Kenya Rift International Seismic Project Research Articles

Chapter 4 Geophysical evidence for the structure of the crust and upper mantle of the Tanzania Craton and the Gregory Rift Valley

The past three decades have seen an upsurge in geophysical fieldwork across the East African Rift Valley in Kenya (KRISP: the Kenya Rift International Seismic Project; Prodehl et al . 1994; Fuchs et al . 1997) and in Ethiopia (Project EAGLE: the Ethiopia Afar Geophysical Lithosphere Experiment; Yirgu et al . 2006). In northern Tanzania, in 1994, A. A. Nyblade and co-workers established an array of broadband receivers in northern Tanzania, the Tanzania Broadband Seismic Experiment, TBSE (Nyblade et al . 1996) that, together with interpretation of earlier commercially acquired data, have provided new insights into the present-day structure of the Tanzania Craton and the Mozambique belt. Although in many experiments data have also been acquired for the Western Rift, attention will be confined in the following section to observations on the northern Tanzania and southern Kenya sectors of the Gregory Rift Valley. The interpretation of gravity data alone does not give a unique solution because Bouguer anomalies can reflect lateral changes in: (i) the density of the crust; (ii) the density of the upper mantle; (iii) the depth to the crust/mantle interface, the Moho; or (iv) a combination of these possible variables. Gravity measurements can, when combined with data from seismic experiments and measured rock densities, lead to integrated models for the structure of the crust and upper mantle, but it is relevant to discuss here the results obtained by individual disciplines. Northern Tanzania lies on the southern margin of a broad, regional, negative Bouguer anomaly around 350±50 km wide and with an amplitude of 800 g.u. relative to a background value of −1200 g.u. (Fig. 4.1). The anomaly, which is highest in Kenya and decreases southwestwards, lies slightly to the west of the present-day rift valley, strikes NNE and is superimposed on a wider 1000 km negative …

An integrated geophysical study of the northern Kenya rift

The Kenyan part of the East African rift is among the most studied rift zones in the world. It is characterized by: (1) a classic rift valley, (2) sheer escarpments along the faulted borders of the rift valley, (3) voluminous volcanics that flowed from faults and fissures along the rift, and (4) axial and flank volcanoes where magma flow was most intense. In northern Kenya, the rift faults formed in an area where the lithosphere was weakened and stretched by Cretaceous-Paleogene extension, and in central and southern Kenya, it formed along old zones of weakness at the contact between the Archean Tanzania craton and the Proterozoic Mozambique orogenic belt. Recent geophysical investigations focused on the tectonic evolution of the East African rift and on exploration for geothermal energy in the southern portion of the Kenyan rift provide considerable information and insight on the structure and evolution of the lithosphere. In the north, a variety of other data exist. However, the lack of an integrated regional analysis of these data was the motivation for this study. Our study began with the collection and compilation of gravity data, and then we used the seismic refraction results from the Kenya Rift International Seismic Project (KRISP), published seismic reflection data, aeromagnetic data, and geologic and drilling data as constraints in the construction of integrated gravity models. These models and gravity anomaly maps provide insight on spatial variations in crustal thickness and upper mantle structure. In addition, they show the distribution of basins and help characterize the distribution of magmatism along the axis of the northern sector of the rift. Our main observations are the following: (1) the region of thinning and anomalous mantle widens northward in agreement with previous studies showing that the crust thins from about 35 km in the south to 20 km in the north; (2) as observed in the south, gravity highs observed along the axis are due to mafic bodies associated with the main volcanic centers, densification of the upper crust due to diking, and horst blocks where Precambrian basement has been brought to or near the surface; (3) the axis of the rift is marked by a series of high amplitude magnetic anomalies whose wavelengths are less than 2.5 km, the positive anomalies coinciding closely with known Quaternary volcanoes.

Geophysical project in Ethiopia studies continental breakup

As continental rift zones evolve to sea floor spreading, they do so through progressive episodes of lithospheric stretching, heating, and magmatism, yet the actual process of continental breakup is poorly understood. The East African Rift system in northeastern Ethiopia is central to our understanding of this process, as it lies at the transition between continental and oceanic rifting [Ebinger and Casey, 2001].We are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts, a region providing an ideal laboratory to examine the process of breakup as it is occurring. EAGLE is a multidisciplinary study centered around the most advanced seismic project yet undertaken in Africa (Figure l). Our study follows the Kenya Rift International Seismic Project [e.g., KRISP Working Group, 1995],and capitalizes on the IRIS/PASSCAL broadband seismic array [Nyblade and Langston, 2002], providing a telescoping view of the East African Rift within this suspected plume province.

