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Russian Geology and Geophysics

2013 year, number 3

1.
THICKNESS OF THE EARTHS CRUST IN THE DEEP ARCTIC OCEAN: RESULTS OF 3D GRAVITY MODELING

V.Yu. Glebovsky, E.G. Astafurova, A.A. Chernykh, M.S. Korneva, V.D. Kaminsky, V.A. Poselov
Keywords: 3D gravity modeling, Mohorovicic discontinuity (Moho), crustal thickness, Arctic Ocean
Pages: 247-262

Abstract >>
The employed method of 3D gravity modeling is based on calculation of the gravity effects of the main density boundaries of the lithosphere, subtraction of these effects from the observed gravity field, and the subsequent conversion of the residual gravity anomalies first to the Moho depth and then to the total thickness of the Earths crust and the thickness of its consolidated part. On the modeling, we also took into account the gravity effects due to an increase in the sediment density with increasing sediment depth and a rise of the top of the asthenosphere beneath the mid-ocean Gakkel Ridge. The resulting 3D models of the Moho topography and crustal thickness are well consistent with the data of deep seismic investigations. They confirm the significant differences in crustal structure between the Eurasian and Amerasian Basins and give an idea of the regional variations in crustal thickness beneath the major ridges and basins of the Arctic Ocean.



2.
POSSIBLE CONTRIBUTION OF LOWER-MANTLE MAGNESIOWSTITE METALLIZATION INTO GEOMAGNETIC DATA

V.V. Plotkin, P.G. Dyadkov, S.G. Ovchinnikov
Keywords: Magnesiowstite, metallization, phase change, lower mantle, conductivity, magnetic susceptibility, global electromagnetic induction, geomagnetic variation, geomagnetic observatories
Pages: 263-271

Abstract >>
Phase change of dielectric magnesiowüstite in the lower mantle may leave signatures in geomagnetic records of the globally distributed array of observatories. We investigate theoretically the assumed contribution of magnesiowüstite metallization to geomagnetic data and how the variations of magnetic susceptibility associated with this phase change may influence the Earths field. The modeling is performed using spherical harmonic analysis (SHA) of mantle electromagnetic (EM) responses in observatory geomagnetic data at periods of decades, 11 years, 1 year, and 27 days. The existence of a lower mantle conductor is checked against monthly means of real observatory records from 1920 through 2009 obtained by preliminary processing.



3.
THE ISOTOPE COMPOSITION OF Hf IN ZIRCON FROM PALEOARCHEAN PLAGIOGNEISSES AND PLAGIOGRANITOIDS OF THE SHARYZHALGAI UPLIFT ( southern Siberian craton): IMPLICATIONS FOR THE CONTINENTAL-CRUST GROWTH

O.M. Turkina, I.N. Kapitonov, S.A. Sergeev
Keywords: Paleoarchean, tonalitetrondhjemitegranodiorite complex, zircon, UPb dating, LuHf isotope composition, crustal growth, Sharyzhalgai uplift
Pages: 272-282

Abstract >>
This paper presents results of UPb dating (SHRIMP-II) and LuHf (LAICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388 11 and 3311 16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalitetrondhjemitegranodiorite (TTG) complexes are characterized mainly by positive values of ε Hf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average ε Hf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the LuHf isotope composition of zircon from the Bulun rocks in the period 3.333.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the LuHf isotope characteristics of zircon and the SmNd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.453.60 and ~3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.



4.
PALEOPROTEROZOIC GRANITOIDS OF THE CHUYA AND KUTIMA COMPLEXES ( southern Siberian craton): AGE, PETROGENESIS, AND GEODYNAMIC setting

T.V. Donskaya, D.P. Gladkochub, A.M. Mazukabzov, S.L. Presnyakov, T.B. Bayanova
Keywords: Granites, trondhjemites, UPb geochronology, geochemistry, accretionary orogen, Paleoproterozoic, Siberian craton
Pages: 283-296

Abstract >>
Detailed geochemical, isotope, and geochronological studies were carried out for the granitoids of the Chuya and Kutima complexes of the Baikal marginal salient of the Siberian craton basement. The obtained results indicate that the granitoids of both complexes are confined to the same tectonic structure (Akitkan fold belt) and are of similar absolute age. UPb zircon dating of the Kutima granites yielded an age of 2019 16 Ma, which nearly coincides with the age of 2020 12 Ma obtained earlier for the granitoids of the Chuya complex. Despite the close ages, the granitoids of these complexes differ considerably in geochemical characteristics. The granitoids of the Chuya complex correspond in composition to calcic and calc-alkalic peraluminous trondhjemites, and the granites of the Kutima complex, to calc-alkalic and alkali-calcic peraluminous granites. The granites of the Chuya complex are similar to rocks of the tonalitetrondhjemitegranodiorite (TTG) series and are close in CaO, Sr, and Ba contents to I -type granites. The granites of the Kutima complex are similar in contents of major oxides to oxidized A-type granites. Study of the Nd isotope composition of the Chuya and Kutima granitoids showed their close positive values of εNd(T) (1.93.5), which indicates that both rocks formed fr om sources with a short crustal history. Based on petrogeochemical data, it has been established that the Chuya granitoids might have been formed through the melting of a metabasic source, wh ereas the Kutima granites, through the melting of a crustal source of quartzfeldspathic composition. Estimation of the PT-conditions of granitoid melt crystallization shows that the Chuya granitoids formed at 735776ºC (zircon saturation temperature) and >10 kbar and the Kutima granites, at 819920ºC and >10 kbar. It is assumed that the granitoids of both complexes formed in thickened continental crust within an accretionary orogen.



