East African Orogen

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Arabic-Nubian shield metamorphic facies
Arabic-Nubian shield with crustal provinces and ages
Mozambique belt with crustal provinces and ages
Mozambique Belt Structural and metamorphic facies

The East African Orogen is an approximately 6,000 km long, neoproterozoic orogen complex , which runs in a north-south direction in the east of Africa and in the adjacent part of the Near East . It begins in the south of Israel , continues through Jordan and the Sinai Peninsula to Mozambique . Parts of the Precambrian basement of Madagascar are also assigned to the East African Orogen.

The geological development took place in the context of Pan-African orogeny and is related to the configuration of the relatively short-lived supercontinent Pannotia after the collapse of its predecessor Rodinia . It extends from around 900 million years ago (in the following text abbreviated as mya ) to 500 mya. In the course of the formation of Pannotia, the largest continuous mountain range of the Neoproterozoic and early Cambrian was unfolded. It was comparable to today's high mountain system of the Alpidic Orogeny , which stretches from the Alps to the Himalayas . Long-lasting weathering and erosion during and after the unfolding led to the erosion ( denudation ) of these high mountains and the deposition of the erosion debris in the surrounding sedimentary basins.

Some of the most significant geological processes that led to the formation of Pannotias and the large southern continent of Gondwana , which was still stable far into the Phanerozoic, belong to the development of the East African orogen . These were especially the

  • Consolidation, d. H. the latest tectono-metamorphic episode in the geological history of the Arab-Nubian Shield
  • Consolidation of the Madagascar basement
  • Collision of "Ur-Madagascar" with "Ur-East Africa" ​​and subsequent docking of the Indian craton to both

Geographical extension

The East African Orogen consists of the Arab-Nubian Shield (ANS) , the Mozambique Belt and finds its geological continuation in Madagascar.

Arabic-Nubian shield

The Arab-Nubian Shield begins in the south of Israel and continues across the Sinai Peninsula to the Arabian Plate . The land masses of the ANS were separated by the Red Sea (RM) 38 my ago.

Seen in north-south direction, the ANS is located west of the RM on the Nubian Shield with the Eastern or Arabian Desert and the Nubian Desert with today's states of Egypt in the north to Ethiopia and southern Kenya in the south.

On the east side of the RM is the Arab shield with the states of Israel in the north to Somalia in the south. To the west, the ANS is bounded by the Sahara Metakraton, the then still connected Congo-São Francisco (Congo-SF) craton and the Tanzania craton. To the east it turns into the Arabic plate.

Mozambique belt

The Mozambique belt connects to the south of the Arab-Nubian shield. It runs from southern Ethiopia, Kenya, Somalia, Tanzania , Malawi to central Mozambique .

In the west it is limited by the Lufifian arch.

Madagascar

The center of Madagascar is the Antananarivo craton. Sedimentary units are the Bemarivo belt in the extreme north, the Vohibory domain in the extreme south, the Androyen unit in the south, the molo area in the southwestern center and the Betsimisaraka suture zone in the eastern areas.

Geological evolution

With the disintegration of Rodinia, diverse ocean floor spreads formed and the Mozambique Ocean opened up. When the Mozambique Ocean began to close again and subducted (submerged) under the African east coast, one or more crustal blocks, terranos or microcontinents of archaic and paleoproterozian origin broke away from the African east coast .

It is believed that subduction was accompanied by the formation of oceanic island arches with backarc and forearc basins . This stretched the African continental crust, which eventually led to the detachment of the crust blocks. ( English slab roll-back ). The period of this separation is not yet clear.

The detached land masses were named after an old name for areas of the African east coast Azania. Azania presumably stretched from present-day Madagascar, Somalia, Ethiopia to the Afif Terrans on the Arab-Nubian Shield.

Arabic-Nubian shield

The Arab-Nubian shield was created from accretion (amalgamation) of a multitude of intra-oceanic plates and possibly oceanic plateaus and other terrances , which in the west with the Sahara metacraton and the cratons Congo-SF and Tanzania and in the east with Azania and the Afif Terrans collided. This merger took place between 890 and 580 mya. The southernmost part of the ANS is probably located on the northern and southern tip of Madagascar with the Bemarivo belt and the Vohibory domain. The Mozambique Ocean joined in the area of ​​the ANS.

