Inversion of a Lower Cretaceous extensional basin, south central Pyrenees, Spain

The structure of the Boixols thrust sheet is dominated by kilometric-scale E-W trending folds, the most important of which is the Sant Corneli-Boixols-Nargo fault-propagation, growth anticline at the leading edge of the thrust sheet. Five megasequences have been identified within the Boixols thrust sheet and these may be related to phases of extension, tectonic quiescence and phases of inversion. The Boixols thrust sheet incorporates the Lower Cretaceous Organyá basin which is bounded by a north-dipping extensional fault that underwent inversion in the Late Campanian-Maastrichtian and was incorporated into the Pyrenean south-vergent thrust system during the Eocene-Oligocene. The inversion history is recorded by the syn-inversion Arén growth sequence that is only preserved in the footwall syncline of the main fault-propagation growth fold. During inversion a short-cut thrust fault developed in the footwall to the Lower Cretaceous extensional fault. This now forms the Boixols thrust at surface. An inversion model for the Boixols thrust sheet and the Organyá basin is proposed and compared to analogue and numerical models of half-graben inversion. This model for inversion in the Boixols thrust sheet may be applied to other thrust structures in the Pyrenees and to other foreland fold and thrust belts.