the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Mar 2023
Hydrothermal activity of the Lake Abhe geothermal field (Djibouti): Structural controls and paths for further exploration
Abstract. Lake Abhe is sitting on the Gob Aad graben within the tectonic Afar triangle in the Republic of Djibouti. It is known for its exposures of massive hydrothermal chimneys on the lake’s eastern shore. The many hydrothermal surface manifestations on this side of the lake, including steam vents, hot springs and carbonate chimney structures, reflect the geothermal field of this area. This study describes the structural settings of the Lake Abhe Geothermal Field (LAGF), using multiscale structural lineament distribution mapping. It also investigates the hydrothermal surface manifestations distribution in order to specify structural controls on local fluid flow and discuss its evolution. Structural features of the LAGF area are dominated by ESE-trending extensional faults that form a series of narrow elongated horst, graben and half-graben structures. Fault interaction and accommodation zones, as well as fault intersections, relay ramps and possible breaching faults are also recognized and may represent interesting structural features in terms of fluid flow pathways. Hydrothermal chimneys and hot springs distribution over the LAGF area is controlled by the main structural trends, and show signs of higher hydrothermal activity located at intersecting structural traces. Field observations, in conjunction with satellite images analysis, suggest a progressive lateral evolution of the LAGF hydrothermal fluid outflows over time. Therefore, this study provides new insights on the local tectonically driven fluid flow of the LAGF, that may support further exploration of this remarkable site and may promote its geothermal development.
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Status: closed
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RC1: 'Comment on egusphere-2023-397', Anonymous Referee #1, 16 Mar 2023
General comment:
The manuscript of Walter et al. aims at determining the relationship between tectonic activity/structures and geothermal surface manifestations (hot springs and carbonate chimneys) in order to better constrain the geothermal potential of the Lake Abhe area. It is based on the analysis of morphological lineaments, field observations and the mapping of hot springs with satellite images. The manuscript suggests that the ascending geothermal fluids are primary channelized by intersecting faults. Albeit such a control of tectonic structures on the flow path is not new, its characterization in the study zone is essential for future geothermal developments.
Yet, the manuscript requires several improvements before publication (see below). I therefore recommend to published it after major revision.
Specific comments:
- Presentation of the tectonic framework:
In the manuscript, the tectonic setting of study area is described in a two sentences in section 2 (lines 70-72 and 75-77) whereas it is judiciously integrated in the discussion to interpret the role of the fault system in the channelization of fluid flow. I recommend to make a more detailed presentation of the regional tectonics, including the southern part of the God Aad basin. The addition of a new figure (regional tectonic framework at a scale between the Figure 1 and its inset) would greatly help to present the study area and support the interpretations, especially the origin of the NNW-trending lineament and section 5.1.
- Lineament naming and interpretation:
The author use two sets of Digital Elevation Models to map “structural” lineaments. I guess that the authors used morphological features like the limit of a crest or the geometry of the valley network to draw these lineaments. Thus, I would encourage the authors to describe these lineaments as they are, i.e. morphological features, in the result section and to interpret them as faults, which in likely, in the discussion.
- Data analysis and presentation:
The authors use several characteristics of the lineaments (orientation and length) to describe the main and secondary structural trends. The values of the mean lineament length suggest strong disparities and I therefore propose to the authors to build a frequency graph for different length populations. I have the feeling it will allow to see a relationship between the length of the lineaments and their orientation. Each trend could later be compared with the regional structural orientations.
- Figures
The figures need to be improved. First the hemisphere must be indicated in the geographic coordinates e.g. 41.90°E and 10.10°N. Moreover, it is not necessary to use three decimal digit in Figures 1, 4 and 9. Second, normal faults are represented with three different symbols. Please choose only one. Third, as mentioned above, you should better use morphological lineaments than structural ones. There are many other specific points to improve listed below.
Other points:
Lines 69-70: (Chorowicz, 2005 ; Fig. 1)
Line 71: The Tendaho graben trends NW-SE instead of NNW-SSE (Acocella et al., 2008)
Line 103: and/or
Line 105: that coats
Lines 115-116: What do the grey and red areas in inset correspond ?
Line 132: the petro-structural description is not used in this manuscript. No need to mention it.
Lines 135-136: It seems that the UAV images were not used in the current study. If it is true, then this part should be removed.
Line 137: studied area, morphological lineaments were mapped using high-resolution remote sensing data and interpreted as faults with knowledge acquired from field geological observations.
Line 139: models
Line 141: ~30 m
Line 143: 0.5 m resolution
Line 145: morphological lineaments
Lines 149-152: I agree that springs can be mapped from satellite images. Yet, how can you be sure that every springs that you identified are hot?
Line 155: mapping of morphological lineaments
Line 156: Ninety
Line 158: fourteen
Lines 158-160: The distinction that is made in subsets is rather hard to see with the rose diagram in Fig. 2a, which instead shows a single trends with a maximum density around N100° and N120°. Maybe should you distinguish the long lineaments (the regional trend) that seem to have a N100-120° orientations and the short ones (secondary fault networks) that seem to have different orientations. See my comment on line 157.
Line 159: (N090-N120°; Fig. 2a). (N070-N090°)
Line 160: (N120-N140°) (N170-180°)
Line 169: A total of 255 morphological lineaments was mapped
Line 171: seventeen
Lines 171-175: Same comment as for lines 158-160. Please modify the notation for the orientation (N090-N120° for instance)
Line 176: Seventy-four
Lines 177-178: Given the rose diagram for the chimney alignments, I would say that you have a main orientation (N110-120°) and two secondary orientations (N090-N100° and N060-N80°).
Line 178: (N060-N090°; Fig. 2c).
Lines 193-194: Here is the interpretation of the morphological lineaments.
Lines 197-198: You should add in Figure 5a elevations for the base and the top of the scarps Line 199: denser fault spacing. Please quantify.
Figure 4: It would be useful to add a color scale for elevation instead of explaining the colors for the min and max elevations.
Line 211: It is said that both profiles have a uniform vertical exaggeration. Please specify the value of vertical exaggeration.in order to easily visualize the tectonic offset.
Line 214: three viewpoints.
Line 238: Ninety one
Line 243: (N350-N020°), (N040-N080°), (N090-N120°)
Line 244: is consistent with the
Line 245: structural lineaments. It is important that you state on the possible origin of the morphological lineaments. Either they are faults or they have another origin. But once it is explained, then you can call them as they are.
