Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

Grobe, Arne; von Hagke, Christoph; Littke, Ralf; Dunkl, István; Wübbeler, Franziska; Muchez, Philippe; Urai, Janos L.

doi:https://doi.org/10.5194/se-10-149-2019

Articles | Volume 10, issue 1

https://doi.org/10.5194/se-10-149-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-10-149-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 10, issue 1

Research article

|

18 Jan 2019

Research article |

| 18 Jan 2019

Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

Arne Grobe, Christoph von Hagke, Ralf Littke, István Dunkl, Franziska Wübbeler, Philippe Muchez, and Janos L. Urai

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Final revised paper (published on 18 Jan 2019)
Supplement to the final revised paper
Preprint (discussion started on 14 Aug 2018)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Comments to Grobe et al. paper', Massimiliano Zattin, 24 Aug 2018
- AC1: 'Author’s response - “Tectono-thermal evolution of Oman’s Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study Jebel Akhdar, Oman” by Arne Grobe et al.', Arne Grobe, 01 Oct 2018
RC2: 'Review of the Manuscript: Se-2018-78', Luca Aldega, 02 Sep 2018
- AC2: 'Author’s response - “Tectono-thermal evolution of Oman’s Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study Jebel Akhdar, Oman” by Arne Grobe et al.', Arne Grobe, 01 Oct 2018
EC1: 'TE comment', Federico Rossetti, 12 Sep 2018

Peer-review completion

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

AR by Arne Grobe on behalf of the Authors (26 Oct 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (29 Oct 2018) by Federico Rossetti

RR by Massimiliano Zattin (05 Nov 2018)

RR by Bruce Levell (12 Nov 2018)

RR by Luca Aldega (15 Nov 2018)

Suggestions for revision or reasons for rejection

This revised version of the manuscript shows an improved organization of the text. Main weakness about explaining the importance of the paper and its scientific significance for the Solid Earth audience has been solved. However, there are still a few points that need further details or should be considered.

1) I do not see too much improvement about describing solid bitumen data. Still there are only four lines for presenting original data and this seems to me too little when compared with fluid inclusion and thermochronology chapters. As solid bitumen data are important for calibrating burial history, I guess that readers would expect some more details.

2) In this manuscript, solid bitumen data are from different stratigraphic units (Sahtan Group, Kharaib Fm., Shu’aiba Fm., Nahr Umr Fm., Natih Fm., Muti Fm.) that cover a long time span from Jurassic to Late Cretaceous. Authors should present their calibrated thermal maturity curve fitting solid bitumen data associated to the 1D burial and thermal models. Using the thermal maturity curves by Grobe et al., 2016 that account only for solid bitumen data from the Natih Formation without implementing them with the new data is a missed opportunity to improve and strenghten the model.

3) I agree with the authors that extensional top-to-NNE shearing might explain that most of the ophiolite units could have been transferred to the Persian Gulf but the southern flank of the Jebel Akhdar dome is also bounded by SW-dipping listric normal faults (see Searle, 2007) that are coeval to extensional top-to-NNE shearing and downthrow ophiolite and Hawasina units against shelf carbonates. Where has all the material coming from the dismantling of 8-10km thick ophiolite overburden gone to the south of Jebel Akhdar dome?

4) I have already commented part of the discussion in my previous report and the arguments provided in this revision are still questionable.
Lines 580-584. Organic matter is more sensitive to temperature changes than clay mineralogy only in hydrothermal/geothermal settings or when the heating event is shorter than 1-2Ma. The burial of the sedimentary succession below the ophiolite (88 to 80 Ma) or the temperature increase prior to exhumation are not the cases. For instance, in the models shown, temperature is higher than 300°C for upper Jurassic rocks from roughly 60 Ma to 50 Ma that is quite long time interval to let both organic and inorganic thermal indicators recording that thermal event.
Also the lack of potassium during the evolution of mixed layered minerals cannot be considered as an explanation for the discrepancy between solid bitumen and clay mineral results because carbonate and siliciclastic rocks of similar stratigraphic ages in Aldega et al, 2017 show similar illite content in mixed layers I-S.
Lines 586-590. I think that apatite fission track (AFT) data by Saddiqui et al. (2006) could have been misinterpreted. AFT data by Saddiqui et al. (2006) from Vendian and Ordovician rocks (sample 8 and 9) have a reset age of 48 and 55 Ma respectively. This means that Vendian and Ordovician rocks should have passed the isotherms 110-120°C of the AFT system (Gleadow and Duddy, 1981) during exhumation at that time and therefore the temperature of the overlying rocks (from upper Jurassic to the Natih Fm) has to be lower than that. During 55 to 48 Ma, most of the burial curves of the 1D models shown in figure 8 intercepts red or green coloured fields indicating that temperature is between 200 and 300°C higher than the reset temperature of the AFT system. Furthermore maximum temperatures experienced by Jurassic rocks in your model are close to or higher than 300°C indicating metamorphic conditions but outcropping rocks are clearly sedimentary. Some clarification is needed.

