LATE CRETACEOUS EXHUMATION OF UNDERPLATED METASEDIMENTARY ROCKS IN THE NORTH AMERICAN CORDILLERA

Underplating and metamorphism of trench and forearc deposits was a common feature in the North American Cordillera during the Late Cretaceous. At that time, a prolonged period of flat-slab subduction (100 to 65 Ma), related to the subduction of buoyant oceanic crust, led to the tectonic removal of the North American mantle lithosphere and the underplating of the Pelona-Orocopia-Rand (POR) schists and Swakane gneiss to the base of the upper plate. Thermochronological data indicate that extant exposures of these rocks remained deep in the crust until mid-Cenozoic exhumation related to San Andreas motion (POR schists) or Eocene extension (Swakane Gneiss) exhumed them to near surface. However, new zircon U-Pb depth-profiling and cathodoluminescence data from the Nanaimo forearc basin and Swakane Gneiss may record an earlier, Late-Cretaceous phase of exhumation of metasedimentary rocks. In the Nanaimo Basin, all Proterozoic grains and some Mesozoic grains exhibit variable and complex zircon rim growth. Dating of the Nanaimo rims indicates that the source area for these grains underwent metamorphism and melting between 100 and 64 Ma (peak metamorphic zircon growth at 86 Ma). Likewise, zircon rim dates on detrital zircons from the youngest known parts of the Swakane Gneiss predate metamorphism and are significantly older than the depositional age of the protolith (81 Ma zircon MDA), indicating a detrital origin. The similarity between detrital zircon populations in the Swakane Gneiss, the POR schists and the Nanaimo Basin and the compatibility of the timing of metamorphism of the source area of the Nanaimo Basin with the timing of underplating, metamorphism and melting of the POR schists suggest a metasedimentary origin for these grains. The youngest metamorphic rims in Nanaimo Basin samples are, on average, only 5 m.y. older than the depositional age of the sample indicating that exhumation of the metasedimentary source rock for the Nanaimo Basin was rapid following metamorphism and began before 84 Ma when the first grains with rims occur in the Nanaimo Basin. Exhumation of the metasedimentary source rock for the Nanaimo Basin predates deposition and underplating of parts of the POR schists and the Swakane Gneiss indicating that underplating and exhumation of metasedimentary rocks was contemporaneous in the Cordillera.