The southeastern Canadian Cordillera: Thrust faulting, tectonic wedging, and delamination of the lithosphere

Abstract

Displacement of tectonic wedges, bounded above and below by zones of thrust faulting with opposing vergence, has produced tectonic delamination at various stratigraphic levels and times in the Rocky Mountain fold and thrust belt of Canada. The ‘triangle zone’, along the eastern edge of the belt, comprises an easterly tapering wedge of imbricate thrust-slices and delaminated Upper Cretaceous molasse deposits. The Porcupine Creek anticlinorium is a fan fold resulting from the insertion of a wedge of Proterozoic rocks along a zone of delamination at the base of the Lower Paleozoic shale succession. In the Selkirk fan structure, which involves the metamorphic infrastructure of the Cordillera, delamination has occurred as the result of the insertion of an apparently allochthonous mass (a ‘suspect terrane’) between the detached parautochthonous supracrustal rocks and the autochthonous basement complex. The diagnostic feature of tectonic wedging and delamination is a reversal in vergence between the bottom and the top of the wedge. Conspicuous tectonic overprinting (‘polyphase folding’) characteristically occurs above the wedge, and is commonly described as ‘backfolding’ and ascribed to polyphase orogeny. Tectonic wedging and delamination are widepread in compressional fold belts of various ages. Examples occur in the Cordilleran tectonic collage of accreted terranes and in early Proterozoic (Wopmay Orogen), Paleozoic (Caledonide-Appalachian) and Cenozoic (Alps) mountain belts. Moreover, tectonic wedging and delamination occur over the whole spectrum of scales from the microscopic to the lithospheric. They are an inherent feature of ‘flake tectonics’ and of the development of large overthrusts such as crystalline basement sheets and obducted ophiolites, all of which comprise a strong upper layer that has been delaminated from the rest of one lithospheric plate, and has overlapped another lithospheric plate that has been wedged beneath it. Geometric and kinematic relationships discernible in well-documented natural prototypes of tectonic wedging and delamination on a small scale provide a basis for elucidating relationships that are more difficult to establish directly on a large scale.