Australian paleo-stress fields and tectonic reactivation over the past 100 Myr R. D. MU ¨ LLER 1 *, S. DYKSTERHUIS 1,2 AND P. REY 1 1 EarthByte Group, School of Geosciences, The University of Sydney, Madsen Building F09, NSW 2006, Australia. 2 ExxonMobil, 12 Riverside Quay, Southbank, VIC 3006, Australia. 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. KEY WORDS: paleo-stress, fault reactivation, plate tectonics, crustal deformation. INTRODUCTION Faults are a key aspect in many of Australia’s sedimen- tary basins that have undergone significant reactivation resulting in the breaching of hydrocarbon traps. In particular, Oligocene collisional processes north of Australia (Cloetingh et al. 1992) and the Miocene separation of the Indo-Australian Plate into two distinct plates along a diffuse plate boundary (Royer & Chang 1991) have played an instrumental role in modifying the intra-plate stress field, resulting for instance in Miocene reactivation in the Timor Sea (O’Brian et al. 1996). The timing of Cenozoic tectonic events on the Northwest Shelf (NWS) compiled by Cloetingh et al. (1992) indicates a fundamental connection between plate tectonics, i.e. changes in plate motions and related in-plane stresses, and basin subsidence/uplift. As many basins of the Australian Northwest Shelf area have been subjected to a number of repeated extensional and compressional tectonic stages, controlling hydrocarbon migration in fault-controlled traps, understanding fault-trap char- ging and integrity requires some knowledge of paleo- stresses. Etheridge et al. (1991) pointed out that, in particular, steeply dipping strike-slip faults develop into wrench-reactivated transfer faults with associated structures that dominate traps in the Carnarvon, Bonaparte and Gippsland Basins. In central and eastern Australia, major reactivation of basin forming struc- tures occurred in the early Late Cretaceous (ca 95 Ma, e.g. Hill 1994; Korsch et al. 2009) when plate motion to the east slowed down before to change towards a northerly direction. While modelling of the contemporary maximum horizontal stress (s H ) regime is useful for improving our understanding of the driving forces of plate tectonics as well as for the planning of deviated drilling during hydrocarbon production, information concern- ing paleo-stress regimes allows for the creation of predictive frameworks for fault reactivation through time. Compilation of stress data under the auspices of the World Stress Map project in the early 1980s (Zoback 1992) showed s H orientations over most continental areas are parallel to the direction of absolute plate motion, leading to the hypothesis that s H orientations are the product of dominant plate driving forces acting along plate boundaries (Zoback 1992). Orientations of s H 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 *Corresponding author: dietmar.muller@sydney.edu.au padmavathym 27/7/11 12:32 TAJE_A_605801 (XML) Australian Journal of Earth Sciences (2011) 00, (1–16) ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2011 Geological Society of Australia DOI: 10.1080/08120099.2011.605801