云南西部地壳深部结构特征

The deep crust structure characteristics beneath western Yunnan

在云南西部,穿过红河、小江断裂带完成了一条长360km、呈北东向的深地震宽角反射/折射剖面.通过对该测线的观测资料进行一维、二维模拟解释,得到了沿剖面的二维地壳速度模型.研究结果显示,沿测线Moho界面埋深横线变化大,其西南侧Moho埋深约35km,东北侧Moho最大埋深可达43km.沿剖面从西南到北东方向,地壳平均P波速度从5.9km/s逐渐增加到6.13km/s,但显著低于全球大陆平均值.结合以往的接收函数和面波联合反演结果,我们推算沿测线从西南到东北,其下方地壳泊松比介于0.23～0.25之间.剖面西南侧上地壳具有异常低的P波速度和泊松比,暗示其下方上地壳以α-相长英质组分为主;而剖面东北上地壳相对较高的P波速度和泊松比则暗示其物质组成以花岗岩-花岗闪长岩为主.研究区下地壳的P波速度和泊松比分别介于6.25～6.75km/s和0.24～0.26km/s之间,暗示其上部组成以花岗岩相的片麻岩为主,而下部组成则以角闪石类岩石为主.红河断裂两侧地壳速度显著不同,从浅到深其速度差异逐渐变弱,但红河断裂两侧地壳厚度变化较大.而小江断裂下方两侧地壳速度和地壳厚度变化并没有红河断裂那么明显.

One two-dimensional P wave velocity model of the crust and uppermost mantle was obtained based on the integrative interpretation of travel-time data and amplitude information from the deep seismic sounding experiment on the Puer-Luxi profile, western Yunnan. Our results show strong lateral variations in crustal thickness along the profile, which varies from about 35 km in the southwest to 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 function and surface wave, average crustal Poisson′s ratio （σ） is calculated and our results show that σ varies from 0.23 in the southwest part of the profile to 0.25 towards the northeast part of the profile beneath the western Yunnan. The low P wave velocity and σ 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.