Earthquake Reaearch in China  2017, Vol. 31 Issue (2): 225-233
New Activity of the Tanlu Fault Zone in the South of Huaihe to the Nvshanhu Segment since the Late Quaternary
Yao Daquan, Zheng Haigang, Zhao Peng, Yang Yuanyuan, Wang Xingzhou, Miao Peng, Fang Zhen, Tao Yuechao, Li Junhui, Wang Jun, Wang Xiaoli     
Earthquake Administration of Anhui Province, Hefei 230031, China
Abstract: Taking the Huaihe to the Nvshanhu segment of the Tanlu (Tancheng-Lujiang) fault zone as the main research target to explore whether there has been new activity since the late Quaternary, and based on the interpretation of remote sensing images and repeated surface investigations, we excavated trenches at the sections where the tectonic landform is significant, identified and recorded the deformation patterns of the fault and analyzed the activity behavior. Samples of new activity and deformation were collected and oriented slices were ground based on the samples original state to make the micro structural analysis and demonstration. All of the above research shows very clear linear tectonic geomorphology along the fault, three trenches across the fault zone all revealed new deformation traces since late Quaternary. The latest stratum dislocated by the fault is the late Quaternary and Holocene. The main slip mode is stick slip, as represented typically by fault scarps, wedge accumulation, the faults and the filled cracks and so on. In general, it shows the characteristics of brittle high-speed deformation and belongs to the prehistoric earthquake ruins. The above understanding was confirmed partially by microscopic analysis. In addition, the similarities and differences and the possible reasons for the characteristics of the latest activities of the Tancheng-Lujiang fault zone in the north and south of the Huaihe River regions are also discussed in this paper.
Key words: Tanlu fault zone     South of Huaihe-Nvshanhu     Late Quaternary     New activity characteristics    

INTRODUCTION

It has been commonly recognized that there has been new activity on the north of the Huaihe segment of the Tanlu (Tancheng-Lujiang) fault zone (Fang Zongjing, et al., 1987; Gao Weiming, et al., 1988; Chao Hongtai, et al., 1994; Wu Zhe, 2001; Jiang Wenliang, et al., 2011; Yao Daquan et al., 2012, 2014; Research Group of Geological Mapping of the Tanlu Fault Zone, 2013; Shen Xiaoqi et al., 2015; Liu Baojin et al., 2015; Liu Bei et al., 2015; Xu Hanggang, et al., 2016). However, the activities of the south of Huaihe segment are still rarely reported, due to the poor exposure of faults, the low level of research and other reasons. Most of the published papers regard the southern Huaihe segment of the Tanlu fault zone as a segment without obvious new activity. Some researchers believe that the Quaternary activity in the Tanlu fault zone is characterized by segmentation, as a general trend, activity increased in the segments from Jiangsu northward to Shandong, and decreased in the southern segments from Jiangsu to Anhui (Xie Ruizheng, et al., 1991). Since the late Pleistocene, fault activity has been strong in the northern Huaihe area, especially in the Yishu fault zone, showing as dextral strike-slip thrust (Cheng Jie., et al., 1996). The Sihong segment in the north of Huaihe has been obviously active since the late Pleistocene. In the Anhui segment in the south of Huaihe, no dislocated Quaternary profiles have ever been found, and the fault crush zones are cemented hard and covered universally by the late Pleistocene alluvial grave. The latest active time of the Anhui segment in the south of Huaihe is inferred to be in the early Quaternary (Zhang Peng et al., 2015).

Since the second half of 2014, from Fushan in the south of Huaihe to Liugudui in the north of Nvshanhu, nearly 20km long along the Tanlu fault zone, we carried out a detailed analysis of remote sensing data and field surveying, and on the basis of the above work, selected the areas with significant tectonic geomorphology to excavate trenches, then carried out logging, sampling and macroscopic and microscopic analysis for each trench. The preliminary conclusion drawn is that there has been activity on this segment of the Tanlu fault zone since the late Quaternary. The key points are summarized as follows:

1 LANDFORM FEATURES

In the satellite images, from Fushan in the south of Huaihe to Liugudui in the north of Nvshanhu, linear low monadnock mountains trending NNE are distributed discontinuously along the Tanlu fault zone. Pre-Quaternary strata is exposed in the mountains, and to the two sides are vast lowlands covered by the Quaternary (Fig. 1).

