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32-745: The Onverwacht Group or Onverwacht Series is a series of greenstone belts and volcanic rock formations from the Archean Eon in the Kaapvaal Craton in South Africa and Eswatini . A well known part of the Onverwacht Series is visible in the Komati valley, located in the east of the Transvaal region. The Onverwacht Group can be divided into two subgroups with six formations: The fossils found in

64-472: A crust which is either oceanic or intermediate between the normal oceanic crust and that typical of continents; heat flow in the basins is higher than in normal continental or oceanic areas. Some arcs, such as the Aleutians, pass laterally into the continental shelf on the concave side of the arc, while most of the arcs are separated from the continental crust. Movement between two lithospheric plates explains

96-483: A distinct curved form, a chain of active or recently extinct volcanoes, a deep-sea trench , and a large negative Bouguer anomaly on the convex side of the volcanic arc. The small positive gravity anomaly associated with volcanic arcs has been interpreted by many authors as due to the presence of dense volcanic rocks beneath the arc. Inactive arcs are a chain of islands which contains older volcanic and volcaniclastic rocks . The curved shape of many volcanic chains and

128-514: A greenstone belt and the granites they abut, and the Proterozoic where greenstone belts sit upon granite-gneiss basements and / or other greenstone belts, and the Phanerozoic where clear examples of island arc volcanism, arc sedimentation and ophiolite sequences become more dominant. This change in nature is interpreted as a response to the maturity of the plate tectonics processes throughout

160-404: Is a plane that dips under the overriding plate where intense volcanic activity occurs, which is defined by the location of seismic events below the arc. Earthquakes occur from near surface to ~660 km depth. The dip of Benioff zones ranges from 30° to near vertical. An ocean basin may be formed between the continental margin and the island arcs on the concave side of the arc. These basins have

192-412: Is now believed that water acts as the primary agent that drives partial melting beneath arcs. It has been shown that the amount of water present in the down-going slab is related to the melting temperature of the mantle. The greater the amount of water present, the more the melting temperature of the mantle is reduced. This water is released during the transformation of minerals as pressure increases, with

224-404: Is related to the age of the subduction zone and the depth. The tholeiitic magma series is well represented above young subduction zones formed by magma from relative shallow depth. The calc-alkaline and alkaline series are seen in mature subduction zones, and are related to magma of greater depths. Andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of

256-491: The Archean greenstone belts. These similarities include a decrease in the amount of ultramafic and mafic rocks as you move up the stratigraphic column , in addition to an increase in pyroclastics , felsic and/or andesite rocks. Also, the rock successions tend to have clastics in the upper portion and tholeiitic suites in the lower. Calc-alkaline dikes are common in these suites. Archaean greenstones are found in

288-819: The Slave craton , northern Canada , Pilbara craton and Yilgarn Craton , Western Australia , Gawler Craton in South Australia , and in the Wyoming Craton in the US. Examples are found in South and Eastern Africa, namely the Kaapvaal craton and also in the cratonic core of Madagascar , as well as West Africa and Brazil , northern Scandinavia and the Kola Peninsula (see Baltic Shield ). Proterozoic greenstones occur sandwiched between

320-588: The Earth's geological history. Archaean plate tectonics did not take place on mature crust and as such the presence of thrust-in allochthonous greenstone belts is expected. By the Proterozoic, magmatism was occurring around cratons and with established sedimentary sources, with little recycling of the crust , allowing preservation of more sediments. By the Phanerozoic, extensive continental cover and lower heat flow from

352-756: The Onverwacht Series are among the oldest found on Earth. This South Africa location article is a stub . You can help Misplaced Pages by expanding it . This Eswatini location article is a stub . You can help Misplaced Pages by expanding it . This article about a specific stratigraphic formation in Africa is a stub . You can help Misplaced Pages by expanding it . Greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from

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384-732: The Pilbara and Yilgarn cratons in Australia, and adjoining the Gawler Craton and within the extensive Proterozoic mobile belts of Australia, within West Africa, throughout the metamorphic complexes surrounding the Archaean core of Madagascar; the eastern United States , northern Canada and northern Scandinavia. The Abitibi greenstone belt in Ontario and Quebec is one of the largest Archean greenstone belts in

416-449: The angle of the descending lithosphere are related. If the oceanic part of the plate is represented by the ocean floor on the convex side of the arc, and if the zone of flexing occurs beneath the submarine trench , then the deflected part of the plate coincides approximately with the Benioff zone beneath most arcs. Most modern island arcs are near the continental margins (particularly in

448-494: The calc-alkaline magmas. Some Island arcs have distributed volcanic series as can be seen in the Japanese island arc system where the volcanic rocks change from tholeiite—calc-alkaline—alkaline with increasing distance from the trench. Several processes are involved in arc magmatism which gives rise to the great spectrum of rock composition encountered. These processes are, but not limited to, magma mixing, fractionation, variations in

480-614: The characteristics of the Barberton Greenstone belt has been used as a primer for other greenstone belts around the world. He noted the existence of pillow lavas , indicating a lava being rapidly cooled in water , as well as the spinifex textures created by crystals formed under rapidly cooling environments, namely water. Island arc Island arcs can either be active or inactive based on their seismicity and presence of volcanoes. Active arcs are ridges of recent volcanoes with an associated deep seismic zone. They also possess

512-401: The crust is neither being consumed nor generated. Thus the present location of these inactive island chains is due to the present pattern of lithospheric plates. However, their volcanic history, which indicates that they are fragments of older island arcs, is not necessarily related to the present plate pattern and may be due to differences in position of plate margins in the past. Understanding

