Tectono-stratigraphic correlations between Northern Evvoia, Skopelos and Alonnisos, and the postulated collision of the Pelagonian carbonate platform with the Paikon forearc basin (Pelagonian-Vardar zones, Internal Hellenides)

Review of

Tectono-stratigraphic correlations between Northern Evvoia, Skopelos and Alonnisos,

and the postulated collision of the Pelagonian carbonate platform with the Paikon forearc basin (Pelagonian-Vardar zones, Internal Hellenides, Greece)

by Rudolph Scherreiks and Marcelle BouDagher-Fadel

General assessment:

This manuscript provides a lot of very important information regarding the geology and tectonics of the Northern Sporades. This is a crucial area regarding for the internal Hellenic orogen that cannot be continuously followed south of Macedonia since it is covered by water of the Gulf of Saloniki. The Sporades are the only place in the Aegean domain where the innermost Hellenides can be studied. The area has been previously investigated by several authors whose publications are often difficult and partially even impossible to access. The only recent study available is that of Porkolab et al. (2019). However this study is limited to the geology of Skopelos island and the authors of this study follow older interpretations of the tectonic position of the NW end of Skopelos island differing from the view of the authors of the present study. The major strength of this manuscript is that an attempt I made to discuss all the islands of the Sporades and, particularly, to compare the geology of these islands with that of Evvia that has been studied in detail in earlier publications of the first author.

The main conclusion of the paper regards (1) the tectonic nature of the contact between a higher nappe consisting of Late Cretaceous limestones (Palouki nappe) above the Pelagonian substrate along a brittle-ductile shear zone (I would regard this zone as a kind of tectonic mélange), and (2) the large erosional gap of the Pelagonian substrate including the originally overlying obducted W-Vardar ophiolites that have been totally eroded during the Cretaceous, i.e. before the thrusting of the Palouki nappe in latest Cretaceous to Paleogene times. In contrast to previous authors and also in contrast to Porkolab et al., the authors of the manuscript deny the existence of a so-called “meso-authochthonous” unit (an awkward and old-fashioned expression for Late Cretaceous strata unconformably deposited onto the Pelagonian margin and/or its overlying ophiolites along an angular discordance), as well as the preservation of sizable remnants of the obducted ophiolites (these were almost completely removed by Late Cretaceous erosion). Personally I am tempted to follow their arguments although I would like to know more about composition and structure of the brittle-ductile shear zone (or mélange) underlying the allochthonous Palouki carbonates.

The major shortcoming of the manuscript regards the poor style of presentation. I also have some problems with the parallelization of the Palouki nappe with the Paikon unit in the boundary area between Greek Macedonia and Northern Macedonia that I comment below.

Style of presentation

I spend very considerable time trying to understand what the authors meant to write because of very poor presentation. I tried to improve the quality of presentation and made many detailed comments in an annotated manuscript that I enclose in my review. Particularly the introduction is very poor and only the informed reader manages to get an idea what the paper is all about. Also the introduction takes away many of the conclusions before the data are presented.

Data presentation is also pretty chaotic and full of repetitions. The text also suffers from nomenclatural difficulties because the authors use some very old-fashioned jargon. For example they us “Eohellenic” also for tectonic units (they are not the only ones) while “Eohellenic” just denotes an orogenic event related to the obduction of ophiolites. They also use “Vardar” in an indiscriminate fashion. “Vardar” is an ill-defined term invented by Kossmat during the first world war and only later became a name for a kind of root zone of ophiolites. The reviewer tried to make some order and refers to West-Vardar ophiolites (obducted onto Adria) and East-Vardar ophiolites (obducted onto the European margin), and used the term Sava suture to denote the suture between Europe and Adria that deformed in the Early Cenozoic. The figures embedded in the pdf have no numbers and the figure legends are incomplete.

