Rift thermal inheritance in the SW Alps (France): insights from RSCM thermometry and 1D thermal numerical modelling

Célini, Naïm; Mouthereau, Frédéric; Lahfid, Abdeltif; Gout, Claude; Callot, Jean-Paul

doi:https://doi.org/10.5194/egusphere-2022-949

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https://doi.org/10.5194/egusphere-2022-949

Preprints

20 Sep 2022

| 20 Sep 2022

Rift thermal inheritance in the SW Alps (France): insights from RSCM thermometry and 1D thermal numerical modelling

Naïm Célini, Frédéric Mouthereau, Abdeltif Lahfid, Claude Gout, and Jean-Paul Callot

Abstract. Conceptual models of orogenic accretionary prisms assume the increase in peak temperatures (T_max) towards the internal domains as crustal rocks are accreted from the lower to the upper plate. Yet, the recognition of pre-orogenic heating events in mountain belts questions the magnitude of thermal overprint during nappe stacking. Using Raman Spectroscopy on Carbonaceous Material (RSCM) to calculate T_max, we have investigated the thermal record of Lower Jurassic to Upper Cretaceous strata exposed along the Digne Nappe, at the front of the SW Alps. Our results highlight two groups of depth-dependent temperatures: (1) a regionally extensive constant T_max up to 300–330 °C measured in the Jurassic succession and (2) regionally variable lower temperature (<150 °C) recorded either in the upper Mesozoic, the nappe stack or the syn-orogenic sequence. Modelling shows that the highest paleotemperatures were achieved during the Early Cretaceous (~130 Ma), during the Valaisan-Vocontian rifting, while the lowermost ones reflect syn-orogenic burial in the Alpine foreland basin. This study provides a striking new example where mid-crustal paleotemperatures measured in lower plate sediments now accreted at the thrust front are inherited. Estimated peak thermal gradient of 80–90 °C/km requires crustal thickness of ~15 km during the Early Cretaceous, hence placing new constraints for tectonic reconstruction of rift domains and geophysical interpretation of current crustal thickness in the SW Alps. These results call for the careful interpretation of paleothermal data when they are used to identify past collisional events. Where details of basin evolution are lacking high-temperature record may be misinterpreted as syn-orogenic, which can in turn lead to overestimating both orogenic thickening and horizontal displacement in mountain belts.

Received: 19 Sep 2022 – Discussion started: 20 Sep 2022

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04 Jan 2023

Rift thermal inheritance in the SW Alps (France): insights from RSCM thermometry and 1D thermal numerical modelling

Naïm Célini, Frédéric Mouthereau, Abdeltif Lahfid, Claude Gout, and Jean-Paul Callot

Solid Earth, 14, 1–16, https://doi.org/10.5194/se-14-1-2023,https://doi.org/10.5194/se-14-1-2023, 2023

Short summary

Naïm Célini et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-949', Anonymous Referee #1, 20 Oct 2022
General comment

I read this work with great interest. The starting hypothesis of recognizing a signal of pre-collisional thermal events through the use of geothermometers in an external sector of the chain (where therefore the Alpine metamorphism may not have overprinted everything) seems interesting and promising, possibly providing important contribution to the reconstruction of the pre-Alpine paleogeography evolution of this sector of the Paleo-European continental margin.

However, I believe that a lot of work still needs to be done in order for it to be published. Some important points need to be addressed by the authors; among these, the most important concern the controversy on the nature and timing of the Valais oceanic domain (ignored by the authors) and the fact that the high temperatures presented in this work should imply, in my opinion, some evidence of metamorphism, in a domain which is, on the contrary, commonly described as non-metamorphic (in my extended comments below you will find much more comments on these points).

Furthermore, in many parts the data presentation can be improved, making it more organized and understandable. In the Interpretation and Discussion sections some sentences are a little obscure and hard to understand, and some points deserve wider explanations and discussions. I got the impression that it was written a bit in haste, without the necessary accuracy and without providing a deep and complete discussion of the reliability of the measured data and of their geological meaning in the Alpine geological context.

For these reasons I think that the manuscript in its actual version can not be published, and recommend major revisions.

Abstract

Lines 14 to 17: you should better specify where the samples come from. The expressions “along the Digne nappe” or “ the nappe stack” are too generic

Introduction

Line 30: no comma needed after “crustal structure”

Line 49: the sentence “…together with issues realted to the magnitude of continental collision” is too generic, you must specify and/ore quote some papers

Line 51: Why “Although”? The second part of the sentence is not in contrast with the first…

Line 54: “carrying”: better “Characterized by”; why “..a thick Jurassic syn-rift section”? The Digne nappe is not made up of uniquely Jurassic successions….

Line 56: what do you mean with “pre-collisional stratigraphic series”? The successions (succession is more appropriate than serie) underlying the Alpine Foreland one? But in some cases you sampled up to the Eocene…..

Geological setting

The authors write about two rifting events, stating that in the Late Jurassic – Early Cretaceous the Valais ocean opened. They describe this in few words, as it was a fact, well documented and acknowledged by all the geological community. But it really is not. There has been a long controversy on the Valaisan, and still there is, that concerns its nature of oceanic basin, its extension, and its timing. The Authors quote the paper by Beltrando et al., 2012, but they do it inappropriately. Beltrando et al. dedicated a lot of space to reviewing the debate concerning the Valais (not by chance the title of the paper start with “The Valaisan controversy revisited…”), and in the discussion they interpreted they original data suggesting that “the crustal thinning in the Valaisan basin may have been kinematically linked with the opening of the Western Tethys” (thus not in the Cretaceous!). Geochronological data are scarse and often contrasting (e.g.: Liati et al. 2005), and many Autors provided a large body of structural and stratigraphic evidence pointing to a Jurassic opening of the Valaisan basin, even in recent papers (see for example Mohn et al. 2010, Ribes et al., 2019, 2020). This is not my research field, and I can not say if the Valais opened in the Jurassic together with the Western Tethys or later in the Cretaceous. But the authors can not ignore that it is still a highly controversial topic, and should report and comment on this. Actually, if the authors introduced it adequately, the data resulting from this paper could give a new contribution to such problem!

Lines 63 to 78: the first sentence is not clear, should be rephrased. In general, the description is not always linear (see for example line 68, with an abrupt jump from the second rift event to the continental breakup, which however is referable to the first one). And the 33.9 Ma age seems to me too precise…. Do the quoted papers really indicate it for a long-lasting event such as the transition between two collisional stages?

Line 81: “hangingwall” is unnecessary

Line 84: “It turns…” refers to the main thrust. But in the previous sentence you were referring to the Digne nappe…. Please rephrase.

Line 85: A nappe is not a thrust front….

Lines 85 to 88: please use succession instead of serie. A description of the succession should be provided. It can be brief, but I think it is necessary, since you collected your samples all over it.

Samples and methods

This section is definitely incomplete and bad organized. As general comments I would say that authors should first of all briefly explain what RSCM thermometry is and how it works, since this journal is not uniquely dedicated to Raman analysts (and please, the first time you cite RSCM wrote it in the extended version). They should then describe which was the goal of the sampling and analysis, and provide a lithological (and possibly a petrographic one too) description of the samples (they wrote carbonate deposits: too generic, I guess thay should be CM-rich rocks…. Are all limestones or shales too?). And some description of the analytic instruments and their operating conditions must be provided. Lines 105 to 112 do not refer to the methods of this work and should be moved to the Results or Discussions sections.

Other specific points:

Line 97: the first time please write the name of the sections in the extended version. The same in the legend of figure 1B

Lines 100 to 102: This sentence (“Because the nappe…. prior to thrusting”) is definitely not clear, please rephrase

Lines 105 to 109: too long sentence, better to split in 2 or 3. The statement “during the passive margin stage coeval to crustal thinning” is wrong: the passive margin stage goes on well after the end of the crustal thinning.

Lines 109 to 112: This is an important point for interpretation of your data and I think deserve more explanations. Anyway, not in this section (see comment before)

Line 116: “temperatures across the Digne thrust front..” did you mean along?

Line 117: convert in place of converted

Line 118: I do not understand “burial in the Alpine foreland”… In this scenario the Tmax is achieved due to the unique effect of the burial history. But during the entire burial history…

Line 120 and line 124: some values of the crust and lithosperic mantle are reported. How did you chose them? At least some citations are needed.

