Latest Pleistocene to Holocene river terrace deformation within the southernmost extent of the little salmon fault zone; geomorphic insights to fault termination and rupture history, Van Duzen River, northern California

The southern Cascadia subduction zone (CSZ) of northwestern California exhibits northeast-directed contraction, transitioning to north-northwest directed translation within the broad San Andreas fault (SAF) transform margin to the south. The Little Salmon fault (LSF) is one of the southern-most, active thrust faults within the onshore fold and thrust belt of the CSZ, and lies proximal to the transition from compressional to dextral stress across the Mendocino triple junction (MTJ). Thus, it is an ideal location to characterize strain associated with this complex region of transitional stress regimes. High precision topographic data (LiDAR) enabled detailed mapping of geomorphic features otherwise obscured by dense vegetation of the area. The Van Duzen fault (VDF), a northwest trending mole track scarp, sub-parallel and south of the main splay of the LSF is observed on LiDAR imagery. This fault exhibits up-to-the-northeast offset and traverses several Van Duzen River terrace risers and treads that range from Pleistocene to potentially Holocene in age. A shallow, exploratory trench was hand-excavated across the VDF. The shallow, roughly 1.5 m-deep, 16 m-long trench exposed imbricated gravels that dip into the base of the trench in the upper end. Coring within the lower end of the trench mapped the southern extent of the unconsolidated, clast-supported gravel deposit revealing vertical separation of 2.5 m, displaying an up-to-the-northeast step. The linear map expression of the VDF across river terraces of varying elevation and age suggests that the fault may be relatively steeply dipping. Exposed offset bedrock display reverse offset along the VDF, but with no stratigraphic constraint to measure offset. River terraces are some of the youngest geomorphic features within the study area. By constraining a sequence of relative ages for terraces associated with the Van Duzen River, in combination with regional uplift assumed to be equivalent to incision rates, rough terrace ages have been determined. Slip rates calculated from estimated net slip along the VDF and inferred terraces ages range from ~ 0.05 to 0.5 mm/yr. The west-northwest orientation of the compressional faults and folds within the study area suggest SAFparallel compression, possibly near the transition from transform to compressional tectonic regimes.