| Summary: | The North Atlantic provides classic examples of volcanic rifted continental margins but in such regions flood basalt sequences provide a challenge to deep seismic imaging. High-quality, wide-angle, ocean-bottom seismometer (OBS) data have been acquired with a low frequency (9 Hz) seismic source across two margins. S-wave arrivals, which are dominantly converted from P- to S-waves at the sediment-top basalt interface, were recorded at 170 four-component OBS locations. Tomographic inversion of over 70,000 converted S- wave crustal diving waves and Moho reflections was successfully performed to produce S-wave velocity models using two methods. A flexible layer-based approach was used for initial modelling and quality control while a more automated grid-based approach, which required correction of the S-wave travel-times to effectively symmetric ray paths, provided more robust models and measures of uncertainty through Monte Carlo analyses. The Vp/Vs ratio structures of the margins were estimated by combining P- and S-wave velocity models. The Vp/Vs ratio provides insight to crustal composition. Across the continent-ocean transition, P-wave velocities change from 6.8 km/s to 7.3 km/s and Vp/Vs ratios increase from 1.70 to 1.78 indicating increased levels of mafic intrusion into the crystalline lower continental crust oceanward. The oldest oceanic crust beneath the profiles has high lower-crustal P-wave velocities (up to 7.5 km/s) and low Vp/Vs ratios (1.75- 1.80) caused by the melt produced from abnormally hot mantle immediately after continental break-up. A sub-basalt P- and S-wave low velocity zone was modelled on the Faroes margin and comparison of the modelled properties with measurements from relevant lithologies reveals that this zone is likely to contain largely sedimentary rock rather than igneous hyaloclastites similar to those found beneath the basalt in the Lopra-1/1A borehole. The presence of a sedimentary basin prior to volcanism, beneath the top of what is now the Fugloy Ridge, is also indicated by the properties of the unit immediately beneath the low velocity zone with Vp/Vs ratios of 1.80-1.85 and P-wave velocities of 5.5-6.0 km/s, consistent with sill-intruded Mesozoic sedimentary rock.
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