The Rhenohercynian Zone or Rheno-Hercynian zone in structural geology describes a fold belt of west and central Europe, formed during the Hercynian orogeny. The zone consists of folded and thrusted Devonian and early Carboniferous sedimentary rocks that were deposited in a back-arc basin along the southern margin of the existing paleocontinent Laurussia; the Rhenohercynian Zone, named for the Rhine River and the Hercynian Forest of Antiquity, forms a narrow zone through western and central Europe, from Cornwall and Ireland in the west to the Harz mountains of central Germany in the east, including the Rhenish Massif. The total length of this ancient basin could have been more than 2500 km. In the east the basin merges with the East-Silesian basin of southern Poland; the sedimentary rocks that were laid down in the basin are weakly metamorphic. Most geologists consider the South-Portuguese zone to be a continuation of the zone to the west. Whether the Rhenohercynian Basin was a continuous feature or rather a string of temporaneously interconnected smaller basins is not well understood, because in many places the Devonian and Carboniferous rock strata are covered with younger deposits.
Parts of the basin have their own names, like the Cornwall basin in Cornwall, the Munster basin in Ireland or the Rhenisch basin in Belgium and Germany. The Rhenohercynian Zone is a part of the northern foreland of the Hercynian orogeny, it has a lower grade of metamorphism than the Saxothuringian Zone to the south, meaning its rocks have been at smaller depths and under lower temperatures. The Subvariscan Zone north of the Rhenohercynian Zone was untouched by Hercynian metamorphism. During the Hercynian orogeny, the Rhenohercynian zone was thrusted internally, it was thrust over the foreland to the north. From the south it was overthrust by the Mid-German Crystalline High, part of the Saxothuringian Zone; the metamorphic grade or degree of metamorphism increases towards the southeast. The southern edge of the Rhenish Massif lies in the Northern Phyllite Zone, which has a higher grade than other parts of the zone; the Rhenohercynian basin was situated north of the Rheic Ocean, the ocean that spread between the continents of Laurussia and Gondwana from the Silurian onwards.
The southern margin of Laurussia was formed during the Caledonian orogeny of the Silurian period, about 420 million years ago. In the Gedinnian/Lochkovian the southern part of the Caledonian mountain belt became a region of north-south extension. An elongated basin was formed parallel to the continental margin, it separated the London-Brabant Massif to the north from the Normannian and Mid-German Highs to the south. In the Middle Devonian a subduction zone existed south of Laurussia, where oceanic lithosphere of the Rheic Ocean subducted beneath the Mid-German/Normannian highs. Volcanism above the subduction zone created a cordillera-type mountain chain, the Ligerian cordillera. In the Siegenian/Pragian and Emsian the Rhenohercynic basin was a back-arc basin behind this cordillera. Tectonic subsidence in a system of horsts and grabens together with basaltic volcanism resulted in the creation of new oceanic lithosphere. In the Middle Devonian a second basin, the Saxothuringian/Armorican basin, developed south of the Rhenohercynian basin.
To the west some crustal convergence took place, the Normannian High was thrust over the sedimentary basin fill of the Rhenohercynian basin. The Rhenohercynian basin disappeared when the continent Gondwana collided with Laurussia in the course of the Carboniferous period; the sedimentary rocks in the basin were thrust in a series of piggyback basins over the northern foreland. These rocks now form the folded sequences of the Ardennes, the Eifel and the Harz. From the Frasnian age the mafic volcanism ended, the basin came locally under compressional stress, which led to folding and thrusting in the sedimentary rocks. Somewhere near the end of the Devonian, a subduction zone developed under the Mid-German/Normannian highs and Rhenohercynian crust began to subduct; this was the short Bretonnic phase of the Hercynian orogeny. It was followed, from the Tournaisian till the end of the Visean by a new period of extension. During the Sudetic phase of the Hercynian orogeny compressional tectonics had the upper hand again.
