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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.

‘Stress and Stress Release in the Lithosphere’ has been studied within a Collaborative Research Centre at Karlsruhe University (SFB 108) as special contribution from Germany to the International Lithosphere Program (ILP). This synopsis is an outline of the research results obtained by the SFB 108 as presented in this special volume of Tectonophysics . It starts with a summary of ideas on stress and stress release in the lithosphere which guided the SFB 108 at its beginning in 1981. It is followed by a review of strategy in cooperative research on rift systems, world stress map and the scales of structure of the continental lithosphere from crust to mantle. The main observations are then summarized and connections are made between key results from the various disciplines to obtain new views in lithospheric tectonics. Finally we address open questions and perspectives for future research. The research addressed various styles of continental rifting from the Afro-Arabian rift system to the Central European rift system with the Rhinegraben and the grabens in the French Massif Central as outstanding features. A number of critical parameters such as thickness, heat flow, rheology, composition, strength of the crust and of the lithospheric mantle and the influence of active mantle plumes were identified and classified. Together they strongly influence the style of rifting. Petrophysical modelling of integrated information on seismic velocities, composition and temperature from xenolith analysis, density, gravity and topography provided essential constraints on plume-lithosphere interaction in the French Massif Central and also in the Kenya rift. The Rhinegraben and its Variscan environment offered the opportunity to study different concepts of upper and lower crust, including the occurrence of deep crustal earthquakes and the role of an Alpine detachment. A special concern of the SFB 108 was the distribution of dimensions of heterogeneities in the lithosphere and its relation to the scales of their generating processes. The crust-mantle boundary was recognized as a major change in scales of heterogeneities by deep seismic sounding (DSS) methods applied to the lower crust and the lithospheric mantle. The study of the tectonic stress in western Europe revealed that the crust is in frictional equilibrium down to midcrustal levels and is also decoupled from the mantle allowing adjustive motions of blocks and independent subduction in the foreland of the Alps. This decoupling is achieved in a thin and hot lithosphere driven by mid-Atlantic ridge push and Alpine continent-continent collision. It is described in a quantitative dynamic model of crust-mantle decoupling. The SFB 108 has shown that earth scientists have to rediscover the importance of pattern recognition if complex influences are to be recognized.

The data of the European part of the World Stress Map (WSM) are used to study the western European stress province north of the Alps and Pyrenees. The study area is characterised by an almost homogeneous N 145 degrees E orientation of the maximum horizont

Advances in Deep-Seismic Sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust-mantle transition. In this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near-vertical reflection and wide-angle refraction experiments. The abrupt termination of near-vertical reflections at the Moho and the coincident presence of a strong coda, accompanying the supercritical PMP reflection phase in record sections of continental seismic wide-angle refraction experiments, reveal a new property of the crust-mantle boundary. The Moho forms a sandwiched mix of crust-mantle material between lower crust and topmost mantle in combination with a step in mean velocity, a significant change in scale of the structural dimensions and of velocity variance. This view is strongly supported by the recognition of the subcrustal lithosphere as a high-frequency (5–10 Hz) scattering wave-guide for long-distance propagation of the Pn phase over more than 3000 km, detected during seismic experiments in Russia using Peaceful Nuclear Explosions. Evidence is brought forward that this high-frequency wave-guide is likely to be present also in the Phanerozoic upper mantle of western Europe. It is furthermore hypothesized that the global propagation of teleseismic Sn or corresponding Pn waves observed on continents and oceans combined with the recording of strong PMP-codas for wave paths on continents points to a world-wide change of scale of structure and velocities near the Moho. A change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the Moho. The global presence of the scattering Pn wave-guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle. Locations of present-day tectonic activity where horizontal decoupling between crust and mantle are likely to occur (e.g., subduction at continent-continent collision zones, change of tectonic regimes and continental transform faults) are discussed.

