THE TILZAPOTLA-BUENAVISTA CALDERA: A TECTONO-MAGMAGMATIC FEATURE ASSOCIATED WITH EOCENE-OLIGOCENE STRIKE-SLIP TECTONICS IN SOUTHERN MEXICO

The Tilzapotla ignimbrites are part of a Tertiary volcanic province located south of the Miocene to Quaternary Trans-Mexican Volcanic Belt. These rocks, together with the coastal plutonic belt of southern Mexico have been associated with the convergence process previous and during the displacement of the Chortis Block. The Eocene-Oligocene silicic volcanic rocks in the Tilzapotla -Buenavista area are associated with a large scale (30 x 20 km) oval-shaped structure, the main features of which indicate a volcanic collapse caldera. The outflow volcanic units are poorly preserved because of the deep erosion, but the distribution and thickness of some outflow ignimbrite remnants as well as the exposed fill thickness suggest a minimum volume of 500 m3. The intracaldera fill is composed of at least three major ignimbrite units as well as slide breccias that include, at the southeastern margin, megablocks embedded in the lower ignimbrite. K-Ar ages of the volcanic sequence range from 35 to 31 Ma. Resurgence is indicated by the central doming inferred from the differences in the altitude between outflow and fill tops as well as the occurrence of lava domes and associated subvolcanic rocks. Thickness variations and the general geometry of the caldera indicate a trap-door-like configuration. The southwestern margin of the caldera, which constitutes one of the long sides of the oval structure, is collinear with a regional fault that in neighboring areas displays left-lateral kinematic indicators. Given the position of the Tilzapotla caldera on the northwest side of this lineament near its southeastern end, we interpret that magma ascent and the later collapse of the caldera was favored by extension associated with the termination of a left lateral fault segment.