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58-657: The most important topographical feature in the Inkomati water management area is the Great Escarpment , which divides the water management area into the Plateau area in the west (elevation of more than 2 000 m above sea level) and the Lowveld in the east. Largely attributable to the topography, distinct differences in climate occur, from temperate Highveld in the west to sub-tropical in the eastern Lowveld . Rainfall

116-732: A collisional orogeny). Orogeny typically produces orogenic belts or orogens , which are elongated regions of deformation bordering continental cratons (the stable interiors of continents). Young orogenic belts, in which subduction is still taking place, are characterized by frequent volcanic activity and earthquakes . Older orogenic belts are typically deeply eroded to expose displaced and deformed strata . These are often highly metamorphosed and include vast bodies of intrusive igneous rock called batholiths . Subduction zones consume oceanic crust , thicken lithosphere, and produce earthquakes and volcanoes. Not all subduction zones produce orogenic belts; mountain building takes place only when

174-506: A delamination of the orogenic root beneath them. Mount Rundle on the Trans-Canada Highway between Banff and Canmore provides a classic example of a mountain cut in dipping-layered rocks. Millions of years ago a collision caused an orogeny, forcing horizontal layers of an ancient ocean crust to be thrust up at an angle of 50–60°. That left Rundle with one sweeping, tree-lined smooth face, and one sharp, steep face where

232-589: A major continent-continent collision, is called an accretionary orogen. The North American Cordillera and the Lachlan Orogen of southeast Australia are examples of accretionary orogens. The orogeny may culminate with continental crust from the opposite side of the subducting oceanic plate arriving at the subduction zone. This ends subduction and transforms the accretional orogen into a Himalayan -type collisional orogen. The collisional orogeny may produce extremely high mountains, as has been taking place in

290-412: A noncollisional orogenic belt, and such belts are sometimes called Andean-type orogens . As subduction continues, island arcs , continental fragments , and oceanic material may gradually accrete onto the continental margin. This is one of the main mechanisms by which continents have grown. An orogen built of crustal fragments ( terranes ) accreted over a long period of time, without any indication of

348-442: A pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults . These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates ) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism . Before

406-626: A very high and rugged appearance, combining steep-sided blocks and pinnacles. The KwaZulu-Natal – Free State Drakensberg escarpment is composed of softer rocks and therefore has a more rounded, softer appearance from below. Generally, the top of the Escarpment is almost table-top flat and smooth, even in Lesotho . The "Lesotho Mountains" are formed away from the Drakensberg escarpment by erosion gulleys that turn into deep valleys that contain

464-420: 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

522-446: Is initiated along one or both of the continental margins of the ocean basin, producing a volcanic arc and possibly an Andean-type orogen along that continental margin. This produces deformation of the continental margins and possibly crustal thickening and mountain building. Mountain formation in orogens is largely a result of crustal thickening. The compressive forces produced by plate convergence result in pervasive deformation of

580-422: Is still in use today, though commonly investigated by geochronology using radiometric dating. Based on available observations from the metamorphic differences in orogenic belts of Europe and North America, H. J. Zwart (1967) proposed three types of orogens in relationship to tectonic setting and style: Cordillerotype, Alpinotype, and Hercynotype. His proposal was revised by W. S. Pitcher in 1979 in terms of

638-473: Is strongly seasonal and occurs mainly in summer. Light snow occasionally falls in the vicinity of the southwestern divide in winter. The mean annual rainfall is in the range from 400 mm to 1 000 mm over most of the water management area, reaching close to 1 500 mm in the mountainous areas along the escarpment. The north-eastern part of the water management area falls within the world-renowned Kruger National Park . The Sabie River which flows through

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696-480: Is taking place today in the Southern Alps of New Zealand). Orogens have a characteristic structure, though this shows considerable variation. A foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep immediately beyond

754-453: The Alpine type orogenic belt , typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure. In terms of recognising orogeny as an event , Leopold von Buch (1855) recognised that orogenies could be placed in time by bracketing between the youngest deformed rock and the oldest undeformed rock, a principle which

