Data report: permeability, compressibility, stress state, and grain size of shallow sediments from Sites C0004, C0006, C0007, and C0008 of the Nankai accretionary complex

Abstract

The Nankai Trough Seismogenic Zone Experiment is a multiexpedition project carried out in several stages by the Integrated Ocean Drilling Program. The first stage of the experiment included two cruises during which coring took place (Expeditions 315 and 316). We have studied grain size distribution in more than 600 specimens of cores from Sites C0001, C0002, C0004, and C0006-C0008 and correlated the distributions to the lithologies recovered, logging and logging-while-drilling data in nearby holes, and the overall tectonic setting. The two sites in the upper slope and Kumano Basin (Sites C0001 and C0002, respectively) show the smallest mean grain sizes, which are smaller in the accreted strata compared to the forearc basin or slope sediments. Further downslope (Sites C0004 and C0008), grain size variability is larger and lithologies range from slope apron, mass transport deposits, accreted mud/mudstones, and fault zone material, to underthrust hemipelagics. Surprisingly, the mass transport deposits at Site C0004 are fine grained, whereas their counterparts at Site C0008 slightly downslope are the coarsest within the recovered succession there. The two sites near the toe of the accretionary prism (Sites C0006 and C0007) provided the coarsest material recovered, namely the trench-slope transition facies and the uppermost accreted trench wedge facies. Mudstones from the underlying upper Shikoku Basin facies are well sorted, uniform, and fine grained. In general, the study provides a solid basis for more specialized sedimentological research and further reveals that (1) results of the measurements taken with the Coulter Counter LS200 laser particle analyzer are in good agreement with earlier onboard visual core description; (2) accuracy and reproducibility of the method is satisfactory given the very narrow error bars of repeat measurements on aliquots of a larger sediment sample; and (3) the mass transport deposits have distinct particle size distribution patterns, whereas fault gouge and other material from structural features cannot easily be distinguished from surrounding sediment by grain size analysis alone.

Authigenesis of magnetic minerals in gas hydrate-bearing sediments in the Nankai Trough, offshore Japan

“…[], type 2 samples are associated with the occurrence of turbidites. However, there is no clear evidence of turbidites in the 93–99 and 107.5–110 m CSF depth intervals, based on core description, clay assemblages [ Guo and Underwood , ] and grain size analyses [ Kopf et al ., ], although these intervals seem coarser. Thus lithological variations are unlikely to be the main factor explaining the distribution of type 1 and type 2 samples, although this cannot be entirely ruled out.…”

Strain decoupling reveals variable seismogenic risk in SEJapan (Nankai Trough)

“…The relatively shallow nature of the Nankai Trough is due in part to subduction of young shallow Shikoku Basin crust, and to the thick sediment cover of the subducting plate [Moore et al, 1990]. This sediment-dominated subduction zone has led to the accumulation of a thick accretionary prism, several hundred kilometers wide Kopf et al, 2010], constrained to the northwest by the Kumano Basin and to the southeast by the Shikoku Basin (Figure 1b).…”

“…The accretionary prism of Nankai consists of accreted and underplated trench-fill turbidite facies Kopf et al, 2010], transported axially along the Nankai Trough as a result of subduction of the Izu-Bonin arc during the Pliocene-Pleistocene [Taira and Niitsuma, 1986]. Hemipelagic mudstones and volcanic ash from the Shikoku Basin have also been drilled [Moore et al, 1990;Brown et al, 2003].…”

Coupled fluid flow and deformation modeling of the frontal thrust region of the Kumano Basin transect, Japan: Implications for fluid pressures and decollement downstepping

“…Detailed methods are provided by Rowe et al [2011]. Samples were generally clayey silt, with a few samples containing significant amounts (<5%) of sand [Kopf et al, 2011]. Permeability of each sample was measured at three effective stresses.…”

“…Permeability of Sites C0006 and C0007 samples plotted as a function of porosity. Samples are grouped as either less than or greater than 5% sand based on grain-size data fromKopf et al [2011]. Permeabilityporosity trends from the Muroto transect and relationships used in the simulation are plotted.…”