the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Nov 2023
Rift and plume: a discussion on active and passive rifting mechanisms in the Afro-Arabian rift based on synthesis of geophysical data
Abstract. The causal relationship between the activity of mantle plumes and continental break-up is still elusive. The Afro-Arabian rift system offers an opportunity to examine these relationship, in which an ongoing continental break-up intersects a large Cenozoic plume related flood basalt series. In the Afar region, the Gulf of Aden, the Red-Sea and the Main Ethiopian Rift form an R-R-R triple junction and separate the Ethiopian and Yemen Traps by ~600 km. We provide an up-to-date synthesis of the available geophysical and geological data from this region. We map the rift architecture in the intersection region of the rifts and review the spatio-temporal constraints in the development of the different features of the plume‐rift system.
We infer two spatial constraints in the development of the rifts: (1) the connection of the Main Ethiopian Rift to the Gulf of Aden and to the Red Sea by its northeastward propagation; (2) the abandonment of an early tectonic connection between the Red Sea and the Gulf of Aden. Additionally, chronological evidence suggests that regional uplift and flood basalt eruptions sufficiently preceded rifting. By this, we infer a progressive development in which the onset of the triple junction marks a tectonic reorganization and was the last feature to develop, after all rift arms were thoroughly developed. We argue that the classical active and passive rifting mechanisms cannot simply explain the progressive development of the Afro-Arabian rift and propose a scenario of plume-induced plate rotation that includes an interaction between active and passive mechanisms. In this scenario, the arrival of the Afar plume provided a push-force that promoted the rotation of Arabia around a nearby pole, enabling rifting and ultimately the break-up of Arabia from Africa.
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CC1: 'Comment on egusphere-2023-2467', Valentin Rime, 29 Nov 2023
Dear authors, reviewers and editors,
As I recently worked on exactly the same topic as some parts of the present paper, namely the rift architecture and synthesis of geological history of the Afar Region (Rime et al., 2023), I hope I can bring some useful comment on this manuscript.
One important focus of the present paper is the discussion of the influence of plume on rifting (in particular triple junctions) and vice and versa (active vs. passive rifting). I think that the scenario of plume-induced rotation is interesting and plausible, but I will focus my comments on the structure and history of the Afar Region as my expertise rather lies in this part.
One of the pitfalls of this manuscript is the lack of consideration of geological data and overreliance and overinterpretation of geophysical data. For the onshore part of the Region, many geological data are available (see reference list in Rime et al., 2023) but have been overlooked in this manuscript.
This is mainly seen in fig. 11 which maps “axial segments” with black lines. As the term “axial segments” has not been defined, I’m not 100% sure what was meant, but I assume it is roughly similar to what we name “rift segment” in our paper, i.e. continuous strain accommodating areas testified by faulting and, in some cases, magmatic activity and/or formation of sedimentary basin. In our paper, we have studied the structure of the Afar Region with the same methods as this manuscript (topography, seismicity, BGA and VGG), but we have combined them with lots of surface geological data from several hundreds of references, and we came out with quite different results.
The lines represented on fig. 11 in fact represent geophysical lineations that do not necessarily represent an axial segment. Geophysical lineaments are good tools in the absence of access to the geology (e.g. Augustin et al., 2021) but in this complex area, the surface or near-surface geology, as well as other geological data need to be taken into account.
I attached a version of this figure with colours and number to be able to discuss individual mapped axial segments:
- This axial segment is cross-cutting magnetic anomalies (Fournier et al., 2010) and it is barely plausible to have an axial segment cross-cutting oceanic crust there. It was apparently traced based on BGA data (fig. 8) but on the same figure (taking into account the obliquity of extension), you see an almost perfect symmetry between both side of the MOR on all dataset (topography, VGG and BGA), I thus don’t see why it is interpreted as an axial segment.
- This axial segment was mapped based on VGG (fig. 10) but only represent a limit between the sediment-filled Western Guban Basin and the Ali Sabieh Block, with occurrence of shallow basement and volcanic cover (Le Gall et al., 2015; Ali, 2015; Rime et al., 2022b, 2022a, 2023). It therefore does not represent an axial segment but a boundary between two geological regions with different characteristics.
- Idem but for the western margin of the Ali Sabieh Block. It just represents the limit between shallow basement with relatively thin volcanic cover on the Ali Sabieh Block and deep, massively intruded basement and thick volcanic cover in the Depression sensu stricto.
- This line (we don’t know from which data it is interpreted) is incompatible with surface data. It cross-cut all structures visible on the surface that are active since the Pliocene (look at fault pattern on our map (Rime et al., 2022a) or any other map of the Depression or just Google Earth).
- This line was mapped based on VGG data (fig. 10) but this lineament just represents the combination of a topographic low and the presence of a low-density sedimentary basin. The axial segments in the southern Danakil Depression are marked by the volcanic ranges and not by the sedimentary basins (Barberi et al., 1970, 1972; Barberi and Varet, 1970; Tazieff et al., 1972; Le Gall et al., 2018; Watts et al., 2020; Schaegis et al., 2021; Rime et al., 2023).
- This axial segment is not curved. It represents two different overlapping segments (the Erta Ale and the Tat’Ali segments in our publication) but both are straights. In the north, the rift axis continues straight into the salt plain (Bastow et al., 2018; Schaegis et al., 2021; Hurman et al., 2023; Rime et al., 2023). Further north, there is indeed a puzzling link between the Danakil Depression and the Howlakil Bay visible on BGA data but surface geology and other geophysical data strongly suggests that the axial segment continues towards the Gulf of Zula until jumping west of the Mt. Ghedem (Viltres et al., 2020; Ruch et al., 2021; Rime et al., 2023).
- This line connects several different geological features but does not represent a continuous axial segment. The southernmost part of the line represents Teru Graben (which we classify as a rift segment) but then it jumps to the western sedimentary basin of the Erta Ale (like for comment on line 5, this is not an axial segment) before finally jumping to the sedimentary basin forming the rift axis in the northern part of the Danakil Depression.
These are the main problems I see on this figure, but other lines are also questionable, and some axial segments presented in the literature could be added (Hammond et al., 2011; Rime et al., 2023).
Comments on Section 7.2: Spatial constraints in the development of the plume-rift system:
While initially presented as spatial constraints, the two tectonic events described (MER connection with the Gulf of Aden / Red Sea system and the abandonment of the Bab-el-Mandeb segment) are then suddenly considered in line 424 (and lines 402-404 indirectly) as a temporal constrain where the former happens after the latter. There is no argumentation in the text for this change between two spatially separated tectonic events and two temporally separated events. We have discussed these events in our paper (section 6) and, based on numerous geological constrains, we show that they happen roughly at the same time. If your interpretation is different, it should be explained.
I also see confusion regarding margin curvature. For a margin to be curved clockwise or counterclockwise, you have to define a direction in which you observe it. E.g., the somalian margin is curved clockwise if considered from E to W but counterclockwise if considered from W to E. As such, the comparison between margin curvature and block rotation is sketchy. I don’t believe that the curvature of the Ethiopian margin (if you mean the curvature between approx. 9°N and 14°N) is linked with the rotation of the Danakil Block but rather with the linkage between the MER and the Gulf of Aden / Red Sea / Afar Rift System.
Other comments:
- Line 251-254: Not completely true. You find pre-rift trap basalts in the Depression and Late Miocene and younger (so syn-rift) lavas on the plateaus (e.g. Rime et al., 2022).
- Lines 340-341: Keep also in mind that there is a microcontinent in between: the Arrata or Danakil Microplate and that the rotation of this microplate is the reason why the structure are not all aligned (Zwaan et al., 2020).
- Lines 341-352: This is why we propose that the Red Sea Rift and Afar Rift are distinct. See discussion on the topic with references to the literature in section 5 of our paper.
- Line 363-365: They are many papers discussing this, and it’s probably not bad to cite a few more (Tazieff et al., 1972; Barberi and Varet, 1977; Acton et al., 1991; Manighetti et al., 2001; Eagles et al., 2002; Audin et al., 2004; Kogan et al., 2012; Pagli et al., 2014; Doubre et al., 2017; Moore et al., 2021; Rime et al., 2023).
- On northward propagation of the MER: this a controversial issue and the literature is not conclusive. See section 5.2.1 of our paper for reference list on the topic.
