Cenozoic deformation in the Otway Basin, southern Australian margin: implications for the origin and nature of post-breakup compression at rifted margins

There is growing recognition that pulses of compressive tectonic structuring punctuate the post-breakup subsidence histories of many 'passive' rifted continental margins. To obtain new insights into the nature and origin of compression at passive margins, we have conducted a comprehensive analysis of the post-breakup (< 43 Ma) deformation history of the offshore Otway Basin, southern Australian margin, using a regional seismic database tied to multiple wells. Through mapping of a number of regional intra-Cenozoic unconformities we have determined growth chronologies for a number of major anticlinal structures, most of which are similar to NE-SW-trending folds that developed during mild inversion of syn-rift normal faults or through buckling of the post-rift succession. These chronologies are supplemented by onshore structural evidence and by thermochronological data from key wells. Whilst our analysis confirms the occurrence of a well-documented pulse of late Miocene-early Pliocene compression, post-breakup deformation is not restricted to this time interval. We highlight the growth of a number of structures during the mid-late Eocene and the Oligocene-early Miocene, with evidence for considerable temporal and spatial migration of strain within the basin. Our results indicate a long-lived similar to NW-SE maximum horizontal stress orientation since the mid-late Eocene, consistent with contemporary stress observations but at variance with previous suggestions that this stress orientation was initiated in the late Miocene by increased coupling of the Australian-Pacific plate boundary. We attribute the observed record of deformation to a compressional intraplate stress field, coupled to the progressive evolution of the boundaries of the Indo-Australian Plate, ensuring that this margin has been subject to ongoing compressional forcing since mid-Eocene breakup. Our results indicate that compressional deformation at passive margins may be more common than is generally assumed, and that passive margin basins with evidence for protracted post-breakup deformation histories can provide useful natural laboratories for obtaining improved understanding of the evolution of intraplate stress fields over geological timescales.