|Review of “Aegean-style extensional deformation in the contractional southern Dinarides: Incipient normal fault scarps in Montenegro” by Biermanns, P., Schmitz B., Mechernich S., Weismüller C., Kujtim Onuzi K., Ustaszewski K. and, Reicherter K in SE.|
First of all, let’s set the context. This review is an additional review of a manuscript that has already received extensive comments on SE from 4 reviewers and for a large part, the revised version has integrated previous constructive comments. Yet, I am still quite puzzled by how the manuscript is structure to some degree, the model ages help define the slip-rates, so it would make sense that these model ages are discussed together with the rates, not the other way around. Furthermore, there are some inconsistencies & approximations in the data analysis that if corrected or improved upon might help to consolidate the conclusions. Overall, following my careful read of the manuscript & revised version, I would encourage the authors to further consider the following comments.
First, why start by stating that the paper presents “two-previously unreported normal faults”, while these faults have already been discussed in your earlier paper (Biermanns et al., 2019) that describe Tectonic geomorphology and Quaternary landscape development in the Albania - Montenegro border region – there are several paragraphs discussing these faults & figure already shown, (e.g. Fig 4h) in the earlier paper. This is clumsy at best. This paper is a follow-up analysis of the kinematic and role of these faults in the region deformation and seismicity. The current manuscript should be consistent in that regard and integrate previous publications, including their own.
Furthermore, the data description & quantitative analysis done on these faults is rather superficial and should probably be done prior to any comparison to other fault systems in Italy or Greece. There is a great diversity in the growth and seismicity of normal faults and an extensive literature for not assuming that they are all following the same activation time & seismic behaviour to cite the two characteristics considered in the paper. Furthermore, most of these normal faults are in extension setting and not associated with some extrado deformation of active fold and trust belts, a setting that likely influence their growth, segmentation & seismicity or lack there is. These faults are particular and although examples of on-shore surface ruptures related to such secondary normal faults are rare, especially at that scale, the noteworthy El Asnam earthquake in 1980 earthquake is one famous example for having highlighted that surface ruptures along normal faults can occur as a result of thrust events. That raise many questions: Are these features only transient, superficial and by-products of the folding, propagation and/or segmentation of the fold & trust belts or can these normal faults develop over time and create longer-term relief? If the thrusts are active, as suggested by the instrumental earthquake focal mechanisms of the region, one might need to integrate & discuss the implication of such setting, especially the geometry of the system that ought to the limit of the potential surface rupture of the normal faults given the superficial depth of ramps in fold & trust systems. In any case, without a detailed analysis of these structures, geometry, segmentation, growth and propagation, the discussion remains quite superficial on that front.
Regarding the estimation of the slip-rates (SR) on these faults, the methodology used here is still rather crude in my opinion. The fact that most direct ages gathered on active normal fault surfaces in the literature are Holocene in age and therefore younger than the Last Glacial Maximum or the Younger Dryas, does not imply that all these faults activated necessarily as a result of any climate forcing. In fact, previous detailed analysis of the seismic cycles of numerous normal faults in Italy concluded that there was no climate control of fault exhumation given the asynchronous high slip-rate periods along these faults (Cowie et al., 2017). Other studies that concluded that their local fresh fault scarps were close in age to the last glacial maximum had some evidences, offset glacial deposits (i.e Papanikolaou et al., 2005), dated surfaces (i.e Tesson et al., 2020) or they fully argue their case (i.e. Tucker et al., 2011).
Long-term slip-rate estimates along active faults require both well-defined offsets and an argumentation on the age of these offsets, which is not always straight forward. Whatever your argumentation is, you have to present it clearly. After reading the manuscript, it was still unclear until section 4, what was the timescale cover by your data, that is the Cl36 model ages. The model ages should be presented together with the SR derived using the Cl-36 data. And there are no 36Cl model ages without discussion on the erosion rate. Assuming that all parameters are the same as that of others studies in the Mediterranean, by default, is rather superficial. Did you extrapolate your Cl36 SR to LGM times? If so, what evidence did you gather at your sites that the fault scarps (free-face or degraded one) are as old as the LGM? That is the key question here, one that needs to be tackled in the manuscript. At this stage, I’m puzzled by the fact that the authors choose to put forward the results from a rather debated correlation that is taken as a given evidence thorough the manuscript rather than discussing first the results based on new data & evidences gathered at their sites. The problem is not that the conclusion is that these normal faults were activated post LGM, but it is an issue that there is very little data analysis that lead to that conclusion in the current manuscript.
