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We present a quantitative examination of the liberation and subsequent deposition of silica at the subduction zone plate interface in the Mugi melange, an exhumed accretionary complex in the Shimanto Belt of southwest Japan. Frequency and thickness measurements indicate that mineralized veins hosted in deformed shales make up approximately 0.4% of the volume of this exposure. In addition, whole-rock geochemical evidence suggests that the net volume of SiO2 liberated from the melange at temperatures of < 200 degrees C was as much as 35%, with up to 40% of the SiO2 loss related to the smectite-illite (S-I) conversion reaction, and the rest attributable to the pressure solution of detrital quartz and feldspar. Kinetic modeling of the S-I reaction indicates active liberation of SiO2 at approximately 70 degrees C to 200 degrees C, with peak SiO2 loss at around 100 degrees C, although these estimates should be slightly shifted toward lower temperature conditions based on X-ray diffraction (XRD) analyses of mixed-layer S-I in the Mugi melange. The onset of pressure solution was not fully constrained, but has been documented to occur at around 150 degrees C in the study area. The deposition in deformed shales of quartz liberated by pressure solution and the S-I reaction is probably linked to seismogenic behavior along the plate interface by (1) progressively enhanced velocity-weakening properties, which are favorable for unstable seismogenic faulting, including very-low-frequency earthquakes and (2) increasing intrinsic frictional strength, which leads to a step-down of the plate boundary decollement into oceanic basalt

Quartz deposition and its influence on the deformation process of megathrusts in subduction zones