the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jan 2022
Strain localized deformation variation of a small-scale ductile shear zone
Abstract. A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level. However, the deformation evolution of strain localization at a small-scale shear zone remains unclear. This study investigated < 10 cm wide shear zones developing in undeformed granodiorites exposed at the boundary of the continental-scale Gaoligong strike-slip shear zone. The small-scale ductile shear zone demonstrated a typical transition from protomylonite, mylonite to extremely deformed ultramylonite, and decreased mineral size from coarse-grained aggregates to extremely fine-grained mixed phase. Shearing senses such as hornblende and feldspar porphyroclasts in the shear zone are the more significantly low-strain zone of mylonite. The microstructure and EBSD results revealed that the small-scale shear zone experienced ductile deformation under medium-high temperature conditions. Quartz aggregates suggested a consistent temperature with an irregular feature, exhibiting a dominated high-temperature prism 
slip system. Additionally, coarse-grained aggregates in the mylonite of the shear zone were deformed predominantly by dislocation creep, while ultra-plastic flow by viscous grain boundary sliding was an essential deformation process in the extremely fine-grained (~50 μm) mixed-phase of ultramylonite. Microstructural-derived strain rates calculated from quartz paleopiezometry were on the order of 10−15 to 10−13 s−1 from low-strain mylonite to high strained ultramylonite. The localization and strain rate-limited process was fluid-assisted precipitation presenting transitions of compositions as hydrous retrogression of hornblende to mica during increasing deformation and exhumation. Furthermore, the potential occurrence of the small-scale shear zone was initiated at a deep-seated crustal dominated by the temperature-controlled formation and rheological weakening.
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Interactive discussion
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RC1: 'Comment on se-2022-2', Anonymous Referee #1, 10 Feb 2022
Review of manuscript se-2022-2.
Unfortunately, the paper cannot be accepted for publication in the present form, and I suggest rejection. My main concerns regard (1) the structure and readability of the whole text; (2) the relevance of the presented topic for Solid Earth journal, (3) sample characterization and data presentation. The presented topic might be better suited for another journal, maybe focussed on the regional geology of the region. In the following I will provide some general comments on the sections of the manuscript. Detailed comments can be found in the attached commented pdf file.
General comments.
Semantics and syntax of many sentences are rather complex, and some sentences are not understandable at all. A check of the sentence structure and meaning by a native English speaker would be advisable in this case. Language grammar and spelling are ok (some typos are still present though), but the structure of sentences and paragraphs make the manuscript very hard to follow and understand.
It is not clear how the proposed aims of the paper (Lines 85-89) can contribute to the research topic proposed in the Introduction. It is not clear either what the main research question is. As it is now the paper consists of a (average quality) EBSD and microstructural data collection, without any clear objective neither in the data presentation nor in the discussion section.
Abstract.
- One of the main points of the abstract regards the rate-limiting fluid-assisted mechanisms, which topic is just briefly discussed in the main text and would need to be unfolded and better discussed.
Introduction.
- The introduction presents a rather complete and up-to-date list of references and a general overview of the topics analysed in the following text. However, it is not clear what contribution the present manuscript yields to the described topics. What is the main research question? What are you aiming at with the description of small-scale shear zone? How come the results of you research work help to answer to the main research question?
- The aims of the paper reported at Lines 85-89 leave the readers asking themselves the unfortunate “so what” question.
Geological settings.
- The geological setting of the sample and host rock (age of emplacement, age of deformation, regional conditions of deformation) need to be better described. As it is now, we just know that the samples come from a granodiorite that preserves its magmatic texture. How old is it? Does it have undergone to any metamorphic cycle? Is just syn-shearing pluton emplacement along the GLG-SZ? This is critical to understand the formation conditions of the sample small-scale shear zones.
- Samples need to be properly located in the geological framework. The reader needs to understand if the presented analyses (microstructural description, EBSD, CL, microchemistry) come from the same sample or different samples. This is not clear at all in this manuscript.
Data presentation and figures.
- Mesoscale description. Ok but please consider rephrasing complex sentences.
- Microstructures. See detailed comments.
EBSD:
- It is not clear whether the EBSD maps come from the same sample, different samples or whatsoever.
- Contoured pole figures need to be presented as one-point-per-grain to be statistically meaningful. One-point-per-pixel pole figures are affected by the area (i.e., grain size translated in number of pixels) occupied by each grain, and thus they represent a biased dataset.