Fig. 1 Topographical position of trenches and geological structures of the study area (a) Regional geological structure; (b)-(f) Geomorphic lineament around each trench
2 COMPREHENSIVE ANALYSIS OF THE TRENCHES

Through repeated comparative field surveys, we designed and excavated 5 trenches at the places where linear fault topography is significant, there are suspected fault scarps and Quaternary overburdens, and the sites are good for construction. From the north to the south respectively, they are Fushan, Santang, Zhuchang, Zhuliu and Liugudui trenches (Fig. 1). In this paper, we make a brief report on the signs of new activity revealed by Santang, Zhuliu and Liugudui trenches.

2.1 The Santang Trench Profile

The Santang trench is excavated perpendicular to the fault scarp on the east side of the scarp slope. The scarp is about 1m high and N-S trending (Fig. 1). The trench is about 15m long, 4m deep and 3m wide, nearly E-W trending. From top to bottom respectively, strata revealed by the north wall of the trench is as follows: Layer ① is dark brown cultivated soil, its thickness is about 0.2m. Layer ② is light gray, light yellow loam, 0.1m-0.3m thick, soft, belonging to late Holocene deposit (according to the dating data). Layer ③ is gray-yellow, gray-brown clay, 0.3-1.0m thick, containing iron and manganese and calcareous nodules, partially green-gray and soft, being the upper Pleistocene according to the regional stratigraphic horizon comparison. Layer ④ is hard brown and brown-red clay, in which white or light colored worm-like bands occurred, being the early and middle Pleistocene deposits. Layer ⑤ is red-brown gravel layer, the gravel is obviously angular, relatively even in particle size from a few mm to 10mm, and consolidated, belonging to the upper Cretaceous red clastic rocks (Fig. 2).

Fig. 2 Log of the northern wall of Santang trench (Labels are before and after comparisons, as below) ① Dark brown soil. ② Light gray, light yellow loam (Holocene). ③ Yellow, gray and brown clay (late Pleistocene). ④ Brown, Brown and red clay (early middle Pleistocene). ⑤ Brown red gravel (late Cretaceous). Stx-1 denotes the location and number of sampling; 1050-1025a means the age determined

Three faults can be clearly identified on the profile: The fault in the east dips steeply to the west, cutting layer ③ and layer ④, that in the west dips to the east, with a moderately to steeply dipping angle, cutting layer ⑤ and ②, the middle fault is east dipping, with a moderate to steep dip angle, forming a wedge-like accumulation together with the east fault, the upper part of the wedge is of gray clay layer. The profile displays that at least, 2 high-speed deformation events have occurred since the late Quaternary, the first phase deformation happened before the accumulation of layer ②, and the second phase deformation occurred after the accumulation of layer ② (Fig. 2). The wedge-like accumulation formed during the deformations is prehistoric earthquake ruins (Hudnut et al, 1989).

14C sample dating results (see Fig. 2 for sample position) are ST-C-1: 1050-1025a B.P. and ST-C-2: 3210-3025a B.P., respectively, both in the late Holocene, this shows that the fault is still active in this period.

The original state samples were collected in the profile, and under the premise of maintaining the original state, the samples were fixed, packed and shipped, and after the natural slow dry of the samples, injected a curing agent into the samples under vacuum conditions, and then placed them under constant temperature conditions for uniform curing, ground oriented slices and observing under the microscope. The results show that the micro deformation traces are rich and varied. On the west fault, microscopic slice observation of sample No.2 (Fig. 3, Stb-2) reveals high-speed deformation phenomena such as local strong fragmentation, jagged rifting and gravel-cutting faults. Multi-period deformation traces as well as slow movement to adjust the traces are observed, which enriches the content of macro research to a certain extent.

