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

摘要：

华北克拉通是中国最古老的克拉通, 是中国大陆的主要构造单元, 从太古代到中生代以来的地质记录较完整, 受到了国际上的广泛关注, 是研究大陆形成和演化的天然实验场地。中生代以后, 华北克拉通发生了一系列复杂的构造运动与演化进程, 东部因岩石圈减薄而形成一系列的裂陷盆地, 地壳结构复杂。而西部岩石圈厚度大, 鄂尔多斯地块的地壳结构相对简单。山西断陷带位于华北克拉通东部地块与西部鄂尔多斯地块之间, 其地壳与岩石圈结构从西部稳定的克拉通结构变化为东部破坏严重的克拉通结构, 过渡特征明显。因此, 揭示山西断陷带及其两侧区域的构造特征对研究华北克拉通的破坏动力学过程有着重要意义。文中利用华北克拉通中西部地区(34°~41°N, 107°~117°E)的150个流动地震台站近3a记录的远震波形资料, 采用P波接收函数的H-κ扫描叠加法和共转换点(CCP)叠加法处理计算, 获得了研究区的地壳速度结构图像。结果表明, 鄂尔多斯地块内的地壳厚度为37~47km, 莫霍面较为平坦。山西断陷带的地壳厚度为34~46km, 在临汾盆地凹陷的正下方, 莫霍面呈现出明显的上隆, 上隆量为4~10km, 推断山西断陷带的形成与地幔物质的运动有着密切关系。通过与该区域已有的布格重力异常资料进行对比, 研究区地壳厚度的分布特征与太行隆起东、 西部地区分别呈现出正、 负的布格重力异常分布特征一致。该区域不同构造单元内的地壳厚度和波速比计算结果表明, 3个构造单元内的波速比均随地壳厚度的增加而不同程度地减小。整体看来, 研究区以111.5°E为界分为东、 西2个区, 111.5°E以西的鄂尔多斯地区的泊松比较111.5°E以东的山西断陷带低, 反映出鄂尔多斯地块东部地区具有稳定的古老地块特征, 地壳结构相对简单; 而山西断陷带下方上地幔物质上涌导致其泊松比比两侧山区的泊松比高。就山西断陷带而言, 以38°N为界可分为南、 北2个区域, 38°N以北的区域内地壳因存在部分熔融而呈现低速特征, 而38°N以南的区域仍然保持着相对稳定的地壳特性而呈现高速特征。山西断陷带分南、 北2个区域的原因可能与山西断陷带不均匀沉降有关, 其有关的地球动力学过程需要更多的资料来进一步综合研究。

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