The canadian cordillera: testing contrasting models of orogenesis

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Plate tectonics provides an explanation for mountain systems: the collision of continents along a convergent margin. The Himalayan-Alpine chain stems from the attempted subduction of Africa, in the west, and India, in the east, beneath Europe-Asia. Explaining the Rocky Mountains of the Canadian Cordillera, is more difficult. Deformation occurred 80 to 50 million years ago. But ancient North American continental crust extends west to the coast, and there was no collision at 80 Ma. Hence the Rocky Mountains are thought to have formed 2000 kms inboard of a plate margin, and in the absence of a continental collision. The Cordilleran crust sits beneath thrust sheets of oceanic character. Accretion of these "terranes" was thought to explain the Rockies. However, terrane accretion ended by 200 Ma, >100 Ma before the Rockies formed. How do you explain an 80 Ma mountain belt as the result of a 200 Ma collision?