the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Aug 2025
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)
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RC1: 'Comment on egusphere-2025-3107', Anonymous Referee #1, 25 Sep 2025
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AC1: 'Reply on RC1', Małgorzata Ponikowska, 29 Nov 2025
We thank Reviewer #1 for the thorough and constructive comments, which have helped us substantially improve the clarity, documentation and robustness of the manuscript.
For Reviewer #1, the primary concerns involved (1) documentation of the constraints on Moho and lower-crustal interfaces, (2) clarification of the modelling strategy, (3) consistency of densities and Moho depths across profiles, and (4) broader geological interpretation.
In the revised manuscript, we now clearly document how the Moho and top of the lower crust were constrained for each profile, explicitly incorporating intersections with deep seismic lines (e.g., PQ2-002; Ponikowska et al., 2024) and regional Moho models (Maystrenko & Scheck-Wenderoth, 2013). The overall modelling strategy has been updated to explain that all shallow horizons were kept fixed, while only the Moho and lower-crustal boundary were adjusted within permissible uncertainty limits to fit gravity and magnetic data. Lower-crustal densities have been updated to ensure consistency with intersecting models by Ponikowska et al. (2024), and inversions have been re-run using revised Moho geometries to eliminate earlier inconsistencies.
The discussion has been significantly expanded to integrate our results with a wider range of regional seismic data and recent literature, and to clarify the implications for the STZ/TTZ, the Sorgenfrei–Tornquist Zone, and the broader lithosphere-scale structure of the East European Craton margin. The revised version also emphasizes the prioritization of magnetic constraints, the application of a minimum-change strategy, and the removal of the simplifying assumption of homogeneous crustal layers.
Figures and supplementary materials have been updated to include crossing-line intersections, corrected figure numbering and captions, larger un-interpreted seismic profiles, and explicit depiction of reference Moho depths. Finally, we now provide RMS errors and depth-uncertainty estimates for both the inversion-based maps and the 2-D models, addressing the reviewer’s request for uncertainty quantification.
Below, we include point by point responses to the Reviewer’s comments:
The main issue is how the top lower crust and Moho depth are constrained, which is not properly documented.
We have clarified the constraints on the Moho and top of the lower crust in the revised manuscript and figures.
BGR16-256: Both the Moho and the top of the lower crust are constrained directly by nerby profile PQ2-002.
BGR16-259 and BGR16-257: The depths to the Moho and the top of the lower crust are constrained by intersections with PQ2-004-005 (for BGR16-259 only), PQ2-002, and BGR16-212.
BGR16-202: The geometry is constrained by intersections with BGR15-212 (Ponikowska et al., 2024) and is further guided by the Moho depths published by Maystrenko and Scheck-Wenderoth (2013). In addition, for BGR16-202 the Moho and the top of the lower crust were adjusted within the uncertainty range to achieve an optimal fit between the observed and calculated gravity and magnetic profiles (the latter for the top of the lower crust).
These constraints are now explicitly shown in the revised figures.The general modelling approach is unclear (what was the basis for the decision-making of which data to fit and which structures to perturb).
We have clarified the modelling strategy in the revised manuscript. In our approach, all horizons from the top of the basement upward were taken directly from the seismic interpretations and their geometries were not altered. Only the Moho and the top of the lower crust were adjusted, and these modifications were made solely to obtain the best possible fit between the observed and calculated gravity and magnetic profiles (the latter constraining the top of the lower crust). Importantly, all adjustments were made within the limits permitted by intersections with, or proximity to, neighbouring seismic profiles (Ponikowska et al., 2024), ensuring that the geometry remained consistent with independent seismic constraints.
One of the four analysed 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.
The lower-crustal densities in our revised models have been updated to ensure consistency with the intersecting seismic–gravity models presented by Ponikowska et al. (2024).
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.
The inversion has been re-run using the revised Moho horizons, which resolves the inconsistencies noted by Reviewer #1.
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.
