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

Kelvin S. Rodolfo, A. Colella, R. N. Hiscott, T. Janecek, K. Marsaglia, A. Nishimura, K. Tazaki, J. Firth, J. B. Gill, K. Kaiho, K. Fujioka, B. Taylor, Leg 126 Shipboard Scientific Party

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 to 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.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990