of such a basin with a negative gravity anomaly indicates that the lens-shaped
Katsuura Basin formed as a result of stretching, probably related to westward
movement of the PHS.
Review of 3D Structure Based on Multichannel Seismic
Profiles and Multibeam Echosounder Data
Iwabuchi et al. ( 1990 ) used multibeam echosounder data and multichannel seismic
profiles to interpret the structural relationships of the three tectonic plates in the
Boso triple junction area (Fig. 4 ). They suggested that the PHS lies beneath of the
NAM to the north of the triple junction, which indicates that northward subduction
of the PHS persisted possibly until the Quaternary. Iwabuchi et al. ( 1990 ) also
identified a northeast-trending normal fault system approximately 50 km northwest
of the triple junction in the southernmost tip of the NAM (northeast-trending dotted
line in Fig. 3 ). The location of this fault system suggests NW-SE extension on the
ocean floor in the southernmost part of the NAM.
We have concluded that development of the Boso triple junction was a compli-
cated process controlled by the configuration of the three plates and their inter-
actions. Local horizontal extension in response to the northwestward motion of the
PHS is also consistent with formation of the Katsuura Basin in the northeastern
corner of the PHS.
If we assume that both the Paleogene-Miocene sequence (as discussed in
Sect. 3.4 ) and part of the PHS remained in the triple junction area, the recent north-
ward movement of the PHS and accretion of the part of the Paleogene-Miocene
sequence that is equivalent to the Izu-Bonin forearc strata provide evidence to sup-
port the view that the triple junction was stable until recent times. This theory is
also supported by new paleontological data presented in Sect. 3 .
Relationship Between the Tectonic and Age Data
To further develop the scenarios for the tectonic development of the Boso triple
junction reviewed above, we need to incorporate geochronological data. The
water depth of the floor of the depocenter within the Boso triple junction reaches
9.4 km. At such depths, the common methods of marine geological and geophysi-
cal exploration cannot generally be used. However, we conducted a successful
dive of the unmanned submersible ROV KAIKO-10K (Japan Marine Science and
Technology Center; now the Japan Agency for Marine-Earth Science and
Technology, JAMSTEC) to the deepest part of the basin near the triple junction.
Video images of seafloor features were recorded during this dive and rock and
sediment samples were collected. The following section describes the age deter-
mination of those samples.