the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jul 2024
On crustal composition of the Sardinia-Corsica continental block inferred from receiver functions
Abstract. Subduction-related geodynamic processes significantly influence plate tectonics and Earth’s evolution, yet their impact on the continental crust remains poorly understood. We investigated the Sardinia-Corsica continental block, situated in the Mediterranean Sea, which has experienced intense subduction-driven geodynamic events. By analyzing P-wave receiver functions from our LiSard seismic network and publicly available stations, we aimed to understand crustal structure and composition. We inferred the Moho depth and examined the P-wave to S-wave velocity ratio (VP /VS ). We interpret our findings considering petrological data, heat flux measurements, and other geophysical information. We found that the Variscan granitoid batholith has the greatest Moho depths in both Sardinia and Corsica. VP /VS ratios (ranging from 1.65 to 1.70) are consistent with average crustal values of SiO2 between 65 % and 70 %. However, in central Corsica, two stations have exceptionally high VP /VS values (>1.80), suggesting the possible presence of serpentinite throughout the crust. In Alpine Corsica, a station exhibited similar high VP /VS values but a shallower Moho depth of 21 km. The western part of Sardinia, where Cenozoic volcanism occurred, also showed a shallower Moho depth (20–25 km) and high VP /VS values. The highest VP /VS value (1.91) is recorded in an area where surface-wave dispersion curves from ambient noise identified the lowest average S-wave velocity and where the highest heat flux has been reported, indicating elevated crustal temperatures and possible presence of melt within the crust. Overall, our results indicate that the recent geodynamic processes have left the granitoid batholith almost intact, with minimal alteration to its composition.
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RC1: 'Comment on egusphere-2024-1515', Tony Lowry, 29 Aug 2024
This paper is a well-written and interesting analysis of crustal Vp/Vs from receiver functions in the Corsica-Sardinia microplate, with potential implications for processes and timescales of metasomatic alteration of continental crust. The authors find low Vp/Vs < 1.75 in the Variscan batholith of most of Sardinia where crust is thicker, and higher Vp/Vs in Corsica and the southwesternmost part of Sardinia. These patterns are interpreted in terms of silica content and mafic to ultramafic, supported by thermodynamical modeling of properties, but perhaps a more fruitful way to interpret these (as detailed further below) is in terms of quartz abundance reflecting a history of metasomatism in low Vp/Vs regions and more mafic lithologies elsewhere.
One caveat worth making in the paper, and remembering in the context of interpretation, is that uncertainties in single-station estimates of Vp/Vs using H-k stacking techniques can be quite large. For example, raw one-sigma uncertainties are greater than 0.1 based on the variance at ~0 distance separation for variograms of USArray data in the automated EARS database (Crotwell & Owens, SRL 2005), see e.g. Fig. 2c of Lowry & Pérez-Gussinyé (Nature 2011). The authors appear to have done a very careful analysis here, and judging by the stacks in Figures 4-7 the younger crust of the study area is less structurally complicated than typical North American continental crust in USArray, but even so uncertainties are likely to be of order 0.07 or larger here and possible impacts of that should be addressed in the discussion.
There are a couple of other issues that it might be worthwhile for the authors to consider in a revision of the paper, described in greater detail in comments tied to §2.1.1 below. One is that there appears to be some sort of bias error in Perple_X outputs of Vp/Vs for crustal mineral assemblages, resulting in much lower modeled values than those measured in the lab for corresponding rocks. The practical significance of this is that an exotic (serpentinite, eclogite, or supersolidus) lithology is not necessary to explain higher Vp/Vs in the study area; a gabbro would be sufficient. However it also means that the absolute values of Perple_X-derived Vp/Vs are less useful for interpreting these results than how Vp/Vs changes for different chemistries and volatile contents. I would also suggest that it’s useful to recognize that Vp/Vs variation is dominated not by %-Si so much as by %-qtz, because of the unique elastic properties of quartz. This enables the use of Vp/Vs as a proxy for metasomatic history, as much of the Vp/Vs difference for dry and hydrated lithologies in Figure 9 is related to breakdown of feldspar to quartz and mica (Ma & Lowry, 2017).
