A possible mechanism for the initiation of the Yinggehai Basin: A visco-elasto-plastic model

• For a layered viscosity, only a narrow basin forms even with a pre-existing fault.
• The initiation of the Yinggehai Basin does not require the Red River fault zone.
• The initial extension of the basin is NE–SW rather than previously suggested NW–SE.
• The Indochina rotation causes NE–SW extension. The basin initiation is active rifting.
• The Red River fault zone affects the basin evolution after its initiation.

The Yinggehai Basin, lying along the trace of the Red River fault zone in the South China Sea, has been related to the movements of the Red River fault zone and the rotation of the Indochina block. However, the tectonic origin of the Yinggehai Basin remains unclear. This paper explores the initiation of the Yinggehai Basin using a visco-elasto-plastic model. This model concentrates on two main aspects: lateral viscosity variations (mainly controlled by temperature) in the lithosphere and internal friction angles of the pre-existing fault zone. Modeling results show that: (1) for a layered viscosity, en echelon faults would likely form when there is no pre-existing strike–slip fault; whereas, the basin would be narrow and deep, if any, when there is a pre-existing fault; (2) for moderate lateral viscosity variations, a large basin forms, even without a pre-existing fault zone; and (3) for strong lateral viscosity variations, a major rift could form over geologic time. Our results indicate that the initiation of the Yinggehai Basin requires moderate lateral viscosity variations (i.e., a pre-existing gentle upwelling of the Mohorovicic discontinuity) but no pre-existing fault. In addition, the initial extension predicted is NE–SW and is generated by the rotation of the Indochina block. This differs from the NW–SE extension that resulted from the movements of the Red River fault zone. This indicates that the left-lateral displacements of the Red River fault zone and the spreading of the South China Sea only influence the basin evolution after its initiation.