Shortening and kinematics of the Late Triassic rocks in the Tolimán area, central Mexico

Abstract

In the Tolimán area, central Mexico, the Late Triassic rocks (El Chilar Formation; López-Ramos, 1985) record two deformation events. The first event (D1A) is characterized by open chevron folds and incipient axial plane cleavage in the less competent rocks (shale). The second deformation event (D2B) is accommodated through of mesoscopic and kilometric reverse faults that cut the first folding. The first deformation event records approximately 30% of shortening. It was obtained from chevron folds that affected a succession of intercalated layers of sandstone and shale. The folds (class 1B and 1C in Ramsay's classification) are symmetric and asymmetric with a vergence to the NNE, while mesoscopic reverse faults related with this first event have tectonic transport toward NNE. The second deformation event is registered in the Late Triassic rocks through reverse faults with a tectonic transport to the ~E. Based on our observations and the information of other authors, we suggest that the Late Triassic rocks in the study area register two shortening events, the first occurred in Early Jurassic time, related to the start of subduction in the paleo pacific margin of Pangea. While the second is associated with the Late Cretaceous-Paleogene orogenic event that produces the Mexican fold and thrust belt.