THE SOUTHERN TERMINATION OF THE WESTERN RANGES AND MAIN RANGES OF THE SOUTHERN CANADIAN ROCKY MOUNTAINS: TECTONIC AND GEOCHRONOLOGIC IMPLICATIONS OF NEW STRATIGRAPHIC AND STRUCTURAL DATA AND DETAILED MAPPING

The Western Ranges and Main Ranges subprovinces of the Southern Canadian Rocky Mountains terminate abruptly at the Crowsnest Pass Cross-Strike Discontinuity (CPCSD). The CPCSD coincides with a 230 km dextral offset in the eastern margin of the Cordilleran miogeocline. It is marked by enigmatic transverse faults, by profound along-strike changes in Paleozoic stratigraphy and in the orientation of structures, and by the local eastward encroachment of post-folding, mid-Cretaceous granitic plutons into the Rocky Mountains. The two subprovinces comprise separate tectonostratigraphic domains, each with a distinctive succession of lower Paleozoic miogeoclinal strata that unconformably overlie Mesoproterozoic strata of the Purcell Supergroup. The Hughes Range domain (Western Ranges) features a thin, condensed sequence of Lower Paleozoic miogeoclinal strata that evidently was deposited on the eastern flank of the Windermere paleo-topographic high. The Tanglefoot domain (Main Ranges) contains a very thick shaly Lower Paleozoic miogeoclinal succession, that includes intercalated volcanic rocks. It was deposited in a deep basin (the White River trough) along the eastern margin of the Cordilleran miogeocline. The abrupt stratigraphic contrasts between the two domains are indicative of large thrust displacement on the intervening Lussier River-St. Mary fault.

Small, mid-Cretaceous, monzonitic plutons crosscut and thermally overprint the Lussier River fault and the east-verging thrust faults in the eastern flank of the Purcell Anticlinorium, and also both an earlier east-verging thrust and later west-verging folds and back-thrusts in the western flank of the Porcupine Creek fan structure. ⁴⁰Ar/³⁹Ar age-spectra from the crosscutting stocks show that thrusting and folding within this segment of the CPCSD occurred prior to 108 Ma. Displacement on the Borgeau thrust fault, the nearest major thrust fault that extends under this region from the east, appears to have occurred after the deposition of late Cenomanian to ? Coniacian (95 to ? 85 Ma) marine shales of Alberta Group that are preserved in the Fernie synclinorium, in the footwall of the Bourgeau thrust. This implies a significant hiatus (>13 Ma) between the thrusting and folding in the Western and Main Ranges and displacement on the Bourgeau thrust.