长江形成于何时?

    When was the Yangtze River formed?

    • 摘要:
      研究目的 长江是亚洲最长的河流,是直接连通青藏高原和太平洋的大河,通过研究长江的形成与演化,可以广泛了解东亚地区的地质构造和气候变化。但是其何时形成和具体的演变过程一直未有定论。
      研究方法 鉴于此,对长江流域目前已经发表的有关长江发育和演化的沉积学、地球化学等数据结果进行系统的搜集、整理和对比,从构造演化和气候变化的大背景入手,讨论长江的演化过程。
      研究结果 结果表明:①晚中生代,四川盆地发育南流的大河,江汉盆地和望江盆地在此时由于断陷作用,成为区域汇水中心。长江流域的上游、中游和下游此时处于分段演化阶段。②江汉盆地、望江盆地、南黄海盆地和东海盆地在60~35 Ma处于强烈断陷阶段,继续成为局部的汇水中心,而青藏高原东南缘在晚始新世逐渐出现的季风气候则有利于维系大型河流的流动。此时发源于青藏高原东南缘、秦岭和黄陵背斜以西的古红河开始流入南海,称为原长江演化阶段。③青藏高原在约20 Ma发生整体垂向隆升和侧向挤出,导致云贵高原发生强烈的剥露过程,引起原先南流的古红河彻底解体。长江三峡在中中新世经历了显著的下切过程,导致江汉盆地、南京地区和东海盆地内堆积了来自长江上游的碎屑物质,此时称为始长江演化阶段。④金沙江、雅砻江、大渡河等河流的下游在上新世出现面积巨大的昔格达古湖,出现河湖共存的现象。长江流域广泛分布的同期砾石层,以及长江三峡地区发育的宽谷,表明此时长江的水动力搬运能力明显增强。华南地区类似现今的山-河-海分布模式在上新世已经出现,此时称为长江的成熟阶段。⑤昔格达古湖在2~1 Ma彻底解体,导致长江的干支流发生重组。长江上游在0.7~0.5 Ma发生的新一期快速下切导致金沙江、大渡河、长江三峡地区再次发育河流阶地。随着东海和黄海在中更新世均被海水覆盖,长江上游与下游侵蚀基准面之间的地势差异显著下降,导致长江下游以堆积过程为主,开始逐渐出现三角洲的形态,表明长江流域在第四纪已经进入老年阶段。
      结论 在青藏高原及周边山脉的隆升过程中,构造过程与气候变化和局部随机效应(如山体滑坡)相互作用,决定了长江流域河流地貌的发展。

       

      Abstract:
      Objective The Yangtze River, as the longest river in Asia, serves as a direct link connecting the Xizang Plateau and the Pacific Ocean. Delving into the formation and evolution of the Yangtze River can provide invaluable insights into the geological structure and climate changes occurring in East Asia. However, there is currently no consensus regarding the precise timing of its formation or the specific details of its evolutionary process.
      Methods To address this research gap, we conducted a comprehensive review, analysis, and comparison of existing sedimentological and geochemical data on the development and evolution of the Yangtze River Basin. Our study begins by examining the context of tectonic evolution and climate change, and subsequently delves into a discussion on the evolutionary process of the Yangtze River.
      Results The results of our analysis reveal the followings: ① During the Late Mesozoic, a large river system formed in the Sichuan Basin that flowed southward. Additionally, tectonic subsidence during this period turned the Jianghan Basin and Wangjiang Basin into regional drainage centers. As a result, the upper, middle, and lower reaches of the Yangtze River Basin experienced segmented evolution. ② Between 60 Ma and 35 Ma, significant subsidence occurred in the Jianghan Basin, Wangjiang Basin, South Yellow Sea Basin, and East China Sea Basin, which continued to function as local drainage centers. During this time, the Jianghan Basin developed extensive salt lake deposits, suggesting the absence of major inflowing or outflowing rivers. However, the Paleo−Red River flowed into the South China Sea, marking the early phase of the evolution of the proto−Yangtze River during this time. ③ Approximately 20 Ma, the Xizang Plateau experienced vertical uplift and lateral extrusion, triggering extensive exhumation processes in the Yungui Plateau. As a result, the Paleo−Red River, which previously flowed southward, disintegrated entirely. In the Middle Miocene, the Three Gorges of the Yangtze River underwent significant incision, leading to the deposition of sedimentary material from the upstream Yangtze River in the Jianghan Basin, Nanjing area, and East China Sea Basin. Consequently, the Yangtze River, with a length exceeding 6000 km, emerged during the Miocene epoch, marking the onset of the evolutionary stage of the Yangtze River. ④ During the Pliocene, the downstream areas of rivers such as the Jinsha River, Yalong River, and Dadu River were affected by heavy rainfall to appear the large−scale landslides, leading to the formation of extensive Paleo−Xigeda Lake. Additionally, the formation of thick gravel layers was observed in various regions, including the Jianchuan Basin, Dadu River and Minjiang River basins, Wangjiang Basin, and the Nanjing area. The wide canyon formed in the Three Gorges region of the Yangtze River. These geological changes suggest a substantial enhancement of the hydraulic transport capacity of the Yangtze River compared to the Middle Miocene. During the Pliocene, the South China region exhibited a similar mountain−river−sea distribution pattern as is observed today, representing a developmental stage of the Yangtze River. ⑤ Between 2 and 1 Ma, Paleo−Xigeda Lake completely disintegrated, resulting in significant reorganization of the main and tributary streams in the Yangtze River. This process brought about notable changes in sediment sources in both the Jianghan Basin and the Yangtze River Delta. Subsequently, a new phase of rapid incision took place in the upstream Yangtze River around 0.7~0.5 Ma, leading to the reestablishment of river terraces in the Jinsha River, Dadu River, and the Three Gorges region. During the middle Pleistocene, as the East China Sea and Yellow Sea became submerged by seawater, the continued incision in the upstream Yangtze River combined with the influence of the seawater in the lower reaches resulted in a significant reduction in the topographic difference between the upstream and downstream erosion levels of the river. Consequently, there was a shift toward deposition processes in the lower reaches, leading to the gradual development of a deltaic landform. These changes indicate the entrance of the Yangtze River Basin into a mature stage during the Quaternary.
      Conclusions The development of fluvial landforms in the Yangtze River Basin was influenced by the interplay of tectonic processes, climate change, and local stochastic effects such as landslides, particularly during the uplift processes of the Xizang Plateau and the surrounding mountains.

       

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