We find, as previous studies, a lateral mantle boundary coinciding with the Sorgenfrei-Tornquist Zone (STZ) in Denmark and south-ern Sweden (Fig.1). The STZ continues as the Tornquist-Teisseyre Zone (TTZ) southeast of our study area, where it separates Phanero-zoic Europe from the Precambrian Easteuropean Platform. Both the STZ (Cotte & Pedersen2002; Shomali & Roberts2002; Shomali et al. 2006) and TTZ (Zielhuis & Nolet 1994; Janutyte et al.
2014) are associated with a strong lateral boundary in the upper mantle.
Across the STZ, crustal studies show an increase in Moho depth, both in the Skagerrak Sea (Lie & Andersson1998), in Denmark (Thybo 2001), and in southern Sweden (Thybo2001). The STZ is, despite the change in crustal thickness, not associated with any marked change in topography.
The STZ and the Skagerrak and Oslo Grabens have a common origin in Carboniferous-Permian extensional tectonics and exten-sive magmatic activity (Thybo2001; Neumann et al.2004; Larsen et al.2008). Despite this, the lateral mantle boundary was expected to continue westwards and not to take a detour into southern Norway following the graben structures, as the extensively reworked Danish Basin (e.g. Thybo2001) has a very different crustal structure than the thicker crust of southern Norway (e.g. Stratford & Thybo2011).
Where the lateral mantle boundary branches north from the Oslo Graben, it is no longer associated with any major crustal structures at the surface. On the contrary, the boundary crosses the north-east trending Caledonides and the Scandinavian Mountains (Fig.1).
Using the STZ as an example, Hieronymus et al. (2007) modelled how edge-driven convection would evolve between lithospheres of contrasting thickness (100 and 250 km) and composition (pyrolite and harzburgite) since the last Triassic rifting events 220 Ma. The modelling is able to explain the sharp lateral contrast in seismic velocities across the STZ due to a stable convection pattern. The modelling also shows that the difference in lithospheric thickness is not necessarily associated with any discernible change in sur-face heat flow, and that the high viscosity of the thicker and older lithosphere stabilizes it against erosion.
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Multiscale P and S tomography 215
The model of Hieronymus et al. (2007) is compatible with the increase in lithospheric thickness inferred from lithospheric mod-elling in southern Norway (Kolstrup et al.2012; Gradmann et al.
2013) and explains how such a configuration of the lithosphere can be stable over time. Pascal et al. (2004) suggest that the Oslo Graben developed at the edge of an already existing stepwise in-crease in lithospheric thickness. It is therefore possible that the sharp contrast in seismic velocities in the upper 50–250 km dates back to even older tectonic events than the Carboniferous-Permian rifting.
Another possible scenario is that the rifting left the originally thick (200 km?) shield mantle below southern Norway partly rehy-drated and refertilized. Rehydration can effectively weaken litho-spheric mantle, as seen in for example mantle xenoliths of the Pro-terozoic Colorado Plateau (Li et al.2008). A lithospheric step would in this case have existed between the Danish Basin and southern Norway, facilitating edge-driven convection. Dynamic modelling shows that small-scale edge-driven convection can erode fertile and hydrated lithosphere (Hieronymus et al.2007; Van Wijk et al.2010;
Kaislaniemi & van Hunen2014). Lithospheric erosion by edge-driven convection has been used to explain Cenozoic episodic uplift and magmatism in for example the Atlas Mountains (Kaislaniemi
& van Hunen 2014) and the Colorado Plateau (Van Wijk et al.
2010). Furthermore, the opening of the North Atlantic and the Ice-landic plume could also have triggered convection and erosion of the southern Norwegian mantle, as regional tomographies point to a connection between the Iceland plume and southern Norway (Weidle & Maupin2008; Rickers et al.2013).
Hence, episodic erosion of an initially thick shield mantle be-low southern Norway to its current thickness of about 100 km could explain some of the episodic uplift inferred from geological observations throughout the Mesozoic and Cenozoic in this area (Gabrielsen et al.2010). Nielsen et al. (2002) estimate that delam-ination of 35 km of lithospheric mantle will result in a long-lived uplift greater than 500 m of the average surface. A thin and weak lithosphere is also a necessary prerequisite for uplift from other mechanisms such as lithospheric folding due to far-field compres-sional stresses (Dor´e et al.2008; Cloetingh & Burov2011).
