Tectonic Evolution of the SE West Siberian Basin (Russia): Evidence from Apatite Fission Track Thermochronology of Its Exposed Crystalline Basement

Round 1

Reviewer 1 Report

Manuscript by Vetrov et al contains new high-quality data that will potentially be a significant contribution. However, a few points need clarification and polishing before acceptance for publication.

1. Vibe et al (2018) are not the only who compiled subsidence curves for the West Siberia basin. Similar studies have been discussed in many papers, e.g. by A.Nikishin, in textbook by Allen & Allen “Basin Analysis” (2013), etc. More discussion on the subsidence history has to be presented.
2. Sampled granite intrusions are postulated to be of Permian-Triassic age, but no evidence is presented. Have they been dated? Some explanation is necessary.
3. According to maps in Fig. 1, the Kalman massif intrudes Ordovician and Silurian rocks, whereas the Novosibirsk massif intrudes Carboniferous and Devonian rocks. If both areas have similar stratigraphy, I would expect more significant erosion of the Kalman massif in comparison with the Novosibirsk massif.
4. “Half-boomerang” plot in Fig 6 shows protracted residence in APAZ during Cretaceous for most samples from the Novosibirsk massif. On the other hand, sample 31-1 from the Ob massif was likely affected by rapid cooling at ca 100 Ma. What tectonic event may be correlated with it?
5. Fig 1. What is “depth of X km” in it? Although there is a discussion in the following text, some explanation has to be presented in caption to figure. What is the difference between rocks (Silurian-Devonian rocks) and deposits (e.g. Ordovician-Silurian deposits). Please make colors for Lower-Middle Triassic granitoids and Eifelian-Givetian deposits more distinct. It is not clear from map 1a if sample 664-4 is from Triassic granite or Eifelian-Givetian deposits.
6. Not sure if climate oscillation curve (Fig. 3) is so necessary. Climate changes anyway do not much affect presented AFT data. I recommend delete it from Fig. 3 and related discussion from the text.

Author Response

Dear Anonymous Reviewer,

thank you for review, your comments were useful. Our response on some remarks is in attached file

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript: Tectonic Evolution of the SE West Siberian Basin (Russia): Evidence from Apatite Fission Track Thermochronology of its Exposed Crystalline Basement

Authors: Evgeny V. Vetrov 1, Johan De Grave, Natalia I. Vetrova, Fedor I. Zhimulev, Simon Nachtergaele, Gerben Van Ranst, and Polina I. Mikhailova

Summary:

This paper presents a new Apatite Fission Track dataset from the granitic basement underlying within the West Siberian Basin to resolve the Mesozoic to Cenozoic thermal and tectonic history of the basin. They supplement their AFT data with inverse modelling to infer the thermal history of each sample and interpret these thermal histories within the regional geological and tectonic framework. They show that the individual granite massifs recorded distinct cooling episodes over Mesozoic and Cenozoic and suggest the cooling is caused by denudation. The advocate that tectonic reactivation in response to far-field stresses during regional tectonic events has driven the episodes of enhanced denudation.

Review:

The aims of the paper are well set out and the sampling strategy and technique used is appropriate to investigate the evolution of the West Siberian Basin. I am confident the data has been acquired using an appropriate methodology and the data they present looks consistent and aligns well with the known geological history. The paper has the potential to be a very nice, concise and focussed piece of work, however, I have several concerns that are outlined below, and I feel require major revisions. I think the paper needs clarity and restructured in places and the modelling approach and interpretation of the models needs to be a little more rigorous. The AFT data is a little concerning because each sample has a very small number of single grains dated (albeit with high track densities) than is typical for analysis of basement samples (n_grains = 20 – 30). I feel the authors need to be more upfront about these low numbers and give a strong argument for why they are sufficient to be interpreted and modelled and ideally take this into consideration when developing their modelling strategy.

Comments:

I have provided detailed comments on certain aspects of the work below. In the attached .pdf I have highlighted and commented on specific parts of the manuscript that I would also like the authors to edit or explain in more detail.