- The conclusion that the Afar does not conform to a typical R-R-R triple junction with rift propagating away from the triple junction was already discussed in several papers (Mohr, 1970, 1972; Girdler, 1991; Manighetti et al., 1997; Hébert et al., 2001; Wolfenden et al., 2004; Purcell, 2018; Khalil et al., 2020; Maestrelli et al., 2022; Rime et al., 2023). It’s probably not bad to mention it.
- It is mentioned several times in the manuscript that the margins of the MER are smooth, continuous and the axial ranges are parallel to the margins (e.g. line 330-331). This is only partly true. The volcanic segments of the MER are en-échellon compared to the margins and thus not completely continuous and parallel (see Corti, 2009 and numerous publications by this author).
- Line 396: Numerous evidence suggest that this rotation started around 10 Ma ago (Eagles et al., 2002; McClusky et al., 2010; Reilinger and McClusky, 2011; Rime et al., 2023).
- Line 393: The authors of the mentioned paper say that “the channel most likely does not have a tectonic origin”. The most likely explanation is a massive inflow event. If a hypothetical graben would be active now, it would contradict your idea that this connection is inactive. If it was inactive since the Late Miocene, there was plenty of time (and sediments) to fill it. However, I don’t discuss the fact that the Mab-el-Mandeb is an abandoned rift segment. See also the geological evidence from other references cited in our paper (sections 4.3.1 and 4.6).
- Figure 1: It’s strange to have a lot of emerged land coloured in blue.
- Figure 3: The Shurka el Sheik Discontinuity is not indicated in the correct place. It should be further W (like on fig. 6).
Overall, I think that the manuscript should be at least corrected regarding the position of the axial segments and some interpretation toned down, removed or better justified, in particular for the temporal relationship between the two events described in section 7.2. Finally, a more thorough literature review would have been useful. Several points raised and discussed in this manuscript were already discussed in the literature since decades and these studies should be mentioned.
Best regards,
Valentin Rime
References
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AC1: 'Reply on CC1', Ran Issachar, 06 Dec 2023
Dear Valentin Rime,
Thanks for your profound comments and congratulation for the Earth-Science Reviews paper that was just published. The updated geological maps looks great and your extensive review and discussion is impressive. Definitely will revise the manuscript according to it. Although your mapping of the rift segments in Afar is very persisted and informative, however, we feel no controversy. We find your work reinforcing our argument that the Tendaho-Goba’ad Discontinuity marks a border between rifting style in Afar. Especially, Fig. 8 in your paper is in line with our arguments for a broad region of diffuse deformation and recent tectonic reorganization NE to TGD. Moreover, you even mark the inactive rift segment along Bab al Mandab Strait. In the future, maybe you could also include the continuation of the orange arrow of the Red Sea rift into Afar through Bay of Beyllul as we argue.
- Regarding Fig 11: Probably it is not well clear in the text, but we did not aim to pin rift axis in figure 11, but to give a more generalized pattern of segmented features from the geophysical data, illustrating differences between the two sides of TGD. We agree that these lines that we recognize in the VGG and BGA are not necessarily rift axes and we will make sure not to mislead and more precisely define the terms used. Thanks for your comments regarding segments 1-7 we will refer it in the text.
- Regarding your comment on Section 7.2, thanks for your sensitive observation we agree that there is a logic jump between spatial and temporal arguments and we are happy that you find arguments that the connection of MER and the abandonment of the Bab-el-Mandeb segment happen roughly at the same time. Again, this helps our argument of the progressive development and late tectonic reorganization of the triple junction area.
- Regarding margin curvature, I guess we are not clear enough in the text. We consider the direction from SW to NE, which is the propagation direction of the MER, and indeed we link it with the linkage to the Gulf of Aden and the Red Sea rifts.
- Regarding your other comments, we appreciate them and will take them in account in future revision.
Still, the main point of the paper is that the progressive development of the triple junction is not in-line with classic active mechanism. On the other hand, the chronology of flood basalt eruptions, uplifting and rifting is not in-line with classical passive mechanisms. Your comments are very helpful and not changing that massage.
Sincerely,
Ran
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC1 -
CC2: 'Reply on AC1', Valentin Rime, 06 Dec 2023
Dear Ran,
Thank you for your answer!
Indeed, I do not dispute that the TGD marks a border between rifting styles in Afar. South of it, the influence of the MER is clear (but it was more influenced by the Red Sea/ Gulf of Aden system in the past). I also don’t discuss the presence of a past rift axis on the Bab el Mandab strait. I’m not sure I would include a rift segment between the Red Sea and the Afar as a continuation of my orange arrow. As we are right on the Euler pole of the rotation between the Arabian Plate and the Arrata/Danakil Microplate, the kinematics along this plate boundary would (theoretically) range from compressive to extensive with some strike-slip component (see Viltres et al.). Bref, quite a mess and I was not confident to classify it as rift (same for the connection north of the Gulf of Zula). I thus stayed quite vague.
Regarding fig. 11: indeed, if what you wanted to show is only the geophysical signature, then it makes more sense. Good if you make it clearer in the text.
Section 7.2: good!
Margin curvature: ok, then it makes sense. I also think has to do with the MER propagating in an area already strongly influenced by the Red Sea/ Gulf of Aden system. But I would not see how the rotation of the Danakil block would have had this effect.
Indeed, my comments on the detailed structure of the Afar region and on timing do not invalidate your argumentation about rifting mechanisms.
Best regards,
Valentin
Citation: https://doi.org/10.5194/egusphere-2023-2467-CC2
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RC1: 'Comment on egusphere-2023-2467', Antonio Schettino, 04 Dec 2023
Review of Rift and plume: a discussion on active and passive rifting mechanisms in the Afro-Arabian rift based on synthesis of geophysical data, by Ran Issachar, Peter Haas, Nico Augustin, and Jörg Ebbing
General comment
This is an interesting paper, which aims to investigate the relationship between mantle plumes and rifting processes from the perspective of the Africa-Arabia-Somalia three-plate system, but that is essentially focused on the structural setting of this area (what authors call rifting architecture) through analysis and interpretation of potential field data. Unfortunately there is very limited geological support to this study and some conclusions seem rather questionable.
Specific Comments
R38-40: “Continental flood basalts (...) are associated with extensive volcanism during short time intervals, brought to the surface by deep-seated mantle plumes”. I am not sure that any LIP is associated with the presence of a deep mantle plume. The authors should consider at least the valuable contributions by Don Anderson (e.g., Anderson, 1994 ; Anderson, 2005).
R41-42: “Observations indicate a close temporal and spatial occurrence between the eruption of flood basalts and continental break-up”. Are the authors suggesting that any eruption of flood basalts is associated with continental break-up? As pointed out by Buiter & Torsvik (2014), this is certainly false.
R42-44: “when reconstructed back to their original plate tectonic configuration, a R-R-R triple junction is found within the flood basalts areas”. This assertion is incorrect if considered as a general statement. We have examples of RRR triple junctions in magma poor conditions (e.g., in the southern North Atlantic during the Cretaceous).
R70: “deep mantle convection and its interaction with the Earth’s crust”. It is more correct to say “Earth’s lithosphere”.
R73-76: “This led Morgan (1971) to speculate that deep mantle convection has a significant role in accelerating the overlying tectonic plates. Nevertheless, it was later realized that slab-pull provides the main driving force for plate motion. Furthermore, plumes are thought to have a major role in plate tectonics, triggering rifting by weakening the upper lithosphere”. This is somewhat confusing. The first sentence seems to recover and accept Morgan’s (1971) proposal that “deep mantle convection has a significant role in accelerating the overlying tectonic plates”. The successive sentence recalls the contrasting modern view that the main driving force of plate tectonics is slab pull, but without citing Forsyth & Uyeda (1975) work. Finally, the last sentence, which starts with “Furthermore” but is unrelated to the previous sentences, suggests that plumes have a major role in plate tectonics because they are responsible for the weakening of the upper lithosphere. This seems to imply that in absence of mantle plumes we would have no weakening of the “upper lithosphere” (why “upper”?, does the lower lithosphere remain strong?). In reality, the continental litosphere is weak (and can be extended) because of the presence of water (for a short discussion on this point see Schettino & Ranalli, 2023). When a mantle plume is present, it causes thinning, that is, a rise of the isotherms, not weakening.
