Linking Faults, Fractures, and Clay Mineral Occurrence With Fluid Transport in the Accretionary Prism of the Nankai Trough, Japan

Abstract

The interpretation of logging while drilling resistivity oriented images and geophysical logs from Hole C0002 drilled during the International Ocean Discovery Program (IODP) Expeditions 338 and 348 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project revealed a complex structure of the accretionary prism characterized by steep bedding and abundant folds and fault zones, indicative of specific areas of deformation within the clay‐dominated prism sediments. Local faults were undoubtedly interpreted on images from Hole C0002P (2,162.5–3,058.4 m below seafloor), whereas folds were identified in both Holes C0002P and C0002F (860–2,005.5 m below seafloor). The clay mineral analysis of rock samples (cuttings) shows that clay mineralization occurs within the range of the faulted and fractured zones. This linkage between local deformation and clay mineralization can have implications for enhanced fluid flow and localized fluid‐rock interaction within the highly fractured areas of the accretionary prism. We conclude that dissolution and neomineralization of illitic and smectitic phases within fractures and faults is a process that probably continues during active creep and fault reactivation and plays a key role in influencing weak fault and creep behavior within the Nankai Trough, and likely in shallow fault systems elsewhere.