Newly acquired marine geophysical data along the Central Basin Spreading Center (CBSC), the extinct spreading axis of the West Philippine Basin (WPB), display along-axis variations of spreading style. We have described and analyzed the tectonic spreading fabric and segmentation patterns along a 1000-km-long section of the fossil ridge between 126Á00ÍE and 133Á30ÍE. ñSlow-spreading featuresî (deep rift valleys and nodal basins, rough abyssal hills on the ridge flanks, and mantle Bouguer anomaly (MBA) lows beneath segment centers) are observed in the eastern CBSC. In contrast, ñfast-spreadingî features (overlapping spreading centers, volcanic axial ridges, and smooth abyssal hill fabric) are identified along the western CBSC. We attribute the large morphological and geophysical variations along the CBSC to higher melt supply in the western region caused by high mantle temperature and/or mantle heterogeneity, which may be related to a relatively small-scale mantle plume forming the oceanic plateaus located in the West Philippine Basin. Another prominent feature of the CBSC is the development of a deep valley oblique to the spreading fabric. Reconstructions of the plate boundary geometry through time, using abyssal hill geometry as well as other measÆFurements, provide evidence for a later stage of amagmatic extension (i.e. reactivation) along the CBSC after the formation of the main basin. This stage is dominated by tectonic deformation with minor amounts of volcanism (mainly located in eastern segments), resulting in the observed surface brittle deformation distributed within a broad zone of ductile deformation.

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