Complexities in the tectonic evolution of the West Philippine Basin

Abstract

The oceanic West Philippine Basin can be subdivided into a main basin, a northwest sub-basin and a southern sub-basin. These basins have undergone different tectonic histories. Magnetic anomalies 21(?)-17 are present in the main basin and indicate that it formed by two-limbed seafloor spreading. The now extinct spreading center is located within a zone of rough topography associated with the northwest-trending Central Basin Fault (CBF). The morphology of the CBF is primarily the result of post-spreading tectonism. The magnetic anomaly pattern of the main basin does not extend into the southern sub-basin or the northwest sub-basin. Geometrical constraints indicate that the southern sub-basin did not form as part of the southern limb of the main basin. The southern sub-basin may have formed immediately before a ridge jump to the main basin spreading axis. Alternatively, possible shallower crustal depth in the southern sub-basin may indicate that the sub-basin is younger than the main basin and formed separately from it. In the northwest sub-basin, which originated as part of the main basin, the dominant topographic trends are perpendicular to those in the main basin. This difference as well as higher heat flow and shallower depth in the northwest sub-basin indicate that it has undergone a deformational event which did not affect the main basin. This deformation may be related to subduction along the east Luzon margin in mid-Tertiary time. The Gagua Ridge, an uplifted sliver of oceanic crust along the western edge of the northwest sub-basin, may have originated during this event.