J. P. Patel

The influence of pre-existing structures on the evolution of the southern Kenya Rift Valley — evidence from seismic and gravity studies

The Kenya Rift is an active continental rift that has developed since the Late Oligocene. Although a thermal origin for the rifting episode is indicated by the scale of volcanism and its relative timing with uplift and faulting, the influence of pre-existing lithospheric structural controls is poorly understood. The interpretation of a 430-km-long seismic refraction and gravity line across the southern part of the Kenya Rift shows that the rift is developed across a transition zone, thought to represent the sheared Proterozoic boundary between the Archaean Nyanza Craton and the mobile Mozambique Belt. This zone of weakness has been exploited by the recent thermal rifting event. The Moho is at a depth of 33 km beneath the Archaean craton in the western part of the profile, and 40 km beneath the Mozambique Belt in the east. A few kilometres of localised crustal thinning has developed across the transition from thin to thick crust. At the surface, brittle faulting has formed an asymmetric rift basin 3.6 km deep, filled with low-velocity volcanic rocks. Basement velocities show a transition across the same area from low velocities (6.0 km s−1) in the Archaean, to high velocities (6.35 km s−1) in the Proterozoic. Mid-crustal layers show no deformation that can be attributed to the rifting event. Poorly constrained upper mantle velocities of 7.8 km s−1 beneath the southern rift confirm the continuation of the axial low-velocity zone imaged in previous seismic experiments. This is interpreted as the effect of small degrees of partial melt caused by elevated mantle temperatures. Gravity modelling suggests a contribution to the Bouguer anomaly from below the Moho, invoking the need for deep density contrasts. The regional gravity gradient necessary to model the Bouguer anomaly is used as supporting evidence for mantle-plume type circulation beneath the uplifted East African Plateau to the west of the Kenya Rift.

Preliminary Report on Radon Concentration in Drinking Water and Indoor Air in Kenya

A screening survey has been carried out to determine activity concentrations of radon (222Rn) in drinking water and indoor air in various locations in Kenya. The concentration of 222Rn in water was measured using a liquid scintillation counter (LSC). Three different passive integrating devices were used in the measurements of 222Rn in air. In the short-term measurements, radon is absorbed in activated charcoal and the analyses were carried out using either LSC or gamma ray spectrometry. The long-term measurements were carried out using solid-state nuclear track detectors (SSNTD). The mean and maximum values of 222Rn concentrations in water are 37 and 410 Bq L−1 and 100 and 1160 Bq m−3, respectively, in air. The highest values were obtained from groundwater sources and in the basements of buildings. When these values are compared with the internationally recommended reference levels, there are indications of existence of radon problems in some of the water sources and the dwellings tested in this survey.

Paleomagnetic results from the Narosura and Magadi volcanics of Kenya

Abstract Results of a paleomagnetic study carried out on the exposed volcanic rocks on the western side of the South Kenya Rift Valley are presented. Nine stratigraphic groups ranging in age from Miocene to Pleistocene were sampled. The rocks consist of basalts, trachytes, nephelinites, melanephelinites, olivine melanephelinites and ignimbrites. Paleomagnetic poles obtained for different age ranges are as follows: Period I (0.64–0.72 Ma), 116°E, 85°N ( A 95 = 6°); Period II (1.6–6.9 Ma), 297°E, 84°N ( A 95 = 4°); Period III (12.0–15.0 Ma), 34°E, 80°N ( A 95 = 9°). The results for Period II show large secular variations which are in disagreement with the model predictions for near-equatorial sites.

