Lithospheric Velocity Structure Beneath Chinese Tienshan

This thesis systematically analyzed and studied the kinematic and dynamic features of S receiver function by the synthetic seismogram calculation. This work further developed the non-linear P wave and S wave receiver functions joint inversion method based on the the non-linear receiver function inversion method ( Liu Qiyuan, 1996) and Bayesian inversion theory.From the telesesimic body wave data recorded by 51 moveable broadband stations carried out along the Dushanzi-Kuche Road by Institute of Geology,China Earthquake Administration,this work obtained the P wave and S wave velocity structure beneath the Chinese Tianshan down to 300 km depth by using the non-linear P wave and S wave receiver functions joint inversion method developed in this thesis. From these results,this work obtained some new understandings about the lithospheric structure beneath the Chinese Tianshan and the orogenic dynamic model.On the P wave and S wave receiver functions joint inversion method,this thesis obtained some new result in the following aspects:(1) Suitable epicentral distance for the S receiver function inversion is about 65 to 80 degrees; Comparing with lithospheric bottom interface with steep velocity variation,the gradient belt type lithosphere bottom boundary would weaken the amplitude of SLP conversion phase,but it is still possible to effectively identify it through migration stack.(2) Beneath a station with sediment layers,the amplitude of S wave receiver function is enhanced,which contributing to identity the conversion phase from the basement whose time depends on the velocity of the basement and the sediment layer and the thickness of the sediment layer;(3) Due to the radial component of S wave receiver function does not conform to the delta pulse assumption,the single complex spectral ratio receiver function estimation method in the frequency domain and the iterative deconvolution receiver function estimation in the time domain, which both dependent on the equivalent source assumption, are not conducive to estimate S receiver function. The maximum probability three component receiver function deconvolution method does not depend on the equivalent source assumption and is a better way to give S receiver function estimation.(4)The numerical test results show, the P wave and S wave receiver function joint inversion method takes full advantages of the complementarity between them, not only contributes to the improvement of the P receiver function explanation, but can further extract the P wave and S wave velocity structure of crust and lithospheric mantle. In the case of initial model parameters deviating from the true model 20%, this method is able to make a good prediction about the model parameters in 300km depth ;(5) The actual data processing experience shows,the data of P wave and S wave receiver functions from the same teleseismic events are in favor of P wave and S wave receiver function joint inversion method. Theoretical calculation and practical data processing results show that the epicenter distance of teleseismic events for the P wave and S wave joint inversion should be limited to 65 to 80 degrees.This thesis obtain the following new understandings about the lithosphere structure and orogenic dynamics beneath the stations of Chinese Tianshan:(1)Firstly obtain the P wave and S wave velocity structure of the crust and upper mantle 300km deep beneath Chinese Tianshan by using the joint inversion method of P wave and S wave receiver functions. The results show that across Chinese Tianshan, the lithosphere velocity structure has the transverse block structure in accordance with surface geology.(2) The lithosphere of the Tarim block dips to the north,with thickness about 100 ~ 130km,diving at the southern margin of the South Tianshan in accordance with crustal structure. The lithosphere of the Junggar block dips to the south,with thickness about 120 ~ 150km,diving at the bottom of the North Tianshan; The LAB depth of the South Tianshan gradually increases from the 80km to 130km,and its crustal thickness increases gradually from the south to the north. The LAB depth of the Center Tianshan gradually increases from 130km to 80km,meanwhile,its crustal thickness increases gradually from north to south.(3) The correlation between earthquakes and the low velocity body in the crust suggests that the low velocity layers in the crust of the Tianshan are the results of relative shear deformation and heat in the upper crust and the lower crust caused by N-S direction crustal shortening process. This is consistent with the opposite diving of the Tarim block and the Junggar block;(4) Beneath the South Tianshan an obviously high velocity body at 200-300km depth corresponds to the suture zone,which is the mark of splitting between the South Tianshan and Center Tianshan with the presence of 270km deep high speed body. Accordingly inferred the high velocity body at 200-300km depth beneath the South Tianshan reflects the small scale convection in the upper mantle caused by lithosphere delamination.(5)Through the analysis of inversion results,this work suggests a dynamics model of the Chinese Tianshan orogenic. The results of this thesis support the basins on both sides of the Tianshan"double subduction discontinuous"model. The Tianshan intracontinental orogenic processes are not confined to the crust,but a small scale mantle convection guiding lithospheric deformon process.