- “Negative/positive correlation between misorientation angle distributions”: it is unclear what this sentence means in the context of distribution comparison.
P-T- estimation - Chemical data for Hbl-Pl
- The compositional data adopted for geothermobarometry are not presented in the text. Please, provide at least a table with the phase compositions. It would be also nice to discuss whether the adopted compositions are at the equilibrium during deformation or not.
- Is this really the emplacement depth? Could it be a re-equilibration metamorphic depth? Need to improve the description of the geological setting of the granodiorite from where the samples have been collected.
Grain sizes: it would be better to retrieve grain sizes from the EBSD dataset, and to adopt the same statistical parameter (mean, median) for the flow-stress computation as those adopted in the piezometric relationship here implemented (see Luan and Paterson 1992 and Hirth 2001 for the details).
Flow stress – Strain rate calculations.
- Temperature need to be properly constrained. There are several contradiction throughout the text regarding temperature of deformation. P-T conditions of emplacement are 650-750°C. Are not these the deformation conditions? Then you have to explain that after emplacement the granodiorite underwent some sort of cooling or exhumation. At Line 561, an arbitrary temperature of 550°C is chosen based on microstructural characteristics. Is this the temperature of deformation of the small-scale shear zone? Why then in the discussion you come back to 650-735°C to explain the development of “foliated” granodiorite (the granodiorite hosting the sample was described as non-foliated in the geological setting…).
- The arbitrary choices of both grain size and temperature deeply affect the flow-stress strain rate estimations.
Discussion regarding deformation mechanisms:
- The authors claim for the activation of certain deformation mechanisms without providing compelling evidence, even if they present a large EBSD dataset.
- The discussion regarding deformation mechanisms is confuse, complex and hard to follow. Among other comments, three main topics are of high concern: (i) dislocation creep activation in Kfs: the evidence provided for the activation of slip systems in the present paper might be due to the presentation of pole figures as one-point-per-pixel, which presentation obscures the real statistics of the pole figure; (ii) fluid-assisted deformation and reactions: this topic is not properly introduced and not even described in the data section. (iii) the following discussion about formation conditions and mechanisms need to be reviewed at the light of the comments above.
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CC1: 'Reply on RC1', Lefan zhan, 13 Feb 2022
Thank you for the careful and critical revision of our manuscript from Referee #1. The comments and suggestions will greatly improve the manuscript. Below, we primarily respond to the comments before the close of open discussion and will have further detailed revision and response as soon as possible.
Response to two main questions and remarks raised from Referee #1 as follows:
1. About the proposed aims and conditions of granodiorite formation: The main proposed aims are discussing the processes of conditions of variation deformation, mineral composition, and fabric transition accompanied by a significant grain-size reduction and progressive phase mixing of minerals with increasing strain of the small-scale shear zones. The outcrop-scale structure, microstructure and EBSD textures are focus on the topic. The host rocks developed small-scale shear zones preserve its magmatic texture and are called unfoliated granitic rocks in our work we have the dated ages by zircon U-Pb dating, which compare the foliated granitic rocks in the GLG-SZ, they have the same ages of 110-120 Ma, however, the foliated granitic rocks in the GLG-SZ also have another group ages of ca. 20 Ma from the zircon rims. Therefore, we suggest that the small-scale shear zones can related the foliated rocks in the GLG-SZ. By the deformed structure and analyzed data e.g., EBSD and geothermometer, it can prove that the ductile deformation of the small-scale shear zone was initiated by the temperature-controlled rheological weakening and expend by fluid-assisted mechanism during increasing deformation and exhumation. We think that it’s useful and meaningful for exploring the deformation of the pluton controlled by the continental-scale strike-slip shear zone in the middle crust. To make the significance of our manuscript clearer, we will further consider revised some contents in the discussion and introduction.
2. About the EBSD datasets: The EBSD datasets are from the same sample, and we will strengthen this point in revised version. The point that one-point-per-grain pole figures are statistically meaningful is right, but we don’t separate the grain in the aggregates and the grain in the matrix, what we get is the dominant orientation of minerals in the Zone A, Zone B or Zone C. A large number of grains in the matrix will have a bad influence for it. Thus, we choose the one-point-per-pixel pole figures, and the deviation could be tolerated. With regard to the term ‘negative/positive’, we will change it to make the meaning clearer.
