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88-487: 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 280 mi (450 km) from Mount Kidd near Calgary, AB in

176-405: A decollement . Extensional decollements can grow to great dimensions and form detachment faults , which are low-angle normal faults with regional tectonic significance. Due to the curvature of the fault plane, the horizontal extensional displacement on a listric fault implies a geometric "gap" between the hanging and footwalls of the fault forms when the slip motion occurs. To accommodate into

264-860: A plate boundary. This class is related to an offset in a spreading center , such as a mid-ocean ridge , or, less common, within continental lithosphere , such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed. Dip-slip faults can be either normal (" extensional ") or reverse . The terminology of "normal" and "reverse" comes from coal mining in England, where normal faults are

352-548: A common basal décollement , the Rocky Mountain basal décollement. The Rocky Mountain thrust and fold belt propagated from west to east, accommodating up to 120 mi (200 km) of horizontal shortening near the Canada and US border, and about 43 mi (70 km) 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

440-582: A fault hosting valuable porphyry copper deposits is northern Chile's Domeyko Fault with deposits at Chuquicamata , Collahuasi , El Abra , El Salvador , La Escondida and Potrerillos . Further south in Chile Los Bronces and El Teniente porphyry copper deposit lie each at the intersection of two fault systems. Faults may not always act as conduits to surface. It has been proposed that deep-seated "misoriented" faults may instead be zones where magmas forming porphyry copper stagnate achieving

528-410: A fault often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel , foundation , or slope construction. The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the seismic shaking and tsunami hazard to infrastructure and people in

616-408: A fault's age by studying soil features seen in shallow excavations and geomorphology seen in aerial photographs. Subsurface clues include shears and their relationships to carbonate nodules , eroded clay, and iron oxide mineralization, in the case of older soil, and lack of such signs in the case of younger soil. Radiocarbon dating of organic material buried next to or over a fault shear

704-461: A gap 15 miles wide, and along which was an old Indian trail, and followed it up on the trail for thirty miles to the southwest, finding no obstruction, except from trees; and at that distance, ascending a lofty hill, saw no mountains in the direction of the stream. On each side the mountains were lofty and rugged, showing, generally, perpendicular rock from within 300 feet of their summits, which were covered with snow; and snow banks were also found on

792-602: A good and practicable pass leading from some branch of the Marias; and at this time I was sanguine that we should find there the best solution of the question of the railroad practicability of the Rocky Mountain range. Governor Stevens’ confident entry for September 5 (written years later) could have been colored by the events of a few days later: From the Little Dog, a prominent chief of the Piegan tribe [of Blackfeet ], and

880-567: 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

968-676: A man of character and probity, I got a very particular description of the Marias Pass we were in search of. From some superstition of the Blackfeet, it has not been used for many years, but formerly it was almost the only thoroughfare made use of by the Indians in passing from one side of the mountains to the other. It is a broad, wide open valley, with scarcely a hill or obstruction on the road, excepting here and there some fallen timber. Undoubtedly feeling pressured by his duty to organize

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1056-626: A railroad, because its approach was broad and open, within a valley ranging from one to six miles wide, and at a gentle grade that would not require extensive excavation or rockwork. Construction of the railroad through the pass began on August 1, 1890, starting east from Fort Assinniboine toward Marias Pass. The railroad followed the Middle Fork of the Flathead River west of the Continental Divide. Stevens also discovered

1144-506: A transcontinental railroad. The project was known as the Pacific Railroad Survey , and Isaac Stevens (who had just been appointed as the first governor of Washington Territory ) was chosen to lead the survey of the northernmost of the four candidate routes. Stevens’ voluminous report of that survey, published six years after the expedition, provides detailed insights. Of September 5, 1853, Stevens writes: Mr. Lander

1232-692: Is 2,337 feet (712 m) lower than the south pass of Fremont . Marias Pass was finally explored and charted in December 1889 by John Frank Stevens , principal engineer of the Great Northern Railway (GN). The location of the pass had been rumored for decades beforehand, but it took Stevens and a Flathead guide named Coonsah, who had been hiding out with the Blackfoot in Browning , to discover it. The pass proved ideal for

