The structural evolution of the northern Viking Graben and its bearing upon extensional modes of basin formation

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Abstract
Recent advances in the understanding of rift basin formation, coupled with the increasing public availability of seismic and well data across the northern Viking Graben (60–61°N), have enabled a detailed analysis of its development. Two rifting episodes can be recognized: ?Late Permian–early Triassic and Bathonian–Ryazanian. Major regional unconformities, thought to be primarily tectonic rather than eustatic in origin, separate and subdivide the rifting episodes. The earlier episode involved extension about a N–S axis; ensuing (Triassic–Mid-Jurassic) thermal subsidence was accommodated on steep faults. During the later episode a new NE–SW fault trend was superimposed on pre-existing patterns. Major block rotation, marking active rifting, ceased at the end of the Ryazanian. During the second post-rift episode there was a progressive migration of active faulting towards the basin margins and, as a result, a widening-with-time of the area undergoing subsidence. Asymmetric subsidence of the central part of the basin was hinged at the western margin of the Horda Platform, and accommodated to the NW on major faults within the Tampen Spur, β factors, for the second rifting episode were calculated both by relating subsidence to extension, and by measuring observed extension. Values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases, but are greater for the latter phase. Subsidence patterns are similar for both rifting and thermal subsidence episodes, so that there is vertical stacking of relatively thick sequences in the axis of the northern Viking Graben. These factors preclude the application of models involving uniform and non-uniform stretching and also preclude oblique extension; depth-dependent stretching is preferred.
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