THE ROLE OF LARGE-SCALE OUT-OF-SEQUENCE THRUSTS AND MULTIPLE DETACHMENT LEVELS IN CONSTRAINING THE SEQUENCE OF THRUSTING AND TECTONIC DEVELOPMENT OF THE FRONT RANGES OF THE CANADIAN ROCKIES

In the easternmost physiographic belt of the North American Cordillera, at the 51st parallel, lie the Front Ranges of the Southern Canadian Rockies. This region is considered a prototypical thin-skinned fold-and-thrust belt supporting the longstanding paradigm of break-forward thrusting involving a single basal detachment. With the Front Ranges acting as a template for our understanding of thin-skinned fold and thrust belts around the world, this model of in-sequence thrusting has become the traditional view of analogous fold and thrust belts around the world.

We present a series of kinematically balanced cross-sections and 3D structural models that challenge this traditional view by defining multiple basal detachments and large-displacement out-of-sequence thrusts. We inform our analysis by refined mapping of structures in the region using high-resolution multispectral satellite imagery and digital elevation models. We employ a semi-automated method to define more than 15,000 measurements of fault and bedding attitudes and use these to guide cross-section and model development using quantitative fault-related folding theories. Our mapping and interpretation of the structures in the Bow River and Kananaskis River Valleys documents the significance of multiple basal detachments and out-of-sequence thrusts in the kinematic development of the Front Ranges.

High-displacement out-of-sequence thrusts occur as imbricated, breakthrough fault-propagation folds that are prevalent throughout our study area and played a pivotal role in the tectonic development of the foreland fold-and-thrust belt. When we place these out-of-sequence thrusts into a regional context, we better constrain regional shortening estimates and tie together previous hypotheses of fault timing based on illite age analysis, foreland deposits, and tear fault geometry. These models imply episodic thrusting in the foreland belt and, at the regional scale, aid our knowledge of the kinematic development of the plate margin.