EAST AFRICAN OROGEN
The East African orogen encompasses the Arabian-Nubian shield in the north and the Mozambique belt in the south. These and several other orogenic
Map of the East African orogen and other Pan-African belts that formed as ocean basins closed to form the supercontinent of Gondwana (modeled from T. Kusky, M. Abdelsalam, R. Tucker, and R. Stern, 2003)
belts are commonly referred to as Pan-African belts, recognizing that many distinct belts in Africa and other continents experienced deformation, metamor-phism, and magmatic activity in the general period of 800-450 Ma. Other definitions of the Pan-African orogens are more restrictive, and consider them to be confined to a complex collisional system between the Congo and Kalahari cratons in this time interval, thus including in Africa the Gareip belt, the Kaoko belt, Damara orogen, Lufilian arc, Zambezi belt, Malawi orogen, Mozambique belt, and Luria arc. Pan-African tectonothermal activity in the Mozambique belt was broadly contemporaneous with magmatism, metamorphism, and deformation in the Arabian-Nubian shield, and the two are broadly equivalent. The difference in lithology and metamorphic grade between the two belts has been attributed to the difference in the level of exposure, with the Mozam-bican rocks interpreted as lower crustal equivalents of the rocks in the Arabian-Nubian shield. Neopro-terozoic closure of the Mozambique ocean collapsed an accretionary collage of arc and microcontinental terranes and sutured east and west Gondwana along the length of the East African orogen.
The formation of Gondwana at the end of the Precambrian and the dawn of the Phanerozoic by the collision of cratons including the Congo, Kalahari, India, Antarctica, and South American blocks represents one of the most fundamental problems being studied in earth sciences today. studies of Gondwana link many different fields, and there are currently numerous and rapid changes in understanding of events related to the assembly of Gondwana. One of the most fundamental and most poorly understood aspects of the formation of Gondwana is the timing and geometry of closure of the oceanic basins that separated the continental fragments that amassed to form the Late Proterozoic supercontinent. Final collision between East and West Gondwana most likely occurred during closure of the Mozambique Ocean, forming the East African orogen.
Recent geochronologic data indicate the presence of two major "Pan-African" tectonic events within East Africa. The East African Orogeny (800-
650 Ma) represents a distinct series of events within the Pan-African of central Gondwana, responsible for the assembly of greater Gondwana. Collectively, paleomagnetic and age data indicate that another later event at 550 Ma (Kuunga Orogeny) may represent the final suturing of the Australian and Antarctic segments of the Gondwana continent.
The Atlas Mountains are a series of mountains and plateaus in northwest Africa extending about 1,500 miles (2,500 km) in southwest Morocco, northern Algeria, and northern Tunisia. The highest peak in the Atlas is Jabel Toubkal, at 13,665 feet (4,168 m) in southwest Morocco. The Atlas Mountains are dominantly folded sedimentary rocks uplifted in the Jurassic, and related to the Alpine system of Europe. The Atlas consists of several ranges separated by fertile lowlands in Morocco, from north to south including the Rif Atlas, Middle Atlas, High Atlas (Grand Atlas), and Anti Atlas. The Algerian Atlas consists of a series of plateaus including the Tell and Saharan Atlas rimming the Chotts Plateau, then converging in Tunisia. The Atlas form a climatic barrier between the Atlantic and Mediterranean basins and the Sahara, with rainfall falling on north-facing slopes but arid conditions dominating on the rain-shadow, south-facing slopes. The Atlas are rich in mineral deposits including coal, iron, oil, and phosphates. The area is also used extensively for sheep grazing, with farming in the more fertile intermountain basins.
Extensional plate tectonic forces are presently breaking Africa apart, with parts of eastern Africa rifting away from the main continent. The rift valley that separates these two sections is known as the East African rift system, or the Great Rift Valley, extending from the Ethiopian Afar region, through two segments known as the eastern and western rifts that bend around Lake Victoria, then extend to the Mozambique Channel.