R127-128: “Six pairs of magnetic stripes are recognized along the Gulf of Aden ridge”. It would be more correct to avoid the textbook term “magnetic stripes” and use instead “magnetic anomalies”. Furthermore, it should be “the Gulf of Aden”, not “the Gulf of Aden ridge”.
R148-160: “Two magnetic isochrons have been recognized in the Tendaho graben, indicating young oceanization in central Afar (Bridges et al., 2012)”. This is really a stretch of Bridges’s et al. (2012) thinking. It seems that the authors have mis-interpreted the important results of Bridges’s et al. (2012) paper.
R179: “no recent data was published”. It should be: “no recent data were published”.
R209-211: “The southern edges of the magnetic chrons suggest that the ridge rapidly propagated southwards, with rates of ~30 mm/yr, between chrons 3 (4.2 Ma) and 2A (2.6 Ma). However, the rapid propagation was halted in the last 2.6 Ma”. I don’t understand this observation. The Red Sea ridge is composed by two independent branches. The southern ridge separates Arabia from Danakil with a rotation pole that is located in the Gulf of Aden. Consequently, the linear velocity increases northwards and the ridge propagated southwards, as correctly stated by the authors. However, the northern ridge separated Arabia from Nubia about a pole that is located ~50 km south of El Alamein. Therefore, in this case the linear velocity increases southwards and the ridge propagated northwards. Incidentally, I also note that this manuscript does not attribute much importance to the role of Danakil.
R289-290: “South of latitude 14.5°, we find geophysical evidence that the rift axis is bent, entering the Afar region at the Bay of Beylul (latitude 13.3°)”. This is a very questionable interpretation. In my opinion, although Fig. 7a and 7b show the presence of two transform segments (in SW-NE direction), the rift axis does not enter the Danakil microplate and continues in SSE direction. The proposed boundary of the Danakil plate shoud be justified by new fieldwork, because simple visual analysis of geophysical maps could not be convincing for many readers.
R297-298: “Nevertheless, this segment is not an active rift axis as no earthquake, volcanic or bathymetrical expression is associated with it (Fig. 3)”. This is not an argument, as the velocity of separation between Arabia and Danakil is very small in this area (less than 10 mm/yr) and very oblique with respect to the axis. I tested the possibility of a NE–SW strike–slip structure that transfers extension from the Red Sea ridge to Afar through Danakil but any kinematic test failed (this is discussed in Schettino et al, 2016).
R332-334: “the architecture of the intersection region northeast to the Tendaho-Goba’ad discontinuity is more complex and is not simply resolved by rigid plate kinematics”. Clearly, a region characterized by stretching and rifting cannot be described in terms of rigid rotations. People involved in plate kinematics studies use rigid rotations to describe the motion of plate interiors, not of deforming margins.
R340-342: “Axial segments are generally sub-parallel to the Red Sea axis and not to the rift margins, which led authors to suggest that this region reflects an evolving discontinuity of the oceanic spreading center in the Red Sea”. I don’t understand this sentence.
R342-344: “we don’t find any evidence for a transform connection between the ridge in the Red Sea and the continuation of the northern Afar axial segments, offshore Gulf of Zula”. This sentence is also confusing. E-W dextral strike-slip faults in the area north of the Gulf of Zula are documented by several CMT fault plane solutions.
R396-397: “reconstructions suggest that the Danakil microplate started to rotate in Oligocene-Miocene when Arabia was already separated from Africa”. This timing is strange. According to several kinematic models (including the ones proposed by me) and to geological evidence, rifting between Arabia and Nubia started between 30 and 27 Ma (early Oligocene), while the Danakil and Sinai microplates formed during the Langhian (~14 Ma) by strain partitioning.
R432: “We propose a scenario in which rifting was triggered by a plume-induced plate rotation”. This is incorrect. Rifting in the Red Sea and the Gulf of Aden was triggered by far-field forces as any other process of continental breakup, although the presence of a mantle plume has certainly exerted some influence on the formation of a triple junction and the separation of Somalia from Nubia.
References
Anderson, D. L. (1994). The sublithospheric mantle as the source of continental flood basalts; the case against the continental lithosphere and plume head reservoirs. Earth and Planetary Science Letters, 123(1-3), 269-280.
Anderson, D. L. (2005). Large igneous provinces, delamination, and fertile mantle. Elements 1 (5), 271–275.
Buiter, S. J., & Torsvik, T. H. (2014). A review of Wilson Cycle plate margins: A role for mantle plumes in continental break-up along sutures?. Gondwana Research, 26(2), 627-653.
Forsyth, D., & Uyeda, S. (1975). On the relative importance of the driving forces of plate motion. Geophysical Journal International, 43(1), 163-200.
Schettino, A., Macchiavelli, C., Pierantoni, P.P., Zanoni, D., and Rasul, N., 2016, Recent kinematics of the tectonic plates surrounding the red sea and gulf of aden: Geophysical Journal International, 207, 457–480.
Schettino, A. & Ranalli, G. (2023). Ultra-slow transverse waves during continental breakup, Evolving Earth, 1, 100009.
A. Schettino
Citation: https://doi.org/10.5194/egusphere-2023-2467-RC1 -
AC2: 'Reply on RC1', Ran Issachar, 13 Dec 2023
Dear Professor Schettino,
We are grateful for your insightful review of our manuscript. Your work has been a significant inspiration to us. We agree with your point about the need for further geological evidence, particularly to support the rift segment at Hanish area. We will incorporate more geological support in the revised version of the manuscript.
Please see our reply for your comments below.
Best regards,
Ran
R38-40: The sentence says that continental flood basalts (not LIP) are associated with deep-seated mantle plumes. Still, we will restrict that statement. “Many times, continental flood basalts are associated… “
R41-42: We do not wish to claim that any eruption of flood basalts is associated with breakup. We wish to say that there are evidence for temporal and spatial coincidence between continental flood basalts and breakup. We will edit the text to avoid confusion.
R42-44: Yes, but the sentence don’t say that any R-R-R triple junction is found within the flood basalts areas, but, that in many cases a R-R-R triple junction is found within areas of flood basalts. We will edit the sentence to avoid confusion.
R70: We agree.
R73-76: Thanks, we see the confusion. We will edit the text and cite the referred works.
R127-128: We agree with your wording and will edit the text.
R148-160: The title of Bridges’s et al. (2012) is “Magnetic stripes of a transitional continental rift in Afar”, and in the paper they correlate these stripes with the Matu- yama Chron. How did we mis-interpreted the results of Bridges’s et al. (2012)?
R179: Thanks.
R209-211: Yes we agree, but still the southward propagation is halted between isochrons 2 and 2A.
R289-290: We agree and following discussions with Neil C. Mitchell we will include field observation of young volcanic features and faults on the Hanish Islands which align with the suggested rift segment.
R297-298: Yes, but a segment is active wouldn’t you expect some kind of activity along it? Viltres et al., (2020; fig. 5) suggest a kinematic model in which this area is within Arabia and there is no plate boundary along the Red Sea south of latitude 14.5°.
R332-334: We agree. This is a citation from Garfunkel and Beyth (2006).
R340-342: It means that the parallelism between the Red Sea Rift and the rifting axes in Afar may suggest that they are linked.
R342-344: Although there are evidence for strike slip they do not provide evidence of first-order transform connections, like those observed along mid-ocean rides. We will edit the text to better explain.
R396-397: Yes we agree, this is a mistake also raised by Valentin Rime. We will correct. Moreover, this later chronology strengthening our argument of progressive development and late tectonic reorganization of the triple junction area.
R432: There is no controversy, this is the point. The plume-induced plate rotation suggest that far field forces are increased by the plume-push. Still, the far field forces are the one that derive riffing.
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC2 -
RC2: 'Reply on AC2', Antonio Schettino, 19 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2467/egusphere-2023-2467-RC2-supplement.pdf
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AC3: 'Reply on RC2', Ran Issachar, 21 Dec 2023
Dear Prof. Schettino,
Thanks for your explanation. Then maybe it is more accurate to say that the southward propagation of the spreading center in the southern Red Sea is significantly slowed between isochrons 2 and 2A. And, as I understand from your insights, the slowdown of the spreading center is the result of the slowdown of Danakil. We can write then: "The magnetic isochrons indicate a significant slow down in the southward propagation of the spreading center between chrons 2A and 2 (2.58 and 1.77 Ma), probably due to the slowdown of Danakil relativ to Arabia (e.g., Schettino et al., 2018)."
