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Russian Geology and Geophysics

2015 year, number 7

1.
EXTRATERRESTRIAL FACTORS AND THEIR ROLE IN THE EARTHS TECTONIC EVOLUTION IN THE EARLY PRECAMBRIAN

M.Z. Glukhovskii1, M.I. Kuzmin2
1Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia
2A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Early Earth, lunar-terrestrial interaction, impact events, tectonic evolution

Abstract >>
The paper is focused on the fundamental problem of influence of extraterrestrial factors on the Earths geologic and tectonic evolution. Extraterrestrial factors played a decisive role in the Earths genesis, the formation of the first Hadean continental crust, and the beginning of the Archean era. Their significant influence persisted in the later epochs: Even in the Phanerozoic, extraterrestrial factors might have had a considerable influence on the environment. The sialic cores of protocontinental crust (4.4-3.9 Ga) with first-generation greenstone zones (3.8-3.2 Ga) and the global system of granite-greenstone belts (3.1-2.7 Ga) formed in the rotation-plume regime, mainly in the subequatorial hot belt. The formation of these global structures was, to a large extent, influenced by asteroid impacts, which caused the impact-triggered genesis of mantle plumes. Dramatic changes in the subsequent geologic history began at 2.7-2.0 Ga; at 2.0 Ga they terminated with the Moons transition to an orbit similar to the present-day one (50 3 Earths radii), accompanied by the abrupt slowdown of the Earths axial rotation, the termination of formation of the layer D″, and the start of recent plate tectonics, which is accompanied by the plume tectonics.



2.
EARLY PRECAMBRIAN EARTH HISTORY: PLATE AND PLUME TECTONICS AND EXTRATERRESTRIAL CONTROLS

N.L. Dobretsov1,2, O.M. Turkina3,2
1pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Hadean, Archean, Moon orbit, impact process, subduction, mantle plume, ophiolite, TTG complexes, komatiite

Abstract >>
The Hadean and Archean geologic history of the Earth is discussed in the context of available knowledge from different sources: space physics and comparative planetology; isotope geochronology; geology and petrology of Archean greenstone belts (GB) and tonalite-trondhjemite-granodiorite (TTG) complexes; and geodynamic modeling review to analyse plate-tectonic, plume activity, and impact processes. Correlation between the age peaks of terrestrial Hadean-Early Archean zircons and late heavy bombardment events on the Moon, as well as the Hf isotope composition of zircons indicating their mostly mafic sources, hint to an important role of impact processes in the Earths history between 4.4 and 3.8 Ga. The earliest continental crust (TTG complexes) formed at 4.2 Ga (Acasta gneisses), while its large-scale recycling left imprint in Hf isotope signatures after 3.75 Ga. The associations and geochemistry of rocks suggest that Archean greenstone belts formed in settings of rifting, ocean floor spreading, subduction, and plume magmatism generally similar to the present respective processes. The Archean history differed in the greater extension of rocks derived from mantle plumes (komatiites and basalts), boninites, and adakites as well as in shorter subduction cycles recorded in alternation of typical calc-alkaline andesite-dacite-rhyolite and adakite series that were generated in a hotter mantle with more turbulent convection and unsteady subduction. The Archean is interpreted as a transient period of small plate tectonics.



3.
BOTTOM SEDIMENTS AND NEAR-BOTTOM CURRENTS IN THE SOUTHWESTERN ATLANTIC

E.M. Emelyanov1,2
1Immanuel Kant Baltic Federal University, ul. A. Nevskogo 14, Kaliningrad, 236041, Russia
2P.P. Shirshov Institute of Oceanology, Atlantic Branch, Russian Academy of Sciences, pr. Mira 1, Kaliningrad, 236022, Russia
Keywords: Bottom sediments, near-bottom currents, sediment erosion, Brazil Basin, Atlantic

