Imprint of the Reguibat Promontory (West Africa) on the Appalachian, Mauritanide and Souttoufide Belts During the Assembly of Pangaea

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

A detailed commentary of the MS can be found in the attached pdf and this part of the review is only concerned with remaining problems in general content and English expression:

1. The revised MS still leaves much to be done to improve the English expression. In some sections of the paper a reader is simply left confused as to what the authors were trying to convey about the geology. For example, the term rock series is vague, and a reader does not know whether the authors are referring to structural or stratigraphic units. It would also help a reader if there was consistency in describing individual rock units or stratigraphy. Too often, only the age of a unit is provided when a reader would like to know more about the lithologies involved and whether the thrust packages described are wholly made up of allochthonous material or formerly comprised part of one or more passive margin sequences that became caught up in the collisional process. These are the sort of rock sequences expected to be caught up during a continental collision and would serve to give more context to the events described.
2. The authors conclude that the region of interest comprises rocks and events that can be subdivided into pre-, syn- and post-collisional stages during the impingement of the West African craton on Laurentia. But on what grounds? The pre-collisional stage is said to involve the shouldering aside of the Senegalese Block and Adrar Souttouf terranes by the Reguibat promontory. Is this not a collisional event and thus part of the syn-collisional stage? Or if the Rheic Ocean had already closed by then as the authors clearly state, then how can the middle stage be described as syn-collisional? This part of the MS and the conclusions in general needs to be revised to avoid such conclusion
3. The focus on deformation on either side of the Reguibat promontory and whether it is symmetrical or asymmetrical appears overdone and insufficient grounds for what is described as a new tectonic interpretation. Surely, the authors have only modified past interpretations and are only justified in calling their model a revision?
4. There are deficiencies or problems in several figures. For example, the broken lines used to represent sutures in Fig. 2 barely match what is portrayed in the legend for these same sutures. In Fig. 5, the cross-section is labelled fig. 7 instead of fig. 6.
5. There needs to be better co-ordination between units identified in aerial photographs and the map figures.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Needs to be improved

Author Response

please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This article aims at deciphering the structures that developed in the northern and southern sides of the Reguibat promontory of W. Africa when it collided with Laurentia, indeed a part of Laurussia (this point must clarified in the text). It sounds reasonable to infer that the indentation of the Reguibat promontory was responsible for the structures developed on both sides of the promontory giving rise to the Souttoufide and Mauritanide fold belts. Such a model is partly a re-interpretation of the model proposed by Lefort (1988). Therefore, the previous model and its problems should be introduced first.

The new field observations must come subsequently.

 

It is of course necessary to locate the study area in the pre-Atlantic orogens framework with respect to the Laurentia, Baltica, and Gondwana continents, as shown in Fig. 1. However, as far as I can evaluate it, this reconstruction presents several inconsistencies.

The term of suture is confusive. A "suture zone" is a plate boundary, often outlined by ophiolites corresponding to the trace of an ancient oceanic area. At the scale of fig1, it should be a line. It cannot be a wide area such as the one shown in this figure that rather represents the part of Gondwana that experienced the Variscan (late Devonian-Carboniferous) deformation.

What is the reason for distinguish a "Balkanic block"? it is also a part of the Variscan orogen with 2 oroclines in Poland and in the Alps. See for instance Geosciences 2022, 12, 65. What is AP basin? Fault names are not appropriate, see comments in the annotated pdf file.

Note that in Western Europe, the boundary between pink and brown areas is called the “eo-variscan” suture of Devonian age, the boundary between the brown and grey is the “Variscan” suture of Early Carboniferous age. Note also that the triangles must be in the opposite sense as Laurussia subducted below Gondwana. The Caledonian suture is not in the right place in W. Europe. In this area, the term Taconic is not used.

 

Section “4. Field observations” is rather short and thus lacks of convincing details. For instance, the three constitutive belts of the Appalachians must be shown in fig. 2.  In which way the Appalachians have been impacted by the Reguibat indentation must be clarified.

The section about the N. Mauritanides is very difficult to follow. The figures, map and cross section of the Mauritanides (Figs 5 and 6), are not consistent one with another. The patterns are different. This 4.2 section must be improved.

 In section 4.3, Two contrasted interpretations are proposed for the structure of the Adrar Souttouf Massif. The second, new interpretation is quite plausible, but here again, additional information for instance on the seismic profile would be very useful.

Some inconsistencies must be checked. For instance, line 258, if the deformed rocks are Ordovician in age, the deformation cannot be ascribed to Cadomian orogen.

The final architecture around the Reguibat massif presented in section 5 is not so different from that of the one represented in Lefort (1988) model. The authors insist on the symmetry or not, but the present tectonic boundaries result of recent erosion, the most important point is the direction of transport direction, ie lineation. Some information on these structures must be provided.

