The lithosphere, LAB, LVZ and Lehmann discontinuity under central Fennoscandia from receiver functions

P- and S-wave velocity profiles (Vp and Vs) from the Earth's surface to a depth of 300km are obtained by simultaneous inversion of P- and S-wave receiver functions for the temporary POLENET/LAPNET array in northern Finland and 5 permanent stations in southern Finland. The obtained Vp/Vs velocity ratio in the uppermost mantle is anomalously low (1.65–1.70 versus the standard 1.8). This ratio can be explained by a high (~30%) fraction of orthopyroxene in the depleted upper mantle. An increase of ~4% in the Vs values is detected at a depth of 110–130km. Under southern Finland the high-velocity mantle keel may extend beyond the depth of 300km, but under northern Finland we detect a low-Vs zone (LVZ) with the top at a depth of 160km. This depth corresponds to the intersection of the geotherm with the wet solidus of peridotite at a temperature near ~1100°C, and suggests partial melting as an explanation for the origin of the LVZ. The bottom of the LVZ (the Lehmann discontinuity) is found at a depth of 240km. The fast direction of seismic azimuthal anisotropy beneath northern Finland in the depth interval from 200–240km to 320km is parallel to the current APM direction (60°) but in the interval from 160km to 200–240km the fast direction (150°) appears to be normal to the APM direction (Vinnik et al., 2014). The present study indicates that the fast direction of 150° confines to the LVZ where the flow in the direction normal to the APM direction is unlikely. More likely, the flow direction is parallel to the APM direction and normal to the fast direction of anisotropy, as suggested by experiments with sheared peridotite-type rocks containing melt.