Crustal evolution of the Eastern Canadian Cordillera

Abstract

The Cordillera is not like the Himalayas. The Himalayas are supported by the Indian continent that was thrust under the Asian continent. The eastern Cordillera is supported not by imbricated continents but by imbricated continental margin of the ancient western North America. That ancient margin was compressed and imbricated as it was confined between the North American craton and the minicontinent of Stikinia. In the eastern Canadian Cordillera, tectonic thickening is in the collision zone and not on either side of it, as for example, in the Tibetan Plateau where the underthrust Indian continent has duplicated the thickness of the crust. In the eastern Canadian Cordillera, Jurassic and early Cretaceous thrusts and faults are directed away from the collision zone toward the converging blocks. The oppositely directed thrust faults outline wedge‐shaped sheets of rock. Sedimentary and volcanic rocks (cover), the crystalline rocks on which they were deposited (basement), and the underlying upper mantle may have been detached from each other during the compression. The basement therefore would have imbricated internally between the cover and the mantle as a duplex structure. Confinement of the ancient margin's basement between the colliding blocks requires that the volume of basement under the margin be the same as that presently between the colliding blocks. The possibility of such an equality has been tested and confirmed in cross section. The cross section of the ancient western margin from that test has many features comparable to cross sections recently drawn through the eastern continental margin of Canada.