The terrain concept of evolution of the deep structure of Earth’s crust and upper mantle in the North-Sea of Okhotsk depression and in the adjacent land

Purpose. Using new data on the deep structure of the crust and upper mantle and on paleogeodynamics, resulting from complex interpretation, a model of platectonic development of the transition zone northern coast of the Sea of Okhotsk – Eurasia was developed. To show the role of terrains in the development of Okhotsk microplate paleosubduction, appearance of Okhotsk-Chukchi volcanogenic belt and its latter closing. The examples demonstrate that subduction zones accumulate significant biomass and thermodynamic conditions are favorable factors for conversion it to hydrocarbons, which creates prerequisites for the formation of oil and gas fields. Based on the results of deep drilling in the North Sea of Okhotsk basin, it is necessary to justify a new direction in search for oil in the lower structural levels. Methods. Fixist ideas about the nature of North-Okhotsk downfold are considered. From that position one can not explain new data, obtained by the methods of deep velocity imaging and DSS-CDP method conducted by geotraverses 2DV and 2DV-M, from South Kuril basin in the Sea of Okhotsk, through the Kony peninsula (Magadan) to Wrangel is. in the Chukchi Sea. Results. The results of absolute dating, obtained in the last decade, and new geophysical and seismological data allow based on the terrain concept to offer different history of this area development from the late Mesozoic to the present. The gravity field feature, seismological imaging data, acoustic basement surface morphology, the structure of the crust and upper mantle indicate the existence of paleosubduction plate which submerged beneath the Eurasian lithosphere plate. Subduction was accompanied by volcanism, the beginning of which dates back to 106 Myr, and the ending – to 40 Myr. The subducting plate had oceanic crust, which was a part of the Sea of Okhotsk terrain. This combination of oceanic and sea-marginal crust is confirmed by the existence of the South Kuril basin, where the crust is of oceanic type. Scientific novelty. Data, obtained on the basis of complex interpretation of geological and geophysical data, indicate that the North-Okhotsk downfold was at the end of the Mesozoic and early Cenozoic a typical near-border subduction deepwater downfold, bounded on the north by Okhotsk-Chukchi volcanic belt. At this stage tectonic processes took place within the downfold, accompanied by increased seismic activity and the formation of the bottom topography, which is mostly formed by horst-like ridges, separated by deeps. About 40 Myr ago the subduction ceased because oceanic crust terrain was replaced by transitional crust, lighter and thicker, and therefore stronger, which was the main obstacle for continuation of the subduction. From that moment the downfold starts being filled by fine sediments, forming the upper structural level. Change in the interaction of lithosphere plates activated faults in the acoustic basement rocks, which ledges were subjected to erosion, accompanied by the formation of sand strata on the periphery of horsts. These areas are of greatest interest as potential reservoirs for the accumulation of oil and gas in the lower structural level of northern Sea of Okhotsk shelf. Modern seismic activity is associated with the horizontal movements along ancient sublatitudinal deep faults. Such kinematics explains the absence of any ledges, embankments and deeps in the seabed, the formation of which must have been accompanied by tsunami. Their traces on the Sea of Okhotsk shore are not known. Practical significance. Terrain approach to the development of the North-Okhotsk downfold allows to reorient oil exploration on the shelf from the study of the upper structural level, where all five of deep exploration wells were "dry", to search for traps in lower strata, where hydrocarbons may be concentrated in the sandy sediments near horst-like ledges of the acoustic foundation. Seismicity of the downfold and the coast should be considered in any economic activity. Its role will increase in case of shelf oil and gas deposits development, as extraction is often accompanied by man-inducted earthquakes of great magnitude.