Research paperGeological structures and controls on half-graben inversion in the western Gunsan Basin, Yellow Sea

•Two-stage inversion caused by the Oligocene and Miocene contraction is proposed.•Fault reactivation, fault-related folds, and syninversion faults accommodated inversion stress.•Along-strike variation in the dip angle of bounding faults affected the style of inversion anticline.•Fault reactivation styles are variable both in different inversion phases and along different fault segments.

This paper focuses on the structural styles of an inverted half-graben in the western Gunsan Basin (Cretaceous-Paleogene) in the Yellow Sea. Detailed seismic interpretation calibrated by well data indicates that the half-graben formed by the Cretaceous to Eocene extension and subsequently underwent contraction which led to the basin inversion. The inversion occurred during the Oligocene and Middle Miocene. In the Oligocene inversion phase, the hanging-wall strata underwent shortening by inverted extensional faults, newly-formed reverse faults, and fault-related folds. The inversion deformation was caused by an NNE-directed regional contraction that is nearly normal to the orientation of earlier extensional faults. The style and distribution of internal deformation was mainly governed by variations in the dip angle of the pre-inversion fault plane. The Early Miocene inversion phase commenced with the development of a broad asymmetric hanging-wall anticline caused by the preferential reactivation of the NW-trending bounding fault. This preferential reactivation suggests an orientation change of inversion stress from the Oligocene NNE-directed to the Miocene NE-directed contraction. The bounding fault continued to be reactivated during the Middle Miocene, which caused the growth of a monocline.