The Main Ethiopian and North-Central Afar rifts are part of the continental East African rift system. These two kinematically distinct rift systems, typical of intracontinental rifting, are at different stages of evolution. In the north and east the continental rifts meet the oceanic rifts of Red Sea and Gulf of Aden, respectively, both of which have propagated into the continent. Seismic refraction and gravity studies indicate that the thickness of the crust in the Main Ethiopian rift is less than or equal to 18.5 miles (30 km). In Afar the thickness varies from 14 to 16 miles (23-26 km) in the south and to 8.5 miles (14 km) in the north. The plateau on both sides of the rift
Image of digital elevation model of East African Rift at Lake Kivu from data generated by Shuttle Radar Topography Mission. Area shown covers parts of the Democratic Republic of the Congo, Rwanda, and Uganda. Elevation is color coded, progressing from green at lower elevations through yellow to brown at higher elevations. A false sun in the NW (upper left, pixelated area) causes topographic shading. Lake Kivu lies in the East African Rift, which forms a smooth lava and sediment-filled trough in the area. Two volcanic complexes are shown in the rift, including the Nyiragongo volcano (the one closer to the lake), which erupted in 2002. Virunga volcanic chain extends east of the rift.
has a crustal thickness of 21.5-27 miles (35-44 km). Geologic and geodetic studies indicate separation rates of 0.1-0.2 inches (3-6 mm) per year across the northern sector of the Main Ethiopian rift between the African and Somali plates. The rate of spreading between Africa and Arabia across the North-Central Afar rift is relatively faster, about 0.8 inches (20 mm) per year. Paleomagnetic directions from Cenozoic basalts on the Arabian side of the Gulf of Aden indicate seven degrees of counterclockwise rotation of the Arabian plate relative to Africa, and clockwise rotations of up to 11 degrees for blocks in eastern Afar. The initiation of extension on both sides of the southernmost Red Sea rift, Ethiopia, and Yemen appear coeval, with extension starting between 22 and 29 million years ago.
The Ethiopian Afar region is one of the world's largest, deepest regions below sea level that is sub-aerially exposed on the continent, home to some of the earliest known hominid fossils. The Afar is a hot, arid region where the Awash River drains northward out of the East African rift system, and is evaporated in Lake Abhe before it reaches the sea. This unique and spectacular region is located in eastern Africa in Ethiopia and Eritrea, between Sudan, Somalia, and across the Red Sea and Gulf of Aden from Yemen. The region is so topographically low because it is located at a tectonic triple junction, where three main plates are spreading apart, causing regional subsidence. The Arabian plate is moving northeastward away from the African plate, and the Somali plate is moving, at a much slower rate, to the southeast away from Africa. The southern Red Sea and north-central Afar Depression form two parallel north-northwest-trending rift basins, separated by the Danakil Horst, related to the separation of Arabia from Africa. Of the two rifts, the Afar depression is exposed at the surface, whereas the Red Sea rift floor is submerged below the sea. The north-central Afar rift is complex, consisting of many grabens and horsts. The Afar Depression merges southward with the northeast-striking Main Ethiopian rift, and eastward with the east-northeast-striking Gulf of Aden. The Ethiopian Plateau bounds it on the west. Pliocene volcanic rocks of the Afar stratoid series and the Pleistocene to recent volcanics of the Axial Ranges occupy the floor of the Afar Depression. Miocene to recent detri-tal and chemical sediments are intercalated with the volcanics in the basins.
South of the Ethiopian rifts, the eastern branch of the main East African rift strikes southward through Kenya, forming Lake Turkana across the Kenya highlands, and forming the famous Ngorongoro crater, where millions of wildlife gather for scarce water in the deep rift valley. The western branch of the main East Africa rift strikes southward from Ethiopia and
Sudan into Uganda, forming a series of deep, steep-sided lakes including Lakes Albert, Edward, Kivu, Tanganyika, and Malawi. Some of these lakes are more than a mile (1.6 km) deep, and are fed by drainage systems that remain on the floor of the rift, while drainage on the rift shoulders carries water away from the central rift.
