the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Nov 2023
Geomorphic expressions of active rifting reflect the role of structural inheritance: A new model for the evolution of the Shanxi Rift, North China
Abstract. Many rifts are influenced by pre-existing structures and heterogeneities during their evolution, a process known as structural inheritance. During rift evolution, these heterogeneities may aid rift nucleation, growth, and segmentation of faults, encourage linkage of various segments, or even inhibit the formation of faults. Understanding how structural inheritance influences early rift evolution could be vital for evaluating seismic risk in tectonically active areas. The Shanxi Rift in the North of China is an active rift system believed to have formed along the trend of the Proterozoic Trans North China Orogen, however, the influence of these pre-existing structures on the present-day rift architecture is poorly known. Here we use tectonic geomorphological techniques, e.g., hypsometric integral (HI), channel steepness (ksn) and local relief to identify the impact of structural inheritance on the formation of the Shanxi Rift. Of these measures, we found that HI was less sensitive to lithology and more valuable in evaluating the tectonic signal. Based on their geomorphic expression we characterise the activity levels of active faults and found that activity is concentrated in two rift interaction zones (RIZ) formed between the sub-basins. Furthermore, we found that many faults formed parallel to inherited structures. Based on these observations we propose a new model for the evolution of the Shanxi Rift where inherited structures play an important part in the initial segmentation of the rift which in turn controls the development of the RIZ structures. Geomorphic indices might prove useful in the study of the evolution of structural inheritance in other active rifts, such as the East African Rift.
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RC1: 'Comment on egusphere-2023-2563', Anonymous Referee #1, 13 Dec 2023
General comments:
The authors try to use tectonic geomorphological techniques (e.g., hypsometric integral (HI), channel steepness (ksn), and local relief) to identify the impact of structural inheritance on the formation of the Shanxi Rift. The manuscript obtains new geomorphological parameters covering a large portion of the Shanxi Rift System, which is useful for understanding the tectonic characteristics of the rift. The manuscript also contains new ideas and interesting analyses regarding the tectonic evolution of the rift. Moreover, the manuscript is well prepared. Its structure is clear and its language is easy to understand. Although I find one major issue and many minor ones (see below), I think this manuscript would be a good contribution to understanding the evolution of the Shanxi Rift System after revision.
Major issue:
The manuscript tries to use tectonic geomorphological techniques (e.g., HI, ksn, and local relief) to study the impact of structural inheritance on the formation of the Shanxi Rift, but the manuscript does not show the relationship between observed geomorphological parameters and preexisting structures. There is even no discussion about the relationship between observed geomorphological parameters and the preexisting structures. The role of structural inheritance is mainly based on analyzing the relationship between the current rift structures and published geologic maps. Such discussion is somewhat similar to published works in recent years, but the authors do not mention them. Collectively, I feel there is a disconnection between the main research technique and the research target.
Specific comments:Line 81: I don't think you need to mention seismic data. There are many seismic reflection profiles in the Shanxi Rift. Previous isopach maps (e.g., Xu and Ma, 1992; Xu et al, 1992) already take full advantage of the seismic data.
Lines 135-135: This is not correct. More and more studies found that the Eocene extension also occurred in western NCC, e.g., Wang et al (2013), Fan et al (2019), and Su et al (2021).
Line 149-151: I think the authors want to describe the thickness of the syn-rift sediments since the late Miocene rather than the sedimentary rocks. The mentioned thickness values are not correct. Please refer to Xu et al (1992) for the syn-rift sediment thickness. For the Linfen Basin, you can refer to Su et al (2023) for the syn-rift sediment thickness.
Lines 345-346: I think the precise description regarding the precipitation would be "...roughly little variation in precipitation...".
Lines 405-406: As the authors discussed regarding the limitations of HI in Lines 296-305, HI values may related to loess landscapes. The Lingshi RIZ is covered by widely distributed loess, which I think should also be considered when analyzing the observed high and varied HI values in the Lingshi RIZ.
