Rayleigh Wave Phase Velocities And Azimuthal Anisotropy Beneath Ordos Block And Its Surrounding Areas

The Ordos block is considered as a residual stable block after the North China craton was destructed in the Mesozoic.In the Ordos block, there are rich in mineral resources and weak activity. In surrounding areas, there are strong tectonic movement and frequent earthquakes. Because of the transition location of the Ordos block form Eastern to Eastern China, it is influenced by dual dynamic. The deep dynamic problem about this area is a focus of the earth science.This paper gives a brief historical overview of tectonic evolution, geology, earthquake activity, motion and stress environment, and the results of geophysical research about the Ordos block and its adjcent areas. The Rayleigh surface wave tomographic imaging method is used to invert phase velocities and azimuthal anisotropy in this paper. So, this paper introduces surface wave characteristics and analysis methods, anisotropy types and formation.There are98temporary and118permanent broadband seismic stations in the Ordos block for surface wave invertion. The high quality surface waveforms of237teleseismic events from January,2010to Decembe,2011are selected. One and two-dimensional phase velocities and azimuthal anisotropy of thirteen periods from20to143s are inverted by the two-plane-wave inversion technique. Then, shear wave velocities are inverted from phase velocities by the nonlinear method developed by Saito. According to these results, deep structures, layer Interaction and dynamic environment are discussed.The results show that:The thickness of the lithosphere beneath the Ordos block is170-180km. The lithosphere with high velocity and weak deformation may retain cratonic lithosphere roots. The fast direction of azimuthal anisotropy in upper lithosphere of the Ordos block is nearly EW and thar in lower lithosphere is NW-SE. The azimuthal anisotropy in upper lithosphere may be frozen anisotropy, and that in lower lithosphere and asthenosphere is recently formed anisotropy caused by deformation due to present-day tectonic plates. The change of deformation happen form80km to100km depth in the lithosphere.The difference deformation with depths in the lithosphere may be mainly related with the variations of temperature with depth. Now the NE direction compression of the Tibetan Plateau plays a major role in the deformation of lower lithosphere beneath the Ordos block.The upwelling of asthenosphere with passive continental rifting mechanism may happen along the weak belt of lithosphere in the northeast of the Ordos block. The central upwelling region locates on the west of Datong basin and southeast of Daihai basin. Due to rapid upwelling of magma thermal mass of asthenospheric along the channel and injection into the bottom of the crust, the mass of the crust and mantle melt because of the thermal erosion. And this results in large-scale low velocity anomaly in the crust and upper mantle of this area. The upwelling location of asthenosphere and difference of magma migration may control the volcano eruption pattern and differences of Cenozoic basalts in the area near Datong. The upwelling of asthenosphere results in further thinning of the lithosphere beneath the area in the Cenozoic. The low velocity anomaly in the lower crust and lithosphere of Weihe rift blet may be closely related to the lateral migration of thermal mass of asthenosphere.In the western Qinling and eastern Qilianshan orogeny, the low velocity anomaly in the crust and lithosphere is mainly correlated with tectonic movements rather than the thermal material upwelling of asthenosphere in this area. Continuous uplifting and northeastward spreading of the Tibetan Plateau result in strongly shear deformation of crust and mantle. And then internal heat generation and plasto-rheological deformation and flow cause the low velocity anomaly. Near the Liupanshan fault, thermal material upwelling of asthenosphere may partially happen.In the southern of Shanxi graben and northwest of Henan, lithosphere thickness is about70km, and the asthenosphere obviously rises, which may show lithospheric thinning of the North China Craton in the Mesozoic. Beneath the central area of Shanxi graben from N37°to N38°, there is a high-velocity lithosphere segment of the Ordos block separating the north and south asthenosphere. This segment may be suffering from thermal erosion of the asthenosphere.Beneath the east-central part of the Qinling orogen, the side of near the Weihe basin between E108°and E109.5°, there is a high-velocity anomaly at the depth of100-150km. And it may be delamination lithosphere of of the underthrust Yangtze block betheath the North China block. The delamination of lithosphere in the region results from the oblique convergence of the Yangtze block and North China block.In conclusion, the northeastward spreading and compression of the Tibetan Plateau play a major role in the deformation and movement of the Ordos block, and the movement of the Ordos block provides new tectonic background for further evolution of surrounding area.