Analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation. The deformation was imaged by X-ray scanner. Three erosion-sedimentation conditions were tested. For all the experiments, a duplex develops between the two decollement levels. For the experiment without erosion and without sedimentation, ramp anticlines form in the upper series. For the model with syntectonic sedimentation and without erosion, the shortening in the upper series is first characterized by fault-related folds and then by a forward propagation of the decollement that induces the development of a piggy-back basin. For the experiment with erosion and sedimentation, a “passive roof duplex” is formed. Relationships between structural style and syntectonic superficial conditions in classical thin-skinned thrust belts are similar to those found in the analogue models: the ramp anticlines of the Jura developed with low erosion and low sedimentation and the large sub-Andean thrust belt is controlled by syntectonic sedimentation, whereas the passive roof duplex of Pakistan is synchronized with intense erosion. This study outlines the crucial effect of the superficial mass-transport phenomena in the tectonic style of thrust belts.

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