The distribution of the mid-to-lower crustal low-velocity zone beneath the northeastern Tibetan Plateau revealed from ambient noise tomography

We collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the Chinese Provincial Digital Seismic Networks, A Seismic Collaborative Experiment of Northern Tibet, and the Northeastern Tibet Seismic experiment. Cross correlations of vertical component records are computed to extract the Rayleigh wave empirical Green's functions. Group and phase velocities are then constructed from the empirical Green's functions in 8 to 50 s period. At periods <= 25 s, more than 10% lower velocities are imaged beneath the Qaidam Basin, and high velocities are observed beneath the nonbasin regions. At periods >= 30 s, up to 10% lower velocities are imaged in the Qiangtang and Songpan-Ganze Terranes. From these group and phase velocity maps, a three-dimensional (3-D) V-sv model of the crust is derived. The model shows that the Qiangtang and Songpan-Ganze Terranes have a very thick crust with a prominent low-velocity zone (LVZ) in the middle crust. The LVZ thins out in the vicinity of the eastern Kunlun Mountains, providing a new constraint on the mode of deformation across the Tibetan Plateau. The northwestern Qilian Orogen, where receiver functions reveal a Moho deeper than the surrounding areas, also features a relatively weak midcrustal LVZ, which we interpret as an intracrustal response associated with the shortening between the North China Craton and the Tibetan Plateau.