INDICATIONS FOR CHANNEL FLOW IN THE OMINECA BELT IN SOUTHEASTERN BRITISH COLUMBIA

Cordilleran deformation and metamorphism in the crystalline Omineca Belt in SE British Columbia have been interpreted as being a result of accretion of the Intermontane and Insular superterranes to the west. Their accretion to the North American craton occurred in the Middle Jurassic and mid-Cretaceous, respectively. Models involving large scale crustal detachments have been proposed to accommodate the shortening within the Omineca Belt. However, parts of the southern Canadian Omineca Belt, such as the Monashee Complex and the overlying Selkirk Allochthon, show penetrative ductile deformation instead of discrete large-scale detachments. Structural, metamorphic and geochronological data in and around this area are shown to be consistent with a channel flow model, which involves mid- to lower-crustal lateral ductile extrusion of material towards the foreland. The channel flow model explains the penetrative ductile structures related to Cordilleran deformation better than models involving discrete large-scale detachments. Four general structural levels can be distinguished in the region, based on the orientation and vergence of folds. In the lowest three levels folds are mostly recumbent, whereas in the uppermost level they are upright. The lowest three levels are interpreted as a channel flow zone. NE-verging folds of the lowest level (e.g. the Monashee Complex) formed during top-to-the-NE detachment flow and/or in the lower part of a channel flow zone. When detachment flow changed to channel flow, the sense of shear changed in the upper part of the channel flow zone, resulting in overprinting of NE-verging folds by SW-verging folds (e.g. most parts of the Shuswap Metamorphic Complex to the west of the Monashee Complex). Temperature was probably increasing, weakening a progressively larger portion of the crust, and the crustal shear zone therefore widened. Thus, in the highest structural levels within the channel flow zone, SW-verging folds developed in areas where no NE-verging folds originally formed (e.g. the Cariboo Mountains). The channel flow model is compatible with many of the ductile structures and accommodates existing metamorphic and geochronological data.