Icelandia

Gillian R. Foulger¹, Laurent Gernigon² & Laurent Geoffroy³

¹Department of Earth Sciences, Durham University, Science Laboratories, South Rd. DH1 3LE, UK; g.r.foulger@durham.ac.uk

²Norges Geologiske Undersøkelse (NGU), Geological Survey of Norway, Leiv Erikssons vei 39, N-7491 Trondheim, Norway; Laurent.Gernigon@ngu.no

³Laboratory Geosciences Ocean, UMR 6538 CNRS, IUEM, Université de Bretagne Occidentale, Place Copernic, 29280 Plouzané, France; Laurent.Geoffroy@univ-brest.fr

This webpage is a summary of: Foulger, G.R., Gernigon, L., and Geoffroy, L., 2021, Icelandia, in Foulger, G.R., Hamilton, L.C., Jurdy, D.M., Stein, C.A., Howard, K.A., and Stein, S., eds., In the Footsteps of Warren B. Hamilton: New Ideas in Earth Science: Geological Society of America Special Paper 553, p. 1–12, https://doi.org/10.1130/2021.2553(04)

Introduction

We propose a new, sunken continent beneath the North Atlantic Ocean that we name Icelandia. The region we identify includes most or all of the Greenland-Iceland-Faroe Ridge and the Jan Mayen Microplate Complex and covers an area of ~600,000 km² (Figures 1 and 2). It is contiguous with the Faroe Plateau, which is known to comprise a continental block overlain by lavas and this, in turn, is contiguous with known submarine continental regions offshore Britain. If these are included in a “Greater Icelandia” the entire area is ~1,000,000 km² in size.

We propose that Icelandia is extensively underlain by continental lithosphere in a variety of forms that may include blocks of full-thickness crust and mantle lithosphere and extended, magma-inflated continental crust forming hybrid continental-oceanic crust. Such crust is often termed "transitional" in the context of passive margins. The continental material may be continuous or show boudinage in character. The percentage of continental and magmatic material may vary during drastic thinning and lateral exhumation of the continental lithosphere.

Figure 1: Regional map of the Northeast Atlantic Realm. Bathymetry is shown in color and topography in land areas in gray. DAS: Davis Strait, CGFZ: Charlie-Gibbs Fracture Zone, RR: Reykjanes Ridge, KR: Kolbeinsey Ridge, JMMC: Jan Mayen Microcontinent Complex, AR: Aegir Ridge, FI: Faroe Islands, Red lines: boundaries of the Caledonian orogen and associated thrusts, blue lines: northern boundary of the Hercynian orogen, both dashed where extrapolated into the younger Atlantic Ocean. Green lines: Nagssugtoqidian orogenic belt, magenta line: Rinkian orogeny, orange line: Ketilidian orogeny [from Foulger et al., 2020].

Figure 2: Bathymetric map of the NE Atlantic Ocean. Magenta line—boundary of continental crust; magenta—Icelandia; magenta + beige—Greater Icelandia; GIR—Greenland-Iceland Ridge; IFR—Iceland-Faroe Ridge; JMMC—Jan Mayen microplate complex; WTR—Wyville-Thompson Ridge; FR—Fugløy Ridge; GBB— George Bligh Bank; LB—Lousy Bank; BBB— Bill Bailey’s Bank; FB—Faroe Bank; RB—Rosemary Bank. Base map: Google Earth [from Foulger et al., 2021].

Mechanism of formation

After continental breakup caused the Northeast Atlantic Ocean to begin forming, normal seafloor spreading developed to the north and south of Iceland [e.g., Gaina et al., 2009; Gernigon et al., 2020]. However, in a ~ 300-km-wide band between ~63.5°N and 66.5°N–the future latitude band of Iceland, a strong, geological structure–the frontal thrust of the Caledonian suture–stretched from Greenland to Britain, across the line of breakup. The new spreading plate boundary could not split this barrier, or lithospheric buffer. As a result, while classical seafloor spreading and new oceanic lithosphere developed to the north and south, the barrier extended diffusely. It stretched via multiple, ephemeral spreading zones with relatively short lifetimes that continuously migrated laterally and propagated longitudinally (Figure 3).

