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Russian Geology and Geophysics

2003 year, number 4

1.
HISTORY OF MULTIWAVE SEISMIC EXPLORATION IN RUSSIA

N.N. Puzyrev
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Multiwave seismic exploration, excitation and recording of shear and converted waves
Pages: 269-275

Abstract >>
The paper presents a historic account of multiwave seismic exploration in Russia for the past fifty years: the origin of the method, its development in different institutions, and principal results concerning excitation and recording of shear and converted waves.
																								



2.
SIMULATION OF STRESSES IN THE HEATING LITHOSPHERE OF THE BAIKAL RIFT AT THE ONSET OF RIFTING

A.N. Adamovich, S.I. Sherman, and S.V. Ivanova
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Stress field, thermal anomaly, finite element simulation, Baikal rift
Pages: 276-284

Abstract >>
Evolution, space and time dynamics of stresses, geometry of basins, and other particular features of the Baikal rift can be explained in the context of stress changes associated with rifting initiation. Rifting may have been triggered by local heating of the 70 km thick lithosphere from a sublithospheric mantle plume. The impact of heating necessary to initiate extension has been estimated in terms of a thermoelastic loading model for rift-orthogonal plane strain. Finite element simulation using the NASTRAN software reveals a mosaic stress field in heating lithosphere. The existence of different stress zones may account for the rift asymmetry, initiation and/or development of major faults, horizontal layering of lithosphere, attenuated crust, etc. The plane strain finite element model of lithosphere subjected to thermal loading provides a sound clue to understanding the driving mechanism for rift formation.
																								



3.
METAMORPHIC ZONING OF THE TONGULAK MOUNTAIN RANGE, ALTAI: MATHEMATICAL MODELING

V.A. Anan'ev, O.P. Polyansky, G.G. Lepezin, and V.V. Reverdatto
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Low-pressure high-temperature metamorphism, magmatic heat source, mineral geothermometry, mathematical simulation
Pages: 285-294

Abstract >>
The Tongulak metamorphic complex lies in southeastern Gorny Altai, in the Bashkaus-Kubadru interfluve. Four zones have been recognized in its composition: chloritic (on the periphery), cordieritic, sillimanitic, and staurolite-free. The metamorphic zoning is symmetrical, with a general width of 13-15 km. Conditions of metamorphism are estimated to have been 500-700
																								



4.
MINERAL REACTIONS IN HIGH-ALUMINA FERRIFEROUS METAPELITIC HORNFELSES: THE PROBLEM OF STABILITY OF RARE PARAGENESES OF CONTACT METAMORPHISM

I.I. Likhanov
Institute of Mineralogy and Petrography, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Metapelites, chloritoid, mineral reactions, petrogenetic grids
Pages: 295-306

Abstract >>
A detailed petrological study has been given to high-alumina ferriferous hornfelses characterized by the development of mineral associations (Cld+Bt, Cld+Bt+And, and Crd+Grt+Ms), quite rare for contact metamorphism. The sequence and stability of the observed parageneses were thoroughly analyzed in terms of the Harte-Hudson, Spear-Cheney, and Powell-Holland petrogenetic grids. It has been shown that in the contact aureole of the Ayakhta Massif: (1) the formation of chloritoid atypical of thermal metamorphism and the stability of rare parageneses (Cld+Bt and Cld+And+Bt) are explained by a rare combination of appropriate pressure (3 kbar) with a specific type of rocks enriched in both Al and Fe; (2) the appearance of the Grt+Crd+Ms paragenesis is caused by specific chemical compositions of Mn-enriched garnet and high-Al high-Fe rocks; (3) the development of Crd-And parageneses and staurolite-free associations at the intermediate steps of contact metamorphism is explained by the simultaneous expansion of the Grt+Chl field with participation of Mn-garnets and narrowing, up to complete disappearance, of the St+Bt stability field; (4) the sequence of the observed parageneses and reactions in high-Al Fe-rich hornfelses of the Ayakhta aureole is in agreement with the Spear-Cheney thermodynamic petrogenetic grid.
																								



