Crustal variations and terrane boundaries in southern Mexico as imaged by magnetotelluric transfer functions

To investigate the complex crustal structure of southern Mexico magnetotell uric (MT) data were acquired at 75 stations along two north-south profiles, 200-250 km apart, crossing several terrane boundaries whose interpretation has been controversial in the literature. Resulting sounding curves can be grouped, according to their shapes, into 10 different sets. Each of them m ay represent a crustal unit with a characteristic depth dependent electrica l conductivity distribution. Often, the spatial extent of such a crustal un it matches well with a geological terrane. However, we also observed transi tion zones or sharp contrasts in crustal electrical properties which do not correspond to any defined terrane boundary. Transition zones are associate d with low angle crustal structures (regional thrust faults or mylonitic zo nes) and/or with areas of plutonism, which are adding heterogeneities to th e crust. In other cases our data suggest that terrane boundaries proposed o n the basis of surface geology are probably shallow structures, which may n ot continue at depth. The comparison of apparent resistivity profiles with the available geologic information indicates that the MT method is capable of clearly defining zones affected by recent and active volcanism and tecto nics as well as the part of the crust with Paleozoic and Precambrian metamo rphic rock assemblages. (C) 2000 Elsevier Science B.V. All rights reserved.