Home – Home – Jornals – Russian Geology and Geophysics 2014 number 4

We consider the primary nature and sources of the protoliths of metamorphic rocks of the Kurai block located in the large Teletskoe-Kurai system of deep faults separating the Gorny Altai and West Sayan structures. It has been established that the protoliths of the Kurai block metapelites were deposits of transitional crust: They lack typical rocks of mature continental crust (arkoses, litharenites) and have reduced (relative to the upper continental crust) contents of lithophile elements and elevated contents of transitional elements. The average Nd model age of the protoliths of the metapelites of the Kurai complex corresponds to the Middle Riphean (1.4–1.6 Ga). The metabasites of the Kurai complex are similar in petrologic composition to metamorphic products of oceanic basalts. It is shown that the formation of metamorphic associations of the Kurai block was caused by tectonometamorphic transformations of a compositionally heterogeneous rock unit (basalts, aluminous and volcanomict sediments), which were, most likely, a fragment of Middle Cambrian–Early Ordovician turbidite basin with an oceanic basement.

Distribution of water among the main rock-forming nominally anhydrous minerals of mantle xenoliths of peridotitic and eclogitic parageneses from the Udachnaya kimberlite pipe, Yakutia, has been studied by IR spectroscopy. The spectra of all minerals exhibit vibrations attributed to hydroxyl structural defects. The content of H₂O (ppm) in minerals of peridotites is as follows: 23–75 in olivine, 52–317 in orthopyroxene, 29–126 in clinopyroxene, and 0–95 in garnet. In eclogites, garnet contains up to 833 ppm H₂O, and clinopyroxene, up to 1898 ppm (~0.19 wt.%). The obtained data and the results of previous studies of minerals of mantle xenoliths show wide variations in H₂O contents both within different kimberlite provinces and within the Udachnaya kimberlite pipe. Judging from the volume ratios of mineral phases in the studied xenoliths, the water content varies over narrow ranges of values, 38–126 ppm. At the same time, the water content in the studied eclogite xenoliths is much higher and varies widely, 391–1112 ppm.

Gravity models of the crust and upper mantle to a depth of 100 km are analyzed to study structural relationships of tectonic and tectonophysical media of different rigidities with the distribution of shallow ore deposits above the Aldan–Zeya plume. The spatial correlation of ore clusters and districts with high crustal viscosity inhomogeneities at depths of 10, 20, and 35 km shows distinct stepwise behavior. On the other hand, media of decreased viscosity are observed in the lower crust (at depths of 25–30 km), subcrustal (40–50 km) layers, and asthenosphere (at a depth below 70 km). They are related to chambers of the complete or partial melting (heat sources) of magmatic and ore occurrences near the Earth’s surface. Lateral metallogenic zoning in the spatial distribution of the ore deposits is due to the spread and redistribution of magmas and ore-forming fluids, shielded by rigid plates in the lower crust. A naturally determined series of ore parageneses is observed from center to flanks of the plume: Au, Mo → Au, Ag, Pb, Zn → Au, Pb, Zn → Au, W → Au, Sb → W, Sn → Sn. The mutual position of the tectonic-magmatic structures of different ranks within the plume head obeys hierarchical and fractal laws.

The paper is focused on the regularities and character of the response of the regional landscapes of the Angara–Lena Plateau to variations in the global climate system during the Holocene. They were revealed by integrated studies of four peat bogs of the plateau — an important area for the understanding of the environmental dynamics in the entire Baikal region. Age models for the records obtained were provided by 16 radiocarbon dates. A spatiotemporal correlation of spore-pollen characteristics with the trend of δ¹⁸O records from global stratotypes was used to find out the possible causes of changes in the landscape conditions and climate of the Angara-Lena Plateau in the context of past changes in the global climate system. The plateau environment showed a dramatically varying response to global climate variations in the Middle–Late Holocene. Moreover, the observed intervals of reorganization in the regional environment took place in a quasi–millennial regime, in accordance with global climate rearrangement. However, not all the studied regions of the Angara–Lena Plateau made a synchronous or analogous response to global environment change. This emphasizes the complicated character of regional climate manifestations in the Holocene and necessitates the use of paleogeographical data from a wider range of territories.

