Imaging igneous rocks on the North Atlantic rifted continental margin

Abstract

SUMMARY Extruded basalt flows with thicknesses of several kilometres occur ubiquitously along the rifted continental margins of the northern North Atlantic. Their total volume exceeds 1 million km 3 , and may reach several million cubic kilometres. Intruded igneous rock comprising the complementary melt fraction to that extruded at the surface should exist in the lower crust with a volume at least as large as that of the extrusive basalts. To image the extrusive and intrusive igneous rocks on the continental margin near the Faroe Islands, in 2002 we shot a 375 km long deep-penetration seismic profile across the margin using 85 ocean bottom seismometers for wide-angle acquisition and shot a separate reflection profile using a 12 km streamer for optimal imaging. By using large airgun sources tuned to produce low frequency energy we were able to constrain the seismic velocity structure of the whole crust. We imaged not only layering and structure within the extrusive basalts, but also lower-crustal intrusions on the continent–ocean transition (COT). Combination of good velocity control on the lower-crust together with direct imaging of the extrusive basalts enables us to constrain the volumes of extrusive and intrusive igneous rocks produced near the Faroe Islands to be 340–420 and 560–780 km 3 per kilometre along strike, respectively. The COT marked both by high-velocity intrusions and lower-crustal layering is surprisingly narrow, with only 50 km separating unstretched continental crust from fully oceanic crust. In contrast, the extruded basalts flow up to 150 km landward at the paleo-surface.