Home – Home – Jornals – Russian Geology and Geophysics 2024 number 9

We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur-Stanovoi superterrane. The Larba block is dominated by basic schists and garnet-biotite-orthopyroxene and garnet-biotite-cordierite-sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P - T parameters. The P - T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7-8 kbar, 800-850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4-5 kbar and T = 630-700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P-T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, t _Nd(DM) = 2.57-2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8-3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.

We have studied the isotopic composition of Nd, Sr, and Pb in Permo-Triassic subalkaline dolerites and Late Cretaceous basanites of the northern part of the Minusa depression. The wide variations in the primary isotope parameters of dolerites (ɛ_Nd = 6.6-8.5, ⁸⁷Sr/⁸⁶Sr = 0.7031-0.7061, ²⁰⁶Pb/²⁰⁴Pb = 18.13-18.72, ²⁰⁷Pb/²⁰⁴Pb = 15.51-15.55, ²⁰⁸Pb/²⁰⁴Pb = 37.88-38.07) and basanites (ε_Nd = 5.3-9, ⁸⁷Sr/⁸⁶Sr = 0.7026-0.7054, ²⁰⁶Pb/²⁰⁴Pb = 18.63-19.09, ²⁰⁷Pb/²⁰⁴Pb = 15.54-15.56, ²⁰⁸Pb/²⁰⁴Pb = 38.40-39.01) indicate both heterogeneity of mantle mafic melts and their partial crust contamination. Doleritic magma was presumably generated predominantly from a substance from a moderately depleted mantle source, which is similar in isotopic composition to the PREMA component of sublithospheric plumes. The basanitic magma might have formed through the melting of the material of the subcontinental lithospheric mantle modified as a result of plume activity in the Paleozoic-early Mesozoic. The similar isotopic compositions of Pb in basanites and the derivates of the enriched lithospheric mantle (EM 2 type) are due to the mixing of different substances of the SCLM substratum.

The generalized chemical composition data on the studied dolerites from the Vilyui-Markha dike swarm (Vilyui paleorift) allowed detecting geochemically anomalous areas among them. It has been found that percentage of TiO₂ and a number of heavy rare-earth elements (Th, Ta, Hf, Y, Nd) in the dolerites of dikes located within the kimberlite fields show almost twofold increase compared to the normal content for these elements in the Vilyui-Markha dolerites. Similar behavior of these elements is also observed in the dolerite dikes near the Kuoika kimberlite field (the Molodo dike swarm, Olenek paleorift). It is inferred that there is a connection between the increase in abundances of titanium and rare-earth elements in the dolerites and that the latter occur within the lithosphere blocks hosting kimberlite fields. The areas of lithospheric mantle roots linked with the formation of proto-kimberlites may have had bearing on the composition of tholeiite melt both in the processes of its generation and during its ascent to the Earth’s surface, which led to its local enrichment in titanium and a number of other elements. While the processes of the alignment of the geochemical composition between the high-titanium basites and the rest of the melt beyond the contours of the lithospheric blocks with kimberlite fields were weakly pronounced, they were responsible for appearance of zones enriched in titanium. Kimberlite fields that formed before or after basite magmas intrusion overlap with placements of high-titanium dolerite dikes. Thus, high-titanium dolerites in dike swarms (belts) can be used as one of the criteria for kimberlite searches. In view of the above, two new potential zones (Tenkelyakh and Kyulenke areas) for kimberlite exploration have been identified within the Vilyui-Markha dike swarm.

Understanding the origin of quartz veins of the mica-pegmatite formation is of great importance for deciphering the emplacement conditions for the of commercially prospective quartz targets. On the example of the Berkutinskaya vein, which is the typical metamorphogenic quartz deposit of the mica-pegmatite formation of the South Urals, we study the source and origin of the mineral-forming fluid, temperatures of quartz formation, the structure of intergranular boundaries, their morphometric feature, the fractal dimension of quartz grains and commercial characteristics of quartz grit. An integrated approach to the study of the typomorphic features of quartz allowed us to assess their impact on the technological characteristics of quartz concentrates. Comparison of data on the content of impurity elements in the Berkutinskaya quartz vein with quartz of the vein No. 175 of OOO Russian Quartz and IOTA-std (Sibelco) showed that the Berkutinskaya quartz vein is promising for commercial development.

