摘要：The West Philippine Basin (WPB) occupies the western part of the Philippine Sea (PHS) plate.The WPB is generally considered to have opened from approximately 50 to 30 Ma at the CBF rift; however, the detailed spreading history of the WPB is not yet clear.In particular, the origin and age of the southern subbasin, the Palau Basin, are unknown.To better understand the initiation and early evolution of the Izu-Bonin (Ogasawara)-Mariana arc, knowing the configuration of the PHS plate at that time is necessary.In this study, we examine the spreading history of the WPB using newly acquired three-component magnetic anomaly and swath bathymetry data, as well as existing datasets.In the WPB south of the CBF rift, the observed magnetic anomalies correspond to Chron C16r to C21n (approximately 36 to 46 Ma).Prevailing models of the WPB reconstruction show a decrease in the spreading rate from 4.4 to 1.8 cm/year since C18n.2n (approximately 39.5 Ma).Our research, however, indicates that the change in the spreading rate is not required to correlate the observed magnetic anomalies to the geomagnetic polarity reversal timescale.The age of the spreading cessation in our interpretation, approximately 36 Ma, is several million years older than in previous estimates, and the spreading ceased progressively from southeast to northwest along the CBF rift.In the Palau Basin, seafloor fabrics and magnetic lineations trend N-S, which indicates E-W seafloor-spreading.Based on 40Ar/39Ar age, we suggest that the magnetic lineations correspond to polarity reversals from C18n.1n to C15r (approximately 38.5 to 35 Ma).The spreading of the Palau Basin may have been coeval with that of the WPB near the CBF rift, although their spreading directions are different.