Regarding the Bridges et al. paper, thanks for refining the significance of this study.
Best,Ran
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC3
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AC3: 'Reply on RC2', Ran Issachar, 21 Dec 2023
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RC2: 'Reply on AC2', Antonio Schettino, 19 Dec 2023
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AC2: 'Reply on RC1', Ran Issachar, 13 Dec 2023
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RC3: 'Comment on egusphere-2023-2467', Derek Keir, 02 Feb 2024
I have read the manuscript, along with the comments and responses already made, so will limit my feedback to items not already discussed and addressed.
Overall I found the manuscript an interesting synthesis of the tectonic and magmatic evolution of Afar and discussion with respect to the interaction between plate forces and plumes in driving rifting. Broadly i thought the synthesis of tectonic, geodynamic, and geochronology literature fairly complete, but with a few commissions mentioned below. This, along with the new analysis of broad scale surface nad subsurface rift morphology were the strongest points of the work. I thought section 7.3 - the discussion of Mechanisms for plume-rift association less strong. It comes across as rater speculative, which is due to the interpretation of plume driven rotation lacking presentation of a quantitative geodynamic model to back it up. See below for more details on this.
Detailed comments
Line 76 - should upper lithosphere be lower lithosphere? The plume impacts the lower lithosphere first.
Line 95 - earth should be upper case Earth
Line 108 - 114 and section 7.3 - i thought a weakness of the work is lack of discussion of quite a bit of geodynamic modelling work on a similar topic to the aim of this discussion paper.
This manuscript has avoided discussing the various models by Stamps for the region in which GNSS, topography and lithosphere and asthenosphere imaging data have been used to guide numerical simulations to isolate driving forces of extension in NE Africa. These works generally found the gravitational potential energy (GPE) from uplift is a major driver of extension, with base of lithosphere traction rather minimally involved.
For example see Stamps et al., 2014 - https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2013JB010717
From what i can see the work by Stamps is somewhat in contradiction to the interpretations being made in the new discussion manuscript. This is fine - but i think the text deserves dealing with this in a more convincing fashion.
Line 141 - 152 - there is a fair body of literature in Afar and rifted margins more generally that discusses the concept that magnetic stripes could form as a result of magma intrusion and volcanism before the continental lithosphere is fully split - ie magnetic striping on the continnet ocean transition, and subtly prior to full seafloor spreading. This is potentially important for interpretting the onset of seafloor spreading. See section 5.1 of Ebinger et al., 2017 - especially the last paragraph of this and references therein https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017TC004526
line 223 - replace "last" with "the early"
Line 247 - Please double check this is a catalog of Quaternary volcanoes , rather than just Holocene volcanoes . Also google earth should be Google Earth
section 6 - consider using the term "escarpment" rather than sharp cliff / cliff. Escarpment is the more globally used term for these topographically prominent rifted margins.
End of section 7.1 - i saw this same point mentioned in one of the other reviews. The concept that strain in Afar is commonly somewhat localised in distinct rift segments but which are set within quite a broad strain field influenced by all the plate motions is not new. e.g. Keir et al., 2010, Tectonics identified a SE-NW striking dike intrusion event in the MER that coupled with structural geology and focal mechanisms, amongst other things, was used to interpret that strain from the NE motion of Arabia occurs in the MER of southern Afar. Also see Doubre et al., 2017 GJI and Pagli et al., 2019 which provide good evidence from geodesy for broad extension in central Afar. See Maestrelli et al., 2022 Tectonics for analog models that use a number of model scenarios to reconstruct potential evolution and distribution of extension and faulting of Afar. These models invoke somewhat broad zones in which extension from the various plates interact, and within which strain has a higher gradient (more localised) in some zones.
Derek Keir - 2 February 2024
Citation: https://doi.org/10.5194/egusphere-2023-2467-RC3 -
AC4: 'Reply on RC3', Ran Issachar, 19 Feb 2024
Dear Dr. Keir,
As a leading scientist in the Afar region, we are very happy to get your review on our work and to hear that you have a generally positive opinion about it. We agree that section 7.3 is rather speculative and should be tested by geodynamic models in future works.
We are grateful for your comments and will follow them to improve the ms. Please see our response below.
I have read the manuscript, along with the comments and responses already made, so will limit my feedback to items not already discussed and addressed.
Overall I found the manuscript an interesting synthesis of the tectonic and magmatic evolution of Afar and discussion with respect to the interaction between plate forces and plumes in driving rifting. Broadly i thought the synthesis of tectonic, geodynamic, and geochronology literature fairly complete, but with a few commissions mentioned below. This, along with the new analysis of broad scale surface and subsurface rift morphology were the strongest points of the work. I thought section 7.3 - the discussion of Mechanisms for plume-rift association less strong. It comes across as rater speculative, which is due to the interpretation of plume driven rotation lacking presentation of a quantitative geodynamic model to back it up. See below for more details on this.
Detailed comments
Line 76 - should upper lithosphere be lower lithosphere? The plume impacts the lower lithosphere first.
- Yes, thanks
Line 95 - earth should be upper case Earth
- Thanks
Line 108 - 114 and section 7.3 - i thought a weakness of the work is lack of discussion of quite a bit of geodynamic modelling work on a similar topic to the aim of this discussion paper.
This manuscript has avoided discussing the various models by Stamps for the region in which GNSS, topography and lithosphere and asthenosphere imaging data have been used to guide numerical simulations to isolate driving forces of extension in NE Africa. These works generally found the gravitational potential energy (GPE) from uplift is a major driver of extension, with base of lithosphere traction rather minimally involved.
For example see Stamps et al., 2014 - https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2013JB010717
From what i can see the work by Stamps is somewhat in contradiction to the interpretations being made in the new discussion manuscript. This is fine - but i think the text deserves dealing with this in a more convincing fashion.
- Thanks for pointing out this significant work by Stamps el al., we will include a discussion with Stamps models in the revised ms. However, we don’t see a contradiction. They calculated the deriving forces for Nubia-Somalia divergence and found that gravitational potential energy is more significant then mantle flow by ~ an order of magnitude. Still, maybe this is not the case for the Nubia (and Somalia) – Arabia dynamics, which is not calculated in their study. If plume push forces are more significant in the context of Arabia breakup, than it could also explain the more rapid divergence of Arabia in comparison to Somalia. This is quite resendable when considering current mantle flow direction beneath that area, see Faccenna et al., 2013 (10.1016/J.EPSL.2013.05.043) and Qaysi et al., 2018 (10.1785/0220180144).
Line 141 - 152 - there is a fair body of literature in Afar and rifted margins more generally that discusses the concept that magnetic stripes could form as a result of magma intrusion and volcanism before the continental lithosphere is fully split - ie magnetic striping on the continnet ocean transition, and subtly prior to full seafloor spreading. This is potentially important for interpretting the onset of seafloor spreading. See section 5.1 of Ebinger et al., 2017 - especially the last paragraph of this and references therein https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017TC004526
- Thanks we will follow these and will change the description of the results by Bridges et al., 2012 according to comments by A. Schettino.
line 223 - replace "last" with "the early"
- “last” is correct.
Line 247 - Please double check this is a catalog of Quaternary volcanoes , rather than just Holocene volcanoes . Also google earth should be Google Earth
- Thanks, we used the Holocene catalog from Smithsonian.
section 6 - consider using the term "escarpment" rather than sharp cliff / cliff. Escarpment is the more globally used term for these topographically prominent rifted margins.
- Thanks, we will.
End of section 7.1 - i saw this same point mentioned in one of the other reviews. The concept that strain in Afar is commonly somewhat localised in distinct rift segments but which are set within quite a broad strain field influenced by all the plate motions is not new. e.g. Keir et al., 2010, Tectonics identified a SE-NW striking dike intrusion event in the MER that coupled with structural geology and focal mechanisms, amongst other things, was used to interpret that strain from the NE motion of Arabia occurs in the MER of southern Afar. Also see Doubre et al., 2017 GJI and Pagli et al., 2019 which provide good evidence from geodesy for broad extension in central Afar. See Maestrelli et al., 2022 Tectonics for analog models that use a number of model scenarios to reconstruct potential evolution and distribution of extension and faulting of Afar. These models invoke somewhat broad zones in which extension from the various plates interact, and within which strain has a higher gradient (more localised) in some zones.
