Continental collision

Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together. Continental collision is known only to occur on Earth.

Continental collision is not an instantaneous event, but may take several tens of millions of years before the faulting and folding caused by collisions stops. The collision between India and Asia has been ongoing for about 50 million years already and shows no signs of abating. Collision between East and West Gondwana to form the East African Orogen took about 100 million years from beginning (610 Ma) to end (510 Ma). Collision between Gondwana and Laurasia to form Pangea occurred in a relatively brief interval, about 50 million years long.

Continental-continental convergence Fig21contcont
Cartoon of a tectonic collision between two continents

Subduction zone: the collision site

The process begins as two continents (different bits of continental crust), separated across a tract of ocean (and oceanic crust), approach each other, while the oceanic crust is slowly consumed at a subduction zone. The subduction zone runs along the edge of one of the continents and dips under it, raising volcanic mountain chains at some distance behind it, such as the Andes of South America today. Subduction involves the whole lithosphere, the density of which is largely controlled by the nature of the crust it carries. Oceanic crust is thin (~6 km thick) and dense (about 3.3 g/cm³), consisting of basalt, gabbro, and peridotite. Consequently, most oceanic crust is subducted easily at an oceanic trench. In contrast, continental crust is thick (~45 km thick) and buoyant, composed mostly of granitic rocks (average density about 2.5 g/cm³). Continental crust is subducted with difficulty, but is subducted to depths of 90-150 km or more, as evidenced by ultra-high pressure (UHP) metamorphic suites. Normal subduction continues as long as the ocean exists, but the subduction system is disrupted as the continent carried by the downgoing plate enters the trench. Because it contains thick continental crust, this lithosphere is less dense than the underlying asthenospheric mantle and normal subduction is disrupted. The volcanic arc on the upper plate is slowly extinguished. Resisting subduction, the crust buckles up and under, raising mountains where a trench used to be. The position of the trench becomes a zone that marks the suture between the two continental terranes. Suture zones are often marked by fragments of the pre-existing oceanic crust and mantle rocks, known as ophiolites.

Deep subduction of continental crust

The continental crust on the downgoing plate is deeply subducted as part of the downgoing plate during collision, defined as buoyant crust entering a subduction zone. An unknown proportion of subducted continental crust returns to the surface as ultra-high pressure (UHP) metamorphic terranes, which contain metamorphic coesite and/or diamond plus or minus unusual silicon-rich garnets and/or potassium-bearing pyroxenes. The presence of these minerals demonstrate subduction of continental crust to at least 90–140 km deep. Examples of UHP terranes are known from the Dabie–Sulu belt of east-central China, the Western Alps, the Himalaya of India, the Kokchetav Massif of Kazakhstan, the Bohemian Massif of Europe, the North Qaidam of Northwestern China, the Western Gneiss Region of Norway, and Mali. Most UHP terranes consist of an imbricated sheets or nappes. The fact that most UHP terranes consist of thin sheets suggests that much thicker, volumetrically dominant tracts of continental crust are more deeply subducted.

Orogeny and collapse

Mountain by reverse fault
Mountain formation by a reverse fault movement

An orogeny is underway when mountains begin to grow in the collision zone. There are other modes of mountain formation and orogeny but certainly continental collision is one of the most important. Rainfall and snowfall increase on the mountains as these rise, perhaps at a rate of a few millimeters per year (at a growth rate of 1 mm/year, a 5,000 m tall mountain can form in 5 million years, a time period that is less than 10% of the life of a typical collision zone). River systems form, and glaciers may grow on the highest peaks. Erosion accelerates as the mountains rise, and great volumes of sediment are shed into the rivers, which carry sediment away from the mountains to be deposited in sedimentary basins in the surrounding lowlands. Crustal rocks are thrust faulted over the sediments and the mountain belt broadens as it rises in height. A crustal root also develops, as required by isostasy; mountains can be high if underlain by thicker crust. Crustal thickening may happen as a result of crustal shortening or when one crust overthrusts the other. Thickening is accompanied by heating, so the crust becomes weaker as it thickens. The lower crust begins to flow and collapse under the growing mountain mass, forming rifts near the crest of the mountain range. The lower crust may partially melt, forming anatectic granites which then rise into the overlying units, forming granite intrusions. Crustal thickening provides one of two negative feedbacks on mountain growth in collision zones, the other being erosion. The popular notion that erosion is responsible for destroying mountains is only half correct - viscous flow of weak lower mantle also reduces relief with time, especially once the collision is complete and the two continents are completely sutured. Convergence between the continents continues because the crust is still being pulled down by oceanic lithosphere sinking in the subduction zone to either side of the collision as well as beneath the impinging continent.