We considered used the grain sizes from the EBSD dataset. However, because of the imitated premise and range of grain size data obtained from EBSD maps. Thus, we think the grain sizes can be counted by manual operation may be more appropriate and also useful. Additionally, the detector distance is not the distance between sample and detector, but the distance between the working position and initial position of detector.
3. Detail comments: Some detail comments will be very helpful for our manuscript, and we will revise carefully. It is appreciated your suggestions and feedback in the open discussion. Thank you for your careful review and constructive advice.Citation: https://doi.org/10.5194/se-2022-2-CC1
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CC1: 'Reply on RC1', Lefan zhan, 13 Feb 2022
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RC2: 'Comment on se-2022-2', Anonymous Referee #2, 24 Feb 2022
The manuscript of Zhan et al. presents a microstructural study of strain localisation in small scale shear zones associated with a major continental strike-slip shear zone. The work includes a systematic attempt at microstructural characterisation and some lovely optical micrographs which indeed show some interesting changes across the shear zone. The authors interpret these to indicate changes in strain and deformation mechanisms. Whilst these highly localised features seem worthy of study in the context of strain localisation processes, I cannot recommend that the manuscript be accepted in its present form. The most significant problems include use of analyses that are not always valid (or not enough information is available to show that they are justified), insufficient results provided to support some of the key interpretations, and generally a lack of clarity in how this work and the results add to existing ideas about strain localisation in shear zones. I’ve listed some detail on these major issues (and others) in the general comments below in the hope that this proves useful.
General comments
- There are several occurrences where the choice of words and grammar needs to be improved in order to increase clarity - there are some apparently key points particularly in the intro and discussion where this causes problems. In general the overall structure of the manuscript is logical.
- There needs to be better description of the context of the studied shear zones used for microstructural analysis. Are there field photos of these shear zones, orientation data, and field maps showing their relationship to nearby structures? How far are they from more major shear zones? It is not at all clear whether all the analysed shear zones (including those in micrographs, EBSD and thermobarometry samples) come from different localities/samples – the map in Fig 1 suggests 3 but locations and setting for each should be described. It is not clear how the samples analysed for thermobarometry relate to the shear zones. Ideally locations of EBSD maps should be shown on optical micrographs, and field photos of the sampled shear zones included.
- Some of the key figures are not presented well enough to allow robust comparisons to support the authors interpretations – particularly EBSD pole figures and grain size histograms, suggested modifications are listed against relevant figure number in specific comments below.
- Methods needs to include more detail about the amphibole-plagioclase thermobarometry. E.g. how many pairs were used, how were they selected, what samples did they come from? Where is the raw compositional data? In the results, the reported P and T ranges do not exactly correlate with the range of points plotted in Fig. 10.
- For the piezometry, the authors need to indicate locations of the areas used for the grain size measurements in order to support the assumption that the quartz grain size is only controlled by differential stress during dynamic recrystallisation, and also need to describe the process used to distinguish recrystallised grains. In all the images of zone C, quartz is distributed in polyphase assemblages which makes pinning of the grain size by secondary phases very likely – this will invalidate the piezometry results. If subgrains can be observed in quartz, the authors may wish to consider a subgrain-based piezometer which can be used in polyphase assemblages (e.g. Goddard et al. 2020) although they should note that EBSD grain size analysis would be necessary here.
- Strain rate calculations do not seem completely valid. Be careful with relying on CPO patterns/microstructures for T estimates, as they are also influenced by strain rate as well. I recommend including a figure of the calculated strain rate curves, which allow clearer consideration of uncertainty in stress and temperature. The authors need to state the deformation mechanisms identified to justify choice of the flow laws – which appear to be for dislocation creep despite the fact that later the authors interpret diffusion creep for zone c, so these flow laws will not be applicable there. The choice of published flow laws also needs to be better justified – a more recent study by Tokle et al. (2019) which reviews previous work could likely be a better choice than the earlier Luan & Paterson study, unless the authors have reasons for their preference. Although the authors state that water fugacity was considered, this consideration needs to be outlined more fully. Make sure that the strain rates are shear strain rates – may need to be converted from axial experimental strain rates with a multiplication by √3 (Paterson & Olgaard, 2000).
