Geometric test for Late Cretaceous-Paleogene intracontinental transform faulting in the Canadian Cordillera

Abstract

The Tintina trench-northern Rocky Mountain trench (TT-NRMT) fault zone and the Fraser River-Straight Creek (FR-SC) fault zone are separate, en echelon, concentric, small-circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the Canadian Cordillera, from the outboard part of a tectonic collage of accreted foreign terranes in the south into the North American preaccretionary continental margin in the north. Most of the 450 km of right-hand slip on the TT-NRMT fault zone was transformed southward during the Late Cretaceous and Paleocene into oblique convergence in the southern Canadian Rockies; the remainder, probably comprising less than 100 km, was transformed southwestward during early and middle Eocene time via a zone of distributed shear and east-west crustal stretching into right-hand slip on the en echelon FR-SC fault zone. These interpretations, based on regional systematic mapping of geologic structures, are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post-mid-Cretaceous, right-hand displacement along the general locus of the TT-NRMT fault zone, involving foreign terranes and the parts of North America to which they had been accreted. This paradox must be resolved.