Lines 397-98, 435: Please indicate the widths of the two swath profiles.
Lines 419-422: I am not sure how you found this area experienced a "recent uplift". Also, can you specify the timescale of the "recent" you mean? For example, ... experienced uplift in ~10, 100 ka, 1, or 10 Ma.
Lines 452-453: This sentence needs a reference to support this idea.
basinLines 576-579 (Figure 11): The timing of the three evolution stages need references.
Lines 589-591: The last sentence of this paragraph should be removed. Do not conduct a discussion in the Conclusion section. Also, the analysis regarding future major earthquakes is wrong. Any active faults including these basin border faults may host large earthquakes in the future.
Technical corrections:
I find many technical issues. Please refer to the attached pdf file. The authors need to check the whole manuscript carefully and avoid such issues.
Related references mentioned:
Fan, L.-G., Meng, Q.-R., Wu, G.-L., Wei, H.-H., Du, Z.-M., & Wang, E. (2019). Paleogene crustal extension in the eastern segment of the NE Tibetan plateau. Earth and Planetary Science Letters, 514, 62–74. https://doi.org/10.1016/j.epsl.2019.02.036Su, P., He, H., Tan, X., Liu, Y., Shi, F., & Kirby, E. (2021). Initiation and evolution of the Shanxi Rift System in North China: Evidence from low-temperature thermochronology in a plate reconstruction framework. Tectonics, 40(3), e2020TC006298. https://doi.org/10.1029/2020TC006298
Su, P., He, H., Liu, Y., Shi, F., Granger, D. E., Kirby, E., et al. (2023). Quantifying the structure and extension rate of the Linfen Basin, Shanxi Rift System since the latest Miocene: Implications for continental magma-poor rifting. Tectonics, 42, e2023TC007885. https:// doi.org/10.1029/2023TC007885
Wang, W., Kirby, E., Zhang, P., Zheng, D., Zhang, G., Zhang, H., et al. (2013). Tertiary basin evolution along the northeastern margin of the Tibetan Plateau: Evidence for basin formation during Oligocene transtension. Geological Society of America Bulletin, 125(3–4), 377–400. https://doi.org/10.1130/B30611.1
Xu, X., & Ma, X. (1992). Geodynamics of the Shanxi rift system, China. Tectonophysics, 208(1–3), 325–340. https://doi.org/10.1016/0040-1951(92)90353-8
Xu, X., Ma, X., & Deng, Q. (1993). Neotectonic activity along the Shanxi Rift System, China. Tectonophysics, 219, 305–325. https://doi.org/10.1016/0040-1951(93)90180-R
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RC2: 'Comment on egusphere-2023-2563', Anindita Samsu, 05 Jan 2024
This contribution presents the use of geomorphic indices to assess fault activity in the Shanxi Rift (China). Fault orientations and activity are compared against pre-existing basement fabrics. The authors show that the overall geometry of the segmented Shanxi Rift, the geometries of the rift interaction zones (RIZs) between the main basins, and fault activity are influenced by pre-existing structures in the (upper) crust and anisotropies in the upper mantle; both of these types of pre-existing structures have been attributed to the amalgamation of the North China Craton and the formation and subsequent reworking of the Trans Northern China Orogen.
The manuscript fits the scope of the journal and special issue very well. The use of geomorphology to investigate the multi-faceted influence of structural inheritance is uncommon and therefore makes valuable addition to structural inheritance research. In general, the manuscript is well-structured. It would benefit from some thorough editing to enhance clarity and emphasize key messages. Below are some general and specific comments that I hope will contribute to the overall improvement of the manuscript. Congratulations to the authors for putting this interesting paper together.
General comments
This manuscript is missing a concise presentation of all the faults and their characteristics, including the lithology of the footwall, the main orientation of pre-existing basement fabrics, perhaps the strength of the geomorphic signal, and most importantly the inferred level/recentness of activity – this could be done as a summary table. The above information is all there already, but it is spread across several maps with different color schemes, which makes it difficult to follow the descriptions in the text and connect the different observations.
