Lithospheric structure and crust-mantle decoupling in the southeast edge of the Tibetan Plateau

Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau, and understanding the associated dynamical processes requires investigation of the structures of the crust and the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adjacent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantle anisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orientation. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of the crust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km beneath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variation across latitude 25-26 degrees N. The depth of the lithosphere-asthenosphere boundary (LAB) ranges from 180 km to less than 70 km, also varying abruptly across latitude 25-26 degrees N, which is consistent with the sudden change of the fast S-wave direction (from NW-SE to E-W across 26-28 degrees N). In the north of the transition belt, the lithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanically coupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plate is absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement. However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the southward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) as shown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustal movement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be different from that of the lithospheric mantle, thus leading to crust-mantle decoupling in Yunnan. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.