IODP Expedition 348: Nankai Trough Seismogenic Zone Experiment Stage 3, Plate Boundary Deep Riser

        The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multidisciplinary investigation of fault mechanics and seismogenesis along subduction megathrusts through reflection and refraction seismic imaging, direct sampling by drilling, in situ measurements, and long-term monitoring in conjunction with laboratory and numerical modeling studies. The fundamental objectives of NanTroSEIZE include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout an active plate boundary system. As part of the NanTroSEIZE program, operations during Integrated Ocean Drilling Program (IODP) Expedition 348 were planned to extend and case riser Hole C0002F, begun during IODP Expedition 326 in 2010 and continued during Expedition 338 in 2012, from 860 to 3600 meters below the seafloor (mbsf).

        Riser operations during Expedition 348 were carried out and deepened the hole to 3056 mbsf, including installation and cementing of 13⅜ inch casing to 2008.9 mbsf and 11¾ inch liner to 2922.5 mbsf. Reaching this depth required two sidetracking operations from the original Hole C0002F, resulting in the designation of Holes C0002N and C0002P for the successively deeper sidetracks. During drilling, a full suite of logging-while-drilling (LWD) and measurement-while-drilling (MWD), mud-gas, and cuttings data were collected over the interval from 2162.5 to 3058.5 mbsf in Hole C0002P, and a partial suite was collected in Hole C0002N. The interval from 2163 to 2218 mbsf was cored with the rotary core barrel (RCB). Reentry during planned future riser drilling operations will deepen the hole to penetrate the megasplay fault at ~4600–5000 mbsf.

        Additionally, a test hole for a prototype slimhole small-diameter RCB (SD-RCB) coring system, Hole C0002M, was drilled in riserless mode near Hole C0002F. The hole was advanced to 475 mbsf, where four cores were collected to 512.5 mbsf.

        Overall, Expedition 348 sampled and logged a deep interval in Holes C0002N and C0002P within the inner accretionary wedge, from 856 to 3056 mbsf, including a never-before sampled zone in the lowermost ~1 km of drilling. Cores were collected over a 55.5 m interval from 2163 to 2218.5 mbsf. The sampled sedimentary rocks are composed of hemipelagic sediment and fine turbidite with rare ash. The entire interval from ~2145.5 to 2945.5 mbsf has a depositional age of 9.56–10.73 Ma based on nannofossil first and last occurrence data, which is consistent with accretion of a middle Miocene section of either lower Shikoku Basin equivalent or Miocene-age trench fill; facies analysis suggests the former. Bedding attitudes were ubiquitously steep, measured at 60°–90° in both cores and resistivity image logs. A range of structural fabrics was sampled, including common development of scaly clay fabrics with polished and slickensided clayey surfaces at many depths throughout the drilled interval. Structural fabrics became progressively stronger with depth, and carbonate cement and veins became prevalent below 2100 mbsf. In the cored interval, a well-developed foliated fault zone was identified at 2204.9–2205.8 mbsf with unknown overall displacement sense or amount. This zone contains abundant carbonate cement and vein fill. Log data interpretation suggests at least one additional significant fault zone at ~2220 mbsf, based on fracture intensity and bedding dip anomalies, including apparent broad folds and overturned bedding. Log data also show that P-wave velocity (VP) and resistivity follow a trend of increasing with depth to ~1600 mbsf but vary little from this depth to the bottom of the hole. Average VP actually decreases slightly with depth over this interval, perhaps due to progressively increasing clay content with depth, increased fracturing or rock damage, or pore fluid overpressure.

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