Crustal and Upper Mantle Structure of the Tien Shan Orogenic Belt From Full-Wave Ambient Noise Tomography

In order to have a better understanding of the lithosphere formation and modification of the Tien Shan orogenic belt, we construct a well-defined shear wave velocity model of the crust and upper mantle with full-wave ambient noise tomographic method. High-quality empirical Green's functions at periods of 7-200s are extracted from the cross correlation of the vertical component of continuous seismic data at 108 stations during 2012-2014. Our tomographic results show remarkable velocity variations between and within the major tectonic units from the crust down to the upper mantle. We observe very slow upper crust beneath the Tarim and Junggar sedimentary basins. The interior of the Tarim Basin shows strong seismic heterogeneities. The high-velocity mantle lithosphere of the Tarim Basin underthrusts northward toward the central Tien Shan. Lithosphere underthrusting could trigger intrusion of hot mantle material and partial melting, in correspondence with the prominent low-velocity anomalies observed in the lower crust and uppermost mantle of the central Tien Shan. In contrast, the connected high-velocity upper mantle structure from Tarim Basin across eastern Tien Shan to Junggar Basin may reflect the convergent effect between the Tarim and Junggar Basins, which consequently prevents the asthenosphere upwelling. We observe low-velocity anomalies in the upper mantle of the western Tien Shan, indicating continental crust of the Eurasia lithosphere that has been subducted toward the western Tien Shan. The observed structural variations along the Tien Shan orogenic belt suggest different tectonic mechanisms for the lithosphere formation and modification of the three segments along strike.