Crustal structure of the southeastern Iceland-Faeroe Ridge (IFR) from wide aperture seismic data

Abstract

The aseismic Iceland-Faeroe-Ridge (IFR) is of central importance in reconstructing the opening of the North Atlantic Ocean. To investigate the crustal structure of the IFR at its southeastern part we conducted a wide aperture seismic survey across the IFR from the Iceland Basin to the Norway Basin. Seismic energy was generated by two 60-litre sleeve guns and recorded by 42 ocean-bottom seismometers (OBS). Kinematic and dynamic forward modeling by two-point ray tracing was applied to develop a 2D velocity-depth model. The accuracy of the model obtained is better than 5% for both velocity and depth of interfaces. We find a Moho depth of 23 km below the crest of the ridge that decreases to about 15 km at either end of the line. The upper part of the crust exhibits P-wave velocities between 5.7 and 6.3 km/s that increase towards the Norway Basin. The lower crust is more homogeneous with vp ranging from 6.6 to 7.0 km/s. Upper and lower parts of the crust are separated by a first order discontinuity. Amplitudes (PmP) and arrival times (Pn) of wide-angle phases indicate upper-mantle velocities of 7.9 km/s. Furthermore we analyzed PS-converted phases for upper crustal depth levels and find a vp/vs ratio of 1.73±0.04 equal to a Poisson's ratio of 0.25 with no significant lateral variations along the seismic line. On the crest of the ridge we identified a basalt layer embedded within the sedimentary sequence that was also penetrated by deep sea drilling and dated to 43±3.3 million years. Below the basalt layer we observed a low-velocity layer that is identified as Mesozoic sediment that was also observed to either end of the basalt. We conclude that the southeastern part of the IFR is composed of stretched continental crust and thus part of Paleo-Europe rather than oceanic crust formed by the Iceland hot spot. This conclusion is supported by the distribution of magnetic anomalies that allow identifying the passive continental margin approximately 100 km to the NW of our seismic line.