Formation and growth of diapirs in contractional settings: the Mediano anticline and Clamosa diapir case study (Southern Pyrenees)

Santolaria, Pablo; Silva-Casal, Roi; Carrera, Núria; Muñoz, Josep A.; Arbués, Pau; Granado, Pablo

doi:https://doi.org/10.5194/egusphere-2025-1742

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https://doi.org/10.5194/egusphere-2025-1742

Preprints

05 May 2025

| 05 May 2025

Formation and growth of diapirs in contractional settings: the Mediano anticline and Clamosa diapir case study (Southern Pyrenees)

Pablo Santolaria, Roi Silva-Casal, Núria Carrera, Josep A. Muñoz, Pau Arbués, and Pablo Granado

Abstract. At the northwestern termination of the South Pyrenean Central Salient, thrust imbrication, detachment folding and diapirism are structurally and genetically related. The La Fueba imbricate system has been folded by the Mediano detachment anticline, while this fold connects with the Clamosa diapir. Together, this structural-stratigraphic trinity provides an excellent example to evaluate the factors controlling purely contractional diapirs in onshore-offshore contractional fold-thrust systems. Our study integrates detailed geological mapping, a large structural dataset, new biostratigraphic data, and interpretations of well-tied depth-converted seismic lines and cross-sections. A combined evolutionary model for contractional diapir formation and evolution is presented. While less common than in extensional settings, the interplay shortening of a salt horizon along with vertical axis rotations and local stretching and erosion of the overburden can lead to purely contractional salt diapirs.

Received: 11 Apr 2025 – Discussion started: 05 May 2025

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Pablo Santolaria, Roi Silva-Casal, Núria Carrera, Josep A. Muñoz, Pau Arbués, and Pablo Granado

Status: closed

CC1:
'Referee Comment on egusphere-2025-1742', Antonio Teixell, 16 Jun 2025

This ms. deals with a complex field area in the Southern Pyrenees with which I have some familiarity, so I read it with interest. It is a well written and illustrated ms. that faces several factors that are at stake I the area, as diapirism, buried/blind thrusting (of enigmatic signature), and vertical-axis rotations, not all of them little understood on themselves, but the merit here is the attempt to account for all in an integrated model.
As a sollicited referee I must make a critical review, and I say beforehand that I have no objections for publication of the ms. with minor to moderate revision. My recommendations would go to avoid potential suspect of overinterpretation (I believe that uniterpreted versions of the seismic lines are necessary, for a reader to judge), or the presentation of some interpretations as facts (the data yield what they yield, if you allow me the spanish expression). This should be aknowledged in the text, which probably needs the style to be smoothed in some cathegorical assertions.
A particular structure that called my attention is the Trillo backthrust, which is not easily visble in the seismics (the drawn intepretation aside) and unfortunately is not cropping out at the surface either. Yet this is a crucial structure that explains the long-standing conundrum of the trace of the Montsec thrust in the study area. The backthust is certainly an ingenious solution that conveniently does its job by accounting for the fate of the Montsec, but as important as it is (and with more than 5 km of slip), it deserves better documentation and discussion. Another contribution of the ms. that deserves more specific emphasis is the role of vertical-axis rotations in opening diapiric space (e.g. compare figs 9 d and e), which is not even mentioned in the conclusions. Other minor comments are indicated in the annotated manuscript that I’m attaching.
Antonio Teixell

Citation: https://doi.org/10.5194/egusphere-2025-1742-CC1
- RC1:
  'CC1 again as RC', Antonio Teixell, 27 Jun 2025
  
  This ms. deals with a complex field area in the Southern Pyrenees with which I have some familiarity, so I read it with interest. It is a well written and illustrated ms. that faces several factors that are at stake I the area, as diapirism, buried/blind thrusting (of enigmatic signature), and vertical-axis rotations, not all of them little understood on themselves, but the merit here is the attempt to account for all in an integrated model.
  As a sollicited referee I must make a critical review, and I say beforehand that I have no objections for publication of the ms. with minor to moderate revision. My recommendations would go to avoid potential suspect of overinterpretation (I believe that uniterpreted versions of the seismic lines are necessary, for a reader to judge), or the presentation of some interpretations as facts (the data yield what they yield, if you allow me the spanish expression). This should be aknowledged in the text, which probably needs the style to be smoothed in some cathegorical assertions.
  A particular structure that called my attention is the Trillo backthrust, which is not easily visble in the seismics (the drawn intepretation aside) and unfortunately is not cropping out at the surface either. Yet this is a crucial structure that explains the long-standing conundrum of the trace of the Montsec thrust in the study area. The backthust is certainly an ingenious solution that conveniently does its job by accounting for the fate of the Montsec, but as important as it is (and with more than 5 km of slip), it deserves better documentation and discussion. Another contribution of the ms. that deserves more specific emphasis is the role of vertical-axis rotations in opening diapiric space (e.g. compare figs 9 d and e), which is not even mentioned in the conclusions. Other minor comments are indicated in the annotated manuscript that I’m attaching.
  Antonio Teixell
  
