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The Canadian Rockies are located between the Rocky Mountain trench on the west and the edge of the disturbed belt on the east; to the north they plunge out near the Yukon-British Columbia boundary and to the south they extend approximately halfway through Montana. The dimensions are 1,065 miles in length and an average width of 80 miles. Structurally, and thus scenically, they are unique as compared to the Mackenzie Mountains to the north and the central and southern Rockies to the south; this striking difference is principally due to an origin of extreme shortening by means of a series of flat, superimposed thrust faults as opposed to an origin dominated by vertical uplift both to the north and to the south.
The age of the Rocky Mountains has been determined as Eocene-Oligocene on the basis of very extensive studies of the derived sediments. By comparison, the age of the plutonization of the Western Cordillera is principally Jurassic-Cretaceous transition on the basis of recorded geological relationships or 100 ± 10 m. y. on the basis of extensive radioactive dating.
The Rockies are made up of shelf sediments aggregating 20,000 feet at their eastern edge; by contrast, the Western Cordillera is typified by extensive plutonization of the thick sediments and volcanics of a eugeosyncline.
Shortening of the shelf sediments across the southern part of the Canadian Rockies is probably in excess of 100 miles, which has been accomplished by stacking of sediments on a rather uniform system of superimposed thrust faults, but without disrupting the underlying shield to any known extent. The restoration of these sediments to their pre-Laramide position requires that the adjacent plutonized complex of the Western Cordillera must also be restored a somewhat similar distance to the west. Such a restoration sets back the indented western continental margin of Canada and the Alaska panhandle and puts it into alignment with the western continental margin of the United States. The realization of such differential movement along the western continental margin of North America in the Te tiary and the attendant tensional junctions explains many anomalous conditions in the Northwestern States and southern Alaska. The cause of such differential movement in the Tertiary is much more speculative. An acceptable explanation appears to be that the rigid, simatic Pacific plate has underthrust the continental margin of the United States, whereas it has pushed the continental margin of Canada ahead of it.
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