A Deep-seated Gravitational Slope Deformation in the northern Monashee Complex, Monashee Mountains, British Columbia, Canada

The Blais Creek area is part of the Monashee Complex. The Monashee complex lies in the southern Omineca Belt, the metamorphic and plutonic hinterland to the Canadian Cordillera that developed during the collision between accreted terranes and the North American craton (Crowley, 1997; Monger et al., 1982). The complex comprises basement gneiss and overlying metasedimentary and metaigneous rocks (the cover sequence), which are exposed in a tectonic window beneath the Monashee decollement (Crowley, 1997; Read and Brown, 1981; Brown et al., 1986; Journeay, 1986), a ductile crustal scale thrust fault correlated with the sole thrust of the Rocky Mountain Foreland Belt (Crowley, 1997; Brown et al., 1992).
Basement gneiss (“core gneisses”) forms the core of the two structural culminations in the Monashee complex, namely Frenchman Cap dome in the North (Crowley, 1997; Wheeler, 1965; Hoy and Brown, 1980; Journeay, 1986) and Thor-Odin dome in the south (Crowley, 1997; Reesor and Moore, 1971; Read, 1980). The “basement assemblage” comprises amphibolitic gneiss, Paleoproterozoic migmatitic paragneiss, amphibolite, minor pelitic and semi-pelitic schist and leucogranite (figure 1.12). The “basement assemblage” contains also granitic rocks that crystallized at around 2077 Ma and 1862 Ma, and tectonite fabrics that developed before 1848 Ma (Price and Monger, 2003). Frenchman Cap basement is also dominated by kilometre-wide granitic orthogneiss bodies dated at ~ 2.27 – 2.07 Ga (Crowley, 1997; Armstrong et al., 1991).
Monzogranitic and granodioritic orthogneiss in Frenchman Cap dome at 1.86 Ga are also considered to be basement because rocks of this age only exist beneath the cover sequence. The paragneiss is considered to be basement because it lies beneath the cover sequence, in a relationship interpreted as unconformable (Crowley, 1997; McMillan, 1973; Journeay, 1986), and because it is thought to have been intruded by the basement orthogneiss (Crowley, 1997; Journeay, 1986). Zircon, monazite and titanite U-Pb data indicate that high-grade tectonism in Monashee basement occurred at ~2.06 Ga and during Cordilleran orogenesis in the early Tertiary (Crowley, 1997).