S-wave anisotropy off Lofoten, Norway, indicative of fluids in the lower continental crust?

Abstract

SUMMARY P- and S-wave velocity models along two 145 and 175 km long perpendicular profiles on the continental shelf off Lofoten, northern Norway, have been obtained from a study of nine three-component ocean-bottom seismographs. The S-wave model has been achieved from a study of the high-quality, horizontal component data. On one OBS the S-wave reflection from the Moho can be followed almost continuously from vertical to wide-angle incidence. This unique observation indicates the presence of about 10 per cent S-wave anisotropy in the lower crust along the NE-SW profile. No such anisotropy is observed along the NW-SE profile, and it is suggested that the inferred anisotropy might be caused by liquid-filled microcracks or pores aligned vertically along this profile. The vertical NW-SE alignment of the microcracks/pores might be a result of the present-day stress-field (the maximum compressive stress trends NW-SE), or it might be influenced by recent ductile strain fabrics or by ductile strain fabrics inherited from earlier deformation episodes. Another possible explanation for the inferred anisotropy might be alignment of anisotropic minerals. Many highly anisotropic minerals like olivine, pyroxene and hornblende can be excluded in this case, since the inferred S-wave anisotropy is at least three times higher than the P-wave anisotropy. Alignment of the mineral kyanite, however, might possibly explain the observations, since this mineral has much higher S-wave anisotropy than P-wave anisotropy. The hypothesis involving fluids is preferred since the very high seismic velocities of kyanite do not seem to be compatible with the estimated lower crustal velocity in this area (V, = 6.8 km s-').