Stratigraphic-structural relationships in the intersection of the fault systems San Luis–Tepehuanes and Aguascalientes graben, central Mexico

Abstract

The Zacatecas-San José de Gracia area, located at southern Zacatecas and northern Aguascalientes in the borderline between the Sierra Madre Occidental and the Mesa Central, is particularly important because it is located at the intersection of two regional fault systems: the San Luis-Tepehuanes fault system and the Aguascalientes graben. We mapped the area and measured the faults that affect the Cenozoic units in order to know the interaction between these two fault systems during the Cenozoic. Eight new Cenozoic litostratigraphic units were identified, four of which were dated, defining two felsic volcanic pulses: the first one occurred during middle Eocene, with the La Bufa ryolite at the base (48.91 ± 0.09 Ma) and La Virgen Formation at the top (36.8 Ma; Ponce y Clark, 1988: Economic Geology, 83, 1668-1682), the second pulse during the Oligocene, comprising rocks with isotopic ages of 28.0 ± 0.8 Ma (Garabato ignimbrite) and 27.0 ± 0.7 Ma (Sierra Fría rhyolite), with a magmatic gap between ~36 and 29 Ma. Two deformation events were recognized: The first one (D1) occurred in the Paleocene-early Eocene producing WNW and NNE normal faults. The second event (D2), of late Oligocene-early Miocene age, comprised two deformational phases (D2a and D2b) that reactivated preexisting structures and formed new ones. The first phase (D2a) activated NNE normal faults and the second phase (D2b) formed WNW normal faults. An horizontal stretching direction of ~N60ºE could have generated simultaneous (D1) and alternating (D2) activity in both fault systems, which can be explained if we consider that the observed fault directions are inherited from older basement structures.

An apparent left-lateral displacement of the Aguascalientes graben is observed at its intersection with the San Luis-Tepehuanes fault system, but new data herein presented indicate that this apparent structural con figuration is inherited from older structures.