Deep velocity sections of the transition zone from the Siberian platform to the Central Asian mobile belt are constructed by teleseismic tomography and P-receiver function techniques. An array of the dense ancient Siberian craton is identified in the velocity sections with areas of high seismic velocity. In the SSW section MOBAL_2003, the surface boundary of the craton corresponds to the southern margin of the Siberian platform and is nearly vertical to a depth of 120 km. At larger depths, the craton slides almost horizontally underneath the Tunka rift area. At depths from 150 to 250 km, it is in contact with the area under the Khamar-Daban mountain range. In the southeast, according to the SE velocity section PASSCAL_1992 across the South Baikal basin and the Khamar-Daban mountain range, the Siberian craton thickness is reduced from 270 to 150 km at the contact of the Siberian platform with the Baikal folded area. In this contact zone, the upper part of the craton is wedge-shaped and has an angle of about 45° with the ground surface; it completely tapers off at a depth of 150 km to the east of Lake Baikal. The vertical configuration of the southern segment of the Siberian craton, which evolved with time, may determine the nature of the Baikal rifting in the Cenozoic.

P and S receiver functions from seismograph stations in the Indian shield, Western Himalaya, Ladakh and Tibet are processed with a method which provides estimates of the P and S velocities and their ratio as a function of depth. The time difference between the P660s and P410s phases in the north of the Indian shield and the Lesser Himalaya is 1.0–1.5s larger than the normal 24s. This is an effect of a low temperature with implication that the consumed material of the Indian shield has reached the transition zone. The waveforms of the P410s and S410p phases at some stations in the Indian shield are indicative of a thin (a few tens of kilometers) low S velocity layer atop the 410-km discontinuity, which is usually related to mantle upwelling. The mantle S velocity under the Indian shield at depths less than 180km is 4.4–4.5km/s, much lower than the 4.7km/s, typical for Precambrian shields. We explain this low S velocity mainly by a recent (Tertiary?) metasomatic alteration of the high-velocity mantle keel. Beneath the western Himalaya, Ladakh and western Tibet (but not eastern Tibet) the S velocity in the mantle at depths less than 100–150km is around 4.7km/s, Vp/Vs is anomalously low, and we argue that this high-velocity layer is a remnant of the mantle lithosphere of the northern Greater India. At most locations in the Indian shield high S velocities (3.5km/s and more) are dominant in the middle and lower crusts, and the elevated S velocity is accompanied by an increased Vp/Vs ratio (1.8–2.1 versus the standard 1.73). In the foothills of the Himalaya, the crust is 50–55km thick and consists almost entirely of a high-S-velocity (3.7km/s and more) rock with the increased Vp/Vs ratio in the middle and the standard Vp/Vs ratio in the lower crust. This observation suggests that the upper crust of the Indian plate is scraped off in the collision zone, whereas the high-velocity lower crust is subducted jointly with the mantle lithosphere. The high velocities are responsible for the P-wave teleseismic travel time anomaly of ~1s relative to Ladakh. Under the Himalaya the Vp/Vs ratio in the crust is normal, which suggests a change in composition relative to the crust of the Indian shield. Under Ladakh and Tibet the anomalously high Vp/Vs ratio in the crust is observed again. Beneath Tibet our analysis reveals a low-velocity crustal zone of partial melt between the 20-km and 45-km depths. Previously, the 45-km discontinuity was interpreted as the effect of eclogitization.

The main chain of SW-NE-striking Cenozoic half-grabens of the Baikal rift zone (BRZ) follows the frontal parts of Early Paleozoic thrusts, which have northwestern and northern vergency. Most of the large rift half-grabens are bounded by normal faults at the northwestern and northern sides. We suggest that the rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during Cenozoic extension.Seismic velocities in the uppermost mantle beneath the whole rift zone are less than those in the mantle beneath the platform. This suggests thinning of the lithosphere under the rift zone by asthenosphere upwarp. The geometry of this upwarp and the southeastward spread of its material control the crustal extension in the rift zone. This NW-SE extension cannot be blocked by SW-NE compression generated by pressure from the Indian lithospheric block against Central Asia.The geochemical and isotopic data from Late Cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes. To distinguish and locate these plumes, we use regional isostatic gravity anomalies, calculated under the assumption that topography is only partially compensated by Moho depth variations. Variations of the lithosphere-asthenosphere discontinuity depth play a significant role in isostatic compensation. We construct three-dimensional gravity models of the plume tails. The results of this analysis of the gravity field are in agreement with the seismic data: the group velocities of long-period Rayleigh waves are reduced in the areas where most of the recognized plumes are located, and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails.The Baikal rift formation, like the Kenya, Rio Grande, and Rhine continental rifts [Achauer, U., Granet, M., 1997. Complexity of continental rifts as revealed by seismic tomography and gravity modeling. In: Jacob, A.W.B., Delvaux, D., Khan, M.A. (Eds.), Lithosphere Structure, Evolution and Sedimentation in Continental Rifts. Proceedings of the IGCP 400 Meeting, Dublin, March 20-22, 1997. Institute of Advanced Studies, Dublin, pp. 161-171], is controlled by the three following factors: (i) mantle plumes, (ii) older (prerift) linear lithosphere structures favorably positioned relative to the plumes, and (iii) favorable orientation of the far-field forces. (C) 2003 Elsevier B.V. All rights reserved.

Crustal and upper-mantle velocity inhomogeneities are studied by the interpretation of the P-wave travel times from local explosions and teleseismic earthquakes recorded in 30 stations on the territory of Bulgaria and some neighbouring regions. Different methods of the solution of inverse 2-D and 3-D kinematic problems are used. The established velocity inhomogeneities are discussed in connection with the major characteristics of some geophysical and seismotectonic fields. It is noted that the velocity patterns of the crust reflect the configuration of Alpine tectonic structures. The subcrustal inhomogeneities are discordant to the main features of the near-surface structures. However, these inhomogeneities are concordant to some seismic lineaments inexplicable from the viewpoint of the first-order tectonic structures. The observed relationship between the seismicity and deep velocity structures show that these lineaments are controlled by the uppermost mantle heterogeneities. It is also noted that the high-velocity zone in the deeper upper mantle indicates deep-seated inhomogeneities beneath the Rhodopean Massif. These inhomogeneities most probably represent palaeo-subduction of a lithosphere slab penetrating deeper than 250 km into the asthenosphere.

The computational effectiveness of travel-time inversion methods depends on the parameterization of a 3-D velocity structure. We divide a region of interest into a few layers and represent the perturbation of wave slowness in each layer by a series of Chebyshev polynomials. Then a relatively complex velocity structure can be described by a small set of parameters that can be accurately evaluated by a linearized inversion of travel-time residuals. This method has been applied to artificial and real data at small epicentral distances and in the teleseismic distance range. The corresponding matrix equations were solved using singular value decomposition. The results suggest that the method combines resolution with computational convenience.