Late Neogene tectonics of the mouth of the Gulf of California Public Deposited

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  • Anomaly timescales for the last 90 million years, derived from marine magnetic profiles and published prior to mid-1979, are summarized, illustrated for comparison, and critically reviewed. A revised timescale is constructed using calibration points which fix the ages of anomalies 2.3', 5.5, 24, and 29. An equation is presented for converting K-Ar dates that is consistent with the recent adoption of new decay and abundance constants. The calibration points used in the revised timescale, named NLC-80, are so converted, as are the boundary ages of geologic epochs within the range of the timescale. NLC-80 is then used, along with recently acquired and rigorously navigated underway geophysical data from the region of the mouth of the Gulf of California, to prepare detailed bathymetric, gravimetric, and seismo-tectonic maps of the area. The basement ages at DSDP Leg 63 drilling sites 471, 472, and 473 are estimated from magnetic anomalies fit to timescale NLC-80. The estimates agree with biostratigraphically determined basement ages and support the proposal that an aborted ridge of about 14 MY age has left a small fragment of the Farallon Plate beneath the Magdalena Fan. Several large inactive faults are identified on the deep-sea floor west of the tip of the peninsula of Baja California. Additional magnetic anomaly profiles and bathymetric profiles across the Rivera Ridge are interpreted. These contradict the existence of a 3.5 MY old aborted spreading center on the Maria Magdalena Rise. Instead, it is proposed that an episode of subduction of the Pacific Plate beneath the southeastern tip of Baja California, concomitant with strike-slip faulting west of the peninsula, occurred and that this subduction may be responsible for the uncentered location of the Rivera Ridge within the mouth of the Gulf of California. A single magnetic anomaly profile obtained northeast of the Tamayo Fracture Zone is used to determine that the rate of Pacific/North American plate motion, for the last 3 MY is 68 km/MY at this location. This result, if correct, indicates that the peninsula of Baja California is separating from mainland Mexico faster than the Rivera Ridge is generating oceanic crust in the wake of opening in the gulf. This, in turn, requires that either slow diffuse extension is occurring presently across the Maria Magdalena Rise, or across the Cabo Corrientes-Colima region, or that the portion of North America south of the trans-Mexican volcanic belt is moving right-slip with respect to the North American Plate at a rate of 10-20 km/MY. Large horsts and many smaller continental fragments are found within the southern gulf. Several of them have active seismic boundaries, while others have apparently foundered. The gulf began to open approximately 14-15 MY ago with slow, diffuse block-faulting and the deposition of the Maria Magdalena Fan at the mouth of the gulf. Oceanic crust was exposed in the gulf by about 9-10 MY, at the same time that the Rivera Ridge began reorienting by clockwise rotation. Strike-slip motion along the Tosco-Abreojos Fault took up some of the Pacific/North American motion with the remainder occurring within the gulf itself. During this period the Pacific Plate forming within the gulf was slowly subducting beneath Baja California. By 4-5 MY subduction ceased and all of the Pacific/North American plate motion was shifted to the Gulf of California fault system. The gulf and peninsula of California are still in the process of adjusting to the change from Pacific/Farallon to Pacific/North American motion.
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