Study Of The Mantle Transition Zone Structure Beneath Alaska-Aleutian Subduction Zone And Tonga-Fiji Subduction Zone

Topographies of the 410-km and 660-km discontinuities have great importance in understanding the lateral variation of the temperature and/or chemical composition in the mantle transition zone,as well as the depth of the subdected slab and mantle convection patterns.Aleutian-Alaska subduction zone and Tonga-Fiji subduction zone,located in the northermost and southwestern Circum-Pacific subduction zone,respectively,are the natural laboratory for studying the interaction between the front edge of subduction plate and the 660-km discontinuity.The topographies of the 410-km and 660-km discontinuities beneath Aleutian-Alaska subduction zone with adjacent areas and Tonga-Fiji areas are imaged by using moveout correction and common midpoint stack of the SS precursors,recorded from the National Digital Seismic Network and a regional seismic array deployed in the NE Tibetan plateau,which is operated by “Chin Array” project.The main results are shown in the following(1)The topographies of the 410-km and 660-km discontinuities appear to be positively correlated beneath the areas adjacent to the Aleutian-Alaska subduction zone,including the Bering Sea,Alaska Peninsula,and the southern and eastern Alaska.Thus a normal transition zone is inferred beneath these areas,indicating that the North Pacific plate has not yet subducted into the mantle transition zone.(2)Beneath the western Alaska,the 660-km discontinuity is strongly depressed and a thicker transition zone is mapped,while the overlying 410-km discontinuity appears to be normal compared to the global model.Such observation may indicate the presence of the remnant Kula plate just above the 660-km discontinuity beneath the western Alaska.(3)Beneath the northern Fiji basin,the 410-km discontinuity remains flat and is similar to the global average depth,but the 660-km discontinuity is strongly depressed and reaches to the depth of ?730 km.The depressed 660-km discontinuity can be attributed to the detached mantle lithosphere of the subdeucted the Australian plate at its front edge.We also found that the 660-km discontinuity beneath the Tonga trench is very complex,suggesting that a tearing occurs when the subducted Pacific plate reaches into the lower part of the mantle transition zone.Some of the slabs may become stagnate in the MTZ,while the others penetrate into the lower mantle.