Investigation of Olate Subduction Process Based on Continuous Precise Measurement of Fluid Ventings at the Southwestern Japan

点击翻译按钮获取中文摘要

Precise positions of deep-sea fluid ventings was located at the landward toe of the Nankai Trough south off southwestern Honshu, Japan and detailed morphology of its bottom expression was revealed by the effort of this project under aegis of KAIKO-NANKAI Program. t was already suggested by the past KAIKO Project in 1985 that commonity of bivalves Calyptogena. is concentrated upon the vent. So far similar evidences have been found at other sites over the world, for example, at Barbados and off Oregon. However, the present result of in situ measurement of bottom temperature gradient first indicated that the fluid venting amounting to a few meters per year is exclusively focused under the bivalve community. Seeping water contains much methane and some sulphur which not only feed clams and their coexistent bacteria but also generate calcium carbonates after oxidation in O_2-rich pore water in the superficial sediment. The CaCO_3 takes part in cementing surrounding sediment to form solidifi … More ed sand and silt under soft surface sediment in venting zones at the deformation front of the accretionary prism (about 3800m deep). A number of sediment "chimneys" were identified by both submersible and deep-towed TV and a sample was collected and chemically analyzed. At this fluid venting site long-term measurement of temperature gradient and electric potential was conducted to yield information on time variations in venting rates.On the other hand, much cemented sediment including clam shells is found exposed in the bottom and eroded by sea water at the crest of back-thrust ridge above the front (2000m deep). Small community of bivalves Calyptogena is also existent here showing occurrence of small-scale venting through the high-angle thrust fault but prevailing phenomenon is uplifting of solidified sediment accreted some while before present. It seems likely that in situ observation at this area corresponds to the process of origin and development of the trench accretionary prism deduced from study of the ancient geological formation exposed on land. Less