- Hydrous retrogression of hornblende to mica seems to be a key interpretation – it is mentioned in the abstract and in the discussion – but I’ve not seen evidence that this is certainly recorded here, especially since biotite and hornblende are both shown to be magmatic phases. The authors reference Fig. 4c but I can’t see any particularly clear relationships between hornblende and biotite there. The lack of hornblende in zone C is indeed interesting and I would encourage the authors to explore this but need to rule out alternative explanations e.g. preferential localisation over a hornblende-free layer in the granite. The authors need to present better thin-section evidence of this reaction occurring, and state the full reaction. Ideally a pseudosection could be created for this rock composition. This could also facilitate calculation of the fluid content, if no other constraints are available – this is also a major but unsupported part of the discussion. Castellan et al. (2021) is a good example of a study noting a potentially similar reaction in granitic shear zones, presenting the types of evidence suggested here.
- The interpretation of thermal heterogeneity localising strain (discussion section 8.4) is not clearly supported, in my opinion. In the introduction it states (L196) ‘They are invariably localized on approximately planar structural and compositional heterogeneities within the protolith’. But in the discussion the authors state that their observations cannot be explained by precursors and I am not sure why – this seems contradictory and more explanation is needed here. Is it related to the sudden changes in apparent strain between the zones, which is an interesting feature here? How does this then lead the authors to their preferred model - ‘hot-to-cold contacts ascribed to thermally enhanced rheological weakening’ (L675) needs more explanation as to the processes involved and the evidence from these samples.
- The conclusions are a bit vague. More generally the authors should work on expanding the context and key points of their work. If strain localisation is the key theme, what are the underlying processes and controls which have caused this to be observed? Are small scale shear zones being used to explore (early?) localisation processes in the development of shear zones generally? If so how representative are they. Is there a reason why these shear zones are small and have remained so highly localised? And regionally what is their significance – why are there differences in the deformation between the foliated and non-foliated granites, what is their role in the wider GLG-SZ system? This needs addressing in the intro/discussion but a more detailed geological context for the samples and the study area is also required.
Specific comments
L153-175 (Methods section)
- How were samples selected and cut – can locations be provided? Do any of the field photos show the sampled shear zones? Were samples cut in the X-Z plane?
- Report the step size for EBSD acquisition. Was any post-acquisition filtering done on the EBSD maps?
- Were twin boundaries removed from rotation axes analyses of quartz and feldspar? Do some plots show 1 point per grain or not? These details need including either here or e.g. in figure captions.
L233-240 - The use of ‘high’ for zone B and ‘strong’ for zone C need additional descriptors to give them some sense – maybe this is strain? In any case I would avoid all interpretation of high/low strain in this section – stick to observed properties that underly these interpretations.
L247 – Fig. 3c needs to be shown at higher magnification to convincingly show GBM, it is difficult to see without zooming in and losing resolution.
L265 I can’t clearly see the proposed core-mantle structure in K-feldspar in this image. An optical image may show this more clearly, or annotate the BSE image. Need to indicate orientation of the shear zone to support next sentence about K-spar long axes. See also L625 where this is mentioned again.
L380 – there are a few circumstances where I think ‘corrected’/’uncorrected’ have possibly been used where it should say ‘correlated’/’uncorrelated’ - or if used deliberately, the correction needs to be explained.
L621 – the CL images have not been described in the context of their CL properties – this needs to be included earlier in the results before discussing here.
L728 - ‘deep seated crust’ – are these SZs very deep? Results seemed to estimate 10 km, but deformation temperatures suggest higher?
Fig 1 –
- Part a contains some abbreviated fault labels that are not explained in the caption. If not needed (in the Himalaya) these could be removed.
- It would be helpful to have a higher-scale map of the studied location in addition to this regional one. Could the distribution of foliated vs unfoliated granites be shown on such a map? I am finding it hard to relate the location/context of the studies shear zones to deformed granites and the GLG-SZ.
Fig 2 –
- Parts a and b – this may be a resolution problem but the shear sense indicators labelled are not entirely convincing – a better resolution image and/or a more zoomed-in image would help here. Make sure that the label ‘Tur’ (part a) is defined in the caption
- in caption, part E – explain what is meant by ‘are like joints’ e.g. is there no shear, are there brittle features?
Fig 3 – Are parts e – g flipped or rotated relative to the main image in part d? The foliation looks a different angle. Part d is a fantastic image!
