Chapala half-graben structure inferred. A magnetometric study

Abstract

Many geologic and tectonic-structural studies refer to the Chapala lake structure in general terms as a graben (or more generally as rift). However, no formal study has addressed its structure. The Chapala Lake is located in a major topographic depression surrounded by broad middle to late Miocene volcanic-capped plateaus. We conducted a magnetic survey comprising five S-N lines crossing the Chapala Lake. One profile comprises a land extension (southwards along roads). Magnetic anomalies obtained are mainly smooth and large scale features associated to the sedimentary infill, local and high amplitude anomalies are due to shallow basaltic and andesitic tilted blocks, and highest amplitudes due to the outcropped basaltic flows of the Los Alacranes and Mezcala islands. The models obtained point to a large structural high located at the central depression portion that separates two structural lows. The structural lows correspond to half-graben type structures. Through the faults delimiting it were emplaced the basaltic flows of the Mezcala and Los Alacranes islands. The presence of half-graben type structures indicates that the crust has been subject to an extensional tectonic.