The revised models and maps are now more consistently integrated with the profiles and interpretations published by Ponikowska et al. (2024), as outlined above. Both our results and those of Ponikowska et al. (2024) indicate that the entire study area lies within the attenuated margin of the East European Craton, rather than comprising separate Precambrian terranes. In the revised manuscript, we have expanded the discussion to highlight how our findings refine the understanding of the STZ/TTZ and the craton margin—from the surface down to the base of the lithosphere.
The paper […] 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.
All elements of the modelling strategy highlighted by Reviewer #1 as insufficiently explained have now been clarified and are described in greater detail in the revised manuscript (see tracked changes).
Only profile (BGR16-256) has a more or less direct deep control from a nearby deep seismic line (PQ2-002) throughout much of the profile length, but this constraint does not appear to be visibly incorporated. The two profiles differ significantly and have inconsistent modelling parameters for gravity and magnetic modelling. The other lines do not have coincident deep seismic coverage, though some deep seismic lines intersect at 1–3 locations. It remains unclear whether these were actually used as constraints or not.
In the revised BGR16-256 model, all structures and parameters have been updated to fully align with the PQ2-002 seismic line and its corresponding gravity and magnetic model. Intersections with additional seismic profiles, including those published by Ponikowska et al. (2024), were also incorporated as described above.
With little constraint on deep crustal structure and the simplifying assumption of homogeneous layers (upper+middle crust, lower crust, upper mantle), it becomes difficult to determine whether gravity anomalies originate from the upper–lower crust interface or the Moho (or density variations in between, which of course are neglected here because of the assumptions made).
In the revised models, we no longer assume homogeneous layers. While the density and susceptibility distributions remain relatively simple, their complexity is constrained by the limitations of the available seismic data (which in most cases extend only to the top of the basement) and by the inherently non-unique nature of potential-field modelling.
In Figure 7 [BGR16-202], the text explains that some of the gravity misfit is related to “uplifts in the lower crust.” I don’t follow this. How are these uplifts constrained? As far as I can tell, they are purely modelling features, and the gravity signal is still not well-matched, after all. From a gravity perspective, these anomalies could just as well stem from Moho undulations or features in the upper crust. A feature at the upper–lower crust boundary might explain the magnetic data, which would not be possible with a varying Moho — but this rationale isn’t laid out clearly. For each seismic line, the workflow for building and refining the model should be better explained.
We agree with Reviewer #1’s observations concerning model BGR16-202. The text has been revised accordingly, with particular emphasis placed on the role of the magnetic data in guiding the interpretation.
I recognize that in a poorly constrained region, modelling requires assumptions. One valid strategy is to apply a minimum number of changes to the starting model. Another approach is to prioritize either gravity or magnetic data and treat the other as secondary. In this study, it seems that fitting the magnetic data was prioritized. This should be clearly stated. In many places, structural adjustments appear to be driven primarily by the magnetic response, resulting in gravity misfits. That’s acceptable, but the fit could be otherwise improved by adjusting Moho depth — a step that doesn’t appear to have been taken.
In the revised manuscript, we explicitly state that magnetic data were given priority during the modelling. At the same time, we emphasize that our approach generally followed a “minimum-change” strategy, whereby modifications to the starting model were kept as limited as possible.
The large differences primarily between the density structures in this paper and those in Ponikowska et al. (2024) are concerning. The density differences are often significant, and such discrepancies should lead to overall different Moho depth levels. The authors should again at least explain/discuss this line of thought. To help evaluate this, the lower crust and Moho interfaces from the crossing lines should be shown in the profiles for comparison. The inversion process uses a constant starting Moho density contrast of 0.4 g/cm³, but values within the profiles used range from 0.33–0.4 g/cm³ (as far as I can tell). While these values may be adjusted during inversion, this variation should affect the Moho depth level and thereby introduce biases.
The differences between the density structures presented here and those in Ponikowska et al. (2024) have been reduced in the revised models. Intersections with the crossing profiles are now shown in the updated figures for direct comparison, and the Moho depth from Maystrenko and Scheck-Wenderoth (2013) is indicated with a dashed blue line for reference. We also note that adopting a constant density contrast between the lower crust and upper mantle is standard practice in studies of this type. Our chosen contrast of 0.4 g/cm³ remains on the lower end of commonly used values, as many published models apply a contrast of approximately 0.5 g/cm³.
