Deep-seated gravitational slope deformation scaling on Mars and Earth: same fate for different initial conditions
By Olga Kromuszczyńska et al.
Note
This is a review of the second, revised version of the manuscript. The original version has been reviewed by two anonymous referees (https://dio.org/10.5194/esurf-2018-27-RC1 and https://dio.org/10.5194/esurf-2018-27-RC2), and I have seen their comments and a detailed, point-to-point response letter by the authors. I have not seen the original version of the manuscript as it was first submitted.
Overview
The authors describe morphometrically two cases of Deep-Seated Gravitational Slope Deformation (DSGSD), one in the Tatra mountains (Earth), and one in the Valles Marineris (Mars). The former is known to be a result of post-glacial destabilization by slope-debuttressing after removal of glacial ice, a well-known mechanism which is also thought to be responsible for the latter. Morphologically, DSGSD is characterized by normal faulting which produces uphill-facing scarps, ridge-top splitting, and summital valleys. The authors performed measurements of scarp offsets along topographic profiles oriented normal to the fault strike, i.e. parallel to the extension. The total height (H) and width or length (L) of the DSGSD-affected ridges was also measured, and their ratio R (i.e. H/L) determined. The authors find that the R value is similar for the two investigated cases of DSGSD and propose that the R value may be considered a measure for “maturity” of the process, i.e. that DSGSD-affected ridges will assume a given R value (~0.24) when DSGSD has ended. This proposal is based on the assumption that DSGSD is not active anymore at the investigated ridges on Earth and Mars, and that the horizontal and vertical deformation was proportionally different, i.e. the original ridge geometry was different.
General comments
The manuscript has already been revised according to critical comments by two reviewers, and it is in good technical shape. The language is clear, the illustrations are well prepared and legible, figure captions contain the necessary information, and referencing is up-to-date. Overall, the topic should be of interest to readers of Earth Surface Dynamics. As such, the manuscript might be considered for publication. However, I share one of the main concerns of the second reviewer of the original version (https://dio.org/10.5194/esurf-2018-27-RC2), and I am not fully convinced be the author’s response. This concern is about the essential take-home message of the manuscript (if it is stripped to its bones), i.e. the proposed use of the R value as a measure of slope maturity of ridges that are subject to DSGSD. Only two study sites are investigated, and I would concur with the previous reviewer #2 that (many) more such sites, especially on Earth but also on Mars, should be investigated before such a conclusion can be made. The response by the authors to this concern is basically only some rewording, but they did not analyze more study sites. In the opinion of this reviewer, it is completely unclear if the similar R-values in the Tatra mountains and in Valles Marineris are only a coincidence, or if they are indeed an indication of the final state of DSGSD-affected topographic ridges. Only more data could help, and I would strongly recommend that the authors collect such data to support (or reject) their weakly based hypothesis.
Other comments
Section 1.1: This is certainly interesting information, but most of it (e.g., the compressional bulging, the decollements, etc) is not really linked to any later part of the manuscript. I still find (following the comment#1 of reviewer #1) that the introduction does not clearly outline the problem and explains the author’s approach. Section 1.1 (newly added) is somehow “glued” to the front of the manuscript, but does not form a consistent explanation of why all this is relevant for the reader.
Page 5, lines 7-8: “DSGSD scarps were studied in the Tatra mountains during two periods of field work.” This sentence is out of context here – I suggest moving it to the Data section (2.1).
Page 5, lines 21-24: It is very speculative (and not “likely”) if “liquid water in a melting mountain permafrost” ever played a role on Mars, let alone in a specific environment such as Valles Marineris. On what observation (or published study) is this notion based? Moreover, permafrost thaws, it does not “melt” (this mistake is made very often, mostly when permafrost is confused with ground ice: permafrost is defined by temperature only, so it thaws - ice melts).
Page 9, line 3-4: I don’t understand this. Please explain more clearly.
Page 11, line 13: Normal fault dips are assumed to be 60° to 70°, and normal faults are mode II fractures, yet here the authors talk about tension fractures (mode I fractures). This seems like a contradiction – I suggest the authors clarify the respective role of tension fracturing and shear fracturing in DSGSD.
Page 16, line 10: If DGPS is used (page 7, lines 12-13), a higher vertical precision than 40 cm should be achievable. It is also unclear how dense the horizontal sampling distance was during field work. The profiles in Figure 8b do not really look that great – from visual inspection of the left part of the profile at site T2, it seems that measurements were only made every few meters (note the long perfectly horizontal section of the profile between distances of ~4 m and ~9 m).
