Sedimentary Fills of Izu-Bonin Fore-arc and Back-arc Rift Basins South of Japan, ODP Leg 126: ABSTRACT

Abstract

From April to June 1989, Leg 126 of the Ocean Drilling Program successfully drilled the Izu-Bonin intraoceanic arc: Sites 787, 792, and 793 in the eastern, western, and central portions of a 40-70-km-wide fore-arc basin; Sites 790 and 791 on the 2-km-deep floor of the Sumizu back-arc rift; and Site 788 on the eastern rift footwall. Basaltic andesite and andesite basement of the fore-arc basin, initially 4-5 km deep, was produced by rifting or spreading that started 31-34 Ma (middle Oligocene) and has since been uplifted 1-2 km. Volcanism and erosion of surrounding highs provided debris flows and turbidites that began to fill the basin 250-300 m/m.y. Sharply declining volcanism and epiclastic supply are recorded in slowly accumulated Oligocene-Miocene (24-13 Ma) hemipelagic sediments. Regional explosive volcanism, renewed after 13 Ma, has left more than 200 thin ash layers in the uppermost (late Pliocene-Holocene) sediments. Total basin fill is 1.5-4 km thick. The Sumisu began to form 3.56-1.1 Ma. Prerift and present-day volcanism has been dominated by rhyolitic pumice eruptions. The eastern rift footwall, now 1.1 km below sea level, has been uplifted 0.2-1.7 km. Basaltic and arc-pyroclastic rift basement was 2 km deep prior to 1.1 Ma. From 1.1 tomore » 0.235 Ma, 100-400 m of predominantly hemipelagic sediment were deposited, although intrarift basaltic eruptions and rhyolitic eruptions were fairly common. Explosive arc volcanism increased dramatically 250 Ka, leaving 165-428 m of fine to coarse pumiceous sediments in layers that are each 30-50 m thick at Site 790.« less