Home – Home – Jornals – Russian Geology and Geophysics 2014 number 7

The structure of mantle convection and spatial fields of superlithostatic pressure and vertical and horizontal stresses in the Earth’s mantle are studied in a 2D numerical model for the mantle with non-Newtonian viscosity and heat sources. The model demonstrates a jump-like motion of subduction zones and reveals abrupt changes in the stress fields depending on the stage of slab detachment. The stresses decrease dramatically in the areas without slabs. The horizontal stresses σ_xx, superlithostatic pressure, and vertical stresses σ_zz in the part of the mantle lacking intense near-vertical flows are approximately equal, varying within ±6, ±8, and ±10 MPa, respectively. However, these fields are stronger in the areas of descending slabs, where the values of the above parameters are about an order of magnitude higher (±50 MPa). This result agrees with the current views of the oceanic slabs as the most important agent of mantle convection. We have found significant differences between the σ_xx, σ_zz, and pressure fields. The pressure field reveals both the vertical and horizontal features of slabs and plumes, clearly showing their long thermal conduits with broader heads. The distributions of σ_xx are sensitive to the near-horizontal features of the flows, whereas the fields of σ_zz reveal mainly their vertical substructures. The model shows the presence of cool remnants of lithospheric slabs in the lower mantle above the thermal boundary layer. Numerous hot plumes penetrating through these high-viscosity remnants, as well as the new descending slabs, induce intense stress fields in the lower mantle, which are strongly inhomogeneous in space and time.

Comprehensive geochemical and geochronological studies were carried out for two-mica granites of the Biryusa block of the Siberian craton basement. U-Pb zircon dating of the granites yielded an age of 1874 ± ± 14 Ma. The rocks of the Biryusa massif correspond in chemical composition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, they are assigned to S-granites. The low CaO/Na2O ratios (<0.3), K2O ≈ 5 wt.%, CaO < 1 wt.%, and high Rb/Ba (0.7-1.9) and Rb/Sr (3.9-6.8) ratios indicate that the two-mica granites resulted from the melting of a metapelite source (possibly, the Archean metasedimentary rocks of the Biryusa block, similar to the granites in εNd(t) value) in the absence of an additional fluid phase. The granite formation proceeded at 740-800ºC (zircon saturation temperature). The age of the S -type two-mica granites agrees with the estimated ages of I- and A-type granitoids present in the Biryusa block. Altogether, these granitoids form a magmatic belt stretching along the zone of junction of the Biryusa block with the Paleoproterozoic Urik-Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that they formed within a high-temperature collision structure. It is admitted that the intrusion of granitoids took place within the thickened crust in collision setting at the stage of postcollisional extension in the Early Proterozoic. This geodynamic setting was the result of the unification of the Neoarchean Biryusa continental block, Paleoproterozoic Urik-Iya terrane, and Archean Tunguska superterrane into the Siberian craton.

The compositions of parageneses including arsenopyrite (Asp), pyrite (Py), and As–pyrite (As–Py) have been calculated by the solution of primal physicochemical modeling problems. The numerical models for the interaction of Py matrix with hydrothermal solution saturated with Asp are considered for three variants of the solution penetration into the Py matrix: percolation, spreading, and tightening at 100–300 ºC and 300 bars. It is shown that Asp forms in the zone of an ore column where fluid (solution) is predominant, with the Py matrix being replaced independently of the type of solution transfer. Prevailing As and Fe complexes are considered. The calculated models for the three types of interaction show that the redox potential in the solution varies fr om –0.055 V at the ends of the Asp-containing ore column to –0.55 V in its central zone. This difference makes an electrochemical geochemical barrier at the interface, wh ere metallic gold is deposited in Asp–Py ores.

Southeastern Russia occupies an area south of the Siberian Platform and east of Lake Baikal, up to the coasts of the Sea of Okhotsk and the Sea of Japan. Most of PGE placers are localized south of the Siberian Platform, mainly within the Baikal-Aldan-Stanovoi megablock. Noble-metal placers formed by PGE minerals in varying amounts are associated with the regional mafic-ultramafic complexes of different ages, namely, layered, zoned (ring) massifs and ophiolite fields. PGE mineralization is also found in layered carbonaceous strata of different ages and in several brown-coal deposits localized in the Cenozoic zones of dispersed rifting and intraplate magmatism. One of the magmatism centers is the Ussuri plume structure. The widespread regional manifestations of plume magmatism of different ages permit the development of a new approach to study the additional factors that affected the formation and localization of PGE mineralization. Based on geological, geophysical, geochemical, and mineralogical data, we have established that the conditions favorable for the formation of platinum-bearing deposits resulted mainly fr om ore-generating plume magmatism. This magmatism gave rise to layered (in the Neoarchean and Proterozoic) and zoned (in the Phanerozoic) mafic-ultramafic massifs, which were later subjected to ore-producing magmatogene-fluid-metasomatic processes. The most favorable conditions for PGE concentration appeared in zones wh ere late granite deposits were superposed on early layered, zoned massifs, ophiolite complexes, and layered carbonaceous strata.

The position of lithospheric magnetic anomalies, detected in total magnetic intensity and the vertical component of the magnetic field, has been determined for Siberia using data from the CHAMP satellite. The paper describes the technique for the satellite data processing and the ways of recognition of regional lithospheric magnetic anomalies from satellite-measured values of the total geomagnetic field, which are obtained from several sources (external and internal with respect to the Earth’s surface). Maps of magnetic-field anomalies of different scales have been constructed for several regions of Siberia depending on the method of areal averaging. The possible geologic and physical nature of the magnetic anomalies and their relationship with deep-seated crustal structures are considered. Preliminary interpretation of the magnetic-field maps shows that the anomalies are connected with the present-day large geologic and geophysical elements of the basement. The features of the lithospheric magnetic field, as a parameter reflecting the present position of tectonic structures and their physical properties, can be used for their contouring in combination with other geological and geophysical methods.

