Regional extension in the western United States began soon after 30 m.y. B.P., the time of first interaction between the Pacific and North American plates. Previous explanations for extension include lithospheric-active processes and asthenospheric-active processes; the former are favored because of coincidence of timing with lithospheric interactions, relative timing of volcanism and deformation, and coincidence with pre-existing structures. Although extension has occurred, in part, in a back-arc setting, it cannot be attributed to age of subducted crust or azimuth of subduction, because both attributes favor back-arc contraction (characteristic behavior before 40 m.y. B.P.
The Mendocino triple junction has been unstable since its inception. The continental margin was relatively straight before 30 m.y. B.P., and it has become more convex westward as the triple junction has migrated northward. The continental margin and arc have been anchored to the subducting slab (Farallon-Juan de Fuca), whereas the triple junction must move parallel to the San Andreas transform. The combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the San Andreas transform relative to the coast. These effects result from the unstable geometry of the Mendocino triple junction. Lack of a subducted slab (slab window) beneath the extended lithosphere enables asthenospheric rise but does not cause extension.