Fig. 3 Micro deformation characteristics of sample No.2 (Stb-2) collected from the north wall of Santang trench (a) Strong local fragmentation, then cut later, *40, (+). (b) Micro fracture, the crushed gravels are aligned slightly along the rupture surface, *40, (+). (c) λ-type structure aside of micro-fracture surface, the crushed gravels are directionally or randomly aligned, *40, (+). (d) Jagged tensional crack, *40, (+). (e) Abrasive belt and multi-period gravel cutting deformation, *40, (+). (f) Gravel cutting, gravel grinding, local deformation and directional fabric coexist, *40, (+). *40 denotes 40 times magnification; (+) denotes orthogonal polarization
2.2 The Zhuliu Trench Profile

The Zhuliu trench is perpendicular to the fault scarp, it is about 0.3m-high, nearly north-south trending fault scarp (Fig. 1). The trench is about 20m long, 3m wide, and about 4.5m deep, nearly W-E trending. The strata revealed by the trench from top to bottom are: Layer ① is light-yellow white sand-fine sand, 0.2m-0.4m thick, considering it is close to the south side of Huaihe River, it is inferred to be the flood deposits of the Huaihe River: Layer ② is brownish red loam, with a pure quality and stable extension. The layer is about 0.5m thick, soft, belonging to the Holocene: Layer ③ is black clay layer, containing light sandy strips, which are penetrated from the upper layers along the cracks, the layer is 0.2m-0.3m thick, soft, thinning out westwards, according to dating data, it belongs to the late Pleistocene to early Holocene sediments: Layer ④ is dark grey clay, containing iron manganese nodules, relatively hard and about 1m thick, according to the dating data, it belongs to the late Pleistocene: Layer ⑤ is pale-yellow clay, containing calcareous nodules and iron manganese nodules, hard, 0.5m thick, belonging to late Pleistocene sediments: Layer ⑥ is brown or red-brown clay containing gravels, 0.5m thick, according to stratigraphic analogy, it belongs to early-middle Pleistocene sediments: Layer ⑦ is brownish-red sandstone, seriously weathered: Layer ⑧ is brown-red mudstone, also seriously weathered (Fig. 4).

Fig. 4 Log of the north wall of Zhuliu trench Zl-c-1 means the sampling site and number; 13460-13280 a B.P. means the age determined

This profile reveals three faults, which reflect two phases of activity, the first phase is of compression, as represented by imbricate thrusting of F1, F2, F3 which faulted the layers ④ and ⑤ and the layer below ④, the time of activity is after the deposition of layer ④ and before layer ③; the second phase of activity is of extension, as manifested by F3 dislocating the layer ③ and its underneath, which should be the new activity on the old fault plane, resulting in subsidence of the black soil of layer ③; layer ⑤ and ⑥ underwent small amplitude displacement, the activity age is after the deposition of layer ③ and before the formation of layer ②; 14C dating of the samples from layer ③ yields ages of 13, 460-13, 280a B.P. and 10, 920-10, 890a B.P., respectively, therefore, the activity of this fault was in the late Pleistocene to early Holocene (Fig. 3), and the latest activity showed subsidence deformation, which are traces inherited from high-speed deformation.

2.3 The Liugudui Trench Profile

The Liugudui trench is also vertical to fault scarp, which is about 0.3m-0.8m high, nearly north-south trending (Fig. 1). The trench is about 40m long in total (the profile length is about 10m), 3.5m wide and about 2.5m deep, trending near east-west. The strata revealed by the trench from top to bottom (Fig. 5) are: clay and bedrocks, clay marked as layer ①, which is yellow clay, dense, hard, rich in iron manganese nodules. There are gray calcareous concretions found at the bottom of this layer, the thickness of which is about 0.3m-0.8m, presumably belonging to the upper Pleistocene: The bedrocks are marked as layer ②, ③, ④ and ⑤, according to the reports of regional geological surveys, and it is inferred that they are all upper Cretaceous. From left to right, layer ② is a highly weathered layer of brown red sandstone, with a reddish brown weathered residual mass: layer ③ is the argillaceous hard weathered crust of purple red sandstone: layer ④ is a brownish red sandstone layer, in which cracks are developed: layer ⑤ is a brownish red sandstone layer, fairly fractured, its middle and upper part is intercalated with gray-white calcareous nodules and a large number of the yellow clay stripes from layer ①.

