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

Серия:

isbn: 9781118561355

isbn:

СКАЧАТЬ

      Evidence of continental uplift preceding the magmatism is plausibly attributed to the arrival (‘impact’) of the buoyant and abnormally hot mantle plume. Uplift and magmatism preceded the continental rupturing that marked the genesis of a new ocean floor. At its earliest arrival the plume axis is inferred to have lain beneath west‐central Greenland but, as Greenland drifted westwards across the axis of the plume, the latter came to underlie a more central site under Greenland by 55 Ma (Lawver and Müller 1994; Saunders et al. 1997).

      After first encountering the continental lithosphere the plume, with a temperature some 150 °C hotter than the normal mantle, spread out rapidly beneath the lithosphere in a crudely circular disposition ~2500 km diameter (White and McKenzie 1995; Smallwood and White 2002). It caused transient crustal uplift, large‐scale volcanism and complementary intrusive activity in conjunction with stretching and thinning of the continental lithosphere. Eventually the continental lithosphere parted, sea‐floor spreading commenced and the North Atlantic Ocean was born.

Photos depict (a) conglomerate from a 2–3 m thick stratum separating Cretaceous shales from the base of the overlying plateau lavas on Wollaston Forland, East Greenland (75° N). Cobbles are of quartzite and muscovite granite. (b) Aerial photograph of the same stratum on Kap Broer Ruys, East Greenland (73°30′N).

      Source: B.G.J. Upton.

      Early in the Eocene mantle temperatures in the head of the plume may have fallen rapidly (Smallwood and White 2002) and the plume is inferred to have assumed a narrower sub‐cylindrical form with a ‘central core’ about a 100 km across. Magmatism due to this ‘plume tail’ marked out the shallow submarine welt of the Greenland–Iceland–Faeroes ridge as the Greenland/American tectonic plate migrated westwards. This ridge is characterized by an abnormal thickness (30–40 km) of oceanic crust and rises to shallow depths. The plume tail itself (the Iceland plume) is now considered to underlie eastern Iceland.

      Before the arrival of the postulated mantle plume, the landscapes of Greenland, Norway and Britain largely consisted of relatively high ground composed of early Palaeozoic and Precambrian metamorphic rocks transected by low‐lying faulted basins. The latter, developed during Mesozoic extensional tectonics, were subject to occasional marine inundation. The faulting had structurally preconditioned a tract of the Laurasian continent to a state ripe for exploitation by later rifting and magmatism in the Palaeocene. The embryonic ocean first appeared at approximately 56 Ma around the Palaeocene–Eocene boundary (currently defined at 55.8 ± 0.2 Ma). For the previous 10 Ma we may visualize rifted landscapes roughly comparable to those of Kenya and Ethiopia at present and which appears to have been well vegetated with an equable climate (Walker 1979).

Schematic illustration of fault pattern in the North Atlantic region on a map restored to continental dispositions prior to ocean opening.

      Source: Jolley and Bell (2002).

An illustration of a map depicting the orogenic belts on either side of the North Atlantic. Note that between Greenland and Norway the North Atlantic is approximately bilaterally symmetrical through the Caledonian Orogenic belt.