TESTING THE SYNCONVERGENT CHANNEL FLOW MODEL IN THE SOUTHEASTERN CANADIAN CORDILLERA

We present several lines of evidence to argue for the synconvergent ductile extrusion of a 10-15 km thick channel in the southeastern Canadian Cordillera. This process would have brought the mid-crustal channel to upper crustal levels (<15 km depth) in the Late-Cretaceous-Paleocene. Rocks of the proposed channel are part of what used to be called the "Shuswap Complex", but are herein referred to as the "Lower Selkirk allochthon" (LSa). Evidence includes: 1) a migmatitic channel (the LSa) that records protracted high-temperature metamorphism and deformation tens of million of years after the cessation of ductile deformation in the surrounding lower-grade rocks; 2) synchronous shearing of foreland-verging, and hinterland-verging shear zones bounding the base and roof of the channel respectively; 3) a trend of cooling ages systematically decreasing from ~90 Ma at the exposed front of the channel to ~55 Ma at the rear, where the shear zones are exposed; 4) P-T-t paths indicating that burial of the underlying rocks of the Monashee Complex to >35 km depth was synchronous with transport of the LSa to upper structural levels.

A serious challenge to the channel flow model is the absence of a mapped shear zone at the front of the proposed channel. Rather, the boundary between lower-amphibolite facies and migmatitic rocks of the proposed channel appears to pass within a stratigraphic sequence that is systematically repeated by three generations of folds that are in turn overprinted by a series of metamorphic isograds.

New mapping in the frontal area of the proposed channel reveals a major melt-laden, foreland-verging shear zone that separates hanging wall rocks of the LSa from footwall rocks with hinterland-verging structures, which are typical Jurassic to Early Cretaceous structures in the area. This shear zone cuts through the apparently continuous folded sequence and is interpreted as the base of the proposed channel. The channel is domed above the northern Monashee Complex such that the newly discovered shear zone is interpreted as the frontal expression of the Monashee décollement, which bounds the base of the channel at its rear. We thus conclude that channel flow and ductile extrusion best explain the tectonic evolution of the LSa and were major tectonic processes in the southeastern Canadian Cordillera.