Fig. 5 Log of the northern wall of Liugudui trench Lg-OSL-1: Sampling location and number

This profile reveals two fault groups in Mesozoic clastic rocks, both trending NNE. One group of faults is west dipping, nearly vertical, and the other is east-dipping with a slightly gentle dip angle. Both fault groups have extended into the overlying yellow clay, which is shown as two steps with different heights on the two walls of the fault, high in the east and low in the west, with the fault at its turning point, which coincides exactly with the surface scarp (Fig. 1 and Fig. 5). This indicates that the faults were active after the deposition of yellow clay layer, the eastern part uplifted, and the western part subsided, displaying a thrust deformation. A series of parallel longitudinal fissure zones in the soil may be explained by extension occurring on the tailing edge during the high-speed deformation process. The date of the latest activity of fault can be determined by dating the yellow clay, which should be late Pleistocene sediments according to the preliminary contrast of the Quaternary data.

3 CONCLUSION AND DISCUSSION

Through the brief analysis of the above 3 trench profiles, the following conclusions are obtained:

(1) The Tanlu fault zone shows very clear linear tectonic landform in the segment between the south of Huaihe to the north shore of Nvshanhu.

(2) The latest faulting dislocated the upper Pleistocene-Holocene strata.

(3) The latest activity of the fault shows both extension and compression as revealed in the profiles, which varies along the strike of fault, indicating the characteristics of strike-slip deformation.

(4) The latest movement of the faults is dominated by stick-slip, as represented typically by linear distribution of surface scarps, wedge-like accumulations, seismic faults and earthquake cracks. Correspondingly, they show up microscopically as local strong fragmentation, jagged rifting and gravel-cutting faults.

The features of fault activities are compared with those in the northern section of Huaihe, and it is found that there are similarities and differences between them. The common points are that they all have typical fault land forms and the latest active times all reached the late Quaternary. However, there are differences in the state of activities as revealed in the trench profiles. To the north of Huaihe, the trench profiles show the feature of compression, such as in the Chishan, Zhangshan and Chonggangshan sections (Yao Daquan et al., 2013, 2014; Shen Xiaoqi et al., 2015; Yang Yuanyuan et al., 2016), while to the south of Huaihe, trench profiles reveal the characteristic of extension. The reason may be that the latest activity of fault is mainly of strike slip, and the two walls of most faults are connected with different strata. Considering that the modern stress field is dominated by NEE compression in this region, the state of latest fault activity still should be of reverse strike-slip as a whole, but the local or surface manifestation patterns are different. In addition, this situation is maybe similar to the four quadrant distribution of strike-slip deformation on both walls of fault as proposed by some scholars (Gao Xiang, 2014). Of course, this remains to be further confirmed.

This paper reports, in the form of research briefing, the latest movement traces recently discovered and preliminary understanding of activity on the segment between the south of Huaihe to the north shore of Nvshanhu on the Tanlu fault zone. At present, microscopic analysis and dating analysis are ongoing, and it is expected that more in-depth systematic analysis and research results will be successively published.

ACKNOWLEDGEMENTS: The authors are grateful to research professors Ran Yongkang, Chai Chizhang, research professors Min Wei, and He Honglin for providing on-site guidance and help.

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郯庐断裂带淮河南到女山湖段晚第四纪以来的新活动
姚大全, 郑海刚, 赵朋, 杨源源, 王行舟, 缪鹏, 方震, 陶月朝, 李军辉, 王俊, 汪小励     
安徽省地震局, 合肥市长江西路558号 230031
摘要:以郯庐断裂带淮河南到女山湖段晚第四纪以来是否具有新活动为主要研究目标,通过遥感影像资料解译及地表反复调查,选择构造地貌显著地段开挖探槽,识别和记录其变形形态,分析其活动习性,定向采集新活动变形物质,在保持原态基础上磨制定向薄片,并进行微观构造分析论证。研究表明,断裂沿线线状构造地貌清晰,横跨断裂带的3个探槽均揭示出晚第四纪以来活动痕迹,断裂最新错断了晚更新世-全新世地层; 滑移方式多表现为粘滑,典型表现为断层陡坎、楔状堆积、断层和充填裂缝等,总体显示为脆性高速变形特征,属于史前地震遗迹。上述认识部分得到微观分析证实。文中还初步探讨了淮河南北郯庐断裂最新活动特征的异同点及其可能的原因。
关键词郯庐断裂带    淮河南-女山湖    晚第四纪    新活动特征