Layer-parallel faults, duplexes, imbricate thrusts and vein structures of the Miura Group: Keys to understanding the Izu fore-arc sediment accretion to the Honshu fore arc

Abstract The Miura Group (Miocene-Pliocene) of south-central Japan shows a number of unique lithological and structural features. The group is composed of volcanic arc-derived marine sediments, and those in the south of the Mineoka Tectonic Belt particularly show various kinds of complex structures such as layer-parallel faults, thrust duplexes, imbricate thrusts and vein structures, yet the degree of compaction of the sediments is still remarkably low. These structures involve deformations at a very early stage and at shallow depths. They arose shortly after sedimentation within the Izu fore arc, and continued during accretion to the Honshu fore arc. The deformational stages are classified here into three stages, the first comprises bedding-parallel faulting associated with gravitational sliding and sediment injection. The first vein structures formed during this stage in the Izu fore arc area. These structures are cut by features developed during the second and third stages: especially thrusting, including duplex and imbricate thrusts. This horizontal shortening occurred during the accretionary prism formation on the subduction plate boundary. The second vein structures formed during this stage in the accretionary prism formation. The origin of the vein structures was discussed both by field observation and laboratory experiments. The latter suggests earthquake origin and the formative process is explained in relation to the field evidence.