V-P V-S RATIO STRUCTURE OF THE LOFOTEN CONTINENTAL-MARGIN, NORTHERN NORWAY, AND ITS GEOLOGICAL IMPLICATIONS/

In 1988, an extensive seismic refraction and wide-angle reflection stu dy was performed at the Lofoten passive continental margin, off northe rn Norway, by using three-component ocean bottom seismographs. The Lof oten margin was formed by continental rifting and sea-floor spreading with extensive volcanism, which is responsible for landward flood basa lts and seaward-dipping reflectors (SDR). The P-wave structure beneath the seaward side of the Voring Plateau escarpment at the Lofoten marg in is characterized by a thick oceanic lower crust and the existence o f an additional high-velocity layer at the bottom of the crust. In thi s study, the distribution of the P-wave velocity (V-p) to S-wave veloc ity (V-s) ratio down to the uppermost mantle is obtained by studying P -to-S converted shear-wave data. V-p/V-s ratios of 3.0-5.5 are obtaine d in the sedimentary layers. These high ratios are attributed to uncon solidated sediments. In the upper crust, a significant difference in V -p/V-s ratios is found between oceanic-side profiles and land-side pro files, 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 t o the difference in extrusive processes: the upper crust of the oceani c side of the SDRs was created by submarine eruptions, while the land side of the SDRs was formed by subaerial eruptions. The V-p/V-s ratios in the middle crust and lower crust are estimated as 1.76-1.78 and 1. 80, respectively. Comparing V-p and V-s data from the lower crust to t hose in previous studies clearly indicates the presence of mafic gneis ses. This result represents new seismic evidence for the hotspot hypot hesis 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 orde r to model these S-wave arrivals, a V-s = 4.8 km s(-1) layer is necess ary 2 km below the Moho.