Crustal properties in the transition from orogenic to cratonic areas from seismological analysis: example of the Baltic shield and the Scandinavian Mountains

The mechanism of support for the present topography of Scandinavia is not well explained by current crustal models. High topography in Norway and Sweden (Scandinavian Alps) is directly in contact with a relatively flat region (the Baltic shield). A crustal root beneath the Scandinavian Alps, expected from Airy isostatic calculations, is not present and the gravity anomaly map suggests lateral variations in density support present day topography. To bring new constrains on the variation of crustal properties we use P-receiver functions and ambient seismic noise. Using these methods we quantify Moho depth and depth-velocity variations across the crust. P-receiver functions indicate the presence of a high velocity layer at the base of the crust, interpreted as magmatic underplating or eclogitization. Ambient seismic noise provides data from Rayleigh wave seismic tomography and identifies the presence of a low velocity layer associated with granite intrusion in the upper-middle crust. Our new crustal thickness map shows that the topography at the surface is not reflected by the Moho topography in this region. We observe a relative thickening of the crust from the Atlantic coast (40 km) to the Gulf of Bothnia (44 km). Secondly, Moho sharpness analysis and 1D depth-velocity profiles show a difference in the transition from crust and upper mantle. A high velocity layer (Vp>7.1 km.s−1) beneath the Baltic shield is missing beneath the Scandinavian mountains. This observation explains the crustal thickness variations beneath the mountain belt and the shield, with magmatic underplating beneath the shield and a delamination of the lower crust due to eclogitization process beneath the mountain range. In addition the presence of a local low density layer (granitic body) in the upper crust beneath the northern mountains seems to be a possible mechanism to explain the presence day topography in this region.