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DEFORMATIONAL BELTS AND SUTURE ZONES IN THE EAST AFRICAN OROGEN: A REVISIT IN LIGHT OF RECENT DATA FROM THE PRECAMBRIAN OF SOUTHERN ETHIOPIA B. Yibas1, W.U. Reimold2, R. Armstrong3 and C. Anhaeusser 2 1 Pulles Howard and de Lange Inc., P. O. Box 861 Auckland Park 2006, South Africa (
[email protected]) 2 School of Geosciences, Univ. of the Witwatersrand, Private Bag 3, 2050, Johannesburg, South Africa 3 Research School of Earth sciences, The Australian National Univ., Canberra, ACT 0200, Australia Introduction: The low-grade Arabian-Nubian shield (ANS) and the high-grade Mozambique belt together form the larger collisional belt known as the East African Orogen (EAO) as defined by [1]. Tibetan-style continent-continent collision between West Gondwana and East Gondwana is thought to have been responsible for the formation of the EAO. Collision with NW-SE directed plate motions was followed by a stage of post-collisional ductile shearing parallel to the plate boundaries [2]. The ANS is the northern juvenile part of the Neoproterozoic EAO, and includes basement outliers well to the south in Ethiopia and Kenya [1]. It represents terranes of arc-arc sutures formed when fragments of arc and backarc crust collided as the Mozambique ocean closed as the result of E and W Gondwana collision ~ 600 Ma ago [3]. Whereas identification and correlation of ophiolites and sutures in the northern part of the ANS is quite advanced, the same cannot be said for the southern part. This is mainly due to the complex interplay of processes that produced pervasive N-S structures and amphibolite facies metamorphism in the southern part of the ANS due to E and W Gondwanaland collision, thick Phanerozoic cover which obscures the transition between the two parts, and the lack of sufficient geochronological, geochemical and structural data for the southern part [3]. This has severely limited the many attempts made to link ophiolites to major arc-arc sutures separating terranes of presumably different ages in the north and in the south (e.g., [4,5]). The purpose of this paper is to revisit these attempts in the light of recently acquired data for the Precambrian of Southern Ethiopia. Deformational belts and sutures in the EAO: Deformational belts in the ANS are divided into (1) those associated with sutures, both arc-arc, and arc-continental, and (2) post-accretionary structures, which include N-trending compressional zones and NW-trending strike-slip faults. Terrane accretion in the ANS took place along arc-arc sutures developed between ~800 and 700 Ma [5, and references therein]. These sutures are orientated E to NE in the northern part of the ANS, and N to NE in the south, and are associated with N- or S-verging ophiolitic nappes in the north and east, or W-verging nappes in the south. The sutures in the south are deformed by upright folds and strike-slip faults related to oblique plate collision between terranes and/or post-accretionary deformation. The arc-continental sutures define the eastern and western boundaries of the ANS and are marked by north-trending deformational belts, formation of which took place upon collision of the ANS with E and W Gondwana at 750-650 Ma [5]. Sutures: The term suture refers to zones along which ocean and back-arc basins have closed. The closing of an oceanic basin is accompanied by a range of geological structures, usually localized along linear zones of high strain. Many attempts have been made to link ophiolites in the ANS to major arcarc sutures separating terranes of different ages. Abdel-Salam and Stern [5] linked the Tulu Dimtu (and Gore-Gambella) suture in western Ethiopia with the Barka suture in Sudan and Eritrea in the north, and with the Sekerr Zone of northwest Kenya in the south, to form one of the three major ANS sutures extending from the Arabian part of the shield to the Mozambique Belt in the south. Shackleton [4] linked up ophiolites of the ANS and proposed five to six Neoproterozoic sutures representing successive collisions of juvenile island arcs, Andean arcs, or continental terranes. Two of these sutures pass through southern Ethiopia. The Nabitah suture is distinguished by strong compressive deformation combined with sinistral