Mozambique belt

The formation of the Mozambique Belt is related to the opening of the Mozambique Ocean and subsequent closure as a result of the collision of Western Gondwana, in particular Africa with its cratons Sahara Metakraton, Congo-São Francisco (Congo-SF), Tanzania, and the Bangweulu block with Greater India (India with NE Madagascar , Sri Lanka and the Seychelles ) from 650 to 620 mya. Neoproterozoic juvenile crustal provinces emerged.

This event was followed between 600 and 550 mya by the collision of East Gondwana, in this case East Antarctica and Australia , with the African cratons Kalahari and Congo-SF, known as Kuunga orogeny. During this event, pre-Neoproterozoic crustal areas were reshaped.

Madagascar

The craton located in central Madagascar was originally part of the East African coast with the cratons Congo-SF and Tanzania as well as the Bangweulu block. The Malagasy craton was also separated from the African continental mass during the subduction of the Mozambique Ocean under Greater India.

After the craton was detached, it was surrounded to the west by the Mozambique Ocean and east by the Malagasy Ocean.

When Greater India moved between 650 and 620 mya towards the East African coast, on the one hand the Antananarivo craton collided again with Africa and Greater India with the craton. The Antogil Block, consisting of the two Kraton fragments Antogil and Masora, was connected to the eastern flank of the Antananarivo Kraton. This block had its origin from the south Indian craton Dharwar Kraton.

The collision of the Antogil block with the Antananarivo craton created the Betsimisaraka geosuture between the two crustal blocks. This is seen as the subduction edge zone of the Malagasy Ocean when this subduced and disappeared.

In connection with the Antananarivo development, the formation of sedimentary units such as the Bemarivo Belt, the Vohibory Domaine, the Androyen Unit and the Molo area can be seen.

It is believed that the Antogil Block, and thus Madagascar, broke away from the Indian Dharwar Kraton by 96 to 84 mya, after India, including Madagascar, had already separated from East Africa by 160 to 158 mya. Since then, Madagascar has been separated from Africa by the Indian Ocean .

Rocks, metamorphoses, facies

Arabic-Nubian shield

The rocks from the Arabian-Nubian Shield are considered to be juvenile crustaceans (newly formed oceanic crusts) that were created by partial melting of the middle to upper mantle as a result of ocean floor spreading. They mostly produced igneous granitoids . At sutures (seams) and continental margins, many ophiolite sequences , oceanic lithospheric components (upper layers of the earth) , 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 (stratified fissure- shaped rock ducts ), pillow lavas (pillow lavas ) and sedimentary rocks (deposit rocks). The oldest rocks are 880 my.

The amalgamation of the ANS crustal parts as well as the collision with the African cratons and the Azania land masses caused compression, thickening, deformation and metamorphoses (rock transformations) of the affected rocks. The crust provinces show different facies (rock properties). From south to north there are granulite facies, green schist facies and amphibolite facies with periods between 650 and 540 mya.

Mozambique belt

The Mozambique Belt contains a multitude of crustal provinces, blocks and orogenes that have emerged and developed differently from one another. What they have in common, however, is the impact of the collision of these structures in the context of Pan-African orogeny.

Neoproterozoic juvenile crustal provinces

To the south of the Arab-Nubian Shield, the Eastern Granulite Nappe and Cabo Delgado Nappe complex (EGCD complex) join. Various tectonic units exist within this complex. It runs from eastern Uganda through Kenya and Tanzania to central Mozambique.

The rocks from the EGCD ​​complex, which formed from approx. 990 mya, are mostly classified as juvenile, i.e. i.e., newly formed oceanic crust similar to that of the Arab-Nubian shield. These crustal parts mainly consist of metamagmatic and overlying metasedimentary components, both of which are metamorphically influenced.

A special feature in this complex is the occurrence of marble- containing metasediments. The age of these sediments, around 800 to 600 mya, indicates that they were deposited on the edge of the Mozambique Ocean.