  Citation: https://doi.org/10.5194/egusphere-2025-1742-RC1
  - AC1: 'Reply on RC1', Pablo Santolaria, 30 Jul 2025
    
    We appreciate the interest and constructive revisions.
    Being aware of the importance of the Trillo backthrust and in light of your comment, we have included in the reviewed version an extended explanation and justification for our interpretation of this structure. The rationale behind proposing the existence of the Trillo back thrust lies in the need to explain the juxtaposition of two structurally and stratigraphically contrasting sequences: a ~4 km-thick, sub horizontal and continuous Jurassic to Cuisian sequence associated with the Montsec thrust sheet, and a ~2 km-thick Upper Cretaceous to Cuisian sequence with Gavarnie–Sierras Marginales affinity; the latter is structurally above the former and fragmented into blocks hundreds of meters wide.
    Moreover, unlike the area north of the Clamosa diapir, the tip of the Montsec thrust sheet is not clearly imaged in the available seismic lines. Instead, we interpret a triangular fragment of the Gavarnie–Sierras units in the footwall position relative to both the Montsec thrust and the Trillo back thrust. Taking all of this into account, the most coherent solution is to propose a back thrust that emplaced the Gavarnie–Sierras units over the Montsec thrust sheet, truncating and exposing at the surface the Montsec tip where it was subsequently eroded.
    The influence of vertical-axis rotation, which characterizes the western termination of the South Pyrenean Central Salient, is already discussed in detail in the main discussion. However, it is true that it was not mentioned in the conclusions; this omission has been addressed in the current version.
    Minor comments have been amended in the revised version.
    Regarding the second comment and as suggested by both reviewers, a new figure with the uninterpreted version of seismic lines is provided as Appendix B in the new version of the manuscript.
    
    Citation: https://doi.org/10.5194/egusphere-2025-1742-AC1
RC2:
'Comment on egusphere-2025-1742', Anonymous Referee #2, 05 Jul 2025

Thank you to the editors and authors for the opportunity to review the manuscript by Santolaria et al. The manuscript provides a new structural interpretation of the evolution of an area in the southern Pyrenees, based on field mapping, interpretation of 2D seismic sections, and on 2D cross section building. The area is structurally complex, located in between two thrust sheets and the focus of the manuscript is on the evolution of an outcrop of Keuper interpreted as a diapir (Clamosa diapir), and the adjacent anticline (Mediano Anticline). I appreciated that the authors contextualize the importance of their interpretation regarding the formation of the Clamosa diapir, by pointing out that diapirs formed purely during the shortening phase in fold-and-thrust belts are not that commonly described in the literature (whereas pre-shortening salt diapirs are well-recorded). The manuscript is very well written and nicely illustrated, and I would recommend it for publication.
I have the following comments related to the interpretation and evolution of the area, and presentation of the results.
1) The authors provide the seismic lines with the interpretation, and it is difficult to assess if the seismic lines provide any constrains for the interpretation of the areas which are key to the story. This is especially important in relation to the Trillo back thrust at depth. The Trillo back-thrust is a critical element of the entire history around the Clamosa diapir, because it would have allowed the shallowing of the Keuper cored structure and hence its erosion. However, it is not possible for the readers to assess if the geometries of Figures 7 and 8 are really supported by data, or are based on the conceptual models of the authors. I would encourage the authors to share the uninterpreted seismic sections.
2) Regarding the salt inflation of Clamosa diapir and Mediano anticline, the way it has been sketched in the Figure 9e-f poses some questions. The authors are implying that salt flowing from somewhere in the Triassic basin, was able to inflate these structures, from their depiction as portions along the surface of the Trillo back in Figure 9e, to the salt cored diapir and anticlines in 9f. Also, the figure implies that salt inflation was able to uplift the entire portion of the thrust sheet between Trillo thrust and Olson anticline. I find it difficult to sustain that salt flow alone would be able to open these vertical and horizontal welded surfaces without being assisted by additional forces. For the inflation of the Clamosa diapir, one could argue that if the Gavarnie thrust starts moving, differential displacement between Gavarnie and Montsec thrusts could aid in opening the diapir, but in that case also, some collapsing of the structures would be observed.
3) Finally, the basis for arguing that the Clamosa diapir was formed purely during the compressional stage is based on discarding the possibility of pre-shortening salt movements within the area, which some authors have proposed earlier. I do not discard any or the other possibility. But it could be argued that the observation that the diapir is covered by different stratigraphic levels (Jurassic in the southern margin vs, cretaceous in northern) could already be pointing to salt movement, although the authors resolve with an earlier structural high.
Some minor comments typos:
Fig 3. Caption, correct “nummulitic”.
Line 162. “Teixell and Barnolas”, year is missing.

