Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

- Two volcanic complexes were emplaced during the Miocene on highly extended crust of the Baiyun Sag.
- Borehole and geochemical data suggests that the volcanic complexes were developed in two distinct phases.
- Prolonged post-rift magmatism reflects progressive continental break-up in the SCS.
- The youngest volcanic episode marks the end of a breakup sequence.
- The breakup sequence records diachronous magma emplacement and complex changes in depositional facies.

Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K-Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean-continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.