LESSONS IN FOREARC BASIN EVOLUTION: RECONSTRUCTION OF THE KUMANO FOREARC BASIN FROM NANTROSEIZE DRILLING AND 3D SEISMIC DATA

Sedimentary deposits in the Kumano Forearc Basin of the Nankai Accretionary Prism, offshore Kii Peninsula, Japan, record complex interactions between sedimentation and deformation processes that control the stratigraphic architecture of the basin. This study correlates regional 3D seismic data with sediment cores from IODP NanTroSEIZE drill sites C0002 and C0009, allowing us to characterize and reconstruct the tectonic evolution of the basin. Imaging an area of 11 by 57 km, 3D seismic data reveal more than 2 km of sediments deposited above an upper Miocene-lower Pliocene (5.0-8.5 Ma) accretionary prism. The dominant acoustic character of the basin’s strata consists of landward-tilted (on the seaward end) to planar parallel (landward end), high-amplitude, laterally-continuous reflections interpreted as onlapping turbidite sequences. The onset of turbidite deposition began when accommodation space was created by subduction related uplift of the outer ridge along the splay fault at ~1.65 Ma. The basin-fill architecture shows that continuation of the uplift, along with growth of the accretionary wedge, migrated the locus of sedimentation landward, expanding the basin from ~10 km in width to > 30 km. A topographic high, formed by continued motion along thrusts in the underlying prism, is onlapped by deformed basin strata. There are numerous small to large scale mass transport deposits (MTDs) throughout the basin which seem to have slid along bedding planes during uplift. The dominant lithology in IODP cores is dark, olive-gray, silty, claystone with minor lithologies including sandstone, sandy siltstone, silty claystone, calcareous claystone, and fine ash. Most sediment samples are dominated by a siliciclastic assemblage of clay, quartz, and feldspar, with variable amounts of pelagic carbonate and a minor, but persistent, component of volcanic glass. The cores have clearly documented stacked turbidite deposits, supporting the seismic interpretations of turbidite deposition. The seismic character of much of the basin strata deeper than 400 meters below seafloor is significantly affected by gas hydrate recycling, suggesting that the presence of gas hydrate deposits in forearc basin sediments may play an overlooked, under investigated role in large scale sedimentary processes in forearc basin development.