Doklady Earth Sciences

This work presents new paleomagnetic data on previously dated Ordovician–Silurian volcanics from four sections in the western framework of the Taratash massif (Southern Urals). The geological data indicate that the region under study was a part of the Baltica paleocontinent since the early Mesoproterozoic. The paleomagnetic data obtained for rocks of the westernmost part of the Ural Fold Belt show no evidence of local and regional rotations relative to Baltica. Consequently, the pole obtained can be extrapolated to the entire platform. The result obtained is characterized by a rather high degree of reliability and can clarify the trajectory of the apparent polar wander path (APWP) for the Baltica paleocontinent during the Late Ordovician–Early Silurian, where reliable paleomagnetic data are not available, and can be used for further paleoreconstructions.

In this work we considered the origin of the Verkhoyansk–Kolyma, Chukotka, and Southern Anyui belts, which are part of the Verkhoyansk–Chukotka folded area. It was shown that the formation of the belts is not associated with a collision impulse of the Siberian and Arctic plates. The first belt was crumpled into the giant horizontal fold (Kolyma loop) due to drift of the Omolon massif along the border of the Siberian craton. The Chukotka belt, which cuts the Kolyma loop, is attributed to the lateral zone of the Arctic tectonic flow in the Eastern Hemisphere of the Earth’s polar area. The Arctic flow shares a border with cut structures of the Verkhoyansk–Kolyma orocline (Kolyma loop) along the Southern Anyui–Pekulnei–Koyukuk transform fault. These structural schemes restrict the schemes of the opening of the Amerasian and Eurasian paleooceanic basins remaining a sublatitudinal vector of spreading of the oceanic lithosphere as the only possible option.

The problems of large-scale contamination of the Shufan volcanic plateau (Primorye) basalts with selective melts from xenoliths are discussed. Silico-alkaline liquids that are selectively extracted from xenoliths mix only to a limited extent with the basalt melt, and form granophyre bands that give lavas a taxitic texture. The unique compositions of buchites (pelitic xenoliths chemically modified with a diffusion interaction with basaltic magma and selectively molten politic xenoliths) are characterized. The mineral associations of buchites are represented by highly ferriferous cordierite (sekaninaite), ultra-ferriferous hercynite, (Al, Zr) Fe-armalcolite, Zr-ilmenite, mullite, sillimanite, high-lanthanum monazite, and a barium–phosphate–aluminosilicate phase. The features of the chemical and mineral composition of buchites reflect the accumulation of refractory elements (Al, Fe, Ti, Zr, Ni, and Cr) in the restitic material of xenoliths, followed by the formation of ultra-aluminous, ultra-ferriferous, and initially immiscible metal-silicate composition. Low Pb isotopic ratios in the Shufan basalts indicate selective contamination with material of the ancient cratonic basement. A steady increase in these values is observed for basalts contaminated with the upper crustal material, which also reflects the isotopic composition of buchites.

The K–Ar isotopic age of the beginning of formation of Ichinsky (0.35) and Khangar (0.4) stratovolcanoes is established for the first time, as well as the age of the final stage of activity of the Lauchan Ridge (about 0.9 Ma) in the Sredinny Range of Kamchatka. Probably the formation of the studied volcanoes may be caused by the generation of the new deep NNE fault within the Sredinny Range of Kamchatka, which in turn was connected to the regional reasons of the increasing endogenous activity at the North-West Pacific.

The phase relations upon eclogitization of basalt and melting of H₂O-bearing eclogite were studied experimentally for basalt–H₂O in the range of P = 3.7–4.0 GPa and Т = 1000–1300°C. It is shown that the phase relations depend on temperature. The formation of a supercritical fluid/melt occurs at T = 1000°C and P = 3.7 GPa; the eclogite‒garnetite transition, at T = 1000–1100°C and P = 3.9 GPa; and partial melting of eclogite with the formation of a Na-alkaline silicate melt and clinopyroxenite restite, at 1150 and 1300°C. the supercritical fluid/melt has a high reactivity, which results in the formation of garnet megacrysts, enrichment of garnet in Ti, and replacement with clinopyroxene, the formation of ilmenite, K-bearing amphibole, and the transformation of eclogite into garnetite due to the mass crystallization of garnet.

