the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2022
New constraints on the tectonic evolution by subduction of the Bangong Co-Nujiang Tethys Oceanic Basin: Insights from magnetic fabric and U-Pb dating of detrital zircon during the Late Jurassic to Early Cretaceous
Abstract. The subduction process between the Bangong Co-Nujiang Tethys Oceanic Basin and the South Qiangtang Block is one of the key issues in the study of the Tethys domain. In order to clarify the evolution process and achieve tectonic constraints, the clastic rock and limestone in the northern margin of the Lhasa block and the southern Qiangtang block were studied in detail through the study of magnetic fabric and zircon U-Pb dating. The results show that the depositional age of the Shamuluo Formation is 131–95 Ma, belonging to the Late Jurassic-Early Cretaceous; the Sowa Formation is earlier (163.5–157.3 Ma), belonging to the Late Jurassic. The detrital provenances of the Shamuluo and Suowa Formations are mainly from the magmatic arc of the Lhasa block and the cyclic orogenic belt of the South Qiangtang block. The fabrics of sandstones in the Shamuluo Formation and some sandstones in the Suowa Formation belong to sedimentary magnetic fabrics related to paleocurrent; the limestone and other sandstones of the Sowa Formation are subject to stress and deformation, and belong to the strain fabric related to the structure. A comprehensive study of detrital zircon U-Pb geochronology, magnetic fabric and petrography shows that the Bangong Co-Nujiang ocean basin is affected by plate fragmentation, and there is a north-south two-way subduction. The southward subduction changes in polarity at 163.5–157.3 Ma and begins the northward subduction. At 145 Ma, the Bangong Co-Nujiang Tethys Ocean closed, but the central remnant of Bangong Co-Nujiang Oceanic Basin continued to subduct until 131–102.9 Ma.
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RC1: 'Comment on egusphere-2022-631', Anonymous Referee #1, 12 Aug 2022
This manuscript carried on magnetic fabric and detrital zircons U-Pb dating of the Shamuluo and Suowa formations, with an aim to investigate the subduction and closure of the Bangong-Nujiang Tethys Ocean. The authors proposed that the Bangong-Nujiang Oceanic Basin has been subducting southward continuously since the Jurassic, the switching time of subduction polarity was at 163.5-157.3 Ma, the Bangong-Nujiang Tethys Ocean was closed at 145 Ma, and the central residual oceanic basin completed a continuous northward subduction at 131-102.9 Ma. However, many conclusions and discussions are lack of logicality and robust evidences. Further studies needed for the available data about the Bangong-Nujiang Tethys Ocean. The interpretations seem unreasonable and I don’t think this version could be accepted. Hence I recommend a major revision for this manuscript.
General comments:
- The Shamuluo Formation belongs to the Bangong-Nujiang suture zone, rather than the Lhasa Block.
- More evidences should be added to constrain the closure time of the Bangong-Nujiang Tethys Ocean.
- How the subduction polarity reversal is inferred?
- Line 81-83: How the subduction in the Middle Jurassic is speculated from the Early Cretaceous OIB-type basalt?
- Line 87-97: These sentences are lack of logic, and the Mugagangri melange resulted from the northward subduction of the Bangong-Nujiang Tethys Ocean.
- Line 121: an island arc block? Please add references.
- Line 123: The Shiquan River-Laguo Co-Yongzhu-Nam Co-Jiali belt is not shown in the Figure 1.
- Line 125: ocean-continent collision?
- Line 129: marine molasses deposits? Please add references.
- Line 154: gas-liquid inclusions should be marked in the Figure 2d.
- Line 156: dissolution harbors and intragranular micro-fractures should be marked in the Figure 2f-g.
- Line 285: The detrital zircon dating of the Suowa Formation should be added.
- Line 462-465: two blocks?
- Line 710-711: This is a serious problem. In this paper, the detrital zircon dating and provenances analysis of the Suowa Formation are missing, and the location of the Suowa Formation is far away from the Lhasa Block, and the debris of the Suowa Formation should not source from the Lhasa Block and Bangong-Nujiang suture zone.
- Line 716-719: The recycling of the sediments should be considered.
- Line 721-722: How did this conclusion is inferred? In addition, the sorting and rounding of sediments are poor, and the transportation distance is short in collision setting.
- Line 725-727: the combination between Bangong Co-Nujiang Tethys Oceanic Basin and the southern margin of Qiangtang?
- Line 746: Bangong-Nujiang oceanic basin.
- Line 753-755 and 791-793: A figure should be added. And the northward subduction of the Bangong-Nujiang Tethys Ocean had existed at least from the Early Jurassic.
