LATE OLIGOCENE TO EARLY MIOCENE EXHUMATION OF THE CONDREY MOUNTAIN SCHIST, KLAMATH MOUNTAINS, NORTHERN CALIFORNIA AND SOUTHERN OREGON

The Klamath Mountains in northern California and southern Oregon consist of Paleozoic to Mesozoic accreted terranes that record Paleogene doming centered on the Condrey Mountain Schist (CMS), the structurally lowest unit in the Klamaths. The CMS is an exhumed subduction complex that occupies a fault-bounded window through overlying Klamath terranes. Recently published Oligo-Miocene ages from apatite (U-Th)/He (AHe) on Klamath plutons record a regional exhumation event, possibly due to regional uplift and erosion or associated with detachment faulting. Our previous CMS research constrained subduction-related metamorphism and underplating to ca. 135 Ma at 30-40 km depth, but lack of low temperature thermochronologic data and limited evidence of exhumation-related structures left CMS exhumation timing - as well as mechanism(s) and relation to the regional Klamath signature - enigmatic.

To constrain CMS exhumation timing, we collected zircon (U-Th)/He (ZHe) dates along a north-south transect from the window margin towards the dome center with 1100 m of elevation difference. ZHe dates range from 20.8 ± 3.0 Ma for the highest sample to 26.1 ± 4.7 Ma for the lowest sample. The younging upward trend is consistent with CMS doming, although uncertainties in the dates limit interpretation. The CMS ZHe dates are comparable to recently published AHe dates (19-23 Ma) from Klamath plutons within the structurally overlying Rattlesnake Creek terrane proximal to the CMS (Ashland to the NE, Slinkard to the SW, and Grayback to the NW). The CMS exhumed through the ZHe closure temperature (180°C) coeval with the plutons passing the AHe closure temperature (70°C) and may indicate a similar pervasive mechanism (e.g., uplift and erosion due to continued underplating in the Cascadia margin). The CMS, however, currently crops out at elevations comparable to the Ashland pluton, indicating faster exhumation after ca. 20 Ma relative to the plutons. This requires normal-sense slip on the NE window-bounding fault or other yet unrecognized faults in order to reconcile CMS and Ashland pluton exhumation histories. Although these new ZHe ages link CMS exhumation to shallow depths to the regional signature recorded in the Klamath plutons, understanding the mechanisms driving late-stage exhumation requires more analysis.