See also Archean; basin, sedimentary basin; convergent plate margin processes; craton; deformation of rocks; deserts; divergent plate margin processes; greenstone belts; Madagascar; orogeny; Precambrian.
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Tesfaye, Sansom, David Harding, and Timothy M. Kusky. "Early Continental Breakup Boundary and Migration of the Afar Triple Junction, Ethiopia." Geological Society of America Bulletin 115 (2003): 1,053-1,067.
de Wit, Maarten J., Chris Roering, Rojer J. Hart, Richard A. Armstrong, Charles E. J. de Ronde, R. W. E. Green, Marian Tredoux, E. Pederdy, and R. A. Hart. "Formation of an Archean Continent." Nature 357 (1992): 553-562.
Andes Mountains The Andes are a 5,000-mile (8,000-km) long mountain range in western South America, running generally parallel to the coast, between the Caribbean coast of Venezuela in the north and Tierra del Fuego in the south. The mountains merge with ranges in Central America and the West Indies in the north, and with ranges in the Falklands and Antarctica in the south. Many snow-covered peaks rise more than 22,000 feet (6,000 m), making the Andes the second-tallest mountain belt in the world, after the Himalayan chain. The highest range in the Andes is the Aconcauga on the central and northern Argentine-Chilean border. The high, cold Atacama desert is located in the northern Chile sub-Andean range, and the high Altiplano Plateau is situated along the great bend in the Andes in Bolivia and Peru.
The southern part of South America consists of a series of different terranes (belts of distinctive rocks) added to the margin of the supercontinent of Gond-wana in late Proterozoic and early Proterozoic times. Subduction and the accretion of oceanic terranes continued through the Paleozoic, forming a 155-mile (250-km) wide accretionary wedge. The Andes developed as a continental margin volcanic arc system on the older accreted terranes, formed above a complex system of subducting plates from the Pacific Ocean. They are geologically young, having been uplifted mainly in the Cretaceous and Tertiary (roughly the past 100 million years), with active volcanism, uplift,
Alpamayo Peak in the Cordilleras Mountains, Peruvian Andes (Galyna Andrushko, Shutterstock, Inc.)
and earthquakes. The specific nature of volcanism, plutonism, earthquakes, and uplift are found to be strongly segmented in the Andes, and related to the nature of the subducting part of the plate, including its dip and age. Regions above places where the subducting plate dips more than 30 degrees have active volcanism, whereas regions above places where the subduction zone is subhorizontal do not have active volcanoes.
The Altiplano is a large, uplifted plateau in the Bolivian and Peruvian Andes of South America. The plateau has an area of about 65,536 square miles (170,000 km2), and an average elevation of 12,000 feet (3,660 m) above sea level. The Altiplano is a sedimentary basin caught between the mountain ranges of the Cordillera oriental on the east and the Cordillera occidental on the west. The Altiplano is a dry region with sparse vegetation, and scattered salt flats. Villagers grow potatoes and grains, and a variety of minerals are extracted from the plateau and surrounding mountain ranges.
Lake Titicaca, the largest high-altitude lake navigable to large vessels in the world, is located at the northern end of the Altiplano. Sitting at 12,500 feet (3,815 m) above sea level, the lake straddles the border between Peru and Boliva. The lake basin is situated between Andean ranges on the Altiplano plateau, and is bordered to the northeast by some of the highest peaks in the Andes in the Cordillera Real, where several mountains rise to over 21,000 feet (6,400 m). Covering 3,200 square miles, Lake Titicaca is the largest freshwater lake in South America, although it is divided into two parts by the Strait of Tiquina. The body of water north of the strait is called Chucuito in Bolivia and Lake Grande in Peru, and south of the strait the smaller body of water is called Lake Huinaymarca in Bolivia and Lake Pequeno in Peru. Most of the lake is 460-600 feet (140-180 m) deep, but reaches 920 feet (280 m) deep near the northeast corner of the lake. The lake is fed by many short tributaries from surrounding mountains, and is drained by the Desaguadero River, which flows into Lake Poopo. However, only 5 percent of water loss is through this single outlet—the remainder is lost by evaporation in the hot, dry air of the Altiplano. Lake levels fluctuate on seasonal and several longer-time cycles, and the water retains a relatively constant temperature of 56°F (14°C) at the surface, but cools to 52°F (11°C) below a thermocline at 66 feet (20 m). Salinity ranges from 5.2-5.5 parts per thousand.
