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In February 1994, the Kenya Rift International Seismic Project crawled 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, RH.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(tief)1, shallows by about 5-10 km in the area beneath Lake Magadi, situated in the rift itself. Correlations for the deeper reflector, denoted de, 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(tief)n calculated from the crustal model, and constraints from previous refraction, teleseismic and gravity studies, to determine the velocity at depth. At the d(tief)1 reflector a reasonable velocity contrast was introduced to produce a reflector for modelling purposes. Beneath the d(tief)l reflector the velocity decreases to the average value over 3 km. Beneath the rift the velocity also rises across dl and again, decreases to the average value over the next 3 km. At the d(tief)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(tief)1 and d(tief)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(tief)1 reflector indicates that the sub-Moho lithosphere has not been substantially disrupted by mantle upwelling, even though probably thinned and stretched.

Crustal and upper-mantle structure of major units of the Afro-Arabian rift system has been progressively investigated by seismic-refraction surveys in the Jordan-Dead Sea rift, the Red Sea, the Afar depression and in the East African rift of Kenya. The two extreme structures are oceanic crust flooring the axial trough of the southern Red Sea rift and 30–35 km thick continental crust underneath the Jordan-Dead Sea rift and its flanks underlain by a mantle with Pn-velocities of 8.0–8.2 km/s. The remainder of the units have a crustal structure of transitional to uncertain nature. In the southeastern Red Sea, the change from 15–18 km thick transitional crust to the 40 km thick continental crust of the Arabian Shield is probably very abrupt and is the major lateral change in structure in this region. In the northern Afar depression, the crust is only 15–20 km thick. In the southern Afar depression, anomalous mantle with 7.2–7.3 km/s velocities occurs at depths greater than 25 km. Also here, an abrupt change to a 40 km thick shield crust with a velocity of 8.0 km/s is evident from seismic and gravity data. Crustal thickness of the East African rift system amounts to 30–36 km. A recent trial experiment in the eastern rift of Kenya indicates a 35–36 km thick crust underlain by a low-velocity upper mantle with 7.5 km/s. At 23–24 km depth a thin high-velocity lens of 7.1 km/s velocity may be present within the 22 km thick lower crustal layer averaging 6.6 km/s velocity. Investigations of local earthquake body waves in the western rift provide information that the crust here is of continental type with a thickness of about 32 km overlying mantle material of 8.0 km/s P-velocity and 4.6 km/s S-velocity. In conclusion, a model of preferential rifting of continents is applied to the Afro-Arabian rift system where rifting seems to progress into areas with thick continental crust as seen for example in the Levant and the East African rift systems instead of using paths in thin crust.

The crustal and uppermost-mantle structure of major units of the Afro-Arabian rift system has been consecutively investigated by seismic-refraction surveys in the Jordan-Dead Sea rift, the Red Sea, the Afar depression and the East African rift of Kenya. With the exception of the Jordan-Dead Sea transform, the entire Afro-Arabian rift system is underlain by anomalous mantle with Pn-velocities less than 8 km/s, while under the rift flanks the velocity is clearly equal to or above 8.0 km/s. Various styles of rifting have been found. Oceanic crust floors the axial trough of the southern Red Sea rift, thinned continental crust underlies the margins of the Red Sea as the Afar depression and the northern Kenya rift. On the other hand, 30–35-km-thick continental crust is found both under the Jordan-Dead Sea rift, where strike-slip rifting is active and thinning towards the Mediterranean occurs, and under the central Kenya rift, where updoming is apparently the controlling feature. While the transition from thinned continental to 5–6-km-thick oceanic crust in the centre of the Red Sea appears to be more gradual, the transition from rift-related structure to undisturbed continental crust of 40 ± 5 km thickness is mostly rather abrupt. The seismic data indicate various stages of rifting evidenced by different styles of crustal strucre and they imply the presence of heated uppermost-mantle under most parts of the rift system, possibly related to plume activity. Local volcanism may disrupt and/or underplate the crust in places, altering in particular the structure of the lower crust. Progressive thickening of the rifted crust away from the oceanized centres in the southern Red Sea and Gulf of Aden towards north and south may be viewed as an evolutionary sequence which, however, may be difficult to explain when viewing the Afro-Arabian rift system as an active rift controlled by plume activity.

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.

A major goal of the Kenya Rift International Seismic Project WRISP) 1990 experiment was the determination of deep li~ospheric structure. In the re~action/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 vyere recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrikals 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 amoungs of extrusive volcanics associated with the rift suggest comp~itional 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 beneaith 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 lith~phere due to stretching in the north.

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.

