晋陕峡谷阶地分布的时空差异及其对黄河形成演化的反映

Fluvial landforms are rich in the information of tectonic movement, climate change and surface processes. The Jinshan Gorge of the Yellow River is considered to be formed by the integration of the two separated basins, the Hetao to the north and the Fenwei to the south, with a distance of more than 500km. It becomes a key region to study the formation and evolution of the Yellow River. However, although the Jinshan Gorge had been studied over a century, scientific problems such as the causes, times and processes of the formation have not been fully answered. Actually, different origins of the terraces have variable temporal and spatial distributions. (1) The terraces originated from uplift of the Ordos Plateau are merely distributed within the Gorge system. (2) The terraces formed because of climate change are distributed widely in a large area. (3) The terraces formed by draining and lowering down of the downstream Feiwei paleo-lake level, the regional base level, which was caused by communicating with the Sanmen Gorge, appear in all rivers that fed into the lake. (4) Terraces created by downcutting because of connection of upstream Hetao paleo-lake are only distributed along the Gorge, and not even in its tributaries. Apart from the situation (3), the remaining situations have one-to-one relationship between terraces and nickpoints. All situations would probably act together or separately, leading to possible and complicated evolution routes of the Gorge. On the basis of our very detailed and careful previous work and above ideas, field laboratory works will be carried out, including dating of OSL, megnatostratigraphy, TCN, furthermore analyses of deposit provenance and climate change will also be adopted. Series pictures of high resolution for both transverse and longitude profiles will be compiled, and then, the formation and evolution of the Gorge in response to tectonic movement, climate change and the drainage both upstream and downstream basins could be better understand.

河流地貌蕴含构造、气候和地表过程的丰富信息。长达五百多公里的晋陕峡谷是黄河全线贯通的难点和关键，晋陕峡谷的研究已持续了一个多世纪，但贯通因素、时间和过程等科学问题并未得到充分解答。不同因素造成的阶地特征和时空分布不同。①鄂尔多斯地块抬升的构造阶地仅局限于峡谷及支流；②气候阶地在峡谷内外大范围分布；③下游三门峡贯通造成的阶地分布于注入汾渭古湖的所有水系；④上游河套古湖下泄造成的阶地仅影响到峡谷本身、支流不受影响。除③外，其余三种情况会出现阶地与裂点一一对应关系。上述几种因素或者分别进行、或者共同作用，使峡谷的演化路径变化多端、纷繁复杂。本项目将在我们前期详细工作的基础上，根据上述原则开展野外工作。结合光释光、古地磁、宇宙核素等多种方法测年以及物源分析、古气候识别等，编制高精度系列纵、横剖面图，探查晋陕峡谷对构造运动、气候变化、上、下游盆地演化的响应，全面而系统地解读晋陕峡谷的发展演化历史。

长达600km多的晋陕峡谷是黄河全线贯通的关键区域。基于前两个国家自然科学基金资助的研究，本研究重点解决晋陕峡谷南部和控制晋陕峡谷蚀积过程的汾渭盆地。为更好地探讨汾渭古湖消长的控制因素，把研究范围拓展到三门峡谷和峡谷下游的平原，把河套盆地—晋陕峡谷—汾渭盆地—三门峡谷—郑州平原这一串珠状分布河湖系统作为整体进行研究，得到了自河套盆地到华北平原长达1000km黄河形成演化的系统认识，恢复了黄河跌宕起伏的演化历史，揭示了预料之外和情理之中的发展历程，显示出大河在构造运动和气候变化共同作用下发展演化的普遍规律。可以说，四年的研究获得了根本性的突破。研究表明，晋陕峡谷—汾渭盆地—三门峡谷的阶地可以分为等时和变时两类；构造抬升造成的裂点溯源后退形成变时阶地，冰期干旱溯源后退慢，间冰期湿润溯源后退快，使覆盖阶地的古土壤明显多于黄土；沟通上游古湖造成湖水下泻形成等时阶地。三门峡沟通汾渭盆地在先（约200ka），在三门峡谷内等时阶地被S2覆盖，并造成三门峡谷上游包括晋陕峡谷在内的所有水系均发生溯源侵蚀，汾渭古湖消失，湖相层软弱，河流下切迅速，形成的阶地近于等时，进入峡谷基岩区后又转为变时阶地；晋陕峡谷沟通河套古湖再后（约100ka），从晋陕峡谷—汾渭盆地—三门峡谷中均形成被S1所覆盖的等时阶地；沟通上游引起的下切只影响主流，支流不受影响，两者下切速度差异造成峡谷两侧广泛发育悬谷；运城盐池是汾渭古湖的残留。晋陕峡谷由宽谷、峡谷和直谷嵌套而成，宽谷以侧蚀为主，下蚀为辅，发育曲流，于上新世末到第四纪初发生较快下切，留下了深切曲流格局，其变时阶地被不同年代的红黏土—黄土覆盖；于早更新世中期（约1.4Ma）快速下切，留下了峡谷和系列黄土—古土壤系列覆盖下变时阶地；沟通河套盆地后河流快速下切，在晋陕峡谷—汾渭盆地—三门峡谷中均形成等时阶地和直谷。晋陕峡谷—三门峡谷早期主要受缓慢构造抬升+周期性气候变化影响，形成系列变时阶地；晚期主要为河湖贯通而引起的流量突变和快速下切作用，形成系列等时阶地。贯通所造成的下切速度远大于构造抬升与气候变化的影响。鄂尔多斯高原发育由西北流向东南的泾河、渭河和无定河深切曲流，反映上新世鄂尔多斯地块由北西向南东构造掀斜，而晋陕和三门两峡谷主要于早更新世中期发生下切，可能暗示鄂尔多斯地块转为由东向西的快速构造掀斜。