RECOGNITION OF TRENCH-FILL TYPE ACCRETIONARY PRISM: THRUST ANTICLINES, DUPLEXES AND CHAOTIC DEPOSITS OF PLIOCENE-PLEISTOCENE CHIKURA GROUP, BOSO PENINSULA, JAPAN

Japanese trench areas have many varieties of trench sedimentation and deformation for accretionary prisms. Such present examples have been well studied from on land and under the sea.

The Pliocene-Pleistocene Chikura Group, southern tip of the Boso Peninsula, central Japan, occurs northeast of the present Sagami trough of the Philippine Sea plate subduction boundary. The group has many bedding parallel shortening structures, including thrust anticlines, duplexes, small scale conjugate sets of thrusts in addition to various kinds of chaotic deposits. The group forms one large synclinorium with smaller scale folds, but its relationship to accretionary prism evolution has not been explained. On the basis of geological structures examined on uplifted coastal benches, we propose that the lower half of the group was deposited on the subduction plate boundary as trench fill. When the trench was filled, the frontal thrust jumped seaward causing landward tilting of the earlier trench fill deposits, after which the upper part of the group was deposited in a slope basin setting.

The key observation to unravel the sedimentation and deformation is the recognition of the chaotic deposits, specifically whether they have methane-bearing fluid supported chemosynthetic biocommunity (Calyptogena) and calcite-cemented sediments (chimneys or pipes). The chaotic deposits which bear such methane related materials suggest that the deposition occurs on the thrust at the landward slope foot, and that the emplacement or depositional mechanism is either as a debris flow or injection (diapir). As a result, it is concluded that at least the lower half of the Chikura Group is a kind of accretionary prism of trench-fill type, similar to the Sagami Basin of the present time. We conclude that the lower half of the Chikura Group records accretionary prism development in a trench fill environment, similar to the present day Sagami Basin.