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doi:10.2204/iodp.proc.348.203.2017

Introduction

Scientific ocean drilling has sampled the Nankai Trough offshore southwest Japan (Fig. F1) numerous times over the past four decades (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986; Shipboard Scientific Party, 1991; Shipboard Scientific Party, 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., 2014a; see the “Expedition 348 summary” chapter [Tobin et al., 2015a]). Integrated Ocean Drilling Program (IODP) Site C0002 is located on the NanTroSEIZE transect near the seaward edge of the Kumano Basin (Fig. F1). Expedition 348 deepened the holes at Site C0002 by riser drilling into the middle 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 in cuttings is clayey siltstone (hemipelagic mudstone), with variable percentages of medium silt to fine sand (turbidites). A lithologic boundary occurs at 1665 mbsf in Hole C0002N and is defined by a sharp reduction of sandstone content (Fig. F3). During Expedition 338 (Strasser et al., 2014a), that same boundary in Hole C0002F was placed at 1740 mbsf (Fig. F3).

This report summarizes the results of X-ray diffraction (XRD) analyses of 73 cuttings samples (1–4 mm) extracted from Hole C0002N. Previous investigators demonstrated that hemipelagic mud(stones) throughout the Nankai region 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). The cuttings from Hole C0002N contain nannofossils that are late Miocene (>5.59 Ma) in age (see the “Site C0002” chapter [Tobin et al., 2015b]). As a rule, mud deposited near Nankai Trough during the Miocene was enriched in expandable clay (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 C0002N conform to that temporal trend. Another fundamental goal of NanTroSEIZE is to document the diagenetic and hydration states of clay minerals (especially the smectite group) within accreted and subducting Nankai strata (i.e., subduction inputs). That task is important because of the clay’s likely influence on fluid production within the accretionary prism and along the landward-dipping plate interface (e.g., Saffer et al., 2008). This report also includes characterization of clay mineral diagenesis over the depth range of Hole C0002N.

The drilling technology during Expedition 338 obscured the stratigraphic details of Hole C0002F by placing a 12¼ inch drill bit at the base of the bottom hole assembly and a 20 inch reamer bit above the logging tools. That configuration of two cutting tools resulted in thorough mixing of cuttings over a vertical interval of at least 43.8 m (Strasser et al., 2014a). Hole C0002N was drilled without the reamer bit, so the vertical extent of mixing was reduced considerably. A third objective of this report is to assess how those differences in mixing might have affected compositional data, through a direct comparison among data from equivalent depths in Holes C0002F and C0002N.

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