Home – Home – Jornals – Russian Geology and Geophysics 2020 number 1

We study Late Jurassic thrusting of the Archean craton basement over Jurassic sediments in Siberia, with the Khamar-Daban terrane as a rigid indenter. The study focuses on deformation and secondary mineralization in Archean and Mesozoic rocks along the thrusting front and the large-scale paleotectonic thrust structure. The pioneering results include the inference that the Angara, Posol’skaya, and Tataurovo thrusts are elements of the Angara-Selenga imbricate fan thrust system and a 3D model of its Angara branch. The history of the Angara-Selenga thrust system consists of three main stages: (I) detachment and folding of the basement under the Jurassic basin and low-angle synclinal and anticlinal folding in the sediments in a setting of weak compression; (II) brecciation and mylonization under increasing shear stress that split the Sharyzhalgai basement inlier into several blocks moving in different directions; formation of an imbricate fan system of thrust sheets that shaped up the thrusting front geometry, with a greater amount of thrusting in the front because of the counter-clockwise rotation of the Sharyzhalgai uplift; (III) strike-slip and normal faulting associated with the origin and evolution of the Baikal rift system, which complicated the morphology of the thrust system.

The origin of the Late Jurassic-Early Cretaceous volcanism within the northern part of the Argun terrane (eastern Transbaikalia) is considered. New data on the geology, age, and composition of late Mesozoic volcanic complexes of the Ust’-Kara basin are presented. Three stages of volcanism have been identified: Tithonian-Berriasian (~150-143 Ma), Valanginian (~140-136 Ma), and Hauterivian (~134-131 Ma), during which volcanic rocks and sediments of three formations (Udyugan, Ust’-Kara, and Shilka, respectively) were deposited. The petrochemical and geochemical characteristics of the rocks of these formations are considered. The compositions of chemically similar rocks evolved toward an increase in the contents of incompatible elements. The rocks of the Ust’-Kara basin are compared with the coeval igneous rocks of the Great Xing’an and East Mongolian belts, which formed in the settings of an active margin and intracontinental rifting, respectively. It is shown that the rocks of the basin are similar in composition to the volcanics of the Great Xing’an belt. A conclusion has been drawn that the late Mesozoic magmatism in the northern part of the Argun terrane was controlled by subduction processes, which led to the formation of the late Mesozoic active continental margin of the Asian continent.

Based on experimental data (T = 650-800 ºC, P = 1-2 kbar) on the solubility of columbite and tantalite in silicate melt and on the distribution of Ta and Nb among coexisting silicate melt, aqueous fluid, and aluminum fluoride melt, we calculated a possible change in the Ta/Nb indicator ratio in residual deeply differentiated granite melts. The Clarke values of these metals in acid rocks of the Earth’s crust were taken as their initial contents. The calculations were made by the mass balance method. It is shown that the separation of fluid in the closed magmatic system rock-forming minerals-silicate melt-water can lead to an approximately twice increase in Ta/Nb in the residual melt as compared with the initial Clarke value. In the system rock-forming minerals-silicate melt-aluminum fluoride melt with the initial content of fluorine close to that in biotite granites, the Ta/Nb ratio in the residual melt can increase to ~1. Successive crystallization of minerals of the isomorphic columbite-tantalite series can lead to Ta/Nb > 2 in the residual melt. Crystallization of biotite causes a significant increase in Ta/Nb and prevents the accumulation of these metals in the residual silicate melt.

Hydrotalcite supergroup minerals stichtite, pyroaurite, iowaite, and woodallite form a complex solid-solution system at the Kyzyl-Uyuk locality (Terekta Ridge, Gorny Altai, Russia). The diversity of these minerals is due to: (1) subdivision by anionic interlayer composition into carbonate (stichtite and pyroaurite) and chloride (iowaite and woodallite) species and (2) isomorphism of M ³⁺ cations, mainly between Cr- (stichtite and woodallite) and Fe³⁺-dominant species (pyroaurite and iowaite), with a quantitative predominance of stichtite and iowaite. Most of the studied samples correspond to stichtite and woodallite with high Fe³⁺ contents or pyroaurite and iowaite with high Cr³⁺ contents. According to vibrational (IR and Raman) spectroscopy data, the interlayer Cl^- is partially substituted by OH^- rather than CO₃^2- groups. We suppose that the presence/absence of a band in the region 1400-1350 cm^-1 in the Raman spectra of stichtite can be explained by the local distortion of triangular CO₃ groups. Stichtite and iowaite occur here in polytypic modifications 3 R and 2 H that are the most widespread for the hydrotalcite supergroup minerals. For both minerals, the polytype 3 R strongly dominates over 2 H ; the lowest 3 R /2 H ratio determined for the Terekta iowaite and strichtitle is 2:1. The Altai stichtite is close in 3 R /2 H to the stichtite from Tasmania (Australia) and differs significantly from that in Transvaal samples (South Africa).

