Tomography And Azimuth Anisotropy In The East Qinling-Dabie Using Ambient Seismic Noise

The Qinling-Dabie orogenic belt is a collision orogenic belt between the North China plate and the Yangtze plate.It rises from the northeastern margin of the Tibet Plateau in the west to the Tanlu fault zone in the east.The Qinling-Dabie orogenic belt has undergone many tectonic evolution,such as the formation and evolution of the Precambrian basement,the Triassic plate collision and the Cenozoic intracontinental orogeny.The Qinling-Dabie orogenic belt is composed of different origins,tectonic backgrounds and ages.And the Qinling-Dabie orogenic belt has complex crustal composition and structure.Since the discovery of ultrahigh pressure metamorphic rocks in the Dabie Mountain,the Qinling-Dabie orogenic belt has quickly become a key area for scholars to study continental dynamics.Previous work on geophysical artificial seismic exploration and earthquake tomography in this area has yielded fruitful results.These results reflect that the Qinling-Dabie orogenic belt retains a large amount of crustal deformation information during collisional orogeny,which provides an ideal place for us to study intracontinental tectonic deformation and continental dynamics.The crustal structure of Qinling-Dabie orogenic belt reflected by the tomography is preserved after underwent multiple tectonic movements.And it cannot well indicate the direction information of specific deformation.In addition,the complex geological and structural conditions in the shallow and deep parts of the Qinling-Dabie orogenic belt.That have resulted in a lack of systematic understanding of the entire orogenic belt,and which has led to divergence and controversy on many issues in the region so far.Seismic anisotropy is an important means to reveal the dynamic mechanism of lithosphere.Especially,the anisotropy of stable continental areas is the "fossil" anisotropy left in the lithosphere after the last large-scale tectonic movement,which reflects the stress environment in the past.The azimuthal anisotropy can acquire the anisotropic characteristics in different depth ranges.Although previous studies in this area have also found that the azimuthal anisotropy in the Qinling-Dabie tectonic belt is related to the convergence and collision of the Yangtze and North China plates,but it is difficult to conduct a more in-depth and detailed study because of the low resolution.In the past decade,with the development of ambient noise,more short-period Rayleigh surface waves can be obtained.Which provides a way for us to obtain high resolution S-wave velocity structure and azimuthal anisotropy in the crust.In order to make up the deficiency in this area,180 stations in the East Qinling-Dabie orogenic belt and surrounding areas have been collected in this paper,2 years of continuous seismic waveform data.The Rayleigh wave empirical Green's function between stations is calculated by using ambient noise data processing method,and the dispersion curve is measured by FTAN.On this basis,the phase velocity distribution and azimuthal anisotropy of Rayleigh wave with 8-35 s period are obtained.Finally,the S-wave velocity is obtained by using the neighborhood algorithm.The main understandings and conclusions obtained are as follows:1.There are different tectonic deformation mechanisms in the western region(Qinling)and Eastern region(Tongbai-Dabie)bounded by gravity gradient zone.The collision orogeny model in the East belongs to hard-collision,which retains the structural information of plate convergence and collision in S-wave velocity images;the collision orogeny model in the West belongs to soft-collision,which retains the principal compressive stress information related to tectonic movement in azimuthal anisotropy.(1)The S-wave high-speed anomalies in the Hefei Basin-Beihuaiyang and Mufu mountain-Susong in the East,which indicate the convergence and collision of the Yangtze and North China plates.(2)The azimuthal anisotropy of the South Qinling Mountain shows the principal compressive stress direction of the Yangtze Plate in two stages.The disintegration of lithospheric mantle in the southern Qinling Mountain reduces its strength,which leads to the soft-collision.Soft-collision make the post-orogenic dynamics relatively less intense,while azimuthal anisotropy reflects the past stress environment.2.In this paper,the azimuthal anisotropy characteristics of different depth ranges of the tectonic units in the orogenic belt are comprehensively analyzed,and the crust-mantle deformation dynamic model of the orogenic belt is constructed.(1)The lower crust and upper mantle of the South Qinling Mountain is decoupled deformation.The NE-trending plate movement in the late(Early-Middle Jurassic)has little effect on the middle and lower crust,that the records of the near NS-trending plate movement in the early(Indosinian-Early Yanshanian)period are preserved in the lower crust.(2)The upper mantle of the Qinling Mountains in the west of gravity gradient zone,the convergence and collision of the North China Plate and the Yangtze Plate is located near 33°N.(3)The gravity gradient zone of the Taihang Mountain acts as a barrier to the compression of SW from the North China Plate,resulting in a series of arc-shaped azimuthal anisotropy in the southern margin of the North China Plate and the North Qinling Mountain.(4)The NW-SE trending fast directions of the Tongbai-Dabie orogenic belt in the middle-lower crustal,which reflects the characteristics of tectonic extrusion.And there is an EW-oriented anisotropic separating zone near the 30°N(Jingzhou-Qianjiang)in the Jianghan basin,which is formed by the tectonic extrusion of Tongbai-Dabie to the southeast and Southeast and the northward extrusion of the Yangtze plate,and the Jingzhou-Qianjiang is the southern boundary of tectonic extrusion.3.The S-wave velocity and azimuthal anisotropy reveal that the Hannan-Micang Mountain wedged eastward into the middle-lower crust of the weak South Qinling as a more capable block,and reconstructed the west of Daba Mountain.4.The crustal shear-waves splitting shows that there is a near EW-trending principal compressive stress in Zigui Basin west of Huangling anticline,which may be the combined effect of the two-way compression of Qinling orogenic belt and Xuefeng Mountain.Finally,the paper discusses the middle-lower crustal flow in the Qinling orogenic belt,the origin of the Daba Mountain arcuate tectonic belt,the eastward extension of the Mianlue suture zone,and the "pressure shadow structure" of the Qiyue Mountain Mountains.The research work in this paper not only provides a new understanding of the crust-mantle deformation model and dynamic mechanism of the Qinling-Dabie orogenic belt,but also provides new constraints on the controversial geodynamic problems.