Second review of "From Strong Plates to Weak Boundaries: Strain Localization in the Lithospheric Mantle with Low- to High-Temperature Dislocation Creep" by Etienne Van Broeck, Fanny Garel, Catherine Thoraval, and D. Rhodri Davies.
By Antonio Manjón-Cabeza Córdoba
The authors have done a substantial re-write of the manuscript to address mine and the other reviewer's comments. I do not think this was needed, and the authors have introduced a few technical typos that could have been avoided had they done a more localized, surgical and subtractive revision. Nonetheless, the conclusions remain robust and I do not think that delaying the publication of this article any longer should be done on the sole base of my opinion.
Unfortunately, when I accepted the second review I was not aware of the level of work put into the new version and I needed to ask for an extension once I realised I could not just check a few boxes. Due to personal reasons, it has been a bit frustrating to follow through with this review and I am not convinced of my objectivity (i.e., I may have been overly picky). For that I apologise to the authors and the editor.
Overall, the article is good and has improved. I still find the article overly long. In some parts it is repetitive and in others discusses issues that do not serve the conclusions. The manuscript features > 600 lines of the main text, 5 Appendixes and supplementary material; all this for conclusions that do not need that much argument (i.e., the more robust the conclusions the less discussion they need). However, some parts are definitely clearer now, and the figures are more synthetic and to the point. At the moment, I debate myself between recommending a technical review or a minor revision, and I pass the responsibility to the editor. Regardless, I think that I have contributed what I could contribute and, should another review be needed, I recommend someone else to do it.
ABSTRACT
I warn against the use of the word ductile altogether. Ductile is a rather phenomenological term that in numerical models can (and probably should) be changed by the word "viscous" whenever possible. If a distinction needs to be made, then I suggest to refer to the specific mechanism. In what the field scientists would classically call the "ductile regime", a combination of viscous creeps may apply (and even combined with some micro-brittle behavior). Note that later in the abstract dislocation creep is addressed explicitly (good).
INTRODUCTION
Plate tectonics describes the motion of rigid lithospheric plates separated by narrow, weak boundaries (e.g. Le Pichon, 1968).
-> I don't understand this sentence. Plate tectonics describes? Maybe add "The hypothesis of plate tectonics"? Or "The theory of plate tectonics"? Or even "The law of plate tectonics"? Perhaps more technically "The paradigm of plate tectonics involves the motion of rigid lithospheric plates ..."?
"The lithosphere is expected to be strong under cold, ductile conditions". Ok so, at the very least, please define ductile. I have a rough idea of what the authors mean but a lot of people, particularly those working on the lithosphere will imagine that cold and ductile are an oxymoron. Hot and cold are relative, ok, but brittle is colder. Again another case for abandoning the word "ductile" altogether.
Line 49, low with respect to what? Maybe better, "lower than those predicted at x depth by assuming that the yield stress represents brittle deformation".
Line 53,54 "Low-temperature ductile deformation has been proposed as an alternative route to limiting lithospheric strength without imposing
an ad-hoc stress cap throughout the ductile regime." This is true but even better with a citation.
Also, low-temperature ductile deformation... is the low-temperature relative to other ductile deformation mechanisms or is ductile used indistinctively from dislocation? I am intentionally being obnoxious precisely to point out the problem with using "ductile" within the context of geodynamic models.
Line 67 "Weinstein and Olson 1992: Thanks for this reference! Did not know it and it is indeed useful.
Line 91. Please be consistent, either sects 5 and 6 or sects Discussion and Conclusions
METHODS
Table 1. "Density" -> Reference density
Figure 1. "Freesurface" -> Free surface? Why? BTW?
Line 162-163. References are inline, probably unintentional.
Table 2. Adisl ^{-7}? Is this correct? Please revise
The tanh label makes sense within the context of the previous publications (Garel 2020), but I'm not sure it is clear here as only one formulation is described in the methods
Line 198. I am a bit confused. Some lines above the authors include an adiabatic correction for the viscosity, but now they claim they solve the problem in the Boussinesq approximation. I'm sorry I missed this in the previous review, but I don't think this is clear in the text.
RESULTS
Line 254. I'm sorry if I missed this in the previous review, but F here is confusing. Earlier, a tectonic force F with units of N m^{-1} is defined but now these ratios seem dimensionless. I would expect the same capital letter should correspond to the same units.
4D evolution actually means 5 D (i.e. 4D in time). I recommend either 3D evolution or 4D models.
Line 309 Formatting cmyr^{-1} -> cm yr^{-1}. And around this paragraph, the space between label terms disappear please be consistent throughout the text (Fig. 5 or Fig.5 but not both).
Figure 4: There may be a label missing in the vertical axis of panel b (km). Maybe this was intentional?
Line 352 Fig.9 -> Fig. 9: Line 355 Fig.9d -> Fig. 9d
Line 370 Fig.9 -> Fig 9: Table3 -> Table 3.
