LANDSCAPE Evolution

Understanding the dynamic interactions between tectonics, erosion, and sedimentation in mountain belts is a difficult challenge because field morphological and structural observations only document a “snapshot” in the long geological history of the range. In addition, they usually deliver sparse datasets in both time and space that are difficult to integrate into comprehensive 4D evolution models. To access relief dynamics, experimental modeling can be used to complement fieldwork investigations. Two types of approaches can be distinguished. First, the “tectonic” approach, commonly called “sandbox modeling”, has been used for a long time to study accretionary wedge and fold-and-thrust belt dynamics. Erosion and sedimentation are mainly modeled in 2D by respectively removing material from high topographies and by sifting fresh material in basins. Second, the “geomorphic” approach is focused mainly on landscape dynamics in response to changes in tectonic, climatic or initial boundary conditions. Erosion and sedimentation are triggered by sprinkling water micro-droplets on the model surface whereas tectonics consists essentially of pure uplift. Model uplift is performed mechanically by lowering the channel outlet to decrease the river base-level or by elevating a central column of material. Model materials in both set-ups are generally granular media (such as sands, beads or powders) because their mechanical properties are suitable to simulate deformation and erosion of rocks in the upper continental crust. In tectonic modeling, materials are mainly dry quartz sand or silts, but other components are also used to weaken or strengthen the sand pack and improve monitoring techniques (garnet sand, silica powders, glass microbeads, aluminum microspheres, etc.). In geomorphic experiments, granular materials are generally sandy particles or fine powders systematically dampened with water (natural sands, sand/silt/clay mixtures, loess, or silica powders, etc.). Graphite powders are also used to model coastal stratigraphy whereas plastic powders are used to study the evolution of submarine canyon morphology.

We developed a new experimental approach to model tectonic deformation and erosion–transport–sedimentation processes in a mountain range piedmont. Because common materials for tectonic and geomorphic experiments were not suitable for this purpose, we tested various granular materials in water-saturated conditions, including glass microbeads “GM”, silica powder “SilPwd”, plastic powder “PVC” and graphite “Graph”. If used pure, GM, SilPwd, and PVC materials did not display suitable deformation styles and erosion morphology so we developed a mixture, called MatIV, made up of 40% GM, 40% SilPwd, 18% PVC, and 2% Graph (percentage in weight). This material represents an interesting compromise because it correctly satisfies the following physical criteria for erosion and deformation inspired by nature:

1. From a tectonic point of view, MatIV satisfies Mohr–Coulomb failure criterion, localizes deformation along shear zones (faults), and presents reasonably downscaled frictional properties.

2. From a morphological point of view, MatIV erodes by channelized processes (fluvial-like incision) and hillslope processes (mass landslides) that reproduce the morphogenetic evolution of natural mountains. It generates topography with morphological features like drainage basins, channel networks, crests, and terraces. Its erosional properties have been investigated in relaxation experiments and measurements of erosion fluxes allowed evaluating exponents of a stream power erosion law. The erosion parameters scale reasonably well with natural catchments, which indicates that erosion–transport processes with MatIV are comparable to natural mass transfer laws in real drainage basins.