Dear authors, dear editors,

I have now reviewed the manuscript “Physical modeling of ice-sheet-induced salt movements using the example of northern Germany” by Hardt et al. submitted to Earth Surface Dynamics”. The study presents the results of a series of analogue models of the response of salt structures to ice-sheet loading. The modelled salt structures are deformed in response to the applied load, a mechanism that has previously only been shown in numerical models. The results help to better understand geomorphic features above real salt structures. The manuscript is generally well written and is a very interesting contribution on a topic that needs better understanding, both from a scientific and applied viewpoint.

However, I think the manuscript could benefit from some modifications and improvements. The main points are:

• Define goals / research questions need to be better defined. At the beginning, several questions are posed and the authors get back to those questions in the discussion. However, there is a mismatch between the questions posed at the beginning and those answered in the discussion and there are actually two separate sets of questions. My suggestion is to better define the main questions and placing the “secondary” questions within short explanations provided with each main question.
• The issue of young salt tectonic activity in the study area should be explained in more detail. A challenge when studying ice-load induced salt movements is how to distinguish them from other (longer-term) movements. The description of the study area should provide a bit more information on the phases of salt tectonic activity. Have those salt structures been rising during the latest Cenozoic prior to the Pleistocene glaciations? Does it matter?
• Ice-sheet load in the models is not dynamically scaled (e. g., Lines 130 / 185ff). I am aware that scaling is a huge challenge for any physical model. However, the displacements in the models are very high in relation to the total thickness of the model section (>10 mm vertical displacement vs 35 mm thick model section). Salt extrusion observed in one run seems another example of extreme deformation. However, if the models are to be compared to natural examples and numerical models (the numerical models may of course have over shortfalls), somewhat more context should be provided and the limitations should be pointed out more clearly. Maybe a comparison to other physical models of salt tectonics helps to provide more context. I think the discussion would benefit from some more consideration here.
• More detailed comments are provided below.

Overall, I would rate this as a minor to moderate revision. I hope my comments are considered helpful by the authors.

Sincerely,

Jörg Lang

Detailed comments:

Line 11: Throughout the manuscript, you are switching between “glacial loading” and “ice-sheet loading / ice loading”. Please check and decide for one term – I think ”ice-sheet loading” is the most appropriate and widely used term.

Line 12: This sentence is a bit misleading and should be split up and modified. The presented model is not only based on the Scandinavian ice sheet, but on the overall geological situation (salt structures, ice margins, etc.) in northern Germany.

Line 18: “salt-source layer” is the more common term.

Line 28: Throughout the manuscript, you are switching between “salt structure”, “salt dome” and “salt diapir”. Please check for consistency and if the terms are used according to their definitions.

Line 35: The surface cracks are only one feature your models can help explain. I recommend making a broader statement here.

Line 48: You should explicitly state here that future glaciations are considered a real issue for such long-term safety considerations.

Lines 50-51: Please rephrase or extent to explain the impact.

Line 52ff: As glacio-isostatic adjustment and it’s effects are important processes in the context of the study, this should be explained a bit better. Also, what are “hydrogeological adaptations”? Please elaborate.

Lines 57-59: This is misleading and oversimplifying: The link between neotectonics and ice loading is not just based on the parallel orientation. This would be a very weak link…

Line 61: “the spatial distribution of” can be deleted.

Lines 62 / 70: Again, there is a switch in terminology: Please use either “Zechstein salt” or “Permian salt”. Permian may be more correct, as some salt structures in northern Germany may also include some Rotliegend salt. However, for your study area, Zechstein seems appropriate.

Line 85ff: As those questions are central to the study, maybe use bullet points or number here to make them more striking. Also, there are only 3 questions here, while the discussion tries to answer 4 questions. It is a bit confusing that 3 questions are presented here and 4 similar, but slightly different questions in the next section. Maybe restructure this section, presenting higher-order questions (I think this is the second set) first, each question followed by a short explanation that may include the lower-order, more detailed questions (don’t forget to modify the questions in the discussion accordingly!) Furthermore, all those questions (especially the first set of questions) should be put into a wider context, as your study is not just about checking some specific model configurations, but is a new approach to an understudied topic.

Line 94-96: This sentence should be placed at the beginning of the next section.

Line 102: Maybe rephrase to “…varies in thickness between…”, as the thickness is the important part here.

Line 105: This is a huge leap from the very general features of the salt structures to the very specific surface cracks. I wonder if the surface expressions of salt structures and their association with younger morphological features should be explained in more detail.

Line 110: I think we don’t need a long description of the regional Quaternary geology here, but at least the term “W2” should be explained – please add just one sentence introducing the Weichselian ice advances into the area.

Line 155: Fault or strain pattern?

Line 156ff: The first part of this section provides a lot of background information on the regional geology. I wonder if this should be better placed in the “Study area” – section, while the model set-up should focus on the model.

