江汉盆地宜昌冲积扇物源解析与区域地貌演化

Yangtze River Three Gorges through analysis of sediment provenance of Yichang alluvial fan.The Yangtze River is the largest drainage system originated from Tibetan Plateau, yet how and when the Yangtze River formed have been debated for more than one century. The Three Gorges is the key capture point, and it is essential for us to determine when the upper Yangtze river was captured by the middle-lower Yangtze. In the previous studies, the Yichang alluvial fan has been proved to be the direct deposit of the formation and incision of the Three Gorges. Based on our former depositional facies analysis, the provenance of the late Cenozoic sediments in the Yichang area will be determined by multi-methods, including characteristic minerals/ detritus, detrital zircon U-Pb age/Hf isotope, 40Ar–39Ar muscovite dating and Pb-in-K-feldspar in this research. In addition, the depositional age of these sediments will also be acquired by several dating approaches, including lithostratigraphy, biostratigraphy, Electron Spin Resonance (ESR) and cosmogenic nuclides burial dating (10Be and 26Al). The aim of this research is to make a contribution to understanding the formation of the Three Gorges, and further to explore the effective provenance methods of understanding the evolution of the large drainage system.

长江的形成是地学界的一个重大科学问题。三峡贯通时代的确定是解决长江形成问题的关键，对此研究长达世纪，但仍悬而未决。前期研究发现，宜昌冲积扇是三峡切割和贯通的直接相关沉积。本申请基于“侵蚀-沉积的相关性原理”和“源-汇分析理论”，突出利用地质学方法解决河流地貌学问题的学术思路。在对宜昌砾石层系统地沉积学研究的基础上，根据长江上游流域的地质特征与前期预研究，选择特征岩石（峨眉玄武岩）和特征矿物（攀枝花型）的示踪分析、锆石U-Pb年龄及Hf同位素示踪、碎屑白云母40Ar-39Ar年龄谱示踪以及碎屑钾长石Pb 同位素示踪等综合源—汇分析，确定长江贯通三峡的地层层位。再通过岩石地层、生物地层、ESR、特别是宇生核素埋藏年龄等多重年代地层学方法，建立宜昌砾石层的年代标尺，准确限定三峡贯通层位的时代，从而查明长江贯通三峡的时代。为长江的形成这一世纪科学问题有所贡献。并探索大河形成演化的有效研究方法。

在长江三峡出口的宜昌以东分布着一套巨厚的松散状砾石层，我们称之为宜昌冲积扇。野外调查发现该砾石层主要分布（出露）于以宜昌、松滋、枝江北为顶点的丘陵、岗地上，分布范围逾1000Km2，最大厚达110m。钻孔资料揭露，受荆门—松滋断层影响该砾石层在松滋—枝江以东隐伏江汉平原之下。经地貌学、第四纪地质学、沉积学（沉积相与沉积环境）等综合分析，认为其为一大型砾质冲积扇体。而该冲积扇就是三峡贯通直接证据。宜昌冲积扇的地质时代。通过岩石地层学、生物地层学和年代地层学（ESR、Cosmogenic Nuclide Burial Dating）的综合分析得出，该冲积扇形成于第四纪早更新世，地质年龄为1.4～0.8maBP。由此推断，长江贯通三峡的应该发生在该时期。本研究还通过碎屑白云母40Ar/30Ar、石英ESR、锆石U-Pb年龄等物源示踪方法对冲积扇砾石层的碎屑矿物的物源进行了示踪分析，表明其来源于长江上游地区。