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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.

A new technique for interpretation of 3-D seismic tomographic models in terms of temperature, degree of partial melt and rock composition is presented and tested. We consider both anharmonic and anelastic temperature effects on seismic velocities as well as the effects of mineral reactions, composition and partial melt. It is shown that composition effect is small (less than 1% of velocity) if there are no strongly depleted, Mg-rich harzburgites. We calculate anharmonic temperature derivatives of seismic velocities from compositions of mantle xenoliths. The parameters of a non-linear frequency and temperature-dependent model of attenuation have been taken from published laboratory experiments and calibrated using global Q observations in the upper mantle. For every block of the tomographic model we calculate the absolute temperature and melt fraction required to fit the observed V p perturbation, the average temperature of the tomographic layer being constrained by the observed surface heat flow. With these temperatures we calculate attenuation, density, V p and V s from petrophysical modelling, using the average for 80 mantle xenoliths samples from the French Massif Central. The technique is applied to a recently published 3-D teleseismic P wave tomographic model of the upper mantle beneath the French Massif Central. The observed velocity perturbations are probably caused there by variations in temperature. Temperature does not reach the dry solidus temperature (except for a few tomographic blocks), although it comes close to it at the depth of 60–100 km below volcanic areas. At high subsolidus temperatures the contribution of anelasticity to velocity perturbations is at least as important as the combined effect of anharmonicity and mineral reactions. Our model is consistent with the Pn velocities from refraction seismic studies, Q S estimations from surface waves, observed gravity, geoid, topography and surface heat flow, as well as with the composition and temperatures derived from mantle xenoliths. We suggest that the lithosphere-asthenosphere boundary is uplifted to 50–60 km depth beneath the main volcanic fields. The central and southern part of the Massif Central is underlain by the hot mantle body (plume?) with a potential temperature that is 100–200°C higher than the average potential temperature of the upper mantle.

We infer the fine structure of a sub-Moho boundary layer (SMBL) at the top of the lithospheric mantle from high-resolution seismic observations of Peaceful Nuclear Explosions (PNE) on superlong-range profiles in Russia. Densely recorded seismograms permit recognition of previously unknown features of teleseismic propagation of the well known Pn and Sn phases, such as a band of incoherent, scattered, high-frequency seismic energy, developing consistently from station to station, apparent velocities of sub-Moho material, and high-frequency energy to distances of more than 3000 km with a coda band, incoherent at 10 km spacing and yet consistently observed to the end of the profiles. Estimates of the other key elements of the SMBL were obtained by finite difference calculations of wave propagation in elastic 2D models from a systematic grid search through parameter space. The SMBL consists of randomly distributed, mild velocity fluctuations of 2% or schlieren of high aspect ratios (≥40) with long horizontal e...

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.

A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth. In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust mantle. Although we cannot provide final models and an explanation of why these different scales exist, we believe that scales of inhomogeneities carry significant information regarding the tectonic processes that have affected the lower crust, the lithospheric and the sublithospheric upper mantle. We focus on four different types of small-scale inhomogeneities: (1) the characteristics of the lower crust, (2) velocity fluctuations in the uppermost mantle, (3) scattering in the lowermost lithosphere and on (4) heterogeneities in the mantle transition zone.

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.

The Rhine Graben, part of the European Cenozoic rift system, deserves special attention because of its location in the foreland of the Alpine orogen. The Phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the Variscan orogeny, late-orogenic crustal re-equilibration to the interference of rifting and Alpine orogeny in Tertiary times. The Rhine Graben is one of the most detailedly studied continental grabens. Prospecting for hydrocarbons in its sedimentary fill and intensive pre-site studies for the Continental Deep Drilling Program (KTB) on its eastern crystalline flank (Black Forest) provided a comprehensive set of geoscience data. Seismic investigations ranging from a deep seismic network of near-vertical reflection to concurrent wide-angle refraction experiments were accompanied by seismological and geological surveys. Therefore, a direct observational comparison between the two depth definitions of crustal subdivisions, elastic vs. strength/rheological, was possible. In large parts of Western Europe deep seismic sounding (DSS) investigation distinguishes between a crystalline upper crust of comparatively low P-wave velocities (5.9–6.0 km/s) with little and discrete reflectivity and a strongly reflective lower crust with higher P-wave velocities (about 6.4–6.8 km/s). Both are separated by the Conrad discontinuity. In contrast, strength and rheological considerations define a brittle upper crust and a ductile lower crust. The boundary between these two realms is usually referred to as the brittle/ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres. The present brittle/ductile transition in the Rhine Graben region, as defined by the deepest hypocentres, does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies. This suggests that the laminated lower crust developed during the crustal re-equilibration phase at the end of the Variscan orogeny. The second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the Variscan lower crust. A remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern Rhine Graben and is possibly induced by the young rifting process. We suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the Alpine orogen. This allows us to propose the distinction between the influence of compression, extension, continental subduction and intra-crustal detachment during graben formation. The dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small-scale collisional escape tectonics associated with an intra-continental transform fault at the southern end of the Rhine Graben, the Rhine-Saoˆne Transform zone.