Line 246: consistent directions with
Line 277-278: How can you be sure that the springs that you mapped with satellite images are all hot? How hot are they? Is there any temperature variation with respect to their location, e.g. close to the chimneys or not? According to Awaleh et al. (2015) some springs are only warm (T around 30°C). Do they consist to the springs that are far from the chimney?
Line 280: associated with
Line 283: How can you say that the chimneys are inactive as they have been formed in subaqueous conditions? Do you see any fumaroles at the top of the “active” ones?
Line 309-310: The explanation is unclear. Tilted block usually develop with increasing amount of extension. A brief look on the tectonic map of the area suggests that extension is more important with the proximity of the God Aad basin. I therefore think that the geometry difference between the northern and southern parts results from a higher amount of extension rather than higher extension rates.
Lines 322-326: The paper would be improve by the addition of an interpretative sketch map of the fault development.
Line 325 NNW instead of NWN
Line 333: Do you really mean extension rate or amount of extension?
Line 338-341: How can you explain the lack of hydrothermal occurrence along these putative N-S faults?
Lines 332-349: You discuss quite in detail the origin of the N-NNW trending faults and their origin related to relay ramps. You also propose that they may be related to larger structure of similar orientations that would separate the graben structures in the east and the LAGF in the west. An important information is missing. Indeed, you should mention the NNW trending faults that are observed south of the God Aad basin (see Figure 2 in Dekov et al. 2014 for instance). More generally, you should introduce, earlier in the manuscript, the regional tectonic structures.
Line 371: How do you discriminate active and inactive chimneys ? Is it only related to the presence of hot springs in their vicinity? If you don’t have hot spring within the chimney or fumaroles, I think they are all inactive.
Line 376: two areas.
Line 383: The age of
Line 399: they were not active
Lines 379-405: You never mention the large shield volcano located west of Lake. Is there any temporal relationship between the lake water level and the periods of activity? Is there any relationship between the age of the chimney and the periods of activity? I think these points are worse to discuss.
Lines 407-424: In this section you propose a main role of the relay ramps in the fluid flow ascent. However, you forget the regional structures as those existing south of the God Aad basin (i.e. the NNW-trending faults). The interaction between a northern continuity of these faults (as suggested by the lineaments observed in the western part of the horst-and-graben area) and the ENE-trending fault seems to be much more likely than the speculative role of relay ramps.
Line 445: NNW instead of NWN
Line 449: at depth
Line 452: the orientation distribution
Line 454-455: What about the role of the adjacent shield volcano?
Citation: https://doi.org/10.5194/egusphere-2023-397-RC1 -
AC1: 'Reply on RC1', Bastien Walter, 03 Jul 2023
We thank the anonymous reviewer for his constructive comments on this paper. Most of them have been taken into account in the new version of the article and our responses are summarized below. We also addressed all the small comments (i.e. typos, missing words, notation for orientation, etc.) pointed out by the reviewer.
Specific comments:
- Presentation of the tectonic framework:
In the manuscript, the tectonic setting of study area is described in a two sentences in section 2 (lines 70-72 and 75-77) whereas it is judiciously integrated in the discussion to interpret the role of the fault system in the channelization of fluid flow. I recommend to make a more detailed presentation of the regional tectonics, including the southern part of the God Aad basin. The addition of a new figure (regional tectonic framework at a scale between the Figure 1 and its inset) would greatly help to present the study area and support the interpretations, especially the origin of the NNW-trending lineament and section 5.1.
- We reworked the presentation of the tectonic setting of the study area in section 2 (description of the regional scale structures, with regard to the main extension direction). As suggested, we added a tectonic map of the area to support this tectonic setting presentation. These additions contribute to reinforce the interpretations in section 5, especially on the role of the N-S structural features.
- Lineament naming and interpretation:
The author use two sets of Digital Elevation Models to map “structural” lineaments. I guess that the authors used morphological features like the limit of a crest or the geometry of the valley network to draw these lineaments. Thus, I would encourage the authors to describe these lineaments as they are, i.e. morphological features, in the result section and to interpret them as faults, which in likely, in the discussion.
- Regarding lineament naming and interpretation, we clarified our methodological approach in section 3. Based on studies using the same approach, we specified in this section how the lineaments were mapped and why they are all immediately considered as structural lineaments.
- Data analysis and presentation:
The authors use several characteristics of the lineaments (orientation and length) to describe the main and secondary structural trends. The values of the mean lineament length suggest strong disparities and I therefore propose to the authors to build a frequency graph for different length populations. I have the feeling it will allow to see a relationship between the length of the lineaments and their orientation. Each trend could later be compared with the regional structural orientations.
- Regarding lineaments length distribution analysis, we think such analysis is not possible with this dataset. A significant number of lineaments is incomplete, their length being cut by the mapping frame. We also think such analysis would require a larger study area, as well as at least one other mapping scale to be able to provide a significative lineament length distribution of the area. We therefore prefer not to add such results in this paper, but we will keep investigating this topic with additional data in the future.
- Figures
The figures need to be improved. First the hemisphere must be indicated in the geographic coordinates e.g. 41.90°E and 10.10°N. Moreover, it is not necessary to use three decimal digit in Figures 1, 4 and 9. Second, normal faults are represented with three different symbols. Please choose only one. Third, as mentioned above, you should better use morphological lineaments than structural ones. There are many other specific points to improve listed below.
- Figures were reworked. All the points were addressed.
Other points:
Lines 115-116: What do the grey and red areas in inset correspond ?
Figure 1 legend has been clarified;
Line 132: the petro-structural description is not used in this manuscript. No need to mention it.
We removed any mention about petrological analysis;
Lines 135-136: It seems that the UAV images were not used in the current study. If it is true, then this part should be removed.
We removed any mention about UAV images and survey, we only kept drone pictures in Fig.7 & 9 to support text description;
Lines 158-160: The distinction that is made in subsets is rather hard to see with the rose diagram in Fig. 2a, which instead shows a single trends with a maximum density around N100° and N120°. Maybe should you distinguish the long lineaments (the regional trend) that seem to have a N100-120° orientations and the short ones (secondary fault networks) that seem to have different orientations. See my comment on line 157.
Agreed, text was modified according to these comments;
Lines 177-178: Given the rose diagram for the chimney alignments, I would say that you have a main orientation (N110-120°) and two secondary orientations (N090-N100° and N060-N80°).
