Technical note36Cl exposure dating of the Mahu Giant landslide (Sichuan Province, China)
Introduction
Landslides with volumes of more than 108 m3 are known for their large scale and serious disastrous effects. They have a complex dynamic mechanism and play an important role in surface erosion and landform evolution in mountainous areas (Shroder et al., 2011). Therefore, these giant landslides have attracted extensive attention worldwide, e.g., the 1786 M7.75 earthquake-triggered a landslide dam on the Dadu River in the Kangding-Luding area in Sichuan, southwestern China (Dai et al., 2005); and the Flims landslide (Ivy-Ochs et al., 2009), the Tortum landslide (Duman, 2009), the Green Lake landslide (Hancox and Perrin, 2009), and a mega-landslide in the northern Caucasus foredeep (Pánek et al., 2012) have also been extensively studied. The study of the formation mechanism and timing of giant ancient landslides enables the back-analysis of paleoearthquakes (Ivy-Ochs et al., 2017; Pinto et al., 2008; Shoaei, 2014; Yuan et al., 2013), paleoclimate changes (Břežný et al., 2018), and glacial activity (Ballantyne et al., 2014). The causes of huge landslides can be used to study ancient earthquakes because large earthquakes in mountainous areas often produce tens of thousands of landslides, including some giant landslides, the deposits of which can exist for thousands or even hundreds of thousands of years (Dai et al., 2011a; Keefer et al., 2006; Ma et al., 2001; Xu et al., 2014). Therefore, old landslides can provide a reference for the analysis of regional seismic risk and regional tectonic activity (Jibson, 1996).
In recent years, the Mahu landslide in Sichuan Province, southwestern China, which covers tens of square kilometers, has received extensive attention because of its large scale and the urgent need for the development of regional hydropower resources and seismic risk assessment. In the early 1960s, researchers from the Chinese Academy of Sciences (CAS) proposed that at one time, an ancient river (the ancient Huanglang River) existed in this area and flowed into the Jinsha River from the southwest to the northeast until the collapse of a bank blocked the river and formed the Mahu barrier lake (Zhang, 1991). Wang and Lu (2000) conducted geological mapping of the Mahu landslide and found that the front margin of the landslide deposits had a height of approximately 900 m a.s.l. Based on the geological mapping, the landslide deposits did not reach the Jinsha River, with a height of 320 m a.s.l. in front of it. The spatial scope of the landslide was defined more accurately determined based on a topographical analysis and borehole explorations of the landslide deposits (Cui et al., 2015). It was concluded that the front edge of the deposits extends to the Jinsha River and the thickness of the deposits is larger than was indicated by previous studies. The landslide volume was calculated using an extrapolation method, and it was concluded to be the largest subaerial landslide found in China at that time (Cui et al., 2018).
Based solely on the dating of a 372 ka calcite vein using the electron-spin-resonance (ESR) method, it was determined that the landslide was formed at 500–100 ka (Wang and Lu, 2000). Furthermore, the landslide was speculated to have been formed by five events based on the stacking of the lithology of the head scarp and accumulation body (Cui et al., 2018). These studies of the characteristics, scale, kinematics, and chronology of the landslide have played an active role in understanding its mechanism and triggering factors. Nevertheless, due to the limitations of the previous research methods, a great deal of controversy exists regarding the age of the landslide.
Samples of the cosmogenic nuclides used for dating (10Be, 26Al, 36Cl, etc.) are easy to obtain because they can be taken from the rock on the ground surface, and the time range they can be used to date is very large. The carrier for the cosmogenic nuclides 10Be and 26Al is sandstone, while the carrier for the 36Cl cosmogenic nuclide is limestone. For landslide dating, 10Be and 26Al exposure dating is widely used all over the world (Hormes et al., 2008; Yuan et al., 2013; Zeng et al., 2019). In recent years, 36Cl exposure dating has become more widely used for landslide dating, especially for landslides in some of the limestone areas of the Alps (Ivy-Ochs et al., 2017; Singeisen et al., 2020). However, because there is no laboratory that can conduct age analyses using the 36Cl cosmogenic nuclide in China, although there are a large number of huge limestone landslides with important scientific research value on the eastern margin of the Tibetan Plateau in western China, few such landslide studies have been reported. The 36Cl exposure dating method was used to obtain the ages of the Mahu landslide; the process and the results are presented in this paper.
Section snippets
Geologic setting
The Mahu landslide is located in the lower reaches and on the left bank of the Jinsha River (103°48′5″E, 28°26′54″N), Mahu Township, Leibo County, Sichuan Province, China (Fig. 1). This region is in the slope zone of the Western Sichuan and Northern Yunnan Plateaus to the Sichuan Basin. The slide area is characterized by middle-high mountains with 500–3000 m high peaks. The Jinsha River flows through the landslide area from south to north and has formed steep terrain. Because of the strong
Landslide investigation
First, we collected Google Earth images, 1:50,000 topographic maps, 1:200,000 geological maps, and data and information from previous studies. Based on these basic data, the characteristics of the Mahu landslide, including its lithology and topographic features, and the geological and landform conditions of the surrounding areas were analyzed. Then, we conducted a detailed field investigation and determined the range, lithology, and topography of the landslide. Two typical cross-sections were
Characteristics of the Mahu landslide
Building on the findings from previous studies (Cui et al., 2018), based on this new landslide investigation, some new characteristics of the Mahu landslide were obtained. The Mahu landslide is very large and the overall panoramic photograph makes a spectacular impression (Fig. 3). The landslide is located on the corner of the ancient Huanglang River's junction with the Jinsha River. After the landslide, the landslide deposits blocked the ancient Huanglang River, and formed the Mahu barrier
The age of the Mahu landslide
According to the inferred cross-section of the landslide (Fig. 6c), the depth of the three samples on the scarp of the landslide was >200 m, i.e., very large, and these three samples on the scarp should not contain inheritance from before the landslide. However, the boulders used for the other six samples may have been located on the ground surface or at a shallower depth below the ground surface before the landslide, and therefore, these samples may contain inheritance. According to the test
Conclusions
Through the landslide investigation conducted in this study, the landslide deposits were divided into two zones: zone Z1 composed of limestone, and zone Z2 composed of basalt. The head scarp was divided into two zones: zone S1 composed of limestone strata, and zone S2 composed of limestone strata and basalt blocks. Based on the lithologic relationships, zone S1 is the source of the material in zone Z1, and zone S2 is the source of the material in zone Z2. Based on the geologic mapping of the
Author statement
Under supervision by Jianhui Deng, Chong Xu performed the methodology, Wanyu Hu, Hua Ge and Jinbing Wei performed the investigation. Jun Zheng performed the data curation, Yulong Cui performed the writing - review & editing. All authors read and contributed to the manuscript.
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “36Cl exposure dating of the Mahu giant landslide (Sichuan Province, China)”.
Acknowledgments
This research was supported by the National Key Research and Development Program of China (2018YFC1505004), the National Natural Science Foundation of China (41807267), the Natural Science Research Project of the Colleges and Universities in Anhui Province (KJ2020ZD34). Sample preparation and AMS measurements were conducted at the Laboratory for Ion Beam Physics, ETH Zurich. We also thank Wenxi Fu, Mengke Yuan, Hongchun Zheng, and Yi Qin of Sichuan University for their field investigation and
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