Seismic tomographic imaging of the crust and upper mantle under the central and western Tien Shan orogenic belt

Abstract

[1] Detailed 3-D tomographic images of P and S wave velocity (Vp, Vs) and Poisson's ratio (σ) under the central and western Tien Shan orogenic belt are determined by using a large number of high-quality P and S wave arrival times from local earthquakes. The results show that under the Tien Shan orogenic belt high-Vp, high-Vs, and low-σ anomalies are revealed in the upper and middle crust, possibly indicating the existence of the Paleozoic crystalline basement rocks, while low-Vp, low-Vs, and high-σ anomalies appear in the lower crust and upper mantle, perhaps suggesting that the hot and wet material is upwelling under the Tien Shan orogenic belt from the mantle. Some high-Vp, high-Vs, and low-σ anomalies are tilted toward the Tien Shan along with the seismicity. These are found in the collision zones between the Tien Shan and the Tarim basin in the south and the Kazakh shield in the north and suggest the underthrusting of the Tarim and Kazakh lithosphere beneath the Tien Shan. Meanwhile, some low-Vp, low-Vs, and high-σ anomalies are imaged in other parts of these collision zones, perhaps indicating the intrusion of the hot and wet material into the crust from the upper mantle. These results indicate that both the upwelling of the hot and wet material and the underthrusting of the Tarim and Kazakh lithosphere may have played an important role in the mountain building. Under the Tarim and Fergana basins, low-Vp, low-Vs, and high-σ anomalies are revealed in the upper crust, while high-Vp, high-Vs, and low-σ anomalies are visible in the lower crust and upper mantle. These may reflect the existence of less compacted sedimentary material in the shallow crust and more highly compacted craton-like structures in the deeper crust and upper mantle under the basins. The Talas-Fergana fault shows an obvious tectonic boundary between central and western Tien Shan. The central Tien Shan displays high-Vp, high-Vs, and low-σ anomalies in the upper and middle crust, while western Tien Shan exhibits low-Vp, low-Vs, and high-σ anomalies. However, the pattern of seismic structure between central and western Tien Shan reverses in the lower crust. Such a correlation may extend down to the upper mantle, suggesting that the Talas-Fergana fault may be a lithospheric-scale boundary. Additionally, a columnar low-Vp and low-Vs anomaly is clearly observed around the turning point of the Talas-Fergana fault from the NWN to NWW trending orientations and may indicate that the fault provides a channel for the hot and wet material upwelling from the mantle to the surface.