Erosion in the Chilean Andes between 27°S and 39°S: Tectonic, climatic and geomorphic control

The effect of mean precipitation rate on erosion is debated. Three hypotheses may explain why the current erosion rate and runoff may be spatially uncorrelated: (1) the topography has reached a steady state for which the erosion rate pattern is determined by the uplift rate pattern; (2) the erosion rate only depends weakly on runoff; or (3) the studied catchments are experiencing different transient adjustments to uplift or to climate variations. In the Chilean Andes, between 27°S and 39°S, the mean annual runoff rates increase southwards from 0.01 to 2.6 m a^-1 but the catchment averaged rates of decadal erosion (suspended sediment) and millennial erosion (¹⁰Be in river sand) peak at c. 0.25 mm a^-1 for runoff c. 0.5 m a^-1 and then decrease while runoff keeps increasing. Erosion rates increase non-linearly with the slope and weakly with the square root of the runoff. However, sediments trapped in the subduction trench suggest a correlation between the current runoff pattern and erosion over millions of years. The third hypothesis above may explain these different erosion rate patterns; the patterns seem consistent with, although not limited to, a model where the relief and erosion rate have first increased and then decreased in response to a period of uplift, at rates controlled by the mean precipitation rate.