P-wave Multi-scale Tomography Of The Crust And Upper Mantle On The Northeastern Margin Of The Tibetan Plateau

About 50 Ma years ago,the India-Eurasia collision,the Indian block subducted below the Eurasian continent,resulting in the uplift of the Tibetan Plateau and the significant thickening of the crust beneath the Tibetan Plateau.The northeastern margin of the Tibetan Plateau,as an important field for studying the mechanism of uplift and deformation of the Tibetan Plateau and its far-reaching impact on the tectonic evolution in eastern China,has also received extensive attention.So far,domestic and foreign scholars have carried out numerous researches on geophysical methods in this field and have produced many results,but they are still controversial.For example,in the middle and lower crust of the northeastern margin,noise imaging results show a wide range of low-velocity anomalies,and the receiver function finds low-velocity layers with obvious azimuthal anisotropy.These results are consistent with the lower crust flow model.However,the results of seismic anisotropy studies suggest that the deformation in the northeastern margin of the Tibetan Plateau is a vertical continuous deformation,and the receiver function found that the area has a lower Poisson’s ratio,which is in contradiction with the crustal flow model.In addition,the northeastern margin of the Tibetan Plateau,located between the Tibetan block,the Alxa block,the Ordos block and the Yangtze block,is obviously a key area for studying the structural evolution and contact deformation between the blocks.As a rapidly growing plateau,its internal tectonic movement is active and earthquakes occur frequently,and there have been many large strong earthquakes in history.For example: the magnitude 8 Pingluo earthquake in 1739,the magnitude 8.5 Haiyuan earthquake in 1920,and the magnitude 8 Gulang earthquake in 1927.Whether the occurrence and occurrence of these large structural anomalies are closely related to the lateral movement and expansion of the northeastern margin of the Tibetan Plateau,and whether the deep expansion has affected and transformed the craton block? What is the direct relationship between the formation and movement mechanism of strong earthquakes and the environment of seismogenesis? These problems still require us to further explore and study the evolution of the structure of the northeastern margin of the deep Tibetan Plateau.The rational use of modern seismological observation methods and data technology to observe and acquire deep underground structures in the area is one of the important research methods.To this end,this paper uses the "China Seismological Array" exploration project to deploy the 670 wideband mobile seismic stations deployed in the northeast margin and the P waves recorded by the regional seismic network in the first arrival.Seismic tomography algorithm was used to invert the high-resolution crust-mantle P-wave velocity structure under the northeastern margin of the Tibetan Plateau,Alxa block,and Ordos block.Attempts are made to further study the deep boundaries of the block and their interaction modes,the occurrence mechanism of strong earthquakes,and the environment of seismogenicity.The results of this study show that:(1)The crust-mantle structure on the northeastern margin of the Tibetan Plateau and its adjacent areas is closely related to the surface structure characteristics.In the upper and middle crust,the velocity structure has significant lateral differences,and there is a wide range of low-velocity anomalies in the lower crust depth.The Alxa block and the Ordos block retain the original craton property,and the velocity is a high-speed integral structure with good stratification and no large-scale deformation features have been found.(2)In the Qilian orogenic belt,the velocity results can be divided into two main parts with the Laji Mountain as the boundary: the middle and upper crust in the northwest part shows low-speed anomaly characteristics,and the middle and upper crust in the southeast part mainly shows high-speed characteristics.The occurrence of this phenomenon may be due to the different tectonic processes of the Qilian orogenic belt in the northwest and southeast.In the northwest,deformation and growth of the northern part of the Tibetan Plateau are realized by the northward subduction of the upper crust and the thickening and expansion of the lower crust;while the deformation and development of the upper crust in the southeast orogenic belt are of a strike-slip type.(3)In the "diamond block" bounded by the Ordos,Alxa,Tibetan and Sichuan blocks,from the velocity structure obtained by our inversion,in the upper crust,it generally appears as a high-speed structure,but in the lower crust,it shows a low-speed anomaly and invades the southwestern margin of Ordos.For this reason,we infer that the lateral expansion of low-velocity material in the middle and lower crust of the northeastern margin of the Tibetan Plateau has reached the southwestern border of Ordos..In addition,as the only squeeze boundary in the Ordos periphery,the lateral expansion of the northeast margin is likely to be one of the main driving forces that cause the block to rotate counterclockwise.(4)From the upper crust to the top of the mantle,the speed difference between the north and south of the Ordos block can be observed almost at 38°N.This difference is probably due to different tectonic movements in the north and south of Ordos.The southern part is mainly affected by the extension and uplift of the Tibetan Plateau,while the northern tectonic movement is mainly affected by Yinshan.The difference in tectonic movement also affects the surface elevation,sedimentation and metallogenic position.Compared with traditional tomography,the multi-scale tomography method used in this paper effectively avoids the problems of poor resolution and spectral leakage caused by the uneven distribution of observation stations and actual earthquakes in the study area.The multi-scale analysis and sparse constraints provided by this method ensure that we obtain stable high-resolution results,which provides important seismic evidence for the study of the dynamics of the northeastern edge of the Tibetan Plateau.