TIMING, EVOLUTION AND VOLCANIC STRUCTURES OF PALEOGENE SILICIC VOLCANIC CENTERS OF THE NORTHERN SIERRA MADRE DEL SUR, SOUTHERN MEXICO

The northern sector of the Sierra Madre del Sur (SMS) displays vestiges of a significant episode of voluminous silicic volcanism spanning in age from 38 to 32 Ma. Typically, the stratigraphy of the Eocene-Oligocene successions in the region range from low volume mafic-intermediate lava and subvolcanic units to more voluminous dacitic-rhyolitic ignimbrites. In some volcanic centers the silicic event was followed by short-lived episodes of intermediate-mafic volcanism.

The Paleogene volcanic record of the northern SMS has been mostly removed by deep incision of the drainage and general erosion since the Oligocene and was mainly preserved where ignimbrites are connected with vestiges of volcanic conduits and feeding dikes. The partial exhumation of the volcanic centers provide an opportunity to examine the volcanic structure and the plumbing system that produced large ignimbrite episodes.

We have studied in detail seven silicic volcanic centers (Muñeca, Valle de Bravo, Paredes, Goleta, Taxco, Tilzapotla and Huautla) that include, in some cases, evidence of caldera collapses with piston, trap-door and downsag styles. In one of them an episode of resurgence was also documented. Accommodation of collapse was controlled in three of the volcanic centers by pre-existing NW and N-S trending regional faults that show evidence of re-activation during and after the main episodes of ignimbrite volcanism.

The deep erosion in the area exposed pyroclastic dyke complexes that reach up to 1 km in width. The examination of mutual relationships of pyroclastic dykes and the contact features with pre-volcanic host rocks suggest the conduit erosion and wall rock collapse as widening mechanisms of volcanic conduits.

We interpret the episode of voluminous silicic volcanism in terms of the progressive thermomechanical maturation of the crust driven by the sustained mantle power input over a subduction zone associated with Farallon-North America plate convergence processes