长江三峡河段下切速率研究

INCISION RATE OF THE THREE GORGES SECTION OF THE CHANGJIANG RIVER

长江三峡河段的阶地主要分布在宽谷河段和长江支流河口部位。文章通过对阶地堆积物沉积相的分析和TL测年,计算出了三峡河段不同地点的下切速率:重庆河段为84.56 cm/ka,涪陵河段为93.9 cm/ka,丰都河段为69.8 cm/ka,忠县河段为77.65 cm/ka,万州河段为75.1 cm/ka,奉节河段为83.8 cm/ka,宜昌三斗坪河段为74.2 cm/ka。整个三峡河段的平均下切速率为81.2 cm/ka。分析得出:1)三峡河段不同地点的下切速率相近;2)近0.4M a以来,长江三峡河段的下切速率有越来越快的趋势,这表明该地区在此期间发生了构造隆升而引起河流下切加剧;3)长江三峡河段的下切速率与金沙江下段相近。

River terraces in the Three Gorges section of Changjiang River mainly occur at either the estuary of tributaries or the wide part of the valley. Based on the analyses of sedimentary facies and sedimentary texture of the fiver terraces in the Three Gorges section, the incision rates are estimated during a specific period by taking account of the relative altitude of those river terraces. The height difference between the sampling position on the river terrace and the position of modern sediments of the same sedimentary facies at the same site is used as the incision depth in this study. Terraces with later deformation, such as landslide, are excluded from the analysis. The incision time is obtained from 17 samples at 8 sites between Chongqing and Yichang which are used for TL datingmeasured at the Thermoluminescence Laboratory, Institute of Crustal Dynamics, China Earthquake Administration. The obtained TL ages, ranging from 127kaB. P. to 7kaB. P., are basically in accord with the stratigraphic sequence. In addition, ^14C ages obtained previously are also used for calculating the incision rates. The result shows that the incision rates are 84.56cm/ka in Chongqing, 93.9cm/ka in Fuling, 69.8cm/ka in Fengdu, 77.65cm/ka in Zhongxian, 75. 1cm/ka in Wanzhou, 83.8cm/ka in Fengjie, and 74.2cm/ka in Yichang. The average incision rate is 81.2cm/ka. Consequently, the following conclusions are drawn. （1） The incision rates at different sites in the Three Gorges section are close to each other. （2） The incision rate of Changjiang River in the Three Gorges section tends to be higher during the past 0. 4Ma, which is possibly caused by the new tectonic uplift and the climate change. （3） The incision rate is also close to that of the lower reaches of Jinshajiang River, implying that the new tectonic uplift in these two regions is consistent.