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Original analytical data on trace elements and radiogenic Nd and Pb isotopes in the volcanic rocks of the Southern and southwestern part of the Northern plateaus of the underwater Vityaz Ridge are presented. Interpretation of these data and a comparison with published materials on the volcanic rocks from the southern and northern parts of the Kuril Island Arc (KIA), which formed on two basement blocks of different genetic nature, allow us to draw the following conclusions. The tholeiite varieties of volcanic rocks of the Southern Plateau and the southern part of the KIA have similar isotope-geochemical features, which point to the similar geodynamic conditions of the formation and the identical influence of low-temperature fluid on magma-generating processes. The geochemistry of the volcanic rocks of the Northern Plateau, which are mainly represented by subalkaline varieties, indicates a more pronounced contribution of the mantle component to the magmagenesis and a greater degree of influence of high-temperature melt compared to the rocks of the Southern Plateau, but a lesser degree compared to the rocks of the northern part of the arc. The volcanics of both plateaus are derivatives of a single mantle source, the MORB of the Indian Ocean (Indian MORB), and were formed together with the rocks of the southern part of the KIA within the lithospheric block transformed by tectonomagmatic processes that accompanied the opening of the Kuril Basin. [ABSTRACT FROM AUTHOR]

A model of the geological evolution of the Japan and Okhotsk seas was developed based on radioisotope age, mineralogical, and isotope-geochemical study of the Late Mesozoic–Cenozoic volcanic rocks. The geodynamic settings of volcanic stages were determined as follows: (1) the Late Cretaceous continental-margin (calc-alkaline), (2) the Eocene transform-margin (adakite) in the Sea of Okhotsk, (3) the Miocene–Pliocene marginal sea (alkaline basaltic) in the Sea of Japan, and (4) the Pliocene–Pleistocene island arc (calc-alkaline) in the southern Sea of Okhotsk. It has been established that parental magmas were derived from subcontinental lithosphere, oceanic asthenosphere, and lower-mantle plume-continental (CAB) and plume-oceanic (OIB) sources. Geodynamic settings changed from the Late Cretaceous subduction to the Maastrichtian–Pliocene transform margin. The transform margin setting involved destruction and extension, maximum oceanic-margin spreading (end of the Early Miocene–beginning of the Middle Miocene), and post-spreading lower mantle plume upwelling (the Middle Miocene – Pliocene). It was completed by the resumption of the Pliocene–Pleistocene subduction of the Pacific plate beneath the Eurasian continent. [ABSTRACT FROM AUTHOR]

An extended and improved model of the origin and evolution of marginal seas in the strike-slip megazone due to the lateral interaction of the Eurasian and Pacific lithospheric plates is proposed. In the megazone, fragmentation of continental and oceanic plates, mantle upwelling, and ascending and descending lithospheric seismic vortexes occur. As a proof of the validity of the proposed model, the paper presents data from factor analysis, results of gravity and termophysical modeling, an analysis of distributions of earthquake magnitudes in 3D space, and geochemical features of volcanism in the Sea of Japan and Sea of Okhotsk. Study of the variability of earthquake magnitudes for the Sea of Okhotsk region revealed traces of vortex processes in the circular orientation in the variability vectors of magnitudes in the subcrustal viscous layer and in the asthenosphere. The head of the mantle plume has a typical mushroom shape, and its central trunk extends into the mantle below a depth of 200 km. Studies of the isotopic age and geochemical features of volcanic rocks in the Sea of Okhotsk and Sea of Japan have allowed researchers to identify the magma sources—lithospheric, asthenospheric, and subasthenospheric (plume); to trace the geochronological sequence in the ascent of mantle substrate; and to determine the underlying formation mechanisms for the marine basins. The origin of marginal seas is discussed and compared to other geodynamic models with different geotectonic paradigms. The origin of marginal seas is discussed and compared with other geodynamic models of different geotectonic paradigms. [ABSTRACT FROM AUTHOR]