the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Jun 2025
Low-velocity and low-resistivity zone detected in the crust of the passive continental margin of south China
Abstract. Passive continental margin has long been regarded as a tectonically and geo-dynamically inactive region with few discoveries of crustal unconformity and hence lacks scientific attention. However, previous seismic studies have hinted the presence of isolated crustal weak zones characterized by low shear-wave velocities in Guangdong-Hong Kong-Macao Greater Bay Area (GBA), north rim of South China Sea. In order to confirm the crustal heterogeneity, a 100 km-long magnetotelluric (MT) survey (ZHMT) was conducted across Zhuhai and Jiangmen (Guangdong Province) to fill in the blank of resistivity property in the region and to validate the crustal low-velocity zone at the research region. 3D inversion of the ZHMT data reveals a striking low-resistivity anomaly below the township of Shuangshui at the depth of 10–15 km depth. As plausible evidence of on-going or past tectonic activities, the Shuangshui Low-resistivity Anomaly (SLRA) is in a good spatial agreement with the low-velocity feature from the ambient seismic noise tomography. Two geological models are proposed for its origin: a fluid-bearing shear zone associated with deep faulting and fluid migration, or partial melting of mid-crustal material, possibly linked to anomalous heat flow. These findings provide new insights into the tectonic evolution of the passive margin at the research region, revealing previously unrecognized mid-crustal activity.
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- RC1: 'Comment on egusphere-2025-2073', Anonymous Referee #1, 21 Jul 2025
-
RC2: 'Comment on egusphere-2025-2073', Anonymous Referee #2, 03 Sep 2025
The authors report the existence of a crustal zone of low seismic velocity on the South China continental margin, which could represent a zone of weakness and explain some phenomena, such as seismicity and high thermal flux values. I consider this topic interesting to the geoscience community. However, the manuscript does not clearly describe the problem. One point, for instance, that requires clarification is why the MT ZHMT profile was not positioned along the OOS2020-1 profile (Xiong et al.), where the 15-20 km depth seismic anomaly occurs. It is also unclear whether the low-velocity zone described by Lu et al. is the same anomaly detected by Xiong et al. Furthermore, figures 1 and 2 in the introductory part of the text lack information and are of low resolution. I believe the following changes would significantly help the reader better understand the purpose of the manuscript:
Fig. 1:
- Indicate the locations of Hong Kong and Macao.
- Indicate the area modeled by Lu et al. My suggestion is to use the boundaries in their Fig. 10(f).
- Plot the earthquake epicenters.
- Plot the boundaries of the GBA.
- Indicated where the profile Bb (shown in Fig. 2a) is.
Fig. 2b:
- It would be better to place it just after Fig. 1, either as Fig. 1b or a new Fig. 2. Its limits should be indicated as a rectangle in Fig. 1. Consider also showing the geology.Citation: https://doi.org/10.5194/egusphere-2025-2073-RC2
Status: closed
-
RC1: 'Comment on egusphere-2025-2073', Anonymous Referee #1, 21 Jul 2025
The manuscript titled “Low-velocity and low-resistivity zone detected in the crust of the passive continental margin of South China” by Feng et al. requires substantial revision prior to consideration for the publication. Authors must present the MT curves, detailed data analysis, and modelling and inversion exercises for finalization of the models, prior to any interpretation. I am providing my detailed comments for the improvement of the manuscript as follows:
Title: please consider reframing the title. Presently it indicates reporting of some data based observation over a region.
Abstract:
Highlights you major results/outcome as well as inferences and implications from the study in the abstract. In its present form it seems study is trying to only complement the finding from previous seismic investigations.
Previous studies and implications of crustal weakness zones: In introduction all the subsections can be merged and most relevant information can be presented in the paper. Anyway all these studies are from other paper, so use information which is relevant to this paper at the right place where it is needed.
Please check equation (3) for the tipper and write it in the correct form.
Please include a detailed geology map with tectonics of the studied region.
Data processing:
Pease mention the approaches along with some images which helps the authors to generate the good quality data, remove the irrelevant part without any supporting data presented in the manuscript.
Please display the MT curves (apparent resistivity and phase) for all sites after various processing steps discussed in the manuscript.
Modelling and Inversion:
Why was 5 % error floor opted in the inversion? Did you vary the error floor values?
Why was a uniform half space of 100 ohm.m opted for the inversion? Did you performed a test with varying uniform half spaces?
On what basis first layer assigned a thickness of 100 m?
Did you use MT full impedance for the inversion? What about the tipper information? Is it included in the inversion?
Did you take care of the MT static shift? Please elaborate on it.
Please include the source information about the bathymetry for ocean effect.
How do you incorporate the effect of sediments in your model? Did you utilize model covariance file of ModEM? Please elaborate on it. Do you have sedimentary thickness from some other data source, plz also include all this information.
Results and Discussion
Subsection 3.1 Electromagnetic Environment needs to be before the data processing, as various processing steps take care of electromagnetic noises up to some extent.
Figure corrections:
Figure 1: this figure looks like a location map of present and available dataset and a few prominent features in the region. This figure requires more details such as geology, tectonic elements and fabrics if any exists for the study area etc.
Figure 2. This figure may be dropped and use the shear wave model later.
Figure 3. Please revise this figure for clarity. It's challenging to visualise phase tensor shapes due to overlapping periods. Exaggerating the scale along both axes will solve the issue.. The real part of the induction arrows can be plotted as part (b).
