Tomographic study of crustal velocity structures in the Yunnan region southwest China

We studied P-wave velocity structures of the Yunnan region through a seismic tomography using arrival data from permanent and temporary stations. Our objective is to analyze velocity anomalies that can reflect fault shearing and regional dynamic process at depths. We also like to locate low-velocity zones within the crust that are associated with lower crustal flows. The results reveal a prominent difference of crustal velocity structures across the Ailao Shan-Red River Fault. Bordered by the fault zone, velocity anomalies in the central Yunnan are parallel to the N-S trending faults like the Xiaojiang fault, the Yuanmou Fault and the Chenghai Fault, which reflect the deep effect of the clockwise rotation of the eastern Tibetan crust. Velocity anomalies in the southeastern Yunnan are parallel to the NW-SE trending faults like the Ailao Shan-Red River Fault, the Wuliang Shan Fault and the Lancang River Fault, which show the effect of the southeastward extrusion of the Indochina Block; whereas low-velocity anomalies along the Nanting River Fault are related to tectonic activities of the eastward motion of the Indo-Burma Block. The low-velocity anomalies within the crust are distributed at various depths and in different regions. They are observed in the upper-mid crust between the Ailao Shan Red River Fault and the Lancang River Fault, and in the mid-lower crust near the Xiaojiang Fault and the Yuanmou Fault. In the lowermost crust and near the Moho, they are distributed throughout the central Yunnan, but limited by the Xiaojiang Fault to the east and by the Ailao Shan-Red River Fault to the west. We estimate that the low velocities in the Panxi region is related to the intersection of the Xiaojiang Fault and the Yuanmou Fault, that would provide heat flow channels for the crust-mantle thermal exchange in the rifting period. In the Ailao Shan-Red River region, besides the southeastward extrusion of the Indochina Block, the collision and eastward subduction of the Indo-Burma Block has also played a certain role in deep crustal deformation. The induced mantle upwelling would lead to a penetration of heat flows into the crust through the fault systems and to create the low-velocity layers within the crust. Therefore, the Ailao Shan-Red River Fault is not only a simple geological boundary between the Indochina Block and the South China Block, but also a border to control the deep crustal deformation and the ductile flow in the eastern and western sides.