LARAMIDE JUXTAPOSITION OF THE CABORCA AND "NORTH AMERICA" PALEOPROTEROZOIC BASEMENT BLOCKS IN THE QUITOVAC REGION, NW SONORA, MEXICO

Restoration of 12-30% Basin and Range extension has allowed for direct interpretation of ductile fabrics associated with a stack of Laramide thrusts in the Quitovac region in NW Sonora. The estimated direction of displacement for these thrusts varies through time from N63°W to N23°E. The thrust faults represent a piggy back sequence of thrusting propagating north, towards the foreland. The average direction and sense of displacement for the thrusts is N28°W, and the cumulative 45-km of estimated northward-directed displacement corresponds to approximately 86% of Laramide crustal shortening.

We propose that the onset of thrusting in the Quitovac region occurred sometime between 75 and 61 Ma, the age of the granites affected by deformation and the oldest age recorded with ⁴⁰Ar/³⁹Ar geochronology of muscovite samples from the ductile fabrics, respectively. Cessation of thrusting occurred at about 39 Ma based on the youngest ⁴⁰Ar/³⁹Ar ages obtained for the same ductile fabrics. This age is the youngest proposed for the end of the Laramide orogeny in NW Sonora. The occurrence of Paleocene-Eocene orogenic gold mineralization, spatially associated with thrusting, strengthens our idea that compressional tectonism associated with the Laramide orogeny is the most important dynamo-metamorphic event recorded in pre-Eocene rocks in the Quitovac region.

Similarities in age, kinematics, and structural stratigraphy indicate that the thrusting in the Quitovac region may be equivalent to the Laramide Quitobaquito thrust in SW Arizona. In both areas Laramide thrusts are involved in the juxtaposition of Paleoproterozoic Caborca and "North America" basement blocks, a feature previously proposed as exclusively related to movements along the Late Jurassic Mojave-Sonora megashear.