Tectonic Transformation, Magmatic Activity and Subsidence Centre Migration of Eocene Half-Grabens: A Case Study of the Northern Pearl River Mouth Basin (PRMB) in the Northern South China Sea

Abstract

The Pearl River Mouth Basin (PRMB) is located in the northern part of the South China Sea. The Palaeogene Wenchang Formation (Fm) was formed at the rift stage and contains the main source rocks. The migration of Wenchang subsidence centres in the western Zhu I Depression and northern Yangjiang-Yitong Fault Zone are controlled by tectonic transformation and partially influenced by magmatic activity. From the Eocene Wenchang (E2WC) to the Eocene and Oligocene Enping (E2+3EP) stages, the regional extension direction rotated clockwise from NW–SE to S–N, and the strike of the regional strike-slip fault was NW–SE. The subsidence centres of the Wenchang Fm in the western subsags of the Zhu I Depression migrated to the Beiweitan Fault in a convergent way. Magmatic activity at the E2WC stage developed mostly along the central edge of the subsags. Local subsidence migrated to the side of the basin-controlling faults. The migration characteristics of the subsidence centre of the Wenchang Fm in each subsag are complex in the northern Yangjiang-Yitong Fault Zone. There was no magmatic activity at the E2WC to E2+3EP stage of the Enping 27 subsag, and the subsidence centre migrated eastwards, which is basically consistent with the migration pattern of the Enping sag. In the eastern Yangjiang sag, the strike of the subsags was ENE. The angle between the extensional direction and subsag strike at the E2WC to E2+3EP stage first increased and then decreased. Magmatic activity at the E2WC stage mostly developed in the subsags. Tectonic transformation and magmatic activity at the E2WC stage led to subsidence centre migration from the Enping 21 subsag to the Enping 20 subsag northwest. From the end of the E2WC stage to the E2+3EP stage, magmatic activity developed at the subsag margins, which resulted in severe denudation. Research on the entire area indicates that tectonic transformation controls subsidence centre migration. Magmatic activity influences the migration of subsidence centres locally or controls this process through tectonic transformation.