JURASSIC TO EARLY EOCENE DEFORMATION PULSES IN THE NORTHERN ROCKIES FOLD-THRUST BELT; NEW EVIDENCE FROM 40Ar/39Ar DATING OF CLAY-RICH FAULT GOUGE IN THE ALBERTA SEGMENT

The Rocky Mountains Fold-Thrust Belt (FTB) of western Canada consists of a series of eastward propagating thrusts that transported miogeoclinal sediments during Mesozoic and early Tertiary deformation. In this study, we present new ages of clay-rich fault gouge from the Main Range, Front Range and Foothills of the Canadian Rockies that preserve a history of >100 my of deformation. Ages of faults were obtained by extrapolating results of four grain-size fractions with varying percentages of detrital (2M1) and authigenic (1Md) illite polytypes and their ⁴⁰Ar/³⁹Ar total gas ages. To the west, in the Main Range, the Jasper Thrust produces an age of 162.8 ± 4.9 Ma. This Mid-to-Late Jurassic age is the oldest faulting age preserved in the foreland, and likely represents the age of initiation of thin-skinned deformation in the Rockies. To the east, in the Front Range, the Greenock Thrust yields an age of 103.3 ± 3.1 Ma. This late Early Cretaceous age is consistent with a major pulse of deformation in the belt, associated with major deposition and subsidence in the foreland basin. In the east, the McConnell, Nikanassin, Brule and Muskeg thrusts, which lie along the eastern Front Range and western Foothills, yield ages around 54 Ma. These Early Eocene ages represent the last phase of contractional deformation in the Rockies fold-thrust belt in southern Canada. These new results, combined with recent results to the south (e.g., ~72 Ma ages of the Rundle thrust system), support the interpretation that the Rockies FTB in Canada and northern US formed through a series of forward propagating deformation pulses from the late Middle Jurassic to the Early Eocene.