Manuscript Structure

I think that the second half of the manuscript needs to be better structured. “Section 4 – results and interpretation” jumps from describing the measured data and then to a quick description of the model output for each massif and then into the “5. Discussion” where you are thrown back into the AFT ages and their ‘boomerang’ relationship. The modelling section is really lost within the discussion of the data. Within this discussion of the data there are qualitative interpretations such as at line 207 and 227 and throughout 5.1. I like including these type of interpretations from the measured data but considering you go on to interpret your models in terms of the timing of events with the regional geological and tectonic history, I feel that the cooling history obtained from your model should really supersede the preliminary interpretations based on the data alone. To this end, I feel that Section 4 would be better split into 4.1 AFT results (where you can present the data from each massif and the boomerang plots, which brings it all together with a nice relationship, and make some preliminary interpretations ) and 4.2 modelling results, where the structure of the models can be described and the features in the different Massifs can be compared and contrasted. The that would set you up nicely for the discussion where you interpret the model features alongside the tectonic history.

Dataset

The dataset seems relatively poor in terms of the number of single grain ages (although the track numbers/densities are reasonable) and lengths (although there is the c-axis parallel lengths issue discussed below). The limitations of this are not appropriately acknowledged or discussed and no consideration is given to whether or not the dataset is still sufficient to model and interpret or whether additional analytical strategies are required.

The strategy of hand-picking grains seems flawed here. I understand that hand-picking may be required if there is a large amount of impure separate and you need to hand-pick the apatite to improve the yield but considering that you are aiming for 20 to 30 single grain ages to count, you should be aiming to pick way more than at least 20. The flawed approach is evident in the number of successfully counted grains being considerably lower than 20.

Can you provide clarification here on the lengths parallel to c-axis? It seems strange to me that at times you are able to measure up to 100 lengths in a sample that are exactly parallel to the c-axis. Typically, lengths are measured and their orientation to the c-axis also measured and then you can correct for anisotropic annealing. I appreciate that this correction isn't universally used/accepted but while you make certain assumptions when making the c-axis correction you make assumptions when only using the unprojected lengths, and assumptions when using only c-parallel lengths. For example, when you say they are c-parallel, is this strictly only ones with 0° deviation from c, or do you allow 1° or 5°? The additional issue with the latter is that you potentially limit the number of lengths you have measured. Perhaps this is why the lengths are so low for some of the samples? Convention has been to measure ‘horizontal’ confined tracks. By initially only stating ‘confined’ and using the Nikon scope and software I had assumed you had corrected for track dip, and maybe this is how you boosted c-axis parallel length numbers. However, in future instances you say ‘horizontal confined tracks’? I am not trying to be critical of the decisions you have made, I just think they need to be spelled out a little clearer because the decisions you have made do have the potential to impact your thermal models and given different FT groups do somethings in different ways (either due to their technical capabilities or methodological views) while others strive for consistency it is critical there is no confusion for the reader in what you have done.

Thermal History Modelling

The modelling needs to be more rigorous.

The authors mention c-axis angles were measured but they should explicitly state whether or not they are presenting and modelling the c-axis projected track lengths or if they are presenting and modelling the unprojected lengths as this decision will have an impact on the output thermal history.

The authors should state and reference the annealing model used.

Given the authors know the intrusion ages of the sampled rocks, they could constrain the starting point of the model with a time-temperature box corresponding to the intrusion age. This may result in redundant sampling in the temperature space between the base of the PAZ (e.g. 120 C) and initial temperature constraint used for the intrusion, but the timing of cooling through the PAZ will be made much clearer in the QTQt plots, rather than the models starting at some temperature <120C as is currently the case. If you decide not to put in an initial constraint, you need to state what is happening prior to that first point shown on the expected path. For example, sample 31-1, the path starts at c. 40C at c. 100 Ma…what is happening before then?