A general comment that could apply to multiple parts of the manuscript: Sometimes I get lost in the details and explanations before understanding what the main statement of the paragraph/section is. It could help to flip the paragraph around, bringing the headline statement to the start of the paragraph, and then elaborating on the headline statement. One example of where this occurs is the first paragraph of the Conclusions.
Specific comments (also highlighted blue in the attached PDF)
Lines 17-20: It would be more accurate to state that the tectonic geomorphological techniques were used evaluate fault activity. The interpretations of the role of inheritance comes later, but inheritance is not directly linked to geomorphological indices (at least not in this paper).
Line 51: What is meant by “oblique striking”? Oblique with respect to what?
Lines 357-363: What does HI tell us about tectonic activity? A short explanation of what HI means with respect to tectonics would be useful here.
Line 61: Striking obliquely with respect to what?
Line 75: What did these previous studies reveal? Are their outcomes helpful in understanding the Shanxi Rift?
Lines 92-98: This is a great summary of the presented work and a strong conclusion to the introduction! It would be good to apply this style and clarity in the other sections.
Line 94: Is it necessary to distinguish between basement heterogeneities and inherited structures? Please see the general comment about the need to clearly define basement structures, pre-existing structures, inherited structures, etc.
Line 171: How is it determined that faults are active? What is your definition of an “active” fault?
Lines 179-181: Is there literature supporting the statement that the three chosen geomorphic indices are the most robust for evaluating tectonic signals?
Lines 218-220: Does a high ksn also indicate a high uplift rate? If so, please specify this as you have done for the other two geomorphic indices.
Lines 229-330: You refer to Fig. 3 here, but the faults are not labelled in Fig. 3.
Lines 234-235: This statement is vague. How is the "general" NE-SW trend distinguished from the "local" ENE-WSW orientation? Where can we see this on the map (e.g., in Fig. 3)?
Lines 279-281: Do you mean that the JC and TG faults have lower ksn and Ri values compared to other faults in the region that are considered active? I think the sentence needs rewriting for clarity.
Section 4.5: Can this section be combined with the previous three sections? Otherwise, we are jumping back and forth between geomorphic indices.
Lines 307-308: I suggest moving this to the methods section.
Line 347: Fig. 3 shows the ages of the units but not the lithologies.
Lines 348-351: Why not make it clear that the two groups you are referring to are crystalline rocks and low-grade metasediments? It would make the comparisons in the subsequent paragraphs easier to follow. Also, were the carbonates also subject to low-grade metamorphism? If so, wouldn’t they technically also be considered metasediments?
Lines 361-364: How do we reconcile this with the limitations of HI for tectonic interpretations (Section 4.4)?
Lines 366-367: What does “similar basement geology” mean? Would erodibility be influenced by factors other than lithology, e.g., the orientations and types of structures in these basement rocks?
Lines 368-373: These sentences seem to repeat what was already described in the previous paragraph.
Lines 375-377: It is difficult to follow which faults are being compared here.
Section 5.2: You could consider splitting up this section into two, with the second section focusing more on fault activity and implications for seismic hazard. Or you could include seismic hazard in the current heading.
Lines 386-388: Edit sentence for clarity. I recommend splitting it into two sentences. Also, in Line 388, replace “metrics” with “observations”.
Lines 388-390: Do these two observations or categories only apply to breached RIZs? If so, please make this clearer. In Line 390, what does “They” refer to? Only breached RIZs or all RIZs? Perhaps it is worth summarizing how the temporal evolution of RIZs are assessed, and what observations are associated with unbreached, partially breached, recently breached, and breached RIZs (according to Kolawole et al., 2022).
Lines 402-406: Here small-scale faults are described, but they are not shown on the maps in Fig. 7. Hence, it is difficult to observe the “complexity and distribution of faulting” in the Lingshi RIZ.
