VERY EARLY STAGE DEFORMATION BANDS IN ARGILLACEOUS ROCKS FROM JAPAN: RECOGNITION OF GRAVITATIONAL SLIDING, SYNSEDIMENTARY ACCRETIONARY PROCESS AND RELATED STRUCTURES

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 (the Miura-Boso accretionary prism), which maintain initial deformation structures and high porosity without strong metamorphism and provide the best opportunity to study the early deformation history in an accretionary prism. This prism frequently contains deformation bands, thin lamination-like dark layer, which are not documented from old accretionary complexes. We defined them as “dark band”. The dark bands are classified into four types (types 1-1, 1-2, 2 and 3) on the basis of distribution, crosscutting relation and internal texture. These dark bands were formed in various stages and settings. Type 1-1 bands are independent particulate flow due to the generation of excess pore pressure just after sedimentation. Type 1-2 bands are flexural-slips during folding in off-scraping or mélange formation. Type 2 bands are sliding planes of submarine landslide. Type 3 bands are thrust faults developed after the off-scraping stage of accretion. Each type reflects physical properties such as degree of consolidation, permeability and stress condition the sediments underwent during deformation. Our observations indicate that type 1-1 and 2 were caused by gravitational slide, and type 1-2 and 3 by lateral compression during accretion.