Evidence for a continental keel beneath the Paleoproterozoic Svecofennian orogen in Finland from surface wave tomography

From August 1998 to May 1999 a passive seismic experiment was carried out in southern Finland to image the structure of the upper mantle using body wave tomography, receiver function analysis, and modelling of seismic surface waves. The main goal was to find out if traces of Archean and Proterozoic plate tectonics are preserved in the old and stable lithosphere of Fennoscandia. Further questions were: is there an asthenosphere beneath the Fennoscandian Shield? If yes, how thick is the lithosphere? Can the Archean-Proterozoic suture crossing southern Finland also be seen in the upper mantle? Is the region of thickened crust south of the Archean-Proterozoic suture with its thick and fast lower crust accompanied by anomalous mantle below? We have analysed the surface wave signals recorded on the broadband stations during the SVEKALAPKO seismic tomography experiment and derived both 1D-average and threedimensional models of S-velocity in the upper 300 km of the mantle. The average model gives a mean crustal thickness of 52 km. Shear velocities in the mantle are generally high. There is no perceptible low-velocity zone in the upper 300 km of the mantle which could be interpreted as seismic asthenosphere. Though S-velocities are already high, the 3D model exhibits a positive velocity anomaly in the mantle beneath central southern Finland which reaches down to about 150 km depth and correlates very well with the region of thickened crust. A similar but still deeper reaching high velocity anomaly has also been found from teleseismic P-wave tomography. The Archean-Proterozoic suture zone does not show up as a perceptible structure in the mantle. Since geological evidence indicates that the lithosphere has been in place since about 1.5 Ga the high velocity anomaly probably represents a continental keel stabilized by compositional differences. These differences may have been created by extraction of melt during the formation of the thick lower crust in this region. It remains enigmatic how the keel survived the long plate tectonic history of Fennoscandia after its formation.