Detachment faulting related to oceanic core complexes (OCCs) has been suggested to be a manifestation of slow seafloor spreading. Although numerical models suggest OCCs form under low magma supply, the specific interaction between magmatism and tectonic faulting remains elusive. This paper examines seismic observations detailing the spatiotemporal interactions between magmatism, high-angle faulting, and detachment faulting at a slow-spreading mid-ocean ridge in the West Philippine Basin. We identified a magma-rich spreading phase at 36 Ma, indicated by a magmatic top basement and normal oceanic crust with shallow-penetrating high-angle faults. An axial valley reveals an along-strike transition from normal to highly tectonized oceanic crust over a distance of 70 km. Two older OCCs with concave-down fault geometries and a younger OCC with steep-dipping faulting suggest sequential detachments with the same polarity. Our findings suggest: (1) slow seafloor spreading is cyclical, alternating between high-angle faulting with a relatively high magma supply and detachment faulting with limited magma supply; (2) sequential development of younger detachments in the footwall of its predecessor leads to an asymmetric split in the newly accreted crust; and (3) the life cycle of OCC ends with high-angle faults that overprint the detachment and act as magma pathways, sealing the OCC. Our study captures the dynamic interaction between high-angle and detachment faults and their concurrent and subsequent relationship to magmatic systems. This reveals that strain distribution along strike is critical to OCC formation, thus enriching our understanding beyond conventional considerations such as spreading rates and melt budgets at mid-ocean ridges.

A three-dimensional (3D) P-wave seismic velocity (Vp) model of the crust at the northern South China Sea margin drilled by IODP Expeditions 367/368/368X has been obtained with first-arrival travel-time tomography using wide-angle seismic data from a network of 49 OBSs and 11 air-gun shot lines. The 3D Vp distribution constrains the extent, structure and nature of the continental, continent to ocean transition (COT), and oceanic domains. Continental crust laterally ranges in thickness from ~8 to 20 km, a ~20 km-width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from ~4.5 to 9 km. A high-velocity (7.0-7.5 km/s) lower crust (HVLC) ranges in thickness from ~1 to 9 km across the continental and COT domains, which is interpreted as a proxy of syn-rift and syn-breakup magma associated to underplating and/or intrusions. Continental crust thinning style is abrupter in the NE segment and gradual in the SW segment. Abrupter continental thinning exhibits thicker HVLC at stretching factor (β) <~3, whereas gentler thinning associates to thinner HVLC at β>~4. Opening of the NE segment thus occurred by comparatively increased magmatism, whereas tectonic extension was more important in the SW segment. The Vp distribution shows the changes in deformation and magmatism are abrupt along the strike of the margin, with the segments possibly bounded by a transfer fault system. No conventional model explains the structure and segmentation of tectonic and magmatic processes. Local inherited lithospheric heterogeneities during rifting may have modulated the contrasting opening styles.