Arabic-Nubian shield

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  • Outcrops of the Arab-Nubian Shield.
    • Satellite image of the Nubian Shield
    Arab-Nubian shield and adjacent regions in Africa: crustal provinces and metamorphic ages
    Metamorphic facies of the Arab-Nubian shield and adjacent regions in Africa

    The Arab-Nubian Shield (abbreviated ANS ) is a Precambrian base mountain unit Northeast and East Africa and the West of the Arabian Peninsula . It is largely made up of rocks that arose during the Pan-African Orogeny in the late Neoproterozoic . In this context, it forms the northern section of the approximately 6000 km long East African Orogen .

    Geographical extent and regional geology

    The largest contiguous outcrop of the Arab-Nubian Shield is in the north of the Basement Province. The Arabian Shield extends from the southern tip of Israel over large parts of the west of the Arabian Peninsula , which is geologically part of a separate lithospheric plate , the Arabian Plate . The Nubian shield lies on the African plate , mainly in the countries of Egypt and Sudan . Originally the Arabic and Nubian shields were related. They were only separated from each other as a result of the opening of the Red Sea , which began about 38 Ma ago.

    The Arabic and Nubian shield correspond to the concept of shield in regional geology insofar as Precambrian rock complexes emerge (bite out) there, with the restriction that, unlike many other continental shields (e.g. the Canadian or Baltic shield ), they are not is tectonically stable and very old (ie crustal ) earth crust. The name Arab-Nubian shield , on the other hand, refers to an extensive area of ​​the Precambrian basement of northeast and east Africa and the Arabian Peninsula, regardless of whether this is exposed or covered by younger sediments. In addition to the Arab and Nubian shields, this area includes other island or shield-like emerging Precambrian massifs in Ethiopia and Kenya as well as presumably parts of the Precambrian basement of Madagascar (the so-called Bemarivo belt in the north and the so-called Vohibory unit in the south ).

    The Arab-Nubian Shield is bounded to the west by the Sahara Metacraton and the Congo and Tanzania Craton. All three cratons are assigned to Western Gondwana . In the south, the Arab-Nubian Shield borders the Mozambique Belt, the southern section of the East African Orogen. The eastern boundary on the Arabian Peninsula runs below the Phanerozoic sediments of the Arabian Platform . It has not yet been clarified whether the rock complexes east of this border are also pan-African or older (East Gondwanian-Cratonic).

    Structural development

    Between 900 and 860 mya, various larger intrusions arose on Rodinia in connection with the beginning of continental rifting . Ocean floor spreads followed from 860 to 830 mya. The Mozambique Ocean opened. Between 830 and 750 mya, island arches , backarc and forearc basins and other terranos were formed that were subject to subduction processes or created these crustal parts.

    The ANS was created by accretion (merging) of a large number of intra-oceanic plates and possibly oceanic plateaus as well as other terranos , which in the west with the Sahara metacraton and the Congo-São Francisco craton (Congo-SF ) and the Tanzania craton collided. To the east they clashed with Azania and the Afif Terrans.

    Named after an ancient name for areas of the East African coast, Azania contains one or more crustal blocks, or microcontinents, that were crushed between the Indian Shield and the Congo SF and Tanzania craton during the Pan-African Orogeny. These crust parts made of archaic and paleoproterozoic rocks can be traced from Madagascar to Somalia, Ethiopia and the Arab Afif terran. The neo-archaic Al-Mahfid bloc in Yemen could represent a link between Azania and the Afif terrans. For the formation of the microcontinent Azania and Afif, it is assumed that as a result of the subduction of the Mozambique Ocean under the "primeval East African" continental margin, continental volcanic arcs first formed, which eventually drifted away from the continental margin due to an expansion of the crust in the Backarc (see Backarc Basin ) . Slab roll-back is assumed to be the cause of the back arc stretch. The period of this separation is not yet clear.

    The accretion of the ANS took place in four time segments. The southern to central part developed between 890 and 710 mya, the northern 760 to 650 mya, the eastern 680 to 640 mya and the further east adjoining area 640 to 580 mya. By 580 mya, the accretion of all crustal parts was largely complete and this was connected to the Sahara metacraton, as well as the Congo-SF and Tanzania craton.

    Rock formation, disturbances in the rock formation

    The rocks of the ANS are considered to be juvenile crustal parts (newly formed oceanic crusts), which were formed by partial melting of the middle to upper mantle as a result of ocean floor spreading. They produced mostly igneous calc-alkaline granitoids, which are typical of island-arch components of subduction zones. At sutures and continental margins, many ophiolite sequences , oceanic lithospheric components, can be detected, which were pushed onto the mainland during ocean-continent collisions. Such sequences occur in the Arabian Desert, Egypt, Sudan, and western Saudi Arabia. They contain peridotites , gabbros , igneous sheet dykes , pillow lavas and sedimentary rocks. The oldest rocks are 880 my.

    The collision processes generated various magmatic intrusions , such as B. granitic plutons or igneous dykes that rose in rock crevices. It was also associated with compression, thickening and deformation. The crust thickness of the ANS is up to 45 km. The arachaic African and Malagasy cratons were intensively metamorphosed at the contact zones with the Azania crustal blocks. On the eastern edge of the Congo-SF craton, Charnockitic and granitoid intrusions developed.

    Many disorders , shear zones , and sutures pervade the ANS. The sutures represent the collision zones of the various crustal structures, while the faults and shear zones indicate tectonically caused structural changes in rock formations.

