面波频散与接收函数联合反演南北地震带北段壳幔速度结构

Joint inversion of surface wave dispersion and receiver functions for crustal and uppermost mantle structure beneath the northern north-south seismic zone

通过对南北地震带北段区域所布设的676个流动地震台站观测资料进行处理,联合反演面波频散与接收函数数据,获得了研究区内地壳厚度、沉积层厚度的分布情况以及地壳上地幔高分辨率S波速度结构成像结果.反演结果显示研究区地壳厚度从青藏高原东北缘向外总体逐渐变薄,秦岭造山带地壳厚度较同属青藏高原东北缘的北祁连块体明显减薄;鄂尔多斯盆地及河套盆地分布有非常厚的沉积层,阿拉善块体部分区域也有一定沉积层分布,沉积层与研究区内盆地位置较为一致;松潘—甘孜块体、北祁连造山带等青藏高原东北缘总体表现为S波低速异常;在中下地壳,松潘—甘孜块体下方的低速体比北祁连造山带下方的低速体S波速度值更小、分布深度更浅,更有可能对应于部分熔融的地壳;鄂尔多斯盆地在中下地壳以及上地幔内有着较大范围的高速异常一直延伸到120km以下,而河套盆地地幔只在80km以上部分有着高速异常的分布,此深度可能代表了河套盆地的岩石圈厚度,来自深部地幔的热物质上涌造成了该区域的岩石圈减薄;阿拉善块体在地壳和上地幔都表现出高低速共存的分布特征,暗示阿拉善块体西部岩石圈可能受青藏高原东北缘的挤压作用发生改造.

We analyze the observational data recorded by the 676 temporary seismic stations deployed in the northern north-south seismic zone.Our purpose is to derive the crustal thickness,sedimentary thickness and a high-resolution shear wave velocity model of the crustal and uppermostmantle structure beneath the research area.The results demonstrate that the crustal thickness becomes thinner from the northeastern Tibetan Plateau to its periphery,and that in the Qinling orogen is thinner than the northwestern Qilian orogen,implying the former probably has experienced thinning.The Ordos and Hetao basins seem to have very thick sediments,so do some parts of the Alxa block.The sedimentary thickness is consistent with the distribution of basins in the study area.The 3-D shear wave velocity model shows that the northeastern Tibetan Plateau,including the Songpan-Garzêblock and northwestern Qilian orogen,is characterized by relatively low velocity anomalies.In the middle crust,the low-velocity zone（LVZ）beneath the Songpan-Garzêblock has a lower shear wave velocity and a shallower depth than that beneath the northwestern Qilian orogen,which may suggest partial melting in the Songpan-Garzêblock.A significantly high velocity anomaly that extends downward at least 120 km is observed beneath the Ordos basin,while the high velocity anomaly below the Hetao basin extends downward 80 km,revealing a thinned lithosphere beneath the Hetao basin presumably associated with deep hot mantle materials.The Alxa block is characterized by the coexistence of low and high velocity anomalies,suggesting that the lithosphere beneath the western Alxa block may have been deformed by the expansion of the northeastern Tibetan Plateau.