2022 Fiscal Year Annual Research Report
Estimating subduction boundary stresses and incorporation in next-generation modeling
| Project/Area Number |
21H01188
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| Allocation Type | Single-year Grants |
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| Research Institution | The University of Tokyo |
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Principal Investigator |
WALLIS R・Simon 東京大学, 大学院理学系研究科(理学部), 教授 (30263065)
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| Co-Investigator(Kenkyū-buntansha) |
纐纈 佑衣 名古屋大学, 環境学研究科, 講師 (20726385)
武藤 潤 東北大学, 理学研究科, 教授 (40545787)
遠藤 俊祐 島根大学, 学術研究院環境システム科学系, 准教授 (60738326)
森 宏 信州大学, 学術研究院理学系, 助教 (80788183)
青矢 睦月 徳島大学, 大学院社会産業理工学研究部(理工学域), 准教授 (90415638)
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| Project Period (FY) |
2021-04-01 – 2026-03-31
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| Keywords | Subduction / Stress / Sanbagawa metamorphism / Temperature / Thermal modeling |
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| Outline of Annual Research Achievements |
The purpose of this research is to use the rock record in subduction-type metamorphic belts, and in particular the well-developed and studied Sanbagawa belt of south west Japan, to estimate the shear stress acting along the subduction interface and incorporate these results in new thermal models of subduction zones. An important part of this study relies on accurate determination of the metamorphic temperature. We have made signifiant progress with this issue. In particular, we have shown that conventional methods for estimating Fe(II)/(III) ratios in the higher grade metamorphic rocks of the Sanbagawa belt are in sufficient to give accurate temperature estimates based on Mg-Fe cation exchange thermometry. We carried out numerous experiments using synchrotron radiation-based Micro XANES methods supported by Moessbauer analyses. There are still several aspects of the method that need to be refined before reliable results can be obtained, but the results of the previous year show considerable promise in the possibility to obtain significantly improved temperature estimates in some high-grade samples. We have also made significant progress in the analyses of microstructures required to derive reliable stress estimates. The progress includes evaluating the sectioning effect in measurements of grain size; something which is not incorporated in most previous studies. We also explored the theoretical background for approaches to use measurements of natural samples deformed under plane stress to laboratory examples deformed under uniaxial shortening.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
We have produced some good results over the last academic year that will form the basis for further development in the next. The main goal is to derive stress estimates. This has not yet been finalized but in combination with the improved temperature estimates that we have been able to develop, we have also made steady progress in the work necessary to analyze and quantify the microstructures of deformed metamorphic rocks that will allow us to estimate stress. We have also carried out thermo-mechanical modeling of convergent margins to investigate time-dependent behavior of the slab. Overall the project is on schedule to produce good results within the time span of the grant.
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| Strategy for Future Research Activity |
We will continue with the work designed to improve our understanding of the thermal record of the Sanbagawa belt and other subduction-type metamorphic domains. This will include adapting the current analytical equipment for the micro XANES analyses and obtaining suitable standards. It is also important to have an estimate of the deformation temperature since the peak metamorphic temperature recorded by mineral assemblages and mineral chemistry is not necessarily the same as that at which the rocks deformed. To obtain the deformation temperature, we have examined the utility of using the geometry of quartz crystallographic orientation patterns. We have obtained useful results consistent with other estimates. However, to verify these empirically-derived estimates we also plan to carry out measurements of Ti content of quartz grains using SIMS and have already obtained the agreement of a lab with relevant experience in the USA. Stress estimates from samples derived from different depths will be important to assess how stress changes down dip of the subduction zone. There is also the possibility or likelihood of along-strike heterogeneity and obtaining samples over a broad area will also make a significant contribution to this study. We intend to expand our collection of suitable material to include samples from appropriate metamorphic grades and positions along strike.
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