河套盆地北缘大青山地区构造地貌特征

GEOMORPHOLOGICAL CHARACTERISTICS OF DAQINGSHAN DRAINAGE AREA IN THE NORTHERN MARGIN OF HETAO BASIN

位于河套盆地北缘的大青山断裂晚第四纪以来活动强烈,发生过多次破坏性地震,对现今地貌的塑造具有重要作用。目前,利用地貌因子对河套盆地大青山段流域进行区域性构造地貌形态量化分析,以揭示该区运动特征的研究仍相对缺乏。文中以该区为例,利用30m分辨率的DEM数据和GIS空间分析技术获取大青山流域水系网络和7个亚流域盆地的地形数据,定量统计各汇水盆地的面积-高程积分(HI)值并拟合其空间分布规律,通过改进的Chi-plot基岩河道分析方法--积分法,得到了大青山流域的河道纵剖面和陡峭指数的空间分布特征。结果显示:大青山流域的HI值中等,地貌发育阶段为壮年期;河道纵剖面揭示了多个河流裂点的存在,河道仍处于不均衡发育状态;河道陡峭指数在山体区呈高值分布。对比分析大青山地区的构造地貌特征与气候、岩性及构造的相关性,指示构造活动是控制大青山隆升及地貌形态发育的主要因素。

The Daqingshan Fault located in the northern margin of the Hetao Basin has experienced intensive activity since late Quaternary,which is of great significance to the molding of the present geomorphology. Since basin geomorphological factors can be used to reflect regional geomorphological type and development characteristics,the use of typical geomorphology characteristics indexes may reveal the main factors that control the formation of topography. In recent years,more successful research experience has been accumulated by using hypsometric integral( HI) values and channel steepness index( ksn) to quantitatively obtain geomorphic parameters to reveal regional tectonic uplift information. The rate of bedrock uplifting can be reflected by channel steepness index,the region with steep gradient has high rate of bedrock uplifting,while the region with slower slope has low rate of bedrock uplifting. The tectonic uplift can shape the geomorphic characteristics by changing the elevation fluctuation of mountains in study area,and then affect the hypsometric integral values distribution trend,thus,the HI value can be used to reflect the intensity of regional tectonic activity,with obvious indicating effect.Knick point can be formed by fault activity,and the information of knick point and its continuous migration to upstream can be recorded along the longitudinal profile of stream. Therefore,it is possible and feasible to obtain the information of tectonic activity from the geomorphic characteristics of Daqinshan area. The research on the quantitative analysis of regional large-scale tectonic activities in the Daqingshan area of the Yellow River in the Hetao Basin is still deficient so far. Taking this area as an example,based on the method of hypsometric integral( HI) and channel steepness index( ksn),we use the DEM data with 30 m resolution and GIS spatial analysis technology to extract the networks of drainage system and seven sub-basins. Then,we calculate the hypsometric integral( HI) values of each sub-basin and fit its spatial distribution characteristics. Finally,we obtain the values of channel steepness index and its fitting spatial distribution characteristics based on the improved Chi-plot bedrock analysis method. Combining the extraction results of geomorphic parameters with the characteristics of fault activity,we attempt to explore the characteristics of drainage system development and the response of stream profile and geomorphology to tectonic activities in the Daqingshan section of the Yellow River Basin.The results show that the values of the hypsometric integral in the Daqingshan drainage area are medium,between 0. 5 ~ 0. 6,and the Strahler curve of each tributary is S-shaped,suggesting that the geomorphological development of the Daqingshan area is in its prime,and the tectonic activity and erosion is strong. Continuous low HI value is found in the tectonic subsidence area on the hanging wall of the Daqingshan Fault. The distribution characteristics of the HI value reveal that the Daqingshan Fault controls the geomorphic difference between basin and mountain. Longitudinal profiles of the river reveal the existence of many knick points. The steepness index of river distributes in high value along the trend of mountain which lies in the tectonic uplift area on the footwall of the Daqingshan Fault. It reflects that the bedrock uplift rate of Daqingshan area is faster. The distribution characteristics of the channel steepness index show that the uplift amplitude of Daqingshan area is strong and the bedrock is rapidly uplifted,which is significantly different from the subsidence amplitude in the depression basin at the south margin of the fault,indicating that the main power source controlling the basin mountain differential movement comes from Daqingshan Fault. Based on the comparison and analysis on tectonic,lithology and climate,there is no obvious corresponding relationship between the difference of rock erosion resistance and the change of geomorphic parameters,and the precipitation has little effect on the geomorphic transformation of Daqingshan area,and its contribution to the geomorphic development is limited. Thus,we think the lithology and rainfall conditions have limited impact on the hypsometric integral,longitudinal profiles of the river and channel steepness index. Lithology maybe has some influences on the channel knick points,while tectonic activity of piedmont faults is the main controlling factor that causes the unbalanced characteristics of the longitudinal profile of the channel and plays a crucial role in the development of the channel knick points. So,tectonic activity of the Daqingshan Fault is the main factor controlling the uplift and geomorphic evolution of the Daqingshan area.