We present results of research into the Holocene sediments of Lake Kiran, one of the small saline lakes with carbonate sedimentation in western Transbaikalia. The sediments were studied by X-ray diffraction (XRD), IR spectroscopy, analysis of stable ¹⁸O and ¹³C isotopes, laser granulometry, element analysis (SRXFA), etc. By the mathematical modeling of complex XRD patterns of lacustrine sediments, Mg-calcites with different Mg contents and excess-Ca dolomites have been determined in the assemblage of endogenic carbonate minerals. The obtained carbonate record gives an insight into the stratigraphic distribution of endogenic carbonates of the calcite-dolomite series, in which the number and proportion of phases with different Mg contents are determined by the Mg/Ca ratio, salinity, and total alkalinity of the lake water in the past. The abundance of excess-Ca dolomites in the sediments indicates that the lake is of shallow playa type. The mineral composition and crystallochemical structure of carbonates are in high correlation with the data of lithological analysis, results of determination of stable isotopes (δ¹⁸O and δ¹³C), and distribution of some geochemical indicators of sedimentation conditions. Several stages of the lake evolution depending on the regional climatic changes have been recognized. It has been established that the most arid environments in southern Buryatia were in the Subatlantic. Despite the highly arid climate in the region in the period under study (from 7 ka to the present), it underwent fluctuations toward humidification, in particular, at the Atlantic stage (the time of the lake formation) and in the recent decades, up to the present.

The paper is concerned with the structure of the Arctic shelf sediments in the Ob’ region in the Paleogene and Neogene, sampled from boreholes drilled on the Yamal Peninsula, in the lower reaches of the Pur and Taz Rivers. The specifics of Paleogene marine sedimentation in the central and northern West Siberian Plain are studied. The effect of abiotic (tectonic) factors on the completeness of the geologic record is considered, as well as the effect of recent (Oligocene-Neogene) tectonic processes on topography and sedimentation. The borehole sections are compared with the main seismic sections of the Kara Sea and Lomonosov Ridge.

The parameters of 15 arbitrary dipoles that, in aggregate, represent the main geomagnetic field (MGF) are estimated to obtain information on the distribution of MGF sources within the Earth in the form of dipoles with an arbitrary position and value of the magnetic moment vector. For an adequate estimation of the results, the method of obtaining the data is described, including: the eccentric dipole model and the derivation of the basic formulas for the magnetic field components of an arbitrary dipole; the method of estimating the parameters of these dipoles, including the computational scheme and optimization method; necessary constraints on the dipole parameters and a justification of the choice of the initial values in the optimization fitting of the parameters. The results are presented as a map of the location of the centers of the dipoles and their northern axial poles for the epochs 1955 and 2005 and plots of changes in all six parameters of 15 dipoles for 50 years. Most of the dipole centers are located in the lower mantle. The results suggest the existence of current systems in the lower mantle that produce dipole magnetic fields. These currents are provided by the high conductivity of wüstite, an important component of the mantle, which, at a depth of 1000–2200 km, transforms to the low-spin state of iron with increased density and electrical conductivity.

Magnetic viscosity of rocks associated with magnetic relaxation of ultrafine ferrimagnetic mineral grains (superparamagnetism) is employed in magnetic grain size measurements. Magnetic viscosity is most often estimated from dual frequency measurements of magnetic susceptibility. The measured susceptibility values bear uncertainty that comprises two components: an instrument error and a drift. The instrument error refers to the accuracy of the measurement system and shows how precise the data are in ideal operation conditions. This error affects especially the low susceptibilities of weak samples, which thus should be measured on a high sensitivity range. Drift is due to external factors, such as changes in the temperature of sensors and/or samples, as well as in the orientation of the samples relative to the sensor, vibration, electromagnetic noise, etc. Drift, more critical for measurements on strong samples, is manageable by the operator. To reduce drift, every effort should be made to maintain suitable ‘quiet’ operation conditions.

An analytical model is suggested to describe the electrostatic field produced by near-surface inhomogeneities responsible for galvanic shift in magnetotelluric (MT) apparent resistivity sounding curves. The near-surface inhomogeneities are modeled in thin-sheet approximation with laterally variable longitudinal conductance and transverse resistance. The model accounts for the TM (transverse magnetic) mode secondary electric field in the conductive layered subsurface below the thin sheet. Equations have been obtained to relate the subsurface geoelectric parameters and the spatial harmonics of the secondary electrostatic field. This secondary field, which is the source of galvanic shift in MT data, turns out to be in-phase with the primary field. The equations derived to simulate galvanic distortions are applicable to long-period MT data acquired by a synchronous array.

This contribution represents the first Russian publication and in-depth analysis of accounts by J.G. Gmelin of earthquakes in Siberia from the late 17th and the first three decades of the 18th centuries. These forgotten accounts, though short and fragmentary, expand the existing database and enable new solutions of some of the problematic aspects in the study of seismicity in this region.