the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2023
Late Pleistocene – Holocene denudation, uplift, and morphology evolution of the Armorican Massif (western Europe)
Abstract. Elevated Plio-Pleistocene coastal and marine markers in stable continental regions are commonly explained by a combination of eustatic sea-level variations and regional geodynamics (e.g., mantle dynamics, active faults). In this study, we test the role of erosion rates on the Late Pleistocene uplift and landform evolution of the Armorican Massif, western France. Denudation rates are estimated for 19 drainage basins using terrestrial cosmogenic nuclide (10Be) measurements in quartz. They range between 3 and 34 m.Ma-1, with a factor of two difference between the western highland region and the central lowland region (16 ± 8 m.Ma-1 vs. 9 ± 6 m.Ma-1). Assuming a thin elastic plate model, the lithosphere flexural isostatic response to these denudation rates produces an overall uplift of the Armorican Peninsula from 12 – 15 m.Ma-1 in the central lowland region to 4 – 10 m.Ma-1 in the western peninsula and along the coastline. We show that these erosion-driven uplift rates can explain the uplifted Late Pleistocene marine terraces along the Armorican Peninsula coastline as well as the elevated Quaternary marine deposits in the central lowland region, without necessitating additional geodynamic processes such as regional compression or local active faults. Our results suggest that, in stable continental regions, long-term erosion should be taken into account as a driver of uplift and deformation before trying to derive global or regional geodynamic or tectonic conclusions.
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CC1: 'Comment on egusphere-2023-2154', Timothée Jautzy, 30 Nov 2023
Hello and congratulations for this work, which nicely complements the CRN-derived denudation rates dataset in Western Europe. We also recently made a similar contribution in the Vosges Massif (also part of the European Variscan Belt). https://doi.org/10.1016/j.epsl.2023.118490
We sampled 22 catchments for 10Be and 26Al measurements, for which 13 catchments are in steady-state and we think that our data could be usefully integrated into your dataset.
We asked Alexandru Codilean to integrate them in the Octopus database, but don't hesitate to ask us if you want it quickly.Cheers,
Tim Jautzy
Citation: https://doi.org/10.5194/egusphere-2023-2154-CC1 -
AC1: 'Reply on CC1', Oswald Malcles, 01 Dec 2023
Hello, thanks for your interest in our work and for the information related to your recent publication.
Data from the Vosges Massif would indeed be of great help for a better understanding of the regional context, especially relative to the steady-state problem and the associated time-mass loss relationship ! We will have a look at it.
Best,
O. Malcles
Citation: https://doi.org/10.5194/egusphere-2023-2154-AC1
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AC1: 'Reply on CC1', Oswald Malcles, 01 Dec 2023
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RC1: 'Comment Review on egusphere-2023-2154', Anne Duperret, 05 Jan 2024
The aim of this paper is to explore if slow uplifts in stable continental regions, evidenced by long-term erosion, is driven by simple isostasic adjustements. Surprisingly, slow coastal uplifts have been attributed to global mantle dynamics, plate tectonics, regional lower crustal flow triggered by glaciations cycles, local fault reactivation or local volcanism, but never to simple isostasic adjustements. The question is thus original and of first interest.
The study is then dedicated to the Britain part of the Armorican Massif, using three main models.
Maximum uplift rates due to denudation appears to be localised in central Brittany lowlands where denudation rates are the lowest. This is a surprising result by comparison with the other denudation rates calculations and regional modelling.
In the discussion, the authors try to resolve this conundrum by comparisons with some others results of the literature, but never discuss the initial assumption. This is the main weakness of the paper.
Nevertheless, denudation rates models established in the Britain part of the Armorican Massif is an original and good piece of work, usefull for the community. It needs thus to be published. Models of vertical deformation due to denudation show globally equivalent rates. The variations observed between highlands and lowlands could thus be explained by additional local processes, that needs to be discussed with various processes (local tectonics ? climatic variations ? crustal and lithospheric variations ? isostasic model adaptation ?)
Please, see the supplement pdf for more details.
accepted subject to revisions.
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RC2: 'Comment on egusphere-2023-2154', Anonymous Referee #2, 16 Jan 2024
This study integrates topographic analysis (e.g., elevation and basin-averaged slope) with denudation rates derived from cosmogenic nuclides to enhance the understanding of controls on landscape evolution in the Armorican Massif mountains. The paper includes new data on 10Be-derived cosmogenic nuclides and models. The newly obtained results are thoroughly compared with existing data (e.g., long-term uplift rate from marine terraces) to decipher processes at depth and identify controlling factors over the long-time scale.
