Anomalous lithosphere beneath the Proterozoic of western and central Australia: A record of continental collision and intraplate deformation?

Abstract

A new surface wave seismic tomography model of Australia is presented which provides a means of investigating the lithospheric structure beneath the Proterozoic regions in the west, north and centre of the continent with improved resolution and reliability. The dominant feature of the model is a region of low seismic wavespeeds in the uppermost mantle, at approximately 75 km depth, beneath central Australia. The zone of slow wavespeeds is underlain by a region of fast wavespeeds, more typical of continental lithosphere. This layered velocity structure, and strong positive wavespeed gradient, makes the shallow anomaly hard to explain in terms of high mantle temperatures and typical steady-state continental geotherms. A possible thermal explanation requires the impact of the redistribution of high heat producing elements within the crust. Alternatively, a mineral or minerals with low seismic velocities, such as amphibole, in the shallowest part of the lithosphere, with a more conventional lithology in the deep continental root below, could explain the seismic wavespeeds. The anomaly is located directly beneath the zone where the Australian cratons amalgamated in the Proterozoic and where, subsequently, there have been periods of intraplate tectonic activity; suggesting a correlation between the prolonged history of deformation and a highly unusual lithospheric structure. In western Australia, the Capricorn Orogen and Pilbara Craton have a similar lithospheric thickness, whereas a thicker lithosphere is observed beneath the Yilgarn to the south. In northern Australia, large regions appear to be underlain by fast wavespeeds, similar to those observed beneath the Yilgarn Craton. Variations in the shear wavespeeds beneath the Arunta also indicate that there is not always an obvious correlation between the overlying surface geology and seismic structures observed in the upper mantle.