Space-time patterns of Cenozoic arc volcanism in central Mexico: From the Sierra Madre Occidental to the Mexican Volcanic Belt

A histogram of 778 isotopic ages of magmatic rocks younger than Eocene in c entral Mexico shows a multimodal distribution with peaks at about 30 Ma, 23 Ma, 10 Ma, and 5 Ma. The sample suite displays systematic spatial variatio ns with age that likely reflect the protracted transition from the north-no rthwest-trending are of the Sierra Madre Occidental to the east-west-trendi ng Mexican Volcanic Belt. The reorientation of the are is accompanied by a change in the dominant composition of the products from silicic ignimbrites and rhyolites to andesitic and basaltic lavas. The observed transition is related to the Miocene reorganization of the subduction system following th e cessation of subduction off Baja California and the eastward motion of th e Caribbean-Farallon-North America triple junction along the southeastern m argin of Mexico. Our data support an early-middle Miocene age for the initi ation of subhorizontal subduction in southern Mexico and confirm that the l ocus of are volcanism was primarily controlled by the geometry of plate bou ndaries and the thermal structure of the subducting slab.