The Geologic Evolution of the Southern Sierra de Guanajuato, Mexico: A Documented Example of the Transition from the Sierra Madre Occidental to the Mexican Volcanic Belt

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Geologic mapping and new K-Ar and 40Ar/39Ar geochronology of the southeastern Sierra Madre Occidental (SMO), at its intersection with the northern margin of the Mexican Volcanic Belt (MVB), indicate the occurrence of three volcanic groups. The oldest group corresponds to the SMO, and includes 29 to 22 Ma voluminous ignimbrites and 30 Ma andesites and rhyolites. The youngest group includes widespread basaltic-andesitic lava plateaus that yielded ages from 14.6 to 8.8 Ma and are interpreted as the beginning of the MVB. From 22 to 14.6 Ma, volcanic activity in the area was significantly reduced, but did not cease entirely. We refer to the third group as transitional volcanism, which is dominated by andesitic and rhyolitic lava domes but also includes high-grade andesitic ignimbrites. We conclude that the change from volcanism proper of the SMO to that of the MVB was gradual with respect to age and drastic with respect to composition and style, from a voluminous-silicic-ignimbrite domain to a widespread basaltic-andesitic-lava plateau domain. This change may have been related to major plate tectonic reorganizations within the interval from 25 to 12 Ma that involved the waning of subduction of the Farallon plate west of northern Mexico and the associated southward migration of the triple junction of the Pacific-Farallon-North America plates, the subsequent break-up of the Farrallon plate into the Guadalupe and Cocos plates, and the counterclockwise and clockwise rapid rotation of the ridge between them around 16 to 12.5 Ma.