Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication

The late Miocene to Pliocene Minobu Formation and its equivalents in the Southern Fossa Magna Region, central Japan, consist of turbidites and associated coarse clastic deposits. The thickness of the Minobu Formation and its equivalents attains up to 3 km. In those sediments, sixteen sedimentary facies are recognized, and they are grouped further into eight facies associations; Conglomeratic, Conglomeratic-sandy, Conglomerate-Sandstone, Sandy, Silty, Sandy-silty, Silty and Pebbly siltstone associations. Among them, the first four associations are interpreted as channel fill sediments. While the second three associations are thought to be interchannel sediments. Interpretation of such facies associations as mentioned above is based on the following points; 1) comparative studies of sedimentary facies and facies association in the study area with those reported from modern and ancient turbidite basins, 2) morphologic analysis of sedimentary body of facies association and 3) dispersal pattern of sediments. The last of those associations is assumed from its characteristic sedimentary features to be slump deposits which flowed down along such steep slope as fault scarp. Result of such interpretations leads further to the conclusion that sedimentary environments of the Minobu Formation and its equivalents is fillings of a trough which extends in N-S direction throughout the area. Furthermore, from sedimentological analysis of the trough fillings, feeder channel extending ENE-WSW direction, western boundary fault scarps and an axial channel of a few kilometres in width are revealed. Then it is named herein Fujikawa Trough. Petrography and dispersal pattern of clastic sediments indicate that the main source area was situated around the Kwanto mountainland, more than 40 kilometres away from Fujikawa Trough. Comparison of Fujikawa Trough with modern trough and trench is made. Consequently, Suruga Trough seems to show the closest similarity by the following reasons of 1) similarity of internal topographic features such as western boundary fault scarps and an axial channel of a few kilometres wide, 2) similarity of the axial direction of trough and 3) similarity of the sedimentary ratio of fill sediments. Such geomorphologic similarity of both trough is important, and suggests that Fujikawa Trough was formed as a consequence of collision of Izu-Bonin Arc into Southwest Japan Arc in the same mannar as Suruga Trough is developing now

(Table 2) CT scan measurements of ODP Site 131-808

X-ray computed tomography (CT) is a promising tool that yields data useful for understanding the fine-scale density structure of partly lithified and tectonically deformed sediments. We conducted 21 CT scans of ODP Leg 131 sediments, including whole-round cores and thin-section chips, obtained from the toe of the Nankai accretionary prism. The samples range from highly deformed pieces from the frontal thrust and décollement to homogeneous and essentially undeformed sediments above the frontal thrust and beneath the décollement. In the CT images, kink-like deformation bands and faults are recognized as obvious bright seams, bands, or stripes with relatively high linear attenuation coefficients. The differences in linear attenuation coefficients relative to the matrix range from 0.021 cm**2/g (kink-like deformation band) to 0.038 cm**2/g (fault). These data suggest a 0.10 g/cm**3 to 0.18 g/cm**3 increase in bulk density within the deformation structures, and they appear to be 13% and 33% more compacted than the nondeformed matrix, respectively. In contrast to the samples from the frontal thrust zone, CT images of the décollement sample exhibit relatively homogeneous textures. The attenuation coefficient of the sample of the décollement indicates bulk density and porosity values of 2.45 g/cm**3 and 18%, respectively. The sample, hence, is approximately 50% more compacted than the sediment outside the décollement zone

Mineralogy and major-element chemistry of volcanic ash of ODP Site 131-808

Mineralogical and major-element compositions of 72 samples of volcanic ash, recovered from Site 808 at Nankai Trough during Leg 131, were analyzed in relation to the early diagenetic alteration. Alteration products are first observed at the following depths: smectite, 200 mbsf; clinoptilolite, 646 mbsf; and analcite, 810 mbsf. Glass decomposition dominates over authigenic mineral formation between 200 and 550 mbsf in the sediment column, whereas mineral formation becomes dominant below 550 mbsf. Based on the X-ray diffraction patterns, a broad and asymmetric peak of 15A suggests a presence of illite/smectite (I/S) mixed-layered minerals in a sample from 646 mbsf. I/S mixed-layered mineral formation, however, rarely occurs even at the bottom of the sediment column (1290 mbsf) at 120° C. This is possibly because zeolite (especially clinoptilolite) formed in the ash interferes with illite formation in the smectite. The formation of alteration minerals affects the major-element chemistry of the ash and the interstitial waters. H4SiO4 concentrations in interstitial waters increase during glass decomposition and decrease with smectite and clinoptilolite formation. K is removed from interstitial water into smectite and/or clinoptilolite. Mg is fixed into smectite (and/or chlorite)

(Table 1) Summary of CT values, attenuation coefficients, density, and porosity of ODP Site 131-808A samples

This paper presents results of an interdisciplinary investigation of the relation between fluids, fluid flow, and deformation in the toe region of the Nankai accretionary prism. The techniques include thin-section petrography, SEM, TEM and microprobe analyses, and X-ray computed tomography as well as laboratory experiments. Together, the data suggest three structural/hydrologic regimes within the prism. These are: (1) the accreting sediments above the décollement zone, (2) the décollement zone, and (3) the underthrust sediments. The regime above the décollement is characterized by sediments that are progressively dewatered through both a penetrative fabric and a pervasive, but apparently poorly interconnected, set of core-scale deformation structures. The décollement is characterized by a relatively high density of structures/meter and is considered to be a regime of low stress but frequent failure. Hydrologically the décollement retards the vertical flow of fluids and enhances the potential for overpressuring in the footwall. Finally, the footwall regime contains very few tectonic structures and is structurally isolated from the stresses related to plate convergence. This regime provides an important component to the tectonics of the Nankai prism, however, because it supplies the overpressured fluids that cause the d?collement to fail at relatively low shear stresses