Crustal structure of Edoras Bank continental margin and mantle thermal anomalies beneath the North Atlantic

Abstract

We discuss the structure of the continent‐ocean transition of the Edoras Bank area of the NE Atlantic margin and the influence of the Iceland mantle plume during continental breakup. Edoras Bank lies close to the present‐day limit of influence of the Iceland plume. Sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well‐developed over much of the margin. They form aerially extensive, dipping sheets, with individual reflectors imaged on margin‐parallel profiles as laterally continuous and subhorizontal features. We interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup. A crustal velocity model for the margin was developed by travel time modeling of wide‐angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling. Oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities. The crustal thickness increases to the SE reaching 17 km beneath the continental fragment of Edoras Bank. The continent‐ocean transition is dominated in the lower crust by a lens up to 8 km thick of high‐velocity material (7.2–7.5 km s−1), which exhibits a very low velocity gradient. It extends over a 60 km wide region beneath the seaward dipping reflectors. The high velocities and densities of the lower crust are probably caused by igneous rock with a high MgO content that underplated the margin or heavily intruded pre‐existing crustal material during continental breakup. Both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by Deep Sea Drilling Project (DSDP) boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup. Comparison of the structure and subsidence history of the Edoras Bank margin with other areas of the NW European margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the Iceland mantle plume during and following continental breakup. We find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle ∼150°C hotter than normal lay beneath the 2000 km long rift, extending from the Lofoten margin off Norway in the north to the Edoras Bank margin in the south. Over a period of 5–10 m.y. following the onset of rifting, the mantle thermal anomaly decayed rapidly to only 50°C or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism.