Deformation Pattern of Well-Preserved High-Pressure Rocks (SE Syros, Cyclades)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript "Deformation pattern of well-preserved high-pressure rocks (SE Syros, Cyclades) " by Nikolaos Gerogiannis, Eirini Aravadinou and Paraskevas Xypolias, reports field data and observations of the Blueschist Unit cropping out in south-east Syros (Cyclades, Greece). The work is generally well structured, well written and presents some interesting field observations. However, I have some major comments about the study (see below), especially on the scale of the field observations and consecutive interpretations on the exhumation mode of the Blueschist Unit within subduction zone.

In my opinion the work needs major re-working and can’t be accepted for publication in its actual form. I would still recommend the authors to resubmit their work for publication after addressing and implementing my remarks and comments written below:

 

 

Major comments:

 

1.     There is a lack of integration of your observations, made from a really restricted area on Syros (<10% of the surface of Syros), with the rest of the geology of the island and more generally with the Blueschist Unit observed on other Cycladic Island. For example, I understand that you haven’t observed intense greenschist deformation in the very restricted area of greenschist that you studied on Syros but there is no discussion at all about the greenschist deformation that has been widely described on other Cycladic Islands (e.g. Tinos and Sifnos) and which is not considered in the proposed exhumation model. It is well documented that the exhumation of the Blueschist Unit within the greenschist facies P-T conditions was accommodated by post-orogenic detachment systems (NCDS and WCDS). These detachment systems don’t crop out on Syros but this is not a reason to not consider this at all in your exhumation model of the Blueschist Unit. Other studies have described greenschist-facies deformation on Syros and Sifnos, for example, and mentioned that deformation tends to be more and more localized during exhumation, which can explain why columnar calcite and HP parageneses can be preserved locally. For example, in our Geochimica et Cosmochimica Acta paper (Laurent et al., 2021), we show that ⁴⁰Ar/³⁹Ar ages measured in overprinted greenschist-facies sample on Syros and Sifnos are controlled by syn-greenschist facies deformation intensity. I think you are correct in saying that fluids played a major role in greenschist overprinting and I don’t think there is a big debate here. Note that it is also commonly accepted that shear zone and deformation in general facilitates fluid infiltration and circulation, which means that pervasive fluid infiltration in the greenschist unit is likely to be enhanced by deformation. Stating that the exhumation of the Blueschist Unit in the greenschist-facies conditions is static (or nearly static) go against the well accepted model of exhumation mostly accommodated by the NCDS and WCDS without even mentioning it.

 

2.     I think the model shown in Figure 9 has a major problem. With the proposed geological structure of the Blueschist Unit and if overprinting was only static, due to fluid infiltration from below, then we would expect to see a gradient of greenschist overprinting from bottom to top and not a sharp transition between rocks that are fully overprinted (or very close to) at the base to suddenly very-well preserved eclogite and blueschist rocks on top. This sharp transition has been proposed as evidence of the presence of a major shear zone between the greenschist unit and what you call the meta-sedimentary structure (Laurent et al., 2016; 2021). You might disagree but still have to explain why we observe a sharp transition (very well illustrated in your Figure 9d) and not a gradient of greenschist overprinting.

 

3.     In lines 375-377 you mention about the ‘abundant presence of preserved blue amphibole needles defining the lineation within greenschist’. Here again, I think your result are biased by the very restricted area of greenschist cropping out on Syros that you mapped. While I agree that locally and rarely HP evidences are preserved (e.g. preservation of glaucophane) as mentioned in previous works (e.g. Laurent et al., 2016, 2018), this is far from being ‘abundant’ in the greenschist unit overall. Note that you are not reporting these HP occurrences in greenschist on your map so your arguments are not evidenced. Same remarks with the ‘extensive occurrences of columnar calcite’ (line 369) within greenschist, while you show one example in Fig. 5e, you are not mapping these occurrences, so your statement is not supported with data.

 

4.     I also have some issues with the model shown in Figure 10. First, I’d like you to comment on the vergence of the subduction represented on your figure with what looks like to be a southwestward plunging slab while it is more commonly accepted an approximative northward plunging slab for the Hellenic Subduction Zone.

