Abstract
This study focuses on the tectonic evolution of the greater Egersund Basin in the Norwegian central North Sea, with special emphasis on Late Paleozoic extensional tectonics following Caledonian collapse and the Variscan Orogeny, and the impact of the basement structural grain on this evolution. The Caledonian collapse likely resulted in development/rejuvenation of the deepest E-W trending structures/depocenters by Late Devonian time. Thus, a late Devonian-?early Carboniferous age can be assigned to the initial extension, which was associated with the development of an E-W striking basin system, to be overprinted by N–S extensional structures of similar age. A phase of regional magmatism at the Carboniferous-Permian transition (≈300 Ma) may be associated with a large igneous province centred on the Skagerrak area. Faulting during late Carboniferous-early Permian was minor within the study area as reflected by uniform sedimentary thicknesses of the uppermost Carboniferous and lower Permian sequences. Major normal faults, mainly trending N–S, were active during a late Permian-Early Triassic rift phase affecting large parts of the central and northern North Sea area. A later phase of extension was initiated in late Middle Jurassic time and the Egersund Basin proper formed during the Late Jurassic-Early Cretaceous. The depocenters that developed during this phase was influenced by the deep Late Paleozoic (sub-salt) structural grain, including strike-slip movements along the Sorgenfrei-Tornquist Zone. Later events include mild inversion along the northern flank of the Egersund Basin, possibly as a Late Cretaceous response to far-field Alpine compression, and Cenozoic regional tilting.
Highlights
The Hummer Graben, which is characterized by a N–S-striking graben axis is separated from the Flekkefjord High on its east by the Flekkefjord fault system (FFS), which consists of planar normal faults (Figs. 4 and 7b,c)
The configuration and geometry of the present Egersund Basin area is the result of multistage deformation where direction of extension has changed from dominantly NW-SE through E-W to NE-SW, involving dextral strike-slip movements
Late Devonian-?early Carboniferous NW-SE extension activated two sets of E-W and N–S trending faults bounding distinct pre-Permian depocenters. Tuffs tied to their associated seismic facies at the Carboniferous-Permian transition (≈300 Ma) can be linked to regionalscale rifting and volcanism, and may be associated with a large igneous province centred on the Skagerrak
Summary
The influence of the basement fabric and fault populations (Prote rozoic and Caledonian), referred here as the structural grain, on the development of the basin system of the Norwegian Shelf has long been debated (e.g. Bartholomew et al, 1993; Blundell et al, 1991; Donato and Tully, 1982; Færseth et al, 1995; Gabrielsen and Ramberg, 1979; Gabrielsen, 1984; Smethurst, 2000; Stewart et al, 1992; Ziegler, 1990). Bartholomew et al, 1993; Blundell et al, 1991; Donato and Tully, 1982; Færseth et al, 1995; Gabrielsen and Ramberg, 1979; Gabrielsen, 1984; Smethurst, 2000; Stewart et al, 1992; Ziegler, 1990) This discussion has been hampered by high-quality deep seismic reflection data being scarse. The basement in the area likely carries the imprint of meso- to neo-Proterozoic mainly Fen noscandian orogeny (Bingen et al, 2005), Caledonian contraction and collapse (Bartholomew et al, 1993; Phillips et al, 2016; Reeve et al, 2014; Rey et al, 1997; Sørensen et al, 1992) and the effects of Variscan collision (Rey et al, 1997; Smit et al, 2016; Sørensen et al, 1992). 2D seismic lines of good quality were analyzed to (i) establish the sediment distribution and structuring of the sub-salt sequences (which is not limited to the immediate sub-salt Rotliegend Group in this study) and the supra-salt structural frame work of the larger Egersund Basin area
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