金沙江上游奔子栏-达日河段属横断山区的干热河谷地带,河谷沿岸大型古泥石流堆积扇广泛发育,其成因却一直没有得到很好的研究.对该区瓦卡大型古泥石流堆积物进行了沉积结构、粒度、地球化学和孢粉等分析,揭示了泥石流的沉积环境及其形成过程.通过粗颗粒石英的光释光单片再生法(SAR)测年研究,获得金沙江上游奔子栏-达日河段古泥石流大规模暴发的年代为12 600～4 500 a BP.丰富的风化碎屑物源、陡峻的地形及雨季降水集中是该区古泥石流形成的主要原因.全新世早期青藏高原东南缘受西南季风加强的影响,气候趋于暖湿,季节性暴雨增加.金沙江上游干热河谷区大型泥石流堆积扇的发育年代暗示其是全新世早期西南季风加强作用下的地貌响应.从地质灾害防治的角度,由于现代气候因素导致泥石流灾害的频度和规模较小,预防该区地质灾害的重点应是防止人工砍伐树木和不合理的人工切坡导致对地表环境的破坏加剧.%The Quaternary debris-flow accumulations can provide rich information for geomorphic evolution, tectonic movement and climate change. There occur many Quaternary debris-flow accumulations in the southeastern (SE) marginal area of the Tibetan Plateau (TP) , especially in valleys of the upper reach of the Jinsha River. The river segment between Benzilan to Dari of the upper Jinsha River belongs to the dry-hot valley type in the Henduan Mountainous region. Major old debris-flow accumulations are well developed in this area, however their formative causes have not been studied. The Waka debris flows is one of the major debris flows in the river segment and has successive series in the exposed sections. Detailed granularity, pollen, geochemical and textural studies reveal the formation processes of Waka debris flows. The age estimates of optically stimulated luminescence (OSL) using SAR protocol show that the occurrence of major debris flows started at around 12.6 ka BP and weakened until 4.5 ka BP in the upper Jinsha River valley. The rich source of weathered detritus, steep landform and concentrated precipitation are the dominant factors cansing the occurrence of debris flows in the area. The seasonal variation of the southwest monsoon circulation over the Tibetan Plateau and its adjacent area results in substantial rainfall during summer in the area. Climate change is thus characterized by the alteration between cold-dry and warm-wet. The most characteristic features of Paleoclimatic records are the sudden establishment of wet conditions around 10.0 ka BP, and maximum aridity around 4 ～ 3 a BP. Objected to the strengthening of the southwest monsoon in the SE margin of the Tibetan Plateau in the early Holocene, the climate in the dry-hot valley trended to be wetter and the rainfall in the summer seasons might increased. The age estimates of optically stimulated luminescence show that the occurrence of major debris flows corresponded to the obvious strengthened phase of the summer monsoons in the region. The formation characteristics and ages of the debris-flow deposits in the upper Jinsha river indicates that the occurrence of a mass of debris flows were a response to the intensified southwest monsoon in the southeast margin of the Tibetan Plateau since the early Holocene.
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