Description: We examine the importance of dispersed volcanic ash as a critical component of the aluminosilicate sediment entering the Nankai Trough, located south of Japan’s island of Honshu, via the subducting Philippine Sea plate. Multivariate statistical analyses of an extensive major, trace, and rare earth element data set from bulk sediment and discrete ash layers at Integrated Ocean Drilling Program (IODP) Sites C0011 and C0012 quantitatively determine the abundance and accumulation of multiple aluminosilicate inputs to the Nankai subduction zone. We identify the eolian input of continental material to both sites, and we further find that there are an additional three ash sources from Kyushu and Honshu, Japan and other regions. Some of these ash sources may themselves represent mixtures of ash inputs, although the final compositions appear statistically distinct. The dispersed ash comprises 38 ± 7 weight percent (wt%) of the bulk sediment at Site C0011, and 34 ± 4 wt% at Site C0012. When considering the entire sediment thickness at Site C0011, the dispersed ash component supplies 38000 ± 7000 g/cm2 of material to the Nankai subduction system, whereas Site C0012 supplies 20000 ± 3000 g/cm2. These values are enormous compared to the ~2500 g/cm2 (C0011) and ~1200 g/cm2 (C0012) of ash in the discrete ash layers. Therefore, the mass of volcanic ash and chemically equivalent alteration products (e.g., smectite) that are dispersed throughout the stratigraphic succession of bulk sediment appears to be up to 15–17 times greater than the mass of discrete ash layers. The composition of the dispersed ash component at Site C0011 appears linked to that of the discrete layers, and the mass accumulation rate for dispersed ash correlates best with discrete ash layer thickness. In contrast, at Site C0012 the mass accumulation rate for dispersed ash correlates better with the number of ash layers. Together, the discrete ash layers, dispersed ash, and clay-mineral assemblages present a complete record of volcanism and erosion of volcanic sources; and indicate that mass balances and subduction factory budgets should include the mass of dispersed ash for a more accurate assessment of volcanic contributions to large-scale geochemical cycling.

Description: During IODP Expedition 322, an interval of Late Miocene (7.6 to ∼9.1 Ma) tuffaceous and volcaniclastic sandstones was discovered in the Shikoku Basin (Site C0011B), Nankai region. This interval consists of bioturbated silty claystone including four 1–7 m thick interbeds of tuffaceous sandstones (TST) containing 57–82% (by volume) pyroclasts. We use major and trace element glass compositions, as well as radiogenic isotope compositions, to show that the tuffaceous sandstones beds derived from single eruptive events, and that the majority (TST 1, 2, 3a) came from different eruptions from a similar source region, which we have identified to be the Japanese mainland, 350 km away. In particular, diagnostic trace element ratios (e.g., Th/La, Sm/La, Rb/Hf, Th/Nb, and U/Th) and isotopic data indicate a marked contribution from a mantle source beneath continental crust, which is most consistent with a Japanese mainland source and likely excludes the Izu-Bonin island arc and back arc as a source region for the younger TST beds. Nevertheless, some of the chemical data measured on the oldest sandstone bed (TST 3b, Unit IIb) show affinity to or can clearly be attributed to an Izu-Bonin composition. While we cannot completely exclude the possibility that all TST beds derived from unknown and exotic Izu-Bonin source(s), the collected lines of evidence are most consistent with an origin from the paleo-Honshu arc for TST 1 through 3a. We therefore suggest the former collision zone between the Izu-Bonin arc and Honshu paleo-arc as the most likely region where the eruptive products entered the ocean, also concurrent with nearby (∼200 km) possible Miocene source areas for the tuffaceous sandstones at the paleo-NE-Honshu arc. Estimating the distribution area of the tuffaceous sandstones in the Miocene between this source region and the ∼350 km distant Expedition 322, using bathymetric constraints, we calculate that the sandstone beds represent minimum erupted magma volumes between ∼1 and 17 km3 (Dense Rock Equivalent (DRE)). We conclude that several large volume eruptions occurred during the Late Miocene time next to the collision zone of paleo-Honshu and Izu-Bonin arc and covered the entire Philippine Sea plate with meter thick, sheet-like pyroclastic deposits that are now subducted in the Nankai subduction zone.

Description: Seismostratigraphy, coring, and logging while drilling during Integrated Ocean Drilling Program Expeditions 319, 322, and 333 (Sites C0011/C0012) show three Miocene submarine fans in the NE Shikoku Basin, with broadly coeval deposits at Ocean Drilling Program Site 1177 and Deep Sea Drilling Project Site 297 (NW Shikoku Basin). The sediment dispersal patterns have major implications for paleogeographies at that time. The oldest, finer-grained (Kyushu) fan has sheet-like geometry; quartz-rich flows were fed mostly from an ancestral landmass in the East China Sea. During prolonged hemipelagic mud deposition at C0011-C0012 (similar to 12.2 to 9.1 Ma), sand supply continued at Sites 1177 and 297. Sand delivery to much of the Shikoku Basin halted during a phase of sinistral strike slip to oblique plate motion, after which the Daiichi Zenisu Fan (similar to 9.1 to 8.0 Ma) was fed by submarine channels. The youngest fan (Daini Zenisu; similar to 8.0 to 7.6 Ma) has sheet-like geometry with thick-bedded, coarse-grained pumiceous sandstones. The pumice fragments were fed from a mixed provenance that included the collision zone of the Izu-Bonin and Honshu Arcs. The shift from channelized to sheet-like flows was favored by renewal of relatively rapid northward subduction, which accentuated the trench as a bathymetric depression. Increased sand supply appears to correlate with long-term eustatic lowstands of sea level. The stratigraphic position and 3-D geometry of the sandbodies have important implications for subduction-related processes, including the potential for focused fluid flow and fluid overpressures above and below the plate boundary fault: In sheet-like sands, pathways for fluid flow have greater horizontal permeability compared with those in channel sands.