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

Abstract

Abstract 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.

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