Contrasts in mantle structure beneath Australia: relation to Tasman Lines?

Surface-wave tomography for the Australian region, using data mostly from portable seismic recorders, reveals a very strong contrast in seismic shear-wave speed beneath central and western Australia and the east of the continent. Shear-wave speeds faster than the continental average extend to at least 200 km depth in the cratonic zone to the west of 140°E. Along an approximately north-south line there is then an eastward step to thinner lithosphere (∼150 km thick) but still with fast shear-wave speeds. A further more irregular transition to the east marks the transition to lowered shear-wave speeds. The eastern transition at 75 km depth is in close agreement with the original Tasman Line, whereas the two more westerly transitions do not bear a simple relation to the more recent group of Tasman Lines defined from crustal information (outcrop and inferred geophysical trends). The westward transition to the thickest coherent lithosphere (near 140°E) may well mark the edge of the ancient core of the continent, but the current mantle structures must bear the scars of the breakups and reassembly that have created the current Australian continent.