Precambrian Research 123 (2003) 81–85
Preface Evolution of the East African and related orogens, and the assembly of Gondwana
Neoproterozoic closure of the Mozambique Ocean the Arabian–Nubian Shield. The difference in lithol- collapsed an accretionary collage of arc and microcon- ogy and metamorphic grade between the two belts tinental terranes and sutured East and West Gondwana has been attributed to the difference in the level of along the length of the East African Orogen (Fig. 1). exposure, with the Mozambican rocks interpreted This special issue of Precambrian Research brings to- as lower crustal equivalents of the juvenile rocks in gether articles on aspects of the East African Orogen’s the Arabian–Nubian Shield. Recent geochronologic tectonic history to provide a better understanding of data indicate the presence of two major Pan-African this ancient mountain belt and its relationships to tectonic events in East Africa. The East African the evolution of crust, climate, and life at the end of Orogeny (800–650 Ma; Stern, 1994) represents a dis- Precambrian time. The formation of Gondwana at the tinct series of events within the Pan-African of central end of the Precambrian and the dawn of the Phanero- Gondwana, responsible for the assembly of greater zoic represents one of the most fundamental problems Gondwana. Collectively, paleomagnetic and age data being studied in Earth Sciences today. It links many indicate that another later event at 550 Ma (Kuunga different ﬁelds, and there are currently numerous and Orogeny) may represent the ﬁnal suturing of the rapid changes in our understanding of events related Australian and Antarctic segments of the Gondwana to the assembly of Gondwana. One of the most fun- continent (Meert and van der Voo, 1996). damental and most poorly understood aspects of the Three main topics are the focus of ongoing stud- formation of Gondwana is the timing and geome- ies by the contributors to this volume. The ﬁrst is to try of closure of the oceanic basins which separated better understand the timing of Gondwana’s amalga- the continental fragments that amassed to form the mation, a topic that is central to the debate-relating Late Neoproterozoic supercontinent. It appears that global-scale tectonics to biologic and climatic change. the ﬁnal collision between East and West Gondwana The second is to constrain the conﬁguration of cratons most likely followed the closure of the Mozambique within Rodinia and mechanisms by which Gondwana Ocean, forming the East African Orogen. The East formed. The third concerns the very nature of the East African Orogen encompasses the Arabian–Nubian African Orogen, the youngest collision zone between Shield in the north and the Mozambique Belt in East and West Gondwana: what are its continental and the south (Fig. 1). These and several other orogenic oceanic constituents? When did it form? What is the belts are commonly referred to as Pan-African belts, geometry of the major collision zones that bound its recognizing that many distinct belts in Africa and accreted terranes? All of these issues are central to a other continents experienced deformation, metamor- current, much-debated hypothesis of Neoproterozoic phism, and magmatic activity spanning the period of geology: Can the dramatic biologic, climatic, and 800–450 Ma. Pan-African tectonothermal activity in geologic events that mark Earth’s transition into the the Mozambique Belt was broadly contemporaneous Cambrian be linked to the distribution of continents with magmatism, metamorphism and deformation in to the breakup and reassembly of a supercontinent?
0301-9268/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-9268(03)00062-7 82 Preface / Precambrian Research 123 (2003) 81–85
Fig. 1. Map of Gondwana at the end of Neoproterozoic time showing the general arrangement of different tectonic elements discussed in this volume. Map modiﬁed after Unrug (1997), Powell et al. (1990, 1994), de Wit et al. (1988).
