Electrical resistivity structure under the western Cosmonauts Sea at the continental margin of East Antarctica inferred via a marine magnetotelluric experiment

The western Cosmonauts Sea, off the coast of East Antarctica, was a site of rifting of the Gondwana supercontinent and subsequent early seafloor spreading. To improve our understanding of the breakup of Gondwana, we conducted a marine magnetotelluric experiment to determine the electrical resistivity structure within the uppermost several hundred kilometers beneath the western Cosmonauts Sea. Magnetotelluric response functions at two sites, obtained after considering possible influences of non-plane magnetic field sources, suggest that these responses include distortions by topographic variations and conductive anomalies around the observation sites. Three-dimensional forward modeling confirmed that these distortions due to topographic variations and a thin (∼2-km thick) conductive layer immediately under the sites (mostly sediments) are severe. Furthermore, three-dimensional forward modeling to investigate the resistivity structure at deeper depths revealed an upper resistive layer (≥300 Ω-m), with a thickness of <100 km, and an underlying conductive half-space (∼10 Ω-m). The upper resistive layer and the underlying conductive structure most likely represent dry and water/melt-rich oceanic upper mantle, respectively. The upper resistive layer may be thinner than anticipated under the old seafloor of the study area (likely >90 Ma), and may suggest a conductive anomaly in the upper mantle produced by mantle convection and/or upwelling.