Non-uniform lithospheric extension of the Qiongdongnan Basin driven by Hainan mantle plume migration: Insights from thermo-mechanical 2D modelling

The classical two end-member classification of continental rifting (magma-rich vs. magma-poor) fails to explain the atypical thermo-mechanical conditions of the Qiongdongnan Basin (QDNB), located southeast of Hainan Island along the northern margin of the South China Sea. The basin is surrounded by Late Cenozoic magmatism commonly linked to deep-seated mantle upwelling associated with the putative Hainan mantle plume. The QDNB exhibits pronounced west–east variations in deformation style, reflecting strong spatial heterogeneity in lithospheric strength. These characteristics suggest that time-dependent mantle-lithosphere interactions played a critical role in controlling rift evolution.

Integrated analyses of multiple seismic profiles reveal strong along-strike contrasts in structural style and subsidence history within the QDNB. Following the onset of South China Sea break-up in the early Oligocene (~32-30 Ma), the QDNB rifted diachronously from east to west and subsequently transitioned into the post-rift stage in the same direction, culminating at ~23 Ma. The western QDNB displays a strongly asymmetric architecture with a mid-crustal detachment system and records a relatively late onset of rapid subsidence at ~5.5 Ma. In contrast, the eastern QDNB is characterized by a more symmetric structure approaching complete continental rupture, accompanied by an earlier phase of rapid subsidence at ~10.5 Ma. These contrasting detachment styles and subsidence histories indicate distinct thermo-mechanical regimes, with rapid extension and cooling promoting lower-crustal embrittlement in the east, whereas more prolonged extension under longer-lived thermal weakening conditions maintained ductile lower-crustal behavior in the west.

To test these interpretations quantitatively, we perform a series of two-dimensional thermo-mechanical numerical models that explicitly incorporate westward migration of a secondary mantle plume associated with the Hainan mantle plume together with slab-pull forces from subduction of the Proto-South China Sea. By systematically varying plume migration contributions, the models evaluate its relative role in generating along-strike heterogeneity in extension style, subsidence history, and lower-crustal rheology within the QDNB. The modelling results highlight that plume migration exerts a first-order control on the thermal field of the QDNB, providing a key mechanism for the observed non-uniform lithospheric extension.