Detailing polyphase oceanic rifting evolution and consequences for microplate formation

Oceanic rifting in back-arc basins reveals how pre-existing lithosphere deforms under extension, distinct from mid-ocean ridge processes. However, existing models invoking rapid magmatic rifting or rigid rotation cannot explain the complex deformation patterns observed. We present new seismic reflection data across the Kyushu-Palau Ridge (KPR), integrated with published seismic and gravity data from the Central Basin Fault (CBF) system in the West Philippine Basin and regional datasets. Our results reveal: (1) significant transtensional deformation along the KPR, which we reinterpret as a reactivated oceanic transform fault; and (2) the CBF as a post-accretionary intra-oceanic rift deforming pre-existing oceanic crust, consistent with but mechanically refining previous interpretations. We propose that a Pacific Plate kinematic reorganization around 32 Ma rendered the fossil trench-transform-ridge configuration kinematically unstable, triggering distributed deformation that progressively localized along inherited lithospheric fabrics. This regional stress change reactivated the favorably oriented KPR transform and the extinct spreading center in the eastern CBF, while initiating new intraplate rifting in the central and western CBF segments, driving a ~ 6 Myr evolution from diffuse to localized deformation (32–26 Ma). Our findings establish that oceanic rifting follows a two-stage process: structural inheritance controls initial rift location and geometry, while magmatism provides critical thermal weakening for complete plate separation. The CBF's arrested development, where the system failed to achieve complete plate separation despite localized extreme extension in the east and magmatic input in the west, exemplifies the critical threshold required for successful microplate formation. This work provides a new framework for understanding how subduction-driven stress changes exploit pre-existing weaknesses to generate complex microplate mosaics, offering a predictive template for identifying similar polyphase systems elsewhere. • Seismic and published data reveal the KPR underwent transtensional deformation. • The CBF represents a post-accretionary rift system deforming pre-existing crust. • Subduction velocity changes triggered destabilization and microplate formation. • Inheritance controls rift initiation/geometry; magmatism enables plate separation.