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 modelled area and are notably absent from the interpolated models of top basement, Moho depth, and crustal thickness.
The revised discussion now more explicitly contrasts our results, together with those of Ponikowska et al. (2024), with earlier interpretations. We emphasize the critical role of the BalTec wide-angle reflection/refraction profile, which provides key constraints for the present study. Notably, several older interpretations—such as Krawczyk et al. (2002)—proposed a significantly shallower Moho southwest of the STZ/TTZ, a view that is not supported by either our results or those of Ponikowska et al. (2024).
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.
As requested by Reviewer #1, we have expanded the discussion and incorporated several recent studies, including Ponikowska et al. (2024), Nguen et al. (2024), Kind et al. (2025), and Mazur et al. (2026).
Figures 6–9: Please add the Moho depth interpretations, and perhaps also the density values from crossing deep seismic lines (Ponikowska et al. 2024 and others) for direct comparison.
We did not show the Moho depths or densities from Ponikowska et al. (2024) because our revised models are fully consistent with their results. Instead, we included intersections with the Moho from Maystrenko and Scheck-Wenderoth (2013), which we consider the most representative and regionally comprehensive reference for this area.
Figure numbering: Starting from section 5.10, figure numbers appear misaligned — perhaps all figures are one number too high? For example, Fig. 12 and Fig. 13 are both labelled “Depth-to-basement” but clearly show something different. Fig. 13 probably shows crystalline crust thickness.
Figures numbering and titles have been corrected.
Uncertainty: There is no mention of uncertainty or resolution in the final models. This is especially important in the inversions. Forward models are one thing, but inversions typically provide at least some uncertainty estimates — this is missing here.
RMS errors and depth uncertainties are provided for both inversion-based maps. In our models, the RMS error for the modelled top-basement and Moho horizons does not exceed 300 m and 500 m, respectively, indicating a satisfactory level of accuracy at the regional scale.Detailed Comments (with possible repetition from general comments):
L22–23: A more detailed discussion of how different Precambrian basement blocks may have reactivated to form this “kink” would be valuable. (not necessarily here)
This problem is addressed in the revised Discussion: “In the Rønne Graben, by contrast, the Moho remains deep despite significant basement subsidence. This pattern, evident in seismic and gravity data (Figs. 8, 10, 12), implies that crustal thickening may have occurred after initial extension. One hypothesis is that the Rønne Graben formed as a pull-apart basin (Deeks and Thomas, 1995) or was influenced by horizontal shear in the lower crust that decoupled upper crustal extension from the deeper lithosphere (Yang et al., 2018). Alternatively, the area may have experienced crustal shortening and thickening during inversion, with lower crustal subversion accommodating compressive strain – a mechanism analogous to that proposed by the BABEL Working Group (1993).”
L34–36: Consider also discussing the mantle lithosphere, which remains poorly understood.
This has been done in the Discussion.
L39–44: Several additional seismic lines (PL1-5600, BASIN9601, BABEL) have been published or re-evaluated recently and should be considered.
Previous studies are now mentioned in lines 39-40 of the revised manuscript.
L50–51: It would be helpful to mention here how the interpretations were constrained.
This has been done later in the methodology and Results sections.
L52: “Constrained by potential field models” is vague. What exactly is being constrained, and how? Are the 2D forward models used as constraints? Perhaps to better call these a model input, as these 2D lines are also a product of relatively unconstrained potential field modelling.
This sentence has been revised by removing the phrase “constrained by,” which makes the intended message clearer.
L54–56: How was the “improved spatial resolution” assessed? I don’t see any uncertainty/resolution analysis.
Information on uncertainty and resolution is provided in the results section. Including these details in the Introduction could be premature, especially since earlier publications do not report comparable information. At the Introduction stage, the intent is primarily to mention a visual impression of the level of detail depicted on the horizon maps.
L84: Repeating an earlier point — include more recent and ‘diverse’ literature, for example, on the mantle lithosphere (e.g., tomography).
The lithospheric study by Kind et al. (2025) was referred to in the revised text.
