the sedimentary cover from the Archean to Paleoproterozoic crystalline crust of North America. This thrust and fold belt has a thin skinned geometry as indicated by the array of thrust faults that interleave and overlap along strike and cut across strata at low to moderate angle that flatten with depth, repeat the same Cambrian to Triassic stratigraphy from thrust sheet to thrust sheet, and merge into a common basal décollement, the Rocky Mountain basal décollement. The Rocky Mountain thrust and fold belt propagated from west to east, accommodating up to of horizontal shortening near the Canada and US border, and about in northern parts of BC and Montana. The eastern boundary of the fold and thrust belt is marked by the easternmost deformed strata known in outcrop and or in the subsurface. Because strata underlying the Alberta plains is gently dipping, it is difficult to pinpoint the edge of deformation on this side of the belt. On the west side, the Rocky Mountains are bounded by the Rocky Mountain Trench, where the trench is interpreted to overlie the western, down-dropped blocks of major normal faults that separate the southern Rocky Mountains from the Purcell mountains. Horizontal shortening of the thin-skinned sediments lying above the detachment fault due to tectonic convergence must accommodate this horizontal shortening and has done so by the formation of major thrust faults with large displacement, the largest of which is the Lewis Thrust. The thrust sheets involved in the Canadian Rocky Mountain foreland thrust and fold belt consist of different aged strata indicative of significant deformation over time. The dominant structure of the deformational belt is a series of thrust faults, which are mostly listric and north-easterly or easterly verging. These thrust faults follow long bedding parallel detachments separated by ramps. As a result, a series of overlying thrust sheets is produced that follow their associated fault detachments. In addition, there is a westward dipping basal detachment that extends into the Cordilleran metamorphic core at mid-crustal levels. Strata from differing depositional environments is thought to have been scraped off of the under-riding North American craton and accreted to the over-riding Intermontane terrane during the Late Jurassic to Paleocene convergence of tectonic plates. Studies and modern dating have found that eastward propagation of thrusting took place in four distinct pulses that are separated by relative tectonic quiescence. 40Ar/39Ar dates indicate that these pulses occurred in the Late