Lines 125 to 130. All this explanation is not completely clear, particularly the last sentence. How could you reconstruct the thickness of the Cretaseous sediments eroded during the formation of the Foreland basin unconformity starting from the AFT data in the Miocene deposits? Please try to better illustrate your reasoning

Line 141: Why did you chose to fix the T at 1333 °C? And which is the reason for a such specific value? It is an assumption, so I do not understand which is the meaning of a such specific value. I mean, 1350 would be uncorrect? Why?

Line 149. In which locations of the SW Alps is reported this thickness of Middle Triassic limestones? In many places (e.g. External Briançonnais) the Middle Triassic is much more thick….

Results

Line 155: “Domain”?? I would replace with trend

Some major points corcernings this section.

It contains a discussion of the RSCM results, and thus in my opinion it should be moved to the Discussion section.

You found very high Temperatures, up to 340°, in the Early-Middle Jurassic beds of the most of the sections. These temperatures are commonly considered to be in the range of metamorphism. Low grade metamorphism (green schist facies), but metamorphism. In the Discussion below you propose an interpretation in which such thermal perturbation is related to two rifting events: this means that such perturbation lasted for tens of million of years. I would expect that sedimentary rocks affected by temperatures above 300°C for such a long time interval in an extensional setting (in which the fluid circulation through the crust is highly favoured) would be transformed in metamorphic rocks. At least they should bear evidence of recrystallization and neo-blastesis. And I guess that many of your samples are shales or rocks with a shaly component, that is the most reactive to metamorphic reactions. You should face this point by providing a petrographic description of your samples. In the case you can not find any evidence of recrystallization and neo-blastesis you should propose a mechanism that hampered these processes at such high temperatures. I am not an analyst or a Raman expert and I do not want to doubt about the RSCM method; but I know that there has been (and probably there is) debate around it. Some authors reported kinetic effects and the occurrence of metastable poorly crystallized graphitic carbon that, in their opinion, would affect the reliability of the RSCM geothermometer (see for example Foustoukos 2012, ). All the more reason you should describe the rocks you analysed, showing the differences between the ones in the upper part of the sections which not experienced high temperatures and the ones in the lower part which were affected by temperatures up to 340°C. Alternatively, if you could not find any evidence, you should discuss how it was possible, in order to exclude that such temperatures are “fake” temperatures due to analytical artefacts.

Line 168. “(para-) autochtonous” I would avoid this old and ambiguous terms: you could more simply use “AFT ages from the Eocene sediments of the Digne nappe”

Lines 173 to 175: I got your reasoning but I think you could make it a bit more explicit

Line 174: Figure 2C, not 2B

Lines 177-178: see comment before too. Why do you talk about syn-orogenic burial? I would say that T values in the CAS section are consistent with a normal and continuous burial history (fromt Triassic to Pliocene)…

Line 185: did you mean figures 4 and 5? In figure 3 results from all the three models are shown

Lines 195-196: “deepest sediments”: which sediments are you referring to? The ones below 2 Km? In this case I would say between 230-340°C, not above

Lines 205 - 209: This is not convincing at all! The CAS section is just 24 km from the CLN section, and owns to the same tectonic unit and the same paleogeographic domain. Moreover, you can not desume a more internal location from the described difference in thickness of the Jurassic succession (and in figure 6 you do not locate it a more inner position….). Continental rifted margins are characterized by significant and abrupt changse in thickness of the syn-rift sediments. In the Dauphinois domain see for example the Ornon fault area (Chevalier et al 2003, and their figure 1c); the Briançonnais domain is characterized by a reduced and condensed Jurassic succession, but it pertained to a much more external position with respect to the study area of this work. I think that another explanation must be given for the “colder” values of the CAS section.

Lines 210 - 215: This is an important point and deserves much more attention. What are the 3D effects? And the kinetics of organic matter maturation? You simply mention these aspects, but I think you should comment them. Some authors do not consider the RSCM geothermometer reliable because of some of these problems (see my comment above): you should comment more extensively.

DISCUSSION

Lines 219 to 226: such a huge lithospheric stretching and very high geothermal gradients should have a large body of evidence, even in the syn-rift stratigraphic successions (paleofaults, hydrothermal products, abrupt thickness changes, large breccia bodies, etc….). Actually some of these evidence have been reported in literature for the Early cretaceous too, both in the Dauphinois Domain and in the adjoining Brianòonnais Domain. I think you should consider this.

Lines 229 to 231: It is not clear which is the old proposed position of the Digne nappe and your new interpretation. Do you think it corresponded to a hyper-thinned domains? And why it should have been located on a transfer zone?

Line 235: “expected to be better preserved”: what should be better preserved? And the European paleomargin represented the lower plate during the collision….

Lines 236 to 240: Ok, but which could be the reason? Please provide an interpretation

Line 250: what do you mean with “differential extension”?

Line 251-251: What the expression “In case of westward propagation…” mean? Do you think a westward propagation occurred or not?

Line 254: “To the east of the Embrun…”: better in the eastern part of the Embrun…

Line 257: why “in contrast”? I would write: “...reflects the overthrusting of the E-U nappes bu also require heating…”

CONCLUSIONS

General comment: some problems and doubts remain concerning the reliability and accuracy of the applied RSCM geothermometer (e.g.. the drop not reproduced by models; the “colder” CAS section; very high temperatures in a non-metamorphic domain). I would thus be more cautious in proposing such extremely high geothermal gradients, and at least I would suggest that it would be necessary and interesting to apply other geothermometers (illite crystallinity index?) in order to confirm or not the presented T values.

Line 272: “foreland deposition”? not clear, please expand and clarify

FIGURES

Figure 1

A) It lacks a legend. And in the external zone you should more clearly distinguish the Digne nappe

B) The geological map of the external zone is hardly readable. It is too detailed, with a lot of not useful (and not homogenously distributed) elements that, given the figure size, make the figure itself hardly readable. In the caption please add that 15 original RSCM analysis are indicated, and report the name of the sampled stratigraphic sections. Delete the second “reconstructed”.

In the legend the symbols for the tectonic contacts are missing. And I would highlight the Digne main thrust with initials

Figure 2

The font size seems to me too small. Particularly the numbers in 2A and the text in 2C, almost unreadable. In 2B it is not specified that the values of the columns are temperatures

Figure 3

This figure is too small, and the text and numbers. Almost unreadable. You could split the 6 diagrams in 3 + 3….

Figure 4

The isotherms lines are too thin, difficult to see them

Figure 5

Specify in the caption that the models are based on the two-rift hypothesis
Citation: https://doi.org/10.5194/egusphere-2022-949-RC1
- AC1: 'Reply on RC1', Naïm Célini, 25 Nov 2022
  
  The co-authors would like to thank the anonymous referee for the thorough comments on the manuscript.
  Please find attached the pdf file with the answers, point by point, to each comment.
  
  Citation: https://doi.org/10.5194/egusphere-2022-949-AC1
RC2:
'Comment on egusphere-2022-949', Yann Rolland, 23 Oct 2022
General comment

I have read this manuscript with interest as it provides a synthesis of recently acquired (and some new) RSCM data at the SW Alps scale and provide a comprehensive modelling approach to explain these data. The form of the manuscript is excellent. It is well organized and concisely written, the figures are well drafted. Though, I am not an expert of modelling, I found this part well presented and understandable at least from the results. I found especially interesting the position of the authors in not considering raman temperatures as just a result of Alpine thickening as previous authors did (see specific discussion points below), but as a result of previous rifting history. Of course, this could be surprising at first glance in metasediments not exhibiting any other metamorphic minerals at temperatures >300°C. However, this is always the case in Low Pressure sediments (like in contact-metamorphism in general), and maybe the impact of (hot) fluid circulations could explain some relatively short-lived thermal reset (note that ore formation is documented at least for the Jurassic phase). This point could be better discussed in the paper. However, I found that the explanations proposed (two rifting episodes) and tested by modelling are very interesting and fit very well with the regional geology where Jurassic and Lower Cretaceous rifting phases are clearly highlighted. Finally, the general model that is presented seems sound. Of course, the relationship with the Valais ocean is not simple, as the paleogeography has been significantly disturbed by the Alpine orogeny, but, however, the proposed reconstruction agrees with the shape of the Vocontian low and the dissymmetry of the Valais unit (not continuing in SW Alps).

