Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone
Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to underst...
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| Format: | Article |
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MDPI AG
2024-02-01
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| Series: | Minerals |
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| Online Access: | https://www.mdpi.com/2075-163X/14/3/229 |
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| author | Hiroto Endo Katsuyoshi Michibayashi Takamoto Okudaira David Mainprice |
| author_facet | Hiroto Endo Katsuyoshi Michibayashi Takamoto Okudaira David Mainprice |
| author_sort | Hiroto Endo |
| collection | DOAJ |
| description | Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 10<sup>17</sup>–10<sup>19</sup> Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line. |
| first_indexed | 2024-04-24T17:59:30Z |
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| id | doaj.art-864b9af1bc87471396deb28902c9e00d |
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| issn | 2075-163X |
| language | English |
| last_indexed | 2024-04-24T17:59:30Z |
| publishDate | 2024-02-01 |
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| series | Minerals |
| spelling | doaj.art-864b9af1bc87471396deb28902c9e00d2024-03-27T13:56:09ZengMDPI AGMinerals2075-163X2024-02-0114322910.3390/min14030229Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear ZoneHiroto Endo0Katsuyoshi Michibayashi1Takamoto Okudaira2David Mainprice3Department of Science, Graduate School of Integrated Science and Technology, Shizuoka University, Shizuoka 422-8529, JapanDepartment of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, JapanDepartment of Geosciences, Osaka Metropolitan University, Osaka 558-8585, JapanGéoscience, Université de Montpellier 2, CNRS, CEDEX 05, 34095 Montpellier, FranceUltramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 10<sup>17</sup>–10<sup>19</sup> Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line.https://www.mdpi.com/2075-163X/14/3/229viscosity contrastflow lawquartzplagioclasecrystal fabricmylonite |
| spellingShingle | Hiroto Endo Katsuyoshi Michibayashi Takamoto Okudaira David Mainprice Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone Minerals viscosity contrast flow law quartz plagioclase crystal fabric mylonite |
| title | Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone |
| title_full | Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone |
| title_fullStr | Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone |
| title_full_unstemmed | Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone |
| title_short | Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone |
| title_sort | effect of low viscosity contrast between quartz and plagioclase on creep behavior of the mid crustal shear zone |
| topic | viscosity contrast flow law quartz plagioclase crystal fabric mylonite |
| url | https://www.mdpi.com/2075-163X/14/3/229 |
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