Study On The Structure Of The Discontinuities Of The Upper Mantle Beneath Southeastern China And Northern South China Sea

The South China Sea(SCS)is located at a convergent region of the Eurasian plate,the Pacific plate,and the Indian-Australian plate with complicated geological evolution history and tectonic activities.Compared with regular oceanic plate,such as the Pacific plate,the SCS is not only affected by multiple subduction activities but also shows possible effects from the Hainan plume,which suggests that the tectonic evolution of the SCS has been largely controlled by the dynamic process of the deep mantle.Thus,a better seismic image of the upper mantle structure beneath the SCS and the continental margin is important for a better understanding of the tectonic history of the SCS.Here,I select P-wave and S-wave triplication data with high-to-noise ratio from 414 earthquakes around the SCS recorded by the China National Seismic Network.Through waveform modeling,we establish one-dimensional velocity models of the upper mantle beneath southeastern China and the northern SCS.Our model provides new constraints on the physical properties of the upper mantle discontinuities including the lithosphere and asthenosphere boundary(LAB),the X-discontinuity,and the 660 km discontinuity.The triplication data sampling beneath southeastern China display strong arrivals with rays turning at the top of the lower mantle at an epicentral distance of 10°.To explain the observations at different frequencies,it is necessary to increase the S-velocity by?8.2%±0.5%across the 660-km discontinuity over less than 10 km.This sharp 660-km discontinuity suggests that harzburgite enrichment exists at the bottom of the 660-km discontinuity and the temperature near 660 km does not exceed 2000 K.Through comparisons with 1-D models for other regions,we find that the S-velocity structure in the mantle transition zone beneath our study area is similar to that beneath oceanic plates.Thus,we suggest that the mantle near the 660-km discontinuity beneath southeastern China currently is locally not affected by either slab or plume.For the SH data of the Philippine earthquakes recorded by the China National Seismic Network,we observe the Lid signal propagating in the fast Lid acting as the first arrival.Through waveform modeling,we find that the average thickness of Lid is?45 km,the depth of the LAB is?70 km,and the S-velocity reduction across the LAB is?6%.The thickness of the lithosphere in northern SCS is comparable to that of the Pacific plate.The lithospheric structure does not show significant lateral variation.However,crustal multiples are strong when rays travel through the western part of the northern SCS and the waveforms become longer period with increasing epicentral distance,which suggests a 2-D effect varying from the thin oceanic crust to the thick continental crust.The P-wave data sampling the northern SCS show an evidence of X-discontinuity at depth of?250 km.The P-velocity increases?2%through a gradual X-discontinuity within a transition thickness of 40 km.X-discontinuity is generally related to the phase transition from coesite to stishovite.However,the SCS is a complex region where subduction and mantle plume coexist.Thus,it is difficult to draw any conclusions about the origin of the X-discontinuity here.