Eastern Canadian Cordillera: Implications of a Balanced Basement

Abstract

The ancient margin of western Canada may have been more than 800 km wide and was compressed to 280 km by continent-“arc” collision. The proposed margin consisted of four horsts of North American craton (basement) bounding one volcanic and three sedimentary basins. The westernmost horst supported sediments and fragmental volcanics. During Jurassic and Cretaceous, the margin’s five sedimentary and volcanic basins were compressed into five thrust sheets that verge east and west and are everywhere detached from the basement. These five thrust sheets are the terranes of the Eastern Cordillera: the Quesnel, Slide Mountain, Selkirk, Cassiar, and Rocky Mountain terranes. The compressed basement of the ancient margin did not underthrust either the area west of the continent-“arc” suture zone or the area east of the eastern limit of the Rocky Mountain thrust belt, as both these areas remained low during the collision. The sialic basement presumably did not subduct into the mantle. Rather, on collision, it became shortened and thickened, possibly by thrust imbrication and flow, above a detachment on the upper mantle. The amount of basement before and after compression can therefore be balanced between structural and palinspastic cross-sections. The crystalline basement, therefore, has a roof and floor thrust and as such thickened as a duplex. Because the thrusts could verge in either direction with respect to the collision, the cover and basement have thickened in wedges. And, because the ancient margin’s cover remains over the ancient margin’s basement after the compression, the cover, in a broad sense, is para-autochthonous.