If low seismic velocities below the passive margins of the North Atlantic are indeed associated with dynamic support of topography from the mantle and Neogene uplift, as suggested by Rickers et al.
(2013), it is quite important to quantify the anomalies in terms of temperature and/or composition and investigate their tectonic implications.
Within the upper subcontinental mantle and away from the hy-drated regions close to subduction zones, seismic velocity anomalies are best explained by variations in temperature (from either
con-vection, changes in lithospheric thickness and/or changes in heat production) and iron content (from melt extraction or metasomatic refertilization; e.g. Goes et al.2000; Poudjom Djomani et al.2001;
Griffin et al. 2009). Separating the effects of the two parameters is notoriously difficult (Cammarano et al.2003) and perhaps only possible in extreme cases where Archean and tectonically young lithospheres are compared (Goes et al.2000).
As anhydrous variations in composition give seismic velocity variations smaller than 1–2 per cent in Proterozoic and younger tectonic regions (Goes et al.2000; Cammarano et al.2003; Griffin et al. 2009; Hieronymus & Goes 2010), we can make an end-member interpretation of the high- and low-velocity anomalies in terms of temperature. Using for example derivatives of VS veloc-ity with respect to temperature from the literature (∂lnVS/∂T ≈
−1 per cent/100 K, Lee2003), VS velocity anomalies of±3 per cent translate into temperature anomalies about∓300◦C.
The main problem with this linear interpretation of seismic high-and low-velocity anomalies into hot high-and cold, is that shear anelas-ticity makes the temperature derivatives of VP and VS strongly temperature-dependent and the absolute value of the derivatives increases with increasing temperature (Cammarano et al.2003).
Velocity anomalies of different sign therefore give temperature
Downloaded from https://academic.oup.com/gji/article-abstract/202/1/190/585392 by NATIONAL TAIWAN UNIVERSITY LIBRARY user on 02 July 2019
Multiscale P and S tomography 211
Figure 23. Synthetic input for a test with one cylinder extending from 150 to 500 km depth and with a diametre of 100 km. The velocity perturbations are δlnVP= −3 per cent and δlnVS= −3 or −5 per cent. Upper left map shows location of profiles and grid discretization.
anomalies of different magnitude, and absolute seismic velocities are needed to make a thermal interpretation (Goes et al. 2000;
Cammarano et al. 2003). In addition, the anelasticity param-eters used in calculating velocities and velocity derivatives (e.g. Berckhemer et al.1982; Sobolev et al.1996; Jackson et al.
2002; Shapiro & Ritzwoller 2004) also have a significant effect on the magnitude of estimated velocities (e.g. Goes et al.2000;
Cammarano et al.2003; Kolstrup et al.2012).
Due to these complications we take a more indirect approach in the interpretation of the velocity anomalies in Figs4–7and compare our results to those obtained by thermal lithosphere modelling in the study area (Kolstrup et al.2012; Gradmann et al.2013).
Kolstrup et al. (2012) modelled the temperature field and pre-dicted seismic velocities in southern Norway using several geo-physical data sets (Moho depth, geoid, surface heat flow), taking into account the dependence of seismic waves on temperature, composi-tion and anelasticity at high temperature and pressure. To limit the range of possible models fitting the data sets, they used a constraint of local isostatic equilibrium for the lithospheric column overly-ing an adiabatic asthenosphere. Kolstrup et al. (2012) inferred a thin and warm lithosphere (∼100 km) in western Norway and a thick and cold lithosphere in eastern Norway and Sweden (∼200 km), and
additionally a slightly thinner and warmer area associated with the Oslo Graben. The maximum relative difference in synthetic seismic shear wave velocity between the thin western lithosphere and the thick eastern lithosphere in Kolstrup et al. (2012) is up to 8.5 per cent at 100 km depth (T ≈ 400◦C) and 5.5 per cent at 150 km depth (T ≈ 300◦C). In our tomographic images of VSwe find a maxi-mum relative anomaly between southeast and northwest of around 7.5 per cent at 100 km depth and around 5 per cent at 150 km depth (Fig. 5).
