Jurassic to Cretaceous structural evolution and subsidence patterns of the Southern Central Graben (North Sea) – a combination of extensional tectonics and salt movement

The Southern Central Graben is a prominent evaporite-influenced rift structure in the Central European North Sea region. The evolution of the graben probably started in the Permian followed by several minor rifting phases during the Mid to Late Triassic. After a phase of regional uplift from the latest Early to Mid Jurassic, a major rifting phase occurred during the Late Jurassic and lasted until Early Cretaceous. In the Late Cretaceous, the graben was partly inverted due to tectonic compression. The southern part of the graben, located in the German and Dutch North Sea sector, is characterized by a half-graben structure bounded by a complex normal fault system at the eastern flank. The internal structure of the graben is differentiated into several sub-basins. More importantly, the Post-Permian graben fill was affected by mobilization of the Upper Permian Zechstein evaporites. Numerous salt diapirs and adjacent depocentres developed due to salt redistribution in particular during the Jurassic and Cretaceous.

Even if the overall tectonic and sedimentary evolution of the graben is well known, a detailed survey on the extensional phases and the influence of salt movement on the subsidence patterns has not yet been carried out. In this study, we intend to investigate these aspects based on interpretation of 2D and 3D seismic data and cross section restoration. With special focus on the Late Jurassic to Early Cretaceous main rift event, we aim to distinguish the effects of extensional tectonics and salt movement on the local subsidence patterns. In the study area (northern Dutch, German and southern Danish North Sea sectors), we mapped 15 stratigraphic Post-Permian horizons in total of which 9 belong to the Jurassic and Early Cretaceous. Based on the depth-converted seismic interpretations, depth and thickness maps were constructed. Subsidence analysis and 2D backstripping of the depocentres enabled us to constrain timing and spacing of the tectonic activity and the salt migration.

The outcomes of this study show that depocentres at the graben edges are mainly controlled by fault movement at the base of the salt. In contrast, structural patterns of post-Permian layers in the graben interior are often decoupled from base-salt features, i.e. faults in the base do not penetrate into supra-salt layer. Instead the post-Permian strata is gently folded, affected by thin-skinned extension, and penetrated by salt diapirs. The subsidence patterns of the Mesozoic strata were controlled by differential movement of sub-salt faults and overprinted by movement of the underlying Zechstein evaporites. We suggest that salt migration and its influence on subsidence patterns were strongest during the Late Jurassic probably accompanied by the initiation and acceleration of salt diapir growth and the lateral migration of depocentres.