兰州东部地区1，240ka以来的河流阶地研究 英文标题:The Studies on Fluvial Terraces in Eastern Lanzhou Region during the Last 1， 240 ka

气候变化和地面上升在河流阶地形成过程中所起的作用一直是河流地貌学界争论的焦点。究竟河流是以哪一种模式发育阶地需要更多的地貌实地资料来检验。兰州地区是国内地貌学界公认的研究河流阶地的理想地区之一，很早以来，就陆续有不少地貌学家对兰州地区河流阶地进行考察研究，并取得了一定成果。然而，由于当时技术条件的限制，以往的研究不仅对黄河支流的阶地涉及甚少，而且连黄河干流阶地的划分、对比和年代测定方面也存在一定疏漏，尤其在黄河阶地发育模式的研究方面至今还没有统一意见。为此我们以兰州东部地区（包括黄河流经的兰州东盆地和宛川河流经的榆中盆地）的黄河阶地和黄河的一级支流宛川河阶地为对象，进行深入研究，以期获得兰州东部地区河流阶地的最新资料，并以此为基础，在河流阶地发育模式、阶地变形等方面的研究中提出新的见解。通过古地磁、14C、光释光测年及古土壤断代法年龄推算，确定了1，240ka以来黄河在兰州东盆地主要有两个阶地发育时期。第一个时期是1240—860kaBP，黄河至少发育了4级阶地，其形成时代分别为：1，240（T7）、1，050（T6）、960（T5）和860ka BP（T4）；后一个时期是最近130ka，黄河发育3级阶地，时代分别为130（T3）、60（T2）和10ka BP（T1）。其中形成时代为1，050ka（T6）和960ka（T5）的阶地在以往的文献中没有报道属于本次研究新发现的阶地，黄河的T4（五一山阶地）形成时代为860ka BP，而不是以往报道的590ka BP，在兰州东盆地迄今没有发现时代为590ka的阶地。黄河的一级支流宛川河在330ka以来发育了至少4级阶地，各级阶地形成年代由高到低分别为：330、130、60和10kaBP。黄河各阶地河漫滩顶部都有一层古土壤发育表明黄河下切形成阶地发生在古土壤开始发育的冰期向间冰期的过度阶段，但是并非1，240ka以来的每次冰期—间冰期的气候交替都能引起黄河下切形成阶地。气候变化只是阶地形成的必要条件之一，不是充分条件。对比黄河下切速率和阶地年代序列发现，地面上升仍然是影响黄河下切的重要因素。只有在地面上升速率达到一定程度的时候（例如：1240—860ka BP．和最近130ka），气候变化才能导致黄河堆积—下切交替形成阶地；而在地面上升缓慢时期（例如：860—130ka BP），即使发生了大幅度的气候变化，黄河也没有阶地记录。由此看来，兰州东盆地黄河阶地序列的发育是冰期—间冰期的气候旋回与地面抬升耦合的结果，我们称这种阶地发育模式为：气候变化与地面抬升耦合发育阶地模式。这一模式不仅在兰州东部地区适用，而且也适用于黄河流域的其它地段甚至西欧的河流阶地发育。兰州东盆地黄河阶地的变形是九州台褶皱隆起所产生的地面差异性抬升的结果，而造成九州台褶皱隆起的直接原因是伴随青藏高原块体侧向挤出的马衔山北缘断裂的左旋走滑。因此兰州东盆地的黄河阶地不仅记录了青藏高原的整体抬升而且还在一定程度反映了高原块体的侧向挤出。

The roles of climate change and ground uplift in the formation of river terraces have long been a focal point of debate in fluvial geomorphology. Which specific pattern fluvial systems develop terraces through requires more field geomorphic data for validation.

The Lanzhou region is recognized by the Chinese geomorphology community as one of the ideal areas for studying river terraces. For a long time, numerous geomorphologists have conducted field investigations and studies on river terraces in this region, achieving certain research outcomes. However, due to technical limitations at the time, previous studies paid little attention to the terraces of the Yellow River's tributaries, and even had certain oversights in the division, correlation, and dating of terraces along the main stream of the Yellow River. In particular, there is still no unified consensus on the research of the development patterns of Yellow River terraces.

