Crustal structure of Yunxian-Ninglang, northwestern Yunnan, China from wide-angle seismic reflection

The Red River fault in western Yunnan, is one of the longest strike-slip faults in China. It has a high seismic potential. To investigate its complicated structure, we carried out a nearly south-north 300-km long seismic refraction/wide-angle reflection survey across the Red River fault from Yunxian to Ninglang. Three chemical explosions with charge from 1ton to 3 tons were fired, more than 150 seismic stations were deployed. The 2-D model of the seismic velocity structure along the profile was obtained by a combination of first arrival travel-time tomography and forward modeling of later arrivals. Our results show strong lateral variations in the crust. Crustal thickness increases from 45 km in the south to 54 km in the north. However, no abrupt crustal thickness change was observed across the Red River fault in our study area. The crust in the study area is thicker than average thickness of continental crust. The crustal thickening is mainly observed in the lower crust, whereas the upper and middle crustal crust possess nearly constant thickness. We observed strong seismic velocity contrast across the Red River fault. All those observations emphasize the role of the fault as an important tectonic boundary between Yangtze block and the Three Rivers geosynclinal system.