Geochronology, Geochemistry, and Tectonic Setting of the Mesozoic Nazas Arc in North-Central Mexico, and its Continuation to Northern South America

Abstract

Volcanic, sedimentary, and granitic plutonic rocks that are part of the early Mesozoic Cordilleran continental magmatic arc are exposed in a belt from the southwestern United States to Guatemala. In the Mexican states of Chihuahua, Coahuila, Durango, Zacatecas, and San Luis Potos, these rocks form a discontinuous southeast-trending belt across north-central Mexico. Whole-rock geochemical analyses of volcanic and intrusive rocks in north-central Mexico indicate a calc-alkaline suite formed in this continental volcanic arc along the convergent margin of western North America. Paleomagnetism, field relations, and isotopic ages (40Ar/39Ar, K-Ar, Rb-Sr, and U-Pb) of 73 volcanic and intrusive rocks document the Late Triassic–Middle Jurassic age of the arc. In the region, isotopic ages commonly are reset, apparently because of thermotectonic events during the Laramide orogeny that led to the development of the Sierra Madre Oriental fold and thrust belt and to deep burial of the arc rocks. Available evidence suggests that the arc underwent two main phases of subsidence. One phase of extensional subsidence created intra-arc basins and a peak of volcanism throughout the arc in the Early–Middle Jurassic. A second phase began in the Oxfordian, with subsidence and initial deposition of the Zuloaga and La Gloria Formations. Continued sedimentation during this phase led to accumulation of 5–7 km of strata above the arc, as Cretaceous seas transgressed westward over inland Mexico.The similarities in age, depositional environment, clastic composition, magma types, and geochemical affinity and, more importantly, the tectonic settings that gave rise to the Nazas Formation in Mexico and La Quinta and Girn Formations in Venezuela and Colombia suggest that these two volcanic-sedimentary sequences, now hundred of kilometers apart, were once part of the Late Triassic–Jurassic continental magmatic arc. This arc extended from Alaska to South America and evolved during simultaneous subduction along the western margin of Pangea, rifting in the Caribbean–Gulf of Mexico region, and associated large-scale transpressive activity.