Study Of Mantle Discontinuities In The Tonga-Fiji Subduction Zone

Exploring the existence and morphological characteristics of the upper mantle intersection can help to better understand the internal structure of the earth,detect the lateral inhomogeneity of the internal temperature of the mantle,understand the morphology of subduction plates and the convective mode of the mantle.Seismology generally agrees that the upper mantle has 410-km and 660-km mantle discontinuities.The Tonga-Fiji subduction zone is located in the southwest of the Pacific Rim subduction zone,and its frequent medium-deep source earthquakes help to better study the structure of the upper mantle intersection below it and its effects.In this paper,the morphological characteristics of the mantle intersection in the Tonga-Fiji region were obtained by using the deep seismic data recorded by the National Seismic Network Data Backup Center from 2007 to 2021,and the influence of the subduction zone on the mantle intersection of the mantle was explored by the oblique superposition of a large number of seismic data and the pickup analysis of the s P precursor wave s410 P phase.Key findings include:(1)We divide the Tonga-Fiji subduction zone into three regions: northern,central and southern,and find that the 410-km velocity interval in the northern,central and southern regions of the Tonga-Fiji subduction zone is sinking.The 410-km discontinuity varies between 415-435 km with an average depth of 427.4 km,among which,the subsidence is more obvious in the northern region,and the depth of the 410-km velocity interval can be as deep as 435 km,the central part has complicated variation,and the southern part is deeper and has less variation.The subsidence of the 410-km discontinuitiy is caused by the mantle column action resulting in the formation of a temperature difference with the surrounding mantle.The temperature difference is caused by the mantle column action,while the subduction plate in Tonga-Fiji area has some difference in the direction along the Tonga Trench,which may be the reason for the spatial difference of the 410-km discontinuitiy interruption surface in the Tonga-Fijian subduction zone..(2)The 300-km mantle discontinuity under the central and southern regions of Tonga-Fiji did not produce significant depth changes due to latitude changes,and were concentrated at depths of 270-300 km.It is possible that the gabbroic material within it is derived from the remnants of oceanic crust composed of mid-ocean ridge basalts that were trapped during the downward subduction of the plate,resulting in the small-scale undulation of the 300-km velocity interruption surface.(3)The depth of LAB beneath the Tonga-Fijian subduction zone is around 85 km,which is thought to be deeper probably due to the fact that the Tonga-Fijian subduction zone is a very active tectonic zone with frequent seismic and volcanic activities and high temperatures at the base of the lithosphere.Our results show that there is obvious spatial variability in the mantle structure(LAB,300-km,410-km mantle discontinuities,etc.)beneath the Tonga-Fijian subduction zone,and this spatial variability is important for a clearer understanding of the Earth’s internal structure and the Earth’s internal dynamical processes as well as the influence of the subduction zone on the mantle discontinuities.In order to better understand the internal structure of the Earth,we need to explore the structural features of the mantle in different regions in depth to provide new clues for a better understanding of the internal structure of the Earth.