Obduction, backfolding and piggyback thrusting in the metamorphic hinterland of the southeastern Canadian Cordillera

Abstract The (paleo-)continental margin of the southeastern Canadian Cordillera was deformed and metamorphosed mainly in the Jurassic and Cretaceous, beginning with the obduction of a composite oceanic terrane, eastwards at least 60 km, onto the western part of the continental margin in the late Early Jurassic. In the Middle Jurassic, W-verging backthrusts and backfolds developed in this region, the folds becoming increasingly overturned and attenuated at deep structural levels. This deformation of the hinterland produced major crustal thickening and regional metamorphism, which peaked in the late Middle Jurassic. The thickening mass of the hinterland curtailed westward vergence. The focus of crustal shortening stepped to the east in the form of E-verging piggyback thrusting, which carried the deformed hinterland eastwards. Displacements propagated from the hinterland into the foreland on major detachments and thrusts. The initial shear zone, the Monashee decollement, decoupled the cover from its basement and accounted for at least 80 km of shortening by the middle Late Jurassic. Continued thrusting on lower shear zones that rooted at the base of the crust, led to the development of a basement duplex and major uplift of the hinterland. We present two balanced cross-sections; each based on available structural, petrologic, geophysical and geochronologic data. Both sections are internally consistent, and demonstrate that development of a basement duplex beneath the hinterland could have accommodated contemporaneous thin-skinned shortening of the Rocky Mountain Foreland.