TECTONIC TERRACES AT THE TRIPLE JUNCTION

The overlapping Cascadia and San Andreas tectonic regimes provides an opportunity to deconvolve the evidences for different tectonic forcing factors. In the Humboldt Bay region, northwest-striking, southwest-vergent thrust faults represent anelastic deformation related to Cascadia convergence. To the south and east of Point Delgada, east-stepping Pacific‐North America dextral shear generates strike‐slip faults that either terminate in uplifted terranes and east‐west striking reverse or thrust faults and folds, or strike-slip faults that penetrate through and past the Humboldt Bay region.

The Russ fault zone, included as a source in the USGS National Seismic Hazard Map, is mapped as north-vergent reverse, displacing late Cretaceous Franciscan and younger Yager rocks over Tertiary Wildcat rocks. However, we locate a topographic scarp adjacent to the Russ fault zone that may represent Holocene slip on a west-striking, south-vergent reverse fault offsetting late Pleistocene to Holocene fluvial terraces. Scarp heights increase on progressively older terraces. Using regionally derived incision rates as a proxy for terrace age, we use topographic swath profiles to measure scarp heights and calculate a late Pleistocene slip rate of about 0.75 mm/yr.

To better understand the stratigraphic setting and to provide relative age control for the geomorphic surfaces offset by the fault, we conduct a terrace mapping campaign. This chronostratigraphic framework will form the basis for updated slip‐rate calculations made for the scarp-forming fault. We use LiDAR-derived slope rasters to delineate fluvial terrace treads using maximum slopes up to 4°. We calculate the relative elevation for the treads using a constructed digital elevation model that represents the modern floodplain. Using the distribution of relative elevations for each tread we correlate terraces along the lower Eel and lower Van Duzen rivers.

Terraces north of the mouth of Van Duzen River display syntectonic deformation in the form of a N20E striking syncline, while there is no apparent folding evident in terrace profiles to the south. Based on preliminary correlations south of this confluence, we find that older terraces may be displaced up to the south, consistent with northward vergence of the Russ fault mapping.