In the Namurian age full-scale continental collision between Laurussia and Gondwana resulted in the destruction of the last oceanic crust of the basin. Its sedimentary fill was, not subducted but instead thrust northward. During the part of the Carboniferous period the Rhenohercynian zone formed the foreland of a fast-developing Hercynian mountainbelt to the south. Isostatic subsidence of the foreland resulted in the development of a deep foreland basin; this filled with the products of erosion in the Hercynian mountains and the contemporaneously uplifted London-Brabant Massif to the north. During the Westphalian, the basin was filled and rose above sea level; the Rhenohercynian basin was filled with Carboniferous sediments. Sedimentation was disrupted by tectonic phases, but the total thickness of the sediments can in some places be more than several kilometers; when a foreland basin was formed in the Rhenohercynian zone, this was filled with upper Carboniferous flysch and molasse s
Back-arc basins are geologic basins, submarine features associated with island arcs and subduction zones. They are found at some convergent plate boundaries, presently concentrated in the western Pacific Ocean. Most of them result from tensional forces caused by oceanic trench rollback and the collapse of the edge of the continent; the arc crust is under extension or rifting as a result of the sinking of the subducting slab. Back-arc basins were a surprising result for plate tectonics theorists, who expected convergent boundaries to be zones of compression, rather than major extension. However, they are now recognized as consistent with this model in explaining how the interior of Earth loses heat. Back-arc basins are very long and narrow; the restricted width of back-arc basins is because magmatic activity depends on water and induced mantle convection and these are both concentrated near the subduction zone. Spreading rates vary from slow spreading, a few centimeters per year, to fast, 15 cm/year.
These ridges erupt basalts. The high water contents of back-arc basin basalt magmas is derived from water carried down the subduction zone and released into the overlying mantle wedge. Additional source of water could be the eclogitization of amphiboles and micas in the subducting slab. Similar to mid-ocean ridges, back-arc basins have hydrothermal vents and associated chemosynthetic communities. Evidence of this spreading came from cores of the basin floor; the thickness of sediment that collected in basin decreased toward the center of the basin. The idea that thickness and age of sediment on the sea floor is related to the age of the oceanic crust was proposed by Harry Hess. Magnetic anomalies of the crust formed in back-arc basins deviated in form from the crust formed at mid-ocean ridges. In many areas the anomalies do not appear parallel; the profiles of the magnetic anomalies in the basin do not show symmetry or a central anomaly as a traditional ocean basin does. This has prompted some to characterize the spreading in back-arc basins to be more diffused and less uniform than at mid-ocean ridges.
The idea that back-arc basin spreading is inherently different that mid-ocean ridge spreading has been debated though the years. Other arguments put forward is that the process of seafloor spreading is the same but the movement of seafloor spreading centers in the basin causes the asymmetry in the magnetic anomalies; this can be seen in the Lau back-arc basin. Though the magnetic anomalies are more complex to decipher the rocks sampled from back-arc basin spreading centers do not differ much from those at mid-ocean ridges; the volcanic rocks of the nearby island arc do differ from those in the basin. Back-arc basins are different from normal mid-ocean ridges because they are characterized by asymmetric seafloor spreading, but this is quite variable within single basins. For example, in the central Mariana Trough current spreading rates are 2-3 times greater on the western flank whereas at the southern end of the Mariana Trough the position of the spreading center adjacent to the volcanic front suggests that overall crustal accretion has been nearly 100% asymmetric there.
This situation is mirrored to the north where a large spreading asymmetry is developed. Other back-arc basins such as the Lau Basin have undergone large rift jumps and propagation events that have transferred spreading centers from arc-distal to more arc-proximal positions although recent spreading rates appear to be symmetric with small rift jumps; the cause of asymmetric spreading in back-arc basins remains poorly understood. General ideas invoke asymmetries relative to the spreading axis in arc melt generation processes and heat flow, hydration gradients with distance from the slab, mantle wedge effects, evolution from rifting to spreading; the extension of the crust behind volcanic arcs is believed to be caused by processes in association with subduction. As the subducting plate descends into the asthenosphere it is heated up causing the volcanism at the island arcs. Another result of this heating is a convection cell; the rising magma and heat in the convection cell cause a rift to form.
