VpIVsratio structure of the Lofoten continental margin, northern Norway, and its geological implications

Abstract

SUMMARY In 1988, an extensive seismic refraction and wide-angle reflection study was performed at the Lofoten passive continental margin, off northern Norway, by using three-component ocean bottom seismographs. The Lofoten margin was formed by continental rifting and sea-floor spreading with extensive volcanism, which is responsible for landward flood basalts and seaward-dipping reflectors (SDR). The P-wave structure beneath the seaward side of the Voting Plateau escarpment at the Lofoten margin is characterized by a thick oceanic lower crust and the existence of an additional high-velocity layer at the bottom of the crust. In this study, the distribution of the P-wave velocity (Vp) to S-wave velocity (Vs) ratio down to the uppermost mantle is obtained by studying P-to-S converted shear-wave data. Vp/Vs ratios of 3.0–5.5 are obtained in the sedimentary layers. These high ratios are attributed to unconsolidated sediments. In the upper crust, a significant difference in Vp/Vs ratios is found between oceanic-side profiles and land-side profiles, i.e. 1.86 oceanward of the SDRs and 1.76 landward of the SDRs. This variation may have been caused by a difference in porosity, due to the difference in extrusive processes: the upper crust of the oceanic side of the SDRs was created by submarine eruptions, while the land side of the SDRs was formed by submarian eruptions. The Vp/Vs ratios in the middle crust and lower crust are estimated as 1.76–1.78 and 1.80, respectively. Comparing Vp and VsV, data from the lower crust to those in previous studies clearly indicates the presence of mafic gneisses. This result represents new seismic evidence for the hotspot hypothesis concerning the formation of passive volcanic continental margins. Clear S-wave upper-mantle refractions can be observed at offsets of up to 100 km, even where P-wave arrivals cannot be identified. In order to model these S-wave arrivals, a VS=4.8 km s-1 layer is necessary 2 km below the Moho.