On the basis of a preliminary experiment in 1985 (KRISP 85), a seismic refraction/wide-angle reflection survey and a teleseismic tomography experiment were jointly undertaken to study the lithospheric structure of the Kenya rift down to depths of greater than 200 km. This report serves as an introduction to a series of subsequent papers and will focus on the technical description of the seismic surveys of the main KRISP 90 effort. The seismic refraction/wide-angle reflection survey was carried out in a 4-week period in January and February 1990. It consisted of three profiles: one extending along the rift valley from Lake Turkana to Lake Magadi, one crossing the rift at Lake Baringo, and one located on the eastern flank of the rift proper. A total of 206 mobile vertical-component seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to about 550 km. During the teleseismic survey an array of 65 seismographs was deployed to record teleseismic, regional and local events for a period of about 7 months from October 1989 to April 1990. The elliptical array spanned the central portion of the rift, with Nakuru at its center, and covered an area about 300 × 200 km, with an average station spacing of 10–30 km. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the Kenya rift, to study the relationship between deep crustal and mantle structure and the development of sedimentary basins and volcanic features within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system, and to answer fundamental questions such as the mode and mechanism of continental rifting.

A finite-element (FE) model was used in this study to investigate the intraplate rifting process in south Kenya, East Africa. The rifting model shows that the important factors influencing the amount of shoulder uplift and rift subsidence include the horizontal deviatoric extensional stresses, the viscosity of the lower crust, and the dimension and density contrast of the low-velocity upper-mantle anomaly. Thus, for instance, a reduced lower-crust viscosity as well as an increased extensional far-field stress favours the subsidence of the rift basin. In turn, for a reduced lower-crust viscosity, the magnitude of the applied extensional stress should be reduced in order to end up with the same topography of the rifted area as can be obtained with a more viscous lower crust combined with a relatively high extensional far-field stress. In addition, it also became obvious that the uplift of the crust-mantle boundary beneath the rift depends predominantly on the size and density contrast of the upper-mantle anomaly. An increased density contrast between the upper-mantle anomaly and normal mantle results in increased buoyancy forces thus producing an increased uplift of the crust-mantle boundary. In turn, in order to avoid too large values for the surface topography, a reduced lower-crust viscosity in the order of η = 1021 Pa s is required. Models without an upper-mantle low-density anomaly have resulted in crustal thickening beneath the rift and not in crustal thinning. In accordance with the geological field evidence, we have modelled the evolution of the South Kenya Rift in two stages. In the first stage, from 16 to 10 Ma ago, a broad regional uplift of a few hundred metres produced by a relatively small but broad low-density upper-mantle anomaly was accompanied by an insignificant amount of faulting. In the second stage, from 10 Ma ago to the present day, significant faulting accompanied by the buoyancy effects of a larger, low-density, low-velocity upper-mantle anomaly concentrated more beneath the rift itself have quite successfully reproduced the rift subsidence, shoulder topography and crust-mantle boundary uplift observed at the present day.

Using the elastic properties of individual minerals the velocity of a rock at a known temperature and pressure can be calculated. Alternatively, from P- and/or S-wave seismic velocities at a particular depth (pressure and temperature) the mineralogical composition of a rock may be estimated. In this study, this procedure is applied to the uppermost mantle beneath the Kenya Rift and its eastern shoulder. It will be shown that, for estimated in-situ temperatures and pressures, the mineralogical compositions derived from mantle xenoliths found on the eastern shoulder are compatible with the uppermost-mantle Pn velocities derived from the KRISP 90 seismic experiment beneath the eastern shoulder. Here the Pn velocities are compatible with an average undepleted spinel peridotite composition at temperatures of 300–400°C, corresponding to surface heat flows of 35–50 mW/m2, at 30–35 km depth and in certain areas with a composition containing a 10–15% increase in olivine at temperatures of 600–670°C, corresponding to a surface heat flow of about 75 mW/m2, at 30–35 km depth. Beneath the Rift itself, where no mantle xenoliths have been found, the mineralogical composition derived from mantle xenoliths found on the eastern shoulder is not compatible with the low observed uppermost-mantle Pn velocities of 7.5–7.7 km/s. Under both the northern Rift and the southern Rift where Moho temperatures of respectively 630°C at 21 km depth (5.6 kbar) and 1000°C at 35 km depth (9.6 kbar) have been estimated, corresponding in both cases to a surface heat flow of 105–120 mW/m2, the observed Pn velocities are 0.1–0.3 km/s lower than the predicted velocities. In order to explain these low observed velocities, the presence of 3–5% basaltic melt rising from greater depths and being trapped just below the Moho is invoked. At depths greater than those penetrated by the Pn-wave, for the northern Rift a temperature-depth profile of 830°C at 35 km depth (10.0 kbar), 940°C at 45 km depth (12.9 kbar), 1030°C at 55 km depth (16.0 kbar) and 1100°C at 63 km depth (18.4 kbar) has been used, while for the southern Rift values of 1200°C at 45 km depth (12.9 kbar), 1300°C at 55 km depth (16.0 kbar) and 1350°C at 63 km depth (18.4 kbar) have been utilized. Beneath the southern Rift the observed low velocities which are less than 7.8 km/s, can again be explained by the presence of basaltic melt which probably exists as in-situ partial melt below 45 km depth. The high-velocity layers beneath the northern Rift with velocities of 8.05–8.15 km/s at a high estimated temperature of about 940°C at 45 km depth and about 8.3 km/s at a high estimated temperature of about 1100°C at 60–65 km depth require some kind of preferred mineral orientation. Possible models include either a transverse isotropic structure in which the slow b-axis of olivine is oriented vertically and the faster a- and c-axes are randomly oriented in the horizontal plane or an orthorhombic structure in which some 40–55% of the olivine has the fast a-axis oriented horizontally along the rift axis. In the case of the layer at 60–65 km depth, if a transverse isotropic structure exists, then depletion of basalt may additionally be required in order to reconcile the observed and predicted seismic velocities.