Abstract >>
On the basis of the authors data on the composition of sediments and seismic cross sections, together with literature data, the bottom topography was described and the main structural features of the top 10-100 m thick sedimentary sequence in the Southwestern Atlantic (Brazil Basin) were identified. The presence of a heavy northward flow of Antarctic bottom water (AABW) and its active erosive activity were confirmed. The AABW caused the erosion or redeposition of red pelagic clays and hemipelagic clays, which accumulated in the Brazil Basin in the Holocene and Pleistocene; the clays contain abundant redeposited Pleistocene diatoms and Neogene and Paleogene discoasters. In most of the sediment columns of the Brazil Basin, the red pelagic clays are of Pleistocene age. Contourites and sandy microbeds have been found in the sediments at the foot of the continental slope of South America; this is the effect of the Deep Western Boundary Current on the ocean floor. The AABW transfers Antarctic diatom species along the continental slope of South America to 10º-5º S. The presence of the Equatorial Midocean Channel with a relative depth of 149 m in the western pelagic equatorial part of the Atlantic was confirmed, and new channels, such as Vavilov and Akademik Ioffe, have been found. The AABW flows northward along the Equatorial Mid-Ocean Channel. Apparently, the Akademik Ioffe Channel is not a proper mid-ocean channel. At 20º S (at a depth of 5000 m), Pleistocene diatomaceous ( Ethmodiscus rex ) ooze containing up to 74% amorphous SiO 2 was detected. On the Amazon-Mid-Atlantic Ridge profile, the AABW flows into the Guyana Basin through only one valley of the Nara Plain, with a depth of 4620 m. Near the Ceara Rise and on the Amazon Fan, no geologic traces of the AABW flow into the Guyana Basin were found. Near the Rio Grande Rise, the AABW might have appeared in the Eocene. The formation of the Vema Channel, which separates the Rio Grande Rise from South America, also began at that time. The AABW flows were the heaviest before the largest glaciations (particularly at isotopic stages 7/6 and 3/2).



4.
MOBILITY OF ELEMENTS IN A CONTINENTAL SUBDUCTION ZONE: EVIDENCE FROM THE UHP METAMORPHIC COMPLEX OF THE KOKCHETAV MASSIF

V.S. Shatsky1,2,3, S.Yu. Skuzovatov1, A.L. Ragozin2, N.V. Sobolev2,3
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Continental subduction, UHP metamorphism, mobility of elements

Abstract >>
We studied clastics of high-alumina garnet-kyanite-mica schists and garnet-kyanite-quartz granofelses, including diamond-bearing ones, found in the eluvial sediments near Lake Barchi. In contents of major elements the studied rocks correspond to argillaceous shales. The garnet-kyanite-quartz granofelses are poorer in K (0.49-1.35 wt.% K2O) than the garnet-kyanite-mica schists (4.9-2.2 wt.% K2O) but have the same contents of other major components. The REE patterns of most of the garnet-kyanite-phengite schists are similar to those of the Post-Archean Australian Shale (PAAS) (xLa/Yb = 13). All garnet-kyanite-quartz rocks are much stronger depleted in LREE (xLa/Yb = 1.4) and other incompatible elements. Our studies show that allanite and monazite are the main concentrators of LREE and Th in the garnet-kyanite-phengite rocks of the Barchi site. Monazite, occurring as inclusions in garnet, contains not only LREE but also Th, U, and Pb. Rutile of the nondepleted rocks is enriched in Fe and Nb impurities only. The garnet-kyanite-quartz granofelses bear rutile, apatite, and xenotime as accessory phases. Rutile of the depleted rocks shows wide variations in contents of Nb, Ta, and V impurities. In places, the contents of Nb and Ta reach 10.5 and 2.3 wt.%, respectively. The rutile decomposes into rutile with Nb (1.4 wt.%) and Fe (0.87 wt.%) impurities and titanium oxide rich in Fe (6.61 wt.%), Nb (up to 20.8 wt.%), and Ta (up to 2.81%) impurities. Based on the measured contents of incompatible elements in differently depleted high-alumina rocks, the following series of element mobility during UHP metamorphism has been established: Th > Ce > La > Pr > Nd > K > Ba > Rb > Cs > Sm > Eu. The contents of U, P, and Zr in the depleted rocks are similar to those in the nondepleted rocks. The studies have shown that metapelites subducted to the depths with diamond stability can be depleted to different degrees. This might be either due to their exhumation from different depths of the subduction zone or to the presence of an external source of water controlling the temperature of dissolution of phengite and the formation of supercritical fluid/melt.



