the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Feb 2023
Kinematics and time-resolved evolution of the main thrust-sense shear zone in the Eo-alpine orogenic wedge (the Vinschgau Shear Zone, Eastern Alps)
Abstract. The Vinschgau Shear Zone (VSZ) is one of the largest and most significant shear zones developed in plastic conditions within the Austroalpine domain, juxtaposing the Ötztal and the Texel units to the Campo, Scharl and Sesvenna units during the building of the Eo-alpine Orogen. The VSZ dominates the structural setting of a large portion of the central Austroalpine Late Cretaceous thrust stack. In order to fully assess the evolution of the VSZ, a multi-faceted approach based on detailed multiscale structural and petrochronological analyses has been carried out across representative transects of the shear zone in the Vinschgau Valley. The research has been performed with a view to characterizing kinematics, PT conditions and age of motion of the VSZ.
Our fieldwork-based analyses suggest that the dip of mylonitic foliation increases from W to E with an E-W trending stretching lineation which dips alternatively to the W and to the E, due to later folding related to the Cenozoic crustal shortening. The dominant top to the W-directed shear sense of the mylonites recognized in the field and confirmed by microstructural analyses is in agreement with the exhumation of the upper Austroalpine nappes in the hanging wall of the shear zone: the Texel unit with Late Cretaceous eclogites, the Schneeberg and Ötztal units both affected by Eo-alpine amphibolite-facies metamorphism. Chemical and microstructural analyses suggest deformation temperatures of ca. 350–400 °C during shearing. Timing of deformation along the VSZ has been constrained for the first time through 40Ar/39Ar dating of syn-shearing micas, which reveal a Late Cretaceous age of the VSZ mylonites with ages ranging between 80 and 97 Ma. A systematic younging trend of deformation occurs towards the central part of the shear zone in the studied transects. Vorticity analysis shows a clear decrease in the simple shear component correlated to the younging direction of mica ages towards the core of the shear zone. This evolution is consistent with the growth of a shear zone where strain localizes into its central part during deformation. The defined evolution of the VSZ sheds new light on how large-scale thrust-sense shear zones act and how much exhumation they can accommodate in the frame of an evolving orogenic wedge.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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AC1: 'Comment on egusphere-2023-126', Chiara Montemagni, 17 Feb 2023
Dear all,
I just realized that I uploaded the wrong version of figure 4.
In Fig. 4c the shear sense arrows point in the wrong direction and in Fig. 4e the fabric is a crenulation cleavage and obviously the shear sense arrows should not be there. The errors will be corrected in the final version of the manuscript.
I apologize for this!
Chiara Montemagni
Citation: https://doi.org/10.5194/egusphere-2023-126-AC1 -
RC1: 'Comment on egusphere-2023-126', Paolo Conti, 06 Mar 2023
The paper is very interesting, clear and well written, and can be published with only minor revisions. The figures are ok (fig 4c and 4e must be corrected). I have some concern using the therm "finite strain" in such rocks: grain size and shape of grains we observe today in rocks has nothing to do with finite strain suffered by rocks, but are only relate to deformation mechanisms and syntectonic (dynamic) recrystallization.
More comments and corrections in the attached PDF.
-
AC2: 'Reply on RC1', Chiara Montemagni, 13 Mar 2023
Dear Prof. Conti,
we wish to sincerely thank you for your interesting and useful comments, which will improve the final version of our paper.
Best regards,
Chiara Montemagni
Citation: https://doi.org/10.5194/egusphere-2023-126-AC2
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AC2: 'Reply on RC1', Chiara Montemagni, 13 Mar 2023
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RC2: 'Comment on egusphere-2023-126', Franz Neubauer, 06 Mar 2023
General remarks:
This is a very interesting manuscript documenting kinematics, white mica and biotite composition and corresponding 40Ar/39Ar ages from the Vinschgau Shear Zone in Eastern Alps and deduce a reasonable model. The Vinschgau shear zone is clearly one of the largest and most important eo-Alpine (Cretaceous) shear zones in the Austroalpine unit of Eastern Alps. This shear zone has a high importance because it exhumes also the eo-Alpine Texel eclogites in the hangingwall unit, for which a SHRIMP U-Pb age of 86 ± 5 Ma. The mixed structural and geochronological approach applied in three sections allow to document that the shear zone was active in an increasingly shallow crustal level. The ages vary over a relatively wide range between 97 and 80 Ma indicating that the western part with older ages was exhumed first. The manuscript represents a significant progress for tectonics in the western Austroalpine units and also includes methodological progress because of the combination of structural and geochronological methods.
