藏东南波堆藏布江谷地古冰碛堰塞湖初步研究

Preliminary study of the paleo-moraine dammed lake in the Bodui Zangbo River Valley,southeastern Tibet

藏东南作为青藏高原内外动力作用最强烈的地区之一,是河流堰塞-溃决洪水链式灾害频发地区。研究该区域古堰塞湖的形成与演化,对认识、评估灾害风险具有重要意义。研究结果表明,波堆藏布江谷地保存有一套湖相沉积地层,通过地貌填图和光释光测年等手段,并结合前人研究成果,推测海因里希事件1(H1)以来波堆藏布江谷地可能发生了一期古堰塞湖事件。该古堰塞湖可能是由波堆藏布江侧蚀白玉沟沟口冰碛垄,致使冰碛物滑塌并阻江形成的,古湖的最大湖面面积约为18.8 km^(2),库容蓄水量约为0.13 km^(3)。该古堰塞湖的形成时间可能介于H1~10 ka之间,其溃决时间可能在~6 ka之后,后期河流溯源侵蚀及下切作用形成了多级以湖相地层为基座的阶地。

As one of the most intensive internal and external dynamics areas of the Tibetan Plateau(TP),southeastern Tibet is a high-risk area for river damming and outburst flood.Thus,it is of great significance to analyze the evolution process of the paleo-dammed lake for understanding and assessing disaster risk from outburst flood caused by river blockage.southeastern Tibet is one of the most extensively glaciated areas of the TP,both now and during the Quaternary,however,few paleo-moraine dammed lake has been reported in this region.During our recent field investigation,widely distributed lacustrine deposits has been first found from the Baiyu Village to Linqiong Village reaches of the Bodui Zangbo River Valley,eastern Nyainqêntanglha Range,southeastern Tibet.This study used optically stimulated luminescence(OSL)technique to date the lacustrine sediments which are preserved on the surfaces of moraine and terraces near to the Linqiong Village.The OSL ages range from~6 to~10 ka,corresponding to the early Holocene.Based on geomorphology mapping and OSL dating,and combined with previous dating results about moraines located at the Baiyu Valley,we speculated that a paleo-dammed events may have occurred during the Heinrich event 1(H1)in the Bodui Zangbo River Valley.The paleo-dammed lake was caused by a debris landslide originating from H1 moraine at the Baiyu Valley,triggered by the intensive downcutting and lateral erosion by the Bodui Zangbo River.The maximum lake area and volume were~18.8 km^(2) and~0.13 km^(3),respectively.According to the dating results from moraines and lacustrine deposits,we inferred that the formation time of this paleolake was about between the H1 and 10 ka,and its failure time would be after 6 ka,implying that the paleolake was at last stable for 4×103 years.The Bodui Zangbo River incision lacustrine sediments later,forming multilevel lacustrine terraces.