Abstract:

A comprehensive seismic profiling study was conducted across the Jizhong depression, Cangxian uplift, Huanghua depression, and Chengning uplift in the North China Plain to investigate crustal fine structure and the relationship between deep and shallow faults. Two profiles were completed: a deep seismic reflection profile spanning approximately 200km and a middle-shallow seismic reflection profile covering about 66km.

Our results indicate a crust thickness of approximately 30 to 35km along the section, with a thin distribution in the east and thick in the west. Notably, there is a significant uplift on the Moho surface beneath the Jizhong depression, with an uplift amplitude of about 2 to 3km. The deep seismic reflection profile reveals distinct upper and lower structural characteristics of the crust. The upper crust displays typical sedimentary layer reflection characteristics, marked by alternating depressions and uplifts. Numerous large-scale faults are concealed beneath the North China Plain, playing a pivotal role in uplift and sag formation. The lower crust’s reflection structure comprises events with significant changes in reflection energy, unstable stratification, and complex occurrences, contrasting with the strong reflection energy and good horizontal continuity of the upper crust reflections. The piedmont fault of the Taihang Mountain, identified by the mid-shallow seismic profile and deep seismic reflection section, is a large shovel-shaped normal fault with a low angle, linked to the large detachment structure at the eastern margin of Taihang Mountain. The detachment structure is developed between the basement and the sedimentary cover layer, and is shown on the profile as a reflection zone consisting of 3 to 4 strong reflection phases, lasting 0.3~0.4 seconds. This detachment structure controls the formation of graben and horst structures. The Jizhong depression may have been an extensional tectonic system formed in the upper crust on the basis of the extensional detachment of the Taihang Mountain front fault. The deep seismic reflection section highlights the North China Basin’s structural features, characterized by alternating depressions and uplifts, with boundaries clearly delineated by faults such as the Cangxi, Cangdong, and Chengxi faults. These faults control the formation of graben and horst structures and are considered concealed active faults since the Quaternary period. The Cangxi fault, as the eastern boundary of the Jizhong depression, developed in the weak zone of the front thrust nappe tectonic area of the detachment slip structure. The fault plane resembles a plow shape, steep at the top and gently sloping at the bottom. It utilized or transformed the early thrust section, which is now manifested as a west-dip normal fault, controlling the basement structure and stratigraphic sedimentation on the west side of the Cangxian uplift. The Cangdong fault is the eastern boundary fault of the Cangxian uplift, which appears as an east-dipping shovel shaped normal fault on the profile, cut through the reflection waves of the Carboniferous-Permian strata, the Cambrian-Ordovician strata, the Proterozoic strata, and the crystalline basement. It terminates at the interface of the upper and lower crust at a depth of about 18km. The Chengxi fault is a west-dipping normal fault, which cuts through the Cenozoic sedimentary layer at a depth of about 600~700m in the shallow section. It terminates at the interface between the upper and lower crust in a shovel shaped normal fault downward. The deep seismic reflection section also clearly shows the coexisting structural morphology of uplift and depression. Multiple secondary faults that tilt in the same direction or opposite direction to the main fault are developed inside the depression, causing the depression to be divided into multiple secondary structural units, resulting in the complexity of the entire fault basin structure.

In conclusion, the development of boundary faults plays a decisive role in the stratigraphic sedimentary and tectonic deformation of the strata within the depression.

The existing deep and shallow structures and tectonic patterns in the Wuji-Yanshan section of the North China Basin are formed by the “graben-horst” structure developed in the upper crust, the complex fault combination style near the surface, the stratified reflection and the upper and lower superimposed reflection structure developed in the lower crust, and the undulating Moho surface. The findings of this study contribute to the seismological understanding of the dynamic processes occurring in the North China Basin, as well as to the analysis of the structural relationship between deep and shallow structures in the region.

Key words: Cangxian uplift, Huanghua depression, Jizhong depression, Deep seismic reflection, Crustal structure and tectonics

摘要：

为研究华北盆地的地壳精细结构和断裂的深、 浅构造关系, 跨冀中坳陷、 沧县隆起、 黄骅坳陷和埕宁隆起完成了1条长约200km的深地震反射剖面和1条长66km的中—浅层地震反射剖面。结果表明, 沿剖面地壳厚30~35km, 且具有东薄西厚的分布特征, 在冀中坳陷下方, 莫霍面出现明显上隆, 其隆起幅度为2~3km。研究区上地壳反射结构具有凹隆相间的展布特征, 多条规模较大的断裂隐伏于华北盆地之下, 对该区隆起和坳陷的形成与发展具有重要的控制作用。剖面上一系列与主断裂同向或反向的次级断裂将研究区的上地壳切割为多个次级小型地堑或半地堑断陷, 导致整个华北盆地的上地壳结构复杂化。下地壳反射结构由一系列反射能量变化较大、 横向上不能稳定成层、 产状复杂多变的反射事件构成, 这与反射能量强、 横向连续性好及产状近水平展布的上地壳反射明显不同, 暗示研究区上、 下地壳物质有着明显不同的形成环境、 变形方式和温压条件。中—浅层地震剖面和深地震反射剖面共同揭示的太行山山前断裂表现为一条低角度的大型铲形正断层, 向下可与太行山东缘向E缓倾的大型滑脱构造(拆离断层)相联系; 沧西断裂、 沧东断裂和埕西断裂作为沧县隆起、 黄骅坳陷、 埕宁隆起的边界断裂, 控制了该区堑、 垒构造的形成与发展, 是第四纪以来的隐伏活动断裂, 推测其深部终止于深约18km的壳内解耦面上。文中研究结果可为进一步理解华北盆地的深部动力学过程及分析研究区深、 浅断裂构造关系提供地震学依据。

关键词: 沧县隆起, 黄骅坳陷, 冀中坳陷, 深地震反射剖面, 地壳结构和构造