Imaging crustal structure variation across southeastern Australia

Abstract

A broad-band seismic network of 28 three-component seismometers was deployed in southeastern Australia to examine variations in crustal thickness across the transition between Precambrian and Phanerozoic lithosphere. Receiver function observations and modelling of P-to-S conversions at the Mohorovičić discontinuity (Moho) have been employed to investigate: (i) the variations in the Moho depth across southeastern Australia, and (ii) the nature of the transition between crust and mantle. Data from temporary deployments were used together with data from the few permanent broad-band stations in the region. The extraction of P-receiver functions from high-quality seismic data recorded on these stations has enabled the determination of the crustal thickness across the region. The crustal thicknesses lie in the range 28–48km. The Moho depth is generally well correlated with the Earth surface elevation in the southeastern Australia. The Moho estimates from receiver functions are in good agreement with results from reflection profiling. The average crustal thickness is found to be around 39km beneath the Precambrian area in the west and even thicker beneath the Lachlan Orogen in the east (~43km). The average crustal thickness in between, beneath the Murray Basin is thinner ~32km. Interestingly, the crust in the Mount Gambier volcanic area is rather thick ~41km, suggesting that the limit between the Delamerian and western Lachlan orogens is located east of Mount Gambier. Our results favour a position for the Tasman Line generally consistent with the interpretation by Direen and Crawford (2003) and thus to the east of the location favoured by many authors. The broader crust–mantle transition and thicker crust beneath the Lachlan Orogen suggest the presence of magmatic underplating at the base of the lower crust. The intermediate nature of the crust–mantle transition also suggests magmatic underplating beneath the Gawler Craton and the Curnamona Province.