In conclusion, I think that this work deserves a rapid publication with some relatively minor revisions.

Minor comments

Figures: in many cases the text in figures is too small (see attached file).

L47 : Altough, these temperatures are similar to those estimated by mineral thermometry in the Briançonnais (e.g. Lanari et al., 2014), this is no the case in the Pelvoux domain where RSCM estimates lie systematically higher, by about 50°C in eastern Pelvoux, (Simon-Labric et al., 2009), and by > 100°C to the west of Pelvoux ECM (especially, considering vitrinite reflectance results from Cretaceous beds, ( Deville and Sassi (2006) ), which led Bellanger et al. to propose a "hypothetical" substractive contact that has not been observed in the field...

L75 :

I would say ‘before 34 Ma’ to be more consistent with error bars of geochronological methods (Ar-ar dating of syn-kinematic phengite)

Top NW deformation is documented in the Eocene: Lanari et al., 2014, Terra Nova: Top NNW syn-kinematic deformation dated at 45 Ma in briançonnais units at the base of the Helminthoid flysch

L271: not sure the concept of "temperature structure" is adequate...? maybe the thermal state/profile?

See attached file for details
Citation: https://doi.org/10.5194/egusphere-2022-949-RC2
- AC2: 'Reply on RC2', Naïm Célini, 25 Nov 2022
  
  The co-authors would like to thank Yann Rolland for reviewing the manuscript.
  Please find attached the pdf file with the answers for each comment.
  
  Citation: https://doi.org/10.5194/egusphere-2022-949-AC2
RC3:
'Comment on egusphere-2022-949', Anonymous Referee #3, 24 Oct 2022

This paper presents a new dataset of temperatures based on Raman Spectroscopy on Carbonaceous Material and thermal modelling in Mesozoic and Cenozoic rocks of the southwestern Alps to unravel the thermal and tectonic evolution of this part of the mountain belt. I find this work interesting, relevant, and worth to be published in EGUsphere after some moderate revision.

However, this manuscript has some errata and contains some misleading information that I summarized below. The main issue that I find is the lack of a proper description of the geological setting, which is necessary to describe and justify the tectonic scenarios (number of rifting stages) tested for the modelling. The information is sometimes a bit disorganized along the text and the methodology description also needs to be improved. It lacks several references along the text which need to be added. In the following, I made some comments and suggestions that in my opinion will contribute to improve the manuscript and make it easier to understand for the reader.

Abstract:

L14: also Cenozoic rocks.

L20: “lower plate sediments”, do you mean sediments deposited in a basement that subducted, accreted sediments from the subducted plate? Please, clarify.

Introduction:

I find the introduction a bit short and that fails in emphasizing the relevance of this study to unravel the processes governing the accretion of sediments during collision in the SW Alps. The authors are referring frequently about the subduction processes and the accretion of sediments to the upper plate but there is none crustal-scale cross-section that shows the structure they are referring to. I also find the map of figure 1a too simplified.

The general description of the Alps and Digne Nappe is insufficient to understand the tectonic context the authors are studying. In this section I would also rather read about an overview of the geology of the western Alps and the rifting episodes with a reference to figures 1a-1c. A proper definition of the Valaisan and Vocontian domains would be useful, as the authors are often referring to them but are not properly described and are relevant to understand fig 6. Regarding the Digne Nappe, it should also be properly described in the geological setting.

It is not very clear what are the previous AFT studies considered in the study. Caption of figure 1b states that they are specified along the text, but until the “samples and method” section, there are no references. I suggest the authors include previous thermochronological studies in the caption of fig. 1, in the introduction as state-of-the-art studies, and maybe also add other reference such as Bogdanoff et al. (2000).

Geological setting:

This section needs to be reorganized. I would prefer to read first the description of the geology of the study area (crystalline massifs, nappes…) and after that, the tectonic evolution since the Jurassic. In addition, to support the 3 scenarios proposed for the numerical modelling, a more detailed explanation of the geology is needed, including a description of the cross-section of figure 1d.

L67: “Alpine arc”, this term has neither been used before nor later, can it be changed to “Alps”?

L65-68: I suggest rewriting this sentence: “The second, Late Jurassic-Early Cretaceous in age, appears synchronous with the rifting of the Bay of Biscay and led to opening the Valaisan domain to the NE of the Alpine arc and renewed extension in the so-called Vocontian Trough of the SW Alps” to avoid confusion about the extensional phases affecting the NE and the SW Alps. In addition, is it relevant to mention the rifting of the Bay of Biscay here? In case it is, you need to provide references.

L83: “Helminthoid flysch” is only used here, is this relevant for the study? If not, I suggest to remove it. "sub-Briançonnais" does not appear in Fig. 1a, 1b, only Briançonnais, what do you mean with “sub”?

Samples and methods:

L98: Add the number of samples in the beginning of the section.

L99: replace “in rare occasions” for something like: “additionally, two samples were collected in Eocene limestone”.

L110: “… of the existing low-temperature fission-track analyses…”, add apatite before fission-track.

Subsection "Numerical modelling with basin model":

I would rather read first about the software used for the modelling and the parameters chosen, than the scenarios tested. As I am not a user of this software, a brief description of the input parameters, processing and outputs would be very useful.

L127-129: - “Mirabeau well” add the reference to figure 1b.

“We infer that it is 3 km at the Digne thrust front (Fig. 1b).” This sentence is a bit odd, re write, please.

L142-143: “crustal basement with homogeneous properties”, in base of what? A continental crust could be very heterogeneous in terms of lithologies, and therefore, feature different density values. A crystalline crust could imply densities ranging from 2.7 to 3.4 g/cm³ (e.g., Barton, 1986; Rudnick and Fountain, 1995). Other type of models assumes a homogeneous crustal density that increases with depth between 2.6 to 3 g/cm³ (Torne et al., 2015). At lease, provide a reference that supports the value chosen.

For table 1 provide also references and justifications for the values chosen.

Results:

L167: define the acronym AFT before. In addition, add a reference of the AFT study in this sentence.

L169: Add the reference for the geothermal gradient assumption. For instance, Bigot-Cormier et al. (2006) considered a present geothermal gradient of 25-30 ºC/km, and Valla et al. (2011) considered a gradient of 25 ºC/km.

L181: replace to something like: “the comparison between the temperatures derived from RSCM data and from the numerical modelling…”.

L185-186: fig. 3 shows the results for the 3 scenarios, and figure 4 shows the two-rift scenario and one site for the no-rift scenario. Please, be precise.

L205-209: the CAS section is here explained as only burial during the Cenozoic, but how does this fit with the tectonic framework of the rest of the sections which are also located in the Digne Nappe and closer to it (e.g., DGN and CLN)? Maybe a cross-section along this area will help to understand the structure and discuss it more thoroughly…

L211: “affecting the samples in nature”, what do you mean?

Discussion:

Here I miss some discussion about the CAS section and why it is assumed to have experienced a constant geothermal gradient of 30ºC/km if it is also located in a rifted margin.

I would like to have some more information/discussion about the paleogeographic reconstruction of fig. 6 regarding the location of the rift axis, and transfer zones. It could also help to include in fig. 6a the location of the study sections. I am aware that they are already included in fig. 6b, but its tentative position in the map would give the reader a better spatial location of the dataset. In addition, a more detailed description of fig. 6b is needed. That would also help to address my previous comment on the explanation of section CAS.

L232. Geothermal gradients of 80-90 ºC/km in the Pyrenees led to high-temperature metamorphism (Ducoux et al., 2021, a reference that the authors cite), and it is accompanied by mantle exhumation to the base of the sedimentary basin or even to the seafloor (e.g., Lagabrielle et al., 2010; DeFelipe et al., 2017, 2019; Teixell et al., 2018). The rift domains defined there, and the role of transfer zones are topics that are being highly discussed. If you want to make a proper comparison with the tectonic setting of the Pyrenees (and Bay of Biscay as they also mention it without any reference in L66), you need to add more refences and discuss all these topics.

