V. M. Kozhevnikov

SKS splitting beneath continental rift zones

We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift-orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift-parallel again, although a small number of measurements are still rift-orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N-S trending rift. All three rifts are underlain by low-velocity upper mantle as determined from teleseismic tomography. Rift-related mantle flow provides a plausible interpretation for the rift-orthogonal fast directions. The rift-parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small-scale mantle convection with rift-parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift-parallel fast directions are caused by oriented magmatic cracks.

Thickness of the lithosphere beneath the Baikal rift zone and adjacent regions

Abstract Methods of separation of gravity anomalies related to mantle density inhomogeneity are developed. Based on the inversion of these anomalies with regard to some constraints obtained from seismic data, the map of lithospheric thickness beneath the Baikal rift zone and adjacent regions was made. The lithospheric thickness beneath the Baikal rift zone is estimated to be 40–50 km, i.e. the lithosphere is thinned here to a crustal thickness. Beneath the Siberian platform the lithospheric thickness increases to 200 km, and underneath the Trans-Baikal region of moderate Cenozoic tectonic activity it ranges from 75 to 160–175 km. Therefore, a wide asthenospheric upwelling was revealed beneath the rift zone. The data available on the configuration of the Baikal depression and that of the crust and the lithosphere as a whole made it possible to estimate the magnitudes of extension at different lithospheric levels. The significant increase in extension with depth implies that rifting in the Baikal zone was caused by asthenospheric diapirism.

The Baikal rift zone: the effect of mantle plumes on older structure

Abstract 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.

Structure of the lithosphere of the Mongolian-Siberian mountainous province

Abstract The Mongolian-Siberian mountainous province distinguished by Florensov (1978) includes the Sayan-Baikal, the Altai-Sayan regions and East Mongolian high ranges, and those of Mongolian Altai, Goby Altai and Khangai. In the Late Cenozoic all this province was involved in an intensive orogeny, the latter occurring both in the area under extension and in that under compression (the Sayan-Baikal domal uplift and the Altai uplift system, respectively). The study of the deep structure of the mountainous province and of relatively stable adjacent regions must contribute to a more complete understanding of the causes of intracontinental orogeny. The main subject of this study was to map the Moho depth and the thickness of the lithosphere as a whole, based on the interpretation of geophysical data covering the Mongolian-Siberian mountainous province, the Southern Siberian platform and the East Mongolian high plains.

Structure of the upper mantle in Asia from phase and group velocities of Rayleigh waves

Dispersion curves of phase velocities of Rayleigh waves are determined by the method of frequency-time analysis in a range of periods of 10–200 s from data of 43 interstation traces in Central Asia. Because the joint use of phase and group velocities significantly decreases the uncertainty in the determination of S wave velocity structures, the same traces were used for calculating group velocities from tomographic reconstructions obtained in [Yanovskaya and Kozhevnikov, 2003, 2006] and determining average velocity structures along these traces. The velocity structures were calculated by the Monte Carlo and linear inversion methods, which gave consistent results. Using velocity values obtained at fixed depths by the 2-D tomography method, lateral variations in velocities at these depths were estimated, which allowed us to construct smoothed vertical velocity structures at some points in the region. The resulting structures were used as initial approximations for constructing local velocity structures solely from previously obtained local dispersion curves of group velocities in the area (32°–56°N, 80°–120°E). Based on these structures, we mapped the lateral distribution of velocity variations at upper mantle depths of 75–400 km and along three vertical profiles. The inferred velocity variations are in good agreement with data on the tectonics of the region.