Early Miocene magmatism in the Baiyun Sag (South China Sea): A view to the origin of intense post-rift magmatism

Abstract

Post-rift magmatism associated with seafloor spreading (∼32 Ma to ∼15 Ma) and post-spreading tectonics (from ∼15 Ma to the present day) is widely recorded in the South China Sea (SCS). However, the phenomena originating this magmatism are still under debate, hindering a full understanding of the SCS’ evolution. In this study, extensive and voluminous magmatic bodies in the northern SCS are investigated based on the integration of 3D/2D seismic and borehole data. Magmatism occurred from ∼19.0 Ma to ∼17.5 Ma, a period of time marking the cessation of seafloor spreading above listric boundary faults formed on the southern shoulder of the Baiyun Sag. Magmatism close to the main basin depocenter (Magmatic Zone 1) is documented in the form of oblique/saucer sills that step upwards. They have fed volcanoes and lava flows to a paleo-seabed via igneous dikes. In contrast, magmatism in the shoulders of the Baiyun Sag (Magmatic zones 2 and 3) occurred above listric boundary faults that formed large magma pathways. The phase of magmatism documented in this work was probably triggered by the retreat of the Hainan Plume and associated mantle readjustments during jumps in spreading-ridge location. These events rejuvenated mantle upwelling underneath the highly stretched Baiyun Sag to generate partial melting. Mantle upwelling also reactivated the listric boundary faults flanking this basin, generating migration pathways for deep-seated magma. This work addresses the syn-spreading magma plumbing system of the Baiyun Sag in detail, and provides evidence for the origin of intense post-rift magmatism in the northern SCS. It also highlights that mantle readjustments triggered by changes in seafloor spreading (ridge jump or cessation of spreading) can generate significant post-rift magmatism.