Structure of the Tasmanian lithosphere from 3D seismic tomography N. RAWLINSON 1 *, H. TKALC ˇ IC ´ 1 AND A. M. READING 2 1 Research School of Earth Sciences, Australian National University, ACT 0200, Australia. 2 School of Earth Sciences and ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tas 7001, Australia. Seismic data from three separate experiments, a marine active source survey with land-based stations, and two teleseismic arrays deployed to record distant earthquakes, are combined in a joint inversion for the 3D seismic structure of the Tasmanian lithosphere. In total, travel-time information from nearly 14 000 source–receiver paths are used to constrain a detailed model of crustal velocity, Moho geometry and upper mantle velocity beneath the entire island. Synthetic reconstruction tests show good resolution beneath most of Tasmania with the exception of the southwest, where data coverage is sparse. The final model exhibits a number of well-constrained features that have important ramifications for the interpretation of Tasmanian tectonic history. The most prominent of these is a marked easterly transition from lower velocity crust to higher velocity crust which extends from the north coast, northeast of the Tamar River, down to the east coast. Other significant anomalies include elevated crustal velocities beneath the Mt Read Volcanics and Forth Metamorphic Complex; thickened crust beneath the Port Sorell and Badger Head Blocks in central northern Tasmania; and distinctly thinner, higher velocity crust beneath the Rocky Cape Block in northwest Tasmania. Combined with existing evidence from field mapping, potential-field surveys and geochemical data, the new results support the contention that east and west Tasmania were once passively joined as far back as the Ordovician, with the transition from lithosphere of Proterozoic continental origin to Phanerozoic oceanic origin occurring some 50 km east of the Tamar River; that the southeast margin of the Rocky Cape Block may have been a former site of subduction in the Cambrian; and that the Badger Head and Port Sorell Blocks were considerably shortened and thickened during the Cambrian Tyennan and Middle Devonian Tabberabberan Orogenies. KEY WORDS: lithospheric structure, seismic tomography, Tasmania, Tyennan Orogen. INTRODUCTION Seismic tomography is a data-inference technique which requires the solution of a large inverse problem to build heterogeneous models of the Earth’s interior that are consistent with observations made from seismic records. From its origins in the mid-late 1970s (Aki & Lee 1976; Aki et al. 1977; Dziewonski et al. 1977), it has developed into a mature technique that is widely used to image subsurface structure at a variety of scales. The use of many sources and receivers with an even geographical distribution is essential for producing a detailed and well-resolved seismic model. Artificial sources, such as explosions, airguns or vibroseis are often used in cross-hole, reflection or wide-angle tomo- graphy, which is favoured in exploration and continen- tal profiling (Bishop et al. 1985; McMechan 1987; Williamson 1990; Zelt & White 1995; Bleibinhaus & Gebrande 2006). Earthquake sources are more common in larger-scale studies which may examine regions of the crust, lithosphere or even the whole Earth (Walck 1988; Benz et al. 1992; Grand et al. 1997; Steck et al. 1998; Simons et al. 1999; Burdick et al. 2008; Priestley et al. 2008). In Australia, 3D seismic tomography experiments began with the continent-wide SKIPPY project (1993– 1998), which involved progressive coverage of Australia using a portable array of broadband seismometers to record regional earthquakes (Zielhuis & van der Hilst 1996). Both surface wave and body wave data from SKIPPY and subsequent deployments have been used to generate 3D tomographic images of compressional and shear wave-speed variations in the upper mantle beneath Australia at a horizontal resolution of between 200 and 250 km (Zielhuis & van der Hilst 1996; Debayle & Kennett 2000; Gorbatov & Kennett 2003; Fishwick et al. 2005). Major results from these studies include the delineation of Archean and Proterozoic Cratons at depth, a pronounced transition from high wave speeds beneath Precambrian western and central Australia to low wave speeds beneath Phanerozoic eastern Austra- lia, and even lower wave speeds beneath the eastern *Corresponding author: nick@rses.anu.edu.au Australian Journal of Earth Sciences (2010) 57, (381–394) ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2010 Geological Society of Australia DOI: 10.1080/08120099.2010.481325 Downloaded By: [University of Tasmania] At: 23:34 24 May 2010