A geometrical approach to defining sub-salt and supra-salt fault coupling relationships across the Southern North Sea using fault and salt structure trend correlations

In the Southern North Sea (SNS), the Zechstein salt locally acts as a detachment horizon that decouples basement-involved deformation from gravity-driven salt tectonics, resulting in a spectrum of fault-coupling relationships. These range from fully coupled (physically linked), through partially coupled (mechanically related but physically disconnected) to fully decoupled (unlinked) sub- and supra-salt fault systems. This study attempts to define zones inside the SNS area characterized by different types of coupling through a qualitative analysis of regional sub-salt and supra-salt fault pattern correlations and salt structural trends and a quantitative analysis of faults and salt structures orientation. Using the MegaSurvey 3D seismic dataset, sub- and supra-salt fault networks were mapped through a combination of seismic attributes and automated vector tracing techniques. Sub- and supra-salt fault pattern correlations were used to identify three zones of mechanical fault coupling: zone A, where correlation is strong and mostly basement-involved salt deformation is observed, zone B, where basement-involved and gravity-driven salt deformation in the cover can be observed and zone C, where correlation is weaker and gravity-driven salt deformation dominates. Additionally, zone C’ is a subset of zone C, where we observe a visible discrepancy in the general trend of salt structures and main basement fault trends. This disparity may imply interactions between NW-SE, N-S and NE-SW basement trends or, alternatively, a northward pure shear stretching of the overburden during basin extension. The study shows that increasing salt thickness favours decoupling across the SNS area; however, in the area of major Upper Jurassic rifting in the Central Graben, fault displacement takes over the role of the dominant factor controlling the degree of sub- to supra-salt fault coupling despite the abundance of salt. • Three zones of mechanical fault coupling defined using fault pattern correlations • Salt thickness controls fault coupling where fault displacement is low to moderate • First regional-scale attempt to map fault coupling relationships using attributes • Systematic trend change between subsalt faults and salt structures observed