Crustal structure and formation of a magnetic quiet zone in the Australian southern margin

Abstract

The crustal structure of the magnetic quiet zone outboard of the Otway Basin of the Australian southern margin is interpreted for the whole crustal section using fully processed multichannel seismic reflection data, seismic refraction data and dredge data collected by the Australian Bureau of Mineral Resources (BMR) vessel Rig Seismic, and extrapolation of well control. The quiet zone is interpreted to consist of relatively thin (10 km), faulted and extended continental crust. The upper and lower crust are interpreted to have been extended separately by listric but approximately subplanar faulting, and planar normal faulting, respectively, during a final mid-Cretaceous extensional phase prior to sea-floor spreading. This brittle extension in both cases is calculated to be, on average, less than about 40%. A discrepancy occurs between the observed crustal thickness and that implied by fault geometry because the interpreted lower crustal thickness is approximately 7 km and the observed faulting cannot account for a thinning of the crust to less than 25 km thickness from an initial continental crustal thickness of around 35 km. Subsidence calculations are, however, consistent with the interpreted crustal thicknesses. These observations require previous crustal thinning, other than by the observed faulting, and argue for two distinct phases in the formation of the continental margin and quiet zone. A model is presented for the mid-Cretaceous formation of the continental margin quiet zone of the Otway Basin. This model involves a second stage rifting of an older (first stage) onshore rift. In the model the ‘lower plate’ of the first stage onshore rift is separated from the upper plate by simple shear, possibly accompanied by a component of pure shear within the lower plate. The lower plate is then subjected to subsidence and overlying sedimentation. This is followed by a second stage of rifting involving separate extension and thinning of the original lower plate and subsequent sediments prior to sea-floor spreading. The extensional faults interpreted from seismic reflection data collected in this study are thought to provide the mechanism. The second stage rifting is modelled as finally forming the ‘lower plate’ continental passive margin and continental magnetic quiet zone. Towards the end of the second stage rifting, high angle normal faults are interpreted to cut the whole crust on the continental quiet zone. They are believed to be associated with earliest thermal subsidence in that region after the onset of sea-floor spreading. These faults are not observed in seismic reflection data over the continental slope. The onset of oceanic crust occurs at a ridge at the southern end of the transect. The ridge corresponds to a free-air gravity high and the beginning of long wavelength magnetic anomalies with amplitudes of approximately 100 nT. The characteristic two-stage rifting model derived here appears to be also valid for the Exmouth Plateau and may have implications for some other magnetic quiet zones and continental margins.