V p/V s-ratios from the central Kolbeinsey Ridge to the Jan Mayen Basin,North Atlantic; implications for lithology,porosity and present-day stress field |
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| Authors: | Mjelde Rolf Aurvåg Roar Kodaira Shuichi Shimamura Hideki Gunnarsson Karl Nakanishi Ayako Shiobara Hajime |
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| Institution: | (1) Institute of Solid Earth Physics, Univ. of Bergen, Allégt. 41, 5007 Bergen, Norway;(2) Institute of Solid Earth Physics, Univ. of Bergen, Allégt. 41, 5007 Bergen, Norway;(3) Institute of Seismology and Volcanology, Hokkaido University, Sapporo, 060, Japan;(4) Institute of Seismology and Volcanology, Hokkaido University, Sapporo, 060, Japan;(5) National Energy Authority, Grensasvegi 9, 108 Reykjavik, Iceland;(6) Institute of Seismology and Volcanology, Hokkaido University, Sapporo, 060, Japan;(7) Institute of Seismology and Volcanology, Hokkaido University, Sapporo, 060, Japan |
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| Abstract: | The horizontal components from twenty Ocean Bottom Seismometers deployed along three profiles near the Kolbeinsey Ridge, North Atlantic, have been modelled with regard to S-waves, based on P-wave models obtained earlier. Two profiles were acquired parallel to the ridge, and the third profile extended eastwards across the continental Jan Mayen Basin. The modelling requires a thin (few 100 m) layer with very high V
p/V
s-ratio (3.5–9.5) at the sea-floor in the area lacking sedimentary cover. The obtained V
p/V
s-ratios for the remaining part of layer 2A, 2B, 3 and upper mantle, correspond to the following lithologies: pillow lavas, sheeted dykes, gabbro and peridotite, respectively. All crustal layers exhibit a decreasing trend in V
p/V
s-ratio away-from-the-axis, interpreted as decreasing porosity and/or crack density in that direction. A significant S-wave azimuthal anisotropy is observed within the thin uppermost layer of basalt near the ridge. The anisotropy is interpreted as being caused by fluid-filled microcracks aligned along the direction of present-day maximum compressive stress, and indicates crustal extension at the ridge itself and perpendicular-to-the-ridge compression 12 km off axis. Spreading along the Kolbeinsey Ridge has most likely been continuous since its initiation ca. 25 Ma: The data do not suggest the presence of an extinct spreading axis between the Kolbeinsey Ridge and the Aegir Ridge as has been proposed earlier. The V
p/V
s-ratios found in the Jan Mayen Basin are compatible with continental crust, overlain by a sedimentary section dominated by shale. |
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