How steady are steady-state mountain belts? A reexamination of the Olympic Mountains (Washington state, USA)

Michel, Lorenz; Glotzbach, Christoph; Falkowski, Sarah; Adams, Byron A.; Ehlers, Todd A.

doi:https://doi.org/10.5194/esurf-7-275-2019

Articles | Volume 7, issue 1

https://doi.org/10.5194/esurf-7-275-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-7-275-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 7, issue 1

Research article

|

14 Mar 2019

Research article |

| 14 Mar 2019

How steady are steady-state mountain belts? A reexamination of the Olympic Mountains (Washington state, USA)

Lorenz Michel, Christoph Glotzbach, Sarah Falkowski, Byron A. Adams, and Todd A. Ehlers

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Final revised paper (published on 14 Mar 2019)
Supplement to the final revised paper
Preprint (discussion started on 03 Sep 2018)
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: 'Review of ”How steady are steady-state mountain belts? – a re-examination of the Olympic Mountains (Washington State, USA)” by Michel et al.', Anonymous Referee #1, 09 Oct 2018
- AC1: 'Response to referee#1', Lorenz Michel, 24 Oct 2018
RC2: 'Review of ”How steady are steady-state mountain belts? – a re-examination of the Olympic Mountains (Washington State, USA)” by Michel et al.', Anonymous Referee #2, 09 Oct 2018
- AC2: 'Response to referee#2', Lorenz Michel, 24 Oct 2018

Peer-review completion

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

AR by Lorenz Michel on behalf of the Authors (23 Dec 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Jan 2019) by David Lundbek Egholm

RR by Anonymous Referee #1 (15 Jan 2019)

Suggestions for revision or reasons for rejection

Review of ”How steady are steady-state mountain belts? – a re-examination of the Olympic Mountains (Washington State, USA)” by Michel et al. (second version)

The authors have in general responded well to all the comments raised by the reviewers. However, there is a tendency that the authors try to explain their way out of the comments instead of making changes to the manuscript. For instance, both reviewers have argued that they find the 2D flux calculation needless. The fix for these comments has been to elaborate even more on the text, which just makes the manuscript longer and protracted. In the interest of making the paper clearer, more transparent and more readable, the authors may want the change this strategy.

Below are a few comments that remained while reading the revised text.

1. Perhaps my comment was not clear earlier, regarding the use of Pecube in 3D. My suggestion was to use Pecube in 3D in inversion mode. However, I agree with the authors that this could be a next step, and something for the future. But such approach would allow you to include all available data, also the ones with only one thermochron, all together. An inversion like that would not be able to fit all data, but would find the best suitable model on the basis of all data.

2. Also related to the thermos-kinematic modeling, the new comment on time-step sensitivity is not very clear (new manuscript lines 255-257). If there is nothing to gain from the sensitivity analysis I don’t see the need to include it. However, that being said, I am not sure that the original comment by reviewer 2 has been adequately met, since the authors only have tested a larger timestep and not a smaller one. But perhaps reviewer 2 has commented on this as well – the original comment by reviewer 2 on this could have been clearer.

3. A minor comment regarding the flux calculations. I appreciate the efforts the authors have made to elaborate on this. However, I still feel that the authors could be a bit more specific, and simply mention explicitly that “the variable with the greatest uncertainty is the sediment thickness” is actually “the variable with the greatest uncertainty is the sediment thickness back in time, that has now been subducted below the Olympic Mountains”. Since the authors mention that this is the variable with the largest uncertainty, I would think that they would include something on this in their new section 5.5 (on the actual numbers used for sediment thickness).

4. It is a matter of taste, and I will leave it up to the authors to decide on this, but I just want to stress once again that you don’t need to include both the 2D and 3D for influx/outflux in order to argue that the 3D is different/better. Of course 3D is different and better when there are large spatial variations in the pattern you are integrating. This is not “a new and unexpected observation” as you phrase it in your reply. Also, it is not necessary to do both 2D and 3D to link with previous work (especially since you do not discuss the actual difference between your 2D and the previous work). Your comparison of 2D/3D ends up being obvious and in fact dispensable. In the interest of making your paper clear and transparent, I would still suggest to leave it out. You can still argue that the 3D is better, just by referring to previous work.

