Estimating subduction boundary stresses and incorporation in next-generation modeling

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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.