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doi:10.2204/iodp.proc.322.205.2013

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 322 is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project and aimed at understanding the initial presubduction conditions because the downdip evolution of those initial properties is what ultimately changes slip behavior along the plate interface from aseismic to seismic (Vrolijk, 1990; Hyndman et al., 1997; Moore and Saffer, 2001; Tobin and Kinoshita, 2006). Expedition 322 sampled and logged the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan) prior to their arrival and burial at the Nankai subduction front (Saito et al., 2009; Underwood et al., 2010). Two sites were drilled: IODP Site C0011 on the northwest flank of the bathymetric high called Kashinosaki Knoll and IODP Site C0012 near the crest of the seamount (Fig. F1). Here we analyze samples from Site C0011.

During the early–middle Miocene the Shikoku Basin formed as part of the Philippine Sea plate (Okino et al., 1994; Kobayashi et al., 1995), which is currently moving toward the northwest beneath the Eurasian plate, roughly orthogonal to the axis of the Nankai Trough, at a rate of ~4–6 cm/y (Seno et al., 1993; Miyazaki and Heki, 2001) (Fig. F1). Deposits within the Shikoku Basin and the overlying Quaternary trench wedge are actively accreting at the deformation front (Tobin et al., 2009).

At Site C0011, logging-while-drilling and measurement-while-drilling data were collected at the end of IODP Expedition 319 in Hole C0011A, whereas a 536 m thick succession was cored in Hole C0011B during Expedition 322 starting at 340 meters below seafloor (mbsf). The age at the bottom of the cored succession is poorly constrained within the range of middle Miocene (~14.0 Ma) (Underwood et al., 2009). The igneous basement lies at ~1050 mbsf.

Understanding compression and permeability behavior as well as overpressure and fluid flow in mudstone is critical for the identification and exploration of petroleum reserves, for any basin modeling studies, and for evaluating the stability of continental slopes. This study benefits NanTroSEIZE by understanding how porosity, permeability, and compressibility of mudstones in their initial presubduction conditions evolve over large effective stress ranges and for varying compositions, which is important for several applications: (1) pore pressure prediction in accretionary prisms, (2) accretionary wedge geometry and morphology, and (3) larger scale deformation and fluid flow in the forearc of convergent margins.

We used a method called resedimentation to prepare homogeneous and undisturbed samples in the laboratory under controlled stress conditions. The key advantages of resedimented over intact core samples are homogeneity, no disturbance, and controlled stress history. Thus, resedimentation is ideal for geotechnical experimentation in situations where drilling-induced disturbance is present and where systematic studies are being performed to understand fundamental sediment behavior. The samples are mixtures of varying proportions of silty claystone, mostly from the lower Shikoku Basin hemipelagic facies from depths between 580.4 and 865.9 mbsf in Hole C0011B, and commercially available silt-size silica. We then conducted Atterberg limits measurements, particle size analyses, and constant rate of strain (CRS) consolidation tests and took scanning electron microscope (SEM) images in order to understand the compositional influence on deformation, flow behavior, and microstructure such as the change in packing and particle alignment.

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