In geology, a forebulge is a flexural bulge in front as a result of a load on the lithosphere, often caused by tectonic interactions and glaciations. An example of forebulge can be seen in the Himalayan foreland basin, a result of the Indian-Eurasian (continent-continent) plate collision, in which the Indian plate subducted and the Eurasian plate created a large load on the lithosphere, leading to the Himalayas and the Ganges foreland basin. [1]
Forebulge is most commonly found with continent-continent convergent collisions, in which the formation of mountain ranges as the plates collide places a large load on the lithosphere below. [2] The lithosphere flexes in response to the load on the mantle, causing depression and subsidence (foredeep) followed by the forebulging in the front. The forebulge area is lifted by a height that is 4% of the depression height caused by the load. [3] It takes roughly 10,000 to 20,000 years for forebulge to fully develop when the mantle flexure to reach isostatic equilibrium, a process that is controlled by mantle viscosity. [3]
Forebulge can be seen during the formation of a mountain range, which creates a large load and crustal thickening that leads to lithospheric flexure. Part of the land sinks under the load (Foredeep) while part of the outer land forebulges, leading to the creation of these foreland basins. Forebulge associated with the formation of these basins is most commonly a result of convergent collision. [2] Foreland basins can occur in convergent subduction, but this is rare. [2] These basins are linked to fold-thrust belts, which are divided into three main types: collisional (peripheral), retroarc, and retreating collisional subduction. [4] Collisional and retroarc thrust belts form in collision convergent plates whereas retreating collisional forms when the subduction rate exceeds the convergence rate of the collision.
The Persian Gulf foreland basin and forebulge was created as a result of the collision of the Arabian and Eurasian plates around 13 million years ago. [5] The Zagros mountains that formed, as a result, created a load on the lithosphere that led to the creation of the modern-day Persian Gulf. [5]
Collisions between tectonic plates and island arcs lead to loading and flexure of the lithosphere. The flexure produces a significant forebulge, which divides a forearc basin and a back arc basin.
The Ganges basin and Himalayan Basin roughly 50 to 70 million years ago as a result of the load of the Himalayan mountains after the collision of the Eurasian and Indian plates. [6]
The Andean foreland basins were created as a result of the lithospheric flexure due to the load of the Andean mountain (fold-thrust belt) and resulting foredeep, forebulge, and backbulge. [4]
Forebulge can also be observed in the flexural plate of the Mariana arc, which was formed when the Pacific plate subducted under the Philippine plate. [7]
One cause for forebulge formation is the loading of the continental lithosphere by ice sheets during continental glaciations. Due to recession of the ice sheets, the formerly-glaciated areas are currently rising in a phenomenon known as post-glacial rebound. Forebulge collapse due to post-glacial rebound is greatest along the east coast of the United States, which lay just under the North American ice sheet. [8] As a result of forebulge collapse due to post-glacial rebound, most of the coast of the Eastern US has slowly been sinking; it is estimated that the area around the Chesapeake Bay will sink a foot (0.305 m) over the next hundred years. [9] Due to the coupling of the mantle with the plates, data from post-glacial rebound are used as a direct probe of the viscosity of the upper mantle. [10] As an ice sheet collapses, the land that was previously depressed rises in isostatic recovery followed by subsidence of forebulge, causing sea levels to rise. [8] Forebulge collapse is also the reason why the Netherlands and parts of southern England are still slowly sinking. [11] One estimate is that the center of the North Sea rose by about 170 m (558 ft) during the Ice Age because of forebulging. [12]
Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.
Orogeny 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 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. A synorogenic process or event is one that occurs during an orogeny.
The Alps form part of a Cenozoic orogenic belt of mountain chains, called the Alpide belt, that stretches through southern Europe and Asia from the Atlantic all the way to the Himalayas. This belt of mountain chains was formed during the Alpine orogeny. A gap in these mountain chains in central Europe separates the Alps from the Carpathians to the east. Orogeny took place continuously and tectonic subsidence has produced the gaps in between.
Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about 50,000 km (31,000 mi) of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 10,920 m (35,830 ft) below sea level.
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
Obduction is a geological process whereby denser oceanic crust is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction, which is less common, normally occurs in plate collisions at orogenic belts or back-arc basins.
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence creates a regional depression that provides accommodation space for accumulation of sediments. Over millions or tens or hundreds of millions of years the deposition of sediment, primarily gravity-driven transportation of water-borne eroded material, acts to fill the depression. As the sediments are buried, they are subject to increasing pressure and begin the processes of compaction and lithification that transform them into sedimentary rock.
Isostasy or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density. This concept is invoked to explain how different topographic heights can exist at Earth's surface. Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes, such as the Hawaiian Islands.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.
Forearc is a plate tectonic term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent plate margins. A back-arc region is the companion region behind the volcanic arc.
The Indian Plate is a minor tectonic plate straddling the equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, the Indian Plate broke away from the other fragments of Gondwana 100 million years ago, began moving north and carried Insular India with it. It was once fused with the adjacent Australian Plate to form a single Indo-Australian Plate, and recent studies suggest that India and Australia have been separate plates for at least 3 million years and likely longer. The Indian Plate includes most of modern South Asia and a portion of the basin under the Indian Ocean, including parts of South China and western Indonesia, and extending up to but not including Ladakh, Kohistan and Balochistan.
In tectonics, vertical displacement refers to the shifting of land in a vertical direction, resulting in uplift and subsidence. The displacement of rock layers can provide information on how and why Earth's lithosphere changes throughout geologic time. There are different mechanisms which lead to vertical displacement such as tectonic activity, and isostatic adjustments. Tectonic activity leads to vertical displacement when crust is rearranged during a seismic event. Isostatic adjustments result in vertical displacement through sinking due to an increased load or isostatic rebound due to load removal.
Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thickening, changes in the density distribution of the crust and underlying mantle, and flexural support due to the bending of rigid lithosphere.
In geology, continental collision is a phenomenon of plate tectonics 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 only known to occur on Earth.
A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure. The width and depth of the foreland basin is determined by the flexural rigidity of the underlying lithosphere, and the characteristics of the mountain belt. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that thin away from the mountain belt. Foreland basins represent an endmember basin type, the other being rift basins. Space for sediments is provided by loading and downflexure to form foreland basins, in contrast to rift basins, where accommodation space is generated by lithospheric extension.
Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces produced by faulting brought about subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.
The Himalayan foreland basin is an active collisional foreland basin system in South Asia. Uplift and loading of the Eurasian Plate on to the Indian Plate resulted in the flexure (bending) of the Indian Plate, and the creation of a depression adjacent to the Himalayan mountain belt. This depression was filled with sediment eroded from the Himalaya, that lithified and produced a sedimentary basin ~3 to >7 km deep. The foreland basin spans approximately 2,000 kilometres (1,200 mi) in length and 450 kilometres (280 mi) in width. From west to east the foreland basin stretches across five countries: Pakistan, India, Nepal, Bangladesh, and Bhutan.
The geology of Sicily records the collision of the Eurasian and the African plates during westward-dipping subduction of the African slab since late Oligocene. Major tectonic units are the Hyblean foreland, the Gela foredeep, the Apenninic-Maghrebian orogen, and the Calabrian Arc. The orogen represents a fold-thrust belt that folds Mesozoic carbonates, while a major volcanic unit is found in an eastern portion of the island. The collision of Africa and Eurasia is a retreating subduction system, such that the descending Africa is falling away from Eurasia, and Eurasia extends and fills the space as the African plate falls into the mantle, resulting in volcanic activity in Sicily and the formation of Tyrrhenian slab to the north.
The Junggar Basin, also known as the Dzungarian Basin or Zungarian Basin, is one of the largest sedimentary basins in Northwest China. It is located in Dzungaria in northern Xinjiang, and enclosed by the Tarbagatai Mountains of Kazakhstan in the northwest, the Altai Mountains of Mongolia in the northeast, and the Heavenly Mountains in the south. The geology of Junggar Basin mainly consists of sedimentary rocks underlain by igneous and metamorphic basement rocks. The basement of the basin was largely formed during the development of the Pangea supercontinent during complex tectonic events from Precambrian to late Paleozoic time. The basin developed as a series of foreland basins – in other words, basins developing immediately in front of growing mountain ranges – from Permian time to the Quaternary period. The basin's preserved sedimentary records show that the climate during the Mesozoic era was marked by a transition from humid to arid conditions as monsoonal climatic effects waned. The Junggar basin is rich in geological resources due to effects of volcanism and sedimentary deposition. According to Guinness World Records it is a land location remotest from open sea with great-circle distance of 2,648 km from the nearest open sea at 46°16′8″N86°40′2″E.
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