Seismicity and deep seismic sounding data gathered mainly in the last 15 years have been combined to provide a picture of crustal and uppermost mantle deformations in the Jordan-Dead Sea transform and its eastern and western flanks. The results show that the main earthquake energy release is connected with the strike-slip mechanism of the transform itself. The seismicity of the area is concentrated almost exclusively in the upper 21 km of the crust, Ihus including the upper crust and possibly the top few kilometres of the lower crust. Relative crustal thinning of 5–8 km with respect to a 40 km thick continental crust, regional uplift of 1 –2 km, extrusion of basalts and possible intrusion of mantle derived material into the crust, and possible existence of fluids in the lower crust are the other major manifestations of upper lithospheric deformation in this area.

The tectonic and magmatic history of the Jizan coastal plain (Tihama Asir, southwest Arabia) suggests a two-stage evolution. A first stage of extension began during the Oligocene and ended with uplift of the Arabian graben shoulder which began about 14 Ma ago. It was followed by a period of approximately 10 Ma characterized by magmatic and tectonic quiescence. A second stage of extension began roughly contemporaneously with the onset of seafloor spreading in the southern Red Sea some 4–5 Ma ago and is still active today. The geometry of faulting in the Jizan area supports a Wernicke model of simple shear for the development of the southern Red Sea. Regional asymmetries of the Red Sea area, such as the distribution of volcanism, the marginal topography and asymmetries in the geophysical signatures are consistent with such a model. Available seismic profiles allow a rough estimate for β-values of the Arabian Red Sea margin and were used to simulate subsidence history and heat flow of the Red Sea for “classical” two-layer stretching models. Neither finite uniform nor finite non-uniform stretching models can account for observed subsidence and heat flow data. Thus, two model scenarios of whole-lithosphere normal simple-shear are presented for the geological history of the southwestern Arabian margin of the Red Sea. These models are limited because of the Serravallian rearrangement in the kinematics of the Red Sea.

Interpretation of a long-range seismic refraction line in Saudi Arabia has shown that beneath the Arabian Shield velocity generally increases with depth, from about 6 km s −1 at the surface to about 7 km s −1 at the top of the crust-mantle transition zone. The base of this transition zone (Moho) occurs at 37–44 km in depth. Intracrustal discontinuities can also be recognized, the most important being in the 10–20 km-depth range and separating the upper from the lower crust. Laterally, the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions. Beneath the Red Sea shelf and coastal plain the crust, including 4 km of sediments, is only 15–17.5 km thick. With the aid of both seismic and gravity data an abrupt, steeply dipping transition from the crust of the Red Sea shelf and coastal plain to that of the Arabian Shield has been derived. With a jump of more than 20 km in Moho depth, this appears to be the major discontinuity between the Red Sea depression and the Arabian continental shield.

The formation of continental rifts with small extension values can be simulated over several tens of millions of years by means of a finite element program able to handle large deformations. As an example an attempt has been made to model the formation of the northern and southern Rhinegraben over the past 40 m.y. Deviatoric extensional stresses of 25–40 MPa were required and, to model the Moho uplift, mass anomalies in the lower lithosphere had to be invoked with density contrasts of −0.15 and −0.30 g / cm 3 for the northern and southern Rhinegraben respectively. Trial models without a mass anomaly in the lower lithosphere resulted in crustal thickening beneath the graben and not crustal thinning. In the northern Rhinegraben the rifting is more passive and uniform and model subsidence values of 4000 m were obtained together with model shoulder and Moho uplifts of 800 m and 1500 m respectively. In the southern Rhinegraben the rifting is more active and episodic and, in fact, between 13 and 10 m.y. ago the model graben floor was uplifted in order to simulate the observed graben floor uplift at around 10 m.y. ago. Model subsidence values of 2000 m were obtained while the model shoulder and Moho uplifts were 1900 m and 7000 m respectively in good agreement with the observed values.

