The Chuxiong-Nanhua fault lies in central Yunnan Province on the Sichuan-Yunnan rhombic block, parallel to the adjacent Honghe fault. The boundary of the Sichuan-Yunnan rhombic block is composed of the Xianshuihe, Anninghe, Zemuhe, Xiaojiang, Honghe and Jinshajiang faults (Fig. 1). Since the Eocene epoch, the collision of the Indian plate and Eurasian plate has led to the formation and intensive uplift of the Qinghai-Tibetan Plateau, and as the Indian plate moves north by east and the Qinghai-Tibetan Plateau continues to uplift, the Sichuan-Yunnan rhombic block escapes toward the southeast. It is the result of strain responses of tectonic plate edge deformation and differential movements caused by the collision of the Indian plate and Eurasian plate and northward movement of the Indian plate (Zhong Dalai et al., 1996, 1989; Molnar et al., 1975; Peltzer et al., 1988; Tapponnier et al., 1977, 1982). As the Sichuan-Yunnan rhombic block moves southeastward, the Xianshuihe fault, Zemuhe fault and Xiaojiang fault along the eastern boundary exhibit left-lateral strike-slip movement, the Honghe fault and Jinshajiang fault along the western boundary show evident dextral strike-slip movement, and many scholars have made extensive studies on this (Ren Jinwei, 1994; Song Fangmin et al., 1998; Guo Shunmin et al., 2001; Xiang Hongfa et al, 2004; Leloup et al., 1993; Tapponnier et al., 1990). These faults are mostly Holocene active faults. Due to intensive activities of boundary faults, strong earthquakes occur frequently along these faults, such as the 1854 Garzê MS7.0 earthquake, the 1923 Luhuo MS7.3 earthquake, the 1725 Kangding MS7.0 earthquake, the 1833 Songming MS8.0 earthquake and the 1925 Dali MS7.0 earthquake. There are also strong earthquakes inside the Sichuan-Yunnan rhombic block, such as the 1996 Lijiang MS7.0 earthquake, the 1976 Yanyuan MS6.7 earthquake and the 1970 Tonghai MS7.8 earthquake, and the occurrences of these earthquakes are closely related to the activities of faulting structures inside the block (Han Zhujun et al., 2004; Zhang Jianguo et al., 1997; Xiang Hongfa et al., 2002). All in all, the Sichuan-Yunnan rhombic block is an active block developing and evolving with the uplift of the Qinghai-Tibetan Plateau, with intense and frequent seismic activities along the boundary of the block and its interior.
The Honghe fault, as a western boundary fault of the Sichuan-Yunnan rhombic block, has long been the concern of many earth scientists. Based on research in the fields of sedimentary formation, magmatism, tectonic deformation and geophysics (Kan Rongju et al, 1986; Zhong Dalai et al., 1989), the Honghe fault is an inter-block tectonic deformation belt that has been undergoing long-term evolution (continent-continent collision, accretion of continental nuclei and extrusion-shearing function). Since the new tectonic period, it again experienced large-scale left-lateral shear movement in the early period (Paleogene) and dextral strike-slip movement in the later period (since the Neogene) (Xiang Hongfa et al., 2004). The 1680 Chuxiong MS6
The geotectonic position of the area where the Chuxiong-Nanhua fault lies belongs to the south-central section of the Xikang-Yunnan axis of the Yangtze paraplatform (Fig. 1). The basement is the lower proterozoic Kunyang group, which is a set of neritic facies flysch formation, and the Jinning movement caused regional metamorphism in the strata. After the Jinning movement, there has been regional uplift, which began to subside until the Triassic epoch, and the overlying strata composed of a Mesozoic group is more than 10, 000m thick. The lower part of the overlaying strata is the carbonate rock formation, the central part is the coal-bearing formation of sea-land inter-bedding facies and land facies, and the upper part is a thickly deposited red formation of land facies. This shows that this area belongs to a large Mesozoic depression basin.
The tectonics of overlaying strata shows a nearly SN orientation in the east, such as the Yuanmou, Lufeng and Tanglang-Yimen faults, and NW orientation in the west, such as the Chuxiong-Nanhua fault. Tectonism of overlaying strata are mainly caused by the Yanshan movement, for traces of Indosinian movement are not obvious. Sichuan movement, at the last phase of Yanshan movement, gave rise to folding and faulting of the whole overlaying strata along with alkaline magmatic activities, thus laying a prototype for tectonic framework in this area. Himalayan movement and the neo tectonic movement period are characterized by ascending movement, with no Paleogene developed, and only a scattered Neogene developed along fault zones and intermontane basins, which caused the Neogene folded and faulted.2 LATE QUATERNARY ACTIVITY OF THE CHUXIONG-NANHUA FAULT 2.1 Geomorphic Behavior of Fault Activity
The Chuxiong-Nanhua fault extends from Dazhuang on the right bank of Luzhijiang to the western margin of Nanhua basin, passing north-westward through Chuxiong, Lvhe and Luojiatun, striking 290°-330°, with a total length of about 100km. It is composed of 2-3 faults which are nearly parallel to each other, and strong deformation, or even the treverse, of Mesozoic and Cenozoic zones can be found in regions where it passes, and it is rather common to find he offset of Pliocene and Quaternary system. Several Quaternary pull-apart basins have developed along the fault, such as the Chuxiong, Nanhua and Ziwu basins. On geomorphologic aspects, along the fault it shows directional alignment of fault triangular facets, thin linear ridges and straight fault valleys (Fig. 2). Satellite imaging shows clear linear features along the fault. Clear fault landform and linear features show the obvious neotectonic activity of the fault.
The faults developed on the 3rd terrace of the Zidian river, a branch of Longchuan river in Lvhe town. The lower part of the terrace is formed by Pliocene semi-solid light gray clayey sand, the upper part is made up of upper Pleistocene maroon loose alluvial-proluvial gravelly clay, and the result of thermoluminescence is dated (25.0±2) ka B.P., indicating the deposits belonging to the the late Pleistocene epoch. It can be seen in the profile that two faults have developed (Fig. 3), with attitude of 335°/SW∠82°and 295°/SW∠78° respectively, and both faults offset the lower Pliocene semi-solid light gray clayey sand and the upper Pleistocene maroon loose gravelly clay strata, indicating obvious movements in the late Pleistocene.
A road construction trench reveals Quaternary fault outcrops on the 2nd terrace in the Nanhua basin in the north of Nanhua County (Fig. 4, Photo 1), with attitude of 90°/S∠70°. The fault offset upper Pleistocene yellow gravelly sand and brown clay strata, of which, gray black muddy clay turns out to be late Pleistocene by 14C dating, which is (14620±90) a B.P. The existence of faulting fully shows that this fault developed in the north of the Nanhua basin, and there are signs of movements in the late Pleistocene, and even Holocene.
The fault developed on the 2nd terrace of Longchuan river, next to the Chuxiong-Dali Highway in Xiasancun, 4km northwest of Chuxiong, with attitude of 300°/SW∠50°. The fault offset the Pliocene semi-solid meatly sand and upper Pleistocene alluvial gravelly clay, and judging from the geomorphologic position and soil characters, the loose alluvial gravelly clay belongs to deposits of the late Pleistocene. Tracing along the fault line, we found many steep fault belts in base rocks.
The above indicates that this fault was active at least in late Pleistocene.2.2 Kinematics Characteristics of Faults
Since the Quaternary, the Chuxiong-Nanhua fault and Honghe fault have displayed similar kinematic features, characterized by obvious right-lateral strike-slip and multiple dextral offsets of rivers along the fault, such as the 500m dextral offset of the primary tributary of the Longchuan river near Xiaotuanshan of Yinjiazui reservoir, 600m dextral offset of the primary tributary of the Longchuan river in the Sanjiatang area, 2km northwest of Chuxiong, and 400m dextral offset of the Zidian river, a branch of Longchuan river, in Lvhe town. From Guanyindong to Fengshanyi southeast of Nanhua basin, synchronous dextral offset of multiple streams were observed, which are 100m, 250m and 30m. From Majunying to Niufenglong in southeast of Nanhua basin, three branches of the Longchuan river are displaced synchronously 100m, 350m and 170m respectively (Fig. 6).
