Toshio Furuta

Widespread submarine tephras around Japan — Petrographic and chemical properties

Abstract In deep-sea sediments around the Japanese Islands, tephra layers are frequently recognized, and some tephras can be identified by petrographic and chemical features and correlated to source volcanoes of both Japan and Korea. Most tephras distributed over more than several hundred kilometers from the source volcanoes were presumably carried away by the dominant westerly wind. Six characteristic marker tephras in deep-sea sediments can be identified and correlated according to their distribution and grain size, petrography, and chemistry. They play an important role in compiling the Late Quaternary chronology of the deep-sea sediments distributed around the Japanese Islands. Petrographic and geochemical studies reveal that the refractive index of volcanic glass shards in tephras is controlled by the amount of total transition metal oxides rather than the amount of SiO 2 . We propose a new equation relating refractive index to glass shard chemistry.

The eastern and western ends of Nankai Trough: results of box 5 and box 7 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the Nankai Trough. The eastern survey covers the transition between the large Izu-Bonin (Izu-Ogasawara) Ridge collision with Honshu and the Nankai Trough subduction. It includes a northeast trending basement ridge, the Zenisu Ridge, to the southeast of Nankai Trough as well as two large channel systems, one following the trench, the Nankai channel and the other coming from the north, the Tenryu Canyon. The Zenisu Ridge is a zone of recent intra-plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast. We interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the Izu-Bonin Ridge and the concentrated shortening along the Nankai Trough. The shortening axis appears to change from north-south along the Zenisu Ridge to northwest-southeast along the Nankai Trough and finally to east-west within Honshu. The western survey covers the junction between the Nankai and Ryukyu Trenches and the Kyushu-Palau Ridge which is subducted without major internal deformation. The ridge appears to act as an indenter upon the margin. The tip of the indenter is presently situated under the upper accretionary prism.

Interpretation of temperature measurements from the Kaiko-Nankai cruise: Modeling of fluid flow in clam colonies

During the Kaiko-Nankai detailed submersible survey, numerous measurements of the temperature gradient inside the sediment were performed on the deepest active zone of fluid venting, which is situated on the anticline related to the frontal thrust, using the Ifremer T-Naut temperature probe operated from the submersibleNautile. We thus obtained the temperature structure below different types of clam colonies associated with fluid venting. We used the finite element method to model the thermal structure and fluid flow pattern of these vents and to determine the velocity of upward fluid flow through the colonies. On a biological basis, four types of clam colonies are defined. Each biological type has distinctive thermal characteristics and corresponds to a particular fluid flow pattern. Darcian flow velocity in the most active type of colony (type A) is of the order of 100 m/a. The total amount of fluid flowing through colonies in the studied area is estimated to be 200 m3 a−1 per metre width of subduction zone. Most of the flow is vented through type A colonies. This value is more than one order of magnitude too high to be compatible with the amount of water available from steady-state compaction of sediments in the whole wedge. Thermal arguments suggest that downwelling of seawater occurs around type A colonies and that seawater is then mixed with upcoming fluids at a depth of 1 or 2 metres. Furthermore, finite element modeling shows that a salinity difference of a few parts per mil between the upcoming fluids and seawater is sufficient to drive convection around the colonies. As water samples from a few vents indicate that the fluid source should actually be significantly less saline than seawater, we propose that the very high fluid flows measured are a consequence of the dilution of the fluid of deep origin with seawater by a factor of 5 to 10.

Nankai Trough and the fossil Shikoku Ridge: results of Box 6 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the Nankai Trough where the fossil Shikoku Ridge is subducted below southwest Japan. The geometry of the oceanic lithosphere bending under the margin as well as the three-dimensional structure of the accretionary prism have thus been determined in detail. Three 350° trending, probably transform faults have been identified in the area of the survey. They do not extend further south and appear to be limited to the last phase of spreading within the Shikoku Basin, probably between 15 and 12 Ma; this last phase of spreading would then have been accompanied by a sharp change in spreading direction from east-west to N 350°. The two eastern transform faults limit a zone of reduced Nankai trench fill of turbidites opposite to the Tosa Bae Embayment. This observation suggests that the Tosa Bae Embayment actually results from this reduced supply of trench fill to the imbricate thrusting process. The accretionary prism can be divided into three different tectonic provinces separated by continuous mappable thrusts, the Lower and Upper Main Thrusts. Surface shortening is limited to the lower accretionary prism south of the Upper Main Thrust (UMT) whereas uplift with possible extension characterizes the prism above the UMT. Deformation, due to the relative plate motion, mostly affects the lower accretionary prism south of the UMT.

Electron probe microanalysis study on processes of low-temperature oxidation of titanomagnetite

Abstract Partially oxidized titanomagnetite grains in various kinds of volcanic rocks were investigated by electron probe microanalyzer (EPMA) in order to clarify the process of oxidation at low temperature. The following results were obtained by the present investigation: (1) Primary composition of titanomagnetite is homogeneous in individual grains, although variation in composition among different grains is observed on each thin section. (2) Migration of Fe cations during low-temperature oxidation is clearly seen in all oxidized grains. Some other constituent cations are also bleached and consequently the relative content of the remaining cations becomes large. (3) The detailed internal structures of titanomagnetite grains are observed as backscattered electron images (BEI) with an electronprobe microanalyzer, and it seems likely that the structures depend upon the degree of low-temperature oxidation. (4) The chemical and physical properties of oxidized titanomagnetites imply that low-temperature oxidation is not a simple process but a complex one. Such an oxidation process is correlatable to both the mobility of cation and the oxidation condition such as a circulation of some active hydrothermal materials at low temperature.

