The internal dynamics of foreland basins

Abstract

Foreland basins form by flexure of the lithosphere under the weight of an adjacent mountain belt. They record the evolution of the mountain topography that is continuously set by the balance between tectonically-driven uplift and climatically-modulated erosion. This record is, however, affected by a range of autogenic processes that affect the growth of the topography in the mountain area and the efficiency and patterns of sediment transport and deposition in the basin. It remains an unresolved question to assess the relative contributions of both external (forcing) and internal (autogenic) processes. In particular, it remains notably difficult to extract periodic climatic signals, such as those at astronomically-tuned MIlankovitch periods, from the sedimentary record because it is also affected by perturbations caused by random or quasi-periodic internal processes.  Here we use a landscape evolution model coupled to a flexural isostatic model to quantify the efficiency of autogenic processes in ''shredding'' the sedimentary record. The landscape evolution model assumes that sediment transport is the result of a balance between erosion and deposition, and therefore allows for a smooth transition between sediment production in the mountain and sediment deposition in the sedimentary basin, while allowing for sediment by-pass out of the system along its base level. The model assumes that water and sediment are passed from each node to all of its neighbours in proportion to their relative slopes leading to the formation of a constantly and rapidly evolving multi-threaded channel system. Of particular interest to us are sedimentary waves that form at the surface of the foreland basin and that appear to be associated with avulsions of the main channels transporting sediment from the mountain across the basin. These waves, in turn, control the relative position of the mountain base level when they reach the boundary between the basin and the orogen, and may cause perturbations in mountain topography and drainage patterns. We use the model to determine the parameters controlling the amplitude and frequency of these waves, and whether they are amplified by flexural isostasy. We also infer the optimum conditions under which they are most likely to affect the sedimentary record.  In addition to the shape of the surface topography and the path of water flow, the model predicts the patterns of sediment deposition/erosion, the basin stratigraphy and the distribution of grain size. This allows us to compare model predictions to natural examples and validate our findings.