Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore SW Iberia

Alves, Tiago M.

doi:https://doi.org/10.5194/egusphere-2023-1671

Preprints

https://doi.org/10.5194/egusphere-2023-1671

Preprints

09 Aug 2023

| 09 Aug 2023

Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore SW Iberia

Tiago M. Alves

Abstract. The structural styles and magnitudes of Alpine tectonic inversion are reviewed for the Atlantic margin of SW Iberia, a region known for its historical earthquakes, tsunamis, and associated geohazards. Reprocessed, high-quality 2D seismic profiles provide new images of 26 faults, which were mapped to a depth exceeding 10 km for the first time in this work. The faults are mostly syn-rift structures accommodating vertical uplift and horizontal advection (shortening) during Alpine tectonics. At a regional scale, tectonic reactivation has been marked by: a) the exhumation of parts of the present-day continental shelf, b) local folding and thrusting of sediment at the foot of the continental slope, and c) oversteepening of syn- and post-rift strata near reactivated faults (e.g. ‘passive uplift’). This work proves, for the first time, that geometric coherence dominated the growth and linkage of offshore faults in SW Iberia; thus, they are prone to reactivate as a kinematically coherent fault network. Importantly, they form 100–250 km long structures, the longest of which may generate earthquakes with a momentum magnitude (Mw) greater than 8.0. Tectonic inversion started in the Late Cretaceous and its magnitude is greater close to where magmatic intrusions are identified at depth. In contrast to previous models, this work postulates that regions where Late Mesozoic magmatism was more intense comprise thickened, harder crust, forming lateral buttresses to NW-SE compression. It shows these structural buttresses to have promoted the development of early stage fold-and-thrust belts – typical of convergent margins – in two sectors of SW Iberia.

Received: 21 Jul 2023 – Discussion started: 09 Aug 2023

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Journal article(s) based on this preprint

29 Jan 2024

Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore southwestern Iberia

Tiago M. Alves

Solid Earth, 15, 39–62, https://doi.org/10.5194/se-15-39-2024,https://doi.org/10.5194/se-15-39-2024, 2024

Short summary

Tiago M. Alves

Interactive discussion

Status: closed

CC1: 'Comment on egusphere-2023-1671', Gang Rao, 08 Sep 2023

Publisher’s note: this comment is a copy of RC1 and its content was therefore removed.

Citation: https://doi.org/10.5194/egusphere-2023-1671-CC1
RC1:
'Comment on egusphere-2023-1671', Gang Rao, 09 Sep 2023

The manuscript submitted by Dr. Alves reports the geometric coherence of inverted normal faults and their seismic implications in SW Iberia, through the analysis of well and seismic dataset.
Mostly, the manuscript is well-written, which provides important insights into the structural evolution of FTBs developed on the early-stage rifts. Obviously, it fits for the standard for publication in the journal Solid Earth. Still, I feel that minor revision is needed before final acceptance. I hope the authors find the comments and suggestions listed below helpful in subsequence modifications.
1) Lines 425-427: For clarity, it would be better to mention the M-L scaling relations when making the statement that the recognition of a >200 km long Fault 3 suggests the potential to generate Mw 8.0 earthquakes in this structure alone.
2) The arrangement of Figure 1: the color bars are too large on the right side, which can be placed within the main figure. What’s the meaning of “share v. 3.3”? who or which organization shared the data. I suppose the black arrow, at the upper right corner, pointed to the north. On the left, the orientation statistical data about what? fault and/or folds?
3) Figure 6, it is unnecessary to repeat the figure caption above the figure “Structural map of SW Iberia superimposed on Total Magnetic Intensity data”.
4) Figure 8, the authors display the amounts of uplift and horizontal advection measured along each fault. But, the orientation of the bars is inconsistent. Most of them are in a W-S direction. But, some of them are perpendicular to the fault traces.
5) The direction of Cenozoic tectonic compression: Lines 433, Cenozoic, in a N-S and NW-SE direction; Line 448, Late Cenozoic, dominated by NW-SE tectonic compression; Is there a temporal change in stress field? Is there any geodetic observation regarding the modern stress? In Figure 10c and d, under the NW-SE direction, localized FTBs formed. In the map view, the NW-SE-trending Fault 16 is also a thrust fault? not a tear fault?
Gang Rao

Citation: https://doi.org/10.5194/egusphere-2023-1671-RC1
- AC1: 'Reply on RC1', Tiago Alves, 21 Sep 2023
  
  Reply to Gang Rao’s comments to Preprint egusphere-2023-1671
  
  Dear reviewer
  
  Thank you for the constructive comments provided with your review. I fell your comments very pertinent and will certainly make the narrative in this paper much more robust. I went a step forward from your assessment and thoroughly revised the text in terms of its flow and grammar. I have simplified some concepts and added information I thought pertinent to the paper – once more following the comments provided below.
  