Dinocyst assemblages are subjected to taxonomic typification based on a qualitative and quantitative assessment of their generic composition for the Northern Hemisphere during the Coniacian-Santonian time in order to identify correlation taxa. The typification is carried out using a cluster analysis according to the Jacquard calculation model (BioDiversity Professional, 1997). Three types of dinocyst assemblages have been identified in the Coniacian. As compared to the Turonian, the relatively cold-water West Siberian basin is characterized by a decrease in the number of cosmopolitan taxa and an increase in species endemism. The only taxa identified at present are those allowing for intrabasin correlations. Three types of dinocyst assemblages have also been established for the Santonian. Free interbasin connections in the Northern Hemisphere contribute to the equalization of the generic composition between the three identified types and to the identification of characteristic taxa that allow for interregional correlation.

The Permian-Triassic (P-T) boundary records the most dramatic events in Phanerozoic history. The character of the boundary differs greatly, so it has been the subject of great discussion. The Barents Sea separates regions having markedly different expressions of the P-T boundary, and it can give an insight into the conditions of formation of this boundary in different parts of the European North. This contribution is based on a combination of regional projects from the Russian and Norwegian sectors, including seismic data across the Barents Sea and well data in marginal zones. Application of new seismic data and the sequence stratigraphy concept provides a novel approach to correlation and interpretation of the P-T boundary beneath the Barents Sea. The study has revealed a distinct regional sequence boundary corresponding to the P-T boundary with conformable and unconformable bedding. Three major types of this stratigraphic boundary are recognized. The distinct “erosional” type of the P-T boundary, with a significantly reduced Permian section, exists in the southeast (Timan-Pechora Basin). The conformable “overcompensated” type of boundary with an additional Lower Triassic section is interpreted in the central part of the Barents Sea. Westward, beneath the Norwegian sector, a relatively conformable “condensed” type of P-T boundary predominates. In addition to these types associated with regional paleostructural and depositional trends, the superposed “structural” subtype caused by local growth of structures at the Permian-Triassic boundary is identified.

Erosion and sedimentary landforms are linked through sediment transport pathways, which forms a source-to-sink system (S2S). The coupling relationship of different components in the clastic sediment system is emphasized by the S2S concept. A new method for characterizing the sedimentary process of continental rift basins is also provided. It has been proven that there is rich exploration potential in the third member of the Shahejie Formation (Es₃) in the Shulu Slope of the Bohai Bay Basin in China but with relatively low production. With the complex structural background of the Es₃ of the Shulu Slope, conventional research methods are ineffective in guiding the current development strategies. Therefore, this study adopts the S2S theory, and its elements in the study are characterized using core, logging, and seismic data. The results suggest that the S2S in this study area was supplied by the Ningjin Uplift in the western region, and a fan delta and lake sedimentary systems were formed by the sand transported through valleys and fault troughs. The S2S coupling model, “the Ningjin Uplift sand supply-fault trough, valley transport-fan delta, and shore-shallow lake sedimentation,” is established. This research field permits prediction of sand bodies in continental rift basins with similar structural backgrounds.

