the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2023
Implications of the tectonic rotation of the South Qiangtang Massif for the subduction closure of the BangongCo - Nujiang Tethys Ocean – A study of magnetic fabric and zircon U-Pb chronology
Abstract. In the BangongCo-Nujiang Tethys Ocean, the timing of closure and subduction polarity are key issues in the study of the Tethys domain. Detailed magnetic fabrics and zircon U-Pb dating of marine carbonates collected in the South Qiangtang Massif and clastic rocks collected in the Ban-Nu Suture Zone were carried out to constrain their subduction and collapse processes, from the Middle to Late Jurassic-Early Cretaceous. The results show that the Shamuluo Formation in the suture zone is 131–95 Ma in age, which belongs to the Late Jurassic-Early Cretaceous and develops a primitive sedimentary magnetic fabric; Under the tectonic stress, the limestones in the Middle Jurassic Buqu Formation developed the strongly cleaved magnetic fabric. Meanwhile. the sandstones in the Late Jurassic Sowa Formation developed the tensile lineation and initial deformaed magnetic fabrics. Integrated magnetic fabrics, zircon U-Pb chronology and petrographic studies suggest a WE to SW-NE anticlockwise sinistral movement in the South Qiangtang Massif from the Buqu Formation to the early Suowa Formation; In the Late Suowa Formation, it began a clockwise dextral movement. The change in the direction of massif rotation was associated with a change in the subduction polarity of the BangongCo-Nujiang Tethys Ocean. The BangongCo-Nujiang Ocean changed from southward subduction to northward subduction, which began at the Buqu Formation and ended at the late Sowa Formation (163.5–157.3 Ma). Afterwards, it began to close at 145 Ma and ended its subduction after 131–109.9 Ma, achieving complete closure of the central BangongCo - Nujiang Tethys Ocean.
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RC1: 'Comment on egusphere-2022-1214', Anonymous Referee #1, 03 Feb 2023
This manuscript focuses on establishing magnetic fabric of the Buqu and Suowa formations in Qiangtang Block and the depositional age of the Shamuluo Formation in Bangong-Nujiang suture zone. With the data, the authors conclude that the Bangong-Nujiang Ocean subducted southwards until 163.5-157.3 Ma and then changes its direction to northwards, before it started to close at 145 Ma and stopped subduction at 131- 109.9 Ma.
Overall this paper seems to discuss about very important problems in the central Tibet, but their data do not support their conclusion. I find the current draft of the manuscript is too rough and also difficult to follow. So I suggest a reject or resubmission after revision.
Major comments:
- The authors misinterpreted the stratigraphy as Shamuluo Formation. They should refer to Kapp et al. (2007), DeCelles et al. (2007) and Ma et al. (2018) in which 125-118 Ma has changed to nomarine environment and the Shamuluo in the area was deposited during the Late Jurassic.
- The language needs substantially improved and many wrong citations through the whole text.
- How can authors prove the magnetic fabric is original? Cenozoic strike-slip fault and normal fault may change the magnetic fabric.
- The Biluoco Formation between Suowa and Buqu formations was interpreted to represent a tectonic event, which seems to be support the authors' interpretation.
- Part 4.5 seems the key point for this paper. Unfortunately, it is quite difficult to follow. There is no evidence provided to support the authors’ view for the closure timing of BanongCo-Nujiang Ocean. Similarly, for the discussion of subduction polarity in Lines 568-584, I cannot follow the relationships between the sea level change and the subduction polarity. Additionally, the citations are needed.
Again, the language needs to be fully improved. I just give some specific comments to the “Abstract“ in the following. Authors need to revise for the whole text...
Lines 24-28: needs reorganized; collapse-collision? Ban-Nu Suture Zone: BangongCo-Nujiang Suture Zone? check the whole text.
Line 32: Late-Upper? Sowa Formation-Suowa Formation?
Line 33: initial deformaed magnetic fabrics?
Lines 35-36: early Suowa Formation/Late Suowa Formation: strange expression; check the whole text.
Line 42: central BangongCo - Nujiang Tethys Ocean?
Citation: https://doi.org/10.5194/egusphere-2022-1214-RC1 -
RC2: 'Comment on egusphere-2022-1214', Anonymous Referee #2, 04 Feb 2023
Comments on EGUsphere MS entitled as “Implications of the tectonic rotation of the South Qiangtang Massif for the subduction closure of the BangongCo -Nujiang Tethys Ocean - A study of magnetic fabric and zircon U-Pb chronology” by Chen et al.
