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the Creative Commons Attribution 4.0 License.
| 10 Jun 2024
Alternating Extensional and Contractional Tectonics in the West Kunlun Mountains during Jurassic: Responses to the Neo-Tethyan Geodynamics along the Eurasian Margin
Abstract. The Tethyan Orogenic Belt records a long-lived geological cycle involving subduction and collision along the southern margin of the Eurasian continent. The West Kunlun Mountains, located at the junction between the Tibetan and Western Asian Tethyan realm, records multiple orogenic events from the Paleozoic to the Cenozoic that shape the northwestern Tibetan Plateau. However, deciphering the complex Mesozoic contractional and extensional tectonics to interpret the broader Tethyan geodynamics remains challenging. To address the tectonic transition following the early Cimmerian (Late Triassic) collision, this study investigates the newly identified Jurassic sedimentary strata and volcanic rocks in the West Kunlun Mountains. Zircon geochronological results of basalts and sandstones reveal that this ~ 2.5-km-thick package was deposited at ca. 178 Ma, rather than the previously reported Neoproterozoic age. The alkaline basalts at the top of the formation exhibit chemical compositions similar to oceanic island basalts, consistent with the intracontinental extension environment revealed by the upward-fining sedimentary pattern. Provenance analysis, integrating conglomerate clast lithologies with detrital zircons, suggests a substantial contribution from adjacent basement sources, likely influenced by the normal faulting during initial rift stage. These findings indicate that the West Kunlun Mountains rapidly transitioned into an extensional setting after suturing with Cimmerian terranes. The regional structure, stratigraphy and magmatism suggest that this Early - Middle Jurassic basin was subsequently inverted during the Late Jurassic and earliest Cretaceous. We propose that the Mesozoic deformational history in the West Kunlun Mountains was related to the northward subduction of the Neo-Tethys Ocean, as it transitioned from southward retreat to northward flat-slab advancement. Comparing with the entire strike-length of the Eurasian Tethyan orogen, we find that the subduction mode varied from the west to the east, reflecting the broad geodynamic changes to, or initial conditions of, the Neo-Tethyan system.
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CC1: 'Comment on egusphere-2024-1670', Johannes Rembe, 11 Jun 2024
Dear Authors,
with great interest, I read your preprint.
I would like to draw your attention to the evidence for extensional or transtensional tectonics in the North Pamir during the Triassic.
This was published in Lithosphere (Rembe et al., 2022). During the Late Triassic, bi-modal back-arc volcanism occurred along a band running from the NE Pamir into the Afghan Hindukush (see also Siehl et al., 2017). There are hints for a latest Triassic (mid-Rhaetian) contractional phase and partial closure of the back-arc basin in the NE Pamir. It would be interesting to discuss whether this extensional back-arc tectonics at least partly transitioned directly into the Jurassic extensional tectonics. I would also recommend updating your figure 9: Profile (5) Qiemugan, should comprise Triassic strata, for instance a more than 800m thick bimodal volcanic sequence cropping out east of Ka Latashi (喀拉塔什) Ulugqat, Kizilsu, Xinjiang. A 200 m thick crystal tuff gave an Anisian zircon U-Pb age of 244.1±1.1 Ma (Rembe et al., 2022).Best Regards,
Johannes Rembe
Rembe, et al. "Geochronology, Geochemistry, and Geodynamic Implications of Permo-Triassic Back-Arc Basin Successions in the North Pamir, Central Asia." Lithosphere 2022.1 (2022).
Siehl, Agemar. "Structural setting and evolution of the Afghan orogenic segment–a review." Geological Society, London, Special Publications 427.1 (2017): 57-88.
Citation: https://doi.org/10.5194/egusphere-2024-1670-CC1 -
AC1: 'Reply on CC1', Hongxiang Wu, 15 Jun 2024
Dear Johannes,
Thanks for your attention and comment.
