Deformation Structures From Splay and Décollement Faults in the Nankai Accretionary Prism, SW Japan(IODP NanTroSEIZE Expedition 316): Evidence for Slow and Rapid Slip in Fault Rocks

Abstract

Abstract Drilling during IODP NanTroSEIZE Expedition 316 led to the recovery of cores from the basal décollement in the frontal part of the Nankai accretionary prism and from a splay fault branching from the décollement at 25‐km landward of the prism toe. The core from the splay fault shows a main shear zone and two secondary shear zones. The main shear zone can be divided into two subzones. The upper subzone consists of a 1.2‐mm thick foliated gouge zone truncated downward by a through‐going fault encompassing a 0.4‐mm thick weakly foliated gouge interval. A nearby 200‐μm thick granular injection vein is interpreted as derived from the fault. The lower subzone consists of a foliated clayey gouge. A 70‐μm thick granular injection vein is also observed along this subzone. In the basal décollement core, microstructures consist of foliated gouge along a flat‐lying shear zone and seven flat‐lying or gently dipping secondary or incipient shear zones above. A redox front lies beneath the main shear zone. The shear zone and the redox front are truncated by a fault surface outlined by microbreccia developed at the expense of the overlying foliated gouge. Foliated gouge from the shear zones is tentatively interpreted as resulting from slow slip or aseismic creep. The weakly foliated gouge, the microbreccia, and the granular injection veins are interpreted as resulting from coseismic slip. The presence of the redox front beneath the main shear zone of the décollement fault core is interpreted as a consequence of oxidizing fluid flow along the microbreccia‐bearing fault.