the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Mar 2024
Fluid-rock interaction in the intraplate active seismic zone: Boon or bane?
Abstract. The Koyna-Warna Seismogenic Region of western India has been recognized as one of the hotspots for reservoir-triggered seismicity (RTS) since 1967. The current study investigates the fluid's interaction with the severely fractured granitoid basement of this area and its potential contribution to the recurring seismicity. The presence of several secondary minerals, such as chlorite, epidote, calcite, illite, etc., along the pre-existing faults and fractures, is revealed by detailed petrologic investigation at mesoscopic and microscopic scales along with XRD analysis. This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration under acidic to neutral conditions (pH 5.5–7) and the temperature of above 200–220 °C up to about 350 °C. Additionally, the transformation of biotite into chlorite due to fluid interaction has been inferred from the microscopic appearance of biotitic remnant within neoformed chlorite which is further supported by the mass loss of K2O and concurrent gain of MgO and FeO, demonstrating the replacement of potassium (K) interlayer sheet by brucite-like [Mg (OH)2] layer during biotite chloritization. However, this released K2O further assists in the formation of illite resulting in the mass gain of K2O at a few certain depths, whereas the dissolution of plagioclase justifies the formation of albite and calcite as evidenced by the gain of Na2O and CaO. The present study also highlights that the recurring nature of the seismicity in this area may be related to clay mineralization along the faults and fractures due to fluid-rock interaction, such as chlorite, illite, etc., in addition to the existing fault geometry and stress build-up due to reservoir impoundment. At increasing stress condition, the anisotropic and weakly bonded, layered crystal structure of chlorite forming ripplocations may develop kink bands and increases the yield strength proportionally with rising pressure up to dehydration temperature. Such visco-elastic behaviour of chlorite may promote aseismic creep in the faults. On the other hand, epidote noticed at certain depths has a contrasting behaviour; it tends to wear at the micron or submicron‐scale asperity contacts and produce fine particles which generate unstable sliding. However, the relatively higher abundance of chlorite in the faults and fractures disrupts the epidote‐epidote contact asperities and prevents such wearing of epidote grains into fine particles. Thus, biotite chloritization in conjunction with relatively less production of epidote along pre-existing faults and fractures helps to release the accumulated stress through a series of small-scale earthquakes and results in the steady fault creep observed in this region during the past 50 years. In this context, fluid-rock interaction along the pre-existing faults and fractures at shallow depth has acted as a blessing for the Koyna-Warna Seismogenic region shielding it from relatively large magnitude earthquakes – a boon for the region.
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RC1: 'Comment on egusphere-2023-2553', Anonymous Referee #1, 02 Apr 2024
The paper presents the results of mineralogical and geochemical analyses to infer fluid-rock interactions and their potential contribution to recurring seismicity in the Koyna-Warna Seismogenic Region of western India. It is a topic of interest for Seismology and Geology, but I am not in the favor of publication of this manuscript in its current form. There are several areas that require clarification, expansion, or improvement to enhance the quality.
L14: XRD, acronyms should be introduced the first time you use the term.
L16: I didn’t find any discussion or interpretation to support the point about the acidic to neutral alteration conditions (pH 5.5-7) in the main text. How did you know the pH value?
L17: Similarly, the inferences about the alteration temperatures are missing.
L156-160: How was the transition from the faulting to normal faulting observed? Some references may need to be cited to support this transition and its potential relation with reservoir impoundment.
L169-200: The authors introduced several faults in the study area. Which fault does the borehole pass through? It is also necessary to specify which part of the faults the samples came from, e.g., the principal slip zone or damage zone?
Figure 3d: Is it fault gouge?
L173, L177: Delete the full stop before the parenthesis of the citations. Please check throughout.
L184: “litholog”→ lithology. Please check throughout for errors.
L270-271: Quartz with chess-board pattern generally reflects dislocation creep during high-temperature deformation. This mismatch with the presence of chlorite and epidote which indicates relatively low-temperature alteration.
L463-478: Given the dissolution of plagioclase and potential release of Al, why not try TiO2 as the immobile element for the isocon plot?
L473: I am not going to agree that the precipitation of calcite is caused by the dissolution of plagioclase. As shown in Figure 6c, calcites are precipitated as calcite veins along fractures. This is generally considered to be related to precipitation from hydrothermal fluids. More evidence is required to indicate the source of calcite.
L647: “although” and “but” cannot be used at the same time. There are many grammar errors here and elsewhere, and authors have to check to ensure that the language used throughout the paper is clear, concise, and grammatically correct.