Japsen et al. (2012) reject edge-driven convection as a possi-ble contributor to the topography of elevated passive continental margins but do not consider newer modelling (e.g. Hieronymus et al.2007; Van Wijk et al.2010; Kaislaniemi & van Hunen2014) performed after the original work of King & Anderson (1998).
They find that only far-field compressive stresses and lithospheric folding can explain their observations of uplift in areas such as southern Norway and eastern Greenland. Our images of strongly varying lithospheric properties suggest that the structure and evo-lution of the mantle must be taken into account as well, and mod-elling of edge-driven convection points to the importance of ero-sive weakening of continental margins. Episodic erosion of the lithosphere could act as a trigger of uplift events and weaken the lithospheric margin, facilitating uplift due to compressive stresses.
6 C O N C L U S I O N S
We present the first finite-frequency P- and S-wave tomographies in southwestern Scandinavia, using a wavelet-based multiscale parametrization that greatly increases the recovery of amplitude and location of seismic velocity anomalies.
We image, as previous studies, low velocities in both VPand VS
below southern Norway and Denmark compared to high veloci-ties below Sweden and towards the central Fennoscandian Shield, but add to this picture important details. The low-velocity region below southern Norway consists of two adjacent anomalies: a shal-low channel-like feature extending from the coastal and western part of southern Norway into Denmark, and a cylindrically shaped anomaly that emerges from the channel further east at 150–200 km depth and extends to a depth of around 350 km. Furthermore, we find that the lateral boundary between the low and high velocities closely follows Carboniferous-Permian rift structures and centres of magmatic activity, that is the Sorgenfrei-Tornquist Zone and the Oslo Rift. Anomalies in the VP/VSratio are robustly constrained in the upper part of the model but depth variations cannot be imaged with confidence. Below Sweden we estimate negative anomalies in the VP/VSratio down to−3 per cent and below southern Norway positive anomalies up to+2 per cent.
The variations inδlnVP,δlnVSandδln(VP/VS) point consistently to higher temperatures below southern Norway and Denmark com-pared to Sweden and also to a higher degree of depletion in the lithospheric mantle below Sweden. The low velocities and higher temperatures in Denmark and Norway can be explained to a large part simply by a thinner lithosphere, but the deeper cylindrical low-velocity anomaly below the northwestern end of the Oslo Graben is more difficult to explain.
The channel-like low-velocity region is associated with a high degree of intraplate seismicity and an estimated contribution to the support of the high topography by low density material in the upper-most mantle. It is possible that the thin lithosphere below southern Norway and the stepwise increase in thickness towards the central Fennoscandian Shield is older than the Carboniferous-Permian rift-ing. On the other hand, it is equally possible that an originally thick but weakened lithosphere has been eroded to its current thickness of about 100 km after the Permian, causing episodic uplift of the area.
A C K N O W L E D G E M E N T S
This work has been done in the framework of the ESF EUROCORES TOPO-EUROPE Program 07-TOPO-EUROPE-FP-014 and was supported by MOST grant,102-2116-M-002-025, in Taiwan (S.-H. Hung). MAGNUS waveforms were recorded with the mobile KArlsruhe BroadBand Array (KABBA) of the Karlsruhe Institute of Technology, Germany as well as with permanent stations of the NORSAR array and the Norwegian National Seismological Net-work. We thank Andy Frassetto for providing us with the DANSEIS data (University of Copenhagen) and Marie Keiding (Geological Survey of Norway) for helping with the FENCAT database. We are grateful to Niels Balling (Univerity of Aarhus) for sharing data from the CALAS experiment with us. Seismological stations of the CALAS project, applied in this study, were from the University of Aarhus instrument pool and from the NERC Geophysical Equip-ment Facility (SEIS-UK). That project was financially supported by the Danish National Science Research Council. Financial sup-port for the MAGNUS experiment was provided by the Universities of Aarhus, Copenhagen, Karlsruhe and Oslo as well as NORSAR.
This work greatly improved from the constructive reviews of Karin Sigloch and an anonymous reviewer. Figures have been prepared us-ing the Generic Mappus-ing Tools (Wessel & Smith1998) and M_Map (Pawlowicz2005).
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216 M.L. Kolstrup, S.-H. Hung and V. Maupin
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