EXAMINING TIME-SPACE PATTERNS IN TERTIARY-PRESENT EXHUMATION AND UPLIFT IN THE KLAMATH MOUNTAINS, SOUTHERN CASCADIA FOREARC

In subduction zones, isolating the signals of earthquake-cycle strain from long-term geological deformation is critical for hazards estimation and characterizing geologic-timescale deformation. In southern Cascadia, our understanding of the long-term deformation in the upper plate is particularly limited, due to the complex transition from subduction to San Andreas transform faulting. Moreover, the reason modern mean elevation in the Klamath Mountains is ~1000m.a.s.l. and greatly diminishes along strike to the Oregon Coast Ranges is unknown. We present six new AHe mean cooling ages along a high-relief transect from the Canyon Creek Pluton in the Klamath Mountains. Mean cooling ages range from 24.7 +/-2.3 Ma (2710m.a.s.l.) to 15.2 +/- 1.0 Ma (1294m.a.s.l.). Synthesizing these data with existing regional thermochronologic datasets suggests a pattern of increasing rock exhumation in mid-to-late Tertiary time: ~0.08 mm/yr from 40 to 25 Ma, ~0.14mm/yr from 25 to 15 Ma, and ~0.19 mm/yr from 15 Ma to present. Increasing rock exhumation from the Oligocene to mid-Miocene may reflect increasing flexural unloading from the La Grange Fault as rebound becomes more efficient as unroofing progresses. However, these exhumation rates are at odds with regional geologic observations. For example, vitrinite reflectance values and detrital AHe ages from sedimentary basin fill in the Reading Creek graben, 25 km south of the Canyon Creek pluton, indicate negligible exhumation since the mid-Miocene. Onlapping marine sediments deposited on uplifted erosional surfaces at the western margin of the Klamath Mountains indicate >1 km of surface uplift since the latest Miocene. Finally, vertical velocity measurements from GPS range from -0.25 +/-1.0 mm/yr in the Weaverville basin, 10 km southeast of the Canyon Creek pluton, to 1.64+/- 0.59 mm/yr at Gasquet in the western Klamath Mountains. Together, these observations suggest differential and perhaps localized Tertiary exhumation and uplift across the forearc. While the La Grange Fault accounts for mid-Tertiary exhumation in the central Klamaths, rock exhumation and surface uplift since 15 Ma may be influenced by regional tectonic mechanisms, such as strong upper plate coupling with the subducting Gorda segment, or by localized deformation along active structures.