华南地貌-水系演化与南海构造-沉积事件的台湾第三纪地层记录

Surface uplift of the Tibetan Plateau and the onset of seafloor spreading in the South China Sea are believed to have had a strong influence on the tectonics, landforms, drainage systems, and climate in East Asia during the Cenozoic. The time at which East Asia began to tilt toward the southeast, as it does today is hotly debated. The evolution of large drainages such as the Yangtze River has been used to constrain the establishment of topography in large area. However the relationship between rivers in the southeast coastal area, such as the Minjiang River and the Pearl River, and large rivers including Yangtze and Red River is unclear. . Previous investigations on the topography and drainage evolution largely focused on records in terrestrial strata, which faced limitations from the discontinuous accumulation and ambiguous depositional ages. Sediments transported by rivers and deposited on the East Asia continental margins may be more reliable tracers of evolving continental erosion because of their continuous record and good potential for the application of biostratigraphic dating techniques. Cenozoic sediments deposited on the northern margin of South China Sea record a change in the landscape of the Pearl and Red River basins and offer insights into the evolution of drainage in southern China and SE Tibet/SW China. Nevertheless, interpretation of provenance data from the northern margin of the South China Sea is still disputed because the difficulty in obtaining samples inhibits a better understanding. . The thick and successive Cenozoic sedimentary sequences on the continental margin of southeast China were deformed, uplifted and exposed along the Western Foothills and Hsuehshan Range of Taiwan during mountain building resulting from arc-continent collision after 6.5 Ma. This region provides an excellent opportunity for sampling without expensive oceanic drilling. Therefore, Taiwan is the best window to gain insights on the evolution of topography and river systems in eastern China.. This study addresses the provenance of sedimentary rocks exposed in the Western Foothills using the geochemistry of water-immobile elements such as Sc, Th, rare earth elements (REE) and their ratios, as well as Nd isotopes. U-Pb chronology and Hf isotope of detrital zircon grains in sandstones will also be analyzed. These proxies have been proved to be effective to trace the sediment source. . Some major and trace elements, such as alkali and alkali earth elements that are water mobile elements and very sensitive to climatic change, which can be used as a valuable proxy of weathering evolution. Elemental ratios of water mobile elements and CIA values will be analyzed to ascertain the degree of source area weathering. . In particular, we address: (1) provenance evolution and its meaning for the transition of landform and drainage systems in South China; (2) the evolution of the rivers in Southeast China; (3) the relationship between regional tectonics and landform evolution in Southeast China.

青藏高原隆升和东亚边缘海扩张是新生代最为引人瞩目的地质事件，对区域构造、地貌、水系及生态等均产生了重大影响。但目前对东亚地貌反转、水系重组及源－汇过程仍存在诸多争议。华南沿海水系（如闽江、珠江等）与长江、红河的演化关系也尚不明确。南海洋壳俯冲及弧－陆碰撞使沉积于南海北缘的第三纪层序完整出露于台湾岛，成为研究华南沿海地貌水系以及南海北缘构造-沉积过程的重要陆上窗口。本项目以台湾西部麓山带的第三纪地层为主要研究对象，在系统分析其沉积层序、地层格架及碎屑组成的基础上，对关键层位开展详细的地球化学、碎屑锆石U-Pb年龄谱和Hf同位素分析：（1）确定台湾西部麓山带第三纪地层沉积物的来源、输送路径及时空演化；（2）分析华南沿海水系，如闽江、瓯江等与长江演化的关系；（3）重建华南沿海与南海北缘整体构造地貌演化格局。将台湾视作“出露的南海”，为深入探讨南海北缘及华南沿海构造地貌演化提供了新的途径和思路。

台湾岛南端的恒春半岛是位于欧亚板块与花东-菲律宾海板块会聚边界的增生楔, 是认识南海北缘构造-沉积演化过程及其控制因素的重要研究对象。本项目以海陆结合的研究思路, 对恒春半岛中新统牡丹层砂岩的碎屑锆石、垦丁混杂岩中的大量变质锆石、增生楔中的增积玄武岩进行了详细的野外调查、岩石学、岩相学、地球化学、同位素地质年代学研究, 并基于此探讨了它们的物源和构造启示, 主要取得了如下认识: . 牡丹层的碎屑沉积物源区主要为华南沿海被动大陆边缘。结合文献报道的研究资料, 认为里龙山层与牡丹层的主体都属于同一深水扇沉积体系, 其中后者为上扇或补给水道沉积, 前者为中扇沉积。乐水层为独立的深水扇沉积体系, 沉积环境为中扇至下扇。U-Pb定年结果进一步显示里龙山层与牡丹层来自福建武夷山的闽江水系, 而乐水层来自九龙江流域。. 恒春半岛的垦丁混杂岩的碎屑物质中含有大量的变质锆石, 这些锆石所在的沉积物同样来自于中国华南陆缘河流向南海的沉积物输入, 变质于汇聚、堆积之后。这些变质锆石最可能的变质场所是南海洋壳沿马尼拉海沟向东俯冲于花东海盆之下形成的俯冲通道。这一重要的新发现表明垦丁混杂岩的形成起始于早中新世。. 恒春半岛出露的增积基性岩块主要包括玄武岩、辉长岩、辉绿岩、斜长角闪岩, 能够提供会聚边界地幔源区、构造演化以及物质运移的关键信息。它们具有显著的地球化学多样性, 可划分为N-MORB、E-MORB和OIB三种类型。N-MORB型岩石占基性岩块的多数, 锆石SIMS U-Pb年代学给出22.86 ± 0.32 Ma和24.2 ± 1.1 Ma, 是亏损地幔端元部分熔融形成的南海洋壳的碎片。OIB亲缘性的碱性玄武岩与南海海山玄武岩具有地球化学相似性, 是剪切破碎并加积进入增生楔的南海海山残片。E-MORB型玄武岩与吕宋岛弧弧前基底(花东海盆)玄武岩具有一致的早白垩世年龄、相似的岩石类型和地球化学特征, 并且都具有显著的IMM型Hf-Nd同位素特征, 表明它们是吕宋弧前基底俯冲消减过程中进入增生楔的碎片。