Upper mantle structure of fennoscandia by finite-frequency body-wave tomography: Analysis and geodynamic implications

Abstract

Tectonic research on the region around the North Atlantic Ocean, including Fennoscandia (in the northwestern part of the East European Craton) and Greenland has led to several enigmatic questions. Despite numerous studies, theories are debated on the causes of the onshore high topography and off-shore sedimentary basins in the passive continental margins in the region. Here, the results of a seismological study of most of Fennoscandia (the Baltic Shield) based on data which was acquired with a nominal station distance of 50 km is presented. We determine a seismic model of upper-mantle P-wave velocity in order to improve the understanding of the long-term evolution of Fennoscandia, and with the specific objective of testing various models for topographic change in the area. The new velocity model contributes to testing if the crust and lithosphere are in isostatic equilibrium or if dynamic forces actively affect the high topography in the region. The seismic model is obtained by application of a method for finite-frequency body-wave tomography. We use P-waves from teleseismic earthquakes at epicentral distances between 30° and 104° with magnitudes of 5.5 or greater, acquired by 174 broadband seismic stations installed in Norway, Sweden and Finland in variable periods between 2012 and 2017. The model is based on inversion of 74,057 traveltime residuals for P-wave velocity perturbations in the upper-mantle in the depth range between 50 and 800 km. The algorithms used for relative traveltime picking and tomography inversion were rearranged to the specific data set, and an extensive test program was carried out to assess the resolution of the resulting P-wave velocity model. The velocity model shows that up to ±6% anomaly contrasts characterize the Fennoscandian upper mantle. The contrasts are stronger than in previous local and regional/global models. Such strong contrasts are surprising for a cratonic area, and we suggest that this finding is made possible by the use of the finite-frequency method, which has higher resolution power than conventional ray-based methods. We identify very strong low velocity anomalies to ~200 km depth beneath the modern Scandinavian mountain range within the Caledonian and Sveconorwegian orogenic belts. These low velocities may be caused by temperature or compositional contrasts, and they are consistent with the presence of low density material, which may contribute substantially to the high topography. We identify a linear correlation between topography and sub-Moho velocity anomalies in most of central Scandinavia, which provide support to the importance of the low-velocity zone for the high topography. We observe exceptionally high velocity anomalies in the central part of Fennoscandia, which suggests that an original cratonic core has survived modification of the surrounding Archaean and Svecofennian mantle. The model indicates that these very high velocity anomalies extend westward below the low-velocity zone below the high topography, which indicates that the Svecofennian lithospheric mantle is present below Caledonian and Sveconorwegian deformed areas. North-dipping high velocity anomalies in the southwestern Baltic Sea may be related to an ancient subduction zone and a hidden terrane in the crust and uppermost mantle.