Citation: https://doi.org/10.5194/egusphere-2025-1742-RC2
- AC2: 'Reply on RC2', Pablo Santolaria, 30 Jul 2025
  
  First of all, we would like to thank you for the time and effort devoted to reviewing our article, as well as for your positive comments.
  Regarding the first comment, and as suggested by both reviewers, we have included a new figure showing the uninterpreted version of the seismic lines. This has been added as Appendix B in the revised version of the manuscript.
  The second comment addresses a critical issue that we also faced during the development of our emplacement model for the Clamosa diapir. While the relative uplift of the entire Gavarnie–Sierras Marginales sequence is well constrained, estimating the elevation of the Lutetian unconformity—and, consequently, the amount of salt inflation—remains essential. Contrary to what is suggested in the review, there is no need to “open [...] vertical and horizontal welded surfaces,” as these surfaces were never welded.
  As illustrated in Figures 9a to 9d, the salt unit is always present, albeit with variable thickness. The nature of this inflation is discussed in detail in the second paragraph of the Discussion section, which is also referenced at the end of Section 4.
  In the Discussion, we also address the role of differential displacement—leading to vertical-axis rotation and extension—in promoting the extrusion and continued growth of the Clamosa diapir through Triassic salt inflation and accumulation.
  Finally, we are not ruling out pre-shortening salt movements in the area. The distribution of Triassic basins and depocenters within our study area is spatially linked to the presence of Jurassic and Cenomanian carbonate units. These units are still preserved today likely because they subsided into salt-rich depocenters during the Mesozoic, thus avoiding erosion. To clarify this point, we have added the following sentence in the revised manuscript:“We propose that the presence of these rocks relates to their deposition over relatively thicker Middle–Upper Triassic depocenters. These depocenters were prone to accommodate the Lower Jurassic carbonates and later the Upper Cenomanian limestones, which may have subsided due to differential loading and were thereby sheltered from erosion.”
  However, we do reject the presence of a pre-existing diapir, as there is no evidence such as growth strata or halokinetic sequences within the pre-contractional sequence (Upper Cretaceous–Ypresian) to support its existence.
  Minor typos have been amended.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1742-AC2

Status: closed

CC1:
'Referee Comment on egusphere-2025-1742', Antonio Teixell, 16 Jun 2025

This ms. deals with a complex field area in the Southern Pyrenees with which I have some familiarity, so I read it with interest. It is a well written and illustrated ms. that faces several factors that are at stake I the area, as diapirism, buried/blind thrusting (of enigmatic signature), and vertical-axis rotations, not all of them little understood on themselves, but the merit here is the attempt to account for all in an integrated model.
As a sollicited referee I must make a critical review, and I say beforehand that I have no objections for publication of the ms. with minor to moderate revision. My recommendations would go to avoid potential suspect of overinterpretation (I believe that uniterpreted versions of the seismic lines are necessary, for a reader to judge), or the presentation of some interpretations as facts (the data yield what they yield, if you allow me the spanish expression). This should be aknowledged in the text, which probably needs the style to be smoothed in some cathegorical assertions.
A particular structure that called my attention is the Trillo backthrust, which is not easily visble in the seismics (the drawn intepretation aside) and unfortunately is not cropping out at the surface either. Yet this is a crucial structure that explains the long-standing conundrum of the trace of the Montsec thrust in the study area. The backthust is certainly an ingenious solution that conveniently does its job by accounting for the fate of the Montsec, but as important as it is (and with more than 5 km of slip), it deserves better documentation and discussion. Another contribution of the ms. that deserves more specific emphasis is the role of vertical-axis rotations in opening diapiric space (e.g. compare figs 9 d and e), which is not even mentioned in the conclusions. Other minor comments are indicated in the annotated manuscript that I’m attaching.
Antonio Teixell

