ZIRCON AND APATITE (U-TH)/HE THERMOCHRONOLOGY REQUIRES TWO-STAGE (EARLY CRETACEOUS AND MIOCENE) UPLIFT OF THE KLAMATH MOUNTAINS PROVINCE (SOUTHERN OREGON-NORTHERN CALIFORNIA)

The Klamath Mountains Province is an accretionary orogen underlain by eight distinct tectonic belts. This regional structure is overprinted by an ~70,000 km² dome, cored by the youngest and lowest-grade rocks in the region - the Condrey Mountain Schist (CMS). There is little consensus on the origin of this structure. This effort aims to constrain the timing and mechanism(s) of the uplift of the Klamath Mountains Province through (U-Th)/He thermochronology of apatite and zircon extracted from plutons flanking the CMS window and zircon from the CMS itself. Pluton-derived zircon and apatite dates range from 140-120 Ma and 25-10 Ma, respectively, and are relatively constant across eU. These patterns point to two separate, relatively rapid pulses of uplift. Inverse modeling confirms this assertion, apparently requiring significant Early Cretaceous cooling to temperatures of ~120 °C plus a second phase of cooling to near-surface temperatures in Miocene time. In contrast, CMS-derived zircon yield ages in the 30-20 Ma range, recording only the latter of the two cooling events and requiring more rapid unroofing compared to flanking plutons. We suggest that the Early Cretaceous pulse of cooling resulted from subduction and accretion of the CMS and associated thickening and erosion of the Klamath wedge, occurring in the waning stages of the Nevadan orogenic event. Miocene rejuvenation of uplift is likely to have formed the Condrey Mountain dome in its current form and came about via convergence between the Juan de Fuca plate and/or the Sierra Nevada block with the Cascadia forearc.