Miocene sedimentary rocks
Paleogene volcanic rocks
Quaternary volcanic rocks
Fig. 2 Bathymetric features and inferable geologic outline of the Ohmachi Seamount (Bathymetry
based on Sakamoto et al. 2001 ). Dive and dredge locations out of the field of Fig. 4 are shown
is the thinnest with about 10 km, and this ocean-like crust splits the northern and
southern Izu-Bonin arc crust (Yuasa 1992 ).
The Ohmachi Seamount has a tadpole-like outline with the main part (body) in the
north and the peninsular part (tail) in the south (Fig. 2 ). The main part comprises a
broad and north-trending elliptical high ~20 km in width and ~30 km in length, whose
summit is ~1,700 mbsl (meters below sea level). The peninsular part is a N-S-trending
ridge with less elevated (~2,300 mbsl at the ridge crest) and asymmetric cross
sections (Fig. 3 ). Its eastern slope is relatively flat and gentle (~6°), whereas the
western slope is much steeper with average dips of ~30°. The western slope comprises
normal fault scarps of the Nishinoshima Rift basin in the west. Single-channel seis-
mic reflection profiles (Yuasa et al. 1991 ) displayed that the rift basin-fill deposits
abut onto the fault scarp, whereas seemingly dragged by the fault in deeper horizons.
The western slope of the peninsular part consists of two alternating domains trending
0-20° and ~340°, probably resulted from a rhombus geometry of the rift normal
faults. To the south of 29°08'N, the western slope has a terrace-like gently-dipping
band at the level of 3,000-3,200 mbsl. The asymmetric bathymetry is likely to have
resulted from tilting and uplifting of initially flat basin floor.
The Ohmachi Seamount shows a broad positive magnetic anomaly (Yamazaki
et al. 1991 ; Yamazaki and Yuasa 1998 ), inferred as an effect of somewhat deeply-
seated magnetized body (Yamazaki et al. 1991 ).