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Site C00021

M. Strasser, B. Dugan, K. Kanagawa, G.F. Moore, S. Toczko, L. Maeda, Y. Kido, K.T. Moe, Y. Sanada, L. Esteban, O. Fabbri, J. Geersen, S. Hammerschmidt, H. Hayashi, K. Heirman, A. Hüpers, M.J. Jurado Rodriguez, K. Kameo, T. Kanamatsu, H. Kitajima, H. Masuda, K. Milliken, R. Mishra, I. Motoyama, K. Olcott, K. Oohashi, K.T. Pickering, S.G. Ramirez, H. Rashid, D. Sawyer, A. Schleicher, Y. Shan, R. Skarbek, I. Song, T. Takeshita, T. Toki, J. Tudge, S. Webb, D.J. Wilson, H.-Y. Wu, and A. Yamaguchi2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site C0002 (proposed Site NT3-01B; Fig. F1) is the centerpiece of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project (Tobin and Kinoshita, 2006). Planned scientific and technical targets for IODP Expedition 338 included collecting logging-while-drilling (LWD), cuttings, and core data in the lower Kumano forearc basin and in the inner wedge of the Nankai accretionary complex and extending riser Hole C0002F to 3600 meters below seafloor (mbsf). This would extend the hole beyond the 20 inch casing point (860 mbsf), which was cemented in place during IODP Expedition 326 in 2010 (Expedition 326 Scientists, 2011) (Fig. F2). Riser drilling with the D/V Chikyu during Expedition 338 was to sample the interior of the accretionary complex in the midslope region beneath the Kumano forearc basin with both cores and drilling cuttings and to collect an extensive suite of LWD and mud-gas data to characterize the formation. Through the installation of two casing strings (16 inch casing from 860 to 2300 mbsf and 13⅜ inch casing from 2300 to 3600 mbsf), Expedition 338 was to prepare Hole C0002F for deeper drilling expected to reach the megasplay target during the 2014 and 2015 International Ocean Discovery Program riser drilling seasons. Because of weather and current conditions that caused the suspension of riser drilling operations (see below), LWD data and cuttings were only obtained from 860 to 2005 mbsf (Figs. F2, F3), and additional riserless coring (200–500, 900–940, and 1100–1120 mbsf) (Fig. F3) was completed at Site C0002 as part of contingency operations.

The uppermost 1400 m section at Site C0002 was characterized with a comprehensive LWD program during IODP Expedition 314 (Hole C0002A) (Fig. F4) (Expedition 314 Scientists, 2009). The intervals 0–204 and 475–1057 mbsf were cored during IODP Expedition 315 (Holes C0002D and C0002B) (Expedition 315 Scientists, 2009b). The Kumano forearc basin sedimentary package extends from the seafloor to ~940 mbsf and is underlain by the deformed inner wedge of the accretionary package. The seismic reflection character of the entire zone from ~940 mbsf to the megasplay reflection at ~5200 mbsf exhibits virtually no coherent reflections that would indicate intact stratal packages, which is in contrast to the outer accretionary wedge seaward of the megasplay fault system (Fig. F2; also see Moore et al., 2009). This seismic character is thought to indicate complex deformation within the inner wedge of the Nankai accretionary prism.

The primary research objectives for the interval drilled during Expedition 338 were

  1. Determination of the composition, age, stratigraphy, and internal style of deformation of the upper forearc basin section, the basin-to-prism transition, and the presumably Miocene accretionary complex;

  2. Reconstruction of the accretionary complex’s thermal, diagenetic, and metamorphic histories and comparison with present pressure and temperature conditions;

  3. Determination of horizontal stress within the deep interior of the inner wedge;

  4. Investigation of the mechanical state and behavior of the formation; and

  5. Characterization of the overall structural evolution of the Nankai accretionary prism and the current state of the upper plate above the seismogenic plate boundary thrust.

Cuttings and core samples were collected for geomechanical experiments to be completed at inferred in situ conditions, which will help constrain mechanical and hydrological properties of the inner wedge materials. Continuous cuttings analyses provided information on the lithologic constituents and their variation with depth in the inner accretionary wedge. Cuttings help ground-truth properties estimated by LWD. Careful consideration must be made with cuttings, however, as there is known mixing because of the reaming-while-drilling (RWD) process (see “Operations”).

Initial riser drilling plans required modification when, because of a newly found risk of riser operations during a quick change in wind direction (e.g., weather front passing) with the fast Kuroshio Current, riser operations were suspended and we were unable to extend Hole C0002F to 3600 mbsf or set any casing strings (see “Operations”). The revised operations plan included LWD with cuttings collection from 872.5 to 2005.5 mbsf (Hole C0002F) and riserless coring of the intervals 200–505 mbsf (Holes C0002K and C0002L), 900–940 mbsf (Hole C0002J), and 1100.5–1120 mbsf (Hole C0002H) (Fig. F3). Relative locations of all Site C0002 holes are shown in Figure F4. This revised plan provided LWD data and cuttings samples from the previously unsampled deeper part of the accretionary prism and core samples across the unresolved unconformity between the Kumano Basin sediment and underlying accretionary prism sediment and across the gas hydrate zone, which was not cored during Expedition 315. Thus, the revised plan enabled us not only to newly explore the inner accretionary wedge to 2005.5 mbsf but also to complement our knowledge of this site.

1 Strasser, M., Dugan, B., Kanagawa, K., Moore, G.F., Toczko, S., Maeda, L., Kido, Y., Moe, K.T., Sanada, Y., Esteban, L., Fabbri, O., Geersen, J., Hammerschmidt, S., Hayashi, H., Heirman, K., Hüpers, A., Jurado Rodriguez, M.J., Kameo, K., Kanamatsu, T., Kitajima, H., Masuda, H., Milliken, K., Mishra, R., Motoyama, I., Olcott, K., Oohashi, K., Pickering, K.T., Ramirez, S.G., Rashid, H., Sawyer, D., Schleicher, A., Shan, Y., Skarbek, R., Song, I., Takeshita, T., Toki, T., Tudge, J., Webb, S., Wilson, D.J., Wu, H.-Y., and Yamaguchi, A., 2014. Site C0002. In Strasser, M., Dugan, B., Kanagawa, K., Moore, G.F., Toczko, S., Maeda, L., and the Expedition 338 Scientists, Proc. IODP, 338: Yokohama (Integrated Ocean Drilling Program).

2Expedition 338 Scientists’ addresses.

Publication: 13 January 2014
MS 338-103