| As the boundary of the Tibetan Plateau,the southeastern margin of the Tibetan Plateau has undergone complex geological evolution and deformation processes.The strong tectonic activities and frequent strong seismic activities in this region make it an ideal area to test different models of the deformation mechanism of the Tibetan Plateau.The Anninghe-Zemuhe-Xiaojiang fault zone is located in the southeastern part of the Tibetan Plateau,at the intersection of the Sichuan-Yunnan block,Songpan-Ganzi block,Sichuan Basin and Yangtze block.And it is connected with the Xianshuihe fault in the north,the Red River fault in the south and the Daliangshan fault in the east.It is the eastern boundary of the Sichuan-Yunnan block,and its unique geographical location and active tectonic activity have attracted many scholars to conduct research.It has accumulated rich seismic data such as deep seismotectonic background and lithospheric structure in the southeastern margin of the Tibetan Plateau.Therefore,studying the subsurface velocity structure and deformation features in this region is important for understanding the tectonic dynamics process of the Tibetan Plateau,and the geological and tectonic background of the Sichuan-Yunnan region,the environment of strong earthquake incubation.A series of geophysical observations and studies have been carried out in this region by previous authors.And two low-velocity zones were found in the middle and lower crust of the southeastern margin of the Tibetan Plateau,suggesting the existence of plateau material flow here as an important result.However,due to sparse distribution of seismic stations,short observation time and different tomography methods,the lateral resolution of imaging results is mostly large.There are few high-resolution velocity structure near the southeastern margin of the Tibetan Plateau.So that the connectivity and genesis of the low-velocity zones in the middle and lower crust,the effect of the high-velocity body of the Anninghe-Zemuhe fault zone on the low-velocity zones,and the uplift mechanism of the Gongga Shan are still unclear.In this paper,the southeastern margin of the Tibetan Plateau is divided into two study areas,north and south,with 27°E as the boundary for separate imaging studies.Firstly,we obtain the different years of Rayleigh wave phase velocity dispersion curves at the periods of 4-25 s using observations from a total of 378 stations from the Chuanxi Array,Himalayan Array,Xichang Array and Anninghe-Zemuhe-Xiaojiang Array were used in the area near the Anninghe-Zemuhe fault zone.The new 3D S-wave velocity model has lateral resolutions of 30 km in 0-35 km depth is obtained by direct inversion of surface wave.Then,in order to further discuss issues such as the connectivity of the low-velocity zones in the study area,we continuou vertical component waveform data from a total of 291 stations from the Himalayan Array Phase I,the Xichang Array,and the Xiaojiang fault Array were used in the area near the Xiaojiang and Red River faults.The 0-45 km 3D S-wave velocity structure with resolutions of about 50 km is obtained using the same method.In order to discuss scientific issues in the region with richer information,we obtain the deformation characteristics of the two regions using the direct inversion for three-dimensional shear wave speed azimuthal anisotropy based on surface wave,with a resolution of about 60 km and 100 km,respectively.Based on the new imaging results,the following insights were obtained:(1)The low-velocity zones in the middle crust of the southeastern margin of the Tibetan Plateau are not continuously distributed along the Xianshuihe-Anninghe/Daliangshan-Xiaojiang fault zone,but are separated by a high-velocity anomaly near Shimian,Mianning,Xichang and Panzhihua.The fast-wave polarization direction of the surface wave azimuthal anisotropy also shows the boundary of the high-velocity body with the different direction.The western side shows a trend of clockwise rotation.The western low-velocity zone is formed by the flow of material from the Tibetan Plateau,which is driven by gravity and shearing movement of the faults.And on the east side,it is distributed in a ring within the Lvzhijiang fault,the northern section of the Xiaojiang fault and the Red River fault.Combined with the shear heating theory calculation of the faults,the formation of this low-velocity zone is a combination of shear heating by the faults and upwelling of mantle material.(2)The upper crust of the velocity structure around the Gongga Shan shows a low velocity anomaly,and the south-east section of the Xianshuihe fault on its east side overlaps with the high-low velocity boundary.Combining the azimuthal anisotropy on both sides of the Xianshuihe fault also shows a different distribution of southwest on the west side and southeast on the east side,it shows that the low-intensity upper crust in the Gongga Shan area is vulnerable to the influence of the bending and compression of the Sichuan-Yunnan Block and Xianshuihe-Anninghe fault zone.At the same time,the low-velocity feature in this region extends to 35 km and is connected to the crustal flow channel.Combined with a high-velocity zone corresponding to the subducted Yangtze block exists in the middle and lower crust on the east side of the Gongga Shan.It suggests that the uplift of Gongga Shan is also related to the continuous accumulation of material from the lower and middle crust of the Tibetan Plateau blocked by the Yangtze block.(3)Near Panzhihua,a near east-west high-velocity anomaly is shown from the surface to 45 km,which is related to the high-density mafic and ultramafic magma from upper mantle.High-strength Panzhihua can act as a barrier to tough materials from the plateau,and makes the mechanical strength and elevation of the two subblocks in Chuanxibei subblock and Dianzhong subblock differ significantly.(4)The Red River fault exhibits a relatively low-velocity in the northwest section with the Honghe-Shiping boundary in the middle and lower crust.Since the fast wave direction is north-south,combined with the positive radial anisotropy,it is believed that the material flow across the Red River fault exists here.Since the fast wave direction is realized as nearly north-south throughout the whole crust,which is consistent with the direction of the maximum horizontal stress in the region.It is believed that the crust is coupled and extruded from the Sichuan-Yunnan block and the Yangtze block. |
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