Lake Titicaca, translated variously as Rock of the Puma or Craig of Lead, has been the center of culture since pre-Inca times (600 years before present [b.p.]), and its shoreline is presently covered by Indian villages and terraced rice fields. Some of the oldest civilizations are preserved in ruins around Lake Titicaca, including those at Tiahuanaco, on the southern end of the lake, and others on the many islands in the lake. Ruins of a temple on Titicaca Island mark the spot where Inca legends claim that Manco Capac and Mama Ocllo, the founders of the Inca dynasty, were sent to Earth by the Sun.
The northern Andes are drained to the east by the world's second-longest river, the Amazon, stretching 3,900 miles (6,275 km) from the foothills of the Andes to the Atlantic ocean. The southern Andes are drained to the east by the Paraná River, and a number of smaller rivers run down the steep western slope of the Andes to the Pacific ocean. The Amazon begins where the Ucayili and Maranon tributaries merge, and drains into the Atlantic near the city of Belém. The Amazon carries the most water and has the largest discharge of any river in the world, averaging 150 feet (45 m) deep. Its drainage basin amounts to about 35 percent of South America, covering 2,500,000 square miles (6,475,000 km2). The Amazon lowlands in Brazil include the largest tropical rain forest in the world. In this region the Amazon is a muddy, silt-rich river with many channels that wind around numerous islands in a complex maze. The delta region of the Amazon is marked by numerous fluvial islands and distributaries, as the muddy waters of the river get dispersed by strong currents and waves into the Atlantic. A strong tidal bore, up to 12 feet (3.7 m) high, runs up to 500 miles (800 km) upstream.
The Amazon River basin occupies a sediment-filled rift basin, between the Precambrian crystalline basement of the Brazil and Guiana shields. The area hosts economic deposits of gold, manganese, and other metals in the highlands, and detrital gold in lower elevations. Much of the region's economy relies on the lumber industry, with timber, rubber, vegetable oils, Brazil nuts, and medicinal plants sold worldwide.
Spanish commander Vincent Pinzon was probably the first European in 1500 to explore the lower part of the river basin, followed by the spanish explorer Francisco de Orellana in 1540-41. Orel-lana's tales of tall, strong female warriors gave the river its name, borrowing from Greek mythology. Further exploration by Pedro Teixeira, Charles Darwin, and Louis Agassiz led to greater understanding of the river's course, peoples, and environment, and settlements did not appear until steamship service began in the middle 1800s.
See also convergent plate margin processes; plate tectonics; South American geology.
Moores, Eldridge, and Robert Twiss. Tectonics. New York:
Antarctica The southern continent, Antarctica, is located nearly entirely below the Antarctic Circle (66° 33' 39") and is distributed asymmetrically around the south pole. The continent covers approximately 5.46 million square miles (14 million km2), is nearly completely covered in ice, and has several large ice shelves extending off the mainland into surrounding oceans. Antarctica is surrounded by relatively isolated waters of the Southern Ocean, comprised of southern reaches of the Atlantic, Pacific, and Indian Oceans. Antarctica is the fifth-largest continent, covering an area equal to 57 percent of North America, or nearly 1.5 times the size of the United States including Alaska. The Russian explorers Mikhail Lazarev and Fabian Gottlieb von Bellingshausen first discovered the continent in 1820, and the Scottish cartographer John Bartholomew named it in 1890. In 1959 12 countries (later joined by more, to bring the total to 46) signed the Antarctic Treaty, prohibiting military activity and mining in Antarctica, and promoting cooperative scientific and environmental work.
