83-713: The Algoman orogeny , known as the Kenoran orogeny in Canada, was an episode of mountain-building ( orogeny ) during the Late Archean Eon that involved repeated episodes of continental collisions , compressions and subductions . The Superior province and the Minnesota River Valley terrane collided about 2,700 to 2,500 million years ago. The collision folded the Earth's crust and produced enough heat and pressure to metamorphose
166-548: A close. The end of the Archean Eon marks a major change in the development of the Earth's crust : the crust was essentially formed and achieved thicknesses of about 40 km (25 mi) under the continents. The collision between terranes folded the Earth's crust, and produced enough heat and pressure to metamorphose then-existing rock. Repeated continental collisions, compression along a north–south axis, and subduction resulted in
249-732: A collisional orogeny). Orogeny typically produces orogenic belts or orogens , which are elongated regions of deformation bordering continental cratons (the stable interiors of continents). Young orogenic belts, in which subduction is still taking place, are characterized by frequent volcanic activity and earthquakes . Older orogenic belts are typically deeply eroded to expose displaced and deformed strata . These are often highly metamorphosed and include vast bodies of intrusive igneous rock called batholiths . Subduction zones consume oceanic crust , thicken lithosphere, and produce earthquakes and volcanoes. Not all subduction zones produce orogenic belts; mountain building takes place only when
332-497: A delamination of the orogenic root beneath them. Mount Rundle on the Trans-Canada Highway between Banff and Canmore provides a classic example of a mountain cut in dipping-layered rocks. Millions of years ago a collision caused an orogeny, forcing horizontal layers of an ancient ocean crust to be thrust up at an angle of 50–60°. That left Rundle with one sweeping, tree-lined smooth face, and one sharp, steep face where
415-429: A high grade of metamorphism, intrusion and basement remobilization typical of Archean terranes. Migmatites, batholithic intrusive and granulitic metamorphic rocks show foliation and compositional banding; the rocks are uniformly hard and so thoroughly deformed that little foliation exists. Most Yellowknife Supergroup metasediments are tightly folded ( isoclinal ) or occur in plunging anticlines . The Archean rocks forming
498-582: A major continent-continent collision, is called an accretionary orogen. The North American Cordillera and the Lachlan Orogen of southeast Australia are examples of accretionary orogens. The orogeny may culminate with continental crust from the opposite side of the subducting oceanic plate arriving at the subduction zone. This ends subduction and transforms the accretional orogen into a Himalayan -type collisional orogen. The collisional orogeny may produce extremely high mountains, as has been taking place in
581-412: A noncollisional orogenic belt, and such belts are sometimes called Andean-type orogens . As subduction continues, island arcs , continental fragments , and oceanic material may gradually accrete onto the continental margin. This is one of the main mechanisms by which continents have grown. An orogen built of crustal fragments ( terranes ) accreted over a long period of time, without any indication of
664-442: A pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults . These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates ) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism . Before
747-596: A roughly northeasterly heading, while the southern border dips south to follow the northeast shore of Lake Superior. The three subprovinces are separated by steeply dipping shear zones caused by continued compression that occurred during the Algoman orogeny. These boundaries are major fault zones. The boundary between the Wabigoon and Quetico subprovinces seems to have been also controlled by colliding plates and subsequent transpressions . This Rainy Lake – Seine River fault zone
830-488: A sequence of events, approximately 75 million years in duration, leading to the formation of a new crustal segment. The oldest rocks, at 2,650 million years old, are basic metavolcanics with largely calc-alkaline characteristics. Radiometric dating indicates ages of 2,640 to 2,620 million years are recorded for the syn-kinematic quartz diorite batholiths and 2,590 to 2,100 million years for the major late-kinematic bodies. Pegmatitic adamellites , at 2,575±25 million years, are
913-482: A single layer may be exposed at the surface many times by subsequent erosion. As the greenstone belts were forming, volcanoes ejected tephra into the air which settled as sediments to become compacted into the greywackes and mudstones of the Knife Lake and Lake Vermilion formations. Greywackes are poorly sorted mixtures of clay , mica and quartz that may be derived from the decomposition of pyroclastic debris;
