Sediment distribution of the Eastern Nankai forearc region; significance of tectonic and sea level control - 東南海前弧地域の堆積物分布; テクトニクスと海面による制御の意味

The purpose of this study is to evaluate the tectonic and climate control on the origin and distribution of sediment on an active margin. The study area is located in the offshore area of Tokai district, southward central Japan. Tectonically, the area is a convergence margin between the Philippine Sea plate which is subducting beneath the Honshu arc. Sediments obtained piston cores, bathymetric data and side-scan sonar imagery collected on the Eastern Nankai Forearc compose the main set of data.

Analyses of the data allowed me to define different types of morpho-sedimentary domains as well as their surface patterns, lithology and origin of surficial deposits, and the sedimentary processes. Four types of domains can be distinguished: upper slope (Domain I), canyon and overbank (Domain II), topographic high (Domain III). and trench slope (Domain IV).

The lithology of Domain I and the Domain II is dominated by silty clay, but occurrence of reworked sediment characterizes the Domain II. Turbidites and silty clay are found in the Domain III and Domain IV. In the Domain III and southwestern part of Domain IV also found debris flow deposit in several cores.

Sea-level change controlled the sedimentary process in the Domain I, Domain II and northeastern part of the Domain IV. Coarser sediment was deposited during low sea level stand, whereas at high sea level stand finer grain was deposited by turbidity current or settled by vertical flux particle. Turbidite layer on the foot of upper slope (the Domain I) was formed related to the formation of deep-sea fan. In the southwestern part, gravity flow occurred more frequently in the Domain IV than in the Domain III. The gravity flow deposit forming in those areas was suggested controlled by tectonic activity such as a great earthquake occurrence.