Large-scale slope failure involving Triassic and Middle Miocene salt and shale in the Gulf of Cadiz (Atlantic Iberian Margin)

Sheets of salt and ductile shale advancing beyond the thrust front of the Gibraltar Arc (Iberian-Moroccan Atlantic continental margin) triggered downslope movements of huge allochthonous masses. These allochthons represent the Cadiz Nappe, which detached from the Gibraltar Arc along low-angle normal faults and migrated downslope from the Iberian and Moroccan continental margins towards the Atlantic Ocean. Extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large-scale westward mass wasting from the shelf and upper slope. Low-angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding, thrusting and sheets of salt and shale in the Gulf of Cadiz. From backstripping analyses carried out on the depocentres of the growth-fault-related basins on the shelf, we infer two episodes of rapid subsidence related to extensional collapses; these were from Late Tortonian to Late Messinian (200-400 m Myr(-1)) and from Early Pliocene to Late Pliocene (100-150 m Myr(-1)). The extensional events that induced salt movements also affected basement deformation and were, probably, associated with the westward advance of frontal thrusts of the Gibraltar Arc as a result of the convergence between Africa and Eurasia. The complexities of salt and/or shale tectonics in the Gulf of Cadiz result from a combination of the deformations seen at convergent and passive continental margins.