Velocity blocks and anisotropy in the crust of the central Fennoscandian Shield

The central Fennoscandian Shield has a complex structure. After several tectonic events, forming up island arcs, progressive thrusting and extensional phases, the crust is composed of varying size blocks and slanting belts. These blocks and belts are visualized by seismic tomography method. The tomography data comprises of P-wave and S-wave arrival data from controlled source and seismic tomography experiments, and from chemical explosions. The inverted seismic velocities vary locally in the whole crust expressing several distinct anomalies. The boundary zone between the Archean and the Proterozoic terranes of the Shield appears as an upper crustal low velocity zone. Lower velocities (Vp/Vs<1.70) are associated with schist belts, and higher velocities (Vp/Vs >1.76) with rapakivi batholiths and with the Central Finland Granitoid Complex suggesting hidden mafic blocks in the lower crust.
In the isotropic tomographic velocity models of the crust, anisotropic component is embedded in the residual component. Horizontal azimuthal dependency of residual component images the horizontal component of tectonic transport. An azimuthal anisotropy in tomographic model is verified when fast P-wave velocity direction is matched with slow one in the orthogonal direction. The resulting anisotropy directions coincide with transport directions drawn from structural observations.