Characterization And Modelling Of The Regional In Situ Stress Field Of Continental Australia
Reynolds, Scott David
Doctor of Philosophy, 2001
University of Adelaide
The Indo-Australian plate (IAP) is unique when compared to most other plates in that the maximum in situ horizontal stress (sHmax) orientation is not uniform and does not parallel the direction of absolute plate velocity. Consequently, the Australian continent provides an ideal setting to study the interaction between tectonic forces and the in situ stress field. The focus of this thesis is on (1) improving current knowledge of the in situ stress field of the Australian continent, largely through the compilation of new data in the Bowen, Sydney and Perth Basins, and (2) the analysis of the origin of the Australian stress field in terms of plate boundary forces.
New in situ stress data based on hydraulic fracturing and overcoring measurements have been compiled for the Bowen and Sydney Basins. The Bowen Basin displays a consistent north-northeast sHmax orientation. In contrast, the Sydney Basin displays a variable sHmax orientation as a result of local sources of stress. The Bowen Basin is relatively aseismic compared to the Sydney Basin, which is relatively seismically active. A correlation exists between the in situ stress measurements and the levels of seismicity in the two basins, despite the shallow depth at which the hydraulic fracturing and overcoring data were obtained. Mohr-Coulomb analysis of the propensity for failure indicates that 41% of the new in situ stress data are indicative of failure in the Sydney Basin, compared with only 13% in the Bowen Basin. This difference in propensity for seismicity in the two basins based on in situ stress data is further emphasised when the pre-existing structural grains of the two areas are considered.
The mean orientation of sHmax inferred from borehole breakout data in the Perth Basin is approximately east-west. This orientation is consistent with sHmax orientations from both deeper earthquake focal mechanisms and shallower hydraulic fracturing and overcoring data from the adjacent Yilgarn Craton. Hence the regional east-west sHmax orientation is consistent across different stress indicators, depths and geological provinces. This robust east-west sHmax orientation is almost orthogonal to the direction of absolute plate velocity and thus this region is of particular significance to our understanding of the in situ stress field of Australia. Anomalous north-south sHmax orientations identified in a number of wells in the Perth Basin can be attributed to local structural effects.
In order to compare observed in situ stresses with those based on modelling, the regional stress field throughout continental Australia has been defined using two different methods: stress province definition and stress trajectory mapping. These techniques have been applied to a greatly increased Australian stress database comprising both existing stress analyses and the new analyses presented here. In general there is broad agreement between the two techniques, which reveal continental scale rotation of the stress field across Australia. Stress orientations in Australia vary from east-west in the western and southern parts of the continent (Perth region, Carnarvon and Cooper Basins) rotating to northeast-southwest across the northern margin (New Guinea, Bonaparte and Canning Basins) and northwest-southeast through southeastern Australia (Otway and Gippsland Basins). The central (Amadeus Basin) and northeastern (Bowen Basin) parts of Australia are characterised by a north-northeast to south-southwest stress field.
The tectonic forces controlling the present day regional intraplate stress field in continental Australia have been evaluated through finite element analysis of the intraplate stresses in the IAP. Constraints for the modelling are provided by the 'observed' regional stress field based on the stress province definition. This observed dataset is greatly improved compared to that used in previous modelling efforts. A weighted 'basis-set' method has been employed to quantitatively assess the misfit between the observed and predicted intraplate stresses, providing an efficient means to evaluate a very large number of boundary and potential energy force combinations acting on the plate.
The modelling results indicate that the major features of the regional stress field in continental Australia can be explained in terms of a geologically plausible array of plate boundary and potential energy forces. These results indicate that modelling of the Australian intraplate stress field is inherently non-unique such that a large number of different boundary force combinations can produce a very similar predicted stress fields. Nevertheless a number of fundamental conclusions can be made about the tectonic settings along the principal plate boundary segments including the following:
- Compressional forces act along the Himalayan and Papua New Guinea boundaries to produce stress focussing normal to those boundaries and stress rotation between them.
- The observed stress field in the Bowen Basin requires compressional forces along the Solomon and New Hebrides subduction zones.
- East-west compression in eastern Australia requires moderate compression acting along the Tonga-Kermadec subduction zone.
- Modelling stresses in southeastern Australian requires compressional forces along the New Zealand, Puysegur Trench and Macquarie Ridge boundary segments.