Abstract:The paper presents the results of deep seismic studies on Geophysical Reference Profile 1-SB (Sredneargunsk -Ust-Karenga -Taksimo -Vitim) in East Transbaikalia, Russia. The 1200 km long profile crosses the major tectonic structures of the Central Asian fold belt: the Argun median massif, the Selenga-Stanovoy and Transbaikalia folded regions, and the Baikal rift zone. Its northwestern fragment extends into the Angara-Lena monocline of the Siberian platform. The southeastern (Transbaikalia) and northwestern (Baikal-Patom) fragments of the profile are based on the spot (differential) seismic sounding technique using explosions and 40-tonne vibrators. The southeastern (Transbaikalia) fragment shows small crustal thickness values (~40 km), an almost horizontal position of the Moho, and high velocities of longitudinal waves (~8.4 km/sec) beneath the Moho. The analysis of parallelism graphs and the dynamic expression of the wave refracted from the Moho suggests a less than 5-10 km thick layer of high velocities and low gradients below Moho. The database on the territory of Transbaikalia includes ~200 wave arrival times from large earthquakes, which were refracted at the Moho at distances of ~200-1400 km. As part of the tomographic interpretation, using additional DSS data on the Moho, the territory of Transbaikalia has been mapped to show the patterns of the threshold velocity values at the Moho. The seismic data was used to contour an area with high velocity values in the mantle in the central part of the Mongolia-Okhotsk orogenic belt and the neighboring fold structures of Transbaikalia. According to the analysis of the seismic and geologic data on the study area, the mantle layer with high velocity values in the Mongolian-Okhotsk orogenic belt may be represented by the eclogitic rock plates.Key words: deep seismic sounding (DSS); velocity of longitudinal waves; hodographs from explosions and earthquakes; Moho; Mongolia-Okhotsk orogenic belt For citation : Soloviev V.M., Chechelnitsky V.V., Salnikov A.S., Seleznev V.S., Liseikin A.V., Galyova N.A., 2017. Specific velocity structure of the upper mantle in the Transbaikalia segment of the Mongolia-Okhotsk orogenic belt. GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E

The recent geodynamics of the Far East region of Russia is considered, where three large tectonic plates converge—Eurasian, North American, and Pacific, as well as several microplates—Okhotsk, Bering, and Amurian—have been hypothesized to exist. The available data on the position of the plate boundaries, the relative plate rotation poles, and the regional seismicity were analyzed, and parameters of plate motion models for northeastern Russia were determined in this study. The regional deep structure was investigated, using data obtained by different geophysical methods. The results of observations of the Magadan–Vrangel Island profile by deep seismic sounding (DSS), common-depth point (CDP) method, and correlation refraction method (CRM) are presented.

Analysis of geological and geophysical data on the boundary zones between the Eurasian plate and other plates shows poor knowledge of the deep structure of the region. This information will help to refine the position of the plate boundaries and the structure of the Earth’s crust and mantle. We present data on the seismicity and deep structure along the deep seismic sounding (DSS) profile running across the boundary between the Eurasian and Okhotsk plates. A comprehensive analysis of the DSS materials and seismic data shows a significant expression of this boundary zone both in the deep Earth’s crust structures and in the Moho. A zone of anomalous seismicity and deep structure extends along the DSS profile for several hundred kilometers. We have refined the position of the main boundary between the Eurasian and Okhotsk plates, which passes approximately along 144° E.

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—A river seismic survey technology based on using pneumatic water sources and autonomous seismic recorders, which are installed on the river bank and configured for continuous seismic recording, is developed at the Geophysical Survey of the Russian Academy of Sciences (GS RAS). In recent years, several thousand kilometers of profiles have been developed by the CDP-2D method on the rivers of East Siberia: Lena, Nizhnyaya Tunguska, and Vitim. Previously, only 6- to 10-s seismograms were used to study the structure of the upper part of the Earth’s crust. At the same time, the deep structure of the crust in the vast territories of East Siberia remains poorly understood because of the high cost of research. The river seismic survey data are used on a profile section acquired in the lower reaches of the Lena River to demonstrate the possibility of studying the crustal structure to the full depth, including the Mohorovičić discontinuity. For this purpose, the archival seismic records are reprocessed with the construction of montages of seismograms of increased duration (up to 23 s). The low-amplitude oscillations of reflected waves from deep boundaries are distinguished owing to multiple summation, which is much larger than in conventional seismic surveys. The equipment used in this study has a sufficient dynamic range, and a high multiplicity is achieved by increasing the binning area. Wind noise leads to the inferior quality of time sections of the upper crust but does not worsen the section at great depths, so this material should not be excluded from processing. The river seismic surveys carried out using the technology developed at the GS RAS on the rivers of East Siberia within about 2700 km contain data that make it possible to build deep sections down to the Mohorovičić discontinuity, and this work needs to be done.