ZIRCON U-PB GEOCHRONOLOGY OF THE CONDREY MOUNTAIN SCHIST (NORTHERN CALIFORNIA-SOUTHERN OREGON) REVEALS A PREVIOUSLY UNDOCUMENTED EARLY CRETACEOUS CORDILLERAN SHALLOW/FLAT-SUBDUCTION EPISODE

The Klamath Mountains province (KMP) of northern California and southern Oregon consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Condrey Mountain schist (CMS) – a composite of outer metavolcanic and inner metasedimentary subunits – violates this structural “rule” of the KMP, cropping out in the core of a dome in the center of the province. We present zircon U-Pb geochronology from each subunit of the CMS in an effort to fit the CMS into the regional tectonic puzzle. Igneous samples from the outer CMS yield U-Pb ages of ca. 175-170 Ma, presumably reflecting the timing of eruption of mafic volcanic protoliths. One detrital sample from the same subunit contains abundant (~54% of analyzed grains) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder, and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an intra-arc rift basin and subsequently underthrust the KMP during the Late Jurassic Nevadan orogeny. Age spectra derived from five samples of inner CMS closely overlap those from the oldest section of the Franciscan subduction complex (South Fork Mountain schist), suggesting that the former represents the inboard equivalent of the latter. The inner CMS yields MDAs ranging from 145 to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a >10 Myr timespan. The age, inboard position, and structural position (i.e. the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggests that these rocks were emplaced during a previously unrecognized Early Cretaceous episode of shallow-angle subduction restricted to the KMP. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the KMP, which may relate to disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction overlaps that of the westward translation of the KMP relative to correlative rocks in the northern Sierra Nevada Range, suggesting that subduction dynamics were responsible for relocating the KMP from the arc to the forearc.