Sandbox and finite-element models have demonstrated that a uniform Coulomb layer that deforms under basal boundary conditions of asymmetric detachment and underthrusting of the underlying substrate develops as a structural fan. The fan has a broad pro-wedge of thrusts and folds that verge toward the "subducting plate" and a narrower retro-wedge of thrusts and folds that develop above the stationary "lithospheerec plate" . The Selkirk fan of the Canadian Cordillera is similar in structure, and we infer that it originated during Jurassic accretion of terranes to the western boundary of cratonic North America by processes analogous to those of the models. Palinspastic restoration of the Selkirk fan to the Jurassic places it within the present coast belt. By implication, the North American plate must have extended this far west in Early Jurassic time. Previously published interpretations of Lithoprobe seismic-reflection profiles support this interpretation and suggest that lower crust underlying the intermontane belt and the eastern edge of the coast belt is the relict of the pre-Jurassic North American plate and that the accreted terranes that currently overlap this plate boundary by up to 300 km are crustal slivers that were detached from their subducting lithosphere, deformed as in the models, and obducted onto the North American plate. This model satisfies geological and geophysical constraints; it suggests a mechanism of crustal deformation involving only minor shortening of the North American plate across the width of the orogen.

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