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| 28 Mar 2024
The influence of vertical lithological contrasts on strike-slip fault behavior: Insights from analogue models
Abstract. This work investigates the influence of rheological contrasts on the nucleation and behavior of strike-slip faults. To achieve this, we have carried out a series of brittle-viscous strike-slip shear analogue models, using quartz sand and microbeads as granular materials with different internal friction and cohesion values. Particle Imaging Velocimetry (PIV) was applied to time-series of surface images to calculate incremental and cumulative strains. Understanding how strike-slip faults nucleate and interact in the heterogeneous upper crust is relevant in seismic hazard analysis and geothermal and hydrocarbon exploration. To reproduce the heterogeneity of the upper crust, three sets of experiments we performed: 1) upper layer composed either of quartz sand or microbeads; 2) upper layer with a vertical contrast i.e., quartz sand surrounded by microbeads and vice-versa; and 3) same set-up as in the previous set but changing the orientation of the vertical contrast. Our study shows that the introduction of an upper crustal vertical contrast influences the behavior and evolution of strike-slip faults. The models containing a vertical contrast were more complex and induced a compartmentalization of the model. The initial fault strike is related to the material’s properties. However, this initial strike changes when faults crosscut the materials with less internal friction angle clockwise, and anticlockwise when the contrast has higher internal friction angle. Areas containing materials with less internal friction angle take longer to localized the deformation, but they show a greater number of faults. The biggest increase in the number of synthetic and antithetic faults occurs with the introduction of vertical contrast. These results were compared with the intraplate fault systems of the NW Iberian Peninsula, focusing on the Penacova-Régua-Verin and Manteigas-Vilariça-Bragança fault systems. They are major left-lateral faults that cross-cut lithologies characterized by vertical rheological contrasts, with deformation patterns similar to those observed in our analogue models.
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RC1: 'Comment on egusphere-2024-852', Roy H. Gabrielsen, 11 Apr 2024
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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)
Citation: https://doi.org/10.5194/egusphere-2024-852-RC1
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