GEOMORPHIC FINGERPRINTS OF ACTIVE FAULTS IN THE SOUTHERN CASCADIA FOREARC AND THEIR GEODYNAMIC SIGNIFICANCE

The distribution, slip rates, and kinematics of active faults within the overriding plate of convergent margins provide insight into the geodynamics of plate boundary deformation and associated seismic hazard. Within the Cascadia forearc, identification and characterization of active faults is made challenging by heavy vegetative cover, scarcity of geologic markers with which to measure displacement, structures inherited from previous deformational regimes, and slip histories that potentially involve long intervals between and/or irregular recurrence of co-seismic events. Here, we present geomorphic observations that provide evidence for active faults within the southern Cascadia forearc associated with relative differences in long-term deformation and vertical motion of across the Coast Ranges. These faults include a system of NNE-striking, high-angle faults within the Western Klamath Terranes that bound a faceted range front west of Cave Junction, Oregon (informally referred to as the Eight Dollar Mountain fault). East of the fault, fluvial networks are characterized by gentle alluviated valleys, whereas west of the fault, steep channels are decorated with flights of strath terraces and perched gravels attesting to recent incision. Spatial variations in incision rate measured from these terraces and watershed-average erosion rates measured from ¹⁰Be in modern sediment both confirm sustained differential rock uplift across this structure. Within the Klamath Mountains Province, inboard of the Coast Ranges, similar differences in geomorphology exist along the west side of Scott Valley, CA and along the range front northwest of Weaverville, CA.

Several competing tectonic drivers for active deformation exist in the region, including the NNW translation of the Klamath terranes driven by northward motion of the Sierra Nevada-Great Valley block, deformation associated with the northward migration of the Mendocino Triple Junction, and coupling along the southern portion of the Cascadia subduction zone. The orientation and relative sense of motion on these newly recognized faults is consistent with the deformation produced by the interaction of these tectonic drivers in the southern Cascadia forearc.