FORMATION OF SUBMARINE LANDSLIDE DEPOSITS IN CONVERGENT MARGIN ACCRETIONARY PRISMS: AN EXAMPLE FROM THE SOUTHERNMOST BOSO PENINSULA, JAPAN

Japan has a unique tectonic setting in which four plates are converging at a trench-trench-trench (TTT)-type triple junction off the Boso Peninsula, southeast of Tokyo. This area includes Middle Miocene and younger accretionary prism units, which are little metamorphosed maintaining their initial deformation structures and high porosity. The coherent Nishizaki Formation of the middle Miocene – early Pliocene Miura Group represents one of these accretionary prism units and contains deformed blocks and chaotic deposits with many folds & faults. These deformed blocks are characterized as follows; 1) the equivalent beds are not found or correlated in or with any strata in the adjacent area, 2) they are highly folded and faulted and generally overturned, 3) the area is divided into six domains bordered by faults, 4) liquefied pumiceous sediments inject along fault zones between the adjacent coherent strata and within the deformed blocks, 5) except for the pumiceous sediments no matrix for the chaotic deposits are identified, and 6) the boundary faults separate each domain with different styles and modes. Based on detailed field mapping including strata correlation, facing identification and structural geometric analysis, we infer that the deformed blocks in each domain were emplaced by submarine landslide, and that all domains were subsequently stacked by thrust faults at the toe of the slide mass. The sliding direction was toward N as inferred from the arrangement and attitude of the fold axes in each domain. The mass movement occurred subaquaeously on a subducted plate from the Izu forearc side across the SW trending proto-Sagami Trough before formation of the accretionary prism.