Stripping the upper crust of the southwestern U.S. during flat-slab subduction

Abstract

The upper crustal responses to Late Cretaceous–Eocene flat-slab subduction beneath the southwestern United States remain poorly understood. The ca. 137–74 Ma McCoy basin near the southern California–Arizona border provides a unique record of extreme (>7 km) subsidence, immediately prior to the underplating of trench fill (Orocopia Schist) beneath the region during flat-slab subduction at ca. 70 Ma. Detrital zircon (U-Th)/He (ZHe) dates indicate that the McCoy basin was heated to T >200 °C after ca. 74 Ma and subsequently exhumed beginning at ca. 65 Ma. ZHe and ⁴⁰Ar/³⁹Ar data from the Maria fold-thrust belt and Mule Mountains thrust—north and south of the McCoy basin, respectively—indicate comparable and coeval cooling after cessation of crustal shortening. The abrupt transition from basin burial to exhumation ca. 65 Ma coincides with underplating of Orocopia Schist at depths of ~35 km beneath the region. Our study shows that between ca. 65 Ma and 45 Ma, >6 km of the upper crust was removed by erosion across the southwestern North American Cordillera during flat-slab subduction.