The present-day stress field of Australia

The unusual present-day crustal stress pattern of the Australian continent has been the subject of scientific debate for over 25 years. The orientation of maximum horizontal present-day stress (SHmax) in continental Australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub-parallel to the direction of absolute plate motion. Previous studies on the stress pattern of Australia revealed that the complex stress pattern of the continent is controlled, at a first-order, by the superposition of plate tectonic forces exerted at the plate boundaries. However, prior analysis of the contemporary Australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern Australia, and has not extensively addressed the numerous smaller scale variations in stress orientation. The recent development of unconventional reservoirs in Australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorly-constrained areas in eastern Australia. In addition, stress analysis in conventional hydrocarbon, mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the Australian Stress Map (ASM). This study presents the new release of the ASM, with a total of 2150 stress data records in Australia (increased from 594 data records in 2003). The 2016 ASM contains 1359 data records determined from the interpretation of drilling-related stress indicators, 650 from earthquake focal mechanism solutions, 139 from shallow engineering measurements and two from geological indicators. The results reveal four distinct regional trends for the SHmax orientation in Australia including a NNE-SSW SHmax trend in northern and northwestern Australia, which rotates to a prevailing E-W orientation in most Western and South Australia. The orientation of SHmax in eastern Australia is primarily ENE-WSW and swings to NW-SE in southeastern Australia. A comparison between the new ASM database and neotectonic features further confirms the role of contemporary stress in recent deformation of the Australian crust. The 2016 ASM reveals significant discrepancies between newly observed SHmax orientations and predictions by published geomechanical-numerical models for the Australian continent. Forces generated at the boundary of Indo-Australian Plate remain the major control on the regional crustal stress pattern in continental Australia, however, the increased data density, particularly in eastern Australia, reveals numerous local perturbations of the stress field that were not previously clearly captured. Hence, the key findings of the new release are that local (intra-plate) stress sources are more significant than previously recognized, particularly in eastern Australian basins, and cause substantial local scale rotations of the present-day crustal stress pattern that have not been factored into existing Australian stress models.Mojtaba Rajabi, Mark Tingay, Oliver Heidbach, Richard Hillis, Scott Reynold