Possible Influences upon Lake Development in the East African Rift Valleys
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Abstract:
The rift valley lakes of East Africa exhibit considerable variety in size, hydrology, hydrochemistry, and sedimentary regime. Although tectonics and climate are ultimately responsible for lake formation, vol-canism may exert a significant influence upon the lake characteristics. In the Eastern Rift (Kenya, Ethiopia), extensive volcanic activity has disrupted drainage and helped create small, shallow lake basins. Weathering of volcanic rocks produces runoff waters rich in and , and smectite is a common clay mineral. In contrast, the Western Rift (Uganda, Mozambique) has localized volcanism; the lakes are large and deep with significant fluvial influents and effluents. Dissolved and are proportionately greater and detrital mineralogy is more variable. Carbonates and evaporites also reflect the hy-drochemical differences. In addition, the topographic elevation of the lake within the rift may determine the composition and rate of sedimentation.Keywords:
East African Rift
Rift valley
Rift zone
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(3)He/(4)He compositions are presented for groundwater samples from the Ethiopian segment of the East-Afrikan Rift and from its northern extension, the adjacent Afar region (Djibuti). Helium isotope data are compared to those obtained previously from the Gregory Rift, south of Ethiopia. The distribution pattern of mantle-derived volatiles along the entire East-African-Rift (-from south Kenya to Djibuti-) is discussed and their sources are identified. Helium isotope ratios (R) for samples from the Ethiopian part of the Rift range from 6.3 to 16.0 times the atmospheric ratio (Ra=1.4 x 10(exp -6) and thus show together with a MOR component a considerable hotspot helium component. These mantle helium concentrations are comparable to those observed in groundwaters and volcanic rocks from the Afar plume region in Djibuti. Here R/Ra values range from 9 to 13 times the atmospheric composition, with mantle-derived helium concentrations being higher than at spreading ocean ridges. R/Ra values from Ethiopia and Djibuti are entirely different from those observed in groundwaters at the southerly extending Gregory Rift in Kenya, where R/Ra values scatter between 0.5 and 6. At the northernmost part of the Gregory Rift, close to Ethiopia mantle helium contents are slightly higher, with R/Ra-values varying between 6.5 and 8.0.
East African Rift
Rift valley
Isotopes of helium
Hotspot (geology)
Mantle plume
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Crustal stress pattern provide important information for the understanding of regional tectonics and for the modelling of seismic hazard. Especially for small rifts (e.g. Upper Rhine Graben) and beside larger rift structures (e.g. Baikal Rift, East African Rift System) only limited information on the stress orientations is available. We refine existing stress models by using new focal mechanisms combined with existing solutions to perform a formal stress inversion. We review the first-order stress pattern given by previous models for the Upper Rhine Graben, the Baikal Rift, and the East African Rift System. Due to the new focal mechanisms we resolve second-order features in areas of high data density. The resulting stress orientations show dominant extensional stress regimes along the Baikal and East African Rift but strike-slip regimes in the Upper Rhine Graben and the interior of the Amurian plate.
East African Rift
Half-graben
Rift zone
Stress field
Extensional tectonics
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The magmatic-tectonic evolution of the EARS has been accompanied, since the initial stages of crustal deformation, by the formation of volcanic structures that formed outside of the main rift depression and are described with the general term of flank or off-rift volcanism. In this contribution, six Quaternary off-rift volcanic fields, from southern Ethiopia to southern Kenya are studied in terms of their map view shape, vent distribution and volcanic structures and constructions. They are, from the North one: Mega, Hurry Hills, Marsabit, Nyambeni Hills, Mount Kenya and Chyulu Hills. These six Quaternary off-rift volcanic fields formed far (> 80 km) from the rift's main border faults and all have an elongated shape having their axis at a high angle to the nearly N-S southern Ethiopian and the nearly N-S trending Kenya rifts and to the transversal, roughly NW-SE-trending, pre- existing Mesozoic Anza Graben. The shape, the spatial distribution of vents as well as the volcanic structures of the fields indicate that each volcanic field records the interaction between the magma dynamics, the crustal structural architecture and the local stress field. The volcanic fields are located at the border of a very cold and stiff lithosphere as inferred by geophysical data. In summary, the location and the evolution of the analysed off-rift volcanic fields are controlled by the inner structures of the EARS whereas their vent spatial distribution and shape are controlled by the local shallow structure of the crust and local stress field.
East African Rift
Rift valley
Lava field
Rift zone
Volcanic plateau
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East African Rift
Rift valley
Rift zone
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Tectonic stress field in rift systems a comparison of Rhinegraben, Baikal Rift and East African Rift
Crustal stress pattern provide important information for the understanding of regional tectonics and for the modelling of seismic hazard. Especially for small rifts (e.g. Upper Rhine Graben) and beside larger rift structures (e.g. Baikal Rift, East African Rift System) only limited information on the stress orientations is available. We refine existing stress models by using new focal mechanisms combined with existing solutions to perform a formal stress inversion. We review the first-order stress pattern given by previous models for the Upper Rhine Graben, the Baikal Rift, and the East African Rift System. Due to the new focal mechanisms we resolve second-order features in areas of high data density. The resulting stress orientations show dominant extensional stress regimes along the Baikal and East African Rift but strike-slip regimes in the Upper Rhine Graben and the interior of the Amurian plate.
East African Rift
Half-graben
Rift zone
Stress field
Extensional tectonics
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