Line 177: Please add: Was the model surface flattened? Were the sand layers compacted before loading?

Line 121: “ice-sheet load” seems more appropriate (see earlier comment).

Line 256: “Between W2 and W2”? Please check! May be rephrase to avoid the regional terminology.

Line 280: Why place the cross sections in the supplement only? I recommend adding them as a regular figure.

Line 281: Discussion: Please see my earlier comments on the questions. If you decide to modify the questions / goals, some reorganization may be necessary here. However, I don’t see any issues with the overall structuring of the discussion with the questions as section headings.

Line 288, 300: Is “connectivity” the appropriate term here? Please consider rephrasing.

Lines 290-291: The last sentence of the section starts the interesting part of the discussion. Please elaborate further.

Line 293: My impression is that the focus of this section is rather the position of the (ice) load margin relative to the salt structure than the type / size of the salt structure. Please also check with the next section. Maybe the section should better be combined.

Lines 294-295: Please define “larger” and “smaller”. Does this refer to the area, volume of salt or else?

Lines 305-307: The resistance to salt flow caused by thinning salt layers is a well known phenomenon in salt tectonics. Some references seem appropriate here.

Line 349: This is not exactly what we wrote. Our point was rather that a relatively small obstacle may initiate the formation of glacitectonic thrusts. Furthermore, I am still skeptical about rising salt structures acting as (significant) obstacles to ice flow, as the ice sheets did transgress other, even higher topographic obstacles, e.g. some low mountain ranges near the maximum extends of the Middle Pleistocene ice sheets in northern Germany.

Line 361: I find your observation of the intense deformation in salt structures that are partly loaded very interesting. The reason we placed the ice-margin 1000 m away from the salt structure in the numerical models was exactly the strong deformation occurring if the ice margin was located exactly on top of the salt structure. This strong and rapid deformation commonly triggered the numerical models to crash. We never really addressed this issue in our papers. I fully agree that such a configuration should result in larger displacements.

Figure 5: in the central part of the figure, it looks like the orientation of the load is different in the cross section and in the map view. The map shows a left-right (west-east?) trending margin of the load. I would understand the cross-section to show a top-bottom (north-south) trending margin. Please clarify.

Figures 6, 8, 9 and 11: I suggest naming the stages above each panel, so the reader does not have to look up what “stage 2” actually represents.

Supplement: I wonder why the supplement (one figure showing cross sections from the model) is not included as a regular figure? You don’t show any cross sections, so this might be a nice addition.

I found this to be an exceptionally interesting study and with important bearings on both salt and ice dynamics with relevance to their respective literatures, and a tantalizing step towards understanding the possible dynamical coupling of the two processes. The paper is very well written and I was not able to find much wrong with it such that I suggest it could be published virtually as it stands. These innovative physical modelling experiments appear to shed new light on the topic and have gone a long way to convincing me that surface cracks in Germany are related to salt dynamics. [Along with colleagues, we also found surface cracks in DEMs and spent some time building an alternative hypothesis relating to tension arising in connection with tunnel valley development]. An important point that comes from the modelling is when the margin is halfway across the salt body we get large effects and the reverse flow and the influence of lobate margins is interesting. The discussion was very helpful in interpreting the results and connecting so well to the various historical notions of salt and ice dynamics in the region.

The limitations and pragmatic aspects of the physical modelling regarding physical and time scalings are well made, and of course relevant to the extent to which we seek to interpret the results. I think two things slip a little between the words, and could helpfully be expanded upon if the authors wish

• Yes you say the applied load is unrealistically high and is mostly to speed up the experiments which seems fine. But maybe you could tell us by what rough factor; take a typical low parabolic surface profile from an ice sheet margin and estimate the likely ice loading.
• The percentage z movements appear large compared to the dimensions of the ‘sediments’ in the sandbox. You tell us that this is likely unrealistic which is why you  don’t fully explore the absolute Z movements, but could you say more on this; the extent to which they likely arise from  viscosity and loading scaling issues vs, static ice margins against mobile ones which might not have enough elapsed time for bulges to develop.

  It would help to tell the reader early in the paper what you actually mean by pillow vs domes, later on I gathered it was combination of factors relating to size and depth, but am still a bit unclear, and yet you often distinguish between them.

 Line 195  stage 1 growth. Suggest to make clearer in wording that this stage is a ‘relaxing’ stage such that the materials initialise prior to your experiment. Use of the word ‘growth’ first implied to me that stuff was responding to an advancing ice load.

Explain what  the white lines are in fig 1b

The salt dome stipple is not very visible in fig 1b

Explain in fig caption what  the grey is in fig6A? no data or a value I cant see in colour bar?

