The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation?

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of “The relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): a new regional interpretation?” by Elter and Mantovani

I read the manuscript entitled “The relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): a new regional interpretation?” with interest. I hope my comments on both the PDF and this file will help (please see the annotated version of the PDF file), and I warmly suggest the authors to thoroughly consider them for future submissions.

I’m convinced that this paper include potential arguments for work that will be a novelty in the framework of the Sardinian belt (e.g., the Andalusite occurrence which has implications for the understanding of the contact metamorphism).

Nevertheless, this paper is not ready to be published

Firstly, the used English must be improved. Often sentences are complex or verbose, and the aim of the sentence is not clear for the reader.

Secondly, I would like to highlight a important general suggestions for each chapters and generally to be taken into account in order to improve the manuscript.

-        Introduction

The introduction lacks a clear statement on the necessity of renewing the tectono-metamorphic subdivision and the nomenclature of the area, as well as an explanation of what is missing and why older geological interpretations are considered insufficient or erroneous. Furthermore, both in the introduction and geological setting citations are incorrectly placed and do not refer to the subject under discussion (see the PDF).

The author employs an excessive number of acronyms and abbreviations that are not consistent with those used in the existing literature. Furthermore, it is unclear what distinguishes the PVSZ (sensu Carosi et al., 2020) from the CIZ. It appears that both can be attributed to the PASZ, as evidenced by numerous publications in the scientific literature, including the Posada Line (cited herein).

Notwithstanding the considerable amount of work and structural data presented, the new interpretations offered by the authors in this study lack sufficient support from the data. Meso- and microscale data without petrology and quantitative structural analysis (such as quantitative estimation of vorticity, strain, etc.) are inadequate for a complete rewriting of the geology of this area. Previous authors have made geological interpretations using quantitative data, including Carosi et al. (2020) for quantification of the amount of pure and simple shear, thus the distribution of the high and the low strain domains.

-        Materials and Methods

This section is of a poor standard. It would be beneficial to gain insight into the methodology employed for cutting the samples to create thin sections. Additionally, it would be advantageous to understand the guidelines utilized for microstructural analysis and kinematic indicator analysis. Furthermore, it would be beneficial to define in this section which sources the authors followed for the mineral abbreviations (e.g., Whitney & Evans, 2010 or Warr, 2021). It is also essential to maintain uniformity with these guidelines throughout the entire manuscript. Lastly, addressing the method for defining new metamorphic complexes is fundamental.

-        Geological Outlines of Sardinia

The authors cited work that never talked about a Condensed Isograds Zone (CIZ). Here the sections where the word “condensed” appears in the references are reported.

i)                ELTER & PANDELI 2005 (ref #43): the Posada Valley Zone is characterized by MT/MP condensed isograds and by a metamorphic evolution syn-kinematic with non-coaxial deformation (Elter et al., 1999, 2004 and references therein). The Posada Valley Zone is here considered a transitional metamorphic complex from the Nappe Zone to HGMC. The northern border of the Posada Valley Zone is represented by a segment of the most important regional tectonic line, the so-called Posada - Asinara Line (Figure 3). This about 3 km-thick, dextral strike-slip shear zone developed in greenschist facies conditions which overprinted the condensed isograds metamorphism.

 

ii)              CAROSI & PALMERI 2002 (ref #41): The rapid increase of the metamorphic grade in northeastern Sardinia over a short distance is not consistent with Barrovian metamorphic gradients. The D2 shear zone was responsible for the stretching of the observed condensed isograds (Franceschelli, Memmi & Ricci, 1982).

 

iii)             CORSI & ELTER 2006: No sections

 

iv)             ELTER et al. 2004: No sections

As stated in the previous comment about the introduction, authors must check all the citations in the text, especially in this section, by checking if they cited the correct work, and if the citation is scientifically coherent with the sentence and is placed in the correct position. It looks like the authors never read the works they have cited in the manuscript.

