The nature and significance of large ‘blind’ thrusts within the northern Rocky Mountains of Canada

Abstract

Summary The northern Canadian Rocky Mountains comprise a rugged, structurally complex Foothills subprovince of large amplitude box and chevron folds, and a structurally diverse Rocky Mountain subprovince in which large mappable thrusts are rare. The boundary between them is, in some regions, defined by the unfaulted E-dipping limbs of an en echelon sequence of large mountain-front anticlines. The lack of thrusts, especially along the mountain front, contrasts with the well exposed linearly continuous thrusts of the Front Ranges structural subprovince within the southern Rocky Mountains, and leads to the impression that little lateral displacement has occurred. Where deep cross-cutting valley erosion combines with increased fold plunge, it is apparent that the frontal anticlines are, in reality, large allochthonous sheets displaced many kilometres eastward relative to the craton on flat thrusts that separate Ordovician shales from underlying Devonian and Mississippian shales. The faults can be traced, in some places, eastward to the mountain front where they cut abruptly through the thick hanging wall successions of carbonate rocks; however, they cannot be mapped further eastward into surface exposures because they terminate within a décollement zone of Devonian and Mississippian shales, where the displacement on them is transformed into disharmonic folds and tectonic thickening of overlying units. The Devonian and Mississippian shale succession is interpreted as a fundamental décollement zone of regional extent that separates a lower structural level of thrustfaulted carbonate rocks from an upper structural level characterized by folded late Palaeozoic and Mesozoic units. The shortening represented by Foothills folds is interpreted to equal the amount of shortening on ‘blind’ thrusts beneath the western margin of the Foothills structural subprovince. A structural reinterpretation across the Muskwa Anticlinorium using the blind thrust interpretation demonstrates that the mountain-front Tuchodi Anticline may represent a large allochthonous thrust sheet folded over a large step in the blind thrust on which it was transported. The northern Rocky Moutains, narrower and less foreshortened than the southern Rocky Mountains, are interpreted as a thin-skinned tectonic regime similar to but orogenically less mature than the southern Rocky Mountains.