Structural, geochronological, and metamorphic discontinuities across the Monashee décollement, southern British Columbia, Canada

The Monashee décollement (MD) is a crustal-scale shear zone within the southern Omineca belt of the Canadian Cordillera. This structure separates autochthonous Precambrian basement rocks and cover sequence of the Monashee complex in the footwall from allochthonous rocks of the Lower Selkirk allochthon in the hanging wall. The MD has been variably interpreted to be a major reverse-sense (top-to-the-east) contractional shear zone, potentially related to the basal detachment of the Rocky Mountain fold and thrust belt; a normal-sense (top-to-the-west) extensional shear zone; or the base of a mid-crustal ductile “channel.” Investigation of an alpine exposure of the MD at Probity Peak, ~50 km northwest of Revelstoke, British Columbia, Canada, presented herein, utilizes geological field mapping, microstructural analyses, quartz c-axis distributions; titanium-in-biotite (Ti-in-Bt) geothermometry, and titanite and monazite petrochronology to quantify the movement history in proximity to the structure. The data outline a structurally complex and protracted tectonometamorphic history in the vicinity of the MD at Probity Peak. Dominantly reverse-sense contractional movement along three distinct structures (the MD, TD1 – previously unrecognized, TD2 – the Anstey Range fault) initiated ca. 90 Ma and concluded before 51 Ma, after which extension-related movement along steep crosscutting normal faults continued until at least ca. 49 Ma. The kinematics of the thrust-sense MD and the associated, structurally lower discontinuities, mirror that noted in the Himalaya and other orogens wherein convergence in the exhuming mid-crust is accommodated by thrust stacking that drives footwall metamorphism.