This rift drives the island arc toward the subduction zone and the rest of the plate away from the subduction zone. This process is known as trench rollback; this is the backward motion of the subduction zone relative to the motion of the plate, being subducted. As the subduction zone and its associated trench pull backward, the overriding plate is stretched, thinning the crust, manifest in the back-arc basin. Therefore, back-arc basins form. In some cases, extension is triggered by the entrance of a buoyant feature in the subduction zone, which locally slows down subduction and induce the subducting plate to rotate adjacent to it; this rotation is associated with overriding plate extension. For back-arc extension to form, a subduction zone is required, but not all subduction zones have a back-arc extension feature. Back-arc basins are found in areas where the subducting plate of oceanic crust is old; the age need to establish back-arc spreading is oceanic lithosphere, 55 million years old or older.
This includes areas like the western pacific where
Iran called Persia, the Islamic Republic of Iran, is a country in Western Asia. With over 81 million inhabitants, Iran is the world's 18th most populous country. Comprising a land area of 1,648,195 km2, it is the second largest country in the Middle East and the 17th largest in the world. Iran is bordered to the northwest by Armenia and the Republic of Azerbaijan, to the north by the Caspian Sea, to the northeast by Turkmenistan, to the east by Afghanistan and Pakistan, to the south by the Persian Gulf and the Gulf of Oman, to the west by Turkey and Iraq; the country's central location in Eurasia and Western Asia, its proximity to the Strait of Hormuz, give it geostrategic importance. Tehran is the country's capital and largest city, as well as its leading economic and cultural center. Iran is home to one of the world's oldest civilizations, beginning with the formation of the Elamite kingdoms in the fourth millennium BCE, it was first unified by the Iranian Medes in the seventh century BCE, reaching its greatest territorial size in the sixth century BCE, when Cyrus the Great founded the Achaemenid Empire, which stretched from Eastern Europe to the Indus Valley, becoming one of the largest empires in history.
The Iranian realm fell to Alexander the Great in the fourth century BCE and was divided into several Hellenistic states. An Iranian rebellion culminated in the establishment of the Parthian Empire, succeeded in the third century CE by the Sasanian Empire, a leading world power for the next four centuries. Arab Muslims conquered the empire in the seventh century CE; the Islamization of Iran led to the decline of Zoroastrianism, by the country's dominant religion, Iran's major contributions to art and science spread within the Muslim rule during the Islamic Golden Age. After two centuries, a period of various native Muslim dynasties began, which were conquered by the Seljuq Turks and the Ilkhanate Mongols; the rise of the Safavids in the 15th century led to the reestablishment of a unified Iranian state and national identity, with the country's conversion to Shia Islam marking a turning point in Iranian and Muslim history. Under Nader Shah, Iran was one of the most powerful states in the 18th century, though by the 19th century, a series of conflicts with the Russian Empire led to significant territorial losses.
The Iranian Constitutional Revolution in the early 20th century led to the establishment of a constitutional monarchy and the country's first legislature. A 1953 coup instigated by the United Kingdom and the United States resulted in greater autocracy and growing Western political influence. Subsequent widespread dissatisfaction and unrest against the monarchy led to the 1979 Revolution and the establishment of an Islamic republic, a political system that includes elements of a parliamentary democracy vetted and supervised by a theocracy governed by an autocratic "Supreme Leader". During the 1980s, the country was engaged in a war with Iraq, which lasted for eight years and resulted in a high number of casualties and economic losses for both sides; the sovereign state of Iran is a founding member of the UN, ECO, NAM, OIC, OPEC. It is a major regional and middle power, its large reserves of fossil fuels – which include the world's largest natural gas supply and the fourth largest proven oil reserves – exert considerable influence in international energy security and the world economy.
The country's rich cultural legacy is reflected in part by its 22 UNESCO World Heritage sites, the third largest number in Asia and 11th largest in the world. Iran is a multicultural country comprising numerous ethnic and linguistic groups, the largest being Persians, Azeris and Lurs. Organizations including Amnesty International and Human Rights Watch have criticized Iran's women's rights record; the term Iran derives directly from Middle Persian Ērān, first attested in a third-century inscription at Rustam Relief, with the accompanying Parthian inscription using the term Aryān, in reference to the Iranians. The Middle Iranian ērān and aryān are oblique plural forms of gentilic nouns ēr- and ary-, both deriving from Proto-Iranian *arya-, recognized as a derivative of Proto-Indo-European *ar-yo-, meaning "one who assembles". In the Iranian languages, the gentilic is attested as a self-identifier, included in ancient inscriptions and the literature of the Avesta, remains in other Iranian ethnic names Alan and Iron.
