Refining the final phase of the seafloor spreading process in the West Philippine Basin through the recalibration of magnetic isochrones

Abstract

The evolution of the West Philippine Basin (WPB), the largest back-arc basin in the northwestern Pacific margin, remains unclear, mainly due to the lack of accurate magnetic chronological constraints. We integrate a newly acquired shipborne magnetic grid and EMAG2 data to investigate the final phase of seafloor spreading and post-spreading magma-poor extension in the WPB during the Eocene and Oligocene. An ∼800 km-long transect located in the eastern part of the WPB was modeled to recalibrate the magnetic isochrones. The results show that the seafloor spreading direction of the WPB rotated counterclockwise from ∼NE–SW to ∼NNE–SSW at ∼44 Ma (Isochrone C20r) and from ∼NNE–SSW to ∼N–S at ∼39 Ma (Isochrone C18n.1n). After the latter rotation, the spreading became unstable, decreased significantly, fluctuated, and finally ceased at ∼33 Ma (Isochrone C13n). The post-spreading magma-poor extension along the Central Basin Rift (CBR) driven by the initial opening of the Parece Vela Basin (PVB) began ∼4 Myr after the cessation of spreading. In the area east of ∼132°E, the CBR inherited the fossil spreading center, but to the west, it manifests a newly formed rift valley intersecting the segmented fossil spreading center. The destabilization of the spreading process in the marginal basin during its extinction phase was predominantly controlled by the dynamic process and stress field imposed by the adjacent plate boundaries.