- Thanks, we will carefully go over these references and bring them into the discussion.Sincerely,
Ran Issachar
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC4
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AC4: 'Reply on RC3', Ran Issachar, 19 Feb 2024
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2023-2467', Valentin Rime, 29 Nov 2023
Dear authors, reviewers and editors,
As I recently worked on exactly the same topic as some parts of the present paper, namely the rift architecture and synthesis of geological history of the Afar Region (Rime et al., 2023), I hope I can bring some useful comment on this manuscript.
One important focus of the present paper is the discussion of the influence of plume on rifting (in particular triple junctions) and vice and versa (active vs. passive rifting). I think that the scenario of plume-induced rotation is interesting and plausible, but I will focus my comments on the structure and history of the Afar Region as my expertise rather lies in this part.
One of the pitfalls of this manuscript is the lack of consideration of geological data and overreliance and overinterpretation of geophysical data. For the onshore part of the Region, many geological data are available (see reference list in Rime et al., 2023) but have been overlooked in this manuscript.
This is mainly seen in fig. 11 which maps “axial segments” with black lines. As the term “axial segments” has not been defined, I’m not 100% sure what was meant, but I assume it is roughly similar to what we name “rift segment” in our paper, i.e. continuous strain accommodating areas testified by faulting and, in some cases, magmatic activity and/or formation of sedimentary basin. In our paper, we have studied the structure of the Afar Region with the same methods as this manuscript (topography, seismicity, BGA and VGG), but we have combined them with lots of surface geological data from several hundreds of references, and we came out with quite different results.
The lines represented on fig. 11 in fact represent geophysical lineations that do not necessarily represent an axial segment. Geophysical lineaments are good tools in the absence of access to the geology (e.g. Augustin et al., 2021) but in this complex area, the surface or near-surface geology, as well as other geological data need to be taken into account.
I attached a version of this figure with colours and number to be able to discuss individual mapped axial segments:
- This axial segment is cross-cutting magnetic anomalies (Fournier et al., 2010) and it is barely plausible to have an axial segment cross-cutting oceanic crust there. It was apparently traced based on BGA data (fig. 8) but on the same figure (taking into account the obliquity of extension), you see an almost perfect symmetry between both side of the MOR on all dataset (topography, VGG and BGA), I thus don’t see why it is interpreted as an axial segment.
- This axial segment was mapped based on VGG (fig. 10) but only represent a limit between the sediment-filled Western Guban Basin and the Ali Sabieh Block, with occurrence of shallow basement and volcanic cover (Le Gall et al., 2015; Ali, 2015; Rime et al., 2022b, 2022a, 2023). It therefore does not represent an axial segment but a boundary between two geological regions with different characteristics.
- Idem but for the western margin of the Ali Sabieh Block. It just represents the limit between shallow basement with relatively thin volcanic cover on the Ali Sabieh Block and deep, massively intruded basement and thick volcanic cover in the Depression sensu stricto.
- This line (we don’t know from which data it is interpreted) is incompatible with surface data. It cross-cut all structures visible on the surface that are active since the Pliocene (look at fault pattern on our map (Rime et al., 2022a) or any other map of the Depression or just Google Earth).
- This line was mapped based on VGG data (fig. 10) but this lineament just represents the combination of a topographic low and the presence of a low-density sedimentary basin. The axial segments in the southern Danakil Depression are marked by the volcanic ranges and not by the sedimentary basins (Barberi et al., 1970, 1972; Barberi and Varet, 1970; Tazieff et al., 1972; Le Gall et al., 2018; Watts et al., 2020; Schaegis et al., 2021; Rime et al., 2023).
- This axial segment is not curved. It represents two different overlapping segments (the Erta Ale and the Tat’Ali segments in our publication) but both are straights. In the north, the rift axis continues straight into the salt plain (Bastow et al., 2018; Schaegis et al., 2021; Hurman et al., 2023; Rime et al., 2023). Further north, there is indeed a puzzling link between the Danakil Depression and the Howlakil Bay visible on BGA data but surface geology and other geophysical data strongly suggests that the axial segment continues towards the Gulf of Zula until jumping west of the Mt. Ghedem (Viltres et al., 2020; Ruch et al., 2021; Rime et al., 2023).
- This line connects several different geological features but does not represent a continuous axial segment. The southernmost part of the line represents Teru Graben (which we classify as a rift segment) but then it jumps to the western sedimentary basin of the Erta Ale (like for comment on line 5, this is not an axial segment) before finally jumping to the sedimentary basin forming the rift axis in the northern part of the Danakil Depression.
These are the main problems I see on this figure, but other lines are also questionable, and some axial segments presented in the literature could be added (Hammond et al., 2011; Rime et al., 2023).
Comments on Section 7.2: Spatial constraints in the development of the plume-rift system:
While initially presented as spatial constraints, the two tectonic events described (MER connection with the Gulf of Aden / Red Sea system and the abandonment of the Bab-el-Mandeb segment) are then suddenly considered in line 424 (and lines 402-404 indirectly) as a temporal constrain where the former happens after the latter. There is no argumentation in the text for this change between two spatially separated tectonic events and two temporally separated events. We have discussed these events in our paper (section 6) and, based on numerous geological constrains, we show that they happen roughly at the same time. If your interpretation is different, it should be explained.
I also see confusion regarding margin curvature. For a margin to be curved clockwise or counterclockwise, you have to define a direction in which you observe it. E.g., the somalian margin is curved clockwise if considered from E to W but counterclockwise if considered from W to E. As such, the comparison between margin curvature and block rotation is sketchy. I don’t believe that the curvature of the Ethiopian margin (if you mean the curvature between approx. 9°N and 14°N) is linked with the rotation of the Danakil Block but rather with the linkage between the MER and the Gulf of Aden / Red Sea / Afar Rift System.
Other comments:
- Line 251-254: Not completely true. You find pre-rift trap basalts in the Depression and Late Miocene and younger (so syn-rift) lavas on the plateaus (e.g. Rime et al., 2022).
- Lines 340-341: Keep also in mind that there is a microcontinent in between: the Arrata or Danakil Microplate and that the rotation of this microplate is the reason why the structure are not all aligned (Zwaan et al., 2020).
- Lines 341-352: This is why we propose that the Red Sea Rift and Afar Rift are distinct. See discussion on the topic with references to the literature in section 5 of our paper.
- Line 363-365: They are many papers discussing this, and it’s probably not bad to cite a few more (Tazieff et al., 1972; Barberi and Varet, 1977; Acton et al., 1991; Manighetti et al., 2001; Eagles et al., 2002; Audin et al., 2004; Kogan et al., 2012; Pagli et al., 2014; Doubre et al., 2017; Moore et al., 2021; Rime et al., 2023).
- On northward propagation of the MER: this a controversial issue and the literature is not conclusive. See section 5.2.1 of our paper for reference list on the topic.
- The conclusion that the Afar does not conform to a typical R-R-R triple junction with rift propagating away from the triple junction was already discussed in several papers (Mohr, 1970, 1972; Girdler, 1991; Manighetti et al., 1997; Hébert et al., 2001; Wolfenden et al., 2004; Purcell, 2018; Khalil et al., 2020; Maestrelli et al., 2022; Rime et al., 2023). It’s probably not bad to mention it.
- It is mentioned several times in the manuscript that the margins of the MER are smooth, continuous and the axial ranges are parallel to the margins (e.g. line 330-331). This is only partly true. The volcanic segments of the MER are en-échellon compared to the margins and thus not completely continuous and parallel (see Corti, 2009 and numerous publications by this author).
- Line 396: Numerous evidence suggest that this rotation started around 10 Ma ago (Eagles et al., 2002; McClusky et al., 2010; Reilinger and McClusky, 2011; Rime et al., 2023).
- Line 393: The authors of the mentioned paper say that “the channel most likely does not have a tectonic origin”. The most likely explanation is a massive inflow event. If a hypothetical graben would be active now, it would contradict your idea that this connection is inactive. If it was inactive since the Late Miocene, there was plenty of time (and sediments) to fill it. However, I don’t discuss the fact that the Mab-el-Mandeb is an abandoned rift segment. See also the geological evidence from other references cited in our paper (sections 4.3.1 and 4.6).
- Figure 1: It’s strange to have a lot of emerged land coloured in blue.
