Metamorphism and deformation in the El Fuerte region: Their role in the tectonic evolution of NW Mexico

The Río Fuerte Formation, cropping out in Sinaloa state of northwestern Mexico, has been ascribed to the Middle to Late Ordovician based on its fossil content. U-Pb detrital zircon study in a rock sample of this unit yielded peaks at 521 Ma, 605 Ma and 881 Ma. Relative-age probability plots in the region suggest that the Río Fuerte Formation deposited in a basin located between Laurentia and an inactive peri-Gondwanan arc. Thermobarometric and petrographic studies of the Río Fuerte Formation indicate an initial low P/T or Buchan type metamorphic event. Structural analysis indicates that this event had a ~N-S shortening direction, and may be related to the collision of peri-Gondwanan blocks during the final amalgamation of Pangaea. In the same region, a granitic clast within an andesitic meta-agglomerate of the Topaco Formation yielded a 151 ± 1 Ma age, which predates a second tectono-metamorphic event. This event is ascribed to a Late Jurassic overprint in the El Fuerte region, which may be related to collision of an ophiolite block against the North American plate and is coeval with the Nevadan Orogeny of the North American Cordillera.