Characteristics of Deep Crust Structure Beneath Western Yunnan

Abstract

AbstractOne two‐dimensional P wave velocity model of the crust and uppermost mantle was established based on the integrative interpretation of travel‐time data and amplitude information from the deep seismic sounding on the Pu'er‐Luxi profile, western Yunnan. Our results show strong lateral variations in crustal thickness along the profile, which is about 35 km in the southwest and 43 km in the northeast Yunnan. Average crustal P velocities are low with respect to the global average, varying from 5.9 to 6.13 km/s. Combined with previous joint inversion results of receiver functions and surface waves, the average crustal Poisson's ratio (σ) is calculated, 0.23 in the southwest part of the profile and 0.25 in the northeast part of the profile beneath the western Yunnan. The low P wave velocity and σ values for the upper crust layer beneath the southwest part of the profile is indicative of felsic rocks rich in quartz in the α state, while the relative high P wave velocity and σ for the upper crust layer under the northeast part of the profile is indicative of rocks of granitic and granodioritic composition. The P wave velocity and σ for the lower crust layer is 6.25∼6.75 km/s and 0.24∼0.26 km/s beneath the profile, which can be explained by felsic gneisses in the upper part of the lower crust transitioning to amphibolite in the lower part. Our crustal velocity model shows remarkable contrasts across the Red River fault and the velocity difference becomes gentler with increasing depth. The crustal thickness across the Red River fault is also significantly different. The variation of the velocity and crustal thickness across the Xiaojiang fault is not so remarkable.