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Teleseismic tomography of the upper mantle beneath the southern Lachlan Orogen, Australia

Nick Rawlinson and Brian Kennett

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

In the 2005-2006 EVA experiment, 50 short period seismic stations were deployed across the southern end of the Great Dividing Range in Victoria (southeast Australia) to record distant earthquakes (see Figure 1). This study forms part of an ambitious long term experiment undertaken by RSES, called Wombat-SE, to use rolling deployments of dense seismic arrays to cover much of Eastern Australia. A total of 7452 relative P-wave arrival time residuals from 169 teleseismic events have been extracted from the continuous records of EVA using an adaptive stacking technique, which exploits the coherency of global phases across the array. These residuals are mapped as 3-D 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 3-D model is a zone of elevated wavespeed beneath the northern end of the array (Figure 2), 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 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 (Figure 2), 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.

 

 

Figure 1.  Location of 50 EVA stations (open triangles) used in this study.
Sites vary in elevation between 9 m (evd6) and 1724 m (evh1).

 

Figure 2. A selection of slices through the solution model with several features of interest highlighted.
(a) Depth slice at 100 km; (b) depth slice at 150 km; (c) E-W cross-section at 36° S;
(d) N-S cross-section at 147° E. NVP = Newer Volcanic Province; PCF = Proterozoic Continental Fragment.