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Age of Fault Movements in Tanzanian Sector of East African Rift System

Naturevolume 247pages354356 (1974) | Download Citation



THE Neogene volcanic province of northern Tanzania is a southerly extension of the more extensive volcanic areas of Ethiopia and Kenya. It stands astride the Eastern Rift Valley and represents a complex interplay of volcanic activity and widespread Earth movements with associated faulting. A general picture is that an older series of basaltic-trachytic shield volcanoes, together with smaller nephelinitic centres, erupted and grew up on the floor of a major late-Tertiary fault-bounded depression; this earlier series includes the basaltic volcanoes of the Crater Highlands, Oldoinyo Sambu, Gelai and Kitumbeine, and the nephelinitic volcanoes Mosonik and Essimingor (Fig. 1). This volcanic area was later disrupted by a major episode of faulting that created the present-day Rift Valley depression. The main manifestation of this faulting is the major Rift Escarpment that runs from Lake Natron in the north to well south of Lake Manyara, this escarpment being the surface expression of the major Manyara–Natron Fault. The Tanzania sector of the Rift Valley is not a classic graben, as in Kenya to the north, and is bounded on its eastern side only by minor faults or down-warping. The major faulting was followed by another phase of volcanic activity, the volcanoes being dominantly nephelinitic, with carbonatite present at some centres; the major volcanoes are Hanang, Ufiome, Meni, Burko, Kerimasi and Oldoinyo Lengai together with many smaller volcanoes1. Previous geochronological studies on volcanic rocks from the Tanzania province have outlined the ages of various volcanic centres2, and also of individual flows in connection with palaeomagnetic determinations3 or anthropological studies4. The work described here has been directed at dating the faulting episodes, particularly the main phase, with a view to setting a maximum age for the onset of the later nephelinite-carbonatite magmatism, and also for the comparison with dated volcanic areas in Kenya to the north5,6. Such basic age information is also necessary for examination of the suggested temporal and spatial association of swelling, rifting and volcanicity (pluming?) with carbonatite magmatism7.

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  1. Department of Applied Geology, University of Strathclyde and Scottish Universities Research and Reactor Centre, East Kilbride

  2. School of Physics, University of Newcastle

    • J. G. MITCHELL
  3. Department of Geology, University of St Andrews

    • J. B. DAWSON


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