A model for Neogene crustal rotations, transtension, and transpression in southern California

Two episodes of crustal rotation have occurred in the regions of southern California and the Mojave Desert since the beginning of Miocene time. An early Miocene episode of clockwise crustal rotation is associated with widespread north-south to north-northeast-south-southwest crustal extension throughout southeast California and western Arizona. This episode is apparently related to changing subduction parameters of the Farallon plate. A middle Miocene and later episode of clockwise rotation in the Transverse Ranges and bordering areas is related to shear between the Pacific and North American plates. In both episodes, clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend. The effects of extension are most pronounced in the earlier episode but clear in the later one also. The Pacific-North American deforming zone began contracting in width during Pliocene time, but clockwise rotations continued and are probably in progress today.