EVIDENCE OF TECTONIC INVERSION IN THE SOUTHERN CANADIAN ROCKIES: CONTRACTIONAL DEFORMATION SUPERIMPOSED ON EXTENSIONAL FAULTS REVEALED THROUGH FINE-SCALE MAPPING

The eastern Main Ranges of the southern Canadian Rocky Mountain thrust-and-fold belt include a network of normal faults (the result of apparent extensional episodes) that occur within a contractional orogen. The origin, timing, and nature of these normal faults remain unresolved. Many investigators have contended that the normal faults developed as a consequence of postcontractional extension that occurred west of the Rocky Mountain Trench during the Paleogene Period and that the the normal-sense dip-slip movement was facilitated by the utilization of pre-existing thrust faults. This model is consistent with the overall tectonic history of the cordillera; a convergent margin, that evolved into a transform system followed by phases of extension. However, a growing body of evidence shows that contractional episodes have deformed the normal faults located in the Main Ranges of the Canadian Rockies. The normal fault surfaces and adjacent strata appear to have undergone contractional deformation after the normal faults formed. The extensional episodes are interpreted to have developed prior to the Late Cretaceous contractional stage and may be as old as Late Devonian to Early Carboniferous periods. Reinterpreting the normal faults to an earlier age leads to new views of the history of the Cordillera, the nature of hydrothermal fluid flows and mineralization events. The preferred reinterpretation (Late Devonian to Early Carboniferous) posits that the network of normal faults found in the eastern Main Ranges may represent a preserved sample of structures found in the Cordillera that developed during the late Paleozoic time period. Deformation by tectonic inversion (extension followed by contraction) can manifest itself through a wide range of structural forms, especially where the mechanical stratigraphy is strongly anisotropic. I will take you on a tour along the Stephen-Cathedral normal fault, near the headwaters of the Yoho River, and show examples of structures that developed from tectonic inversion in the study area.