Review:

The influence of vertical lithological contrasts on strike- slip fault behavior: Insights from analogue models.

Sandra González-Muñoz, Guido Schreurs, Timothy Schmid, Fidel Martín-González

Submitted for publication in Solid Earth

Summary and conclusions

This is an interesting contribution to the study of mechanically composite strike-slip systems as commonly found in nature, and as such, is relevant to the readers of Solid Earth. I, however,  question whether the title of the paper fully reflects this complexity (see specific comment below).

The architecture of the paper is logical and appropriate. The experiments are well constructed and are well performed and reasonably well documented, although a keen reader may like to see some photography and blow-ups of some details (that should be selected by the authores).

The paper still needs substantial improvements on several crucial ponts:

• In the Description and Discussion only the the effects of horizontally distributed mechanical contrasts are focused upon. This selection may be appropriate because these are the effects that are studied, but the infuence of the vertical stratification should at lest be mentioned and evaluated, not least because structres (faults) may be expected to nucleate at different levels resulting a hybrid thin-skin/thick-skin system. The authors need to explain why this selection was done, alternatively why the vertical stratification is neglectible. It is my view that this is a methodological basal point that must be mended.
• I have a feeling that the descriptions should be expanded fully to realize the full potential of the experments. This view is supported by that the authors include som additional observations in the Discussion that were not included in the Description/Result-chapter. I recommend that it is left to the authors to deside.
• The description of the faults in the Iberian Massif should be moved forward of the Discussion and the analogue value of the experiments should be demonstated more clearly.
• The Discussion chapter is the weakest part of thes paper. It fails in displaying to the reader what are the new discoveries obtained by these experiments and to discuss this in light of current knowledge and literature in any depth. Also the observations that are comparable to that of the experments should be included and discussed her. I therefore recommend that a complete rewriting of the Discussion-chapter is demanded before accepetance of the paper.
• Although presented in a logical sequence many phrasings and statements are unneccessarily complex, less precise and even amibiguous. I have made somesuggstions for improvements below, but I suggest that the authors seek the advice of a person whose first language is English (mine is not).

Please note that no technical/formal aspscts of the references (format, references, Solid Earth format standards etc.), nor the  consistency between references given in the main text and the reference list have been checked by me.

Conclusion: This is a work that would be of interest to the readers of Solid Earth and deserves publication. At present, the manuscript, however, has several profound weaknesses that need to be mended before publication.

Specfic comments

Title of paper

The experiments were constructed with both vertical and horizontal mechanical stratification and structuring. Thuis is not fully relected in the title of the paper, and I would suggest that the authors consuíder whether or not an axpansion of the title would be benefitial.  

Abstract

The format and content of the abstract is good in that it dfeines the roblem, displays the methods and enpasize the findings/conclusions. It can still be improved om certain points:

Lines 14-15: Perhaps the sentence “Understanding how strike-slip …” should be moved up-front? Furthermore, the defined problem is generic and should be defined as such before mentioning consequences in applied science.

Lines 16ff: Suggest an expansion of this paragraph to avoid telegram style in the abstract, e.g.: “three experiment series were performed. The first experiment series  utilized…… The second series of experiments ….., whereas the third ……”

Furthermore: I understand from the Methods-section that four experimental series (A;B:C and D) were performed? (although two were reference models)

I think the phrase “verical contrast” should be specified to include “mechanical strength” or “rheological” throughout the remainder of the abstract.  Perhaps this could be specified (e.g. “weak-strong”, “strong-weak”,strong-weak-strong”)

It is acknowledged that the authors display clearly in the abstract where conclusions are given (“Our study shows that…..”). Perhaps this important part of the abstract should be emphasized even more strongly, e.g. by introducing anew paragraph?

Line 19ff: Care should be taken so that the conclusions gave a consequent grammatical time sence in distinguishing between what was observed in the experiments and what is generally valid for such systems (see comments below).

Introduction

Lines 54-55: Altough involving time-dependent shear-configurations, Gabrielsen et al. (2023) touched upon on this problem in the  analogue study of the Barents Shear Margin (Gabrielsen,R.H., Gianennas,P.A., A. Sokoutis,D.,Ernst Willingshofer,E.,Hassaan,M.&Faleide,J.I., 2023: Analogue experiments on releasing and restraining bends and their application to the study of the Barents Shear Margin, Solid Earth, 14, 961–983, 2023, https://doi.org/10.5194/se-14-961-2023).

