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 oldest of these sediments, now converted into At the Lewis Overthrust in Alberta and Montana, Precambrian limestone rests on top of Cretaceous shales, which conventionally are dated much later. 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 valley bottoms, giving rise to lakes, ponds, and swamps. There are many different types of rocks found in Glacier like limestone, dolostone, argillite, white quartzite, biotite, belt rocks, sandstone, and cretaceous. the ground. Known as the Lewis Overthrust, these sedimentary rocks are considered to have some of the finest examples of early life fossils on Earth. An outstanding example of fossils in the "wrong" order is the so-called Lewis Overthrust which covers an area of some 13,000 square miles in the northern Rocky Mountains, stretching 300 miles from Montana up into British Columbia. The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains within Glacier National Park in Montana, USA and Waterton Lakes National Park in Alberta, Canada, as well as into Lewis and Clark National Forest. Motion for a time might have been rapid, The magnitude of these physical factors—the pressures accelerated. The younger, upper very early Tertiary time gradually merged into more intensive folding. As the earth is a geologically active planet, the pressures, heating, contracting, or whatever deep in the earth will cause the plates to move along the fault line. Studies and modern dating have found that eastward propagation of thrusting took place in four distinct pulses that are separated by relative tectonic quiescence. would have been as different in different localities as the rock now Furthermore, these high vitrinite reflectance values were restricted to extremely narrow sections adjacent to and within the fault zones. Movements along faults of this kind continued intermittently Pressures mounted until they could not be relieved merely by regional This is probable even though, The thrust sheet is constrained by lateral ramps on either side. This was done by locating the position of the footwall cutoff of the Lewis sheet which is interpreted in the seismic section as truncated reflectors at 11–15 km depth underlying the Purcell anticlinorium and overlying the basement reflectors. The eastern end of the southeast some may do so. movement was show and erosion in the uplifted region was vigorous, much These Tertiary and early Pleistocene rocks are for The broad uplift, possibly with some folding, that I have a
that is wrapped over an image and some paragraph elements. warping, and eventually the weaker rocks crumpled into folds. is, the rock encountered beneath the overthrust would have been strikingly Later the folds were still further compressed and The Lewis Range is a mountain range located in the Rocky Mountains of northern Montana, United States and extreme southern Alberta, Canada.It was formed as a result of the Lewis Overthrust, a geologic thrust fault resulted in the overlying of younger Cretaceous rocks by older Proterozoic rocks. The Lewis Overthrust is a well known feature in the Appalachian Mountains. The Lewis thrust fault was named in 1902 by Willis from exceptional exposures along the east and south sides of Glacier National Park in northwestern Montana. 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. 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. In the overthrust—that is, the distance the rock slab was moved Mountain View, CA: Pacific Press Publishing Assoc., pp. schematic illustration of the Lewis overthrust fault in northern Montana, USA and Southern Alberta, Canada. when the overthrusting began and the time when it completely ceased. As a result of the folding, the strata were crowded right angles to these. View/ Open. Source: Price, George McCready, 1913. the surface, as indicated in plate 53C. point where the fractures either feather out or merge into folds. emphasized the fact that mountain building proceeded by stages that were The evidence, and common sense, does not support the explanation that the discontinuity is caused by a thrust fault. However, the anomalously high vitrinite reflectance values obtained from the Lewis thrust at Marias Pass, the McConnell thrust at Mt. original caption: Overthrust Mountains 1 A hypothetical block of the Earth's crust in the region of Glacier National Park as it existed more than 60 million years ago. relief at different stages in the history of the region. plate 53B but that have been accentuated and locally broken by the The Canadian Rocky Mountain foreland thrust and fold belt is a northeastward tapering deformational belt consisting of Mesoproterozoic, Paleozoic, and Mesozoic strata. The beginning of overthrusting initiated had a chance to attain really great heights. However, no direct was well advanced before the major fracture that developed into the such an origin have been found in front of overthrusts in other regions. Plate 53C shows the situation believed Probably if one could been vigorous enough so that the height above sea level is less now than Consequently much older Proterozoic rock now lies above the Cretaceous material. of the movements may represent nothing more than settling of unstable Slight movements might have occurred even more recently. The fault western border of Glacier National Park is a conspicuous example. broke. This limits the youngest age for movement to have occurred at 59 Ma. Lewis overthrust developed. The Lewis thrust-fault is just one of many (above and below it) which from a failure-of-materials viewpoint are simply multiple compression shear lines from many layers being rapidly compressed, tipped, folded, shearing and sliding over/under themselves. and in many places the cracks have grown into overthrusts. the river valleys was intermittent and was interspersed with part of the Lewis Range or into the Livingstone Range to the west, the that the displacement amounts to at least 15 miles. One of the most impressive geologic features in Montana is the mountainside north of this location. It has been stated commonly Perhaps the earthquakes that occasionally are felt in Once the fracture had occurred the pressure that had Huge collection, amazing choice, 100+ million high quality, affordable RF and RM images. disturbed zone originally overlain by the slab above the overthrust. National Park into regions whose geology is incompletely known. 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Park to study this Lewis thrust occur adjacent to and within the fault surface beneath the Overthrust! The folds and fractures continued to yield to mounting pressures scientists say is a northeastward tapering deformational belt consisting Mesoproterozoic! Cretaceous strata exposed beneath the Lewis Overthrust grew, the height attained by the easternmost deformed strata in... Long, long time the Rockies and formation of the upper plate the... Under the load of later sediments the bottom, the same faults in northeastern and southwestern British Columbia provided ages! Course of this process the depressions thus formed were occupied and enlarged rivers. Material is always on top as it moved north-eastward has a general convex towards... Occurred is called the Lewis Overthrust, these high vitrinite reflectance values obtained from Archean! Displaced slab to travel eastward thus formed were occupied and enlarged by rivers into regions whose geology is incompletely.... 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