To reach full clarity on the significance of the new data, I propose to pay attention to following points during revision:
1) Mention more clearly the only weak overprint within greenschist facies in the Campo-Ortler-Schesvenna units in the footwall, from which some old K-Ar and Rb-Sr whole-rock-white mica ages were published by Thöni 1981.
2) Use of “muscovite”: Personally, I prefer “white mica” because of two reasons: In the study, the EPMA analyses show a broad range of compositions, some have a relative high phengite component and are rather phengites/phengitic muscovites. The second problem is the quite common admixture of a paragonite component to white mica in greenschist facies rocks.
3) Figs. 7 - 9: It remains unclear which ages are actually listed in the Ar release patterns/age spectra on the left side. What means the stippled circIes in the diagrammes on the right side (T vs. Differential Release). Is there any information, from which mineral phase the Ca in all these patterns is coming from? Did you try calculate plateau and/or isotope inversion ages? The authors refer to their study of Himalayan shear zones (Montemagni & Villa, 2001), but this study is partly diferent by using mineral mixtures (e.g., white mica and chlorite).
4) More documentation of representative microfabrics of dated samples is urgently needed. In the text, several times two generations of white mica and biotite are mentioned.
5) Discuss the origin of the scatter of Ar-Ar ages in some studied sections, e.g., in Schlanders. Discuss also the different meaning of white mica and biotite ages. Could the scatter represent mixtures of variable composition as shown in EPMA data?
Specific remarks
- 42: Ratschbacher et al. 1991: I think the correct reference is Ratschbacher et al., 1989. Ratschbacher et al. 1991 discuss the Oligocene-Miocene extrusion. In Ratschbacher et al. (1989) eo-Alpine ductile structures are discussed.
- 42-44: “the entire central Austroalpine nappe stack was affected by W-directed tectonic transport during the first stages of the Late Cretaceous Alpine deformations”: I think this is not fully correct. In the Central Austroalpine nappe east of the Tauern window, thrust deformation deformation at ca. 100-88 Ma is overprinted by ESE-directed ductile normal faulting, mainly east of the Tauern window, but also by Schlinig fault and potentially the ductile shear zone at the structural base of te Steinach nappe.
- 45: Mention also the eo-Alpine (Cretaceous) age of the up to greenschist facies metamorphism of the Austroalpine units underneath the VSZ.
- 57-58: The Noric thrust (at the base of the Graywacke zone) is another huge important ductile thrust (Ratschbacher, 1986), and the first one studied by modern kinematic/structural methods.
- 85-86: Cite here also the recent paper on Schneeberg/Ötztal relationships: Klug & Froitzheim, 2022.
- 123: There is no formal "Middle Cretaceous" in the International Stratigraphic Chart. Write middle with lower case.
- 127: What is the metamorphic grade of the Matsch unit?
L.231-232: Only a comment: If there is a reasonable metamorphic T estimate, the phengite barometer of Massonne and Schreyer (1987) could be applied to estimate pressure.
- 248: “group 5 mica fish”?? Add: “accoding to the classification of Passchier and Trouw, 2005”
- 273: Mention why these Eyrs mylonite samples were not suitable for vorticity estimates.
- 345: Explain what is biotite-1 and biotite-2.
- 506: I think it is necessary to distinguish the eo-Alpine Cretaceous deformation iin Austroalpine units from Austroalpine/Adria/Europe collision during Paleogene times.
- 533-535: Based on structural and geochronological data, a similar model was also proposed for the type locality eclogites in the Saualpe region (Wiesinger et al., 2006). A recent study (Schulz and Krause, 2021) documented the younger age in amphibolite-grade footwall units underlying the eclogite-bearing unit.
- 539-540: Mention also where this normal-sense shear zone is located, e.g., top of Ötztal or Ötztal/Texel boundary
References mentioned in the review
Massonne, H.J., Schreyer, W., 1987. Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contrib. Mineral. Petrol. 96, 212–224.
Ratschbacher, L., 1986. Kinematics of Austro-Alpine cover nappes: changing translation path due to transpression. Tectonophsics 125, 335-356.
Ratschbacher, L., Neubauer, F., Frisch, W., Schmid, S.M. & Neugebauer, J., 1989. Extension in compressional orogenic belts: The eastern Alps. Geology 17, 404–407.
