SHEAR-WAVE VELOCITY STRUCTURE IN THE CRUST AND UPPER MANTLE BENEATH THE CENTRAL TIEN SHAN FROM SURFACE-WAVE TOMOGRAPHY
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The Tien Shan in central Asia is the world’s largest and most active intracontinental orogen, with a shortening rate of 20 mm/yr. Its tectonic reactivation started 20-25 Ma, which has been attributed to the India-Eurasia collision. Despite many geological and geophysical studies in the Tien Shan, the mechanism of building this intraplate mountain belt remains debatable. Here I constructed a 3-D shear-wave velocity model beneath the central Tien Shan using Rayleigh-wave data recorded at 41 broadband stations in the study area. Phase velocities are obtained from the two-plane-wave inversion at periods of 20 to 133 s and from the ambient noise tomography at 8 to 30 s and are used to determine shear-wave velocity variations beneath the central Tien Shan. Low-velocity anomalies in the upper crust correlate well with thick sediments in the Tarim basin, Lake Issky Kul, and Naryn basin. The lowest velocity in the lower crust is beneath the central high range, suggesting a weak crust. A fast velocity anomaly is imaged beneath the northern Tarim block to the Naryn basin from 30 to 70 km, indicating the underthrusting of the Tarim basement. These observations support that the formation of the Tien Shan is due to both a strong Tarim and a weak Tien Shan lithosphere. A circular slow anomaly to 150 km depth is observed beneath the Naryn basin in the west, while the eastern central Tien Shan is characterized by a fast anomaly. In addition, the fast anomaly that represents the Tarim lithosphere stops sharply at the Naryn basin but continues and deepens under the eastern part. Such variation of velocity structure along the Tien Shan strike implies that the western and eastern range is undergoing different mountain building procedures. A favorable interpretation is that the subducted Tarim lithosphere was delaminated in the west and the slow anomaly there reflects the hot asthenosphere. In contrast, the Tarim lithosphere is still present beneath the eastern part, which may be removed in the future to develop the scenario as in the western part.