K-Ar illite-mica age constraints on the formation and reactivation history of the El Doctor fault zone, central Mexico
The tectonic history of the Mexican Fold and Thrust Belt exposed in the Sierra Madre Oriental can be assessed in detail by dating the activation of the major shortening structures. The aim of this paper is to present mineralogical and K-Ar geochronologic data of illite-mica from the El Doctor thrust fault, a kilometer-scale structure exposed in the central-western Mexican Fold and Thrust Belt. Such a structure is a brittle-ductile shear zone characterized by subgreenschists facies peak conditions. The crystallization ages of neoformed clay-micaceous minerals from the core of this shear zone are used to constrain the tectonic activity along the selected shear zone. A total of 17 lutite samples were collected from different structural levels of the shear zone. The samples were characterized petrographically and detailed mineralogy was determined by X-ray diffraction. Grain-size subfractions were separated from 11 selected samples, which were subsequently used for K-Ar dating. The mesostructural and microstructural analyses, supported by K-Ar geochronologic data, permitted reconstruct of the crystallization history of phyllosilicate phases related to the tectonic activity of the shear zone. Our data indicate that the El Doctor thrust fault records a complex structural evolution, which is represented by at least three superposed deformation phases. The first phase consists of the shear zone activation, which produced a main foliation pervasive at the submillimeter-scale defined by neoformed illite-mica. This phase is constrained at 80–75 Ma, which is the range of K-Ar ages obtained by illite-mica from the coarse-grained subfractions collected at the core of the shear zone. Subsequent tectonic phases produced the reactivation of the El Doctor shear zone, which produced folding and crenulation of the previous foliation, successively cut by meter- scale extensional shear bends. The most recent reactivation phases, including diagenesis, are constrained to an age of 50–40 Ma obtained from the finest illite subfractions. Based on our data, we conclude that the El Doctor thrust fault recorded a complex, multi-phase structural evolution, characterized by a phase of activation during the Campanian period, and at least one post-Cretaceous reactivation event. The whole history of orogenic wedge construction is preserved in the El Doctor fault.