High Resolution and Three-Dimensional Investigation of Crustal Structure of an Island Arc-Toward the Deliberation of Stratified Llthospheric Tectonices(Stratotectonics)

1992年对伊豆-小笠原（小笠原）弧沿纬32‰的深部地壳结构进行了地震成像研究。现场调查包括东京大学海洋研究所所属的三艘船：R/V Hakuho-maru和Tansei-maru，以及一艘用于爆破作业的包租船。进行了两次船舶地震试验。白虎丸是发送船，淡井丸是接收船。这些实验使我们能够获得更深层地壳图像的数据。沿双船地震试验路线共布置了50台海底地震仪。OBS阵列记录了包括炸药产生的地震波在内的各种信号，获得了岛弧地壳下上地幔地震速度结构的信息。初步资料处理结果表明，该位置的伊豆-小asawara岛弧地壳由4个地震层组成：1)2-3.5km/sec纵波速度层，以覆盖火山层序为主（A1层），2)4-5km/sec层，解释为片状脉状复岩（A2层），3)上地壳6.1-6.3km/sec层（A3层），4)7.1-7.3km/sec层对应下地壳（a4层）。A3层在弧前区域下厚5km，在中央裂谷带下增厚7 ~ 8km。A4层厚度相对均匀：弧前厚度约8km，中央裂谷带厚度约7km。A3层为典型的“花岗质层”地震速度范围。考虑到伊豆碰撞带的坦泽岩体很可能是一个暴露的例子，我们将A3层解释为长英质侵入层，可能由闪长岩和花岗闪长岩组成。这个长英质层可能是由下地壳基性层（包括之前存在的海洋地壳？）的再熔融而来的。A4层可能由下镀基性-超基性侵入物组成。

The investigation conducted in 1992 focused on seismic imaging of deep crustal structure of the Izu-Ogasawara(Bonin) Arc along the latitude 32ﾟ.The field investigation included three vessels: R/V Hakuho-maru and Tansei-maru which belong to Ocean Research Institute of the University of Tokyo and a chartered vessel used for explosive operation. Two ship seismic experiments were conducted. Hakuho-maru being as transmitting ship and Tansei-maru as receiving ship. The experiments enabled us to obtain the data for deeper crustal images. Total fifty ocean bottom seismometers (OBS) were deployed along the line of two-ship seismic experiments. The OBS array recorded various signals including dinamite-generated seismic waves and yielded information on upper mantle seismic velocity structure underneath the island arc crust.The results of preliminary data processing indicate that the Izu-Ogasawara Island Arc crust at this location is composed of four seismic layers: 1)2-3.5km/sec P-wave velocity layer which is dominantly composed of cover volcanicalastic sequences(Layer A1),2)4-5km/sec layer which is interpreted to be sheeted dike complex(Layer A2),3) 6.1-6.3km/sec layer of the upper crust(Layer A3),4)7.1-7.3km/sec layer which corresponds to the lower crust(Layer a4).Layer A3 is 5km thick under the fore arc region and increases thickness under the central rift zone(7-8km thick). Layer A4 shows relatively uniform thickness: about 8km thick under the fore arc and 7km thick under the central rift zone.Layer A3 shows a range of seismic velocity typical to "granitic layer". Taking also account that the Tanzawa pluton in the Izu collision zone could well be an exposed example we interpreted Layer A3 as felsic intrusive layer possibly being composed of tonalite and granodiorite. This felsic layer might be derived from remelting of the lower crust mafic layer (including pre-existed oceanic crust ?) Layer A4 is probably composed of underplated mafic-ultra mafic intrusives.