The lithospheric structure of the Kenya Rift as revealed by wide-angle seismic measurements

Abstract The Kenya Rift International Seismic Project (KRISP) seismic refraction-wideangle reflection experiments carried out between 1985 and 1994 show abrupt changes in Moho depths and Pn phase velocities as the rift boundaries are crossed. Beneath the rift flanks, normal Pn phase velocities of 8.0–8.3 km s−1 are observed, except for the Chyulu Hills volcanic field, east of the rift, where it is 7.9–8.0 km s−1. Also to the east, some of the thickest crust (38–44km) encountered so far beneath Kenya has been observed over a distance of c. 300km. However, beneath the surface expression of the rift itself, the uppermost mantle velocity of the Pn phase is anomalously low at 7.5-7.8 km s−1 throughout its length. Beneath the rift itself, there are major differences in crustal thickness, extension and upper mantle velocity structure between the north and the south. Beneath the section from the centre of the Kenya Dome southwards, where the extension is estimated to be 5–10km, the crust is thinned by c. 10 km to a thickness of 35 km, and the narrow low-velocity zone in the mantle extends to a depth of at least 65 km. However, in the north beneath Turkana, where the extension is 35–40km, the crust is only c. 20km thick and two layers with velocities of 8.1 and 8.3 km s−1 are embedded in the low velocity mantle material at depths of 40–45 km and 60–65 km. This mantle velocity structure indicates that the depth to the onset of melting is at least 65 km beneath the northern part of the rift and is thus not shallower than the corresponding depth (45–50 km) in the south. These results, taken together with those from teleseismic studies, petrology and surface geology, have been used to deduce that anomalously hot mantle material appeared below the present site of the Kenya Rift c. 20–30 Ma ago. This led to widespread volcanism along the whole length of the rift and modification of the underlying crust by mafic igneous underplating and intrusion.

OPTIMIZATION OF X-RAY FLUORESCENCE ELEMENTAL ANALYSIS : AN EXAMPLE FROM KENYA

Abstract A systematic study and application of a radioisotope excited energy dispersive X-ray fluorescence analysis method to multi-elemental determination in Kenyan rock samples whose supporting matrices are of light elements is presented. The method consists of instrumentation and analytical parameter optimization, and an empirical background stripping model for reduced dark matrix scatter effects. The method was applied to trace, minor and major elemental determination in rock samples collected from various parts of Kenya in order to test it for routine mineral and geochemical prospecting in light element geological matrices. A correlative analysis of the samples revealed a common presence of elements: K, Ca, Ti, Cr, Mn, Fe, Cu, Zn, Ga, Au, Pb, Br, Rb, Sr, Nb, Zr, As and Y in all five types of rocks, down to 3.0 μg g −1 for Br, with major differences revealed in the rock tectonic setting, origin, age and related palaeomagnetic parameters, and mineral enrichment prospects.

Estimation of annual effective dose and radiation hazards due to natural radionuclides in Mount Homa, southwestern Kenya

The radiological hazard of naturally occurring radioactive material in Mount Homa in southwestern Kenya was investigated after 210 point measurements and 44 samples were analysed. In situ measured average outdoor absorbed dose rate in air using survey meters was found to vary from 154.8 to 2280.6 nGy h(-1). The mean (range) values of radioactive concentrations measured using an HpGe detection system for (40)K, (226)Ra and (232)Th were 915 ± 3 (64-3017), 195 ± 8 (17-1447) and 409 ± 4 (23-1369) Bq kg(-1), respectively. The calculated range of the annual effective dose for a person living in Homa Mountain area varied from 28.6 to 1681.2, with a mean of 470.4 µSv. All calculated average radiological indices, namely Radium equivalent, Representative level, Gamma activity, External and Internal hazard, were higher than the limits set by various national and international bodies. These results imply that Mount Homa region is a high background radiation area.

