Biogeography in the Sub-Arctic. Группа авторов

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.

An illustration of a map depicting the break-up pattern showing the eastward displacement of the central section separating the Vøring Plateau and the Faeroe Plateaux.

      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.

An illustration of a map depicting distribution of the early Paleogene lavas, subaerial and submarine.

      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.

      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).

      Photo depicts flat-lying basalts of the Geikie Plateau Formation, Gåseland, East Greenland, looking towards 1980 m summit. Photo depicts flat-lying basalts of the Geikie Plateau Formation, Gåseland, East Greenland, looking towards 1980 m summit.

      Source: Photo by W.S. Watt.

      (b)


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