GEODYNAMICS OF THE NORTH-EASTERN ASIA IN MESOZOIC AND CENOZOIC TIME AND THE NATURE OF VOLCANIC BELTS

Abstract

Geodynamics and tectonic evolution of the North-Eastern Asia was determined by the establishment of those structural elements in the geological sections which are typical of the modern active continental margins.The Neogene island arcs on the majority of their strike coincide with the modern ones (the Kuril-Kamchatka) but locally they are broken up (the Western Sakhalin). They are conjugated with the back, front and interarc troughs. The Paleogene island arc occurs, probably, in the northwestern Kamchatka and on the Academy of Sciences uplift in the Sea of Okhotsk. The Early Mesozoic Uda-Murgal island arc marks the southeastern boundary of the ancient Eastern Siberia megablock. Along its southern and northern boundaries the island arc complexes of the same age are distinguished. Position of Eugeosynclinal zones on the outer side of the island arcs indicates the isolation of the Eastern Siberia megablock from the Bureya-Ehanka, Okhotomorskiy and Chukotka ones the basement of which is composed by the Early Precambrian metamorphic complexes of sialic composition and granitoids. Tectonic movements at the end of Neocomian are expressed in the formation of the Andian type active continental margin with the Okhotsk-Chukotka margin-continental volcanic belt originated on a site of the Uda-Murgal island arc. This process was accompanied by the geometrical changes of the Benioff zone determined by the analysis of K2O content in volcanites on the base of Dickinson and Hatherton's diagrams -the angle of dip became more gentle, the width along the dip and depth of the magma active part of the Benioff zone were increased. Simultaneously with the development of the Okhotsk-Chukotka volcanic belt there was formed the island arc on the East of the Sikhote-Alin. Owing to the Pre-Senonian movements on a site of this arc the Eastern Sikhote-Alin margin-continental volcanic belt similar to the Okhotsk-Chukotka one originated.Many of the island arcs and margin-continental volcanic belts occur along the margins of the ancient sialic megablocks. Paleotectonic reconstructions prove the fact that there were significant horizontal displacements of the sialic megablocks causing the crash of the oceanic basins situated between them. The enlargement of the eastern part of the Asian continent took place not only at the expense of the island arcs displacement towards the ocean, but also due to the sialic blocks joining.Geodynamics and tectonic evolution of the continental margin is revealed by the determination of those structural elements in the geological sections which are typical of the modern and Late Cenozoic geosynclinal systems (island arcs). The most significant attention is paid to the tectonic position and nature of volcanic belts of different types and ages widely distributed in the North East of Asia. They help to reconstruct the former plate boundaries and to determine the kinematics of their movements accroding to the plate tectonics models. Among them we distinguish: (1) the inner island arcs or volcanic geanticlines, (2) margin-continental volcanic belts, (3) plateau-basalts coinciding with the margin-continental belts in the space but being the independent formations.