5) figure and tables needs some revision (see points line by line below)

Abstract
Lines 18-19. I would delete these two lines.
Lines 21-23. Please state what is the main factor responsible for peak temperature evolution of passive margin units. Obduction? Heat advection?

Introduction
Line 56 – time-temperature history of what? Better specify

Tectonic setting
Line 126. Replace “constrains” with “constraints”

Stratigraphic sequence
Line 162-163 – Please provide a more detailed description of the Hawasina deposits. Many ZHe ages and some data for deriving peak temperatures for the southern flank of the Jebel Akhdar dome come from Hawasina deposits. Which unit and formation did you analyze? Which lithology?

Previous paleothermal data of the authocton
Line 179. Locate “Al Hamra” in figure 1
Line 187. Replace “shows” with “show”

Fluid inclusion thermometry
Line 246- Are FI-M1 and 2 quartz or calcite veins? Authors report both compositions in the text. In table 3 the hosting mineral is quartz. Please check it.
Line 255- delete “sedimentary” before “Hawasina nappe” and “°” before “km”

Numerical basin modeling
Line 298 – What do you mean for “The south of the foothills is unaffected by foredeep”? I would suggest to replace the sentence as follows if it is consistent with authors’ thought: “The area to the south of the Adam foothills is unaffected by foredeep sedimentation”
Actually for this area, available paleothermal data are from the Fahud and Natih fields. Please modify the sentence.

Basin modeling
Line 466- replace “at least 4-4.5 km” with “maximum 4 km”. Figure 9a and figure 10a should be corrected setting temperature at 140°C for the Jebel Qusaybah area and Ro% at 1.1. For the Jebel Qusaybah area, only data from Mozafari et al., 2015 are available.
Lines 503-504- provide reference

Burial history
Line 529- How can petrographical data constrain the thickness of sedimentary units? I would replace “petrographical” with “geological”
Lines 533-535. Replace “at least 4 to 4.5 km” with “maximum 4 km”. See comment above.
Line 542- Replace “was achieved” with “are available”
Line 555-556. Expand this part. Readers cannot be aware of models by Lutz et al. (2004)
Line 560. Pressure value of 320 MPa calculated from 10 km thick ophiolite units is different from that reported in table 3 (340 MPa). Please revise
Line 564-565. What is the mechanism or process that favours the temperature increase prior to exhumation? Heat advection? I would not talk about thermal equilibration. From your 1D models the temperature increase between maximum burial and exhumation is more than 100°C. Please discuss and provide further details. Furthermore replace “uplift” with “exhumation”.
Line 575. Replace “mixed illite-smectite layers” with “mixed layers illite-smectite”
Line 582. Quantify “burial was short enough” using your modelling results.
Line 592 -64 ± 4 Ma does not mark the time of deepest burial. Please revise.

Exhumation history
Line 649 – replace “has seen” with “experienced”

Conclusions
Line 695 – is 300°C a burial temperature due to ophiolite obduction or a peak temperature prior to exhumation?

Figures and tables
Figure 2. Replace “white circles” with “black circle” in the figure caption

Figure 3. Delete “*” and “+” in front of authors’ names in the figure caption. Please enlarge the peak temperature interval of figure 3 in order to contain the error bar of the Fiqa/Muti Fm.

Figure 5. Authors could plot also the data for the Natih and Muti Fms. showing the pressure correction needed.

Figure 9a and figure 10 a – The grey area for temperature constraints should be a single line set at 140°C as only 1.1 Vr% value (Mozafari et al., 2015) is available for the Jebel Qusaybah area. Modify the text accordingly.
Figure 9c. The grey area for temperature constraints should be between 289±3 °C (see table 1)
Figure 10c. Grey area derived from calculated VR values (6.69%) is missing in figure 10c (see table 1).

Figure 11. Please add temperature and vitrinite-equivalent ranges to the figure intercepting the degree of serpentinization and heat flow variations lines.

Figure 12. delete in the figure caption “* indicate times of overpressure formation”

Table 1. Replace “mean Temp.” in the last column with “temperature range” as you do not always provide a mean value. Please provide a range of calculated Vr values when they are associated to a temperature range. Why did not you convert bitumen reflectance data by Fink et al., 2015 into calculated Vr values and then into a temperature range? One data is between brackets. Why? Define it in the figure caption. Peak temperature ranges for the the northern flank of Jebel Akhdar dome in figure 3 are different from those reported in table 1 where a mean value is shown, Please modify values of table 1 accordingly.
Delete “measured at RWTH” from the reference by Mozafari et al., 2015.