The paper presents the results of paleomagnetic and geochronological studies of the Late Paleozoic granites of the Angara-Vitim batholith as well as Vendian-Early Cambrian sedimentary rocks and Late Devonian subvolcanic rocks of the Patom margin of the Siberian Platform. Primary and metachronous magnetization in the rocks of the study region was used to calculate an Early Permian (~290 Ma) paleomagnetic pole, which is proposed as a reference pole for the Siberian Platform in paleomagnetic reconstructions, plotting of the apparent polar-wander path curve, and other magnetotectonic studies. The published and obtained paleomagnetic data and analysis of the geological data confirm the Late Paleozoic age of the final folding in the Baikal-Patom area. Possible causes of deformations and large-scale granite formation in the Baikal-Patom area and Transbaikalia in the Late Paleozoic are considered.

The paper presents the results of detailed paleomagnetic studies of the Paleogene-Neogene continental sediments stripped by borehole 8 in southwestern West Siberia (Russkaya Polyana district, Omsk Region), near the Kazakhstan frontier. According to the previous biostratigraphic data, the sediments under study formed from Rupelian to Ruscinian. The results of stepwise thermal demagnetization and alternating-field demagnetization were used to carry out a component analysis of natural remanent magnetization, which revealed characteristic (primary) remanent magnetization (ChRM). The compiled paleomagnetic section, which includes seven regional horizons and same-named formations (Oligocene Atlym, Novomikhailovka, and Zhuravka Formations and Neogene Abrosimovka, Beshcheul, Tavolzhan, and Novaya Stanitsa Formations), was compared with the Cenozoic polarity scale for the West Siberian Plate. This made it possible to assess the completeness of the geologic section of Paleogene and Neogene continental sediments in borehole 8 and to record the magnetozones and their fragments missing from the magnetostratigraphic section (for some intervals, in absolute chronology). The comparison shows that the magnetostratigraphic section of the studied sediments at the edges of the Om’ basin is approximately twice shorter than that of the basin center.

Modeling of the seismic, thermal, and density structure of the Siberian craton lithospheric mantle at depths of 100–300 km has been performed along the superlong Meteorite and Rift seismic profiles. The 2D velocity sections reflect the specific features of the internal structure of the craton: lateral inhomogeneities, seismic–boundary relief at depths of ~100, 150, 240, and 300 km, velocities of 8.3–8.7 km/s, and the lack of low-velocity zone in the lower lithosphere. Mapping of the thermal state along the Meteorite and Rift profiles shows a significant temperature decrease in the cratonic mantle as compared with the average temperatures of the surrounding Phanerozoic mantle (≥300ºC) estimated from the global reference model AK135. Lateral temperature variations, reflecting the thermal anomalies in the cratonic keel, are observed at depths of <200 km (with some decrease in temperature in the central part of the craton), whereas at depths of >200 km, temperature variations are negligible. This suggests the preservation of residual thermal perturbations at the base of the lithosphere, which must lead to the temperature equalization in the transition zone between the lithosphere and the asthenosphere. Variations in chemical composition have a negligible effect on the thermal state but affect strongly the density structure of the mantle. The results of modeling admit a significant fertilization of matter at depths more than 180–200 km and stratification of the cratonic mantle by chemical composition. The thicknesses of chemical (petrologic) and thermal boundary layers beneath the Siberian craton are estimated. The petrologic lithosphere is localized at depths of ~ 200 km. The bottom of the thermal boundary layer is close to the 1450 ºC isotherm and is localized at a depth of 300 km, which agrees with heat flow and seismic-tomography data.

Based on the proposed electrical model of a system for subsurface sensing by an induction transient electromagnetic method, a numerical experiment was performed to evaluate the limitations of the method in shallow-depth studies. During the experiment, the theoretical and pseudoexperimental emf curves are compared. Based on the degree of their convergence, the capability of the system to adequately record the response from the excited space at depth is determined. The experimental results are used to plot dependences of the measured parameters of the system on its geometrical dimensions. Recommendations on the use of small-size systems are given.

A “difference spectra” method is proposed for the qualitative assessment of changes of illite-smectite structures accompanying the flooding of oil reservoirs. The method permits one to get an open system and to reduce the application of procedures based on the Markov’s chains formalism. A computer simulation is made to obtain spectra by subtracting the spectrum of a glycol–saturated object from the spectrum of air-dried preparation throughout the range of concentrations of illite and smectite components with the structure short–range order factor R = 0 or R = 3. It has been established that in the case of filtration, the maximum and minimum of the spectra in the range of 12.5–9.4 Å are complicated by a number of local extremes, whose position is specified by the structure of the intermediate phases. The flooding process initially involves mixed–layered phases with R = 0, leading to a partial segregation of the structures into phases with one and two grids of interlayer H₂O. When the secondary particles of mica break, phases with R = 3 appear at the boundaries of nanoblocks, first, only with 1H₂O and then, only with 2H₂O in the labile interlayers. Their coexistence with the phases with R = 0 in the sample proves the existence of percolation effects due to the two-phase filtration in the porous medium. In the fully flooded reservoir, a mechanical mixture of illite-smectite phases of different nature with R = 0 and with different ratios of components is always predominant. Transformation of mica that can drastically reduce oil production begins long before the appearance of flooding zones, which are revealed by standard logging methods.