As proposed, the NE Pamir evolved from a Permian-Triassic back-arc setting to a collisional orogeny during the latest Triassic to the earliest Jurassic. Recent identification of the Triassic back-arc belt extending from northern Afghanistan to the NE West Kunlun Mountains in China (Rembe et al., 2022) suggests a more complex pre-Cenozoic evolution of the Paleo-Tethys in the Pamir segment than previously understood. However, the precise timing of the closure of this back-arc system remains unclear.
No direct geochronological constraints have been reported for this significant contractional event in the back-arc system. Regional low-temperature thermochronological records indicate uplift and cooling events in the Triassic and Late Jurassic (Yang et al., 2017). The closure of the Paleo-Tethys and the subsequent collisional orogeny likely happened in the latest Triassic, as evidenced by Triassic thrusts and stratal deformation observed in the western Tarim Basin (Wu et al., 2021). Notably, the Permian- Triassic volcanic rock only outcropped in the north segment (Qiemugan-Gaizi River) of the West Kunlun, whereas the south segment (Kyzyltau-Aertashi) was dominated by the carbonate sequence of platform sedimentation. The Triassic thrust belt and paleo-uplifts developed only in the south segment (Kyzyltau-Aertashi; Wu et al., 2021). Additionally, Permian-Triassic detrital zircons are absent from the Early-Middle Jurassic Kyzyltau basin in the West Kunlun Mountains (Fig. 6), implying sediment communication occurred only from the Late Jurassic to Early Cretaceous (Fig. 10). These lines of information support the hypothesis that partial closure of the Paleo-Tethys and back-arc seaway in the Pamir occurred during the latest Triassic.
We propose that the evolution of the Paleo-Tethys Ocean and the back-arc basin behaved differently between the NW Pamir segment (Qiemugan-Gaizi River) and the E North Pamir segment (Kyzyltau-Keliyang) (Fig. 1c). It is likely that the Triassic back-arc basin did not extend into the E North Pamir segment, and the closure of the Paleo-Tethys varied between its northern and southern segments, influenced by the Late Paleozoic embayment in the Pamir region (Li et al., 2020).
Overall, the Mesozoic and Cenozoic evolution of the North Pamir region is more complex than current tectonic models suggest and requires further study.
Additionally, we are pleased to revised the stratigraphic column in Fig. 9 to present the thick Triassic volcanic sequence in the NW Pamir.
Thanks for your valuable suggestion.
Best Regards,
Hongxiang Wu
Rembe, J., Sobel, E. R., Kley, J., Terbishalieva, B., Musiol, A., Chen, J., and Zhou, R.: Geochronology, Geochemistry, and Geodynamic Implications of Permo-Triassic Back-Arc Basin Successions in the North Pamir, Central Asia, Lithosphere, 2022, 10.2113/2022/7514691, 2022.
Yang, Y.-T., Guo, Z.-X., and Luo, Y.-J.: Middle-Late Jurassic tectonostratigraphic evolution of Central Asia, implications for the collision of the Karakoram-Lhasa Block with Asia, Earth-Science Reviews, 166, 83-110, https://doi.org/10.1016/j.earscirev.2017.01.005, 2017.
Wu, H., Cheng, X., Chen, H., Chen, C., Dilek, Y., Shi, J., Zeng, C., Li, C., Zhang, W., Zhang, Y., Lin, X., and Zhang, F.: Tectonic Switch From Triassic Contraction to Jurassic-Cretaceous Extension in the Western Tarim Basin, Northwest China: New Insights Into the Evolution of the Paleo-Tethyan Orogenic Belt, Frontiers in Earth Science, 9, 10.3389/feart.2021.636383, 2021.
Li, Y.-P., Robinson, A. C., Gadoev, M., and Oimuhammadzoda, I.: Was the Pamir salient built along a Late Paleozoic embayment on the southern Asian margin?, Earth and Planetary Science Letters, 550, 116554, https://doi.org/10.1016/j.epsl.2020.116554, 2020.
