Gariep belt
Gariep belt ( English Gariep Belt ) referred to in the regional geology Africa an approximately north-south trending , Neoproterozoic folds - and thrust belt on the Atlantic coast, starting in Lüderitz in the southwest Namibia to Kleinzee on the north west coast of South Africa , over a length of about 400 km and a width of about 80 kilometers. It is named after the river Orange , which is also called Gariep in South Africa .
The Gariep belt is the Damara - Orogen attributed and can be considered a southern continuation of the Damara Belt. In the south it goes into Saldania the buckles on the southwestern edge Kalahari - craton over.
The formation of the Gariep belt occurred in the course of Pan-African orogeny . Its genesis covers the period 780 to 520 million years ago ( mya ).
development
Between 780 and 740 mya, the then still intact Congo-São Francisco craton (Congo-SF) and the Kalahari craton broke apart on the “African” side , and both broke away from the “South American” Río-de-la-Plata- Kraton . The continental rift phase was followed by ocean floor spreading . The corresponding ocean basin is called the Adamastor Ocean. The oldest igneous evidence of this stretching tectonics are 741 million year old rhyolites and basalts , which are part of the sedimentary sequence of the Rosh-Pina-Formation. This formation came to be deposited in a part of the fossil rift system, which did not develop into an ocean basin, but whose expansion came to a standstill at some point ( Aulakogen , also called "failed rift"). The 717 million year old mafic Gannakouriep gang swarm (English: Gannakouriep mafic dyke swarm ) is a testimony to the transition from the continental rift stage to the ocean stage (English: "rift-to-drift", see also → Wilson cycle ) ) on the southwestern edge of the Kalahari craton, which, in contrast to the basalts of the Rosh Pina formation classified as sub-alkaline, is also characterized by alkaline rock.
The formation of oceanic crust at the bottom of the Adamastor Ocean is evidenced by approximately 700 million-year-old basalts with the MORB signature that occur in the Marmora Terran (see below ). Basalts also occur there, which, due to their OIB-like geochemistry, are interpreted as remains of deep sea mountains (seamounts) or aseismic ridges, which were formed as a result of the plate drift over a hotspot at that time.
With the beginning of subduction under the Rio-de-la-Plata-Kraton and thus the closure of the southern Adamastor Ocean from 600 mya, the Gariep orogeny began. The rock complexes of the Marmora Terran accreted between 580 and 570 mya.
The final folding and thrusting processes in the Gariep Belt took place during the collision of the Kalahari and the Río-de-la Plata craton at around 545 mya. The oldest molasses in the Nama Basin, the eastern foreland basin of the orogen, is dated 540 million years ago. Around 520 mya the Gariep belt had largely consolidated, as old post-kinematic plutons show. However, eastward thrust tectonics in the Nama foreland basin can still be detected up to 496 mya.
Today's geological situation developed with the opening of the South Atlantic from the early Cretaceous period .
Structures, rocks
Basement
The Gariep Belt is based with the Port Nolloth Zone on the basement of the Namaqua Natal Belt, in particular the western Namaqua section with the regions Bushmanland, Northern Cape and Richtersveld . The Namaqua-Natal belt runs across the lower South Africa from Namaqualand on both sides of the Oranje River lower reaches on the Atlantic coast to the province of KwaZulu-Natal on the Indian Ocean . Geologically it has contact with the southern flank of the Kaapvaal craton .
The reason for the formation of the Namaqua-Natal belt was the formation of rifts. The belt developed mainly in two periods around 2,200 and 1,400 mya from partially melted ( partial melt ) material of the earth's mantle . Components from older crust sources are only slightly present.
The rocks in the two regions relevant to the Gariep Belt were formed between 2,000 and 1,750 mya and consist of calc-alkine lavas, granitoids , para and orthogneiss .
Structures
The Gariep Belt is divided into two main zones: the parautochthonous with only slightly displaced rock units of the Port Nolloth Zone and to the west of it on the Atlantic coast the allochthonous , predominantly mafic Marmora Terrane with widely displaced crustal blocks. The port Nolloth zone and the Marmora Terrane are by Schakal mountains separated -Disturbance. During the collision of the Rio de la Plata craton with the Kalahari craton, the Marmora terranes were pushed along this fault in a south-easterly direction towards the Port Nolloth zone.
