A seismic interpretation of the Danish North Sea
This study is based on a regional grid of seismic data acquired by Merlin Geophysical. The main emphasis of this thesis was on the detailed structural mapping of the Danish Central Graben, since this area has the highest density of well and seismic data. The principal objective of the study was to explain why the North Sea Rift changes strike in the Danish Sector, from the N-S trending Dutch Central Graben, to the NW-SE trending U.K/Norwegian Central Graben. The Danish Central Graben opened initially in the Late Carboniferous, as part of a regional dextral transtensile deformation that affected much of Europe. The initial extensional structures developed by reactivation of a pre-existing basement fabric. The NNW trending Coffee Soil Fault bounding the rift, is interpreted as a planar structure transecting the entire crust, and is thought to have developed by extensional reactivation of an east-verging Caledonian thrust. Basement shear zones identified on the rift shoulders on the Ringkobing-Fyn High are interpreted as the along-strike continuation of the Caledonides of Southern Norway, offset to the east by syn-orogenic transform motion along fracture zone elements of the Tornquist Zone. The WNW trending fault zones that dominate the structural grain in the Danish Central Graben are shown to be closely related to WNW trending fracture zones on the Ringkobing-Fyn High, which are regarded as splay shears of the Tornquist Zone. The WNW trending transverse fault zones segment the Danish Central Graben. The segmentation exerted a fundamental influence on the structural and stratigraphic development of the rift, in that individual segments were free to subside at different rates, in different styles, at different times. Two main phases of rifting are recognised, Permo-Triassic, and Middle and Upper Jurassic. These two phases have contrasting patterns of subsidence, and contrasting structural expression, particularly as regards the extent of the involvement of the transverse fault zones. The Permo-Triassic phase is characterised by parallelism of stratal configurations, and horizontal subsidence of the graben floor, whereas the Jurassic phase is characterised by strongly divergent configurations and asymmetric, rotational subsidence directed towards and controlled by the Coffee Soil Fault. Active rifting ceased at the end of the Jurassic, and Lower Cretaceous sediments are found to onlap extant fault scarps in a passive infill of the rift basin. The major bounding structures of the rift change strike abruptly in several incremental steps across the points of intersection with the transverse segment boundaries. The re-orientation of the rift is therefore explained as a consequence of the underlying presence of the earlier transform dominated basement fabric. A model for the formation of the North Sea Rift is proposed, which draws heavily on observations of the fracture patterns in continental rifts such as the Oslo Rift and the Rio Grande Rift, and is a development of the Megashear-Rhombochasm concept of S.W.Carey.