The primary results are presented on the dissolution of plane-faced diamond crystals of octahedral habit in a Fe–Ni–S melt under 3.5 GPa and 1400°C. It was found that the dissolution resulted in the transformation of plane-faced into curve-faced individuals of morphological features characteristic for kimberlite diamonds. It was concluded that the diamond forms as such might have formed in reduced domains of the Earth’s mantle before becoming involved in the kimberlite magma.

The location of earthquake epicenters in the central and northern parts of the East European Platform in 2009–2016 has been analyzed. The earthquake epicenters have been recorded by seismic stations of the Geophysical Survey of the Russian Academy of Sciences and using the Mikhnevo Small-Aperture Array of the Institute of Geosphere Dynamics, Russian Academy of Sciences. The results obtained are indirectly indicative of the seismic activity of the regional Riphean structures complicating the foundation surface and of their possible activation at present. The available data on the historical earthquakes also confirm their confinement to paleorifts. It seems appropriate to take into account the position of ancient aulacogens in assessing the seismic hazard of the East European Platform.

We consider a relaxation equation for turbulence wavenumber in semi-empirical turbulence closures. It is shown that the well-known phenomenological equation for the dissipation rate of turbulent kinetic energy can be related to this relaxation equation as a close approximation of the latter for stably stratified quasi-stationary flows. The proposed approach makes possible clarification of turbulent closures in the boundary layers of the atmosphere and ocean by specifying the equilibrium states and relaxation relations consistent with the direct and large eddy simulation data.

Overview maps of tsunami hazards for the Far Eastern coast of the Russian Federation are created. The methodological principles of the PTHA (Probabilistic Tsunami Hazard Assessment) approach are discussed, as are the problems of constructing seismotectonic models of the main tsunamigenic zones, mathematical models and algorithms for calculating probability estimates of tsunami hazards, and some problems of applying the PTHA methods related both to the lack of observation data and the complexity in performing a large volume of numerical scenario simulations. Examples of overview maps of tsunami hazards for various recurrence intervals, constructed using the PTHA methodology and presented using the “WTMap” application, are given.

The Osborn Plateau is a large intraplate rise in the eastern part of the Indian Ocean, which has been poorly studied by the geological and geophysical methods. In cruise SO258/1 on the R/V Sonne, new data were collected using Parasound seismic profiling and a multibeam echo-sounder survey. Faults in the sedimentary cover, which extend to the bottom surface, indicate high neotectonic activity in the Osborn Plateau area. It may continue up to the present, as well as in the adjacent segment of the Ninetyeast Ridge, where strong earthquakes have been recorded. Two reflectors in the upper part of the sedimentary cover mark the global lowering of the World Ocean level at the Miocene/Pliocene and Pliocene/Pleistocene boundaries. The reflector in the sediments at the Lower/Upper Pliocene boundary is associated with a change in the regional hydrodynamic regime that occurred at that time in the eastern Indian Ocean. The rocks dredged on Osborn Plateau are identical to some volcanic rocks of the Ninetyeast Ridge, confirming their assumed genetic link, but they are more similar to the basalts of the Kerguelen Plateau. Extremely altered vitroclastic tuffs appear to have been formed as a result of explosive volcanic eruptions of alkali basalts or foidites under subaeral or relatively shallow water conditions and represent the most recent eruptions in the region.

This paper reports on the nature and history of the mineral-rich frost heave mounds—lithalsas—developed on the first terrace of the Sentsa River in Okinsky District of Buryatia. An ice core of the lithalsa was exposed by drilling of a 20-m borehole. The most typical feature of the isotopic diagrams, which we obtained for the ice core, is their cyclical pattern illustrating isotope minima at depths of about 3–5 m, 9–12 m, and 18–21 m, separated by two distinct isotope maxima. Most likely, this demonstrates the cyclicity of the triple flooding of the growing frost heave mound and the subsequent active evaporation of lake-march waters, which are the main source of moisture for the lithalsa ice core. Based on the detailed radiocarbon dating of organic material from the upper horizons of the highest and surrounding lithalsas, we determined the time of their formation. Organic material in drained frost-susceptible soils was intensely accumulated during the period from 0.5 to 0.2 ka BP. This is the time of the beginning of the formation of permafrost, active ice-formation, and the associated frost heave and lithalsa growth. The age of at least three of four mounds studied is not older than 200 years, it coincides with the cooling at the beginning of the 19th century.