I have checked the work you have cited for the CIZ and I did not find any definition of such a zone. Can you please explain better what do you mean with CIZ and how it relates to the existing literature? I want to stress that the authors did not cite in a scientifically correct way the previous works, and it looks like they did not read well the cited works, therefore putting them in the wrong sentence.

3.4. The Regional Mylonitic Complex (RMC) or High Grade Metamorphic Complex (HGMC)

The authors [14] hypothesized that the High Grade Metamorphic Complex (HGMC; 180 [78–80]) cannot be considered as the Axial Zone of the Sardinian Variscan chain but instead as a Regional Mylonitic Complex (RMC) associated with the top-right East Variscan Shear Zones (EVSZ, [81–83]) .

This sentence is interpretation, it is not data! The geological setting must contain a detailed and clear description of the geological framework extrapolated from all the works from previous authors, trying to be as objective as possible, separating previous data from current interpretations. Interpretations, rising from new data reported in this work and the older reported by previous work must be reported in the discussions section.  

-        Results

In this section, I wish to draw attention to the following comments and issues:

In the stereographic projections, the number of data points and the method used to create the contour (which algorithm? Kamb?) are absent from the description. Additionally, the authors employ the concept of "poles" in the lineation. The data are delineated in accordance with the geographic position, commencing from the south and progressing towards the north. Nevertheless, it is challenging for those with limited knowledge of Sardinian geology to rapidly ascertain the measures' positions, as no accompanying figures or maps are provided to indicate these.

The terminology used to describe the structural data is not always accurate. For instance, a foliation may be delineated using the values of dip direction and dip angle, or strike. The authors occasionally employ the terms "inclination toward the SE" (line 260) and "Lm shows a SE orientation," yet fail to elucidate the precise nature of the phenomenon in question, whether it be the dip direction, the dip angle or the strike.

 

The figures, at both the meso- and microscales, lack the requisite orientations (as cardinal points). The author provided a description of the orientation in the text, but the orientation itself was absent. It is therefore challenging for the reader to comprehend the orientation of the topographic feature if the orientation of the outcrop surface is not clearly defined. Furthermore, in some figures, the line used to highlight the foliation (e.g., Sm or S2) is not consistently well drawn and does not follow any discernible orientation (e.g., Figure 8b; see PDF for detailed comment).

The authors report the paragenesis for each unit/rock type, but the link between the minerals and the microstructures is not always evident, or there are insufficient references (if the authors are referring to previous works in the literature). The authors create new tectono-metamorphic units with a new nomenclature. Nevertheless, this subdivisions is not entirely supported by the data or by the explanation. In fact, a tectono-metamoprhic unit can be defined only if it is possible to distinghish different P-T-t conditions and a tectonic contact separating from others units (see definition of tectono-metamoprhic unit, sensu Spalla et al., 2005, https://doi.org/10.1144/gsl.sp.2005.243.01.16). The lithological creteria alone is not sufficient to discriminate between different tectono-metamorphic units.

Sincerely,

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English used must be improved. Often sentences are complex or verbose, and the sentence's aim is unclear for the reader.

Author Response

Dear Reviewer 1,

We would like to express our sincere gratitude for your valuable comments and suggestions, which have significantly contributed to the improvement of our paper. Your feedback has been instrumental in refining the content and enhancing the overall quality of our manuscript.

In response to your comments, we have made the following revisions:

1. Language and Terminology: We have carefully revised the entire manuscript to improve the English language, using more appropriate terminology and correcting the grammar to make the text more fluent and readable.

2. Introduction: We have rewritten the introduction section, incorporating your insights to strengthen the background and context of our study.

3. Materials and Methods: This section has been enhanced by including the specific suggestions you provided, ensuring a clearer and more comprehensive description of our approach.

4. New Chapter on Sardinian Shear Zones: We have added a dedicated chapter on the Shear Zones of Sardinia, addressing the gaps highlighted in your review.