The pace of mountain building associated with the collision is measured by radiometric dating of igneous rocks or units that have been metamorphosed during the collision and by examining the record of sediments shed from the rising mountains into the surrounding basins. The pace of ancient convergence can be determined with paleomagnetic measurements, while the present rate of convergence can be measured with GPS.

Far-field effects

The effects of the collision are felt far beyond the immediate site of collision and mountain-building. As convergence between the two continents continues, the region of crustal thickening and elevation will become broader. If there is an oceanic free face, the adjacent crustal blocks may move towards it. As an example of this, the collision of India with Asia forced large regions of crust to move south to form modern Southeast Asia. Another example is the collision of Arabia with Asia, which is squeezing the Anatolian Plate (present day Turkey). As a result, Turkey is moving west and south into the Mediterranean Sea and away from the collision zone. These far-field effects may result in the formation of rifts, and rift valleys such as that occupied by Lake Baikal, the deepest lake on Earth.

Fossil collision zones

Continental collisions are a critical part of the supercontinent cycle and have happened many times in the past. Ancient collision zones are deeply eroded but may still be recognized because these mark sites of intense deformation, metamorphism, and plutonic activity that separate tracts of continental crust having different geologic histories prior to the collision. Old collision zones are commonly called "suture zones" by geologists, because this is where two previous continents are joined or sutured together.

References

  • Ernst, W.G. (2006). "Preservation/exhumation of ultrahigh-pressure subduction complexes". Lithos. 92 (3–4): 321–335. Bibcode:2006Litho..92..321E. doi:10.1016/j.lithos.2006.03.049.
  • Ernst, W.G.; Maruyama, S. Wallis; Wallis, S. (1997). "Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust". Proceedings of the National Academy of Sciences. 94 (18): 9532–9537. Bibcode:1997PNAS...94.9532E. doi:10.1073/pnas.94.18.9532. PMC 23212. PMID 11038569.
  • O'Brien, P.J. (2001). "Subduction followed by collision; Alpine and Himalayan examples". Physics of the Earth and Planetary Interiors. 127 (1–4): 277–291. Bibcode:2001PEPI..127..277O. doi:10.1016/S0031-9201(01)00232-1.
  • Toussaint, G.; Burov, E.; Avouac, J.-P. (2004). "Tectonic evolution of a continental collision zone: A thermomechanical numerical model". Tectonics. 23 (6): TC6003. Bibcode:2004Tecto..23.6003T. doi:10.1029/2003TC001604.
  • Song, S.G. (2014). "Continental orogenesis from ocean subduction, continent collision/subduction, to orogen collapse, and orogen recycling: The example of the North Qaidam UHPM belt, NW China". Earth-Science Reviews. 129 (3–4): 59–84. Bibcode:2014ESRv..129...59S. doi:10.1016/j.earscirev.2013.11.010.

External links

1950 Assam–Tibet earthquake

The 1950 Assam–Tibet earthquake, also known as the Assam earthquake, occurred on 15 August and had a moment magnitude of 8.6. The epicentre was located in the Mishmi Hills, known in Chinese as the Qilinggong Mountains (祁灵公山), south of the Kangri Garpo and just east of the Himalayas in the North-East Frontier Agency part of Assam, India. This area, south of the McMahon Line and now known as Arunachal Pradesh, is today disputed between China and India.

