The Monashee decollement of the southern Canadian Cordillera: a crustal-scale shear zone linking the Rocky Mountain Foreland belt to lower crust beneath accreted terranes

Abstract

The Monashee decollement, a crustal-scale shear zone in the hinterland (Omineca belt) of the southern Canadian Cordillera is interpreted as correlating with the sole thrust of the Rocky Mountain Foreland belt. LITHOPROBE seismic reflection data indicate that the shear zone is rooted in the lower crust beneath the Intermontane Superterrane. The decollement is exposed on the margins of the Monashee complex, an elongate domal culmination within the Omineca belt. This complex includes Early Proterozoic gneisses of North American basement. The hangingwall of the decollement consists of deformed and metamorphosed North American continental margin and oceanic accreted terranes of the Selkirk allochthon. Layered reflections observed in LITHOPROBE seismic reflection data correlate with sheared and transposed rocks in both the hangingwall and the footwall of the Monashee decollement; the decollement is imaged as reflections, cutoffs and correlatable reflection segments. Field relations, kinematics and geochronology indicate that the shear zone has experienced a complex history that included protracted easterly directed shear of hangingwall rocks and imbrication of basement rocks in its footwall. U-Pb zircon dating of syn- and post-kinematic leucogranites in the decollement zone demonstrates that the final stages of thrusting occurred in the Late Palaeocene. The cessation of thrusting and the onset of crustal extension in the southern Omineca belt correspond to the end of thrusting in the Foreland belt.