Between 650 and 620 mya they were subject to tectonic, thermal processes and now show highly metamorphic and deformed granulite facies.

Pre-Neoproterozoic reshaped crustal areas

As a result of the collision processes during the East African and Kuunga Orogenes, pre-Neoproterozoic crustal areas on the eastern to southwestern edges of the cratons Congo-SF, Tanzania and Bangweulu as well as the northern and eastern edges of the cratons Kalahari and Zimbabwe were reworked tectonically, thermally and structurally in different ways. This occurred in the Western Granulite Belt, the Usagaran – Ubendian Belt, the Irumide Belt, Southern Irumide Belt, Unango and Marrupa Complex and in the Nampula Block.

The Western Granulite Belt is located in SW Kenya and S Tanzania and contacts the Congo-SF and Tanzania cratons to the west. The basement has an age of 3,100 to 2,500 mya, comparable to that of the Tanzania Kraton. Magmatic and sedimentary events occurred between 843 and 665 mya. The Pan-African influences produced a metamorphic granulite-amphibolite facies, 580 to 540 mya.

The Usagaran and Ubendian belts run across Tanzania and Malawi. They lie between the southern area of ​​the Tanzania Craton and the northwest of the Bangweulu Block. The base of this belt has an archaic age of 3,100 to 2,800 mya and corresponds to that of the Tanzania craton. Both belts show similar igneous periods from 2,100 to 1,730 mya and show a Pan-African metamorphosis with green slate to sub-green slate facies, 570 to 530 mya.

The Irumide Belt stretches from central Zambia through northern Malawi to southern Tanzania and connects to the Bangweulu Block to the northwest. The basement of this belt is formed by granitoid gneiss with an age of 2,050 to 1,930 mya, which is overlaid by a metasedimentary cover, 1,880 to 1,550 mya. According to the Pan-African Metamorph, the rocks show a green slate to sub-green slate facies, 570 to 530 mya.

The Southern Irumide Belt is located in southern Tanzania. The south side encloses the northern flank of the Kalahari Kraton. It represents a complex zone of accretion of terrans that come from the Usagaran and Ubendian belts, the Irumide belt and other parts of the crust. The igneous rocks date from 1,400 to 1,040 mya. The Pan-African metamorphic influence produced granulite amphibolite facies, 616 to 563 mya.

The Unango and Marrupa complex is located in the north and center of Mozambique. The rock units, 1,062 to 946 mya, probably developed in continental rift fractures and united with the cratons Tanzania and Congo-SF. Pan-African deformations and high-grade metamorphoses occurred between 527 and 569 mya, producing amphibolite to granulite facies.

The Nampula Block lies in the north of Mozambique. The oldest rocks, 1,127 mya, contain a polydeformed sequence of upper amphibolite volcanic rocks composed of gray gneiss and migmatites. The geochemistry of these rocks suggests formation as a juvenile crust in the island arc milieu. Various terranos formed, which then merged. As a result of the Pan-African orogeny, the old rocks were transformed tectonically and thermally metamorphically and are present as amphibolite facies, 550 to 500 mya.

The Lurio Belt, also known as the Lurio Shear Zone, is located in the north of Mozambique and separates the Nampula Block from the Southern Irumide Belt to the north and the Cabo Delgado Nappe Complex in the west. The Lurio belt originated during the Kibara orogeny, from 1,400 mya, and is related to the formation of the Nampula belt.

Madagascar

Paleoproterozoic Kraton areas

The Antananarivo Kraton is made up of granitoids, 2,550 to 2,500 mya. Between 824 and 719 mya, voluminous volcanic rocks made of granite, syenite and gabbro penetrated . The chemism of these volcanic rocks corresponds to magmas that arise in areas of subduction zones. The entire craton was thermally and tectonically affected between 700 and 532 mya. This gave the original rocks a granulite facies with the development of a gneiss-like arrangement.

On the Antananarivo Kraton, rocks of the Tsaratanana plate have been deposited, which formed between 2,750 and 2,490 mya and were deformed by 2,500 mya. The plate is separated from the underlying kraton by mylonite zones.

