CONSTRUCTION AND WIDENING MECHANISMS OF PYROCLASTIC DIKE COMPLEXES IN THE EOCENE GOLETA CALDERA, SOUTHERN MEXICO

The late Eocene Sultepec-Goleta Volcanic Center (SGVC) is part of the SMS volcanic province which is located south of the Trans-Mexican Volcanic Belt. The SGVC volcanic succession and volcanic structure records an event of partial collapse and the emission of voluminous ignimbrites.

One of the most remarkable features of the SGVC is the conspicuous exposure of tabular and non-tabular subvolcanic pyroclastic intrusions, interpreted to represent the remnants of the conduits that fed volcanic activity in the area. The almost absolute dominance of pyroclastic dikes over coherent igneous ones indicates that exhumation has not in general exposed dikes below the depths at which magma fragmented. The intrusions are almost continuously distributed in different areas of the northern and southern sectors of the SGVC and in most cases they display textural and compositional variations interpreted to represent different stages of pyroclastic injection. Pyroclastic dike complexes reach up to one with in the ring fracture associated with the collapse.

The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes. The fact that contact zones between individual pyroclastic dike facies do not contain abundant blocks or fragments that might have split off from earlier dikes suggests that these dikes remained hot during their development, thereby preventing brittle behavior.