L88: Please clarify why the strike-slip model doesn’t align with the apparent STZ–TTZ connection.
The strike-slip model does not account for the divergence between the TTZ and the Caledonian Deformation Front (CDF) in the southern Baltic Sea, nor does it explain the apparent linkage between the TTZ and the Sorgenfrei–Tornquist Zone (STZ) near Bornholm, the latter being widely regarded as an intracratonic structure. This explanation has been added to the text.
L94: Unclear reference — what does “It” refer to?
‘It” was replaced with “the area”.
L102: The lithospheric thickness change should be discussed for the TTZ as well.
The revised manuscript now clarifies that, unlike the relatively narrow TTZ, lithospheric mantle thinning extends across a much broader zone between the TTZ and the margin of the Bohemian Massif (Mazur et al., 2026).
L111: Should this be “The crustal keel has been…” or “Crustal keels have been…”?
The correct phrasing is “The crustal keel has been…”. The text was corrected accordingly.
L126: The “bend” of the CDF toward the TTZ is hard to locate — please clarify.
The sentence in question was rewritten for clarity: “It bends south-eastward toward the TTZ along a NW–SW trend (Fig. 1) and subcrops onshore in northern Poland (Dadlez et al., 1994).”
L213: Densities are said to be derived from seismic velocities — but which ones/which velocity models? Please specify. Also, why does only BGR16-202 have a different lower crustal density? It intersects BGR16-212 (Ponikowska et al. 2024), which has higher values. This inconsistency is not addressed, and similar discrepancies exist with BGR16-259 and other profiles crossing PQ2-002 and PQ2-004-005.
Density values were harmonized across all intersecting profiles. Seismic velocities were primarily adopted from the BalTec refraction profile (Janik et al., 2022), which coincides with the BGR16-212 seismic reflection line. For the remaining seismic reflection profiles, interval velocities corresponding to the relevant model units were used in the calculations.
L217: The Moho and top of lower crust aren’t really primary — they’re in fact the only interfaces, if I understand right. How was the top of the lower crust chosen? It seems as unconstrained as other interfaces. This modelling rationale needs documentation.
This issue stems from imprecise wording. The term ‘primary’ has been replaced with ‘key’. Magnetic data and profile intersections were used chiefly to constrain the geometry of the top of the lower crust.
L218: Again, which seismic velocities were used for density conversion?
Seismic velocities were primarily adopted from the BalTec refraction profile (Janik et al., 2022), which coincides with the BGR16-212 seismic reflection line. For the seismic reflection profiles, interval velocities corresponding to the relevant model units were used in the calculations.
L237: More detail is needed on how previous studies were used to constrain the initial top lower crust and Moho horizons. Were they interpolated? Were intersections with deep lines used? Please include these surfaces in the 2-D profiles.
Both the Moho and the top of the lower crust were constrained using intersections with deep seismic reflection profiles PQ2-002 and PQ2-004-005 (Ponikowska et al., 2024) and the BalTec refraction profile coincident with BGR16-212 (Janik et al., 2022).
L249: Why weren’t other lines like BABEL, BASIN9601, POLONAISE, PL1-5600 used, even though they cross the area?
In the revised inversion, additional constraints were incorporated from profile PL1-5600 (Mazur et al., 2016) and from the northernmost segment of BASIN9601. The POLONAISE profiles, including the pre-POLONAISE LT-7 line, lie too far from the study area to provide useful constraints. As for BABEL Line A, depth-converted data and an accompanying velocity model were not available to us.
L294: Maybe refer to Fig. 2 for the Ustka Fault. It can be traced to PQ2-004-005, but not clearly further east (BGR16-212 and PL1 5600) – why?
A reference to Figure 2 has been added in the revised text. The Ustka Fault is still visible on line BGR16-212 (just east of the A8-1/83 well), but it represents only one of several comparable faults in this part of the section.
Figure 5: “Cr” is used for Cretaceous; should be “K.”
Corrected.
L356: Cretaceous inversion structures are mentioned but not described. There is evidence — please elaborate.
The inversion structures are now clearly pointed out in the revised text.