This is an important question, broad in scope and in Madagascar, where a controversy surrounds issues fundamental in nature. The range of papers in this related to the timing of collision, location of sutures, issue cover these topics across a geographic area that vergence of early-generation structures, dip of sub- extends over more than 7000 km, and a period of time duction zones, and afﬁnity (East or West Gondwana, that spans more than 500 million years. Many work- or neither) of sedimentary sequences. Fernandez and ers have integrated ﬁeld, structural, geochronological, others present a ﬁeld-based structural and geochrono- and satellite remote sensing studies with other data logic synthesis of the critical Itremo Group, suggesting to derive the unique view of the East African Orogen it is disposed in a westward-directed fold-and-thrust represented by this volume. belt that thrust relatively low-grade Itremo Group This special issue of Precambrian Research is rocks over higher-grade rocks to the west. Fernandez arranged geographically from south to north, which and his co-workers suggest that contacts between also imparts a general arrangement from the high- circa 800 Ma plutons are in the form of early thrusts, est metamorphic grade rocks, generally deformed and recognize that younger, circa 565 Ma plutons cut by the earliest tectonic structures, through to later, the tectonic fabrics, Collins and others synthesize lower-grade metamorphic rocks and younger struc- their ﬁeld-based studies of the same region. They tures in the northern part of the orogen. The ﬁrst suggest that the region experienced E–W contraction three papers focus on the part of the orogen exposed during a deformation event that occurred between 637 Preface / Precambrian Research 123 (2003) 81–85 83 and 560 Ma, followed by a non-coaxial deformation (40 km), suggesting that crustal thicknesses reached that occurred between 530 and 515 Ma in the form 70 km during the height of orogenesis. Rapid ex- of E-verging thrusting. Collins and his co-workers humation between 640 and 545 Ma is interpreted to suggest that the boundary between the Itremo Group represent orogenic collapse. and underlying Archean–Proterozoic gneisses (their Neoproterozoic rock assemblages of the Arabian– Antanarivo Block) is an extensional structure named Nubian Shield cover approximately 3 × 106 km2 in the Betsileo shear zone. They also suggest that the northeastern Africa, western Arabia and Sinai. The main suture between East and West Gondwana lies to Nubian Shield encompasses parts of Egypt, Sudan, the east of the Itremo Group along the Betsimisaraka Ethiopia, Eritrea, and Somalia, whilst the Arabian suture, across which the Dharwar Craton of India Shield stretches in Saudi Arabia, Yemen, and Oman. was separated from the Congo/Tanzania/Bangweulu The Saharan metacraton occupies a poorly exposed Craton of Africa, and the Antanarivo Block of Mada- region west of the Nubian Shield in western Su- gascar. Goncalves and others present a structural dan, Chad, Central African Republic, Cameroon, synthesis of the maﬁc–ultramaﬁc Andriamena unit Nigeria, Niger, and Libya. Abdelsalam and others in north-central Madagascar. They suggest that the describe the geology of this region, and relate it to Andriamena unit is a lower crustal fragment of a con- events in the East African Orogen. They recognize tinental margin magmatic arc generated by closure of three Neoproterozoic deformation events including the Mozambique Ocean, then thrust to the east over early (700–650 Ma) emplacement of S- to SE-verging gneissic basement after 630 Ma. Structures related to nappes consisting of ophiolites and passive margin continued E–W shortening, occurred after emplace- sediments. E–W shortening produced by collision ment of the Andriamena unit, are suggested to be of the Saharan metacraton with the Arabian–Nubian related to convergence of the Madagascar–India–Sri Shield at 650–590 Ma formed N- to NE-trending Lanka block with the Australia–Antarctica block into folds in the eastern part of the craton. North to the Cambrian. NNW-trending sinistral strike-slip shear zones devel- Hargrove and others synthesize the tectonic evolu- oped at 590–550 Ma, and are kinematically linked tion of the Zambezi orogenic belt in northern Zim- to N-striking fold belts. North striking extensional babwe. The E–W-trending Zambezi orogen records shear zones may be related to late (post-550 Ma) interactions of the Congo and Kalahari cratons, and extensional collapse of the orogen. connects at a high angle to the N-trending East The main units in the Arabian–Nubian Shield African Orogen. In the eastern part of the orogen, comprise Neoproterozoic metasedimentary rocks and S-verging, thick-skinned thrusting has inverted a migmatites, metavolcanic suites, serpentinites and circa 795 Ma Neoproterozoic