Abstract:

Chishan is located in the northeastern Sixian county, Anhui Province. The western branch fault of Tangcheng-Lujiang Fault zone passes through this area. According to the previous researches, the Chishan segment of Tancheng-Lujiang Fault zone is obviously active since Quaternary. Trenches excavated perpendicular to the Chishan segment of Thancheng-Lujiang Fault zone for this study have revealed many prehistoric earthquake remains-the multi-phase reverse faulting colluvial wedges, represented as the western brick-red sandstone of the late Cretaceous or maize gravel stratum of the mid Pleistocene of the hanging wall of the fault overlapping eastward the mid-late Pleistocene brown clay. In the base of the wedges, steep NW-dipping faults were found, and upward, the steep fault planes turned to gently dipping collapse planes. As revealed by the trenches, the connection line of the breaking points strikes NNE in general, heaving landforms are preserved at most part of the tailing edge of the hanging wall in the places the fault passes through, and some EW-trending gullies were offset by right-lateral faulting. Evidences of collapse under west-to-east extrusion are revealed unanimously in several trenches. Among them, Tc1-Tc4 show that the brick-red limestone (K₂) overthrust and collapsed on the yellow-brown clay containing ferro-manganese nodules (Q_p^2-3); Tc5 reveals that the yellowish-white gravel (Q_p²) and the limestone (K₂) overthrust and collapsed on the aforementioned clay. Reverse faulting colluvial wedges are found on both walls of each of the 8 trenches, but the number of wedges revealed in different trenches is different: there is 1 wedge in some trenches, and 2 wedges in Tc1 and Tc3, 3 wedges in Tc2, Tc4 and Tc5, and in individual trenches, few wedges are revealed, this may be related to the trench's location, depth and height of the remaining denudation. From the analysis of the trenches and the thermoluminescence dating results, we can conclude preliminarily that multiple large-scale reverse faulting stick-slip events took place on the Chishan segment of the Tancheng-Lujiang Fault zone during mid-late Pleistocene, that is to say, many pre-historic strong earthquakes occurred.

Key words: Tanlu Fault zone, Chishan, mid-late Pleistocene, pre-historic earthquake

摘要：

赤山地处安徽省泗县东北部,为郯庐断裂带西支断裂通过处。以往研究认为,郯庐断裂带赤山段第四纪以来有较为明显的活动。此次在垂直于赤山段开挖的多个探槽中均揭露出史前地震遗迹(多期逆断型崩积楔),具体表现为断层上盘砖红色上白垩统砂岩或黄白色中更新统砾石层向东叠置于褐黄色中、上更新统黏土之上。在楔状体根部均发现向NW陡倾的断裂,陡立的断面向上转变为缓倾的上翻垮落面。多条探槽揭示的断裂点的连线总体呈NNE走向,断裂通过处的上盘后缘大部分残留了凸起地貌,部分EW向冲沟被断裂右旋扭错。多条探槽两壁都一致显示出从西向东的推挤垮塌痕迹。其中Tc1～Tc4表现为砖红色砂岩(K₂)逆冲垮落于黄褐色含铁锰结核黏土(Q_p^2-3)之上,Tc5则表现为红砂岩(K₂)和黄白色砾石层(Q_p²)逆冲垮塌于上述的黏土之上。在几个探槽的逆断型崩积楔上均揭示出此段曾有多次逆断型崩积事件发生,个别探槽揭露楔体较少,可能与其所处的位置、揭露深度和剥蚀残存的高度有关。对开挖探槽及光释光测年结果的详细分析初步得出,郯庐断裂带赤山段在中、晚更新世之交曾多次发生大规模逆断型黏滑事件,即曾多次发生史前大地震。

关键词: 郯庐断裂带, 赤山, 中晚更新世, 史前地震