Abstract:Nixu landslide is located in the middle of the Yadong-Gulu Rift Valley in southern Tibet. The accumulation of Nixu landslide is the landform of the flat river valley. It is a typical “flat” rocky long-runout landslide developed in the back-land of the Qinghai–Tibet Plateau. The landslide accumulation area has a complete accumulation structure and typical debris flow patterns. The investigation and research on Nixu landslide will help to reveal the mechanism of movement and accumulation differentiation of this type of “flat” high-speed debris flow. A detailed ground survey of the Nixu landslide and its components in the accumulation area was carried out, combined with geophysical methods to reveal the failure characteristics of the sliding surface at the bottom of the source area as well as the mechanism of the formation of the landslide.The high resolution remote sensing interpretation and particle test were used to reveal the spatial distribution rules of various stacked structures. According to the research of the evolution of the debris flow accumulation structure and spatial morphology of Nixu landslide, it is proposed that the formation and evolution of “flat” high-speed debris flow can be divided into three sections: Structure fragmentation in the slope section of the source area, the material structure debrisification in the sliding zone, and the debris flow fluidization movement and accumulation in the leading edge. The structure of the corresponding sliding mass had the characteristics of step development of fragmentation-debrisification-fluidization. In the sliding zone and leading edge, the high-speed sliding mass movement was affected by the friction of the base, and a horizontal layered shear fracture surface could be generated inside, so that the debris flow continued to move and expand longitudinally under the “dilution effect”. The process of debris flow at the bottom transition from “dry” high-speed fluidization to solid and still was extremely rapid, and thus it maintained the accumulation morphology under the special fluid properties such as continuously distributed ridge and trunking. According to the research, it was proposed that the disintegration and destruction of Nixu landslide after earthquake were controlled by the slope of the source area, and they slided from front to back in sequence, controlled by their own potential energy conditions. The sliding body from the source area was stacked in the front edge accumulation area from left to right. It was the main reason for the deflection of the stacking plane to the downstream.