ALBERT

Description: Much information is available on crustal structure in the area of southern Scandinavia, but little high-resolution information has been presented on structural variations in the upper mantle. We present relative P-wave travel time residuals (P-residuals) from distant earthquakes at 48 temporary seismological stations and 15 permanent seismological stations covering areas of southern Norway, northern Denmark and southwestern Sweden. We investigate whether high topography in southern Norway may be associated with an anomalous upper mantle, and we look for the southwestern boundary of thick shield lithosphere. Differences in station means of P-residuals corrected for topography and contributions from the crust are up to about 1 s across the study area. Early arrivals are observed to the east in Sweden in shield areas associated with thick lithosphere, and around the Oslo Graben (OG). Late arrivals southwest of the Sorgenfrei-Tornquist Zone in the Norwegian-Danish Basin are interpreted as being associated with thinner lithosphere related to the basin formation. Late arrivals are also observed in the southern part of Norway west of the Oslo Graben area. This indicates reduced P-wave velocity and reduced density in the upper mantle and perhaps some regional isostatic buoyancy from the upper mantle. However, a clear spatial correlation with areas of high topography is not observed. The combined information of mean P-residuals and the azimuthal dependence of single P-residuals indicate the presence of a narrow lithospheric transition around the Sorgenfrei-Tornquist Zone extending from the southeast into the northernmost part of Jutland. From here it may continue northwards and connect with the transition we observe in southern Norway in about the position of the Oslo Graben.

Description: Structure and dynamics of the upper mantle is important in understanding timing and mechanisms shaping prensent day topography and near surface geology. Debate persists regarding the geological age of the Scandes Mountains. We contribute by imaging upper mantle structure beneath southern Scandinavia using teleseismic P-wave travel time tomography (P-tomography). We include data from mobile stations deployed in projects CALAS, CENMOVE, MAGNUS, SCANLIPS and Tor. Permanent stations included are those available from the University of Uppsala, NORSAR and GEUS. P-wave arrival times generally show differences of up to 1 second across the study area. Upper mantle velocities are relatively high in southern Sweden and southern Norway east of the Oslo Graben. Lower velocities are observed in the Norwegian-Danish Basin southwest of the Sorgenfrei-Tornquist Zone(STZ) and in the southwestern part of Norway. We interpret the southwestern boundary of thick Baltic Shield lithosphere where we observe the highest horizontal P-wave velocity gradient. Thus, we find the boundary of thick lithosphere to more or less coinside with the STZ in the southeastern part of the study area, extending from southern Sweden into the northern part of Jutland. From here it turns north, passing through the Oslo Graben area to about 60_N then turning northwest, approaching the Norwegian west coast around 65_N. Thus, as compared to Baltic Shield, upper mantle velocity is significantly reduced beneath deep sedimentary basins of Denmark and northern Germany.