Line 57: “ behavior through changes…..”, but perhaps better: “Rheological properties of the upper crust is of particular importance for the dynamics and geometry in strike-slip fault systems.”

Line 63: Rather: The results from the experiments were compared …”

Methods

The Methods-chapter is well designed and complete, but contains some formulations that could be improved/clarified:

Line  69: Is “utilized” better than “comprises” here? (It should be  grammatical past, since the experiments definitely are finalized). Some of my colleagus with English as their first language (mine is not) would say this is a matter of taste, whereas others would sagree with this distintion.

Line 71: Rather: 60 idependent and moveable plexiglass bars … were positioned above the two basal plates”

Line 88: “…. consist of reactivated lithological boundaries”. Actually they represent contact surfaces of materials of contrasting mechanical strength. Do you mean “were analogues to lithological boundaries” (as actually stated in the chapter on Analogue Materials)? Please state the relative strength between the quartz sand and the (glass?) microbeads already at this stage (perhaps referring to the Aanaolgue Materials-chapter).

Line 90: Can the N-direction be indicated in Figure 1 for clarity? This means that displacement was oriented transverse to N in series A (i.e. E-W). For the continuation, I would suggest: “ Three model sets with the quartz layer oriented with contrasting orientation relative to N were tested, namely……..”.

Line 102: Suggest reformulation: “Pictures were taken by fixed intervals of 30 secs for the experimental run of 120 minutes, totalling 240 pictures for each experiment”

Line 104: Suggestion: “This software allows for camera calibration” (please say which parametres were calibrated for: only camera functions like exposure time etc or something else?). It also supports image correlation and …..”.

Lines 107-108: Suggestion: “Each  picture covered…..”. Why is the pixel coverage average? Did it vary from exposure to exposure, or is this an affect of changing perpspective? Please say. Also: X and Y axes” are plural, so rather X-axis and Y-axis. Again, why is resolution an avarage value?

Line 112: Should it say “neutral” or “parallel”strike-slip set-up (to make it clear that no transpression or transtension was involved)?

Lines 112-113: The band of contrasting mechanical properties ia aoriented at a high angle to the main strainvector in several experiments. This is likely to cause vetical displacement. Please comment in text on Methods.

Analogue materials and Scaling

This sections are complete and well written.

Lines 142-143 Suggestion: “was sieved ….” etc. See comment to line 69.

Results

I think a short text here reminding the reader about the difference/similarities between the three (four?) series would be in place here.

There is some confusion when the description of the different segments are concerned, shifting between (eastern, central and western) domain, zone, band, contrast. I suggest that doain is used consequently as actually spelled out in lines 89-93. 

Series A

I would suggest a slightly different introduction here, e.g.: “Series A1 and A2 were conducted to investigate the effects of the use of pure quartz sand (serie A1 and microbeads A2). As expected these experimentseries were rather similar, but incremental strain panels for the quartz sand showed ….., whereas similar  imagery for the microbeads……   ”

Lines 171 ff: see comment to line 69.

Lines 185-187: There seems to be a confusion between the major text, Figure 3 itself and the figure caption here.

Series B

Line 189: Should rather read something like: “ ….. has a 5cm wide central segment with contrasting mechanical strength (microbeads embedded in quartz sans (B1) or quartz sand embebbed in microbeads (B2). This bamd of mechanical strength devides the model into an eastern and a western segment ……”

Lines 170-200: These sequencec contain some not clear phrasings/less good language and also contains some dubious statements. It should be rewritten.

Examples:

Lines 178ff: Deformation cannot be compartmentalized, but strain can be distributed among several fault compartments.

Line 193-194ff: Since this is a primary contact, I would say “activation” rather than “reactivation”.

Line 193: I think this phrasing is the correct description of this boundary: “In both models, initial simple shear results in reactivation of the N-S rheological boundaries” (though there is a superfluous comma, and since this boundary was induced in the model construction, it was activated and not reactivated).