Agreed, text was modified according to these comments;
Lines 197-198: You should add in Figure 5a elevations for the base and the top of the scarps Line 199: denser fault spacing. Please quantify ; Figure 4: It would be useful to add a color scale for elevation instead of explaining the colors for the min and max elevations ; Line 211: It is said that both profiles have a uniform vertical exaggeration. Please specify the value of vertical exaggeration.in order to easily visualize the tectonic offset.
Done;
Lines 149-152: I agree that springs can be mapped from satellite images. Yet, how can you be sure that every springs that you identified are hot? ; Line 277-278: How can you be sure that the springs that you mapped with satellite images are all hot? How hot are they? Is there any temperature variation with respect to their location, e.g. close to the chimneys or not? According to Awaleh et al. (2015) some springs are only warm (T around 30°C). Do they consist to the springs that are far from the chimney? ; Line 283: How can you say that the chimneys are inactive as they have been formed in subaqueous conditions? Do you see any fumaroles at the top of the “active” ones? ; Line 371: How do you discriminate active and inactive chimneys ? Is it only related to the presence of hot springs in their vicinity? If you don’t have hot spring within the chimney or fumaroles, I think they are all inactive.
Hot spring description was reworked, based on literature in section 2 and on our field observations in section 4. According to Awaleh et al (2015), all the 16 springs measured in the Lake have T>70°C. The other ones mentioned in this paper are actually not located in this area according to the GPS coords. We confirm that we didn’t observe any colder springs in the area during fieldwork. Therefore, we make the assumption that all the springs, observed on the field and with satellite images in the LAGF have similar temperatures and can be all considered as hot springs. We also clarified in section 4 and 5 the link between active chimneys and springs. We only considered active chimneys when hot springs are found at the base of these chimneys. We were therefore able to distinguish those inactive (mostly in the eastern part of the LAGF) to the active ones and discuss the lateral evolution of the the hydrothermal field.
Line 309-310: The explanation is unclear. Tilted block usually develop with increasing amount of extension. A brief look on the tectonic map of the area suggests that extension is more important with the proximity of the God Aad basin. I therefore think that the geometry difference between the northern and southern parts results from a higher amount of extension rather than higher extension rates. ; Line 333: Do you really mean extension rate or amount of extension?
It was indeed a mistake from us, we didn’t mean extension rate in these paragraphs, but the overall amount of extension produced by the faults between the northern and southern areas;
Line 325 NNW instead of NWN
Done;
Line 445: NNW instead of NWN
Done;
Line 338-341: How can you explain the lack of hydrothermal occurrence along these putative N-S faults?
Lines 332-349: You discuss quite in detail the origin of the N-NNW trending faults and their origin related to relay ramps. You also propose that they may be related to larger structure of similar orientations that would separate the graben structures in the east and the LAGF in the west. An important information is missing. Indeed, you should mention the NNW trending faults that are observed south of the God Aad basin (see Figure 2 in Dekov et al. 2014 for instance). More generally, you should introduce, earlier in the manuscript, the regional tectonic structures.
Lines 407-424: In this section you propose a main role of the relay ramps in the fluid flow ascent. However, you forget the regional structures as those existing south of the God Aad basin (i.e. the NNW-trending faults). The interaction between a northern continuity of these faults (as suggested by the lineaments observed in the western part of the horst-and-graben area) and the ENE-trending fault seems to be much more likely than the speculative role of relay ramps.
The N-trending structures about 40km south to the study area (i.e. E-WFB) have been described in section 2. The influence of these structures with the N-S lineaments of the LAGF and the breaching faults has been discussed. Based on literature, we also discussed the fault kinematics with respect to the stress field over the area. N-S structures may therefore play as shear structures and represent permeability barrier for fluid flow. This may explain the lack of hydrothermal manifestation according to this orientation.
Line 445: NNW instead of NWN
Done;
Citation: https://doi.org/10.5194/egusphere-2023-397-AC1
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RC2: 'Comment on egusphere-2023-397', Anonymous Referee #2, 29 Mar 2023
This study integrate remote sensing data and field work to map the structures in God Aad graben. The mapped the linear structures, chimneys, and hot springs and suggested that the hydrohermal fluid evolution is controlled by structures. I personally like the work with validation from field observation. The wide spread hydrothermal feature could have some implication on active tectonics. Or the other way around. However, I have a few concerns on the methods and tectonic background of this work.
Surprisingly, tectonic background is not fully descried in section 2. located in Afar depression, God Aad graben and Lake Abhe have complex tectonic history of interaction between volcanism and extension, which definitely affect the evolution of the lake and control the structure development. This part is not talked about in the section2, nor in the discussion section. How does the structural evolution in the God Aad graben is related to the regional extensional phases and volcanism? What time scale do we estimate these structures, same as the 1-4 Ma basalts?
My second concern is that the Methods section is over simplified. I hope to see some details on data acquisition and interpretation and fault extraction. For example, it was mentioned that “lineament mapping was carried out on two digital elevation model”. But what criteria used to map the lineament, like offset, length, color contrast of the image? For faults with orientation changes, are they considered as two lineaments or one? There are multiple algorithm can be used to extract linear structure from dem or combination of optical images and dem (e.g., Ahmadi& Pekkan, 2021; Echeverria et al., 2022). Also, information of the UAV derived data is also missing. Is it optical image or point cloud derived dem? How was it processed. What is the coverage of the data?
Line 22 over what time scale?
Line 26: I believe not only EARS, south Africa also has geothermal potential.
Line 39: This is situation for Sub-Saharan Africa, how about Ethiopia-Djibouti?
Line 55: Without a map showing all these ridges and rifts, readers cannot locate them. I suggested to include a regional geological map cover all these geologic units and regional structure to help reader understand the geologic settings in the region.
Line 80: From Fig. 1, Lake Abhe seems to be bordered by rift faults, how does the fault development influence the the lake. How do you see it compared to rift lakes in East Africa?
Figure 1: How are these structural lineaments mapped? What does the red region in the inset map represent?
Line 148: How the lineaments is weighted by length?
Line 165: How to distinguish between lineament and chimneys alignments in the DEM or optical map?
Line 157: Will the faults cut by the mapping frame be considered in the rose diagram? How are they weighted by length?
Figure 3: Some of the lineaments are streams rather than structures. How does that influence the results?
Line 190: Field observation are mentioned here. How did the field work help validate the lineament extracted from DEM?
Line 210: How are the dipping direction of these faults decided? Based on field work, Google EARTH(Fig. 5c) or DEM.
