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Modal Analysis of Sand from the Nankai Accretionary Prism

Emily E. Comer
University of Texas at Austin

During the Cenozoic, complex plate interactions between the Philippine, Eurasian, and Pacific plates produced the Nankai Accretionary Prism off the southeast coast of Japan. The prism is currently experiencing rapid sedimentation from both onshore and trench-derived sources. As the Philippine plate subducts beneath the Eurasian plate, sediment from the Izu collision zone northeast of the Nankai Trough travels down the trench and accretes into the prism. This sediment is rich in feldspar and volcanic lithics. More quartz-rich onshore sediment from the Kii peninsula is also sedimented upon this prism by transportation down numerous submarine water canyons such as the Suruga Canyon.

Although the prism has been deformed by numerous thrust faults, provenance identification is possible through grain identification. Previous studies by Marsaglia 1992, Fergusson 1995, and others have categorized grain data into QFL diagrams to determine the relative contributions of onshore vs. trench sediment within the prism. In this study approximately 70 samples will be thinsectioned, point-counted, and analyzed. These samples were obtained from the Chikyu drilling vessel during NanTroSEIZE Expedition 316. In future expeditions, NanTroSEIZE will drill to deeper depths, even reaching the bedrock beneath the main decollement of the subduction zone, in hopes of a better understanding of rock-property evolution in this earthquake-causing region near Japan. Data from this study on the initial composition of the sediments in the prism will provide important base-line information on the reactions that occur during deep burial with in the prism.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas