Description: 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.

Description: The seismic refraction-wide-angle of reflection experiments carried out between 1985 and 1994 in the Kenya Rift (KRISP ’85, KRISP ’90 and KRISP ’84) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The profile completed in 1990 across the rift of the Kenya dome at the latitude of Lade Baringo and the profile completed in 1994 across the rift south of the Kenya dome at the latitude of Lake Magadi both show that the low uppermost mantle P (tief)n velocity of 7.5-7.7 km/s and crustal thining of 5-10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P (tief)n velocities of 8.0-8.2 km/s occur. East of the rift, the profile completed in 1994 through the Chyulu Hills Quaternary volcanic field reveals some of the thickest crust (38-44 km) encountered so far beneath Kenya over a distance of volcanic field are of a reverberatory nature from immediately behind the first arrivals to beyond the Moho reflection P(tief)M P. This reverberatory nature could possibly be caused by equivalents of the volcanic outpourings in the Chyulu Hills. Beneath the Chyulu Hills uppermost mantle P(tief)n velocity is 7.9-8.0 km/s. Below the 600 km long axial rift profile, P(tief)n velocities are low being 7.5-7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low velocity mantle material at 40-45 km and 60-65 km depth respectively. In contrast, the wide-angle data show that beneath the Kenya dome in the southern part of the rift low mantle velocities occur down to at least 60 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45-50 km) to the onset of melting under the Kenya dome to the south. The above results taken together with results from teleseismic studies, petrology and surface geology indicate anomalously hot mantle material appearing below the present site of the Kenya Rift about 20-30 Ma ago. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion especially into the basal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5-10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift a greater amount of extension (35-40 km) has taken place and the crustal thickness is small (20 km) although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

Description: Wide-angle reflection and refraction data acquired as part of the URSEIS ’95 geophysical exsperiment across the southern Uralide orogen provide evidence for a 12 to 15-kilometer-thick crustal root, yielding a total crustal thickness of 55 to 58 kilometers. Strong reflections from the Mohorovičić discontinuity (Moho) at relatively small precritical distances suggest that the crust-mantle transition beneath the crustal root is a sharp feature. The derived P- and S-wave velocity models constrain key physical properties of the crust, including the depth of the mafic rocks of the Magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the East Uralian zone.

Description: INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by whe structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

Description: During the Kenya Rift International Seismic Project (KRISP 90) a 450 km long E-W seismic-refraction/wide-angle-reflection profile involving the deployment of 250 instruments was shot across the Kenya Rift. A reflected phase recorded between distances of 260 and 350 km from a 1000 kg shot at the western end of the line in Lake Victoria has been interpreted as originating from about 60 km beneath the western margin of the rift. Detailed processing of this phase has resulted in defining its polarity in relation to the first-arrival diving wave at the same range. Extensive kinematic and dynamic modelling shows there is a high-velocity zone at depths below 60 km under the western flank of the rift. We cannot exclude the presence of a layered alternating high-low-velocity structure as found in the upper mantle beneath the northern part of the N-S seismic profile along the rift axis. Constraints from xenolith studies indicate that anisotropy may explain the high velocity found beneath the reflecting horizon (≥8.40km s(hoch)-1). Petrological modelling shows that if the anisotropy is due to the preferred orientation of olivine crystals, then either a transverse isotropic structure, in which the 'a' and 'c' axes are randomly orientated in the horizontal plane, or an orthorhombic structure, in which the fast 'a' axis is orientated along the direction of the E-W seismic line, is possible. The reflection could also be caused by a pre-rift structure associated with the Proterozoic collisional orogen involving the Mozambique Orogenic Belt and the Archaean Nyanza Craton, whose contact is subparallel to and lies about 70 km to the west of the Tertiary rift. The evidence presented here delimits the lateral extent of the upper-mantle region of anomalously low-velocity material that is confined to below the surface expression of the rift itself at depths below 60 km.

Description: A two-dimensional velocity structure for the mantle down to a depth of 700 km derived from super long-range nuclear explosion seismic data recorded along the Quartz profile in northern Eurasia is presented. The profile extends from the Russian Platform, across Western Siberia to the Altai mountains. A detailed structural image of the lithosphere and asthenosphere shows strong lateral variations along the profile. The subcrustal lithosphere to a depth of 100 km is characterized by lateral variations of the P-wave velocity between 7.7 and 8.7 km/sec. The seismic data are consistent with the Ural mountains having a crustal root. Another important finding in the data is an additional discontinuity in the mantle transition zone at 530 km. This discontinuity could represent one or more phase transitions known to occur at about this depth. At present, the nuclear-seismic records are believed to represent a key data set to prove the existence of this much debated upper mantle discontinuity.

Description: Synthetic 1-D models of 3-component data recorded during the BABEL experiment by two land stations F01 and F04 are presented. The stations are located in the Bothnian Gulf, at the southern end and to the east of the BABEL line 1, respectively. As the noise, both random and coherent, has similar characteristics on both the in-line (F01) and the quasi-fan (F04) seismic profiles, the same processing procedure (frequency bandpass, frequency-wavenumber and coherence filtering and vertical stacking) has been applied. The observed shear-wave phases help to put constraints on the reconstruction of the P and S-wave field. Synthetic modeling has been performed using the reflectivity method assuming that both the P and PS sea-bottom converted waves are propagating directly from the source and omitting the laterally inhomogeneous first layer, in which the lateral contact between the sea water and the granitoid rocks of the land occurs. The velocity-depth function has been reconstructed taking into account 2-D effects to explain misfits between the theoretical and the observed traveltime curves. A Poisson´s ratio crustal model is also proposed.