Northeastward extrusion and extensional exhumation of crystalline rocks of the Monashee complex, southeastern Canadian Cordillera

North American basement and cover rocks of the Monashee complex (MC) are exposed through a tectonic window within the hinterland of the southeastern Canadian Cordillera. The complex records a history of Late Cretaceous to early Tertiary crustal thickening (F1/F2) related to emplacement of the Selkirk allochthon (SA). Part of the MC and overlying SA then formed a mid-crustal wedge that was extruded towards the foreland from the overthickened hinterland (F3, F4). Finally, the complex was exhumed by Eocene ductile and brittle extensional deformation (D5). Rocks along the studied west flank of the MC are thoroughly transposed by F1/F2 (into S2). The extent of the transposition, and a well-developed northeast-trending L2 lineation, indicate intense strain during F1/F2 throughout the studied portion of the MC (4–5 km thickness exposed) and overlying SA. Ductile flow continued, resulting in northeast-verging F3 folds in the MC, west-southwest-verging F3 folds in the SA and broad F4 warps in the MC and SA. A shear zone records significant reactivated slip on S2 during D5. Rapid exhumation of the MC is attributed to ductile flow during extrusion and extensional deformation; this ductile flow is correlated with foreland thrusting, which ended in the Early Eocene.