Volcanism straddling the Mio-Pliocene boundary on Patmos (East Aegean Sea): Insights from new 40Ar/39Ar ages
Abstract. The island of Patmos, in the eastern Aegean Sea, consists almost entirely of late Miocene to Pliocene volcanic rocks. The magmatism in the Aegean is governed by subduction of the African plate below the Eurasian plate, back-arc extension, slab roll-back, slab edge processes and westward extrusion of central Anatolia to the west along the Northern Anatolian Fault into the Aegean domain, The evolution of the Aegean basin is that of a back arc setting, with a southerly trend in the locus of both convergent tectonics, and back arc stretching, allowing intermittent upwelling of arc, lithospheric and asthenospheric magmas.
Here, we present new 40Ar/39Ar age data for Patmos and the nearby small island of Chilomodi to place this volcanism in a new high resolution geochronological framework. High resolution geochronology provides a key to understanding the mechanisms of both the tectonic and magmatic processes that cause the extrusion of magma locally, and sheds light on the tectonic evolution of the larger region of the back-arc basin as a whole.
The volcanic series on Patmos is alkalic, consistent with a back arc extensional setting and ranges from trachybasalt, to phonolites, trachytes and rhyolites, with SiO2 ranging from 51.6–80.5 wt.% and K2O from 2–11.8 wt.% with extrusion ages ranging from 6.59 ± 0.14 Ma–5.17 ± 0.11 Ma. Volcanism on Patmos and adjacent Chilomodi can be understood by a combination of mantle and crustal tectonic processes including influence of transform faults and rotational crustal forces that also caused the opening of the south Aegean basin due to roll back of the subducting slab south of Crete.
Katharina M. Boehm et al.
Status: open (until 21 Jun 2023)
- RC1: 'Comment on gchron-2023-11', Uwe Ring, 03 Jun 2023 reply
Katharina M. Boehm et al.
Katharina M. Boehm et al.
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The manuscript by Boehm et al. describes and analyses volcanic rocks from the Island of Patmos in the eastern Aegean Sea region, provides Ar/Ar ages for the volcanic rocks and then attempts to interpret the data in the context of Aegean tectonics. I am not an expert on the geochemistry of volcanic rocks and Ar dating. I assume another reviewer will have the expertise to evaluate those aspects of the manuscript.
My main concern is how the authors describe and interpret the tectonics of the wider eastern Mediterranean region. I truly believe that they need to seriously improve on that for being able to put their, presumably good and detailed, geochemical and geochronologic results into a tectonic context. Therefore, I am suggesting major rewriting and hence major revisions.
In my opinion, the authors need to add a section on ‘Regional Setting’, or something like that, and need to better understand the regional structure and tectonic development of the region. They would then need to decide, which tectonic processes are important for discussing their data. I would think that the various tectonic units they haphazardly introduced in section 4.5. in the Discussion are not needed. This section is utterly confusing and impossible to follow. They would also need to be clear about the ‘asthenospheric window’ that underlies western Turkey and how it may, or may not, relate to the Patmos volcanic rocks (as Patmos is about 150 km west of this anomaly). Western Turkey underwent a distinctly different tectonic evolution then the adjacent Aegean Sea region (see review in for instance Gessner et al. 2013, Gond.Res., https://doi.org/10.1016/j.gr.2013.01.005). One could argue that those different evolutions need some sort of transition zone in between them and Patmos might be part of this transition zone? Jolivet et al. 2015, Tectonophysics, https://doi.org/10.1016/j.tecto.2015.08.004, propose a different model (which in my opinion is harder to reconcile with the ages Boehm et al. report). The authors do not need to buy these models, but they may provide a starting point to put their data into a tectonic perspective.
Finally, I consider it also important to better review and define critical age data of the tectonic processes that may help to interpret their data. Usually, literature is cited that does not report any geochronologic data.
I hope my comments below will help to rewrite the manuscript.
Stockholm, 3 June 2023
l.20: please be clear what ‘rotational crustal forces’ are.
Intro: I understand that most of the authors are not experts of eastern Mediterranean geology. Ok. However, they need to be a bit more careful with their review of the tectonics of the region, especially the ages of processes. In the submitted manuscript this aspect is not dealt with adequately. I made a number of comments that may help to rewrite the Introduction.
l.24: I would mention slab tearing (apparently later in the manuscript referred to as slab fracturing, which is a term not really be used) in the second sentence, which also needs references (Biryol et al. for instance). The next sentence is too simplifying as not only continental platform sediments were subducted. There is also Carboniferous basement, Triassic granites, presumably late Cretaceous oceanic lithosphere etc. subducted to high-P conditions at various times between about 55 and 30 Ma (e.g., Glodny and Ring 2022, ESR, 10.1016/j.earscirev.2021.103883).
l.36: Please note that the central Aegean Sea is made up of numerous core complexes, it is NOT one single core complex. Western Turkey is slightly different but the Menderes Massif is also NOT a giant core complex (e.g., review in Gessner et al., 2013).
l.38. I wonder where ‘middle Miocene age’ for extreme thinning is coming from? The references provided are not adequate as not a single of these studies reports age data. There are numerous fission-track cooling ages in the central Aegean Sea region (e.g., summaries in Ring et al. 2010 (DOI: 10.1146/annurev.earth.050708.170910), 2017, op cit.); the onset of extension in the Aegean and western Turkey dates back to about 23-34 Ma (see for instance review in Gessner et al., 2013, and references therein).
