Cenozoic basement-involved rifting of the northern South China Sea margin

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The northern South China Sea (SCS) margin developed on a thinning crust comprising basement-involved structures inherited from the post-orogenic extension of the Cathaysia Block. While, the pre-rift structural configuration and its influence on the geometry and development of the Cenozoic rift basins are ambiguous. Based on 3D high-resolution seismic reflection data in the western Pearl River Mouth Basin (PRMB) and onshore fieldwork, we delineated the intrabasement boudinage structure which is constituted by the low-angle E–W-striking Indosinian fold-thrust belts and the steeply dipping NE-striking Yanshanian thrusts. It is concluded that the pre-rift structure played a key role in controlling the architecture and two-phase early rifting of the northern SCS margin. (1) During the Early Eocene syn-rift phase, the Indosinian thrusts were rejuvenated first and controlled the formation of a series of ENE-striking grabens. (2) During the Late Eocene syn-rift phase, both the Indosinian and Yanshanian thrusts were reactivated. The Indosinian thrusts continued to maintain normal faulting and even evolved into the detachment faults. The Yanshanian thrusts were inverted into the NE-striking dextral strike-slip faults, which controlled the formation of the pull-apart basin group en échelon and even the SCS opening. The lithospheric thinning mechanism of the SCS continental margin experienced a major change from the Early Eocene pure shear to the Late Eocene simple shear.