Upper mantle structure beneath the Southern Scandes Mountains and the Northern Tornquist Zone revealed by P-wave travel time tomography
Medhus, A.B. et al., Geophys. J. Int., 189, 1315–1334, 2012.
This study images upper-mantle structure beneath different tectonic and geomorphological provinces in southern Scandinavia by P-wave traveltime tomography based on teleseismic events. We present results using integrated data from several individual projects (CALAS, MAGNUS, SCANLIPS, CENMOVE and Tor) with a total of 202 temporary seismological stations deployed in southern Norway, southern Sweden, Denmark and the northernmost part of Germany. These stations, together with 18 permanent stations, yield a high density data coverage and enable presentation of the first high resolution 3D seismic velocity model for the upper mantle for this region, which includes the entire northern part of the prominent Tornquist Zone and the Southern Scandes Mountains.
P-wave arrival time residuals of up to ±1 s are observed indicating large seismic velocity contrasts at depths. Relative regional as well as absolute global tomographic inversion is carried out and consistently show upper-mantle velocity variations relative to the ak135 global reference model of up to ±2–3 per cent corresponding to P-wave velocity differences of 0.4–0.5 kms–1 from depths of about 100 km to more than 300 km. High upper-mantle velocities are observed to great depth to the east in Baltic Shield areas of southwestern Sweden suggesting the existence of a deep lithosphere keel. Lower velocities are found to the west and southwest beneath the Danish and North German sedimentary basins and in most of southern Norway. Awell defined, generally narrowand deep boundary is observed between areas of contrasting upper-mantle seismic velocity. In the southern part of the study area, this boundary is localized along and east of the Sorgenfrei–Tornquist Zone. It seems to follow the eastern boundary of a zone of significant Late Carboniferous–Permian volcanic activity from southwestern Sweden to the Oslo Graben area. To the north, it crosses shield units, Caledonides as well as areas of high topography. Supported by independent results of surface wave studies, we interpret this velocity boundary as a first order lithosphere boundary representing the southwestern edge of thick shield lithosphere.
In basin areas to the southwest, low upper-mantle velocities are associated with asthenosphere beneath thinned lithosphere and velocity contrasts are likely to arise mainly from temperature differences. To the north structural and geodynamic relations are more complex and both temperature and compositional differences may play a part. Reduced upper-mantle velocity beneath southern Norway also seems, despite relatively low heat flow, to be associated with areas of thinned lithosphere, pointing towards increased temperatures and reduced density in the upper mantle. This feature extends over large areas and seems not directly correlated to the shorter wavelength high topography of the Scandes Mountains, but may contribute with some isostatic buoyancy on a regional scale. For this northern area, there is no obvious geodynamic explanation to reduced upper-mantle velocity. A number of candidates are available including deep transient thermal influence from basin areas to the southwest.