THE NORTH AMERICAN CORDILLERA: ANOMALY OR MODEL OROGEN?

The North American Cordillera contains most of the elements of Phanerozoic collisional orogens, like the Alps, Appalachians, Himalayas, and Urals [foreland fold-thrust belt, metamorphic core(s), accreted volcanic-plutonic arcs and other terranes, etc.], but without a terminal collisional event in the past or within sight in the future. The western North American margin has faced open ocean since the breakup of Rodinia ~750 Ma, producing first a rifted margin that evolved into a Paleozoic platform. Parts of this platform were disturbed during the Paleozoic and early Mesozoic by construction of the Ancestral Rockies and by the Antler and Golconda orogenies. By Mid-Mesozoic time, subduction had begun off the western margin that led to arc formation, offshore amalgamation of composite terranes, terrane accretion, then destruction of the margin by the Cretaceous to early Tertiary. Plate realignment during the Tertiary yielded the northward transport of westernmost components and Basin and Range extension. This orogen, together with the Andes, served as the type “cordilleran” mountain chain in Dewey and Bird's early plate tectonic view of mountain chains that form on continental margins above subduction zones. It was subsequently recognized that most collisional mountain chains underwent a cordilleran phase prior to collision—how else would the ocean(s) separating the colliding elements be eliminated? The potential collision of Australia with Asia may constitute an end-member model for orogenic belts, involving a continent sweeping up a number of island arcs and terranes on its way to closing all of the oceanic realms between two colliding continents. The Cordillera may represent another end member without continent-continent collision. The Appalachians and Scandinavian Caledonides may represent intermediate products that involved fewer terranes, subduction of varied amounts of ocean crust along strike, and had a terminal event.