Shrimp detrital zircon geochronology of forearc basins: A study of the Great Valley Group of California and the Methow Basin of Washington

Detrital zircon geochronology of the Great Valley and Methow forearc basins provides improved resolution of the provenance and evolution of active margin basins and establishes a basis for developing sampling protocols for similar studies.; In the Cretaceous Great Valley Group of California, detrital zircon age data indicate that the Klamath-Sierran arc drainage divide was nearly coincident with the active volcanic front, except where complicated by uplift and pre-existing structural trends in the northern Sierra Nevada terranes. Drainage systems feeding the Great Valley forearc depocenter evolved from localized to extensive catchment basins, coincident with the broadening of the shelf and increased dissection of the arc.; The temporal and spatial variability of detrital zircon age distributions from the Methow Basin in northern Washington and southern British Columbia demonstrates that the degree of homogenization of detrital zircon age distributions in a sedimentary succession is directly related to depositional environment. Sediment deposited in fluvial and marginal marine settings contains more variable detrital zircon age distributions than sediment deposited in deep-marine turbidite settings. Variability in age distributions results from different degrees of sediment mixing within the two depositional environments, and directly affects the sampling density required to characterize each sedimentary succession completely. The detrital zircon age signature determined from this detailed sampling of the Methow Basin matches best with source regions in the southern Canadian Cordillera and provides evidence against large-scale tectonic translation of the Methow Basin.; In both the Great Valley and Methow forearc basins, strata deposited during arc magmatism display narrowly-distributed detrital zircon ages, but the age distributions broaden dramatically when are volcanism ends. This observation suggests that the active magmatic arc forms a topographic high which is rapidly dissected when magmatism stops. Therefore, where provenance determination is the primary goal of detrital zircon analysis, sampling must include the youngest strata, which display the widest detrital zircon age distributions and probably contain the most representative fingerprint of ages in the source region. Where the goals of detrital zircon geochronology include basin analysis, sampling should span the entire stratigraphy to permit recognition of shifting sediment dispersal patterns recorded by changing detrital zircon patterns recorded by changing detrital zircon distributions.