The authors reported magnetic fabrics and zircon U-Pb chronology from the late Mesozoic strata in Tibet in attempts to clarify the rotation history of the southern Qiangtang Block and to constrain the evolution of the Bangong-Nujiang Tethys Ocean. Although the topic seems to be interesting and important, the overall research is not solid. The English expression is poor and the use of terminology is improper in many places.The methodology used is flawed. As the major conclusions are not erected by data, the logic of the whole story is hard to follow. I believe a lot of work is needed before the work could be considered for publication. My major concerns are as follows:
- The statistical analysis is flawed. Since the magnetic fabrics were given on site level, the authors should list the statistical parameters of each site/unit so that the readers can intuitively evaluate the quality of the AMS data.
- The rotation patterns inferred from AMS data are not reliable. Because the 3 units were collected at different profiles in different regions, a large number of local faults contribute significantly to rotation.
- The relationship between the stress state and subduction polarity is speculative. The selected sections may not be representative and recorded stresses may vary from section to section. Moreover, as the acquisition of magnetic fabric postdates deposition, magnetic fabric is not necessarily acquired in stratigraphic order.
Citation: https://doi.org/10.5194/egusphere-2022-1214-RC2 -
RC3: 'Comment on egusphere-2022-1214', Anonymous Referee #3, 07 Feb 2023
This manuscript mainly focuses on geochronlogy and magnetic fabric measurement on samples collected from the South QIangtang and Bannu suture zone. According to these data, an evolutionary model has been proposed to depict the Banggongco-Nujiang ocean history, which is meanlingful to understand the tethyan history.
The zircon data is robust and has been interpreted properly. However, the AMS data and related magnetic fabric interpreation is the biggest flaw of this manuscript. Major concerns regariding the AMS fabric and its interpretaion are list below:
1. The K-T curve, applied to distinguish magnetic carrier, is of poor quality. Generally, most Km values (except diamagnetic carrier) should be positive, such as the paramagnetic carrier of mica and chlorite. To the contary, all km values are negative in Figure 7 are negative, which makes these results questionable. One cannot interpret the AMS fabric properly until the magnetic carriers detected. Low Km of most sites, suggest paramagnetic, even diamagnetic carrier contributing to the magnetic fabric. The diamagnetic carrier, such as calcite, show completely inverse correlation between the Kmax and the X axis, which would lead an totally different interpretation of the magnetic fabric.
2. The author claim that some of the AMS fabric link to deformation, but no direct evidence presented in the manuscript. All interpretations are speculated and most of them are overinterpreted. Anything shall yield an magnetic ellipsoid, plenty work should be carried out to prove the correaltion between the magnetic fabric and the strain ellipsoid. First, is there any microphotographs show clear mineral alighment corresponding to the magnetic foliaiton? Some of the Kmax axis are highly concentrated, which could be the result of paleocurrent rather than deformation. Delicate comparision between the field fabric and magnetic fabric is suggest here.
3. Many supreme references regarding basic AMS theory are missing from the manuscript, some of the citation are inapproprite, please see details in the attached pdf file.
Status: closed
-
RC1: 'Comment on egusphere-2022-1214', Anonymous Referee #1, 03 Feb 2023
This manuscript focuses on establishing magnetic fabric of the Buqu and Suowa formations in Qiangtang Block and the depositional age of the Shamuluo Formation in Bangong-Nujiang suture zone. With the data, the authors conclude that the Bangong-Nujiang Ocean subducted southwards until 163.5-157.3 Ma and then changes its direction to northwards, before it started to close at 145 Ma and stopped subduction at 131- 109.9 Ma.
Overall this paper seems to discuss about very important problems in the central Tibet, but their data do not support their conclusion. I find the current draft of the manuscript is too rough and also difficult to follow. So I suggest a reject or resubmission after revision.
Major comments:
- The authors misinterpreted the stratigraphy as Shamuluo Formation. They should refer to Kapp et al. (2007), DeCelles et al. (2007) and Ma et al. (2018) in which 125-118 Ma has changed to nomarine environment and the Shamuluo in the area was deposited during the Late Jurassic.
- The language needs substantially improved and many wrong citations through the whole text.
- How can authors prove the magnetic fabric is original? Cenozoic strike-slip fault and normal fault may change the magnetic fabric.
- The Biluoco Formation between Suowa and Buqu formations was interpreted to represent a tectonic event, which seems to be support the authors' interpretation.
