Downloaded from http://jgs.lyellcollection.org/ at University of Rochester on March 23, 2015 Journal of the Geological Society, London, Vol. 154, 1997, pp. 689–700, 4 figs, 2 tables. Printed in Great Britain
Tectonic development of the northern Tanzanian sector of the East African Rift System A. FOSTER 1 , C. EBINGER 2 , E. MBEDE 3 & D. REX 2 Bullard Laboratories, University of Cambridge, Madingley Rise, Cambridge, CB3 0EZ, UK (e-mail
[email protected]) 2 Department of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK 3 Department of Geology, University of Dar es Salaam, PO Box 35052, Dar es Salaam, Tanzania 1
Abstract: The Eastern Branch of the East African Rift System diverges from a single, c. 50 km wide rift in southern Kenya to a c. 200 km wide zone in northern Tanzania, where it is comprised of three distinct rifts with different orientations. The western part of this zone contains two rift branches: the Natron– Manyara–Balangida and Eyasi–Wembere rifts. Each rift contains individual basins that are defined here on the basis of structural and geophysical interpretations. These basins are shallow (4.0 as reported by the International Seismological Centre (ISC). Lower hemisphere projection earthquake focal mechanisms and centroid depths are constrained by body wave inversion (900515/15:21 and 900515/16:24 from Foster 1997; 640507 from Nyblade & Langston 1995) or are taken from the Harvard Centroid Moment Tensor (CMT) catalogue (depths starred and fixed at 15 km). Each mechanism is accompanied by its date, origin time, mb and centroid depth in km (yymmdd/hh:mm/mb/depth). Inset shows the regional, long-wavelength topography produced by applying a 100 km full-width median filter to the ETOPO5 dataset, and gridding the output at 5 minute intervals. Darker shading indicates topography above 1000 m. The Eastern Branch lies on the eastern edge of the uplifted East African plateau. Area of main figure is shown by box.
Downloaded from http://jgs.lyellcollection.org/ at University of Rochester on March 23, 2015 TECTONIC DEVELOPMENT: EAST AFRICAN RIFT
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System are c. 25–35 km (Bechtel et al. 1987; Ebinger et al. 1989, 1997). In other areas of continental extension (e.g. North Sea, Greece, western USA, Tibet), Te is usually 10 to 15 km or less (Burov & Diament 1995; Cloetingh & Burov 1996; and references therein). The East African lithosphere thus maintains significant flexural strength during rifting. Since Te is strongly dependent on the thermal structure of the lithosphere (e.g. Karner et al. 1983; Burov & Diament 1995), the greater values in East Africa suggest that the lithosphere is colder and stronger than that found in other regions of continental extension. In parts of East Africa earthquake centroid depths indicate a seismogenic thickness of c. 35 km; approximately double that found in other actively extending continental regions (Wagner & Langston 1988; Jackson & White 1989; Nyblade & Langston 1995; Foster 1997). Compilations of the epicentres of recent earthquakes in northern Tanzania and southern Kenya (Fig. 1) show that most seismicity is associated with the Tanzanian sector of the Eastern Branch. Focal mechanisms indicate normal faulting on planes approximately parallel to the riftbounding faults (Fig. 1). The current seismicity divides northern Tanzania into two seismogenic parts; eastern and western areas separated by an aseismic block (the Masai Plateau) (Fig. 1). Seismic refraction data in southern Kenya indicate a crustal thickness of 30 to 40 km beneath the rift and its flanks (Maguire et al. 1994). No measurements of crustal thickness are available for northern Tanzania, but receiver function studies elsewhere in Tanzania indicate that Archaean and Proterozoic crustal thicknesses are 35–38 km and 31–38 km respectively (Last et al. 1995). Seismic reflection and refraction data from the Lake Magadi area of southern Kenya (Fig. 1) indicate that the crust has been thinned from 40 km beneath the eastern flank, and 34 km beneath the western (cratonic) flank, to c. 30 km beneath the Magadi Basin (Birt et al. 1995). Estimates of crustal stretching across parts of the Eastern Branch in Kenya, based on reconstructions of fault geometries and on crustal thicknesses, give an extension factor, â, of c. 1.1 (Strecker et al. 1990; Green et al. 1991). The Miocene–Recent extension across the Eastern Branch is E–W to NW–SE (Strecker et al. 1990), consistent with earthquake focal mechanisms (Fig. 1) and the predicted Africa–Somalia plate motion (Jestin et al. 1994). In Kenya the rift lies across the N- to NW-trending Proterozoic (650–475 Ma) Mozambique orogenic belt (Shackleton 1986; Smith & Mosley 1993). Structural, geophysical and heat flow data indicate that during a late Proterozoic collisional event the eastern margin of the Archaean Tanzanian craton was reworked, overthrust from the east and effectively buried by these tectonically emplaced Mozambique Belt rocks. The contact between these two terranes is not well exposed. The buried margin of the craton may extend up to 100 km eastwards from the mapped outcrop (Smith & Mosley 1993; Birt et al. 1995). In the Eyasi–Wembere Rift the sheared contact between the Proterozoic Mozambique Belt and the Archaean Tanzanian craton is exposed (Fig. 2).
cline) on its eastern side. Each rift is divided into a number of basins. The rifts are bounded by steeply dipping, planar, normal fault systems which produce the present-day rift escarpments. Between the rifts, the uplifted Mbulu Plateau contains a number of small, NE-trending basins, bounded by normal faults (including the Yaida Depression) (Fig. 1). These basins, not studied here, are regularly spaced across the plateau at c. 30 km intervals. They are not bounded by major escarpments and do not contain significant thicknesses of syn-rift sediments or volcanics. Individual basins are elongate topographic depressions bordered on one or both sides by a major border fault system which has greater than 500 m throw and uplifted flanks. Such border fault systems define the topographic depression and flank uplift within a basin, and may be segmented into a number of individual faults (e.g. Crone & Haller 1991).
Basin structure
Wembere Basin
Figure 2 shows the regional morphology of the Natron– Manyara–Balangida and the Eyasi–Wembere rifts. Each rift is bounded on its western side by a major border fault system and by a largely unfaulted basement flexural warp (or mono-
The c. 120 km by c. 40 km Wembere Basin is structurally ill-defined, and trends N–S, approximately orthogonal to the Eyasi Basin (Fig. 2). The basin is bounded to the east by the block-faulted Iramba Plateau, and to the west by a broad
Eyasi Basin The c. 100 km by c. 30 km Eyasi Basin trends SW from the volcanic constructs of the Neogene Crater Highlands to the Iramba Plateau, encompassing both Lake Eyasi and Lake Kitingiri (Fig. 2). Gravity data show a NW-trending positive feature which crosses the basin where the surface expression of the Eyasi Fault ends (Ebinger et al. 1997). This gravity anomaly (and associated magnetic anomaly) may result from a basement high beneath the basin, and separates the Eyasi Basin into two distinct sub-basins: the East Eyasi Basin (bounded by the Eyasi Fault) and the West Eyasi Basin (bounded to the south east by the Iramba Plateau) (Fig. 2). The Eyasi Fault bounds the NW side of the East Eyasi Basin, and appears to be a single fault segment of c. 100 km length. Faults bounding the opposite margin of the basin are short (
