Lewis Overthrust

The Canadian Rocky Mountain foreland thrust and fold belt is a northeastward tapering deformational belt consisting of Mesoproterozoic, Paleozoic, and Mesozoic strata. The Lewis thrust sheet is one of the major structures of the foreland thrust and fold belt extending over 450 km from Mount Kidd near Calgary, AB in the Southeast Canadian Cordillera to Steamboat Mountain, located west of Great Falls, Northwest Montana in the United States. The Lewis overthrust provides scientific insight into geologic processes happening in other parts of the world, like the Andes and the Himalaya Mountains.

 The onset of Cordilleran orogenesis began in the Middle Jurassic time, as a result of the breakup of Pangea and North American plate motion toward subduction zones at the western margin. Most of the Canadian Cordillera today consists of numerous tectonostratigraphic terranes that were accreted to the stable margin of North America from the Jurassic to Early Tertiary as a result of eastward and northward drifting island arcs that collided with the continental lithosphere of North America. These terranes were accreted due to upper-crustal rocks being detached from the denser lower-crustal and proto-Pacific upper mantle lithosphere that was subducted under the North American craton. The allochthonous upper crustal terranes were juxtaposed over top of each other and over the western margin of the North American craton along a system of interconnected, northeast and southwest verging major thrust faults.

The onset of deformation of the Rocky Mountain fold and thrust belt was due to collisional tectonic forces that occurred on the west edge of the North American craton. This thrust and fold belt was uplifted east of the Canadian Cordillera and formed between the Middle Jurassic and Early Eocene within an easterly tapering wedge of Mesoproterozoic to early Cenozioc sedimentary rocks that were deposited in the Western Canada sedimentary basin. A profound unconformity separates the sedimentary cover from the Archean to Paleoproterozioc 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 200 km of horizontal shortening near the Canada and US border, and about 70 km 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 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.

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