The paleo-lithospheric structure and tectono-magmatic processes of the plate-edge type passive margin on the northern South China Sea
Zhao Z., Qiu N., Sun Z., Lin J., Zhang J., Li F., Zhang J., Sun L., Lee E.Y.
Published in
Gondwana Research, July 2023
The northern South China Sea has complex rifting-magmatic styles, which are significantly distinct from the Atlantic end-member passive margins that initiated rifting in a craton, and was argued to originate from a subducted setting. Here we propose a new kinematic method to calculate the paleo-lithospheric structure and reconstruct the rifting processes along a ∼ 400 km seismic line in the northern South China Sea margin. The detailed paleo-tectonic setting and its linkage with the rifting-magmatic processes are explored. Seismic-gravity analysis reveals a “boudin-type” passive margin with a wide distal domain (>200 km) comprising discrete hyperextended zones of the Baiyun Sub-basin, Liwan Sub-basin and a narrow continent-ocean transition (<30 km). The calculated pre-rift lithospheric thickness declines abruptly from ∼ 100–80 km in the proximal margin to ∼ 40 km in the distal margin, implying that the pre-rift margin was affected by a flat to steep slab of the subducted paleo-Pacific. The steep slab lying beneath the proximal margin formed an arc where the high-density lower crust probably has a pre-rift age. The flat slab beneath the Baiyun Sub-basin to the southern conjugate margin formed a forearc. As the slab steepened and retreated to beneath the Baiyun Sub-basin in rift phase 1 (∼56–38 Ma) and Liwan Sub-basin in rift phase 2 (∼38–33 Ma), the mantle convection and slab dehydration weakened the above lithosphere successively, promoting seaward migration and younging of strong faulting, magmatic underplating, intrusion, and eruption. When slab broke suddenly at ∼ 33 Ma in the continent-ocean transition, the deep hot asthenosphere upwelled to rupture the lithosphere abruptly and induced intense magmatism. Modelling results further show that the paleo-crustal thickness and basement erosion are both fundamentally important in assessing and understanding the crustal thinning.
Highlights
- New method offered to model paleo-lithospheric structure and multiple-rift processes.
- Step-wisely thinned pre-rift lithosphere infer presence of flat to steep subduct slab.
- Subsequent slab steeping and retreating explain well the margin evolution.