Slab tearing beneath western Tibet and its geodynamic implications: Insight from anisotropic tomography

Radial anisotropy and velocity anomalies beneath a 2-D seismic array in western Tibet reflect obviously E-W trending structural variations. Two distinct anomalies are separated by a layered belt between 80°E and 81°E, which reflect the advancing Indian lithospheric mantle, the weaker and thinner Eurasian or Tibetan lithospheric mantle, and upwelling asthenosphere. In the eastern part of the study area, a high-velocity zone with positive radial anisotropy is detected, which reflects the underthrusting Indian lithosphere. To the west of 80°E, a layered radial anisotropic structure with a relatively high velocity is revealed from ~70 km to 300 km depth (the bottom of our model), which probably indicates foundering lithosphere. Combining our results with geological, geochemical and geophysical findings to date, we propose that these local features may result in tearing in the northward advancing Indian lithospheric mantle. Significant east-west variations of seismic velocity and radial anisotropy are revealed in western Tibet, suggesting that not only the Indian lithospheric mantle but also the Eurasian or the Tibetan lithosphere have contributed to the slab tearing. In addition, the slab tearing has no direct relation to the formation of the Ya-Ri rift and the Puran graben in western Tibet.