Teleseismic P-wave Tomography For Imaging The Upper Mantle Velocity Structure Beneath NE China

Northeast(NE)China and its adjacent regions belong to the easternmost part of the Central Asian Orogenic belt,and are one of the largest accretionary terranes in the world which formed from Paleozoic to early Mesozoic.In the Mesozoic,NE China experienced large-scale volcanism and extension due to the subduction of the Paleo-Pacific Ocean;in the Cenozoic,it also experienced extensive alkaline volcanism and the associated subduction of the Pacific plate.Today,NE China is still experiencing the plate extrusion and intraplate volcanism.The existing seismic tomography and thermo-mechanical simulation results show that there are extensive stagnant slabs in the mantle transition zone beneath the most part of the study region,and the formation of the volcano could be related to the dehydration of the subducted stagnant plate and the mantle convection,though the location distribution of volcanoes cannot be explained very well based on the dehydration model.However,since the NECESSArray data obtained by a collaborative project between China,U.S.and Japan has been published,seismic tomography and receiver function studies have shown that there is a gap through the mantle transition zone beneath the Changbai volcano groups.According to this clear gap image,a new model was proposed that lower mantle upwells along the gap and provides the heat source of the upper mantle,but this model cannot prove if the upwelling of the lower mantle itself can heat the upper mantle in such a large region.Therefore,the source of deep thermal material in the Northeast still deserves debating.The study region is more than 1000 km away from the Japanese trench.Teleseismicities can provide more data than the regional ones and are suitable for tomographic studies.As a reliable and robust tomography approach,FMTT(Fast Marching Teleseismic Tomography)can be used to obtain the velocity structure of the upper mantle beneath the study region,and to explore the sources of volcanoes,plate subduction,and mantle rheology.In this study,in order to explore the velocity structure of the upper mantle beneath the study region and obtain the distribution of deep thermal materials,the waveform data from 286 seismic stations provided by IRIS were picked with the help of two waveform correlation methods.A totality of 72,300 relative travel time residuals were picked.Using these residuals,the P-wave velocity structure down to 750 km below the study region is retrieved.The results show that there are low-velocity anomalies through the mantle transition zone below the Changbai volcanic group,Jingpohu volcano and Aershan volcano.These low-velocity anomalies extend from the upper edge to the shallow part.Based on this study,it is speculated that the Cenozoic volcanoes in the study region are caused both by dehydration and the upwelling of the lower mantle.The low-velocity anomily beneath northern Songliao Basin doesn't penetrate the mantle transition zone,which may be related to mantle convection and basin delamination.The underground resolution of the Wudalianchi volcanic group is insufficient,but low-speed anomaly images from the lower mantle can still be obtained,and it is speculated that its formation is not only due to mantle convection.According to the low-velocity anomalies widely distributed in the upper mantle and the low-velocity bodies passing through the mantle transition zone below the volcanoes,we believe that the Cenozoic volcanoes in the northeast region have their kinship and have a common formation mechanism.