[S-CG67_1PM2] 海洋底地球科学

[SCG67-08] 地磁気三成分異常に基づくパラオ海盆の起源と西フィリピン海盆拡大モデルの改良

佐々木 智弘¹、*山崎 俊嗣²、石塚 治³ (1.筑波大学大学院生命環境科学研究科、2.東京大学大気海洋研究所、3.産業技術総合研究所地質情報研究部門)

キーワード：Philippine Sea plate, West Philippine Basin, Palau Basin, magnetic anomaly, seafloor spreading

The western part of the Philippine Sea (PHS) plate was occupied by the West Philippine Basin (WPB) in the north and the Palau Basin (PB) in the south. The WPB is generally considered to have opened from about 50 to 30 Ma, but the details are still unclear; in particular the origin and age of the PB was unknown. Studying the history of the WPB is important for understanding better the initiation and evolution of the IBM Arc. Here we discuss the spreading history of the WPB using new data on three-component magnetic anomaly and swath bathymetry acquired in the PB as well as those obtained previously by JAMSTEC fleets in the southern WPB. NS-trending magnetic boundaries and seafloor fabrics occur in the PB, indicating the formation by EW seafloor spreading. With a constraint from a 40Ar/39Ar age of 40.4 Ma obtained from the northernmost part of the PB, we interpret that the observed magnetic anomalies correspond to polarity reversals from C16n1r to C18n/C18r (35.6 to 38/39 Ma). Previous models of WPB spreading incorporated a spreading-rate decrease around 40 Ma from about 4.4 to 1.8 cm/year. Our study in the southern WPB, however, suggests that the decrease is unnecessary for correlating observed three-component anomalies to the GPTS. A typical profile along 130E corresponds to C16r to C21 (36.3 to 45.3 Ma). The cessation age of the spreading in our interpretation, about 35 to 37 Ma, is older than the previous estimation (about 30 to 33 Ma). It was difficult to constrain rotation of the PHS plate from the magnetic anomaly skewness.