Ferrari, Luca, Valencia-Moreno, Martín and Bryan, Scott (2005) Magmatismo y tectónica en la Sierra Madre Occidental y su relación con la evolución de la margen occidental de Norteamérica. Boletín de la Sociedad Geológica Mexicana, 57(3), pp. 343-378. ISSN (online) 1405-3322

Abstract

The Sierra Madre Occidental (SMO) is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America and to the opening of the Gulf of California. The stratigraphy of the SMO consists of fi ve main igneous complexes: (1) Late Cretaceous to Paleocene plutonic and volcanic rocks and (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called “Lower Volcanic Complex” (LVC); (3) silicic ignimbrites mainly emplaced during two pulses of Oligocene (~32-28 Ma) and early Miocene (~24-20 Ma) ages and grouped into the “Upper Volcanic Supergroup”; 4) transitional basaltic-andesitic lavas that erupted after each ignimbritic pulse and correlated with the Southern Cordillera Basaltic-Andesite (SCORBA) of the southwestern USA; and 5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the late Miocene, Pliocene and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapped, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages beneath the rest of the SMO. The oldest rocks of the LVC (~101 to ~89 Ma) in Sinaloa, and Maastrichtian volcanics of the LVC in central Chihuahua are affected by moderate contractile deformation ascribed to the Laramide orogeny. In the fi nal stages of this deformation cycle, during the Paleocene and Early Eocene, ~E-W to ENE-WSW-trending tension fractures formed within the LVC, along which world-class porphyry copper deposits of the SMO were emplaced. Extensional tectonics began as early as the Oligocene in the whole eastern half of the SMO, forming grabens bounded by high-angle normal faults, traditionally referred to as the southern (or Mexican) Basin and Range province. In the early to middle Miocene, extension migrated westward. In northern Sonora the deformation was suffi ciently intense to exhume lower crustal rocks, whereas in the rest of SMO crustal extension did not exceed 20%. In the late Miocene time, extension became focussed in the westernmost part of the SMO, adjacent to the Gulf of California, where NNW normal fault systems produced both ENE and WSW tilt domains separated by transverse accomodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon was still active off Baja California. Geochemical data show that the SMO rocks form a typical calc-alkaline rhyolite suite with intermediate to high K and relatively low Fe content. late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are dominant. Initial 87Sr/86Sr ratios mostly range between 0.7041 and 0.7070, and initial εNd values are generally intermediate between crust and mantle values (+2.3 and -3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from a few sites in the SMO, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron and others considered the mid-Tertiary SMO silicic magmas to have formed by fractional crystallization of mantle-derived mafi c magmas with little (<15%) or no participation of the crust. In contrast, other workers have suggested the rhyolites, taken to the extreme case, could be entirely the result of partial melting of the crust in response to thermal and material input from basaltic underplating. Several lines of evidence suggest that SMO ignimbrite petrogenesis involved large-scale mixing and assimilation-fractional crystallization processes of crustal and mantle-derived melts. Geophysical data indicate that the crust in the unextended core of the northern SMO is ~55 km-thick, but thins to ~40 km to the east. The anomalous thickness in the core of the SMO suggests that the lower crust was largely intruded by mafi c magmas. In the westernmost SMO adjacent to the Gulf of California crustal thickness is ~25 km, implying over 100% of extension. However structures at the surface indicate no more than about 50%. The upper mantle beneath the SMO is characterized by a low velocity anomaly, typical of the asthenosphere, which also occurs beneath the Basin and Range province of western U.S. The review of the magmatic and tectonic history presented in this work suggests that the SMO is the result of the Cretaceous-Cenozoic evolution of the western North America subduction system. In particular, the Oligo-Miocene SMO is viewed as a silicic large igneous province formed as the precursor to the opening of the Gulf of California during the fi nal stages of the subduction of the Farallon plate. The mechanism responsible for the generation of the ignimbrite pulses seems related to the removal of the Farallon plate from the base of the North American plate after the end of the Laramide orogeny. The rapid increase in the subduction angle due to slab roll-back resulted in extension of the continental margin and, possibly, the detachment of the deeper part of the subducted slab as younger and buoyant oceanic lithosphere arrived at the paleotrench, eventually leading to direct interaction between the Pacific and North American plates.

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