SEISMIC REFLECTION IMAGING OF FOLD DEFORMATION ABOVE THE WEST TRACY FAULT NEAR BYRON, CALIFORNIA

The blind West Tracy fault (WTF) strikes northwest, dips southwest, and underlies the southwestern Sacramento-San Joaquin Delta region approximately between the towns of Byron and Tracy. Previous studies have proposed that the long-term average rate of reverse slip on the WTF is about 0.1-0.3 mm/yr and that the fault may generate ~M6.75 earthquakes (Unruh and Hitchcock, 2015). A recent seismic reflection survey conducted by the California Department of Water Resources (DWR) near Byron images a NE-vergent fault-propagation fold above the blind tip of the WTF. Based on stratigraphic-structural relations interpreted in the seismic data, the fold has grown in late Cenozoic time by progressive migration of the synformal hinge at the base of the forelimb into flat-lying strata in the footwall as the fault tip propagates updip to the northeast. A distinctive NW-trending geomorphic lineament near Byron associated with the WTF is interpreted to be a fold scarp; i.e., a zone of localized surface uplift and NE-tilting when the fold grows incrementally during a moderate to large earthquake, rather than discrete fault offset of the ground surface. The reflection data image a late Pleistocene unconformity at depths of about 18 m (60 ft) in the hanging wall and 30 m (100 ft) in the footwall that is folded into a monocline above the WTF. It is possible that the folded unconformity formed during one of the major low stands of sea level and fluvial incision in the Delta region during glacial advances, and associated with deposition of the late Pleistocene Modesto or Riverbank Formations (Shlemon, 1971). If it is assumed that the unconformity is associated with the 130ka low stand during marine oxygen isotope stage 6 (i.e., the end of Riverbank deposition), then the implied vertical separation rate on the Pleistocene unconformity above the WTF is about 0.1 mm/yr. The separation rate could be higher if the unconformity is associated with the ~20ka Modesto Formation. DWR plans additional studies of late Pleistocene stratigraphy near Byron to reduce uncertainty in the WTF slip rate.