Systematic variation in shear-induced physical properties and fabric in shallow part of accretionary prism

Middle Miocene to early Pliocene on-land example of shallow part of an accretionary prism consisting of non-metamorphosed and high-porous sediments is well represented in the southern Miura and Boso Peninsulas, central Japan. Neogene deep marine volcaniclastic sedimentary sequences are accreted from the Izu forearc side to the Honshu forearc during the Izu arc collision. Based on the mode of deformation, this off-scraped body could be divided into three parts ascendingly; gouge zone of the basal detachment fault, thrust fault-dominated unit just above the gouge zone in the lower part of the off-scraped body, upper coherent unit in the upper part of the off-scraped body. Sediment-fabrics obtained from measurement of anisotropy of magnetic susceptibility (AMS) and physical properties such as porosities and pore size distributions are quite variable in the respective three parts. Sediments in the upper coherent unit show layer-parallel flattening fabrics. Comparing with this, sediments in the thrust fault-dominated zone show constriction field of less magnetic foliation, and those in the gouge zone flattening fabrics parallel to the detachment fault plane, with stronger magnetic foliation and lineation. Sudden 7 to 9 % down drop of porosity is recognized at the boundary between the coherent upper unit and the thrust faults dominated lower unit. Similar down drop was also recognized in the contact between the thrust fault-dominated zone and gouge zone. Mercury porosimetries indicate that pore size distribution in the thrust fault-dominated zone is characterized by a small amount of large pores, whereas that in the coherent unit by a large amount of large pores. These analyses indicate the strain history during accretionary prism process in its shallow part. At the first stage, sediments suffer vertical compaction before accretion in general. When horizontal compressional stress is exerted during accretionary prism formation, the previous compaction fabric is disturbed accompanied with porosity decrease in several tens meters thick, before the displacement occurs. This porosity decrease is caused by large pore collapse and associated dewatering induced by the tectonic stress. At the last stage, displacement of thrusting occurs in a restricted zone where strain is localized. Flattening fabric parallel to the thrust fault and further porosity reduction occur associated with this thrusting only in the gouge zone.