Литосфера (Aug 2017)
The structure of the lithosphere in area of junction between the Okhotsk sea and the continental tectonic structures
Abstract
The area of junction of the Ul'in volcano-tectonic structure of the Okhotsk-Chukotka volcanic belt and tectonic structures of Okhotsk Sea has both fundamental interest - for understanding tectonics and geodynamics on the outskirts of the continent, and practical - for assessing the metallogenic potential of the territory, primarily for gold. The main subject of the present studies was the deep unhomogeneities and disjunctive tectonics, clarification of the block structure of the lithosphere. The creation of a deep base was carried out by means of a complex interpretation of geophysical data (maps of gravity and magnetic fields), data of the Deep Seismic Sounding profile of the north-eastern section of the 3-DV profile (Skovorodino-Tommot-Khandyga-Myakit), geophysical and geological information. The main tool for geopotential fields data processing was the computer technology “KOSKAD 3D” intended for processing of three-dimensional digital geoinformation by the methods of probability-statistical analysis. Three types of 3D gravity field models up to a depth of 90 km are calculated: 3D distribution of low-frequency characteristics; 3D distribution of high-frequency characteristics; density 3D models. An analysis of petrophysical information was carried out on a formalized basis with subsequent compilation of specialized petrodensity and petromagnetic maps. The obtained 3D models allowed to construct sections and slices of the gravity field components, density of the lithosphere at different depth levels. Cartographic, geophysical and petrophysical materials formed the basis for a map of faulty tectonics in the region.The main disruptive disorders are ranked in the direction (northeastern, northwestern, sublatitudinal, submeridional), by the depth of deposition (mantle, lower crust, medium-crustal, upper crustal, near-surface), by their specialization - (blocking, magma-leading). According to the density model, the mantle part of the lithosphere is dense enough both for the continent and for the Okhotsk Sea. In the earth's crust, the Dzhugdzhur-Okhotsk and Mongol-Okhotsk fault systems divide the continental part of the territory into three regions: high-density - western and eastern (coastal), and central - decompressed ones. The Dzhugdzhur-Okhotsk northeastern system of disturbances is a regional boundary between a dense and reduced density lithosphere on the continent. Geologically, it can be regarded as the boundary of the ancient (Jurassic-Cretaceous) paleosubduction, during which a belt of a granitized decompressed lithosphere formed. The boundary of the junction of marine and continental structures does not coincide with the coast line: marine structures rise under continent, indicating a collision between the Okhotsk Sea and the Eurasian plate. Probably, the Jurassic-Cretaceous subduction regime on the outskirts of the Eurasian plate was replaced by a collision one in the Cenozoic, as a result of which the Ul'in volcano-tectonic structure was divided by northwestern faults into transverse blocks. The subsequent intraplate marginal rifting caused the Coastal (Mongol-Okhotsk) fault to “creep” of continental deposits onto “marine” tectonic structures. The results of the performed studies made it possible to better understand the nature of the interaction of the tectonic structures of the southwestern margin of the Okhotsk Sea. The deep-seated schemes of fault tectonics and the transformant of the gravitational field can be used for palinspastic reconstructions. The constructed petrophysical maps have prospects for further multi-functional application. The received structural basis can become the basis of studying laws of formation of mineral deposits.
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