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Integrated Ocean Drilling Program (IODP) Expeditions 322 and 333 are part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). One goal of that project is to determine how geologic differences affect mechanical properties, permeability, fluid flow, pore pressure, shear strength, and earthquake rupture processes within the subduction zone (Tobin and Kinoshita, 2006). Material on the plate approaching the Nankai Trough was sampled during Expeditions 322 and 333 in order to quantify the inputs to the subduction system.

We focus on amorphous silica within the sedimentary section on the incoming plate at Sites C0011 and C0012. Amorphous silica may cement grain contacts, strengthening the sediment and influencing deformation in the NanTroSEIZE transect (Spinelli et al., 2007). Portions of the sedimentary section approaching the Nankai Trough subduction zone at Ocean Drilling Program (ODP) Sites 1173 and 1177 have anomalous porosity, seismic velocity, and rigidity caused by small amounts (~1 wt%) of amorphous silica cement (Spinelli et al., 2007; White et al., 2011). These anomalies are consistent with experimental work demonstrating that even a very small volume of grain-coating cement can greatly increase sediment strength (e.g., Clough et al., 1981; Karig, 1993). Amorphous silica is rare in Shikoku Basin sediment; it is ~1% of the shallow sediment at Sites 1173 and 1177 and Deep Sea Drilling Project (DSDP) Sites 297, 442, 443, and 444 (Spinelli et al., 2007; White et al., 2011). Although biogenic silica (i.e., opal-A) is usually the most common form of amorphous silica in seafloor sediment (Barron and Whitman, 1981), the dominant constituent of amorphous silica in Shikoku Basin sediment is volcanic glass (White et al., 2011).

The strength of sediment entering a subduction zone controls the nature and distribution of sediment deformation (Karig and Morgan, 1994). Determining the amorphous silica content at Sites C0011 and C0012 helps quantify the distribution and abundance of silica cement along the Nankai Trough subduction zone system.