ELECTRICAL IMAGE OF THE SUBDUCTING COCOS PLATE FROM MAGNETOTELLURIC OBSERVATIONS

Magnetotelluric data acquired over the subducting Cocos plate in south ern Mexico image the top of the oceanic plate for at least 150 km inla nd from the coast. Although fluid expulsion occurs in the accretionary prism, enough fluid appears to remain at the top of the subducting pl ate, because of sealing of pores and fractures on the underside of the continental plate, to produce the conductivity contrast necessary for electrical mapping. The results are supported by those of previous gr avity and seismic refraction surveys that suggest the presence of more porous material on top of a denser, subducting oceanic crust. Earthqu ake epicenters also confirm the location of the top of the plate. Magn etotelluric data could, therefore, be used to map the Cocos plate alon g the entire Middle America Trench, where it is apparently broken into separate segments that subduct at different rates and dip at differen t angles.