Evolution of the triangle zone in the Rocky Mountain Foothills near Coalspur, central Alberta

A triangle zone, which commonly occurs along the external margin of a foreland thrust and fold belt with a buried thrust front, is underlain by a subhorizontal, blind, foreland-verging thrust that ends against a foreland-dipping, hinterland-verging thrust. These contemporaneous thrusts, active towards the end of orogenesis, enclose an intercutaneous wedge that moved towards the foreland. During orogenesis, a triangle zone evolves through periodic replacement of faults bounding the active wedge. Replacements occur in cycles during each of which a lower fault tends to be replaced by one in a lower stratigraphic horizon, an upper fault by one farther away from the foreland. Each cycle ends with the lower fault moving to a younger horizon where it joins a new, more external upper fault.Near Coalspur, the triangle zone exposes the remnants of several wedges involving Upper Cretaceous and Paleocene molasse. Most of these wedges developed during the last cycle but one and have a combined displacement of about 5 km. Within this cycle, the younger the wedge, the older the strata at its extremity. The upper fault of each wedge cuts the lower fault of the preceding wedge. The upper fault of one of the wedges has a lateral ramp.