Role of the transform plate boundary. As seen in the two adopted plate reconstructions (Fig. S1), continental rifting of the PRMB was initiated along a passive margin where the PSCS was anchored to South China (Fig. 5a). Contemporaneously, back-arc extension started36, 47–48 along other East Asian margins where the subduction of the Izanagi Plate continued. In Model 1, a NW-trending transform plate boundary developed along the eastern boundary of the fossilized PSCS at the latest Cretaceous and the earliest Cenozoic26. The subducted flat Izanagi slab was transported by the lateral mantle flow (Fig. S2a) to the upper mantle southwest of this transform plate boundary below the PRMB (Figs. 3a and 5a). Therefore, we suggest the northward dipping fast anomaly below the present northern SCS region revealed by tomographic images18–19 likely represents a piece of this displaced Izanagi slab9, 53. Accompanying the slab hole formed between the sinking slab and the transform boundary, the asthenosphere flowed into the mantle wedge under South China, resulting in the observed toroidal mantle flow (Figs. 4a and 5a). To the southwest of the transform boundary, the toroidal mantle flow pushed the overriding continental lithosphere northwestward (Fig. 5a). Meanwhile, the continental margin was trying to move away in the SE direction following trench retreat, offset by the sinsitral strike-slip movement along the transform plate boundary. These two motions in opposite directions stretched the overriding plate in the NW-SE direction, finally triggering the continental rifting and the formation of narrow rifts in the PRMB (Fig. 5a). To the northeast of the transform plate boundary, the toroidal mantle flow pushed the Izanagi slab to retreat, leading to back-arc extension and the formation of the ECSB in the overriding plate (Fig. 5a). Overall, the continental rifting in both the northern SCS passive margin and the East China Sea active margin were induced by the toroidal mantle flow around the slab edge between these two regions.
To illustrate the role of the toroidal mantle flow in dominating the continental rifting, we compared these results with those in Model 2. Model 2 generated a smooth and unfragmented flat slab at the same location as that in Model 1 prior to 40 Ma (Fig. S2), when the transform plate boundary in Model 1 was replaced by a subduction zone based on the adopted reconstruction in Ref.25. The broad flat slab usually induced lithospheric compression, as seen in the Laramide Orogeny54, and was used to explain the earliest Cenozoic surface uplift and tectonic inversion in East Asian sedimentary basins41. Therefore, we proposed that the NW-trending transform plate boundary suggested in the new reconstruction of Ref.26 is the key reason for the formation of the toroidal mantle flow and the resulting continental rifting. This NW-trending transform plate boundary was well recorded by the prominent NW-trending Yushan-Kume Fault Zone55, which is located between the present PRMB and the ECSB (Figs. 1 and 5a). The Yushan-Kume Fault Zone was sinistrally strike-slipping at the latest Cretaceous56, as it absorbed the large fractions of relative plate movement along the subducting transform plate boundary.
Role of the Izanagi-Pacific mid-ocean ridge. Through basal traction, the toroidal mantle flow in Model 1 dragged the buoyant Izanagi-Pacific mid-ocean ridge to continuously subduct westward36. The slab hole eventually grow into a broad trench-parallel slab window at ~ 40 Ma (Figs. 4b and 5b), associated with the complete subduction of the Izanagi-Pacific mid-ocean ridge under East Asia36. The broad trench-parallel slab window provided a pathway that connected the mantle wedge beneath South China with the approaching asthenosphere below the Pacific Plate, forming the strong landward mantle wind (Figs. 3a, 4b and 5b). The high-velocity mantle wind exerted a strong landward basal traction on the thick overriding continental lithosphere east of the North-South Gravity Lineament, stretching the narrow PRMB rift into an asymmetric wide rift that is characterized by the highly extended crust and the seaward dipping detachment faults (Fig. 5b). Therefore, the trench-parallel subduction of the Izanagi-Pacific mid-ocean ridge is the key reason in forming the regional mantle wind and the resulting wide rift36, which is also observed in Model 2 (Fig. 3b, Fig. 5b and Fig. S2b).
The slab hole and the subsequent slab window also allowed the occurrence of mantle upwelling and magmatism. The magmatism was weak during the narrow rifting phase (Fig. 5a), which was previously attributed to a strong or weak hot stretched lithosphere57. The volcanism ceased at ~ 50 Ma58 as recorded in the northern SCS margin. The subsequent broad slab window during the wide rifting phase led to intense mantle upwelling (Fig. 5b), which could explain the 45–33 Ma magmatism along the northern SCS margin and even a pulse of the Hainan mantle plume59–60. The plume impingement in turn weakened the lithosphere, increased the lithospheric ductility and promoted the wide rifting to evolve into continental breakup and rapid onset of seafloor spreading as observed. The transition from narrow rift with weak magmatism to wide rift with intense magmatism along the northern SCS margin may represent an intermediate rifted margin between the magma-poor and the magma-rich end-members.
Involvement of pre-existing basement faults. As shown above, the landward mantle flow from the Pacific subduction system can explain the geodynamic discrepancies between two syn-rift phases in the northern SCS margin. In addition, the differences in structural architectures between the two syn-rift phases suggest the involvement of the pre-existing basement faults, which are common in rifted margins. The intra-basement boudinage structure (Figs. 2a-b), consisting of the ENE-striking thrusts and the NE-striking transpressional faults onshore-offshore the South China Block, have been delineated in the previous works7, 27–28. During the Early Eocene rifting, strains were mainly localized on the brittle upper crust and no basement structures were reactivated, leading to the formation of the ENE-striking short normal faults with a small crustal stretching factor of 1.2. During the Late Eocene rifting, lithospheric ductility was increased and strains migrated to the ductile lower crust, facilitated by the hot mantle upwelling through the broad slab window. So, the upper crustal ENE-striking normal faulting penetrated into the crystalline basement, resulting in the formation of the detachment faults and the reactivation of the pre-existing ENE-striking thrusts. Simultaneously, the pre-existing NE-striking transpressional faults were reversed into transtensional faults under the extension driven by the landward mantle wind. Reactivation of both the intrabasement ENE-striking and NE-striking faults led to the occurrence of the Late Eocene pull-apart basins en échelon (Fig. 2d), with a rhomb shape inherited from the pre-existing boudinage configuration.