  Your comments, and further changes made to the paper, are addressed below:
  
  1) Lines 425-427: For clarity, it would be better to mention the M-L scaling relations when making the statement that the recognition of a >200 km long Fault 3 suggests the potential to generate Mw 8.0 earthquakes in this structure alone.
  
  Reply: The revised paper now stresses the M-L scaling relationships referred to by the reviewer and cites the published articles where these relationships are made clear. As indicated in the work of Trippetta et al. (2019), faults with a length of 150+ km are able to cause, at least, Mw 8.0 earthquakes. I added more detail on this relationship in the text and ‘toned down’ the abstract and main text to refer to a maximum Mw value of 8.0, potentially generated by the faults studied in this paper, and not one that exceeds Mw 8.0.
  
  2) Arrangement of Figure 1: the color bars are too large on the right side, which can be placed within the main figure. What’s the meaning of “share v. 3.3”? who or which organization shared the data. I suppose the black arrow, at the upper right corner, pointed to the north. On the left, the orientation statistical data about what? fault and/or folds?
  
  Reply: The rose diagrams showing the trends of main offshore faults were deleted from Figure 1. Also deleted was the reference to ‘share v. 3.3’, which was posted together with the legend. The sources of these data are indicated in the figure caption. Thank you for spotting this incoherence in the figure.
  
  3) Figure 6, it is unnecessary to repeat the figure caption above the figure “Structural map of SW Iberia superimposed on Total Magnetic Intensity data”.
  
  Reply: Agreed. The title on top of Figure 6 was deleted. Also changed was the map itself, which now shows Differential Reduction to the Pole (DRTP) data. This change was based on an offline comment by M. Neres from IPMA, Portugal, who recently characterised the shape and distribution of magmatic bodies in SW Iberia. The DRTP data is much akin to the distribution of magnetic anomalies published in Neres et al. (2023b). Tectonics, doi: 10.1029/2022TC007637. Further details on Neres et al. (2023b) work and its main findings are now included in the Introduction and Discussion sections of the revised article.
  
  4) Figure 8, the authors display the amounts of uplift and horizontal advection measured along each fault. But, the orientation of the bars is inconsistent. Most of them are in a W-S direction. But, some of them are perpendicular to the fault traces.
  
  Reply: This was done purposefully as: a) the bars will be almost parallel to the fault lines (as shown on the map), near their fault tips, in all faults with a NE-SW strike. This will make the amounts of uplift and horizontal advection confusing to the readers, as they will not be able to separate fault from the quantitative bars; b) the bars oriented at 30o on the map actually highlight the structures with a strike approaching N45, i.e. formed obliquely to the trend of the continental margin. Most of these faults are thrusts and steep reverse faults that seem to differ in length and style from the remaining faults to the east, nearer to the continental slope and shelf.
  
  5) The direction of Cenozoic tectonic compression: Lines 433, Cenozoic, in a N-S and NW-SE direction; Line 448, Late Cenozoic, dominated by NW-SE tectonic compression; Is there a temporal change in stress field? Is there any geodetic observation regarding the modern stress? In Figure 10c and d, under the NW-SE direction, localized FTBs formed. In the map view, the NW-SE-trending Fault 16 is also a thrust fault? not a tear fault?
  
  Reply: Yes. There are multiple observations that corroborate a NW-SE direction of tectonic compression, at present. Perhaps the most detailed information on present-day stresses required has been published in Ribeiro et al. (1990), Ribeiro et al. (1996) and more recently in Somoza et al. (2021). Detail derived from these papers were added to the Results and Discussion sections of the paper. Unfortunately, there is no published information on palaeo-stress directions by the simple fact that Miocene-Recent tectonics overprints, structurally speaking, the directions of compression associated with the Pyrenean and older tectonic phases.
  
  Other changes completed:
  
  A) Figures were improved and the text on key seismic sections made larger. All figures were made tidier and clearer to the readers.
  
  B) The introduction was revised, and information was added to improve its flow. Parts of some paragraphs were also merged or reviewed in relation to the narrative written before and after their appearance. In other words, there was a thorough effort to review the text for typos and small inconsistencies in this first revision round.
  
