Along‐Strike Tectonic Deformation of the SE Nansha Block in Response to the Southwestward Seafloor Spreading of the South China Sea

The southeastern Nansha Block (Dangerous Grounds) in the South China Sea (SCS) lies along the southern margin of the southwestern‐propagating SCS spreading center. It was also situated north of the Proto–South China Sea (PSCS) oceanic crust, which was completely subducted southeastward beneath the northwestern Borneo by the Middle Miocene. Subsequently, the northern Borneo landmass began to uplift during the Middle‐Late Miocene. However, the along‐strike (NE‐SW) lithospheric extensional and flexural deformation response to these geodynamic processes remain poorly constrained. Here, we establish a refined tectono‐stratigraphic framework based on newly acquired drilling data and long cable seismic profiles, enabling quantitative analyses of cumulative fault heave, tectonic subsidence, and two‐dimensional flexural inversion. Our results reveal a southwestward decreasing gradient of extensional strain throughout the opening of the SCS. This pattern is evidenced by: (a) southwestward decreasing syn‐rift cumulative fault heave (from 45.42 to 20.27 km); (b) southwestward increasing tectonic subsidence rates in both the syn‐rift and drift stages; and (c) southwestward decreasing net shear force during the Nansha Block‐Borneo collision (from 1.53 × 10 12 N to 0.89 × 10 12 N), after correcting for sedimentary loading and thus representing subduction‐related driving forces. These patterns may be attributed to diachronous subduction of the PSCS slab. Earlier and deeper subduction in the northeast likely generated stronger slab‐pull forces, triggering earlier continental breakup and seafloor spreading. In contrast, later and shallower subduction in the southwest produced weaker extensional regimes and more prolonged continental rifting.