812-648: The Andes Mountains in South America are being formed today. The Cape Fold Mountains have been re-exposed by erosion of the coastal plain below the Great Escarpment (see "Geological origin", above), after having been covered by sediments originating from an even higher and more extensive range of mountains, comparable to the Himalayas , that developed during the assembly of Gondwana to

870-584: The Himalayas for the last 65 million years. The processes of orogeny can take tens of millions of years and build mountains from what were once sedimentary basins . Activity along an orogenic belt can be extremely long-lived. For example, much of the basement underlying the United States belongs to the Transcontinental Proterozoic Provinces, which accreted to Laurentia (the ancient heart of North America) over

928-691: The San Andreas Fault , restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that look as if they are not necessarily on present tectonic-plate boundaries, but they are essentially the product of plate tectonism. Likewise, uplift and erosion related to epeirogenesis (large-scale vertical motions of portions of continents without much associated folding, metamorphism, or deformation) can create local topographic highs. Eventually, seafloor spreading in

986-647: The late Devonian (about 380 million years ago) with the Antler orogeny and continuing with the Sonoma orogeny and Sevier orogeny and culminating with the Laramide orogeny . The Laramide orogeny alone lasted 40 million years, from 75 million to 35 million years ago. Orogens show a great range of characteristics, but they may be broadly divided into collisional orogens and noncollisional orogens (Andean-type orogens). Collisional orogens can be further divided by whether

1044-585: The 22° S parallel, and from there veers west to Mokopane , where it is known as the Strydpoort Mountains. The absence of the Great Escarpment for approximately 450 km (280 mi) to the north of Tzaneen (to reappear on the border between Zimbabwe and Mozambique in the Chimanimani Mountains ) is due to a failed westerly branch of the main rift that caused Antarctica to start drifting away from southern Africa during

1102-518: The Burke's, Vanrhyns, Bloukrans , Gannaga, Ouberg , Verlatekloof, Teekloof, Molteno , Goliatskraal, Daggaboersnek , Katberg , Nico Malan , and Barkly passes. Nevertheless, the escarpment to the south and west of the plateau lacks the grandeur of the Mpumalaga and Lesotho Drakensberg, on the one hand, and the extremely rugged, intricately-folded, ranges of Cape Fold Mountains that run parallel to

1160-759: The Great Escarpment, the most well-known section being the Drakensberg (diagram on the right). The Schwarzrand and edge of the Khomas Highland in Namibia , as well as the Serra da Chela in Angola, are also well-known names. About 180 million years ago, a mantle plume under southern Gondwana caused bulging of the continental crust in the area that would later become southern Africa. Within 10–20 million years, rift valleys formed on either side of

1218-685: The Kalahari sands) are younger than 180 million years. The youngest rocks that remain cap the plateau in Lesotho and form the steep sides of the Great Escarpment in this region. These are the Clarens Formation laid down under desert conditions about 200 million years ago, topped by a 1,600 m (5,200 ft) thick layer of lava that erupted and covered most of southern Africa, and indeed large parts of Gondwana, approximately 180 million years ago. Erosional retreat means that

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1276-641: The N1 highway between Beaufort West , below the escarpment, and just beyond Three Sisters on the way to Richmond or Victoria West , on the plateau; and also where the Orange River has eroded a broad valley through the escarpment in the west before it flows into the Atlantic Ocean ). Along most of its extent, however, it forms a 400–800 m high mountain-like ridge that roads into the interior have to negotiate, sometimes up steep winding passes, such as

1334-497: The acceptance of plate tectonics , geologists had found evidence within many orogens of repeated cycles of deposition, deformation, crustal thickening and mountain building, and crustal thinning to form new depositional basins. These were named orogenic cycles , and various theories were proposed to explain them. Canadian geologist Tuzo Wilson first put forward a plate tectonic interpretation of orogenic cycles, now known as Wilson cycles. Wilson proposed that orogenic cycles represented

