V. V. Mordvinova

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.

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.

The East Siberia Transect

The new East Siberia transect, constructed by synthesizing recently analyzed geological and geophysical data, runs in a broken line through the vicinities of the towns of Nizhneangarsk, Chita, and Borzya, traversing the Siberian Platform margin and the Baikal and Mongolia-Okhotsk fold areas. The transected region encompasses a number of terranes, most of which once were arc-trench systems with fore-arc and back-arc basins. The island arcs involved Early Precambrian blocks that were sialic cores of some of their islands. The terranes have complex spatial relations in the present tectonic framework, with most of the rocks belonging to magmatic arcs, whereas the associated basins comprise allochthons thrust over continental margins. Accretion of the terranes to the Siberian continent occurred in two stages, the Early Proterozoic and the Early Paleozoic. Middle Paleozoic and Late Paleozoic–Early Mesozoic subduction on the periphery of the Mongolia-Okhotsk ocean, which actually was an enormous guK of the Pacif...

Structure of the Earth's Crust and Upper Mantle of the North Muysk Segment of the Baikal Rift Zone According to Seismic-gravity Data

Summary The performed interpretation of seismic-gravimetric data in the North-Muysky area of the Baikal rift zone makes it possible to estimate the thickness, extent and direction of sinking of the sole of large geological bodies of acidic composition and to isolate in the earth’s crust large-amplitude thrusts with a length of hundreds of kilometers.

The seismic-gravity transect of Earth's crust beneath the Siberian platform and Central Mongolia and its geological implication

Low-velocity layers in the Earth’s crust, related to large-scale thrusts corresponding to the suture zones, can be as the sign of perspective of this territory on the deposits of minerals and recommendation to more detailed researches