- Figure 3: The Shurka el Sheik Discontinuity is not indicated in the correct place. It should be further W (like on fig. 6).
Overall, I think that the manuscript should be at least corrected regarding the position of the axial segments and some interpretation toned down, removed or better justified, in particular for the temporal relationship between the two events described in section 7.2. Finally, a more thorough literature review would have been useful. Several points raised and discussed in this manuscript were already discussed in the literature since decades and these studies should be mentioned.
Best regards,
Valentin Rime
References
Acton, G.D., Stein, S., and Engeln, J.F., 1991, Block rotation and continental extension in Afar: A comparison to oceanic microplate systems: Tectonics, v. 10, p. 501–526, doi:10.1029/90TC01792.
Ali, M.Y., 2015, Petroleum geology and hydrocarbon potential of the Guban Basin, Northern Somaliland: Journal of Petroleum Geology, v. 38, p. 433–457, doi:10.1111/jpg.12620.
Audin, L., Quidelleur, X., Coulié, E., Courtillot, V., Gilder, S., Manighetti, I., Gillot, P.-Y., Tapponnier, P., and Kidane, T., 2004, Palaeomagnetism and K-Ar and 40 Ar/ 39 Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr: Geophysical Journal International, v. 158, p. 327–345, doi:10.1111/j.1365-246X.2004.02286.x.
Augustin, N., van der Zwan, F.M., Devey, C.W., and Brandsdóttir, B., 2021, 13 million years of seafloor spreading throughout the Red Sea Basin: Nature Communications, v. 12, p. 2427, doi:10.1038/s41467-021-22586-2.
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AC1: 'Reply on CC1', Ran Issachar, 06 Dec 2023
Dear Valentin Rime,
Thanks for your profound comments and congratulation for the Earth-Science Reviews paper that was just published. The updated geological maps looks great and your extensive review and discussion is impressive. Definitely will revise the manuscript according to it. Although your mapping of the rift segments in Afar is very persisted and informative, however, we feel no controversy. We find your work reinforcing our argument that the Tendaho-Goba’ad Discontinuity marks a border between rifting style in Afar. Especially, Fig. 8 in your paper is in line with our arguments for a broad region of diffuse deformation and recent tectonic reorganization NE to TGD. Moreover, you even mark the inactive rift segment along Bab al Mandab Strait. In the future, maybe you could also include the continuation of the orange arrow of the Red Sea rift into Afar through Bay of Beyllul as we argue.
- Regarding Fig 11: Probably it is not well clear in the text, but we did not aim to pin rift axis in figure 11, but to give a more generalized pattern of segmented features from the geophysical data, illustrating differences between the two sides of TGD. We agree that these lines that we recognize in the VGG and BGA are not necessarily rift axes and we will make sure not to mislead and more precisely define the terms used. Thanks for your comments regarding segments 1-7 we will refer it in the text.
- Regarding your comment on Section 7.2, thanks for your sensitive observation we agree that there is a logic jump between spatial and temporal arguments and we are happy that you find arguments that the connection of MER and the abandonment of the Bab-el-Mandeb segment happen roughly at the same time. Again, this helps our argument of the progressive development and late tectonic reorganization of the triple junction area.
- Regarding margin curvature, I guess we are not clear enough in the text. We consider the direction from SW to NE, which is the propagation direction of the MER, and indeed we link it with the linkage to the Gulf of Aden and the Red Sea rifts.
- Regarding your other comments, we appreciate them and will take them in account in future revision.
Still, the main point of the paper is that the progressive development of the triple junction is not in-line with classic active mechanism. On the other hand, the chronology of flood basalt eruptions, uplifting and rifting is not in-line with classical passive mechanisms. Your comments are very helpful and not changing that massage.
Sincerely,
Ran
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC1 -
CC2: 'Reply on AC1', Valentin Rime, 06 Dec 2023
Dear Ran,
Thank you for your answer!
Indeed, I do not dispute that the TGD marks a border between rifting styles in Afar. South of it, the influence of the MER is clear (but it was more influenced by the Red Sea/ Gulf of Aden system in the past). I also don’t discuss the presence of a past rift axis on the Bab el Mandab strait. I’m not sure I would include a rift segment between the Red Sea and the Afar as a continuation of my orange arrow. As we are right on the Euler pole of the rotation between the Arabian Plate and the Arrata/Danakil Microplate, the kinematics along this plate boundary would (theoretically) range from compressive to extensive with some strike-slip component (see Viltres et al.). Bref, quite a mess and I was not confident to classify it as rift (same for the connection north of the Gulf of Zula). I thus stayed quite vague.
Regarding fig. 11: indeed, if what you wanted to show is only the geophysical signature, then it makes more sense. Good if you make it clearer in the text.
Section 7.2: good!
Margin curvature: ok, then it makes sense. I also think has to do with the MER propagating in an area already strongly influenced by the Red Sea/ Gulf of Aden system. But I would not see how the rotation of the Danakil block would have had this effect.
Indeed, my comments on the detailed structure of the Afar region and on timing do not invalidate your argumentation about rifting mechanisms.
Best regards,
Valentin
Citation: https://doi.org/10.5194/egusphere-2023-2467-CC2
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RC1: 'Comment on egusphere-2023-2467', Antonio Schettino, 04 Dec 2023
Review of Rift and plume: a discussion on active and passive rifting mechanisms in the Afro-Arabian rift based on synthesis of geophysical data, by Ran Issachar, Peter Haas, Nico Augustin, and Jörg Ebbing
General comment
This is an interesting paper, which aims to investigate the relationship between mantle plumes and rifting processes from the perspective of the Africa-Arabia-Somalia three-plate system, but that is essentially focused on the structural setting of this area (what authors call rifting architecture) through analysis and interpretation of potential field data. Unfortunately there is very limited geological support to this study and some conclusions seem rather questionable.
Specific Comments
R38-40: “Continental flood basalts (...) are associated with extensive volcanism during short time intervals, brought to the surface by deep-seated mantle plumes”. I am not sure that any LIP is associated with the presence of a deep mantle plume. The authors should consider at least the valuable contributions by Don Anderson (e.g., Anderson, 1994 ; Anderson, 2005).
R41-42: “Observations indicate a close temporal and spatial occurrence between the eruption of flood basalts and continental break-up”. Are the authors suggesting that any eruption of flood basalts is associated with continental break-up? As pointed out by Buiter & Torsvik (2014), this is certainly false.
R42-44: “when reconstructed back to their original plate tectonic configuration, a R-R-R triple junction is found within the flood basalts areas”. This assertion is incorrect if considered as a general statement. We have examples of RRR triple junctions in magma poor conditions (e.g., in the southern North Atlantic during the Cretaceous).
R70: “deep mantle convection and its interaction with the Earth’s crust”. It is more correct to say “Earth’s lithosphere”.
R73-76: “This led Morgan (1971) to speculate that deep mantle convection has a significant role in accelerating the overlying tectonic plates. Nevertheless, it was later realized that slab-pull provides the main driving force for plate motion. Furthermore, plumes are thought to have a major role in plate tectonics, triggering rifting by weakening the upper lithosphere”. This is somewhat confusing. The first sentence seems to recover and accept Morgan’s (1971) proposal that “deep mantle convection has a significant role in accelerating the overlying tectonic plates”. The successive sentence recalls the contrasting modern view that the main driving force of plate tectonics is slab pull, but without citing Forsyth & Uyeda (1975) work. Finally, the last sentence, which starts with “Furthermore” but is unrelated to the previous sentences, suggests that plumes have a major role in plate tectonics because they are responsible for the weakening of the upper lithosphere. This seems to imply that in absence of mantle plumes we would have no weakening of the “upper lithosphere” (why “upper”?, does the lower lithosphere remain strong?). In reality, the continental litosphere is weak (and can be extended) because of the presence of water (for a short discussion on this point see Schettino & Ranalli, 2023). When a mantle plume is present, it causes thinning, that is, a rise of the isotherms, not weakening.
R127-128: “Six pairs of magnetic stripes are recognized along the Gulf of Aden ridge”. It would be more correct to avoid the textbook term “magnetic stripes” and use instead “magnetic anomalies”. Furthermore, it should be “the Gulf of Aden”, not “the Gulf of Aden ridge”.
