Revisit of the Fennoscandian Shield along the UPPLAND seismic profile: competitive velocity models

This paper presents a comprehensive re-analysis of seismic data collected along the UPPLAND profile in the Fennoscandian Shield, focusing on the competitive velocity models for P-wave (Vp) and S-wave (Vs) velocities and the Vp/Vs ratio. The initial data collection was conducted in 2017, and the first interpretation was published by Buntin et al. (2021). This study reveals that, while both the previous and current models exhibit similar velocities up to a depth of approximately 35 km, significant discrepancies arise in the lower-crust and upper-mantle velocities and in the depth of the Moho boundary. The preferred model obtained by 2D forward ray-tracing modelling indicates Vp values of approximately 7.05–7.17 km s−1 in the lower crust (LC) and 8.05 km s−1 in the upper mantle (UM), contrasting with the earlier model's values of 7.25–7.4 and 8.0–8.5 km s−1, respectively. The Moho depth varies between 43–50 km in the new model, compared to 45–52 km in the previous one. In addition, we present two, possibly overlapping, tectonic interpretations to explain the new model. The main crustal structure formed during W-vergent crustal stacking at ca. 1.86 Ga, followed by N–S crustal shortening at 1.82–1.80 Ga. The bulging of the high-velocity upper mantle is either related to extension at 1.89–1.87 Ga in a continental back-arc or was formed during extensional magmatism at 1.6/1.7/1.8 Ga. The findings highlight the complexities in determining lower-crustal and upper-mantle properties from ambiguous seismic data and suggest that the interpretations presented may require a more cautious approach, allowing alternative explanations.