Experimental evidence that viscous shear zones generate periodic pore sheets

Gilgannon, James; Waldvogel, Marius; Poulet, Thomas; Fusseis, Florian; Berger, Alfons; Barnhoorn, Auke; Herwegh, Marco

doi:https://doi.org/10.5194/se-12-405-2021

Articles | Volume 12, issue 2

https://doi.org/10.5194/se-12-405-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-12-405-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 2

Short communication

|

18 Feb 2021

Short communication |

| 18 Feb 2021

Experimental evidence that viscous shear zones generate periodic pore sheets

James Gilgannon, Marius Waldvogel, Thomas Poulet, Florian Fusseis, Alfons Berger, Auke Barnhoorn, and Marco Herwegh

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Final revised paper (published on 18 Feb 2021)
Preprint (discussion started on 19 Aug 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Comment to Gilgannon et al. - SE2020-137', Alberto Ceccato, 19 Sep 2020
- AC1: 'Replies to referees' comments', James Gilgannon, 06 Nov 2020
RC2: 'Referee Comment', Lars Hansen, 20 Sep 2020
- AC1: 'Replies to referees' comments', James Gilgannon, 06 Nov 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by James Gilgannon on behalf of the Authors (06 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (06 Nov 2020) by Federico Rossetti

RR by Alberto Ceccato (04 Dec 2020)

Suggestions for revision or reasons for rejection

I reviewed a previous version of the manuscript, and I am glad to see that the Authors answered in an exhaustive and very detailed way to the concerns of both Reviewers. The Authors’ modification to the previous version of the manuscript effectively improved both the readability and the scientific content and structure of the paper. The overall length of the paper is right at the word count limit for a Short Communication. I wish to see these data and discussions published as soon as possible. Unfortunately, I still have some minor comments which the Authors may want to consider. The manuscript already fulfil the high-quality standards of Solid Earth and it deserves to be published after some very minor corrections. I provide below some comments and suggestions which I hope might help the Authors in shortening the text and clarify even more its content.
General comments:
1) Title: as it is now, the title is rather inconsistent with the whole bulk of topics discussed in the manuscript. I am referring to “focus mass transport”, which subject is rather limited in the Discussion section. This comment relates also to the General comment 3) about Section 3.1.
Perhaps a slight rewording of the title would make it effectively reflect the content of the whole manuscript (e.g.: “Experimental evidence that viscous shear zones generate periodic pore sheets: effects on fluid redistribution and mechanical behaviour”; something that include both topics. By the way, this is only a suggestion.)
2) Introduction and Discussions: there are two seminal papers, in my opinion, from Neil Mancktelow (2002, “How ductile are ductile shear zones?” Geology https://doi.org/10.1130/G22260.1; and 2008 Lithos https://doi.org/10.1016/j.lithos.2007.09.013) which must be discussed (or at least cited) in your manuscript. Both papers cover exactly the topics and paradigms you wish to discuss, and thus I am quite surprised in not seeing them even cited in your manuscript.
Briefly, Mancktelow (2002) shows that there is the necessity of a “pressure-sensitive plastic deformation” component during viscous deformation of ductile shear zones to explain melt–fluid flow within ductile shear zones from a continuum mechanics point-of-view.
Indeed, including this point-of-view in the Introduction (Lines 20-32) would strengthen and support your claim for the necessity of a “reappraisal of the community’s perception of how viscous deformation in rocks proceeds with time”, alongside with natural and experimental results. It would also support your discussion in Sections 3.1 and 3.2.3 (Lines 225-227).
3) “How mylonites could focus mass transport”. I would like to see a clear discussion and separation between what is observed and inferred from the experimental data and microstructures and what is then extrapolated to occur in natural shear zones. I’ll explain myself. The presented experimental data and microstructures show that there is the formation of a systematic, periodic and anisotropic pore network which allows for the mass redistribution within your sample, rather than an effective mass transport. The deforming sample in conjunction with the deformation apparatus cell constitute a “closed” chemical system.
Given that the “system definition” is a matter of scale, if one considers the deforming sample and the confining medium as two separate entities, the ingress of Ar from the confining medium is a clear evidence for the occurrence of an effective mass transport between two media. However, this cannot be demonstrated with the presented data and I completely understand that demonstrating Ar mobility is far beyond the scope of the present manuscript.
By contrast, mass transport in natural shear zones implies either gain or loss of chemical components in an “open” chemical system, which commonly includes two media: the shear zone and the some other rock (host rock, subducted or nearby tectonic units, e.g. Selverstone et al., 1991 JMG; Barnes et al., 2004 JMG) which act as either source or sink of the transported mass.
Therefore, I would suggest the Authors to clearly state that in the experimental case the porosity allows for a mass redistribution within the sample, which sample can be probably treated as a closed system. Then, if this process is extrapolated and adapted to the natural “open system” shear zones, where the deforming dynamic-porosity-bearing medium communicates with another medium, it can effectively promote mass transport. This can be easily addressed by the Authors with some rewording of the paragraph.
4) In my opinion, it would be better to present first the comparison with other experiments and then discuss how the porous anisotropy may affect the mechanics of such experiments (swop the order between section 3.2.1 and 3.2.2). This would also allow to extend the discussion about the Generalised Thermodynamic model to the other experiments and thus discuss the differences between the experimental results. Indeed, when reading the section comparing experimental results one question comes up: what about the boundary conditions (constant force vs. constant velocity) in these different experiments? Does it relates to the mechanics? This is only a personal suggestion, but it would probably ease the reading of the manuscript and its logical structure.
Otherwise, the Authors need to consider the boundary conditions as one of the “difference and similarities” between experiments (Lines 183-197), given that they show the important role of boundary conditions on the mechanics and microstructure of experiments in the previous section (3.2.1).
5) Section 3.3: I really like the discussion about veining and fluid flux, which perfectly fits with the above-discussed “mass transfer” capability of shear zones related to creep cavitation and the dynamic fluid pump model. The reference to “recent experiments on calcite gouges” fits perfectly with the “earthquakes and tremors” topic discussed at the end of the paragraph and the discussion of experimental mechanics above. However, the discussions concerning dyking and frictional melting seem a bit out of place, they are still speculative (as stated by the Authors) and a bit disconnected to the rest of the paper in my opinion (Delete Lines 230-233; 236-240). Therefore, I would suggest the Authors to limit the discussion about “speculative” topics, also in order to shorten the main text. A rapid rewording and swop of sentences within the paragraph will easily satisfy this suggestion, if the Authors agree on that.