Citation: https://doi.org/10.5194/egusphere-2023-2553-RC1 -
AC1: 'Reply on RC1', Piyal Halder, 04 Apr 2024
Respected Sir/Ma’am,
We appreciate the time and effort that you have dedicated and are grateful for your insightful comments which have improved the quality of the manuscript as well as helped us to identify many silly mistakes. We are also sorry for such mistakes and will keep in mind in future. We are responding to all of your queries and concerns for your kind consideration. As there is no provision to upload the revised MS at this stage, the revised MS will be uploaded after the completion of the review process. We shall incorporate all of the suggestions and modifications in the revised MS. We are requesting you kindly consider our response. Your kind approval regarding the acceptance of the submitted MS will be highly appreciated. We shall be grateful for your kind perusal.Regards,
Yours Sincerely,
Piyal Halder (First author) & Dr. Anupam Sharma (Corresponding author)
RESPONSE TO THE SUGGESTIONS AND QUERIES
L14: XRD, acronyms should be introduced the first time you use the term.
- Thanks for pointing out such a mistake. The backronym of XRD i.e., X-Ray Diffraction has been inserted in the revised manuscript which will be uploaded shortly after the completion of the review process.
L16: I didn’t find any discussion or interpretation to support the point about the acidic to neutral alteration conditions (pH 5.5-7) in the main text. How did you know the pH value?
L17: Similarly, the inferences about the alteration temperatures are missing.
- Thanks for pointing out this flaw, otherwise it would mislead the readers. Actually, the temp. and pH mentioned in the text are characteristics of the propylitic grade of hydrothermal alteration (Fulignati, 2020). Hence, instead of mentioning a particular temp. and pH, a range has been provided. Relevant Citations have been already mentioned in the discussion part of the submitted MS file (L498-L500). However, to avoid any confusion, the line relevant to this statement in the abstract section has been modified in the following manner-
“This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration which characterizes acidic to neutral conditions (pH 5.5-7) and the temperature of above 200-220 °C up to about 350 °C”
L156-160: How was the transition from the faulting to normal faulting observed? Some references may need to be cited to support this transition and its potential relation with reservoir impoundment.
- The evidence of fault transition from normal to strike-slip pattern and vice versa has been obtained from the inversion of 50 focal mechanism solutions of earthquakes that occurred during the last 45 years in the Koyna–Warna region (Rao and Shashidhar, 2016). The relevant citation has been included in the text of the revised MS.
L169-200: The authors introduced several faults in the study area. Which fault does the borehole pass through? It is also necessary to specify which part of the faults the samples came from, e.g., the principal slip zone or damage zone?
- I am overwhelmed by your detailed review. These details are required for a reader to understand the entire research perspective. I am extremely sorry for such incompleteness in the information. The borehole KBH1 has cut numerous small-scale faults of the Koyna River fault Zone (KRFZ) which is already shown in 1 of the submitted manuscript. Additionally, the same has been inserted in the sampling section of the revised MS, which will be uploaded later after the completion of the review process. The altered samples are collected from the slip surface of these small-scale faults (fault core) as mentioned in the L192, L194, L206, and L286. The same has also been inserted in the above-mentioned line for a clearer understanding as per your suggestions.
“The altered samples from the slip planes of the small scale-faults of the KRFZ have been collected through mesoscopic observations”
Figure 3d: Is it fault gouge?
- The figure is the mesoscopic observation of the sample KBH1_379 collected from 1073 m depth. The physical appearance as well as the presence of clay minerals, identified in the XRD (Fig. 7c & d), infers that the sample may be a fault gouge or clayey gouge. Hence, the above-mentioned line has been modified in the revised MS as below-
“Clayey fault gouge reflecting the mechanical disintegration during fault slip followed by the clay formation due to fluid-rock interaction”.
The relevant line in the (L195-L196) main text has also been modified accordingly.
L173, L177: Delete the full stop before the parenthesis of the citations. Please check throughout.
- Thanks for pointing out such silly mistakes. Such mistakes have been noted and rectified as per your suggestions.
L184: “litholog” → lithology. Please check throughout for errors.
- Here, the “litholog” of the studied borehole has been shown at the side of the schematic diagram. The litholog exhibits the distribution of the porous and vesicular Deccan basalt upto 932 m depth which is underlain by the granitoid basement rocks.
L270-271: Quartz with chess-board pattern generally reflects dislocation creep during high-temperature deformation. This mismatch with the presence of chlorite and epidote which indicates relatively low-temperature alteration.
- I agree with your observations. However, the basement rocks in the study area have undergone three phases of deformation. The first of which corresponds to the development of gneissosity in the granitic rocks, the second was the shearing events that caused pre-existing fabrics (S1) to be overprinted and transposed by the high-strain S2 fabrics, and the third was the emergence of anastomosing fractures as a function of brittle deformation such as faulting. The fractures cutting the plagioclase grains and fillings of chlorite and calcite throughout the network of fractures, as documented in our study (Fig. 4f, g), indicate that the percolation of fluid and subsequent formation or precipitation of secondary minerals along the fractures took place after the third deformation event in which faulting occurred and fractures were developed. So, the chessboard pattern of quartz as well as the marker offset within plagioclase indicates the third deformation event which gave rise to fractures. Later, fluid percolated through these fractures and resulted in the formation of secondary minerals. Hence, the presence of chlorite, illite as well as precipitation of calcite can’t be correlated with the deformation pattern of quartz. (Please see L556-L570 in the submitted MS)
L463-478: Given the dissolution of plagioclase and potential release of Al, why not try TiO2 as the immobile element for the isocon plot?