5. References and Citations: We thoroughly reviewed and corrected all citations and references. Additionally, we have incorporated all the suggested papers into the bibliography to provide a more robust scientific context.

6. Geological Map: We have included a new geological map of the study area, detailing lithotypes and locations as suggested.

7. Figures and Images: All figures and images have been improved for better clarity and presentation, following your recommendations.

For a detailed account of how each comment was addressed, please refer to the resubmitted paper, where responses to the reviewer's comments are highlighted in red for Reviewer 1 and blue for Reviewer 2.

We truly appreciate your insightful feedback, which has greatly enhanced the quality of our work. We hope that the revisions meet your expectations, and we look forward to your further feedback.

Thank you once again for your time and effort.

Sincerely,

Franco Marco Elter

Federico Mantovani

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

I have carefully read your work on the transition zone between the INZ and the RMC in the metamorphic-structural evolution of the northeastern Sardinia Variscan orogen. The reinterpretation of the CIZ in your study gives new insights into the tectonic evolution of the Variscan orogeny and its geodynamic implication for the Sardinia geological history. The work is well structured even if a bit articulated. The literature data are sometimes recalled and sometimes they are confused with the original ones. Furthermore, I have some doubts about the correspondence between the data of schistoty and shear milonitic fabric orientations and their representation in the final scheme of the various metamorphic complexes. I have corrected errors and some oversights in the text and suggested some improvements.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 2,

We would like to sincerely thank you for your time, valuable advice, and insightful suggestions, which have greatly contributed to the improvement of our paper. Your thorough review has been extremely helpful in refining our manuscript.

We have carefully considered and accepted all of your comments and corrections, implementing them throughout the paper to enhance its overall quality.

For a detailed account of how each comment was addressed, please refer to the attached resubmitted paper, where our responses to the reviewer’s comments are highlighted in red for Reviewer 1 and in blue for Reviewer 2.

We truly appreciate your insightful feedback, which has greatly enhanced the quality of our work. We hope that the revisions meet your expectations, and we look forward to your further feedback.

Thank you once again for your time and effort.

Sincerely,

Franco Marco Elter

Federico Mantovani

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

I am grateful for the revised manuscript, which has been significantly enhanced following the initial round of revisions. The authors have demonstrated a commendable commitment to addressing the feedback provided, offering detailed responses to each comment. I am sincerely appreciative of this effort.

However, there are additional aspects that require attention before the manuscript can be considered for publication.

-            Posada Valley shear zone vs Posada Asinara shear zone.

I can't find in Elter et al., 1990 where they separate the PVSZ from the PASZ. However, I understood this separation from your comments that explain: “We refer to the Posada Valley Shear Zone sensu Elter et al., 1990, i.e. geographically limited to the Posada valley”. “

Nevertheless, I can’t understand why the authors said: “The PVSZ is subsequent to the East Variscan Shear Zone”. If the PVSZ is a geographically separated sector of the PASZ, a branch of the EVSZ, how can be not a part of the EVSZ?

-            CIZ

Introdcued by Corsi, B.; Elter, F.M. Eo-Variscan (Devonian?) Melting in the High Grade Metamorphic Complex of the NE Sardinia Belt (Italy). Geodin. Acta 2006, 19, 155–164, doi:10.3166/ga.19.155-164 and followed by Elter, F.M.; Corsi, B.; Cricca, P.; Muzio, G. The South-Western Alpine Foreland: Correlation between Two Sectors of the Variscan Chain Belonging to “Stable Europe”: Sardinia(-)Corsica and the Maures Massif (South-Eastern France). Geodin. Acta 2004, 17, 31–40, doi:10.3166/ga.17.31-40.”

We agree about the provided references, they discussed the presence of a condensed metamorphic zonation and telescoped isograds, but no one discussed and introduced a NEW ZONE. Thus, why authors must create the CIZ?  Is it necessary?

-            Regional Mylonitic Complex,

As the authors confirm, only one paper used this kind of terminology. Is it possible to avoid marginal terms and new complexes? I prefer to use a more common and discussed nomenclature.

