【作者】 苏怀；

【导师】 潘保田；

【作者基本信息】 兰州大学 ， 自然地理学， 2006， 博士

【摘要】 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。更多还原

【Abstract】 The competing role of surface uplift and climatic change on terrace formation remains controversy in geomorphic community. It needs more geomorphic evidences to solve the problem. Lanzhou area is an acknowledged perfect site for studies of fluvial terraces. In the last century, a lot of people in succession have investigated the terraces in Lanzhou area and many of their work are successful. However, previous researches on terrace mapping, correlation and dating are not very exact, and especially, few people investigated the terraces of the tributaries of Yellow River. Therefore, we chose the Eastern Lanzhou basin and Yuzhong Basin as a field lab for study of terrace in order to provide new information about the total number, distribution and ages of the terraces, and then, on this basis, put new ideas about the model of terrace formation and the role of the Qinghai-Tibet plateau tectonic on development and deformation of Yellow River terrace.Based on field investigations, loess-paleosol sequences, paleomagnetic ¹⁴C and optically stimulated luminescence （OSL） dating, the terrace sequence of the Yellow River over the past 1,200 ka in Lanzhou basin has been revealed. The ages of Yellow River terraces focus only in two relative short time periods. One is 1,240—860ka BP when 4 terraces were formed, their ages are respectively 1,240 （T7）, 1,050 （T6）, 960 （T5） and 860ka BP （T4）. The other is 130ka BP-the present when 3 terraces were formed, their ages are respectively 130 （T3）, 60 （T2） and 10 （Tl）ka.BP. The 1,050 ka terrace （T6） and 960 ka terrace （T5） were not reported in previous research The age of the fourth Yellow River terrace is 860ka BP rather than 590ka BP which were the previously believed. Up to now 590ka Terrace was not found in East Lanzhou basin. This result answers a long-term question in the geomorphology community of whether there exists a ⁸00ka terrace in Lanzhou area. The research results of Wangchuang river terrace shows that 4 terraces have developed in Yuzhong basin, whose ages are 330, 130, 60 and 10ka BP, respectively.All the Yellow River terraces are strikingly similar. They have several meters of paleosol developed on the top of overbank silt, suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates and there may be a link between the production of terraces and glacial-interglacial climatic cycles. However, it is not every glacial-interglacial transition during the last 1,240ka that terrace can form and preserve. That implies climate change is not sufficient condition of terrace development but necessary condition. Comparison of the terrace sequence of Yellow River with its average incision rates that is used as the proxy of surface uplift reveals that the formation and preservation of Yellow River terrace was strongly related to surface uplift and the realization of effect of climatic changes on the formation of terraces depends on the rate of rock uplift. It is when the uplift reaches a certain rate （e.g. 1,240-860ka BP and 130-0ka BP）, the climatic changes can result in river incision and the terraces are preserved. If the uplift is very slow, no terrace can be preserved. In conclusion, the formation of Lanzhou Yellow River terrace is attributed to coupling effect of climatic changes and uplift. We named this model of terrace formation as "climate-uplift coupling model" This model not only can explain the formation of Lanzhou Yellow River terraces but also can explain the terrace formation of other area in the reach of the Yellow River.The differential uplift due to the sinistral strike-slip movement of the northern marginal fault of Maxianshan Mountains is responsibility for the deformation of Yellow River terraces. The strike-slip movement of the fault is associated with the lateral extrusion of plateau terranes therefore, both the uplift of Qindhai-Tibet Plateau and the lateral extrusion of plateau terranes are recorded by Yellow River terraces in Lanzhou east basin.更多还原