page_title: Lewis Overthrust
section_title: Lewis Thrust System
hierarchical_section_title: Lewis Overthrust / Lewis Thrust System
caption_attribution_description: English: Diagram showing progressive formation of a thrust duplex
context_page_description: 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 170 million years ago that drove a several mile thick wedge of rock 50 mi eastwards, causing it to overlie softer Cretaceous age rock that is 400 to 500 million years younger.
context_section_description: 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 (in the southern Canadian Rocky Mountains Foothills and Eastern Front ranges) 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 7.5–8.4 mi (12–13.5 km) thick when thrusting commenced.
caption: A simple duplex structure showing successive stacking of thrust faults.
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