Line 230-240: These describe on how to measure the alignment are great.
Line 267-290: The hot spring part seems not to be relevant to the topic of the work, the structural control of thermal features.
Line 309: Any evidence supporting this explanation? Like InSAR or GNSS data? Like Doubre et al.(2017). What kind of extension is refereed to here in Afar. If there were earthquakes, what about the focal mechanism?
Line 330: So how depth are the faults?
Line 341: Okay, the volcanic series. As a big player in Afar, it is worthy to discuss the role of volcanism in fracture development and thermal activity here.
Line 375: As there are multiple intersections, statistic analysis can be carried out to check the percentage of intersection is associated with chimney to support this point.
References
Ahmadi, H., & Pekkan, E. (2021). Fault-based geological lineaments extraction using remote sensing and GIS—a review. Geosciences, 11(5), 183.
Cécile Doubre, Aline Déprez, Frédéric Masson, Anne Socquet, Elias Lewi, Raphaël Grandin, Alexandre Nercessian, Patrice Ulrich, Jean-Bernard De Chabalier, Ibrahim Saad, Ahmadine Abayazid, Gilles Peltzer, Arthur Delorme, Eric Calais, Tim Wright, Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements, Geophysical Journal International, Volume 208, Issue 2, February 2017, Pages 936–953, https://doi.org/10.1093/gji/ggw434
Villalta Echeverria, M. D. P., Viña Ortega, A. G., Larreta, E., Romero Crespo, P., & Mulas, M. (2022). Lineament Extraction from Digital Terrain Derivate Model: A Case Study in the Girón–Santa Isabel Basin, South Ecuador. Remote Sensing, 14(21), 5400.
Citation: https://doi.org/10.5194/egusphere-2023-397-RC2 -
AC2: 'Reply on RC2', Bastien Walter, 03 Jul 2023
We thank the anonymous reviewer for his constructive comments on this paper. Most of them have been taken into account in the new version of the article and our responses are summarized below (in italics). We also addressed all the small comments (i.e. typos, missing words, notation for orientation, etc.) pointed out by the reviewer.
Surprisingly, tectonic background is not fully descried in section 2. located in Afar depression, God Aad graben and Lake Abhe have complex tectonic history of interaction between volcanism and extension, which definitely affect the evolution of the lake and control the structure development. This part is not talked about in the section2, nor in the discussion section. How does the structural evolution in the God Aad graben is related to the regional extensional phases and volcanism? What time scale do we estimate these structures, same as the 1-4 Ma basalts?
We reworked the presentation of the tectonic setting of the study area in section 2 (description of the regional scale structures, with regard to the main extension direction). As suggested, we added a tectonic map of the area to support this tectonic setting presentation. These additions contribute to reinforce the interpretations in section 5, especially on the role of the N-S structural features identified in the area. The kinematics of the fault structures is discussed with respect to the extensional axis of the deformation tensor over the area. The enhancement of the tectonic background in the paper helps to discuss the structural control on the LAGF fluid flow.
My second concern is that the Methods section is over simplified. I hope to see some details on data acquisition and interpretation and fault extraction. For example, it was mentioned that “lineament mapping was carried out on two digital elevation model”. But what criteria used to map the lineament, like offset, length, color contrast of the image? For faults with orientation changes, are they considered as two lineaments or one? There are multiple algorithm can be used to extract linear structure from dem or combination of optical images and dem (e.g., Ahmadi& Pekkan, 2021; Echeverria et al., 2022). Also, information of the UAV derived data is also missing. Is it optical image or point cloud derived dem? How was it processed. What is the coverage of the data?
We clarified our methodological approach in section 3. We specified in this section how the structural lineaments were manually mapped (“ tracing lineaments based on the observation of linear or slightly curvilinear shapes, elevation offsets highlighted by color contrasts or streams / wadis radical course changes”). We then clarified how the lineaments are considered (sets of segments with different orientation) and how they are processed (analysis of the distribution of the directions of all the line segments for a lineament dataset). We couldn’t use such algorithm mentioned but we thank the reviewer for this comment.
As mentioned by the reviewer, we also clarified in the paper how field observations (scarps morphology and topography) were used afterwards with satellite images to interpret the normal fault distribution in the area.
Also, no UAV derived data were used in this study. This was a miswriting, drone was only used for aerial pictures.
_________________
- Line 22 over what time scale?
No data to answer this question.
- Line 55: Without a map showing all these ridges and rifts, readers cannot locate them. I suggested to include a regional geological map cover all these geologic units and regional structure to help reader understand the geologic settings in the region ; What does the red region in the inset map represent?
A new figure to introduce regional tectonic structures was added. Figure 1 legend has been clarified, the red region representing Djibouti.
- Line 148: How the lineaments is weighted by length? ; Line 157: Will the faults cut by the mapping frame be considered in the rose diagram? How are they weighted by length?
Section 3 was rewritten. The accumulated length of the line segments for each direction bin (intervals of 10°) determines the size of each bin in the rose diagrams. Segments of faults cut by the mapping frames are included in the rose diagrams without distinction, in order to describe the lineament distribution in the mapping frames.
- Line 165: How to distinguish between lineament and chimneys alignments in the DEM or optical map?
The chimney alignments are clearly recognizable as they represent the only relief above the sedimentary flats. They can therefore be easily distinguished from the structural lineaments mapped in the basalt series.
- Figure 3: Some of the lineaments are streams rather than structures. How does that influence the results?
According to our interpretation, we think they are all structural lineaments (linear or slightly curvilinear shapes marked by or streams / wadis radical course changes)
-Line 190: Field observation are mentioned here. How did the field work help validate the lineament extracted from DEM? ; Line 210: How are the dipping direction of these faults decided? Based on field work, Google EARTH(Fig. 5c) or DEM.
Landscape observations helped to determine location of faults in the field, as well as their kinematics/dipping direction (cf. graben picture for instance). According to these observations and interpretations, we attributed normal fault features to some of the lineaments.
- Line 267-290: The hot spring part seems not to be relevant to the topic of the work, the structural control of thermal features.
The hot spring part of the paper contributes to extend the hydrothermal mapping area. those of the northwestern part of the LAGF being aligned with the main structural direction, without being associated to any chimney. It also contributes to discuss the spatial evolution of the fluid flow in the area, to help for geothermal development.
