An intrusive andesile rock was found for the first time in the Miocene Hayama Group, Miura Peninsula, northeast of the Sagami Trough, Central Japan. This andesite has a chilled contact with the surrounding mudstone of the lowermost part of the Hayama Group. Major and trace element chemical analyses of the andesite verified that it is a high-K andesite of arc affinity. A K-Ar age of 23.4±0.8 Ma shows that the intrution of the andesite magma took place just after the deposition of the mudstone of the Hayama Group. This data suggest that the Hayama Group probably represents earliest Miocene fore-arc sedimentation associated with andesitc fore-arc volcanism.

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Multichannel seismic reflection survey was performed at southeast of the Izu Oshima in March, 1987. Based on the multichannel seismic reflection profile, we can iendify at least five units. Those are units A, B, C, D, and E. Units C and D are considered as trough filled sediments judging from their distribution and are correlated with the Shiozawa Formation (0.4-1.0 Ma B. P.) and the Neishi, Seto, and Hata Formation (1.4-2.0 Ma B. P.) of the Ashigara Group respectively. Many normal faults are developed in unit D and acoustic basement (the Shirahama Group), however scarcely in upper units. Unit D is stepped down toward the Sagami Trough by the faults with gradual increase in its thickness. The normal faults are closely related to formation of the present morphology of the Sagami Trough. Therefore, we interpret that unit D was deposited during formation of the Sagami Trough. Both units A and B are correlated with volcaniclastic sediments in the upper Pleistocene and the Holocene. On the western slope of the Sagami Trough, upper part of unit A is sliding down toward the Sagami Trough with a length of approx. 7 km. An instantaneous amplitude of the seismic profile indicates that the Habu Spur is composed of many igneous intrusions. The Habu Spur has a trend of NW-SE direction which is consistent with that of lateral volcanoes of the Izu Oshima. It is considered that the Habu Spur is the southeastern extension of lateral volcanoes of the Izu Oshima.

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The precise age for the Oki trachyte-rhyolite group was determined by Rb-Sr whole rock isochron method. This trachyte-rhyolite group was formed by a large scale ring fissured eruption on the Oki Dogo Island in the Japan Sea. A regression of the Rb-Sr isotopic data yields an age of 6.8±0.2 Ma with an initial Sr isotopic ratio (Sri) of 0.707176±0.000080 (2a). This age is consistent with the K-Ar age of 6.3 Ma determined by KANEOKA (1970), and is similar to the activity ages of alkali basalts occurring in Oki Dozen Islands near the Oki Dogo Island. The fission track age of 3.6 Ma by SUZUKI (1970) may be caused by some late annealing events. The Sri determined for the Oki trachyte-rhyolite group is very close to that of basic granulite xenoliths included in some alkali basalts. The lower crust beneath the Oki Dogo Island is considered to be composed of these basic granulites which are very poor in alkali metals. It is proposed that the magma of the Oki trachyte-rhyolite group was formed by partial melting of the lower crust with leaving the basic granulite as a residue.

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