The ice sheet covering Antarctica is the world's largest reservoir of fresh water (although frozen) and averages more than a mile (1.6 km) thick. The weight of this ice causes the underlying continent to be depressed by more than 1.5 miles (2.5 km). A small amount of rock is exposed in the Transantarc-tic Mountains and in the Dry Valleys area.
The Transantarctic Mountains divide the continent in two, stretching from the Ross Sea to the Wedell Sea. Western Antarctica (using the Greenwich meridian that runs nearly along the Transantarctic Mountains) is covered by the West Antarctic Ice Sheet, which some climatologists warn could collapse, raising sea levels by 10 feet (3 m) or more in a short time. The Antarctic Peninsula, Marie Byrd Land, and the area east and north of the Transant-arctic Mountains are part of West Antarctica. The eastern part of Antarctica is a large Precambrian cra-ton known as East Antarctica, with ages extending to at least 3 billion years. The rocks of the ancient craton, however, were reworked in younger mountain-building events, including an Early Paleozoic event during which East Antarctica was incorporated into Gondwana.
Most of western Antarctica was built up through the accretion of microplates that include the Ellsworth Mountains terrane, the Antarctic Peninsula, Marie Byrd Land, and an unnamed block of igneous and metamorphic rocks. Compared with the subdued (subglacial) topography of East Antarctica, western Antarctica has relatively rugged, mountainous topography.
The Transantarctic Mountains are up to 15,000 feet (4,570 m) high, and were formed during the Ross orogeny 500 million years ago. In contrast, the Ellsworth Mountains reach 16,000 feet (4,880 m) and were formed about 190 million years ago in the Early Mesozoic. The Antarctic Peninsula is the youngest addition to Antarctica, formed mostly in the Late Mesozoic to Early Cenozoic Andean orogeny (80-60 million years ago). Most activity in the Transantarctic Mountains was in the period of the breakup of Gondwana, as this region became a convergent margin continuous with the Andes of South America.
GEOLOGY, PALEONTOLOGY, AND PALEOCLIMATE
The Precambrian basement rocks of East Antarctica comprise the East Antarctic craton, with Archean cores surrounded by Proterozoic orogenic belts with younger deformation and metamorphism than the Archean blocks. These Archean cores include coastal areas of western Dronning Maud Land, Enderby Land, the Prince Charles Mountains, and the Vest-fold Hills. The rocks in the Vestfold Hills include 2.5 billion-year-old gneisses derived from igneous rocks, and some indications of 2.8 billion-year-old gneisses in the area. The best known part of the East Antarctic craton is the Napier Complex in Enderby Land. This Archean granulite-gneiss belt includes 2.8 to 3.0 billion-year-old metamorphosed igneous gneisses, with some indication that initial igneous activity in the area may extend back to 3.8 billion years. Several deformation and metamorphic events are recorded at about 2.8 billion years ago, and a very high temperature metamorphic event recorded at 2.51-2.47 billion years ago. Additional deformation is recorded
as East Antarctica became part of Gondwana, near the end of the Precambrian.
East Antarctica was part of Gondwana in the Early Paleozoic, resting in equatorial latitudes and accepting marine deposits of fossiliferous limestones from the tropical seas. These limestones are rich in trilobite and invertebrate fossils. West Antarctic was in the northern hemisphere in the Paleozoic, and not yet sutured with East Antarctica. By the Devonian, Gondwana, with East Antarctica, had drifted into southern latitudes and experienced a cooler climate, but still has a good fossil record of land plants in sandstone and siltstone beds exposed in the Ellsworth and Pensacola Mountains. The end of the Devonian (360 million years ago) witnessed a major glaciation as Gondwana became centered on the South Pole. Despite the glaciation, East Antarctica remained vegetated, and by the Permian many swamps across Antarctica were flourishing with the fernlike Glos-sopteris fauna, known throughout much of Gond-wana. By the end of the Permian the climate over much of Gondwana had turned hot and dry.