Lines 194ff: Synthetic and antithetic are relative terms. These faults must be syn- or antithetic relative to something. Say what. And if they did not exist as faults on beforehand, they are activated, not reactivated. There are two sets of structures that potentially can be/are sources for strain concentration and contrasts in deformatioal style here. These have transverse orientations: One is defined by the basement (sinistral) shear fault and the other is the rheologic contrasts affiliated with the central segment, and they are located at different levels in the experiment. One should expect structres related to these structures to be nuclated at different levels and to develop (primary) thick-skin and thin-skin structures respectively. These tentatively belong to separate stress (depth-dependent) regimes. Again: for the surface structures: syn-/antithetic relative to what?

General for this section: Many authors use the terms syn- and antithetic structures for R-, R’- and P-structures. Considering the set-up here, I assume the authors mean something else with the nomencature applied here. Please comment and explain.

Lines 200-211: This is a complex and  uclear phrasing. Please cosider something like: “Both in Model B1 and in Model B2 dominant fault strike for the dextral faults are different when the western and eastern domains are compared, striking …. and …..  respectively. The new faults have an inclination of … degs with the border between the quartz sand and the microbeads.” Hereafter: Specify structure and dynamics and for each domain (referring to the rose diagrams in Figure 4) for one domain at a time and high-light the most significant obaservations. Please avoid over-qualification of statements like: “dextral strike-slip faults”. All faults with dextral dispacements are strike-slip faults in this setting.

Series C and D

Lines 221-265: Please straighten description of series C and D in accordance with series A and B.

Also these section on Series C and D contain some unclear statements, for axample “There are more sinistral faults in model C2 than in model C1 (Fig. 5c and h), and both cut the central band.” (both what??) I guess you mean: The central domain is transected by the sinistral fault set in both cases”.

“…change its strike counterclockwise in the central domain” Why not: “The faults in the central domain deviate by xxx degs (counterclockwise) as compared to ….”. Actually, this is actually precisely phrased for the C-series in lines 227-229.

Discussion

The discussion is not mature for publication: It has the flare of a continued description, and several points and statements should be transferred to the Results chapter.

I see of course the point on the removal of the cardboard sheets, but cannot see that this is appropriate for the intro to the discussion. Furthermore, I would hardly call this a recativation, which is consequently stated in this paper (see comments to the text). The border was perhaps slightly disturbed, but it hardly affects the de facto function of this surface as a a primary zone of mechanical strength contrast.

Ths part should rather be transferred to the experimental set-up section.

Introductory remarks to Discussion

I do endorse a general introduction to the Discussion chapter, but I think the introduction rather should be used to high-light the genereic (experimental and geological) problems that are addressed in this paper, stressing the original aspects of these experiments and mentioning in which geological centext(s) the results will be useful. Perhaps some elements from the Introduction of the paper should be repeated or moved here.

The main body of the Discussion

After mentioning the generic problems rised before and materialized by the current experiments, these should be addressed one by one and discussed in light of previous knowledge by the authors (the second author is a world-leading scholar on strike-slip faults) and published literature. The items addressed in chapters 4.1 and 4.2 could be singled out as generic problems in this context and incorporated here.

In this context, field observaions would be crucial, and I find it strange that data from the Iberian Massif ae presented after the Discussion (see comment below). Rather, these observations should be part of the Discussion. I Recommen a full re-writng of the Discussion capter, and have therefor not commentd upon this in detail. (But there are many statements in the thant would nedda brush-up).

Figures

The figures in this paper are well taken and well presented, but it seems that more attention should be given to Figure 2, its general description in the main text and its Figure caption. The figures do, however, contain som details that easily can be lost if the present lay-out/size is maintained in the final publication.  

I suspect that an additional figure would benefit the Discussion chapter if ths is rewritten to include what is suggested by me above. The presnet Figure 7 is more a summary figure than a figure that would high-light a more profound discussion.

References

References have not been checked by me other than in the cases I had to look up references given in the main text.

Oslo April 8, 2024,

Roy H. Gabrielsen

(sign)

The paper presents interesting strike-slip experiments where the direction of applied simple shear is orthogonal and oblique to zones of either increasing or decreasing strength. The results are novel and will be of interest to many readers – especially as many crustal strike-slip faults cross materials of different strength. The pattern of faulting depends on the spatial heterogeneity of material strength. The experiments and analysis of the experimental data are strong and the figures are effective but the text needs revision. The manuscript has many instances of awkward English grammar that can be rewritten for clarity. I highlighted many (but not all!) in the specific suggestions. 

I also provide some general comments that can strengthen both the presentation of the experiments and the interpretations.