1320-681: Is a mountain pass in the Rocky Mountains in the western US state of Montana . Lying on the southern border of Glacier National Park , it is traversed by US Highway 2 and by the BNSF Hi-Line Subdivision . The pass is the lowest crossing of the Continental Divide between Canada and central New Mexico , and is the northernmost pass in the US open to automobile traffic year-round. Marias Pass traverses

1408-415: Is also the line commonly plotted on geologic maps to represent a fault. A fault zone is a cluster of parallel faults. However, the term is also used for the zone of crushed rock along a single fault. Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed. Due to friction and

1496-514: Is characterized by broad open folds in relatively undeformed rocks, and a fairly thin lower level consisting of stacked imbricate, southwest-facing, sigmoidal thrust fault slices, bounded below by the Lewis thrust, and above by a separate bedding-parallel thrust called the Tombstone thrust. These culminations progressively stack up and accommodate significant lateral crustal shortening associated with

1584-591: Is limited by the age of normal faults that cut the thrust and the associated sediments found within these normal faults. 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. U-Pb dating of zircons from various deformed and crosscutting mid-crustal granitic rocks in south-central British Columbia provided cooling ages of 59 Ma. Furthermore,

1672-543: Is often critical in distinguishing active from inactive faults. From such relationships, paleoseismologists can estimate the sizes of past earthquakes over the past several hundred years, and develop rough projections of future fault activity. Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits. An example of

1760-434: Is released in part as seismic waves , forming an earthquake . Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing , whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. A fault in ductile rocks can also release instantaneously when

1848-613: Is seen in an area that extends from the Kootenay Pass north of the border to Marias Pass found in Montana. 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. Two windows in this section showing exposures of Upper Cretaceous strata exposed beneath

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1936-475: 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. There is controversy over how the overthrust movement occurred and the effect this movement had on

2024-487: Is south of the pass and Glacier National Park is to the north. As a low pass through the Rocky Mountains, Marias Pass was undoubtedly known to Native Americans for countless years. And while the pass was likely known by early fur trappers , its potential for use beyond foot traffic wasn’t recognized until the middle of the nineteenth century. In 1853 Congress approved funding to survey possible routes for

2112-400: 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. The Youngest movement along the fault or in other words, the end of movement for thrust movement is based on the stratigraphic and structural characteristics of early Eocene deposits and

2200-479: 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. Ar/Ar dates indicate that these pulses occurred in

2288-463: The Chesapeake Bay impact crater . Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by ring dikes . Synthetic and antithetic are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in

2376-810: The Continental Divide in the Lewis Range , along the boundary between the Lewis and Clark National Forest and the Flathead National Forest . The pass forms the southern limit of the Continental Ranges , a major grouping of the Rocky Mountains which extends as far north as McGregor Pass in the Northern Rockies of the Canadian province of British Columbia . The Great Bear Wilderness in Lewis and Clark National Forest

2464-541: 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. The Lewis thrust Is

2552-661: 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

2640-538: The Late Jurassic (163-146 Ma), middle Cretaceous (103-99 Ma), Late Cretaceous (76-68 Ma), and late Paleocene- early Eocene (57-51 Ma), 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

2728-480: The Lewis Thrust occur adjacent to the Flathead Fault. Within these windows, the Lewis thrust is folded along with overlying and underlying strata in a series of northwest-trending anticlinal culminations that extend the length of the west side of the salient. Furthermore, two distinct structural levels can be seen in this section, an upper level comprising the majority of the mass of the Lewis thrust sheet which

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2816-415: 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. This study acted as a reinforcement to previous work and was very consistent with previous data collected. Additionally, this seismic data presented

2904-535: The Lewis thrust sheet was 7.5–8.4 mi (12–13.5 km) thick when thrusting commenced. Thrust faults often associate three types of structures, imbricate fan structures, ramp-flat structures, and duplex structures, all of which are seen within the Lewis thrust and the Rocky Mountain thrust and fold belt. Duplex structures are common and have been located in numerous locations along the Lewis thrust. These structures are distinct due to their structurally overlapping, lenticular stacked thrust fault slices. A prime example

