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doi:10.2204/iodp.proc.314315316.202.2012

Introduction

The Nankai Trough subduction zone is the product of convergence between the Philippine Sea plate and the Eurasian plate (Fig. F1). Many sites have been drilled and cored in this region over the past four decades, including those of Deep Sea Drilling Project (DSDP) Legs 31 and 87 (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986) and Ocean Drilling Program (ODP) Legs 131, 190, and 196 (Taira et al., 1992; Moore et al., 2001, 2005). Expeditions 314, 315, and 316 of the Integrated Ocean Drilling Program (IODP) focused on a new transect—the Kumano transect—during Stage 1 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) (see the “Expedition 314 summary,” “Expedition 315 summary,” and “Expedition 316 summary” chapters [Tobin et al., 2009; Ashi et al., 2009; Screaton et al., 2009]). Moore et al. (2009) have provided a summary of structural and tectono-stratigraphic framework for the six Stage 1 coring sites. The three principal domains are the Kumano forearc basin (Site C0002), the shallow megasplay fault (Sites C0001, C0004, and C0008), and the frontal fault of the accretionary prism (Sites C0006 and C0007) (Fig. F2).

Previous investigations of clay minerals in the vicinity of the Nankai Trough and the Shikoku Basin demonstrated that the hemipelagic mud(stones) change in composition largely as function of depositional age (Cook et al., 1975; Chamley, 1980; Chamley et al., 1986; Fagel et al., 1992; Underwood et al., 1993a, 1993b; Steurer and Underwood, 2003; Underwood and Steurer, 2003). Miocene strata throughout the region tend to contain higher percentages of smectite, whereas Pliocene and Pleistocene deposits are more enriched in illite and chlorite. Clay diagenesis (particularly the smectite-to-illite reaction) is accentuated along the Muroto transect, where proximity to the paleospreading center of the subducting Shikoku Basin is responsible for higher heat flow (Underwood and Pickering, 1996; Masuda et al., 1996, 2001; Steurer and Underwood, 2003; Spinelli and Underwood, 2005; Saffer et al., 2008). To expand the documentation of clay composition into the Kumano transect area, we analyzed the clay mineral assemblages from 702 samples of hemipelagic mud and mudstone using X-ray diffraction (XRD). This report documents how the common clay minerals (smectite, illite, chlorite, and kaolinite) change in relative abundance as a function of depositional age, structural position, and lithostratigraphy. We also test whether or not smectite-to-illite diagenesis has progressed in a systematic pattern at shallow depths of accretionary prism.

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