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

在KAIKO-NANKAI项目的支持下，本项目确定了日本本州西南以南南开海槽向陆端的深海流体喷口的精确位置，并揭示了其底部表达的详细形态。早在1985年，KAIKO项目就已经提出了双壳类动物的共性。集中在通风口上。到目前为止，在世界各地的其他地方也发现了类似的证据，例如在巴巴多斯和俄勒冈州附近。然而，目前对海底温度梯度的现场测量结果首先表明，每年达几米的流体喷涌只集中在双壳类动物群落之下。渗漏水中含有大量的甲烷和少量的硫，这些甲烷和硫在浅层沉积物中富含o_2的孔隙水氧化后生成碳酸钙，不仅可以作为蛤蜊及其共生菌的饲料。CaCO_3参与胶结周围沉积物，在增生棱柱变形前缘（约3800m深）的通风带软面沉积物下形成固化砂粉。通过潜水和深拖曳电视识别了许多沉积物“烟囱”，并收集了样本并进行了化学分析。在这个流体排气点，进行了温度梯度和电势的长期测量，以获得有关排气速率时间变化的信息。另一方面，包括蛤壳在内的大量胶结沉积物暴露在底部，并在前缘以上的逆冲脊顶（2000m深）被海水侵蚀。这里还存在着小的双壳类动物群落，显示出通过高角度逆冲断层的小规模喷发，但主要的现象是之前一段时间沉积的固化沉积物的隆升。该地区的现场观测似乎符合从陆地上暴露的古地质构造研究中推断出的海沟增生棱镜的起源和发展过程。少

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