Iran has been referred to as Persia by the West, due to the writings of Greek historians who referred to all of Iran as Persís, meaning "land of the Persians", while Persis itself was one of the provinces of ancient Iran, today defined as Fars. As the most extensive interaction the Ancient Greeks had with any outsider was with the Persians, the term persisted long after the Greco-Persian Wars. In 1935, Reza Shah requested the international community to refer to the country by its native name, effective March 22 that year; as The New York Times explained at the time, "At the suggestion of the Persian Legation in Berlin, the Tehran government, on the Persian New Year, March 21, 1935, substituted Iran for Persia as the official name of the country." Opposition to the name change led to the reversal of the decision, Professor Ehsan Yarshater, editor of Encyclopædia Iranica, propagated a move to use Persia and Iran interchangeably. Today, both Iran and Persia are used in cultural contexts, while Iran remains irreplaceab
Southeast Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China and Japan, east of India, west of Papua New Guinea, north of Australia. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, to the south by Australia and the Indian Ocean; the region is the only part of Asia that lies within the Southern Hemisphere, although the majority of it is in the Northern Hemisphere. In contemporary definition, Southeast Asia consists of two geographic regions: Mainland Southeast Asia known as Indochina, comprising parts of Northeast India, Laos, Thailand and West Malaysia. Maritime Southeast Asia known as Nusantara, the East Indies and Malay Archipelago, comprises the Andaman and Nicobar Islands of India, East Malaysia, the Philippines, East Timor, Christmas Island, the Cocos Islands. Taiwan is included in this grouping by many anthropologists; the region lies near the intersection of geological plates, with both heavy seismic and volcanic activities.
The Sunda Plate is the main plate of the region, featuring all Southeast Asian countries except Myanmar, northern Thailand, northern Laos, northern Vietnam, northern Luzon of the Philippines. The mountain ranges in Myanmar and peninsular Malaysia are part of the Alpide belt, while the islands of the Philippines are part of the Pacific Ring of Fire. Both seismic belts meet in Indonesia, causing the region to have high occurrences of earthquakes and volcanic eruptions. Southeast Asia covers about 4.5 million km2, 10.5% of Asia or 3% of earth's total land area. Its total population is about 8.5 % of the world's population. It is the third most populous geographical region in the world after East Asia; the region is culturally and ethnically diverse, with hundreds of languages spoken by different ethnic groups. Ten countries in the region are members of ASEAN, a regional organization established for economic, military and cultural integration amongst its members; the region, together with part of South Asia, was well known by Europeans as the East Indies or the Indies until the 20th century.
Chinese sources referred the region as 南洋, which means the "Southern Ocean." The mainland section of Southeast Asia was referred to as Indochina by European geographers due to its location between China and the Indian subcontinent and its having cultural influences from both neighboring regions. In the 20th century, the term became more restricted to territories of the former French Indochina; the maritime section of Southeast Asia is known as the Malay Archipelago, a term derived from the European concept of a Malay race. Another term for Maritime Southeast Asia is Insulindia, used to describe the region between Indochina and Australasia; the term "Southeast Asia" was first used in 1839 by American pastor Howard Malcolm in his book Travels in South-Eastern Asia. Malcolm only included the Mainland section and excluded the Maritime section in his definition of Southeast Asia; the term was used in the midst of World War II by the Allies, through the formation of South East Asia Command in 1943.
