Passive diapirism in the west Iberian Atlantic margin: examples from the Lusitanian Basin

The Lusitanian Basin is a Mesozoic salt-floored basin located at the west Iberian Atlantic margin. Subsidence initiated during Triassic times with the development of basement-involved extensional faults related to the opening of the Atlantic Ocean. The faults were then covered by Rhaetian to Hettangian salt. Subsequent Jurassic and Cretaceous deformation within the detached overburden was largely decoupled from the basement and was influenced by the rising of salt structures, particularly during an Oxfordian to Kimmeridgian rifting peak. Prior studies view the Jurassic and Cretaceous salt structures as salt pillows, which only pierced the topographic surface during later shortening related to mainly Cenozoic Alpine deformation. This interpretation is largely incompatible with current studies of deformational processes related to salt movement. In this example, an examination of published cross-sections reveal clear halokinetic geometries on the Jurassic strata flanking the diapirs, which suggest a protracted phase of passive diapirism not previously recognised. Moreover, the postulated piercing of diapirs during Cenozoic shortening is thought very unlikely, unless the anticlines were deeply eroded, or pre-existing diapirs were re-activated.

In this presentation, we propose a new model for the evolution of salt tectonics in the Lusitanian Basin. We hypothesise that salt movement initiated shortly after the deposition of the overburden, and localised above the underlying basement-involved extensional faults. Salt flowed from the structural highs towards the structural lows and the overburden was draped over the footwalls of the faults, resulting in forced folds. Salt kinematics were primarily controlled by the orientation of the basement-involved faults, relative to the regional NW-SE direction extension, and to some extent by the thickness of both the salt and the overburden. In areas with a structural trend orthogonal to the direction of extension, thin-skinned extensional faults formed at the hinges of the forced folds, and triggered the development of reactive diapirs. Later in the Jurassic, salt pierced the thinned overburden and formed passive diapirs with associated halokinetic sequences and megaflaps. Later burial of the salt diapirs by clastic sedimentation occurred in the Cretaceous. In areas with a structural trend sub-parallel to the direction of extension, salt did not break through the overburden, and salt anticlines with shallow dipping limbs developed.

The structural style during subsequent Alpine deformation was strongly influenced by the original geometry of the salt structures. The passive diapirs were squeezed, and some salt walls formed near-vertical thrust-welds, which now constitute linear fault zones connecting the remaining diapirs. In contrast, the shortening of the salt anticlines generated new faults, generally oblique to the thrust-welds and related diapirs. Accordingly, the current map-view pattern of these structures reflects a polygonal network of salt diapirs and anticlines inherited from the pre-existing basin. In conclusion, our new model suggests that passive salt diapirs existed during almost the entire evolution and infill of the Lusitanian Basin, not only since the Late Jurassic, providing an excellent structural analogue for the evolution of salt structures in the west Iberian Atlantic margin.