伊豆・小笠原孤の横断掘削　ＯＤＰ１２６節成果報告

The Izu-Bonin intra-oceanic island arc-trench system is a product of the subduction of Pacific lithosphere since the Eocene. The purposes of Ocean Drilling Program (ODP) Leg 126 were to study the following three important and poorly understood aspects of this system, namely (1) the origin and evolution of the forearc basin, investigated by drilling a series of holes through the sediment piles and into the basement of the forearc basin (Sites 787 and 792-793); (2) the process and products of arc rifting, investigated by drilling holes into the center (Sites 790/791) and eastern footwall (Sites 788/789) of the Sumisu backarc depression ; and (3) recycling of subducted lithosphere and evolution of the mantle, investigated indirectly from the composition of the volcanic rocks as well as volcaniclastics recovered from all sites.<BR>The principal drilling results of Leg 126 show that the forearc basin formed about 35-30 Ma by separation of the formerly contiguous frontal and outer arc highs. Igneous basement beneath the center of the forearc basin includes high-Mg andesites, andesites with boninitic affinities, and low-Mg tholeiitic lavas. The forearc basin was rapidly buried immediately after its formation with volcaniclastic turbidites and debris flow deposits in the late Oligocene age. Following a minimum of volcanic output between 24 and 13 Ma, there has been a steady increase in explosive volcanic materials in the forearc with a dramatic increase in the late Quaternary. Paleomagnetic evidence shows that the forearc has been northwardly translated about 15° since 30 Ma. Benthic foraminiferal assemblage data suggest 1-2km of basement uplift has occurred since the mid-Oligocene. Pore waters in the Oligocene volcanogenic sediments of the forearc basin are the most extensively altered pore water of seawater origin ever sampled by DSDP/ODP. Low-temperature alteration of the volcanogenic sediments has produced fluids extremely enriched in Ca and depleted in Mg, Si and sulfate.<BR>The present stage of rifting together with deep sea explosive volcanic eruption of the basaltic mousse in the backarc Sumisu Rift at 31°N began between 3.56 and 1.1 Ma with both present-day and pre-rift volcanism along the volcanic front dominated by rhyolitic pumice eruptions. The footwall of the Sumisu Rift has been uplifted 200-1700m, and rift basement depth prior to 1.1 Ma exceeded 2 km. The basement of the rift is formed by early rift basaltic lavas and intrusives, as well as by arc pyroclastics metamorphosed to zeolite to greenschist facies. Bimodal volcanic activity with intra-rift basaltic eruptions (basaltic mousse) and rhyolitic eruptions was common but explosive arc volcanic activity dramatically increased 200 ka. at the Sumisu and South Sumisu Calderas along the volcanic front. Unlike the forearc region, fluids other than sea water are not circulating locally through the sediments in the Sumisu Rift.