Asymmetric electrical structure in the mantle beneath the East Pacific rise at 17 degrees S

The magnetotelluric component of the Mantle Electromagnetic and Tomography (MELT) Experiment measured the electrical resistivity structure of the mant le beneath the fast-spreading southern East Pacific Rise (EPR). The data re veal an asymmetric resistivity structure, with lower resistivity to the wes t of the ridge. The uppermost 100 kilometers of mantle immediately to the e ast of the ridge is consistent with a dry olivine resistivity structure ind icating a mantle depleted of melt and volatiles. Mantle resistivities to th e west of the ridge are consistent with a Low-melt fraction (about 1 to 2 p ercent interconnected melt) distributed over a broad region and extending t o depths of about 150 kilometers, The asymmetry in resistivity structure ma y be the result of asymmetric spreading rates and a westward migration of t he ridge axis and suggests distinct styles of melt formation and delivery i n the mantle beneath the two plates.