Receiver Function Mapping of the Mantle Transition Zone Beneath the Tian Shan Orogenic Belt

This study provides new constraints on the mantle transition zone (MTZ) structure under the Tian Shan orogenic belt in the context of the double-sided subduction of the Junggar lithosphere and the Tarim lithosphere. The 410- and 660-km discontinuities bordering the MTZ under the Tian Shan and its adjacent areas are mapped by stacking 68,361 receiver functions from 4,122 events recorded by 100 broadband seismic stations. Regional and large-scale 3-D velocity models are used to explain the effect of the laterally heterogeneous velocity anomalies. We identify a thickened MTZ of about 9.3 km under the eastern Tian Shan and the Darbut belt. This thickening indicates a lower temperature, which is correlated with the broken-off subducted lithosphere or the delamination of the eastern Tian Shan lithosphere. We document the thinning of the MTZ caused by the depressed 410-km discontinuity and the uplifted 660-km discontinuity across the Tian Shan orogenic belt, Junggar Basin and Tarim Basin. This thinning corresponds to a + 100 K thermal anomaly in the MTZ, and we suggest that it may have been produced by thermal upwelling originating from the lower mantle. The slightly thinned MTZ beneath the Altai Mountains was caused by the uplift of these two discontinuities, which is related to the high-velocity anomalies in the upper mantle and MTZ. In contrast, the thinned MTZ below the easternmost segment of the Tian Shan orogenic belt, formed by the depression of 6.5 km for the 410-km discontinuity may indicate the presence of small-scale mantle upwelling.