An evaluation of models for the kinematic evolution of thrust and fold belts: structural analysis of a transverse fault zone in the Front Ranges of the Canadian Rockies north of Banff, Alberta

The prevailing ‘piggyback’ conceptual model for the kinematics of ‘thin-skinned’ thrust and fold belts maintains that the main faults develop sequentially from the hinterland to the foreland, and from the top to the bottom of the accretionary wedge. Moreover, it presumes that when younger thrust faults originate, overlying older thrust faults become inactive and are carried forward passively. This appears to contradict the prevailing mechanical model for the evolution of ‘thin-skinned’ thrust and fold belts, the critical Coulomb wedge model, which requires that lateral growth of the wedge must be accompanied by vertical thickening of the wedge. Crosscutting relationships along a transverse fault zone in the Front Ranges of the Canadian Rockies north of Banff, Alberta, and patterns of overprinting of thrust-related folding on pre-existing thrust sheets, demonstrate substantial overlap in the times of displacement on four major thrust faults in this part of the Front Ranges. The presumption that displacement on one major thrust fault ends when displacement on a younger underlying thrust begins is a fallacy. There is no contradiction between the ‘piggyback’ conceptual kinematic model and the critical Coulomb wedge mechanical model for the evolution of ‘thin-skinned’ foreland thrust and fold belts. The main faults do originate sequentially from the hinterland to the foreland, and from the top to the bottom of the evolving wedge; but displacement occurs simultaneously on several major faults.