Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

Abstract

Geomagnetic field measurements have been made at three new seafloor locations off the coast of southern Australia, to extend a magnetometer array deployed by White and Polatayko (1978). Geomagnetic depth sounding (GDS) and vertical gradient sounding (VGS) estimates provide a measure of both the TE and TM mode response of the continent-ocean boundary, which is relatively two-dimensional (2D) over hundreds of kilometres. 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 shelf. A 2D inversion of the GDS and VGS estimates to determine the sub-seafloor conductivity structure suggests that (a) the oceanic crust and continental crust have conductivities of 0.05 and 0.005 S·m-1 respectively to a depth of 10 km, (b) the upper mantle above 140 km has conductivity 0.01-0.003 S·m-1, (c) the upper mantle between 140 and 390 km has 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 between oceanic and continental lithosphere conductivity can be discerned. The conductivity structure beneath the coastline of southern Australia is somewhat less conductive than the preferred model of Kellett et al.(1991) for southeastern Australia.