Lines 410-411: Rewrite sentence for clarity.
Lines 413-414: Why is the Lingshi RIZ classified as “recently” breached? Because breaching is thought to have occurred in the Late Pleistocene? See previous suggestion about clarifying how RIZs are classified.
Line 416: Is the Shilingguan Fault the breaching fault? If so, please make this clear. Also, the Jiaocheng Fault needs to be labelled on the map (Fig. 8a).
Lines 421-422: “Recent uplift” was already mentioned earlier in the same paragraph. Combine the two for conciseness.
Lines 422-424: Refer to Fig. 2 here?
Lines 441-442: What is the general extension direction? Or is this a general statement about oblique faults (not limited to the Shanxi Rift)?
Lines 454-455: Could you please rewrite this sentence for clarity?
Line 461-465: Specify the length of the Huoshan Piedmont Fault here for comparison to the shorter faults in the Shilingguan RIZ. Also, this is a long sentence that should be split into at least 2 sentences.
Lines 466-468: This sentence feels a bit out of place and could be connected to a previous statement about shorter RIZ faults.
Line 473: Samsu et al. (2023) is now published.
Lines 478-480: Rewrite this sentence for clarity.
Lines 486-487: This headline statement could be moved further up to make the discussion easier to follow, e.g., to Line 477.
Line 496: Upon observing Fig. 9, Hengshan Fault does not appear to follow the basement trend (at least not along its entire length), contrary to what is written in the text.
Lines 505-507: This is another headline statement that could be moved to the start of the paragraph.
Lines 513-515: Zig-zag faults have been described in other studies (e.g., Lezzar et al., 2002; Corti, 2009; Hodge et al., 2018).
Lines 517-520: Here it would be good to mention the orientation of the regional strain (and cite the work that supports this, e.g., Middleton et al., 2017?). Also, have you considered whether (a) the geometry and relative timing of faults is influenced by the mechanical contrasts or weaknesses in the basement rocks, (b) rift-related faults that are still active and perturbing the stress or strain field as the RIZ faults form and grow, (c) we have a combination of a and b? Is there existing evidence of the main basin-bounding rift faults forming before the RIZ faults?
Line 548: Molnar et al. (2020) presents analogue models, not numerical models.
Lines 554-556: I appreciate that this is beyond the scope of this contribution, but I think it is useful to discuss in a bit more detail why the pre-existing lithospheric and crustal scale structures are not parallel, especially as you mention that this “could have either occurred during transpressional accretion of the Trans-North China Orogen or during later reworking by the polyorogenic event that formed the Trans-North China Orogen” (Lines 550-551) but do not explain this in more detail.
Lines 558-559: Agostini et al. (2009) demonstrated this en-echelon arrangement of faults above a weak zone.
Line 576: The last sentence is not a complete sentence.
Lines 584: Why is it a problem that lithology has a strong influence on the geomorphic signal? I think you could be more explicit that one of the aims is to see which geomorphic index is most useful for understanding the interaction between tectonics and landscapes – then this sentence would have a clearer meaning. I think you could also explicate this aim in the Discussion (Section 5.1).
Figure 588-589: I don’t recall where it was explained that linkage of sub-basins progressed towards the north.
Lines 589-591: This sentence should directly follow the first sentence of the paragraph.
Figure 3: Label faults on this map. It would be good it the colors of the geological units here more closely matched the ones on Fig. 6.
Figure 6: The colors of the different faults are a bit distracting, as they appear arbitrary here and do not correspond with colors on any of the maps. On the other hand, the background colors that indicate Mesozoic-Paleozoic sedimentary rocks vs. Proterozoic basement is quite useful.
Figure 7: Label Taiyuan and Linfen basins and Fen River on map (e.g., Fig. 7a).
Figure 8: Label basins in Fig. 8a and 8d. What are the blue arrows in Fig. 8a showing?
