SEA-FLOOR MAGNETOTELLURIC SOUNDING ABOVE AXIAL SEAMOUNT

Axial Seamount is a large, active, ridge axis volcano located on the c entral segment of the Juan de Fuca Ridge in the northeast Pacific Ocea n. Magnetotelluric (MT) data have been collected at three sites, appro ximately 4 km apart around the eastern rim of the volcano, during a 65 -day deployment. MT responses, in the bandwidth of 10(2) - 10(5) s, ar e almost isotropic, with a weakly-defined principal direction of strik e parallel to the main topographic trends of Axial Seamount, and are r elatively flat over the whole bandwidth. Apparent resistivities are of the order of 7 - 20 Omega m, and phases are as low as 30 degrees at t he short periods. Diagonal terms of the MT tensor are an order of magn itude smaller than the off-diagonal terms, suggesting that three-dimen sional effects on the data are minimal. Two-dimensional inversions sug gest that seafloor bathymetry and the distant coastlines have a surpri singly small effect on the MT response, and one-dimensional inversions fit the MT data to within the errors with no serial correlation in th e residuals. A low crustal resistivity is the most robust part of the model, probably due to seawater in fractures and possibly due to a mag ma chamber. An electrical asthenosphere, although less well constraine d, exists over a depth range 30 - 60 km, and the resistivity of this r egion is compatible with about 8% fraction of melt.