Crustal Structure of Southwest China and Northern Vietnam From Ambient Noise Tomography: Implication for the Large-Scale Material Transport Model in SE Tibet

Several models have been proposed to describe the tectonic evolution of SE Tibet and its marginal areas. Hence, high-resolution crustal velocity models are essential to address this controversy. With waveform data from 73 broadband stations in southwest China and northern Vietnam, we invert for a 3-D shear wave velocity model of the crust and uppermost mantle from ambient noise tomography. Our model reveals that the midlower crustal low-velocity zone in the Xiaojiang Fault Zone extends farther southward across the Red River Fault to Vietnam and is approximately bounded by the Xiaojiang and Dien Bien Phu faults to the east. We suggest that the observed low-velocity zone represents a mechanically weak zone in the mid-lower crust, which may serve as a channel for efficient southward material transport in SE Tibet. With our results and previous evidence, we propose a combined model that integrates rigid block extrusion and crustal channel flow to describe the large-scale material transport in SE Tibet. We further propose a two-phase material transport model in SE Tibet after the India-Eurasia plate collision: (1) rigid block extrusion between the right-lateral Sagaing Fault and left-lateral Red River Fault during the early Oligocene-early Miocene and (2) a combined model of rigid block extrusion and material channel flow in the mid-lower crust from the late Miocene to the present. The southward crustal material transport is likely to be diverted along two major channels around the more rigid crust beneath the inner zone of the Emeishan large igneous province.