ABSTRACT

After three decades of research and hydrocarbon exploration in the Nordkapp Basin (Norwegian Barents Sea), the dynamics of Mesozoic salt mobilization is still poorly understood. Both progradational loading and basement-involved extension have been proposed as triggers of salt mobilization, where the latter is most accepted. This study combines two-dimensional and three-dimensional seismic reflection data, borehole data, isochore maps, and structural restorations to (1) provide a tectonostratigraphic evolution of the Nordkapp Basin, (2) indicate which triggering mechanisms fit the observed structural styles, and (3) determine the geological controls that influenced the along-strike distribution of salt structures in the basin. Our results indicate that a combination of Early–Middle Triassic thick-skinned extension and sediment loading induced the differential loading and mobilization of the underlying salt, generating a series of northwest-shifting minibasins bounded by salt walls, ridges, and stocks. Sediment loading and the distribution of salt structures were strongly conditioned by rheology variations within the salt layer and subsalt fault activity, which (1) created tectonically induced depressions that became preferential areas of infill and differential loading; (2) caused faulting and extension of the overburden, allowing the preferential growth of reactive diapirs, which later on evolved into passive diapirs; and (3) acted as effective barriers of salt expulsion, enhancing salt inflation and growth of salt above the subsalt faults. Early Triassic differential loading occurred diachronically along strike, causing early passive diapirism, salt welding, and salt depletion in the eastern and central subbasins because of the diachronous subsalt activity and the closer proximity of these basins with respect to the sediment source, the Uralides. Although most of the salt was depleted by the end of the Middle Triassic, the ongoing extension created across-fault thickness variations and sagging of some of the west-northwest–east-southeast salt walls in the central subbasin. The rest of the structures in the Nordkapp Basin continued growing until the end of the Mesozoic by minor evacuation of the remaining salt and thin-skinned gliding and subsequent shortening triggered by subsalt fault activity. Finally, salt structures were rejuvenated and eroded during Cenozoic contraction and uplift. These results have implications for the four-dimensional understanding of the Nordkapp Basin and its petroleum system, and they can be used as an analog to decipher other confined salt-bearing basins alike.

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