We process ambient noise data from seismic stations deployed in central Asia to determine the crustal shear wave velocity structure beneath the Tianshan Mountians and surrounding area.About 748 inter-station Rayleigh wave empirical Green's functions have been recovered to estimate the phase velocity dispersions over periods from 6 to 50 s using the image transformation technique.Results show that for short periods(6-20 s),the distribution of Rayleigh wave phase velocities is generally consistent with surface geology,with high velocities corresponding to mountain ranges and low velocities to sedimentary basins.Along two profiles,which trend from NE-SW and NW-SE,the shear wave velocity shows a pair of high velocity anomalies dipping in opposite directions beneath the Tianshan Mountains.At shallow depths,those high velocity anomalies roughly correlate with areas where the mountain front and the surrounding basin are connected.The profiles also show a narrow zone beneath the Tianshan Mountains,which may represent a route for the upwelling from upper mantle.Those observations suggest that the underthrusting of the Tarim Basin and Kazakh Shield combine with the weakness of the crust,which is heated by the upwelling from upper mantle,may play an important role on the reactivation of the Tianshan Mountains associated with the India-Eurasia collision.
