Geophysical constraints on the nature of lithosphere in central and eastern Tibetan plateau

Abstract

The structure and rheology of lithosphere are fundamental to understanding lithospheric deformation operating within the Tibetan plateau and hence holds significant implications for continental dynamics. Yet, some of these aspects remain less well constrained in Tibet. In this study we construct new models of isotropic-average shear-wave velocity and radial anisotropy from previously published Rayleigh-wave and Love-wave phase velocities at periods of 8–143 s. Integrating with other previously published geophysical data, we demonstrate that a weak mid-to-lower crust is pervasive within the plateau, as directly inferred from seismically low velocity and high conductivity at these depths along with high Moho temperature. When combining with the observation of positive radial anisotropy patterns (Vsh > Vsv), we speculate that this weak mid-to-lower crust could probably flow towards southeastern Tibet, whereas in the Qinling Orogen this crustal flow is absent. This weak mid-to-lower crust in Tibet could decouple upper crust and lower crust/upper mantle, accommodating active deformation in response to the Indian-Asian convergence. Additionally, our results show that the subduction front of Indian lithospheric mantle (LM) could reach to the Bangong-Nujiang-Suture (BNS) in central Tibet and to the Jingsha-River-Suture (JRS) in eastern Tibet. We find no expected high velocity beneath northern Tibet to support southward subduction of the Asian LM. Instead, the northern Tibet is characterized with a continuous low-velocity-anomaly from Moho to 200 km. Coupled with the observations of shallow Curie-point depths and high Moho temperature, we infer that the Tibetan upper mantle in the north is hotter than that in the south, probably resulting from mantle upwelling due to either the Indian LM subduction, or thickened Tibetan lithosphere delamination with some contributions of shear heating and radioactive heating from the thickened Tibetan crust. This study highlights the nature of lithosphere, associated dynamic processes and the overall deformation within the Tibetan plateau.