Three-dimensional S Wave Velocity Structure And Radial Anisotropy Of The Tarim Basin And Its Surrounding Regions

Due to the complex topography and tectonic structure, northwest of China has become theideal site to study the effects of mountain-basin coupling. In this paper, we construct the3-Dshear wave velocity structure and radial anisotropy in the crust and uppermost mantle in theTarim basin and its surrounding regions by ambient noise seismic tomography. The data include9months (2009January to2009September) three-component continuous data recorded at74seismic stations of China Provincal Digital Seismic Networks and regional Kyrgyzstan andKazakhstan Networks. Empirical Rayleigh and Love wave Green’s functions are obtained frominterstation cross-correlations. Group velocity and phase velocity dispersion curves between8and50s periods are measured for each interstation path by applying the Time-Frequency analysismethod with phase-matched processing. The group velocity and phase velocity maps show clearlateral variations which correlate well with major geological structures and tectonic units in thestudy regions. At short periods (<20s), the basins show low group velocity and phase velocity,but the orogenic belt shows high velocity; At long periods(>30s), high velocity is observedbeneath the basins, on the other hand, the Tianshan orogenic belt stands out as low velocity.SV and SH wave velocity structures of the crust and upper mantle are inverted from Rayleighand Love wave dispersion maps. Because of the thick sedimentary in the basins, S wavevelocities of the Tarim and Junnggar basins are relatively low at shallow depth, while the S wavevelocities of the mountain area are high because of the widely out of magmatic rocks, In themiddle and lower crust, the basins are clearly featured with high velocities and the Tianshanorogenic belt stands out as low velocity. Then we compute the radial anisotropy by measuringthe differences between SH wave and SV wave velocity. The Tianshan orogenic belt showsnegative radial anisotropy, which reflects dominantly vertical movement of the deep crustal materials during the process of crustal shortening and thickening beneath the Tianshan orogenicbelt. Meanwhile, western Tianshan display stronger anisotropy than eastern Tianshan, whichmay be related to different blocking effects due to the Tarim block. Positive radial anisotropy isthe major characteristic of the Tarim basin in the upper crust, and the velocity of SH wave ofhorizontal polarization faster than the SV wave of vertical polarization is mainly affected by thesedimentary layer. In the stable block, the radial anisotropy represents fossil anisotropy left inthe lastest large-scale tectonic movement. Since the whole Tarim block’s movement or shiftresulted in mineral crystals arrangement dominated in a horizontal direction, the middle crust ofthe Tarim basin shows positive radial anisotropy.