The upper mantle under Europe: an interpretation of some preliminary results from the NARS project

G. Nolet; B. Dost; H. Paulssen

G. Nolet Department of Theoretical Geophysics, institute of Earrh Sciences, University of Utrecht, P. 0 . Box 80.021, 3508 TA Utrecht, The Netherlands
B. Dost Department of Theoretical Geophysics, institute of Earrh Sciences, University of Utrecht, P. 0 . Box 80.021, 3508 TA Utrecht, The Netherlands
H. Paulssen Department of Theoretical Geophysics, institute of Earrh Sciences, University of Utrecht, P. 0 . Box 80.021, 3508 TA Utrecht, The Netherlands

Keywords: lithosphere, upper mantle, surface waves, European platform, Scandinavian shield

Abstract

Using teleseismic recordings of 5 strong Japanese earfhquakes in 1983/84, Dost (in press) has recently determined average higher mode phase velocities with the broad-band digital NARS array. These are the first such results ever to be obtained for a single geophysical province, in this case the west European platform. Moreover, the frequency range has been doubled with respect to earlier higher mode phase velocity determinations. It is to be expected that we will refine and extend our measurements in the coming years. No strong earthquake at intermediate depth has so far occurred since the installation of the NARS network, which leaves important gaps in velocity measurements at several frequencies. Nevertheless we have made a preliminary interpretation of the data now available, giving us guidelines for future observational efforts. lnversion of the phase velocities for an average velocity/density structure for the west European platform results in an upper mantle with a low velocity channel between 120 and 250 km that is not very pronounced (average Vs is 4.35 km/s) and with remarkable layer of high velocity and density at roughly 100km depth. Comparison with older higher mode data enables us to determine the difference in shear velocity between the Scandinavian shield and the west European platform: shield Vs is 4.49 km/s averaged over 120-250km. The velocity difference between shield and platform can be explained by a temperature difference of at least 250 degrees, or effects of partial melting under the platform. We have extrapolated laboratory values of shear velocity and density of single crystal specimens of 4 different rock types (olivine, orthopyroxene, clinopyroxene and garnet) to ambient upper mantle pressure and temperature. Despite the uncertainty inherent to such a procedure, comparison of these curves with the inversion results suggests strongly that a layer of eclogitic composition is present between 80 and 120 km depth.