Assessment of the tectonic role of the Triassic evaporites in the North Toulon fold-thrust belt

Abstract

The eastern Beausset Syncline and Toulon Belt, in the southern France, represents the easternmost remnant of the Pyrenean Apto-Albian Rift System and of the Pyrenean-Provençal orogen (Late Cretaceous-Eocene). Detailed structural and stratigraphic field mapping as well as the integration of published structural and stratigraphic data, are used to reconstruct the Jurassic to Late Cretaceous tectonic evolution of this area. A layered evaporite sequence, composed of a succession of evaporitic units interbedded with more competent lithologies, behaved as the main decoupling horizon and source of diapiric bodies. Structural and lithostratigraphic observations in the Mont Caumes area are interpreted as halokinetic in origin (wedges, flaps, welds, thrust welds, highly localized depocentes). These were controlled by the sinuous Mont Caumes salt wall that grew along the southern flank of the eastern Beausset Syncline and interacted with regional tectonic stresses from Early Jurassic to latest Santonian times. Jurassic and Lower Cretaceous carbonate units thin toward the Mont Caumes salt wall, recording deposition in salt controlled broad synclinal depocentres controlled by early salt mobilization. Inverted relics of Apto-Albian rift depocenters are aligned along the northern margin of the Toulon Belt and the adjacent Bandol Belt to the west. In the Turonian-Coniacian Revest depocenter, stratal thickness variations, progressive unconformities lateral depocentre, and the westward increase in stratal, overturning of a flap on the basin’s southern margin all record localized strong asymmetrical growth of the 3D Mont Caumes salt wall. During Pyrenean-Provençal N-S convergence starting in Early Campanian, the salt wall was squeezed and reactivated as a thrust weld. The upper part of the flap was sheared and thrust north over the Beausset Syncline (Mont Caumes imbricate). Further to the west, the Saint-Cyr salt extrusion associated with the Santonian-Muschelkalk unconformity, indicates that the Bandol salt wall extruded at the same time as the main diapiric activity of the Mont Caumes salt wall. Compressional reactivation further extruded the salt body leading to emplacement of the Beausset Klippe onto the Beausset Syncline. The Toulon salt structures can be correlated with other examples of contractional salt structures in the external Alps and Pyrenees.