In the Eurasian part of the World Stress Map almost the whole region east of the Tornquist-Teisseyre line is terra incognita. The closure of this information gap is of fundamental importance to the understanding of the geodynamics of the Eurasian continent. A detailed analysis of stress-induced wellbore breakouts has been performed over a 4.1-km-long depth interval in the Vorotilov drillhole (VGS). The borehole is located in the central part of the Russian platform, right in the center of the Vorotilov meteorite impact crater 60 km to the NNE of the city of Nizni Novgorod. An ultrasonic borehole televiewer (BHTV) was used to obtain high-resolution acoustical images from the borehole wall. With an interactive system for analyzing BHTV data the azimuth and shape of borehole breakouts occurring in the depth range of 1.3–4.8 km were analyzed. A statistical analysis of the resulting orientation profile of the breakout azimuths yields an overall direction of the maximum horizontal principal stress S H of N 137°E ± 15°. Variations of breakout orientation with depth ranging from a few degrees up to more than 90° are seen on various depth scales. The observed stress direction of N 137°E agrees very well with the average S H orientation of N 145°E in Central Europe. If this measurement is taken as representative for the Russian platform, the stress field in Russia is only slightly rotated in comparison to Central Europe. This can possibly be interpreted as indicative for the stress field to be governed by broad scale tectonic forces, such as a strong contribution from the forces exerted by the collision zone in the Alpine-Himalayan belt and by the Mid-Atlantic ridge.

We present the results of a finite-element modelling study of pull-apart basin formation related to left-stepping left lateral strike-slip faults. The modelling quantifies the relationship between fault geometry (i.e., fault overlap and separation) and pull-apart basin formation. Two depocentres (subbasins) separated by a broad zone of relative uplift in between may develop if the strike-slip domain is characterized by fault underlap. For overlapping faults migration of the subbasins is predicted by the models. Deep subbasins in a large area of subsidence which spans the entire inner fault zone may form if fault overlap is about three times the fault separation. The models suggest that a topographic asymmetry within the fault zone may arise due to a different displacement ratio of the strike-slip faults. The modelling results show that this asymmetry in topography becomes more pronounced towards the more active fault. Thus, basin deepening occurs progressively towards the fault characterized by the largest amount of lateral displacement. Moreover, the results indicate that the smaller the fault separation (less than basin length) the less pronounced the topographic asymmetry. The models provide quantitative estimates for the effects of changes in elastic material properties, the magnitude of the compressive far-field stress and the coefficient of friction of the faults on the resulting topography. Comparison of the modelling results with field observations from the Cerro Blanco-El Barranquete (CBB) subbasin located in the Internal Zone of the Betic Cordillera, southeastern Spain support an interpretation in which the interplay of major faults has formed the CBB subbasin.

Controlled shear-wave sources (a horizontal vibrator and dynamite) were used in near-vertical reflection studies to explore the nature of the reflective laminated lower crust in southwest Germany. The data reveal strong S-wave reflections from the lower crust. These lower crustal S-reflections were not observed by previous wide-angle surveys in the same region. A laminated crustal model with isotropic layers (and changing Poisson's ratio) cannot explain the differing near- and far-offset S-response. By introducing alternating anisotropic and isotropic lamellae, this discrepancy may be resolved. This Seismic model is consistent with a petrological model of deformed amphibolites containing 10–30% of preferentially orientated hornblende. Observed amplitude differences in P- and S-wave reflections from the same depth in the lower crust suggest regions with a changing Poisson's ratio.

The recent acquisition of high-quality seismic refraction data in the Jordan—Dead Sea rift and adjacent areas has made possible the investigation of the dynamic properties of seismic P-waves refracted and reflected at the crust—upper mantle boundary. These waves cause high-amplitude arrivals near the outer cusp of the travel-time curve which are followed by an abrupt decrease in amplitudes at increasing distances beyond the cusp. It has been shown that such amplitude distributions can only be the result of a smooth rapid increase of velocity with depth. In the case of the Jordan—Dead Sea rift the amplitude distribution indicates the presence of a transition zone between the lower crust and upper mantle in which the velocity increases smoothly. The interpretation of seismic refraction data in the Rhinegraben indicates the existence of a similar transition zone. In both rifts the crust—mantle boundary outside the rift is represented by sharp velocity discontinuity. The comparison of the velocity structure of the crust—upper mantle boundary suggests that a smooth transition zone at the base of the lower crust is a characteristic property of continental rifts which could be interpreted in terms of crust—mantle interaction.

