KINEMATIC EVOLUTION OF THE FRONT RANGES OF THE CANADIAN ROCKIES INCLUDING THE ROLE OF FAULT PROPAGATION FOLD BREAKTHROUGHS AND MULTIPLE DETACHMENTS IN DEFINING THE THRUSTING SEQUENCE

Over the past 50 years, some of the most significant advances in our knowledge of thin-skinned fold and thrust belts have come from field observations in the Front Ranges of the Canadian Rockies and other well-exposed areas of the North American Cordillera. These field data constrain geologic cross-sections that illustrate structures that form the framework of fold and thrust belts, including detachments, thrust ramps, and fault-related folds (e.g., Bailey et al., 1966; Burchfiel and Davis, 1972; Suppe, 1980; and Price and Fermour, 1985). Over the past several decades, remote sensing methods have substantially evolved, and characterize fold and thrust belt geology in unprecedented detail.

The Canadian Rockies are generally interpreted as a break-forward imbricate system involving a single basal detachment (Price and Fermour, 1985). Utilizing the richness of these remote datasets, we challenge and refine elements of this interpretation of the Front Ranges. The frontal thrust in a typical break-forward thin-skinned fold and thrust belt will generally have less slip than older thrust sheets in the interior of the mountain belt (Moore et al., 1990). In contrast, the McConnell Thrust is the frontal thrust in the Front Ranges and has the greatest amount of slip (~35 km of slip, Price and Mountjoy, 1970). To explain this observation, we explore alternative models for the timing and kinematics of the McConnell thrust sheet that involve multiple basal detachments. These datasets also illustrate that several thrust sheets in the Front Ranges reach upper detachments, which further indicate that this is a multiple detachment system. Our analysis suggests that individual thrust sheets exhibit a complex history of fault propagation folding with subsequent breakthroughs. These breakthrough faults are out-of-sequence thrusts that have significant slip and complicate the patterns of structural imbrication.

We present a set of kinematically balanced cross-sections and models that show that the Front Ranges are a multiple detachment system that evolved with a complex sequence of break forward and break backward imbricate thrusting. By kinematically restoring these sections, we present a new interpretation of the paleo-stratigraphic architecture of the passive margin of western North America that pre-dated orogenesis.