Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia

Gürer, Derya; van Hinsbergen, Douwe J. J.; Özkaptan, Murat; Creton, Iverna; Koymans, Mathijs R.; Cascella, Antonio; Langereis, Cornelis G.

doi:https://doi.org/10.5194/se-9-295-2018

Articles | Volume 9, issue 2

https://doi.org/10.5194/se-9-295-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-9-295-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 9, issue 2

Research article

|

21 Mar 2018

Research article |

| 21 Mar 2018

Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia

Derya Gürer, Douwe J. J. van Hinsbergen, Murat Özkaptan, Iverna Creton, Mathijs R. Koymans, Antonio Cascella, and Cornelis G. Langereis

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Final revised paper (published on 21 Mar 2018)
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Preprint (discussion started on 03 Jul 2017)
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Interactive discussion

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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'review', Anonymous Referee #1, 01 Aug 2017
RC2: 'Comments', Anonymous Referee #2, 15 Aug 2017
AC1: 'author final response se-2017-66', Derya Gürer, 27 Sep 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Derya Gürer on behalf of the Authors (23 Nov 2017) Author's response

ED: Referee Nomination & Report Request started (29 Nov 2017) by Federico Rossetti

RR by Aral Okay (23 Dec 2017)

RR by Anonymous Referee #3 (29 Dec 2017)

Suggestions for revision or reasons for rejection

The manuscript “Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia” presents an impressive amount of new paleomagnetic data for a very interesting and very complex area. I think their approach of obtaining as many paleomagnetic sites as possible represents the only way in dealing with such a complicated tectonic area and definitely deserves publication. It is well written, I think the geological introduction is sufficiently detailed, the structure might need improvement, especially in terms of a summary of the results, but I agree that the topic and results are quite numerous and complex.
On the other hand, however, I think there still remains a problem in the data treatment. My only major concern is the already mentioned treatment of individual sample directions rather than site mean directions. I agree with the authors that always applying the standard paleomagnetic procedures is not always the best thing to do and new approaches are sometimes reasonable and it makes sense to explore those. However, I have the feeling that this area and the presented data might not be the perfect place to introduce a “break with paleomagnetic tradition”. In my opinion, using individual directions rather than site mean directions is reasonable when treating with inclination shallowing, or when studying different parts of a continuous section. In such a tectonic complex area, however, local rotations might vary a lot on a small scale, which would require site mean directions. Then, a site with a small number of samples and high alphas (or As) is sometimes very important and equally necessary than another site with high quality results and a large number of samples. To exaggerate, treating 2 separated sites, one with 100 directions, and the other with 10, even if they yield mean directions which are statistically different, would yield a mean direction equal to the first site. The authors talk a bit about that, but I think a much more rigorous introduction of this approach would be necessary to justify it. A way to get around this, would be the presentation of regional mean directions based on both site means and individual means. A small table 2, which shows that the two methods are comparable, but the error is smaller (?) would leave everyone with the choice what to choose. I would say it is necessary to present the standard method at first, and then the alternative. The missing propagation of errors is true, but on one hand, error propagation exists and, I think, this would be a better way to treat this problem. The presented alternative approach does not do propagation either. One thing, which is also absolutely necessary for the presented method, is a clear discussion about structural data. Which sites have which bedding? So combining individual directions is based on sites with similar bedding? This needs further discussion.
In this context, I would like an additional discussion about block rotations versus more continuous deformation? I am not so familiar with the region, but it seems that there is an orocline in the north and one in the south with continuously changing declination (figure 8), but a more blocky behavior in the center. This might well be true, but a small discussion would be nice. Also the white block boundary in figure 8 might be a bit speculative. Maybe the red stripes for coherently rotating domains should be restricted to the center?
In the end, I think the authors have to decide what they want to do. Either they present a statement in favor of their alternative statistical approach, which would require more comparisons, a more straight forward discussion for the two different approaches, and a more simple tectonic setup. Or they tackle the tectonic history of the presented area, which would require at least also to show the site mean directions and do it the classical way.
It is definitely an interesting discussion the authors present here. Also in my opinion the Deenen et al. paper is a major step forward. However, if site mean directions do not fulfil the requirements, using individual directions need to be further justified. I don’t think that increasing n and decreasing error is always better.
Two more minor points are first, I miss a discussion about inclination shallowing. Because the authors are using arguments based on inclination, some discussion including figures about shallowing would be good. The directions do not seem to show much elongation, but given the amount of results, a site by site inclination shallowing inspection might yield additional information about primary or secondary (if shallowing is present, remagnetization is less likely).
Also, it is not easy for the reader to assess the actual outcomes of the study. It would be nice to see some additional summarizing figures, like e.g. D versus time or a sketch showing blocks rotating? Most of the rotation arrows in the last figure are already in the first figure. Figure 2 and 3 show all the results (right?). So, what did change with the presented results? Is there a consistent rotation over time? What is the rotation rate?
In summary, as I said, I think this is a very interesting data set, which deserves publication after some small revision (most of my comments are rather suggestions and I think not much more modification is needed anymore). I hope my comments help to improve the manuscript.

