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doi:10.2204/iodp.proc.314315316.215.2011

Introduction

The Nankai Trough is formed by subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate (Fig. F1). Here the plate motion of the Philippine Sea plate relative to southwest Japan is commonly quoted as ~4 cm/y, but the rate may be as high as 6.5 cm/y (Seno et al., 1993; Miyazaki and Heki, 2001). The convergence direction is approximately normal to the trench, and sediments of the Shikoku Basin are actively accreting at the deformation front (Kimura et al., 2007). Integrated Ocean Drilling Program (IODP) Expedition 316 is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) complex drilling project. The fundamental goal of NanTroSEIZE is to sample and instrument the plate boundary system at several locations offshore the Kii Peninsula of southwest Japan, where violent, large-scale earthquakes have occurred repeatedly throughout history (Ando, 1975; Tobin and Kinoshita, 2006a, 2006b). The last major earthquakes in this region occurred in 1944 and 1946 (Fig. F1), and the next earthquake is anticipated in the middle of this century.

During Expedition 316, cores were collected from four sites along a transect lying almost perpendicular to the Nankai Trough offshore the Kii Peninsula, central/southwestern Japan (Fig. F2). Two major thrust fault systems were sampled at relatively shallow depths: (1) the shallow portion of the megasplay system (Sites C0004 and C0008) and (2) the initial faulting (or frontal thrust) Sites C0006 and C0007.

Drilling at Site C0004, which is located along the slope of the accretionary prism landward of the inferred intersection of the megasplay fault zone with the seafloor (Fig. F3), revealed that sediments consist of slowly deposited marine sediments and redeposited material from further upslope. This redeposited material provides information about past slope failures, which may be related to past megasplay movement, earthquakes, and tsunamigenesis. Cores from the megasplay fault zone record a complex history of deformation based on structural observations and two age reversals suggested by nannofossil evidence (Kimura et al., 2008). Our postexpedition study suggests that splay fault activity varies through time, with alternating high-activity periods during which splay fault thrusting accommodates a large part of the plate convergence (Strasser et al., 2009). This implies periods of overall accretionary prism mechanical stability alternating with periods of prism instability, the forcing factor(s) of which remain to be discovered.

Site C0008 was drilled at the slope basin seaward of the megasplay fault (Fig. F3). This basin records the history of fault movement. In addition, sediment layers within this basin provide a reference for the sediment underthrust beneath Site C0004. A simplified summary of lithostratigraphic units recovered at Sites C0004 and C0008 is shown in Figure F4.

Drilling at Sites C0006 and C0007 examined the frontal thrust region of the Nankai Trough (Fig. F5). At Site C0006, several fault zones within the prism were reached and sampled. At Site C0007, the plate boundary frontal thrust was penetrated and thrust fault material ranging from breccia to fault gouge was successfully recovered (Kimura et al., 2008). Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth (Kimura et al., 2008). Site C0006 lithostratigraphy consists of three units (Kimura et al., 2008); Site C0007 lithostratigraphy consists of four (Fig. F6). Turbidite deposits below the slope apron units at both Sites C0006 and C0007 were interpreted by the Expedition 316 scientists as trench deposits. Based on our anisotropy of magnetic susceptibility (AMS) results, Kitamura et al. (2010) proposed a new model for the structural evolution at the toe of the prism, with the following elements:

• Underthrusting sediments induce horizontal stress in the lower part of the wedge, which reduces the effective stress and forms a high–pore pressure anomaly and zones of fracture.

• The frontal thrust is bent geometrically and ceases activity in response to an increase of friction that triggers initiation of the next-generation frontal thrust.

• The upper part of the wedge tilts accordingly, resulting in an unstable slope.

During Expedition 316, pass-through magnetometer measurements were taken on all split-core archive sections. In order to isolate the characteristic remanent magnetization (ChRM), cores were subjected to alternating-field (AF) and thermal demagnetization. Several lithologies have very high coercivity (i.e., resistance to demagnetization by alternating fields) and maximum unblocking temperatures (i.e., the temperature at which natural remanence is completely removed by heating and cooling in a zero-field environment) close to 580°C. Other lithologies are dominated by softer magnetization, with unblocking temperatures of 300°–400°C. Stable ChRM components are observed throughout the majority of the recovered cores, following removal of a low-stability drilling-induced remanence. In some cases, however, the ChRM is not resolved. Several intervals in the frontal thrust sites (e.g., 43.00–138.18 mbsf in Hole C0007B) show little drilling-induced overprinting both in natural remanent magnetization (NRM) intensity and directions. The recovered materials have very high NRM intensities. Preliminary shipboard paleomagnetic results suggested that additional and more detailed paleomagnetic and rock magnetic data were needed from shore-based laboratories to interpret the magnetic behavior of Expedition 316 cores.

Here we present new results from such a study, which involves detailed paleomagnetic and rock magnetic measurements on samples from megasplay system Sites C0004 and C0008 and frontal thrust Sites C0006 and C0007. The magnetic results from this study will help to more fully understand the nature and origin of the remanence carriers, the history of the sedimentation process, and the faulting activities of the Nankai Trough seismogenic zone.

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