- The mass balance calculation and isocon plots have been executed following Grant (2005). The equation of an isocon is,
CA=(MO/MA) CO
[CA= concentration of an element in altered rock
CO= concentration of an element in unaltered rock
MO= Mass before alteration
MA= Mass after alteration]
Here, the MO/MA is the slope of the isocon and the line with this slope indicates no gain and no loss in terms of mass. The equation is similar to the straight-line equation with zero intercept. So, this method employs the plotting of isocon in two different ways-
(i) The first method involves clustering CA and CO values along the y and x axis respectively. Later, the best-fit line is drawn through the origin by graphically searching the elements that lie on or near the line. This line is the isocon line and is named in the present study as the Best-fit isocon (Grant, 1986). So, there is no way to fit the isocon manually in this method. However, if we see the best-fit isocons in all the altered samples, we notice that most of the best-fit isocons have passed near the TiO2. So, it is evident that TiO2 is almost immobile but not completely immobile.
(ii) However, there is another method for drawing the isocon line, in which the line is extended by clustering and selecting the elements that show minimum gain/loss with respect to other elements, i.e. [(CA-Co)/ CO] will be minimum. This isocon has been named in the present study as the Immobility Isocon. For this purpose, an average of the concentrations of each element in all the unaltered samples (Co) is calculated. Then gain or loss of that particular element in the altered sample is estimated with respect to the Co. For example, the Co value of Al2O3 is 15.12 and for TiO2 is 0.39. In one altered sample, the concentration of Al2O3 is 21.34 and 1.46. So, the gain of Al2O3 and TiO2 are 6.22 and 1.07 respectively. But with respect to Co, there are gains of 0.41 and 2.73 for Al2O3 and TiO2 respectively [Please see table S1 in the supplementary file]. The same approach applies to other altered samples. In all the cases, Al2O3 has shown the minimum gain or loss with respect to Co [Please see Table S1 in the supplementary file]. Hence, immobility isocon has been drawn mostly through Al2O3 rather than TiO2. In the latter case (isocon drawing through TiO2), a priori assumption is needed regarding the immobility of TiO2. Such priori assumption may give misinformation regarding mass gain or loss. In fact, isocon drawn exactly through TiO2 will show a loss of CaO in samples KBH1_378 and KBH1_379 which is contrary to the evidence of calcite precipitation. Similarly, in KBH1_381, if the isocon is drawn through TiO2, it will represent the loss of CaO, MgO and FeO, which will contradict the findings of chlorite in XRD.
Moreover, this method provides a holistic view of the mobility of elements throughout the geologic section or system (so an average is taken). Also, Al activity in the hydrothermal fluid is restricted, it is mobile on the mineral scale, and it is immobile on a rock scale (Parneix et al., 1985). Most importantly, the findings emanated from immobility isocon match with the findings found in the case of the best-fit isocon method, which is completely statistically derived and doesn’t involve any manual interference. That is why, the isocons have been drawn through Al2O3 is more reliable rather than through TiO2.
L473: I am not going to agree that the precipitation of calcite is caused by the dissolution of plagioclase. As shown in Figure 6c, calcites are precipitated as calcite veins along fractures. This is generally considered to be related to precipitation from hydrothermal fluids. More evidence is required to indicate the source of calcite.
I agree with your view that calcite is precipitated by the hydrothermal fluid. In our study, the heat derived from the faulting and fracturing must have contributed to this hydrothermal alteration (because the temperature at the depth of this study is less than 150 °C; See L501-L505 of the preprint file). So, there may be two sources of Ca in the hydrothermal fluid-
(i) One source is the overlying basalt which contains CaO of 8.76–11.7 wt% (Banerjee and Mondal, 2021). The meteoric water percolated through the basaltic rocks dissolves this Ca and transports it to the fracture or permeable faults, where the fluid deposits it in the form of calcite by degassing after heating.
(ii) Another source is the Ca2+ released due to plagioclase dissolution in the granitoids. In this case, the Ca2+ released due to plagioclase dissolution is mixed up with the descending meteoric water which gets hot by the heat generated by the faulting and fracturing and leads to the precipitation of calcite by degassing.
This is why, the probability of plagioclase dissolution as the source of calcite can’t be ruled out. Hence, the above-mentioned line will be modified in a below-cited manner in the revised ms-
“Moreover, during this process along with Al3+, Ca2+ is also released which may act as one of the sources of calcite precipitated and elucidates the gain of CaO in the altered samples. However, considering the CaO content (8.76–11.7 wt%) of the overlying basalt and the basalt’s high weathering potential, the contribution of the Deccan Trap basalt in the introduction of Ca2+ in the hydrothermal fluid must be taken into account”
L647: “although” and “but” cannot be used at the same time. There are many grammar errors here and elsewhere, and authors have to check to ensure that the language used throughout the paper is clear, concise, and grammatically correct.
- I am sorry for such carelessness. The same has been noted with care and will be rectified in the revised MS which will be submitted after the completion of the review.
