Evidence of a Low-Velocity Layer beneath the Accretionary Prism of the Nankai Trough: Inferences from a Synthetic Sonic Log

Aoki, Yasuhiro; Kinoshita, Hajimu; Kagami, Hideo

doi:10.2973/dsdp.proc.87.120.1986

Cited by 13 publications

(13 citation statements)

References 2 publications

(2 reference statements)

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“…Active sediment accretion has taken place since the Miocene period in the Nankai Trough, which is one of the bestdocumented modern accretionary prisms (for example, Aoki et al 1982Aoki et al , 1986Kalko Research Group 1986;Moore et al 1990). …”

Section: Geologic Setting Of Samplesmentioning

confidence: 99%

Relation Between Interlayer Composition of Authigenic Smectite, Mineral Assemblages, I/S Reaction Rate and Fluid Composition in Silicic Ash of the Nankai Trough

Masuda

1996

Clays and clay miner.

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Abstract--The compositions, fabrics and structures of authigenic minerals that formed recently from silicic volcanic ash layers from a 1300-meter sediment column obtained at ODP Site 808 of the Nankai Trough were studied using XRD, STEM, AEM and SEM. Smectite and zeolites were first detected as alteration products of volcanic glass with increasing depth, as follows: smectite at 200 m below seafloor (mbsf) (20 ~ clinoptilolite at 640 mbsf (60 ~ and analcime at 810 mbsf (75 ~ A primitive clay precursor to smectite was observed as a direct alteration product of glass at 366 mbsf (approximately 30 ~ High defect smectite with lattice fringe spacings of 12 to 17 A and having a cellular texture filling pore space between altering glass shards occurs at 630 mbsf. Packets of smectite become larger and less disordered with increasing depth and temperature. The smectite that forms as a direct alteration product of volcanic glass has K as the dominant interlayer cation.With increasing depth, smectite becomes depleted in K as the proportion of clinoptilolite increases, and then becomes depleted in Na as the proportion of analcime increases. The composition of the exchangeable interlayer of smectite appears to be controlled by the formation first of K-rich clinoptilolite and then Na-rich analcime, via the pore fluid, giving rise at depth to Ca-rich smectite. Smectite reacted to form illite in the interbedded shales but not in the bentonites. Paucity of K in smectite and pore fluids, due to formation of clinoptilolite under closed system conditions, is believed to have inhibited the reaction relative to shales.

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Section: Geologic Setting Of Samplesmentioning

confidence: 99%

Relation Between Interlayer Composition of Authigenic Smectite, Mineral Assemblages, I/S Reaction Rate and Fluid Composition in Silicic Ash of the Nankai Trough

Masuda

1996

Clays and clay miner.

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“…Judging from its structural style, this horizontal reflector that develops in the middle of the hemipelagic section can be interpreted as a décollement (detachment surface). The reflected seismic wavelet is of reverse polarity and a lowvelocity zone beneath it has been observed (Aoki et al, 1986). Toward the arc, the clear seismic image within the accretion-ary prism is lost and replaced by vague, landward-dipping reflectors.…”

Section: Morpho-tectonic Features Of the Nankai Troughmentioning

confidence: 95%

Sediment Deformation and Hydrogeology at the Nankai Accretionary Prism: Synthesis of ODP Leg 131 Shipboard Results

