相模湾とその周辺の地質 : KT88-1次航海報告

• Main Authors: Fujioka, Kantaro, Kinoshita, Masataka, Soh, Wonn, Tsukawaki, Shinji, Ashi, Juichiro, Akimoto, Kazumi, Watanabe, Masaharu
• Format: Publication
• Language: English
• Published: Earthquake Research Institute, University of Tokyo 25.12.1989; 東京大学地震研究所

1988年2月相模湾の地質・地球物理・地球化学等の総合的な調査が東京大学海洋研究所の淡青丸によって行なわれた.日本列島周辺で最大の深海生物群集である相模湾西部の初島の南にある初島生物群集では最高1680mW/m2に達する高地殻熱流量の異常が観察された.それとともにメタン(CH4)の高異常を示す海底直上の海水が得られた.これらの結果は初島生物群集の立地について新しいアイディアを提出させることになった.初島生物群集周辺の地形,表層堆積物,堆積物中の底生有孔虫の結果や地球物理・地球化学的性質は,初島生物群集が地下のマグマに関係した海底温泉によって形成されている可能性を強く示唆している.相模湾の東部にあり北西-南東方向の配列をしている浅瀬,沖ノ山堆列では地形と地質の調査が行なわれ,火山性堆積物を含む大量の泥岩がドレッジによって得られている.従来海上保安庁水路部により提出されている地磁気異常と全磁力の分布とから沖ノ山堆は火山の列ではなく,中新世から鮮新世にかけての火山フロント近くから供給された火山性物質が陸側斜面に付加してできた可能性が高い. Geological and geophysical characteristics of Sagami Bay, 80 km west off Tokyo, were studied on KT88-1 cruise by R/V Tansei-Maru, Ocean Research Institute, University of Tokyo, February 1988. In this paper, we summarized the results of the geological and geophysical studies, and discussed two topics: 1) The Hatsushima deep-sea community which is one of the largest communities around the Japanese Islands is located in the western part of the Sagami Trough. Heat flow anomalies up to 1,680 mW/m2 and high methane anomalies of the surrounding seawater were discovered there. We deduce that the deep sea community has most likely originanted and is developed by submarine hot springs related to magmatic activity. 2) Pebbly mudstones including various sizes and kinds of volcanic materials were obtained by dredge hauls at several sites of the Okinoyama Bank Chain. No in-situ volcanic material was obtained. The magnetic anomalies along the Okinoyama Bank Chain give no evidence of volcanic seamounts. We believe that the Okinoyama Bank Chain is not underlain by volcanic seamounts, but rather by large blocks of accreted sediments, including large amounts of volcaniclastic materials deposited in the forearc basins of the Izu-Bonin Ridge near the volcanic front. The mutual relation between the Sagami Trough and the Bank Chain should be similar to the relation between the So-o Trough and the southern offshore Boso area.