OROGENIC PULSES IN THE CANADIAN ROCKY MOUNTAINS; RESULTS FROM REGIONAL DATING OF FAULT GOUGE
In the Main Ranges, the Pyramid thrust is 158.0 ± 2.3 Ma, coinciding with a well-defined unconformity and the transition from shale to sandstone deposition. This reflects initiation of thin-skinned deformation from Jurassic terrane accretion and thickening of supracrustal rocks. Ages from the Simpson Pass (155.4 ± 4.2 Ma) and Johnson Creek (148.6 ± 2.9 Ma) thrusts show that initial contraction continued with Late Jurassic deposition of Kootenay-Nikanassin-Minnes clastics. In the Front Ranges, the Greenock thrust is 103.9 ± 0.3 Ma, coinciding with a major unconformity in the immediate foreland; to the northwest, the Broadview/Snake Indian thrust is 97.2 ± 3.3 Ma, contemporaneous with development of Cenomanian deltaic deposits in the foreland. Three thrusts in the Front Ranges are 75.0 ± 2.3 Ma (Rocky Pass thrust), 72.4 ± 4.7 Ma (Sulphur Mountain thrust), and 74.2 ± 6.7 Ma (Clearwater thrust), overlapping with previously reported ages of ~72 Ma for thrusts to the south, and represent the shallow expression of the Late Cretaceous basal detachment of the belt. This Campanian pulse of tectonic loading led to the last major transgression in the southern portion of the Alberta foreland. Along the eastern margin of the Front Ranges and western Foothills, gouge samples from four major thrusts yield Paleocene to early Eocene ages. The McConnell thrust, which is the eastern boundary of the Rocky Mountains, is 53.6 ± 1.1 Ma, matching previous ages farther south. In the Foothills, from north to south, the Muskeg, Brule and Nikanassin thrusts are 61.9 ± 1.9 Ma, 56.6 ± 1.2 Ma, and. 53.2 ± 0.5Ma, respectively, recording the last phase of contraction.
Ages from regionally-distributed fault gouge show that the Canadian Rocky Mountains formed through a series of four, forward-propagating deformation pulses, Late Jurassic, Middle Cretaceous, Late Cretaceous and Paleocene-early Eocene, separated by relatively long periods (>>10 Ma) of tectonic quiescence.