雅鲁藏布江水系反转问题的探讨

Preliminary Study on the Inversion of Yalungzangbo River

雅鲁藏布江63条主要支流中有24条支流与干流逆交,16条支流与干流直交,其中雅鲁藏布江中游37条支流中有12条直交支流和21条逆交支流。通过野外地质调查、遥感图像分析和年代学研究,认为藏南雅鲁藏布江存在流向反转现象,上新世以来发育的近东西向雅鲁藏布江袭夺了始新世-中新世形成的近南北向河流,形成直交水系结构。东西向雅鲁藏布江中游发育上新世至中更新世吉隆、定日、定结-岗巴等湖盆,它们向东有序迁移,上新世至早更新世湖盆发育期的构造沉降为雅鲁藏布江西流提供了地貌基础和容水空间,更新世以来的古大湖区约1 000 m高的隆升造成上述盆地由湖相转为河流相,晚更新世东构造结东侧与南侧河流的贯通和雅鲁藏布江大峡谷的形成导致雅鲁藏布江反转,这在米林-直白湖盆和孟加拉盆地沉积中有所反映。雅鲁藏布江的反转过程反映了青藏高原南部南北向和东西向的差异隆升、基岩剥蚀作用、河流切割作用的耦合关系以及板内伸展构造对板块碰撞构造的叠加和改造作用。

Among 63 anabranehes of Yalungzangbo River, 24 branches are inverse cross rivers,and 16 branches are orthogonal rivers. In central Yalungzangbo River,there are 12 orthogonal rivers and 21 inverse cross rivers in the 37 anabranehes. The reversion of this river is expounded by the field geological survey, remote sensing image analysis and chronological study. Nearly east-west Yalungzangbo River developed in the Pliocene and captured the previous north-south rivers which developed in Eocene-Miocene, thus forming the modern orthogonal structure. There are lacustrine basins, such as Gillon Basin, Dingri Basin, and Dingjie-Gamba Basin,developing eastward in order. The tectonic subsidence of these basins from Pliocene to early Pleistocene provides geomorphological possibility and water container. While the uplifting of the Pleistocene ancient great lakes to about 1 000 m turned the lacustrine basin into a fluvial facies. In late Pleistocene the run-through of the east river and the south river of eastern Himalayan syntaxis and the formation of Yalungzangbo Grand Canyon caused the inversion, which is reflected by the sediments in Milin- Zhibai Basin and Bengal Basin. The reversal process reflects the different uplifting between north and south and between east and west of the south Qinghai-Tibet Plateau,denudation of bedrock,river incision, the superposition and transformation from plate collision tectonics to intraplate extensional tectonics.