Effect of subduction of the mendocino fracture zone on tertiary sedimentation in Southern California

The age and paleogeographic setting of middle to late Tertiary fluvial deposits in southern California are consistent with the theory that subduction of the Mendocino fracture zone produced a north-facing, north-moving slope and zone of deformation in the North American plate. The Mendocino fracture zone was a 1 km high, north-facing scarp in the Farallon plate that moved northward at about 3 cm yr −1 . Subduction of this scarp produced isostatic stress in the overlying North American plate. If this stress were expressed in the crust as a north-facing slope, then a given area should have experienced the following sedimentary evolution: (1) normal (probably oceanward) drainage when the slope was far to the south; (2) reorientation to northward drainage as the slope moved to a position immediately south of the area; (3) deposition of coarse sedimentary units, possibly accompanied by down-to-the-north faulting, when the slope reached the area; (4) uplift, deformation, and erosion of previously deposited sediments as the area was uplifted along the slope; and (5) reestablishment of normal drainage. Compilation of data from sedimentary sequences in southern California supports this model. In particular, the time of fracture-zone passage was marked by strong dominance of northward stream flow. These relationships suggest that middle Tertiary fluvial sedimentation in the southern California area was strongly influenced by passage of the fracture zone.