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.

Following two previous experiments in 1985 (KRISP 85) and 1989–1990 (KRISP 90), a series of geophysical experiments was undertaken in 1993–1995 (KRISP 94) to study the lithospheric structure of the southern Kenya rift down to depths of greater than 100 km, with special emphasis on the Chyulu Hills, a complex of volcanic centres on the eastern flank of the rift. KRISP 94 involved a teleseismic tomography experiment of the Chyulu Hills area in July and August 1993, a seismic refraction-wide-angle reflection survey across southern Kenya from Lake Victoria to the Indian Ocean in February 1994, seismicity studies of southern Kenya from 1993 to 1995, a special seismicity study of the Lake Magadi area in February 1994, a gravity study along the seismic-refraction lines before and after the seismic-refraction study, and a magnetotelluric study of southern Kenya in February 1995. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the southern Kenya rift and its flanks for several 100 km to the west and to the east and their evolution, to study the relationship between deep crustal and uppermost mantle structure, to learn more on the development of sedimentary basins and volcanic features on the flanks and its relation to the Kenya rift, to obtain information on the temperature conditions underneath the rift and its flanks, to perform a particular integrated and calibrative study of seismological and petrological data in the Chyulu Hills, and to understand the processes which are producing extension, uplift, and extensive magmatism. This report is an introduction to a series of subsequent papers. It focuses on the technical description of the main seismic surveys of the KRISP 94 effort and summarizes the key results. During the teleseismic survey an array of 31 seismographs was deployed to record teleseismic, regional and local events for a period of about 3 months from June to August 1993. The elliptical array covered an area about 150 km (N-S) × 100 km (E-W) and spanned the central portion of the Chyulu Hills and its surroundings, with an average station spacing of 10–30 km. The seismic refraction-wide-angle reflection survey was carried out in a 2-week period in February 1994. It consisted of two profiles: one extending from Lake Victoria across the western flank and the southernmost Kenya rift at Lake Magadi, the other extending from Athi River near Nairobi across the eastern flank of the rift, traversing the Chyulu Hills and terminating at the Indian Ocean near Mombasa. A total of 204 mobile seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to 730 km. Key results are as follows: (1) The crust reaches a maximum thickness of up to 44 km under the Chyulu Hills. (2) Only a minor upwarping of the crust-mantle boundary is seen under the rift proper in the Lake Magadi area. (3) To the west the crust shallows to about 34 km near Lake Victoria, in contrast to the thickening of the crust further north from the central part of the rift near Lake Baringo towards the west. (4) There is a steep rise of the Moho east of the Chyulu Hills towards the Indian Ocean. (5) P-wave velocities in the uppermost mantle are above 8 km/s except under the rift proper and under the Neogene volcanic centre of the Chyulu Hills, where the velocity is 7.9–8.0 km/s. (6) Under the Chyulu Hills, the Moho is replaced by a gradual crust-mantle transition, and the low velocities near the crust-mantle boundary extend to greater depths as evidenced by teleseismic tomography which indicates a velocity decrease of 3–5%, i.e. from 8.1–8.2 km/s to at least 7.9 km/s. Both effects are probably caused by the local recent volcanic activity, but cannot be interpreted as due to plume activity which is assumed to be present under the Nyanza craton further west. (7) Gravity modelling and first preliminary results of the magnetotelluric measurements support the seismic-refraction and tomographic results. Furthermore, under the western flank the magnetotelluric and gravity data indicate increased conductivity and decreased density in the uppermost mantle below 60–80 km depth.

Since 1985 the KRISP working group has recorded a number of seismic profiles and carried out array experiments in the Kenya Rift valley. Long range profiling has shown that across the Rift at latitude 0.5°, the crust thins from 40 km beneath the western flank to 30 km beneath the graben proper, and thickens again to 35 km beneath the eastern flank. The crustal thickness beneath the graben decreases northwards from 35 km beneath the culmination of the Kenya dome to 20 km under Lake Turkana, suggesting a change to a highly extended terrain in the northern Kenya Rift. Furthermore, the anomalous low mantle velocities of 7.5–7.6 km/s suggest that the upper mantle immediately beneath the Rift may contain reservoirs of magmas, which were generated at greater depth. The anomalous mantle under the graben proper, as seen by the tomographic imaging, is characterized by a large, steep walled low velocity zone with a velocity decrease of at least 10% in the central part between 65 and 100 km depth which reaches down to more than 150 km. A comparison of the shape and size of the observed Bouguer anomaly with the LVZ suggests that the upper mantle beneath the Rift probably includes some 5% partial melt, concentrated in the areas with lowest velocities. These results provide new constraints for geodynamic models of the evolution of 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 Moho depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and Pan-African 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.

This paper presents those results from the 1974 Lithospheric Seismic Profile in Britain (LISPB) which relate to the compressional velocity structure of the crust and uppermost mantle beneath Northern Britain. A combination of interpretation techniques suitable for modelling laterally inhomogeneous media, including two-dimensional ray-tracing and time-term analysis, has resulted in a detailed seismic cross-section across the Caledonian orogenic belt. The main features of this section are a possible horizontal discontinuity in the Pre-Caledonian basement, a change in the relationship between the lower crust and the uppermost mantle from north to south and a considerable thickening of the crust beneath the Caledonian fold belt. These results place considerable constraints upon tectonic models for the evolution of the Caledonides in particular in their implication of differing crustal structures north and south of the Southern Uplands and their indication of the primary significance of the Southern Uplands Fault.

Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (< 6.3 km/s). We suggest that a horizontal discontinuity may exist in the pre-Caledonian basement between the Southern Uplands fault and the Stublick Line though we cannot yet determine the exact nature of this discontinuity.

The planning, execution and preliminary results of a major Anglo-German explosion seismic project are presented in this, paper I of a series. This Lithospheric Seismic Profile in Britain (LISPB) was planned as a reversed 1000 km line between two major sea-shot points off Cape Wrath in Scotland and one in the English Channel; additional sea-shots and intermediate land-shots were fired to give reversed and overlapping crustal coverage (to 180–400 km distance) along the line. In all, 29 shots were fired and 60 mobile magnetic tape stations recorded three-components of ground motion. The resulting 14 crustal and three long-range profiles have observations at intervals of typically 2–4 km. Recordings have been digitized and four examples of filtered, computer-plotted record sections are presented to illustrate data quality. In a preliminary analysis, phase correlations are discussed and some models presented; the latter especially are more relevant to future interpretations than to geological or tectonic problems. However, significant variations in crustal thickness and in the nature of the crust-mantle transition do seem to occur beneath the British Isles.