2992-527: The Lewis thrust sheet. 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. 75 km East along the profile is the exposure of the Lewis fault in the Waterton area, which directly links to the most displaced portion of

3080-459: 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

3168-520: The Lewis thrust. Results yielded the pre-deformational paleogeothermal gradient to a range of <30 to 11 °C/km compared to 18–22 °C/km during peak coalification and maximum temperatures. These results are indicative that the Lewis thrust sheet succession was overlain by at least 3 km of additional Late Cretaceous strata, along with the 8 km thick succession indicating that the Lewis thrust sheet had an approximate thickness of 12–13.5 km prior to thrust movement. Geophysical methods in

3256-473: 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

3344-790: 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

3432-598: The Survey results, James G. Swan , early resident of Washington Territory, wrote in The Northwest Coast , In 1853, Governor Isaac I. Stevens, the first Governor of the Territory, surveyed a route for a Northern Pacific Rail-road and discovered a pass near the sources of Maria’s River suitable for a rail-road, estimated to be 2500 feet lower than the south pass of Fremont. By modern measurements, Marias Pass

3520-605: The compression along the Lewis thrust fault. Another extremely similar section of this duplexing is seen at another outcrop in the Waterton Lakes area in Southwest Alberta. 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

3608-414: The crust. A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds. A section of a hanging wall or foot wall where a thrust fault formed along a relatively weak bedding plane is known as a flat and a section where the thrust fault cut upward through

Lewis Overthrust - Misplaced Pages Continue

3696-489: 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

3784-420: 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 . The onset of deformation of the Rocky Mountain fold and thrust belt

3872-402: The determined Isaac Stevens made one final attempt to locate the elusive pass, requesting survey party member James Doty , left behind at Fort Benton , attempt to find it from the east. Stevens dutifully reported Doty’s not-quite-successful results in the Survey report: Turning back on the 8th of June [1854], Mr. Doty retraced his route as far as the Marias, already referred to as issuing from

3960-433: The direction of extension or shortening changes during the deformation but the earlier formed faults remain active. The hade angle is defined as the complement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault. Ring faults , also known as caldera faults , are faults that occur within collapsed volcanic calderas and the sites of bolide strikes, such as

4048-409: The fault (called a piercing point ). In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector. The two sides of a non-vertical fault are known as the hanging wall and footwall . The hanging wall occurs above the fault plane and the footwall occurs below it. This terminology comes from mining: when working a tabular ore body,

4136-459: The fault is the horizontal component, as in "Throw up and heave out". The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault. Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault. The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of

4224-465: The fault movement. Faults are mainly classified in terms of the angle that the fault plane makes with the Earth's surface, known as the dip , and the direction of slip along the fault plane. Based on the direction of slip, faults can be categorized as: In a strike-slip fault (also known as a wrench fault , tear fault or transcurrent fault ), the fault surface (plane) is usually near vertical, and

4312-506: The fault place the overall movement of the Lewis thrust fault to have occurred over a span of about 15 Ma in the Late Cretaceous to Early Paleocene periods between 75 and 59 Ma. 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

4400-478: The fault zones. This is indicative that the high temperatures were fairly short-lived. Thus, the high temperatures are interpreted to be the result of frictional heating during stick slip faulting. Evidence for the local high temperatures within the fault zone indicate that local areas of frictional stress must have existed, with the possibility of this occurring due to ramps in the fault plane where drainage of high pore pressures may have occurred. Moreover, samples from

4488-438: 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

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4576-428: The footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults and those with right-lateral motion as dextral faults. Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault. A special class of strike-slip fault is the transform fault when it forms

4664-531: The footwall. The dip of most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees. A downthrown block between two normal faults dipping towards each other is a graben . A block stranded between two grabens, and therefore two normal faults dipping away from each other, is a horst . A sequence of grabens and horsts on the surface of the Earth produces a characteristic basin and range topography . Normal faults can evolve into listric faults, with their plane dip being steeper near

