How a stationary knickpoint is sustained: new insights into the formation of the deep Yarlung Tsangpo Gorge

Abstract:

Researches on south-east Tibet suggested that Yarlung Tsangpo Gorge was resulted either from glacial dam (climatic control) or differential rock uplift (tectonic control). Most of these available studies agreed that this huge deepest gorge on Earth is a stationary knickpoint. Although different hypotheses were proposed to explain its origin, little attempt focused on the critical condition to sustain its stability in a theoretical and quantitative way.

Here we derived a universal formula to express the upstream migration velocity of knickpoints via stream incision model. This formula was suitable for many conditions, for example, it allowed spatially varied channel uplift rates , linear or non-linear correlation between erosion and channel gradient. According to the formula and two primary viewpoints, a stationary knickpoint occured where incision rates of gorge matched the upstream deposition rates (glacial dam) or the difference of basement uplift rates between the upstream and downstream reaches of the knickpoint (tectonic control).

We calculated the rock exhumation rates of the Yarlung Tsangpo Gorge via published thermochronological age and one-dimension steady-state thermal diffusion equation, the channel steepness from digital elevation model, and the erodibility by dividing exhumation rates by channel steepness. Combining these parameters, we obtained river incision rates of the gorge. We found the incision rates were much larger than the upstream deposition rates but showing good accordance with the difference of basement uplift rates between the upstream and downstream reaches of the knickpoint. Our results shown here supported the conclusion that the stationary knickpoint on the Yarlung Tsangpo Gorge was resulted from tectonic control rather than glacial dam. Therefore, we speculated that the Yarlung Tsangpo Gorge was controlled by tectonics more than climate. Our finding thus theoretically and quantitatively outlined how this stationary huge knickpoint was sustained along the Yarlung Tsangpo river, where tectonics and climate are interacted and modulate the landscape and dynamic evolution near eastern Himalayan.