Abstract

The Witch Ground Graben is a NW-SE trending extensional basin in the central part of the northern North Sea, lying between the Moray Firth and the Viking Graben. NE-SW extension occurred from Triassic to Cretaceous times. Extension took place on listric normal faults along the SW margin, with downthrow to the NE, giving rise to the asymmetry of the basin. One of the principal phases of fault-controlled subsidence occurred in the Lower Cretaceous, from the Valanginian to Albian. Compilation of biostratigraphic data from this interval shows the development of an increasing number of stratigraphic breaks across the basin from SW to NE, these breaks increasing in time span also in this direction. This is a result of rotation of the subsiding fault block on the listric extensional faults. The rotation is recorded in dipmeter data, and occurred in several distinct episodes—the Triassic, the Volgian and the Valanginian to Aptian. In reconstructing a cross-section of the basin, the interpreted fault curvature is constrained by the observed rotation of bedding surfaces, and the amount of extension across the listric faults is constrained by the observed basin subsidence. A depth to décollement for the faults of 4 km below sea level and an extension across the basin of 40% are estimated.

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