Figure 9: Why does this figure appear so late in the text? I think this figure could really aid earlier explanations, including Section 5.1. Also, what do “high”, “medium”, and “low” geomorphic signals mean? Can these signals be quantified? Definitions of the abbreviations of fault names are missing in the caption.
Figure S3 (incorrectly labelled as S2): Why is this map in the supplementary information? It nicely shows the spatial relationship between earthquakes and faults (especially in the RIZs), which would be a useful visual aid for setting up the research problem (Introduction) and the discussion in Section 5.2. Is there a way to elegantly combine this map with Fig. 2?
Technical comments
Some letters as capitalized or written in lower case when they shouldn’t be. Commas have been used rather sparingly, making many sentences unwieldy.
The manuscript contains many other technical or grammatical errors – please refer to the attached PDF.
The supplementary material also contains several errors – please check for these.
References
Agostini, A., Corti, G., Zeoli, A., Mulugeta, G., 2009. Evolution, pattern, and partitioning of deformation during oblique continental rifting: Inferences from lithospheric-scale centrifuge models. Geochemistry, Geophysics, Geosystems 10. https://doi.org/10.1029/2009GC002676
Corti, G., 2009. Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth-Science Reviews 96, 1–53. https://doi.org/10.1016/j.earscirev.2009.06.005
Hodge, M., Fagereng, Å., Biggs, J., Mdala, H., 2018. Controls on Early-Rift Geometry: New Perspectives From the Bilila-Mtakataka Fault, Malawi. Geophysical Research Letters 45, 3896–3905. https://doi.org/10.1029/2018GL077343
Lezzar, K.E., Tiercelin, J.-J., Le Turdu, C., Cohen, A.S., Reynolds, D.J., Le Gall, B., Scholz, C.A., 2002. Control of normal fault interaction on the distribution of major Neogene sedimentary depocenters, Lake Tanganyika, East African rift. AAPG Bulletin 86, 1027–1059.
Molnar, N., Cruden, A., Betts, P., 2020. The role of inherited crustal and lithospheric architecture during the evolution of the Red Sea: Insights from three dimensional analogue experiments. Earth and Planetary Science Letters 544, 116377. https://doi.org/10.1016/j.epsl.2020.116377
Samsu, A., Micklethwaite, S., Williams, J.N., Fagereng, Å., Cruden, A.R., 2023. Structural inheritance in amagmatic rift basins: Manifestations and mechanisms for how pre-existing structures influence rift-related faults. Earth-Science Reviews 246, 104568. https://doi.org/10.1016/j.earscirev.2023.104568
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RC3: 'Comment on egusphere-2023-2563', Folarin Kolawole, 13 Feb 2024
The study by Froemchen et al. investigated the active tectonics of the Shanxi Rift System using geomorphic indices to ‘to identify the impact of structural inheritance on the formation of the Shanxi Rift System’. The geomorphic indices highlight the focusing of active deformation in the rift interaction zones (RIZs) separating the active rift basins: Linfen and Taiyuan Basins are separated by the Lingshi RIZ, and the Taiyuan and Xinding Basin are separated by the Shilingguan RIZ. The Lingshi RIZ shows a through-going fault system, a through-going axial stream with unidirectional southward flow, and a southward down-stepping longitudinal surface relief shape that suggests a recently breached RIZ. The Shilinggian RIZ retains a high relief, lacks a through-going stream, but hosts evolving active faults that suggest a partially Breached RIZ morphology. The authors went further to assess the basement fabric trends across the region, and their relationship between faults bounding the rift basins and those extending into the RIZS, and found that the NE-trending basin-bounding faults follow the basement fabrics indicating structural inheritance control, whereas the RIZ faults form zigzag geometries suggesting strain re-orientation in the RIZs. The authors conclude that ‘geomorphic indices might prove useful in the study of the evolution of structural inheritance in other active rifts’.
I think the topic of the paper is of broad interest to the continental rifting community and a good fit for EGU-Solid Earth’s audience. However, while I enjoyed reading the manuscript, I have two major criticisms of the paper which I explain below and provide suggestions on how the issues can be fixed.
