Distribution of the low velocity bulk in the middle-to-lower crust of southern Tibet: implications for formation of the north–south trending rift zones

We conducted the ambient noise tomography to image the shallow crustal structure of southern Tibet. The2D maps of phase velocity anomalies at the periods of10–16 s show that the low velocities are mainly confined along or near some of the rift zones. While the maps at the periods of 18–25 s show that the coherent patterns that the low velocities expand outside of the rift zones. It means that the low velocities are prevailing in the middle crust of southern Tibet. According to the previous study of surface wave tomography with teleseismic data,we find that the low velocities from the lower crust to the lithospheric mantle are also restricted to the same rift zones. Thus,the integrated knowledge of the distribution of the low velocities in southern Tibet provides some new insight on the formation of the north–south trending rift zones. Compiling the multidiscipline evidences,we conclude that the rifting was an integrated process of the entire lithosphere in the early stage（＊26–10 Ma）,but mainly occurred within the upper crust due to the weakening a decoupling in the low velocity middle crust in the late stage（later than ＊8 Ma）.