ACTIVE CRUSTAL SHORTENING IN THE SOUTHWESTERN SACRAMENTO VALLEY, CALIFORNIA

The 1983 Coalinga earthquake, which occurred on a blind thrust fault beneath the western margin of the San Joaquin Valley, prompted a reassessment of active tectonics and seismic hazards along the eastern edge of the California Coast Ranges. As part of this reassessment, Unruh and Moores (1992) studied Quaternary deformation in the southwestern Sacramento Valley, the epicentral region of the 1982 Winters-Vacaville earthquake sequence that previous workers (Eaton, 1986; Bennett, 1987) suggested may have occurred on a blind thrust fault like the 1983 Coalinga event. Unruh and Moores (1992) documented evidence that actively growing folds in the southwestern Sacramento Valley (e.g., the Rumsey Hills and Dunnigan Hills anticlines) originally formed in early Tertiary time and accommodated arcward-vergent shortening of the ancestral Great Valley forearc region, similar to models for active tectonic wedging of the Tobago trough forearc basin in the Lesser Antilles (Unruh et al. 1991; 1995). Subsequent work by O'Connell et al. (2001) and Unruh et al. (2004) found that west-dipping blind thrust faults underlying the reactivated folds exhibit a right-stepping en echelon geometry, suggesting that the structures presently accommodate transpressional shearing along the eastern edge of the San Andreas fault system, rather than shortening normal to the plate boundary. The right-stepping pattern appears to limit the dimensions of the faults such that earthquakes on individual thrust segments are likely to be M6.5 events or smaller. Because the thrust faults and the right-stepping geometry predate the onset of late Cenozoic transpression in western California, the structures may have originally accommodated net dextral motion as well as shortening during oblique plate convergence in late Cretaceous-early Tertiary time.