strike-slip [8], by metamorphism up to amphibolite facies, and by voluminous granite-tonalite emplacement. Ophiolitic rocks from the Nabitah Zone have been dated at ~709-750 Ma [9] and collision in the suture is believed to have occurred at about 680-640 Ma [10] and 720-680 [5]. The Nabitah suture separates two isotopically distinct suture-bounded terranes – a Neoproterozoic juvenile arc to the west and a crust with Palaeoproterozoic or Archean signatures to the east [9]. Outcropping Palaeoproterozoic (>1800 Ma) crust has been mapped in the southern Arabian Afif Terrane [4]. Isotopic data from Yemen [11], for northern Somalia [12], and from the Hirna-Harar area in eastern Ethiopia [13] confirm that east of the Nabitah Zone and its southern projection across the Red Sea rift, the underlying crust is still Palaeoproterozoic or Archaean [4]. Shackleton [4] also envisaged the projection of the Nabitah Zone to the southern part of the East African Orogen through Ethiopia under the Cainozoic volcanics in central Ethiopia, to the west of the Precambrian of southern Ethiopia, and linked it with the N-S trending 620-570 Ma Barsaloi Shear Zone in 242
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north-central Kenya. The Urd-Al Amar Zone, in the westernmost part of the Arabian Shield, was linked with the Al Bayda arc in Yemen and with the Sokka Group in Harar, eastern Ethiopia, and then correlated, under a 200 km stretch of lava, with the Adola and Moyale ophiolites in southern Ethiopia and northern Kenya. Due to lack of sufficient geochronological and transport direction data, Shackleton [4] doubted the presence of a suture along the ophiolites in southern Ethiopia and suggested that these ophiolites represent obducted complexes. The Tulu-Dimtu-Gore Gambella and the Adola-Moyale sutures in Ethiopia have been interpreted as arc-arc sutures [5] or arc-cratonic (collision between the ANS and the Nile Craton) sutures by Bonavia and Chorowicz [14]. Revisit of suture configuration in the light of recent geological data from southern Ethiopia: The Precambrian of southern Ethiopia, which consists of high-grade quartzofeldspathic gneisses, lowgrade ophiolitc belts, and granitoid complexes, occupies an important position in the southern part of the ANS and northern part of the Mozambique Belt. Yibas [15] and Yibas et al. [6,7] presented extensive structural, geochronological and geochemical data for the Precambrian of southern Ethiopia and demonstrated the presence of at least three ophiolite belts with ages of >880, ~790 and 700 Ma. The closure of the Megado (Adola) and Moyale oceanic basins was related to two different collisional orogenies referred herein as the Megado and Moyale Tectonothermal Events, which occurred at about 760-720 Ma and 660-600 Ma. These two events are associated with two distinct structural phases (D1 and D2) and are separated by rifting (extension), as evidenced by granitic magmatism in an attenuated continentalarc crustal setting (720-700 Ma). Therefore, these two ophiolites represent two different sutures, which are distinct in space and time. The structures associated with the two earlier closures (suturing), however, could not be preserved due to subsequent deformation that culminated in oblique transpressive (strike-slip) deformations (D3-4) at about 600-550 Ma, as a result of the final collision between East and West Gondwana. Shackleton [4] suggested that the Nabitah and Urd-Al Amar zones represent a single collisional event, in spite of the fact that the deformational age of the Urd-Al Amar Zone is younger (680-640 Ma) than that of the Nabitah Zone (720-680 Ma). Whereas the imaginary suture line projected through central Ethiopia, underneath Cainozoic volcanics to link the Nabitah Zone in the northwest with the Baragoi Zone in the south is speculative, the Baragoi ophiolite is structurally and lithologically similar to the Megado ophiolite. Therefore, these two ophiolites can be linked taking the effect of the sinistral Akobo Shear Zone into account. The other alternative, namely to link the Baragoi Zone with the Tulu DimtuBarka suture, is possibly supported by the similar deformation ages for the Baragoi Zone [15]. The available structural, geochronological and geochemical evidence, and the