Geology and structure of Guanajuato and Codornices ranges, Mesa Central, Mexico

Abstract

To increase the knowledge of tectonic evolution of central Mexico during the Cenozoic, it was considered to study the structure and uplift history of the Guanajuato and Codornices ranges. They form a tectonic element, elevated up to 700 m relative to the El Bajio basin, and where the Jurassic-Cretaceous basement is exposed. The study area is the limit between the Mesa Central and the central-northern part of the Transmexican Volcanic Belt. It is located in a key place to study the Cenozoic tectonics of central Mexico. There are two important orographic entities in the study area: (1) the Sierra de Guanajuato and (2) the Sierra de Codornices. Together they form the raised block of an Oligocene-Eocene half-graben, which bounds the El Bajio basin. By compiling the geological maps of the study area, we recognized six lithologic groups: Jurassic-Cretaceous rocks, Eocene conglomerates, Ypresian volcanic and plutonic rocks, Rupelian volcanic rocks, Chattian rocks and Miocene rocks. With this information, we elaborated 27 geological sections, configuring a complete model of the geometry and distribution of stratigraphic units and Cenozoic faults. The Eocene volcanic rocks are only found in the NW sector of the study area as remnants of a more extensive and eroded cover. The Rupelian volcanic activity has a difference in its distribution: along the SW margin of the study area, pyroclastic rocks were deposited, while throughout the NE front large volumes of effusive volcanic rocks were emplaced. The Oligocene and Miocene activity of the normal faults occurred in successive phases forming the stepped configuration to the SW margin of the Sierras of Guanajuato and Codornices due to the migration of the active zones towards the basin. In contrast, normal faults are scarce and a steep mountainous front is not well defined along the entire NE front of the study area. In this part, the slopes gently dip towards the NE giving the whole area the shape of a tilted block. The Sierra de Guanajuato and the Sierra de Codornices experienced three major phases of uplift-exhumation: the first in the early-middle Eocene, the second in the Oligocene and the third in the Miocene, when acquired the current configuration. The relative rise with respect to El Bajio basin was always higher in the Sierra de Guanajuato and lower in the Sierra de Codornices.