Seismic signature of the mantle transition zone beneath eastern Tibet and Sichuan Basin

Abstract

The strong interaction between the eastward flow escaping from Tibet and the rigid Sichuan Basin resulted in the rise of the Longmenshan. However, the detailed dynamics in the mantle remains controversial. In this study, the structure of the mantle transition zone (MTZ) beneath eastern Tibet and Sichuan Basin is investigated using 5080 receiver functions from 51 broadband stations. The depth of the 410km discontinuity is close to the global average, except for the Longmenshan where the 410 and 660-km discontinuities are found to be depressed by up to 10–25km and 5–10km, respectively. The observed simultaneous depressions of the 410 and 660-km discontinuities distributed along the LMS, together with proofs from tomography and regional tectonics, suggest that asthenospheric flow sinks into the MTZ, resulting in a high velocity zone, as well as variation in the MTZ thickness. The depressions are not from the traditional Clapeyron slopes or temperature variation. Also, the depression of the 410km discontinuity and the dehydration of wadsleyite are syngenetic, both of which originate from the dry mantle flow traveling across the old 410km interface.