R148-160: “Two magnetic isochrons have been recognized in the Tendaho graben, indicating young oceanization in central Afar (Bridges et al., 2012)”. This is really a stretch of Bridges’s et al. (2012) thinking. It seems that the authors have mis-interpreted the important results of Bridges’s et al. (2012) paper.
R179: “no recent data was published”. It should be: “no recent data were published”.
R209-211: “The southern edges of the magnetic chrons suggest that the ridge rapidly propagated southwards, with rates of ~30 mm/yr, between chrons 3 (4.2 Ma) and 2A (2.6 Ma). However, the rapid propagation was halted in the last 2.6 Ma”. I don’t understand this observation. The Red Sea ridge is composed by two independent branches. The southern ridge separates Arabia from Danakil with a rotation pole that is located in the Gulf of Aden. Consequently, the linear velocity increases northwards and the ridge propagated southwards, as correctly stated by the authors. However, the northern ridge separated Arabia from Nubia about a pole that is located ~50 km south of El Alamein. Therefore, in this case the linear velocity increases southwards and the ridge propagated northwards. Incidentally, I also note that this manuscript does not attribute much importance to the role of Danakil.
R289-290: “South of latitude 14.5°, we find geophysical evidence that the rift axis is bent, entering the Afar region at the Bay of Beylul (latitude 13.3°)”. This is a very questionable interpretation. In my opinion, although Fig. 7a and 7b show the presence of two transform segments (in SW-NE direction), the rift axis does not enter the Danakil microplate and continues in SSE direction. The proposed boundary of the Danakil plate shoud be justified by new fieldwork, because simple visual analysis of geophysical maps could not be convincing for many readers.
R297-298: “Nevertheless, this segment is not an active rift axis as no earthquake, volcanic or bathymetrical expression is associated with it (Fig. 3)”. This is not an argument, as the velocity of separation between Arabia and Danakil is very small in this area (less than 10 mm/yr) and very oblique with respect to the axis. I tested the possibility of a NE–SW strike–slip structure that transfers extension from the Red Sea ridge to Afar through Danakil but any kinematic test failed (this is discussed in Schettino et al, 2016).
R332-334: “the architecture of the intersection region northeast to the Tendaho-Goba’ad discontinuity is more complex and is not simply resolved by rigid plate kinematics”. Clearly, a region characterized by stretching and rifting cannot be described in terms of rigid rotations. People involved in plate kinematics studies use rigid rotations to describe the motion of plate interiors, not of deforming margins.
R340-342: “Axial segments are generally sub-parallel to the Red Sea axis and not to the rift margins, which led authors to suggest that this region reflects an evolving discontinuity of the oceanic spreading center in the Red Sea”. I don’t understand this sentence.
R342-344: “we don’t find any evidence for a transform connection between the ridge in the Red Sea and the continuation of the northern Afar axial segments, offshore Gulf of Zula”. This sentence is also confusing. E-W dextral strike-slip faults in the area north of the Gulf of Zula are documented by several CMT fault plane solutions.
R396-397: “reconstructions suggest that the Danakil microplate started to rotate in Oligocene-Miocene when Arabia was already separated from Africa”. This timing is strange. According to several kinematic models (including the ones proposed by me) and to geological evidence, rifting between Arabia and Nubia started between 30 and 27 Ma (early Oligocene), while the Danakil and Sinai microplates formed during the Langhian (~14 Ma) by strain partitioning.
R432: “We propose a scenario in which rifting was triggered by a plume-induced plate rotation”. This is incorrect. Rifting in the Red Sea and the Gulf of Aden was triggered by far-field forces as any other process of continental breakup, although the presence of a mantle plume has certainly exerted some influence on the formation of a triple junction and the separation of Somalia from Nubia.
References
Anderson, D. L. (1994). The sublithospheric mantle as the source of continental flood basalts; the case against the continental lithosphere and plume head reservoirs. Earth and Planetary Science Letters, 123(1-3), 269-280.
Anderson, D. L. (2005). Large igneous provinces, delamination, and fertile mantle. Elements 1 (5), 271–275.
Buiter, S. J., & Torsvik, T. H. (2014). A review of Wilson Cycle plate margins: A role for mantle plumes in continental break-up along sutures?. Gondwana Research, 26(2), 627-653.
Forsyth, D., & Uyeda, S. (1975). On the relative importance of the driving forces of plate motion. Geophysical Journal International, 43(1), 163-200.
Schettino, A., Macchiavelli, C., Pierantoni, P.P., Zanoni, D., and Rasul, N., 2016, Recent kinematics of the tectonic plates surrounding the red sea and gulf of aden: Geophysical Journal International, 207, 457–480.
Schettino, A. & Ranalli, G. (2023). Ultra-slow transverse waves during continental breakup, Evolving Earth, 1, 100009.
A. Schettino
Citation: https://doi.org/10.5194/egusphere-2023-2467-RC1 -
AC2: 'Reply on RC1', Ran Issachar, 13 Dec 2023
Dear Professor Schettino,
We are grateful for your insightful review of our manuscript. Your work has been a significant inspiration to us. We agree with your point about the need for further geological evidence, particularly to support the rift segment at Hanish area. We will incorporate more geological support in the revised version of the manuscript.
Please see our reply for your comments below.
Best regards,
Ran
R38-40: The sentence says that continental flood basalts (not LIP) are associated with deep-seated mantle plumes. Still, we will restrict that statement. “Many times, continental flood basalts are associated… “
R41-42: We do not wish to claim that any eruption of flood basalts is associated with breakup. We wish to say that there are evidence for temporal and spatial coincidence between continental flood basalts and breakup. We will edit the text to avoid confusion.
R42-44: Yes, but the sentence don’t say that any R-R-R triple junction is found within the flood basalts areas, but, that in many cases a R-R-R triple junction is found within areas of flood basalts. We will edit the sentence to avoid confusion.
R70: We agree.
R73-76: Thanks, we see the confusion. We will edit the text and cite the referred works.
R127-128: We agree with your wording and will edit the text.
R148-160: The title of Bridges’s et al. (2012) is “Magnetic stripes of a transitional continental rift in Afar”, and in the paper they correlate these stripes with the Matu- yama Chron. How did we mis-interpreted the results of Bridges’s et al. (2012)?
R179: Thanks.
R209-211: Yes we agree, but still the southward propagation is halted between isochrons 2 and 2A.
R289-290: We agree and following discussions with Neil C. Mitchell we will include field observation of young volcanic features and faults on the Hanish Islands which align with the suggested rift segment.
R297-298: Yes, but a segment is active wouldn’t you expect some kind of activity along it? Viltres et al., (2020; fig. 5) suggest a kinematic model in which this area is within Arabia and there is no plate boundary along the Red Sea south of latitude 14.5°.
R332-334: We agree. This is a citation from Garfunkel and Beyth (2006).
R340-342: It means that the parallelism between the Red Sea Rift and the rifting axes in Afar may suggest that they are linked.
R342-344: Although there are evidence for strike slip they do not provide evidence of first-order transform connections, like those observed along mid-ocean rides. We will edit the text to better explain.
R396-397: Yes we agree, this is a mistake also raised by Valentin Rime. We will correct. Moreover, this later chronology strengthening our argument of progressive development and late tectonic reorganization of the triple junction area.
R432: There is no controversy, this is the point. The plume-induced plate rotation suggest that far field forces are increased by the plume-push. Still, the far field forces are the one that derive riffing.
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC2 -
RC2: 'Reply on AC2', Antonio Schettino, 19 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2467/egusphere-2023-2467-RC2-supplement.pdf
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AC3: 'Reply on RC2', Ran Issachar, 21 Dec 2023
Dear Prof. Schettino,
Thanks for your explanation. Then maybe it is more accurate to say that the southward propagation of the spreading center in the southern Red Sea is significantly slowed between isochrons 2 and 2A. And, as I understand from your insights, the slowdown of the spreading center is the result of the slowdown of Danakil. We can write then: "The magnetic isochrons indicate a significant slow down in the southward propagation of the spreading center between chrons 2A and 2 (2.58 and 1.77 Ma), probably due to the slowdown of Danakil relativ to Arabia (e.g., Schettino et al., 2018)."
Regarding the Bridges et al. paper, thanks for refining the significance of this study.