5.
A MAGMATIC EPISODE IN THE WESTERN RIFT OF EAST AFRICA (19-17 Ma)

S.V. Rasskazov, N.A. Logachev+, A.V. Ivanov, A.A. Boven*, M.N. Maslovskaya, E.V. Saranina, I.S. Brandt, and S.B. Brandt
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
* Vrije Universiteit Brussel, 2, Pleinlaan, Brussels, 1050, Belgium
Keywords: East African rift system, Cenozoic, volcanic rocks, 40Ar/39Ar, K-Ar, and Rb-Sr geochronology
Pages: 307-314

Abstract >>
A brief 19-17 Ma episode that marks the onset of magmatism has been dated from Rb-Sr isochron and 40Ar/39Ar geochronology of lavas from the Rungwe volcanic province. According to 40Ar/39Ar and preliminary K-Ar geochronology, volcanism and rifting in East Africa initiated in the Eocene in the Turkana depression between the Ethiopian and East African plateaus. A strong thermal impact from a mantle plume impinged on the lithosphere beneath the Ethiopian plateau caused numerous volcanic eruptions along the central segment of the Ethiopian rift about 30 Ma ago and sublithospheric thermal erosion on the periphery of the East African plateau, since 23 Ma in the Kenya rift and since 19 Ma in the Western rift.
																								



6.
GEOLOGY, CHEMISTRY, AND GENESIS OF Ba-Sr-BEARING (

N.V. Vladykin and I.I. Tsaruk*
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
* Sosnovgeologiya Enterprise, 53 ul. Gogolya, Irkutsk, 664039, Russia
Keywords: Carbonatites, alkaline magmatism, differentiation processes, isotope geochemistry
Pages: 315-330

Abstract >>
The Malyi Murun Massif is a unique rock formation: This is the largest alkaline K-agpaite massif having no analogs in the world. Almost all its rocks are promising for various ores. Therefore, it is of great importance to exactly reconstruct the subsequence of magmatic events here, elucidate the factors responsible for the great variety of rocks in the massif, and determine the PT-conditions of their formation.
The proposed new scheme of magmatism of the Murun Massif explains the formation of a complete series of K-alkaline rocks - from alkaline-ultrabasic through basic and normal to silicic (alkali granites) - and a residual silicate-carbonate differentiate here. We suggest that these rocks resulted from magmatic differentiation and liquid segregation of a peralkaline magma. The residual fluxed melt is also subjected to segregation; it gave rise to unique charoite rocks and
																								



7.
DEPOSITS OF PRECIOUS OPAL: GENESIS AND SEARCH CRITERIA

D.V. Kalinin and V.V. Serdobintseva
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Opal-forming solutions; generation, growth, and supramolecular crystallization of silica spheres; search criteria for deposits
Pages: 331-337

Abstract >>
Origin, growth, and steady existence of monodisperse silica spheres (MSS) composing precious opal are possible solely in concentrated alkaline solutions of silica. In Australian deserts, such solutions result from dissolution of alkaline dust in periodic shower pre- cipitation. The dust forms on decomposition of alkaline minerals and accumulates on their surface under conditions of hot and dry climate with drastic daily temperature variations. Alkaline solutions percolate down through high-porosity and fracture zones and thus give rise to MSS, which then grow, form suspensions, and undergo gravitational separation by size. At depths of 20-30 m, the suspensions are subjected to sedimentational concentration in fractures and cavities, followed by their supramolecular crystallization to opal structures. Subsequent silicification and transformation of these structures into solid stone are due to the action of colloidal solutions of silica that form from MSS suspensions. Zones of depthward percolation of alkaline solutions are always conjugate with evaporation zones of ascending flows of solutions responsible for the surficial silicification and deposition of Solonchak salts. Search for precious-opal deposits must be based first of all on alkali mapping of weathered surficial rocks in deserts as well as on geomorphological and mineralogical reconstruction of the local hydrogeologic environment within alkali spots during the period of opal formation.
																								



8.
MICROPHYTOFOSSILS AND MAGNETOSTRATIGRAPHY OF UPPER CRETACEOUS AND CENOZOIC DEPOSITS OF THE SOUTHEASTERN WEST SIBERIAN PLAIN

O.B. Kuz'mina, V.S. Volkova, Z.N. Gnibidenko*, and N.K. Lebedeva
United Institute of Geology, Geophysics, and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
*Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Palynology, dinoflagellates, spores, pollen, magnetostratigraphy, magnetozone, Upper Cretaceous, Cenozoic, Upper Ob' region, West Siberia
Pages: 338-352

