Tectonic and Climatic Controls of Alluvial fan Geometry

Alluvial fans are useful natural laboratories for the study of the tectonic, climatic, and lithologic boundary conditions that constrain landscape form. We evaluate the importance of these boundary conditions by building on the analysis advanced by Whipple and Trayler (1996, Basin Res., 8:351-366), and thus in terms of mass conservation and a simple stream-power erosion law applied to fan-catchment systems at topographic steady state. Our new analysis links fan area with rates of uplift and precipitation, area and relief of the eroding catchment supplying debris to the fan. The explicit relationships that emerge from this analysis provide a useful rationale for summary measures of fan-catchment geometries of previously-studied and environmentally wide-ranging systems in California, Argentina, Italy and Spain. In particular, we predict and observe relationships between the ratio of rock-uplift rate and precipitation rate and, respectively, the average relief of upland, source catchments and the ratio of fan area to the product of catchment area and relief.