| As a typical example of land-land collision,the eastern margin of Tibetan Plateau is regarded as the golden key to open the door of geodynamics,and it is also one of the research hotspots in international geoscience at present.Among them,the “Y”-shaped structural fault zone(composed of three fault zones: Xianshuihe fault,,Anning River fault and Longmenshan fault)is the most active,and the Songpan-Ganzi block,the Sichuan-Yunnan block and the Yangtze block adjacent to the three fault zones are also of concern.In this paper,the ambient noise continuous records recorded by 99 broadband stations from January 2015 to December 2015 are collected,and the empirical Green’s function is obtained by using cross-correlation superimposition and other techniques.Then the phase velocity dispersion of Rayleigh wave and Love wave in the study area is obtained by multiple filtering method.Finally,the S wave velocity structure in the study area is obtained by direct inversion technique.In this study,the velocity structure of the transitional zone from Songpan-Ganzi block and Sichuan-Yunnan rhombic block to Yangtze platform in the study area is explained dynamically based on previous research results.The conclusions are as follows:(1)The thickness of the whole set of sediments in Sichuan Basin is about 8km.The surrounding area can reach 10 km,forming the characteristics of thick edge and thin middle.At the depth of 50 km,the relative high-speed anomaly in Sichuan Basin is generally 4.12km/s-4.24km/s,and in Songpan-Ganzi block and Sichuan-Yunnan rhomboid block appear partially,and the velocity is about in the range of 4.03km/s-4.14km/s.Combined with various indications,the rapid rate of change in the Sichuan basin at the depth of 50 km is due to the fact that the Sichuan basin began to shift from the crust to the mantle.The results show that the crustal thickness is about 50 km,but some areas of Songpan-Ganzi block and Sichuan-Yunnan rhombic block begin to be in the upper mantle environment gradually,indicating that they are of more complex geological structure.(2)The Songpan-Ganzi block does have a low velocity body in the middle and lower crust.However,near the Longmenshan fault zone,the low velocity body of the middle and lower crust disappears.Combined with the results of previous studies,it is concluded that the low velocity body material in the middle and lower crust of the central and eastern Tibetan Plateau is not only hindered to a certain extent by the Xianshuihe fault in the eastward escape process,but also blocked by the hard rock of the middle and lower crust of the Sichuan Basin.And when it is close to the Longmenshan fault zone,under the influence of certain forces,the original direction of motion has been changed and distributed along the Longmenshan fault zone,thus accumulating stress here.However,whether the crust flow model can explain the low velocity body phenomenon in the middle and lower crust of Songpan-Ganzi block still needs further in-depth study and higher resolution research evidence.(3)From the section CC’,it is found that the low velocity anomaly body is found only in the middle and lower crust of the Sichuan-Yunnan rhombic block,but not in the middle and lower crust of the southern Sichuan Basin.Based on the analysis of GPS observations,the difference may be due to the eastward migration of the low-velocity body.Some of them were diverted to the south,and the other part was blocked by the Anning River fault and Xiaojiang fault.(4)Longmenshan fault zone and Daliangshan fault zone obviously separate the velocity difference between Sichuan basin and its surrounding areas,and have obvious control effect on Sichuan basin. |
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