Seafloor Electromagnetic Measurements above Axial Seamount, Juan de Fuca Ridge

Magnetotelluric (MT) data were collected at three sites around the eastern rim of the caldera of Axial Seamount, on the Juan de Fuca Ridge. The seamount has been observed to be volcanically and hydrothermally active over the last ten years, and is therefore an excellent target for electromagnetic induction studies on the seafloor. This paper follows an initial interpretation by Heinson et al. (1996) with a more complete analysis of the MT data, to investigate both oceanographic induction effects and the resistivity structure beneath the seamount. From time series analysis of electric field data using multitaper methods, coherences between electric field data from different sites are significant at the 95% confidence level at periods less than 1 day, and generally greater than 0.8 at solar and ocean tidal periods. Spectral peaks at 16.7 hours and 4 days are observed; the former is due to inertial currents in the area, and the latter is probably a ridge-trapped Rossby wave. Robust MT impedance tensors are derived using a remote-reference, and tensor decomposition shows that there is no galvanic distortion and almost isotropic responses at each site. The MT data are inverted for 1D structure, and more complex 3D forward models used to assess the lateral extent of the resistivity structure. 1D inversions show that the data are consistent with a crust with a very high electrical conductance of 1200 ± 200 S and an asthenosphere of 5-50 Ω·m at a depth of 40 km, connected by a low resistivity lithosphere of 50-100 Ω·m. The low resistivity lithosphere acts as a leakage path to the mantle for induced currents in the ocean. 3D forward modelling suggests that this region may be present only beneath Axial Seamount, surrounded by a resistive lithosphere of 500-50, 000 Ω·m. The tectonic implications from these models are that a small fraction of melt is presently migrating from the melt source in the mantle to a crustal magma chamber beneath Axial Seamount. Bulk estimates of melt fractions are 1-10% for the asthenosphere, and 1% between the asthenosphere and the crustal magma chamber, although melt may be concentrated in fractures or pipes.