- Line 309: Any evidence supporting this explanation? Like InSAR or GNSS data? Like Doubre et al.(2017). What kind of extension is refereed to here in Afar. If there were earthquakes, what about the focal mechanism?
We rewrote this part of the paper, as well as section 2, with regard to Doubre et al (2017) work on the extension direction in this area and its influence on the local tectonic structures.
- Line 330: So how depth are the faults?
No data to answer this question.
- Line 375: As there are multiple intersections, statistic analysis can be carried out to check the percentage of intersection is associated with chimney to support this point.
We thank the reviewer for this comment. We would need however more time and a larger funding in order to acquire higher quality satellite images to be able to do such statistical analysis over the whole area.
Citation: https://doi.org/10.5194/egusphere-2023-397-AC2
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AC2: 'Reply on RC2', Bastien Walter, 03 Jul 2023
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-397', Anonymous Referee #1, 16 Mar 2023
General comment:
The manuscript of Walter et al. aims at determining the relationship between tectonic activity/structures and geothermal surface manifestations (hot springs and carbonate chimneys) in order to better constrain the geothermal potential of the Lake Abhe area. It is based on the analysis of morphological lineaments, field observations and the mapping of hot springs with satellite images. The manuscript suggests that the ascending geothermal fluids are primary channelized by intersecting faults. Albeit such a control of tectonic structures on the flow path is not new, its characterization in the study zone is essential for future geothermal developments.
Yet, the manuscript requires several improvements before publication (see below). I therefore recommend to published it after major revision.
Specific comments:
- Presentation of the tectonic framework:
In the manuscript, the tectonic setting of study area is described in a two sentences in section 2 (lines 70-72 and 75-77) whereas it is judiciously integrated in the discussion to interpret the role of the fault system in the channelization of fluid flow. I recommend to make a more detailed presentation of the regional tectonics, including the southern part of the God Aad basin. The addition of a new figure (regional tectonic framework at a scale between the Figure 1 and its inset) would greatly help to present the study area and support the interpretations, especially the origin of the NNW-trending lineament and section 5.1.
- Lineament naming and interpretation:
The author use two sets of Digital Elevation Models to map “structural” lineaments. I guess that the authors used morphological features like the limit of a crest or the geometry of the valley network to draw these lineaments. Thus, I would encourage the authors to describe these lineaments as they are, i.e. morphological features, in the result section and to interpret them as faults, which in likely, in the discussion.
- Data analysis and presentation:
The authors use several characteristics of the lineaments (orientation and length) to describe the main and secondary structural trends. The values of the mean lineament length suggest strong disparities and I therefore propose to the authors to build a frequency graph for different length populations. I have the feeling it will allow to see a relationship between the length of the lineaments and their orientation. Each trend could later be compared with the regional structural orientations.
- Figures
The figures need to be improved. First the hemisphere must be indicated in the geographic coordinates e.g. 41.90°E and 10.10°N. Moreover, it is not necessary to use three decimal digit in Figures 1, 4 and 9. Second, normal faults are represented with three different symbols. Please choose only one. Third, as mentioned above, you should better use morphological lineaments than structural ones. There are many other specific points to improve listed below.
Other points:
Lines 69-70: (Chorowicz, 2005 ; Fig. 1)
Line 71: The Tendaho graben trends NW-SE instead of NNW-SSE (Acocella et al., 2008)
Line 103: and/or
Line 105: that coats
Lines 115-116: What do the grey and red areas in inset correspond ?
Line 132: the petro-structural description is not used in this manuscript. No need to mention it.
Lines 135-136: It seems that the UAV images were not used in the current study. If it is true, then this part should be removed.
Line 137: studied area, morphological lineaments were mapped using high-resolution remote sensing data and interpreted as faults with knowledge acquired from field geological observations.
Line 139: models
Line 141: ~30 m
Line 143: 0.5 m resolution
Line 145: morphological lineaments
Lines 149-152: I agree that springs can be mapped from satellite images. Yet, how can you be sure that every springs that you identified are hot?
Line 155: mapping of morphological lineaments
Line 156: Ninety
Line 158: fourteen
Lines 158-160: The distinction that is made in subsets is rather hard to see with the rose diagram in Fig. 2a, which instead shows a single trends with a maximum density around N100° and N120°. Maybe should you distinguish the long lineaments (the regional trend) that seem to have a N100-120° orientations and the short ones (secondary fault networks) that seem to have different orientations. See my comment on line 157.
Line 159: (N090-N120°; Fig. 2a). (N070-N090°)
Line 160: (N120-N140°) (N170-180°)
Line 169: A total of 255 morphological lineaments was mapped
Line 171: seventeen
Lines 171-175: Same comment as for lines 158-160. Please modify the notation for the orientation (N090-N120° for instance)
Line 176: Seventy-four
Lines 177-178: Given the rose diagram for the chimney alignments, I would say that you have a main orientation (N110-120°) and two secondary orientations (N090-N100° and N060-N80°).
Line 178: (N060-N090°; Fig. 2c).
Lines 193-194: Here is the interpretation of the morphological lineaments.
Lines 197-198: You should add in Figure 5a elevations for the base and the top of the scarps Line 199: denser fault spacing. Please quantify.
Figure 4: It would be useful to add a color scale for elevation instead of explaining the colors for the min and max elevations.
Line 211: It is said that both profiles have a uniform vertical exaggeration. Please specify the value of vertical exaggeration.in order to easily visualize the tectonic offset.
Line 214: three viewpoints.
Line 238: Ninety one
Line 243: (N350-N020°), (N040-N080°), (N090-N120°)
Line 244: is consistent with the
Line 245: structural lineaments. It is important that you state on the possible origin of the morphological lineaments. Either they are faults or they have another origin. But once it is explained, then you can call them as they are.
Line 246: consistent directions with
Line 277-278: How can you be sure that the springs that you mapped with satellite images are all hot? How hot are they? Is there any temperature variation with respect to their location, e.g. close to the chimneys or not? According to Awaleh et al. (2015) some springs are only warm (T around 30°C). Do they consist to the springs that are far from the chimney?
Line 280: associated with
Line 283: How can you say that the chimneys are inactive as they have been formed in subaqueous conditions? Do you see any fumaroles at the top of the “active” ones?
Line 309-310: The explanation is unclear. Tilted block usually develop with increasing amount of extension. A brief look on the tectonic map of the area suggests that extension is more important with the proximity of the God Aad basin. I therefore think that the geometry difference between the northern and southern parts results from a higher amount of extension rather than higher extension rates.
