ACCRETION CYCLES IN DUCTILE WEDGES WITH A FRONTAL RAMP OBSERVED IN SANDBOX MODELS

The localization and accumulation of strain in scaled physical experiments of non-Coulomb wedges is analyzed applying a Particle Image Velocimetry (PIV) technique. Model consists of a basement represented by a mixture of quartz and corindon sand, a weak and confined viscous décollement horizon made of SGM36 silicone, and a relatively competent brittle cover sequence of dry quartz sand. We maintain constant the layer thickness and material properties, and the factors under investigation are the effects of the brittle/ductile mechanical coupling in the evolution of deformation, and the influence of pre-existing high strength contrasts (ramps) in the structural styles. We have varied the experimental strain rate and the presence of one or two basement ramps confining the ductile layer. Syntectonic sedimentation on the surface is included in one model. Photographs for image analysis were recorded at one of the lateral plexiglas walls. Because the models are affected by similar friction boundary conditions, their comparison resulted useful to individuate the first order differences due to setup changes. Our results confirm previous observations stating that folding and faulting are different responses of the same idealized material to different loading conditions and properties. Shortening deformation occurs in cycles characterized by forward advance and fold amplification and/or thrust growth. Strain rate also influence the structures formed. In the case of higher strain rate, folding predominates while for lower values deformation propagated forward from the ramp forming diapirs in the direction of shortening. Finally, the results may be applied to understand the salt tectonics deformation that occurred in the Gulf of Mexico (GoM) and other orogens. The GoM basin is remarked by the presence of a thick salt layer that pinch out towards deep water. Above the pinch out zone the structural style of a chain of foldbelts is best exemplified by the low strain rate models.