supracrustal sequence dismembered ophiolite complexes, gabbro-diorite– that is now structurally overlain by circa 1830 Ma tonalite complexes, and unmetamorphosed vol- lower-crustal granulites consisting of metagabbro and canic and pyroclastic sequences that are extensively meta-anorthosite. Granitic orthogneisses in the thrust intruded, especially in the north, by batholithic stack have ages of 1050 and 870 Ma. U–Pb zircon and monzonite–granodiorite–granite complexes (Stern, titanite metamorphic ages indicate that the S-directed 1994). The recognition of ophiolites and their dis- thrusting occurred at 550–530 Ma, during the ﬁnal membered fragments, together with the identiﬁcation assembly of Gondwana. of chemically distinct island-arc volcanic and plutonic Beyth and others investigated aspects of late- complexes in the Arabian–Nubian Shield, has led to orogenic evolution of the East African Orogen in general agreement that this part of the continental northeast Ethiopia and Eritrea using ﬁeld work and crust developed through horizontal crustal accretion orbital remote sensing studies. They examine a group during closure of the Mozambique Ocean. The pres- of 800 Ma metasediments and metavolcanics struc- ence of highly dismembered ophiolite suites which turally overlying an 850 Ma high-grade assemblage deﬁne suture zones that can be traced southward from of metasediments and orthogneiss in eastern Eritrea. the Arabian–Nubian Shield into the Mozambique Belt They estimate that metamorphic conditions in the supports the idea that collision was the dominant oro- high-grade assemblage reached 700 ◦C at 12 kbar genic mechanism throughout the East African Orogen. 84 Preface / Precambrian Research 123 (2003) 81–85
The Arabian–Nubian Shield ophiolites demonstrate accretion of the metasediments (accretionary prism?) that oceanic crust formed and was partially preserved between colliding arcs. El-Shafei and Kusky consid- and that ophiolite and other nappes traveled tens to ered a younger N-striking generation of upright folds hundreds of kilometers, indicating that horizontal to be related to E–W shortening and NW-directed translation was an important aspect of crustal shorten- tectonic escape along NW-striking shear zones, per- ing. Stein presents an alternate but not incompatible haps reﬂecting a deep crustal level of the Najd Fault view on the evolution of the Arabian–Nubian Shield System. Interestingly, whereas El-Shafei and Kusky based primarily on geochemical studies of volcanic relate the late-orogenic structures to folding associ- and plutonic rocks. He suggests that the high rate of ated with the Najd Fault System, Brooijimans and crustal growth inferred for the Arabian–Nubian Shield others relate the similar dome in the Wadi Kid area may be attributable to the accretion of magmas orig- to core complex formation. inally derived from a mantle plume head, and swept Jarrar and others examine the abundant late- and into Mozambique Ocean subduction zones during the post-orogenic intrusives of Jordan, in the north- accretion of oceanic plateaus. This mechanism would ernmost segment of the East African Orogen. The transform plume-mantle into juvenile lithospheric Aqaba Suite (600–640 Ma) ranges from gabbro to mantle via the subduction process, reconciling plume high-silica granite, follows a high-K differentiation and subduction models for the growth of the shield. trend and represents the main crust-forming stage in Volesky and others integrate ﬁeld and orbital remote SW Jordan. Jarrar and his co-workers use geochemi- sensing studies for the Wadi Bidah mineral district cal modeling to show that the Aqaba Suite represents in southwestern Saudi Arabia. They demonstrate the melts generated in a subduction zone environment, utility of Landsat Thematic Mapper and Advanced with variable amounts of melting of spinel lherzo- Space-Borne Thermal Emission Radiometer remote lite (for the gabbro) and high degrees of melting of sensing data for regional structural syntheses. They subducted oceanic crust for the granitoids. The vol- show that massive sulﬁde mineral deposits in the umetrically smaller Araba Suite (560–600 Ma) cuts area are stratabound and related to a group of felsic a regional unconformity, and has characteristics of volcanics. within-plate magmatism. Jarrar and others further From the Sinai, Brooijmans and others present note the change in magma types in Jordan from con- geothermobarometric evidence to support an earlier vergent margin magmas (Aqaba Suite), to a bimodal model for a metamorphic core complex in the Wadi suite of rift-related magmas characterized by shallow Kid area. Uplift of the core occurred during an upper (feldspar controlled) fractionation. greenschist to upper amphibolite metamorphic condi- Collision between East and West Gondwana left tions. They postulate that this