Home – Home – Jornals – Russian Geology and Geophysics 2013 number 1

Remote-sensing, cosmogeological, tectonic, geophysical, structural, compositional, isotopic, and geochronological criteria permit identifying the Kotuikan ring structure in the northern Siberian Platform as a Paleoproterozoic large astrobleme, close in age to the Vredefort and Sudbury impact structures. Also, indirect evidence for two more large impact structures was obtained here. This confirms widely hypothesized massive bombardment of the early Earth by asteroids and a possible effect of large impact events on the Earth’s mantle dynamics and rotation regime, that is, the tectonic evolution of our planet, including plate tectonics.

The structure, composition, and age of Vendian–Early Cambrian plagiogranitoid associations composing the Kshta and Taraskyr massifs of the Yenisei pluton in the Altai–North Sayan island-arc belt are considered. We have established that these associations formed within 550–520 Ma and differ in petrographic composition and sources. Two stages of island-arc plagiogranitoid magmatism are recognized: early (550–540 Ma, formation of plagiogranitoids of the Kshta (545 ± 8 Ma) and Taraskyr (545 ± 7 Ma) massifs) and late (525–520 Ma, formation of plagiogranitoids of the Maina complex of the Yenisei (524 ± 2 Ma) and Tabat plutons). By petrochemical composition and geochemical characteristics, the rocks of the Kshta massif are high-alumina plagiogranitoids similar to adakites. They might have been produced through the melting of metabasites compositionally similar to N-MORB in equilibrium with garnet-containing restite during the subduction of oceanic slab at ≥15 kbar. The rocks of the Taraskyr massif are low-alumina plagiogranites. They formed through the melting of metabasites located in the lower layers and(or) the basement of the island-arc system in equilibrium with plagioclase-bearing restite at 3–8 kbar. The low-alumina plagiogranitoids of the Yenisei pluton melted out under the same conditions. Isotope-geochemical studies showed that the Vendian–Early Cambrian plagiogranitoids formed at the early stage are characterized by high positive ε_Nd(T) values (7.5–4.9), Late Riphean model Nd-age (T_Nd(DM) = 0.64–0.98 Ga), and Sr isotope ratio varying from 0.7040 to 0.7053. These data point to the juvenile parental melts of the rocks and the varying content of ancient crustal material in the magma generation zone.

Two island arcs of different ages have been reconstructed in the Neoproterozoic history of southeastern East Sayan: Dunzhugur and Shishkhid. According to earlier concepts, the Dunzhugur arc formed at ~1020 Ma and underwent collision with the Siberian(?) continent at ~810 Ma. The Shishkhid arc formed somewhat earlier than 800 Ma and existed till the end of the Late Baikalian (~600 Ma, from indirect data). This primitive geologic history, when each arc existed for 200 Myr, was suggested because of the deficit of direct data, and its reconstruction cast doubt. In this work we present results of preliminary dating of detrital zircons extracted from the volcaniclastic rocks composing the above arcs. We analyzed 12 zircon crystals from the Dunzhugur volcanic clastics, whose ²⁰⁶Pb/²³⁸U age varies from 844 ± 8 to 1048 ± 12 Ma (1σ). Five most ancient zircons form a concordant cluster with an age of 1034 ± 9 Ma (2σ). Hence, the arc formed earlier than it was assumed and existed for a long time, most likely, till its collision with the continent. We also studied two zircon samples from the volcaniclastic rocks of the Oka accretionary prism, which probably formed in the Shishkhid arc. All ten crystals of the first sample form a concordant cluster with an age of 813 ± 7 Ma (2σ). The analyzed zircons of the second sample arrange in two clusters, with an age of 775 ± 8 Ma (2σ, nine crystals) and 819 ± 17 Ma (three crystals). Thus, the Shishkhid arc formed earlier than it was assumed, at the end of the Early Baikalian, and underwent active volcanism at least till 775 Ma. Dating of detrital zircons from the volcaniclastic rocks generated at the mature stage of the Shishkhid-arc evolution will help to reconstruct partly or completely its history in the period 775–600 Ma.

New data are presented on the petrogeochemical composition, age, and formation conditions of the Late Neoproterozoic metarhyolite–basalt association of the Glushikha trough (Yenisei Ridge). The association is localized within the subaerial and shallow-water terrigenous-carbonate sediments of the Orlovka Group, which overlies Proterozoic rocks with unconformity. The felsic volcanics are essentially potassic and enriched in Rb, U, Th, and Fe. They show a weakly fractionated REE pattern with a prominent negative Eu anomaly. The basalts and picrite basalts have higher contents of Ti, Fe, P, HFSE, REE, U, Th, Ba, and Sr, and their spidergrams show no Nb or Ta depletion with respect to Th and LREE. These rocks have the petrochemical parameters of intraplate magmatic associations in continental rift zones. New geochronological data (SHRIMP II) on single zircon grains from the felsite porphyry of the metarhyolite–basalt association (717 ± 15 Ma) indicate Late Neoproterozoic volcanism in the Yenisei part of the Central block of the Trans-Angara region. According to Sm–Nd isotopic data, the rhyolites originate from Paleoproterozoic crust (T_Nd(DM) = 1757 Ma; T_Nd(DM-2st) = 1651 Ma; ε_Nd(T) = –2.7). The Orlovka volcanosedimentary rocks are rift-related, as evidenced by the following facts: (1) localization of the volcanosedimentary rocks in a narrow fault-line trough; (2) bimodal rhyolite-basaltic composition of the volcanics; and (3) petrology and geochemistry of the picrite basalts and basalts, typical of intraplate environments. The studies show that Late Neoproterozoic rifting and intraplate plume magmatism took place not only in the Tatarka–Ishimba fault zone but also in the Yenisei fault zone of the Yenisei Ridge.

