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

• Evidence of the LVZ beneath central Fennoscandia
• The LAB at a depth of 160 km and the Lehmann discontinuity at a depth of 240 km
• Evidence of anomalous azimuthal anisotropy in the LVZ
• Very low (~ 1.7) Vp/Vs ratio in the mantle lithosphere
• Evidence of a mid-lithospheric discontinuity at a depth of 120 km

P- and S-wave velocity profiles (Vp and Vs) from the Earth's surface to a depth of 300 km 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 110–130 km. Under southern Finland the high-velocity mantle keel may extend beyond the depth of 300 km, but under northern Finland we detect a low-Vs zone (LVZ) with the top at a depth of 160 km. 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 240 km. The fast direction of seismic azimuthal anisotropy beneath northern Finland in the depth interval from 200–240 km to 320 km is parallel to the current APM direction (60°) but in the interval from 160 km to 200–240 km 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.