Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)
, Sara Sibil Giuseppina Guerini 1, Francesca Rotondo 1, Andrea Festa 2, Gianni Balestro 2, Gray E. Bebout 3, Enrico Cannaò 1,4, Gabe S. Epstein 3 and Marco Scambelluri 5
Javier Escuder-Viruete
Round 1
Reviewer 1 Report
REVIEW to Manuscript Number: GEOSCIENCES 556153
Title: Superposed tectonic and sedimentary block-in-matrix fabrics in a subducted serpentinite mélange (High-pressure Zermatt Saas ophiolite, Western Alps),
by Tartarotti et al.
In the paper GEOSCIENCES 556153 Tartarotti et al. present valuable field, petrographic, microstructural and metamorphic mineral assemblage data of selected metasedimentary rocks from the Composite Chaotic Unit of the Zermatt Saas ophiolite, outcropping in the Aosta Valley region of Western Alps. These data allow the characterization of preserved internal stratigraphy of the unit, relicts textures and mineralogy of the oceanic caotic sedimentary protolith, and stable metamorphic mineral assemblages present in sequential deformative phases formed during burial and exhumation of the Alpine tectonometamorphic cycle.
Although Authors describe in great detail the mesostructures observed in the field and the mineral assemblages that constitute the fabrics formed during each Alpine deformative stage, there is nevertheless a special emphasis in the work to demonstrate the existence of clastic textures and serpentinite-rich compositions, formed during the previous sedimentation of the protolith.
The results are discussed in a regional geodynamic context, reviewing the previous studies and some proposed new models for the evolution of the Jurassic Ligurian-Piedmont ocean basin of the western Alps. Following Tartarotti et al., the most reliable geodynamic setting for the formation of sedimentary deposits with a block-in-matrix fabric of the Composite Chaotic Unit could be in the seafloor of the oceanic basin, in relation to the tectonic exhumation of the mantle basement. In this sense, a nice plate-scale model for formation and evolution of the Composite Chaotic Unit of the Zermatt Saas ophiolite compatible with the data is finally proposed.
Overall, I find the purpose of the work very interesting and the results of it can be an outstanding contribution in the knowledge of the subducted sedimentary serpentinite mélanges in the western Alps during the Cretaceous. The very detailed field and petrographic data exposed in the manuscript are of high quality, and the tectono-sedimentary and tectono-metamorphic models that support their interpretations are well constructed.
I affirm therefore that the manuscript can be published in Geosciences. However, I think that it requires a previous revision, since I find two major problems.
The first problem is the interpretation of some presented microtextures as clastic. A field geologist may easily recognize the sedimentary origin of little-deformed serpentinite-matrix mélanges, but mélanges within accretionary prisms or subduction complexes have undergone significant deformation and metamorphic recrystallization of the blocks and the matrix. This seems to be the case of the Composite Chaotic Unit of the Zermatt Saas ophiolite, where the sedimentary characteristics of the protolith and, therefore, of the mass transport processes that generated it, are better discernible in less deformed domains at outcrop scale than to the microscale. Therefore, the data presented by the Authors as doubtful or problematic in the manuscript should perhaps be eliminated.
The second main problem that I find in the manuscript is the frequent complex (and some poor) writing of the text, on which depends an adequate presentation of data, the construction of arguments and the interpretations. The petrographic descriptions are very long and difficult to understand for the reader. There is material that is repeated two or three times in the text. There are also sections in the text where the exposure of the data is mixed with its interpretation. Therefore, it is necessary to simplify the text. The presentation of data by tables can help a lot. Also improve the organization of the text, taking all interpretations to the Discussion section.
Some minor points for review are included in the annotated manuscript (PDF file). These comments are related to aspects in the text. Some figures also require minor corrections to increase the size of letters. I found text that can be improved in writing; but as I am not a native English speaker, I suggest that the final text will be revised in this sense.
Finally, I hope that the Authors consider my review as critical constructive.