-            Old Gneiss Complex

Franceschelli does not use the name  Old Gneiss Complex, but the data relating to the areas where those rocks outcrops, are the same. So why authors must create a new nomenclature?

-            Figure 7

 

Figure B does not provide sufficient clarity regarding the S-C' fabric, wherein the S is not deflected and the C' plane is observed to be quite subvertical. This is a rather anomalous observation, particularly in the context of the C' planes. Figure C is believed to be accurate by the author, however, to identify a clast between sigma or delta, it is necessary to observe the entire clast, rather than just half of it.

 

Author Response

Dear Reviewer,

we would like to express our deepest gratitude for your precise, meticulous, and thoughtful comments and suggestions. Without your valuable insights, our paper would not have reached its current state. It is always stimulating to engage with a reviewer who is rigorous yet honest, and open to discussion and equitable exchange. Your feedback has significantly contributed to the improvement of our work, and we truly appreciate the opportunity to address your comments.

We hope that our responses have clarified your concerns. However, we remain at your disposal for any further clarifications or future discussions.

Below, we have provided detailed responses to your questions and annotations.

Thank you once again for your invaluable input.

Sincerely,

Franco Marco Elter & Federico Mantovani

 

• Posada Valley shear zone vs Posada Asinara shear zone.

Reviewer Question

I can't find in Elter et al., 1990 where they separate the PVSZ from the PASZ. However, I understood this separation from your comments that explain: “We refer to the Posada Valley Shear Zone sensu Elter et al., 1990, i.e. geographically limited to the Posada valley”.

Answer:

We are pleased that we were able to clarify in our comments what we mean by “sensu Elter et al., 1990”. We would like to add this clarification: in Elter et al. (1990), the authors describe the kinematics and deformational features of the Posada Valley Shear Zone and the Mt. Grighini Shear Zone. They do not extend their study area further west beyond the Posada Valley. In the discussions and conclusions, the authors suggest a possible correlation between these shear zones and those reported in the Maures-Tanneron Massif. However, they do not investigate the Asinara and Anglona regions.

Reviewer Question

Nevertheless, I can’t understand why the authors said: “The PVSZ is subsequent to the East Variscan Shear Zone”. If the PVSZ is a geographically separated sector of the PASZ, a branch of the EVSZ, how can be not a part of the EVSZ?

Answer:

We do not physically or geographically separate the Posada Valley Shear Zone (PVSZ) from the Posada Asinara Shear Zone (PASZ) and the East Variscan Shear Zone (EVSZ), nor do we question the branching system of the EVSZ as presented in Simonetti et al. (2020) and Carosi et al. (2022). However, we do distinguish them chronologically. Field data from Mantovani & Elter (2024) in the P.ta Orvili area (see Figure 12 in that paper) and from this study in the Sedda Eneas area identify geometric overlap relationships, where the Sm of the PVSZ cuts the S3. In this study, we do not have precise geochronological datings, unlike Carosi et al. (2022). As shown in the summary table of Variscan Shear Zone ages (Table 1 in Carosi et al., 2022), the timing for the PASZ is indicated as 325–300 Ma. The difference compared to our chronological considerations (Table 2 of this paper) lies in our observation of the described geometric overlaps, which suggest that the activity of the PVSZ sensu Elter et al., 1990, occurred after the events that generated the S3.