L371: I might get it wrong, but I have the impression that it’s the only profile to the south/west of the fault.
Yes, this is correct. The profile lies SW of the Koszalin Fault.
L398: Perhaps rewrite: “in the first 20 km of the profile.” Also, please mark the Kolobrzeg Anticline or refer to the figure where it is shown.
Both corrections are implemented in the revised manuscript.
L430: Is top basement not well constrained? Instead, it probably indicates heterogeneity in the (upper) crust. Furthermore, the observed gravity anomaly shows the opposite of what is expected – a low where there should be a high above the shallow basement. The modelled gravity shows the exact opposite behaviour.
This is an observational feature that cannot be disputed. The revised text now includes an additional clarification: “These shorter-wavelength signals are more difficult to reproduce in the model because of limited resolution, the complexity of the basement structure, and the influence of 3-D effects and lateral density variations in the upper crust and sedimentary cover.”
L432: This statement is confusing. If misfits align with lower crust uplifts, are these independently constrained features or just model features? Could the misfit be diminished by introducing Moho variations instead? The modelling approach seems to prioritize fitting magnetics. That’s fine, but if magnetics allow for lateral susceptibility variation, this could explain some of the misfits, instead of changing the top lower crust too much. Moho adjustments should not affect magnetics but could improve gravity fit. Why were they not attempted?
We agree with the reviewer’s criticism. However, reproducing the observed anomalies by varying the Moho depth would be difficult, as such changes would generate longer-wavelength signals than those observed due to the considerable depth of the Moho. The remaining misfits are more likely related to upper-crustal density variations not accounted for in the model, as well as the model’s orientation, which is nearly parallel to the strike of the gravity anomalies (Fig. 3a). This along-strike configuration may introduce significant 3-D effects.
To summarise: What exactly was the modelling 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.
The additional explanation was added to the current section: “The depths to the Moho and the lower crust were constrained through intersection with the BalTec and BGR16-212 seismic refraction and reflection profiles, respectively (Janik et a., 2022; Ponikowska et al., 2024). The geometry of the Moho for the profile BGR16-202 is also guided by previously interpreted Moho depths published by Maystryenko and Scheck-Wenderoth (2013).” The priority given to magnetic data is addressed in the methodological section.
L459–460: Saying “where the model predicts a basement uplift” is misleading — the uplift is part of the modelling, used to fit geophysical data.
The paragraph in question has been rewritten: “As in the previous models, several short-wavelength anomalies in the synthetic gravity response – primarily resulting from the detailed basement geometry – do not correspond closely with the observed data. For example, the observed gravity shows a low between 0 and 35 km along the profile, whereas the synthetic response indicates a mass excess from 0 to 20 km and a mass deficit from 20 to 35 km. Another mismatch appears near the 65 km mark, where the synthetic response suggests a basement uplift that is not evident in the observed gravity data, possibly reflecting an over-smoothed satellite gravity signal in this part of the profile.”
L484: “…and density” is unclear — the lower crustal density is fixed.
The reviewer’s point is valid; however, the revised model now incorporates some variation in lower-crustal density. As a result, the original wording is now more appropriate for the updated model interpretation.
L505: Figure numbering seems off.
The figure numbering has been corrected.
L523: Should this be FA instead of BA? Probably refers to Fig. 3a. Also, what would be expected from the BA or FA here?
Thank you for pointing out this error. The correct reference here should be to the Free Air gravity (Fig. 3a).
L524–525: Please elaborate. Why would later tectonic activity explain this?
This effect can be explained by the influence of younger structural features on the gravity field, most notably those formed during Late Cretaceous inversion tectonics. In several parts of the study area, gravity highs correspond well with subcrops of Triassic and Jurassic strata uplifted within the cores of inversion-related anticlines, as well as with basement highs located in the hanging walls of inversion-related faults.
L525: The ENE–WSW trend also appears in the FA (Fig. 3b).
We apologise for the confusion caused by the incorrect reference to Bouguer gravity. Figure 3b presents a magnetic anomaly map.
L548–549: The “depression west of Bornholm” isn’t clear — possibly meant east? Please also clarify where exactly the Moho uplift is beneath the NW Mid-Polish Trough.
