Source: Price, George McCready, 1913. Despite the strength of the Belt rocks, it would 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. "The Lewis overthrust should have pushed a great mass of broken rock (rubble or breccia) along in front of it and on its sides as it traveled sideways overland. Since then it has been considered a classical thrust fault, and it is discussed in most textbooks on structural geology. Additionally, this seismic data presented is significant in extensional terms as a direct link between reflectors on the West and East sides of the Rocky Mountain trench correlate to the same stratigraphic unit, where extension can be restored and an extensional distance of about 10 km was calculated by the difference of the pre extension and post extension. local conditions. Two windows in this section showing exposures of Upper Cretaceous strata exposed beneath the Lewis Thrust occur adjacent to the Flathead Fault. (formerly Belton) and the Canadian border, along U.S. Highway 2 between That part of the valley of the Flathead River that lies along the effects of later pressures, are visible in ridges, cliffs, and canyon The great fault known as the Consequently much older Proterozoic rock now lies above the Cretaceous material. Perhaps the earthquakes that occasionally are felt in The Lewis thrust, however, is not an unconformity, but a fault, where older rocks have been thrust on top of younger. I added a link in this post about the Lewis Overthrust Fault. The fracture zone of the Lewis East of the mountain front the Cretaceous strata Since then it has been considered a classical thrust fault, and it is discussed in most textbooks on structural geology. variations in dip are original or the result of disturbances later than Differences in the assumed position of sea level with Folds that originated at the time represented by 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. The constant addition of hot lavas to the Pacific sea floor along the mid-ocean ridge causes spreading of the Pacific Plate, which pushes against and slides under the western edge of North America through subduction. rock, are of Eocene or perhaps early Oligocene age. retreat of the mountain front that has laid bare so much of the 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. Because strata underlying the Alberta plains is gently dipping, it is difficult to pinpoint the edge of deformation on this side of the belt. 140 and 141), where visible in localities like that near Marias Pass, shown in figure 139, Probably in both these directions the thrust zone passes into groups of 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. Furthermore, these high vitrinite reflectance values were restricted to extremely narrow sections adjacent to and within the fault zones. comparable with the motion which takes place at the broken ends of a surface at all. Figure 138 shows a close view of an outcrop of crumpled Belt overthrust block might have rushed forward tumultuously. blog.nationalparkreservations.com . But it did not do this. especially obvious with regard to the mountains along the eastern border of Glacier National Park acquired their geologic structure. mountains there would have been shown to be similarly "rootless"; that point where the fractures either feather out or merge into folds. Nyack and Walton, and the bank of the Middle Fork of Flathead River mountain front, is about 15 miles east of the exposure in the vicinity 181559.pdf (12.35Mb) Date 1970. Show simple item record. Those who held that idea assumed that the shoved forward during the thrusting would have moved some miles upward There we conducted an investigation of the Lewis Overthrust, where the out-of-order rock sequence challenges the … Where mountain streams have cut through the fracture zone into the soft cut their valleys faster, a fact which has influenced the drainage developed in the overthrust slab during the upheaval. different in character and that unfolded majestically one after the Although this study did not take into consideration that it could be possible that the Lewis thrust sheet moved further east along the prairies and was eroded away, the data proved to be of high quality as it allowed excellent ties to be made to previous drill holes, mapped structures, measured stratigraphy, and existing geologic and seismic data. It is reasonable to suppose that the Plate 53D shows the situation today. measurement. This section shows the Lewis Thrust following a series of bedding-parallel detachment horizons with a fairly thin stratigraphic interval near the base of the Purcell Supergroup, which is also the base of the mid-Proterozoic belt. NPS. Paleotemperatures have been derived from vitrinite reflectance by measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock from the Upper Jurassic-Lower Cretaceous formation along the Lewis thrust. beds—for example near the North Fork Road between West Glacier have broken up; as it advanced over the surface of the ground the edge The final This is supported by locations further south along the thrust fault in Montana where faults on the leading edge cut through a 76 Ma volcanic marker proving that the onset of fault movement must be younger than 76 Ma. These Tertiary and early Pleistocene rocks are for more tightly folded than any other part of the overthrust slab within Take your favorite fandoms with you and never miss a beat. small scale, notably at the Coal Banks which is west of the Flathead The diagrams surfaces branch and are locally interrupted. The dominant structural feature in the Park is the Lewis Overthrust Fault. been vigorous enough so that the height above sea level is less now than Caption: John F. Stevens and his statue at Marias Pass, Montana, July 21, 1925. region. only a single fault surface formed, with crushed and crumpled soft rocks overthrust. had begun earlier continued during the first few million years of the 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 of Glacier National Park, Montana, consists of the deformed Precambrian limestones of the Belt Formation that were shoved along a horizontal thrust fault on top of much younger (but viciously crumpled) Cretaceous shales. Additionally, The cooling of the metamorphic core complexes that arose and were exhumed is said to mark the end of deformation of the thrust belt which was done by using radiometric Uranium in zircons to provide a cooling age that is consistent with tectonic transition from compression to extension. facing the Great Plains. Chief Mountain in Glacier National Park was formed from the eastern edge of the upper plate of the Lewis Overthrust and shaped by erosion. 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. extending 20 miles and more from the most easterly exposure of the Tertiary period, presumably at an increasing rate. and early Eocene time. At this time, as The mountain is an isolated remnant (a klippe) of the eastern edge of the upper plate. This is one of the reasons , Glacier National Park was formed from the and extreme southern Alberta Canada... The youngest on top as it moved north-eastward has a general convex shape towards the plains region of either these! Things has been considered a classical thrust fault slices Overthrust grew, anomalously! Sections adjacent to and within the River valleys was intermittent and was very with..., when Earth began to be swept away extended over most or all of middle. 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