  Sincerely yours,
  
  Tiago M. Alves
  
  Citation: https://doi.org/10.5194/egusphere-2023-1671-AC1
RC2:
'Comment on egusphere-2023-1671', Oscar Fernandez, 06 Oct 2023

please see attached document

Citation: https://doi.org/10.5194/egusphere-2023-1671-RC2
- AC2: 'Reply on RC2', Tiago Alves, 19 Oct 2023
  
  Thank you for your constructive comments. Please, see the attached .pdf file with a detailed reply.
  Sincerely, Tiago
  
  Citation: https://doi.org/10.5194/egusphere-2023-1671-AC2

Interactive discussion

Status: closed

CC1: 'Comment on egusphere-2023-1671', Gang Rao, 08 Sep 2023

Publisher’s note: this comment is a copy of RC1 and its content was therefore removed.

Citation: https://doi.org/10.5194/egusphere-2023-1671-CC1
RC1:
'Comment on egusphere-2023-1671', Gang Rao, 09 Sep 2023

The manuscript submitted by Dr. Alves reports the geometric coherence of inverted normal faults and their seismic implications in SW Iberia, through the analysis of well and seismic dataset.
Mostly, the manuscript is well-written, which provides important insights into the structural evolution of FTBs developed on the early-stage rifts. Obviously, it fits for the standard for publication in the journal Solid Earth. Still, I feel that minor revision is needed before final acceptance. I hope the authors find the comments and suggestions listed below helpful in subsequence modifications.
1) Lines 425-427: For clarity, it would be better to mention the M-L scaling relations when making the statement that the recognition of a >200 km long Fault 3 suggests the potential to generate Mw 8.0 earthquakes in this structure alone.
2) The arrangement of Figure 1: the color bars are too large on the right side, which can be placed within the main figure. What’s the meaning of “share v. 3.3”? who or which organization shared the data. I suppose the black arrow, at the upper right corner, pointed to the north. On the left, the orientation statistical data about what? fault and/or folds?
3) Figure 6, it is unnecessary to repeat the figure caption above the figure “Structural map of SW Iberia superimposed on Total Magnetic Intensity data”.
4) Figure 8, the authors display the amounts of uplift and horizontal advection measured along each fault. But, the orientation of the bars is inconsistent. Most of them are in a W-S direction. But, some of them are perpendicular to the fault traces.
5) The direction of Cenozoic tectonic compression: Lines 433, Cenozoic, in a N-S and NW-SE direction; Line 448, Late Cenozoic, dominated by NW-SE tectonic compression; Is there a temporal change in stress field? Is there any geodetic observation regarding the modern stress? In Figure 10c and d, under the NW-SE direction, localized FTBs formed. In the map view, the NW-SE-trending Fault 16 is also a thrust fault? not a tear fault?
Gang Rao

Citation: https://doi.org/10.5194/egusphere-2023-1671-RC1
- AC1: 'Reply on RC1', Tiago Alves, 21 Sep 2023
  
  Reply to Gang Rao’s comments to Preprint egusphere-2023-1671
  
  Dear reviewer
  
  Thank you for the constructive comments provided with your review. I fell your comments very pertinent and will certainly make the narrative in this paper much more robust. I went a step forward from your assessment and thoroughly revised the text in terms of its flow and grammar. I have simplified some concepts and added information I thought pertinent to the paper – once more following the comments provided below.
  
  Your comments, and further changes made to the paper, are addressed below:
  
  1) Lines 425-427: For clarity, it would be better to mention the M-L scaling relations when making the statement that the recognition of a >200 km long Fault 3 suggests the potential to generate Mw 8.0 earthquakes in this structure alone.
  
  Reply: The revised paper now stresses the M-L scaling relationships referred to by the reviewer and cites the published articles where these relationships are made clear. As indicated in the work of Trippetta et al. (2019), faults with a length of 150+ km are able to cause, at least, Mw 8.0 earthquakes. I added more detail on this relationship in the text and ‘toned down’ the abstract and main text to refer to a maximum Mw value of 8.0, potentially generated by the faults studied in this paper, and not one that exceeds Mw 8.0.
  
  2) Arrangement of Figure 1: the color bars are too large on the right side, which can be placed within the main figure. What’s the meaning of “share v. 3.3”? who or which organization shared the data. I suppose the black arrow, at the upper right corner, pointed to the north. On the left, the orientation statistical data about what? fault and/or folds?
  
  Reply: The rose diagrams showing the trends of main offshore faults were deleted from Figure 1. Also deleted was the reference to ‘share v. 3.3’, which was posted together with the legend. The sources of these data are indicated in the figure caption. Thank you for spotting this incoherence in the figure.
  
  3) Figure 6, it is unnecessary to repeat the figure caption above the figure “Structural map of SW Iberia superimposed on Total Magnetic Intensity data”.
  