Therefore, in your study area, how was the rift system? Which rift domains are described? Is there any evidence of mantle exhumation? To the east of their study area, ophiolite complexes include serpentinized peridotites with ophicalcite (e.g., Lafay et al., 2017).

In Figure 6a, thinned continental crust is divided into “thinner” and “thicker”, can you provide a thickness estimate? Can you also indicate this in fig. 6b? How is it related to the domains of a rifted margin? (e.g., Tugend et al., 2015, a reference that the authors cite).

Figure 6 can also be enlarged.

Conclusions:

The authors would better summarize their main results here: samples collected, paleotemperature data, scenarios modelled and chosen as representative, and geothermal gradient estimations.

Figures:

Figure 1: please reorganize the figure to have image “1a” in the top left part of the figure.

1b: - What do broken red lines indicate?

- Please, change the colour of the stratigraphic sections (DEV, SLC, …). They have the same colour as the reconstructed isopachs.

- Add the definition of the acronym AFT for apatite fission track in the caption.

Figure 2: rewrite the caption. Suggestion: “stratigraphic sections along the front of the Digne Nappe with the RSCM-derived peak temperatures”.

Figure 3: define the tectonic models as two-rifts, one-rift, no-rift along the text to homogenize terms. Place this figure after it is first called (so after L180). Check spelling of color/colour in the caption.

Figure 4: change the tables of each diagram to something like: “thickness of the crust” and “thickness of the lithospheric mantle”. Otherwise, it looks confusing (thickness vs. time).

Figure 6: in the last two lines of the caption: “dashed lines”, can you provide a tentative value for each isotherm colour?

In fig. 6b the stratigraphic sections are projected for reference, but I suggest projecting them also on the map of fig. 6a. In the legend, separate the “cover” box into Jurassic and Cretaceous.

Other (minor) comments:

L25: “ Where details of basin evolution are lacking high-temperature record…”, add a comma after lacking.

L55: “This study combines 80 new RSCM measurements…”, remove the word new.

L79: “currently running between the variscan…”, remove currently.

Along the text, the Digne Nappe is referred to with different names: “front of the Digne Nappe”, “Digne thrust front”, “Digne frontal thrust”, “Digne thrust sheet”, and “Digne main thrust”, please, unify. Please, do the same for “Vocontian Trough”, “Vocontian domain”, “Vocontian-Valaisan rifting”, “Valaisan extensional domain”, “Valaisan rift”, “Valaisan domain”.

L198: “the Early-Middle Jurassic” add the "y".

L221: add geothermal before “gradients are about 80-90ºC/km”.

L225-226: from where does the β-factor comes from? Literature or your own modelling?

L276: remove the in “between the Europe and Adria”.

References:

Barton, P. J. (1986). The relationship between seismic velocity and density in the continental crust — a useful constraint?, Geophysical Journal International, Volume 87, Issue 1, October 1986, Pages 195–208, https://doi.org/10.1111/j.1365-246X.1986.tb04553.x

Bigot-Cormier, F., Sosson, M., Poupeau, G., Stéphan, J.-F., and Labrin, E. (2006) The denudation history of the Argentera Alpine External Crystalline Massif (Western Alps, France-Italy): an overview from the analysis of fission tracks in apatites and zircons, Geodinamica Acta, 19:6, 455-473, DOI: 10.3166/ga.19.455-473

Bogdanoff, S., Michard, A., Mansour, M. and Poupeau, G. (2000), Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps. Terra Nova, 12: 117-125. https://doi.org/10.1046/j.1365-3121.2000.123281.x

DeFelipe, I., Pedreira, D., Pulgar, J.A., Iriarte, E., Mendia, M., 2017. Mantle exhumation and metamorphism in the Basque-Cantabrian Basin (N Spain): Stable and clumped isotope analysis in carbonates and comparison with ophicalcites in the North-Pyrenean Zone (Urdach and Lherz). Geochemistry, Geophys. Geosystems 18. https://doi.org/10.1002/2016GC006690

DeFelipe, I., Pedreira, D., Pulgar, J. A., van der Beek, P. A., Bernet, M., & Pik, R. (2019). Unraveling the Mesozoic and Cenozoic tectonothermal evolution of the eastern Basque-Cantabrian zone–western Pyrenees by low-temperature thermochornology. , 38, 3436-3461, https://doi.org/10.1029/2019TC005532

Lafay, R., Baumgartner, L.P., Schwartz S., Picazo S., Montes-Hernandez G., Vennemann T. (2017). Petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment (Chenaillet ophicalcites, Western Alps, France): Processes of carbonate cementation, Lithos, Volumes 294–295, Pages 319-338, https://doi.org/10.1016/j.lithos.2017.10.006.

Lagabrielle, Y., Labaume P., de Saint Blanquat, M. (2010), Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies, Tectonics, 29, TC4012, doi:10.1029/2009TC002588.

Rudnick, R. L., and Fountain, D. M. (1995), Nature and composition of the continental crust: A lower crustal perspective, Rev. Geophys. 33(3), 264-309, doi:10.1029/95RG01302.

Teixell, A., Labaume, P., Ayarza, P., Espurt, N., de Saint Blanquat, M., Lagabrielle, Y., 2018. Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data. Tectonophysics 724–725, 146–170. https://doi.org/10.1016/j.tecto.2018.01.009

Torne, M., Fernàndez, M., Vergés, J., Ayala, C., Salas, M.C., Jimenez-Munt, I., Buffett, G.G., Díaz, J., 2015. Crust and mantle lithospheric structure of the Iberian Peninsula deduced from potential field modeling and thermal analysis. Tectonophysics 663, 419–433. https://doi.org/10.1016/j.tecto.2015.06.003

Valla, P. G., van der Beek, P. A., and Braun, J. (2011). Rethinking low-temperature thermochronology data sampling strategies for quantification of denudation and relief histories: A case study in the French western Alps, Earth and Planetary Science Letters, Volume 307, Issues 3–4, Pages 309-322, https://doi.org/10.1016/j.epsl.2011.05.003.

Citation: https://doi.org/10.5194/egusphere-2022-949-RC3
- AC3: 'Reply on RC3', Naïm Célini, 25 Nov 2022
  
  The co-authors would like to thank this anonymous referee for the review and the thorough comments on the manuscript.
  Please find attached the pdf file with all the answer to each comment.
  
  Citation: https://doi.org/10.5194/egusphere-2022-949-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-949', Anonymous Referee #1, 20 Oct 2022
General comment

I read this work with great interest. The starting hypothesis of recognizing a signal of pre-collisional thermal events through the use of geothermometers in an external sector of the chain (where therefore the Alpine metamorphism may not have overprinted everything) seems interesting and promising, possibly providing important contribution to the reconstruction of the pre-Alpine paleogeography evolution of this sector of the Paleo-European continental margin.

However, I believe that a lot of work still needs to be done in order for it to be published. Some important points need to be addressed by the authors; among these, the most important concern the controversy on the nature and timing of the Valais oceanic domain (ignored by the authors) and the fact that the high temperatures presented in this work should imply, in my opinion, some evidence of metamorphism, in a domain which is, on the contrary, commonly described as non-metamorphic (in my extended comments below you will find much more comments on these points).

Furthermore, in many parts the data presentation can be improved, making it more organized and understandable. In the Interpretation and Discussion sections some sentences are a little obscure and hard to understand, and some points deserve wider explanations and discussions. I got the impression that it was written a bit in haste, without the necessary accuracy and without providing a deep and complete discussion of the reliability of the measured data and of their geological meaning in the Alpine geological context.

For these reasons I think that the manuscript in its actual version can not be published, and recommend major revisions.

Abstract

Lines 14 to 17: you should better specify where the samples come from. The expressions “along the Digne nappe” or “ the nappe stack” are too generic

Introduction

Line 30: no comma needed after “crustal structure”

Line 49: the sentence “…together with issues realted to the magnitude of continental collision” is too generic, you must specify and/ore quote some papers

Line 51: Why “Although”? The second part of the sentence is not in contrast with the first…

Line 54: “carrying”: better “Characterized by”; why “..a thick Jurassic syn-rift section”? The Digne nappe is not made up of uniquely Jurassic successions….

Line 56: what do you mean with “pre-collisional stratigraphic series”? The successions (succession is more appropriate than serie) underlying the Alpine Foreland one? But in some cases you sampled up to the Eocene…..