Fig. 4 – I don’t think these images have a consistent reference orientation, can the shear zone orientation be annotated on each part to make this clear (or X-Z arrows, presuming that is the cut surface)
Fig. 5 – the bins should be the same size and interval for all the histograms in this figure, otherwise it is impossible to compare different subsets properly.
Fig. 6 –
- the M.U.D scales need to be consistent in order to be comparable i.e. the range should be the same across all pole figures. This will help avoid over-interpreting clustering when in fact the max M.U.D is very low. The scales need more values labelled – sometimes it is not clear whether the M.U.D should be increasing or decreasing when only one value is listed, some scales have no values! Is it possible to use a colour scale other than the rainbow one used here?
- Can the locations of the EBSD maps be shown on earlier figures?
Fig. 8 – is the X direction vertical in this map? Add annotation to show the reference frame on all EBSD maps.
Citation: https://doi.org/10.5194/se-2022-2-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on se-2022-2', Anonymous Referee #1, 10 Feb 2022
Review of manuscript se-2022-2.
Unfortunately, the paper cannot be accepted for publication in the present form, and I suggest rejection. My main concerns regard (1) the structure and readability of the whole text; (2) the relevance of the presented topic for Solid Earth journal, (3) sample characterization and data presentation. The presented topic might be better suited for another journal, maybe focussed on the regional geology of the region. In the following I will provide some general comments on the sections of the manuscript. Detailed comments can be found in the attached commented pdf file.
General comments.
Semantics and syntax of many sentences are rather complex, and some sentences are not understandable at all. A check of the sentence structure and meaning by a native English speaker would be advisable in this case. Language grammar and spelling are ok (some typos are still present though), but the structure of sentences and paragraphs make the manuscript very hard to follow and understand.
It is not clear how the proposed aims of the paper (Lines 85-89) can contribute to the research topic proposed in the Introduction. It is not clear either what the main research question is. As it is now the paper consists of a (average quality) EBSD and microstructural data collection, without any clear objective neither in the data presentation nor in the discussion section.
Abstract.
- One of the main points of the abstract regards the rate-limiting fluid-assisted mechanisms, which topic is just briefly discussed in the main text and would need to be unfolded and better discussed.
Introduction.
- The introduction presents a rather complete and up-to-date list of references and a general overview of the topics analysed in the following text. However, it is not clear what contribution the present manuscript yields to the described topics. What is the main research question? What are you aiming at with the description of small-scale shear zone? How come the results of you research work help to answer to the main research question?
- The aims of the paper reported at Lines 85-89 leave the readers asking themselves the unfortunate “so what” question.
Geological settings.
- The geological setting of the sample and host rock (age of emplacement, age of deformation, regional conditions of deformation) need to be better described. As it is now, we just know that the samples come from a granodiorite that preserves its magmatic texture. How old is it? Does it have undergone to any metamorphic cycle? Is just syn-shearing pluton emplacement along the GLG-SZ? This is critical to understand the formation conditions of the sample small-scale shear zones.
- Samples need to be properly located in the geological framework. The reader needs to understand if the presented analyses (microstructural description, EBSD, CL, microchemistry) come from the same sample or different samples. This is not clear at all in this manuscript.
Data presentation and figures.
- Mesoscale description. Ok but please consider rephrasing complex sentences.
- Microstructures. See detailed comments.
EBSD:
- It is not clear whether the EBSD maps come from the same sample, different samples or whatsoever.
- Contoured pole figures need to be presented as one-point-per-grain to be statistically meaningful. One-point-per-pixel pole figures are affected by the area (i.e., grain size translated in number of pixels) occupied by each grain, and thus they represent a biased dataset.
- “Negative/positive correlation between misorientation angle distributions”: it is unclear what this sentence means in the context of distribution comparison.
P-T- estimation - Chemical data for Hbl-Pl
- The compositional data adopted for geothermobarometry are not presented in the text. Please, provide at least a table with the phase compositions. It would be also nice to discuss whether the adopted compositions are at the equilibrium during deformation or not.
- Is this really the emplacement depth? Could it be a re-equilibration metamorphic depth? Need to improve the description of the geological setting of the granodiorite from where the samples have been collected.
Grain sizes: it would be better to retrieve grain sizes from the EBSD dataset, and to adopt the same statistical parameter (mean, median) for the flow-stress computation as those adopted in the piezometric relationship here implemented (see Luan and Paterson 1992 and Hirth 2001 for the details).