Given the poor sample size of the single grain fission track ages, I strongly encourage the authors to consider combining samples together for modelling. For example, the samples from the Novosibirsk Massif have generally 6-8 grains (this number is far too low for adequate interpretation), they are from the same unit, the Dpar (i.e. composition) is essentially the same (average from these samples 2.43, c. 5% variation), even the MTL is similar, so I think you could justify combining these samples together and treating them as one. If you want to keep the samples together then I feel it is necessary to model them together, using their vertical offset, such that you find a mutually consistent thermal history for all of the samples in the same unit. You state that the “near-horizontal Tt-paths with some increase of the temperature that allows interpreting this as a slight reheating episode.” But the 95% credible intervals on this are very large, and you can see that none of the posterior models show reheating and the amount of heating is very slight, and so stating this allows interpretation of reheating is very weakly justified. Perhaps a joint model would reinforce and better constrain the need for early cooling and slight reheating before a final cooling episode.

You need to improve the quality of the modelling images and show the Maximum Posterior and Maximum Likelihood clearer. I don’t feel the maximum mode model is very informative and could probably be removed. Alternatively, since you only interpret the expected thermal history you should only show that and confine the other models to the supplementary information.

You need to show the data fits for the models. Show observed vs predicted data for AFT ages, and MTL and show the fit of the track length distribution.

I feel that the modelling section could be rounded off and summarised by using some forward models that show the authors preferred thermal history for each massif (and how well this fits the data) rather that the overlay of the individual expected Tt paths in figure 7, which look messy particularly at the time of the Neogene cooling event.

Interpretations

Given the controversy surrounding peneplains formation/existence, described by Phillips, 2002, (Geomorphology) and exemplified in work such as Pedersen et al., 2016 (EPSL) and work by Lidmar-Bergstrom et al., 2013 (Global and Planetary Change) amongst others. Have you considered alternative explanations for planar surfaces forming at elevations between 250 - 500 m? This is important because you use the dissection of this expansive peneplain as support for an Oligocene event, which is poorly constrained by the thermal history models.

The reheating in the Novosibirsk really isn't convincing enough and constrained enough to be and interpretable feature. You can see the posterior model (the simplest one that fits the data) really doesn't need it. However, your explanation for reheating or residence in the PAZ for this region does seem reasonable. While I agree with the initial approach of modelling your data unconstrained, considering the uncertainty on the expected model, you aren’t learning much other than it was at reasonably cold temperatures for a long time. Multi-sample modelling may tighten the solution up a little, or you could present a forward model that is plausible given the uncertainties of the inversion and consistent with your hypothesis that this region was buried.

Regarding the Neogene cooling event, I think you are too quick to acknowledge and then dismiss the artifact because the presence of it supports your tectonic model. It is known and possible that it is a consequence of the annealing algorithm. This needs to be explored further, perhaps by interrogating the maximum posterior model or by running forward models with and without this cooling event to see how much the data fit is impacted.

I would like to congratulate the authors on the good work done so far and look forward to receiving a revised version of the manuscript in due course.

Regards.

Comments for author File: Comments.pdf

Author Response

Dear Anonymous Reviewer,

thank you for review, your comments were useful. Our response on some remarks is in attached file

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript describes an apatite fission track (AFT) thermochronology study from three late Paleozoic crystalline basement blocks, similar in intrusion age and petrology, exposed in the southeastern West Siberian Basin. Inverse thermal history modelling of the data (which all yield Cretaceous ages) generate time-temperature cooling histories indicating different cooling histories for each block, and these are considered to reflect variations in denudation and exhumation patterns.  The possibility of intraplate mantle plume activity as a trigger for the discrete cooling episodes identified is largely discounted and these are rather attributed as mainly being a response to the transmission of far field forces over hundreds of kilometres resulting from stresses created largely by collisions between different continental blocks. Seismic evidence in the Novosibirsk massif indicating a shallower crustal depth suggests that it may have undergone more pronounced denudation (also supported by evidence from mineralization) compared to the Ob massif which shows evidence for deeper plutonic roots.  The influence of sea level fluctuations on changing base level and the possible influence of climate are also considered as a minor factor affecting denudation.

