We present new bathymetric, seismic and gravity data on the southwestern tip of the South China Sea oceanic basin, where propagation of continental break-up occurred before c. 15 Ma. The oceanic domain has a V-shape typical of oceanic propagating rifts. The tectonic fabric of its margins shows that the main stretching direction was slightly oblique to that of the rift axis. A 2D gravity anomaly inversion, corrected for the thermal effect, is used to estimate the crustal structure. At the continent-ocean boundary, the continental crust is stretched by a factor of about four, rapidly decreasing to about two over a few tens of kilometres, a distance corresponding to just over 1 Ma of break-up propagation. Thus, strain localization occurs at the tip of the propagating oceanic crust just before break-up. The along-axis variation in continental crustal stretching is in good agreement with the kinematics of the oceanic crust derived from magnetic anomalies. This analysis suggests that break-up propagates toward the pole of relative rotation and is primarily controlled by the amount of stretching of the continental crust before oceanization.