Occurring on a Tuesday evening at 7:39 pm India Standard Time, the earthquake was destructive in both Assam (India) and Tibet (China), and approximately 4,800 people were killed. The earthquake is notable as being the largest recorded quake caused by continental collision rather than subduction, and is also notable for the loud noises produced by the quake and reported throughout the region.

Alleghanian orogeny

The Alleghanian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 1957.

The Alleghanian orogeny occurred approximately 325 million to 260 million years ago over at least five deformation events in the Carboniferous to Permian period. The orogeny was caused by Africa colliding with North America. At the time, these continents did not exist in their current forms: North America was part of the Euramerica super-continent, while Africa was part of Gondwana. This collision formed the super-continent Pangaea, which contained all major continental land masses. The collision provoked the orogeny: it exerted massive stress on what is today the Eastern Seaboard of North America, forming a wide and high mountain chain. Evidence for the Alleghanian orogeny stretches for many hundreds of miles on the surface from Alabama to New Jersey and can be traced further subsurface to the southwest. In the north, the Alleghanian deformation extends northeast to Newfoundland. Subsequent erosion wore down the mountain chain and spread sediments both to the east and to the west.

Amazonian Craton

The Amazonian Craton is a geologic province located in South America. It occupies a large portion of the central, north and eastern part of the continent. The Guiana Shield and Central Brazil Shield (Guaporé Shield) constitutes respectively the northern and southern exhumed parts of the craton. Between the two shields lies the Amazon Rift, a zone of weakness within the craton. Smaller cratons of Precambrian rocks south of the Amazonian Shield are the Río de la Plata Craton and the São Francisco Craton, which lies to the east.

The Río Apa Craton at the Paraguay-Brazil border is considered be likely just the southern part of the Amazonian Craton. The rocks of Río Apa were deformed during the Sunsás orogeny.It has been suggested that the Late Mesoproterozoic–Early Neoproterozoic aged Sveconorwegian Orogen in Fennoscandia could have been caused by a continent–continent collision between the Amazonia and Baltica. The question is open if Telemarkia terrane in Norway was derived from the Amazonian Craton but this possibility does not imply necessarily that there was a continental collision.

Anorogenic magmatism

In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny. This contrasts with orogenic magmatism that occurs at convergent plate boundaries where continental collision, subduction and orogeny are common.

Balcones Fault

The Balcones Fault or Balcones Fault Zone is a tensional structural system in the U.S. state of Texas that runs approximately from the southwest part of the state near Del Rio to the north central region near Dallas along Interstate 35. The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and horsts. One of the obvious features is the Mount Bonnell Fault.The location of the fault zone may be related to the Ouachita Mountains, formed 300 million years ago during a continental collision. Although long since eroded away in Texas, the roots of these ancient mountains still exist, buried beneath thousands of feet of sediment. These buried Ouachita Mountains may still be an area of weakness that becomes a preferred site for faulting when stress exists in the Earth's crust.

The Balcones Fault has remained inactive for nearly 15 million years, with the last activity being during the Miocene epoch. This activity was related to subsidence of the Texas Coastal Plain, most likely from the large amount of sediment deposited on it by Texas rivers. The Balcones Fault is in one of the lowest-risk zones for earthquakes in the United States.The surface expression of the fault is the Balcones Escarpment, which forms the eastern boundary of the Texas Hill Country and the western boundary of the Texas Coastal Plain and consists of cliffs and cliff-like structures. Subterranean features such as Wonder Cave and numerous other smaller caves are found along the fault zone.

Many cities are located along this fault zone. Springs such as San Pedro Springs, Comal Springs, San Marcos Springs, Barton Springs and Salado Springs are found in the fault zone and provide a source of fresh water and a place for human settlement.

The Balcones Fault Zone is a demarcation line for certain ecological systems and species distributions, e.g., the California fan palm (Washingtonia filifera) is the only species of palm tree that is native to the continental United States west of the Balcones Fault.

