Revisiting the Lusitanian Basin: Exposed Jurassic Passive Diapirs and Halokinetic Sequences at the Western Iberian Atlantic Margin

The Lusitanian Basin is a Mesozoic salt-floored extensional basin located at the western Iberian Atlantic margin. It exposes several seismic-scale diapirs, which have arisen from a thick Rhaetian to Hettangian salt layer. In the literature, these diapirs are interpreted as Jurassic salt pillows (i.e. subcircular upwellings of salt having a concordant overburden), which pierced the overburden during Cenozoic shortening related to Alpine deformation. However, the postulated scenario is incompatible with recent insights into the deformational processes associated with salt tectonics.

Based on field research and reassessment of published seismic-based cross-sections, we propose a new model for the salt tectonic evolution of the Lusitanian Basin. We identified distinct Jurassic halokinetic sequences on exposed diapir flanks, which suggest a protracted phase of passive diapirism (i.e. syndepositional growth of a diapir whose exposed crest rises while sediments accumulated around it). This phase of passive diapirism was not identified in previous studies, and forms the base of our hypothesis.

We suggest salt movement initiated shortly after the deposition of an Early Jurassic overburden, localised above active, basement-involved, extensional faults. Simultaneously, overburden draped over the footwalls of these faults formed forced folds and developed extensional faults at the hinges of these folds. The thin-skinned faults affecting the overburden were restricted to areas where folding was orthogonal to the direction of extension and allowed the salt to pierce thinned overburden and trigger the development of passive diapirs with associated halokinetic sequences. Deposition of clastic sedimentary sequences during the Cretaceous buried these passive diapirs. In areas where folding was sub-parallel to the extension direction, salt did not pierce the overburden but developed salt anticlines instead.

Shortening related to subsequent Alpine contractional deformation reactivated both the diapirs and the salt anticlines. Most diapirs were squeezed and became narrower during the Alpine event but the associated halokinetic sequences were largely unaffected by this deformation. In some cases, however, the diapirs were weld shut and the salt walls formed near-vertical thrust-welds, which now connect the remaining diapirs. In contrast, reactivation of the salt anticlines during the Alpine event generated new faults sub-parallel to their axes, and further folding. Several of the anticlines were eroded, forming new diapirs. In conclusion, our new model suggests that passive salt diapirs the Lusitanian Basin existed since the Early Jurassic, and thus during almost the entire evolution of the basin. Today’s polygonal network of salt diapirs and anticlines is inherited from the original basin geometry.

Understanding the salt tectonic evolution of the Lusitanian Basin helps constrain petroleum system models for the basin. The timing and nature of salt diapirism impacts on the distribution of source rocks, reservoirs, seals, structural traps and fluid migration pathways, as well as on the thermal maturity of surrounding rocks. Moreover, the Lusitanian Basin provides the best onshore structural analogue for potentially equivalent salt-influenced offshore hydrocarbon provinces, such as the Peniche or Alentejo basins.