RESEARCH ON THE BASIS OF ACCRETIONARY PRISM FORMATION IN VIEW OF DEFORMATION AND DIAGENESIS OF PELAGIC SEDIMENTS

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These three year project on deep-sea sediments (pelagic sediments) has done the research on the process of sedimentation and accretion to the accretionary prism. We mainly focused the project on the mechanism and the role of the pelagic sediments when they are accreted. The main target is the textural change during accretion, particularly how the sediments work during the diagenesis and deformation ; for the original pelagic sediments on the open ocean suffer compaction due to vertical loading, while they get horizontal stress with higher temperature and pressure within the accretionary prisms. The sediments have their own depositional environments and age control, so that the biostratigraphic data by means of microfossils, and provenance determination are also important. In addition the chemical and mineralogical change of the minerals and bulk rocks are also related to the physical change. Many methods were used, but the SEM is above all the most important way to know the texture of … More the sediments. We obtained many results as follows ;The representative pelagic sediment minerals are clays, and they are locally different ; smectite is largely deposited in front of volcanic arcs ; siliceous microfossils are dominant within the pelagic sediments, and they are useful for determination of the ages ; smectite-bearing horizon is most important for the accretionary prism formation ; this zone becomes the decollement (the mechanical boundary) of the prism ; siliceous deposits in the open ocean have only very slow change of texture and chemical/mineralogncal diagenesis, while they vastly change into chert during incorporation into the accretionary prism ; this means the pelagic sediments have the most important role for the formation of the accretionary prism ; pelagic sediments are responsible for the large structures which are characterized by duplex structures ; reconstruction of the accretionary prism need radiolarian biostratigraphy and chromian spinel sandstone petrography. Less