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#1733085603528996-501: A succession of metamorphosed volcanic and sedimentary rocks called greenstone belts . Most Archean volcanic rocks are concentrated within greenstone belts; the green color comes from minerals, such as chlorite , epidote and actinolite that formed during metamorphism. After metamorphism occurred, these rocks were folded and faulted into a system of mountains by the Algoman orogeny. The volcanic beds are 8 to 9 km (26,000 to 30,000 ft) thick. About 2,700 million years ago
1079-649: A trench during the collision of several island arcs (greenstone belts). Boundaries between the gneiss belt and the flanking greenstone belts to the north and south are major fault zones, the Vermilion and Rainy Lake – Seine River fault zones. The Wawa subprovince is a formerly active volcanic island chain, consisting of metamorphosed greenstone belts which are surrounded by and cut by granitic plutons and batholiths. These greenstone belts consist of felsic volcanics, felsic batholiths, felsic plutons and sediments aged from 2,700 to 2,670 million years old. The predominant rock type
1162-536: A zone of partial melting which is possible only under high temperature and pressure conditions. It is visible as a 500 m (1,600 ft) wide belt. Most of the flattened large crystals in the fault indicate a simple compression rather than a wrenching, shearing or rotational event as the two subprovinces docked. This provides evidence that the Quetico and Wawa subprovinces were joined by the collision of two continental plates, about 2,690 million years ago . Structures in
1245-461: Is a major northeast–southwest trending strike-slip fault zone; it trends N80°E to cut through the northwest part of Voyageurs National Park in Minnesota and extends westward to near International Falls , Minnesota and Fort Frances , Ontario. The fault has transported rocks in the greenstone belt a considerable distance from their origin. The greenstone belt is 2 to 3 km (0 to 0 mi) wide at
1328-524: Is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of planetary accretion , the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust. This crust was likely destroyed by large impacts and re-formed many times as the Era of Heavy Bombardment drew to a close. The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are
1411-500: Is a white, coarse-grained, foliated hornblende tonalite. Minerals in the tonalite are quartz, plagioclase, alkali feldspar and hornblende. In extensive regions of the Slave province of northern Canada, the magma that later became batholiths heated the surrounding rock to create metamorphic regions called aureoles about 2,575 million years ago. These regions are typically 10 to 15 km (6 to 9 mi) wide. The creation of aureoles
1494-491: Is debated. The anorthosite highlands of the Moon are primary crust, formed as plagioclase crystallized out of the Moon's initial magma ocean and floated to the top; however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water. The Martian meteorite ALH84001 might represent primary crust of Mars; however, again, this is debated. Like Earth, Venus lacks primary crust, as
1577-420: Is essentially a large downfold or downfaulted block. The areas between individual belts are fault zones consisting of granite or granitic gneiss. Its western part contains a regional pattern of east–west-trending 100 to 200 km (60 to 120 mi) wide granitic greenstone and metasedimentary belts (subprovinces). Western Superior province's mantle has remained intact since the 2,700-million-year-ago accretion of
1660-587: Is estimated at 50 to 100 million years. The current boundary between these terranes is known as the Great Lakes tectonic zone (GLTZ). This zone is 50 km (30 mi) wide and extends in a line roughly 1,200 kilometers long from the middle of South Dakota , east through the middle of the Upper Peninsula of Michigan , into the Sudbury, Ontario region. The region remains slightly active today. Rifting in
1743-446: Is initiated along one or both of the continental margins of the ocean basin, producing a volcanic arc and possibly an Andean-type orogen along that continental margin. This produces deformation of the continental margins and possibly crustal thickening and mountain building. Mountain formation in orogens is largely a result of crustal thickening. The compressive forces produced by plate convergence result in pervasive deformation of
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#17330856035281826-490: Is needed to create tertiary crust, and Earth is the only planet in the Solar System with plate tectonics. Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere , a division of Earth's layers that includes the crust and the upper part of the mantle . The lithosphere is broken into tectonic plates that move, allowing heat to escape from
1909-582: Is still debated, but if modern-day tectonics were operative in the Archean, higher heat fluxes might have caused tectonic processes to be more active. As a result, plates and continents may have been smaller. No broad blocks as old as 3 Ga are found in Precambrian shields . Toward the end of the Archean, however, some of these blocks or terranes came together to form larger blocks welded together by greenstone belts . Two such terranes that now form part of
