Effect of the Western Pacific plate subduction on upper mantle in the eastern segment of the Central Asian Orogenic Belt: Revealed by long-period magnetotelluric data

Abstract

Ever since the Mesozoic era, the eastern segment of the Central Asian Orogenic Belt (CAOB) has been significantly affected by the subduction of the Pacific plate. In order to better understand the subduction effect of the Pacific plate at different distances, we set up 65 magnetotelluric stations to acquire a three-dimensional inversion resistivity structure. The results illustrate that: there are three large-scale high-conductivity anomalies C1, C2 and C3 affecting the crust and mantle of the eastern segment of the Central Asian Orogenic Belt. The high-conductivity anomaly C1 extends from the surface to the asthenosphere and is connected with C2, which exists in the asthenosphere below the Xing'an block. The high-conductivity anomaly C3 under the Songliao block extends downward from the surface to the asthenosphere. We interpret that these anomalies are caused by aqueous fluid and partial melting based on petrophysical analysis. In the light of the high-temperature high-pressure theory, the estimated water content and melting fraction of C3 is considerably higher than that of C2. We infer that the lateral heterogeneity of the fluid-related properties is caused by the variable distance to the Western Pacific subduction zone. The C3 anomaly is located above the stagnant plate and controlled by the medium-distance effects of Pacific plate subduction, which have been influenced by intense dehydration and seriously transforming. The C2 anomaly is located at the end of the stagnant plate and controlled by the long-distance effects of Pacific plate subduction with slight dehydration and transformation.