Magnetic fabric analyses of ocean floor sediments: characterizing depositional processes in the Nankai Trough and Shikoku Basin

Magnetic fabric analysis was conducted on accretionary prism sediments and on sediments taken from the Shikoku Basin during Integrated Ocean Drilling Program (IODP) Expedition (Exp) 333 in order to characterize the sedimentary processes and depositional mechanisms active along the Nankai Trough. Sedimentary ages, in the form of a magnetostratigraphy, were also determined for these sediments during the course of this study. IODP Exp 333 focused coring efforts off the coast of the Kii Peninsula in Japan on three sites, one site within the Nankai Trough accretionary prism (Site C0018) and two sites seaward of the Nankai Trough in the Shikoku Basin (Sites C0011 and C0012). Exp 333 was designed to characterize the sedimentary sequence on the down-going plate and the uppermost igneous basement prior to their arrival at the Nankai Trough subduction front (Sites C0011 and C0012) and to constrain the submarine landslide history along the forearc slope as it relates to the megasplay fault (Site C0018). Sedimentary ages obtained from the magnetostratigraphy of the 314 m section at Site C0018 range from 0 to approximately 1.7 Ma (Pleistocene to Holocene). Sediments from Site C0011, a 380 m section, range in age from 0 to 7.12 Ma (Miocene to Holocene) and the sediments from Site C0012, a 180 m section, range in age from 0 to 8.25 Ma (Miocene to Holocene). Anisotropy of magnetic susceptibility (AMS) measurements have been used to identify sediment disturbance and compaction disequilibria, to characterize the depositional mechanisms for each site, and to identify sediment transport directions. Oblate AMS fabrics are dominant at all three locations. At Site C0018 significant sediment disturbance due to submarine landslide events was recognized. Current deposition is an important mechanism at Sites C0018 and C0011. Depositional mechanisms at Site C0012 are dominated by slope gravity deposition, which is gravity-controlled deposition of sediment in the water column onto an inclined surface, and the interaction of slope gravity with grain collision. The sediment transport direction at Site C0018 is predominantly to the SE. The paleocurrent at Site C0011 has had two distinct stages: SW from ~7.12 to ~5.24 Ma, and predominantly NW-SE from ~5.24 Ma to present, altering the accumulation of sediment at Site C0011. Site C0012 sediment transport directions are generally to the SE from 7.6 to 7.5 Ma and ESE from 7.5 Ma to 7.14 Ma. There is no distinct sediment transport direction recorded in the sediments from 7.14 to present at Site C0012. Compaction disequilibria were noted at Sites C0011 and C0012 and have been attributed to the presence of silica cement in the sediments. Analysis of the paleomagnetic record with depth has been used to determine the effects of compaction and slumping on the paleomagnetic signal. In order to accurately define a relationship between inclination and compaction in these sediments, it is necessary to understand the depositional history and consolidation history at each site. Due to sediment disturbance caused by submarine landslide events at Site C0018, inclination values have been significantly altered and therefore are unusable in paleomagnetic reconstructions. Significant inclination shallowing is not recorded at Sites C0011 and C0012, which may be due to reduced compaction caused by the possible presence of silica cement in the sediments from both sites. Inclination error is not universally found in sediments and therefore sediments can record the geomagnetic field without significant bias