Abstract
The relatively flat-lying rocks of the upper volcanic supergroup exposed between Durango City and Mazatlán in western Mexico consist of two distinct sequences. Near Durango about 800 m of ignimbrites and associated lavas and tuffs were emplaced between 32.1 and 28.3 m.y. ago (23 K-Ar dates). About 100 to 200 km to the west, at least 1,000 m of ignimbrites, lavas, and tuffs were emplaced 23.3 ± 0.3 m.y. ago, as indicated by 8 K-Ar dates. The two calc-alkalic sequences have significant but slight chemical and mineralogic differences, the older and eastern one being more alkalic. In the Durango vicinity, distinctive alkali basalts were erupted during normal faulting about 12 m.y. ago. Tholeiitic basalt of Quaternary age is also widespread there. Neither the normal faulting nor the basalts are prominent to the west, except near the Gulf of California.
The upper volcanic supergroup commonly rests upon an older volcanic-intrusive complex approximately 100 to 45 m.y. old. Between 45 and 32 m.y. ago little or no magmatism occurred in the Sierra Madre Occidental, but major rift-related alkalic volcanic rocks were erupted then in Trans-Pecos Texas. Voluminous rhyolitic activity occurred between 32 and 23 m.y. ago in western Mexico. Intense volcanism began in southern Baja California 20 m.y. ago. The shifting position, nature, and amount of volcanism may correspond to changes in lithospheric plate motions in the east Pacific region, and 45 m.y., 32 m.y., and 23 m.y. ago appear to be key times of plate-boundary reorganization.
