Iceland is always referred to as an oceanic island on an oceanic ridge protruded above the Atlantic sea level. Under the aegis of Plate Tectonic Concepts(PTC), this island is considered as the real time evidence of lithospheric spreading. In the wake of new geophysical data, apprehensions are more about the spreading concepts of lithosphere and related geodynamics.  This paper looks into a few important facts, which negate the PTC on Iceland.

The ideas of lateral drift and sea floor spreading took hold in the late 1960s, followed which the unexpected xenoliths in surface volcanics of Iceland had been observed. But after Surtsey eruption of 1963, the light-coloured xenoliths were readily spotted in contrasting lavas and black volcanic ashes, which made Sigurdsson (1968) to carry out a more extensive search of xenoliths in Iceland and he found that they were mainly granites.

This has triggered a petrological question about the extent of the acidic material as the product of differentiation from a parental basic magma. Alternatively, these xenoliths could be looked at as the eruption related ejected out portion of a granitic/granulitic crust torn out by fusion of a deep seated granitic/ granulitic crust. But, Sigurdsson(1968) stick on to magmatic differentiation process, as likely mechanism / source of the acidic material, although Sigurdsson (1968) considered the above conclusion too as problematic from the point of its volume, because the acid rocks make up more than 10% of the exposed succession in eastern Iceland.

In addition to this conclusion, Sigurdsson (1968) has pointed to the fact that some granite xenoliths have a composition that could not be placed conformable with those granites raised out as a consequence of crystal fractionation. Consistent with this latter aspect, Sigurdsson mentions the discovery, in the volcanic ashes of Surtsey, of a xenolith consisting almost entirely of recrystallized dolomite.  Another xenolith from a basalt flow on Bredadalsheidi was described as consisting entirely of quartz grains,with a texture resembling that of quartzite (Storetvedt and Longhinos, 2011).  Taken at their face value, these foreign fragments are suggestive of ancient metamorphic sediments at depth, indicating that the deep crust of Iceland is continental – capped by an unknown succession of Tertiary and younger volcanic.

Darbyshire et al. (2000), Allen et al. (2002), Foulger et al. (2003),Gudmundsson (2003), Björnsson et al. (2005) and Foulger (2006) have brought out the geophysical character of the Icelandic crust and established that, it underlies an anomalous thick crust - thickness ranging upward to ~40km. Storetvedt and Longhinos (2011) considers that the bowl-shaped crust of the island, with maximum thicknesses in the centre, is an  most important feature for understanding crustal evolution – in Iceland and elsewhere.  

Foulger (2006)suggests that at least part of the thick crust might be continental –representing a southerly extension of the Jan Mayen micro-continent. Storetvedt(2003) looks Icelandic situation as the test lab to understand thickness variation, sub-crustal eclogitization and gravity driven loss of crust into mantle during basification. Concordant with that proposition, Anderson (2005)has suggested the low velocity zone at depth may constitute of eclogite layers,which were delaminated and sunk with the crust into the mantle to a level ofneutral buoyancy, a level anywhere between 300-650km depth. Storetvedt and Longhinos (2011) argues that the variable eclogitization/delamination of the lower crust is likely to be the most fundamental mechanism behind

1) the generation of deep oceanic basins, 

2) the presence of partly thinned continental crust in deep sea settings,and

3) thinned crust beneath major sedimentary basins on land.

In the North Atlantic, for example, the shallow and a seismic trans-oceanic Shetland-Faeroe-Iceland-Greenland Ridge is likely to constitute a moderately thinned and subsided continental fragment, making the sea floor spreading hypothesis a physical impossibility.

The majority view holds that Iceland is a part of Mid-Atlantic Ridge, whichruns usually -2000m msl, is subjected to be fed by ‘hotspot’ volcanism rising from the deep mantle, resulting in thickened crust. But, on the other hand,seismic tomography of the upper mantle seems to contradict this idea –referring the concentrated volcanic activity of Iceland to an unusually fertileregion within a relatively cool shallow upper mantle (Foulger&Anderson2005).  

Storetvedt (2010c) applied the susceptibility-contrast model of  Luyendyk&Melson (1967) and Luyendyk etal. (1968) to redefine the linear marine magnetic system (LMMS) and concluded that LMMS is resulted out of lateral, fault-controlled, variation in iron-oxidemineral alteration and associated magnetic susceptibility and the linear marine magnetic belts outline prevailing tectonic shear grains, in which the fault-controlled topographic lows are likely to be associated with the stronger shearing and hence the more advanced degree of dynamo-metamorphic processes –converting iron-titanium oxides (carrying original fossil magnetizations) into secondary silicates without remanence properties.

Storetvedt (2010c) contendsthe prevailing usage of linear marine magnetic anomalies for determining rock-age and geomagnetic polarity sequences. The North Atlantic region displays an irregular magnetic anomaly matrix – with a number of sharp discontinuities and related changes of the magnetic anomaly trend, as well as regions without appreciable magnetic lineation, trans-oceanic Shetland-Faeroe-Iceland Ridge being an example.

The evolving knowledge therefore denies any chance for lithospheric spreading in Iceland, rather crustal basification as an alternative explanation. It places Iceland as yet another case of a remnant continental crust, failed to be eaten up by mantle during Cainozoic basification, which is comparatively weaker to those in Mesozoic and Palaeozoic. 

It is yet another case akin to Azorean region and sitting on trans-oceanic Shetland-Greenland ridge. Looking from the angle of ‘crustal basification’, the presence of xenoliths of granites, dolomites and other sedimentary rocks from Icelandic eruption, the thickened crustal profiles in gravity and seismic tomography are geologically self explanatory. 

 References:

Allen, R.M.G. et al. 2002. Plume driven plumbing and crustal formation inIceland. J. Geophys. Res., v. 107, doi: 10.1029/ 2001JB000584

Anderson, D.L.  2005. Eclogite in the mantle.www.MantlePlumes.org

Bjőrnsson, A., Eysteinsson, H. and Beblo, M., 2005. Crustal formation andmagma genesis beneath Iceland:magnetotelluric constraints. In: Plates, Plumes, and Paradigms. Geol.Soc. Am., p.665-686

Darbyshire, F.A., White, R.S. and Priestley, K.F., 2000.  Structure of the crust and uppermost mantleof Iceland from combined seismic and gravity study. Earth Planet. Sci.Letters, v. 181, p. 409-428

Foulger, G.R., Du, Z. and Julian, B.R., 2003. Icelandic-type crust. Geophys.J. Int., v. 155, p.567-590.

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Luyendyk, B.P., Mudie, J.D.and Harrison, C.G.A., 1968. Lineations ofmagnetic anomalies in the Northeast Pacific observed near the ocean floor. J.Geophys. Res., v. 73, p. 5951-5957

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Storetvedt, K.M., 2010.World Magnetic Anomaly Map and Global Tectonics. New Concepts in GlobalTectonics Newsletter, no. 57, p. 27-52

Storetvedt, K.M. and Longhinos, B. (2011) Evolutionof the North Atlantic: Paradigm Shift in the Offing, Concepts in Global Tectonics Newsletter, No.59, pp 9-48