The Mesa Central (MC) of Mexico presents a noticeable feature as is the three-dimensional (3D) deformation of Cenozoic volcanic rocks. To figure out when this unusual deformation began, we constructed a geological map at 1:100,000 scale, with detailed stratigraphy, and thirteen new U-Pb ages in the southern MC. The mapped area is dominated by silicic volcanic rocks with ages from ca. 34.4 Ma to ca. 23.5 Ma affected by coeval normal faulting. An angular unconformity evidences a shift in the deformation style that occurred around 30 Ma, from an NW-trending fault system in domino-style to a polymodal fault system, which also temporally coincided with the emplacement of lava dome complexes. The structural and stratigraphic data give new insight for understanding how the polymodal fault system developed in the southern MC, suggesting that the change in the deformation and volcanism regime played an important role.

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It is widely accepted that the polymodal fault patterns accommodate triaxial deformation. Multiphase extension and inherited structural fabrics have been counted as common features for the development of polymodal faults in extensional systems. This research documents the development of a polymodal normal fault system in the Mesa Central of Mexico, focusing on the evolution of the deformation field. The fault system comprises NW and NE grabens and N-S and E-W faults. These faults affected silicic volcanic rocks in two Oligocene extensional phases. In the Rupelian (~31 Ma), a NW trending domino fault system was formed, associated with fault domain boundaries (transfer or accommodation zones) oriented NW, NE, N-S, and E-W. The domino faults tilted the early Rupelian rocks, and the deformation was biaxial with the principal extension NE-SW. Subsequently, the Rupelian structures were buried by the middle and late Rupelian volcanism. The second extensional phase occurred in the Chattian (after 28 Ma). In this phase, the Rupelian fault domain boundaries acted as faulting zones forming a polymodal fault system in the volcanic cover. The resulting deformation was triaxial with ENE and NNW horizontal principal extensions. From revision of cases in the literature and the deformation documented in the Mesa Central, we propose a general way to form a brittle triaxial deformation zone, requiring (1) the presence of planes of weakness in the upper crust, (2) the deposition of a cover, and (3) a subsequent faulting event reactivating the underlying anisotropies and producing new faults in the cover. Since the pioneering work of Oertel (1965), a few theoretical and experimental studies have been published regarding fault systems formed under triaxial deformation. Most of the geometric, dynamic, and kinematic models have been proposed for orthorhombic fault systems from centimetric to kilometric scales (e.g., Aydin

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 Bajío 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 Bajío 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 Bajío basin was always higher in the Sierra de Guanajuato and lower in the Sierra de Codornices.