TOPOGRAPHIC EXPRESSION OF ACTIVE FAULTING ALONG THE GREAT VALLEY THRUST SYSTEM NEAR WINTERS, NORTHERN CALIFORNIA

Seismic sources and their associated hazards within the Sacramento-San Joaquin Delta region of north-central California (‘Delta’) are relatively poorly characterized, in part because faults have slow slip rates, likely < 5 mm/yr. These faults also have a variety of kinematics, more so than the principal plate boundary faults of the San Andreas Fault system to the west, ranging from dextral to reverse in an transpressional regime. In addition, erosion and deposition within the northern Delta by the Sacramento River and its tributaries through the late Pleistocene and Holocene may obscure any active tectonic signal in the landscape. Together, these processes lead to a record of faulting and folding with poor surface expression that is often low amplitude and/or long wavelength, locally concealed or dissected, and thus difficult to identify on the ground.

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.