两万年来岷江汶川古堰塞湖事件研究

A Paleo-lake in the Minjiang River since 20,000 Years before present in Wenchuan,China

位于构造活动频繁的高山峡谷地区,常发生滑坡堵江,进而形成堰塞湖。堰塞湖对河流自然演化及人类文明发展均有着重要影响,非稳态堰塞湖形成蓄水后往往进一步溃决引发特大洪水,致使下游河道严重冲刷并导致大规模基础设施破坏,而稳态堰塞湖会抬高上游侵蚀基准面,抑制上游河床下切,改变河流纵剖面形态。对于古堰塞湖事件的研究不仅可以更好地理解现代堰塞湖灾害,也可指示地质历史上的极端气候事件与构造运动事件。经野外考察,在青藏高原东缘,岷江上游汶川芤山发现古堰塞坝残留体,以及对应的上游湖相沉积物和下游溃坝特大洪水沉积物,标志着堰塞湖形成—稳定—溃决过程。研究表明,该堰塞湖形成于1.4万年前,初期发生小部分漫顶溃决,后稳定存在近万年,由于地震作用,距今3.8~6.9 ka间溃决消失。深入研究这类地区古堰塞湖形成与溃决,对该地区河流地貌演化研究、现代堰塞湖风险、特大滑坡灾害链防治和理解古人类文明的发展与消亡,均有着重要的指导意义。

Landslide-dammed lakes are usually formed by landslides and rock falls that block the river in high-relief areas with active tectonics.Landslide-dammed lakes are an important disturbance factor both in natural fluvial process of orogenic belts and human civilization.On the one hand,when an unsteady landslide-dammed lake formed,the outburst flood strongly washing the downstream regions would be occur within a few days.On the other hand,Steady landslide-dammed lakes continuing several thousand years or more could elevate the base level of upstream region.As a result,such landslide-dammed lakes could restrain downcutting of the upstream riverbed and develop knickpoint in large spatial scale of river longitudinal profile.Thus,researches on paleo-dammed lake are essential to deepen the understanding of chains of landslide hazards and river morphologies in mountain areas.A relic landslide dam,outburst flood deposits,and lacustrine sediments were found in the field survey of the upper reaches of the Minjiang River,located at the eastern margin of the Tibetan Plateau,China.This indicates that at least one round of the formation-stability-outburst process had occurred.Five sections were chosen and collected 7 optically stimulated luminescence(OSL)samples which respectively from lacustrine sediments sections and flood deposits sections.Two 14C samples were collected from flood deposits sections.These dating results can be used to establish when the date of landslide-dammed lake formed and disappeared.Our study show that the landslide-dammed lake was formed 14000 years ago.A small part of the dam was breached as water overtopped it,while the other part of the dam stood for about 10000 years.Eventually the dam experienced an outbursted and disappeared at 3.8~6.9 ka.The in-depth study of the formation and breaching of paleo-dammed lakes in similar areas was critical to prevent the risk in existing dammed lakes and to understand the evolution of fluvial geomorphology and vicissitude of ancient human civilizations.