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.