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 Earthquakes and tsunamis are high-impact geohazard events that can be extremely destructive when they occur at large magnitudes and intensities, although their causes and potential locations are, for the most part, predictable within the framework of plate tectonics. Amongst the main reasons for their high impact include enormous numbers of casualties, extensive property damage in vast areas and significant social and economic disruption in urban settings where populous residential areas, global banking centres, industrial factories and critical facilities (nuclear power plants, dams) may be located. In order to reduce the impact of these geohazards, nations, societies, professional organizations and governments need to collaborate to prepare more effective seismic and tsunami risk assessments, disaster management plans, educational and training programmes for increased preparedness of the public, and strategic plans and objectives for capacity building, skill and knowledge transfer, and building of societal resilience. Improved building design and construction codes and emergency preparedness and evacuation plans should be part of disaster management plans in countries where destructive earthquakes and tsunamis have occurred. The rapid increases in population along coastal corridors in developing and developed countries is likely to escalate the social and economic impacts of these geohazards exponentially in the future. The chapters in this book present case studies of some of the most salient earthquake and tsunami events in historical and modern times, their origins and manifestations, and efforts and the most effective practices of risk assessment and disaster management implemented by various governments, international organizations and inter-governmental agencies following these events. New methods of computing probabilistic seismic hazard risks, delineating respect distance and damage zones along and across seismically active faults and recognizing tsunamigenic and submarine landslides on the seafloor are introduced. The conclusions presented in the chapters show that: (1) scientific understanding of the characteristics of seismically active faults is paramount; (2) increased local (community), national and global resilience is necessary to empower societal preparedness for earthquake and tsunami events; and, (3) all stakeholders, including policy-makers, scientists, local, state and national governments, media and world organizations (UNESCO, IUGS, GeoHazards International, Global Geodetic Observing System, National Earthquake Hazards Reduction Program) must work together to disseminate accurate and timely information on geohazards, to develop effective legislation for risk reduction and to prepare realistic and practical hazard mitigation and management measures.

We investigated the field relations, metamorphic and deformation conditions, age, and chemistry of basaltic, plutonic, and metamorphic blocks in the Mineoka ophiolite mélange belt, Boso Peninsula, central Japan, to clarify their emplacement mechanisms. We considered internal and external deformation of the blocks in the context of the complicated processes by which the ophiolite mélange belt was formed in a forearc setting. A two-stage history leading to the present-day forearc sliver fault zone was revealed: an early stage of deep ductile deformation followed by an episode of brittle deformation at shallower levels. Both stages were the result of transpressional stress conditions. The first stage produced subduction-related schistosity with microfolding and mylonitization and then brecciation during exhumation in the intraoceanic subduction zone, from a maximum depth of garnet-amphibolite facies or eclogitic facies. The second stage was characterized by strong, brittle shear deformation as the rocks were incorporated into the present-day fault zone. The first incorporation of the oceanic plate to the side of the Honshu arc might have occurred during the Miocene, and was followed by right-lateral oblique subduction that has continued ever since the Boso triple junction arrived at its present-day position, thus forming the paleo-Sagami trough plate boundary.

Two types of thrust duplex structures were identified in excellent exposures of the deep level of the Jurassic to Cretaceous accretionary complex in the Kanto Mountains, central Japan, and the thickening ratio and shortening ratio were calculated. Simple (S type) and composite (C type) duplexes are mapped in an excavation site 100 × 40 m in extent. The beds of the S type and C type duplexes were thickened by factors of 5.8 and 6.0, respectively; however, the C type duplex includes four orders of smaller duplexes within it that underwent their own shortening. Thus the total thickening factor may attain at least 6–13, indicating a comparable degree of thickening at the level of greenschist facies conditions (approximately10 km or more in depth) in the accretionary prism.