Thus, there are only two possible solutions:

1. PVSZ activity occurred within the time range of the EVSZ (325-300 Ma according to Carosi et al., 2022). Consequently, S3 and S2 must have been pre-dated by an earlier “tectonic step”. However, this does not align with all the known geochronological data available in the literature.
2. The PVSZ is subsequent to the EVSZ activity (325-300 Ma according to Carosi et al., 2022) and thus falls between the end of the EVSZ activity and the emplacement of late Variscan granites. This scenario is more reliable than Solution 1 and is consistent with Carosi et al. (2022), which states: “Furthermore, data documented that the EVSZ is not a simple late Variscan (or even Permian) strike-slip shear zone developed synchronously along all the different strands, but rather a crustal-scale system of transpressive shear zones with a complex and progressive evolution over time… A detailed investigation of other shear zones, potentially linked to the EVSZ, could lead to the recognition of other generations of shear zones and more complex architectures of a possible anastomosing pattern of ductile shear zones”. We added tihis point in the conclusion (line 754-760)

This suggests that further investigation into shear zones related to the EVSZ may reveal additional generations and more intricate structural patterns, aligning with our interpretation that the PVSZ postdates the activity of the PSVZ itself.

 

• CIZ

Reviewer Question

We agree about the provided references, they discussed the presence of a condensed metamorphic zonation and telescoped isograds, but no one discussed and introduced a NEW ZONE. Thus, why authors must create the CIZ?  Is it necessary?

Answer

It is true that the term “Condensed Isogrades Zone” has never been formally introduced or adopted as official terminology in metamorphic zonation. However, experts in the Variscan Sardinia context are well acquainted with the term and its location. Since this paper focuses on the evolution of this “narrow” metamorphic zone, we believed that the intrinsic concept behind the term would help especially those less familiar with Sardinia's metamorphic zonation to better understand the context. The intention of this paper is not to introduce new terminology, complexes, or zones but rather to use an already familiar term to convey the metamorphic zonation concept effectively to a broader audience.

 

• Regional Mylonitic Complex

Reviewer Question

As the authors confirm, only one paper used this kind of terminology. Is it possible to avoid marginal terms and new complexes? I prefer to use a more common and discussed nomenclature.

Answer

To maintain consistency with what was published by the authors in Mantovani & Elter (2024), we have used this terminology. We have already agreed in the previous round of revisions that the newly introduced term might be confusing for both experts on Sardinia and those less familiar with the area. For this reason, we have repeatedly included the classical terminology (HGMC and Axial Zone) throughout the text to provide clarity.

 

• Old Gneiss Complex

Reviewer Question

Franceschelli does not use the name Old Gneiss Complex, but the data relating to the areas where those rocks outcrops, are the same. So why authors must create a new nomenclature?

Answer

The nomenclature was introduced by Elter et al. (2010; The Emplacement of Variscan HT Metamorphic Rocks Linked to the Interaction between Gondwana and Laurussia: Structural Constraints in NE Sardinia (Italy). Terra Nova 2010, 22, 369–377, doi:10.1111/j.1365-3121.2010.00959.x). For us, it is crucial that this geographic area is distinguished into two different complexes: an older complex (Old Gneiss Complex), which is incorporated within a younger mylonitic complex (New Gneiss Complex). The metamorphic data from Franceschelli refer specifically to this geographic area but only to the lithotypes belonging to the Old Gneiss Complex. Therefore, it is essential to distinguish between the two to accurately represent the metamorphic characteristics and structural evolution of the area.

 

• Figure 7 

Reviewer Question

Figure B does not provide sufficient clarity regarding the S-C' fabric, wherein the S is not deflected and the C' plane is observed to be quite subvertical. This is a rather anomalous observation, particularly in the context of the C' planes.

Answer

The shear bands in question are localized within augen gneisses. It is unlikely that the deformation in these rocks reaches the levels observable in other lithologies such as micaschists and paragneisses. However, it is important to note that these shear bands still display kinematic indicators consistent with those observed in other lithotypes. This consistency helps validate the deformation patterns across different rock types within the studied area.

Reviewer Question

Figure C is believed to be accurate by the author, however, to identify a clast between sigma or delta, it is necessary to observe the entire clast, rather than just half of it.

Answer

We agree that the quality of the photo is not excellent, and unfortunately, it was cut in this manner during sample preparation. However, when comparing the structures observed in this photo with those in Figures 11 B, C, and D, the same structures can be seen at different scales. This consistency across different images supports the interpretation of the structural features presented in the study.