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AFRICA Pan-African Orogeny Pan-African Orogeny North African Phanerozoic Rift Valley A KroВЁ ner, UniversitaВЁ t Mainz, Mainz, Germany R J Stern, University of Texas-Dallas, Richardson TX, USA © 2005, Elsevier Ltd. All Rights Reserved. Introduction The term 'Pan-African' was coined by WQ Kennedy in 1964 based on an assessment of available Rb-Sr and K-Ar ages in Africa. The Pan-African event was interpreted as a tectono-thermal event some 500 Ma ago, during which mobile belts formed surrounding older cratons. This concept was extended to the Gondwana continents (Figure 1) with regional names such as Brasiliano for South America, Adelaidean for Australia, and Beardmore for Antarctica. The thermal event was later recognized as part of an orogenic cycle leading to orogenic belts resulting from the amalgamation of continental domains during the period $870 to $550 Ma. Pan-African is now used to describe tectonic, magmatic, and metamorphic activity of Neoproterozoic to earliest Palaeozoic age, especially for crust that was once part of Gondwana. Due to its tremendous geographical and temporal extent, the Pan-African cannot be a single orogeny but must reflect the opening and closing of large oceanic realms as well as accretion and collision of buoyant crustal blocks. Pan-African events culminated in the formation of the Late Neoproterozoic supercontinent Gondwana (Figure 1). The Pan-African orogenic cycle is time-equivalent with the Cadomian Orogeny in western and central Europe and the Baikalian in Asia; these parts were probably part of Gondwana in pre-Palaeozoic times as well as small Neoproterozoic crustal fragments identified in Turkey, Iran, and Pakistan (Figure 1). Within Pan-African domains, two broad types of orogenic or mobile belts can be distinguished. One type consists predominantly of Neoproterozoic supracrustal and magmatic assemblages with juvenile origins, similar to Phanerozoic collision and accretion belts. These belts expose upper to middle crustal levels and contain features such as ophiolites, subduction- or collision-related granitoids, island-arc or passive continental margin assemblages, and exotic terranes (Figure 2). Examples include the Arabian-Nubian shield of Arabia and north-east Africa, Damara-Kaoko-Gariep Belt and Lufilian Arc of south-central and south-western Africa, West Congo Belt of Angola and Congo Republic, Trans-Sahara Belt of West Africa, and Rokelide and Mauretanian belts along the western part of the West African Craton (Figure 1). The other type contains polydeformed high-grade metamorphic assemblages exposing middle to lower crustal levels. Protoliths consist predominantly of much older Mesoproterozoic to Archaean continental crust reworked during the Neoproterozoic. Examples are the Mozambique Belt of East Africa, including Madagascar (Figure 2), extending into western Antarctica, Zambezi Belt of northern Zimbabwe and Zambia, and possibly little-known migmatitic terranes in Chad, Central African Republic, Tibesti Massif in Libya, and western parts of Sudan and Egypt (Figure 1). It has been proposed that the latter type represents deeply eroded collisional orogeny with two Pan-African belt types not fundamentally different but constituting different crustal levels. The term East African Orogen has been proposed for the combined upper crustal Arabian-Nubian Shield and lower crustal Mozambique Belt (Figure 2). Figure 1 Map of Gondwana at the end of Neoproterozoic time ($540 Ma) showing general arrangement of Pan-African belts. AS, Arabian Shield; BR, Brasiliano; DA, Damara; DM, Dom Feliciano; DR, Denman Darling; EW, Ellsworth-Whitmore Mountains; GP, Gariep; KB, Kaoko; MA, Mauretanides; MB, Mozambique Belt; NS, Nubian Shield; PM, Peterman Ranges; PB, Pryolz Bay; PR, Pampean Ranges; PS, Paterson; QM, Queen Maud Land; RB, Rokelides; SD, Saldania; SG, Southern Granulite Terrane; TS, Trans-Sahara Belt; WB, West Congo; ZB, Zambezi. (Reproduced with permission from Kusky et al., 2003.) The Pan-African system of orogenic belts in Africa, Brazil and eastern Antarctica has been interpreted as a network surrounding older cratons (Figure 1) resulting from closure of several major Neoproterozoic oceans: Mozambique Ocean between East Gondwana (Australia, Antarctica, southern India), West Gondwana (Africa, South America), Adamastor Ocean between Africa and South America, Damara Ocean between Kalahari and Congo cratons, and Trans-Sahara Ocean between the West African Craton and poorly known pre-Pan-African terrane in north-central Africa variously known as Nile or Sahara Craton (Figure 1). Arabian-Nubian Shield (ANS) A broad region was uplifted during Cenozoic rifting to form the Red Sea, exposing a large tract of mostly juvenile Neoproterozoic crust. These exposures comprise the Arabian-Nubian Shield (ANS). The ANS makes up the northern half of the East African orogen and stretches from southern Israel and Jordan south as far as Ethiopia and Yemen, transitioning into the Mozambique Belt (Figure 2). The ANS is distinguished by its juvenile nature, relatively low-grade metamorphism, and abundance of island-arc rocks and ophiolites. Defined ANS extends about 3000 km north to south and >500 km on either side of the Red Sea (Figure 3). It is flanked westward by a broad tract of older crust remobilized during Neoproterozoic time, known as the Nile Craton or 'Saharan Metacraton'. Juvenile Neoproterozoic crust to the east in Arabia's subsurface is not well defined but appears Pan-African underlies most regions. Scattered outcrops in Oman yielded mostly Neoproterozoic radiometric ages for igneous rocks, with no evidence of significant pre-Pan-African crust underlying this region. The ANS is truncated northward due to Precambrian-Cambrian boundary rifting generating crustal fragments preserved in southeast Europe, Turkey, and Iran. The ANS is the largest tract of mostly juvenile Neoproterozoic crust among Pan-African-affected regions (Figure 4). It formed through a multistage process producing juvenile crust above intra-oceanic convergent plate boundaries (juvenile arcs) or oceanic plateaux (ca. 870-630 Ma), which collided and coalesced to form larger composite terranes. Older continental crust, including Mesoproterozoic Afif terrane in Arabia and Palaeoproterozoic-Archaean crust in Yemen, was overprinted by Pan-African tectonomagmatic events (Figure 2). ANS terrane boundaries are frequently defined by suture zones marked by ophiolites, with tonalitic to granodioritic plutons stitching the terranes together. Most ANS ophiolites have trace element compositions suggesting formation above a convergent plate margin, either as part of a back-arc basin or fore-arc setting. Boninites identified in Sudan and Eritrea suggest a fore-arc setting for some ANS sequences. Sediments are immature sandstones and wackes derived from nearby arc volcanoes. Neoproterozoic 'snowball Earth' episodes recognized in parts of the ANS, with banded iron formations in the northern ANS possibly deep-water expressions (Figure 3). Due to its Sahara and Arabian desert location, the ANS has almost no vegetation or soil, making it excellently exposed for remote sensing satellite imagery. Juvenile crust of the ANS was sandwiched between continental tracts of East and West Gondwana. Collision timing is still unresolved but occurred after $630 Ma when high-magnesium andesite 'schistose dykes' were emplaced in southern Israel, before $610 Ma post-tectonic 'Mereb' granites in northern Ethiopia. By analogy with India-Asia collision, the terminal Gondwana collision may have continued for a few tens of millions of years. ANS deformation ended by Cambrian time but locally continued farther south in Africa. Most intense collision occurred south of the ANS in the Mozambique belt (Figure 4). Compared to strong... Ключевые слова: e, r, o