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 deg discontinuity
at a depth of ca. 100 km. Travel time inversion of P- and S-wave
first arrivals shows that the 8 deg discontinuity represents the
top of a zone with negative or very small vertical velocity gradients
and a Vp/Vs 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 deg 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 Vp/Vs 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.
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