Schulz, B. & Krause, J. (2021): Electron probe petrochronology of polymetamorphic garnet micaschists in the lower nappe units of the Austroalpine Saualpe basement (Carinthia, Austria). Z. Dt. Ges. Geowiss. DOI: 10.1127/zdgg/2021/0247.
Thöni, M., 1981. Degree and Evolution of the Alpine Metamorphism in the Austroalpine Unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr Age Determinations on Micas. Jahrbuch der Geologischen Bundesanstalt, 124, 111–174.
Wiesinger, M., Neubauer, F., Handler, R.: Exhumation of the Saualpe eclogite unit, Eastern Alps: constraints from 40Ar/39Ar ages. Mineralogy and Petrology 88, 149–180.
Technical/editorial problems (see also few further remarks in the enclosed annotated pdf-file)
- 77: shear zone develops (singular)
- 163: Correct: “The paragneiss displays” or “Paragneisses display”
- 188: Correct: frquent
- 208: Explain abbreviation a.s.l.
- 237: Correct: ortogneiss
- 248: “group 5 mica fish”?? Add according to the classification of Passchier and Trouw, 2005.
- 251-252: Add: "lower temperatures during shearing than in the Juval section".
- 278: correct to: method reveals
- 321: explain abbreviation apfu for non-specialists
- 342: Fig. Xd = (Fig. 6d)?
- 345: Explain what is biotite-1 and biotite-2.
- 438: either: "mechanisms (Passchier and Trouw, 2005), which lead" or "mechanism (Passchier and Trouw, 2005), which leads"
- 442: Correct to: may also weaken
- 718: correct to: Rb-Sr
Fig. 1b: Where is the Naif thrust in the map? Furthermore, “calcschist with ophiolite”: misfit of color between legend and map.
Fig. 2: In lower hemisphere diagrams: increase size of Lm and poles to Sm.
Fig. 3: If possible, add approximate field orientations to all field photographs. Fig. 3h: Enlarge inset, it is too small to recognize calcite. Fig. 3d: The amphibolitic boudin does not look greenish (or arrow head is not exactly located?).
Fig. 4: Write sample nos. on these photomicrographs.
Figs. 7 - 9: It remains unclear which ages are actually listed in the Ar release patterns/age spectra on the left side. What means the stippled circIes in the diagrammes on the right side (T vs. Differential Release). Is there any information, from which mineral phase the Ca in all these patterns is coming from? Did you try calculate plateau and/or isotope inversion ages? Explain abbreviation EMPA in section 4.1.
Fig. 7: You could save much space writing the sample nos. insude of diagrams. A minor point: Use same scale in the age vs. Ca/K diagrams. Add information, which sort of 40Ar/39Ar ages are listed on diagrams and was the younger age of the first steps mean. Explain abbreviation EMPA in section 4.1.
-
AC3: 'Reply on RC2', Chiara Montemagni, 13 Mar 2023
Dear Prof. Neubauer,
we wish to sincerely thank you for your interesting and useful comments and corrections, which will improve the final version of our paper.
Best regards,
Chiara Montemagni
Citation: https://doi.org/10.5194/egusphere-2023-126-AC3
Interactive discussion
Status: closed
-
AC1: 'Comment on egusphere-2023-126', Chiara Montemagni, 17 Feb 2023
Dear all,
I just realized that I uploaded the wrong version of figure 4.
In Fig. 4c the shear sense arrows point in the wrong direction and in Fig. 4e the fabric is a crenulation cleavage and obviously the shear sense arrows should not be there. The errors will be corrected in the final version of the manuscript.
I apologize for this!
Chiara Montemagni
Citation: https://doi.org/10.5194/egusphere-2023-126-AC1 -
RC1: 'Comment on egusphere-2023-126', Paolo Conti, 06 Mar 2023
The paper is very interesting, clear and well written, and can be published with only minor revisions. The figures are ok (fig 4c and 4e must be corrected). I have some concern using the therm "finite strain" in such rocks: grain size and shape of grains we observe today in rocks has nothing to do with finite strain suffered by rocks, but are only relate to deformation mechanisms and syntectonic (dynamic) recrystallization.
More comments and corrections in the attached PDF.
-
AC2: 'Reply on RC1', Chiara Montemagni, 13 Mar 2023
Dear Prof. Conti,
we wish to sincerely thank you for your interesting and useful comments, which will improve the final version of our paper.