How do you know that Patmos had thinned lithosphere (note that lithosphere involves the crust) by the end of the Miocene?
l.40ff. The Cretan Sea basin formed earlier in the middle Miocene, see Drooger, C.W., Meulenkamp, J.E. (1973). Stratigraphic contributions to geodynamics in the Mediterranean area: Crete as a case history. Bulletin of the Geological Society of Greece, 10, pp. 193-200.
l.42ff: The central Menderes metamorphic core complex formed in the Pliocene (e.g., Gessner et al. 2001 (Geology 29 (7), 611-614).
The authors take it a bit too far here. Judging from their Fig.3, ages from Patmos are, within error, up to about 7.1 Ma. Saying in l.205 ‘no evidence for >6.0 Ma’ is therefore simply wrong. To me, the ages of Boehm et al. are in agreement with the earlier work.
Section 4.3. is also a bit arm-waving. Apparently, the phonolites are considered important. Only because they are silica undersaturated? Or because they usually occur in intracontinental settings (note that Patmos represents an intracontinental setting)? The only age that is being used in this section is the 14.12 Ma of Altunkaynak et al. 2010 from Foca, an island about 150 km NNE of Patmos. Maybe this age should be reported in the Intro and section 4.3. dropped?
I am a bit confused. In section 4.2., it is mentioned that Fytikas et al. (1976) reported ages of about 7 Ma for trachytes. In section 4.4., the authors state that phonolites are the oldest volcanic rocks, followed by trachytes, but the trachytes apparently provide the oldest ages.
Referring to the very young, intra-plate Kula volcanics is a bit haphazard here. The authors are mixing, also in previous sections of the Discussion, volcanic rocks that developed above the slab tear (asthenospheric window) mapped by Biryol et al. 2011 and the Patmos volcanics, which formed about 150 km W of the slab tear.
The authors should be a bit more careful with their terminology. ‘What is ‘slab fracturing’? I assume the authors refer to the ca. 300 km wide ‘asthenospheric window’ in western Turkey, which a slow wave speed anomaly that is commonly interpreted as a tear in the African plate (Biryol et al. 2011).
In l.248-254 a tectonic subdivision of the Aegean/Menderes region is casually weaved in and terms like Lower and Upper Cycladic Blueschist Nappe, Amorgos unit, Menderes cover sequence, Ören unit, Afyon unit, Dilek Nappe and Trans Cycladic thrust are being used without any context and explanation. This is absolutely not acceptable and utterly confusing. The Lower (and Upper) Cycladic Blueschist Unit (and the Trans Cycladic Thrust) are a concept, first introduced by Grasemann et al. 2018, GSAB (DOI: 10.1130/B31731.1) and these two units and the thrust are defined in the western Cyclades. Whether or not the Lower and Upper Cycladic Blueschist Units can also be distinguished in the eastern Aegean Sea region is unknown and the speculative correlations by Roche et al. (2019, op. cit.) are not being backed-up by data (see Glodny and Ring 2022 for a different view). Menderes cover sequence is something that has been introduced in the middle of the last century but not being a sound concept these days anymore. The standard reader does not have a clue what Amorgos unit refers to (Laskari et al. 2022, https://doi.org/10.1016/j.gr.2022.02.007). All these units would need to be properly introduced and discussed. In the current manuscript the reader is wholly confused and lost.
L.256: Now exhumation is early to middle Miocene in age. In the Intro ‘extension’ was supposed to be middle Miocene in age. Note that most of the exhumation of the Cycladic blueschists occurred much before the Miocene and had nothing to do with extensional deformation.
ln the following (l.256-264), things become more than vague. The phase of relatively young ‘N-S shortening’ does most probably not exist and is not being accepted by people working in the Menderes massif. Why is the North Anatolian fault and the Peleponnese important here? There is no tectonic context provided and all things of things are thrown into the mix.
L.260: see Gessner et al. 2001 (Geology, see above) for actual age data of this renewed phase of N-S extension, which created the Central Menderes Metamorphic core complex. Neither Westaway nor Bozkurt and Mittwede report any age data.
L.267. The E-W shortening phase in Samos is actually well dated between <8.6 and c. 9 Ma by Ring et al. (1999, op cit.).
l.266ff: Most of the info here is literature review and should go into an introductory section on ‘Regional Setting’.
As mentioned above, I believe the authors need to completely rewrite section 4.5. if they wish to provide a tectonic interpretation of the Patmos volcanics (see general comments above).
Fig.1: The map is largely ok; however, there are a few structures and tectonic subdivisions that would need to be discussed properly as they are controversial. The Mid-Cycladic lineament (MCL in Fig.1) is a concept first proposed (if I remember correctly) by Walcott and White 1988, https://doi.org/10.1016/S0040-1951(98)00182-6. Nobody has ever seen this lineament and there is no direct evidence for its existence. It is just a concept a few people believe in and use, while most people simply do not believe in its existence. This needs to be made clearer. One cannot create a fact out of something that is not understood and controversial. The IBTZ (not explained in the figure caption) is a similar problem. The same is true for Lower (and Upper) Cycladic Blueschist Unit (see above).