Topographic disequilibrium, landscape dynamics and active tectonics: an example from the Bhutan Himalaya

Simoes, Martine; Sassolas-Serrayet, Timothée; Cattin, Rodolphe; Le Roux-Mallouf, Romain; Ferry, Matthieu; Drukpa, Dowchu

doi:https://doi.org/10.5194/esurf-9-895-2021

Articles | Volume 9, issue 4

https://doi.org/10.5194/esurf-9-895-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-9-895-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 9, issue 4

Research article

|

03 Aug 2021

Research article |

| 03 Aug 2021

Topographic disequilibrium, landscape dynamics and active tectonics: an example from the Bhutan Himalaya

Martine Simoes, Timothée Sassolas-Serrayet, Rodolphe Cattin, Romain Le Roux-Mallouf, Matthieu Ferry, and Dowchu Drukpa

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Final revised paper (published on 03 Aug 2021)
Supplement to the final revised paper
Preprint (discussion started on 18 Dec 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Reviewer Comment', Anonymous Referee #1, 14 Jan 2021
- AC1: 'Reply on RC1', Martine Simoes, 04 Mar 2021
RC2: 'Review', Wolfgang Schwanghart, 05 Feb 2021
- AC2: 'Reply on RC2', Martine Simoes, 04 Mar 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Martine Simoes on behalf of the Authors (08 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (18 Apr 2021) by Sebastien Castelltort

RR by Anonymous Referee #1 (04 May 2021)

Suggestions for revision or reasons for rejection

I have complete my review of the revised version of “Topographic disequilibrium, landscape dynamics and active tectonics: an example from the Bhutan Himalayas”. Generally the manuscript is greatly improved and most of my large scale comments were suitably addressed from my initial review. I have a few minor thoughts for the authors, but nothing that should preclude the acceptance of this after a few tweaks (or maybe without a few tweaks depending on how the authors feel about my thoughts).

L324: This is a pretty minor quibble, but I’m not sure how smoothing is significant for the calculation of chi? Ksn, sure, but as long as the smoothing doesn’t change the drainage area accumulation (which CRS shouldn’t, since it operates exclusively on the extracted stream network), smoothing should have zero impact on the calculation of chi. I think you could just be vague here since you haven’t introduced all of the metrics yet and just leave it at saying that smoothing is important for some of the metrics you’ll use.

L793-794: Though for the area-loss mechanism, would you necessarily expect the spatial consistency that you note? You end up essentially using this to favor a tectonic origin, but it seems reasonable to “kill” this idea here, i.e., the extent to which an area loss mechanism is actually consistent.

L895-890: I’m curious how much of an issue you actually expect this to be? You cite this multiple times and imply that it can be quite significant generally and for the Bhutan dataset specifically, but from the cited work, this seems the most problematic for very small basins (e.g., <10 km^2) that are eroding very slowly (with the added caveats of things like erodibility, diffusivity, etc. playing a role). For the data from Adams et al 2016, while some basins are eroding reasonably slow, most of the basins (both the slow and fast eroding ones) are within the size range where this previous analysis suggests that the E/U ratio should be quite close to 1. It’s fair to point out that this could be an issue given the documented divide instability you provide here, but it’s also important to be honest about how much of an effect you actually expect this to impart, i.e. is the level of distrust you indicate we should have toward this aspect of prior work warranted? At least a qualitative assessment of this should be quite doable since many of the same authors appear on the 2019 paper as this one. This has a little bit of overlap with my comments on the first go around, i.e., that some of the criticism here of prior work seems a little overly harsh. Your study easily stands on its own merits without casting unnecessary aspersions on others work. If you can demonstrate that the majority of basins in previous published datasets may be dramatically influenced by the changes in drainage area and systematically would change the result of what was argued previously, that's one thing, but I think you should either (1) attempt to do this or (2) tone down your criticisms of this aspect.

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RR by Wolfgang Schwanghart (11 May 2021)

Suggestions for revision or reasons for rejection

First of all, I like to apologize for the delay of my review. In my review of the revised version, I went through the reply to reviewer comments. In addition, I focused on rereading the parts of the text that were changed.

I thank the authors for providing careful revisions in response to the comments by the reviewers. I think that Martine Simoes and coauthors have addressed all comments and made changes to the earlier draft of the manuscript where necessary. Overall, these edits have improved the manuscript considerably.

With 47 pages, the manuscript is still quite long. This is not to be taken as criticsm, but rather I'd like to encourage the authors to condense the text where possible.

Wolfgang Schwanghart

Minor comments

10: consider replacing 'most often' with 'commonly'
11: may however be rare in nature
13: these drainage dynamics
13f: 'particular case example' seems like three words meaning the same thing. Why not simply: Here, we document these drainage dynamics in the Bhutan Himalayas, where out-of-equilibrium morphologies have been noticed from major river knickpoints and high-altitude low-relief regions in the mountain hinterland.
19: these dynamics. Moreover, either landscape response is rapid, or time scale is short. Consider removing time scale
28: remove "of uplifted terranes".
320: In order to let others replicate your analysis, please provide tau (the quantile) and K (the smoothness parameter) used in the CRS function.
324: The computation of chi is actually not influenced by smoothing. Other geomorphometric attributes such as stream gradient or ksn, however, are.
326: Perhaps also note here the tolerance value applied in the knickpointfinder function.
328: Is there a reference supporting the 3800 m cutoff?

758: consider to replace "whatever" with "regardless"
765: see comment above
857: Make sure to format references correctly
860: remove somehow
915: "is consistent" rather than "fits"

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ED: Publish subject to minor revisions (review by editor) (12 May 2021) by Sebastien Castelltort

AR by Martine Simoes on behalf of the Authors (21 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Jun 2021) by Sebastien Castelltort

ED: Publish as is (15 Jun 2021) by Niels Hovius (Editor)

AR by Martine Simoes on behalf of the Authors (18 Jun 2021) Manuscript

Short summary

Elevated low-relief regions and major river knickpoints have for long been noticed and questioned in the emblematic Bhutan Himalaya. We document the morphology of this region using morphometric analyses and field observations, at a variety of spatial scales. Our findings reveal a highly unstable river network, with numerous non-coeval river captures, most probably related to a dynamic response to local tectonic uplift in the mountain hinterland.