What is the Palouki nappe and to what unit does it belong; what is the nature of the brittle-ductile shear zone; what about the NW tip of Skopelos island

The Palouki nappe undoubtedly tectonically overlies the formerly obducted West-Vardar ophiolites that are still preserved as small bodies enclosed in the tectonic mélange (or brittle ductile shear zone as the authors call that tectonic mix below the Palouki-carbonates. In this sense, the position of the Palouki nappe is very similar to that of the Paikon unit. However, I would avoid using the term Paikon unit in the case of the Sporades, because these two areas are very distant from each other. The typical Jurassic-age island-arc sequences of the Paikon unit are absent in the Sporades. Instead we have Late Cretaceous carbonates that, admittedly, are also present in the Paikon area, but only at the contact with the Almopias ophiolites. The derivation of the Palouki carbonates from the European side (“Rhodopian” in the paper by Porkolab et al, Eastern Vardar and Circum-Rhodope units in the sense of Schmid et al. 2020) is indeed a valuable working hypothesis, but no more. It does not justify using the name “Paikon-Palouki” nappe. The plate tectonic cartoons are o.k. but, as I wrote, just speculation at the moment.

What I miss most is a better description of the “brittle-ductile shear zone” that should by no means be called a “formation”. I infer from the rather lengthy description that this tectonic mélange encloses real ophiolitic material (e.g. serpentinites) as well as continental material (i.e. phyllites, or better “phyllonites”). Hence I would call it mélange because it mixes rocks of different origin. This typically occurs either at the base of an obducted ophiolite (probably not the case in the example of the Sporades) or in a suture zone such as the Sava suture zone of Schmid et al. (2008 and 2020). It would also be nice to know the direction of transport (top SW according to Porkulab et al, but no data are given by the authors of this manuscript.

I also miss clear evidence for the statement of the authors that the Glossa series of NE Skopelos island represents the older substrate of the Pelagonian Mesozoic rather that a “Eohellenic” klippe. Looking at Google Earth I have the impression that the authors of this manuscript are correct but they do not provide good arguments nor are their map and cross section 1 on Skopelos explicit about this issue.

Biostratigraphy

Fossil findings are presented but the importance of these findings in the general geological and tectonic context is not given at all. Table 1 stands isolated on its own and the findings are not integrated. Even the ages inferred are not given in some cases (locality A is Cretaceous, but what does updated mean?; localities B,C and D just give fossil findings and the reader is left alone in finding out the age of the strata analysed).   From which tectonic units are these fossils anyway?

Summary

I would really be glad if the authors would improve the manuscript a second time because the scientific content and importance of the conclusions merit such improvement without any doubt.

Stefan Schmid                   Zürich, February 3 2020

Review: Tectono-stratigraphic correlations between Northern Evvoia, Skopelos and Alonnisos, and the postulated collision of the Pelagonian carbonate platform with the Paikon forearc basin (Pelagonian-Vardar zones, Internal Hellenides)

The manuscript presents important new fieldwork mainly on the islands Skopelos and Alonnisos and reviews the available literature for the tectonic model and stratigraphy. Oceanic crust rocks on the islands of Alonnisos and Skopelos have been reinterpreted to be part of a large shear zone that extends over the islands of the northern Sporades. The paper also reinterprets basement rocks on Skopelos as Triassic similar to Evvoia. The main conclusion is that the Cretaceous carbonate platform (Paikon-Palouki Nappe) was thrusted over the old basement and is not in sedimentary contact. This has implications for the tectonic history and the paleogeography, and new ideas are presented to explain the observed findings. I consider the work presented as very valuable, showing the importance of new fieldwork and analytical work. The stratigraphy of the islands is controversial in the literature with various different interpretations that are well summarised in the manuscript, which enables the reader to follow critical new interpretations.

However, I have a few points I would like to raise in this review. Punctuation and English sentence structure need a further proof read. I have suggested some changes for the authors to consider. I also would suggest that the authors add more context and background so that the reader can better follow the steps from the results presented, to the interpretation, and then to the conclusions. The manuscript also would benefit from a better organisation. At present some conclusions are already anticipated in earlier parts of the manuscript.

Introduction

Page 1: Basement in Glossa. It would be useful to modify Fig. 1b to give more attention to the basement in Glossa and its correlation with Evvoia.

Please help the reader in the Introduction what the study will be about and why. Without going into interpretation or conclusions.

Style of indention for new paragraph sections is not consistent. Most have an indent but, for example, the Geological background (please correct to lower case) section has not.

Geological Background and previous research

The section geological background could be expanded to present a better introduction of the Vardar zone and the tectonic history of the region, so that an audience not familiar with the current subduction model of Pelagonia and the Almopias ocean could better follow the text. For this section, a paleogeography map might help.

Page 2 last section on background: please move the brackets and change to (see Discussion-Pelagonian Basement) before the comma and delete the point after the brackets.