Citation: https://doi.org/10.5194/egusphere-2025-1742-CC1
- RC1:
  'CC1 again as RC', Antonio Teixell, 27 Jun 2025
  
  This ms. deals with a complex field area in the Southern Pyrenees with which I have some familiarity, so I read it with interest. It is a well written and illustrated ms. that faces several factors that are at stake I the area, as diapirism, buried/blind thrusting (of enigmatic signature), and vertical-axis rotations, not all of them little understood on themselves, but the merit here is the attempt to account for all in an integrated model.
  As a sollicited referee I must make a critical review, and I say beforehand that I have no objections for publication of the ms. with minor to moderate revision. My recommendations would go to avoid potential suspect of overinterpretation (I believe that uniterpreted versions of the seismic lines are necessary, for a reader to judge), or the presentation of some interpretations as facts (the data yield what they yield, if you allow me the spanish expression). This should be aknowledged in the text, which probably needs the style to be smoothed in some cathegorical assertions.
  A particular structure that called my attention is the Trillo backthrust, which is not easily visble in the seismics (the drawn intepretation aside) and unfortunately is not cropping out at the surface either. Yet this is a crucial structure that explains the long-standing conundrum of the trace of the Montsec thrust in the study area. The backthust is certainly an ingenious solution that conveniently does its job by accounting for the fate of the Montsec, but as important as it is (and with more than 5 km of slip), it deserves better documentation and discussion. Another contribution of the ms. that deserves more specific emphasis is the role of vertical-axis rotations in opening diapiric space (e.g. compare figs 9 d and e), which is not even mentioned in the conclusions. Other minor comments are indicated in the annotated manuscript that I’m attaching.
  Antonio Teixell
  
  Citation: https://doi.org/10.5194/egusphere-2025-1742-RC1
  - AC1: 'Reply on RC1', Pablo Santolaria, 30 Jul 2025
    
    We appreciate the interest and constructive revisions.
    Being aware of the importance of the Trillo backthrust and in light of your comment, we have included in the reviewed version an extended explanation and justification for our interpretation of this structure. The rationale behind proposing the existence of the Trillo back thrust lies in the need to explain the juxtaposition of two structurally and stratigraphically contrasting sequences: a ~4 km-thick, sub horizontal and continuous Jurassic to Cuisian sequence associated with the Montsec thrust sheet, and a ~2 km-thick Upper Cretaceous to Cuisian sequence with Gavarnie–Sierras Marginales affinity; the latter is structurally above the former and fragmented into blocks hundreds of meters wide.
    Moreover, unlike the area north of the Clamosa diapir, the tip of the Montsec thrust sheet is not clearly imaged in the available seismic lines. Instead, we interpret a triangular fragment of the Gavarnie–Sierras units in the footwall position relative to both the Montsec thrust and the Trillo back thrust. Taking all of this into account, the most coherent solution is to propose a back thrust that emplaced the Gavarnie–Sierras units over the Montsec thrust sheet, truncating and exposing at the surface the Montsec tip where it was subsequently eroded.
    The influence of vertical-axis rotation, which characterizes the western termination of the South Pyrenean Central Salient, is already discussed in detail in the main discussion. However, it is true that it was not mentioned in the conclusions; this omission has been addressed in the current version.
    Minor comments have been amended in the revised version.
    Regarding the second comment and as suggested by both reviewers, a new figure with the uninterpreted version of seismic lines is provided as Appendix B in the new version of the manuscript.
    
    Citation: https://doi.org/10.5194/egusphere-2025-1742-AC1
RC2:
'Comment on egusphere-2025-1742', Anonymous Referee #2, 05 Jul 2025

