Early Tertiary lithospheric breakup between Eurasia and Greenland was
accompanied by a transient (approximately 3 m.y.) igneous event emplac
ing both the onshore flood basalts of the North Atlantic Volcanic Prov
ince (NAVP) and huge extrusive complexes along the continent-ocean tra
nsition on the rifted continental margins. Seismic data show that volc
anic margins extend >2600 km along the early Eocene plate boundary, in
places underlain by high-velocity (7.2-7.7 km/s) lower crustal bodies
. Quantitative calculations of NAVP dimensions, considered minimum est
imates, reveal an areal extent of 1.3 x 10(6) km2 and a volume of floo
d basalts of 1.8 x 10(6) km3, yielding a mean eruption rate of 0.6 km3
/yr or 2.4 km3/yr if two-thirds of the basalts were emplaced with 0.5
m.y. The total crustal volume is 6.6 x 10(6) km3, resulting in a mean
crustal accretion rate of 2.2 km3/yr. Thus NAVP ranks among the world'
s larger igneous provinces if the volcanic margins are considered. The
velocity structure of the expanded crust seaward of the continent-oce
an boundary differs from standard oceanic and continental crustal mode
ls. Based on seismic velocities this ''volcanic margin'' crust can be
divided into three units of which the upper unit corresponds to basalt
ic extrusives. The regionally consistent velocity structure and geomet
ry of the crustal units suggest that the expanded crust, including the
high-velocity lower crust which extends some distance landward of the
continent-ocean boundary, was emplaced during and subsequent to break
up. The volcanic margin crust was formed by excess melting within a wi
de zone of asthenospheric upwelling, probably reflecting the interacti
on of a mantle plume and a lithosphere already extending.