11:45 〜 12:00
[SCG39-05] Constraints on silica transport along subduction boundaries based on volume change estimates of metamorphic rocks
キーワード：体積変化、変形脈、スロー地震、シリカ移動
Transport of significant quantities of SiO2 along subduction boundaries has been highlighted as important in processes such as silicification of the lower crust that may influence the time scales of slow earthquakes and the formation of a broad antigorite-rich domain along the base of the wedge mantle. The source of such silica is thought to be subducted quartz-rich sediments and water-rich fluids transported along the subduction boundary have the potential to transport silica over large distances. However, there are few studies that can help place bounds of the amount of silica that has been transported in subducted rocks. The removal of silica from one domain and its redeposition in another should be related to large changes in rock volume. The changes in volume that are important are those that take place at depth where the metamorphic temperature is the greatest and where the rocks are influenced by deep-sourced fluids. Previously proposed methods to estimate the volume change of rocks can be broadly classified into two categories: geometric methods based on strain estimates including absolute stretching, and chemical methods based on the bulk chemical compositions. However, application of the geometric methods to slate belts generally indicates large volume decreases not recognized by chemical methods. Possible reasons for this discrepancy are: uncertainties associated with assumptions about the original composition and the nature of immobile elements may and lack of good methods to assess the contribution of grain boundary sliding to bulk rock deformation. This study examines how orientations and deformation types of deformed mineral vein sets can be used to estimate volume change. This possibility of using this approach has been previously recognized but its practical application has been little studied. The deformed vein set analysis method is potentially more reliable than other geometric approaches because veins develop on scales much larger than individual grains and should record the otherwise hidden contribution to deformation by grain boundary sliding. In this study we developed a new approach that incorporates a statistical analysis to evaluate appropriate uncertainties. The newly developed approach was applied to analyze deformed metagreywacke in the Del Puerto canyon of the Franciscan belt. The analysis allows constraints to be placed on all three components of finite deformation: strain, mean vorticity number and volume change. The results are compatible with negligible volume change but also allow for significant volume increase. The results are not compatible with previously estimated significant amounts of volume reduction in the same region based on grain-scale analysis. The lack of prominent overgrowths around individual grains suggests that any volume increase is limited. Strain analysis using the Rf /φ method applied to individual grains showed maximum shortening was about 50%, implying that about 10% of strain was provided by grain boundary sliding. Furthermore, comparing the results of the Rf /φ method with those of the deformed vein sets method, it is suggested that the deformation during subduction is negligible compared to that developed during exhumation. The lack of volume change implies a lack of large-scale silica transport in the region either due to a limited fluid flux and/or silica saturation. This implies that there is not sufficient silica to form antigorite domains with kilometer-scale thicknesses or forming major silicification of the lower crust. Our data suggest that zones of antigorite-rich domains along the base of the mantle wedge will be more limited than generally thought and processes other than silica deposition may control time scales of slow earthquakes.