AN OLIGOCENE CALDERA AT THE SOUTHERN PART OF THE MEXICAN MESA CENTRAL, SAN LUIS POTOSÍ, MEXICO

The San Luis Potosí Volcanic Field (SLPVF), is located to the southwestern part of San Luis Potosí city. This field is a volcanic sequence formed almost by rhyolites with small volume of basaltic andesites and basalts, interlayered. The sequence belongs to Oligocene age (32.7±1.0 to 27.0±0.7 Ma), and some other small units are of Miocene age (Cerro Reina Rhyolite 21.8±0.3 Ma, Cabras Basalt 21.5±0.5 Ma, and Los Castillo Traquite 20.3±0.5 Ma). The main tectonic frame in the SLPVF corresponds to normal faults (NW strike), and to two semicircular concentric features. Three rhyolitic ignimbrites (Santa Maria, Cantera and Panalillo), of different age and character, are inserted between different events of lava emissions. Santa Maria ignimbrite has been proposed associated to the Milpa Grande Caldera; Panalillo ignimbrite is associated to extensional volcanism; and Cantera ignimbrite is associated to the development of the San Luis Potosí caldera. The arguments to assign a caldera origin to this ignimbrite are: mapping of volcanic facies to define the location of emission sources of the piroclastic material; and results of a Anisotropy of Magnetic Susceptivility (AMS) study. The volcanic facies of the Cantera ignimbrite suggests three emission zones of piroclastic material: two of these sources are asociated to the semicircular major feature and the third one to the main extensional faults located at the northeastern part of the SLPVF. Flow directions inferred from AMS analysis indicate several ignimbrite sources located along selected NW-SE linear features (mainly El Potosino fault) as well as along the rim of the caldera structure. The results obtained with both methods are coherent. The geometry of volcanic outcrops, the NW-SE faulting fracture systems as well as the AMS results suggest clearly that this is a caldera structure resembling the trapdoor-type (Lipman, 1997).