purported metamorphic an oceanic free-face in the north. Extension of the core complex formed during late-orogenic extension Arabian–Nubian Shield toward this free-face led to post-dating orogenic collapse. The doming is thought the formation of rifted basins in NE Africa and Arabia to have occurred along with widespread intrusion during latest Neoproterozoic time. The continental of post-orogenic granitic rocks. Similar evolution of collision ﬁrst led to crustal thickening and uplift, and core complexes have been described for metamor- continued with escape tectonics until the end of Pre- phic domes in the SE Desert of Egypt. El-Shafei cambrian. Burke and Sengor (1986) suggested that and Kusky present a detailed structural analysis of the NW-trending Najd Fault System was associated the Feiran–Solaf metamorphic belt in the southern with escape tectonics, due to the collision between Sinai Peninsula. High-grade predominantly gneiss East and West Gondwana along the East African and migmatites of sedimentary origin are folded by Orogen. However, the concentrations of granulitic early NW-striking intrafolial isoclinal folds and cut by rocks in the Mozambique Belt and their absence from early mylonite zones, then refolded by NW-striking the Arabian–Nubian Shield support the interpretation recumbent folds. The high grade metamorphic rocks that crustal thickening, erosion, and intensity of de- form the core of a dome with migmatites in the formation increased to the south. On the other hand, center, surrounded by amphibolite-facies rocks. The occurrences of granulite as far north as central Su- early generations of folds are interpreted as related to dan suggests at least two episodes of granulite-facies Preface / Precambrian Research 123 (2003) 81–85 85 metamorphism, Archean and Neoproterozoic (Stern Powell, C.Mc.A., Preiss, W.V., Gatehouse, C.G., Krapez, B., and Dawoud, 1991). Li, Z.X., 1994. South Australian record of a Rodinian Johnson presents a comprehensive review of post- epicontinental basin and its mid-Neoproterozoic breakup to form the Palaeo-Paciﬁc Ocean. Tectonophysics 237, 113–140. amalgamation basins in the Arabian–Nubian Shield, Stern, R.J., 1994. Arc assembly and continental collision in all formed after the main accretionary events in the the Neoproterozoic East African Orogen: implications for shield. The circa 670–650 Ma Murdama basins are up consolidation of Gondwanaland. Annu. Rev. Earth Planet. Sci. to 8 km thick and cover more than 70,000 km2, perhaps 22, 319–351. reﬂecting deposition in foreland and intermontane col- Stern, R., Dawoud, A.S., 1991. Late Precambrian (740 Ma) charnockite, enderbite, and granite from Jebal Moya, Sudan: lisional basins, as well as in incipient pull-apart basins a link between the Mozambique Belt and the Arabian–Nubian of the Bani Ghayy Group. The Murdama basins were Shield? J. Geol. 99, 648–659. folded at 650 Ma then unconformably overlain by Unrug, R., 1997. Rodinia to Gondwana: the geodynamic map of 640–620 Ma rocks of the Jurdhawiyah Group and Hib- Gondwana supercontinent assembly. GSA Today 7 (1), 1–6. shi Formation in fault-controlled basins. E–W shorten- de Wit, M.J., Jeffry, M. Bergh, Nicolayson, L., 1988. Geological map of sectors of Gondwana, reconstructed to their disposition ing formed additional N-trending folds in these basins. ∼150 Ma, American Association of Petroleum Geologists and This was followed by the deposition of the Jibalah the University of Witwatersrand, Tulsa. Group in pull-apart basins along the Najd Fault System between 580 and 570 Ma, with sedimentation includ- ing isolated lake deposits. The Jibalab basins record Timothy M. Kusky∗ the transition from the Neoproterozoic into the Cam- Department of Earth and Atmospheric Sciences brian, marked in most places in the Arabian Shield Saint Louis University, 3507 Laclede Ave. by an unconformity overlain by the Saq sandstone. St. Louis, MO 63103, USA ∗ Corresponding author References Mohamed Abdelsalam, Robert J. Stern Burke, K., Sengor, C., 1986. Tectonic escape in the evolution of Geosciences Department the continental crust. In: M. Barazangi, L. Brown (Ed.), Reﬂec- University of Texas at Dallas, Box 830688 tion Seismology: The Continental Crust. Am. Geophys. Union, Richardson, TX 75083-0688, USA Geodyn. Ser. 14, Am. Geophys. Union, Washington, DC. Meert, J.G., van der Voo, S., 1996. Paleomagnetic and 40Ar/39Ar study of the Sinyai Dolerite, Kenya: implications for Gondwana Robert D. Tucker assembly. J. Geol. 104, 131–142. Department of Earth and Planetary Sciences Powell, C.Mc.A., Li, Z.X., Thrupp, G.A., Schmidt, P.W., Washington University, Campus Box 1169 1990. Australian Palaeozoic palaeomagnetism and tectonics—1: One Brookings Drive Tectonostratigraphic terrane constraints from the Tasman fold St. Louis, MO 63130-4899, USA belt. J. Struct. Geol. 12, 553–565.