Lewis Overthrust

Geologic thrust fault of the Rocky Mountains
The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains found within the bordering national parks of Glacier in Montana, United States and Waterton Lakes in Alberta, Canada. The structure was created due to the collision of tectonic plates about 59 75 million years ago that drove a several mile thick wedge of Precambrian rock 50 mile (80 km) eastwards, causing it to overlie softer Cretaceous age rock that is 1300 to 1400 million years younger. The décollement can be seen from Marias Pass as a thin tan line of rock roughly halfway up Summit Mountain and Little Dog Mountain.

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 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 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 Cenozoic sedimentary rocks that were deposited in the Western Canada sedimentary basin. 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. The thrust sheet is constrained by lateral ramps on either side. In the south this occurs near Marias Pass, Montana, where the ramp geometry is parallel to the direction of sheet movement. Similarly, the thick succession of Mesoproterozoic strata consisting of the Belt Purcell supergroup followed the same sequence of events leading to the structural culmination seen in the southern end of the Purcell anticlinorium. In addition to duplexes seen in windows, the Lewis thrust also shows isolated remnants of the eastern edge of the upper plate( klippes) located at Chief Mountain in Montana and Crowsnest Mountain in Alberta. Erosion over time has shaped the mountains into their characteristic shape, where they tower over the associated prairies.

The fault motion of the Lewis thrust is dated based on the oldest age for motion being defined by the youngest sediments on the footwall, which are said to be about 65 million years old. Fission track analysis of Uranium bearing minerals such as Zircons and Apatite which involves dating the radioactive uranium found in sediments along the Lewis thrust using isotopic ratios of Uranium provides constraints of the late pre deformational paleogeothermal gradient and thickness of the Lewis sheet. This data, after being calibrated into geologic ages, led to the conclusion that maximum burial and heating in the Lewis thrust occurred during the Campanian over a time interval of less than 15 million years prior to the start of movement of the thrust sheet. The Apatite fission track data showed abrupt change in paleotemperatures from high to low temperatures and associated changes in uranium concentrations as burial and heating came to a halt and movement and exhumation began, which showed that displacement of Mesoproterozoic strata of the Belt–Purcell Supergroup along the Lewis thrust fault was in motion by approximately 75 Ma. In addition, the same faults in Montana that cut a volcanic marker were also found to be cut by 59 Ma porphyry dikes. This shows solid agreement with the evolution of the Canadian Rocky mountain foreland thrust and fold belt, including the Lewis thrust sheet which has been interpreted to have developed and commenced movement in pulses.