Lithospheric Structure of Scandinavia derived from Ambient Noise and Surface Waves

The western rim of Scandinavia is characterized by the Scandes mountain range with topography up to 2500 m. Since this region lacks recent compressional tectonic forces, it provides a great opportunity to study the geodynamic evolution of crustal and upper mantle structures at today's passive continental margins. Together with the ScanArray network we use data from previous and permanent projects, in total more than 200 stations, for a surface wave tomography of the area. We aim to present a 3D SV-velocity model.

In previous studies, an unusually thin crust and shallow lithosphere-asthenosphere boundary (LAB) have been found beneath the high-topography of western Norway, where a clear crustal mountain root seems to be absent. The lower topography regions of eastern Norway and Sweden, however, reveal a thicker crust, in contrast to the principles of Airy isostasy. Lower seismic velocities than expected have been found with a sharp transition to higher velocities beneath Sweden across the upper-mantle velocity boundary (UMVB).

Beamforming of Rayleigh surface waves yields average phase velocities for all Scandinavia and several of its sub-regions. A remarkable cos(1Θ) phase velocity variation with azimuth is estimated for periods >35sec. Since this effect can be seen in northern and southern Scandinavia but not in the central area this might indicate a dipping LAB related to varying topography from North to South. Phase velocity maps derived with the two plane wave method show a similar transition from slow to fast velocities across the UMVB but less pronounced in the central part. However, the Lofoten peninsula is imaged with lower velocities at crustal and lithospheric depths. From ambient noise, trans-dimensional MCMC Bayesian inversion yields strong small-scale phase velocity variations beneath the Lofoten peninsula at shallow depths. Aside from the Lofoten, we find large-scale high velocities beneath the Baltic shield for periods <50sec which is consistent with our surface wave findings.