A new model of upper mantle P-wave velocity below the Baltic Shield: indication of partial melt in the 95 to 160 km depth range

Abstract

The subcrustal mantle to about a depth of 160 km below the Baltic Shield can be divided seismically into two main intervals. The upper interval 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-layered model of intermixed high- and low-velocity layers. The lower interval is a ca. 40-km-thick zone of relatively low average velocity at a depth increasing from 95 km in the southern part to 120 km in the northern part of the shield. Seismic sections show that isolated bodies in this 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 interval. Our model is based on a general gap in correlation of seismic arrivals at offsets between 800 and 1100 km. At larger offsets we observe continuous linear first arrivals, indicating a relatively homogenous layer of high (> 8.6 km/s) velocity below a depth of 160 km. Petrologically, the lower reflective interval is explained by partially melted small bodies with patches of carbonatitic or kimberlitic magma, of horizontal extent less than 20 km. This new model of the upper mantle is based on seismic data from the Baltic Shield and there is clear indication that it is equally valid for other shield and platform areas.