ACTIVE AND ANCIENT SUBDUCTION-RELATED OROGENESIS AND LOWER CRUSTAL MODIFICATION IN THE KLAMATH MOUNTAINS PROVINCE, CA-OR

The Klamath Mountains Province (KMP) is significant to understanding accretionary orogens and modern subduction zone systems for many reasons including: 1) The 26,000 km² province is underlain by a collage of lithotectonic units that record over 500 million years of episodic subduction and arc magmatism, marginal basin formation, collisional orogenesis, uplift, and tectonic and erosional denudation. 2) The present-day KMP may be defined by an anomalous high-relief topographic culmination that forms at the convergence of Gorda plate subduction, Cascades arc volcanism, San Andreas transform-related seismo-tectonics, and Basin-and-Range extension. As such, the KMP preserves a unique record of active and ancient subduction-related orogenesis. In the western KMP, on-going geologic and isotopic studies suggest that an extensive calc-alkaline magmatic arc initiated at ~ 167 Ma and was ruptured by mafic magmatism that evolved into the 162 Ma Josephine ophiolite (JO). The Jurassic arc was active to at least 158 Ma in board of the JO marginal basin. Preliminary LA-ICPMS U/Pb zircon and previous published U/Pb zircon ages for arc magmatism associated with the Chetco batholith/Rogue volcanics along the rifted western margin of the JO range from 165 – 151 Ma. Therefore, the JO may represent marginal basin lithosphere that evolved from a forearc to intra-arc environment during younger, westward arc magmatism. New Hf-in-zircon data presented at this meeting indicate a spatial variation in metasedimentary vs. mantle sources for the Jurassic plutonic rocks. Combined with regional considerations, these data are consistent with transfer of metasedimentary material from the subduction channel into the mantle wedge and incorporation into arc magmas. Interestingly, the recent evolution of the KMP may also be interpreted in terms of lower crustal modification during subduction. Neogene exhumation has generated steeply incised river drainages and rock uplift resulting in the uplift of a Plio-Pleistocene erosional surface in excess of 2 km. This may be due to the combined effects of lower crustal “relamination” of buoyant material (e.g., sediments, serpentinized oceanic crust, serpentinized forearc mantle) in the Cascadian subduction channel or intracrustal duplexing along reactivated Jurassic-Cretaceous thrusts.