Figure 4. Show MT curves for all the sites. Occasionally, the Pseudo section misleads us with unusual information.
Figure 5. More tests are required before concluding this is a final model (See modelling and Inversion comments).
Figure 6. I can see a classical case of static shift effect in the MT data [as apparent resistivity components (derived from impedance magnitude) are statically shifted against the period, whereas phases are unaffected by it].
After achieving the final model, repeat this test for the sites with coverage of the anomaly.
Figure 7. Please include the sedimentary thickness in (b).
Figure 8. It may be merged with 9. Please calibrate you model with known surface geology? I couldnot see any geology map in the manuscript. Please include the same.
Figure 9. Figure 2a and Figure 8 resistivity-section can be interpreted jointly and possible inferences of this joint interpretation.
Citation: https://doi.org/10.5194/egusphere-2025-2073-RC1 -
RC2: 'Comment on egusphere-2025-2073', Anonymous Referee #2, 03 Sep 2025
The authors report the existence of a crustal zone of low seismic velocity on the South China continental margin, which could represent a zone of weakness and explain some phenomena, such as seismicity and high thermal flux values. I consider this topic interesting to the geoscience community. However, the manuscript does not clearly describe the problem. One point, for instance, that requires clarification is why the MT ZHMT profile was not positioned along the OOS2020-1 profile (Xiong et al.), where the 15-20 km depth seismic anomaly occurs. It is also unclear whether the low-velocity zone described by Lu et al. is the same anomaly detected by Xiong et al. Furthermore, figures 1 and 2 in the introductory part of the text lack information and are of low resolution. I believe the following changes would significantly help the reader better understand the purpose of the manuscript:
Fig. 1:
- Indicate the locations of Hong Kong and Macao.
- Indicate the area modeled by Lu et al. My suggestion is to use the boundaries in their Fig. 10(f).
- Plot the earthquake epicenters.
- Plot the boundaries of the GBA.
- Indicated where the profile Bb (shown in Fig. 2a) is.
Fig. 2b:
- It would be better to place it just after Fig. 1, either as Fig. 1b or a new Fig. 2. Its limits should be indicated as a rectangle in Fig. 1. Consider also showing the geology.Citation: https://doi.org/10.5194/egusphere-2025-2073-RC2
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The manuscript titled “Low-velocity and low-resistivity zone detected in the crust of the passive continental margin of South China” by Feng et al. requires substantial revision prior to consideration for the publication. Authors must present the MT curves, detailed data analysis, and modelling and inversion exercises for finalization of the models, prior to any interpretation. I am providing my detailed comments for the improvement of the manuscript as follows:
Title: please consider reframing the title. Presently it indicates reporting of some data based observation over a region.
Abstract:
Highlights you major results/outcome as well as inferences and implications from the study in the abstract. In its present form it seems study is trying to only complement the finding from previous seismic investigations.
Previous studies and implications of crustal weakness zones: In introduction all the subsections can be merged and most relevant information can be presented in the paper. Anyway all these studies are from other paper, so use information which is relevant to this paper at the right place where it is needed.
Please check equation (3) for the tipper and write it in the correct form.
Please include a detailed geology map with tectonics of the studied region.
Data processing:
Pease mention the approaches along with some images which helps the authors to generate the good quality data, remove the irrelevant part without any supporting data presented in the manuscript.
Please display the MT curves (apparent resistivity and phase) for all sites after various processing steps discussed in the manuscript.
Modelling and Inversion:
Why was 5 % error floor opted in the inversion? Did you vary the error floor values?
Why was a uniform half space of 100 ohm.m opted for the inversion? Did you performed a test with varying uniform half spaces?
On what basis first layer assigned a thickness of 100 m?
Did you use MT full impedance for the inversion? What about the tipper information? Is it included in the inversion?
Did you take care of the MT static shift? Please elaborate on it.
Please include the source information about the bathymetry for ocean effect.
How do you incorporate the effect of sediments in your model? Did you utilize model covariance file of ModEM? Please elaborate on it. Do you have sedimentary thickness from some other data source, plz also include all this information.
Results and Discussion
Subsection 3.1 Electromagnetic Environment needs to be before the data processing, as various processing steps take care of electromagnetic noises up to some extent.
Figure corrections:
Figure 1: this figure looks like a location map of present and available dataset and a few prominent features in the region. This figure requires more details such as geology, tectonic elements and fabrics if any exists for the study area etc.
Figure 2. This figure may be dropped and use the shear wave model later.
Figure 3. Please revise this figure for clarity. It's challenging to visualise phase tensor shapes due to overlapping periods. Exaggerating the scale along both axes will solve the issue.. The real part of the induction arrows can be plotted as part (b).
Figure 4. Show MT curves for all the sites. Occasionally, the Pseudo section misleads us with unusual information.
Figure 5. More tests are required before concluding this is a final model (See modelling and Inversion comments).
Figure 6. I can see a classical case of static shift effect in the MT data [as apparent resistivity components (derived from impedance magnitude) are statically shifted against the period, whereas phases are unaffected by it].
After achieving the final model, repeat this test for the sites with coverage of the anomaly.
Figure 7. Please include the sedimentary thickness in (b).
Figure 8. It may be merged with 9. Please calibrate you model with known surface geology? I couldnot see any geology map in the manuscript. Please include the same.
Figure 9. Figure 2a and Figure 8 resistivity-section can be interpreted jointly and possible inferences of this joint interpretation.