Lithosphere-Asthenosphere Interaction Beneath the Southeastern Tibetan Plateau From Joint Rayleigh and S Wave Tomography

Abstract

The ongoing convergence between the Indian and the Eurasian plates has caused significant lithospheric shortening and eastward expansion of the Tibetan Plateau. The Southeastern Tibetan Plateau (SETP), bordered by the Yangtze craton to the east and the subduction of the Indian plate beneath Myanmar to the southwest, plays a crucial role in accommodating this lateral growth. In this study, we construct a new upper mantle S-wave velocity model beneath SETP by jointly inverting broadband (5–140 s) surface wave dispersion curves and teleseismic S-wave traveltimes. Our model reveals two prominent high-velocity anomalies in the upper mantle, differing from previous models' vertical extent. One anomaly beneath the Yangtze craton exhibits a separated two-layered feature; while the other, beneath the South Chuan-Dian Block (SCDB), extends continuously from the uppermost mantle to 300 km depth. These two deep-rooted high-velocity anomalies likely represent mechanically strong blocks modulating the plateau's asthenospheric flow. Two low-velocity features that might be related to such mantle flow are imaged. One is identified at 100–200 km depth near the northwestern Sichuan basin, suggesting potential intrusion of asthenospheric material into the margins of the cratonic lithosphere. The other, a north-south low-velocity channel, is observed at 200–350 km depth beneath the western margin of the SCDB, indicating the southeastward mantle flow may be deflected by plume-enhanced lithosphere beneath the SCDB, or suggesting a component of toroidal flow around the Eastern Himalayan Syntaxis. Our new model has significant implications for understanding the lithosphere-asthenosphere interactions in the Tibetan Plateau and adjacent blocks.