1-D electrical conductivity structure beneath the Philippine Sea: Results from an ocean bottom magnetotelluric survey

Eight-months of observation using Ocean Bottom Electro-Magnetometers (OBEMs) have allowed us to estimate the regional electrical conductivity structure beneath the Philippine Sea. Six OBEMs were deployed along a line crossing the Philippine Sea from NW to SE and five of them recorded useful data. The raw time series data were cleaned up before we estimated the magnetotelluric (MT) impedance tensor. Conductivity structure at five sites is estimated using 1-D Occam's inversion to fit the determinant average of each MT impedance tensor after a correction for the effect of topography. We examined effect from two dimensionalities on the 1-D conductivity structure and the robustness of solutions. The results of the 1-D conductivity structural model are strongly related to tectonic setting and the crustal age beneath each site. The structure beneath the spreading axis of the Mariana Trough shows a distinct low conductivity structure at depths of 50-150 km and it probably reflects the upwelling dynamics operating beneath the spreading axis. These low values are comparable with that of olivine with low hydrogen content, implying that (1) the melting process extracts water from minerals such as olivine, and (2) the melt beginning depth in the Mariana Trough is deeper than that of the typical MORB source region. The off-axis conductivity profiles infer the existence of a high conductivity peak or a conductivity gradient change at mid-depth. The depth level of the peak increases with crustal age, suggesting that the conductivity structure is related to a geothermal structure and that these conductivity profiles are explained by the temperature gradient change, possibly combined with the presence of partial melt. Our results suggest that further ocean bottom EM study has high potential to investigate the temperature gradient change and amount of hydrogen (water) and melt in the upper mantle.