Cameron's Line

Cameron's Line is an Ordovician suture fault in the northeast United States which formed as part of the continental collision known as the Taconic orogeny around 450 mya. Named after Eugene N. Cameron, who first described it in the 1950s, it ties together the North American continental craton, the prehistoric Taconic Island volcanic arc, and the bottom of the ancient Iapetus Ocean.

East African Orogeny

The East African Orogeny (EAO) is the main stage in the Neoproterozoic assembly of East and West Gondwana (Australia–India–Antarctica and Africa–South America) along the Mozambique Belt.

Hebridean Terrane

The Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archaean and Paleoproterozoic gneisses of the Lewisian complex, unconformably overlain by the Neoproterozoic Torridonian sediments, which in turn are unconformably overlain by a sequence of Cambro–Ordovician sediments. It formed part of the Laurentian foreland during the Caledonian continental collision.

Messalonskee Lake

Messalonskee Lake is a body of water in the Belgrade Lakes region of Maine. It is bordered by the towns of Oakland, Sidney, and Belgrade. The lake is a 9 mile long, narrow, natural creation, resulting from continental collision and glacial scouring. A dam originally built in the town of Oakland in 1905 increased the lake's size.

In the first part of the 20th century, Messalonskee Stream provided waterpower for Oakland's Cascade Woolen Mill, as well as for a number of factories responsible for Oakland's long-defunct title as "axehead capital of the world."

Messalonskee Lake is home to a great variety of wildlife, including great blue herons, bass, yellow perch, white perch, sunfish, painted and snapping turtles, loons, and occasionally Bald Eagles can be seen soaring above the lake. The surrounding community has recently formed the Messalonskee Lake Association in the interest of its protection and preservation.

Like some Maine lakes, Messalonskee Lake has seen infestations of Milfoil.The lake is also more commonly called "Snow Pond", a reference to Philip Snow who settled in the area in 1774.Many summer camps and related tourist accommodations are located on Messalonskee Lake, among them the New England Music Camp, which was founded in 1937.

Nappe

In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi) or 5 km (3.1 mi) above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced (or "thrust") over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent folds, shearing along the fault plane, imbricate thrust stacks, fensters and klippe.

The term stems from the French word for tablecloth in allusion to a rumpled tablecloth being pushed across a table.

Obduction

Obduction was originally defined by Coleman to mean the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere.

Subsequently, this definition has been broadened to mean the emplacement of continental lithosphere by oceanic lithosphere at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.

Obduction occurs where a fragment of continental crust is caught in a subduction zone with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction may occur where a small tectonic plate is caught between two larger plates, with the lithosphere (both island arc and oceanic) welding onto an adjacent continent as a new terrane. When two continental plates collide, obduction of the oceanic lithosphere between them is often a part of the resulting orogeny.Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.

Pampean orogeny

The Pampean orogeny (Spanish: orogenia pampeana) was an orogeny active in the Cambrian in the western margin of the ancient landmass of Gondwana. The orogens remains can now be observed in central Argentina, in particular at the Sierras de Córdoba and other parts of the eastern Sierras Pampeanas. It is uncertain if the orogeny involved at some point a continental collision. The Pampean orogen can be considered both part of the larger Terra Australis orogen and of the Brasiliano orogeny. The Pampean orogeny was succeeded by the Famatinian orogeny further west.

Parkstein

Parkstein is a district in the municipality of Neustadt an der Waldnaab in Bavaria in Germany. In 2006, it counted approximately 2500 citizens within its district. The origins of its castle, built atop a conical shaped mountain, also called the Parkstein, date back to around the year 1000. A first written account of its existence can be traced back to the year 1053 in the documentations of the monks of Niederalteich of the Reichstag in Merseburg. Parkstein was chartered in 1435.Most likely in November 1796, Alexander von Humboldt called the 24-million-year-old basalt formation the most beautiful he had encountered in Europe. According to the Bavarian State Geology Office, during the Cenozoic, from Paleocene to the Pliocene epochs, a number of active volcanoes produced liquid magma in Northern Bavaria, mostly due to the continental collision of Europe and Africa. As a result, not only the Alps but a number of fissures and cracks began to form throughout central Europe where magma could rise. Most of the Parkstein's magma cooled below the surface, leading to the crystalline column formation that is now exposed as a result of erosion.

