Differential Extrusion of Indochina‐South China Block and Its Interplay With Slab‐Pull Tectonics: Evidence for End‐Oligocene Basin Inversion in the Beibuwan Basin.

The Beibuwan Basin is situated at the northwest margin of the South China Sea (SCS) and thereby provides valuable insights into the complex relationship between extrusion and slab‐pull tectonics. Through borehole‐constrained seismic interpretation and quantitative analyses of inversion magnitude, broadly developed end‐Oligocene basin inversion structures have been identified, with their intensity gradually diminishing toward the southeast. The mechanisms underlying the inversion cannot be fully explained by either the extrusion of Indochina Block and South China Block (SCB) or the slab‐pull of Proto‐South China Sea alone. Therefore, we propose an innovative hybrid model suggesting that extrusion tectonics have played a more dominant role than slab‐pull tectonics over time. More specifically, during the Oligocene, extrusion tectonics, as evidenced by the thrusting along the Longmenshan Fault, exerted compressional stress on the western part of the SCB. Concurrently, slab‐pull tectonics induced extension in the SCS, with the extensional force impacting the southern part of the SCB as well. This resulted in a northeast‐oriented stress neutral line within the SCB, delineating the boundary between extensional and compressional stress. At the end of the Oligocene, the initiation of the Sabah Orogeny weakened the effects of slab‐pull tectonics. Consequently, the stress neutral line shifted southeastward to the south of the Qiongdongnan Basin and the northwest sub‐basin of the SCS, triggering the development of inversion structures in the study area and the southward jump of the spreading ridge. This model highlights the significance of compressional stress during the SCS evolution, marking a fundamental difference from the traditional models. Plain Language Summary: The opening of the South China Sea (SCS) is attributed to either the extrusion of the Indochina Block (extrusion model) or the subduction of the Proto‐South China Sea (P‐SCS, slab‐pull model). Both models emphasize that extensional forces dominate during the SCS's opening. However, seismic interpretations have identified end‐Oligocene compressional structures at the northwest margin of the SCS. These structures cannot be accounted for by either the traditional model. Consequently, we introduce a new model that integrates plausible elements from both aforementioned models. Our new model is likened to a moving train, where India applies the push from behind and the P‐SCS functions as the locomotive. During the Early‐Late Oligocene, the "locomotive" moved at a faster pace, resulting in widespread rifting in the SCS. However, the Sabah Orogeny and the counter‐clockwise rotation of Borneo at the end of the Oligocene moderated the "locomotive's" speed and correspondingly the velocity of the SCS. Meanwhile, the SCB sustained its motion or even accelerated, prompting compressional structures in the study area. This innovative model underscores the impact of compressional stress on the SCS's evolution, thereby providing a contrast to the previously accepted archetype model and offering valuable insights into the evolution of marginal sea basins globally. Key Points: We study the geometry, distribution, and magnitude of end‐Oligocene basin inversion structures at the Beibuwan Basin. Further study shows that compressional structures are pervasive which traditional South China Sea (SCS) evolution models fail to explain, necessitating a new model to elucidate its dynamicsThe hybrid model highlights how extrusion‐induced compressional stress overtakes slab‐pull induced extensional stress as the SCS evolvesThe new model explains major observations in the SCS, such as the southward jump of the spreading ridge and earthquake focal mechanism [ABSTRACT FROM AUTHOR]