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 v p ranging from 6.6 to 7.0 km/s. Upper and lower parts of the crust are separated by a first order discontinuity. Amplitudes (P m P) and arrival times (P n ) 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 v p /v s ratio of 1.73 AE 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 43AE 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. #

Moho and basement depth in the NE Atlantic Ocean based on seismic refraction data and receiver functions

“…Two lines were completely removed from the Moho dataset. The first line is the IFR profile (Bohnhoff & Makris 2004) across the Iceland -Faroe Ridge, which has a substantially lower Moho depth (23 km) than the FIRE offshore line (Richardson et al 1998) along the ridge (.30 km) (Fig. 4).…”

The stratigraphy and structure of the Faroese continental margin

“…However, seismic refraction data provide some indications of the nature and character of the sub-volcanic succession, and information has also been compiled from various studies (e.g. Pálmason 1965;Richardson et al 1998;Smallwood & White 2002;White et al 2003;Bohnhoff & Makris 2004;Raum et al 2005) that intersect the chosen transect of this study (Figs 1-3) where important information such as e.g. the top of the crust has been plotted.…”

“…1), whereas the magnetic chrons stop when they reach the ridge. In transect section 1, on the SE part of the IFR, Bohnhoff & Makris (2004) interpreted the crust to be about 15 km thick and of a stretched continental character, with the top at a depth of 3 km (Fig. 4).…”

“…In terms of the pre-volcanic stratigraphy, some information is derived from a NE-SW refraction seismic line orientated approximately perpendicular to the trend of the IFR (Fig. 1b), and which crosses the shallow crustal transect in the southern part of section 2 and at the boundary between sections 3 and 4, where DSDP site 336 is located (Bohnhoff & Makris 2004). At these two intersecting points, a low-velocity layer beneath the volcanic sequence has been proposed, which has been suggested to represent Mesozoic sedimentary rocks, with a thickness of 0.5 and 1.5 km, respectively (Figs 1b & 4).…”

“…Although a number of sub-basins and highs have been interpreted in the western half (Faroese sector) of the FaroeShetland Basin, it is not possible at presentowing to the limited subsurface imaging of the basalt and the lack of well penetrations -to get a clear picture of the underlying pre-volcanic succession. Refraction seismic data have been used to suggest that Mesozoic and Palaeozoic sedimentary rocks might occur between the basement and the volcanic strata (Richardson et al 1998Smallwood & White 2002;White et al 2003White et al , 2008Bohnhoff & Makris 2004;Raum et al 2005), but this remains unsubstantiated as no well in the Faroese sector has penetrated strata older than Paleocene. Thus, the presence of Upper Palaeozoic and Mesozoic rocks in the Faroese sector of the Faroe -Shetland Basin remains largely inferred.…”