2km northwest of Shangdongshan village in Nanhua, where the fault passes, ridges show negative relief, and synchronous dextral offset of ridges and streams are also observed (Fig. 7). The stream terraces are offset about (10±1)m. 14C samples are collected on the terraces, and the dating results are (5500±30)a BP, and the dextral strike-slip rate of fault is estimated to be 1.6-2.0mm/a, according to the amount of displacement.
In the case of strike-slip faults, regional tensional zones are often generated in fault step-overs and curved segments of faults, forming pull-apart basins. In places where dextral strike-slip faults curve to the right, pull-apart basins are formed due to stress tension (Fig. 8(a)), in right-step areas produced between dextral strike-slip faults, pull-apart basins are also formed due to fault movements in opposite directions (Fig. 8(b)) (Zhu Zhicheng, 1999). Pull-apart basins are often formed in the curved segments at the end of faults because of similar mechanisms.
The Chuxiong Quaternary basin, located in the middle section of the fault, 16km long, 3km-4km wide, is a pod-shaped basin bending SW, with Quaternary deposits of sand and gravel of 120m thick approximately. Under the action of dextral strike-slip movement of fault, the Chuxiong pull-apart basin is formed in the pressure the tensional area of the right-curved part (Fig. 1). Similarly, Ziwu basin, on the west flank of Chuxiong, is a pull-apart basin formed in the right-curved part of a branch fault of the Chuxiong-Nanhua fault. It is also a pull-apart basin developed in the end of a branch fault, but its formation mechanism is consistent with that of Chuxiong basin. However, the Nanhua basin, in the western end of the fault, is located in the right-step zone between the faults on the north and south, which is a pull-apart basin formed in the step-over and its formation mechanism is as shown in Fig. 8(b).
From the analysis of the formation mechanism of the basin, all of these basins were formed by dextral strike-slip movements of the Chuxiong-Nanhua fault.3 FAULT ACTIVITY AND THE 1680 CHUXIONG MS6
In addition, there have been many moderate-strong earthquakes along the fault in history, such as the Xiaobaila MS5.6 earthquake in Chuxiong of Yunnan on January 12, 1975, the Chuxiong MS5.0 earthquake on May 22, 1511, the Chuxiong MS5.0 earthquake on August 24, 1615, the Taoyuan MS5.3 earthquake in Chuxiong on July 10, 2001 and the Nanhua MS5.0 earthquake on November 20, 1964. All these earthquakes are generated by the activity of the Chuxiong-Nanhua fault.4 CONCLUSION AND DISCUSSION
(1) Multiple faults in Luhe town displaced strata and lower river terraces of the later period of the late Pleistocene, suggesting that the Chuxiong-Nanhua fault is a late Quaternary active fault. The fault shows a clear landform. Multiple Quaternary pull-apart basins developed along the fault, such as the Chuxiong, Nanhua and Ziwu basin. The fault is characterized by dextral strike-slip movements, with horizontal strike-slip rate of 1.6-2.0mm/a. In history, the 1680 Chuxiong MS6
(2) The Chuxiong-Nanhua fault, adjacent to the Honghe fault, is an important seismic structure in the Sichuan-Yunnan rhombic block, and has the same dextral strike-slip features. From the perspective of a much wider area, it is composed of a group of oblique NW-striking active faults together with the Qujiang fault in the east and Shiping-Jianshui fault, and the motion characteristics are all mainly dextral strike slip. The motion characteristics, like the Honghe fault of the Sichuan-Yunnan Rhombic Block southwestern boundary, is concerned with the eastward escaping movement of the Sichuan-Yunnan rhombic block. The difference is that the Honghe fault, as a boundary fault of an active block, has larger slip rates. However, all these faults were strongly active in the late Quaternary, and were major seismotectonics in the block.
(3) The central Yunnan where the Chuxiong-Nanhua fault lies is large depression basin formed in Mesozoic, which has been in a state of uplift since the Sichuan movement, at the last phase of Yanshan movement, with no development of Cenozoic erathem, which shows that this region is a long-term neotectonic uplift. Several MS6.0-6.5 earthquakes have occurred in the Dayao and Yaoan areas in the north of the Chuxiong-Nanhua fault, and no large-scale active tectonics were found in the vicinity of the epicenters. Thus, the causes of these earthquakes may be concerned with this neotectonic uplift, or the basement folding.
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