References:
Banerjee, R. and Mondal, S.K., 2021. Petrology and geochemistry of the Deccan basalts from the KBH-7 borehole, Koyna Seismic Zone (Western Ghats, India): Implications for nature of crustal contamination and sulfide saturation of magma. Lithos, 380-381: 105864.
Fulignati, P., 2020. Clay Minerals in Hydrothermal Systems. Minerals, 10(10): 919.
Grant, J., 2005. Isocon analysis: A brief review of the method and applications. Physics and Chemistry of the Earth, Parts A/B/C, 30.
Parneix, J.C., Beaufort, D., Dudoignon, P. and Meunier, A., 1985. Biotite chloritization process in hydrothermally altered granites. Chemical Geology, 51(1): 89-101.
Rao, N. and Shashidhar, D., 2016. Periodic variation of stress field in the Koyna–Warna reservoir triggered seismic zone inferred from focal mechanism studies. Tectonophysics, 679.
[The same has been uploaded in .pdf format.
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AC1: 'Reply on RC1', Piyal Halder, 04 Apr 2024
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RC2: 'Comment on egusphere-2023-2553', Anonymous Referee #2, 12 Apr 2024
In the article titled "Fluid-rock interaction in the intraplate active seismic zone: Boon or bane?" the authors have implemented several analytical techniques to assess the chemo-mechanical evolution of fluid infiltrated reactivated faults in the Koyena-Warna Seismogenic region of Western India. The objective of the article is to discuss the impact of secondary mineralization on the frictional behavior of reactivated faults.
While the authors have presented quite an exhaustive mineralogical analysis of the KBH-1 borehole from different depths, the discussion and interpretations from the analytical results could be more enriching in terms of the mineralogical control over the frictional evolution of fluid infiltrated fault zones. The velocity strengthening behavior of phyllosilicates and related aseismicity is a well-established fact, and simply discussing the effects of phyllosilicate content on the friction at fault interfaces based on existing literature does not contribute much towards the current understanding of the rate-and-state friction laws or aseismic/seismic slip. Thus, the article, at its present state, is not novel enough to be acceptable for publication. However, the topic itself is exciting and the field area is of high geological importance. A few suggestions in this regard may help the authors to better shape their manuscript
- Reaction path modeling: The authors have used two alteration metrices to distinguish the type of alteration at different depths along the borehole. However, these alteration indices are largely empirical. Therefore, more emphasis should be placed on interpreting the alteration mechanisms obtained from these empirical values. For example, in 10c, the authors have shown two different alteration mechanisms (Carbonization and Hydrothermal alteration) along different depth profiles of the borehole. The discussion should focus on the interplay between these two mechanisms, especially when the infiltrating fluid is believed to be reservoir water percolating through the basement rock during dam impoundment. Hence, the authors can think of incorporating reaction path modeling (e.g., Klein et al., 2022) in their work with a known fluid species and its consequent evolution during percolation through the host rock and altered zones at different depths (i.e., at different P-T conditions).
- Discussion on frictional evolution: The interpretation of seismic to aseismic transition just on the basis of mineralogical observation is oversimplifying, especially when a complex chemical kinetics is involved. Fault plane asperity plays a major role in this transition. The number of contact points between the two interfaces of a fault plane determines the nature of the slip. With higher asperity, the number of contact points decreases, resulting in high stress accumulation at each point. Which may activate the seismic slip, through stick-slip mechanism. However, if secondary fluid infiltrates through the conduits of the fault planes and precipitates secondary minerals such as chlorite or clays, the cryptocrystalline secondary grains would collectively increase the number of contact points and consequently, the nature of the slip may evolve to aseismic type. The two critical parameters that determine this evolution are the amount of shear stress at grain boundaries, and the rate of reaction through which the secondary minerals precipitate (Yarushina and Bercovici, 2013). If the reaction and nucleation rates of the secondary minerals are higher than the inter-seismic period of the region, then the altered minerals would inhibit the seismic slip and undergo sub-seismic to aseismic slip. Therefore, it is important to estimate the rate of alterations. While experimental observations (e.g., Bullock et al., 2015) have suggested that >10% “wet” phyllosilicate content in the fault gauge would decrease the interface friction and accommodate the stress by steady state sliding, that largely decreases the possibilities of seismic slip, “dry” phyllosilicate, on the other hand, can show velocity weakening behavior at high strain-rates that can facilitate seismic slip. The authors have stated that the semi-brittle deformation mechanisms such as kinking and delamination of phyllosilicates can promote the velocity strengthening behavior, however, it is also advised to note that these microscopic deformation mechanisms can create secondary pore spaces in the altered zones, that may not only increase the pore-pressure and hinder the alteration reaction, but also decrease the effective stress acting on the fault interface, causing embrittlement. Embrittlement, a dominant mechanism known to facilitate seismic slip. Hence, the discussion on “Boon or Bane” is critical to assess and involves a consideration of the aforementioned factors. The authors should address the alteration rate, the saturation, and activity of percolating fluids in the fractured zones.
