Trench‐Parallel Fluid Migration and Its Transient Discharge in a Cold Subduction Zone Decoded by Geochemistry of Subducted Cherts

Abstract The presence of fluid plays a critical role in controlling the fault slips and the seismogenesis in subduction zones. However, the origins and pathways of the fluid remain poorly understood. In this study, we examined the Inuyama sequence in the Mino belt of Japan, which is an on‐land analog of a cold subduction zone such as the Japan Trench. In this region, white chert layers are intercalated within red chert layers, and the white chert layers were originally fluid conduits within subducting sediments along the plate interface according to the mass balance of the precipitated silica that constitutes the white chert. Trace element concentrations and Sr and Nd isotopic compositions of the chert layers suggest that the seawater expelled from subducting sediments due to compaction was the main carrier of silica. The temperature for the silica precipitation was approximately 60°C based on the O isotopic composition of the white chert layers, equivalent to the depth condition of approximately 5 km below the seafloor based on the analogy with the modern Japan Trench. As the major element concentrations suggest that the compaction and dehydration of the surrounding sediments could not fulfill the fluid volume required for supplying silica to fluid conduits, there may be trench‐parallel fluid migration to one location where silica was precipitated. Furthermore, considering the fluid volume that would lead to silica precipitation, the permeability of the fluid conduit could be higher than that of the typical subducting siliceous sediments, suggesting that fluid flow toward shallow depths was transient.