Nordfjord-Sogn Detachment

Low-angle fault contact between Devonian sediments of the Kvamshesten Basin and mylonites of the underlying shear zone
Lihesten near Hyllestad made of well-jointed Devonian conglomerates (part of the Solund Basin) in fault contact with mylonites of the detachment

The Nordfjord—Sogn Detachment (NSD) is a major extensional shear zone up to 6 km in thickness, which extends about 120 km along strike from Nordfjord to Sognefjord, bringing Devonian continental coarse clastic sedimentary rocks into close contact with eclogite facies metamorphic rocks of the Western Gneiss Region. It formed towards the end of the Caledonian Orogeny and was mainly active during the Devonian. It has an estimated displacement of at least 70 km and possible as much as 110 km.[1] It was reactivated during the Mesozoic and may have influenced the development of fault structures in the North Sea rift basin.[2]

Extent

The NSD is recognised from the western end of the Sognefjord through to the northern shore of Bremanger, a distance of about 120 km. The exposure is fairly continuous, broken occasionally by fjords and some later high-angle faults, such as the Standal Fault.[3] There is evidence from seismic reflection data that the structure continues offshore to the west.

Geometry

The NSD has an overall low westward dip, although its sinuous outcrop shows that it has a folded geometry with a series of west-plunging antiforms and synforms, with Devonian sediments preserved in four of the larger synforms as the Solund, Kvamshesten, Håsteinen and Hornelen basins. It places rocks of the Upper Plate in tectonic contact with rocks of the Lower Plate.

Upper plate

The upper plate of the NSD consists of rocks of the highest tectonostratigraphic levels of the Norwegian Caledonides, the upper allochthon, unconformably overlain by conglomerates and sandstone of Devonian age.[2][3]

Lower plate

The lower plate of the NSD is formed by the Western Gneiss Region.

References

  1. ↑ Hacker B.R., Andersen T.B., Root D.B., Mehl L., Mattinson J.M. & Wooden J.L. (2003). "Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway" (PDF). Journal of Metamorphic Geology. 21 (6): 613–629. doi:10.1046/j.1525-1314.2003.00468.x.
  2. 1 2 Johnston S., Hacker B.R. & Ducea M.N. (2007). "Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway" (PDF). Bulletin of the Geological Society of America. 119 (9-10): 1232–1248. doi:10.1130/B26172.1.
  3. 1 2 Fossen H. "Extensional tectonics in the North Atlantic Caledonides: a regional view". In Law R.D.; Butler R.W.H.; Holdsworth R.E.; Krabbendam M.; Strachan R.A. Continental Tectonics and Mountain Building: The Legacy of Peach and Horne (PDF). Special Publications. 335. Geological Society. ISBN 9781862393004.
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