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S.B. Nielsen

Cenozoic structures and the tectonic evolution of the eastern North Sea

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Abundant seismic sections and well data from the Cenozoic succession in the eastern North Sea area generally
reveal normal faulting, salt tectonics and localized tectonic inversion. However, inferences on the Cenozoic
dynamic evolution of the region require thorough analysis of interactions between detachment surfaces withinthe
sedimentary succession and basement structures. Here we define basement structures by offsets in the pre
Zechstein succession. Cover structures are confined to the post Zechstein succession, or part hereof, and detach
internally along surfaces in the post Zechstein succession. These structures have in the last decades been an
integrated part of the discussions about subsidence and uplift of not only the interior of the basin but also of
the basin margin. Abundant 2D and 3D seismic data and new depositional models enable detailed analysis and
reinterpretation of where and when basement or cover tectonism took place. Our objectives are thus 1) to analyze
the interaction between basement and cover structures, and if possible 2) to relate the structures to the regional
tectonic evolution.
The Zechstein evaporites pinch out onto the Ringkøbing-Fyn High, which in the eastern North Sea is a Paleozoic-
Mesozoic structural high separating the Northern and Southern Permian embayments. This pinch-out controls the
presence of detaching cover structures and salt structures in the Norwegian Danish Basin, and the conspicuous absence
of structures above the Ringkøbing-Fyn High (Clausen & Huuse 1999). Furthermore, prograding Oligocene
and Miocene units in combination with thermal and loading induced differential subsidence between the basins
and the Ringkøbing-Fyn High controlled the Cenozoic reactivations of the main coverfaults. The detaching cover
faults generated additional accommodation space, which influencede.g. Miocene deposition and controlled the
generation of second order faults. The latter detached along the top Chalk Group due to the topography generated
during faulting, i.e. they are second order detachment surfaces. We conclude that the regional tectonic significance
of the Cenozoic structures is of small importance, but that halokinetics and the rheological characteristics of
Zechstein salt and Paleocene clays controlled the structural history along the Ringkøbing-Fyn High. A Cenozoic
tectonic inversion of the Ringkøbing-Fyn High does not seem to be supported by structures.
Tectonic inversion was restricted to the Sorgenfrei-Tornquist Zone (STZ) and the Central Graben (Vejbæk &
Andersen 2002). The marginal troughs of the STZ allow identification of compressional inversion during the late
Cretaceous and relaxation inversion in the mid Paleocene (Nielsen et al., 2005). Structural reconstruction including
decompaction in the Central Graben along the Arne-Elin trend shows that two phases of basement related inversion
took place duringthe Paleocene-Eocene and the Oligocene. Halokinetics and differential compaction across the
Paleogene inversion structure explain later tectonic signals, and the Neogene inversion reported by e.g. Rasmussen
(2009) is therefore not required.
Our structural interpretation, which considers halokinetics and differential compaction, is consistent with the
established late Cretaceous compressional inversion history of the European continent and the mid-Paleocene
phase of relaxation inversion (Nielsen et al. 2005). In conclusion, the Cenozoic structures in the North Sea area do
not generally support ideas on Neogene basement tectonism.
References:
Clausen, O. R. and M. Huuse (1999). "Topography of the Top Chalk
surface on- and offshore Denmark." Marine and Petroleum
Geology,16(7): 677-691.
Nielsen, S. B., E. Thomsen, et al. (2005). "Plate-wide stress
relaxation explains European Palaeocene basin inversions."
Nature, 435(7039): 195-198.
Rasmussen, E. S. (2009). "Neogene inversion of the Central Graben and
Ringkøbing-Fyn High, Denmark." Tectonophysics, 465(1-4): 84-97.
Vejbæk, O. V. and C. Andersen (2002). "Post mid-Cretaceous inversion
tectonics in the Danish Central Graben." Bulletin of the
Geological Society of Denmark, 49: 129-144.
Original languageEnglish
JournalGeophysical Research Abstracts
Volume13
Issue7039
Number of pages1
ISSN1607-7962
Publication statusPublished - 3 Apr 2011

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