Radioisotope photon-excited energy dispersive x-ray fluorescence technique for the analysis of organic matrices

A semi-empirical method for improved trace multi-element determination in organic matrices based on 109Cd radioisotope energy-dispersive x-ray Nuorescence (EDXRF) spectrometry is presented. It involves optimizing the geometry and electronics, interpretation and transformation of the analytical performance indices to practical quanti‐cation, an approach to model the dark matrix, estimation of the background and scatter in order to enhance matrix correction and the systematic optimization of x-ray spectral data deconvolution using the AXIL‐ QXAS techniques. The objective was to establish an optimum procedure for routine multi-elemental determination of trace levels in organic matrices using radioisotope-excited EDXRF. Examples of analyses for some matrices of importance in toxicology, medicine and agriculture are presented, with results indicating that the radioisotope photon-excited EDXRF technique is very versatile for the analysis of light element matrices of the organic type. Background and scatter, and the dark matrix problem, can be modelled. 1998 John Wiley & Sons, Ltd.

Palaeomagnetic studies of the Kapiti phonolite of Kenya

Abstract Specimens of Kapiti phonolite from sixteen sites were treated in an alternating field up to 900 Oe peak. Four sites were discarded on the basis of Watsons Criterion for randomness. Variations in various magnetic parameters along a vertical section of the rock indicate that a hard secondary component of magnetization of high coercive force is present in the surface samples of the rock — a possible cause for the poor grouping of directions of the remaining sites. Weathering may have introduced this component in the surface samples. The Kapiti phonolite possesses both normal and reverse polarities. The Pole position for the Upper Miocene is calculated at 81°N, 118°E with a circle of confidence of 17°.

Radiological survey and assessment of associated activity concentration of the naturally occurring radioactive materials (NORM) in the Migori artisanal gold mining belt of southern Nyanza, Kenya.

A radiological survey and assessment was carried out at selected sites (Osiri, Mikei, Masara and Macalder) in the Migori gold mines of southern Nyanza, Kenya to determine the levels of exposure of the artisanal miners to the naturally occurring radioactive materials (NORM) and dust. The activity concentrations of (40)K and the decay products of (232)Th and (226)Ra were obtained using an innovative method in single channel NaI(Tl) gamma-ray spectrometry. The counts for both the sample and the reference material in a specific window for a particular radionuclide were compared to arrive at the activity concentration of the radionuclide in the sample. Measurement of dust loading at various crushing sites was carried out by trapping the dust particles on a 0.45 μm cellulose acetate filter paper (47 mm diameter) using a vacuum pump. The activity concentration levels range widely 80-413, 12-145 and 21-258 Bq/kg for (40)K, (232)Th and (226)Ra, respectively. The calculated absorbed dose in air range from 16 to 178 nGy/h (with a mean of 42 nGy/h). Dust loading was found to range from 1.3 to 3.7 mg/m(3). Although the activity concentration of the radionuclides and the calculated annual absorbed dose is below the worlds average, the dust level at the mines was relatively high. The results obtained show that the artisanal miners are exposed to various levels of radionuclides and dust and necessary precautions need to be taken.

A crustal seismic refraction line along the axis of the S.Kenya Rift

Abstract The first phase of the Kenya Rift International Seismic Project was carried out in 1985 (KRISP 85) and included both earthquake and explosion programmes. In the explosion programme, 13 shots ranging in size between 100 kg and 1140 kg were fired in lakes and boreholes and recorded by 50 3-component stations deployed at 3.5 km intervals along the southern half of the rift axis and at 1 km intervals across it near Susua. Phases identified on the normalized and true amplitude record sections for the axial line include P g , P m , P n , and intracrustal reflections. Time term and ray-tracing methods were used to interpret the data which suggest a new crustal model for this part of the rift. The velocity at the top of the basement is 6.05 km/s and its depth varies between 2 and 6 km. The intracrustal reflections are best explained by velocity increases to 6.5 km/s at about 15 km depth and a high velocity layer, about 4 km thick, at a depth of about 25 km. An anomalous upper mantle velocity of 7.5 km/s was observed at a depth of 34–35 km. The crustal structure appears to be rather different from that suggested by the 1968 low-resolution experiment in the northern part of the rift and emphasises the need for further seismic studies of the Kenya rift.