1392-414: The active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems. The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are mainly derived from the erosion of the actively uplifting rocks of

1450-673: The breakup of Gondwana about 150 million years ago. The lower Limpopo River and Save River drain into the Indian Ocean through what remains of this relict incipient rift valley, which now forms part of the South African Lowveld. The eastern portion of the Great Escarpment within the borders of South Africa (see the accompanying map, above) is referred to as the Drakensberg (meaning "Dragon Mountains"). The Limpopo , Mpumalanga and Lesotho Drakensberg have hard erosion-resistant upper surfaces and therefore have

1508-583: The central bulge and flooded to become the proto-Atlantic Ocean and proto-Indian Ocean more or less along the present southern African coastline and separating the Southern Cape from the Falkland Plateau . The stepped, steep walls of these rift valleys formed escarpments that surrounded the newly formed Southern African subcontinent. During the past 20 million years, southern Africa has experienced further massive uplifting, especially in

1566-405: The coast on the seaward side of the Great Escarpment. (The parallel ranges of mountains, to the south of the escarpment, can clearly be seen on the accompanying satellite image of South Africa , especially when compared to the diagram on the left, which shows the course of the Great Escarpment.) The fold mountains were formed about 330 million years ago, and therefore pre-date the formation of

1624-635: The collision is with a second continent or a continental fragment or island arc. Repeated collisions of the later type, with no evidence of collision with a major continent or closure of an ocean basin, result in an accretionary orogen. Examples of orogens arising from collision of an island arc with a continent include Taiwan and the collision of Australia with the Banda arc. Orogens arising from continent-continent collisions can be divided into those involving ocean closure (Himalayan-type orogens) and those involving glancing collisions with no ocean basin closure (as

1682-596: The course of 200 million years in the Paleoproterozoic . The Yavapai and Mazatzal orogenies were peaks of orogenic activity during this time. These were part of an extended period of orogenic activity that included the Picuris orogeny and culminated in the Grenville orogeny , lasting at least 600 million years. A similar sequence of orogenies has taken place on the west coast of North America, beginning in

1740-539: The creation of new continental crust through volcanism . Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere ( crust and uppermost mantle ). A synorogenic (or synkinematic ) process or event is one that occurs during an orogeny. The word orogeny comes from Ancient Greek ὄρος ( óros )  'mountain' and γένεσις ( génesis )  'creation, origin'. Although it

1798-427: The crust of the continental margin ( thrust tectonics ). This takes the form of folding of the ductile deeper crust and thrust faulting in the upper brittle crust. Crustal thickening raises mountains through the principle of isostasy . Isostacy is the balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by

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1856-575: The dense underlying mantle . Portions of orogens can also experience uplift as a result of delamination of the orogenic lithosphere , in which an unstable portion of cold lithospheric root drips down into the asthenospheric mantle, decreasing the density of the lithosphere and causing buoyant uplift. An example is the Sierra Nevada in California. This range of fault-block mountains experienced renewed uplift and abundant magmatism after

1914-754: The development of geologic concepts during the 19th century, the presence of marine fossils in mountains was explained in Christian contexts as a result of the Biblical Deluge . This was an extension of Neoplatonic thought, which influenced early Christian writers . The 13th-century Dominican scholar Albert the Great posited that, as erosion was known to occur, there must be some process whereby new mountains and other land-forms were thrust up, or else there would eventually be no land; he suggested that marine fossils in mountainsides must once have been at

1972-643: The east the escarpment extends northward to form the border between Mozambique and Zimbabwe , continuing on beyond the Zambezi river valley to form the Muchinga Escarpment in eastern Zambia. In the west, it extends northward into Namibia and Angola . It is the combination of this escarpment and the aridity of Southern Africa that leads to the lack of navigable rivers in South Africa. Different names are applied to different stretches of

