On plate tectonics and the geologic evolution of southwestern North America
Authors: Ward Peter L. 
Published: February 2008

Very rapid subduction of the Farallon plate under southwestern North America between 60 and 40 Ma was accompanied by a relatively low volume of magmatism throughout the southwestern United States and northern Mexico. Between 40 and 20 Ma, when subduction slowed significantly and in one area may have even stopped, magmatism became widespread and voluminous from Nevada and Utah to central Mexico. This correlation of rapid subduction with a relatively low volume of magmatism can be explained by the observation that subduction‐related andesitic arc volcanism, often formed in a Laramide‐style compressional regime, is relatively low volume compared to continental volcanism. The shallow roots of arc volcanic systems are clearly exposed in the porphyry copper deposits found in currently active arcs and common throughout southwestern North America between 60 and 50 Ma. By 43 Ma, worldwide plate motions changed, the Pacific plate began moving away from North America, and subduction of the Farallon plate slowed. By around 36 Ma, the easternmost part of the East Pacific Rise, which was located between the Pioneer and Murray fracture zones, approached the trench and the young, hot, buoyant lithosphere appears to have clogged part of the subduction zone. Uplift on land became widespread. Voluminous continental magmatism formed the Sierra Madre Occidental (SMO) of Mexico, one of the largest batholiths in the world, as well as volcanic centers now exposed in the San Juan Mountains of Colorado and the Rio Grande Rift of New Mexico. Vectors of motion of the Pacific plate relative to the North American plate determined by Stock and Molnar (1988) are consistent with formation of a transtensional environment along the plate boundary sufficient to create a 100‐ to 200‐km‐wide void just landward of the old volcanic arc. While the SMO batholith was forming within this void, the Monterey and Arguello microplates just offshore to the west were broken off from the Farallon plate and rotated so that the East Pacific Rise in this immediate area became nearly perpendicular to the trench and perpendicular to the vector of motion of the Pacific plate relative to North America. Formation of the SMO batholith was followed between 24 and 20 Ma by a major increase in the rate of subduction of the Guadalupe plate, a fragment of the former Farallon plate, and by increasing mylonitization, extension, and uplift in the metamorphic core complexes that extend northwestward through southern Arizona from the northern end of the SMO batholith. The plate margin underwent another major change between 12.5 and 10 Ma when subduction again stopped, strike slip faulting became dominant along the coast, the Basin and Range Province opened, and numerous tectonostratigraphic terranes in southern California underwent large rotations. By 3 Ma a large, new terrane had been severed from North America immediately west of the SMO batholith as the Gulf of California opened. These observations can be explained by a model for the weakening and ultimate falling apart of the uppermost part of the subducted oceanic plate in the 20–30 m.y. after the end of rapid subduction. As the plate falls apart, not only is compressional stress relieved, but significant backslip along the old subduction zone is also possible, perhaps bringing blueschists rapidly upward from 20‐ to 30‐km depths.