the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Deep crustal structure of the southern Baltic Sea in the light of seismic and potential field data
Abstract. The southern Baltic Sea lies within a critical transitional zone between two major geological provinces of Europe: the Precambrian East European Platform and the Palaeozoic Platform of Western Europe. While the shallow expression of this boundary is generally marked by the Caledonian deformation front, the deeper crustal configuration remains contentious due to thick Phanerozoic cover. This study integrates seismic interpretation with 2-D gravity and magnetic modelling to investigate the deep crustal architecture beneath the southern Baltic Sea. Four new seismic profiles (BGR16-256, BGR16-202, BGR16-257, BGR16-259), acquired during the BalTec (MSM52) expedition, were analysed alongside borehole and legacy seismic data. Seismic imaging reveals that the upper crust was primarily shaped by Permian–Mesozoic extension and Late Cretaceous inversion. Extensional basins such as the Mid-Polish Trough and Rønne Graben accumulated up to 4 km of sediments, later uplifted and folded during inversion, which caused displacements of 1.5–2 km and produced asymmetrical marginal troughs with NE-directed compressional vergence. The gravity and magnetic models, constrained by seismic horizons, enable imaging of deeper crustal levels including the top of the lower crust and the Moho, which lies between 38 and 42 km depth. These data reveal that thick Baltica-type crust extends south-westward beyond the Teisseyre-Tornquist Zone, contradicting interpretations that propose a sharp lithospheric boundary along this zone. A key finding is the identification of a NE–SW-trending crustal lineament, likely inherited from Precambrian lithospheric fabric. Furthermore, evidence of pre-Triassic tilting and erosion of Silurian strata suggests a significant tectonic event, possibly related to early Carboniferous uplift. The combined data provide new insights into the complex tectonic evolution of the region, supporting a model of Baltica crustal affinity beneath the southern Baltic Sea and emphasising the interplay of inherited Precambrian structures, Permian-Mesozoic extension, and Late Cretaceous inversion.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Solid Earth.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3107', Anonymous Referee #1, 25 Sep 2025
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RC2: 'Comment on egusphere-2025-3107', Anonymous Referee #2, 01 Oct 2025
This study presents four new deep seismic lines from the Baltic Sea and interprets them in the broader tectonic context of the southwestern Baltic Sea. New fault and trends are described. In addition, potential field modelling is carried out along the 2D profiles in addition to gravity inversions to derive regional maps of the top basement and Moho depth.
This paper provides new insights and a valuable contribution to the discussion on the crustal nature and tectonic history of the southwestern Baltic Sea. It is well written and structured and addresses the necessary background information and assumptions. The seismic data presented are of very high quality (although the reader does not get to see them in full resolution) and the figures are overall clear. The methods are well presented and the scientific results sound, though I think that some results need to be more carefully presented and discussed. In particular the derived grids of top basement and Moho thickness need a better evaluation. I have listed 5 major points that I think need to be addressed. In addition, I am attaching an annotated manuscript with several minor comments.
- Seismic horizons
- It is often not clear how the geological units are assigned to the interpreted seismic reflectors and horizons. I don’t think the interpretations need to be modified, but the reader needs to get a better understanding where these interpretations come from. The few boreholes shown are hardly sufficient to interpret the entire seismic sections. I suggest to either refer to previously interpreted seismic profiles from the region and/or to show close-ups of the seismic characteristics of the geological units to let the reader understand how the interpretation was guided.
- A larger, non-interpreted version of the seismic profiles would be much appreciated by the inclined reader. Could these be added as an appendix?
- 2D Potential field modelling
- On profile BGR16-202 (Figure 7), the modelled gravity signal clearly does not fit the observed one. Instead of arguing for too smooth gravity data, I would invoke 3D structures and a perhaps unlucky positioning of the profile. This profile runs roughly parallel to several basement folds (which profile BGR16-256 crosses perpendicular). Hence, basement offsets to the left and right of the profile may be present and would be reflected in the gravity signal but not in the 2D seismic line. Here, a 3D model would be necessary to build an accurate gravity model.
- It is not clear how the top and base of the lower crust are defined in the 2D models. There may be very large uncertainties here, I assume. These must be discussed. The magnetic data seems overfitted. Here, numerous crustal blocks are used. Some of the magnetic trends (long-wavelength signal) could perhaps be modelled with the upper crustal thickness. The long-wavelength signal of the gravity signal should be provided by the Moho. Perhaps these could explain some of the large residuals at the model ends?
