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reworked species were seen from either the late Miocene Thalassionema
schraderi Zone (NPD6B) or Rouxia californica Zone (NPD7A). The reworked
Pliocene species were mostly Neodenticula kamtschatica, but not its descendant
N. koizumii, which is limited to the late Pliocene, suggesting that the reworked
Pliocene species were mainly from lower Pliocene sediments. There were no
reworked early Pleistocene species.
In summary, all the diatoms in the seven Pleistocene to Holocene samples
were from the Neodenticula seminae Zone (NPD 12), and four of them correlated
to the Pseudoeunotia doliolus Zone (NTD 17), which implies an age of 0-0.3 Ma.
Sample #151 R-1 was the only markedly older sample; it was assigned to the
Denticulopsis lauta Zone (NPD 4A), which is of early middle Miocene age
(15.2-15.4 Ma).
Correlation to the Marine Rocks of the Izu Forearc
A submarine geologic map (Fig. 10 ) published by the Geological Survey of Japan
( 1990 ) covers the area of the Boso triple junction, and the accompanying cross sec-
tion passes through the triple junction. The seafloor ages shown on the map were
interpreted from numerous single-channel seismic profiles and dredged samples.
The Oligocene to lower Miocene sedimentary rocks (marked Pg) lie on the Taito
Spur, which borders the Katsuura Basin on the landward side of the triple junction.
The Pg rock unit can be traced from the Bonin Islands in the south to the Boso
Peninsula in the north. The map also shows a small area of rocks (marked N and D
in Fig. 10 ) that represent the present-day accretionary prism. This rock assemblage
might correlate with the rock sampled during our ROV dives.
Tectonic Synthesis: Summary and Conclusion
Diatomaceous mudstone samples of middle Miocene age (15.2-15.4 Ma) were
recovered from the toe of the landward slope on the Honshu-arc side of the Boso
triple junction. Other samples of slope sediments and rocks were much younger
(0-0.3 Ma, Quaternary). The Quaternary sediments and rocks are probably part of
the present-day accretionary prism. According to the subsea video footage, these
strata have undergone complex deformation; there is an alternating sequence of
steep and shallow dips that suggests a collisional thrust-and-fold structure.
The deep-sea rocks of Miocene age that we sampled are diatomaceous mudstones
of the same age as the Kinone Formation on Boso Peninsula. The deep-sea sediments
may have been deposited over a wide area from the Boso Peninsula to the forearc of
the Izu-Bonin trench. The rocks in the latter area may have been incorporated in the
triple junction area during formation of an accretionary prism. A Miocene formation
such as that we sampled, or its equivalent, may be distributed over a wide area from
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