SEAC popularised the use of the term "Southeast Asia," although what constituted Southeast Asia was not fixed. However, by the late 1970s, a standard usage of the term "Southeast Asia" and the territories it encompasses had emerged. Although from a cultural or linguistic perspective the definitions of "Southeast Asia" may vary, the most common definitions nowadays include the area represented by the countries listed below. Ten of the eleven states of Southeast Asia are members of the Association of Southeast Asian Nations, while East Timor is an observer state. Papua New Guinea has stated that it might join ASEAN, is an observer. Sovereignty issues exist over some territories in the South China Sea; some southern parts of Mainland China, as well as Hong Kong and Taiwan, are considered as part of Southeast Asia by some authors. * Administrative centre in Putrajaya. Southeast Asia is geographically divided into two subregions, namely Mainland Southeast Asia and Maritime Southeast Asia. Mainland Southeast Asia includes: Maritime Southeast Asia includes: The Andaman and Nicobar Islands of India are geographically considered part of Maritime Southeast Asia.
Eastern Bangladesh and Northeast India have strong cultural ties with Southeast Asia and sometimes considered both South Asian and Southeast Asian. Sri Lanka has on some occasions been considered a part of Southeast Asia because of its cultural ties to mainland Southeast Asia; the rest of the island of New Guinea, not part of Indonesia, Papua New Guinea, is sometimes included, so are Palau and the Northern Mariana Islands, which were all part of the Spanish East Indies with strong cultural and linguistic ties to the region the Philippines. The eastern half of Indonesia and East Timor are considered to be biogeographically part of Oceania due to its distinctive faunal features. New Guinea and its surrounding islands are geologically considered as a part of Australian continent, connected via the Sahul Shelf; the region
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 proposed by H. P. Woodward in 1957; the Alleghanian orogeny occurred 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; the immense region involved in the continental collision, the vast temporal length of the orogeny and the thickness of the pile of sediments and igneous rocks known to have been involved are evidence that at the peak of the mountain-building process, the Appalachians once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded. As the continents collided, the rock material trapped in-between was forced upward. With nowhere to go, rocks along the eastern margin of the North American continent were shoved far inland. Close to the boundary between the colliding plates, tectonic stresses contributed to the metamorphism of the rock; the sedimentary rock in the eastern Appalachian Basin region was squeezed into great folds that ran perpendicular to the direction of forces. The greatest amount of deformation associated with the Alleghanian orogeny occurred in the Southern Appalachians.
In that region, a series of great faults developed in addition to the folds. As the two continents collided, large belts of rock bounded by thrust faults piled one on top of another, shortening the crust along the eastern edge of North America in the North Carolina and Tennessee region by as much as 200 miles; the relative amount of deformation diminishes as one travels northward. The fold belt extends northward through Pennsylvania and fades in the vicinity of the New York border; the Kittatinny Mountains in northwestern New Jersey mark the Northeastern-most extension of the high ridges of the Valley and Ridge Province. The influence of Alleghanian deformation on the regions east of the Valley and Ridge Province must have been more intense. Rocks of Mississippian and Permian age are missing along the Eastern Seaboard; the mountains formed by the Alleghanian orogeny were once rugged and high, but in our time are now eroded into only a small remnant: the heavily-eroded hills of the Piedmont. Sediments that were carried eastward formed the coastal part of the continental shelf.
Thus, the coastal plain and Piedmont are the byproducts of erosion that took place from 150+ million years ago to the present. Sediments that were carried westward formed the Allegheny and Cumberland Plateau, which in some areas are popularly called mountains, but are simply uplifted and eroded plateaus. A portion of the Alleghanian mountain system departed with Africa when Pangaea broke up and the Atlantic Ocean began to form. Today, this forms the Anti-Atlas mountains of Morocco; the Anti-Atlas have been geologically uplifted in recent times, are today much more rugged than their Alleghanian relatives. Geology of the Appalachians Central Pangean Mountains Mauritanide Belt Inliers and outliers
South Asia or Southern Asia, is a term used to represent the southern region of the Asian continent, which comprises the sub-Himalayan SAARC countries and, for some authorities, adjoining countries to the west and east. Topographically, it is dominated by the Indian Plate, which rises above sea level as Nepal and northern parts of India situated south of the Himalayas and the Hindu Kush. South Asia is bounded on the south by the Indian Ocean and on land by West Asia, Central Asia, East Asia, Southeast Asia; the current territories of Afghanistan, Bhutan, Nepal, India and Sri Lanka form South Asia. The South Asian Association for Regional Cooperation is an economic cooperation organisation in the region, established in 1985 and includes all eight nations comprising South Asia. South Asia covers about 5.2 million km2, 11.71% of the Asian continent or 3.5% of the world's land surface area. The population of South Asia is about 1.891 billion or about one fourth of the world's population, making it both the most populous and the most densely populated geographical region in the world.
