DEEP STRUCTURE OF THE EURASIA-PACIFIC TRANSITION ZONE

Abstract

The deep structure of the Eurasia-Pacific transition zone was investigated under the Geotraverse International Project along the deep sections of the tectonosphere, including the lithosphere and the asthenosphere, based on the complex interpretation of geological and geophysical data. The first geotraverse, investigated in cooperation with Japanese geoscientists, crossed the region of the Japan Sea. The second geotraverse, investigated in cooperation with Japanese and Chinese geoscientists, crossed the region of the Philippine Sea and the North China Plain. The third geotraverse crossed the region of the Okhotsk Sea. The total length of the geotraverses amounted to a few thousand kilometers with a depth of 100 km. The structure of the study region is distinguished by the fact that its upper mantle includes an asthenosperic layer with its diapirs of hot anomalous mantle, responsible for the formation of a transitional zone. The asthenosphere has a depth of 50-80 km under the old Paleogene basins, such as the West Philippine Basin, a roughly 30-km depth under the Neogene basins, such as the Parece Vela Basin of the Philippine Sea or the Kuril Basin of the Okhotsk Sea, and a merely 20-10-km depth under the Pliocene-Quaternary inter-arc basins, where it caused the break-up of the lithosphere, the formation of rifts, basalt magma flow, and hydrothermal activity. The sedimentary basins of the marginal seas are distinguished by an abnormal deep structure characterized by the localization of asthenospheric diapirs under these basins, the development of rifts and spreading centers at their bases, volcanic activity during the early phase of their formation, associated with hydrothermal processes and sulfide formation, and the high heat flow caused by the rise of the asthenosphere toward the surface. It appears that the asthenospheric diapirs with the partial melting of rocks represent channels by which hot mantle fluids from the asthenosphere penetrate to the sedimentary basins. Based on these geotraverses, researchers from the Geophysical Center of the Russian Academy of Sciences created a database including the deep geological and geophysical sections of the lithosphere under the transition zone from Eurasia to the Pacific and the related primary geological and geophysical data, the results of the bathymetric, magnetic, and gravity surveys, heat flow measurements, deep seismic sounding, tomography, seismology, the results of studying the fine structure of the Benioff zone, some data on the chemistry and age of the rocks, and the results of deep-sea drilling and dredging.