PALEOGENE-NEOGENE EXTENSION AND TOPOGRAPHY BUILDING IN THE SOUTHERN CASCADIA FOREARC RECORDED BY DEPOSITION OF THE WEAVERVILLE FORMATION

Topographic generation of the southern Klamath Mountains Province (KMP) in the Southern Cascadia forearc occurred from the late Cretaceous through Miocene, although paucity of a sedimentary rock record has precluded detailed timing of tectonics, topography building, and sediment dispersal during this time frame. Based on preserved remnants of marine Great Valley Group, the KMP was at sea level in the mid-Cretaceous. Early-mid Miocene sand of the terrestrial Weaverville Formation (Tw) preserved within five isolated basins in the southern KMP provides geologic evidence for uplift and extension. However, timing of deposition is assigned based on previous paleobotanical, palynological, and paleoecological studies (MacGinitie, 1937; Barnett 1989), and is poorly correlated across modern basins. We present detrital zircon U-Pb ages from the fluvial section of four Tw basins: Hyampom, Hayfork, Lowden Ranch, and Weaverville; and use DZStats (Sundell and Saylor, 2016) to determine statistical likeness between basins. First, U-Pb age distributions from all four basins can be explained entirely by local KMP sources. Common regional sources such as Sierra Nevada and Idaho Batholith are absent, implying the KMP experienced locally elevated topography which prevented input from outside sources during deposition. Second, two grain-ages from the Lowden Ranch basin provide a preliminary estimate of a mid-Miocene maximum depositional age (10.6 Ma and 16.7 Ma; with positive epsilon-Hafnium values, likely indicating ashfall from Cascade volcanoes). Third, cross-correlation coefficients between each basin suggest the basins were fed by distinctively different localized sources (all values less than 0.5; 1=identical and 0=maximum dissimilarity). Finally, detrital zircon age distributions show that the mid-Miocene fluvial system must have included source rocks to the north of the modern Trinity River drainage area and that drainage basin reorganization occurred post Tw deposition to its current configuration. A possible mechanism for reorganization is mid-late Miocene extension of the La Grange Fault and other more localized extensional structures. Alternatively, tectonic changes associated with northward migration of the Southern Cascadia-San Andreas Fault system boundary over the last few Ma could explain drainage reorganization.