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 119

29 o 07'19"

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1
2
3
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Nishinoshima Rift
basin-fill deposits

Ogasawara Trough
basin-fill deposits

Massive serp.

Schistose serp.

0 5 10 15 20 25 30 35

Distance (km)

Fig. 10 Geologic cross section schematically projected from the south onto the west-east traverse
crossing the localities of float blocks of amphibole schist

a difference in altitude of ca. 200 m from the ridge crest. At 6K#341 route, it is deeper
than 2,700 mbsl with a difference in altitude exceeding 300 m from the ridge crest.
It is thus inferred that the Miocene (possibly with younger sediments) becomes thicker
towards south. On the contrary, the thickness of the Paleogene volcanic sequence is
assumed to become thinner towards south, from a thickness of 600-700 m at 6K#575
to less than 400 m at 6K#341. Contrasting lithofacies between shallow-marine (partly
reefal) limestone in the Paleogene and the Miocene turbidites imply a disconformable
boundary accompanied by significant subsidence, and an abut relation of the Miocene
onto a Paleogene volcano is assumed based on the opposite trends of the southward-
thinning Paleogene and the southward-thickening Miocene formations.

6 Structural Characteristics of Sub-crustal Origins

Dive and dredge results from the Ohmachi Seamount showed that the mode of
occurrences of serpentinites are quite different from those of well-known serpen-
tinite seamounts in the Mariana and Bonin forearcs (“forearc serpentinite seamounts”)
located much nearer to the trench. The forearc serpentinite seamounts occur in two
different modes: mud volcanoes and horsts (Fryer 1992 ; Fryer et al. 1995 ). The mud
volcanoes are piles of serpentine mud flows, which contain small fragments of
serpentinized peridotites, gabbro, volcanic rocks, and high-pressure metamorphic
rocks (Maekawa et al. 1992, 1993 ). Serpentinites consist dominantly of low-T spe-
cies (chrysotile and lizardite: Saboda et al. 1992 ), although antigorite-olivine equi-
libria suggesting greenschist to amphibolite grades are recently found by Murata
et al. ( 2009 ). Rocks of the horsts are ophiolitic consisting of massive serpentinite
accompanied by significant amounts of gabbro and volcanic rocks (Ishii 1985 ;
Ishiwatari et al. 2006 ). High-pressure metamorphic rocks are rarely associated
(Maekawa et al. 2004 ). Antigorite schist was not found in both types of the forearc
serpentinite, and they are either massive or highly fractured enough to be fluidized.
These structures obviously crosscut or disturbed the Paleogene volcanic formation
including boninite. They are generally considered as diapirs by buoyancy (Ishii et al.
1992 ) or hydration expansion (Fryer and Fryer 1987 ; Fryer 1992 ), and fluidized
effusives by over-pressured pore fluids (Fryer 1992 ).