Abstract >>
A combined palynological and paleomagnetic study of core samples of Upper Cretaceous and Cenozoic deposits from four boreholes drilled in the Upper Ob' region has been carried out to provide materials for the State Geological Map-200. Palynological and paleomagnetic descriptions have been performed. Fourteen spore-and-pollen assemblages have been recognized in the deposits under study. They characterize the marine Eocene (Lyulinvor and Tavda Horizons), continental Paleogene (Atlym, Novomikhailovka, and Zhuravka Horizons), and Neogene (Abrosimovka, Beshcheul, Tavolzhan, and Pavlodar Horizons). Features of flora changes at Cenozoic borderlines (Eocene-Oligocene, Oligocene-Miocene, and Middle-Late Miocene) and their stratigraphic significance are outlined. A paleomagnetic section has been compiled for borehole 10. It includes 19 major magnetozones of different polarities: nine in the Paleogene, eight in the Neogene, and two in the Quaternary. They correspond to the rank of orthozones, paleobotanically characterized and referred to the regional stratigraphic scheme. Some of these orthozones are referred to the Berggren time scale. All these data allowed subdivision of the Upper Cretaceous and Cenozoic deposits under consideration, estimation of the completeness of the section, rock dating, and refinement of the stratigraphic volume of the Upper Cretaceous and Cenozoic formations and regional horizons exposed in the boreholes.
																								



9.
THE PLEISTOCENE SUBAERIAL SEQUENCE AND ENVIRONMENTAL HISTORY OF WEST SIBERIA

I.A. Volkov
Institute of Oil and Gas Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Stratigraphy, paleogeography, glaciation, radiocarbon dating, paleopedology, oceanic muds, West Siberia
Pages: 353-360

Abstract >>
The Pleistocene subaerial sequence of West Siberia and other regions of temperate climate formed in a continuous-discontinuous mode during late stages of the Matuyama paleomagnetic epoch and the whole Brunhes epoch. This sequence has a cyclic structure. Each cyclite includes three climatoliths, which reflect changes from warm to cold environment. Comparison of the structures of subaerial and other deposits indicates that the subaerial sequence reflects multiple cyclic changes of environment from warm-arid (loess deposits) to warm humid (soil) and then to cold humid (overmoistening and cryogenic deposits).
The recent upper cyclite, belonging to deglaciation and the Holocene, is not complete. The present epoch of soil formation will give way to cooling with the appearance of permafrost in the nearest geologic future. Comprehensive investigation of subaerial deposits is the main line of inquiry in the development of the glaciation theory. It is important to correlate these deposits with the processes occurring in the off-glacier, near-glacier, and glacier zones. All predictions of the future climate should take into account the knowledge of subaerial deposits.
																								



10.
HOLOCENE DEFORMATION IN THE WESTERN END OF THE TUNKA SYSTEM OF RIFT BASINS (Southwestern Baikal Rift)

A.V. Arzhannikova, C. Larroque*, and S.G. Arzhannikov
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664 033, Russia
*Geosciences Azur CNRS UMR Scientific Affiliation, Valbonne, 6526, France
Keywords: Tectonic deformation, fault kinematics, earthquake, strain field
Pages: 361-367

Abstract >>
We investigate the fault geometry and kinematics of Holocene active compressional structures in the Mondy basin and its surroundings (western termination of the Tunka system of rift basins) and continue the discussion on the M = 7 Mondy earthquake of 4 April 1950. The Holocene reverse-oblique motions along the W-E trending Mondy and Ikh-Ukhgon faults are interpreted in the context of the Late Cenozoic evolution of a flower structure, on the basis of deciphered digital SPOT (10 m resolution) and Landsat (30 m resolution) imagery and field geomorphological measurements.
																								



11.
LATE HOLOCENE VEGETATION AND CLIMATE IN THE EAST SAYAN MOUNTAINS

E.V. Bezrukova, K.E. Vershinin*, L.A. Orlova*, P.P. Letunova, S.M. Krapivina, V.V. Chepinoga**, A.V. Verkhozina***, N.V. Dudareva***, and A.A. Abzaeva
Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
*United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
**Irkutsk State University, 1 ul. Marksa, Irkutsk, 664003, Russia
***Siberian Institute of Physiology and Biochemistry of Plants, Siberian Branch of the RAS,
132 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Holocene, plant communities, humidity changes, short climatic episodes, highlands of southern East Siberia
Pages: 368-372

Abstract >>
Late Holocene vegetation and climate have been reconstructed from pollen spectra and plant remnants in peat core section near Lake Yarma, 1320 m asl (Oka Ridge, East Sayan mountains, southern East Siberia), and 14C dating. The reconstructed evolution of local and regional vegetation records alternating humid and arid climate cycles through the past 6 kyr. The reconstruction of short climatic episodes in the historic (Fernau) stage of glacier activity about 2750 years BP and in the Little Ice Age about 500 years BP have been obtained for the first time for this territory.