Geology of the West Margin of the Shuswap Terrane near Sicamous Implications for Tertiary Extensional Tectonics

Recent field and geochronological investigations. in conjunction with LITHOPROBE seismic surveys, have led to the recognition of crust&scale, low to moderate-angle, Eocene extensional faults and shear ZOIIL‘S in the southern Omineca Belt. The present study focuses on documentation of the Eagle River fault, an extensional shear zone that delineates the western boundary of the Shuswap metamorphic terune (Figure l-6-l). The gcomctry and the tectonic history of the Eagle river fault place important constraints on crustal cmsssections through the southern Omineca Belt. The Shuswap termne consists of polydeformed rocks that have been metamorphosed in the sillimanite zone of the amphibolite facies. The western margin of the Shuswap terrane is a sharp metamorphic discontinuity, for which various interpretations have been offered. Jones (1959) mapped the boundary as a high-angle fatlt. Campbell (1971) documented closely spaced isograds, but no fault. Okulitch (1979) showed the boundary between rocks of contrasting metamorphic grade to be cut locally by high-angle faults. Fyson (1970) suggested that the high-grade rocks of the Shuswap terrane formed as a hot metamorphic infrastructure, separated from cold, brittly deformed upper-crustal rocks by a steep geothermal gradient. A detailed study of the structure and metamorphism in the Mara Lake area by Nielsen t 1978, 1982) seemed to support the hypothesis of a strep metamorphic gradient with no major structural break. These studies were completed before the concept of crustal-scale, low-angle normal faults. for example, Wemicke t 1981). had gained much attention. More recently. low-angle normal faults have been documented in the Columbia River valley (Read and Brown, 1981: Lane, 1984). the Okanagan Valley (Bardoux, 1985; Parkinson. 1985~1, l9RSb: Parrish en/., 1985;TemplemawKluit and Parkinson, 1986). and on both east and west flanks of the Valhalla Complex (Parrish, 1984; Car, 1986; Carret a/., 1987) (Figure I -6-l). Uranium-lead zircon ages of synkinematic granites (Solberg, 1976; Carr ef al., 1987; Parrish et al., 1988) have established Eocene histories of ductile shearing and subsequent brittle deformation in each of these fault zones. Potassium-argon mineral dates consistently record an early Tertiary thermal history in the high-grade footwall rocks, whereas the low-grade, brittly defomtcd hangingwall rocks preserve Mesozoic cooling ages (see Parrish cf al., 1988, for a summary). The present investigation was prompted by rccwmaiswnce onn i mapping hy Joumeay and Brown (1986) which led to the discovery of mylonites in the Eagle River valley shear Sicamous