Analysis of Marine Magnetic Field Anomaly and Multi-beam Bathymetry Profiles of the West Philippine Basin and Its Style of Opening

Abstract

The West Philippine Basin (WPB), located on the Philippine Sea Plate, is considered to have undergone a rapid opening during the Eocene. However, the detailed opening of the WPB and its relationship with surrounding basins were rather uncertain in the previous plate reconstruction models because of, for instance, (1) the limited marine magnetic data coverage, (2) highly skewed and weak magnetic anomalies by complex basin formation, (3) impossible to analyze subducted part of the basin. This study re-examines the opening of the WPB with the sea surface marine magnetic anomalies and multi-beam echo sounder data that were added to the database over the last several decades. The shipboard three component magnetometer (STCM) is an efficient instrument not only to obtain data without additional sensor manipulations but also to compare the observed magnetic anomaly with 2-D synthetic model because the vertical component magnetic anomaly is not affected by skewed horizontal magnetic field. However, the STCM data is complicated to remove the noise caused by position of the ship and its induced magnetization. In order to corroborate the validity of the STCM data, therefore, the magnetic boundary strike diagrams (MBSD) were computed to compare with the seafloor fabric trends and magnetic isochrons, which showed good connections. Based on this reliability, the magnetic isochrons could be identified to chron 22y and detailed rotation poles for different stages were computed by Gplates program. The calculated kinematic values indicate that the WPB started opening in ENE-WSW direction in chron 22y (> 49 Ma) but changed gradually to N-S direction in chron 21y (45 Ma). The time dependent rms spreading speed of this study is faster than that of previous reports. The opening suddenly slowed down in 37 Ma then finally ceased in 26 Ma. According to our analysis, the observed asymmetric spreading between north and south became more pronounced towards the end of the opening. Although marine magnetic anomaly data are too ambiguous to discern if plate is subductedor under complex formation, it provides vital constraints for a reconstructing plate kinematic model.