Rhyolitic volcanism in extension zone associated with strike-slip tectonics in the Taxco region, southern Mexico

Abstract

The Taxco Volcanic Field (TVF) is part of a broad magmatic province in southern Mexico. It constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of Mesozoic sedimentary and volcano-sedimentary units. A thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the TVF. This volcanic sequence is distributed within a well defined zone, it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults. Geochronologic data indicate that most rhyolitic volcanism in the area is Oligocene in age and synchronous with episodes of strike-slip faulting. We document two successive phases of strike-slip faulting for the late Eocene–early Oligocene interval, the first with NNW extension and the second with NE extension. In both cases pre-existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults. The stratigraphic sequence gives evidence that the TVF is located in an extensional basin associated to strike-slip faults. The evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism. The result of this change was the development of a volcanic pile with elevations higher than the surrounding Mesozoic rocks. According to the fault kinematics, stratigraphy and the volume of volcanic rocks, the rhyolitic volcanism was emplaced in the area of maximum extension, showing that magma flowed into low pressure zones. The small number of faults within the Oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike-slip faulting.