TECTONIC EVOLUTION OF THE PACIFIC MARGIN AND THEIR INFLUENCE IN THE STRATIGRAPHY AND PALEOGEOGRAPHY OF EARLY MESOZOIC UNITS IN CENTRAL TO NORTHEASTERN MEXICO

The results of the present research, allow us to recognize a relationship between the tectonic processes that were given along the paleo-pacific margin of Mexico and the Mesozoic stratigraphy, as well as the paleogeographic evolution of central and northeastern Mexico. A similar situation seems to extend toward the northwest, until Sonora and toward the south, until Guerrero. It allows to reconstruct the approximate position of this ancient continental margin of Mexico. Crtography of key areas, petrography, structural analysis, geochemistry and geochronology, as well as the study of the scarce fossil material, have allowed an interpretation of the tectonic evolution, as well as sedimentary and magmatic processes, in such a way that it is possible to interpret, with more details the paleogeography and to relate the main stratigraphic Triassic and Early Jurassic units to the main tectonic events: The Late Triassic is represented in the central-western part of Mexico (San Luis Potosí and Zacatecas) by a siliciclastic marine sequence (Zacatecas formation), that has been interpreted as an ancient submarine fan. Toward the west (Michoacán, Baja California) appear vestiges of an oceanic basin and toward the northeast (Nuevo León and Tamaulipas), a continental fluviatil sequence, (lower Huizachal Group). While the Triassic marine sequence shows a polideformation, the continental sediments shows only structures produced during a laramidic event as well as later extension episodes. Volcanic and volcaniclastic rocks are overlaying the marine Triassic and partly also the continental sediments, they correspond in age and tectono-magmatic setting to an Early Jurassic continental volcanic arc (Nazas Formation). Some graben structures as a result of the early extension that caused the opening of the Gulf of Mexico basin were filled with continental deposits (La Boca Formation). Finally red beds as products of erosion during the opening of the Gulf of Mexico (La Joya Formation)represent the bottom of the Late Jurassic marine transgression. During the Cretaceous time toward the east, there were deposited the marine gulf sequences that include extensive carbonated paltforms; toward the pacific, at this time intraoceanic volcanic arcs were developed. With the approach of these arc complexes to the continent,40% of the actual mexican territory was consolidated.