Structural characteristics of shallowly buried accretionary prism: Rapidly uplifted Neogene accreted sediments on the Miura‐Boso Peninsula, central Japan

Abstract

The upper Miocene Misaki and Nishizaki formations on the Miura and Boso peninsulas in central Japan preserve the deformation features of an off‐scraped accretionary prism. The spatial distribution, geometry, and style of accretion‐related deformation with paleotemperature and burial depth estimation are elucidated in this study. The deformation structures and textures are similar to those of modern accretionary prisms. The low maximum paleotemperature (<50°C) and high preserved porosity of the sediments (30–50%) imply a maximum burial depth of less than 1000 m. On the basis of the mode of deformation, this off‐scraped body is divided into an imbricate thrust, a thrust unit, and an upper coherent unit in ascending order. The imbricate thrust corresponds to a branch from the basal décollement zone and is subdivided into a brecciated zone, a main gouge zone, and a shear band zone. The thrust unit hosts a concentration of thrust systems that form various orders of duplex structures, while the upper coherent unit is characterized by gravitational instability‐ and earthquake‐induced deformation without thrust faulting. The duplex distribution, shear strain, and fluid migration associated with the off‐scraping processes are clearly localized within the imbricate thrust and thrust unit. This accretion‐related deformation occurred before lithification of the sediment and under high fluid pressure induced by shear deformation, thickening of the sedimentary sequence, and earthquake‐induced liquefaction. These processes are inferred to control the effect on the mode of deformation and the location of thrusting during early deformation of the accretionary prism.