2: Mesozoic-Cenozoic Cordilleran plate tectonics

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Abstract
Motion of the Africa plate with respect to Tristan da Cunha since 135 m.y. B.P. and motion of the Pacific plate with respect to Hawaii since 80 m.y. B.P. are combined with relative motion between the North America and Africa plates and between the Kula-Farallon and Pacific plates to compute plate reconstructions and relative motion between the North American Cordilleran margin and Farallon-Kula-Pacific plates during Mesozoic-Cenozoic time. Cordilleran tectonic timing and distribution of petro-tectonic assemblages appear grossly consistent with inferred plate reconstructions and interactions. Major tectonic transition from Paleozoic modes characterized by rifting and by arcs colliding with the quiet continental margin to the Mesozoic-Cenozoic mode of active Andean-type continental margins coincides with initiation of disruption of Pangea and opening of the Atlantic Ocean-Gulf of Mexico in Late Triassic-Early Jurassic time. North America moved northwest‐ward during Jurassic-Early Cretaceous time, apparently overriding the Farallon plate from Alaska southward. Transition from the Sevier-Columbian orogeny to the Laramide orogeny about 80 m.y. ago coincides with the end of a major magnetic quiet period, initiation of separation of North America from Eurasia, and a general reorganization of plates. The Laramide orogeny (70 to 45 m.y. B.P.) progressed during more rapid westward motion of North America, accompanied by accelerated northeast-southwest convergence between the North America and Farallon plates. The end of the Laramide orogeny is broadly synchronous throughout the Cordillera and coincides with the age of the Hawaii-Emperor “elbow” and a drop in the North America-Farallon convergent rates. From 40 to about 20 m.y. B.P., vast ignimbrite eruptions in the southern Cordillera correlate with final subduction of the Farallon plate. Widespread basalt eruption, block faulting, and collapse of the Basin and Range province occurred since 20 m.y. B.P. and coincided with cessation of subduction, growth of the San Andreas-Queen Charlotte transform faults, and interaction between the North America and Pacific plates.

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