the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Nov 2023
Magma-poor continent-ocean transition zones of the southern North Atlantic: a wide-angle seismic synthesis of a new frontier
Abstract. Magma-poor rifted margins, and their corresponding potential zones of exhumed serpentinized mantle, represent a unique class of tectonic boundaries with enormous promise for advancing the energy transition, such as with hydrogen production, carbon sequestration, and in the search for critical minerals. In this study, a synthesis of the results from seismic refraction/wide-angle reflection profiling, and resulting velocity models, across the continent-ocean transitions of the southern North Atlantic Ocean is presented. The models are assessed and compared to understand characteristic basement types and upper mantle behaviour across the region and between conjugate margin pairs. Ultimately, this work highlights the variable nature of continent-ocean transition zones, even within the magma-poor rifted margin end-member case, and points to avenues for future research to fill the knowledge gaps that will accelerate the energy transition.
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J. Kim Welford
Status: open (until 31 Dec 2023)
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RC1: 'Comment on egusphere-2023-2656', Anonymous Referee #1, 24 Nov 2023
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Dear Editor,
In the contribution “Magma-poor continent-ocean transition zones of the southern North Atlantic: a wide-angle seismic synthesis of a new frontier”, J. K. Welford (J.K.W.) provides a synthesis of the velocity models derived from seismic refraction and wide-angle reflection profiling along the rifted margins of the southern parts of the North Atlantic plus Labrador Sea. The author uses the interpretation of these profiles to compare the along-dip extent of the domains made of thinned continental crust, exhumed and non-exhumed serpentinized mantle, and thin, normal and thick oceanic crust among the different margins, and speculates on the potential causes of their diversity among margins.
I found the manuscript very well written and referenced. It is convenient to have all the velocity models for the southern North Atlantic margins displayed with a same color scale and a same vertical exaggeration throughout the paper, and at similar vertical and horizontal scales within each figure, however I think that this contribution could benefit from a certain number of precisions and clarifications.
My main points of concern are the following:
- It is not clear from the manuscript whether the interpreted extent of the different “basement type” and “mantle type” domains in the rifted margin seismic sections are from J.K.W. or from the authors of the original contribution.
- Given that the present contribution aims to provide a coherent synthesis, the best may be to use one and the same methodology to do all the interpretations. If that is what was done, then the methodology needs to be presented. For instance, if the extent of the exhumed mantle corresponds to a specific velocity interval, this interval must be stated. However, when I compare the Moho trend vs the velocity model plotted in profiles 12 and 14, with the Moho trend vs the velocity model in the other profiles, I suspect that the same rule was not applied in all profiles.
- It is not clear how each of these “basement type” and “mantle type” domains is defined. It should be specified whether the interpreted extent of the different domains is only based on the velocity model, or whether other data like gravimetry analyses, magnetic anomaly analyses, seismic reflection images, drilling/dredging, etc. are included. When a different approach has been used for interpreting the domains extent in the different profiles (for instance because the same data were not necessarily available everywhere), then the evidence on which the interpretation is based must be specified for each profile.
- According to Schön, 2015, Péron-Pinvidic et al., 2016; Osmundsen et al., 2016; Karner et al., 2021, amongst others, and implied by the author in l 66-67, velocity models do not bear only one solution with respect to the petrological nature of the rocks. It would be useful to further develop and discuss this aspect, especially because the sections discussed in this contribution appear to be only discussed from the seismic velocity model perspective. If that is indeed the case, then the proposed interpretations would not be very convincing, and I would recommend to at least display some sort of error bar.
- The comparison and analysis of the extent of serpentinized, exhumed or non-exhumed mantle along the southern North Atlantic undertaken in the discussion section looks weak to me because of the lack of convincing arguments to accurately identify exhumed mantle domains (distinguish them from magmatic additions, for example).
Additional minor points are listed below:
L 20: Why mentioning the Wilson Cycle here? it is never discussed in the manuscript.
L 53-54: I’d specify that wide tracts of exhumed serpentinized mantle can display variable amounts of magmatic products (e.g.: Bronner et al., 2011; Epin et al., 2019).
L 59: Reston (2009) states "Magma-poor, magma-dominated and transform are all approximately equally numerous" (see caption of his fig. 1), but according to the more recent map by Haupert et al. (2016), magma-poor margins may be more common than magma-rich ones.
L 66-67: As said in the point 4) above, I think this would merit some further development and referencing, since the present paper is about one of these geophysical methods that provide non-unique solutions. The occurrence of a “S-reflector” or the lack of Moho reflectivity on seismic reflection sections can for instance be used to support interpretations of exhumed mantle.
L 113: Please explicit how the continent-ocean transition zone (COTZ) is defined: from the figures, it appears to correspond to the zone where serpentinized mantle occurs, but this is never stated in the text.
L 155: How is “transitional crust” defined in the present contribution?
L 160-161: Thinon et al. (2003) rely on a gravity analysis and reflection seismic data in addition to velocity models to support their interpretation. I think this should be mentioned.
L 178: Please specify how "oceanic crust" is defined in this study. Is it based on the only seismic velocity profile? For instance, what is the difference between exhumed mantle with scarce magmatic additions and thin oceanic crust?
L 193-194: The interpretation by Merino et al. doesn't only rely on a seismic velocity model but also on a gravity analysis. I think this should be mentioned.
L 260-261: I don't think this is correct, this process requires only the remaining crust to be fully brittle, and yet the upper part of the crust is always brittle, as shown by the systematic presence of faults on seismic reflection profiles. This may well be because an "infinite" amount of less than 10 °C oceanic water constantly cools the upper part of the crust.
L 270: Another (easy to observe) definition of LaLOC was proposed by Sauter et al. (2023), who describe it as the inner edge of tabular magmatic crust (top basement and Moho are flat and parallel). How does this definition compare to the (velocity-based?) definition in the present manuscript?
L 282-283: This is inaccurate: Manatschal et al. (2015) state that deformation in the proximal and necking domains may be controlled by their inherited, and thus highly variable among margins, rheology, while deformation in the hyperextended domain may be rather controlled by a much more uniform rheology among margins, acquired through pervasive fluid alteration.
L 285-286: An alternative explanation could simply be that mantle exhumation continues as long as extension continues and not enough magma is formed to accommodate extension.
L 289-291: This sounds contradictory with the statement on L 260-261, where it is stated that mantle serpentinization requires ultra-slow extension. Besides, it is not clear to me what is the logical link between the two parts of this sentence: "this piece-wise acceleration [...] may be sufficient to explain why unexhumed mantle serpentinization is so widespread". This should be clarified.
L291-292: This would imply that no "failed rift" basin would display exhumed mantle. I find this statement a bit bold because the occurrence of exhumed mantle in the southern part of the Porcupine Basin cannot be ruled out.
Citation: https://doi.org/10.5194/egusphere-2023-2656-RC1
J. Kim Welford
J. Kim Welford
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