The Itremo plate joins the Antananarivo Kraton in the SW. It consists of sedimentary rocks that are unevenly supported by amphibolites and gneisses, which in turn are comparable to the orthogneiss of the Antananarivo Kraton. These sedimentary rocks may have come from East African sources and were deposited between 1,700 and 804 mya.

Neoproterozoic juvenile crustal provinces

The broad Betsimisaraka suture zone separates the cratons Antananarivo and Antogil. It consists of metasediments and contains numerous ultramafic and mafic rocks, which are considered to be the remains of an oceanic lithosphere (upper geological layers of the earth). From this it is concluded that this suture was the subduction edge zone of the Malagasy Ocean between the two cratons. The sediments come from the Indian Dharwar Kraton and were deposited between 800 and 550 mya.

The Bemarivo Belt contains two different juvenile terranos that developed as island arch components in different ocean areas. In the southern, older terran, highly metamorphosed paragneiss developed from metasedimentary, i.e. that is, transformed sedimentary rocks formed Paleoproterozoic age. The rocks of the southern terran have upper amphibolite to granulite facies. The northern, younger terran originated in the eastern area of ​​Azania and contains mainly metamorphic supracrustal rocks (volcanic rocks or sedimentary rocks deposited on the surface) from igneous and igneous-sedimentary sequences, 750 to 740 and 720 mya. The facies of the northern block are marked as amphibolite to green schist. These can be associated with the Seychelles and NW India. The amalgamation of the two crust blocks took place between 563 and 532 mya and both with the northern Madagascar craton area around 540 to 520 mya.

The Vohibory-Domaine consists of juvenile crust with mafic granulites, amphibolites and sedimentary rocks, which arose from island arch components under intra-oceanic conditions between 910 and 760 mya. Between 630 and 600 mya they were subject to tectonic and thermal changes when the crustal components collided. The Vohibory Domaine is similar to the Eastern Granulite Belt in Tanzania, so that the juvenile crust in the Mozambique Belt stretches from southern Kenya to Tanzania to Madagascar.

The Androyene Unit consists of highly transformed metasediments. The conversion took place at ultra high temperatures. An archaic basement under the sediment cover has not yet been confirmed. The sediments were deposited between 620 and 560 mya.

In the Molo area, metasediments were deposited in a triangular zone between the Itremo plate and the Androyene unit. The parent rocks with an age of 620 to 560 mya come from a basin that separated central Madagascar from East Africa.

climate

The East African orogen had a significant influence on the flow conditions in the atmosphere and the oceans. It formed an approximately 6,000 km long and high barrier for the prevailing global westerly wind zone , comparable to today's Andes and Rocky Mountains.

In the cryogenium there was an ice age worldwide between 660 and 635 mya , the Marino Ice Age , which led to the snowball earth . Glacial sediments, such as B. Boulder clay , dropstones , varves and diamictites as well as rock scratches and paleomagnetic reconstructions clearly indicate glaciation up to the equator.

After the end of the cryogenium, temperatures rose again globally.

literature

  • Robert J. Stern: Arc Assembly and Continental Collision in the Neoproterozoic East African Orogen: Implications for the Consolidation of Gondwanaland. In: Annual Review of Earth and Planetary Sciences. Vol. 22, 1994, pp. 319-351 ( utdallas.edu PDF).
  • Alan S. Collins, Sergei A. Pisarevsky: Amalgamating eastern Gondwana: The evolution of the Circum-Indian Orogens. In: Earth Science Reviews, Vol. 71, No. 3-4, 2005, pp. 229-270 doi: 10.1016 / j.earscirev.2005.02.004 .
  • Joseph G Meert: A synopsis of events related to the assembly of eastern Gondwana. In: Tectonophysics, Vol. 362, No. 1-4, 2003, pp. 1-40 doi: 10.1016 / S0040-1951 (02) 00629-7 .

