ACTIVE DEFORMATION AND THE ROLE OF INHERITED STRUCTURES IN THE SACRAMENTO-SAN JOAQUIN DELTA, NORTHERN CALIFORNIA, CONSTRAINED USING A COMPILATION OF GEOMORPHIC, GEOLOGIC, AND GEODETIC DATASETS

Seismic sources and associated hazards within the Sacramento-San Joaquin Delta region of north-central California (‘Delta’) are relatively poorly characterized. This is in part because faults in the ~35-km-wide region have slow slip rates that likely combine to accommodate less than 5 mm/yr. In addition, these faults exhibit a variety of kinematics, ranging from dextral to reverse in a transpressional regime. Finally, erosion and deposition by the Sacramento and San Joaquin Rivers and their tributaries through the late Pleistocene and Holocene may have obscured 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.

We address these challenges by integrating surface observations with a compilation of subsurface, bedrock geology, and geodetic datasets from the Delta and from the Coast Ranges and Diablo Range to the northwest and southwest, respectively. Structural geologic data from the surrounding region highlights the significant influence that Tertiary-age forearc structures exert on the geometry and kinematics of major Quaternary-active structures within the Delta. These inherited structures include both extensional and reverse faults associated with deformation within the Farallon subduction system forearc prior to the initiation of the transform plate boundary. They have been reactivated as part of the modern plate boundary, and exhibit a range of geometries and kinematics in a region generally dominated by right-lateral strike-slip Quaternary faulting. The approximately plate-boundary-parallel Pittsburg-Kirby Hills and Midland faults which formed as extensional structures in the early Tertiary exhibit a component of reverse displacement in the Quaternary based upon geomorphology and subsurface datasets. A clearer understanding of subsurface geometries and structural relationships, built upon the regional tectonic history, provides insight into modern deformation accommodated on older structures and may help inform interpretations of seismic hazard within the Delta, where traditional paleoseismological investigations are of limited use.