Geological setting

The authors write about two rifting events, stating that in the Late Jurassic – Early Cretaceous the Valais ocean opened. They describe this in few words, as it was a fact, well documented and acknowledged by all the geological community. But it really is not. There has been a long controversy on the Valaisan, and still there is, that concerns its nature of oceanic basin, its extension, and its timing. The Authors quote the paper by Beltrando et al., 2012, but they do it inappropriately. Beltrando et al. dedicated a lot of space to reviewing the debate concerning the Valais (not by chance the title of the paper start with “The Valaisan controversy revisited…”), and in the discussion they interpreted they original data suggesting that “the crustal thinning in the Valaisan basin may have been kinematically linked with the opening of the Western Tethys” (thus not in the Cretaceous!). Geochronological data are scarse and often contrasting (e.g.: Liati et al. 2005), and many Autors provided a large body of structural and stratigraphic evidence pointing to a Jurassic opening of the Valaisan basin, even in recent papers (see for example Mohn et al. 2010, Ribes et al., 2019, 2020). This is not my research field, and I can not say if the Valais opened in the Jurassic together with the Western Tethys or later in the Cretaceous. But the authors can not ignore that it is still a highly controversial topic, and should report and comment on this. Actually, if the authors introduced it adequately, the data resulting from this paper could give a new contribution to such problem!

Lines 63 to 78: the first sentence is not clear, should be rephrased. In general, the description is not always linear (see for example line 68, with an abrupt jump from the second rift event to the continental breakup, which however is referable to the first one). And the 33.9 Ma age seems to me too precise…. Do the quoted papers really indicate it for a long-lasting event such as the transition between two collisional stages?

Line 81: “hangingwall” is unnecessary

Line 84: “It turns…” refers to the main thrust. But in the previous sentence you were referring to the Digne nappe…. Please rephrase.

Line 85: A nappe is not a thrust front….

Lines 85 to 88: please use succession instead of serie. A description of the succession should be provided. It can be brief, but I think it is necessary, since you collected your samples all over it.

Samples and methods

This section is definitely incomplete and bad organized. As general comments I would say that authors should first of all briefly explain what RSCM thermometry is and how it works, since this journal is not uniquely dedicated to Raman analysts (and please, the first time you cite RSCM wrote it in the extended version). They should then describe which was the goal of the sampling and analysis, and provide a lithological (and possibly a petrographic one too) description of the samples (they wrote carbonate deposits: too generic, I guess thay should be CM-rich rocks…. Are all limestones or shales too?). And some description of the analytic instruments and their operating conditions must be provided. Lines 105 to 112 do not refer to the methods of this work and should be moved to the Results or Discussions sections.

Other specific points:

Line 97: the first time please write the name of the sections in the extended version. The same in the legend of figure 1B

Lines 100 to 102: This sentence (“Because the nappe…. prior to thrusting”) is definitely not clear, please rephrase

Lines 105 to 109: too long sentence, better to split in 2 or 3. The statement “during the passive margin stage coeval to crustal thinning” is wrong: the passive margin stage goes on well after the end of the crustal thinning.

Lines 109 to 112: This is an important point for interpretation of your data and I think deserve more explanations. Anyway, not in this section (see comment before)

Line 116: “temperatures across the Digne thrust front..” did you mean along?

Line 117: convert in place of converted

Line 118: I do not understand “burial in the Alpine foreland”… In this scenario the Tmax is achieved due to the unique effect of the burial history. But during the entire burial history…

Line 120 and line 124: some values of the crust and lithosperic mantle are reported. How did you chose them? At least some citations are needed.

Lines 125 to 130. All this explanation is not completely clear, particularly the last sentence. How could you reconstruct the thickness of the Cretaseous sediments eroded during the formation of the Foreland basin unconformity starting from the AFT data in the Miocene deposits? Please try to better illustrate your reasoning

Line 141: Why did you chose to fix the T at 1333 °C? And which is the reason for a such specific value? It is an assumption, so I do not understand which is the meaning of a such specific value. I mean, 1350 would be uncorrect? Why?

Line 149. In which locations of the SW Alps is reported this thickness of Middle Triassic limestones? In many places (e.g. External Briançonnais) the Middle Triassic is much more thick….

Results

Line 155: “Domain”?? I would replace with trend

Some major points corcernings this section.

It contains a discussion of the RSCM results, and thus in my opinion it should be moved to the Discussion section.

You found very high Temperatures, up to 340°, in the Early-Middle Jurassic beds of the most of the sections. These temperatures are commonly considered to be in the range of metamorphism. Low grade metamorphism (green schist facies), but metamorphism. In the Discussion below you propose an interpretation in which such thermal perturbation is related to two rifting events: this means that such perturbation lasted for tens of million of years. I would expect that sedimentary rocks affected by temperatures above 300°C for such a long time interval in an extensional setting (in which the fluid circulation through the crust is highly favoured) would be transformed in metamorphic rocks. At least they should bear evidence of recrystallization and neo-blastesis. And I guess that many of your samples are shales or rocks with a shaly component, that is the most reactive to metamorphic reactions. You should face this point by providing a petrographic description of your samples. In the case you can not find any evidence of recrystallization and neo-blastesis you should propose a mechanism that hampered these processes at such high temperatures. I am not an analyst or a Raman expert and I do not want to doubt about the RSCM method; but I know that there has been (and probably there is) debate around it. Some authors reported kinetic effects and the occurrence of metastable poorly crystallized graphitic carbon that, in their opinion, would affect the reliability of the RSCM geothermometer (see for example Foustoukos 2012, ). All the more reason you should describe the rocks you analysed, showing the differences between the ones in the upper part of the sections which not experienced high temperatures and the ones in the lower part which were affected by temperatures up to 340°C. Alternatively, if you could not find any evidence, you should discuss how it was possible, in order to exclude that such temperatures are “fake” temperatures due to analytical artefacts.

Line 168. “(para-) autochtonous” I would avoid this old and ambiguous terms: you could more simply use “AFT ages from the Eocene sediments of the Digne nappe”

Lines 173 to 175: I got your reasoning but I think you could make it a bit more explicit

Line 174: Figure 2C, not 2B

Lines 177-178: see comment before too. Why do you talk about syn-orogenic burial? I would say that T values in the CAS section are consistent with a normal and continuous burial history (fromt Triassic to Pliocene)…

Line 185: did you mean figures 4 and 5? In figure 3 results from all the three models are shown

Lines 195-196: “deepest sediments”: which sediments are you referring to? The ones below 2 Km? In this case I would say between 230-340°C, not above

Lines 205 - 209: This is not convincing at all! The CAS section is just 24 km from the CLN section, and owns to the same tectonic unit and the same paleogeographic domain. Moreover, you can not desume a more internal location from the described difference in thickness of the Jurassic succession (and in figure 6 you do not locate it a more inner position….). Continental rifted margins are characterized by significant and abrupt changse in thickness of the syn-rift sediments. In the Dauphinois domain see for example the Ornon fault area (Chevalier et al 2003, and their figure 1c); the Briançonnais domain is characterized by a reduced and condensed Jurassic succession, but it pertained to a much more external position with respect to the study area of this work. I think that another explanation must be given for the “colder” values of the CAS section.

Lines 210 - 215: This is an important point and deserves much more attention. What are the 3D effects? And the kinetics of organic matter maturation? You simply mention these aspects, but I think you should comment them. Some authors do not consider the RSCM geothermometer reliable because of some of these problems (see my comment above): you should comment more extensively.

DISCUSSION

Lines 219 to 226: such a huge lithospheric stretching and very high geothermal gradients should have a large body of evidence, even in the syn-rift stratigraphic successions (paleofaults, hydrothermal products, abrupt thickness changes, large breccia bodies, etc….). Actually some of these evidence have been reported in literature for the Early cretaceous too, both in the Dauphinois Domain and in the adjoining Brianòonnais Domain. I think you should consider this.

Lines 229 to 231: It is not clear which is the old proposed position of the Digne nappe and your new interpretation. Do you think it corresponded to a hyper-thinned domains? And why it should have been located on a transfer zone?

Line 235: “expected to be better preserved”: what should be better preserved? And the European paleomargin represented the lower plate during the collision….

