Biogeography in the Sub-Arctic. Группа авторов
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separation of the western and eastern plates lay close to the present east Greenland coastline. Continental separation often results in formation of micro‐continents, representing ‘splinters’ that remain isolated. The Rockall Plateau became such a micro‐continent, as did also a fragment further north between Iceland and Jan Mayen.
Figure 7 Map showing the break‐up pattern showing the eastward displacement of the central section separating the Vøring Plateau and the Faeroe Plateaux. The shading indicates the outcrop of the flood basalts on Greenland.
Source: Larsen et al. (1989).
Evolution of the oceanic lithosphere was attended by development of the transform faults. At least some of these were probably inherited from faulting on the continents prior to separation. Such faults not only separate different crustal segments, but the rifting style and average magma characteristics are liable to change across them. To the north of one of these, the Jan Mayen Fracture Zone, the mid‐ocean ridge is called the Mohns Ridge. The active Kolbeinsey and the extinct Ægir Ridge lay between this and the evolving Iceland–Faeroe Fracture Zone, whilst south of that (and Iceland) the mid‐ocean ridge is called the Reykjanes Ridge.
Magmatism Heralding the Birth of the New Ocean
Volcanism preceding continental break‐up is generally confined to a narrow zone along which new oceanic crust is generated and is typically subdued. Although phenomena comparable to that of the North Atlantic area are known from some other oceanic openings (e.g. the Red Sea), such voluminous magmatism is the exception rather than the rule. For the North Atlantic the huge quantities of intrusive and extrusive igneous rocks that heralded the opening of the new ocean are now attributed to the ascent within a few million years of abnormally hot proto‐Icelandic plume. The contrast between the much smaller degree of magmatism attending the southern parts of the Atlantic and that attending the North Atlantic opening is ascribed to the major influence of a mantle plume in the latter. The opening of the North Atlantic, and Baffin Bay, was presaged by magma generation along the zones attenuated as a result of Mesozoic continental extension. Whilst the surviving volcanic products can be seen in the eastern coastal regions of Greenland, the Faeroes, north‐western UK as well as in central West Greenland and eastern Baffin, most of the basaltic rocks are submerged and lie off the Greenland and Norwegian shelves, the Faeroe and Rockall plateaux and the Jan Mayen ridge (Figure 8, see Plate section).
Figure 8 Distribution of the early Paleogene lavas, subaerial and submarine. The current spreading centres are marked in red.
Source: Based on Larsen et al. (1994).
The magmatism occurred in two principal periods, (a) 62–58 Ma and (b) 56–52 Ma (Saunders et al. 1997; Fitton and Larsen 2001). The latter period was characterized by higher eruption rates and greater magmatic volumes, accompanied by rapid thinning and rupture of the continental lithosphere. The bulk of the magmatism may have been accomplished within only two to three million years (White 1988).
The start of magmatism in the early stages of the first period typically involved sediments rich in volcanic particles (i.e. volcanogenic sedimentation) and the accumulation of sequences of pillow lavas and hyaloclastite breccias in shallow (non‐marine) waters. Subsequently, sub‐aerial eruptions dominated. During the earlier period a large volume of basalt lavas was erupted in West Greenland and Baffin Island, but within the Northern Irish and Hebridean region activity was more subdued. Eroded remnants of the prodigious quantities of lavas erupted in the second period are preserved along much of the coastal region of East Greenland between latitudes 67.5° and 75°N (Figure 8) and on the Faeroe Islands.
Flood Basalt Eruptions
The lavas were erupted at elevations little above sea‐level and the typical magma channels are inferred to have been fissures up to tens of kilometres long. However, in other cases the magma conduits became more highly localized and, rather than producing an elongate fissure volcano, a ‘shield volcano’ of more or less circular geometry provided the lava source. In both cases the highly fluid lavas flowed widely and sub‐horizontally, in some cases for many tens of kilometres. In East Greenland the lavas cover over 65,000 km2 with sequences up to 7 km thick (Brooks 2011; Figures 8 and 9, see Plate section). Lavas in the Faeroes, approximately coeval with those of East Greenland, are at least 5.5 km thick (Larsen 1988).
Lavas in the British Isles were far less voluminous and rarely built up successions more than 1.5 km thick. What were considered to be the earliest lavas attributable to the proto‐Icelandic plume are the (Danian) basaltic lavas and trachytic tuffs of the Eigg Lava Formation in the Hebridean ‘Small Isles’ (Pearson et al. 1996). However, the Lower Basalt Formation in Antrim (Northern Ireland) appears to be still older, being dated at 62.6 ± 0.3 Ma (Ganerød et al. 2010). Since the chemical composition of the Hebridean–Irish basalts differs significantly from that of the younger lavas of Faeroes and Greenland it is surmised that they arose from mantle with a composition distinct from those of other parts of the North Atlantic.
A central‐type volcano developed early at Rum (Inner Hebrides), in which high‐temperature (picritic) magmas attained shallow crustal levels. It has been argued that these came from a hot outer sheath of mantle that surrounded the plume itself although geochemical characteristics suggest that it arose from a compositionally distinctive part of the plume (Upton et al. 2002). Rapid erosion of the Rum volcano (Emeleus et al. 1996; Emeleus and Bell 2005) was followed by eruption of the Skye lava field (Hamilton et al. 1998).
In response to loading by the lava successions there was synchronous subsidence of the underlying crust. Thus, it appears that successive eruptions could build up lava sequences kilometres thick while each eruptive site was never far from sea‐level. The intervals between one eruption and the next are inferred to have lasted hundreds to thousands of years. With relatively fast weathering under warm, humid conditions, surface features of the lavas were rarely preserved. However, dendritic drainage patterns of river systems developed on the lava plains and inter‐flow fluviatile sediments are preserved (e.g. in the Hebrides). After each eruption, colonization of the lava surface by plants would have been rapid, probably ferns in the first stages followed by forest growth. Jointing patterns in the lavas provide evidence for chilling against trees; some relic fossils of these remain vertical as with the celebrated McCulloch's Tree in the south west of Mull in the Hebrides.
Figure 9 (a) Flat‐lying basalts of the Geikie Plateau Formation, Gåseland, East Greenland, looking towards 1980 m summit.
Source: Photo by W.S. Watt.
(b)