The influence of salt layers on cover faulting during thick-skinned extensional faulting is examined using seismic examples from the salt basins offshore UK. The ratio of salt layer thickness to basement fault displacement is a key geometric parameter governing spatial offset between cover and basement fault segments. The influence of geological factors such as stratigraphic variation in salt thickness, basement fault zone geometry and basement fault growth through time on this parameter are individually examined. 3D models are constructed to illustrate the spectra of possible fault geometries which result from variations in these factors. Lateral stratigraphic variations within individual ‘salt’ layers are also considered, as are complications introduced by the addition of further salt layers into cover stratigraphy. Since diapiric intrusions occur after cover fault geometries are established and are localized by such faulting, an understanding of basement-cover fault relationships also illuminates salt diapir-basement fault relationships. An example from the Central North Sea diapir province is subject to such a genetic analysis, relating the diapir and its location to an underlying basement fault. The influence of salt on cover fault reactivation during basin inversion is discussed, focusin on the southern North Sea, where inhibition of reverse fault propagation from basement to cover is noted. It is suggested that analysis of local basement-cover relationships should form a first step in any attempt at a genetic classification of salt bodies.
Scaled analogue models provide a powerful tool for investigating progressive deformation. By varying experimental parameters it is possible to produce a wide variety of diapir morphologies and overburden responses.This paper describes the results of one of a series of experiments which investigates the geometry of overburden sediments around diapirs. The roles of basement faulting and differential sediment loading in the growth of diapirs are assessed. The template for this experiment is provided by analysis of a regional seismic line across the Central Graben of the North Sea.The model produced two diapir types which formed in different ways. Type 1 diapirs are initiated by a large fault smearing the salt along the fault plane which creates a path for later upward migration of salt to produce small diapiric walls. Type 2 diapirs can be triggered by either tectonic or sedimentary differential loading. They are predicted to be longer lived and more voluminous than Type 1 diapirs and will tend to form isolated circular structures.
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