the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jul 2025
Polyphase tectonic, thermal and burial history of the Vocontian basin revealed by U-Pb calcite dating
Abstract. The Vocontian Basin in southeastern France records a long-lived history of subsidence and polyphase deformation at the junction of Alpine and Pyrenean orogenic systems. This study aims to reconstructed the geodynamical evolution of this basin, based on new U–Pb dating of calcite from veins and faults combined with RSCM thermometry and stratigraphy-based burial models. Three main generations of calcites are dated: (1) Late Cretaceous to Paleocene dates related to Pyrenean-Provençal convergence (~84–50 Ma); (2) Oligocene dates linked to the West European Rift extension (~30–24 Ma); and (3) Miocene dates ascribed to strike-slip and compression associated with Alpine collision (~12–7 Ma). No older ages related to the Jurassic and Early Cretaceous rifting phases are obtained suggesting limited syn-rift fluid circulation or subsequent dissolution of early calcite mineralization. RSCM data highlight a pronounced E–W thermal gradient, with peak temperatures exceeding 250 °C in the eastern basin, consistent with crustal thinning and/or salt diapirism. These results emphasize the large-scale impact of the opening of the West European Rift in SE France and underscore the possible mismatch between the large-scale tectonics and the tectonic history inferred from calcite U–Pb dating, which is sensible to presence of fluids and the physical conditions for their preservations.
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RC1: 'Comment on egusphere-2025-3332', Anonymous Referee #1, 07 Sep 2025
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Review of manuscript entitled
“Polyphase tectonic, thermal and burial history of the Vocontian basin revealed by U-Pb calcite dating”
by Boschetti et al.
General comments
The authors aim to describe the tectonic, thermal and polyphase burial history of the Vocontian Basin. However, this very ambitious undertaking is not entirely successful, and the resulting discussion is unconvincing. Although the authors present a wealth of new data, making this a potentially interesting and publishable contribution asks for more work. The main problem with the current manuscript is that the data report is largely disconnected from the discussion. There are numerous inconsistencies and assumptions, and the reconciliation of the different data sets remains clumsy. It would be a shame to publish the current version, as the data, the authors' experience and the subject matter easily allow for improvement, but this would require rewriting parts of the current manuscript. The main point is that the discussion and main conclusions should instead be based on the data presented. I am not sure that the data presented allow for a large-scale interpretation of the geodynamic evolution of the Vocontian (the data are too sporadic and scattered for that), but I am convinced that they can provide interesting information and insights, which are not, however, fully discussed in the report. As presented, the manuscript resembles a data report accompanied by a discussion that, while not insignificant, is largely disconnected from the data. I therefore suggest major revisions to this manuscript. If the authors were to rewrite parts of the present manuscript, the following points could be taken into consideration
- The objectives are vague and need to be better defined
- The data are overinterpreted (particularly the paleostress data)
- The links between the different data are not established, the presentation of the data lacks clarity, and the data points are difficult to locate (especially the RSCM data)
- The correlations are arbitrary, and the scaling of the data and the link to large-scale events are not constrained and are often more assumed than corroborated by actual observations (a more realistic and data-driven discussion would be welcome)
- The final interpretation of the various data seems fragmented, and the link between the data and their integration into a broader geodynamic framework remains unconvincing.
Reading the manuscript, one gets the impression that a set of data is attempting to solve a geological problem without really being able to define it and offer a well-defined discussion of the data. In addition, the manuscript is repetitive and, in many places, the English could be improved.
More general comments that may help with rewriting the manuscript (at this stage, I am not referring to detailed comments, as I consider that the current version requires significant improvement and rewriting)
Abstract:
It needs to be rewritten. I strongly recommend focusing on the data and highlighting their implications for understanding the Vocontian BasinIntroduction:
The large-scale geodynamic context and the small-scale/sample scale should be better linked. In the current version, it is not clear how the point data on veins, palaeostresses and thermal state can be linked to large-scale interpretations. It is clear that the authors see the link, but the reader should be able to follow their reasoning. An attempt should be made to define the actual objective of this study more precisely and to remain focused on that objective.Geological context:
It is not clear how stratigraphic logs can be defined from maps alone. Why did the authors not rely on existing logs and/or drilling data (both of which exist and are available)? In addition, the geological context would benefit from being better structured. Why not start by describing the location/geodynamic context and its evolution (some parts of which are included in the introduction)? A second section could present the tectono-stratigraphy (which constitutes the bulk of the current geological context). A third section could summarise the existing data sets and interpretations, a section that is missing in the current geological context.Sampling strategy:
The analysis of palaeostresses is based on the assumption that structures record the large-scale tectonic stress field that controls large-scale deformation. However, in a salt basin, the sedimentary sequences covering the salt may be decoupled and the deformation structures may not necessarily record the larger-scale stress field. The authors do not address this point, which could call into question the validity of the data. Another weakness of the study is that the history of burial, as presented in the manuscript, is not really well defined. How, for example, is palaeobathymetry determined and how are uplift events (e.g. the Barremian/Aptian event) treated in the approach?Discussion:
There is a significant disconnect between the discussion and the data set presented. It is difficult to understand how the new age provided actually constrains the geodynamic evolution of the Vocontian and how the few point data can allow for a discussion of the geodynamic evolution of the Vocontian basin. Indeed, if the aim of the article is to discuss the geodynamic evolution, a figure similar to Figure 12 should be inserted in the geological setting and questions should be formulated that in turn can be answered with the here presented more accurate data.The evolution of the burial is difficult to understand solely on the basis of the manuscript presented in this article.
The authors conclude that rifting is not limited by the U-Pb geochronology of calcite: if I understand correctly (but I am not sure), U-Pb dating of calcite was only performed on lithologies from the Upper Jurassic to the Lower Cretaceous. If this is true, it is not surprising that the veins do not record Jurassic rifting, as the rocks containing the veins are post-rift and therefore the veins cannot record Jurassic rifting events.
The discussion of Ca-rich fluid circulation is somewhat superficial: either it is retained, but then needs to be substantiated, or it is omitted.
Age correlations: it appears that the analytical ages correlate with large scale tectonic events, but do these ages actually record and result from a large scale tectonic event or is it just locale deformation? The U-Pb ages of calcite, particularly if the veins are polyphase, as seems to be the case for some of them, are probably more complex, and the presentation here seems to be an oversimplification of the ages, which are in reality more difficult to interpret. A more detailed discussion of the ages would be desirable.
Citation: https://doi.org/10.5194/egusphere-2025-3332-RC1
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