Lines 322-326: The paper would be improve by the addition of an interpretative sketch map of the fault development.
Line 325 NNW instead of NWN
Line 333: Do you really mean extension rate or amount of extension?
Line 338-341: How can you explain the lack of hydrothermal occurrence along these putative N-S faults?
Lines 332-349: You discuss quite in detail the origin of the N-NNW trending faults and their origin related to relay ramps. You also propose that they may be related to larger structure of similar orientations that would separate the graben structures in the east and the LAGF in the west. An important information is missing. Indeed, you should mention the NNW trending faults that are observed south of the God Aad basin (see Figure 2 in Dekov et al. 2014 for instance). More generally, you should introduce, earlier in the manuscript, the regional tectonic structures.
Line 371: How do you discriminate active and inactive chimneys ? Is it only related to the presence of hot springs in their vicinity? If you don’t have hot spring within the chimney or fumaroles, I think they are all inactive.
Line 376: two areas.
Line 383: The age of
Line 399: they were not active
Lines 379-405: You never mention the large shield volcano located west of Lake. Is there any temporal relationship between the lake water level and the periods of activity? Is there any relationship between the age of the chimney and the periods of activity? I think these points are worse to discuss.
Lines 407-424: In this section you propose a main role of the relay ramps in the fluid flow ascent. However, you forget the regional structures as those existing south of the God Aad basin (i.e. the NNW-trending faults). The interaction between a northern continuity of these faults (as suggested by the lineaments observed in the western part of the horst-and-graben area) and the ENE-trending fault seems to be much more likely than the speculative role of relay ramps.
Line 445: NNW instead of NWN
Line 449: at depth
Line 452: the orientation distribution
Line 454-455: What about the role of the adjacent shield volcano?
Citation: https://doi.org/10.5194/egusphere-2023-397-RC1 -
AC1: 'Reply on RC1', Bastien Walter, 03 Jul 2023
We thank the anonymous reviewer for his constructive comments on this paper. Most of them have been taken into account in the new version of the article and our responses are summarized below. We also addressed all the small comments (i.e. typos, missing words, notation for orientation, etc.) pointed out by the reviewer.
Specific comments:
- Presentation of the tectonic framework:
In the manuscript, the tectonic setting of study area is described in a two sentences in section 2 (lines 70-72 and 75-77) whereas it is judiciously integrated in the discussion to interpret the role of the fault system in the channelization of fluid flow. I recommend to make a more detailed presentation of the regional tectonics, including the southern part of the God Aad basin. The addition of a new figure (regional tectonic framework at a scale between the Figure 1 and its inset) would greatly help to present the study area and support the interpretations, especially the origin of the NNW-trending lineament and section 5.1.
- We reworked the presentation of the tectonic setting of the study area in section 2 (description of the regional scale structures, with regard to the main extension direction). As suggested, we added a tectonic map of the area to support this tectonic setting presentation. These additions contribute to reinforce the interpretations in section 5, especially on the role of the N-S structural features.
- Lineament naming and interpretation:
The author use two sets of Digital Elevation Models to map “structural” lineaments. I guess that the authors used morphological features like the limit of a crest or the geometry of the valley network to draw these lineaments. Thus, I would encourage the authors to describe these lineaments as they are, i.e. morphological features, in the result section and to interpret them as faults, which in likely, in the discussion.
- Regarding lineament naming and interpretation, we clarified our methodological approach in section 3. Based on studies using the same approach, we specified in this section how the lineaments were mapped and why they are all immediately considered as structural lineaments.
- Data analysis and presentation:
The authors use several characteristics of the lineaments (orientation and length) to describe the main and secondary structural trends. The values of the mean lineament length suggest strong disparities and I therefore propose to the authors to build a frequency graph for different length populations. I have the feeling it will allow to see a relationship between the length of the lineaments and their orientation. Each trend could later be compared with the regional structural orientations.
- Regarding lineaments length distribution analysis, we think such analysis is not possible with this dataset. A significant number of lineaments is incomplete, their length being cut by the mapping frame. We also think such analysis would require a larger study area, as well as at least one other mapping scale to be able to provide a significative lineament length distribution of the area. We therefore prefer not to add such results in this paper, but we will keep investigating this topic with additional data in the future.
- Figures
The figures need to be improved. First the hemisphere must be indicated in the geographic coordinates e.g. 41.90°E and 10.10°N. Moreover, it is not necessary to use three decimal digit in Figures 1, 4 and 9. Second, normal faults are represented with three different symbols. Please choose only one. Third, as mentioned above, you should better use morphological lineaments than structural ones. There are many other specific points to improve listed below.
- Figures were reworked. All the points were addressed.
Other points:
Lines 115-116: What do the grey and red areas in inset correspond ?
Figure 1 legend has been clarified;
Line 132: the petro-structural description is not used in this manuscript. No need to mention it.
We removed any mention about petrological analysis;
Lines 135-136: It seems that the UAV images were not used in the current study. If it is true, then this part should be removed.
We removed any mention about UAV images and survey, we only kept drone pictures in Fig.7 & 9 to support text description;
Lines 158-160: The distinction that is made in subsets is rather hard to see with the rose diagram in Fig. 2a, which instead shows a single trends with a maximum density around N100° and N120°. Maybe should you distinguish the long lineaments (the regional trend) that seem to have a N100-120° orientations and the short ones (secondary fault networks) that seem to have different orientations. See my comment on line 157.
Agreed, text was modified according to these comments;
Lines 177-178: Given the rose diagram for the chimney alignments, I would say that you have a main orientation (N110-120°) and two secondary orientations (N090-N100° and N060-N80°).
Agreed, text was modified according to these comments;
Lines 197-198: You should add in Figure 5a elevations for the base and the top of the scarps Line 199: denser fault spacing. Please quantify ; Figure 4: It would be useful to add a color scale for elevation instead of explaining the colors for the min and max elevations ; Line 211: It is said that both profiles have a uniform vertical exaggeration. Please specify the value of vertical exaggeration.in order to easily visualize the tectonic offset.
Done;
Lines 149-152: I agree that springs can be mapped from satellite images. Yet, how can you be sure that every springs that you identified are hot? ; Line 277-278: How can you be sure that the springs that you mapped with satellite images are all hot? How hot are they? Is there any temperature variation with respect to their location, e.g. close to the chimneys or not? According to Awaleh et al. (2015) some springs are only warm (T around 30°C). Do they consist to the springs that are far from the chimney? ; Line 283: How can you say that the chimneys are inactive as they have been formed in subaqueous conditions? Do you see any fumaroles at the top of the “active” ones? ; Line 371: How do you discriminate active and inactive chimneys ? Is it only related to the presence of hot springs in their vicinity? If you don’t have hot spring within the chimney or fumaroles, I think they are all inactive.