We present results of analysis of the morphology and chemical composition of platinum-group minerals from chromite-platinum orebodies of zoned clinopyroxenite-dunite massifs in the Middle Urals (Nizhnii Tagil, Svetlyi Bor, Veresovyi Bor, and Kamenushka). Study of more than 500 grains has given an insight into the sequence of formation of platinum-group minerals in chromitites of  the studied massifs. Three assemblages of platinum-group minerals have been revealed: magmatic (Os-Ir-Ru intermetallic compounds, isoferroplatinum, ferroplatinum, sulfides of the isomorphous series laurite-erlichmanite and kashinite-bowieite, and thiospinels of the series cuproiridsite-cuprorhodsite-malanite); postmagmatic (with a predominance of tulameenite, tetraferroplatinum, and ferronickelplatinum, resulted from serpentinization of dunites); and latest secondary (minerals with a predominance of platinum-group sulfides, arsenides, sulfoantimonides, sulfoarsenides, plumbides, and amalgams).

We discuss the structure of the Earth’s crust, the seismic stratigraphy, thermal evolution, and stretching of the lithosphere in the sedimentary basin of the Cooperation Sea located on the continental margin of Antarctica in the south of the Indian Ocean. The sedimentary basin includes intracontinental and marginal rifts; the former is located on the shelf, and the latter, in the deep-water area. A seismostratigraphic analysis has revealed four sequences in the sedimentary cover of the intracontinental rift, which formed fr om middle Permian to late Cenozoic, and nine sequences in the sedimentary cover of the marginal rift, which have been deposited since the Middle Jurassic. One-dimensional numerical modeling of the thermal regime and the basement subsidence was performed over 18 points along the profile crossing the basin from the shelf to the lower continental rise. Based on the results of modeling and seismostratigraphic analysis, we have established variations in rock temperature with depth and in the degree of lithosphere stretching throughout the basin rift history. The modeling showed that the depth of the basement subsidence and the thickness of the crystalline part of the basin crust are governed by the lithosphere stretching before sedimentation. The maximum stretching is reached at the depocenters of rift structures (wh ere the crust is the thinnest), being 2.8 before sedimentation and 1.16 after it in the intracontinental rift and 4.6 and 1.4, respectively, in the marginal rift.

The potentialities of the 3D inversion program WSINV3DMT were estimated on the model [ Zm ] and observed [ Zob ] components of the impedance tensor and its invariants during the construction of geoelectrical models for the southern part of the Altai-Sayan region characterized by a three-dimensional distribution of electrical conductivity. These components were obtained for a 3D model constructed by the method of interactive selection of the 3D model induction curves for the experimental ones. Testing of the WSINV3DMT program on the model magnetotelluric data showed the possibility of not only isolation of blocks of high electrical conductivity but also of the appearance of conductivity anomalies different from the model ones. The paper presents a 3D geoelectrical model for the southern focal zones of the region, constructed with the use of the WSINV3DMT program and the proposed method of interpretation. The isolated blocks of low electrical resistivity are correlated with the location of earthquake foci, deep faults, and regions of high absorption of earthquake exchange waves.

The development of strong seismic activations in the northern Baikal area in 1999-2007 is considered. Based on maps of earthquake epicenter density, it has been shown that each activation is a separate group of seismic shocks (a cluster), whose scale and spatio-temporal pattern depend strongly on the stress-strain state of the crust. Estimates of seismic-moment tensors for strong earthquakes within the clusters demonstrate that the most numerous groups of shocks form in the rift stress field. With moving away to the southeast fr om the conventional axis of the Baikal rift, this field changes under amplification of the compressional stresses in Transbaikalia. Simultaneously, we observe a decrease in the number of seismic events and in their energy level. Totally, the considered seismic activations prove the small-scale block structure of the crust in the northern Baikal area and reflect the main features of the modern geotectonic development of this area related to the adjacent morphostructural zone, wh ere most of fragments are involved in rifting and the other are affected by the activation of positive-sign block movements. The obtained results should be taken into account in the assessment of the seismic hazard of the studied territory.

A general procedure is suggested for calculating the upper (Voigt) and lower (Reuss) bounds of the average elastic constants of an anisotropic medium from crystallographic directions. The elastic tensors of Hooke’s law can be expanded into irreducible representations of the rotation group. The Voigt/Reuss-averaged elastic constants depend on the second and fourth moments of the distribution function rather than on the entire function used for the averaging. In this case, the distribution function depends on one angle, while the elastic constants depend on two variables. The limitations imposed by the probability theory on the moment values are investigated and used to derive general constraints on the Voigt (Reuss) bounds of elastic constants.