Paleothermal and seismic constraints on late Miocene-Pliocene uplift and deformation in the Torquay sub-basin, southern Australian margin

The passive southern margin of the Australian continent contains a rich record of late Miocene-Pliocene neotectonic deformation and uplift that continues to the present day as witnessed by unusually high levels of seismicity for a so-called ‘stable continental region’. To date however, few studies have sought to estimate the magnitude of exhumation triggered by this deformation and uplift. Here we combine apatite fission track analysis (AFTA), apatite (U-Th)/He dating and vitrinite reflectance (VR) data from the Nerita-1 well in the Torquay sub-basin with seismic reflection data from this basin and the adjoining Otway Ranges to constrain the magnitude and driving mechanisms of exhumation in this part of the southern Australian margin. The Cenozoic succession in this basin has been deformed by folding and reverse faulting and contains a major, low-angle mid-Miocene unconformity that can be traced for distances of ~1500 km along the margin. Palaeothermal data from Nerita-1 show that the sub-mid-Miocene succession has been more deeply buried by ~1 km of now missing section, and indicate that exhumation began between 10 and 5 Ma, in excellent agreement with stratigraphic constraints. Our estimates of removed section and higher than previous estimates based on extrapolation of seismic reflectors, but are corroborated by AFTA results from nearby wells. Seismic data show that late Miocene-onwards intraplate deformation in the Torquay-sub-basin and Otway Ranges has been accomplished by reverse-reactivation of normal faults within Cretaceous-early Palaeogene syn-rift successions, resulting in folding of overlying post-rift late Palaeogene-Neogene sediments. The probable cause of this deformation and uplift is increased levels of intraplate stress induced by enhanced coupling of the Indo-Australian and Pacific plates ~10 Myr ago, and our results thus demonstrate the key role that plate boundary-controlled stress fields play in intraplate uplift and deformation.