I miss a discussion of the potential implications of DSGSD on the thermal nature of Martian glaciers. If DSGSD requires slopes to be oversteepened (so they get unstable after debuttressing by melting or sublimating of ice), this implies glacial erosion of slopes. Typically, glacial erosion is effective for polythermal or “warm-based” glaciers, while cold-based glaciers are frozen to their beds and do not cause significant erosion (but some erosion is also caused by cold-based glaciers). If this consideration is correct, the observation of DSGSD on Mars would potentially imply warm-based glaciers, a contradiction to most models which predict that glaciers on Mars in most climatic scenarios should be cold-based. The authors are encouraged to elaborate on this possibility - although it is probably beyond the original scope of this manuscript, it may add some “beef” to it. As it is now, the scientific content is limited to a few topographic measurements and a weakly based speculation (the use of R). More scientific discussion would add to the value of the manuscript.
Page 21, line 11-12: Is it not possible to determine rather than inferring the initial slopes of Valles Marineris walls? Maybe the authors can make some measurements elsewhere in VM, where DSGSD has not affected the slopes.
Page 1, lines 27-29: The last sentence of the abstract is largely redundant (cf. lines 21-22) and could be deleted.
Figure 2: I would appreciate if the compressive bulging (page 2, line 6) is illustrated in Figure 2a.
Figures 8a and 8b: If I understood the text correctly, the term “reconstruction” is not really appropriate. The fault traces are just plotted where the slope suggests their position, and the dips are actually assumed, not “reconstructed”.
Figure 9: What is the vertical extent of the grey bars? I understand that it represents the range of values as given in Table 2 (?). But if this true, why is the range of Dh for site 2(a) from 0.007 to ~0.12 (estimated from Figure 9a), whereas Table 2 gives values for Dh for site 2(a) of 0.010 (a=60°) and 0.007 (a=70°)? Is there a typo in the table (0.010 instead of 0.11)? I also can’t see that the values for the Tatra sites are between 0.002 to 0.005 – from Figure 9a, I would estimate values between ~0.02 and ~0.03. Same for Dv: Why is a values for VM (0.056-0.204) larger than the values for the Tatra mountains (0.20 – 0.34)? To me, it looks smaller. I may have missed the point in comparing Table 2 with Figure 9 – any clarification would be helpful. This was already a source of confusion for previous reviewer #2, and I strongly recommend to double-check the numbers.
Page 22, line 5: Laskar et al. (2004) is not an appropriate reference here.
Formal issues
Page 1, line 19: replace “two orders” by “two orders of magnitude”
Page 1, line 26: “Martian obliquity”
Page 1, lines 25-26: “activity of faults” – does a landform (a fault) have an activity? I would suggest writing “activity of the faulting”, because a process (faulting) does have duration of activity.
Page 3, line 24: remove blank after DSGSD.
Page 3, lines 24-30: Something is wrong with the sentences here (a period seems to be missing, splitting this long text into two or more sentences). Please check and rephrase.
Page 3, line 31: remove comma after “retreat”
Page 4, line 10: replace “fully similar” by “self-similar” (?).
Page 5, lines 15-16: “some instances […] sometimes” remove “sometimes”
Page 5, line 17: “are not against” this reads awkward. Perhaps reword like “[…] based on observations by Mège and Bourgeois (2011) it is not possible to exclude”
Page 7, line 3: replace “done” by “made”
Page 11, line 15: replace “with” by “by”
Page 18, line 9: Add comma after “displacement Dh (a)”
Figure 6: Th caption says the fault scarps are marked by black lines, however, in the figure they are red.
Page 16, line 2: “[…] graben at, e.g., Site M1”
Table 2: The lines for Dh and Dv are offset with respect to lines for x(m) and z(m).
Figure 9: On the x-axis, write site “M1”; “T4” etc. instead of just “1”, “4”, etc.
Page 18, line 10: “intervening” reads awkward. Please try to reword.
Page 19, line 4: “gravitational acceleration”
Page 19, line 14: replace “much deep” with just “deep”
Page 21, Table 3, 1st line: “ex” is offset with respect to the other columns.
Page 21, line 13: “does not carry an indication” (insert “an”)
Page 21, line 14: “scarps” (plural)
References
Despite this is already a revised version, there are many flaws in the reference list. I recommend a thorough double-checking of the entire list. Some mistakes that I could find:
Page 22, line 27: 1935-2017 (insert dash)
Page 23, line 7: Provide full page range (4067–4091)
Page 23, line 13: The author name is “Sutton”, not “Suton”
Page 23, line 23: Geomorphology (typo)
Page 24, line 12: Superscript “10” before “Be”
Page 24, line 13: Insert line break before “Howard” (=next reference)
Page 25, line 3: The author name is Dromart, not Dromard
Page 26, line 7: It should be “sackung”, not “sacking”
Page 26, line 9: I think “Würmian” should be capitalized.
Page 26, lines 25-26: The correct title is “Ancient groundwater flow in the Valles Marineris on Mars inferred from fault trace ridges”, and the page range is 181-183. |
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