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    Review of 'The closure of the Vardar ocean (the western domain of the northern Neotethys) from early Middle Jurassic to Paleocene time, based on surface geology of eastern Pelagonia and the Vardar zone, biostratigraphy, and seismic-tomographic images of the mantle below the Central Hellenides'

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    The closure of the Vardar ocean (the western domain of the northern Neotethys) from early Middle Jurassic to Paleocene time, based on surface geology of eastern Pelagonia and the Vardar zone, biostratigraphy, and seismic-tomographic images of the mantle below the Central HellenidesCrossref
    Attempt to link surface geology with mantle structure of NE Greece to infer tectonic history
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    The closure of the Vardar ocean (the western domain of the northern Neotethys) from early Middle Jurassic to Paleocene time, based on surface geology of eastern Pelagonia and the Vardar zone, biostratigraphy, and seismic-tomographic images of the mantle below the Central Hellenides

     Rudolph Scherreiks,  Marcelle Boudagher-Fadel (corresponding) (2021)
    Seismic tomographic images of the mantle below the Hellenides indicate that the Vardar ocean probably had a composite width of over 3000 kilometres. From surface geology we know that this ocean was initially located between two passive margins: Pelagonian Adria in the west and Serbo-Macedonian-Eurasia in the east. Pelagonia was covered by a carbonate platform that accumulated, during Late Triassic to Early Cretaceous time, where highly diversified carbonate sedimentary environments evolved and reacted to the adjacent, converging Vardar ocean plate. We conceive that on the east side of the Vardar ocean, a Cretaceous carbonate platform evolved from Aptian to Maastrichtian time in the forearc basin of the Vardar supra-subduction volcanic arc complex. The closure of the Vardar ocean occurred in one episode of ophiolite obduction and in two episodes of intra-oceanic subduction.
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      Review information

      10.14293/S2199-1006.1.SOR-EARTH.ABCEAF.v1.RQLZLN

      This work has been published open access under Creative Commons Attribution License CC BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at www.scienceopen.com.

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      Review text

      This paper presents evidence from field-based geology and related laboratory studies, with interpretation of mantle structure, for the closure of part of the Tethyan oceans formerly in the region of present-day NE Greece. It is many years since I worked in the eastern Mediterranean and surrounding region and I am not up-to-date on current models and interpretations. I found the paper reasonably easy to follow and it is clear that the authors are presenting some views that challenge some earlier interpretations. I consider that if I were to embark on a project in this region that this paper would provide a useful introduction to different views and an insight into the literature. However, I did feel that the material would benefit from some reorganisation. I cannot judge if the differences from earlier models are convincing, but they are reasonably clear. I have annotated the manuscript pdf with a few comments, identified minor typos, and made some suggestions.

      In the introductory sections the authors explain some terminology concerning the region and their usage of certain technical terms such as ophiolite and melange. Incidentally, it would be helpful to define “Eohellenic”; the term may be very familiar to workers in Greece but not so for other readers – me included, and is not explained.  They present a summary of the regional geological evolution which I assume is broadly agreed. They then present an interpretation of mantle structure in which they argue there are two major slabs, mainly in the lower mantle, which were subducted in the Mesozoic. This is important but seemed out of place in its present position.

      As I read and re-read the manuscript I felt it would benefit from a significant reorganisation. It would benefit from a consideration of what conclusions are to be presented at the end of the paper and then bringing together the critical evidence for each of the important points in the conclusions before the discussion. Here is a suggestion for an example layout, based on what is already in the paper, which could be re-ordered`: 1. An introductory section explaining terminology and key features of the regional geology. 2. A section on ophiolites, including geochemistry; where they are and what they are. 3. Carbonates: Pelagonian and Vardar – ages and differences. 4. Cataclasites versus sedimentary conglomerates. Why is this important and what are the implications for stratigraphy and deformation events. 5. Seismic tomography: Implications for the size of the Vardar ocean and the way in which it was eliminated by obduction and subduction. 6. Discussion. 7 Conclusions.

