The Pacific and North America plates are among the world’s largest, and most of the active tectonics in North America takes place on their mutual plate boundary, extending from the Aleutians down the west coast of British Columbia through the Pacific Northwest to California and Baja California. The diffuse plate boundary extends north across the Denali fault into central Alaska, northeast into the Canadian Yukon Territory, east through the Basin and Range Province of the United States and northwestern México to the Wasatch fault of Utah and the Rio Grande Rift of New Mexico and adjacent regions, and southwest across the California Continental Borderland.
Earlier in the Cenozoic, nearly all the western margin of North America was a subduction zone, with the offshore East Pacific Rise separating the Pacific Plate from the subducting Farallon Plate on the east and the Kula Plate on the north (Atwater, 1989). (For a recent synthesis, see Madsen et al., 2006.) The North America Plate moved westward as sea-floor spreading increased the width of the Atlantic Ocean. Some time after 30 Ma, parts of the East Pacific Rise reached the margin of the continent and were subducted so that the Pacific Plate came into direct contact with the North America Plate (Figure 2.1). Pacific Plate motion was northwest relative to North America, parallel to the boundary at the base of the continental slope, so that the resulting boundary was strike-slip, the beginning of the San Andreas transform-fault system in California. The remnants of the Farallon Plate, the Juan de Fuca Plate in the north and the Cocos Plate in the south, continue to subduct beneath North America, the former as the Cascadia subduction zone, and the latter as the Middle America subduction zone. The Juan de Fuca Plate developed two satellite microplates at its northern and southern ends, the Explorer and Gorda plates, respectively, and the Cocos Plate spun off the Rivera Plate in the mouth of the Gulf of California. An oceanic plateau within the Pacific Plate became entrained near the plate boundary, colliding with southern Alaska as the Yakutat Microplate.