On the other hand, the lineation trend is sub-parallel to the STL and the spreading
direction of the WPB. This implies that the exhumation occurred along with one of
the back-arc spreading centers earlier than the Miocene.
In addition to the geochronology of the metamorphic rocks, one of the important
points remaining unsolved is the contact relationship between the serpentinite body
and the overlying Paleogene volcanic rocks, as previously mentioned. If it is an
unconformity, the exhumation exclusively occurred before the Late Eocene, within
the WPB or the Cretaceous remnant arcs as a different subduction system from the
IBM. If it is a detachment fault, at least the final stage of the exhumation should
have occurred after the Early Oligocene, i.e., related to one of the back-arc spread-
ing centers of the IBM system.
Acknowledgments This paper is dedicated to the late Prof. T. Watanabe, who conducted the
scientific investigations of the YK01-04 cruise, and guided us to geological and petrological studies
on the Ohmachi Seamount. The authors thank Captains O. Yukawa (YK01-04), U. Fujita (KT04-08)
and S. Ryono (YK08-05), crews and on-shore staffs of R/V Yokosuka and Tansei-maru , commanding
officers Y. Imai and T. Sakurai and the Shinkai 6500 operation team for their operation and admin-
istrations, and Ms. M. Aoki and M. Imamura for her assistance of onboard works in R/V Yokosuka .
The Shinkai 6500 operation team also helped us to develop clinometer plate. Successful dredges of
KT04-28 cruise owe greatly to operation and assistance by Prof. T. Ishii, Drs. S. Haraguchi and S.
Machida, and other members of KT04-28 scientific party with fruitful discussions. Dr. A. Nishimura
helped us to review results of GH84-1 and GH86-1 cruises. Constructive reviews by Drs. J.
Wakabayashi and T. Tsujimori greatly helped to improve the manuscript. H. Ueda was financially
supported by MEXT grant-in aid for JSPS Fellows 01403900 and 17740329, and for twenty-first
Century COE Program “Neo-Science of Natural History” (Hokkaido University).
Appendix: In Situ Measurement of Geological Surface
Orientation by Submersible
Foliation planes by schistosity were measured by a newly developed clinometer
plate at three localities during the dive 6K#1064. Bedding plane of semi-
consolidated mudstone was also measured at a locality of the dive 6K#1067 by the
same method. Here we describe the outline of the method, and more detailed
method, calibration, and evaluation of its reliability will be given in another paper.
The clinometer plate is an acryl disc 24 cm in diameter with a steal vertical bar
12 cm long at the center (Fig. 12 ). Both the disc and the vertical bar are scaled at 1 cm
intervals by painting. The disc is mounted on a steel disc of the same diameter, on
which a handle is equipped. At the outcrop, the clinometer plate was laid on, or set
parallel to the surface structure of interest, handled by the manipulator of the submers-
ible. And it was photographed by a camera, whose focal length and orientation relative
to the submersible were logged. Also logged was orientation of the submersible.
The photographs were analyzed by the following procedures. First, the angle j
of the vertical bar to the line of sight (connecting the lens and disc centers) was
calculated by a simple graphical analysis shown in Fig. 13 . The lengths b and s in
Fig. 13a was determined reading the scale of the point where scales of the disc and