5.
ON THE COMPOSITION AND ORIGIN DEPTH OF BASALTIC MAGMA IN THE UPPER MANTLE

V.N. Sharapov, Yu.V. Perepechko, G.V. Kuznetsov, K.E. Sorokin
Keywords: Basalt, mantle, melting, numerical modeling
Pages: 297-311

Abstract >>
The depths of mantle melting zones can be constrained by forward (in terms of physicochemical thermodynamics) or inverse (in terms of equilibrium thermodynamics) modeling. However, there is discrepancy in this respect between fluid-dynamic models of decompression melting in convecting upper mantle and thermodynamic models of basaltic magma sources beneath mid-ocean ridges. We investigate the causes of the mismatch in melting depth predictions with reference to the magmatic systems of the Basin and Range Province in the western margin of North America. The inverse solutions turn out to represent melts from different substrates (depth facies) in the lithospheric mantle, while modeling decompression melting in convecting fertile upper mantle refers to the depths the faults in spreading zones never reach. The discrepancy between forward and inverse solutions may be due to the fact that the respective depth estimates correspond to different levels of the same mantlecrust magmatic systems.



6.
TRACES OF PALEOEARTHQUAKES IN THE QUATERNARY DEPOSITS OF INTERMONTANE BASINS IN CENTRAL GORNY ALTAI

E.V. Deev, I.D. Zolnikov, S.V. Goltsova, G.G. Rusanov, A.A. Emanov
Keywords: Neotectonics, earthquakes, seismites, Neopleistocene, Holocene, central Gorny Altai
Pages: 312-323

Abstract >>
Study of the sections of NeopleistoceneHolocene deposits filling the basins in central Gorny Altai has revealed earthquake-induced soft-sediment deformations (seismites). They formed as a result of the brittle deformation of deposits and liquefaction of loose water-saturated sediments under vibration seismic impact. The paleoearthquakes resulting in such seismites had the minimum intensity I = 6 and magnitude M = 56. Hence, the study region underwent strong earthquakes in the NeopleistoceneHolocene.



7.
NEW DATA ON THE MAGNETOSTRATIGRAPHY OF THE JURASSICCRETACEOUS BOUNDARY INTERVAL, NORDVIK PENINSULA ( northern East Siberia)

V.Yu. Bragin, O.S. Dzyuba, A.Yu. Kazansky, B.N. Shurygin
Keywords: JurassicCretaceous boundary, northern Siberia, magnetostratigraphy, ammonites, belemnites, buchias, TethyanBoreal correlation
Pages: 335-348

Abstract >>
The results of this study were used to identify a reversed polarity magnetozone, referred to as M17r, in Berriasian sections of the Nordvik Peninsula (northern East Siberia) within the normal polarity magnetozone (M18n) from previous studies. The new magnetozone embraces the VolgianRyazanian boundary (Chetaites chetae/C. sibiricus zonal boundary). It was also found that the former magnetozone M17r at Nordvik, which includes the C. sibiricus/Hectoroceras kochi zonal boundary, should correspond to magnetozone M16r. Using magnetostratigraphic and biostratigraphic criteria proves that the Boreal C. sibiricus Zone is correlated with at least the major part of the Tethyan Tirnovella occitanica Zone, and the Boreal H. kochi Zone is correlated with the lower part of the Malbosiceras paramimounum Subzone of the Tethyan Fauriella boissieri Zone.



8.
SOLVING UNSOLVABLE PROBLEMS IN STRATIGRAPHY (Comments on the paper New data on the magnetostratigraphy of the JurassicCretaceous boundary interval, Nordvik Peninsula ( northern East Siberia) by V.Yu. Bragin, O.S. Dzyuba, A.Yu. Kazansky, and B.N. Shurygin)

A.Yu. Guzhikov
Keywords: JurassicCretaceous boundary, biostratigraphy, magnetostratigraphy, TethyanBoreal correlation
Pages: 349-354

Abstract >>
In this study we analyze the importance of new magnetostratigraphic data on the Nordvik section for solving the problem of detailed TethyanBoreal correlation around the JurassicCretaceous boundary with a special emphasis on the aspects of interpretation of the paleomagnetic data in magnetostratigraphic studies and the need for the integrated (paleontological and paleomagnetic) approach to recognition of the base of the Berriasian.