TERRANE CONFIGURATION OF CENTRAL-WESTERN MEXICO AND THE MESOZOIC EVOLUTION OF THE CONTINENTAL MARGIN

Previous terrane configurations for central and western Mexico were defined base upon the differences in Jurassic-Cretaceous stratigraphy. However detailed studies of the few exposures of older rocks suggest a complex pre-Jurassic accretionary history. Thus, terranes as defined up to date are in fact composite, including the Guerrero terrane. Overall the basement of western Mexico is made up of more juvenile material than the east and north. The oldest terranes are the Oaxaquia block and Coahuila (Delicias) terranes that have a Precambrian/Paleozoic? basement. To the west, thrusting over the Oaxaquia block, is the Potosi allochthon of Paleozoic(?) - Triassic age. To the west is the Central terrane (Zacatecas state), which basement remains unknown, and was accreted to Oaxaquia sometime between Late Permian-Early Jurassic, as indicated by the accretionary complex that is exposed in Pico de Teyra. The Parral terrane (Durango State) is made up of Paleozoic arc-related rocks that were deformed during mid-Paleozoic. The Cortez terrane (Sonora and Sinaloa states) is made up of metamorphic Ordovician volcano-sedimentary rocks. The basement of the Guerrero Composite terrane is made up of Triassic oceanic back-arc rocks (Arteaga subterrane), deformed during Early Jurassic and intruded by mid-Jurassic granitoids. Northwestern Guerrero terrane contains Paleozoic sedimentary rocks of the San Jose de Gracia subterrane. Contact relationships among the Cortez, Parral, and Central terranes are unknown. Late Jurassic Caopas-Nazas arc was the first overlapping unit deposited on the Oaxaquia, Central and Parral terranes. These three terranes were displaced, and seem to be in fault contact with the Coahuila terrane. The data obtained suggest that the continental margin during Late Paleozoic to Early Mesozoic time was located to the west of the Potosi allochthon. Its deformation seems to be related to the collision of the Central, Parral, and Cortez terranes, and the basement assemblages of the Guerrero terrane during Early Jurassic time. Strike slip and regional extension caused dispersion of these terranes during mid Jurassic to mid Cretaceous time. A complex arc-back arc system developed in western Mexico (Guerrero terrane) during this time. During the Laramide orogeny the arc was placed against eastern Mexico.