Two assumptions provide the basis for a new generalized reconstruction of California and Baja California prior to the major Tertiary strike-slip faulting of the San Andreas system. (1) The contrasting Franciscan and batholith basement types originally formed two parallel belts such as are found elsewhere around the Pacific margin; and (2) rhombochasms or zones of extension must be found to account quantitatively for any major displacement of crustal blocks along these strike-slip faults, and conversely, zones of Tertiary extension must be closed in any Mesozoic reconstruction. The Gulf of California rhombochasm of Carey (1958) and Hamilton (1961) is large enough to account for all of the apparent offset in southern California but only half of that in central and northern California. It is here proposed that this additional offset may be accounted for in the deep-water region of the continental borderland of northern Baja California. This region is thus considered to be analogous to the Gulf of California rhombochasm. This implies a two-stage movement history of the San Andreas system with an intervening change in direction of block movement by about 25° and related left-lateral faulting in the Transverse Ranges. Salinia would have moved twice for a total of approximately 600 km and would once have lain off southern California and northern Baja California, while southern and Baja California would have moved only once for approximately 300 km.

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