I also had difficulties to understand how the model describe in the text relate to the model shown in Fig.10. In the text you seem to consider that the Vari Detachment only accommodated the exhumation in the greenschist facies P-T conditions (i.e. not the HP exhumation in eclogite to blueschist facies), but on your figure, this detachment seems to be a major structure that would also accommodate exhumation at high-pressure (looks like the roof of the subduction channel for me even if it is not clear if you consider the Upper Unit as the overriding plate or as a subducted nappe on the figure). This is not clear for me. There is a need to significantly improve this model before publication. You need to make a clear step-by-step reconstruction of what you think is the model of exhumation of the Blueschist Unit, from peak P-T conditions, to GS-facies exhumation and up to present day. You need to draw these reconstructions at slightly larger scale to show the full extent of the subduction zone. Clearly explain on what evidences your reconstructions are based (e.g. which observations confirm the existence of a top-to-NE basal thrust between the Blueschist- and Basal Unit). As mentioned above, your model needs to consider or discuss previously published data constraining the exhumation history of the Blueschist Unit (field structural, petrological, geochronological…). You need to much better discuss other models and clearly explain why you disagree or agree.

 

I have lots of other more minor comments and remarks to make on the text and on the figures that I am not going to list in this review as I recommend the authors to first address the major comments raised above.

Comments on the Quality of English Language

English language is good overall with only some minor editing that would be required.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This nicely written and illustrated paper reports new structural observations on the famous Cycladic Blueschists cropping out on Syros Island, in the Cyclades. The work is focused on the SE part of the island where the complete section can be observed. A new map is proposed and a new interpretation of the exhumation mechanism discussed. The rationale is based upon (1) a reinterpretation of the stratigraphy of the metamorphic units and (2) an analysis of the deformation history.

 

The authors draw detailed cross-sections involving large-scale folds refolding the original stratigraphy with a metasedimentary sequence on top of a meta-igneous sequence. This superposition of these two simple sequences allows drawing these folds. The final model involves an extrusion wedge within a top-NE thrusting within a SW-dipping continental subduction, which departs from previously published models, including those involving top-NE syn-orogenic detachments (like those our team has long proposed).

 

Although the manuscript is clean and easy reading with nice informative illustrations, I am not convinced by the new interpretation. 

 

(1)   The superposition of just two simple metasedimentary and meta-igneous units is an assumption, not a fact. The field indeed shows alternations of these two lithologies that the authors interpret with isoclinal folds of the same contact. But these alternations could very well be partly original, just as the alternations of metapelites with marbles. With this in mind we have always avoided drawing large folds, except where they can be clearly seen in the field like in the very north of the island. Isoclinal folds are indeed everywhere, but I do not think one can prove that no original alternation exist, which makes the cross-sections doubtful. This is why we have always based our interpretations upon the mapping of foliation, lineations and kinematic indicators, together with metamorphic grades. The distinction we make between the Kampos, Chroussa and Posidonia units are based on these criteria. They also agree with the overall differences in lithologies with lots of metabasites and well-preserved eclogites in Kampos unit at the top, alternations of marbles and metapelites with minor metabasites in the intermediate Chroussa unit with a good preservation of the blueschist-facies parageneses with minor eclogites, and, finally, mostly metapelites and minor metabasites, with some basement below, in the lower Posidonia unit, almost totally retrograded in the greenschist-facies. If the authors wish to stick to their interpretation of the original stratigraphy, they should explain the observed distribution of the protoliths and metamorphic parageneses. They have to explain the strong contrasts in metamorphic evolution seen across the main contacts we interpret as detachments with a global increase of retrogression downward. One such top-east contact goes across the area studied in this paper and is, in my opinion, overlooked, like in other recent interpretations.

 

(2)   The structural data, other than the folds, in terms of phases and kinematics of deformation are not new. The main top-east shearing within the blueschist-facies has long been described. It actually is mainly associated with the blueschist facies, nothing really shows that it was already active in the eclogite-facies.

 

(3)   The authors’ conclusion of a static retrogression in the greenschist-facies in the lower unit is not compatible with the many evidence of stretching of greenschists-facies parageneses and associated top-east sense of shear. See between Kini and Delfini for instance.

 

(4)   The presence of “columnar calcite” which are probably former aragonite, does not mean that no deformation has happened afterward. The deformation is obviously localized along shear zones, and more and more during exhumation, thus leaving entire domains preserving older structures. Then, this peculiar facies is seen all over the place, sometimes as layers, sometimes within former pebbles, like in Tinos. But I agree that the Kampos unit is poorly affected by the greenschist-facies deformation. At the time when the Posidonia unit was exhuming and deforming in the greenschist facies, the Kampos unit, and part of the Chroussa unit were rigid because strain had localized underneath.

