Physics of the Earth and Planetary Interiors 167 (2008) 84–97 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi Teleseismic tomography of the upper mantle beneath the southern Lachlan Orogen, Australia N. Rawlinson ∗ , B.L.N. Kennett Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia article info Article history: Received 18 August 2007 Received in revised form 29 January 2008 Accepted 19 February 2008 PACS: 91.30.-f 91.30.Cd 91.30.Jk 91.30.Wx 91.35.Gf Keywords: Teleseismic tomography Non-linear inversion Southeast Australia Lachlan Orogen Lithospheric structure abstract 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 150km, 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Prior to the breakup of the Meso-Neoproterozoic superconti- nent Rodinia, the Australian land-mass comprised an assemblage of the north, west and south Australian cratons. This amalgamation of Archean and Proterozoic terranes now represents the western two-thirds of the Australian continent, and formed as a result of continent–continent collision, accretion along several subduction fronts, mafic underplating associated with vertical accretion, and several episodes of lithospheric extension (Betts et al., 2002). Dis- tinct blocks of Archean lithosphere contained within this region include the Pilbara and Yilgarn Cratons in Western Australia, and the Gawler Craton in South Australia. ∗ Corresponding author. Tel.: +61 2 6125 5512; fax: +61 2 6257 2737. E-mail address: nick@rses.anu.edu.au (N. Rawlinson). URL: http://rses.anu.edu.au/∼nick (N. Rawlinson). Following the break-up of Rodinia, disassembled elements of the former supercontinent reformed during the Late Neoproterozoic to Early Cambrian to create Gondwana (Meert, 2001). Subsequent convergence along the proto-Pacific margin of east Gondwana formed a series of orogenic belts that date from the Middle Cam- brian through to the onset of the break-up of Gondwana and Pangea, at around 227 Ma (Glen, 2005). The Tasman Orogen or “Tasman- ides” (Foster and Gray, 2000) comprises the eastern one-third of the present day Australian continent that was accreted along the east- ern margin of Precambrian Australia as a result of this process. The Delamerian, Lachlan, New England, Thomson and North Queens- land Orogens represent separate components of this Phanerozoic terrane. In southeast Australia (Fig. 1), the Mid-Late Cambrian Delame- rian Orogen, which extends from the mainland into Tasmania (Glen, 2005; Rawlinson et al., 2006a), formed as a result of west-directed subduction along the proto Pacific margin (Betts et al., 2002). The underlying basement consists of Proterozoic lithosphere, which is thought to partially extend beneath the Lachlan Orogen to the 0031-9201/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2008.02.007