## 4 Results  
### Characteristics of the Conaxue Co Landslide Dam  
Field investigations revealed that the Conaxue Co landslide dam formed by the high-speed and long-distance movement of the biotite granite slope rock mass from the Late Yanshan period (\(\gamma\beta_{5}\)3) on both sides under strong earthquakes, ultimately resulting in a collision in the Kangrilongba gully that obstructed the river (Figure 7A). Numerous boulders resulting from this movement are distributed across the dam surface (Figure 7B), with maximum boulder sizes reaching up to 6 m (Figure 7C). The dam blocked the third-order tributary of the Jinsha River, the Kangrilongba River, with an upstream catchment area of \(\sim\)69.86 km2 and an average annual runoff of \(\sim\)1.5 m3/s at the dam site. Dammed lake discharges primarily through the spillway via surface runoff. The Conaxue Co landslide dam features a saddle-shaped profile along the river channel, characterized by elevated sides and a lower central section. The crest topography exhibits notable variation: the left bank reaches an elevation of 4397 m, while the right bank stands at 4379 m, and the crest spillway is positioned at 4331 m (Figure 6). The dam extends approximately 1420 m along the river channel, with an upstream slope ratio of approximately 1:8 and a downstream slope ratio of 1:7. The Conaxue Co landslide dam covers an area of approximately 0.77 km2 and has a volume of about 6060 \(\times\) 104 m3, classifying it as an ultra-large landslide dam. The Conaxue Co landslide dam deposit exhibits a distinct inverse grading structure (Figure 8). Specifically, the dam surface is predominantly composed of large boulders, while the lower part mainly consists of gravel, breccia, and fine-grained particles. The dam structure is characterized by a distinct carapace facies and body facies formed by a high-speed and long-distance landslide. Notably, the basal facies at the lower part remain unexposed due to limited excavation depth of the highway cutting profile. The carapace layer of the dam is predominantly composed of chaotically stacked coarse-grained materials, such as large boulders, which exhibit low matrix content and a skeleton-supported structure among particles (Figure 8B). The body facies constitute the largest portion of the dam, characterized by a marked decrease in grain size. They are predominantly composed of fine-grained materials such as gravel, coarse sand, and fine sand, which exhibit a predominantly matrix-supported fabric among particles (Figure 8C). Additionally, jigsaw structures were observed within the coarse-grained layer on the dam surface (Figure 9). Figure 7: Characteristics of the Conaxue Co landslide dam. (**A**)—Overview of the landslide dam; (**B**)—dense distribution of the isolated boulders on the dam surface; (**C**)—isolated boulder with a particle size of 6 m; (1)—direction of debris deposit movement; (2)—flow direction; (3)—scarp; (4)—landslide dam; (5)—dammed lake. (**A**,**C**) were adapted from [PERSON] et al. [17]. The grain composition of Layer 1 (upper layer) and Layer 2 (lower layer) in the typical profile of the Conaxue Co landslide dam exhibits a distinct left-to-right shift in the gradation curves (Figure 10). This observation quantitatively demonstrates that the structure of landslide dam exhibits pronounced vertical sorting, characterized by an upward coarsening trend in grain size along the profile. The grain size composition of each layer in the profile is specifically characterized as follows: Layer 1 deposits are predominantly composed of boulders (d \(>\) 200 mm), which account for 80%, followed by gravel (2 mm \(<\) d \(<\) 60 mm) at 10%, while crushed rock and sandy soil collectively comprise approximately 10%. Layer 2 deposits exhibit a significant reduction in blocky fragments (d \(>\) 60 mm), which drop to 47%. In this layer, boulders (d \(>\) 200 mm) account for 25%, while particles exceeding 500 mm are notably absent. Gravel (2 mm \(<\) d \(<\) 60 mm) and sandy soil constitute 33% and 20% of this layer, respectively.