- Moho & Top-basement grids
- The process to derive a regional Moho and top-basement grid is described in detail but it leaves some doubts whether one can invert the same dataset once for a Moho horizon and once for a top basement horizon without doubly interpreting some of the gravity anomalies. If I understood it correctly, it was not the residual of the Moho inversion that was used for the top-basement inversion, neither the original gravity anomalies, but something in between. Be that as it may, the paper needs a proper assessment of the Moho (and top basement) uncertainties. This needs to be given (a) for the 2D models and (b) for the regional grids. In addition, there needs to be a discussion on how the new grids differ from previously published grids. Are the tectonic implications derived from the Moho and crustal thickness maps consistent with previously published maps?
- Data availability
- Will the new data (i.e. the top basement and Moho grids) be available in a data repository?
- Seismic horizons
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EC1: 'Comment on egusphere-2025-3107', Christopher Juhlin, 09 Oct 2025
Dear Authors,
You have received two thorough reviews of your manuscript and both reviewers are positive. However, they also request greater clarity in the presentation of the data and interpretation. Please provide responses to the two reviews on a point by point basis. I agree with reviewer #1 that it would be good with uninterpreted versions of the profiles. These can be included in a supplement.
Best Regards,
Chris Juhlin, Guest editor
Citation: https://doi.org/10.5194/egusphere-2025-3107-EC1
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This is a well-written and, generally, well-executed study of additional shallow seismic lines in the southern Baltic Sea, supplemented by gravity and magnetic modeling. The results support a complex transition zone between the East European Craton and Phanerozoic Europe — a concept that appears to have increasingly gained traction in recent years.
While I generally follow the seismic interpretation and understand the technical aspects of the potential field modeling, I have some concerns regarding the gravity and magnetic models. Some of these concerns may possibly be resolved by adding appropriate clarification or documentation; others may require some more significant changes. The main issue is how the top lower crust and Moho depth are constrained, which is not properly documented. Furthermore, the general modelling approach is unclear (what was the basis for the decision-making of which data to fit and which structures to perturb). One of the four analyzed profiles in this paper is assigned a different density in the lower crust (Why?), and all densities appear to be inconsistent with those published in Ponikowska et al. (2024), which are crossing the profiles. All of these profiles are later used together in the same inversion scheme. As a result, I argue that the Moho depths across the various profiles are not directly comparable due to these inconsistencies. Additionally, I would expect to see better integration with other seismic lines in the region and a more in-depth discussion of why different Precambrian terranes may have responded differently to rifting. It would also be helpful to integrate and discuss how this affects our understanding of the STZ/TTZ — or more broadly, the margin of the EEC from the surface to the base of the lithosphere.
General Comments
Gravity/Magnetic Modeling Approach:
I have several questions about the overall modeling approach. The paper provides a general explanation of how gravity and magnetic modeling were performed and work, but does not describe the specific strategy adopted in this study. It’s also unclear what constraints were applied and how/to what degree — beyond the shallow seismic reflection lines, there should be other, sometimes deep adjacent seismic lines, and how the parameters or parameter ranges were selected.
Other Seismic Lines in the Study Area
In discussing and comparing crustal structure, other seismic lines beyond Ponikowska et al. (2024) may be discussed in more detail — potentially PL1-5600, PQ2-91, BASIN9601, the BalTec refraction line, and the BABEL lines. These lines intersect or approach the modeled area and are notably absent from the interpolated models of top basement, Moho depth, and crustal thickness.
EEC Margin, STZ–TTZ, Trans-European Suture Zone
For a complete discussion of the EEC boundary's width and complexity, the authors should include the most recent literature — including work by some co-authors of this paper — and extend the discussion into the sub-crustal domain. Consider incorporating the Sorgenfrei–Tornquist Zone and the so-called “Tornquist Fan,” which represents an equally complex transition. This is critical, as lithosphere-scale processes can reveal structural complexity that’s not visible in crustal data alone.
Other General Comments
Detailed Comments (with possible repetition from general comments):
To summarise: What exactly was the modeling approach? How were the Moho and lower crust interfaces defined and adjusted? Was priority given to gravity or magnetics, and why? Why were Moho depths not perturbed when unconstrained? Many long-wavelength gravity anomalies could be addressed with Moho changes. The approach may be fine, but it needs to be explicitly laid out.