Experimental and field study of the effects of lithological contrasts on thrust-related deformation

The influence of mechanical contrasts related to stratigraphy on thrust-related deformation are investigated by sandbox modeling using combinations of granular materials possessing different mechanical properties and by structural analysis of a natural example from the Spanish Pyrenees. The models used in this study consist of loose sand and glass microbeads, in varied stratigraphic thicknesses and sequences, and the natural example consists of a weakening-upward sequence of sandstones and carbonates. In both cases, layers of different composition had a mechanical significance and accommodated shortening differently. Model sand and natural sandstone exhibited most shortening by thrust imbrication irrespective of stratigraphic position, whereas glass microbeads and carbonates exhibited wider deformation zones and enhanced folding, accompanied by displacement reduction along thrust surfaces cutting them. Transitions of structural style were guided by the stratigraphy rather than by a simple unidirectional propagation from a step up. Layer-parallel shortening was accommodated by diverse mechanisms but did not vary significantly in layers which had different properties. The magnitude with which mechanical contrasts were expressed in the models was influenced by the strength at the décollement zone. The effect of mechanical contrast was more profound in models shortened above a low-friction décollement.