Even though a multitude of observations suggest time-dependent regional tectonic reactivation of the Australian Plate, its large-scale intraplate stress field evolution remains largely unexplored. This arises because intraplate paleo-stress models are difficult to construct, and that observations of tectonic reactivation are often hard to date. However, because the Australian plate has undergone significant changes in plate boundary types and geometries since the Cretaceous, we argue that even simple models can provide some insights into the nature and timing of crustal reactivation through time. We present Australian intraplate stress models for key times from the Early Cretaceous to the present, and link them to geological observations for evaluating time-dependent fault reactivation. We focus on the effect time-dependent geometries of mid-ocean ridges, subduction zones and collisional plate boundaries around Australia have on basin evolution and fault reactivation through time by reconstructing tectonic plates, restoring plate boundary configurations, and modelling the effect of selected time-dependent plate driving forces on the intraplate stress field of a rheologically heterogeneous plate. We compare mapped fault reactivation histories with paleo-stress models via time-dependent fault slip tendency analysis employing Coulomb-Navier criteria to determine the likelihood of strain in a body of rock being accommodated by sliding along pre-existing planes of weakness. This allows us to reconstruct the dominant regional deformation regime (reverse, normal or strike-slip) through time. Our models illustrate how the complex interplay between juxtaposed weak and strong geological plate elements and changes in far-field plate boundary forces have caused intraplate orogenesis and/or tectonic reactivation in basins and fold belts throughout Australia.

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[1] Although the low-order present stress field of most continents is fairly well established, information on paleostress fields is generally sparse. Knowledge of paleostresses is crucial for understanding brittle tectonic reactivation through time. The Indian-Australian plate lends itself well to a reconstruction of paleostresses, as it has undergone enormous changes in plate-driving forces through the Tertiary, and there is a rich record of fault reactivation from sedimentary basins. We reconstruct the plate boundary configuration and age-area distribution of ocean crust around Australia through time to obtain estimates for ridge push, slab pull, and collisional forces acting on the Indian-Australian plate since the Eocene. Other model parameters we explore are the effects of the Australian-Antarctic discordance and the mechanical strength of the Australian continental margin. We apply these constraints to model the orientation of the maximum horizontal compressive stress (S Hmax ) regime for the present, early Miocene, and early Eocene using the commercial software ABAQUS 2 along with the optimization software Nimrod/O. We use an elastic two-dimensional plane stress finite element model with a resolution of $0.2°in both longitude and latitude. Realistic elastic parameters representing different rock types and geologic provinces for the Australian continent have been included to model the stress field of a heterogeneous plate. We show that spatially significant rotations of S Hmax directions can be modeled as a consequence of perturbations of S Hmax in areas of juxtaposed rigid and compliant rheologies. The absence of the collisional Papua New Guinea boundary in the Miocene and reduced ridge push force from the south result in stress directions considerably different from the present. Stress directions over the northern Australian continent in the early Miocene in particular show large disparity with present stress directions. Stress orientations for the Australian plate during the early Eocene are controlled predominantly by ridge push forces arising from spreading in the Wharton Basin in the Indian Ocean and vary substantially with stress directions in the early Miocene and the present because of the drastically different plate geometry and boundary configurations. Fault reactivation histories observed on the northwest shelf of Australia and in the Bass Strait region are consistent with modeled changes in stress directions through time.

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