Protracted (>100 m.y.) deep crustal orogenesis revealed by in situ monazite petrochronology in the Shuswap Metamorphic Complex, British Columbia, Canada

Abstract

The Shuswap Metamorphic Complex in southeastern British Columbia, Canada, exposes penetratively deformed rocks exhumed from deep crustal levels (∼25−30 km depth) of the Canadian Cordillera. Existing models describing its tectonometamorphic evolution are not directly linked to absolute age constraints acquired through modern petrochronological methods and, therefore, remain ambiguous. To differentiate between proposed models, here we apply U-Th-Pb monazite petrochronology, petrological modeling, and microstructural analysis to quantify conditions and timing of deformation across a transect of the Shuswap Metamorphic Complex. Our results demonstrate that metamorphism decreases in age with increased structural depth associated with progressive localization of NE-directed shearing toward the base of the Shuswap Metamorphic Complex. Monazite U-Th-Pb age data from the structural level exposed in the study area are consistent with protracted northeast-directed compression from ca. 167 Ma to ca. 59 Ma, after which west-directed ductile extension continued until at least ca. 49 Ma and was progressively localized along the Okanagan Valley fault system. New data presented are consistent with a model of basal accretion in front of a foreland-propagating ductile thrust system, followed by exhumation of the Shuswap Metamorphic Complex facilitated in large part by crustal-scale extension.