Disputed and modified stratigraphies of Alonnisos and Skopelos

Page 2 last section: add the b figure to …previous stratigraphies are compared to the new stratigraphy (Fig. 3a and b).

Page 3 line 14: ‘The conglomerate contains…’

Page 3 line 15: please add ‘and is in turn overlain by…’

Page 3: the laterite deposits are not in the stratigraphy Figure 3a. If the laterites are interpreted to be a product of metabasite alteration, it has to be stated.

The Paikon-Palouki nappe and the brittle-ductile shear-zone formation

It would help the reader to first establish what the Paikon-Palouki nappe is… e.g. Cretaceous carbonate platform, and not at the end of the paragraph. If the previous recognised Palouki nappe is introduced, it has to be explained what this is.

Page 3 line 36: please add the numerical age of the new Ar data for the reader.

Porkoláb et al. (2019) dated micas in the shear zone. Why would the micas in the shear zone give an age for deformation in the overlying nappe? Please expand to explain that the shear zone formation occurred during thrusting after deposition of the Cretaceous carbonate. Or was the nappe later thrusted over the shear zone? Also, it hasn’t been established in the manuscript that the nappe is deformed.

Page 3 please change to: …rocks on Alonnisos and Skopelos have previously been observed, including…and/or were interpreted as…

Page 3 please change to: A greenschist of Kira Panagia has been interpreted as a metabasite or as arc-tholeiite basalt-rhyodacite…

The Brittle-Ductile Shear-Zone Formation

Page 4: it is implied that the shear zone grades into the Cretaceous carbonate. Please clarify what the structural contact is. Is the carbonate part of the shear zone, or does it overlie it with a fault contact?

Page 4 line 26: insert brackets for Higgins reference.

Description of the geologic maps and cross sections of Alonnisos and Skopelos

Geology and tectonic sections of Alonnisos

Please remind the reader what the Eohellenic nappe is, that had been removed.

Page 4 please change: Our new geological map of Alonnisos mainly differs in two areas from the previous map(s)…

I assume the two areas are the disputed areas marked red on the map. If so please refer to those in the text. It would also help to state what was corrected in these areas.

The next two sentences which start with Frank (1997) need to be rewritten. What is the Mourtero series that is referred to here? Please explain. Why is this series in the mapping section of Alonnisos especially named, but no other stratigraphic unit discussed?

Please move the Skopelos map sentence to the Skopelos section below.

The Pelagonian basement of Skopelos

Page 4: Please give context to what the two series are (at present the reader has to look into the figure).

Page 5 please add: “Eohellenic ophiolite outliers on Alonnisos”

Page 5 typo:  ‘occurred’

Page 5 typo: Early Cretaceous

Page 5 line 11 typo: spelling of Porkoláb et al. (2019).

Page 5: please mark the important thrust fault on the map more clearly.

Description of the tectonic sections

Page 5: Barker reference in blue

Page 5: What are the Vardar formations?

The allochthonous Cretaceous and subjacent shear zone

Page 5: remove brackets around the reference name

Biostratigraphy

Please help the reader in adding formation or series names to the analysed microfossils and consider reordering from old to young. It could follow a short summary showing which formation has which age.

Page 6: Cladocoropsis (in italics) finding.

Page 6: Matarangas (1992) reported Actinoporella podolica Alth in…

Please add sentence end point after (BouDagher-Fadel 2008, 2015).

Radiolitidae and Hippuritidae in italics

Geochemistry of the shear zone formation

Page 7 line 10, please add: and (before the mineral grains of plg)

Fig. 4b: please change the symbol for Glossa or the shear-zone metabasites (hard to differentiate)

Please clarify: …mostly of a MORB origin but many indicating IAB provenance (a few indicate IAB?)

Discussion

The shear zone formation

phyllic alteration (lower case)

The geotectonic evolution of the study area

A paleogeography map (not just the cross section presented) would help the reader to understand where the carbonate platform was formed.

Subductions of two slabs of oceanic lithosphere

What was the original interpretation of Bijwaard et al. and Hafkenscheid for the two anomalies?

As far as I understand the figures X was an earlier subduction phase (or contemporaneous to Y), but did the slab break-off and subduction cease while Y was still subducting? If so, I would like to ask the authors to add a few sentences to better explain their model for an audience that might not be too familiar with the subduction history of the area.

Page 10 line 5: bracket before Fieldwork is not closed.

Tim Breitfeld