Enigmatic Paleozoic and Precambrian sequences rich in volcanic and plutonic rocks form discrete terranes along the outer border of the North American Cordillera. These borderland terranes occur in California in the northern Sierra Nevada and Klamath Mountains, in central Oregon and eastern Oregon–westernmost Idaho, in Washington in the Northern Cascade Mountains and San Juan Islands, in British Columbia in Vancouver Island, and in Alaska in the Alexander Archipelago. The difficulty in relating the geology of the borderland terranes to that of the North American continent, the recognition of ophiolites and suture zones separating the terranes from the continent, plus the Asiatic affinity of certain of the borderland faunas indicate that the terranes are allochthonous relative to the North American continent. Furthermore, major differences in stratigraphy, magmatic activity, tectonic activity, metamorphism, and particularly the ages and types of basement between the terranes — when considered together with discordant paleomagnetic data — suggest that at least six lithospheric plates are represented.
The terranes in the Klamath Mountains have an Ordovician ultramafic rock (ophiolite) basement. The Oregon terranes have ultramafic complexes (ophiolites) in close association with volcanic rocks (volcanic arcs) that form the basement. In the Northern Cascade Mountains and San Juan Islands, the terranes have, respectively, Precambrian and Ordovician crystalline metaplutonic (magmatic arc) basement. The terrane in the southern part of the Alexander Archipelago has a Precambrian crystalline meta-volcanic-metasedimentary (remnant arc) basement, but an Ordovician basaltic-andesitic basement (initial deposits of an upward-shoaling island arc) appears farther north. Transcurrent faults segment and truncate parts of the Cordillera, but since the borderlands are in themselves composed of several plates, models of a single allochthonous plate are difficult to apply. More likely, during Precambrian and Paleozoic time, multiple microcontinental plates and volcanic arcs moved outboard and inboard (away from and toward North America) to accommodate a succession of marginal ocean basins opening and closing behind migrating arcs. This was followed in Mesozoic and Cenozoic time by large-scale northwestward drift.