Crustal and Upper Mantle Velocity Structure of SE Tibet From Joint Inversion of Rayleigh Wave Phase Velocity and Teleseismic Body Wave Data

Since the continental collision between the Indian and Eurasian plates began about 50 Ma ago, southeastern Tibet (SET) has undergone complex tectonic deformation. In this study, we investigate fine scale structural features of the crustal and upper mantle depths (<250 km) beneath SET, which hold important clues to understanding the dynamic processes related to this collision. A 3D shear velocity model is constructed through jointly inverting Rayleigh wave phase velocity and teleseismic body wave data from more than 650 stations. Our 3D model identifies three independent low-velocity zones (LVZs) in the mid-lower crust with unprecedented details. More specifically, we observe a prominent LVZ beneath the North Chuan-Dian Block, which is well separated from another LVZ beneath the Tengchong volcano in the south. This LVZ beneath the volcano represents a focused magma reservoir in the crust whose origin is potentially linked to the mantle upwelling associated with the eastward subduction of the Indian plate. The third LVZ, observed around the Xiaojiang Fault, likely represents a separated and mechanically weak layer in the mid-lower crust due to the combined effects of regional crustal thickening under the southeastward plateau expansion, mantle upwelling, and shear heating of strike-slip faults. In the upper mantle, we observe strong velocity reductions both in localized areas beneath the Tibetan Plateau and the broad region south of 26 degrees N. These low velocity anomalies are sitting above high velocity anomalies at deeper depths, suggesting their association with lithospheric thickening and delamination processes. Plain Language Summary Southeastern Tibet (SET) has undergone complex geological evolution due to the plate collision. To better understand the related dynamic processes, we present a high-resolution 3D S-wave velocity model from the earth surface to the depth of 250 km by using surface wave and body wave data. Our model shows three independent crustal low-velocity zones (LVZs). Especially, the one beneath the Tibetan Plateau is seen to be well separated from the one beneath the Tengchong volcano which represents the hot magma upwelling related to the eastward subduction of the Indian plate. In addition, another LVZ around the Xiaojiang Fault probably represents a separated and mechanically weak layer that is attributed to the regional crustal thickening under the southeastward plateau expansion, mantle upwelling, and shear heating of strike-slip faults. In the upper mantle, we observe strong velocity reductions both in localized areas beneath the Tibetan Plateau and the broad region south of 26 degrees N. Many of these low velocity anomalies are sitting above high velocity anomalies at deeper depths, suggesting their origin of lithospheric thickening and delamination