The end-Permian warming caused the polar ice caps on Eastern Gondwana, including Antarctica, to melt, and the continent became a vast desert. still, seed ferns and giant reptiles, including Lystrosau-rus, inhabited the land, and thick beds of sandstone and shale were deposited on the East Antarctic platform. The Antarctic Peninsula was forming during the Jurassic (206-146 million years ago), and beech trees began to take over the floral assemblage. West Antarctica had accreted to the East Antarctic craton and was covered in conifer forests through the Cretaceous, gradually replaced by the beech trees toward the end of the period. The seas around Antarctica were inhabited by ammonites.
Modern-day Antarctica began to take shape in the Cenozoic. The Antarctic Peninsula and Western Antarctica are an extension of the Andes of south America.
The climate of Antarctica is the coldest, driest, and windiest on Earth, with the lowest recorded temperature being -129°F (-89°C) from the Vostok weather station, located at two miles (three km) elevation in Antarctica. Despite being covered in ice, the climate of Antarctica is best described as a dry or polar desert, since the amount of precipitation is so low, with the South Pole receiving fewer than four inches (10 cm) of rainfall equivalent each year. Although it is covered in ice, interior Antarctica is technically the largest desert on Earth. Temperatures show a considerable range, from -112°F to -130°F (-90°C to -90°C) in interior winters, to 41°F to 59°F (5°C to 15°C) along the coastline in summer. In general, the eastern
Ice calving from an ice front off Adelaide Island, Antarctica (British Antarctic Survey/Photo Researchers, Inc.)
part of the continent is colder than the western part, because it has a higher elevation.
Although 98 percent of Antarctica is covered in ice, a few places are ice-free. The Dry Valleys are the largest area on Antarctica not covered by ice. The Dry Valleys, located near McMurdo Sound on the side of the continent closest to New Zealand, have a cold desert climate and receive only four inches (10 cm) of precipitation per year, overwhelmingly in the form of snow. The Dry Valleys are one of the coldest, driest places on Earth and are used by researchers from the National Aeronautics and Space Administration (NASA) as an analog for conditions on Mars. No vegetation exists in the Dry Valleys, but a number of unusual microbes live in the frozen soils and form cyanobacterial mats in places. In the Southern Hemisphere summer, glaciers in the surrounding Transantarctic Mountains release significant quantities of meltwater so that streams and lakes form over the thick permafrost in the valleys.
The edge of the continent is often hit by strong katabatic winds, formed when high-density air forms over the ice cap, and then moves rapidly downhill, typically along glaciated valleys, at times reaching hurricane strength in force. High-density air often forms over the ice cap because the ice cools the air through radiative cooling effects, making it denser. This dense air then finds the lowest points to flow downhill, and because of the high elevation of central Antarctica the winds pick up enormous speed through gravitational energy, until they roar out of the coastal valleys as exceptionally cold channels of hurricane-force winds.
The Antarctic ice cap is huge, containing more than 70 percent of the fresh water on the planet. It is about the same size as the Laurentide ice sheet that covered the northern part of North America in the last ice age. If the ice in the Antarctic ice cap all melted, sea levels would rise by 230 feet (70 m), yet there is no evidence that the south polar ice cap is melting, and it has been stable for about the past 5 million years. Many models predict that global warming may increase precipitation in Antarctica and actually cause the ice cap to increase in volume and lower sea levels by 0.04 inches (0.09 cm) per year.
The ice cap consists of a vast area of ice more than one mile (1.6 km) thick, covering nearly all of East Antarctica in Queen Maud Land and Wilkes Land. The geology of this region is understood through nunataks, isolated peaks piercing through the ice cap mostly near the coast, and in the Trans-antarctic Mountains. Likewise, most of West Antarctica is covered by ice, including Marie Byrd Land, Ellsworth Land, Palmer Land, and the Antarctic Peninsula. The large Ross Ice Shelf is located between Marie Byrd Land and the Transantarctic Mountains, while on the other side of the continent, the Ronne Ice Shelf fills the space between the Antarctic Peninsula and the Transantarctic Mountains.