Overall, it accounts for about 39.49% of Asia's population, over 24% of the world's population, is home to a vast array of people. In 2010, South Asia had the world's largest population of Hindus and Sikhs, it has the largest population of Muslims in the Asia-Pacific region, as well as over 35 million Christians and 25 million Buddhists. The total area of South Asia and its geographical extent is not clear cut as systemic and foreign policy orientations of its constituents are quite asymmetrical. Aside from the central region of South Asia part of the British Empire, there is a high degree of variation as to which other countries are included in South Asia. Modern definitions of South Asia are consistent in including Afghanistan, Pakistan, Sri Lanka, Nepal and Maldives as the constituent countries. Myanmar is included in Southeast Asia by others; some do not include Afghanistan, others question whether Afghanistan should be considered a part of South Asia or the Middle East. The current territories of Bangladesh and Pakistan, which were the core of the British Empire from 1857 to 1947, form the central region of South Asia, in addition to Afghanistan, a British protectorate until 1919, after the Afghans lost to the British in the Second Anglo-Afghan war.
The mountain countries of Nepal and Bhutan, the island countries of Sri Lanka and Maldives are included as well. Myanmar is added, by various deviating definitions based on substantially different reasons, the British Indian Ocean Territory and the Tibet Autonomous Region are included as well; the common concept of South Asia is inherited from the administrative boundaries of the British Raj, with several exceptions. The Aden Colony, British Somaliland and Singapore, though administered at various times under the Raj, have not been proposed as any part of South Asia. Additionally Burma was administered as part of the Raj until 1937, but is now considered a part of Southeast Asia and is a member state of ASEAN; the 562 princely states that were protected by but not directly ruled by the Raj became administrative parts of South Asia upon joining Union of India or Dominion of Pakistan. Geopolitically, it had formed the whole territory of Greater India,The South Asian Association for Regional Cooperation, a contiguous block of countries, started in 1985 with seven countries – Bangladesh, India, the Maldives, Nepal and Sri Lanka – and added Afghanistan as an eighth member in 2007.
China and Myanmar have applied for the status of full members of SAARC. This bloc of countries include two independent countries that were not part of the British Raj – Nepal, Bhutan. Afghanistan was a British protectorate from 1878 until 1919, after the Afghans lost to the British in the Second Anglo-Afghan war; the World Factbook, based on geo-politics and economy defines South Asia as comprising Afghanistan, Bhutan, British Indian Ocean Territory, Maldives, Nepal and Sri Lanka. The South Asia Free Trade Agreement incorporated Afghanistan in 2011, the World Bank grouping of countries in the region includes all eight members comprising South Asia and SAARC as well, the same goes for the United Nations Children's Fund; the United Nations Statistics Division's scheme of sub-regions include all eight members of the SAARC as part of Southern Asia, along with Iran only for statistical purposes. Population Information Network includes Afghanistan, Burma, Nepal and Sri Lanka as part of South Asia.
Maldives, in view of its characteristics, was admitted as a member Pacific POPIN subregional network only in principle. The Hirschman–Herfindahl index of the United Nations Economic and Social Commission for Asia and the Pacific for the region includes only the original seven signatories of SAARC; the British Indian Ocean Territory is connected to the region by a publication of Jane's for security considerations. The region may include the disputed territory of Aksai Chin, part of the British Indian princely state of Jammu and Kashmir, but is now administered as part of the Chinese autonomous region of Xinjiang; the inclusion of Myanmar in South Asia is without consensus, with many considering it a part of Southeast Asia and others including it within South Asia. Afghanistan was of importance to the British colonial empire after the Second Anglo-Afghan War over 1878–1880. Afghanistan remained a British protectorate until 1919, when a treaty with Vladimir Lenin included the granting of independe
A transform fault or transform boundary is a plate boundary where the motion is predominantly horizontal. It is connected to another transform, a spreading ridge, or a subduction zone. Most of these faults are hidden in the deep ocean, where they offset divergent boundaries in short zigzags resulting from seafloor spreading, the best-known being those on land at the margins of continental tectonic plates. A transform fault is the only type of strike-slip fault, classified as a plate boundary; these faults are known as conservative plate boundaries, since they neither create nor destroy lithosphere. Geophysicist and geologist John Tuzo Wilson recognized that the offsets of oceanic ridges by faults do not follow the classical pattern of an offset fence or geological marker in Reid's rebound theory of faulting, from which the sense of slip is derived; the new class of faults, called transform faults, produce slip in the opposite direction from what one would surmise from the standard interpretation of an offset geological feature.
