A reappraisal of the tectonic significance of early Tertiary low-angle shear zones exposed in the Vernon map area (82 L), Shuswap metamorphic complex, southeastern Canadian Cordillera

Detailed geological mapping across the Shuswap metamorphic complex between latitudes 50 degrees 00'N and 50 degrees 45'N reveals that superstructure forms a semicontinuous carapace across the complex, with minimal evidence of internal thinning. Near the western margin of the complex, superstructure and infrastructure are juxtaposed across low-angle, similar to 2 km thick, ductile shear zones spatially associated with Paleocene to Early Eocene syn-kinematic granitic rocks. The shear zones, which yield upper plate to the west shear-sense indicators, are interpreted as the northern extension of the Okanagan Valley fault. Farther east, near the north-south axis of the complex, superstructure and infrastructure are separated by an attenuated metamorphic section, but evidence of noncoaxial strain is lacking. Discrete detachments were not found. Steeply dipping normal faults cut low-angle shear zones and do not merge with them at depth. Middle Eocene volcanic and sedimentary rocks rest Unconformably on metamorphic basement. The continuity of superstructure indicates that infrastructure was not exhumed by crustal-scale detachments. The results provide the basis for a complete reinterpretation of the tectonic significance of low-angle shear zones exposed in the Vernon area. It is proposed that Late Cretaceous to early Tertiary partial melting of the middle crust resulted in the development of a zone of channel flow. As the channel was underthrust by a crustal-scale ramp in underlying, more competent Paleoproterozoic basement, it was exhumed from depths of 20-30 km and thinned vertically. Shear zones between infrastructure and superstructure are interpreted as being a transient theological interface at the upper boundary of the channel.