 

(5)   The model of an extruding wedge can be discussed. This is what we have done when we interpret the Eocene exhumation, preserving the HP-LT parageneses, as “syn-orogenic” with a thrust at the base, observed in Ios (see Huet et al.) and a top-east detachment at the top below the Vari Unit. I do not understand how the proposed model can explain the main top-east shearing deformation acknowledged by the authors. This top-east shearing is seen until the very base of the Vari Unit and even within its base along Fabrika Beach with the retrograde amphibolites.

 

(6)   This model does not either explain the observed P-T-time paths with younger ages in Posidonia unit, and a general downward younging known since the work of Wijbrans et al. and precised in Laurent et al. more recently. The distribution of metamorphic parageneses and the geochronological record is, in my opinion, best explained by Eocene syn-orogenic detachments, grading into Oligocene and Miocene post-orogenic ones for the youngest deformation in the lowermost Posidonia unit. In order to better convince the reader, the authors should draw a series of reconstructions in cross-section view showing the successive stages and the progressive exhumation of the various units, sticking to the published P-T-time paths and kinematics of deformation.

 

(7)   My impression also is that the authors observations are in general excellent and the map good, but that they should consider the larger-scale context, at the scale of the whole island, and further at the scale of the Cyclades, including the P-T-time-deformation history at different scales. Important observations have been published at the scale of the entire Cycladic Blueschists showing the superposition of two main nappes, the upper one with eclogites and the lower one without eclogite, which suggests a north-dipping accretionary wedge and thus a N-dipping subduction (see the works of Grasemann et al., Roche et al…

 

My final impression is thus mixed. The observations are indeed interesting and the author’s assumption of simple original stratigraphy allowing to draw folds is worth testing. But the structural observations, other than these folds are not really knew and the model does not, in my opinion, explain some first-order observations in terms of metamorphism, kinematics and timing.

 

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The new version is clearly an improvement and several of my questions were addressed. I still have some disagreements though but I do not want to block the publication of the paper that brings a nice new map of an important area of the Cyclades where exhumation mechanisms have long been discussed. The two points which I disagree with are:

(1)  The fact that some parts of the greenschist-facies unit (Posidonia unit) are locally not deformed or little deformed is not an argument to rule out the deformation of that unit in the greenschist-facies in general. The characteristic of greenschist-facies is that deformation is localized because the temperature is low. The local presence of preserved blueschist-facies, or even eclogite-facies parageneses in the Posidonia unit or the Chroussa unit has been observed for a long time but it simply means that some regions have escaped low-temperature deformation, not that there was no deformation at all. Even in the high-strain greenschists facies shear zone of Tinos, very close to the Tinos detachment, can we find perfectly preserved lenses of blueschists. Even in the migmatitic gneiss with strong amphibolite-facies and then greenschist-facies deformation along one of the largest detachments worldwide in Norway can we find preserved boudins of eclogite that have locally escaped deformation. We have observed evidence of greenschist-facies deformation everywhere and we have also observed those preserved chunks of preserved blueschists and eclogites.

(2)  The second debatable point is the generalization of folding. Folds indeed are present everywhere as I already mentioned in my first review but some of the interpretations presented here are doubtful for me. The fact that a layer terminates at some point does not mean that we face a fold hinge. Many of these terminations are due to boudinage in Syros, a fact that is often neglected in Syros. The stratigraphy is also not simple and alternating lithologies can be original not by folding.

 

Some more minor issues:

Figure 8c: the columnar fabric is not clearly visible here, please chose a better photograph.

Lines 367-368: the new lineation maps is in perfect agreement in terms of directions with the one we published earlier (Laurent et al, 2016) so I do not see any contradiction here.

That said, I do not want to delay the publication of this paper any longer. My review is already very late and I apologize for this situation due to an overload of work. The authors have their right to prefer their conclusions, whether I agree or not. So, I leave it to the editor to decide the fate of this manuscript, I can easily accept that the authors do not wish to modify their conclusions. I then just only suggest to address the minor issues above before the paper is accepted. It is not necessary that I see the paper again.

 

 

Author Response

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Author Response File: Author Response.pdf