Teleseismic tomography of the upper mantle beneath the southern Lachlan Orogen

With the goal of better understanding the deep structure and tectonic setting of the Lachlan Orogen, 50 short period seismic stations were deployed across the southern end of the Great Dividing Range in Victoria (southeast Australia) between 2005 and 2006 to record distant earthquakes. A total of 7452 relative P-wave arrival time residuals from 169 teleseismic events have been extracted from the continuous records using an adaptive stacking technique, which exploits the coherency of global phases across the array. These residuals are mapped as three-dimensional perturbations in P-wavespeed in the upper mantle beneath the array using a recently developed iterative non-linear tomographic procedure, which combines a grid based eikonal solver and a subspace inversion technique. The capability of the new scheme to include interface geometry is utilised in order to investigate the effects of a priori Moho topography on the resolution of upper mantle structure. The resultant images show a pattern of P-wavespeed anomalies that lacks a predominant orientation, and therefore does not favour a purely W E subduction accretion model for the formation of the Lachlan Orogen. One of the main features of the three-dimensional model is a zone of elevated wavespeed beneath the northern end of the array, which extends to a depth of approximately 150 km, and contrasts with significantly lower wavespeeds to the south. This anomaly, which does not appear to be an artifact of relative arrival time residual contributions from the adjoining mountainous terrane, may reflect the presence of a substantial piece of Proterozoic lithosphere incorporated within the Phanerozoic subduction accretion setting of the Lachlan Orogen. Another key feature of the solution model is a zone of relatively low velocity beneath the newer volcanic province northwest of Melbourne, which extends from the crust to a depth of approximately 200 km. This is likely to represent the signature of elevated temperatures associated with a diffuse mantle source for the Quaternary volcanism in Victoria.