关键词: 构造地貌, 隐伏构造, 盆地, 全新世构造, 水系密度

Abstract: In order to identify the Holocene activity of the buried tectonic structures in the down-faulted Weihe basin. Northwest China (Fig. 1) , the authors conducted a semi-quantitative morphotectonic study by using a special method of morphometric analysis of the drainage density on topographic maps (1: 50000) of the whole basin, created by themselves, in combination with interpretation of satellite imagery and field investigation.This method of morphometric analysis involves 1. By using the grid network on the large-scale topographic map to measure the value of drainage (rivulet. stream and river) length in per unit area such as 9 ((from 3 ? 3) . 25 (from 5 ?5) or 100 (from 10 ? 10) sq. km, which is the drainage length density. and the value of drainage area (length ? width of rivulet, stream and river) in per unit area , which is the drainage area density. 2. To use these values as two kinds of point values, to plot them respectively on two small-scale topographic maps (1 : 50000) . then to link them by isolines respectively on these two maps, and eventually to obtain two kinds of specific maps. One of them is called the map of drainage length density (Fig. 2) and the other is called the map of drainage area density (Fig. 3) 3. To analysis the geomorphic expressions of the Holocene activity of buried tectonic structures in the form of uplifting, subsiding, block-faulting, tilting etc. , according to the figuration and patterns of those two kinds of isolines. 4. To identify the activation mode. pattern and intensity of the buried tectonic structures in the recent Holocen time by the combination of analysing the geophysical. geological and geodatic data, interpreting satellite imagery and aerophotograph, and field checking.The method of morphometric analysis of drainage density is based on the following principle: In the subsiding region, especially during its early stage of subsidence while the intensity of subsidence being equal to or less than that of deposition. water currents will flow slowly. causing the growth of tributories and the formation of well developed drainage network. That will result in the increase of total length of drainage in per unit of area, which is the value of drainage length density. In the subsiding region during its middle or late stage while the intensity of subsidence is large than that of deposition, river courses will become much wider and will develop meandring. That will lead to an increase of the total drainage area in per unit area, namely the value of drainage area density. but the drainage length density may be somewhat reduced.In the uplifting region, especially during tis early stage while the intensity of uplift is larger than that of river downcutting. the stream and river will rapidly deepen and narrow their courses.some of them will decline and even disappear. That will result in formation of a scattered drainage network. Therefore. in this case the drainage length density will significantly decline. However, in the region with uplifting during its middle or late stage while the intensity of uplift is equal to or less than that of river downcutting, a new gully and rivulet network will develop on the margin around an uplifting block due to their backward recessive erosion. That will lead to the formation of some radial and more densely drainage pattern with many narrow and downcutting rivulets. In such cases, the drainage length density may become much higher. producing a false image of tectonic subsiding. But such an image can be precluded by analysing the map of drainage area density (Fig. 3) . because in this region all the river courses in the uplifting area are very narrow, taking up only a rather smaller area and hence the value of drainage area density dn the map is small. Thus, in order to clarify the true status of tectonic activity in such a region. it is very important to compare the above -mentioned two kinds of maps.The results obtained by this study are well supported by the data of drilling holes and repeated geodatic surveying as well as the isopath amp of Quate

Key words: Testonic landform, Buried tectonics, Holocene tectonics, Drainage density, Morphometric analysis