An integrated geophysical analysis of the upper crust of the southern Kenya rift

SUMMARY Previous interpretations of seismic data collected by the Kenya Rift International Seismic Project (KRISP) experiments indicate the presence of crustal thickening within the rift valley area beneath the Kenya dome, an uplift centred on the southern part of the Kenya rift. North of the dome, these interpretations show thinning of the crust and an increase in crustal extension. To the south near the Kenya/Tanzania border, crustal thinning associated with the rift is modest. Our study was aimed at further investigating crustal structure from this dome southwards via a detailed analysis focused on upper crustal structure. We used results from surface geological mapping, drill hole data from water wells and geothermal exploration wells, KRISP 85 seismic data for a profile across the rift, KRISP 85 and 90 seismic data for a profile along the rift axis and KRISP 94 seismic data for a profile crossing southernmost Kenya to constrain gravity modelling and construction of integrated models of crustal structure. Our integrated analysis produced the following results concerning the structure and evolution of the southern Kenya rift: (1) the graben master faults are consistently located along the western margin of the rift valley, and there is no evidencefor half-graben polarity reversals for a distance of about 300 km; (2) there is no axial (north‐south) crustal symmetry with respect to the apex of the Kenya dome, and the crustal thickness may be as much related to pre-rift crustal type and thickness as it is to crustal thickening and modification by magmatic processes; (3) the pre-existing lithospheric contrast between the Archaean and Proterozoic basement terranes played a significant role in the location and structural geometry of the rift; (4) south of latitude 1uS, low velocities and densities observed under the western flank of the rift probably represent reworked Archaean Tanzanian craton; (5) magmatic modification of the upper crust is modest except near the major Quaternary volcanic centres that produce a series of isolated axial gravity highs; (6) the other element of the axial gravity high is an intrarift horst block that extends along the axis of most of the rift valley.

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 P n phase velocities as the rift boundaries are crossed. Beneath the rift flanks, normal P n 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 P n 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.

Upper crustal structure in the vicinity of Lake Magadi in the Kenya Rift Valley region

In 1994, the Kenya Rift International Seismic Project (KRISP) conducted a large-scale seismic experiment which extended across southern Kenya from Lake Victoria to the Indian Ocean. One major goal of the KRISP 94 effort was to determine the upper crustal structure of the southern Kenya Rift Valley near Lake Magadi. Thus in this area, the seismic refraction experiment was designed to obtain better resolution of shallow structures by employing a shorter seismic station spacing (1.5 km). In this study, these seismic data were used simultaneously with surface geology, drill hole data and gravity measurements to construct a relatively detailed model of the upper crust in the Lake Magadi region. This integrated analysis provided several significant results. At Lake Magadi, the rift floor is covered by low density-low velocity (2400 kg m −3 and 4.2 km s −1) sediments and volcanics that form a complex, asymmetric graben whose deepest portion (about 3.5 km) is found adjacent to the Nguruman Escarpment master fault to the west. This basin is about 2.0 km deep along its eastern margin and is divided into two sub-basins by a horst that runs along its axis, creating a localised gravity high. This simple structure was sufficient to produce the observed geophysical anomalies, so no pervasive magmatic modification of the upper crust was required. Beneath the rift basin, a 4 km thick layer, with relatively low velocities and densities, was found to extend from the eastern rift valley margin across the western rift flank to the vicinity of the Oloololo Scarp. This basement layer, with an average velocity of 5.6 km s −1 and a density of 2600 kg m −3, is intetpreted to represent a combination of quartzites and rocks fractured by the Neoproterozoic (Pan-African) collision of the Mozambique Belt and Tanzania Craton. Although Proterozoic rocks of the Mozambique Belt outcrop on both sides of the rift valley, the velocities, densities and thicknesses of the layers constituting the crystalline upper crust on the western rift flank are less than those on the eastern flank and similar to values found within the Tanzania Craton elsewhere. This suggests that the two rift flanks are located in different structural regimes (craton and mobile belt) with rifting having been localised along the boundary between them. From a deep crustal perspective, this boundary appears to be the western margin of the Mozambique Belt terrane, although rocks of this belt are found at the surface further west.

