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Základy africké geologie

19.03.2006  |  128× přečteno      vytisknout článek

African geology The continent of Africa is made up of a vast stable crystalline basement of very old rocks, mainly of Precambrian age.

Superimposed on this basement are later, largely flat-lying cover successions; along the E, N, and W coasts there are sediments of Mesozoic and Tertiary age, deposited in marginal marine basins.Other major features are the Atlas Mountains in the extreme NW, a part of the Alpine fold belt of Europe, and the Palaeozoic Cape fold belt in the extreme S. In the E, the continent is rifted N-S along the lines of the East African Rift System, and there are huge outpourings of volcanic rocks associated with the Great Rift Valley. The Precambrian basement can be divided into three large masses or cratons; these are the Kalahari, Congo, and West African cratons. They are separated from each other by a number of mobile belts active in late Precambrian and early Palaeozoic times.The Kalahari craton occupies much of southern Africa and contains some of the oldest known rocks and microfossils in the world. The oldest rocks occur in the Transvaal province; in this area the basement consists of granites, gneisses, and migmatites around 3,400 million years old. Within this granitic basement are a number of greenstone belts where the rocks have been less highly metamorphosed and contain many primary features. These rocks comprise the Swaziland system. They form a thick pile of basic volcanics and cherts, which pass up into turbidites and sandstones. Gold ores (the first to be worked in South Africa) occur in fracture systems in the greenstone belts. The whole of this ancient basement became stabilised around 3,000 million years ago, and was then covered by a thick sequence of shallow-water sediments, lavas, and igneous intrusions. Although of great age (deposited between 2,800 and 2,000 million years ago), the rocks are remarkably well preserved, and are of great economic importance. The Witwatersrand system at the base is the world's most productive source of gold; the younger Transvaal system contains important iron ores, and the Bushveld igneous complex contains platinum, chromium, titanium-iron, and tin ores.To the north of the Transvaal province, and separated from it by the younger Limpopo belt, is the Rhodesian province. This is composed of a similar granitic basement with narrow greenstone belts; within the greenstone belts, the sequence consists of basic volcanics, covered by greywackes, shales, and conglomerates. The volcanic rocks are the main gold-bearing rocks of Zimbabwe.The Limpopo belt runs E-NE separating the Rhodesian and Transvaal massifs, and is a belt of high-grade metamorphic rocks involved in a long cycle of metamorphism and deformation that ended 2,000 million years ago, after the stabilisation of the adjacent massifs.The final component of the Kalahari craton is the much younger Orange River belt, which loops around the southern margin of the Transvaal massif. This belt of high-grade metamorphic rocks became stabilised around 1,000 million years ago.The Congo craton occupies a large part of central southern Africa; its oldest rocks occur in the Tanzania province, an area of granitic basement and greenstone belts similar in structure to the Rhodesian province. Two younger belts of metamorphic rocks, the Ubendian and Toro belts, represent mobile belts active about 1,800 million years ago. Another distinctive but younger mobile belt is the Karagwe-Ankole belt, which runs NE-SW for at least 1,500 km/932 mi from Uganda to Zambia. It was active 1,400-1,000 million years ago and suffered several periods of deformation.The West African craton comprises practically the whole of western Africa from the Gulf of Guinea to the Anti-Atlas Mountains of Morocco. It is bounded E, W, and N by much younger mobile belts, while in the Sahara area it is often covered by later Phanerozoic sediments. The oldest rocks occur as scattered masses of highly metamorphosed rocks, metamorphosed during the period 2,900-2,500 million years ago. Rocks of the younger Eburian province outcrop over a wide area, occurring in Ghana, Ivory Coast, and Sierra Leone. This province consists of a group of sediments and volcanics, the Birrimian group, which has been folded, metamorphosed, and intruded by a large suite of granites, the Eburian granites (1,900-2,200 million years old). Some of these granites in Ghana are associated with gold-quartz reefs. The final stages of activity in the craton were marked by the deposition of late Precambrian sediments, laid down unconformably on the stabilised craton.Late Precambrian and early Palaeozoic mobile belts: A number of well-defined mobile belts became established in the continent in late Precambrian times; these were the Mozambique belt, which stretches