Lines ~360-374. Rather than a friction coefficient, the authors impose a temperature limit. Fine. But then they should not analyse a stiff layer that arises precisely because of this cutoff! A friction coefficient should indeed result on a stiffer part of the lithosphere, but the one in this manuscript is simply imposed. I think the authors overinterpret their results here (which they don't need at all).
Figure 9: Notice that HT and LT-HT are not equivalent formulations at high t. The viscosity scale is logarithmic and therefore the difference is noticeable. Not sure I agree with the authors both cases are comparable (this does not hurt the general conclusions of the paper, but does put a limit on the strength of their remarks on LT-HT vs HT, which admittedly, they tuned down). Just to be clear, I do like the author's LT-HT formulation better than just HT, my comment is just on what's shown in the manuscript
DISCUSSION
"Taken together, our results show that lithospheric-scale strain localization in this extensional setting is controlled not simply
by the integrated strength of the plate, but by where weakening occurs and how rheology couples deformation focusing
to asthenospheric upwelling through rheology-modulated feedbacks linking deformation focusing, plate weakening and asthenospheric
upwelling." -> This sentence is key and it is too hard to read. I would suggest dividing in 2-3 sentences. Otherwise readers may miss the main point of this work. What about... [...] our results show that [...] strain localization [...] is controlled by the integrated strength of the plate and by where weakening occurs. In addition, a key controlling effect is the rheological coupling of deformation focusing and asthenospheric upwelling [...] ...? (I'm sorry if I did not capture the original meaning, but at the very least I should have shown this wasn't easy to understand).
Line 443 Potentially unintentional inline citation (Ulvrova, 2019).
Line 474 Fig.9 -> Fig. 9
Line 483 Fig.6c -> Fig. 6c
Line 574 "[...] approaches should distinguish [...]". Should distinguish for what? I understand that there is something missing when not including dislocation creep or a "ductile yield stress", but are the authors suggesting that all models that do not include this should be automatically scrapped? I would not agree with that, nor I think the authors have the results to make such statement. Instead, this sentence would be better written as "Models that would like to accurately estimate the absolute timescale of localization should distinguish [...]". That way this statement is precise and with real actionable implications.
I apologise to the authors if this comment seems pedantic, but this sort of comments are too common and of little help in our community (For example, I work on melting calculations in this sort of models, I could argue that extension tectonics of this sort usually happens under strong magmatism, therefore casting doubt on the authors' results because "Models should include the effect of melting". But I honestly think that the authors' models are better because they are simplified and therefore could properly study creep-based localisation without the effect of melting.
Line 577 "result" -> aspect (not technically a result).
Line 585. I understand that the authors acted to address a reviewer's comment. But I wanted to highlight that by no means I believe this is the main limitation of the study. No action needed here.
CONCLUSIONS
Line 627 "In all simulations that successfully localize" Localize what? I may be overly picky here, the sentence should be understandable for most readers, but an object is missing.
GENERAL COMMENTS
I suggest you review the order of the citations. This is not important, but at the moment, they are not ordered neither chronologically nor alphabetically in some parts of the text. When they do, they do so chronologically, which I like, so I suggest to be consistent throughout the text. Perhaps does not make that much of a change, but easier on the reader.
Many of the labels for the Figures do not have a space between the "Fig." and the number. Others do. I have pointed some out, but I am sure I missed some. This is just a technical issue but appears throughout the manuscript, please revise. |
Dear editor,
this technical paper presents a series of numerical experiments to address the role of rheology on deformation localization during extensional tectonics. While several detailed models of extension and rifting exist, evaluation of localization and adequate rheological mechanism is mostly absent in modern literature. In addition, they add the role of low-temperature plasticity, a continuation of their own work (Gouriet 2019; Garel 2020)
They show that considering multiple rheological mechanisms, in particular non-linear rheologies, is important to reproduce localization processes as we imagine them on Earth. They could specify a bit more on whether this was ever called into question, or what are the consequences of ignoring one rheology or the other, not just for their models, but for the wider community that will be forced to take simplifying assumptions regardless.
The manuscript is quite technical but the methodology is well explained. I have little to say there. Because Solid Earth has a wide focus - inasmuch as the deep Earth can be called a wide field – the article would benefit for a slightly more careful contextualization (e.g. what advances or new evidence motivate this study? How does this work compare with other studies? What are the implications for other geophysicists/geologists? etc). But this is not a criticism of the science of the manuscript itself.
Overall, I have few important comments to make and the majority of them are oriented to increase the reader interest on the manuscript rather than a criticism of the science of the manuscript. As a matter of taste, I find that they describe too many details of their results; these are simplified far-from-reality numerical experiments, and describing too many details may have little relevance. Perhaps summarizing a bit would improve the manuscript (i.e. in some sections there are some details that would be obvious to most people, like with faster extension comes faster thinning).
Attached, you can find the main comments to be addressed by the authors, then you can find a series of minor suggestions that the authors are free to address or not, or even ignore during review/reply process.