4752-570: The form of seismic analysis has also been used to determine movement along the thrust sheet. In one study, seismic data recorded along the 49°N parallel (the border between Canada and the US) was recorded from the Rocky Mountain trench fault in the foreland belt to the east flank of the Moyie anticline of the Purcell anticlinorium, which is thought to be the location where the Lewis fault tapers off. Seismic data produced showed total displacement of 115 km of

4840-429: The geometric gap, and depending on its rheology , the hanging wall might fold and slide downwards into the gap and produce rollover folding , or break into further faults and blocks which fil in the gap. If faults form, imbrication fans or domino faulting may form. A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of

4928-621: The hanging wall collected in close proximity to the fault plane show no evidence for heating during progressive burial of sediments. This absence of evidence for heating during faulting is indicative of low frictional stress and therefore, low rates of slip. 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. Marias Pass Marias Pass ( elevation 5,213 feet (1,589 m))

5016-446: The hanging wall. Measuring the distance between the footwall cutoff and the fault exposure at the Earth's surface, total movement of the Lewis thrust sheet was determined. Results showed that there was 75 km of direct movement of the thrust sheet along the Lewis fault, and an additional 40 km of transport by the formation of footwall-domain duplexes. Although this study did not take into consideration that it could be possible that

5104-491: The implied mechanism of deformation. A fault that passes through different levels of the lithosphere will have many different types of fault rock developed along its surface. Continued dip-slip displacement tends to juxtapose fault rocks characteristic of different crustal levels, with varying degrees of overprinting. This effect is particularly clear in the case of detachment faults and major thrust faults . The main types of fault rock include: In geotechnical engineering ,

5192-464: The largest faults on Earth and give rise to the largest earthquakes. A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault . Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, and others occur where

5280-418: The megathrust faults of subduction zones or transform faults . Energy release associated with rapid movement on active faults is the cause of most earthquakes . Faults may also displace slowly, by aseismic creep . A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace

5368-408: The miner stood with the footwall under his feet and with the hanging wall above him. These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. Distinguishing between these two fault types is important for determining the stress regime of

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5456-435: The most common. With the passage of time, a regional reversal between tensional and compressional stresses (or vice-versa) might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement (fault inversion). In such a way, a normal fault may therefore become a reverse fault and vice versa. In a normal fault, the hanging wall moves downward, relative to

5544-432: 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

5632-639: The newly-formed Washington Territory, Governor Stevens pressed on across the Rockies without locating Marias Pass. After crossing the Continental Divide at Cadotte Pass (elevation 6,073 feet (1,851 m)), Stevens sent the civilian engineer Abiel Tinkham back to look for the pass. But approaching from the steeper western side of the divide, Tinkham missed Marias Pass, and instead crossed at today’s Cut Bank Pass (elevation 7,850 feet (2,390 m)), roughly 12 miles (19 km) northwest of Marias Pass. Upon hearing Tinkham’s report, and realizing Tinkham’s error,

5720-591: The north side of many hills. It is to be regretted that Mr. Doty did not continue on, and ascertain where the trail issued on the western side of the mountains. This is the true Marias Pass, described to me by the Little Dog at Fort Benton in September, 1853, and formerly used by the Indians in crossing the mountains. News of such a pass, even unexplored, was too important to wait for publication. Ever self-confident, Isaac Stevens must have spread word of Marias Pass by mouth: in 1857, three years before publication of

5808-494: The opposite direction. These faults may be accompanied by rollover anticlines (e.g. the Niger Delta Structural Style). All faults have a measurable thickness, made up of deformed rock characteristic of the level in the crust where the faulting happened, of the rock types affected by the fault and of the presence and nature of any mineralising fluids . Fault rocks are classified by their textures and