N-S along the E side of the continent from Egypt to Mozambique; the Katanga belt, which runs E-W between the Congo and Kalahari cratons; and two other less well-defined belts running through the Cape province and along the W Zaire coast. At this time Africa was joined to the other southern continents as part of the super-continent of Gondwanaland, and these late Precambrian belts can be traced across into the other continents.The Mozambique belt is a 4,000-km/2,400-mi long belt of high-grade metamorphic rocks, representing a mobile belt which began forming in Precambrian times and underwent several phases of folding and metamorphism before its final phase of regional metamorphism some 600 million years ago at the end of the Precambrian.The Katanga belt is a broad curved zone of late Precambrian orogenic activity (see orogeny), which passes through the copper belt of Zambia and the mining district of Katanga (Shaba). The rocks forming the Katanga system are a thick succession of shallow-water sediments, up to 10 km thick. The copper deposits are sulphide ores lying mainly within shale horizons. The copper-bearing rocks have only suffered low-grade metamorphism, but towards the centre of the mobile belt metamorphism is of a higher grade. Activity in this belt ceased around 570 million years ago, with the intrusion of mantled gneiss domes, reactivated in the final stage of the orogenic cycle. The West Congo belt is a sedimentary succession, similar to the Katanga system, which runs N-S for 1,000 km/620 mi along the W side of the Congo craton. The orogenic activity in all these belts had ceased by about 500 million years ago, and by this time the whole continent of Gondwanaland, with Africa at its core, had become a single stabilised craton encircled by zones of continuing mobile belt activity.The Mauritanide belt in NW Africa is a mobile belt active in Palaeozoic times and having its final phase of folding in Hercynean times. Much of it is covered by younger sediments. The Cape fold belt cuts across the southern tip of Africa, and is another belt of Palaeozoic activity, stabilised in Upper Palaeozoic times.Glaciation: Towards the end of the Upper Palaeozoic new basins of deposition were initiated within the continent and on its margins. This phase began with a widespread glaciation in Permo-Carboniferous times. Huge thicknesses of glacial deposits - tillites, varved clays, and sandstones - were laid down; the Dykwa glacial sequence of the Karroo basin is 800-900 m/2,600-2,950 ft thick, suggesting a lengthy glaciation. Sedimentation in the continental Karroo basin continued without interruption through Permian, Triassic, and Jurassic times. The Permian glacial deposits were closely followed by the deposition of coal measures (of economic importance in the Wankie basin of Zimbabwe), followed in turn by shales, sandstones, and finally the Karroo basalts. Large volumes of plateau basalts, up to 1 km/0.62 mi thick, poured out in late Triassic and Jurassic times, and marking the beginning of the splitting up of the continent of Gondwanaland.During the initial phases of continental disruption in late Jurassic and early Cretaceous times, narrow marine troughs developed at the sites of the future continental margins, and faulting and flexing in the basement brought about the formation of the East African Rift System. At the same time, basic and alkaline igneous activity was widespread.In Cretaceous times the continents began to move apart, and since that time Africa has remained a major land mass, the only orogenic activity being that on the extreme NW corner of the continent, in the Atlas belt, which is of Alpine age. Fluviatile and lacustrine sediments accumulated in a number of interior basins, while thin cover sequences were laid down on the continental margins in some regions.The East African Rift System, which is part of a 5,000-km/3,100 mi fracture zone extending from the Limpopo valley in the south to the Jordan valley in the north, came into existence in its southern part in the late Mesozoic, and was associated with voluminous igneous activity. Further extensive igneous activity is seen in the Tertiary to recent volcanics of the northern part of the rift, which originated in mid-Tertiary times. Fault movement forming the rift valley took place mainly in Miocene and Pleistocene times, and the area is still marked by seismic activity, volcanism, and high heat flow through the crust.The Red Sea : During Oligocene and Miocene times a new marine trough was developed on the site of the present Red Sea; clays, marls, evaporites, and limestones accumulated in Miocene and Pliocene times, and then the two sides began to split apart as new sea floor was generated at the floor of the trough. The Red Sea therefore seems to be a new ocean basin in the making.ZDROJ: www.fortunecity.com

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