A NEW MODEL OF UPPER-MANTLE P-WAVE VELOCITY BELOW THE BALTIC SHIELD -INDICATION OF PARTIAL MELT IN THE 95-KM TO 160-KM DEPTH RANGE

The subcrustal mantle to about a depth of 160 km below the Baltic Shie ld can be divided seismically into two main intervals, The upper inter val between ca. 45 and 100 km is characterized by refracted waves with velocities between 8.0 and 8.6 km/s, interpreted as a sandwich-layere d model of intermixed high- and low-velocity layers. The lower interva l is a ca. 40-km-thick zone of relatively low average velocity at a de pth increasing from 95 km in the southern part to 120 km in the northe rn part of the shield. Seismic sections show that isolated bodies in t his interval generate scattered reflections with substantial amplitude variation over short distance. The maximum velocity difference at the reflectors is 1.0 km/s which is larger than in the upper main interva l. Our model is based on a general gap in correlation of seismic arriv als at offsets between 800 and 1100 km, At larger offsets we observe c ontinuous linear first arrivals, indicating a relatively homogenous la yer of high (> 8.6 km/s) velocity below a depth of 160 km. Petrologica lly, the lower reflective interval is explained by partially melted sm all bodies with patches of carbonatitic or kimberlitic magma, of horiz ontal extent less than 20 km. This new model of the upper mantle is ba sed on seismic data from the Baltic Shield and there is clear indicati on that it is equally valid for other shield and platform areas.