Best,Ran
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC3
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AC3: 'Reply on RC2', Ran Issachar, 21 Dec 2023
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RC2: 'Reply on AC2', Antonio Schettino, 19 Dec 2023
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AC2: 'Reply on RC1', Ran Issachar, 13 Dec 2023
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RC3: 'Comment on egusphere-2023-2467', Derek Keir, 02 Feb 2024
I have read the manuscript, along with the comments and responses already made, so will limit my feedback to items not already discussed and addressed.
Overall I found the manuscript an interesting synthesis of the tectonic and magmatic evolution of Afar and discussion with respect to the interaction between plate forces and plumes in driving rifting. Broadly i thought the synthesis of tectonic, geodynamic, and geochronology literature fairly complete, but with a few commissions mentioned below. This, along with the new analysis of broad scale surface nad subsurface rift morphology were the strongest points of the work. I thought section 7.3 - the discussion of Mechanisms for plume-rift association less strong. It comes across as rater speculative, which is due to the interpretation of plume driven rotation lacking presentation of a quantitative geodynamic model to back it up. See below for more details on this.
Detailed comments
Line 76 - should upper lithosphere be lower lithosphere? The plume impacts the lower lithosphere first.
Line 95 - earth should be upper case Earth
Line 108 - 114 and section 7.3 - i thought a weakness of the work is lack of discussion of quite a bit of geodynamic modelling work on a similar topic to the aim of this discussion paper.
This manuscript has avoided discussing the various models by Stamps for the region in which GNSS, topography and lithosphere and asthenosphere imaging data have been used to guide numerical simulations to isolate driving forces of extension in NE Africa. These works generally found the gravitational potential energy (GPE) from uplift is a major driver of extension, with base of lithosphere traction rather minimally involved.
For example see Stamps et al., 2014 - https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2013JB010717
From what i can see the work by Stamps is somewhat in contradiction to the interpretations being made in the new discussion manuscript. This is fine - but i think the text deserves dealing with this in a more convincing fashion.
Line 141 - 152 - there is a fair body of literature in Afar and rifted margins more generally that discusses the concept that magnetic stripes could form as a result of magma intrusion and volcanism before the continental lithosphere is fully split - ie magnetic striping on the continnet ocean transition, and subtly prior to full seafloor spreading. This is potentially important for interpretting the onset of seafloor spreading. See section 5.1 of Ebinger et al., 2017 - especially the last paragraph of this and references therein https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017TC004526
line 223 - replace "last" with "the early"
Line 247 - Please double check this is a catalog of Quaternary volcanoes , rather than just Holocene volcanoes . Also google earth should be Google Earth
section 6 - consider using the term "escarpment" rather than sharp cliff / cliff. Escarpment is the more globally used term for these topographically prominent rifted margins.
End of section 7.1 - i saw this same point mentioned in one of the other reviews. The concept that strain in Afar is commonly somewhat localised in distinct rift segments but which are set within quite a broad strain field influenced by all the plate motions is not new. e.g. Keir et al., 2010, Tectonics identified a SE-NW striking dike intrusion event in the MER that coupled with structural geology and focal mechanisms, amongst other things, was used to interpret that strain from the NE motion of Arabia occurs in the MER of southern Afar. Also see Doubre et al., 2017 GJI and Pagli et al., 2019 which provide good evidence from geodesy for broad extension in central Afar. See Maestrelli et al., 2022 Tectonics for analog models that use a number of model scenarios to reconstruct potential evolution and distribution of extension and faulting of Afar. These models invoke somewhat broad zones in which extension from the various plates interact, and within which strain has a higher gradient (more localised) in some zones.
Derek Keir - 2 February 2024
Citation: https://doi.org/10.5194/egusphere-2023-2467-RC3 -
AC4: 'Reply on RC3', Ran Issachar, 19 Feb 2024
Dear Dr. Keir,
As a leading scientist in the Afar region, we are very happy to get your review on our work and to hear that you have a generally positive opinion about it. We agree that section 7.3 is rather speculative and should be tested by geodynamic models in future works.
We are grateful for your comments and will follow them to improve the ms. Please see our response below.
I have read the manuscript, along with the comments and responses already made, so will limit my feedback to items not already discussed and addressed.
Overall I found the manuscript an interesting synthesis of the tectonic and magmatic evolution of Afar and discussion with respect to the interaction between plate forces and plumes in driving rifting. Broadly i thought the synthesis of tectonic, geodynamic, and geochronology literature fairly complete, but with a few commissions mentioned below. This, along with the new analysis of broad scale surface and subsurface rift morphology were the strongest points of the work. I thought section 7.3 - the discussion of Mechanisms for plume-rift association less strong. It comes across as rater speculative, which is due to the interpretation of plume driven rotation lacking presentation of a quantitative geodynamic model to back it up. See below for more details on this.
Detailed comments
Line 76 - should upper lithosphere be lower lithosphere? The plume impacts the lower lithosphere first.
- Yes, thanks
Line 95 - earth should be upper case Earth
- Thanks
Line 108 - 114 and section 7.3 - i thought a weakness of the work is lack of discussion of quite a bit of geodynamic modelling work on a similar topic to the aim of this discussion paper.
This manuscript has avoided discussing the various models by Stamps for the region in which GNSS, topography and lithosphere and asthenosphere imaging data have been used to guide numerical simulations to isolate driving forces of extension in NE Africa. These works generally found the gravitational potential energy (GPE) from uplift is a major driver of extension, with base of lithosphere traction rather minimally involved.
For example see Stamps et al., 2014 - https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2013JB010717
From what i can see the work by Stamps is somewhat in contradiction to the interpretations being made in the new discussion manuscript. This is fine - but i think the text deserves dealing with this in a more convincing fashion.
- Thanks for pointing out this significant work by Stamps el al., we will include a discussion with Stamps models in the revised ms. However, we don’t see a contradiction. They calculated the deriving forces for Nubia-Somalia divergence and found that gravitational potential energy is more significant then mantle flow by ~ an order of magnitude. Still, maybe this is not the case for the Nubia (and Somalia) – Arabia dynamics, which is not calculated in their study. If plume push forces are more significant in the context of Arabia breakup, than it could also explain the more rapid divergence of Arabia in comparison to Somalia. This is quite resendable when considering current mantle flow direction beneath that area, see Faccenna et al., 2013 (10.1016/J.EPSL.2013.05.043) and Qaysi et al., 2018 (10.1785/0220180144).
Line 141 - 152 - there is a fair body of literature in Afar and rifted margins more generally that discusses the concept that magnetic stripes could form as a result of magma intrusion and volcanism before the continental lithosphere is fully split - ie magnetic striping on the continnet ocean transition, and subtly prior to full seafloor spreading. This is potentially important for interpretting the onset of seafloor spreading. See section 5.1 of Ebinger et al., 2017 - especially the last paragraph of this and references therein https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017TC004526
- Thanks we will follow these and will change the description of the results by Bridges et al., 2012 according to comments by A. Schettino.
line 223 - replace "last" with "the early"
- “last” is correct.
Line 247 - Please double check this is a catalog of Quaternary volcanoes , rather than just Holocene volcanoes . Also google earth should be Google Earth
- Thanks, we used the Holocene catalog from Smithsonian.
section 6 - consider using the term "escarpment" rather than sharp cliff / cliff. Escarpment is the more globally used term for these topographically prominent rifted margins.
- Thanks, we will.
End of section 7.1 - i saw this same point mentioned in one of the other reviews. The concept that strain in Afar is commonly somewhat localised in distinct rift segments but which are set within quite a broad strain field influenced by all the plate motions is not new. e.g. Keir et al., 2010, Tectonics identified a SE-NW striking dike intrusion event in the MER that coupled with structural geology and focal mechanisms, amongst other things, was used to interpret that strain from the NE motion of Arabia occurs in the MER of southern Afar. Also see Doubre et al., 2017 GJI and Pagli et al., 2019 which provide good evidence from geodesy for broad extension in central Afar. See Maestrelli et al., 2022 Tectonics for analog models that use a number of model scenarios to reconstruct potential evolution and distribution of extension and faulting of Afar. These models invoke somewhat broad zones in which extension from the various plates interact, and within which strain has a higher gradient (more localised) in some zones.
- Thanks, we will carefully go over these references and bring them into the discussion.Sincerely,
Ran Issachar
Citation: https://doi.org/10.5194/egusphere-2023-2467-AC4
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AC4: 'Reply on RC3', Ran Issachar, 19 Feb 2024
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Ran Issachar
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