ABSTRACT Deformation in a subduction zone and the related transition from smectite to illite within the aseismic-seismic transition zone (2–4 km below the seafloor) were analyzed by studying an onland accretionary complex that was previously buried to a depth of just 2–4 km. The early to middle Miocene Hota accretionary complex of central Japan is an excellent example of an accretionary complex that records shallow underthrusting at the updip end of the seismogenic zone. Two types of subduction-related deformation are preserved in the complex: phacoidal deformation (D1) characterized by rhombus-shaped fragments of mudstone with a random fabric and a thin rim of clay minerals with a preferred orientation, similar to the deformation features of the primary décollement zone at the toe of modern accretionary prisms (as revealed by ocean drilling); and block-in-matrix deformation (D2) characterized by an asymmetric S-C foliation with shear bands and an intense shape-preferred orientation of clay minerals, similar to the deformation features of tectonic mélange in ancient, mature décollement zones. D2 is marked by a large reduction in the amount of smectite and a corresponding increase in illite. During D2, the shear zone increased in strength due to the disappearance of weak smectite, which has a low friction coefficient, and due to an increase in the cohesion of sediments associated with a reduction in porosity and the development of a preferred orientation of clay minerals. Such strain hardening represents a fundamental mechanical/chemical change in the properties of sediments immediately before entering the seismogenic zone.

ABSTRACT Discrimination between gravity slides and tectonic fold-and-thrust belts in the geologic record has long been a challenge, as both have similar layer shortening structures resulting from single bed duplication by thrust faults of outcrop to map scales. Outcrops on uplifted benches within the Miocene to Pliocene Misaki accretionary unit of Miura-Boso accretionary prism, Miura Peninsula, central Japan, preserve good examples of various types of bedding duplication and duplex structures with multiple styles of folds. These provide a foundation for discussion of the processes, mechanisms, and tectonic implications of structure formation in shallow parts of accretionary prisms. Careful observation of 2-D or 3-D and time dimensions of attitudes allows discrimination between formative processes. The structures of gravitational slide origin develop under semi-lithified conditions existing before the sediments are incorporated into the prism at the shallow surfaces of the outward, or on the inward slopes of the trench. They are constrained within the intraformational horizons above bedding-parallel detachment faults and are unconformably covered with the superjacent beds, or are intruded by diapiric, sedimentary sill or dike intrusions associated with liquefaction or fluidization under ductile conditions. The directions of vergence are variable. On the other hand, layer shortening structure formed by tectonic deformation within the accretionary prism are characterized by more constant styles and attitudes, and by strong shear features with cataclastic textures. In these structures, the fault surfaces are oblique to the bedding, and the beds are systematically duplicated (i.e., lacking random styles of slump folds), and they are commonly associated with fault-propagation folds. Gravitational slide bodies may be further deformed at deeper levels in the prism by tectonism. Such deformed rocks with both processes constitute the whole accretionary prism at depth, and later may be deformed, exhumed to shallow levels, and exposed at the surface of the trench slope, where they may experience further deformation. These observations are not only applicable in time and space to large-scale thrust-and-fold belts of accretionary prism orogens, but to small-scale examples. If we know the total 3-D geometry of geologic bodies, including the time and scale of deformational stages, we can discriminate between gravitational slide and tectonic formation of each fold-and-thrust belt at the various scales of occurrence.

Abstract The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate affinity. High-Fe and -Ti tholeiitic basalt and within-plate alkali-basalt have Ar/Ar ages of 49 ± 13 Ma and 19.62 ± 0.90 Ma, respectively. Three plutonic rocks have K-Ar ages of c. 25, 35 and 40 Ma. These ages are inconsistent with the known ages from the Pacific or Philippine Sea Plate. We infer that the Mineoka ophiolitic assemblage was part of another Tertiary oceanic plate, the ‘Mineoka Plate’, which underwent island-arc volcanism in the Miocene as a result of subduction initiation at a fracture zone or a transform fault system due to a change in the position of the Euler rotation pole of the Pacific Plate at c. 43 Ma. Eruption of within-plate type alkali basalts on the Mineoka Plate took place near the palaeo-Japan continental arc just before the emplacement of the Mineoka ophiolite into the Japanese continental margin.