2-DIMENSIONAL ELECTRICAL-CONDUCTIVITY STRUCTURE ACROSS THE SOUTHERN COASTLINE OF AUSTRALIA

Geomagnetic field measurements have been made at three new seafloor lo cations off the coast of southern Australia, to extend a magnetometer array deployed by White and Polatayko (1978). Geomagnetic depth soundi ng (GDS) and vertical gradient sounding (VGS) estimates provide a meas ure of both the TE and TM mode response of the continent-ocean boundar y, which is relatively two-dimensional (2D) over hundreds of kilometre s. The seafloor data from the continental shelf, mid-continental slope and at the edge of the abyssal plain show a strong geomagnetic coast effect which can largely be accounted for by the sea-water and a thick wedge of sediments in a rifted marginal basin on the continental shel f. A 2D inversion of the GDS and VGS estimates to determine the sub-se afloor conductivity structure suggests that (a) the oceanic crust and continental crust have conductivities of 0.05 and 0.005 S.m-1 respecti vely to a depth of 10 km, (b) the upper mantle above 140 km has conduc tivity 0.01-0.003 S.m-1, (c) the upper mantle between 140 and 390 km h as conductivity less than 0.003 S.m-1, and (d) the lower mantle below 390 km has conductivity less than 0.3 S.m-1. No distinct boundary betw een oceanic and continental lithosphere conductivity can be discerned. The conductivity structure beneath the coastline of southern Australi a is somewhat less conductive than the preferred model of Kellett et a l. (1991) for southeastern Australia.