the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2024
Geomorphic analysis and fluvial incision rates from valley-filling lava flows: implications for the Quaternary morphotectonic evolution in the Moroccan Massif Central and Middle Atlas
Abstract. Fluvial dynamics is one of the main surface processes that shape the Earth’s topography. Geomorphic records, such as fluvial terraces, play a crucial role in reconstructing the history of landscapes and deciphering the complex interactions among tectonic activity, lithology, and surface processes, which are primarily controlled by climate. This is also valid in valleys characterized by the emplacement of effusive volcanic rocks that are generally more resistant to erosion, and hence have a high preservation potential, and are easier to date than alluvial deposits. Valley-filling volcanic rocks, thus, represent ideal geomorphic markers to estimate the magnitude and the spatio-temporal pattern of fluvial incision and associated forcing mechanisms.
In this study, we combine fluvial incision rates on dated lava flows emplaced in the valleys of the Moroccan Massif Central and Middle Atlas with DEM-based geomorphic analysis to gain insights into the Quaternary landscape evolution. The results show that incision rates are in the order of 0.01 and 0.1 mm yr-1 for the Middle Atlas and the Massif Central respectively. This spatial discrepancy in incision rates agrees with geomorphic metrics, with lower rates within the low topographic relief landscape and higher rates (up to one order of magnitude) along its margins that are highly dissected by fluvial incision. The comparison between our data and published incision rates in the northeastern flank of the Middle Atlas suggests that the eastern flank of the Middle Atlas accommodates active tectonic shortening. Furthermore, our analysis indicates that lithology and climate may not be the primary factors controlling the observed spatial variation in incision rates between the Middle Atlas and the Massif Central. Instead, surface uplift, which is probably related to forebulge flexural uplift enhanced by dynamic mantle-related uplift, could have triggered relatively high incision rates in the Massif Central. Ultimately, we conclude that a significant proportion of the topographic relief in our study area has been generated before the lava emplacement (i.e., earlier than 2.85 Ma).
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Status: open (until 13 Dec 2024)
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RC1: 'Comment on egusphere-2024-1746', Anonymous Referee #1, 13 Sep 2024
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The study by Yaaqoub et al. presents a geomorphic analysis of the Moroccan Massif, and Central and Middle Atlas. This is combined with post-lava emplacement incision rates to study the drivers of landscape evolution. The incision rates presented are interesting, especially when compared to the previously published, faster estimates of regional uplift. However, the study lacks a clear research question or hypothesis and the writing is often goes into excessive detail, making it read more like a textbook than a research article. The analysis is similarly unfocused, with a lot of plots and text, but limited information being conveyed. Despite these issues, the incision data appear robust, and there are several noteworthy observations that, if properly emphasized, could significantly strengthen the paper. Below, I provide a list of suggested improvements (in no particular order), along with detailed line-by-line comments that I hope the authors will find constructive for future versions of this manuscript.
Introduction and abstract do not state a hypothesis or research question, beyond one general statement of “gaining insights into the Quaternary landscape evolution”. Currently, the beginning of the abstract and the entire introduction read like a geomorphology text book instead of a research paper. As a reader, I am left wondering what the aim of the study is. These sections need a complete overhaul.
The background section includes a large amount of unnecessary information, which is more distracting than useful. Also, because the reader is not informed about the research question/hypothesis/approach of the study in the introduction, it is hard to judge what information is relevant while reading. Please, only include information relevant to the study at hand.
The methods section also includes A LOT of unnecessary detail. This level of detail is not required for an ESurf submission. The authors should focus on the important details, such as the parameters used, etc. Some examples are found in lines 183-184, 187-194, 202-208
In the case where lava flows pour into valleys of weaker bedrock, stream incision may try to avoid the hard volcanic rocks and focus on the contact between non-volcanic bedrock and the lava flow. Does this occur here too? Nothing like this was mentioned in the text, but I’d assume that the sedimentary section of the plateau is easier to erode than the lava flow.
Throughout the paper, especially in the result section, a lot of reference is made to local town names. As someone unfamiliar with the region that makes the text harder to follow and provides an unnecessary amount of detail. It would be better to explain the large-scale patterns using the names of just a few domains: Folded Middle Atlas, Tabular Middle Atlas, etc., Lines 280-193 are a good example, where the paragraph starts fine and then dives into a lot of detail, without a take-home message.
The results section goes through several river catchments in a lot of detail, but it remains unclear what knowledge is gained from the individual analysis. The paper would be much stronger if all the geomorphic analysis was condensed to 2-3 figures and paragraphs that lay out the general patterns, which then can be compared to the incision rate patterns.
The incision rates are the most interesting part of the paper. However, they are currently underused, while the geomorphic metrics are overused. For instance, there is no map showing the spatial distribution of incision rates. Incision rates from lava flows could be compared to the Ksn map to evaluate if the topographic pattern matches the actual incision rates.
All figures are underlain by a dark shading, which makes it harder to see things on the maps.
Line 17-19: The wording in the second sentence should be revised. Currently, it is unclear what noun is referred to when the authors write: are primarily controlled by climate. I assume they refer to the fluvial terraces but this is unclear.
Line 30: How does this comparison point to active shortening? No reason is stated.
Line 33-37: No data or arguments are presented to support the arguments. As a reader, I am left in the dark.
Line 67: This sentence is very confusing. The late Miocene and Pliocene are part of the Neogene. Therefore, I am confused what “in the Neogene before the late Miocene and during the Pliocene” means. Were there two phases of mountain building, one in the early-middle Miocene, and one in the Pliocene?