Global warming is not significantly affecting most of Antarctica, since the interior of the continent is isolated from the global climate system. The ice cap in central Antarctica is presently growing in volume, whereas some of the peripheral ice shelves, such as along the northern parts of the Antarctic Peninsula, are losing volume. For instance, in 2003 parts of the Larsen ice shelf on the northern Antarctic Peninsula began collapsing from a combination of global warming and other cyclical processes. Farther south on the Peninsula, the Wilkins ice shelf lost 220 square miles (570 km2) of ice in 2008, but it is still not well established whether these giant collapses result from global warming or whether similar processes have existed for many thousands of years. In support of the latter idea is the observation that the overall amount of sea ice around Antarctica has remained stable over the past 30 years, although there is considerable variation month to month and year to year.
See also convergent plate margin processes; craton; glacier, glacial systems; global warming; Gondwana, Gondwanaland.
Craddock, Campbell. Antarctic Geoscience. Madison: University of Wisconsin Press, 1982. McKnight, T. L., and Darrel Hess. "Katabatic Winds." In Physical Geography: A Landscape Appreciation, 131— 32. Upper Saddle River, N.J.: Prentice Hall, 2000. Stonehouse, B., ed. Encyclopedia of Antarctica and the Southern Oceans. New York: John Wiley & Sons, 2002.
Arabian geology The Arabian Peninsula can be classified into two major geological provinces, including the Precambrian Arabian shield and the Phanerozoic cover. The Arabian shield comprises the core and deep-lying rocks of the Arabian Peninsula, a landmass of near trapezoidal shape bounded by three water bodies. The Red Sea bounds it from the west, the Arabian Sea and the Gulf of Aden from the south, and the Arabian Gulf and Gulf of Oman bound it on the east.
The Precambrian Shield is located along the western and central parts of the peninsula. It narrows in the north and the south but widens in the central part of the peninsula. The shield lies between latitudes 12° and 30° north and between longitudes 34° and 47° east. The Arabian shield is considered part of the Arabian-Nubian shield formed in the upper Proterozoic Era and stabilized in the Late Proterozoic around 600 million years ago. The shield has since subsided and been covered by thick deposits of Pha-nerozoic continental shelf sediments along the margins of the Tethys ocean. Later in the Tertiary the Red Sea rift system rifted the Arabian-Nubian shield into two fragments.
Phanerozoic cover rocks unconformably overlie the eastern side of the Arabian shield, forming the Tuwaiq Mountains, and these rocks dip gently toward the east. Parts of the Phanerozoic cover are found overlying parts of the Precambrian shield, such as the Quaternary lava flows of Harrat Rahat in the middle and northern parts of the shield, as well as some sandstones, including the Saq, Siq, and Wajeed sandstones in different parts of the shield. The Pha-nerozoic rocks are well exposed again in tectonic uplifts in the Oman (Hajar) Mountains in the east, where the geology is well known.
The Arabian shield includes an assemblage of Middle to Late Proterozoic rocks exposed in the western and central parts of the Arabian Peninsula and overlapped to the north, east, and south by Phanero-zoic sedimentary cover rocks. Several parts of the shield are covered by Tertiary and Quaternary lava flows that were extruded along with rifting of the Red Sea starting about 30 million years ago. Rocks of the Arabian shield may be divided into assemblages of Middle to Late Proterozoic stratotectonic units, volcanosedimentary, and associated mafic to intermediate intrusive rocks. These rocks are divided into two major categories, the layered rocks and the intrusive rocks. Researchers variously interpret these assemblages as a result of volcanism and magmatism in continental basins or above subduction zones. More recently workers suggested that many of these assemblages belong to Late Proterozoic volcanic-arc
American Landsat image of Arabia. The Rub'a Khali (Empty Quarter) desert forms the great yellow sand sheet in the southern part of the peninsula, the Arabian shield forms the dark-colored terrane in the west, and the Semail ophiolite (oceanic crust and lithosphere) forms the dark area in the southeast. The fertile Mesopotamia area (in dark green, between the Tigris and Euphrates Rivers) separates Arabia from the Zagros Mountains of Iran. (Earth Satellite Corporation/Photo Researchers, Inc.)
systems that comprise distinct tectonic units or ter-ranes, recognized following definitions established in the North America cordillera.