The Nama Group joins the Port Nolloth Zone to the east as a sedimentation area of the Gariep Belt.
Port Nolloth Zone
The Port Nolloth Zone developed as a result of crust expansion from an intra-continental rift (rifting) to a passive continental margin on the southwestern area of the Kalahari craton. It consists of metasediments and metavulcanites (each with metamorphic overprinted rocks). There are several models of the stratigraphic structure, e.g. B. the following:
The Lower Gariep Group is formed by the Stinkfontein Subgroup, which is divided into several formations. It contains feldspar - quartzite , intermediate and acidic volcanic rocks , calcareous feldspathic quartzites, phyllites and limestones . These rocks were deposited during continental rifting combined with rift formation. Their formation began from 780 mya, which was accompanied by intrusions of alkali feldspar granites .
The upper Gariep group is structured into the Hilda, Numees and Holgat subgroups, each with several formations. The rocks that occur in it consist of marbles , meta pelitschichten , quartzites Diamiktiten with varves layered pelites, dropstones and bands ores and turbiditic Meta arkose and meta graywackes . It was deposited on a passive continental margin in the shelf area and as deep-sea sediment. Around 741 mya, massive Felsic lava flows and pyroclastic flows spread north of the Orange River in the lowest formation of the Hilda subgroup.
The diamictites were formed during the Sturtic Ice Age from around 715 mya and the Marino Ice Age from around 650 mya. These are assigned to the hypothesis about global glaciations ( snowball earth ).
The rocks of the Stinkfontein and Hilda subgroups as well as those of the basement were penetrated by the Gannakouriep mafic dyke swarm in 717 mya. (see also Dyke ). These intrusions extend between South Africa and Namibia in the lower Orange River region. The Dyke swarm is characterized by mafic , tholeiitic rocks and metamorphic amphiboles .
Nama group
The Nama Group represents a sedimentation area in a foreland basin on the eastern edge of the Gariep Belt. The sediments overlay the rocks of the Port Nolloth Zone. The lowest layers, however, come from the basement. Most of the sediment input was provided by the erosion processes of the Gariep Belt, which started from around 540 mya. The highest age is about 600 mya and is characterized by the eponymous Nama community of the Ediacara fauna .
The tectonic-sedimentary phase of the Port Nolloth Zone and the Nama Group ended around 520 mya.
Several post-orogenic plutons of alkali feldspar granites and syenite intruded into the central areas of the Port-Nolloth Zone around 507 mya along the SW-NE trending Kuboos-Bremen Line south of the Oranje River . They were not subject to any deformation or metamorphosis of the Gariep orogeny and can therefore only have arisen after the end of the tectonic-sedimentary phase. It is believed that these magmatic events are related to subduction processes on the western edge of Gondwana .
Marmora terran
The Marmora Terran can be divided into three tectono-stratigraphic complexes: the Jackalsberg Complex, the Oranjemund Complex and the Chameis Complex. The complexes are separated by thrusts . The complexes represent the neoproterozoic sea floor and are interpreted in their entirety as a fossil accretion wedge that was formed in the course of the subduction of the Adamastor Ocean and was pushed into the Port Nolloth zone during the subsequent collision of the cratons in an east to south-east direction. The southeastern areas of the terran are dominated by pure compression structures, while the northeastern areas are transpressive in character.
The Schakalsberg complex is the southernmost and tectonically least stressed unit. It contains relics of seamounts (seamounts) and Ocean islands and aseismic back, covered by reef - Dolomite . The lower formation (Grootderm formation) is a typical ophiolite with meta gabbros and metabasalts with different structures in which cushion structures have been preserved. Hyaloclastites are also common in this rock suite . The overlying Gais Formation contains stromatolite and oolithic dolomites .
The Oranjemund complex lies between the other two complexes and is the largest in extent. The degree of tectonic stress is also between that of the two neighboring complexes. Meta greywacke , phyllites and quartzites are dominant , although turbiditic sequences can still be recognized in only slightly deformed areas .
