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A characteristic feature of the surface geology in the southern Boso Peninsula,
Honshu, Japan, is the existence of outcrops of ophiolitic rocks in the Mineoka Belt (Fig. 1).
The Mineoka Ophiolite Belt consists mainly of pillow lavas,
serpentinized ultramafic and mafic rocks and their brecciated cumulates.
The main body of outcrops is restricted to a narrow band with a length of about 10 km in east-west extent,
starting at the east coast of the peninsula,
and with a north-south width of less than 1 km (e.g. Kanehira et al., 1968; Kanehira, 1976).
Ophiolitic rocks crop out intermittently in the Hota or Hayama Belt from the Boso Peninsula to the Miura Peninsula (e.g. Uchida and Arai, 1978; Arai et al., 1990).
The ages of the Mineoka, Hota,
and Hayama belts range from Paleogene to early Miocene and are older than the surrounding Miocene to Pliocene geological belts (GSJ, 1982).
The age of basaltic rocks sampled at the Mineoka Ophiolite is estimated to be 30-50 Ma from K-Ar and Ar-Ar dating (Takigami et al., 1980; Kaneoka et al., 1981).
The line connecting the ophiolite outcrops is assumed to be a paleo plate boundary.
The mechanism of emplacement of the ophiolitic rocks in the present location,
between the northern tip of the Philippine Sea Plate and the Honshu landmass,
must relate to the evolution of the trench-trench-trench type triple junction,
located southeast of the Boso Peninsula (Fig. 1),
and thus are important for understanding the paleo plate tectonics around the Japanese islands.
Many arguments have been made about the origin and emplacement of the ophiolitic rocks and the related paleo plate tectonics (e.g. Tonouchi and Kobayashi,
1983; Ogawa and Fujioka, 1985; Ogawa and Taniguchi, 1987; Arai, 1991; Arai and Okada, 1991; Soh et al., 1991).
Ogawa and Taniguchi (1987) proposed the existence of a Mineoka Microplate which developed through collision of the Izu island-arc with the Honshu landmass. Taira et al. (1989),
and Arai and Okada (1991) suggested that the Mineoka Belt was formed by accretion of serpentine diapirs in the forearc region of the Izu-Bonin Ridge. Soh et al.
(1991) proposed that the Mineoka Belt was produced by tectonic fragmentation of the crust of the Izu-Bonin Arc during arc-arc collision.
Tectonic and petrographical studies of the Mineoka Ophiolite Belt and its surrounding area indicate that the Mineoka ultramafic body is a group of isolated
and fragmented small bodies resulting from intrusion of serpentinized gabbroic sheets in the Cenozoic era (Arai, 1991; Arai and Okada, 1991).
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| In spite of this accumulation of information, however,
stratigraphic correlation between individual outcrops is poorly known because their outcrops are limited in size.
Detailed subsurface structure of the southern Boso Peninsula can give an insight into the tectonics.
Magnetic anomaly analysis is an effective tool for investigating the subsurface structure of this ophiolite belt because ophiolitic rocks carry strong magnetization in contrast to sedimentary layers.
Aeromagnetic surveys have been carried out by a number of institutions and organizations (GSJ, 1980; JICA, 1983; NEDO, 1983; for compiled maps, Nakai et al., 1987; GSJ and CCOP, 1996).
A remarkable feature on the aeromagnetic anomaly maps is a magnetic low anomaly of -300 nT in amplitude extending in the SE-NW direction from off the Boso Peninsula to the Miura Peninsula (Fig. 2).
This low anomaly belt traces the south edge of the ophiolitic rock outcrops, suggesting a relation between the magnetic anomaly and the structure of the ophiolitic belts.
Tonouchi and Kobayashi (1983) discussed the Mineoka Ophiolite as the source of this low magnetic anomaly belt and proposed the subsurface structure.
However, aeromagnetic anomalies are not of sufficient resolution for discussion of the magnetic structure of the Mineoka Ophiolite Belt.
These aeromagnetic surveys, flown at 10,500 ft (3,200 m) above sea level, are mode at a great distance from the magnetic source,
and are not able to detect the magnetic anomalies due to the ophiolitic belt.
The structure may be as small as the size of outcrops of the Mineoka Ophiolite;
limited to a few kilometers. Because high resolution magnetic anomaly data were required to resolve this structure,
we carried out ground magnetic surveys onshore and offshore of the southern Boso Peninsula to clarify the magnetic crustal structure.
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