The Moho depression is indeed situated west of Bornholm. A reference to Figure 12 has also been added, as this figure best illustrates the distribution of Moho uplifts and depressions.
L598: Indicate where this fault lies (I think ~100–105 km). The basement appears to outcrop beneath the Cenozoic, so a fault still seems possible.
Yes, we agree that the presence of a fault is till not excluded. The text has been modified accordingly.
L603–604: The phrase “westward increase in thickness of the Caledonian accretionary wedge” is confusing. It seems to thicken eastward — please clarify.
The sentence in question has been rewritten to clarify this issue: “Because this profile is located farthest to the west among the study profiles, the greater thickness of lower Palaeozoic indicates a westward increase in the thickness of the Caledonian accretionary wedge.”
L616: Expand the TTZ and EEC margin discussion using other results covering neighbouring areas and incorporating mantle lithosphere structure.
Discussion has been expanded: “A key unresolved issue concerns the extent and configuration of the EEC beneath the southwestern Baltic Sea and its relationship to the TTZ. Several models propose that the EEC crust extends south-westward beneath the North German–Polish Caledonides, reaching as far as the Elbe Lineament (Tanner and Meissner, 1996; Bayer et al., 2002; Mazur et al., 2015, 2016a, b; Smit et al., 2016; Ponikowska et al., 2024). This interpretation is supported by deep seismic reflection and refraction data, and gravity and magnetic modelling, all consistently indicating the presence of a thick, reflective lower crust of likely EEC affinity. Comparable crustal signatures have been documented farther west beneath northern Germany (e.g., DEKORP and EUGENO-S profiles; Aichroth et al. 1992; Bayer et al., 2002; Mazur et al., 2026), suggesting that craton-derived lower-crustal material may underlie parts of the Caledonian belt in a broad suture zone rather than terminating along a discrete boundary.
In contrast, alternative models argue for an abrupt termination of the EEC at the TTZ, interpreted as a major strike-slip suture formed during the Ordovician–Silurian accretion of Avalonian terranes (Dadlez et al., 2005). However, a sharp Moho- and crustal-scale boundary at the TTZ is difficult to reconcile with several independent observations. First, the geometry of the CDF and the onshore thin-skinned Caledonian structures in NW Poland imply that deformation was decoupled from the crystalline basement (Mazur et al., 2016b; Ponikowska et al., 2024), inconsistent with a rigid, abrupt cratonic termination beneath the deformation front. Second, the TTZ is associated with a broad zone of heterogeneous basement properties, rather than a single sharply defined suture; recent potential-field inversions and magnetotelluric studies reveal an up to 120 km wide transition zone characterised by mixed crustal signatures, inherited faults, and variable lower-crustal reflectivity (Smit, 2016; Mazur et al., 2021, 2024, 2026; Jóźwiak et al. 2022).
Further constraints come from mantle-lithosphere structure. Recent seismological studies show that the Scandinavian Mid-Lithospheric Discontinuity (MLD), a hallmark of the stable cratonic mantle, continues southward beneath the Caledonides and even reaches the Bohemian Massif (Kind et al., 2025). This implies that craton-derived lithospheric mantle extends significantly farther southwest than traditionally assumed, supporting models of a EEC transition zone beneath the southwestern Baltic Sea. Additionally, recent tomography models reveal high-velocity mantle lithosphere beneath NE Germany and the Baltic Sea, consistent with craton-affinity lithosphere overstepping the surface trace of the TTZ (Vecsey et al., 2014; Zhu et al., 2015). In contrast, low-velocity zones and lithospheric thinning occur farther southwest beneath the Variscan front, indicating that the major lithosphere–asthenosphere transition lies well beyond the TTZ.
L635: The abrupt 90-degree shift in the thick crust boundary is notable and contrasts with the STZ–TTZ trend. A more detailed discussion of possible controlling factors (e.g., rheology, stress regime, inheritance) would be valuable.
