The effects of salt tectonics in the evolution of a fold and thrust belt, south-west Alps

Abstract

The sub-Alpine chains of south-east France have been the subject of geological inquiry for over a century. While some salt structures have previously been identified in the area, our understanding of salt tectonics has since matured significantly, driven by observations from seismic data in offshore basins worldwide and by analogue modelling. Armed with this new understanding, this study identifies new salt structures in the sub-Alpine chains, and reinterprets structures previously attributed to Alpine shortening as pre-Alpine salt-related structures. The area near the Dôme de Barrot highlights the influence of Permian to Triassic pre-salt faults on the deposition of the Triassic salt section, and how the reactivation of the Permian structural fabric locally controlled the evolution of Jurassic to Cretaceous diapirs. Along the Rouaine-Daluis strike-slip fault system, multiple periods of salt tectonic rise have now been identified, including the extrusion of a salt sheet to surface in the Aptian-Albian, and the development of a Roho-like listric normal fault associated with salt evacuation into the active fault zone during early Alpine shortening. The evolution of the salt wall and diapir along the Rouaine-Daluis fault system is analogous to that of the Svend structure of the South Arne field in the Danish Central Graben, and the two areas represent different endmembers for salt wall-diapir pairs affected by varying magnitudes and orientations of shortening. Salt structures in the sub-Alpine chains are strongly associated with Tethyan (Early-Mid Jurassic) rifting and with the palaeotopography of the Dauphinois (Jurassic) and Vocontian (Cretaceous) basins. They strongly influenced the regional structural fabric with the pre-existing salt structures utilized as thrusts and strike-slip faults as Alpine shortening emplaced the basinal units over the carbonate platform. The importance of salt in the structural evolution of the area may therefore necessitate reassessment of previous regional shortening estimates.