Permeability is one of the most significant and challenging parameters to estimate when characterizing an oil reservoir. Several empirical methods with geophysical borehole logs have been employed to estimate it indirectly. They include the Timur model, which uses conventional logs, and the Timur-Coates model, which uses the nuclear magnetic resonance log. The first goal of this study was to evaluate porosity, because it directly impacts permeability estimates. Deterministic and stochastic inversions were then carried out, as the main objective of this work was to estimate the permeability in a carbonate reservoir of the Campos Basin, Southeastern Brazil. The ridge regression scheme was used to invert the Timur and Timur-Coates equations deterministically. The stochastic inversion was later solved using fuzzy logic as the forward problem, and the Monte Carlo method was utilized to assess uncertainty. The goodness of fit for the estimations was all checked with porosity and permeability laboratory data using the Pearson correlation coefficient ( R ), root mean square error (RMSE), mean absolute error (MAE), and Willmott’s agreement index ( d ). The results for the Timur model were R = 0.41; RMSE = 333.28; MAE = 95.56; and d = 0.55. These values were worse for the Timur-Coates model, with R = 0.39; RMSE = 355.28; MAE = 79.35; and d = 0.51. The Timur model with flow zones had R = 0.55; RMSE = 210.88; MAE = 116.66; and d = 0.84, which outperformed the other two models. The deterministic inversion showed, thus, little ability to adapt to the significant variations of the permeability values along the well, as can be seen from comparing these three approaches. However, the stochastic inversion using three bins had R = 0.35; RMSE = 320.27; MAE = 190.93; and d = 0.73, looking worse than the deterministic inversion. In the meantime, the stochastic inversion with six bins successfully adjusted the set of laboratory observations, because it provides R = 0.87; RMSE = 156.81; MAE = 74.60; and d = 0.92. This way, the last approach has proven it can produce a reliable solution with consistent parameters and an accurate permeability estimation.

Information on the distribution of earthquake hypocenters for many seismically active zones of Siberia remains insufficient, which is associated with sparse networks of seismological observations. The paper presents the results of determining the earthquake depths in several seismogenic areas in the Altai-Sayan region, Cisbaikalia, Transbaikalia, and Yakutia using the travel time of longitudinal refracted waves from the Mohorovičič discontinuity ( P_n waves) from earthquakes and the recently obtained information about the deep structure of these regions. The depth determination algorithm is tested using data from aftershocks of large earthquakes occurring in Tuva in 2011 and 2012 ( M_L = 6.7 and 6.8), recorded both by the regional seismological network and by a local group of seismic stations. Different methods are applied to reveal that some aftershock depths have a close match, including those for the main shocks - the Tuva-1 and Tuva-2 earthquakes. Another good match of earthquake depths is obtained using Р_n waves with materials from regional and detailed studies of the Baikal branch of the Geophysical Survey of the Russian Academy of Sciences in the Muyakan activation site in the Baikal rift zone. The resulting data complement the information on the hypocenter of the main shock and confirm the change of the Muyakan activation cluster from large depths to shallow ones since the onset of activation in 2014. New information on the earthquake depths using Р_n waves is obtained in Yakutia along the border of the largest Eurasian and Okhotsk tectonic plates. It is revealed that they decrease to 6-12 km as compared to higher depths of 20-30 km in adjacent areas. The resulting series of new information on the distribution of earthquake hypocenters using Р_n waves is extremely important mostly because it indicates the possibility of identifying and redefining earthquake hypocenters using previous seismological observations in seismically active zones of Siberia.

Earthquake focal mechanisms that are atypical for the South Baikal basin, which is under the extension of the Earth’s crust in the NW-SE direction, are analyzed. Atypical mechanisms are understood as focal solutions of strike-slip and reverse fault types, as well as solutions with normal fault movements along NW-trending planes transverse to the main structures of the basin. Whereas normal faults along NE-trending planes dominate, 29% of solutions from the sample of focal mechanisms are of non-normal fault type, of which 18% account for strike-slip faults and their combinations with other types of displacements (with a normal or reverse component) and reverse faults (with a strike-slip component) - 11%. Such displacements occur predominantly along NW-trending planes, as well as along submeridional and sublatitudinal ones, and strike-slip movements are characterized by right-lateral displacement along NW and submeridional planes, and, accordingly, left-lateral displacement along sublatitudinal and some NE planes. Earthquakes with atypical mechanisms are distributed almost throughout the entire basin, but it is necessary to note an increase in their number on its southwestern termination (the Kultuk segment) and on the eastern side of the Central Basin. In the current field of crustal extension, transverse shears play the role of transfer faults, accommodating differences in the rates and vectors of deformation of local blocks within the basin, and on a regional scale between neighboring rift basins.