- The temperature range provided (200-350 °C) for propylitic alteration should be accompanied by pressure estimates and comparisons with typical geothermal gradients, as these conditions significantly affect mineral stability fields.
The article is written well. However, it can be improved. Below I share some suggestions. The authors may consider these and all other similar possibilities.
Abstract, Line 2: Consider changing "The current study investigates the fluid's interaction..." to "The current study investigates fluid interaction..."
Abstract, Line 5: "This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration under acidic to neutral conditions..." could be clearer. Consider: "This interaction and the subsequent propylitic-grade hydrothermal alteration occur under acidic to neutral conditions..."
Abstract, Line 10-12: The sentence starting "However, this released K2O..." is slightly confusing and could be simplified for clarity. Consider splitting it into two sentences: "This released K2O further assists in the formation of illite, resulting in a mass gain of K2O at certain depths. Additionally, the dissolution of plagioclase justifies the formation of albite and calcite, as evidenced by the gain of Na2O and CaO."
Introduction, Line 1: "Faults and fractures act as fluid migration pathways during seismic rupture as well as the inter-seismic creep periods..." could be improved by removing "the" before "inter-seismic."
Citation: https://doi.org/10.5194/egusphere-2023-2553-RC2
Status: closed
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RC1: 'Comment on egusphere-2023-2553', Anonymous Referee #1, 02 Apr 2024
The paper presents the results of mineralogical and geochemical analyses to infer fluid-rock interactions and their potential contribution to recurring seismicity in the Koyna-Warna Seismogenic Region of western India. It is a topic of interest for Seismology and Geology, but I am not in the favor of publication of this manuscript in its current form. There are several areas that require clarification, expansion, or improvement to enhance the quality.
L14: XRD, acronyms should be introduced the first time you use the term.
L16: I didn’t find any discussion or interpretation to support the point about the acidic to neutral alteration conditions (pH 5.5-7) in the main text. How did you know the pH value?
L17: Similarly, the inferences about the alteration temperatures are missing.
L156-160: How was the transition from the faulting to normal faulting observed? Some references may need to be cited to support this transition and its potential relation with reservoir impoundment.
L169-200: The authors introduced several faults in the study area. Which fault does the borehole pass through? It is also necessary to specify which part of the faults the samples came from, e.g., the principal slip zone or damage zone?
Figure 3d: Is it fault gouge?
L173, L177: Delete the full stop before the parenthesis of the citations. Please check throughout.
L184: “litholog”→ lithology. Please check throughout for errors.
L270-271: Quartz with chess-board pattern generally reflects dislocation creep during high-temperature deformation. This mismatch with the presence of chlorite and epidote which indicates relatively low-temperature alteration.
L463-478: Given the dissolution of plagioclase and potential release of Al, why not try TiO2 as the immobile element for the isocon plot?
L473: I am not going to agree that the precipitation of calcite is caused by the dissolution of plagioclase. As shown in Figure 6c, calcites are precipitated as calcite veins along fractures. This is generally considered to be related to precipitation from hydrothermal fluids. More evidence is required to indicate the source of calcite.
L647: “although” and “but” cannot be used at the same time. There are many grammar errors here and elsewhere, and authors have to check to ensure that the language used throughout the paper is clear, concise, and grammatically correct.
Citation: https://doi.org/10.5194/egusphere-2023-2553-RC1 -
AC1: 'Reply on RC1', Piyal Halder, 04 Apr 2024
Respected Sir/Ma’am,
We appreciate the time and effort that you have dedicated and are grateful for your insightful comments which have improved the quality of the manuscript as well as helped us to identify many silly mistakes. We are also sorry for such mistakes and will keep in mind in future. We are responding to all of your queries and concerns for your kind consideration. As there is no provision to upload the revised MS at this stage, the revised MS will be uploaded after the completion of the review process. We shall incorporate all of the suggestions and modifications in the revised MS. We are requesting you kindly consider our response. Your kind approval regarding the acceptance of the submitted MS will be highly appreciated. We shall be grateful for your kind perusal.Regards,
Yours Sincerely,
Piyal Halder (First author) & Dr. Anupam Sharma (Corresponding author)
RESPONSE TO THE SUGGESTIONS AND QUERIES
L14: XRD, acronyms should be introduced the first time you use the term.
- Thanks for pointing out such a mistake. The backronym of XRD i.e., X-Ray Diffraction has been inserted in the revised manuscript which will be uploaded shortly after the completion of the review process.
L16: I didn’t find any discussion or interpretation to support the point about the acidic to neutral alteration conditions (pH 5.5-7) in the main text. How did you know the pH value?
L17: Similarly, the inferences about the alteration temperatures are missing.