Detailed comments:
Line 80: If you are not considering the mineral-filled pores in your quantification, then specify that these values are minimum estimates.
Line 165: Please specify what is the related boundary conditions in the Generalised Thermodynamic model (i.e. constant velocity = constant thermodynamic flux; constant force = ?).

Bologna, 04.12.2020
Alberto Ceccato

Referee Report: PDF

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RR by Lars Hansen (18 Dec 2020)

Suggestions for revision or reasons for rejection

The revised manuscript is greatly improved, and I thank the authors for taking the time to consider my previous comments. The introduction and discussion are much clearer in the revised version, and I find the inclusion of the absolute porosity changes to be intriguing. There are a couple of primary points I’d like to raise related to the revised text and to the response to my initial comments. I still think the content of the article is timely and worth publishing. My comments relate to the fact that I’m having trouble making some logical connections among different parts of the paper, and therefore some clarification would be welcome in a revision.

I admit that I did find the expanded discussion on the thermodynamics of creep to be difficult to follow at times. I appreciate the emphasis on the boundary conditions, and the possibility for constant velocity boundary conditions to prevent localization. However, I’m not sure I understand why a macroscopic response related to the porosity evolution must be linked to localization. There should still be some effect on the mechanical response associated with a 1% increase in the pore fraction, even if there isn’t localization. This effect is well documented in the literature on partially molten rocks (e.g., see part 1 and part 2 of Hirth and Kohlstedt, 1995). Because of the reduction in grain-grain contact area, the local stresses are higher, and therefore higher strain rates are generated for the same macroscopic stress. The details of this reduction in apparent viscosity clearly depend on the specific microstructure, but for the basalt-olivine system in dislocation creep, you’d expect a reduction in viscosity of about 10% (factor of 1.5 increase in strain rate at constant stress, or a factor of 1.1 decrease in stress at constant strain rate, for a stress exponent of 3.5). Some of the experiments in Barnhoorn et al. (2004) may have weakening of this amount at strains >5, but the majority seem pretty stable.

I find it particularly interesting that the porosity doesn’t increase until after gamma = 5. Considering that you are only counting open pores (as documented in the revised manuscript), this must mean the sample has increase in volume by 1% between a gamma of 5 and a gamma of 10. I understand the hypothesis that porosity is generated by recrystallization, but there is still plenty of recrystallization (perhaps even more) at low strains. So it remains unclear to me why there is a porosity increase only after very large strains (and as noted above, in association with essentially no change in flow stress). I think it would be useful to include some explanation, even if speculative, for this phenomenon.