Observations made in different continental rift systems (European, Red Sea-Gulf of Aden, and East African Rift Systems) were investigated in terms of the influence of different parameters on the style of rifting. Apart from the lithospheric thermal regime at the time of rift initiation, the process of rifting seems to be mainly controlled by the far-field stress regime and the presence or absence of a mantle plume. In a hot lithosphere the low viscosity of the lower crust enables the upper crust to be detached from the mantle and be deformed independently under far-field stresses. Therefore, in western Europe the main rifts could open obliquely to the direction of mantle movement in crustal levels without appreciable extension in the lithospheric mantle. In contrast, the colder lithosphere of Arabia did not allow detachment of crust and mantle. Therefore, despite being in a similar tectonic situation as in western Europe, i.e. rifting in front of an orogen, the whole lithosphere deformed congruently. Rift opening occurred parallel to mantle movement, i.e. parallel to the direction of extensional stress in the lithospheric mantle induced by the pull of the subducting slab at the orogenic front. The forces needed to extend the whole relatively cool Arabian lithosphere could, however, not be produced by slab pull alone. Additional forces and weakening of the lithosphere were produced by the Afar mantle plume. Mantle plumes are generally not able to break very thick cratonic lithosphere but they deflect sidewards when hitting this kind of lithosphere. Warmer (but still relatively cool) lithosphere like in the surroundings of the East African Tanzania craton or in Arabia can, by the buoyancy of a plume, be bent strongly enough to break. As a consequence, long linear rift structures develop with generally high shoulders. The presence of a plume explains thus the position of the East African and Red Sea-Gulf of Aden rifts. Under far-field compression, rifts will open only a small amount, whereas under far-field extension continental break-up may occur. A plume hitting a hot lithosphere may penetrate it without producing long linear rifts. Instead, crustal deformation will be distributed in parallel basins over a wide area with only minor amounts of rift shoulder uplift as has happened in northern Kenya and the French Massif Central.

Seismic recordings from Peaceful Nuclear Explosions along six long-range profiles in northern Eurasia reveal the presence of an intermediate discontinuity in the mantle transition zone (410–660 km depth) at 530–555 km depth. In addition, between 200 and 300 km depth beneath the younger East European platform, Uralides and West Siberian basin a prominent low velocity zone (perhaps the asthenospheric layer) is identified which seems to be absent under the older East Siberian platform.

Models derived from controlled source seismology (reflection and refraction data) and long-period seismology provide the impression that the upper mantle is rather homogeneous. For near-vertical reflections the mantle appears transparent if compared to the reflective lower crust. Mantle reflections are sometimes observed, but as discrete reflections rather than spatially distributed reflectivity. Refraction data allow the observation of a Pn phase which serves to define a sub-Moho velocity or velocity gradient. Long-range observations define discrete velocity contrasts in the mantle, but again the models display a mantle which is mostly homogeneous on the scale of many wavelengths.