consideration of the geology to the west of the Megado ophiolite-arc complex, suggest that the Megado basin was closed due to collision involving a micro-continental block from the west and the Kenticha-Bulbul arc/back-arc terrane to the east [7]. Therefore, it plausibly represents a continent-arc-arc suture. Does the final collision zone between east and west Gondwana pass through southern Ethiopia? The Moyale ophiolite represents one of the youngest (~700 Ma old) oceanic basins so far recognized in the Arabo-Nubian Shield. It was closed due to oblique continent arc-continent collision at about 660 Ma [7]. Its closure was followed by final terrane accretion, which emplaced the ANS between the continental blocks of east and west Gondwana between 650 and 550 Ma, along arc-continent sutures [6,15]. The Moyale ophiolitic fold and thrust belt is a transpressive deformational belt, associated with arctype plutonic rocks, dated at ~670 Ma [6,7]. Its western margin is the N-S trending Roukka Shear Zone, which was strongly reactivated by the transpressive regional D3-4 deformation, the age of which is reasonably constrained by the 580 to 550 Ma granites that were deformed by this transpressive deformation event [6]. The deformational style of the Roukka Shear Zone and its apparent northward continuation, the Wadera Shear Zone [15,6, 7] bears a strong resemblance to the structural styles of both the Nabitah and Urd-Al Amar zones in the northwestern part of the Arabian-Nubian Shield [5]. Moreover, the ophiolites of the Urd-Al Amar Nabitah zones have ages (694±8 Ma and 709-750 Ma, respectively [4,9]) similar to those of the Moyale ophiolite (700±10 Ma, [6,13]). Their estimated times of collision (Nabitah, 720-680 Ma, Urd-Al Amar, 680-640 Ma, [5]) are similar to the age of subduction for the Moyale basin at about 670 Ma (SHRIMP U-Pb zircon age for the Moyale granodiorite, [6,15]), although that of the Nabitah Zone is slightly older. One of the strongest arguments by Shackleton [4] to support that the Nabitah and the Urd-Al Amar deformational belts are major suture zones between East and West Gondwana is the contrasting age results for the terranes on both sides of these zones. Such contrasting age signatures were also obtained to the west and east of the D3 Wadera-Roukka Shear Zone: xenocrystic zircon ages ranging between ~2050 to 1362 Ma were obtained (SHRIMP, zircon [6]) from pan-African granitoids west of the Wadera Shear Zone. 243
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All the evidence discussed above strongly suggests that the Moyale-Roukka deformation belt forms a major part of a suture zone between East and West Gondwana. If the interpretation that the Moyale-Wadera Zone is correlative with the Urd-Al Amar Zone is correct (this interpretation is favored on account of the amphibolite grade of metamorphism and deformation ages and structural style), then it follows that this suture forms an important part of the final collisional suture between East and West Gondwana, which occurred between 670 and 550 Ma. This suture zone, herein referred to as the Urd-Al Amar-Moyale Suture, would separate the juvenile PanAfrican crust (partly reworked) to the west from the Palaeo- to Mesoproterozoic crust to the east. References: [1] Stern R.J. (1994) Ann. Rev Earth Planet. Sci., 22, 319-333. [2] Shackleton, R.M. (1986) Geol. Soc. London, Spec. Publ., 19, pp. 329-349. [3] Stern R.J. et al. (2004) Development in Precamb Geol., 13, 95-128. [4] Shackleton, R.M. (1997) J. Afr. Earth Sci., 23, 289-310. [5] Abdel-Salam M.G. and Stern, R.J. (1997) J. Afr. Earth Sci., 23, 289-310. [6] Yibas et al. (2002) J. Afr. Earth Sci., 34,57-84. [7] Yibas et al. (2003) Precambr. Res., 121, 157-183. [8] Quick J.E. et al. (1991) Precambr. Res., 53, 119-147. [9] Pallister J.S. et al. (1988) Precambr. Res., 38, 1-54. [10] Stoesser D.B. (1986) J. Afr. Earth Sci., 4, 21-46. [11] Windley B.F. et al. (1996) Geology, 24, 131-134. [12] Kröner A. and Sassi F.P. (1996) J. Afr. Earth Sci., 22, 1-15. [13] Teklay M. (1998) J. Afr. Earth Sci., 26, 207-227. [14] Bonavia F.F. and Chorowicz J. (1993) Precambr. Res., 62, 307-322. [15] Yibas B. (2000), PhD Thesis, Univ. of the Witwatersrand, Johannesburg, 448pp.
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