Tectonic Evolution of Southeast Australia: Insights From Body Wave Tomography

The Tasman Orogen or "Tasmanides" occupies the eastern third of the Australian continent, and is distinct from the Precambrian cratonic terrane to the west both in terms of its subduction accretion style of emplacement, and its Phanerozoic age. From west to east, the southern Tasmanides comprises the Middle Cambrian Delamerian Orogen, and the Late Cambrian to Devonian Lachlan Orogen. The tectonic evolution of this region is not well understood, and a variety of models have been proposed to explain its development. The relationship between Tasmania, which lies at the southeastern tip of the Australian continent, and the mainland Phanerozoic terranes, is also unresolved. In particular, the presence of exposed Precambrian blocks in western Tasmania is in marked contrast to the Phanerozoic outcrops that characterize the Lachlan Orogen. One possible explanation is that Tasmania evolved separately from the mainland. In this paper, results from several recent body wave tomography studies of southeast Australia will be presented. These include two teleseismic experiments targeting the mainland, and a joint wide-angle and teleseismic study of Tasmania. In each case, 3-D variations in seismic structure were obtained from traveltime data using a new iterative non-linear inversion scheme. The new scheme makes use of a robust grid based wavefront tracking technique to solve the forward problem of data prediction, and a subspace inversion method to reconcile these predictions with the observations by adjusting the model parameters. Analysis of the tomographic results reveals a distinctly faster upper mantle beneath the Delamerian Orogen compared to the western Lachlan Orogen. This suggests that the former is underlain by Proterozoic lithosphere, and the latter by Palaeozoic lithosphere. An easterly dipping zone of high velocity beneath western Tasmania may be related to remnant subduction of oceanic lithosphere associated with the mid- Cambrian Tyennan Orogeny, which was largely responsible for the formation of western Tasmania. This is significant, as the Delamerian Orogeny is thought to have involved easterly dipping subduction, and recent dating evidence points to boninites in western Tasmania and the Delamerian Orogen being similar in age and type. Consequently, the tectonic evolution of Tasmania may well be intrinsically linked to the formation of the Tasmanides.