Hot spring description was reworked, based on literature in section 2 and on our field observations in section 4. According to Awaleh et al (2015), all the 16 springs measured in the Lake have T>70°C. The other ones mentioned in this paper are actually not located in this area according to the GPS coords. We confirm that we didn’t observe any colder springs in the area during fieldwork. Therefore, we make the assumption that all the springs, observed on the field and with satellite images in the LAGF have similar temperatures and can be all considered as hot springs. We also clarified in section 4 and 5 the link between active chimneys and springs. We only considered active chimneys when hot springs are found at the base of these chimneys. We were therefore able to distinguish those inactive (mostly in the eastern part of the LAGF) to the active ones and discuss the lateral evolution of the the hydrothermal field.
Line 309-310: The explanation is unclear. Tilted block usually develop with increasing amount of extension. A brief look on the tectonic map of the area suggests that extension is more important with the proximity of the God Aad basin. I therefore think that the geometry difference between the northern and southern parts results from a higher amount of extension rather than higher extension rates. ; Line 333: Do you really mean extension rate or amount of extension?
It was indeed a mistake from us, we didn’t mean extension rate in these paragraphs, but the overall amount of extension produced by the faults between the northern and southern areas;
Line 325 NNW instead of NWN
Done;
Line 445: NNW instead of NWN
Done;
Line 338-341: How can you explain the lack of hydrothermal occurrence along these putative N-S faults?
Lines 332-349: You discuss quite in detail the origin of the N-NNW trending faults and their origin related to relay ramps. You also propose that they may be related to larger structure of similar orientations that would separate the graben structures in the east and the LAGF in the west. An important information is missing. Indeed, you should mention the NNW trending faults that are observed south of the God Aad basin (see Figure 2 in Dekov et al. 2014 for instance). More generally, you should introduce, earlier in the manuscript, the regional tectonic structures.
Lines 407-424: In this section you propose a main role of the relay ramps in the fluid flow ascent. However, you forget the regional structures as those existing south of the God Aad basin (i.e. the NNW-trending faults). The interaction between a northern continuity of these faults (as suggested by the lineaments observed in the western part of the horst-and-graben area) and the ENE-trending fault seems to be much more likely than the speculative role of relay ramps.
The N-trending structures about 40km south to the study area (i.e. E-WFB) have been described in section 2. The influence of these structures with the N-S lineaments of the LAGF and the breaching faults has been discussed. Based on literature, we also discussed the fault kinematics with respect to the stress field over the area. N-S structures may therefore play as shear structures and represent permeability barrier for fluid flow. This may explain the lack of hydrothermal manifestation according to this orientation.
Line 445: NNW instead of NWN
Done;
Citation: https://doi.org/10.5194/egusphere-2023-397-AC1
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RC2: 'Comment on egusphere-2023-397', Anonymous Referee #2, 29 Mar 2023
This study integrate remote sensing data and field work to map the structures in God Aad graben. The mapped the linear structures, chimneys, and hot springs and suggested that the hydrohermal fluid evolution is controlled by structures. I personally like the work with validation from field observation. The wide spread hydrothermal feature could have some implication on active tectonics. Or the other way around. However, I have a few concerns on the methods and tectonic background of this work.
Surprisingly, tectonic background is not fully descried in section 2. located in Afar depression, God Aad graben and Lake Abhe have complex tectonic history of interaction between volcanism and extension, which definitely affect the evolution of the lake and control the structure development. This part is not talked about in the section2, nor in the discussion section. How does the structural evolution in the God Aad graben is related to the regional extensional phases and volcanism? What time scale do we estimate these structures, same as the 1-4 Ma basalts?
My second concern is that the Methods section is over simplified. I hope to see some details on data acquisition and interpretation and fault extraction. For example, it was mentioned that “lineament mapping was carried out on two digital elevation model”. But what criteria used to map the lineament, like offset, length, color contrast of the image? For faults with orientation changes, are they considered as two lineaments or one? There are multiple algorithm can be used to extract linear structure from dem or combination of optical images and dem (e.g., Ahmadi& Pekkan, 2021; Echeverria et al., 2022). Also, information of the UAV derived data is also missing. Is it optical image or point cloud derived dem? How was it processed. What is the coverage of the data?
Line 22 over what time scale?
Line 26: I believe not only EARS, south Africa also has geothermal potential.
Line 39: This is situation for Sub-Saharan Africa, how about Ethiopia-Djibouti?
Line 55: Without a map showing all these ridges and rifts, readers cannot locate them. I suggested to include a regional geological map cover all these geologic units and regional structure to help reader understand the geologic settings in the region.
Line 80: From Fig. 1, Lake Abhe seems to be bordered by rift faults, how does the fault development influence the the lake. How do you see it compared to rift lakes in East Africa?
Figure 1: How are these structural lineaments mapped? What does the red region in the inset map represent?
Line 148: How the lineaments is weighted by length?
Line 165: How to distinguish between lineament and chimneys alignments in the DEM or optical map?
Line 157: Will the faults cut by the mapping frame be considered in the rose diagram? How are they weighted by length?
Figure 3: Some of the lineaments are streams rather than structures. How does that influence the results?
Line 190: Field observation are mentioned here. How did the field work help validate the lineament extracted from DEM?
Line 210: How are the dipping direction of these faults decided? Based on field work, Google EARTH(Fig. 5c) or DEM.
Line 230-240: These describe on how to measure the alignment are great.
Line 267-290: The hot spring part seems not to be relevant to the topic of the work, the structural control of thermal features.
Line 309: Any evidence supporting this explanation? Like InSAR or GNSS data? Like Doubre et al.(2017). What kind of extension is refereed to here in Afar. If there were earthquakes, what about the focal mechanism?
Line 330: So how depth are the faults?
Line 341: Okay, the volcanic series. As a big player in Afar, it is worthy to discuss the role of volcanism in fracture development and thermal activity here.
Line 375: As there are multiple intersections, statistic analysis can be carried out to check the percentage of intersection is associated with chimney to support this point.
