青藏高原东北缘循化-贵德盆地及邻区更新世时期沉积与后期侵蚀样式研究

PLEISTOCENE DEPOSITION AND SUBSEQUENT EROSION DISTRIBUTION AROUND XUNHUA -GUIDE BASIN,NORTH EASTERN TIBETAN PLATEAU

利用残存的地貌标志恢复原始地貌形态是地貌研究的难点之一。青藏高原东北缘循化-贵德地区晚新生代构造活动强烈,晚新生代黄河在本区发育,其后期演化记录了青藏高原隆升扩展的详细信息,同时黄河侵蚀下切过程本身也是值得深入研究的重要科学问题。由于黄河水系的发育,晚更新世以后循化-贵德盆地地区实现由盆地加积向退积的凋整,盆地地区逐渐开始遭受黄河水系的侵蚀下切,并逐渐形成现今青藏高原东北缘的地貌形态。野外地质调查发现更新统的变形程度较弱,由于区域构造隆升与河流强烈下切的共同作用,现今保存的更新统已经成为盆地内部的分水岭,如龙羊峡地区。本研究正是选取循化-贵德盆地及其邻区更新统地层为古地貌重建的标志,基于数字高程模型(DEM)空间分析技术,构建了青藏高原东北缘循化-贵德盆地地区更新世古地貌形态,并进行了初步分析,主要认识有:1)秦岭北缘断裂带构成其南西向北东方向地形快速降低的边界带;2)在北西南东方向上,西秦岭、黄河、拉脊山、湟水河以及祁连山等总体上构成了向形-背形相间的地貌格局。同时以古地形为基础,定量计算了盆地区更新世以来的侵蚀分布图像,定量结果表明:1)剥蚀量的分布形态与高原东北缘盆山地貌系统之间有一定相关耦合性,盆地地区的剥蚀量比较大,而相邻山脉地区的剥蚀量都比较小;2)剥蚀量比较大的盆地地区剥蚀量与盆地内部河流形态之间也具有明显的关联特征,盆地内部剥蚀量最大的区域往往是盆地内部独立河流的中游地区。

Reconstruction of paleo-landforms of long time scale has been a key issue for geomorphologists. The present landforms of the Earth are only part of remnant topography, which resulted from the competition between tectonics and surface processes. Therefore, these remnants have preserved the information about the geological evolution history,and further are reliable source for advanced geological analysis. Reconstruction of original landforms from limited topographic remnant markers, however, is especially difficult, and increasing studies began to challenge this geomorphological problem. We first summarizes some recent techniques utilized for paleosurface reconstruction and then discussed the limitation of each technique. All of the markers for reconstruction are relatively young strata; most of them are Late Cenozoic and Quaternary. Further these markers are not deformed or the deformation is relatively little and they are typically fluvial sediments. Recent studies around Xunhua - Guide Basin along the margin of north Eastern Tibetan Plateau indicates this region has experienced intense tectonic deformation during Late Cenozoic, and the introduction and consequent evolution of the Yellow River also occurred during this time. All of these events provide ideal laboratory to study the upward and outward growth of the plateau and incision processes of the Yellow River. Due to the formation and evolution of the Yellow River,transition from aggradation to degradation occurred during Late Pleistocene around the Xunhua- Guide Basin,and erosion and incision processed then dominated within the basin and finally constructed the present landforms along north Eastern Tibetan Plateau. Field investigations indicate that deformation within the Pleistocene strata is very weak. And because of the regional uplift and associated erosion, the remnant Pleistocene strata are already part of catchment divide between the basins,as seen around the Longyanxia gorge area. Based on this observation,we take the remnant Pleistocene strata within Xunhua - Guide Basin and adjacent region as the markers for reconstructing the paleosurface. By applying the spatial analysis techniques on Digital Elevation Model （DEM）, the Pleistocene landform around Xunhua - Guide Basin was reconstructed. Preliminary analysis indicates that （ 1 ） the North West Qinling fault zone is a topographic boundary between southwest plateau and northeast of the basin-range area, and （2）topography across West Qinling,Yellow River,Lajishan range,Huangshuihe River and South Qilianshan range is characteristic of coexistence of anticlinal and synclinal landforms. The paleodrainage networks of the Huangshuihe River have been extracted to determine the tectonic influence on the drainage evolution. Comparison of those drainage networks indicates that the river valley has been deflected to the south by 10 -26kin after Pleistocene. This might be indicative for the tilt of the channel due to the uplift of the Qilianshan to the north. Besides the above paleosurface reconstruction,we also try to quantify the post Pleistocene erosion distribution within the study area. Finally we found that （1）erosion magnitude is coupled with the typically large-scaled basin and range landforms. Erosion with the basins is more intense than that along the adjacent mountain ranges and the erosion is usually around 1000 - 1500m above sea level. （2）erosion distribution within the basins is also coupled with drainage network features, as the maximum erosion is usually near the middle stream of the Yellow River and its individual tributaries, such as along the Longwu River within the Tongren Basin, Huangshuihe River in the Xi＇ning- Minhe Basin. Considering all the previous geological and geomorphic studies together, we conclude that the topography around the Xunhua - Guide Basin area is in pre-steady state,and the erosion signal therefore has not been propagated into the upstream area, and the erosion processes of all the river channels are dominated along the middle streams.