Abstract:

The Tanlu fault zone is the most active fault zone in eastern China. It has been active mainly along the Anqiu-Juxian Fault(AJF)since the Quaternary. Predecessors have done a lot of research on the age, paleoearthquake and geometry structure of the AJF, but most of them focus on the exposed area of the fault, and relatively few studies on the buried section. Using field geological survey, shallow seismic exploration, drilling, and paleoearthquake trench, this paper focuses on the geometry structure of the Xinyi section(the buried section)of the AJF, and analyzes its geometry distribution characteristics in the plane and the structural relationship between the deep and the shallow parts, thus filling the gap of the activity characteristics of the Xinyi section of the AJF. The results show that the Xinyi section of the AJF can be divided into three sections from north to south: the Beimalingshan-Guanzhuang section, the Guanzhuang-Tangdian section and the Tangdian-Xindian section.
The Xinyi section of the AJF, mainly manifested as strike-slip and normal faulting, has a right-handed and right-step distribution. The step-over zone with~900m in width and~16km in length is dominated by extension, leaving a length-width ratio of 18:1, much larger than the traditional pull-apart basin ratio of 3:1. According to the shallow seismic profile, the shallow seismic line in the Guanzhuang-Tangdian section revealed the extensional fault depression basin on the north side of the terrace, and the bedrock top of the basin gradually became shallower toward the north. The top of the bedrock in the shallow seismic survey line on the north side of the Nanmalingshan suddenly became deeper, and the NNE-trending compressional near-EW basins of the Nanmalingshan and Tashan developed. The two basins were formed from different origin. With the activity of the Anqiu-Juxian Fault and the erosion and deposition of the Shu River, the two basins gradually developed and merged into a composite basin, and the basin structure was consistent with the Quaternary stratigraphic isopach.
The Xinyi section of the Anqiu-Juxian Fault presents the deformation characteristics of the same genesis and coordinated geometric structure in the deep and superficial layers, showing a single branch in the deep, cutting through the Cretaceous strata, extending and rupturing upward along the contact interface between the bedrock mountains and the Quaternary soft soil layer in the superficial layer. The fault is shown as a single branch in the north and south Maling Mountains, and ruptured to the surface in many places. In the pull-apart basin in the middle of the fault, the thickness of the Quaternary system is more than 300m. When the Anqiu-Juxian Fault ruptures to the upper part, it divides into two branches, the east and the west, which are concealed and stand opposite to each other in the shape of “Y”, forming the Anqiu-Juxian Fault. On the east-west boundary of the fault, the latest activity is along the west branch of the fault, which is a Holocene active fault. When it extends to the basement rock mass of the Maling Mountains in the north and south, the depth of the upper fault point gradually becomes shallower until it is exposed.
The vertical movement of the Xinyi section of the AJF shows the four quadrants characteristics of uplift and subsidence. The extensional area forms a pull-apart basin, while the compressive area constitutes an uplift. The vertical bedrock offset of the Guanzhuang-Tangdian section, with the maximum vertical offset of~230m, gradually decreases to both sides. It can be concluded that the Xinyi section of the AJF presents a spiral-like pivot movement.

Key words: Xinyi segment of Anqiu-Juxian Fault, geometrical structure, active fault

摘要：

郯庐断裂带是中国东部活动性最强的断裂带, 第四纪以来主要沿安丘-莒县断裂活动。前人针对安丘-莒县断裂的活动时代、 古地震事件及几何结构进行了大量研究, 但多集中于断裂出露区, 对断裂隐伏区的研究相对较少。文中利用野外地质调查、 浅层地震勘探、 钻孔联合地质剖面及古地震探槽等深、 浅、 表立体式多层次研究方法, 重点开展安丘-莒县断裂新沂段的几何结构特征研究, 分析其平面几何展布特征和深、 浅、 表构造关系, 填补安丘-莒县断裂新沂段隐伏区活动特征的研究空白。结果表明, 安丘-莒县断裂新沂段自北向南可分为北马陵山-官庄段、 官庄-唐店段和唐店-新店段3段; 中段以双支的走滑兼正断活动为主, 向南、 北两端转变为单支的走滑兼逆冲; 基岩顶垂直位移在中部最大, 约为230m, 向两侧逐渐变小, 表现为螺旋状的枢纽运动。安丘-莒县断裂新沂段的深、 浅表呈现成因相同和几何结构协调的变形特征, 在深部表现为单支, 切穿白垩纪地层向上延伸, 在浅表层沿基岩山与第四系软土层接触面向上破裂, 在南马陵山和北马陵山处表现为单支, 多处破裂至地表。在断裂中部的拉分盆地内, 第四系厚度>300m。安丘-莒县断裂向上部破裂时分为东、 西2支, 呈隐伏状, 以“Y”字形相对而立, 构成了安丘-莒县断裂的东、 西边界, 最新活动沿断裂的西支发生。安丘-莒县断裂的西支为全新世活动断层, 向南马陵山、 北马陵山基岩山体延伸时上断点埋深逐渐变浅, 直至出露。

关键词: 安丘-莒县断裂新沂段, 几何结构, 活动断层