    Crustal domains, metamorphoses and rock facies

    The ANS contains i. w. The following crustal domains, the rocks of which have the following metamorphic facies : Granulite facies in southern Kenya, 650 to 615 mya, which were strongly compressed and thickened as a result of the approach of Azania to the cratons Congo-SF and Tanzania, 580 to 540 mya.

    Further north follow granulite facies, 580 to 540 mya, up to about the middle of the Nubian Shield, which in places contain amphibolite deposits, 650 to 605 mya. Greenschist rocks, 650 to 605 mya, formed on both sides of the Red Sea , followed by amphibolite facies older than 900 mya in the Sudan area. The arachaic African and Malagasy cratons were metamorphically overprinted at the contact zones with the Azania crustal blocks.

    Erosion and sedimentation

    The erosion products of the ANS deposited large, km-thick sediment masses in neighboring marine and terrestrial basins of NE-North Africa, the Arabian Peninsula and the Arabian Plate. Some geological structures and deposits suggest the hypothesis “ snowball earth ”.

    Natural resources

    The Arab-Nubian Shield is rich in economically minable gold-copper deposits. Ancient Egypt owed its gold wealth to these deposits . Especially due to the deposits in Nubia ( nub = gold), Egypt was the main producer of gold in early antiquity . Gold mining reached its peak around 1300 BC. Chr.

    Further information

    • Arabian Shield website Saudi Geological Survey accessed 9/2/2016 sgs.org.sa
    • Peter R. Jonhson & Beraki Woldehaimanot: Development of the Arabian-Nubian Shield: perspectives on accretion and deformation in the northern East African Orogen and the assembly of Gondwana. In Saudi Geological Survey utdallas.edu
    • PR Johnsona, A. Andresen, AS Collins, AR Fowler, H. Fritz, W. Ghebreab, T. Kusky, RJ Stern: In Late Cryogenian – Ediacaran history of the Arabian – Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the losing stages of the northern East African Orogen doi: 10.1016 / j.jafrearsci.2011.07.003
    • Pierre Nehlig, Antonin Genna and Fawzia Asfirane In: A review of the Pan-African evolution of the Arabian Shield GeoArabia, Vol. 7, No. 1, 2002 utdallas.edu

    Individual evidence

    1. ↑ a b H. Fritz, M. Abdelsalam, KA Ali, B. Bingen, AS Collins, AR Fowler, W. Ghebreab, CA Hauzenberger, PR Johnson, TM Kusky, P. Macey, S. Muhongo, RJ Stern, G. Viola: "Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution" doi: 10.1016 / j.jafrearsci.2013.06.004
    2. ↑ Hans-Joachim Pachur, Norbert Altmann: The Eastern Sahara in the late Quarter . Springer, 2006, ISBN 978-3-540-47625-2 , pp. 15 .
    3. ^ RJ Thomas, B. De Waele, DI Schofield, KM Goodenough, M. Horstwood, R. Tucker, W. Bauer, R. Annells, K. Howard, G. Walsh, M. Rabarimanana, JM Rafahatelo, AV Ralison, T. Randriamananjara: Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar . In: Precambrian Research . tape 172 , no. 3–4 , 2009, pp. 279-300 , doi : 10.1016 / j.precamres.2009.04.008 .
    4. ^ Alan S. Collins, Peter D. Kinny, Théodore Razakamanana: Depositional age, provenance and metamorphic age of metasedimentary rocks from southern Madagascar . In: Gondwana Research . tape 21 , no. 2–3 , 2012, pp. 353-361 , doi : 10.1016 / j.gr.2010.12.006 .
    5. ↑ Mohamed G Abdelsalama, Jean-Paul Liégeois, Robert J Star: The Saharan Metacraton . In: Journal of African Earth Sciences . tape 34 , no. 3–4 , 2002, pp. 119-136 , doi : 10.1016 / S0899-5362 (02) 00013-1 .
    6. ^ Arabian Shield. Saudi Geological Survey, accessed April 18, 2016
    7. ↑ Fernando F. Alkmim, Stephen Marshak, Marco A. Fonseca: Assembling West Gondwana in the Neoproterozoic: Clues from the São Francisco craton region, Brazil . In: Geology . tape 29 , no. 4 , 2001, p. 319-322 , doi : 10.1130 / 0091-7613 (2001) 029 <0319: AWGITN> 2.0.CO; 2 .
    8. ↑ Tomoeki Nakakuki, Erika Mura: Dynamics of slab rollback and induced back-arc basin formation . In: Earth and Planetary Science Letters . tape 361 , 2013, p. 287-297 , doi : 10.1016 / j.epsl.2012.10.031 .
    9. ^ Robert J. Stern, Mohamed G. Abdelsalam: Formation of juvenile continental crust in the Arabian-Nubian shield: evidence from granitic rocks of the Nakasib suture, NE Sudan . In: Geologische Rundschau . tape 87 , no. 1 , 1998, p. 150-160 , doi : 10.1007 / s005310050196 .
    10. ↑ B. Blasband, S. White, P. Brooijmans, H. de Boorder, W. Visser: Late Proterozoic extensional collapse in the Arabian-Nubian Shield . In: Journal of the Geological Society . tape 157 , no. 3 , 2000, pp. 615–628 , doi : 10.1144 / jgs.157.3.615 ( PDF ).
    11. ↑ Edmund O. von Lippmann : Origin and Spread of Alchemy: A Contribution to Cultural History . Springer-Verlag, 1919, ISBN 978-3-642-50949-0 , p. 263 ( Google Books ).
    12. ^ Bernhard Neumann : The Metals: History, Occurrence and Extraction . Verlag von Wilhelm Knapp, Halle 1904, chapter "Gold" ( PDF ).