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doi:10.2204/iodp.proc.338.104.2014

Site C0012¹

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) Expeditions 322 and 333 were designed to characterize the sedimentary and upper igneous basement inputs to the Nankai Trough subduction zone (Expedition 333 Scientists, 2012a; Underwood et al., 2010). IODP Site C0012 is located in the Shikoku Basin on the crest of a prominent basement high (Kashinosaki Knoll) on the subducting Philippine Sea plate (Figs. F1, F2). This location provides access to the uppermost igneous crust with modest penetration below the seafloor. The rotary core barrel (RCB) was used during Expedition 322 to collect samples to 576 meters below seafloor (mbsf), which included sampling sediment and basement (Underwood et al., 2010). During Expedition 333, the hydraulic piston coring system (HPCS), the extended punch coring system (EPCS), the extended shoe coring system (ESCS), and the RCB were used to sample sediment and igneous basement to 630.5 mbsf (Expedition 333 Scientists, 2012a). As part of its contingency operations, IODP Expedition 338 used logging while drilling (LWD)/measurement while drilling (MWD) to collect geophysical logs to 710 mbsf. Together, these drilling operations provided core and logging data through the entire sedimentary succession and into the uppermost igneous basement.

Previous coring during Expeditions 322 and 333 revealed that the sedimentary succession at Site C0012 is dominated by silty clay or silty claystone with minor lithologies, including ash, volcaniclastic sandstone, and tuff (Expedition 333 Scientists, 2012a; Underwood et al., 2010). The base of the sedimentary section is reddish-brown pelagic claystone that is older than 18.9 Ma. Interstitial water in sediment above the igneous basement shows distinct trends that are indicative of diffusive processes, chemical reactions, and interaction with chemical species in the basement. Chlorinity values start at seawater values at the seafloor but increase to >600 mM near the top of basement. This increase is influenced by hydration reactions in the sediment and diffusional exchange with basement fluid. This chlorinity profile is in contrast to the profile at IODP Site C0011, where chlorinity decreases with depth. Therefore, it is interpreted that Site C0012 is less affected by the focused flow of fluids that originate beneath the trench wedge and frontal accretionary prism. The sulfate-reduction zone at Site C0012 is at ~250 mbsf, deeper than at other sites along the Nankai margin. Shear strength is relatively low in the upper 100 mbsf, which correlates to nearly constant high porosity. Below this depth, shear strength increases and porosity decreases. Porosity reaches a minimum just above the basement. Approximately 100 m of the igneous basement has been sampled and is divided into Units I (pillow lavas) and II (sheet flows with pillow lava interlayers) (Expedition 333 Scientists, 2012a). Alteration of Unit I pillow lava compositions is indicative of a reducing, low-temperature environment. Alteration of Unit II suggests oxidizing and reducing conditions, which most likely occurred at different times. Characterization of the sediment and igneous basement provide insights into the composition and origin of Kashinosaki Knoll.

The specific questions motivating the study of input Sites C0011 and C0012 include

• Is fluid circulation in basement and permeable sedimentary layers influencing heat flow and diagenesis?
• How does silica diagenesis affect physical properties at Sites C0011 and C0012?
• Was magmatic activity heterogeneous in composition and age on the backarc basin basement high (Kashinosaki Knoll)?
• How does heterogeneous alteration of oceanic crust influence geochemical and fluid budgets at the inputs in the subduction zone?
• Which factor(s) control(s) the décollement’s position near the toe of the Nankai accretionary prism and the location of ramps and flats and mechanical behavior throughout?
• Does the plate boundary fault, near its updip limit of seismicity, shift position from a sediment/sediment interface (stable sliding) to a sediment/basalt interface (stick-slip)? If so, what are the causes?

To address these objectives, LWD activities in Hole C0012H were conducted as contingency operations during Expedition 338 in order to provide a full suite of petrophysical logs that can be combined with analysis of cores and seismic data for a comprehensive characterization of the subduction zone inputs. LWD data generated high-resolution sampling of sediment and 180 m of the upper basement characteristics (i.e., extending basement penetration by ~80 m), including resistivity, resistivity images, natural gamma radiation, and compressional wave velocity. Specific objectives are to understand how compressional velocity relates to compaction state and fluid sources, how chemical profiles relate to physical and chemical properties, how the structures of the sedimentary section relate to the slumping, and how structures in igneous basement relate to the alteration state. Ultimately, these analyses will help define the fluid and chemical budgets of the subduction inputs, which are important to the understanding of fluid in the accretionary prism and the subducted materials.

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 C0012. In ‘, Proc. IODP, 338: Yokohama (Integrated Ocean Drilling Program). doi:10.2204/iodp.proc.338.104.2014

² Expedition 338 Scientists’ addresses.

Publication: 13 January 2014
MS 338-104

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