The Chyulu Hills, a 1.4 Ma B.P. to Holocene volcanic field located about 150 km to the east of the Kenya rift, is one of the few locations on Earth for which detailed geochemical (volcanic rocks), thermobarometric (xenoliths), seismological and gravity data are available. This paper combines these data to achieve an integrated seismic-petrological model for the deep structure of this volcanic field. Results of a wide-angle reflection and refraction experiment reveal an average crustal thickness of 40 km and a thickness of 20 km for the lower crust. Beneath the volcanic field, the crust thickens to about 44 km. In this region a low-velocity body (LVZ) is modelled which extends from about 30 ± ; 5 km depth to the Moho. The LVZ is characterised by an increased vP/vS-ratio ranging from 1.81 to 1.93 depending on the possible extents of this body. This is in contrast to the surrounding crust where a ratio of only about 1.76 is observed. In the same area, the results of a teleseismic tomography study show a P-wave low-velocity anomaly of − 3%. The seismic data can be explained by either an anorthositic body directly above the Moho in the region of the Chyulu Hills or by the presence of partial melt. Directly beneath the Chyulu Hills, a P-wave velocity of 7.9 km/s is determined for the uppermost mantle ; this velocity is 0.2– 0.3 km/s lower than that of the surrounding mantle region. The teleseismic tomography model suggests a P-wave low-velocity anomaly of − 2.5 to − 3.5% in the uppermost mantle (

This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the P(tief)n phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the P(tief)n phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P-wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s(hoch)-1 at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed P(tief)n amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s(hoch)−1 in the fast direction to 0 s(hoch)−1 in the slow direction of horizontal P-wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.

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.

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.

High-resolution seismic sounding of the mantle using two Peaceful Nuclear Explosions (PNEs) along the 3950 km long Quartz profile in northern Eurasia reveal major unexpected vertical and lateral mantle inhomogeneities. In contrast to standard seismological earth models such as PREM and IASP91 three, rather than two, mantle discontinuities are found between 400 and 700 km depth. They are located at about 420, 550 and 670 km depth. Lateral variations in depths amount to 10–20 km, and associated P-wave velocities vary by 0.1 km s(hoch)-1. In the upper mantle, the uppermost layer through which P(tief)n propagates extends down to about 100 km depth. The mantle below this depth, but above the 410 km discontinuity, is characterized by alternating high and low P-wave velocities in the depth range between 100 and 200 km. The velocity fluctuates between 8.3 and 8.7 km s−1 and shows variations from shotpoint to shotpoint. Average velocities are clearly higher than standard seismological reference models. This part of the mantle overlies a low-velocity zone between about 200 and 300 km depth. This channel also displays substantial lateral variation in depth which could be interpreted as a laterally variable asthenosphere. Beneath the West Siberian basin the asthenosphere seems to be well established whereas it is only poorly defined under the Uralian belt and the tectonic units to the northwest. With the addition of more data from this and other profiles, future aims include two-dimensional and three-dimensional tomographic modelling, as well as modelling of petrological composition, of the mantle down to 850 km depth beneath northern Eurasia.

Indications for the possible presence of a fluid accumulation below the drillbit of the KTB (German Continental Deep Drilling Project)‐Hauptbohrung are provided by comparisons of seismic P‐wave and S‐wave reflection sections. A bright reflection with negative polarity at approx. 8 km depth is seen only on P‐wave sections. It is absent on S‐wave sections in contrast to deeper reflections. This observation can be explained by the following geological models or combinations of them: (1) brine accumulation within a strongly fractured rock reservoir, (2) liquid/gas interface within strongly fractured rocks, (3) rock composition with increased quartz content and corresponding low Poisson's ratio, and (4) varying symmetry system of seismic anisotropy due to rock foliation. Predictions concerning the nature and location of this reflector are expected to be verified by drilling and direct probing in spring‐summer, 1993.

The World Stress Map (WSM), published in April 2007 by the Commission for the Geological Map of the World and the Heidelberg Academy of Sciences and Humanities, displays the tectonic regime and the orientation of the contemporary maximum horizontal compressional stress at more than 12,000 locations within the Earth's crust. The Mercator projection is a scale of 1:46,000,000. The WSM provides insight into large-scale patterns of stress orientations (i.e., first-order stress patterns due to plate boundary forces and second-order stress patterns due to topography), large lateral density variations, and deglaciation effects. Furthermore, the WSM contains a number of regions with high data resolution that enable users to investigate variations in stress orientations on local scales and to discuss factors controlling third-order stress patterns such as active faults, local inclusions, detachment horizons, and density contrasts. Forces resulting from these geological subsurface structures control the stress field orientations especially when magnitudes of the horizontal stresses are close to each other.