Characteristics and genesis mechanisms of flexural tectonic deformation along the southeastern margin of the Nansha Block

Located along the southern continental margin of the South China Sea, the Nansha Block has experienced a complex tectonic evolution involving the subduction and demise of the Proto-South China Sea and the subsequent spreading of the South China Sea. The flexural deformation along its southeastern margin is crucial for understanding the evolutionary processes of the South China Sea. We integrated stratigraphic and structural interpretations from four new wells and three NW-SE–oriented multichannel seismic profiles: 13nh0004, Das-3, and ns93-10 from NE to SW, in total length of 1000 km. Combined with quantitative fault displacement analysis, tectonic subsidence rates calculation, and flexural deformation assessment, we investigated the spatiotemporal variations and mechanisms of flexural deformation. Three unconformities (ROU: rift onset unconformity; BU: breakup unconformity; RU: red unconformity) were recognized, representing three stages of flexural forebulge evolution. During the syn-rift stage (65 Ma to 27～28 Ma, ROU–BU), deformation was dominated by NW-trending listric normal faults forming half-graben structures, and the fault-block tilting decreased from NE to SW. Most faults on Das-3 terminate at BU. The drift stage (27～28 Ma to 15.2 Ma, BU–RU) was marked by fault-controlled sagging, with 13nh0004 and ns93-10 faults mainly ending at RU, including newly formed secondary and reactivated bounding faults. The post-drift stage (15.2～0 Ma, RU–SB) features stable draping sedimentation. Fault activity decreased markedly from the syn-rift to the post-drift stage, during which the maximum vertical displacement decreased from 2750.02 m to 718.44 m; and spatially diminished from NE to SE. Drift-stage subsidence rates were mostly negative (–3.77 m/Ma to –1.48 m/Ma), while the post-drift stage showed significantly higher average subsidence (～85 m/Ma), with fitted curves indicating a prominent flexural geometry. The differential flexural morphology revealed by the fitted curves of tectonic subsidence rates along the profiles is consistent with seismic interpretations and fault displacement patterns, indicating that the flexural deformation formed during mainly the Middle Miocene, induced by lithospheric flexure resulting from slab subduction. This flexural deformation exhibits a spatial variation in intensity from NE to SW, characterized as “strong at both ends (NE and SW) and weak in the central part”. Integrated with the regional tectonic framework and the evolutionary history of the South China Sea, we inferred that flexural deformation in the northeastern area was mainly controlled by slab pulling from proto-South China Sea subduction, whereas the southwestern area was dominated by tectonic loading from the collision between the Nansha Block and Borneo, and the central segment represents likely a transitional zone where these two dynamic mechanisms are weakened or decoupled.