THE ANCIENT PACIFIC MARGIN OF MEXICO DURING THE LATEST PALEOZOIC EARLY MESOZOIC TIME

Remnants of the ancient pacific margin of Mexico can be follow from Sonora to eastern Guerrero state through central Mexico, based on distribution of Triassic continental and marine sedimentary facies and the presence of continental slope deposits in the states of Durango, Coahuila, Zacatecas, San Luis Potosí and Queretaro. Lower Jurassic, mostly subaerial volcanic rocks rest on both, Late Triassic marine sedimentary rocks in western San Luis Potosí and Zacatecas, as well as on Precambrian-Paleozoic rocks and their Triassic sedimentary cover to the east, in Nuevo León and Tamaulipas, in what was once the westernmost part of Pangea. Last discussions about the origin of Jurassic volcanism in Mexico, disagree with the previously proposed model of a continuation to the south of the cordilleran volcanic arc and propose instead a tectonic setting related to extension during brake up of Pangea and opening of the Gulf of Mexico. In each case, the proximity of this volcanic province to the active margin of Pangea should be considered, if not with the presence of subduction towards the end of the Triassic, then as a transform system along a segment of that margin. The occurrence of subduction in front of the paleo pacific margin of Mexico towards the end of the Triassic and beginning of the Jurassic is not well documented, however, in all the outcrops of Triassic rocks in the province of the Mesa Central of Mexico, there are structures resulting from a notable contractive deformation whose origin would have to be elucidated. This deformation, consisting of folding, thrusting and pervasive foliation where low degrees of metamorphism can be reached, does not affect or in some cases may only affect the basal part of the Lower Jurassic volcanic successions. In addition, the unconformity where Jurassic subaerial volcanic rocks rest on Triassic marine sequences implies tectonic processes previously to volcanism, that would have caused contractive deformation and uplift of sedimentary rocks initially deposited as turbiditic sequences in the deep sea until their emergence above sea level. In any case, we observe a scenario influenced by processes related to extensional tectonics during the opening of the Gulf of Mexico and to a greater or lesser extent also by tectonic processes along the active paleo-pacific margin.