Table 3. Please replace “sedimentary nappe” with “sedimentary units” or “tectonic units” in the figure caption. A Nappe has a tectonic origin and cannot be associated to sedimentation

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ED: Reconsider after major revisions (28 Nov 2018) by Federico Rossetti

Dear Authors,

Thanks for the revision of the manuscript.

Your revised version was reviewed by the previous reviewers (Rev#1 and Rev#2) and by a new one (Rev#3). Based on their reports and my own assessment, your manuscript has been improved and it is potentially a valuable contribution, but it still needs a further work of revision.

Critical issues deal with (i) the thermal structure and boundary conditions assumed for the basin modelling (Rev#3) and (ii) (still) the thermal maturity data, their cross-correlation and interpretation (Rev#2). These points are essential to better constrain and support the tecto-thermal evolution of the continental margin. On this regard, both reviews provide an effective guide to prepare a suitable revised version of the manuscript.

I would also suggest to place in the main text some of the representative T-t paths as obtained from the forward modeling of the thermochronology data. Some details are necessary to better illustrate the T-t modeling results of the different units shown in S4 and S5: which the boundary conditions? what relevant for the tectono-thermal evolution of the continental margin? Did the T-t forward modeling presented in S4 and S5 take into consideration also the available thermochronological data set shown in Figure 3? All these points are critical when discussing the "burial" and "exhumation history" in sections 5.1 and 5.2 and should be integrated in the burial/exhumation history of the continental margin presented in Figure 12 (that actually does not only describe the burial history as stated in the Figure caption of Figure 12, but also the exhumation history). In other words, Figure 12 (and the related text, lines 597-604) is the synthesis of the entire data set, which should also take into consideration the thermochronological data (exhumation history). See also comments from Rev#2 on this regard.

Since the amount of required revision is significant, I am returning back your manuscript with a decision of major revision.

When submitting a revised version, please also include a "Author's response file" that lists changes and describes how the manuscript has/has not been modified following the Reviewer's comments.

Looking forward for a suitable revised version,

sincerely,

Federico Rossetti

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AR by Arne Grobe on behalf of the Authors (19 Dec 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Dec 2018) by Federico Rossetti

RR by Bruce Levell (31 Dec 2018)

Suggestions for revision or reasons for rejection

The following two quotations are from the authors response to a discussion of the validity of the modelling approach using an initially cold ophiolite slab.

"As correctly stated above, we are not modelling the thermal evolution within the ophiolite (which is not necessary as the rocks of the carbonate platform show no thermal overprinting from above; a potential heating by a potentially warm ophiolite would in a maximal case effect the units of the sedimentary nappes only)."

"We have to keep in mind, that the manuscript focusses on the thermal evolution of thepassive margin and not the one of the sedimentary nappes or the ophiolite. Therefore, as stated by the reviewer, the “emplacement” approach is useful. However, if future work aims to focus on the ophiolite, a different modelling approach needs to be tested.
To avoid misunderstanding we changed the methods part and added: “Modelling ophiolite obduction as rapid deposition accounts for burial related heat effects in the carbonate platform underneath, but this approach does not allow temperature modelling within the ophiolitic or sedimentary nappes.”and “ This is based on the assumption that a possible heat source represented by the ophiolite itself is not affecting the temperature of the top of the carbonate platform (see discussion).”

I accept that the new data and the previous work summarised in the paper suggest a modest temperature increase in the autochthonous platform sequence and that this is reproduced by the model, but I am still left with the question as to how the thermal model adds value to the field observations and laboratory data. The model has been steered to produce a maximum temperature result consistent with the data from the autochthon, and it is interesting how this can be done in terms of the physical parameter set. But, by the author's own admission this does not explain the actual physical processes responsible ie the processes controlling the time-temperature evolution of the overlying nappes during obduction. Perhaps someone else will take up this challenge !

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ED: Publish as is (31 Dec 2018) by Federico Rossetti

ED: Publish as is (31 Dec 2018) by Federico Rossetti (Executive editor)

AR by Arne Grobe on behalf of the Authors (01 Jan 2019) Manuscript

Short summary

The Mesozoic sequences of the Oman mountains experienced only weak post-obduction overprint and deformation, and thus they offer a unique natural laboratory to study obduction. We present a study of pressure and temperature evolution in the passive continental margin under the Oman Ophiolite using numerical basin models calibrated with thermal maturity data, fluid-inclusion thermometry, and low-temperature thermochronology.