Geology topics

Research about Kenya

Source-linked reports with geographic coverage including Kenya.

25 records · Page 2Linked to original sources

USGS1994

The East African rift system in the light of KRISP 90

On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100–150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study.

Tectonophysics↗

USGS1991

Large-scale variation in lithospheric structure along and across the Kenya rift

The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

Nature↗

USGS1989

Impact of fenthion on nontarget birds during quelea control in Kenya

Two nesting colonies of Quelea quelea in Kenya, 10 and 40 ha in size, were sprayed with 40 and 100 L of Queletox (60% fenthion ), respectively. Predatory and insectivorous birds were exposed to short-lived, but severe and potentially harmful, residues. In both colonies, quelea died during a 4-7 day period. In the larger colony, dead and dying birds were found over an area of ≤17 days after spraying, only 2 were found moribund, but based on cholinesterase depression, 16 of 23 raptors examined after spraying were thought to have been exposed. -from Authors

Wildlife Society Bulletin↗

USGS1987

Geochemistry of high-silica peralkaline rhyolites, Naivasha, Kenya rift valley

The Recent (<15000 y) volcanic complex of southwest Naivasha, Kenya, consists of mildly peralkaline (comenditic) rhyolite domes, lava flows, air fall pumices, and lake sediments, with minor, peripheral, basalts and hawaiites. The comendites are either aphyric or sparsely porphyritic, few samples containing >5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon. Ferrohedenbergite and zircon are restricted to less peralkaline, and amphibole, aenigmatite, and biotite to more peralkaline, rocks.The comendites show unusually strong enrichment in Cs, F, Hf, Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletion in Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched, with large, negative Eu anomalies. Values of LIL/HFS element ratios, such as Th/Ta and Rb/Zr, are unusually high for peralkaline rhyolites, and are consistent with a substantial crustal component in the comendites. Parameters such as LREE/HREE and Zr/Nb ratios indicate that the Naivasha rhyolites represent several pulses of closely related, but subtly different, magmas. Sanidine/glass partition coefficients for Ba, Pb, Rb, Sr, U, and the REE are presented for one specimen.Major and trace element modelling, and feldspar-rock relationships, show that closed system crystal fractionation cannot alone account for the overall compositional variations in the comendites. A model involving partial melting of variable crustal source rocks and migration of dissolved volatile-metal complexes may be appropriate at Naivasha. © 1987 Oxford University Press.

Journal of Petrology↗

USGS1976

Fission-track dating of pumice from the KBS Tuff, East Rudolf, Kenya

Fission-track dating of zircon separated from two pumice samples from the KBS Tuff in the Koobi Fora Formation, in Area 131, East Rudolf, Kenya, gives an age of 2.44±0.08 Myr for the eruption of the pumice. This result is compatible with the previously published K–Ar and 40 Ar/ 39 Ar age spectrum estimate of 2.61±0.26 Myr for the KBS Tuff in Area 105, but differs from the more recently published K–Ar date of 1.82±0.04 Myr for the KBS Tuff in Area 131. This study does not support the suggestion that pumice cobbles of different ages occur in the KBS Tuff

Nature↗

USGS1974

East African seminar/workshop trip report

Reports activities of U.S.G.S project leader and officer of AID sponsor, in Kenya, on trip made for preparation of plans for conduct of a East Africa Seminar/Workshop on applications of remote sensing and especially ERTS-1 data to resources analysis and management. Activities included orientation of officials of Kenyan agencies responsible for natural resources development and management, foreign affairs, finance and planning, of the University of Nairobi, of US AID and USGS resident in Kenya, the planning for use of facilities and other support by the Government of Kenya, the development of a schedule for the Seminar/Workshop and an agenda.

Open-File Report↗

USGS1968

Gels composed of sodium-aluminum silicate, Lake Magadi, Kenya

Sodium-aluminum silicate gels are found in surficial deposits as thick as 5 centimeters in the Magadi area of Kenya. Chemical data indicate they are formed by the interaction of hot alkaline springwaters (67° to 82°C; p H, about 9) with alkali trachyte flows and their detritus, rather than by direct precipitation. In the process, Na 2 O is added from and silica is released to the saline waters of the springs. Algal mats protect the gels from erosion and act as thermal insulators. The gels are probably yearly accumulates that are washed into the lakes during floods. Crystallization of these gels in the laboratory yields analcite; this fact suggests that some analcite beds in lacustrine deposits may have formed from gels. Textural evidence indicates that cherts of rocks of the Pleistocene chert series in the Magadi area may have formed from soft sodium silicate gels. Similar gels may have acted as substrates for the accumulation and preservation of prebiological organic matter during the Precambrian.

Lake Magadi↗