Impact of backstop thickness lateral variations on the tectonic architecture of orogens: insights from sandbox analogue modeling and application to the Pyrenees

We performed sandbox analogue models to investigate the influence of along strike tapering of the backstop on the three-dimensional architecture of natural doubly vergent thrust-fold belts, applying the results to a natural example as the Pyrenees. Available geological and geophysical data indicate that the European crust, that provided the backstop in the Pyrenean orogen, has a westward tapering on both the upper (westward thickening of the Mesozoic cover rocks) and the lower (westward rising of the Moho) envelope surfaces. Accordingly, we performed laboratory models characterised by backstops with an along strike tapering of either the upper or the lower surface. Our results indicate a significant impact of backstop lateral tapering in thrust wedge architecture in terms of the outward propagation of the deformation fronts, the width and elevation of the axial zone, and the timing of thrust polarity reversal along strike. When compared to the Pyrenees, our results support the inferred westward tapering of the European basement and its commonly accepted backstop role in the Pyrenean orogen. Moreover, comparison between the map pattern of the Pyrenees and of laboratory experiments suggests that the along strike tapering of the bottom envelope surface of the backstop played a most important role on the three-dimensional thrust wedge architecture than the tapering of the upper envelope surface.