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doi:10.2204/iodp.proc.348.204.2017

Introduction

The Nankai Trough offshore southwest Japan has been targeted by scientific ocean drilling numerous times over the past four decades (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986; Shipboard Scientific Party, 1991, 2001; Moore et al., 2005). The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is the latest of those efforts (Ashi et al., 2009; Screaton et al., 2009; Tobin et al., 2009; Underwood et al., 2010; Expedition 333 Scientists, 2012; Strasser et al., 2014; see the “Expedition 348 summary” chapter [Tobin et al., 2015a]). Integrated Ocean Drilling Program (IODP) Site C0002 is located within the NanTroSEIZE transect near the seaward edge of the Kumano Basin (Fig. F1). IODP Expedition 348 used riser drilling to deepen the holes at Site C0002 into the middle of the inner accretionary prism (Fig. F2), reaching a record depth of 3058 meters below seafloor (mbsf) see the “Expedition 348 summary” chapter [Tobin et al., 2015a]). The common lithology there is hemipelagic mudstone with variable percentages of medium siltstone to fine sandstone (turbidites). A lithologic boundary occurs at 2625 mbsf, as defined by a reduction of sandstone content and a shift from silty claystone above to fine silty claystone below (Fig. F2). Bedding planes within the accretionary prism dip at consistently steep angles.

This report summarizes the results of X-ray diffraction (XRD) analyses of 102 cuttings samples (1–4 mm) extracted from Hole C0002P. The cuttings from Hole C0002P contain nannofossils that are late Miocene (9.56–10.764 Ma) in age (see the “Site C0002” chapter [Tobin et al., 2015b]). Previous investigators of the Nankai region demonstrated that hemipelagic mud(stones) change in composition largely as function of depositional age (Cook et al., 1975; Chamley, 1980; Chamley et al., 1986; Underwood et al., 1993a, 1993b; Steurer and Underwood, 2003; Underwood and Steurer, 2003; Guo and Underwood, 2012; Underwood and Guo, 2013; Underwood and Song, 2016). As a rule, deposition of mud near the Nankai Trough during the Miocene was dominated by expandable clay minerals (smectite group), whereas proportions of detrital illite and chlorite increased steadily during the Pliocene and Quaternary. The primary objective of this report is to show whether or not samples from Hole C0002P conform to that temporal trend. A second goal is to document the diagenetic and hydration states of clay minerals (especially the smectite group). That task is important because of the clay’s likely influence on fluid production within deeper levels of the accretionary prism, as well as along the landward-dipping plate interface (e.g., Saffer et al., 2008).

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