Taira¹,

Hill²,

Firth³

1991

Proceedings of the Ocean Drilling Program, 131 Initial Reports

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The Nankai accretionary prism is arguably the best-known clastic prism in the world. The main objective of ODP Leg 131 was to provide data on the deformational processes and associated hydrogeology of the prism. Such information can be incorporated into the existing geophysical and geological data set to characterize the evolution of sediments in this tectonic setting. The objective was approached by drilling Site 808, at which the frontal thrust and décollement of the prism toe were penetrated. Drilling also succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, and with good core recovery. Both the frontal thrust and the décollement were clearly recognizable in the cored material. Bedding dips, faults, and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to northwest-southeast shortening. Below the décollement no significant deformation features were observed. Physical properties data show major discontinuities at the décollement, notably an increase in porosity below the décollement. This might indicate excess pore pressure in the subducted section. An increase in porosity below the frontal thrust is less marked and might reflect only the youthfulness of this feature. Attempts to make downhole measurements were severely hampered by unstable hole conditions, but useful constraints have been placed on the thermal regime, some calibration of laboratory physical properties to in-situ conditions has been provided, and in-situ stress and pore pressure were measured in the uppermost sediments. Evidence of channelized fluid flow is inconclusive because no vein structures were observed in the cores, no unequivocal geochemical anomalies indicative of flow were found, and thermal measurements are not significantly different from those predicted by a purely conductive heat-flow model. We conclude that dewatering probably occurs mainly through diffuse flow at this site.

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“…4. It was found by AOKI et al (1986) and by NISHIYAMA (1988) that reflectivity patterns within the sedimentary part of the crust change from a laminar configuration seaward to a smeared structure landward from the trough. This change in acoustic impedance profiles is explained as an initiation of the destruction of laminations and a dewatering process within accretionary materials.…”

Section: Crustal Structurementioning

confidence: 99%

“…Tentative estimates give a positioning ambiguity as large as 300 meters. Distances between OBSH and sound sources were determined assuming the sound speed in sea water to be 1.5 km/ s. Also, the sound speed of the topmost unconsolidated sediment cover is assumed to be 1.8-2.0 km/ s with a gradient downward referring to the multichannel reflection patterns (AOKI et al, 1986;NISHIYAMA,1988). Seismic structures obtained by iteration of the models along a track line perpendicular to the trough axis is shown in Fig.…”

Section: Crustal Structurementioning

confidence: 99%

Seismic structure and geomagnetic anomaly in the Nankai Trough related to subduction of the Philippine Sea plate.

Kinoshita

Matsuda

1989

J. geomagn. geoelec

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Decay of a magnetic anomaly pattern to the north of the Nankai Trough, off Shikoku, Japan is discussed based upon studies of the seismic structure of the oceanic crust of the region. A simple subduction model of the lithosphere with a temperature gradient in the crust to reduce the resultant amplitude of the total magnetic field anomaly on the sea surface is not sufficient to explain the large decay rate of the total force anomaly versus landward distance from the Trough axis. Assumptions are made that some kind of destruction of the topmost part of the magnetic basement is causing the rapid decay. Effects of the geometrical disordering of the magnetic layer to reduce the resultant field intensity is studied by model experiments. Several models were tested to conclude that the destruction of the topmost part (ca. 1 km) of the oceanic basement, the magnetic source of the anomaly field, may affect the decay rate in two ways; i.e., randomization of natural remanent magnetization as well as the subduction of the rest of the nondisturbed magnetic basement. Subduction of the Philippine Sea plate along the Nankai Trough area and strong friction may enhance the destruction of the magnetism. Some of the present-day destruction and subsequent dismembering of the part of the oceanic basement seem to be occurring at intermediate depths between the crust and subducting lithosphere.

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Evidence of a Low-Velocity Layer beneath the Accretionary Prism of the Nankai Trough: Inferences from a Synthetic Sonic Log

Cited by 13 publications

References 2 publications

Relation Between Interlayer Composition of Authigenic Smectite, Mineral Assemblages, I/S Reaction Rate and Fluid Composition in Silicic Ash of the Nankai Trough

Relation Between Interlayer Composition of Authigenic Smectite, Mineral Assemblages, I/S Reaction Rate and Fluid Composition in Silicic Ash of the Nankai Trough

Sediment Deformation and Hydrogeology at the Nankai Accretionary Prism: Synthesis of ODP Leg 131 Shipboard Results

Seismic structure and geomagnetic anomaly in the Nankai Trough related to subduction of the Philippine Sea plate.

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