Crustal structure of the southeastern flank of the Kenya rift deduced from wide-angle P-wave data

The 420-km-long northwest-southeast KRISP (Kenya Rift International Seismic Project) flank line F, part of the KRISP 94 experiment, extends from Athi River (30 km southeast of Nairobi) to the Indian Ocean near Mombasa, Kenya. Line F crosses the Chyulu Hills area, a young Quaternary volcanic field, surrounded by the basement of the Mozambique belt. The basement is at the surface almost along the entire profile with two exceptions, one in the Chyulu Hills area due to the presence of volcanic pyroclastics, the other next to the Indian Ocean where sediments reach a thickness of about 8 km. Below the basement the crust can be divided into three layers. The upper layer extends to about 10 km depth with P-wave velocities of roughly 6.25-6.4 km/s. The mid-crustal layer reaches a depth of about 20 km where P-wave velocities range from 6.55 km/s to 6.7 km/s. The lower crust in the area of the Chyulu Hills is unexpectedly thick (>20 km) and generates strong refracted phases with velocities of about 7.0 km/s. Crustal thickness is about 40 km but thickens in the area of the Chyulu Hills and thins towards the Indian Ocean to 22 km. P n-phases, refracted waves travelling through the uppermost mantle, are identified on four record sections and give good control over the upper mantle velocities which are slightly reduced from 8.1–8.2 km/s to 7.9 km/s underneath the Chyulu Hills area. The upper and lower crust in the region of the Chyulu Hills are significantly heterogeneous, producing strong signal-induced noise which masks secondary arrivals in the corresponding distance range. Underneath the Chyulu Hills P MP-reflections are hard to identify indicating that the crust-mantle boundary is a transition zone, rather than a first-order discontinuity.

Further remarks on the interpretation of the KRISP 90 cross-rift line

During the Kenya Rift International Seismic Project (KRISP 90) a 450-km east-west seismic profile was shot across the rift in the vicinity of the Equator. Reflectivity modelling of some of the P- and S-phases combined with results derived from the ray-trace forward model enable more information to be extracted from the data set. The Pg phase at the western end of the profile required a different velocity-depth gradient in the Archaean and the Proterozoic terranes crossed by the profile. This, combined with structural and seismic velocity differences derived from the forward model, provides the means for the two units to be distinguished throughout the whole crust. A phase in the PmP coda from shot points within the rift has been successfully modelled as a PmP multiple reflected from the base of or from within the rift infill. The clearest S-phase on the cross-rift profile, SmS, has been reproduced by reflectivity modelling and compared to the same phase from the KRISP 90 flank line: combining these observations with the results from the spectral analysis of the principal P- and S-phases observed from all the shot points on the cross-rift profile supports the contention that most of the seismic attenuation occurs immediately beneath the rift in the upper crust or in the rift infill, with little attenuation occurring in the lower crust.

Seismic structure of the upper mantle beneath the southern Kenya Rift from wide-angle data

In February 1994, the Kenya Rift International Seismic Project carried out two wide-angle reflection and refraction seismic profiles between Lake Victoria and Mombasa across southern Kenya. Our investigation of the data has revealed evidence for the presence of two upper mantle reflectors beneath southwestern Kenya, sometimes at short range, from seven shotpoints. Two-dimensional forward modelling of these reflectors using a pre-existing two-dimensional velocity-depth model for the crust [Birt, C.S., Maguire, P.H.K., Khan, M.A., Thybo, H., Keller, G.R., Patel, J., 1997. The influence of pre-existing structures on the evolution of the Southern Kenya Rift Valley — evidence from seismic and gravity studies. Tectonophysics 278, 211–242], has shown them to lie at depths of approximately 51 and 63 km. The upper reflector, denoted d 1, shallows by about 5–10 km in the area beneath Lake Magadi, situated in the rift itself. Correlations for the deeper reflector, denoted d 2, are sparse and more difficult to determine, so it was not possible to define any shallowing corresponding to the surface expression of the rift. Only limited control exists over the upper mantle velocities used in the modelling. Immediately beneath the Moho we use a value of P n calculated from the crustal model, and constraints from previous refraction, teleseismic and gravity studies, to determine the velocity at depth. At the d 1 reflector a reasonable velocity contrast was introduced to produce a reflector for modelling purposes. Beneath the d 1 reflector the velocity decreases to the average value over 3 km. Beneath the rift the velocity also rises across d 1 and again, decreases to the average value over the next 3 km. At the d 2 reflector a similar model is used. This model accounts for the presence of the mantle reflectors seen in the data by using layers of thin higher velocity in a lower background velocity. Due to the uncertainty in the velocities the absolute position of both d 1 and d 2 could vary, but the relative upwelling beneath the rift is reasonably well constrained and data from four different shotpoints which indicate the shallowing show good agreement. A significant result of this study is that the continuity of the d 1 reflector indicates that the sub-Moho lithosphere has not been substantially disrupted by mantle upwelling, even though probably thinned and stretched.

