the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Nov 2023
Along-strike variation of volcanic addition controlling post breakup sedimentary infill: Pelotas margin, Austral South Atlantic
Abstract. We investigate the lateral variability of breakup volcanic addition along-strike of the Pelotas segment of the Austral South Atlantic rifted margin and its control on post-rift accommodation space and sediment deposition. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of ~300 km. Using 2D flexural backstripping we determine the post-rift accommodation space above top volcanics. In the north, where SDRs are thickest, the Torres High shows SDRs up to ~ 20 km thick and post-breakup water-loaded accommodation space is ~2 km. In contrast, in the south where magmatic addition is normal and SDRs are thinner, post-breakup water-loaded accommodation space is ~ 3–4 km. We show that post-breakup accommodation space correlates inversely with SDR thickness, being less for magma-rich margins and more for magma normal/intermediate margins. The Rio Grande Cone, with large sediment thickness, is underlain by small SDR thicknesses allowing large post-breakup accommodation space. A relationship is observed between the amount of volcanic material and the TWTT of first volcanics; first volcanics are observed at 1.25s TWTT for the highly magmatic Torres High profile while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed at 4.2s TWTT or deeper. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions by a simple isostatic model of continental lithosphere thinning and decompression melting during breakup.
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RC1: 'Comment on egusphere-2023-2584', Valentin Rime, 11 Dec 2023
Dear authors, dear editor,
Thank you for this nice manuscript!
Overall, the manuscript is novel, of clear scientific significance, useful for the community and well argued. The aim, the focus of the paper and the conclusions are clear. It is overall well written, and the figures are clear (and look nice!).
Nevertheless, there is still room for some improvement to improve the readability and strengthen the argumentation. Precisions or corrections are definitely needed for the 4s vs 6s TWTT of first volcanics and for the generation of the results of the model in time domain (see details in the PDF). Some interpretations might be restructured. Some precisions seem to be needed for some methods. There are several potential small improvements to the figures and the text. Finally, I also propose a series of comments that are more proposition than corrections and that the authors might elect not to follow.
After these improvements/precisions/corrections, I’m sure that the manuscript will be a great contribution to the understanding of passive margins and a great addition to Solid Earth.
You'll find my detailed review in the supplementary PDF. Do not hesitate to contact me if something I wrote is unclear or if you have questions about my review.
Best regards,
Valentin Rime
- AC1: 'Reply on RC1', Marlise Colling Cassel, 14 Mar 2024
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RC2: 'Comment on egusphere-2023-2584', Anonymous Referee #2, 31 Jan 2024
Review of “Along strike variations of volcanic additional controlling post breakup sedimentary infill”
This manuscript provides a useful insight into the ongoing evolution of magmatic margin and their associated sedimentary basins. Although the focus is the Pelotas margin, there are wider implications.
There are number of details comments below, but there are three specific aspects that need to be considered before the manuscript is appropriate for publication.
- Throughout the study there is no clear differentiation of basement and no discrimination between oceanic and continental crust. For margin development I think it is critical that more consideration is put into this component. While the data itself may not be present to define this, there are sufficient publications on similar data that more detail could be included. I also think more coverage of SDR formation could be included. This is important because it is fundamental to how we interpret such margins and especially as SDRS are an intrinsic part of margin formation. The way the manuscript comes across it appears that SDRs are emplaced onto ‘basement’, rather than the emplacement of SDRS (or at least outer SDRs) are simply the shallower component of an over thickened magmatic/oceanic crust development. Whether the authors agree or disagree with this it is essential that this argument is presented. This is essential because 1) where SDRs are limited is this just because there is greater proportion of sub-SDR magmatism hence along trend magma volumes are not simply a function of SDR thickness. 2) When rotation is undertaken magmatic crust thickness is not simply a function of present day vertical thickness – what impact does this have on estimations of magma supply?
- Current SDRs models invoke emplacement at sealevel; the backstripping suggests significant water depth when backstripped. This is incompatible with SDR emplacement but is not discussed. A more comprehensive discussion of this has to be included as to how appropriate is backstripping for such margin and what the role of isostacy and thermal perturbations are when backstripping is undertaken.
- A central premise of the manuscript is the control of the margin on overburden and along margin variation– as noted in (1) a much more compressive discussion of crustal thickness (continental vs oceanic) has to be undertaken including what sits beneath SDRs. In addition there is no discussion of the role of paleogeography and sediment supply, eg. Drainage and sediment entry in any substantial way. There is a discussion that the backstripping sediment can be used to estimate thickness but this needs to be correlated with additional sediment supply data.
- Suggest there should be consistency of using depth converted data Currently starts with TWT then Z(m) – suggest more consistency having depth conversion from the outset
Specific comments
Introduction – this really needs more on SDR and magmatic margin formation processes that addresses crustal nature below SDRs – need to define what basement is at each part of the section,
This is specifically relevant for Line 44 – it is the volume of magma that is important not just SDRs.
Line 45 – how representative is this and what implications are appropriate for other margins?
Line 85 – basement characterisation in text and associated figures is required
Line 88 – “top basement is a smooth horizon onto which the SDRs downlap” this is not substantiated by the data presented and is very different interpretation than current models of SDR formation.
Line 91-2 – 1st mention of basement shape – tapering, box shape. No discussion of what this means, how it is defined and what relevance is. Also how much does this geometric configuration rely upon definition of continental or oceanic crust?
Line 95 – this seems slightly unnecessary for this paper
Line 100 Figure 2 caption is incorrect
~Line 105 – suggest this is all depth converted from the outset
Figure 3 – this highlights the importance of defining basement and a more detailed discussions SDR and its sub-crust. For example in b if SDR and magmatic crust is considered as a single crustal type (ie overthickened oceanic crust) this has a very different implication.
Line 124 – very difficult to make a discussion of sediment supply with more extensive discussion, and indeed from a 1D perspective rather than 2D at least.
Line 129- volcanic addition would be fine if it was all onto continental crust but as defined basement includes oceanic crust therefore need further discussion of SDR formation
Line 134 – where is crustal basement, how would this change?
Figure 5 – generally assumed that SDRs are sub aerial but backstripping does not restore these the SDR packages to sea level. This must be discussed further
Line 145 – if accommodation space is estimated from backstripping, then need to consider the point raised above – i.e. SDRs at sea level. This comment applies to this whole section.
Line 165-170 – there are some significant assumptions made in this statement. This needs further justification.
Line 188- magmatic volume - needs further justification as to whether extrusive SDR is equivalent to magmatic volume which again goes back to defining basement.
Line 228-233 – what is the evidence of significantly underfilled sun-rift basins? This would be apparent from the geometry of the basin fill
Line 240 – not sure how this fits with the original premise of the paper?
Citation: https://doi.org/10.5194/egusphere-2023-2584-RC2 - AC2: 'Reply on RC2', Marlise Colling Cassel, 14 Mar 2024
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