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