Home – Home – Jornals – Russian Geology and Geophysics 2003 number 5

The morphology, structural setting, and evolution of the Baikal rift system has been controlled by its position at the junction of the Siberian craton and the Central Asian mobile belt, two major tectonic units with contrasting thermomechanical properties. Rifting initiated in South Baikal, in place of the present Selenga delta, where first Late Cretaceous-Paleocene pulses of extension produced a large basin. The basin began to draw in the regional surface runoff as the Selenga valley broke through the Khamar-Daban Ridge and captured the drainage of Western Transbaikalia and Northern Mongolia inherited from the Late Mesozoic. Rifting propagated on both sides off South Baikal as far as the youngest rift basins and faults in Mongolia in the southwest and in the Olekma region in the northeast. The Baikal basin includes two rather than three structurally equal subbasins, South and North Baikal, separated by a diagonal link of Olkhon island - submerged Akademichesky Ridge - Ushkan'i isles. The South Baikal basin is in turn bisected by the Selenga saddle, the oldest and largest deposition center filled with about 10,000 m thick sediments strongly deformed in Pliocene-Quaternary time. The neotectonics, crustal thickness, and 3D velocity structure of the region between the rift and the India/Eurasia collision front indicate that rifting in East Siberia evolved under the joint effect of local and far-field geodynamic mechanisms.

The bottom sediments of Lake Baikal were studied, with the emphasis placed upon the conditions of sedimentation in the Holocene and material composition of the sediments. On the basis of study of spatial distribution of sediments in all parts of the lake, six characteristic zones of sedimentation have been recognized. Lithological types of sections of the modern bottom deposits of Baikal are described in detail. Much attention is given to the description of turbidite sedimentation as one of the main mechanisms of formation of bottom sediments in the Holocene. Some features distinguish turbidites from sediments accumulated under calm conditions. It has been established that buried relics of oxidized Fe-Mn formations occur not only in the northern basin of the lake but also in the southern basin.

A hierarchic systematics of granitic pegmatites is proposed, which is based on the factors of different ranks responsible for characteristics of pegmatite fields as a whole and of individual pegmatite bodies. Three main levels of classification are as follows: formations (subformations) - minerogenic (geochemical) evolution sequences - paragenetic types of pegmatites. The pressure under which pegmatites began to crystallize was taken as the main discriminating factor for pegmatitic formations (subformations). In general, five pegmatitic formations are recognized, combined into three groups. The low-pressure group (<2.5 kbar) includes crystal-bearing and rare-metal-rare-earth formations. The crystal-bearing formation is subdivided into fluorite-rock-crystal-bearing and subrare-metal subformations. The rare-metal formation of moderate pressure (2-5 kbar) is subdivided into petalite and spodumene subformations. The mica-bearing and feldspar formations are included into the group of high-pressure formations (>5 kbar). The mica-bearing formation is divided into rare-metal-muscovite and muscovite subformations. Several minerogenic (geochemical) evolution sequences in every formation (subformation) are recognized. And every evolution sequence combines several paragenetic types of pegmatites - from simple (barren) to most productive for a certain mineral.
Pegmatites with primary (residual) mineralized cavities (miaroles, pockets) are understood as miarolitic facies, peculiar to a varying degree to every pegmatitic formation. The intensity of miarolitic facies manifestation decreases from low-pressure crystal-bearing to high-pressure mica-bearing and feldspar formations. The presence of residual miaroles is not sufficient evidence for referring pegmatites to low-pressure (shallow) formations.

We studied interstitial glasses and melt inclusions in minerals of peridotites from Pliocene basanites of the Vitim volcanic field, Dzhilinda River basin. The glasses can be divided into two groups: A and B. The glasses of group A resulted from deep-level partial melting of mantle, and those of group B are the interaction products of the host basanite melt and peridotites. The heated glasses of group A in xenolith minerals are compositionally similar to experimental and model partial (<10%) melts of peridotites obtained at 10 kbar. They contain (wt.%): SiO₂ = 51-56, TiO₂ = 0.5-0.6, Al₂O₃ = 12-16, Cr₂O₃ = 0-0.5, FeO = 2-4, MgO = 7-10, CaO = 6-12, Na₂O = 3-7, and K₂O < 1.5. Glasses from Ti-rich peridotites contain 1.9-3.5 wt.% TiO₂. Glasses of group B have similar contents of SiO₂, Al₂O₃, and Na₂O, high TiO₂ and K₂O contents, and low Mg-number. Interstitial glasses of this group are abnormally enriched in K₂O (up to 10 wt.%). Glasses in spinel harzburgite are SiO₂-richest (up to 71 wt.%); their alkali concentration decreases with increasing SiO₂ content. These data are consistent with the model for the reaction of xenolith orthopyroxene with basaltic (or hybride) melt, producing olivine and medium- and high-silica glass. The regular compositional variations of olivine microlites with high Mg-number (up to 94) and coexisting glass suggest that they are in equilibrium. The coefficients of Fe/Mg partition between them are 0.25-0.33.

The available data show that the hydrogeology of the Nyurol'ka basin is characterized by widespread connate waters, in places diluted by ancient percolating waters. This basin has vertical hydrodynamic and hydrogeochemical zoning. Combined with elision, this favors oil and gas formation. We have established that the Lower-Middle Jurassic sediments are fed by their own saline waters as well as by the waters of Paleozoic and Upper Jurassic complexes at the sites where permeable sediments exist. This explains mixing of different genetic types of waters in the sediments. We have first revealed the existence of vertical water cross-flows between aquiferous complexes, which generally does not disturb their hydrodynamic isolation.

This is the second paper on the history of multiwave seismic exploration in Russia in the past fifty years. It considers the behavior of compressional, shear, and converted waves and their synthetic interpretation. Multiwave seismic exploration is analyzed in application to various geological targets, including petroleum prospecting.

A new teleseismic tomography approach (RR-R scheme), based on joint processing of differential travel times of P or S refracted rays and corresponding PP or SS rays with bounce points in a selected area, allows three-dimensional velocity images of upper mantle in aseismic regions with poor seismological coverage. The approach makes it possible to avoid the difficulty of source and station corrections which cause problems in teleseismic tomography. The RR-R scheme has been applied to more than 10,000 ray pairs from the ISC database to investigate a large territory from the North Arctic Ocean to northern China and Mongolia. Inversion was carried out in several separate blocks that cover the territory, and velocity anomalies were computed in grid nodes distributed in the 3D region according to ray density. The results from different blocks were synthesized into general maps and cross sections. The lithospheric structure imaged by the inversion includes positive P-wave velocity anomalies associated with the Precambrian Siberian craton and Tuva, negative anomalies corresponding to the thin lithosphere of the West Siberian Plate, and an intense negative anomaly beneath the Hangayn mountains, possibly, produced by a mantle plume.

Some stratigrapho-paleogeographic discrepancies have been removed from the interpretation of spatial-temporal relationship between leading factors of morpholithogenesis in the Late Pleistocene of West Siberia. New data obtained from mammoth remains indicate that there was no ice-dammed basin on the West Siberian Plain in the Sartan time. A hypothesis has been formulated that the Karga marine deposits occur in overdeepened lows, tens of meters below the paleolevel of the ocean, which formed when the ice-dammed waters broke the ice dam to run northward. This refines the configuration of the Karga marine transgression whose deposits are mapped on the north of West Siberia in accordance with a recent stratigraphic scheme. The Late Pleistocene and Holocene of West Siberia include relatively long stages of stable and metastable ecogeological settings alternating with short epochs of drastically contrasting transformations of paleolandscapes.