Unified geophysical investigations of the lithosphere in the Rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process. The new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust: 1. (1) the lower crust as the layer beneath the Conrad discontinuity with a P-wave velocity of about 6.5 km/s or greater (refraction seismics); 2. (2) the laminated band of reflections, as seen in the near-vertical reflection seismic experiments in many parts of the continents; 3. (3) the ductile part of the crust below the brittle-ductile transition, devoid of earthquakes in seismically active regions; 4. (4) the electrical conductivity of the lower crust indicative of dry or wet conditions, or still unknown conduction phenomena. In the Rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process, in which the crust of the Rhinegraben rift system has been subjected to three different natural dynamic processes: (1) Uplift by 2 to 3 km with subsequent erosion of the Rhinegraben shoulders (Black Forest and Vosges Mountains) caused decompression possibly leading to the formation of a low-velocity/high-electrical-conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the Black Forest. (2) The brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust, which became about 5 to 7 km thinner below the graben than below the shoulders. (3) The deepest hypocentres in the Black Forest (Dinkelberg area), if projected onto the neighbouring reflection profile, would be located 7 to 8 km within the laminated lower crust beneath the southern Black Forest, indicating a discrepancy between the top of the lower crust as defined by the brittle-ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band. The Rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations.

Long-range explosion seismic profiles on continents with penetration to a depth of about 100 km revealed both vertical and lateral variations of structure and physical properties in the subcrustal lithosphere. Findings from all available continental long-range profiles are compiled. The most important result is the discovery of unexpectedly high in-situ velocities of up to 8.7 km/s and larger, which is indirect evidence of the existence of anisotropic minerals with a preferred orientation in the subcrustal lithosphere. There is also direct evidence for the presence of anisotropy in the topmost mantle of some regions as large as observed in the oceans. The vertical gradient of velocity is larger than the gradient due to self-compression and is different in various parts of the continents. This is indicative of lateral variations in the continental subcrustal lithosphere.

The previously published results of a deep seismic refraction study of the Dead Sea—Gulf of Elat rift show crustal thinning underneath the rift and the presence of a 5 km thick velocity transition zone in the lower crust along the rift. The structural interpretation of the first-arrival data was revised using the detailed velocity-depth distribution. The revised crustal thicknesses are 35 km near Elat and 27 km, 160 km south of Elat. The crustal thinning and the presence of the velocity transition zone are interpreted as being the result of intrusion of upper mantle material into the lower crust, possibly representing the initial shape of the processes which have been active further south in the Red Sea since earlier times.

The seismic activity following the March 4, 1977 earthquake in Romania was observed by fixed and supplementary mobile seismic stations. Epicenter and hypocenter determinations in March and April provided new information on the structure of the seismic zone of Vrancea, with intermediate foci at depths between 7o and 13o km, a seismic gap between 4o and 7o km, and shallow crustal foci. Seismic activity migrated into the southwestern foredeep, into the Carpathian mountains and towards Bucharest at different velocities of km/minute, km/hour, and km/day, respectively. The viscosity of the asthenosphere under the Carpathian arc is estimated from this migration to be 1017 to l018 Poise, a value encountered only at island arcs. A new model for the mechanism of Continent-continent collision at the Eastern Carpathians is proposed, which tries to explain the isolated occurrence of the intermediate seismic zone in Vrancea, at the edge of a paleoseismic zone beneath the Eastern Carpathian mountains. +)Paper of the Romanian–German Seismological Working Group which was formed after the earthquake of March 4 by the above mentioned institutes. Contribution No. 158, Geophysical Institute, University of Karlsruhe

The geophysical investigation of the continental rift system of the Rhinegraben by various methods has progressed to such a stage that details of structure, physical properties and seismic activity and relations between these features can be recognised. Differences in the nature of the crust-mantle boundary inside and outside the graben proper seem to be a typical property of continental rifts. Asymmetries of structure, seismic activity, temperature in the crust and physical properties in the lithosphere are an important clue to the understanding of rift dynamics. These asymmetries indicate that the formation of the rift cannot be explained by the updoming of an asthenolith alone but that taphrogenesis took place in the foreland of the continent—continent collision in the Alpine—Mediterranean zone. A number of future key experiments is proposed.