Influence of differential compaction above basement steps on salt tectonics in the Ligurian-Provençal Basin, northwest Mediterranean

Abstract

Detailed mapping of the Ligurian-Provençal Basin (northwestern Mediterranean) indicates that salt diapirs in the deep basin are restricted to an area whose upslope boundary forms reentrants located above deep crustal transfer zones associated with the opening of the basin in Oligo–Miocene times. Because these basement faults were no longer active in Messinian and post-Messinian times, the geographic correlation between diapirs and basement faults cannot be attributed to slip along these basement faults coeval with salt tectonics. Using three physical experiments, we examine how dormant (i.e. inactive) basement steps can affect the development of the overlying salt structures during combined gravity-driven gliding and spreading. Where a basement step trends obliquely with respect to the direction of the slope and initially offsets the base salt, grabens and salt ridges form above and downdip from the basement step, in turn forming a reentrant pointing upslope. Where the basement step is buried under pre-Messinian compactable sediments, loading by Messinian and post-Messinian sediments causes differential subsidence above the step, forcing grabens and salt ridges to form above and updip of the basement step. There, too, the salt structures are confined in a triangular, reentrant-shaped area pointing upslope. The combination of these two mechanisms with passive diapir growth during Plio-Pleistocene times explains the striking geographic correlation between salt diapirs and basement structures in the Gulf of Lion.