The electrical conductivity structure of the oceanic lithosphere beneath the Tasman Sea

Abstract

Seafloor magnetotelluric observations of natural fluctuations of the Earth's magnetic and electric fields provide information about the electrical structure of the suboceanic lithosphere and upper mantle. However, such measurements may be distorted by three-dimensional induction effects in the overlying ocean, associated with the geomagnetic coast-effect and changes in bathymetry. In the present paper electromagnetic three-dimensional modelling is applied to the Tasman Sea. A thin-sheet algorithm is used, and calculated magnetotelluric parameters are compared to observations made during the Tasman Project of Seafloor Magnetotelluric Exploration (TPSME). The model calculations suggest that, away from coastlines, three-dimensional induction in the ocean distorts the electromagnetic signature of the sub-oceanic lithosphere and upper mantle in a frequency-dependent but smooth manner. The model calculations thus permit the observed TPSME responses to be corrected for distortion, to give improved estimates of the electromagnetic response of the solid-Earth underlying the observation sites. Inversion of such (corrected) response estimates for one-dimensional electric structure reveals that the structures beneath 55 and 70 Ma seafloor sites are very similar. Furthermore, constraints imposed by the thin-sheet modelling indicate that to replicate the TPSME observations, a poorly conductive (less than 10-3 S/m) upper lithosphere is required. This result implies that electric currents which are induced and 'trapped' in the ocean may contaminate magnetotelluric observations far inland.