Age and evolution of thin-skinned deformation in Zacatecas, Mexico: Sevier orogeny evidence in the Mexican Fold-Thrust Belt

Abstract

Integrating U-Pb ages from zircons of syn- and postectonic intrusives emplaced in folded pre- and synorogenic marine sedimentary rocks, it is proposed that thin-skinned deformation in the Concepción del Oro salient of the Mexican Fold Thrust Belt in northern Zacatecas, Mexico, was active between 92 and 71.6 Ma. The intrusives Pico de Teyra and El Peñuelo (U-Pb zircon ages: 76.9 and 72.5 Ma) show internal tectonic foliations and horizontal shear zones that cut off aplitic veins, which apparently developed syntectonically to thin-skinned deformation. Other intrusives like Saltillito (71.6 Ma) and Concepción del Oro are clearly postectonic because they are undeformed internally, cut regional structures and are younger than syntectonic plutons. Biostratigraphic ages reported for synorogenic sediments (Concepción del Oro and Parras formations) indicate that regional thin-skinned deformation was active between Early Turonian and Late Campanian, which is in agreement with syn and postectonic intrusive emplacement ages in the area. Nevertheless, the thin-skinned structures are disrupted by a younger NNW-SSE high angle reverse and normal faults that uplifts the San Julián Block in the west and truncate the Concepción del Oro salient, suggesting a post-Paleocene thick-skinned stage of deformation. In this work, we propose that style and age of thin-skinned deformation is similar to the Sevier orogeny in the Rocky Mountains.