Ages and genesis of terrace flights in the middle reaches of the Y arlung Z angbo R iver, T ibetan P lateau, C hina

Field investigation has shown that the middle reaches of the Y arlung Z angbo R iver, in T ibet (known as the B hramaputra R iver in reaches downstream) has 11 terraces. The ages of these terraces were determined as follows: T 11 >2000 ka, T 10  = 1783 ka, T 9  = 1238 ka, T 8  = 683 ka, T 7  = 382 ka, T 6  = 150 ka, T 5  = 82 ka, T 4  = 67 ka, T 3  = 43 ka, T 2  = 28 ka and T 1  = 10 ka. These terraces formed during several stages of the Q uaternary when the T ibetan P lateau was uplifting, recording tectonic–climatic cycles or subcycles of 1.2, 0.4, 0.1, 0.04 and 0.02 million years duration. The tectonic–climatic cycles identified by this study are correlated with red soil and palaeosol records. The preservation of the O ligocene to E arly M iocene conglomeratic D azhuka F ormation, close to the present altitude of the main planation surface of the T ibetan P lateau and occurring only in the Y arlung Z angbo valley, suggests that this river has existed since at least 7 Ma. Discoveries of old and thick fluvial gravel layers extending to within 630 m of the valley floor show that the Y arlung Z angbo R iver was already incised by 2000 m or more below the surface of the T ibetan P lateau, that much of the incision from plateau to valley floor had been accomplished and that the plateau was already substantially uplifted prior to the Q uaternary. In addition to its importance for understanding the evolution of the T ibetan P lateau, this study documents the impact of varying rates of tectonic uplift on river development.