Geomagnetic induction in the Tasman Sea and electrical conductivity structure beneath the Tasman Seafloor

During the Tasman Project of Seafloor Magnetotelluric Exploration recordings were made of the natural magnetic and electric field variations along a line of seafloor sites crossing the Tasman Sea. Using these data and additional magnetic field recordings made on the Australian continent the pattern of geomagnetic induction in the Tasrnan Sea Region is examined. An investigation of the seafloor magnetotelluric impedance tensor indicates the geomagnetic induction is influenced by the large-scale 3-D conductivity structure of the region. This result is confirmed by a study of impedance estimates obtained using the vertical gradient sounding method (in which the seafloor impedance is determined from the attenuation of the horizontal magnetic field variations through the ocean). Inversion of the Epolarization impedance component from sites in the central Tasman Sea indicates the conductivity structure at depths greater than about 100 km is laterally homogeneous in this region. It is probable that the conductivity structure at these sites includes a high-conducting layer similar to one observed in the Pacific Ocean. One seafloor site, lying on the Tasmantid Seamount Chain, exhibits an increased conductivity at depths of less than 100 km. The increased conductivity may be due to heating of the lithosphere by a hotspot, one proposed source of the seamount chain.