Quaternary blind thrusting in the southwestern Sacramento Valley, California

Abstract

Patterns of microearthquakes and Quaternary surface deformation suggest that the tectonic setting of the SW Sacramento Valley is similar to areas of the western San Joaquin Valley known to be underlain by seismogenic blind thrust faults. On the basis of previous work and analysis of geologic and seismic reflection data, the following late Cenozoic tectonic features and processes are identified: (1) uplift of the northern Coast Ranges beginning approximately 3.4 Ma, and eastward propagation of uplift into the southwestern Sacramento Valley by 1.0 Ma; (2) uplift and homoclinal flexure of Plio‐Pleistocene strata at the eastern Coast Ranges mountain front; (3) uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern Sacramento Valley. Similar associations of structures and processes have been observed in thrust belts in Pakistan, the Peruvian Andes, and the Canadian Cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone. By using other thrust belts as analogs, the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern Coast Ranges mountain front. Seismic reflection profiles reveal that (1) the triangle zone consists primarily of east‐vergent blind thrusts and (2) west‐vergent backthrusts exposed in the the eastern Coast Ranges and southwestern Sacramento Valley are rooted in the east‐vergent thrusts. Transfer of slip from the east‐vergent blind thrusts to the west‐vergent backthrusts occurs locally beneath the southwestern Sacramento Valley. Fault‐bend folding in the hanging walls of the backthrusts has created a north‐northwest striking chain of low hills approximately 15 km east of the mountain front. The folds deform 3.4–1.0 Ma fluvial sediments and thus are middle Pleistocene in age or younger. Local variations in strike suggest that the fold chain is segmented, like the New Idria‐Coalinga‐Kettleman Hills segmented fold chain in the southwestern San Joaquin Valley (Stein and Ekström, 1989). These data have implications for seismic hazard assessment. Anecdotal accounts indicate that two M = 6.0+ events of the 1892 Winters‐Vacaville earthquake sequence probably occurred beneath the eastern Coast Ranges (Dale, 1977; Toppozada et al., 1981). Ground cracking was observed following the main shocks along the mountain front in the southwestern Sacramento Valley. We propose that the earthquakes were generated by slip on a blind thrust beneath the Coast Ranges, and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone. The 1892 earthquake sequence suggests that blind thrusts beneath the southwestern Sacramento Valley are active and capable of generating moderate to large magnitude earthquakes.