Seismic tomography of the crust and upper mantle structure underneath the Chinese Tianshan

From the travel time data of the P-arrivals recorded by the passive seismic array across the Chinese Tianshan and the regional seismic network, the P wave velocity structure of the crust and upper mantle down to 400 km along Kuqa-Kuytun profile is reconstructed by using seismic tomography technique. Our results demonstrate that the crust along the profile has an obvious block structure and strong lateral crustal distortion. These manifest that the Tianshan crust is compressed strongly by the Tarim block. Beneath the Tarim and Junggar block, there are high-velocity anomalies with the thickness of 60-90 km at the top of the upper mantle. The high-velocity anomaly below the Tarim-South Tianshan has a clear curved distortion, and that below the Junggar-Northern Tianshan thrusts down to the depth of 300 km at the south side of the central. Tianshan. Both of them construct an asymmetric bilateral thrusting system. Beneath the Tarim and Junggar block, in the depth range from 150 km to 400 km, exist low-velocity anomalies. One of them on the Tarim side rises to bellow the south Tianshan. In the upper mantle, at the depth of 200-300 km beneath the Tarim-South Tianshan, exists a high-velocity anomaly, which could be the detached lithosphere of the Tarim melted by the up-welling hot material of the upper mantle. All of our observations manifest that the lithosphere of Tarim block is involved in the northward subduction, but its front is limited to the north border of the south Tianshan. The long-range effect caused by the uplift of the Tibetan plateau could drive not only the subduction of the Tarim lithosphere, but also the upwelling of the upper mantle in the south side of the Tianshan orogenic belt. The low-velocity anomalies widely existed in the Tianshan upper mantle should facilitate the mantle deformation, and the large difference between the high and low velocity anomalies in the upper mantle may promote the small-scale mantle convection. According to the crustal and upper mantle velocity structure beneath the Tianshan orogenic belt, it could be inferred that the main source driving the quick uplift of the Tianshan since Miocene comes from the long-range effect caused by the uplift of the Tibetan plateau, and the relatively soft upper mantle beneath the Tianshan creates the necessary condition for accelerating the uplift and distortion of the crust.