Transitional continental-oceanic structure beneath the Norwegian Sea from inversion of surface wave group velocity data

We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone, Using the multiple filter analysis technique, we isolated the Rayleig h and Love wave group velocities for periods between 10 and 50 s. The surfa ce wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inv erted and produced shear wave velocity models that correspond to a quasi-co ntinental oceanic structure in the Greenland-Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18 km in the Norwegian Sea area and 9 km in the region between Sva lbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedi mentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9 k m s(-1) for most paths. An average lithospheric thickness of 60 km was obse rved, which is lower than expected for oceanic-type structure of similar ag e. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30 km b eneath the surface can account for the thin lithosphere and observed low ve locities. Anisotropy coefficients of 1-5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization ani sotropy in the region.