THE ROLE OF BACKSTOP THICKNESS LATERAL VARIATIONS ON THE TECTONIC ARCHITECTURE OF OROGENS: AN EXPERIMENTAL APPROACH AND COMPARISON WITH THE PYRENEES

The geometry of the backstop (i.e. the region of the upper plate that is significantly stronger than adjacent regions in the lower plate) represents a major parameter controlling the evolution of accretionary orogens. To analyse the impact of the backstop geometry on the along-strike structural and sedimentary variability characterising thrust wedges, three-dimensional sandbox with an along strike tapering of the backstop (either in the upper or lower surface) are compared with the crustal architecture of the Pyrenees.

The commonly accepted geodynamic setting for the Pyrenean orogen consists of a delaminated Iberian lower crust underthrusted below the European crust, that behaves as the backstop of the orogenetic system. Available geological and geophysical data indicate that the European crust thins westward, with a progressive tapering on both the upper (westward thickening of the Mesozoic cover rocks located over the Upper Triassic detachment level) and the lower (westward rising of the Moho) envelope surfaces.

Our results indicate a significant impact of backstop lateral tapering in thrust wedges 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 crust and its commonly accepted backstop role in the Pyrenean orogen.