces. For example fault-parallel stress rotation perpendicular to the regional stress field occurs in the Perth Basin (Reynolds & Hillis 2000). Such secondorder stress variations are likely caused by structural, topographic and density variations within the lithosphere. Whilst these variations are important on a local scale, their effects are not clearly witnessed in regional stress provinces and stress trajectories determined herein. Determining the source of local stress variations is often problematic due to the lack of detail in the stress/structural datasets. We acknowledge that excluding geology to model the Australia stress field is an oversimplification, although it is justifiable for the modelling of first-order tectonic stresses. COMPARISON OF REGIONAL STRESS ORIENTATION IN THE AUSTRALIAN CONTINENT WITH OTHER GEOPHYSICAL DATABASES Several geophysical databases for the Australian continent are discussed in this volume. These databases hold the key to our understanding of the evolution and dynamics of the Australian continent. However, in order to elucidate the evolution and dynamics of the Australian continent, it may be possible, indeed necessary, to categorise such databases into those whose dominant response reflects the evolution (geological history) of the Australian continent and those whose dominant response reflects its (present-day) dynamics. The magnetic and gravity anomaly images of Australia can be divided into geophysical domains that, where basement is exposed, correspond with geologically mapped cratons and blocks (Wellman 1998). These domains comprise large contiguous areas with a common geological history. The clear expression of the Tasman Line (the eastern boundary of the Australian Proterozoic craton along which late Neoproterozoic rifting and breakup and subsequent Phanerozoic collision occurred) in the potential field images (Milligan et al. 2003) clearly illustrates that the response of these datasets is controlled by the structural evolution of the Australian continent. Although long-wavelength components of both the magnetic and gravity anomaly images of Australia do reflect present-day dynamics (isostasy and depth to Curie temperature in the case of the gravity and magnetic images respectively), the dominant response reflects the structural evolution of the different geological domains of Australia, and the variation in physical properties frozen into these different domains as a result of their different geological histories. The in situ stress data do not, at least at the first-order, reflect the different geological domains of the Australian continent. Indeed, as discussed above, stress orientations

9 In situ stress field of Australia 51 are insensitive to major tectonic boundaries such as that between the Yilgarn Craton and the Perth Basin, and to variation in regional structural trends or geological age along the northern Australian margin. Nor is there a firstorder control on stress orientations in the eastern-third of Australia by the dominantly north south trends of the Tasman fold-belt system. Reynolds et al. (2002) show that the first-order variation of in situ stress orientations in the Australian continent can be successfully accounted for by forces acting on the present-day boundaries of the plate. Hence, as in other continental areas (Richardson 1992), the in situ stress field of Australia is controlled by the present-day dynamics rather than the structural evolution of the lithosphere. The pattern of stress orientation in the Australian continent shows distinct similarities to the pattern of shear (SV-) wave azimuthal anisotropy from Rayleigh wave inversion (Debayle & Kennett 2002). The lateral resolution of the determinations of seismic anisotropy is of the order of a few hundred kilometres and is thus similar to that of the stress provinces defined herein. The fit between stress orientations and SV-wave azimuthal anisotropy is good in the Bowen, Gippsland, Otway and Cooper Basins of eastern Australia (Figure 7). The fit is poor in the Amadeus Basin. However, the broad variation in direction of seismic anisotropy from essentially north south in the Bowen Basin to east west in the Cooper Basin to north south west of the Amadeus Basin is very similar to the variation exhibited by the stress data. The fit between stress orientations and SV-wave azimuthal anisotropy is also good in the Canning Basin and Northern and Southern Bonaparte Basins (Figure 7). The fit is poor in western Australia, where stress directions are east west and the seismic anisotropy poorly developed, but essentially north south (Figure 7). However, due to the distribution of earthquakes along the plate boundaries surrounding Australia, seismic anisotropy is relatively poorly constrained west of 125 E (Debayle & Kennett 2002). Two distinct layers of seismic anisotropy are observed in the Australian continent (Debayle & Kennett 2002). The upper layer is characterised by varying directions of anisotropy as shown in Figure 7. However, at depths greater than 150 km the pattern of anisotropy is smoother with the dominant north south direction subparallel to the absolute direction of plate motion (Debayle & Kennett 2002). Debayle & Kennett (2002) suggest that the anisotropy in the upper layer reflects frozen deformation in the lithosphere, while in the lower layer the smoother pattern is more likely to reflect present-day deformation due to shearing at the bottom of the northward moving Australian Plate. We agree that the anisotropy in the lower layer probably reflects processes in the asthenosphere associated with northward motion of the Australian