The AFT data were acquired in a well-established laboratory and the analytical methodology appears to be robust. The overall subject matter is of potential interest to an international readership because not much low thermochronology data has been reported from the study area, but it would be more so if the implications of the work could be expanded to place the reported findings in a broader regional context. As it now stands however, the work flow could be better organised and considerable English editing is required. Several other improvements could be made in the work, which call for major revision. These are listed below in no particular order.

• The introduction could be better constructed by some rearrangement of the text as follows; statement of the problem, introduction to the study area and scope of the present paper.
• The Geological Setting could be shortened and the tectonic maps in Fig. 2, particularly panels c to f, are arguably more relevant to the Discussion and interpretation of data.
• Lines 90-99 describing the samples studied (and reference to Table 1) would fit better in a section 3 renamed something like Apatite Fission Track Thermochronology: Samples and Methods.
• Lines 106-108 – is there any information on the possible P-T conditions for this mineralization, which might be useful here in terms of depth?
• Line 189 - the number of grains analysed (varying between 5-12) is low by modern standards, yet the spontaneous track counts are quite high. What was the minimum threshold for grain size chosen? Can the U content of the apatite grains be indicated in Table 2.
• Section 4 – this would read better with a title Results containing two sub sections AFT data and Inverse Thermal History Modelling. Rather than just presenting model outputs more background information should be presented on the model set-up conditions (this has been debated in recent papers and discussions, e.g. Vermeesch & Tian and Gallagher & Ketcham) and whether any constraints were used (or taken into account).
• 4 – an explanation is needed for the coloured shading and model output information is lacking. Indicate in the caption that the black lines are best fit t-T paths (presumably that is the case) and present age information on how well they match the observed age data and for lengths the predicted length track distributions can be compared by a red curve with the light blue bins – the length data shown is presented in Table 2, but for comparison the predicted MTL values based on the modelling are not.
• Section 5.2 – it would be very useful for the reader to be able to visually compare via an additional figure select backstripping models for the southeastern basin area with the AFT inverse thermal history models. If the trigger for cooling is indeed the transmission of far-field stresses then one would expect that basin sections would not be immune from such effects in time and space.
• Line 363 – heading should read ‘Geodynamic implications’
• 5 shows that from a more regional point of view there is wealth of published AFT data from the Central Asia Orogenic Belt showing a similar age range to that reported in the present work. How do the inverse models published from those areas compare with those in Fig. 4 and how are those cooling histories explained? Such comparisons could also be shown visually in an expanded Fig. 7. Do they also show a pronounced Neogene cooling episode from temperatures which are lower than the top of the AFT partial annealing zone and for which annealing kinetics are rather sluggish?
• The term reactivation is used in several places in the manuscript, in particular in the paragraph covering lines 387-395 and this is alluded to in terms of inherited basement structures. No mention is made of such structures in the Geological Setting and if thought to be present how was sampling in the quarries related to such features.
• Following FAIR protocol, which is now becoming mandatory for many journals it is essential that the full fission track data for each sample be listed in a Supplementary Data section, in sufficient detail (single grain age counts, track lengths and Dpar data) so that anyone could attempt to emulate the models presented at a later time.

Author Response

Dear Anonymous Reviewer,

thank you for review, your comments were useful. Our response on some remarks is in attached file

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you to the authors for providing a response to my previous comments. I have made a very small number of minor comments on the revised manuscript. After these have been resolved the manuscript should be suitable for publication.

Comments for author File: Comments.pdf

Author Response

We have taken in consideration your new comments and revised the manuscript

Lines 64-65. The sentence has been rephrased: ‘…This method represents a geological dating technique in which the damage tracks result-ing from 238U spontaneous fission in apatite are sensitive to elevated temperatures…’

Lines 157-158. The sentence with minor clarification has been added. ‘…We suggest the planar surfaces were formed on sea level or even below during stable sea level period…’

Lines 226-227. The sentence has been modified according to your recommendation. ‘…The expected t-T models (the black paths) are an average of all accepted models weighted to their posterior probability…’

Reviewer 3 Report

The authors have now addressed most of the points raised in my review satisfactorily. There are still several typos though and some light editing is still required. Some points are listed below, but are by no means comprehensive.