Figure 3: The Greenland-Iceland-Faroe Ridge (GIFR) and surrounding areas showing bathymetry and tectonic features. JMMC: Jan Mayen Microcontinent Complex. Thick black lines: axes of Reykjanes and Kolbeinsey Ridges, thin gray lines on land: outlines of neovolcanic zones, dark gray: currently active extensional volcanic systems, dashed black lines: extinct rifts on land, thin black lines: individual faults of the South Iceland Seismic Zone (SISZ), white: glaciers. WVZ, EVZ, NVZ: Western, Eastern, Northern Volcanic Zones, TFZ: Tjörnes Fracture Zone comprising two main shear zones and one (dotted) known only from earthquake epicenters. Thick red lines: extinct rift zones from Hjartarson et al. [2017].

A block of continental crust, probably broken off East Greenland, was trapped in this diffuse extensional zone and transported into the developing ocean [Geoffroy et al. 2015; Geoffroy et al., 2020]. It is probably a southerly extension of the Jan Mayen Microcontinent Complex (Figures 1, 2 & 3). On either side of this block, to the east and west of present-day Iceland, conjugate pairs of volcanic margins developed. These margins have similar structures to the current Vøring Plateau off the Norwegian coast, with seaward-dipping lava flows overlying stretched, magma-inflated continental crust flowing towards the central block (Figure 4) (Yuan et al., 2020).

Figure 4: Volcanic passive margin (Geoffroy et al., 2020). A necking zone is associated with thick Inner-Seaward-Dipping Reflectors (SDRs) and a steeply dipping Moho. The distal ‘continental spreading zone’ has gentler Moho dips and Outer-SDRs (or flat-lying lava flows) on top of an exhumed and magma-injected high-velocity middle/lower crust. In C, LC1 and LC2 (Lower-Crust-1 and Lower-Crust-2) refer to distinct seismic characteristics within the volcanic passive margin high-velocity lower crust.

For the first ~40 Myr of opening, the entire zone from Greenland to the Faroe Islands was subaerial and formed a landbridge spanning the so-called ocean. The parts of this ancient landbridge to the west and east of Iceland subsequently subsided below sea level, but passive-margin-style volcanism continues in Iceland to the present day. There, multiple, volcanic zones are still active simultaneously along with widespread volcanism further afield. This widely dispersed volcanism testifies that the Greenland-Iceland-Faroe continental bridge still has not fully broken up and given way to classical, mono-axial seafloor spreading.

Our developing understanding of Iceland

Interpretations of the geological structure of Iceland and surrounding regions have, for many years, been dominated by the assumption that Iceland was built by essentially the same processes as occur at normal spreading plate boundaries. In this view, the crust is oceanic, and entirely constructed of basaltic volcanic rocks. Its extraordinary thickness, ~6 x that of normal oceanic crust, has been traditionally explained by unusually high temperatures.

Our new study was motivated by the inability of this model to explain several fundamental features of Iceland the surrounding shallow sea floor, and numerous other regional factors. The greatest difficulties were:

These problems, and many others, vanish if a large part of the crust and lithosphere on Icelandia is dominantly continental. Much less melt is then required, dispensing with the need for high temperatures. A relatively low, sub-solidus temperature and the absence of melt in the deeper crust is expected if much of it is old continental crust. There is no difficulty at finding continental, lower-crustal petrologies that match the density and seismic velocity. As regards the great lithosphere thickness observed, only continental lithosphere can provide an explanation.

Testing Icelandia

The existence of Icelandia needs to be tested. There are four ways in which this could be approached:

It is furthermore timely to reappraise the geology of Iceland. The questions needing to be answered are a) where does older crust underlie surface lavas [Foulger, 2006], and b) can the geology of Iceland be interpreted as an assemblage of seaward-dipping reflector–draped blocks of magma-inflated continental crust?

Implications

The theory that extensive continental crust underlies the North Atlantic Ocean raises several important issues:

References