1992-422: Is still in use today, though commonly investigated by geochronology using radiometric dating. Based on available observations from the metamorphic differences in orogenic belts of Europe and North America, H. J. Zwart (1967) proposed three types of orogens in relationship to tectonic setting and style: Cordillerotype, Alpinotype, and Hercynotype. His proposal was revised by W. S. Pitcher in 1979 in terms of
2075-473: Is taking place today in the Southern Alps of New Zealand). Orogens have a characteristic structure, though this shows considerable variation. A foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep immediately beyond
2158-449: The Alpine type orogenic belt , typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure. In terms of recognising orogeny as an event , Leopold von Buch (1855) recognised that orogenies could be placed in time by bracketing between the youngest deformed rock and the oldest undeformed rock, a principle which
2241-571: The Canadian shield collided about 2,700 to 2,500 million years ago . These were the Superior province and the large Minnesota River Valley terrane, the former composed mainly of granite and the latter of gneiss . This led to the mountain-building episode known as the Algoman orogeny in the U. S. (named for Algoma , Kewaunee County, Wisconsin ), and the Kenoran orogeny in Canada. Its duration
2324-613: The Himalayas for the last 65 million years. The processes of orogeny can take tens of millions of years and build mountains from what were once sedimentary basins . Activity along an orogenic belt can be extremely long-lived. For example, much of the basement underlying the United States belongs to the Transcontinental Proterozoic Provinces, which accreted to Laurentia (the ancient heart of North America) over
2407-683: The San Andreas Fault , restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that look as if they are not necessarily on present tectonic-plate boundaries, but they are essentially the product of plate tectonism. Likewise, uplift and erosion related to epeirogenesis (large-scale vertical motions of portions of continents without much associated folding, metamorphism, or deformation) can create local topographic highs. Eventually, seafloor spreading in
2490-421: The adiabatic rise of mantle causes partial melting. Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways: The only known example of tertiary crust is the continental crust of the Earth. It is unknown whether other terrestrial planets can be said to have tertiary crust, though the evidence so far suggests that they do not. This is likely because plate tectonics
2573-921: The crust is the outermost solid shell of a planet , dwarf planet , or natural satellite . It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle). The crusts of Earth , Mercury , Venus , Mars , Io , the Moon and other planetary bodies formed via igneous processes and were later modified by erosion , impact cratering , volcanism, and sedimentation. Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types: continental crust and oceanic crust . These two types have different chemical compositions and physical properties and were formed by different geological processes. Planetary geologists divide crust into three categories based on how and when it formed. This
Algoman orogeny - Misplaced Pages Continue
2656-669: The igneous and sedimentary rocks, heating and pressing the rocks to metamorphose them into hard greenish greenstones. They began with fissure eruptions of basalt , continued with intermediate and felsic rocks erupted from volcanic centers and ended with deposition of sediments from the erosion of the volcanic pile. The rising magma was extruded under a shallow ancient sea where it cooled to form pillowed greenstones. Some of Minnesota's pillows probably cooled at depths as great as 1,000 m (3,300 ft) and contain no gas cavities or vesicles . Most greenstone belts, with all of their components, have been folded into troughlike synclines ;
2739-634: The late Devonian (about 380 million years ago) with the Antler orogeny and continuing with the Sonoma orogeny and Sevier orogeny and culminating with the Laramide orogeny . The Laramide orogeny alone lasted 40 million years, from 75 million to 35 million years ago. Orogens show a great range of characteristics, but they may be broadly divided into collisional orogens and noncollisional orogens (Andean-type orogens). Collisional orogens can be further divided by whether
2822-598: The GLTZ began about 2,500 million years ago at the end of the Algoman orogeny. The orogeny affected adjacent regions of northern Minnesota and Ontario in the Superior province as well as the Slave and the eastern part of the Nain province , a far wider region of influence than in subsequent orogenies. It is the earliest datable orogeny in North America and brought the Archean Eon to
2905-601: The Nain province of northeastern Canada and Greenland are separated from the Superior terrane by a narrow band of remobilized rocks. Greenland separated from North America less than 100 million years ago and its Precambrian terranes align with Canada's on the opposite side of Baffin Bay. The southern tip of Greenland is part of the Nain Province, this means it was connected to North America at
