In situ stress field of Australia

Abstract

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 llthosphere. It is suggested that both datasets are predominantly controlled by present- day plate dynamics.