Sincerely,
Reviewer
Comments for author File: 
Comments.pdf
Author Response
Reply to Review 1 (blue highlighter in the marked text file)
Page 2: “serpentinite mélange”: changed to “serpentinite-matrix mélange”
Page 3, Figure 1: this figure has been improved also according the suggestion of Reviewer 2
Page 4, comment: “Probably, there exits a progradation of the youngest ages of the thermal peak of metamorphism towards the NE/E”. We do not reply to this comment because we are not convinced of a progradation of the “thermal peak “(in case it is rather a Pressure peak) towards NE. We think that the time span of the HP peak metamorphism is due to the different applied geochronological methods, and to the different structural fabrics on which dating has been performed (not always specified in publications). Anyway such discussion is beyond the aim of this paper.
Page 5: All corrections were made in the “Methods” section.
Chapter 3. Methods: In order to short this section as suggested by Rev. 1 we cut and moved part of this section to the Supplementary Text S1 file.
Page 6: comment “really, would have to be called as "serpentinite-group minerals"”. We prefer to list the mineral assemblage of the studied serpentinites since not all serpentinites retain a common mineralogy.
Page 6, comments: “The clastic character has been preserved despite the Alpine pervasive ductile deformation?” We deleted the words “rounded clasts” because boudinaged portions are dominant. In the metasomatic horizon there are not relict clastic textures.
Page 6, comment “"Chaotic" should refer to the sedimentary characteristics more or less preserved in metamorphic rocks”. We deleted this part.
Page 6, comment “If they are units formally defined according to the International Code of Stratigraphic Nomenclature, their denomination is uppercase; if they are informal units are refered in lowercase”. These units are not formally defined thus, we corrected into lowercase.
Figure 2: we added a legend box for the white parts in the map.
Figure 3: made changes accordingly
Page 9 (text and Figure 4 caption): comment “internal normal? grading. In the text we added: “both normal and reverse internal grading, probably resulting from tectonic repetition”.
Page 9, comment “Are clasts that still preserve the clastic character of the protolith, or are porphyroclasts product of the ductile boudinage and/or disruption/fragmentation of the original stratification?” We think that the two circumstances may occur, either separately or superposed. For examples, serpentinite clasts shown in Figure 10B, 10D likely preserve the clastic character of the protolith (see explanation in the text), whereas in other cases clasts may result from pure tectonic transposition from a serpentiite rocks or deformation of original clasts. This issue is the real topic of this study. We thus prefer to use the term “clast” with a general (or descriptive) meaning.
Page 10: the paragraph describing boudins has been shortened and simplified.
Page 10: corrected “heteropic”
Page 12. all corrections made.
Figure 7 caption. All corrections made.
Page 13. The sentence ..”Among all the type of boudins, particular attention must be given to the asymmetrical ones as they can be interpreted as shear sense indicators” has been deleted.
Page 14. Comment “The shear sense provided by kinematic indicators is regionally consistent?”. We corrected as follows: “Asymmetric or sigmoidal blocks and boudins within the matrix attest for a shear component during D1/D2 deformation phases, indicating both top-to NW and top-to NE shear sense, as observed in all the high Champorcher valley.”
Page 14. The paragraph has be shortened and simplified (Page 16 of the “marked” file).
Page 15, Comment: “Perhaps, it would be better to define Srp as serpentinite group minerals and Atg as antigorite (Lhz as lizardite, etc.)”. According to the mineral abbreviations by [104], the abbreviation “Srp” means “Serpentine group”. In our study, no diffraction analyses are reported, although in the Discussion we refer to two Master Theses reporting analyses of antigorite. We have now deleted any mention to “antigorite” and use the general term “Srp” which includes the type Antigorite, Lizardite and Chrisotile.
Figure 9: corrections were made. See also corrections after suggestions of Rev 2 (Figure B and D have been switched).
