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

Abstract

To investigate the complex crustal structure of southern Mexico magnetotelluric (MT) data were acquired at 75 stations along two north–south profiles, 200–250km 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 may represent a crustal unit with a characteristic depth dependent electrical conductivity distribution. Often, the spatial extent of such a crustal unit matches well with a geological terrane. However, we also observed transition zones or sharp contrasts in crustal electrical properties which do not correspond to any defined terrane boundary. Transition zones are associated with low angle crustal structures (regional thrust faults or mylonitic zones) and/or with areas of plutonism, which are adding heterogeneities to the crust. In other cases our data suggest that terrane boundaries proposed on the basis of surface geology are probably shallow structures, which may not 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 tectonics as well as the part of the crust with Paleozoic and Precambrian metamorphic rock assemblages.