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2006 year, number 10

1.
BARGUZIN MICROCONTINENT ( Baikal mountain area ): THE PROBLEM OF OUTLINING

V.G. Belichenko, N.K. Geletii, and I.G. Barash
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Microcontinent, terrane, basement, sedimentary cover, island arc
Pages: 1035-1045

Abstract >>
The term Barguzin microcontinent is well-known in the geological literature and is widely used in paleogeodynamic reconstructions of the Paleoasian ocean, though the problem of its outlining is still debatable. Different authors draw the boundaries of the Barguzin microcontinent in different ways. Most of the authors believe that the Barguzin microcontinent occupies the central part of the Baikal orogenic system. Therefore, it seems reasonable to examine the structure of the Baikal orogen in terms of terrane analysis and, on the basis of the obtained data, to outline the Barguzin microcontinent. It is shown that the distinguished Barguzin microcontinent is related to the Baikal orogen terranes. The Baikal-Muya terrane, which was accreted to the craton before the Vendian, is not considered part of the Barguzin microcontinent. Its main components might be the Barguzin and Ikat terranes, but their specific structure precludes their identification with a representative reference, the Tuvino-Mongolian microcontinent. The Barguzin and Ikat terranes were, most likely, formed under the conditions of back-arc basin.



2.
THE NATURE OF CENOZOIC UPPER MANTLE PLUMES IN EAST SIBERIA ( Russia ) AND CENTRAL MONGOLIA

Yu.A. Zorin, E.Kh. Turutanov, V.M. Kozhevnikov, S.V. Rasskazov, and A.I. Ivanov
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Upper mantle plumes, subducting slab, mantle transition zone, fertilized peridotite, asthenospheric convection
Pages: 1046-1059

Abstract >>
We discuss the space relationship between upper mantle plumes revealed earlier from analysis of long-wavelength isostatic gravity anomalies and the subducting Pacific slab. According to global seismic tomography, the oceanic slab in its segments corresponding to the Japan and Izu-Bonin island arcs flattens out at the bottom of the mantle transition zone, extends horizontally far beneath Eurasia, and then resumes sinking into the lower mantle. The upper mantle plumes are located beyond the western endpoint of the slab sector that advances the farthest beneath the continent.
A considerable part in the plume material may belong to fertilized (enriched with incompatible elements) peridotite. A layer of fertilized peridotite forms at depths between 200 and 600 km under the effect the melts produced by partial melting of the slab oceanic crust cause on the overlying depleted mantle. The peridotite layer integrates into the slab and heats up by friction along the slab top during the horizontal motion of the latter in the transition zone where the mantle material is of relatively high strength. Portions of hot fertilized peridotite detach from the slab as it sinks into the lower mantle, rise by buoyancy through the upper part of the transition zone, and become entrained into an elongate asthenospheric convection cell which arises beneath the continent behind the subduction zone. The ascending convection flow splits into separate streams which are the upper mantle plumes.



3.
EMPIRICAL GARNET THERMOBAROMETER FOR MANTLE PERIDOTITES

I.V. Ashchepkov
Institute of Geology and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Thermobarometry, mantle, kimberlites, xenoliths, peridotites, garnet, xenoliths
Pages: 1060-1074

Abstract >>
A new version of garnet thermobarometer is proposed. It ensures better correlations of TP-estimates with results of pyroxene-pyrope thermobarometry than the previous version proposed by Ryan et al. The dependence of KD of the internal exchange reaction, KD = MgO · TiO2/ ((CaO+ MgO)2 · FeO · Al2O3), on temperature estimated with the Brey and Kohler orthopyroxene thermometer ( R = 0.783) is as follows: T (



4.
ELECTRON PROBE MICROANALYSIS OF PYROPE FOR NICKEL TRACES AS APPLIED TO STUDY OF THE GEOTHERMOMETRY OF PERIDOTITES

Yu.G. Lavrent'ev, V.N. Korolyuk, L.V. Usova, and A.M. Logvinova
Institute of Geology and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Geothermometry, electron probe microanalysis, peridotites, pyropes
Pages: 1075-1078

Abstract >>
Nickel-pyrope geothermometer, based on proton microprobe determination of Ni contents in pyropes, has been reproduced by applying electron probe microanalysis (EPMA). The measurements were made on a JEOL JXA-8100 microprobe. The detection limit of 6ppm was reached, which is nearly a record value for EPMA. Precision standard deviation is 3ppm. The accuracy of determination, estimated as the difference between the data obtained by the PIXE and EPMA methods, is 9 ppm, which corresponds to a temperature standard deviation of about 50



5.
AN INSTABILITY GRAVITY MODEL FOR THE STRUCTURE AND STRESS-STRAIN STATE OF LITHOSPHERE IN THE BAIKAL RIFT

S.V. Gol'din, V.D. Suvorov, P.V. Makarov*, and Yu.P. Stefanov*
Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Institute of Physics of Strength and Materials Science, Siberian Branch of the RAS,
2/1 prosp. Akademichesky, 634021, Tomsk, Russia
Keywords: Lithosphere, seismic velocities, density, gravity instability, elastic-brittle-plastic strain, Baikal rift
Pages: 1079-1090

Abstract >>
The stress-strain state of undeformed but gravity-strained lithosphere is simulated in a 2D numerical model in the elastic-brittle-plastic approximation using combined seismic and density cross sections to depths of 80 km. The cross sections follow DSS profiles across the Baikal rift (Ust'-Uda-Khilok) and along the northeastern flank of the rift system (Ust'-Kut-Nizhneangarsk-Chara). Gravity instability is produced by a zone of anomalous mantle and the 10-12 km deep Baikal basin which have low elastic moduli and density.
We estimated the depth-dependent variations in shear strength and internal friction coefficient required for strain in the upper crust to fit the crustal density and velocity distributions and the observed Cenozoic shallow geological and tectonic structures. The greatest stress-strain contrasts are attributed to zones of most prominent density contrasts and show up in the horizontal stress component to 0.15 GPa. Relative strain in the crust and at the Moho reaches 10-15%. Gravity instability may have contributed significantly to lithospheric deformation and rift evolution.



6.
METHOD OF CONTRACTING MAPPING FOR ESTIMATING GREEN'S TRANSFER FUNCTIONS IN ELECTRICAL PROSPECTING

L.F. Moskovskaya
Sevmorgeo Association, 36 ul. Rozenshteina, St. Petersburg, 198095, Russia
Keywords: Electromagnetic field, approximation of functions, contracting mapping
Pages: 1091-1097

Abstract >>
The method of contracting mapping of the function space is suggested as a tool of spatial approximation of electromagnetic fields. This transformation is physically justified being associated with the dynamics of space field variation. Formulated are the basic requirements for scale transformation of the solution space. Three basic types of contracting mapping are applied to some linear-symmetry VES problems. A special algorithm is suggested for smoothness control of signals and automated filling of the space with additional reference points in intervals of strongly varying field components. The new approach allows high-performance modeling in problems that require high-density recovery of functions.



7.
THE POSSIBLE EXISTENCE OF AN ENDOGENOUS SOURCE OF FRESH WATERS IN RIFT SETTINGS

Yu.N. Didenkov, V.A. Bychinsky*, and I.S. Lomonosov*
Irkutsk State Technical University, 83 ul. Lermontova, Irkutsk, 664074, Russia
* Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Rift structures, structure-hydrogeological analysis, hydrocarbon fluid, physicochemical modeling
Pages: 1098-1102

Abstract >>
The rifting in the Baikal region predetermined the existence of three types of hydrogeologic structures: rift depressions - hydrogeologic basins; rift