A three-dimensional (3D) P-wave seismic velocity (Vp) model of the crust in 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) ∼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

21 pages, 10 figures, supporting information https://doi.org/10.1029/2022JB026171.-- Data Availability Statement: The raw data used for P-wave travel-time tomographic inversion in this study is available at Science Data Bank via https://www.scidb.cn/anonymous/ckVyTVpm with CC0 (Zhang J., Zhao, M., & Qiu X., 2022)

This work was financially supported by National Natural Science Foundation of China (Grants 41730532, 91958212, and U20A20100)

With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

Peer reviewed