- Part 4.5 seems the key point for this paper. Unfortunately, it is quite difficult to follow. There is no evidence provided to support the authors’ view for the closure timing of BanongCo-Nujiang Ocean. Similarly, for the discussion of subduction polarity in Lines 568-584, I cannot follow the relationships between the sea level change and the subduction polarity. Additionally, the citations are needed.
Again, the language needs to be fully improved. I just give some specific comments to the “Abstract“ in the following. Authors need to revise for the whole text...
Lines 24-28: needs reorganized; collapse-collision? Ban-Nu Suture Zone: BangongCo-Nujiang Suture Zone? check the whole text.
Line 32: Late-Upper? Sowa Formation-Suowa Formation?
Line 33: initial deformaed magnetic fabrics?
Lines 35-36: early Suowa Formation/Late Suowa Formation: strange expression; check the whole text.
Line 42: central BangongCo - Nujiang Tethys Ocean?
Citation: https://doi.org/10.5194/egusphere-2022-1214-RC1 -
RC2: 'Comment on egusphere-2022-1214', Anonymous Referee #2, 04 Feb 2023
Comments on EGUsphere MS entitled as “Implications of the tectonic rotation of the South Qiangtang Massif for the subduction closure of the BangongCo -Nujiang Tethys Ocean - A study of magnetic fabric and zircon U-Pb chronology” by Chen et al.
The authors reported magnetic fabrics and zircon U-Pb chronology from the late Mesozoic strata in Tibet in attempts to clarify the rotation history of the southern Qiangtang Block and to constrain the evolution of the Bangong-Nujiang Tethys Ocean. Although the topic seems to be interesting and important, the overall research is not solid. The English expression is poor and the use of terminology is improper in many places.The methodology used is flawed. As the major conclusions are not erected by data, the logic of the whole story is hard to follow. I believe a lot of work is needed before the work could be considered for publication. My major concerns are as follows:
- The statistical analysis is flawed. Since the magnetic fabrics were given on site level, the authors should list the statistical parameters of each site/unit so that the readers can intuitively evaluate the quality of the AMS data.
- The rotation patterns inferred from AMS data are not reliable. Because the 3 units were collected at different profiles in different regions, a large number of local faults contribute significantly to rotation.
- The relationship between the stress state and subduction polarity is speculative. The selected sections may not be representative and recorded stresses may vary from section to section. Moreover, as the acquisition of magnetic fabric postdates deposition, magnetic fabric is not necessarily acquired in stratigraphic order.
Citation: https://doi.org/10.5194/egusphere-2022-1214-RC2 -
RC3: 'Comment on egusphere-2022-1214', Anonymous Referee #3, 07 Feb 2023
This manuscript mainly focuses on geochronlogy and magnetic fabric measurement on samples collected from the South QIangtang and Bannu suture zone. According to these data, an evolutionary model has been proposed to depict the Banggongco-Nujiang ocean history, which is meanlingful to understand the tethyan history.
The zircon data is robust and has been interpreted properly. However, the AMS data and related magnetic fabric interpreation is the biggest flaw of this manuscript. Major concerns regariding the AMS fabric and its interpretaion are list below:
1. The K-T curve, applied to distinguish magnetic carrier, is of poor quality. Generally, most Km values (except diamagnetic carrier) should be positive, such as the paramagnetic carrier of mica and chlorite. To the contary, all km values are negative in Figure 7 are negative, which makes these results questionable. One cannot interpret the AMS fabric properly until the magnetic carriers detected. Low Km of most sites, suggest paramagnetic, even diamagnetic carrier contributing to the magnetic fabric. The diamagnetic carrier, such as calcite, show completely inverse correlation between the Kmax and the X axis, which would lead an totally different interpretation of the magnetic fabric.
2. The author claim that some of the AMS fabric link to deformation, but no direct evidence presented in the manuscript. All interpretations are speculated and most of them are overinterpreted. Anything shall yield an magnetic ellipsoid, plenty work should be carried out to prove the correaltion between the magnetic fabric and the strain ellipsoid. First, is there any microphotographs show clear mineral alighment corresponding to the magnetic foliaiton? Some of the Kmax axis are highly concentrated, which could be the result of paleocurrent rather than deformation. Delicate comparision between the field fabric and magnetic fabric is suggest here.
3. Many supreme references regarding basic AMS theory are missing from the manuscript, some of the citation are inapproprite, please see details in the attached pdf file.
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