- Thanks for pointing out this flaw, otherwise it would mislead the readers. Actually, the temp. and pH mentioned in the text are characteristics of the propylitic grade of hydrothermal alteration (Fulignati, 2020). Hence, instead of mentioning a particular temp. and pH, a range has been provided. Relevant Citations have been already mentioned in the discussion part of the submitted MS file (L498-L500). However, to avoid any confusion, the line relevant to this statement in the abstract section has been modified in the following manner-
“This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration which characterizes acidic to neutral conditions (pH 5.5-7) and the temperature of above 200-220 °C up to about 350 °C”
L156-160: How was the transition from the faulting to normal faulting observed? Some references may need to be cited to support this transition and its potential relation with reservoir impoundment.
- The evidence of fault transition from normal to strike-slip pattern and vice versa has been obtained from the inversion of 50 focal mechanism solutions of earthquakes that occurred during the last 45 years in the Koyna–Warna region (Rao and Shashidhar, 2016). The relevant citation has been included in the text of the revised MS.
L169-200: The authors introduced several faults in the study area. Which fault does the borehole pass through? It is also necessary to specify which part of the faults the samples came from, e.g., the principal slip zone or damage zone?
- I am overwhelmed by your detailed review. These details are required for a reader to understand the entire research perspective. I am extremely sorry for such incompleteness in the information. The borehole KBH1 has cut numerous small-scale faults of the Koyna River fault Zone (KRFZ) which is already shown in 1 of the submitted manuscript. Additionally, the same has been inserted in the sampling section of the revised MS, which will be uploaded later after the completion of the review process. The altered samples are collected from the slip surface of these small-scale faults (fault core) as mentioned in the L192, L194, L206, and L286. The same has also been inserted in the above-mentioned line for a clearer understanding as per your suggestions.
“The altered samples from the slip planes of the small scale-faults of the KRFZ have been collected through mesoscopic observations”
Figure 3d: Is it fault gouge?
- The figure is the mesoscopic observation of the sample KBH1_379 collected from 1073 m depth. The physical appearance as well as the presence of clay minerals, identified in the XRD (Fig. 7c & d), infers that the sample may be a fault gouge or clayey gouge. Hence, the above-mentioned line has been modified in the revised MS as below-
“Clayey fault gouge reflecting the mechanical disintegration during fault slip followed by the clay formation due to fluid-rock interaction”.
The relevant line in the (L195-L196) main text has also been modified accordingly.
L173, L177: Delete the full stop before the parenthesis of the citations. Please check throughout.
- Thanks for pointing out such silly mistakes. Such mistakes have been noted and rectified as per your suggestions.
L184: “litholog” → lithology. Please check throughout for errors.
- Here, the “litholog” of the studied borehole has been shown at the side of the schematic diagram. The litholog exhibits the distribution of the porous and vesicular Deccan basalt upto 932 m depth which is underlain by the granitoid basement rocks.
L270-271: Quartz with chess-board pattern generally reflects dislocation creep during high-temperature deformation. This mismatch with the presence of chlorite and epidote which indicates relatively low-temperature alteration.
- I agree with your observations. However, the basement rocks in the study area have undergone three phases of deformation. The first of which corresponds to the development of gneissosity in the granitic rocks, the second was the shearing events that caused pre-existing fabrics (S1) to be overprinted and transposed by the high-strain S2 fabrics, and the third was the emergence of anastomosing fractures as a function of brittle deformation such as faulting. The fractures cutting the plagioclase grains and fillings of chlorite and calcite throughout the network of fractures, as documented in our study (Fig. 4f, g), indicate that the percolation of fluid and subsequent formation or precipitation of secondary minerals along the fractures took place after the third deformation event in which faulting occurred and fractures were developed. So, the chessboard pattern of quartz as well as the marker offset within plagioclase indicates the third deformation event which gave rise to fractures. Later, fluid percolated through these fractures and resulted in the formation of secondary minerals. Hence, the presence of chlorite, illite as well as precipitation of calcite can’t be correlated with the deformation pattern of quartz. (Please see L556-L570 in the submitted MS)
L463-478: Given the dissolution of plagioclase and potential release of Al, why not try TiO2 as the immobile element for the isocon plot?
- The mass balance calculation and isocon plots have been executed following Grant (2005). The equation of an isocon is,
CA=(MO/MA) CO
[CA= concentration of an element in altered rock
CO= concentration of an element in unaltered rock
MO= Mass before alteration
MA= Mass after alteration]
Here, the MO/MA is the slope of the isocon and the line with this slope indicates no gain and no loss in terms of mass. The equation is similar to the straight-line equation with zero intercept. So, this method employs the plotting of isocon in two different ways-
(i) The first method involves clustering CA and CO values along the y and x axis respectively. Later, the best-fit line is drawn through the origin by graphically searching the elements that lie on or near the line. This line is the isocon line and is named in the present study as the Best-fit isocon (Grant, 1986). So, there is no way to fit the isocon manually in this method. However, if we see the best-fit isocons in all the altered samples, we notice that most of the best-fit isocons have passed near the TiO2. So, it is evident that TiO2 is almost immobile but not completely immobile.
