Unstable spreading owing to extra magmatism in the central West Philippine Sea Basin

The central West Philippine Basin (WPB) comprises three contrasting geomorphologic provinces, the mantle-plume type Benham Rise in west, the volcanic arc-type Kyushu-Palau Ridge (KPR) at the east limit, and the in-between central rift hosting the deepest (~7874 m) seafloor. Their morphic transit and dynamic interactions remain elusive owing to the insufficiency of relevant data. Incorporating new and previous multibeam bathymetry data fully enveloping the central rift enables us to synthesize a 3-band, 3-segment seafloor morphic fabric of the central WPB and deduce its formation mechanisms. It features prominent across- and along-axis variations and variable landform assemblages, including: 1) overall trends of abyssal hill lineation swing counter-clockwise from ~N100/105°E at a distance of 100 km from the central rift valley to ~N85/95°E at a distance of ~30/50 km, then clockwise to ~N100/140°E in the valley, which recognizes three irregularly embedded bands reflecting a rotatory spreading fabric; 2) the valley narrows westward from ~85 km wide near the KPR to ~25 km at Centric Deep, and then fades out to further west, synchronously from shallow, volcano-rich to deep, volcano-poor then to failed, indicating a northwestward propagating rift driven by the KPR arc volcanism. Moreover, it is found in the middle segment that three major transform faults bend southwesterly in the southern band with their concaves consistently pointing to the Benham Rise. Behaving as pseudo-faults, they are interpreted as result of propagating rift triggered by the former magmatism of the Benham Rise. Lying just at the intersection of a relict nodal basin and the western tip of the later propagating rift, the depth maxima (~7874 m) might have resulted from intersection of two rifts from west and east, respectively. Thus, the geomorphic fabric featuring high spreading instability reflects strong magma-tectonic intervention by excessive magmatism of mantle plume and subduction arc.

This study was funded by the China Geological Survey Project (grant No. DD20230642), NSFC (grant No. 42574132 and U1901217).