Relationship between basement-linked and gravity-driven fault systems in the UKCS salt basins

Abstract

Extension of basement below significant detachment layers such as salt results in the formation of structures in the cover strata above the detachment layer in two ways. ‘Thick-skinned’ cover structures evolve to balance basement stretching. These are linked to the basement faults via detachment in the salt layer, or are directly linked to the basement structure, depending on the amount of basement fault displacement relative to salt layer thickness. Basement extension also results in rotation of major basement fault blocks, tilting the detachment layer and precipitating gravity-driven, ‘thin-skinned’ slip of the cover involving detached extensional faulting and folding. Detachment layers consisting specifically of salt play two roles in facilitating the development of thin-skinned systems: salt constitutes the detachment layer and also lends buoyancy to the cores of anticlinal folds, reducing resistance to shortening. The detachment layer may become tilted away from the basement fault scarp, resulting in ‘synthetic slip systems’ with the same polarity as the underlying basement fault, or the detachment may be tilted toward the basement scarp, giving ‘antithetic slip systems’, of the opposite polarity. In both cases the slides may terminate downdip in detachment folds and/or reverse faults. Both types of thin-skinned system will interact with those cover structures directly balancing basement extension and all of these components must be considered during section restoration. Tilt associated with 3D basement fault geometry (e.g. relay zones) may result in decoupled cover faults locally trending at high angles to the trend of underlying basement faults. Contrasting strain rates of basement faulting and related cover slip can lead to continuation of cover structural evolution after cessation of basement extension, giving for example cover faults and folds of Lower Cretaceous age in the Central North Sea, though basement extension had occurred in the Upper Jurassic. Synthetic systems may include footwall-derived olistostromes emplaced above basement hangingwalls—these commonly occur above basement relay ramps on segmented basin margins. Examples from the Central Graben and Channel Basin are discussed. Examples of antithetic systems from the Irish Sea, Central Graben and a field example from the Bristol Channel Basin are discussed.