The last section (section 6) is a tectonic model. It is rather simple and elegant, but some flaws appear also. The strike-slip faults of the Adrar Souttouf in the pre-collisional stage are not shown. It cannot be said that during the collisional stage “the Rheic ocean was already closed” as the collision precisely corresponds to the ocean closure. This is just a question of terminology. Note that there is a difference in the sense of subduction of the Rhéic ocean. In the study area, the ocean subducted below Laurussia whereas in the Western European Variscan belt, from Iberia to Poland, the subduction was below Gondwana. The late-collisional nappes of Galicia in Spain, Massif central in France and S. Bohemia in Tchekia and Poland are antithetic with respect to the subduction. This is not at all a problem as the Rheic ocean subduction might have difference subduction senses. It might just be mentioned for a possible comparison. 

 

In conclusion, in spite of its interest, the manuscript is difficult to read for someone unfamiliar with the regional geology. All cited names are not always in the maps. There are many acronyms. In each caption, the acronym  list must be provided in alphabetical order.  The inconsistencies in the text and figures as mentioned here and in the annotated manuscript must be corrected before considering publication.

 A major revision is recommended.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

see some comments in the annotated file

Author Response

please see the attachment 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Editors, dear Authors,

Please find my review in the attached PDF.

Regarding the recommendation below, I find it hard to tell whether I should suggest "Accept after minor revision" or "Reconsider after major revision". Indeed, there is for me a major issue regarding the structural, tectonic and geodynamic interpretation (or implicit implications from the text and figures) in this paper.

Because in their first reply, the authors said "I do not intend to develop the relationships between the Laurentia and Gondwana and particularly the evolution of the Peri-Gondwana terranes", the paper could be accepted after "simply" removing most parts that refer to those relationships and correcting some inconsistencies in the text and figures related to that.

Now, if the authors want to keep their view, why not. But they then have to much develop and argue for their opinion, and that would need major revision to include it all.

I recall here I am convinced and satisfied with the core of the paper stating that the Reguibat Promontory imprinted the Appalachian Belt. I'm just unhappy with the tectonic/geodynamic implications suggested by the paper to arrive at that state.

I hope my review will be helpful.

Best regards

Comments for author File: Comments.pdf

Author Response

please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Many more comments have been made on the attached pdf and here I concentrate on what I perceive to be weaknesses in the MS:

1. The persistent use of terminology like “unit” to describe geological packages or tectonic elements does not help a reader come to terms with the proposed tectonic model or regional geology. Why not use terms like formation, group or supergroup to describe genetically or stratigraphically related sediment-dominated rock packages? This can be done even where the protoliths have been metamorphosed. Terms like western or eastern units are too vague and invariably a reader is asking east or west of what particular reference point or geological marker, and are we referring to Laurentian or Gondwana affinity.
2. It was unclear whether any of the sequences described form part of a passive margin sequence even though this seems likely given the tectonic setting of the southern Mauritanides and their northern counterparts. Is the Neoproterozoic-early Paleozoic sequence overlying Archean basement a good candidate to be a passive margin sequence that was then overthrust by the ophiolitic rocks and Laurentian terranes?
3. Map legends and cross-sections so not always match in terms of geological units shown or in colour tones. For example, in figures 5 and 6, the upper quartzite and main quartzite unit are not always differentiated or shown separately.
4. Just where does the gabbro/ophiolite unit (Dayet-Lawda; Figure 8) fit into the overall stratigraphy or scheme of things? If a fragment of oceanic crust as implied, could it not mark an important boundary between terranes of Laurentian as opposed to Gondwana affinity? Several cross-sections are included but it would help if stratigraphic columns were drawn up for the geology of the two lobes on either side of the Reguibat Shield to show how similar or different the geology is in the AKL vs ASL; owing to their position, they presumably have very similar geologies and stratigraphies that are largely if not entirely Gondwanan in affinity.
5. The overall geometry of the impacted margins is one of tectonic escape accommodated by strike-slip faulting as Gondwana impinges on (or indents) the Laurentian margin. Similar indentor models have been suggested for parts of the eastern Himalaya and are not too far removed from what Lefort was originally proposing for the Reguibat. Contrary to what is suggested in the MS about the degree of asymmetry in the ASK and ASL on either side of the Reguibat, the new model proposed by the authors is not that different from that of Lefort save for a modest change in the amount of clockwise rotation for the ASL block/lobe on the northern side of the Reguibat Shield. To this reviewer, that makes for far less interest than the history of indentation and the resulting Variscan thrust and fold geometry produced on the west African craton. As such, the title of the MS still captures the key aim of the authors and that is to explain how the fold and thrust belts of west Africa came into being.
6. In light of the previous comment, the use of terms like indentor, indentation might make a useful substitute for words like imprint and imprintor.
7. The different schools of thought about tectonic models in the opening sections of the MS appear too detailed and might be better discussed in the closing sections of the paper. And is it necessary to include two very different maps of the ASL (Figures 7 & 8) which might simply confuse a reader as to which one to focus on. Figure 8 is much more detailed and gives a reader a better appreciation of the geology than Figure 7.
8. Lastly, the MS would have benefitted from greater attention to detail as far as editing and the level of English are concerned.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English needs improvement

Author Response

1-I used the term of « Units »  because this term is commonly used in  the bibliography of the Adrar Souttouf and also in West Africa (see Lecorché’thesis). In the revised version, I tried to precise the contains of these units.

