Inverted turtle salt anticlines in the Eastern Basque-Cantabrian basin, Spain

Halokinetic structures are sometimes masked by subsequent compression in inverted salt-related basins making their study difficult. The Eastern Basque-Cantabrian Basin is an outstanding area to study salt tectonics and sedimentation relationships because precursor salt structures have maintained undeformed geometries after the Alpine contraction phase. Based on the interpretation of subsurface data with surface geology, a structural model of some of the area's salt structures is proposed. According to the model, the formation of listric faults and raft tectonics during the first extensional rifting stage resulted in the sinking and collapse of the Late Jurassic-Early Cretaceous formations, giving rise to a turtle-back anticline structure with a semicircular geometry. The Upper Triassic Keuper salt migrated towards the outer zones of the structure and peripherial troughs were subsequently filled with Albian-Late Cretaceous post-rift sediments. During the Eocene Alpine contraction, the whole area was folded and moved southwards, maintaining much of the geometry of the previously formed salt anticlinal structures. Traditionally, the presence of an NNE-SSW alignment of Triassic diapirs in the eastern zone of the basin has been used to place the southern segment of the so-called Pamplona fault. Rather than being associated with fault, these diapirs are interpreted to rise along the eastern edge of the collapsed extensional turtle structure existing below the Eocene Urbasa-Andia synclines. No evidence of fault affecting the Mesozoic cover series is observed in several composite profiles that cross the Pamplona fault alignment. It is suggested that the alignement of the Triassic diapirs is due to the incidence of the later Alpine compression in a previous expulsion rollover, rather than the effect of a major, Alpine strike-slip fault.