References
Ahmadi, H., & Pekkan, E. (2021). Fault-based geological lineaments extraction using remote sensing and GIS—a review. Geosciences, 11(5), 183.
Cécile Doubre, Aline Déprez, Frédéric Masson, Anne Socquet, Elias Lewi, Raphaël Grandin, Alexandre Nercessian, Patrice Ulrich, Jean-Bernard De Chabalier, Ibrahim Saad, Ahmadine Abayazid, Gilles Peltzer, Arthur Delorme, Eric Calais, Tim Wright, Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements, Geophysical Journal International, Volume 208, Issue 2, February 2017, Pages 936–953, https://doi.org/10.1093/gji/ggw434
Villalta Echeverria, M. D. P., Viña Ortega, A. G., Larreta, E., Romero Crespo, P., & Mulas, M. (2022). Lineament Extraction from Digital Terrain Derivate Model: A Case Study in the Girón–Santa Isabel Basin, South Ecuador. Remote Sensing, 14(21), 5400.
Citation: https://doi.org/10.5194/egusphere-2023-397-RC2 -
AC2: 'Reply on RC2', Bastien Walter, 03 Jul 2023
We thank the anonymous reviewer for his constructive comments on this paper. Most of them have been taken into account in the new version of the article and our responses are summarized below (in italics). We also addressed all the small comments (i.e. typos, missing words, notation for orientation, etc.) pointed out by the reviewer.
Surprisingly, tectonic background is not fully descried in section 2. located in Afar depression, God Aad graben and Lake Abhe have complex tectonic history of interaction between volcanism and extension, which definitely affect the evolution of the lake and control the structure development. This part is not talked about in the section2, nor in the discussion section. How does the structural evolution in the God Aad graben is related to the regional extensional phases and volcanism? What time scale do we estimate these structures, same as the 1-4 Ma basalts?
We reworked the presentation of the tectonic setting of the study area in section 2 (description of the regional scale structures, with regard to the main extension direction). As suggested, we added a tectonic map of the area to support this tectonic setting presentation. These additions contribute to reinforce the interpretations in section 5, especially on the role of the N-S structural features identified in the area. The kinematics of the fault structures is discussed with respect to the extensional axis of the deformation tensor over the area. The enhancement of the tectonic background in the paper helps to discuss the structural control on the LAGF fluid flow.
My second concern is that the Methods section is over simplified. I hope to see some details on data acquisition and interpretation and fault extraction. For example, it was mentioned that “lineament mapping was carried out on two digital elevation model”. But what criteria used to map the lineament, like offset, length, color contrast of the image? For faults with orientation changes, are they considered as two lineaments or one? There are multiple algorithm can be used to extract linear structure from dem or combination of optical images and dem (e.g., Ahmadi& Pekkan, 2021; Echeverria et al., 2022). Also, information of the UAV derived data is also missing. Is it optical image or point cloud derived dem? How was it processed. What is the coverage of the data?
We clarified our methodological approach in section 3. We specified in this section how the structural lineaments were manually mapped (“ tracing lineaments based on the observation of linear or slightly curvilinear shapes, elevation offsets highlighted by color contrasts or streams / wadis radical course changes”). We then clarified how the lineaments are considered (sets of segments with different orientation) and how they are processed (analysis of the distribution of the directions of all the line segments for a lineament dataset). We couldn’t use such algorithm mentioned but we thank the reviewer for this comment.
As mentioned by the reviewer, we also clarified in the paper how field observations (scarps morphology and topography) were used afterwards with satellite images to interpret the normal fault distribution in the area.
Also, no UAV derived data were used in this study. This was a miswriting, drone was only used for aerial pictures.
_________________
- Line 22 over what time scale?
No data to answer this question.
- Line 55: Without a map showing all these ridges and rifts, readers cannot locate them. I suggested to include a regional geological map cover all these geologic units and regional structure to help reader understand the geologic settings in the region ; What does the red region in the inset map represent?
A new figure to introduce regional tectonic structures was added. Figure 1 legend has been clarified, the red region representing Djibouti.
- Line 148: How the lineaments is weighted by length? ; Line 157: Will the faults cut by the mapping frame be considered in the rose diagram? How are they weighted by length?
Section 3 was rewritten. The accumulated length of the line segments for each direction bin (intervals of 10°) determines the size of each bin in the rose diagrams. Segments of faults cut by the mapping frames are included in the rose diagrams without distinction, in order to describe the lineament distribution in the mapping frames.
- Line 165: How to distinguish between lineament and chimneys alignments in the DEM or optical map?
The chimney alignments are clearly recognizable as they represent the only relief above the sedimentary flats. They can therefore be easily distinguished from the structural lineaments mapped in the basalt series.
- Figure 3: Some of the lineaments are streams rather than structures. How does that influence the results?
According to our interpretation, we think they are all structural lineaments (linear or slightly curvilinear shapes marked by or streams / wadis radical course changes)
-Line 190: Field observation are mentioned here. How did the field work help validate the lineament extracted from DEM? ; Line 210: How are the dipping direction of these faults decided? Based on field work, Google EARTH(Fig. 5c) or DEM.
Landscape observations helped to determine location of faults in the field, as well as their kinematics/dipping direction (cf. graben picture for instance). According to these observations and interpretations, we attributed normal fault features to some of the lineaments.
- Line 267-290: The hot spring part seems not to be relevant to the topic of the work, the structural control of thermal features.
The hot spring part of the paper contributes to extend the hydrothermal mapping area. those of the northwestern part of the LAGF being aligned with the main structural direction, without being associated to any chimney. It also contributes to discuss the spatial evolution of the fluid flow in the area, to help for geothermal development.
- Line 309: Any evidence supporting this explanation? Like InSAR or GNSS data? Like Doubre et al.(2017). What kind of extension is refereed to here in Afar. If there were earthquakes, what about the focal mechanism?
We rewrote this part of the paper, as well as section 2, with regard to Doubre et al (2017) work on the extension direction in this area and its influence on the local tectonic structures.
- Line 330: So how depth are the faults?
No data to answer this question.
- Line 375: As there are multiple intersections, statistic analysis can be carried out to check the percentage of intersection is associated with chimney to support this point.
We thank the reviewer for this comment. We would need however more time and a larger funding in order to acquire higher quality satellite images to be able to do such statistical analysis over the whole area.
Citation: https://doi.org/10.5194/egusphere-2023-397-AC2
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AC2: 'Reply on RC2', Bastien Walter, 03 Jul 2023
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