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RR by Anonymous Referee #1 (06 Jan 2018)

Suggestions for revision or reasons for rejection

The paper reports a large number of new paleomagnetic data which are of great interest for interpreting the tectonic evolution of the different Anatolia blocks.
However, the data analyses and interpretation are, in my opinion, not adequate and should be completely reconsidered before the paper be acceptable for publication. In the following I briefly discuss my main concern about data analysis.

1) Orthogonal diagrams show in many cases (Fig. 5f, h, o, p, r, s) no-linear path to the origin, whereas the selected magnetic component is fixed to the origin. Furthermore the MAD values, which appear quite large in some diagrams, are never reported in the text or in the figures. In the text the authors report that they interpret the demagnetization diagrams using “common sense and consistency of results”, which do not seem a reliable criteria for a scientific paper.

2) The calculation of the mean direction for each locality is done using the approach suggested by Deenen et al., 2011, which averages all the directions obtained in each locality and not the mean direction for each site. I completely disagree with this approach. In fact, as correctly stated by the Authors, in this method the weight of each site depends from the number of samples. As a consequence a site with 100 samples weights 10 times more than a site with 10 samples. If this is the approach I do not understand why, for each locality, the author decided to sample many sites with few samples instead to concentrate their sampling in a single site with a large number of samples. For example, in Aktoprak locality they sampled several sites with about 20 samples each, but the locality mean direction was in any case pre determined by the results obtained by the Mejiers et al section, which contains 313 directions. I completely miss the utility of this approach (except to apparently increase the statistical quality of the data). The use of standard approach in averaging site mean directions is essentially due to the fact that giving the same weight to each site of a locality, independently from the number of samples, we avoid the risk that the presence of one site with a very large number of samples pre determine the locality mean direction. The classical approach (to distribute a large number of sites, each one with a sufficient number of samples to avoid PSV concerns, in a wide area) allows to have a locality mean direction which be really representative of the locality (or basin) and not of a single site simply because we collected more samples from it.
3) I disagree to use a 45° cut-off for calculating the site mean direction. In most of the sites all the directions are within the 45°, but in some of them the cut-off eliminate more than 60% of the obtained direction. I suspect that the directions obtained from these sites are not really reliable.
4) I completely miss the criteria used to define if a site (locality) is magnetically overprinted or not. I totally disagree that a low A95 value be a criteria to assume that the site is remagnetized, unless the authors be very aware of the remagnetization mechanism, which does not seem the case. Magnetic overprint is a very complicated, and not fully understood, process which not necessary occurs in a time span not sufficient to average the PSV. This assumption seems also at odd with the fact that in some localities (Sivas, Kemah) or areas (Tauride) the remagnetized sites show both normal and reverse polarities which suggest that magnetic overprint occurred during a long time, which should be sufficient to average the PSV. I also disagree that a low inclination be a criteria to decide if a site (locality) be magnetically overprinted. Inclination flattening is a very known process, which can easily explain the low inclination values measured in some sites.
5) I think that the Author can produce a much better tectonic model than that reported in Fig. 8, which does not give any significant information about the tectonic evolution of the area.

I think that the approach used to analyse the data is substantially wrong and hide the strong scientific value that this large set of new paleomagnetic data could have for interpreting the tectonic evolution of the area.
Before the paper be considered for publication in Solid Earth I strongly suggest to the Authors to use a classical approach to analyse the data (using MAD with a well defined threshold value and not “common sense”; averaging site mean directions and not samples directions for each locality; do not use the 45° cut-off; to use classical field methods to define magnetic overprint instead of strange and unreliable criteria).

Please report the name of each site in Fig. 2 and Fig. 3.

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ED: Reconsider after major revisions (09 Jan 2018) by Federico Rossetti

AR by Derya Gürer on behalf of the Authors (17 Jan 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Jan 2018) by Federico Rossetti

RR by Giovanni Muttoni (06 Feb 2018)

ED: Publish subject to technical corrections (07 Feb 2018) by Federico Rossetti

ED: Publish as is (07 Feb 2018) by Fabrizio Storti (Executive editor)

AR by Derya Gürer on behalf of the Authors (14 Feb 2018) Manuscript

Short summary

Central and Eastern Anatolia (present-day Turkey) accommodated Africa–Eurasia convergence in Cenozoic times. As a result, the region underwent distributed deformation and rotation. We provide a paleomagnetic dataset from sedimentary basins and assess the timing and amount of rotations. The obtained rotation patterns together with known fault zones suggest that south-central Turkey represents a coherently counterclockwise-rotating domain.