Using interplay of sub- and supra salt structures for an extensive formation model of the Central Graben, North Sea

Abstract

This research aims to develop a more complete model of how the Central Graben, which is located in the North Sea, formed as a result of overprinting of Paleozoic and Mesozoic tectonics. The first interpretations of Paleozoic stratigraphy and structures under the CG are shown. The Central graben is a passive rift, meaning extension took place through mechanical stretching rather than volcanic activity driving the extension. The earliest structures influencing the formation of the Central Graben formed during the Paleozoic and are likely related to Caledonian structures. Thus it is important to incorporate the Paleozoic grain in the subsequent tectonic events. The most important phases of rifting forming the CG occurred in the Mesozoic. Cretaceous and Cenozoic inversion phases caused reactivation of older faults. During all these different phases of deformation halokenesis of Zechstein salts also had a major effect on the formation of the Central Graben as it is observed today. Most reconstructions or models do not account for the structures located below the Zechstein evaporites, as the thick sequence of salt makes imaging of these structures difficult. However, recent data acquisitions have made these structures more clear, allowing for a more accurate interpretation of the deeper structures. These sub-salt structures are kinematically linked with structures observed in the supra-salt sediments. They are thus of importance when studying the basin development. This research shows the first interpretations on the Paleozoic units and structures under the CG. This research may aid in the further understanding of the effects of pre-existing crustal structures on the evolution of salt-influenced rifts. An application of this study is that earlier unrecognized hydrocarbon migration paths or trapping structures could be identified with these findings. Existing burial graphs, extension velocity models or calculated sedimentation rates may be reviewed with the results of this research in future work. This research finds that large basement faults within the Paleozoic units and the detachment between Avalonia and Baltica have been of major importance for the formation of the Central Graben. After the Caledonian Orogeny finished, the accretionary complex collapsed, separating Avalonia and Baltica. The old suture between these 2 paleocontinents is currently recognized as the Thor Suture Zone. As the detachment weakened the crust strain localisation occurred, reactivating basement faults during later tectonic phases. This shows the it is fundamental to study basement structures when studying basin formation. Salt layers prevented the faults from propagating into the supra-salt cover. The CG is thus an asymmetric soft-linked half-graben.