Nature of mid-crustal low velocity zones in central and southern Tibetan Plateau from joint inversion of surface wave dispersion, receiver function, and Pn station delay

Abstract:

We introduce Pn station delay time into the joint inversion of surface wave dispersions and P receiver functions to reduce trade-offs of model parameters. The method is implemented using a global search-based algorithm and a flexible parameterization of a sedimentary layer and spline-based parameterization that can represent sharp discontinuities. Extensive tests using synthetic and real data suggest that the method is suitable and robust for a variety of crustal velocity structures, and can simultaneously provide crustal Vp/Vs ratio and better-constrained Moho depth. The better-constrained crustal structure also improves the resolution of the mantle velocities. We apply the joint inversion scheme to all available portable stations in central and southern Tibet. A stacked receiver function after harmonic corrections (H-κ-c; Li et al., 2019) is used for each station. The results show considerable improvements on crustal Vs structures with additional information of crustal Vp/Vs ratios. The high Vp/Vs regions correlating with low S velocities suggest possible partial melting in the mid-crust in parts of the Tibetan Plateau. Their correlation with low seismicity further suggests different deformation pattern (ductile vs. brittle) in different blocks of the plateau. The results do not support channel flow model to surface in southern Tibet. The distinct Lhasa block (high Vs, low Vp/Vs, high seismicity relative to other blocks) may be related to hypothesis of proto-Tibetan Plateau and outward growth of the plateau.