Slip along transform faults does not increase the distance between the ridges it separates. This hypothesis was confirmed in a study of the fault plane solutions that showed the slip on transform faults points in the opposite direction than classical interpretation would suggest. Transform faults are related to transcurrent faults and are confused. Both types of fault are side-to-side in movement. In addition, transform faults have equal deformation across the entire fault line, while transcurrent faults have greater displacement in the middle of the fault zone and less on the margins. Transform faults can form a tectonic plate boundary, while transcurrent faults cannot; the effect of a fault is to relieve strain, which can be caused by compression, extension, or lateral stress in the rock layers at the surface or deep in the Earth's subsurface. Transform faults relieve strain by transporting the strain between ridges or subduction zones, they act as the plane of weakness, which may result in splitting in rift zones.
Transform faults are found linking segments of mid-oceanic ridges or spreading centres. These mid-oceanic ridges are where new seafloor is created through the upwelling of new basaltic magma. With new seafloor being pushed and pulled out, the older seafloor slides away from the mid-oceanic ridges toward the continents. Although separated only by tens of kilometers, this separation between segments of the ridges causes portions of the seafloor to push past each other in opposing directions; this lateral movement of seafloors past each other is where transform faults are active. Transform faults move differently from a strike-slip fault at the mid-oceanic ridge. Instead of the ridges moving away from each other, as they do in other strike-slip faults, transform-fault ridges remain in the same, fixed locations, the new ocean seafloor created at the ridges is pushed away from the ridge. Evidence of this motion can be found in paleomagnetic striping on the seafloor. A paper written by geophysicist Taras Gerya theorizes that the creation of the transform faults between the ridges of the mid-oceanic ridge is attributed to rotated and stretched sections of the mid-oceanic ridge.
This occurs over a long period of time with the spreading center or ridge deforming from a straight line to a curved line. Fracturing along these planes forms transform faults; as this takes place, the fault changes from a normal fault with extensional stress to a strike slip fault with lateral stress. In the study done by Bonatti and Crane and gabbro rocks were discovered in the edges of the transform ridges; these rocks are created deep inside the Earth's mantle and rapidly exhumed to the surface. This evidence helps to prove that new seafloor is being created at the mid-oceanic ridges and further supports the theory of plate tectonics. Active transform faults are between faults. Fracture zones represent the active transform-fault lines, which have since passed the active transform zone and are being pushed toward the continents; these elevated ridges on the ocean floor can be traced for hundreds of miles and in some cases from one continent across an ocean to the other continent. The most prominent examples of the mid-oceanic ridge transform zones are in the Atlantic Ocean between South America and Africa.
Known as the St. Paul, Romanche and Ascension fracture zones, these areas have deep identifiable transform faults and ridges. Other locations include: the East Pacific Ridge located in the South Eastern Pacific Ocean, which meets up with San Andreas Fault to the North. Transform faults are not spreading centers; the best example is the San Andreas Fault on the Pacific coast of the United States. The San Andreas Fault links the East Pacific Rise off the West coast of Mexico to the Mendocino Triple Junction off the coast of the Northwestern United States, making it a ridge-to-transform-style fault; the formation of the San Andreas Fault system occurred recently during the Oligocene Period between 34 million and 24 million years ago. During this period, the Farallon plate, followed by the Pacific plate, collided into the North American plate; the collision led to the subduction of the Farallon plate underneath the North American plate. Once the spreading ce