Regarding the link to partially molten rocks, I understand the authors’ point that to properly treat that connection is a major undertaking. And I understand that this comparison has been made already by Spiess et al. (2012). However, I still feel that something needs to be said in this manuscript, even if it is just a single sentence referring the reader to Spiess et al. I should also note that the authors’ second reason for not including this topic (that these experiments represent single phase aggregates) does not sound quite right to me. If there is a fluid phase in the pores (as the authors suggest there is in the revised version), then this is a multiphase system. Much of the relevant literature on partially molten rocks focuses on a single solid phase and a single fluid phase.

I also appreciate the expanded discussion on how failure associated with cavity formation is not easily predicted in existing experiments in rocks. My comment here is intended to point out that there is an extensive literature on failure by cavity formation in metals, and the time to failure is a primary concern for engineering applications. Correspondingly, there are a variety of efforts to establish constitutive equations that predict the time to failure, which is exactly what the authors seem to be advocating for. Some well cited examples include Ashby and Dyson (1984, https://doi.org/10.1016/B978-1-4832-8440-8.50017-X) and Kowalewski et al. (1994, https://doi.org/10.1243/03093247V294309). As with the discussion of partially molten rocks, it seems unfortunate to not, at least, acknowledge that significant effort has already been made on this front outside the geological literature and refer the reader to this vast resource.

Minor comments:

Line 26: “by” instead of “with”

Line 27: Perhaps it is worth starting a new paragraph at “In contrast” since the topic sentence of this paragraph is about existing models. Also, “that” instead of “which”.

Line 29: Why capitalize “Generalized Thermodynamic”? This can’t be the only generalized thermodynamic model.

Lines 29 to 30: Is it really a paradigm if much remains to be tested? I guess it sounds more like a hypothesis to me.

Lines 54 to 55: I’m not sure I fully understand this statement. Indeed, the energetics are critical to consider, but they are, to some extent, already included in the mechanics. For example, the typical quantities to measure in tests of viscous materials are stress and strain rate. The product of these two quantities is the total energy dissipation (in Watts per unit volume). Therefore, the energetics of the system are generally being measured and considered in traditional treatments of these data. Perhaps some additional clarification is necessary here.

Lines 58 to 59: T should be italicized.

Line 75: I think a slight rewording is needed to clarify that it is the clusters (rather than the pores) that are systematically oriented.

Line 89: Perhaps include “dominant” before “spatial”.

Line 120: This statement caught me off guard a bit. I suppose my instinct is that the evolution of the porosity and porosity distribution will depend large on the nature of the pore fluid and the pore-fluid pressure. I think the only way to say that these features of the porosity depend on bulk material properties like the elastic modulus is to demonstrate that the porosity is different for different materials, rather than the suggestion that the porosity reaches steady state, as is implied here.

Line 129: “affected” → “affect”

Line 131: Remove “concerning” or “of”

Line 148: Is “flux” the right word here? What is there a flux of? This discussion is about the boundary conditions, but flux refers to some sort of transport. Is this the flux of energy into the system? Some clarification would be useful.

Line 195: “evolved”→ “evolve”

Line 198: Again, shouldn’t the pore-fluid pressure factor into this discussion?

Line 200: Remove “a” prior to “fracture”

Line 230: An “e.g.” makes sense before the citation to Hobbs since there are many documented observations of frictional melting in the geological record.

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ED: Publish subject to minor revisions (review by editor) (22 Dec 2020) by Federico Rossetti

AR by James Gilgannon on behalf of the Authors (08 Jan 2021) Author's response Manuscript

ED: Publish as is (12 Jan 2021) by Federico Rossetti

ED: Publish as is (12 Jan 2021) by Federico Rossetti (Executive editor)

AR by James Gilgannon on behalf of the Authors (14 Jan 2021)

Short summary

Using experiments that simulate deep tectonic interfaces, known as viscous shear zones, we found that these zones spontaneously develop periodic sheets of small pores. The presence of porous layers in deep rocks undergoing tectonic deformation is significant because it requires a change to the current model of how the Earth deforms. Emergent porous layers in viscous rocks will focus mineralising fluids and could lead to the seismic failure of rocks that are never supposed to have this occur.