2030-411: The east, with the result that most of the plateau lies above 1,000 m (3,300 ft) despite extensive erosion. The plateau is tilted such that it is highest in the east and slopes gently downward toward the west and south. Typically, the elevation of the edge of the eastern escarpments is in excess of 2,000 m (6,600 ft). It reaches its highest point of over 3,000 m (9,800 ft) where

2088-520: The edge of the uplifted layers are exposed. Although mountain building mostly takes place in orogens, a number of secondary mechanisms are capable of producing substantial mountain ranges. Areas that are rifting apart, such as mid-ocean ridges and the East African Rift , have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography . In strike-slip orogens, such as

2146-534: The escarpment by nearly 200 million years. The two events are geologically unrelated to one another. They also represent two very different geological processes: the Great Escarpment resulted from rifting, and the tearing apart of the Gondwana super-continent, whereas the Cape Fold Mountains resulted from the collision of tectonic plates , during the assembly of Gondwana, in the same way that

2204-547: The escarpment forms part of the international border between Lesotho and the South African province of KwaZulu-Natal . With the widening of the Atlantic, Indian, and Southern oceans, southern Africa became tectonically quiescent. Earthquakes rarely occur in the region, and there has been no volcanic or orogenic activity for approximately 50 million years. An almost uninterrupted period of erosion continues to

2262-409: The final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and which dip away from the orogenic core. An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis. Orogens are usually long, thin, arcuate tracts of rock that have

2320-456: The following Dams: Primary drainage region X. Great Escarpment, Southern Africa The Great Escarpment is a major topographical feature in Africa that consists of steep slopes from the high central Southern African plateau downward in the direction of the oceans that surround southern Africa on three sides. While it lies predominantly within the borders of South Africa , in

2378-611: The less resistant overlying sediments were removed, ultimately to form the parallel formations that protrude from the coastal plain of the south and southwest Cape. In the main, the rocks of the KwaZulu-Natal Midlands belong to the Beaufort and Ecca Groups (of the Karoo Supergroup ), aged 220–310 million years. The eastern portion of the Great Escarpment goes as far north as Tzaneen at approximately

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2436-543: The mountain range, although some sediments derive from the foreland. The fill of many such basins shows a change in time from deepwater marine ( flysch -style) through shallow water to continental ( molasse -style) sediments. While active orogens are found on the margins of present-day continents, older inactive orogenies, such as the Algoman , Penokean and Antler , are represented by deformed and metamorphosed rocks with sedimentary basins further inland. Long before

2494-416: The ocean basin comes to a halt, and continued subduction begins to close the ocean basin. The closure of the ocean basin ends with a continental collision and the associated Himalayan-type orogen. Erosion represents the final phase of the orogenic cycle. Erosion of overlying strata in orogenic belts, and isostatic adjustment to the removal of this overlying mass of rock, can bring deeply buried strata to

2552-463: The original rift valley walls) that surrounds the subcontinent, creating the present-day coastal plain. The rate of the erosion of the escarpment in the Drakensberg region is said to average 1.5 m (5 ft) per 1000 years, or 1.5 millimetres ( 1 ⁄ 16  in) per year. Because of erosion throughout most of the Mesozoic and Cenozoic eras, none of the plateau's surface rocks (except

2610-659: The park, is one of the ecologically most important rivers in the country, whilst the Crocodile River forms the southern border to the park. Several private game reserves adjoin the Kruger National Park, with other conservation areas throughout in the water management area. 4 sub-management area are made, they are: The WMA Includes the following major rivers: the Nwanedzi River , Sabie River , Crocodile (East) River and Komati River , and covers

2668-416: The periodic opening and closing of an ocean basin, with each stage of the process leaving its characteristic record on the rocks of the orogen. The Wilson cycle begins when previously stable continental crust comes under tension from a shift in mantle convection . Continental rifting takes place, which thins the crust and creates basins in which sediments accumulate. As the basins deepen, the ocean invades

2726-415: The present, removing layers many kilometers thick from the surface of the plateau and moving the present position of the escarpment approximately 150 kilometres (93 mi) inland from the original fault lines that formed the walls of the rift valley along the coastline during the break-up of Gondwana. Consequently, a thick layer of marine sediment was deposited onto the continental shelf (the lower steps of

