青藏高原东缘龙门山构造带晚第四纪构造隆升作用的河流地貌响应

摘要

基于ASTER GDEM数据,提取了青藏高原东缘龙门山地区12条基岩河道纵剖面,通过河流纵剖面形态的数学函数拟合、坡度-面积双对数函数关系以及基岩水力侵蚀模型的分析,探讨了龙门山北、中、南段不同河流水系地貌对其晚第四纪构造隆升运动的响应过程.研究表明:1)龙门山地区的河流纵剖面拟合形态中段多为对数型、指数型,南段多为指数型、直线型,北段均为对数型,表明了龙门山中段和南段的河流受构造运动的控制作用强烈,隆升较快,而北段隆升相对较慢.2)河流水力侵蚀模型中段多呈直线型和上凸型,南段均为上凸型,北段则呈直线型,表明了龙门山地区的河流水系地貌具有由SW向NE逐渐从前均衡状态向均衡状态转换的特征,指示了其构造隆升速率也由SW向NE逐渐递减.3)河流地貌的参数值表明了龙门山北段的河流地貌处于均衡状态,而龙门山中、南段的河流地貌则受构造隆升运动的影响较强;反映了青藏高原东缘向东扩展的时空格局.%The Longmen Shan(Longmen meaning Dragon's Gate,Shan meaning mountains)striking NE to SW is located in the eastern margin of Tibetan Plateau.This mountain does not only have the steepest topographic gradient in any margin around the modern-day Tibetan Plateau,but also is the most representative active fault showing strong activity characteristics of Late Quaternary along the eastern margin of Tibetan Plateau in the Bayanhar Block.In addition,it is an area in which the change of river geomorphology has a sensitive and direct response to tectonic activity.In recent years,the apparent seismic activities around Longmen Shan have included the Wenchuan Earthquake (Ms8.0),the Lushan Earthquake (Ms7.0),and the Jiuzhaigou Earthquake (Ms7.0),which occurred in 2008,2013,2017,respectively.Especially,the development of water system pattern is the most meaningful geological event that was accurately and completely recorded during the uplift process of Late Quaternary in the area with the most active tectonic movements.Thus,Longmen Shan area has been one of the best places to study the relationships among tectonic,geomorphy and river system.Based on the ASTER GDEM data,this study extracts 12 longitudinal profiles of bedrock river channels,including Yaque River (R1),Linguan River (R2),Chujiang River (R3),Minjiang River (R4),Jianjiang River (R5),Jinhe River (R6),Mianyuan River (R7),Anchang River (R8),Tongkou River (R9),Pingtong River (R10),Fujiang River(Rll),and Qingzhu River(R12) in Longmen Shan area by using simple mathematical functions to match the rivers' longitudinal profiles,and tries to analyze the geomorphological features of rivers system (including S-A double logarithmic curve,values of the concavity index (θ) and steepness index log (ks)),which responded to uplift process in different segments of Longmen Shan.Firstly,the result shows that there are three different types of the fitting results for the longitudinal profiles of rivers in Longmen Shan.The first one is logarithmic function which is fitted to the longitudinal profile of 8 rivers(R3,R5,R6,and R8～ R12),the second one is exponential function which is fitted to the longitudinal profile of 3 rivers(R1,R2,and R7),and the third one is linear function which is matched with the longitudinal profile of 1 river(R4).It indicates that there is a high uplift rate in Longmen Shan area,which has a strong river erosion rate in this area.Secondly,the double logarithmic curves (LogS-LogA figures,the slope of bedrock channels is S and the catchment area is A) of these rivers fall into three types:The convex line (up convex),the notching line (down convex) and the straight line.The double logarithmic curves of the rivers developed in the south segment (R 1 ～ R4)and middle segment(R5～ R8)of Longmen Shan are up convex line and straight line,the rivers in the north segment (R9 ～ R 12) showing straights line.The results indicate that the south segment and middle segment of Longmen Shan have a higher uplift rate than the north segment.In addition,the values of the concavity index(θ) and steepness index(log(ks))are affected by many factors,such as watershed area,glacial melt water,differential uplifting tectonic movement and so on.In total,the value of concavity index(θ) in the north segment(R9～ R12)of Longmen Shan is about ＞0.40,and the value of steepness index(log(ks)) is 0.97～ 1.37,less than the value of steepness index(log(ks)),which is 1.23 ～ 2.40 in the middle segment(R5 ～ R8)and south segment(R1～ R4)of Longmen Shan.It shows that the topography relief is with equilibrium stage in the north segment of Longmen Shan,and the value of the steepness index(log(ks)) increases gradually from north to south,indicating the tectonic uplift rate gradually becomes strong.So it is not only a response of tectonic geomorphology to active tectonics of Longmen Shan in the Late Quaternary,but also is reflected to the difference of uplift process of Longmen Shan in the eastern margin of Tibetan Plateau.