Using a more complete 3-D modelling approach of the litho-sphere in southwestern Scandinavia, Gradmann et al. (2013) es-timated an abrupt change in lithospheric thickness from 100 to 200 km across the Oslo Graben, but also needed a change in com-position between a fertile southern Norwegian mantle and a depleted Fennoscandian Shield to fit absolute VSV velocities in southern Norway (Maupin2011) and Sweden (Cotte & Pedersen2002).
Both modelling approaches show that a lithosphere in local iso-static equilibrium can exhibit strong variations in seismic velocities just from variations in lithospheric thickness and associated temper-ature variations. It is therefore not necessary to invoke a plume or diapir in the uppermost mantle below southern Norway (Rohrman
& van der Beek1996; Rohrman et al.2002) to explain the low
Downloaded from https://academic.oup.com/gji/article-abstract/202/1/190/585392 by NATIONAL TAIWAN UNIVERSITY LIBRARY user on 02 July 2019
212 M.L. Kolstrup, S.-H. Hung and V. Maupin
Figure 24. Recovered P-wave model using the 3-D multiscale parametrization with the synthetic model shown in Fig.23and a velocity perturbationδlnVP=
−3 per cent. Upper left map shows location of profiles and grid discretization.
seismic velocities. A direct translation of the relative velocity anomalies in Figs4and5into relative temperature anomalies could easily be taken as evidence for anomalously hot temperatures and diapiric upwelling of the asthenosphere into the lithosphere.
Neither the lithospheric modelling of Kolstrup et al. (2012) nor Gradmann et al. (2013) is able to explain the details in our tomo-graphic images, but suggests that the low velocities in the channel-like structure between Denmark and Norway may be due to a thin but stable lithosphere of about 100 km overlying convecting as-thenosphere. The high velocities in Sweden and the sharp transition from low to high velocities can likewise be explained by a rapid increase in lithospheric thickness, possibly associated with a higher degree of depletion of the lithospheric mantle below Sweden.
What we cannot explain by variations in lithospheric thickness and mantle depletion is the deep cylindrical low-velocity anomaly that does not fit at all with the uniform asthenosphere assumed below the lithosphere in Kolstrup et al. (2012) and Gradmann et al.
(2013). Above 200 km depth, the cylinder merges with the low-velocity channel and could be ascribed to lithospheric thickness
variations, but between depths of 200 and 350 km it is a much stronger feature than the deep part of the channel and it has its own distinct geometry. The location of the structure is slightly west of the northern end of the Oslo Graben and it might therefore have an origin in the extensive Permian magmatism of the Oslo Graben (Neumann et al.2004; Larsen et al.2008; Torsvik et al.2008).
The perhaps most natural explanation for such a structure is a centralized small-scale upwelling, but numerical simulations using a uniform composition for the asthenosphere indicate that upwelling would not give a velocity anomaly as strong as−3 per cent in VSand 1.5–2 per cent in VP(Hieronymus et al.2007). Hence, the narrow cylinder needs to be anomalously hot or to have an anomalous composition, with for example a higher water content or increased heat production.
The interpretations in this section are based mainly on the anoma-lies in VS. The covariation of VP and VSin δln(VP/VS) provides additional information on the causes of the velocity anomalies, es-pecially on compositional variations, and will be explored in the following section.
Downloaded from https://academic.oup.com/gji/article-abstract/202/1/190/585392 by NATIONAL TAIWAN UNIVERSITY LIBRARY user on 02 July 2019
Multiscale P and S tomography 213
Figure 25. The same as Fig.4for theδlnVPmodel obtained using only the high-frequency P-wave data set and a ray-theoretical approach. The same 3-D multiscale parametrization and damping value as in Fig.4is used in the inversion.
Figure 26. The same as Fig.5for theδlnVSmodel obtained using only the high-frequency S-wave data set and a ray-theoretical approach. The same 3-D multiscale parametrization and damping value as in Fig.5is used in the inversion.
Downloaded from https://academic.oup.com/gji/article-abstract/202/1/190/585392 by NATIONAL TAIWAN UNIVERSITY LIBRARY user on 02 July 2019
214 M.L. Kolstrup, S.-H. Hung and V. Maupin
5.4 Tectonophysical implications—δln(VP/VS)