Alan D. Chapman, Jennifer Grischuk, Meghan Klapper, William Schmidt, Todd LaMaskin

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 KMP more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g, rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic driving mechanisms surrounding this profound Mesozoic growth of the KMP are poorly understood. In this effort, we show that formation of the Condrey Mountain schist (CMS) of the central KMP spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the KMP. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175-170 Ma, reflecting the timing of eruption of volcanic protoliths. One detrital sample from the same subunit contains abundant (~54% of analyzed zircon 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 outboard rift basin and subsequently underthrust the KMP during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 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. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS each closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices, sequentially underplated beneath the KMP. 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 one or more previously unrecognized episodes 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 we relate to disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes 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. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternates with slab shallowing due to recurrent subduction of buoyant oceanic features.