The Sierra Madre in Mexico, a geologic summary

The Sierra Madre region is known for its extensive volcanic cover, same that was developed between the Cretaceous and Miocene, but often it is not taken into account that its basement consists of four different stratotectonic terranes, namely North America, Caborca, Cortes and Guerrero (see figure). According to Centeno-Garcia (2008, 2011), the North America terrane is defined by the crystalline complexes of Grenvillian age. The Caborca terrane ​​has a Proterozoic basement older than 1.7 Ga, covered by a thick sedimentary succession which has been interpreted as a displaced Nevada miogeosinclinal block, transported southwards through the proposed Mohave-Sonora megashear. The Cortes terrane is interpreted as native to North America, same that probably evolved outward the Caborca terrane (Stewart et al., 1990), with its sedimentary units interpreted as deposits on the slope of the continental margin. These deformed Paleozoic rocks are overlain by Triassic marine and continental sedimentary rocks. In turn, the Triassic rocks are overlain by red beds and Cretaceous volcanic rocks. Contact relations between Cortes and the composite Guerrero terrane have not been well defined, but the contact is inferred as a Late Cretaceous thrust. The Guerrero terrane is composed by at least five sub-terranes, of which only the Tahue sub-terrane lies within the Sierra Madre Region. This sub-terrane hosts the oldest rocks of the Guerrero terrane, with Ordovician age submarine arc volcanic rocks and late Paleozoic siliciclastic turbidites. These rocks are covered or intruded by volcanic rocks of the lower Jurassic and lower to late Cretaceous.

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The existence of a Triassic-Jurassic magmatic arc is established by isolated outcrops of clastic sediments interbedded with volcanic rocks, besides the presence of several intrusive bodies from northern Sonora to the south of Chihuahua and north of Durango. The presence of a magmatic arc of Cretaceous to Eocene age has been in the literature of the Sierra Madre Occidental for more than 30 years, starting with the proposal by McDowell and Keizer (1977) of the Lower Volcanic Complex, later changed to Lower Volcanic Group . This group is composed of large diorite and quartz-diorite batholiths, alkaline granites (Valencia-Moreno et al., 2001) and by volcanic sequence with a predominantly andesitic composition. This volcanic sequence has a top member of dacitic and rhyodacitic tuffs, intercalated with sedimentary rocks. More recently the age of this arc has been constrained between the Early-Late Cretaceous to the Paleocene, while the Eocene magmatism has been assigned to another magmatic arc (L. Ferrari, 2005). In the northern part of the Sierra Madre Occidental, the magmatic arc’s volcanism is mainly exposed in the deepest canyons of western Chihuahua. This episode begins 46 Ma ago, but its first pulses are difficult to distinguish from the more felsic upper member of the Cretaceous-Paleocene arc.

At the beginning of the Oligocene, and with surprising synchrony, the first and most extensive ignimbrite pulse occurred; within a period of three million years that led to deposition of more than one kilometer thickness of ignimbrite layers in the heart of the Sierra Madre (see picture below, near Batopilas, Chihuahua). A second pulse occurred in the Early Miocene, although it was more important at the southern end of the Sierra Madre Occidental. These two pulses created one of the largest volcanic provinces in the world, with an average width of 250 kilometers and a length of 1,200 kilometers. Basaltic to andesitic composition flows of Miocene age crown many mountaintops of the Sierra Madre Occidental.DSCN5504

 

Epithermal deposits in the Sierra Madre Occidental appear in three discrete pulses (Camprubi et al., 2003) that roughly coincide with the main magmatic pulses that affected the region: (1) between ~ 48 and ~ 40 Ma, (2) between ~ 36 and ~ 27 Ma and (3) between ~ 23 and 18 Ma.

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