A planation hypothesis is proposed to explain landform evolution of the Tibet Plateau.A denudation threshold(T),the maximum potential denudation rate for a certain type of rock,is introduced to explain the combined effects of lithology and tectonics on landform evolution.If the tectonic uplifting rate(U)is equal to or less than the threshold rate(U≤T),the tectonic uplifting and terrain denudation are in dynamic equilibrium,and landforms are in a steady state.The end product should be planation surfaces whether the original landforms are flat plains or deeply dissected mountains.If U>T,uplift and denudation are not able to reach a dynamic equilibrium state.The plateau surface is mostly underlain by soft rocks,such as the Mesozoic epimetamorphic argillites and Tertiary sedimentary rocks,while the mountain ranges comprise hard rocks,such as granite,gneiss and limestone.In soft rock regions,hills are low with a relative relief of mostly less than 100m and the slopes are gentle at a gradient of <20o.In contrast,hills can maintain steep slopes in hard rock regions.The Tibet Plateau has been under an equilibrium condition between tectonic uplifting and denudation except for the mountain ranges.The plateau might have reached the present altitudes before the Quaternary.
