WEDGING THE FLOW! THE CRITICAL EVOLUTION OF AN OROGENIC CHANNEL IN THE SOUTHEASTERN CANADIAN CORDILLERA

The main characteristics of the Lower Selkirk allochthon in the metamorphic core-zone of the southeastern Canadian Cordillera were interpreted as the products of Late Cretaceous channel flow. A c. 15 km-thick panel of completely transposed, upper-amphibolite facies rocks, laden with melt and sheared leucogranite is bounded above and below by a normal-sense shear zone and by a thrust-sense shear zone, respectively. The latter, which contributed to the exhumation of the proposed channel to a depth of less than 15 km, is exposed foreland-ward as the Hellroar Creek shear zone. This reverse zone of high strain records a protracted history of synkinematic leucogranite emplacement and coincides, at least for some of its length, with a sillimanite-kyanite isograd. The footwall of this shear zone presents contrasting characteristics. Rocks are not transposed and preserve primary structures, sheared leucogranite and leucosome are much less abundant, there is a Late Cretaceous metamorphic field gradient, and the timing of metamorphism and deformation becomes younger structurally downward. These features are diagnostics of critical taper and were, therefore, brought forward against the channel flow model. A model involving the incorporation of the channel into the foreland wedge taper after it intruded the upper crust foreland-ward reconciles the controversy, and supports the view that “channel flow” and “critical taper” constitute end-members of a continuum rather than a dichotomy.