PATTERNS OF EROSION IN THE THREE RIVERS AREA, EASTERN TIBET

Since the early Cenozoic, India has been moving northward into and underneath Eurasia at a rate of 36 to 40 mm/yr. This convergence of buoyant crustal material necessarily results in high rates of uplift and substantial thickening of the crust. One unusual feature of the collision is the asymmetry of the Plateau. The Tibetan Plateau decreases in elevation from the southwest to the northeast, and the western margin of the Plateau does not have the extreme relief that is seen on the eastern margin. On the eastern margin of the plateau, in the Three Rivers Region (TRR), the rivers flow within 30 to 50 kilometers of each other for nearly 300 kilometers. At the narrowest, the distance from the Salween to the Yangtze is only 100 kilometers, with two 3,000-meter mountain ranges and the Mekong River in between. Nowhere else on Earth do three rivers of this size flow so close or so parallel to one another.

We opportunistically collected over 50 sand samples from catchments of varying size and relief throughout the TRR during two field trips in 2005 and 2006. To date we have measured 10Be erosion rates for 22 of these basins. The Mekong basins have erosion rates between 0.08 and 0.2 mm-yr-1 and average 10-km scale mean local relief of between 900 and 1600 m. The Salween basins have erosion rates between 0.07 and 0.4 mm/yr for average basin relief between 1150 and 2000 m. The Yangtze basins, with the exception of one high relief basin (1900 m) eroding at 0.1 mm/yr, are eroding the slowest with erosion rates between 0.004 and 0.05 mm/yr for basins with average relief of 600 to 1800 m. These data plot below the classic Ahnert Relation (i.e., R = 0.2E, where R is local relief in km and E is erosion in mm/yr), suggesting that, despite predictions of extremely high rates of erosion based on the unit stream power, the region is eroding at rates below those expected for tectonically inactive regions.