Upper-mantleP- andS-wave velocities across the Northern Tornquist Zone from traveltime tomography

Abstract

S U M M A R YThis study presents P-and S-wave velocity variations for the upper mantle in southern Scandinavia and northern Germany based on teleseismic traveltime tomography. Tectonically, this region includes the entire northern part of the prominent Tornquist Zone which follows along the transition from old Precambrian shield units to the east to younger Phanerozoic deep sedimentary basins to the southwest. We combine data from several separate temporary arrays/profiles (276 stations) deployed over a period of about 15 yr and permanent networks (31 stations) covering the areas of Denmark, northern Germany, southern Sweden and southern Norway. By performing an integrated P-and S-traveltime analysis, we obtain the first high-resolution combined 3-D V P and V S models, including variations in the V P /V S ratio, for the whole of this region of study. Relative station mean traveltime residuals vary within ±1 s for P wave and ±2 s for S wave, with early arrivals in shield areas of southern Sweden and later arrivals in the Danish and North German Basins, as well as in most of southern Norway. In good accordance with previous, mainly P-velocity models, a marked upper-mantle velocity boundary (UMVB) is accurately delineated between shield areas (with high seismic mantle velocity) and basins (with lower velocity). It continues northwards into southern Norway near the Oslo Graben area and further north across the Southern Scandes Mountains. This main boundary, extending to a depth of at least 300 km, is even more pronounced in our new S-velocity model, with velocity contrasts of up to ±2-3 per cent. It is also clearly reflected in the V P /V S ratio. Differences in this ratio of up to about ±2 per cent are observed across the boundary, with generally low values in shield areas to the east and relatively higher values in basin areas to the southwest and in most of southern Norway. Differences in the V P /V S ratio are believed to be a rather robust indicator of upper-mantle compositional differences. For the depth interval of about 100-300 km, thick, depleted, relatively cold shield lithosphere is indicated in southern Sweden, contrasting with more fertile, warm mantle asthenosphere beneath most of the basins in Denmark and northern Germany. Both compositional and temperature differences seem to play a significant role in explaining the UMVB between southern Norway and southern Sweden. In addition to the main regional upper-mantle velocity contrasts, a number of more local anomaly features are also outlined and discussed.

Deep structure of the northern North Sea and southwestern Norway based on 3D density and magnetic modelling

“…Later on, according to teleseismic P-wave and/or S-wave tomography and surface wave analysis, the shape of reduced upper-mantle velocities beneath SW Norway has been reported in more detail (Maupin, 2011;Köhler et al, 2015;Medhus et al, 2012;Maupin et al, 2013;Wawerzinek et al, 2013). This reduction in seismic velocities is also supported by the recent results of Hejrani et al (2015Hejrani et al ( , 2017 and Kolstrup et al (2015), who have shown, based on P-and S-wave velocities tomography, that there is a prominent lowvelocity zone within the upper mantle of SW Norway. Moreover, based on isostatic modelling, a low-density mantle beneath the southern Scandes has already been proposed by Olesen et al (2002) and Ebbing et al (2012) in order to explain the relatively high topography of the Scandes mountain chain in SW Norway.…”

“…Some atypical values of 3204-3217 (3301-3316) kg/m 3 have been assigned in places where the mantle low-velocity anomaly has been identified (Hejrani et al, 2015(Hejrani et al, , 2017Kolstrup et al, 2015;Köhler et al, 2015). Finally, an average density of 3160 kg/m 3 has been assigned to the uppermost asthenosphere in the case of the 'light' model and 3240-3265 kg/m 3 for the 'heavy' one.…”

“…2), our model area is bounded by the Sorgenfrei-Tornquist and the Lithosphere Transition zones which separate lithospheric blocks with different thicknesses and velocities (Berthelsen, 1992;Thybo, 2001;Cotte et al, 2002;Gregersen et al, 2005;Shomali et al, 2006;Medhus et al, 2009Medhus et al, , 2012Hejrani et al, 2015Hejrani et al, , 2017Kolstrup et al, 2015;Köhler et al, 2015). These deeply seated, lithosphere-scale, fault zones may possibly represent Precambrian suture zones (Olesen et al, 2004), Maystrenko et al, 2012.…”

Upper-mantle velocities below the Scandinavian Mountains fromP- andS-wave traveltime tomography

“…The main purpose of this study is to present new high-resolution seismic velocity models (P-and S-wave velocities) of the upper mantle below the Scandes Mountains and adjacent shield areas. Previous regional studies with comparable resolution covered only the southern part of this study area (Medhus et al 2012a;Wawerzinek et al 2013;Hejrani et al 2015;Kolstrup et al 2015). Regional studies with less station coverage and lower resolution are available and cover larger adjacent areas (e.g.…”

Two-decade seismicity of the SE Alborz region, Iran: Relocation of earthquake hypocenters using cross-correlation based time corrections

“…Careful manual assessment of the seismic arrivals reduces the number of outliers in the arrival time data (e.g. Muksin et al, 2013;Hejrani et al, 2014;Mousavi et al, 2015;Hejrani et al, 2015;Hejrani et al, 2017;Lange et al, 2018;Hejrani et al, 2019). One of the problems that can be studied in the seismic data is the existence of more than one event on the seismic waveform record that may cause mistakes in the identification of seismic signals.…”