(ii) However, there is another method for drawing the isocon line, in which the line is extended by clustering and selecting the elements that show minimum gain/loss with respect to other elements, i.e. [(CA-Co)/ CO] will be minimum. This isocon has been named in the present study as the Immobility Isocon. For this purpose, an average of the concentrations of each element in all the unaltered samples (Co) is calculated. Then gain or loss of that particular element in the altered sample is estimated with respect to the Co. For example, the Co value of Al2O3 is 15.12 and for TiO2 is 0.39. In one altered sample, the concentration of Al2O3 is 21.34 and 1.46. So, the gain of Al2O3 and TiO2 are 6.22 and 1.07 respectively. But with respect to Co, there are gains of 0.41 and 2.73 for Al2O3 and TiO2 respectively [Please see table S1 in the supplementary file]. The same approach applies to other altered samples. In all the cases, Al2O3 has shown the minimum gain or loss with respect to Co [Please see Table S1 in the supplementary file]. Hence, immobility isocon has been drawn mostly through Al2O3 rather than TiO2. In the latter case (isocon drawing through TiO2), a priori assumption is needed regarding the immobility of TiO2. Such priori assumption may give misinformation regarding mass gain or loss. In fact, isocon drawn exactly through TiO2 will show a loss of CaO in samples KBH1_378 and KBH1_379 which is contrary to the evidence of calcite precipitation. Similarly, in KBH1_381, if the isocon is drawn through TiO2, it will represent the loss of CaO, MgO and FeO, which will contradict the findings of chlorite in XRD.
Moreover, this method provides a holistic view of the mobility of elements throughout the geologic section or system (so an average is taken). Also, Al activity in the hydrothermal fluid is restricted, it is mobile on the mineral scale, and it is immobile on a rock scale (Parneix et al., 1985). Most importantly, the findings emanated from immobility isocon match with the findings found in the case of the best-fit isocon method, which is completely statistically derived and doesn’t involve any manual interference. That is why, the isocons have been drawn through Al2O3 is more reliable rather than through TiO2.
L473: I am not going to agree that the precipitation of calcite is caused by the dissolution of plagioclase. As shown in Figure 6c, calcites are precipitated as calcite veins along fractures. This is generally considered to be related to precipitation from hydrothermal fluids. More evidence is required to indicate the source of calcite.
I agree with your view that calcite is precipitated by the hydrothermal fluid. In our study, the heat derived from the faulting and fracturing must have contributed to this hydrothermal alteration (because the temperature at the depth of this study is less than 150 °C; See L501-L505 of the preprint file). So, there may be two sources of Ca in the hydrothermal fluid-
(i) One source is the overlying basalt which contains CaO of 8.76–11.7 wt% (Banerjee and Mondal, 2021). The meteoric water percolated through the basaltic rocks dissolves this Ca and transports it to the fracture or permeable faults, where the fluid deposits it in the form of calcite by degassing after heating.
(ii) Another source is the Ca2+ released due to plagioclase dissolution in the granitoids. In this case, the Ca2+ released due to plagioclase dissolution is mixed up with the descending meteoric water which gets hot by the heat generated by the faulting and fracturing and leads to the precipitation of calcite by degassing.
This is why, the probability of plagioclase dissolution as the source of calcite can’t be ruled out. Hence, the above-mentioned line will be modified in a below-cited manner in the revised ms-
“Moreover, during this process along with Al3+, Ca2+ is also released which may act as one of the sources of calcite precipitated and elucidates the gain of CaO in the altered samples. However, considering the CaO content (8.76–11.7 wt%) of the overlying basalt and the basalt’s high weathering potential, the contribution of the Deccan Trap basalt in the introduction of Ca2+ in the hydrothermal fluid must be taken into account”
L647: “although” and “but” cannot be used at the same time. There are many grammar errors here and elsewhere, and authors have to check to ensure that the language used throughout the paper is clear, concise, and grammatically correct.
- I am sorry for such carelessness. The same has been noted with care and will be rectified in the revised MS which will be submitted after the completion of the review.
References:
Banerjee, R. and Mondal, S.K., 2021. Petrology and geochemistry of the Deccan basalts from the KBH-7 borehole, Koyna Seismic Zone (Western Ghats, India): Implications for nature of crustal contamination and sulfide saturation of magma. Lithos, 380-381: 105864.
Fulignati, P., 2020. Clay Minerals in Hydrothermal Systems. Minerals, 10(10): 919.
Grant, J., 2005. Isocon analysis: A brief review of the method and applications. Physics and Chemistry of the Earth, Parts A/B/C, 30.
Parneix, J.C., Beaufort, D., Dudoignon, P. and Meunier, A., 1985. Biotite chloritization process in hydrothermally altered granites. Chemical Geology, 51(1): 89-101.
Rao, N. and Shashidhar, D., 2016. Periodic variation of stress field in the Koyna–Warna reservoir triggered seismic zone inferred from focal mechanism studies. Tectonophysics, 679.
[The same has been uploaded in .pdf format.
