FORMATION OF CHAOTIC ROCK-UNITS DURING PRIMARY ACCRETION PROCESSES: LIQUEFACTION AND MASS TRANSPORT DEPOSITS IN THE MIURA BOSO ACCRETIONARY PRISM, CENTRAL JAPAN

Chaotic rock-units are characteristic components in accretionary complex and trench-slope basins in subduction zone. They preserve detailed tectonic information related to the periods before, during, and immediately after accretion. The Miura and Boso area, central Japan is composed of two accretionary complexes: the early to middle Miocene Hota accretionary complex and the upper Miocene Miura–Boso accretionary prism, and several trench-slope basins. They uplifted rapidly due to the Izu-Bonin island arc collision, therefore have never been experienced deeper burial, thermal and physical overprinting, and give us great opportunity to study primary accretion processes and mélange-formation process in shallow portion of subduction zone.

We identified 5 types of chaotic rock-units formed during primary accretion processes, on the basis of the grain sizes and characteristics of blocks and surrounding matrices and cross cutting relations. The chaotic rock-units composed of silt matrices and sandy to pebbly blocks (Type 3) formed by gravity-driven slumping upon the seafloor. The slumping occurred contemporaneously with deposition. Vertical variations in the direction of slump vergence represent successive changes from an initially flat seabed to tilting to the NW and finally to the SE. Slumping with a NW vergence indicates landward tilting of the seafloor immediately prior to accretion, whereas vergence to the SE reflects oceanward tectonic tilting that occurred once the sediments had crossed the deformation front. These slumps and sedimentary sequence were cut by subsequent thrust systems during off-scraping accretion at the toe of the prism. These accretion-related thrusts concentrate in the lower part of the accretionary prism while rare in the upper part. Chaotic rock-units that have matrices abundant in sand and pebble (Type 1, 2) formed as a result of subsurface liquefaction and injection associated with large earthquakes that occurred during and after accretion of the sediments. When liquefaction occurred on the tilted seafloor, typically at trench-slope basin, liquefied sand and sedimentary unit deposited above were easy to slide on the slope, and make another type of chaotic unit (Type 4: rotated blocks in sand/pebble matrices). Imbricate thrust and/or plate-boundary décollement zone in relatively deeper part (2-4 km) made tectonic mélange representing block-in-matrix with intense S-C structure (Type 5: another talk in this session). These chaotic rock-units are useful in examining surface/subsurface changes such as tectonic tilting of the sea floor, earthquake events, and subduction-related deformations during the initial accretion process.