The evolution of a gravity-driven system accompanied by diapirism under the control of the prograding West Luconia Deltas in the Kangxi Depression, Southern South China Sea

Abstract

Gravitational collapse structures are commonly observed in shelf-margin deltas underlain by mobile shales. However, these structures are rarely accompanied by mud diapirs. This paper presents an updated study of the gravity-driven system in the West Luconia Deltas, a shelf-margin delta system, in the Kangxi Depression, southern South China Sea. Compared to the classical shale-detachment model, the syn-collapse deformation in the contractional domain in this study is accommodated mainly by thrust faults combined with mud diapirs rather than simply imbricated thrusts. Based on seismic interpretation and structural analysis, this gravity-driven system is divided into three domains, the extensional domain, the contractional domain and the transitional domain. All of these domains are intruded by mud diapirs. The quantitative analysis of the amounts of extension and contraction suggests that these structures mainly resulted from gravitational collapse rather than the tectonic compression. Quantification of the relative contributions of gravity spreading and gliding indicates that the gravitational collapse was mainly driven by gravity spreading. Two episodes of collapse are suggested by the analysis of the progradation of the West Luconia Deltas and the features of the syn-collapse structures. The first episode was minor and not accompanied by diapirism, whereas the second episode was major and accompanied by diapirism. The entire evolution of the GDS is divided into five stages: (1) the first episode of the gravitational collapse, lasted from the earliest Middle Miocene to the earliest Pliocene; (2) the deposition of an interval between the syn-collapse strata and the mobile shale, occurred in the Early Pliocene.; (3) the initiation of the second episode of the gravitational collapse, lasted from the Early Pliocene to the Late Pliocene; (4) the attenuation and basinward migration of the gravity-driven deformation, lasted from the Late Pliocene to the Early Pleistocene; and (5) the ending of the gravitational collapse, lasted from the Early Pleistocene to the present. The last four stages were accompanied by intensive diapirs which pierced the overlying strata and became targets for the hydrocarbon exploration.