GEOMORPHOLOGY OF THE MINJIANG DRAINAGE SYSTEM (SICHUAN, CHINA) AND ITS STRUCTURAL IMPLICATIONS
岷江水系流域地貌特征及其构造指示意义*

Compared with the remarkably flat feature of the main body of the Tibetan Plateau,geomorphology of the peripheral areas varies greatly.As an example,the eastern margin of Tibetan Plateau is more diffuse and irregular than the northern and southern ones.The Minshan Mountain along with its adjacent regions form a series of escarpments,become one of the key continental escarpments.The Minjiang drainage basin is part of this series and is adjacent to the Sichuan Basin.Knowledges of the geomorphic features of the Minjiang drainage networks and drainage sub-basins,of the stream morphology and the morphometric relation between the slope and elevation of the Minjiang drainage basin are still lacking.The recently developed digital elevation model(DEM) makes it possible to rapidly quantify the topographic characteristics of areas ranging from local drainage basins to entire continents.A DEM-based topographic analysis of mountain belts have been recently carried out along cross-sections perpendicular to the main structures of different orogens,which has shown that the Minjiang drainage basin has distinct geomorphic characteristics.Results from SRTM-DEM-determined Minjiang drainage sub-basins and channel profiles have shown that the eastern and western tributaries of the Minjiang River are obviously asymmetric.Almost all the analyzed characters(such as the areas,perimeters and total channel lengths) of the eastern sub-basins(except 32~(#),46~(#) and 58~(#)) are smaller that those of the western ones.Areas and perimeters of most the western sub-basins are more than 100km~(2) and 100km respectively and the area of 15~(#)sub-basin even reaches up to 7202km~(2) or so.The total channel lengths within many western sub-basins are above 200km.Besides the above features,the eastern streams show a little lower bifurcation ratios within 3.0~4.0;however,these parameters of the western ones are almost between 4.1~5.0.Longitudinal channel profiles of western and eastern 5~(th) order streams indicate that the lengths of the longest channels within almost all the eastern sub-basins are less than 20km,but those of western ones are normally more than 20km.The relative higher drops and gradients presented by eastern longest streams show the existence of the higher incision rate along this region.Taking it into account that the same outcropping and climate conditions control the whole Minjiang drainage basin,it can be concluded that lower perimeter and area of drainage sub-basins,total channel length,bifurcation ratio and higher channel gradient within eastern sub-basins along the Minjiang mainstream maybe result from the differential uplift of the Minshan Mountain region within Late Cenozoic.The shorter streams and lower bifurcation ratios maybe indicate the undergrowth and newborn features of these eastern streams,which are also representative for the eastern uplift of the Minshan Mountain.