Rock cycle

The rock cycle is a basic concept in geology that describes the transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. As the adjacent diagram illustrates, each of the types of rocks is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

Slab detachment

In plate tectonics, slab detachment or slab break-off may occur during continent-continent or arc-continent collisions. When the continental margin of the subducting plate reaches the oceanic trench of the subduction zone, the more buoyant continental crust will in normal circumstances experience only a limited amount of subduction into the asthenosphere. The slab pull forces will, however, still be present and this normally leads to the breaking off or detachment of the descending slab from the rest of the plate. The isostatic response to the detachment of the downgoing slab is rapid uplift. Slab detachment is also followed by the upwelling of relatively hot asthenosphere to fill the gap created, leading in many cases to magmatism.The uncritical use of the slab-detachment model to explain disparate observations of magmatism, uplift and exhumation in continental collision zones has been criticised.

Supercontinent cycle

The supercontinent cycle is the quasi-periodic aggregation and dispersal of Earth's continental crust. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being reconfigured. One complete supercontinent cycle is said to take 300 to 500 million years. Continental collision makes fewer and larger continents while rifting makes more and smaller continents.

Tectonics

Tectonics (from Latin tectonicus; from Ancient Greek τεκτονικός (tektonikos), meaning 'pertaining to building') is the process that controls the structure and properties of the Earth's crust and its evolution through time. In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Tectonics also provides a framework for understanding the earthquake and volcanic belts that directly affect much of the global population. Tectonic studies are important as guides for economic geologists searching for fossil fuels and ore deposits of metallic and nonmetallic resources. An understanding of tectonic principles is essential to geomorphologists to explain erosion patterns and other Earth surface features.

Timanide Orogen

The Timanide Orogen (Russian: Ороген Протоуралид-Тиманид, literally: "Protouralian–Timanide Orogen") is a pre-Uralian orogen that formed in northeastern Baltica during the Neoproterozoic in the Timanide orogeny. The orogen is about 3000 km long. Its extreme points include the southern Urals in the south and the Polar Urals, the Kanin and Varanger peninsulas in the north. The Timan Ridge is the type area of the orogen. To the west, at the Varanger Peninsula, the north-west oriented Timanide Orogen is truncated by the younger Scandinavian Caledonide Orogen that has an oblique disposition. The northeastern parts of the orogen are made up of volcanic and sedimentary rocks, granitoids and few ophiolites. In contrast the southwestern part of the orogen is made up mostly of sedimentary rocks. I and A type granitoids and volcanic rocks are common in the orogen.From the Late Neoproterozoic o the Middle Cambrian the Timanide Orogen was associated to a subduction zone that existed to the northeast of it. Most studies interpret subduction as going inward (subducted plate moving southwest) albeit one suggest the opposite (subducted plate moving to the northeast).

In the Cambrian the Timanide Orogen is believed to have developed in a continental collision context as Baltica and Arctida collided between 528 and 510 million years ago. Some researchers do however dissent from this view suggesting there was never such a collision.Erosion of the Timanide Orogen have produced sediments that are now found in the East European Platform, including the Cambrian Sablino Formation near Lake Ladoga. Studies of sediments points that its likely that the erosion of the orogen was beginning in the Cambrian and then became stronger in Ordovician.The first geologists to study the orogen where Wilhelm Ramsay and Feodosy Tschernyschev who published works in 1899 and 1901 respectively. Hans Reusch compiled the existing knowledge on the orogen in 1900.

Variscan orogeny

The Variscan or Hercynian orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.

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