Volume 49/2

Frederiksen, S. 2002–12–02: Quantitative dynamic modelling of basin development in the central and eastern North Sea region – coaxial stretching and strain localization. Bulletin of the Geological Society of Denmark. Vol. 49, pp. 95–108. Copenhagen.

A new two-dimensional dynamic lithosphere model is used to simulate the Late Palaeozoic to end Danian evolution of the Norwegian-Danish Basin and the post Permian evolution of the Central North Sea including the Central Graben. The transient heat equation and the equations of motion are solved using the finite element method. The lithosphere deforms by brittle and ductile processes through an elasto-visco-plastic rheology depending on temperature, pressure, strain-rate and material parameters. Strain softening dependent on accumulated strain is incorporated. Deposition, erosion and compaction of sediments are simulated. Results show that it is possible to satisfy observations of crustal structure, sediment thickness and surface heat flow for both basins taking all major tectonic and thermal events into consideration. The evolution of the Norwegian-Danish Basin is modelled using a Late Carboniferous – Early Permian thermal event, main rift phase in Early Permian and a minor extensional phase in Triassic. For the Central North Sea two thermal and three tectonic events are simulated: Late Carboniferous – Early Permian and Middle Jurassic thermal events, Early Triassic and Late Jurassic extension, and Late Cretaceous compression. Results show that strain softening may lead to strain localization during extension and therefore may explain observations of upper mantle dipping reflectors in the North Sea. A pure shear dominated extensional regime may change into a simple shear system.

Key words: dynamic model, extension, strain softening, strain localization, the Norwegian-Danish Basin, the Central North Sea.

Susanne Frederiksen [ ], Department of Earth Sciences, University of Aarhus, Finlandsgade 8, DK-8200 Århus, Denmark. 15 August 2001.

A Model for the evolution of the Weald

Hansen, D.L., Blundell, D.J. & Nielsen, S.B. 2002–12–02. A Model for the evolution of the Weald Basin. Bulletin of the Geological Society of Denmark, Vol. 49, pp. 109–118. Copenhagen.

The Weald Basin developed through the Jurassic–Lower Cretaceous as an extensional basin founded upon E–W trending low-angle faults that were probably Variscan thrusts, subsequently reactivated as normal faults. Later, the basin was inverted and uplifted into a broad dome, whilst the London Basin to the north, and the Hampshire–Dieppe Basin to the south, subsided as flanking basins during the late Palaeocene–Eocene. Seismic sections across the Weald indicate that inversion resulted from north-directed stress. A stratigraphic reconstruction based on a N–S profile across the Weald and flanking basins serves as a template for a forward, 2D thermo-mechanical model that simulates the evolution of the Weald Basin through crustal extension and its inversion, and subsidence of the flanking basins, through compression. The model provides a physical explanation for this sequence of events, requiring a region of crust of reduced strength relative to its flanks. This weak region is the location of crustal-scale Variscan thrusts that have been reactivated subsequently. The strong crust on the flanks is essential for the development of flanking basins during inversion and uplift of the Weald.

Key words: Basin inversion, lithosphere, thermo-mechanical modelling, finite elements, visco-elastic- plastic, sedimentation, erosion.

David Lundbek Hansen [ ] & Søren Bom Nielsen, Department of Earth Sciences, University of Aarhus, Finlandsgade 6, DK-8200 Aarhus N. Derek J. Blundell, Department of Geology, Royal Holloway, University of London, England. 15 January 2001.

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Differential vertical movements in the eastern North Sea area from 3-D thermo-mechanical finite elemental modelling

Gemmer, L., Nielsen, S.B. & Lykke-Andersen, H. 2002–12–02: Differential vertical movements in the eastern North Sea area from 3-D thermo-mechanical finite elemental modelling. Bulletin of the Geological Society of Denmark, Vol. 49, pp. 119–128. Copenhagen.

The response of a heterogeneous lithosphere to a compressional stress field is studied using a three-dimensional thermo-mechanical finite element model. Weak zones in the lithosphere thicken and act as loads that pull down the lithosphere in regions around the weak zones. Strong zones are subjected to less lithospheric thickening than the surroundings and produce surface depressions and uplift in the surrounding areas. The model is used to study the Late Cretaceous and Paleocene differential vertical movements in the eastern North Sea area. The Sorgenfrei-Tornquist Zone is assumed to be a pre-existing weak crustal zone, which inverts during compression and produces marginal basins by loading the lithosphere. The area of the Silkeborg Gravity High is an example of a pre-existing strong crustal zone which subsides during compression. Moho topography in the area gives rise to lateral strength variations, which result in surface uplift where Moho is deep and subsidence where Moho is shallow. These effects, together with the lateral variations of the thermal structure and the stress field, determine the overall Late Cretaceous-Paleocene distribution of vertical movements of the area. This has implications for the pattern of erosion, sediment transport and the distribution of sediment facies.

