Home – Home – Jornals – Russian Geology and Geophysics 2022 number 1

Neotectonic activity in the area of the Salair Ridge (southern West Siberia) rejuvenated a system of large arc-shaped faults separating the Salair tectonic arc from the adjacent tectonic units. These regional faults, which make up the general tectonic framework of the Altai-Sayan folded area, originated in the late Paleozoic and were repeatedly reactivated in the Mesozoic. The deformation within the major Salair thrust sheet is mainly brittle and follows small fault planes that crosscut the margins of Paleozoic thrusts. The neotectonic faulting controlled the erosion pattern of the territory and produced a reticulate drainage system. The Salair tectonic unit is a single 80 × 250 km block comprising multiple neotectonic blocks, with a relative vertical offset of no more than 100 m in the block interior and 100-200 m at its southern, northern, and eastern borders. The northwestern and southeastern border faults have reverse slip geometry, while the motions on the en-echelon northeastern fault boundary include reverse and right-lateral strike-slip components. The thickness of the Salair thrust sheet estimated from magnetotelluric (MT) data increases in the western direction from 5 to 15 km in the northern block part and from 10 to >20 km in the south. The allochthon base is delineated by a low-resistivity zone interpreted as a horizontal detachment. This boundary formed in the Mesozoic and was rejuvenated at the neotectonic stage. The lithology and deformation of Jurassic sediments filling piedmont basins around the Salair Ridge indicate that the Cenozoic fault pattern generally inherits the Mesozoic framework but differs in about ten times smaller vertical offset.

We present data on the most complete Neogene sections and wells of northern Ustyurt and the Cis-Aral area. The Miocene deposits in the northern chinks and the North Ustyurt depression are composed of sediments of the Tarkhanian, Chokrakian, Karaganian, Konkian, and Sarmatian regional stages of the Eastern Paratethys, which are overlapped by lower Pontian beds. The distribution of thicknesses and facies shows that the North Ustyurt depression remained the main synclinal structure of the region, along which the waters of all Miocene transgressions were spread. Sandy material was brought by the rivers from the north: A sandy lens of Chokrakian age is traced along the Shomishtykol Sor almost to the axial part of the depression. Transgressions were pulsating, and the lower and middle Sarmatian deposits were most widespread. These data are presented as a geologic profile near Beineu Village and as schematic maps of facies for the middle Miocene regional stages.

We present data on the geochemical and Sr-Nd isotope compositions of rocks and on the Lu-Hf isotope composition of magmatic and xenogenic zircons from granitoids and gabbroids of the late Neoproterozoic island arc structure of the Lake Zone. Plagiogranitoids, gabbroids, and quartz diorites (559-542 Ma) formed at the Vendian subduction stage of magmatism, and two-feldspathic granites (~483 Ma) mark Cambrian-Ordovician accretion-collision processes. We have established that the volcanic rocks of the late Neoproterozoic island arc and/or its oceanic base, which formed from the depleted mantle, were the mafic source of plagiogranitoids. This is proved by the overlapping positive εNd values of plagiogranitoids and the host volcanic rocks and by the commensurate εHf values of magmatic zircons from the plagiogranitoids and depleted mantle. The lower εNd values of gabbro and quartz diorites from the Tavan Hayrhan and Shuthuyn plutons, the lower εHf values of zircons from these rocks, and the high (87Sr/86Sr)0 ratios and K2O, Rb, and Th contents point to the generation of these rocks from a less depleted mantle source, namely, mantle wedge peridotites. The isotope composition of the latter changed at the previous subduction stage under the impact of fluids and with the contribution of subducted sediments. The least radiogenic Hf isotope composition of magmatic and xenogenic zircons from Ordovician accretion-collisional two-feldspathic granites of the Ih Zamiin pluton suggests their formation through the melting of the late Neoproterozoic-Cambrian island arc crust with the contribution of more differentiated crustal sources enriched in Th, Nb, and LREE and characterized by low εNd values. The age of xenogenic zircons (≤716 Ma) in the studied granitoids and gabbroids and their similarity in Hf isotope composition to magmatic zircons from the same rocks confirm the formation of the late Neoproterozoic island arc of the Lake Zone in an intraoceanic setting far from ancient continental sources similar to the Dzavhan microcontinent.

We present results of petrographic, geochemical, and isotope-geochemical (Sm-Nd) studies of the Paleoproterozoic terrigenous rocks of the Urik-Iya graben, which formed during three successive stages of extension. We have established that these rocks are both petrogenic (Ingashi and Daldarma formations) and lithogenic (Ermosokha Formation) sediments. It is concluded that the rocks in the lower and, partly, middle parts of the Urik-Iya graben section (Ingashi Formation and Lower Daldarma Subformation) resulted mostly from the disintegration of felsic igneous rocks. The terrigenous rocks in the middle part of the section (Upper Daldarma Subformation) might have formed through the disintegration of both felsic and mafic igneous rocks. The rocks in the upper part of the section (Ermosokha Formation) probably formed from the underlying terrigenous rocks of the Ingashi and Daldarma formations. The Nd model age (2.3-2.5 Ga) estimated for the rocks of the three studied sections points to a predominance of rocks of the Neoarchean and Paleoproterozoic upper continental crust in the provenance.

