Seismic ambient noise tomography and azimuthal anisotropy of the central Tien Shan

Tien Shan is situated in central Asia and surrounded by the relatively stable Kazakh shield and Junggar basin in north and Tarim basin in south and is suggested as far-field effects of India-Eurasia collision. Detailed tomographic imaging will help us better understand interaction of these tectonic blocks and associated orogenic and crustal deformation processes in this complicated region. In this study, we apply ambient noise tomography to image azimuthally anisotropic structures of the crust beneath the Tien Shan. This study is the first attempt to investigate crustal anisotropy structure of the central Tien Shan. We use continuous ambient noise recorded by temporal broadband deployment of TianShan Continental Dynamic project (GHENGIS) consisting of 28 stations uniformly distributed in Tien Shan during 1997-2000. Our results show prominent Rayleigh wave phase velocity anomalies and heterogeneity in Tien Shan at periods up to 35s. At shorter periods, mountain ranges of metamorphic, igneous exposure are associated with high velocities, while sedimentary filled basins in and around Tien Shan are associated with low velocities. Heterogeneity of velocity distribution at deep depths beneath the central Tien Shan and along the south and north boundary faults is also observed. Our results indicate that azimuthal anisotropy in the Tien Shan is depth dependent. The fast direction in the central Tien Shan is in NEE-SWW at shorter periods, in response to compressional stress due to ongoing N-S shortening, whereas the fast direction in the Tarim basin and Kazakh shield is complex at shorter periods and becomes uniformly NE-WS oriented in Kazakh shield and SE-NW oriented in Tarim basin at longer periods. At the depths of lower crust and upper mantle, the fast directions become normal or sub-normal to the orogen for most part of central Tien Shan. The relative simple patterns of azimuthal anisotropy may suggest that the observed anisotropy originates from LPO of strong crustal anisotropic minerals and olivine as a consequence of northward and southward subduction of Tarim basin and Kazakh shield and flow of the Tien Shan lithospheric mantle. This study highlights the potential of probing seismic anisotropy for regional tectonic studies using ambient noise tomography.