Along-strike evolution of folding, stretching and breaching of supra-salt strata in the Plataforma Burgalesa extensional forced fold system (northern Spain)

The Plataforma Burgalesa is a partly exposed extensional forced fold system with an intermediate salt layer, which has developed along the southern portion of the Basque-Cantabrian Basin from Malm to Early Cretaceous as part of the Bay of Biscay-Pyrenean rift system. Relationships between syn- and pre-rift strata of the supra-salt cover sequence and distribution of intra-cover second-order faults are observed both along seismic sections and at the surface. These relationships indicate an along-strike variability of the extensional structural style. After a short period of salt mobilization and forced folding, high slip rates in the central portion of the major basement faults have rapidly promoted brittle behaviour of the salt layer, preventing further salt mobilization and facilitating the propagation of the fault across the salt layer. In contrast, at the tip regions of basement faults, slower slip rates have facilitated ductile salt behaviour, ensuring its further evaporite evacuation, preventing fault propagation across the salt layer and, in essence, allowing for a long-living forced folding process. Our results indicate the important effect of along-strike variation in displacement and displacement rates in controlling evaporite behaviour in extensional basins. Amount of displacement and displacement rates are key factors controlling the propagation of basement faults across evaporite layers. In addition, growth strata patterns are recognized as a powerful tool for constraining the up-dip propagation history of basement faults in extensional fault-related fold systems with intermediate decollement levels.