  Reply: Agreed. The title on top of Figure 6 was deleted. Also changed was the map itself, which now shows Differential Reduction to the Pole (DRTP) data. This change was based on an offline comment by M. Neres from IPMA, Portugal, who recently characterised the shape and distribution of magmatic bodies in SW Iberia. The DRTP data is much akin to the distribution of magnetic anomalies published in Neres et al. (2023b). Tectonics, doi: 10.1029/2022TC007637. Further details on Neres et al. (2023b) work and its main findings are now included in the Introduction and Discussion sections of the revised article.
  
  4) Figure 8, the authors display the amounts of uplift and horizontal advection measured along each fault. But, the orientation of the bars is inconsistent. Most of them are in a W-S direction. But, some of them are perpendicular to the fault traces.
  
  Reply: This was done purposefully as: a) the bars will be almost parallel to the fault lines (as shown on the map), near their fault tips, in all faults with a NE-SW strike. This will make the amounts of uplift and horizontal advection confusing to the readers, as they will not be able to separate fault from the quantitative bars; b) the bars oriented at 30o on the map actually highlight the structures with a strike approaching N45, i.e. formed obliquely to the trend of the continental margin. Most of these faults are thrusts and steep reverse faults that seem to differ in length and style from the remaining faults to the east, nearer to the continental slope and shelf.
  
  5) The direction of Cenozoic tectonic compression: Lines 433, Cenozoic, in a N-S and NW-SE direction; Line 448, Late Cenozoic, dominated by NW-SE tectonic compression; Is there a temporal change in stress field? Is there any geodetic observation regarding the modern stress? In Figure 10c and d, under the NW-SE direction, localized FTBs formed. In the map view, the NW-SE-trending Fault 16 is also a thrust fault? not a tear fault?
  
  Reply: Yes. There are multiple observations that corroborate a NW-SE direction of tectonic compression, at present. Perhaps the most detailed information on present-day stresses required has been published in Ribeiro et al. (1990), Ribeiro et al. (1996) and more recently in Somoza et al. (2021). Detail derived from these papers were added to the Results and Discussion sections of the paper. Unfortunately, there is no published information on palaeo-stress directions by the simple fact that Miocene-Recent tectonics overprints, structurally speaking, the directions of compression associated with the Pyrenean and older tectonic phases.
  
  Other changes completed:
  
  A) Figures were improved and the text on key seismic sections made larger. All figures were made tidier and clearer to the readers.
  
  B) The introduction was revised, and information was added to improve its flow. Parts of some paragraphs were also merged or reviewed in relation to the narrative written before and after their appearance. In other words, there was a thorough effort to review the text for typos and small inconsistencies in this first revision round.
  
  Sincerely yours,
  
  Tiago M. Alves
  
  Citation: https://doi.org/10.5194/egusphere-2023-1671-AC1
RC2:
'Comment on egusphere-2023-1671', Oscar Fernandez, 06 Oct 2023

please see attached document

Citation: https://doi.org/10.5194/egusphere-2023-1671-RC2
- AC2: 'Reply on RC2', Tiago Alves, 19 Oct 2023
  
  Thank you for your constructive comments. Please, see the attached .pdf file with a detailed reply.
  Sincerely, Tiago
  
  Citation: https://doi.org/10.5194/egusphere-2023-1671-AC2

Peer review completion

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

AR by Tiago Alves on behalf of the Authors (25 Oct 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (27 Oct 2023) by Yang Chu

RR by Gang Rao (12 Nov 2023)

ED: Publish as is (16 Nov 2023) by Yang Chu

ED: Publish as is (20 Nov 2023) by Federico Rossetti (Executive editor)

AR by Tiago Alves on behalf of the Authors (27 Nov 2023) Manuscript

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Tiago Alves on behalf of the Authors (24 Jan 2024) Author's adjustment Manuscript

EA: Adjustments approved (24 Jan 2024) by Yang Chu

Journal article(s) based on this preprint

29 Jan 2024

Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore southwestern Iberia

Tiago M. Alves

Solid Earth, 15, 39–62, https://doi.org/10.5194/se-15-39-2024,https://doi.org/10.5194/se-15-39-2024, 2024

Short summary

Tiago M. Alves

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https://doi.org/10.5194/egusphere-2023-1671-supplement

Tiago M. Alves

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Alpine tectonic inversion is reviewed for SW Iberia, known for its historical earthquakes and tsunamis. High-quality 2D seismic data image 26 faults mapped to a depth exceeding 10 km. Normal faults accommodated important vertical uplift and shortening. They are 100–250 km long and may generate earthquakes with Mw > 8.0. Regions of Late Mesozoic magmatism comprise thickened, harder crust, forming lateral buttresses to compression, promoting the development of fold-and-thrust belts.


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