Biogeography in the Sub-Arctic. Группа авторов

1 The Opening of the North Atlantic

       Brian G. J. Upton

       School of GeoSciences, University of Edinburgh, UK

      The northern landmasses, namely North America, Greenland and Europe/Asia were part of one global super‐continent in the lower Palaeozoic, approximately 420–430 million years (Ma) ago. This super‐continent (Pangaea) resulted from continental collisions. Driven by convective flow deep in the interior of the Earth it is the nature of continents to break apart, re‐join and to come apart again. Such a separation and amalgamation constitutes ‘the Wilson Cycle’, which takes several hundred million years to run its full course. No sooner had Pangaea come into existence than it became subject to tectonic stresses that tended to disrupt it. The North Atlantic Ocean is a late product of the disintegration of Laurasia, a part of Pangaea, which split to form North America, Greenland, Europe and Asia. Continental separation had begun in the south Atlantic region at ca 130 Ma and spread north by 60–50 Ma.

Plate Tectonic Résumé

      Before considering the birth and growth of the North Atlantic, a brief résumé concerning plate tectonics is in order. Volumetrically the greater bulk of the planet is composed of the mantle. The latter, itself covered by thin veneers of crust, hydrosphere and atmosphere, extends down to a depth of 2885 km, i.e. to the outer boundary of the core from which it receives heat. The mantle is composed of various magnesium‐rich silicates and oxides and is deduced to behave as a ductile material that is in constant slow convective motion, flowing whilst remaining (almost entirely) in the solid state. The flowage is due to variations in its composition and temperature (principally the latter) that confer different densities to some parts. Consequently, the relative buoyancy of those parts with lower density causes them to rise while, simultaneously, other denser parts sink to take their place.

Whereas most of the mantle is thought to behave in a ductile manner, a relatively thin outer layer differs in being mechanically rigid and is known as the lithosphere. The lower and larger part of this comprises the lithospheric mantle whilst the upper layer (the crust) consists of less magnesian and more siliceous and aluminium‐rich rocks. The lithosphere is sub‐divided into some 30 major tectonic plates and a host of micro‐plates. These tectonic plates, floating on the convecting underlying mantle, move relative to each other in one of three ways. They may (a) slide past each other without colliding, (b) collide and under‐ or over‐ride another plate or (c) just move apart. Typically, the continental lithosphere has a thickness of >100 km but under the oceans the lithosphere is much thinner, ranging from zero (at the mid‐ocean ridges) to ~100 km. Older, colder and thicker parts of the oceanic lithosphere sink back into the deeper mantle at subduction zones where they undergo re‐cycling. This loss of oceanic lithosphere is counterbalanced by continuous growth of new lithosphere along the ‘mid‐ocean ridges’ or ‘constructive plate boundaries’, where the plates move apart. This juvenile lithosphere is formed from material arising from the underlying ductile mantle. In the context of oceanic lithosphere, ‘old’ means having ages of up to ~200 Ma whereas the rocks of the continental lithosphere have ages of anything up to 4000 Ma.

      Continental lithosphere differs from its oceanic counterpart not only in being older and thicker but in composition, complexity and overall lower density. The last of these causes it to ‘float’ higher so that most rises above sea‐level and the remainder is covered only by shallow seas and constitutes the continental shelves. By contrast, because of its higher density, the oceanic lithosphere lies at lower levels than its continental counterpart and its surface is almost invariably submarine, with the ocean floors generally lying at depths of ~4 km. The oceanic lithosphere, with its constructive plate boundaries (mid‐ocean ridges) and corresponding destructive plate boundaries (along subduction zones, generally demarcated by deep ocean trenches), covers some 5/7th of the Earth's surface. The continental shelves slope steeply down from depths of ~2 km to the deep ocean floors. Consequently, the submarine 2 km contour approximates the change‐over from one type of lithosphere to the other. On the western side of the North Atlantic the continental shelf is very narrow, contrasting with the European side where it is much broader.

The first mid‐ocean ‘ridge’ to be recognized was the ‘mid‐Atlantic Ridge’ along the Atlantic axis (Heezen et al. 1959: Figure 1, see Plate section). This was subsequently shown to be merely a part of a great circum‐global ridge system, some ~80 000 km long, rising to heights of 3 km or more from the deep (‘abyssal’) oceanic plains. Typically, these ridges bear a rift‐valley along their crests that can be up to 10–20 km wide and with a relief of ~1 km (Bown and White 1994).

      The ocean floor moves away symmetrically on either side of the mid‐ocean ridges as new lithosphere is generated along them. The process of ocean‐floor spreading causes a reduction of pressure along the axes which in turn promotes partial melting, to the extent of some 10–15%, of the asthenospheric mantle that underlies the lithosphere. The melt product is basaltic magma that, being less dense than the residual solid mantle, ascends towards the surface. Losing heat as it does so, most crystallizes to form intrusive rocks within the oceanic crust and the remainder erupts as lava on the ocean floor. Although the volume of lava erupted from the mid‐ocean ridges is estimated to be between 2 and 3 km³/year, a much greater volume is estimated to crystallize to form the underlying intrusions.

      The spreading process confers bilateral symmetry to the mid‐ocean ridges as juvenile material is welded on either side to the pre‐existing lithosphere. The mantle rocks that are not consumed in the melting remain behind, contributing to the lower part of the oceanic lithosphere. Sea‐floor spreading occurs at rates that can exceed 100 mm/year, whilst that in the North Atlantic is roughly 25 mm/year (Bown and White 1994).

When extensional stresses acting on a ridge sector become too great, rupturing (rifting) occurs along its crest and the new‐formed magma rises up a vertical fissure to form an intrusive ‘dyke’. The dykes cool to form tabular, near‐vertical intrusions, typically not more than a few metres wide. Their repeated generation produces a ‘sheeted complex’ in which the youngest are those newly formed along the spreading axis and the oldest are those furthest away. Dykes can compose virtually the entirety of the sheeted complexes lying beneath the lavas extruded on the ocean floor.