Citation: https://doi.org/10.5194/egusphere-2024-1670-AC1
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AC1: 'Reply on CC1', Hongxiang Wu, 15 Jun 2024
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RC1: 'Comment on egusphere-2024-1670', Anonymous Referee #1, 13 Aug 2024
This work reports field-based, geochronological and geochemical data for the Jurassic volcaniclastic sequences in West Kunlun. New results provide constraints on the Mesozoic tectonic history of the central junction of the Tethyan Orogenic Belt, of importance to understand the Tethyan geodynamics. The data is of good quality and the figures are well presented. Minor revision is suggested before publication, especially regarding age and tectonic setting explanations. English also needs polishing. My major and minor comments are presented in the following.
Major comments:
- “Alternating” in the title may not be quite appropriate, as it means multiple changes. But the main finding of this work is a transition?
- The section 2 concerning geological framework is supposed not to describe too many debates (Lines 190-207).
- Is there any corresponding relationship among morphology, internal structure under CL, and age of zircons from the basalt sample? These zircons were sub-categorized into two groups based on the presence of oscillatory zoning, but whether the two group zircons have distinctive ages and Th/U ratios have not been clearly mentioned. Besides, the zircons from basalt are not supposed to have oscillatory zoning, which is a typical feature of zircons from felsic magma. Please reconsider the above-mentioned issues and add more discussion.
- Why is there a significant gap between the youngest zircon age and the depositional age of the sedimentary rocks? Except for the age of basalt, is there any other evidence to indicate their Jurassic depositional age?
- The length of the text is suggested to be largely reduced. For example, there are some overlaps between 6.1 concerning the tectonic setting of the Jurassic volcanism and 6.2 about the setting under which the Jurassic basin formed.
- 3 is discussed largely based on previous work. This contribution is suggested to be highlighted in this section. Besides, better add some summary sentences to conclude the main findings.
Minor comments:
- Better add the columns or sections of the Jurassic strata in the studied area in Figure 2.
- Better add some photos to show the different clast compositions of Jurassic conglomerate.
- Figure 4: the various colored circles are suggested to be annotated to represent different age explanations.
- Figure 5b: what are dots 19, 25, 28, and 30?
- The ~195 Ma basalts in the bimodal volcanicsuite in Karakoram are plotted in Figures 7-8, but comparison has not been made with the studied samples. What is their significance? Better add some discussion.
- Line 517: slightly negative
- Lines 587-592: lowercase following (1), (2), (3), and (4).
- Condense 6.2 and add some summary sentences for this section.
Citation: https://doi.org/10.5194/egusphere-2024-1670-RC1 -
AC2: 'Reply on RC1', Hongxiang Wu, 30 Sep 2024
Thank you very much for your time and valuable suggestions. We will address your queries individually below. Additionally, our co-authors, two American academics, Yildrim and Andrew, have contributed to revising and refining the language.
About major comments:
1. We agree with your suggestion and have revised our title to: ' Switching Extensional and Contractional Tectonics in the West Kunlun Mountains During Jurassic.’
2. We have removed the discussion regarding the interpretation of Jurassic molasse deposits; please refer to line 191. We suggest retaining the first sentence in this paragraph, as the first-order geodynamic mechanisms in the mid-Mesozoic are the most important focus of this paper.
3. Based on a detailed classification and statistical analysis of zircon characteristics in Table S2, we identified a strong correlation among the morphology, internal structure, and age of zircons from the basalt. Type 1 zircons, typically euhedral to sub-euhedral with clear oscillatory zoning, have older ages ranging from 405 Ma to 911 Ma. In contrast, type 2 zircons, which exhibit subrounded external shapes, show uniform Jurassic ages between 168 Ma and 193 Ma. Please refer to discussion in line 335-340.
We have assigned the younger ages (type 2) to represent the crystallization age of this basaltic rock sample, while the older ages (type 1) are interpreted as inherited from the country rocks. Notably, the older ages in the basalt sample are consistent with the detrital zircon ages found in the study region.
4. The provenance of the Jurassic sedimentary rocks is locally sourced from the North West Kunlun Mountains, which are primarily composed of Early Paleozoic sedimentary rocks (lines 616-635). Younger detrital zircons, dating of the Carboniferous and Triassic ages, in the South West Kunlun Mountains, were transported into the Tarim Basin beginning in the Late Jurassic and Early Cretaceous (Fig. 10). As a result, a significant gap between the youngest zircon age and the actual depositional age can be observed.
