Three-dimensional finite-element modelling of the tectonic stress field in continental Australia

Abstract

Traditionally, intraplate stress orientations have been modelled using an isotropic elastic plate. For the Australian Plate this method has been applied successfully to model the first-order pattern of stress orientations. However, the distribution of intraplate earthquakes and the juxtaposition of strong, cold with hotter, younger lithosphere in many areas suggest that the spatial variation in mechanical strength of the plate may result in substantial regional anomalies in stress orientations and magnitudes. We explore this idea with a three-dimensional finite-element model to investigate the regional response of the Australian continent to tectonic forces. The model covers the area of -40 to -10° (S) and 111 to 155° (E) with a spatial resolution of 90 x 90 x 50 km. The relative magnitudes of the ridge-push and boundary forces, which act on the Australian continent, are estimated through an inversion analysis of in situ stress data. The differences between modelled and observed stress orientations are minimised in a least-squares sense. Major tectonic blocks and the differences in their elastic strength are included in the model, and the initial estimates of Young's modulus for the tectonic blocks are adapted from a published coherence analysis of gravity and topographic data. The values of Young's modulus are adjusted in the inversion analysis to best fit the stress orientations observed on the Australian continent. The inversion analysis of rheological parameters is most efficient for estimating Young's modulus for the Northern Lachlan Fold Belt, the New England Fold Belt, and the Southern Lachlan Fold Belt. The adjusted values for the flexural rigidity are 0.040 x 10 2 5 Nm for the Northern Lachlan Fold Belt, 0.037 x 10 2 5 Nm for the New England Fold Belt, and 0.040 x 10 2 5 Nm for the Southern Lachlan Fold Belt, which correspond to an effective elastic thickness of about 30 km. Based on the optimised body and boundary forces acting on the plate, a map of maximum principal-stress distribution is constructed so that variations of the relative magnitude of tectonic stresses can be assessed. We find a good match between predicted zones of stress concentration and the distribution of major belts of seismicity in Australia. The results show that while the overall pattern of stress orientations in the Australian continent is controlled by the forces which drive the Indo-Australian Plate, the maximum horizontal stress orientations and the pattern of the stress concentration manifested by seismicity are modulated by local/regional geological structures.