The South Anyui suture zone consists of late Paleozoic-Jurassic ultramafic rocks and Jurassic-Cretaceous pre-, syn-, and postcollisional sedimentary rocks. It represents the closure of a Mesozoic ocean basin that separated two microcontinents in northeastern Russia, the Kolyma-Omolon block and the Chukotka block. In order to understand the geologic history and improve our understanding of Mesozoic paleogeography of the Arctic region, we obtained U-Pb ages on pre-and postcollisional igneous rocks and detrital zircons from sandstone in the suture zone. We identified four groups of sedimentary rocks: (1) Triassic sandstone deposited on the southern margin of Chukotka;(2) Middle Jurassic volcanogenic sandstone that was derived from the Oloy arc, a continental margin arc, along the Kolyma-Omolon block, south of the Anyui Ocean, a sample of which yielded no pre-Jurassic zircons and a single peak at 164 Ma; (3) suture zone sandstone that yielded Late Jurassic maximum depositional ages and likely predated the collision; and (4) a Mid-Cretaceous syncollisional sandstone that had a maximum depositional age of 125 Ma. These rocks were intruded by postkinematic plutons and dikes with ages of 109 Ma and 101 Ma that postdate the collision. We present a seismic-reflection line through the South Anyui suture zone that indicates south-vergence of thrusting of the Chukotka block over the Kolyma-Omolon block, opposite of most existing models and opposite of the vergence in the Angayucham suture zone, the postulated along-strike equivalent in Alaska. This suggests that Chukotka and Arctic Alaska may have different pre-Cretaceous histories, which could solve space problems with existing reconstructions of the Arctic region. We combine our detrital zircon data and interpretations of the seismic line to construct a new GPlates model for the Mesozoic evolution of the region that decouples Chukotka and Arctic Alaska to solve space problems with previous Arctic reconstructions.

An integrated geological and geophysical study was performed to investigate the region of junction of the eastern part of the Central Asian Fold Belt and the Siberian Platform in the Skovorodino–Tommot 3-DV reference profile line (52°–60° N, 122°–129° E), where the belt is separated from the Aldan–Stanovoi Shield of the Platform by a series of deep faults. The main results are as follows: Seismic, density, and geoelectric characteristics of rocks were obtained and used to determine (refine) the intracrustal boundaries of tectonic structures; large-block structure of the Earth’s crust, caused by mantle faults, and the difference between the layered structure of the crust for the shield and fold regions were established; and available paleomagnetic data were used to perform palinspastic reconstructions for 180 and 140 million years, the most productive metallogenic epoch in the region, coeval with collision processes at the closure of the Mongol-Okhotsk paleobasin.

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

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.

The tectonic structure of the junction of the eastern Central Asian Fold Belt and the Siberian Platform, along with the deep structure of the Earth’s crust and lithosphere in this region, has been described on the basis of new geological and geophysical data (seismic, geoelectric, and space-structural studies as well as new-generation geological maps), combined with new interpretation techniques (processing of the previous data by special software). The data suggest the existence of oblique collision during the convergence of the tectonic plates and, correspondingly, tectonic units composing these plates, when the Mongol–Okhotsk paleobasin closed. Such a scenario within the Aldan–Stanovoi Shield is evidenced by areas of syn- and postcollisional magmatism, with their deep-level and geochemical characteristics, and by the presence of a Late Mesozoic fold–thrust zone. Deep “traces” of these tectonomagmatic events, detected in the course of geological and geophysical modeling, are manifested in inclined deep boundaries between the crustal and lithospheric blocks. On the Earth’s surface, they correspond to large fault systems: the Dzheltulak, North and South Tukuringra, Gulyui, and Stanovoi. It has been found that the influence of collision decreases northward with distance from the junction of the eastern Central Asian Fold Belt and the Siberian Platform (Dzheltulak and North Tukuringra transcrustal faults).