On the upper mantle beneath the Kenya Rift

SUMMARY During the Kenya Rift International Seismic Project (KRISP 90) a 450 km long E-W seismic-refractionlwide-angle-reflection profile involving the deployment of 250 instruments was shot across the Kenya Rift. A reflected phase recorded between distances of 260 and 350 km from a 1000 kg shot at the western end of the line in Lake Victoria has been interpreted as originating from about 60 km beneath the western margin of the rift. Detailed processing of this phase has resulted in defining its polarity in relation to the first-arrival diving wave at the same range. Extensive kinematic and dynamic modelling shows there is a high-velocity zone at depths below 60 km under the western flank of the rift. We cannot exclude the presence of a layered alternating high-lowvelocity structure as found in the upper mantle beneath the northern part of the N-S seismic profile along the rift axis. Constraints from xenolith studies indicate that anisotropy may explain the high velocity found beneath the reflecting horizon (2 8.40 km s-’). Petrological modelling shows that if the anisotropy is due to the preferred orientation of olivine crystals, then either a transverse isotropic structure, in which the ‘a’ and ‘c’ axes are randomly orientated in the horizontal plane, or an orthorhombic structure, in which the fast ‘a’ axis is orientated along the direction of the E-W seismic line, is possible. The reflection could also be caused by a pre-rift structure associated with the Proterozoic collisional orogen involving the Mozambique Orogenic Belt and the Archaean Nyanza Craton, whose contact is subparallel to and lies about 70 km to the west of the Tertiary rift. The evidence presented here delimits the lateral extent of the upper-mantle region of anomalously low-velocity material that is confined to below the surface expression of the rift itself at depths below 60 km.

Group takes a fresh look at the lithosphere underneath southern Kenya

Since the turn of the century the well‐developed Kenya rift has been a crucial location for studying the interrelationships between extension, uplift, and magmatism. In 1989–1990, an experiment conducted by the Kenya Rift International Seismic Project (KRISP) focused on the central and northern portions of Kenya (Figure 1) and provided a rich base of information regarding the structure and evolution of the rift, which answered many key questions and raised others. Does the crust thin to the south of the Kenya dome as it does to the north?, and if it does, which crustal layer would be mainly affected? How is the Chyulu Hills Quaternary volcanism related to the rifting process? What is the relationship between the crust and lithospheric mantle during extension?

Crustal tomography of the central kenya rift

During the Kenya Rift International Seismic Project 1990 (KRISP90) experiment the shots of the seismic refraction programme were recorded by a two-dimensional teleseismic network. This was installed in the central part of the Kenya Rift and on its shoulders, from Lake Baringo in the north to Lake Naivasha in the south P-wave first arrivals were used to determine lateral crustal velocity heterogeneities with a three-dimensional inversion programme. Mainly high-velocity zones were found which can be correlated with other geophysical anomalies and geological features. This comparison shows that the high-velocity bodies found within the crust are likely indicators for mafic cumulates or magma chambers. These evolved during the major magmatic episodes which are typical indicators for an active rift.

Seismic structure of the uppermost mantle beneath the Kenya rift

A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05–8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60–65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north.

