New Insights Into the Rift-To-Drift Process of the Northern South China Sea Margin Constrained by a Three-Dimensional Wide-Angle Seismic Velocity Model

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 similar to 8 to 20 km, a similar to 20 km-width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from similar to 4.5 to 9 km. A high-velocity (7.0-7.5 km/s) lower crust (HVLC) ranges in thickness from similar to 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 (beta) < similar to 3, whereas gentler thinning associates to thinner HVLC at beta > similar to 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. Plain Language Summary Rapid transition from continent to ocean at the northern South China Sea has been found by IODP Expeditions 367/368/368X through seven drilling sites, but where and how the transition occurred along the strike of the rifted margin is still debated. A three-dimensional (3D) P-wave seismic velocity (Vp) model is therefore constructed with first-arrival travel-time tomography using wide-angle seismic data from a network of 49 OBSs and 11 air-gun shot lines. In combination with other geophysical and geological data, we reveal the geometry of a comparatively narrow continent to ocean transition zone of similar to 20 km in width and the significant along-strike variation in the continental crust thinning style and magma generation. Furthermore, a transfer fault system has been inferred to possibly separate the NE tectono-magmatic segment from the SW tectonic-dominated segment. The segmentation in tectonic and magmatic domains supports that local inherited lithospheric heterogeneities may have led to the different opening styles during rift-to-drift process.

Automated summary

A three-dimensional (3D) P-wave seismic velocity (Vp) model of the crust in the northern South China Sea margin reveals the extent, structure, and nature of continental, continent to ocean transition (COT), and oceanic domains. The continental crust ranges in thickness from approximately 8 to 20 km, the COT contains no exhumed mantle, and the oceanic crust ranges from approximately 4.5 to 9 km. The Vp distribution shows differences in deformation and magmatism along the margin, possibly separated by a transfer fault system.