Biogeography in the Sub-Arctic. Группа авторов
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Название: Biogeography in the Sub-Arctic
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781118561355
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Source: Gee et al. 2008.
Figure 6 A coastal section on Cape Searle Island, Baffin Island, Canada, showing a succession similar to that of parts of the East Greenland coast. Palaeocene non‐marine sediments form the basal third, with white sandstones and dark coals and organic‐rich shales. The central part (pale brownish and not well stratified) is of sub‐aqueous volcanic rocks (picritic hyaloclastites and pillow breccias). The upper third consists of subaerially erupted picritic lavas.
Source: B.G.J.Upton
The Geographical Pattern of Break‐Up
The bathymetry of the ocean shows that the continental shelf alongside east Greenland is narrow compared with that on the European side. Spreading started in the south and propagated northwards at a rate of ~1 m/year (Larsen 1988). Initially the line of opening lay close to the present East Greenland coastline, apart from the region between Kangerdlugssuaq and Hold with Hope where it had a large deflection, convex to the east. Thus, the line of parting was divided into three (Figure 7). To the north and south of this easterly deflection spreading has been simple and continuous up to the present. By contrast, in the deflected mid‐section spreading was complex, with repeated shifts in position of the spreading axis. The mid‐section produced the lavas of central East Greenland and, after continental breaking, the lavas of Iceland and the now extinct Ægir Ridge–Jan Mayen spreading axis (Larsen et al. 1989).
Micro‐continent Formation
The initial 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 СКАЧАТЬ