Lithospheric structure in southern Sweden—results from FENNOLORA

Abstract

The major objective of FENNOLORA, the Fennoscandian Long Range Seismic Project of 1979, was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the Fennoscandian Shield. In the component represented by this study, data recorded at stations in southern Sweden from three shotpoints, one in northern Germany (profile WN) and two separated by 300 km in southern Sweden (profiles BN and CS), are used to derive a two-dimensional lithosphere structure model extending over 600 km. From north to south, the profile crosses the Svecofennides, the Småland-Värmland Granite Belt, Paleozoic cover rocks lying unconformably on the Fennoscandian Shield, the Baltic Sea (where there were no stations) and into the Caledonian tectonic province of northern Germany where one shotpoint but no stations were located. Interpretation of the three record sections was based on a two-dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms. Lack of data from 0–150 km, for the shotpoint in Germany to the first station in southern Sweden, precluded derivation of any detailed crustal structure for this region. Crustal thickness was determined to be 32 km. A rapid increase in velocity from 8.0 to 8.35 km/s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km. The reversed profiles BN and CS in southern Sweden are fundamentally different. Interpretation shows a difference in crustal thickness of about 12 km, with the northern segment having a deeper Moho, a feature resulting from a thicker lower crust and a crust-mantle transition zone rather than a rapid increase in velocity. In the southern segment of the reversed profile, a crustal low velocity zone of limited extent is indicated. The most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern Sweden occurs within a transition zone of limited lateral extent, less than a few tens of kilometers. We suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the Småland-Värmland Granite Belt and the Svecofennides.