Key words: Sorgenfrei-Tornquist Zone, inversion, Silkeborg Gravity High, eastern North Sea, Moho, Cenozoic, finite element.

Lykke Gemmer [ ], Søren B. Nielsen & Holger Lykke-Andersen, Department of Earth Sciences, University of Aarhus, Finlandsgade 6-8, DK-8200 Århus, Denmark. 15 September 2001.

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Post mid-Cretaceous inversion tectonics in the Danish Central Graben – regionally synchronous tectonic events?

Vejbæk, O.V. & Andersen, C. 2002–12–02: Post mid-Cretaceous inversion tectonics in the Danish Central Graben – regionally synchronous tectonic events? Bulletin of the Geological Society of Denmark, Vol. 49, pp. 129–144. Copenhagen.

Structural analysis of the Upper Cretaceous to Palaeogene succession in the Danish Central Graben suggests continuous inversion heralded in the Late Hauterivian and continuing into Palaeogene times. The following phases of increased intensity are identified: 1) latest Santonian, 2) Mid Campanian, 3) late Maastrichtian, 4) Late Paleocene – Eocene, and 5) Early Oligocene. Phases 1 through 3 are Sub-Hercynian, phase 4 is Laramide, and phase 5 is Pyrenean according to Alpine Orogen nomenclature. A temporal change in structural style is noted from early inversion confined to narrow zones associated with reverse faulting along pre-existing normal faults to late inversion dominated by gentle basinwide flexuring and folding. Inversion phases in the Danish Central Graben seem to be synchronous with inversion phases along the Sorgenfrei-Tornquist Zone. The location of inversion is generally spatially linked to Upper Jurassic – Lower Cretaceous depocentres, whereas older depocentres generally have remained intact. The origin of the compressional stress field is generally based on suggested compressional stresses transmitted into the foreland from the Alpine Orogen. In the Sub-Hercynian phase, orogenic compression dominated the Eastern Alps and Northern Carpathians to produce a likely NW oriented compression. However, structures in Denmark rather suggest a transpressional environment resulting from NNE–SSW compression. Furthermore, transmission of Alpine orogenic stresses into the foreland commenced in the Turonian, a considerable time after the Late Hauterivian and later inversion precursors. Ridge-push forces transmitted from sea-floor spreading south of the Charlie-Gibbs fracture zone, particularly from the Goban Spur SW of Ireland, acting in conjunction with Alpine orogenic stresses are suggested as the cause for the stress field.

Key words: Inversion, Central Graben, Upper Cretaceous, Chalk Group, Sorgenfrei-Tornquist Zone.

Ole Valdemar Vejbæk [ ] & Claus Andersen [ ], Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark. 11 March 2000.

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Late Cenozoic palaeogeography of the eastern North Sea Basin: climatic vs tectonic forcing of basin margin uplift and deltaic progradation

Huuse, M. 2002–12–02: Late Cenozoic palaeogeography of the eastern North Sea Basin: climatic vs tectonic forcing of basin margin uplift and deltaic progradation. Bulletin of the Geological Society of Denmark, Vol. 49, pp. 145–170. Copenhagen.

The late Eocene to middle Pleistocene development of the eastern North Sea Basin is described by a series of palaeogeographic maps. The maps are based on published information integrated with recent investigations of seismic and well data from the eastern North Sea. The maps provide overviews of the basin geometry at late Eocene, late Oligocene, middle Miocene, late Miocene, late Pliocene and middle Pleistocene time. In post-Eocene time, the eastern and central North Sea Basin was progressively filled by large deltas, which built out from the eastern basin margin. These deltas were fed by ancient rivers from southern Norway (late Paleocene-Oligocene and Pliocene), southern Norway and Sweden (early Miocene), the Baltic region (middle Miocene-early Pleistocene), and finally by rivers flowing northward through the northwest European lowland (middle Pleistocene). It is argued that the Cenozoic evolution of the eastern North Sea Basin may be explained by a ‘self-perpetuating’ passive model. This model involves isostatic uplift of source areas due to erosional unloading of a relief generated by early Palaeogene uplift. The erosional unloading accelerated at the Eocene/Oligocene transition, in the middle Miocene and in the Plio-Pleistocene corresponding to periods of global climatic cooling and long-term eustatic lowering as indicated by δ18O records. The passive model diminishes the need for hypothetical Neogene tectonic events, although the influence of tectonic events cannot be excluded. Previous estimates of Neogene uplift and erosion of the northeastern Danish North Sea of the order of 500–1000 m do not agree with seismic geometries or with the regional palaeogeographic development. This indicates that previous estimates of Neogene uplift and erosion of the northeastern Danish North Sea may be several hundred metres too high.

