雅鲁藏布江大拐弯峡谷的地貌特征和成因

雅鲁藏布江下游作发夹形的奇特大拐弯,并构成深峻的峡谷。本文根据实地考察的资料,阐明大峡弯的地貌特征,并结合地质构造的分析,认为包括大峡弯在内的雅鲁藏布江是适应不同方向断裂构造发育的先成河。大峡弯不是河流的袭夺弯,雅鲁藏布江曾经由东向西流的看法是没有根据的。

THE TOPOGRAPHIC FEATURES AND THE ORIGIN OF THE GREAT BEND VALLEY AT THE YARLUNGZANGBO RIVER IN TIBET

In the lower reaches of the yarlungzangbo river there exhibits a marvellous Hairpin-like Great Bend Gorge with valley in valley. The channel of the Great-Bend valley cut down into the rock flows through many interlocksed epurs. It is obvious that the Bend is a beautiful incised meander. The width of the channel is only about 70-80 meters.The diagram of the longitudinal profile of the river bed is convex upward with the gradient break like a flight of stairs. The slope of the channel and the velocity of flow can reach locally 44.9‰ and 16 meters per second. The canyon terraces along the valley are distributed in odd pieces. Owing to the high altitude and precipitous valley slopes and strong physical weathering, the rock debris on the slopes moved down quikly.These topographic features of the Great Bend valley above-mentioned reflect controls exerted by geological structure system, which shows the great intensity of the new tectonic movement. A violent earthquake (8.5 grade), occurring in Cayu on August 15th, 1950, had a great influence on the geomorphology of Great Bend valley.As regards the origin of the Great bend valley, we can classify the lower reaches of the Yarlungzangbo river into two parts. One is above the Parlungzango river. The valley is developed in a huge arcuate flalt zone with a big angle, where the northward drifting of the Indian Plate was thrusted under the Euasian Plote. And below it, the valley adapts itself to a group of north-south to east-north trending fault zone. The Great Bend valley is an antecedent gorge developed along the tectonic fractional zone of different directions. It is erroneous to consider that the Yarlungzangbo river flowed westwards in the geologicol times and the Great Bend was an elbow of capture, as postulated by some authors.