基于释光年代约束的北天山金钩河河道演化历史

On the Tianshan piedmont, three rows of fold-and-thrust belts with significant gradient and pattern in tectonic activity align roughly in parallel with the Tianshan Mountain to the south. Fed by the melting water from high range of Tianshan, more than ten transverse rivers flow across the fold belts, either incising into the fold or deflected by the fold. Such geological and geographical settings make these rivers be ideals natural laboratory for investigating the mechanisms on how the fluvial evolution would respond to tectonic and climate forcing. Jingou River is of such geomorphic significance, which incised into the Huoerguosi anticline and was then deflected by the Anjihai anticline. However, the lack of chronological data hinders the investigation on its evolution. In this project, we propose to employ systematic, spatially and temporally sequential luminescence chronology of the river terraces to facilitate our understanding of evolution of Jingou River. Based on time series of rivers incision at more than ten sites along the ~50 km long river, which are obtained by luminescence dating using both quartz and feldspars, the spatial-and-temporal sequences of river incision will be reconstructed by compiling the individual time series. By further including the elevation, width and slope data of the terrace surface, as well as the warp of terrace surface at the anticline, we aim to unveil the distinct features of river incision within the structural framework of two adjacent active folds, and explore the mechanism for driving the Jingou river incising into Huoerguosi anticline and being deflected by Anjihai anticline. Finally, we will attempt to reconstruct the channel evolution history of Jingou River and understand the processes how tectonic and climate involve in.

天山北麓山前自南向北依次发育了构造活动性呈现显著空间规律的三排逆断裂-背斜带。源于天山的十余条河流流过背斜带汇入准噶尔盆地。天山的冰雪融水控制着这些河流的流量，流经不同背斜时，河流或下切穿过，或改变流向绕行斜切，使得这些河流成为探讨河流发育与构造气候关系的天然实验场，但是年代数据的欠缺制约了相关研究的深入。本项目以穿过霍尔果斯背斜，绕过安集海背斜，缺乏系统测年数据的金钩河为研究对象，开展成时空序列、系统的阶地释光年代学研究。将对近50 km长的河道上，十余个地点的阶地序列，通过石英和长石两种释光测年技术进行测年，结合阶地平面图和位相图，建立金钩河下切的时空序列。结合阶地拔河高度、宽度、坡度以及背斜区阶地拱曲变形等数据，探讨在两个背斜活动的共同影响下，河流下切作用的特征，以及造成金钩河穿过霍尔果斯背斜而绕过安集海背斜的原因。最后将尝试恢复金钩河河道演化历史，探讨其如何受控于构造气候要素。

本项目基于释光年代学方法，以金沟河为例，探索褶皱逆冲带河道演化特征与其对分布式构造变形和气候变化的响应机制。项目结合野外调查、地形测量、DEM地形分析、释光年代测量和数值模拟等诸多方法，调查分析了金沟河阶地形态及沉积特征，有针对性地改进了钾长石红外激发后红外释光和难纯化石英释光测年方法，测量了洪积相/河流相沉积物和上覆黄土的释光年代，模拟了构造变形造成的岩性沿河道变化和多个背斜协同活动对河道演化的影响。.项目研究厘清了金沟河河流阶地分布特征与形态；揭示了超过250度的热处理将导致钾长石受辐照时陷阱电子俘获概率降低20%，引发的初始灵敏度变化使得钾长石红外释光测年结果存在不确定性；在25度进行的脉冲绿光激发，可显著降低不纯净石英释光信号中长石的贡献。在此基础上测得的沉积物释光年代揭示，金沟河在130ka-36ka之间经历了一次或多次“填充-下切”旋回，自36ka-18ka发生快速堆积，直至形成现今最为连续的地貌面，而后废弃于18ka-8ka间，并快速下切至少70m至现代河床；而安集海背斜在过去300ka中，由于构造抬升引起的地层掀斜不小于5度，其变形速率远大于先前认识；数值模拟实验则揭示了沿河道方向的岩性差异可导致金沟河高达20m的额外下切，该下切量与气候变化和构造抬升均无直接关系；当金沟河下游安集海背斜抬升持续时间超过20ka后，安集海背斜抬升引起的河流下切可传播至霍尔果斯背斜。.通过项目实施，优化了石英和长石释光测年方法；通过年代学方法，清晰揭示了褶皱逆冲带河流的“填充-下切”旋回，提示了对沉积和废弃阶段的绝对定年对研究褶皱逆冲带冲积型河道演化、提取构造和气候变化信息的重要意义；揭示了褶皱逆冲带冲积型河道的复杂演化特征(系统外部驱动和系统内部复杂响应)，对于理解碎屑物质自源区向汇区的输运过程及其对气候变化和构造活动信息的记录具有重要科学意义。