The East African rift system
Introduction
The concept of ‘East African rift fracture’ was established by Suess (1891), following the explorations and discoveries of Livingstone, Stanley, Fischer, Thomson, Teleki and Von Höhnels during the XIXth century. Gregory in 1896 named it the ‘Great Rift Valley of East Africa’, and in 1921 he described a system of graben basins including the Red Sea and Dead Sea systems, forming the Afro-Arabian rift system. The present paper is focused on the African part, trying to update major knowledge and concepts. Several synthetic papers have been written before, e.g., Willis, 1936, Quennel, 1960, McConnel, 1967, McConnel, 1972, Baker et al., 1972, Mohr et al., 1972, Mohr, 1982, Girdler, 1991, Morley, 1995, Burke, 1996, Frostick, 1997, Schlüter, 1997. This new overall presentation is from a structural geologist who, since 1979, has been carrying out fieldwork and remote sensing analysis along the entire 6000 km north–south stretch of the rift, and has been almost everywhere.
The history of geological researches dedicated to the East African rift system (EARS) comprises several steps. After the discovery expeditions, detailed local geological studies span from the beginning of the 1930s to the end of the 1960s. These essential works were focused on mapping (e.g., Smith, 1931, McCall, 1957, McConnel, 1959, Harkin, 1960). The second step began during the1960s, lasting in the 1970s, with more regional scale studies of some aspects of the geology, especially tectonics, volcanism and acquisition of geophysical data (e.g., Mohr, 1962, Harris, 1969, Wohlenberg, 1969).
Research was boosted in the 1970s by the introduction of the concept of global tectonics, considering that oceans and related accretion of oceanic lithosphere begin with an intra-continental rift stage (Baker, 1970, Girdler and Sowerbutts, 1970, Mohr, 1970, Searle, 1970, Vail, 1970, Baker et al., 1971, McKenzie et al., 1972, Fairhead and Girdler, 1972, Chorowicz and Mukonki, 1979). This was also the time of first acquisitions of space imagery, covering large scenes with good ground resolution, a powerful tool for structural analysis of large areas (Mohr, 1974, Chorowicz and Mukonki, 1979).
In the last step, since the 1980s, the EARS remains the archetypal continental rift and a widely analogue to the early stages of evolution of passive continental margins preceding oceanic opening. Numerous papers concern the various aspects of rift evolution (e.g., Mohr, 1983, Bosworth, 1985, Mougenot et al., 1986a, Mougenot et al., 1986b, Chorowicz et al., 1987, Ebinger et al., 1987, Daly et al., 1989, Strecker et al., 1990, Kampunzu and Lubala, 1991, KRISP Working Group, 1991, Rosendahl et al., 1992, Tiercelin et al., 1992a, Tiercelin et al., 1992b). The 1980’s also saw the first reflection seismic profiling of many of the rift basins (e.g., Ebinger et al., 1984, Le Fournier et al., 1985, Rosendahl et al., 1986, Ebinger et al., 1987, Morley, 1988, Morley, 1989, Morley, 1999, Morley et al., 1992a, Morley et al., 1992b). The number of papers is still increasing, making it difficult for non specialists to have a clear view of the rift evolution, and justifying this synthesis.
The particularity of this new overview is to consider the East African rift system as an intra-continental ridge system, comprising an axial rift, prelude of oceanic opening. Section 2 describes the structural organization of the rift, including overall morphology, cross-sections, the morphotectonics, the main tectonic features that can be found, and volcanism. Section 3 focuses on the kinematics, with discussion of the directions of movements, in the frame of the significance of transform fault, transfer and accommodation zones. Section 4 is an attempt to reconstitute the rift system evolution through time and space, considering various aspects such as rift propagation, relationships with ancient structures, mechanism of formation, tectonics and sedimentation, and relationships of the ‘cradle of Mankind’ with human evolution.
Section snippets
Overall morphology
The East African rift system shows up at the surface as a series of several thousand kilometers long aligned successions of adjacent individual tectonic basins (rift valleys), separated from each other by relative shoals and generally bordered by uplifted shoulders (Fig. 1). Each basin is controlled by faults and forms a subsiding graben or trough, near one hundred kilometers long, a few tens kilometers wide, empty or filled with sediments and/or volcanic rocks.
