PALEOZOIC TECTONIC EVOLUTION OF NORTH-CENTRAL AND WESTERN TERRANES OF MEXICO

Reconstructing the geological evolution of central and western Mexico during the Paleozoic is very difficult due to a lack of exposures. The few outcrops available, and indirect information obtained from geophysical and geochemical data, suggest that Central and Western Mexico are made up of a mosaic of pre-Jurassic terranes. Most of those terranes were allochthonous with respect to North America, during Early Paleozoic and were not far from their present position by the Carboniferous. Several authors suggest that the Coahuila and Sierra Madre terranes (Oaxaquia block), were part of Gondwana during Early Paleozoic, and collided with North America during Late Paleozoic time. However, their Mississippian faunas are of North American affinity, suggesting that the collision of Oaxaquia occurred earlier. A continental arc with a paleo-Pacific, east-dipping subduction zone evolved from Carboniferous to Early Permian time in eastern Mexico (Oaxaquia), and was in part contemporaneous to deformation in the Ouachita belt. Detrital zircons from the Rara Formation of Chihuahua, which has been interpreted as the southern continuation of the Ouachita belt into Mexico, indicate both Gondwanan and Carboniferous arc-volcanic sources, since they yield age clusters at 330, 530, 590, and 970 Ma. However some influence from North America is suggested by the wide range of Proterozoic ages that show a peak at 2,140 Ma. Further to the west in the present coastal region of the Gulf of California, the exposures of Paleozoic rocks of the Tahue Terrane suggest a similar evolution of pre Carboniferous allochthony. There, Paleozoic outcrops are restricted to the El Fuerte and San José de Gracia areas. Detrital zircon ages obtained by Vega-Granillo (in press), from middle-late Ordovician Rio Fuerte Formation, show Gondwanan provenance. In contrast our data from overlying Carboniferous San José de Gracia Formation show prominent peaks at 1,885 and 2,730 Ma, similar to the “Peace River Arch signature” present in miogeoclinal rocks of Nevada, Sonora and Baja California.