2784-441: The rift zone, and as the continental crust rifts completely apart, shallow marine sedimentation gives way to deep marine sedimentation on the thinned marginal crust of the two continents. As the two continents rift apart, seafloor spreading commences along the axis of a new ocean basin. Deep marine sediments continue to accumulate along the thinned continental margins, which are now passive margins . At some point, subduction

2842-789: The rocks exposed on the coastal plain are, with very few and small exceptions, older than those that cap the escarpment. The rocks found in the Lowveld below the Mpumalanga portion of the Great Escarpment are more than 3 billion years old. As the escarpment retreated inland, the Cape Fold Mountains that had formed 150 million years earlier and been buried under sediments from the Himalaya -sized range of Gondwana mountains were gradually re-exposed. Being composed of erosion-resistant quartzitic sandstone , they remained as

2900-491: The sea-floor. Orogeny was used by Amanz Gressly (1840) and Jules Thurmann (1854) as orogenic in terms of the creation of mountain elevations, as the term mountain building was still used to describe the processes. Elie de Beaumont (1852) used the evocative "Jaws of a Vise" theory to explain orogeny, but was more concerned with the height rather than the implicit structures created by and contained in orogenic belts. His theory essentially held that mountains were created by

2958-399: The sills are thinner or absent then, like the portion of the Drakensberg between KwaZulu-Natal – Free State , the escarpment is composed of softer rocks, aged between 250 and 300 million years old. This means that in these regions the scarp has a more rounded appearance, or has eroded away to such an extent that the scarp may no longer be evident (for instance along the route taken by

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3016-411: The south of the present African continent, on the portion of Gondwana called the " Falkland Plateau", the remnants of which are at present located far to the southwest of southern Africa close to southern tip of South America . 32°12′00″S 22°38′24″E  /  32.2000°S 22.6400°E  / -32.2000; 22.6400 Orogeny Orogeny ( / ɒ ˈ r ɒ dʒ ə n i / )

3074-414: The squeezing of certain rocks. Eduard Suess (1875) recognised the importance of horizontal movement of rocks. The concept of a precursor geosyncline or initial downward warping of the solid earth (Hall, 1859) prompted James Dwight Dana (1873) to include the concept of compression in the theories surrounding mountain-building. With hindsight, we can discount Dana's conjecture that this contraction

3132-423: The subduction produces compression in the overriding plate. Whether subduction produces compression depends on such factors as the rate of plate convergence and the degree of coupling between the two plates, while the degree of coupling may in turn rely on such factors as the angle of subduction and rate of sedimentation in the oceanic trench associated with the subduction zone. The Andes Mountains are an example of

3190-460: The surface. The erosional process is called unroofing . Erosion inevitably removes much of the mountains, exposing the core or mountain roots ( metamorphic rocks brought to the surface from a depth of several kilometres). Isostatic movements may help such unroofing by balancing out the buoyancy of the evolving orogen. Scholars debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics ). Thus,

3248-480: The tributaries that flow into the Orange River . There are so many of these tributaries that it gives the Lesotho Highlands a very rugged mountainous appearance, both from the ground and from the air. Along the southern extent of the central plateau some of the thicker, hard, erosion-resistant dolerite sills form large parts of the upper edge of the escarpment (see illustration on the right), but where

3306-451: Was due to the cooling of the Earth (aka the cooling Earth theory). The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, fiercely contested by proponents of vertical movements in the crust, or convection within the asthenosphere or mantle . Gustav Steinmann (1906) recognised different classes of orogenic belts, including

3364-499: Was used before him, the American geologist G. K. Gilbert used the term in 1890 to mean the process of mountain-building, as distinguished from epeirogeny . Orogeny takes place on the convergent margins of continents. The convergence may take the form of subduction (where a continent rides forcefully over an oceanic plate to form a noncollisional orogeny) or continental collision (convergence of two or more continents to form

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