Lower-crustal intrusion on the North Atlanticcontinental margin

Abstract

When continents break apart, the rifting is sometimes accompaniedby the production of large volumes of molten rock. The totalmelt volume, however, is uncertain, because only part of it haserupted at the surface. Furthermore, the cause of the magmatism isstill disputed—specifically, whether or not it is due to increasedmantle temperatures. We recorded deep-penetration normalincidenceand wide-angle seismic profiles across the Faroe andHatton Bank volcanic margins in the northeast Atlantic. Herewe show that near the Faroe Islands, for every 1 km along strike,360–400 km3 of basalt is extruded, while 540–600 km3 is intrudedinto the continent–ocean transition. We find that lower-crustalintrusions are focused mainly into a narrow zone 50 km wideon the transition, although extruded basalts flow more than100 km from the rift. Seismic profiles show that the melt isintruded into the lower crust as sills, which cross-cut the continentalfabric, rather than as an ‘underplate’ of 100 per cent melt, ashas often been assumed. Evidence from the measured seismicvelocities and from igneous thicknesses are consistent with thedominant control on melt production being increased mantletemperatures, with no requirement for either significant activesmall-scale mantle convection under the rift or the presence offertile mantle at the time of continental break-up, as has previouslybeen suggested for the North Atlantic Ocean.