Salt tectonics in salt-bearing rift basins: Progradational loading vs extension

Confined salt flow in narrow (<100 km) and deep (ca. 10 km) rift basins with an initially thick (>3 km) layer of evaporites, results in a complex structural style consisting of vertical and closely-spaced diapirs surrounded by thick (6-10 km) and narrow minibasins with intricate internal patterns recording salt withdrawal. These basins are usually explained as the result of thick-skinned extension, while other important factors such as sediment loading and subsalt basin configuration are not commonly addressed. We use sandbox models, 3D reconstruction of the models' final stage, and structural restoration to evaluate the impact of sediment loading, thick-skinned extension, and different basin configurations on the suprasalt structural style and the spatial and temporal distribution of salt structures in these basins. Models simulating sediment loading display expulsion rollovers and younger salt structures in the progradation direction. In contrast, thick-skinned extension models display stacked depocenters near basin-boundary faults and opposite timing of diapir growth. Along strike changes in basin configuration and subsalt faults impact salt flow and salt structures as follows: (1) Faults perpendicularly facing the sediment progradation contribute to the formation of salt diapirs, (2) Oblique faults favour salt flow along strike, (3) Fault intersections cause salt inflation and salt diapirs, and (4) Basin confinement by graben narrowing or intrabasinal faults produces along-strike differences in the timing of salt mobilization. The sandbox models show close resemblance and are directly comparable to seismic sections through the Nordkapp Basin (Norwegian Barents Sea) and other salt-bearing rift basins with similar configuration.