CRUSTAL-SCALE STRUCTURE BENEATH THE NORTHWESTERN SACRAMENTO VALLEY INTERPRETED FROM SEISMIC REFLECTION DATA: IMPLICATIONS FOR ANCESTRAL WESTERN CALIFORNIA CONVERGENT MARGIN TECTONICS

Analysis of a comprehensive suite of 2-D seismic reflection profiles (two strike lines and eight dip lines; total about 270 line-km of data) reveals that the northwestern Sacramento Valley and Coast Range foothills are underlain by a system of blind, west-dipping thrust faults. The thrust faults are rarely imaged as discrete reflectors, but recognized by systematic patterns of dip discordance and truncated reflectors. Structure contours based on interpretation of depth-migrated sections indicate that the blind thrust system is divided into a series of discrete 10- to 20-km-long segments. Homoclinally east-dipping and folded Mesozoic Great Valley Group rocks (marine forearc strata) exposed along the western valley margin define the forelimbs of northeast-vergent fault-propagation folds developed in the hanging walls of the thrusts. Exhumed coherent blueschists of the Franciscan accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern Coast Ranges are in the hanging wall of the blind thrust system, and have been displaced from their roots in the footwall. Deep, east-dipping reflectors in the footwall of the thrust system are correlative with strongly magnetic rocks (Godfrey et al. 1997), and may be fragments of ophiolitic basement. Restoration of slip on the thrusts suggests that the Coast Range fault, which is the exposed structural contact between the coherent blueschists and attenuated ophiolite, originally dipped east and probably is associated with the east-dipping reflectors in the footwall. Our interpretation of the reflection data supports the two-stage model for blueschist uplift proposed by Wakabayashi and Unruh (1995): (1) blueschist exhumation relative to the forearc basin occurred by attenuation of the ophiolitic basement along the east-dipping Coast Range fault system in late Cretaceous; (2) blueschists, attenuated ophiolite, and forearc strata subsequently were uplifted and folded in the hanging wall of the blind thrust system beginning in latest Cretaceous-early Tertiary, generally coeval with the Laramide orogeny to the east. The blind thrust system probably rooted in, and was antithetic to, the ancestral east-dipping subduction zone beneath California.