§ 2.1.1 Thermodynamical (Perple_X) modeling:
The choice here to examine various bulk compositions but only use one constant (0.25wt-% H2O) volatile state unfortunately obscures one of the most significant potential takeaways for interpretation of the results. Namely, the primary factor in determining bulk crustal Vp/Vs is the abundance of the mineral quartz (Christensen, JGR 1996; Lowry & Pérez-Gussinyé, Nature, 2011). This does of course depend to some degree on SiO2 content, but it is much more sensitive to whether water is present to react with the bulk constituents, which breaks down feldspar into quartz and mica (Ma & Lowry, Tectonics, 2018). Hydration reactions that break down feldspar also presumably depend on whether CO2 is present to buffer those reactions (Yardley, J. Geol. Soc. Lond. 2009). From that perspective, it seems to me that a more useful approach to examining Vp/Vs with Perple_X is to use the bulk compositions from the rock environment of interest but vary the volatile mix, and then interpret the variations primarily in terms of hydration history.
Also, as an aside: There is a problem of some sort in the elastic parameter database of Perple_X, because it gives Vp/Vs seismic velocity ratio estimates that are consistently about 0.05 to 0.1 lower than the corresponding values from Christensen’s (JGR 1996) measurements. This becomes apparent if one compares the 1.71 to 1.86 range of Christensen’s measurements of granite to gabbro in Fig. 1a of Ma & Lowry to predictions in Figure 9 of this paper. For that reason, Ma & Lowry did not show absolute Vp/Vs in their Fig. 14, but rather the perturbations with water versus without water present in Perple_X thermodynamical modeling that used a similar database to this paper. Xiaofei Ma spent significant time and effort trying to figure out where the problem may be coming from during his dissertation studies, but we were unable to track it down. Since attenuation effects are likely to be larger for Vs than Vp, that is one candidate for the discrepancy, but I am somewhat skeptical that attenuation would be that significant for small-scale room temperature samples like those in Christensen’s (JGR 1996) database. Because of this, it is perhaps safer to use Perple_X as a tool to examine relative Vp/Vs for different choices of composition or state than for purposes that assign meaning to the absolute Vp/Vs. For purposes of this paper, the conclusions regarding likely mineral assemblages are probably still valid (within the large uncertainties that are inherent in single-station H-k stacking estimates of Vp/Vs), but note that robust measurements of whole-crustal averaged Vp/Vs less than ~1.7 are extremely rare except when errors are present due to the perturbations of amplitude stacks by other reflectivity, dipping structure and anisotropy, as this paper notes can be present. Vp/Vs exceeding 1.8 on the other hand does not require an unusual composition like that of eclogite; it simply requires lower abundance of quartz. In fact, the mean Vp/Vs for the USArray footprint in the United States is about 1.79 (Ma & Lowry, Tectonics 2017) and Vp/Vs exceeding 1.85 is possible in crust where lithologies are gabbroic.
Lines 368-369: The change in Vp/Vs due to adding water to the chemistry is not primarily because of the reduction of the solidus, as inferred here, but because hydration reactions reduce the feldspar content of the mineral assemblage and instead favor formation of quartz and mica (Ma & Lowry, Tectonics 2017). The abundance of quartz dominates crustal Vp/Vs variations because quartz has a very unusual Poisson’s ratio translating to a Vp/Vs less than 1.5 (Lowry & Pérez-Gussinyé, Nature 2011, based on measurements in Christensen, 1996).
Lines 415-418: As noted above, a high Vp/Vs coupled with high Vs does not require serpentinite or some other exotic mineralogy to explain; rather a Vp/Vs up to 1.88 and high shear velocity can be expected for a common mafic lithology. Where Vp/Vs exceeds 1.88, it can probably be attributed to the large uncertainties expected for single-station H-k stacking estimates of Vp/Vs.
