NEW ACTIVITY PHENOMENA REVEALED BY TRENCH ON THE NORTH SIDE OF NÜSHAN LAKE IN THE TANLU FAULT ZONE AND DISCUSSION

doi:10.3969/j.issn.0253-4967.2023.02.003

Abstract

Abstract:

The Tan-Lu fault zone is a huge seismic-tectonic belt in the eastern China. It can be generally divided into three segments: the north, the middle, and the south segment. Among them, recent activity of the middle segment has been most thoroughly studied. The junction section between Jiangsu and Anhui Province is located in the transition zone between the middle and the south segment of the fault zone. Due to the complex tectonic structure, unevenly distributed Quaternary deposits and severely transformed surface landscape, it is difficult to study the recent Quaternary activity of the fault. Research in recent years have shown that the faults in the Fushan and Ziyang areas to the south of the Huaihe River were active during late Pleistocene-early Holocene, and their activities were characterized by thrusting, normal faulting, tension and twisting. How is the fault activity extending southwards to Nüshan Lake and whether the late Quaternary activity occurred at Nüshan Lake are issues worthy of attention.

Geomorphology of the study area is characterized by slope plains and uplands. The uplands mostly extend in near north-south direction and are obviously controlled by the faults. In the remote sensing satellite images, linearity features of the fault from Huaihe River to Nüshan Lake are distinct. Field investigations confirmed that in the farmland to the east of Liugudui Village, north of Nüshan Lake, there are scarps extending in NNE direction and distributing intermittently due to faulting. In this study, we chose relatively clear scarps and excavated trenches across the fault. The trench revealed abundant faulting phenomena. The trench wall revealed a fault deformation zone as wide as 2~4 meters, consisting of 3 fault branches. Among them, faults f₁ and f₃ are the boundary faults while fault f₂ is developed within the deformation zone. The latest activity of fault f₃ on the west side has ruptured the overlying horizon of late Pleistocene strata, and the rupture extended upwards to the surface. OSL dating samples were collected in the uppermost layer of the faulted horizons. Dating results show that the fault has been active at least in late Pleistocene. The scratches and steps developed on the fault plane indicate that the fault has experienced thrusting and dextral faulting. The deformation zone appears dark brown, which is conspicuously different from the horizons on both sides. Materials in the fault zone are compacted, crumpled and deformed, and the alignment direction is consistent with the fault. The deformation zone contains gravels and calcium tuberculosis of different sizes. Two brownish-yellow clay masses in irregular shape are deposited near the upper part of the fault plane. Among them, the clay mass tk1 on the south wall of the trench is quite clear, with the upper part connected with f₁ and the middle part obliquely cut by f₂. OSL dating samples were collected from clay masses from two trench walls. The dating results are consistent with the late Pleistocene horizons, indicating that the brownish-yellow clay masses were involved in the fault zone when faulting occurred in the middle-late Pleistocene, and the faulting event occurred roughly between(50.92±4.65)kaBP and(27.12±2.26)kaBP. Our research shows that late Quaternary activity of the most active fault of the eastern branch of the Tanlu fault zone extended southwards to Nüshan Lake in Mingguang, but intensity of the fault activity has weakened.

The segment from Sihong in Jiangsu Province to Mingguang in Anhui Province is the structural node between the middle segment and the southern segment of the Tanlu fault zone. Trench exposures in Wangqian, Sunpaifang, Dahongshan in Sihong and Santang, Ziyang, Zhuliu in Mingguang and other places revealed a variety of faulting phenomena such as wedges, wedge-shaped mass, normal faulting, negative flower-shaped structure, clay mass, etc. These show that faults that were dominantly thrusting led to the local and abundant phenomena near surface in this region. The reasons for these different phenomena may be related to the influence of regional complex stresses and their changes on large-scale fault systems at different time and spaces scales.

Key words: Tanlu fault zone, north of Nüshan Lake, clay mass, later period of late Pleistocene, trench

摘要：

郯庐断裂带苏皖交界段处于断裂中段与南段的过渡区, 断层新活动较为复杂。近几年的研究成果显示, 在淮河南侧浮山、紫阳一带断层晚第四纪仍有较强活动, 该断层向S至女山湖的活动情况如何、晚第四纪活动是否延伸至女山湖, 是值得关注的问题。在明光女山湖北侧六谷堆村东开挖的地质探槽揭示了宽2~4m的断层变形带, 带内沿断层卷入多个棕黄色黏土团; 断层的新活动错动了上覆晚更新世黏土层, 断层面延伸至地表呈“通天”状, 地层年代数据表明其最新活动时代至少为晚更新世晚期; 断层面发育的擦痕及阶步显示断层经历过逆右旋活动。以上成果表明, 郯庐断裂带东支最为活动的断层晚第四纪的活动向S延伸至明光女山湖。泗洪-明光是郯庐断裂带中-南段的构造“节点”, 多个断层剖面存在楔状土、正断、黏土团等现象, 显示以逆走滑为主要运动方式的断层在地表局部存在丰富的伴生现象, 而产生这些现象的原因可能与断层在不同时、空所受的区域复杂应力及其变化有关。

关键词: 郯庐断裂带, 女山湖北侧, 黏土团, 晚更新世晚期, 探槽

CLC Number:

P315.2

ZHAO Peng, LI Jun-hui, TAO Yue-chao, SHU Peng, FANG Zhen. NEW ACTIVITY PHENOMENA REVEALED BY TRENCH ON THE NORTH SIDE OF NÜSHAN LAKE IN THE TANLU FAULT ZONE AND DISCUSSION[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(2): 338-354.

赵朋, 李军辉, 陶月潮, 疏鹏, 方震. 郯庐断裂带女山湖北侧探槽的新活动现象及讨论[J]. 地震地质, 2023, 45(2): 338-354.

Figures/Tables 13

Fig. 1 Location of the study area.

Fig. 2 Satellite image of Doushan-Liugudui section(Google Earth).

Fig. 3 Slope scarp at the Liugudui trench site.

Fig. 4 Photos of Liugudui trench.

Fig. 5 Photos and profile of the south wall of Liugudui trench.

Fig. 6 Photos and profile of the north wall of Liugudui trench.

Table1 OSL sample dating results of Liugudui trench

编号	采样位置	测量方法	环境剂量率/Gy·ka^-1	年龄/ka
LGD1-1	南壁f₂黏土团	SMAR	3.74±0.24	52.97±8.75
LGD1-3	南壁U3层下部	SMAR	3.51±0.22	22.19±2.4
LGD1-6	南壁U2层上部	SMAR	3.63±0.24	50.92±4.65
LGD1-2-3	南壁U2层上部	SMAR	3.52±0.23	51.87±5.3
LGD1-8	北壁U2层上部	SMAR	3.56±0.24	53.3±6.41
LGD1-9	北壁f₁黏土团	SMAR	4.25±0.28	52.09±6.98
LGD1-10	北壁U3层下部	SMAR	4.08±0.27	27.12±2.26

Fig. 7 Clay mass in fault deformation zone of the southern wall.

Fig. 8 Fault F3 on the north and south walls of the trench(local exhumation in fault).

Fig. 9 Photos of f3 fault staggered to the surface.

Fig. 10 Photos of fault slickensides.

Fig. 11 Partial view of f3 fault plane profile.

Fig. 12 Profiles of fault activity phenomena in Sihong-Mingguang section of Tanlu fault zone.

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