Kenji Konishi

Deep scientific dives in the Japan and Kuril Trenches

Abstract In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and Kuril Trenches were made in the newly launched submersible “Nautile”. The sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2–3 km which is consistent with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor.

The Southwest Ryukyu Arc is a migrating microplate (forearc sliver)

Abstract We propose a new interpretation for the tectonism of the Southwest Ryukyu Arc, based on inland stress field analyses and the interpretation of seismic reflection profiles. The recent stress field in the Southwest Ryukyu Arc is under tension, the axial direction of which is parallel to the trench axis, but an E-W dextral slip shear zone is developing at its western extremity, Yonaguni, and westwards to Taiwan. These features are closely related to the degree of the oblique subduction of the Philippine Sea Plate and the strength of coupling between it and Southwest Ryukyu Arc. We regard the Southwest Ryukyu Arc as a migrating microplate. The neotectonics of the Southwest Ryukyu Arc are attributed to the westward migration of the microplate, which is bounded by the Miyako Depression to the east and by Taiwan to the west. The microplate obducting onto the Eurasian Plate at Taiwan started to migrate westward at about 4 Ma, when the relative motion between two plates, Philippine Sea and Eurasian, changed direction from S-N to SE-NW in region.

Limestone of the Daiichi Kashima Seamount and the fate of a subducting guyot: fact and speculation from the Kaiko Nautile dives

Abstract The Daiichi-Kashima Seamount subducting in the Japan Trench has two reef-capped flat tops with different depths, which are bounded by a nearly straight scarp. The western (inboard) crest is 5300–5450 m deep, and the eastern (outboard) one is 3880–4000 m deep. A variety of shallow-water reefoid limestones studied by the Kaiko “Nautile” dives not only confirms the similarity in thickness and lithologies, but also establishes an approximate biostratigraphic correlation between the two reefoid caps, based for the first time on the orbitolinid foraminifers ( Orbitolina (Mesorbitolina) parva from the western block and O. (M.) texana from the eastern block). This conclusion supports the interpretation that the present topography of the seamount has resulted from a subduction-induced faulting of a once single reef-capped guyot. A plausible scenario of the fate of a subducting Early Cretaceous guyot in the Western Pacific is outlined as exemplified by the Daiichi-Kashima Seamount.

High-resolution rock-magnetic variability in shallow marine sediment: a sensitive paleoclimatic metronome

Abstract An outer shelf deposit in central Japan centered on the Olduvai normal polarity event in the reversed Matuyama chron reveals a close correlation of both the magnetic susceptibility and remanent intensity with the sedimentary cyclicities apparent in lithologies and molluscan assemblages. Two sedimentary cycles are characterized by distinctly similar, but double-peaked magnetic cyclicities. The rock-magnetic variability is primarily attributed to the relative abundance of terrigenous magnetic minerals, and the double peak of the variability is characterized by the concentration of finer-grained magnetic minerals. The concentration is suspected to be controlled by both climatic change and shifting proximity of the shoreline as a function of rise and fall of the sea level due to glacio-eustasy. Rock-magnetic study reveals the record of a 21 ka period of orbital precession cycles within the sedimentary cyclicity attributable to a 41 ka period of orbital obliquity forcing.