16:30 〜 16:45
[SCG45-31] Stress distribution in a subduction channel associated with rapid subduction: evidence from quartz piezometry applied to the Sanbagawa subduction-type metamorphic belt
キーワード：石英、転位クリープ、動的再結晶、応力、沈み込みプレート境界、深部スロー地震
Shear stress at the subduction plate interface is used for understanding earthquake phenomena and as an important input in subduction zone modeling. However, it is difficult to measure and is estimated by simulation or a combination of other parameters, with large associated uncertainties. Rocks originally located deep in subduction zones can record information about deformation processes, such as shear stress conditions, occurring in regions which cannot be directly accessed. Analysis of such samples has the potential to provide improved stress estimates. In this study, we estimated shear stress along the subduction plate interface by using samples from the Sanbagawa belt, in which blocks of mantle wedge-derived serpentinite are widely distributed and in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the ancient subduction plate interface.
To obtain estimates of shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks—quartz is the main component of the rocks we collected and its deformation stress is assumed to be representative of the region. Shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c-axis fabric opening-angle thermometer), and apparent grain size of dynamically recrystallized quartz on a thin section to an appropriate piezometer. Combined with sample deformation depth, which is estimated from PT path and deformation temperatures, it is suggested that shear stress does not change much in the estimated depth range of 20 to 27 km, ranging from 18 to 36 MPa when calculated for uniaxial compression and from 21 to 42 MPa when calculated for plane stress conditions. In addition to down-dip changes in stress we also examined possible along-strike stress heterogeneity. Samples collected over a 20×20 km area were analyzed to examine heterogeneities in shear stress along the subduction plate interface.
The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Some samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquakes domain. Estimated shear stress may represent the initial conditions from which slow earthquakes in the same domain nucleated.
To obtain estimates of shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks—quartz is the main component of the rocks we collected and its deformation stress is assumed to be representative of the region. Shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c-axis fabric opening-angle thermometer), and apparent grain size of dynamically recrystallized quartz on a thin section to an appropriate piezometer. Combined with sample deformation depth, which is estimated from PT path and deformation temperatures, it is suggested that shear stress does not change much in the estimated depth range of 20 to 27 km, ranging from 18 to 36 MPa when calculated for uniaxial compression and from 21 to 42 MPa when calculated for plane stress conditions. In addition to down-dip changes in stress we also examined possible along-strike stress heterogeneity. Samples collected over a 20×20 km area were analyzed to examine heterogeneities in shear stress along the subduction plate interface.
The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Some samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquakes domain. Estimated shear stress may represent the initial conditions from which slow earthquakes in the same domain nucleated.