Seismic upper mantle discontinuities beneath Southeast Tibet and geodynamic implications

• Receiver function shows LAB depth from 70 to 180 km beneath Southeast Tibet.
• The 410 and 660 km interfaces are mostly flat, with depression by 10–20 km.
• Geodynamical mode in Yunnan is different from that beneath LMS.

In this study, a total of 13,080 P receiver functions and 2800 S receiver functions recorded by 116 stations in Southeast Tibet, are used to determine the depths of the Moho, the lithosphere–asthenosphere boundary (LAB), and the 410 km and 660 km discontinuities. The results show that the LAB depth increases from 80–100 km beneath southern Yunnan to 140–180 km beneath Sichuan. Most of the 410 km discontinuity is flat and at 410 km depth, with a slight depression of 10–20 km beneath the Longmenshan (LMS) Fault, the central Sichuan–Yunnan (SY) diamond-shaped block, and southeastern Yunnan. For the 660 km discontinuity, the greatest depth occurs beneath the LMS and southern Yunnan, reaching 670–680 km. The deepening of 660 km discontinuity is attributed to compositional heterogeneity within the MTZ or dynamic pressure on the discontinuity. Additionally, the velocity of movement in the crust and upper mantle beneath SY is different. In Yunnan, crustal movement is southeastward, whereas across the Red-River fault, part of the crustal motion is redirected southwest by the westward rollback of the mantle beneath Myanmar. The interaction between the asthenosphere moving northeastward from Myanmar and southeastward from Tibet near latitude 26°N causes the crust and lithospheric mantle beneath southern Yunnan to have different movement directions.

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