TOPOGRAPHIC EXPRESSION OF ACTIVE FAULTING ALONG THE GREAT VALLEY THRUST SYSTEM NEAR WINTERS, NORTHERN CALIFORNIA
The expression of active surface deformation along a strand of the Great Valley thrust system (GVTS) in the northwestern Delta is apparent from geomorphic mapping both in the field and using a recent lidar dataset. Apparent uplift on the east side of the fault, which we call the Bigelow fault, is indicated by various landforms, including wind gaps, discontinuous fluvial terraces and floodplains along a north-south trend across the eastward-draining Putah Creek. These features align with a linear escarpment, striking ~340° and ~30 m-high, along the western edge of the Bigelow Hills, west of Winters, CA, suggesting a protracted history of east-side-up motion. Geomorphic markers along the Bigelow fault do not appear to record any obvious strike-slip displacement (i.e., right-lateral stream deflections or shutter ridges).
Integration of these observations with previous interpretations of this bedrock structural system ~50 km to the north leads us to the interpretation that the Bigelow fault acts as a west-vergent, secondary backthrust in the primarily east-vergent GVTS. In this model, the Bigelow fault and GVTS accommodate plate-boundary-normal convergence along the eastern flank of the Coast Ranges system in the western Sacramento Valley. This interpretation implies that the east-side-up Bigelow fault may be actively accommodating shortening, an example of strain partitioning in a region of dominantly right-lateral faulting.