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AC1: 'Reply on RC1', Piyal Halder, 04 Apr 2024
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RC2: 'Comment on egusphere-2023-2553', Anonymous Referee #2, 12 Apr 2024
In the article titled "Fluid-rock interaction in the intraplate active seismic zone: Boon or bane?" the authors have implemented several analytical techniques to assess the chemo-mechanical evolution of fluid infiltrated reactivated faults in the Koyena-Warna Seismogenic region of Western India. The objective of the article is to discuss the impact of secondary mineralization on the frictional behavior of reactivated faults.
While the authors have presented quite an exhaustive mineralogical analysis of the KBH-1 borehole from different depths, the discussion and interpretations from the analytical results could be more enriching in terms of the mineralogical control over the frictional evolution of fluid infiltrated fault zones. The velocity strengthening behavior of phyllosilicates and related aseismicity is a well-established fact, and simply discussing the effects of phyllosilicate content on the friction at fault interfaces based on existing literature does not contribute much towards the current understanding of the rate-and-state friction laws or aseismic/seismic slip. Thus, the article, at its present state, is not novel enough to be acceptable for publication. However, the topic itself is exciting and the field area is of high geological importance. A few suggestions in this regard may help the authors to better shape their manuscript
- Reaction path modeling: The authors have used two alteration metrices to distinguish the type of alteration at different depths along the borehole. However, these alteration indices are largely empirical. Therefore, more emphasis should be placed on interpreting the alteration mechanisms obtained from these empirical values. For example, in 10c, the authors have shown two different alteration mechanisms (Carbonization and Hydrothermal alteration) along different depth profiles of the borehole. The discussion should focus on the interplay between these two mechanisms, especially when the infiltrating fluid is believed to be reservoir water percolating through the basement rock during dam impoundment. Hence, the authors can think of incorporating reaction path modeling (e.g., Klein et al., 2022) in their work with a known fluid species and its consequent evolution during percolation through the host rock and altered zones at different depths (i.e., at different P-T conditions).
- Discussion on frictional evolution: The interpretation of seismic to aseismic transition just on the basis of mineralogical observation is oversimplifying, especially when a complex chemical kinetics is involved. Fault plane asperity plays a major role in this transition. The number of contact points between the two interfaces of a fault plane determines the nature of the slip. With higher asperity, the number of contact points decreases, resulting in high stress accumulation at each point. Which may activate the seismic slip, through stick-slip mechanism. However, if secondary fluid infiltrates through the conduits of the fault planes and precipitates secondary minerals such as chlorite or clays, the cryptocrystalline secondary grains would collectively increase the number of contact points and consequently, the nature of the slip may evolve to aseismic type. The two critical parameters that determine this evolution are the amount of shear stress at grain boundaries, and the rate of reaction through which the secondary minerals precipitate (Yarushina and Bercovici, 2013). If the reaction and nucleation rates of the secondary minerals are higher than the inter-seismic period of the region, then the altered minerals would inhibit the seismic slip and undergo sub-seismic to aseismic slip. Therefore, it is important to estimate the rate of alterations. While experimental observations (e.g., Bullock et al., 2015) have suggested that >10% “wet” phyllosilicate content in the fault gauge would decrease the interface friction and accommodate the stress by steady state sliding, that largely decreases the possibilities of seismic slip, “dry” phyllosilicate, on the other hand, can show velocity weakening behavior at high strain-rates that can facilitate seismic slip. The authors have stated that the semi-brittle deformation mechanisms such as kinking and delamination of phyllosilicates can promote the velocity strengthening behavior, however, it is also advised to note that these microscopic deformation mechanisms can create secondary pore spaces in the altered zones, that may not only increase the pore-pressure and hinder the alteration reaction, but also decrease the effective stress acting on the fault interface, causing embrittlement. Embrittlement, a dominant mechanism known to facilitate seismic slip. Hence, the discussion on “Boon or Bane” is critical to assess and involves a consideration of the aforementioned factors. The authors should address the alteration rate, the saturation, and activity of percolating fluids in the fractured zones.
- The temperature range provided (200-350 °C) for propylitic alteration should be accompanied by pressure estimates and comparisons with typical geothermal gradients, as these conditions significantly affect mineral stability fields.
The article is written well. However, it can be improved. Below I share some suggestions. The authors may consider these and all other similar possibilities.
Abstract, Line 2: Consider changing "The current study investigates the fluid's interaction..." to "The current study investigates fluid interaction..."
Abstract, Line 5: "This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration under acidic to neutral conditions..." could be clearer. Consider: "This interaction and the subsequent propylitic-grade hydrothermal alteration occur under acidic to neutral conditions..."
Abstract, Line 10-12: The sentence starting "However, this released K2O..." is slightly confusing and could be simplified for clarity. Consider splitting it into two sentences: "This released K2O further assists in the formation of illite, resulting in a mass gain of K2O at certain depths. Additionally, the dissolution of plagioclase justifies the formation of albite and calcite, as evidenced by the gain of Na2O and CaO."
Introduction, Line 1: "Faults and fractures act as fluid migration pathways during seismic rupture as well as the inter-seismic creep periods..." could be improved by removing "the" before "inter-seismic."
Citation: https://doi.org/10.5194/egusphere-2023-2553-RC2
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