A DEFORMATION ANALYSIS OF THE LITTLE SALMON FAULT AT STRONG’S CREEK, HUMBOLDT COUNTY, CALIFORNIA

Through paleoseismic trench investigation, we investigate the relation between folding and faulting within the Little Salmon fault (LSF), a thrust fault within the fold and thrust belt of the Cascadia Subduction Zone (CSZ), at Strong's Creek in Humboldt County, California. The LSF has a regional strike of ~N45°W and a slip rate of 3 to 5 mm/yr. The 24 m long trench exposed largely fine grained fluvial overbank deposits. Based on deformation of soils, scarp-derived sedimentation and cross-cutting fault relations, we infer three or possibly four deformation events since the late Pleistocene (radiocarbon ages pending). Total vertical separation due to a combination of faulting and folding at this site is 2.5 m. The fault zone has a shear zone width of ~1 m and the width of folding is ~6 m. The average strike and dip of the fault in the trench is N20°W, 45°E, with the dip ranging from 30 to 60 degrees. Slip along the fault produced drag folds in both the hanging wall and footwall. In general, these folds have axes with a near horizontal (~5°) plunge and parallel to the fault plane. Because fold axes are aligned with the strike of the fault plane and are mutually parallel, we conclude that this portion of the LSF exhibits nearly pure dip slip motion. Therefore over the time period recorded by the folding, deformation on this portion of the LSF has a principle axis of compression subparallel to the PAC/JDF plate convergence direction. We infer that strain characteristic of the northward encroaching San Andreas transform margin is not evident in the fold and fault deformation recorded at the LSF trench site at Strong's Creek.