Line 89-90: Check language. Maybe “and is part of the Cenozoic Alpine belt”
Line 94: What does alpine-type refer to?
Line 152: Typo. Messinian.
Line 152-155: This sentence is written as if there was a contradiction between this and the previous statement. It seems like the authors meant to say uplift may have started pre-Messinian. But the sentence says “in the late Cenozoic”, which would include the Messinian.
Line 219: Why modelling? It seems like the actual profiles are being analyzed and no modelling takes place. Please, rephrase.
Line 220-223: This is only true for detachment-limited bedrock rivers. Please, also add a statement that addresses whether detachment-limitation is a reasonable assumption in your study area.
Line 224: What is “the later model”. Do you mean “latter”? If so, please add that the equation that is being shown only emerges if the stream power incision model is combined with a conservation of mass statement.
Line 231: Only if you solve for slope.
Line 234: Typo. Empirical.
Line 235: The s of ks is not in the subscript.
Line 254: change capital K in Ksn to lower case k.
Line 238: Deviations from what form? Do the authors mean when ks changes along profile? That can also be linked to numerous other factors, e.g., a change in K.
Line 258: Which knickpoints? So far there was no mentioning of knickpoint mapping. No indication has been given of how they were mapped, and what criteria were used to map them.
Line 260 and following: references are made to colors, but here is no reference to the relevant figure.
Line 275: What dating methods were used for these lava flows?
Line 473-476: It depends whether the landscape it thought to be near steady state or not. Here, one could use the river profile to argue for transience, as well as pointing out that the velocity of knickpoints is dependent on river discharge. Hence, climate may play a role.
Line 504-505: The period of observation stated in the figure caption is 1901-1981 and therefore different than what is said here.
Section 5.2: It would be more informative to plot incision rate against mean annual precipitation instead of a qualitative map interpretation. Also, what about climate change? Climate is only addressed in terms of spatial patterns, but what about temporal changes? Later in the paper the difference between incision rates from lava’s (shorter time-scale) and from uplifted sediments (longer time scale) is discussed. Could the slower lava incision rates be related to increasing aridity?
Section 5.3 It is extremely hard to follow these dense paragraphs loaded with specific place names. A lot of the information presented here can be streamlined.
Line 620: This is an interesting hypothesis and should be explained in more detail. Maybe also a schematic.
Line 625: Wait. How does the drainage capture no come into play? Why is the drainage capture as mechanism mentioned and then immediately neglected in the concluding statement of the paragraph?
Line 640: Doesn’t this support the hypothesis of aridification leading to a decrease in incision rates? Could this be an interesting research question to ask?
Fig. 1 Area of thinned lithosphere and drainage divides are hard to see. Either use lighter colors for these labels or a lighter color map to increase contrast. My suggestion would be to replace the color map with one that is perceptually uniform and color blind friendly.
Fig. 2: Studied valleys with lava flows and dating locations should be added to this figure.
Figures 12 and 13 are great and I wish they would have come much earlier in the manuscript!
Please, describe what can be seen in figure 13D.
Figure 14: Can you split this into 2 cross-sections? The change in direction of the profile makes it harder to understand.
Line 672: What is geomorphic complexity?
Citation: https://doi.org/10.5194/egusphere-2024-1746-RC1 -
RC2: 'Comment on egusphere-2024-1746', Jo De Waele, 11 Nov 2024
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Dear authors,
I read your ms with interest, and while doing so made many corrections on the pdf file (attached). Most are minor things, that will make you help "shaving" the ms a bit.
I know the areas you studied, so I more or less got through the ms comfortably, but I am sure several readers will get lost in the jungle of names and descriptions. Not sure how to deal with this. I agree with the other reviewer on many points he (or she) rises, and will not come back to these...I am also not an expert in fluvial geomorphology...
The methods you use appear to be adequate, and it all is based on an extensive literature review. I have some major concerns, which I list below.
1. my major one: you never take into account the karstic nature of parts of the study area. In karst areas, water mainly follows underground routes, and the erosional potential is focussed along subterranean flow routes, rather than at the surface in streams. This is completely ignored! The Middle Atlas is entirely karstic, so of course erosion rates in surface rivers is lower than in the siliciclastic bedrocks. This undermines a big part of your discussion and conclusions: what you see is really (entirely) due to differential uplift between areas? Are lithology and climate completely uninfluential on fluvial incision rates? I would need to be clearly convinced this is the case...
Please: acknowledge that karst needs to be considered; describe the areas for their karst; maybe put some data on present surface runoff (flowrates) in the rivers you have investigated (explain hydrology of aquifers and rivers more in detail); put this also in a geochronological and climatic times scale and perspective (rainfall and runoff has not been constant during the Quaternary, influencing also karst!). Then convince me that the fact certain areas are karstic has no influence on fluvial erosion rates...!!
2. My second comments regard the SRTM data and their resolution (altitudes). Are they precise enough to draw reliable conclusions? Can you better describe in methods how useful these data can be in your analysis?
3. Scenarios: your conclusions are very focussed, with no description of different possible explanations: I ask you to describe the different possible scenarios, describing pros and cons of each, and then "promote" the most probable one. Just an example (but there could be many more): the Plio-Quaternary volcanic activity has brought lavas to the surface, creating voids underneath that might have caused the same areas to subside... is this possible? Why or why not? Has it been considered? During ice ages, the Middle Atlas was covered with glaciers: has this caused glacio-isostatic rebound? Many other possible vertical movements would need to be discussed....
Best wishes
Jo De Waele
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