2-D Crustal Velocity Structure And Structual Feature From Yunxian-ninglang Profile In Western Yunnan, China

Western Yunnan has a complex geological structure and a history of frequent large earthquakes. It also connects geologically and tectonically with southern Tibet. Thus, western Yunnan has long been regarded as an important research area by geophysicists. The Red River fault system, a long tectonic zone in western Yunnan, clearly plays a key role in the geological activity. Many seismic imaging studies have been completed in this area. However, structural feature in different part of Red River fault have not been constrained yet.In order to obtain detailed crust and upper mantle structure beneath the Red River fault and neighboring regions, we recorded a 300-km nearly south-north wide-angle reflection/refraction seismic profile in western Yunnan. Our profile extended from Yunxian to Ninglang. The middle-south segment of profile crossed the Red River fault, while the northern part paralleled the Chenghai fault. The south portion of the profile intersected the Nanding River fault, Lancang River fault and Wuliangshan fault. The 2-D P wave velocity structure model along the profile was obtained using 1-D and 2-D seismic analysis. An excellent fit of the observed and modeling data was obtained by combining first arrival travel-time tomography and forward modeling.The results show several key features. The entire crust can be divided into three layers:upper crust, middle crust and lower crust. Seismic boundaries within the crust and at the Moho vary in some regions. The depth range are:14.5-15.8km for upper crust; 27.1-31.0km for middle crust; 45.0-54.0km for lower crust (Moho). The range of velocity variations are:6.02-6.23km/s for upper crust; 6.20-6.55km/s for middle crust; 6.49-6.90km/s for lower crust.A relative large lateral variation exists in the depth of Moho, which becomes deeper from south to north, ranging from 45km to as deep as 54km. Compared to other typical continental crusts, our study area’s demonstrates a striking thickening Moho to the north. It should be mentioned that the boundary depths of the upper and middle crust have only a slight change. This demonstrates that the crustal thickening in this area mainly due to the increasing thickness of lower crust. There is an obvious lateral change in the crust structure across the Red River fault, suggesting that this fault is an important tectonic boundary between Yangtze paraplatform and Three Rivers geosynclinal system. The Moho depth has no significant change when crossing Red River fault. This result differs from previous studies of the Red River fault in nearby area, and indicates that there is a notable discrepancy between different parts of Red River fault.