Chris Clark 23/11/2023

Review of the paper by Hardt et al.: Physical modeling of ice-sheet-induced salt movements using the example of northern Germany

The submitted paper presents the results of analogue modelling of glacial loading and unloading on salt structures. The modelling replicates existing salt structures in the northern German subsurface and a series of different scenarios are tested. The goals of the modelling are to investigate the role of the size and shape of the salt structures, the relation between variations of shape/orientation of the salt and variations of the ice margin, and to investigate intra-salt flow patterns. In addition to this, the interpretations of the modelling results aim at getting a better understanding of the known surface cracks in the landscape above the salt structures in northern Germany.

The paper is an important contribution to the understanding of the impact on salt structures from the loading and unloading of the Pleistocene ice sheets and the possible effect on the surface morphology. The modelling is nicely structured and presented. The paper is a pleasant read and I am happy to recommend it for publication. I have some comments and suggestions listed below which most likely could be handled in a minor revision.

General comments:

• In the Introduction it is mentioned (lines 56 to 59) that in areas affected by GIA fault zones can be reactivated (‘glacially induced faults’). However, this is the only place in the paper where this is mentioned. Obviously, it is not in the scope of the laboratory modelling to include the effect on the upper mantle, but I suggest that a sentence or two are added to the Discussion, where it is discussed whether/to which extent this mechanism could have affected the area chosen as study area.
• The ‘Experimental methodology and setup’ section starts with ‘Remarks on the selection of model parameters’ followed by ‘Modeling materials and data capture’ and finally ‘Model design’. I suggest that the ‘Remarks’ section be moved to the end because the reader cannot necessarily relate to remarks on the individual stages of the modelling before the model has been described. Also, consider moving the sentences of lines 185 to 193 to the end of the ‘Remarks’ section as it, in my opinion, fits better here.
• In the ‘Results’ section you mention the crestal grabens that form above the modelled salt structures. As the figures generally are small, please refer more specifically to where on the figures the crestal grabens can be seen (i.e. with arrows).
• As the timescales of salt flow and ice flow are very different, I agree that it is obvious that the loading in the modelling can only be stationary. It is also understandable that the modelling cannot be weighted, and that the ice load in the model setup has to be exaggerated. However, when specifically evaluating the effect of a lobate ice margin, I feel - due to the factors mentioned – that the uncertainties on the model results here must be quite large. I suggest more elaboration on this in the discussion, for instance as a separate part of the discussion dealing with uncertainties.
• It is mentioned as a result, that ‘the reversed vertical displacement after the unloading, caused by the flow reversal of the salt system accounts for only up to roughly 50 % of the vertical displacement that occurred during the loading stage’ (sentence from the Conclusions). But there are no suggestions as to why this is happening. Please elaborate on why the system does not return to the pre-loading situation but instead establishes a new equilibrium, and to which extent is it believed that the chosen model setup can be responsible for some of the observed differences (the static load, a non-weighted model, the extrusion etc.)?

Specific comments:

Line 57-59: Please re-think this sentence and the argument it contains: The orientation of faults parallel to the Pleistocene ice margins does not document a link between neotectonic activity and ice sheet loading. 

Line 71: Delete ‘at the surface’.

Line 90: The loading/unloading processes are unrelated to the size/shape of the salt structure. Consider writing ‘during the loading and unloading processes’.

Line 91: I would prefer to write ‘explore the relation between’.

Line 94: Consider ‘Northern Germany constitute an ideal study area, as it is….’ instead of ‘Northern Germany provides the ideal model region for our study, as it is….’

Line 103: With ‘low-lying’, do you mean ‘deep-seated’?

Line 107: Consider using ‘terrain surface’ instead of ‘free surface’

Line 136: Write ‘silicone flow’ rather than ‘salt flow’.

Line 159: Please explain what you mean with: ‘The GS and KH have a heterogeneous geometry with several peaks’.

Line 162: Should the sentence ‘…stimulating the debate of the relationship between salt structures and ice sheet extent’ be moved to the discussion? In my opinion it is irrelevant here.

Line 166. Consider deleting ‘…thus providing a promising modeling scenario’, because it is a subjective evaluation at this stage. If what you mean is that it would be interesting to model a scenario like this because the salt structures were partly transgressed, please rephrase.

Line 179: ‘Front edge’ instead of ‘leading edge’?

Line 212: Consider reducing ‘covered by the glacial load during the loading stage’ to simply ‘loaded’.

Line 282: ‘Here, we will……….attempt to discuss…’. Delete ‘attempt to’.

Line 325: ‘Keeping the ice dynamics of the two different Weichselian ice advances……in mind,….’ What is meant here apart from the spatial extent of the ice advance?

Line 355: ‘….the advancing ice sheet would push an intrasalt ‘bowwave’ in front of it, giving rise to the structures in front of it’: As modelling does not include the dynamics of the ice sheet and given the large differences in time scales of salt movement and ice-sheet movement, I find that this conclusion is difficult to make based on the modelling. 

Line 514: The Sandersen & Jørgensen (2022) reference is missing.

______________________

Peter Sandersen

Geological Survey of Denmark and Greenland (GEUS)