2988-571: The Seven Sisters Islands; to the west the greenstone interfingers with pods of anorthositic gabbro . Radiometric dating from the Rainy Lake area in Ontario show an age of about 2,700 million years old, which favors a moving tectonic plate model for the formation of the boundary. The largest fault is the Vermilion fault separating the Quetico and Wawa subprovinces. It has a N40°E trend and
3071-626: The Southern province. Immediately to the south, the Quetico subprovince extends as far west in north-central Minnesota, and extends further to the northeast. It is completely interrupted by a narrow band of the 1,100- to 1,550-million-year-old Southern province to the northeast of Thunder Bay . The Wawa subprovince is the most southerly of the three; it begins in central Minnesota, continues northeast to Thunder Bay, Ontario, Canada, (where its southern border just skims north Thunder Bay) and then extends east beyond Lake Superior. The northern boundary continues in
3154-497: The acceptance of plate tectonics , geologists had found evidence within many orogens of repeated cycles of deposition, deformation, crustal thickening and mountain building, and crustal thinning to form new depositional basins. These were named orogenic cycles , and various theories were proposed to explain them. Canadian geologist Tuzo Wilson first put forward a plate tectonic interpretation of orogenic cycles, now known as Wilson cycles. Wilson proposed that orogenic cycles represented
3237-462: The action of metamorphism on volcanic and sedimentary rock. The areas between individual belts consist of granites or granitic gneisses that form fault zones . These two types of belts can be seen in the Wabigoon, Quetico and Wawa subprovinces ; the Wabigoon and Wawa are of volcanic origin and the Quetico is of sedimentary origin. These three subprovinces lie linearly in southwestern- to northeastern-oriented belts about 140 km (90 mi) wide on
3320-414: The active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems. The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are mainly derived from the erosion of the actively uplifting rocks of
3403-404: The belt are schists and gneisses produced by intense metamorphism of greywackes and minor amounts of other sedimentary rocks. The sediments, alkalic plutons and felsic plutons are aged from 2,690 to 2,680 million years. The metamorphism is relatively low-grade on the margins and high-grade in the center. The low-grade components of the greywackes were derived primarily from volcanic rocks;
Algoman orogeny - Misplaced Pages Continue
3486-631: The collision is with a second continent or a continental fragment or island arc. Repeated collisions of the later type, with no evidence of collision with a major continent or closure of an ocean basin, result in an accretionary orogen. Examples of orogens arising from collision of an island arc with a continent include Taiwan and the collision of Australia with the Banda arc. Orogens arising from continent-continent collisions can be divided into those involving ocean closure (Himalayan-type orogens) and those involving glancing collisions with no ocean basin closure (as
3569-578: The course of 200 million years in the Paleoproterozoic . The Yavapai and Mazatzal orogenies were peaks of orogenic activity during this time. These were part of an extended period of orogenic activity that included the Picuris orogeny and culminated in the Grenville orogeny , lasting at least 600 million years. A similar sequence of orogenies has taken place on the west coast of North America, beginning in
3652-539: The creation of new continental crust through volcanism . Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere ( crust and uppermost mantle ). A synorogenic (or synkinematic ) process or event is one that occurs during an orogeny. The word orogeny comes from Ancient Greek ὄρος ( óros ) 'mountain' and γένεσις ( génesis ) 'creation, origin'. Although it
3735-427: The crust of the continental margin ( thrust tectonics ). This takes the form of folding of the ductile deeper crust and thrust faulting in the upper brittle crust. Crustal thickening raises mountains through the principle of isostasy . Isostacy is the balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by
3818-423: The crust ranges between about 20 and 120 km. Crust on the far side of the Moon averages about 12 km thicker than that on the near side . Estimates of average thickness fall in the range from about 50 to 60 km. Most of this plagioclase-rich crust formed shortly after formation of the Moon, between about 4.5 and 4.3 billion years ago. Perhaps 10% or less of the crust consists of igneous rock added after
3901-562: The dense underlying mantle . Portions of orogens can also experience uplift as a result of delamination of the orogenic lithosphere , in which an unstable portion of cold lithospheric root drips down into the asthenospheric mantle, decreasing the density of the lithosphere and causing buoyant uplift. An example is the Sierra Nevada in California. This range of fault-block mountains experienced renewed uplift and abundant magmatism after