Thank you to the editors and authors for the opportunity to review the manuscript by Santolaria et al. The manuscript provides a new structural interpretation of the evolution of an area in the southern Pyrenees, based on field mapping, interpretation of 2D seismic sections, and on 2D cross section building. The area is structurally complex, located in between two thrust sheets and the focus of the manuscript is on the evolution of an outcrop of Keuper interpreted as a diapir (Clamosa diapir), and the adjacent anticline (Mediano Anticline). I appreciated that the authors contextualize the importance of their interpretation regarding the formation of the Clamosa diapir, by pointing out that diapirs formed purely during the shortening phase in fold-and-thrust belts are not that commonly described in the literature (whereas pre-shortening salt diapirs are well-recorded). The manuscript is very well written and nicely illustrated, and I would recommend it for publication.
I have the following comments related to the interpretation and evolution of the area, and presentation of the results.
1) The authors provide the seismic lines with the interpretation, and it is difficult to assess if the seismic lines provide any constrains for the interpretation of the areas which are key to the story. This is especially important in relation to the Trillo back thrust at depth. The Trillo back-thrust is a critical element of the entire history around the Clamosa diapir, because it would have allowed the shallowing of the Keuper cored structure and hence its erosion. However, it is not possible for the readers to assess if the geometries of Figures 7 and 8 are really supported by data, or are based on the conceptual models of the authors. I would encourage the authors to share the uninterpreted seismic sections.
2) Regarding the salt inflation of Clamosa diapir and Mediano anticline, the way it has been sketched in the Figure 9e-f poses some questions. The authors are implying that salt flowing from somewhere in the Triassic basin, was able to inflate these structures, from their depiction as portions along the surface of the Trillo back in Figure 9e, to the salt cored diapir and anticlines in 9f. Also, the figure implies that salt inflation was able to uplift the entire portion of the thrust sheet between Trillo thrust and Olson anticline. I find it difficult to sustain that salt flow alone would be able to open these vertical and horizontal welded surfaces without being assisted by additional forces. For the inflation of the Clamosa diapir, one could argue that if the Gavarnie thrust starts moving, differential displacement between Gavarnie and Montsec thrusts could aid in opening the diapir, but in that case also, some collapsing of the structures would be observed.
3) Finally, the basis for arguing that the Clamosa diapir was formed purely during the compressional stage is based on discarding the possibility of pre-shortening salt movements within the area, which some authors have proposed earlier. I do not discard any or the other possibility. But it could be argued that the observation that the diapir is covered by different stratigraphic levels (Jurassic in the southern margin vs, cretaceous in northern) could already be pointing to salt movement, although the authors resolve with an earlier structural high.
Some minor comments typos:
Fig 3. Caption, correct “nummulitic”.
Line 162. “Teixell and Barnolas”, year is missing.

Citation: https://doi.org/10.5194/egusphere-2025-1742-RC2
- AC2: 'Reply on RC2', Pablo Santolaria, 30 Jul 2025
  
  First of all, we would like to thank you for the time and effort devoted to reviewing our article, as well as for your positive comments.
  Regarding the first comment, and as suggested by both reviewers, we have included a new figure showing the uninterpreted version of the seismic lines. This has been added as Appendix B in the revised version of the manuscript.
  The second comment addresses a critical issue that we also faced during the development of our emplacement model for the Clamosa diapir. While the relative uplift of the entire Gavarnie–Sierras Marginales sequence is well constrained, estimating the elevation of the Lutetian unconformity—and, consequently, the amount of salt inflation—remains essential. Contrary to what is suggested in the review, there is no need to “open [...] vertical and horizontal welded surfaces,” as these surfaces were never welded.
  As illustrated in Figures 9a to 9d, the salt unit is always present, albeit with variable thickness. The nature of this inflation is discussed in detail in the second paragraph of the Discussion section, which is also referenced at the end of Section 4.
  In the Discussion, we also address the role of differential displacement—leading to vertical-axis rotation and extension—in promoting the extrusion and continued growth of the Clamosa diapir through Triassic salt inflation and accumulation.
  Finally, we are not ruling out pre-shortening salt movements in the area. The distribution of Triassic basins and depocenters within our study area is spatially linked to the presence of Jurassic and Cenomanian carbonate units. These units are still preserved today likely because they subsided into salt-rich depocenters during the Mesozoic, thus avoiding erosion. To clarify this point, we have added the following sentence in the revised manuscript:“We propose that the presence of these rocks relates to their deposition over relatively thicker Middle–Upper Triassic depocenters. These depocenters were prone to accommodate the Lower Jurassic carbonates and later the Upper Cenomanian limestones, which may have subsided due to differential loading and were thereby sheltered from erosion.”
  However, we do reject the presence of a pre-existing diapir, as there is no evidence such as growth strata or halokinetic sequences within the pre-contractional sequence (Upper Cretaceous–Ypresian) to support its existence.
  Minor typos have been amended.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1742-AC2

Pablo Santolaria, Roi Silva-Casal, Núria Carrera, Josep A. Muñoz, Pau Arbués, and Pablo Granado

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Short summary

Among sedimentary rocks, evaporites (as salt) have a particular behavior when deformed under geological forces: they flow while the others break. Such behavior controls the evolution of mountain building events. By mapping the distribution of rocks and interpreting the subsurface architecture of geological structures we were able to reconstruct the mountain building processes of an area in the Southern Pyrenees and how those evaporites flowed and accumulated where we find them nowadays.


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