2- I don’t know if parts of this terranes belong to a passive margin but the Adrar Souttouf and the Akjoujt lobes come from the Mauritanide belts which include two Panafrican belts (Panafrican I and II) reworked during the Carboniferous times by the collision between the Senegalese block and the West African craton.

3-These figure have several times reworked following the differents reviews. In figure 5 and 6 we would like to be in accordance with the Lecorché’figures who worked in this area.

4-The Dayet Lawda unit with basaltic and ophiolitic rock is still controversed. According to Bea et al. these basic rock are related to a « rift » but according to O. Guillou et al. (in press) these rocks could be linked to a volcanic arc. In our first appoach we favor a back-arc basin. In any, case this unit will be studied in the Omar Guillou’thesis which is still in progress and I expect a stratigraphic column of this unit. Concerning the Akjoujt lobe, I did not work in this area and Lecorché did not publish a stratigraphic column of the area.

5- I agree that the Lefort model presented in 1988 is a key for the interpretation of this area but it has been neglected because the northern Adrar Souttouf lobe with a N-S orientation have not been evidenced.  But since 2006, with a lot of radiometric data, we understood that the Adrar Souttouf lobe belongs to the Mauritanide belts. Thus, the comparisons with the Lefort’model was not used before owing to the recently study of the Adrar Souttouf where military mines coming from the battle with the « Polisario », are still working.

6- The terms « indentor » or « indentation » are not familiar. Lefort use « imprint ».

7-We would like to present the different interpretations. Fig.7 is an aerial photography study and fig.8 the result of field trips.

8- The English language has been improved many times by the German and American co-authors.

NB : The modifications in the text are in red colour .

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I appreciate this revised verssion. The authors considerd well the reviewer's comments and suggestions. The paper is now acceptable for publication.

There are some minor formal flaws underlined in the attached ms

 

Comments for author File: Comments.pdf

Author Response

 

Corrections asked by reviewer 2 are in blue in the file submitted

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1. In general, corrections made by the authors make the manuscript much improved, and I consider the paper should now be accepted for publication with minor final editing (see comments in the PDF, and the points raised below).
2. Minor editing concerns in particular figure captions, and Figure.9.a, which is very impossible to read. I think however, a new capture of this Google Map view is easy to do with a much higher resolution.
3. The questions I had regarding the position of the “main” (ophiolitic) suture – in particular in Figure.3 - has not really been addressed. I still don’t know whether they want to have it in front of the Senegalese Block as per Figure.11, and thus corresponding to where the two lobes (AKL and ASL) are; or whether they want to have it back of the Senegalese Block (along the thrust) as implied by Figure.3.

Anyway. Because I am probably the only reader in the world concerned by this issue (nobody will notice), and because it is not the main topic of the paper, I guess we can ignore this point. In addition, the authors replied in their letter that:

I agree that theorical geodynamic cross-section presented in fig.3 could be presented as « Working hypothesis » even this interpretation has already been published in a previous paper devoted to the Variscan network (GSL, 2023).

And I am fine with that.

4. I’d like the terms “pre-collisional stage” (line.323) to be changed. It makes no sense to talk about collision prior to collision; better use early-collision stage. I acknowledge terms have already been modified in the sub-titles 6.2.1 (line.334) and 6.2.2 (line.340) regarding the “main-collision stage” and the “late-(or post-)collision stage”: good so.

Comments for author File: Comments.pdf

Author Response

Please see the attached file

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Some of the ideas and concepts in this MS are difficult to fully comprehend owing to poorly expressed English. Did all authors review the revised MS before submission, including those with English as a first language? The MS, particularly the sections attempting to resolve competing ideas about tectonic evolution, is no easier to follow than previously and could be greatly improved by further editing. Parts of the text in need of obvious revision are highlighted in the attached pdf but there are many others that are in need of attention.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The MS falls short in English expression to the effect that a reader struggles to understand the geology and differences between competing interpretations at the core of the paper.

Author Response

 

The Manuscript has been corrected following the recommendations of the reviewer for the 3eme round.

I also added my answers to the reviewer in a separate file.

Author Response File: Author Response.docx