Best regards,
Chiara Montemagni
Citation: https://doi.org/10.5194/egusphere-2023-126-AC2
-
AC2: 'Reply on RC1', Chiara Montemagni, 13 Mar 2023
-
RC2: 'Comment on egusphere-2023-126', Franz Neubauer, 06 Mar 2023
General remarks:
This is a very interesting manuscript documenting kinematics, white mica and biotite composition and corresponding 40Ar/39Ar ages from the Vinschgau Shear Zone in Eastern Alps and deduce a reasonable model. The Vinschgau shear zone is clearly one of the largest and most important eo-Alpine (Cretaceous) shear zones in the Austroalpine unit of Eastern Alps. This shear zone has a high importance because it exhumes also the eo-Alpine Texel eclogites in the hangingwall unit, for which a SHRIMP U-Pb age of 86 ± 5 Ma. The mixed structural and geochronological approach applied in three sections allow to document that the shear zone was active in an increasingly shallow crustal level. The ages vary over a relatively wide range between 97 and 80 Ma indicating that the western part with older ages was exhumed first. The manuscript represents a significant progress for tectonics in the western Austroalpine units and also includes methodological progress because of the combination of structural and geochronological methods.
To reach full clarity on the significance of the new data, I propose to pay attention to following points during revision:
1) Mention more clearly the only weak overprint within greenschist facies in the Campo-Ortler-Schesvenna units in the footwall, from which some old K-Ar and Rb-Sr whole-rock-white mica ages were published by Thöni 1981.
2) Use of “muscovite”: Personally, I prefer “white mica” because of two reasons: In the study, the EPMA analyses show a broad range of compositions, some have a relative high phengite component and are rather phengites/phengitic muscovites. The second problem is the quite common admixture of a paragonite component to white mica in greenschist facies rocks.
3) Figs. 7 - 9: It remains unclear which ages are actually listed in the Ar release patterns/age spectra on the left side. What means the stippled circIes in the diagrammes on the right side (T vs. Differential Release). Is there any information, from which mineral phase the Ca in all these patterns is coming from? Did you try calculate plateau and/or isotope inversion ages? The authors refer to their study of Himalayan shear zones (Montemagni & Villa, 2001), but this study is partly diferent by using mineral mixtures (e.g., white mica and chlorite).
4) More documentation of representative microfabrics of dated samples is urgently needed. In the text, several times two generations of white mica and biotite are mentioned.
5) Discuss the origin of the scatter of Ar-Ar ages in some studied sections, e.g., in Schlanders. Discuss also the different meaning of white mica and biotite ages. Could the scatter represent mixtures of variable composition as shown in EPMA data?
Specific remarks
- 42: Ratschbacher et al. 1991: I think the correct reference is Ratschbacher et al., 1989. Ratschbacher et al. 1991 discuss the Oligocene-Miocene extrusion. In Ratschbacher et al. (1989) eo-Alpine ductile structures are discussed.
- 42-44: “the entire central Austroalpine nappe stack was affected by W-directed tectonic transport during the first stages of the Late Cretaceous Alpine deformations”: I think this is not fully correct. In the Central Austroalpine nappe east of the Tauern window, thrust deformation deformation at ca. 100-88 Ma is overprinted by ESE-directed ductile normal faulting, mainly east of the Tauern window, but also by Schlinig fault and potentially the ductile shear zone at the structural base of te Steinach nappe.
- 45: Mention also the eo-Alpine (Cretaceous) age of the up to greenschist facies metamorphism of the Austroalpine units underneath the VSZ.
- 57-58: The Noric thrust (at the base of the Graywacke zone) is another huge important ductile thrust (Ratschbacher, 1986), and the first one studied by modern kinematic/structural methods.
- 85-86: Cite here also the recent paper on Schneeberg/Ötztal relationships: Klug & Froitzheim, 2022.
- 123: There is no formal "Middle Cretaceous" in the International Stratigraphic Chart. Write middle with lower case.
- 127: What is the metamorphic grade of the Matsch unit?
L.231-232: Only a comment: If there is a reasonable metamorphic T estimate, the phengite barometer of Massonne and Schreyer (1987) could be applied to estimate pressure.
- 248: “group 5 mica fish”?? Add: “accoding to the classification of Passchier and Trouw, 2005”
- 273: Mention why these Eyrs mylonite samples were not suitable for vorticity estimates.
- 345: Explain what is biotite-1 and biotite-2.
