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Description
The Pacific/Farallon plate - North American plate relative motion resulting from combining the respective absolute plate motions shows a rapid rate of convergence during Late Cretaceous-early Cenozoic. This resulted in subduction of the Farallon plate in a N55°E ± 10° direction at a rate of 14.0 ± 0.8 cm/yr for 70 - 56 myBP and a N19°E ± 3° subduction orientation at 24.3 ± 0.9 cm/yr for 56 - 45 myBP. This later subduction orientation produced oblique plate convergence which promoted the initial right-lateral displacement of the Salinian block in central California. The shallow subduction which resulted from this rapid rate caused an increase in the arc-trench gap, demonstrated by the inland shifting of arc-related magmatic activity. The shallow subduction angle also allowed a more efficient trans-mission of compressive stress between the overriding and subducting plate which resulted in the widespread 'Laramide' deformation. Plate motion reorganizations about 40 myBP dramatically altered the plate interactions. Subduction continued in a northeast direction due to the geometry of motion about the spreading ridge though at the greatly reduced rate of 7.7 ± 0.5 cm/yr. This caused an increase in the subduction angle which resulted in a migration of the zones of arc-related magmatism back towards the trench as well as a termination of compressive deformation associated with the 'Laramide' orogeny. The oceanic crust between the Mendocino and Murray fracture zones was shifted eastward so that it is 20 -25 my younger than the adjacent crust to the north. This section of crust was opposite northern Mexico after 40 myBP. Subduction of this relatively young crust may account for the resurgence of magmatic activity in southern Arizona and eastwards into southern New Mexico. The spreading ridge had a continuous though variable eastward translation while the continent moved westward. The reconstructions show their initial contact occurred 25 - 27 myBP in the vicinity of the present California - Mexico border. The subsequent contact of the Pacific and North American plates resulted in relative motion in a northwest-southeast direction promoting strike-slip displacement and initiating development of the Continental Borderland structures. Growth of the zone of strike-slip displacement occurred with migration of the triple junctions with a total right-lateral strike-slip displacement for the past 27 my of 1250 -1360 kilometers.
