THRUST GEOMETRY CONTROLLED BY EROSION AND SEDIMENTATION - A VIEW FROMANALOG MODELS

Analogue sandbox models have been set up to study thrust systems forme d above two decollements and affected by erosion and sedimentation. Th e deformation was imaged by X-ray scanner. Three erosion-sedimentation conditions mere tested. For all the experiments, a duplex develops be tween the two decollement levels. For the experiment without erosion a nd without sedimentation, ramp anticlines form in the upper series. Fo r the model with syntectonic sedimentation and without erosion, the sh ortening in the upper series is first characterized by fault related f olds and then by a forward propagation of the decollement that induces the development of a piggy-back basin. For the experiment with erosio n and sedimentation, a ''passive roof duplex'' is formed. Relationship s between structural style and syntectonic superficial conditions in c lassical thin-skinned thrust belts are similar to those found in the a nalogue models: the ramp anticlines of the Jura developed with low ero sion and low sedimentation and the targe sub-Andean thrust belt is con trolled 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.