5.
POLYGENESIS OF MAFIC-ULTRAMAFIC COMPLEXES: ISOTOPE-GEOCHRONOLOGICAL AND GEOCHEMICAL EVIDENCE FROM ZIRCONS OF THE BEREZOVKA MASSIF ROCKS (Sakhalin Island)

F.P. Lesnov1, V.V. Khlestov1,2, V.G. Galversen3, S.A. Sergeev4,5
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3Sakhalin Geological-Prospecting Expedition, ul. A. Matrosova 28, Yuzhno-Sakhalinsk, 693005, Russia
4A.P. Karpinsky Russian Geological Research Institute, Srednii prosp. 74, St. Petersburg, 199106, Russia
5St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia
Keywords: Zircons, U-Pb age, SHRIMP II, geochemistry, LA-ICP-MS, gabbroids, ultramafic rocks, ophiolites, Berezovka massif, Sakhalin Island

Abstract >>
Results of comprehensive isotope-geochronological (U-Pb and SHRIMP II dating) and geochemical (LA-ICP-MS) studies of zircons from different rocks of the Berezovka polygenetic mafic-ultramafic massif of the East Sakhalin ophiolite association are presented. The massif includes three proximal but genetically autonomous structure-lithologic complexes of different ages: protrusion of ultramafic rocks of restite nature, gabbroid intrusion breaking through it, and contact reaction zone located along their boundaries. The isotopic age of zircons in the massif as a whole and in its individual rocks varies over a broad range of values. The zircons belong to several populations according to their age (Ma) and other features: relict and xenogenous (~3100-990, 70-410, and ~395-210) and syngenetic (~200-100, ~90-65, and ~30-20). They differ in grain size and morphology, optical and cathodoluminescence images, and trace-element patterns. By morphology, the grains are divided into short-prismatic crystals with well-developed faces and edges, long-prismatic crystals with well-developed faces and edges, prismatic crystals with slightly resorbed faces and edges, prismatic crystals with strongly resorbed faces and edges, and intensely resorbed grains totally or partly lacking faceting. The ages of zircons depend inversely on the contents of La, Ce, and Yb, total contents of REE, (Ce/Ce*) n , and (Eu/Eu*) n . Some grains are characterized by abnormal REE and trace-element patterns due to their epigenetic redistribution. The wide scatter of intermediate ages of the relict and xenogenous zircon grains, their resorption and disturbed optical and geochemical features are probably due to the nonuniform rejuvenation of their isotope systems and variations in other parameters, caused by the effect of younger mafic melt and its fluids, whose crystallization gave rise to a gabbroid intrusion dated at 170-150 Ma. The obtained data on the isotopic age and other properties of zircons from the Berezovka massif rocks agree with the geological model of its polygenesis.



6.
RELICS OF BIOLIPIDS IN KEROGEN OF CAMBRIAN SIBERIAN PLATFORM OIL SHALE

V.V. Savelev1, V.F. Kamyanov1, A.K. Golovko1,2
1Institute of Petroleum Chemistry, Siberian Branch of the Russian Academy of Sciences, Akademicheskii pr. 4, Tomsk, 634021, Russia
2Tomsk Department of A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Akademicheskii pr. 4, Tomsk, 634021, Russia
Keywords: Organic components of sediments, kerogens, thermolysis products

Abstract >>
Products of thermal cracking of kerogen of Lower-Middle Cambrian oil shale are studied by modern instrumental analytical methods (FTIR, gas chromatography, mass spectrometry, etc.). The analyses demonstrate that the geopolymer kerogen matrix inherited fragments of lipids synthesized by living organisms, including aliphatic fat acids, both free or bounded into glycerol esters, i.e., animal or vegetable fats. It is shown that vegetable pigments (carotenes and xanthophylls) play a crucial role in the formation of monocyclic structures (including molecular alkylbenzene, alkylphenol, and phenylalkane fragments) in kerogen. Since the studied kerogen is rich in esters, it is worth performing alkaline hydrolysis of its macromolecules (ester saponification) before their thermolysis, which inevitably distorts the initial structure of molecular skeletons.



7.
DIELECTRIC-PERMITTIVITY SPECTRA OF OIL-WATER-SATURATED SAND-CLAYEY ROCKS OF DIFFERENT MINERALOGICAL COMPOSITIONS AND RELAXATION PROPERTIES OF WATER IN THESE ROCKS

P.P. Bobrov1, V.L. Mironov2,3, A.V. Repin1
1Omsk State Pedagogical University, Naberezhnaya im. Tukhachevskogo 14, Omsk, 644099, Russia
2L.V. Kirenskii Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk, Akademgorodok 50, str. 38, 660036, Russia
3M.F. Reshetnev Siberian State Aerospace University, pr. im. Gazety Krasnoyarskii Rabochii 31, Krasnoyarsk, 660014, Russia
Keywords: Oil-bearing deposits, sand-clayey rocks, dielectric permittivity, conductivity, multifrequency dielectric relaxation