Lines 236 to 240: Ok, but which could be the reason? Please provide an interpretation

Line 250: what do you mean with “differential extension”?

Line 251-251: What the expression “In case of westward propagation…” mean? Do you think a westward propagation occurred or not?

Line 254: “To the east of the Embrun…”: better in the eastern part of the Embrun…

Line 257: why “in contrast”? I would write: “...reflects the overthrusting of the E-U nappes bu also require heating…”

CONCLUSIONS

General comment: some problems and doubts remain concerning the reliability and accuracy of the applied RSCM geothermometer (e.g.. the drop not reproduced by models; the “colder” CAS section; very high temperatures in a non-metamorphic domain). I would thus be more cautious in proposing such extremely high geothermal gradients, and at least I would suggest that it would be necessary and interesting to apply other geothermometers (illite crystallinity index?) in order to confirm or not the presented T values.

Line 272: “foreland deposition”? not clear, please expand and clarify

FIGURES

Figure 1

A) It lacks a legend. And in the external zone you should more clearly distinguish the Digne nappe

B) The geological map of the external zone is hardly readable. It is too detailed, with a lot of not useful (and not homogenously distributed) elements that, given the figure size, make the figure itself hardly readable. In the caption please add that 15 original RSCM analysis are indicated, and report the name of the sampled stratigraphic sections. Delete the second “reconstructed”.

In the legend the symbols for the tectonic contacts are missing. And I would highlight the Digne main thrust with initials

Figure 2

The font size seems to me too small. Particularly the numbers in 2A and the text in 2C, almost unreadable. In 2B it is not specified that the values of the columns are temperatures

Figure 3

This figure is too small, and the text and numbers. Almost unreadable. You could split the 6 diagrams in 3 + 3….

Figure 4

The isotherms lines are too thin, difficult to see them

Figure 5

Specify in the caption that the models are based on the two-rift hypothesis
Citation: https://doi.org/10.5194/egusphere-2022-949-RC1
- AC1: 'Reply on RC1', Naïm Célini, 25 Nov 2022
  
  The co-authors would like to thank the anonymous referee for the thorough comments on the manuscript.
  Please find attached the pdf file with the answers, point by point, to each comment.
  
  Citation: https://doi.org/10.5194/egusphere-2022-949-AC1
RC2:
'Comment on egusphere-2022-949', Yann Rolland, 23 Oct 2022
General comment

I have read this manuscript with interest as it provides a synthesis of recently acquired (and some new) RSCM data at the SW Alps scale and provide a comprehensive modelling approach to explain these data. The form of the manuscript is excellent. It is well organized and concisely written, the figures are well drafted. Though, I am not an expert of modelling, I found this part well presented and understandable at least from the results. I found especially interesting the position of the authors in not considering raman temperatures as just a result of Alpine thickening as previous authors did (see specific discussion points below), but as a result of previous rifting history. Of course, this could be surprising at first glance in metasediments not exhibiting any other metamorphic minerals at temperatures >300°C. However, this is always the case in Low Pressure sediments (like in contact-metamorphism in general), and maybe the impact of (hot) fluid circulations could explain some relatively short-lived thermal reset (note that ore formation is documented at least for the Jurassic phase). This point could be better discussed in the paper. However, I found that the explanations proposed (two rifting episodes) and tested by modelling are very interesting and fit very well with the regional geology where Jurassic and Lower Cretaceous rifting phases are clearly highlighted. Finally, the general model that is presented seems sound. Of course, the relationship with the Valais ocean is not simple, as the paleogeography has been significantly disturbed by the Alpine orogeny, but, however, the proposed reconstruction agrees with the shape of the Vocontian low and the dissymmetry of the Valais unit (not continuing in SW Alps).

In conclusion, I think that this work deserves a rapid publication with some relatively minor revisions.

Minor comments

Figures: in many cases the text in figures is too small (see attached file).

L47 : Altough, these temperatures are similar to those estimated by mineral thermometry in the Briançonnais (e.g. Lanari et al., 2014), this is no the case in the Pelvoux domain where RSCM estimates lie systematically higher, by about 50°C in eastern Pelvoux, (Simon-Labric et al., 2009), and by > 100°C to the west of Pelvoux ECM (especially, considering vitrinite reflectance results from Cretaceous beds, ( Deville and Sassi (2006) ), which led Bellanger et al. to propose a "hypothetical" substractive contact that has not been observed in the field...

L75 :

I would say ‘before 34 Ma’ to be more consistent with error bars of geochronological methods (Ar-ar dating of syn-kinematic phengite)

Top NW deformation is documented in the Eocene: Lanari et al., 2014, Terra Nova: Top NNW syn-kinematic deformation dated at 45 Ma in briançonnais units at the base of the Helminthoid flysch

L271: not sure the concept of "temperature structure" is adequate...? maybe the thermal state/profile?

See attached file for details
Citation: https://doi.org/10.5194/egusphere-2022-949-RC2
- AC2: 'Reply on RC2', Naïm Célini, 25 Nov 2022
  
  The co-authors would like to thank Yann Rolland for reviewing the manuscript.
  Please find attached the pdf file with the answers for each comment.
  
  Citation: https://doi.org/10.5194/egusphere-2022-949-AC2
RC3:
'Comment on egusphere-2022-949', Anonymous Referee #3, 24 Oct 2022

This paper presents a new dataset of temperatures based on Raman Spectroscopy on Carbonaceous Material and thermal modelling in Mesozoic and Cenozoic rocks of the southwestern Alps to unravel the thermal and tectonic evolution of this part of the mountain belt. I find this work interesting, relevant, and worth to be published in EGUsphere after some moderate revision.

However, this manuscript has some errata and contains some misleading information that I summarized below. The main issue that I find is the lack of a proper description of the geological setting, which is necessary to describe and justify the tectonic scenarios (number of rifting stages) tested for the modelling. The information is sometimes a bit disorganized along the text and the methodology description also needs to be improved. It lacks several references along the text which need to be added. In the following, I made some comments and suggestions that in my opinion will contribute to improve the manuscript and make it easier to understand for the reader.

Abstract:

L14: also Cenozoic rocks.

L20: “lower plate sediments”, do you mean sediments deposited in a basement that subducted, accreted sediments from the subducted plate? Please, clarify.

Introduction:

I find the introduction a bit short and that fails in emphasizing the relevance of this study to unravel the processes governing the accretion of sediments during collision in the SW Alps. The authors are referring frequently about the subduction processes and the accretion of sediments to the upper plate but there is none crustal-scale cross-section that shows the structure they are referring to. I also find the map of figure 1a too simplified.

The general description of the Alps and Digne Nappe is insufficient to understand the tectonic context the authors are studying. In this section I would also rather read about an overview of the geology of the western Alps and the rifting episodes with a reference to figures 1a-1c. A proper definition of the Valaisan and Vocontian domains would be useful, as the authors are often referring to them but are not properly described and are relevant to understand fig 6. Regarding the Digne Nappe, it should also be properly described in the geological setting.

It is not very clear what are the previous AFT studies considered in the study. Caption of figure 1b states that they are specified along the text, but until the “samples and method” section, there are no references. I suggest the authors include previous thermochronological studies in the caption of fig. 1, in the introduction as state-of-the-art studies, and maybe also add other reference such as Bogdanoff et al. (2000).

Geological setting:

This section needs to be reorganized. I would prefer to read first the description of the geology of the study area (crystalline massifs, nappes…) and after that, the tectonic evolution since the Jurassic. In addition, to support the 3 scenarios proposed for the numerical modelling, a more detailed explanation of the geology is needed, including a description of the cross-section of figure 1d.

L67: “Alpine arc”, this term has neither been used before nor later, can it be changed to “Alps”?

L65-68: I suggest rewriting this sentence: “The second, Late Jurassic-Early Cretaceous in age, appears synchronous with the rifting of the Bay of Biscay and led to opening the Valaisan domain to the NE of the Alpine arc and renewed extension in the so-called Vocontian Trough of the SW Alps” to avoid confusion about the extensional phases affecting the NE and the SW Alps. In addition, is it relevant to mention the rifting of the Bay of Biscay here? In case it is, you need to provide references.

L83: “Helminthoid flysch” is only used here, is this relevant for the study? If not, I suggest to remove it. "sub-Briançonnais" does not appear in Fig. 1a, 1b, only Briançonnais, what do you mean with “sub”?

