Relationship between valley morphology and thrust tectonics : insights from analogue models

Valleys parallel to the direction of the main thrusts are quite common in thrust belts such as the Alps. Analogue models were used to investigate the relationship between valley morphology and thrust evolution. Scaled 3D sandbox models were constructed using dry quartz sand in a 320 cm x 90 cm rig with lateral glass walls. Convergence was simulated by moving a mylar sheet underneath the homogeneous layered sandpack to produce a doubly-vergent thrust wedge. Model evolution was recorded using digital photography and laser scanning. The photographs were edited to measure wedge geometric parameters and displacement along thrusts and to animate the models. Laser scans were elaborated to get 3D views and the evolution of topographic profiles. As the model ran, a thrust sequence, accompanied by along strike elongated topographic depressions on the back of each thrust sheet, formed. After the wedge was well developed, synkinematic continuous erosion (5 erosional events) was introduced by deepening one of those depressions up to the original level of the sandpack, to simulate a symmetric V-shaped valley. After the first erosional event the valley was not preserved, its base was uplifted soon and its shape passed from symmetric to asymmetric till it was completely deleted; the eroded thrust sheet was well active and deformation mainly concentrated along its surface. After the third erosional event the valley was preserved but its morphology mitigated; the eroded thrust sheet was still active, but new thrusts developed in the forewedge and deformation was no longer concentrated only along the eroded one. At the end of the evolution the valley was not modified any more but fossilized and simply transported without any change; the eroded thrust sheet was still active, but a very little displacement occurred along its surface; deformation moved in the forewedge along the external thrusts. The model evolution shows the relationship between the thrust activity and the morphology of the valley incised on its back: till the displacement along the thrust is high the valley shape is quickly modified, when the deformation moved externally the valley is preserved and its shape no longer changes. This model is applied to selected areas in the Alps in order to find out the relationship between thrust chronology and associated valley shapes, trying to get a model of topography development based on tectonic evolution and erosional history of relieves.