§ 5 Conclusions
This section does not tie back strongly to the information presented in the previous sections. It seems to me that the modeling in the discussion section does reaffirm earlier work suggesting that hydration lowers Vp/Vs, and so– given that Vp/Vs in Corsica and southwestern Sardinia remains high– however-much hydration may contribute to regional volcanism, it does not appear to translate to substantial metasomatic modification of the crustal lithology in those particular locations (which is slightly different than what is said in lines 425-430). I also don’t know that I would strongly emphasize evidence for a thermal anomaly in Sardinia, given that Vp/Vs is insensitive to temperature. (And although it may be sensitive to partial melt, in practice the crustal averages of Vp/Vs derived from H-k stacking are not.)
Figures 2 and 3: These are a bit hard to interpret because the y-axis back-azimuth for the left and center panels is nonlinear, and must be inferred by looking back and forth to the right panel. It would make more sense to bin and sum, or if showing individual traces is preferred, to plot each trace as a linear function of back-azimuth (even if they then overlap). This would aid in identifying patterns expected for dipping layer boundaries and/or layer anisotropy (e.g., Schulte-Pelkum & Mahan, EPSL 2014).
Citation: https://doi.org/10.5194/egusphere-2024-1515-RC1 -
AC2: 'Reply on RC1', Fabio Cammarano, 09 Nov 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1515/egusphere-2024-1515-AC2-supplement.pdf
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AC2: 'Reply on RC1', Fabio Cammarano, 09 Nov 2024
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RC2: 'Comment on egusphere-2024-1515', Anonymous Referee #2, 20 Sep 2024
Review of the manuscript “On crustal composition of the Sardinia-Corsica continental block inferred from receiver functions” by Fabio Cammarano, Henrique Berger Roisenberg, Alessio Conclave, Islam Fadel, and Mark van der Meijde submitted for publications to Solid Earth.This work presents a newest addition to the investigation of the complex crustal structure of Sardinia-Corsica continental block using the P receiver function method at 21 seismic stations. For this the authors use data from both permanent stations and temporary ones from the project LiSard.The topic of investigation is interesting, and the methods used are sound and my main critique of the manuscript is mostly about the presentation of the results and the usage of the receiver function method. The language used is appropriate and reads easily but the overall impression is that of the manuscript lacks a bit more depth in the analysis of the results. The authors compare the receiver function modeling results with crustal composition obtained by thermodynamical modeling but a bit more discussion on various influences of complex structure on receiver functions would make the main conclusions more robust.Bellow, please find the comments that I suggest authors address before the manuscript can be considered for publication.Comments:-Introductory section is well written with abundant information about tectonics and previous investigations in the area, but, in my opinion, accompanying Figure 1. is rather poor with part of the text in both images a) and b) not readable. Also, image showing seismic stations used should be plotted in more presentable way. Ttry some of the plotting packages like GMT, Matplotlib basemap or variety of similar packages. Also, please put in the Figure caption what are red and what are black station markings in the station plot.The authors should invest a bit of time to fix this as good introductory image will help reader connect conclusions with research aims.Line 54: change “The average P-wave velocity…” to “The average crustal P-wave velocity…”- Data and methods sectionLines 99-100: In this sentence It is a bit unclear what is deconvolved from what. Please make it more concise.Table 1: Why is the part of the caption above and part of it bellow the table? I suggest authors correct this and put everything above the table.Also, it is not clear which of the six groups is part of the 3 provinces mentioned in the caption?Line 178: Links to the software packages should go either in the Code and the data availability part after the main text or in the References section.Table 3 should be moved to Supporting material as it will clutter the main body of the text without bringing any substantially relevant information that is not already shown in Figure 1.Line 209: “…as indicated in Table 1.” The authors probably mean Table 3 not Table 1.Line 212: Correct reference from Zhu and H (2000) to Zhu and Kanamori (2000) and do the same in the Reference section.Line 215: Please provide reference for the statement in this sentence.Figure 2 and Figure 3 (and figures S1 – S17) These figures should be made more presentable as currently some of the important details are not visible. The amplitudes of the main Ps arrival and reverberation are not visible at all. I suggest that authors make figures bigger by removing unnecessary text on Y-axis (listing of event) and remove last image showing Back-azimuth for each event and sort both R and T receiver functions by Back-azimuth. Why is currently only T-component sorted by back-az and R not?In this way figure will be bigger and details clearer.- Results sectionLines 234 – 235: “…would be only made SV (S-wave in the vertical plane) energy.” Wording here is a bit unclear. Please make it more concise and clearer.Figure 2-3 (and S1 – S17) Authors show T-component RFs but are not actively discussing them. TRFs are crucial in interpreting possibly 3D structural complexities (dip, anisotropy, etc.) especially important in H-Kappa stacking and interpretation of these results. Authors should spend some time to at least try to interpret some of the signal on T-component for stations that do not have clear maximum in H-k stacking (e.g. station IV.DGI or IV.AGLI).Overall, discussion about possible influences on interpretation of RFs H-k stacking is thin and should be done more thoroughly as conclusions are based on these results that could be overinterpreted.Lines 241 – 242 “…those show a noisy pattern.” How is this estimated? How do the authors see that something is noisy and other stations are not? This needs to be more concise.Figure 4-5-6-7:In my opinion there are too many figures for showing H-k stacking. It would be better if the authors show only 4 relevant H-k stackings for stations and the rest can be moved to supporting information. Additionally, if there is a discussion about some the problems at particular stations with H-k that (or those) stations can be showed in separate Figure indicating possibly problems.Also, text in the Figures and on both axes is small and hard to read.Line 260:Authors are using station UT.011 in discussion and deeming that station as showing intricate features as on the other hand they dismissed that station on Line 241. as noisy? Why use it if it is noisy?Line 281:“…for station UT.009, the search had to be extended to k = 1.5.” Why it had to be extended to 1.5 when the maximum is at 1.7? There are several such inconsistencies through the manuscript connected with h-k that needs to be dealt with. Why hasn’t all been calculated in the same broad range?Lines 289-290: “…deeper Moho depth.” wording depth is redundant. Put “…deeper Moho.”Lines 290 – 291: remove “indicating a substantial Moho depth for both locations.” as it is already stated in the previous sentence.Lines 294 – 295:“Additionally, it is worth noting that the CORF station exhibits an indistinct Ps phase, implying the presence of intricate wave propagation characteristics specific to the Central Corsica region.”Or more likely complex structure.Line 307: “…situated at a depth of 111 m…” borehole or underground cave station? Give a couple of words to describe it.Lines 307 - 311: Authors indicate possible problems with this station that may be connected with orientation problems. Please check.Lines 312 - 317: In the case of station IV.DGI if only Ps phase was used for H-k stack then the authors should not use the resulting Vp/Vs values from that stacking as a viable results as the uncertainty is too great.Line 341: “Additionally, we have computed, the P-T properties of the upper-, middle- and lower-crust from the global compilation of Rudnick and Gao (Table 2).”The sentence is a bit misleading as one expects that the authors show P-T properties in Table 2. Correct this please.Line 376: “…average Vs in the eastern part.” Figure 10 shows that western side has lower Vs values?Line 383: “…slightly from 3.75 to 3.80…” In Figure 11. one can see that these values are approx. 3.85 to 3.90.Citation: https://doi.org/
10.5194/egusphere-2024-1515-RC2 -
AC1: 'Reply on RC2', Fabio Cammarano, 09 Nov 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1515/egusphere-2024-1515-AC1-supplement.pdf
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AC1: 'Reply on RC2', Fabio Cammarano, 09 Nov 2024
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