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

The Baltic Shield is located in the northern part of Europe. It formed by amalgamation of a series of terranes and microcontinents during the Archean to the Paleoproterozoic, followed by significant modification in Neoproterozoic to Paleozoic time by the Sveconorwegian (Grenvillian) and the Caledonian orogenies. The Baltic Shield includes an up to 2500 m high northeast-southwest oriented mountain range, the Scandes, which mainly coincides with the Caledonian and Sveconorwegian deformed parts along the western North Atlantic coast, despite being located far from any active plate boundary.

We present a crustal scale seismic model along the WNW to ESE directed Silver Road profile in northern Scandinavia between 8^oE and 20^oE. This profile extends south of Lofoten for ~300km across the Norwegian shelf in the Atlantic Ocean and for ~300km across the onshore Caledonides and Baltic Shield proper. 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 into the oceanic environment. The results of this experiment will provide information on the origin of the anomalous onshore topography and offshore bathymetry at the edge of the North Atlantic Ocean.

We present results from ray tracing modeling and tomographic inversion of the seismic velocity structure along the profile. The crustal structure is uniform with a thickness of 45 km along the whole onshore profile including both the Caledonides and the shield part. The crust thins abruptly to ~25 km thickness towards the shelf around the coastline. Pn velocity is only ~7.6-7.8 km/s below the high topography areas with Caledonian nappes, and extending into the offshore part, whereas it is 8.4 km/s below the shield proper. By gravity modelling we find that the low Pn zone has a low density of 3.20 g/cm³, which we interpret as partially eclogitizised lower crust. The Svecofennian unit has a very high density of 3.48 g/cm³ in the shield with low topography. Isostasy to 60 km depth, as suggested by Receiver Functions, indicates a ~2 km topography which is ~1 km higher than observed. However, recent results from high-resolution seismic tomography shows a velocity change between the two onshore zones down to 120 km depth. Including this observations into the calculations allows us to explain the observed topography by isostasy in the crust and lithospheric mantle.