Line 24 – ‘interpret’ not interprete

Line 28 – ‘possibly moderately reactivated’

Line 34 – ‘are to a large extent’

Line 65 – better to say ‘damage tracks resulting from ²³⁸U spontaneous fission in apatite’

Line 85 – as a result of collisional events

Line 125 – ‘WSB subsidence started in’ no need for ‘The’ here

Line 138 – ‘sediments representing shallow marine and lagoon’

Line 141 – ‘alluvial plain’

Line 150 – ‘Numerical ages shown are according to’

Lines 201-202 – why don’t you add a sentence explain why this was the case as you indicated in your response letter when referring to Line 189 in the previous manuscript.

Line 209 – need to rephrase this sentence – also do you mean ‘inclination angle’ or better ‘dip angle’ – see also caption for Table 2 (line 358).

Line 217 – ‘performed on each sample’

Line 218 – ‘insufficient’

Line 369 – What about citations for references for the 10 Ma to recent cooling here. Are they the same as for the Cretaceous AFT ages?

Line 370 - Caption for Fig. 6, you introduce the term apatite U-Th-He dating here but haven’t mentioned the significance of such ages anywhere in the paper – so perhaps mention something briefly here

Line 373 – ‘occurrence’

Line 421 – ‘studied’

Line 432 – ‘within the aforementioned interval’

Figure 6 – the caption

Supplementary Material S1) ‘AFT age data’ there is more than ‘density’ in this table which is not a good term here.

S2) Suggested heading ‘AFT length data’

Line 542 – Acknowledgments should indicate that this work has been reviewed.

Author Response

We have taken in consideration your new comments and revised the manuscript

(Line 24 – ‘interpret’ not interprete) done

(Line 28 – ‘possibly moderately reactivated’) done

(Line 34 – ‘are to a large extent’) done

(Line 65 – better to say ‘damage tracks resulting from 238U spontaneous fission in apatite’) done

(Line 85 – as a result of collisional events) done

(Line 125 – ‘WSB subsidence started in’ no need for ‘The’ here) done

(Line 138 – ‘sediments representing shallow marine and lagoon’) done

(Line 141 – ‘alluvial plain’) done

(Line 150 – ‘Numerical ages shown are according to’) done

(Lines 201-202 – why don’t you add a sentence explain why this was the case as you indicated in your response letter when referring to Line 189 in the previous manuscript.) The sentences with minor clarification has been added. ‘…Some of analyzed apatite grains contained zircon microinclusions, which produced a high number of spontaneous tracks and greatly distorted the results for single grains and the sample as a whole. Therefore, we have managed to analyze a few (5 to 12) grains for each sample; nevertheless, we have counted a sufficient number of spontaneous tracks (more than 1000)…’

(Line 209 – need to rephrase this sentence – also do you mean ‘inclination angle’ or better ‘dip angle’ – see also caption for Table 2 (line 358).) done

(Line 217 – ‘performed on each sample’) done

(Line 218 – ‘insufficient’) done

(Line 369 – What about citations for references for the 10 Ma to recent cooling here. Are they the same as for the Cretaceous AFT ages?) References for the 10 Ma are the same as for the Cretaceous AFT ages.

(Line 370 - Caption for Fig. 6, you introduce the term apatite U-Th-He dating here but haven’t mentioned the significance of such ages anywhere in the paper – so perhaps mention something briefly here). Brief remark has been added in Lines 370-371: ‘…apatite U-Th-He dating (U-Th-He system with a closure temperature range between ~45-75 °C is more sensitive to near-surface process than AFT system) and thermal modeling…’

(Line 373 – ‘occurrence’) done

(Line 421 – ‘studied’) done

(Line 432 – ‘within the aforementioned interval’) done

Figure 6 – the caption

(Supplementary Material S1) ‘AFT age data’ there is more than ‘density’ in this table which is not a good term here S2) Suggested heading ‘AFT length data’.) The terms have been modified according to your recommendations

(Line 542 – Acknowledgments should indicate that this work has been reviewed.) Done. ‘…The editors and three anonymous reviewers are thanked for their constructive comments all of which greatly improved this manuscript…’