Seismic Tomography of the Crust and Upper Mantle Structure Underneath the Chinese Tianshan

Abstract

From the travel time data of the P-arrivals recorded by the passive seismic array across the Chinese Tianshan and regional seismic network, the P wave velocity structure of the crust and upper mantle down to 400km along Kuqa-Kuytun profile is reconstructed by using seismic tomography technique. Our results demonstrate that the crust along the profile has an obvious blocked structure and manifest strong lateral crustal distortion. These demonstrate that the Tianshan crust is compressed strongly by the Tarim block. There are high-velocity anomalies with a thickness of 60~90 km on the top of the upper mantle beneath the Tarim and Junggar basins. 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, at the depth ranging from 150 km to 400 km, exist low-velocity anomalies, one of which on the side of Tarim up-wells 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 detachment of the lithosphere of the Tarim melted by the up-welling hot material of the upper mantle. All of our observations mentioned above illustrate that the lithosphere of the Tarim block is involved in the northward subduction, but its front is bounded merely to the north border of the south Tianshan. The long-range effect caused by the uplift of the Tibetan plateau drives not only the subduction of the Tarim lithosphere, but also the upwelling of the upper mantle on 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 could prompt 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.