ABSTRACT: Evolution of the Izu-Bonin Forearc Basin

Patricia A. Cooper

Three sites within the Izu-Bonin intra-oceanic forearc were drilled during Leg 126; two penetrated basement. The 7 km wide forearc basin, defined by thick (1.5-4 km) sequences of relatively undeformed coarse-grained volcaniclastic and fine-grained hemipelagic strata, occupies a long belt between the Izu-Bonin frontal arc to the west and the outer-arc high to the east. Based on correlations of logging, physical properties measurements, and lithostratigraphic data to regional multichannel seismic surveys, four major seismostratigraphic sequences, representing four major developmental phases, are recognized: (1) Initial mid-Eocene to early Oligocene tholeiitic and boninitic volcanism formed a basement with highly variable acoustic characteristics--basement near the frontal a c high is well defined and lacks coherent reflectors, while nearer the outer-arc high, basement is often seismically stratified and cut by dipping reflectors. (2) Mid-Oligocene rifting of the forearc and late Oligocene volcanism; the lowermost sedimentary units lap out onto basement to the east. Chaotic, subparallel reflectors are capped by strong, continuous reflectors, parallel and conformable to the underlying units, representing coarse, volcaniclastic turbiditic basin in-fill. The entire late Oligocene sequence is extensively faulted. (3) Miocene spreading in Shikoku basin; a volcanic minimum in the forearc region (27-13 Ma) resulted in dominantly nannofossil-rich hemipelagic sedimentation. (4) Pliocene-Quaternary volcanism, increasing to present maximum; this upper sequence thickens and is downfaulted towards the arc. It consists of many thin packets of strong reflectors, repeatedly interrupted by canyon cutting and filling.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990