Crustal Velocity Structure in Northeastern Tibet and its Tectonic Implications

The Tibetan Plateau has been the prime site to understand the processes of continental collision, mountain building, and the interaction between tectonics. More detailed studies of crustal structure in northeast Tibet can be of great benefit to the understanding of crustal deformation and plateau growth mechanisms. To test the low-crust flow models responsible for the growth of northeast Tibet, Ambient noise tomography in northeast Tibetan Plateau and adjacent regions are conducted by using continuous broadband seismic data from regional seismic network operated by China Earthquake Administration and 32 temporary seismic stations deployed by the Northeast Tibet Seismic (NETS) experiment. Empirical Rayleigh and Love wave Green's functions are obtained from interstation cross-correlations. Group velocity dispersion curves for Rayleigh wave between 10 and 50 s and for Love wave between 10 and 30 s are obtained by using the multiple-filter analysis method with phase-matched processing. The group velocity variations of Rayleigh and Love waves overall correlate well with the major geological structures and tectonic units in the study region.Shear wave velocity structures were then inverted from Rayleigh and Love wave dispersion maps. The results show that the Qilian Orogen is characterized by a high velocity at shallow depth (< 30 km), however, at depth deeper than 30 km, it is dominated by a low velocity. The Qaidam Basin appears as a low-velocity zone at depth shallower than 20 km due to its thick sedimentary layer. The Songpan-Ganzi Terrane is associated with a low velocity at deeper depth (≥ 20 km). There is no clear evidence of the low-velocity mid-lower crust beneath the Qaidam Basin and southern Qilian Orogen as the crustal flow model predicts.