Magnetic properties and sand composition at DSDP Site 87-582 and Hole 87-583D

At Site 582, DSDP Leg 87, turbidites about 560 m thick were recovered from the floor of the Nankai Trough. A turbidite bed is typically composed of three subdivisions: a lower graded sand unit, an upper massive silt unit, and an uppermost Chondrites burrowed silt unit. The turbidites intercalate with bluish gray hemipelagic mud which apparently accumulated below the calcite compensation depth. In order to investigate the nature and provenance of the turbidites, we studied the grain orientation, based on magnetic fabric measurements and thin-section grain counting, and grain size, using a photo-extinction settling tube and detrital modal analysis.
The following results were obtained: (1) grain orientation analysis indicates that the turbidity current transport parallels the trench axis, predominantly from the northeast; (2) Nankai Trough turbidites generally decrease in grain size to the southwest; (3) turbidite sands include skeletal remains indicative of fresh-water and shallow-marine environments; and (4) turbidites contain abundant volcanic components, and their composition is analogous to the sediments of the Fuji River-Suruga Bay area. Considering other evidence, such as physiography and geometry of trench fill, we conclude that the turbidites of Site 582 as well as Site 583 were derived predominantly from the mouth of Fuji River and were transported through the Suruga Trough to the Nankai Trough, a distance of some 700 km. This turbidite transport system has tectonic implications: (1) the filling of the Nankai Trough is the direct consequence of the Izu collision in Pliocene- Pleistocene times; (2) the accretion of trench fill at the trench inner slope observed in the Nankai Trough is controlled by collision tectonics; and (3) each event of turbidite deposition may be related to a Tokai mega-earthquake.

在深海钻探计划（Deep Sea Drilling Project, DSDP）第87航次的582号站位，研究人员从南海海槽（Nankai Trough）洋底采集到厚度约560米的浊积岩。典型浊积岩层通常由三个次级单元组成：下部粒序砂岩单元、上部块状粉砂岩单元，以及最顶部被芽枝迹（Chondrites）潜穴改造的粉砂岩单元。该浊积岩与蓝灰色半远洋泥岩互层，此类泥岩显然沉积于钙补偿深度（Calcite Compensation Depth, CCD）之下。
为探究浊积岩的成因与物源，研究人员基于磁组构测量与薄片颗粒计数开展颗粒定向分析，并利用光消沉降管与碎屑模式分析开展粒度研究。
本次研究获得以下结果：（1）颗粒定向分析结果显示，浊流搬运方向平行于海槽轴部，主要来自东北方向；（2）南海海槽浊积岩的粒度整体向西南方向逐渐变细；（3）浊积岩砂岩中含有指示淡水与浅海环境的生物骨骼遗迹；（4）浊积岩中富含火山物质，其成分组成与富士河-骏河湾（Fuji River-Suruga Bay）区域的沉积物相似。
结合海槽充填物的地貌特征与几何形态等其他证据，研究人员得出结论：582号站位与583号站位的浊积岩主要源自富士河河口，并通过骏河槽（Suruga Trough）搬运至南海海槽，搬运距离约700千米。
这套浊流搬运系统具有构造学启示意义：（1）南海海槽的充填作用是上新世-更新世伊豆碰撞事件的直接产物；（2）南海海槽内坡观测到的海槽充填物增生作用受碰撞构造调控；（3）每一次浊积岩沉积事件可能均与东海大地震（Tokai mega-earthquake）相关。