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Abstract

Using fundamental mode Rayleigh waves from the INDEPTH-IV and Namche-Barwa seismic experiments for periods between 20 and 143 s, we have investigated the lithospheric structure beneath eastern Tibet. We have found a ∼200-km-wide high velocity body, starting at ∼60 km depth and roughly centered beneath the Bangong-Nijuang Suture, which is most likely a piece of the underthrusting Indian continental lithosphere. The sub-horizontal underthrusting of the Indian lithosphere beneath eastern Tibet appears to be accompanied by its lateral tearing into at least two fragments, and subsequent break-off of the westernmost portion at ∼91°E-33°N. The uppermost mantle low velocity zone we observe beneath the N. Qiangtang and Songpan-Ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern Tibet. We attribute the low velocity zones concentrated along the northern and southern branches of the eastern Kunlun fault at lithospheric depths to strain heating caused by shearing. The azimuthal fast directions at all periods up to 143 s (∼200 km peak sensitivity depth) beneath the N. Qiangtang and Songpan-Ganzi terranes are consistent, suggesting vertically coherent deformation between crust and uppermost mantle. Furthermore, the low velocity zone below the Kunlun Shan reaching down to >200 km argues against a present southward continental subduction along the southern margin of Qaidam Basin.