Steep topographic slope preservation by anisotropic diffusion: An example from the Neogene Têt fault scarp, eastern Pyrenees
Résumé
Mapping active faults based on large-scale landscape analysis might suffer from large uncertainties because rocks on both sides of an ancient fault trace may respond differently to erosion, producing a contrasting relief which can be misleadingly interpreted as due to active faulting. Identifying the bedrock response to erosion is thus of crucial interest to address the question of seismic hazard, especially in slow deforming areas where evidences of recent fault motions are scarce. Hillslope erosion models based on the diffusion equation predict progressive slope decay with time, suggesting that steep slopes along a fault trace indicate recent tectonic activity. However, this law is generally unable to take into account neither lateral nor vertical variations in the erodibility of the bedrock, such as those resulting from the existence of a rock planar fabric (RPF). Field evidence of structural surfaces indicates that steep topographic slopes can be preserved from erosion over the long term, suggesting that hillslope erosion might become less efficient when the rock planar fabric is favorably oriented with respect to the escarpment slope. We present a 2D model accounting for possible diffusion anisotropy due to pre-existing RPF, which is used to determine in which conditions high topographic slopes can be preserved. This model is then used to reproduce the last 500 ka evolution of a part of the Têt normal-fault escarpment in the Eastern Pyrenees. We have collected 42 points on a fault-perpendicular profile using a portable GPS device, with a resolution compatible with the numerical model. We show that present-day fault activity is not needed to explain its good state of preservation, which can be reproduced by considering the anisotropic response to hillslope erosion of hard, north-dipping mylonites which constitute the underlying bedrock.