Lewis Overthrust
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 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.
Geography
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 , 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.Geology and tectonics
Tectonic initiation and the Canadian Cordillera
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.Rise of the Rockies and formation of the Rocky Mountain Thrust and Fold Belt
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. A profound unconformity separates 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 Jurassic, middle Cretaceous, Late Cretaceous, and late Paleocene- early Eocene, separated by quiescent periods of >40 Ma, >20 Ma, and >10 Ma respectively.
The Rockies were uplifted during the Laramide Orogeny which occurred between 80 and 55 million years ago during the Late Cretaceous to the Early Paleocene as a result of subduction of the Kula and Farallon plates beneath the North American continent. Furthermore, the first radiometric ages obtained from direct dating of thrust-fault gouge from the front ranges of the southern Canadian Rockies identified two distinct deformation episodes named the "Rundle pulse" and the "McConnell pulse". These pulses were dated and are interpreted to have occurred at 72 Ma and 52 Ma respectively.
Lewis thrust system
The Lewis thrust Is a low-angle thrust fault in which Precambrian sediments have been thrust over younger Cretaceous sediments. 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. In the north, the thrust sheet is forced up and over an oblique ramp near the Kootenay Pass region in British Columbia. The overall shape of the thrust sheet as it moved north-eastward has a general convex shape towards the foreland.The Lewis sheet is carried by the Lewis thrust fault where the compression and thrusting was associated with oblique, right-hand convergence between the Intermontane terrane and the North American craton. This transpression in the Late Cretaceous led to the tectonic inversion of the Cordilleran miogeocline and the Belt-Purcell basin as the Lewis sheet began to buckle and fold, where strata was then overturned until a break or fault was formed. This involved thick successions of Paleozoic rocks that make up the Corilleran miogeocline and the underlying Neoproterozoic rocks to become detached from the crystalline basement; displaced up the passive-margin ramp along which they had accumulated; and juxtaposed over the flat surface of the North American craton to form the structural culmination that defines the Main Ranges of the Canadian Rockies. 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.
The Lewis thrust is cut by two major extensional fault systems, the Flathead fault and the Rocky Mountain trench fault system. Both of which are late Eocene to Miocene in age. However, the amount of shortening that has taken place on the thrust is not connected to Eocene extension due to the Rocky Mountain trench fault system and the Flathead fault having no influence positionally on the footwall and hanging wall cutoffs of the Lewis thrust. Instead, this transpression was replaced with transtension in the early Eocene involving east–west crustal extension and tectonic exhumation, which brought up mid-crustal metamorphic rocks to the surface to be exposed. Additionally, this transition from transpression to transtension resulted in rapid cooling of the metamorphic core complexes as they were exhumed and brought to the surface. Dextral transtension on intracontinental strike-slip faults in northeastern and southwestern British Columbia culminated with the mid Eocene extensional exhumation of midcrustal metamorphic core complexes. This leads to the basal décollement being exposed and the association with north–south faulting, dyke emplacement, and voluminous magmatism, which in turn, marked cessation of crustal shortening. Paleotemperatures and geothermal gradients indicate that the Lewis thrust sheet was thick when thrusting commenced.