MEXICAN LATE PALEOZOIC-EARLY MESOZOIC REMNANT BASIN AT THE WESTERNMOST CULMINATION OF THE OUACHITA-MARATHON BELT

After closure of the Rheic Ocean during Mississippian time, gondwanan Mexico was placed at the equatorial western margin of Pangea. The oldest sedimentary rocks that conform the Lower Mesozoic succession from western Mexico were deposited in a remnant basin, partially floored by oceanic crust at the westernmost culmination of the Ouachita Marathon belt. The basin was bounded by a transform margin at southern Laurentia and during Late Paleozoic-Early Mesozoic time by an east dipping subduction along the western margin of gondwanan Mexico. Successive subduction zones, displaced progressively westward from the Permian margin, located today in eastern Mexico, produced westward younging sedimentation, magmatism and deformation cycles, up to the present Pacific margin. Detrital zircon geochronology of Paleozoic and Mesozoic siliciclastic rocks from central and northeastern Mexico allow interpretation of provenances and maximal depositional ages, whose tectonic implications reveal an evolving continental margin that during Permian to Early Cretaceous time resulted in the consolidation of approximately the western half of the actual Mexican territory. Early Permian subduction produced during a first high-stress stage deformation and greenshist facies metamorphism (Granjeno Schist in northeastern Mexico), as well as during a low stress stage the Permo-Triassic east Mexico magmatic arc. Upper Triassic strata of the “Potosí submarine Fan”, exposed in the Mesa Central, are strongly deformed and locally registered low-grade metamorphism and lithofacies assemblages interpreted as parts of a subduction complex, suggesting that a subduction zone was active in a high stress stage during the latest Triassic time. Arc-volcanism occurred during a subsequent low-stress stage of this subduction process, and is recorded by predominantly subaerial volcanic deposits of the Nazas Formation. Finally, during the Late Jurassic and Early Cretaceous, the intraoceanic volcanic arc activity of the Guerrero superterrane was initiated, as a consequence of repeated east and west dipping subduction events.