关键词: 安丘-莒县断裂新沂段, 几何结构, 活动断层

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