Line 106 were selected to collect samples for 36Cl dating (site BFSN only; see following Section 3.1.2) and to estimate low-resolution long- term (post-Last Glacial Maximum, LGM) slip rates based on topographic profiles across the fault scarps for comparison
Since the Cl-36 data are not of LGM ages, the sentence is puzzling. The Cl-36 SR are based on low-resolution sampling data, and covering a shorter timescale than that of LGM. The link between your data and that timescale need to be done first. Clarify or just remove the () at this stage.
Line 115 two sets of generalized post-LGM (18 ± 3 kyr, e.g., Papanikolaou et al., 2005; for discussion see Section 5.3) slip rates for each site: (i) A conservative one, only considering slip on the visible free-face and (ii) a progressive one, incorporating the degraded NFS in prolongation of the free-face (Table S10)
Maximum ages lead to minimum SR whether define using the free-face or older eroded fault surface. It would be more logical to first define the ages, then the offsets to estimate SR before projecting the SR to longer timescales such as LGM times. In the Cl-36 section, the free-face is 8.8m and model age is 6.4ka, the 22m degraded scarp have a modelled age of 14.8 ka assuming a constant SR & constant low erosion of 1mm/ka, not 18ka. That’s an issue that should be resolve before defining SRs using the same 18ka LGM climatic correlation on other fault scarps.
Line 120 Despite these weaknesses and the availability of an absolute 36Cl dating for site BFSN (see Section 3.1.2), we still consider the comparison of topography-based slip rate estimations a reasoned benchmark in the frame of our study.
The Cl36 ages are not absolute ages; they are modelled based on assumptions on the erosion rate, scaling schemes, 36Cl production rates… but at least, they are model ages derived from the studied sites that can be fully discussed.
Line 161 36Cl scarp modelling method and parameters
The model age analysis is missing in this section and should be presented at the very least together with the SR defined using the code of Schlagenhauf et al. (2010). There are several codes available to calculate the Cl-36 model ages such as https://stoneage.ice-d.org/math/Cl36/v3/v3_Cl36_age_in.html, pending the shielding correction is gather form the codes provided by Schlagenhauf et al. (2010).
Line 178 and integrates over a time-span of 17.3 kyr, which is appropriate for our postglacial focus
Why 17.3ka? is your Cl-36 model ages reach this timescale? Best is to estimate the model ages first and correct for the production rate for the proper time-span of each sample, or at least the older Cl-36 model age at ~6ka.
Line 180 Scaling with respect to latitude and elevation was performed using the Stone (2000) scaling scheme assuming a constant geomagnetic field intensity.
You mention the opposite earlier in line 174 “Furthermore, production rates have to be scaled appropriately to the local and distant shielding of the site from cosmic rays and to changes of production through time due to geomagnetic field effects.” Explain or clarify.
Line 188 Assuming this as the minimum amount of erosion and using a preliminary calculated 36Cl age of ~15 kyr at 8.8 m height, we estimate an erosion rate of ~1 mm/kyr at our sample locations
Why 15ka? Preliminary, do you mean zero-erosion model age? is your Cl-36 zero-erosion model ages reach this timescale? If you have considered 2 to 8 mm in 15ka, that would give 0.13 to .53 mm/ka of erosion rate pending the sampled free-face reach the 15ka timescale. If you older zero-erosion model is in fact ~ 6ka, then the erosion rate would be higher and ranging from 0.33 to 1.3 mm/ka. Why not discuss & use your estimates instead of ~ 1mm/yr? Please update.
Line 211 The total throw of the NFS is estimated to max. ~200 - 400 m. The 200-m frame is based on the offset of stratigraphic markers across KFS (see cross sections in Fig. 4 C).
This is unclear how the throw rate is estimated. Is it really from the low-resolution topographic profile of a geological cross-section? This is not the classic way of doing it, please explain/clarify.
Line 212 The 400-m frame is based on an analysis of topographic cross sections across BFS, where a clearly perceptible knickpoint (~850 m a.s.l.) marks the NFS ~400 m below the overlying highest parts of the Rumija ridge (~ 1250 m a.s.l.)
Any figure to refer to?
Line 249 sets of conservative and progressive minimum slip rates
Both are based on a climatic correlation implying a maximum age for the free-face surfaces leading to a minimum SR… “conservative” & “progressive” adjectives in that context are rather subjective.
Line 261 The modelling of the 36Cl concentrations on the BFSN free-face highlights that the measured 36Cl pattern can be generated by a constant slip rate of 1.5 ± 0.1 mm/yr (Fig. 7 A).
The SR rate based on your data should probably come before the climate correlation together with an argument for projecting your CL-36 rate to the longer LGM timescale…
Line 267 The retrieved slip rate suggests that the 8.8 m-high free-face was most likely exhumed within the last 5.9 ± 0.4 kyr (Fig. 7 B) and the according fault scarp age is presumably 14.8 ± 1.0 kyr (Fig. 7 B).