3984-738: The development of geologic concepts during the 19th century, the presence of marine fossils in mountains was explained in Christian contexts as a result of the Biblical Deluge . This was an extension of Neoplatonic thought, which influenced early Christian writers . The 13th-century Dominican scholar Albert the Great posited that, as erosion was known to occur, there must be some process whereby new mountains and other land-forms were thrust up, or else there would eventually be no land; he suggested that marine fossils in mountainsides must once have been at
4067-512: The edge of the uplifted layers are exposed. Although mountain building mostly takes place in orogens, a number of secondary mechanisms are capable of producing substantial mountain ranges. Areas that are rifting apart, such as mid-ocean ridges and the East African Rift , have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography . In strike-slip orogens, such as
4150-437: The emplacement of granite at the core of the thermal dome. This phase occurred at lower pressure because of erosional unloading, but the temperatures were more extreme, ranging up to about 700 °C (1,300 °F). With deformation complete, the thermal dome decayed; minor mineralogical changes occurred during this decay phase. The region has since been effectively stable. Geochronology of several Archean rock units establishes
4233-567: The end of the Kenoran orogen. Orogeny Orogeny ( / ɒ ˈ r ɒ dʒ ə n i / ) is a mountain - building process that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges . This involves a series of geological processes collectively called orogenesis . These include both structural deformation of existing continental crust and
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#17330856035284316-453: The entire planet has been repeatedly resurfaced and modified. Secondary crust is formed by partial melting of mostly silicate materials in the mantle, and so is usually basaltic in composition. This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the lunar maria . On Earth secondary crust forms primarily at mid-ocean spreading centers , where
4399-409: The final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and which dip away from the orogenic core. An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis. Orogens are usually long, thin, arcuate tracts of rock that have
4482-463: The formation of the initial plagioclase-rich material. The best-characterized and most voluminous of these later additions are the mare basalts formed between about 3.9 and 3.2 billion years ago. Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago. There is no evidence of plate tectonics . Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth. Although
4565-415: The greenstone belt caused movement horizontally along several faults and moved huge blocks of the crust vertically relative to adjacent blocks. This combination of folding, intrusion and faulting built mountain ranges throughout northern Minnesota, northern Wisconsin, Michigan's Upper Peninsula and southernmost Ontario. Igneous and high-grade metamorphic rocks are associated with the orogeny. By extrapolating
4648-609: The greenstone belt was subjected to new stresses that caused movement along several faults. Faulting on both small and large scales is typical of greenstone belt deformation. These faults show both vertical and horizontal movement relative to adjacent blocks. Large-scale faults typically occur along the margins of the greenstone belts where they are in contact with enclosed granitic rocks. Vertical movement may be thousands of meters and horizontal movements of many kilometers occur along some fault zones. Some time before 2,600 million years ago , masses of magma intruded under and within
4731-456: The high-grade rocks are coarser-grained and contain minerals that reflect higher temperatures. The granitic intrusions within the high-grade metasediments were produced by subduction of the ocean crust and partial melting of metasedimentary rocks. Immediately south of Voyageurs National Park and extending to the Vermilion fault is a broad transition zone that contains migmatite. The Quetico gneiss belt represents an accretionary wedge that formed in
4814-491: The igneous mechanisms that formed them. This is because it is difficult to study: none of Earth's primary crust has survived to today. Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed all rocks older than about 4 billion years , including whatever primary crust Earth once had. However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury's highlands might represent primary crust, though this
4897-416: The interior of Earth into space. A theoretical protoplanet named " Theia " is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon. As the Moon formed, the outer part of it is thought to have been molten, a " lunar magma ocean ". Plagioclase feldspar crystallized in large amounts from this magma ocean and floated toward
4980-754: The migmatite include folds and foliations ; the foliations cut across both limbs of earlier-phase folds. These cross-cutting foliations indicate that the migmatite has undergone at least two periods of ductile deformation. The Wabigoon subprovince is a formerly active volcanic island chain, made up of metavolcanic-metasedimentary intrusions. These metamorphosed rocks are volcanically derived greenstone belts, and are surrounded and cut by granitic plutons and batholiths. The subprovince's greenstone belts consist of felsic volcanics, felsic batholiths and felsic plutons aged from 3,000 to 2,670 million years old. The Quetico gneiss belt extends some 970 km (600 mi) across Ontario and parts of Minnesota. The dominant rocks within