Chaper 6. All this chapter has been simplified and reduced according to Rev. 1’s suggestions. We deleted several paragraphs, among which the following (that Rev. 1 suggested to move to the Discussion: “…
Summing up, in spite of pervasive carbonation and tectonic transposition that partially mask the BrFm2 fabric, the occurrence of partially preserved clastic texture in the ultramafic domains (horizons and boudins) allows us to interpret the ultramafic portions as original ultramafic (i.e., serpentinized peridotite) metasandstones. Such metasandstone layers clearly alternate with carbonate-rich domains that show a strongly recrystallized fabric including several generations of calcite and dolomite. These carbonatic layers also include transposed ultramafic levels, and show activation of shear zones mostly concentrated along the contacts between the ultramafic and the carbonatic portions. All these evidence point to a general fabric of the BrFm2 characterized by a strong compositional layering, that probably inherits an original assemblage made of ultramafic sandstone levels alternating with carbonatic levels (see Discussion).
All these evidences point to a sequence of textural evolution starting from an original mantle peridotite affected by hydration-serpentinization (oceanic and early Alpine D1) and successive carbonation (D2).
Chapter 7 (Discussion): all corrections made.
Page 25, comment “Really, Atg is the high-P mineral of the serpentinite group”. Antg is commonly found in subducted ultramafic rocks, however, Antg has also been obeserved in modern oceanic lithosphere (e.g., Früh-Green et al., 2004 – AGU Monograph series 144-and refrs. Therein) suggesting that Antg is sensitive to T more than P. We corrected the phrase into: …”high temperature/high pressure”…
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript presents an interesting and important case study of a subducted serpentinite mélange in the Zermatt-Saas ophiolite (Western Alps). The paper presents a complete lithological, meso and microstructural analysis of ophiolitic units and a remarkable interpretation of their possible origin, and evolution during the Alpine subduction. The distinctive characters of the units are clearly presented as well as the deformation phases affecting the rocks. The resulting interpretations are strong and fully supported by geological data. Figures are adequate and sufficiently clear. Overall, I find this manuscript sufficiently concise, well organized and written with some interesting data. However, I have some issues concerning references and figures of the Introduction and some minor comment about other parts of the manuscript. Therefore, I recommend publication after some minor revisions.
I here attach a commented pdf files of the manuscript which includes 41 comments.
Good luck with your revisions.
Comments for author File: Comments.pdf
Author Response
Reply to Review 2 (yellow highlighter in the marked text)
Page 2, comment #1: we adopt the term “Jurassic Tethyan ocean” throughout the paper
Page 2, comments #2,3,4: we added all the suggested references and adjusted consequently the ref. Numbers. We replaced the suggested ref. “Caby et al., 1981” (regarding the Hoggar region (Algeria) with Caby et al. 1978 (ref. [56]).
Page 3, comments #1-7: Figure 1 has been improved by adopting all the suggested corrections. Comment 4: we deleted the terms “Combin zone” and “Zermatt Saas Zone” from the Legend.
Page 4: corrections made and references added
Page 5 (Page 6 of the Rev.): comma added
Page 7 comment #1: “exotic”. The meaning of this term, is “extra-basinal” with respect to the host successions (e.g., Cowan and Pini, 2001; Camerlenghi and Pini, 2009). The term “exotic” is commonly used in sedimentary mélange and it is counterposed to “native”. In the “Methods” section we added a sentence explaining that we refer to Festa et al. (2019a; see reference [4]) for the used mélange terminology. The “Method” chapter is partly moved to Supplementary material.
Page 8: corrections made on Figure 2. Although difficult in many instances, we tried to define the boundary among different units of the CCU by a dashed black line.
Page 10: correction made on Figure 4. We added a lithostratigraphic log for the BrFm1 in contact with overlying calcschists as specified in the text (see correction at Page 10.
Page 11: we added a sentence explaining the relation between BrFm1 and BrFm2 which have never been seen in direct contact (…”whereas the BrFm1 has never been observed at the base of BrFm2.”)
Page 12: see correction made at Page 7.
Page 12: correction made to Figure 6.
Page 15, comment #1: reference to Figure 7B has been added at present Page 14 of the text.
Page 15, comment #2: added the explanation as requested, at present Page 15 of the text.
Page 17: corrections made to Figure 8 at to the text.
Page 19, comments #1-3: corrections made in Table 1 (now Table S1).
Page 21, comments 1,2: corrections made to text and to Figure 9.
Page 28, comment #1: correction made to the text.
Author Response File: Author Response.docx
