Home – Home – Jornals – Russian Geology and Geophysics 2009 number 3

We suggest a more rigorous approach to paleogeodynamic reconstructions of the Sayan-Baikal folded area proceeding from update views of the origin and evolution of island arcs and back-arc basins. Modern island arcs and attendant back-arc basins form mainly by trench rollback caused by progressive subduction of negatively buoyant thick and cold oceanic slabs. Slab stagnation upsets the dynamic equilibrium in the subduction system, which accelerates the rollback. As a result, a continental volcanic arc transforms into an island arc, with oceanic crust production in the back-arc basin behind it. As subduction progresses, the island arc and the back-arc basin may deform, and fold-thrust structures, with the involved back-arc basin and island arc complexes, may accrete to the continent (accretion and collision) without participation of large colliding blocks. When applied to the Sayan-Baikal area, the model predicts that the Riphean and Vendian-Early Paleozoic back-arc basins were more active agents in the regional geologic history than it was thought before. They were deposition areas of sedimentary and volcanosedimentary complexes and then became the scene of collision and accretion events, including folding, metamorphism, and plutonism.

We report the first data on the contents of main oxides and REE in rocks and the compositions of pyropes and almandines from the Maiskoe kimberlite body recently discovered in the Nakyn field of the Yakutian kimberlite province.

A comparative analysis of morphology and geochemistry was made for gold from the primary ores and weathering crust of the Suzdal' gold deposit, Eastern Kazakhstan. The deposit is localized in Carboniferous carbonaceous-terrigenous strata and is of gold-sulfide type. Study of gold from primary ores showed that it occurs mainly in two species: free and so-called invisible. Free gold is crystallomorphic segregations and irregular-shaped grains up to tens of microns in size; it occurs in intergrowths with sulfides, quartz, carbonate, and mica-chlorite aggregate. Most of gold particles have a fineness of 930-980

The Berezitovoe gold-polymetallic deposit is localized in the west of the Selenga-Stanovoi superterrane composed of a wide spectrum of Early and Late Precambrian igneous rocks and abundant Paleozoic and Mesozoic intrusive and volcanoplutonic associations. The ⁴⁰Ar/³⁹Ar ages determined for metasomatites bearing gold-polymetallic mineralization are as follows: garnet-quartz-muscovite-sericite-K-feldspathic metasomatites (129.7±3.2-127.3±4.4 Ma); muscovite-quartz-sericite metasomatites (132.0±2.9-131.3±2.3 Ma). According to the age and general scheme of evolution of the Early Cretaceous magmatism in the Selenga-Stanovoi superterrane, the metasomatites of the Berezitovoe deposit are nearly coeval to the intrusive rocks of the Amudzhikan complex (132-128 Ma). The revealed platinum potential of gold-polymetallic ores and metasomatites permits ranking the Berezitovoe deposit as a specific complex gold-polymetallic-platinum deposit, which considerably increases its commercial value.

We have obtained formulas to calculate the field components of an eccentric dipole (ED) with an arbitrarily directed moment from specified coordinates of the dipole center and from its moment vector. With these formulas, the model dipole position was fitted to the observed geomagnetic field, and the approximation accuracy was estimated according to the standard deviation from IGRF along the X , Y , and Z components; the fitting procedure was checked in several tests.
We computed the ED parameters (position and moment orientation) and harmonic coefficients of the ED field for 50 years using surface IGRF components, discovered some changes, and compared them with those according to the Schmidt eccentric dipole model. It was found that the nondipolar contributions to the geomagnetic field increase as the dipole field decays with time, and the dipole center drifts north- and eastward away from the Earth's gravity center.
The main contribution of the dipole part in the Gaussian spherical harmonic expansion of the geomagnetic field turned out to come from the terms with n of 1 to 5 rather than from two first terms, and the contribution of higher harmonics increases with time. Therefore, the Schmidt ED approximation based on the first eight Gauss coefficients ( n ≤ 2) must have only relative significance indicating changes of the parameters. On the other hand, world magnetic anomalies (WMA) show up already in two first Gauss terms rather than since the third term as it has been commonly assumed.

We investigated space-time lithospheric stress patterns of the Baikal rift system according to the hierarchy of earthquakes using mechanisms of 265 K _P ≥ 10 events recorded from 1950 to 1998 and seismic moments of 802 K _P ≥11 events from 1968 to 1994. The lithosphere of the region was confirmed to undergo rifting with mostly normal-slip events, while local areas of frequent strike-slip and reverse motions may record stress heterogeneity. The dominance of rifting, although being evident in the stress dynamics, is unstable, which is indicated by increase in strike-slip and reverse motions to as many as normal slip events in the latest 1980s-earliest 1990s. The lithospheric stress patterns inferred from seismic-moment data are generally consistent with those derived from the classical focal mechanism method. The suggested approach of seismic zoning according to earthquake slip geometry may provide a more reliable background for successful mitigation of seismic hazard in the region.

The mechanics of the ice cover of Lake Baikal has been studied through monitoring of its deformation and seismic effects and full-size uniaxial compression and shear tests in 2005-2007. We measured the shear strength of ice specimens and large in situ blocks (σ = 0.2-1.9 MPa) and investigated it as a function of air temperature and ice structure. Deformation was analyzed in terms of various natural controls, such as air temperature and pressure, wind, sub-ice currents, and local earthquakes. Precise strain measurements along ice cracks were used to explore the strain behavior of ice, including the cases of dynamic failure (ice shocks). Measurements by seismic station Baikal-12 were used to monitor diurnal background microseismicity variations and to record an ice quake with its magnitude ( M = 0.3-0.8; E = 10⁴-10⁵ J) comparable to a medium-size rock burst or a small earthquake. Ice quakes were studied in terms of their nucleation, dynamics, and aftereffects, as well as the strain and seismic responses of the ice, using sub-ice explosions in the latter case. The natural conditions of deformation in the elastoviscoplastic Baikal ice are similar to lithospheric processes and thus can be employed in tectonophysical modeling with scientific and practical implications for hazard mitigation.

Ungrounded horizontal loop responses at low frequencies and/or late times can be modeled in terms of an equivalent circuit with lumped elements, but a loop in a general case is a distributed system. At high frequencies and/or early times, the wire in combination with the underlying earth makes a transmission line in which current behaves according to the wave equation. Solving the equation for current turn-off is quite difficult because the primary parameters of the wire-earth system depend, in an intricate way, on earth conductivity (resistivity) and frequency (or time). In modeling the current turn-off process, the loop was simulated as a symmetrical combination of two identical transmission lines with shorted outputs. Modeling was performed in the frequency domain with subsequent transformation into the time domain. Comparison of measured and computed transient self responses showed that good fit requires taking into account (1) interaction of each line with its own image current, (2) mutual inductance of the two lines, and (3) skin effect in the wire. As a result of mutual inductance, the parameters of the lines and, hence, of the whole loop depend on local conductivity, which, at least in principle, may allow one to infer the resistivity of shallow subsurface from current turn-off responses. A real ungrounded horizontal loops lacks symmetry at early times and its magnetic field differs from that predicted by the conventional methods of induction soundings.