Samples and methods:

L98: Add the number of samples in the beginning of the section.

L99: replace “in rare occasions” for something like: “additionally, two samples were collected in Eocene limestone”.

L110: “… of the existing low-temperature fission-track analyses…”, add apatite before fission-track.

Subsection "Numerical modelling with basin model":

I would rather read first about the software used for the modelling and the parameters chosen, than the scenarios tested. As I am not a user of this software, a brief description of the input parameters, processing and outputs would be very useful.

L127-129: - “Mirabeau well” add the reference to figure 1b.

“We infer that it is 3 km at the Digne thrust front (Fig. 1b).” This sentence is a bit odd, re write, please.

L142-143: “crustal basement with homogeneous properties”, in base of what? A continental crust could be very heterogeneous in terms of lithologies, and therefore, feature different density values. A crystalline crust could imply densities ranging from 2.7 to 3.4 g/cm³ (e.g., Barton, 1986; Rudnick and Fountain, 1995). Other type of models assumes a homogeneous crustal density that increases with depth between 2.6 to 3 g/cm³ (Torne et al., 2015). At lease, provide a reference that supports the value chosen.

For table 1 provide also references and justifications for the values chosen.

Results:

L167: define the acronym AFT before. In addition, add a reference of the AFT study in this sentence.

L169: Add the reference for the geothermal gradient assumption. For instance, Bigot-Cormier et al. (2006) considered a present geothermal gradient of 25-30 ºC/km, and Valla et al. (2011) considered a gradient of 25 ºC/km.

L181: replace to something like: “the comparison between the temperatures derived from RSCM data and from the numerical modelling…”.

L185-186: fig. 3 shows the results for the 3 scenarios, and figure 4 shows the two-rift scenario and one site for the no-rift scenario. Please, be precise.

L205-209: the CAS section is here explained as only burial during the Cenozoic, but how does this fit with the tectonic framework of the rest of the sections which are also located in the Digne Nappe and closer to it (e.g., DGN and CLN)? Maybe a cross-section along this area will help to understand the structure and discuss it more thoroughly…

L211: “affecting the samples in nature”, what do you mean?

Discussion:

Here I miss some discussion about the CAS section and why it is assumed to have experienced a constant geothermal gradient of 30ºC/km if it is also located in a rifted margin.

I would like to have some more information/discussion about the paleogeographic reconstruction of fig. 6 regarding the location of the rift axis, and transfer zones. It could also help to include in fig. 6a the location of the study sections. I am aware that they are already included in fig. 6b, but its tentative position in the map would give the reader a better spatial location of the dataset. In addition, a more detailed description of fig. 6b is needed. That would also help to address my previous comment on the explanation of section CAS.

L232. Geothermal gradients of 80-90 ºC/km in the Pyrenees led to high-temperature metamorphism (Ducoux et al., 2021, a reference that the authors cite), and it is accompanied by mantle exhumation to the base of the sedimentary basin or even to the seafloor (e.g., Lagabrielle et al., 2010; DeFelipe et al., 2017, 2019; Teixell et al., 2018). The rift domains defined there, and the role of transfer zones are topics that are being highly discussed. If you want to make a proper comparison with the tectonic setting of the Pyrenees (and Bay of Biscay as they also mention it without any reference in L66), you need to add more refences and discuss all these topics.

Therefore, in your study area, how was the rift system? Which rift domains are described? Is there any evidence of mantle exhumation? To the east of their study area, ophiolite complexes include serpentinized peridotites with ophicalcite (e.g., Lafay et al., 2017).

In Figure 6a, thinned continental crust is divided into “thinner” and “thicker”, can you provide a thickness estimate? Can you also indicate this in fig. 6b? How is it related to the domains of a rifted margin? (e.g., Tugend et al., 2015, a reference that the authors cite).

Figure 6 can also be enlarged.

Conclusions:

The authors would better summarize their main results here: samples collected, paleotemperature data, scenarios modelled and chosen as representative, and geothermal gradient estimations.

Figures:

Figure 1: please reorganize the figure to have image “1a” in the top left part of the figure.

1b: - What do broken red lines indicate?

- Please, change the colour of the stratigraphic sections (DEV, SLC, …). They have the same colour as the reconstructed isopachs.

- Add the definition of the acronym AFT for apatite fission track in the caption.

Figure 2: rewrite the caption. Suggestion: “stratigraphic sections along the front of the Digne Nappe with the RSCM-derived peak temperatures”.

Figure 3: define the tectonic models as two-rifts, one-rift, no-rift along the text to homogenize terms. Place this figure after it is first called (so after L180). Check spelling of color/colour in the caption.

Figure 4: change the tables of each diagram to something like: “thickness of the crust” and “thickness of the lithospheric mantle”. Otherwise, it looks confusing (thickness vs. time).

Figure 6: in the last two lines of the caption: “dashed lines”, can you provide a tentative value for each isotherm colour?

In fig. 6b the stratigraphic sections are projected for reference, but I suggest projecting them also on the map of fig. 6a. In the legend, separate the “cover” box into Jurassic and Cretaceous.

Other (minor) comments:

L25: “ Where details of basin evolution are lacking high-temperature record…”, add a comma after lacking.

L55: “This study combines 80 new RSCM measurements…”, remove the word new.

L79: “currently running between the variscan…”, remove currently.

Along the text, the Digne Nappe is referred to with different names: “front of the Digne Nappe”, “Digne thrust front”, “Digne frontal thrust”, “Digne thrust sheet”, and “Digne main thrust”, please, unify. Please, do the same for “Vocontian Trough”, “Vocontian domain”, “Vocontian-Valaisan rifting”, “Valaisan extensional domain”, “Valaisan rift”, “Valaisan domain”.

L198: “the Early-Middle Jurassic” add the "y".

L221: add geothermal before “gradients are about 80-90ºC/km”.

L225-226: from where does the β-factor comes from? Literature or your own modelling?

L276: remove the in “between the Europe and Adria”.

References:

Barton, P. J. (1986). The relationship between seismic velocity and density in the continental crust — a useful constraint?, Geophysical Journal International, Volume 87, Issue 1, October 1986, Pages 195–208, https://doi.org/10.1111/j.1365-246X.1986.tb04553.x

Bigot-Cormier, F., Sosson, M., Poupeau, G., Stéphan, J.-F., and Labrin, E. (2006) The denudation history of the Argentera Alpine External Crystalline Massif (Western Alps, France-Italy): an overview from the analysis of fission tracks in apatites and zircons, Geodinamica Acta, 19:6, 455-473, DOI: 10.3166/ga.19.455-473

Bogdanoff, S., Michard, A., Mansour, M. and Poupeau, G. (2000), Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps. Terra Nova, 12: 117-125. https://doi.org/10.1046/j.1365-3121.2000.123281.x

DeFelipe, I., Pedreira, D., Pulgar, J.A., Iriarte, E., Mendia, M., 2017. Mantle exhumation and metamorphism in the Basque-Cantabrian Basin (N Spain): Stable and clumped isotope analysis in carbonates and comparison with ophicalcites in the North-Pyrenean Zone (Urdach and Lherz). Geochemistry, Geophys. Geosystems 18. https://doi.org/10.1002/2016GC006690

DeFelipe, I., Pedreira, D., Pulgar, J. A., van der Beek, P. A., Bernet, M., & Pik, R. (2019). Unraveling the Mesozoic and Cenozoic tectonothermal evolution of the eastern Basque-Cantabrian zone–western Pyrenees by low-temperature thermochornology. , 38, 3436-3461, https://doi.org/10.1029/2019TC005532

Lafay, R., Baumgartner, L.P., Schwartz S., Picazo S., Montes-Hernandez G., Vennemann T. (2017). Petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment (Chenaillet ophicalcites, Western Alps, France): Processes of carbonate cementation, Lithos, Volumes 294–295, Pages 319-338, https://doi.org/10.1016/j.lithos.2017.10.006.

Lagabrielle, Y., Labaume P., de Saint Blanquat, M. (2010), Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies, Tectonics, 29, TC4012, doi:10.1029/2009TC002588.

