KINEMATIC EVOLUTION OF THE OKANAGAN VALLEY DETACHMENT FAULT NEAR OLIVER, BRITISH COLUMBIA

The Okanagan Valley Fault (OVF) is an Eocene, low-angle, crustal-scale extensional fault that stretches more than 400 km from the northern USA through southern British Columbia (BC). Field mapping techniques and microstructural analysis are being used to investigate the heterogeneous high strain zone that is in excess of 1 km thick below the brittle detachment fault in the Green Lake region north of Oliver, BC. Preliminary mapping results of this metamorphic core complex reveal that it can be subdivided into two different metamorphic and structural zones. The first zone is an ultramylonite proximal to the OVF that is tightly folded around steep folds. The host rocks are melanocratic (chlorite- and biotite-rich) to leucocratic (quartz- and feldspar-rich) amphibolite facies schists showing significant grain-size reduction and abundant shear sense indicators. The second zone is a classic mylonite that occurs further away from the OVF (i.e., deeper) and consists mainly of igneous protoliths (orthogneiss) with lesser amounts of sedimentary protoliths (paragneiss). These mostly garnet-bearing orthogneisses are typically very coarse grained with mineral composition and abundance varying strongly depending on location. There is evidence for at least three deformation events. Deformation is not homogeneous as the main upright folding event is only observed within the mylonite zone, which indicates that subsequent deformation occurred after intrusion of another igneous protolith, responsible for a later stage orthogneiss. A recumbent fold fabric verging to the southeast overprints the entire area and has been weakly deformed by a large-scale shallow, open fold fabric (i.e., the majority of the compositional banding is sub-horizontal). This fabric is also observed in felsic veins throughout the region. Large-scale fault structures run parallel to the shallowly dipping footwall of the north/south-striking detachment fault and they are either syn-extensional or erosional features. Microstructural work will aid in better constraining evolution of the OVF and the deformation mechanisms associated with fault movement.