Fracture zones in the Gulf of California are charted from data of reflection profiling and bathymetry. Positions of active spreading centers are located through magnetic anomalies at the mouth, positions of deep troughs in the central gulf, and locations of oceanic ridge-type earthquake swarms in the northern gulf and Salton Trough. Assuming the 6-cm-per-yr spreading rate determined by earlier magnetic anomaly studies has been constant and that spreading has been symmetrical and has not involved transpeninsular faulting, areas of new and old crust within the province are delineated. With the same assumptions, translation of Pacific Plate terrain 240 km back along the 306° azimuth of fracture zones shows the initial Pacific-North American Plate boundary and results in geography clearly requiring a protogulf prior to the current episode of plate separation. Interpretations of reflection profiles and faunal studies of dredged rocks imply that sediments of the central protogulf were deposited in depths significantly greater than 1,000 m and that, in early stages of rifting, they may have been uplifted and subsided as much as 1,200 m. Poorly developed, semi-coherent stratification of postrifting new crustal area is contrasted to well-developed strata of the protogulf areas. This contrast, plus the general absence of ponded turbidites and recording of normal growth-faults at a trailing plate-edge trough, lead to the hypothesis of “clastic compensation.” As continental, or intermediate, crusts are pulled apart by plate motions in the presence of high or moderate clastic sediment sources, crustal growth and resulting structure are controlled by the balance between separation and the flow of sediment into the gaps. Partial isostatic compensation is achieved by supraplate injection of elastics and subplate rising of mantle-derived basalts. Very high supplies of elastics, as in the northern Gulf of California province fed by the Colorado River, preclude the formation of oceanic-type crust as a result of plate separation and lead instead to the formation of intermediate crust, typical neither of oceanic nor continental realms.

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