Web links

Individual evidence

  1. ^ H. Fritz and others: Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution. In: Journal of African Earth Sciences. Volume 86, October 2013, pp. 65-106 doi: 10.1016 / j.jafrearsci.2013.06.004 .
  2. ↑ Peter R. Johnson & 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 PDF).
  3. ↑ Mohamed G Abdelsalam and others: The Saharan Metacraton. In: Journal of African Earth Sciences. Volume 34, No. 3-4, April-May 2002, pp. 119-136 doi: 10.1016 / S0899-5362 (02) 00013-1 .
  4. ↑ Fernandez-Alonso and others: THE PROTEROZOIC HISTORY OF THE PROTO-CONGO CRATON OF CENTRAL AFRICA. , Royal Museum for Central Africa, Tervuren
  5. ↑ GEOLOGICAL FRAMEWORK AND REGIONAL METALLOGENY OF TANZANIA. ( Memento of the original from April 26, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / www.kilimanjarominingcompany.com
  6. ↑ AB Kampunzu, J. Cailteux: Tectonic Evolution of the Lufilian Arc (Central Africa Copper Belt) During Neoproterozoic Pan African Orogenesis. In: Gondwana Research . Volume 2, No. 3, July 1999, pp. 401-421, doi: 10.1016 / S1342-937X (05) 70279-3 .
  7. ^ Alan S. Collins: Madagascar and the amalgamation of Central Gondwana. In: Continental Evolution Research Group, Geology and Geophysics. Received August 4, 2005; accepted October 25, 2005, January 10, 2006 ( adelaide.edu: PDF).
  8. ↑ Tomoeki Nakakuki, Erika Mura: Dynamics of slab rollback and induced back-arc basin formation. In: Earth and Planetary Science Letters. 361, pp. 287–297 • January 2013, doi: 10.1016 / j.epsl.2012.10.031 .
  9. ↑ LS Andersen, R. Unrug: Geodynamic evolution of the Bangweulu Block, northern Zambia. In: Precambrian Research. Volume 25, Nos. 1-3, August 1984, pp. 187-212 doi: 10.1016 / 0301-9268 (84) 90032-9 .
  10. ↑ Armin Zeh and others: 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. doi: 10.1093 / petrology / egp027 .
  11. ^ H. Fritz and others: East African and Kuunga Orogenies in Tanzania - South Kenya. bibcode : 2012EGUGA..14.8754F .
  12. ↑ Geology of Dharwar Craton (PDF) on shodhganga.inflibnet.ac.in ( Shodhganga )
  13. ^ John C. Briggs: The biogeographic and tectonic history of India. In: Journal of Biogeography. Volume 30, No. 3, March 2003, pp. 381-388, doi: 10.1046 / j.1365-2699.2003.00809.x .
  14. ↑ V. Tenczet and other: Anorthosites in the Eastern Granulites of Tanzania-New SIMS zircon U-Pb age data, petrography and geochemistry. In: Precambrian Research. Volume 148, No. 1-2, 20 July 2006, pp. 85-114 doi: 10.1016 / j.precamres.2006.03.004 .
  15. ^ G. Viola and others: Growth and collapse of a deeply eroded orogen: Insights from structural, geophysical, and geochronological constraints on the Pan-African evolution of NE Mozambique. In: Tectonics. Volume 27, TC5009, doi: 10.1029 / 2008TC002284 .
  16. ↑ TM Kusky: Tectonic setting and terrane accretion of the Archean craton Zimbabwe. In: Geology. Department of Earth Sciences, Boston University, Boston, Massachusetts 02215, Vol. 26, No. 2, doi : 10.1130 / 0091-7613 (1998) 026 <0163: TSATAO> 2.3.CO; 2 , pp. 163-166 tgrc.cug .edu.cn PDF ( Memento of the original from February 19, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.tgrc.cug.edu.cn
  17. ^ Andreas Möller and others: Crustal Age Domains and the Evolution of the Continental Crust in the Mozambique Belt of Tanzania: Combined Sm – Nd, Rb – Sr, and Pb – Pb Isotopic Evidence. In: Oxford Journals Science & Mathematics Journal of Petrology Volume 39, No. 4, pp 749-783, Received April 1, 1997. Accepted December 3, 1997. J. Petrology (1998) 39 (4): 749-783. doi: 10.1093 / petroj / 39.4.749 ( oxfordjournals.org ).
