Tectonic Control of the Volcano-Sedimentary Sequence of the Chapala Graben,Western Mexico

The Chapala graben forms part of a regional system of intra-arc and half-grabens located along the western and central portions of the Mexican Volcanic Belt. Results of a stratigraphic and tectonic study of the thick, widespread volcano-sedimentary sequence around the Chapala graben are reported. The study has concentrated on the distribution of lacustrine deposits and on the stratigraphy, geochronology, and deformation of the sequence. The volcano-sedimentary record suggests a probable link between the Tepic-Zacoalco and Chapala grabens through a basin system developed during late Miocene and early Pliocene time. The stratigraphic sections show a southward spatial migration of the basin (and lake) system. The main unit of the volcano-sedimentary succession is the Chapala Formation, which has been affected by northeast tilting, unlike the lacustrine deposits to the east in the Chapala plain (e.g., Ixtlan area), where the sequence is essentially flat lying. Two distinct units with different structural attitudes, separated by an angular unconformity, can be distinguished in the Chapala Formation. Based on differences between the stratigraphic sections and structural attitudes, we propose a model for development of the Chapala graben that involves a combination of left-lateral and extensional deformation, together with volcanic and erosional processes, all of which have contributed to shape the basin. Furthermore, we suggest that Lake Chapala is the remnant of a large Jalisco paleo-lake in west-central Mexico.