1) Drainage and topographic geomorphic indices cannot directly show the impact of structural inheritance on active rift tectonics. In other words, one cannot look at a map of hypsometric integral, local, relief, channel steepness index etc. and interpret that areas with high values are controlled by structural inheritance. Where there is a relationship, it is always indirect. The reason is simply because geomorphic indices of drainage systems are primarily sensitive to surface processes and controlling factors like vertical motions and variability in erodibility of rocks. Whereas structural inheritance is a mechanical process that is controlled by the interaction between the geometry and strength of inherited mechanical anisotropy and the tectonic stress field. Thus, while I think the tectonic geomorphology analysis and structural inheritance investigations in this study are great, the interpretation of a causative relationship between both is problematic. This implied causative relationship weakens the impact of the paper and should be avoided. The main point is that geomorphic indices can highlight the zones of intense active tectonic deformation which in the case of this study are the deforming RIZs within which active breach faulting is influenced by the inherited basement fabrics. However, I think it can be easily fixed by rewording key sections of the texts that alludes to an interpretation of direct causative relationship which includes the manuscript title, the abstract (lines 18-19, 24-25), introduction (lines 92-93), and many sections of the discussion (see my in-text suggested edits).
2) On interpreting mantle anisotropy to represent inherited mantle fabric: Due to a lack of any age constraint on the timing of development of the anisotropy beneath this region, I do not think the interpretation of mantle shear wave anisotropy to be an inherited mantle fabric is appropriate. The mantle underlying active Continental rifts commonly develop anisotropic fabric due to mantle flow induced by the rifting, and this anisotropy is not always parallel to the rift axis (see East African Rift studies e.g., Tepp et al. 2018; doi.org/10.1029/2017JB015409). I don’t think there is a need for this interpretation. The study has presented a strong case for the control of crustal inheritance. I think this is sufficient unless there is strong data on the age of the mantle anisotropy.
In addition to these two criticisms, I have a lot of minor recommended edits which are mostly typographical errors in the text and missing references. I have attached my in-text comments to this review.
I would note two of the recommended edits:
1) Include some discussion text on the relatively different stages of development of the Lingshi and Shilingguan RIZs, which I think can be explained by their contrasting RIZ geometries. This is well supported by our recently accepted paper Kolawole et al. (2024, AGU Books, in print) where we find that RIZ geometries may influence the pace of breaching of active RIZs as they influence tectonic stress distribution across the interacting RIZ bounding faults. See the ESSOAr preprint: https://www.researchgate.net/publication/370066205_Rapid_Versus_Delayed_Linkage_and_Coalescence_of_Propagating_Rift_Tips
Reference:
Kolawole, F., Xue, L., Dulanya, Z. Rapid Versus Delayed Linkage and Coalescence of Propagating Rift Tips. Accepted, in press at AGU Books: Extensional Tectonics: Continental Breakup to Formation of Oceanic Basins. Preprint: 10.22541/essoar.168167202.29986035/v2.2) Adding a panel to Figure 7 showing the longitudinal elevation profile of the Fen He River itself from the center of the Taiyuan Basin to the center of Linfen Basin. I did a quick plot of the profile, and it shows the very nice 'down-stepping' shape that is typical of 'recently breached RIZs'. I think a plot like this would make this figure stronger.
Data sets
Geomorphic expressions of active rifting reflect the role of structural inheritance: A new model for the evolution of the Shanxi Rift, North China Malte Froemchen, Ken McCaffrey, Mark Allen, Jeroen van Hunen, Tom Phillips, Xu Yueren https://doi.org/10.5281/zenodo.10058449
Model code and software
R_Hypsometry Malte Froemchen, Ken McCaffrey, Mark Allen, Jeroen van Hunen, Tom Phillips, Xu Yueren https://github.com/MFroemchen/R_Hypsometry
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