关键词: 接收函数, 鄂尔多斯地块, 山西断陷带, H-κ扫描叠加, 共转换点叠加, 地壳结构

Abstract:

The North China Craton is the oldest craton in China and also the main tectonic unit of the Chinese mainland. The geological marks from Archean to Mesozoic era are complete and have attracted scientists all over the world. It has been the natural experimental site for the study of continental formation and evolution. A series of complex tectonic movement and evolution processes occurred in the North China Craton since Mesozoic. A series of rift basins were formed due to the thinning of lithosphere in its eastern part, so its crust structre is complicated. But the lithosphere is thick in its western part, so the crust structure of the Ordos block is simple. Shanxi rift zone is located between the eastern block of North China Craton and the western Ordos block. The crust and lithosphere structure of Shanxi rift zone is changed from stable craton structure in the west to severely damaged craton structure in the east, showing obvious transition characteristics. Therefore, it is of great significance to study the structural characteristics of the Shanxi rift zone and its two sides so as to reveal the failure dynamics of the North China Craton. Based on the teleseismic waveform data recorded by 150 mobile seismic stations in the central and western part of the North China Craton(107°E~117°E; 34°N~41°N)in the recent three years, the crustal velocity structure images of the study area are obtained by using the H-κ stacking method of P-wave receiver function and the common conversion point(CCP)superposition method. Our research results show that the crustal thickness in the Ordos block is between 37km and 47km, the Moho surface is relatively flat. The crust thickness of Shanxi rift zone is between 34km and 46km. Under the depression of Linfen Basin, Moho surface shows obvious uplift, and the uplift amount is between 4km and 10km. It is inferred that the formation of Shanxi rift zone is closely related to the movement of mantle materials. Compared with the existing Bouguer gravity anomaly data in this area, the distribution characteristics of crustal thickness in the study area are consistent with the distribution characteristics of positive and negative Bouguer gravity anomalies in the eastern and western Taihang uplift, respectively. The calculation results of crustal thickness and wave velocity ratio in different tectonic units in this region show that the wave velocity ratio in the three tectonic units decreases with the increase of crustal thickness. On the whole, the study area is divided into east and west areas with 111.5°E as the boundary. The Poisson's ratio of Ordos area to the west is lower than that of Shanxi rift zone to the east of 111.5°E, which reflects that the eastern part of Ordos block has the characteristics of stable ancient block and the crustal structure is relatively simple; however, the upwelling of upper mantle material under the Shanxi rift zone leads to higher Poisson's ratio than the mountainous areas on both sides. As far as the Shanxi rift zone is concerned, it is divided into north and south regions with 38°N as the boundary. The crust to the north of 38°N is characterized by low velocity due to partial melting, while the area south of 38°N still maintains a relatively stable crust and presents high-velocity characteristics. The difference of crustal structure and material composition between the north and the south of Shanxi rift zone may be related to the uneven subsidence of Shanxi rift zone, and more data are needed for further comprehensive study on the related dynamic process.

Key words: receiver function, Ordos block, Shanxi rift zone, H-κ stacking, common conversion point stacking, crustal structure