Selkirk fan structure, southeastern Canadian Cordillera: Tectonic wedging against an inherited basement ramp

摘要

A revised cross section through the Selkirk fan structure provides the basis for a new model for the Middle Jurassic tectonic evolution of the southern Omineca belt, Canadian Cordillera. Palinspastic restoration of this cross section shows that the southwest-verging structures along the west flank of the Selkirk fan structure formed as a result of tectonic wedging of distal North American strata (Clachnacudainn complex) beneath more proximal North American strata, and that the Selkirk fan structure developed outboard from a crustal ramp (Dogtooth high) inherited from Late Proterozoic–early Paleozoic rifting along the western margin of North America. The first episode of Mesozoic deformation in southeastern British Columbia occurred between 187 and 173 Ma and involved the northeastward juxtaposition of the Intermontane superterrane over the outer part of the North American continental terrace wedge. It resulted in deep burial (20–25 km) of the outer margin of North America. A crustal ramp, localized along the western edge of the Late Proterozoic–early Paleozoic Dogtooth high, impeded the northeastward propagation of the orogenic wedge comprising the Intermontane superterrane and the imbricate, underlying northeast-verging thrust sheets of North American supracrustal rocks. Tectonic wedging, involving southwest-verging deformation, occurred within the orogenic wedge, and the resulting crustal thickening established sufficient topography and gravitational potential to drive the propagation of the deformation eastward into the Dogtooth Range and the Rocky Mountains. The southwest-verging structures along the west flank of the Selkirk fan developed between approximately 173 and 168 Ma concurrent with synorogenic extension and 10 km of exhumation. The initial subsidence of the foreland basin during Kimmeridgian time (ca. 154 Ma) provides the first indication of tectonic loading and lithospheric flexure of the North American plate. It is interpreted to mark the time at which the orogenic wedge overrode the crustal ramp of the Dogtooth high and advanced onto relatively thick and rigid continental lithosphere. The tectonic model proposed for the Selkirk fan structure illustrates how the configuration of the rifted margin influenced the style of crustal thickening during subsequent compressional deformation.