Tectonic context of fluid venting at the toe of the eastern Nankai accretionary prism: Evidence for a shallow detachment fault

During the Kaiko-Nankai diving cruise the peak of the venting activity was located near the top of the very first anticline. The most prominent morphological feature between the mid-slope (3870 m) and the apex of the fold (3770 m) is a 20 m high cliff cutting through subhorizontal massive mudstones affected by numerous joints. The trend of this scarp is oblique to the fold axis and structurally controlled along two sharply defined NNE-SSE and E-W directions. Fresh talus and blocks found locally suggest active tectonics and recent erosion. Intense deformation is evident from strongly tilted strata restricted to the base of the cliff that we interpret as an upslope thrust. At the scale of Seabeam mapping, this thrust can be followed eastward for more than 5 km along the 3820 m isobath. Two seismic lines recorded during one of the pre-site surveys show deformation at shallow depth, including small-scale folding and thrusting affecting only the wedge-shaped top sequence. Deeper layers can be traced continuously below this sequence. We conclude that the boundary between the “piggy-back” basin and the frontal fold turbidites acts as a shallow detachment fault, and interpret the base of the cliff as the outcrop of the fault. Dense colonies ofCalyptogena clams and strongly nonlinear thermal gradients locate the major peak of fluid activity at the edge of the plateau above the main cliff. Scattered biological colonies as well as white bacterial mats and cemented chimneys were also found in a narrow belt along the base of the cliff. Fluid activity is thus closely related to the shallow detachment fault, fluid being expelled both at the outcrop of the fault and above it through the overlying strata, possibly using the very dense joint network.

Deep-tow survey in the KAIKO-Nankai cold seepage areas

Deep-sea biological communities associated with cold venting were located within the eastern portion of the Nankai Trough using a deep-towed TV system. Five continuous lines (approximately 200 km long) were observed. Bivalve communities were found both in the Tenryu Canyon outlet region and at the outcrop of the frontal thrusts of the accretionary wedge off Cape Omaezaki. The former is a site at which biological communities had been found during the KAIKO dives in 1985. However, the second site appears to be associated with larger scale venting and was chosen for the 1989 KAIKO-Nankai submersible cruise. Three small mud volcanoes with diameters of several hundred meters and heights of several tens of meters are found in the Seabeam topography map, but the present survey showed neither evidence of recent venting nor biological communities.

Sub-sea-floor hydrothermal alteration in the Galápagos Spreading Center

Abstract Oceanic crust was drilled at Hole 504 B on the southern crest of Costa Rica Rift, Galapagos Spreading Center. This is the first hole that contains hydrothermally altered rocks containing secondary minerals of greenschist facies in D.S.D.P. holes drilled at the typical mid-oceanic ridges. Altered rocks under high-temperature water-rock interaction are recovered below 890-m subbottom depth. Based on the secondary minerals, the distribution of veins, and the trace-element contents, the cores are classified into three zones: zone 1 (from 274 to 890 m); zone 2 (from 890 to 1050 m); and zone 3 (from 1050 to 1350 m). In zone 1 the rocks remain fresh or are partly altered to secondary minerals (smectite and saponite, etc.), while in zones 2 and 3 laumontite, chlorite, epidote, quartz and actinolite appear. Veins are more frequent in zone 2 than in zone 3 . Some samples in zone 2 are enriched in Mn and Zn. Part of zones 2 and 3 is considered to be the discharge zone in a hydrothermal convection system, based upon the chemical composition of actinolite, whole-rock composition and Sr isotope data. The estimated alteration temperature changes sharply between zones 2 and 3 . It is lower than 100°C in zone 1 , while it is higher than 200°C in zones 2 and 3 . This gap corresponds to the sharp change of hydrothermal alteration, and other geophysical properties. The most plausible mechanism for the formation of this gap is the time lag. Intensive hydrothermal activity occurred during the formation of oceanic crust in the rift valley. Another lava flow covered the rocks in zones 2 and 3 .

Geophysical study of the mafic belts along the margins of the japanese islands

Abstract Along the margins of the Japanese Islands there are large belts of mafic rocks that are now deeply buried. The origin of the belts is unknown: they could be either remnants of oceanic crust now trapped within the continental area, or they could be igneous bodies of continental origin. In this paper a discussion of this problem is given and new results presented.

Is the oceanic crust over 1 km necessary for the source of marine magnetic anomalies

Abstract A paleomagnetic vertical profile over 1 km through the oceanic crust is constructed by magnetic properties of 256 DSDP samples from Hole 504B, the Costa Rica Rift. The results of magnetic measurements of the relatively young oceanic crust indicate that the thickness of the marine magnetic source layer is a 0.6 km thick pillow lava layer of the uppermost oceanic crust.