Influence of variable lithology on landscape evolution

Landscapes evolve as a result of drivers (e.g. tectonics and climate) that provide the energy for geomorphic work and the resistance of the Earth's surface to that work. Lithology, a major factor in resisting erosion, is spatially variable on many scales across the Earth's surface. Even in relatively 'homogeneous' landscapes, variability in mineral content and joint spacing give rise to spatial variation in rock erodibility. In this study we quantify the significance of lithologic variability in the evolution of mountain topography using a well-constrained geologic setting and numerical modeling. To address this problem, we focus on the eastern Jura Mountains in northern Switzerland. Intense geologic study has produced detailed 3D lithologic maps from a combination of surface mapping at a 1:25000 scale and hundreds of boreholes in a region of only a few 10's of km2. Lithologies range from relatively soft shale to hard calcite-dolomites units providing a strong spatial contrast in erodibility. This well documented lithologic heterogeneity makes this region a viable natural experiment to explore the implications of lithologic variability on landscape evolution. The CHILD landscape evolution model is used after modification to allow for spatially variable rock erodibility. We choose a minimum and maximum fluvial erodibility that scales linearly with rock tensile strength to assign each mapped unit a value. Modern topography is scaled to have a total relief of 5 m. This scaled-down DEM is used as the initial topographic condition. Simulations assume that the beds extend vertically from the mapped geometry. We explore the sensitivity of modeled results to the assumed minimum and maximum erodibility values. The range (difference between maximum and minimum values) is varied from ½ to 2 orders of magnitude but mean erodibility is held constant. Next, the sensitivity of these different scenarios is explored for different tectonic and climate environments to quantify the relative roles of tectonics, climate, and lithology on the evolved landscape. Results are quantified in terms of changes in hillslope angle, river steepness indices, relief, drainage density, and time to steady-state. All results are compared with a baseline scenario of uniform lithology as well as modern topography. We find that spatial variations in modern topography are strongly correlated with lithology. Reproducing such variations in the modeled landscape requires an ~1 order of magnitude variation in rock erodibility between stratigraphic units. Both bed thickness and the range of erodibilities are important parameters in controlling the impacts of lithologic heterogeneity on landscape evolution. The model results are useful in understanding the spatial and temporal variations in landscape processes in lithologically complex terrain.