5896-465: The pass across the Cascades in central Washington which bears his name . A memorial to President Theodore Roosevelt was constructed along the Continental Divide at the top of the pass. Designs for the memorial were made by Kimball , Steele & Sandham of Omaha, Nebraska. Construction began in 1930, with the original intent to produce a granite archway across the highway, and a cornerstone

5984-542: 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

6072-412: The right time for—and type of— igneous differentiation . At a given time differentiated magmas would burst violently out of the fault-traps and head to shallower places in the crust where porphyry copper deposits would be formed. As faults are zones of weakness, they facilitate the interaction of water with the surrounding rock and enhance chemical weathering . The enhanced chemical weathering increases

6160-423: The rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The regions of higher friction along a fault plane, where it becomes locked, are called asperities . Stress builds up when a fault is locked, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy

6248-463: 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

6336-404: The strain rate is too great. Slip is defined as the relative movement of geological features present on either side of a fault plane. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault concerning the other side. In measuring the horizontal or vertical separation, the throw of the fault is the vertical component of the separation and the heave of

6424-416: The stratigraphic sequence is known as a ramp . Typically, thrust faults move within formations by forming flats and climbing up sections with ramps. This results in the hanging wall flat (or a portion thereof) lying atop the foot wall ramp as shown in the fault-bend fold diagram. Thrust faults form nappes and klippen in the large thrust belts. Subduction zones are a special class of thrusts that form

6512-566: 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

6600-459: The summit of Marias Pass. Fault (geology)#Listric fault In geology , a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth 's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as

6688-400: The surface, then shallower with increased depth, with the fault plane curving into the Earth. They can also form where the hanging wall is absent (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults. The fault panes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along

6776-471: 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

6864-700: The surrounding geology. More specifically, attempting to determine if the thrust movement was continuous or if movement was subject to a more stick-slip style of movement remains inconclusive. However, the anomalously high vitrinite reflectance values obtained from the Lewis thrust at Marias Pass, the McConnell thrust at Mt. Yamnuska, the Coleman fault at Wintering Creek and several others indicate temperatures of 350–650 °C were generated during thrusting. Furthermore, these high vitrinite reflectance values were restricted to extremely narrow sections adjacent to and within

6952-526: 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

7040-462: 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

7128-467: 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. This

7216-541: The transition from thrusting and folding to crustal stretching led to rapid cooling of the Priest River metamorphic core complexes where cooling ages found in biotite gave ages of >55 Ma through K-Ar and Ar/Ar dating methods. In addition, the same faults in Montana that cut a volcanic marker were also found to be cut by 59 Ma porphyry dikes. This limits the youngest age for movement to have occurred at 59 Ma. Together, dates revealed for oldest and youngest movement along

7304-582: The vicinity. In California, for example, new building construction has been prohibited directly on or near faults that have moved within the Holocene Epoch (the last 11,700 years) of the Earth's geological history. Also, faults that have shown movement during the Holocene plus Pleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such as power plants, dams, hospitals, and schools. Geologists assess

7392-533: 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

7480-410: Was busily occupied to-day in fitting out his party for the survey of Marias Pass, in reference to which I also gained some general information that satisfied me of the existence of a pass in that quarter. Since I have given my attention to the passes of these mountains, I have been greatly impressed with the fact, from the course of the streams and the general deportment of the country, that there must be

7568-547: 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

7656-633: Was later decided to build an obelisk instead of an archway; it was completed 93 years ago in 1931. Today, U.S. Route 2 uses the pass, along with the BNSF Railway , a successor to the Great Northern Railway. The railway line, which sees freight traffic, as well as Amtrak 's Empire Builder , is part of BNSF's Northern Transcon line linking Chicago and the Pacific Northwest . A statue of Stevens stands at

7744-471: Was placed to the southeast of the highway. Eleanor Roosevelt was later on hand to place a small copper box filled with a copy of the bill that appropriated funds for the memorial as well as some other documents into the cornerstone. The obelisk is sixty feet (18 m) in height and extends nineteen feet (5.8 m) into the ground. It has a tapering cement core covered on all sides with seven-inch (18 cm) slabs of Montana granite quarried near Helena . It

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