South Pyrenean fold and thrust belt: The role of foreland evaporitic levels in thrust geometry

Abstract

Combined surface and subsurface data were used to determine the geometry of the foreland fold and thrust belt in the southeastern Pyrenees. In the south Pyrenean foreland basin, three evaporitic horizons were deposited: the Beuda, the Cardona and the Barbastro Formations. These evaporites range from Lutetian to Lower Oligocene in age, and their depocentres shifted successively to the SW in front of the advancing Pyrenean thrust sheets. From Late Eocene to Late Oligocene, a salt-basal foreland fold and thrust belt developed over a detachment level with a staircase geometry controlled by the arrangement of the evaporitic basins. Synchronously, the central Pyrenean thrust sheets were displaced southwards over the same detachment level. The arrangement of the evaporitic basins, mainly the Cardona basin, the N-S transport direction of the south Pyrenean thrust sheets, and the change in trend of the South Pyrenean main thrust (SPMT), were all responsible for the different trend of structures in the foreland fold and thrust belt. The limits of these folded regions coincide with the boundaries of the Cardona evaporitic basin. The southernmost outcropping frontal structure of the Pyrenees is a backthrust related to the edge of the Cardona massive salt level and is located in some places 40 km southwards of the SPMT. Well-constrained timing of deformation permits the interpretation that the shortening occurred from Upper Eocene times and has minimum values of between 21 km and 25 km. These values represent more than 15% of the 147 km of shortening deduced for the overall Pyrenean chain.