- Line 763-766: How is this speculated? The northward subduction was existed at this time.
- Line 790: The polarity transition time should be discussed in detail.
- Line 816-817: The provenance of the Suowa Formation should be the Qiangtang Block and the Longmuco-Shuanghu suture zone.
Citation: https://doi.org/10.5194/egusphere-2022-631-RC1 -
RC2: 'Comment on egusphere-2022-631', Anonymous Referee #2, 07 Sep 2022
Major comments:
1. How can we get the conclusions that “ the Bongong Co-Nujiang ocean basin is affected by plate fragmentation, and there is a north-south two way subduction”? The data and analyzing results in this study seems not closely related to the conclusions. I suggest the authors to focus on the new data and draw a practical conclusion.
2. The magnetic fabric data and figures are the key results in this study, but not detailed illustrated and analyzed.
3. The English-writing should be significantly improved.
Minor comments:
Line 32: The detritals of the Shamuluo and Suowa formations are mainly from the magmatic arc…
Line 34-36: Magnetic fabric analyses indicated that the sandstones in the Shamuluo Formation and some sandstones in the Showa Formation show sedimentary magnetic fabric pattern which was possibly influenced by paleocurrent.
Line 37: the limestone and other sandstones of the Shuowa Formation are in a structural strain fabric.
Line 327: the K-T analyses are not enough to draw the conclusions that the mineral transition is because of the sulfide of iron. It must be careful to tell the existence of pyrrhotite, which is typically Fe7S8. The curie temperature of pyrrhotite is ~325 degree, so the susceptibility signal above 350 degree is not contributed by pyrrhotite. It will be much better to divide the samples into several groups based on their rock magnetic characteristics and discuss them one by one.
Line 333-336: the Hopkinson peak is because of the transition of small single domain magnetic particles to superparamagnetic(SP) state during the heating process, so not related to the MD particles. How do you know the magnetic minerals in sample J3A0104 and J3A0402 is MD particles? It seems not the truth.
Section 4.4.2: it must be noted that the magnetic anisotropy is generally not significant in limestones. Besides, you only show six measurements in this figure, it is hard to give a clear fabric result.
Line 429: you can say “in stratigraphic coordinate”, or “tilt corrected”, or “with stratigraphic/tilt correction”, but not a noun phrases “stratigraphic correction”. So it is below.
Line 438: if you want to show the magnetic lineation is parallel to the strike direction of the stratigraphic beddings, it will be better to draw the strike line in figure 11b. So in this case, geographic coordinate is better to show the relationship between the magnetic fabrics and the strike directions and so it is for 11d and 11c.
Line 441: where is the orientation of magnetic lineation? NE or NW? They are absolutely different.
Line 568-574: Based on the analyses of magnetic fabric results above, only Figure 11a show a clear paleocurrent direction. How the authors obtained the other paleocurrent directions and make so important inferences in the discussion? It must be clearly and detailed documented. In deformed structures like figure. 11c, the fabric indicates the strain state, not the sedimentary conditions, it cannot be used to infer the water flow.
Citation: https://doi.org/10.5194/egusphere-2022-631-RC2
Status: closed
-
RC1: 'Comment on egusphere-2022-631', Anonymous Referee #1, 12 Aug 2022
This manuscript carried on magnetic fabric and detrital zircons U-Pb dating of the Shamuluo and Suowa formations, with an aim to investigate the subduction and closure of the Bangong-Nujiang Tethys Ocean. The authors proposed that the Bangong-Nujiang Oceanic Basin has been subducting southward continuously since the Jurassic, the switching time of subduction polarity was at 163.5-157.3 Ma, the Bangong-Nujiang Tethys Ocean was closed at 145 Ma, and the central residual oceanic basin completed a continuous northward subduction at 131-102.9 Ma. However, many conclusions and discussions are lack of logicality and robust evidences. Further studies needed for the available data about the Bangong-Nujiang Tethys Ocean. The interpretations seem unreasonable and I don’t think this version could be accepted. Hence I recommend a major revision for this manuscript.
General comments:
- The Shamuluo Formation belongs to the Bangong-Nujiang suture zone, rather than the Lhasa Block.
- More evidences should be added to constrain the closure time of the Bangong-Nujiang Tethys Ocean.
- How the subduction polarity reversal is inferred?
- Line 81-83: How the subduction in the Middle Jurassic is speculated from the Early Cretaceous OIB-type basalt?
- Line 87-97: These sentences are lack of logic, and the Mugagangri melange resulted from the northward subduction of the Bangong-Nujiang Tethys Ocean.