To address this issue, we conducted an in-depth study focusing on the terraces of the Yellow River and its first-order tributary, the Wanchuan River, in the eastern Lanzhou region (including the eastern Lanzhou Basin traversed by the Yellow River and the Yuzhong Basin traversed by the Wanchuan River). We aimed to obtain the latest data on river terraces in eastern Lanzhou, and based on these data, put forward new insights into the research of fluvial terrace development patterns, terrace deformation, and other related aspects.

Using paleomagnetic dating, 14C dating, optically stimulated luminescence (OSL) dating, and age estimation via the paleosol dating method, we determined that there have been two main terrace development periods for the Yellow River in the eastern Lanzhou Basin since 1240 ka. The first period spans 1240–860 ka BP, during which the Yellow River developed at least four terraces, with formation ages of 1240 ka BP (T7), 1050 ka BP (T6), 960 ka BP (T5), and 860 ka BP (T4), respectively; the latter period is the most recent 130 ka, during which the Yellow River developed three terraces, with ages of 130 ka BP (T3), 60 ka BP (T2), and 10 ka BP (T1), respectively. Notably, the terraces formed at 1050 ka BP (T6) and 960 ka BP (T5) have not been reported in previous literature, representing newly discovered terraces in this study. The formation age of the Yellow River's T4 (Wuyishan Terrace) is 860 ka BP, rather than the previously reported 590 ka BP, and no terraces dated to 590 ka BP have been found in the eastern Lanzhou Basin to date.

The Wanchuan River, a first-order tributary of the Yellow River, has developed at least four terraces since 330 ka BP, with formation ages from high to low being 330, 130, 60, and 10 ka BP, respectively.

The presence of a paleosol layer atop the floodplain of each Yellow River terrace indicates that the incision of the Yellow River to form terraces occurred during the glacial-to-interglacial transition period when paleosols began to develop. However, not every glacial-interglacial climate alternation since 1240 ka BP has triggered the Yellow River's incision and terrace formation. Climate change is only one of the necessary conditions for terrace formation, not a sufficient one. A comparison of the Yellow River's incision rates and terrace chronology reveals that ground uplift remains an important factor influencing the Yellow River's incision. Only when the ground uplift rate reaches a certain threshold (e.g., 1240–860 ka BP and the most recent 130 ka) can climate change trigger aggradation-incision cycles of the Yellow River to form terraces. During periods of slow ground uplift (e.g., 860–130 ka BP), even with significant climate changes, there are no terrace records preserved for the Yellow River.

Thus, the development of the Yellow River terrace sequence in the eastern Lanzhou Basin is the result of the coupling of glacial-interglacial climate cycles and ground uplift. We term this terrace development pattern the "Climate Change and Ground Uplift Coupled Terrace Development Model". This model is not only applicable to the eastern Lanzhou region, but also to other sections of the Yellow River basin and even the development of river terraces in Western Europe.

The deformation of the Yellow River terraces in the eastern Lanzhou Basin is the result of differential uplift caused by the fold uplift of Jiuzhoutai. The direct cause of the Jiuzhoutai fold uplift is the left-lateral strike-slip movement of the northern margin fault of the Maxian Mountains, which accompanies the lateral extrusion of the Tibetan Plateau block. Therefore, the Yellow River terraces in the eastern Lanzhou Basin not only record the overall uplift of the Tibetan Plateau, but also reflect, to a certain extent, the lateral extrusion of the plateau block.

搜集汇总

背景与挑战

背景概述

该数据集聚焦于兰州东部地区过去1,240ka以来的河流阶地研究，旨在探究气候变化与地面抬升在阶地形成中的作用。通过多种测年方法，研究确定了黄河在该区域的两个主要阶地发育时期，并发现了新的阶地记录，修正了已有阶地年代。最终提出阶地发育是冰期—间冰期气候旋回与地面抬升耦合的结果，这一模式适用于更广泛区域。