Abstract

Radiometric ages from regionally distributed thrust-fault gouge show that the eastward propagation of the southern Rocky Mountain fold-and-thrust belt in Canada occurred in four orogenic pulses that correlate with tectonic events of the Cordilleran interior and with depositional patterns in the adjacent foreland. In the Main Ranges, the Pyramid (163.0 Ma), Simpson Pass (161.7 Ma), and Johnson Creek (145.7 Ma) thrusts were related to the initiation of thin-skinned deformation from Jurassic terrane accretion and were partly contemporaneous with development of the first clastic wedge in the foreland basin. In the Front Ranges, the emplacement of the Greenock thrust (103.1 Ma) and Broadview–Snake Indian thrust (99.2 Ma) was contemporaneous with development of Cenomanian deltaic deposits in the immediate foreland. Three thrusts in the Front Ranges, Rocky Pass (74.8 Ma), Sulfur Mountain (75.6 Ma), and Clearwater (74.2 Ma) thrusts, define a Campanian phase of tectonic loading that led to the last major transgression in the southern portion of the Alberta foreland. Along the eastern margin of the Front Ranges, the McConnell thrust (54.0 Ma), together with the Muskeg (52.4 Ma), Brule (53.9 Ma), and Nikanassin (52.1 Ma) thrusts in the Foothills, recorded the last phase of regional contraction. The Late Jurassic, mid-Cretaceous, Late Cretaceous, and early Eocene deformation pulses are separated by relatively long periods (>40 m.y., >20 m.y., and >10 m.y.) of tectonic quiescence. These spatially and temporally restricted fault motion pulses contrast with gradual, forward fault propagation, while regional eastward progression of deformation is preserved.

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