Geoscience Reference
In-Depth Information
0
(A) 29
o
16'20"
2
Main part
4
0
(B) 29
o
11'58"
2
Main part
4
0
(C) 29
o
07'19"
2
peninsular part
4
0
(D) 29
o
05'10"
2
peninsular part
4
0
(E) 29
o
02'35"
2
Intra-arc rift basin
Forearc basin
(OgasawaraTrough)
4
peninsular part
(Nishinoshima Rift)
0
5
10
15
20
25
30
35
Distance (km)
Fig. 3
East-west topographic cross sections of the Ohmachi Seamount. Reference
broken lines
tilting
at 6° to the east are drawn for the interpretation of flat surfaces on the eastern slopes of the peninsular
part. Also shown are inferable projections of the normal faults by recent intra-arc rifting
3
Lithology
The Ohmachi Seamount is composed mainly of the basement serpentinite with
trace amphibole schist, Paleogene volcanic rocks, and Miocene sedimentary rocks
(Figs.
2
and
4
). Common lithological characteristics are described here, and their
occurrences in each dive tracks will be described in the next section.
3.1
Serpentinites
Serpentinites in the Ohmachi Seamount are classified into two major groups: massive
and schistose serpentinites (Figs.
5
and
6
), in terms of development of mineral folia-
tion. The massive serpentinite occasionally accompanies ultramafic dikes.
3.1.1
Massive Serpentinite
The massive serpentinites are characterized by lacking or very weak mineral
foliation (Fig.
5c-d
). They are jointed or fractured to varying extent, and
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