We present results of study of Holocene and Late Pleistocene deposits recovered on the underwater Akademicheskii Ridge in Lake Baikal. The change in mineral composition and grain size in the bottom sediment core is closely consistent with the change of major diatom complexes marking the Holocene–Late Pleistocene boundary. A high content of chloritoid (up to 14.6 %) has been found among the heavy minerals of the sand fraction of Late Pleistocene clays. The concentration of chloritoid in Holocene mud is no higher than 1.2 %. The source of chloritoid is chloritoid shales of the Anaya Formation (Upper Proterozoic), widespread in the watershed of the Primorskii Ridge in the upper reaches of the Lena and Anaya Rivers. Chloritoid was transported to the area of the Akademicheskii Ridge by predominant western and northwestern winds, which is also evidenced from the absence of mechanical impacts on the surface of its grains. The high contents of chloritoid in the Late Pleistocene sediments are due to the more intense eolian transportation at that time as compared with the Holocene.

Kuh-e Mond Field is a conventional heavy-oil resource in the Zagros Foreland Basin, Iran, produced from the fractured carbonates partially filled with dolomite, calcite, and anhydrite cement. Vitrinite reflectance data from carbonate reservoir suggest low-maturation levels corresponding to paleotemperatures as low as 50ºC. The observed maturation level (<0.5% R_max) does not exceed values for simple burial maturation based on the estimated burial history. Oil inclusions within fracture-filled calcite and dolomite cement indicate the key role of these fractures in oil migration.
Fluid inclusion temperature profiles constructed from the available data revealed the occurrence of petroleum in dolomite, calcite, and anhydrite and characterize the distinct variations in the homogenization temperatures (T_h). Fluid inclusions in syntectonic calcite veins homogenize between 22ºC and 90ºC, showing a salinity decrease from 22 to 18 eq. wt.% NaCl. Fluid inclusions in anhydrite homogenize at 50ºC, showing that the pore fluids became warmer and more saline during burial. The T_h range in calcite-dolomite cement depicts a change in water composition; therefore, we infer these cements precipitated from petroleum-derived fluids. The petroleum fluid inclusions microthermometry data suggest that the reservoir became filled with heavy black oils and high-salinity waters and indicate that undersaturated oil was present in a hydrostatically pressured reservoir.
The T_h data do not support vertical migration of hot fluids throughout the section, but extensive lateral fluid migration, most likely, drove tectonically dewatering in the south or west of the pool.

The role of fracture and faulting zones in the filtration inhomogeneity of productive formations in oil and gas fields has been considered. The absence of fracture or faulting zones from geological and, consequently, hydrodynamical models reduces the flooding efficiency and the oil recovery factor in general.
The current situation results from the underdevelopment of methods for the mapping and classification of fracture zones based on seismic-prospecting data. For example, the use of the seismic-horizon gradient method makes it possible to describe only complete rock fracturing at tectonic-block boundaries.
We propose to use the structural inhomogeneity of the deformation field of seismic horizonts for a more complete description and classification of fracture zones by the faulting regime. This parameter is suitable not only for late but also for early stages of faulting and describes block-boundary deformations more accurately than gradients. The example of a West Siberian field showed that the production wells from the J₁ productive formation in the faulting zones with the calculated post-Cenomanian structural inhomogeneity of the deformation field have considerably higher discharges owing to fracture permeability.

Local tectonic stress field has been calculated based on aftershock focal mechanisms, using materials of epicentral studies with temporary networks of stations in the epicentral area of the 2003 Chuya earthquake. The originality of this work is that the stress state is studied invoking earthquakes with large differences in energy and using highly accurate data on the location and depth in the analysis of events. The main result is the selection of two hierarchical levels of the stress field. It is shown that the stress state determined from data on large earthquakes is homogeneous (horizontal shear) throughout the aftershock zone, and the stress state determined from data on small events varies according to the block structure of the aftershock region.

Magnetotelluric sounding (MT) measurements are conducted, as a rule, along a single profile or mutiple profiles, and data are interpreted using 1D and 2D earth models. In a real situation, the earth is three-dimensional, and the primary field may vary laterally. This implies that 1D and 2D inversions of MT data contain some error, up to the appearance of false structures. In this study, analytical methods are used to obtain expressions for the effect of small deviations of the earth and the primary field from the Tikhonov–Cagniard model on the impedance and tipper tensor components (sensitivity functions determining the size of the zone near the sounding point within which the perturbations affect the impedance and tipper at this point).

We propose a heuristic method for the analytical solution of electrical-prospecting problems for direct currents and 3D isotropic media. The corresponding parameters of the medium (conductivity, magnetic permeability) are determined from Maxwell’s equations by the assignment of electric-or magnetic-field intensity in the analytical form. The application of this method is illustrated with examples.

The development of the predictive estimation of drainage waters of sulfide-containing wastes from mining industry attracts much attention. Investigation of particular objects permits the development of empirical dependences for analysis of various situations in similar technogenic systems. In this contribution, the acid and neutralization potentials of the waste rocks from the Veduginskoe and Taseevskoe gold deposits were calculated from the measured contents of sulfide sulfur and CO₃^2–. Accelerated-leaching experiments (peroxide and dynamic) were carried out, and pH values, acidity and alkalinity (titrated and calculated), and metal (Fe, Zn, Cu, Pb, Ni, Co) concentrations were determined. The values of net acid generation potential were compared with the chemical composition of model drainage waters. The data were correlated with the physicochemical processes in the water-rock system. The value of net acid generation potential was proposed to use for estimating the chemical composition of mine waters.