Expedition 316 summary

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 316 is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). This coordinated multiexpedition drilling project is designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies. Expedition 316 was designed to evaluate the deformation, inferred depth of detachment, structural partitioning, fault zone physical characteristics, and fluid flow at the frontal thrust and at the shallow portion of the megasplay system. To accomplish these objectives, drilling was conducted at two sites in the megasplay region, one within the fault zone and one in the slope basin seaward of the megasplay. Two sites were also drilled within the frontal thrust region. During IODP Expedition 314, two of these sites were characterized using logging while drilling. IODP Site C0004 is located along the slope of the accretionary prism landward of the inferred intersection of the megasplay fault zone with the seafloor. Drilling at this site examined the youngest sediments on the slope overlying the accretionary prism; these sediments consist of slowly deposited marine sediments and redeposited material from upslope. This redeposited material provides information about past slope failures, which may be related to past megasplay movement, earthquakes, and tsunamigenesis. The accretionary prism was sampled, and the megasplay fault zone was successfully drilled. The top of the prism corresponds to a prominent unconformity (age gap = ~1 m.y.) that displays mineralization by pyrite and other minerals. Structural observations of core material from the fault zone and two age reversals suggested by nannofossils indicate a complex history of deformation. Sediments under the fault zone were sampled to understand their deformation, consolidation, and fluid flow history. Drilling at IODP Site C0008 targeted the slope basin seaward of the megasplay fault. This basin records the history of fault movement. In addition, sediment layers within this basin provide a reference for sediment underthrusting the splay fault zone at Site C0004. Drilling at IODP Sites C0006 and C0007 allowed examination of the frontal thrust region. At Site C0006, several fault zones within the prism were penetrated before drilling was stopped because of poor conditions. The plate boundary frontal thrust was successfully drilled, and thrust fault material ranging from breccia to fault gouge was successfully recovered at Site C0007.