Deep seismic sounding in the Turkana depression, northern Kenya Rift

Existing models of the structure and evolution of the Kenya Rift such as pure shear lithospheric extension, extension by simple shear or rift development by diapiric upwelling of an asthenolith, were based upon surface geology and a few geophysical (mainly gravity and seismic) data sets. Since the knowledge of the lithospheric structure plays an important role to distinguish between these different models, the Kenya Rift International Seismic Project was conducted in 1990 in the area of the Kenya Rift. The project involved a detailed multi-fold refraction wide-angle reflection line in the northern Kenya Rift along the western shore of Lake Turkana which is the most prominent feature of the so-called Turkana depression. Under-water explosions were used as sources and a good signal-to-noise ratio was obtained allowing the identification of arrivals from the crust and upper mantle. The most important result of modelling the data is a crustal thickness of 21 km in the area of Lake Turkana, whereas in the area of the Kenya dome the crustal thickness is 34 km. This points to an extension not previously expected in the northern Kenya Rift. The Moho discontinuity appears as a transition zone in the northern part of the profile, whereas in the southern part it changes to a first-order discontinuity. The largest vertical and lateral heterogeneities are observed in the rift infill displaying basins of varying thickness. The upper and lower crust which are separated by the Conrad discontinuity (the velocity changes from 6.2 to 6.4 km/s here) show very little lateral heterogeneity. The P-wave velocity in the upper mantle was modelled to be 7.7 km/s and thus can be distinguished from that in the southern Kenya Rift, where a velocity of 7.5 km/s was observed. The crustal structure of the graben supports a model of a southerly propagation of the rift.

A study of the Kenya rift using delay-time tomography analysis and gravity modelling

A combined study of 2-D delay-time tomography analysis and gravity modelling was carried out for a profile across the Kenyan rift at about 1°S. The results of the P-wave delay-time tomography analysis along the profile, carried out in 1985 within the framework of KRisp81 (Kenya Rift International Seismic Project), reveals a low-velocity zone (LVZ) down to depths of at least 200 km beneath the rift, slightly dipping to the west. To the east of the rift a second LVZ can be observed restricted to the upper mantle. Crustal “stripping” was applied to look in more detail at the delays related to the sub-Moho structure. The resulting model shows an even more pronounced low-velocity zone in the upper mantle, with a velocity contrast of up to 10%. The resulting seismic image was then used to construct a gravity model fitting not only the observed Bouguer gravity anomaly but also the seismic image in shape and size. To match the amplitudes of the Bouguer anomaly, as well as the observed seismic structure, anomalously large values for the ratio of velocity contrast to density contrast are required. To explain this, the occurrence of partial melt under the rift is proposed. The amount of melt is estimated to be about 5%. The low velocity zone east of the rift, centered at about 60–100 km depth, is consistent with petrological studies.

Tomographic study of local earthquake data from the Lake Bogoria region of the Kenya Rift Valley

The 3-D P-wave and S-wave velocity structure in the upper crust beneath the Lake Bogoria region of the Kenya Rift Valley is obtained by the simultaneous inversion of local earthquake arrival-time data for velocity and hypocentral parameters. The data were obtained from seismograms recorded by the KRISP85 (Kenya Rift International Seismic Project 1985) Lake Bogoria earthquake network and inverted using a modified version of Thurber's 3-D iterative simultaneous inversion program. The results are consistent with the inversion of teleseismic traveltime residuals from the same network and correlate with the mapped surface geological features. There is a high-velocity anomaly located at depth beneath the Goituimet volcanic region in the southwest of the study area which may be associated with basic igneous intrusives or metamorphism. There is a low-velocity anomaly in the north of the study area which can be correlated with the geothermal activity around the southern shores of the lake. The main feature is the variation in velocity across the Legisianana—Emsos—Bogoria fault, with velocities up to 5 per cent higher than the average to the east and up to 2.5 per cent lower to the west.

Monitoring recent crustal movements in the Kenya rift valley by global positioning system (GPS) — a proposal

There is evidence that the Kenya Rift is active. 1990 witnessed the execution of the Kenya Rift International Seismic Project to study the deep structure of the Kenya Rift. Yet there is no actual measurement of the rate of its spreading. Estimation based on volumes of volcanic rocks extruded over a given period has led to a wide range of estimates (0.2–2 mm/a). There is a need to establish geometrically the real spreading rate at present. The new technology (Global Positioning System) in its precise mode gives a promise to solve the problem. A preliminary network is proposed to consist of 4 points on each of the Rift walls. The logistics of site location, configuration of the network, observation method, frequency, etc. are discussed. The project requires international cooperation for execution.

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

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.