The rift valleys form two main
Example of the Malawi rift region
The cross-section of Lake Malawi (Nyasa) presents the main characteristics of most of the other graben basins (Fig. 3). This rift is at an early stage of development. The seismic reflection profile A (Specht and Rosendahl, 1989, Rosendahl et al., 1992) shows that thickness of the sediments is ∼3000 m. The graben structure is asymmetric. (1) Almost all the tilted blocks dip in the same westward direction. (2) Accordingly, all the extensional faults have the same 65–70° eastward dip (antithetic
Afar region
The Afar triangle (Fig. 4) is the triple junction between the African, Arabian and Somalian plates. It is floored by Mio-Pliocene “Red Bed” unit (Kazmin, 1976) dated from 24 to 5.4 Ma and Quaternary tholeiitic volcanic rocks (Barberi et al., 1972). Elevations are low, some under sea level (−176 m), the basement being made of high density oceanic and thin continental lithospheres. It is not yet clearly established which parts of the Afar depression are of oceanic type and which are thinned
Main tectonic features
The morphotectonics of the EARS are under control of divergent movements, inducing localized extensional strain in the continental lithosphere. The brittle crust has reacted by faulting and subsidence, forming elongate, narrow rifts, while the lithospheric mantle is subjected to sharply define ductile thinning, inducing ascension of asthenospheric mantle.
The most characteristic features in the rift system are then these narrow elongate zones of thinned lithosphere related to deep intrusions of
Distribution and types of volcanism
Cenozoic volcanism in the EARS is widespread in the north—especially eastern branch, but sparse in the south (Fig. 15). In other rift systems of similar length, such as the West European rift, volcanism is much scarcer. Abundant volcanism in Northeast Africa is related to plume occurrence (Schilling, 1973, Schilling et al., 1992, Keller et al., 1994). The scenario of north to south migration of plume activity (Bonavia et al., 1995) is consistent with the thin lithosphere shown by a large N–S
Directions of movements
The EARS is a lithospheric opening in the African continent, which in terms of plate tectonics results from the divergence of large, regional-scale blocks. Movements can be deduced from focal mechanisms of earthquakes, borehole breakout in exploratory wells, structural analysis, strike of dykes, elongate volcanoes, volcanic chains, tension fractures and neo-formed normal faults (e.g., Shudofsky, 1985, Morley, 1988, Bosworth et al., 1992, Chorowicz et al., 1994, Haug and Strecker, 1995, Korme et
Transform fault zones
NW-striking large faults linking rift basins are parallel to the main extensional movement as interpreted in the preceding chapter, and consequently can be regarded as intracontinental transform fault zones belonging to the rift system (Fig. 16). First presented by Chorowicz and Mukonki, 1979, Kazmin, 1980, this model was also supported by Tiercelin et al., 1988, Chorowicz, 1989, Daly et al. (1989), Kilembe and Rosendahl, 1992, Wheeler and Karson, 1994. The model predicted that these
Rift propagation
First rift manifestation is the formation of open fractures at ∼30 Ma in the Afar and Ethiopian plateau, due to hot spot activity, responsible for emission of trap lava flows (Hoffman et al., 1997, Mège and Korme, 2004), accompanied by initiation of three converging graben structures, forming a triple junction at lake Tana (Chorowicz et al., 1998). Rifting in the Gulf of Aden began at 29.9–28.7 Ma, and in the southernmost Red Sea at 27.5–23 Ma (Hughes et al., 1991). The Afar depression seems to
Conclusions
The East African rift system can be regarded as a unique succession of graben basins linked by intracontinental transforms and segmented by transfer zones and accommodation zones. This is a lithospheric scale structure, characterized by uplift of hot asthenospheric elongate diapirs across the upper mantle, responsible for shoulder uplifts forming an intracontinental ridge system, several hundreds of kilometers wide, several thousands of kilometers long, quite equivalent to an oceanic ridge
Acknowledgements
This work started in Eastern Africa at end of the 1970s with support from Centre National de la Recherche Scientifique and Centre National d’Etudes Spatiales, because for a scientist specialized in the Alps, it was necessary to have a better knowledge of the early basin and transform fault history of belts. I have beneficiated largely from scientific advices of Jean Aubouin. The analysis of space imagery, which I was one of the first to use, was powerful to enter such a new and large study
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