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 Theological 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 IO 25 Nm for the Northern Lachlan Fold Belt, 0.037 x IO 25 Nm for the New England Fold Belt, and 0.040 x IO 26 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.

Present‐day stress orientation in the Clarence‐Moreton Basin of New South Wales, Australia: a new high density dataset reveals local stress rotations

“…Until recently, most of our information about the state of stress in eastern Australian in general, and the C‐M Basin in particular, are from different published stress models of the continent (Cloetingh & Wortel, , ; Coblentz et al ., , ; Reynolds et al ., ; Hillis & Reynolds, ; Reynolds et al ., ; Zhao & Müller, ; Burbidge, ; Dyksterhuis et al ., , b; Dyksterhuis & Müller, ; Müller et al ., ). Almost all of these models evaluated the relative importance of various boundary forces acting on the Indo‐Australian Plate for the pattern of the S Hmax orientation in the Australian continent.…”

“…The inability of numerical models to predict the observed stress orientations in the basin, when combined with the numerous local stress perturbations, suggests that second and third‐order stress sources probably play a greater role in the Australian stress pattern than previously recognised. Previously published models are 2D (except Zhao & Müller () which is 3D but has only two element layers over 100 km thickness) and may not replicate the real lithospheric behaviour of the continent and hence the state of stress. The stress orientation in the basin, as well as that observed in some major basins in northern, central and north‐eastern Australia, indicates that, local perturbations due to presence of different geological features are significant and can lead to substantial changes in the S Hmax orientation, particularly at the basin scale.…”

“…() by 2D elastic models predict a generally northwest‐southeast S Hmax orientation for most eastern part of Australia which is not in agreement with the observed S Hmax orientation in different basins of eastern Australia. Also the 3D model of Australia from Zhao & Müller () could not resolve these regional to local deviations from the general trend.…”

“…Most of our information and understanding about the state of stress in eastern Australia is based on stress observations in the Bowen and Sydney Basins (Hillis et al ., ), which are mostly from hydraulic fracture and overcoring test measurements. These limited set of observational stress indicators have been used in numerous geomechanical‐numerical models to constrain and predict the contemporary state of stress in the Australian continent (Cloetingh & Wortel, , ; Coblentz et al ., , ; Reynolds et al ., , ; Zhao & Müller, ; Burbidge, ; Dyksterhuis et al ., ; Müller et al ., ). However, all these large scale models have been unable to explain the pattern of S Hmax orientation particularly in north‐eastern Australia and New South Wales (NSW).…”

Paleostress field evolution of the Australian continent since the Eocene

“…Up to now most approaches for modeling Indian‐Australian intraplate stress orientations have been based on applying forces to homogeneous elastic plates [ Coblentz et al , 1995, 1998; Hillis et al , 1999; Reynolds et al , 2002]. While these studies model the first‐order pattern of stress over the Australian continent well, more recently, Zhao and Müller [2003] studied the second‐order effects of homogeneous material properties on the stress field of eastern Australia, determining through inverse analysis that inclusion of realistic material properties improves the regional fit between modeled and measured S Hmax orientations. Here we include realistic elastic parameters representing different rock types and geologic provinces for the Australian continent to model the stress field of a heterogeneous plate.…”