Key words: Cenozoic, North Sea Basin, Climate, Eustasy, Tectonics.

Mads Huuse [ ], Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark. 24 March 2000.

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Mid-Paleocene palaeogeography of the Danish area

Clausen, O. R. & Huuse, M. 2002–12–02: Mid-Paleocene palaeogeography of the Danish area. Bulletin of the Geological Society of Denmark, Vol. 49, pp. 171–186. Copenhagen.

At the Danian/Selandian transition the North Sea Basin experienced a marked change, from deposition of almost pure carbonate during the Upper Cretaceous and Danian to deposition of greensand, marl and clay during the Selandian. Erosional features at the Top Chalk surface and the occurrence of an overlying conglomerate (transgressive lag deposit) indicates that large parts of the Danish area were subaerially exposed at the Danian/Selandian transition, probably due to regional tectonic uplift. Tectonically induced inversion of fault trends in the Central Trough and the Sorgenfrei- Tornquist Zone and differential relative subsidence between the Ringkøbing-Fyn High and the Norwegian-Danish Basin strongly affected the distribution of the lower Selandian sediments. Three palaeogeographic maps are presented to illustrate the various stages of the early Selandian transgression in order to demonstrate the mid-Paleocene evolution of the Danish area.

Key words: Paleocene, Palaeogeography, Denmark.

Ole R. Clausen [ ] & Mads Huuse, Department of Earth Sciences, University of Aarhus, DK- 8000 Århus C, Denmark. 6 June 2002.

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A post mid-Cretaceous North Sea model

Nielsen, S.B. 2002–12–02: A post mid-Cretaceous North Sea model. Bulletin of the Geological Society of Denmark. Vol. 49, pp. 187–204. Copenhagen.

The relative importance and causal relationship between tectonics, eustacy and sediment charge is investigated for the post mid-Cretaceous North Sea. Guided by the predictions of a quantitative model and previously published work it is argued that the geological evolution comprises 1) continued subsidence of the central North Sea and its more marginal basin areas, related to Palaeozoic and Mesozoic lithospheric and deeper processes, 2) Cenozoic uplift of the British Isles (Scotland) and western Fennoscandia initiated by the Iceland plume in Paleocene/Eocene, 3) inversion zone dynamics induced by in-plane stress variations from plate boundary processes (the African-Eurasian collision and Atlantic spreading), and 4) denudation controlled by the availability of topography and erosional base level changes. The processes are consistent with the general present day sediment structure of the North Sea. Furthermore, they produced the pattern and amplitude of the burial anomalies in the North Sea region: 1) the continued subsidence of the central North Sea ensured maximum burial here at the present day, 2) Cenozoic uplift of the coastal areas of Scotland and western Fennoscandia, including southern Norway, produced over-burial that decreases away from the coast with a gradient that depends on the flexural strength of the lithosphere, 3) inversion zones (in particular the Sole Pit High and the Sorgenfrei-Tornquist Zone) developed a two phase burial anomaly, firstly by erosion of the narrow strip of topography in the central zone of inversion, and secondly by more regional erosion of the post-compressional rebound topography, involving both the central inversion zone and the marginal troughs. The falling eustatic level generated a burial anomaly by erosional unloading and isostatic uplift of the basin margins. Deep Quaternary erosion enhanced the burial anomaly in, for example, the Farsund Basin and the Skagerrak. The post-compressional rebound of the Sorgenfrei-Tornquist Zone is identified as the Neogene tectonic mechanism in the eastern North Sea area.

Key words: modelling, lithosphere, basin inversion, erosion, North Sea basin, Neogene uplift.

Søren B. Nielsen [ ], Department of Earth Sciences, The University of Aarhus, Finlandsgade 8, DK-8200 Aarhus N, Denmark. 25 November 2002.

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Bulletin of the Geological Society of Denmark • Vol. 49/2

Contents

Quantitative dynamic modelling of basin development in the central and eastern North Sea region – coaxial stretching and strain localization