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RR by Philippe Steer (23 Jan 2019)

Suggestions for revision or reasons for rejection

Dear authors,

I appreciate the amount of work you did to respond in a fair and correct manner to the comments raised by reviewer 1 and myself. This has helped increase the quality of the manuscript. Thanks for that. I therefore see no a priori major reason to preclude the publication of this interesting manuscript.

Minor Comments:

1) I think there is a mistake in Equation 2. I believe this equation < Vsed=nu.d.t.u > is incorrect (with nu the porosity), as you are here estimating the volume of pores entering the wedge. This equation should be < Vsed=(1-nu).d.t.u > so that you compute the volume of rocks accreted in the wedge. I believe there are three possibilities:
- I am wrong.
- This is simply a typo, and the correct equation was used in the model, then all is fine. This is what I believe to be the most likely option, and in turn, this is why I consider this is simply a minor comment.
- This is not simply a typo, and your model also solves for the wrong equation. Then, this is a major issue with large implications, as this means you have underestimated the accretionary flux by a factor ~2 (nu=0.31) to ~4 (nu=0.22). This would completely change your results and their interpretation.
I am sorry I have not seen that during the first round of review.

2) It is not clear to me if you correct for porosity (of exhumed and eroded rocks) when assessing volumes of denudation? If not, there is a consistency issue. Indeed, you should either consider a bulk volume, in both the accretionary and denudation fluxes (porosity is therefore not needed), or a rock volume (without pores), and in this case you need to multiply the denudation by (1-nu), similarly to the accretionary flux. Moreover, the best would be probably to consider a mass budget with fluxes given in mass per time, as the density of accreted sedimentary rocks (without pores) is probably different than the density of eroded rocks (still without pores).

Very minor comments:

Lines 699-703: These two sentences suggest that Batt et al. (2001) and Brandon et al. (1998) use a circular argument (assuming steady-state to ... interpret steady-state). Please rephrase as this is probably incorrect. For instance, in Batt et al. (2201) a convergence rate of 36 mm/yr is assumed based on geological observations.

Equation 2: It is not really satisfying to use the word "volume" for Vsed as Vsed as a unit of [km2]. This is also annoying later in the text when you compare 2D and 3D "volumes". Maybe the easier would be to multiply this equation by a unit length (in the 2D case) to make a 3D column, and by the wedge length in the 3D case (this is done later in the manuscript anyway)

Equation 3: Please, precise the unit of z is [km] and not [m], if I am correct

Lines 1253-1255 - I would have said exactly the opposite. Generally, we expect the influx (or uplift) to set the outflux (or erosion) at steady-state. This is for instance what occurs in numerical models of landscape evolution. I am not aware of papers clearly demonstrating the opposite - that outflux sets the influx (but maybe this illustrates my lack of knowledge about sediment recycling in accretionary wedges).

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ED: Publish subject to minor revisions (review by editor) (30 Jan 2019) by David Lundbek Egholm

AR by Lorenz Michel on behalf of the Authors (08 Feb 2019) Author's response Manuscript

ED: Publish as is (20 Feb 2019) by David Lundbek Egholm

ED: Publish as is (20 Feb 2019) by Niels Hovius (Editor)

AR by Lorenz Michel on behalf of the Authors (22 Feb 2019) Author's response Manuscript

Short summary

Mountain-building processes are often investigated by assuming a steady state, meaning the balance between opposing forces, like mass influx and mass outflux. This work shows that the Olympic Mountains are in flux steady state on long timescales (i.e., 14 Myr), but the flux steady state could be disturbed on shorter timescales, especially by the Plio–Pleistocene glaciation. The contribution highlights the temporally nonsteady evolution of mountain ranges.