|Spreading center type:||Large-scale extinct MOR|
|Time of cessation:||28.2 Ma, Chron C10n (Gernigon et al., 2012)|
|Subsequent active spreading center:||Kolbeinsey Ridge|
|Cessation style:||Slowed to ultra-slow spreading before cessation (Gernigon et al., 2012)|
|Later deformation/volcanism:||None reported|
The Aegir Ridge is a well-studied extinct ridge situated within the Norwegian Sea. It was the active spreading center between the Norwegian margin and Greenland before a ridge-jump to the west of the Jan Mayen microcontinent. The ridge was active during the early Eocene to the Oligocene and represented a corridor of spreading between Norway (the Eurasian plate) and the Jan Mayen microcontinent, to the east of Greenland (Breivik et al., 2006). Crust formed at the Aegir ridge crust is wider to the north, demonstrating a fanning shape that is suggestive of a nearby pole of rotation, as was observed by Nunns (1983). It has also been argued that rotation was balanced by rotation on the simultaneously active Kolbeinsey ridge between the Faroe Islands and Greenland, for at least some of its lifespan (Nunns, 1983; Greenhalgh and Kusznir, 2007).
The Aegir ridge is reported to have ceased spreading at 33 Ma, after a sharp reduction in spreading rate at around Chron C13n associated with formation of a deep axial valley (Gernigon et al., 2012; Jung and Vogt, 1997). The Kolbeinsey ridge then became the dominant spreading center to the west of the Jan Mayen microcontinent, leading to isolation of this fragment.
Comprehensive bathymetric, gravity and magnetic studies have been undertaken at the ridge (Talwani and Eldholm, 1977; Jung and Vogt, 1997), in addition to a detailed seismic refraction survey (Breivik et al., 2006), which provide valuable information on the crustal structure about the ridge. While there is good data coverage available, interpretations of crustal thickness in the region have varied, as have interpretations of the tectonic events in this corridor. Thinned crust within the basin is argued by Jung and Vogt (1997) and Breivik et al. (2006), who both propose that formation of the voluminous Iceland-Faroe ridge was likely to have locally depleted the mantle and decreased melt availability. Greenhalgh and Kusznir (2007) further propose that anomalously cold mantle in the early Eocene could provide an explanation for the thinned crust in this region. If regional mantle depletion at the Aegir Ridge was indeed a significant factor for very slow spreading at this ridge it may have also provided the impetus for the subsequent relocation of spreading away from the Aegir ridge as a more fertile source to the northwest was preferentially tapped.
Both formation and subsequent extinction of the ridge are believed to have been strongly influenced by the presence of the Iceland hotspot, and possibly the Jan Mayen hotspot, within the region (Müller et al., 2001). Breivik et al., (2006) interpreted “V-shaped” gravity anomalies to estimate the volume of asthenospheric flow from the Iceland plume at the time to have been 3-6 mm/yr. Whilst other small magnitude ridge jumps and minor relocations resulting in crustal capture have been reported elsewhere in proximity to the Iceland plume (Kristjansson and Jonsson, 1998; Smallwood and White, 2002), the Aegir ridge cessation resulted in the most significant reorganization of this spreading corridor. Gernigon et al. (2012) propose that northward propagation of the Reykjanes Ridge may have led to the isolation of the Jan Mayen microcontinent and favour a hotspot influence for the cessation of spread at the Aegir Ridge in favour of the Kolbeinsey Ridge (Gernigon et al., 2012)
The Aegir Ridge has a clearly defined axial valley morphology, although the westernmost tip of the spreading center is less well-resolved. The prominent transform fault to the north of the ridge that was likely to have been active during the lifetime of the Aegir spreading center and fanned shape of crust bears some similarity to that of a rotating microplate, such as the Magellan Trough (Tamaki and Larson, 1988; Schouten and Dick, 2005) or the active Easter microplate (Naar and Hey, 1991), although on a much larger scale. Axial relief is between 500 and 1500 m, increasing to the east, away from the Iceland plume. The gravity anomaly seen at the ridge axis is between 57-90 mGals, also increasing in magnitude to the east.
Breivik, A. J., Mjelde, R., Faleide, J. I. and Murai, Y., 2006, Rates of continental breakup magmatism and seafloor spreading in the Norway Basin–Iceland plume interaction, Journal of Geophysical Research, v. 111, no. B07102, doi: 10.1029/2005JB004004.
Gernigon, L., Gaina, C., Olesen, O., Ball, P. J., Péron-Pinvidic, G. and Yamasaki, T., 2012, The Norway Basin revisited: From continental breakup to spreading ridge extinction, Marine and Petroleum Geology, v. 35, no. 1, p. 1–19.
Greenhalgh, E. E., and Kusznir, N. J., 2007, Evidence for thin oceanic crust on the extinct Aegir Ridge, Norwegian Basin, NE Atlantic derived from satellite gravity inversion, Geophysical Research Letters, v. 34, p. 1–5.
Jung, W. and Vogt, P. R., 1997, A gravity and magnetic anomaly study of the extinct Aegir Ridge, Norwegian Sea, Journal of Geophysical Research, v. 102, no. B3, p. 5065–5089.
Kristjansson, L. and Jonsson, G., 1998, Aeromagnetic results and the presence of an extinct rift zone in western Iceland, Journal of Geodynamics, v. 25, p. 99-108.
Müller, R. D., Gaina, C., Roest, W. and Hansen, D. L., 2001, A recipe for microcontinent formation, Geology, v. 29, no. 3, p. 203–206.
Nunns, A. G., 1983, Plate tectonic evolution of the Greenland-Scotland ridge and surrounding regions, In, Bott, M.H.P., Saxov, S., Talwani, M., Thiede, J., eds., Structure and Development of the Greenland-Scotland Ridge: New methods and concepts, Plenum Press, New York, p. 1-30.
Schouten, H., and Dick, D. K., 2005, Counter-rotating Magellan and Trinidad microplates at the Mesozoic Pacific-Phoenix-Farallon triple junction, In, American Geophysical Union, Fall Meeting 2005, abstract #T51D-1382.
Smallwood, J. R. and White, R. S., 2002, Ridge-plume interaction in the North Atlantic and its influence on continental breakup and seafloor spreading, Geological Society, London Special Publication, v. 197, p. 15-37.
Talwani, M., and Eldholm, O. 1977, Evolution of the Norwegian-Greenland Sea, Geological Society of America Bulletin, v. 88, p. 969-999.