Velocity Anomalies Around the Mantle Transition Zone Beneath the Qiangtang Terrane, Central Tibetan Plateau From Triplicated P Waveforms

The Qiangtang terrane exhibits many unique features compared with surrounding areas in the Tibetan plateau. It has been largely remodeled by the northward downwelling of the Indian lithosphere and the vertical upwelling of the Eurasian asthenosphere. Here, we investigate the P-wave velocity around the mantle transition zone (MTZ) beneath the Qiangtang terrane using high-frequency triplicated waveforms recorded by the China Digital Seismic Network. We found a low-velocity layer (LVL) atop the MTZ and a high-velocity anomaly (HVA) in the lower part of the MTZ. They are characterized by thickness of 36 km with velocity drop of -2.0% and 155 km with velocity increment of 1.2%, respectively. The 410 and 660 km discontinuities are depressed by different depths of 12 and 30 km, suggesting a thickened MTZ. In combination with previous multi-disciplinary observations, we infer that the LVL is a hydrous partial melt affected by dehydration and high temperature, and the HVA is due to water and low temperature conditions. The downwelling cold continental lithosphere may have accumulated in the lower MTZ and induced the hydrous hot MTZ materials to upwelling until the shallow depths of the upper mantle beneath the Qiangtang terrane. Plain Language Summary The Qiangtang terrane is located in front of the northward downwelling Indian plate. Whether there are material exchanges between the upper mantle and the mantle transition zone (MTZ) is on debate. Our observations of P-wave triplicated waveforms reveal a low-velocity layer atop the MTZ and a high-velocity anomaly in the lower MTZ. Different scales of depressed 410 and 660 km discontinuities suggest a thickened MTZ. We suggest a dynamic model of mantle convection involving the asthenosphere and the MTZ for the evolution of the Tibetan plateau.