This paper presents some data and results from a seismic refraction experiment, completed mainly in 1979 in the Rhenish Massif, Federal Republic of Germany and extending through Luxembourg and Belgium into the Paris Basin in France. Velocity-depth functions have been derived for each record section independently, based on the assumption that velocity varies only with depth: these models are being improved upon by time-term and ray-tracing methods capable of handling laterally varying velocity structures and by calculating synthetic seismograms. The Pg phase which is observed very clearly on all record sections represents a refracted wave, with velocity generally > 6 km/s, from depths below 1.5–5.5 km. Along the 600 km long main profile one intracrustal reflection can usually be recognized, while from the three shorter crossing profiles in the massif two intracrustal reflectors can always be seen. Beneath much of the main profile the crust-mantle boundary is either a first order discontinuity or thin (< 1 km) transition zone at ~30 km depth. However, beneath the Ardennes and West Eifel there is a 6–12 km thick transition zone before a velocity of 8.1 km/s is reached at ~36 km depth. Beneath the crossing profiles, there is generally a transition zone < 3 km thick between crust and mantle. In some cases, there can be recognized at the top of the mantle a thin high velocity layer which is underlain by a low velocity layer which, in turn, is underlain by a reflector 4–11 km below the crust-mantle boundary.

The interpretation of a deep seismic refraction study in Jordan, performed in May 1984, shows that much of the country is underlain by continental crust, 32–35 km thick, and normal mantle with a velocity of 8.0–8.2 km/s. In the Aqaba region, southwest and central Jordan, east of Wadi Araba, the crustal thickness is of the order of 32–35 km, while in the Amman region it is not less than 35 km. In southeast Jordan the crust thickens to at least 37 km in what is probably the transition to the Arabian Shield type of crust. The boundaries between the upper and lower crust at about 20 km depth and the lower crust and uppermost mantle are probably transition zones. The upper crystalline crust has velocities of 5.8–6.5 km/s while the lower crust has velocities greater than or around 6.65 km/s. While the crystalline basement is exposed in southwest Jordan and is at a depth of 2–2.5 km north of Amman, it is at a depth of not less than 5 km in central Jordan. A comparison of the crustal type and structure of Jordan and the adjacent Dead Sea rift with that of the Black Forest and the Rhine valley yields a striking resemblance. The situation of the Jordan-Dead Sea rift is explained in terms of the continental crust of Arabia rifting in preference to the thin (?oceanic) crust of the Mediterranean Sea.

As part of the geophysical investigations into the uplift of the Rhenish Massif, a long-range seismic refraction experiment with areal coverage was carried out during 1978–1979 to determine the velocity structure of the crust and uppermost mantle beneath the massif (Mechie et al., 1983). The experiment included a 600 km long main profile extending from the Paris Basin in the southwest, across the massif itself, to the Hessian Depression in the northeast, and three cross profiles, up to 170 km long, almost completely located in the massif itself. Interpretation of the data, including ray-tracing through laterally inhomogeneous media and calculation of synthetic seismograms, has yielded the following velocity-depth information. The Pg phase represents the refracted wave, with velocity 6.0–6.4 km/sec, from the upper crust below an average depth of 3–5 km. Other intracrustal phases, of variable lateral extent and occurring in the depth range 9–24 km, can also be recognized. The crust-mantle boundary structure beneath the profiles also displays marked lateral variability. Between shot-points A and B, beneath the Paris Basin, there exists a sharp crust-mantle boundary at 32 km depth. Along the main profile, between shot-points B and D, beneath the Southern Ardennes, an upper mantle velocity of 8.1 km/sec is reached at about 37 km depth, at the base of a 7.5 km thick transition zone. Northeast of shot-point D, beneath the massif itself, there is a thin (1 km) high-velocity layer (V = 8.1 km/sec) beneath a sharp crust-mantle boundary at 29–31 km depth. Below the thin high-velocity layer, there is a 4–6 km thick transition zone in which the velocity increases from low values (6.3–7.0 km/sec) to upper mantle values (8.4 km/sec). Along the cross profiles, mainly towards the northwest of the main profile, it appears that the crust-mantle boundary structure is a 2–4 km thick transition zone, with an upper mantle velocity of 8.1 km/sec being reached at 30–33 km depth. An attempt is made to explain the results obtained from the seismic experiment in terms of a petrological model. In particular, the transition zone from low velocities (6.3–7.0 km/sec) to high velocities (8.4 km/sec) in the depth range 30–35 km along the main profile northeast of shot-point D may be explained in terms of magmatic material rising from greater depths and being trapped at the level of the crust mantle boundary structure.