PROCESSES OF DENUDATION FROM CATCHMENT TO CONTINENT

Appraising erosion processes relevant to continental scale and to geological times is a key issue before tackling the interplay between tectonic and geomorphic processes. However despite the numerous data and models, the erosion processes are only vaguely known, and the factors that are supposed to control the long-term erosion law are still debated. Climatic variability makes difficult the definition of long-term trends in both sediment fluxes and controlling hydraulic (discharge) and structural (slope) factors; and relevant data at geological time and length scales (sediment accumulations, "cosmogenic" erosion rates, etc...) are still sparse compared to the spatial variability of erosion rates.

Interpreting topography in terms of erosion dynamics is a way to tackle this problem. It is not a trivial issue because of the large diversity of (non-linear) erosion processes and rates that entail a complex organization of relief and of drainage paths. Moreover the boundary conditions (initial topography, tectonic rates and erodibility distribution) of system dynamics are often poorly constrained. The guess is that topography has reached some equilibrium stages, and that these stages are interpretable in terms of erosion dynamics.

We illustrate some elements of this dynamics with a stochastic simulator that models erosion processes from diffusive terms operating on hillslopes to non-linear advective fluvial processes. We first discuss the most basic parameters that should be incorporated in erosion and transport laws. We then calculate the upscaled erosion law by integrating local processes over space and time. We show that the large-scale erosion law has the characteristics of an abnormal diffusion whose basic time-length exponent is related to parameters of the fluvial transport law. Results are found consistent with the observed negative correlation between current denudation rates and drainage areas in world-wide fluvial watersheds. We then discuss the response of geomorphic systems to some exemplary tectonic deformations, and the parameters that control the dynamics with a special emphasis on resulting equilibrium stages, characteristic time and length scales.