Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block

Abstract

The geology of a large portion of western Mexico, including the entire northern boundary of the Jalisco block, has been compiled at a regional scale. Revision of previous works and new geological surveys allowed the construction of a 1:250,000 scale map with 28 informal geologic units, 25 for the Tertiary and Quaternary volcanic succession of the Sierra Madre Occidental (SMO) and the Mexican Volcanic Belt (MVB). In this region, the MVB covers the boundary between the SMO volcanic province and the Cretaceous to Paleocene batholith and volcano-sedimentary sequences of the Jalisco block (JB). The 1000-m-thick succession of the SMO was emplaced during three discrete volcanic episodes of Eocene, Oligocene, and Early Miocene, separated by volcano-sedimentary deposits or unconformities. This succession, dominated by silicic ash flows, is absent in the deep geothermal wells of La Primavera, San Marcos and Ceboruco areas, indicating that the southern boundary of the SMO is located north of these sites. Within most of the study area, the MVB is clearly separated from the SMO by a tectonic unconformity produced in Middle Miocene. The first episode belonging to the MVB is represented by a widespread and volumetrically significant mafic volcanism, sometimes with an alkaline composition, which occurred at 11‐8 Ma in central-eastern Nayarit and in the Guadalajara region. A period of reduced volcanic activity between 7.2 Ma and 5.5 Ma was followed by the emplacement of large volumes of rhyolites and minor ignimbrites in Early Pliocene. Intermediate to mafic volcanism, also with alkaline composition, becomes again dominant between ~4.5 Ma and the Present, although large rhyolitic and dacitic dome complexes were emplaced between Guadalajara and Tepic. In Late Pliocene and Quaternary times, large andesitic to dacitic strato-volcanoes were built in the northern part of the arc, whereas basaltic shield volcanoes and cinder cones characterize the volcanic front.