巨型裂点形成与河谷演化的沉积学证据-以东喜马拉雅构造结雅鲁藏布江大拐弯为例

The Namjagbarwa syntaxis of east Himalaya is an area with rapid stream network evolution related to tectonics and climate interaction. The river course veers greatly when the Yarlu Tsangpo River flows through the Namjagbarwa, and the valley geomorphology exhibits a sharp contrast at the entrance of the great turning in the gorge of the Yarlu Tsangpo River. Taking the vertex of the gorge as knick point, morphology of the valley in the upstream reach appears "U" shape whereas intensely incised in the downstream reach. At the upstream reach of the entrance, multiple glacial dammed lake sediments distributed at different heights, and the latest drilling work revealed more than 500m thick riverbed covers mainly consisting of fine Quaternary deposits which provide sedimentary records for the uplifting history of Namjagbarwa syntaxis in the east Himalaya, Quaternary glacial process as well as glacial weir dam as knick point on valley landform evolution. This study intends to apply combination of OSL,14C,ESR,26Al/10Be exposure dating methods,and environmental indicators analysis on the outcrop and drilling cores about 700 meters thickness deposition, together with the measurements of the deposition section and morphology of the valleyto establish geochronological framework of valley deposition and landform evolution of the gorge and upper stream, investigate the shaping process of valley-knick point-gorge valley geomorphology of the Yalur Tsangpo River, and provide the geochronological constraints for understanding the immensely landform evolution process of the east syntaxis caused by rapid uplifting of Tibetan Plateau in the late Cenozoic and the interaction among tectonics, climate, and other factors.

东喜马拉雅构造结是构造与气候因素交织下河流系统演变十分迅速的地区。雅鲁藏布江流经南迦巴瓦峰时河道大拐弯，大拐弯入口河段河谷地貌反差巨大，以裂点为界，上游为宽谷，下游为大峡谷。在上游河段，分布着高度不等的多期冰川堰塞湖沉积，最近的钻探又揭示出厚度超过500m的河床覆盖层，这一巨厚的以细粒为主的第四纪沉积，可为探知南迦巴瓦峰的隆升过程、第四纪冰川历史以及冰川堰塞坝作为裂点对河谷地貌演化的作用等问题提供沉积记录。本项目拟对露头剖面和钻探岩芯所揭示的厚约700余米的沉积进行系统的OSL、14C、ESR和26Al/10Be测年和环境代指标测试，结合河谷沉积剖面和河谷形态的大比例测量，建立大峡谷及其上游河段河谷沉积和地貌演化的年代学框架，探讨雅鲁藏布江的宽谷－裂点－大峡谷的河谷地貌的塑造过程，为理解青藏高原晚新生代快速隆升造成的东构造结的地貌巨变过程及构造、气候等因素的相互作用等科学问题供时间约束。

对于地质构造演化、气候变化和地表侵蚀过程的相互关系这样一个地球科学研究的前沿命题，雅鲁藏布江河流沉积和地貌过程的研究能够从一个独特的视角来提供证据。本项目通过对大峡谷入口处的地貌学、沉积学和年代学的综合研究，获得了河谷沉积的年代学框架，为深入开展东喜马拉雅南迦巴瓦构造结地区的构造、气候和地表过程的相互作用及其过程提供了时间约束。迄今为止，我们获得了大峡谷河谷演化的几个重要时间节点：（1）至少在 ~2.5 Ma以前，雅鲁藏布江就已经深入侵蚀到高原内部，其后南迦巴瓦峰开始的加速构造隆升堵塞了古峡谷，形成了分隔上游冲填楔和下游陡峭大峡谷的巨型裂点，大峡谷内的高侵蚀速率是岩石快速抬升的直接反馈。（2）~1 Ma南迦巴瓦峰冰川启动，冰川坝堵江堰塞导致河谷沉积由河流相转为河湖相，其后十余次短时间尺度的河-湖相沉积转换可能与中更新世气候转型以来冰期-间冰期（或冰阶-间冰阶）的气候波动相关联。（3）末次冰期以来的冰川堰塞湖、溃坝洪水所构建的极端气候-灾害事件序列，对雅鲁藏布江大峡谷现今河谷地貌和流域生态环境有重要影响。另一方面，我们对大峡谷河流地貌进行了测绘和数值地貌分析，通过对雅鲁藏布江及其支流的面积－高程积分和河流陡峭系数的分析，结合区域降雨量及岩性分布特征，探讨了不同区段地貌参数变化所蕴含的构造活动信息，认为加查裂点与南迦巴瓦裂点均位于隆升块体的西边界，横向构造隆升可能控制了裂点的形成和稳定。本项目的工作将为青藏高原与高原边缘差异性隆升机制、河流对高原物质的传输、藏东南地区第四纪地层年表的建立以及极端气候-灾害事件的研究奠定坚实的基础。