Enigmatic extinct spreading center in the West Philippine backarc basin unveiled

The Central Basin fault in the center of the West Philippine Basin was firs t discovered similar to 50 yr ago. It is a 1000-km-long ridge oriented nort hwest to southeast and is cut by north-south-trending fracture zones. Hypot heses about the origin and development of the Central Basin fault have rema ined unresolved until recently. Submersible observations and SeaBeam survey s show that the Central Basin fault is a segmented spreading ridge having a morphology similar to that of a slow spreading ridge, with a nontransform offset, a nodal deep, and an inside corner high. The distance from the ridg e versus the depth of the sea floor, the obliqueness of sets of small troug h and ridge structures, and heat-flow values both of the crestal and off-ax is areas of the Central Basin fault suggest that the fault is not a simple spreading center, but rather underwent multiple spreading episodes. The tex ture and chemistry of basalts obtained from the ridge suggest that the lava s were formed in a backarc basin setting. These data confirm that the Centr al Basin fault is a slow backarc spreading center that has a more complicat ed evolutionary history than previously realized.