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Geological Society, London, Special Publications; 2000; v. 167; p. 105-131;
DOI: 10.1144/GSL.SP.2000.167.01.06
© 2000 Geological Society of London

Intra-Plate Rifting and Basin Formation

Detachments and low-angle faults in the northern North Sea rift system

Haakon Fossen1, Tore Odinsen1, Roald B. Færseth2 & Roy H. Gabrielsen1

1 Department of Geology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway
2 Norsk Hydro, 1321 Stabekk, Norway

Several master faults in the North Sea basin tend to flatten to give low dips at depth, and in this sense form detachments in the rift system. Such low angle faults are identified in the western flank of the Viking Graben (Tampen Spur area), where they occur as both intra- and supra-basement detachments. Interference between detachments and steeper faults results in ramp-flat-ramp geometries. In the eastern part of the Gullfaks fault block, a supra-basement detachment is probably associated with anomalously high late Jurassic extension in the Gullfaks Field area. The low-angle Gullfaks detachment also helps explain the presence of sets of parallel east-dipping faults (domino systems), a common feature in the collapsed hanging wall to low-angle detachments. Similar detachments probably exist beneath the Gullfaks Sør block and SE of the Visund fault block. All of these are interpreted as late Jurassic collapse structures directly related to active late Jurassic extensional tectonics. Strong indications of intra-basement detachments are also found in the Tampen Spur area. These detachments are formed by major normal faults that flatten in the basement, as seen beneath the Visund fault block. This geometry may to some extent be related to fault rotation during repeated phases of extension in the Palaeozoic-Early Mesozoic period. However, abrupt flattening of some of the faults in the basement indicates that the master faults follow some of the many pre-existing mechanically weak zones in the basement, primarily low-angle Devonian extensional shear zones or Caledonian thrusts.





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