在构造和气候因素制约下雅鲁藏布江的演化

雅鲁藏布江位于印度和欧亚大陆汇聚带内，其形成受到冈底期山和喜马拉雅山差异性抬升的控制。冈底期山抬升在先，发生在中生代晚期至新生代早期。一系列起源于冈底期山和青藏高原的水系向南先是流主特提斯海。在特提斯海关闭后流入印度次大陆。喜马拉雅山构造抬升要晚于冈底斯山，大规模抬升发生在中新世早期，其抬升阻断了这些河流的通道，水流开始汇聚在这两个造山带之间，牙鲁藏布江由此形成。在雅鲁藏布江大拐弯地区，在海拔4500m处存在一个平坦的侵蚀面，并构成雅鲁藏布江大峡谷最高的一级谷肩，这表明雅鲁藏布江在下切前就在该面上流动，而且流速不大。在大拐弯以南，雅鲁藏布江的下游－布拉马普特拉河位于印度洋热带季风带内，其下切和源侵蚀速率很大。印度洋热带季风形成于6－9MaB.P.。因此，该河流很可能形成于该时期，要比雅鲁藏布江年轻，它在向北的溯源侵蚀的过程中袭夺了雅鲁藏布江，袭夺处可能就大拐弯的北端，因此大拐弯是袭夺成因。

TECTONIC AND CLIMATIC ELEMENT-CONTRLLED EVOLUTION OF THE YALUNGZANGBU RIVER IN SOUTHERN TIBET

The Yalungzangbu River runs from the west to the east along the trace of the Yalungzangbu suture zone in southern Tibet, flanked by the Himalayan range on the south and Gangdese range on the north. It turns a corner at the core area of the eastern syntaxis of the Himalayas. Further towards the south, the Brahmaputra River, its lower reaches, flows perpendicularly through the Himalayas into the Bay of Bengal. The Gangdese range formed as a magmatic arc and marked the southern edge of the Tibetan Plateau prior to the uplift of the Himalayas, across which streams, derived from the Tibetan Plateau to the north, flow southward directly into the Indian subcontinent. As the convergence between the Indian and Eurasian continents proceed in early Miocene time, the Himalayas began to uplift because of the occurrence of south-directed thrusting. Consequently, all streams derived from the Gangdese range began to be enclosed between the Himalayan and Gangdese ranges, and thus the rudimentary Yalungzangbu River has appeared. The Indian monsoon initiated in late Miocene time, caused by the uplift of the Himalayas, and the increased precipitation has accelerated the headward erosion of the Brahmaputra River. By the accelerated head erosion, the Brahmaputra River captured the Yalungzangbu River from the south at the core of the northeastern syntaxis of the Himalayas. Originally, the Yalungzangbu River might have run farther to the east Tibet into the Three River region.