Crustal Structure across Central Scandinavia along the Silver-Road refraction profile

The western edge of the Baltic Shield is covered by the northeast – southwest oriented, 2500 m high mountain range, the Scandes at the northwestern Atlantic Ocean. This mountain range is located far from any active plate boundary and lack of sedimentary sequences precludes direct knowledge of the timing of uplift.

We present a crust and upper mantle scale velocity model, obtained along thea 600 km long Silver-Road seismic profile, which extends in a WNW to ESE direction in the northeastern Baltic Shield perpendicular to the coast between 8^oE and 20^oE. The profile has a 300 km long offshore section on the continental shelf and the deep ocean as well as a 300 km onshore section across Caledonian to Svecofennian units. The seismic data were acquired with 5 onshore explosive sources and offshore air gun shots from the vessel Hakon Mosby along the whole offshore profile. Data was acquired by 270 onshore stations at nominally 1.5 km distance and 16 ocean bottom seismometers on the shelf, slope and oceanic environment. The results of this study will provide new input to interpretation of the anomalous topography the Scandes and continental shelf in the northeast Baltic Shield.

We present results of ray tracing and gravity modeling along the profile. The vertical crustal structure in the upper, middle and lower crust are almost constant across the Caledonian and Svecofennian parts of the profile. The crust is 45 km thick along the whole onshore profile and abruptly thins to 25 km thickness in the continental shelf. Pn velocity is low ~7.6-7.8 km/s below the high topography areas with Caledonian nappes, whereas it is 8.4 km/s below the Svecofennian parts. Our gravity models, based on the seismic velocity structure, suggest a low density 3.20 g/cm³ for the low Pn zone below the high Caledonian topography in contrast to the very high density 3.48 g/cm³ below the Svecofennian parts with relatively low topography. We interpret these bodies as eclogitizised basaltic crustal material at different metamorphic grades. Isostatic calculation with a 60 km depth compensation depth predicts 2 km high topography which is ~1 km higher than observed. We therefore propose that the low-grade metamorphic unit below the high topography is underlain by a sequence with relatively high mantle density to 120 km depth.