S-wave structure of the Lofoten Margin, N. Norway, from wide-angle seismic data: A review
pp. 231-244

The seismic S-wave structure of the Lofoten Margin, N. Norway, has been obtained from the modelling of three-component Ocean Bottom Seismograph (OBS) data. The relatively high VPVs-ratios estimated for the sediments in most of the area suggest the abundant presence of shales, with the exception of the basin south of the Røst High, where the very low VP/Vs-ratio indicates a higher sand/shale ratio. The VPVs-ratio in the crystalline crust is estimated to 1.75 beneath the continental shelf and seawards to the Vøring Escarpment, and 1.9 beneath the seaward-dipping reflectors (SDR) seaward of the Vøring Escarpment. These values confirm that the crust is of oceanic origin beneath the SDR, and that the crust landward of the Vøring Escarpment is continental. On the continental shelf, a strong S-wave anisotropy is observed in the sediments and in the lower crust, and the modelling of the S-waves also reveals a strong P- and S-wave anisotropy in the upper mantle. The sedimentary anisotropy can be explained by the presence of liquid-filled microcracks aligned vertically along the direction of the present-day maximum horizontal compressive stress in the area, whereas the lower crustal anisotropy might be related to the ( steep) alignment of pore space influenced by ductile strain fabrics inherited from Jurassic-Cretaceous and older extension episodes, or by present ductile strain fabrics in case of a still active deformation. The upper mantle anisotropy can be explained by the steep alignment of the strongly anisotropic mineral olivine. The steep inclination of the minerals might possibly be caused by shearing close to a postulated master fault cutting the crust and upper mantle in this area. The OBS-experiment on the Lofoten Margin has shown that S-waves can be easily included in the modelling, and that they provide valuable geological information that cannot be extracted from P-wave studies alone.

R. Mjelde & M. A. Sellevoll, Institute of Solid Earth Physics, Allégt. 41, University of Bergen, 5007 Bergen, Norway.