西藏加查拉岗村巨型古滑坡发育特征与形成机理研究

青藏高原地质构造活跃,内外动力作用强烈,加之气候异常变化,区内大型滑坡发育。以雅鲁藏布江断裂附近新发现的拉岗村古滑坡为研究对象,在现场调查、槽探揭露、地质测年和工程地质分析等基础上,对其发育特征及成因机制进行了分析研究。研究表明,(1)拉岗村滑坡属巨型岩质滑坡,体积达3.6×107 m3,最大水平滑动距离约3050m,滑坡后壁与堆积体前缘高差达965m,最大运动速率达78.1m/s,具明显高速远程特征;(2)受冷冻风化和冰体"楔劈"作用影响,滑坡后部岩体崩裂,全新世以来气候变化冰川逐渐消退,融雪降水入渗加剧劣化岩体结构,降低岩体强度;(3)根据14 C和10Be测年结果,拉岗村古滑坡形成于距今4140~9675a,沿雅鲁藏布江断裂发生的强震可能是该滑坡的直接诱因,岩体受到地震抛掷力作用,原有节理裂隙和新生破裂面发生张剪-拉裂破坏迅速贯通,首先沿断裂附近碎裂结构岩体发生破坏,上部岩体随之失稳并高速下滑。该研究可为认识青藏高原断裂带内大型古滑坡的形成机理提供借鉴。

Characteristics and Formation Mechanisms of the Lagangcun Giant Ancient Landslide in Jiacha, Tibet

Active tectonic activity and intense internal and external dynamic effects, together with abnormal climate change, result in occurrence of many large landslides in the Tibetan plateau. With the newly discovered Lagangcun landslide near the Yarlung Zangbo fault as an example, this study analyzed the characteristics and formation mechanism using field investigation, geological dating, exploratory trench and engineering geological analysis. The results are listed as follow. (1) The Lagangcun landslide is a giant rocky landslide with a total volume of 3.6×107 m3, the maximum sliding distance of 3050 m, and the elevation difference of approximately 965 m between the back scarp and the accumulation zone at the leading edge, and the maximum speed of 78.1 m/s, indicating that the rocky landslide is characterized with long runout distance and high speed. (2) Influenced by freezing weathering and wedge splitting effect from ice, cracks occur in the rock at the back of the landslide. The glaciers had receded since the Holocene, and melting snow and rainfall infiltration promoted the deterioration of the rock structure, reducing the strength of rock. (3) 14C and 10Be age dating shows that the Lagangcun landslide occurred at about 4140~9675 a BP. The strong earthquake caused by the Yarlung Zangbo fault may be a direct trigger factor of the landslide. The rock mass in landslide area was affected by the throwing force of earthquake, resulting in tensile shear failure and rapid cut through of original joint fissures and new fracture surfaces, with failing firstly in broken rock mass near, followed by the upper rock mass losing stability and sliding rapidly downslope. This study can provide references for the research on formation mechanism of giant ancient landslides in the fault zone of the Tibetan plateau.