The Pliocene-Pleistocene Chikura Group, southern tip of the Boso Peninsula, central Japan, occurs northeast of the present Sagami trough of the Philippine Sea plate subduction boundary. This group has many bedding-parallel shortening structures, including thrust-anticlines, duplexes, and small-scale conjugate sets of thrusts in addition to various kinds of chaotic deposits. The group forms one large synclinorium with smaller scale folds, but its relationship to accretionary prism evolution has not been explained. On the basis of geological structures examined on uplifted coastal benches, we propose that the lower half of the group was deposited on the subduction plate boundary as trench fill. When the trench was filled, the frontal thrust jumped seaward, causing landward tilting of the earlier trench fill deposits, after which the upper part of the group was deposited in a slope basin setting. The key observation to unravel the sedimentation and deformation is the recognition of the chaotic deposits, specifically whether they have a methane-bearing, fluid-supported chemosynthetic biocommunity ( Calyptogena ) and calcite-cemented sediments (chimneys or pipes). The chaotic deposits that bear such methane-related materials suggest that the deposition has occurred on the thrust at the landward slope foot, and that the emplacement or depositional mechanism is either as a debris flow or an injection (diapir). As a result, it is concluded that at least the lower half of the Chikura Group is a kind of accretionary prism of the trench-fill type, similar to the Sagami Basin at the present time. We conclude that the lower half of the Chikura Group records accretionary prism development in a trench-fill environment, similar to the present day Sagami Basin.

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.

Abstract A belt of disrupted ophiolitic rocks occurs on the Boso Peninsula (Japan), currently located north of the oblique subduction boundary between the Philippine Sea and North American Plates, under which the Pacific Plate has been subducting westwards. This ophiolitic belt (Mineoka Belt) is composed of mafic-ultramafic rocks together with Tertiary chert and limestone and island-arc volcaniclastic rocks. Our detailed structural studies in and around the basaltic rock bodies within the ophiolite reveal three phases of deformation. The first phase is further divided into three stages, all related to oblique normal faulting associated with extensional tectonics at or near a spreading axis. Fluid pressures appear to have fluctuated in association with faulting and veining during this phase. The second phase of deformation is characterized by thrust-related shear zones with a significant strike-slip component and is probably related to the final emplacement of the ophiolite by oblique subduction-obduction processes. The third and final phase of deformation affected not only the ophiolite but also later terrigenous and island-arc pyroclastic rocks. This deformation involved large-scale transpressional dextral slip on forearc sliver faults, which are still active today.

Abstract The mid to late Miocene Misaki Formation sediments of the Miura and Boso peninsulas, south central Japan, were deposited in the Pacific-facing forearc region of the proto Izu-Bonin arc at bathyal depths. The hemipelagic background facies, composed mainly of calcareous biogenic and pumiceous voicaniclastic material, are interbedded with thin to thick scoriaceous beds of turbiditic and pyroclastic fall origin. Careful study in the field and in the laboratory of these fine-grained background sediments has revealed the marked influence of bottom currents at certain horizons in producing intervals with distinct muddy contourite characteristics. These include: a general absence of primary structures due to intense bioturbation, some diffuse layering, irregular concentrations of coarser-grained material, sharp and erosive top and bottom lamina contacts, rare micro-cross-lamination disrupted by bioturbation that was continuous with deposition, and a mixed pelagic biogenic (commonly fragmented), volcaniclastic and terrigenous composition. Small-scale cyclicity of variations in grain-size and structural features can be related in part to episodic volcaniclastic input and in part to fluctuation in bottom current strength. Evidence for bottom current reworking of the tops of thin-bedded sandy turbidites is equivocal, and further work is required to resolve this debate. The recognition of Miocene-age contourites from the NW Pacific provides further evidence for the existence of active deep-ocean circulation in the Pacific at this time. However, it is not possible to determine which current system, Antarctic Bottom Water or deep Kuroshio Current, was responsible for these outcrop examples of fossil contourites.

Full article available in PDF version.