The Chameis complex is the northernmost and most heavily tectonically stressed unit. It contains a tectonically repeated sequence of serpentinites , as well as metamorphic gabbros, basalts, basaltic breccias, hyaloclastites and tuffs, overlaid by pelites , an alternation of dolomites and quartzites and various phyllites . Partly serpentinized ultramafic and mafic "blocks" with dimensions in the decimeter to kilometer range are embedded in tuffs and metasediments. These "blocks", like the other mafic rocks of the Sequent, are interpreted as relics of oceanic crust and therefore as an ophiolite complex.
Deformations
In the main deformation of the Gariep-belt, which is dated on 546-542 mya, the oceanic crust of Marmora Terrane with a sinistral transpressional were autopsy ( thrust ) to the passive edge of the pushed-Port Nolloth zone. Thrust structures trending from NNE to SSE formed in the central and northern part of the belt. In the southern belt area they run from NE to SW.
The thrust structures contain folds of about 1 to 200 m wavelengths with complex shapes and slopes, mostly in easterly and southeasterly directions. Some structures were overprinted and folded several times. The most notable structure is formed by the Jackalsberg Fault.
Metamorphoses
The Gariep Belt is characterized by a different polyphase metamorphic development in the Port Nolloth Zone and the Marmora Terrans.
The Port Nolloth Zone underwent a barrow-type metamorphosis characterized by a succession of mineral zones under the conditions of the upper greenschist facies to lower amphibolite facies . The age of the main metamorphosis was dated to 546 mya, which is synonymous with the main deformation phase.
In the Marmora- terranes , there were three events metamorphosis: the first phase M1 is as hydrothermal ocean floor metamorphism at very low pressures up to amphibolite facies interpreted high temperatures. It occurred from around 700 mya during the opening of the Adamastor Ocean. The second, later metamorphosis M2 took place at about the same temperatures, but slightly higher pressures than M1, as a result of subduction of the Adamastor Ocean with accretion of the terrane between 580 and 570 mya. The hornblende surrounding it with barrosite are characteristic . The very low to low M3 degrees of metamorphosis have only local distribution. This produced actinolite - amphiboles and chlorites . It took place at the same time as the metamorphosis in the eastern Port Nolloth zone around 546 mya.
Natural spaces
Remarkable natural spaces are assigned to the Gariep Belt or are in its immediate vicinity, for example:
The Great Edge Level is a steep drop and a layer level in southern Africa, which delimits the central plateau from the coastal plains to the Atlantic and Indian Oceans.
The Namib is an approximately 2,000 km long and 160 km wide dry desert that stretches along the Atlantic coast from central Angola to South Africa. The largest section is on the territory of Namibia. In the east, the Namib connects to the Great Peripheral Step. To the southeast it merges into the Kalahari .
The Fish River Canyon in the western ǁKaras region in southern Namibia.
Natural resources
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The mining of diamonds and various ores are of particular economic importance.
Diamonds
Diamond is a mineral and the cubic modification of carbon with the greatest hardness of all natural substances. It usually forms octahedral crystals .
Natural diamonds were formed in the earth's mantle under high pressures and temperatures, typically at depths greater than 150 kilometers at temperatures of 1200 to 1400 ° C. These conditions were mostly only given in the upper mantle from the Archean .
Diamond host rocks in the earth's mantle are peridotite and eclogite . The latter is often caused by subducted oceanic crust. Gas-rich volcanic rocks, so-called kimberlites, transport fragments of the earth's mantle with the diamonds it contains to the earth's surface when they erupt , where they can be found in solidified volcanic eruptive vents .
Diamonds have been mined in German Southwest Africa since the times . Initially, it was extracted in an open-cast mine centered around the city of Kolmanskop or Kolmanskop. Both the opencast mine and Kolmanskop have since been abandoned and Kolmanskop has become a ghost town .
Subsequently, the dismantling took place in today's Tsau-ǁKhaeb- (restricted area) national park . This restricted diamond area extends along the Atlantic coast between the towns of Lüderitz and Oranjemund at the mouth of the Orange River on the border with South Africa. Currently, most of the Namibian diamonds - up to two thirds - are mined offshore in alluvial soils (young alluvial soils on sea coasts) . These soils were created by the Orange River, which deposited sediments with the diamonds from inland on the Atlantic coast. The dismantling is carried out by the monopoly Namdeb Holding . The Namibian diamonds are by far the purest and most expensive in the world.
