Changes in the lithospheric structure across the Sorgenfrei-Tornquist Zone inferred from dispersion of Rayleigh waves

Abstract

Surface wave analysis was used to study possible lateral changes in the lithospheric/upper mantle structure across the Sorgenfrei-Tornquist Zone (STZ), which is part of the transition zone between the ancient Baltic Shield and younger western Europe. Records of long period surface waves from earthquakes at teleseismic distances were analysed. The data originated from long period permanent stations in southeastern Norway and from broadband NARS stations installed in Denmark between 1983 and 1986. This configuration made it possible to compare the lithospheric structure north and south of the STZ by analysis of surface wave dispersion between pairs of closely spaced stations in the same tectonic unit. Phase velocities of fundamental mode Rayleigh waves were determined by calculating cross spectra of pairs of records. The phase velocity dispersion curves show significant differences between the two tectonic units north and south of STZ. Linearized inversion of the observed dispersion curves, to obtain models of shear wave velocity in the crust and upper mantle, shows a low velocity zone (LVZ) at a depth of about 110 km beneath the western part of Denmark, just south of the STZ. A similar LVZ was not identified beneath southeastern Norway, north of the STZ. If a low velocity zone is present beneath the Baltic Shield in this area it must be very weak. The lateral changes in lithospheric-asthenospheric structures take place over a distance of less than 500 km. Our results therefore suggest that the STZ coincides with a transition zone in the upper mantle.