Shionomisaki Canyon, off Kii Peninsula, SW Japan, cuts through five E–W-trending ridges developed in the Nankai accretionary prism. Using the submersible SHINKAI 6500, we investigated outcrops along the eastern canyon wall in the landward-most ridge. Four dives yielded information on lateral variations in the structure, texture, and physical properties of sediments along the splay fault zone in the accretionary prism. The ridge itself is disrupted by E–W-trending gullies that continue to bifurcations in the splay fault. Southward (seaward) dipping strata are predominant in gently folded, commonly steeply inclined sediments of gravity flow origin. The south-younging sequence is consistent with the radiolarian biostratigraphy. The sediments in this region are Pliocene to Recent in age (<4.3 Ma). Detailed observations reveal soft-sediment deformation structures such as web structure, vein structure, and deformation bands. The distribution of structures and chemosynthetic biocommunities (vesicomyid bivalves and tube worms) marks the position of faults that accompany active cold seepages. Porosity decreases southward toward the splay fault, showing a negative correlation with the age of the sedimentation (i.e., older sediments have higher porosity). This finding indicates progressive tectonic compaction toward the splay fault. The uniaxial compressive strength of sandstone, as calculated from a needle penetration test, shows a marked increase above the deduced fault zones. We attribute this finding to the precipitation of carbonate cement from CaCO3-saturated fluids that migrated along the splay fault and its bifurcations. Sandstones with high pore-connectivity and permeability, located above the faults, acted as a channel through which CaCO3-saturated fluids migrated. The related cementation acted to strengthen the frontal part of the ridge, which may act as an indenter for the inner wedge and as a backstop for the outer wedge. The proposed model accounts for the southward decrease in porosity behind the ridge front.
Four samples of the Setouchi volcanic rocks from two different Miocene formations on Shodo-Shima Island, SW Japan, were dated multi-chronologically using whole-rock and mineral K-Ar and Ar-Ar, zircon fission-track, and zircon U-Pb techniques, in order to better estimate their eruption ages. Statistical and chronological analyses of the results suggest that the lower and upper formations formed at 14.3-14.4 Ma and 13.1-13.4 Ma, respectively, which is consistent with geological relationships. U-Pb dating tends to show a reliable eruption age for analyses of magmatic zircon, and fission-track dating is suitable for determining the eruption age if the sample is largely free of thermal alteration. A careful examination of the contribution of old basement rocks and alteration is required to constrain the eruption age based on Ar-Ar plateau and K-Ar age data. The ubiquitous occurrence of xenocrystic zircon, even in the least-differentiated high-Mg andesite, indicates the involvement of pre-existing crust in the magma. The U-Pb ages of these zircons reveal the involvement of Late Cretaceous granitic basement, as well as older components.
A Late Cretaceous ammonite was discovered from the lower section of the Mugi Melange of the Shimanto Belt in the Kusunoura area, Mugi, Tokushima Prefecture, Japan. The ammonite is identified with Gaudryceras cf. tombetsense reported from the K2 unit (Maastrichian) of the Krasnoyarka Formation, Makarov area, southern Sakhalin, Russia. The lower section of the Mugi Melange is assigned to the early Paleogene, based on a recently obtained U-Pb zircon age. It is reasonable to assume that the ammonite from the Kusunoura area was trapped as an exotic block within the lower section of the Mugi Melange.
K-Ar ages of 0.07±0.02Ma, 0.09±0.02Ma, 0.50±0.31Ma, and 0.09±0.04Ma were newly obtained for the Ponmoshirito lava dome, Oshidomari pon’yama lava dome, Oshidomari lava and Minamihama lava respectively, from the Rishiri Volcano off north Hokkaido, Japan. Data for three of the lavas (not the Oshidomari lava) confirm an eruptive history in which the initial stage of volcanic activity lasted from 130 to 70ka.
Luminescence dating involves the observation of naturally accumulated radiation damage, caused by radioisotopes such as U and Th, in the form of the glow stimulated by heating or exposure to light. The emission color of luminescence varies among samples. A thermoluminescence color image (TLCI) can be recorded using a digital camera after irradiation with gamma rays, which results in significant radiation damage, thereby enhancing the luminescence. A new application was developed to handle RGB (red-green-blue) information for each pixel. The application enables the automatic plotting of pixel data on a CIE (Commission Internationale de l’Eclairoge) chromaticity diagram and counting of the number of pixels within a color zone, as defined by dividing the color space into five color zones. The application helps to handle many digital TLCIs in a short time and is useful for statistical color analyses.