  18. ^ H. Fritz and others: Central Tanzanian tectonic map: A step forward to decipher Proterozoic structural events in the East African Orogen. In: Tectonics. Volume 24, TC6013, 2005 doi: 10.1029 / 2005TC001796 ( erdwissenschaften.uni-graz.at PDF).
  19. ^ JL Lenoir and others: The Palaeoproterozoic Ubendian shear belt in Tanzania: geochronology and structure. In: Journal of African Earth Sciences. Volume 19, No. 3, pp. 169-184, 1994, Received August 5, 1994: accepted January 16, 1995 0899-5362 (95) 00026-7 ( africamuseum.be PDF).
  20. ^ B. De Waele and others: High-temperature, low-pressure tectono-thermal evolution of the Irumide Belt, central, Southern Africa: Lithosphere delamination during arc-accretion. In: Frontier Research on Earth Evolution Report 2002-2004, Volume 2. 9p. ( bdewaele.be PDF).
  21. ↑ CA Hauzenberger and others: Termination of the Southern Irumide Belt in Tanzania: Zircon U / Pb geochronology "ResearchGate. In: Precambrian Research (Impact Factor: 5.66). 12/2014; 255. doi: 10.1016 / j.precamres.2014.09. 021 .
  22. ↑ B. Bingen and others: Geochronology of the Precambrian crust in the Mozambiquebelt in NE Mozambique and implications for Gondwana assembly. In: ScienceDirect Precambrian Research, Volume 170, No. 3-4, May 2009, pp. 231-255, doi: 10.1016 / j.precamres.2009.01.005 .
  23. ^ PH Macey et al: Mesoproterozoic geology of the Nampula Block, northern Mozambique: Tracing fragments of Mesoproterozoic crust in the heart of Gondwana. In: Precambrian Research Volume 182, No. 1-2, September 2010, pp. 124-148, doi: 10.1016 / j.precamres.2010.07.005
  24. ^ R. Sacchi and others: Pan-African reactivation of the Lurio segment of the Kibaran Belt system: a reappraisal from recent age determinations in northern Mozambique. In: Journal of African Earth Sciences, Vol. 30, No. 3, April 2000, pp. 629-639 doi: 10.1016 / S0899-5362 (00) 00042-7
  25. ^ A. Kröner and others: Kibaran magmatism and Pan-African granulite metamorphism in northern Mozambique: single zircon ages and regional implications. In: ScienceDirect Journal of African Earth Sciences, Volume 25, No. 3, October 1997, pp. 467-484, doi: 10.1016 / S0899-5362 (97) 00117-6
  26. ↑ DI Schofield and others: Geological evolution of the Antongil Craton, NE Madagascar. In: ScienceDirect, Precambrian Research Volume 182, No. 3, October 1, 2010, pp. 187-203, doi: 10.1016 / j.precamres.2010.07.006 .
  27. ↑ Tsilavo Raharimahefa, Timothy M. Kusky: Structural and remote sensing analysis of the Betsimisaraka Suture in northeastern Madagascar. In: Gondwana Research, Volume 15, No. 1, February 2009, pp. 14-27, doi: 10.1016 / j.gr.2008.07.004 .
  28. ↑ RJ Thomas and others: Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar. In: Precambrian Research. Volume 172, No. 3-4, August 2009, pp. 279-300 doi: 10.1016 / j.precamres.2009.04.008 .
  29. ↑ LD Ashwal and others: Petrogenesis of Neoproterozoic Granitoids and Related Rocks from the Seychelles: the Case for an Andean-type Arc Origin. In: Journal of Petrology 1/2002, pp. 45-83 doi: 10.1093 / petrology / 43.1.45
  30. ^ Alan S. and others: Depositional age, provenance and metamorphic age of metasedimentary rocks from southern Madagascar. In: Gondwana Research, Volume 21, No. 2-3, March 2012, pp. 353-361, Special No .: Western Gondwana doi: 10.1016 / j.gr.2010.12.006