Tectonic framework, magmatic diapirism, and evolution of the Heshan Sag in the ultra-deepwater Pearl River Mouth Basin

The Heshan Sag is located in the ultradeep-water zone of the Pearl River Mouth Basin, a region of intense crustal extension on the northern continental margin of the South China Sea, which is characterized by low exploration maturity and complex structural features. To clarify its structural characteristics, we conducted detailed structural interpretation using 3D seismic data covering the main area of the sag, integrated with 2D seismic profiles. Delineation of structural units clarifies the tectonic framework, thereby facilitating analysis of the evolutionary characteristics. Result indicates that the Heshan Sag could be subdivided into 6 major subsags based on the nature of boundary faults and morphological characteristics: ① half-graben type, ② detachment type, and ③ graben type. Widespread magmatic activity was observed throughout the area, influenced by three phases of magmatic diapirism: Wenchang, Enping, and Zhuhai, of which the Enping phase exhibited the most extensive distribution and highest intensity. The Heshan Sag exhibits differential tectonic evolutionary stages. The Northern and Northeastern Sub-sags underwent a complete three-phase evolution: rift stage, rift-sag transition stage, and sag stage. In contrast, the Main Subsag and Eastern Subsags, strongly modified by intense magmatic diapirism, experienced a distinct evolution in a sequence comprising the rift stage, magmatic diapir modification stage, and sag stage. Meanwhile, the eastern part of the Southwestern Subsag developed a single-fault rift stage followed by a double-fault graben stage.