Ocean-continent transition architecture and breakup mechanism at the mid-northern South China Sea

Abstract The Ocean-Continent Transition (OCT) located between the edge of the continental and unequivocal oceanic crusts is an ideal laboratory to understand one of the most fundamental processes of Plate Tectonics, namely the formation mechanism of a new plate boundary, also referred to as lithospheric breakup. However, the location and architecture of the OCT and the processes governing the rupture of continental lithosphere and creation of new oceanic crust remain debated. In this paper, we present newly released high-resolution seismic reflection profiles that image the complete transition from unambiguous continental to oceanic crusts in the mid-northern South China Sea (SCS), accompanied with IODP drill holes and gravity data, with the aim to map the OCT and explore where, when and how lithospheric breakup occurred. Based on observations and interpretations of seismic reflection data, we define the limits of the SCS OCT. The results show that the OCT basement corresponds to hybrid crust resulting from the complex interaction between crustal thinning along detachment systems and emplacement of new syn-tectonic igneous materials. The lithospheric breakup in the northern SCS and the conjugate margin occurred asymmetrically and was accomplished by core-complex type structures related to an oceanward transition from tectonic to magma-controlled processes during plate separation. Additionally, the observations suggest a sharp along-strike transition from a lower to an upper plate rifted margin setting over a lateral distance of 30 km. The strong variability in the basement architecture and the abrupt flip in detachment polarity imply a transfer zone to explain the segmentation of the margin. Such segmentation may result from inherited pre-rift crustal and/or lithospheric heterogeneities. Notably, the segmentation did not control breakup and subsequent oceanic accretion.