Ores
Significant is the Rosh Pinah formation in the Hilda subgroup of the Port Nolloth zone. It is a sedimentary-exhalative deposit and contains stratified accumulations with zinc , lead , copper and other ores in non- sulphide and sulphide compounds. The zinc content in the rocks can be up to 18%, that of lead up to 6% and that of copper up to 0.8%.
The Rosh Pinah formation was created during a rift formation between the cratons Kalahari and Rio de la Plata. This occurred u. a. different volcanic activities and high heat generation at anomalies with the formation of hydrothermal solutions .
These ores are mainly contained in clastic, not Carbonate -containing arkose and minor in volcanic quartz - sericite -Schiefern, were deposited in shallow or deep water zones.
The minerals were precipitated in primary, hydrogen or secondary, supergenic phases. In the primary phase, sulfidic ores are usually formed , while the supergenic phase generates non-sulfides by means of a weathering solution in water near the surface (above the groundwater level) and renewed precipitation from the sulfides .
These are mined in the vicinity of the mining town of Rosh Pinah north of the Orange River , particularly in the open-cast zinc mine Skorpion Zinc . After this deposit, the rare non-sulphide mineral scorpionite with an exceptionally high zinc content of up to 29% was discovered and mined. Other zinc ores that occur are z. B. smithsonite or hemimorphite .
Evolutionary fauna development
The Ediacara fauna in the period from 580 to 540 mya is a significant evolutionary phase of the development of the fauna . It is assumed that this evolution is related to the end of the Marionian Ice Age around 635 mya, which was the last global glaciation ( Snowball Earth ) . Changes in the marine paleoecology , in particular the increase in the oxygen content even in deep waters, could have favored the development of fauna.
It is believed that the Ediacara fauna are predominantly very simple, but multicellular animals (Metazoa). The Nama community emerged in the Namibian Nama group ( type locality ) and other global regions . The Nama community includes the most recent Ediacaran fossils , from about 548 to 543 mya.
Typical representatives are the fern-like Swartpuntia , Ernietta and the skeletal Cloudina and Namacalathus . There are also tubular fossils, which are believed to be buried in the sediment. The finds of the Nama community lie far apart, but mostly on a narrow band within the tropics . Many of the finds come from carbonate rocks. The fossils of this community appear to have lived in moving water in relatively deep water on continental slopes.
literature
- David R. Gray and others: 40Ar / 39Ar thermochronology of the Pan-African Damara Orogen, Namibia, with implications for tectonothermal and geodynamic evolution. In: Precambrian Research (Impact Factor: 5.66). 10/2006; 150 (1): 49-72. DOI: 10.1016 / j.precamres.2006.07.003 , [5]
- MJU Jasper, JG Stanistreet and EG Charlesworth: Preliminary results of a study of the structural and sedimentological evolution of the late Proterozoidearly Palaeozoic Gariep Belt, southern Namibia. In: Department of Geology, University of the Witwatersrand, Private Bag 3 - WITS 2050, Johannesburg, RSA, Communs geol. Surv. Namibia, 8 (1992/93), 105-126 mme.gov.na PDF
- H. E Frimmel, W. Frank: Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia and South Africa. In: Precambrian Research, Volume 90, Issue 1-2, June 30, 1998, Pages 1-28 doi: 10.1016 / S0301-9268 (98) 00029-1 .
- Rudolf Nagel: One billion years of geological development on the northwest edge of the Kalahari Craton. Dissertation to obtain the doctoral degree of the mathematical and natural science faculties of the Georg-August-Universität zu Göttingen, Faculty of Earth Sciences and Geography, Göttingen 1999 online
Web links
- Regional Geological Setting, Gariep Belt
- Rocks of the Richtersveld, Geological description and illustrations of rock formations.