544-559: The deepest features of ocean basins; the deepest being the Mariana trench (approximately 11,000 m or 36,000 ft). They are formed by flexing of the oceanic lithosphere, developing on the ocean side of island arcs. Back-arc basin : They are also referred to as marginal seas and are formed in the inner, concave side of island arcs bounded by back-arc ridges. They develop in response to tensional tectonics due to rifting of an existing island arc. Benioff zone or Wadati-Benioff zone : This

576-535: The granite and gneiss events, because the vast majority of greenstones are interpreted as altered basalts and other volcanic or sedimentary rocks . As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaean geological history. Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes . Greenstone belts are primarily formed of volcanic rocks, dominated by basalt , with minor sedimentary rocks inter-leaving

608-406: The greater volume of otherwise homogeneous granite - gneiss within a craton contains a significantly larger degree of heterogeneity and complications and forms a tectonic marker far more distinct than the much more voluminous and homogeneous granites. Additionally, a greenstone belt contains far more information on tectonic and metamorphic events, deformations, and paleogeologic conditions than

640-400: The green hue imparted by the colour of the metamorphic minerals within the mafic rocks: The typical green minerals are chlorite , actinolite , and other green amphiboles . Greenstone belts also often contain ore deposits of gold , silver , copper , zinc , and lead . A greenstone belt is typically several dozens to several thousand kilometres long. Typically, a greenstone belt within

672-466: The major features of active island arcs. The island arc and small ocean basin are situated on the overlying plate which meets the descending plate containing normal oceanic crust along the Benioff zone. The sharp bending of the oceanic plate downward produces a trench. There are generally three volcanic series from which the types of volcanic rock that occur in island arcs are formed: This volcanic series

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704-431: The mantle has seen greater preservation of sediments and greater influence of continental masses. Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with ' metabasalt ' et cetera; greenschist , whiteschist and blueschist are all terms spawned from the study of greenstone belts. The West African early Proterozoic greenstone belts are similar to

736-419: The mantle wedge. If hot material rises quickly enough so that little heat is lost, the reduction in pressure may cause pressure release or decompression partial melting . On the subducting side of the island arc is a deep and narrow oceanic trench, which is the trace at the Earth's surface of the boundary between the down-going and overriding plates. This trench is created by the downward gravitational pull of

768-526: The margins of continents. Below are some of the generalized features present in most island arcs. Fore-arc : This region comprises the trench, the accretionary prism, and the fore-arc basin. A bump from the trench in the oceanward side of the system is present (Barbados in the Lesser Antilles is an example). The fore-arc basin forms between the fore-arc ridge and the island arc; it is a region of undisturbed flat-bedded sedimentation. Trenches : These are

800-446: The mineral carrying the most water being serpentinite . These metamorphic mineral reactions cause the dehydration of the upper part of the slab as the hydrated slab sinks. Heat is also transferred to it from the surrounding asthenosphere. As heat is transferred to the slab, temperature gradients are established such that the asthenosphere in the vicinity of the slab becomes cooler and more viscous than surrounding areas, particularly near

832-487: The northern and western margins of the Pacific Ocean). However, no direct evidence from within the arcs shows that they have always existed at their present position with respect to the continents, although evidence from some continental margins suggests that some arcs may have migrated toward the continents during the late Mesozoic or early Cenozoic . They are also found at oceanic-oceanic convergence zones, in which case

864-622: The older plate will subduct under the younger one. The movement of the island arcs towards the continent could be possible if, at some point, the ancient Benioff zones dipped toward the present ocean rather than toward the continent, as in most arcs today. This will have resulted in the loss of ocean floor between the arc and the continent, and consequently, in the migration of the arc during spreading episodes. The fracture zones in which some active island arcs terminate may be interpreted in terms of plate tectonics as resulting from movement along transform faults , which are plate margins where

896-458: The relatively dense subducting plate on the leading edge of the plate. Multiple earthquakes occur along this subduction boundary with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Benioff zone . Island arcs can be formed in intra-oceanic settings, or from the fragments of continental crust that have migrated away from an adjacent continental land mass or at subduction-related volcanoes active at

928-429: The source of heat that causes the melting of the mantle was a contentious problem. Researchers believed that the heat was produced through friction at the top of the slab. However, this is unlikely because the viscosity of the asthenosphere decreases with increasing temperature, and at the temperatures required for partial fusion, the asthenosphere would have such a low viscosity that shear melting could not occur. It

960-420: The upper part of the slab. This more viscous asthenosphere is then dragged down with the slab causing less viscous mantle to flow in behind it. It is the interaction of this down-welling mantle with aqueous fluids rising from the sinking slab that is thought to produce partial melting of the mantle as it crosses its wet solidus . In addition, some melts may result from the up-welling of hot mantle material within

992-422: The volcanic formations. Through time, the degree of sediment contained within greenstone belts has risen, and the amount of ultramafic rock (either as layered intrusions or as volcanic komatiite ) has decreased. There is also a change in the structure and relationship of greenstone belts to their basements between the Archaean where there is little clear relationship, if any, between basalt- peridotite sheets of

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1024-707: The world. In Antarctica, the Proterozoic-aged Fisher Massif closely resembles the composition and structure of a greenstone belt. One of the best known greenstone belts in the world is the South African Barberton greenstone belt , where gold was first discovered in South Africa . The Barberton Greenstone belt was first uniquely identified by Prof Annhauser at the University of the Witwatersrand , Johannesburg . His work in mapping and detailing

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