| The East Qinling-Dabie orogenic belt is located in the east of the Central Orogenic Belt in China.It is an orogenic belt formed by the long-term collision between the Yangtze plate and the North China plate.It has experienced multi-stage tectonic movements and has a complex lithospheric structure.Several complex structural belts developed in the area,such as Dabashan arc structural belt,Tongbai Hong’an orogenic belt and Dabie high pressure and ultra-high pressure belt.And the orogenic belt is also one of the largest and best preserved HP/UHP metamorphic rock outcrops in the world.Therefore,this area is one of the crucial research areas to study the process of continental deep subduction and continental collision.Based on the continuous waveform data of 91 seismic stations in the East-Qinling Dabie orogenic belt and its adjacent area,this paper constructs the S-wave velocity and azimuth anisotropy structure in the lithosphere of the study area using ambient noise imaging technology,.On this basis,the relevant dynamic problems in the area are discussed.The main research work and results obtained in this study are as follows:(1)The Rayleigh wave phase velocity dispersion curves in 5-70 s frequency band were extracted based on the continuous waveform data of 84 permanent stations in recent 5 years and 7 portable stations in recent 2 years.The S-wave velocity structure of the crust and upper mantle in the east Qinling-Dabie orogenic belt and its surrounding areas is constructed.Based on the S-wave velocity structure,the azimuthal anisotropic structure varying with depth is further constructed.The structures constructed in this paper provide new seismological support and constraints for the study of regional geodynamic process such as the structure of lithosphere and the deformation characteristics of crust and upper mantle in the East Qinling Dabie orogenic belt and its adjacent areas.(2)According to the existence of large-scale low-velocity anomalies and highvelocity anomalies distributed in blocks in the upper mantle of the Yangtze plate in the study area,this paper infers that the lithospheric mantle of the Yangtze plate has also been damaged and thinned,which is bounded by the North-South Gravity lineament.According to the low-velocity anomaly in the lower crust,combined with the relatively high Vp/Vs value and surface heat flow value,the lower crust may have partially melted to a certain extent,and the directional arrangement of melt results in the NW-SE fast wave direction of the lower crust and the top of the upper mantle.The weak anisotropy zone and the near E-W fast wave direction in the upper mantle of the Yangtze plate in the study area,combined with the research results of magmatism in the Cenozoic Jianghan Basin,indicate that there is an eastward asthenospheric flow caused by the subduction of the Indian plate in the mantle of the Yangtze plate in the study area,accompanied by the upwelling of mantle flow.(3)Dabashan arc tectonic belt is a "thin skin" thrust nappe structure with lowvelocity middle upper crust and high-velocity lower crust.The change of azimuth anisotropy features of the upper crust and middle to lower crust incidate that the slip layer is existed in the crust.Hannan Micang uplift,Shennong Huangling uplift and Chengkou Fangxian fault played a key role in the formation of Dabashan arc structural belt.The NE-SW compression formed in the west of the orogenic belt caused by the clockwise large angle rotation of the Yangtze plate during subduction,provided an important dynamic source for the formation of Dabashan arc tectonic belt,and resulted in the NE-SW fast wave direction in the northeast margin of Sichuan Basin and Dabashan lithosphere.(4)Based on a series of characteristics such as low-velocity lower crust in the East Qinling Dabie orogenic belt,low-velocity uppermost mantle,banded high-velocity anomaly distributed along the orogenic belt in the upper mantle,deflection of fast wave direction at the uppermost mantle and consistency of fast wave direction between the upper mantle and the lower crust,combined with observation results such as Vp/Vs value and surface heat flow value.This paper argue that the orogenic belt has undergone the delamination of the lower crust in the past,resulting in the transformation of the mafic lower crust into felsic,and the low-velocity anomaly of the lower crust,the lowvelocity anomaly at the uppermost mantle,the transformation of the fast wave direction,the lower Vp/Vs value and surface heat flow value in the crust,and the banded highspeed anomaly in the upper mantle.(5)Based on the characteristics of S-wave velocity structure and azimuth anisotropy in the orogenic belt,combined with relevant research results such as petrology,mineralogy and teleseismic imaging.I infer that the exhumation process of HP/UHP metamorphic rocks in the orogenic belt from the upper mantle to the surface can be divided into two stages: at the first stage,under the setting of continuous subduction and rotation of the Yangtze plate,the ancient ocean plate broke away from the subducted Yangtze lithosphere and caused the upwelling of the asthenosphere,resulting in the exhumation of HP/UHP metamorphic rocks to the middle and lower crust.At the second stage,the continuous collision between the Yangtze plate and the North China plate result in the continuous thickening of the lower crust.At the same time,the asthenosphere upwelling caused by plate fracture continued to heat the lithospheric mantle and weaken the lithospheric mantle below the orogenic belt.With the thickening of the lower crust and the phase transformation of eclogite,the delamination of the lower crust occurred,which large-scale magmatism,resulting in the exhumation of HP/UHP metamorphic rocks from the middle and lower crust to the surface.(6)The characteristics of S-wave velocity and azimuth anisotropy have changed significantly between the east and west sides of the North-South Gravity lineament in the study area.This significant difference is due to the different thickness of the lithosphere on the east and west sides and the clockwise large angle rotation of the Yangtze plate during subduction.There is a strong anisotropic layer in the upper mantle below the North-South Gravity lineament,and the fast wave squareness is similar to the S-N direction,which is quite different from the anisotropic characteristics in the crust,indicating that the deformation characteristics at the lithospheric thinning boundary are more complex.Under the strong anisotropic layer,the anisotropy intensity decreases and the fast wave direction deflects.It is speculated that it may be caused by the eastward mantle flow passing through the gravity lineament. |
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