East African Rift Valley

If we look at the geographical distribution of early hominids in Africa, we recognize that many of the famous sites are aligned in a chain along eastern Africa from Ethiopia via Kenya and Tanzania, to Malawi in the southeast. Localities like Hadar, Middle Awash, Omo, Lake Turkana, Olduvai, Laetoli, and others have yielded a great number of hominid specimens during the last 50 years. These sites are positioned in a geological megastructure, the EARV (O Figure 11.1). The EARV extends about 6,400 km from northeast to southeast.

The suture of the EARV follows a fault zone in the Continental crust. Plate tectonics explains the process of break-apart and rifting. The Nubian and

O Figure 11.1

The geological megagraben of the EARV (gray area) with its famous hominid sites, extends over more than 6,400 km from the Gulf of Aden and the Red Sea in the north into the Indian Ocean in the southeast at the height of Madagascar. The main rift splits into eastern and western branches just north of Lake Victoria. A schematic transverse section (below) through the Rift Valley shows that the graben shoulders are composed of a stepped block-system separated by normal faults. The center is lowered due to crustal extension and divergence and uplift of the shoulders

Somalian plates diverge along the main rift axes and result in a great graben with an average width of 30-40 km. The flanks are uplifted, while subsidence forms the graben floor. Active east-west extension leads to an enlargement of the accumulation space. A sedimentary fault basin develops as soon as the subsidence is great enough that an initial drainage system grows along the main faults. Minor rivers and alluvial fans transport material into the basin from the rift shoulders. A complex erosion-sedimentation interaction system starts in the active rift zone.

The expression "rift," or "rift valley,'' can be traced back to the definition of Gregory (1896). Accordingly, a rift valley, like the Malawi Rift, is a parallel-sided down-faulted valley some tens of kilometers in width and at least a few hundred kilometers in length (Ring and Betzler 1995). Several smaller-scaled tectonic structures, like halfgrabens, horsts, warped blocks, major faults, transform faults, and pull-apart basins can occur within a rift system.

All these geological structures provide space for accumulation, and erosional processes to bury and unearth skeletal remains.

In the history of a mature rift system, diverse environmental settings can be recorded, representing a wide range of habitats. Rifting may end in a marine transgression phase and the birth of an ocean, when the continental breakup continues and rift floor subsidence extends toward the coast, as we can observe happening in the Red Sea and the Gulf of Aden. In other cases, the rifting dynamics may stop after some active phases.

The EARV is probably one of the most attractive rift systems, reflecting several stages of rift development in its longitudinal extension. It therefore provides an important field of research for understanding crust breakup in a continental setting. Over the last 110 years, extensive research has been focused on the EARV (Gregory 1896; Wegener 1912; Krenkel 1922), making the East African rift system one of the best documented continental rift systems on Earth (Ring and Betzler 1995). Two areas have been of major interest: the Ethiopian Afar Triangle (Baker et al. 1972; Mohr 1987) in the Northeast at the horn of Africa, because of its special rifting situation as a triple junction of three crustal plates; and the Kenyan or Gregory Rift (Baker et al. 1972, 1988; King 1978; Crossley 1979; Strecker et al. 1990), as the eastern branch of the graben system.

Extensive magmatism and volcanism accompanies sedimentation in the graben. The oldest rifting phases are recognized in the northeastern parts at the western boundary of the Sinai Peninsula marking the boundary of the Arabian plate. The Red Sea and the Gulf of Aden are probably the oldest segments of the rift system, starting with its initial uplifts and doming of the crust, probably in the Oligocene. Initial volcanism is evident as early as the Eocene (e.g., Trap series), producing widespread lava. In the Miocene, thick flood basalts produced by large shield volcanoes filled the proto-rift in the Afar region. A highly active rifting phase followed in the Miocene and produced accumulation space for extensive sedimentation of the erosional material from the rift shoulders and the rift volcanism. Sedimentation was always interrupted or accompanied by flood basalts and other volcanic eruptions, changing the landscape and sedimentation environments within the basins.

There is no doubt that due to the spreading process, the EARV witnessed some habitat changes in eastern Africa. Faulting, uplifts, and subsidence support the development of all common fluvial processes, with meandering and braided rivers, flood plains, alluvial fans, oxbow lakes, and natural levees. Major rift lakes, like Lake Turkana, developed permanent water sources during long-term transgression phases. Regression/transgression cycles are evident through changes in sedimentary formations.

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