Ridge.
Iceland is generally presented as arising from the interaction between a thermal anomaly in the upper mantle, interpreted as a hot spot at the top of a plume, and a major axis of oceanic expansion, the Mid-Atlantic Ridge (MAR ) (Figure 1.4).
The physical, chemical and dynamic characteristics of the Icelandic crust and lithosphere, and more generally of the Northeast Atlantic domain within which it is located, do not fit easily into simple models of continental break-up and accretion of the oceanic lithosphere. Understanding the origin of Iceland requires going back to the history of continental fragmentation between the North American and Eurasian plates in the Meso-Cenozoic (from 250 My). This history was characterized by the temporary individualization of a tectonic plate, Greenland, within a domain deeply marked by the heritage of the Caledonian collision (440–410 My).
1.2. Components of the North Atlantic domain
The major components of the North Atlantic domain are the MAR, the North Atlantic Igneous Province, the Icelandic hot spot and the GFIR.
1.2.1. The Mid-Atlantic Ridge
The MAR is a succession of ridge segments that range from the South Atlantic near Bouvet Island (latitude 54° S) to the North Atlantic, south of the Arctic Circle (latitude 87° N). In its southern part, it marks the boundary between the South American and African plates and in its northern part, it defines the boundary between the Eurasian and North American plates.
These segments are significantly offset by transform faults or zones. The two most important transform zones in the North-MAR are the Jan Mayen Fault Zone (latitude 71° N) and the Charlie–Gibbs Transform Zone (latitude 53° N), which limits to the south the North Atlantic oceanic domain sensu stricto (Figure 1.4). The Icelandic Rift, which represents the part of the ridge that emerged in Iceland, is itself currently shifted about 100 km eastward from the axis of the MAR (Chapter 2).
The North-MAR is a slow to ultra-slow ridge whose rate of expansion decreases toward the north (Le Breton et al. 2012). This rate varies from about 21 mm/year at the axis of the Reykjanes Ridge (slow ridge southwest of Iceland) to 16 mm/year at the Mohns Ridge (north of Jan Mayen). It becomes close to only 6 mm/year in the cold Arctic Ocean at the end of the ultra-slow Gakkel Ridge (Jokat et al. 2003), near the pole of rotation between the Eurasian and North American plates, located in the extreme East of Siberia. In Iceland, the rate of expansion at the axis of the ridge is of the order of 20 mm/year. The complex genesis of the North-MAR and its evolution during the Cenozoic are presented in Chapter 3 (section 3.2).
1.2.2. The North Atlantic Igneous Province
The North Atlantic Igneous Province (NAIP) is a basaltic province of the North Atlantic formed during the Paleogene. Before the opening of the North Atlantic, it extended over a large area (Saunders et al. 1997). Significant remnants of it exist mainly in Northern Ireland, Scotland, the Faroe Islands, and western Greenland (Figure 1.5 and Chapter 3).
At the end of the Cretaceous, the North America–Greenland–Europe region had no significant volcanic activity. Oceanic expansion was taking place between Canada and the southern Labrador Sea. Around 62 My, volcanic eruptions began in a vast region with extensive magma activity (intrusive and extrusive), notably in western Greenland (Chauvet et al. 2019) and in the sector of the Hebrides Islands (Wilkinson et al. 2016) and continued until 56 My.
1.2.3. The Icelandic hot spot
The concepts of hot spot and mantle plume, born at the same time as plate tectonics, are due to John Tuzo Wilson (1963) and Jason Morgan (1971). The first suggested, based on the case of Hawaiian volcanoes, that oceanic volcanic chains could constitute the trace left on a moving lithospheric plate of a fixed magma source located beneath this plate; the second proposed that the feeding of this fixed source was an abnormally hot plume rising from the base of the mantle (layer D’’). Over the next half-century, geophysical and geochemical measurements, numerical modeling and laboratory experiments have refined and consolidated Wilson–Morgan’s theory.
The link between plumes and the vast lava flows that constitute the large igneous provinces (traps and oceanic plateaus) was proposed in the 1980s (Courtillot et al. 1989), the idea being that, when the plumes come close to the surface, they thermally thin the lithosphere and generate very large magmatism. The volumes and rates of magmatic production are of the same order of magnitude as those of the ridges or arcs of the subduction zones. The NAIP would thus have been classically fed by a plume precursor to the one underlying the Icelandic hot spot.
Fluid mechanics have developed a “standard model” of a mantle plume, in which the plume is in the form of a long, narrow conduit – the tail of the plume – topped by a mushroom-shaped head below the lithosphere. Nevertheless, there are several variants of this model, some suggesting that the plumes flow toward and along the ridges through pipe-like channels, others through pancake-like gravity currents (see references to these works in Ito et al. 2003). The thickness of the oceanic crust in the Northeast Atlantic is abnormally high, as is the thickness of the mafic crust located between Greenland and the Faroe Islands. This is classically interpreted as a demonstration of abnormally high mantle melting rate, associated with a warmer than “normal” asthenospheric mantle (White and McKenzie 1989).
Figure 1.5. The North Atlantic Igneous Province (from Storey et al. 2007; Thodarson and Larsen 2007; Chauvet et al. 2019)
COMMENT ON FIGURE 1.5.– GIR: Greenland–Iceland Ridge; IFR: Iceland–Faroe Islands Ridge, connecting the Icelandic hot spot with volcanic accumulations in the British Tertiary Volcanic Province (BTVP) south of the Faroe Islands; CGFZ: Charlie–Gibbs Fracture Zone; JMFZ: Jan Mayen Fracture Zone.
Icelandic magmatism is commonly genetically related to the Paleogene magmatism of NAIP. It seems therefore related to a unique mantle plume that has existed for more than 60 My (Figure 1.6). Iceland could thus be interpreted as a residual plume axis (tail) whose “cap” is cooled.
Figure 1.6. Seismic imaging of the upper mantle at the Icelandic hot spot
COMMENT ON FIGURE 1.6.– Imagery illustrating the narrow cylindrical shape of the mantle plume (A), with a radius of about 150 km (low velocity anomaly to a depth of at least 400 km) (from Wolfe et al. 1997). Vertical sections in the ICEMAN model by Allen et al. (2002) perpendicular (B) and parallel (C) to the rift. These sections illustrate the vertical conduit between 400 and 200 km deep and the horizontal plume
