Tomographic inversion of seismic P- and S-wave velocities from the Baltic Shield based on FENNOLORA data

Abstract

Tomographic travel time inversion of seismic compressional (P) and shear (S) wave data from the long-range deep seismic sounding experiment Fennoscandian Long Range (FENNOLORA) reveals the velocity structure of the crust and upper mantle in the Baltic Shield. Pronounced scattering and delay in travel times of seismic P- and S-wave phases together with strong attenuation of S-wave phases beyond ca. 800-km offset are attributed to a low-velocity zone (LVZ) below the 8° discontinuity at a depth of ca. 100 km. Travel time inversion of P- and S-wave first arrivals shows that the 8° discontinuity represents the top of a zone with negative or very small vertical velocity gradients and a V p/ V s ratio of 1.74–1.77. We observe clear, linear refracted P-wave phases (i.e. the Lehmann refraction) at offsets beyond 1100–1300 km, which suggest that the base of the low-velocity zone is at ca. 150-km depth in the Baltic Shield. No refracted S-wave phases are observed beyond 1200-km offset, which we attribute to strong S-attenuation within the zone below the 8° discontinuity. These features are interpreted by the presence of small amounts of partial melts, almost molten rocks or possibly free fluids, in the 100–150-km depth interval. Local variations in the V p/ V s ratio of the crust and uppermost mantle correlate with Proterozoic terranes, which collided and were amalgamated during the Precambrian plate tectonic events that led to the assemblage of the Baltic Shield.