Additionally, the Early Jurassic basalts are scattered throughout this region, making them insufficient to serve as a major source. Zircon is also difficult to crystallize in these mafic magmas due to their silicon unsaturation. To better understand the Jurassic depositional age of this sedimentary package, we present several lines of evidence as follows: (1) the age spectrum of the sandstone from Kandilik is remarkably similar to that of the Early Jurassic strata (Fig. 6); (2) the structural compatibility of this new stratigraphic scheme was we demonstrated (line 487-499); (3) The clastic member in Kandilik is primarily composed of gray-black carbonaceous slate and siltstone, similar to the Jurassic coal-bearing sequences.
5. In this revised version, we have significantly shortened the discussion about the tectonic setting in the section 6.1 and 6.2. Please refer to line 548-574.
6. We briefly discussed the comparable geological history between the Pamir-West Kunlun and the Tibetan Plateau and added a summary of this in the conclusion section.
About minor comments:
1. We have added a field geological section to show the regional strata and deformation in Figure 2c.
2. We have added several field photos of conglomerate clast lithologies from Oytag, Gaizi and Tamu regions in Figure 2.
3. We have explained the meaning of the different colored circles in the caption of Figure 4.
4. The subscale numbers in Figure 5b represent the zircon test sites shown in Figure 5a that were not used to calculate the weighted mean age. We have deleted these subscale numbers in this revised version.
5. We have highlighted the chemical differences among magmatic rocks in the discussion section 6.3 (lines 685-690). In particular, we emphasize the back-arc MORB affinity of the basalts in Tanshuihai and the OIB-affinity (within-plate) of the basalts in West Kunlun, which is comparable to the tectonic setting of the active margin of the western Pacific (line 713-718).
6. We have corrected this mistake.
7. We have corrected this mistake.
8. We have shortened both the section 6.1 and 6.2 in this revised manuscript, and have added a summary of basin evolution for the section 6.2 (line 640-645).
Citation: https://doi.org/10.5194/egusphere-2024-1670-AC2
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RC2: 'Comment on egusphere-2024-1670', Anonymous Referee #2, 01 Sep 2024
The authors investigate the Jurassic basins in the West Kunlun Mountains. Based on the presence of oceanic island basalts and an upward-fining sedimentary pattern, they argue for an extensional setting in the Early to Middle Jurassic in this region, before basin inversion occurred in the Late Jurassic. They propose that the basin evolution was related to the northward subduction of the Neo-Tethys Ocean. Overall, I find this study data-rich and well-written, and it should be published after minor revisions. Below, please find several comments that I hope may be useful for the authors to improve the manuscript.
Lines 141-142: The South Qiangtang and North Qiangtang terranes are larger than most other terranes you mentioned. They should not be omitted here. Additionally, some authors believe North Qiangtang had a Cathaysian affinity.
Figure 3h: No layers can be identified. Also, in which strata are these basalts found?
Figure 5: I have concerns about the zircons. From the CL image, they appear to originate from felsic rocks. Additionally, there are multiple age clusters, which seems unusual to me. Also, how do you explain the Th/U values of zircons being lower than 0.1?
The lack of Jurassic and the presence of very few Triassic detrital zircons in the Jurassic strata, with a strong 440 Ma peak, suggests there was no widespread arc magmatism in the region during the Late Silurian to Triassic. Therefore, detrital zircon ages may not be that useful for determining depositional ages. Do you have any other evidence for the depositional ages?
The Pamirs can be subdivided further and then correlated with Tibet. Although there is considerable debate on the correlation due to the presence of the Karakorum fault, the authors should not overlook this in their discussion. They should consider the role of the Meso-Tethys Ocean in Tibet and the Pamirs in more detail. From what I know of the Tibetan part, the Jurassic was an important period during which the Bangong-Nujiang Meso-Tethys Ocean was subducting and experiencing microcontinent assemblage (Ma et al., 2023, Tectonophysics 862, 229957). The authors mentioned the regional unconformity beneath the Late Jurassic conglomerate, which is important, as nearly synchronous unconformities also exist in the South Qiangtang and Bangong-Nujiang suture zones (Ma et al., 2017, Journal of Geophysical Research: Solid Earth 122(7), 4790-4813; 2018, Palaeogeography, Palaeoclimatology, Palaeoecology 506, 30-47). It would be interesting if the authors could provide more details about the basin inversion and further discuss the relationship with Meso-Tethys geodynamics.
