Part of the Emi Group, a Miocene accretionary prism in the Boso Peninsula, Japan, displays an extensive west-trending duplex array. The intense faulting there is divided into five stages. The first stage, involving layer-parallel to subparallel faulting, is primarily responsible for thickening and horizontal shortening of the strata. This stage thickened the sequence by a factor of 3.5. Duplex formation is here interpreted to be essential to thicken trench turbidites at a relatively early stage of deformation just after sedimentation, even in shallow levels of accretionary prisms. The east to west vergence of duplex formation might be attributed to the unique tectonics around the Boso trench-trench-trench–type triple junction.