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. This group has many bedding-parallel shortening structures, including thrust-anticlines, duplexes, and 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 a methane-bearing, fluid-supported chemosynthetic biocommunity (Calyptogena) and calcite-cemented sediments (chimneys or pipes). The chaotic deposits that bear such methane-related materials suggest that the deposition has occurred on the thrust at the landward slope foot, and that the emplacement or depositional mechanism is either as a debris flow or an 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 the trench-fill type, similar to the Sagami Basin at 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.