      In the introduction the authors say they use the term “ophiolite” in the strict sense of the “Steinmann Trinity”, but following this throughout the paper they seem to use the term in a more modern way – as oceanic lithosphere. Some attention is given later to the distinction between MORB and IAT which would seem to permit that some of the preserved ophiolite fragments could be arc or forearc lithosphere rather than ocean crust formed at a mid-ocean ridge (and is consistent with the model of an eastern arc). In the section on “Evvoia and Northern Sporades“ the authors report that parts of a large obducted ophiolite sheet (c.200 km wide) are present in Evvoia, but in the Northern Sporades there is no serpentinite and the former ophiolite sheet is inferred to have been removed by erosion. Is this supported by any evidence of the former ophiolite? Or is this based simply on the assumption that the ophiolite sheet was originally present all along the Vardar zone? Incidentally, as a reader only moderately familiar with the region I would appreciate inset maps showing locations of e.g. Figs. 4, 6 and 8.

      It would be helpful if Fig. 3 was moved in the MS closer to the section headed “Palaeogeography of the Vardar ocean decerned [should be discerned] from seismic tomographic images of the mantle below the Hellenides”. Given that this is an e-manuscript there would seem to be plenty of room for more and improved figures. 3b is too small to assess and would benefit from rotating the depth slices to the same orientation as 3a and made larger. The slices also need a colour velocity scale. 3d purports to show the two slabs in different other places, further east, but it is really not clear where these sections are, and what is their orientation. One slice (e.g. Eastern Anatolia) would probably be enough to make this point with some sort of map indicating its position. On re-reading his paper for the purposes of this review I felt that the seismic tomography could be improved by some reorganisation, in particular bringing all the mantle structure sections (section from line 204 to 220; section from line 609 to 668) together into a single section.

      One of the challenges presented by the authors is the suggestion that conglomerates in Almopias(?) are cataclasites not transgressional sediments. It would be very interesting to see some images of the conglomerates which allow the reader to judge the different interpretations. Plate 2 does not allow this. Their significance is apparently important in terms of regional tectonics. It would be helpful to spell this out.

      The discussion section should lead up to the model of Fig. 9. Figs. 4, 6 and 8 are critical in this respect. I did work quite hard to unravel the connections between these figures and was not satisfied I had succeeded. A-A’ on Fig. 6 refers back to a section of Fig. 4 – why not include a section for A-A’ on Fig. 6 like B-B’? I admit that readers who are working in this region may grasp more quickly the implications of these diagrams. Fig. 8 raised some questions for me. How do the sections a and b relate to one another? A inset regional map showing their locations would help. The arc rocks mentioned in the paper seem to represent the Vardar zone arc in the east. Why is there no arc associated with the long and large (1200 km shown on 9a) Eohellenic subduction? The conclusions seem to brush this aside as a “Late Jurassic a supra-subduction volcanic island arc and back-arc complex”.

      I am glad that I read this paper. I am sure that readers more familiar with this region will understand better than me many of the implications of the work and the challenges presented by the authors. However, writing as someone who has worked on arcs and subduction in SE Asia and the western Pacific for more than 30 years I have to say the ophiolites, subduction history and collisions in the Middle East Tethyan belt (where I once worked) continue to puzzle me. They bear little or no resemblance to the arcs and subduction history of the Far East. Thousands of kilometres of lithosphere in NE Greece are interpreted to have been subducted leaving little or no trace, and the traces that are left of this interval seem to be the predominantly carbonate remnants of platforms and post-arc build-ups. And it has become conventional wisdom that hundreds of kilometres of oceanic lithosphere have been obducted – by mysterious processes now unquestioned as there is a term “obduction” for this magic – and in my view a feature not seen anywhere in the arcs of SE Asia and West Pacific. It is good to see an attempt to link surface geology with mantle structure, but for me the Tethyan belt remains a mystery.

      My comments here are accompanied by a marked pdf which I hope can be made available to the authors and readers of the paper.

      Comments

      We are so indebted for Prof. Robert Hall for reviewing our paper and for his very constructive comments and corrections. We have followed them and the paper had been enriched by his input.

      2021-07-08 10:05 UTC
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