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 scarprelated 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.

Salt thickness and composition influence rift structural style, northern North Sea, offshore Norway

“…Well 17/12–1R, which is located 15 km to the east of 17/12–2 and in the hangingwall of the Sele High Fault System, penetrates the upper 100 m of a 450 ms TWT ( c . 1,013 m) thick salt pillow (Figure b; see also Jackson & Lewis, ). The ZSG in the well is composed predominantly of halite (69%), although the upper 27 m is dominated by anhydrite and claystone; by assuming the salt pillow below the termination of the well is halite‐dominated, we tentatively place 17/12–1R in DZ4 (Figure ).…”

“…Seismic data thus suggest a first‐order positive relationship between the present thickness and mobility of the ZSG (e.g. thick ZSG is mobile; thin ZSG is immobile; Jackson & Lewis, ). Furthermore, basement‐involved normal faults appear to exert a primary control on the ZSG thickness, with the unit being thinnest on basin margin or intra‐basin, fault‐bound structural highs (e.g.…”

“…Compared to the UK sector, almost nothing is known about basin‐scale compositional variations in the ZSG in the Norwegian sector of the North Sea Basin (Jackson & Stewart, ). Jackson and Lewis () use 3D seismic and reflection data from the Sele High Fault System, eastern Sele High, to document rapid across‐fault variations in salt thickness and composition, demonstrating the footwall apex of this large‐displacement fault system (>2 km) is capped by relatively thin (58 m), largely immobile carbonate and claystone, whereas relatively thick (>200 m) and mobile halite occurs in the adjacent hangingwall. The study of Jackson and Lewis () covers only a relatively small area ( c .…”

“…Jackson and Lewis () use 3D seismic and reflection data from the Sele High Fault System, eastern Sele High, to document rapid across‐fault variations in salt thickness and composition, demonstrating the footwall apex of this large‐displacement fault system (>2 km) is capped by relatively thin (58 m), largely immobile carbonate and claystone, whereas relatively thick (>200 m) and mobile halite occurs in the adjacent hangingwall. The study of Jackson and Lewis () covers only a relatively small area ( c . 3,600 km 2 ) however and to date there has been no systematic regional study of basin‐scale compositional variability in the ZSG.…”

Balancing sub- and supra-salt strain in salt-influenced rifts: Implications for extension estimates

“…(ii) a lack of borehole data that cannot demonstrate the spatial variations in salt rheology and 386 lithology, and their impact on the distribution and evolution of supra-salt structures, or (iii) a lack 387 of hard-linkage between sub-and supra-salt faults, hence there are no growth packages to constrain 388 sub-salt fault activity (Jackson and Lewis, 2016). Given that we interpret the fold overlying the 389 sub-salt faults in Figure 5c and Figure 10a-b as a forced fold (sensu Stearns, 1978), and observe 390 progressive onlapping by Upper Jurassic -Lower Cretaceous strata, we suggest that the sub-salt 391 faults were active during the Late Jurassic -Early Cretaceous.…”