Vol 52, No 6 (2018)

Based on obtained data, the paper considers the structure of the sedimentary cover and basement in the continent–ocean transition zone. We analyze the structure of modern tectonic activity zones in the Laptev Sea and structurally similar zones in the Novosibirsk Trough and the De Long Massif. Three sedimentary Anisin–Laptev, Amundsen, and West Laptev basins separated by basement uplifts are distinguished in sedimentary cover. The Anisin–Laptev Basin is separated from the West Laptev Basin by the North Laptev Horst and from the Amundsen Basin by an uplift stretching from the Lomonosov Ridge and covered by the Neogene–Quaternary deposits. The modern tectonic activity zone, marked by a rift valley and earthquakes, stretches across the continental slope from the Gakkel Ridge above a sedimentary rock sequence possessing many-kilometers thickness. The zone reached its present-day position in the Pliocene. Near the shelf boundary, the zone bifurcates, with one branch departing into the West Laptev Basin, and the other branch departing into grabens that developed to the west of New Siberian Islands forming the Laptev microplate.

Our research of the modern structures of Siberian Craton aims to reveal regional regularities in the seismotectonic destruction of the Earth’s crust and to clarify the dynamics of the formation of the focal zones of strong earthquakes. Analysis of activated structures and marginal suture zones located in the study area was based on structural and geophysical data, the modern structural plan, and the quantitative characteristics of modern and recent tectonic movements, active faults, and tectonic stress fields identified by tectonophysical analysis of deformation and seismological parameters. Unambiguous correlation between the seismic activity level of modern structures and the rates of modern and recent tectonic movements were not determined in our study. The most active structures of the Siberian Craton located in zones of the dynamic influence of marginal sutures are contrasted against the gradient field of modern vertical tectonic movements and characterized by the mosaic field of the mean and low rates of modern movements. The kinematics of seismotectonic deformation and levels of seismic activation of suture zones of the Siberian Craton are governed by global geodynamic processes taking place at the boundaries between the Eurasian, North American and Amur lithospheric plates. The activated structures in the northern regions of Siberian platform, which are characterized by the highest rates of modern movements, are heterogeneous. Both fluid processes and glacioisostatic movements may have influenced the dynamics of the formation of these high-gradient deformation zones. In seismotectonic studies aimed at determining levels of the potential seismic hazard of modern structures, we find it important to take into account errors in geodetic data and ensure more correct reference to the rates of tectonic movements at the neotectonic stage. In order to correctly assess the degree of geodynamic activity of modern structures, special consideration should be given to the fluid-geodynamic factor that controls most geodynamic processes, including tectonic stress accumulation, the formation of earthquake focal zones, and the intensity of seismic events.

According to obtained results of profiling by microseismic sounding method along the line in submeridional direction crossing the eastern cone of the Elbrus volcano and a magmatic chamber was found under the Elbrus volcano in the range of depths of 7‒13 km below sea level with horizontal size of ≈ 7 km. At the depths of 18‒40 km, a narrow linear magmatic focus steeply dipping in the southern direction is deduced, its horizontal size in the cross section is ≈5 km underlain by another magmatic focus at the depths of 45‒55 km, penetrating the upper mantle. The found magmatic chamber and earthquake center in the crust section are formed by isometric and linear low-velocity bodies. The sections of two profiles show deep faults correlated with morphological features revealed on the surface. To the south of Tyrnyauz town at the depths of 17‒40 km below sea level, a low-speed steeply sinking in the northern direction linear body was found and interpreted as cool volcanic channel approximately at the size of Elbrus. Having carried out geophysical research we obtained data on 7 ancient earthquakes with magnitude M = 6.5‒7.0 occurred over past 7000 years in the vicinity of the volcano. In that historic period five strong eruptions of Elbrus volcano had happened. We settled the absence of time commonality in activations of volcanism and seismicity. Currently Elbrus volcano is in a phase of deep seismic abate. In 2004‒2017 we have fulfilled geological, geophysical and paleoseismological research that confirms Elbrus is not completely extinct volcano.