TECTONIC EVOLUTION AND CRUSTAL GROWTH OF THE WESTERN MARGIN OF MEXICO DURING THE MESOZOIC (Invited Presentation)

Assembling of Pangea was followed by east facing subduction along the western margin of Mexico at the end of the Paleozoic.

During the breakup of Pangea, Mexico became a narrow and elongated piece of crust, protruding from the large North American continent. It was limited on its eastern side by the Atlantic/Gulf of Mexico rifting-drifting processes; on its western margin by Pacific subduction-related tectonics; and to the south by a rifting/transcurrent-related tectonics, linked to the Caribbean plate evolution. The question of how these tectonic scenarios interacted in time and space, and to which of them magmatism and sedimentation is related, arises for each Mesozoic outcrop throughout all of Mexico. Some models on this evolution and interactions are presented below

A large Triassic submarine fan formed along the western margin of Mexico (Potosi fan) and extended into the ocean. Sedimentation was Carnian–Norian in age (based on biostratigraphy). Most of detrital zircon (DZ) U/Pb ages are no younger than Permian–Early Triassic, except for two localities where a few DZ Late Triassic ages have been reported. These ages suggest that subduction ceased and an Early Mesozoic passive margin formed along the Pacific margin. Subduction resumed at the end of Triassic–Early Jurassic time.

Rocks of the Potosí fan were deformed in a subduction zone, originating an accretionary complex that contains blocks of oceanic rocks. The age of deformation is unknow. The oldest rocks that overlie the accretionary complex are Middle Jurassic. This is the only stage of the Mesozoic tectonic evolution of western Mexico when allochthony and oceanic-arc collision could have occurred, but this has not been proven. Whether the complex formed in an east-facing or west-facing subduction zone is still unknown. There is weak evidence for both.

Evidence suggests a long-lived, east-facing subduction along the Pacific coast, from Middle Jurassic to date. When rifting of the Gulf of Mexico began, an arc was built on the accretionary complex (Callovian to Kimmeridgian), placing the rocks attributed to the “Nazas arc” in a back-arc position. Subduction continued to roll back, and the Arperos back-arc basin formed during Late Jurassic–Early Cretaceous time.

The Arperos Basin closed in late Cretaceous, and contraction continued (Sevier-Laramide deformations) while magmatism remained active in western Mexico at the end of the Cretaceous.