Previous   |    Next

doi:10.2204/iodp.proc.322.202.2013

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 sites from 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). Integrated Ocean Drilling Program (IODP) Expeditions 314, 315, and 316 focused on a new transect—the Kumano transect—during Stage 1 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) (Ashi et al., 2009; Screaton et al., 2009; Tobin et al., 2009). Subsequent drilling during IODP Expedition 322 concentrated on inputs to the subduction zone by coring two sites in the Shikoku Basin (see the “Expedition 322 summary” chapter [Underwood et al., 2010]). IODP Sites C0011 and C0012 are located on the northwest flank and at the summit of a subducting basement high known as Kashinosaki Knoll (Ike et al., 2008) (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). Underwood (2007) summarized findings from the so-called reference sites along the Muroto and Ashizuri transects (ODP Sites 1173 and 1177) (Fig. F1), which are of particular interest for comparisons with Sites C0011 and C0012. Miocene strata throughout the Nankai-Shikoku region tend to contain higher percentages of smectite, whereas Pliocene and Pleistocene deposits are more enriched in illite and chlorite. This temporal trend also exists in the shallow accretionary prism and forearc basin of the Kumano transect (Guo and Underwood, 2012).

The abundance and hydration state of expandable clay minerals are important factors to consider during the NanTroSEIZE project because of their influence on fluid production as depth increases along the plate interface (Saffer et al., 2008). Clay diagenesis (particularly the smectite-to-illite reaction) is more advanced along the Muroto transect (Site 1173), 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). In contrast, there is no evidence for presubduction diagenesis at Site 1177 (Steurer and Underwood, 2003). To document clay composition in the Kumano transect area, particularly for those stratigraphic units that eventually affect conditions near the subduction megathrust, we analyzed the clay mineral assemblages from 292 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 and lithostratigraphy. We also test whether or not smectite-to-illite diagenesis has progressed to any measurable extent prior to the arrival of sedimentary strata at the subduction front.

Top of page   |    Previous   |    Next