1. The zones of heterogeneous strength are referred to as ‘vertical contrast’ through much of the paper and ‘horizontal contract’ on line 296. I found this descriptor very confusing. Because ‘contrast’ as noun means the difference between two materials and the term ‘vertical contrast’ doesn’t make sense to describe a zone of one material. Also, I thought that ‘vertical contrast’ meant that the strength was varying with depth. I suggest describing these features as vertical zones of different strength from the surround material.  
2. The text describing the experimental results switches between past and present tense. I recommend past tense since you are describing particular experiments that have occurred at a specific time in the past. In the discussion, where you infer general fault behavior from the experiment results, you can switch to present tense because you may be talking about general fault behavior that is not linked to a specific experiment.
3. The two materials were described as having different relative strength based on their internal friction angle (lines 140-141). This parameter informs the angle of fault development but not strength. The cohesion values, which you also have, provide better measure of material strength.
4. The results often described the greater or lesser number of faults. How many faults? Is the difference statistically significant? Do the experiment only have few active faults at any one time but these reorganize so that new faults grow? Within the experiments with fewer faults, did the faults accommodate greater shear strain?
5. The greater number of faults that develop in the lower cohesion material makes a lot of sense because new faults will grow in this material at lower levels of stress – it is easier to grown new faults in weaker material. This point should be more clearly presented in the discussion. Right now, the discussion lines (287-290) refers to grain size and packing but the more relevant impact of grain size and packing is the difference in cohesion/strength. At least some of the citations on lines 289-290 refer to deformation within lithified rocks– this is not the same as the faults within a loose granular media. Note: Li et al., 2021 is missing from reference list.
6. The discussion missed the opportunity explain/postulate why the different fault patterns emerged within the experiments.
   1. The removal of the cardboard facilitated development of antithetic faults – Why? I suspect that the cardboard removal produces zones if increased dilation and lower strength that facilitate faulting, In fact, Bellahsen and Daniel (2005) used this technique to introduce normal faults in a sandpack (https://doi.org/10.1016/j.jsg.2004.12.003).
   2. Lines 328-329 gently speculates that the differences in fault orientation could be related to friction angle. Because the friction angle is well established to have direct impact on fault orientation, this relationship can be explored more rigorously. The two materials have ~10˚ difference in friction angle and the difference in fault orientation within the two materials is often 10˚˚. This is not a coincidence.  The paper missed the opportunity to explore situations where the difference in fault angle is not 10˚ , Those situations could provide interesting insights into how the stress states might different near regions of heterogeneity.
   3. Related to point b, the discussion could summarize the strain state by showing a composite of all the rose diagrams. We can then see if the differences of fault orientation between beads and sand are systematic.
   4. The discussion missed the opportunity to explain why the stronger material develops the first faults at lower applied strain than the weaker material. This is not an expected result at all and is a very interesting finding. I suspect it may have to do with the different dilatancy of the two materials.
   5. Why do antithetic faults develop only on one side of the experiments B1 and B2. Why does the side switch when the central zone has weaker material or stronger material? This significant result has not been adequately explained.   Is this pattern repeatable?
7. When citing papers that are just a few examples of papers that make the point, one should add ‘e.g.’ before the citation. Here are some places where you could add e.g.: Lines 41, 45, 46, two on 47, 156, 316, 329, 332 etc

Specific suggestions:

Line 37: I recommend reading two more papers on strike-slip development within laboratory experiments: Hatem et al. 2017 (https://doi.org/10.1016/j.jsg.2017.06.011) and Visage et al. 2023 (https://doi.org/10.1016/j.tecto.2023.229704)

Line 50: awkward What is ‘they’?

Line 57-58: Awkward. I think that you mean to say “… strike-slip faults may change their expression as they cross regions of different rheology and display different orientation or different number of active faults’’.

Line64: I suggest changing ‘natural’ to ‘crustal’. To say that the experiments are compared to natural faults implies that the experiments are unnatural. In English this can mean that they are spiritual, magical or fabrications. The experiments are natural systems that develop within the laboratory.

Line 70: This is a good place to explain the role of the plexiglass bars to distribute the basal shear.  For example, you don’t put the viscous layer directly on the basal plates because the viscous layer alone will not distribute shear all the way across the box.

