TECTONIC MODEL FOR PALEOPROTEROZOIC ORTHOQUARTZITE DEPOSITION IN THE SOUTHWESTERN UNITED STATES

Thick (1-2 km) successions of Paleoproterozoic orthoquartzite exposed throughout the southwestern United States represent a key component in understanding the tectonic history of the Yavapai and Mazatzal orogenies and the growth and stabilization of continental lithosphere of southern Laurentia. New and existing age constraints suggest that orthoquartzite and interlayered schist were deposited in short-lived basins at two times (ca. 1.70 and 1.65 Ga) during crustal assembly. Unimodal detrital zircon populations suggest local sources and first-cycle origin of orthoquartzites within relatively short time intervals during unroofing of local underlying basement. Basin development and the onset of sedimentation were accompanied by voluminous extrusion of high-silica rhyolite throughout the region and ongoing granitic magmatism at deeper crustal levels. Thickening patterns observed in the Mazatzal Group and restored fault geometries show that depositional basins were bounded by growth faults that were active during the accumulation of ca. 1.5 km of strata. Our model for syntectonic deposition involves two major episodes of extensional basin development followed by thrust closure due to opening and closing of slab-rollback basins related to outboard subduction. An episode of slab rollback following the culmination of the Yavapai Orogeny ca. 1.70 Ga would have facilitated extension throughout the newly accreted lithosphere, resulting in rapid exhumation of basement granitoids, basin development, and widespread magmatism. The accretion of the Mazatzal province to the south would have caused basin closure and inversion through fold-and-thrust-style deformation between ca. 1.68 and 1.65 Ga. Renewed rollback along the southern margin of the Mazatzal province could have facilitated a second episode of basin development and magmatism throughout the region, with subsequent closure during the final stages of the Mazatzal Orogeny ca. 1.60 Ga. This model accounts for the regional extent of quartzite exposures, contemporaneous sedimentation and felsic magmatism due to crustal extension, and the transient nature of basin development and inversion during protracted accretionary orogenesis along the southern Laurentian margin.