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

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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.