COMPARISON OF THE SELKIRK FAN STRUCTURE WITH MECHANICAL MODELS - IMPLICATIONS FOR INTERPRETATION OF THE SOUTHERN CANADIAN CORDILLERA

Sandbox and finite-element models have demonstrated that a uniform Cou lomb 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 t hat verge toward the ''subducting plate'' and a narrower retro-wedge o f thrusts and folds that develop above the stationary ''lithospheric p late.'' The Selkirk fan of the Canadian Cordillera is similar in struc ture, and we infer that it originated during Jurassic accretion of ter ranes to the western boundary of cratonic North America by processes a nalogous to those of the models. Palinspastic restoration of the Selki rk fan to the Jurassic places it within the present coast belt. By imp lication, the North American plate must have extended this far west in Early Jurassic time. Previously published interpretations of Lithopro be seismic-reflection profiles support this interpretation and suggest that lower crust underlying the intermontane belt and the eastern edg e of the coast belt is the relict of the pre-Jurassic North American p late and that the accreted terranes that currently overlap this plate boundary by up to 300 km are crustal slivers that were detached from t heir subducting lithosphere, deformed as in the models, and obducted o nto the North American plate. This model satisfies geological and geop hysical constraints; it suggests a mechanism of crustal deformation in volving only minor shortening of the North American plate across the w idth of the orogen.