The North American portion of the Cordilleran orogen extends continuously from Alaska to southern Mexico, and from east to west over much of its length the orogen comprises an easterly vergent fold-thrust belt, a complexly deformed metamorphic hinterland that collapsed gravitationally, and an interlaced mosaic of exotic terranes. Although most models for the development of the Cordilleran orogen invoke Late Jurassic–Cretaceous intraplate, backarc shortening above an eastwardly dipping subduction zone, a simple collisional model in which the leading edge of North America was subducted to the west, beneath a segmented, arc-bearing microcontinent, better fits the data.
During the early Mesozoic, Panthalassic Ocean crust was subducted westward beneath a ribbon continent named Rubia, where it created a generally low-standing continental arc. At about 124 Ma, the widespread deposition of intraformational gravels and conglomerates atop the passive margin marked the passage of the North American shelf over the outer bulge of the trench and its entry into the subduction zone. Loading by the “bulldozed” and thickened accretionary wedge—as well as the overlying eastern edge of the Rubian ribbon continent—depressed the lithosphere to create the Cretaceous foredeep, which migrated eastward during progressive convergence. As the westernmost edge of North America was subducted, the dewatering of slope-rise and rift deposits abruptly created voluminous melts that rose to thicken and assimilate the overlying exotic crust, where they formed Cordilleran-type batholiths. Owing to the difficulty of subducting an old craton, convergence slowed to a halt by 80–75 Ma, causing the shutdown of Cordilleran-type magmatism, and finally, during the Maastrichtian, break-off of the North American plate. The first segment to fail was likely the Great Basin segment, located south of the Lewis and Clark lineament and north of the Sonoran segment. There, slab failure rates were apparently slow enough that there was considerable lithospheric necking, and so slab-failure magmas were prevented from rising into the overriding plate. The diachronous break-off caused a catastrophic stress inversion in both upper and lower plates. Released from its oceanic anchor, the partially subducted edge of the North American craton rose rapidly, causing its stress regime to change from extensional to compressional, which, along with continued convergence, generated the thick-skinned Laramide deformation. Uplift and gravitational collapse of the overlying Rubian plate formed the linear belt of Paleocene-Eocene metamorphic core complexes within the orogenic hinterland. In the Canadian segment, located north of the Lewis and Clark line, the Coast plutonic complex was uplifted rapidly as asthenosphere rose through the torn lower-plate lithosphere to invade Rubia with a 1500-km linear belt of break-off–generated magmas. Within the Sonora segment to the south, break-off magmatism was also prevalent. Both the Canadian and Sonoran segments have abundant porphyry copper mineralization temporally and spatially associated with the break-off magmas, which suggests a genetic link between slab failure and porphyry copper mineralization. By 53 Ma, eastwardly dipping subduction of Pacific Ocean crust was generating arc magmatism on the amalgamated Cordilleran collision zone in both the Canadian and Sonoran segments. Oceanic schists, such as the Orocopia-Pelona-Rand, were formed in the ocean basin west of Rubia and accreted during initiation of the new easterly dipping subduction zone.
A major transform fault, called the Phoenix fault, connects the Sevier fold-thrust belt at the California-Nevada border with that in eastern Mexico and separates the Great Basin and Sonoran segments. It juxtaposes the Sierra-Mojave-Sonora block alongside the Transition Zone of the Colorado Plateau.
Cordilleran events affected the subsequent development of western North America. For example, the structural Basin and Range Province appears to coincide with the region where exotic allochthons sit atop North American crust in both the Great Basin and Sonoran segments. Also, within the triangular Columbia embayment, large segments of Rubia appear to have escaped laterally during the Cordilleran orogeny to create a lithospheric “hole” that was later filled by basalt of the Columbia River and Modoc plateaux.
