Structure Of Discontinuities Below The Southeast Margin Of The Tibetan Plateau Revealed By Seismic Daylight Imaging

The structure and physicochemical properties of the lithosphere not only reflect the structural characteristics and interrelationships of the deep earth,but also provide reliable information and effective constraints on the deep dynamic evolution process and mechanism.The internal structure of the lithosphere,especially the discontinuity structure,is one of the key ways to understand the formation and evolution of the earth's continents.If these discontinuities can be delineated,it will be helpful to further understand the geodynamic processes such as mantle convection,plate movement and lithospheric evolution.The southeastern margin of the Tibetan Plateau is located in the transition zone between the Tibetan Plateau and the Yangtze Platform with frequent seismicity,The study of the lithospheric discontinuity structure in the southeastern margin of the Tibetan Plateau is helpful to understand the dynamic problems such as tectonic evolution and deep warning signs of an earthquake in the region.In this paper,the lithospheric discontinuity structure in the southeastern margin of the Tibetan Plateau is studied by seismic daylight imaging method using the teleseismic P-wave waveform data recorded by the stations of the China Science Array Exploration Project,and the accuracy and stability of this method are discussed.The results show that in Yunnan Province(within longitude 97~°~106~°),it can be seen that the depth of Moho is thinner in the west and thicker in the east in each section,while the depth of Moho continuing to reach the west of Yangtze plate decreases again;In the north-south direction(from section C to section A),the depth of Moho gradually changes from 30-40km of C section to 40-50km of section B,the Moho depth in section A is also about40-50km,and there is a transition zone of Moho depth at latitude of 25~°;the lithospheric thickness in the southeastern margin of the Tibetan Plateau is thinner in the West and thicker in theeast.The thinnest lithospheric thickness near Tengchong volcanoin Yunnan-Myanmar-Thailand block is about 60 km.The thinner lithospheric thickness may be caused by the upwelling of asthenosphere mantle material;The lithospheric thickness of the Yangtze Block is gradually thinning from Sichuan Basin to the south,especially the thickest below the Sichuan Basin,whose lithospheric thickness can reach about 190 km.Obvious discontinuities have been detected at a depth of about 150 km beneath Tengchong volcano,which may be the original magma source of volcano.The continuity of the structure of the Indochina block and the Yunnan-Burma-Thailand block further provides seismological evidence for the eastward spillover of low-velocity materials from Tengchong volcano caused by the pushing of the Indian plate;Local high-speed anomaly and complex discontinuity structure(section A)found in the Sichuan-Yunnan rhombic block in the depth range of 50-250km may imply the location of the mantle plume in Emeishan large igneous province.The high-speed anomaly with uneven distribution above the block may be caused by magma preserving at different depths in the form of intrusive rocks and intrusive rocks during volcanic eruption,and may also be related to the later regional tectonic activities.