Individual evidence
- ^ A b c Hartwig E. Frimmel, Chris JH Hartnady, Friedrich Koller: Geochemistry and tectonic setting of magmatic units in the Pan-African Gariep Belt, Namibia. In: Chemical Geology 130 (1996) 101-121, June 5, 1995; December 4, 1995 [1]
- ^ Report: Preliminary results of a study of the structural and sedimentological evolution of the late Proterozoidearly Palaeozoic Gariep Belt, southern Namibia. Geological Survey of Namibia, 1992/1993.
- ↑ Kleinzee, Namakwa In: Website SA-Venues.com [2]
- Jump up ↑ A Rozendaala, PG Gresseb, R Scheepersa, JP Le Roux: Neoproterozoic to Early Cambrian Crustal Evolution of the Pan-African Saldania Belt, South Africa. In: ScienceDirect, Precambrian Research, Volume 97, Issues 3-4, September 1999, Pages 303-323 doi: 10.1016 / S0301-9268 (99) 00036-4 .
- ↑ Fernandez-Alonso and others: The Proterozoic History of the Proto-Congo Craton of Central Africa. In: Department of Earth Sciences, Royal Museum for Central Africa, B-3080 Tervuren, Belgium. africamuseum.be PDF
- ↑ Armin Zeh, Axel Gerdes and Jackson M. Barton, Jr .: Archean Accretion and Crustal Evolution of the Kalahari Craton — the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton / Swaziland to the Francistown Arc. In: Oxford Journals, Science & Mathematics Journal of Petrology, Advance Access10.1093 / petrology / egp027, Received November 11, 2008, Accepted April 8, 2009. doi: 10.1093 / petrology / egp027 , [3]
- ↑ Pedro Oyhantçabal, Siegfried Siegesmund and Klaus Wemmer: The Río de la Plata Craton: a review of units, boundaries, ages and isotopic signature. In: International Journal of Earth Sciences, April 2011, Volume 100, Issue 2, pp 201-220 doi: 10.1007 / s00531-010-0580-8 .
- ↑ DL Reid, IGD Ransome, TC Onstott, CJ Adams: Time of emplacement and metamorphism of Late Precambrian mafic dykes associated with the Pan-African Gariep orogeny, Southern Africa: implications for the age of the Nama Group. In: Journal of African Earth Sciences (and the Middle East), Volume 13, Issues 3-4, 1991, Pages 531-541 doi: 10.1016 / 0899-5362 (91) 90116-G .
- ↑ David R. Gray, David Foster, JG Meert, BD Goscombe, Richard Armstrong, RAJ Trouw, CW Passchier: A Damara Orogen perspective on the assembly of southwestern Gondwana. Pp. 257-278 in RJ Pankhurst, RAJ Trouw, BB De Brito Neves, MJ De Wit (Eds.): West Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region. Geological Society, London, Special Publications, Vol. 294, 2008, doi: 10.1144 / SP294.14 (alternative full text access : ResearchGate )
- ^ BM Eglington: Evolution of the Namaqua-Natal Belt, southern Africa - A geochronological and isotope geochemical review. In: Journal of African Earth Sciences 46 (2006) 93-111, Received September 15, 2005; January 15, 2006, Retrieved July 10, 2006 doi: 10.1016 / j.jafrearsci.2006.01.014 , PDF
- ^ Hetu C. Sheth, Ignacio S. Torres-Alvarado, Surendra P. Verma: What Is the "Calc-alkaline Rock Series"? In: International Geology Review 44 (8): 686-701 August 2002, DOI: 10.2747 / 0020-6814.44.8.686
- ^ David L. Reid: Alkaline rocks in the Kuboos-Bremen igneous province, southern Namibia: The Kanabeam multiple ring complex. In: Communs geol. Surv. Namibia, 7 (1991) 3 - 13, Department of Geochemistry, University of Cape Town, Rondebosch 7700, South Africa mme.gov.na PDF
- ↑ Katrin Kärner: webdoc.sub.gwdg.de The Metallogenesis of the Skorpion Non-Sulphide Zinc Deposit, Namibia. Dissertation, Halle (Saale), July 4, 2006
- ↑ Supergene Deposits In: Web page Mineralienatlas - Fossilatlas [4]
- ↑ website of Vedanta Zinc International