Regional distribution and geometry of salt diapirs and supra-Zechstein Group faults in the western and central North Sea

Abstract

Regional mapping of major faults above the Zechstein Group salt in the western and central parts of the North Sea basin has shown that there exist at least three discrete salt tectonic zones. On the platform west of the Central Graben there is Zone 1, an area of extensional faulting above the salt, roughly parallel to the margin of the Central Graben, c. 60 km wide and continuous along strike for more than 180 km. Within Zone 1, extension (c. 6%) has created arcuate and en echelon listric growth faults, which sole within the salt layer beneath and bound blocks typically 3–7 km wide, 2–3 km thick and 7–10 km long. Zone 2 occurs in the Central Graben area, where there is considerably less faulting above the Zechstein Group salt, and a much greater thickness of salt. Tall and narrow salt diapirs occur exlusively in the three deepest parts of the Central Graben and some penetrate high into the Cenozoic section. Between Zone 1 and Zone 2 there is a third area, Zone 3, characterised by having relatively few supra-Zechstein faults, and no major salt diapirism. Zone 1 formed mainly due to gravity-driven break-up of the supra-Zechstein sedimentary wedge. Diapirism in Zone 2 resulted from increased relative bouyancy of salt at greater depths and a relatively greater supply of salt. Zone 3 represents a relatively stable region, with possibly some movement of salt and supra-salt sediments down the regional slope.