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doi:10.2204/iodp.proc.322.212.2016

Introduction

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a complex drilling project of the Integrated Ocean Drilling Program (IODP) focused on understanding the mechanics of seismogenics and rupture propagation along subduction plate boundary faults (Tobin and Kinoshita, 2006). Two sites were cored during IODP Expeditions 322 and 333 on the subducting Philippine Sea plate in order to characterize the sediment section and upper igneous basement prior to deformation at the subduction front (see the “Expedition 322 summary” chapter [Underwood et al., 2010]; Expedition 333 Scientists, 2012). IODP Sites C0011 and C0012 sampled the Shikoku Basin 100 km southeast of the Kii Peninsula at the summit and on the northwest flank of the Kashinosaki Knoll, a prominent basement high (Fig. F1). Coring reached 876 meters below seafloor (mbsf) at Site C0011 by using the hydraulic piston coring system (HPCS) from 21 to 184 mbsf and the extended shoe coring system (ESCS) from 207 to 360 mbsf during Expedition 333 and by using rotary core barrel (RCB) drilling from 340 to 876 mbsf during Expedition 322. Drilling at Site C0012 penetrated into the igneous basement to 630.5 mbsf, recovering the sediment/basalt interface intact at 537.8 and 525.7 mbsf during Expeditions 322 and 333, respectively (see the “Expedition 322 summary” chapter [Underwood et al., 2010]; Expedition 333 Scientists, 2012). At Site C0012, core samples were obtained from RCB drilling during Expedition 322.

Oxygen isotope geochemistry has been widely applied to the study of sedimentation and diagenetic processes in continental and oceanic environments because the isotopic fractionation depends on both the temperature and the chemical composition of the minerals undergoing diagenesis. Examples of applications of the ¹⁸O/¹⁶O ratio in pore fluids include ion filtration processes, alteration of volcanogenic tephras and oceanic crust, evidence of diagenetically evolved fluid fluxes, gas hydrate dynamics, opal transformations to cristobalite, and quartz and clay dehydration reactions (Coplen and Hanshaw, 1973; Lawrence et al., 1975; Kastner et al., 1993; Tomaru et al., 2006; Kashiwaya et al., 2013; Kim et al., 2013). Isotopic geothermometry laws based on experimental, empirical, and theoretical data are available in scientific literature for most carbonate, silicate, and oxide minerals (Friedman and O’Neil, 1977; Savin and Lee, 1988; Sheppard and Gilg, 1996). Thus, the ¹⁸O/¹⁶O ratio in pore fluids is a valuable tracer, which, in the context of a full gamut of geochemical tracers, provides information on the origin of the fluid and on the nature of water–rock interactions (Destrigneville et al., 1991; Wilkinson et al., 1992). Here, we focus on the oxygen isotopic signature of pore fluids sampled from the sediment column prior to subduction.

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