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Bullard Laboratories, Department of Earth Sciences, Madingley Road, Cambridge CB3 OEZ, UK
Enormous variations exist along rifted margins of the North Atlantic in the magmatic activity that accompanied continental breakup. The North Atlantic margins exhibit a full range of magmatic behaviour. One end of the spectrum (commonly termed ‘volcanic’ margins) is found in the northern North Atlantic off Greenland, Norway and northwest Britain, where huge volumes of igneous rock were added to the continental crust as it rifted, and the adjacent oceanic crust is considerably thicker than normal. The other end of the spectrum (so-called ‘non-volcanic’ margins) occurs further south off France and Spain, where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust.
Results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced. The volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle, and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere. Large quantities of igneous rock were produced as the continents broke up to form the North Atlantic where the newly initiated Iceland plume brought abnormally hot mantle beneath the rift. Much smaller quantities of melt were produced further south, where mantle temperatures were normal during rifting.
The distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust. Where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years, as is common during the initial stages of rifting, then the rising asthenospheric mantle has time to cool by conduction. Heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region. As a consequence of the reduced mantle temperatures, considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts. This explains the abnormally thin oceanic crust found adjacent to the non-volcanic rifted continental margins in the North Atlantic. As the new ocean widens, the spreading centre becomes isolated from the continental margins on either side by newly formed, warm oceanic lithosphere, and the rate of rifting generally increases. The oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces, and all the melt formed by decompression of the mantle solidifies close to the spreading axis.
Basaltic melts can intrude continental crust far more easily than oceanic crust. Basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts. This redistributes melt laterally towards the continent in both volcanic and, to a lesser extent, in non-volcanic margins and may be a contributory factor in leaving unusually thin crust in the first-formed ocean immediately adjacent to some rifted margins. It also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the Rockall area northwest of Britain.
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