- Line 121: an island arc block? Please add references.
- Line 123: The Shiquan River-Laguo Co-Yongzhu-Nam Co-Jiali belt is not shown in the Figure 1.
- Line 125: ocean-continent collision?
- Line 129: marine molasses deposits? Please add references.
- Line 154: gas-liquid inclusions should be marked in the Figure 2d.
- Line 156: dissolution harbors and intragranular micro-fractures should be marked in the Figure 2f-g.
- Line 285: The detrital zircon dating of the Suowa Formation should be added.
- Line 462-465: two blocks?
- Line 710-711: This is a serious problem. In this paper, the detrital zircon dating and provenances analysis of the Suowa Formation are missing, and the location of the Suowa Formation is far away from the Lhasa Block, and the debris of the Suowa Formation should not source from the Lhasa Block and Bangong-Nujiang suture zone.
- Line 716-719: The recycling of the sediments should be considered.
- Line 721-722: How did this conclusion is inferred? In addition, the sorting and rounding of sediments are poor, and the transportation distance is short in collision setting.
- Line 725-727: the combination between Bangong Co-Nujiang Tethys Oceanic Basin and the southern margin of Qiangtang?
- Line 746: Bangong-Nujiang oceanic basin.
- Line 753-755 and 791-793: A figure should be added. And the northward subduction of the Bangong-Nujiang Tethys Ocean had existed at least from the Early Jurassic.
- Line 763-766: How is this speculated? The northward subduction was existed at this time.
- Line 790: The polarity transition time should be discussed in detail.
- Line 816-817: The provenance of the Suowa Formation should be the Qiangtang Block and the Longmuco-Shuanghu suture zone.
Citation: https://doi.org/10.5194/egusphere-2022-631-RC1 -
RC2: 'Comment on egusphere-2022-631', Anonymous Referee #2, 07 Sep 2022
Major comments:
1. How can we get the conclusions that “ the Bongong Co-Nujiang ocean basin is affected by plate fragmentation, and there is a north-south two way subduction”? The data and analyzing results in this study seems not closely related to the conclusions. I suggest the authors to focus on the new data and draw a practical conclusion.
2. The magnetic fabric data and figures are the key results in this study, but not detailed illustrated and analyzed.
3. The English-writing should be significantly improved.
Minor comments:
Line 32: The detritals of the Shamuluo and Suowa formations are mainly from the magmatic arc…
Line 34-36: Magnetic fabric analyses indicated that the sandstones in the Shamuluo Formation and some sandstones in the Showa Formation show sedimentary magnetic fabric pattern which was possibly influenced by paleocurrent.
Line 37: the limestone and other sandstones of the Shuowa Formation are in a structural strain fabric.
Line 327: the K-T analyses are not enough to draw the conclusions that the mineral transition is because of the sulfide of iron. It must be careful to tell the existence of pyrrhotite, which is typically Fe7S8. The curie temperature of pyrrhotite is ~325 degree, so the susceptibility signal above 350 degree is not contributed by pyrrhotite. It will be much better to divide the samples into several groups based on their rock magnetic characteristics and discuss them one by one.
Line 333-336: the Hopkinson peak is because of the transition of small single domain magnetic particles to superparamagnetic(SP) state during the heating process, so not related to the MD particles. How do you know the magnetic minerals in sample J3A0104 and J3A0402 is MD particles? It seems not the truth.
Section 4.4.2: it must be noted that the magnetic anisotropy is generally not significant in limestones. Besides, you only show six measurements in this figure, it is hard to give a clear fabric result.
Line 429: you can say “in stratigraphic coordinate”, or “tilt corrected”, or “with stratigraphic/tilt correction”, but not a noun phrases “stratigraphic correction”. So it is below.
Line 438: if you want to show the magnetic lineation is parallel to the strike direction of the stratigraphic beddings, it will be better to draw the strike line in figure 11b. So in this case, geographic coordinate is better to show the relationship between the magnetic fabrics and the strike directions and so it is for 11d and 11c.
Line 441: where is the orientation of magnetic lineation? NE or NW? They are absolutely different.
Line 568-574: Based on the analyses of magnetic fabric results above, only Figure 11a show a clear paleocurrent direction. How the authors obtained the other paleocurrent directions and make so important inferences in the discussion? It must be clearly and detailed documented. In deformed structures like figure. 11c, the fabric indicates the strain state, not the sedimentary conditions, it cannot be used to infer the water flow.
Citation: https://doi.org/10.5194/egusphere-2022-631-RC2
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