Figure 10: I suggest adding the log of the West Kunlun here for comparison.
Citation: https://doi.org/10.5194/egusphere-2024-1670-RC2 -
AC3: 'Reply on RC2', Hongxiang Wu, 30 Sep 2024
Thank you very much for your consideration and suggestion. We will address your questions and make the following revision.
1. We have mentioned the South Qiangtang and North Qiangtang terranes in this paragraph.
2. Due to the exposure conditions and harsh terrain, we were unfortunately unable to capture clear photographs showing the contact between the basalt and surrounding strata. These basalts were found in the upper layer of volcanic breccia, and detailed profiles were measured by geologists as early as the 1990s (Ma et al., 1991). To provide readers with a clearer understanding of the strata formation and rocks we collected, we have added a field outcrop profile of the area in Fig. 2c.
3. In this revised version, we have conducted a detail classification and statistical analysis of zircon characteristics in Table S2. Based on the morphology, internal structure (CL), and age of zircons from the basalt, we have divided the 36 zircons we tested into two groups. The type 1 zircons, typically euhedral to sub-euhedral with clear oscillatory zoning, have older ages ranging from 405 Ma to 911 Ma. In contrast, the type 2 zircons, exhibiting subrounded external shapes with no clear oscillatory zoning, show uniform Jurassic ages between 168 Ma and 193 Ma.
The older ages in the basalt sample are consistent with the detrital zircon ages found in the country rocks in the study region (Fig. 6). Accordingly, we have assigned the older ages (type 1) as inherited from the country rocks, while he younger ages (type 2) to represent the crystallization age of this basaltic rock sample.
The low Th/U ratios, occurred only within the zircon group of type 2, which has a “polished” shape with nebulous or patchy-zoned centers. This may result from moderate resorption either during the evolution of the magma chamber when the magma is oversaturated with respect to zircon or a certain degree of metamorphism (Corfu et al., 2003). Despite this, most zircons still display high Th/U ratios, indicating a clear magmatic origin. Therefore, we believe the Jurassic age represents the crystallization age of the volcanic rocks. Please refer to discussion in line 335-340.
4. To better understand the Jurassic depositional age of this sedimentary package, we present several lines of evidence as follows: (1) the age spectrum of the sandstone from Kandilik is remarkably similar to that of the Early Jurassic strata (Fig. 6); (2) the structural compatibility of this new stratigraphic scheme was we demonstrated (line 487-499); (3) The clastic member in Kandilik is primarily composed of gray-black carbonaceous slate and siltstone, similar to the Jurassic coal-bearing sequences.
5. We have added more information about this event in the South Qiangtang and Bangong-Nujiang suture zone. We also cited these important papers in our revised manuscript (line 233 and 626). In the final discussion section, we further discussed the relationship between the basin inversion in South Qiangtang and the evolution of the Meso-Tethys.
6. Jurassic strata are entirely absent in the West Kunlun Mountains. Only in some region, such as east of Tashkurghan, the Lower Cretaceous reddish sandstones unconformably overlie Paleozoic strata and Triassic granitoids.
References:
Ma, S., Wang, Y., and Fang, X.: Basic characteristics of Proterozoic Eonothem as a table cover on northern slope, Xinjiang Geology, 9, 59-71, 1991 (in Chinese with English abstract).
Corfu, F., Hanchar, J. M., Hoskin, P. W. O., and Kinny, P.: Atlas of Zircon Textures, Reviews in Mineralogy and Geochemistry, 53, 469-500, 10.2113/0530469, 2003.
Citation: https://doi.org/10.5194/egusphere-2024-1670-AC3
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AC3: 'Reply on RC2', Hongxiang Wu, 30 Sep 2024
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