Flow stress – Strain rate calculations.
- Temperature need to be properly constrained. There are several contradiction throughout the text regarding temperature of deformation. P-T conditions of emplacement are 650-750°C. Are not these the deformation conditions? Then you have to explain that after emplacement the granodiorite underwent some sort of cooling or exhumation. At Line 561, an arbitrary temperature of 550°C is chosen based on microstructural characteristics. Is this the temperature of deformation of the small-scale shear zone? Why then in the discussion you come back to 650-735°C to explain the development of “foliated” granodiorite (the granodiorite hosting the sample was described as non-foliated in the geological setting…).
- The arbitrary choices of both grain size and temperature deeply affect the flow-stress strain rate estimations.
Discussion regarding deformation mechanisms:
- The authors claim for the activation of certain deformation mechanisms without providing compelling evidence, even if they present a large EBSD dataset.
- The discussion regarding deformation mechanisms is confuse, complex and hard to follow. Among other comments, three main topics are of high concern: (i) dislocation creep activation in Kfs: the evidence provided for the activation of slip systems in the present paper might be due to the presentation of pole figures as one-point-per-pixel, which presentation obscures the real statistics of the pole figure; (ii) fluid-assisted deformation and reactions: this topic is not properly introduced and not even described in the data section. (iii) the following discussion about formation conditions and mechanisms need to be reviewed at the light of the comments above.
-
CC1: 'Reply on RC1', Lefan zhan, 13 Feb 2022
Thank you for the careful and critical revision of our manuscript from Referee #1. The comments and suggestions will greatly improve the manuscript. Below, we primarily respond to the comments before the close of open discussion and will have further detailed revision and response as soon as possible.
Response to two main questions and remarks raised from Referee #1 as follows:
1. About the proposed aims and conditions of granodiorite formation: The main proposed aims are discussing the processes of conditions of variation deformation, mineral composition, and fabric transition accompanied by a significant grain-size reduction and progressive phase mixing of minerals with increasing strain of the small-scale shear zones. The outcrop-scale structure, microstructure and EBSD textures are focus on the topic. The host rocks developed small-scale shear zones preserve its magmatic texture and are called unfoliated granitic rocks in our work we have the dated ages by zircon U-Pb dating, which compare the foliated granitic rocks in the GLG-SZ, they have the same ages of 110-120 Ma, however, the foliated granitic rocks in the GLG-SZ also have another group ages of ca. 20 Ma from the zircon rims. Therefore, we suggest that the small-scale shear zones can related the foliated rocks in the GLG-SZ. By the deformed structure and analyzed data e.g., EBSD and geothermometer, it can prove that the ductile deformation of the small-scale shear zone was initiated by the temperature-controlled rheological weakening and expend by fluid-assisted mechanism during increasing deformation and exhumation. We think that it’s useful and meaningful for exploring the deformation of the pluton controlled by the continental-scale strike-slip shear zone in the middle crust. To make the significance of our manuscript clearer, we will further consider revised some contents in the discussion and introduction.
2. About the EBSD datasets: The EBSD datasets are from the same sample, and we will strengthen this point in revised version. The point that one-point-per-grain pole figures are statistically meaningful is right, but we don’t separate the grain in the aggregates and the grain in the matrix, what we get is the dominant orientation of minerals in the Zone A, Zone B or Zone C. A large number of grains in the matrix will have a bad influence for it. Thus, we choose the one-point-per-pixel pole figures, and the deviation could be tolerated. With regard to the term ‘negative/positive’, we will change it to make the meaning clearer.
We considered used the grain sizes from the EBSD dataset. However, because of the imitated premise and range of grain size data obtained from EBSD maps. Thus, we think the grain sizes can be counted by manual operation may be more appropriate and also useful. Additionally, the detector distance is not the distance between sample and detector, but the distance between the working position and initial position of detector.
