Structural style and evolution of a salt-influenced rift basin margin; the impact of variations in salt composition and the role of polyphase extension

Abstract

Because salt can decouple sub- and supra-salt deformation, the structural style and evolution of salt-influenced rifts differs from those developed in megoscopically homogenous and brittle crust. Our understanding of the structural style and evolution of salt-influenced rifts comes from scaled physical models, or subsurface-based studies that have utilised moderate-quality 2D seismic reflection data. Relatively few studies have used high-quality 3D seismic reflection data, constrained by borehole data, to explicitly focus on the role that along-strike displacement variations on sub-salt fault systems, or changes in salt composition and thickness, play in controlling the four-dimensional evolution of supra-salt structural styles. In this study, we use 3D seismic reflection and borehole data from the Sele High Fault System (SHFS), offshore Norway to determine how rift-related relief controlled the thickness and lithology of an Upper Permian salt-bearing layer (Zechstein Supergroup), and how the associated variations in the mechanical properties of this unit influenced the degree of coupling between sub- and supra-salt deformation during subsequent extension. Seismic and borehole data indicate that the Zechstein Supergroup is thin, carbonate-dominated and immobile at the footwall apex, but thick, halite-dominated and relatively mobile in high accommodation areas, such as near the lateral fault tips and in the immediate hangingwall of the fault system. We infer that these variations reflect bathymetric changes related to either syn-depositional (i.e. Late Permian) growth of the SHFS or underfilled, fault scarp-related relief inherited from a preceding (i.e. Early Permian) rift phase. After a period of tectonic quiescence in the Early Triassic, regional extension during the Late Triassic triggered halokinesis and growth of a fault-parallel salt wall, which was followed by mild extension in the Jurassic and forced folding of Triassic overburden above the fault systems upper tip. During the Early Cretaceous, basement-involved extension resulted in noncoaxial tilting of the footwall, and the development of an supra-salt normal fault array, which was restricted to footwall areas underlain by relatively thick mobile salt; in contrast, at the footwall apex, no deformation occurred because salt was thin and immobile. The results of our study demonstrate close coupling between tectonics, salt deposition and the style of overburden deformation for >180 Myr of the rift history. Furthermore, we show that rift basin tectono-stratigraphic models based on relatively megascopically homogeneous and brittle crust do not appropriately describe the range of structural styles that occur in salt-influenced rifts.