Morphotectonic Evolution of an Alluvial Fan: Results of a Joint Analog and Numerical Modeling Approach

Round 1

Reviewer 1 Report

The authors present the results of a very interesting study aimed at investigating the role of surface processes and tectonics on the evolution of the alluvial fan morphology. In my opinion, the topic is of great interest to the readers of Geosciences especially for those scientists dealing with active surface processes. Data are correctly presented and interpreted, the quality of the figures is good, and the text is fairly well written. I strongly appreciated the use of analog models combined with numerical simulation,  an approach rarely developed in scientific papers. For the reasons above, I’m convinced that the paper from Garcia-Estève and coauthors deserve publication as it stands.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper suggests the intriguing possibility of teasing the tectonic signature of a fault from the topographic characteristics of an alluvial fan. Unfortunately, it ultimately fails to provide a convincing argument in support of the initial statement and remains as a nice exercise of modeling, using a join approach of analog and numerical methods.

I focused my revive on the tectonic geomorphology aspects, results and implications, with the assumption that the modeling component is robust.

Some important points need clarification, and some guidance is provided here below, both as general statements and a few, in-line comments.

• A discussion on time and transients is needed to key the morpho-tectonic evolution to real world dynamics. While the paper is a modeling approach, results need to have some linkage to the natural world.
• A justification of the setting chosen for the modeling is needed. Why an alluvial fan crossed by a thrust fault rather than any other possible setting?
• Are the expected results generalizable to all tectonic settings?
• Is there a natural world example that will “fit” the model and can provide a validation for the results?
• The authors chose not to include the signal from “external” regional base level variation. It is apparent that this choice greatly influences (and in my opinion drives and limits) the results and the geometry of the “footwall” in their model. Regional base level falls should not be disregarded. It is well accepted that the comparison between terraces in footwall and hanging wall is a key observable for long-term geodesy (see Pazzaglia et al., and reference therein https://0-www-journals-uchicago-edu.brum.beds.ac.uk/doi/10.1086/712636).
• A complete review of available literature on the topic of tectonic forcing on terrace formation is advisable, including autogenic vs. external forcing on sediment transport.

 

Introduction and Abstract:

• Please don’t use a future tense. The study has happened and you are providing the results.
• Line 41-42: well known processes – please clarify and add references.
• Line 67-68: change topography analysis to analysis of topography.

Methods:

• Line 82: change unique to single (?)
• Line 87: change deposit to depositional
• Line 91: check figure citation – 1 or S1?
• Line 95: change dipping to dip
• Line 99 to 102 + Figure 2: agree on 1 term – Stable, continuous or creep. Checj all document
• Line 100: sedimentation rate at 1cm/h. is this uniform across the whole model?
• Line 111: change tectonic characteristics to model set-up.
• Line 115: arrow in figure represent direction, not velocity
• Line 138: update references
• Line 186: this is potentially a high rate, especially when assumed constant over a long-time interval. See Perouse & Wernicke 2017 – Geosphere

Results:

• Line 406: change compartments to blocks (?)
• Line 406: change deposited sediments to sediments deposited
• Line 406-408: control is exerted by slip rate, not fan elevation, which is indeed the point of the experiment.
• Line 408: sediment transport is one of the driving factors but likely not the only one.

Conclusions:

• Line 631: move better before quantifying
• Line 641-646: this is not a robust enough statement. There are other autogenic factors that affect “real world” fan geometry that are not considered in the model, including degradation and creep. Given the ambiguity of the results, a thorough discussion is worth. Ambiguity in the footwall geometry will be limited by accounting for regional base level fall, which will provide an actual record of usable geomorphic marker which are “buried” in this particular setting.
• Line 664: correct hyphen in dis-cussion

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Revision of the manuscript n. 1301279

Many thanks for the opportunity to revise this manuscript, which I found very interesting. The manuscript entitled “Morpho-tectonic evolution of an alluvial fan: results of a joint analog and numerical modelling approach” by Garcia-Estève et al. was deals with a combination of two different approaches, the analog and the numerical one, in the study of an alluvial fan morphological evolution. The manuscript is well organized and the methodology, results, and discussion section are clear. I think that the paper represents a good first method in the analysis of the fan evolution and if in the future it will be jointed with field data, it will furnish interesting information about that topics.

The paper is clear and only a few improvements to the text are required. The introduction is good and my suggestion is to add some previous studies on the analog and numerical approaches discussing their relevance in the international scientific audience. The author's citations in the text are not formatted according to the reference style suggested by the journal. Please, authors may follow the reference format as suggested by the journal. In conclusion, the paper is suitable for publication in the journal after minor revision of the text.

Specific comments in the text are reported below.

Line 2: In geomorphology, the term morphotectonic is more used than the morpho-tectonic. I suggest changing it in the text and also in the title of the paper.

Lines 24-26: Take into account that also volcanism produce relief and then control landscape morphology. Within what categories of tectonics, sedimentation, and erosion it is included? Please revise more correctly the sentence.

Line 37:  the sentence is not clear, please rewrite it.

Lines 41-44: with this aim, I suggest reading the following papers:

Giano S.I., Pescatore E., Agosta F., Prosser G. (2018) - Geomorphic evidence of Quaternary tectonics within an underlap fault zone of southern Apennines, Italy. Geomorphology, 303, 172-190.

Giano S.I., Schiattarella M. (2014) – Age constraints and denudation rate of a multistage fault line scarp: an example from Southern Italy. Geochronometria, 41(3), 245-255. DOI 10.2478/s13386-013-0158-x.

Giano S.I. (2011) - Quaternary alluvial fan systems of the Agri intermontane basin (southern Italy): tectonic and climatic controls. Geologica Carphatica, 62 (1), 65-76. DOI: 10.2478/v10096-011-0006-y.

I hope that they could be useful for your discussion because they analyse in a quantitative way what you said in your sentence.

Line 44:  please, delete “…for the sake of simplicity…”. It is not useful in your discussion. Furthermore, please use the term morphotectonic than morpho-tectonic.

Line 45:  Considering the caption of figure 1 the fault is a reverse fault that cuts the fan. Please, could you use these terms in the text?

Line 74: could be better the term topography than the surface morphology.

Line 142: please, see the comment of line 44.

Lines 400-403: can you stress in more detail the formation of the new fan after the tectonics? Considering that uplift of the hanging wall generates a shifting of the intersection point of the fan could you better explain the depositional dynamics of that shifting?

Lines: 472-473: It is not important that you introduce the next section at the end of the previous one. I suggest deleting it.

Lines 526-527: please, see the above comment.

Line 533: please, see the comment of line 44.

Lines 537-538: Please, which are the active fault zones do you refer? Can you specify in the text?

Lines 575-580: Please, take into account that the return period of an earthquake is very difficult to demonstrate. It has been realized in the field with the palaeoseismological approach and furnished not very exact results. Then, I suggest caution when you trying to provide a return period only by a model and without field inspection of that fault.

Line 597: could be better “the accommodation space of deformation?…

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Thank you for considering my comments.