Plate. However, given the correspondence between the anisotropy in the upper layer and in situ stress directions, we believe that the seismic anisotropy in the upper layer reflects in situ stresses associated with present-day plate dynamics, possibly due to the influence of stress-aligned fluid-saturated microcracks (cf. extensive dilatancy anisotropy: Crampin et al. 1984). Both the upper and lower layers of seismic anisotropy in the Australian continent may thus reflect present-day plate dynamics, with the difference between the two layers witnessing decoupling between lithospheric and asthenospheric stresses. Continental areas that exhibit consistent maximum horizontal stress orientations that are parallel to the direction of absolute plate motion, such as western Europe, South America and mid-plate North America, may provide a test of the processes controlling the upper layer, lithospheric seismic anisotropy. If present-day plate dynamics/in situ stress control both lithospheric and asthenospheric seismic anisotropy (as is suggested above for Australia) then in these continental areas both lithospheric and asthenospheric seismic anisotropy would be expected to be consistent and parallel to the direction of plate motion. If lithospheric seismic anisotropy is controlled by pre-existing structure then these continental areas would be expected to show variable directions of seismic anisotropy in the lithosphere, as is seen in Australia. In order to elucidate the evolution and dynamics of the Australian Plate it is necessary to distinguish datasets controlled by present-day dynamics from those such as the magnetic and gravity anomaly images of Australia that are dominantly controlled by the structural history of the crust. SUMMARY AND CONCLUSIONS (1) The Australian stress map comprises 549 stress indicators of which 331 yield reliable, A C-quality information on the orientation of horizontal, tectonic stresses. Compared to the World stress map database (Mueller et al. 2000), data based on earthquake focal mechanism solutions are relatively underrepresented in the Australian stress map, due largely to the relatively low levels of seismicity in Australia. Data based on borehole breakouts and drillinginduced tensile fracture in hydrocarbon exploration wells are relatively over-represented. Engineering-type measurements are also well represented, especially in eastern Australia. There are no reliable stress indicators based on young geological features, but we consider that the increased recognition of neotectonic activity in the Australian continent will lead to the future incorporation of such data into the Australian stress map. (2) Sixteen stress provinces have been defined within the Australian continent. Fifteen of the 16 provinces show statistically significant mean stress orientations at the 95% or greater confidence level. The consistency of stress orientations in individual provinces indicates that statistically significant regional orientations are being resolved. However, both the stress provinces and stress trajectory mapping reveal systematic, continental-scale rotations in stress orientation. Hence, unlike most other continental areas, stress orientations in the Australian continent are variable and do not parallel the north to north-northeast absolute motion direction of the Indo-Australian Plate. (3) Regional stress orientations in the Australian continent are not affected to a first-order by either tectonic province, regional structural trends, geological age, or by the depth at which orientations are sampled. A number of locally anomalous stress orientations appear influenced by second-order sources of stress such as structure, topography and density heterogeneities. Despite the absence of parallelism between absolute plate motion and stress orientations, the regional pattern of stress orientation in the

10 52 R. R. Hillis and S. D. Reynolds Australian continent is consistent with control by plateboundary forces, if the complex nature of the northeastern boundary of the Indo-Australian Plate, and stress focusing by collisional segments of the boundary, is recognised (see also Reynolds et al. 2002). (4) in situ stress orientations show a strong correlation with the direction of seismic anisotropy in the lithosphere. It is suggested that both datasets are, to a first-order, controlled by present-day plate dynamics. ACKNOWLEDGEMENTS The Australian stress map is an ongoing project originally funded by the Australian Research Council ( ). For updates on the Australian stress map see < David Denham and Chris Windsor are thanked for their support of the project. Jim Enever is thanked for providing an extensive engineering-based stress database from eastern Australia, and for collaborative work on that data. Much of the data on the Australian North West Shelf was compiled as part of a PhD project undertaken by Scott Mildren and funded by Geoscience Australia. CSIRO s Division of Petroleum Resources is thanked for its collaboration with the project. The following companies 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 are also grateful for data provided by the South Australian, Western Australian and Northern Territory Departments of Mines and Energy and the Australian Bureau of Resource Sciences. REFERENCES BRUDY M. & ZOBACK M. D Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude. International Journal of Rock Mechanics and Mining Sciences 36, COBLENTZ D. D. & RICHARDSON R. M Statistical trends in the intraplate stress field. Journal of Geophysical Research 100, COBLENTZ D. D., ZHOU S., HILLIS R. 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