Monsoon-driven incision and exhumation of the Eastern Tibetan Plateau

The formation and uplift history of the Tibetan Plateau, driven by the India-Eurasia collision, is the subject of intense research. We analyse the link between climate and tectonics in the central and eastern Tibetan Plateau using geomorphic indices of surface roughness (SR) hypsometric integral (HI) and elevation-relief ratio (ZR) and mean annual precipitation, thermochronology and erosion rate data. Geomorphic indices capture the landscape response to competition between climate and tectonics and reflect the spatial distribution of erosion. This is a region where competing tectonic models suggest either early Cenozoic plateau growth, or a late phase of crustal thickening, surface uplift and plateau growth driven by lower crustal flow (“channel flow”). Swath profiles of rainfall, elevation and the geomorphic indices were constructed, orthogonal to the internal drainage boundary. Each profile was analysed to find the location of maximum change in trend. We identify a broad ˜WSW-ENE trending transition in the landscape where changes in landscape and precipitation are grouped and in alignment. It represents, from east to west, a sharp decline in precipitation (interpreted as the western extent of the East Asian monsoon), a change to a low relief landscape at 4500-5000 m elevation, an increase in ZR and a transition to low HI and SR. This zone cuts across structural boundaries and is not a drainage divide: the main rivers have their headwaters further West, in the interior of the plateau. We argue that this geomorphic-climatic transition zone represents a change from incised to non-incised landscapes, the location of which is controlled by the western extent of the monsoon. Modern erosion rates are lower in the non-incised region, west of the monsoon extent (mean 0.02 mm/yr), than the incised region (mean 0.26 mm/yr). Compiled thermochronology data shows an increase in exhumation from ˜25 Ma in the incised area but no evidence of this increased exhumation in the non-incised area. This pattern supports a model of early Cenozoic growth of the eastern Tibetan Plateau, superimposed by incision driven by Miocene monsoon intensification. Our results do not support the channel flow model, which would predict an eastwards wave of surface uplift and therefore erosion and exhumation during the Miocene, which are not present in the data.