Efforts in suggesting models for the evolution of the Arabian shield started in the 1960s. Early workers suggested that the Arabian shield experienced three major orogenies in the Late Proterozoic Era. They also delineated four classes of plutonic rocks that evolved in chemistry from calc-alkaline to peralkaline through time. In the 1970s a great deal of research emerged concerning models of the tectonic evolution of the Arabian shield. Two major models emerged from this work, including mobilistic plate tectonic models and a nonmobilistic basement-tectonic model.
The main tenet of the plate tectonic model is that the evolution of the Arabian shield started and took place in an oceanic environment, with the formation of island arcs over subduction zones in a huge oceanic basin. on the contrary, the basement-tectonic model considers that the evolution of the Arabian shield started by the rifting of an older craton or continent to form intraoceanic basins that became the sites of island arc systems. In both models, late stages of the formation of the Arabian-Nubian shield are marked by the sweeping together and collision of the island arcs systems, thrusting of the ophiolites onto continents, and cratonization of the entire orogen, forming one craton attached to the African craton. Most subsequent investigators in the 1970s supported one of these two models and tried to gather evidence to support that model.
As more investigations, mapping, and research were carried out in the 1980s and 1990s, a third model invoking microplates and terrane accretion
Simple map showing the geology of the Arabian Peninsula was suggested. This model suggests the existence of ; an Early to mid-Proterozoic (2.0-1.63 billion-year- ] old) craton that was extended, rifted, then dispersed, ; causing the development of basement fragments that were incorporated as allochthonous microplates into younger tectonostratigraphic units. The tectonostrati- ] graphic units include volcanic complexes, ophiolite complexes, and marginal-basin and fore-arc strato-tectonic units that accumulated in the intraoceanic to continental-marginal environments that resulted from rifting of the preexisting craton. These rocks, including the older continental fragments, constitute five large and five small tectonostratigraphic terranes ]
accreted and swept together between 770 and 620 million years ago to form a neocraton on which younger volcanosedimentary and sedimentary rocks were deposited. Most models developed in the period since the early 1990s represent varieties of these three main classical models, along with a greater appreciation of the formation of the supercontinent of Gondwana in the formation of the Arabian-Nubian shield.
GEOLOGY OF THE ARABIAN SHIELD
Peter Johnson of the U.S. Geological Survey and his coworkers have synthesized the geology of the
Arabian shield and proposed a general classification of the geology of the Arabian shield that attempts to integrate and resolve the differences between the previous classifications. According to this classification, the layered rocks of the Arabian shield are divided into three main units separated by periods of regional tectonic activity (orogenies). This gives an overall view that the shield was created through three tectonic cycles. These tectonic cycles include early, middle, and late upper Proterozoic tectonic cycles.
The early upper Proterozoic tectonic cycle covers the period older than 800 million years and includes the oldest rock groups that formed before and up to the Aqiq orogeny in the south and up to the Tuluhah orogeny in the north. In this general classification the Aqiq and Tuluhah orogenies are considered part of one regional tectonic event, or orogeny, that is given a combined name of Aqiq-Tuluhah orogeny.
The middle upper Proterozoic tectonic cycle is considered to have taken place between 700 and 800 Ma. it includes the Yafikh orogeny in the south and the Ragbah orogeny in the north. These two orogenies were combined into one regional orogeny, the Yafikh-Ragbah orogeny.
The late upper Proterozoic tectonic cycle took place in the period between 700 and 650 ms. it includes the Bishah orogeny in the south and the Rimmah orogeny in the north. These two orogenies are combined into one regional orogeny, the Bishah-Rimmah orogeny.
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