CRUSTAL FAULTING AT THE MENDOCINO TRIPLE JUNCTION INFERRED FROM RIVER AND MARINE TERRACES IN THE MATTOLE RIVER WATERSHED

Spatial distribution and elevation of abandoned bedrock river terraces may provide insight into the incision history of the river system and, by proxy, the tectonic uplift history within the watershed. In the Mendocino Triple Junction region, it is difficult to directly map surface expressions of active faults due to the steep, heavily vegetated terrain that is prone to landsliding, and the weak, heterogeneous highly deformed bedrock that allows the possibility of diffuse active deformation. However, the Mattole River and tributaries have extensive flights of uplifted bedrock strath terraces that may provide insight into the tectonic history of this complex deformation zone. We map and evaluate terraces and longitudinal stream profiles to identify the elevation and extent of previous river levels. Patterns in these data suggest there are two active fault systems affecting the Mattole watershed with discrete deformation, the King Range Thrust and the Honeydew Fault. In tributary watersheds west of the mainstem Mattole, the King Range Thrust coincides with a knickzone. Upstream of this knickzone, the modern river profile projects above any terraces in either the tributary or the main Mattole River valley, suggesting the upstream position of these watersheds are being uplifted at a higher rate than the downstream reach of the Mattole. Along the mainstem Mattole River, the Honeydew Fault is indicated by a distinct change in terrace elevations and continuity between Honeydew and Petrolia. Upstream of the fault there are multiple sets of closely spaced terraces at low to moderate elevations, but where the fault intersects with the river there is a single set of low terraces, few moderate elevation terraces, and multiple terraces and abandoned meanders at high elevation. Luminescence ages from marine and fluvial terraces near the mouth of the Mattole River and on opposite sides of the Honeydew fault indicate that terraces with >100 m difference in elevation have similar ages of ~105 ka, suggesting they have been differentially affected by tectonic uplift. Constraining the location, temporal activity, and slip rate estimates along these faults will improve seismic hazard analyses for this tectonically complex region.