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doi:10.2204/iodp.proc.322.203.2012

Introduction

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multistage, multiplatform drilling project within the Integrated Ocean Drilling Program (IODP) designed to shed light on fault mechanics and controls on seismogenesis along the Nankai Trough subduction megathrust (Tobin and Kinoshita, 2006). The project includes in situ measurements, long-term monitoring, and direct sampling of sediments and crustal rocks from the Nankai Trough subduction zone complex offshore the Kii Peninsula, southeast Japan (Fig. F1).

During the NanTroSEIZE project’s second stage, the sediments approaching the subduction zone were penetrated during IODP Expedition 322 approximately 15 km seaward of the trench. The purpose of this expedition within the project’s framework was to sample sediments, fluids, and basement rocks and to serve as reference sites for boreholes in the accretionary complex (see the “Expedition 322 summary” chapter [Underwood et al., 2010]). In 2009, Expedition 322 drilled two sites with the D/V Chikyu that were selected for their contrasting seafloor relief. The sites are on the northwest flank (Site C0011) and on the crest (Site C0012) of the Kashinosaki Knoll (Fig. F1). Five distinct sedimentary units were penetrated with well-correlated unit boundaries between the two sites (see the “Site C0011” and “Site C0012” chapters [Expedition 322 Scientists, 2010a, 2010b]) (Fig. F2). Visual core description documented predominantly silty claystone of Quaternary to middle Miocene age, but numerous sandstone interbeds were sampled in Units II (volcanic turbidite facies), IV (silty turbidite facies), and V (volcaniclastic-rich facies).

In the present study, we focus on geotechnical testing of recovered sediment samples with differences in lithology, which may exert fundamental control over compressibility, fluid flow, and excess pore pressures (Saffer et al., 2011). To predict these parameters in the subduction zone, it is vital to account for lithologic heterogeneity in consolidation behavior and hydrologic properties. To investigate the role of lithology, we tested five samples of silty claystone and six samples of sandstone from Sites C0011 and C0012 from lithologic Units II–V of the subduction inputs (Table T1). Incremental loading and constant rate of strain (CRS) consolidation tests were conducted on intact and remolded samples to determine maximum past effective stress and in situ intrinsic permeability as well as compressibility and hydraulic conductivity during uniaxial deformation. NanTroSEIZE-related studies that focus on coupled fluid flow and sediment deformation would benefit from these measurements, which describe a lithologically diverse section of sediment inputs.

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