The 600-km-long Salinian block is petrologically similar to the plutonic-volcanic arc complexes of the Sierra and Peninsular Ranges and is bounded on the east by the San Andreas fault. Neogene motion of only 300 km on the fault is insufficient to explain the present offset position of the Salinian block, assuming that it was originally contiguous with the Sierra and Peninsular Ranges. Models involving earlier pre-Eocene movements on an “ancestral” San Andreas fault can account for the basement offset, but a reconstruction based on such models leaves the Salinian block standing at least 100 km west of its counterparts. In addition, it does not explain how the present length of 600 km fits in a 300-km gap between the Sierra and Peninsular Ranges. Our model postulates that significant slivering and northwestward extension have occurred within the block during Neogene time, and that the block formed a westward-protruding bulge in the North American continental margin prior to collision of the Pacific and North American plates. A reconstruction of plate motions indicates approximately 1,000 km of relative North American–Pacific displacement in the last 30 m.y.; this displacement could easily account for the block's extension and basement offset.

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