Line 80: Awkward wording

Line 91: Awkward wording (contrast)

Line 108: the resolution of the cameras is not so relevant as the resolution of your horizontal velocity data points.  What does ‘average resolution of 300 px’ refer to?

Line 129: What is the displacement increment in 30 seconds? 

Lines 155 and 158. These are very standard equations so you might not need to be included (in my opinion).

End of the section 2.2: The scaling section gives us a lot of values of scaling parameters but doesn’t take the opportunity to explain the implications of these values. How much crustal time does 1 minute of the experiment represent?  How much crustal length does 1 cm of the experiment represent? Do some of these values suggest that the experiment does not exactly scale to the crust?

Line 173:  What is the difference of the displacement required to develop the first faults in the Quartz sand and the micro beads? Is this difference significant? The earlier development of faults in the stronger material is not intuitive and warrants explanation in the discussion.

Line 185-187: Figure caption for 3 and other figures should explain the rose diagrams. Is the length of the rose indicating number of faults or degree of strain accommodation?  Why not make the sinistral and dextral faults different colors on the rose diagrams.  Also, larger rose diagrams will be easier to see.

Line 210-211: Awkward. Located in the microbeads? What spacing?

Line 229: Awkward. “the same occurs”

Line 237 Awkward.  Change to “In this series of models” and change ‘behavior of both models’ to ‘affect the fault pattern that emerged within the two experiments”

Line 238 Awkward: There are no domain. Domains of what?

Line 239 Awkward: Change “both cut,,” to “both cross…” Change  “…when these faults…” to “…where these faults…”

Line 241: Unclear: What does it mean specifically that deformation is less localized within the band?

Line 242: Unclear: What does ‘strike counterclockwise’ mean?  You might have a counter-clockwise change in strike from one position to another but one strike can’t be counterclockwise.  I see a sigmoidal shape that has one change from the western region to the central zone and a different change from the central region to the eastern region.  But I see this in more in model C1 than model C2 so I’m not sure I understand what the text means.

Line 252: change to “models of series B…”

Line 260: Instead of ‘takes longer’ change to ‘required greater applied displacement’. Because the time involved depends on the rate of loading.  As far as I’m aware of these materials are not sensitive to loading rate so the deformation is more a factor of the amount of applied strain than the duration of the experiment. (same comment for line 282)

Line 294: “they have different rupture criteria”. Both materials grow new faults following Mohr-Coulomb failure and the rupture criteria is the same.  It is not the case that one material fails in tension and the other shear.

Line 298: awkward -> “heterogeneity of the models impacts the structure and …”. By the way, I prefer to describe the experiments as ‘experiments’ rather than ‘models’ because it seems that many geoscientists (in the US) think of numerical models when we say ‘models’.

Line 302: awkward -> ‘compared to’

Line 306: These are parallelograms or trapezoids but not rectangles

Line 306-307: The model is not compartmentalized, the faulting or the deformation is compartmentalized within the experiment.

Line 348-349: Explain why you think that the region of slates is similar to having a central region of the experiments with microbeads.  What about the angle of the faulting?

-Michele Cooke

Amherst, MA

The manuscript presents findings from four pairs of comparative experiments to discuss the impact of lithological contrasts on secondary faults along major strike-slip faults. It highlights the lithological variations in nature, contrasting with simplifications in previous modeling efforts, which often utilized uniform quartz sand to represent the brittle upper crust. This research contributes to both physical modeling and the field of structural geology along strike-slip faults, showcasing distinctive phenomena. The results are clearly described and thoughtfully discussed. There are a few questions need to be modified in the manuscript or replied.

The term "Vertical" lithological contrasts might potentially lead to misconceptions regarding the depth-related aspects of lithological contrasts. It is recommended to consider revising this term throughout the text to avoid confusion and enhance clarity.

To facilitate comprehension for readers unfamiliar with physical modeling, it would be beneficial to include top-view photos in Section 3 displaying the final structures and interpreted results. Integrating all results in one figure is an option.

The discussion of the natural example in Section 4.3 appears too brief. I suggest incorporating an additional figure (e.g., Figure 9) comparing experimental results, as depicted in Figure 7, with typical structures observed in nature, as illustrated in Figure 8. Accompanying this with detailed descriptions would enhance the discussion and enrich the manuscript.

Additionally, Table 1 should be reformatted into a table format like Table 2, rather than being presented as a low-resolution image.