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Drainage network dynamics in an accretionary wedge: an experimental approach

Abstract

Erosion rates in mountain ranges control the flux of clastic sediments transported by the drainage network and deposited in foreland basins. Changes in clastic sedimentary flux are classically interpreted as variations of erosion rates driven by Tectonics and/or climate. Using an experimental approach, we tested the interactions between deformation, rainfall rate and the intrinsic dynamics of a geomorphological system defined by an accretionary prism submitted to erosion and sedimentation. We show that rising structures in the frontal part of a prism may deviate channels leading to the formation of longitudinal reaches (parallel to the main structures) later incorporated passively in the prism. Ongoing shortening leads to the formation of transverse channels that incise the external slopes of uplifting thrust units. Experiments show that headward erosion in these transverse channels can result in the capture of perched longitudinal channels previously transported in the prism interior. Such captures induce a sudden lowering of the base level in the captured channels, which results in an increase of the downstream slopes at the capture point, i.e., in the formation of a knickpoint that later propagates upstream in the drainage network. A capture also induces an increase of the drainage area and discharge of the captor channel. The increase of drainage area and the wave of erosion associated to knickpoint migration result in a substantial increase of the sediment supply at the outlet of a transverse channel. Our experimental results confirm the view that early longitudinal-dominated networks are progressively replaced by transverse-dominated rivers during mountain building (Babault et al., 2012; 2013). They also suggest that not only Tectonics and/or climate but also the intrinsic dynamics of the drainage network can modulate the clastic sedimentary flux that fills the foreland basins.