Kohsaku Arai

High-density surveys conducted to reveal active deformations of the upper forearc slope along the Ryukyu Trench, western Pacific, Japan

AbstractIn this study, which was conducted to investigate present-day geological deformations occurring off the central Ryukyu Arc along the Ryukyu Trench (Okinawa-jima) in the vicinity of Japan, we obtained and examined dense, high-resolution, 2D grid (high-density) multichannel seismic reflection data. The forearc slope of the central Ryukyu Arc lacks a large forearc basin but has a steep narrow slope that dips to the southeast in the direction opposite to the plate convergence. However, while the seismic profiles obtained in our investigation do not reveal any active structures indicative of compressional stress in this convergent subduction zone, recent tectonic deformations are characterized by normal faults that strike perpendicular to the Ryukyu Trench axis. Examinations of high-density geological data show that the recent relative motion of the overriding plate has resulted in an active, arc-parallel extensional field. Furthermore, the results of our observations indicate two stages of extensional stress beneath the forearc slope of the central Ryukyu Arc that resulted from back-arc rifting in the Okinawa Trough.

Determining sources of deep-sea mud by organic matter signatures in the Sunda trench and Aceh basin off Sumatra

The content, optically determined properties, and stable isotope composition of organic carbon in fine-grained sediment cores were analyzed to investigate the origins of deep-sea sediments deposited in the Aceh forearc basin and on the Sunda trench floor off Sumatra from the late Pleistocene to the Holocene. In the Aceh basin, the depositional frequency of turbidite mud decreased as sea level rose during the deglaciation. The terrigenous organic carbon content was high at the end of the last glacial period, whereas during the deglaciation most of the organic carbon was of marine origin. In the Sunda trench, the Holocene turbidites consisted of remobilized slope sediments from two different sources: sediments derived from the old Bengal/Nicobar fan included thermally matured organic fragments, whereas those derived from the trench slope contained little terrigenous organic carbon.

Carbonate sediments of the Bahama Platform

ODP(国際深海掘削計画)の第166次航海は, バハマトランセクトの炭酸塩堆積シークエンスというテーマで, 海水準変動とバハマ炭酸塩プラットフォームの形成史の関係を解明するために, 1996年2月～4月にかけて行われた(Eberli,Swart et al.,1997).1997年10月に第2回ポストクルーズミーティングがバハマのグレートエクスーマ (Great Exuma) 島のジョージタウンで開催され, それぞれの試料の分析結果を持ち寄って討論がなされた. 特に, 本航海で得られた, 更新統と中新統の“サイクル”に多くの研究者が重点をおいて, 試料の解析を行っていた. また, シークエンス境界についても, 航海後, 計算し直した年代値が提案された. このポストクルーズミーティングの後, 主席研究員の2人であるマイアミ大学Gregor P. Eberli教授, Peter K Swart 教授らによって準備された巡検がエクスーマ諸島の一つであるリーストッキング (Lee Stocking) 島で行われ, バハマプラットフォームの現世堆積物を目の当たりにすることができた. ここでは, 潮下帯(subtidal)で発達したストロマトライトや, ウウイドの瀬(ooid shoals)など,その一部を紹介する.