Abstract Analyses of consolidation state, fabrics and physical properties were conducted on rock samples from the Plio-Pleistocene Boso forearc basin, central Japan. Consolidation tests identified that the trend in consolidation yield stress was systematically 8 MPa smaller than expected for the overburden from the sediment thickness of the Kazusa Group. An excess fluid pressure interval was also identified in the lower part of the basin fill, where several large-scale (several kilometres in length and several tens of metres thick) mass-transport deposits (MTDs) are intercalated. This interval is characterized by high porosity and small consolidation yield stresses, indicating that consolidation had been retarded by the excess fluid pressure. The estimated excess fluid pressure was c. 5–7 MPa. In addition, outcrop-scale fluidization and minor liquefaction features were identified within and below the high fluid pressure interval. The excess fluid pressure reduced the effective stress in the Boso forearc basin and, subsequently, the stability of the slope, allowing small tectonic events to generate submarine landslides. Therefore, the formation of these large-scale MTDs was probably related to the excess fluid-pressure generation.

Thin, planar, dark, lamination-like bands are found in host siltstones in the Miocene-Pliocene metamorphosed Miura-Boso accretionary prism, southern Boso Peninsula, Japan. We classified the bands into four types on the basis of distribution, crosscutting relations, and internal textures. Type 1-1 dark bands are developed parallel to the bedding plane and do not include crushed or deformed grains within the band. Type 1-2 bands are also developed parallel to the bedding plane, but grain alignment within the band cuts obliquely across that in the host rock. Type 2 bands include ductilely deformed grains similar to an S-C′ structure, whereas type 3 bands have cataclastic grains. All the dark bands except type 1-1 (being an open fracture with little displacement) are shear bands or slip planes formed from sedimentation to accretion, although the formation mechanisms between the four types are different. These deformation bands are affected by the state of consolidation and magnitude of stress during formation, reflecting the deformation processes. Type 1-1 bands show evidence of independent particulate flow from excess pore-fluid-pressure generation, which occurs just after sedimentation. Type 1-2 bands are flexural-slip faults formed during formation of folds; type 2 bands are sliding planes formed from submarine landslides, whereas type 3 bands are thrust faults formed during accretion.

The NW corner of the Pacific Ocean is a place of unique Tertiary tectonism, which provides one of the clearest examples of arc-arc collision. Voluminous Cretaceous rhyolitic-granitic magmatism along the continental margin continues into the Paleogene. In contrast, Miocene island arc volcanism follows Eocene boninitic magmatism in the Izu-Mariana Arc, in association with the opening of backarc basins, including those in the Philippine and Japan Seas. The triple junction between the Eurasian, Philippine Sea, and Pacific plates arrived in the area south of Tokyo during the Miocene, just as the Japan Sea was opening. After the beginning of Philippine Sea plate subduction to the north, the Izu Island Arc began to collide obliquely with the Honshu Arc. As a result, this unique tectonic setting in the NW Pacific has produced a miniature Alpine-type orogenic belt (Tanzawa) in the collisional center, whereas in the eastern part of the Izu Arc sediment has been actively accreting in that forearc. Such settings have resulted in systematic accretionary prism formation from the early Miocene in the Boso-Miura peninsular area to the present in the Sagami Trough area. We modeled the tectonics by a simple sandbox experiment. Systematic fault and fracture patterns of the oblique subduction type are predicted to occur during arc-arc collision.