Seismic structure of the southeast Australian lithosphere from surface and body wave tomography

Abstract

Teleseismic arrival time residuals from the WOMBAT transportable seismic array experiment are inverted to construct a high-resolution 3-D P-wave velocity model of the upper mantle beneath southeast Australia. In order to address one of the principal limitations of teleseismic tomography – that long wavelength structure is filtered out when data from multiple arrays operating at different times are used – an initial model with lower spatial resolution, derived from surface tomography, is constructed to preserve the broad scale features that would otherwise be lost. Although the absolute velocities of the final model are not strongly constrained due to the assumption of radial Vp/Vs ratios and differences in regularisation, the relative variations appear robust across all scales. These reveal a wealth of features that can be related to the geology and tectonic history of the region, the most significant being (1) an easterly dipping velocity transition zone which involves a higher velocity Delamerian Orogen extending beneath the Western Subprovince of a lower velocity Lachlan Orogen; (2) a distinct region of low velocity in the upper mantle north of Melbourne, which can be associated with recent Quaternary hot-spot volcanism; (3) a gradual east-southeast decrease in velocity towards the coast, which is consistent with lithospheric stretching and thinning near a passive margin; and (4) a zone of high velocity north of Adelaide that may correspond to the presence of the Palaeoproterozoic Curnamona Province at depth. These results have important implications for the Palaeozoic evolution of the east margin of Gondwana, the subsequent break-up of Australia and Antarctica and opening of the Tasman Sea.