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doi:10.2204/iodp.proc.348.101.2015 Expedition 348 summary1H. Tobin, T. Hirose, D. Saffer, S. Toczko, L. Maeda, Y. Kubo, B. Boston, A. Broderick, K. Brown, A. Crespo-Blanc, E. Even, S. Fuchida, R. Fukuchi, S. Hammerschmidt, P. Henry, M. Josh, M.J. Jurado, H. Kitajima, M. Kitamura, A. Maia, M. Otsubo, J. Sample, A. Schleicher, H. Sone, C. Song, R. Valdez, Y. Yamamoto, K. Yang, Y. Sanada, Y. Kido, and Y. Hamada2AbstractThe Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multidisciplinary investigation of fault mechanics and seismogenesis along subduction megathrusts and includes 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 are to characterize 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 3058 mbsf, a new maximum depth record in scientific ocean drilling. Operations included installation and cementing of 13⅜ inch casing to 2008.9 mbsf and an 11¾ inch liner to 2922.5 mbsf, stabilizing the borehole for future deepening. 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 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). Planned future riser drilling operations will deepen the hole to penetrate the plate boundary fault at ~4600–5200 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 3058.5 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 turbidites with rare ash. The entire interval from ~2145.5 to 2945.5 mbsf has an apparent depositional age of 9.56–10.73 Ma based on nannofossil first and last occurrence data. These ages are 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 resistivity follows a trend of increasing with depth to ~1600 mbsf but varies little from this depth to the bottom of the hole. P-wave velocity (VP) also increases to ~1600 mbsf and then decreases slightly with depth to the bottom of the hole, perhaps due to progressively increasing clay content with depth, increased fracturing or rock damage, or pore fluid overpressure. |
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