- 506: I think it is necessary to distinguish the eo-Alpine Cretaceous deformation iin Austroalpine units from Austroalpine/Adria/Europe collision during Paleogene times.
- 533-535: Based on structural and geochronological data, a similar model was also proposed for the type locality eclogites in the Saualpe region (Wiesinger et al., 2006). A recent study (Schulz and Krause, 2021) documented the younger age in amphibolite-grade footwall units underlying the eclogite-bearing unit.
- 539-540: Mention also where this normal-sense shear zone is located, e.g., top of Ötztal or Ötztal/Texel boundary
References mentioned in the review
Massonne, H.J., Schreyer, W., 1987. Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contrib. Mineral. Petrol. 96, 212–224.
Ratschbacher, L., 1986. Kinematics of Austro-Alpine cover nappes: changing translation path due to transpression. Tectonophsics 125, 335-356.
Ratschbacher, L., Neubauer, F., Frisch, W., Schmid, S.M. & Neugebauer, J., 1989. Extension in compressional orogenic belts: The eastern Alps. Geology 17, 404–407.
Schulz, B. & Krause, J. (2021): Electron probe petrochronology of polymetamorphic garnet micaschists in the lower nappe units of the Austroalpine Saualpe basement (Carinthia, Austria). Z. Dt. Ges. Geowiss. DOI: 10.1127/zdgg/2021/0247.
Thöni, M., 1981. Degree and Evolution of the Alpine Metamorphism in the Austroalpine Unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr Age Determinations on Micas. Jahrbuch der Geologischen Bundesanstalt, 124, 111–174.
Wiesinger, M., Neubauer, F., Handler, R.: Exhumation of the Saualpe eclogite unit, Eastern Alps: constraints from 40Ar/39Ar ages. Mineralogy and Petrology 88, 149–180.
Technical/editorial problems (see also few further remarks in the enclosed annotated pdf-file)
- 77: shear zone develops (singular)
- 163: Correct: “The paragneiss displays” or “Paragneisses display”
- 188: Correct: frquent
- 208: Explain abbreviation a.s.l.
- 237: Correct: ortogneiss
- 248: “group 5 mica fish”?? Add according to the classification of Passchier and Trouw, 2005.
- 251-252: Add: "lower temperatures during shearing than in the Juval section".
- 278: correct to: method reveals
- 321: explain abbreviation apfu for non-specialists
- 342: Fig. Xd = (Fig. 6d)?
- 345: Explain what is biotite-1 and biotite-2.
- 438: either: "mechanisms (Passchier and Trouw, 2005), which lead" or "mechanism (Passchier and Trouw, 2005), which leads"
- 442: Correct to: may also weaken
- 718: correct to: Rb-Sr
Fig. 1b: Where is the Naif thrust in the map? Furthermore, “calcschist with ophiolite”: misfit of color between legend and map.
Fig. 2: In lower hemisphere diagrams: increase size of Lm and poles to Sm.
Fig. 3: If possible, add approximate field orientations to all field photographs. Fig. 3h: Enlarge inset, it is too small to recognize calcite. Fig. 3d: The amphibolitic boudin does not look greenish (or arrow head is not exactly located?).
Fig. 4: Write sample nos. on these photomicrographs.
Figs. 7 - 9: It remains unclear which ages are actually listed in the Ar release patterns/age spectra on the left side. What means the stippled circIes in the diagrammes on the right side (T vs. Differential Release). Is there any information, from which mineral phase the Ca in all these patterns is coming from? Did you try calculate plateau and/or isotope inversion ages? Explain abbreviation EMPA in section 4.1.
Fig. 7: You could save much space writing the sample nos. insude of diagrams. A minor point: Use same scale in the age vs. Ca/K diagrams. Add information, which sort of 40Ar/39Ar ages are listed on diagrams and was the younger age of the first steps mean. Explain abbreviation EMPA in section 4.1.
-
AC3: 'Reply on RC2', Chiara Montemagni, 13 Mar 2023
Dear Prof. Neubauer,
we wish to sincerely thank you for your interesting and useful comments and corrections, which will improve the final version of our paper.
Best regards,
Chiara Montemagni
Citation: https://doi.org/10.5194/egusphere-2023-126-AC3
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Chiara Montemagni
Stefano Zanchetta
Martina Rocca
Igor Maria Villa
Corrado Morelli
Volkmar Mair
Andrea Zanchi
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(3578 KB) - Metadata XML
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Supplement
(119 KB) - BibTeX
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- Final revised paper