The discussion has been expanded: “This suggests that the transition is rooted in the Precambrian architecture of the EEC and likely represents the preserved signature of a long-lived structural grain formed during early Proterozoic crustal assembly. Similar NE–SW-striking basement domains and shear zones are documented in the adjacent part of the craton (Bogdanova et al., 2008), supporting the interpretation that the observed crustal step reflects inherited lithospheric heterogeneity rather than a Caledonian or younger tectonic boundary.”Citation: https://doi.org/10.5194/egusphere-2025-3107-AC1
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AC1: 'Reply on RC1', Małgorzata Ponikowska, 29 Nov 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?
-
AC2: 'Reply on RC2', Małgorzata Ponikowska, 29 Nov 2025
We thank Reviewer #2 for the constructive and detailed comments, which have led to substantial improvements in the clarity, documentation, and presentation of our results.
The revised manuscript now includes a more comprehensive discussion of the top-basement and Moho geometries, together with clearer explanations of how these horizons are constrained. We have expanded the description of the seismostratigraphic framework by explicitly referencing borehole data and intersecting seismic profiles from Ponikowska et al. (2024). To further assist the reader, we have added large, non-interpreted versions of all seismic profiles to the Supplementary Information.
Model BGR16-202 has been revised following the reviewer’s suggestions, including a clearer explanation of potential 3-D effects related to the profile orientation. We also clarify how the top and base of the lower crust were defined, emphasizing the role of seismic intersections in limiting modelling uncertainty. Uncertainty estimates for both the 2-D models and the regional Moho and top-basement grids have been added (RMS errors and depth ranges), and we now discuss how our results compare with previously published Moho and crustal-thickness maps.
Finally, as requested, the newly derived grids of top basement and Moho depth will be made available as text files in the Supplementary Information.Below, we include point by point responses to the Reviewer’s comments:
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.
The revised manuscript now provides a more detailed and coherent discussion of both the top-basement geometry and Moho depth.
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.
As the reviewer correctly notes, the seismostratigraphic and structural interpretation used in this study is grounded in borehole data and further constrained by intersections with, and comparisons to, adjacent seismic profiles published by Ponikowska et al. (2024). These constraints are now described more explicitly in the revised manuscript.
A larger, non-interpreted version of the seismic profiles would be much appreciated by the inclined reader. Could these be added as an appendix?
A larger, non-interpreted version of the seismic profiles has now been added to the Supplementary Information, as suggested by Reviewer #2.
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.
The description of model BGR16-202 has been revised in accordance with the suggestions provided by Reviewer #2.
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?
In the revised models, the top of the lower crust and the Moho were adjusted to obtain the best possible fit between the observed and calculated gravity and magnetic data. These adjustments were consistently made within the limits imposed by intersections with, and proximity to, neighbouring seismic profiles (Ponikowska et al., 2024). In many cases, these seismic constraints significantly reduce the degree of freedom available when matching geophysical anomalies to crustal structures.
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?
Uncertainties for the Moho and top-basement depths have been added to the revised manuscript in the form of RMS errors and corresponding depth estimates. For the 2-D models, uncertainties are expressed using the residuals in mGal and nT. We have also expanded the discussion to address the consistency of our results with previously published Moho and crustal-thickness maps.
Will the new data (i.e. the top basement and Moho grids) be available in a data repository?
Yes, these grids will be provided as text files in the Supplementary Information.
All corrections and suggestions marked by Reviewer #2 on the annotated pdf copy of the manuscript have been included in the revision.
Citation: https://doi.org/10.5194/egusphere-2025-3107-AC2
- 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 -
AC3: 'Reply on EC1', Małgorzata Ponikowska, 29 Nov 2025
We would like to thank you and the Anonymous Reviewers for your comments. They have helped us significantly improve the quality of our manuscript. We have provided detailed, point-by-point responses to both reviews. In accordance with suggestion from Reviewer #1, we have added large, non-interpreted versions of all seismic profiles to the Supplementary Information.
Sincerely,
Małgorzata Ponikowska (corresponding author)
on behalf of all co-authorsCitation: https://doi.org/10.5194/egusphere-2025-3107-AC3
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AC3: 'Reply on EC1', Małgorzata Ponikowska, 29 Nov 2025
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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.