Rated 3 of 5.
Level of importance:
Rated 4 of 5.
Level of validity:
Rated 3 of 5.
Level of completeness:
Rated 3 of 5.
Level of comprehensibility:
Rated 2 of 5.
|Keywords:||Eastern Pelagonia; Paikon collision; ocean floor mélange; shear zone formation; Slab break-off, The Environment, Climate|
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)
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.
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?
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