PREVIOUSLY UNRECOGNIZED REGIONALLY EXTENSIVE PRE-WINDERMERE SUPERGROUP FOLDING IN NORTHWESTERN CANADA

Parautochthonous Neoproterozoic strata of northwestern Canada preserve a record of episodic extension and deposition in intracratonic and extensional basins. A newly delineated folding event affecting Tonian strata in the western Northwest Territories adds complexity to the extensional record of the proto-Pacific margin. This folding event was identified from bedrock mapping data in the eastern Wernecke Mountains and Mackenzie Mountains combined with interpretation of reflection-seismic data within Mackenzie Plain and Franklin Mountains, an area approximately 300 km x 500 km. With a wavelength of approximately 100 km and amplitude of up to 5 km, the folding is expressed subtly beneath younger strata, typically as a low-angle unconformity where exposed in the Mackenzie Mountains. The strata involved in folding belong to the Coates Lake Group, the Mackenzie Mountains Supergroup and lateral equivalents (Hematite Creek Group and Shaler Supergroup), and underlying Meso- and Paleoproterozoic strata. Windermere Supergroup strata overlying the Coates Lake Group are not similarly folded, suggesting that folding occurred between deposition of the Coates Lake Group and deposition of the overlying Rapitan Group, supporting stratigraphic arguments that the Coates Lake Group be excluded from the Windermere Supergroup. Current dating of these units brackets the timing of folding between 732 Ma and 717 Ma. Localized smaller folds associated with minor west-directed thrusts affecting the same stratigraphic interval to the west in east-central Yukon have previously been assigned to an enigmatic event dubbed the Corn Creek orogeny. The recognition of regional-scale folding in western Northwest Territories suggests that this deformation event may have affected a radically larger area than was recognized previously. It also suggests that the interplay of extensional and contractional events just prior to the onset of Windermere Supergroup deposition was more complex than is suggested by existing models.