5063-489: The mountain range, although some sediments derive from the foreland. The fill of many such basins shows a change in time from deepwater marine ( flysch -style) through shallow water to continental ( molasse -style) sediments. While active orogens are found on the margins of present-day continents, older inactive orogenies, such as the Algoman , Penokean and Antler , are represented by deformed and metamorphosed rocks with sedimentary basins further inland. Long before
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#17330856035285146-432: The now-eroded and tilted beds upward, geologists have determined that these mountains were several kilometers high. Similar projections of the tilted beds downward, coupled with geophysical measurements on the greenstone belts in Canada, suggest the metavolcanic and metasedimentary rocks of the belts project downward at least a few kilometers. The action of metamorphism on the border between granite and gneiss bodies produces
5229-416: The ocean basin comes to a halt, and continued subduction begins to close the ocean basin. The closure of the ocean basin ends with a continental collision and the associated Himalayan-type orogen. Erosion represents the final phase of the orogenic cycle. Erosion of overlying strata in orogenic belts, and isostatic adjustment to the removal of this overlying mass of rock, can bring deeply buried strata to
5312-459: The original basaltic rock, which was on the bottom, occurs on the outer margins of the trough. The overlying, younger rock units – rhyolites and greywackes – occur closer to the center of the syncline. The rocks are so intensely folded that most have been tilted nearly 90°, with the tops of layers on one side of the synclinal belt facing those on the other side; the rock sequences are in effect lying on their sides. The folding can be so complex that
5395-416: The periodic opening and closing of an ocean basin, with each stage of the process leaving its characteristic record on the rocks of the orogen. The Wilson cycle begins when previously stable continental crust comes under tension from a shift in mantle convection . Continental rifting takes place, which thins the crust and creates basins in which sediments accumulate. As the basins deepen, the ocean invades
5478-601: The presence of this debris suggests that some explosive volcanic activity had occurred in the area earlier. The volcanism took place on the surface and the other deformations took place at various depths. Numerous earthquakes accompanied the volcanism and faulting. The Superior province forms the core of both the North American continent and the Canadian shield, and has a thickness of at least 250 km (160 mi). Its granites date from 2,700 to 2,500 million years ago. It
5561-496: The relationship to granite occurrences. Cawood et al. (2009) categorized orogenic belts into three types: accretionary, collisional, and intracratonic. Both accretionary and collisional orogens developed in converging plate margins. In contrast, Hercynotype orogens generally show similar features to intracratonic, intracontinental, extensional, and ultrahot orogens, all of which developed in continental detachment systems at converged plate margins. Crust (geology) In geology ,
5644-441: The rift zone, and as the continental crust rifts completely apart, shallow marine sedimentation gives way to deep marine sedimentation on the thinned marginal crust of the two continents. As the two continents rift apart, seafloor spreading commences along the axis of a new ocean basin. Deep marine sediments continue to accumulate along the thinned continental margins, which are now passive margins . At some point, subduction
5727-519: The rock. Blocks were added to the Superior province along a 1,200 km (750 mi) boundary that stretches from present-day eastern South Dakota into the Lake Huron area. The Algoman orogeny brought the Archean Eon to a close, about 2,500 million years ago ; it lasted less than 100 million years and marks a major change in the development of the Earth's crust. The Canadian shield contains belts of metavolcanic and metasedimentary rocks formed by
5810-491: The sea-floor. Orogeny was used by Amanz Gressly (1840) and Jules Thurmann (1854) as orogenic in terms of the creation of mountain elevations, as the term mountain building was still used to describe the processes. Elie de Beaumont (1852) used the evocative "Jaws of a Vise" theory to explain orogeny, but was more concerned with the height rather than the implicit structures created by and contained in orogenic belts. His theory essentially held that mountains were created by
5893-663: The southern portion of the Superior Province. The Slave province and portions of the Nain province were also affected. Between about 2,000 and 1,700 million years ago these combined with the Sask and Wyoming cratons to form the first supercontinent , the Kenorland supercontinent. Through most of the Archean Eon, the Earth had a heat production at least twice that of the present. The timing of initiation of plate tectonics
