Seafloor Geology of the Basement Serpentinite Body in the Ohmachi Seamount (Izu-Bonin Arc) as Exhumed Parts of a Subduction Zone Within the Philippine Sea - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 111

4.1 Dive 6K#341

This dive was the first research that discovered the occurrence of serpentinites. The
route was set at 29°04.33-4.12'N climbing eastward from the Nishinoshima Rift
basin floor to the ridge crest of the peninsular part. Dive results have been reported
by Yuasa et al. ( 1998, 1999 ), as briefly reviewed here. The lower slope between
3,487 mbsl (edge of basin floor) and ~3,180 mbsl was covered by slope debris
containing serpentinite blocks and clasts, whose amounts and sizes increased
upslope. Between the depths of 3,180 and 3,080 mbsl foliated rocks occurred,
which were not sampled. The foliated rocks were first described as presumable
metamorphic rocks, however, now they are considered to be schistose serpentinite
taking into account results from the following dive investigations. The slopes
became gentler between 3,080 and 2,800 mbsl and were covered by mud. Slopes
between the depths of 2,800 and 2,600 mbsl were composed mainly of talus con-
sisting of blocks of Miocene turbidite slid from cliffs above. A near-vertical cliff
appeared at ~2,500 mbsl, where Miocene conglomerates and turbidites were
exposed. Foreign pumice was collected on the ridge crest at 2,393 mbsl.

4.2 Dive 6K#570

This one and the following dives 6K#571 and 6K# 575 were performed during
YK00-08 cruise, whose results were reported in JAMSTEC ( 2000 ), Fujiwara et al.
( 2001 ) and Niida et al. ( 2001 ). Dive 6K#570 climbed the western fault scarp on the
main part of the seamount at ~29°10.5'N. The lower slope between 3,460 (basin
floor) and 2,990 mbsl comprised landslide debris and talus deposits covered by soft
muddy sediments. In the upper slopes (2,990-2,670 mbsl) occurred dacite and
dioritic porphyrite partly with platy and/or columnar joints. Volcanic breccia
occurred between 2,670 and 2,570 mbsl. The uppermost outcrops (2,526-2,483
mbsl) at the edge of a terrace at ~2,500-2,400 mbsl consisted of well stratified
volcanic sandstone containing pumiceous layers and foraminifers.

4.3 Dive 6K#571

This dive route roughly tracked the lower section (3,506-3,079 mbsl) of the dive
6K#341. Less fractured massive serpentinite and associated clinopyroxenite
appeared from the foot of the fault scarp at 3,480-3,300 mbsl as sporadic outcrops
among debris blocks and soft sediment covers. Serpentinites between 3,300 and
3,200 mbsl were massive but highly fractured enclosed in clayey matrices, occur-
ring within slope debris. This fracturing probably resulted from a landslide, on
whose bathymetric feature the submersible crossed at these depths. Hornblendite