Biogeography in the Sub-Arctic. Группа авторов

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СКАЧАТЬ 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) View of the steep east coast of Greenland (the Blosseville coast) between ca. 67° and 69° N, composed of horizontal basalt lavas, west of the seaward flexing.

      Source: Photo by B.G.J. Upton.

Seaward Dipping Reflectors

In the Kangerdlugssuaq region of East Greenland transition from flat‐lying lavas into a zone in which the lavas acquire increasingly steep dips towards the coast was first recorded by Wager (1934, 1947) and described by Larsen and Jakobsdottir (1988) (Figure 10, see Plate section). The zone of down‐warping is accompanied by an increase in the intensity of the coast‐parallel dyke swarm, culminating in an underlying ‘sheeted complex’ comparable to those seen in ophiolite occurrences regarded as uplift portions of oceanic crust (Nielsen and Brooks 1981; Muttervet al. 1982; Brooks 2011). It came to be realized that what marine seismologic surveyors referred to as ‘seaward‐dipping reflectors’, occurring as belts up to 150 km wide, occur on both sides of the Atlantic and also in west Greenland, were the off‐shore equivalents of these down‐warped lavas.

Figure 10 Kap Hammer on the East Greenland coast (67°40′N), within the zone showing maximum seaward flexing. The cliffs here consist almost entirely of dykes, thus composing a ‘sheeted complex’. Since the flexure here is eastwards, the dykes have a corresponding westward dip. The greater the inclination, the older the dyke. A few (young) near‐vertical dykes are seen in the near cliffs.

      Source: Photo by B.G.J. Upton.

The lavas acquired their seaward dips long after the time of their eruption, which had been sub‐aerial (Brooks 2011). Since the total thickness of lavas and their associated intrusions on the rifted margins reaches 25–35 km (Reid et al. 1997; Smallwood and White 2002), these great quantities of basaltic materials on the thinned continental lithosphere would have very significantly increased its bulk density causing the down‐warping. Figure 11 (see Plate section) shows subaerial exposures in West Greenland analogous to those of East Greenland where initially sub‐horizontal lavas were down‐warped towards a new ocean floor in Baffin Bay. As demonstrated in East Greenland, the down‐warping was achieved, not by ductile folding but by displacements along a myriad normal faults dipping away from the developing ocean (Nielsen 1975; Nielsen and Brooks 1981).

Ash Beds of Western and Central Europe

Palaeocene–Eocene basaltic ashes occur widely across north‐west and central Europe (Knox and Morton 1988; Egger et al. 2000). It has been concluded that the ashes were the products of extremely energetic fountaining of magma close to the time of ocean‐opening (Fitton and Larsen 2001; Larsen et al. 2003). The continental crust thinned and subsided, eventually sinking beneath sea‐level as it became ever more laden with lavas and mafic intrusions. At this critical stage, close to parting and formation of the embryonic ocean, the rising magmas interacted with shallow sea waters, producing cataclysmic steam‐driven ash eruptions.

Figure 11 Sub‐aerial (picritic) lavas on the Svartenhuk peninsula, West Greenland (71° 30′N). These lie within the flexed zone, dipping westwards towards the Baffin Bay spreading centre. Such ‘seaward‐dipping reflectors’ are generally sunk below sea‐level but in this instance they are well seen sub‐aerially.

      Source: Photo by T.C.R. Pulvertaft.

Ashes from these eruptions travelled hundreds of kilometres eastwards and are reported from as far away as Austria (Egger et al. 2000). Those falling into the shallow seas that covered much of western Europe were generally diluted by normal terrestrial sedimentation and are thus hard to recognize. However, in western Denmark in a shallow marine basin in which diatomite sediment was being very slowly accumulated, some two hundred ash layers contrast strikingly with the intervening white sediments (Figure 12, see Plate section). The alternation of the ‘ash’ beds and white clay reflects the repose periods between eruptions (Fitton and Larsen 2001; Larsen et al. 2003).

Whilst the older ash layers are inferred to have come from sub‐aerial volcanoes on the thinning continental lithosphere, the climactic (‘Stage 4’) ashes are attributed to a time when the locus of the proto‐Iceland plume had shifted away from the Greenland continent into the sea‐covered opening rift. Interaction of incandescent magmas and sea water caused the change from relatively quietly effusive to violently explosive eruptions.

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