Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic-Cretaceous orogenic structure

The eastward transition from penetrative ductile deformation, metamorphism, and plutonism in the hinterland of the southern Canadian Cordillera to the thin-skinned'' deformation of the foreland represents a significant change in tectonic style and process. The transition is also marked by a zone of structural divergence across which SW vergent hinterland structures pass into NE vergent foreland structures. The Selkirk fan within the Selkirk Mountains of the southern Canadian Cordillera is considered a type locality for this zone. This structure trends SE-NW for more than 120 km in greenschist to upper amphibolite facies rocks that have at least three generations of superposed deformation. The kinematic development of the Selkirk fan has been controversial, but its age generally has been inferred to be Middle to Late Jurassic. However, new U-Th-Pb dates obtained by ID-TIMS and SHRIMP indicate a more protracted history. The data demonstrate that a protofan developed in the Middle Jurassic (172-167 Ma) and that the west flank of the fan was exhumed to upper crustal levels at this time. Conversely, the eastern flank was progressively buried to > 25 km depth and pervasively overprinted by Cretaceous deformation (104-84 Ma) and Cretaceous-Paleocene metamorphism (144-56 Ma). A new tectonic model is proposed to reconcile the composite nature and protracted development (> 100 Ma) of the Selkirk fan within an orogenic system that evolved in response to periods of terrane accretion on the western margin of the North American craton.