The approach is well-explained, and the interpretation aligns with the presented results. I have some major concerns regarding the geomorphology of the study area, along with suggestions for the figures and the method-results sections. I am convinced that the paper can be significantly improved by adding additional information and details.
I believe that the suggested changes require moderate-to-major revisions. Overall, most components of the manuscript are in pretty good shape, and the authors should be able to address my comments fairly easily, as my recommendations do not involve substantial additional analyses or changing interpretations.
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RC3: 'Comment on egusphere-2023-2154', Andrew Wickert, 01 Feb 2024
Dr. Malcles and coauthors present an argument that the recent uplift of the Armorican Massif is entirely due to erosional isostasy. I think that their argument seems plausible. However, I do not think that they (yet) demonstrate this. I would be glad to see a revised manuscript draft in which they carefully describe their data-analysis and modeling approaches, including more careful (and sometimes more accurate) descriptions of both their cosmogenic 10Be approaches and their flexural isostatic modeling.
This review is a bit limited becuase I am not familiar with the geological setting and therefore cannot evaluate its accuracy.
Line-by-line comments, some substantial, follow:
36-38. I do not understand how a region could be affected only by its own local erosion unless it is fully bounded by inviscid lithosphere-spanning vertical channels. Meaning: you get yourselves into some trouble further on, which I discuss.39. Because of the hierarchical structure of watersheds, I think that all landscapes would have small watersheds. Therefore, I do not know that this is an advantage specific to your field area.
49. Europe --> European
52. No spaces needed around this endash (and others like it: X and Y, X date through Y date). Note: Spaces are used when the endash is being used like a comma, to separate a subordinate clause.
95. Remove parentheses around Bessin ref
98. Minor note: "rasa" is a new word to me. Probably don't mind this; I imagine that it is broadly used in your community.
117-118. No parentheses around Bonnet ref. Could you provide some quick context around the archaeological findings -- what is it that linked these to differential uplift rates?
112-124. This paragraph is close to "X1 says Y1 about Z1, X2...", especially when the final sentence seemingly strongly opposes a number of prior sentences. I think it is *okay* as is, but I wonder if there is a way to start with a stronger synthesis or to split the paragraph. Some ideas:
"Evidence for recent relative or absolute uplift are below the rate of geodetic data....",
And perhaps a separate paragrpah discussing the geomorphic evidence since that likely developed over ca 10^5+ years, and so might be more relevant for the geological record.
There is no need to take my suggestions, but at their heart, I see a mix of time scales, methods, and discussion here that could be better clarified.
Figure 2: I trust that you will have a nicer vector-graphics map set for any final published version.
146 "mu" failed to print properly.
Section 3.2: Would it be appropriate to cite Granger et al. (1996) here? Their paper has seemed to me to be an important central one in developing the method that you use.
168. Here you have jumped straight to erosion rate. However, I do not yet know some critical information that you must use to calculate this. This information includes:
* Rock density
* Chosen attenuation length
* Are you making an assumption of spatially uniform erosion?
* Are your quartz sources well distributed across the watershed?
I further cannot find this information in Table 1.Therefore, could you please add some significant methodological information to make your study more understandable and reproducible before moving towards your desired results (the erosion rates)?
I will review the rest of the paper assuming that you did this work reasonably/correctly.
181-183. This is a surprisingly short description of the pebble vs. sand denudation rates. If you are going to make this comparison, I think that you will have to do two things:
* Note which rate was higher (sand vs. gravel) rather than just noting them to be different
* Connect your findings to prior work. I've been thinking of the Andes recently, and a couple papers came to mind / I checked into them -- plus, I used to work with Taylor Schildgen (so use these papers with an active warning about possible nepotism of convenience -- I know her work!):
- Schildgen et al. (2016, see Fig. 7). On the top of p. 407, they note that these pebbles may have lower 10Be concentrations because they could be produced during mass-wasting events. They provide a set of references that could be relevan
- Tofelde et al. (2018) much more deeply analyzed this effect. I think that this reference is very relevant to your findings, and hesitate only because the "Wickert" in the coauthor spot is me... again, cite if you find useful187-189. You are so efficient in going to your results that I don't know what you did! Could you please note how you used them to obtain erosion rates? Fortunately, ESurf is a long-format journal.
