CENOZOIC ACTIVITY OF THREE N-S TRENDING BASEMENT FAULTS IN SOUTHERN MEXICO

The Taxco-San Miguel de Allende, the Caltepec and the Oaxaca faults are N-S trending basement faults located in southern Mexico. The three faults have the same distance from the North American trench but only one has had major activity during Cenozoic time: the Oaxaca fault. The Cenozoic fragile deformation in southern Mexico has been mainly liberated by NW-SE trending left-handed strike-slip faults. We studied the evolution of these faults in order to explain why parallel N-S faults have been activated in different ways and why only one of them propitiated the partitioning of deformation.

The Taxco-San Miguel Allende fault was originated as a boundary between a marine basin and platform in Late Jurassic; later, it was the eastern boundary of the accretion of the Guerrero terrane in the Early Cretaceous time and at the end of Cretaceous, during the Laramide orogeny, the fragile deformation was liberated by many parallel thrust faults (Teloloapan fault). During the Cenozoic time, only few minor N-S faults were activated near Taxco city (Taxco, Coapango and Acamixtla faults).

The Caltepec fault was originated in Permian time as a flower structure due to the juxtaposition of the Acatlan and the Oaxaca complexes. During the Jurassic time the fault was located in the western continental margin; during the Laramide Orogeny many N-S folds were formed and anhydrites were injected along the Tamzulapan fault (southern extension of the Caltepec fault). During the Cenozoic time minor N-S normal fault was formed in the western flank of the Tamazulapam anticline. It is remarkable that the deformation was heterogeneous in the eastern block limited by the Tamazulapan fault due to the anhydrite beneath it.

The Oaxaca fault was formed in the Permian, in the the Jurassic was reactivated as right-handed strike-slip fault during the opening of the Gulf of Mexico, the roots of the Oaxaca fault was exposed in the Neocomian time, during the Laramide orogeny it was activated as thrust fault exposing the middle crust; during Cenozoic this fault acted as normal fault, synchronous with the oblique NW-SE Donaji fault.

During the Cenozoic tectonic history of the Sierra Madre del Sur, the deformation has been concentrated only in the Oaxaca fault because it was transformed in a weakness shear zone during the opening of the Gulf of Mexico.