The area that surrounds the Gulf of California was intensely faulted during the late Cenozoic prior to opening of the gulf. In a representative 10,000-km2 area of southern Sinaloa, the faults consist of a predominant north-northwest-striking set having normal displacement and a subsidiary east-northeast set having largely strike-slip displacement. Two domains are recognized: one in which both fault trends are abundant and one in which east-northeast faults are minor. In the former, north-northwest faults form a series of mostly southwest-tilted half-grabens filled with upper Tertiary sediments. Displacement on individual faults is commonly as much as several kilometers. East-northeast faults probably represent accommodation zones between areas of differential extension. In the second domain, north-northwest faults form an extensive graben system. Major faults, dipping 40° to 70° into the graben and having several kilometers of cumulative displacement, are spaced every 5 to 10 km. Bedding attitudes indicate that fault blocks are rotated as much as 65°. Dependent upon assumptions about subsurface geometry of the faults, total extension may range from 20% to 50%. Fault geometry and stress orientations calculated from fault-slip data indicate that the least principal stress was east-northeast.

K-Ar ages of three north-northwest-striking dikes indicate that east-northeast extension began as early as 32 Ma. Ages of tilted volcanic rocks, however, indicate that measurable tilting, and therefore most faulting, began after about 17 Ma.

Faulting having similar timing, style, and orientation occurred throughout the area surrounding the Gulf of California, as far south as Nayarit on the Mexican mainland and as far north as areas in Sonora that are unequivocally within the Basin and Range province. This continuity and similarity in characteristics of faulting around the gulf to those of early Basin and Range faulting in the United States indicate that the area around the gulf is part of the Basin and Range province. This Basin and Range faulting probably created the widely recognized but debated proto-Gulf of California. The present Gulf of California opened by ocean-floor spreading and transform faulting in a zone already weakened by Basin and Range extension. The lack of correspondence between areas of extension and likely areas of crustal thickening related either to Laramide contraction or magmatism argues against a model of Basin and Range extension related solely to spreading of overthickened crust.

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