This paper presents the first data on the mineralogy, geochemistry, and quantitative distribution of suspension determined with the use of sedimentation traps installed in Lake Onego (the exposure time was one year (2019)). The obtained data on the mineralogical and geochemical composition of the recent suspension of the Onego water column are compared with the data for the surface layer of the lake bottom sediments (0-10 cm). Data on the sedimentation rates for the substance determined with the sedimentation traps have been obtained for the first time for Lake Onego. The estimates of the sedimentation rates differ slightly from the results of calculation of the sedimentation rates by radioisotope dating based on the distribution of ²¹⁰Pb activity in the upper layer of bottom sediments in combination with the data on ¹³⁷Cs. It has been established that the mineral part of the dispersed sedimentary matter that entered the lake reaches the bottom sediment without significant changes. The geochemistry of the sedimentation traps is similar in many respects to the geochemistry of the upper part of the bottom sediments (0-10 cm) in different areas of Lake Onego. The dispersed sedimentary material and the bottom sediment differ in the amount of the biogenic part (in the bottom sediment layer (0-10 cm), the biogenic part loses a significant amount of organic matter). The bottom sediments are characterized by a strong predominance of the ferruginous varieties of illite and chlorite, in contrast to the suspension with the prevailing Mg-Fe varieties of these minerals. Degraded mica minerals brought by rivers are regenerated to normal ferruginous illites and chlorites directly in the uppermost part of the bottom sediments. The bottom sediments are characterized by higher manganese and molybdenum contents, and the material from the sedimentation traps, by high mercury contents.

The 2003 Chuya earthquake aftershocks are studied using the data obtained during experiments with dense networks of stations. Density maps of the foci of more than 50,000 aftershocks are compared with the day surface faults and the block structure and tectonics of the focal area. The large shearing strain caused by the Chuya earthquake is accompanied by a spatially intermittent aftershock structure stretching along it. The density maps of long-lasted aftershocks differ in structure from the maps of seismic activity in the initial aftershock area. The study has revealed a relationship between the block structure of the epicentral area and the structure of the aftershock process. The nodes of the intersection of faults with the aftershock area are characterized by reduced aftershock activity. The aftershock process is only partly confined to the block-separating faults. In many cases, the aftershock process is shifted relative to these faults or wanders from them.

A model of the deep electrical conductivity of the Altai-Sayan folded region is presented, which was compiled from the results of three-dimensional inversion of the invariant values of the impedance matrix and three-dimensional impedance data fitting, using the trial-and-error procedure. The obtained electrical-resistivity values were used to estimate the fluid saturation of the Earth’s crust by applying the Shankland-Waff equation. The NaCl salinity of the aqueous solution is taken equal to 170 g/L, for which the fluid saturation is most consistent with the seismic data. The electrical conductivity and fluid saturation of focal zones of earthquakes, activated blocks of the region, and deep faults were studied. Most of the earthquake hypocenters are located above the top of conductors and near deep low-resistivity faults. The position of low-resistivity anomalies correlates with the position of domains with the enhanced attenuation of converted earthquake waves and reduced velocities of compressional waves. The Teelin, Samagaltai, and Kaa-Khem earthquake sources are characterized by maximum fluid contents (0.5-0.9 %), and the Altai, Shapshal, Shagonar, and Bolsheporoshskii sources, by minimum ones (0.1-0.2 %). The fluid saturation of deep faults reaches 1.2 %. The faults characterized by tensile stresses oriented orthogonally to their strike, show the highest fluid contents. A similar pattern is observed for the crustal blocks located beneath depressions. The high fluid contents beneath the Kyzyl Basin and its surroundings correlate with the major compressional-wave velocity deficit beneath it.

Electrical resistivity tomography has been widely applied in prospecting and exploring gold deposits, but its potential for studying placers has not been sufficiently revealed. It is shown on the basis of numerical simulation and the example of field studies that linear anomalies with high electrical resistivity identified in floodplains correspond to gold-promising stream pool and riffle facies. It is concluded from working with alluvial placers in Kamchatka, Altai, and the Baikal Region that electrical resistivity tomography has a high resolution for solving geological problems, which means that it can be used as the main method for prospecting and exploring placers.

The efficiency of the development of an oil and gas field is largely determined by the knowledge of its geologic structure. In the recent decade, complex fractured carbonate reservoirs have attracted more and more attention. This paper is concerned with a new technology for constructing 3D images of complex reservoirs, based on Gaussian beam processing of scattered seismic waves. This technology was developed at OOO RN-KrasnoyarskNIPIneft’ in cooperation with the Trofimuk Institute of Petroleum Geology and Geophysics. To test it, a special synthetic model was constructed, which is analogous to one of the licensed objects of PAO NK Rosneft’. For this purpose, a full-scale 3D seismic modeling was performed, which provided us with synthetic wave fields and made it possible to carry out well-controlled numerical experiments for reconstructing the geologic structure of the object of study. One of the distinctive features of the constructed digital model (digital twin) is the presentation of faults not as some ideal slip surfaces but as 3D geologic bodies filled with tectonic breccias. A series of numerical experiments was performed to simulate such breccias, the geometry of these bodies, and the geomechanical processes of fault formation. To select the parameters of the used method of discrete elements, we used the information obtained by geophysical studies in horizontal wells crossing the fault within the geologic prototype of the constructed digital model.