Thanks for sharing the finally timescale of your data... So, the fault scarp is ~15ka based on the Cl-36 SR define over the last ~6ka, assuming a constant rate over time. First, you use a preliminary (Line 188), i.e zero-erosion model age for the age of fault scarp, adding some erosion to the model ought to get an older age, yet the preliminary and corrected ages appear to be the same. How do you justify using 18ka for the minimum SR, since you have an estimate of ~15ka? Why not estimate a long-term erosion rate using the 6ka-SR together with the geometry of the fault plane, pending the geometry is well established (See Tucker et al., 2011)? Then speculate on the longer timescale SR using some revised erosion rates? Please consolidate and clarify.
Line 268 Since the slip rate of 1.5 ± 0.1 mm/yr is very high
Is it? Compare to what? maybe the erosion rate use for defining the 6ka-SR is underestimate. Carbonates in the Mediterranean can have erosion rate up to ~25mm/yr (i.e. Ryb et al.,
2014), so there is room to discuss this parameter and revise a SR to more reasonable values.
Line 274 To be as open-minded as possible, we used any hypothetical scenario, without correlation to the local mapping.
Line 280 Hence, we highlight that the slip rate of the Bar fault during the last ~6 kyr was surely higher than ~1.15 mm/yr and presumably around 1.5 ± 0.1 mm/yr.
Sure, neglecting the inheritance would increase the timescale and thus reduce the rate, but the inheritance is derived from the data gathered from the colluvium wedge, so that’s not really a reasonable option. However, the erosion is more open to debate as pointed out earlier.
Line 281 Also, alternative interpretations of the few 36Cl data points is conceivable, but they are significantly more complicated, not underlined by field findings and hence considered less likely.
If you don’t present these alternatives, there is no point in that statement.
Line 338 To enable a comparison of the different (structurally and exposure-related distinct) sections of the fault scarps nonetheless – and to provide at least one benchmark for the obtained 36Cl dating results – we invoke the rather simplistic technique of fault scarp profiling (see also Sections 3.1.1 and 4.2.1) for slip rate derivation. For this technique, it is assumed that the preservation of NFS initiated around the LGM.
The topographic profile perpendicular to the fault scarp are done to gather the geometry (dip angles) of the fault & to define the offsets. To use of the LGM age as the age of all these offsets, there is a need to first justify the use of LGM age on the free-face sampled for Cl-36 analysis.
Line 411 capable of triggering earthquakes up to Mw≈7 ± 0.5 , yet within the text the “Derived magnitudes range from Mw≈5.3 to 6.5.”
Given the logarithmic scale used for earthquake magnitudes, it is best to stay within the range of Mw calculated or use the mean Mw of 6.3 ± 0.1 as defined in table S9. A Mw 7 event release ~15 times more energy than a Mw 6.5…
Biermanns, P., Schmitz, B., Ustaszewski, K., Reicherter, K., 2019. Tectonic geomorphology and Quaternary landscape development in the Albania - Montenegro border region: An inventory. Geomorphology 326, 116–131. https://doi.org/10.1016/j.geomorph.2018.09.014
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Papanikolaou, I.D., Roberts, G.P., Michetti, A.M., 2005. Fault scarps and deformation rates in Lazio–Abruzzo, Central Italy: Comparison between geological fault slip-rate and GPS data. Tectonophysics 408, 147–176. https://doi.org/10.1016/j.tecto.2005.05.043
Ryb, U., Matmon, A., Erel, Y., Haviv, I., Benedetti, L., Hidy, A.J., 2014. Styles and rates of long-term denudation in carbonate terrains under a Mediterranean to hyper-arid climatic gradient. Earth and Planetary Science Letters 406, 142–152. https://doi.org/10.1016/j.epsl.2014.09.008
Schlagenhauf, A., Gaudemer, Y., Benedetti, L., Manighetti, I., Palumbo, L., Schimmelpfennig, I., Finkel, R., Pou, K., 2010. Using in situ Chlorine-36 cosmonuclide to recover past earthquake histories on limestone normal fault scarps: a reappraisal of methodology and interpretations: Using 36Cl to recover past earthquakes. Geophysical Journal International no-no. https://doi.org/10.1111/j.1365-246X.2010.04622.x
Tesson, J., Benedetti, L., Godard, V., Novaes, C., Fleury, J., n.d. Slip rate determined from cosmogenic nuclides on normal-fault facets, 2020, Geology 49, 5.
Tucker, G.E., McCoy, S.W., Whittaker, A.C., Roberts, G.P., Lancaster, S.T., Phillips, R., 2011. Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls: Fault scarps and Climate. J. Geophys. Res. 116, n/a-n/a. https://doi.org/10.1029/2010JF001861