3. Detail comments: Some detail comments will be very helpful for our manuscript, and we will revise carefully. It is appreciated your suggestions and feedback in the open discussion. Thank you for your careful review and constructive advice.Citation: https://doi.org/10.5194/se-2022-2-CC1
-
CC1: 'Reply on RC1', Lefan zhan, 13 Feb 2022
-
RC2: 'Comment on se-2022-2', Anonymous Referee #2, 24 Feb 2022
The manuscript of Zhan et al. presents a microstructural study of strain localisation in small scale shear zones associated with a major continental strike-slip shear zone. The work includes a systematic attempt at microstructural characterisation and some lovely optical micrographs which indeed show some interesting changes across the shear zone. The authors interpret these to indicate changes in strain and deformation mechanisms. Whilst these highly localised features seem worthy of study in the context of strain localisation processes, I cannot recommend that the manuscript be accepted in its present form. The most significant problems include use of analyses that are not always valid (or not enough information is available to show that they are justified), insufficient results provided to support some of the key interpretations, and generally a lack of clarity in how this work and the results add to existing ideas about strain localisation in shear zones. I’ve listed some detail on these major issues (and others) in the general comments below in the hope that this proves useful.
General comments
- There are several occurrences where the choice of words and grammar needs to be improved in order to increase clarity - there are some apparently key points particularly in the intro and discussion where this causes problems. In general the overall structure of the manuscript is logical.
- There needs to be better description of the context of the studied shear zones used for microstructural analysis. Are there field photos of these shear zones, orientation data, and field maps showing their relationship to nearby structures? How far are they from more major shear zones? It is not at all clear whether all the analysed shear zones (including those in micrographs, EBSD and thermobarometry samples) come from different localities/samples – the map in Fig 1 suggests 3 but locations and setting for each should be described. It is not clear how the samples analysed for thermobarometry relate to the shear zones. Ideally locations of EBSD maps should be shown on optical micrographs, and field photos of the sampled shear zones included.
- Some of the key figures are not presented well enough to allow robust comparisons to support the authors interpretations – particularly EBSD pole figures and grain size histograms, suggested modifications are listed against relevant figure number in specific comments below.
- Methods needs to include more detail about the amphibole-plagioclase thermobarometry. E.g. how many pairs were used, how were they selected, what samples did they come from? Where is the raw compositional data? In the results, the reported P and T ranges do not exactly correlate with the range of points plotted in Fig. 10.
- For the piezometry, the authors need to indicate locations of the areas used for the grain size measurements in order to support the assumption that the quartz grain size is only controlled by differential stress during dynamic recrystallisation, and also need to describe the process used to distinguish recrystallised grains. In all the images of zone C, quartz is distributed in polyphase assemblages which makes pinning of the grain size by secondary phases very likely – this will invalidate the piezometry results. If subgrains can be observed in quartz, the authors may wish to consider a subgrain-based piezometer which can be used in polyphase assemblages (e.g. Goddard et al. 2020) although they should note that EBSD grain size analysis would be necessary here.
- Strain rate calculations do not seem completely valid. Be careful with relying on CPO patterns/microstructures for T estimates, as they are also influenced by strain rate as well. I recommend including a figure of the calculated strain rate curves, which allow clearer consideration of uncertainty in stress and temperature. The authors need to state the deformation mechanisms identified to justify choice of the flow laws – which appear to be for dislocation creep despite the fact that later the authors interpret diffusion creep for zone c, so these flow laws will not be applicable there. The choice of published flow laws also needs to be better justified – a more recent study by Tokle et al. (2019) which reviews previous work could likely be a better choice than the earlier Luan & Paterson study, unless the authors have reasons for their preference. Although the authors state that water fugacity was considered, this consideration needs to be outlined more fully. Make sure that the strain rates are shear strain rates – may need to be converted from axial experimental strain rates with a multiplication by √3 (Paterson & Olgaard, 2000).
- Hydrous retrogression of hornblende to mica seems to be a key interpretation – it is mentioned in the abstract and in the discussion – but I’ve not seen evidence that this is certainly recorded here, especially since biotite and hornblende are both shown to be magmatic phases. The authors reference Fig. 4c but I can’t see any particularly clear relationships between hornblende and biotite there. The lack of hornblende in zone C is indeed interesting and I would encourage the authors to explore this but need to rule out alternative explanations e.g. preferential localisation over a hornblende-free layer in the granite. The authors need to present better thin-section evidence of this reaction occurring, and state the full reaction. Ideally a pseudosection could be created for this rock composition. This could also facilitate calculation of the fluid content, if no other constraints are available – this is also a major but unsupported part of the discussion. Castellan et al. (2021) is a good example of a study noting a potentially similar reaction in granitic shear zones, presenting the types of evidence suggested here.
