Home – Home – Jornals – Russian Geology and Geophysics 2006 number 8

We suggest a model of elastic rebound for the M = 7.3-7.5 Chuya earthquake of 27 September 2003 based on 3D deformation data from the Gorny Altai. We report processed data from yearly resurveyed Altai GPS stations through 2000-2004. The geodynamic network of the Institute of Geophysics (Novosibirsk) consists of 21 stations, including the Klyuchi reference station (world network code IGS-NVSK DOMES N12319 M001); it covers the structural elements of Gorny Altai and extends into the southern West Siberian plate and the West Sayan mountains. Displacement of the NVSK station in the system of permanent stations of Asia and displacement of Central Asian stations were processed to obtain a map of GPS velocities for the Gorny Altai. The velocities are in a range from 0.2 to 11 mm/yr. The velocity field shows signature of a preseismic process before the Chuya event in the southern Gorny Altai. Measurements through 2003-2004 give a pattern of postseismic deformation indicating a right-lateral strike slip in the epicentral area. The amount of displacement decreases away from the nodal plane from 300 mm at 15 km to 25 mm at 90 km far from the main shock. This regularity is explained in terms of elastic rebound in a 2D dislocation model. Modeling with the use of experimental data showed a 4MPa stress release and a 2 m displacement along the fault in the epicenter, at an average fault depth of 8 km. The shear strain related to the earthquake was estimated to be 10^-6 within 100 km away from the rupture.

We report locally nonlinear approximation algorithms for processing high-frequency induction logging data in axisymmetrical models. Forward problems for high-frequency logging are solved using the Born and Rytov approximations. Space conductivity variations in the high-frequency range change only the phase components of the electric or magnetic fields. The Rytov locally nonlinear approximation is the most accurate (1-2% error). Numerical modeling and comparison of high-frequency induction logging diagrams are applied to typical 2D models of terrigenous reservoirs.

Based on microprobe data on the contents and distribution of Cl and F in halogen-containing minerals of enclosing intrusive rocks and their hydrothermally altered varieties, the role of metasomatic processes in the balance of volatiles of the ore-forming system is discussed by the example of porphyry Cu-Mo deposits of Siberia and Mongolia. Two groups of deposits are considered: essentially copper (Erdenetiyn-Ovoo, northern Mongolia, and Aksug, Tuva), with prevailing propylitization and quartz-sericite alteration, and essentially molybdenum (Sora, Kuznetsk Alatau), with prevailing K-metasomatism (orthoclase-biotite association). All types of hydrothermal transformations lead to a drastic decrease in Cl contents in metasomatic minerals as compared with halogen-containing magmatic minerals. In chlorites and sericites, which are the most widespread in metasomatites of essentially copper deposits, no Cl and Cl-containing minerals were detected by microprobe analysis, whereas F was found in large amounts in some metasomatic minerals: secondary biotite, actinolite, chlorite, and sericite. At the studied deposits, huge volumes of enclosing rocks (hundreds of mln tons) are involved in metasomatism. The large amounts of thus released halogens (particularly Cl) might have seriously influenced the regime of volatiles during the ore formation, including the increase in the salinity of hydrothermal solutions.

First data on the U-Pb age of island-arc gabbroids and granitoids of the Dzhida Caledonide zone are presented. It has been established that these rocks formed at nearly the same time (506 ± 1 Ma, gabbroids, and 504 ± 2 Ma, granitoids) at the final stage of the evolution of ensimatic island arc. The low ⁸⁷Sr/⁸⁶Sr (0.7031-0.7033) and high positive ε _Nd (T) (+6.7...+7.6) values and localization of sample points at the same isochron evidence that the studied igneous rocks belong to the same differentiated gabbro-diorite-tonalite series, which resulted from the melting of a primary depleted mantle source with the subordinate role of a long-lived crustal component.

By the example of West Siberia, it has been shown that paleocarpology permits quite a detailed division of the continental Cenozoic of closed territories, which is unattainable by other paleontological methods. In the range from Aquitanian to Gelasian, ten biostratigraphic zones have been recognized, whose sum covers the complete stratigraphic interval of the section.

Palynological studies were given to the Holocene lacustrine-palustrine sediments of Lake Beloe (Novosibirsk Region). On the basis of cores obtained from two boreholes, spore-and-pollen diagrams were compiled and four palynozones were recognized to characterize the vegetation for the last 5000 years. It has been established that the lake has been surrounded by birch forests for this period. The pollen of Pinus sylvestris L. is a component brought from other zones and reflects a remote flora. The local vegetation was subject to changes. In the early sub-Boreal period the climate was wet and somewhat warmer than it is now. Diverse hydrophytes, sedge, and reed were quite abundant around and within the water body. In the second half of the sub-Boreal period, the lake was surrounded by swamped areas, where ferns, horsetails, and palustrine Thelipteris grew. Sapropels began to form at around the onset of the sub-Atlantic period. The dwarf birch dispersed on the shores of the lake. The amount and diversity of hydrophytes were drastically diminished. Most likely, some cooling took place. Since the second half of the sub-Atlantic period, the vegetation of Lake Beloe has acquired the modern character. The lake is surrounded with reeds, canes, and sedge communities. Pondweed, duckweed, Myriophyllum, and other aquatic plants grow in the water body.

We briefly report the history of studying Baikal gas hydrates, their discovery in the areas of new structures, and the specific composition of Baikal hydrate-containing subsurface sediments. The structures with a volcanic breccia were referred to as mud volcanoes. The first geological search criteria for the presence of gas hydrates in the Baikal subsurface sediments are presented.