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doi:10.2204/iodp.proc.314315316.214.2011

Introduction

Accretionary prisms contain saturated sediments that are subject to intense deformation as a result of lithospheric plate convergence (e.g., Carson and Screaton, 1998). To better understand how the accretionary prism behaves in the Nankai Trough subduction zone, we measured the hydrological properties of whole-round specimens recovered at two Integrated Ocean Drilling Program (IODP) Expedition 316 sites (Fig. F1) (see the “Expedition 316 summary” chapter [Screaton et al., 2009a]). Permeability influences sediment consolidation and shear strength by governing pore fluid pressure and effective normal stress (Moore and Vrolijk, 1992; Saffer and Bekins, 2006). Elevated pore pressures (i.e., values greater than hydrostatic) play a critical role in the evolution of accretionary complexes, including the development of the décollement zone (Gamage and Screaton, 2006) and the taper angle of the accretionary wedge (Saffer and Bekins, 2002, 2006). By comparing hydrological properties within fault blocks and fault zones at various depths, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) will examine how geologic structures and permeability in the frontal Nankai accretionary prism might influence one another over a large range of effective stress values.

Previous laboratory tests of natural clay–rich sediment and shale reveal large ranges in values of intrinsic permeability (k) and hydraulic conductivity (K) because of differences in the material’s mineral composition, texture, and porosity (Bennett et al., 1989; Neuzil, 1994; Dewhurst et al., 1999; Yang and Aplin, 2007; Gamage et al., 2011). The hydrological properties of sediments and sedimentary rocks depend on many factors inherited from the time of deposition, including grain size and shape, sorting, type of particle association and arrangement, magnitude and strength of the forces between particles, and different scales of fabric elements (Moon and Hurst, 1984; Bennett et al., 1989, 1991; Mitchell, 1993). Fabric, in small scales, is directly related to the aggregation of particles and very small pores. These aggregated particles and pores control the fluid flow (Olsen, 1960; Delage and Lefebvre, 1984), and the fabric can be highly anisotropic (Anandarajah and Kuganenthira, 1995). The anisotropy of permeability (e.g., Clennell et al., 1999; Bolton et al., 2000) is particularly important in studies of subduction zones because it influences a variety of behaviors that are related to tectonic loading and fault-induced deformation.

To establish the extent of anisotropy, comparisons are made between horizontal (cross-core) permeability (k_h) and vertical (along-core) permeability (k_v) at the same sampling depth. When k_h = k_v, the sediment is isotropic. In many cases, preferred alignment of platy mineral grains in sedimentary deposits results in k_h > k_v. The ratio of horizontal to vertical permeability (k_h/k_v) for soils can range from <1 to >10 (Schwartz and Zhang, 2003). Permeability anisotropy usually varies with the thickness of sedimentary layers (varves, laminae, beds, etc.), depth of burial, and the magnitude of applied effective stress. As a general rule, long-term burial loading and chemical diagenesis impart changes in the volume and orientation of platy clay minerals in sedimentary basins. With deeper burial, the alignment of platy grains becomes almost perpendicular to the maximum principal effective stress (Sintubin, 1994; Kim et al., 1999; Aplin et al., 2006). Permeability should become more anisotropic in response to this evolving grain fabric because fluids physically seek the easiest flow path, which is usually along rather than across the direction of grain alignment.

In this report, we document the results of constant-flow permeability tests completed at the University of Missouri (USA). Two holes were cored at Site C0006 and two holes were cored at Site C0007 during Expedition 316 (see the “Expedition 316 Site C0006” [Expedition 316 Scientists, 2009b] and “Expedition 316 Site C0007” [Expedition 316 Scientists, 2009c] chapters). Expedition 316 was designed to evaluate the deformation, inferred depth of detachment, structural partitioning, fault zone physical characteristics, and fluid flow at the frontal thrust and at the shallow portion of the megasplay system (Screaton et al., 2009b). At Site C0006, several subsidiary fault zones within the prism were penetrated before drilling was stopped because of poor conditions. The frontal thrust was successfully drilled at Site C0007, and fault material ranging from breccia to fault gouge was recovered (see the “Expedition 316 summary” chapter [Screaton et al., 2009a]). The 11 samples that we tested are from the hanging wall of the frontal thrust (see Moore et al., 2009), with subbottom depths ranging from 34 to 564 m coring depth below seafloor (CSF) (Fig. F2). The main purpose of this report is to document the anisotropy of permeability in the hanging wall of the frontal thrust.

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