- The interpretation of thermal heterogeneity localising strain (discussion section 8.4) is not clearly supported, in my opinion. In the introduction it states (L196) ‘They are invariably localized on approximately planar structural and compositional heterogeneities within the protolith’. But in the discussion the authors state that their observations cannot be explained by precursors and I am not sure why – this seems contradictory and more explanation is needed here. Is it related to the sudden changes in apparent strain between the zones, which is an interesting feature here? How does this then lead the authors to their preferred model - ‘hot-to-cold contacts ascribed to thermally enhanced rheological weakening’ (L675) needs more explanation as to the processes involved and the evidence from these samples.
- The conclusions are a bit vague. More generally the authors should work on expanding the context and key points of their work. If strain localisation is the key theme, what are the underlying processes and controls which have caused this to be observed? Are small scale shear zones being used to explore (early?) localisation processes in the development of shear zones generally? If so how representative are they. Is there a reason why these shear zones are small and have remained so highly localised? And regionally what is their significance – why are there differences in the deformation between the foliated and non-foliated granites, what is their role in the wider GLG-SZ system? This needs addressing in the intro/discussion but a more detailed geological context for the samples and the study area is also required.
Specific comments
L153-175 (Methods section)
- How were samples selected and cut – can locations be provided? Do any of the field photos show the sampled shear zones? Were samples cut in the X-Z plane?
- Report the step size for EBSD acquisition. Was any post-acquisition filtering done on the EBSD maps?
- Were twin boundaries removed from rotation axes analyses of quartz and feldspar? Do some plots show 1 point per grain or not? These details need including either here or e.g. in figure captions.
L233-240 - The use of ‘high’ for zone B and ‘strong’ for zone C need additional descriptors to give them some sense – maybe this is strain? In any case I would avoid all interpretation of high/low strain in this section – stick to observed properties that underly these interpretations.
L247 – Fig. 3c needs to be shown at higher magnification to convincingly show GBM, it is difficult to see without zooming in and losing resolution.
L265 I can’t clearly see the proposed core-mantle structure in K-feldspar in this image. An optical image may show this more clearly, or annotate the BSE image. Need to indicate orientation of the shear zone to support next sentence about K-spar long axes. See also L625 where this is mentioned again.
L380 – there are a few circumstances where I think ‘corrected’/’uncorrected’ have possibly been used where it should say ‘correlated’/’uncorrelated’ - or if used deliberately, the correction needs to be explained.
L621 – the CL images have not been described in the context of their CL properties – this needs to be included earlier in the results before discussing here.
L728 - ‘deep seated crust’ – are these SZs very deep? Results seemed to estimate 10 km, but deformation temperatures suggest higher?
Fig 1 –
- Part a contains some abbreviated fault labels that are not explained in the caption. If not needed (in the Himalaya) these could be removed.
- It would be helpful to have a higher-scale map of the studied location in addition to this regional one. Could the distribution of foliated vs unfoliated granites be shown on such a map? I am finding it hard to relate the location/context of the studies shear zones to deformed granites and the GLG-SZ.
Fig 2 –
- Parts a and b – this may be a resolution problem but the shear sense indicators labelled are not entirely convincing – a better resolution image and/or a more zoomed-in image would help here. Make sure that the label ‘Tur’ (part a) is defined in the caption
- in caption, part E – explain what is meant by ‘are like joints’ e.g. is there no shear, are there brittle features?
Fig 3 – Are parts e – g flipped or rotated relative to the main image in part d? The foliation looks a different angle. Part d is a fantastic image!
Fig. 4 – I don’t think these images have a consistent reference orientation, can the shear zone orientation be annotated on each part to make this clear (or X-Z arrows, presuming that is the cut surface)
Fig. 5 – the bins should be the same size and interval for all the histograms in this figure, otherwise it is impossible to compare different subsets properly.
Fig. 6 –
- the M.U.D scales need to be consistent in order to be comparable i.e. the range should be the same across all pole figures. This will help avoid over-interpreting clustering when in fact the max M.U.D is very low. The scales need more values labelled – sometimes it is not clear whether the M.U.D should be increasing or decreasing when only one value is listed, some scales have no values! Is it possible to use a colour scale other than the rainbow one used here?
- Can the locations of the EBSD maps be shown on earlier figures?
Fig. 8 – is the X direction vertical in this map? Add annotation to show the reference frame on all EBSD maps.
Citation: https://doi.org/10.5194/se-2022-2-RC2
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