Abstract >>
Experimental measurements of dielectric permittivity and equivalent conductivity of sand-clayey samples (a mixture of river sand with bentonite or kaolin) saturated with salt-solution-diesel-fuel emulsions were performed in the frequency range from 10 kHz to 1 GHz at temperatures of 25-65 C. It is shown that when the content of the salt solution in the saturating fluid does not exceed 10 %, the dielectric permittivity in the frequency range from 1 MHz to 1 GHz depends little on the mineral composition of the sample and on the concentration of the saturating solution. When the portion of water is 33.3 % or higher, an increase in the concentration of the salt solution leads to an increase in the equivalent conductivity and the real part of the complex dielectric permittivity. Using the refractive model of the complex dielectric permitivity, we have estimated the dielectric properties of bound water, which depend on temperature and the type of clay (bentonite or kaolin) in the sand-clay mixture but are independent of the water saturation and the amount of clay in the sample.



8.
LONG-TERM PREDICTION FOR SEISMIC HAZARD FOR RADIOACTIVE WASTE DISPOSAL

B.T. Kochkin, V.A. Petrov
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017, Russia
Keywords: Seismic hazard, long-term prediction, radioactive waste, geological disposal, safety

Abstract >>
We consider possible approaches to the long-term prediction for seismic hazard in relation to the practical need for the safety of geological disposal of long-lived radioactive waste. The required period of prediction significantly exceeds the one reflected in the set of maps of General Seismic Zoning of the territory of the Russian Federation (GSZ-97). The first geological repository in Russia is planned to be set up in the Nizhnii Kan granite massif in the Krasnoyarsk Krai. This region is an intraplate territory with a relatively high seismic activity. We summarize the analysis of the known empirical generalizations and theoretical principles underlying the seismic hazard prediction. Real seismic events constantly violate forward-looking statements even for relatively short periods of time. These and other arguments suggest that the hypothesis of stationarity of the seismic regime, which is the basis of long-term prediction today, has limited and uncertain applicability in time. Intraplate earthquake prediction is especially uncertain because of the uncertainty in the factor responsible for generating tectonic stresses in these regions. The short horizon of the prediction, based on statistical methods, can be attributed to the nonlinearity of seismic geodynamic processes. Fundamental laws of tectonic processes should be used as the scientific basis for long-term predictions for seismic hazard at the sites chosen for geological disposal of long-lived radioactive waste. These processes can be reflected in models for the migration of the seismically active boundaries of lithospheric plates and the occurrence of seismic activity in intraplate regions.



9.
ACCOUNTING FOR NEAR-SURFACE INHOMOGENEITIES OVER A HORIZONTALLY LAYERED SECTION IN MAGNETOTELLURIC SOUNDING

V.V. Plotkin, D.I. Gubin
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Magnetotelluric sounding, near-surface inhomogeneity, galvanic distortions, electric conductivity, horizontally layered medium, boundary conditions

Abstract >>
This paper presents a numerical model for the effect of near-surface inhomogeneities over a one-dimensional horizontally layered geoelectric section and the distortions they cause during magnetotelluric sounding (MTS). The electromagnetic field within the layer of near-surface inhomogeneities is calculated using the Trefftz method. Expressions are derived for the boundary conditions on the day surface and on the roof of the underlying inhomogeneity of a horizontally layered medium. These boundary conditions allow for the excitation of TM-mode fields by subsurface inhomogeneities and their penetration into the atmosphere and the underlying medium. The spatial distribution and characteristics of galvanic and inductive distortions over different time periods during MTS have been studied. Experimental data show that accounting for galvanic distortions is possible with synchronous recording of the distribution of components of the electric and magnetic fields in a limited area of the Earths surface.



10.
LOCAL PRESSURE LOWS AS POSSIBLE SINKS OF FLUIDS IN GEOLOGIC STRUCTURES

B.P. Sibiryakov1,2, E.B. Sibiryakov1
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Pressure change across interface, layered-homogeneous medium, pressure low, boundary integral equation

Abstract >>
Local pressure lows in layered sections perturbed by anticlinal structures are studied analytically using equations for pressure change across interfaces. They are simple equations of pressure difference for low-angle structures and boundary integral equations for steeply dipping anticlines. Pressure may decrease locally near the crests of anticlines, as well as away from them at distances commensurate to the anticline height. Predicting stress patterns, which are specific for different groups of geologic structures, is a difficult task. However, some components of the stress field, such as low-pressure zones which may act as sinks for fluids, are relatively easy to constrain. Stress in these zones depends on the dip of anticlines and their curvature at each surface point. Negative curvature causes additional lateral extension and promotes further decrease of overburden pressure around the crests.