Continental break-up in the northern North Atlantic was accompanied by massive volcanism. This produced sequences of basalt flows erupted close to sea-level, which are imaged as seaward-dipping reflectors on seismic reflection profiles. They comprise the upper 4-5 km of transitional crust in both the Edoras Bank and Hatton Bank areas of the Rockall continental margin. Detailed surveys show that the reflectors are convex-upward on dip lines but are sub-horizontal on strike lines. They are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis. Along the Rockall margin two sequences of seaward-dipping reflectors occur, separated by an 'outer high' near the foot of the continental margin. The landward set of basalt flows are emplaced above stretched and intruded continental crust, while the seaward set span the region from highly distended continental crust to fully oceanic crust. The outer high is capped by basalt flows but exhibits little coherent internal reflectivity. We discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of Iceland.

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