189. I think that Small et al. (1999) might also be worth a look for high-relief rocky surfaces eroding more slowly than regolith-mantled hills; see the end of their article.
Table 1, caption. [comparing to main text] "Pebble" and "cobble" are different grain sizes. Please indicate which these are. Numbers are fine too, to avoid the vagaries of the Udden-Wentworth grain scale.
233. "complete" is a bit final. "augment" ?
278. "Fleuve Machne river"... River Manche river? Are you just worried that they might not know French :) ?
282-284. This is a really sudden jump to, presumably, isostatic impacts of sedimentation. You have not introduced this at all.
290-295. Same as above. I would suggest organizing the article to first provide information on sedimentation rates, then introduce the isostatic processes (presumably), and then discuss deposition and (presumably I'm about to arrive at it) erosion.
307. Time scale depends on the rheology at the location (e.g., Iceland, S. Patagonia, are responding to load changes over years to decades.
310-312. There are a few problems with this statement. First, the elastic plate is about the wavelength of the deformation and not about the time scale. Second, I think that most humans would consider the mantle to be quite high viscosity.
312-315. How about the elastic component of mantle rheology? Is this something worth considering, and why? (Your time scales compared to something like the Maxwell time of the mantle might give you an answer.)
317. gFlex is a 2D model: it does not resolve variations in the vertical.
318. gFlex cannot provide velocities since it has no viscous-type rheology component or time dependence. Could you please describe what you have done?
My first guess is that you provide some time (when?) to remove the load, or perhaps some range of times (provide a constant erosion rate and see what the spatial pattern of isostatic uplift should be). But you shouldn't leave the reader guessing... or worse, the reviewer!320. Could you discuss these data sources and how they relate to the rather wide range of Te that you tested?
Figure 6 (or text around): Which boundary conditions and solver methods did you use with gFlex?
Figure 6 (itself): How did you choose where to mark your "border effects": I imagine that these should extend uniformly from 1 flexural wavelength or so from the edge of your domain. However, they seem to be irregularly drawn. Relatedly, do you think that the decrease in erosion rates to the N, S, and E outside of the grayed-out area, and possibly the maximum magnitude of uplift predicted, could also be produced by such edge effects?
Towards this, I wonder if it might be worthwhile to extend your erosion-rate modeling efforts at least one flexural wavelength inland.
350. I think that the fact that your random model has a similar shape to the fit model is because of two reasons:
1. Your cells are much smaller than a flexural wavelength, so you will see a mean signal when flexurally filtered. Towards this, I would like to see how you came up with your "50 km" number for "minimum sensitivity length", and if you can define this in a more precise/formal way.
2. Your model outputs are affected by the boundaries, both the sea (where you assume that depositional effects are minimal) and the inland regions (where you might be artificially affecting the domain).Section 6 and onwards: Unfortunately, I am not able to evaluate your Discussion and Conclusions. I do not know how you converted the deflections into uplift rates and think that your model boundary conditions may play a role in setting the pattern that you see. This said, I find your idea of all of the recent uplift to be erosional/isostatc to be quite plausible, and I think that this phase of your data analysis + modeling points in that direction.
REFERENCES
Schildgen, T. F., Robinson, R. A., Savi, S., Phillips, W. M., Spencer, J. Q., Bookhagen, B., ... & Strecker, M. R. (2016). Landscape response to late Pleistocene climate change in NW Argentina: Sediment flux modulated by basin geometry and connectivity. Journal of Geophysical Research: Earth Surface, 121(2), 392-414.
Small, E. E., Anderson, R. S., & Hancock, G. S. (1999). Estimates of the rate of regolith production using 10Be and 26Al from an alpine hillslope. Geomorphology, 27(1-2), 131-150.
Tofelde, S., Düsing, W., Schildgen, T. F., Wittmann, H., Alonso, R. N., & Strecker, M. R. (2017, December). Changes in denudation rates and erosion processes in the transition from a low-relief, arid orogen interior to a high-relief, humid mountain-front setting, Toro Basin, southern Central Andes. In AGU Fall Meeting Abstracts (Vol. 2017, pp. EP33B-1946).
Citation: https://doi.org/10.5194/egusphere-2023-2154-RC3
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