3-D lithospheric structure beneath southern Tibet from Rayleigh-wave tomography with a 2-D seismic array

P>A 3-D shear wave velocity model has been established for the lithosphere of southern Tibet through Rayleigh-wave tomography. The teleseismic data are acquired from a 2-D seismic array selected out of the second phase of the Hi-CLIMB project. We first inverted for a 2-D phase velocity model using two-plane-wave tomography approach, and then inverted for the 3-D shear wave velocity model with the 2-D phase velocity model and the map of crustal thickness derived from receiver functions. The results reveal a pervasive low velocity anomaly in the middle crust of the Lhasa block. We interpret this low velocity anomaly as the presence of wholesale mid-crustal channel flow. Prominent low velocity anomalies from the lower crust to the mantle lithosphere are observed along the north-south trending rifts: the Tangra Yum Co rift and the Pumqu Xianza rift. We propose a possible scenario for the formation of these rifts: the entire lithosphere is involved in the early-stage rifting, while the late-stage rifting is restricted in the upper crust. In the non-rift regions, a slab-like high velocity anomaly is traced beneath both the Himalayas and Lhasa block. Combining all the observations, we suggest that the central part of southern Tibet is currently underlain by the broken Indian lower crust and mantle lithosphere.