5976-414: The squeezing of certain rocks. Eduard Suess (1875) recognised the importance of horizontal movement of rocks. The concept of a precursor geosyncline or initial downward warping of the solid earth (Hall, 1859) prompted James Dwight Dana (1873) to include the concept of compression in the theories surrounding mountain-building. With hindsight, we can discount Dana's conjecture that this contraction
6059-423: The subduction produces compression in the overriding plate. Whether subduction produces compression depends on such factors as the rate of plate convergence and the degree of coupling between the two plates, while the degree of coupling may in turn rely on such factors as the angle of subduction and rate of sedimentation in the oceanic trench associated with the subduction zone. The Andes Mountains are an example of
6142-425: The subprovinces. Both folding and faulting can be seen in the Wabigoon, Quetico and Wawa subprovinces. These three subprovinces lie linearly in southwestern- to northeastern-oriented belts of about 140 km (90 mi) wide (see figure on right). The northernmost and widest province is the Wabigoon. It begins in north-central Minnesota and continues northeasterly into central Ontario; it is partially interrupted by
6225-444: The surface. The cumulate rocks form much of the crust. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite ): the lower part of the crust may contain a higher percentage of ferromagnesian minerals such as the pyroxenes and olivine , but even that lower part probably averages about 78% plagioclase. The underlying mantle is denser and olivine-rich. The thickness of
6308-460: The surface. The erosional process is called unroofing . Erosion inevitably removes much of the mountains, exposing the core or mountain roots ( metamorphic rocks brought to the surface from a depth of several kilometres). Isostatic movements may help such unroofing by balancing out the buoyancy of the evolving orogen. Scholars debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics ). Thus,
6391-604: The uprising of the Algoman Mountains. This was followed by intrusions of granite plutons and batholithic domes within the gneisses about 2,700 million years ago ; two examples are the Sacred Heart granite of southwestern Minnesota and the Watersmeet Domes metagabbros (metamorphosed gabbros ) that straddle the border of Wisconsin and Michigan's Upper Peninsula . After the intrusions solidified, new stresses on
6474-409: The youngest plutonic units. Metagreywackes and meta pelites from two areas traversing one of these aureoles near Yellowknife have been studied. Most of the Slave province rocks are granitic with metamorphosed Yellowknife metasedimentary and volcanic rocks. Isotopic ages of these rocks is around 2,500 million years ago , the time of the Kenoran orogeny. Rocks comprising the Slave province represent
6557-441: Was a continuous process, but three recognizable metamorphic phases can be correlated with established deformational phases. The cycle began with a deformation phase unaccompanied by metamorphism. This evolved into the second phase accompanied by broad regional metamorphism as thermal doming began. With continued updoming of the isotherms, the third phase produced minor folding but caused major metamorphic recrystallization, resulting in
6640-438: Was caused by the introduction of masses of magma. The Vermilion fault can be traced westward to North Dakota. It has had a 19 km (12 mi) horizontal movement with the northern block moving eastward and upward relative to the southern block. The junction between the Quetico and Wawa subprovinces has a zone of biotite -rich migmatite , a rock that has characteristics of both igneous and metamorphic processes; this indicates
6723-399: Was due to the cooling of the Earth (aka the cooling Earth theory). The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, fiercely contested by proponents of vertical movements in the crust, or convection within the asthenosphere or mantle . Gustav Steinmann (1906) recognised different classes of orogenic belts, including
6806-591: Was formed by the welding together of many small terranes , the ages of which decrease away from the nucleus. This progression is illustrated by the age of the Wabigoon, Quetico and Wawa subprovinces, discussed in their individual sections. Later terranes docked on the periphery of continental masses with geosynclines developing between the fused nuclei and oceanic crust. In general the Superior province consists of east–west-trending belts of predominately volcanic rocks alternating with belts of sedimentary and gneissic rocks. Due to down warping along elongate zones, each belt
6889-494: Was used before him, the American geologist G. K. Gilbert used the term in 1890 to mean the process of mountain-building, as distinguished from epeirogeny . Orogeny takes place on the convergent margins of continents. The convergence may take the form of subduction (where a continent rides forcefully over an oceanic plate to form a noncollisional orogeny) or continental collision (convergence of two or more continents to form
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