Rudnick, R. L., and Fountain, D. M. (1995), Nature and composition of the continental crust: A lower crustal perspective, Rev. Geophys. 33(3), 264-309, doi:10.1029/95RG01302.

Teixell, A., Labaume, P., Ayarza, P., Espurt, N., de Saint Blanquat, M., Lagabrielle, Y., 2018. Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data. Tectonophysics 724–725, 146–170. https://doi.org/10.1016/j.tecto.2018.01.009

Torne, M., Fernàndez, M., Vergés, J., Ayala, C., Salas, M.C., Jimenez-Munt, I., Buffett, G.G., Díaz, J., 2015. Crust and mantle lithospheric structure of the Iberian Peninsula deduced from potential field modeling and thermal analysis. Tectonophysics 663, 419–433. https://doi.org/10.1016/j.tecto.2015.06.003

Valla, P. G., van der Beek, P. A., and Braun, J. (2011). Rethinking low-temperature thermochronology data sampling strategies for quantification of denudation and relief histories: A case study in the French western Alps, Earth and Planetary Science Letters, Volume 307, Issues 3–4, Pages 309-322, https://doi.org/10.1016/j.epsl.2011.05.003.

Citation: https://doi.org/10.5194/egusphere-2022-949-RC3
- AC3: 'Reply on RC3', Naïm Célini, 25 Nov 2022
  
  The co-authors would like to thank this anonymous referee for the review and the thorough comments on the manuscript.
  Please find attached the pdf file with all the answer to each comment.
  
  Citation: https://doi.org/10.5194/egusphere-2022-949-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Naïm Célini on behalf of the Authors (25 Nov 2022) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (28 Nov 2022) by Stefano Tavani

Dear Authors,
after reading the revised manuscript and your responses to the reviewers’ comments, I have concluded that an additional round of revision is required.
Below the four points you need to address:

1) You haven’t provided any response for a major point raised during the first round of revision. I’m reporting it below and I strongly encourage you to address this issue in the manuscript, not only in the response letter.
You found very high Temperatures, up to 340°, in the Early-Middle Jurassic beds of the most of the sections. These temperatures are commonly considered to be in the range of metamorphism. Low grade metamorphism (green schist facies), but metamorphism. In the Discussion below you propose an interpretation in which such thermal perturbation is related to two rifting events: this means that such perturbation lasted for tens of million of years. I would expect that sedimentary rocks affected by temperatures above 300°C for such a long time interval in an extensional setting (in which the fluid circulation through the crust is highly favoured) would be transformed in metamorphic rocks. At least they should bear evidence of recrystallization and neo-blastesis. And I guess that many of your samples are shales or rocks with a shaly component, that is the most reactive to metamorphic reactions. You should face this point by providing a petrographic description of your samples. In the case you can not find any evidence of recrystallization and neo-blastesis you should propose a mechanism that hampered these processes at such high temperatures. I am not an analyst or a Raman expert and I do not want to doubt about the RSCM method; but I know that there has been (and probably there is) debate around it. Some authors reported kinetic effects and the occurrence of metastable poorly crystallized graphitic carbon that, in their opinion, would affect the reliability of the RSCM geothermometer (see for example Foustoukos 2012, American Mineralogist). All the more reason you should describe the rocks you analysed, showing the differences between the ones in the upper part of the sections which not experienced high temperatures and the ones in the lower part which were affected by temperatures up to 340°C. Alternatively, if you could not find any evidence, you should discuss how it was possible, in order to exclude that such temperatures are “fake” temperatures due to analytical artefacts

2) In your work you report the occurrence of a temperature shift of about 100° across the Middle/Upper Jurassic boundary. This shift occurs between samples processed with different calibrations (Lahfid et al. (2010) for temperatures ranging between 200 and 340°C, and Saspiturry et al. (2020) for lower temperatures between 100 and 180°C). Can you rule out that this shift is due to a methodological issue? Having an overlap area (let’s say 150-250°) in which samples are processed with both calibrations would greatly help to convince the reader.

3) Again concerning the shift. If confirmed, such a remarkable temperature shift intuitively requires a thermal event at the end of the Middle Jurassic, rather than during the Cretaceous. Comparison between models and data, indeed, shows that the two-rifting scenario fails to reproduce the abrupt temperature change for 4 of the 6 analyzed sections.

4) The Cretaceous tectonic framework of the SE France basin is slightly different from what you are reporting in figure 6. The Vocontian basin, the durance Isthmus, and the South Provence basin form E-W elongated horst and graben structures and, fully in agreement, mid Cretaceous faults in the area point to N-S extension. Crustal scale mid Cretaceous faults in the Brianconnais domain also point to N-S extension, rather than NW-SE oriented extension as you infer.

The above points should be solved or at least you should add a section in the discussion, in which you list the limitations/discrepancies of your model

Stefano Tavani

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AR by Naïm Célini on behalf of the Authors (01 Dec 2022) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (02 Dec 2022) by Stefano Tavani

Dear Chief Editor and Authors.

This paper has passed trough a first round of revision, with three contrasting reports provided by three independent reviewers. Then, I have reviewed the revised version of the paper, for which I have indicated four points to be addressed.
The authors’ response to my four points is only partly satisfying, and because of this I have to ask for another round of revision. I will try to explain more in detail the motivation of my “Editor’s requests”.

Point 1 arises from the curiosity of the first reviewer about your temperatures of 300° obtained in a sedimentary sequence that is commonly considered by geologist working in that area as non metamorphic. I share this curiosity and I suspect that many readers will do the same. I recognize your effort in providing a response to this question but it is not fully clear why you do not want to add petrographic description of the “hotter” samples, as suggested by the reviewer #1. I would really appreciate having in the supplementary material images of thin sections of the samples for which you have obtained temperatures around 300°.

Point 2
As the authors know, Raman derived temperatures in diagenetic environments are problematic, and the application of the qualitative method by Saspiturry et al. (2020) could be considered questionable by many readers. Moreover, the fact that your thermal jump occurs at the transition between temperatures estimated by two different methods (one of which is by Saspiturry et al.), strongly calls the attention. In order to demonstrate the occurrence of the temperature jump, if you do not want to use the two methods in an overlap area, in figure 2 you should at least add the graphs for some raman spectra parameters, such as the RBS (Raman band separation) or the R1 (intensity ratio between D and G).

Point 3
Com’on! If you tune properly the parameters a polynomial function of degree N compared to a polynomial function of degree N-1 will always provide a better fit. So, if you add a third rifting event your RMS will surely decrease. The point is that you should make crystal clear to the reader that despite you are adding a second rifting event (i.e. despite you are increasing the degree of the polynomial function) the temperature shift across Middle-Late Jurassic boundary still cannot be modeled.

Point 4
I understand that in this paper you do not want to discuss about the connection between the NE-SW striking Valaisian structures and the WNW-ESE striking “Pyrenean” structures of the SE France basin/Brianconnais. However, your figure 6 suggests the two sets of structures form part of the same framework. Either you tone down such a graphic suggestion (i.e. removing the faults in Vocontian basin and the entire Provence platform from your figure 6) or you discuss the bigger picture and explicate you view on how the Pyrenean and Valesian rifts connects.

Stefano Tavani

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AR by Naïm Célini on behalf of the Authors (07 Dec 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (08 Dec 2022) by Stefano Tavani

ED: Publish as is (09 Dec 2022) by Federico Rossetti (Executive editor)

AR by Naïm Célini on behalf of the Authors (09 Dec 2022) Manuscript

Journal article(s) based on this preprint

04 Jan 2023

Rift thermal inheritance in the SW Alps (France): insights from RSCM thermometry and 1D thermal numerical modelling

Naïm Célini, Frédéric Mouthereau, Abdeltif Lahfid, Claude Gout, and Jean-Paul Callot

Solid Earth, 14, 1–16, https://doi.org/10.5194/se-14-1-2023,https://doi.org/10.5194/se-14-1-2023, 2023

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We investigate the peak temperature of sedimentary rocks of the SW Alps (France), using the Raman Spectroscopy on Carbonaceous Material. This method provides an estimate of the peak temperature achieved by organic-rich rocks. To determine the timing and the tectonic context at the origin of these temperatures we use 1D thermal modelling. We find that the high temperatures up to 300 °C were achieved during pre-collisional extensional events, not during tectonic burial in the Western Alps.


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